JP7559733B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel plate, and in particular to a thick steel plate suitable for use at cryogenic temperatures, which can stably ensure excellent cryogenic toughness over a wide range of plate thicknesses, and to a method for manufacturing the same. The steel plate of the present invention can be suitably used for structures used in cryogenic environments, such as liquefied gas storage tanks for ships and land use.

When hot-rolled steel plates are used in structures such as liquefied gas storage tanks, the environment in which they are used is extremely low, so the steel plates are required to have not only excellent strength but also excellent toughness at extremely low temperatures (cryogenic toughness). For example, when hot-rolled steel plates are used in liquefied natural gas storage tanks, they must have excellent toughness at extremely low temperatures of -164°C or lower, which is the boiling point of liquefied natural gas. If the steel material has poor cryogenic toughness, there is a risk that it will not be possible to maintain safety as a cryogenic storage structure, so there is a high demand for improved cryogenic toughness of the steel plates used. To meet this demand, 7% Ni or 9% Ni steel plates have traditionally been used.

A 7% Ni steel sheet is proposed in, for example, Patent Document 1.
Patent Document 1 discloses a steel plate for cryogenic use that contains more than 5.0% and less than 10.0% Ni and predetermined amounts of C, Si, Mn, and Al. In the steel plate disclosed in Patent Document 1, the average absorbed energy per unit area vE-196 is 1.25 J/mm2 ^or more over a plate thickness range of 6 to 50 mm.

JP 2011-219848 A

The inventors conducted extensive research into high Ni steel plate (hereinafter also referred to as 7% Ni steel plate) with Ni: 6.0 to 7.5% and found that when the plate is quenched and tempered directly after rolling in order to improve productivity, the absorbed energy in Charpy tests and the like decreases and the risk of brittle fracture increases. However, Patent Document 1 does not address these issues, particularly the occurrence of brittle fracture.

The present invention has been made in consideration of the above circumstances, and aims to provide a high-strength steel plate that has excellent cryogenic toughness and brittle crack suppression performance, on the premise that it is manufactured using a reduced process that utilizes direct quenching and tempering.

In order to solve the above problems, the present inventors have conducted extensive research into the composition and structure of 7% Ni steel sheet and have obtained the following findings.
(a) The above-mentioned decrease in absorbed energy is due to the formation of Mn-enriched and -depleted regions, which causes cracks (herein also referred to as separation) that are perpendicular to the fracture surface and parallel to the rolled surface.
(b) The above-mentioned brittle crack initiation originates in Mn-enriched regions, particularly from unstable austenite (hereinafter also referred to as γ) that is likely to form there.
(c) In order to stably increase the absorbed energy, Mn should be limited to less than 0.20%, the concentration of the band-like Mn-enriched region should be reduced, and the occurrence of separation caused by Mn segregation bands should be reduced.
(d) In order to reduce the occurrence of brittle cracks, in addition to ensuring the Charpy absorbed energy in the low temperature range, it is necessary to limit the amount of Mn and reduce the concentration of the band-like Mn-enriched region while suppressing the formation of γ, which is the cause of the occurrence of brittle cracks.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[1] In mass%,
C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.05% or more and less than 0.20%;
Ni: 6.0% or more and 7.5% or less,
Cr: 0.01% or more and 1.00% or less,
Mo: 0.01% or more and 0.50% or less,
P: 0.03% or less,
S: 0.005% or less,
N: 0.0010% or more and 0.0080% or less; Al: 0.008% or more and 0.100% or less; and the balance being Fe and unavoidable impurities.
A steel plate in which the amount of retained austenite at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is less than 1.7%.

[2] The composition further includes, in mass%,
Cu: 0.40% or less,
Nb: 0.05% or less,
V: 0.05% or less,
The steel plate according to the above [1], containing one or more selected from Ti: 0.03% or less and B: 0.0030% or less.

[3] The composition further comprises, in mass%,
Ca: 0.007% or less,
The steel sheet according to the above [1] or [2], containing one or more selected from REM: 0.010% or less and Mg: 0.070% or less.

[4] A method for producing a steel plate, comprising the steps of: hot rolling a steel material having the component composition according to any one of [1] to [3] above, at a cumulative reduction rate of 15 to 75% at 870°C or less and a final rolling end temperature of 830 to 700°C at the surface temperature of the steel plate to form a steel plate; directly quenching the steel plate at a temperature at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction, the average cooling rate in a temperature range of 600°C or less and 300°C or more being 3°C/s or more and the cooling end temperature being 300°C or less; and tempering the steel plate at a temperature of 550°C or more and less than Ac ₁ point; and the amount of retained austenite at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is less than 1.7%.

According to the present invention, a steel plate having excellent cryogenic toughness and brittle crack suppression ability can be provided with high productivity. By using the steel plate of the present invention for steel structures used in low temperature and cryogenic environments, such as liquefied gas storage tanks, for example, LNG tanks and liquefied _CO2 tanks, the safety of the steel structures can be improved, which is an industrially significant advantage.

The following is a detailed description of an embodiment of the present invention. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited thereto.

[Component composition]
The steel plate of the present invention has a predetermined composition. In addition, it is preferable that the steel material used for manufacturing the steel plate of the present invention also has the above-mentioned predetermined composition. Each element contained in this composition will be explained below. In this specification, "%" as a unit of the content of each element means "mass%" unless otherwise specified.

C: 0.01% or more and 0.15% or less C is an element that has the effect of improving the strength of the steel plate. To obtain this effect, the C content is set to 0.01% or more. Preferably, it is set to 0.03% or more. On the other hand, if the C content exceeds 0.15%, the cryogenic toughness of the steel plate decreases. Therefore, the C content is set to 0.15% or less. Preferably, it is set to 0.12% or less.

Si: 0.01% or more and 0.50% or less Si is an element that contributes to improving the strength of the steel plate and also acts as a deoxidizer. In order to exert these effects, the Si content is set to 0.01% or more. Preferably, it is set to 0.03% or more. On the other hand, if the Si content is excessively high, the toughness decreases. Therefore, the Si content is set to 0.50% or less. Preferably, it is set to 0.30% or less.

Mn: 0.05% or more and less than 0.20% Mn is an element that is effective in increasing the hardenability of steel and increasing the strength of steel plate. To obtain this effect, Mn is added at 0.05% or more. Preferably, it is 0.10% or more. On the other hand, if Mn is contained at 0.20% or more, the probability of austenite being generated locally increases, and absorbed energy may decrease, so it is limited to less than 0.20%.

Ni: 6.0% or more and 7.5% or less Ni is an element that is extremely effective in improving the cryogenic toughness of steel plate. To achieve this, the Ni content is set to 6.0% or more. Preferably, it is set to 6.5% or more. More preferably, it is set to 7.0% or more. On the other hand, since Ni is an expensive element, the higher the Ni content, the higher the steel plate cost. Therefore, in the present invention, the Ni content is set to 7.5% or less.

Cr: 0.01% or more and 1.00% or less Cr is an element that can improve the strength of the steel plate without significantly impairing the cryogenic toughness. To obtain the above effect, the Cr content is set to 0.01% or more. Preferably, it is set to 0.30% or more. However, if the Cr content exceeds 1.00%, the cryogenic toughness of the steel plate decreases. Therefore, the Cr content is set to 1.00% or less. Preferably, it is set to 0.80% or less.

Mo: 0.01% or more and 0.50% or less Like Cr, Mo is an element that can improve the strength of the steel plate without significantly impairing the cryogenic toughness. For this purpose, the Mo content is set to 0.01% or more. Preferably, it is more than 0.10%. On the other hand, if the Mo content exceeds 0.50%, the cryogenic toughness is rather reduced. For this reason, the Mo content is set to 0.50% or less. Preferably, it is 0.30% or less. More preferably, it is 0.25% or less.

P: 0.03% or less P is an inevitable impurity and a harmful element that adversely affects the cryogenic toughness of steel plates. For example, in order to obtain a sound base material and welded joint when a steel plate is welded to form a welded structure, it is preferable to reduce the P content as much as possible. Therefore, the P content is suppressed to 0.03% or less. In addition, from the viewpoint of cryogenic toughness, the lower the P content, the better, so the lower limit is not particularly limited and may be 0%, but even in that case, it is acceptable to contain it as an inevitable impurity. On the other hand, excessive reduction causes an increase in cost, so from the viewpoint of cost, it is preferable to set the lower limit of the P content to 0.001%.

S: 0.005% or less S forms MnS in steel and significantly deteriorates cryogenic toughness, so the upper limit is set to 0.005%, and it is desirable to reduce the S content as much as possible. The S content is preferably set to 0.002% or less. On the other hand, the lower the S content, the better, so there is no particular restriction on the lower limit, and it may be 0%, but even in that case, it is permitted to contain S as an inevitable impurity.

N: 0.0010% or more and 0.0080% or less N forms precipitates in steel, and if the content exceeds 0.0080%, it causes a decrease in the toughness of the base material. However, N is also an element that contributes to the refinement of the base material by forming AlN, and such an effect can be obtained by making the N content 0.0010% or more. Therefore, the N content is made 0.0010% or more and 0.0080% or less. The N content is preferably 0.0020% or more. It is preferably 0.0060% or less.

Al: 0.008% or more and 0.100% or less Al is an element contained in a deoxidizer. If the Al content is less than 0.008%, the effect as a deoxidizer is poor. In addition, Al is also an element that contributes to the refinement of the base material by forming AlN. Therefore, the Al content is set to 0.008% or more. Preferably, it is set to 0.020% or more. On the other hand, if the Al content exceeds 0.100%, the cleanliness of the steel is impaired, and the toughness, especially the Charpy absorbed energy at extremely low temperatures, is reduced. Therefore, the Al content is set to 0.100% or less. Preferably, it is set to 0.050% or less.

The composition of the components in one embodiment of the present invention can be a composition consisting of the above-mentioned elements in the specified amounts, with the remainder consisting of Fe and unavoidable impurities.

In another embodiment of the present invention, the above composition may further contain, optionally, one or more elements selected from Cu, Nb, V, Ti, and B, preferably in the amounts described below.

Cu: 0.40% or less Cu is an element that has the effect of increasing the strength of the steel plate by improving the hardenability. However, if the Cu content exceeds 0.40%, the cryogenic toughness of the steel plate decreases, and the properties of the steel (slab) surface after casting deteriorate. Therefore, when Cu is added, the Cu content is preferably 0.40% or less. More preferably, it is 0.30% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but in order to obtain the above effect, it is preferable that the Cu content is 0.10% or more.

Nb: 0.05% or less Nb is an effective element for increasing the strength of steel plate by precipitation strengthening. However, if the Nb content is excessively high, the cryogenic toughness of the steel plate decreases. Therefore, when Nb is added, the Nb content is preferably 0.05% or less. More preferably, it is 0.03% or less. On the other hand, the lower limit of the Nb content is not particularly limited, but in order to obtain the above effect, the Nb content is preferably 0.010% or more.

V: 0.05% or less Like Nb, V is an effective element for increasing the strength of steel plate by precipitation strengthening. However, if the V content is excessively high, the cryogenic toughness of the steel plate decreases. Therefore, when V is added, the V content is preferably 0.05% or less, and more preferably 0.04% or less. On the other hand, the lower limit of the V content is not particularly limited, but in order to obtain the above effect, the V content is preferably 0.010% or more.

Ti: 0.03% or less Ti is an element that has the effect of increasing the toughness of the weld without decreasing the mechanical properties of the base material when steel plates are welded to form a welded structure. For this purpose, it is preferable to add 0.003% or more. On the other hand, if the content exceeds 0.03%, the toughness is decreased, so it is preferable to contain Ti in the range of 0.03% or less.

B: 0.0030% or less B is an element that improves hardenability when added in small amounts. To effectively exert this effect, it is preferable to contain B at 0.0003% or more. On the other hand, if the B content exceeds 0.0030%, toughness deteriorates. Therefore, when B is contained, the content is preferably 0.0030% or less.

In another embodiment of the present invention, the above composition may further contain one or more elements selected from Ca, REM, and Mg, preferably in the amounts described below.

Ca: 0.007% or less Ca is an element that has the effect of improving the cryogenic toughness of the steel plate by controlling the form of inclusions in the steel. However, if Ca is excessive, it impairs the cleanliness of the steel and reduces the Charpy absorbed energy (hereinafter also referred to as Charpy toughness) at cryogenic temperatures. Therefore, when Ca is added, the Ca content is preferably 0.007% or less. More preferably, it is 0.004% or less. On the other hand, the lower limit of the Ca content is not particularly limited, but in order to obtain the above effect, it is preferably 0.001% or more.

REM: 0.010% or less Like Ca, REM (rare earth metal) is an element that has the effect of improving the cryogenic toughness of steel plate by controlling the form of inclusions in steel. However, if the REM is excessive, the cleanliness of the steel is impaired and the Charpy toughness is reduced. Therefore, when REM is added, the REM content is preferably 0.010% or less. More preferably, it is 0.008% or less. On the other hand, the lower limit of the REM content is not particularly limited, but in order to obtain the above effect, the REM content is preferably 0.001% or more.
Here, REM is a collective term for 17 elements including 15 lanthanoid elements, Y, and Sc, and these elements may be contained alone or in combination. The content of REM means the total content of these elements.

Mg: 0.070% or less
Like Ca and REM, Mg is an element that controls the morphology of inclusions in steel and improves the cryogenic toughness of steel plates. However, excessive Mg impairs the cleanliness of steel. Therefore, when Mg is added, the Mg content is preferably 0.070% or less, and more preferably 0.004% or less. On the other hand, the lower limit of the Mg content is Although not particularly limited, in order to obtain the above effects, the Mg content is preferably 0.001% or more.

[Microstructure]
The steel plate of the present invention has a structure in which the amount of retained austenite (hereinafter also referred to as γ) at a depth position of ¼ of the plate thickness t (hereinafter also referred to as ¼t) from the surface of the steel plate in the plate thickness direction is less than 1.7%. In other words, if the amount of retained γ is 1.7% or more, brittle cracks are likely to occur.

In addition, it is preferable that the structure of the steel plate is mainly composed of martensite and bainite, specifically, that the total area ratio of bainite and martensite is 98.3% or more. As mentioned above, a structure mainly composed of bainite + martensite makes it easier to obtain sufficient strength while ensuring excellent cryogenic toughness. The ratio of bainite to martensite can be any ratio.

The thickness of the steel plate is not particularly limited and can be any thickness. For example, it is preferable that it be 6 mm or more and 50 mm or less.

[Mechanical properties]
(Tensile strength)
The lower limit of the tensile strength of the steel sheet does not need to be particularly limited, but it is preferable that the lower limit is 690 MPa, and more preferably 720 MPa or more. On the other hand, the upper limit of the tensile strength does not need to be particularly limited, but it is preferable that the upper limit is 930 MPa, and more preferably 900 MPa or less.
The tensile strength can be measured by the method described in the Examples section below.

(Cryogenic toughness)
The toughness value of the steel plate does not need to be particularly limited, but the Charpy absorbed energy at -196°C (vE _-196°C ) is preferably 200 J or more in a full-size Charpy impact test. On the other hand, it is preferably 350 J or less. More preferably, it is 280 J or less. Also, in a half-size Charpy impact test, it is preferable that vE _-196°C is 100 J or more. On the other hand, the upper limit is less than 200 J. More preferably, it is 150 J or less.

(Brittle crack generation suppression ability)
As for the brittle crack initiation suppression performance of a steel plate, it is preferable that in a CTOD test, the maximum load point is reached without a sudden drop in load.

[Production method]
Next, a method for producing a steel sheet of the present invention will be described. In the following description, unless otherwise specified, the temperature refers to the temperature at the center of the sheet thickness. The temperature at the center of the sheet thickness can be calculated by heat transfer calculation from the surface temperature of the steel sheet measured by a radiation thermometer, for example.
That is, the steel sheet of the present invention can be suitably manufactured by sequentially carrying out the following steps (1) to (4).
(1) Heating the steel material (2) Hot rolling (3) Quenching (accelerated cooling)
(4) Tempering

(1) Heating of steel material First, it is preferable to heat a steel material having the above-mentioned composition to a temperature of 900°C or more and 1200°C or less. The method for producing the steel material is not particularly limited, but for example, the steel material can be produced by melting molten steel having the above-mentioned composition by a normal method and casting it. Melting can be carried out by any method, such as a converter, an electric furnace, or an induction furnace. Furthermore, from the viewpoint of productivity, it is preferable to carry out casting by a continuous casting method, but it can also be carried out by an ingot casting-breakdown rolling method. For example, a steel slab can be used as the steel material.
Here, the heating of the steel material may be carried out after the steel material obtained by a method such as casting is once cooled, or the obtained steel material may be directly subjected to heating without being cooled.

If the heating temperature of the steel material is less than 900°C, the deformation resistance of the steel material is high, which may increase the load on the rolling mill in the subsequent hot rolling, making it difficult to perform the hot rolling. Therefore, it is preferable to heat the steel material to 900°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1200°C, oxidation of the steel becomes significant, and the loss caused by removing the oxide film due to oxidation increases, which may result in a decrease in yield. Therefore, it is preferable to heat the steel material to 1200°C or lower.

(2) Hot rolling [reduction rate: cumulative reduction rate at 870°C or less is 15 to 75%]
In hot rolling, if the cumulative reduction is less than 15% in the austenite non-recrystallization temperature range of 870°C or less, the structure is not sufficiently refined and the toughness is reduced. On the other hand, if the cumulative reduction exceeds 75%, rolling at the following finishing temperatures becomes difficult. Therefore, the cumulative reduction at 870°C or less is set to 15 to 75%, and preferably 30 to 70%.
[Final rolling end temperature: 700 to 830°C]
In hot rolling, if the final rolling end temperature is less than 700°C, separation due to texture is likely to occur, and toughness is reduced. On the other hand, if the final rolling end temperature exceeds 830°C, sufficient rolling reduction in the non-recrystallized region becomes difficult, a fine texture cannot be obtained, and strength is reduced. The final thickness of the hot-rolled steel sheet is not particularly limited, but as described above, it is preferably 6 mm or more and 50 mm or less.

(3) Quenching (accelerated cooling)
The hot-rolled steel sheet after the hot rolling is directly quenched. It is essential that the average cooling rate in the temperature range of 600° C. or less and 300° C. or more at the temperature at the ¼t position of the steel sheet during the direct quenching is 3° C./s or more.

That is, in direct quenching, if the average cooling rate is less than 3°C/s, it is difficult to obtain the desired transformed structure, and it is difficult to obtain sufficient strength and toughness. On the other hand, although there is no particular upper limit to the average cooling rate, if the average cooling rate is higher than 200°C/s, it becomes difficult to control the temperature at each position in the steel plate, and material variations are likely to occur in the plate width direction and rolling direction. As a result, material properties such as tensile properties and toughness are likely to vary. Therefore, it is preferable to set the average cooling rate to 200°C/s or less.

In addition, in direct quenching, if the cooling end temperature is higher than 300°C at (1/4)t, unstable residual gamma is likely to be generated. Therefore, the cooling end temperature is set to 300°C or less at (1/4)t. By performing accelerated cooling under these conditions, the hot-rolled steel sheet is well quenched.

The cooling process in direct quenching is not particularly limited and can be carried out by any method. For example, air cooling and/or water cooling can be used. For water cooling, any cooling method using water (e.g., spray cooling, mist cooling, laminar cooling, etc.) can be used.

(4) Tempering Next, the hot-rolled steel sheet after quenching is tempered. The tempering temperature is 550°C or higher and lower than Ac ₁ point. If the tempering temperature is lower than 550°C, the tempering is insufficient and the Charpy toughness is reduced. If the tempering temperature is Ac ₁ point or higher, the strength is reduced and unstable γ is generated, which reduces the ability to suppress the occurrence of brittle cracks.

The Ac ₁ point can be determined by the following formula (1).
A _C1 point (°C) = 750.8-26.6×C+17.6×Si-11.6×Mn-22.9×Cu-23×Ni+24.1×Cr+22.5×Mo-39.7×V-5.7×Ti +232.4×Nb-169.4×Al...(1)
In the above formula (1), the element symbols represent the content (mass%) of each element, and when the element is not contained, the value is set to 0.

Any heating method can be used for the heating in the tempering process as long as the heating temperature can be controlled as described above. One example of a heating method is furnace heating. There are no particular limitations on the furnace heating, and a general heat treatment furnace can be used.

After the tempering temperature is reached, the material may be held at the tempering temperature for any period of time, after which any cooling may begin. When holding at the tempering temperature, the holding time is not particularly limited, but is preferably 5 minutes or more.

Steel plates were manufactured according to the procedure described below, and their properties were evaluated.
First, molten steel having the composition shown in Table 1 was melted in a converter and a steel slab (thickness: 200 mm) was produced as a steel material by a continuous casting method. The _AC1 point (°C) calculated by the above formula (1) is also shown in Table 1.

Next, the obtained steel slabs were heated and hot rolled according to the conditions shown in Table 2 to produce hot-rolled steel sheets of each thickness (final thickness), which were then quenched and tempered to produce the product steel sheets.

The microstructure, amount of retained γ, tensile strength (TS), Charpy absorbed energy at −196° C. (vE _{−196° C.} ), and brittle crack initiation suppression ability (CTOD test) of each of the product steel sheets thus obtained were evaluated according to the following methods. The evaluation results are shown in Table 2.

[Microstructure]
A test piece for microstructure observation was taken from each steel sheet so that the 1/4t position was the observation position. The test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror polished. Next, after performing nital etching, the structure was observed with a scanning electron microscope at magnifications of 2000 and 10000 times and images were taken. The obtained images were analyzed to identify the microstructure.

[Residual γ content after deep-cooling treatment]
Five test pieces for X-ray diffraction were taken from each steel plate parallel to the plate surface from the 1/4t position, and after immersion in liquid nitrogen for 30 minutes, the test pieces were ground and chemically polished so that the 1/4t position became the measurement surface, and then subjected to X-ray diffraction. The diffraction intensities of the (200), (211) planes of α-Fe and the (200), (220), and (311) planes of γ-Fe appearing in the symmetric reflection X-ray diffraction pattern were obtained, the volume fraction of γ-Fe was calculated, and the average value of the five test pieces was obtained to obtain the amount of residual γ (volume fraction).

(Tensile strength)
A JIS No. 4 tensile test piece was taken from the 1/4t position of the steel plate. Using this tensile test piece, a tensile test was carried out in accordance with the provisions of JIS Z 2241 to evaluate the tensile strength (TS) of the steel plate. If the tensile strength was 690 MPa or more, the steel plate was deemed to have high strength and passed the test.

(Cryogenic toughness)
A V-notch test piece was taken from the 1/4t position of the steel plate in accordance with the provisions of JIS Z 2202. Using this V-notch test piece, a Charpy impact test was performed in accordance with the provisions of JIS Z 2242 to determine the Charpy absorbed energy at -196°C (vE _-196°C ). The Charpy absorbed energy can be regarded as an index of the cryogenic toughness of the steel plate. For the Charpy impact test, three test pieces were taken along the rolling direction at the front end, rear end, and center of each steel plate in the rolling direction, and measurements were performed on each test piece. The individual measurement results and average values are shown in Table 2. In this full-size Charpy impact test, if the vE _-196°C of each test piece was 200J or more, it was evaluated as having excellent Charpy toughness and was passed.

(Ability to suppress brittle fracture occurrence)
For steel plates with a thickness of 15 mm, a test piece of 10 x 10 x 55 mm is taken from the depth position of 1/2 of the plate thickness (1/2t), and for other steel plates, a test piece of 10 x 10 x 55 mm is taken from the depth position of 1/4t according to ISO 12135. The specimens were collected and tested in accordance with ISO 12135. The test temperature was -165°C. Those that did not experience unstable fracture (brittle fracture) were deemed to have passed the test.

Claims (4)

In mass percent,
C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.05% or more and less than 0.20%;
Ni: 7.0 % or more and 7.5% or less,
Cr: 0.01% or more and 1.00% or less,
Mo: 0.01% or more and 0.50% or less,
P: 0.03% or less,
S: 0.005% or less,
N: 0.0010% or more and 0.0080% or less; Al: 0.008% or more and 0.100% or less; and the balance being Fe and unavoidable impurities.
A steel plate in which the amount of retained austenite at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is less than 1.7%. The composition further includes, in mass%,
Cu: 0.40% or less,
Nb: 0.05% or less,
V: 0.05% or less,
The steel plate according to claim 1, further comprising one or more selected from the group consisting of Ti: 0.03% or less and B: 0.0030% or less. The composition further comprises, in mass%,
Ca: 0.007% or less,
The steel sheet according to claim 1 or 2, containing one or more selected from the group consisting of REM: 0.010% or less and Mg: 0.070% or less. A method for producing a steel plate, comprising the steps of: hot rolling a steel material having the component composition according to any one of claims 1 to 3 at a cumulative reduction rate of 15 to 75% at 870°C or less and a final rolling end temperature of 830 to 700°C at the surface temperature of the steel plate to obtain a steel plate; quenching the steel plate at a temperature at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction, the average cooling rate in a temperature range of 600°C or less and 300°C or more being 3°C/s or more and the cooling end temperature being 300°C or less, and the steel plate being tempered to a temperature range of 550°C or more and less than Ac ₁ point; and an amount of retained austenite at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is less than 1.7%.