CN115398018A - Steel sheet and method for producing same - Google Patents

Abstract

A steel plate having a chemical composition in mass % of C: 0.040-0.160%, Si: 0.01-0.50%, Mn: 0.70-2.50%, P: 0.030% or less, S: 0.020% or less, Al: 0.001-0.100% %, N: 0.0010～0.0080%, Nb: 0.003～0.050%, Ti: 0.003～0.050%, balance: Fe and impurities, in the C section, the metallographic structure at the position of 1/4t contains more than 80 area% of The average length of the bainitic ferrite constituting bainite in the major axis direction is 10 μm or less, and the average length of the prior-austenite grains at the 1/4t position in the thickness direction of the L section is 20 μm or less , the average aspect ratio is more than 2.5, the grain boundary density in the C section is 500-1100mm/mm ² at the 1/10t position, 400-1000mm/mm ² at the 1/4t position, and 1/2t position 300-900 mm/mm ² .

Description

Steel plate and manufacturing method thereof

technical field

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

Background technique

Examples of applications of steel plates include welded structures such as ships, high-rise buildings, other buildings, bridges, marine structures, LNG storage tanks and other large tanks, and line pipes (for example, refer to Patent Documents 1 to 5). In recent years, in order to increase the loading weight of container ships, etc., the enlargement of welded structures has progressed. Accordingly, increased plate thickness and increased strength are required for steel sheets. Furthermore, in the above-mentioned welded structure, further improvement of low-temperature toughness and fracture toughness becomes a problem from the viewpoint of further safety and reliability.

Furthermore, welded structures are required to have brittle crack growth arresting properties (hereinafter referred to as "crack arrestability") that stop brittle cracks in the base metal even if brittle cracks occur in the welded joint.

prior art literature

patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2019-023322

Patent Document 2: Japanese Patent Laid-Open No. 2019-023323

Patent Document 3: Japanese Patent Laid-Open No. 2019-023324

Patent Document 4: Japanese Patent Laid-Open No. 2019-035107

Patent Document 5: International Publication No. 2019/069771

Contents of the invention

The problem to be solved by the invention

However, there is usually a so-called trade-off relationship between strength and low-temperature toughness, so it is not easy to balance them. In addition, improvement of crack arrestability is not easy and becomes an important issue. Furthermore, the current situation is that improvement of fracture toughness has hardly been studied so far.

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

solutions to problems

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

(1) A steel plate, wherein the chemical composition of the steel plate is calculated as

C: 0.040～0.160%,

Si: 0.01 to 0.50%,

Mn: 0.70～2.50%,

P: 0.030% or less,

S: 0.020% or less,

Al: 0.001～0.100%,

N: 0.0010～0.0080%,

Nb: 0.003 to 0.050%,

Ti: 0.003～0.050%,

Balance: Fe and impurities,

In the section perpendicular to the rolling direction of the aforementioned steel plate, when the thickness of the aforementioned steel plate is t, the metallographic structure at a position where the distance from the surface of the aforementioned steel plate is 1/4t contains 80% or more of shellfish by area %. and the average length in the major axis direction of the bainitic ferrite constituting the aforementioned bainite is 10 μm or less,

In a section parallel to the rolling direction and the thickness direction of the steel sheet, the average length of the prior-austenite grains in the thickness direction at a distance of 1/4t from the surface of the steel sheet is 20 μm or less, and the average aspect ratio is 2.5 or more,

In the section perpendicular to the rolling direction of the aforementioned steel plate,

The grain boundary density at a position at a distance of 1/10t from the surface of the steel sheet is 500 to 1100 mm/mm ² ,

The grain boundary density at a position at a distance of 1/4t from the surface of the steel sheet is 400 to 1000 mm/mm ² ,

The grain boundary density at a position at a distance of 1/2t from the surface of the steel sheet is 300 to 900 mm/mm ² .

(2) The steel sheet according to (1) above, wherein the chemical composition contains, in mass %, a compound selected from the group consisting of

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: At least one or more of the group consisting of 0.0050% or less.

(3) The steel sheet according to the above (1) or (2), wherein the chemical composition contains, in mass %, a compound selected from the group consisting of

Mg: 0.0100% or less,

Ca: 0.0100% or less, and

REM: at least one or more of the group consisting of 0.0100% or less.

(4) The steel sheet according to any one of the above (1) to (3), wherein the chemical composition contains, in mass %, a steel sheet selected from the group consisting of

Zr: 0.0100% or less, and

Te: at least one or more of the group consisting of 0.0100% or less.

(5) The steel sheet according to any one of (1) to (4) above, wherein the chemical composition contains, in mass %, a compound selected from the group consisting of

W: 1.00% or less, and

Sn: at least one or more of the group consisting of 0.50% or less.

(6) The steel sheet according to any one of the above (1) to (5), wherein the chemical composition satisfies the following formula (i),

1.7≤Ti/N≤3.4(i)

In addition, the symbol of the element in the said formula represents content (mass %) of each element contained in a steel plate, and 0 is substituted when it does not contain.

(7) The steel sheet according to any one of the above (1) to (6), wherein the chemical composition satisfies the following formula (ii),

In a section perpendicular to the rolling direction of the steel sheet, the average circle-equivalent diameter of the TiN particles at a position 1/10t away 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,

Ti×N≥3.0×10 ^-5 (ii)

In addition, the symbol of the element in the said formula represents content (mass %) of each element contained in a steel plate, and 0 is substituted when it does not contain.

(8) A method for manufacturing a steel sheet, which is the method for manufacturing a steel sheet according to any one of (1) to (6),

In the manufacturing method, a steel slab having the chemical composition described in any one of the above (1) to (6) is sequentially subjected to a heating step, a hot rolling step, and an accelerated cooling step, wherein,

In the aforementioned heating step, the aforementioned steel slab is heated to a heating temperature of 950-1050° C.,

The aforementioned hot rolling process comprises rough rolling and finish rolling,

The aforementioned rough rolling is carried out in the range where the surface temperature of the aforementioned steel slab is _Trex or higher,

The cumulative reduction ratio in the aforementioned rough rolling is set to 10 to 75%,

The above-mentioned finish rolling is carried out in the range where the surface temperature of the above-mentioned steel slab is not less than Ar ₃ and lower than _Trex ,

The cumulative rolling reduction in the aforementioned finish rolling is set to 65 to 90%, and the time between passes is set to 15 seconds or less,

The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is 50 seconds or less,

In the aforementioned accelerated cooling process, water cooling to 0 to 550 °C is carried out under the condition that the cooling start temperature is set to be _Trex -10 °C or less, and the average cooling rate from the start of cooling to the end of cooling is 5 to 50 °C/sec. °C cooling stop temperature,

Among them, Ar ₃ is obtained by the following formula (iii), _{and Trex} is obtained by the following formula (iv). It should be noted that the element symbols in the following formulas represent the contents of each element contained in the steel sheet (mass % ), substitute 0 if it does not contain,

Ar ₃ ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo(iii)

T _rex ＝-91900[Nb*] ² +9400[Nb*]+770(iv)

However, when the amount of solid solution Nb (mass %) obtained by the following formula (v) is sol.Nb,

In the case of Nb≥sol.Nb, [Nb*]=sol.Nb,

In the case of Nb<sol.Nb, [Nb*]=Nb,

sol.Nb＝(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N)(v)

In addition, T in the said formula represents the heating temperature (degreeC) of the steel slab in a heating process.

(9) A method for producing a steel plate, which is the method for producing a steel plate as described in (7) above,

The production method includes: a refining process for producing molten steel; and a continuous casting process for continuously casting the molten steel to produce a slab having the chemical composition described in any one of the above (1) to (6), and the obtained The aforementioned billets are sequentially subjected to a heating process, a hot rolling process and an accelerated cooling process, wherein,

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 process, the average cooling rate during the surface temperature of the steel slab is 1200-900° C. is set to 0.1-0.5° C./second,

In the aforementioned heating step, the aforementioned steel slab is heated to a heating temperature of 950-1080°C,

The aforementioned hot rolling process comprises rough rolling and finish rolling,

The aforementioned rough rolling is carried out in the range where the surface temperature of the aforementioned steel slab is _Trex or higher,

The cumulative reduction ratio in the aforementioned rough rolling is set to 10 to 75%,

The above-mentioned finish rolling is carried out in the range where the surface temperature of the above-mentioned steel slab is not less than Ar ₃ and lower than _Trex ,

The cumulative rolling reduction in the aforementioned finish rolling is set to 65 to 90%, and the time between passes is set to 15 seconds or less,

The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is 50 seconds or less,

In the aforementioned accelerated cooling process, water cooling to 0 to 550 °C is carried out under the condition that the cooling start temperature is set to be _Trex -10 °C or less, and the average cooling rate from the start of cooling to the end of cooling is 5 to 50 °C/sec. °C cooling stop temperature,

Here, Ar ₃ is obtained by the following formula (iii), _{and Trex} is obtained by the following formula (iv). It should be noted that the element symbols in the following formulas represent the contents (by mass) of each element contained in the steel sheet. %), substitute 0 if not included,

Ar ₃ ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo(iii)

T _rex ＝-91900[Nb*] ² +9400[Nb*]+770(iv)

However, when the amount of solid solution Nb (mass %) obtained by the following formula (v) is sol.Nb,

In the case of Nb≥sol.Nb, [Nb*]=sol.Nb,

In the case of Nb<sol.Nb, [Nb*]=Nb,

sol.Nb＝(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N)(v)

In addition, T in the said formula represents the heating temperature (degreeC) of the steel slab in a heating process.

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

The effect of the invention

According to the present invention, a steel plate having high strength and excellent low-temperature toughness, fracture toughness, and crack arrestability can be obtained.

Detailed ways

The inventors of the present invention conducted detailed studies on the above-mentioned problems, and as a result obtained the following findings.

As mentioned above, there is a so-called trade-off relationship between strength and low temperature toughness. Furthermore, as a result of research by the inventors of the present invention, it has been found that it is not easy to balance strength and fracture toughness. Therefore, first, the inventors of the present invention studied a method for achieving both high strength and improvement of low-temperature toughness and fracture toughness. As a result, it can be seen that by making the metallographic structure mainly bainite, the strength is increased, and the bainitic ferrite that constitutes bainite is also The miniaturization not only suppresses the decrease in low-temperature toughness but also suppresses the decrease in fracture toughness.

In addition, the refinement and flattening of the bainite structure and the refinement of the bainitic ferrite can be achieved by controlling the heating temperature before hot rolling to be low and performing finish rolling at a high pressure reduction rate in the non-recrystallized region. .

Next, as a result of studying methods for improving crack arrestability, it was found that crack arrest in a direction parallel to the steel plate surface, for example, in a direction perpendicular to or parallel to the rolling direction, can be improved by controlling the grain boundary density in the thickness direction of the steel plate. sex.

The present invention is made based on the above findings. Each requirement of the present invention will be described in detail below.

(A) chemical composition

The reason for limitation of each element is as follows. In addition, in the following description, "%" with respect to content means "mass %". In addition, in this specification, "-" which shows a numerical range is used in the meaning which includes the numerical value described before and after it as a lower limit and an upper limit, unless there is special notice.

C: 0.040～0.160%

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, when the C content exceeds 0.160%, it becomes difficult to ensure good low-temperature toughness and fracture toughness, so the C content is made 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%. In addition, the C content is 0.160% or less, preferably 0.140% or less, more preferably 0.120% or less.

Si: 0.01 to 0.50%

Si is effective as a deoxidizing element and a strengthening element, so 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 fracture toughness will deteriorate significantly, so the Si content is made 0.50% or less. Therefore, the Si content is 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. In addition, 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: 0.70～2.50%

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, if the Mn content exceeds 2.50%, the central segregation becomes prominent, and the low-temperature toughness and fracture toughness of the portion where the central segregation occurs deteriorates, so the Mn content is made 2.50% or less. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, more preferably 1.20% or more. In addition, 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 ensure low-temperature toughness and fracture toughness stably, the content of P is set to 0.030% or less. Preferably it is 0.020% or less, More preferably 0.015% or less. The lower limit is 0%, but considering the cost for reducing the P content, the P content may be set to 0.0001% or more.

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 extending in the central segregation portion is formed, and the low-temperature toughness, fracture toughness, and ductility deteriorate. Therefore, the S content is made 0.020% or less. Preferably it is 0.010% or less. The smaller the S content is, the more preferable it is, so the lower limit is not particularly defined, but from the viewpoint of production cost, the S content may be 0.0001% or more.

Al: 0.001～0.100%

Al is usually an element actively contained as a deoxidizing element, and the Al content is made 0.001% or more. However, if the Al content is excessive, the formation of coarse clustered alumina (Al ₂ O ₃ )-based inclusions is promoted, and the low-temperature toughness and fracture toughness deteriorate. Accordingly, the Al content is 0.100% or less, preferably 0.050% or less.

N: 0.0010～0.0080%

N has the effect of forming Ti nitrides and suppressing the increase of the austenite grain size when the slab is heated, so N is contained in an amount of 0.0010% or more. However, if the N content exceeds 0.0080%, the steel sheet becomes brittle, so the N content is made 0.0080% or less. Therefore, the N content is 0.0010% or more, preferably 0.0015% or more, more preferably 0.0020% or more. In addition, the N content is 0.0080% or less, preferably 0.0065% or less, more preferably 0.0060% or less.

Nb: 0.003～0.050%

Nb can improve the strength and toughness of the steel plate. In addition, in order to obtain a predetermined microstructure, it is necessary to roll the non-recrystallized austenite region, but Nb is an element effective for expanding the non-recrystallization temperature range, and increasing the rolling temperature also contributes to productivity improvement. . In order to obtain this effect, 0.003% or more is contained. However, if the content of Nb exceeds 0.050%, low-temperature toughness, fracture toughness, and weldability will decrease, so the content of Nb is made 0.050% or less. Therefore, the Nb content is 0.003% or more, preferably 0.005% or more, more preferably 0.008% or more. In addition, the Nb content is 0.050% or less, preferably 0.025% or less, more preferably 0.018% or less.

Ti: 0.003～0.050%

Ti can improve the strength and toughness of the steel plate. In addition, by containing Ti, TiN is formed, and an increase in the austenite grain size is suppressed when the slab is heated. When the austenite grain size increases, the grain size of the transformation structure also increases, so it is difficult to obtain a predetermined grain boundary density, and the toughness and crack arrestability decrease. In order to obtain the effect achieved by TiN, 0.003% or more of Ti is contained.

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

In addition, the Ti content preferably satisfies the following formula (i) in relation to the N content. By setting the value of Ti/N to 1.7 or more, solid-solution N can be fixed and crack arrestability can be improved. It should be noted that when the amount of solid solution N is excessive, it is considered to be caused by, for example, promotion of fracture sensitivity due to solid solution N, acceleration of grain boundary embrittlement due to solid solution N, The resulting formation of MA, embrittlement due to Fe nitrides of fixed dislocations, etc., reduces the crack arrestability.

On the other hand, by setting the value of Ti/N to 3.4 or less, formation of coarse TiN, TiC, etc. is suppressed, and crack arrestability can be improved. The value of Ti/N is preferably 2.0 to 3.0, more preferably 2.3 to 2.7.

1.7≤Ti/N≤3.4(i)

In addition, the symbol of the element in the said formula represents content (mass %) of each element contained in a steel plate, and 0 is substituted when it does not contain.

Furthermore, the Ti content preferably satisfies the following formula (ii) in relation to the N content. By setting the value of Ti×N to 3.0×10 ^-5 or more, as described later, at a position where the distance from the surface of the steel plate is 1/10t, an average equivalent circle diameter of 60 nm or less and an area ratio of 0.0001 are obtained. More than % TiN particles contribute to the improvement of crack arrest. The value of Ti×N is preferably 4.0×10 ^-5 to 10.0×10 ^-5 , more preferably 5.0×10 ^-5 to 8.0×10 ^-5 .

Ti×N≥3.0×10 ^-5 (ii)

In addition, the symbol of the element in the said formula represents content (mass %) of each element contained in a steel plate, and 0 is substituted when it does not contain.

In the chemical composition of the steel sheet of the present invention, in addition to the above-mentioned elements, at least one element selected from the group consisting of Cu, Ni, Cr, Mo, V, and B may be contained within the range shown below in order to improve the strength. more than one species. The reason for limitation of each element is demonstrated.

Cu: 1.50% or less

Since Cu has the effect of improving the strength and toughness of the steel sheet, it may be contained as necessary. However, if Cu is contained excessively, the improvement of the performance corresponding to the increase of alloy cost is not found, and it may become a cause of a surface crack on the contrary. Therefore, the Cu content is 1.50% or less, preferably 1.20% or less, more preferably 1.00% or less. When it is desired to obtain the above effect more reliably, the Cu content is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.050% or more.

Ni: 2.50% or less

Since Ni is an element having an effect of improving the strength of the steel sheet, it may be contained as necessary. In addition, Ni is an element that has an effect of improving the toughness of a steel matrix (slab) in a solid solution state. However, if Ni is contained in excess, the low-temperature toughness, fracture toughness, and weldability will deteriorate. Therefore, the Ni content is 2.50% or less, preferably 1.00% or less, more preferably 0.50% or less, further preferably 0.30% or less. When it is desired to obtain the above effect more reliably, the Ni content is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.050% or more.

Cr: 1.00% or less

Since Cr is an element having an effect of improving the strength of the steel sheet, it may be contained as necessary. However, if Cr is contained in excess, the low-temperature toughness, fracture toughness, and weldability will deteriorate. 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 it is desired to obtain the above effect more reliably, the Cr content is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.050% or more.

Mo: less than 1.00%

Since Mo is an element having an effect of improving the strength of the steel sheet, it may be contained as necessary. However, if Mo is contained in excess, the low-temperature toughness, fracture toughness, and weldability will deteriorate. 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 it is desired to obtain the above effect more reliably, the Mo content is preferably 0.001% or more, more preferably 0.005% or more, and 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 as necessary. However, if V is contained in excess, low-temperature toughness, fracture toughness, and weldability deteriorate. 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 it is desired to obtain the above effect more reliably, the V content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more.

B: 0.0050% or less

B is an element that improves the hardenability and contributes to the improvement of the strength of the steel sheet, so it can be contained as needed. However, when excessive B is contained, the low temperature toughness and fracture toughness will fall. Therefore, the B content is 0.0050% or less, preferably 0.0040% or less, more preferably 0.0030% or less. When it is desired to obtain the above effect more reliably, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and 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 may be contained within the range shown below in order to control inclusions. The reason for limitation of each element is demonstrated.

Mg: 0.0100% or less

Mg is a deoxidizing element, and is an element that suppresses the generation of coarse inclusions by forming sulfides, and suppresses the generation of harmful inclusions by forming fine oxides. Therefore, it can contain as needed. However, if Mg is contained in excess, coarse oxides, sulfides, and oxysulfides are easily formed, and the low-temperature toughness and fracture toughness decrease. Therefore, the Mg content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. When it is desired to obtain the above effect more reliably, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

Ca: 0.0100% or less

Ca is a deoxidizing element, and is an element that suppresses the generation of coarse inclusions by forming sulfides and suppresses the generation of harmful inclusions by forming fine oxides. Therefore, it can contain as needed. However, if Ca is contained in excess, coarse oxides, sulfides, and oxysulfides are easily formed, and the low-temperature toughness and fracture toughness decrease. Therefore, the Ca content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. When it is desired to obtain the above effect more reliably, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

REM: 0.0100% or less

REM is a deoxidizing element, and is an element that suppresses the generation of coarse inclusions by forming sulfides and suppresses the generation of harmful inclusions by forming fine oxides. Therefore, it can contain as needed. However, if REM is contained in excess, coarse oxides, sulfides, and oxysulfides are likely to be formed, and the low-temperature toughness and fracture toughness decrease. Therefore, the REM content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. When it is desired to obtain the above effect more reliably, the REM content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

Here, in the present invention, REM refers to a total of 17 elements including Sc, Y, and lanthanoid elements, and the aforementioned REM content refers to the total content of these elements. It should be noted that lanthanides are industrially added in the form of mixed rare earth alloys.

In addition to the above elements, the chemical composition of the steel sheet of the present invention may further contain at least one selected from the group consisting of Zr and Te within the range shown below in order to refine the metallographic structure. The reason for limitation of each element is demonstrated.

Zr: 0.0100% or less

Zr is an element that contributes to improving toughness by refining the structure of the steel sheet. In addition, Zr also functions as a deoxidizing element. Therefore, it can contain as needed. However, if Zr is contained excessively, the low-temperature toughness and fracture toughness will fall. Therefore, the Zr content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. When it is desired to obtain the above effect more reliably, the Zr content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

Te: 0.0100% or less

Te is an element that contributes to toughness improvement by microstructure of the steel sheet, so it may be contained as necessary. However, even if Te is contained in excess, the above effects are saturated. Therefore, the Te content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. When it is desired to obtain the above effect more reliably, the Te content is preferably 0.0001% or more, more preferably 0.0005% or more, and 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 within the range shown below in order to improve corrosion resistance. The reason for limitation of each element is demonstrated.

W: 1.00% or less

W is an element that dissolves and adsorbs to rust as oxyacid ions WO ₄ ^- , inhibits the permeation of chloride ions in the rust layer, and improves corrosion resistance, so it can be contained as needed. However, even if an excessive amount of W is contained, the above-mentioned effects are saturated, and the low-temperature toughness and fracture toughness may be lowered. Therefore, the W content is 1.00% or less, preferably 0.75% or less. When it is desired to obtain the above effect more reliably, the W content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more.

Sn: 0.50% or less

Sn is an element having an action of forming Sn ²⁺ to be dissolved and inhibiting corrosion by an inhibitor action in an acidic chloride solution. In addition, Sn has the effect of suppressing the anodic dissolution reaction of steel and improving corrosion resistance. Therefore, it can contain as needed. However, even if an excessive amount of Sn is contained, the above effects are saturated, and rolling cracks in the steel sheet are likely to occur. Therefore, the Sn content is 0.50% or less, preferably 0.30% or less. When it is desired to obtain the above effect more reliably, the Sn content is preferably 0.001% or more, more preferably 0.005% or more, and 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, "impurities" refer to components that are mixed in due to various factors such as raw materials such as ores and scraps, and production processes during the industrial production of steel sheets, and are allowed within the range that does not adversely affect the present invention. In the steel sheet, O may be mixed as an impurity, but the O content is allowed if it is 0.0040% or less.

(B) Metallographic structure of steel plate

The metallographic structure of the steel sheet of the present invention will be described. In addition, in the following description, "%" means "area %". In addition, in the present invention, when the thickness of the steel plate is t, the position at a distance of 1/4t from the surface of the steel plate in the cross section perpendicular to the rolling direction of the steel plate is referred to as "1/4t in the C cross section. position", the position at a distance of 1/4t from the surface of the steel plate in a section parallel to the rolling direction and the thickness direction of the steel plate is called "1/4t position in the L section". Furthermore, the said "rolling direction" means the rolling direction in finish rolling.

Bainite: more than 80%

In the present invention, the main body of the metallographic structure is bainite. Specifically, the strength of the steel sheet can be secured by setting the area ratio of bainite at the 1/4t position in the C section to 80% or more. The area ratio of bainite is preferably 90% or more. It should be noted that there is no need to set an upper limit to the area ratio of bainite, that is, it may be a single phase of bainite.

It should be noted that ferrite, pearlite, and martensite/austenite mixed phase (MA phase) may be mixed in as the remainder structure, but they are allowed if their total area ratio is 20% or less. The above-mentioned total area ratio is preferably 10% or less. These total area ratios are preferably small, and the lower limit is not particularly limited. For example, the above-mentioned total area ratio may be 0%. In addition, it may exceed 0%, and may be 1% or more.

As described above, in addition to making bainite the main body, the bainite structure is refined and flattened, and the bainitic ferrite is further refined, so that the strength, low-temperature toughness, and fracture toughness of the steel sheet can be balanced. Specifically, the bainite structure needs to satisfy the following requirements.

Average length of bainitic ferrite: 10 μm or less

The average length in the long-axis direction of bainitic ferrite constituting bainite at the 1/4t position in the C section is set to be 10 μm or less. Fracture toughness can be ensured by making the bainitic ferrite constituting bainite finer. The average length of bainitic ferrite is preferably 8 μm or less.

Average length of prior austenite grains in the thickness direction: 20 μm or less

Average aspect ratio of prior austenite grains: 2.5 or more

The refinement of the bainite structure can be achieved by controlling the heating temperature before hot rolling to be low, and performing finish rolling at a high reduction rate in the non-recrystallized region. That is, the prior-austenite grains of bainite are elongated in the rolling direction. Therefore, the average length of prior-austenite grains in the thickness direction at the 1/4t position in the L cross section is set to be 20 μm or less, and the average aspect ratio is set to be 2.5 or more. The average length of the prior-austenite grains in the thickness direction is preferably 15 μm or less. In addition, the average aspect ratio of the prior austenite grains is preferably more than 2.5, more preferably 4.0 or more.

Here, in the present invention, the area ratio of the metallographic structure is obtained as follows. First, a sample is collected from the steel plate so that the 1/4t position in the C section becomes the observation surface. Next, the observation surface was etched with nital etching solution, and after etching, 8 fields of view were photographed at 500 times using an optical microscope. Next, image analysis was performed on the obtained photograph of the structure, ferrite was seen when it was white, and pearlite was seen when it was black, and the area ratios of each were calculated.

Next, LePera etching was performed on the portion etched by the nital etching solution, image analysis was performed on the gray portion etched by the nital etching solution, and the area ratio was calculated for the white portion as the MA phase.

The average length of bainitic ferrite and the area ratio of bainite were calculated by KAM (Kernel Average Misorientation) analysis using electron backscatter diffraction (EBSD, Electron Back Scatter Diffraction). In the KAM analysis, in the structure judged to be ferrite, a region with a local misorientation exceeding 1.0° is bainitic ferrite. In addition, at the time of measurement, the bainitic ferrite whose length in the long-axis direction was 1 micrometer or more was made into object. In addition, the area ratio of bainite is obtained by summing up the area ratios of bainitic ferrite.

The average length in the thickness direction of the prior austenite grains and the average aspect ratio were measured in accordance with JIS G0551:2013. First, a sample is collected from the steel plate so that the 1/4t position in the L section becomes the observation surface. Next, after mirror-polishing the observation surface, it was etched by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. Black grains appear as prior austenite grains by corrosion.

The observation surface where prior austenite grains appeared was observed with an optical microscope, and eight or more fields of view (a total of 0.40 mm ² or more) were photographed for fields of 0.05 mm ² or more in area. Next, the thickness of the prior austenite grains was measured by the intercept method based on the microstructure photograph taken with an optical microscope, and the average value thereof was taken as the average length in the thickness direction of the prior austenite grains. In addition, in the measurement, prior-austenite grains having a length in the thickness direction of 1 μm or more were used as objects.

In addition, the maximum length in the major axis direction and the maximum length in the minor axis direction perpendicular to the major axis direction were respectively measured for each prior-austenite grain from the above-mentioned structure photograph, and the ratio (maximum major axis length/short axis direction) was obtained. shaft length). The average value is then taken as the aspect ratio average of the prior austenite grains. It should be noted that when finish rolling is performed at a high reduction rate in the non-recrystallized region, the prior austenite grains show a shape elongated in the rolling direction, so the major axis direction is the rolling direction and the minor axis direction is the plate thickness direction (so-called ND direction).

In the case where prior austenite grains cannot sufficiently appear by the above-mentioned method, through "Research on high-precision reconstruction method of austenite structure facing steel (Steel no ostenite structure reconstruction method high-precision ni To けた検议)" (Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kono, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), p.24-30), specific Hara Oku The average length and aspect ratio of the prior austenite grains in the thickness direction are obtained.

Grain boundary density at 1/10t position in C section: 500～1100mm/mm ²

Grain boundary density at 1/4t position in C section: 400～1000mm/mm ²

Grain boundary density at 1/2t position in C section: 300～900mm/mm ²

As a dominant factor in the improvement of crack arrestability, the contribution of the grain boundary, which is an obstacle to the growth of brittle cracks, is large. In the grain boundary, since the crystal orientation is different between adjacent crystal grains, the crack propagation direction changes in this portion. Therefore, an unfractured region is generated, and stress is dispersed through the unfractured region to form crack closing stress. Therefore, the driving force for crack growth is reduced, and the arrestability is improved. In addition, the unfractured regions eventually fail ductilely, thus absorbing the energy required for brittle fracture. Therefore, crack arrestability is improved.

So far, it has been considered that in order to increase the grain boundaries, it is necessary to reduce the grain diameter. The structure mainly composed of ferrite is as described above, but the utilization of bainite is indispensable for thick and high-strength steel. This bainite differs from ferrite in that the shape of the underlying structure is complex, so it is extremely difficult to define crystal grains. Therefore, even if the relationship between the crystal grain diameter and the crack arrestability is obtained by converting it into the equivalent circle diameter, the variation is large, and it is difficult to determine the crystal grain diameter required for the improvement of the crack arrestability.

Therefore, it can be seen that if returning to the basic principle that grain boundaries become obstacles to crack propagation, defining the total length of grain boundaries per unit area (hereinafter referred to as "grain boundary density"), and using the relationship between its arrangement and crack arrestability, then Most relevant. Here, the "grain boundary density" refers to "the total length per unit area of grain boundaries having crystal orientation differences of 15° or more". The reason why the crystal orientation difference is 15° or more is that when it is less than 15°, the grain boundary is less likely to become an obstacle to the propagation of brittle cracks, and the effect of improving crack arrestability decreases.

By setting the grain boundary density in the C 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 , excellent crack arrestability can be obtained. Furthermore, in order to stably improve the arrestability, the grain boundary density in the C section is preferably 600 mm/mm ² or more at the 1/10t position, preferably 500 mm/mm ² or more at the 1/4t position, and 500 mm/mm 2 or more at the 1/2t position. Preferably it is 400 mm/mm ² or more.

Crack arrestability improves as the grain boundary density increases, but an excessive increase leads to an increase in rolling load, which in turn reduces productivity. Therefore, the grain boundary density in the C 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 at the 1/2t position the following. The grain boundary density in the C section is preferably 1000 mm/mm ² or less at the 1/10t position, preferably 900 mm/mm ² or less at the 1/4t position, and preferably 800 mm/mm ² or less at the 1/2t position.

It should be noted that in order to improve the crack arrestability of extremely thick materials, it is necessary to increase the grain boundary density over the entire plate thickness. In the production method described later, the grain boundary density at the 1/2t position is mainly controlled. For other plate thickness positions, the temperature is necessarily low and the cooling rate is increased, so there is a tendency for the grain boundary density 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 is generated in the thickness direction, for example, at the 1/4t position and the 1/2t position, and the grain boundary density may be reversed. Therefore, in the present invention, the grain boundary densities at the 1/10t position, the 1/4t position, and the 1/2t position are specified as representative values of the thickness-average grain boundary density.

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

TiN particles at 1/10t position

Average equivalent circle diameter: below 60nm

Area ratio: 0.0001% or more

At 1/10t, if the TiN particles are finely dispersed, the pinning effect by the TiN particles is effectively exhibited, and the coarsening of prior austenite is suppressed. As a result, the grain boundary density at the 1/10t position increases and the crack arrestability of the steel sheet is further improved. Therefore, it is preferable that the average circle-equivalent diameter of the TiN particles present at the 1/10t position is 60 nm or less, and the area ratio is 0.0001% or more.

The average equivalent circle diameter of the TiN particles is more preferably 50 nm or less, 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. In addition, the area ratio of the TiN particles is more preferably 0.0002% or more, still more 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 the TiN particles were measured by the following methods. First, extract the replica from the 1/10t position of the steel plate, and use a TEM equipped with an energy dispersive X-ray analyzer (EDX) with a magnification of 30,000 times or more to set the observation area of one field of view to 15 μm ² or more. Particles with a size of 15 to 200 nm were observed. All observed particles were 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 were identified as TiN particles.

It should be noted that the TEM used for quantitative analysis of the particles has an electron beam diameter of 1 to 20 nm and an observation magnification of 50,000 to 1,000,000 times to perform quantitative analysis on an arbitrary position within the particles. The average circle-equivalent diameter of the TiN particles is an arithmetic mean of the circle-equivalent diameters (diameters) that form the same area as the area of each TiN particle discriminated above. The area ratio of the TiN particles is a value obtained by dividing the sum of the areas of the TiN particles identified above by the area of the observed field of view.

Here, with a magnification of 30,000 times or more, the observation area of one field of view is set to 15 μm2 ^or more, and particles with a size of 15 to 200 nm are observed. When the number of TiN particles identified is less than 100, another field of view is confirmed. , and continue to observe until the total number of TiN particles is more than 100. At this time, the average circle-equivalent diameter of the TiN particles is the arithmetic mean of the circle-equivalent diameters (diameters) of the respective identified TiN particles as described above. The area ratio of the TiN grains is a value obtained by dividing the sum of the areas of the TiN grains observed until the number of TiN grains is 100 or more by the total area of the field of view observed so far. In addition, when the additional observation of the continuous field of view, the number of fields of continuous observation reaches 50 fields of view, and the cumulative observation area is 750 μm ² or more, when the total number of identified TiN particles is less than 100, it is considered that there are no TiN particles. outside the scope of this application.

(C) Mechanical properties of the steel plate

The mechanical properties of the steel sheet of the present invention are not particularly limited, and the steel sheet of the present invention has high strength and is excellent in low-temperature toughness, fracture toughness, and crack arrestability. Specifically, it is preferable that the yield stress (YS) is 460 to 860 MPa, and the tensile strength (TS) is 570 to 980 MPa. In addition, it is preferable that the fracture transition critical temperature (vTrs), which is an index of low-temperature toughness, be -60°C or lower. Furthermore, it is preferable that the crack tip opening displacement (Crack Tip Opening Displacement: CTOD) value at -10 degreeC used as an index of fracture toughness is 0.50 mm or more.

In addition, tensile strength (TS) and yield stress (YS) were measured based on JIS Z 2241:2011, using the No. 1B tensile test piece collected from the thickness center part in the direction at right angles to the rolling direction. Specifically, the yield stress (YS) is the resistance of the permanent elongation method when the permanent elongation is 0.2%. In addition, the fracture transition critical temperature (vTrs) was evaluated in accordance with JIS Z 2242:2005, and the test piece was set as a V-notch test piece, and it collected so as to include the 1/4t position of the steel plate. Furthermore, in accordance with ISO 15653:2018, the CTOD test piece with the total thickness in the plate thickness direction of the base metal set at the notch position of 3-point bending was collected, and the CTOD value at -10°C was measured.

Furthermore, the brittle crack growth arrest toughness value Kca at a test temperature of -10°C in the temperature gradient type ESSO test (hereinafter referred to as "crack arrest toughness value Kca _-10°C ") is preferably 6000 N/mm ^1.5 or more, more preferably 8000N/mm ^1.5 or more. By satisfying this characteristic, the steel sheet has excellent crack arrestability.

The crack arrest toughness value Kca _-10℃ is based on the "Inspection essentials for temperature gradient type ESSO test and temperature gradient type double tensile test" in Appendix K3.12.2-1. (2016) of NK Classification Association Steel Ship Rules Inspection Essentials (Temperature matching type ESSO test 験 and び temperature matching type double introduction test 験に关する検查数)" for determination.

In addition, the non-ductile transition temperature (hereinafter referred to as "NDT temperature") in the NRL drop weight test is preferably -100°C or lower, more preferably -110°C or lower. By satisfying this characteristic, the steel sheet has excellent crack arrestability.

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

Then, the NRL drop weight test was implemented based on ASTM E208-06 using the said test piece. Specifically, first, a weld extending in a direction parallel to the longitudinal direction of the test piece was formed on the outermost surface of the steel plate perpendicular to the thickness direction of the test piece. At this time, as the welding material, a welding material with low toughness specified in ASTM E208 was used. Adjustment is made so that the length of the weld bead is within a range of 60 to 70 mm, and the width is within a range of 12 to 16 mm. Next, a notch parallel to the width direction of the test piece was formed on the weld. At this time, the width of the notch was 1.5 mm or less, and it was adjusted so that the distance between the groove bottom of the notch and the test piece was within the range of 1.8 to 2.0 mm.

Next, the surface of the test piece on which the weld bead was formed was directed downward, and both ends in the longitudinal direction were supported, and then an impact bending load by a falling weight was applied to the surface on the opposite side to the side where the weld bead was formed. Then, the growth state of the brittle crack generated by the notch in the test piece was investigated, and Break (crack growth) or No Break (no crack growth) was judged. When the brittle cracks generated by the notch spread on the surface of the test piece in the width direction of the test piece and progressed to the end, the test result was judged to be Break (there was crack growth). When the cracks did not reach the ends in the width direction, the test result was judged as No Break (no crack growth).

For the above-mentioned drop weight test, using two test pieces each, starting from -100°C, for example, while changing the test temperature at intervals of 5°C (decrease by 5°C in the case of No Break, increase by 5°C in the case of Break) The process was carried out, and the temperature 5° C. lower than the lowest test temperature at which No Break was obtained for both test pieces was defined as the no-ductility transition temperature.

(D) Thickness of steel plate

The thickness of the steel sheet of the present invention is not particularly limited, and when used as a welded structure, the sheet thickness is preferably 10 to 70 mm, more preferably 20 to 60 mm. In addition, the improvement effects of the low-temperature toughness and fracture toughness in the present invention are remarkably exhibited when the thickness is less than 50 mm.

(E) Manufacturing method of steel plate

The production conditions of the steel sheet of the present invention are not particularly limited, and can be produced, for example, by sequentially performing a refining process, a continuous casting process, a heating process, a hot rolling process, and an accelerated cooling process under the conditions shown below. Each step will be described.

(a) Refining process

The refining process is a process of producing molten steel. The conditions of the refining step are not particularly limited, and conventional methods may be used. However, when it is desired to suppress the generation of _Ti2O3 and finely disperse TiN, specifically, 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. In this case, it is preferable to add Ti after vacuum degassing is performed and the dissolved O concentration in 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 generation of Ti ₂ O ₃ . Ti can be added, for example, in a reflux type degasser.

(b) Continuous casting process

The continuous casting process is a process of continuously casting molten steel to manufacture a slab having the chemical composition described above. The conditions of the continuous casting process are not particularly limited, and conventional methods may be used. However, when the average equivalent circle diameter of the TiN particles at the 1/10t position is to be 60 nm or less and the area ratio is to be 0.0001% or more, it is preferable to averagely cool the slab while the surface temperature is 1200 to 900°C. The speed is set at 0.1 to 0.5°C/sec. When 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 process

The heating step is a step of contributing to the control of the structure of the austenite phase by heating the steel slab. In the heating step, the steel slab is heated to a heating temperature of 950 to 1080°C. What is necessary is just to perform a heating process using a heating furnace. It should be noted that heating the steel slab to 950-1080°C refers to heating so that the average temperature of the total thickness of the steel slab at the time of extraction from the heating furnace is in the range of 950-1080°C. In this specification, the total thickness of the steel slab is The thickness average temperature is called the heating temperature of the billet. In addition, the total thickness average temperature can be obtained by calculation from the temperature in the heating furnace, the heating time, and the surface temperature of the slab.

When the heating temperature is lower than 950° C., the austenitization is insufficient and the austenite grains are refined, which lowers the hardenability, making it difficult to form a thick and high-strength steel sheet. Furthermore, recrystallization during finish rolling is promoted by miniaturization of austenite grains, thereby reducing the aspect ratio of prior austenite grains. In addition, when the heating temperature exceeds 1080° C., the austenite grains are coarsened, and it becomes difficult to refine the bainite structure in the final structure. The range of preferable heating temperature is 1000-1050 degreeC.

As described above, TiN can be finely dispersed by appropriately controlling the timing of adding Ti in the refining process and by appropriately controlling the average cooling rate in the continuous casting process from 1200 to 900°C, thereby controlling the grain boundary density. within the above range. At this time, the heating temperature of the slab may be 1080° C. or lower.

On the other hand, even when TiN is not positively and effectively utilized, by adjusting the heating temperature of the slab in the heating step to be low, the coarsening of austenite can be suppressed, and the grain boundary density can be controlled within the above-mentioned range. At this time, the heating temperature of the slab is set to 1050° C. or lower.

(d) Hot rolling process

The hot rolling process includes rough rolling and finish rolling. Rough rolling is performed in the range where the surface temperature of the slab is _Trex or higher. That is, rough rolling starts when the surface temperature of the steel slab is _Trex or higher, and ends rough rolling when the surface temperature of the steel slab is higher than _Trex . By performing rough rolling in the range of _Trex or higher, the recrystallization of austenite grains can be used to achieve miniaturization. In addition, the surface temperature at the end of rough rolling may be higher than the surface temperature at the start of rough rolling. This is considered to be due to the influence of processing heat generated by rough rolling and the influence of heat transfer in the thickness direction of the steel slab due to the fact that the internal temperature is higher than the surface temperature.

In addition, the accumulated rolling reduction in rough rolling is in the range of 10 to 75%. The cumulative rolling reduction in rough rolling refers to a value obtained by dividing a value obtained by subtracting the thickness after rough rolling from the thickness at the start of rough rolling by the thickness at the start of rough rolling. When the cumulative reduction ratio during rough rolling is less than 10%, it is difficult to refine the austenite by recrystallization, internal cracks may remain due to pores, and ductility and toughness may deteriorate. In addition, if the cumulative reduction ratio exceeds 75%, the austenite grains are excessively refined, so the recrystallization during finish rolling is promoted, thereby reducing the aspect ratio of the prior austenite grains, and the number of passes increases. Reduced productivity. The preferred cumulative reduction ratio is 30 to 60%. In addition, in the following description, the rough-rolled steel slab is called a steel plate.

The subsequent finish rolling is carried out in the range where the surface temperature of the steel sheet is not less than Ar ₃ and less than _Trex . That is, cooling is completed after rough rolling, finish rolling is started when the surface temperature of the steel sheet is Ar ₃ or higher and lower than _Trex , and finish rolling is completed when the surface temperature of the steel sheet is Ar ₃ or higher and lower than _Trex . By performing finish rolling in a range lower than _Trex , strain can be imparted to austenite grains without recrystallization. This makes it possible to refine the bainite in the final structure. When the final working temperature is performed in the range where the surface temperature is equal to or higher than _Trex , recrystallization is promoted, and the aspect ratio of prior-austenite grains is reduced. On the other hand, if finish rolling is performed in a range where the surface temperature is lower than Ar ₃ , processed ferrite is formed, and there is a possibility that a bainite-based structure cannot be formed in the final structure.

In addition, the cumulative rolling reduction in finish rolling is in the range of 65 to 90%. Cumulative rolling reduction in finish rolling refers to the value obtained by subtracting the plate thickness after finish rolling from the plate thickness at the start of finish rolling (after rough rolling) and dividing it by the plate thickness at the start of finish rolling value. By setting the cumulative reduction ratio in finish rolling to 65% or more, sufficient strain can be imparted to the austenite grains. If the cumulative reduction ratio is less than 65%, the strain imparted to the austenite grains is insufficient, and the flattening of the austenite grains is not promoted, resulting in a decrease in the aspect ratio. In addition, when the cumulative reduction ratio exceeds 90%, recrystallization is promoted, the aspect ratio of the prior austenite grains decreases, and the number of passes increases to decrease productivity. The preferred cumulative reduction rate is 70 to 80%.

Furthermore, the time between passes in finish rolling is set to 15 seconds or less. If the time between passes exceeds 15 seconds, the strain applied by the processing recovers, and the bainite in the final structure cannot be sufficiently refined, and recrystallization is promoted, and the aspect ratio of the prior austenite grains decreases. The shorter the inter-pass time is, the more preferable it is, so there is no need to set a lower limit, but it is preferably 3 seconds or more from the viewpoint of operability. In addition, finish rolling is usually performed by reverse rolling. The time between passes in finish rolling refers to the time when the steel plate is rolled forward by the rolling roll, the rear end of the steel plate passes through the rolling roll, until the direction of travel of the steel plate changes to the rear, and the rear end of the steel plate The time until it is bitten by the roll again.

Next, the time from the completion of the finish rolling to the start of cooling in the accelerated cooling step described later is set to be 50 seconds or less. If the time from the completion of finish rolling to the start of cooling exceeds 50 seconds, the bainite in the final structure cannot be sufficiently refined due to the recovery of the strain imparted by processing, and recrystallization is promoted, and the original austenite grains The aspect ratio is reduced. The shorter the time from the completion of finish rolling to the start of cooling, the better. Therefore, there is no need to set a lower limit, but it is preferably 5 seconds or more from the viewpoint of operability. It should be noted that the time from the completion of finish rolling to the start of cooling refers to the time from the time when the front end of the forward-moving steel plate passes through the rolling rolls in the final pass until water cooling starts.

In the above description, Ar ₃ refers to the transformation start temperature at which austenite grains start to transform into ferrite grains during the cooling process, and is obtained by the following formula (iii). In addition, T _rex refers to the lowest temperature at which equiaxed recrystallized grains can be formed and grown, that is, the recrystallization temperature, and is obtained by the following formula (iv). In addition, the element symbol in the following formula represents content (mass %) of each element contained in a steel plate, and 0 is substituted when it does not contain.

Ar ₃ ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo(iii)

T _rex ＝-91900[Nb*] ² +9400[Nb*]+770(iv)

However, when the amount of solid solution Nb (mass %) obtained by the following formula (v) is sol.Nb,

In the case of Nb≥sol.Nb, [Nb*]=sol.Nb,

In the case of Nb<sol.Nb, [Nb*]=Nb.

sol.Nb＝(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N)(v)

In addition, T in the said formula represents the heating temperature (degreeC) of the steel slab in a heating process.

(e) Accelerated cooling process

In the accelerated cooling step, the steel sheet after finishing rolling is water-cooled. At this time, water cooling to a cooling stop temperature of 0 to 550°C is carried out under the conditions that the cooling start temperature is set to be T _rex -10°C or less, and the average cooling rate from the start of cooling to the end of cooling is 5 to 50°C/sec. .

Even if finish rolling is carried out in the range of Ar ₃ or more and lower than _Trex , if the cooling start temperature exceeds _Trex -10°C due to subsequent reheating, the recovery of the strain imparted by working will be accelerated, and the final structure cannot be formed. The bainitic ferrite of bainite is sufficiently refined.

In addition, by water cooling at an average cooling rate of 5 to 50° C./sec to a cooling stop temperature of 0 to 550° C., the final structure can be made into a bainite-based structure. It should be noted that the average cooling rate and the cooling stop temperature were adjusted according to the value of Ceq in the chemical composition of the steel sheet, and were set under conditions where martensitic transformation did not proceed.

(f) Tempering process

After the accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650° C. may be further included. By performing the tempering process, it is possible to reduce the dislocation density that has increased excessively due to cooling. In addition, since the self-tempering effect can be acquired when the cooling stop temperature in an accelerated cooling process is high, it is not necessary to perform a tempering process. On the other hand, in the accelerated cooling step, for example, when cooling to about room temperature, it is preferable to perform a tempering step.

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

Example

The molten iron from the blast furnace is subjected to desulfurization treatment through molten iron pretreatment, and after the P and C removal treatment is performed in a converter type refining vessel, it is received in a ladle. When tapping, alloying elements are added, and surface slag for heat preservation is added.

Next, the molten steel in the ladle is decompressed using the RH vacuum degasser. Properly collect molten steel samples during smelting for analysis to obtain molten steel components. The temperature of molten steel moves 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 reflux treatment was performed for uniform mixing.

After processing by the RH vacuum degasser, billets having the chemical compositions in Tables 1 and 2 were produced by the continuous casting method. In continuous casting, the average cooling rate during the period when the surface temperature of the slab is 1200-900°C is properly adjusted. Tables 3 and 4 show the dissolved O concentration (mass %) in molten steel when Ti is added, and the average cooling rate (° C./sec) during 1200 to 900° C. in continuous casting. Furthermore, steel plates with a plate thickness of 10 to 70 mm were trial-produced under the production conditions in Table 5 and Table 6 using the above-mentioned steel slabs.

[Table 1]

[Table 2]

[table 3]

table 3

[Table 4]

Table 4

[table 5]

table 5

[Table 6]

Table 6

The metallographic structure of the obtained steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, a sample is collected from the steel plate so that the 1/4t position in the C cross section becomes the observation surface. Then, the observation surface was etched with nital etching solution. After etching, an optical microscope was used to take 8 fields of view at a magnification of 500, and image analysis was performed on the obtained tissue photos. When it was seen as white, it was ferrite, and when it was black, it was pearlite. Find the respective area ratios.

Next, LePera etching was performed on the portion etched by the nital etching solution, image analysis was performed on the gray portion etched by the nital etching solution, and the area ratio was calculated for the white portion 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 KAM analysis, in the structure judged to be ferrite, a region with a local misorientation exceeding 1.0° is regarded as bainitic ferrite. In addition, at the time of measurement, the bainitic ferrite whose length in the long-axis direction is 1 micrometer or more was made into object. In addition, the area ratio of bainite is obtained by summing up the area ratios of bainitic ferrite.

Furthermore, the measurement of the average length in the thickness direction of prior-austenite grains and the average aspect ratio was performed in accordance with JIS G0551:2013. First, a sample is collected from the steel plate so that the 1/4t position in the L section becomes the observation surface. Next, after mirror-polishing the observation surface, it was etched by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. Pre-austenite grains appear.

The observation surface where prior austenite grains appeared was observed with an optical microscope, and eight or more fields of view (a total of 0.40 mm ² or more) were photographed for fields of 0.05 mm ² or more in area. Next, the thickness of the prior austenite grains was measured by the intercept method based on the microstructure photograph taken with an optical microscope, and the average value thereof was taken as the average length in the thickness direction of the prior austenite grains. In the measurement, prior austenite grains having a length in the thickness direction of 1 μm or more were used as objects.

In addition, the maximum length in the major axis direction and the maximum length in the minor axis direction perpendicular to the major axis direction were respectively measured for each prior-austenite grain from the above-mentioned structure photograph, and the ratio (maximum major axis length/short axis direction) was obtained. The maximum length of the axis), and its average value is taken as the average of the aspect ratio of the prior austenite grains.

Furthermore, the measurement of the average circle-equivalent diameter and area ratio of TiN particles was performed using TEM with EDX. First, an extraction replica was made from the 1/10t position of the steel plate, and particles with a size of 15 to 200 nm were observed by TEM with a magnification of 30,000 times or more and an observation area of 1 field of view of 15 μm ² or more. All observed particles were 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 were identified as TiN particles.

In addition, the electron beam diameter of TEM is 1-20 nm, and the observation magnification is set to 50,000 times to 1,000,000 times, and quantitative analysis is performed on an arbitrary position in the particle. The average circle-equivalent diameter of the TiN particles is an arithmetic mean of the circle-equivalent diameters (diameters) that form the same area as the area of each TiN particle discriminated above. The area ratio of the TiN particles is a value obtained by dividing the sum of the areas of the TiN particles identified above by the area of the observed field of view.

Next, the measurement of the grain boundary density was performed by the EBSD method. Specifically, by the EBSD method, a region of 500 μm×500 μm at the 1/10t position, 1/4t position, and 1/2t position was measured at a pitch of 1 μm, and the boundary with a crystal orientation difference of 15° or more from the adjacent crystal grain was defined as For the grain boundary, the grain boundary density can be obtained by dividing the total length of the grain boundary at this time by the measurement area.

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

[Table 7]

[Table 8]

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

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

Next, in accordance with ISO 15653:2018, the CTOD test piece with the total thickness in the plate thickness direction of the base metal at the notch position of 3-point bending was collected, and the CTOD value at -10°C was measured. The test was carried out 3 times, and their minimum values were recorded in the table. In this example, the case where the minimum value of the CTOD value at -10° C. was 0.50 mm or more was considered excellent in fracture toughness.

In addition, according to NK Classification Association Steel Ship Rules Inspection Essentials K, Appendix K3.12.2-1. (2016) "Inspection Essentials for Temperature Gradient ESSO Test and Temperature Gradient Double Tensile Test (Temperature Matching ESSO Test And びTemperature collusion type double tension test 験に关する検検查结构) "Determination of crack arrest toughness value Kca _{-10 ℃} . Next, a test was performed in accordance with the NRL drop weight test method specified in ASTM E208-06 to obtain the NDT temperature. In this example, the case where the crack arrest toughness value Kca _-10°C is 6000 N/mm ^1.5 or more and the NDT temperature is -100°C or less is regarded as having excellent crack arrestability.

The measurement results are shown in Tables 9 and 10.

[Table 9]

Table 9

[Table 10]

Table 10

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

Specifically, in Test No. 30, the concentration of dissolved O in the refining process when Ti was added was high, and the heating temperature in the heating process was high. In Test No. 31, the average cooling rate in the continuous casting process was high, In none of them, TiN particles were not precipitated, and the grain boundary density could not be optimized, so the arrestability 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 appropriated, so the crack arrestability deteriorated.

In Test No. 33, since the C content was excessive, low-temperature toughness and fracture toughness deteriorated. In Test No. 34, the C content was low, the bainite-main structure was not formed, the strength was insufficient, and the low-temperature toughness and fracture toughness were deteriorated. In Test No. 35, since the Si content was excessive, the low-temperature toughness and fracture toughness deteriorated. In Test No. 36, since the Mn content was excessive, low-temperature toughness and fracture toughness deteriorated. In Test No. 37, the Mn content was low and the strength was insufficient.

In Test No. 38, the P and S contents were excessive, in Test No. 39, the Al content was excessive, and in Test No. 40, the N content was excessive, so low-temperature toughness and fracture toughness deteriorated. In Test No. 41, the N content was low, the BF length and prior austenite grains became coarse, and the grain boundary density could not be optimized, so low-temperature toughness, fracture toughness, and crack arrest performance deteriorated.

In Test No. 42, since the Nb content was excessive, the low-temperature toughness and fracture toughness deteriorated. In test No. 43, the Nb content was low, the BF length and the prior austenite grains were coarsened, and the aspect ratio of the prior austenite grains was reduced, so that the grain boundary density could not be optimized. Therefore, the low temperature toughness, Deterioration of fracture toughness and crack arrest. In Test No. 44, the Ti content was excessive, so the low-temperature toughness and fracture toughness deteriorated. Furthermore, the TiN particles are coarsened and the heating temperature in the heating step is also high, so that the grain boundary density cannot be optimized, and the crack arrestability is also deteriorated. In Test No. 45, since the Ti content was low, the BF length and prior austenite grains were coarsened, and the grain boundary density could not be optimized, so low-temperature toughness, fracture toughness, and crack arrest performance deteriorated.

For Test Nos. 46 and 47, the heating temperature in the heating process was high, the BF length and prior austenite grains were coarsened, and the grain boundary density could not be properly optimized. Therefore, low temperature toughness, fracture toughness and crack arrest Sexual deterioration. In Test No. 48, since the heating temperature was low and the area ratio of bainite decreased, the strength was insufficient, and the low-temperature toughness and fracture toughness deteriorated. In Test No. 49, the finish temperature of rough rolling was lower than _Trex , the BF length, and the coarsening of prior austenite grains, and the grain boundary density could not be optimized, so low-temperature toughness, fracture toughness, and crack arrest performance deteriorated.

In Test No. 50, the cumulative reduction ratio of rough rolling was high, the BF length and prior austenite grains were coarsened, and the aspect ratio of prior austenite grains was reduced, so that the grain boundary density could not be optimized. Low-temperature toughness, fracture toughness, and crack arrestability are therefore deteriorated. On the other hand, in Test No. 51, the cumulative reduction ratio was low, the BF length and prior austenite grains were coarsened, and the grain boundary density could not be optimized, so low-temperature toughness, fracture toughness, and crack arrest performance deteriorated.

In Test No. 52, the start temperature of finish rolling was Trex _or higher, the BF length and prior austenite grains were coarsened, and the aspect ratio of prior austenite grains was reduced, so that the grain boundary density could not be optimized , so the low temperature toughness, fracture toughness and crack arrestability deteriorate. In Test No. 53, the finishing temperature of the finish rolling was lower than Ar ₃ , so that excessively formed ferrite was formed, the strength was insufficient, and the low-temperature toughness and fracture toughness deteriorated.

For Test No. 54, the cumulative reduction ratio of finish rolling was high, and for Test No. 55, the cumulative reduction ratio was low, the BF length and prior austenite grains were coarsened, and the prior austenite grains All of the aspect ratios were lowered, and the grain boundary density could not be properly adjusted, so the low-temperature toughness, fracture toughness, and crack arrestability all deteriorated. For test number 56, the time between passes was long, and for test number 57, the time from the completion of finish rolling to the start of cooling was long, the BF length and prior austenite grains were coarsened, and the prior austenite The aspect ratio of the bulk crystal grains is all reduced, and the grain boundary density cannot be appropriated, so the low-temperature toughness, fracture toughness, and crack arrestability are all deteriorated.

In Test No. 58, since the cooling rate in the accelerated cooling step was high, the MA phase was excessively produced, and low-temperature toughness and fracture toughness deteriorated. In Test No. 59, the cooling rate was low, the structure mainly of bainite was not formed, the strength was insufficient, and the low-temperature toughness and fracture toughness were deteriorated. In Test No. 60, the cooling stop temperature was high, so the structure mainly of bainite was not formed, the strength was insufficient, and the low-temperature toughness, fracture toughness, and crack arrestability deteriorated. In Test No. 61, the cooling start temperature exceeded _Trex -10° C., and the BF length was coarsened. Therefore, although the low-temperature toughness was good, the result was obtained that the fracture toughness deteriorated.

Industrial availability

According to the present invention, a steel plate having high strength and excellent low-temperature toughness, fracture toughness, and crack arrestability can be obtained. Therefore, the steel plate of the present invention can be suitably used as a raw 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 (10)

1. A steel plate, wherein the chemical composition of the steel plate is calculated as C: 0.040～0.160%, Si: 0.01 to 0.50%, Mn: 0.70～2.50%, P: 0.030% or less, S: 0.020% or less, Al: 0.001～0.100%, N: 0.0010～0.0080%, Nb: 0.003 to 0.050%, Ti: 0.003～0.050%, Balance: Fe and impurities, In the section perpendicular to the rolling direction of the steel plate, when the thickness of the steel plate is t, the metallographic structure at a position where the distance from the surface of the steel plate is 1/4t contains 80% by area % The above bainite, and the average length of the bainitic ferrite constituting the bainite in the major axis direction is 10 μm or less, In the cross section of the steel plate parallel to the rolling direction and the thickness direction, the average length of the prior austenite grains in the thickness direction at a position at a distance of 1/4t from the surface of the steel plate is 20 μm or less, and the length and width than the average of 2.5 or more, In the section perpendicular to the rolling direction of the steel plate, The grain boundary density at a position at a distance of 1/10t from the surface of the steel sheet is 500 to 1100 mm/mm ² , The grain boundary density at a position at a distance of 1/4t from the surface of the steel sheet is 400 to 1000 mm/mm ² , The grain boundary density at a position at a distance of 1/2t from the surface of the steel sheet is 300 to 900 mm/mm ² . 2. The steel sheet according to claim 1, wherein the chemical composition contains, in mass %, a group selected from the group consisting of 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: At least one or more of the group consisting of 0.0050% or less. 3. The steel sheet according to claim 1 or claim 2, wherein the chemical composition contains, in mass %, a portion selected from the group consisting of Mg: 0.0100% or less, Ca: 0.0100% or less, and REM: at least one or more of the group consisting of 0.0100% or less. 4. The steel sheet according to any one of claims 1 to 3, wherein the chemical composition contains, in mass %, a portion selected from the group consisting of Zr: 0.0100% or less, and Te: at least one or more of the group consisting of 0.0100% or less. 5. The steel sheet according to any one of claims 1 to 4, wherein the chemical composition contains, in mass %, a portion selected from the group consisting of W: 1.00% or less, and Sn: at least one or more of the group consisting of 0.50% or less. 6. The steel plate according to any one of claims 1 to 5, wherein the chemical composition satisfies the following formula (i), 1.7≤Ti/N≤3.4(i) However, the element symbols in the above formula represent the mass % content of each element contained in the steel sheet, and 0 is substituted when not contained. 7. The steel plate according to any one of claims 1 to 6, wherein the chemical composition satisfies the following formula (ii), In a section perpendicular to the rolling direction of the steel sheet, the average circle-equivalent diameter of the TiN particles at a position at a distance of 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 %above, Ti×N≥3.0×10 ^-5 (ii) However, the element symbols in the above formula represent the mass % content of each element contained in the steel sheet, and 0 is substituted when not contained. 8. A method for manufacturing a steel plate, which is a method for manufacturing a steel plate according to any one of claims 1 to 6, In the manufacturing method, a heating process, a hot rolling process, and an accelerated cooling process are sequentially performed on a steel slab having the chemical composition described in any one of claims 1 to 6, wherein, In the heating process, the steel slab is heated to a heating temperature of 950-1050° C., The hot rolling process includes rough rolling and finish rolling, The rough rolling is carried out in the range where the surface temperature of the slab is T _rex or higher, The cumulative rolling reduction in the rough rolling is set to 10 to 75%, The finish rolling is carried out in the range where the surface temperature of the slab is not less than Ar ₃ and lower than _Trex , The cumulative rolling reduction in the finish rolling is set to 65% to 90%, and the time between passes is set to be 15 seconds or less, The time from the completion of the finish rolling until the start of cooling in the accelerated cooling step is 50 seconds or less, In the accelerated cooling process, water cooling to 0 to 0°C is carried out under the condition that the cooling start temperature is set to be _Trex -10°C or less, and the average cooling rate from the start of cooling to the end of cooling is 5°C/sec. 550°C cooling stop temperature, Here, Ar _is obtained by the following formula (iii), _{and Trex} is obtained by the following formula (iv). It should be noted that the element symbols in the following formulas represent the mass % content of each element contained in the steel plate, Substitute 0 if not included, Ar ₃ ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (iii) T _rex ＝-91900[Nb*] ² +9400[Nb*]+770 (iv) However, when the amount of solid-solution Nb in mass % obtained by the following formula (v) is sol.Nb, In the case of Nb≥sol.Nb, [Nb*]=sol.Nb, In the case of Nb<sol.Nb, [Nb*]=Nb, sol.Nb＝(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) (v) In addition, T in the said formula represents the heating temperature of the steel slab in a heating process, and the unit of the said heating temperature is degreeC. 9. A method for manufacturing a steel plate, which is the method for manufacturing a steel plate according to claim 7, The production method comprises: a refining process for producing molten steel; and a continuous casting process for continuously casting the molten steel to produce a slab having the chemical composition described in any one of claims 1 to 6, and the obtained The billets are sequentially subjected to a heating process, a hot rolling process and an accelerated cooling process, wherein, 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 process, the average cooling rate during the period when the surface temperature of the steel slab is 1200-900°C is set to 0.1-0.5°C/sec, In the heating process, the steel billet is heated to a heating temperature of 950-1080° C., The hot rolling process includes rough rolling and finish rolling, The rough rolling is carried out in the range where the surface temperature of the slab is T _rex or higher, The cumulative rolling reduction in the rough rolling is set to 10 to 75%, The finish rolling is carried out in the range where the surface temperature of the slab is not less than Ar ₃ and lower than _Trex , The cumulative rolling reduction in the finish rolling is set to 65% to 90%, and the time between passes is set to be 15 seconds or less, The time from the completion of the finish rolling until the start of cooling in the accelerated cooling step is 50 seconds or less, In the accelerated cooling process, water cooling to 0 to 0°C is carried out under the condition that the cooling start temperature is set to be _Trex -10°C or less, and the average cooling rate from the start of cooling to the end of cooling is 5°C/sec. 550°C cooling stop temperature, Here, Ar ₃ is obtained by the following formula (iii), _{and Trex} is obtained by the following formula (iv). It should be noted that the element symbols in the following formulas represent the mass % content of each element contained in the steel sheet , substitute 0 if it does not contain, Ar ₃ ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (iii) T _rex ＝-91900[Nb*] ² +9400[Nb*]+770 (iv) However, when the amount of solid-solution Nb in mass % obtained by the following formula (v) is sol.Nb, In the case of Nb≥sol.Nb, [Nb*]=sol.Nb, In the case of Nb<sol.Nb, [Nb*]=Nb, sol.Nb＝(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) (v) In addition, T in the said formula represents the heating temperature of the steel slab in a heating process, and the unit of the said heating temperature is degreeC. 10 . The method for manufacturing a steel sheet according to claim 8 or claim 9 , wherein a tempering step of heating to a temperature range of 350 to 650° C. is further performed after the accelerated cooling step. 11 .