TWI869860B - Hot-rolled steel plate, square steel pipe, manufacturing method thereof and building structure - Google Patents

Abstract

The object of the present invention is to provide a hot-rolled steel plate, a square steel pipe, a manufacturing method thereof and a building structure having excellent strength and low-temperature toughness. A hot-rolled steel plate has a predetermined component composition, and the steel structure at the center of the plate thickness and the back of the plate surface has: a main phase composed of granular iron, and a second phase whose area ratio of the sum of pulex and pseudopulex is 6-25% and the area ratio of upper ductile iron is less than 5%, and when the orientation difference of adjacent crystals at the center of the plate thickness is more than 15°, the steel structure at the center of the plate thickness and the back of the plate surface has: a main phase composed of granular iron, and a second phase whose area ratio of the sum of pulex and pseudopulex is 6-25% and the area ratio of upper ductile iron is less than 5%. When the area surrounded by the boundary is regarded as the crystal grain, the average crystal grain size of the steel structure in the central part of the plate thickness is 10.0~30.0μm, the area ratio of crystal grains with an equivalent circular diameter of 40.0μm or more is less than 20%, and the number of crystal grains with a major diameter to minor diameter ratio (= (major diameter)/(minor diameter)) of 3.0 or more is ^less than 30/mm2, and the hardness difference between the position within 1.0mm from the back of the steel plate surface toward the plate thickness direction and the center of the plate thickness is less than 40HV.

Description

Hot-rolled steel plates, square steel pipes, methods for manufacturing the same and building structures

The present invention relates to a hot-rolled steel plate, and a square steel pipe (corner column) manufactured by cold rolling using the hot-rolled steel plate as a raw material. In particular, it relates to a square steel pipe suitable for a building structural member of a large building. And it relates to a building structure obtained by using the square steel pipe.

In recent years, building structural components used in large buildings such as factories, warehouses, and commercial facilities (hereinafter referred to as buildings) have been continuously strengthened in order to reduce construction costs by reducing weight. In particular, square steel pipes (corner columns) with flat plates and corners used as columns for buildings require high strength for the flat plates. At the same time, square steel pipes used for building structural components are also required to have excellent toughness from the perspective of earthquake resistance.

Square steel pipes are usually made of hot-rolled steel plates (hot-rolled steel strips) or thick steel plates, and then manufactured by cold forming the materials. As the cold forming method, there are cold stamping forming method or cold roll forming method.

字形(U字形)，將該等藉由埋弧熔接進行接合而加以製造。 The square steel tube is produced by roll-forming the material (hereinafter referred to as roll-formed square steel tube), and the hot-rolled steel plate is roll-formed in the cold to form a cylindrical open tube, and then the butt joints are resistively welded. Then, the cylindrical open tube (round steel tube) is drawn by several % in the tube axis direction by rollers arranged above, below, left and right of the open tube, and then formed into a square to produce a square steel tube. On the other hand, the square steel tube is produced by stamping and bending the material (hereinafter referred to as stamping-formed square steel tube), and the thick steel plate is cold stamped to form the cross-sectional shape into a square (rectangular) or square (square) shape.

The U-shaped ones are produced by joining them by submerged arc welding.

Compared with the manufacturing method of stamping square steel pipes, the manufacturing method of rolling square steel pipes has high productivity and has the advantage of being able to be manufactured in a short time. However, in the stamping square steel pipe, the flat part is not cold-formed, and only the corners are work-hardened. In contrast, in the rolling square steel pipe, especially when cold-forming into a cylindrical shape, a large work strain is introduced in the axial direction of the tube throughout the entire circumference of the steel pipe. Therefore, the rolling square steel pipe has the problem of high yield ratio and low toughness in the axial direction of the tube not only in the corners but also in the flat parts.

Moreover, for roll-formed square steel tubes, the thicker the wall, the greater the work hardening during roll forming, so the yield ratio becomes higher and the toughness becomes lower. Therefore, especially when manufacturing thick-walled roll-formed square steel tubes, it is necessary to select a material that can withstand the increase in yield ratio and decrease in toughness caused by roll forming.

For such a requirement, for example, in Patent Document 1, when calculated by weight%, a steel billet containing less than 0.20% C and further containing one or two of Mn: 0.40~0.90%, Nb: 0.005~0.040% and Ti: 0.005~0.050% is subjected to hot rolling in the non-recrystallization temperature range with a reduction ratio of 55% or more, a rolling end temperature of 730~830℃, and a coiling temperature of 550℃ or less as a hot rolling process for the coil, and the circumference is drawn to less than 3 times the plate thickness outside the steel pipe forming process, and a square steel pipe with a yield ratio of less than 90% and a Charpy absorption energy of more than 27J at a test temperature of 0℃ is proposed.

In patent document 2, when calculated by mass%, steel containing 0.07-0.18% C and 0.3-1.5% Mn is heated to a heating 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 start temperature of 1100-850°C and a finish rolling end temperature of 900-750°C, and then subjected to primary cooling and air cooling until the surface temperature is cooled to a cooling stop temperature of 550°C or above. By performing secondary cooling of 3 to 15 seconds and tertiary cooling of cooling at an average cooling rate of 4 to 15°C/s in the temperature range of 750 to 650°C in the middle of the plate thickness to below 650°C, the value of the second phase frequency contained in the steel structure is set to 0.20 to 0.42, thereby proposing a square steel pipe having mechanical properties showing a low yield ratio of less than 80% and an absorbed energy of more than 150J in the Charpy impact test at a test temperature of 0°C.

Patent document 3 proposes that, when calculated by mass%, steel containing 0.07-0.18% C and 0.3-1.5% Mn is heated to a heating 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 start temperature of 1100-850°C and a finish rolling end temperature of 900-750°C, so that the surface temperature is 750-65 The average cooling rate in the temperature range of 0°C is less than 20°C/s, the time until the temperature of the center of the plate thickness reaches 650°C is within 35s, and the average cooling rate in the temperature range of 750~650°C in the center of the plate thickness is 4~15°C/s. A square steel pipe is proposed, which has mechanical properties of a yield ratio of less than 80%, a test temperature of 0°C, and an absorbed energy of more than 150J in the Charpy impact test.

In Patent Document 4, when calculated by mass%, steel containing C: 0.07-0.20%, Mn: 0.3-2.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.01-0.06%, N: 0.006% or less and having a composition consisting of residual Fe and inevitable impurities is heated to a heating temperature of 110 After 0~1300℃, the rough rolling with the end temperature of rough rolling: 1150~950℃ and the finishing rolling with the start temperature of finishing rolling: 1100~850℃ and the end temperature of finishing rolling: 900~750℃ are carried out. Then, the average cooling rate from the start of cooling to the end of cooling becomes 4~25℃/s, and cooling is carried out until the cooling stop temperature: 5 The steel structure of the central part of the plate thickness is composed of a main phase of granular iron and a phase composed of one or more types selected from pulverized iron, pseudo-pulverized iron and upper tantalum iron. The steel structure of the main phase and the second phase has an average grain size of 7~20μm, and the steel structure on the back of the plate thickness is a single phase of granular iron or a single phase of ductile iron granular iron, with an average grain size of 2~20μm, showing a low yield ratio of less than 90%, and a square steel tube with mechanical properties of more than 27J of absorption energy in the Charpy impact test at a test temperature of 0℃.

[Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Publication No. 9-87743

[Patent Document 2] Japanese Patent Publication No. 5594165

[Patent Document 3] Japanese Patent Publication No. 5589885

[Patent Document 4] Japanese Patent No. 6388091

Here, the larger the wall thickness of the square steel pipe manufactured by cold rolling becomes, or the smaller the length of the side becomes, the greater the processing strain introduced into the square steel pipe, and the greater the increase in yield ratio and the decrease in toughness become. Therefore, the hot-rolled steel plate as the raw material needs to have a steel structure that suppresses the increase in yield ratio during forming and excellent low-temperature toughness that can withstand the deterioration of toughness caused by large processing strain. However, in the square steel pipe manufactured by the method disclosed in the above-mentioned patent documents 1 to 3, especially when the plate thickness exceeds 25mm, there is a problem that the yield ratio becomes higher and cannot meet the yield ratio of less than 90%.

In the technology described in Patent Document 4, in order to obtain a low yield ratio and high toughness, the steel structure on the back of the plate surface must be a single phase of granular iron or a single phase of ductile iron-granular iron. In order to suppress the formation of wave iron or pseudo-wave iron, the cooling process when manufacturing hot-rolled steel plates is subjected to sudden cooling, etc., and there are issues that are restricted by the equipment and conditions that can be manufactured.

This invention is developed in view of the above-mentioned problems, and its purpose is to provide a hot-rolled steel plate with high yield strength and tensile strength, low yield ratio, and excellent low-temperature toughness, a square steel pipe using the hot-rolled steel plate, the manufacturing method thereof, and a building structure using the aforementioned square steel pipe.

Here, (1) high yield strength, (2) high tensile strength, and (3) low yield ratio refer to the following conditions: (1) yield strength of 250 MPa or more, (2) tensile strength of 400 MPa or more, and (3) yield ratio of 0.75 or less, in order, using a JIS No. 5 tensile test piece taken in a manner in which the tensile direction is parallel to the rolling direction, and in accordance with the tensile test specified in JIS Z2241 (2011).

In addition, the so-called excellent low-temperature toughness means that according to the provisions of JIS Z2242 (2018), at the t/2 position of the plate thickness t (the center of the plate thickness), a V-notch standard test piece is used with the test piece length direction parallel to the rolling direction. The Charpy impact test is carried out at the test temperatures: -80℃, -60℃, -40℃, -20℃, and 0℃. The Charpy absorbed energy at -20℃ is more than 100J and the ductility-brittleness transition temperature is below -20℃.

In addition, the square steel pipe of the present invention refers to a JIS No. 5 drawing test piece taken in a manner in which the drawing direction is parallel to the tube axis direction, and the yield strength of the flat plate portion is 295 MPa or more, the tensile strength of the flat plate portion is 400 MPa or more, and the yield ratio of the flat plate portion is 0.90 or less by a drawing test in accordance with the provisions of JIS Z2241 (2011), and in accordance with the provisions of JIS Z2242 (2018), at the t/4 position of the wall thickness t from the outer surface of the tube, a V-notch standard test piece is taken in which the length direction of the test piece is parallel to the tube axis direction, and the Charpy impact test is performed at the test temperatures: -60°C, -40°C, -20°C, 0°C, and +20°C, and the Charpy absorbed energy of the flat plate portion at -20°C is 40 J or more. Square steel tubes with a flat plate ductility-brittle transition temperature below -5°C.

The inventors of the present invention have conducted careful research to solve the above-mentioned problems. As a result, they have obtained the following insights (1)~(3).

(1) In order to make the hot-rolled steel plate meet the yield strength and tensile strength required for the purpose of the present invention, the C content needs to be made 0.07 mass% or more. In addition, the main structure (main phase) of the hot-rolled steel plate at the center of the plate thickness and the surface and back of the plate needs to be granular iron.

(2) In order to further obtain the low temperature toughness and yield ratio required for the purpose of the present invention, the hot-rolled steel sheet, in addition to the above (1), needs to have a second phase consisting of one or more selected from pulverized iron, pseudo-pulverized iron and variable-toughness iron, the area ratio of the total pulverized iron and pseudo-pulverized iron being set to 6-25%, the area ratio of the upper variable-toughness iron being set to less than 5%, and the area ratio of the upper variable-toughness iron being set to less than 5%. When the area surrounded by the boundary where the orientation difference of adjacent crystals at the center of the plate thickness is 15° or more is taken as the crystal grain, the average crystal grain size of the steel structure including the main phase and the secondary phase is 10.0~30.0μm, the area ratio of crystal grains with an equivalent circular diameter of 40.0μm or more is 20% or less, and the number 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 , and the hardness difference between the position within 1.0mm in the plate thickness direction from the back of the steel plate surface and the center of the plate thickness is 40HV or less.

(3) In order to obtain the aforementioned steel structure, it is necessary to adjust the component composition to the correct range, control the content of Mn and Si within a specific range, and adjust the cooling stop temperature and coiling temperature of the cooling process after the hot rolling process to the correct range.

This invention is developed based on such knowledge and consists of the following gist.

[1] When calculated by mass%, it contains C: 0.07% to 0.20%, Si: 0.40% to 0.40%, Mn: 0.20% to 1.00%, P: 0.100% to 0.100%, S: 0.050% to 0.050%, Al: 0.005% to 0.100%, N: 0.0100% to 0.0100%, or further contains 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%, B: 0.0003% to 0.0100%, one or two selected from the following, the contents of Mn and Si satisfy the following formula (1)

The remaining part has a composition composed of Fe and inevitable impurities. The steel structure at the center of the plate thickness and the back of the plate surface has a main phase composed of ferrous iron, a second phase with an area ratio of 6-25% of the total area ratio of pulex and pseudopulex and an area ratio of less than 5% of upper ductile iron. When the orientation difference of the adjacent crystals at the center of the plate thickness is more than 15°, When the area surrounded by the boundary is regarded as the crystal grain, the average crystal grain size of the steel structure in the center of the plate thickness is 10.0~30.0μm, the area ratio of the crystal grains with an equivalent circular diameter of 40.0μm or more is less than 20%, the number of the crystal grains with a ratio of major diameter to minor diameter (= (major diameter)/(minor diameter)) of 3.0 or more is less than 30/ ^mm2 , and the hardness difference between the position within 1.0mm from the back of the steel plate surface in the plate thickness direction and the center of the plate thickness is less than 40HV.

1.0≦%Mn/%Si≦3.5． ． ． (1)

Here, %Mn and %Si are the contents (mass %) of each element.

[2] The hot-rolled steel sheet as described in [1] above, wherein the yield strength is 250 MPa or more, the tensile strength is 400 MPa or more, the yield ratio is 0.75 or less, the Charpy absorbed energy at -20°C is 100 J or more, and the ductility-brittleness transition temperature is -20°C or less.

[3] The hot-rolled steel plate as described in [1] or [2] above, wherein the steel structure has 5 to 15% of the above-mentioned false wave iron when calculated by area ratio.

[4] A hot-rolled steel plate as described in any one of [1] to [3] above, wherein the plate thickness is greater than 20 mm.

[5] A method for manufacturing a hot-rolled steel plate, wherein the hot-rolled steel plate is any one of the hot-rolled steel plates described in any one of [1] to [4], characterized by comprising: heating the steel material at a heating temperature of 1100°C to 1300°C, and then hot rolling the steel material at a rough rolling end temperature of 850°C to 1150°C, a finish rolling end temperature of 750°C to 850°C, and a total reduction ratio of 40% to 59% at a temperature below 930°C; and then cooling the steel material at an average temperature of the center of the plate thickness to reduce the average temperature of the hot-rolled steel plate. The cooling rate Vc (℃/s) and the average cooling rate Vs (℃/s) of the plate surface from the back of the plate surface to a depth of 1.0mm in the thickness direction meet the following equations (2) and (3), and the hot rolled steel plate is air-cooled for more than 5 seconds from the start of cooling to the end of cooling, and the cooling is carried out at the cooling stop temperature of the center of the plate thickness: 550℃ to 680℃; and then, the coiling process is carried out at the center temperature of the plate thickness: 550℃ to 680℃.

2≦Vc≦15． ． ． (2)

Vs/Vc≦2.0． ． ． (3)

[6] A square steel pipe, which is made of the hot-rolled steel plate as described in any one of [1] to [4] above.

[7] A method for manufacturing a square steel pipe, comprising cold-rolling the hot-rolled steel plate of any one of [1] to [4] to obtain a square steel pipe.

[8] A building structure using the square steel pipe as mentioned above [6] as column material.

According to the present invention, it is possible to provide a hot-rolled steel plate with high yield strength and tensile strength, low yield ratio, and excellent low-temperature toughness, a square steel pipe using the hot-rolled steel plate, a manufacturing method thereof, and a building structure using the aforementioned square steel pipe.

1: Square steel pipe

4: Main beam

5: Small beam

6: Diaphragm

7: Intermediate columns

[Figure 1] is a three-dimensional diagram schematically showing an example of a building structure using the square steel pipe of the present invention.

[Figure 2] is a schematic diagram showing the location of the flat plate pulling test piece implemented by the present invention.

[Figure 3] is a schematic diagram showing the location of the Charpy test piece implemented by the present invention.

The following describes in detail the implementation of the present invention.

<Hot-rolled steel plate>

The hot-rolled steel sheet of the present invention has the following composition, that is, when calculated by mass%, it contains C: 0.07% to 0.20%, Si: 0.40% to 1.00%, Mn: 0.20% to 1.00%, P: 0.100% to 0.100%, S: 0.050% to 0.050%, Al: 0.005% to 0.100%, N: 0.0100%, the contents of Mn and Si satisfy the following formula (1), the remainder has a steel structure composed of Fe and inevitable impurities, and the steel structure at the center of the plate thickness and the surface and back of the plate is a main phase composed of ferrous iron, and pulex and pseudo-pulex. The second phase has an area ratio of 6 to 25% of the total iron and an area ratio of 5% or less of the upper ductile iron. When the area surrounded by the boundary where the orientation difference of the adjacent crystals in the center of the plate thickness is 15° or more is taken as the crystal grain, the average crystal grain size of the steel structure in the center of the plate thickness is 10.0 to 30.0μm and the equivalent circle diameter is 40.0μm or more. The number of crystal grains with a ratio of the major diameter to the minor diameter (ratio of major diameter to minor diameter (= (major diameter)/(minor diameter)) of 3.0 or more in the steel structure in the center of the plate thickness is 20% or less, and the number of crystal grains is 30/mm ² or less, and the hardness difference between the position 1.0mm inside the back of the steel plate in the plate thickness direction and the center of the plate thickness is 40HV or less. In addition, "hot-rolled steel plate" includes hot-rolled steel plate and hot-rolled steel strip.

1.0≦%Mn/%Si≦3.5． ． ． (1)

Here, %Mn and %Si are the contents (mass %) of each element.

The following is a description of the composition of the hot-rolled steel sheet of the present invention. In addition, unless otherwise specified, the "%" indicating the steel composition is "mass %".

C: 0.07% or more and 0.20% or less

C is an element that improves the strength of the steel plate by solid solution strengthening. In addition, C is an element that contributes to the formation of pulex and pseudopulex, which are one of the second phases. In order to ensure the strength and yield ratio required for the purpose of the present invention, it is necessary to contain 0.07% or more of C. However, if the C content exceeds 0.20%, the proportion of the hard phase becomes high and the toughness decreases. In addition, if the yield ratio exceeds 0.90, it becomes impossible to obtain the desired yield ratio. In addition, the weldability will also deteriorate. Therefore, the C content is set to be between 0.07% and 0.20%. The C content is preferably 0.08% or more, and more preferably 0.10% or more. In addition, the C content is preferably 0.18% or less, and more preferably 0.17% or less.

Si: less than 0.40%

Si is an element that improves the strength of steel plates by solid solution strengthening, and can be contained as needed. In order to obtain such an effect, it is expected to contain more than 0.01% Si. However, if the Si content exceeds 0.40%, oxides are easily generated in the resistance welded portion, resulting in reduced weld properties. In addition, the toughness of the base material portion other than the resistance welded portion is also reduced. Therefore, the Si content is set to 0.40% or less. The Si content is preferably 0.01% or more, and more preferably 0.05% or more. In addition, the Si content is preferably 0.37% or less, and more preferably 0.35% or less.

Mn: 0.20% or more and 1.00% or less

Mn is an element that improves the strength of steel plates by solid solution strengthening. In addition, Mn is an element that contributes to the refinement of the structure by lowering the starting temperature of the ferrous iron transformation. In order to ensure the strength and structure required for the purpose of the present invention, it is necessary to contain 0.20% or more of Mn. However, if the Mn content exceeds 1.00%, the yield ratio exceeds 0.90 due to the increase in the amount of ductile iron generated, and the desired yield ratio cannot be obtained. In addition, if the Mn content exceeds 1.00%, the hardness of the central segregation portion increases, which may become a cause of cracking during welding. Therefore, the Mn content is set to be above 0.20% and below 1.00%. The Mn content is preferably above 0.25%, and more preferably above 0.30%. In addition, the Mn content is preferably below 0.95%, and more preferably below 0.90%.

P: less than 0.100%

Since P segregates at grain boundaries and causes material inhomogeneity, it is ideal to reduce it as much as possible as an inevitable impurity, but it can be allowed to be up to 0.100%. Therefore, the P content is set within the range of 0.100% or less. The P content is ideally 0.030% or less, and more ideally 0.020% or less. In addition, the lower limit of P is not specifically limited, but excessive reduction will lead to increased smelting costs, so P is ideally set to 0.002% or more.

S: less than 0.050%

S generally exists in the form of MnS in steel, but MnS is thinly stretched during the hot rolling process, which has an adverse effect on ductility. Therefore, in the present invention, it is better to reduce S as much as possible, but it can be allowed to be up to 0.050%. Therefore, the S content is set to less than 0.050%. The S content is ideally less than 0.015%, more ideally less than 0.010%, and even more ideally less than 0.008%. In addition, the lower limit of S is not specifically limited, but excessive reduction will lead to high smelting costs, so S is ideally set to more than 0.001%.

Al: 0.005% or more and 0.100% or less

Al is an element that acts as a strong deoxidizer. In order to obtain such an effect, it is desirable to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates, and the alumina inclusions increase, and the surface properties deteriorate. In addition, the toughness of the welded portion also decreases. Therefore, the Al content is set to 0.005% or more and 0.100% or less. The Al content is preferably 0.010% or more, and more preferably 0.015% or more. In addition, the Al content is preferably 0.070% or less, and more preferably 0.050% or less.

N: 0.0100% or less

N is an inevitable impurity, an element that has the effect of reducing toughness by firmly fixing dislocation movement. Therefore, in the present invention, it is better to reduce N as an impurity as much as possible, but the N content can be allowed to be up to 0.0100%. Therefore, the N content is set to less than 0.0100%. The N content is ideally less than 0.0080%, more ideally less than 0.0040%, and even more ideally less than 0.0035%. Furthermore, excessive reduction will lead to high smelting costs, so the N content is ideally set to more than 0.0010%, and more ideally more than 0.0015%.

1.0≦%Mn/%Si≦3.5． ． ． (1)

Here, %Mn and %Si are the contents (mass %) of each element.

In the present invention, the content of Mn and Si needs to be set to the above range and meet the equation (1) of 1.0≦%Mn/%Si≦3.5. By meeting this relationship, a steel structure having a second phase with an area ratio of 6-25% of Porphyry and/or pseudo-Porphyry and an area ratio of 5% or less of upper tantalum can be obtained, and the strength, yield ratio, Charpy absorbed energy, and ductility-brittleness transition temperature required for the purpose of the present invention can be obtained. %Mn/%Si is preferably 1.2 or more, and more preferably 1.4 or more. In addition, %Mn/%Si is preferably 3.2 or less, and more preferably 3.0 or less.

The remainder is Fe and unavoidable impurities. However, within the range that does not damage the effect of the present invention, it may contain 0.005% or less of O (oxygen).

In addition, regarding Nb, Ti, V, Cr, Mo, Cu, Ni, Ca, and B, which are optional elements described below, Nb: more than 0% and less than 0.005%, Ti: more than 0% and less than 0.005%, V: more than 0% and less than 0.01%, Cr: more than 0% and less than 0.01%, Mo: more than 0% and less than 0.01%, Cu: more than 0% and less than 0.01%, Ni: more than 0% and less than 0.01%, Ca: more than 0% and less than 0.0005%, and B: more than 0% and less than 0.0003% are regarded as unavoidable impurities.

The above components are the basic components of the hot-rolled steel sheet of the present invention. The properties of the present invention can be obtained with the above necessary elements, but the following elements can be contained as needed.

Select one or more of 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%, 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 are elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through precipitation strengthening. In order to obtain such an effect, when Nb is contained, 0.005% or more of Nb is contained. In addition, when Ti is contained, 0.005% or more of Ti is contained.

On the other hand, Nb and Ti, respectively, have a content exceeding 0.020% that may form coarse carbides and nitrides, leading to a decrease in toughness. Therefore, when Nb is contained, the Nb content is 0.005% to 0.020%, and when Ti is contained, the Ti content is 0.005% to 0.020%. For Nb and Ti, respectively, it is ideal to be 0.007% or more, and more ideally 0.009% or more. In addition, for Nb and Ti, respectively, it is ideal to be 0.018% or less, and more ideally 0.016% or less.

V: 0.01% to 0.10%, Cr: 0.01% to 0.50%, Mo: 0.01% to 0.50%

V, Cr, and Mo are elements that improve the hardenability of steel and increase the strength of steel, and can be contained as needed. In order to obtain the above effects, when V, Cr, and Mo are contained, the V content is set to 0.01% or more, the Cr content is set to 0.01% or more, and the Mo content is set to 0.01% or more. The ideal V content is 0.02% or more, the Cr content is 0.05% or more, and the Mo content is 0.05% or more. More ideally, the V content is 0.03% or more, the Cr content is 0.08% or more, and the Mo content is 0.08% or more.

On the other hand, excessive inclusion may lead to reduced toughness and deterioration of weldability. Therefore, when V, Cr, and Mo are contained, the V content is set to less than 0.10%, the Cr content is set to less than 0.50%, and the Mo content is set to less than 0.50%. The ideal V content is less than 0.08%, the Cr content is less than 0.40%, and the Mo content is less than 0.40%. More ideally, the V content is less than 0.07%, the Cr content is less than 0.35%, and the Mo content is less than 0.35%.

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 improve the strength of steel sheets by solid solution strengthening and can be contained as needed. In order to obtain the above-mentioned effects, when Cu and Ni are contained, the Cu content is set to 0.01% or more and the Ni content is set to 0.01% or more. It is ideal that the Cu content is 0.02% or more and the Ni content is 0.02% or more. It is more ideal that the Cu content is 0.10% or more and the Ni content is 0.10% or more. On the other hand, excessive inclusion may lead to reduced toughness and deterioration of weldability. Therefore, when Cu and Ni are contained, the Cu content is set to 0.30% or less and the Ni content is set to 0.30% or less. It is ideal that the Cu content is 0.20% or less and the Ni content is 0.20% or less. It is more ideal that the Cu content is 0.15% or less and the Ni content is 0.15% or less.

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 thinly stretched during the hot rolling process, and can be contained as needed. In order to obtain such an effect, when Ca is contained, it is desirable to contain 0.0005% or more of Ca. However, if the Ca content exceeds 0.0100%, Ca oxide clusters may be formed in the steel, and the toughness may deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.0100% or less. Furthermore, the Ca content is preferably 0.0005% or more. More preferably, the Ca content is 0.0010% or more. The Ca content is preferably 0.0050% or less.

B: 0.0003% or more and 0.0100% or less

B is an element that contributes to the refinement of the structure by lowering the starting temperature of the transformation of ferrous iron. In order to obtain such an effect, when B is contained, it is desirable to contain 0.0003% or more of B. However, if the B content exceeds 0.0100%, the yield ratio may increase. Therefore, when B is contained, it is ideal to be 0.0100% or less. More ideally, the B content is 0.0005% or more. Ideally, the B content is 0.0050% or less.

Next, the reasons for limiting the steel structure of the hot-rolled steel plate of the present invention are explained.

In the steel structure of the hot-rolled steel plate of the present invention, the steel structure at the center of the plate thickness and the surface and back of the plate has a main phase composed of granular iron, and a second phase whose area ratio of the sum of pulex and pseudo-pulex is 6 to 25% and the area ratio of upper ductile iron is 5% or less. In the center of the plate thickness, when the area surrounded by the boundary with the orientation difference of adjacent crystals being 15° or more is taken as the grain, the plate The average crystal grain size of the steel structure in the center of the thickness is 10.0~30.0μm, the equivalent circular diameter is 40.0μm or more, the aforementioned grains are 20% or less relative to the entire steel structure in the center of the plate thickness when calculated by area ratio, and the number of crystal grains with a major diameter to minor diameter ratio (ratio of major diameter to minor diameter (= (major diameter)/(minor diameter)) of 3.0 or more is 30/mm2 or ^less , and the hardness difference between the position within 1.0mm in the plate thickness direction from the back surface of the plate and the center of the plate thickness is 40HV or less.

Furthermore, in the present invention, the equivalent circle diameter (grain diameter) refers to the diameter of a circle with the same area as the grains of interest. In addition, the steel structure is the center of the plate thickness and the plate surface (plate surface back) of the hot-rolled steel plate. Here, the plate surface (plate surface back) refers to the position within 1.0 mm from the surface of the steel plate in the plate thickness direction.

Main phase: Fertilizer iron

Granulated iron is a soft structure. In order to obtain the desired strength and low yield ratio, it is used as the main phase in the present invention. The ideal granulated iron is 70% or more when calculated by area ratio, and more preferably 72% or more. If the area ratio of granulated iron exceeds 94%, the strength will decrease, and the desired yield strength and tensile strength may not be obtained. Therefore, the ideal granulated iron is 94% or less when calculated by area ratio, and more preferably 92% or less.

The total area ratio of the wave iron and the pseudo wave iron is 6~25%, and the area ratio of the upper part of the variable toughness iron is less than 5%

Porphyry and pseudo-Porphyry are hard structures, and are the most important steel structures in order to increase the strength of steel and obtain a low yield ratio. In order to obtain the yield strength, tensile strength, and yield ratio required by the present invention, the total area ratio of Porphyry and pseudo-Porphyry needs to be set to 6% or more. The area ratio of this total is ideally 7% or more, and more ideally 9% or more. If the area ratio of Porphyry and pseudo-Porphyry exceeds 25%, the toughness may deteriorate. Therefore, the total area ratio of Porphyry and pseudo-Porphyry needs to be less than 25%. The area ratio of this total is ideally 23% or less, and more ideally 21% or less.

In addition, the area ratio of pseudo-corrosion is ideally 5% or more. If pseudo-corrosion exists at an area ratio of 5% or more, the yield ratio can be suppressed when manufacturing square steel pipes, so better earthquake resistance can be obtained. In order to make the area ratio of pseudo-corrosion exceed 15%, it is necessary to cool the temperature range generated by corrosion in the cooling process during hot rolling, so the manufacturing conditions are limited. Therefore, the area ratio of pseudo-corrosion is ideally below 15%.

The upper tungsten iron has a hardness structure between ferrous iron and pulverized iron, which increases the strength of the steel. However, if the area ratio of the upper tungsten iron exceeds 5%, the low yield ratio of the present invention cannot be obtained. Therefore, the area ratio of the upper tungsten iron needs to be less than 5%. Ideally, it is less than 4%. The upper tungsten iron can also be 0%.

Furthermore, the area ratio of granular iron, pulex iron, pseudo-pulex iron, and upper ductile iron can be measured by the following method.

In the center of the plate thickness, when the area surrounded by the boundary where the orientation difference (crystal orientation difference) of the adjacent crystals is 15° or more is taken as the crystal grains, the average crystal grain size of the steel structure in the center of the plate thickness is 10.0~30.0μm, the equivalent circle diameter is 40.0μm or more, the aforementioned crystal grains are less than 20% when calculated by area ratio, and the ratio of the major diameter to the minor diameter (= (major diameter)/(minor diameter)) is 3.0 or more, and the number of the aforementioned crystal grains is less than 30 pieces/ ^mm2 .

As described above, the steel structure of the present invention is made into a steel mixed with soft structure and hard structure (hereinafter referred to as "composite structure steel") in order to obtain the yield ratio, yield strength and tensile strength of the present invention. However, the toughness of composite structure steel is poorer than that of single structure steel. Therefore, in the present invention, since the aforementioned mechanical properties and excellent toughness coexist, when the area surrounded by the boundary with a crystal orientation difference of more than 15° is regarded as a crystal grain, the crystal grain size of the steel structure containing the main phase and the secondary phase, the area ratio of the coarse crystal grains and the number of elongated crystal grains are specified.

If the average crystal grain size (equivalent circular diameter) of the steel structure containing the main phase and the second phase is less than 10.0μm, the yield ratio increases and the yield ratio of the present invention cannot be obtained. On the other hand, if the average crystal grain size of the steel structure containing the main phase and the second phase exceeds 30.0μm, the toughness will deteriorate. Therefore, the average crystal grain size of the steel structure containing the main phase and the second phase must be 10.0~30.0μm. It is ideally 11.0μm or more, and more ideally 12.5μm or more. In addition, the average crystal grain size is ideally 28.0μm or less, and more ideally 26.0μm or less.

However, even if the average grain size is within the range of 10.0~30.0μm, the yield ratio and Charpy absorption energy of the present invention may not be obtained. The inventors of the present invention have carefully examined and found that in order to obtain the toughness of the present invention, the grains with an equivalent circular diameter of 40.0μm or more need to be less than 20% when calculated as the area ratio of the entire steel structure in the center of the plate thickness, and in order to obtain the yield ratio of the present invention, the number of grains with a ratio of major diameter to minor diameter (= (major diameter)/(minor diameter)) of 3.0 or more needs to be less than 30/ ^mm2 .

Furthermore, the crystal orientation difference, average crystal grain size, and the area ratio of crystal grains with a crystal grain size (equivalent circular diameter) of 40.0 μm or more can be measured by the SEM/EBSD method. Here, the measurement can be performed by the method described below.

In the structural observation, test pieces were taken from the center of the width direction of the hot-rolled steel plate and the position of the plate thickness t/2 (t: plate thickness) (the plate thickness center of the present invention) and the plate surface (the back side of the steel plate (the lower surface during hot rolling). After polishing in such a way that the observation surface becomes a cross section in the rolling direction during hot rolling, nitrate etching was performed to produce it.

The structure observation is to observe and take images of the structure of the hot-rolled steel plate at the plate thickness t/2 position (plate thickness center) and the back side (plate surface) using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times). The area ratios of ferrite, pulex, pseudopulex, and upper ductile iron are obtained from the obtained optical microscope images and SEM images.

The area ratio of each structure is observed in 5 fields of view and calculated as the average value of the values obtained in each field of view. Here, the area ratio obtained by tissue observation is used as the area ratio of each structure. The area ratio of ferrite, pulex, pseudopulex, and upper ferrite is calculated by using an optical microscope image or SEM image to distinguish each phase by the shape and color shown below, and then dividing it by the area of the entire optical microscope image or SEM image to calculate the area ratio of each phase.

Here, ferrite is a product produced by diffusion transformation, and has a low dislocation density and a basically restored structure. Polygonal ferrite and pseudo-polygonal ferrite are included here. Pole iron is a structure in which ferrite and ferrite are arranged in layers, and pseudo-porre iron is a structure in which ferrite is confirmed to be arranged in a point array in ferrite. In addition, upper ferrite is a complex structure of layered ferrite and ferrite with high dislocation density. In addition to the above-mentioned shapes, it can be distinguished from the fact that ferrite is white, ferrite is black, pseudo-porre iron is black or gray, and upper ferrite is white or gray.

In addition, the average crystal grain size (average equivalent circular diameter) is measured at the plate thickness t/2 position (plate thickness center) using the SEM/EBSD method. The measurement area is 500μm×1000μm (= ^0.5mm2 ), and the measurement step size is 0.5μm. The crystal grain size is determined by finding the orientation difference between adjacent crystal grains, and the boundary with an orientation difference of 15° or more is measured as the crystal grain boundary. From the obtained crystal grain boundary, the grain size (equivalent circular diameter) of each crystal grain is calculated, and the arithmetic average is calculated as the average crystal grain size (average equivalent circular diameter). In addition, the total area of crystal grains with an equivalent circular diameter of 40.0 μm or more was calculated and then divided by the area of the measurement area (0.5 mm ² ) to calculate the area ratio of crystal grains with an equivalent circular diameter of 40.0 μm or more. The above-mentioned equivalent circular diameter and crystal grain area were obtained by analyzing the data obtained by SEM/EBSD measurement using OIM Analysis manufactured by EDAX.

In addition, the major diameter and minor diameter of the crystal grains were measured by the method of JIS R 1670 (2006), and the major diameter to minor diameter ratio (= (major diameter)/(minor diameter)) was calculated. The number of crystal grains with a major diameter to minor diameter ratio (= (major diameter)/(minor diameter)) of 3.0 or more was measured and divided by the area of the measurement region (0.5 mm ² ), thereby calculating the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more (pieces/mm ² ). In the crystal grain size analysis and the measurement of the number of crystal grains, crystal grains with a diameter of less than 2.0 μm were excluded from the analysis object as measurement noise.

The hardness difference between the position 1.0mm inside the back of the steel plate in the thickness direction and the center of the plate thickness: less than 40HV.

The hardness difference between the position 1.0mm inside from the back of the plate surface toward the plate thickness direction and the center of the plate thickness of the hot-rolled steel plate of the present invention is 40HV or less. When the hardness difference between the position 1.0mm inside from the back of the plate surface toward the plate thickness direction and the center of the plate thickness exceeds 40HV, when manufacturing a square steel pipe, there is a risk that the yield ratio of the flat plate portion of the square steel pipe will exceed 0.90. In addition, there is also a case of toughness deterioration. The hardness difference between the position 1.0mm inside from the back of the plate surface toward the plate thickness direction and the center of the plate thickness is preferably 35HV or less, and more preferably 30HV or less.

Here, the hardness difference between the position 1.0mm inside from the back of the plate surface to the plate thickness direction and the center of the plate thickness is greater. Therefore, in the present invention, the hardness of the position 1.0mm inside from the surface of the steel plate to the plate thickness direction and the hardness of the position 1.0mm inside from the back of the plate to the plate thickness direction are measured, and the greater hardness is taken as the hardness of the position 1.0mm inside from the back of the plate surface to the plate thickness direction.

In addition, as a method for measuring the aforementioned hardness, first, in accordance with the provisions of JIS Z 2244 (2020), the Vickers hardness is measured with a test force of 9.8N (1kgf). Measure 10 points each, calculate the average value of the 8 points excluding the maximum and minimum values, and take the average value (total of the Vickers hardness of the 8 points/8) as the hardness of the test piece. The test piece uses the test piece for the aforementioned tissue observation, and after mirror polishing from the back of the steel plate surface toward the position 1.0mm inside in the plate thickness direction, the hardness is measured. In addition, the difference between the hardness of the back of the plate surface and the hardness of the center of the plate thickness (hardness difference) is calculated.

By meeting the aforementioned component composition and steel structure, and further meeting a specific hardness difference, a hot-rolled steel plate having the strength, yield ratio, and toughness (Charpy absorbed energy at -20°C, ductility-brittleness transition temperature) that are the purpose of the present invention can be obtained.

Specifically, the hot-rolled steel sheet of the present invention can have a yield strength of 250 MPa or more, a tensile strength of 400 MPa or more, a yield ratio of 0.75 or less, a Charpy absorbed energy of -20°C of 100 J or more, and a ductility-brittleness transition temperature of -20°C or less.

The hot-rolled steel plate of the present invention can be ideally used as a hot-rolled steel plate for square steel pipes with a low yield ratio, and can obtain square steel pipes with a yield ratio described below.

The thickness of the hot-rolled steel plate of the present invention is preferably 20 mm or more. In addition, the thickness of the hot-rolled steel plate of the present invention is preferably 20 to 32 mm.

<Method for manufacturing hot-rolled steel plate>

Next, as a method for manufacturing a hot-rolled steel plate of the present invention, a method for manufacturing a hot-rolled steel plate of one embodiment of the present invention is described.

The manufacturing method of the hot-rolled steel plate of the present invention, for example, in the hot rolling process, after heating the steel material having the above-mentioned composition to a heating temperature of 1100°C to 1300°C, implements hot rolling at a rough rolling end temperature of 850°C to 1150°C, a finish rolling end temperature of 750°C to 850°C, and a total reduction ratio of 40% to 59% at a temperature of 930°C.

Secondly, in the cooling step, the average cooling rate Vc (℃/s) of the center of the plate thickness and the average cooling rate Vs (℃/s) of the plate surface at a depth of 1.0 mm from the plate surface to the thickness direction meet the following equations (2) and (3). The hot-rolled steel plate is air-cooled for more than 5 seconds from the start of cooling to the end of cooling, and the cooling is performed at a cooling stop temperature of 550℃ to 680℃ at the center of the plate thickness. Then, in the coiling process, the hot-rolled steel plate is coiled at a plate center temperature of 550℃ to 680℃ to produce a hot-rolled steel plate.

2≦Vc≦15． ． ． (2)

Vs/Vc≦2.0． ． ． (3)

Furthermore, in the following description of the manufacturing method, unless otherwise specified, "℃" related to temperature indicates the surface temperature of the steel material or steel plate (hot-rolled plate, raw steel plate).

These surface temperatures can be measured with a radiation thermometer or the like. In addition, the temperature at the center of the steel plate thickness can be obtained by calculating the temperature distribution in the steel plate cross section using heat transfer analysis and correcting the result based on the surface temperature of the steel plate. In addition, "hot-rolled steel plate" includes hot-rolled steel plate and hot-rolled steel strip.

In the present invention, the melting method of the steel material (steel plate block) is not particularly limited, and any of the known melting methods such as converter, electric furnace, vacuum melting furnace, etc. is suitable. The casting method is also not particularly limited, but it can be manufactured into the desired size by the known casting method such as continuous casting method. Furthermore, the block-block roll method can also be applied to replace the continuous casting method without any problem. For the molten steel, secondary refining such as ladle refining can also be implemented.

(Hot rolling process)

Secondly, in the hot rolling process, the obtained steel material (steel plate block) is heated to a heating temperature of 1100°C to 1300°C, and then rough rolling is performed with the rough rolling end temperature set at 850°C to 1150°C, and finishing rolling is performed with the finishing temperature set at 750°C to 850°C, and hot rolling is performed at a total reduction rate of 40% to 59% at a temperature below 930°C to produce hot rolled steel plates.

Heating temperature: above 1100℃ and below 1300℃

When the heating temperature is less than 1100°C, the deformation resistance of the rolled material increases, making rolling difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite particles become coarse, making it impossible to obtain fine austenite particles in the subsequent rolling (rough rolling, finish rolling), making it difficult to ensure the average grain size of the steel structure of the hot-rolled steel plate for the purpose of the present invention. In addition, it is difficult to suppress the generation of coarse tungsten and it is difficult to suppress the area ratio of grains with a grain size of 40.0μm or more within the range of the purpose of the present invention. Therefore, the heating temperature of the hot rolling process is set to be above 1100°C and below 1300°C. The ideal heating temperature for the hot rolling process is above 1120°C. In addition, the ideal heating temperature for the hot rolling process is below 1280°C.

Furthermore, in the present invention, in addition to the conventional method of temporarily cooling the steel block (plate) to room temperature after manufacturing and then heating it again, the energy-saving process of directly feeding the steel block (plate) without cooling it to room temperature and directly loading it into the heating furnace in the warm state can be applied without any problem.

Rough rolling end temperature: above 850℃ and below 1150℃

When the rough rolling end temperature is less than 850℃, the surface temperature of the steel sheet becomes below the starting temperature of the phase transformation of ferrous iron in the subsequent finishing rolling, and a large amount of ferrous iron is generated, forming processed ferrous iron particles that extend in the rolling direction, which becomes the cause of the increase in the yield ratio. On the other hand, if the rough rolling end temperature exceeds 1150℃, the reduction in the austenite non-recrystallization temperature region is insufficient, and fine austenite particles cannot be obtained. As a result, the steel structure of the hot-rolled steel plate of the present invention cannot be obtained. When the area surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more is taken as the crystal grains, it is difficult to obtain a steel structure in which the average crystal grain size is 10.0 to 30.0 μm, the ratio of the major diameter to the minor diameter (= (major diameter)/(minor diameter)) is 3.0 or more, the number of crystal grains is 30 or less/ ^mm2 , and when calculated by area ratio, the equivalent circle diameter is 40.0 μm or more, and the aforementioned crystal grains account for less than 20% of the entire steel structure.

In addition, it is difficult to suppress the formation of coarse tungsten iron. Therefore, the rough rolling end temperature is set to 850°C or more and 1150°C or less. The rough rolling end temperature is preferably 860°C or more, and more preferably 870°C or more. In addition, the rough rolling end temperature is preferably 1100°C or less, and more preferably 1050°C or less.

Finishing rolling end temperature: above 750℃ and below 850℃

When the finishing temperature of the finishing rolling is less than 750°C, the surface temperature of the steel plate becomes below the starting temperature of the phase transformation of the ferrous iron during the finishing rolling, forming processed ferrous iron particles that extend in the rolling direction, and there is a possibility of reduced workability. On the other hand, if the finishing temperature of the finishing rolling exceeds 850°C, the amount of reduction in the temperature region where the austenite is not recrystallized is insufficient, and fine austenite particles cannot be obtained. As a result, the crystallized grains become coarse, and it is difficult to ensure the strength for the purpose of the present invention. In addition, it is not easy to suppress the formation of coarse tampered iron. Therefore, the finishing temperature of the finishing rolling is set to be above 750°C and below 850°C. The finishing temperature of the finishing rolling is preferably above 770°C, and more preferably above 780°C. In addition, the finishing temperature of the finishing rolling is ideally below 830°C, and more ideally below 820°C.

Total reduction rate below 930℃: above 40% and below 59%

In the present invention, in the hot rolling process, the subgrains in the austenite are refined, and the granular iron and ductile iron generated in the subsequent cooling process and coiling process are refined, so as to obtain a steel structure of a hot rolled steel plate having the strength and toughness for the purpose of the present invention. In order to refine the subgrains in the austenite in the hot rolling process, it is necessary to increase the reduction rate in the austenite non-recrystallization temperature region to introduce sufficient processing strain. However, if the total reduction rate exceeds 59%, it is easy to generate crystallized grains with a large ratio of long diameter to short diameter, resulting in reduced toughness. Therefore, in the present invention, the total reduction rate below 930°C is set to below 59%. The total reduction rate below 930℃ is ideally 57% or less, and more ideally 55% or less. If the total reduction rate below 930℃ is less than 40%, the grain size of granular iron, ductile iron, etc. will increase, resulting in reduced toughness. Therefore, the total reduction rate below 930℃ is set to 40% or more. The total reduction rate below 930℃ is ideally 42% or more, and more ideally 45% or more.

Furthermore, the temperature is set below 930℃ because if it exceeds 930℃, the austenite will recrystallize during the rolling process, causing the dislocation introduced by the rolling process to disappear, and it is impossible to obtain fine austenite.

The total reduction ratio mentioned above refers to the plate thickness _TE (mm) before the start of rolling and the final plate thickness _TO (mm) after the end of rolling in the rolling channel in the temperature range below 930°C. It can be calculated by the following formula.

Total reduction rate (%) = 100 × ( _{TE -TO} ) / _TE

Furthermore, when the plate is hot rolled, the total reduction rate below 930°C can be set to 40% to 59% in both the rough rolling and the finish rolling. Alternatively, the total reduction rate below 930°C can be set to 40% to 59% in the finish rolling only. In the latter case, if the total reduction rate below 930°C cannot be set to 40% to 59% in the finish rolling only, the plate can be cooled during the rough rolling and the temperature can be set to 930°C or below, and then the total reduction rate below 930°C in both the rough rolling and the finish rolling can be set to 40% to 59%.

In the present invention, the upper limit of the processed plate thickness (the plate thickness of the steel plate after finish rolling) is not specifically specified, but from the perspective of ensuring the necessary reduction ratio, steel plate temperature management, etc., it is ideal to set the processed plate thickness to 32 mm or less.

(Cooling process)

2.0，在從冷卻開始到冷卻停止間將熱軋鋼板進行5秒以上的空冷，再將板厚中心部的冷卻停止溫度設為550℃以上680℃以下進行冷卻。 After the hot rolling process, the hot rolled plate (raw material steel plate for hot rolled steel plate, hereinafter referred to as raw material steel plate) is cooled in the cooling process. In the cooling process, the average cooling rate Vc from the center of the plate thickness to the cooling stop temperature is 2℃/s or more and 15℃/s or less, and the average cooling rate Vs from the back of the plate surface to the cooling stop temperature of the plate surface to a depth of 1.0mm in the thickness direction and Vc meet the Vs/Vc ratio.

2.0, the hot rolled steel plate is air-cooled for more than 5 seconds from the start of cooling to the stop of cooling, and then the cooling stop temperature at the center of the plate thickness is set to 550°C or more and 680°C or less.

Average cooling rate Vc from the start of cooling to the cooling stop temperature (550~680℃) at the center of the plate thickness: 2℃/s or more and 15℃/s or less

When the average cooling rate Vc of the hot-rolled steel plate in the temperature range from the start of cooling to the cooling stop temperature (550~680℃) described later is less than 2℃/s, the nucleation frequency of ferrous iron decreases and the ferrous iron particles coarsen, so the desired strength cannot be obtained. In addition, it is not easy to control the area ratio of the crystal grains with an average crystal grain size of 40.0μm or more as the purpose of the present invention within the desired range.

On the other hand, if the average cooling rate Vc exceeds 15℃/s, a large amount of upper ductile iron will be generated, and the yield ratio of the present invention cannot be obtained. The average cooling rate Vc is preferably above 4℃/s, and more preferably above 5℃/s. It is ideal to be below 12℃/s, and more preferably below 10℃/s.

Vs/Vc≦2.0

When the average cooling rate Vs (℃/s) from the start of cooling to the cooling stop temperature (550~680℃) of the plate surface exceeds 2.0 times the average cooling rate Vc (℃/s) of the plate thickness center until the cooling stop, a large amount of tough iron is generated in the plate surface, and the steel structure as the purpose of the present invention cannot be obtained, and the expected yield ratio and Charpy absorption energy are not obtained.

Vs/Vc is ideally ≤ 1.8, and Vs/Vc is more ideally ≤ 1.7. Although the lower limit is not specifically specified, since the hot-rolled steel plate is cooled from the front and back sides, the average cooling rate Vs from the plate surface to the cooling stop becomes a value greater than the average cooling rate Vc from the plate thickness center to the cooling stop (1.0 ≤ Vs/Vc). Here, the average cooling rate Vs from the plate surface to the cooling stop is the larger value of the average cooling rate from the plate surface or the plate back side to the position 1.0 mm inside.

Furthermore, in the present invention, from the perspective of suppressing the coarsening of the crystal grain size, it is better to start cooling immediately after the finish rolling is completed.

Air cooling time: more than 5s

In the cooling process, by air cooling the hot-rolled steel plate for more than 5 seconds, the temperature difference between the surface of the hot-rolled steel plate and the center of the plate thickness becomes smaller, and a hot-rolled steel plate having the mechanical properties of the present invention can be obtained. If the air cooling time is less than 5 seconds, the temperature difference between the surface of the plate and the center of the plate thickness becomes larger, and the desired yield ratio cannot be obtained. The air cooling time is preferably more than 10 seconds. Although the upper limit is not specifically limited, if the air cooling time exceeds 100 seconds, the productivity is significantly reduced. Therefore, the air cooling time is preferably less than 100 seconds. More preferably, it is less than 90 seconds. In addition, air cooling is performed from the start of cooling to the stop of cooling.

Although air cooling is not specifically limited, it means cooling from 0.01 to 0.90℃/s in the surface of the plate.

Cooling stop temperature: above 550℃ and below 680℃

When the cooling stop temperature of the hot rolled plate (raw material steel plate) is less than 550°C, temperature distribution is likely to occur in the length direction and/or width direction of the hot rolled plate during cooling, and there is a possibility of uneven mechanical properties. On the other hand, if the cooling stop temperature exceeds 680°C at the plate thickness center temperature of the hot rolled plate, the granular iron particles will coarsen and the desired average crystal grain size cannot be obtained. The cooling stop temperature at the plate thickness center temperature of the hot rolled plate is preferably 560°C or more, and more preferably 580°C or more. It is ideally below 660°C, and more preferably below 650°C.

Furthermore, in the present invention, the average cooling rate is a value obtained by ((temperature of the hot-rolled steel plate before cooling (at the start of cooling) (℃) - temperature of the hot-rolled steel plate up to the cooling stop temperature (550~680℃) (℃) / cooling time (s), which can be calculated from the temperature distribution in the cross section of the hot-rolled steel plate obtained by heat transfer analysis. The cooling time used to calculate this average cooling rate also includes the time for air cooling.

As cooling methods, water cooling such as water spraying from a nozzle, cooling by spraying cooling gas, etc. can be cited. In the present invention, in order to cool both sides of the hot-rolled plate under the same conditions, it is preferred to perform cooling operation (treatment) on both sides of the hot-rolled plate. In addition, in order to obtain the aforementioned cooling speed, the amount, pressure, spraying time, angle of cooling water or cooling gas, and the transportation speed of the hot-rolled steel plate are adjusted. If cooling water or cooling gas is constantly sprayed at a specific position on the surface of a hot-rolled steel plate, or a large amount of cooling water or cooling gas is sprayed at the surface of a hot-rolled steel plate instantly, the surface of the steel plate is rapidly cooled, and therefore, the difference in cooling speed between the surface layer of the plate and the center of the plate thickness becomes larger. Therefore, in order to prevent a specific position of the hot-rolled steel plate from being rapidly cooled, the difference in cooling speed between the surface layer of the plate and the center of the plate thickness can be controlled within a predetermined range by correctly arranging the position of the nozzle for spraying cooling water or cooling gas, and correctly adjusting the spraying amount and time of cooling water or cooling gas. In order to obtain the cooling rate specified in the present invention, after heat transfer analysis is performed in advance to determine the conditions for the cooling treatment of the hot-rolled steel plate, the conditions can be reflected in the manufacturing conditions.

(Rolling process)

After the cooling step, the hot rolled steel plate is coiled in the coiling process and then cooled. In the coiling process, from the perspective of the steel plate structure, the hot rolled steel plate is coiled at a coiling temperature of 550°C or more and 680°C or less at the center temperature of the plate thickness. When the coiling temperature is less than 550°C, a large amount of upper tough iron is generated on the surface of the steel plate, and the area ratio may exceed 5%. When the coiling temperature exceeds 680°C, the fat iron particles are coarsened and the desired crystal grain size cannot be obtained. The coiling temperature is more preferably 570°C or more, and more preferably 580°C or more. In addition, the coiling temperature is preferably below 660°C, and more preferably below 650°C.

<Square steel pipe>

The square steel pipe of the present invention is made of the hot-rolled steel plate of the present invention. The square steel pipe of the present invention can be made as follows, that is, in the axial direction of the tube, in the flat plate part, the yield strength is 295MPa or more, the tensile strength is 400MPa or more, and the yield ratio is 0.90 or less. In the flat plate part, the absorbed energy of the Charpy impact test at a test temperature of -20°C is 40J or more, and the ductility-brittleness transition temperature is -5°C or less. Low temperature toughness, for example, can be suitable as a structural member of a building in a low temperature environment such as a cold area with a temperature below zero.

As a manufacturing method of the square steel pipe of the present invention, in the pipe making process, the hot rolled steel plate is formed into a cylindrical open pipe (ball-shaped steel pipe) by roller rolling, and the butted parts are subjected to electric resistance welding. Then, by using rollers arranged above, below, left and right relative to the round steel pipe, a few % of drawing is applied to the pipe axis direction in the cylindrical state, forming it into a square shape to obtain a square steel pipe.

For example, the coil-shaped hot-rolled steel plate of the present invention is formed into a round shape by a roll forming method using a roll in a cold mill to manufacture a round steel pipe, and then the round steel pipe is formed into a square shape by a roll forming method using a roll to manufacture a square steel pipe. If the round steel pipe is rolled in a cold mill, since a large processing strain is introduced in the axial direction of the pipe, there is a problem that the yield ratio in the axial direction of the pipe is easily increased and the toughness is easily reduced. However, in the square steel pipe of the present invention, since the hot-rolled steel plate of the present invention is used as the raw material, the increase in the yield ratio can be suppressed, and for example, even if the wall thickness is more than 20 mm, it can be made into one with a low yield ratio and low-temperature toughness.

Furthermore, the square steel tube of the present invention is not limited to a square steel tube whose respective side lengths are equal (the value of (long side length/short side length) is 1.0) when viewed from a cross-sectional angle perpendicular to the tube axis, but also includes a square steel tube whose value of (long side length/short side length) exceeds 1.0.

However, if the value of (long side length/short side length) of the square steel pipe exceeds 2.5, local buckling is likely to occur on the long side, and the compressive strength in the pipe axis direction is reduced. Therefore, the value of (long side length/short side length) of the square steel pipe is preferably set to 1.0 or more and 2.5 or less. The value of (long side length/short side length) is more preferably 1.0 or more and 2.0 or less.

The square steel pipe of the present invention is manufactured in the above manner. According to the present invention, a square steel pipe having a yield strength of 295 MPa or more, a tensile strength of 400 MPa or more, a yield ratio of 0.90 or less, a Charpy absorbed energy of 40 J or more at -20°C, and a ductility-brittleness transition temperature of -5°C or less can be obtained. Since the ductility-brittleness transition temperature of the square steel pipe of the present invention is less than 0°C, it can be ideally used as a structural member of buildings in cold regions with temperatures below zero.

<Building structures>

FIG1 is a schematic diagram showing an example of a building structure of the present invention.

The building structure of the present invention uses the aforementioned square steel pipe (low yield ratio square steel pipe) 1 of the present invention as a column material. Symbols 4, 5, 6, and 7 represent the main beam, small beam, diaphragm, and intermediate column in sequence.

As mentioned above, the square steel pipe of the present invention has excellent mechanical properties in the flat plate portion. Therefore, the building structure of the present invention using the square steel pipe as a column material can exhibit excellent earthquake resistance.

[Implementation example]

The present invention is described in further detail below based on the embodiments. Furthermore, the present invention is not limited to the following embodiments.

Molten steel having the composition shown in Table 1 is cast to form a plate. The obtained plate is subjected to a hot rolling process, a cooling step, and a coiling process under the conditions shown in Table 2 to form a hot rolled steel plate. After the coiling process, the pipe making process shown below is performed. In addition, the total reduction rate below 930°C specified in the hot rolling process is the total reduction rate for only finishing rolling.

The obtained hot-rolled steel plate is formed into a cylindrical round steel pipe by roller forming, and then the butt joint is subjected to electric resistance welding. Then, the round steel pipe is formed into a square shape (square shape in the vertical section view in the pipe axis direction) by rollers arranged on the top, bottom, left and right sides, and a roller-formed square steel pipe with corners and flat parts and the side length (mm) and wall thickness (mm) shown in Table 4 is obtained.

Test pieces were taken from the obtained hot-rolled steel plates and subjected to the following structural observations, hardness measurements, tensile tests, and Charpy impact tests.

[Organizational Observation]

The test piece for structural observation is collected from the center of the width direction of the hot-rolled steel plate and the position of the plate thickness t/2 (t: plate thickness) (the center of the plate thickness of the present invention) and the back of the steel plate (the lower surface during hot rolling) in the back of the plate surface. The structure of the steel plate surface (upper surface during hot rolling) and the back of the steel plate (lower surface during hot rolling) are the same, and the test piece for structural observation can be collected from either the surface or the back of the steel plate, but here, the sampling position of the test piece for structural observation is unified on the back of the steel plate (lower surface during hot rolling). After grinding in a manner that the observation surface becomes a vertical section parallel to the rolling direction during hot rolling (the normal direction of the observation surface becomes a section in the plate width direction), nitrate etching is performed to produce it.

Furthermore, regarding which of the two sides of the finally obtained steel plate is the back side, since the hot-rolled surface side (upper surface side) is rolled in the coiling after cooling, if a part of the hot-rolled steel plate after rolling is cut, an arc-shaped steel plate can be obtained. Therefore, the outer side and the inner side of the hot-rolled coil are determined from the shape of the arc-shaped steel plate, and the front and back sides during rolling are determined.

The structural observation is to observe and photograph the structure of the hot-rolled steel plate at the plate thickness t/2 position (plate thickness center) and the back side of the steel plate (plate surface and back side) using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times). The area ratio of ferrite, pulex, pseudopulex, and upper ductile iron is obtained from the obtained optical microscope image and SEM image.

The area ratio of each structure is observed in 5 fields of view and calculated as the average value of the values obtained in each field of view. Here, the area ratio obtained by tissue observation is used as the area ratio of each structure. The area ratio of ferrite, pulex, pseudopulex, and upper ferrite is calculated by using an optical microscope image or SEM image to distinguish each phase by the shape and color shown below, and then dividing it by the area of the entire optical microscope image or SEM image to calculate the area ratio of each phase.

Here, ferrite is a product produced by diffusion transformation, and has a low dislocation density and a basically restored structure. Polygonal ferrite and pseudo-polygonal ferrite are included here. Pole iron is a structure in which ferrite and ferrite are arranged in layers, and pseudo-porre iron is a structure in which ferrite is confirmed to be arranged in a point array in ferrite. In addition, upper ferrite is a complex structure of layered ferrite and ferrite with high dislocation density. In addition to the above-mentioned shapes, it can be distinguished from the fact that ferrite is white, ferrite is black, pseudo-porre iron is black or gray, and upper ferrite is white or gray.

In addition, the average crystal grain size (average equivalent circular diameter) is measured at the plate thickness t/2 position (plate thickness center) using the SEM/EBSD method. The measurement area is 500μm×1000μm (= ^0.5mm2 ), and the measurement step size is 0.5μm. The crystal grain size is determined by finding the orientation difference between adjacent crystal grains, and the boundary with an orientation difference of 15° or more is measured as the crystal grain boundary. From the obtained crystal grain boundary, the grain size (equivalent circular diameter) of each crystal grain is calculated, and the arithmetic average is calculated as the average crystal grain size (average equivalent circular diameter). In addition, the total area of crystal grains with an equivalent circular diameter of 40.0 μm or more was calculated and then divided by the area of the measurement area (0.5 mm ² ) to calculate the area ratio of crystal grains with an equivalent circular diameter of 40.0 μm or more. The calculation of the equivalent circular diameter and the area of crystal grains was obtained by analyzing the data obtained by the SEM/EBSD method using OIM Analysis manufactured by EDAX.

In addition, the major diameter and minor diameter of the crystal grains were measured by the method of JIS R 1670 (2006), and the major diameter to minor diameter ratio (= (major diameter)/(minor diameter)) was calculated. The number of crystal grains with a major diameter to minor diameter ratio (= (major diameter)/(minor diameter)) of 3.0 or more was measured and divided by the area of the measurement region (0.5 mm ² ), thereby calculating the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more (pieces/mm ² ). In the crystal grain size analysis and the measurement of the number of crystal grains, crystal grains with a diameter of less than 2.0 μm were excluded from the analysis object as measurement noise.

[Pull test]

From the obtained hot-rolled steel plate, a JIS No. 5 drawing test piece is taken in such a way that the drawing direction becomes parallel to the rolling direction. For the drawn drawing test piece, the drawing test is carried out in accordance with the provisions of JIS Z2241 (2011), and the yield strength YS and the tensile strength TS are measured, and the yield ratio defined as (yield strength)/(tensile strength) is calculated. In addition, the number of test pieces is 2, and the average value is calculated to obtain YS, TS, and yield ratio.

[Charpy impact test]

At the t/2 position (center of the plate thickness) of the obtained hot-rolled steel plate, a V-notch standard test piece in accordance with the provisions of JIS Z2242 (2018) was used, and the length direction of the test piece was parallel to the rolling direction. In accordance with the provisions of JIS Z 2242 (2018), the Charpy impact test was carried out at the test temperatures of -80℃, -60℃, -40℃, -20℃, and 0℃. In addition, the number of test pieces was set to 3 at each test temperature, and the average values of the ductility-brittleness transition temperature and the absorbed energy (J) were calculated.

[Hardness]

According to the provisions of JIS Z 2244 (2020), the Vickers hardness is measured with a test force of 9.8N (1kgf). Measure 10 points each, calculate the average value of the 8 points excluding the maximum and minimum values, and take the average value (total of the Vickers hardness of the 8 points/8) as the hardness of the test piece. The test piece uses the test piece for the above-mentioned tissue observation, and the hardness is measured after mirror polishing at the position 1.0mm inside from the back of the steel plate surface in the plate thickness direction and the center of the plate thickness. Regarding the hardness from the back of the steel plate surface to the position 1.0mm inside in the plate thickness direction, the greater hardness is set as the hardness from the back of the steel plate surface to the position 1.0mm inside in the plate thickness direction.

In addition, the difference in hardness between the position 1.0 mm inside the plate thickness direction from the back of the steel plate surface and the center of the plate thickness (hardness difference) is calculated.

The results are shown in Table 3.

In addition, test pieces were taken from the obtained square steel pipe (rolled square steel pipe) and subjected to the following pulling test and Charpy impact test.

[Pull test]

Figure 2 is a schematic diagram showing the location of the pull test piece on the flat plate.

As shown in Figure 2, the pulling test is performed by taking a JIS No. 5 pulling test piece from the flat plate of the square steel pipe in such a way that the pulling direction is parallel to the tube axis direction. For the pulled test piece taken, the pulling test is carried out in accordance with the provisions of JIS Z2241 (2011), and the yield strength YS and the pulling strength TS are measured, and the yield ratio defined by (yield strength)/(pulling strength) is calculated. In addition, the pulling test piece of the flat plate is taken from the position of the central part of the width of the flat plate of the edge of the 3 o'clock position when the welded part of the square steel pipe is set to the 12 o'clock direction (refer to the symbol X in Figure 2). In addition, the number of test pieces is 2 each, and the average value is calculated to obtain YS, TS, and yield ratio.

[Charpy impact test]

Figure 3 is a schematic diagram showing the locations where the Charpy test pieces were taken.

As shown in Figure 3, the Charpy impact test is performed using a V-notch standard test piece taken from the flat plate of a square steel pipe and in accordance with the provisions of JIS Z2242 (2018) at the position t/4 from the outer surface of the square steel pipe to the wall thickness t, so that the length direction of the test piece is parallel to the pipe axis direction (refer to the symbol Y in Figure 3). In accordance with the provisions of JIS Z2242 (2018), the Charpy impact test is carried out at the test temperatures of -60℃, -40℃, -20℃, 0℃, and +20℃. In addition, the number of test pieces is set to 3 at each test temperature, and the average values of the ductility-brittleness transition temperature and the absorbed energy (J) are obtained.

The results are shown in Table 4.

In Tables 3 and 4, Steel No. 1 to 20 are examples of the present invention, and Steel No. 21 to 45 are comparative examples.

As shown in Table 3, in the steel structure of the central portion of the plate thickness and the back portion of the plate surface of any one of the hot-rolled steel plates of the present invention, when the total area ratio of the pulvinar and pseudo-pulvinar is 6 to 25% and the area ratio of the upper tantalum is 5% or less, and in the central portion of the plate thickness, the area surrounded by the boundary where the orientation difference of the adjacent crystals is 15° or more is taken as the crystal grain, the area ratio of the crystal grains with an average crystal grain size of 10.0 to 30.0 μm and an equivalent circle diameter of 40.0 μm or more in the steel structure of the central portion of the plate thickness is 20% or less, and the number of crystal grains with a ratio of the major diameter to the minor diameter (= (major diameter)/(minor diameter)) of 3.0 or more is 30 grains/mm ² or less, and the hardness difference between the steel plate surface and the center of the plate thickness is 40HV 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 Charpy absorbed energy at -20℃ is 100J or more, and the ductility-brittleness transition temperature is -20℃ or less.

In addition, as shown in Table 4, the square steel pipes made using the hot-rolled steel plate of the present invention have a yield strength of 295 MPa or more in the flat plate, a tensile strength of 400 MPa or more in the flat plate, a yield ratio of 0.90 or less in the flat plate, a Charpy absorbed energy of 40 J or more at -20°C in the flat plate, and a ductility-brittleness transition temperature of -5°C or less in the flat plate.

The comparison example No. 21 (steel U) has Mn/Si=0.8, which is not within the scope of the present invention. Therefore, the total area ratio of pulex and pseudo-pulex is less than 6%, and the tensile strength and yield ratio do not reach the expected values.

Comparative example No. 22 (Steel V) has Mn/Si=7.9, which is not within the scope of the present invention. Therefore, the total area ratio of pulex and pseudo-pulex exceeds 25%, and the Charpy absorption energy at -20℃ and the ductility-brittleness transition temperature do not reach the expected values.

In the comparative example No. 23 (Steel W), the C content exceeds the range of the present invention, so the area ratio of the second phase is out of the range of the present invention. In addition, the area ratio of crystal grains with a grain size (equivalent circular diameter) of 40.0 μm or more exceeds 20%, and the yield ratio, Charpy absorbed energy at -20°C, and ductility-brittleness transition temperature do not reach the expected values.

The Si content of the comparative example No. 24 (Steel X) exceeds the range of the present invention, so the area ratio of pseudo-wave iron increases excessively, and the Charpy absorbed energy at -20℃ and the ductility-brittleness transition temperature do not reach the expected values.

In the comparative example No. 25 (steel Y), the Mn content exceeded the range of the present invention, so the area ratio of the upper ductile iron exceeded 5%. As a result, the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more was 30/ ^mm2 , the hardness difference exceeded 40 HV, and the yield ratio did not reach the desired value.

Comparative Example No. 26 (Steel Z) has a Nb content exceeding the range of the present invention, so the amount of upper ductile iron exceeds 5%. As a result, the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more is 30/ ^mm2 , the hardness difference exceeds 40HV, and the Charpy absorbed energy at -20℃ and the ductility-brittleness transition temperature do not reach the expected values.

In the comparative example No. 27 (Steel AA), the Ti content exceeded the range of the present invention, so the area ratio of the upper ductile iron exceeded 5%, and coarse carbides, nitrides, etc. were formed. As a result, the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more was 30/ ^mm2 , and the hardness difference exceeded 40HV. As a result, the Charpy absorbed energy at -20℃ and the ductility-brittleness transition temperature did not reach the expected values.

In the comparative example No. 28 (Steel AB), the V content exceeded the range of the present invention, so the area ratio of the upper ductile iron exceeded 5%. As a result, the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more was 30/ ^mm2 , the hardness difference exceeded 40HV, and the Charpy absorbed energy at -20℃ and the ductility-brittleness transition temperature did not reach the expected values.

In the comparative example No. 29 (steel AC), the Cr content exceeded the range of the present invention, so the area ratio of the upper ductile iron exceeded 5%. As a result, the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more was 30/ ^mm2 , the hardness difference exceeded 40HV, and the Charpy absorbed energy at -20℃ and the ductility-brittleness transition temperature did not reach the expected values.

In the comparative example No. 30 (steel AD), the Mo content exceeded the range of the present invention, so the area ratio of the upper ductile iron exceeded 5%. As a result, the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more was 30/ ^mm2 , the hardness difference exceeded 40HV, and the Charpy absorbed energy at -20℃ and the ductility-brittleness transition temperature did not reach the expected values.

In the comparative example No. 31 (steel AE), the Cu content exceeds the range of the present invention, so there is coarse precipitation of Cu. As a result, the Charpy absorption energy at -20℃ and the ductility-brittleness transition temperature do not reach the expected values.

In the comparative example No. 32 (Steel AF), the Ni content exceeded the range of the present invention, so the area ratio of the upper ductile iron exceeded 5%. As a result, the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more was 30/ ^mm2 , the hardness difference exceeded 40 HV, and the ductility-brittleness transition temperature did not reach the desired value.

In the comparative example No. 33 (Steel AG), since the Ca content exceeds the range of the present invention, Ca oxide clusters are formed. As a result, the Charpy absorption energy at -20°C and the ductility-brittleness transition temperature do not reach the expected values.

In the comparative example No. 34 (Steel AH), the B content exceeded the range of the present invention, so the area ratio of the upper ductile iron exceeded 5%. As a result, the number of crystal grains with a major diameter to minor diameter ratio of 3.0 or more was 30/ ^mm2 , the hardness difference exceeded 40HV, and the yield ratio did not reach the desired value.

In the comparative example No.35 (steel AI), since the C content exceeds the range of the present invention, the total area ratio of pulex and pseudo-pulex is less than 6%, the average grain size exceeds 30.0μm, and the area ratio of grains with a grain size (equivalent circular diameter) of 40.0μm or more exceeds 20%. The yield strength, tensile strength, yield ratio, Charpy absorbed energy at -20℃ and ductility-brittleness transition temperature do not reach the expected values.

In the comparative example No. 36 (steel C), the plate heating temperature exceeded the range of the present invention, the crystal grains coarsened, the average crystal grain size exceeded 30.0μm, and the area ratio of crystal grains with a crystal grain size (equivalent circular diameter) of 40.0μm or more exceeded 20%. As a result, the yield strength, tensile strength, Charpy absorbed energy at -20℃, and ductility-brittleness transition temperature did not reach the expected values.

In the comparative example No.37 (steel C), the finishing temperature of the finishing rolling exceeds the range of the present invention. Therefore, the total reduction rate below 930°C is lower than the range of the present invention, and the formation of coarse tungsten iron cannot be suppressed. The average grain size exceeds 30.0μm, and the area ratio of grains with a grain size (equivalent circular diameter) of 40.0μm or more exceeds 20%. As a result, the Charpy absorption energy at -20°C and the ductility-brittleness transition temperature did not reach the expected values.

In the comparative example No. 38 (Steel C), the total reduction ratio at 930°C or below exceeded the range of the present invention, resulting in the formation of coarse ductile iron elongated in the rolling direction, and the number of crystal grains with a major diameter/minor diameter ratio of 3.0 or more exceeded 30/mm ² . As a result, the ductility-brittleness transition temperature did not reach the desired value.

In the comparison example No. 39 (steel C), the average cooling rate Vc of the center of the plate thickness exceeds the range of the present invention, and the air cooling time is lower than the range of the present invention. Therefore, the area ratio of the upper ductile iron exceeds 5%, which is not within the range of the present invention. As a result, the hardness difference exceeds 40HV, and the yield ratio does not reach the expected value.

In the comparative example No. 40 (steel C), since Vs/Vc exceeds the range of the present invention, the area ratio of the upper tantalum exceeds 5%, and a large amount of upper tantalum is generated on the surface of the steel plate. As a result, the hardness difference exceeds 40HV, and the yield ratio, Charpy absorbed energy at -20℃, and ductility-brittleness transition temperature do not reach the expected values.

In the comparative example No. 41 (steel C), since the cooling stop temperature and the coiling temperature are lower than the range of the present invention, the area ratio of the upper ductile iron exceeds 5%. As a result, the hardness difference exceeds 40HV, and the yield ratio does not reach the expected value.

In the comparative example No. 42 (steel C), the cooling stop temperature and the coiling temperature exceeded the range of the present invention, so the average grain size exceeded 30.0 μm. As a result, the yield strength, tensile strength, Charpy absorbed energy at -20°C, and ductility-brittleness transition temperature did not reach the expected values.

In the comparative example No. 43 (steel C), the average cooling rate Vc in the center of the plate thickness is lower than the range of the present invention, so the average crystal grain size exceeds 30.0μm, and the area ratio of grains with a grain size of 40.0μm or more exceeds 20%. In addition, the total area ratio of pulex and pseudo-pulex in the center of the plate thickness is less than 6%, and the yield strength, tensile strength, Charpy absorbed energy at -20℃ and ductility-brittle transition temperature do not reach the expected values.

For the comparative example No. 44 (Steel C), the total reduction ratio below 930°C exceeds the range of the present invention, and the average cooling rate Vc at the center of the plate thickness greatly exceeds the range of the present invention. Therefore, the area ratio of the upper ductile iron exceeds 5%, and the average grain size is less than 10.0μm, and the yield ratio does not reach the expected value.

In the comparative example No. 45 (steel C), since the air cooling time is lower than the range of the present invention, it can be estimated that the temperature difference between the surface of the plate and the center of the plate thickness becomes larger. As a result, a large amount of ductile iron is generated on the surface of the steel plate, and as a result, the hardness difference exceeds 40HV, and the yield ratio does not reach the expected value.

1: Square steel pipe

4: Main beam

5: Small beam

6: Diaphragm

7: Intermediate columns

Claims (9)

A hot-rolled steel plate, which, when calculated by mass%, contains C: 0.07% to 0.20%, Si: 0.40% to 0.40%, Mn: 0.20% to 1.00%, P: 0.100% to 0.100%, S: 0.050% to 0.050%, Al: 0.005% to 0.100%, N: 0.0100% to 0.0100%, or further contains Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, V: 0.01% to 0.005%, .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, B: 0.0003% or more and 0.0100% or less, the contents of Mn and Si satisfy the following formula (1): the remainder has a composition consisting of Fe and inevitable impurities, The steel structure at the center of the plate thickness and the back of the plate surface has: a main phase composed of ferrous iron; and a second phase whose area ratio of the sum of pulex and pseudopulex is 6 to 25%, and the area ratio of upper ductile iron is 5% or less. When the area surrounded by the boundary where the orientation difference of adjacent crystals on the center of the plate thickness is 15° or more is taken as the crystal grain, the average crystal grain size of the steel structure at the center of the plate thickness is 10.0 to 30.0 μm, and the number of the crystal grains with an equivalent circular diameter of 40.0 μm or more is 20% or less calculated by area ratio, and the number of the crystal grains with a ratio of major diameter to minor diameter (= (major diameter)/(minor diameter)) of 3.0 or more is 30/mm. ² or less, the hardness difference between the position 1.0mm inside the back of the steel plate in the plate thickness direction and the center of the plate thickness is 40HV or less, and the ferrous iron, when calculated by area ratio, is 70% or more and 94% or less, 1.0≦%Mn/%Si≦3.5. . . (1) Here, %Mn and %Si are the contents of each element (mass %). The hot-rolled steel plate of claim 1, wherein the yield strength is 250 MPa or more, the tensile strength is 400 MPa or more, the yield ratio is 0.75 or less, the Charpy absorbed energy at -20°C is 100 J or more, and the ductility-brittleness transition temperature is -20°C or less. As in claim 1, the hot-rolled steel plate, wherein the aforementioned steel structure, when calculated by area ratio, has 5 to 15% of the aforementioned false wave iron. As in claim 2, the hot-rolled steel plate, wherein the aforementioned steel structure, when calculated by area ratio, has 5 to 15% of the aforementioned false wave iron. A hot-rolled steel plate as claimed in any one of claims 1 to 4, wherein the plate thickness is 20 mm or more. A method for manufacturing a hot-rolled steel plate, comprising: heating a steel material at a heating temperature of 1100°C to 1300°C, and then performing a hot rolling process with a rough rolling end temperature of 850°C to 1150°C, a finish rolling end temperature of 750°C to 850°C, and a total reduction ratio of 40% to 59%; and then, cooling the steel plate at an average cooling rate Vc (°C/s) at the center of the plate thickness. and the average cooling rate Vs (℃/s) of the plate surface from the back of the plate surface to a depth of 1.0 mm in the thickness direction, which satisfies the following equations (2) and (3), and the hot rolled steel plate is air-cooled for more than 5 seconds from the start of cooling to the stop of cooling, and the cooling process is carried out at a cooling stop temperature of 550℃ to 680℃ at the center of the plate thickness; and secondly, the coiling process is carried out at a temperature of 550℃ to 680℃ at the center of the plate thickness, 2≦Vc≦15． ． ． (2) Vs/Vc≦2.0． ． ． (3). A square steel pipe made of a hot-rolled steel plate as described in any one of claims 1 to 5. A method for manufacturing a square steel pipe is provided by cold rolling a hot-rolled steel plate as in any one of claims 1 to 5 to obtain a square steel pipe. A building structure using square steel pipes as claimed in claim 7 as column materials.

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