Hot-rolled steel sheet, square steel tube, method for producing same, and building structure

Abstract

Provided is a low-yield-ratio hot-rolled steel sheet having excellent strength and low-temperature toughness. The steel sheet has a prescribed component composition, the steel structure in the sheet thickness center portion has a ferrite main phase and a second phase in which the total area ratio of pearlite and pseudo pearlite is 6-25% and the area ratio of upper bainite is 5% or less, and when a region surrounded by a boundary in which the orientation difference between adjacent crystals is 15 DEG or more is defined as a crystal grain, the thickness of the steel sheet is 10-30 [mu] m, and the thickness of the steel sheet is 10-30 [mu] m. The average crystal grain diameter of a steel structure containing a main phase and a second phase is 10.0-30.0 [mu] m, the area ratio of crystal grains having a crystal grain diameter within + / -5.0 [mu] m of the average crystal grain diameter is 35% or more, and the number of crystal grains having a ratio (long diameter) / (short diameter) of the long diameter to the short diameter of 3.0 or more is 30 / mm2 or less.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel plate with a low yield ratio for square steel pipes, a square steel pipe (square column) having a low yield ratio and low-temperature toughness manufactured by cold rolling and roll forming using the hot-rolled steel plate as a blank, and a method for manufacturing the same. In particular, the present invention relates to a square steel pipe suitable for use as a building structural component of a large building. In addition, the present invention relates to a building structure using the square steel pipe. Background Art

[0002] In recent years, building structural members used in large buildings (hereinafter referred to as "buildings") such as factories, warehouses, and commercial facilities have been increasingly being strengthened in order to reduce construction costs by reducing weight.

[0003] In particular, a square steel pipe (square column) having a flat plate portion and corner portions used as a column material for a building is required to have high strength in the flat plate portion and excellent toughness from the viewpoint of earthquake resistance.

[0004] Square steel pipes are usually manufactured by cold rolling hot rolled steel sheets (hot rolled steel strips) or thick steel sheets as blanks. As the cold rolling forming method, there are cold rolling press bending forming method or cold rolling roll forming method.

[0005] Regarding the square steel pipe manufactured by stamping and bending the blank (hereinafter sometimes referred to as "stamped square steel pipe"), thick steel plates are stamped and bent by cold rolling to form the cross-sectional shape into a "ロ" shape (quadrilateral shape) or a "コ" shape (U shape) and they are joined by submerged arc welding.

[0006] On the other hand, in the case of a square steel pipe manufactured by roll-forming a blank (hereinafter sometimes referred to as a "roll-formed square steel pipe"), a hot-rolled steel plate is roll-formed by cold rolling to form a cylindrical open pipe, and the butted portions are resistance-welded to manufacture a round steel pipe. Then, the cylindrical round steel pipe is drawn by several % in the pipe axial direction by rollers arranged above, below, left and right of the round steel pipe, and then formed into a square to manufacture a square steel pipe.

[0007] Compared with the method of manufacturing a press-formed square steel pipe, the method of manufacturing a roll-formed square steel pipe has the advantages of high productivity and capable of manufacturing in a short period of time.

[0008] However, in the case of press-formed square steel pipes, the flat portion is not cold-formed but the corner portion is cold-formed, so only the corner portion is work-hardened. In contrast, in the case of roll-formed square steel pipes, particularly in the front section of forming the square steel pipe, i.e., when cold-forming into a cylindrical shape, a large processing strain is introduced along the tube axis direction over the entire circumference of the steel pipe. Therefore, the roll-formed square steel pipe has the problem of high yield ratio and low toughness in the tube axis direction not only in the corner portion but also in the flat portion.

[0009] In addition, for roll-formed square steel pipes, the thicker the wall, the greater the work hardening during roll forming, so the yield ratio is higher and the toughness is lower. Therefore, especially when manufacturing thick-walled roll-formed square steel pipes, it is necessary to select a billet that can withstand the increase in yield ratio and the decrease in toughness caused by roll forming.

[0010] In response to such a demand, for example, Patent Document 1 proposes a square steel pipe, wherein a steel billet containing 0.20% or less of C and further containing one or two of Mn: 0.40 to 0.90%, Nb: 0.005 to 0.040% and Ti: 0.005 to 0.050% by weight is subjected to a hot rolling process to obtain a coil by hot rolling with a reduction rate of 55% or more in a non-recrystallization temperature range, a rolling end temperature of 730 to 830°C, and a coiling temperature of 550°C or less, the hot rolled coil is formed and welded to form an electric resistance welded steel pipe, and then cold worked to form a square steel pipe, wherein the outer circumference reduction in the steel pipe forming process is made to be 3 times or less of the plate thickness, thereby achieving a yield ratio of 90% or less and a Charpy absorbed energy of 27 J or more at a test temperature of 0°C.

[0011] Patent document 2 proposes a square steel pipe, wherein steel containing, by mass%, 0.07 to 0.18% C and 0.3 to 1.5% Mn is heated to a heating temperature of 1100 to 1300°C, and then subjected to rough rolling with a rough rolling end temperature of 1150 to 950°C and a finish rolling start temperature of 1100 to 850°C and a finish rolling end temperature of 900 to 750°C, and then subjected to primary cooling in which the surface thermometer cooling stop temperature is 550°C or higher, and air cooling for 3 to 6 hours. The thick-walled hot-rolled steel sheet is obtained by secondary cooling for 15 seconds and tertiary cooling at an average cooling rate of 4 to 15°C / second in the temperature range of 750 to 650°C at the center of the plate thickness to below 650°C, so that the value of the second phase frequency contained in the steel structure is 0.20 to 0.42. The thick-walled hot-rolled steel sheet is cold-formed to have mechanical properties showing a low yield ratio of less than 80% and an absorbed energy of more than 150J in a Charpy impact test at a test temperature of 0°C.

[0012] Patent document 3 proposes a square steel pipe, wherein steel containing, by mass%, 0.07 to 0.18% C and 0.3 to 1.5% Mn is heated to a heating temperature of 1100 to 1300°C, followed by rough rolling at a rough rolling end temperature of 1150 to 950°C and finishing rolling at a finish rolling start temperature of 1100 to 850°C and a finish rolling end temperature of 900 to 750°C, and then the average cooling rate is 0.02% in a temperature range of 750 to 650°C at a surface temperature. The hot-rolled steel sheet is cooled to a coiling temperature of 500 to 650°C at an average cooling rate of 4 to 15°C / second in a temperature range of 750 to 650°C at the center of the plate thickness, and the steel sheet is cold-formed as a billet to have mechanical properties such as a low yield ratio of less than 80% and an absorbed energy of more than 150J in a Charpy impact test at a test temperature of 0°C.

[0013] Patent document 4 proposes a square steel pipe, wherein steel having a composition containing, by mass%, C: 0.07-0.20%, Mn: 0.3-2.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.01-0.06%, N: 0.006% or less, and the remainder consisting of Fe and inevitable impurities is heated to a heating temperature of 1100-1300°C, rough rolling with a rough rolling end temperature of 1150-950°C and finish rolling with a finish rolling start temperature of 1100-850°C and a finish rolling end temperature of 900-750°C is performed, and then cooling is performed at an average cooling rate of 4-25°C / sec from the start of cooling to the stop of cooling to a cooling stop temperature of 580°C at a plate thickness center temperature. ℃ or less, there is one or more cooling step of 0.2 seconds or more and less than 3.0 seconds in the initial cooling step 10s after the start of cooling, and then coiling is performed at a coiling temperature of 580℃ or less, and then cooling is performed, so that the steel structure of the center part of the plate thickness has a main phase composed of ferrite and a second phase composed of one or more selected from pearlite, pseudo pearlite and upper bainite with an area ratio of 8-20%, the average grain size of the steel structure including the main phase and the second phase is 7-20μm, the steel structure on the surface and back of the plate thickness is a ferrite single phase or a bainitic ferrite single phase, the average grain size is 2-20μm, and it has a low yield ratio of 90% or less, and the absorbed energy of the Charpy impact test at a test temperature: 0℃ is 27J or more. Mechanical properties.

[0014] Prior art literature

[0015] Patent Literature

[0016] Patent Document 1: Japanese Patent Application Laid-Open No. 9-87743

[0017] Patent Document 2: Japanese Patent Application Publication No. 2012-153963

[0018] Patent Document 3: Japanese Patent Application Publication No. 2012-132088

[0019] Patent Document 4: International Publication No. 2018 / 110152 Summary of the invention

[0020] Here, as described above, the larger the wall thickness of the roll-formed square steel pipe is and the smaller the length of the side is, the greater the processing strain introduced into the square steel pipe is, and the greater the increase in yield ratio and the decrease in toughness become.

[0021] Therefore, a hot-rolled steel sheet as a base material is required to have a steel structure that suppresses an increase in the yield ratio during forming and excellent low-temperature toughness that can withstand deterioration in toughness due to large working strain.

[0022] However, in the square steel pipes produced by the methods disclosed in Patent Documents 1 to 3, there is a problem that the yield ratio becomes too high, particularly when the wall thickness exceeds 25 mm, and the yield ratio cannot be satisfied to be 90% or less.

[0023] In addition, in order to obtain a low yield ratio and high toughness using the technology described in Patent Document 4, it is necessary to make the steel structure on the surface and back of the plate thickness a single phase of ferrite or a single phase of bainitic ferrite. In order to obtain such a steel structure, a cooling step needs to be provided in the cooling step, that is, an additional step is required, so there is a problem that the manufacturing process becomes complicated.

[0024] The present invention is proposed in view of the above-mentioned problems, and its purpose is to provide a hot-rolled steel plate that can be used for square steel pipes, which has high yield strength and tensile strength, low yield ratio, excellent low-temperature toughness in the pipe axial direction and the pipe circumferential direction, and work hardening property, as well as a square steel pipe using the hot-rolled steel plate and their manufacturing methods, and a building structure using the square steel pipe.

[0025] Here, for the hot-rolled steel sheet in the present invention, (1) the yield ratio is low, (2) the yield strength is high, and (3) the tensile strength is high means that, using a JIS No. 5 tensile test piece collected in a manner such that the tensile direction is parallel to the rolling direction, a tensile test is carried out in accordance with the provisions of JIS Z 2241 (2011), and the results are, respectively, (1) the yield ratio is not more than 0.75, (2) the yield strength is not less than 250 MPa, and (3) the tensile strength is not less than 400 MPa.

[0026] The work hardening property is an index for evaluating uniform elongation (plastic elongation at the highest load point in a tensile test) and refers to the work hardening index n defined in JIS Z 2253 (2011). 3－7That is, if the work hardening index n of the hot-rolled steel sheet is 3－7 If it is less than 0.20, when a square steel pipe is manufactured, the uniform elongation of the flat plate portion of the square steel pipe is reduced, the earthquake resistance is reduced, and the yield ratio of the flat plate portion of the square steel pipe may exceed 0.90.

[0027] 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 at the t / 2 position of the plate thickness t (the center of the plate thickness) with the long side direction of the test piece parallel to the rolling direction, and the Charpy impact test is carried out at the test temperatures of -80°C, -60°C, -40°C, -20°C, and 0°C respectively. The Charpy absorbed energy at -20°C is 100J or more, and the ductile-brittle transition temperature is below -20°C.

[0028] In addition, in the present invention, the low yield ratio square steel pipe refers to the following square steel pipe: using a JIS No. 5 tensile test piece collected in a manner parallel to the tube axis direction, a tensile test is performed in accordance with the provisions of JIS Z 2241 (2011), and the yield strength of the flat plate portion is 295 MPa or more, the tensile strength of the flat plate portion is 400 MPa or more, the yield ratio of the flat plate portion is 0.90 or less, and the uniform elongation of the flat plate portion is 5.0% or more, and using a V-notch standard test piece collected from the flat plate portion of the square steel pipe at a position from the outer surface of the tube to 1 / 4t of the wall thickness t in a manner parallel to the tube axis direction, a Charpy impact test is performed in accordance with the provisions of JIS Z 2242 (2018) at test temperatures: -60°C, -40°C, -20°C, and 0°C, and the Charpy absorbed energy at -20°C in the tube axis direction of the flat plate portion is 60 J or more, and the ductile-brittle transition temperature of the flat plate portion is -10°C or less.

[0029] In addition, in the present invention, a more excellent low yield ratio square steel pipe refers to the following square steel pipe: in accordance with the provisions of JIS Z2242 (2018), a V-notch standard test piece collected at the t / 4 position from the outer surface of the pipe to the wall thickness t in a manner such that the long side direction of the test piece is parallel to the circumferential direction of the pipe is used, and the Charpy absorbed energy at -20°C in the pipe axial direction and the pipe circumferential direction of the flat plate portion is measured respectively, 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.

[0030] Where, P = (Charpy absorbed energy at -20°C in the tube circumferential direction) / (Charpy absorbed energy at -20°C in the tube axial direction)

[0031] The present inventors have conducted intensive studies to solve the above-mentioned problems and have obtained the following findings (1) to (3).

[0032] (1) In order for the hot-rolled steel sheet to satisfy the yield strength and tensile strength targeted by the present invention, the C content must be 0.07 mass % or more and the main phase in the center of the hot-rolled steel sheet must be ferrite.

[0033] (2) In order for the hot-rolled steel sheet to obtain the low-temperature toughness and yield ratio targeted by the present invention, it is necessary that: in addition to the main phase described in (1) above, a second phase consisting of one or more of pearlite, pseudo-pearlite and upper bainite is present in the center of the plate thickness, the total area ratio of pearlite and pseudo-pearlite is 6 to 25%, the area ratio of upper bainite is 5% or less, in the center of the plate thickness, when the region surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more is defined as grains, the average grain size of the grains including the main phase and the second phase is in the range of 10.0 to 30.0 μm, the area ratio of grains having a grain size within ±5.0 μm of the average grain size of the grains is 35% or more, and the number density of grains having a major axis to minor axis ratio (= (major axis) / (minor axis)) of 3.0 or more in the grains is 30 grains / mm 2 the following.

[0034] (3) In order to obtain the steel structure described in (1) and (2) above, it is necessary to adjust the component composition to an appropriate range, especially to control the content of Mn and Si to a specific range, and to start finish rolling after a specified time has passed after the completion of rough rolling in the hot rolling process, and further, to maintain the specified temperature range for a specified time after coiling.

[0035] The present invention has been completed based on this knowledge and further research. That is, the gist of the present invention is as follows.

[0036] 1. A hot-rolled steel sheet having the following composition: containing, by mass%, C: 0.07% to 0.20%, Si: 0.40% or less, Mn: 0.20% to 1.00%, P: 0.100% or less, S: 0.050% or less, Al: 0.005% to 0.100% and N: 0.0100% or less, the remainder being Fe and unavoidable impurities, and the contents of Mn and Si satisfy the following formula (1),

[0037] The steel structure at the center of the plate thickness has a main phase of 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.

[0038] In the steel structure at the center of the plate thickness, when the region surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more is defined as the grains, the average grain diameter of the grains is 10.0 to 30.0 μm, the area ratio of the grains having a grain diameter within ±5.0 μm of the average grain diameter among the grains is 35% or more, and the number density of the grains having a ratio of the major diameter to the minor diameter, i.e., (major diameter) / (minor diameter) of 3.0 or more among the grains is 30 grains / mm 2 the following.

[0039] 1.0≤%Mn / %Si≤3.5···(1)

[0040] Here, %Mn and %Si are the contents (mass %) of the respective elements in the steel sheet.

[0041] 2. The hot-rolled steel sheet according to item 1 above, wherein the above-mentioned component composition further contains, in terms of mass %, one or more selected from the group consisting of Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, V: 0.01% to 0.10%, Cr: 0.01% to 0.50%, Mo: 0.01% to 0.50%, Cu: 0.01% to 0.30%, Ni: 0.01% to 0.30%, Ca: 0.0005% to 0.0100% and B: 0.0003% to 0.0100%.

[0042] 3. The hot-rolled steel sheet according to 1 or 2 above, wherein the sheet thickness is 12 mm or more.

[0043] 4. A method for producing a hot-rolled steel sheet, comprising heating a steel billet having the composition described in 1 or 2 above to a heating temperature of 1100° C. to 1300° C.,

[0044] Next, the hot rolling is performed as follows: rough rolling is performed at a rough rolling end temperature of 850°C to 1150°C, and finish rolling is started 15 seconds or more after the rough rolling is completed, the finish rolling end temperature is 750°C to 850°C, and the total reduction ratio of the hot rolling process is 40% to 59% below 930°C.

[0045] Next, the steel sheet obtained by the hot rolling is cooled so that the average cooling rate Vc (°C / sec) at the center of the plate thickness satisfies the following formula (2) and the cooling stop temperature at the center of the plate thickness is 550°C to 680°C.

[0046] Next, the steel plate is coiled at a center temperature of 550°C to 680°C.

[0047] Next, the coiled steel sheet obtained by the above coiling is subjected to second cooling by retaining the steel sheet in a temperature range of 400° C. to 300° C. for 1.0 to 10.0 hours.

[0048] 4≤Vc≤20···(2)

[0049] 5. A square steel pipe using the hot-rolled steel plate according to any one of 1 to 3 above as a base material.

[0050] 6. The square steel pipe according to 5 above, wherein a ratio P of the Charpy absorbed energy at -20°C in the pipe circumferential direction to the pipe axial direction is in the range of 0.5 to 1.2.

[0051] Where, P = (Charpy absorbed energy at -20°C in the tube circumferential direction) / (Charpy absorbed energy at -20°C in the tube axial direction)

[0052] 7. A method for producing a square steel pipe, comprising: forming a square steel pipe by cold rolling a hot-rolled steel sheet obtained by the method for producing a hot-rolled steel sheet according to 4 above.

[0053] 8. A building structure comprising the square steel pipe described in 5 or 6 above as a column material.

[0054] According to the present invention, there is provided a technique for obtaining a hot-rolled steel sheet which can be used for a low-yield ratio square steel pipe and has high yield strength and tensile strength, a low yield ratio, and excellent low-temperature toughness and work hardenability.

[0055] Furthermore, a building structure using the square steel pipe of the present invention as a column material can obtain more excellent earthquake resistance performance than a building structure using a square steel pipe manufactured by conventional cold forming. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is a schematic diagram showing the temperature measurement position of the coiled steel sheet.

[0057] Figure 2 This is a perspective view schematically showing an example of a building structure using the square steel pipe of the present invention.

[0058] Figure 3 This is a schematic diagram showing the positions at which test pieces were collected for the flat plate tensile test performed in Examples.

[0059] Figure 4 This is a schematic diagram showing the positions at which test pieces were collected for the Charpy impact test performed in Examples. DETAILED DESCRIPTION

[0060] Hereinafter, the present invention will be described.

[0061] <Low yield ratio hot rolled steel sheet>

[0062] The low yield ratio hot rolled steel sheet (hereinafter also referred to as "hot rolled steel sheet") used for the low yield ratio square steel pipe (hereinafter also referred to as "square steel pipe") of the present invention has the following composition: containing, by mass%, C: 0.07% to 0.20%, Si: 0.40% or less, Mn: 0.20% to 1.00%, P: 0.100% or less, S: 0.050% or less, Al: 0.005% to 0.100% and N: 0.0100% or less, the remainder being Fe and inevitable impurities, and the contents of Mn and Si satisfy the following formula (1). In addition, in the above hot rolled steel sheet, the steel structure at the center of the plate thickness has a main phase of ferrite, and a second phase having a total area ratio of pearlite and pseudo-pearlite of 6 to 25% and an area ratio of upper bainite of 5% or less. In addition, the hot-rolled steel sheet is characterized in that, in the steel structure at the center of the plate thickness, when a region surrounded by a boundary where the orientation difference between adjacent crystals is 15° or more is defined as a grain, the average grain diameter of the grain is 10.0 to 30.0 μm, the area ratio of grains having a grain diameter within ±5.0 μm of the average grain diameter among the grains is 35% or more, and the number density of grains having a ratio of major diameter to minor diameter, i.e., (major diameter) / (minor diameter) of 3.0 or more among the grains is 30 grains / mm 2 Hereinafter, the "hot-rolled steel sheet" in the present invention includes a hot-rolled steel sheet and a hot-rolled steel strip.

[0063] 1.0≤%Mn / %Si≤3.5···(1)

[0064] Here, %Mn and %Si are the contents (mass %) of the respective elements in the steel sheet.

[0065] Hereinafter, the chemical composition of the hot-rolled steel sheet of the present invention will be described. It should be noted that, unless otherwise specified, "%" indicating the steel composition means "mass %".

[0066] C: 0.07%~0.20%

[0067] 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 one of the second phases. In order to ensure the strength and yield ratio that are the targets of 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 the proportion of the hard phase increases and the toughness decreases, but also the yield ratio exceeds 0.90, and the desired yield ratio cannot be obtained. In addition, weldability also deteriorates. Therefore, the C content is set to 0.07% to 0.20%. 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.

[0068] Si: 0.40% or less

[0069] Si is an element that increases the strength of steel by solid solution strengthening. There is no particular lower limit for Si (usually more than 0%), but in order to obtain such an effect, it is preferably contained at least 0.01% Si. The Si content is more preferably at least 0.05%. On the other hand, if the Si content exceeds 0.40%, oxides are easily generated in the resistance welding weld, and the weld characteristics are reduced. In addition, the toughness of the base material portion other than the resistance welding weld is also reduced. Therefore, the Si content is set to 0.40% or less. Preferably it is 0.37% or less, and more preferably it is 0.35% or less.

[0070] Mn: 0.20%~1.00%

[0071] 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 start temperature. In order to ensure the strength and structure that are the targets of the present invention, it is necessary to contain more than 0.20% Mn. On the other hand, if the Mn content exceeds 1.00%, the yield ratio exceeds 0.90 due to the excessive amount of bainite generated, and the desired yield ratio cannot be obtained. In addition, if the Mn content exceeds 1.00%, the hardness of the center segregation portion increases, which may cause fracture during welding. Therefore, the Mn content is set to 0.20% to 1.00%. 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 more preferably 0.90% or less.

[0072] P: 0.100% or less

[0073] Since P segregates at grain boundaries and causes inhomogeneity in the material, it is preferably reduced as much as possible, but a content of 0.100% is acceptable. 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. There is no particular lower limit for P (usually more than 0%), but excessive reduction will lead to a surge in refining costs, so P is preferably 0.002% or more.

[0074] S: 0.050% or less

[0075] S usually exists in the form of MnS in steel, but MnS is stretched thinner during the hot rolling process, which has an adverse effect on ductility. Therefore, in the present invention, it is preferred to reduce S as much as possible, but a content of 0.050% is allowed. 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 further preferably 0.008% or less. It should be noted that there is no special lower limit for S (usually more than 0%), but excessive reduction will cause a surge in refining costs, so S is preferably 0.0002% or more.

[0076] Al: 0.005% to 0.100%

[0077] Al is an element that acts as a strong deoxidizer. In order to obtain such an 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 number of alumina inclusions increases, and the surface properties deteriorate. In addition, the toughness of the weld also decreases. Therefore, the Al content is set to 0.005% to 0.100%. 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.

[0078] N: 0.0100% or less

[0079] N is an element that has the effect of reducing toughness by firmly fixing the movement of dislocations. In the present invention, N is expected to be reduced as much as possible, but the N content can be allowed to be up to 0.0100%. Therefore, the N content is set to be less than 0.0100%. The N content is preferably less than 0.0080%, more preferably less than 0.0040%, and further preferably less than 0.0035%. It should be noted that there is no particular lower limit for N (usually more than 0%), but excessive reduction will cause a surge in refining costs, so the N content is preferably more than 0.0010%, more preferably more than 0.0015%.

[0080] 1.0≤%Mn / %Si≤3.5

[0081] Here, %Mn and %Si in the above formula are the contents (mass %) of the respective elements in the steel sheet.

[0082] In the present invention, the contents of Mn and Si need to be within the above ranges and further satisfy the relationship of 1.0≤%Mn / %Si≤3.5.

[0083] By satisfying this relationship, a steel structure having a second phase with an area ratio of pearlite and / or pseudo-pearlite of 6 to 25% and an area ratio of upper bainite of 5% or less as described below can be obtained, and the strength, yield ratio, Charpy absorbed energy, and ductile-brittle transition temperature targeted 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.

[0084] In the above-mentioned component composition, the remainder is Fe and inevitable impurities. However, O may be contained in an amount of 0.005% or less within a range not impairing the effects of the present invention. It should be noted that the O refers to the total oxygen including O in the form of oxides.

[0085] In addition, in the present invention, for Nb, Ti, V, Cr, Mo, Cu, Ni, Ca and B described later as arbitrary elements, 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% can all be included in the inevitable impurities.

[0086] The above components are the basic components of the hot-rolled steel sheet in the present invention. The properties targeted by the present invention can be obtained based on the above components, but the components may contain the following elements as necessary.

[0087] Specifically, one or more selected from the group consisting of Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, V: 0.01% to 0.10%, Cr: 0.01% to 0.50%, Mo: 0.01% to 0.50%, Cu: 0.01% to 0.30%, Ni: 0.01% to 0.30%, Ca: 0.0005% to 0.0100% and B: 0.0003% to 0.0100%.

[0088] Nb: 0.005%~0.020%, Ti: 0.005%~0.020%

[0089] Nb and Ti are elements that form fine carbides and nitrides in steel and contribute to the improvement of steel strength through precipitation strengthening. In order to obtain such an effect, when Nb is contained, it is preferably 0.005% or more. In addition, when Ti is contained, it is preferably 0.005% or more. On the other hand, when more than 0.020% of Nb and Ti are contained, coarse carbides and nitrides may be formed, resulting in a decrease in toughness.

[0090] Therefore, when Nb is contained, the content is set to 0.020% or less, and when Ti is contained, the content is set to 0.020% or less. The content of each of Nb and Ti is more preferably 0.007% or more, and further preferably 0.009% or more. In addition, the content of each of Nb and Ti is preferably 0.018% or less, and preferably 0.016% or less.

[0091] V: 0.01% to 0.10%, Cr: 0.01% to 0.50%, Mo: 0.01% to 0.50%

[0092] V, Cr, and Mo are elements that improve the hardenability and strength of steel and can be contained as needed. In order to obtain the above effects, when V, Cr, and Mo are contained, preferably 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.

[0093] On the other hand, excessive content may lead to a decrease in toughness and deterioration in weldability. Therefore, when V, Cr, and Mo are contained, V is set to 0.10% or less, Cr is set to 0.50% or less, and Mo is set to 0.50% or less, respectively. Preferably, V is set to 0.08% or less, Cr is set to 0.40% or less, and Mo is set to 0.40% or less, respectively.

[0094] Cu: 0.01% to 0.30%, Ni: 0.01% to 0.30%

[0095] Cu and Ni are elements that improve the strength of steel by solid solution strengthening and can be contained as needed. In order to obtain the above-mentioned effect, when Cu and Ni are contained, Cu: 0.01% or more and Ni: 0.01% or more are preferred. More preferably, Cu: 0.10% or more and Ni: 0.10% or more are preferred. On the other hand, excessive inclusion may lead to a decrease in toughness and deterioration in weldability. Therefore, when Cu and Ni are contained, Cu: 0.30% or less and Ni: 0.30% or less are respectively set. Preferably, Cu: 0.20% or less and Ni: 0.20% or less are preferred.

[0096] Ca: 0.0005%~0.0100%

[0097] Ca is an element that helps improve the toughness of steel by spheroidizing sulfides such as MnS that are stretched thin in the hot rolling process, and can be contained as needed. In order to obtain such an 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, if the Ca content exceeds 0.0100%, Ca oxide clusters are sometimes formed in the steel, and the toughness deteriorates. Therefore, when Ca is contained, it is set to 0.0100% or less. The preferred Ca content is 0.0050% or less.

[0098] B: 0.0003%~0.0100%

[0099] B is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. In order to obtain such an effect, when B is contained, it is preferably 0.0003% or more. More preferably, the B content is 0.0005% or more. On the other hand, if the B content exceeds 0.0100%, the yield ratio sometimes increases. Therefore, when B is contained, it is set to 0.0100% or less. Preferably, the B content is 0.0050% or less.

[0100] Next, the reasons for limiting the steel structure of the hot-rolled steel sheet of the present invention will be described.

[0101] In the hot-rolled steel sheet of the present invention, the steel structure at the center of the plate thickness has a main phase of 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 at the center of the plate thickness, when a region surrounded by a boundary in which the orientation difference of adjacent crystals is 15° or more is set as a grain, the average grain diameter of the grain is 10.0 to 30.0 μm, the area ratio of grains having a grain diameter within ±5.0 μm of the average grain diameter among the grains is 35% or more, and the number density of grains in which the ratio of the major diameter to the minor diameter in the grains, i.e., (major diameter) / (minor diameter) is 3.0 or more is 30 grains / mm 2 the following.

[0102] In addition, in the present invention, the crystal grain diameter refers to the diameter of a circle having an area equal to that of the target crystal grain (equivalent circle diameter).

[0103] Ferrite (main phase)

[0104] Ferrite is a soft structure, and in order to obtain the desired yield strength and low yield ratio, it is set as the main phase in the present invention. It should be noted that the "main phase" refers to an area ratio of 50% or more. If the area ratio of ferrite is less than 50%, the yield stress becomes too large, and the work hardening index becomes smaller, and sometimes the desired yield ratio cannot 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 sometimes the desired yield strength and tensile strength cannot be obtained. Therefore, the ferrite is calculated as an area ratio of 94% or less, and preferably the ferrite is calculated as an area ratio of 92% or less.

[0105] Total area ratio of pearlite and pseudo-pearlite: 6-25%, area ratio of upper bainite: 5% or less (second phase)

[0106] Pearlite and pseudo-pearlite are hard structures, and are the most important steel structures for improving the strength of steel and obtaining a low yield ratio. In order to obtain the yield strength, tensile strength, and yield ratio that are the targets of the present invention, the total area ratio of pearlite and pseudo-pearlite needs to be 6% or more. Preferably, it is 7% or more, and more preferably, it is 9% or more. On the other hand, if the total area ratio of pearlite and pseudo-pearlite exceeds 25%, the toughness sometimes deteriorates. Therefore, the total area ratio of pearlite and pseudo-pearlite must be 25% or less. Preferably, it is 23% or less, and more preferably, it is 21% or less.

[0107] It should be noted that the area ratio of the pseudo pearlite is preferably 5% or more. If the pseudo pearlite is present in an area ratio of 5% or more, the yield ratio is suppressed to a low level when manufacturing a square steel pipe, thereby obtaining a more excellent earthquake resistance. On the other hand, in order to make the area ratio of the pseudo pearlite exceed 15%, it is necessary to perform rapid cooling in the temperature range in which pearlite is generated in the cooling process of hot rolling, and the manufacturing conditions are limited. Therefore, the area ratio of the pseudo pearlite is preferably 15% or less.

[0108] In addition, upper bainite is a structure having an intermediate hardness between ferrite and pearlite, and improves the strength of steel. However, if the area ratio of upper bainite exceeds 5%, the low yield ratio targeted in the present invention cannot be obtained. Therefore, the area ratio of upper bainite must be 5% or less. Preferably, it is 4% or less. The upper bainite can be 0%.

[0109] In addition, in the present invention, the structure other than the above-mentioned main phase and the above-mentioned second phase is austenite and martensite.

[0110] The area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite can be measured by the method described later.

[0111] In the steel structure at the center of the plate thickness, when the region surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more is defined as a grain, the average grain diameter of the grain is 10.0 to 30.0 μm, the area ratio of the grains having a grain diameter within ±5.0 μm of the average grain diameter among the above-mentioned grains is 35% or more, and the number density of the grains having a ratio of the major diameter to the minor diameter, i.e., (major diameter) / (minor diameter) of 3.0 or more among the above-mentioned grains is 30 grains / mm 2 the following

[0112] In order to obtain the low yield ratio, yield strength and tensile strength targeted by the present invention in the steel structure of the present invention, a steel in which a soft structure and a hard structure are mixed (hereinafter referred to as "composite structure steel") is used.

[0113] However, the toughness of the composite structure steel is inferior to that of the single structure steel. Therefore, in order to achieve both the above-mentioned mechanical properties and excellent toughness, in the present invention, when the region surrounded by the boundary with a crystal orientation difference of 15° or more is set as the grain, the grain diameter of the steel structure including the main phase and the secondary phase in the plate thickness center of the steel plate, the area ratio of the coarse grains, and the number density of the elongated grains are specified.

[0114] If the average grain diameter (average equivalent circle diameter) of the above-mentioned grains is less than 10.0μm, the yield ratio increases and the yield ratio targeted in the present invention cannot be obtained. On the other hand, if the above-mentioned average grain diameter exceeds 30.0μm, the toughness deteriorates. Therefore, the average grain diameter of the steel structure containing the main phase and the secondary phase must be in the range of 10.0 to 30.0μm. The above-mentioned average grain diameter is preferably greater than 11.0μm, and more preferably greater than 12.5μm. In addition, the above-mentioned average grain diameter is preferably less than 28.0μm, and more preferably less than 26.0μm.

[0115] Here, in the research of the present inventors, even if the average grain size is within the range of 10.0 to 30.0 μm, the yield ratio and Charpy absorbed energy targeted by the present invention may not be obtained. Therefore, as a result of further research 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 grains having a grain size within ±5.0 μm of the above average grain size and the number density of elongated grains are extremely important.

[0116] Specifically, the center of the steel plate thickness must have an area ratio of 35% or more and a ratio of major diameter to minor diameter (major diameter) / (minor diameter) of 3.0 or more, and a number density of 30 grains / mm. 2 The following organizations.

[0117] The crystal orientation difference, the average grain size, and the area ratio of grains having a grain size within ±5.0 μm of the average grain size can be measured by SEM / EBSD method. In the present invention, they can be measured by the method described below.

[0118] By satisfying the above-mentioned chemical composition and steel structure, a hot-rolled steel sheet having the strength, yield ratio, and toughness (Charpy absorbed energy at −20° C., ductile-brittle transition temperature) targeted by the present invention can be obtained.

[0119] That is, in the hot-rolled steel sheet of the present invention, the yield strength can be made greater than 250 MPa, the tensile strength can be made greater than 400 MPa, the yield ratio can be made less than 0.75, the work hardening index with a plastic strain of 3 to 7% can be made greater than 0.20, the Charpy absorbed energy at -20°C can be made greater than 100 J, and the ductile-brittle transition temperature can be made less than -20°C.

[0120] Furthermore, by using this hot-rolled steel sheet, a square steel pipe described later can be obtained.

[0121] In addition, the hot-rolled steel sheet of the present invention preferably has a sheet thickness of 12 mm or more, and more preferably has a sheet thickness in the range of 12 to 32 mm.

[0122] Next, as a method for producing a hot-rolled steel sheet of the present invention, a method for producing a hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0123] The method for manufacturing a hot-rolled steel sheet of the present invention, for example, first heats a steel slab having the above-mentioned composition to a heating temperature of 1100°C to 1300°C (heating step). Then, as hot rolling, the following hot rolling is performed: rough rolling is performed at a rough rolling end temperature of 850°C to 1150°C, and finish rolling is started more than 15 seconds after the end of the rough rolling, and the finish rolling end temperature is 750°C to 850°C, and the total reduction ratio of 930°C or less is 40% to 59% in the hot rolling process as a whole (hot rolling step). Next, the slab steel sheet obtained in the above-mentioned hot rolling is cooled so that the average cooling rate Vc (°C / second) at the center of the plate thickness satisfies the following formula (2) and the cooling stop temperature at the center of the plate thickness is 550°C to 680°C (cooling step). Next, the above-mentioned slab steel sheet is coiled at a plate thickness center temperature of 550°C to 680°C (coiling step). Next, the coiled steel sheet obtained in the coiling step is subjected to second cooling (second cooling step) by retaining the steel sheet at a temperature range of 400° C. to 300° C. for 1.0 to 10.0 hours. Thus, the hot-rolled steel sheet of the present invention is obtained.

[0124] 4≤Vc≤20···(2)

[0125] The following is a further detailed description of the method for manufacturing a hot-rolled steel plate. It should be noted that in the following description of the manufacturing method, "°C" indicates that, unless otherwise specified, the temperature is the surface temperature of a steel billet, a steel plate (hot-rolled plate, a billet steel plate, a hot-rolled steel plate) (hereinafter also referred to as a steel plate, etc.). These surface temperatures can be measured using a radiation thermometer or the like. In addition, the temperature at the center of the thickness of a steel plate, etc. can be calculated by performing a heat transfer analysis on the temperature distribution in the cross section of the steel plate, etc., and the result can be corrected using the surface temperature of the steel plate, etc.

[0126] In the present invention, the smelting method of the steel billet (steel billet) is not particularly limited, and known smelting methods such as converters, electric furnaces, and vacuum melting furnaces are suitable. The casting method is also not particularly limited, and the desired size can be produced by known casting methods such as continuous casting. It should be noted that there is no problem in applying the ingot casting-opening rolling method instead of the continuous casting method. The molten steel can also be further subjected to secondary refining such as ladle refining.

[0127] Next, as a heating process, the obtained steel billet (steel billet) is heated to a heating temperature of 1100° C. to 1300° C. Next, as a hot rolling process, hot rolling is performed as follows: rough rolling is performed at a rough rolling end temperature of 850° C. to 1150° C., and finish rolling is started after 15 seconds or more after the rough rolling is completed, and the finish rolling end temperature is 750° C. to 850° C., and the total reduction ratio of 930° C. or less in the entire hot rolling process is 40% to 59%, thereby obtaining a billet steel plate for hot-rolled steel plate.

[0128] Heating temperature: 1100℃~1300℃

[0129] In the heating process, when the heating temperature is less than 1100°C, the deformation resistance of the rolled material becomes larger and rolling becomes difficult. On the other hand, if the heating temperature exceeds 1300°C, the austenite grains coarsen, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), and it is difficult to ensure the average grain size of the steel structure of the hot-rolled steel plate targeted in the present invention. In addition, it is difficult to suppress the formation of coarse bainite, and it is difficult to control the area ratio of grains with a grain diameter within ±5.0 μm of the average grain diameter to the range targeted in the present invention. Therefore, the heating temperature in the heating process is set to 1100°C to 1300°C. More preferably, it is above 1120°C. In addition, the heating temperature in the heating process is more preferably below 1280°C.

[0130] It should be noted that in the present invention, in addition to the existing method of temporarily cooling the steel billet (slab) to room temperature after manufacturing and then heating it again, the energy-saving process of direct rolling in which the steel billet is loaded into a heating furnace in a hot sheet state without cooling to room temperature can also be applied without any problems.

[0131] Rough rolling end temperature: 850℃~1150℃

[0132] In the hot rolling process, when the rough rolling end temperature is less than 850°C, the surface temperature of the steel plate becomes below the ferrite transformation start temperature in the subsequent finish rolling, a large amount of ferrite is generated, and the area ratio of pearlite and pseudo-pearlite is reduced, so it is difficult to obtain the low yield ratio square steel pipe targeted by the present invention. On the other hand, if the rough rolling end temperature exceeds 1150°C, the reduction amount within the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the steel structure of the hot-rolled steel plate targeted by the present invention cannot be obtained, and it is difficult to obtain a grain having an average grain diameter of 10.0 to 30.0 μm and a number density of 30 grains / mm when the area surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more in the center of the plate thickness of the steel plate is set as the grain. 2The steel structure with a grain size within the average grain size of ±5.0 μm and an area ratio of 35% or more is less than 100 μm. In addition, it is difficult to suppress the formation of coarse bainite. Therefore, the rough rolling end temperature is set to 850°C to 1150°C. It is preferably 860°C or more, and more preferably 870°C or more. In addition, the rough rolling end temperature is preferably 1100°C or less, and more preferably 1050°C or less.

[0133] Time from the end of rough rolling to the start of finish rolling: 15 seconds or more

[0134] In the hot rolling process, when the time from the end of rough rolling to the start of finish rolling is less than 15 seconds, the deviation of the grain size of austenite becomes large, and it is difficult to achieve the goal of the present invention that the area ratio of grains with a grain diameter within ±5.0 μm of the average grain diameter in the center of the plate thickness of the steel plate is 35% or more. In addition, it is difficult to obtain a low yield ratio square steel pipe in which the ratio P of the Charpy absorption energy at -20°C between the circumferential direction of the tube and the axial direction of the tube (hereinafter referred to as "energy ratio P") is 0.5 to 1.2. The above time is preferably 18 seconds or more, and more preferably 20 seconds or more. There is no particular upper limit on the time from the end of rough rolling to the start of finish rolling, but from the perspective of productivity, it is preferably 300 seconds or less, and more preferably 280 seconds or less.

[0135] Finishing rolling end temperature: 750℃~850℃

[0136] In the hot rolling process, when the finishing temperature of the finishing rolling is less than 750°C, the surface temperature of the steel plate becomes below the ferrite transformation start temperature during the finishing rolling, forming ferrite elongated in the rolling direction, which may reduce the workability. On the other hand, if the finishing temperature of the finishing rolling exceeds 850°C, the reduction amount within the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the grains become coarse, and it is difficult to ensure the strength targeted in the present invention. In addition, it is difficult to suppress the formation of coarse bainite. Therefore, the finishing temperature of the finishing rolling is set to 750°C to 850°C. The finishing temperature of the finishing rolling is preferably above 770°C, and more preferably above 780°C. In addition, the finishing temperature of the finishing rolling is preferably below 830°C, and more preferably below 820°C.

[0137] Total reduction below 930°C: 40% to 59%

[0138] In the present invention, the subgrains in austenite are refined in the hot rolling process of the rough rolling and the finish rolling, so that the ferrite and bainite generated in the subsequent cooling process and the coiling process are refined, and the steel structure of the hot-rolled steel sheet having the strength and toughness targeted in the present invention is obtained. In order to refine the subgrains in austenite in the hot rolling process, it is necessary to increase the reduction rate in the austenite non-recrystallization temperature range to introduce sufficient processing strain. However, if the total reduction rate exceeds 59%, it is easy to generate grains with a large ratio of major diameter to minor diameter, resulting in a decrease in toughness. Therefore, in the present invention, the total reduction rate below 930°C is set to 59% or less. Preferably, it is 57% or less, and more preferably, it is 55% or less. On the other hand, if the total reduction rate below 930°C is less than 40%, the grain size of ferrite and bainite becomes large, resulting in a decrease in toughness. Therefore, the total reduction rate below 930°C is set to 40% or more. Preferably, it is 42% or more, and more preferably, it is 45% or more.

[0139] The reason why the total reduction ratio is set to 930° C. or less is that in the rolling process, when the temperature exceeds 930° C., austenite recrystallizes and dislocations introduced by rolling disappear, so that fine austenite cannot be obtained.

[0140] The total rolling reduction ratio is the sum of the rolling reduction ratios in the respective rolling passes within a temperature range of 930° C. or less.

[0141] In the present invention, when hot rolling is performed on the heated steel billet (slab), hot rolling may be performed so that the total reduction ratio at 930°C or below is 40% to 59% across both the rough rolling and the finish rolling. Alternatively, the total reduction ratio at 930°C or below may be 40% to 59% only by the finish rolling. That is, when the total reduction ratio at 930°C or below cannot be 40% to 59% only by the finish rolling, the steel billet (slab) may be cooled in the middle of the rough rolling so that the temperature is 930°C or below, and then the rolling may be performed so that the total reduction ratio at 930°C or below in the rough rolling and the finish rolling is 40% to 59%.

[0142] After the hot rolling process, as a cooling process, the blank steel plate for hot rolled steel plate (hereinafter also referred to as blank 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 (°C / second) at the center of the plate thickness satisfies the following formula (2) and the cooling stop temperature at the center of the plate thickness is 550°C to 680°C.

[0143] 4≤Vc≤20···(2)

[0144] Average cooling rate Vc at the center of plate thickness: 4℃ / sec~20℃ / sec

[0145] In the cooling process, if the average cooling rate Vc at the center of the plate thickness is less than 4°C / sec, the nucleation frequency of ferrite is reduced, and the ferrite grains are coarsened, so the desired strength cannot be obtained. On the other hand, if the above average cooling rate Vc exceeds 20°C / sec, a large amount of upper bainite is generated, and the yield ratio targeted in the present invention cannot be obtained. The average cooling rate Vc is preferably 6°C / sec or more, more preferably 8°C / sec or more. In addition, it is preferably 18°C / sec or less, and more preferably 16°C / sec or less.

[0146] In the present invention, from the viewpoint of suppressing the coarsening of the crystal grain size, it is preferred to start cooling immediately after the finish rolling.

[0147] Cooling stop temperature at the center of plate thickness: 550℃~680℃

[0148] 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 length direction and / or width direction of the blank steel plate during cooling, and there is a possibility that the mechanical properties will deviate. On the other hand, if the cooling stop temperature at the center of the plate thickness exceeds 680°C, the ferrite grains will coarsen and the desired grain diameter cannot be obtained. It should be noted that the cooling stop temperature at the center of the plate thickness is preferably 560°C or more, more preferably 580°C or more. In addition, it is preferably 660°C or less, and more preferably 650°C or less.

[0149] In the present invention, the average cooling rate Vc (°C / sec) at the center of the plate thickness is the average cooling rate within 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 billet steel plate before cooling - temperature of the billet steel plate after cooling) / cooling time), which can be calculated from the temperature distribution in the cross section of the billet steel plate obtained by heat transfer analysis.

[0150] Examples of the cooling method include water cooling such as spraying water from a nozzle and cooling by injecting a cooling gas.

[0151] In the cooling process, it is preferred that both sides of the billet steel plate (hot-rolled plate) be cooled under the same conditions. In addition, in order to obtain the above cooling rate, the amount, pressure, injection time and angle of cooling water or cooling gas and the conveying speed of the billet steel plate can be adjusted. In particular, in order to obtain the cooling rate specified in the present invention, heat transfer analysis can be performed in advance to determine the conditions for the cooling treatment of the billet steel plate, and then the conditions can be reflected in the manufacturing conditions.

[0152] After the above-mentioned cooling process, the blank steel plate is coiled as a coiling process. In this coiling process, from the perspective of the steel plate structure, the blank steel plate is coiled at a plate thickness center temperature (coiling temperature): 550°C to 680°C. If the coiling temperature is less than 550°C, a large amount of upper bainite is sometimes generated on the surface of the steel plate, and the area ratio exceeds 5%. On the other hand, if the coiling temperature exceeds 680°C, the ferrite grains are coarsened and the desired grain diameter cannot be obtained. The coiling temperature is preferably above 570°C, and more preferably above 580°C. In addition, the coiling temperature is preferably below 660°C, and more preferably below 650°C.

[0153] After the above-mentioned coiling process, a second cooling process is implemented to cool the coiled steel plate obtained in the above-mentioned coiling process. In the second cooling process, the coiled steel plate obtained in the above-mentioned coiling process is retained in a temperature range of 400°C to 300°C for a time of 1.0 hour to 10.0 hours. If the retention time in the temperature range of 400°C to 300°C is less than 1.0 hour, the desired work hardening index cannot be obtained, and the desired yield ratio and toughness cannot be obtained. If the retention time in the temperature range of 400°C to 300°C exceeds 10.0 hours, the desired yield strength and tensile strength may not be obtained. The retention time in the temperature range of 400°C to 300°C is preferably 1.5 hours or more, and more preferably 2.0 hours or more. The retention time in the temperature range of 400°C to 300°C is preferably 9.0 hours or less, and more preferably 8.5 hours or less.

[0154] It should be noted that how the temperature of the coiled steel plate (coiled steel plate) changes varies depending on the thickness, width, length, etc. of the blank steel plate. Therefore, in the second cooling step, it is preferred to perform a heat transfer analysis in advance to calculate the temperature change of the coiled steel plate, and to cool the steel plate by covering the periphery of the steel plate with an insulating material or the like, or by using cooling water, cooling gas, etc., so that the retention time in the temperature range of 400°C to 300°C is within the range of the present invention. Since the properties such as the steel structure and strength hardly change within the temperature range of less than 300°C, the cooling from 300°C to room temperature may be any cooling such as rapid cooling or cooling.

[0155] In addition, the temperature of the coiled steel sheet (coiled steel sheet) varies depending on the position. Figure 1 As shown, the temperature of the side surface of the coiled steel plate (finally the hot-rolled steel plate) is measured at three locations (symbol 10. The outer surface, inner surface, and center of the coiled steel plate), and the average value is calculated as the temperature of the coiled steel plate. It should be noted that the temperature of the coiled steel plate is measured every 20 minutes, and the residence time from 400°C to 300°C is calculated. The temperature of the coiled steel plate can be measured using a non-contact thermometer such as a radiation thermometer or a contact thermometer such as a thermocouple.

[0156] <Low Yield Ratio Square Steel Pipe>

[0157] The low yield ratio square steel pipe of the present invention uses the above-mentioned low yield ratio hot-rolled steel plate as its base material.

[0158] The low yield ratio square steel pipe can not only have a yield strength of more than 295MPa, a tensile strength of more than 400MPa and a low yield ratio of less than 0.90 in the flat plate portion along the tube axis direction, but also has a uniform elongation of more than 5.0%, an absorbed energy of more than 60J in the Charpy impact test at a test temperature of -20°C, an energy ratio P of 0.5 to 1.2, and a ductile-brittle transition temperature of less than -10°C low-temperature toughness.

[0159] Furthermore, since the steel sheet has excellent low-temperature toughness not only in the tube axial direction but also in the tube circumferential direction, it can be suitably used as a structural member of a building in a low-temperature environment such as in a cold region where the temperature drops below freezing.

[0160] Regarding the low yield ratio square steel pipe, in the pipe making process, the hot rolled steel plate is formed into a cylindrical open pipe by roll forming, and the butt joint is subjected to electric resistance welding to produce a round steel pipe. Then, the round steel pipe is drawn by several % in the pipe axis direction by rolls arranged vertically and horizontally, and formed into a square shape to obtain a square steel pipe.

[0161] In the present invention, the cylindrical shape refers to a shape obtained by forming a hot-rolled steel sheet into a circular shape by roll forming, and refers to a state where the end portions of the hot-rolled steel sheet are not subjected to resistance welding.

[0162] A round steel pipe is manufactured by forming a coiled low yield ratio hot rolled steel sheet into a round shape by roll forming using a roll by cold rolling, and then the round shape is formed into a square shape by roll forming using a roll to manufacture a square steel pipe.

[0163] When a round steel pipe is roll-formed by cold rolling in this way, since a large processing strain is introduced in the pipe axial direction and the pipe circumferential direction, there is a problem that the yield ratio in the pipe axial direction and the pipe circumferential direction is likely to increase and the toughness is likely to decrease. However, in the low yield ratio square steel pipe of the present invention, since the above-mentioned low yield ratio hot-rolled steel plate is used as the blank, the increase in the yield ratio is suppressed, and even if the wall thickness is, for example, 12 mm or more, it can have a low yield ratio.

[0164] In addition, as described later, the square steel pipe of the present invention has a Charpy absorbed energy ratio P at -20°C of 0.5 to 1.2, and has 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 specified in the present invention, it is necessary to start finish rolling more than 15 seconds after the end of rough rolling. By starting finish rolling more than 15 seconds after the end of rough rolling, a hot-rolled steel sheet with a small difference in steel structure between the plate width direction and the plate length direction (rolling direction) can be obtained. By roll-forming the hot-rolled steel sheet, a low yield ratio square steel pipe with the above-mentioned Charpy absorbed energy ratio P at -20°C of 0.5 to 1.2 can be obtained.

[0165] The above method can produce a low yield ratio square steel pipe with a Charpy absorbed energy ratio P at -20°C of 0.5 to 1.2, but for more stable production, a round steel pipe can be produced by roll forming and then heat treated, and then formed into a square to produce a square steel pipe. By performing such a heat treatment, the toughness in the circumferential direction of the pipe is improved, and a square steel pipe with a Charpy absorbed energy ratio P at -20°C of 0.5 to 1.2 as the target in the present invention can be obtained more advantageously.

[0166] The heat treatment temperature of the heat treatment is preferably 100°C to 550°C. If the heat treatment temperature is less than 100°C, the toughness will not be improved. On the other hand, if the heat treatment temperature exceeds 550°C, the steel structure will coarsen, and the strength and toughness will deteriorate. The heat treatment temperature is more preferably above 150°C. In addition, the heat treatment temperature is more preferably below 500°C. The heat treatment time is preferably more than 30 seconds, more preferably more than 1 minute. There is no particular upper limit, but from the viewpoint of suppressing the cost of heat treatment, it is preferably less than 10 minutes, more preferably less than 5 minutes.

[0167] The method of heat treating the square steel pipe is not limited, and a known heat treatment device (heating device) such as combustion of combustible gas, heating with an electric heater, heating with IH (induction heating) or the like can be used.

[0168] It should be noted that the square steel pipe in the present invention is not limited to a square steel pipe with equal sides (the ratio of the long side length to the short side length (long side length / short side length) is 1.0), but also includes a square steel pipe with a ratio exceeding 1.0. However, if the ratio exceeds 2.5, local buckling is likely to occur on the long side, resulting in a decrease in the compressive strength in the tube axis direction. Therefore, the ratio is preferably in the range of 1.0 to 2.5. The ratio is more preferably 1.0 to 2.0.

[0169] The square steel pipe of the present invention is manufactured in this way. According to the present invention, a square steel pipe having excellent mechanical properties of the flat plate portion is obtained. More specifically, according to the present invention, a square steel pipe having a yield strength of 295 MPa or more in the flat plate portion, a tensile strength of 400 MPa or more in the flat plate portion, a yield ratio of 0.90 or less in the flat plate portion, a uniform elongation of 5.0% or more, a Charpy absorbed energy of 60 J or more at -20°C in the flat plate portion, a ductile-brittle transition temperature of -10°C or less in the flat plate portion, and a Charpy absorbed energy ratio P at -20°C of 0.5 to 1.2 is obtained.

[0170] In addition, the square steel pipe of the present invention has a ductile-brittle transition temperature of less than 0°C, and has excellent toughness not only in the axial direction but also in the circumferential direction of the pipe. Therefore, it can be suitably used as a structural component of buildings in cold areas where the air temperature or room temperature reaches below freezing point. Even in the event of a huge earthquake, the building structure is unlikely to collapse, and excellent earthquake resistance can be ensured.

[0171] <Architectural structures>

[0172] Figure 2 This is a schematic diagram showing an example of the building structure of the present invention.

[0173] The building structure of the present invention comprises the square steel pipe (low yield ratio square steel pipe) 1 of the present invention as a column material. Reference numerals 4, 5, 6, and 7 represent a beam, a small beam, a partition, and a column, respectively.

[0174] As described above, the flat plate portion of the square steel pipe of the present invention has excellent mechanical properties. Therefore, the building structure of the present invention using the square steel pipe as a column material exhibits excellent earthquake resistance.

[0175] Example

[0176] Hereinafter, the present invention will be described in further detail based on examples. However, the present invention is not limited to the following examples.

[0177] Molten steel having the component composition shown in Table 1 was cast to obtain a slab. 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 to obtain a hot-rolled steel sheet. In Table 1, a dash (-) indicates that the content is 0 (zero) or corresponds to an impurity.

[0178]

[0179] [Table 2]

[0180] Table 2

[0181]

[0182] Then, the following pipe manufacturing steps are performed.

[0183] That is, the hot rolled steel sheet is formed into a cylindrical shape by roll forming, and the butted portions are subjected to electric resistance welding to produce a round steel pipe. Then, the round steel pipe is formed into a square shape (a square shape when viewed in a cross section perpendicular to the pipe axis direction) by rollers arranged above, below, and left and right of the round steel pipe, to obtain a roll-formed square steel pipe having corners and flat portions and having side lengths (mm) and wall thicknesses (mm) shown in Table 4 described later.

[0184] In addition, the test piece was collected from the said hot-rolled steel plate, and the structure observation, tensile test, and Charpy impact test shown below were carried out.

[0185] [Organization Observation]

[0186] The test piece for microstructure observation was collected from the central part of the width of the hot-rolled steel plate in a manner that includes a position of 1 / 2t of the plate thickness (t: plate thickness) and a range of 5 mm in the thickness direction from the position of 1 / 2t of the plate thickness. The observation surface was made into a cross section in the rolling direction during hot rolling, and was made by nitric acid etching after grinding.

[0187] In the structural observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) was used to observe and photograph the structure within a range of ±1 mm in the thickness direction from the plate thickness 1 / 2t position of the hot-rolled steel plate. The area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite were calculated from the obtained optical microscope images and SEM images.

[0188] The area ratio of each tissue was calculated by observing in five fields of view and taking the average value of the values ​​obtained in each field of view. Here, the area ratio obtained by the tissue observation was defined as the area ratio of each tissue.

[0189] Here, ferrite is a product of diffusion phase transformation, and its dislocation density is low and has a basically restored structure. It includes polygonal ferrite and pseudo-polygonal ferrite. Pearlite is a structure in which cementite and ferrite are arranged in layers, and pseudo-pearlite is a structure in which cementite arranged in a point array is confirmed in ferrite. In addition, upper bainite is a multiphase structure of lath-shaped ferrite and cementite with high dislocation density. In addition to the above shapes, ferrite is white, pearlite is black, pseudo-pearlite is black or gray, and upper bainite is white or gray, so it can be distinguished.

[0190] In addition, the average grain size (average equivalent circle diameter) is measured using the SEM / EBSD method in the range of ±1 mm in the thickness direction from the plate thickness t / 2 position including the plate thickness t / 2 position (in the present invention, it refers to the plate thickness center). The measurement area is 500 μm × 1000 μm (= 0.5 mm 2), the measurement step length is 0.5μm. The grain diameter is measured by finding the orientation difference between adjacent grains and taking the boundary with an orientation difference of 15° or more as the grain boundary. The grain size (equivalent circle diameter) of each grain is calculated based on the obtained grain boundary, and the arithmetic average is calculated to be the average grain diameter (average equivalent circle diameter). In addition, the sum of the areas of grains with a grain diameter within ±5.0μm of the average grain diameter is calculated and divided by the area of ​​the measurement area (0.5mm 2 ), and thereby calculate the area ratio of grains having a grain diameter within ±5.0 μm of the average grain diameter.

[0191] The major diameter and minor diameter of the grains were measured by the method described in JIS R 1670 (2006), and the ratio of major diameter to minor diameter, i.e., (major diameter) / (minor diameter), was calculated. The number of grains with a major diameter to minor diameter ratio, i.e., (major diameter) / (minor diameter) of 3.0 or more was divided by the area of ​​the measurement region (0.5 mm 2 ), and the number of grains with a ratio of major diameter to minor diameter, i.e., (major diameter) / (minor diameter) of 3.0 or more (pieces / mm) was calculated. 2 ). It should be noted that in the grain size analysis and the measurement of the number of grains, grains having a grain size of less than 2.0 μm were excluded from the analysis object as measurement noise.

[0192] [Tensile test]

[0193] From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece is collected in a manner that the tensile direction is parallel to the rolling direction. The collected tensile test piece is subjected to a tensile test in accordance with the provisions of JIS Z 2241 (2011), and the yield strength YS and tensile strength TS are measured, and the yield ratio defined by (yield strength) / (tensile strength) is calculated. It should be noted that the number of test pieces is 2, and the average value is calculated to obtain YS, TS, and yield ratio. In addition, the work hardening index of 3 to 7% plastic strain is calculated by the method (2-point method) described in JIS Z 2253 (2011).

[0194] [Charpy impact test]

[0195] A V-notch standard test piece according to JIS Z 2242 (2018) was used, which was collected at the 1 / 2t position (center of the plate thickness) of the obtained hot-rolled steel plate in such a way that the long side direction of the test piece was parallel to the rolling direction. According to the provisions of JIS Z2242 (2018), the Charpy impact test was carried out at the test temperatures of -80°C, -60°C, -40°C, -20°C, and 0°C. It should be noted that the number of test pieces was 3 at each test temperature, and the average values ​​(J) of the ductile-brittle transition temperature and the absorbed energy were obtained.

[0196] The obtained results are shown in Table 3.

[0197] [Table 3]

[0198] Table 3

[0199]

[0200] In addition, test pieces were collected from the obtained square steel pipe (roll-formed square steel pipe), and the following tensile test and Charpy impact test were performed.

[0201] [Tensile test]

[0202] Figure 3 It is a schematic diagram showing the position of collecting the tensile test piece of the flat plate.

[0203] Tensile test piece Figure 3 As shown in X, a JIS No. 5 tensile test piece is collected from the flat plate of the square steel pipe 1 in a manner that the tensile direction is parallel to the pipe axis direction. The collected tensile test piece is subjected to a tensile test in accordance with the provisions of JIS Z 2241 (2011), and the yield strength YS and tensile strength TS are measured, and the yield ratio defined by (yield strength) / (tensile strength) is calculated. In addition, the plastic elongation (uniform elongation) at the highest load point (A of JIS Z 2241 (2011)) is also measured. g It should be noted that the tensile test piece of the flat plate is obtained by welding the square steel pipe to the welded part ( Figure 3 W) is set to the position of the width center of the flat plate portion at the 3-point side when the 12 o'clock direction is set ( Figure 3 It should be noted that the number of test pieces was 2, and the average value was calculated to obtain YS, TS, and yield ratio.

[0204] [Charpy impact test]

[0205] Figure 4 This is a schematic diagram showing the location of collecting the Charpy test piece.

[0206] In the Charpy impact test, Figure 4 As shown in Y, a V-notch standard test piece according to the provisions of JIS Z2242 (2018) was collected from the flat plate of the square steel pipe 1 at a position from the outer surface of the square steel pipe 1 to 1 / 4t of the wall thickness t in a manner such that the long side direction of the test piece is parallel to the pipe axis direction. According to the provisions of JIS Z 2242 (2018), the Charpy impact test was carried out at test temperatures of -60°C, -40°C, -20°C, and 0°C. It should be noted that the number of test pieces was 3 at each test temperature, and the average values ​​(J) of the ductile-brittle transition temperature and the absorbed energy were obtained.

[0207] In addition, in order to determine the Charpy absorbed energy in the circumferential direction of the tube, Figure 4As shown in Z, a V-notch standard test piece according to the provisions of JIS Z 2242 (2018) was collected from the flat plate of the square steel pipe 1 at a position from the outer surface of the square steel pipe 1 to 1 / 4t of the wall thickness t in a manner such that the long side direction of the test piece is parallel to the circumferential direction of the pipe. In accordance with the provisions of JIS Z 2242 (2018), the Charpy impact test was carried out at a test temperature of -20°C. It should be noted that the number of test pieces was 3, and the average value of the absorbed energy (J) was calculated. In addition, the ratio P of the Charpy absorbed energy at -20°C in the circumferential direction of the pipe and the axial direction of the pipe was calculated.

[0208] The obtained results are collectively recorded in Table 4.

[0209] [Table 4]

[0210] Table 4

[0211]

[0212] 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. The Steel Plate No. in Table 4 indicates a square steel pipe manufactured using the steel plate of Table 3 with the same number. For example, Steel Plate No. 1 in Table 4 is a square steel pipe manufactured using Steel Plate No. 1 in Table 3.

[0213] As shown in Table 3, the hot-rolled steel sheets of the examples of the present invention all have a steel structure in the center of the plate thickness that includes a main phase of ferrite, pearlite and pseudo-pearlite with a total area ratio of 6 to 25%, and upper bainite (second phase) with an area ratio of 5% or less, and when a region surrounded by a boundary with an orientation difference of 15° or more is defined as a grain, in the steel structure in the center of the plate thickness, the average grain size of the steel structure including the main phase and the second phase is 10.0 to 30.0 μm, the area ratio of grains having a grain size within ±5.0 μm of the average grain size among the above-mentioned grains is 35% or more, and the number density of grains having a ratio of major diameter to minor diameter, i.e., (major diameter) / (minor diameter) of 3.0 or more is 30 grains / mm 2 the following.

[0214] In addition, the yield strength is 250MPa or more, the tensile strength is 400MPa or more, the yield ratio is 0.75 or less, and the work hardening index n of the plastic strain is 3-7%. 3－7 The ductile-brittle transition temperature is below -20°C and the Charpy absorbed energy is above 100 J.

[0215] In addition, as shown in Table 4, the square steel pipes manufactured using the hot-rolled steel plates of the present invention examples all have a yield strength of the flat plate portion of 295 MPa or more, a tensile strength of the flat plate portion of 400 MPa or more, a yield ratio of the flat plate portion of 0.90 or less, a uniform elongation of the flat plate portion of 5.0% or more, a Charpy absorbed energy of the flat plate portion in the tube axis direction at -20°C of 60 J or more, an energy ratio P of 0.5 to 1.2, and a ductile-brittle transition temperature of the flat plate portion of -10°C or less.

[0216] In contrast, in the comparative example No. 21 (steel U), since Mn / Si=0.8 and outside the scope of the present invention, the total area ratio of pearlite and pseudo-pearlite is less than 6%, and the work hardening index of the plastic strain of 3 to 7% and the like do not reach the desired values. In addition, the uniform elongation of the square steel pipe manufactured using the hot-rolled steel sheet is reduced, and the yield ratio and the like do not reach the desired values.

[0217] 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%, and the Charpy absorbed energy at -20°C and the like do not reach the desired values ​​for the hot-rolled steel sheet. In addition, the Charpy absorbed energy in the pipe axis direction at -20°C and the like for the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0218] Since the C content of No. 23 (Steel W) of Comparative Example is lower than the range of the present invention, the total area ratio of pearlite and pseudo-pearlite is outside the range of the present invention, and the yield strength and tensile strength of the hot-rolled steel sheet do not reach the desired values. In addition, the yield strength and tensile strength of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0219] Comparative Example No. 24 (Steel X) has a C content higher than the range of the present invention, so the area ratio of the second phase is outside the range of the present invention, and the yield ratio and the Charpy absorbed energy at -20°C do not reach the desired values. In addition, the Charpy absorbed energy in the pipe axis direction at -20°C of the square steel pipe manufactured using the hot-rolled steel sheet does not reach the desired values.

[0220] Comparative Example No. 25 (Steel Y) has a Si content higher than the range of the present invention, so the area ratio of pseudo pearlite is excessively increased, and the Charpy absorbed energy at -20°C and the like do not reach the desired values. In addition, the Charpy absorbed energy in the tube axis direction at -20°C of the square steel pipe manufactured using the hot-rolled steel sheet does not reach the desired values.

[0221] Comparative Example No. 26 (Steel Z) has a Mn content higher than the range of the present invention, so the amount of upper bainite is excessively increased, and the yield ratio and the like do not reach the desired values. In addition, the yield ratio and the like of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0222] Comparative Example No. 27 (Steel AA) has an excessively increased amount of upper bainite because the Nb content is higher than the range of the present invention. As a result, the number density of grains with a major axis to minor axis ratio of 3.0 or more is outside the range of the present invention, and the Charpy absorbed energy at -20°C and other properties of the hot-rolled steel sheet do not reach the desired values. In addition, the Charpy absorbed energy in the tube axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0223] Comparative Example No. 28 (Steel AB) has a Ti content higher than the range of the present invention, so it is considered that an excessive increase in the amount of upper bainite and the formation of coarse carbides and nitrides occurred. As a result, the hot-rolled steel sheet did not reach the desired values ​​for the Charpy absorbed energy at -20°C and the like. In addition, the Charpy absorbed energy at -20°C in the tube axis direction and the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet did not reach the desired values.

[0224] Comparative Example No. 29 (steel AC) has a V content higher than the range of the present invention, so the amount of upper bainite is outside the range of the present invention. As a result, the Charpy absorbed energy at -20°C and other properties of the hot-rolled steel sheet do not reach the desired values. In addition, the Charpy absorbed energy in the tube axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0225] Comparative Example No. 30 (Steel AD) has a Cr content higher than the range of the present invention, so the amount of upper bainite is outside the range of the present invention. As a result, the Charpy absorbed energy at -20°C and other properties of the hot-rolled steel sheet do not reach the desired values. In addition, the Charpy absorbed energy at -20°C in the tube axis direction and the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0226] Comparative Example No. 31 (Steel AE) has a Mo content higher than the range of the present invention, so the amount of upper bainite is outside the range of the present invention. As a result, the Charpy absorbed energy at -20°C and other properties of the hot-rolled steel sheet do not reach the desired values. In addition, the Charpy absorbed energy in the tube axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0227] Comparative Example No. 32 (Steel AF) has a Cu content higher than the range of the present invention, so it is considered that Cu is coarsely precipitated. As a result, the Charpy absorbed energy at -20°C and other properties of the hot-rolled steel sheet did not reach the desired values. In addition, the Charpy absorbed energy in the tube axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet did not reach the desired values.

[0228] Comparative Example No. 33 (Steel AG) has a Mn content lower than the range of the present invention and a Ni content higher than the range of the present invention, so the amount of upper bainite is outside the range of the present invention. As a result, the ductile-brittle transition temperature and the like do not reach the desired values ​​for the hot-rolled steel sheet. In addition, the Charpy absorbed energy in the tube axis direction at -20°C and the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0229] Comparative Example No. 34 (Steel AH) has a Ca content higher than the range of the present invention, so it is considered that Ca oxide clusters are formed. As a result, the Charpy absorbed energy at -20°C does not reach the desired value for the hot-rolled steel sheet. In addition, the Charpy absorbed energy at -20°C in the tube axis direction and the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0230] Comparative Example No. 35 (Steel AI) has a B content higher than the range of the present invention, so the amount of upper bainite is outside the range of the present invention, and the yield ratio of the flat plate portion does not reach the desired value. In addition, the yield ratio of the square steel pipe manufactured using the hot-rolled steel sheet does not reach the desired value.

[0231] The slab heating temperature of No. 36 (steel T) of the comparative example was higher than the range of the present invention, the grains were coarsened, and the average grain diameter and the area ratio of grains with a grain diameter within ±5.0 μm of the average grain diameter were outside the range of the present invention. As a result, the yield strength, tensile strength, and Charpy absorbed energy at -20°C of the hot-rolled steel sheet did not reach the desired values. In addition, the yield strength and tensile strength of the square steel pipe manufactured using the hot-rolled steel sheet did not reach the desired values.

[0232] Comparative Example No. 37 (Steel T) has a finishing temperature higher than the range of the present invention, so the total reduction ratio below 930°C is lower than the range of the present invention, and the formation of coarse upper bainite cannot be suppressed, and the average grain size is outside the range of the present invention. As a result, the yield strength, tensile strength, and Charpy absorbed energy at -20°C of the hot-rolled steel sheet do not reach the desired values. In addition, the yield strength and tensile strength of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0233] Comparative Example No. 38 (Steel T) has a total reduction ratio below 930°C that is higher than the range of the present invention, so coarse upper bainite elongated in the rolling direction is generated, the average grain size is lower than the range of the present invention, and the number density of grains with a major diameter / minor diameter ratio of 3.0 or more is outside the range of the present invention. As a result, the ductile-brittle transition temperature and other characteristics of the hot-rolled steel sheet do not reach the desired values. In addition, the Charpy absorbed energy and ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet in the tube axis direction at -20°C did not reach the desired values.

[0234] Comparative Example No. 39 (Steel T) has an average cooling rate at the center of the plate thickness higher than the range of the present invention, so the area ratio of upper bainite exceeds 5% and is outside the range of the present invention. As a result, the yield ratio does not reach the desired value and the work hardening index of the plastic strain of 3 to 7% does not reach the desired value. In addition, the uniform elongation of the square steel pipe manufactured using the hot-rolled steel plate is less than 5.0%, and the yield ratio and the like do not reach the desired value.

[0235] Since the time from the end of rough rolling to the start of finish rolling in Comparative Example No. 40 (Steel T) is below the range of the present invention, the area ratio of grains having a grain diameter within ±5.0 μm of the average grain diameter is outside the range of the present invention. As a result, the yield ratio of the hot-rolled steel sheet and the square steel pipe manufactured using the hot-rolled steel sheet did not reach the desired value. In addition, since the work hardening index of the hot-rolled steel sheet with a plastic strain of 3 to 7% did not reach the desired value, the uniform elongation of the square steel pipe was less than 5.0%.

[0236] Comparative Example No. 41 (Steel T) has a cooling stop temperature and a coiling temperature lower than the range of the present invention, so the area ratio of upper bainite and the area ratio of grains having a grain diameter within ±5.0 μm of the average grain diameter are outside the range of the present invention. As a result, the yield ratio of the hot-rolled steel sheet and the square steel pipe manufactured using the hot-rolled steel sheet did not reach the desired value. In addition, since the work hardening index of the hot-rolled steel sheet with a plastic strain of 3 to 7% did not reach the desired value, the uniform elongation of the square steel pipe was less than 5.0%.

[0237] Comparative Example No. 42 (Steel T) has a low average cooling rate at the center of the plate thickness, and the cooling stop temperature and coiling temperature are higher than the range of the present invention, so the average grain size is outside the range of the present invention. As a result, the yield strength, tensile strength, and Charpy absorbed energy at -20°C of the hot-rolled steel sheet do not reach the desired values. In addition, the yield strength and tensile strength of the square steel pipe manufactured using the hot-rolled steel sheet do not reach the desired values.

[0238] Since the residence time in the temperature range of 400°C to 300°C of Comparative Example No. 43 (Steel T) is lower than the range of the present invention, the area ratio of upper bainite exceeds 5%, and the work hardening index, yield ratio, and Charpy absorbed energy at -20°C of the plastic strain of 3 to 7% do not reach the desired values ​​for the hot-rolled steel sheet. In addition, the Charpy absorbed energy in the pipe axis direction at -20°C of the square steel pipe manufactured using the hot-rolled steel sheet does not reach the desired values.

[0239] Since the residence time in the temperature range of 400°C to 300°C of Comparative Example No. 44 (Steel T) is higher than the range of the present invention, the average grain size is outside the range of the present invention, and the yield strength and tensile strength of the hot-rolled steel sheet do not reach the desired values. In addition, the yield strength and tensile strength of the square steel pipe manufactured using the hot-rolled steel sheet also do not reach the desired values.

[0240] Since the rough rolling finishing temperature of No. 45 (steel T) of the comparative example is higher than the range of the present invention, the number density of grains having a ratio of major diameter to minor diameter (major diameter) / (minor diameter) of 3.0 or more is outside the range of the present invention. As a result, the ductile-brittle transition temperature of the hot-rolled steel sheet does not reach the desired value. In addition, the ductile-brittle transition temperature of the square steel pipe manufactured using the hot-rolled steel sheet does not reach the desired value.

[0241] Comparative Example No. 46 (Steel T) has a rough rolling end temperature and a finish rolling end temperature lower than the range of the present invention, so a large amount of ferrite is generated, and the total area ratio of pearlite and pseudo-pearlite is less than 6%, resulting in a hot-rolled steel sheet having a work hardening index of 3 to 7% of plastic strain, etc. that does not reach the desired values. In addition, the uniform elongation of the square steel pipe manufactured using the hot-rolled steel sheet is less than 5.0%, and the tensile strength and yield ratio do not reach the desired values.

[0242] In Comparative Example No. 47 (steel T), since the slab heating temperature was lower than the range of the present invention, the deformation resistance of the rolled material increased, and rolling became difficult, so the rolling was interrupted in the middle of the rough rolling. Therefore, hot-rolled steel sheets and square steel pipes could not be produced.

[0243] Comparative Example No. 48 (Steel T) has a finish rolling end temperature lower than the range of the present invention, so ferrite is formed that is elongated in the rolling direction. As a result, the Charpy absorbed energy at -20°C and other properties of the hot-rolled steel sheet do not reach the desired values. In addition, the Charpy absorbed energy at -20°C in the pipe axis direction of the square steel pipe manufactured using the hot-rolled steel sheet does not reach the desired values.

[0244] Comparative Example No. 49 (Steel T) had an average cooling rate lower than the range of the present invention, so the ferrite coarsened and the average grain size exceeded 30.0 μm. As a result, the yield strength, tensile strength, and Charpy absorbed energy at -20°C of the hot-rolled steel sheet did not reach the desired values. In addition, the Charpy absorbed energy in the tube axis direction at -20°C of the square steel pipe manufactured using the hot-rolled steel sheet did not reach the desired values.

[0245] Explanation of symbols

[0246] 1 Square steel pipe

[0247] 4 Beam

[0248] 5 Trabeculae

[0249] 6. Partition

[0250] 7 Column

[0251] 10 Temperature measurement position of hot rolled steel plate

Claims

1. A hot-rolled steel sheet having the following composition: containing, by mass%, C: 0.07% to 0.20%, Si: 0.40% or less, Mn: 0.20% to 1.00%, P: 0.100% or less, S: 0.050% or less, Al: 0.005% to 0.100% and N: 0.0100% or less, the remainder being Fe and unavoidable impurities, and the contents of Mn and Si satisfy the following formula (1), The steel structure at the center of the plate thickness has a main phase of 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. In the steel structure at the center of the plate thickness, when the region surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is set as the grain, the average grain diameter of the grain is 10.0 to 30.0 μm, the area ratio of the grains having a grain diameter within ±5.0 μm of the average grain diameter among the grains is 35% or more, and the number density of the grains having a ratio of major diameter to minor diameter, i.e., major diameter / minor diameter of 3.0 or more among the grains is 30 grains / mm 2 the following, 1.0≤%Mn / %Si≤3.5···(1) Here, %Mn and %Si are the contents of the respective elements in the steel sheet, and the unit is mass %.

2. The hot rolled steel sheet according to claim 1, wherein: The component composition further contains, in mass%, one or more selected from the group consisting of Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, V: 0.01% to 0.10%, Cr: 0.01% to 0.50%, Mo: 0.01% to 0.50%, Cu: 0.01% to 0.30%, Ni: 0.01% to 0.30%, Ca: 0.0005% to 0.0100% and B: 0.0003% to 0.0100%.

3. The hot rolled steel sheet according to claim 1 or 2, wherein: The plate thickness is 12 mm or more.

4. A method for producing a hot-rolled steel sheet, comprising heating a steel billet having the composition of claim 1 or 2 to a heating temperature of 1100°C to 1300°C, Next, the hot rolling is performed as follows: rough rolling is performed at a rough rolling end temperature of 850°C to 1150°C, and finish rolling is started 15 seconds or more after the rough rolling is completed, the finish rolling end temperature is 750°C to 850°C, and the total reduction ratio of the hot rolling process is 40% to 59% below 930°C. Next, the steel plate obtained by the hot rolling is cooled at an average cooling rate Vc at the center of the plate thickness satisfying the following formula (2) and a cooling stop temperature at the center of the plate thickness of 550° C. to 680° C., wherein the unit of Vc is ° C. / sec. Next, the steel plate is coiled at a plate thickness center temperature of 550°C to 680°C. Next, the coiled steel sheet obtained in the coiling is subjected to a second cooling process in which the coiled steel sheet is retained at a temperature range of 400° C. to 300° C. for 1.0 to 10.0 hours. 4≤Vc≤20···(2).

5. A square steel pipe using the hot-rolled steel plate according to claim 1 or 2 as a blank.

6. A square steel pipe using the hot-rolled steel plate according to claim 3 as a blank.

7. The square steel tube according to claim 5, wherein: The ratio P of the Charpy absorbed energy at -20°C in the tube circumferential direction to the tube axial direction is in the range of 0.5 to 1.

2. Here, P = (Charpy absorbed energy at -20°C in the tube circumferential direction) / (Charpy absorbed energy at -20°C in the tube axial direction).

8. The square steel tube according to claim 6, wherein: The ratio P of the Charpy absorbed energy at -20°C in the tube circumferential direction to the tube axial direction is in the range of 0.5 to 1.

2. Here, P = (Charpy absorbed energy at -20°C in the tube circumferential direction) / (Charpy absorbed energy at -20°C in the tube axial direction).

9. A method for producing a square steel pipe, comprising forming the square steel pipe by roll-forming the hot-rolled steel sheet obtained by the method for producing a hot-rolled steel sheet according to claim 4 by cold rolling.

10. A building structure comprising the square steel pipe according to claim 5 as a column material.

11. A building structure comprising the square steel pipe according to claim 6 as a column material.

12. A building structure comprising the square steel pipe according to claim 7 as a column material.

13. A building structure comprising the square steel pipe according to claim 8 as a column material.

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