JP2026001848A - Steel plate and its manufacturing method - Google Patents

Abstract

The present invention provides a thick steel plate having high tensile strength, excellent corrosion resistance in chloride-containing environments, and improved lamellar tear resistance, and a method for manufacturing the same.
The chemical composition of the steel sheet is, in mass%, C: 0.04-0.15%, Si: 0.05-1.20%, Mn: 0.50-2.00%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.10-2.00%, Nb: 0.0030-0.0400%, Ti: 0.003-0.040%, Al: 0.010-0.080%, Sn: 0.0200-0.4000%, N: 0.0010-0.0070%, O: 0.00 0.05 to 0.0040%, Ca: 0.0001 to 0.0080%, and the balance: Fe and impurities, Ceq: 0.32 to 0.47, SnEQ: 0.10 or more, SC: 0.3 to 15.0, the steel plate has a thickness of 41 mm or more and less than 75 mm, a tensile strength at the 1/4 position of the plate thickness is 570 MPa or more, and at the center of the plate thickness of the C cross section of the steel plate, the maximum Mn segregation degree is 1.30 or less and the maximum defect length is 0.30 mm or less.
[Selection diagram] None

Description

The present invention relates to a steel sheet and a manufacturing method thereof.

In recent years, there has been a demand for larger and longer-lasting structures such as bridges, leading to an increasing demand for thick steel plates with high tensile strength. Furthermore, bridges and other structures constructed in coastal areas also require excellent corrosion resistance in the severely corrosive environments caused by chlorides.

Generally, in environments where chloride corrosion is a problem, steel materials are painted to prevent corrosion, and the progress of corrosion is monitored through regular inspections. If corrosion exceeds the control standard, the steel is repainted. However, in the case of bridges and other structures, it is necessary to use gondolas for high-altitude work or set up scaffolding, and the cost of repainting is enormous. Furthermore, because painting has an environmental impact, it is best to keep it to a minimum.

As examples of high-strength thick steel plates with excellent corrosion resistance in such chloride environments, Patent Document 1 discloses a steel plate containing Sn, in which the Sn concentration at the grain boundaries and within the grains is controlled, for use in large structures such as bridges. Furthermore, Patent Document 2 discloses a steel plate for marine structures that contains Sn and is composed of ferrite and a hard second phase.

International Publication No. 2019/116520 JP 2012-144799 A

In cross joints, T-joints, corner joints, and the like, the volumetric shrinkage of the weld metal that accompanies cooling after welding generates tensile stress in the steel plate in the plate thickness direction. This tensile stress in the plate thickness direction causes cracks to form inside the steel plate. This type of crack is called lamellar tearing. Lamellar tear resistance is an issue for steel plates used in bridges and other structures, as beams and other components are welded to their surfaces. However, the steel plates disclosed in Patent Documents 1 and 2 still have room for improvement in terms of lamellar tear resistance.

The present invention aims to solve the above problems by providing a thick steel plate that has high tensile strength, excellent corrosion resistance in chloride-containing environments, and improved lamellar tear resistance, as well as a manufacturing method for the same.

The present invention was made to solve the above problems, and is summarized as follows:

(1) The chemical composition of the steel plate is, in mass%,
C: 0.04-0.15%,
Si: 0.05-1.20%,
Mn: 0.50-2.00%,
P: 0.015% or less,
S: 0.0024% or less,
Ni: 0.10-2.00%,
Nb: 0.0030-0.0400%,
Ti: 0.003 to 0.040%,
Al: 0.010-0.080%,
Sn: 0.0200-0.4000%,
N: 0.0010-0.0070%,
O: 0.0005-0.0040%,
Ca: 0.0001 to 0.0080%, and the balance: Fe and impurities;
Ceq represented by the following formula (i) is 0.32 to 0.47,
The SnEQ represented by the following formula (ii) is 0.10 or more,
The SC represented by the following formula (iii) is 0.30 to 15.00,
The thickness of the steel plate is 41 mm or more and less than 75 mm,
The tensile strength at a quarter-thickness position of the steel plate is 570 MPa or more,
At the center of the thickness of a cross section perpendicular to the rolling direction of the steel plate,
The maximum Mn segregation degree is 1.30 or less,
The maximum defect length is 0.30 mm or less.
steel plate.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
SnEQ=Sn+W+Ni/10+Mo/4...(ii)
SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

(2) The chemical composition of the steel plate is, in mass%,
C: 0.04-0.15%,
Si: 0.05-1.20%,
Mn: 0.50-2.00%,
P: 0.015% or less,
S: 0.0024% or less,
Ni: 0.10-2.00%,
Nb: 0.0030-0.0400%,
Ti: 0.003 to 0.040%,
Al: 0.010-0.080%,
Sn: 0.0200-0.4000%,
N: 0.0010-0.0070%,
O: 0.0005-0.0040%,
Ca: 0.0001-0.0080%,
and further containing one or more selected from the group consisting of the following Group A, Group B, and Group C:
The balance is Fe and impurities.
Ceq represented by the following formula (i) is 0.32 to 0.47,
The SnEQ represented by the following formula (ii) is 0.10 or more,
The SC represented by the following formula (iii) is 0.30 to 15.00,
The thickness of the steel plate is 41 mm or more and less than 75 mm,
The tensile strength at a quarter-thickness position of the steel plate is 570 MPa or more,
At the center of the thickness of a cross section perpendicular to the rolling direction of the steel plate,
The maximum Mn segregation degree is 1.30 or less,
The maximum defect length is 0.30 mm or less.
steel plate.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
SnEQ=Sn+W+Ni/10+Mo/4...(ii)
SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.
[Group A] One or more selected from the group consisting of Cu: 0.80% or less, Mo: 0.60% or less, V: 0.150% or less, B: 0.0050% or less, Zr: 0.050% or less, and Ta: 0.05% or less. [Group B] One or more selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group C] One or more selected from the group consisting of Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, and REM: 0.010% or less.

(3) The steel sheet according to (2) above, wherein the chemical composition contains one or more elements selected from Group A.

(4) The steel sheet according to (2) above, wherein the chemical composition contains one or more elements selected from Group B.

(5) The steel sheet according to (2) above, wherein the chemical composition contains one or more elements selected from Group C.

(6) The method for producing a steel sheet according to (1) above,
a soaking process in which the slab is heated and then soaked;
a hot rolling step of hot rolling the slab to form a steel plate,
The chemical composition of the cast piece is, in mass%,
C: 0.04-0.15%,
Si: 0.05-1.20%,
Mn: 0.50-2.00%,
P: 0.015% or less,
S: 0.0024% or less,
Ni: 0.10-2.00%,
Nb: 0.0030-0.0400%,
Ti: 0.003 to 0.040%,
Al: 0.010-0.080%,
Sn: 0.0200-0.4000%,
N: 0.0010-0.0070%,
O: 0.0005-0.0040%,
Ca: 0.0001 to 0.0080%, and the balance: Fe and impurities;
Ceq represented by the following formula (i) is 0.32 to 0.47,
The SnEQ represented by the following formula (ii) is 0.10 or more,
The SC represented by the following formula (iii) is 0.30 to 15.00,
In the soaking step, the cast slab is soaked at 1050 to 1250°C,
In the hot rolling step, the cumulative reduction rate in a temperature range of 900°C or higher is set to 52 to 70%.
Steel plate manufacturing method.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
SnEQ=Sn+W+Ni/10+Mo/4...(ii)
SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the slab, and 0 is substituted if the element is not contained.

(7) The method for producing a steel sheet according to (2) above,
a soaking process in which the slab is heated and then soaked;
a hot rolling step of hot rolling the slab to form a steel plate,
The chemical composition of the cast piece is, in mass%,
C: 0.04-0.15%,
Si: 0.05-1.20%,
Mn: 0.50-2.00%,
P: 0.015% or less,
S: 0.0024% or less,
Ni: 0.10-2.00%,
Nb: 0.0030-0.0400%,
Ti: 0.003 to 0.040%,
Al: 0.010-0.080%,
Sn: 0.0200-0.4000%,
N: 0.0010-0.0070%,
O: 0.0005-0.0040%,
Ca: 0.0001-0.0080%,
and further containing one or more selected from the group consisting of the following Group A, Group B, and Group C:
The balance is Fe and impurities.
Ceq represented by the following formula (i) is 0.32 to 0.47,
The SnEQ represented by the following formula (ii) is 0.10 or more,
The SC represented by the following formula (iii) is 0.30 to 15.00,
In the soaking step, the cast slab is soaked at 1050 to 1250°C,
In the hot rolling step, the cumulative reduction rate in a temperature range of 900°C or higher is set to 52 to 70%.
Steel plate manufacturing method.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
SnEQ=Sn+W+Ni/10+Mo/4...(ii)
SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the slab, and 0 is substituted if the element is not contained.
[Group A] One or more selected from the group consisting of Cu: 0.80% or less, Mo: 0.60% or less, V: 0.150% or less, B: 0.0050% or less, Zr: 0.050% or less, and Ta: 0.05% or less. [Group B] One or more selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group C] One or more selected from the group consisting of Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, and REM: 0.010% or less.

(8) The method for manufacturing a steel sheet according to (7) above, wherein the chemical composition contains one or more elements selected from Group A.

(9) The method for manufacturing a steel sheet according to (7) above, wherein the chemical composition contains one or more elements selected from Group B.

(10) The method for manufacturing a steel sheet according to (7) above, wherein the chemical composition contains one or more elements selected from Group C.

The present invention provides a thick steel plate that has high tensile strength, excellent corrosion resistance in chloride-containing environments, and improved lamellar tear resistance.

FIG. 1 is a diagram for explaining a method for preparing a test piece for evaluating lamellar tear resistance.

The inventors conducted a detailed study into the effects of chemical composition and metal structure on the tensile strength, corrosion resistance, and lamellar tear resistance of thick steel plates with a thickness of 41 mm or more and less than 75 mm used in bridges and other structures, and as a result, they obtained the following findings.

1. Tensile Strength In order to increase tensile strength, it is effective to add alloying elements to improve hardenability. Therefore, the value of Ceq, defined as C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, needs to be set to 0.32 to 0.47.

2. Corrosion Resistance Steel plate corrosion occurs when iron ions are eluted from water adhering to the surface of the steel plate, and these iron ions combine with oxygen in the air to form iron oxide, causing rust. The rust is then hydrolyzed in the adhering water, generating hydrogen ions, which create an acidic aqueous solution. However, in an environment containing chlorides, the adhering water contains chloride ions, making the aqueous solution highly acidic. As a result, corrosion of the steel plate progresses more rapidly.

Therefore, it is necessary to include Sn as an essential additive element and to set the SnEQ value, defined as SnEQ = Sn + W + Ni/10 + Mo/4, to 0.10 or higher. When W, Ni, and Mo are added to the steel sheet in addition to Sn, the elution of iron ions is suppressed, slowing down the rate of rust formation. As a result, it has been discovered that excellent corrosion resistance can be achieved even in environments containing chlorides.

3. Lamellar Tearing Resistance However, Sn, which is added to improve corrosion resistance, has a small solid-liquid distribution coefficient. Therefore, Sn easily migrates to the liquid phase during slab solidification from the surface during casting. As a result, Sn concentrates in the center of the slab, which in turn promotes the concentration of other elements such as Mn, lowering the melting point of the slab center. In particular, in the central segregation zone, which is the final solidification zone, the melting point is significantly lowered due to the concentration of alloying elements, especially in the area where elements such as Sn are segregated (hereinafter referred to as the "Sn-segregated zone"). Therefore, the Sn-segregated zone promotes the formation of casting defects such as porosity and promotes hardening due to its increased hardenability. As a result, in evaluation tests for lamellar tearing resistance, it was found that cracks originated from the Sn-segregated zone.

As such, when it is necessary to improve tensile strength and corrosion resistance while adding alloying elements, ensuring lamellar tear resistance becomes difficult. In particular, thick steel plates contain a large amount of alloying elements to achieve high tensile strength, and Sn is added to achieve corrosion resistance, which promotes the formation of casting defects in Sn segregated areas and hardening due to the concentration of alloying elements. Therefore, ensuring lamellar tear resistance becomes extremely difficult. Therefore, the inventors conducted further detailed research to improve lamellar tear resistance while maintaining the tensile strength and corrosion resistance of thick plates, and as a result, they discovered the following.

The impurity element S has a particularly low solid-liquid distribution coefficient, and is an element that tends to concentrate, along with Sn, in the center of the steel plate in the thickness direction (hereinafter referred to as the "center of the plate thickness"). S is an element that significantly lowers the melting point, and is a factor that promotes casting defects such as porosity. S also combines with Mn to form MnS. When MnS is stretched during rolling, it behaves in the same way as unbonded porosity areas, thereby degrading lamellar tear resistance.

We therefore discovered that Ca is an essential additive element, and that the SC value, defined as SC = S/Ca, must be in the range of 0.30 to 15.00. Ca is an element that reduces the S content in the liquid phase by forming sulfides during the solidification process. This suppresses the concentration of S in the center of the plate thickness, thereby suppressing the formation of casting defects and the concentration of alloying elements in the center of the plate thickness, thereby improving lamellar tear resistance.

The inventors further investigated manufacturing methods to reduce large casting defects that cause cracks. As a result, they discovered that by increasing the reduction rate in the high-temperature range of 900°C or higher during the hot rolling process, casting defects can be consolidated and their size reduced.

The present invention was made based on the above findings. Each of the requirements of the present invention will be explained in detail below.

(A) Chemical Composition The reasons for limiting the content of each element are as follows: In the following description, "%" for the content means "% by mass."

C: 0.04-0.15%
C is an element necessary to ensure the strength of steel sheets. However, excessive C content increases the hardness of Sn-segregated areas, reducing lamellar tear resistance and significantly reducing weldability. Furthermore, as the C content increases, the amount of cementite produced, which acts as a cathode and promotes corrosion in a pH-decreasing environment, decreases, resulting in reduced corrosion resistance. Therefore, the C content is set to 0.04 to 0.15%. The C content is preferably 0.05% or more, more preferably 0.06% or more. The C content is preferably 0.13% or less, more preferably 0.12% or less.

Si: 0.05-1.20%
Silicon is added for the purpose of deoxidation. Silicon is also an element necessary for ensuring strength because it has the effect of suppressing temper softening. However, excessive silicon content impairs the toughness of the base material and welded joints. Therefore, the silicon content is set to 0.05 to 1.20%. The silicon content is preferably 0.10% or more, and more preferably 0.20% or more. The silicon content is preferably 0.90% or less, and more preferably 0.70% or less.

Mn: 0.50-2.00%
Mn is an element that enhances the hardenability and thereby increases the strength of steel sheets. However, Mn is an element that easily concentrates in the liquid phase during casting, and excessive Mn content leads to excessive Mn concentration in the center segregation region. As a result, the hardenability becomes excessive, and the hardness increases, resulting in a deterioration in lamellar tear resistance. Therefore, the Mn content is set to 0.50 to 2.00%. The Mn content is preferably 0.60% or more, and more preferably 0.70% or more. The Mn content is preferably 1.80% or less, and more preferably 1.70% or less.

P: 0.015% or less P is an element present as an impurity in steel sheet. P is an element that reduces acid resistance, and in chloride corrosion environments where the pH of the corrosion interface decreases, it reduces corrosion resistance. Furthermore, P is an element that easily segregates at grain boundaries, and an increase in the amount of grain boundary segregation reduces the lamellar tear resistance and toughness of the steel sheet, so the lower the content, the better. Therefore, the P content is set to 0.015% or less. The P content is preferably 0.013% or less, and more preferably 0.011% or less.

S: 0.0024% or less S is an element present as an impurity in steel sheet. S is an element with a very small solid-liquid distribution coefficient, and when concentrated in the liquid phase during casting, it forms coarse MnS in the central segregation region, thereby degrading toughness. Furthermore, when MnS is stretched during rolling, it spreads in a planar shape in the rolling direction, significantly degrading lamellar tear resistance. Therefore, it is preferable to reduce the S content as much as possible. To ensure toughness and lamellar tear resistance, the S content is set to 0.0024% or less. The S content is preferably 0.0020% or less, and more preferably 0.0018% or less.

Ni: 0.10-2.00%
Ni improves strength by increasing hardenability and improves the toughness of the matrix structure. Ni also has the effect of improving low-temperature toughness. Ni is also an element that improves corrosion resistance by suppressing the anodic dissolution of steel in low-pH environments. However, Ni is an expensive element, and even if it is contained in an amount exceeding 2.00%, not only does the effect saturate but it also leads to a significant increase in cost. Therefore, the Ni content is set to 0.10 to 2.00%. The Ni content is preferably 0.20% or more, and more preferably 0.30% or more. Furthermore, the Ni content is preferably 1.70% or less, and more preferably 1.40% or less.

Nb: 0.0030-0.0400%
Nb is an element that improves hardenability and strength. Furthermore, Nb has the effect of widening the unrecrystallized region, so rolling in that temperature range can introduce high-density dislocations. Furthermore, increasing the number of transformation nucleation sites can refine the structure of the steel sheet. As a result, toughness can be improved. However, excessive Nb content increases hardenability by concentrating in the center segregation region during casting, and also forms coarse Nb carbides, deteriorating lamellar tear resistance. Therefore, the Nb content is set to 0.0030 to 0.0400%. The Nb content is preferably 0.0080% or more, more preferably 0.0120% or more. The Nb content is preferably 0.0350% or less, more preferably 0.0300% or less.

Ti: 0.003-0.040%
Ti contributes to increasing strength. It also combines with N to form TiN, which acts as pinning particles to suppress austenite grain growth during reheating and quenching. This refines the austenite grains, which is also effective in improving toughness. However, excessive Ti content causes coarse TiN to form in the central segregation region, degrading lamellar tear resistance. Therefore, the Ti content is set to 0.003 to 0.040%. The Ti content is preferably 0.005% or more, more preferably 0.008% or more. The Ti content is preferably 0.035% or less, more preferably 0.030% or less.

Al: 0.010-0.080%
Al is an effective element for deoxidizing steel. However, excessive content of Al not only reduces corrosion resistance in low pH environments, thereby reducing corrosion resistance in chloride corrosion environments, but also causes nitrides to coarsen, resulting in a decrease in toughness. Therefore, the Al content is set to 0.010 to 0.080%. The Al content is preferably 0.015% or more, and more preferably 0.020% or more. Furthermore, the Al content is preferably 0.070% or less, and more preferably 0.060% or less.

Sn: 0.0200-0.4000%
Sn is an element that improves the corrosion resistance of steel. In addition, the inclusion of Sn in a steel sheet makes it possible to form a Sn oxide layer on the surface of the steel sheet in advance. The Sn oxide layer significantly suppresses the anodic dissolution reaction and hydrogen evolution reaction of steel in a low-pH chloride environment, thereby significantly improving corrosion resistance in a chloride corrosion environment.

However, if added in excess, not only will the above effects saturate, but as mentioned above, Sn itself not only segregates during casting, but also promotes the segregation of other alloying elements, resulting in reduced lamellar tear resistance. Furthermore, the toughness of the base material also decreases. For this reason, the Sn content is set to 0.0200-0.4000%. The Sn content is preferably 0.0300% or more, and more preferably 0.0800% or more. Furthermore, the Sn content is preferably 0.3500% or less, and more preferably 0.3000% or less.

N: 0.0010-0.0070%
N forms nitrides with Ti, suppressing grain coarsening during reheating and quenching, thereby contributing to improved toughness. Furthermore, N dissolves as ammonia, suppressing pH reduction due to hydrolysis of Fe ³⁺ in environments with high airborne salt content, thereby improving the corrosion resistance of steel sheets in chloride corrosion environments. However, excessive N content not only saturates the effect, but also leads to the formation of coarse AlN and TiN, reducing the toughness of the steel sheet. Therefore, the N content is set to 0.0010 to 0.0070%. The N content is preferably 0.0020% or more, more preferably 0.0025% or more. The N content is preferably 0.0060% or less, more preferably 0.0050% or less.

O: 0.0005-0.0040%
O is added to remove impurities during the refining process, increasing the cleanliness of the steel sheet and ensuring the toughness of the steel sheet. However, O forms oxides such as SnO and _SnO2 . Therefore, excessive O content makes it difficult to ensure a sufficient Sn content in the steel. Furthermore, these oxides act as starting points for corrosion, reducing the corrosion resistance of the steel sheet. Therefore, the O content is limited to 0.0005 to 0.0040%. The O content is preferably 0.0007% or more, more preferably 0.0009% or more. The O content is preferably 0.0030% or less, more preferably 0.0025% or less.

Ca: 0.0001-0.0080%
Ca forms sulfides during the casting process of slabs, suppressing the concentration of S in the center of the plate thickness, thereby suppressing a decrease in melting point. As a result, Ca suppresses casting defects such as porosity and the formation of MnS elongated in the rolling direction. It also suppresses the concentration of alloying elements in the center of the plate thickness, improving lamellar tear resistance. Ca also exists in the form of oxides in steel, suppressing a decrease in pH at the interface in corrosion reaction zones and suppressing the acceleration of corrosion. However, excessive oxide content causes coarsening and degrades toughness. Therefore, the Ca content is limited to 0.0001 to 0.0080%. The Ca content is preferably 0.0005% or more, more preferably 0.0009% or more. The Ca content is preferably 0.0070% or less, more preferably 0.0060% or less.

The steel sheet according to the present invention has the above chemical composition, with the balance consisting of Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

Ceq: 0.32-0.47
As described above, in order to improve hardenability and tensile strength, Ceq, defined by the following formula (i), is set to 0.32 or more. On the other hand, if Ceq exceeds 0.47, not only toughness and ductility but also weldability will decrease. Furthermore, lamellar tear resistance may decrease. Therefore, Ceq is set to 0.32 to 0.47. Ceq is preferably 0.34 or more and 0.45 or less.

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

SnEQ: 0.10 or more By adding W, Ni, and Mo to the steel sheet in addition to Sn, the elution of iron ions is suppressed, thereby improving corrosion resistance in chloride-containing environments. Therefore, the SnEQ defined by the following formula (ii) is set to 0.10 or more. The SnEQ is preferably 0.13 or more. Although there is no particular upper limit for the SnEQ, in the chemical composition of the present invention, the substantial upper limit for the SnEQ is 1.59. The SnEQ is preferably 1.00 or less. Furthermore, since the effect of improving corrosion resistance saturates when the SnEQ exceeds 0.40, it is unnecessary to include any further alloying elements for the purpose of corrosion resistance. Therefore, the SnEQ is preferably 0.40 or less. However, when SnEQ is added for the purpose of ensuring tensile strength and low-temperature toughness, there is no problem even if the SnEQ exceeds 0.40.

SnEQ=Sn+W+Ni/10+Mo/4...(ii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

SC:0.30~15.00
As described above, Ca forms sulfides during the solidification process and can improve lamellar tear resistance by suppressing the concentration of S in the center of the plate thickness. However, an extreme reduction in the S content leads to increased steelmaking costs. Therefore, the SC, defined as the ratio of the S content to the Ca content, as shown in the following formula (iii), is set to 0.30 or more. On the other hand, if the SC exceeds 15.00, the S content relative to the Ca content is too high, so even if Ca is added, the concentration of S in the center of the plate thickness cannot be controlled, and lamellar tear resistance decreases. Therefore, the SC is set to 0.30 to 15.00. The SC is preferably 0.50 or more, preferably 10.00 or less, and more preferably 6.00 or less.

SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass %) of each element contained in the steel sheet.

In order to improve strength, the chemical composition of the steel sheet of the present invention may further contain one or more elements selected from the following Group A (Cu: 0.80% or less, Mo: 0.60% or less, V: 0.150% or less, B: 0.005% or less, Zr: 0.050% or less, Ta: 0.05% or less) within the ranges shown below. Note that these elements are not necessarily essential for the steel sheet of the present invention, so the lower limit of their content is 0%. The reasons for limiting each element are explained below.

Cu: 0.80% or less Cu is an element that enhances the hardenability of steel sheets. It can be added as needed because it inhibits the anodic dissolution of steel in low pH environments, thereby improving corrosion resistance. However, excessive addition not only saturates the effect but also causes embrittlement. Therefore, the Cu content is set to 0.80% or less. The Cu content is preferably 0.50% or less, and more preferably 0.30% or less. To stably obtain the above effects, the Cu content is preferably 0.001% or more, 0.005%, 0.010% or more, or 0.050% or more, and more preferably 0.10% or more.

Mo: 0.60% or less Mo is an element that enhances the strength of steel sheet without reducing lamellar tear resistance. Specifically, Mo improves strength by precipitating Mo carbides during tempering. Mo also dissolves and adsorbs to rust in the form of oxyanions MoO ₄ ^2- , inhibiting the permeation of chloride ions through the rust layer. Therefore, Mo can be added as needed. However, excessive Mo content not only saturates the effect, but also leads to excessive strength of the steel sheet, resulting in a deterioration of toughness. Therefore, the Mo content is set to 0.60% or less. The Mo content is preferably 0.50% or less, and more preferably 0.40% or less. To stably obtain the above effects, the Mo content is preferably 0.03% or more, and more preferably 0.05% or more.

V: 0.150% or less V is an element that improves hardenability and increases the strength of the steel sheet. Specifically, V improves strength by precipitating V carbonitrides during tempering. Furthermore, like Mo, V dissolves and exists in the form of oxyanions, inhibiting the permeation of chloride ions through rust layers. Therefore, V can be added as needed. However, excessive V content not only saturates the effect, but also increases alloying costs because V is an expensive alloying element. Therefore, the V content is set to 0.150% or less. The V content is preferably 0.130% or less, and more preferably 0.110% or less. To stably obtain the above effects, the V content is preferably 0.020% or more, and more preferably 0.030% or more.

B: 0.0050% or less B is an element that improves hardenability and increases strength, so it can be added as needed. However, if added in excess, the effect of increasing strength saturates, and there is a significant tendency for the toughness of both the base material and the HAZ to decrease. Therefore, the B content is set to 0.0050% or less. In order to stably obtain the above effect, it is preferable that the B content be 0.0003% or more.

Zr: 0.050% or less Like Ti, Zr is an element that forms oxides and contributes to refining crystal grains and improving strength, so it may be contained as needed. However, if Zr is contained in excess, the oxides become coarse and mechanical properties deteriorate. Therefore, the Zr content is set to 0.050% or less. The Zr content is preferably 0.030% or less. In order to more reliably obtain the above effects, the Zr content is preferably set to 0.001% or more, and more preferably 0.005% or more.

Ta: 0.05% or less Ta is an element that contributes to improving strength and also contributes to improving corrosion resistance, although the mechanism is not necessarily clear. Therefore, it may be contained as needed. However, Ta is an expensive element, and the inclusion of a large amount of Ta increases the steelmaking cost. Therefore, the Ta content is set to 0.05% or less. The Ta content is preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. Note that, to more reliably obtain the above effects, the Ta content is preferably 0.001% or more, more preferably 0.005% or more.

In order to improve corrosion resistance, the chemical composition of the steel sheet of the present invention may further contain one or more elements selected from the following Group B (Cr: 0.20% or less, W: 0.80% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Sb: 0.10% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, Hf: 0.05% or less) within the ranges shown below. Note that these elements are not necessarily essential for the steel sheet of the present invention, and therefore the lower limit of their content is 0%. The reasons for limiting each element are explained below.

Cr: 0.20% or less Cr is an element that generally improves corrosion resistance and has the effect of increasing hardenability and improving strength, so it can be added as needed. However, Cr reduces acid resistance, which reduces the corrosion resistance in chloride-rich environments, which is the problem to be solved by the present invention. On the other hand, since a content of 0.20% or less does not result in a decrease in acid resistance, the Cr content is set to 0.20% or less. The Cr content is preferably 0.15% or less, and more preferably 0.12% or less. In order to stably obtain the above effects, the Cr content is preferably 0.005% or more, or 0.01% or more, and more preferably 0.02% or more.

W: 0.80% or less Like Mo, W dissolves and exists in the form of oxyanions WO ₄ ^2-. This element has the effect of inhibiting the permeation of chloride ions through rust layers, and can be added as needed. However, excessive W content not only saturates the effect, but also significantly increases the cost of the steel sheet. Therefore, the W content is set to 0.80% or less. The W content is preferably 0.60% or less, and more preferably 0.40% or less. In order to stably obtain the above effects, the W content is preferably 0.01% or more, and more preferably 0.02% or more.

Sb: 0.10% or less Sb forms sulfides with S and is an element effective in improving corrosion resistance in an acid corrosion environment, so it may be contained as needed. However, if Sb is contained in excess, toughness decreases. Therefore, the Sb content is set to 0.10% or less. The Sb content is preferably 0.08% or less, and more preferably 0.05% or less. Note that, to more reliably obtain the above effects, the Sb content is preferably 0.005% or more, and more preferably 0.010% or more.

As: 0.05% or less Although As does not have a significant effect compared to Sb, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be contained as needed. However, if As is contained in excess, toughness decreases. Therefore, the As content is set to 0.05% or less. The As content is preferably 0.04% or less, and more preferably 0.03% or less. Note that, to more reliably obtain the above effects, the As content is preferably 0.003% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

Bi: 0.05% or less Although Bi does not have a significant effect compared to Sb, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be contained as needed. However, if Bi is contained in excess, toughness decreases. Therefore, the Bi content is set to 0.05% or less. The Bi content is preferably 0.04% or less, and more preferably 0.03% or less. Note that, to more reliably obtain the above effects, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

Se: 0.05% or less Although Se does not have a significant effect compared to Sb, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be contained as needed. However, if Se is contained in excess, toughness decreases. Therefore, the Se content is set to 0.05% or less. The Se content is preferably 0.04% or less, and more preferably 0.03% or less. Note that, to more reliably obtain the above effects, the Se content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

Te: 0.05% or less Although Te does not have a significant effect compared to Sb, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be contained as needed. However, if Te is contained in excess, toughness decreases. Therefore, the Te content is set to 0.05% or less. The Te content is preferably 0.04% or less, and more preferably 0.03% or less. Note that, to more reliably obtain the above effects, the Te content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

Zn: 0.05% or less Ga: 0.05% or less Zn and Ga form sulfides with S and are effective elements for improving corrosion resistance in acid corrosion environments, so they may be contained as needed. However, if Zn and Ga are contained in excess, toughness decreases. Therefore, the Zn and Ga contents are each set to 0.05% or less. The Zn and Ga contents are each preferably set to 0.04% or less, and more preferably set to 0.03% or less. Note that, to more reliably obtain the above effects, the Zn and Ga contents are each preferably set to 0.005% or more, and more preferably set to 0.010% or more.

Ge: 0.05% or less Ge forms sulfides with S and is an element effective in improving corrosion resistance in an acid corrosion environment, so it may be contained as needed. However, if Ge is contained in excess, toughness decreases. Therefore, the Ge content is set to 0.05% or less. The Ge content is preferably 0.04% or less, and more preferably 0.03% or less. Note that, to more reliably obtain the above effects, the Ge content is preferably 0.005% or more, and more preferably 0.010% or more.

Co: 0.50% or less Co is an element that forms oxides to improve corrosion resistance, so it may be contained as needed. However, excessive Co content reduces economic efficiency. Therefore, the Co content is set to 0.50% or less. The Co content is preferably 0.30% or less, and more preferably 0.20% or less. To more reliably obtain the above effects, the Co content is preferably 0.005%, 0.01% or more, or 0.05% or more, more preferably 0.08% or more, and even more preferably 0.10% or more.

Hf: 0.05% or less Hf is an element that forms oxides to improve corrosion resistance, so it may be contained as needed. However, if excessive Hf is contained, economic efficiency decreases. Therefore, the Hf content is set to 0.05% or less. The Hf content is preferably 0.04% or less, and more preferably 0.03% or less. Note that, to more reliably obtain the above effects, the Hf content is preferably 0.002% or more, and more preferably 0.005% or more.

In order to control the cleanliness of the steel sheet and inclusions, the chemical composition of the steel sheet of the present invention may further contain one or more elements selected from the following Group C (Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, REM: 0.010% or less) within the ranges shown below. Note that these elements are not necessarily essential for the steel sheet of the present invention, so the lower limit of their content is 0%. The reasons for limiting each element are explained below.

Mg: 0.010% or less Mg suppresses a decrease in pH at the interface in the corrosion reaction zone, so it can be added as needed. However, if added in excess, the effect saturates. Therefore, the Mg content is set to 0.010% or less. The Mg content is preferably set to 0.005% or less. In order to stably obtain the above effect, the Mg content is preferably set to 0.0002% or more, and more preferably set to 0.0005% or more.

Sr: 0.010% or less Ba: 0.010% or less Sr and Ba may be added as needed to form fine oxides. However, excessive addition of Sr and Ba increases steelmaking costs. Therefore, the Sr and Ba contents are each set to 0.010% or less. The Sr and Ba contents are each preferably 0.008% or less, and more preferably 0.005% or less. To more reliably obtain the above effects, the Sr and Ba contents are each preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

REM: 0.010% or less REM (rare earth elements) have the effect of improving the weldability of steel, so they can be added as needed. However, if added in excess, the effect saturates, so the REM content is set to 0.010% or less. The REM content is preferably 0.0050% or less. In order to stably obtain the above effect, the REM content is preferably 0.0002% or more, and more preferably 0.0005% or more.

Here, REM is a collective term for 17 elements, including 15 lanthanide elements plus Y and Sc, and one or more of these elements can be contained. The REM content refers to the total content of these elements.

(B) Plate Thickness The thickness of the steel plate according to the present invention is set to 41 mm or more and less than 75 mm, since it is used for structures such as bridges.

(C) Maximum Mn Segregation and Maximum Defect Length In the steel sheet according to the present invention, in a cross section perpendicular to the rolling direction of the steel sheet (hereinafter referred to as "C cross section"), the maximum Mn segregation at the center of the sheet thickness is 1.30 or less, and the maximum defect length is 0.30 mm or less. The reasons for limiting each requirement will be explained.

Maximum Mn segregation degree: 1.30 or less. Since alloy elements such as Sn are segregated in the Sn-segregated area, the melting point is lower than that of the surrounding area, and the liquid phase state is maintained during the solidification process of the slab. When the area around the Sn-segregated area subsequently shrinks due to solidification, the Sn-segregated area is pulled, forming cavities. This can result in casting defects. Furthermore, since the casting defects are elongated by rolling, when tensile stress acts in the thickness direction, stress concentrates at both ends of the casting defects in the rolling direction. As a result, cracks can occur from the casting defects.

As described below, the maximum Mn segregation degree can be reduced by applying a soft reduction during final solidification in the casting process. In particular, the maximum Mn segregation degree is set to 1.30 or less. While various alloying elements segregate in the Sn-segregated areas, Mn is the most susceptible to segregation, and an evaluation method for this has been established. Therefore, it was decided to measure the Mn segregation degree as an indicator of the segregation of alloying elements.

In addition, in the present invention, the "center of plate thickness" refers to the center of the steel plate in the plate thickness direction, and is the part of the steel plate where elements such as Sn and Mn tend to concentrate. Specifically, when the plate thickness of the steel plate is t, it is the region between 9/20t and 11/20t.

In the present invention, the "Sn segregation area" refers to a portion of the central segregation area where elements such as Sn are segregated. When observing the metallographic structure at the center of the plate thickness after nital etching (described below), this portion is observed as a band-like or block-like black distorted metallographic structure compared to the surrounding metallographic structure. Furthermore, when it is difficult to distinguish from the metallographic structure, it can be distinguished by also conducting Mn mapping measurement using an electron probe microanalyzer (EPMA).

The maximum Mn segregation degree is measured using the following method. A 20 mm wide x 20 mm thick specimen is taken from the center of the steel sheet thickness, with the C-section of the steel sheet serving as the observation surface. The observation surface is then mirror-polished, and the Mn content is measured using an EPMA with an acceleration voltage of 15 kV, a beam diameter of 50 μm, an irradiation time of 30 ms, and a measurement pitch of 50 μm. Next, to accurately measure the Mn content in the most Mn-enriched area (Mn-enriched zone), the Mn content is measured in a 1 mm wide x 1 mm thick region with an acceleration voltage of 15 kV, a beam diameter of 2 μm, an irradiation time of 30 ms, and a measurement pitch of 2 μm. The region is set to a width of 1 mm x a thickness of 1 mm, with the Mn-enriched zone at its center. The ratio of the maximum Mn content in the observation field to the Mn content of the steel sheet is then calculated, and this is defined as the maximum Mn segregation degree.

Maximum defect length: 0.30 mm or less. Normally, central segregation occurs in the center of a slab. However, the liquid phase remaining in the center at the end of solidification during casting is subjected to tensile strain due to the shrinkage of the surrounding solid phase as the temperature drops. As a result, the liquid phase opens, forming voids and eventually casting defects. Elements such as Sn have a small solid-liquid distribution coefficient, so they significantly concentrate in the liquid phase as solidification progresses, lowering the melting point. As the melting point of the liquid phase decreases, the time from the start to the end of solidification increases. As the solidification time increases, the shrinkage strain of the solid phase surrounding the liquid phase increases, resulting in larger casting defects.

These casting defects disappear when they are stretched and pressed during the subsequent hot rolling process; however, if the size of the casting defect before rolling is large, or if the required steel plate thickness is large and a sufficient reduction ratio cannot be ensured, the defect may not be completely pressed, leaving an unpressed defect. Normally, if this defect is small, it does not affect the tensile properties or Charpy impact properties, but in tensile tests in the plate thickness direction, stress may concentrate at the tip of the casting defect, causing fracture.

As described below, it is desirable to heat the slab soaking temperature to 1050-1250°C and to roll it so that the cumulative reduction in temperatures above 900°C during the hot rolling process is 52-70%, thereby reducing the size of casting defects. However, as the plate thickness increases and the tensile strength of the steel plate increases, it becomes more difficult to reduce casting defects through rolling, but the maximum defect length should be 0.30 mm or less. Preferably, the maximum defect length should be 0.20 mm or less.

The maximum defect length is measured using the following method. Test specimens for observation are taken from the center of the plate thickness at each of the 1/4W, 1/2W, and 3/4W width positions, so that the C-section of the steel plate becomes the observation surface. The observation surface is then mirror-polished, and the observation surface is continuously photographed across the width at 10 to 15 mm using an optical microscope. Based on the photographed structure, areas that appear black are determined to be casting defects, and each casting defect is enlarged and photographed to measure its maximum length. The largest casting defect among these is taken as the maximum defect length.

(D) Mechanical Properties The steel plate of the present invention has a tensile strength of 570 MPa or more at the 1/4 position of the plate thickness. The use of a steel plate with a tensile strength of 570 MPa or more enables designs with increased load capacity, making it easier to increase the size of buildings. Furthermore, the higher the tensile strength of the steel plate of the present invention, the larger the size of buildings becomes possible, so there is no upper limit to the tensile strength. However, excessive tensile strength causes a decrease in toughness and lamellar tear resistance, so the tensile strength is preferably 760 MPa or less. In the following description, when the plate thickness is t, the 1/4 position of the plate thickness is denoted as "1/4t."

Tensile strength is measured in accordance with JIS Z 2241:2022 using a No. 4 round bar tensile test specimen taken from a 1/4 t specimen so that the width direction of the specimen coincides with the longitudinal direction of the specimen.

(E) Anticorrosion Coating The steel sheet described above exhibits good corrosion resistance even when used as is. However, when its surface is subjected to an anticorrosion treatment, specifically when the surface is covered with an anticorrosion coating made of an organic resin or metal, the durability of the anticorrosion coating improves compared to conventional steel sheets, and the corrosion resistance is further improved.

Here, examples of corrosion-resistant coatings made from organic resins include vinyl butyral-based, epoxy-based, urethane-based, and phthalic acid-based resin coatings. Furthermore, examples of corrosion-resistant coatings made from metals include plated coatings of Zn, Al, Zn-Al, etc., and thermally sprayed coatings of Zn, Al, Al-Mg, etc.

The improved durability of the corrosion-protective coating is believed to be due to the fact that corrosion of the underlying steel sheet of the present invention is significantly suppressed, thereby suppressing swelling or peeling of the corrosion-protective coating caused by corrosion of the underlying steel sheet from defects in the corrosion-protective coating.

(F) Manufacturing Method The manufacturing method of the steel plate according to the present invention includes a soaking step of soaking a slab having the above-described chemical composition, and a hot rolling step of hot rolling the slab to produce a steel plate. Each step will be described in detail below.

<Casting process>
In the casting process, steel having the above-described chemical composition is melted by a conventional method and then continuously cast into a slab. In the final solidification stage of the continuous casting process, the slab solidifies and shrinks from the outside. As a result of solidification shrinkage, a gap is formed in the center of the slab, allowing unsolidified molten steel with concentrated elements such as Sn to move. As a result, elements such as Sn segregate in the center of the slab. Therefore, in the casting process, soft reduction is performed at the final solidification position of the slab. This reduces the gap formed in the center of the slab due to solidification shrinkage, thereby reducing the maximum Mn segregation degree. The soft reduction conditions may be well-known.

<Soaking process>
In the soaking process, the slab having the above-mentioned chemical composition is soaked. The soaking temperature is 1050 to 1250°C. By bringing the entire slab to the soaking temperature before starting rolling, the temperature at the center of the steel sheet remains high during rolling, which is advantageous for crimping of casting defects. The soaking time varies depending on the operating conditions and is not particularly limited, but it is preferably 200 minutes or more after the slab is placed in the heating furnace. Here, "soaking" means that the temperature at the thickness center of the slab reaches the soaking temperature and then is maintained at the same temperature. The temperature at the thickness center of the slab is calculated from the temperature inside the heating furnace by performing a heat conduction calculation simulation of the slab.

<Hot rolling process>
In the hot rolling process, the slab is hot rolled to produce a steel plate. In the hot rolling, the cumulative reduction in the temperature range of 900°C or higher is set to 52 to 70%. By increasing the reduction in the high temperature range, the maximum defect length can be reduced.

<Quenching process>
The steel sheet after hot rolling may be quenched. In the quenching process, the steel sheet after the hot rolling process may be quenched from a temperature range of 750°C or higher without lowering the temperature of the steel sheet below 750°C. Direct quenching after hot rolling provides a steel sheet having the above-mentioned tensile strength. On the other hand, if the cooling start temperature exceeds 900°C, quenching occurs in a state where the γ grain size is coarse, which may result in a decrease in toughness and lamellar tear resistance. Therefore, the cooling start temperature may be 900°C or lower.

In addition, from an economic standpoint, it is preferable to perform the above-mentioned direct quenching during the quenching process, but reheating and quenching may also be performed. The heating temperature before quenching may be in the range of 850 to 950°C. Quenching results in a steel plate with the above-mentioned tensile strength. The heating time may be adjusted depending on the plate thickness, and may be 180 to 240 minutes in order to heat the center of the steel plate until it reaches the same temperature as the heating temperature.

<Tempering process>
The steel sheet after quenching may be tempered. The tempering temperature may be in the temperature range of 570 to 700°C. By tempering in this temperature range, it is possible to suppress local hardening of the Sn segregated portions while maintaining the tensile strength of the entire steel sheet.

The coating process for the above-mentioned anticorrosion coating can be carried out using standard methods. It is not necessary to apply an anticorrosion coating to the entire surface of the steel plate; it is sufficient to apply anticorrosion treatment to only one side of the steel plate that is exposed to a corrosive environment, or the outer or inner surface in the case of a steel pipe, i.e., only a portion of the steel plate surface.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

Steel having the chemical composition shown in Table 1 was melted and soaked under the conditions shown in Table 2, then hot-rolled with the cumulative reduction shown in Table 2 to the required thickness, and then water-cooled to produce steel plate. Note that the cumulative reduction in Table 2 refers to the cumulative reduction in a temperature range of 900°C or higher.

<Method for measuring maximum Mn segregation>
The maximum Mn segregation degree was measured by the following method. An observation specimen measuring 20 mm in width and 20 mm in thickness was taken from the center of the steel sheet so that the C-section of the steel sheet served as the observation surface. The observation surface was then mirror-polished, and the Mn content was measured using an EPMA at an acceleration voltage of 15 kV, a beam diameter of 50 μm, an irradiation time of 30 ms, and a measurement pitch of 50 μm. Next, in order to accurately measure the Mn content of the most Mn-enriched part (Mn-enriched portion), the Mn content of a region measuring 1 mm in width and 1 mm in thickness was measured at an acceleration voltage of 15 kV, a beam diameter of 2 μm, an irradiation time of 30 ms, and a measurement pitch of 2 μm. At this time, a region measuring 1 mm in width and 1 mm in thickness was set so that the Mn-enriched portion was at the center. The ratio of the maximum Mn content in the observation field to the Mn content of the steel sheet was then calculated, and this was taken as the maximum Mn segregation degree.

<Method for measuring maximum defect length>
The maximum defect length was measured by the following method. Test specimens for observation were taken from the center of the plate thickness at each of the width positions of 1/4W, 1/2W, and 3/4W so that the C-section of the steel plate was the observation surface. The observation surface was then mirror-polished, and the observation surface was continuously photographed in the width direction at 10 to 15 mm using an optical microscope. Based on the photographed structure, areas that appeared black were determined to be casting defects, and each casting defect was enlarged and photographed to measure its maximum length. The largest casting defect among them was taken as the maximum defect length.

<Evaluation of tensile strength>
The tensile strength was measured using No. 4 tensile test pieces taken from 1/4t so that the plate width direction coincided with the longitudinal direction of the test piece, in accordance with JIS Z 2241:2022. A test piece having a tensile strength of 570 MPa or more was judged to be acceptable.

<Evaluation of corrosion resistance>
Corrosion resistance was evaluated using each test material. The corrosion test was performed using the SAE (Society of Automotive Engineers) J2334 test. The J2334 test is an accelerated test consisting of 6 hours of wet (50°C, 100% RH), 0.25 hours of salt exposure (immersion in _a 0.5% NaCl, 0.1% CaCl2, 0.075% _NaHCO3 aqueous solution), and 17.75 hours of dry (60°C, 50% RH) in one cycle (total 24 hours), and the corrosion pattern is said to be similar to a corrosive environment containing chlorides (Hiroo Nagano, Masato Yamashita, Hitoshi Uchida: Environmental Materials Science, Kyoritsu Shuppan (2004), p. 74).

After 40 cycles of the J2334 test, the rust layer on the surface of each test specimen was removed and the thickness reduction (mm) was measured. In this invention, if the thickness reduction in the above test was 0.15 mm or less, it was judged as "○: Excellent corrosion resistance," and if it was more than 0.15 mm, it was judged as "×: Poor corrosion resistance."

<Evaluation of lamellar tear resistance>
The lamellar tear resistance was evaluated using the following method. FIG. 1 is a diagram illustrating a method for preparing a test piece 10 for evaluating lamellar tear resistance. The test piece 10 was prepared by processing in the order of FIGS. 1(a) to 1(f). First, as shown in FIG. 1(a), a section 1 for evaluating lamellar tear resistance was cut from the steel plate at a width position corresponding to ¼ W, so as to include the plate thickness Z (see FIG. 1(b)). Then, as shown in FIG. 1(c), blocks 2 were pressed against one side and the other side of the section 1 in the plate thickness direction to prepare a blank 3 for evaluating lamellar tear resistance. A round bar 4 for evaluating lamellar tear resistance was then cut from the blank 3 (see FIG. 1(d) and FIG. 1(e)), and further processed to prepare the test piece 10. Specifically, as shown in FIG. 1(f), the gripping portion 5 was threaded, and the length Lc of the parallel portion 6 was set equal to the plate thickness Z. The diameter D ₀ of the parallel portion was set to 10 mm, and the length of the test piece 10 in the longitudinal direction was set to 200 to 300 mm.

A tensile test was conducted using test piece 10 in accordance with JIS G 3199:2021, and the reduction in area (%) was calculated. A reduction in area of 35% or greater was considered to be acceptable.

Table 3 summarizes the measurement results for maximum Mn segregation and maximum defect length, as well as the evaluation results for tensile strength, corrosion resistance, and lamellar tear resistance.

As shown in Table 3, Tests No. 1 to 25, which satisfied all of the specifications of the present invention, achieved excellent results in all performance evaluations. In contrast, Tests No. 26 to 34, which are comparative examples, resulted in a deterioration in at least one of the properties: tensile strength, corrosion resistance, and lamellar tear resistance.

The present invention provides a thick steel plate that has high tensile strength, excellent corrosion resistance in chloride-containing environments, and improved lamellar tear resistance.

REFERENCE SIGNS LIST 1 Section 2 Block 3 Material 4 Round bar 5 Grip portion 6 Parallel portion 10 Test piece

Claims (10)

The chemical composition of the steel plate is, in mass%,
C: 0.04-0.15%,
Si: 0.05-1.20%,
Mn: 0.50-2.00%,
P: 0.015% or less,
S: 0.0024% or less,
Ni: 0.10-2.00%,
Nb: 0.0030-0.0400%,
Ti: 0.003 to 0.040%,
Al: 0.010-0.080%,
Sn: 0.0200-0.4000%,
N: 0.0010-0.0070%,
O: 0.0005-0.0040%,
Ca: 0.0001 to 0.0080%, and the balance: Fe and impurities;
Ceq represented by the following formula (i) is 0.32 to 0.47,
The SnEQ represented by the following formula (ii) is 0.10 or more,
The SC represented by the following formula (iii) is 0.30 to 15.00,
The thickness of the steel plate is 41 mm or more and less than 75 mm,
The tensile strength at a quarter-thickness position of the steel plate is 570 MPa or more,
At the center of the thickness of a cross section perpendicular to the rolling direction of the steel plate,
The maximum Mn segregation degree is 1.30 or less,
The maximum defect length is 0.30 mm or less.
steel plate.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
SnEQ=Sn+W+Ni/10+Mo/4...(ii)
SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained. The chemical composition of the steel plate is, in mass%,
C: 0.04-0.15%,
Si: 0.05-1.20%,
Mn: 0.50-2.00%,
P: 0.015% or less,
S: 0.0024% or less,
Ni: 0.10-2.00%,
Nb: 0.0030-0.0400%,
Ti: 0.003 to 0.040%,
Al: 0.010-0.080%,
Sn: 0.0200-0.4000%,
N: 0.0010-0.0070%,
O: 0.0005-0.0040%,
Ca: 0.0001-0.0080%,
and further containing one or more selected from the group consisting of the following Group A, Group B, and Group C:
The balance is Fe and impurities.
Ceq represented by the following formula (i) is 0.32 to 0.47,
The SnEQ represented by the following formula (ii) is 0.10 or more,
The SC represented by the following formula (iii) is 0.30 to 15.00,
The thickness of the steel plate is 41 mm or more and less than 75 mm,
The tensile strength at a quarter-thickness position of the steel plate is 570 MPa or more,
At the center of the thickness of a cross section perpendicular to the rolling direction of the steel plate,
The maximum Mn segregation degree is 1.30 or less,
The maximum defect length is 0.30 mm or less.
steel plate.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
SnEQ=Sn+W+Ni/10+Mo/4...(ii)
SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.
[Group A] One or more selected from the group consisting of Cu: 0.80% or less, Mo: 0.60% or less, V: 0.150% or less, B: 0.0050% or less, Zr: 0.050% or less, and Ta: 0.05% or less. [Group B] One or more selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group C] One or more selected from the group consisting of Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, and REM: 0.010% or less. The steel sheet according to claim 2, wherein the chemical composition contains one or more elements selected from Group A. The steel sheet according to claim 2, wherein the chemical composition contains one or more elements selected from Group B. The steel sheet according to claim 2, wherein the chemical composition contains one or more elements selected from Group C. The method for producing a steel sheet according to claim 1,
a soaking process in which the slab is heated and then soaked;
a hot rolling step of hot rolling the slab to form a steel plate,
The chemical composition of the cast piece is, in mass%,
C: 0.04-0.15%,
Si: 0.05-1.20%,
Mn: 0.50-2.00%,
P: 0.015% or less,
S: 0.0024% or less,
Ni: 0.10-2.00%,
Nb: 0.0030-0.0400%,
Ti: 0.003 to 0.040%,
Al: 0.010-0.080%,
Sn: 0.0200-0.4000%,
N: 0.0010-0.0070%,
O: 0.0005-0.0040%,
Ca: 0.0001 to 0.0080%, and the balance: Fe and impurities;
Ceq represented by the following formula (i) is 0.32 to 0.47,
The SnEQ represented by the following formula (ii) is 0.10 or more,
The SC represented by the following formula (iii) is 0.30 to 15.00,
In the soaking step, the cast slab is soaked at 1050 to 1250°C,
In the hot rolling step, the cumulative reduction rate in a temperature range of 900°C or higher is set to 52 to 70%.
Steel plate manufacturing method.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
SnEQ=Sn+W+Ni/10+Mo/4...(ii)
SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the slab, and 0 is substituted if the element is not contained. The method for producing a steel sheet according to claim 2,
a soaking process in which the slab is heated and then soaked;
a hot rolling step of hot rolling the slab to form a steel plate,
The chemical composition of the cast piece is, in mass%,
C: 0.04-0.15%,
Si: 0.05-1.20%,
Mn: 0.50-2.00%,
P: 0.015% or less,
S: 0.0024% or less,
Ni: 0.10-2.00%,
Nb: 0.0030-0.0400%,
Ti: 0.003 to 0.040%,
Al: 0.010-0.080%,
Sn: 0.0200-0.4000%,
N: 0.0010-0.0070%,
O: 0.0005-0.0040%,
Ca: 0.0001-0.0080%,
and further containing one or more selected from the group consisting of the following Group A, Group B, and Group C:
The balance is Fe and impurities.
Ceq represented by the following formula (i) is 0.32 to 0.47,
The SnEQ represented by the following formula (ii) is 0.10 or more,
The SC represented by the following formula (iii) is 0.30 to 15.00,
In the soaking step, the cast slab is soaked at 1050 to 1250°C,
In the hot rolling step, the cumulative reduction rate in a temperature range of 900°C or higher is set to 52 to 70%.
Steel plate manufacturing method.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(i)
SnEQ=Sn+W+Ni/10+Mo/4...(ii)
SC=S/Ca...(iii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the slab, and 0 is substituted if the element is not contained.
[Group A] One or more selected from the group consisting of Cu: 0.80% or less, Mo: 0.60% or less, V: 0.150% or less, B: 0.0050% or less, Zr: 0.050% or less, and Ta: 0.05% or less. [Group B] One or more selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group C] One or more selected from the group consisting of Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, and REM: 0.010% or less. The method for manufacturing a steel sheet according to claim 7, wherein the chemical composition contains one or more elements selected from Group A. The method for manufacturing a steel sheet according to claim 7, wherein the chemical composition contains one or more elements selected from Group B. The method for manufacturing a steel sheet according to claim 7, wherein the chemical composition contains one or more elements selected from Group C.