WO2022045352A1

WO2022045352A1 - Steel sheet and method for producing same

Info

Publication number: WO2022045352A1
Application number: PCT/JP2021/031921
Authority: WO
Inventors: 啓介中井; 大貴今城; 真吾中村; 祥晃新宅; 清孝中島
Original assignee: 日本製鉄株式会社
Priority date: 2020-08-31
Filing date: 2021-08-31
Publication date: 2022-03-03
Also published as: JP7127753B2; JPWO2022045352A1; CN115398018B; CN115398018A; KR20220147126A

Abstract

Provided is a steel sheet having a chemical composition of, in mass percentage, 0.040-0.160% of C, 0.01-0.50% of Si, 0.70-2.50% of Mn, 0.030% or less of P, 0.020% or less of S, 0.001-0.100% of Al, 0.0010-0.0080% of N, 0.003-0.050% of Nb, 0.003-0.050% of Ti, and a balance of Fe and impurities, wherein the metal structure at a 1/4t point in the C-section contains bainite occupying 80% or more of the area thereof, the average length of bainitic ferrite forming said bainite, in the direction of the long axis thereof, is 10μm or less, the average length of prior austenite grains at a 1/4t point in the L-section, in a thickness direction, is 20μm or less with the average aspect ratio thereof being 2.5 or more, and the crystal grain boundary density at a 1/10t point, a 1/4t point, and a 1/2t point in the C-section is 500-1100mm/mm2, 400-1000mm/mm2, and 300-900mm/mm2, respectively.

Description

Steel plate and its manufacturing method

The present invention relates to a steel sheet and a method for manufacturing the same.

Applications of steel sheets include ships, high-rise buildings, other buildings, bridges, marine structures, LNG storage tanks and other large tanks, welded structures such as line pipes (see, for example, Patent Documents 1 to 5). .. In recent years, the size of welded structures has been increasing due to the increase in the load weight of container ships. Along with this, steel sheets are required to be thicker and stronger. In addition, in the welded structure as described above, further improvement of low temperature toughness and fracture toughness is an issue from the viewpoint of further safety and reliability.

Further, the welded structure is required to have a brittle crack propagation stopping property (hereinafter referred to as "arrest property") in which the brittle crack is stopped by the base metal even if a brittle crack is generated at the welded joint. Be done.

Japanese Unexamined Patent Publication No. 2019-0233222 Japanese Unexamined Patent Publication No. 2019-0233323 Japanese Unexamined Patent Publication No. 2019-0233224 Japanese Unexamined Patent Publication No. 2019-035107 International Publication No. 2019/069771

However, in general, there is a so-called trade-off relationship between strength and low temperature toughness, so it was not easy to achieve both. In addition, it was not easy to improve the arrest property, which was an important issue. Furthermore, the current situation is that almost no studies have been made on the improvement of fracture toughness.

An object of the present invention is to solve the above-mentioned problems and to provide a steel sheet having high strength and excellent low temperature toughness, fracture toughness and arrest property, and a method for producing the same.

The gist of the present invention is the following steel sheet and its manufacturing method.

(1) The chemical composition of the steel sheet is mass%.
C: 0.040 to 0.160%,
Si: 0.01-0.50%,
Mn: 0.70 to 2.50%,
P: 0.030% or less,
S: 0.020% or less,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0080%,
Nb: 0.003 to 0.050%,
Ti: 0.003 to 0.050%,
Remaining: Fe and impurities,
In the cross section perpendicular to the rolling direction of the steel sheet, when the thickness of the steel sheet is t, the metallographic structure at a position 1/4 t from the surface of the steel sheet is formed.
In% area, it contains more than 80% bainite and
The average length of the bainite ferrite constituting the bainite in the major axis direction is 10 μm or less.
In the cross section parallel to the rolling direction and the thickness direction of the steel sheet, the average length of the former austenite grains at a position 1 / 4t from the surface of the steel sheet in the thickness direction is 20 μm or less, and the average aspect ratio is 2. .5 or more,
In the cross section perpendicular to the rolling direction of the steel sheet,
The grain boundary density at a position 1/10 t from the surface of the steel sheet is 500 to 1100 mm / mm ² ,
The grain boundary density at a position 1 / 4t from the surface of the steel sheet is 400 to 1000 mm / mm ² ,
The grain boundary density at a position 1 / 2t from the surface of the steel sheet is 300 to 900 mm / mm ² .
Steel plate.

(2) The chemical composition is, instead of a part of the Fe, by mass%.
Cu: 1.50% or less,
Ni: 2.50% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 0.150% or less, and B: 0.0050% or less,
It contains at least one selected from the group consisting of
The steel sheet according to (1) above.

(3) The chemical composition is, instead of a part of the Fe, by mass%.
Mg: 0.0100% or less,
Ca: 0.0100% or less, and REM: 0.0100% or less,
It contains at least one selected from the group consisting of
The steel sheet according to (1) or (2) above.

(4) The chemical composition is, instead of a part of the Fe, by mass%.
Zr: 0.0100% or less, and Te: 0.0100% or less,
It contains at least one selected from the group consisting of
The steel sheet according to any one of (1) to (3) above.

(5) The chemical composition is, instead of a part of the Fe, by mass%.
W: 1.00% or less, and Sn: 0.50% or less,
It contains at least one selected from the group consisting of
The steel sheet according to any one of (1) to (4) above.

(6) The chemical composition satisfies the following formula (i).
The steel sheet according to any one of (1) to (5) above.
1.7 ≤ Ti / N ≤ 3.4 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.

(7) The chemical composition satisfies the following formula (ii).
In the cross section perpendicular to the rolling direction of the steel sheet, the average circle equivalent diameter of the TiN particles at a position 1 / 10t from the surface of the steel sheet is 60 nm or less, and the area ratio of the TiN particles is 0.0001% or more. ,
The steel sheet according to any one of (1) to (6) above.
Ti × N ≧ 3.0 × ^10-5 ... (ii)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.

(8) The method for manufacturing a steel sheet according to any one of (1) to (6) above.
In a method for manufacturing a steel sheet, a heating step, a hot rolling step, and an accelerated cooling step are sequentially performed on a steel piece having the chemical composition according to any one of (1) to (6) above.
In the heating step, the steel pieces are heated to a heating temperature of 950 to 1050 ° C.
The hot rolling step includes rough rolling and finish rolling.
The rough rolling was carried out in a range where the surface temperature of the steel pieces was _Trex or higher.
The cumulative rolling reduction in the rough rolling is 10 to 75%.
The finish rolling was carried out in a range where the surface temperature of the steel piece was Ar ₃ or more and less than _Trex .
The cumulative rolling reduction in the finish rolling is 65 to 90%, and the time between passes is 15 seconds or less.
The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is set to 50 seconds or less.
In the accelerated cooling step, the cooling stop temperature is 0 to 550 ° C. under the condition that the cooling start temperature is _Trex -10 ° C or lower and the average cooling rate from the cooling start to the cooling end is 5 to 50 ° C / sec. Water-cooled to
Steel sheet manufacturing method.
However, Ar ₃ is obtained by the following formula (iii), and _Trex is obtained by the following formula (iv). The element symbol in the following formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.
Ar ₃ = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iii)
TRex = _-91900 [Nb *] ² +9400 [Nb *] +770 ... (iv)
However, the amount of solid solution Nb (mass%) obtained by the following formula (v) is determined by sol. When it is Nb,
Nb ≧ sol. In the case of Nb, [Nb *] = sol. Nb
Nb <sol. In the case of Nb, [Nb *] = Nb
And.
sol. Nb = (10 ^{(-6770 / (T + 273) + 2.26)} ) / (C + 12/14 × N) ・・・ (v)
In addition, T in the above formula represents the heating temperature (° C.) of the steel piece in the heating step.

(9) The method for manufacturing a steel sheet according to (7) above.
Obtained by comprising a refining step for producing molten steel and a continuous casting step for continuously casting the molten steel to produce a steel piece having the chemical composition according to any one of (1) to (6) above. In a method for manufacturing a steel plate, a heating step, a hot rolling step, and an accelerated cooling step are sequentially performed on the steel pieces.
In the refining step, Ti is added after the dissolved O concentration in the molten steel becomes 0.0050% or less.
In the continuous casting step, the average cooling rate when the surface temperature of the steel pieces is between 1200 and 900 ° C. is 0.1 to 0.5 ° C./sec.
In the heating step, the steel pieces are heated to a heating temperature of 950 to 1080 ° C.
The hot rolling step includes rough rolling and finish rolling.
The rough rolling was carried out in a range where the surface temperature of the steel pieces was _Trex or higher.
The cumulative rolling reduction in the rough rolling is 10 to 75%.
The finish rolling was carried out in a range where the surface temperature of the steel piece was Ar ₃ or more and less than _Trex .
The cumulative rolling reduction in the finish rolling is 65 to 90%, and the time between passes is 15 seconds or less.
The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is set to 50 seconds or less.
In the accelerated cooling step, the cooling stop temperature is 0 to 550 ° C. under the condition that the cooling start temperature is _Trex -10 ° C or lower and the average cooling rate from the cooling start to the cooling end is 5 to 50 ° C / sec. Water-cooled to
Steel sheet manufacturing method.
Here, Ar ₃ is obtained by the following formula (iii), and _Trex is obtained by the following formula (iv). The element symbol in the following formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.
Ar ₃ = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iii)
TRex = _-91900 [Nb *] ² +9400 [Nb *] +770 ... (iv)
However, the amount of solid solution Nb (mass%) obtained by the following formula (v) is determined by sol. When it is Nb,
Nb ≧ sol. In the case of Nb, [Nb *] = sol. Nb
Nb <sol. In the case of Nb, [Nb *] = Nb
And.
sol. Nb = (10 ^{(-6770 / (T + 273) + 2.26)} ) / (C + 12/14 × N) ・・・ (v)
In addition, T in the above formula represents the heating temperature (° C.) of the steel piece in the heating step.

(10) After the accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650 ° C. is further performed.
The method for manufacturing a steel sheet according to (8) or (9) above.

According to the present invention, it is possible to obtain a steel sheet having high strength and excellent low temperature toughness, fracture toughness and arrest property.

As a result of detailed studies on the above problems, the present inventors have obtained the following findings.

As mentioned above, there is a so-called trade-off relationship between strength and low temperature toughness. In addition, as a result of the studies by the present inventors, it was found that it is not easy to achieve both strength and fracture toughness. Therefore, the present inventors first investigated a method for achieving both high strength and improvement of low temperature toughness and fracture toughness. As a result, the strength is increased by using bainite as the main component of the metal structure, and in addition to the miniaturization and flattening of the bainite structure, the bainite ferrite constituting the bainite is refined not only to have low temperature toughness. It was found that the decrease in fracture toughness can be suppressed.

In addition, by controlling the heating temperature before hot rolling to a low level and performing finish rolling at a high pressure reduction rate in the unrecrystallized region, miniaturization and flattening of the bainite structure and miniaturization of bainitic ferrite are achieved. I found out what I could do.

Next, as a result of examining a method for improving the arrest property, by controlling the crystal grain boundary density in the plate thickness direction of the steel sheet, the direction parallel to the surface of the steel sheet, for example, the direction perpendicular to or parallel to the rolling direction. It was found that the arrest property can be improved.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%". Further, in the present specification, "-" indicating a numerical range is used to mean that the numerical values described before and after the numerical range are included as the lower limit value and the upper limit value unless otherwise specified.

C: 0.040 to 0.160%
C is contained in an amount of 0.040% or more in order to secure the strength of the steel sheet. On the other hand, if the C content exceeds 0.160%, it becomes difficult to secure good low temperature toughness and fracture toughness, so the C content is set to 0.160% or less. Therefore, the C content is 0.040% or more, preferably 0.050% or more or more than 0.050%, more preferably 0.060% or more or more than 0.075%. The C content is 0.160% or less, preferably 0.140% or less, and more preferably 0.120% or less.

Si: 0.01-0.50%
Since Si is effective as a deoxidizing element and a strengthening element, it is contained in an amount of 0.01% or more. On the other hand, if the Si content exceeds 0.50%, the low temperature toughness and the fracture toughness are significantly deteriorated, so the Si content is set to 0.50% or less. Therefore, the Si content is 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. The Si content is 0.50% or less, preferably 0.40% or less, more preferably 0.35% or less, still more preferably 0.30% or less.

Mn: 0.70 to 2.50%
Mn is contained in an amount of 0.70% or more in order to economically secure the strength of the steel sheet. On the other hand, when the Mn content exceeds 2.50%, the central segregation becomes remarkable and the low temperature toughness and the fracture toughness of the portion where the central segregation occurs deteriorates. Therefore, the Mn content is set to 2.50% or less. .. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, and more preferably 1.20% or more. The Mn content is 2.50% or less, preferably 2.00% or less, more preferably 1.80% or less, still more preferably 1.60% or less.

P: 0.030% or less P is an element present in steel as an impurity. In order to stably secure low temperature toughness and fracture toughness, the content of P is 0.030% or less. It is preferably 0.020% or less, more preferably 0.015% or less. The lower limit is 0%, but the P content may be 0.0001% or more in consideration of the cost for reducing the P content.

S: 0.020% or less S is an element present in steel as an impurity. When the S content exceeds 0.020%, a large amount of MnS stretched in the central segregation portion is generated, and the low temperature toughness, fracture toughness and ductility deteriorate. Therefore, the S content is set to 0.020% or less. It is preferably 0.010% or less. The lower the S content is, the more preferable it is, so the lower limit is not particularly specified, but the S content may be 0.0001% or more from the viewpoint of manufacturing cost.

Al: 0.001 to 0.100%
Al is generally an element positively contained as a deoxidizing element, and the Al content is 0.001% or more. However, when the Al content becomes excessive, the formation of coarse cluster-like alumina (Al ₂ O ₃ ) -based inclusions is promoted, and the low temperature toughness and the fracture toughness deteriorate. Therefore, the Al content is 0.100% or less, preferably 0.050% or less.

N: 0.0010 to 0.0080%
Since N has the effect of forming a Ti nitride and suppressing an increase in the austenite particle size when the steel piece is heated, it is contained in an amount of 0.0010% or more. However, if the N content exceeds 0.0080%, the steel sheet becomes embrittlement, so the N content is set to 0.0080% or less. Therefore, the N content is 0.0010% or more, preferably 0.0015% or more, and more preferably 0.0020% or more. The N content is 0.0080% or less, preferably 0.0065% or less, and more preferably 0.0060% or less.

Nb: 0.003 to 0.050%
Nb can improve the strength and toughness of the steel sheet. Further, in order to obtain a predetermined microstructure, rolling in the unrecrystallized austenite region is required, but Nb is an effective element for expanding the unrecrystallized temperature region, and raises the rolling temperature. It also contributes to productivity improvement. In order to obtain this effect, it is contained in an amount of 0.003% or more. However, if the Nb content exceeds 0.050%, the low temperature toughness, fracture toughness and weldability deteriorate, so the Nb content is set to 0.050% or less. Therefore, the Nb content is 0.003% or more, preferably 0.005% or more, and more preferably 0.008% or more. The Nb content is 0.050% or less, preferably 0.025% or less, and more preferably 0.018% or less.

Ti: 0.003 to 0.050%
Ti can improve the strength and toughness of the steel sheet. Further, by containing Ti, TiN is formed, which suppresses the increase in austenite grain size when the steel piece is heated. As the austenite grain size increases, the crystal grain size of the transformed structure also increases, making it difficult to obtain a predetermined grain boundary density, and the toughness and arrest property deteriorate. In order to obtain the effect of TiN, Ti is contained in an amount of 0.003% or more.

However, if the Ti content exceeds 0.050%, TiC is formed and the HAZ toughness decreases, so the Ti content should be 0.050% or less. Therefore, the Ti content is 0.003% or more, preferably 0.006% or more, and more preferably 0.008% or more. The Ti content is 0.050% or less, preferably 0.020% or less, and more preferably 0.015% or less.

Further, it is preferable that the Ti content satisfies the following formula (i) in relation to the N content. By setting the Ti / N value to 1.7 or more, the solid solution N can be fixed and the arrest property can be improved. When the solid solution N is excessive, for example, the solid solution N promotes the susceptibility to cleft fracture, the solid solution N promotes grain boundary brittleness, the solid solution N forms MA, and the Fe nitride that fixes dislocations causes brittleness. It is considered that the arrest property is lowered due to the phenomenon such as conversion.

On the other hand, by setting the Ti / N value to 3.4 or less, it is possible to suppress the formation of coarse TiN, TiC and the like, and improve the arrest property. The Ti / N value is preferably 2.0 to 3.0, more preferably 2.3 to 2.7.
1.7 ≤ Ti / N ≤ 3.4 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.

Further, the Ti content preferably satisfies the following equation (ii) in relation to the N content. By setting the value of Ti × N to 3.0 × ^10-5 or more, as will be described later, the average circle equivalent diameter is 60 nm or less and the area ratio is 0. At the position 1/10 t from the surface of the steel sheet. TiN particles of 0001% or more can be obtained, which contributes to the improvement of arrest property. The value of Ti × N is preferably 4.0 × 10 ^-5 to 10.0 × 10 ^-5 , and more preferably 5.0 × 10 ^-5 to 8.0 × 10 ^-5 .
Ti × N ≧ 3.0 × ^10-5 ... (ii)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, at least one selected from the group consisting of Cu, Ni, Cr, Mo, V and B for the purpose of improving the strength is described below. It may be contained in the range shown. The reason for limiting each element will be described.

Cu: 1.50% or less Cu has the effect of improving the strength and toughness of the steel sheet, and may be contained as necessary. However, if Cu is contained in an excessive amount, the performance is not improved in proportion to the increase in alloy cost, but rather it may cause surface cracking. Therefore, the Cu content is 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less. When the above effect is to be obtained more reliably, the Cu content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.050% or more.

Ni: 2.50% or less Ni is an element having an effect of improving the strength of the steel sheet, and may be contained as necessary. Further, Ni is an element having an effect of increasing the toughness of the steel matrix (fabric) in the solid solution state. However, if Ni is excessively contained, the low temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Ni content is 2.50% or less, preferably 1.00% or less, more preferably 0.50% or less, still more preferably 0.30% or less. When the above effect is to be obtained more reliably, the Ni content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.050% or more.

Cr: 1.00% or less Cr is an element having an effect of improving the strength of the steel sheet, and may be contained as necessary. However, if Cr is excessively contained, low temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, still more preferably 0.30% or less. When the above effect is to be obtained more reliably, the Cr content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.050% or more.

Mo: 1.00% or less Mo is an element having an effect of improving the strength of the steel sheet, and may be contained as necessary. However, if Mo is contained in an excessive amount, low temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Mo content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, still more preferably 0.30% or less. When the above effect is to be obtained more reliably, the Mo content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.

V: 0.150% or less Since V is an element having an effect of improving the strength of the steel sheet, it may be contained if necessary. However, if V is excessively contained, low temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the V content is 0.150% or less, preferably 0.100% or less, more preferably 0.070% or less, still more preferably 0.050% or less. When the above effect is to be obtained more reliably, the V content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.

B: 0.0050% or less B is an element that enhances hardenability and contributes to improving the strength of the steel sheet, and may be contained as necessary. However, if B is contained in an excessive amount, the low temperature toughness and the fracture toughness are lowered. Therefore, the B content is 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less. When the above effect is to be obtained more reliably, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, at least one selected from the group consisting of Mg, Ca and REM is further contained in the range shown below for the purpose of controlling inclusions. You may. The reason for limiting each element will be described.

Mg: 0.0100% or less Mg is a deoxidizing element, which suppresses the formation of coarse inclusions by forming sulfides and suppresses the formation of harmful inclusions by forming fine oxides. It is an element that does. Therefore, it may be contained as needed. However, if Mg is excessively contained, coarse oxides, sulfides, and acid sulfides are likely to be formed, and low temperature toughness and fracture toughness are deteriorated. Therefore, the Mg content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is to be obtained more reliably, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.

Ca: 0.0100% or less Ca is a deoxidizing element, which suppresses the formation of coarse inclusions by forming sulfides and suppresses the formation of harmful inclusions by forming fine oxides. It is an element to be used. Therefore, it may be contained as needed. However, if Ca is excessively contained, coarse oxides, sulfides, and acid sulfides are likely to be formed, and low temperature toughness and fracture toughness are deteriorated. Therefore, the Ca content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is to be obtained more reliably, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.

REM: 0.0100% or less REM is a deoxidizing element, which suppresses the formation of coarse inclusions by forming sulfides and suppresses the formation of harmful inclusions by forming fine oxides. It is an element that does. Therefore, it may be contained as needed. However, if REM is excessively contained, coarse oxides, sulfides, and acid sulfides are likely to be formed, and low temperature toughness and fracture toughness are deteriorated. Therefore, the REM content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is desired to be obtained more reliably, the REM content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.

Here, in the present invention, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements. Lanthanoids are industrially added in the form of misch metal.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, at least one selected from the group consisting of Zr and Te is further contained in the range shown below for the purpose of miniaturizing the metal structure. May be good. The reason for limiting each element will be described.

Zr: 0.0100% or less Zr is an element that contributes to the improvement of toughness by miniaturizing the structure of the steel sheet. Zr also functions as a deoxidizing element. Therefore, it may be contained as needed. However, excessive Zr content reduces low temperature toughness and fracture toughness. Therefore, the Zr content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is to be obtained more reliably, the Zr content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.

Te: 0.0100% or less Te is an element that contributes to the improvement of toughness by refining the structure of the steel sheet, and may be contained as necessary. However, even if Te is excessively contained, the above effect is saturated. Therefore, the Te content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is to be obtained more reliably, the Te content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, at least one selected from the group consisting of W and Sn may be contained in the range shown below for the purpose of improving corrosion resistance. .. The reason for limiting each element will be described.

W: 1.00% or less W is an element that dissolves and adsorbs to rust in the form of oxygen acid ion WO _4- ^, suppresses the permeation of chloride ions in the rust layer, and improves corrosion resistance, so it is necessary. It may be contained according to the above. However, even if W is excessively contained, not only the above effect is saturated, but also low temperature toughness and fracture toughness may be lowered. Therefore, the W content is 1.00% or less, preferably 0.75% or less. When the above effect is to be obtained more reliably, the W content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.

Sn: 0.50% or less Sn is an element that dissolves as Sn ²⁺ and has an action of suppressing corrosion by an inhibitory action in an acidic chloride solution. In addition, Sn has an effect of suppressing the anode melting reaction of steel and improving corrosion resistance. Therefore, it may be contained as needed. However, even if Sn is contained in an excessive amount, not only the above effect is saturated, but also rolling cracks of the steel sheet are likely to occur. Therefore, the Sn content is 0.50% or less, preferably 0.30% or less. When the above effect is to be obtained more reliably, the Sn content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.

In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component mixed with raw materials such as ore and scrap and various factors in the manufacturing process when the steel sheet is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something. O can also be mixed in the steel sheet as an impurity, but it is permissible if the O content is 0.0040% or less.

(B) Metallic structure of steel sheet The metal structure of the steel sheet of the present invention will be described. In the following description, "%" means "area%". Further, in the present invention, when the thickness of the steel sheet is t, the position 1 / 4t from the surface of the steel sheet in the cross section perpendicular to the rolling direction of the steel sheet is referred to as "1 / 4t position in the C cross section". The position 1 / 4t from the surface of the steel sheet in the cross section parallel to the rolling direction and the thickness direction of the steel sheet is referred to as "1 / 4t position in the L cross section". Further, the above-mentioned "rolling direction" means a rolling direction in finish rolling.

Bainite: 80% or more In the present invention, the metal structure is mainly bainite. Specifically, by setting the area ratio of bainite at the 1 / 4t position on the C cross section to 80% or more, it is possible to secure the strength of the steel sheet. The area ratio of bainite is preferably 90% or more. It is not necessary to set an upper limit on the area ratio of bainite, that is, it may be bainite single phase.

Ferrite, pearlite, and martensite / austenite mixed phase (MA phase) may be mixed as the residual structure, but it is permissible if the total area ratio of these is 20% or less. The total area ratio is preferably 10% or less. It is preferable that the total area ratio of these is small, and the lower limit is not particularly limited. For example, the total area ratio may be 0%. Further, it may be more than 0% or 1% or more.

As described above, in addition to using bainite as the main component, by making the bainite structure finer and flatter, and further making the bainite ferrite finer, it is possible to achieve both the strength of the steel sheet and the low temperature toughness and fracture toughness. can. Specifically, the bainite organization must meet the following provisions.

Average length of bainitic ferrite: 10 μm or less At the 1 / 4t position in the C cross section, the average length of bainite ferrite constituting bainite in the major axis direction shall be 10 μm or less. By refining the bainitic ferrite that constitutes bainite, it is possible to secure fracture toughness. The average length of bainitic ferrite is preferably 8 μm or less.

Average length in the thickness direction of the old austenite grains: 20 μm or less Average aspect ratio of the old austenite grains: 2.5 or more The miniaturization of the bainite structure controls the heating temperature before hot rolling to a low level and does not recrystallize. This can be achieved by performing finish rolling at a high-pressure reduction ratio in the region. That is, the old austenite grains of bainite have a shape elongated in the rolling direction. Therefore, at the 1 / 4t position in the L cross section, the average length of the old austenite grains in the thickness direction is 20 μm or less, and the average aspect ratio is 2.5 or more. The average length of the old austenite grains in the thickness direction is preferably 15 μm or less. Further, the average aspect ratio of the old austenite grains is preferably more than 2.5, more preferably 4.0 or more.

Here, in the present invention, the area ratio of the metal structure is calculated as follows. First, a sample is taken from the steel plate so that the 1 / 4t position on the C cross section is the observation surface. Then, the observation surface is night-game-etched, and after etching, eight fields of view are photographed at a magnification of 500 using an optical microscope. Then, image analysis is performed on the obtained tissue photograph, and the area ratio of each is obtained by using ferrite as the one that looks white and pearlite as the one that looks black.

Next, the night-game-etched part is repeller-etched, the part that looks gray by night-game etching is image-analyzed, and the area ratio is obtained with the part that looks white as the MA phase.

The average length of bainite ferrite and the area ratio of bainite are calculated by KAM (Kernel Average Missionation) analysis using EBSD (Electron Back Scatter Diffraction). In the structure determined to be ferrite in KAM analysis, the region where the local orientation difference exceeds 1.0 ° is bainitic ferrite. In the measurement, bainitic ferrite having a length in the major axis direction of 1 μm or more is targeted. The area ratio of bainite is the sum of the area ratios of bainite ferrite.

The average length and aspect ratio of the old austenite grains in the thickness direction are measured according to JIS G 0551: 2013. First, a sample is taken from the steel plate so that the 1 / 4t position on the L cross section is the observation surface. Next, after the observation surface is mirror-polished, it is corroded by the Behcet-Beaujard method using a saturated aqueous solution of picric acid. The grains that appear black due to corrosion are called old austenite grains.

The observation surface on which the old austenite grains are exposed is observed with an optical microscope, and a field of view having an area of 0.05 mm ² or more is photographed with 8 fields or more (total 0.40 mm ² or more). Then, the thickness of the old austenite grains is measured by a cutting method based on the tissue photograph taken by an optical microscope, and the average value thereof is taken as the average length in the thickness direction of the old austenite grains. In the measurement, the old austenite grains having a length of 1 μm or more in the thickness direction are targeted.

Further, from the above microstructure photograph, the maximum length in the major axis direction and the maximum length in the minor axis direction orthogonal to the major axis direction were measured for each old austenite grain, and the ratio (maximum length / short axis in the major axis direction) was measured. Axis maximum length) is calculated. Then, the average value is taken as the average aspect ratio of the old austenite grains. When finish rolling is performed in the unrecrystallized region at a high pressure reduction rate, the old austenite grains show a shape extended in the rolling direction, so the major axis direction is the rolling direction and the minor axis direction is the plate thickness direction ( The so-called ND direction).

If the old austenite grains cannot be sufficiently expressed by the above method, "Study for improving the accuracy of the method for reconstructing the austenite structure of steel" (Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kono, Nippon Steel & Sumitomo Metal) The old austenite grains are identified by the reconstruction method described in Bulletin No. 404 (2016), p.24-30), and the average length and aspect ratio of the old austenite grains in the thickness direction are obtained. ..

Grain boundary density at 1 / 10t position in C cross section: 500 to 1100 mm / mm ²
Grain boundary density at 1 / 4t position in C cross section: 400-1000 mm / mm ²
Grain boundary density at 1 / 2t position in C cross section: 300-900 mm / mm ²
As a controlling factor in improving arrestability, the contribution of grain boundaries, which hinder brittle crack propagation, is large. At the grain boundaries, the crystal orientation differs between adjacent crystal grains, so the direction in which cracks propagate changes at this portion. Therefore, an unbroken region is generated, and the stress is dispersed by the unbroken region, resulting in a crack closing stress. Therefore, the driving force for crack propagation is reduced, and the arrest property is improved. Further, since the unbroken region is finally ductile fractured, the energy required for brittle fracture is absorbed. Therefore, the arrest property is improved.

Until now, it was thought that it was necessary to make the crystal grain size finer in order to increase the grain boundaries. This is true for ferrite-based structures, but for thick, high-strength steels, the use of bainite is essential. Unlike ferrite, this bainite has a complicated substructure shape, so it is extremely difficult to define crystal grains. Therefore, even if the relationship between the crystal grain size and the arrest property is obtained in terms of the equivalent circle diameter, the variation is large, and it is difficult to determine the crystal grain size required for improving the arrest property.

Therefore, returning to the basic principle that the grain boundaries hinder crack propagation, the total length of the grain boundaries per unit area (hereinafter referred to as "grain boundary density") is defined, and the arrest property is used. It was found that the correlation was the best when the relationship with was organized. Here, the "crystal grain boundary density" means "the total length per unit area of the crystal grain boundaries having a crystal orientation difference of 15 ° or more". The reason why the crystal orientation difference is set to 15 ° or more is that if the crystal orientation difference is less than 15 °, the grain boundaries are unlikely to interfere with brittle crack propagation, and the effect of improving arrestability is reduced.

Excellent arrest by setting the grain boundary density in the C cross section to 500 mm / mm ² or more at the 1 / 10t position, 400 mm / mm ² or more at the 1 / 4t position, and 300 mm / mm ² or more at the 1 / 2t position. Sex is obtained. In order to further stably improve the arrest property, the grain boundary densities in the C cross section are 600 mm / mm ^{2 or more at the 1 / 10t position, 500 mm / mm 2} ^or more at the 1 / 4t position, and 1 / 2t, respectively. The position is preferably 400 mm / mm ² or more.

As the grain boundary density increases, the arrestability improves, but an excessive increase leads to an increase in the rolling load, which in turn lowers the productivity. Therefore, the grain boundary density in the C cross section is set to 1100 mm / mm ² or less at the 1 / 10t position, 1000 mm / mm ^{2 or less at the 1 / 4t position, and 900 mm / mm 2} ^or less at the 1 / 2t position. The grain boundary densities in the C cross section are preferably 1000 mm / mm ^{2 or less at the 1 / 10t position, 900 mm / mm 2 or less at the 1 / 4t position, and 800 mm / mm 2} ^or ^less at the 1 / 2t position, respectively.

It is necessary to increase the grain boundary density of the entire plate thickness in order to improve the arrest property of the extra-thick material. In the production method described later, the grain boundary density at the 1 / 2t position is mainly controlled. At other plate thickness positions, the temperature is inevitably low and the cooling rate is high, so that the grain boundary density tends to increase. Therefore, it is often sufficient to specify only the grain boundary density at the 1 / 2t position. However, depending on the heating method, a large temperature gradient may occur in the plate thickness direction, and the grain boundary density may be reversed at, for example, the 1 / 4t position and the 1 / 2t position. Therefore, in the present invention, the grain boundary densities at the 1 / 10t position, the 1 / 4t position, and the 1 / 2t position are defined as the representative values of the grain boundary densities of the average plate thickness.

In the present invention, the grain boundary density is measured by the electron backscatter diffraction (EBSD) method. Specifically, by the EBSD method, the 500 μm × 500 μm region at the 1 / 10t position, 1 / 4t position, and 1 / 2t position is measured at a pitch of 1 μm, and the boundary where the crystal orientation difference from the adjacent grain is 15 ° or more is defined. It is defined as a grain boundary and can be obtained by dividing the total length of the crystal grain boundary at that time by the measured area.

TiN particles average circle equivalent diameter at 1 / 10t position: 60 nm or less Area ratio: 0.0001% or more When TiN particles are finely dispersed at 1 / 10t, the pinning effect of TiN particles is effectively exhibited. The coarsening of old austenite is suppressed. As a result, the grain boundary density at the 1 / 10t position increases, and the arrest property of the steel sheet is further improved. Therefore, it is preferable that the average circle equivalent diameter of the TiN particles existing at the 1 / 10t position is 60 nm or less and the area ratio is 0.0001% or more.

The average circle equivalent diameter of the TiN particles is more preferably 50 nm or less, and further preferably 40 nm or less. The lower limit of the average circle equivalent diameter of the TiN particles is not particularly limited, and may be, for example, 10 nm or more. Further, the area ratio of the TiN particles is more preferably 0.0002% or more, further preferably 0.0003% or more. The upper limit of the area ratio of TiN particles is not particularly limited, and may be, for example, 0.0020% or less.

The average circle-equivalent diameter and area ratio of TiN particles are measured by the following methods. First, an extraction replica is prepared from the 1 / 10t position of the steel plate, and with a TEM equipped with an energy dispersive X-ray analyzer (EDX), the observation area of one field of view is set to 15 μm ² or more at a magnification of 30,000 times or more, and 15 Observe particles with a size of ~ 200 nm. All observed particles are analyzed using EDX, and particles containing 1% by mass or more of Ti, less than 1% by mass of O (oxygen), and 1% by mass or more of N are discriminated as TiN particles. do.

The electron beam diameter of the TEM used for quantitative analysis of the particles is 1 to 20 nm, the observation magnification is 50,000 to 1,000,000 times, and any position in the particles is quantitatively analyzed. The average circle equivalent diameter of TiN particles is an arithmetic mean of the area of each TiN particle determined above and the equivalent diameter (diameter) of a circle having the same area. The area ratio of the TiN particles is a value obtained by dividing the total area of the individual TiN particles determined above by the area of the observed visual field.

Here, when the observation area of one field of view is 15 μm ² or more at a magnification of 30,000 times or more, particles having a size of 15 to 200 nm are observed, and the number of discriminated TiN particles is less than 100, Check another field of view and continue observing until the total number of TiN particles is 100 or more. In this case, the average circle-equivalent diameter of the TiN particles is the arithmetic mean of the circle-equivalent diameters (diameters) of the individual identified TiN particles, as described above. The area ratio of TiN particles is a value obtained by dividing the total area of TiN particles that have been observed until the number of TiN particles reaches 100 or more by the total area of the visual field observed so far. Further, when the number of visual fields continued to be observed became 50 and the cumulative observation area became 750 μm ² or more, the total number of identified TiN particles was less than 100. In this case, it is considered that TiN particles do not exist, and it is out of the scope of the present application.

(C) Mechanical Properties of Steel Sheet The mechanical properties of the steel sheet according to the present invention are not particularly limited, but the steel sheet according to the present invention has high strength and is excellent in low temperature toughness, fracture toughness and arrest property. Specifically, it is preferable that the yield stress (YS) is 460 to 860 MPa and the tensile strength (TS) is 570 to 980 MPa. Further, it is preferable that the fracture surface transition temperature (vTrs), which is an index of low temperature toughness, is −60 ° C. or lower. Further, it is preferable that the Crack Tip Opening Displacement (CTOD) value at −10 ° C., which is an index of fracture toughness, is 0.50 mm or more.

The tensile strength (TS) and yield stress (YS) are measured using a No. 1B tensile test piece collected from the center of the plate thickness in the direction perpendicular to the rolling direction based on JIS Z 2241: 2011. Specifically, the yield stress (YS) is the proof stress of the permanent elongation method at 0.2% permanent elongation. The evaluation of the fracture surface transition temperature (vTrs) is based on JIS Z 2242: 2005, and the test piece is a V-notch test piece and is collected so as to include the 1 / 4t position of the steel plate. Further, according to ISO 15653: 2018, a CTOD test piece having the total thickness in the plate thickness direction of the base metal as the notch position of 3-point bending is collected, and the CTOD value at −10 ° C. is measured.

Further, in the temperature gradient type ESSO test, the brittle crack propagation stop toughness value Kca (hereinafter referred to as “arest toughness value Kca _{-10 ° C} ”) at a test temperature of −10 ° C. is 6000 N / mm ^1.5 or more. It is preferably 8000 N / mm ^1.5 or more, and more preferably 8000 N / mm 1.5 or more. By satisfying this characteristic, the steel sheet has excellent arrest property.

Arrest toughness value Kca _{-10 ° C} is NK Ship Class Association Steel Ship Regulation Inspection Procedure K Edition Annex K3.12.2-1. Measurements are performed in accordance with (2016) "Inspection Guidelines for Temperature Gradient ESSO Test and Temperature Gradient Double Tensile Test".

Further, the non-ductile transition temperature (hereinafter referred to as “NDT temperature”) in the NRL drop test is preferably −100 ° C. or lower, and more preferably −110 ° C. or lower. By satisfying this characteristic, the steel sheet has excellent arrest property.

The NDT temperature is determined by conducting a test in accordance with the NRL drop weight test method specified in ASTM E208-06. The NRL drop test method will be described in detail. First, a type P3 test piece specified in ASTM E208 is collected so as to include the outermost surface of the steel plate. The type P3 test piece is a test piece having a length of 130 mm, a width of 50 mm, and a thickness of 16 mm. At this time, the sample is collected so that the thickness direction of the test piece coincides with the plate thickness direction of the steel sheet and the longitudinal direction of the test piece coincides with the rolling direction of the steel sheet.

Then, using the above test piece, carry out an NRL drop test in accordance with ASTM E208-06. Specifically, first, a weld bead extending in a direction parallel to the longitudinal direction of the test piece is formed on the outermost surface of the steel plate perpendicular to the thickness direction of the test piece. At that time, the welding material having low toughness specified in ASTM E208 is used. The length of the weld bead is adjusted to be in the range of 60 to 70 mm and the width is adjusted to be in the range of 12 to 16 mm. Then, a notch parallel to the width direction of the test piece is formed on the weld bead. At this time, the width of the notch is set to 1.5 mm or less, and the distance between the groove bottom of the notch and the test piece is adjusted to be in the range of 1.8 to 2.0 mm.

Then, after the surface on which the weld bead is formed of the test piece is directed downward and both ends in the length direction are supported, the impact bending load due to the drop weight is applied to the surface opposite to the surface on which the weld bead is formed. Add. Then, by examining the state in which the brittle crack generated from the notch propagates to the test piece, Break (with crack propagation) or No Break (without crack propagation) is determined. If the brittle crack generated from the notch propagates on the surface of the test piece in the width direction of the test piece and progresses to the end thereof, the test result is determined to be Break (with crack propagation). If the crack does not reach the end in the width direction, the test result is determined to be No Break (no crack propagation).

The above drop test is performed using two test pieces, for example, starting from the condition of -100 ° C and changing the test temperature at 5 ° C intervals (in the case of No Break, the temperature drops by 5 ° C, Break's (In the case of an increase of 5 ° C.), the temperature 5 ° C. lower than the lowest test temperature at which No Break was obtained for both of the two test pieces is defined as the non-ductile transition temperature.

(D) Thickness of Steel Plate The thickness of the steel plate according to the present invention is not particularly limited, but when used as a welded structure, the thickness is preferably 10 to 70 mm, preferably 20 to 60 mm. Is more preferable. Further, the effect of improving the low temperature toughness and the fracture toughness in the present invention is remarkably exhibited when the thickness is less than 50 mm.

(E) Method for manufacturing steel plate The manufacturing conditions for the steel plate according to the present invention are not particularly limited, but for example, the refining step, the continuous casting step, the heating step, the hot rolling step and the accelerated cooling step are sequentially performed under the conditions shown below. By doing so, it can be manufactured. Each process will be described.

(A) Refining process The refining process is a process for producing molten steel. The conditions of the refining process are not particularly limited, and a conventional method may be used. However, we want to suppress the formation of Ti ₂ O ₃ and finely disperse TiN. Specifically, we want the average circle equivalent diameter of TiN particles at the 1 / 10t position to be 60 nm or less and the area ratio to 0.0001% or more. In this case, it is preferable to perform vacuum degassing and add Ti after the dissolved O concentration in the molten steel becomes 0.0050% by mass or less. If Ti is added in a state where the dissolved O concentration exceeds 0.0050% by mass, it becomes difficult to suppress the formation of Ti ₂ O ₃ . The addition of Ti can be performed, for example, in a recirculation type degassing device.

(B) Continuous Casting Step The continuous casting step is a step of continuously casting molten steel to produce steel pieces having the above-mentioned chemical composition. The conditions of the continuous casting process are not particularly limited, and a conventional method may be used. However, if the average circle equivalent diameter of TiN particles at the 1 / 10t position is 60 nm or less and the area ratio is 0.0001% or more, the average cooling rate when the surface temperature of the steel pieces is between 1200 and 900 ° C. is set. It is preferably 0.1 to 0.5 ° C./sec. If the average cooling rate is less than 0.1 ° C./sec, the TiN particles may be coarsened, and if it exceeds 0.5 ° C./sec, the area ratio of TiN may decrease.

(C) Heating step The heating step is a step that contributes to the microstructure control of the austenite phase by heating the steel pieces. In the heating step, the above steel pieces are heated to a heating temperature of 950 to 1080 ° C. The heating step may be performed in a heating furnace. Note that heating the steel pieces to 950 to 1080 ° C. means heating the steel pieces so that the average temperature of the total thickness of the steel pieces when extracted from the heating furnace is in the range of 950 to 1080 ° C., and is described in the present specification. The average temperature of the total thickness of the steel pieces is referred to as the heating temperature of the steel pieces. Further, the total thickness average temperature can be calculated from the temperature in the heating furnace, the heating time, and the surface temperature of the steel piece.

If the heating temperature is less than 950 ° C., austeniticization becomes insufficient and hardenability is lowered due to the miniaturization of austenite grains, so that it is difficult to obtain a thick steel sheet and high strength steel sheet. Further, the miniaturization of the austenite grains promotes recrystallization during finish rolling, so that the aspect ratio of the old austenite grains is lowered. Further, when the heating temperature exceeds 1080 ° C., the austenite grains become coarse and it becomes difficult to make the bainite structure finer in the final structure. The preferred heating temperature range is 1000-1050 ° C.

As described above, TiN can be finely dispersed by appropriately controlling the timing of adding Ti in the refining process and appropriately controlling the average cooling rate between 1200 and 900 ° C. in the continuous casting process. As a result, the grain boundary density can be controlled within the above range. In this case, the heating temperature of the steel pieces may be 1080 ° C. or lower.

On the other hand, even when TiN is not actively utilized, by adjusting the heating temperature of the steel pieces in the heating process to a low level, coarsening of austenite is suppressed and the grain boundary density is controlled within the above range. Is possible. In that case, the heating temperature of the steel pieces is 1050 ° C. or lower.

(D) Hot rolling process The hot rolling process includes rough rolling and finish rolling. Rough rolling is carried out in the range where the surface temperature of the steel pieces is _Trex or higher. That is, the rough rolling is started when the surface temperature of the steel pieces is _Trex or higher, and the rough rolling is finished when the surface temperature of the steel pieces is _Trex or higher. By carrying out rough rolling in the range of _Trex or higher, miniaturization becomes possible by recrystallization of austenite grains. The surface temperature at the end of rough rolling may be higher than the surface temperature at the start of rough rolling. It is considered that this is due to the effect of processing heat generation due to rough rolling and the effect of heat transfer in the plate thickness direction of the steel piece due to the internal temperature being higher than the surface temperature.

In addition, the cumulative rolling reduction in rough rolling shall be in the range of 10 to 75%. The cumulative rolling reduction in rough rolling is a value obtained by subtracting the plate thickness after the end of rough rolling from the plate thickness at the start of rough rolling and dividing by the plate thickness at the start of rough rolling. If the cumulative rolling reduction during rough rolling is less than 10%, it is difficult to make the austenite finer by recrystallization, and porosity may remain to cause internal cracking, resulting in deterioration of ductility and toughness. In addition, when the cumulative rolling reduction rate exceeds 75%, the austenite grains become excessively fine, and recrystallization during finish rolling is promoted, so that the aspect ratio of the old austenite grains decreases and the number of passes increases. As a result, productivity decreases. The preferred cumulative reduction rate is 30-60%. In the following description, the steel piece after rough rolling is referred to as a steel plate.

Subsequent finish rolling is carried out in the range where the surface temperature of the steel sheet is Ar ₃ or more and less than _Trex . That is, it is cooled after the rough rolling is completed, the finish rolling is started when the surface temperature of the steel sheet is Ar ₃ or more and less than _Trex , and the finish rolling is finished when the surface temperature of the steel sheet is Ar ₃ or more and less than _Trex . .. By performing the finish rolling in the range of less than _Trex , it becomes possible to impart strain to the austenite grains without recrystallization. This makes it possible to miniaturize bainite in the final structure. When the finishing temperature is set in the range where the surface temperature is _Trex or higher, recrystallization is promoted and the aspect ratio of the old austenite grains is lowered. On the other hand, if the finish rolling is performed in the range where the surface temperature is less than Ar ₃ , processed ferrite may be generated and the final structure may not have a bainite-based structure.

In addition, the cumulative rolling reduction in finish rolling shall be in the range of 65 to 90%. The cumulative rolling reduction in finish rolling is a value obtained by subtracting the plate thickness after the end of finish rolling from the plate thickness at the start of finish rolling (after the end of rough rolling) and dividing by the plate thickness at the start of finish rolling. By setting the cumulative rolling reduction in finish rolling to 65% or more, it is possible to impart sufficient strain to the austenite grains. When the cumulative reduction rate is less than 65%, the strain is not sufficiently applied to the austenite grains, the flattening of the austenite grains is not promoted, and the aspect ratio is lowered. Further, when the cumulative reduction rate exceeds 90%, recrystallization is promoted, the aspect ratio of the old austenite grains is lowered, the number of passes is increased, and the productivity is lowered. The preferred cumulative reduction rate is 70-80%.

Furthermore, the time between passes in finish rolling shall be 15 seconds or less. When the inter-pass time exceeds 15 seconds, the strain applied by the processing is recovered, the bainite in the final structure cannot be sufficiently refined, recrystallization is promoted, and the aspect ratio of the old austenite grains is lowered. The shorter the inter-pass time, the more preferable it is. Therefore, it is not necessary to set a lower limit, but it is preferably 3 seconds or more from the viewpoint of operability. In general, finish rolling is performed by reverse rolling. The time between passes in finish rolling means that the steel sheet is rolled by a rolling roll while moving forward, the rear end of the steel sheet comes out of the rolling roll, the traveling direction of the steel sheet is reversed backward, and the rear end of the steel sheet is again. Means the time it takes for the roll to be bitten into the rolling roll.

Then, the time from the completion of finish rolling to the start of cooling in the accelerated cooling process described later is set to 50 seconds or less. When the time from the completion of finish rolling to the start of cooling exceeds 50 seconds, the strain applied by the processing is recovered, bainite in the final structure cannot be sufficiently refined, recrystallization is promoted, and the old austenite grains are promoted. The aspect ratio of is reduced. The shorter the time from the completion of finish rolling to the start of cooling, the more preferable it is. Therefore, it is not necessary to set a lower limit, but it is preferably 5 seconds or more from the viewpoint of operability. The time from the completion of finish rolling to the start of cooling means the time from when the tip of the steel sheet traveling forward passes through the rolling roll in the final pass to the start of water cooling.

In the above description, Ar ₃ means the transformation start temperature at which the transformation from the austenite particles to the ferrite particles starts in the temperature lowering process, and is obtained by the following equation (iii). Further, Trex means the recrystallization temperature which is the lowest temperature at which _equiaxial recrystallized grains can be generated and grown, and is obtained by the following equation (iv). The element symbol in the following formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.

Ar ₃ = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iii)
TRex = _-91900 [Nb *] ² +9400 [Nb *] +770 ... (iv)
However, the amount of solid solution Nb (mass%) obtained by the following formula (v) is determined by sol. When it is Nb,
Nb ≧ sol. In the case of Nb, [Nb *] = sol. Nb
Nb <sol. In the case of Nb, [Nb *] = Nb
And.
sol. Nb = (10 ^{(-6770 / (T + 273) + 2.26)} ) / (C + 12/14 × N) ・・・ (v)
In addition, T in the above formula represents the heating temperature (° C.) of the steel piece in the heating step.

(E) Accelerated cooling process In the accelerated cooling process, the steel sheet that has been finished rolled is water-cooled. At this time, water cooling is performed to a cooling stop temperature of 0 to 550 ° C. under the condition that the cooling start temperature is _Trex -10 ° C. or lower and the average cooling rate from the cooling start to the cooling end is 5 to 50 ° C./sec. ..

Even if finish rolling is carried out in the range of Ar ₃ or more and less than _Trex , when the cooling start temperature exceeds _Trex -10 ° C due to the subsequent reheating, recovery of strain applied by processing is promoted and bainite in the final structure is promoted. It becomes impossible to sufficiently miniaturize the bainitic ferrite constituting the above.

In addition, the final structure can be made mainly bainite by water cooling to a cooling stop temperature of 0 to 550 ° C at an average cooling rate of 5 to 50 ° C / sec. The average cooling rate and the cooling stop temperature are adjusted according to the value of Ceq in the chemical composition of the steel sheet, and are set to conditions under which martensitic transformation does not occur.

(F) Tempering step After the accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650 ° C. may be further provided. By performing the tempering step, it is possible to reduce the dislocation density that has become excessively high due to cooling. When the cooling stop temperature in the accelerated cooling step is high, the self-tempering effect can be obtained, so that the tempering step does not have to be performed. On the other hand, in the accelerated cooling step, for example, when the cooling is performed to about room temperature, it is preferable to perform a tempering step.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

The hot metal ejected from the blast furnace was desulfurized by hot metal pretreatment, de-P and de-C treated in a converter type refining vessel, and then steel was received in a ladle. At the time of steel ejection, alloying elements were added and cover slag for heat insulation was added.

Subsequently, the molten steel in the ladle was depressurized with an RH vacuum degassing device. During melting, molten steel samples were taken as appropriate and subjected to analysis to obtain molten steel components. The molten steel temperature changed from 1560 ° C to 1610 ° C. Vacuum degassing was performed in the first half of the RH treatment to adjust the dissolved O concentration. The dissolved O concentration was measured using an oxygen concentration probe. Then, Ti was added and a reflux treatment was performed to mix them uniformly.

After treatment with an RH vacuum degassing device, steel pieces having the chemical compositions shown in Tables 1 and 2 were produced by a continuous casting method. In continuous casting, the average cooling rate was appropriately adjusted when the surface temperature of the steel pieces was between 1200 and 900 ° C. Tables 3 and 4 show the dissolved O concentration (% by mass) in the molten steel when Ti is added, and the average cooling rate (° C./sec) between 1200 and 900 ° C. in continuous casting. Further, using the above steel pieces, a steel plate having a plate thickness of 10 to 70 mm was prototyped according to the production conditions shown in Tables 5 and 6.

The metallographic structure of the obtained steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, a sample was taken from the steel plate so that the 1 / 4t position on the C cross section was the observation surface. Then, the observation surface is nital-etched, and after etching, eight fields of view are photographed at a magnification of 500 using an optical microscope, and image analysis is performed on the obtained microstructure photograph. Was taken as pearl light, and the area ratio of each was calculated.

Next, the part that had been etched by night game was subjected to repera etching, and the image analysis was performed on the part that looked gray by night game etching, and the area ratio was calculated with the part that looked white as the MA phase.

The average length of bainitic ferrite and the area ratio of bainite were calculated by KAM analysis using EBSD. In the structure judged to be ferrite in the KAM analysis, the region where the local orientation difference exceeds 1.0 ° was defined as bainitic ferrite. In the measurement, bainitic ferrite having a length in the major axis direction of 1 μm or more was targeted. The area ratio of bainite is the sum of the area ratios of bainite ferrite.

Furthermore, the average length and the average aspect ratio of the old austenite grains in the thickness direction were measured according to JIS G 0551: 2013. First, a sample was taken from the steel plate so that the 1 / 4t position on the L cross section was the observation surface. Next, after the observation surface was mirror-polished, it was corroded by the Behcet-Beaujard method using a saturated aqueous solution of picric acid to reveal old austenite grains.

The observation surface on which the old austenite grains appeared was observed with an optical microscope, and a field of view with an area of 0.05 mm ² or more was photographed for 8 fields or more (total 0.40 mm ² or more). Then, the thickness of the old austenite grains was measured by a cutting method based on the tissue photograph taken by an optical microscope, and the average value was taken as the average length in the thickness direction of the old austenite grains. In the measurement, old austenite grains having a length of 1 μm or more in the thickness direction were targeted.

In addition, from the above microstructure photograph, the maximum length in the major axis direction and the maximum length in the minor axis direction orthogonal to the major axis direction were measured for each old austenite grain, and the ratio (maximum length / short axis) was measured. The maximum axis length) was calculated, and the average value was taken as the average aspect ratio of the old austenite grains.

Furthermore, the average circle-equivalent diameter and area ratio of TiN particles were measured using a TEM with EDX. First, an extraction replica was prepared from the 1 / 10t position of the steel plate, and particles having a size of 15 to 200 nm were observed by TEM with an observation area of 15 μm ² or more in one field of view at a magnification of 30,000 times or more. All observed particles are analyzed using EDX, and particles containing 1% by mass or more of Ti, less than 1% by mass of O (oxygen), and 1% by mass or more of N are discriminated as TiN particles. did.

The electron beam diameter of the TEM was 1 to 20 nm, the observation magnification was 50,000 to 1,000,000 times, and an arbitrary position in the particle was quantitatively analyzed. The average circle equivalent diameter of TiN particles is an arithmetic mean of the area of each TiN particle determined above and the equivalent diameter (diameter) of a circle having the same area. The area ratio of the TiN particles is a value obtained by dividing the total area of the individual TiN particles determined above by the area of the observed visual field.

Subsequently, the grain boundary density was measured by the EBSD method. Specifically, by the EBSD method, the 500 μm × 500 μm region at the 1 / 10t position, 1 / 4t position, and 1 / 2t position is measured at a pitch of 1 μm, and the boundary where the crystal orientation difference from the adjacent grain is 15 ° or more is defined. It was defined as a grain boundary and obtained by dividing the total length of the crystal grain boundary at that time by the measured area.

The measurement results are shown in Tables 7 and 8. In the table, the ferrite area ratio is "F fraction", the pearlite area ratio is "P fraction", the bainite area ratio is "B fraction", and the MA phase area ratio is "MA fraction". , The average length of bainitic ferrite in the major axis direction is referred to as "BF length".

Furthermore, the tensile strength (TS) and the yield stress (YS) were measured based on JIS Z 2241: 2011. The test piece was measured using a No. 1B tensile test piece collected with the direction perpendicular to the rolling direction (width direction) from the center of the plate thickness as the longitudinal direction. The yield stress (YS) was the proof stress of the permanent elongation method when the permanent elongation was 0.2%. In this example, those having a YS of 460 MPa or more and a TS of 570 MPa or more are considered to have high strength.

In addition, V-notch test pieces were collected so as to include the 1 / 4t position of the steel plate, and the fracture surface transition temperature (vTrs) was evaluated in accordance with JIS Z 2242: 2005. At this time, two V-notch test pieces were taken so that the longitudinal direction of the test pieces coincided with the rolling direction and the width direction of the steel sheet. In this example, the two test pieces having vTrs of −60 ° C. or lower were considered to have excellent low temperature toughness.

Then, according to ISO 15653: 2018, CTOD test pieces having the total thickness in the plate thickness direction of the base metal as the notch position of 3-point bending were collected, and the CTOD value at −10 ° C. was measured. The test was performed 3 times and the minimum values are shown in the table. In this example, those having a minimum CTOD value of 0.50 mm or more at −10 ° C. are considered to have excellent fracture toughness.

In addition, NK Ship Class Association Steel Ship Regulations Inspection Guidelines K Edition Annex K3.12.2-1. The arrest toughness value Kca- _{10 ° C.} was measured in accordance with the "Inspection Procedure for Temperature Gradient ESSO Test and Temperature Gradient Double Tensile Test" of (2016). Next, the test was performed in accordance with the NRL drop weight test method specified in ASTM E208-06, and the NDT temperature was determined. In this example, those having an arrest toughness value of Kca _{-10 ° C} of 6000 N / mm ^1.5 or more and an NDT temperature of -100 ° C or less are considered to have excellent arrest properties.

The measurement results are shown in Tables 9 and 10.

As can be seen from Tables 7 to 10, in the examples of the present invention (test numbers 1 to 29) satisfying the provisions of the present invention, the results were high in strength and excellent in low temperature toughness, fracture toughness and arrest property. On the other hand, in Comparative Examples (Test Nos. 30 to 61), at least one of strength, low temperature toughness and arrest property was deteriorated.

Specifically, test number 30 has a high dissolved O concentration when Ti is added in the refining step, and the heating step in the heating step is high, and test number 31 has a high average cooling rate in the continuous casting step. Since the TiN particles did not precipitate and the grain boundary density could not be optimized, the arrest property deteriorated. In Test No. 32, since the average cooling rate in the continuous casting process was low, coarse TiN particles were precipitated and the grain boundary density could not be optimized, so that the arrest property was deteriorated.

Test number 33 had an excessive C content, so that the low temperature toughness and fracture toughness deteriorated. Test No. 34 had a low C content, did not have a bainite-based structure, had insufficient strength, and deteriorated low temperature toughness and fracture toughness. In Test No. 35, the low temperature toughness and the fracture toughness deteriorated due to the excessive Si content. In Test No. 36, the low temperature toughness and the fracture toughness deteriorated due to the excessive Mn content. Test No. 37 had a low Mn content and insufficient strength.

Test number 38 had an excessive content of P and S, test number 39 had an excessive content of Al, and test number 40 had an excessive content of N, so that low temperature toughness and fracture toughness deteriorated. In Test No. 41, the N content was low, the BF length and the old austenite grains became coarse, and the grain boundary density could not be optimized, so that the low temperature toughness, fracture toughness and arrest property deteriorated.

Test number 42 had an excessive Nb content, so that the low temperature toughness and fracture toughness deteriorated. In Test No. 43, the Nb content was low, the BF length and the austenite grains were coarsened, the aspect ratio of the austenite grains was small, and the grain boundary density could not be optimized. Therefore, low temperature toughness and fracture occurred. Toughness and arrestability deteriorated. In Test No. 44, the low temperature toughness and the fracture toughness deteriorated due to the excessive Ti content. In addition, since the TiN particles are coarsened and the heating temperature in the heating step is high, the grain boundary density cannot be optimized and the arrest property is also deteriorated. In Test No. 45, since the Ti content was low, the BF length and the old austenite grains were coarsened, and the grain boundary density could not be optimized, so that the low temperature toughness, the fracture toughness and the arrest property were deteriorated.

In test numbers 46 and 47, the heating temperature in the heating step was high, the BF length and the old austenite grains were coarsened, and the grain boundary density could not be optimized. Therefore, low temperature toughness, fracture toughness and arrest property were obtained. Deteriorated. In Test No. 48, the heating temperature was low and the bainite area ratio was low, so that the strength was insufficient and the low temperature toughness and fracture toughness deteriorated. In test number 49, the end temperature of rough rolling was less than _Trex , the BF length and the old austenite grains were coarsened, and the grain boundary density could not be optimized, so that the low temperature toughness, fracture toughness and arrest property deteriorated. did.

In test number 50, the cumulative reduction rate of rough rolling was high, the BF length and the old austenite grains were coarsened, the aspect ratio of the old austenite grains was lowered, and the grain boundary density could not be optimized, so that the temperature was low. Deteriorated toughness, fracture toughness and arrestability. On the other hand, in Test No. 51, the cumulative reduction rate was low, the BF length and the old austenite grains were coarsened, and the grain boundary density could not be optimized, so that the low temperature toughness, fracture toughness and arrest property deteriorated.

In test number 52, the start temperature of finish rolling is _Trex or higher, the BF length and the old austenite grains are coarsened, the aspect ratio of the old austenite grains is lowered, and the grain boundary density cannot be optimized. Therefore, the low temperature toughness, fracture toughness and arrest property deteriorated. In Test No. 53, since the finish rolling end temperature was less than Ar ₃ , processed ferrite was excessively generated, the strength became insufficient, and the low temperature toughness and fracture toughness deteriorated.

Test No. 54 has a high cumulative reduction rate of finish rolling, and Test No. 55 has a low cumulative reduction rate, both of which coarsen the BF length and the former austenite grains, reduce the aspect ratio of the former austenite grains, and further. Since the grain boundary density could not be optimized, the low temperature toughness, fracture toughness and arrest property deteriorated. In test number 56, the time between passes is long, and in test number 57, the time from the completion of finish rolling to the start of cooling is long, both the BF length and the old austenite grains become coarse, and the aspect ratio of the old austenite grains decreases. Furthermore, since the grain boundary density could not be optimized, the low temperature toughness, fracture toughness and arrest property deteriorated.

In Test No. 58, since the cooling rate in the accelerated cooling step was high, the MA phase was excessively generated, and the low temperature toughness and the fracture toughness deteriorated. Test No. 59 had a low cooling rate, did not have a bainite-based structure, had insufficient strength, and deteriorated low temperature toughness and fracture toughness. Since the cooling stop temperature of the test number 60 was high, the structure was not mainly composed of bainite, the strength was insufficient, and the low temperature toughness, the fracture toughness and the arrest property were deteriorated. In Test No. 61, the cooling start temperature exceeded _Trex -10 ° C. and the BF length became coarse, so that the low temperature toughness was good, but the fracture toughness deteriorated.

According to the present invention, it is possible to obtain a steel sheet having high strength and excellent low temperature toughness, fracture toughness and arrest property. Therefore, the steel plate according to the present invention can be suitably used as a material for welded structures such as ships, high-rise buildings, other buildings, bridges, marine structures, LNG storage tanks and other large tanks, and line pipes. ..

Claims

The chemical composition of the steel sheet is mass%,
C: 0.040 to 0.160%,
Si: 0.01-0.50%,
Mn: 0.70 to 2.50%,
P: 0.030% or less,
S: 0.020% or less,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0080%,
Nb: 0.003 to 0.050%,
Ti: 0.003 to 0.050%,
Remaining: Fe and impurities,
In the cross section perpendicular to the rolling direction of the steel sheet, when the thickness of the steel sheet is t, the metallographic structure at a position 1/4 t from the surface of the steel sheet is formed.
In% area, it contains more than 80% bainite and
The average length of the bainite ferrite constituting the bainite in the major axis direction is 10 μm or less.
In the cross section parallel to the rolling direction and the thickness direction of the steel sheet, the average length of the former austenite grains at a position 1 / 4t from the surface of the steel sheet in the thickness direction is 20 μm or less, and the average aspect ratio is 2. .5 or more,
In the cross section perpendicular to the rolling direction of the steel sheet,
The grain boundary density at a position 1/10 t from the surface of the steel sheet is 500 to 1100 mm / mm 2 ,
The grain boundary density at a position 1 / 4t from the surface of the steel sheet is 400 to 1000 mm / mm 2 ,
The grain boundary density at a position 1 / 2t from the surface of the steel sheet is 300 to 900 mm / mm 2 .
Steel plate.
The chemical composition is, in mass%, instead of a portion of the Fe.
Cu: 1.50% or less,
Ni: 2.50% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 0.150% or less, and B: 0.0050% or less,
It contains at least one selected from the group consisting of
The steel plate according to claim 1.
The chemical composition is, in mass%, instead of a portion of the Fe.
Mg: 0.0100% or less,
Ca: 0.0100% or less, and REM: 0.0100% or less,
It contains at least one selected from the group consisting of
The steel sheet according to claim 1 or 2.
The chemical composition is, in mass%, instead of a portion of the Fe.
Zr: 0.0100% or less, and Te: 0.0100% or less,
It contains at least one selected from the group consisting of
The steel plate according to any one of claims 1 to 3.
The chemical composition is, in mass%, instead of a portion of the Fe.
W: 1.00% or less, and Sn: 0.50% or less,
It contains at least one selected from the group consisting of
The steel plate according to any one of claims 1 to 4.
The chemical composition satisfies the following formula (i).
The steel plate according to any one of claims 1 to 5.
1.7 ≤ Ti / N ≤ 3.4 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.
The chemical composition satisfies the following formula (ii).
In the cross section perpendicular to the rolling direction of the steel sheet, the average circle equivalent diameter of the TiN particles at a position 1 / 10t from the surface of the steel sheet is 60 nm or less, and the area ratio of the TiN particles is 0.0001% or more. ,
The steel plate according to any one of claims 1 to 6.
Ti × N ≧ 3.0 × 10-5 ... (ii)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.
The method for manufacturing a steel sheet according to any one of claims 1 to 6.
In a method for manufacturing a steel sheet, in which a heating step, a hot rolling step, and an accelerated cooling step are sequentially performed on a steel piece having the chemical composition according to any one of claims 1 to 6.
In the heating step, the steel pieces are heated to a heating temperature of 950 to 1050 ° C.
The hot rolling step includes rough rolling and finish rolling.
The rough rolling was carried out in a range where the surface temperature of the steel pieces was Trex or higher.
The cumulative rolling reduction in the rough rolling is 10 to 75%.
The finish rolling was carried out in a range where the surface temperature of the steel piece was Ar 3 or more and less than Trex .
The cumulative rolling reduction in the finish rolling is 65 to 90%, and the time between passes is 15 seconds or less.
The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is set to 50 seconds or less.
In the accelerated cooling step, the cooling stop temperature is 0 to 550 ° C. under the condition that the cooling start temperature is Trex -10 ° C or lower and the average cooling rate from the cooling start to the cooling end is 5 to 50 ° C / sec. Water-cooled to
Steel sheet manufacturing method.
However, Ar 3 is obtained by the following formula (iii), and Trex is obtained by the following formula (iv). The element symbol in the following formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.
Ar 3 = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iii)
TRex = -91900 [Nb *] 2 +9400 [Nb *] +770 ... (iv)
However, the amount of solid solution Nb (mass%) obtained by the following formula (v) is determined by sol. When it is Nb,
Nb ≧ sol. In the case of Nb, [Nb *] = sol. Nb
Nb <sol. In the case of Nb, [Nb *] = Nb
And.
sol. Nb = (10 (-6770 / (T + 273) + 2.26) ) / (C + 12/14 × N) ・・・ (v)
In addition, T in the above formula represents the heating temperature (° C.) of the steel piece in the heating step.
The method for manufacturing a steel sheet according to claim 7.
The said product obtained by comprising a refining step for producing molten steel and a continuous casting step for continuously casting the molten steel to produce a steel piece having the chemical composition according to any one of claims 1 to 6. In a steel plate manufacturing method in which a heating process, a hot rolling process, and an accelerated cooling process are sequentially performed on a steel piece.
In the refining step, Ti is added after the dissolved O concentration in the molten steel becomes 0.0050% or less.
In the continuous casting step, the average cooling rate when the surface temperature of the steel pieces is between 1200 and 900 ° C. is 0.1 to 0.5 ° C./sec.
In the heating step, the steel pieces are heated to a heating temperature of 950 to 1080 ° C.
The hot rolling step includes rough rolling and finish rolling.
The rough rolling was carried out in a range where the surface temperature of the steel pieces was Trex or higher.
The cumulative rolling reduction in the rough rolling is 10 to 75%.
The finish rolling was carried out in a range where the surface temperature of the steel piece was Ar 3 or more and less than Trex .
The cumulative rolling reduction in the finish rolling is 65 to 90%, and the time between passes is 15 seconds or less.
The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is set to 50 seconds or less.
In the accelerated cooling step, the cooling stop temperature is 0 to 550 ° C. under the condition that the cooling start temperature is Trex -10 ° C or lower and the average cooling rate from the cooling start to the cooling end is 5 to 50 ° C / sec. Water-cooled to
Steel sheet manufacturing method.
Here, Ar 3 is obtained by the following formula (iii), and Trex is obtained by the following formula (iv). The element symbol in the following formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.
Ar 3 = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iii)
TRex = -91900 [Nb *] 2 +9400 [Nb *] +770 ... (iv)
However, the amount of solid solution Nb (mass%) obtained by the following formula (v) is determined by sol. When it is Nb,
Nb ≧ sol. In the case of Nb, [Nb *] = sol. Nb
Nb <sol. In the case of Nb, [Nb *] = Nb
And.
sol. Nb = (10 (-6770 / (T + 273) + 2.26) ) / (C + 12/14 × N) ・・・ (v)
In addition, T in the above formula represents the heating temperature (° C.) of the steel piece in the heating step.
After the accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650 ° C. is further performed.
The method for manufacturing a steel sheet according to claim 8 or 9.