EP3467130A1

EP3467130A1 - High-tensile steel plate having excellent low-temperature toughness

Info

Publication number: EP3467130A1
Application number: EP16903957.5A
Authority: EP
Inventors: Fumitoshi Takamine; Mitsuru Sawamura; Norimasa Kawabata; Toshiaki NAMBA; Naoki Saitoh
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-05-31
Filing date: 2016-05-31
Publication date: 2019-04-10
Anticipated expiration: 2036-05-31
Also published as: EP3467130A4; KR102184966B1; KR20180096782A; WO2017208329A1; CN108603258A; JPWO2017208329A1; EP3467130B1; JP6631702B2; CN108603258B

Abstract

A high tensile strength steel plate has a chemical composition including, by mass%: C: 0.08% to 0.15%; Mn: 0.80% to 1.60%; Ni: 3.00% to 4.50%; Cr: 0.50% to 1.00%; Mo: 0.50% to 1.00%; Al: 0.020% to 0.085%; N: 0.0020% to 0.0070%; and B: 0.0005% to 0.0020%, in which a plate thickness t mm is more than 200 mm and not more than 300 mm, in the chemical composition, Ts is 380 to 430, Ceq is 0.80 to 1.05, Ac1 is 580 to 647, and x is 46 to 90, a total amount of martensite and bainite is 99% to 100% by area%, a tensile strength is 780 MPa to 930 MPa, and an absorbed energy of a thickness middle portion obtained by a charpy impact test at -60°C is 69 J or more.

Description

[Technical Field of the Invention]

The present invention relates to a high tensile strength steel plate having excellent low temperature toughness and a large plate thickness. More specifically, the present invention relates to a steel plate having a plate thickness of more than 200 mm, a tensile strength of 780 MPa or more, and an absorbed energy of a thickness middle portion at -60°C of 69 J or more. The steel plate is suitably used for structures such as offshore structures, pressure vessels, penstocks, and large cranes for a ship.

[Related Art]

In the structures mentioned above, a steel plate used as a base metal is generally required to have low temperature toughness in order to ensure the safety of the structures. The scale of the structures in recent years has been significantly increased, and there is a trend toward using a steel plate having a large plate thickness and has a high strength for such structures.
In the structures mentioned above, a 780 MPa class high tensile strength steel plate is generally used. In order to obtain a tensile strength of 780 MPa or more, in the high tensile strength steel plate, a microstructure primarily containing low temperature transformation products such as bainite and martensite is formed by quenching such as a direct quenching method. However, as the plate thickness is increased, the cooling rate inside the steel plate during quenching is decreased, so that it is difficult to form a low temperature transformation structure. Therefore, alloying elements such as C, Mn, Cr, Mo, and V for improving hardenability have been added to the steel in an appropriate amount so that sufficient low temperature transformation products can be obtained even when the cooling rate is decreased. As a result, a tensile strength of 780 MPa or more has been achieved even when the plate thickness is increased to about 150 mm. However, in a steel plate having a plate thickness of more than 200 mm, the effect of transformation heat on the actual cooling rate during quenching is significant, so that transformation proceeds at a high temperature, and low temperature transformation products cannot be sufficiently obtained.
For example, Patent Document 1 discloses a high tensile strength steel plate in which Ceq is 0.80 or less, the C content, the P content, the Mn content, the Ni content, and the Mo content satisfy a predetermined equation, and the ratio (HVmax/HVave) of the hardness of a center segregation portion of the steel plate to the average value of the hardness of a certain region of the steel plate, the C content, and the plate thickness satisfy a predetermined equation. In addition, in Patent Document 1, it is disclosed that the plate thickness of the steel plate is 60 mm to 150 mm. Patent Document 2 discloses a high tensile strength steel plate in which Ceq is CeqM or less, and the plate thickness is 75 mm to 200 mm. Patent Document 3 discloses a high tensile strength steel plate with high toughness, in which a parameter x determined by the amounts of elements is 26 to 42 and the plate thickness is 75 mm to 200 mm. However, in these three patent documents, if the plate thickness of the steel plate exceeds 200 mm, the desired effect cannot be exerted on the steel plate.
Patent Document 4 discloses a high tensile strength steel plate in which the C content is 0.005% to 0.02%, and the plate thickness is 50 mm to 200 mm. Patent Document 5 discloses a high tensile strength steel plate in which the C content is 0.02% to 0.05%, and the plate thickness is 75 mm to 200 mm. Patent Documents 4 and 5 disclose a method of requiring rapid cooling at which the cooling rate of a thickness middle portion during quenching is 1.1 °C/s or more. However, when the plate thickness of the steel plate exceeds 200 mm, it is industrially impossible to increase the cooling rate of the thickness middle portion to 1.1 °C/s or more. Therefore, when the plate thickness of the steel plate exceeds 200 mm, the method disclosed in Patent Documents 4 and 5 cannot be realized.
Patent Document 6 discloses a method in which, in order to obtain fine austenite grains, the cumulative rolling reduction in a temperature range of Ar₃ point to 900°C during hot rolling is increased to 50% or more, and the heating temperature for quenching is limited to a temperature range of Ac₃ point to (Ac₃ point + 100°C). In addition, Patent Document 6 discloses a high tensile strength steel plate having a plate thickness of 40 mm to 65 mm. However, as the plate thickness of the steel plate is increased, the effect of rolling is decreased at the center in a plate thickness direction of the steel plate. Therefore, when the plate thickness of the steel plate exceeds 100 mm, the effect of low temperature rolling on grain refinement is small. Therefore, even though there is an attempt to refine grains through low temperature rolling, when the plate thickness of the steel plate exceeds 200 mm, the desired effect cannot be exerted on the steel plate. In addition, low temperature rolling causes an increase in deformation resistance and makes it difficult to bury voids inside the steel plate. Therefore, low temperature rolling is not suitable for the manufacturing of a steel plate having a plate thickness of more than 200 mm.
Patent Document 7 discloses a high tensile strength steel plate in which Ceq is 0.50 to 0.80, a parameter β determined by the amounts of elements is 8.45 to 15.2, the average grain size in a thickness middle portion of the steel plate is 35 µm or less, and the plate thickness is 25 mm to 200 mm. In addition, Patent Document 7 discloses a method of increasing the cumulative rolling reduction in a temperature range of 900°C to 1150°C to 50% or more so as to achieve an average grain size of 35 µm or less. However, as described above, as the plate thickness of the steel plate is increased, the effect of rolling is decreased at the center in a plate thickness direction of the steel plate. Furthermore, as disclosed in Patent Document 7, when the plate thickness of the steel plate exceeds 200 mm, the cooling rate of the thickness middle portion significantly is decreased, resulting in grain coarsening. Therefore, in Patent Document 7, when the plate thickness of the steel plate exceeds 200 mm, the desired effect cannot be exerted on the steel plate.
Patent Document 8 discloses a method of performing a quenching treatment twice or more so that fine and uniform austenite grains can be obtained by recrystallization. However, as shown in Non-Patent Document 1 and Non-Patent Document 2, in a low-alloy steel, as the heating speed is decreased, the effect of reheating on grain refinement is decreased. In addition, Patent Document 8 discloses a high tensile strength steel plate having a plate thickness of 50 mm. However, as the plate thickness of the steel plate is increased, the heating speed is decreased. Therefore, in the manufacturing of a steel plate having a plate thickness of more than 200 mm, grains are rarely refined even when two or more quenching treatments are performed, and only an increase in the manufacturing cost is caused. Therefore, in the method disclosed in Patent Document 8, when the plate thickness of the steel plate exceeds 200 mm, the desired effect cannot be exerted on the steel plate.
A method of increasing the toughness of a steel plate by concentrating Ni in fine residual austenite grains and thus stabilizing the residual austenite is known. For example, Patent Document 9 and Patent Document 10 disclose a high tensile strength steel plate in which the plate thickness is 150 mm to 200 mm, the amount of residual austenite is 1% to 10%, and a property of stopping propagation of brittle fracture (crack) is good. In these patent documents, a method of tempering the steel plate in a temperature range (a temperature range higher than Ac1) in which austenitic transformation can be achieved so as to form fine residual austenite. However, in a case where the plate thickness of the steel plate exceeds 200 mm, the grain size of austenite becomes coarse at a thickness middle portion of the steel plate and the concentration of Ni in the austenite becomes insufficient. Therefore, the stability of the residual austenite is decreased, and the toughness of the thickness middle portion of the steel plate is decreased. In addition, in order to increase the stability of the residual austenite, there is a need to increase the amount of Ni, and there is a tendency toward an increase in the cost. Furthermore, Patent Document 9 discloses a method in which the temperature range of finish rolling is limited to 700°C to 850°C to obtain fine austenite and the cumulative rolling reduction in this temperature range is limited to 25% to 75%. As described above, in Patent Document 9, since low temperature rolling is used, the method of Patent Document 9 is not suitable for the manufacturing of a steel plate having a plate thickness of more than 200 mm.
As described above, in the methods of the related art, when the plate thickness of a steel plate exceeds 200 mm, a high tensile strength steel plate having a tensile strength of 780 MPa or more and excellent low temperature toughness cannot be obtained.

[Prior Art Document]

[Patent Document]

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2013-91845
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2011-202214
[Patent Document 3] Japanese Patent No. 2662409
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2013-104065
[Patent Document 5] Japanese Patent No. 5552967
[Patent Document 6] Japanese Unexamined Patent Application, First Publication No. H6-240353
[Patent Document 7] Japanese Patent No. 5590271
[Patent Document 8] Japanese Unexamined Patent Application, First Publication No. H10-265846
[Patent Document 9] Japanese Patent No. 3336877
[Patent Document 10] Japanese Patent No. 3327065

[Non-Patent Document]

[Non-Patent Document 1] "Effect of Ni on the Behavior of Austenite Grain in Ni-Cr-Mo-V Steels" by Ryosuke HONMA, Tetsu-to-Hagané, Vol. 58 (1972), No. 1, p. 119
[Non-Patent Document 2] "Reverse Transformation of Low-carbon Low Alloy Steels" by Shoichi MATSUDA et al., Tetsu-to-Hagané, Vol. 60 (1974), No. 2, p. 60

[Disclosure of the Invention]

[Problems to be Solved by the Invention]

The present invention has been made taking the foregoing problems into consideration, and an object of the present invention is to provide a steel plate having a plate thickness of more than 200 mm, excellent low temperature toughness, and high strength.

[Means for Solving the Problem]

The inventors found a novel chemical composition and a microstructure capable of imparting high strength and high low temperature toughness to a thickness middle portion of a steel plate even when the plate thickness of the steel plate exceeds 200 mm. In addition, the inventors found that this novel chemical composition is different from the chemical composition that imparts high strength and high low temperature toughness to a steel plate of the related art, and it is suitable to apply a novel method which is different from the method of the related art to steel having the novel chemical composition.
The present invention has been made based on the above findings, and the gist of thereof is as follows.

(1) According to an aspect of the present invention, a steel plate having a chemical composition including, by mass%: C: 0.08% to 0.15%; Mn: 0.80% to 1.60%; Ni: 3.00% to 4.50%; Cr: 0.50% to 1.00%; Mo: 0.50% to 1.00%; Al: 0.020% to 0.085%; N: 0.0020% to 0.0070%; B: 0.0005% to 0.0020%; P: 0.000% to 0.010%; S: 0.000% to 0.003%; Si: 0.00% to 0.30%; Cu: 0.00% to 0.50%; V: 0.000% to 0.050%; Nb: 0.000% to 0.050%; Ti: 0.000% to 0.020%; Ca: 0.0000% to 0.0030%; Mg: 0.0000% to 0.0030%; REM: 0.0000% to 0.0030%; and a remainder including Fe and impurities, in which a plate thickness t mm is more than 200 mm and not more than 300 mm, in the chemical composition, Ts defined by Equation 1 is 380 to 430, Ceq defined by Equation 2 is 0.80 to 1.05, Ac1 defined by Equation 3 is 580 to 647, and x defined by Equation 4 is 46 to 90, a total amount of martensite and bainite is 99% to 100% by area%, and a tensile strength is 780 MPa to 930 MPa, and an absorbed energy of a thickness middle portion obtained by a charpy impact test at -60°C is 69 J or more: $Ts = 750 - 4240 \times {(t / 2)}^{- 1.4} \times (80 \times C + 10 \times Mn + 7 \times Ni + 13 \times Cr + 13 \times Mo - 40 \times Si)$
$Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5$
$Ac1 = 720 - 25 \times C + 22 \times Si - 40 \times Mn - 30 \times Ni + 20 \times Cr + 25 \times Mo$
$x = C^{1 / 2} \times (1 + 0.64 \times Si) \times (1 + 4.10 \times Mn) \times (1 + 0.27 \times Cu) \times (1 + 0.52 \times Ni) \times (1 + 2.33 \times Cr) \times (1 + 3.14 \times Mo)$
(2) In the steel plate according to (1), the chemical composition may further satisfy Ti/N ≤ 3.4.
(3) In the steel plate according to (1) or (2), the chemical composition may further satisfy C: 0.09% to 0.13%.
(4) In the steel plate according to any one of (1) to (3), the chemical composition may further satisfy Mn: 0.80% to 1.30%.
(5) In the steel plate according to any one of (1) to (4), the chemical composition may further satisfy Ni: 3.60% to 4.50%.
(6) In the steel plate according to any one of (1) to (5), the chemical composition may further satisfy Cr: 0.75% to 1.00%.
(7) In the steel plate according to any one of (1) to (6), the chemical composition may further satisfy Mo: 0.70% to 1.00%.
(8) In the steel plate according to any one of (1) to (7), the chemical composition may further satisfy Si: 0.00% to 0.10%.
(9) In the steel plate according to any one of (1) to (8), the chemical composition may further satisfy V: 0.020% to 0.050%.
(10) In the steel plate according to any one of (1) to (9), the chemical composition may further satisfy Ti: 0.000% to 0.004%.
(11) In the steel plate according to any one of (1) to (10), the chemical composition may further satisfy a condition where the Ts is 395 to 415.
(12) In the steel plate according to any one of (1) to (11), the chemical composition may further satisfy a condition where the Ceq is 0.85 to 1.05.

[Effects of the Invention]

According to the present invention, it is possible to provide a steel plate having a plate thickness of more than 200 mm and having excellent low temperature toughness and high strength. Therefore, the safety of a structure having a larger scale can be further increased.

[Brief Description of the Drawings]

FIG. 1 is a view showing an example of the relationship between Ts and vE_-60°C.
FIG. 2 is a view showing an example of the relationship between Ceq and vE_-60°C.
FIG. 3 is a view showing an example of the relationship between x and vE_-60°C.
FIG. 4 is a photograph showing the microstructure of a high strength steel plate according to an embodiment of the present invention.
FIG. 5 is a view schematically showing an effect of Ts on a quenched structure as an example.

[Embodiments of the Invention]

Hereinafter, a steel plate (high tensile strength steel plate) according to an embodiment of the present invention will be described.
First, the chemical composition of the steel plate according to the embodiment will be described. Hereinafter, the amount (%) of each element is represented in mass%.

C: 0.08% to 0.15%

C increases the hardness of the microstructure of a steel plate after quenching and is thus effective for improving strength. Therefore, the C content is needed to be 0.08% or more. On the other hand, when the C content is excessive, the toughness is impaired, and thus the C content is needed to be 0.15% or less. Therefore, the C content is 0.08% to 0.15%. In order to further increase the strength, the C content is preferably 0.09% or more or 0.10% or more. In order to further increase the toughness, the C content is preferably 0.14% or less, more preferably 0.13% or less, or more preferably 0.12% or less.

Mn: 0.80% to 1.60%

Mn is effective for both deoxidation and improvement in hardenability. In order to increase the hardenability of the steel and improve the strength, the Mn content is needed to be 0.80% or more. The Mn content may be set to 0.85% or more, 0.90% or more, 0.95% or more, 1.00% or more, 1.05% or more, or 1.10% or more. On the other hand, when the Mn content is excessive, the hardenability is excessive, and the microstructure becomes hard. In addition, since tempering embrittlement is promoted when the Mn content is excessive, the toughness of steel is decreased due to the synergistic effect between the hard microstructure and the tempering embrittlement. Therefore, the Mn content is needed to be 1.60% or less. Therefore, the Mn content is 0.80% to 1.60%. In order to further increase the toughness, the Mn content is preferably 1.50% or less, more preferably 1.40% or less, and most preferably 1.35% or less or 1.30% or less. If necessary, the Mn content may be set to 1.25% or less or 1.20% or less.

Ni: 3.00% to 4.50%

Ni is effective for improving the strength and toughness of steel, and the Ni content is needed to be 3.00% or more. When the Ni content is excessive, it is necessary to lower the tempering temperature by decreasing Ac1, resulting in an increase in the tempering time. In addition, since Ni stabilizes austenite, there is concern that residual austenite may be remained. Furthermore, Ni is expensive. Therefore, when the Ni content is excessive, the manufacturing cost is deteriorated. Therefore, the Ni content is needed to be 4.50% or less. Accordingly, the Ni content is 3.00% to 4.50%. In a case where the strength and toughness of the steel is further increased, the Ni content is preferably 3.15% or more, 3.30% or more, 3.40% or more, or 3.50% or more, and more preferably 3.60% or more. The Ni content may be set to 4.30% or less, 4.15% or less, 4.00% or less, 3.90% or less, or 3.80% or less.

Cr: 0.50% to 1.00%

Mo: 0.50% to 1.00%

Cr and Mo improve the hardenability of steel and improve the strength. The Cr content is needed to be 0.50% or more, and the Mo content is needed to be 0.50% or more. On the other hand, when the amount of Cr or the amount of Mo is excessive, the toughness is decreased due to the formation of alloy carbides. Therefore, the Cr content is needed to be 1.00% or less, and the Mo content is needed to be 1.00% or less. Therefore, the Cr content is 0.50% to 1.00%, and the Mo content is 0.50% to 1.00%. In order to stably increase the strength of the steel, the Cr content is preferably 0.60% or more, and more preferably 0.65% or more, 0.70% or more, 0.75% or more, or 0.80% or more. The Cr content may be set to 0.96% or less, 0.94% or less, or 0.91% or less. Similarly, the Mo content is preferably 0.60% or more, and more preferably 0.70% or more, 0.75% or more, 0.80% or more, or 0.85% or more. The Mo content may be 0.96% or less, 0.94% or less, 0.92% or less, or 0.90% or less.

Al: 0.020% to 0.085%

Al is effective for deoxidation, and is bonded to solute N in steel to form AlN. This AlN makes grains fine and reduces the amount of solute N in steel, whereby the effect of B on the hardenability of the steel is stabilized. Therefore, the Al content is needed to be 0.020% or more. On the other hand, when the Al content is excessive, the size of the AlN is too large, and thus the toughness is decreased, resulting in cracking in a slab. Therefore, the amount of AlN is needed to be 0.085% or less. Accordingly, the Al content is 0.020% to 0.085%. In order to further increase the effect for improving the hardenability of B, the Al content may be set to 0.030% or more, 0.040% or more, or 0.045% or more. In order to more reliably prevent the formation of coarse AlN, the upper limit of the Al content may be 0.070%, 0.065% or 0.060%.

N: 0.0020% to 0.0070%

N is bonded to alloying elements to form compounds (nitrides and carbonitrides), thereby making grains fine. Therefore, the N content is needed to be 0.0020% or more. On the other hand, when the N content is excessive, the solute N in the steel becomes excessive, or the compounds (nitrides and carbonitrides) become coarse, so that the toughness of the steel is decreased. Therefore, the N content is needed to be 0.0070% or less. Therefore, the N content is 0.0020% to 0.0070%. The N content may be set to 0.0025% or more, 0.0030% or more, or 0.0040% or more, and may be set to 0.0065% or less, or 0.0060% or less.

B: 0.0005% to 0.0020%

When the steel contains a small amount of B, the hardenability of the steel is improved and the strength is improved. Therefore, the B content is needed to be 0.0005% or more. However, in a case where the B content is excessive, metal carbon boride is formed and the hardenability is decreased. Therefore, the B content is needed to be 0.0020% or less. Accordingly, the B content is 0.0005% to 0.0020%. In order to further increase the hardenability, the B content may be set to 0.0007% or more or 0.0008% or more. In order to further optimize the hardenability, the B content may be set to 0.0018% or less, 0.0016% or less, or 0.0014% or less.
The steel plate of the embodiment contains the above eight elements (C, Mn, Ni, Cr, Mo, Al, N, and B) as essential elements. In addition to these essential elements, the steel may contain the following elements as optional elements.

P: 0.000% to 0.010%

P is an impurity in the steel and decreases the toughness by promoting intergranular embrittlement. As described above, since P is harmful to the toughness of the steel, the P content is preferably as small as possible. Therefore, the P content is needed to be 0.010% or less. The P content may be 0.000%. Therefore, the P content is 0.000% to 0.010%. The P content may be set to 0.007% or less or 0.005% or less. When the P content is reduced, the refining cost is increased and the productivity is decreased. Therefore, the P content may be set to 0.0005% or more, or 0.001% or more.

S: 0.000% to 0.003%

S is an impurity in the steel, and segregation of S and sulfides reduce the toughness. Therefore, the S content is preferably as small as possible. Therefore, the S content is needed to be 0.003% or less. The S content may be 0.000%. Therefore, the S content is 0.000% to 0.003%. The S content may be set to 0.002% or less. When the S content is reduced, the refining cost is increased or the productivity is decreased. Therefore, the S content may be set to 0.0004% or more, or 0.0006% or more.

Si: 0.00% to 0.30%

When the Si content is excessive, S promotes the tempering embrittlement and decreases the toughness. Therefore, the Si content is needed to be 0.30% or less. On the other hand, the Si content may be 0.00%. Therefore, the Si content is 0.00% to 0.30%. In addition, since Si is effective for both deoxidation and improvement in strength, the steel may optionally contain Si. The Si content may be 0.01% or more, 0.02% or more, or 0.03% or more in order to increase the deoxidation efficiency in refining molten steel. In addition, in order to more stably increase the toughness, the Si content is preferably 0.25% or less, and more preferably 0.20% or less, or 0.15% or 0.10% or less.

Cu: 0.00% to 0.50%

When the Cu content is excessive, cracking occurs during hot working, and metal Cu is precipitated, resulting in the decrease in toughness. Therefore, the Cu content is needed to be 0.50% or less. When the Cu content is 0.50% or less, the strength of the steel can be increased without impairing low temperature toughness. In addition, when the Cu content is increased, Ceq is increased, and thus the formation of ferrite at the time of quenching can be more stably suppressed. Therefore, the steel may optionally contain Cu. However, the effect of Cu on the strength of steel and Ceq can be obtained even when Cu is replaced by another alloying element. Therefore, the Cu content may be 0.00%. Accordingly, the Cu content is 0.00% to 0.50%. In a case where Cu is contained in molten steel used as crude material, it is difficult to reduce the Cu content to 0.00% by refining, and thus the Cu content may be set to 0.01% or more, 0.02% or more, or 0.06% or more. The Cu content may be set to 0.45% or less, 0.40% or less, 0.35% or less, or 0.030% or less.

V: 0.000% to 0.050%

When the V content is excessive, the toughness is decreased due to the formation of alloy carbides. Therefore, the V content is needed to be 0.050% or less. On the other hand, V forms carbides or improves hardenability, thereby improving the strength of steel. In addition, when the V content is increased, Ceq is increased, and thus the formation of ferrite at the time of quenching can be more stably suppressed. Therefore, the steel may optionally contain V. However, the effect of V on the strength of steel and Ceq can be obtained even when V is replaced by another alloying element. Therefore, the V content may be 0.000%. Accordingly, the V content is 0.000% to 0.050%. In a case where V is contained in molten steel used as crude material, it is difficult to reduce the V content to 0.000% by refining, and thus the V content may be set to 0.003% or more, or 0.005% or more. In order to stably increase the strength of the steel, the V content is more preferably 0.010% or more, and the V content is most preferably 0.020% or more. The upper limit of the V content may be 0.045%, 0.040% or 0.035%.

Nb: 0.000% to 0.050%

Nb forms carbonitrides and makes grains inside the steel fine. Therefore, the steel may optionally contain Nb. On the other hand, the Nb content may be 0.000%. However, when the Nb content is excessive, the size of the carbonitride is increased and the toughness is decreased. Therefore, the Nb content is needed to be 0.050% or less. Accordingly, the Nb content is 0.000% to 0.050%. In a case where Nb imparts an effect for refining grains to steel, the Nb content may be 0.001%. In this case, the upper limit of the Nb content may be 0.040%, 0.035%, 0.030% or 0.025%. In a case where the effect for refining grains by Nb is unnecessary or the like, the intentional addition of Nb may not be performed.

Ti: 0.000% to 0.020%

Ti forms stable nitrides and makes grains fine. Therefore, the steel may optionally contain Ti. On the other hand, the Ti content may be 0.000%. However, when the Ti content is excessive, the size of the nitride is increased and the toughness is decreased. Therefore, the Ti content is needed to be 0.020% or less. Accordingly, the Ti content is 0.000% to 0.020%. In a case where Ti imparts an effect of refining grains to steel, the Ti content may be 0.001% or more. In addition, since grain refinement can also be achieved by AlN, the Ti content may be 0.010% or less, 0.004% or less, or 0.002% or less. In a case where the effect for refining grains by Ti is unnecessary or the like, the intentional addition of Ti may not be performed.

Ca: 0.0000% to 0.0030%

Mg: 0.0000% to 0.0030%

REM: 0.0000% to 0.0030%

Any of Ca, Mg, and REM is bonded to harmful impurities such as S and form harmless inclusions, thereby improving the mechanical properties of steel. Therefore, the steel may optionally contain at least one selected from the group consisting of Ca, Mg and REM. On the other hand, the Ca content, the Mg content, and the REM content may all be 0.0000%. When the amounts of these elements are excessive, refractory materials such as casting nozzles are melted down. Therefore, any of the Ca content, the Mg content, and the REM content is needed to be 0.0030% or less. Therefore, any of the Ca content, the Mg content, and the REM content is 0.0000% to 0.0030%. In a case where Ca, Mg, and REM impart an effect on the mechanical properties of the steel to the steel, it is preferable that any of the Ca content, the Mg content, and the REM content is 0.0001% or more. This effect is saturated when the amount of each element is reached 0.0030%. The intentional addition of Ca, Mg, REM may not be performed.
Some other elements may be contained in the steel plate of the embodiment as long as the elements do not substantially have a disadvantageous effect on the properties of the steel plate of the embodiment. For example, as the allowable amounts, the W content is 0.00% to 0.10%, the Co content is 0.00% to 0.10%, the Sb content is 0.000% to 0.010%, the As content is 0.000% To 0.010%, the Sn content is 0.000% to 0.010%, and the Pb content is 0.000% to 0.050%. These elements may be incorporated into the molten steel from, for example, scrap. Each of the W content and the Co content may be set to 0.05% or less, 0.02% or less, 0.01% or less, or 0.005% or less.
The steel plate of the embodiment has a chemical composition containing the above eight essential elements and the remainder including Fe and impurities or a chemical composition containing the above eight essential elements, at least one selected from the group consisting of the above optional elements, and the remainder including Fe and impurities. In the steel plate according to the embodiment, furthermore, the chemical composition is needed to satisfy the following conditions.

Ts: 380 to 430

Ts is defined by Equation 5 below and has a relatively strong correlation with the microstructure of the steel plate after the steel plate having a thickness of more than 200 mm is quenched by water cooling. In a case where Ts is excessively low, the microstructure primarily contains martensite, and the toughness of the steel plate is decreased. Therefore, as shown in FIG. 1, Ts is needed to be 380 or more. On the other hand, in a case where Ts is excessively high, the microstructure primarily contains upper bainite, and the strength and toughness of the steel plate are decreased. Therefore, as shown in FIG. 1, Ts is needed to be 430 or less. Accordingly, the range of Ts is 380 to 430. As described above, since the range of Ts is defined as 380 to 430, Ts itself is a dimensionless quantity. Therefore, there is no need to limit the unit of Ts. If a unit is given to Ts, the unit of Ts is mm^-1.4.%. In order to more stably increase the toughness of the steel plate, Ts is preferably 385 or more, 390 or more, 395 or more, or 400 or more. For the same reason, Ts is preferably 425 or less, 420 or less, 415 or less, or 412 or less. $Ts = 750 - 4240 \times {(t / 2)}^{- 1.4} \times (80 \times C + 10 \times Mn + 7 \times Ni + 13 \times Cr + 13 \times Mo - 40 \times Si)$
Here, t is the plate thickness mm of the steel plate, and each element symbol is the amount % of the corresponding element.

Ceq: 0.80 to 1.05

Ceq is defined by Equation 6 below and represents the hardenability of the steel. When Ceq is too low, ferrite is crystallized, and the strength and low temperature toughness of the steel plate are not sufficient. Therefore, as shown in FIG. 2, Ceq is needed to be 0.80 or more. On the other hand, when the Ceq is too high, the strength of the steel plate becomes too high and the toughness of the steel plate significantly is decreased. Therefore, as shown in FIG. 2, Ceq is needed to be 1.05 or less. Therefore, the range of Ceq is 0.80 to 1.05. As described above, since the range of Ceq is defined as 0.80 to 1.05, Ceq itself is a dimensionless quantity. Therefore, there is no need to limit the unit of Ceq. If a unit is given to Ceq, the unit of Ceq is %. In order to further increase the strength and low temperature toughness of the steel plate, Ceq is preferably more than 0.80, and Ceq is more preferably 0.85 or more, 0.86 or more, 0.87 or more, or 0.89 or more. The upper limit of Ceq may be 1.02, 0.99, 0.96, or 0.94. $Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5$
Here, each element symbol is the amount % of the corresponding element.

x: 46 to 90

x is defined by Equation 7 below and represents the hardenability of the steel. When x is too low, the amount of the upper bainite is increased, and the low temperature toughness of the steel plate is not sufficient. Therefore, as shown in FIG. 3, x is needed to be 46 or more. On the other hand, when x is too high, the amount of martensite is too large, and thus the low temperature toughness of the steel plate is not sufficient. Therefore, as shown in FIG. 3, x is needed to be 90 or less. Therefore, the range of x is 46 to 90. As described above, since the range of x is defined as 46 to 90, x itself is a dimensionless quantity. Therefore, there is no need to limit the unit of x. If a unit is given to x, the unit of x is %^6.5. The lower limit of x may be 50, 53, 56, 59, 61, or 63, and the upper limit of x may be 85, 82, 79, 76, or 73. $x = C^{1 / 2} \times (1 + 0.64 \times Si) \times (1 + 4.10 \times Mn) \times (1 + 0.27 \times Cu) \times (1 + 0.52 \times Ni) \times (1 + 2.33 \times Cr) \times (1 + 3.14 \times Mo)$
Here, each element symbol is the amount % of the corresponding element.

β

β is defined by Equation 8 below and represents the hardenability of the steel. When β is too low, the quenched structure primarily contains upper bainite, and the strength and low temperature toughness of the steel plate are not sufficient. Therefore, β is needed to be 22 or more. On the other hand, when β is too high, the quenched structure primarily contains martensite, and the low temperature toughness of the steel plate is not sufficient. Therefore, β is needed to be 60 or less. Therefore, the range of β is 22 to 60. However, in the embodiment, since the Si content is 0.00% to 0.30% and x is 46 to 90, the range of β is always 22 to 60. Therefore, there is no need to limit the range of β. In addition, since the range of β is defined as 22 to 60, β itself is a dimensionless quantity. Therefore, there is no need to limit the unit of β. If a unit is given to β, the unit of β is %^6.5. The lower limit of β may be 25, 28, 31, or 34, and the upper limit of β may be 56, 53, 50, or 48. $β = 0.65 \times C^{1 / 2} \times (1 + 0.27 \times Si) \times (1 + 4.10 \times Mn) \times (1 + 0.27 \times Cu) \times (1 + 0.52 \times Ni) \times (1 + 2.33 \times Cr) \times (1 + 3.14 \times Mo)$
Here, each element symbol is the amount % of the corresponding element.

Ac1: 580 to 647

Ac1 represents the temperature at which austenitic transformation starts when the steel is heated and is defined by Equation 9 below. In a steel having a microstructure including tempered martensite and tempered bainite, when Ac1 is lower than 580, impurities are segregated at grain boundaries and the low temperature toughness of the steel is not sufficient. Therefore, Ac1 is needed to be 580 or more. In the embodiment, since the C content, the Si content, the Mn content, the Ni content, the Cr content, and the Mo content are needed to be in the above-mentioned ranges, Ac1 is 647 or less. Therefore, the range of Ac1 is 580 to 647. As described above, since the range of Ac1 is defined as 580 to 647, Ac1 itself is a dimensionless quantity. Therefore, there is no need to limit the unit of Ac1. In a case where a unit is given to Ac1, the unit of Ac1 is °C. The upper limit of Ac1 may be set to 640, 635, 630, or 625, and the lower limit thereof may be set to 585, 590, or 595. $Ac 1 = 720 - 25 \times C + 22 \times Si - 40 \times Mn - 30 \times Ni + 20 \times Cr + 25 \times Mo$
Here, each element symbol is the amount % of the corresponding element.

Ti/N

In a case where Ti is added to steel, Ti is bonded to N to form TiN. In this reaction, when the ratio of Ti to N is smaller than the stoichiometric ratio (3.4), Ti can be prevented from being bonded to an element other than N (for example, C). Therefore, the effect of TiN on grain refinement can be stably obtained, and the low temperature toughness can be further increased. Therefore, it is preferable that the chemical composition of the steel satisfies Ti/N ≤ 3.4.
Next, the microstructure of the steel plate according to the embodiment will be described.

Total Amount of Martensite and Bainite: 99% To 100%

Martensite and bainite increase the strength of the steel plate. Therefore, the total amount of martensite and bainite is needed to be 99% to 100%. There may be cases where the remainder of the microstructure contains ferrite, pearlite, and residual austenite. The amount of the remainder (the total amount of ferrite, pearlite, and residual austenite) is 0% to 1%. The amount of the remainder may be 0.5% or less, 0.2% or less, or 0.1% or less. That is, the total amount of martensite and bainite may be 99.5% or more, 99.8% or more, or 99.9% or more. It is most preferable that the amount of the remainder is 0%, that is, the total amount of martensite and bainite is 100%.
There is a possibility that a microstructure may contain martensite, bainite, pearlite, ferrite, and residual austenite. In the embodiment, since the total amount of martensite and bainite is 99% or more, it is extremely difficult to directly specify the total amount of these two structures. Therefore, the amount of the remainder, that is, the total amount of ferrite, pearlite, and residual austenite is previously determined by the following method. Thereafter, the total amount of martensite and bainite is calculated by subtracting the total amount of these three structures from 100%.
The amount of ferrite and the amount of pearlite are expressed in area fraction (area%) and are determined from a photograph taken with an optical microscope at a magnification of 500-fold. A sample is taken from a thickness middle portion at a position more than 100 mm away from the edge of the steel plate. The longitudinal section of this sample (a plane including a plate thickness direction and a rolling direction; a plane perpendicular to a width direction) is etched by Nital and the three visual fields are taken from this etched surface. The three visual fields are determined such that there is no overlapping region. For example, the amount of ferrite is determined by integrating white regions (regions of ferrite) in an optical micrograph, thereafter dividing the integrated area by a measurement area, and averaging the obtained area fractions.
The amount of residual austenite is expressed in volume fraction (volume%) and is measured by an X-ray diffraction method. The sample is taken from the thickness middle portion at a position more than 100 mm away from the edge of the steel plate. X-rays are caused to incident on the longitudinal section of this sample (the plane including the plate thickness direction and the rolling direction; the plane perpendicular to the width direction), and the volume fraction of residual austenite is determined from the obtained data. The volume fraction (volume%) of the austenite is identified with the area fraction (area%) of the residual austenite such that the area fraction of the residual austenite is determined. In a case where the amount of the residual austenite is a trace amount and cannot be quantified, the amount of the residual austenite is regarded as 0%. Therefore, the total amount of martensite and bainite is also expressed in area fraction (area%). In the steel plate according to the embodiment, there is almost no possibility that a quantifiable amount of residual austenite will occur in most of chemical composition ranges that can be employed. In such a case, the measurement according to the X-ray diffraction method can be omitted.
The thickness middle portion (t/2 portion) means a position in the steel plate which is half the plate thickness in the plate thickness direction away from the surface of the steel plate. It is the most difficult for martensite and bainite to be generated at the thickness middle portion. Therefore, when the total amount of martensite and bainite is in the range of 99% to 100% at the thickness middle portion, the total amount of martensite and bainite can be regarded as 99% to 100% over the entire steel plate excluding a decarburized layer having a depth (thickness) of about 1 mm from the surface of the steel plate. Therefore, it is sufficient to evaluate the structure only for the thickness middle portion.
FIG. 4 shows an example of the microstructure of the steel plate according to the embodiment. In this figure, ferrite and pearlite are not observed. In a case where the residual austenite cannot be quantified by the X-ray diffraction method, the total amount of ferrite, pearlite, and residual austenite is 0%, so that the total amount of martensite and bainite is 100%.

Tensile Strength: 780 MPa to 930 MPa

Absorbed Energy of Thickness Middle Portion obtained by Charpy Impact Test at -60°C: 69 J or More

In the embodiment, the tensile strength of the steel plate is needed to be 780 MPa to 930 MPa and the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is needed to be 69 J or more. The reason for this will be described below.
Tempered lower bainite most effectively increases the strength and low temperature toughness of the steel plate. Tempered martensite also increases the strength and low temperature toughness of the steel plate. The tempered martensite further increases the strength of the steel plate compared to the tempered lower bainite and does not increase the low temperature toughness of the steel plate as much as the tempered lower bainite does. Therefore, it is most preferable that the steel plate has a microstructure containing tempered lower bainite or a microstructure containing tempered lower bainite and tempered martensite. When the total amount of the tempered lower bainite and the tempered martensite is sufficient, the steel plate may include tempered upper bainite. However, the tempered upper bainite does not increase the strength and low temperature toughness of the steel plate as much as the tempered lower bainite or tempered martensite does. Therefore, it is preferable that the amount of the tempered upper bainite is as small as possible. On the other hand, virgin (untempered) martensite, virgin (untempered) upper bainite, and virgin (untempered) lower bainite greatly decrease the low temperature toughness. Therefore, there is a need to reduce as much as possible the untempered martensite, the untempered upper bainite, and the untempered lower bainite. In the steel plate according to the embodiment, in a case where the steel is tempered, as long as a tempering temperature, which will be described later, does not exceed Ac1, the untempered martensite, the untempered upper bainite, and the untempered lower bainite are not present. That is, in order not to generate the untempered martensite, the untempered upper bainite, and the untempered lower bainite, a heat treatment (tempering) may be performed so as not to cause the tempering temperature, which will be described later, to exceed Ac1. It is preferable that the total amount of the untempered martensite, the untempered upper bainite, and the untempered lower bainite is 0%.
Therefore, there is a need to appropriately control the amounts of the tempered martensite, the tempered upper bainite, the tempered lower bainite, the untempered martensite, the untempered upper bainite, and the untempered lower bainite in the martensite and the bainite described above. However, it is extremely difficult to identify the tempered martensite, the tempered upper bainite, the tempered lower bainite, the untempered martensite, the untempered upper bainite, and the untempered lower bainite in order to measure the fraction of the microstructure using an optical microscope, which is typically used. Therefore, it is substantially impossible to appropriately measure the amounts of the tempered martensite, the tempered upper bainite, the tempered lower bainite, the untempered martensite, the untempered upper bainite, and the untempered lower bainite. However, when the chemical composition of the steel plate satisfies the above conditions, the tensile strength of the steel plate is 780 MPa to 930 MPa, and the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is 69 J or more, the amounts of the six structures can be regarded as being appropriate.
For example, Ts has a relatively strong correlation with the quenched structure, and as shown in FIG. 5, a considerable part of the quenched structure (the amounts of the martensite, the lower bainite, and the upper bainite) is achieved by adjusting Ts. However, Ts alone does not completely represents the quenched structure or determine the structure after being tempered. In addition, the morphology of precipitates (for example, carbides or nitrides) in the microstructure after being tempered (final structure) cannot be expressed only by the chemical composition. However, in the embodiment, since there may be cases where precipitates are extremely fine and the grain size distribution is very wide, measurement of the precipitates is extremely difficult. Therefore, the amounts of the six structures and the morphology of the precipitates are expressed by a combination of the chemical composition, the tensile strength, and the charpy impact test. Accordingly, as described above, the tensile strength of the steel plate is needed to be 780 MPa to 930 MPa and the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is needed to be 69 J or more. The upper limit of the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is not needed to be limited, and may be 400 J or less. The tempered martensite and the untempered martensite are subordinate concepts of martensite, and the tempered upper bainite, the tempered lower bainite, the untempered upper bainite, and the untempered lower bainite are subordinate concepts of bainite.
In order to further optimize the amounts of the six structures and the morphology of the precipitates in the steel plate, the tensile strength of the steel plate is preferably less than 930 MPa. Preferable upper limits of the tensile strength are 900 MPa, 880 MPa, and 870 MPa, which are arranged in order toward the most preferable strength. Similarly, the yield strength of the steel plate is preferably 880 MPa or less. Preferable upper limits of the yield strength are 850 MPa, 830 MPa, and 810 MPa, which are arranged in order toward the most preferable strength. In addition, the yield strength of the steel plate is preferably 665 MPa or more, or 685 MPa or more.
The tensile strength is measured by a tensile test specified in JIS Z 2241. In this test, a No. 14 tensile test piece specified in JIS Z 2201 is taken from a t/4 portion. The longitudinal direction (tensile direction) of the No. 14 tensile test piece is a transverse direction (T direction), that is, a direction (C direction) perpendicular to a rolling direction. The t/4 portion means a position in the steel plate, which is 1/4 of the plate thickness away from the surface of the steel plate in the plate thickness direction.
The absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is measured by the charpy impact test specified in JIS Z 2242. In this test, a charpy impact test piece specified in JIS Z 2242 is taken from the thickness middle portion. The longitudinal direction of the charpy impact test piece is a transverse direction (T direction), that is, a direction (C direction) perpendicular to the rolling direction. In addition, the depth direction of a V notch is the rolling direction. The absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is sometimes abbreviated to vE_-60°C.

Plate Thickness: More Than 200 mm and Not More Than 300 mm

In order to further increase the safety of future large-scale structures, it is preferable that the plate thickness of the steel plate is as large as possible as long as the steel plate can be produced and handled. Therefore, the plate thickness is needed to be more than 200 mm, and preferable lower limits of the plate thickness are 210 mm, 215 mm, 220 mm, 225 mm, and 230 mm, which are arranged in order toward the most preferable thickness. On the other hand, when the plate thickness becomes too large, it becomes more difficult to produce a steel plate having high strength and excellent low temperature toughness, and moreover, the effect of the chemical composition described above on high strength and excellent low temperature toughness is decreased. Therefore, the plate thickness is needed to be 300 mm or less, and preferable upper limits of the plate thickness are 290 mm, 280 mm, 270 mm, and 260 mm, which are arranged in order toward the most preferable thickness. For the above reasons, the plate thickness is needed to be more than 200 mm and not more than 300 mm.
The steel plate according to the embodiment is suitably manufactured by a manufacturing method of a steel plate according to an embodiment described below from the viewpoint of reducing the manufacturing cost.
Next, the manufacturing method of the steel plate (high tensile strength steel plate) according to the embodiment will be described.
First, molten steel having the chemical composition described above is cast to obtain a slab. The slab may also be obtained by continuous casting or by blooming an ingot using a blooming mill.
In a case where the slab is not soaked at a temperature of 1200°C or higher before hot rolling, coarse AlN (AlN of 1.5 µm or more) is remained in the steel, and this coarse AlN lowers the toughness of the steel plate. Therefore, the slab is soaked at 1200°C to 1380°C before hot rolling. In order to further reduce the absolute maximum value of the grain size of the AlN in the thickness middle portion, the soaking temperature is preferably 1250°C or higher. In order to further improve the productivity, the soaking temperature is preferably 1300°C or lower. It is extremely difficult to determine that AlN of 1.5 µm or more is rarely present. For example, although AlN of 1.5 µm or more can be observed with a transmission electron microscope, the region observed by the transmission electron microscope is very small. Therefore, it is impossible to determine that AlN of 1.5 µm or more is rarely present with a realistic number of measurements. On the other hand, it can be confirmed by the absorbed energy (69 J or more) of the thickness middle portion obtained by the charpy impact test at -60°C that AlN of 1.5 µm or more is rarely present.
After the soaking, the slab is hot rolled to obtain a hot rolled steel plate having a plate thickness of more than 200 mm and not more than 300 mm as an intermediate product. Except for the target plate thickness, the hot rolling conditions are not limited. In order to sufficiently add the effect of reduction on the grain size and the like to the thickness middle portion while properly maintaining the quality of the surface of the steel plate, it is preferable to start the hot rolling from a temperature of 950°C to 1250°C.
In order to obtain a microstructure in which the total amount of martensite and bainite is 99% or more, in a quenching treatment, the steel plate is reheated to a temperature of Ac3°C or higher and is water cooled to a temperature of lower than 300°C. When the steel plate is reheated to a temperature of Ac3°C or higher in the quenching treatment, the microstructure of the steel plate is transformed into a single phase of austenite. When the microstructure of the single phase of austenite is quenched, austenite is transformed into martensite or bainite such that the microstructure of the steel plate becomes uniform. In the quenching treatment, in order to obtain a sufficient amount of martensite and lower bainite, the average water cooling rate in the thickness middle portion, while the temperature of the thickness middle portion decreases from 800°C to 500°C, is needed to be 0.4 °C/s to 0.8 °C/s. In addition, the temperature and the water cooling rate in the thickness middle portion can be determined by heat transfer calculation. Ac3 is defined by Equation 10 below. $Ac 3 = 937.2 - 476.2 \times C + 56 \times Si - 19.7 \times Mn - 16.3 \times Cu - 26.6 \times Ni - 4.9 \times Cr + 38.1 \times Mo + 124.8 \times V + 198.4 \times Al + 3315 \times B - 19.1 \times Nb + 136.3 \times Ti$
Here, each element symbol is the amount % of the corresponding element.
In order to increase the toughness of the hot rolled steel plate, in a tempering treatment, the steel plate after the quenching is heated to a temperature of 580°C to Ac1°C, and thereafter water cooled from the temperature of 580°C to Ac1°C to a temperature of lower than 300°C. When the steel plate is heated to a temperature of higher than Ac1°C, austenite is generated in the steel plate, and untempered bainite remains after the tempering treatment, so that the toughness of the steel plate is decreased. On the other hand, when the tempering temperature is lower than 580°C, a sufficient amount of tempered structure cannot be obtained, or tempering embrittlement occurs. Therefore, the toughness of the steel plate is not sufficient. Accordingly, the tempering temperature is needed to be 580°C to Ac1°C. In addition, Ac1 is defined by Equation 9 described above.
In the embodiment, since the plate thickness of the hot rolled steel plate exceeds 200 mm, segregation proceeds and embrittlement occurs during cooling in the tempering treatment. The temperature range in which the embrittlement occurs is mainly 300°C to 500°C. Therefore, the steel plate is needed to pass through this temperature range as rapidly as possible after hot rolling. Accordingly, in the tempering treatment, the average water cooling rate in the thickness middle portion, while the temperature of the thickness middle portion decreases from 500°C to 300°C, is needed to be set to 0.3 °C/s to 0.7 °C/s. In addition, the temperature and the water cooling rate in the thickness middle portion can be determined by heat transfer calculation. In addition, in order to prevent embrittlement on the surface of the steel plate, the temperature of the surface of the steel plate is needed to be set to 580°C or higher when water cooling is started. The temperature of the surface of the steel plate is measured with a radiation-type thermometer.

[Examples]

Steel pieces obtained by melting steels having the chemical compositions shown in Tables 1 to 3 were soaked at soaking temperatures shown in Table 5 and thereafter hot rolled and cooled to room temperature, thereby obtaining hot rolled steel plates as intermediate products. Furthermore, under the conditions shown in Table 5, the steel plates were heated again and quenched to room temperature. Thereafter, the quenched steel plates were tempered under the conditions shown in Table 6 and cooled to room temperature, thereby obtaining hot rolled steel plates (Nos. 1 to 50) as final products. Tables 5 to 6 show the temperatures at which the steel pieces were soaked, the temperatures at which the steel plates were heated for quenching, the average water cooling rates from 800°C to 500°C during quenching, the tempering temperatures, and the temperatures at which water cooling started immediately after tempering (the temperatures of the surfaces of the steel plates), and the average water cooling rates from 500°C to 300°C during water cooling immediately after tempering. The plate thickness of the hot rolled steel plate was 210 mm to 270 mm.
Subsequently, No. 14 tensile test pieces specified in JIS Z 2201 were taken from t/4 portions of all the steel plates so that the longitudinal directions thereof were coincident with the T direction, and a tensile test specified in JIS Z 2241 was performed. In addition, charpy impact test pieces specified in JIS Z 2242 were taken from thickness middle portions of all the steel plates so that the longitudinal directions thereof were coincident with the T direction, and the test was performed. The results are shown in Table 7.
Furthermore, test pieces were taken from the thickness middle portions, and the test pieces were etched with Nital. The etched test piece was observed in the width direction perpendicular to the rolling direction using an optical microscope. The magnification of the optical microscope was 500-fold, and the measurement visual fields were three in number. In addition, the samples were moved only in the rolling direction so that the visual fields did not overlap, and optical micrographs of the three visual fields were taken. The area fractions of ferrite and pearlite were determined from the optical micrographs. As a result, no pearlite was detected in all of Nos. 1 to 50, and the amount of pearlite was 0%. In Nos. 12, 29, 35, and 41, the amount of ferrite was 0.5% or more and less than 1.0%, and in Nos. 37 and 38, the amount of ferrite was 4.5% or more and less than 5.0%. Table 4 shows the amount of ferrite rounded off to the first decimal place.
A test piece was taken from a separate thickness middle portion, the volume fraction of austenite was measured by an X-ray diffraction method, and the volume fraction was assumed to be equal to the area fraction. In the X-ray diffraction method, X-rays were caused to be incident in the width direction of the test piece. Residual austenite was detected in all of Nos. 1 to 50. However, the amount of the residual austenite was a trace amount, and could not be quantified. Therefore, the amount of residual austenite was 0% in all of Nos. 1 to 50.

Cells with underlines in the following tables indicate that the essential conditions of the present invention are not satisfied.

[Table 1]

No	Chemical composition (mass%) [remainder: Fe and impurities]
No	C	Si	Mn	P	S	Cu	Ni	Cr
1	0.10	0.15	1.39	0.0023	0.0010	0.34	3.21	0.56	Example
2	0.09	0.03	1.22	0.0057	0.0009	0.40	4.40	0.96	Example
3	0.11	0.25	1.05	0.0058	0.0014	0.25	3.31	0.86	Example
4	0.11	0.25	1.05	0.0047	0.0020	0.35	3.26	0.97	Example
5	0.11	0.05	1.05	0.0031	0.0008	0.28	3.99	0.98	Example
6	0.10	0.21	1.49	0.0032	0.0020	0.34	3.56	0.95	Example
7	0.14	0.18	0.85	0.0019	0.0006	0.01	3.78	0.82	Example
8	0.12	0.13	0.81	0.0055	0.0010	0.50	3.02	0.91	Example
9	0.13	0.05	1.00	0.0075	0.0014	0.00	4.39	0.77	Example
10	0.15	0.03	0.85	0.0088	0.0006	0.00	4.29	0.84	Example
11	0.13	0.25	1.02	0.0063	0.0016	0.31	3.99	0.92	Example
12	0.06	0.20	1.05	0.0074	0.0010	0.27	3.89	0.99	Comparative Example
13	0.16	0.25	1.10	0.0094	0.0014	0.30	3.45	0.78	Comparative Example
14	0.12	0.40	1.42	0.0047	0.0019	0.23	4.08	0.69	Comparative Example
15	0.13	0.17	0.72	0.0088	0.0010	0.50	3.13	0.90	Comparative Example
16	0.11	0.26	1.74	0.0061	0.0011	0.08	3.30	0.54	Comparative Example
17	0.11	0.26	1.59	0.0120	0.0020	0.19	3.03	0.51	Comparative Example
18	0.08	0.17	1.00	0.0063	0.0036	0.34	3.82	0.66	Comparative Example
19	0.12	0.13	1.41	0.0049	0.0016	0.61	3.12	0.60	Comparative Example
20	0.10	0.03	1.57	0.0043	0.0017	0.08	2.68	0.62	Comparative Example
21	0.15	0.21	1.21	0.0098	0.0015	0.41	3.70	0.40	Comparative Example
22	0.09	0.22	0.82	0.0015	0.0008	0.47	3.17	1.15	Comparative Example
23	0.12	0.22	1.13	0.0021	0.0016	0.43	3.98	0.91	Comparative Example
24	0.10	0.24	0.82	0.0060	0.0011	0.25	3.17	0.66	Comparative Example
25	0.09	0.27	1.23	0.0067	0.0012	0.10	3.53	0.75	Comparative Example
26	0.09	0.25	0.89	0.0094	0.0014	0.00	4.03	0.59	Comparative Example
27	0.10	0.22	1.12	0.0050	0.0012	0.40	3.15	0.95	Comparative Example
28	0.10	0.15	1.45	0.0093	0.0012	0.05	3.32	0.58	Comparative Example
29	0.11	0.20	1.27	0.0084	0.0006	0.13	3.24	0.66	Comparative Example
30	0.11	0.26	1.38	0.0013	0.0009	0.36	3.22	0.85	Comparative Example
31	0.11	0.03	0.88	0.0033	0.0019	0.16	3.80	0.91	Comparative Example
32	0.10	0.22	1.42	0.0043	0.0007	0.41	3.90	0.52	Comparative Example
33	0.15	0.26	1.03	0.0034	0.0008	0.02	3.10	0.59	Comparative Example
34	0.11	0.05	1.21	0.0053	0.0014	0.17	4.11	0.80	Comparative Example
35	0.09	0.27	0.93	0.0092	0.0016	0.27	4.20	0.93	Comparative Example
36	0.14	0.17	0.91	0.0069	0.0019	0.38	4.49	0.66	Comparative Example
37	0.09	0.14	0.89	0.0096	0.0014	0.22	4.14	0.54	Comparative Example
38	0.11	0.15	0.86	0.0078	0.0009	0.16	3.45	0.78	Comparative Example
39	0.15	0.11	1.58	0.0084	0.0006	0.46	3.81	0.94	Comparative Example
40	0.10	0.22	1.42	0.0043	0.0007	0.41	3.90	0.52	Comparative Example
41	0.09	0.05	1.02	0.0056	0.0015	0.05	3.98	0.70	Comparative Example
42	0.13	0.22	1.28	0.0049	0.0008	0.36	3.92	0.89	Comparative Example
43	0.11	0.04	0.86	0.0031	0.0017	0.45	3.60	0.54	Comparative Example
44	0.11	0.03	1.49	0.0063	0.0018	0.31	3.63	0.97	Comparative Example
45	0.10	0.15	1.39	0.0023	0.0010	0.34	3.21	0.56	Comparative Example
46	0.09	0.03	1.22	0.0057	0.0009	0.40	4.40	0.96	Comparative Example
47	0.10	0.15	1.39	0.0023	0.0010	0.34	3.21	0.56	Comparative Example
48	0.09	0.03	1.22	0.0057	0.0009	0.40	4.40	0.96	Comparative Example
49	0.11	0.25	1.05	0.0047	0.0020	0.35	3.26	0.97	Comparative Example
50	0.10	0.15	1.39	0.0023	0.0010	0.34	3.21	0.56	Comparative Example

[Table 2]

No	Chemical composition (mass%) [remainder: Fe and impurities]
No	Mo	V	Al	N	B	Nb	Ti	Others
1	0.88	0.027	0.055	0.0053	0.0006	0.034	0.000	Ca 0.0028	Example
2	0.95	0.030	0.085	0.0059	0.0013	0.000	0.018		Example
3	0.72	0.041	0.075	0.0065	0.0013	0.000	0.000		Example
4	0.83	0.049	0.055	0.0043	0.0009	0.014	0.008		Example
5	0.86	0.030	0.040	0.0031	0.0015	0.000	0.000	REM 0.0021	Example
6	0.60	0.011	0.081	0.0038	0.0008	0.024	0.000	Mg 0.0024	Example
7	0.95	0.000	0.048	0.0033	0.0020	0.047	0.000		Example
8	0.81	0.018	0.065	0.0045	0.0012	0.000	0.012	Ca 0.0014 REM 0.0010	Example
9	0.91	0.000	0.025	0.0029	0.0010	0.000	0.000		Example
10	0.54	0.016	0.081	0.0060	0.0017	0.000	0.000		Example
11	0.88	0.044	0.068	0.0038	0.0009	0.014	0.018		Example
12	0.65	0.023	0.059	0.0067	0.0011	0.038	0.000		Comparative Example
13	0.61	0.000	0.045	0.0043	0.0018	0.043	0.000		Comparative Example
14	0.79	0.018	0.052	0.0068	0.0015	0.044	0.016	Ca 0.0015	Comparative Example
15	0.77	0.000	0.058	0.0061	0.0016	0.021	0.020		Comparative Example
16	0.99	0.048	0.039	0.0035	0.0007	0.033	0.000		Comparative Example
17	0.80	0.015	0.024	0.0036	0.0013	0.035	0.000		Comparative Example
18	0.94	0.044	0.027	0.0027	0.0015	0.018	0.000		Comparative Example
19	0.74	0.021	0.031	0.0041	0.0012	0.034	0.012	Mg 0.0009	Comparative Example
20	0.71	0.043	0.082	0.0021	0.0019	0.018	0.000		Comparative Example
21	0.61	0.000	0.031	0.0038	0.0019	0.023	0.000		Comparative Example
22	0.83	0.000	0.081	0.0062	0.0012	0.010	0.014	REM 0.0018	Comparative Example
23	0.40	0.044	0.028	0.0042	0.0013	0.050	0.009		Comparative Example
24	1.21	0.000	0.075	0.0062	0.0020	0.015	0.000		Comparative Example
25	0.94	0.069	0.073	0.0030	0.0008	0.031	0.000		Comparative Example
26	0.99	0.044	0.014	0.0040	0.0017	0.020	0.000	Ca 0.0013	Comparative Example
27	0.63	0.016	0.090	0.0048	0.0014	0.036	0.008		Comparative Example
28	0.75	0.031	0.076	0.0010	0.0019	0.000	0.000		Comparative Example
29	0.65	0.047	0.025	0.0087	0.0015	0.028	0.008		Comparative Example
30	0.80	0.016	0.042	0.0047	0.0002	0.000	0.014	Mg 0.0018	Comparative Example
31	0.73	0.000	0.073	0.0034	0.0031	0.000	0.000		Comparative Example
32	0.65	0.017	0.062	0.0042	0.0013	0.020	0.000	Ca 0.0019	Comparative Example
33	0.79	0.000	0.029	0.0046	0.0020	0.000	0.000		Comparative Example
34	0.90	0.000	0.068	0.0026	0.0014	0.008	0.000		Comparative Example
35	0.84	0.023	0.083	0.0033	0.0006	0.031	0.000		Comparative Example
36	0.76	0.029	0.035	0.0067	0.0008	0.014	0.000		Comparative Example
37	0.71	0.000	0.050	0.0055	0.0006	0.000	0.000		Comparative Example
38	0.55	0.028	0.070	0.0059	0.0013	0.038	0.000		Comparative Example
39	0.93	0.025	0.076	0.0046	0.0015	0.014	0.000		Comparative Example
40	0.65	0.017	0.062	0.0042	0.0013	0.020	0.000	Ca 0.0019	Comparative Example
41	0.60	0.045	0.072	0.0053	0.0014	0.000	0.000		Comparative Example
42	0.87	0.035	0.064	0.0031	0.0011	0.000	0.000		Comparative Example
43	0.58	0.020	0.081	0.0053	0.0014	0.000	0.000		Comparative Example
44	0.91	0.032	0.037	0.0038	0.0010	0.022	0.000		Comparative Example
45	0.88	0.027	0.055	0.0053	0.0006	0.034	0.000	Ca 0.0028	Comparative Example
46	0.95	0.030	0.085	0.0059	0.0013	0.000	0.018		Comparative Example
47	0.88	0.027	0.055	0.0053	0.0006	0.034	0.000	Ca 0.0028	Comparative Example
48	0.95	0.030	0.085	0.0059	0.0013	0.000	0.018		Comparative Example
49	0.83	0.049	0.055	0.0043	0.0009	0.014	0.008		Comparative Example
50	0.88	0.027	0.055	0.0053	0.0006	0.034	0.000	Ca 0.0028	Comparative Example

[Table 3]

No	Ti/N	Ts	Ceq	Ac1	x	β	Plate thickness (mm)
1	0.0	392	0.86	602	59	36	210	Example
2	3.1	424	1.00	581	86	55	270	Example
3	0.0	417	0.85	617	58	35	210	Example
4	1.9	402	0.90	623	71	42	210	Example
5	0.0	410	0.94	598	73	47	250	Example
6	0.0	421	0.92	590	74	45	230	Example
7	0.0	386	0.89	613	64	39	220	Example
8	2.7	399	0.84	635	52	32	210	Example
9	0.0	381	0.93	584	67	43	240	Example
10	0.0	400	0.86	585	46	29	240	Example
11	4.7	428	0.96	602	85	51	240	Example
12	0.0	399	0.84	600	48	29	210	Comparative Example
13	0.0	398	0.87	605	63	38	210	Comparative Example
14	2.4	401	0.94	580	89	51	210	Comparative Example
15	3.3	408	0.83	635	50	31	210	Comparative Example
16	0.0	381	0.94	590	81	48	210	Comparative Example
17	0.0	420	0.85	599	61	36	210	Comparative Example
18	0.0	391	0.85	604	52	32	210	Comparative Example
19	2.9	387	0.88	600	62	38	210	Comparative Example
20	0.0	383	0.82	605	46	30	210	Comparative Example
21	0.0	407	0.83	585	48	29	210	Comparative Example
22	2.3	408	0.87	638	59	36	210	Comparative Example
23	2.1	392	0.87	585	54	32	210	Comparative Example
24	0.0	417	0.84	638	55	33	210	Comparative Example
25	0.0	402	0.89	607	67	40	210	Comparative Example
26	0.0	406	0.83	603	49	29	210	Comparative Example
27	1.7	417	0.84	618	56	34	210	Comparative Example
28	0.0	392	0.84	594	52	32	210	Comparative Example
29	0.9	416	0.82	603	50	30	210	Comparative Example
30	3.0	397	0.91	608	79	47	210	Comparative Example
31	0.0	395	0.85	605	50	32	230	Comparative Example
32	0.0	421	0.86	575	56	34	220	Comparative Example
33	0.0	447	0.81	619	51	31	220	Comparative Example
34	0.0	340	0.94	585	73	47	220	Comparative Example
35	0.0	447	0.90	600	67	40	240	Comparative Example
36	0.0	352	0.91	581	62	38	210	Comparative Example
37	0.0	400	0.78	590	37	23	210	Comparative Example
38	0.0	418	0.77	612	37	23	210	Comparative Example
39	0.0	384	1.08	583	130	81	250	Comparative Example
40	0.0	421	0.86	575	56	34	220	Comparative Example
41	0.0	396	0.80	588	38	24	220	Comparative Example
42	0.0	392	0.99	592	98	59	230	Comparative Example
43	0.0	423	0.75	601	32	20	220	Comparative Example
44	0.0	394	1.00	592	95	61	250	Comparative Example
45	0.0	392	0.86	602	59	36	210	Comparative Example
46	3.1	424	1.00	581	86	55	270	Comparative Example
47	0.0	392	0.86	602	59	36	210	Comparative Example
48	3.1	424	1.00	581	86	55	270	Comparative Example
49	1.9	402	0.90	623	71	42	210	Comparative Example
50	0.0	392	0.86	602	59	36	210	Comparative Example

[Table 4]

No	Fraction (%) of martensite + bainite	Remainder (%)	Other structures
1	100	0		Example
2	100	0		Example
3	100	0		Example
4	100	0		Example
5	100	0		Example
6	100	0		Example
7	100	0		Example
8	100	0		Example
9	100	0		Example
10	100	0		Example
11	100	0		Example
12	99	1	1% ferrite	Comparative Example
13	100	0		Comparative Example
14	100	0		Comparative Example
15	100	0		Comparative Example
16	100	0		Comparative Example
17	100	0		Comparative Example
18	100	0		Comparative Example
19	100	0		Comparative Example
20	100	0		Comparative Example
21	100	0		Comparative Example
22	100	0		Comparative Example
23	100	0		Comparative Example
24	100	0		Comparative Example
25	100	0		Comparative Example
26	100	0		Comparative Example
27	100	0		Comparative Example
28	100	0		Comparative Example
29	99	1	1% ferrite	Comparative Example
30	100	0		Comparative Example
31	100	0		Comparative Example
32	100	0		Comparative Example
33	100	0		Comparative Example
34	100	0		Comparative Example
35	99	1	1% ferrite	Comparative Example
36	100	0		Comparative Example
37	95	5	5% ferrite	Comparative Example
38	95	5	5% ferrite	Comparative Example
39	100	0		Comparative Example
40	100	0		Comparative Example
41	99	1	1% ferrite	Comparative Example
42	100	0		Comparative Example
43	100	0		Comparative Example
44	100	0		Comparative Example
45	100	0		Comparative Example
46	100	0		Comparative Example
47	100	0		Comparative Example
48	100	0		Comparative Example
49	85	15	15% ferrite	Comparative Example
50	100	0		Comparative Example

[Table 5]

No	Ac3 (°C)	Steel piece soaking temperature (°C)	Reheating temperature (°C)	Water cooling rate (°C/s) between 800°C and 500°C
1	826	1250	930	0.8	Example
2	807	1250	930	0.5	Example
3	834	1300	980	0.8	Example
4	833	1300	930	0.8	Example
5	801	1250	910	0.6	Example
6	810	1350	910	0.7	Example
7	811	1380	910	0.7	Example
8	830	1250	950	0.8	Example
9	792	1250	910	0.8	Example
10	777	1200	910	0.7	Example
11	811	1250	930	0.6	Example
12	829	1250	930	0.7	Comparative Example
13	790	1250	910	0.8	Comparative Example
14	808	1250	910	0.8	Comparative Example
15	823	1250	930	0.8	Comparative Example
16	827	1250	930	0.8	Comparative Example
17	823	1250	930	0.8	Comparative Example
18	830	1300	930	0.8	Comparative Example
19	805	1300	930	0.8	Comparative Example
20	839	1300	930	0.8	Comparative Example
21	782	1300	910	0.8	Comparative Example
22	846	1300	950	0.8	Comparative Example
23	784	1300	910	0.8	Comparative Example
24	863	1300	980	0.8	Comparative Example
25	847	1250	950	0.8	Comparative Example
26	832	1250	950	0.8	Comparative Example
27	834	1250	950	0.8	Comparative Example
28	831	1250	930	0.8	Comparative Example
29	821	1250	930	0.8	Comparative Example
30	820	1250	930	0.8	Comparative Example
31	814	1300	910	0.8	Comparative Example
32	804	1300	910	0.7	Comparative Example
33	817	1300	930	0.7	Comparative Example
34	800	1300	910	0.7	Comparative Example
35	823	1250	930	0.7	Comparative Example
36	775	1250	910	0.6	Comparative Example
37	807	1250	910	0.8	Comparative Example
38	820	1250	930	0.8	Comparative Example
39	786	1250	930	0.6	Comparative Example
40	804	1300	910	0.8	Comparative Example
41	814	1250	910	0.7	Comparative Example
42	802	1250	930	0.7	Comparative Example
43	810	1250	910	0.7	Comparative Example
44	800	1250	910	0.6	Comparative Example
45	826	1250	930	0.8	Comparative Example
46	807	1250	910	0.5	Comparative Example
47	826	1250	930	0.8	Comparative Example
48	807	1150	930	0.7	Comparative Example
49	833	1300	930	0.3	Comparative Example
50	826	1250	930	0.8	Comparative Example

[Table 6]

No	Tempering temperature (°C)	Steel plate surface temperature (°C) at time of start of water cooling	Water cooling rate (°C/s) between 500°C and 300°C
1	600	594	0.6	Example
2	580	580	0.3	Example
3	610	603	0.6	Example
4	620	617	0.6	Example
5	595	590	0.4	Example
6	590	585	0.5	Example
7	610	608	0.5	Example
8	625	623	0.6	Example
9	580	580	0.6	Example
10	585	584	0.5	Example
11	600	595	0.4	Example
12	590	585	0.5	Comparative Example
13	600	591	0.6	Comparative Example
14	580	580	0.6	Comparative Example
15	600	599	0.6	Comparative Example
16	585	582	0.6	Comparative Example
17	590	590	0.6	Comparative Example
18	600	600	0.6	Comparative Example
19	590	588	0.6	Comparative Example
20	600	584	0.6	Comparative Example
21	580	580	0.6	Comparative Example
22	630	629	0.6	Comparative Example
23	580	580	0.6	Comparative Example
24	630	621	0.6	Comparative Example
25	600	600	0.6	Comparative Example
26	595	595	0.6	Comparative Example
27	615	613	0.6	Comparative Example
28	590	590	0.6	Comparative Example
29	600	594	0.6	Comparative Example
30	600	600	0.6	Comparative Example
31	600	597	0.6	Comparative Example
32	580	580	0.5	Comparative Example
33	610	604	0.5	Comparative Example
34	580	580	0.5	Comparative Example
35	590	590	0.5	Comparative Example
36	580	580	0.4	Comparative Example
37	590	590	0.6	Comparative Example
38	610	610	0.6	Comparative Example
39	580	580	0.4	Comparative Example
40	570	570	0.6	Comparative Example
41	585	580	0.5	Comparative Example
42	590	585	0.5	Comparative Example
43	580	580	0.5	Comparative Example
44	590	590	0.4	Comparative Example
45	550	550	0.6	Comparative Example
46	610	605	0.3	Comparative Example
47	600	560	0.6	Comparative Example
48	580	580	0.5	Comparative Example
49	620	617	0.6	Comparative Example
50	600	590	0.2	Comparative Example

[Table 7]

No	Yield strength (MPa)	Tensile strength (MPa)	vE_-60°C (J)
1	790	841	107	Example
2	708	795	97	Example
3	799	880	83	Example
4	791	868	131	Example
5	763	851	120	Example
6	739	812	104	Example
7	793	867	118	Example
8	738	809	167	Example
9	803	913	81	Example
10	765	883	92	Example
11	813	898	70	Example
12	649	741	65	Comparative Example
13	850	925	12	Comparative Example
14	776	870	33	Comparative Example
15	691	762	63	Comparative Example
16	796	883	11	Comparative Example
17	778	829	27	Comparative Example
18	709	799	41	Comparative Example
19	798	857	31	Comparative Example
20	701	785	58	Comparative Example
21	727	771	66	Comparative Example
22	786	851	26	Comparative Example
23	711	769	66	Comparative Example
24	807	883	44	Comparative Example
25	780	842	38	Comparative Example
26	791	773	15	Comparative Example
27	780	858	31	Comparative Example
28	748	820	15	Comparative Example
29	746	769	59	Comparative Example
30	685	733	65	Comparative Example
31	717	767	32	Comparative Example
32	695	854	16	Comparative Example
33	737	757	53	Comparative Example
34	857	892	20	Comparative Example
35	765	770	36	Comparative Example
36	832	885	14	Comparative Example
37	673	768	56	Comparative Example
38	641	735	63	Comparative Example
39	879	956	13	Comparative Example
40	696	871	10	Comparative Example
41	730	848	42	Comparative Example
42	857	922	51	Comparative Example
43	722	767	61	Comparative Example
44	818	913	31	Comparative Example
45	804	923	12	Comparative Example
46	660	844	25	Comparative Example
47	699	839	31	Comparative Example
48	712	810	55	Comparative Example
49	666	740	53	Comparative Example
50	801	854	11	Comparative Example

In Nos. 1 to 11, the final products had the chemical composition and microstructure of the present invention and had excellent low temperature toughness and high strength. As can be seen from Nos. 1 to 11, when Ti/N is reduced to 3.4 or less, the low temperature toughness can be further enhanced.
In No. 12, since the C content was low, the tensile strength and impact absorbed energy were low. On the other hand, in No. 13, since the C content was high, the impact absorbed energy was very low. In No. 14, since the Si content was high, the impact absorbed energy was low.
In No. 15, since the Mn content was low, the tensile strength and impact absorbed energy were low. On the other hand, in No. 16, since the Mn content was high, the impact absorbed energy was very low.
In No. 17, since the P content was high, the impact absorbed energy was low. In No. 18, since the S content was high, the impact absorbed energy was low.
In No. 19, since the Cu content was high, the impact absorbed energy was low. In No. 20, since the Ni content was low, the impact absorbed energy was low.
In No. 21, since the Cr content was low, the tensile strength and impact absorbed energy were low. On the other hand, in No. 22, since the Cr content was high, the impact absorbed energy was low. In No. 23, since the Mo content was low, the tensile strength and the impact absorbed energy were low. On the other hand, in No. 24, since the Mo content was high, the impact absorbed energy was low. In No. 25, since the V content was high, the impact absorbed energy was low.
In No. 26, since the Al content was low, the tensile strength and the impact absorbed energy were low. On the other hand, in No. 27, since the Al content was high, the impact absorbed energy was low. In No. 28, since the N content was low, the impact absorbed energy was low. On the other hand, in No. 29, since the N content was high, the tensile strength and impact absorbed energy were low. In No. 30, since the B content was low, the tensile strength and impact absorbed energy were low. On the other hand, in No. 31, since the B content was excessive, the tensile strength and impact absorbed energy were low.
In No. 32, since Ac1 was low, the impact absorbed energy was low. In Nos. 34 and 36, since Ts was low, the impact absorbed energy was low. In Nos. 33 and 35, since Ts was high, the tensile strength and impact absorbed energy were low. In Nos. 37 and 38, since Ceq was low, the tensile strength and the impact absorbed energy were low. In No. 39, since Ceq was high, the tensile strength was excessively high and the impact absorbed energy was low. In No. 40, since Ac1 was low, the impact absorbed energy was low. In No. 40, a low tempering temperature was used so that the steel would not be tempered in the two-phase region.
In No. 41, since x was low, the impact absorbed energy was low. In No. 42, since x was high, the impact absorbed energy was low. In No. 43, since β as well as x was low, the tensile strength and the impact absorbed energy were low. In No. 44, since β as well as x was high, the impact absorbed energy was low.
In No. 45, since the tempering temperature was lower than 580°C, the impact absorbed energy was low.
In No. 46, since the tempering was performed at a temperature of higher than Ac1°C, the impact absorbed energy was low. In No. 47, since the temperature of the surface of the steel plate was lower than 580°C when water cooling was started, the impact absorbed energy was low.
In No. 48, since the soaking temperature of the slab was lower than 1200°C, the impact absorbed energy was low. In No. 49, the water cooling rate in the thickness middle portion while the temperature of the thickness middle portion decreased from 800°C to 500°C during the quenching was lower than 0.4 °C/s. Therefore, the tensile strength and the impact absorbed energy were low. In No. 50, the water cooling rate in the thickness middle portion while the temperature of the thickness middle portion decreased from 500°C to 300°C during the tempering was lower than 0.3 °C/s. Therefore, the impact absorbed energy was low.

[Industrial Applicability]

According to the present invention, since a high tensile strength steel plate having excellent low temperature toughness and a plate thickness of more than 200 mm is provided, the safety of a structure having a larger scale can be further increased. Therefore, the industrial applicability of the present invention is great.

Claims

A steel plate having a chemical composition comprising, by mass%:
C: 0.08% to 0.15%;

Mn: 0.80% to 1.60%;

Ni: 3.00% to 4.50%;

Cr: 0.50% to 1.00%;

Mo: 0.50% to 1.00%;

Al: 0.020% to 0.085%;

N: 0.0020% to 0.0070%;

B: 0.0005% to 0.0020%;

P: 0.000% to 0.010%;

S: 0.000% to 0.003%;

Si: 0.00% to 0.30%;

Cu: 0.00% to 0.50%;

V: 0.000% to 0.050%;

Nb: 0.000% to 0.050%;

Ti: 0.000% to 0.020%;

Ca: 0.0000% to 0.0030%;

Mg: 0.0000% to 0.0030%;

REM: 0.0000% to 0.0030%; and

a remainder including Fe and impurities,

wherein a plate thickness t mm is more than 200 mm and not more than 300 mm,

in the chemical composition, Ts defined by Equation 1 is 380 to 430, Ceq defined by Equation 2 is 0.80 to 1.05, Ac1 defined by Equation 3 is 580 to 647, and x defined by Equation 4 is 46 to 90,

a total amount of martensite and bainite is 99% to 100% by area%, and

a tensile strength is 780 MPa to 930 MPa, and an absorbed energy of a thickness middle portion obtained by a charpy impact test at -60°C is 69 J or more. $Ts = 750 - 4240 \times {(t / 2)}^{- 1.4} \times (80 \times C + 10 \times Mn + 7 \times Ni + 13 \times Cr + 13 \times Mo - 40 \times Si)$
$Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5$
$Ac 1 = 720 - 25 \times C + 22 \times Si - 40 \times Mn - 30 \times Ni + 20 \times Cr + 25 \times Mo$
$x = C^{1 / 2} \times (1 + 0.64 \times Si) \times (1 + 4.10 \times Mn) \times (1 + 0.27 \times Cu) \times (1 + 0.52 \times Ni) \times (1 + 2.33 \times Cr) \times (1 + 3.14 \times Mo)$
The steel plate according to claim 1,
wherein the chemical composition further satisfies
Ti/N ≤ 3.4.
The steel plate according to claim 1 or 2,
wherein the chemical composition further satisfies
C: 0.09% to 0.13%.
The steel plate according to any one of claims 1 to 3,
wherein the chemical composition further satisfies
Mn: 0.80% to 1.30%.
The steel plate according to any one of claims 1 to 4,
wherein the chemical composition further satisfies
Ni: 3.60% to 4.50%.
The steel plate according to any one of claims 1 to 5,
wherein the chemical composition further satisfies
Cr: 0.75% to 1.00%.
The steel plate according to any one of claims 1 to 6,
wherein the chemical composition further satisfies
Mo: 0.70% to 1.00%.
The steel plate according to any one of claims 1 to 7,
wherein the chemical composition further satisfies
Si: 0.00% to 0.10%.
The steel plate according to any one of claims 1 to 8,
wherein the chemical composition further satisfies
V: 0.020% to 0.050%.
The steel plate according to any one of claims 1 to 9,
wherein the chemical composition further satisfies
Ti: 0.000% to 0.004%.
The steel plate according to any one of claims 1 to 10,
wherein the chemical composition further satisfies a condition where the Ts is 395 to 415.
The steel plate according to any one of claims 1 to 11,
wherein the chemical composition further satisfies a condition where
the Ceq is 0.85 to 1.05.