Thick steel plate for square steel pipe

Abstract

To provide a thick steel plate for a square steel pipe, the thick steel plate having high strength and ductility and enabling the occurrence of constriction thereof to be suppressed.SOLUTION: A thick steel plate for a square steel pipe is provided, having two rolled surface and a chemical composition, in mass %, C: 0.04% or more and less than 0.13%, Si: 0.10 to 0.36%, Mn: 1.00 to 1.90%, P: 0.030% or less, S: 0.010% or less, Al: 0.010 to 0.100%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.030%, N: 0.010% or less, and the balance: Fe and impurities, wherein the carbon equivalent CeqWES is 0.36 to 0.50%, the weld crack susceptible composition Pcm is 0.17 to 0.25%, the uniform elongation uELs of the surface layer portion on at least one of the rolled surfaces is 5.0% or more, the tensile strength TS is 550 to 740 MPa, the yield stress YS is 385 to 540 MPa, and the plate thickness is 33 to 70 mm.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a thick steel plate for square steel pipes, in particular to a 55k or 60k class thick steel plate for square steel pipes having a tensile strength of 550 to 740 MPa and a yield stress of 385 to 540 MPa, which is used for architectural structures such as columns of high-rise buildings. [Background technology]

[0002] Square steel pipes used in building structures and the like are manufactured by cold pressing thick steel plates. Specifically, as shown in Figure 1, the thick steel plate is pressed to be bent at 90 degrees to form a corner (hereinafter also referred to as the "bent R part") and make it U-shaped. Next, the ends of the two U-shaped thick steel plates are welded together to make a square steel pipe.

[0003] Square steel pipes used in building structures and the like are sometimes required to have various properties such as strength, ductility, toughness, etc., from the viewpoint of the strength of the building, etc. For this reason, as described in Patent Documents 1 to 4, steel plates for square steel pipes and square steel pipes with the above-mentioned properties have been developed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-11439 A [Patent Document 2] JP 2012-102393 A [Patent Document 3] JP 2014-31546 A [Patent Document 4] JP 2011-94214 A Summary of the Invention [Problem to be solved by the invention]

[0005] Conventionally, the mainstream steel pipes have been made of relatively thin and low-strength steel plates, as described in Patent Document 1. However, in recent years, as architectural structures have become larger, there is a demand for steel pipes made of thicker and stronger steel plates with improved properties such as strength and ductility.

[0006] In order to manufacture a square steel pipe using a thick and strong steel plate, the steel plate needs to be press-formed. During this press-formation, a recess of about 50 to 250 μm, called a "necking," may be formed along the bend radius. Although the necking does not significantly reduce the properties of the square steel pipe, such as strength, it is undesirable from the viewpoint of aesthetics because it is formed on the outer surface of the steel pipe. However, Patent Documents 1 to 4 do not consider this necking. Therefore, the steel plates and steel pipes of these documents have room for further improvement in terms of suppressing necking.

[0007] An object of the present invention is to solve the above problems and to provide a thick steel plate for square steel pipes which has high strength and ductility and can suppress the occurrence of necking. [Means for solving the problem]

[0008] The present invention has been made to solve the above problems, and the gist of the present invention is the following thick steel plate for square steel pipes.

[0009] (1) A steel plate having two rolled surfaces, Chemical composition, in mass%, C: 0.04% or more and less than 0.13% Si: 0.10-0.36%, Mn: 1.00-1.90%, P:0.030% or less, S: 0.010% or less, Al: 0.010 to 0.100%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.030%, N: 0.010% or less, The balance is Fe and impurities. The carbon equivalent CeqWES calculated by the following formula (i) is 0.36 to 0.50%, The weld crack susceptibility composition P is calculated using the following formula (ii) cm is 0.17-0.25%, The uniform elongation uELs of the surface layer portion on at least one of the rolled surfaces is 5.0% or more; The tensile strength TS is 550 to 740 MPa, The yield stress YS is 385 to 540 MPa, Thick steel plate for square steel pipes, with a thickness of 33 to 70 mm. CeqWES(%)=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (i) P cm (%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B ···(ii) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero.

[0010] (2) The chemical composition contains, in mass%, a part of the Fe replaced by Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: less than 0.20%, and V: 0.10% or less, The thick steel plate for square steel pipes according to the above (1), comprising one or more selected from the group consisting of:

[0011] (3) The chemical composition contains, in mass%, a part of the Fe replaced by Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, The thick steel plate for square steel pipes according to the above (1) or (2), comprising one or more selected from the group consisting of:

[0012] (4) The chemical composition contains, in mass%, a part of the Fe replaced by Zr: 0.0050% or less, The thick steel plate for square steel pipes according to any one of the above (1) to (3), which contains: Effect of the Invention

[0013] According to the present invention, it is possible to obtain a thick steel plate for square steel pipes which has high strength and ductility and is capable of suppressing the occurrence of necking. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing the process of manufacturing a square steel pipe by cold pressing. [Diagram 2] FIG. 2 is a schematic diagram showing the positions at which tensile test pieces for measuring the uniform elongation of the surface layer were taken. [Diagram 3] FIG. 3 is a schematic diagram showing the positions at which tensile test specimens for measuring tensile strength TS, yield stress YP, and elongation EL were taken. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present inventors have investigated the mechanism by which necking occurs and have obtained the following findings (a) to (d).

[0016] (a) Even when cold press working is performed on thick steel plates with the same characteristics, necking does not always occur at the same location. For this reason, the inventors performed an analysis using the finite element method (FEM) to examine the cause of necking. This analysis was performed when cold press working a thick steel plate with a notch on the steel plate surface.

[0017] (b) According to FEM analysis, when the notch depth is approximately 30 μm or more, i.e., when the surface roughness becomes rough, it is found that the strain concentrates at the bottom of the notch and the notch depth increases. From this, it was thought that one of the factors that cause necking is the surface roughness of the thick steel plate used for the square steel pipe, and that necking is more likely to occur in areas with rough surface roughness. However, even if it is possible to improve the surface roughness of the thick steel plate, in order to control the surface roughness to 100 μ or less, it is necessary to take into account the effects of the growth of surface scale, etc., and to suppress the roughness that can occur in the normal manufacturing process, especially the rolling process, to the utmost extent possible, which imposes limitations in terms of manufacturing costs.

[0018] (c) Therefore, the inventors further investigated the FEM analysis and found that if strain is concentrated on the surface of the thick steel plate and the surface of the steel plate is elongated beyond the limit of uniform deformation during cold press working, necking may occur. Therefore, the inventors found that it is effective to control the surface of the thick steel plate so that uniform deformation is easily generated, and specifically, it is effective to improve the uniform elongation near the surface of the thick steel plate.

[0019] (d) Here, in order to suppress necking, it is more effective to control the uniform elongation near the surface than to control the uniform elongation over the entire steel plate as described in Patent Document 1. This is because necking itself occurs on the surface of the steel pipe, whereas attempting to control the uniform elongation over the entire steel plate may result in a decrease in other important properties required for the steel pipe, such as tensile strength.

[0020] In addition, when it comes to high-strength steel of 55k class or more, the microstructural composition is different from that of 50k class steel as described in Patent Document 1. For this reason, when considering performing cold press working while ensuring strength, high-strength and thick steel plates of 55k class or more are difficult to bend by cold press working. Therefore, in order to prevent necking from occurring when manufacturing square steel pipes using such thick steel plates, it is effective to improve the uniform elongation near the surface of the steel plate, which is the position corresponding to the bottom of the notch, as is clear from the results of FEM analysis, rather than the uniform elongation throughout the entire thickness of the steel plate.

[0021] The present invention has been made based on the above findings. A steel plate according to one embodiment of the present invention is a steel plate having two rolled surfaces. Each requirement will be described below.

[0022] 1.Chemical composition The reasons for limiting the content of each element are as follows. In the following description, "%" for the content means "mass %."

[0023] C: 0.04% or more and less than 0.13% C is an important element for improving strength. In order to stably secure the desired strength, the C content is set to 0.04% or more. In order to more stably increase the strength, the C content is preferably set to 0.05% or more, more preferably set to 0.06% or more, and even more preferably set to 0.08% or more. However, if C is contained in excess, toughness, i.e., ductility, decreases. For this reason, the C content is set to less than 0.13%. The C content is preferably set to 0.12% or less, and more preferably set to 0.11% or less.

[0024] Silicon: 0.10 to 0.36% Silicon is an important element for deoxidation. Appropriate deoxidation improves toughness and ductility. For this reason, the silicon content is set to 0.10% or more. However, excessive silicon content promotes the formation of island martensite, which adversely affects toughness, and the strength becomes excessively high, decreasing the uniform elongation uELs of the surface layer. For this reason, the silicon content is set to 0.36% or less. The silicon content is preferably set to 0.34% or less, and more preferably set to 0.32% or less.

[0025] Mn: 1.00-1.90% Mn is an element effective for improving strength and is an element effective for economically ensuring strength. For this reason, the Mn content is set to 1.00% or more. From the viewpoint of stably ensuring strength, the Mn content is preferably set to 1.20% or more. However, if Mn is excessively contained, the central segregation of the slab becomes significant, and in addition to increasing the hardness, it promotes the formation of island martensite and reduces toughness. It also reduces the uniform elongation uELs of the surface layer. For this reason, the Mn content is set to 1.90% or less. From the viewpoint of increasing the hardness and suppressing the formation of island martensite, the Mn content is preferably set to 1.80% or less, and more preferably set to 1.70% or less.

[0026] P:0.030% or less P is an impurity element contained in steel, and if contained in excess, it reduces toughness. It also reduces the uniform elongation uELs of the surface layer. For this reason, the P content is set to 0.030% or less. On the other hand, if the P content is reduced excessively, the steelmaking cost increases, which is economically disadvantageous. For this reason, the P content is preferably set to 0.003% or more.

[0027] S: 0.010% or less Like P, S is an impurity element contained in steel. Furthermore, if the S content is high, MnS, which elongates during rolling and reduces toughness, is more likely to form. For this reason, the S content is set to 0.010% or less. On the other hand, like P, if S is reduced too much, the steelmaking cost increases, which is economically disadvantageous. For this reason, the S content is preferably set to 0.001% or more.

[0028] Al: 0.010 to 0.100% Al is an element that acts as a deoxidizer, reducing O (oxygen) and improving the cleanliness of steel. Appropriate deoxidization improves toughness and ductility. In order to stably and reliably exert this effect, the Al content is set to 0.010% or more. However, excessive Al content not only reduces toughness, but also mixes with the weld metal during welding, reducing the toughness of the weld metal. It also reduces the uniform elongation uELs of the surface layer. For this reason, the Al content is set to 0.100% or less. The Al content is preferably set to 0.090% or less, and more preferably set to 0.080% or less.

[0029] Nb: 0.005 to 0.050% Nb is an element that is effective in increasing the non-recrystallization temperature of austenite, maximizing the effect of controlled rolling during hot rolling, and preventing softening of the heat-affected zone during welding and cutting. Nb also hardens steel by precipitation, contributing to improving strength. For this reason, the Nb content is set to 0.005% or more. The Nb content is preferably set to 0.007% or more. However, excessive Nb content leads to deterioration of toughness. It also reduces the uniform elongation uELs of the surface layer. For this reason, the Nb content is set to 0.050% or less. The Nb content is preferably set to 0.040% or less, and more preferably set to 0.035% or less.

[0030] Ti: 0.005 to 0.030% Ti has an effect of improving toughness. This is because Ti combines with N and finely precipitates in the slab as TiN, suppressing the coarsening of austenite grains during heating and refining the rolled structure. In addition, such a refining effect may improve the uniform elongation uELs of the surface layer. In addition, the fine TiN present in the steel sheet also has the effect of refining the HAZ structure during welding. For this reason, the Ti content is set to 0.005% or more. The Ti content is preferably set to 0.010% or more. However, excessive Ti content forms TiC, which reduces low-temperature toughness and weldability. In addition, the strength becomes excessive, and the uniform elongation uELs of the surface layer is also reduced. For this reason, the Ti content is set to 0.030% or less. The Ti content is preferably set to 0.025% or less.

[0031] N: 0.010% or less N is an impurity element contained in steel, which causes strain aging after cold working and reduces the toughness of the steel. It also reduces the uniform elongation uELs of the surface layer. For this reason, the N content is set to 0.010% or less. The N content is preferably set to 0.008% or less. It is preferable to reduce the N content as much as possible, but excessive reduction of the N content increases the manufacturing cost. For this reason, from an economical point of view, the N content is preferably set to 0.001% or more.

[0032] In addition to the above elements, one or more elements selected from Cu, Ni, Cr, Mo, V, Ca, Mg, REM, and Zr may be contained within the ranges shown below. The reasons for limiting each element will be explained below.

[0033] Cu: 1.00% or less Cu contributes to improving solid solution strengthening and hardenability, and has the effect of improving the strength of steel. Therefore, it may be contained as necessary. However, if Cu is contained in excess, the surface properties deteriorate. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.30% or less, and more preferably set to 0.20% or less. On the other hand, in order to obtain the above effect, the Cu content is preferably set to 0.01% or more, more preferably set to 0.05% or more, and even more preferably set to 0.10% or more.

[0034] Ni: 1.00% or less Ni has the effect of improving the strength of steel without reducing toughness. Therefore, it may be contained as necessary. However, if Ni is contained in excess, not only the effect becomes saturated, but also the alloy cost increases, which is economically disadvantageous. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably set to 0.40% or less, and more preferably set to 0.30% or less. On the other hand, in order to obtain the above effect, the Ni content is preferably set to 0.01% or more, more preferably set to 0.05% or more, and even more preferably set to 0.10% or more.

[0035] Cr:0.50% or less Cr contributes to improving hardenability and has the effect of improving the strength of steel. Therefore, it may be contained as necessary. However, if Cr is contained in excess, not only will the effect saturate, but the alloy cost will also increase, which is economically disadvantageous. Therefore, the Cr content is set to 0.50% or less. The Cr content is preferably set to 0.30% or less, and more preferably set to 0.20% or less. On the other hand, in order to obtain the above effect, the Cr content is preferably set to 0.01% or more, and more preferably set to 0.10% or more.

[0036] Mo: Less than 0.20% Mo contributes to improving hardenability and has the effect of improving the strength of steel. Therefore, it may be contained as necessary. However, if Mo is contained in excess, the base material toughness and HAZ toughness decrease. Therefore, the Mo content is less than 0.20%. The Mo content is preferably 0.15% or less, and more preferably 0.10% or less. On the other hand, in order to obtain the above effect, the Mo content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0037] V: 0.10% or less V has the effect of improving the strength of steel by precipitation strengthening. Therefore, it may be contained as necessary. However, if an excessive amount of V is contained, the base material toughness and HAZ toughness decrease. Therefore, the V content is set to 0.10% or less. The V content is preferably set to 0.08% or less, and more preferably set to 0.06% or less. On the other hand, in order to obtain the above effect, the V content is preferably set to 0.01% or more, more preferably set to 0.02% or more, and even more preferably set to 0.04% or more.

[0038] Ca: 0.0050% or less Ca contributes to morphological control of sulfides, oxides, and oxysulfides, and has the effect of improving the toughness and ductility of steel. Therefore, it may be contained as necessary. However, if Ca is contained in excess, the toughness of the base material decreases. Therefore, the Ca content is set to 0.0050% or less. The Ca content is preferably set to 0.0040% or less, and more preferably set to 0.0030% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably set to 0.0003% or more, more preferably set to 0.0005% or more, and even more preferably set to 0.0010% or more.

[0039] Mg: 0.0050% or less Mg contributes to morphological control of sulfides, oxides, and oxysulfides, and has the effect of improving the toughness and ductility of steel. Therefore, Mg may be contained as necessary. However, if Mg is contained in excess, the toughness of the base material decreases. Therefore, the Mg content is set to 0.0050% or less. The Mg content is preferably set to 0.0040% or less, and more preferably set to 0.0030% or less. On the other hand, in order to obtain the above effect, the Mg content is preferably set to 0.0003% or more, more preferably set to 0.0005% or more, and even more preferably set to 0.0010% or more.

[0040] REM: 0.0050% or less REM contributes to morphological control of sulfides, oxides, and oxysulfides, and has the effect of improving the toughness and ductility of steel. Therefore, it may be contained as necessary. However, if REM is contained in excess, the toughness of the base material decreases. Therefore, the REM content is set to 0.0050% or less. The REM content is preferably set to 0.0040% or less, and more preferably set to 0.0030% or less. On the other hand, in order to obtain the above effect, the REM content is preferably set to 0.0003% or more, more preferably set to 0.0005% or more, and even more preferably set to 0.0010% or more.

[0041] REM refers to a total of 17 elements, including Sc, Y and lanthanides, and the above REM content refers to the total content of these elements. REM is added industrially in the form of misch metal.

[0042] Zr: 0.0050% or less Zr, like Ca, Mg, and REM, forms oxides, sulfides, and oxysulfides to suppress the generation of coarse inclusions and enhance the toughness of the base material and HAZ toughness. Therefore, it may be contained as necessary. However, if Zr is contained in excess, the number of Zr-based inclusions that become the starting point of brittle fracture increases. Therefore, the Zr content is set to 0.0050% or less. The Zr content is preferably set to 0.0045% or less, and more preferably set to 0.0040% or less. On the other hand, in order to obtain the above effect, the Zr content is preferably set to 0.0001% or more. The Zr content is preferably set to 0.0003% or more, more preferably set to 0.0015% or more, and even more preferably set to 0.0020%.

[0043] In the chemical composition of the thick steel plate according to one embodiment of the present invention, the balance is Fe and impurities. Here, "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel, and are acceptable within a range that does not adversely affect the present invention. Note that various elements are considered as impurities, and B is one example. The content of B as an impurity is not particularly limited, but is usually considered to be 0.0005% or less.

[0044] Carbon equivalent CeqWES: 0.36-0.50% The carbon equivalent CeqWES calculated by the following formula (i) is an index of the strength of steel, and the higher the value, the higher the strength. Therefore, in the steel plate according to one embodiment of the present invention, the carbon equivalent CeqWES is limited in order to obtain the desired strength.

[0045] CeqWES(%)=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (i) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero.

[0046] If the carbon equivalent CeqWES is less than 0.36%, even if the thermomechanical processing method (also referred to as "TMCP") described below is applied, the tensile strength TS and yield stress YS of the thick steel plate required as the material for square steel pipes cannot be stably secured. For this reason, the carbon equivalent CeqWES is set to 0.36% or more. The carbon equivalent CeqWES is preferably set to 0.37% or more, and more preferably 0.38% or more.

[0047] On the other hand, if the carbon equivalent CeqWES exceeds 0.50%, the base material and HAZ will harden or martensite will form, making it impossible to stably ensure the toughness of the base material and HAZ. It will also reduce the uniform elongation uELs of the surface layer. For this reason, the carbon equivalent CeqWES is set to 0.50% or less. The carbon equivalent CeqWES is preferably set to 0.48% or less, and more preferably set to 0.45% or less.

[0048] Weld crack susceptibility composition P cm :0.17~0.25% The weld crack susceptibility composition P is calculated using the following formula (ii) cm is an index showing the strength of steel, similar to the carbon equivalent CeqWES, and is also an index showing the susceptibility to cold cracking during welding. Therefore, in the steel plate according to one embodiment of the present invention, in order to obtain the desired strength and weld cracking resistance, the weld cracking susceptibility composition P cm Control.

[0049] P cm (%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B ···(ii) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero.

[0050] Weld crack susceptibility composition P cm However, if it is less than 0.17%, the required tensile strength TS and yield stress YS of the steel plate cannot be stably secured, even if the TMCP described later is applied, as with the carbon equivalent CeqWES.cm The weld crack susceptibility component P is 0.17% or more. cm It is preferable that the content of N is 0.175% or more.

[0051] On the other hand, the weld crack susceptibility composition P cm If P exceeds 0.25%, the base material and HAZ will harden or martensite will form, making it impossible to stably ensure the toughness of the base material and HAZ. It also reduces the uniform elongation uELs of the surface layer. For this reason, the weld crack susceptibility composition P cm The content of weld crack susceptibility P is 0.25% or less. cm is preferably 0.24% or less, and more preferably 0.22% or less.

[0052] 2. Uniform elongation of the surface layer In the thick steel plate according to one embodiment of the present invention, as described above, the uniform elongation uELs of the surface layer portion is controlled in order to suppress the occurrence of necking. Necking is likely to occur starting from a portion with a high surface roughness, but the occurrence of necking can be suppressed by improving the uniform elongation uELs of the surface layer portion.

[0053] Here, the steel plate has two rolling surfaces, an upper one and an lower one, because it is sandwiched between rolls from above and below and rolled. At least one of these rolling surfaces corresponds to the outer surface of the square steel pipe when cold press-formed. Therefore, in order to suppress necking of the outer surface, the uniform elongation uELs of the surface layer portion of at least one of the rolling surfaces is controlled to be 5.0% or more.

[0054] If the uniform elongation uELs of the surface layer is less than 5.0%, necking is likely to occur from the portion with poor surface quality (portion with rough surface). Therefore, the uniform elongation uELs of the surface layer is set to 5.0% or more. The uniform elongation uELs of the surface layer is preferably set to 5.5% or more. The upper limit of the uniform elongation uELs of the surface layer is not particularly limited, but is usually 12.0% or less.

[0055] Here, the uniform elongation of the surface layer refers to the uniform elongation measured by taking a tensile test piece from the rolled surface near the surface of the thick steel plate. Specifically, as shown in Figure 2, a JIS No. 5 tensile test piece is taken from the rolled surface of the thick steel plate at a thickness of 3 mm in the direction perpendicular to the plate thickness from the surface, with the tensile direction perpendicular to the rolling direction, and the uniform elongation of the surface layer is measured. As described above, it is sufficient to take a tensile test piece from the rolling surface that will become the outer surface of the steel pipe out of the two rolling surfaces, upper and lower, of the steel plate and perform the measurement, and the uniform elongation of the surface layer may be different between the upper and lower rolling surfaces.

[0056] 3.Strength The tensile strength TS of the thick steel plate according to one embodiment of the present invention is 550 to 740 MPa. The tensile strength TS of the thick steel plate is preferably 570 MPa or more, and more preferably 600 MPa or more. The tensile strength TS is preferably 720 MPa or less, and more preferably 700 MPa or less.

[0057] The yield stress YS is 385 to 540 MPa. The yield stress YS is preferably 400 MPa or more, and more preferably 420 MPa or more. The yield stress YS is preferably 520 MPa or less, and more preferably 500 MPa or less.

[0058] These strengths (tensile strength and yield stress) are what are known as 55k and 60k class strengths, and when a square steel pipe is manufactured using the steel pipe of the present invention, it is the strength required for a pillar (also called a "column") of a building structure. In this way, the thick steel plate has a strength higher than that of the 50k class steel plate that was previously mainstream.

[0059] The above tensile strength and yield stress may be calculated by performing a tensile test using a round bar test piece taken from 1 / 4t. Here, t indicates the plate thickness (total thickness), and 1 / 4t means 1 / 4 of the plate thickness. In addition, as a steel plate used in architectural structures, the yield ratio (= yield stress / tensile strength) is preferably 80% or less. In the thick steel plate of the present invention, the elongation (total elongation) of the steel plate is not particularly specified, but when a tensile test is performed by the above method, the total elongation is preferably more than 20.0%.

[0060] 4. Plate thickness The thickness of the thick steel plate according to one embodiment of the present invention is 33 to 70 mm. If the thickness is less than 33 mm, even if the steel plate has a portion with partially deteriorated surface properties, large strain will not concentrate in that portion, and the problem of necking is unlikely to occur. For this reason, the thickness is 33 mm or more. On the other hand, if the thickness exceeds 70 mm, it becomes difficult to ensure an outer bending radius (mm) of 4.0t or less for the plate thickness t (mm) by cold press working, and it becomes impossible to process into a desired rectangular shape. For this reason, the thickness is 70 mm or less.

[0061] 5. Manufacturing method A preferred method for manufacturing the steel plate according to one embodiment of the present invention will be described. The steel plate according to one embodiment of the present invention can obtain the effects as long as it has the above-mentioned configuration regardless of the manufacturing method, but it can be stably manufactured, for example, by the following TMCP manufacturing method.

[0062] A slab having the above chemical composition is manufactured by continuous casting. The manufactured slab is preferably heated to 1100 to 1200°C. If the heating temperature of the slab is less than 1100°C, it is difficult to obtain a thick steel plate with the desired strength. Therefore, the heating temperature of the slab is preferably 1100°C or higher.

[0063] On the other hand, if the heating temperature of the slab exceeds 1200°C, the crystal grains become coarse and the toughness is likely to decrease. In addition, the hardenability increases too much, and the surface hardness of the thick steel plate increases. As a result, it becomes difficult to achieve a uniform elongation uELs of 5.0% or more in the surface layer. For this reason, the heating temperature of the slab is preferably 1200°C or less, and more preferably 1180°C or less.

[0064] Next, the heated slab is hot-rolled. Hot rolling is classified into rough rolling and finish rolling. In particular, in the finish rolling, rolling in the non-recrystallization temperature range of austenite is effective in suppressing coarsening of the structure, so the start temperature (hereinafter simply referred to as "finish rolling start temperature") is preferably 870°C or less. If the finish rolling start temperature exceeds 870°C, the structure becomes coarse, making it difficult to obtain the desired toughness. In addition, the surface hardness increases, making it difficult to make the uniform elongation uELs of the surface layer 5.0% or more. For this reason, the finish rolling start temperature is preferably 870°C or less, more preferably 860°C or less.

[0065] In addition, the rolling temperature of the final pass of hot rolling is preferably 680°C or higher. If the rolling temperature of the final pass is less than 680°C, ferrite is generated during or immediately after rolling, and the ferrite is coarsened, resulting in a decrease in the toughness of the surface layer. In addition, it becomes difficult to achieve a uniform elongation uELs of 5.0% or more in the surface layer. For this reason, the rolling temperature of the final pass of hot rolling is preferably 680°C or higher, and more preferably 700°C or higher.

[0066] In addition, other conditions in the hot rolling are not particularly limited. They may be adjusted as necessary. For example, the rolling reduction rate may be adjusted as necessary to obtain a specified plate thickness.

[0067] It is preferable to perform accelerated cooling after hot rolling. The thick steel plate according to the embodiment of the present invention has a thick plate thickness of 33 mm or more. In such a case, the cooling rate becomes faster at the surface of the thick steel plate when water-cooled, and quenching tends to occur easily and the uniform elongation tends to become small. For this reason, it is preferable to perform accelerated cooling and control the cooling rate in a specific temperature range in order to make the uniform elongation uELs of the surface layer 5.0% or more. It is sufficient to perform accelerated cooling on at least one of the rolling surfaces, and it is not necessarily required to perform it on both sides, and it is sufficient to perform it on the rolling surface where it is desired to control the uniform elongation uELs of the surface layer.

[0068] Accelerated cooling is a cooling method in which the water density is controlled. Specifically, the cooling rate in the temperature range of 600°C or higher at 1 / 4t is preferably 15°C / s or less. If the cooling rate exceeds 15°C / s, the surface is cooled rapidly, hardening advances, and it becomes difficult to achieve a uniform elongation uELs of 5.0% or more in the surface layer. For this reason, the cooling rate is preferably 15°C / s or less, and more preferably 11°C / s or less. The lower limit of the cooling rate is not particularly limited, but is usually 5°C / s or more from the viewpoint of manufacturing.

[0069] In addition, the cooling rate below 600°C in 1 / 4t is preferably 8°C / s or more. If the cooling rate is less than 8°C / s, island martensite is generated and toughness is reduced. Therefore, the cooling rate is preferably 8°C / s or more.

[0070] In addition, the accelerated cooling stop temperature is preferably 300°C or less. If the accelerated cooling stop temperature exceeds 300°C, island martensite is generated and the toughness is reduced. Therefore, the accelerated cooling stop temperature is preferably 300°C or less, and more preferably 200°C or less.

[0071] After the accelerated cooling is stopped, a tempering treatment may be further applied, for example, to improve the uniform elongation of the surface layer. The heat treatment temperature in this case is, for example, in the range of 300 to 500°C.

[0072] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. EXAMPLES

[0073] A 240 mm thick slab having the chemical composition shown in Table 1 was produced by continuous casting.

[0074] [Table 1]

[0075] The obtained slab was heated to a temperature range of 1100 to 1200 ° C. as shown in Table 2, the start of finish rolling was set to 870 ° C. or less, and the rolling temperature of the final pass was set to 730 ° C. or more. After that, the water flow density was adjusted according to the plate thickness so that the cooling rate in the temperature range of 600 ° C. or more was 15 ° C. / s or less and the cooling rate below 600 ° C. was 8 ° C. / s or more at 1 / 4t. The accelerated cooling stop temperature was set to 300 ° C. or less, and a thick steel plate with a thickness of 35 to 70 mm was manufactured. Note that some thick steel plates were manufactured so that the rolling temperature of the final pass or the cooling rate in the temperature range of 600 ° C. or more did not fall within the above-mentioned conditions.

[0076] [Table 2]

[0077] Tensile tests were performed on each of the manufactured steel plates, and the tensile strength TS, yield stress YP, elongation EL, and uniform elongation uELs of the surface layer were measured according to the following procedure. In addition, in order to evaluate the susceptibility to necking, cold pressing was performed according to the following procedure.

[0078] (Measurement of tensile strength TS, yield stress YP, and elongation EL) The tensile strength TS, yield stress YP, and elongation EL were determined by taking bar-shaped test pieces perpendicular to the rolling direction from 1 / 4t of the plate thickness as shown in Figure 3, and conducting tensile tests in accordance with JIS Z 2241:2011.

[0079] (Measurement of uniform elongation of the surface layer uELs) One side of the manufactured thick steel plate was ground, and as described above, a plate-shaped JIS No. 5 tensile test piece was taken, finished to a thickness of 3 mm, leaving the surface layer as shown in Figure 2, and a tensile test was performed in accordance with JIS Z 2241:2011 to obtain the values.

[0080] (Evaluation of waistline) Apart from the tensile tests, each thick steel plate was subjected to cold pressing. In the cold pressing, pressing was performed so that the outer bending radius r (mm) = 3.5t for the plate thickness t, corners were formed, and the plate was shaped into a U-shape. Since corners are prone to necking, the presence or absence of necking at the corners was confirmed visually. In addition, for some examples, after processing, the ends of two thick steel plates were butted together and the inner surface was carbon dioxide arc welded and the outer surface was submerged arc welded to produce square steel pipes, and it was confirmed that there were no problems in the production of the steel pipes. The results are shown in Table 3 below.

[0081] [Table 3]

[0082] Nos. 1 to 14, which satisfy the requirements of the present invention, had good strength and were able to suppress the occurrence of necking, whereas Nos. 15 to 35, which do not satisfy the requirements of the present invention, were inferior in one of the desired properties.

Claims

1. A thick steel plate having a first rolled surface which becomes an outer surface of a steel pipe and a second rolled surface which becomes an inner surface of the steel pipe, The chemical composition, in mass%, is C: 0.04% or more and less than 0.13%; Si: 0.10-0.36%, Mn: 1.00-1.90%, P: 0.030% or less, S: 0.010% or less, Al: 0.010-0.100%, Nb: 0.005-0.050%, Ti: 0.005 to 0.025%, N: 0.010% or less, The balance is Fe and impurities. The carbon equivalent CeqWES calculated by the following formula (i) is 0.37 to 0.50%, The weld crack susceptibility composition P calculated by the following formula (ii) ｃｍ is 0.17 to 0.25%, The uniform elongation uELs measured using a JIS No. 5 tensile test piece having a tensile direction perpendicular to the rolling direction taken from a thickness position 3 mm in a plate thickness direction perpendicular to the surface of the first rolled surface is 5.0% or more; The tensile strength TS is 550 to 740 MPa; The yield stress YS is 385 to 540 MPa; A thick steel plate for square steel pipes having a thickness of 33 to 70 mm. CeqWES (%) = C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14...(i) P ｃｍ (%)=C+Si / 30+Mn / 20+C / / 20+Ni / 60+Cr / 20+M / / 15+V / 1+5B ・・・ (ii) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero.

2. The chemical composition is, in mass %, replacing a part of the Fe, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: less than 0.20%, and V: 0.10% or less, The thick steel plate for square steel pipes according to claim 1, comprising one or more selected from the group consisting of:

3. The chemical composition is, in mass %, replacing a part of the Fe, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, The thick steel plate for square steel pipes according to claim 1 or 2, comprising one or more selected from the group consisting of:

4. The chemical composition is, in mass %, replacing a part of the Fe, Zr: 0.0050% or less, The thick steel plate for square steel pipes according to any one of claims 1 to 3, comprising:

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