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

Abstract

Provided are: a hot-rolled steel sheet having excellent strength and low-temperature toughness; a rectangular steel pipe; a method for producing the same; and a building structure. The steel sheet has a prescribed component composition, and the steel structure of the sheet thickness center part and the front and back surfaces of the sheet has a main phase comprising ferrite 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. When a region surrounded by a boundary where the orientation difference between adjacent crystals is 15 degrees or more is defined as crystal grains, the average crystal grain size of the steel structure in the plate thickness center portion is 10.0-30.0 [mu] m, the area ratio of crystal grains having an equivalent circle diameter of 40.0 [mu] m or more is 20% or less, and the number of crystal grains having a ratio of major diameter to minor diameter (= (major diameter) / (minor diameter)) of 3.0 or more is 30 / mm2 or less. The hardness difference between a position within 1.0 mm from the front and back surfaces of the steel plate in the plate thickness direction and the plate thickness center portion is 40 HV or less.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel plate and a rectangular steel tube (square column) manufactured by cold rolling using the hot-rolled steel plate as a raw material. In particular, it relates to a rectangular steel tube suitable for use as a building structural member of a large building. In addition, it relates to a building structure obtained by using the rectangular steel tube. Background Art

[0002] In recent years, for building structural members used in large buildings (hereinafter referred to as buildings) such as factories, warehouses, and commercial facilities, the strength of the members has been promoted to reduce the construction cost by reducing the weight. In particular, in rectangular steel pipes (square columns) having a flat plate portion and a corner portion used as column materials for buildings, high strength is required for the flat plate portion. At the same time, rectangular steel pipes used in building structural members are also required to have excellent toughness from the perspective of earthquake resistance.

[0003] Rectangular steel pipes are usually manufactured by cold forming a hot rolled steel sheet (hot rolled steel strip) or a thick steel plate as a raw material. As a method for cold forming, there are a method of cold stamping bending forming or a method of cold rolling forming.

[0004] The rectangular steel tube manufactured by roll-forming the raw material (hereinafter sometimes referred to as the roll-formed rectangular steel tube) is formed by cold-rolling the hot-rolled steel plate into a cylindrical open tube and then resistance-welding the butt joints. Then, the cylindrical open tube (circular steel tube) is drawn by several percent in the tube axis direction by rollers arranged above, below, left and right of the open tube, and then formed into a rectangle to manufacture the rectangular steel tube. On the other hand, the rectangular steel tube manufactured by stamping and bending the raw material (hereinafter sometimes referred to as the stamped rectangular steel tube) is formed by cold-stamping and bending the thick steel plate into a square-shaped (quadrilateral) or square-shaped cross-section. The U-shaped parts are manufactured by joining them by submerged arc welding.

[0005] Compared with the manufacturing method of the punch-formed rectangular steel pipe, the manufacturing method of the roll-formed rectangular steel pipe has the advantages of high productivity and can be manufactured in a short period of time. However, in the punch-formed rectangular steel pipe, cold forming is not applied to the flat portion, and only the corner portion undergoes work hardening. In contrast, in the roll-formed rectangular steel pipe, a large work strain is introduced in the tube axis direction throughout the entire circumference of the steel pipe, especially when cold forming into a cylindrical shape. Therefore, the roll-formed rectangular steel pipe has the problem of high yield ratio in the tube axis direction and low toughness not only in the corner portion but also in the flat portion.

[0006] In addition, for roll-formed rectangular steel pipes, the greater the wall thickness, the greater the work hardening during roll forming, so the yield ratio becomes higher and the toughness further decreases. Therefore, in the case of manufacturing thick-walled roll-formed rectangular steel pipes, it is necessary to select a raw material that can withstand the increase in yield ratio and the decrease in toughness caused by roll forming.

[0007] In response to such a demand, for example, Patent Document 1 proposes a rectangular steel pipe in which a steel raw material 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 in which a reduction ratio 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 is hot rolled to form a coil, and the outer circumference in the steel pipe forming process is drawn 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.

[0008] Patent document 2 proposes a rectangular steel pipe, wherein steel containing C: 0.07-0.18% and Mn: 0.3-1.5% by mass is heated to a heating temperature of 1100-1300°C, rough rolling is performed with the rough rolling end temperature set to 1150-950°C and finishing rolling is performed with the finishing rolling start temperature set to 1100-850°C and the finishing rolling end temperature set to 900-750°C, and then cooling is performed at a surface thermometer stop temperature of 550°C or more. The steel is subjected to primary cooling by cooling in a manner of cooling in a manner of cooling in air for 3 to 15 seconds, secondary cooling by air cooling for 3 to 15 seconds, and tertiary cooling by cooling to below 650°C at an average cooling rate of 4 to 15°C / s in a temperature range of 750 to 650°C based on the temperature of the central part of the plate thickness, so that the value of the second phase frequency contained in the steel structure is 0.20 to 0.42, thereby showing a low yield ratio of less than 80%, and having mechanical properties of an absorbed energy of more than 150J in a Charpy impact test at a test temperature of 0°C.

[0009] Patent document 3 proposes a rectangular steel pipe, wherein steel containing C: 0.07-0.18% and Mn: 0.3-1.5% by mass is heated to a heating temperature of 1100-1300°C, followed by rough rolling with a rough rolling end temperature set to 1150-950°C and finish rolling with a finish rolling start temperature set to 1100-850°C and a finish rolling end temperature set to 900-750°C, and then cooled to a coiling temperature of 500-650°C in such a manner that the average cooling rate in the temperature range of 750-650°C as a surface temperature is 20°C / s or less, the time until the temperature of the center of the plate thickness reaches 650°C is within 35s, and the average cooling rate in the temperature range of 750-650°C in the center of the plate thickness is 4-15°C / s, thereby showing a low yield ratio of 80% or less and having mechanical properties of an absorbed energy of 150J or more in a Charpy impact test at a test temperature of 0°C.

[0010] Patent document 4 proposes a rectangular 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 balance consisting of Fe and inevitable impurities is heated to a heating temperature of 1100-1300°C, followed by rough rolling with a rough rolling end temperature set to 1150-950°C and finishing rolling with a finishing rolling start temperature set to 1100-850°C and a finishing rolling end temperature set to 900-750°C, and then cooled to 580°C at an average cooling rate of 4-25°C / s from the start of cooling to the stop of cooling based on the center temperature of the plate thickness. ℃ or below, there is one or more cooling steps of 0.2s or more and less than 3.0s in the initial cooling step during the 10s after the start of cooling, and then after coiling at a coiling temperature of 580℃ or below, cooling is performed. As a result, the steel structure at the center 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 and 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 front and back sides of the plate thickness is a ferrite single phase or a bainitic ferrite single phase, the average grain size is 2-20μm, it shows a low yield ratio of less than 90%, and has mechanical properties of an absorbed energy of 27J or more in a Charpy impact test at a test temperature of 0℃.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 9-87743

[0014] Patent Document 2: Japanese Patent No. 5594165

[0015] Patent Document 3: Japanese Patent No. 5589885

[0016] Patent Document 4: Japanese Patent No. 6388091 Summary of the invention

[0017] Problems to be solved by the invention

[0018] Here, for rectangular steel pipes manufactured by cold rolling, the greater the wall thickness or the smaller the length of the side, the greater the processing strain introduced into the rectangular steel pipe, and the greater the increase in yield ratio and the decrease in toughness. Therefore, for the hot-rolled steel plate used as the raw material, it is necessary to have a steel structure that suppresses the increase in yield ratio during forming and excellent low-temperature toughness that can withstand the deterioration of toughness caused by large processing strain. However, for rectangular steel pipes manufactured by the methods disclosed in the above-mentioned patent documents 1 to 3, especially when the plate thickness exceeds 25 mm, there is a problem that the yield ratio becomes high and cannot meet the yield ratio of less than 90%.

[0019] In the technology described in Patent Document 4, in order to obtain a low yield ratio and high toughness, the steel structure on the front and back sides of the plate needs to be a ferrite single phase or a bainitic ferrite single phase. In order to suppress the formation of pearlite and pseudo-pearlite, rapid cooling is performed in the cooling process when manufacturing the hot-rolled steel plate, and there is a problem that the equipment and conditions that can be manufactured are restricted.

[0020] The present invention is completed in view of the above-mentioned problems, and its purpose is to provide a hot-rolled steel plate with high yield strength and tensile strength, low yield ratio and excellent low-temperature toughness, a rectangular steel pipe using the hot-rolled steel plate, their manufacturing methods and a building structure using the above-mentioned rectangular steel pipe.

[0021] Here, (1) high yield strength, (2) high tensile strength, and (3) low yield ratio mean that, using a JIS No. 5 tensile test piece cut in a manner such that the tensile direction is parallel to the rolling direction, through a tensile test in accordance with the provisions of JIS Z 2241 (2011), (1) the yield strength is 250 MPa or more, (2) the tensile strength is 400 MPa or more, and (3) the yield ratio is 0.75 or less, respectively.

[0022] In addition, excellent low-temperature toughness means: using a V-notch standard test piece cut at the t / 2 position of the plate thickness t (the center of the plate thickness) in accordance with the provisions of JIS Z 2242 (2018) with the length direction of the test piece parallel to the rolling direction, a Charpy impact test is carried out at test temperatures of -80°C, -60°C, -40°C, -20°C, and 0°C, and the Charpy absorbed energy at -20°C is 100 J or more and the ductile-brittle transition temperature is -20°C or less.

[0023] In addition, the rectangular steel pipe in the present invention refers to a rectangular steel pipe as described below: using a JIS No. 5 tensile test piece cut in a manner parallel to the tube axis direction, through a tensile test in accordance with the provisions of JIS Z 2241 (2011), the yield strength of the flat plate portion is 295 MPa or more, the tensile strength of the flat plate portion is 400 MPa or more, and the yield ratio of the flat plate portion is 0.90 or less. Furthermore, using a V-notch standard test piece cut in a manner parallel to the tube axis direction at a position t / 4 of the wall thickness t from the outer surface of the tube in accordance with the provisions of JIS Z 2242 (2018), a Charpy impact test is performed under the conditions of test temperatures of -60°C, -40°C, -20°C, 0°C, and +20°C, the Charpy absorbed energy of the flat plate portion at -20°C is 40 J or more, and the ductile-brittle transition temperature of the flat plate portion is -5°C or less.

[0024] Methods used to solve problems

[0025] The present inventors have conducted intensive studies to solve the above-mentioned problems and have obtained the following findings (1) to (3).

[0026] (1) In order to satisfy the target yield strength and tensile strength of the present invention, the hot-rolled steel sheet needs to contain 0.07 mass % or more of C. Furthermore, the main structure (main phase) of the hot-rolled steel sheet at the center of the sheet thickness and at the front and back surfaces needs to be ferrite.

[0027] (2) For a hot-rolled steel sheet, in order to further obtain the target low-temperature toughness and yield ratio of the present invention in a steel structure satisfying the above (1), it is necessary to have, in addition to the above (1), a second phase consisting of one or more selected from pearlite, pseudo-pearlite and bainite, such that the total area ratio of pearlite and pseudo-pearlite is 6 to 25%, the area ratio of upper bainite is 5% or less, and when the region surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more in the center of the plate thickness is taken as the grain, 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 with an equivalent circle diameter of 40.0 μm or more is 20% or less, and the number of grains with a major axis to minor axis ratio (= (major axis) / (minor axis)) of 3.0 or more is 30 grains / mm 2 Next, the hardness difference between a position 1.0 mm inside from the front and back surfaces of the plate in the plate thickness direction and the center of the plate thickness is set to 40 HV or less.

[0028] (3) In order to obtain the above steel structure, it is necessary to adjust the chemical composition to an appropriate range, control the contents of Mn and Si to a specific range, and further adjust the cooling stop temperature and coiling temperature in the cooling process after the hot rolling process to an appropriate range.

[0029] The present invention has been accomplished based on these findings and is constituted by the following gist.

[0030] [1] A hot-rolled steel sheet comprising, in mass%, C: 0.07% to 0.20%, Si: 0.40% to 0.40%, Mn: 0.20% to 1.00%, P: 0.100% to 0.100%, S: 0.050% to 0.005% to 0.100%, N: 0.0100% to 0.0100%, or further comprising Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, V: 0.01% to 0.005% The composition is a composition in which the content of Mn and Si satisfies the following formula (1), and the balance is Fe and unavoidable impurities.

[0031] The steel structure at the center of the plate thickness and the surface and back of the plate has a main phase composed of ferrite, a second phase with a total area ratio of pearlite and pseudo-pearlite of 6 to 25% and an area ratio of upper bainite of 5% or less,

[0032] In the center of the plate thickness, the region surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more is regarded as a crystal grain.

[0033] The average grain size of the steel structure in the center of the plate thickness is 10.0 to 30.0 μm.

[0034] The area ratio of the crystal grains having an equivalent circle diameter of 40.0 μm or more is 20% or less, and

[0035] The number of the above-mentioned grains having a ratio of the major diameter to the minor diameter, i.e., (major diameter) / (minor diameter) of 3.0 or more is 30 grains / mm 2 the following,

[0036] The hardness difference between the position 1.0 mm inside from the front and back sides of the plate in the plate thickness direction and the center of the plate thickness is 40 HV or less.

[0037] 1.0≤%Mn / %Si≤3.5 …(1)

[0038] Here, %Mn and %Si are the contents (mass %) of the respective elements.

[0039] [2] The hot-rolled steel sheet according to [1] above, wherein

[0040] The yield strength is above 250MPa.

[0041] The tensile strength is above 400MPa.

[0042] The yield ratio is below 0.75.

[0043] The Charpy absorption energy at -20℃ is more than 100J.

[0044] The ductile-brittle transition temperature is below -20°C.

[0045] [3] The hot-rolled steel sheet according to [1] or [2], wherein the steel structure has 5 to 15% of the pseudo pearlite by area ratio.

[0046] [4] The hot-rolled steel sheet according to any one of [1] to [3] above, wherein the sheet thickness is 20 mm or more.

[0047] [5] A method for producing a hot-rolled steel sheet, which is the method for producing a hot-rolled steel sheet according to any one of [1] to [4] above, comprising:

[0048] A hot rolling process of heating a steel material under the condition of a heating temperature of 1100° C. to 1300° C., and then performing hot rolling with a rough rolling end temperature of 850° C. to 1150° C., a finish rolling end temperature of 750° C. to 850° C., and a total rolling reduction of 40% to 59% at a temperature of 930° C. or less;

[0049] Then, a cooling step is performed under the conditions that the average cooling rate Vc (°C / s) of the center of the plate thickness and the average cooling rate Vs (°C / s) of the plate surface layer from the front and back sides of the plate to a depth of 1.0 mm in the thickness direction satisfy the following equations (2) and (3), the hot-rolled steel plate is air-cooled for more than 5 seconds from the start of cooling to the stop of cooling, and the cooling stop temperature of the center of the plate thickness is 550°C to 680°C; and

[0050] Next, a coiling step is performed under the condition that the plate thickness center temperature is 550° C. or higher and 680° C. or lower.

[0051] 2≤Vc≤15 …(2)

[0052] Vs / Vc≤2.0 …(3)

[0053] [6] A rectangular steel pipe using the hot-rolled steel plate according to any one of [1] to [4] as a raw material.

[0054] [7] A method for producing a rectangular steel pipe, comprising: forming the rectangular steel pipe by cold rolling the hot-rolled steel sheet according to any one of [1] to [4] above.

[0055] [8] A building structure using the rectangular steel pipe described in [6] as a column material.

[0056] Effects of the Invention

[0057] According to the present invention, there are provided a hot-rolled steel sheet having high yield strength and tensile strength, low yield ratio and excellent low-temperature toughness, a rectangular steel pipe using the hot-rolled steel sheet, a method for producing the same and a building structure using the rectangular steel pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 This is a perspective view schematically showing an example of a building structure using the rectangular steel pipe of the present invention.

[0059] Figure 2 It is a schematic diagram showing the cutting position of the flat plate tensile test piece implemented in the present invention.

[0060] Figure 3 This is a schematic diagram showing the cutting position of the Charpy test piece implemented in the present invention. DETAILED DESCRIPTION

[0061] Hereinafter, the present invention will be described in detail.

[0062] <Hot-rolled steel sheet>

[0063] The hot rolled steel sheet of the present invention is characterized in that it has a composition containing, by mass%, C: 0.07% to 0.20%, Si: 0.40% to 1.00%, Mn: 0.20% to 1.00%, P: 0.100% to 0.100%, S: 0.050% to 0.050%, Al: 0.005% to 0.100%, N: 0.0100%, the contents of Mn and Si satisfy the following formula (1), and the balance is Fe and inevitable impurities, and the steel structure at the center of the plate thickness and the front and back surfaces of the plate has a main phase composed of ferrite and pearlite and pseudo-pearlite The invention discloses a second phase in which the total area ratio of the upper bainite is 6 to 25% and the area ratio of the upper bainite is 5% or less, and in the center of the plate thickness, when the area surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is taken as the grain, the average grain size of the steel structure in the center of the plate thickness is 10.0 to 30.0 μm, the area ratio of the grains having an equivalent circle diameter of 40.0 μm or more relative to the entire steel structure in the center of the plate thickness is 20% or less, and the number of grains having a ratio of the major diameter to the minor diameter (ratio of the major diameter to the minor diameter (= (major diameter) / (minor diameter)) of 3.0 or more is 30 grains / mm 2 Hereinafter, the hardness difference between the position 1.0 mm inside from the front and back sides of the plate in the plate thickness direction and the center of the plate thickness is 40 HV or less. It should be noted that the "hot-rolled steel plate" includes hot-rolled steel plates and hot-rolled steel strips.

[0064] 1.0≤%Mn / %Si≤3.5 …(1)

[0065] Here, %Mn and %Si are the contents (mass %) of the respective elements.

[0066] 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 %".

[0067] C: 0.07% or more and 0.20% or less

[0068] 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 target strength and yield ratio in the present invention, it is necessary to contain more than 0.07% C. However, when the C content exceeds 0.20%, the proportion of hard phases increases, the toughness decreases, and the yield ratio exceeds 0.90, and the desired yield ratio cannot be obtained. In addition, weldability is also deteriorated. Therefore, the C content is set to more than 0.07% and less than 0.20%. The C content is preferably more than 0.08%, more preferably more than 0.10%. In addition, the C content is preferably less than 0.18%, more preferably less than 0.17%.

[0069] Si: 0.40% or less

[0070] Si is an element that increases the strength of steel by solid solution strengthening, and can be contained as needed. In order to obtain such an effect, it is preferably contained 0.01% or more Si. However, when the Si content exceeds 0.40%, it is easy to generate oxides in the resistance welding portion, and the weld characteristics are reduced. And the toughness of the parent material portion other than the resistance welding portion is also reduced. Therefore, the Si content is set to 0.40% or less. The Si content is preferably 0.01% or more, more preferably 0.05% or more. In addition, the Si content is preferably 0.37% or less, more preferably 0.35% or less.

[0071] Mn: 0.20% or more and 1.00% or less

[0072] 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 starting temperature of ferrite transformation. In order to ensure the target strength and structure in the present invention, it is necessary to contain more than 0.20% Mn. However, when the Mn content exceeds 1.00%, the amount of bainite generated becomes too much, and thus the yield ratio exceeds 0.90, and the desired yield ratio cannot be obtained. In addition, when the Mn content exceeds 1.00%, the hardness of the central segregation portion increases, which may become the cause of cracks during welding. Therefore, the Mn content is set to more than 0.20% and less than 1.00%. The Mn content is preferably more than 0.25%, more preferably more than 0.30%. In addition, the Mn content is preferably less than 0.95%, more preferably less than 0.90%.

[0073] P: 0.100% or less

[0074] P segregates at grain boundaries, resulting in uneven materials, so it is preferably reduced as much as possible as an inevitable impurity, but can be allowed to be up to 0.100%. Therefore, the P content is set within a range of 0.100% or less. The P content is preferably 0.030% or less, and more preferably 0.020% or less. It should be noted that there is no particular lower limit for P, but excessive reduction leads to a rise in smelting costs, so P is preferably set to 0.002% or more.

[0075] S: 0.050% or less

[0076] S usually exists in the form of MnS in steel, but MnS is stretched very thin in 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 it can be allowed to be up to 0.050%. Therefore, the S content is set to less than 0.050%. The S content is preferably less than 0.015%, more preferably less than 0.010%, and further preferably less than 0.008%. It should be noted that the lower limit of S is not particularly specified, but excessive reduction leads to a surge in smelting costs, so S is preferably set to more than 0.001%.

[0077] Al: 0.005% or more and 0.100% or less

[0078] Al is an element that acts as a strong deoxidizer. In order to obtain such an effect, it is necessary to contain more than 0.005% Al. However, when the Al content exceeds 0.100%, the weldability deteriorates, and the number of alumina inclusions increases, and the surface properties deteriorate. And the toughness of the weld is also reduced. Therefore, the Al content is set to be more than 0.005% and less than 0.100%. The Al content is preferably more than 0.010%, more preferably more than 0.015%. In addition, the Al content is preferably less than 0.070%, more preferably less than 0.050%.

[0079] N: 0.0100% or less

[0080] N is an inevitable impurity and an element that has the effect of reducing toughness by firmly fixing the movement of dislocations. In the present invention, N is preferably reduced as much as possible as an impurity, but the N content can be allowed to be up to 0.0100%. Therefore, the N content is set to less than 0.0100%. The N content is preferably less than 0.0080%, more preferably less than 0.0040%, and further preferably less than 0.0035%. It should be noted that excessive reduction leads to a rise in smelting costs, so the N content is preferably set to more than 0.0010%, and more preferably set to more than 0.0015%.

[0081] 1.0≤%Mn / %Si≤3.5 …(1)

[0082] Here, %Mn and %Si are the contents (mass %) of the respective elements.

[0083] In the present invention, the contents of Mn and Si need to be within the above ranges, and further, 1.0≤%Mn / %Si≤3.5 as the formula (1) needs to be satisfied. 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 later can be obtained, and the target strength, yield ratio, Charpy absorbed energy, and ductile-brittle transition temperature in the present invention can be obtained. %Mn / %Si is preferably greater than 1.2, and more preferably greater than 1.4. In addition, %Mn / %Si is preferably less than 3.2, and more preferably less than 3.0.

[0084] The balance is Fe and inevitable impurities. However, the inclusion of 0.005% or less of O (oxygen) is not rejected within the range that does not impair the effects of the present invention.

[0085] In addition, with respect to Nb, Ti, V, Cr, Mo, Cu, Ni, Ca, and B which are optional elements described later, Nb: greater than 0% and less than 0.005%, Ti: greater than 0% and less than 0.005%, V: greater than 0% and less than 0.01%, Cr: greater than 0% and less than 0.01%, Mo: greater than 0% and less than 0.01%, Cu: greater than 0% and less than 0.01%, Ni: greater than 0% and less than 0.01%, Ca: greater than 0% and less than 0.0005%, and B: greater than 0% and less than 0.0003% are treated as unavoidable impurities.

[0086] The above components are the basic component composition of the hot-rolled steel sheet in the present invention. The target properties in the present invention can be obtained by the above essential elements, but the following elements may be contained as needed.

[0087] 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%, B: 0.0003% to 0.0100%

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

[0089] Nb and Ti are elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through precipitation strengthening. In order to obtain such an effect, when Nb is contained, 0.005% or more of Nb is contained. In addition, when Ti is contained, 0.005% or more of Ti is contained.

[0090] On the other hand, when Nb and Ti are contained in excess of 0.020%, coarse carbides and nitrides may be formed, resulting in a decrease in toughness. Therefore, when Nb is contained, the content of Nb is set to be greater than 0.005% and less than 0.020%, and when Ti is contained, the content of Ti is set to be greater than 0.005% and less than 0.020%. Nb and Ti are preferably greater than 0.007%, more preferably greater than 0.009%, respectively. In addition, Nb and Ti are preferably less than 0.018%, more preferably less than 0.016%, respectively.

[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 of steel and increase the strength of steel, and can be contained as needed. In order to obtain the above-mentioned effect, when containing V, Cr, and Mo, the V content is set to be more than 0.01%, the Cr content is set to be more than 0.01%, and the Mo content is set to be more than 0.01%. Preferably, the V content is more than 0.02%, the Cr content is more than 0.05%, and the Mo content is more than 0.05%, and more preferably the V content is more than 0.03%, the Cr content is more than 0.08%, and the Mo content is more than 0.08%.

[0093] On the other hand, excessive inclusion may lead to a decrease in toughness and deterioration in weldability. Therefore, when V, Cr, and Mo are contained, the V content is set to 0.10% or less, the Cr content is set to 0.50% or less, and the Mo content is set to 0.50% or less, respectively. Preferably, the V content is 0.08% or less, the Cr content is 0.40% or less, and the Mo content is 0.40% or less, and more preferably, the V content is 0.07% or less, the Cr content is 0.35% or less, and the Mo content is 0.35% or less.

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

[0095] Cu and Ni are elements that increase the strength of steel by solid solution strengthening and can be contained as needed. In order to obtain the above effect, when Cu and Ni are contained, the Cu content is set to 0.01% or more and the Ni content is set to 0.01% or more, respectively. Preferably, the Cu content is 0.02% or more and the Ni content is 0.02% or more. More preferably, the Cu content is 0.10% or more and the Ni content is 0.10% or more. On the other hand, excessive inclusion may lead to a decrease in toughness and deterioration of weldability. Therefore, when Cu and Ni are contained, the Cu content is set to 0.30% or less and the Ni content is set to 0.30% or less, respectively. Preferably, the Cu content is 0.20% or less and the Ni content is 0.20% or less. More preferably, the Cu content is 0.15% or less and the Ni content is 0.15% or less.

[0096] Ca: 0.0005% or more and 0.0100% or less

[0097] Ca is an element that contributes to the improvement of the toughness of steel by spheroidizing sulfides such as MnS that are stretched very 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 preferred to contain more than 0.0005% Ca. However, when the Ca content exceeds 0.0100%, Ca oxide clusters are sometimes formed in the steel, and the toughness deteriorates. Therefore, when Ca is contained, the Ca content is preferably set to less than 0.0100%. It should be noted that the Ca content is preferably set to more than 0.0005%. More preferably, the Ca content is more than 0.0010%. Preferably, the Ca content is less than 0.0050%.

[0098] B: 0.0003% or more and 0.0100% or less

[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 contained at least 0.0003% B. However, when the B content exceeds 0.0100%, the yield ratio sometimes increases. Therefore, when B is contained, it is preferably set to 0.0100% or less. More preferably, the B content is 0.0005% or more. 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] The steel structure at the center of the plate thickness and the front and back surfaces of the hot-rolled steel plate of the present invention comprises a main phase composed 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 the center of the plate thickness, when a region surrounded by boundaries in which the orientation difference of adjacent crystals is 15° or more is taken as a grain, the average grain size of the steel structure at the center of the plate thickness is 10.0 to 30.0 μm, the area ratio of the grains having an equivalent circle diameter of 40.0 μm or more relative to the entire steel structure at the center of the plate thickness is 20% or less, and the number of grains having a ratio of major diameter to minor diameter (ratio of major diameter to minor diameter (=(major diameter) / (minor diameter)) of 3.0 or more is 30 grains / mm 2 Below, the hardness difference between a position 1.0 mm inside from the front and back surfaces of the plate in the plate thickness direction and the center of the plate thickness is 40 HV or less.

[0102] It should be noted that in the present invention, the equivalent circle diameter (grain size) is set to the diameter of a circle having an area equal to that of the target grain. In addition, the steel structure targets the plate thickness center and plate surface (plate surface and back) of the hot-rolled steel plate. Here, the plate surface (plate surface and back) refers to the position within 1.0 mm from the surface of the steel plate in the plate thickness direction.

[0103] Main phase: Ferrite

[0104] Ferrite is a soft structure, and in order to obtain the desired yield strength and low yield ratio, it is used as the main phase in the present invention. Preferably, ferrite is 70% or more, more preferably 72% or more, in terms of area ratio. When the area ratio of ferrite exceeds 94%, the strength is sometimes reduced, and the desired yield strength and tensile strength cannot be obtained. Therefore, preferably, ferrite is 94% or less in terms of area ratio, and more preferably, ferrite is 92% or less in terms of area ratio.

[0105] Total area ratio of pearlite and pseudo-pearlite: 6-25%, area ratio of upper bainite: less than 5%

[0106] Pearlite and pseudo-pearlite are hard structures and are the most important steel structures for increasing the strength of steel and obtaining a low yield ratio. In order to obtain the target yield strength, tensile strength, and yield ratio in the present invention, it is necessary to set the total area ratio of pearlite and pseudo-pearlite to 6% or more. The total area ratio is preferably 7% or more, and more preferably 9% or more. When the area ratio of pearlite and pseudo-pearlite exceeds 25%, toughness sometimes deteriorates. Therefore, the total area ratio of pearlite and pseudo-pearlite needs to be 25% or less. The total area ratio is preferably 23% or less, and more preferably 21% or less.

[0107] In addition, the area ratio of pseudo pearlite is preferably 5% or more. When the pseudo pearlite is present in an area ratio of 5% or more, the yield ratio is suppressed to be low when manufacturing a rectangular steel pipe, so that better shock resistance can be obtained. In order to make the area ratio of pseudo pearlite exceed 15%, it is necessary to rapidly cool the temperature range in which pearlite is generated in the cooling process during hot rolling, so the manufacturing conditions are limited. Therefore, the area ratio of pseudo pearlite is preferably 15% or less.

[0108] Upper bainite is a structure with a hardness intermediate between ferrite and pearlite, which increases the strength of steel. However, when the area ratio of upper bainite exceeds 5%, the target low yield ratio in the present invention cannot be obtained. Therefore, the area ratio of upper bainite needs to be 5% or less. Preferably, it is 4% or less. The upper bainite can be 0%.

[0109] In addition, the area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite can be measured by the method described below.

[0110] In the center of the plate thickness, when the region surrounded by the boundary where the orientation difference (crystal orientation difference) of adjacent crystals is 15° or more is taken as the grain, the average grain size of the steel structure in the center of the plate thickness is 10.0 to 30.0 μm, the area ratio of the above-mentioned grains with an equivalent circle diameter of 40.0 μm or more is 20% or less, and the number of the above-mentioned grains with a ratio of major diameter to minor diameter (= (major diameter) / (minor diameter)) of 3.0 or more is 30 grains / mm 2 the following

[0111] As described above, in order to obtain the target low yield ratio, yield strength and tensile strength in the present invention, the steel structure of the present invention is set to a steel in which soft structure and hard structure are mixed (hereinafter referred to as "composite structure steel"). However, the toughness of composite structure steel is inferior to that of single structure steel. Therefore, in the present invention, in order to take into account both the above-mentioned mechanical properties and excellent toughness, when the area surrounded by the boundary with a crystal orientation difference of 15° or more is used as the grain, the crystal grain size, the area ratio of coarse grains and the number of elongated grains of the steel structure containing the main phase and the secondary phase are specified.

[0112] When the average grain size (equivalent circle diameter) of the steel structure containing the main phase and the secondary phase is less than 10.0 μm, the yield ratio increases and the target yield ratio in the present invention cannot be obtained. On the other hand, when the average grain size of the steel structure containing the main phase and the secondary phase exceeds 30.0 μm, the toughness deteriorates. Therefore, the average grain size of the steel structure containing the main phase and the secondary phase needs to be 10.0 to 30.0 μm. It is preferably 11.0 μm or more, and more preferably 12.5 μm or more. In addition, the average grain size is preferably 28.0 μm or less, and more preferably 26.0 μm or less.

[0113] However, even if the average grain size is within the range of 10.0 to 30.0 μm, the target yield ratio and Charpy absorbed energy of the present invention may not be obtained. The present inventors conducted in-depth research and found that in order to obtain the target toughness of the present invention, the area ratio of grains with an equivalent circle diameter of 40.0 μm or more relative to the entire steel structure at the center of the plate thickness should be 20% or less, and in order to obtain the target yield ratio of the present invention, the number of grains with a ratio of major diameter to minor diameter (= (major diameter) / (minor diameter)) of 3.0 or more should be 30 / mm. 2 the following.

[0114] It should be noted that the crystal orientation difference, the average crystal grain size, and the area ratio of crystal grains having a crystal grain size (equivalent circle diameter) of 40.0 μm or more can be measured by the SEM / EBSD method. Here, the measurement can be performed by the method described below.

[0115] In the microstructure observation, test pieces were cut from the center of the hot-rolled steel sheet in the width direction and at the position of t / 2 of the plate thickness (t: plate thickness) (the center of the plate thickness in the present invention) and the plate surface (the back side of the steel plate (the lower surface during hot rolling)). The observation surface was made into a cross section in the rolling direction during hot rolling, and was prepared by grinding and then etching with nitric acid.

[0116] Regarding 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 of the hot-rolled steel plate at the plate thickness t / 2 position (plate thickness center) and the back side of the steel plate (plate surface). Based on the obtained optical microscope images and SEM images, the area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite were calculated.

[0117] The area ratio of each structure was observed in 5 fields of view and calculated as the average value of the values ​​obtained in each field of view. Here, the area ratio obtained by the organization observation is used as the area ratio of each structure. Regarding the area ratio of ferrite, pearlite, pseudo-pearlite, and upper bainite, an optical microscope image or SEM image is used to distinguish each phase using the shape and color shown below and calculate the area of ​​each phase, and then the area ratio of each phase is calculated by dividing it by the overall area of ​​the optical microscope image or SEM image.

[0118] Here, ferrite is a product obtained by diffusion phase transformation, presenting a low dislocation density and basically restored structure. Ferrite includes polygonal ferrite and quasi-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 complex phase structure of lath-shaped ferrite and cementite with high dislocation density. In addition to the above-mentioned shapes, it can be distinguished by the fact that ferrite is white, pearlite is black, pseudo-pearlite is black or gray, and upper bainite is white or gray.

[0119] In addition, the average grain size (average equivalent circle diameter) was measured using the SEM / EBSD method at the plate thickness t / 2 position (plate thickness center). The measurement area was set to 500 μm × 1000 μm (= 0.5 mm 2 ), and the measurement step length is set to 0.5μm. Regarding the grain size, the orientation difference between adjacent grains is determined, and the boundary with an orientation difference of more than 15° is measured 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 as the average grain size (average equivalent circle diameter). In addition, the sum of the areas of grains with an equivalent circle diameter of more than 40.0μm is calculated and divided by the area of ​​the measurement area (0.5mm 2 ), and the area ratio of the grains with an equivalent circle diameter of 40.0 μm or more was calculated. The equivalent circle diameter and the area of ​​the grains were obtained by analyzing the data obtained by the SEM / EBSD method using OIM Analysis manufactured by EDAX.

[0120] In addition, the major diameter and minor diameter of the grains were measured by the method described in JIS R 1670 (2006), and the ratio of the major diameter to the minor diameter (= (major diameter) / (minor diameter)) was calculated. The number of grains with a major diameter to minor diameter ratio (= (major diameter) / (minor diameter)) of 3.0 or more was divided by the area of ​​the measurement area (0.5 mm 2 ), and the number of grains with a major diameter to minor diameter ratio of 3.0 or more (pieces / mm 2). It should be noted that in the crystal grain size analysis and the measurement of the number of crystal grains, crystal grains with a crystal grain size of less than 2.0 μm are excluded from the analysis object as measurement noise.

[0121] The hardness difference between the position 1.0mm inside the back surface of the plate and the center of the plate thickness in the plate thickness direction: 40HV or less

[0122] The hardness difference between the position 1.0 mm from the back surface of the plate in the plate thickness direction and the center of the plate thickness in the hot rolled steel plate of the present invention is 40 HV or less. When the hardness difference between the position 1.0 mm from the back surface of the plate in the plate thickness direction and the center of the plate thickness exceeds 40 HV, when a rectangular steel pipe is manufactured, the yield ratio of the flat plate portion of the rectangular steel pipe may exceed 0.90. In addition, toughness is sometimes deteriorated. The hardness difference between the position 1.0 mm from the back surface of the plate in the plate thickness direction and the center of the plate thickness is preferably 35 HV or less, more preferably 30 HV or less.

[0123] Here, among the hardness at a position 1.0 mm from the front and back sides of the plate in the plate thickness direction and the hardness at the center of the plate thickness, the hardness at a position 1.0 mm from the front and back sides of the plate in the plate thickness direction is greater. Therefore, in the present invention, the hardness at a position 1.0 mm from the surface of the steel plate and at a position 1.0 mm from the back side of the steel plate in the plate thickness direction are measured, and the greater hardness is taken as the hardness at a position 1.0 mm from the front and back sides of the steel plate in the plate thickness direction.

[0124] In addition, as a method for measuring the above-mentioned hardness, first, according to the provisions of JIS Z 2244 (2020), the Vickers hardness is measured with a test force of 9.8N (1kgf). Measure 10 points each, calculate the average value of 8 points other than the maximum and minimum values, and take the average value (total of the Vickers hardness of 8 points / 8) as the hardness of the test piece. The test piece uses the above-mentioned tissue observation test piece, and the position 1.0mm inside the back of the steel plate in the thickness direction is mirror-polished, and then the hardness is measured. Then, the difference (hardness difference) between the hardness of the back of the plate and the hardness of the center of the plate thickness is calculated.

[0125] By satisfying the above-mentioned component composition and steel structure and further satisfying a specific hardness difference, a hot-rolled steel sheet having the target strength, yield ratio and toughness (Charpy absorbed energy at -20°C, ductile-brittle transition temperature) of the present invention can be obtained.

[0126] Specifically, the hot-rolled steel sheet of the present invention can have a yield strength of 250 MPa or more, a tensile strength of 400 MPa or more, a yield ratio of 0.75 or less, a Charpy absorbed energy of 100 J or more at -20°C, and a ductile-brittle transition temperature of -20°C or less.

[0127] The hot-rolled steel sheet of the present invention can be suitably used as a hot-rolled steel sheet for use in a low-yield ratio rectangular steel pipe, and a rectangular steel pipe having a low yield ratio described below can be obtained.

[0128] The hot-rolled steel sheet of the present invention preferably has a thickness of 20 mm or more. In addition, the hot-rolled steel sheet of the present invention preferably has a thickness of 20 to 32 mm.

[0129] <Method for producing hot-rolled steel sheet>

[0130] 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.

[0131] In the method for manufacturing a hot-rolled steel sheet of the present invention, for example, in a hot rolling process, a steel raw material having the above-mentioned component composition is heated to a heating temperature of 1100° C. to 1300° C., and then hot rolling is performed with a rough rolling end temperature of 850° C. to 1150° C., a finish rolling end temperature of 750° C. to 850° C., and a total reduction ratio of 40% to 59% at a temperature of 930° C. or less.

[0132] Next, in the cooling process, the average cooling rate Vc (°C / s) at the center of the plate thickness and the average cooling rate Vs (°C / s) at the surface of the plate from the surface of the plate to a depth of 1.0 mm in the thickness direction satisfy the following equations (2) and (3), the hot-rolled steel plate is air-cooled for more than 5 seconds from the start of cooling to the stop of cooling, and the cooling stop temperature at the center of the plate thickness is 550°C to 680°C. Next, in the coiling process, the plate thickness center temperature is 550°C to 680°C to form a hot-rolled steel plate.

[0133] 2≤Vc≤15 …(2)

[0134] Vs / Vc≤2.0 …(3)

[0135] In the following description of the production method, "°C" related to temperature refers to the surface temperature of a steel material or a steel plate (hot-rolled plate, raw steel plate) unless otherwise specified.

[0136] These surface temperatures can be measured using a radiation thermometer or the like. In addition, the temperature of the center of the steel plate thickness is obtained by calculating the temperature distribution in the cross section of the steel plate through heat transfer analysis, and then modifying the result using the surface temperature of the steel plate. In addition, "hot-rolled steel plate" includes hot-rolled steel plate and hot-rolled steel strip.

[0137] In the present invention, the smelting method of the steel raw material (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 manufactured by known casting methods such as continuous casting. It should be noted that there is no problem in applying the ingot casting-blowing rolling method instead of the continuous casting method. The molten steel can be further subjected to secondary refining such as ladle refining.

[0138] (Hot rolling process)

[0139] Next, in the hot rolling process, the heating temperature is set to be higher than 1100°C and lower than 1300°C to heat the obtained steel raw material (steel billet), and then rough rolling is carried out with the rough rolling end temperature set to be higher than 850°C and lower than 1150°C, and finishing rolling is carried out with the finishing temperature set to be higher than 750°C and lower than 850°C, and hot rolling is carried out at a total reduction rate of higher than 40% and lower than 59% at a temperature below 930°C to produce a hot-rolled steel sheet.

[0140] Heating temperature: 1100°C or higher and 1300°C or lower

[0141] When the heating temperature is lower than 1100°C, the deformation resistance of the rolled material becomes larger and rolling becomes difficult. On the other hand, when 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 target hot-rolled steel sheet 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 size of 40.0 μm or more within the target range of the present invention. Therefore, the heating temperature in the hot rolling process is set to be above 1100°C and below 1300°C. The heating temperature in the hot rolling process is preferably above 1120°C. In addition, the heating temperature in the hot rolling process is preferably below 1280°C.

[0142] It should be noted that in the present invention, after the steel billet (steel billet) is manufactured, it is temporarily cooled to room temperature and then heated again. In addition to such existing methods, an energy-saving process of direct rolling can be applied without problem, in which the steel billet is not cooled to room temperature but is loaded into a heating furnace in a warm sheet state.

[0143] Rough rolling end temperature: 850℃ or higher and 1150℃ or lower

[0144] When the rough rolling end temperature is lower than 850°C, the surface temperature of the steel plate becomes lower than the ferrite transformation start temperature in the subsequent finish rolling, and a large amount of ferrite is generated, which becomes processed ferrite grains elongated in the rolling direction, and becomes the reason for the increase in yield ratio. On the other hand, when 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 in the present invention cannot be obtained, and it is difficult to obtain an average crystal grain size of 10.0 to 30.0 μm when the area surrounded by the boundary with an orientation difference of 15° or more of adjacent crystals is used as the grain, and the number of grains with a major axis to minor axis ratio (= (major axis) / (minor axis)) of 3.0 or more is 30 / mm 2 The following is a steel structure in which the crystal grains having an equivalent circle diameter of 40.0 μm or more account for 20% or less of the area ratio of the entire steel structure.

[0145] In addition, it is difficult to suppress the formation of coarse bainite. Therefore, the rough rolling end temperature is set to 850°C or more and 1150°C or less. The rough rolling end temperature is preferably 860°C or more, more preferably 870°C or more. In addition, the rough rolling end temperature is preferably 1100°C or less, more preferably 1050°C or less.

[0146] Finish rolling end temperature: 750℃ or higher and 850℃ or lower

[0147] When the finishing temperature of the finishing rolling is lower than 750°C, the surface temperature of the steel plate during the finishing rolling becomes lower than the ferrite transformation start temperature, ferrite elongated in the rolling direction is formed, and the workability may be reduced. On the other hand, when 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 target strength 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 be above 750°C and below 850°C. The finishing temperature of the finishing rolling is preferably above 770°C, and more preferably above 780°C. In addition, the finishing temperature of the finishing rolling is preferably below 830°C, and more preferably below 820°C.

[0148] Total reduction ratio below 930°C: 40% or more and 59% or less

[0149] In the present invention, the steel structure of the hot-rolled steel sheet having the target strength and toughness in the present invention is obtained by refining the subgrains in the austenite in the hot rolling process, thereby refining the ferrite and bainite generated in the subsequent cooling process and coiling process. In order to refine the subgrains in the austenite in the hot rolling process, it is necessary to increase the reduction rate in the austenite non-recrystallization temperature range and introduce sufficient processing strain. However, when 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. The total reduction rate below 930°C is preferably 57% or less, and more preferably 55% or less. When the total reduction rate below 930°C is less than 40%, the grain size of ferrite and bainite becomes larger, resulting in a decrease in toughness. Therefore, the total reduction rate below 930°C is set to 40% or more. The total reduction ratio at 930° C. or lower is preferably 42% or more, more preferably 45% or more.

[0150] The temperature is set to 930° C. or lower because, if the temperature exceeds 930° C., austenite recrystallizes during the rolling process, dislocations introduced by rolling disappear, and fine austenite cannot be obtained.

[0151] The above-mentioned total reduction ratio can be used in the rolling pass at a temperature range of 930°C or less using the plate thickness T before the start of rolling. E (mm), final plate thickness after rolling T O (mm) is calculated by the following formula.

[0152] Total reduction rate (%) = 100 × (T E -T O ) / T E

[0153] It should be noted that when the steel slab is hot rolled, the total reduction ratio at 930°C or less in both the rough rolling and the finish rolling may be set to 40% or more and 59% or less. Alternatively, hot rolling may be performed in which the total reduction ratio at 930°C or less is set to 40% or more and 59% or less only in the finish rolling. In the latter case, if the total reduction ratio at 930°C or less cannot be set to 40% or more and 59% or less by the finish rolling alone, the steel slab is cooled in the middle of the rough rolling to a temperature of 930°C or less, and then the total reduction ratio at 930°C or less in both the rough rolling and the finish rolling is set to 40% or more and 59% or less.

[0154] In the present invention, the upper limit of the finished rolled plate thickness (plate thickness after finish rolling) is not particularly specified, but from the viewpoint of ensuring the necessary rolling reduction and controlling the temperature of the steel plate, the finished rolled plate thickness is preferably set to 32 mm or less.

[0155] (Cooling process)

[0156] After the hot rolling process, the hot rolled plate (raw steel plate for hot rolled steel plate, hereinafter referred to as raw steel plate) is cooled by a cooling process. In the cooling process, cooling is performed as follows: the average cooling rate Vc of the center of the plate thickness to the cooling stop temperature is set to 2°C / s or more and 15°C / s or less, the average cooling rate Vs of the plate surface layer from the back surface of the plate to a depth of 1.0 mm in the thickness direction to the cooling stop temperature satisfies Vs / Vc≤2.0, the hot rolled steel plate is air cooled for more than 5 seconds from the start of cooling to the stop of cooling, and the cooling stop temperature of the center of the plate thickness is set to 550°C or more and 680°C or less.

[0157] Average cooling rate Vc from the start of cooling to the cooling stop temperature (550-680°C) at the center of the plate thickness: 2°C / s or more and 15°C / s or less

[0158] When the average cooling rate Vc in the temperature range from the start of cooling to the cooling stop temperature (550-680°C) described later is less than 2°C / s using a thermometer at the center of the plate thickness of the hot-rolled steel plate, the nucleation frequency of ferrite is reduced, and the ferrite grains are coarsened, so the desired strength cannot be obtained. In addition, it is difficult to control the area ratio of grains with an average crystal grain size of 40.0 μm or more as the target in the present invention within the desired range. On the other hand, when the average cooling rate Vc exceeds 15°C / s, a large amount of upper bainite is generated, and the target yield ratio in the present invention cannot be obtained. The average cooling rate Vc is preferably greater than 4°C / s, and more preferably greater than 5°C / s. Preferably less than 12°C / s, more preferably less than 10°C / s.

[0159] Vs / Vc≤2.0

[0160] When the average cooling rate Vs (°C / s) of the surface of the plate from the start of cooling to the cooling stop temperature (550-680°C) exceeds 2.0 times the average cooling rate Vc (°C / s) of the center of the plate thickness until the cooling stop, a large amount of bainite is generated in the surface of the plate, and the target steel structure of the present invention cannot be obtained, and the desired yield ratio and Charpy absorbed energy cannot be obtained.

[0161] Preferably, Vs / Vc≤1.8, more preferably Vs / Vc≤1.7. The lower limit is not particularly specified. Since the hot-rolled steel plate is cooled from the front and back sides, the average cooling rate Vs of the plate surface until the cooling stops is a value greater than the average cooling rate Vc of the plate thickness center until the cooling stops (1.0≤Vs / Vc). Here, the average cooling rate Vs of the plate surface until the cooling stops uses the larger value of the average cooling rate at a position within 1.0 mm from the plate surface or the plate back.

[0162] In addition, 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 is completed.

[0163] Air cooling time: more than 5s

[0164] In the cooling process, the hot-rolled steel plate is air-cooled for more than 5 seconds, thereby reducing the temperature difference between the surface layer of the hot-rolled plate and the center of the plate thickness, and a hot-rolled steel plate having the target mechanical properties of the present invention can be obtained. When the air-cooling time is less than 5 seconds, the temperature difference between the surface layer of the plate and the center of the plate thickness becomes large, and the desired yield ratio cannot be obtained. The air-cooling time is preferably more than 10 seconds. There is no particular upper limit, but when the air-cooling time exceeds 100 seconds, the productivity is significantly reduced, so the air-cooling time is preferably less than 100 seconds. More preferably, it is less than 90 seconds. It should be noted that air cooling is carried out from the start of cooling to the stop of cooling.

[0165] The air cooling is not particularly limited, but refers to cooling at a rate of 0.01 to 0.90° C. / s at the surface layer of the plate.

[0166] Cooling stop temperature: 550°C or higher and 680°C or lower

[0167] When the cooling stop temperature is lower than 550°C, the temperature is easily uneven in the length direction and / or width direction of the hot rolled plate during cooling, and the mechanical properties may be deviated. On the other hand, when the cooling stop temperature exceeds 680°C, the ferrite grains are coarsened and the desired average crystal grain size cannot be obtained. The cooling stop temperature is preferably 560°C or higher, more preferably 580°C or higher, based on the plate thickness center temperature of the hot rolled plate. It is preferably below 660°C, more preferably below 650°C.

[0168] It should be noted that in the present invention, the average cooling rate is a value obtained by ((temperature of the hot-rolled steel plate before cooling (at the start of cooling) (°C) - temperature of the hot-rolled steel plate up to the cooling stop temperature (550-680°C) (°C)) / cooling time (s)), which can be calculated based on the temperature distribution in the cross section of the hot-rolled steel plate obtained by heat transfer analysis. The cooling time used when calculating the average cooling rate also includes the time for air cooling.

[0169] The cooling method may include water cooling such as spraying water from a nozzle, cooling by spraying a cooling gas, etc. In the present invention, it is preferred to implement cooling operation (treatment) on both sides of the hot rolled plate in a manner that both sides of the hot rolled plate are cooled under the same conditions. In addition, in order to obtain the above-mentioned cooling rate, the amount, pressure, spraying time and angle of cooling water or cooling gas and the conveying speed of the hot rolled steel plate are adjusted. If cooling water or cooling gas is always sprayed to a specific position on the surface of the hot rolled steel plate, or a large amount of cooling water or cooling gas is instantly sprayed to the surface of the hot rolled steel plate, the surface of the steel plate is rapidly cooled, so the difference in cooling rate between the surface layer of the plate and the center of the plate thickness becomes larger. Therefore, by appropriately configuring the position of the nozzle spraying cooling water or cooling gas in a manner that a specific position of the hot rolled steel plate is not rapidly cooled, and appropriately adjusting the amount and time of spraying cooling water or cooling gas, the difference in cooling rate between the surface layer of the plate and the center of the plate thickness can be controlled within a specified range. After the conditions for the cooling treatment of the hot rolled steel plate are determined by pre-heat transfer analysis in a manner to obtain the cooling rate specified in the present invention, the conditions are reflected in the manufacturing conditions.

[0170] (Coiling process)

[0171] After the cooling process, the hot-rolled steel sheet is coiled in the coiling process and then cooled. In the coiling process, from the perspective of the steel sheet structure, the coiling temperature is set to be above 550°C and below 680°C based on the plate thickness center temperature of the hot-rolled steel sheet for coiling. When the coiling temperature is lower than 550°C, a large amount of upper bainite is sometimes generated on the surface of the steel sheet, and the area ratio exceeds 5%. When the coiling temperature exceeds 680°C, the ferrite grains are coarsened and the desired grain size cannot be obtained. The coiling temperature is more preferably above 570°C, and further preferably above 580°C. In addition, the coiling temperature is more preferably below 660°C, and further preferably below 650°C.

[0172] <Rectangular steel pipe>

[0173] The rectangular steel pipe of the present invention uses the hot-rolled steel plate of the present invention as a raw material. The rectangular steel pipe of the present invention has a yield strength of 295 MPa or more and a tensile strength of 400 MPa or more in the flat plate portion in the pipe axis direction, and shows a low yield ratio of 0.90 or less. In the flat plate portion, the absorbed energy of the Charpy impact test at a test temperature of -20°C is 40 J or more and the ductile-brittle transition temperature is -5°C or less. The flat plate portion can be suitably used as a structural member of a building in a low temperature environment such as a cold area with a temperature below freezing.

[0174] As a method for manufacturing a rectangular steel pipe of the present invention, in the pipe making process, a hot rolled steel plate is formed into a cylindrical open pipe (round steel pipe) by roll forming, and the butted portions are resistance welded. Then, the round steel pipe is formed into a rectangular shape by drawing by several percent in the pipe axial direction while maintaining the cylindrical shape by rollers arranged above, below, left and right of the round steel pipe to obtain a rectangular steel pipe.

[0175] For example, after the hot rolled steel sheet of the present invention in coil form is formed into a circle by cold rolling forming method using rollers to manufacture a circular steel pipe, the circular steel pipe is formed into a rectangle by roll forming method using rollers to manufacture a rectangular steel pipe. If cold rolling forming is performed on the circular steel pipe, a large processing strain is introduced in the pipe axis direction, so there is a problem that the yield ratio in the pipe axis direction is easily increased and the toughness is easily reduced. However, for the rectangular steel pipe of the present invention, since the hot rolled steel sheet of the present invention is used as the raw material, the increase in the yield ratio is suppressed, and for example, even a thick-walled steel pipe of more than 20 mm can have a low yield ratio and low-temperature toughness.

[0176] It should be noted that the rectangular steel tube of the present invention is not limited to rectangular steel tubes whose sides are all equal in length (the value of (long side length / short side length) is 1.0) in the vertical cross-sectional view in the tube axis direction, but also includes rectangular steel tubes whose value of (long side length / short side length) exceeds 1.0. However, when the value of (long side length / short side length) of the rectangular steel tube exceeds 2.5, local buckling is likely to occur on the long side, and the compressive strength in the tube axis direction is reduced. Therefore, the value of (long side length / short side length) of the rectangular steel tube is preferably set to be greater than 1.0 and less than 2.5. The value of (long side length / short side length) is more preferably greater than 1.0 and less than 2.0.

[0177] The rectangular steel pipe of the present invention is manufactured as described above. According to the present invention, a rectangular 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 Charpy absorbed energy of 40 J or more at -20°C in the flat plate portion, and a ductile-brittle transition temperature of -5°C or less in the flat plate portion can be obtained. The ductile-brittle transition temperature of the rectangular steel pipe of the present invention is lower than 0°C, and therefore it can be suitably used as a structural member of a building in a cold region where the temperature is below freezing.

[0178] <Building Structures>

[0179] Figure 1 This is a schematic diagram showing an example of the building structure of the present invention.

[0180] The building structure of the present invention uses the rectangular steel pipe (low yield ratio rectangular steel pipe) 1 of the present invention as a column material. Reference numerals 4, 5, 6, and 7 represent beams, small beams, partitions, and columns, respectively.

[0181] As described above, the flat plate portion of the rectangular steel pipe of the present invention has excellent mechanical properties. Therefore, the building structure of the present invention using the rectangular steel pipe as a column material exhibits excellent earthquake resistance.

[0182] Example

[0183] Hereinafter, the present invention will be described in further detail based on examples. It should be noted that the present invention is not limited to the following examples.

[0184] Molten steel having the component composition shown in Table 1 is cast to form a steel slab. The obtained steel slab is subjected to a hot rolling process, a cooling process, and a coiling process under the conditions shown in Table 2 to form a hot rolled steel sheet. After the coiling process, the pipe making process shown below is performed. It should be noted that the total reduction ratio of 930°C or less specified in the hot rolling process is the total reduction ratio in the finish rolling only.

[0185]

[0186]

[0187] The obtained hot-rolled steel sheet was formed into a cylindrical round steel pipe by roll forming, and the butted portions were resistance welded. Then, the round steel pipe was formed into a rectangular shape (a square in a cross-sectional view perpendicular to the pipe axis direction) by rollers arranged above, below, and on the left and right sides of the round steel pipe, to obtain a roll-formed rectangular steel pipe having a corner portion and a flat plate portion and having a side length (mm) and a wall thickness (mm) shown in Table 4.

[0188] Test pieces were cut out from the obtained hot-rolled steel sheets, and the following microstructure observation, hardness measurement, tensile test, and Charpy impact test were performed.

[0189] [Organization Observation]

[0190] The test piece for tissue observation is cut from the central part of the hot-rolled steel plate in the width direction and the position of the plate thickness t / 2 (t: plate thickness) (the center part of the plate thickness in the present invention) and the position of the back side of the steel plate (the lower surface during hot rolling). The structure of the steel plate surface (the upper surface during hot rolling) and the back side of the steel plate (the lower surface during hot rolling) are the same, and the test piece for tissue observation can be cut from either side of the steel plate surface or back side. Here, the cutting position of the test piece for tissue observation is unified as the back side of the steel plate (the lower surface during hot rolling). After grinding in a manner that the observation surface is a vertical section parallel to the rolling direction during hot rolling (the normal direction of the observation surface is the section in the plate width direction), it is corroded with nitric acid alcohol solution to produce it.

[0191] It should be noted that regarding which of the two surfaces of the finally obtained steel plate is the back surface, in the coiling after cooling, the hot-rolled surface side (upper surface side) is coiled to be the outer side of the hot-rolled coil. Therefore, if a part of the hot-rolled steel plate after coiling is cut, an arc-shaped steel plate is obtained. Therefore, the outer side and the inner side of the hot-rolled coil are distinguished according to the shape of the arc-shaped steel plate, and the front and back sides during rolling are determined.

[0192] Regarding 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 of the hot-rolled steel plate at the plate thickness t / 2 position (plate thickness center) and the back side of the steel plate (plate surface and back side). Based on the obtained optical microscope images and SEM images, the area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite were calculated.

[0193] The area ratio of each structure was observed in 5 fields of view and calculated as the average value of the values ​​obtained in each field of view. Here, the area ratio obtained by the organization observation is used as the area ratio of each structure. Regarding the area ratio of ferrite, pearlite, pseudo-pearlite, and upper bainite, an optical microscope image or SEM image is used to distinguish each phase according to the shape and color shown below and calculate the area of ​​each phase, and then the area ratio of each phase is calculated by dividing it by the overall area of ​​the optical microscope image or SEM image.

[0194] Here, ferrite is a product obtained by diffusion phase transformation, presenting a low dislocation density and a basically restored structure. Ferrite includes polygonal ferrite and quasi-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 lath-shaped complex phase structure of ferrite and cementite with high dislocation density. In addition to the above-mentioned shapes, it can be distinguished by ferrite being white, pearlite being black, pseudo-pearlite being black or gray, and upper bainite being white or gray.

[0195] In addition, the average grain size (average equivalent circle diameter) was measured using the SEM / EBSD method at the plate thickness t / 2 position (plate thickness center). The measurement area was set to 500 μm × 1000 μm (= 0.5 mm 2 ), and the measurement step length is set to 0.5μm. Regarding the grain size, the orientation difference between adjacent grains is determined, and the boundary with an orientation difference of more than 15° is measured 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 as the average grain size (average equivalent circle diameter). In addition, the sum of the areas of grains with an equivalent circle diameter of more than 40.0μm is calculated and divided by the area of ​​the measurement area (0.5mm 2), and the area ratio of grains with an equivalent circle diameter of 40.0 μm or more was calculated. The equivalent circle diameter and the area of ​​the grains were calculated by analyzing the data obtained by the SEM / EBSD method using OIM Analysis manufactured by EDAX.

[0196] In addition, the major diameter and minor diameter of the grains were measured by the method described in JIS R 1670 (2006), and the ratio of the major diameter to the minor diameter (= (major diameter) / (minor diameter)) was calculated. The number of grains with a major diameter to minor diameter ratio (= (major diameter) / (minor diameter)) of 3.0 or more was divided by the area of ​​the measurement area (0.5 mm 2 ), and the number of grains with a major diameter to minor diameter ratio of 3.0 or more (pieces / mm 2 ). It should be noted that in the crystal grain size analysis and the measurement of the number of crystal grains, crystal grains with a crystal grain size of less than 2.0 μm are excluded from the analysis object as measurement noise.

[0197] [Tensile test]

[0198] A JIS No. 5 tensile test piece was cut from the obtained hot-rolled steel sheet in such a way that the tensile direction was parallel to the rolling direction. For the cut tensile test piece, a tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011), and the yield strength YS and tensile strength TS were measured, and the yield ratio defined by (yield strength) / (tensile strength) was calculated. It should be noted that the number of test pieces was set to 2 each, and their average values ​​were calculated to obtain YS, TS, and yield ratio.

[0199] [Charpy impact test]

[0200] A V-notch standard test piece according to the provisions of JIS Z 2242 (2018) was cut at the t / 2 position (center of the plate thickness) of the obtained hot-rolled steel plate in such a way that the length direction of the test piece is parallel to the rolling direction. According to the provisions of JIS Z2242 (2018), the Charpy impact test was carried out under the conditions of 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.

[0201] [hardness]

[0202] According to the provisions of JIS Z 2244 (2020), the Vickers hardness is measured with a test force of 9.8N (1kgf). 10 points are measured each, and the average value of 8 points other than the maximum and minimum values ​​is calculated, and the average value (total of the Vickers hardness of 8 points / 8) is used as the hardness of the test piece. The test piece uses the above-mentioned tissue observation test piece, and the position inside 1.0mm from the back of the steel plate in the thickness direction and the center of the thickness is mirror-polished, and then the hardness is measured. For the hardness of the position inside 1.0mm from the back of the steel plate in the thickness direction, the larger hardness is used as the hardness of the position inside 1.0mm from the back of the steel plate in the thickness direction.

[0203] In addition, the difference (hardness difference) between the hardness at a position 1.0 mm inside from the front and back sides of the steel plate in the plate thickness direction and the hardness at the center of the plate thickness was calculated.

[0204] The obtained results are shown in Table 3.

[0205]

[0206] In addition, test pieces were cut out from the obtained rectangular steel pipe (roll-formed rectangular steel pipe), and the following tensile test and Charpy impact test were performed.

[0207] [Tensile test]

[0208] Figure 2 It is a schematic diagram showing the cutting position of the tensile test piece of the flat plate portion.

[0209] In the tensile test, Figure 2 As shown in FIG. 1 , a JIS No. 5 tensile test piece is cut from the flat plate portion of the rectangular steel pipe in a manner that the tensile direction is parallel to the tube axis direction. For the cut tensile test piece, a tensile test is carried out 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 tensile test piece of the flat plate portion is taken from the position of the width center of the flat plate portion in the 3 o'clock side portion when the welded portion of the rectangular steel pipe is set to the 12 o'clock direction ( Figure 2 It should be noted that the number of test pieces was set to 2 each, and their average values ​​were calculated to obtain YS, TS, and yield ratio.

[0210] [Charpy impact test]

[0211] Figure 3 It is a schematic diagram showing the cutting position of the Charpy test piece.

[0212] In the Charpy impact test, the Figure 3The V-notch standard test piece (shown in FIG. 1 ) is cut from the flat plate of a rectangular steel pipe at a position t / 4 of the wall thickness t from the outer surface of the rectangular steel pipe so that the length direction of the test piece is parallel to the pipe axis direction according to the provisions of JIS Z 2242 (2018) Figure 3 In the figure, refer to the symbol Y). 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, 0°C, and +20°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 calculated.

[0213] The obtained results are shown in Table 4.

[0214]

[0215] In Tables 3 and 4, Steel Nos. 1 to 20 are examples of the present invention, and Steel Nos. 21 to 45 are comparative examples.

[0216] As shown in Table 3, the hot-rolled steel sheets of the examples of the present invention are all: the steel structure in the center of the plate thickness and the surface of the plate contains pearlite and pseudo-pearlite with a total area ratio of 6 to 25% and upper bainite with an area ratio of 5% or less, and when the area surrounded by the boundary of adjacent crystals with an orientation difference of 15° or more in the center of the plate thickness is taken as the grain, the average grain size of the steel structure in the center of the plate thickness is 10.0 to 30.0 μm, the area ratio of grains with an equivalent circle diameter of 40.0 μm or more is 20% or less, and the number of grains with a ratio of major axis to minor axis (= (major axis) / (minor axis)) of 3.0 or more is 30 grains / mm 2 The hardness difference between the steel plate surface and the center of the plate thickness is 40 HV or less. In addition, the yield strength is 250 MPa or more, the tensile strength is 400 MPa or more, the yield ratio is 0.75 or less, the Charpy absorbed energy at -20°C is 100 J or more, and the ductile-brittle transition temperature is -20°C or less.

[0217] In addition, as shown in Table 4, the rectangular steel pipes manufactured using the hot-rolled steel plates of the examples of the present invention all have: the yield strength of the flat plate portion is greater than 295 MPa, the tensile strength of the flat plate portion is greater than 400 MPa, the yield ratio of the flat plate portion is less than 0.90, the Charpy absorbed energy of the flat plate portion at -20°C is greater than 40 J, and the ductile-brittle transition temperature of the flat plate portion is less than -5°C.

[0218] In Comparative Example No. 21 (Steel U), Mn / Si=0.8, which is outside the range of the present invention, so the total area ratio of pearlite and pseudo-pearlite is less than 6%, and the tensile strength and yield ratio do not reach the desired values.

[0219] In Comparative Example No. 22 (Steel V), Mn / Si=7.9, which is outside the range of the present invention, the total area ratio of pearlite and pseudo-pearlite exceeds 25%, and the Charpy absorption energy at -20°C and the ductile-brittle transition temperature do not reach the desired values.

[0220] In the comparative example No. 23 (steel W), the C content exceeds the range of the present invention, so the area ratio of the second phase is outside the range of the present invention. In addition, the area ratio of grains with a grain size (equivalent circle diameter) of more than 40.0 μm exceeds 20%, and the yield ratio, Charpy absorption energy at -20°C and ductile-brittle transition temperature do not reach the expected values.

[0221] In Comparative Example No. 24 (Steel X), the Si content exceeded the range of the present invention, so the area ratio of pseudo-pearlite increased excessively, and the Charpy absorption energy at -20°C and the ductile-brittle transition temperature did not reach the desired values.

[0222] In the comparative example No. 25 (steel Y), the Mn content exceeds the range of the present invention, so the area ratio of upper bainite exceeds 5%. As a result, the number of grains with a major diameter to minor diameter ratio of 3.0 or more exceeds 30 / mm. 2 , the hardness difference exceeds 40HV, and the yield ratio does not reach the expected value.

[0223] In the comparative example No. 26 (steel Z), the Nb content exceeds the range of the present invention, and the amount of upper bainite exceeds 5%. As a result, the number of grains with a major diameter to minor diameter ratio of 3.0 or more exceeds 30 / mm. 2 , the hardness difference exceeds 40HV, and the Charpy absorption energy and ductile-brittle transition temperature at -20℃ do not reach the expected values.

[0224] In the comparative example No. 27 (steel AA), the Ti content exceeded the range of the present invention, so the area ratio of upper bainite exceeded 5%, and it is believed that coarse carbides and nitrides were formed. As a result, the number of grains with a major diameter to minor diameter ratio of 3.0 or more exceeded 30 / mm. 2 , the hardness difference exceeds 40HV, as a result, the Charpy absorption energy and ductile-brittle transition temperature at -20℃ do not reach the expected values.

[0225] In the comparative example No. 28 (steel AB), the V content exceeds the range of the present invention, so the area ratio of upper bainite exceeds 5%. As a result, the number of grains with a major diameter to minor diameter ratio of 3.0 or more exceeds 30 / mm. 2 , the hardness difference exceeds 40HV, and the Charpy absorption energy and ductile-brittle transition temperature at -20℃ do not reach the expected values.

[0226] In the comparative example No. 29 (steel AC), the Cr content exceeds the range of the present invention, so the area ratio of upper bainite exceeds 5%. As a result, the number of grains with a major axis to minor axis ratio of 3.0 or more exceeds 30 / mm. 2 , the hardness difference exceeds 40HV, and the Charpy absorption energy and ductile-brittle transition temperature at -20℃ do not reach the expected values.

[0227] In the comparative example No. 30 (steel AD), the Mo content exceeds the range of the present invention, so the area ratio of upper bainite exceeds 5%. As a result, the number of grains with a major diameter to minor diameter ratio of 3.0 or more exceeds 30 / mm. 2 , the hardness difference exceeds 40HV, and the Charpy absorption energy and ductile-brittle transition temperature at -20℃ do not reach the expected values.

[0228] In Comparative Example No. 31 (Steel AE), the Cu content exceeded the range of the present invention, so it is considered that Cu was coarsely precipitated. As a result, the Charpy absorption energy at -20°C and the ductile-brittle transition temperature did not reach the desired values.

[0229] In the comparative example No. 32 (steel AF), the Ni content exceeds the range of the present invention, so the area ratio of upper bainite exceeds 5%. As a result, the number of grains with a major diameter to minor diameter ratio of 3.0 or more exceeds 30 / mm. 2 , the hardness difference exceeds 40HV, and the ductile-brittle transition temperature does not reach the expected value.

[0230] In Comparative Example No. 33 (Steel AG), the Ca content exceeded the range of the present invention, and therefore Ca oxide clusters were considered to have formed. As a result, the Charpy absorption energy at -20°C and the ductile-brittle transition temperature did not reach the desired values.

[0231] In the comparative example No. 34 (steel AH), the B content exceeds the range of the present invention, so the area ratio of upper bainite exceeds 5%. As a result, the number of grains with a major axis to minor axis ratio of 3.0 or more exceeds 30 / mm. 2 , the hardness difference exceeds 40HV, and the yield ratio does not reach the expected value.

[0232] In the comparative example No. 35 (steel AI), the C content is lower than the range of the present invention, so the total area ratio of pearlite and pseudo-pearlite is less than 6%, the average grain size exceeds 30.0 μm, and the area ratio of grains with a grain size (equivalent circle diameter) of 40.0 μm or more exceeds 20%, and the yield strength, tensile strength, yield ratio, Charpy absorption energy at -20°C and ductile-brittle transition temperature do not reach the desired values.

[0233] In the comparative example No. 36 (steel C), the billet heating temperature exceeded the range of the present invention, the grains were coarsened, the average grain size exceeded 30.0 μm, and the area ratio of grains with a grain size (equivalent circle diameter) of 40.0 μm or more exceeded 20%. As a result, the yield strength, tensile strength, Charpy absorption energy at -20°C, and ductile-brittle transition temperature did not reach the expected values.

[0234] In the comparative example No. 37 (steel C), the finishing temperature of the finish rolling exceeds the range of the present invention, so the total reduction rate below 930°C is lower than the range of the present invention, the formation of coarse bainite cannot be suppressed, the average grain size exceeds 30.0 μm, and the area ratio of grains with a grain size (equivalent circle diameter) of 40.0 μm or more exceeds 20%. As a result, the Charpy absorption energy and ductile-brittle transition temperature at -20°C did not reach the desired values.

[0235] In the comparative example No. 38 (steel C), the total reduction ratio at 930°C or less exceeded the range of the present invention, and coarse bainite was generated that extended in the rolling direction. The number of grains with a major axis / minor axis ratio of 3.0 or more exceeded 30 / mm. 2 As a result, the ductile-brittle transition temperature does not reach the desired value.

[0236] In the comparative example No. 39 (steel C), the average cooling rate Vc of the plate thickness center portion exceeds the range of the present invention, and the air cooling time is less than the range of the present invention, so the area ratio of upper bainite exceeds 5%, which is outside the range of the present invention. As a result, the hardness difference exceeds 40 HV, and the yield ratio does not reach the desired value.

[0237] In the comparative example No. 40 (steel C), Vs / Vc exceeds the range of the present invention, so the area ratio of upper bainite exceeds 5%, and it is considered that a large amount of upper bainite is generated in the surface layer of the steel plate. As a result, the hardness difference exceeds 40 HV, and the yield ratio, Charpy absorbed energy at -20°C, and ductile-brittle transition temperature do not reach the expected values.

[0238] In the comparative example No. 41 (steel C), the cooling stop temperature and the coiling temperature were lower than the range of the present invention, so the area ratio of upper bainite exceeded 5%. As a result, the hardness difference exceeded 40 HV, and the yield ratio did not reach the desired value.

[0239] In Comparative Example No. 42 (Steel C), the cooling stop temperature and the coiling temperature were outside the range of the present invention, so the average grain size exceeded 30.0 μm. As a result, the yield strength, tensile strength, Charpy absorbed energy at -20°C, and ductile-brittle transition temperature did not reach the desired values.

[0240] In the comparative example No. 43 (steel C), the average cooling rate Vc in the center of the plate thickness is lower than the range of the present invention, so the average grain size exceeds 30.0 μm, and the area ratio of grains larger than 40.0 μm exceeds 20%. In addition, the total area ratio of pearlite and pseudo-pearlite in the center of the plate thickness is less than 6%, and the yield strength, tensile strength, Charpy absorbed energy at -20°C and ductile-brittle transition temperature do not reach the expected values.

[0241] In the comparative example No. 44 (Steel C), the total reduction rate below 930°C exceeds the scope of the present invention, and the average cooling rate Vc in the center of the plate thickness greatly exceeds the scope of the present invention. Therefore, the area ratio of upper bainite exceeds 5%. In addition, the average grain size is less than 10.0 μm, and the yield ratio does not reach the expected value.

[0242] In the comparative example No. 45 (steel C), the air cooling time is shorter than the range of the present invention, so it is estimated that the temperature difference between the surface layer of the plate and the middle layer of the plate thickness becomes larger. As a result, a large amount of bainite is generated in the surface layer of the steel plate, resulting in a hardness difference exceeding 40 HV, and the yield ratio does not reach the desired value.

[0243] Explanation of symbols

[0244] 1 Rectangular steel tube

[0245] 4 Beam

[0246] 5 Trabeculae

[0247] 6. Partition

[0248] 7 columns

Claims

1. A hot-rolled steel sheet, comprising, by mass%, C: 0.07% to 0.20%, Si: 0.40% to 0.40%, Mn: 0.20% to 1.00%, P: 0.100% to 0.100%, S: 0.050% to 0.050%, Al: 0.005% to 0.100%, N: 0.0100% to 0.0100%, or further comprising Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, V: 0.01% The composition is a composition in which the content of Mn and Si satisfies the following formula (1), and the balance is Fe and unavoidable impurities. The steel structure at the center of the plate thickness and the surface and back of the plate has a main phase composed of ferrite, a second phase with a total area ratio of pearlite and pseudo-pearlite of 6 to 25% and an area ratio of upper bainite of 5% or less, When the region surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more in the center of the plate thickness is regarded as a crystal grain, The average grain size of the steel structure in the center of the plate thickness is 10.0 to 30.0 μm, The crystal grains having an equivalent circle diameter of 40.0 μm or more account for 20% or less in terms of area ratio, and The number of the crystal grains having a ratio of the major diameter to the minor diameter, i.e., (major diameter) / (minor diameter) of 3.0 or more is 30 grains / mm 2 the following, The hardness difference between the position 1.0mm inside the back surface of the plate and the center of the plate thickness in the plate thickness direction is less than 40HV. 1.0≤%Mn / %Si≤3.5 …(1) Here, %Mn and %Si are the contents (mass %) of the respective elements.

2. The hot rolled steel sheet according to claim 1, wherein: The yield strength is above 250MPa. The tensile strength is above 400MPa. The yield ratio is below 0.

75. The Charpy absorption energy at -20℃ is more than 100J. The ductile-brittle transition temperature is below -20°C.

3. The hot rolled steel sheet according to claim 1 or 2, wherein: The steel structure has 5 to 15% of the pseudo pearlite in terms of area ratio. 4 . The hot-rolled steel sheet according to claim 1 , which has a sheet thickness of 20 mm or more.

5. A method for producing a hot-rolled steel sheet, which is the method for producing a hot-rolled steel sheet according to any one of claims 1 to 4, comprising: A hot rolling process of heating a steel material under the condition of a heating temperature of 1100° C. to 1300° C., and then performing hot rolling with a rough rolling end temperature of 850° C. to 1150° C., a finish rolling end temperature of 750° C. to 850° C., and a total rolling reduction of 40% to 59% at a temperature of 930° C. or less; Then, a cooling step is performed under the conditions that the average cooling rate Vc (°C / s) of the center of the plate thickness and the average cooling rate Vs (°C / s) of the plate surface layer from the front and back sides of the plate to a depth of 1.0 mm in the thickness direction satisfy the following equations (2) and (3), the hot-rolled steel plate is air-cooled for more than 5 seconds from the start of cooling to the stop of cooling, and the cooling stop temperature of the center of the plate thickness is 550°C to 680°C; and Next, the coiling step is performed under the condition that the plate thickness center temperature is 550°C or higher and 680°C or lower. 2≤Vc≤15 …(2) Vs / Vc≤2.0 …(3).

6. A rectangular steel pipe using the hot-rolled steel plate according to any one of claims 1 to 4 as a raw material.

7. A method for manufacturing a rectangular steel tube, wherein: A rectangular steel pipe is obtained by cold rolling the hot-rolled steel sheet according to any one of claims 1 to 4.

8. A building structure using the rectangular steel pipe according to claim 6 as a column material.

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