TWI873680B - Hot-rolled steel plate, square steel pipe, manufacturing method thereof and building structure - Google Patents

Abstract

The present invention provides a hot-rolled steel plate with excellent strength and low temperature toughness and low yield ratio. The hot-rolled steel plate has a predetermined composition, and the steel structure in the center of the plate thickness has: granular iron as the main phase, and as the second phase, the total area ratio of 6-25% of the pulex and pseudo-pulex, and the area ratio of less than 5% of the upper ductile iron. When the area surrounded by the boundary with the orientation difference of adjacent crystals of more than 15° is regarded as the crystal grain, At the center of the thickness, the average grain size of the steel structure including the main phase and the secondary phase is 10.0~30.0μm, the area ratio of the grains with a grain size within ±5.0μm of this average grain size is more than 35%, and the number density of the grains with a ratio of major diameter to minor diameter (major diameter/minor diameter) of more than 3.0 is less than 30/ ^mm2 .

Description

Hot-rolled steel plate, square steel pipe, manufacturing method thereof and building structure

The present invention relates to a hot-rolled steel plate with a low yield ratio used for a square steel pipe, and a square steel pipe (square column) with a low yield ratio and low temperature toughness manufactured by using the hot-rolled steel plate as a material through roll forming in a cold room (i.e., at room temperature), and a manufacturing method thereof. In particular, the present invention also relates to a square steel pipe that is very suitable for use as a building structural member of a large building. In addition, the present invention also relates to a building structure using the square steel pipe.

In recent years, building structural members used in large buildings such as factories, warehouses, and commercial facilities (hereinafter referred to as "buildings") have been increasingly strengthened in order to reduce construction costs by reducing weight. In particular, square steel pipes (square columns) with flat plates and corners used as building columns are required to have high strength in the flat plate and excellent toughness in addition to earthquake resistance.

Square steel pipes are generally made of hot-rolled steel plates (hot-rolled steel strips) or thick steel plates, which are formed in the cold room (i.e., at room temperature, hereinafter referred to as the cold room). The forming methods in the cold room include: cold-pressing bending forming method or cold-pressing rolling forming method.

Square steel pipes manufactured by press-bending materials (hereinafter sometimes referred to as "press-formed square steel pipes") are manufactured by first press-bending thick steel plates in the cold mill to form semi-finished products with a cross-sectional shape of a R (square) or U (U) shape, and then joining these semi-finished products together using arc welding.

In addition, the square steel pipe manufactured by roll forming the material (hereinafter, sometimes referred to as "roll-formed square steel pipe") is firstly roll-formed the hot rolled steel plate in the cold room into a cylindrical unsealed tube, and then the butted parts are welded together by electric welding to make a round steel pipe. Then, the cylindrical round steel pipe is reduced by several % along the tube axis direction by rollers arranged above, below, left and right of the round steel pipe, and then formed into a square (angular) shape to make a square steel pipe.

Compared with the manufacturing method of square steel pipes by stamping, the manufacturing method of square steel pipes by roll forming has the advantages of higher productivity and can be manufactured in a shorter time. However, the square steel pipes by stamping do not perform cold forming processing on the flat part, but only on the corners, so only the corners will have work hardening phenomenon. In contrast, the square steel pipes by roll forming, especially when they are cold formed into a cylindrical shape in the front process of forming into square steel pipes, introduce a large processing deformation along the tube axis direction on the entire circumference of the steel pipe. Therefore, the square steel tube formed by roll forming has a relatively high yield ratio in the tube axis direction not only at the corners but also at the flat plate, resulting in a problem of low toughness.

In addition, the thicker the wall thickness of the roll-formed square steel tube, the greater the work hardening during roll-forming, resulting in a higher yield ratio and lower toughness. Therefore, especially when manufacturing roll-formed square steel tubes with thicker wall thickness, it is necessary to select materials that can withstand the increase in yield ratio and the decrease in toughness caused by roll-forming.

In view of this requirement, for example, the square steel pipe of the technical solution disclosed in Patent Document 1 is a steel material having a C content of less than 0.20% by weight, and containing one or two of the following: a Mn content of 0.40-0.90%, a Nb content of 0.005-0.040%, and a Ti content of 0.005-0.050%. The steel material is subjected to a hot rolling process in a non-recrystallization temperature range, with a reduction ratio of more than 55%, a rolling end temperature of 730-830°C, and a coiling temperature of less than 550°C to make a hot rolled steel strip coil. The steel strip is then hot rolled to form a hot rolled steel strip coil. After the coil is formed and welded to form an electric-sewn steel pipe, when it is formed into a square steel pipe by cold working, the reduction in the outer circumference in the steel pipe forming process is set to less than 3 times the thickness of the steel plate, so as to manufacture a square steel pipe with a yield ratio of less than 90% and an impact absorption energy of more than 27J in the summer impact test at a test temperature of 0℃.

The square steel pipe of the technical solution disclosed in Patent Document 2 is a steel having a C content of 0.07-0.18% and a Mn content of 0.3-1.5% by mass, which is heated to a temperature of 1100-1300°C, and then subjected to rough rolling with a rough rolling end temperature of 1150-950°C and a finish rolling start temperature of 1100-850°C and a finish rolling end temperature of 900-750°C, and then subjected to: a primary cooling treatment with a cooling stop temperature of 550°C or above based on the surface temperature, a secondary cooling treatment of 3-15 seconds in air, and a temperature range of 750-650°C based on the temperature of the center of the plate thickness. The steel sheet is subjected to three cooling processes, wherein the average cooling rate is 4-15°C/second, and the steel sheet is cooled to below 650°C for three times, so as to control the value of the frequency of the second phase in the steel structure within the range of 0.20-0.42, thereby manufacturing a thick hot-rolled steel sheet, and then the thick hot-rolled steel sheet is subjected to cold forming to manufacture a square steel tube having a low yield ratio of less than 80% and a mechanical property of having an impact absorption energy of more than 150J in a Charpy impact test at a test temperature of 0°C.

The square steel pipe of the technical solution disclosed in Patent Document 3 is a steel having a C content of 0.07-0.18% and a Mn content of 0.3-1.5% by mass, which is heated to a temperature of 1100-1300°C, and then subjected to rough rolling at a rough rolling end temperature of 1150-950°C and a finish rolling at a finish rolling start temperature of 1100-850°C and a finish rolling end temperature of 900-750°C, and then cooled at an average cooling rate of 20°C/sec in a temperature range of 750-650°C based on the surface temperature. The time taken for the temperature of the center of the plate thickness to reach 650°C is within 35 seconds and the average cooling rate of the center of the plate thickness in the temperature range of 750~650°C is 4~15°C/second, so as to cool to a coiling temperature of 500~650°C to make a thick hot-rolled steel plate, and the thick hot-rolled steel plate is used as a material for cold forming to produce a square steel pipe with a low yield ratio of less than 80% and a mechanical property of an impact absorption energy of more than 150J in a Charpy impact test at a test temperature of 0°C.

The square steel pipe disclosed in the technical solution of Patent Document 4 is a steel having a composition, by mass%, of 0.07-0.20% C, 0.3-2.0% Mn, 0.03% or less P, 0.015% or less S, 0.01-0.06% Al, 0.006% or less N, and the remainder being Fe and unavoidable impurities, heated to 110 After the temperature of 0~1300℃, the rough rolling with the end temperature of rough rolling at 1150~950℃ and the finishing rolling with the start temperature of finishing rolling at 1100~850℃ and the end temperature of finishing rolling at 900~750℃ were implemented, and then the average cooling rate from the start of cooling to the end of cooling was 4~25℃/second based on the center temperature of the plate thickness until the cooling stop temperature was below 580℃, and In the initial cooling process of 10 seconds from the start of cooling, there is one or more cooling process of 0.2 seconds or more and less than 3.0 seconds, and then, after coiling at a coiling temperature of 580°C or less, by cooling, the steel structure in the center of the plate thickness has: a main phase composed of granular iron and a surface composed of one or more selected from pulex, pseudo-pulex and upper tantalum. A square steel pipe having a secondary phase with an area ratio of 8 to 20%, and a steel structure containing a primary phase and a secondary phase with an average grain size of 7 to 20 μm, a steel structure on the surface and bottom of the plate thickness being a single phase of granulated iron or a single phase of ductile granulated iron, and an average grain size of 2 to 20 μm, having a low yield ratio of less than 90%, and having mechanical properties of an impact absorption energy of more than 27 J in a Charpy impact test at a test temperature of 0°C. [Prior technical literature] [Patent literature]

Patent document 1: Japanese Patent Publication No. 9-87743 Patent document 2: Japanese Patent Publication No. 2012-153963 Patent document 3: Japanese Patent Publication No. 2012-132088 Patent document 4: International Publication No. 2018/110152

[The problem the invention is trying to solve]

As mentioned above, the larger the tube wall thickness or the smaller the side length of the square steel tube formed by roll forming, the greater the processing deformation introduced into the square steel tube, the greater the yield ratio increase and the degree of toughness reduction. Therefore, the hot-rolled steel plate used as the material is required to have a steel structure that can suppress the yield ratio increase during forming and excellent low-temperature toughness that can withstand the toughness deterioration caused by large processing deformation.

However, the square steel tube manufactured by the method disclosed in the aforementioned patent documents 1-3, especially when the tube wall thickness exceeds 25 mm, will have a too high yield ratio, and will fail to meet the technical problem of a yield ratio of less than 90%.

Furthermore, if one wishes to obtain a low yield ratio and high toughness by using the technology disclosed in Patent Document 4, the steel structure on the surface and bottom of the plate thickness must be changed to a single phase of granulated iron or a single phase of tough granulated iron. In order to obtain such a steel structure, a cooling process must be provided in the cooling process. In other words, an additional process must be added, and there is a technical problem that the manufacturing process becomes complicated.

The present invention has been developed in view of the above technical problems, and its purpose is to provide: a hot-rolled steel plate that can be used in square steel tubes, which has high yield strength and tensile strength, low yield ratio, excellent low-temperature toughness and work hardening properties in the tube axial direction and the tube circumferential direction; a square steel tube using the hot-rolled steel plate and a manufacturing method thereof; and a building structure using the square steel tube.

The statement in this specification that the hot-rolled steel sheet of the present invention has the following characteristics: (1) a very low yield ratio, (2) a very high yield strength, and (3) a very high tensile strength. The result of a tensile test in accordance with the provisions of Japanese Industrial Standard JIS Z 2241 (2011) using a tensile test piece in accordance with Japanese Industrial Standard JIS No. 5 taken in a manner in which the tensile direction is kept parallel to the rolling direction shows, in order, that (1) the yield ratio is less than 0.75, (2) the yield strength is more than 250 MPa, and (3) the tensile strength is more than 400 MPa.

In addition, the so-called "work hardening" is an indicator used to evaluate the uniform elongation (plastic elongation at the highest load point of the tensile test), which means that the work hardening index n _3-7 specified in the Japanese industrial standard JIS Z 2253 (2011) reaches 0.20 or more. In other words, if the work hardening index n _3-7 of the hot-rolled steel plate does not reach 0.20, when manufacturing square steel pipes, the uniform elongation of the flat plate of the square steel pipe will decrease and the seismic resistance will be reduced. Sometimes, the yield ratio of the flat plate of the square steel pipe will exceed 0.90.

In addition, the so-called "excellent low-temperature toughness" means: in accordance with the provisions of Japanese Industrial Standards JIS Z 2242 (2018), a V-groove standard test piece is taken from the t/2 position of the plate thickness t (the center of the plate thickness) in a way that the long side direction of the test piece is parallel to the rolling direction. The Charpy impact test is carried out at test temperatures of -80℃, -60℃, -40℃, -20℃, and 0℃. The impact absorption energy of the Charpy impact test at -20℃ is more than 100J and the ductile-brittle transition temperature is below -20℃.

In addition, the "square steel pipe with low yield ratio" referred to in the present invention means: using a Japanese Industrial Standard JIS No. 5 tensile test piece taken in a manner that the tensile direction is parallel to the tube axis direction, the yield strength of the flat plate portion is 295MPa or more, the tensile strength of the flat plate portion is 400MPa or more, the yield ratio of the flat plate portion is 0.90 or less, the uniform elongation of the flat plate portion is 5.0% or more, and using a V-groove standard test piece taken from the flat plate portion of the square steel pipe at a position of 1/4t of the thickness t calculated from the outer surface of the steel pipe in a manner that the long side direction of the test piece is parallel to the tube axis direction, and the result of the tensile test in accordance with the provisions of Japanese Industrial Standard JIS Z 2241 (2011) According to the provisions of 2242(2018), the results of the Charpy impact test at test temperatures of -60℃, -40℃, -20℃ and 0℃ are as follows: the impact absorption energy of the flat plate portion in the Charpy impact test at -20℃ in the tube axis direction is 60J or more, and the ductile-brittle transition temperature of the flat plate portion is below -10℃.

In addition, the "more excellent square steel pipe with low yield ratio" referred to in the present invention refers to: in accordance with the provisions of Japanese Industrial Standard JIS Z 2242 (2018), a V-groove standard test piece is taken at the t/4 position of the thickness t from the outer surface of the steel pipe, with the long side direction of the test piece parallel to the circumferential direction of the pipe, and the impact absorption energy of the Charpy impact test at -20°C in the axial direction and circumferential direction of the flat plate is measured respectively, and the ratio P of the impact absorption energy of the Charpy impact test at -20°C in the circumferential direction of the pipe to the axial direction of the pipe is 0.5~1.2. Here P = (impact absorption energy of Charpy impact test at -20℃ in the circumferential direction of the tube) / (impact absorption energy of Charpy impact test at -20℃ in the axial direction of the tube) [Technical means to solve the problem]

The inventors of the present invention have made continuous efforts to solve the above technical problems and have obtained the following (1) to (3) findings.

Original idea (1): In order for the hot-rolled steel plate to meet the yield strength and tensile strength desired by the present invention, the C content must be set to 0.07 mass % or more, and the main phase of the hot-rolled steel plate at the center of the plate thickness must be granular iron.

Innovation (2) In order for a hot-rolled steel sheet to obtain the desired low-temperature toughness and yield ratio of the present invention, in addition to the main phase described in the above innovation (1), the following necessary conditions are required: a second phase consisting of one or more of Porphyry, Porphyry and upper tantalum is present at the center of the sheet thickness, the total area ratio of Porphyry and Porphyry is 6-25%, and the area ratio of upper tantalum is less than 5%, and when the area surrounded by the boundary with the orientation difference of adjacent crystals of 15° or more at the center of the sheet thickness is regarded as a crystal grain, the average crystal grain size of the crystal grain including the main phase and the second phase is 10.0-30.0 μm, and the average crystal grain size of the crystal grain having the above-mentioned crystal grain is 10.0-30.0 μm. The area ratio of crystal grains with a grain size within ±5.0 μm is 35% or more, and the number density of crystal grains with a ratio of major diameter to minor diameter (= (major diameter)/(minor diameter)) of 3.0 or more is 30/ ^mm2 or less.

Innovation (3) In order to obtain the steel structure described in the above innovations (1) and (2), in addition to adjusting the composition to an appropriate range, it is also necessary to control the content of Mn and Si within a specific range, and to start finish rolling only after a predetermined time has passed after the rough rolling in the hot rolling process is completed, and to maintain the steel within a predetermined temperature range for a predetermined time after coiling.

The present invention is developed based on the above-mentioned insights and further examined. In other words, the gist of the present invention is as follows. 1. A hot-rolled steel plate, wherein the composition, in mass%, comprises C: 0.07% to 0.20%, Si: 0.40% to 1.00%, Mn: 0.20% to 1.00%, P: 0.100% to 0.050%, Al: 0.005% to 0.100%, and N: 0.0100%, the remainder being Fe and inevitable impurities, and the contents of Mn and Si satisfy the relationship of the following formula (1); the steel structure at the center of the plate thickness comprises: granular iron as a main phase, and pulex and pseudo-pulex as a secondary phase with a total area ratio of 6 to 25%, and upper ductile iron with an area ratio of less than 5%, In the steel structure at the center of the plate thickness, when the area surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is regarded as crystal grains, the average crystal grain size of such crystal grains is 10.0 to 30.0 μm, the area ratio of crystal grains having a crystal grain size within ±5.0 μm of the average crystal grain size of the aforementioned crystal grains is 35% or more, and the number density of crystal grains having a ratio of major diameter to minor diameter (major diameter/minor diameter) of 3.0 or more among the aforementioned crystal grains is 30 grains/ ^mm2 or less,・・・Formula (1) In formula (1), %Mn and %Si represent the content (mass %) of each element in the steel plate.

2. A hot-rolled steel plate as described in 1 above, wherein the above-mentioned components, in terms of mass%, also contain one or more selected from the following elements: Nb: 0.005% or more and 0.020% or less, Ti: 0.005% or more and 0.020% or less, V: 0.01% or more and 0.10% or less, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 0.30% or less, Ni: 0.01% or more and 0.30% or less, Ca: 0.0005% or more and 0.0100% or less, and B: 0.0003% or more and 0.0100% or less.

3. A hot-rolled steel plate as described in 1 or 2 above, wherein the plate thickness is greater than 12 mm.

4. A method for manufacturing a hot rolled steel plate, comprising heating a steel material having the composition described in 1 or 2 above to a temperature of 1100°C or higher and 1300°C or lower; then hot rolling, wherein the hot rolling is firstly rough rolling with a final temperature of 850°C or higher and 1150°C or lower, and after more than 15 seconds after the rough rolling is completed, finishing rolling is started, and the final finishing temperature is set at 750°C or higher and 850°C or lower, and the total reduction ratio below 930°C in the entire hot rolling process is set at 40% or higher and 59% or lower; Next, the raw steel plate obtained by the hot rolling is cooled under the condition that the average cooling rate Vc (°C/second) at the center of the plate thickness satisfies the relationship of the following formula (2), and the cooling stop temperature at the center of the plate thickness is 550°C or more and 680°C or less; Next, the raw steel plate is coiled under the condition that the temperature at the center of the plate thickness is 550°C or more and 680°C or less; Next, the coiled steel plate obtained by the coiling is subjected to a second cooling, which is to retain the coiled steel plate in a temperature range of 400°C to 300°C for 1.0 hour or more and 10.0 hours or less, ……Formula (2).

5. A square steel pipe, which is made of the hot-rolled steel plate described in any one of 1 to 3 above.

6. A square steel pipe as described in 5 above, wherein the ratio P of the impact absorption energy of the Charpy impact test in the circumferential direction of the square steel pipe to the impact absorption energy of the Charpy impact test in the axial direction at -20°C is within the range of 0.5 to 1.2, where P = (impact absorption energy of the Charpy impact test in the circumferential direction at -20°C) / (impact absorption energy of the Charpy impact test in the axial direction at -20°C).

7. A method for manufacturing a square steel pipe, comprising: performing a roll forming process in a cold room on the hot rolled steel plate obtained by the method for manufacturing a hot rolled steel plate described in 4 above to make a square steel pipe.

8. A building structure having the square steel pipes described in 5 or 6 above as column materials. [Effects of the invention]

According to the present invention, it is possible to provide a technology for obtaining a hot-rolled steel plate having high yield strength and tensile strength, low yield ratio, excellent low-temperature toughness and work hardening properties, which can be used in square steel pipes with low yield ratios. In addition, a building structure using the square steel pipe of the present invention as a column material can obtain better earthquake resistance than a building structure using a square steel pipe manufactured by traditional cold forming.

The present invention will be described below. <Hot-rolled steel plate with low yield ratio> The composition of the hot-rolled steel plate with low yield ratio (hereinafter also referred to as "hot-rolled steel plate") used for the square steel pipe with low yield ratio (hereinafter also referred to as "square steel pipe") of the present invention contains, by mass%, C: not less than 0.07% and not more than 0.20%, Si: not more than 0.40%, Mn: not less than 0.20% and not more than 1.00%, P: not more than 0.100%, S: not more than 0.050%, Al: not less than 0.005% and not more than 0.100%, and N: not more than 0.0100%, and the rest is Fe and unavoidable impurities, and the contents of Mn and Si satisfy the relationship of the following formula (1). In addition, the steel structure of the hot-rolled steel plate at the center of the plate thickness includes: granular iron as the main phase, and pulex and pseudo-pulex as the second phase with a total area ratio of 6-25%, and upper tantalum with an area ratio of less than 5%. Furthermore, the hot-rolled steel plate is characterized in that: in the steel structure at the center of the plate thickness, when the region surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is regarded as crystal grains, the average crystal grain size of such crystal grains is 10.0 to 30.0 μm, the area ratio of crystal grains having a crystal grain size within ±5.0 μm of the average crystal grain size of the crystal grains is 35% or more, and the number density of crystal grains having a ratio of major diameter to minor diameter (major diameter/minor diameter) of 3.0 or more among the crystal grains is 30 grains/ ^mm2 or less, ...Formula (1), %Mn, %Si in formula (1) represent the content (mass %) of each element in the steel plate. In addition, the "hot-rolled steel plate" of the present invention includes: hot-rolled steel plate and hot-rolled steel strip.

The following will describe the composition of the hot-rolled steel sheet of the present invention. In addition, unless otherwise stated, the "%" indicating the composition of the steel is "mass %".

C: 0.07% or more and 0.20% or less C is an element that can improve the strength of steel by solid solution strengthening. Furthermore, C is an element that helps to form pulex and pseudo-pulex in the second phase. In order to ensure the desired strength and yield ratio of the present invention, the C content must be set to 0.07% or more. On the other hand, if the C content exceeds 0.20%, the ratio of the hard phase is too high, so not only the toughness will be reduced, but the yield ratio will also exceed 0.90, and the desired yield ratio cannot be obtained. In addition, weldability will also deteriorate. Therefore, the C content is set to 0.07% or more and 0.20% or less. The C content is preferably set to 0.08% or more. In addition, the C content is preferably set to 0.18% or less, and it is better to set it to 0.17% or less.

Si: 0.40% or less Si is an element that can improve the strength of steel by solid solution strengthening. Although the lower limit of Si is not specifically specified (usually more than 0%), in order to obtain this effect, it is advisable to set the Si content to more than 0.01%. The Si content is more preferably set to more than 0.05%. On the other hand, if the Si content exceeds 0.40%, oxides are easily generated in the electric welding part, and the characteristics of the welding part will deteriorate. In addition, the toughness of the base material part other than the electric welding part will also deteriorate. Therefore, the Si content is set to less than 0.40%. It is better to set it to less than 0.37%, and more preferably to set it to less than 0.35%.

Mn: 0.20% or more and 1.00% or less Mn is an element that can improve the strength of steel by solid solution strengthening. In addition, Mn is an element that can reduce the temperature at which ferrous iron begins to transform and contribute to the refinement of the structure. In order to ensure the strength and structure desired by the present invention, the Mn content must be set to 0.20% or more. On the other hand, if the Mn content exceeds 1.00%, too much ductile iron is generated, so the yield ratio will exceed 0.90, and the desired yield ratio cannot be obtained. In addition, if the Mn content exceeds 1.00%, the hardness of the center segregation portion will increase, which may become the cause of cracks during welding. Therefore, the Mn content is set to 0.20% or more and 1.00% or less. The Mn content is preferably set at 0.25% or more, and more preferably at 0.30% or more. Furthermore, the Mn content is preferably set at 0.95% or less, and more preferably at 0.90% or less.

P: 0.100% or less P will segregate at the grain boundaries and cause material inhomogeneity, so it is better to reduce the P content as much as possible, but the allowable content is up to 0.100%. Therefore, the P content is set below 0.100%. The P content is better to be set below 0.030%, and more preferably below 0.020%. Although the lower limit of the P content is not specifically specified (usually more than 0%), if it is reduced too much, it will lead to an increase in the manufacturing cost, so it is better to set the P content above 0.002%.

S: 0.050% or less S usually exists in the form of MnS in steel. MnS will be stretched to a very thin layer during the hot rolling process, which will have an adverse effect on ductility. Therefore, in the present invention, although it is advisable to reduce the S content as much as possible, the allowable content is up to 0.050%. Therefore, the S content is set below 0.050%. The S content is better to be set below 0.015%, more preferably below 0.010%, and most preferably below 0.008%. In addition, although the lower limit of the S content is not specifically specified (usually more than 0%), if it is reduced too much, it will lead to an increase in the manufacturing cost, so it is advisable to set the S content above 0.0002%.

Al: 0.005% or more and 0.100% or less Al is an element that can act as a strong deoxidizer. In order to obtain this effect, the Al content must be set to 0.005% or more. On the other hand, if the Al content exceeds 0.100%, the weldability will deteriorate, and the alumina inclusions will become too much, and the surface properties will deteriorate. In addition, the toughness of the weld will also decrease. Therefore, the Al content is set to 0.005% or more and 0.100% or less. The Al content is preferably set to 0.010% or more, and more preferably to 0.015% or more. In addition, the Al content is preferably set to 0.070% or less, and more preferably to 0.050% or less.

N: 0.0100% or less N is an element that can firmly fix the movement of dislocations and has the effect of reducing toughness. In the present invention, it is advisable to reduce the N content as much as possible. However, the allowable content of N is limited to 0.0100%. Therefore, the N content is set to less than 0.0100%. The N content is better to be set to less than 0.0080%, more preferably to be set to less than 0.0040%, and most preferably to be set to less than 0.0035%. In addition, although the lower limit of the N content is not specifically specified (usually more than 0%), if it is reduced too much, it will lead to an increase in the manufacturing cost, so it is advisable to set the N content to more than 0.0010%, and more preferably to more than 0.0015%.

1.0≦%Mn/%Si≦3.5 In addition, %Mn and %Si in the above relationship represent the content (mass %) of each element in the steel plate. The present invention sets the content of Mn and Si within the above range, and must also meet the relationship of 1.0≦%Mn/%Si≦3.5. By setting the content of Mn and Si to meet the conditions of this relationship, a steel structure having the second phase described below, in which the area ratio of Pole iron and/or pseudo-Pole iron is 6-25% and the area ratio of upper ductile iron is less than 5%, can be obtained, so that the strength, yield ratio, impact absorption energy of the Charpy impact test, and ductile-brittle transition temperature desired by the present invention can be obtained. Therefore, the ratio of %Mn/%Si is preferably above 1.2, and more preferably above 1.4. Moreover, the ratio of %Mn/%Si is preferably below 3.2, and more preferably below 3.0.

The remainder of the above components is Fe and inevitable impurities. However, it is not excluded to contain 0.005% or less of O as long as it does not impair the effect of the present invention. In addition, this O refers to the total oxygen content including O as an oxide.

In the present invention, the elements added according to the requirements described below, namely, Nb, Ti, V, Cr, Mo, Cu, Ni, Ca and B, are regarded as unavoidable impurities if they are within the ranges of Nb: less than 0.005%, Ti: less than 0.005%, V: less than 0.01%, Cr: less than 0.01%, Mo: less than 0.01%, Cu: less than 0.01%, Ni: less than 0.01%, Ca: less than 0.0005% and B: less than 0.0003%.

The above components are the basic components of the hot-rolled steel sheet of the present invention. Although the desired properties of the present invention can be obtained according to the above components, the components may further contain the following elements as necessary.

Specifically, it is one or more selected from Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, V: 0.01% to 0.10%, Cr: 0.01% to 0.50%, Mo: 0.01% to 0.50%, Cu: 0.01% to 0.30%, Ni: 0.01% to 0.30%, Ca: 0.0005% to 0.0100%, and B: 0.0003% to 0.0100%.

Nb: 0.005% or more and 0.020% or less, Ti: 0.005% or more and 0.020% or less Nb and Ti form fine carbides and nitrides in steel and are elements that can help improve the strength of steel through precipitation strengthening. In order to obtain this effect, if Nb is contained, it must be set to 0.005% or more. If Ti is contained, it must be set to 0.005% or more. On the other hand, if the content of Nb and Ti exceeds 0.020% respectively, coarse carbides and nitrides will be formed, which may lead to a decrease in toughness. Therefore, if you want to contain Nb, set it to a range of 0.020% or less, and if you want to contain Ti, set it to a range of 0.020% or less. Regarding the content of each Nb and Ti, it is better to set it at 0.007% or more, and it is more preferably set at 0.009% or more. In addition, regarding the content of each Nb and Ti, it is better to set it at 0.018% or less, and it is more preferably set at 0.016% or less.

V: 0.01% or more and 0.10% or less, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less V, Cr, and Mo are elements that can improve the quenching hardenability and strength of steel, and can be contained as needed. If V, Cr, and Mo are contained to obtain the above effects, it is preferable to set the contents to V: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, respectively. It is better to set the contents to V: 0.02% or more, Cr: 0.10% or more, and Mo: 0.10% or more, respectively. On the other hand, if contained excessively, there is a concern that toughness may be reduced and weldability may be deteriorated. Therefore, if you want to contain V, Cr, and Mo, set the content to less than 0.10% for V, less than 0.50% for Cr, and less than 0.50% for Mo. It is better to set the content to less than 0.08% for V, less than 0.40% for Cr, and less than 0.40% for Mo.

Cu: 0.01% or more and 0.30% or less, Ni: 0.01% or more and 0.30% or less Cu and Ni are elements that can improve the strength of steel by solid solution strengthening, and can be contained as needed. If Cu and Ni are contained to obtain the above effects, it is preferable to set the content to Cu: 0.01% or more and Ni: 0.01% or more, respectively. It is better to set the content to Cu: 0.10% or more and Ni: 0.10% or more, respectively. On the other hand, excessive inclusion may lead to reduced toughness and poor weldability. Therefore, if you want to contain Cu and Ni, set the content to Cu: 0.30% or less and Ni: 0.30% or less, respectively. It is better to set the content to Cu: 0.20% or less and Ni: 0.20% or less, respectively.

Ca: 0.0005% or more and 0.0100% or less Ca is an element that helps improve the toughness of steel by spheroidizing sulfides such as MnS that are stretched to a very thin layer during the hot rolling process, and can be contained as needed. If Ca is contained to achieve this effect, it is preferable to set the content to 0.0005% or more. It is better to set the Ca content to 0.0010% or more. On the other hand, if the Ca content exceeds 0.0100%, clusters of Ca oxides may be formed in the steel, resulting in poor toughness. Therefore, if you want to contain Ca, set the Ca content to 0.0100% or less. It is better to set the Ca content to 0.0050% or less.

B: 0.0003% or more and 0.0100% or less B is an element that can reduce the temperature at which granular iron begins to transform and contribute to the refinement of the structure. If B is contained in order to obtain this effect, it is preferable to set the content to 0.0003% or more. A better B content is set to 0.0005% or more. On the other hand, if the B content exceeds 0.0100%, it may sometimes lead to an increase in the yield ratio. Therefore, if you want to contain B, set the B content to 0.0100% or less. It is better to set the B content to 0.0050% or less.

Next, the reason for limiting the steel structure of the hot-rolled steel plate of the present invention is explained. The steel structure of the hot-rolled steel plate of the present invention at the center of the plate thickness is composed of: granular iron as the main phase, and pulex and pseudo-pulex as the second phase, with a total area ratio of 6 to 25%, and upper ductile iron with an area ratio of less than 5%. In the steel structure at the center of the plate thickness, the region surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is regarded as the crystal structure. When the crystal grains are formed, the average crystal grain size of the crystal grains is 10.0 to 30.0 μm, the area ratio of the crystal grains having a crystal grain size within ±5.0 μm of the average crystal grain size of the crystal grains is 35% or more, and the number density of the crystal grains having a ratio of the major diameter to the minor diameter (major diameter/minor diameter) of 3.0 or more among the crystal grains is 30 grains/ ^mm2 or less. In addition, the "crystal grain size" referred to in the present invention refers to the diameter of a circle having the same area as the crystal grains considered as the object (circle equivalent diameter).

Granulated iron (main phase) Granulated iron is a soft structure. In order to obtain the desired yield strength and low yield ratio, the main phase in the present invention is granulated iron. In addition, the so-called "main phase" refers to: the area ratio is more than 50%. If the area ratio of granulated iron is less than 50%, sometimes the yield stress will become too large, and the work hardening index will become too small, and the desired yield ratio cannot be obtained. In addition, based on the above-mentioned yield stress and yield ratio viewpoints, the area ratio of granulated iron is better to be more than 70%, and more preferably more than 72%. On the other hand, if the area ratio of granulated iron exceeds 94%, sometimes the strength will be reduced and the desired yield strength and tensile strength cannot be obtained. Therefore, the area ratio of granulated iron is set below 94%, and it is better to set the area ratio of granulated iron below 92%.

Total area ratio of Porphyry and Pseudo-Porphyry: 6~25%, area ratio of upper toughened iron: less than 5% (second phase) Porphyry and Pseudo-Porphyry are hard structures and are the most important steel structures for increasing the strength of steel and obtaining a low yield ratio. In order to obtain the desired yield strength, tensile strength, and yield ratio of the present invention, the total area ratio of Porphyry and Pseudo-Porphyry must be set to 6% or more. It is better to set it to 7% or more, and more preferably to set it to 9% or more. On the other hand, if the total area ratio of Porphyry and Pseudo-Porphyry exceeds 25%, toughness may sometimes deteriorate. Therefore, the total area ratio of pulverized iron and pseudo-pulverized iron must be set below 25%. It is better to set it below 23%, and more preferably below 21%. In addition, the area ratio of the pseudo-pulverized iron is preferably 5% or more. If the area ratio of the pseudo-pulverized iron is 5% or more, the yield ratio can be controlled to be lower when manufacturing square steel pipes, so better earthquake resistance can be obtained. On the other hand, in order to make the area ratio of the pseudo-pulverized iron exceed 15%, the temperature range of the generated pulverized iron must be rapidly cooled in the cooling process during the hot rolling process, and its manufacturing conditions are limited. Therefore, the area ratio of the above-mentioned pseudo-Bolai iron should be less than 15%.

In addition, the upper tantalum is a hard structure with intermediate properties between ferrous iron and pulverized iron, which can improve the strength of steel. However, if the area ratio of the upper tantalum exceeds 5%, the low yield ratio desired by the present invention cannot be obtained. Therefore, the area ratio of the upper tantalum must be set below 5%. It is better to set it below 4%. The upper tantalum can also be 0%. In addition, in the present invention, the structure other than the aforementioned main phase and the aforementioned second phase is austenite and matterite.

The area ratios of granular iron, pulex iron, pseudo-pulex iron, and upper ferrite iron can be measured by the method described below.

In the steel structure at the center of the plate thickness, when the area surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is regarded as crystallized grains, the average crystallized grain size of such crystallized grains is 10.0~30.0μm, the area ratio of crystallized grains having a crystallized grain size within ±5.0μm of such average crystallized grain size of the aforementioned crystallized grains is 35% or more, and the number density of crystallized grains having a ratio of major diameter to minor diameter (major diameter/minor diameter) of 3.0 or more among the aforementioned crystallized grains is 30/ ^mm2 or less. The steel structure of the present invention is a steel in which a soft structure and a hard structure are mixed together (hereinafter referred to as "composite structure steel") in order to obtain the low yield ratio, yield strength, and tensile strength desired by the present invention. However, such a composite structure steel has poorer toughness than a single structure steel. Therefore, in order to have both the above-mentioned mechanical properties and excellent toughness, the present invention specifies the conditions for the grain size of the steel structure containing the main phase and the secondary phase at the center of the plate thickness of the steel plate, the area ratio of the coarse grains, and the number density of the elongated grains when the area surrounded by the boundary where the orientation difference of the adjacent crystals is 15° or more is regarded as the grains.

If the average grain size (average equivalent circular diameter) of the aforementioned crystal grains is less than 10.0 μm, the yield ratio will increase and the desired yield ratio of the present invention cannot be obtained. On the other hand, if the aforementioned average grain size exceeds 30.0 μm, the toughness will deteriorate. Therefore, the average grain size of the steel structure containing the main phase and the secondary phase must fall within the range of 10.0~30.0 μm. The above-mentioned average grain size is preferably greater than 11.0 μm, and more preferably greater than 12.5 μm. Furthermore, the above-mentioned average grain size is better to be less than 28.0 μm, and more preferably less than 26.0 μm.

During the review process conducted by the inventors, it was found that even if the average crystal grain size falls within the range of 10.0~30.0μm, sometimes the yield ratio and impact absorption energy of the Charpy impact test desired by the present invention cannot be obtained. Therefore, as a result of further review by the inventors, it was found that in order to obtain the toughness and yield ratio desired by the present invention, the area ratio of crystal grains having a crystal grain size within ±5.0μm of the above-mentioned average crystal grain size and the number density of the crystal grains after elongation are extremely important. Specifically, the center of the thickness of the steel plate must have a structure in which the above-mentioned area ratio accounts for more than 35%, the ratio of the major diameter to the minor diameter, i.e., the ratio of (major diameter)/(minor diameter) is more than 3.0, and the number density of crystal grains is less than 30/ ^mm2 .

The crystal orientation difference, the average crystal grain size, and the area ratio of the crystal grains with a crystal grain size within ±5.0 μm of the average crystal grain size can be measured using a scanning electron microscope/backscattered electron diffraction method (SEM/EBSD method). In addition, the present invention can be measured using the method described below.

By having the above-mentioned components and steel structure, a hot-rolled steel plate having the desired strength, yield ratio and toughness (impact absorption energy of the Charpy impact test at -20°C, ductility-brittle transition temperature) of the present invention can be obtained. That is, the hot-rolled steel plate according to the present invention can achieve the following characteristics: yield strength of 250MPa or more, tensile strength of 400MPa or more, yield ratio of 0.75 or less, work hardening index of 0.20 or more when plastic deformation is 3-7%, impact absorption energy of 100J or more in the Charpy impact test at -20°C, and ductility-brittle transition temperature of -20°C or less. And by using such a hot-rolled steel plate, a square steel pipe described later can be made. In addition, the thickness of the hot-rolled steel plate of the present invention is preferably above 12 mm, and it is better to set the thickness in the range of 12 to 32 mm.

Next, a method for manufacturing a hot-rolled steel plate according to one embodiment of the present invention is described as the method for manufacturing a hot-rolled steel plate of the present invention. The method for manufacturing a hot-rolled steel plate of the present invention includes, for example, firstly heating a steel material having the above-mentioned composition to a temperature of 1100° C. or higher and 1300° C. or lower (heating step). Next, hot rolling is performed. The hot rolling is performed by first performing rough rolling with a rough rolling end temperature of 850°C or higher and 1150°C or lower. After the rough rolling is completed, finish rolling is performed more than 15 seconds later. The finish rolling end temperature is set to 750°C or higher and 850°C or lower, and the total reduction ratio below 930°C in the entire hot rolling process is set to 40% or higher and 59% or lower (hot rolling process). Next, the raw steel plate obtained by the hot rolling is cooled under the conditions that the average cooling rate Vc (°C/second) at the center of the plate thickness meets the relationship of the following formula (2), and the cooling stop temperature at the center of the plate thickness is 550°C or higher and 680°C or lower (cooling process). Next, the raw steel plate is coiled at a plate thickness center temperature of 550°C or higher and 680°C or lower (coiling process). Next, the coiled steel plate obtained by the coiling is subjected to a second cooling process, which is to retain the coiled steel plate in a temperature range of 400°C to 300°C for 1.0 hour or more and 10.0 hours or less (second cooling process). In this way, the hot rolled steel plate of the present invention can be obtained.・・・Formula (2)

The following is a more specific description of the manufacturing method of hot-rolled steel plates. In addition, in the following description of the manufacturing method, the temperature "℃" refers to the surface temperature of the steel material and/or steel plate (hot-rolled plate, material steel plate, hot-rolled steel plate) (hereinafter, also referred to as steel plate, etc.) unless otherwise stated. The surface temperature of these steel plates can be measured using a radiation thermometer, etc. In addition, the temperature at the center of the plate thickness of the steel plate, etc. can be obtained by first calculating the temperature distribution in the cross section of the steel plate, etc. using the analytical heat conduction method, and then correcting the result according to the surface temperature of the steel plate, etc.

In the present invention, the melting method of the steel material (steel billet) is not particularly limited, and a known melting method such as a converter, an electric furnace, a vacuum melting furnace, etc. can be adopted. The casting method is also not particularly limited, and a known casting method such as a continuous casting method can be adopted to produce the desired size. In addition, a block-segment roll method can be adopted to replace the continuous casting method. The molten steel can also be subjected to secondary refining such as ladle refining.

Next, as a heating process, the obtained steel material (steel billet) is heated to a temperature of 1100°C to 1300°C. Next, as a hot rolling process, rough rolling is first performed at a rough rolling end temperature of 850°C to 1150°C. After the rough rolling is completed, finish rolling is started more than 15 seconds later, and the finish rolling end temperature is set at 750°C to 850°C, and the total reduction ratio at 930°C or less in the entire hot rolling process is set at 40% to 59%. By performing this hot rolling process, a raw steel plate for hot rolled steel plate is produced.

Heating temperature: 1100°C or higher and 1300°C or lower If the heating temperature in the heating process is lower than 1100°C, the deformation resistance of the rolled material is too large and rolling is difficult. On the other hand, if the heating temperature exceeds 1300°C, the austenite particles become coarse and fine austenite particles cannot be obtained in the subsequent rolling (rough rolling, finish rolling) process, making it difficult to ensure the average grain size of the steel structure of the hot-rolled steel plate desired by the present invention. In addition, it is difficult to suppress the generation of coarse tungsten, making it difficult to control the area ratio of the grains having a grain size within the average grain size ±5.0μm within the range desired by the present invention. Therefore, the heating temperature of the heating process is set to be above 1100°C and below 1300°C. It is better to be above 1120°C. Moreover, the heating temperature of the heating process is better to be below 1280°C.

In addition, in addition to the traditional method of cooling the steel billet (billet) to room temperature and then heating it again after manufacturing, the present invention can also adopt the "energy-saving process of directly sending it to the roll" in which the hot steel sheet is directly sent to the heating furnace without cooling it to room temperature.

Rough rolling end temperature: 850℃ or more and 1150℃ or less If the rough rolling end temperature in the hot rolling process is lower than 850℃, the surface temperature of the steel plate will drop below the temperature at which the granular iron begins to transform in the subsequent finish rolling, thereby generating a large amount of granular iron, and the area ratio of pulverized iron and pseudo-pulverized iron will decrease, so it will be difficult to obtain the square steel pipe with a low yield ratio desired by the present invention. On the other hand, if the rough rolling end temperature exceeds 1150℃, the reduction amount in the austenite non-recrystallization temperature range is insufficient, and fine austenite particles cannot be obtained. As a result, the desired steel structure of the hot-rolled steel plate of the present invention cannot be obtained, and it is difficult to obtain a steel structure in the center of the plate thickness of the steel plate in which, when the region surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is regarded as crystal grains, the average crystal grain size of such crystal grains is 10.0 to 30.0 μm, the number density of crystal grains having a ratio of the major diameter to the minor diameter (major diameter/minor diameter) of 3.0 or more among such crystal grains is less than 30 grains/ ^mm2 , and the area ratio of crystal grains having a crystal grain size within ±5.0 μm of the average crystal grain size is 35% or more. In addition, it is difficult to suppress the formation of coarse ductile iron. Therefore, the rough rolling end temperature is set to 850°C or higher and 1150°C or lower, more preferably 860°C or higher, and more preferably 870°C or higher. Furthermore, the rough rolling end temperature is more preferably 1100°C or lower, and more preferably 1050°C or lower.

Time from the end of rough rolling to the start of finish rolling: 15 seconds or more In the hot rolling process, if the time from the end of rough rolling to the start of finish rolling is less than 15 seconds, the inconsistency of the grain size of the austenite will become larger, and it is difficult to obtain the steel structure in which the area ratio of the grains with a grain size within ±5.0 μm of the average grain size in the center of the plate thickness of the steel plate is 35% or more as desired by the present invention. In addition, it is difficult to obtain a square steel pipe with a low yield ratio of 0.5 to 1.2 in the Charpy impact test at -20°C in the circumferential direction of the tube to the axial direction of the tube (hereinafter referred to as "shock absorption energy ratio P"). The above time is preferably more than 18 seconds, and more preferably more than 20 seconds. Although the upper limit of the time from the end of rough rolling to the start of finish rolling is not specifically specified, it is preferably less than 300 seconds, and more preferably less than 280 seconds, based on the perspective of productivity.

Finishing rolling end temperature: 750℃ or more and 850℃ or less If the finishing rolling end temperature in the hot rolling process is lower than 750℃, the surface temperature of the steel plate will drop below the temperature at which the granular iron begins to deform during the finishing rolling process, and there is a possibility that granular iron will be formed that elongates in the rolling direction, thereby reducing the workability. On the other hand, if the finishing rolling end temperature exceeds 850℃, the amount of reduction within the austenitic non-recrystallization temperature range is insufficient, and fine austenitic iron particles cannot be obtained. As a result, the crystallized grains become coarse, making it difficult to ensure the strength desired by the present invention. In addition, it will be difficult to suppress the formation of coarse ductile iron. Therefore, the finishing rolling end temperature is set to be above 750℃ and below 850℃. The finishing temperature of the finishing rolling is preferably above 770°C, and more preferably above 780°C. Moreover, the finishing temperature of the finishing rolling is preferably below 830°C, and more preferably below 820°C.

Total reduction ratio below 930°C: 40% or more and 59% or less The present invention refines the secondary grains in the austenite during the hot rolling process of the aforementioned rough rolling and the aforementioned fine rolling, so that the fat iron and ductile iron generated in the subsequent cooling process and coiling process are also refined, thereby obtaining a steel structure of a hot-rolled steel plate having the strength and toughness desired by the present invention. In order to refine the secondary grains in the austenite during the hot rolling process, the reduction ratio must be increased within the austenite non-recrystallization temperature range in order to introduce sufficient processing deformation. However, if the total reduction rate exceeds 59%, it is easy to generate grains with a large ratio of long diameter to short diameter, resulting in a decrease in toughness. Therefore, the present invention sets the total reduction rate below 930°C to below 59%. It is better to set it below 57%, and more preferably to set it below 55%. On the other hand, if the total reduction rate below 930°C is less than 40%, the grain size of the granular iron and/or the ductile iron becomes too large, which will lead to a decrease in toughness. Therefore, the total reduction rate below 930°C is set to more than 40%. It is better to set it to more than 42%, and more preferably to set it to more than 45%. In addition, the reason why the total reduction ratio is set below 930℃ is that if the temperature exceeds 930℃ during the rolling process, austenite will recrystallize, and the dislocations originally introduced by rolling will disappear completely, and it is impossible to obtain fine austenite. The above-mentioned "total reduction ratio" refers to the total reduction ratio of each rolling pass within the temperature range below 930℃.

The present invention can adopt the hot rolling of the heated steel material (bill) to achieve a total reduction ratio of 40% or more and 59% or less at 930°C or less across the two processes of rough rolling and finish rolling. Alternatively, it can also adopt the hot rolling to achieve a total reduction ratio of 40% or more and 59% or less at 930°C or less based on the finish rolling only. In other words, if the total reduction ratio below 930℃ in finish rolling cannot be controlled within 40% to 59%, the steel material (blanket) can be cooled to below 930℃ during rough rolling, and then roll rolling can be performed to control the total reduction ratio below 930℃ of both rough rolling and finish rolling within 40% to 59%.

As a cooling process after the hot rolling process, the raw steel plate for hot rolled steel plate (hereinafter also referred to as raw steel plate or hot rolled plate) is cooled. In this cooling process, the average cooling rate Vc (°C/second) at the center of the plate thickness satisfies the relationship of the following formula (2), and the cooling stop temperature at the center of the plate thickness is 550°C or more and 680°C or less.・・・Formula (2)

Average cooling rate Vc at the center of the plate thickness: 4°C/s or more and 20°C/s or less In the cooling process, if the average cooling rate Vc at the center of the plate thickness is lower than 4°C/s, the nucleation frequency of granular iron will decrease, and the granular iron particles will become coarse, so the desired strength cannot be obtained. On the other hand, if the above average cooling rate Vc exceeds 20°C/s, too much upper ductile iron will be generated, so the yield ratio desired by the present invention cannot be obtained. The average cooling rate Vc is preferably set at 6°C/s or more, and more preferably at 8°C/s or more. It is also better to set it at 18°C/s or less, and more preferably to set it at 16°C/s or less. In the present invention, from the perspective of suppressing the coarsening of the crystal grain size, it is advisable to start cooling immediately after the finish rolling.

Cooling stop temperature at the center of plate thickness: 550°C or more and 680°C or less In the cooling process, if the cooling stop temperature at the center of plate thickness is lower than 550°C, it is easy to produce inconsistent temperature distribution in the length direction and/or width direction of the raw steel plate during cooling, which may lead to inconsistent mechanical properties. On the other hand, if the cooling stop temperature at the center of plate thickness exceeds 680°C, the fat iron particles will coarsen and the desired crystal grain size cannot be obtained. In addition, the cooling stop temperature at the center of plate thickness is preferably set at 560°C or more, and more preferably at 580°C or more. It is also better to set it below 660°C, and more preferably below 650°C.

The average cooling rate Vc (°C/second) at the center of the plate thickness in the present invention refers to the average cooling rate in the temperature range from the start of cooling to the end of cooling at the center of the plate thickness. In addition, the above average cooling rate is a value obtained based on ((temperature of the raw steel plate before cooling - temperature of the raw steel plate after cooling) / cooling time), which can be calculated from the temperature distribution in the cross section of the raw steel plate obtained by analyzing heat conduction. The cooling method can be: a water cooling method in which water is sprayed from a nozzle, an air cooling method in which cooling gas is sprayed from a nozzle, etc.

In the cooling process, it is preferable to cool both sides of the raw steel plate (hot rolled plate) under the same conditions. In addition, in order to obtain the above cooling speed, the amount and/or pressure of cooling water or cooling gas, the spraying time and angle, and the conveying speed of the raw steel plate can be adjusted. In particular, in order to obtain the cooling speed specified in the present invention, the conditions for the cooling treatment of the raw steel plate can be determined by performing thermal analysis in advance, and then such conditions can be reflected in the manufacturing conditions.

After the above cooling process, the raw steel plate is coiled as a coiling process. In this coiling process, based on the viewpoint of the steel plate structure, the raw steel plate is coiled under the condition that the plate thickness center temperature (coiling temperature) is 550°C or more and 680°C or less. If the coiling temperature is lower than 550°C, a large amount of upper tungsten iron will be generated on the surface of the steel plate, and the area ratio may exceed 5%. On the other hand, if the coiling temperature exceeds 680°C, the fat iron particles will become coarse and the desired grain size cannot be obtained. The coiling temperature is preferably set at 570°C or more, and more preferably at 580°C or more. Furthermore, the coiling temperature is more preferably set to 660° C. or lower, and more preferably set to 650° C. or lower.

After the coiling process, a second cooling process is performed to cool the coiled steel plate obtained in the coiling process. In the second cooling process, the coiled steel plate obtained in the coiling process is retained in a temperature range of 400°C to 300°C for more than 1.0 hour and less than 10.0 hours. If the retention time in the temperature range of 400°C to 300°C is less than 1.0 hour, the desired work hardening index cannot be obtained, and the desired yield ratio and toughness cannot be obtained. If the retention time in the temperature range of 400°C to 300°C exceeds 10.0 hours, the desired yield strength and tensile strength may not be obtained. The retention time in the temperature range of 400°C to 300°C is preferably set to 1.5 hours or more, more preferably to 2.0 hours or more. The retention time in the temperature range of 400°C to 300°C is preferably set to 9.0 hours or less, more preferably to 8.5 hours or less.

In addition, how the temperature of the rolled steel plate (rolled steel plate) will change depends on factors such as the thickness, width, and length of the raw steel plate. Therefore, in the second cooling process, a thermal analysis is performed in advance to calculate the temperature change of the rolled steel plate, and then the retention time in the temperature range of 400°C to 300°C is implemented within the scope of the present invention: the steel plate is wrapped with an insulating material or the steel plate is cooled with cooling water and/or cooling gas. In the temperature range below 300℃, the properties of steel structure and/or strength hardly change. Therefore, cooling from 300℃ to room temperature can be done by any method, such as rapid cooling or cooling.

Furthermore, the rolled steel plate (rolled steel plate) has different temperature distributions depending on different places on the steel plate. As shown in Figure 1, the temperature of the side surface of the rolled steel plate (which will eventually be a hot-rolled steel plate) is measured at three different places (symbol 10. The outer surface, inner side surface, and center of the rolled steel plate), and the average value is calculated as the temperature of the rolled steel plate. In addition, the temperature of the rolled steel plate is measured every 20 minutes to calculate the residence time from 400°C to 300°C. The temperature of the rolled steel plate can be measured using a non-contact thermometer such as a radiation thermometer or a contact thermometer such as a thermocouple.

<Square steel pipe with low yield ratio> The square steel pipe with low yield ratio of the present invention uses the above-mentioned low yield specific heat rolled steel plate as the material of the steel pipe. The square steel pipe with low yield ratio not only shows a low yield ratio of 295MPa or more, a tensile strength of 400MPa or more, and 0.90 or less in the flat plate portion along the tube axis direction, but also has: an average elongation of 5.0% or more, an impact absorption energy of 60J or more in the Charpy impact test at a test temperature of -20°C, an impact absorption energy ratio P of 0.5~1.2, and a ductile-brittle transition temperature of -10°C or less. Low temperature toughness. In addition, it has excellent low-temperature toughness not only in the tube axis direction but also in the tube circumferential direction. Therefore, it can be used as a structural member of buildings in low-temperature environments such as cold regions where the temperature is below freezing point (0°C).

In the pipe making process of the square steel pipe with low yield ratio, the hot-rolled steel plate is firstly rolled and formed into a cylindrical unsealed tube in the cold room, and then the butt joints are welded together by electric welding to make a round steel pipe. Then, the cylindrical round steel pipe is reduced by several % along the axial direction of the tube by rollers arranged on the top, bottom, left and right sides of the round steel pipe, and then formed into a square (angular) shape to make a square steel pipe. In addition, the "cylindrical shape" referred to in the present invention refers to the shape obtained by forming the hot-rolled steel plate into a circular shape by roll forming, and the end portions of the hot-rolled steel plate have not yet been welded together by electric welding.

First, the coiled low-yield specific heat rolled steel plate is formed into a round shape by a roll forming method using a roller in the cold room (at room temperature) to produce a round steel pipe. Then, the round shape is formed into a square shape by a roll forming method using a roller to produce a square steel pipe. In the process of cold (at room temperature) hot-rolled steel plate being roll-formed into a round steel pipe, a large processing deformation will be introduced in the axial direction and the circumferential direction of the pipe, so there will be problems such as the yield ratio in the axial direction and the circumferential direction of the pipe is easy to increase and the toughness is easy to decrease. However, the square steel pipe with low yield ratio of the present invention uses the above-mentioned low yield specific heat rolled steel plate as the steel pipe material, so the increase in yield ratio can be suppressed, and even a thick-walled steel pipe of, for example, more than 12 mm can still have a low yield ratio.

Furthermore, as described later, the impact absorption energy ratio P of the square steel tube of the present invention in the Charpy impact test at -20°C is 0.5~1.2, and it has excellent low-temperature toughness in both the tube axial direction and the tube circumferential direction. However, in order to obtain such toughness, it is not only necessary to comply with the components specified in the present invention, but also necessary to wait for more than 15 seconds after the rough rolling before starting the finish rolling. Because the finish rolling starts more than 15 seconds after the rough rolling, the hot rolled steel plate with small difference in steel structure between the plate width direction and the plate length direction (rolling direction) can be obtained. By performing roll forming on the hot rolled steel plate, a square steel tube with a low yield ratio of 0.5~1.2 in the above-mentioned Charpy impact test at -20℃ can be obtained.

Although the above method can be used to manufacture a square steel pipe with a low yield ratio of 0.5 to 1.2 in the Charpy impact test at -20°C, in order to manufacture more stably, it is also possible to manufacture a round steel pipe by roll forming, perform heat treatment, and then form it into a square to manufacture a square steel pipe. By performing such heat treatment, the toughness in the circumferential direction of the pipe can be improved, and the square steel pipe with a ratio P of 0.5 to 1.2 in the Charpy impact test at -20°C desired by the present invention can be manufactured more efficiently. The heat treatment temperature of this heat treatment is preferably above 100°C and below 550°C. If the heat treatment temperature is lower than 100°C, the toughness cannot be improved. On the other hand, if the heat treatment temperature exceeds 550°C, the steel structure becomes coarse and the strength and toughness will deteriorate. The heat treatment temperature is preferably set at 150°C or higher. In addition, the heat treatment temperature is preferably set at 500°C or lower. The heat treatment time is preferably set at more than 30 seconds, and more preferably at more than 1 minute. Although the upper limit is not specifically stipulated, it is preferably set at less than 10 minutes, and more preferably at less than 5 minutes, from the perspective of suppressing the heat treatment cost. This method for heat treating square steel pipes is not particularly limited, and known heat treatment equipment (heating equipment) such as using burning combustible gas and/or using electric heaters for heating, and using IH (induction heating) for heating can be used.

In addition, the square steel tube of the present invention is not limited to square steel tubes with equal lengths of each side (the ratio of the long side length to the short side length (long side length/short side length) is 1.0), but also includes square steel tubes with a ratio exceeding 1.0. However, if this ratio exceeds 2.5, local buckling is more likely to occur on the long side, and the compressive strength in the axial direction of the tube will decrease. Therefore, it is appropriate to set this ratio to a range of greater than 1.0 and less than 2.5. It is better to set this ratio to greater than 1.0 and less than 2.0.

In this way, the square steel pipe of the present invention can be manufactured. According to the present invention, a square steel pipe with excellent mechanical properties of the flat plate portion can be manufactured. More specifically, according to the present invention, a square steel pipe can be manufactured with: the yield strength of the flat plate portion is 295MPa or more, the tensile strength of the flat plate portion is 400MPa or more, the yield ratio of the flat plate portion is 0.90 or less, the uniform elongation is 5.0% or more, the impact absorption energy of the flat plate portion in the Charpy impact test at -20°C is 60J or more, the ductile-brittle transition temperature of the flat plate portion is below -10°C, and the ratio P of the impact absorption energy of the Charpy impact test at -20°C is 0.5~1.2.

Furthermore, the square steel tube of the present invention has a ductile-brittle transition temperature lower than 0°C, and has excellent toughness not only in the tube axis direction but also in the tube circumferential direction. Therefore, it is very suitable for use as a structural member of buildings in cold regions where the air temperature or room temperature falls below the freezing point (0°C). Even when a huge earthquake occurs, the building structure is not likely to collapse, and excellent earthquake resistance can be ensured.

<Building structure> Figure 2 is a schematic diagram showing an example of the building structure of the present invention. The building structure of the present invention has the aforementioned square steel pipe (square steel pipe with low yield ratio) 1 of the present invention as a column material. Symbols 4, 5, 6, and 7 represent: main beam, small beam, partition, and intermediate column in sequence. The square steel pipe of the present invention has excellent mechanical properties of the flat plate part as mentioned above. Therefore, the building structure of the present invention using this square steel pipe as a column material can exert excellent earthquake resistance. [Example]

The present invention will be described in more detail below with reference to examples. In addition, the present invention is not limited to the following examples. Molten steel having the components shown in Table 1 is cast to produce a billet. The produced billet is subjected to a heating process, a hot rolling process, a cooling process, a coiling process, and a second cooling process after the coiling process under the conditions shown in Table 2 to produce a hot-rolled steel plate. The hyphen (-) in Table 1 means that the content of the element is 0 (zero) or is equivalent to an impurity.

Then, the pipe making process described below is carried out. That is, the aforementioned hot-rolled steel plate is formed into a cylindrical shape by roll forming, and then the butt joints are welded to produce a round steel pipe. Then, the round steel pipe is rolled into a square shape (square shape when viewed from a vertical section in the pipe axis direction) by rollers arranged on the top, bottom, left and right sides of the round steel pipe, and a roll-formed square steel pipe having corners and flat portions and having a side length (mm) and thickness (mm) as shown in Table 4 described below is obtained. In addition, test pieces were taken from the aforementioned hot-rolled steel plate, and the following structural observations, tensile tests, and Charpy impact tests were carried out.

[Tissue observation] The test piece for tissue observation is taken from the width center of the hot-rolled steel plate, including the position of 1/2t of the plate thickness (t is the plate thickness), and covers the range extending 5mm from the position of 1/2t of the plate thickness in the thickness direction. The cross section in the rolling direction during hot rolling is used as the observation surface of the test piece, and after grinding, it is etched with nitric acid etching solution to make the test piece. Tissue observation is to observe and photograph the structure within the range of ±1mm in the thickness direction from the position of 1/2t of the plate thickness of the hot-rolled steel plate using an optical microscope (magnification of 1000 times) or a scanning electron microscope (SEM, magnification of 1000 times). From the optical microscope photos and scanning electron microscope photos obtained, the area ratios of ferrite, phoenic iron, pseudo-phoenic iron, and upper ferromagnetic iron were obtained. The area ratio of each tissue was calculated by observing 5 fields of view and then calculating the average value from the values obtained in each field of view as the area ratio. Here, the area ratio obtained by tissue observation is regarded as the area ratio of each tissue.

The ferrite is a product of diffusion metamorphosis, so it is a structure that has almost returned to its original state with a very low dislocation density. Polygonal ferrite and quasi-polygonal ferrite are also included in this structure. Pleione is a structure in which ferrite and ferrite are arranged in layers. Pleione can be seen as a structure in which ferrite is arranged in a matrix in ferrite. In addition, the upper metamorphic iron is a complex phase structure of ferrite and ferrite in a network with a higher dislocation density. In addition to the shapes mentioned above, you can distinguish them by their colors: granular iron is white, wave iron is black, pseudo-wave iron is black or gray, and upper tough iron is white or gray.

The average crystal grain size (average equivalent circle diameter) is measured using a scanning electron microscope/backscattered electron diffraction method (SEM/EBSD method) with the position of plate thickness t/2 and the range of ±1mm in the thickness direction from the position of plate thickness t/2 (in the present invention, it means: the center of the plate thickness) as the object. The measurement area is 500μm×1000μm (= ^0.5mm2 ) and the measurement step length is 0.5μm. The crystal grain size is measured by first finding the orientation difference between adjacent crystal grains and taking the boundary with an orientation difference of 15° or more as the crystal grain boundary. The grain size (equivalent circle diameter) of each grain is calculated from the measured grain boundaries, and the arithmetic mean is calculated as the average grain size (equivalent circle diameter). In addition, the total area of the grains with a grain size within ±5.0 μm of the average grain size is calculated and divided by the area of the measurement area (0.5 mm ² ) to calculate the area ratio of the grains with a grain size within ±5.0 μm of the average grain size.

The major diameter and minor diameter of the crystallized grains were measured according to the method described in Japanese Industrial Standard JIS R 1670 (2006), and the ratio of major diameter to minor diameter (major diameter)/(minor diameter) was calculated. Then, the number of crystallized grains with a ratio of major diameter to minor diameter (major diameter)/(minor diameter) of 3.0 or more was measured and divided by the area of the measurement area (0.5 mm ² ), and the number of crystallized grains with a ratio of major diameter to minor diameter (major diameter)/(minor diameter) of 3.0 or more per unit area was calculated (pieces/mm ² ). In addition, when analyzing the crystal grain size and measuring the number of crystal grains, crystal grains with a crystal grain size of less than 2.0 μm were considered as measurement noise and excluded from the analysis object.

[Tensile test] From the hot-rolled steel plate, a Japanese Industrial Standard JIS No. 5 tensile test piece was taken in a manner that the tensile direction was kept parallel to the rolling direction. The tensile test piece was subjected to a tensile test in accordance with the provisions of Japanese Industrial Standard JIS Z 2241 (2011), and its yield strength YS and tensile strength TS were measured, and the yield ratio defined as (yield strength)/(tensile strength) was calculated. In addition, the number of test pieces was 2 each, and the average value of the two pieces was calculated first, and then YS, TS, and yield ratio were calculated. In addition, the work hardening index of 3~7% plastic deformation was calculated according to the method (2-point method) described in Japanese Industrial Standard JIS Z 2253 (2011).

[Charpy impact test] From the 1/2t position (center of plate thickness) of the hot-rolled steel plate, a V-groove standard test piece was taken out in a manner that the length direction of the test piece is parallel to the rolling direction and is manufactured in accordance with the Japanese Industrial Standard JIS Z 2242 (2018). In accordance with the provisions of the Japanese Industrial Standard JIS Z 2242 (2018), Charpy impact tests were conducted at test temperatures of -80°C, -60°C, -40°C, -20°C, and 0°C. In addition, the number of test pieces was 3 for each test temperature, and the average values (J) of the ductile-brittle transition temperature and the impact absorption energy were calculated. The obtained results are shown in Table 3.

In addition, test pieces were taken from the produced square steel pipe (roll-formed square steel pipe) and subjected to the following tensile test and Charpy impact test.

[Tensile test] Figure 3 is a schematic diagram showing the location of the tensile test piece of the flat plate. As indicated by X in Figure 3, the tensile test piece is taken from the flat plate of the square steel pipe 1 in such a way that the tensile direction is parallel to the tube axis direction, and a tensile test piece conforming to Japanese Industrial Standard JIS No. 5 is produced. The tensile test piece taken was subjected to a tensile test in accordance with the provisions of Japanese Industrial Standard JIS Z 2241 (2011), and its yield strength YS and tensile strength TS were measured, and the yield ratio defined as (yield strength)/(tensile strength) was calculated. In addition, the plastic elongation (average elongation) at the highest load point (Ag of Japanese Industrial Standard JIS Z 2241 (2011)) was also measured. In addition, the tensile test piece of the flat plate is taken from the center of the width of the flat plate (X in Figure 3) when the welded part of the square steel pipe (W in Figure 3) is regarded as the 12 o'clock direction and the edge is at the 3 o'clock direction. In addition, the number of test pieces is 2 pieces each, and the average value of these two pieces is calculated to obtain YS, TS, and yield ratio.

[Charpy impact test] Figure 4 is a schematic diagram showing the location of the specimen for the Charpy impact test. The Charpy impact test uses: as shown by Y in Figure 4, a V-groove standard specimen in accordance with the Japanese Industrial Standard JIS Z 2242 (2018) is taken from the flat plate portion of the square steel pipe 1, at a position 1/4t of the thickness t measured from the outer surface of the square steel pipe 1, in such a way that the length direction of the specimen is parallel to the pipe axis direction. In accordance with the provisions of the Japanese Industrial Standard JIS Z 2242 (2018), the Charpy impact test was carried out at test temperatures of -60°C, -40°C, -20°C, and 0°C. In addition, three test pieces were used for each test temperature, and the average values of the ductile-brittle transition temperature and the impact absorption energy (J) were calculated.

In order to measure the impact absorption energy of the Charpy impact test in the circumferential direction of the tube, as shown in Z in FIG4 , a V-groove standard test piece conforming to the provisions of Japanese Industrial Standard JIS Z 2242 (2018) was prepared by taking a test piece from the flat plate portion of the square steel tube 1 and at a position 1/4t of the thickness t calculated from the outer surface of the square steel tube 1 in such a way that the length direction of the test piece is parallel to the circumferential direction of the tube. In addition, in accordance with the provisions of Japanese Industrial Standard JIS Z 2242 (2018), a Charpy impact test was carried out at a test temperature of -20°C. In addition, the number of test pieces was 3, and the average impact absorption energy (J) was calculated. In addition, the ratio P of the impact absorption energy in the Charpy impact test at -20°C between the tube circumferential direction and the tube axial direction is also shown in Table 4.

In Table 3 and Table 4, steel plates No. 1 to 20 are examples of the present invention, and steel plates No. 21 to 49 are comparative examples. The steel plate No. in Table 4 means: a square steel pipe manufactured using the steel plate of the same number in Table 3. For example: Steel plate No. 1 in Table 4 is a square steel pipe manufactured using steel plate No. 1 in Table 3. As shown in Table 3, the steel structure of the hot-rolled steel plate of the present invention example at the center of the plate thickness includes granular iron as the main phase, pulex and pseudo-pulex as the second phase with a total area ratio of 6 to 25%, and upper ductile iron with an area ratio of less than 5%, and when the area surrounded by the boundary with an orientation difference of 15° or more is regarded as the crystal grain, the average crystal grain size of the steel structure including the main phase and the second phase at the center of the plate thickness is 10.0 to 20.0. 30.0μm, the area ratio of the crystal grains with a crystal grain size within ±5.0μm of the average crystal grain size among the above crystal grains is 35% or more, and the number density of the crystal grains with a ratio of major diameter to minor diameter, that is, (major diameter)/(minor diameter) of 3.0 or more is 30/ ^mm2 or less. In addition, the yield strength is 250MPa or more, the tensile strength is 400MPa or more, the yield ratio is 0.75 or less, the work hardening index _n3-7 at 3~7% plastic deformation is 0.20 or more, the impact absorption energy of the Charpy impact test at -20℃ is 100J or more, and the ductile-brittle transition temperature is -20℃ or less.

Furthermore, as shown in Table 4, the square steel pipes produced using the hot-rolled steel plates of the present invention examples all have a yield strength of 295 MPa or more in the flat plate portion, a tensile strength of 400 MPa or more in the flat plate portion, a yield ratio of 0.90 or less in the flat plate portion, an average elongation of 5.0% or more in the flat plate portion, an impact absorption energy of 60 J or more in the Charpy impact test in the pipe axis direction at -20°C in the flat plate portion, a ratio P of the impact absorption energy of 0.5 or more and 1.2 or less, and a ductile-brittle transition temperature of the flat plate portion of -10°C or less.

In contrast, in the hot-rolled steel plate of Comparative Example No. 21 (Steel U), Mn/Si=0.8 is outside the scope of the present invention, so the total area ratio of pulverized iron and pseudo-pulverized iron is less than 6%, and the work hardening index at 3-7% plastic deformation cannot reach the desired values. In addition, the square steel pipe manufactured using this hot-rolled steel plate has a low average elongation and the yield ratio cannot reach the desired values.

In the hot-rolled steel plate of Comparative Example No. 22 (Steel V), Mn/Si=14.7 is outside the scope of the present invention, so the total area ratio of pulex and pseudo-pulex exceeds 25%, and the impact absorption energy of the Charpy impact test at -20°C cannot reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the pipe axis direction at -20°C of the square steel pipe manufactured using this hot-rolled steel plate cannot reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 23 (Steel W) has a C content lower than the range of the present invention, so the total area ratio of pulverized iron and pseudo-pulverized iron falls outside the range of the present invention, and the yield strength and tensile strength do not reach the expected values. In addition, the yield strength and tensile strength of the square steel pipe manufactured using this hot-rolled steel sheet also cannot reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 24 (Steel X) has a C content higher than the range of the present invention, so the area ratio of the second phase falls outside the range of the present invention, and the yield ratio and the impact absorption energy of the Charpy impact test at -20°C do not reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the pipe axis direction at -20°C of the square steel pipe manufactured using this hot-rolled steel sheet also cannot reach the expected values.

In the hot-rolled steel sheet of Comparative Example No. 25 (Steel Y), the Si content is higher than the range of the present invention, so the area ratio of pseudo-Pierre iron is excessively increased, and the impact absorption energy of the Charpy impact test at -20°C does not reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the tube axis direction at -20°C of the square steel pipe manufactured using this hot-rolled steel sheet also cannot reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 26 (Steel Z) has a Mn content higher than the range of the present invention, so the amount of upper ductile iron increases too much, and the yield ratio and the like do not reach the desired values. In addition, the yield ratio and the like of the square steel pipe manufactured using this hot-rolled steel sheet also cannot reach the desired values.

In the hot-rolled steel sheet of Comparative Example No. 27 (Steel AA), the Nb content is higher than the range of the present invention, so the amount of upper ductile iron increases excessively. As a result, the number density of crystal grains with a ratio of major diameter to minor diameter of 3.0 or more falls outside the range of the present invention, and the impact absorption energy of the Charpy impact test at -20°C and the like do not reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the tube axis direction at -20°C of the square steel pipe manufactured using this hot-rolled steel sheet and the ductile-brittle transition temperature also cannot reach the expected values.

The hot-rolled steel plate of Comparative Example No. 28 (Steel AB) has a Ti content higher than the range of the present invention, so the amount of upper ductile iron increases excessively and coarse carbides and/or nitrides are formed. As a result, the impact absorption energy of the Charpy impact test at -20°C and the like do not reach the expected values. In addition, the impact absorption energy of the Charpy impact test at -20°C in the pipe axis direction and the ductile-brittle transition temperature of the square steel pipe manufactured using this hot-rolled steel plate also do not reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 29 (Steel AC) has a V content higher than the range of the present invention, so the amount of upper ductile iron is outside the range of the present invention. As a result, the impact absorption energy of the Charpy impact test at -20°C did not reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the tube axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using this hot-rolled steel sheet did not reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 30 (Steel AD) has a Cr content higher than the range of the present invention, so the amount of upper ductile iron falls outside the range of the present invention. As a result, the impact absorption energy of the Charpy impact test at -20°C and the like did not reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the pipe axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using this hot-rolled steel sheet also did not reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 31 (Steel AE) has a Mo content higher than the range of the present invention, so the amount of upper ductile iron falls outside the range of the present invention. As a result, the impact absorption energy of the Charpy impact test at -20°C and the like did not reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the pipe axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using this hot-rolled steel sheet also did not reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 32 (Steel AF) has a Cu content higher than the range of the present invention, so coarse Cu crystals are precipitated. As a result, the impact absorption energy of the Charpy impact test at -20°C and the like do not reach the expected values. In addition, the impact absorption energy of the Charpy impact test at -20°C in the tube axis direction and the ductile-brittle transition temperature of the square steel tube manufactured using this hot-rolled steel sheet also do not reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 33 (Steel AG) has a Mn content lower than the range of the present invention and a Ni content higher than the range of the present invention, so the amount of upper ductile iron falls outside the range of the present invention. As a result, the ductile-brittle transition temperature and the like do not reach the desired values. In addition, the impact absorption energy and the ductile-brittle transition temperature of the square steel pipe manufactured using this hot-rolled steel sheet in the pipe axis direction at -20°C in the Charpy impact test also do not reach the desired values.

The hot-rolled steel sheet of Comparative Example No. 34 (Steel AH) has a Ca content higher than the range of the present invention, so clusters of Ca oxides are formed. As a result, the impact absorption energy of the Charpy impact test at -20°C did not reach the expected value. In addition, the impact absorption energy of the Charpy impact test in the tube axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using this hot-rolled steel sheet also did not reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 35 (Steel AI) has a B content higher than the range of the present invention, so the amount of upper ductile iron falls outside the range of the present invention, and the yield ratio of the flat plate portion does not reach the desired values. In addition, the yield ratio of the square steel pipe manufactured using this hot-rolled steel sheet also cannot reach the desired values.

In the hot-rolled steel plate of Comparative Example No. 36 (Steel T), the billet heating temperature is higher than the range of the present invention, the crystal grains become coarse, and the average crystal grain size and the area ratio of the crystal grains with a crystal grain size within ±5.0μm of the average crystal grain size are outside the range of the present invention. As a result, the yield strength, tensile strength and impact absorption energy of the Charpy impact test at -20℃ did not reach the expected values. In addition, the yield strength and/or tensile strength of the square steel pipe manufactured using this hot-rolled steel plate could not reach the expected values.

In the hot-rolled steel sheet of Comparative Example No. 37 (Steel T), the finishing temperature of the finish rolling is higher than the range of the present invention. Therefore, the total reduction ratio below 930°C is lower than the range of the present invention, and the formation of coarse upper ductile iron cannot be suppressed, and the average grain size falls outside the range of the present invention. As a result, the yield strength, tensile strength, and impact absorption energy of the Charpy impact test at -20°C do not reach the expected values. In addition, the yield strength and/or tensile strength of the square steel pipe manufactured using this hot-rolled steel sheet also cannot reach the expected values.

The total reduction ratio of the hot-rolled steel sheet of Comparative Example No. 38 (Steel T) below 930°C is higher than the range of the present invention, so a coarse upper ductile iron extending in the rolling direction is generated, the average grain size is lower than the range of the present invention, and the number density of grains with a major diameter/minor diameter ratio of 3.0 or more falls outside the range of the present invention. As a result, the ductile-brittle transition temperature and the like do not reach the desired values. In addition, the impact absorption energy and the ductile-brittle transition temperature of the square steel pipe manufactured using this hot-rolled steel sheet in the pipe axis direction at -20°C in the Charpy impact test also cannot reach the desired values.

In the hot-rolled steel plate of Comparative Example No. 39 (Steel T), the average cooling rate at the center of the plate thickness is higher than the range of the present invention, so the area ratio of the upper ductile iron exceeds 5%, which is outside the range of the present invention. As a result, the yield ratio does not reach the desired value, and the work hardening index at a plastic deformation of 3-7% does not reach the desired value. In addition, the average elongation of the square steel pipe manufactured using this hot-rolled steel plate is less than 5.0%, and the yield ratio and the like cannot reach the desired values.

In the hot-rolled steel sheet of Comparative Example No. 40 (Steel T), the time from the end of rough rolling to the start of finish rolling is less than the range of the present invention, so the area ratio of the crystal grains with a crystal grain size within ±5.0μm of the average crystal grain size is outside the range of the present invention. As a result, the yield ratio of the hot-rolled steel sheet and the square steel pipe manufactured using the hot-rolled steel sheet cannot reach the desired value. In addition, the work hardening index of the hot-rolled steel sheet at a plastic deformation of 3-7% does not reach the desired value, so the average elongation of the square steel pipe is less than 5.0%.

In the hot-rolled steel plate of Comparative Example No. 41 (Steel T), the cooling stop temperature and the coiling temperature are lower than the range of the present invention, so the area ratio of the upper tungsten iron and the area ratio of the crystal grains with a crystal grain size within ±5.0μm of the average crystal grain size are outside the scope of the present invention. As a result, the yield ratio of the hot-rolled steel plate and the square steel pipe manufactured using the hot-rolled steel plate cannot reach the desired value. In addition, the work hardening index of the hot-rolled steel plate at the time of plastic deformation of 3~7% did not reach the desired value, so the uniform elongation of the square steel pipe was less than 5.0%.

In the hot-rolled steel plate of Comparative Example No. 42 (Steel T), the average cooling rate at the center of the plate thickness is very low, and the cooling stop temperature and the coiling temperature are higher than the range of the present invention, so the average crystal grain size falls outside the range of the present invention. As a result, the yield strength, tensile strength, and impact absorption energy of the Charpy impact test at -20°C do not reach the expected values. In addition, the yield strength and/or tensile strength of the square steel pipe manufactured using this hot-rolled steel plate cannot reach the expected values.

The hot-rolled steel plate of Comparative Example No. 43 (Steel T) has a retention time in the temperature range of 400°C to 300°C that is shorter than the range of the present invention, so the area ratio of the upper ductile iron exceeds 5%, and the work hardening index, yield ratio, and impact absorption energy of the Charpy impact test at -20°C when the plastic deformation is 3-7% do not reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the pipe axis direction at -20°C of the square steel pipe manufactured using this hot-rolled steel plate also cannot reach the expected values.

The hot-rolled steel sheet of Comparative Example No. 44 (Steel T) has a retention time in the temperature range of 400°C to 300°C that is longer than the range of the present invention, so the average grain size falls outside the range of the present invention, and the yield strength and tensile strength do not reach the desired values. In addition, the yield strength and tensile strength of the square steel pipe manufactured using this hot-rolled steel sheet also do not reach the desired values.

In the hot-rolled steel sheet of Comparative Example No. 45 (Steel T), the rough rolling end temperature is higher than the range of the present invention, so the number density of crystal grains with a ratio of major diameter to minor diameter (major diameter)/(minor diameter) of 3.0 or more falls outside the range of the present invention. As a result, the ductile-brittle transition temperature does not reach the desired value. In addition, the ductile-brittle transition temperature of the square steel pipe manufactured using this hot-rolled steel sheet also does not reach the desired value.

In the hot rolled steel plate of Comparative Example No. 46 (Steel T), the rough rolling end temperature and the finish rolling end temperature are lower than the range of the present invention, so a large amount of granular iron is generated, resulting in a total area ratio of pulverized iron and pseudo-pulverized iron of less than 6%, and the work hardening index at a plastic deformation of 3-7% does not reach the desired value. In addition, the average elongation of the square steel pipe manufactured using this hot rolled steel plate is less than 5.0%, and the tensile strength and yield ratio cannot reach the desired values.

In Comparative Example No. 47 (Steel T), since the billet heating temperature is lower than the range of the present invention, the deformation resistance of the rolled material is too large, and rolling is difficult to perform, and rolling is interrupted in the middle of rough rolling. Therefore, hot rolled steel plates and square steel pipes cannot be manufactured.

The hot rolled steel sheet of Comparative Example No. 48 (Steel T) has a finishing temperature lower than the range of the present invention, so granular iron is formed that is elongated in the rolling direction. As a result, the impact absorption energy of the Charpy impact test at -20°C does not reach the expected value. In addition, the impact absorption energy of the Charpy impact test in the pipe axis direction at -20°C of the square steel pipe manufactured using this hot rolled steel sheet also cannot reach the expected value.

The average cooling rate of the hot-rolled steel sheet of Comparative Example No. 49 (Steel T) is lower than the range of the present invention, so the granular iron becomes coarse and the average grain size exceeds 30.0 μm. As a result, the yield strength, tensile strength, and impact absorption energy of the Charpy impact test at -20°C do not reach the expected values. In addition, the impact absorption energy of the Charpy impact test in the pipe axis direction at -20°C of the square steel pipe manufactured using this hot-rolled steel sheet also cannot reach the expected values.

1: Square steel pipe 4: Main beam 5: Small beam 6: Partition 7: Spacer 10: Temperature measurement position of hot-rolled steel plate

[Figure 1] is a schematic diagram showing the temperature measurement position of the rolled steel plate. [Figure 2] is a schematic three-dimensional diagram showing an example of a building structure using the square steel pipe of the present invention. [Figure 3] is a schematic diagram showing the sampling position of the test piece for the flat plate tensile test performed in the embodiment. [Figure 4] is a schematic diagram showing the sampling position of the test piece for the Charpy impact test performed in the embodiment.

10: Temperature measurement position of hot-rolled steel plate

Claims (13)

A hot-rolled steel plate, the composition of which, in terms of mass%, comprises C: 0.07% or more and 0.20% or less, Si: 0.40% or less, Mn: 0.20% or more and 1.00% or less, P: 0.100% or less, S: 0.050% or less, Al: 0.005% or more and 0.100% or less, and N: 0.0100% or less, the remainder being Fe and inevitable impurities, and the contents of Mn and Si satisfy the relationship of the following formula (1), the steel structure at the center of the plate thickness comprises: granular iron as a main phase, and pulex and pseudo-pulex as a second phase with a total area ratio of 6 to 25%, and upper ductile iron with an area ratio of less than 5%, In the steel structure at the center of the plate thickness, when the area surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is regarded as crystal grains, the average crystal grain size of such crystal grains is 10.0 to 30.0 μm, the area ratio of crystal grains having a crystal grain size within ±5.0 μm of the average crystal grain size of the aforementioned crystal grains is 35% or more, and the number density of crystal grains having a ratio of major diameter to minor diameter (major diameter/minor diameter) of 3.0 or more among the aforementioned crystal grains is 30 grains/ ^mm2 or less,・・・Formula (1) In formula (1), %Mn and %Si represent the content (mass %) of each element in the steel plate. The hot-rolled steel sheet as described in claim 1, wherein the above-mentioned components, in terms of mass %, further contain one or more selected from the following elements, Nb: 0.005% or more and 0.020% or less, Ti: 0.005% or more and 0.020% or less, V: 0.01% or more and 0.10% or less, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 0.30% or less, Ni: 0.01% or more and 0.30% or less, Ca: 0.0005% or more and 0.0100% or less, and B: 0.0003% or more and 0.0100% or less. The hot-rolled steel plate as described in claim 1 or claim 2 has a plate thickness of 12 mm or more. A method for manufacturing a hot-rolled steel plate comprises heating a steel material having the composition described in claim 1 or claim 2 to a temperature of 1100°C or higher and 1300°C or lower; then hot rolling is performed, wherein the hot rolling is performed by first performing a rough rolling with a final temperature of 850°C or higher and 1150°C or lower, and after the rough rolling is completed, finishing rolling is performed more than 15 seconds later, the final finishing temperature is set to 750°C or higher and 850°C or lower, and the total reduction ratio below 930°C in the entire hot rolling process is set to 40% or higher and 59% or lower; Next, the raw steel plate obtained by the hot rolling is cooled under the condition that the average cooling rate Vc (°C/second) at the center of the plate thickness satisfies the relationship of the following formula (2), and the cooling stop temperature at the center of the plate thickness is 550°C or more and 680°C or less; Next, the raw steel plate is coiled under the condition that the temperature at the center of the plate thickness is 550°C or more and 680°C or less; Next, the coiled steel plate obtained by the coiling is subjected to a second cooling, which is to retain the coiled steel plate in a temperature range of 400°C to 300°C for 1.0 hour or more and 10.0 hours or less, ……Formula (2). A square steel pipe is made of the hot-rolled steel plate described in claim 1 or claim 2. A square steel pipe is made of the hot-rolled steel plate described in claim 3. For the square steel tube as described in claim 5, the ratio P of the impact absorption energy of the square steel tube in the circumferential direction and the axial direction of the Charpy impact test at -20°C falls within the range of 0.5 to 1.2, where P=(impact absorption energy of the Charpy impact test at -20°C in the circumferential direction)/(impact absorption energy of the Charpy impact test at -20°C in the axial direction). For the square steel tube as described in claim 6, the ratio P of the impact absorption energy of the square steel tube in the circumferential direction and the axial direction of the Charpy impact test at -20°C falls within the range of 0.5 to 1.2, where P=(impact absorption energy of the Charpy impact test at -20°C in the circumferential direction)/(impact absorption energy of the Charpy impact test at -20°C in the axial direction). A method for manufacturing a square steel pipe comprises forming a hot-rolled steel plate obtained by the method for manufacturing a hot-rolled steel plate described in claim 4 into a square steel pipe by roll forming in a cold room. A building structure having the square steel pipes as claimed in claim 5 as columns. A building structure having the square steel pipes as claimed in claim 6 as columns. A building structure having the square steel pipes as claimed in claim 7 as columns. A building structure having the square steel pipes as claimed in claim 8 as columns.

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