WO2024071422A1

WO2024071422A1 - Steel plate

Info

Publication number: WO2024071422A1
Application number: PCT/JP2023/035791
Authority: WO
Inventors: 信幸吉村; 遼太郎白石; 竜一本間; 史寿高峰; 弘宜若松; 武史大久保
Original assignee: 日本製鉄株式会社
Priority date: 2022-09-30
Filing date: 2023-09-29
Publication date: 2024-04-04
Also published as: JP7469734B1

Abstract

This steel plate has a given chemical composition and has an α value of 1.00-1.50 mass%, a β value of 10.0-15.0, a γ value of 0.70-1.50 mass%, a Ceq value of 0.550-0.620 mass%, a yield strength of 670-870 N/mm2, a tensile strength of 780-940 N/mm2, and a Charpy absorption energy at -65°C of 100 J or greater. When the steel plate is examined for hardness distribution at a pitch of 0.05 mm with respect to 121 portions of 1 mm × 1 mm located at 1/4 the plate thickness, the average hardness is 265-290 Hv and the standard deviation is 20 or less. The steel plate has a thickness of 10-60 mm.

Description

Steel Plate

The present invention relates to a steel sheet.
This application claims priority based on Japanese Patent Application No. 2022-157410, filed on September 30, 2022, the contents of which are incorporated herein by reference.

In recent years, as a measure against the climate change problem, there is a strong demand for reducing greenhouse gases, and CCS (Carbon Dioxide Capture and Storage), a technology for capturing and storing carbon dioxide (hereinafter referred to as _CO2 ), has been attracting attention as a technology for achieving carbon neutrality (CCS: Carbon Dioxide Capture and Storage). In CCS, _CO2 emitted from _CO2 emission sources such as refineries, power plants, and chemical plants is separated and captured, and injected and stored in a deep underground reservoir. When a capture facility for separating and capturing _CO2 and a storage facility for injecting and storing _CO2 in an underground reservoir are distant from each other, it is necessary to transport the separated and captured _CO2 between these facilities by pipelines, ships, etc.

When transporting _CO2 by ship, liquefied _CO2 is filled in a transport tank installed on the ship and transported. This improves the efficiency of transporting _CO2 . However, in order to prevent _CO2 from solidifying (becoming dry ice) in the transport tank, it is necessary to transport it while maintaining a pressure of about 2 MPa. In addition, in order to maintain _CO2 in a liquid state at a pressure of about 2 MPa, it is necessary to keep _CO2 at about -35°C. Furthermore, in order to reduce the weight of the ship, there is a demand to make the thickness of the transport tank as thin as possible by increasing the strength of the steel plates used.

Therefore, the steel plate used as the material for the transport tank is required to have high strength and excellent low-temperature toughness. For example, the strength is required to have a tensile strength of 780 N/ ^mm2 or more. In addition, the low-temperature toughness is required to be excellent in the most severe condition, evaluated by a Charpy test at minus 65°C (-65°C), for a plate thickness of 20 to 60 mm used as a transport tank, although this depends on the plate thickness. The test temperature of the Charpy test is minus 65°C because the Charpy test is a small-scale test and is generally evaluated at a certain temperature lower than the usage temperature depending on the plate thickness.

Furthermore, in large welded structures such as transportation tanks, stress relief annealing may be performed on the welds to reduce the possibility of fracture. Stress relief annealing is a heat treatment method in which the welds of a welded structure are heated to a temperature below the Ac1 transformation point and then slowly cooled in order to reduce the residual stress caused by welding. However, when stress relief annealing is applied to high-tensile steel with a tensile strength of 780 N/mm2 or ^more , alloy carbides are selectively precipitated at the grain boundaries, and these alloy carbides cause grain boundary embrittlement, which significantly reduces the toughness of the area where stress relief annealing is performed. This phenomenon is generally called SR (Stress Relieving) embrittlement. In particular, SR embrittlement is highly likely to occur in high-tensile steel that contains B and is manufactured by quenching and tempering. In such high-tensile steel, not only the embrittlement of the base material but also the embrittlement of the weld heat-affected zone obtained when a welded joint is created using this high-tensile steel is significant.

Therefore, in order to ensure high safety in transport tanks manufactured using such high-tensile steel, it is preferable that the base material and welds (especially the weld heat-affected zones) have excellent low-temperature toughness even after stress relief annealing is performed.

Several technical proposals have been made from the above perspective. For example, Patent Document 1 shows a high-strength steel plate characterized by adjusting the chemical composition and setting the average crystal grain size to 15 μm or less. However, the low-temperature toughness at -65°C of the steel plate described in Patent Document 1 has not been evaluated, and there is room for further improvement in low-temperature toughness.

Japanese Patent No. 5590271

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a steel plate suitable for a liquefied _CO2 transport tank, which is excellent in the strength of the base material and the low-temperature toughness of the base material and the welded heat-affected zone, and further excellent in the strength of the base material after stress relief annealing and the low-temperature toughness of the base material and the welded heat-affected zone.

In order to solve the above problems, the present invention employs the following configuration.
[1] A steel sheet according to one embodiment of the present invention is
The chemical composition, in mass%, is
C: 0.07 to 0.11%,
Si: 0.10 to 0.15%,
Mn: 0.70 to 1.20%,
Ni: 1.00 to 2.50%,
Cr: 0.20 to 0.80%,
Mo: 0.20 to 0.80%,
V: 0.005 to 0.070%,
Al: 0.010 to 0.100%,
B: 0.0005 to 0.0030%,
N: 0.0015 to 0.0050%,
P: 0.006% or less,
S: 0.0030% or less,
Cu: 0 to 1.00%,
Nb: 0 to 0.030%,
Ti: 0 to 0.010%,
Ca: 0 to 0.0030%,
Mg: 0 to 0.0030%,
REM: 0 to 0.0030%,
O: 0.0040% or less,
The balance is Fe and impurities.
an α value defined by the following formula (1) of 1.00 to 1.50 mass%,
A β value defined by the following formula (2) is 10.0 to 15.0,
A γ value defined by the following formula (3) is 0.70 to 1.50 mass%,
The Ceq value defined by the following formula (4) is 0.550 to 0.620 mass%,
Yield strength is 670 to 870 N/mm ² ,
Tensile strength of 780 to 940 N/mm ² ,
Charpy absorbed energy at -65°C is 100 J or more,
In a hardness distribution measurement at 1/4 thickness position, 1 mm x 1 mm, 0.05 mm pitch, the average hardness at 121 measurement points is 265 Hv to 290 Hv, with a standard deviation of 20 or less.
The plate thickness is 10 to 60 mm.
α = [C] + 6 × [Si] + 100 × [P] ... (1)
β = 0.65 × [C] ^1/2 × (1 + 0.64 × [Si]) × (1 + 4.10 × [Mn]) × (1 + 0.27 × [Cu]) × (1 + 0.52 × [Ni]) × (1 + 2.33 × [Cr]) × (1 + 3.14 × [Mo]) ... (2)
γ = [Mn] + 20 × [Nb] + 36 × [Ti] ... (3)
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ... (4)
In the formulas (1) to (4), [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti], and [V] represent the contents (mass %) of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti, and V, respectively, and elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0.
[2] In the steel sheet according to [1], [fB] calculated by the following formulas (A) to (E) may be 0.0003 mass% or more.
[fB] = [B] - 0.77 x [fN] ... (A)
[fN] = [N] - 0.29 x [fTi] - 0.52 x [fAl] ... (B)
[fTi] = [Ti] - 2 × [fO] ... (C)
[fAl] = [Al] - 1.125 × [fO] ... (D)
[fO] = [O] - 0.4 x [Ca] - 0.66 x [Mg] - 0.11 x [REM] ... (E)
In the formulas (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively. Elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0, and when the calculated values of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.
[3] In the steel sheet according to [1] or [2], when a region surrounded by a grain boundary having a crystal orientation difference of 15° or more determined by performing a crystal orientation analysis using an electron beam backscatter diffraction pattern analysis is defined as a crystal grain, the circle equivalent grain size of the crystal grain is defined as the crystal grain size, and a value calculated as an area-weighted average weighted by the area of each crystal grain is defined as an average crystal grain size, the average crystal grain size at a 1/4 thickness position may be 15.0 μm or less.
[4] In the steel sheet according to any one of [1] to [3], when the steel sheet is subjected to stress relief annealing at a holding temperature of 600°C for 2 hours and a heating rate and a heating rate of 55°C/hr or less in a temperature range of 425°C or more, the steel sheet may have a yield strength of 670 to 870 N/ ^mm2 , a tensile strength of 780 to 940 N/ ^mm2 , and a Charpy absorbed energy at -40°C of 27 J or more at a location where the stress relief annealing has been performed.

According to the above-mentioned aspect of the present invention, it is possible to provide a steel plate having excellent strength of the base material and low-temperature toughness of the base material and the welded heat affected zone, and further having excellent strength of the base material and low-temperature toughness of the base material and the welded heat affected zone after stress relief annealing. This steel plate is suitable for use in liquefied _CO2 transport tanks.

Hereinafter, a steel sheet according to one embodiment of the present invention (steel sheet according to this embodiment) will be described in detail.
In this embodiment, "stress relief annealing" means stress relief annealing conforming to the contents specified in JIS Z 3700:2022 "Post-weld heat treatment method" unless otherwise specified. In this embodiment, "welding" means welding with a welding heat input of 1.1 to 4.5 kJ/mm unless otherwise specified. These conditions are general conditions in the technical field to which the present invention belongs. However, even if stress relief annealing or welding is performed under conditions different from the above conditions, the same effect as stress relief annealing or welding performed under the above conditions can be obtained. Therefore, the steel plate according to this embodiment may be subjected to stress relief annealing or welding under conditions different from the above conditions.

First, the content of each element constituting the chemical composition of the steel plate according to this embodiment and the reasons for limiting it will be described. In the following, unless otherwise specified, "%" regarding the content of an element means mass %.

(C: 0.07 to 0.11%)
C is an element that improves the strength of the base material. In order for the steel plate according to this embodiment to achieve the desired strength, the C content is set to 0.07% or more. The C content is preferably 0.08% or more.
On the other hand, when a large amount of C is contained, the hardness of the welded heat affected zone increases and at the same time its toughness decreases, so the C content is set to 0.11% or less, preferably 0.10% or less, and more preferably less than 0.10%.

(Si: 0.10 to 0.15%)
Silicon is an element that is generally contained in steel in many cases as a deoxidizing element. In order to contain silicon for the purpose of deoxidization, the silicon content is set to 0.10% or more.
On the other hand, Si is an element that reduces the toughness of steel after stress relief annealing. In addition, in order to suppress the decrease in the toughness of the welded heat affected zone after stress relief annealing (SR), it is preferable that the Si content is low. Therefore, in the steel plate according to this embodiment, the Si content is set to 0.15% or less. The Si content is preferably set to 0.14% or less, more preferably 0.13% or less, and even more preferably 0.12% or less.

(Mn: 0.70 to 1.20%)
Mn is an element effective for deoxidization and also improves the strength of steel, therefore the Mn content is set to 0.70% or more, preferably 0.90% or more.
On the other hand, if Mn is contained in excess, there is a risk that the toughness of the steel after stress relief annealing may be impaired due to temper embrittlement. Therefore, the Mn content is set to 1.20% or less, and preferably 1.10% or less.

(Ni: 1.00 to 2.50%)
Ni is an element effective for improving the hardenability and toughness of steel, and therefore the Ni content is set to 1.00% or more, and preferably 1.20% or more.
On the other hand, if Ni is contained excessively, the toughness of the steel after stress relief annealing may decrease. Also, the toughness of the welded heat affected zone after stress relief annealing may deteriorate. Therefore, the Ni content is set to 2.50% or less. The Ni content is preferably set to 2.00% or less.

(Cr: 0.20 to 0.80%)
Cr is an element effective for improving the hardenability of steel and improving the strength of steel by precipitation strengthening during tempering. Therefore, the Cr content is set to 0.20% or more, and preferably 0.40% or more.
On the other hand, if Cr is excessively contained, there is a risk that the toughness of the base metal and the welded heat affected zone after stress relief annealing may decrease. Therefore, the Cr content is set to 0.80% or less, and preferably 0.70% or less.

(Mo: 0.20 to 0.80%)
Like Cr, Mo is an element effective for improving hardenability and strength of steel by precipitation strengthening during tempering. Therefore, the Mo content is set to 0.20% or more. The Mo content is preferably set to 0.30% or more, more preferably 0.35% or more, and further preferably 0.40% or more.
On the other hand, if Mo is contained in excess, Mo carbides may precipitate at grain boundaries after stress relief annealing, reducing the toughness of the base material and the welded heat affected zone, and the impact on the welded heat affected zone is particularly large. Therefore, the Mo content is set to 0.80% or less, and preferably 0.60% or less.

(V: 0.005 to 0.070%)
V, like Cr and Mo, is an element effective for improving hardenability and improving the strength of steel by precipitation strengthening during tempering. Therefore, the V content is set to 0.005% or more, and preferably 0.010% or more.
On the other hand, if an excessive amount of V is contained, there is a risk that the toughness of the base material and the toughness of the welded heat affected zone may decrease after stress relief annealing. Therefore, the V content is set to 0.070% or less, and preferably 0.050% or less.

(Al: 0.010 to 0.100%)
Al is an element useful for deoxidization, and also an element that refines the grain size during quenching by forming nitrides. In addition, it is an essential element for ensuring [fB] by forming nitrides. Therefore, in the steel sheet according to the present embodiment, the Al content is set to 0.010% or more. When the N content is high, the Al content is preferably 0.030% or more, more preferably 0.040% or more, in order to fix N and ensure fB.
On the other hand, if Al is contained in excess, Al may form coarse nitrides, which may reduce the toughness of the base material and the welded heat affected zone. Therefore, the Al content is set to 0.100% or less, and preferably 0.080% or less.

(B: 0.0005 to 0.0030%)
In the steel sheet according to the present embodiment, B is an element that improves the hardenability of the steel by being contained in a small amount. Therefore, the B content is set to 0.0005% or more. The B content may be set to 0.0006% or more, 0.0008% or more, or 0.0010% or more.
On the other hand, if B is contained in excess, B may form coarse nitrides and/or carbides, which may reduce the toughness of the base metal. Therefore, the B content is set to 0.0030% or less. The B content may also be set to 0.0020% or less or 0.0010% or less.

(N: 0.0015 to 0.0050%)
N is an element that forms nitrides to refine the crystal grain size of the base material and improve toughness. Therefore, the N content is set to 0.0015% or more. The N content may be set to 0.0030% or more or 0.0035% or more.
On the other hand, if N is contained excessively, the nitrides become coarse and the toughness of the weld heat affected zone in the as-welded state decreases. Therefore, the N content is set to 0.0050% or less.

(P: 0.006% or less)
(S: 0.0030% or less)
P and S are impurity elements contained in steel, and the lower the content, the better. Therefore, the lower limit of the P content and the S content is 0%. In the steel plate according to the present embodiment, in order to improve the base material toughness and the toughness of the base material and the welded part after stress relief annealing, the P content is set to 0.006% or less, and the S content is set to 0.0030% or less. The P content is preferably set to 0.005% or less. The S content may be set to 0.0020% or less.

(Cu: 0 to 1.00%)
Cu is not an essential element in the steel plate according to this embodiment, so the lower limit of the Cu content is 0%. However, since Cu has an effect of improving the strength of steel, it can be contained as necessary. When Cu is contained, in order to utilize the effect, the Cu content is preferably 0.10% or more, and more preferably 0.20% or more. If necessary, the Cu content may be 0.25% or more or 0.30% or more.
On the other hand, if Cu is contained excessively, there is a risk that the toughness of the base material will decrease due to the occurrence of cracks on the steel sheet surface and the precipitation of Cu. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.80% or less. If necessary, the Cu content may be set to 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less.

(Nb: 0 to 0.030%)
Since Nb is not an essential element in the steel sheet according to this embodiment, the lower limit of the Nb content is 0%. However, since Nb is an element that refines crystal grains during quenching, it can be contained as necessary. When Nb is contained, in order to utilize its effect, the Nb content is preferably 0.001% or more.
On the other hand, if Nb is contained in excess, Nb may form coarse carbonitrides and reduce the toughness of the base material. Therefore, the Nb content is set to 0.030% or less. Since the toughness of the welded heat affected zone is improved with a smaller Nb content, the Nb content may be set to 0.020% or less, 0.010% or less, or 0.005% or less.

(Ti: 0 to 0.010%)
Ti is not an essential element in the steel plate according to the present embodiment, so the lower limit of the Ti content is 0%. However, Ti may refine the crystal grains when the steel is heated to a high temperature by slab heating, etc., so it may be contained as necessary. When Ti is contained, in order to utilize its effect, it is preferable that the Ti content is 0.001% or more.
On the other hand, if Ti is contained excessively, Ti may form coarse carbonitrides, as with Nb, and may reduce the toughness of the base material. Therefore, the Ti content is set to 0.010% or less. If necessary, the Ti content may be set to 0.005% or less or 0.002% or less.

(Ca: 0 to 0.0030%)
(Mg: 0 to 0.0030%)
(REM: 0 to 0.0030%)
The steel sheet according to the present embodiment may contain one or more of Ca, Mg, and REM. Since Ca, Mg, and REM are not essential elements, the lower limit of the content of Ca, Mg, and REM is 0%.

Ca has the effect of spheroidizing sulfides in the steel sheet, thereby reducing the influence of MnS that reduces the toughness of the steel sheet. In order to obtain this effect, the Ca content may be set to 0.0001% or more.
On the other hand, if a large amount of Ca is contained, there is a risk that the weldability of the steel may be impaired, so the Ca content is set to 0.0030% or less. If necessary, the Ca content may be set to 0.0015% or less, 0.0010% or less, 0.0005% or less, or 0.0002% or less.

Mg and REM form oxides to improve the toughness of the weld heat affected zone. To obtain this effect, the Mg and REM contents may each be 0.0001% or more.
On the other hand, if Mg and REM are contained in large amounts, coarse oxides are formed, which may reduce the toughness of the steel. Therefore, the Mg content and the REM content are each set to 0.0030% or less. If necessary, the Mg content and the REM content may be set to 0.015% or less, 0.010% or less, 0.005% or less, or 0.002% or less, or less than 0.0015%. REM is a general term for rare earth metals including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Compared to other additive elements, REM is characterized by its strong deoxidizing effect and forms stable oxides in steel.

(O: 0.0040% or less)
Oxygen (O) is an impurity element contained in steel, and in many cases, it forms oxides of several μm to several tens of μm in size together with Ca, Mg, REM, Al, Ti, etc., which have strong deoxidizing power. When coarse oxides are contained or when the number density of oxides is high, they may become the starting point of brittle fracture, so the lower the O content, the more preferable it is. For this reason, the lower limit of the O content is 0%. In the steel plate according to this embodiment, the O content is set to 0.0040% or less, preferably 0.0030% or less, in order to improve the toughness of the welded portion.

(balance: Fe and impurities)
The steel sheet according to the present embodiment is composed of the above-mentioned components, with the balance being Fe and impurities. Here, the impurities refer to components that are mixed in due to various factors in raw materials such as ores or scraps, or in the manufacturing process, during industrial production of steel material, and are permissible within a range that does not adversely affect the present invention.

Furthermore, in the steel plate according to the present embodiment, it is preferable that [fB] calculated by the following formulas (A) to (E) is 0.0003 mass% or more. [fB] represents the amount of B dissolved in the steel. By setting [fB] to 0.0003 mass% or more, the hardenability of the steel can be improved in high-tensile steel having a yield strength of 670 to 870 N/mm ² and a tensile strength of 780 to 940 N/mm ^2. B easily forms nitrides in the steel. In addition, Ti and Al easily form nitrides and oxides. Therefore, the amount of B dissolved in the steel [fB] is calculated by the following formulas (A) to (E). [fB] may be 0.0005 mass% or more, or may be 0.0015 mass% or more. In addition, [fB] may be 0.0025 mass% or less, or may be 0.0018 mass% or less.

[fB] = [B] - 0.77 x [fN] ... (A)
[fN] = [N] - 0.29 x [fTi] - 0.52 x [fAl] ... (B)
[fTi] = [Ti] - 2 × [fO] ... (C)
[fAl] = [Al] - 1.125 × [fO] ... (D)
[fO] = [O] - 0.4 x [Ca] - 0.66 x [Mg] - 0.11 x [REM] ... (E)

In formulas (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] respectively represent the content (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM in the steel plate. Elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0, and when the calculated values of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.

In addition to limiting the content of each element, the ranges of the α, β, and γ values calculated from the content of each element are also limited as follows for the steel plate according to this embodiment.

(α value: 1.00 to 1.50% by mass)
The α value is expressed by the following formula (1).
α = [C] + 6 × [Si] + 100 × [P] ... (1)
Here, [C], [Si] and [P] are the contents (mass%) of C, Si and P in the steel plate, respectively.

In this embodiment, the α value is set to 1.50 mass% or less. This is a necessary condition for improving the toughness of the coarse-grained portion of the base material or the welded heat-affected zone after stress relief annealing, and it is necessary to adjust C, Si, and P within a range that satisfies this condition. After SR treatment, the grain boundary segregation concentration of P increases, making brittle fracture at the grain boundaries more likely to occur, but brittle fracture can be controlled by P, C, and Si. Considering the manufacturing process, P is inevitably contained in steel, and since it significantly reduces grain boundary strength due to grain boundary segregation, it is a representative element that causes SR embrittlement and has the highest coefficient. C and Si are also elements that are inevitably contained in steel, and if these elements increase, they will cause embrittlement due to cementite formed at the grain boundaries. It is desirable to reduce both elements, but there are cases where a certain amount is contained due to characteristics or specifications. In order to improve the toughness after SR, it is preferable to set the α value to 1.40 mass% or less. The α value is 1.00 mass% or more. This lower limit is a range determined by the component constraints in the specifications of the field of application and the limits of element control in manufacturing, and is calculated by substituting the above-mentioned lower limits of the C, Si, and P contents and the realistic minimum values in manufacturing into formula (1). The preferred lower limit of the α value can be calculated from the preferred lower limits of the C, Si, and P contents. The α value may be more than 1.10 mass%, or may be 1.30 mass% or more.

(β value: 10.0 to 15.0)
The β value is calculated by the following formula (2).
β = 0.65 × [C] ^1/2 × (1 + 0.64 × [Si]) × (1 + 4.10 × [Mn]) × (1 + 0.27 × [Cu]) × (1 + 0.52 × [Ni]) × (1 + 2.33 × [Cr]) × (1 + 3.14 × [Mo]) ... (2)
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel.

In the steel plate according to this embodiment, the β value is in the range of 10.0 to 15.0. The β value is an index showing the hardenability of the steel material, and the higher the β value, the more reliably the formation of upper bainite structure, which has a poor balance of strength and toughness, can be avoided, but if the β value is too high, the strength of the steel material increases and the toughness deteriorates. In other words, it also serves as an index showing the target range of the content of alloy elements required to improve the toughness of the welded heat-affected zone as it is. If necessary, the β value may be 11.0 or more. At the same time, the β value may be 14.0 or less.

(γ value: 0.70 to 1.50% by mass)
The γ value is calculated by the following formula (3).
γ = [Mn] + 20 × [Nb] + 36 × [Ti] ... (3)
[Mn], [Nb], and [Ti] are the contents (mass%) of Mn, Nb, and Ti in the steel plate.
In the steel plate according to the present embodiment, the γ value is in the range of 0.70 to 1.50 mass%. Mn, Nb, and Ti are all elements that promote grain boundary embrittlement after stress relief annealing, and by setting the γ value to 1.50 mass% or less, it is possible to suppress the decrease in the toughness of the base material and the welded heat affected zone after stress relief annealing. There are several possible mechanisms by which these elements promote grain boundary embrittlement, including a decrease in grain boundary strength due to grain boundary segregation and embrittlement due to carbonitrides formed at the grain boundaries. The γ value may be 1.40 mass% or less.
On the other hand, in order to ensure a certain degree of hardenability and obtain a microstructure having an advantageous balance of strength and toughness, it is preferable to add certain amounts of Mn, Nb and Ti, and the γ value is set to 0.70 mass% or more. The γ value may be 0.75 mass% or more.

By satisfying the numerical ranges for the α, β and γ values, it is possible to provide a wider range of fields in which steel having excellent low-temperature toughness in the welded zone (weld heat-affected zone) can be provided, both as-welded and after stress relief annealing.
Although it is possible to manufacture steel with excellent low-temperature toughness of the welded portion both as-welded and after stress relief annealing by controlling the α value and β value within a certain range, depending on the application, the α value cannot be freely set due to the chemical composition regulations of the applicable standards, and there are cases where the α value has to be increased. In contrast, in the steel plate according to the present embodiment, it is possible to widen the allowable range of the α value by newly setting a γ value, so that it is possible to provide a steel with excellent low-temperature toughness of the welded portion both as-welded and after stress relief annealing, even in fields where the chemical composition regulations of the applicable standards are strict as described above.

In addition, in the steel plate according to this embodiment, the carbon equivalent Ceq, which is an index showing the hardenability of steel and is calculated by the following formula (4), is set to 0.550 to 0.620 mass%.

Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ... (4)
In formula (4), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] are the contents (mass%) of C, Mn, Cu, Ni, Cr, Mo, and V in the steel plate, respectively.

If Ceq is less than 0.550 mass%, the strength of the steel plate may be insufficient. If necessary, Ceq may be set to 0.570 mass% or more, or 0.600 mass% or more. If Ceq exceeds 0.620 mass%, the toughness of the steel plate may decrease. If necessary, Ceq may be set to 0.600 mass% or less.

(Yield strength: 670 to 870 N/ ^mm2 )
(Tensile strength: 780 to 940 N/ ^mm2 )
In the steel plate according to this embodiment, the yield strength is set to 670 to 870 N/mm ² , and the tensile strength of the steel plate is set to 780 to 940 N/mm ^2. In order to reduce the weight of a large welded structure such as a transport tank for liquefied CO ₂ , a steel plate that can ensure the strength of the structure even if the plate thickness is thin is required. Usually, the steel plate selected for such applications is a steel plate having the above-mentioned yield strength and tensile strength, so the yield strength and tensile strength of the steel plate according to this embodiment are also set to the above ranges. If necessary, the yield strength may be set to 690 N/mm ² to 830 N/mm ^2. The tensile strength may be set to 800 N/mm ² to 900 N/mm ² .

(Charpy absorbed energy of 100 J or more at -65°C)
Furthermore, in order to ensure high toughness, the steel plate according to this embodiment needs to have a Charpy absorbed energy of 100 J or more at -65°C. This makes it possible to ensure the safety of a transportation tank made of the steel plate according to this embodiment. The Charpy absorbed energy at -65°C is a value measured at a position that is 1/4 of the plate thickness from the surface in the plate thickness direction (sometimes referred to as the t/4 position or 1/4 thickness position).

(δ value of 0.10 mm or more in CTOD test at -35°C)
In addition, in order to ensure the safety of welded structures such as transportation tanks, fracture resistance characteristics of welded structures are evaluated and incorporated into designs using fracture mechanics evaluation methods. Specifically, as the initiation characteristics of brittle fracture, a crack opening displacement (hereinafter abbreviated as δc) called a CTOD value is obtained as a fracture mechanics parameter by a CTOD test (Crack Tip Opening Displacement test) specified by the Japan Welding Engineering Society standard WES1108, and it is often evaluated whether δc satisfies the design criteria.

In order to improve the δc of a material, it is necessary to improve the characteristics of the material from a different perspective than before. Conventionally, the Charpy impact test has been used as a method for evaluating the brittle fracture resistance of a material. The value obtained from the Charpy impact test represents the average toughness of the evaluation target area. However, in the CTOD test, even if the average toughness of the evaluation target area is good, if there is even a slightly brittle part in the evaluation target area, the existence of the part is reflected in δc. Since δc has such properties, in order to obtain a high δc value, it is necessary to reduce the localized brittle area as much as possible, especially in areas where the microstructure of the steel material has changed in a non-uniform and complex manner, such as the weld heat affected zone.
In order to ensure high toughness, the steel plate according to this embodiment preferably has a δ value of 0.10 mm or more in a CTOD test at −35° C. In this case, the safety of a transportation tank made of the steel plate according to this embodiment is further improved.

(Average hardness, standard deviation)
In the steel plate according to the present embodiment, the average value of the hardness at 121 measurement positions must be 265Hv to 290Hv and the standard deviation must be 20 or less in the hardness distribution measurement at 0.05mm pitch in the range of 0.5mm x 0.5mm at the 1/4 thickness position. The structure of the steel plate according to the present embodiment is preferably a mixed structure of martensite structure and lower bainite structure, which is superior in strength-toughness balance, but due to localized γ grain size and microsegregation variations, the hardenability may be partially reduced, and an upper martensite structure with inferior strength-toughness balance may be formed. If an upper bainite structure is present, the distribution of hardness may become nonuniform, and the toughness of the base material may deteriorate. If the average value of the hardness is less than 265Hv or the standard deviation exceeds 20, the upper bainite structure may be included, and the toughness of the base material cannot be ensured. On the other hand, if the average value exceeds 290Hv, the strength may become too high, and the toughness may decrease.

The above-mentioned hardness distribution measurement is performed by taking a micro sample with a surface (L-section) parallel to the rolling direction and thickness direction of the steel plate as the observation surface, and using a micro Vickers hardness tester. The measurement area is a 0.5 mm x 0.5 mm range centered on any t/4 position within the micro observation surface, with a measurement pitch of 0.05 mm and a measurement load of 25 gf, and measurements are taken at 11 vertical points x 11 horizontal points, for a total of 121 points. The average and standard deviation are calculated from the obtained measurement values.

(Thickness: 10-60mm)
In the case of welding steel plates with a thickness of less than 10 mm, stress relief annealing (SR) is generally not required. However, the steel plate according to the present embodiment is intended for steel plates that require SR, and therefore the thickness is set to 10 mm or more. The thickness is preferably 25 mm or more. On the other hand, a steel plate with a thickness of more than 60 mm is not preferred because it makes little contribution to reducing the weight of the transport tank to which it is applied. Therefore, the thickness of the steel plate according to the present embodiment is set to 60 mm or less.

Furthermore, the steel plate according to this embodiment may have the configuration described below.

(Organization)
In order to satisfy the above-mentioned average value and standard deviation of the hardness at 121 points, the steel plate according to this embodiment preferably has a structure at a 1/4 thickness position of the cross section in the plate thickness direction that is a mixed structure of martensite and lower bainite. The martensite and lower bainite structures may occupy 85 area % or more in total.

(Average grain size at 1/4 thickness position of steel plate: 15.0 μm or less)
In the steel plate according to the present embodiment, the average grain size at the 1/4 thickness position may be 15.0 μm or less. In order to improve the base material toughness and the base material toughness after SR, the average grain size may be 14.5 μm or less, or 14.0 μm or less, as necessary. Since it is preferable that the average grain size at the 1/4 thickness position of the steel plate is small, there is no need to specify a lower limit. Usually, the average grain size is about 10.0 μm at the smallest.

The average crystal grain size is defined as follows.
A sample in which the L-section of the steel sheet can be observed is prepared, and the 1/4 thickness position of the L-section is used as the observation area. A crystal orientation analysis using an electron beam backscatter diffraction pattern analysis method (EBSD method) using a scanning electron microscope is performed in a range of 200 μm in the sheet thickness direction and 250 μm in the rolling direction at a pitch of 0.5 μm. From the results of the crystal orientation analysis, the region surrounded by grain boundaries with a crystal orientation difference of 15° or more is defined as a crystal grain, the circle-equivalent grain size of the crystal grain is defined as the crystal grain size, and the value calculated by the area-weighted average weighted by the area of each crystal grain is defined as the average crystal grain size.

(Charpy absorbed energy of 27 J or more at -40°C after stress relief annealing)
In order to prevent breakage, the steel plate according to this embodiment is subjected to stress relief annealing of the welded portion after assembly into a transport tank, during which not only the welded portion but also the base material is heated. When the base material is heated, the toughness of the base material tends to decrease. Although the cause is not clear, it is presumed that the diffusion of P (phosphorus) to grain boundaries and the growth or aggregation of inclusions in the structure cause a decrease in brittleness and a decrease in toughness. Therefore, the steel plate according to this embodiment preferably has a Charpy absorbed energy of 27 J or more at -40°C after stress relief annealing. In this case, safety can be further improved.

The Charpy absorbed energy at -40°C after stress relief annealing is measured at the location where stress relief annealing was performed when the steel plate is subjected to stress relief annealing at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and cooling rate of 55°C/hr or less in a temperature range of 425°C or higher.

(Yield strength after stress relief annealing is 670-870N/ ^mm2 , tensile strength is 780-940N/ ^mm2 )
The steel plate according to this embodiment preferably has a yield strength of 670 to 870 N/mm ² and a tensile strength of 780 to 940 N/mm ² after stress relief annealing. This ensures sufficient strength in a transport tank for liquefied CO ₂ that has been subjected to stress relief annealing.

Furthermore, it is preferable that the steel sheet according to this embodiment has a δ value of 0.10 mm or more in a CTOD test at -35°C, even after stress relief annealing. In this case, safety is further improved.

The steel plate according to this embodiment has the above-mentioned configuration, and therefore has excellent toughness in the weld heat affected zone (as welded and after stress relief annealing).
The toughness of the weld heat affected zone is not limited, but as a target value, in the as-welded case, the Charpy absorbed energy at -65°C is preferably 70 J or more, and in the case after stress relief annealing, the Charpy absorbed energy at -65°C is preferably 70 J or more.
Moreover, it is more preferable that the weld heat affected zone has a δ value of 0.10 mm or more in a CTOD test at −35° C., whether as welded or after stress relief annealing.

Next, a method for manufacturing a steel sheet according to this embodiment will be described below.
As long as the steel sheet according to this embodiment has the above-mentioned characteristics, the effects can be obtained regardless of the manufacturing method. However, it is preferable to use the method described below, since it can be manufactured stably.

In order to produce steel having the above-mentioned chemical composition as a steel plate, a commonly used method for producing steel products may be used. That is, for example, steel produced by a converter process or an electric furnace process and refined in a secondary refining facility is made into a slab by continuous casting or ingot casting. The slab thickness is sufficient as long as it can reduce segregation and improve the material properties by reducing porosity, and for this purpose, the slab thickness is preferably 150 mm or more. There is no particular upper limit for the slab thickness, but the slab thickness may be, for example, 600 mm or less or 400 mm or less.
The slab is then heated to about 950 to 1250°C in a slab heating furnace, and is then rolled to a predetermined thickness by hot rolling under the conditions described below to obtain a steel sheet. This steel sheet is then quenched and tempered to obtain a steel sheet (final steel sheet) having the desired properties.

The steel plate according to this embodiment needs to have its P content reduced to 0.006% or less. There are cases where it is not possible to reduce the P content to 0.006% or less using normal dephosphorization methods, but in such cases measures can be taken such as extending the dephosphorization treatment time.

When hot rolling, it is desirable to set the cumulative reduction ratio at 50% or more when the rolling temperature is in the range of 1150 to 900°C. There is no particular need to specify an upper limit for the cumulative reduction ratio in the above temperature range, but the cumulative reduction ratio may be 80% or less, or 70% or less.

Regarding quenching and tempering, when the plate thickness is 50 mm or less or less than 50 mm, the reheating and quenching process described below may be omitted by performing a direct quenching process in which the plate is immediately water-cooled after hot rolling. When performing a direct quenching process, the cooling start temperature is set to Ar3 point or more, and water cooling is performed to 300 ° C or less. The average cooling rate during water cooling is preferably 5 ° C / sec or more in the range of 700 ° C to 300 ° C in the cooling temperature history of the front and back surfaces of the steel plate. There is no particular limit to the upper limit of the average cooling rate, but the average cooling rate may be, for example, 100 ° C / sec or less, 50 ° C / sec or less, or 20 ° C / sec or less. In addition, reheating may be performed after direct quenching, and further quenching may be performed.
The Ar3 point is calculated using the following formula.
Ar3 = 910 - 310 x [C] - 8 x [Mn] - 20 x [Cu] - 15 x [Cr] - 55 x [Ni] - 80 x [Mo] + 0.35 x (t-8)
In the formula, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo in the steel plate, respectively, in mass%, and t is the thickness of the steel plate in mm.

For plate thicknesses of 50 mm or more, it is preferable to perform quenching by cooling the steel plate once after rolling and then reheating it. For plate thicknesses of 50 mm or more, if reheating and quenching is performed, direct quenching after hot rolling may be omitted, or quenching may be performed directly.

When reheating is performed, the heating temperature during quenching (i.e., quenching temperature) is desirably 925°C or less, and may be 920°C or less, 915°C or less, or 910°C or less. This is because the metal structure of a thick steel plate may not be sufficiently refined after rolling. If the quenching temperature for a steel plate in which the metal structure is not sufficiently refined exceeds 925°C, the reverse transformation γ structure formed by heating becomes coarse, and the average crystal grain size of the final structure after γ/α transformation by subsequent cooling also becomes coarse.
On the other hand, the lower limit of the quenching temperature is not preferable if it is slightly above the Ac3 point (for example, within a temperature range of not less than the Ac3 point and not more than the Ac3 point + 20°C), because this may cause variations in reverse transformation γ grain size and insufficient solid solution of carbides containing B, resulting in insufficient hardenability. Therefore, the quenching temperature is preferably 880°C or higher, and more preferably 890°C or higher. In the above description of the quenching treatment conditions, it is assumed that the steel plate has a thickness of 50 mm or more, but these quenching treatment conditions are also applied when reheating and quenching are performed on a steel plate having a thickness of less than 50 mm.

In this embodiment, tempering is performed after quenching (after direct quenching or reheating quenching, or after reheating quenching if both are performed). The heating temperature during tempering (i.e., the tempering temperature) is desirably 660°C or lower. If the tempering temperature exceeds 660°C, the tempering effect becomes excessive, making it difficult to ensure the yield stress and tensile strength, and toughness may decrease. The tempering temperature is 500°C or higher, preferably 600°C or higher. If the tempering temperature is too low, tempering becomes insufficient, making it difficult to ensure the specified yield stress and tensile strength.

When cooling is performed after reheating and quenching or tempering, it is preferable to cool the steel plate by water cooling (accelerated cooling) rather than air cooling in order to prevent a decrease in the toughness of the base material due to temper embrittlement. In this case, it is preferable to set the average cooling rate to 300°C to 0.1°C/sec or more or 0.5°C/sec or more.

The steel plate according to the present embodiment is suitable as a steel plate for a liquefied _CO2 transport tank. For example, it can be used as a transport tank mounted on a ship. When transporting _CO2 by ship, liquefied _CO2 is filled into a transport tank installed on the ship and transported, but in order to prevent _CO2 from solidifying (becoming dry ice) in the transport tank, it is preferable to transport it while maintaining a pressure of about 2 MPa. In addition, in order to maintain _CO2 in a liquid state at a pressure of about 2 MPa, it is preferable to keep _CO2 at about minus 35°C. The steel plate according to the present embodiment can be suitably used for such applications.

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

The molten iron that had completed the blast furnace treatment was tapped into a molten iron ladle, and after preliminary treatment such as desulfurization was carried out, the molten iron was then inserted into a converter. Next, dephosphorization was carried out in the converter, and the P content was adjusted to 0.006% or less.

The dephosphorized molten steel was further subjected to composition adjustment, and then slabs having the chemical compositions shown in Tables 1A and 1B were cast.
Thereafter, the slab was heated in a heating furnace to the heating temperature shown in the table, and then hot-rolled to a predetermined plate thickness to obtain a steel plate.
Furthermore, the steel plate was quenched and tempered to obtain a steel plate (final steel plate) having the specified properties. Table 2 shows the heating temperature before rolling, the cumulative reduction ratio of hot rolling at 1150 to 900°C, the plate thickness after rolling, the quenching temperature, and the tempering temperature. Cooling after reheating and quenching or after tempering was performed by water cooling, and the average cooling rate to 300°C was set to 0.1°C/sec or more. In addition, for some steel plates, direct quenching treatment was performed in which the steel plate was immediately water-cooled after hot rolling. The cooling start temperature, cooling end temperature, and average cooling rate in this case are shown in the table.

Tables 1A and 1B show the chemical composition, α value, β value, γ value, fB value, and carbon equivalent Ceq of the steel plate. In addition, the column before SR of the base material properties in Table 3A shows the average hardness (average Hv) of the base material at 121 measurement positions, the average grain size (EBSD grain size), the yield strength (MPa), the tensile strength (MPa), the yield ratio, the Charpy absorbed energy (J) at -65°C, and the δ value (mm) of the CTOD test at -35°C.
The EBSD grain size was measured by preparing a sample in which the L-section of the steel sheet could be observed, observing the 1/4 thickness position of the L-section, and performing crystal orientation analysis using an electron beam backscatter diffraction pattern analysis method (EBSD method) using a scanning electron microscope in a range of 200 μm in the sheet thickness direction and 250 μm in the rolling direction at a pitch of 0.5 μm. From the results of the crystal orientation analysis, the region surrounded by grain boundaries with a crystal orientation difference of 15° or more was defined as a crystal grain, the circle-equivalent grain size of the crystal grain was defined as the crystal grain size, and the value calculated by the area-weighted average weighted by the area of each crystal grain was defined as the average crystal grain size.

The tensile test was conducted in accordance with JIS Z 2241:2011, using JIS No. 4 round bar test pieces with a parallel section of φ14 mm taken from the 1/4 thickness position in the C direction. The yield strength and tensile strength are each the average of two test pieces. The yield strength was taken as 0.2% proof stress. The yield ratio is the ratio of the yield strength YS to the tensile strength TS, and is expressed as a percentage, i.e., 100 x (YS/TS). The yield ratio is expressed in %.

For hardness distribution measurements, a micro sample was taken with the L-section parallel to the rolling direction and thickness direction of the steel plate as the observation surface, the observation surface was wet polished, and then the mirror surface was buffed using 1.0 μm diamond particles, and measurements were taken using a micro Vickers hardness tester. The measurement area was randomly selected within a 0.5 mm x 0.5 mm range centered on the 1/4t position within the micro observation surface, and measurements were taken at a total of 121 points (11 vertical x 11 horizontal) with a measurement pitch of 0.05 mm and a measurement load of 25 gf. The average value and standard deviation were calculated from the obtained measurements.

In addition, a semi-automatic welded joint with a weld line parallel to the rolling direction was produced and evaluated. Specifically, a K groove was produced, argon gas containing 20% _CO2 was used as the shielding gas, the welding wire was YM-69F manufactured by Nippon Steel Welding Industry Co., Ltd., the heat input was set to 2.0 kJ/mm, and the preheating was set to 100°C, and multi-layer gas shielded arc welding (GMAW) was performed to produce a welded joint.

After the microstructure of the welded joint was revealed at the C-section from the welded part (As weld), Charpy test pieces were taken from a position centered at 6.5 mm below the surface (referred to as surface sampling in the table) and a position centered at the plate thickness center (referred to as t/2 sampling in the table) with the front I-side fusion line (FL) as the center of each notch.
The test pieces were subjected to a Charpy test at −65° C. to determine the absorbed energy. The results are shown in the Asweld column of the joint properties in Table 3B.
After the microstructure of the weld was revealed, a full-thickness CTOD test piece was taken with the front I-side fusion line positioned at the center of each notch, and a CTOD test was performed at -35°C to determine the δ value. The results are shown in the Asweld column of the joint properties in Table 3B.

Then, stress relief annealing (SR) was performed on the base material and welded parts. Stress relief annealing was performed at a holding temperature of 600°C for 2 hours, with heating and cooling rates of 55°C/hr or less in the temperature range of 425°C or higher.

The yield strength and tensile strength of the base material after SR were determined in the same manner as before SR.
In addition, a Charpy test was performed at -40°C on a test piece taken in the C direction at the t/4 position of the base material after SR to determine the Charpy absorbed energy. A CTOD test was also performed at -35°C to determine the δ value.
These results are shown in the column after SR of the base material properties in Table 3A.

Furthermore, after the microstructure was revealed from the welded portion after SR at the C-section, Charpy test pieces were taken with the I-side fusion line on the front side as the center of each notch, and the center positions were 6.5 mm below the surface and t/2, and the Charpy absorbed energy at -40°C obtained in the test is shown. In addition, after the microstructure of the welded portion after SR was revealed, full-thickness CTOD test pieces were taken with the I-side fusion line on the front side as the center of each notch, and the δ value of the CTOD test at -35°C obtained in the test is shown.
These results are shown in the column after SR of the joint properties in Table 3B.

The Charpy absorbed energy of the base material and welded joints was measured by taking three V-notch test pieces from each of the base material and welded joints and conducting a Charpy impact test at a specified temperature. The V-notch test pieces were full-size test pieces as specified in JIS Z 2242:2005 taken in the C direction from each plate thickness position. The Charpy impact test was also conducted in accordance with JIS Z 2242:2005.

The δ value (δc) in the CTOD test was measured in accordance with BS7448 (British Standard) Part 1 (1991) and BS7448 (British Standard) Part 2 (1997).
For the base material, evaluation was performed in the C direction (sheet width direction) in which the longitudinal direction of the test piece was perpendicular to the rolling direction.
In the welded joint, gas shielded arc welding was performed at a heat input of 35 kJ/mm on the butted steel plate of the K-groove, and the tip of the fatigue notch of the CTOD test piece of the weld was processed to be the center of the plate thickness of the I-side fusion line (FL) of the weld, and the CTOD test was performed at a specified temperature. The welded joint was evaluated only in the L direction (rolling direction). In the evaluation of the CTOD of the welded joint, the test piece was taken so that the tip of the fatigue crack corresponds to the weld bond. Three tests were performed at each test temperature, and the minimum value of the measured data obtained was taken as the δ value of the CTOD test. The unit of CDOD shown in Tables 3A and 3B is mm.

As shown in Tables 1A to 3B, all of the inventive examples Nos. 1 to 14 had excellent strength and toughness. In particular, they showed excellent low-temperature toughness even after the SR treatment. In addition, the yield strength after the SR treatment was 670 to 870 N/ ^mm2 and the tensile strength was 780 to 940 N/ ^mm2 , which were good values.
In addition, in Nos. 1 to 14, the Charpy absorbed energy in the weld heat affected zone of the welded joint exceeded 70 J both before the SR treatment (−65° C.) and after the SR treatment (−40° C.), and the low temperature toughness was good.

On the other hand, as shown in Tables 1A to 3B, in the comparative examples Nos. 15 to 45 and 55, the chemical composition of the steel plate (element content or α value, β value, γ value, Ceq.) was outside the range specified in the present invention, and therefore the toughness of at least the base material or the welded heat-affected zone was deteriorated.

In addition, although the chemical composition of Nos. 46 to 54 met the range of ingredients of the present invention, the manufacturing conditions did not meet the preferred manufacturing conditions. As a result, the toughness was deteriorated. In other words, the Charpy absorbed energy at -65°C at least at the t/4 position was less than 100 J, and in some examples, other toughness was also inferior.

According to the present invention, a steel plate having excellent strength and low-temperature toughness, and further having excellent strength and low-temperature toughness after stress relief annealing, can be provided. This steel plate is suitable for use in liquefied _CO2 transport tanks and has high industrial applicability.

Claims

The chemical composition, in mass%, is
C: 0.07 to 0.11%,
Si: 0.10 to 0.15%,
Mn: 0.70 to 1.20%,
Ni: 1.00 to 2.50%,
Cr: 0.20 to 0.80%,
Mo: 0.20 to 0.80%,
V: 0.005 to 0.070%,
Al: 0.010 to 0.100%,
B: 0.0005 to 0.0030%,
N: 0.0015 to 0.0050%,
P: 0.006% or less,
S: 0.0030% or less,
Cu: 0 to 1.00%,
Nb: 0 to 0.030%,
Ti: 0 to 0.010%,
Ca: 0 to 0.0030%,
Mg: 0 to 0.0030%,
REM: 0 to 0.0030%,
O: 0.0040% or less,
The balance is Fe and impurities.
an α value defined by the following formula (1) of 1.00 to 1.50 mass%,
A β value defined by the following formula (2) is 10.0 to 15.0,
A γ value defined by the following formula (3) is 0.70 to 1.50 mass%,
The Ceq value defined by the following formula (4) is 0.550 to 0.620 mass%,
Yield strength is 670 to 870 N/mm 2 ,
Tensile strength of 780 to 940 N/mm 2 ,
Charpy absorbed energy at -65°C is 100 J or more,
In a hardness distribution measurement at 1/4 thickness position, 1 mm x 1 mm, 0.05 mm pitch, the average hardness at 121 measurement points is 265 Hv to 290 Hv, with a standard deviation of 20 or less.
The plate thickness is 10 to 60 mm.
Steel plate.
α = [C] + 6 × [Si] + 100 × [P] ... (1)
β = 0.65 × [C] 1/2 × (1 + 0.64 × [Si]) × (1 + 4.10 × [Mn]) × (1 + 0.27 × [Cu]) × (1 + 0.52 × [Ni]) × (1 + 2.33 × [Cr]) × (1 + 3.14 × [Mo]) ... (2)
γ = [Mn] + 20 × [Nb] + 36 × [Ti] ... (3)
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ... (4)
In the formulas (1) to (4), [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti], and [V] represent the contents (mass %) of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti, and V, respectively, and elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0.
[fB] calculated by the following formulas (A) to (E) is 0.0003 mass% or more,
The steel sheet according to claim 1.
[fB] = [B] - 0.77 x [fN] ... (A)
[fN] = [N] - 0.29 x [fTi] - 0.52 x [fAl] ... (B)
[fTi] = [Ti] - 2 × [fO] ... (C)
[fAl] = [Al] - 1.125 × [fO] ... (D)
[fO] = [O] - 0.4 x [Ca] - 0.66 x [Mg] - 0.11 x [REM] ... (E)
In the formulas (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively. Elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0, and when the calculated values of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.
When a region surrounded by a grain boundary having a crystal orientation difference of 15° or more, as determined by performing a crystal orientation analysis using an electron beam backscatter diffraction pattern analysis method, is defined as a crystal grain, the circle equivalent diameter of the crystal grain is defined as the crystal grain size, and a value calculated by an area weighted average weighted by the area of each crystal grain is defined as an average crystal grain size, the average crystal grain size at the 1/4 thickness position is 15.0 μm or less,
The steel sheet according to claim 1 or 2.
When the steel sheet is subjected to stress relief annealing at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a cooling rate of 55°C/hr or less in a temperature range of 425°C or more, the yield strength of the part subjected to the stress relief annealing is 670 to 870 N/ mm2 , the tensile strength is 780 to 940 N/ mm2 , and the Charpy absorbed energy at -40°C is 27 J or more.
The steel sheet according to claim 1 or 2.
When the steel sheet is subjected to stress relief annealing at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a cooling rate of 55°C/hr or less in a temperature range of 425°C or more, the yield strength of the part subjected to the stress relief annealing is 670 to 870 N/ mm2 , the tensile strength is 780 to 940 N/ mm2 , and the Charpy absorbed energy at -40°C is 27 J or more.
The steel sheet according to claim 3.