JP7633510B2 - Thick steel plate for square steel pipe - Google Patents

Description

The present invention relates to thick steel plates for square steel pipes. In particular, it relates to 55k or 60k class thick steel plates for square steel pipes, which are used in architectural structures such as columns of high-rise buildings and have a tensile strength of 550 to 740 MPa and a yield stress of 385 to 540 MPa.

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 bend it at 90 degrees, forming a corner (hereinafter also referred to as the "bent R part") and giving it a U-shape. Next, the ends of the two U-shaped thick steel plates are welded together to make a square steel pipe.

Square steel pipes used in building structures and the like are required to have various properties such as strength, ductility, and toughness in terms of the strength of the building. For this reason, as described in Patent Documents 1 to 4, steel plates for square steel pipes and square steel pipes with improved properties have been developed.

JP 2016-11439 A JP 2012-102393 A JP 2014-31546 A JP 2011-94214 A

Conventionally, square steel pipes made from relatively thin, low-strength steel plates, as described in Patent Document 1, were mainstream. However, in recent years, as architectural structures have become larger, there is a demand for square steel pipes made from thicker, stronger steel plates with improved strength, ductility, and other properties.

In order to manufacture square steel pipes using thick, high-strength steel plate, the steel plate must be pressed. During this press work, depressions of about 50 to 250 μm in size, known as "necking," can form along the bend radius. Although necking does not significantly reduce the properties of the square steel pipe, such as strength, it is undesirable from an aesthetic standpoint because it is formed on the outer surface of the steel pipe. However, Patent Documents 1 to 4 do not consider necking. Therefore, the steel plates and steel pipes in these documents have room for further improvement in terms of suppressing necking.

The present invention aims to solve the above problems and provide a thick steel plate for square steel pipes that has high strength and ductility and can suppress the occurrence of necking.

The present invention was made to solve the above problems, and the gist of the invention is the following thick steel plate for square steel pipes.

(1) A steel plate having two rolling surfaces,
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: 0.005-0.030%,
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 _cm calculated by the following formula (ii) 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;
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+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.

(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 the above (1), comprising one or more selected from the group consisting of:

(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:

(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 (1) to (3) above, containing:

According to the present invention, it is possible to obtain thick steel plates for square steel pipes that have high strength and ductility and can suppress the occurrence of necking.

FIG. 1 is a schematic diagram showing a process for manufacturing a square steel pipe by cold pressing. FIG. 2 is a schematic diagram showing the positions at which tensile test pieces for measuring the uniform elongation of the surface layer are taken. 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.

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

(a) Even when cold pressing is performed on thick steel plates with the same characteristics, necking does not always occur in the same place. For this reason, the inventors performed an analysis using the finite element method (FEM) to investigate the causes of necking. This analysis was performed when cold pressing a thick steel plate with a notch on the surface of the steel plate.

(b) According to FEM analysis, it was found that when the notch depth is approximately 30 μm or more, i.e., when the surface roughness becomes rough, 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 to make 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 surface scale growth, etc., and to suppress the roughness that can occur in normal manufacturing processes, particularly the rolling process, to the utmost extent possible, which poses limitations in terms of manufacturing costs.

(c) Therefore, the inventors further investigated the FEM analysis. They found that if strain concentrates 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 can occur. Therefore, the inventors found that it is effective to control the surface of the thick steel plate so that uniform deformation is more likely to occur, and specifically, it is effective to improve the uniform elongation near the surface of the thick steel plate.

(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 of the entire thick 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 of the entire steel plate may result in a decrease in other important properties required for the steel pipe, such as the tensile strength.

In addition, when it comes to high-strength steel of 55k class or more, the microstructural structure 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.

The present invention was made based on the above findings. The steel plate according to one embodiment of the present invention is a steel plate having two rolled surfaces. Each requirement will be explained below.

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

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.

Si: 0.10-0.36%
Si is an important element for deoxidation. By performing deoxidation appropriately, toughness and ductility are improved. For this reason, the Si content is set to 0.10% or more. However, excessive Si 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 Si content is set to 0.36% or less. The Si content is preferably set to 0.34% or less, and more preferably set to 0.32% or less.

Mn: 1.00-1.90%
Mn is an element effective for improving strength and is an element effective for economically ensuring strength. Therefore, 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. Therefore, 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.

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.

S: 0.010% or less Like P, S is also an impurity element contained in steel. In addition, if the S content is high, MnS, which is elongated by rolling and reduces toughness, is likely to be generated. 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.

Al: 0.010-0.100%
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.

Nb: 0.005-0.050%
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 precipitation hardens the steel and contributes 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.

Ti: 0.005-0.030%
Ti has the 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.

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.

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 are explained below.

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 will 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.

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 will the effect saturate, but the alloy cost will also increase, 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.

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.

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.

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 V is contained excessively, 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.

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.

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. For this reason, it 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.

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. For this reason, 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.

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. Industrially, REM is added in the form of misch metal.

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 the 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%.

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 the raw materials such as ores and scraps and the manufacturing process when steel is industrially produced, and are acceptable within a range that does not adversely affect the present invention. Note that various elements are considered as impurities, but 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.

Carbon equivalent CeqWES: 0.36 to 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 thick steel plate according to one embodiment of the present invention, the carbon equivalent CeqWES is limited in order to obtain the desired strength.

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.

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.

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. It is preferable that the carbon equivalent CeqWES is set to 0.48% or less, and more preferably to 0.45% or less.

Weld crack susceptibility composition P _cm : 0.17 to 0.25%
The weld crack susceptibility composition _Pcm calculated by the following formula (ii) 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 thick steel plate according to one embodiment of the present invention, the weld crack susceptibility composition _Pcm is controlled in order to obtain the desired strength and weld cracking resistance.

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.

If the weld crack susceptibility composition _Pcm is less than 0.17%, the required tensile strength TS and yield stress YS of the thick steel plate cannot be stably secured, similarly to the carbon equivalent CeqWES, even if the TMCP described below is applied. Therefore, the weld crack susceptibility composition _Pcm is set to 0.17% or more. The weld crack susceptibility composition _Pcm is preferably set to 0.175% or more.

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

2. Uniform elongation of the surface layer (uELs)
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.

Here, the thick steel plate is rolled by being sandwiched between rolls from above and below, so it has two rolling surfaces, an upper one and a lower one. At least one of these rolling surfaces corresponds to the outer surface of the square steel pipe when it is cold pressed. Therefore, in order to suppress necking of the outer surface, the uniform elongation uELs of the surface layer on at least one of the rolling surfaces is controlled to be 5.0% or more.

If the uniform elongation uELs of the surface layer is less than 5.0%, necking is likely to occur in areas with poor surface properties (areas with high surface roughness). For this reason, the uniform elongation uELs of the surface layer is set to 5.0% or more. It is preferable that the uniform elongation uELs of the surface layer is set to 5.5% or more. There is no particular limit to the upper limit of the uniform elongation uELs of the surface layer, but it is usually 12.0% or less.

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 at a thickness of 3 mm from the surface of the thick steel plate in the direction perpendicular to the plate thickness, with the tensile direction perpendicular to the rolling direction, and the uniform elongation of the test piece is measured. As mentioned above, of the two rolled surfaces, the top and bottom, of the steel plate, it is sufficient to take a tensile test piece and perform the measurement on the rolled surface that will become the outer surface of the steel pipe, and the uniform elongation of the surface layer may be different between the top and bottom rolled surfaces.

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.

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.

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") in an architectural structure. In this way, it is a thick steel plate with a strength that exceeds that of the 50k class steel plates that were previously mainstream.

The above tensile strength and yield stress can 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, for steel plates used in architectural structures, the yield ratio (= yield stress/tensile strength) is preferably 80% or less. For the thick steel plate of the present invention, the elongation (total elongation) of the steel plate is not particularly specified, but it is desirable that the total elongation is more than 20.0% when a tensile test is performed using the above method.

4. Plate thickness The plate thickness of the thick steel plate according to one embodiment of the present invention is 33 to 70 mm. If the plate thickness is less than 33 mm, even if the steel plate has a part with a partially deteriorated surface property, a large strain is not concentrated in that part, and the problem of necking is unlikely to occur. For this reason, the plate thickness is 33 mm or more. On the other hand, if the plate thickness exceeds 70 mm, it becomes difficult to secure 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 plate thickness is 70 mm or less.

5. Manufacturing method A preferred manufacturing method of the thick steel plate according to one embodiment of the present invention will be described. The thick 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 manufacturing method using TMCP.

A slab having the above chemical composition is manufactured by continuous casting. The manufactured slab is preferably heated to 1100-1200°C. If the slab is heated to a temperature below 1100°C, it becomes difficult to obtain a thick steel plate with the desired strength. For this reason, it is preferable to heat the slab to a temperature of 1100°C or higher.

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.

The heated slab is then hot rolled. Hot rolling is classified into rough rolling and finish rolling. In particular, in finish rolling, rolling in the austenite non-recrystallization temperature range is effective in suppressing coarsening of the structure, so the start temperature (hereinafter simply referred to as the "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 increase in surface hardness makes it difficult to achieve a uniform elongation uELs of the surface layer of 5.0% or more. For this reason, the finish rolling start temperature is preferably 870°C or less, and more preferably 860°C or less.

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 will form during or immediately after rolling, causing coarsening and reducing the toughness of the surface layer. It will also be difficult to achieve a uniform elongation uELs of 5.0% or higher 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.

In addition, other conditions for hot rolling are not particularly limited. They may be adjusted as appropriate and necessary. For example, the reduction rate and the like may be adjusted as appropriate to obtain the specified plate thickness.

After hot rolling, accelerated cooling is preferably performed. The thick steel plate according to the embodiment of the present invention has a thickness of 33 mm or more. In such a case, the cooling rate during water cooling becomes faster toward the surface of the thick steel plate, which tends to make it easier for quenching to occur and to reduce the uniform elongation. 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. Accelerated cooling only needs to be performed on at least one of the rolling surfaces, and does not necessarily need to be performed on both surfaces. It is sufficient to perform accelerated cooling on the rolling surface on which it is desired to control the uniform elongation uELs of the surface layer.

Accelerated cooling is a cooling method that controls the water density. 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 will be cooled rapidly, hardening will progress, and it will be 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 from a manufacturing standpoint, it is usually 5°C/s or more.

In addition, the cooling rate below 600°C at 1/4t is preferably 8°C/s or more. If the cooling rate is less than 8°C/s, island martensite will form and toughness will decrease. For this reason, the cooling rate is preferably 8°C/s or more.

The accelerated cooling stop temperature is preferably 300°C or less. If the accelerated cooling stop temperature exceeds 300°C, island martensite will form and toughness will decrease. For this reason, the accelerated cooling stop temperature is preferably 300°C or less, and more preferably 200°C or less.

After the accelerated cooling is stopped, a further tempering process may be 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.

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

A 240 mm thick slab with the chemical composition shown in Table 1 was produced by continuous casting.

As shown in Table 2, the obtained slab was heated to a temperature range of 1100-1200°C, finish rolling was started at 870°C or less, and the rolling temperature of the final pass was set at 730°C or more. After that, at 1/4t, 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. The accelerated cooling stop temperature was set at 300°C or less, and thick steel plates with thicknesses of 35-70 mm were 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.

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

(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 ¼t of the plate thickness as shown in FIG. 3 and conducting tensile tests in accordance with JIS Z 2241:2011.

(Measurement of uniform elongation uELs of surface layer)
One side of the manufactured thick steel plate was subjected to a grinding process, and as described above, a plate-shaped JIS No. 5 tensile test piece was taken that was finished to a thickness of 3 mm while leaving the surface layer as shown in Figure 2, and a tensile test was performed in accordance with JIS Z 2241:2011 to determine the strength.

(Evaluation of waistline)
Then, each thick steel plate was subjected to cold pressing apart from the tensile test. 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-shaped member. Since corners are prone to necking, the presence or absence of necking at the corners was confirmed by visual inspection. In addition, for some examples, after processing, the ends of two thick steel plates were butted together, and the inner surface was carbon dioxide gas arc welded and the outer surface was submerged arc welded to produce a square steel pipe, and it was confirmed that there were no problems in the production of the steel pipe. The results are shown in Table 3 below.

Nos. 1 to 14, which satisfy the requirements of the present invention, have good strength and are able to suppress the occurrence of necking. On the other hand, Nos. 15 to 35, which do not satisfy the requirements of the present invention, are inferior in one of the desired properties.

Claims (4)

A steel plate having a first rolled surface that becomes an outer surface of a steel pipe and a second rolled surface that 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: 0.005-0.025 %,
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 _cm calculated by the following formula (ii) 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+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. 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: 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: 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:

Images