KR20250035570A - Hot rolled steel plates, square steel pipes and their manufacturing methods and building structures - Google Patents

Abstract

A low-yield hot-rolled steel sheet having excellent strength and low-temperature toughness is provided. The steel sheet has a predetermined component composition, and a steel structure in the center of the plate thickness has a main phase that is ferrite, and a second phase in which the area ratio of the sum of pearlite and pseudo-pearlite is 6 to 25% and the area ratio of upper bainite is 5% or less, and when a region surrounded by boundaries in which the orientation difference between adjacent crystals is 15° or more is defined as a crystal grain, the average crystal grain size of the steel structure including the main phase and the second phase in the center of the plate thickness is 10.0 to 30.0 ㎛, and the area ratio of crystal grains having a crystal grain size within the average crystal grain size ±5.0 ㎛ is 35% or more, and further, the number of crystal grains in which the ratio of the major axis to the minor axis (major axis)/(minor axis) is 3.0 or more is 30/mm2 or less.

Description

Hot rolled steel plates, square steel pipes and their manufacturing methods and building structures

The present invention relates to a hot-rolled steel sheet having a low yield ratio used for a square steel pipe or tube, and a square steel pipe (corner column) manufactured by cold roll forming using the hot-rolled steel sheet as a material and having a low yield ratio and low-temperature toughness, and a method for manufacturing the same. In particular, the present invention relates to a square steel pipe suitably used as a structural member of a large building. Furthermore, the present invention relates to a building structure using such a square steel pipe.

Recently, for example, building structural members used in large buildings such as factories, warehouses, and commercial facilities (hereinafter referred to as “buildings”) are being strengthened in order to reduce construction costs through weight reduction.

In particular, square steel pipes (corner columns) having a flat portion and a corner portion used as pillar materials for buildings are required to have high strength in the flat portion, and at the same time, are required to have excellent toughness from the viewpoint of earthquake resistance.

Square steel pipes are generally manufactured by cold forming hot-rolled steel plates (hot-rolled steel strips) or thick steel plates. Cold forming methods include cold press bending forming and cold roll forming.

A square steel pipe manufactured by press-bending a material (hereinafter sometimes referred to as a “press-formed square steel pipe”) is manufactured by cold-press-bending a steel plate to form a cross-sectional shape into an ㅁ-shape (square shape) or a ㄷ-shape (U-shape), and joining these by submerged arc welding.

Meanwhile, a square steel pipe manufactured by roll forming a material (hereinafter, sometimes referred to as a "roll-formed square steel pipe") is manufactured by cold roll forming a hot-rolled steel plate into a cylindrical open pipe, and electric resistance welding its butting parts to manufacture a round steel pipe. Thereafter, by applying a few percent of drawing in the pipe axis direction to the cylindrical round steel pipe by rolls arranged on the upper, lower, left, and right sides of the round steel pipe, and then forming it into a square shape to manufacture a square steel pipe.

The method for manufacturing roll-formed square steel pipes has the advantages of high productivity and short-term manufacturing compared to the method for manufacturing press-formed square steel pipes.

However, since press-formed square steel pipes are cold-formed not at the flat surface but at the corners, only the corners are work-hardened. On the other hand, in roll-formed square steel pipes, especially when cold-forming into a cylindrical shape that is the shear for forming into a square steel pipe, a large working strain is introduced in the pipe-axis direction throughout the entire circumference of the steel pipe. Therefore, roll-formed square steel pipes have the problem that the yield ratio in the pipe-axis direction is high and the toughness is low not only at the corners but also at the flat surface.

In addition, since the work hardening during roll forming increases as the thickness of the roll-formed square steel pipe increases, the yield ratio increases and the toughness decreases. Therefore, especially when manufacturing thick-walled roll-formed square steel pipes, it is necessary to select a material that can withstand the increase in yield ratio and the decrease in toughness due to roll forming.

For this requirement, for example, in Patent Document 1, a steel material containing C in an amount of 0.20% or less in wt% and additionally containing one or two of Mn: 0.40 to 0.90%, Nb: 0.005 to 0.040%, and Ti: 0.005 to 0.050% is hot rolled into a coil at a reduction ratio of 55% or more in a non-recrystallization temperature range, a rolling finish temperature of 730 to 830°C, and a coiling temperature of 550°C or less, and the resulting hot-rolled coil is then formed into an electric resistance steel pipe by welding, and then cold worked into a square steel pipe, wherein by making the outer peripheral length drawing in the steel pipe forming process 3 times the plate thickness or less, a square steel pipe having a yield ratio of 90% or less and a Charpy absorbed energy of 27 J or more at a test temperature of 0°C is obtained. It has been proposed.

In Patent Document 2, steel containing, in mass%, C: 0.07 to 0.18% and Mn: 0.3 to 1.5% is heated to a heating temperature of 1100 to 1300°C, and then rough rolling is performed at a rough rolling end temperature of 1150 to 950°C and finishing rolling is performed at a finishing rolling start temperature of 1100 to 850°C and a finishing rolling end temperature of 900 to 750°C. Then, first cooling is performed so that the cooling stop temperature becomes 550°C or higher at the surface temperature, second cooling is performed so that the cooling stop temperature becomes 550°C or higher at the air cooling for 3 to 15 s, and third cooling is performed so that the average cooling rate in the temperature range of 750 to 650°C at the center of the plate thickness becomes 4 to 15°C/s to 650°C or lower, and the value of the second phase frequency included in the steel structure is measured. A square steel pipe is proposed which has mechanical properties such that a yield ratio of 80% or less and an absorbed energy of 150 J or more in a Charpy impact test at a test temperature of 0°C is obtained by obtaining a thick-walled hot-rolled steel sheet by setting the thickness to 0.20 to 0.42, and by cold forming the thick-walled hot-rolled steel sheet.

In Patent Document 3, a steel containing, in mass%, C: 0.07 to 0.18% and Mn: 0.3 to 1.5% is heated to a heating temperature of 1100 to 1300°C, and then rough rolling is performed at a rough rolling end temperature of 1150 to 950°C and finish rolling is performed at a finish rolling start temperature of 1100 to 850°C and a finish rolling end temperature of 900 to 750°C, and then a thick-walled hot-rolled steel sheet is obtained by cooling to a coiling temperature of 500 to 650°C so that the average cooling rate in the temperature range from the surface temperature to 750 to 650°C is 20°C/s or less, the time until the temperature at the center of the plate thickness reaches 650°C is within 35 s, and further, the average cooling rate in the temperature range from 750 to 650°C at the center of the plate thickness is 4 to 15°C/s, thereby performing cold forming, A square steel pipe having mechanical properties such that it exhibits a yield strength ratio of 80% or less and an absorbed energy of 150 J or more in a Charpy impact test at a test temperature of 0°C is proposed.

In Patent Document 4, steel having a component composition containing, in mass%, C: 0.07 to 0.20%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, with the remainder being Fe and unavoidable impurities, is heated to a heating temperature of 1100 to 1300°C, and then rough rolling is performed at a rough rolling end temperature of 1150 to 950°C and finish rolling is performed at a finish rolling start temperature of 1100 to 850°C and a finish rolling end temperature of 900 to 750°C, and then cooling is performed to a cooling stop temperature of 580°C or less at a cooling rate such that an average cooling rate from the start of cooling to the stop of cooling is 4 to 25°C/s at the center temperature of the plate thickness, and in an initial cooling process for 10 seconds from the start of cooling, A square steel pipe is proposed which has at least one natural cooling process of 0.2 s or more and less than 3.0 s, and then coils at a coiling temperature of 580°C or less, and then cools to obtain a steel structure in the center of the plate thickness, which has a main phase composed of ferrite and a second phase having an area ratio of 8 to 20% composed of one or more types selected from pearlite, pseudo-pearlite, and upper bainite, and has an average grain size of 7 to 20 ㎛ of the steel structure including the main phase and the second phase, and has mechanical properties such that the steel structure on the front and back surfaces of the plate thickness is a ferrite single phase or a bainitic ferrite single phase, has an average grain size of 2 to 20 ㎛, exhibits a yield ratio of 90% or less, and has an absorbed energy of 27 J or more in a Charpy impact test at a test temperature of 0°C.

Japanese Patent Publication No. 9-87743 Japanese Patent Publication No. 2012-153963 Japanese Patent Publication No. 2012-132088 International Publication No. 2018/110152

Here, as described above, in the case of the roll-formed square steel pipe, as its thickness increases and, furthermore, as the side length decreases, the processing strain introduced into the square steel pipe increases, and the degree of increase in yield ratio and decrease in toughness becomes greater.

For this reason, the hot-rolled steel sheet used as the material is required 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 due to large processing deformation.

However, in the square steel pipes manufactured by the methods disclosed in the above-mentioned patent documents 1 to 3, there is a problem that the yield ratio becomes excessively high, especially when the thickness exceeds 25 mm, and thus it is impossible to satisfy a yield ratio of 90% or less.

In addition, in order to obtain a high yield strength and high toughness by the technique described in Patent Document 4, it is necessary to make the steel structure of the front and back surfaces of the plate thickness into a ferrite single phase or a bainitic ferrite single phase, and in order to obtain such a steel structure, it is necessary to form a cooling process during the cooling process, i.e., an additional process is required, so there is a problem that the manufacturing process becomes complicated.

The present invention has been made in consideration of the above-mentioned problems, and it is an object of the present invention to provide a hot-rolled steel sheet that can be used for making square steel pipes, which has high yield strength and tensile strength, a low yield ratio, and excellent low-temperature toughness in the pipe-axial direction and pipe-circumferential direction and work hardening property, together with a square steel pipe using the hot-rolled steel sheet, a method for manufacturing the same, and an architectural structure using the square steel pipe.

Here, with respect to the hot-rolled steel sheet in the present invention, (1) having a low yield ratio, (2) having a high yield strength, and (3) having a high tensile strength are all those results of a tensile test in accordance with the provisions of JIS Z 2241 (2011) using a JIS No. 5 tensile test piece collected so that the tensile direction is parallel to the rolling direction, in that order, (1) having a yield ratio of 0.75 or less, (2) having a yield strength of 250 MPa or more, and (3) having a tensile strength of 400 MPa or more.

In addition, the work hardening property is an index for evaluating the uniform elongation (plastic elongation at the maximum load point of the tensile test), and refers to a work hardening index n _3-7 stipulated by JIS Z 2253 (2011) being 0.20 or more. That is, if the work hardening index n _3-7 of the hot-rolled steel plate does not satisfy 0.20, when manufactured into a square steel pipe, the uniform elongation of the flat plate portion of the square steel pipe decreases, which lowers the earthquake resistance, and in some cases, the yield ratio of the flat plate portion of the square steel pipe exceeds 0.90.

In addition, excellent low-temperature toughness means that, in accordance with the provisions of JIS Z 2242 (2018), a V-notch standard test piece is collected so that the longitudinal direction of the test piece is parallel to the rolling direction at the t/2 position (center of the plate thickness) of the plate thickness t, and a Charpy impact test is performed at each of test temperatures: -80°C, -60°C, -40°C, -20°C, and 0°C, and the Charpy absorbed energy at -20°C is 100 J or more and the ductile-to-brittle transition temperature is -20°C or less.

In addition, in the present invention, a low yield ratio square steel pipe is a steel pipe having a tensile test specimen of JIS No. 5 collected so that the tensile direction is parallel to the pipe axis direction, and the results of a tensile test in accordance with the provisions of JIS Z 2241 (2011) are that the yield strength in the flat portion is 295 MPa or more, the tensile strength in the flat portion is 400 MPa or more, the yield ratio in the flat portion is 0.90 or less, the uniform elongation in the flat portion is 5.0% or more, and, at a position 1/4t of the thickness t from the outer surface of the pipe, the Charpy impact test in accordance with the provisions of JIS Z 2242 (2018) is performed using a V-notch standard test specimen collected from the flat portion of the square steel pipe so that the longitudinal direction of the test specimen is parallel to the pipe axis direction at test temperatures of -60°C, -40°C, -20°C, and 0°C, and the yield ratio in the flat portion is 0.90 or less. - Refers to a square steel pipe having a Charpy absorbed energy of 60J or more at -20℃ and a ductile-to-brittle transition temperature of the flat plate of -10℃ or less.

In addition, in the present invention, a square steel pipe having a better yield strength refers to a square steel pipe in which the Charpy absorbed energy at -20°C in the pipe axial direction and the pipe circumferential direction in the flat plate is measured using a V-notch standard test piece collected in accordance with the provisions of JIS Z 2242 (2018) at a position t/4 of the thickness t from the outer surface of the pipe so that the longitudinal direction of the test piece is parallel to the pipe circumferential direction, and the ratio P of the Charpy absorbed energy at -20°C in the pipe circumferential direction to the pipe axial direction is 0.5 to 1.2.

However, P = (Charpy absorption energy at -20℃ in the pipe circumferential direction) / (Charpy absorption energy at -20℃ in the pipe axial direction)

The inventors of the present invention conducted a thorough review to solve the above problems. As a result, the following findings (1) to (3) were obtained.

(1) In order for the hot-rolled steel sheet to satisfy the yield strength and tensile strength aimed at in the present invention, it is necessary that the C content be 0.07 mass% or more and that the main phase in the center of the thickness of the hot-rolled steel sheet be ferrite.

(2) In order for the hot-rolled steel sheet to obtain the low-temperature toughness and yield ratio aimed at in the present invention, in addition to the main phase described in (1) above, in the central portion of the plate thickness, a second phase composed of one or more types of pearlite, pseudo-pearlite, and upper bainite is provided, the area ratio of the sum of pearlite and pseudo-pearlite is 6 to 25%, the area ratio of upper bainite is 5% or less, and when a region surrounded by boundaries in which the orientation difference between neighboring crystals is 15° or more in the central portion of the plate thickness is defined as a crystal grain, the average grain size of these crystal grains including the main phase and the second phase is in the range of 10.0 to 30.0 µm, and the area ratio of the crystal grains having a crystal grain size within ±5.0 µm of this average crystal grain size is 35% or more, and further, among the crystal grains, the number density of the crystal grains having a ratio of the major axis to the minor axis (= (major axis) / (minor axis)) is 3.0 or more. It is necessary to keep it to 30/㎟ or less.

(3) In order to obtain the steel structure described in (1) and (2) above, after adjusting the component composition to an appropriate range, it is necessary to control the content of Mn and Si in particular to a specific range, and further, in addition to starting finish rolling after a predetermined period of time has elapsed after the completion of rough rolling in the hot rolling process, it is necessary to maintain a predetermined temperature range for a predetermined period of time after coiling.

The present invention has been completed through further review based on this recognition. That is, the gist of the present invention is as follows.

1. As a hot rolled steel plate,

In mass %,

C: 0.07% or more and 0.20% or less,

Si: 0.40% or less,

Mn: 0.20% or more and 1.00% or less,

P: 0.100% or less,

S: 0.050% or less,

Al: 0.005% or more and 0.100% or less and

N: 0.0100% or less

, the remainder being Fe and inevitable impurities, and further having a composition of components satisfying the following formula (1) with a content of Mn and Si:

The steel structure in the center of the plate thickness has a main phase of ferrite, a second phase in which the area ratio of the sum of pearlite and pseudo-pearlite is 6 to 25%, and an area ratio of upper bainite is 5% or less.

A hot-rolled steel sheet, wherein, in the steel structure at the center of the above plate thickness, when a region surrounded by boundaries where the orientation difference between neighboring crystals is 15° or more is defined as a crystal grain, the average crystal grain size of these crystal grains is 10.0 to 30.0 ㎛, and among the crystal grains, the area ratio of crystal grains having a crystal grain size within the average crystal grain size ±5.0 ㎛ is 35% or more, and further, among the crystal grains, the number density of crystal grains having a ratio of the major axis to the minor axis, (major axis)/(minor axis), of 3.0 or more is 30/mm2 or less.

1.0≤％Mn/％Si≤3.5… (1)

Here, %Mn and %Si represent the content (mass%) of each element in the steel plate.

2. The above composition of ingredients, additionally, in mass%,

Nb: 0.005% or more and 0.020% or less,

Ti: 0.005% or more and 0.020% or less,

V: 0.01% or more and 0.10% or less,

Cr: 0.01% or more and 0.50% or less,

Mo: 0.01% or more and 0.50% or less,

Cu: 0.01% or more and 0.30% or less,

Ni: 0.01% or more and 0.30% or less,

Ca: 0.0005% or more and 0.0100% or less and

B: 0.0003% or more and 0.0100% or less

A hot-rolled steel sheet as described in 1 above, comprising one or more types selected from the group consisting of:

3. Hot-rolled steel plate having a thickness of 12 mm or more, as described in 1 or 2 above.

4. Heat the steel material having the composition described in 1 or 2 above at a heating temperature of 1100℃ or higher and 1300℃ or lower,

Next, as hot rolling, rough rolling is performed at a rough rolling end temperature of 850°C or more and 1,150°C or less, and finish rolling is started after 15 seconds or more have elapsed after the completion of the rough rolling, and the finish rolling end temperature is 750°C or more and 850°C or less, and the hot rolling is performed at a total reduction ratio of 40% or more and 59% or less at 930°C or less throughout the entire hot rolling process.

Next, for the material steel sheet obtained from the hot rolling, cooling is performed such that the average cooling rate Vc (℃/s) at the center of the plate thickness satisfies the following equation (2), and the cooling stop temperature at the center of the plate thickness is 550℃ or more and 680℃ or less.

Next, for the above material steel plate, coiling is performed at a plate thickness center temperature of 550℃ or higher and 680℃ or lower,

Next, a method for manufacturing a hot-rolled steel sheet, wherein a second cooling is performed on the coiled steel sheet obtained from the above coiling, wherein the second cooling is performed at a temperature range of 400°C to 300°C for 1.0 h or more and 10.0 h or less.

4≤Vc≤20 … (2)

5. A square steel pipe made of a hot-rolled steel plate as described in any one of 1 to 3 above.

6. A square steel pipe as described in 5 above, wherein the ratio P of Charpy absorbed energy at -20℃ in the circumferential direction of the pipe to the pipe axis direction is in the range of 0.5 to 1.2.

However, P = (Charpy absorption energy at -20℃ in the pipe circumferential direction) / (Charpy absorption energy at -20℃ in the pipe axial direction)

7. A method for manufacturing a square steel pipe by cold roll forming the hot rolled steel sheet obtained in the method for manufacturing a hot rolled steel sheet described in 4 above into a square steel pipe.

8. A building structure having the square steel pipe described in 5 or 6 above as a pillar material.

According to the present invention, a technique is provided for obtaining a hot-rolled steel sheet which can be used for a low-yield square steel pipe, and which has high yield strength and tensile strength, a low yield ratio, and excellent low-temperature toughness and work hardening properties.

In addition, a building structure using the square steel pipe of the present invention as a column material can obtain better earthquake-resistant performance compared to a building structure using a square steel pipe manufactured by conventional cold forming.

Figure 1 is a schematic diagram showing the temperature measurement location of the steel plate after coiling.
FIG. 2 is a perspective view schematically showing an example of a building structure using a square steel pipe of the present invention.
Figure 3 is a schematic diagram showing the locations of specimens taken for the flat plate tensile test performed in the example.
Figure 4 is a schematic diagram showing the sampling location of a test piece used in a Charpy impact test performed in an embodiment.

(Form for carrying out the invention)

Hereinafter, the present invention will be described.

<Resistance-compensation hot-rolled steel plate>

The low-yield hot-rolled steel sheet (hereinafter also simply referred to as a “hot-rolled steel sheet”) used in the low-yield square steel pipe of the present invention (hereinafter also simply referred to as a “square steel pipe”) contains, in mass%, C: 0.07% or more and 0.20% or less, Si: 0.40% or less, Mn: 0.20% or more and 1.00% or less, P: 0.100% or less, S: 0.050% or less, Al: 0.005% or more and 0.100% or less, and N: 0.0100% or less, the remainder being Fe and inevitable impurities, and has a component composition in which the contents of Mn and Si satisfy the following formula (1). In addition, the hot-rolled steel sheet has a main phase in which the steel structure in the center of the plate thickness is ferrite, and a second phase in which the area ratio of the sum of pearlite and pseudo-pearlite is 6 to 25%, and the area ratio of upper bainite is 5% or less. In addition, the hot-rolled steel sheet is characterized in that, in the steel structure in the center of the plate thickness, when a region surrounded by boundaries in which the orientation difference between adjacent crystals is 15° or more is defined as a crystal grain, the average crystal grain size of these crystal grains is 10.0 to 30.0 µm, and among the crystal grains, the area ratio of crystal grains having a crystal grain size within the average crystal grain size ±5.0 µm is 35% or more, and further, among the crystal grains, the number density of crystal grains in which the ratio of the major axis to the minor axis, (major axis)/(minor axis), is 3.0 or more is 30/mm2 or less. In addition, the "hot-rolled steel sheet" of the present invention includes a hot-rolled steel sheet and a hot-rolled steel strip.

1.0≤％Mn/％Si≤3.5… (1)

Here, %Mn and %Si represent the content (mass%) of each element in the steel plate.

Below, the composition of the hot-rolled steel sheet of the present invention is described. In addition, unless otherwise specified, "%" indicating the steel composition is "mass%."

C: 0.07% or more and 0.20% or less

C is an element that increases the strength of steel by solid solution strengthening. In addition, C is an element that contributes to the formation of pearlite and pseudo-pearlite, which are two phases. In order to secure the strength and yield ratio targeted in the present invention, it is necessary to contain 0.07% or more of C. On the other hand, if the C content exceeds 0.20%, not only does the ratio of the hard phase increase, which lowers the toughness, but also the yield ratio exceeds 0.90, so that the desired yield ratio cannot be obtained. In addition, the weldability also deteriorates. Therefore, the C content is set to 0.07% or more and 0.20% or less. The C content is preferably 0.08% or more. In addition, the C content is preferably 0.18% or less, and more preferably 0.17% or less.

Si: 0.40% or less

Si is an element that increases the strength of steel by solid solution strengthening. The lower limit of Si is not specifically stipulated (usually more than 0%), but in order to obtain this effect, it is preferable to contain 0.01% or more of Si. The Si content is more preferably 0.05% or more. On the other hand, if the Si content exceeds 0.40%, oxides are likely to be generated in the electric resistance welding portion, which deteriorates the weld properties. In addition, the toughness of the base material portion other than the electric resistance welding portion also deteriorates. Therefore, the Si content is set to 0.40% or less. It 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 increases the strength of steel by solid solution strengthening. In addition, Mn is an element that contributes to the refinement of the structure by lowering the ferrite transformation initiation temperature. In order to secure the strength and structure targeted in the present invention, it is necessary to contain 0.20% or more of Mn. On the other hand, if the Mn content exceeds 1.00%, the amount of bainite formed becomes excessively large, so that the yield ratio exceeds 0.90, and the desired yield ratio cannot be obtained. In addition, if the Mn content exceeds 1.00%, the hardness of the central segregation area increases, which may cause cracks during welding. Therefore, the Mn content is set to 0.20% or more and 1.00% or less. The Mn content is preferably 0.25% or more, and more preferably 0.30% or more. In addition, the Mn content is preferably 0.95% or less, and further preferably 0.90% or less.

P: 0.100% or less

P is preferably reduced as much as possible because it segregates at grain boundaries and causes material inhomogeneity, but a content of up to 0.100% is permissible. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less, and more preferably 0.020% or less. Although the lower limit of P is not specifically specified (usually it is over 0%), since excessive reduction causes a sharp increase in refining costs, it is preferable that P be 0.002% or more.

S: 0.050% or less

S is usually present as MnS in steel, but MnS is thinly stretched during the hot rolling process and has a negative effect on ductility. Therefore, in the present invention, it is preferable to reduce S as much as possible, but a content of up to 0.050% is permissible. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.008% or less. In addition, although the lower limit of S is not specifically stipulated (usually it is greater than 0%), since excessive reduction causes a sharp increase in smelting costs, it is preferable that S is set to 0.0002% or more.

Al: 0.005% or more and 0.100% or less

Al is an element that acts as a strong deoxidizer. In order to obtain this effect, it is necessary to contain 0.005% or more of Al. On the other hand, if the Al content exceeds 0.100%, the weldability deteriorates, and the alumina-based inclusions increase, which deteriorates the surface appearance. In addition, the toughness of the weld also deteriorates. 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

Nitrogen is an element that has the effect of lowering the toughness by firmly fixing the dislocation motion. In the present invention, it is preferable to reduce N as much as possible, but the N content is permissible up to 0.0100%. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0040% or less, and still more preferably 0.0035% or less. In addition, the lower limit of N is not specifically stipulated (usually it is over 0%), but since excessive reduction causes a sharp increase in the refining cost, the N content is preferably set to 0.0010% or more, and more preferably 0.0015% or more.

1.0≤％Mn/％Si≤3.5

Here, %Mn and %Si in the above formula represent the content (mass%) of each element in the steel plate.

In the present invention, it is necessary to set the contents of Mn and Si within the above-mentioned ranges, and additionally satisfy the relationship of 1.0≤%Mn/%Si≤3.5.

By satisfying this relation, it becomes possible to obtain a steel structure having a second phase in which the area ratio of pearlite and/or pseudo-pearlite is 6 to 25% and the area ratio of upper bainite is 5% or less, as described below, and the strength, yield ratio, Charpy absorbed energy, and ductile-brittle transition temperature aimed at in the present invention can be obtained. The value of %Mn/%Si is preferably 1.2 or more, more preferably 1.4 or more. In addition, the value of %Mn/%Si is preferably 3.2 or less, more preferably 3.0 or less.

In the above composition of ingredients, the remainder is Fe and inevitable impurities. However, it is not denied to contain O of 0.005% or less within a range that does not impair the effects of the present invention. In addition, this O refers to total oxygen including O as an oxide.

In addition, in the present invention, with respect to Nb, Ti, V, Cr, Mo, Cu, Ni, Ca and B, which are optional elements described later, the ranges of Nb: less than 0.005%, Ti: less than 0.005%, V: less than 0.01%, Cr: less than 0.01%, Mo: less than 0.01%, Cu: less than 0.01%, Ni: less than 0.01%, Ca: less than 0.0005% and B: less than 0.0003% are all considered to be included in unavoidable impurities.

The above components are the basic component composition of the hot-rolled steel sheet in the present invention. The properties targeted by the present invention are obtained with the above component composition, but the component composition may include the following elements as necessary.

Specifically, it is 1 type or 2 or more types selected from among Nb: 0.005% or more and 0.020% or less, Ti: 0.005% or more and 0.020% or less, V: 0.01% or more and 0.10% or less, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 0.30% or less, Ni: 0.01% or more and 0.30% or less, Ca: 0.0005% or more and 0.0100% or less, and B: 0.0003% or more and 0.0100% or less.

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 these effects, when containing Nb, it is preferable to be 0.005% or more. Also, when containing Ti, it is preferable to be 0.005% or more. On the other hand, for each of Nb and Ti, if the content exceeds 0.020%, coarse carbides and nitrides may be formed, which may lead to a decrease in toughness.

For this reason, when containing Nb, it is set to a range of 0.020% or less, and when containing Ti, it is set to a range of 0.020% or less. Regarding each content of Nb and Ti, it is more preferably 0.007% or more, and even more preferably 0.009% or more. In addition, regarding each content of Nb and Ti, it is preferably 0.018% or less, and preferably 0.016% or less.

V: 0.01% or more and 0.10% or less, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less

V, Cr, and Mo are elements that increase the hardenability of steel and increase the strength of steel, and can be contained as needed. In order to obtain the above-mentioned effect, when V, Cr, and Mo are contained, it is preferable that V: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, respectively. More preferably, V: 0.02% or more, Cr: 0.10% or more, and Mo: 0.10% or more, respectively.

On the other hand, excessive content may cause a decrease in toughness and a deterioration in weldability. Therefore, when containing V, Cr, and Mo, V: 0.10% or less, Cr: 0.50% or less, and Mo: 0.50% or less, respectively. Preferably, V: 0.08% or less, Cr: 0.40% or less, and Mo: 0.40% or less, respectively.

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 increase the strength of steel by solid solution strengthening and can be contained as needed. In order to obtain the above effect, when Cu and Ni are contained, it is preferable that Cu: 0.01% or more and Ni: 0.01% or more, respectively. More preferably, Cu: 0.10% or more and Ni: 0.10% or more. On the other hand, there is a concern that excessive content may cause a decrease in toughness and a deterioration in weldability. Therefore, when Cu and Ni are contained, Cu: 0.30% or less and Ni: 0.30% or less, respectively. Preferably, Cu: 0.20% or less and Ni: 0.20% or less.

Ca: 0.0005% or more and 0.0100% or less

Ca is an element that contributes to improving the toughness of steel by spherodizing sulfides such as MnS that are thinly elongated in the hot rolling process, and can be contained as needed. In order to obtain this effect, when Ca is contained, it is preferably 0.0005% or more. More preferably, the Ca content is 0.0010% or more. On the other hand, when the Ca content exceeds 0.0100%, Ca oxide clusters are formed in the steel, and the toughness may deteriorate. Therefore, when Ca is contained, it is 0.0100% or less. Preferably, the Ca content is 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 ferrite transformation initiation temperature. In order to obtain this effect, when B is contained, it is preferable to make it 0.0003% or more. More preferably, the B content is 0.0005% or more. On the other hand, when the B content exceeds 0.0100%, the yield ratio may increase. Therefore, when B is contained, it is made 0.0100% or less. Preferably, the B content is 0.0050% or less.

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

The hot-rolled steel sheet of the present invention has a steel structure in the central portion of the plate thickness, a main phase which is ferrite, and a second phase in which the total area ratio of pearlite and pseudo-pearlite is 6 to 25% and the area ratio of upper bainite is 5% or less, and in the steel structure in the central portion of the plate thickness, when a region surrounded by boundaries in which the orientation difference between adjacent crystals is 15° or more is defined as a crystal grain, the average crystal grain size of these crystal grains is 10.0 to 30.0 µm, and among the crystal grains, the area ratio of crystal grains having a crystal grain size within the average crystal grain size ±5.0 µm is 35% or more, and further, among the crystal grains, the number density of crystal grains in which the ratio of the major axis to the minor axis, (major axis)/(minor axis), is 3.0 or more is 30/mm2 or less.

Additionally, in the present invention, the crystal grain size is defined as the diameter of a circle (equivalent circle diameter) having the same area as the target crystal grain.

Ferrite (main phase)

Ferrite is a soft structure, and in order to obtain the desired yield strength and yield ratio, it is made into a main phase in the present invention. In addition, "main phase" means having an area ratio of 50% or more. If the area ratio of ferrite is less than 50%, the yield stress becomes excessively large, and further, the work hardening index becomes small, so that the desired yield ratio may not be obtained. In addition, from the viewpoint of the above-mentioned yield stress and yield ratio, the area ratio of ferrite is preferably 70% or more, and more preferably 72% or more. On the other hand, if the area ratio of ferrite exceeds 94%, the strength decreases, and the desired yield strength and tensile strength may not be obtained. Therefore, the area ratio of ferrite is 94% or less, and preferably, the area ratio of ferrite is 92% or less.

Area ratio of the sum of pearlite and pseudo-pearlite: 6 to 25%, Area ratio of upper bainite: 5% or less (second phase)

Pearlite and pseudo-pearlite are hard structures, and are the most important steel structures for increasing the strength of steel and also obtaining the yield ratio. In order to obtain the yield strength, tensile strength, and yield ratio aimed at in the present invention, it is necessary that the area ratio of the sum of pearlite and pseudo-pearlite be 6% or more. Preferably, it is 7% or more, and more preferably, it is 9% or more. On the other hand, if the area ratio of the sum of pearlite and pseudo-pearlite exceeds 25%, the toughness may deteriorate. Therefore, it is necessary that the area ratio of the sum of pearlite and pseudo-pearlite be 25% or less. Preferably, it is 23% or less, and more preferably, it is 21% or less.

In addition, it is preferable that the area ratio of the pseudo-pearlite is 5% or more. If the pseudo-pearlite exists at an area ratio of 5% or more, the yield ratio is suppressed low when a square steel pipe is manufactured, so that better earthquake resistance is obtained. On the other hand, in order to make the area ratio of the pseudo-pearlite exceed 15%, it is necessary to rapidly cool the temperature range in which pearlite is generated in the cooling process in hot rolling, so that the manufacturing conditions are limited. Therefore, the area ratio of the pseudo-pearlite is preferably 15% or less.

In addition, upper bainite is a structure having a hardness between ferrite and pearlite, and increases the strength of the steel. However, if the area ratio of upper bainite exceeds 5%, the yield strength aimed at in the present invention is not obtained. Therefore, it is necessary that the area ratio of upper bainite is 5% or less. It is preferably 4% or less. Upper bainite may be 0%.

Additionally, in the present invention, structures other than the main phase and the second phase are austenite and martensite.

Additionally, the area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite can be measured by the method described below.

In the steel structure at the center of the plate thickness, when a region surrounded by boundaries in which the orientation difference between neighboring crystals is 15° or more is defined as a crystal grain, the average crystal grain size of these crystal grains is 10.0 to 30.0 ㎛, and among the crystal grains, the area ratio of crystal grains having a crystal grain size within the average crystal grain size ±5.0 ㎛ is 35% or more, and further, among the crystal grains, the number density of crystal grains having a ratio of the major axis to the minor axis, (major axis)/(minor axis), of 3.0 or more is 30/mm2 or less.

The steel structure of the present invention is a steel (hereinafter referred to as “composite structure steel”) that mixes soft and hard structures in order to obtain the yield strength, yield strength, and tensile strength aimed at in the present invention.

However, such composite grain steels have poor toughness compared to single grain steels. Therefore, in the present invention, in order to achieve both the above mechanical properties and excellent toughness, when the region surrounded by boundaries with a crystal orientation difference of 15° or more is defined as a crystal grain, the grain size of the steel grain including the main phase and the second phase, the area ratio of coarse grains, and the number density of elongated grains are specified in the center of the thickness of the steel plate.

If the average grain size (average circle diameter) of the above-mentioned crystal grains is less than 10.0 ㎛, the yield ratio increases and the yield ratio aimed at in the present invention is not obtained. On the other hand, if the average grain size exceeds 30.0 ㎛, the toughness deteriorates. Therefore, the average grain size of the steel structure including the main phase and the second phase needs to be in the range of 10.0 to 30.0 ㎛. The above-mentioned average grain size is preferably 11.0 ㎛ or more, more preferably 12.5 ㎛ or more. In addition, the above-mentioned average grain size is preferably 28.0 ㎛ or less, more preferably 26.0 ㎛ or less.

Here, during the review by the present inventors, there were cases where the yield ratio and Charpy absorbed energy targeted by the present invention could not be obtained even when the average crystal grain size was within the range of 10.0 to 30.0 ㎛. Therefore, as a result of further review by the present inventors, it was found that in order to obtain the toughness and yield ratio targeted by the present invention, the area ratio of crystal grains having a crystal grain size within ±5.0 ㎛ of the average crystal grain size and the number density of elongated crystal grains are very important.

Specifically, in the center of the thickness of the steel plate, it is necessary that the structure has a grain density of 30/mm2 or less in which the area ratio is 35% or more and the ratio of the major diameter to the minor diameter, (major diameter)/(minor diameter), is 3.0 or more.

The crystal orientation difference, the average crystal grain size, and the area ratio of crystal grains having a crystal grain size within the average crystal grain size of ±5.0 ㎛ can all be measured by the SEM/EBSD method. In addition, in the present invention, the measurement can be made by the method described below.

By satisfying the above-mentioned component composition and steel structure, it becomes possible to obtain a hot-rolled steel sheet having the strength, yield ratio, and toughness (Charpy absorbed energy at -20°C, ductile-to-brittle transition temperature) intended for the present invention.

That is, in the hot-rolled steel sheet of the present invention, the yield strength can be 250 MPa or more, the tensile strength can be 400 MPa or more, the yield ratio can be 0.75 or less, the work hardening index at a plastic strain of 3 to 7% can be 0.20 or more, the Charpy absorbed energy at -20°C can be 100 J or more, and the ductile-brittle transition temperature can be -20°C or less.

And, by using this hot-rolled steel plate, it is possible to obtain the square steel pipe described later.

In addition, the hot-rolled steel plate of the present invention preferably has a plate thickness of 12 mm or more, and more preferably has a plate thickness in the range of 12 to 32 mm.

Next, as a method for manufacturing a hot-rolled steel sheet of the present invention, a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention will be described.

The method for manufacturing a hot-rolled steel sheet of the present invention comprises, for example, first, heating a steel material having the above-described component composition to a heating temperature of 1100°C or more and 1300°C or less (heating process). Next, rough rolling is performed as hot rolling at a rough rolling end temperature of 850°C or more and 1150°C or less, and after 15 s or more have elapsed after the end of this rough rolling, finish rolling is started, and the finish rolling end temperature is 750°C or more and 850°C or less, and further, hot rolling is performed at a total reduction ratio of 40% or more and 59% or less at 930°C or less throughout the hot rolling process (hot rolling process). Next, for the material steel sheet obtained from the hot rolling, cooling is performed (cooling process) at an average cooling rate Vc (℃/s) at the center of the plate thickness that satisfies the following equation (2), and a cooling stop temperature at the center of the plate thickness: 550℃ or more and 680℃ or less. Next, for the material steel sheet, coiling is performed at a center of the plate thickness temperature: 550℃ or more and 680℃ or less (coiling process). Next, for the coiled steel sheet obtained in the coiling process, a second cooling is performed (second cooling process) in which the steel sheet is kept in a temperature range of 400℃ to 300℃ for 1.0 h or more and 10.0 h or less. Thus, the hot rolled steel sheet of the present invention is obtained.

4≤Vc≤20 … (2)

Hereinafter, a method for manufacturing a hot-rolled steel plate will be described in more detail. In addition, in the description of the manufacturing method below, the indication of "℃" regarding temperature refers to the surface temperature of a steel material or steel plate (hot-rolled plate, raw steel plate, hot-rolled steel plate) (hereinafter also referred to as steel plate, etc.) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer, etc. In addition, the temperature at the center of the thickness of the steel plate, etc. can be obtained by calculating the temperature distribution within the cross section of the steel plate, etc. by heat transfer analysis and correcting the result by the surface temperature of the steel plate, etc.

In the present invention, the method for melting the steel material (steel slab) is not particularly limited, and all known melting methods such as a converter, an electric furnace, and a vacuum melting furnace are suitable. The casting method is also not particularly limited, but the steel is manufactured into a desired size by a known casting method such as a continuous casting method. In addition, there is no problem in applying the ingot-slab rolling method instead of the continuous casting method. In addition, secondary refining such as ladle refining may be performed on the molten steel.

Next, as a heating process, the obtained steel material (steel slab) is heated to a heating temperature of 1100°C or more and 1300°C or less. Next, as a hot rolling process, rough rolling is performed at a rough rolling end temperature of 850°C or more and 1150°C or less, and finish rolling is started after 15 seconds or more have elapsed after the end of such rough rolling, and the finish rolling end temperature is 750°C or more and 850°C or less, and further, hot rolling is performed at a total reduction ratio of 40% or more and 59% or less at 930°C or less throughout the entire hot rolling process, thereby obtaining a raw steel sheet for a hot rolled steel sheet.

Heating temperature: 1100℃ or higher, 1300℃ or lower

In the heating process, 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 grains coarsen, and in the subsequent rolling (rough rolling, finish rolling), fine austenite grains are not obtained, making it difficult to secure the average grain size of the steel structure of the hot rolled steel sheet aimed at in the present invention. In addition, it becomes difficult to suppress the formation of coarse bainite, making it difficult to control the area ratio of grains having an average grain size of ±5.0 ㎛ or less within the range aimed at in the present invention. Therefore, the heating temperature in the heating process is set to 1100°C or more and 1300°C or less. It is more preferably 1120°C or more. In addition, the heating temperature in the heating process is more preferably 1280°C or less.

In addition, in the present invention, in addition to the conventional method of manufacturing a steel slab (slab), first cooling it to room temperature, and then reheating it, an energy-saving process of direct rolling in which the slab is loaded into a heating furnace while still hot without cooling it to room temperature can also be applied without any problem.

Rolling end temperature: 850℃ or higher and 1150℃ or lower

In the hot rolling process, if the rough rolling end temperature is lower than 850°C, the surface temperature of the steel sheet becomes lower than the ferrite transformation initiation temperature during subsequent finish rolling, a large amount of ferrite is generated, and the area ratio of pearlite and pseudo-pearlite decreases, so it becomes difficult to obtain the low-yield ratio square steel pipe aimed at in the present invention. On the other hand, if the rough rolling end temperature exceeds 1150°C, the amount of reduction in the austenite non-recrystallization temperature range is insufficient, so that fine austenite grains are not obtained. As a result, the steel structure of the hot-rolled steel sheet aimed at in the present invention is not obtained, and when a region surrounded by boundaries where the orientation difference between neighboring crystals is 15° or more in the center of the thickness of the steel sheet is defined as a crystal grain, it becomes difficult to obtain a steel structure in which the average crystal grain size of these crystal grains is 10.0 to 30.0 µm, and further, the number density of the ratio of the major axis to the minor axis (major axis)/(minor axis) among these crystal grains is 3.0 or more is 30/mm2 or less, and the crystal grains having a crystal grain size within the average crystal grain size ±5.0 µm have an area ratio of 35% or more. In addition, it becomes difficult to suppress the formation of coarse bainite. For this reason, the rough rolling finish temperature is set to 850°C or more and 1150°C or less. It is preferably 860°C or more, and more preferably 870°C or more. In addition, the rough rolling finish temperature is preferably 1100°C or less, and more preferably 1050°C or less.

Elapsed time from the end of rough rolling to the start of final rolling: 15s or more

In the hot rolling process, if the time from the end of rough rolling to the start of finish rolling is less than 15 s, the variation in the grain size of the austenite increases, and it becomes difficult to make the area ratio of grains having an average grain size of ±5.0 µm or less in the center of the steel plate thickness, which is the purpose of the present invention, 35% or more. In addition, it becomes difficult to obtain a low-yield square steel pipe having a ratio P of Charpy absorbed energy at -20°C in the pipe circumferential direction to the pipe axial direction described later (hereinafter referred to as "energy ratio P") of 0.5 to 1.2. The time is preferably 18 s or more, and more preferably 20 s or more. The upper limit of the time from the end of rough rolling to the start of finish rolling is not specifically defined, but from the viewpoint of productivity, it is preferably 300 s or less, and more preferably 280 s or less.

Finishing rolling end temperature: 750℃ or higher and 850℃ or lower

In the hot rolling process, when the finish rolling end temperature is less than 750°C, the steel sheet surface temperature during the finish rolling becomes lower than the ferrite transformation initiation temperature, and ferrite elongated in the rolling direction is formed, which may result in reduced workability. On the other hand, when the finish rolling end temperature exceeds 850°C, the amount of reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains are not obtained. As a result, the grains become coarse, making it difficult to secure the strength aimed at in the present invention. In addition, it becomes difficult to suppress the formation of coarse bainite. Therefore, the finish rolling end temperature is set to 750°C or more and 850°C or less. The finish rolling end temperature is preferably 770°C or more, and more preferably 780°C or more. In addition, the finish rolling end temperature is preferably 830°C or less, and more preferably 820°C or less.

Total pressure reduction below 930℃: 40% or more and 59% or less

In the present invention, in the hot rolling process of performing the above rough rolling and the above finish rolling, by refining the subgrains in the austenite, the ferrite and bainite generated in the subsequent cooling process and coiling process are refinished, thereby obtaining the steel structure of the hot rolled steel sheet having the strength and toughness targeted by the present invention. In order to refining the subgrains in the austenite in the hot rolling process, it is necessary to increase the reduction ratio in the austenite non-recrystallization temperature range and to introduce sufficient processing strain. However, if the total reduction ratio exceeds 59%, grains having a large ratio of major axis to minor axis are likely to be generated, resulting in a decrease in toughness. Therefore, in the present invention, the total reduction ratio of 930°C or less is set to 59% or less. It is preferably 57% or less, and more preferably 55% or less. On the other hand, if the total reduction ratio of 930°C or less is less than 40%, the crystal grain size of ferrite or bainite becomes large, resulting in a decrease in toughness. For this reason, the total pressure reduction ratio below 930℃ was set to 40% or more. Preferably, it is 42% or more, and more preferably, it is 45% or more.

In addition, the reason why the total reduction ratio is set to 930℃ or less is because, in the rolling process, when the temperature exceeds 930℃, austenite recrystallizes and the dislocation introduced by rolling is lost, so refined austenite is not obtained.

The above-mentioned total reduction ratio refers to the sum of the reduction ratios of each rolling pass in a temperature range of 930℃ or less.

In the present invention, when hot rolling a heated steel material (slab), hot rolling may be performed such that the total reduction ratio of 930°C or less across both the rough rolling and the finish rolling is 40% or more and 59% or less. Alternatively, the total reduction ratio of 930°C or less may be 40% or more and 59% or less across only the finish rolling. That is, when the total reduction ratio of 930°C or less cannot be 40% or more and 59% or less across only the finish rolling, the steel material (slab) may be cooled during the rough rolling to make the temperature 930°C or less, and then rolled such that the total of the reduction ratios of 930°C or less across both the rough rolling and the finish rolling is 40% or more and 59% or less.

After the hot rolling process, as a cooling process, the material steel plate for hot rolled steel plate (hereinafter also simply referred to as the material steel plate, also referred to as hot rolled plate) is cooled. In this cooling process, cooling is performed under the conditions that the average cooling rate Vc (℃/s) at the center of the plate thickness satisfies the following equation (2), and the cooling stop temperature at the center of the plate thickness is 550℃ or more and 680℃ or less.

4≤Vc≤20 … (2)

Average cooling rate Vc at the center of plate thickness: 4℃/s or more and 20℃/s or less

In the cooling process, if the average cooling rate Vc at the center of the plate thickness is less than 4°C/s, the frequency of nucleation of ferrite decreases and the ferrite grains coarsen, so that the desired strength cannot be obtained. On the other hand, if the average cooling rate Vc exceeds 20°C/s, a large amount of upper bainite is generated, so that the yield ratio targeted by the present invention cannot be obtained. The average cooling rate Vc is preferably 6°C/s or more, more preferably 8°C/s or more. Furthermore, it is preferably 18°C/s or less, more preferably 16°C/s or less.

In the present invention, from the viewpoint of suppressing the crystal grain size from becoming coarser, it is preferable to start cooling immediately after the end of the finishing rolling.

Cooling stop temperature at the center of plate thickness: 550℃ or more and 680℃ or less

In the cooling process, if the cooling stop temperature at the center of the plate thickness is less than 550°C, temperature unevenness is likely to occur in the longitudinal and/or transverse directions of the material steel plate during cooling, which may cause deviations in mechanical properties. On the other hand, if the cooling stop temperature at the center of the plate thickness exceeds 680°C, the ferrite grains coarsen, and the desired crystal grain size is not obtained. In addition, the cooling stop temperature at the center of the plate thickness is preferably 560°C or higher, more preferably 580°C or higher. In addition, it is preferably 660°C or lower, more preferably 650°C or lower.

In the present invention, the average cooling rate Vc (℃/s) at the center of the plate thickness is the average cooling rate in the temperature range from the start of cooling to the stop of cooling at the center of the plate thickness. In addition, the average cooling rate is a value obtained by ((temperature of the material steel plate before cooling - temperature of the material steel plate after cooling) / cooling time), and can be calculated from the temperature distribution within the cross-section of the material steel plate obtained by heat transfer analysis.

Cooling methods include water cooling, such as spraying water from a nozzle, or cooling by spraying cooling gas.

In the cooling process, it is preferable to perform a cooling operation (process) on both sides of the material steel plate (hot rolled sheet) so that both sides of the material steel plate are cooled under the same conditions. In addition, in order to obtain the above-mentioned cooling rate, the amount or pressure of the cooling water or cooling gas, the injection time and angle, and the return speed of the material steel plate can be adjusted. In particular, in order to obtain the cooling rate specified in the present invention, a heat transfer analysis is performed in advance to determine the conditions for the cooling treatment of the material steel plate, and then these conditions can be reflected in the manufacturing conditions.

After the above cooling process, as a coiling process, the material steel plate is coiled. In this coiling process, from the viewpoint of the steel plate structure, coiling is performed at a temperature at the center of the plate thickness of the material steel plate (coiling temperature): 550°C or more and 680°C or less. When the coiling temperature is less than 550°C, a large amount of upper bainite is generated on the surface of the steel plate, and the area ratio may exceed 5%. On the other hand, when the coiling temperature exceeds 680°C, the ferrite grains coarsen and the desired crystal grain size is not obtained. The coiling temperature is preferably 570°C or more, and more preferably 580°C or more. In addition, the coiling temperature is preferably 660°C or less, and more preferably 650°C or less.

After the above coiling process, a second cooling process is performed to cool the coiled steel sheet obtained in the above coiling process. In this second cooling process, the coiled steel sheet obtained in the above coiling process is allowed to remain in a temperature range of 400°C to 300°C for 1.0 h or more and 10.0 h or less. If the residence time in the temperature range of 400°C to 300°C is less than 1.0 h, the desired work hardening index is not obtained, and the desired yield ratio and toughness are not obtained. If the residence time in the temperature range of 400°C to 300°C exceeds 10.0 h, the desired yield strength and tensile strength may not be obtained. The residence time in the temperature range of 400°C to 300°C is preferably 1.5 h or more, and more preferably 2.0 h or more. The residence time in the temperature range of 400°C to 300°C is preferably 9.0 h or less, and more preferably 8.5 h or less.

In addition, depending on the thickness, width, length, etc. of the material steel plate, how the temperature of the steel plate after coiling (coiled steel plate) changes varies. Therefore, in the second cooling process, heat transfer analysis is performed in advance to calculate the temperature change of the steel plate after coiling, and it is preferable to perform heat insulation treatment by covering the periphery of the steel plate with an insulating material, etc., or to cool the steel plate with cooling water or cooling gas, etc. so that the residence time in the temperature range of 400°C to 300°C falls within the range of the present invention. Since there is almost no change in properties such as steel structure or strength in a temperature range below 300°C, any type of cooling, such as rapid cooling or allowing to cool, may be performed for cooling from 300°C to room temperature.

In addition, since the steel plate after coiling (coiled steel plate) has a temperature deviation depending on the location, as shown in Fig. 1, the temperature of the side of the steel plate after coiling (ultimately a hot-rolled steel plate) was measured at three locations (symbol 10. the outer surface, inner surface, and center of the steel plate after coiling), and the average value was calculated and used as the temperature of the coiled steel plate. In addition, the temperature of the coiled steel plate was measured every 20 minutes, and the residence time of 400°C to 300°C was calculated. The temperature of the coiled steel plate can be measured with a non-contact thermometer such as a radiation thermometer or a contact thermometer such as a thermocouple.

<Resistance ratio square steel pipe>

The low-yield ratio square steel pipe of the present invention uses the above low-yield ratio hot-rolled steel plate as its material.

A low-yield square steel pipe can exhibit, in the pipe-axis direction, a yield strength of 295 MPa or more, a tensile strength of 400 MPa or more, and a low-yield ratio of 0.90 or less in a flat section, as well as a uniform elongation of 5.0% or more, an absorbed energy of 60 J or more in a Charpy impact test at a test temperature of -20°C, an energy ratio P of 0.5 to 1.2, and a low-temperature toughness of a ductile-to-brittle transition temperature of -10°C or less.

In addition, since it has excellent low-temperature toughness not only in the pipe-axial direction but also in the pipe-circumferential direction, it can be suitably used as a structural member of a building in a low-temperature environment such as a cold region where the temperature is below freezing, for example.

In the pipe manufacturing process, a hot rolled steel plate is roll-formed into a cylindrical open pipe, and its mating parts are welded to manufacture an annular steel pipe. Thereafter, a drawing of several percent is applied in the pipe axis direction by rolls arranged up, down, left, and right with respect to the annular steel pipe, and the pipe is formed into a corner shape to obtain a square steel pipe.

In addition, in the present invention, the cylindrical shape means a shape obtained by forming a hot-rolled steel plate into a circle by roll forming, and means a state in which the ends of the hot-rolled steel plate are not welded.

A coil-shaped low-yield hot-rolled steel plate is cold-formed into a circular shape by a roll forming method using rolls to manufacture an annular steel pipe, and then the annular shape is formed into a square shape by a roll forming method using rolls to manufacture a square steel pipe.

When roll forming into an annular steel pipe is performed in the cold in this way, a large processing strain is introduced in the pipe-axial direction and the pipe-circumferential direction, so there is a problem that the yield ratio in the pipe-axial direction and the pipe-circumferential direction tends to increase and the toughness tends to decrease. However, in the low-yield ratio square steel pipe of the present invention, since the low-yield ratio hot-rolled steel plate is used as the material, the increase in the yield ratio, etc. is suppressed, and even if the wall thickness is 12 mm or more, for example, the low-yield ratio can be provided.

In addition, as described later, the square steel pipe of the present invention can have a ratio P of Charpy absorbed energy at -20°C of 0.5 to 1.2 and excellent low-temperature toughness in the pipe-axial direction and the pipe circumferential direction. However, in order to obtain such toughness, in addition to satisfying the component composition stipulated in the present invention, it is necessary to start finish rolling 15 s or more after the end of rough rolling. By starting finish rolling 15 s or more after the end of rough rolling, a hot-rolled steel sheet having a small difference in the steel structure in the sheet width direction and the sheet length direction (rolling direction) is obtained, and by roll-forming the hot-rolled steel sheet, a low-yield square steel pipe having a ratio P of Charpy absorbed energy at -20°C of 0.5 to 1.2 is obtained.

Although it is possible to manufacture a square steel pipe having a Charpy absorption energy ratio P of 0.5 to 1.2 at -20°C by the above method, in order to manufacture it more stably, a round steel pipe may be manufactured by roll forming, then heat treating it, and then forming it into a square shape to manufacture a square steel pipe. By performing such heat treatment, the toughness in the circumferential direction of the pipe is improved, and the square steel pipe having a Charpy absorption energy ratio P of 0.5 to 1.2 at -20°C aimed at by the present invention is more advantageously obtained.

The heat treatment temperature of this heat treatment is preferably 100°C or higher and 550°C or lower. When the heat treatment temperature is lower than 100°C, the toughness does not improve, while when the heat treatment temperature exceeds 550°C, the steel structure coarsens and the strength and toughness deteriorate. The heat treatment temperature is more preferably 150°C or higher. In addition, the heat treatment temperature is more preferably 500°C or lower. The heat treatment time is preferably 30 seconds or longer, and more preferably 1 minute or longer. Although the upper limit is not specifically specified, from the viewpoint of suppressing the heat treatment cost, it is preferably 10 minutes or shorter, and more preferably 5 minutes or shorter.

The method of heat treating these square steel pipes is not limited to any one method, but any known heat treatment facility (heating facility) can be used, such as combustion of combustible gas, heating by an electric heater, or heating by IH (induction heating).

In addition, the square steel pipe in the present invention is not limited to a square steel pipe in which each side length is the same (the ratio of the long side length to the short side length (long side length/short side length) is 1.0), and also includes a square steel pipe in which this ratio exceeds 1.0. However, if this ratio exceeds 2.5, local buckling is likely to occur on the long side, and the compressive strength in the pipe axis direction decreases. Therefore, this ratio is preferably in the range of 1.0 or more and 2.5 or less. This ratio is more preferably 1.0 or more and 2.0 or less.

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

In addition, since the square steel pipe of the present invention has a ductile-brittle transition temperature of less than 0°C and excellent toughness not only in the pipe-axial direction but also in the pipe-circumferential direction, it can be suitably used as a structural member of a building in a cold region where the air temperature or room temperature is below freezing point, and even if a large earthquake occurs, the building structure is unlikely to collapse, so excellent earthquake resistance can be secured.

<Architecture Structure>

Figure 2 is a schematic diagram showing an example of an architectural structure of the present invention.

The building structure of the present invention comprises the square steel pipe (low-yield square steel pipe) (1) of the present invention as a pillar material. Reference numerals 4, 5, 6, and 7 represent a main beam, a sub-beam, a diaphragm, and a stud, in that order.

As described above, the square steel pipe of the present invention has excellent mechanical properties in the flat portion. Therefore, the building structure of the present invention using this square steel pipe as a column material exhibits excellent seismic performance.

Example

Hereinafter, the present invention will be further described in detail based on examples. In addition, the present invention is not limited to the following examples.

A slab was obtained by casting molten steel having the composition shown in Table 1. The obtained slab was subjected to a heating process, a hot rolling process, a cooling process, a coiling process, and a second cooling process after the coiling process under the conditions shown in Table 2, thereby obtaining a hot-rolled steel sheet. In Table 1, a hyphen (-) means that the content is 0 (zero) or corresponds to an impurity.

After that, the following process was performed.

That is, the hot-rolled steel plate was formed into a cylindrical shape by roll forming, and its mating portions were welded to manufacture an annular steel pipe. Thereafter, the annular steel pipe was formed into a corner shape (a square shape when viewed in a cross-section perpendicular to the pipe axis) by rolls arranged on the upper, lower, left, and right sides, thereby obtaining a roll-formed square steel pipe having a corner portion and a flat portion and having side lengths (mm) and thicknesses (mm) shown in Table 4 below.

In addition, test pieces were collected from the hot-rolled steel plate mentioned above, and the following structural observations, tensile tests, and Charpy impact tests were performed.

[Organizational observation]

Test pieces for tissue observation were taken from the central part of the width of the hot-rolled steel plate, including the position of 1/2t of the plate thickness (t: plate thickness), and covering a range of 5 mm in each thickness direction from the position of 1/2t of the plate thickness. The specimens were polished so that the observation surface would be a cross-section in the rolling direction during hot rolling, and then etched with nital to produce them.

For tissue observation, the tissue was observed and captured in a range of ±1 mm in the thickness direction from the position of 1/2t of the hot-rolled steel plate thickness using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times). The area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite were obtained from the obtained optical microscope images and SEM images.

The area ratio of each tissue was calculated by observing from five fields of view and taking the average of the values obtained from each field of view. Here, the area ratio obtained by tissue observation was taken as the area ratio of each tissue.

Here, ferrite refers to a product by diffusion transformation and indicates a low dislocation density and almost recovered organization. Polygonal ferrite and pseudo-polygonal ferrite are included. Pearlite is a organization in which cementite and ferrite are arranged in a layered shape, and pseudo-pearlite is a organization in which cementite arranged in a dot row shape is confirmed among ferrite. In addition, upper bainite is a multi-phase organization of ferrite and cementite in a lath shape with a high dislocation density. In addition to the above shapes, ferrite was determined to be white, pearlite was black, pseudo-pearlite was black or gray, and upper bainite was white or gray.

In addition, the average grain size (average equivalent circular diameter) was measured using the SEM/EBSD method over a range of ±1 mm in the thickness direction from the plate thickness t/2 position including the plate thickness t/2 position (meaning the center of the plate thickness in the present invention). The measurement area was 500 µm × 1000 µm (= 0.5㎟), and the measurement step size was 0.5 µm. The grain size was measured by obtaining the orientation difference between adjacent grains, and defining the boundary at which the orientation difference was 15° or more as the grain boundary. The grain size (equivalent circular diameter) of each grain was calculated from the obtained grain boundaries, and the arithmetic mean thereof was obtained and defined as the average grain size (average equivalent circular diameter). In addition, the total area of crystal grains having a crystal grain size within the average crystal grain size of ±5.0 ㎛ was calculated and divided by the area of the measurement area (0.5㎟), thereby calculating the area ratio of crystal grains having a crystal grain size within the average crystal grain size of ±5.0 ㎛.

The major axis and minor axis of the crystal grains were measured by the method described in JIS R 1670 (2006), and the ratio of the major axis to the minor axis, (major axis)/(minor axis), was calculated. The number of crystal grains whose ratio of the major axis to the minor axis, (major axis)/(minor axis), was 3.0 or more was measured, and by dividing it by the area of the measurement region (0.5㎟), the number of crystal grains (numbers/㎟) whose ratio of the major axis to the minor axis, (major axis)/(minor axis), was 3.0 or more. In addition, in the analysis of the crystal grain size and the measurement of the number of crystal grains, crystal grains with a grain size of less than 2.0 ㎛ were excluded from the analysis target as measurement noise.

[Tensile test]

From the obtained hot-rolled steel sheet, JIS No. 5 tensile test specimens were collected so that the tensile direction was parallel to the rolling direction. For the collected tensile test specimens, a tensile test was performed in accordance with the provisions of JIS Z 2241 (2011), the yield strength YS and the tensile strength TS were measured, and the yield ratio defined as (yield strength)/(tensile strength) was calculated. In addition, the number of test specimens was set to two each, and their average values were calculated to obtain YS, TS, and the yield ratio. In addition, the work hardening index at a plastic strain of 3 to 7% was calculated by the method (two-point method) described in JIS Z 2253 (2011).

[Charpy impact test]

A V-notch standard test piece conforming to the provisions of JIS Z 2242 (2018) was used, which was sampled at a position 1/2t (center of plate thickness) of the plate thickness t of the obtained hot-rolled steel plate so that the longitudinal direction of the test piece was parallel to the rolling direction. In compliance with the provisions of JIS Z 2242 (2018), a Charpy impact test was performed at test temperatures: -80°C, -60°C, -40°C, -20°C, and 0°C. In addition, the number of test pieces was three at each test temperature, and the average value (J) of the ductile-brittle transition temperature and the absorbed energy was obtained.

The results obtained are shown in Table 3.

In addition, test specimens were collected from the obtained square steel pipe (roll-formed square steel pipe), and the tensile test and Charpy impact test shown below were performed.

[Tensile test]

Figure 3 is a schematic diagram showing the locations of the tensile test specimens of the flat plate.

As shown in X of Fig. 3, the tensile test specimen was collected as a JIS No. 5 tensile test specimen from the flat plate portion of the square steel pipe (1) so that the tensile direction was parallel to the pipe axis direction. For the collected tensile test specimen, a tensile test was performed in accordance with the provisions of JIS Z 2241 (2011), the yield strength YS and the tensile strength TS were measured, and the yield ratio defined as (yield strength) / (tensile strength) was calculated. In addition, the plastic elongation (uniform elongation) at the maximum load point was also measured (A _g of JIS Z 2241 (2011)). In addition, the tensile test specimen of the flat plate portion was collected from the position (X of Fig. 3) of the center of the width of the flat plate portion at the 3 o'clock edge when the welded portion (W of Fig. 3) of the square steel pipe is in the 12 o'clock direction. In addition, the number of test specimens was set to two each, and their average values were calculated to obtain YS, TS, and yield ratio.

[Charpy impact test]

Figure 4 is a schematic diagram showing the sampling location of Charpy test pieces.

The Charpy impact test used a V-notch standard test piece conforming to the provisions of JIS Z 2242 (2018) taken from the flat surface of the square steel pipe (1) at a position 1/4t of the thickness t from the outer surface of the square steel pipe (1) so that the longitudinal direction of the test piece was parallel to the pipe axis direction, as shown in Y of Fig. 4. In compliance with the provisions of JIS Z 2242 (2018), the Charpy impact test was performed at test temperatures: -60°C, -40°C, -20°C, and 0°C. In addition, the number of test pieces was three at each test temperature, and the average value (J) of the ductile-brittle transition temperature and the absorbed energy was obtained.

In addition, in order to measure the Charpy absorbed energy in the circumferential direction of the pipe, as shown in Z of Fig. 4, a V-notch standard test piece that complies with the provisions of JIS Z 2242 (2018) was used, which was taken from the flat surface of the square steel pipe (1) at a position 1/4t of the thickness t from the outer surface of the pipe so that the longitudinal direction of the test piece was parallel to the circumferential direction of the pipe. In compliance with the provisions of JIS Z 2242 (2018), a Charpy impact test was performed at a test temperature of -20°C. In addition, the number of test pieces was three, and the average value (J) of the absorbed energy was obtained. In addition, the ratio P of the Charpy absorbed energy at -20°C in the circumferential direction of the pipe to the pipe axis direction was obtained.

The results obtained are shown in Table 4.

In Tables 3 and 4, steel plates No. 1 to 20 are examples of the present invention, and steel plates No. 21 to 49 are comparative examples. Steel plate No. in Table 4 means a square steel pipe manufactured using the steel plate of Table 3 of the same number. For example, steel plate No. 1 in Table 4 is a square steel pipe manufactured from steel plate No. 1 in Table 3.

As shown in Table 3, the hot-rolled steel sheet of the present invention all has a steel structure in the central portion of the plate thickness that includes a main phase that is ferrite, pearlite and pseudo-pearlite having a total area ratio of 6 to 25%, and upper bainite (second phase) having an area ratio of 5% or less, and further, when a region surrounded by boundaries having an orientation difference of 15° or more is defined as a crystal grain, in the steel structure in the central portion of the plate thickness, the average crystal grain size of the steel structure including the main phase and the second phase is 10.0 to 30.0 µm, and among the crystal grains, the area ratio of crystal grains having a crystal grain size within the average crystal grain size ±5.0 µm is 35% or more, and the number density of crystal grains having a ratio of the major diameter to the minor diameter, (major diameter)/(minor diameter), of 3.0 or more is 30/mm2 or less.

In addition, the yield strength was 250 MPa or more, the tensile strength was 400 MPa or more, the yield ratio was 0.75 or less, the work hardening index n _3-7 at 3 to 7% plastic strain was 0.20 or more, the Charpy absorbed energy at -20°C was 100 J or more, and the ductile-to-brittle transition temperature was -20°C or less.

In addition, as shown in Table 4, the square steel pipe manufactured using the hot-rolled steel plate of the present invention all had a yield strength in the flat portion of 295 MPa or more, a tensile strength in the flat portion of 400 MPa or more, a yield ratio in the flat portion of 0.90 or less, a uniform elongation in the flat portion of 5.0% or more, a Charpy absorbed energy in the pipe axis direction at -20°C of 60 J or more, an energy ratio P of 0.5 or more and 1.2 or less, and a ductile-brittle transition temperature of the flat portion of -10°C or less.

In this regard, Comparative Example No. 21 (steel U) has Mn/Si=0.8, which is outside the scope of the present invention, so the total area ratio of pearlite and pseudo-pearlite is less than 6%, and the hot-rolled steel sheet does not reach the desired value in terms of the strain hardening index at a plastic strain of 3 to 7%. In addition, the square steel pipe manufactured using this hot-rolled steel sheet had low uniform elongation, and the yield ratio did not reach the desired value.

Comparative Example No. 22 (steel V) has Mn/Si=14.7, which is outside the scope of the present invention, so the total area ratio of pearlite and pseudo-pearlite exceeds 25%, resulting in a hot-rolled steel sheet in which the Charpy absorption energy at -20°C and the like do not reach the desired value. In addition, a square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired value in the Charpy absorption energy in the pipe axis direction at -20°C and the like.

Comparative Example No. 23 (steel W) had a C content below the range of the present invention, so the area ratio of the sum of pearlite and pseudo-pearlite was outside the range of the present invention, resulting in a hot-rolled steel sheet whose yield strength and tensile strength did not reach the desired values. In addition, a square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in yield strength and tensile strength.

Comparative Example No. 24 (steel X) had a C content exceeding the range of the present invention, so the area ratio of the second phase was outside the range of the present invention, resulting in a hot-rolled steel sheet in which the yield ratio and the Charpy absorbed energy at -20°C did not reach the desired values. In addition, a square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in the Charpy absorbed energy in the pipe axis direction at -20°C.

Comparative Example No. 25 (steel Y) was a hot-rolled steel sheet in which, because the Si content exceeded the range of the present invention, the area ratio of pseudo-pearlite increased excessively, and the Charpy absorption energy at -20°C and the like did not reach the desired values. In addition, a square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in the Charpy absorption energy in the pipe axis direction at -20°C and the like.

Comparative Example No. 26 (steel Z) was a hot-rolled steel sheet in which the yield ratio and the like did not reach the desired values because the amount of upper bainite increased excessively due to the Mn content exceeding the range of the present invention. In addition, a square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in terms of the yield ratio and the like.

In comparative example No. 27 (steel AA), since the Nb content exceeded the range of the present invention, the amount of upper bainite increased excessively. As a result, the number density of crystal grains having a ratio of the major axis to the minor axis of 3.0 or more fell outside the range of the present invention, resulting in a hot-rolled steel sheet in which the Charpy absorbed energy at -20°C and the like did not reach the desired values. In addition, a square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in the Charpy absorbed energy in the pipe axis direction at -20°C and the ductile-to-brittle transition temperature.

Comparative Example No. 28 (steel AB) is thought to have caused an excessive increase in the amount of upper bainite and the formation of coarse carbides and nitrides because the Ti content exceeded the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired values, such as Charpy absorption energy at -20°C. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values, such as Charpy absorption energy and ductile-to-brittle transition temperature in the pipe axis direction at -20°C.

Comparative Example No. 29 (steel AC) had a V content exceeding the range of the present invention, and therefore, the amount of upper bainite was outside the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired values, such as Charpy absorbed energy at -20°C. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values, such as Charpy absorbed energy and ductile-to-brittle transition temperature in the pipe axis direction at -20°C.

In comparative example No. 30 (steel AD), since the Cr content exceeded the range of the present invention, the amount of upper bainite was outside the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired values in Charpy absorption energy at -20°C and the like. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in Charpy absorption energy and ductile-to-brittle transition temperature in the pipe axis direction at -20°C.

In Comparative Example No. 31 (steel AE), since the Mo content exceeded the range of the present invention, the amount of upper bainite was outside the range of the present invention. As a result, the hot-rolled steel sheet was obtained in which the Charpy absorbed energy at -20°C and the like did not reach the desired values. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in the Charpy absorbed energy and the ductile-to-brittle transition temperature in the pipe axis direction at -20°C.

In comparative example No. 32 (steel AF), it is thought that Cu was coarsely precipitated because the Cu content exceeded the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired values, such as Charpy absorption energy at -20°C. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values, such as Charpy absorption energy and ductile-to-brittle transition temperature in the pipe axis direction at -20°C.

Comparative Example No. 33 (steel AG) had an Mn content below the range of the present invention and an Ni content above the range of the present invention, so that the amount of upper bainite fell outside the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired values, such as the ductile-to-brittle transition temperature. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values, such as the Charpy absorbed energy in the pipe axis direction at -20°C and the ductile-to-brittle transition temperature.

Comparative Example No. 34 (steel AH) is thought to have formed Ca oxide clusters because the Ca content exceeded the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired value in Charpy absorption energy at -20°C. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in Charpy absorption energy and ductile-to-brittle transition temperature in the pipe axis direction at -20°C.

Comparative Example No. 35 (steel AI) was a hot-rolled steel sheet in which the yield ratio, etc. of the flat portion did not reach the desired value because the B content exceeded the range of the present invention, and thus the amount of upper bainite was outside the range of the present invention. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired value, such as the yield ratio.

Comparative Example No. 36 (steel T) had a slab heating temperature exceeding the range of the present invention, and the crystal grains were coarsened, so that the average crystal grain size and the area ratio of the crystal grains having a crystal grain size within the average crystal grain size ±5.0 ㎛ were outside the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired values, such as yield strength, tensile strength, and Charpy absorbed energy at -20°C. In addition, a square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values, such as yield strength and tensile strength.

In comparative example No. 37 (steel T), since the finishing rolling end temperature exceeded the range of the present invention, the total reduction ratio at 930°C or lower fell below the range of the present invention, the formation of coarse upper bainite could not be suppressed, and the average grain size became outside the range of the present invention. As a result, the hot-rolled steel sheet was obtained in which the yield strength, the tensile strength, and the Charpy absorbed energy at -20°C did not reach the desired values. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in the yield strength, the tensile strength, and the like.

In Comparative Example No. 38 (steel T), since the total reduction ratio at 930°C or lower exceeded the range of the present invention, coarse upper bainite elongated in the rolling direction was generated, the average crystal grain size was below the range of the present invention, and the number density of crystal grains having a major axis/minor axis ratio of 3.0 or more was outside the range of the present invention. As a result, the hot-rolled steel sheet was obtained in which the ductile-to-brittle transition temperature, etc. did not reach the desired values. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in the Charpy absorbed energy in the pipe axis direction at -20°C and the ductile-to-brittle transition temperature.

In Comparative Example No. 39 (steel T), since the average cooling rate at the center of the plate thickness exceeded the range of the present invention, the area ratio of the upper bainite exceeded 5%, falling outside the range of the present invention. As a result, the hot-rolled steel sheet was obtained in which the yield ratio did not reach the desired value and the work hardening index at a plastic strain of 3 to 7% did not reach the desired value. In addition, the square steel pipe manufactured using this hot-rolled steel sheet had a uniform elongation of less than 5.0%, and the yield ratio and the like did not reach the desired values.

In comparative example No. 40 (steel T), since the time from the end of rough rolling to the start of finish rolling was below the range of the present invention, the area ratio of crystal grains having an average crystal grain size of ±5.0 ㎛ or less fell outside the range of the present invention. As a result, neither the hot-rolled steel sheet nor the square steel tube manufactured using this hot-rolled steel sheet reached the desired yield ratio. In addition, since the work hardening index at a plastic strain of 3 to 7% of the hot-rolled steel sheet did not reach the desired value, the uniform elongation of the square steel tube was less than 5.0%.

Comparative Example No. 41 (steel T) had a cooling stop temperature and a coiling temperature below the range of the present invention, so the area ratio of upper bainite and the area ratio of grains having an average grain size of ±5.0 ㎛ or less fell outside the range of the present invention. As a result, neither the hot-rolled steel sheet nor the square steel tube manufactured using the hot-rolled steel sheet had a yield ratio that reached the desired value. In addition, since the work hardening index at a plastic strain of 3 to 7% of the hot-rolled steel sheet did not reach the desired value, the uniform elongation of the square steel tube was less than 5.0%.

Comparative Example No. 42 (steel T) had a low average cooling rate at the center of the plate thickness, and furthermore, the cooling stop temperature and coiling temperature exceeded the range of the present invention, so that the average grain size fell outside the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired values, such as yield strength, tensile strength, and Charpy absorbed energy at -20°C. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values, such as yield strength and tensile strength.

Comparative Example No. 43 (steel T) had a residence time in the temperature range of 400°C to 300°C that was below the range of the present invention, so that the area ratio of upper bainite exceeded 5%, and the hot-rolled steel sheet did not reach the desired values in the strain hardening index at a plastic strain of 3 to 7%, the yield ratio, and the Charpy absorbed energy at -20°C. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in the Charpy absorbed energy in the pipe axis direction at -20°C.

Comparative Example No. 44 (steel T) had a residence time in the temperature range of 400°C to 300°C exceeding the range of the present invention, so that the average crystal grain size fell outside the range of the present invention, resulting in a hot-rolled steel sheet whose yield strength and tensile strength did not reach the desired values. In addition, a square steel pipe manufactured using this hot-rolled steel sheet also did not reach the desired values in yield strength and tensile strength.

In Comparative Example No. 45 (steel T), since the rough rolling end temperature exceeded the range of the present invention, the number density of crystal grains having a ratio of the major diameter to the minor diameter (major diameter)/(minor diameter) of 3.0 or more fell outside the range of the present invention. As a result, the hot-rolled steel sheet did not reach the desired value of the ductile-brittle transition temperature. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired value of the ductile-brittle transition temperature, etc.

In Comparative Example No. 46 (steel T), since the rough rolling end temperature and the finish rolling end temperature were below the range of the present invention, a large amount of ferrite was generated, and the total area ratio of pearlite and pseudo-pearlite became less than 6%, resulting in a hot-rolled steel sheet in which the work hardening index and the like at a plastic strain of 3 to 7% did not reach the desired values. In addition, a square steel pipe manufactured using this hot-rolled steel sheet had a uniform elongation of less than 5.0%, and the tensile strength and yield ratio did not reach the desired values.

In the comparative example No. 47 (steel T), since the slab heating temperature was below the range of the present invention, the deformation resistance of the rolled material increased, making rolling difficult, and thus rolling was stopped in the middle of rough rolling. Therefore, hot-rolled steel plates and square steel pipes could not be manufactured.

In Comparative Example No. 48 (steel T), since the finishing rolling end temperature was below the range of the present invention, ferrite elongated in the rolling direction was formed. As a result, the hot-rolled steel sheet was obtained in which the Charpy absorption energy at -20°C and the like did not reach the desired value. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired value in the Charpy absorption energy in the pipe axis direction at -20°C and the like.

In Comparative Example No. 49 (steel T), since the average cooling rate was below the range of the present invention, the ferrite coarsen and the average grain size exceeded 30.0 ㎛. As a result, the hot-rolled steel sheet did not reach the desired values in yield strength, tensile strength, and Charpy absorption energy at -20°C. In addition, the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values in Charpy absorption energy in the pipe axis direction at -20°C.

1: Square steel pipe
4: Girder
5: Beam
6: Diaphragm
7 : Stud
10: Temperature measurement location of hot rolled steel plate

Claims (13)

As a hot rolled steel plate,
In mass %,
C: 0.07% or more and 0.20% or less,
Si: 0.40% or less,
Mn: 0.20% or more and 1.00% or less,
P: 0.100% or less,
S: 0.050% or less,
Al: 0.005% or more and 0.100% or less and
N: 0.0100% or less
, the remainder being Fe and inevitable impurities, and further having a composition of components satisfying the following formula (1) with a content of Mn and Si:
The steel structure in the center of the plate thickness has a main phase of ferrite, a second phase in which the area ratio of the sum of pearlite and pseudo-pearlite is 6 to 25%, and the area ratio of upper bainite is 5% or less.
A hot-rolled steel sheet, wherein, in the steel structure at the center of the above plate thickness, when a region surrounded by boundaries where the orientation difference between neighboring crystals is 15° or more is defined as a crystal grain, the average crystal grain size of these crystal grains is 10.0 to 30.0 ㎛, and among the crystal grains, the area ratio of crystal grains having a crystal grain size within the average crystal grain size ±5.0 ㎛ is 35% or more, and further, among the crystal grains, the number density of crystal grains having a ratio of the major axis to the minor axis, (major axis)/(minor axis), of 3.0 or more is 30/mm2 or less.
1.0≤％Mn/％Si≤3.5… (1)
Here, %Mn and %Si represent the content (mass%) of each element in the steel plate. In the first paragraph,
The above composition of ingredients, additionally, in mass%,
Nb: 0.005% or more and 0.020% or less,
Ti: 0.005% or more and 0.020% or less,
V: 0.01% or more and 0.10% or less,
Cr: 0.01% or more and 0.50% or less,
Mo: 0.01% or more and 0.50% or less,
Cu: 0.01% or more and 0.30% or less,
Ni: 0.01% or more and 0.30% or less,
Ca: 0.0005% or more and 0.0100% or less and
B: 0.0003% or more and 0.0100% or less
Hot rolled steel sheet comprising one or more types selected from the following. In claim 1 or 2,
Hot rolled steel plate with a thickness of 12mm or more. A steel material having the composition described in clause 1 or clause 2 is heated at a heating temperature of 1100℃ or higher and 1300℃ or lower,
Next, as hot rolling, rough rolling is performed at a rough rolling end temperature of 850℃ or more and 1150℃ or less, and finishing rolling is started after 15 seconds or more after completion of the rough rolling, and the finishing rolling end temperature is 750℃ or more and 850℃ or less, and the hot rolling is performed at a total reduction ratio of 40% or more and 59% or less at 930℃ or less throughout the entire hot rolling process.
Next, for the material steel sheet obtained from the hot rolling, cooling is performed such that the average cooling rate Vc (℃/s) at the center of the plate thickness satisfies the following equation (2), and the cooling stop temperature at the center of the plate thickness is 550℃ or more and 680℃ or less.
Next, for the above material steel plate, coiling is performed at a plate thickness center temperature of 550℃ or higher and 680℃ or lower,
Next, a method for manufacturing a hot-rolled steel sheet, wherein a second cooling is performed on the coiled steel sheet obtained from the above coiling, wherein the second cooling is performed at a temperature range of 400°C to 300°C for 1.0 h or more and 10.0 h or less.
4≤Vc≤20 … (2) A square steel pipe made of hot-rolled steel plate as described in Article 1 or 2. A square steel pipe made of hot-rolled steel plate as described in Article 3. A square steel pipe as described in Article 5, wherein the ratio P of Charpy absorbed energy at -20℃ in the circumferential direction of the pipe to the pipe axis direction is in the range of 0.5 to 1.2.
However, P = (Charpy absorption energy at -20℃ in the pipe circumferential direction) / (Charpy absorption energy at -20℃ in the pipe axial direction) A square steel pipe as described in Article 6, wherein the ratio P of Charpy absorbed energy at -20℃ in the circumferential direction of the pipe to the pipe axis direction is in the range of 0.5 to 1.2.
However, P = (Charpy absorption energy at -20℃ in the pipe circumferential direction) / (Charpy absorption energy at -20℃ in the pipe axial direction) A method for manufacturing a square steel pipe, comprising cold roll forming a hot rolled steel sheet obtained by the method for manufacturing a hot rolled steel sheet described in Article 4 into a square steel pipe. A building structure having the square steel pipe described in Article 5 as a column material. A building structure having the square steel pipe described in Article 6 as a pillar material. A building structure having the square steel pipe described in Article 7 as a pillar material. A building structure having the square steel pipe described in Article 8 as a pillar material.