JP7396507B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel plate, and in particular to a high-strength thick steel plate suitable for use at cryogenic temperatures, capable of stably ensuring excellent cryogenic toughness and brittle crack initiation suppressing performance over a wide range of plate thicknesses, and a method for producing the same. . The steel plate and manufacturing method of the present invention can be suitably used for structures used in cryogenic environments, such as liquefied gas storage tanks for ships and land.

When hot-rolled steel plates are used in structures such as liquefied gas storage tanks, the environment in which they are used is at extremely low temperatures, so not only the strength of the steel plates but also their toughness at extremely low temperatures (cryogenic toughness) are required. It is required to be excellent at. For example, when a hot-rolled steel plate is used in a liquefied natural gas storage tank, it is necessary to ensure excellent toughness at extremely low temperatures below -164° C., which is the boiling point of liquefied natural gas. If the cryogenic toughness of the steel material is poor, there is a risk that the safety of the structure for cryogenic storage cannot be maintained, so there is a high demand for improving the cryogenic toughness of the applied steel plate. To meet this requirement, conventionally, 7% Ni or 9% Ni steel plates have been used.

A 7% Ni steel plate is proposed in Patent Document 1, for example. Patent Document 1 discloses a thick steel plate for cryogenic use that contains more than 5.0% to less than 10.0% Ni and predetermined amounts of C, Si, Mn, and Al. In the thick steel plate disclosed in Patent Document 1, the average value of absorbed energy vE-196 per unit area is 1.25 J/mm ² or more over the plate thickness of 6 to 50 mm.

JP2011-219848A

As a result of intensive investigation by the present inventors on so-called 7% Ni steel plates (hereinafter also referred to as 7% Ni steel plates) containing approximately 6.0 to 7.5% Ni, we found that rolling It has been found that when the steel sheet is manufactured by directly quenching and tempering, the risk of unstable fracture (brittle fracture) of the steel sheet due to a decrease in absorbed energy (toughness) and the occurrence of brittle cracks in Charpy tests etc. increases. However, Patent Document 1 does not consider these problems, particularly the occurrence of brittle fracture.

The present invention was made in view of the above-mentioned circumstances, and an object of the present invention is to provide a high-strength steel plate that has excellent cryogenic toughness and brittle crack suppression performance, on the premise that it is manufactured in a reduced process using direct quenching and tempering. shall be.

In order to solve the above problems, the present inventors conducted intensive research on the composition and structure of a 7% Ni steel plate, and obtained the following knowledge. That is,
(a) The above-mentioned decrease in absorbed energy is mainly due to separation that occurs in the Mn-enriched region due to the formation of a Mn-enriched region and a diluted region. Here, separation is a result of brittle fracture that occurs perpendicular to the fracture surface and parallel to the rolling surface during toughness measurement tests such as the Charpy impact test and CTOD test. means a crack in the
(b) The above-mentioned brittle crack generation originates from the Mn-enriched region, particularly from unstable austenite (hereinafter also referred to as γ) that tends to form there.
(c) To increase the absorption energy, limit Mn to 0.40% or less, lower the Mn concentration in the Mn-concentrated region (Mn segregation band) that forms in a band shape, and cause separation due to the Mn segregation band. to reduce
(d) To reduce the occurrence of brittle cracks, limit Mn to 0.40% or less, reduce the Mn concentration in the Mn-enriched region that forms in a band shape, and reduce the instability that causes brittle cracks. To suppress the production of γ.

The present invention was completed based on the above findings, and the gist thereof is as follows.
[1] In mass%,
C: 0.01-0.15%,
Si: 0.01 to 0.50%,
Mn: 0.05-0.40%,
Ni: 6.0% or more and less than 7.0%,
Cr: 0.01-1.00%,
Mo: 0.01-0.50%,
P: 0.030% or less,
S: 0.0050% or less,
N: 0.0010~0.0080% and Al: 0.008~0.100%
, with the remainder consisting of Fe and unavoidable impurities,
The amount of retained austenite at a depth of 1/4 of the plate thickness in the thickness direction from the surface of the steel plate is less than 1.7% in terms of volume percentage,
A steel plate whose Charpy absorbed energy at -196°C in a Charpy impact test in accordance with JIS Z 2242 is 200 J or more when a full-size test piece is used and 100 J or more when a half-size test piece is used.

[2] The component 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 [1] above, containing one or more selected from Ti: 0.03% or less and B: 0.0030% or less.

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

[4] A steel material having the composition according to any one of [1] to [3] above, with a cumulative rolling reduction of 870°C or less of 15 to 75% and a final rolling end temperature of 830 to 700 at the steel plate surface temperature. After hot-rolling the hot-rolled sheet at a temperature of 600°C or less, the hot-rolled sheet is heated at a temperature of 600°C or less at a depth of 1/4 of the thickness in the thickness direction from the surface of the hot-rolled sheet. Steel plate subjected to direct quenching with an average cooling rate of 3°C/s or more in a temperature range of 3°C or higher and a cooling stop temperature of 300°C or lower, and then tempered to a temperature range of 550°C or higher and less than the Ac ₁ transformation point. The amount of retained austenite at a depth of 1/4 of the thickness in the thickness direction from the surface of the plate is less than 1.7% in terms of volume percentage, and the Charpy absorption at -196°C was determined by the Charpy impact test in accordance with JIS Z 2242. A method for producing a steel plate, wherein the energy is 200 J or more when a full-size test piece is used, and 100 J or more when a half-size test piece is used.

According to the present invention, a high-strength steel plate with excellent cryogenic toughness and brittle crack generation suppression performance can be provided with high productivity. By applying the steel plate of the present invention and its manufacturing method to steel structures used in cryogenic environments, such as liquefied gas storage tanks, the safety of the steel structures can be improved, resulting in a significant industrial improvement. brings about the effect of

Embodiments of the present invention will be specifically described below. Note that the following description shows preferred embodiments 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. Further, the steel material used in the manufacturing method of the present invention, which is suitably used for manufacturing the steel plate of the present invention, also has the above-described predetermined composition. Each element included in this component composition will be explained below. In addition, unless otherwise specified, in this specification, "%" as a unit of content of each element means "mass %".

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. In order to obtain this effect, the C content is set to 0.01% or more. Preferably it is 0.03% or more. On the other hand, when the C content exceeds 0.15%, the cryogenic toughness of the steel plate decreases. In addition, the brittle crack generation suppression performance also decreases. Therefore, the C content is set to 0.15% or less. Preferably it is 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 is also an element that acts as a deoxidizing agent. In order to exhibit these effects, the Si content is set to 0.01% or more. Preferably it is 0.03% or more. On the other hand, when the Si content becomes excessively high, the toughness decreases. In addition, the brittle crack generation suppression performance also decreases. Therefore, the Si content is set to 0.50% or less. Preferably it is 0.30% or less.

Mn: 0.05% or more and 0.40% or less Mn is an element effective in improving the hardenability of steel and increasing the strength of steel sheets. In order to obtain this effect, Mn is added in an amount of 0.05% or more. Preferably it is 0.10% or more. On the other hand, if Mn is contained in an amount exceeding 0.40%, separation tends to occur and the toughness decreases, so the content is limited to 0.40% or less. Furthermore, if Mn is contained in an amount exceeding 0.40%, the Mn concentration in the Mn segregation band tends to increase, and the brittle crack generation suppressing performance also decreases, so the content is limited to 0.40% or less. The amount of Mn is preferably 0.35% or less, more preferably less than 0.35%, and still more preferably less than 0.20%.

Ni: 6.0% or more and less than 7.0% Ni is an extremely effective element for improving the cryogenic toughness of a steel plate, and also contributes to improving brittle crack generation suppression performance. For this purpose, the Ni content is set to 6.0% or more. Preferably it is 6.5% or more. On the other hand, since Ni is an expensive element, the cost of the steel sheet increases as its content increases. Therefore, in the present invention, the Ni content is set to less than 7.0%.

Cr: 0.01% or more and 1.00% or less Cr is an element that can improve the strength of a steel plate without significantly impairing cryogenic toughness. In order to obtain the above effects, the Cr content is set to 0.01% or more. Preferably it is 0.30% or more. However, when the Cr content exceeds 1.00%, the cryogenic toughness of the steel sheet decreases. In addition, the brittle crack generation suppression performance also decreases. Therefore, the Cr content is set to 1.00% or less. Preferably it is 0.80% or less.

Mo: 0.01% or more and 0.50% or less Mo, like Cr, is an element that can improve the strength of a steel plate without significantly impairing 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, when the Mo content exceeds 0.50%, the cryogenic toughness is rather reduced. In addition, the brittle crack generation suppression performance also decreases. Therefore, 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: 0.030% or less P is an unavoidable impurity and a harmful element that adversely affects the cryogenic toughness and brittle crack generation suppression performance of the steel plate. For example, in order to obtain a healthy base material and welded joint when welding steel plates into a welded structure, it is preferable to reduce the P content as much as possible. Therefore, the P content is suppressed to 0.030% 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 unavoidable impurity. Ru. On the other hand, excessive reduction causes an increase in cost, so from the viewpoint of cost, the lower limit of the P content is preferably 0.001%.

S: 0.0050% or less S forms MnS in steel and significantly deteriorates cryogenic toughness and brittle crack initiation suppressing performance, so it is desirable to reduce S as much as possible, with an upper limit of 0.0050%. The S content is preferably 0.0020% or less. On the other hand, since the lower the S content, the better, the lower limit is not particularly limited and may be 0%, but even in that case, it is allowed to be included as an unavoidable impurity.

N: 0.0010% or more and 0.0080% or less N forms precipitates in steel, and when its content exceeds 0.0080%, it causes a decrease in the toughness of the base metal. In addition, the brittle crack generation suppression performance also decreases. However, N is also an element that contributes to grain refinement of the base material by forming AlN, and such an effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content is set to 0.0010% or more and 0.0080% or less. The N content is preferably 0.0020% or more. Preferably it is 0.0060% or less.

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

The component composition in one embodiment of the present invention may be such that, in addition to the above-mentioned predetermined amount of elements, the balance may be Fe and unavoidable impurities.

In another embodiment of the present invention, the component composition optionally further contains one or more selected from Cu, Nb, V, Ti, and B, preferably in the amounts described below. I can do it.

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, when the Cu content exceeds 0.40%, not only the cryogenic toughness of the steel plate decreases, but also the surface properties of the steel material (slab) after casting deteriorate. Therefore, when adding Cu, 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 effects, the Cu content is preferably 0.10% or more.

Nb: 0.05% or less Nb is an effective element that increases the strength of steel sheets through precipitation strengthening. However, when the Nb content becomes excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when adding Nb, 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 effects, the Nb content is preferably 0.01% or more.

V: 0.05% or less Like Nb, V is an effective element that increases the strength of steel sheets through precipitation strengthening. However, when the V content becomes excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when adding V, the V content is preferably 0.05% or less. More preferably, it is 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 effects, the V content is preferably 0.01% or more.

Ti: 0.03% or less Ti is an element that has the effect of increasing the toughness of the welded part without reducing the mechanical properties of the base metal when welding steel plates to form a welded structure. For this purpose, it is preferable to add 0.003% or more. On the other hand, if it exceeds 0.03%, the toughness will be reduced, so Ti is preferably contained in a range of 0.03% or less.

B: 0.0030% or less B is an element that improves hardenability when added in a small amount. In order to effectively exhibit this effect, it is preferable to contain B in an amount of 0.0003% or more. On the other hand, when the B content exceeds 0.0030%, toughness deteriorates. Therefore, when B is contained, it is preferable that the content is 0.0030% or less.

Further, in another embodiment of the present invention, the above component composition may optionally further contain one or more 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 a steel plate by controlling the form of inclusions in steel. However, when Ca becomes excessive, the cleanliness of the steel is impaired and the Charpy absorbed energy at extremely low temperatures (hereinafter also referred to as Charpy toughness) is reduced. Therefore, when adding Ca, 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 effects, it is preferably 0.001% or more.

REM: 0.010% or less REM (rare earth metal), like Ca, is an element that has the effect of improving the cryogenic toughness of a steel sheet by controlling the form of inclusions in steel. However, excessive REM impairs the cleanliness of the steel and reduces its Charpy toughness. 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 effects, the REM content is preferably 0.001% or more.
Here, REM is a general term for 17 elements including 15 elements of lanthanoids, Y and Sc, and these elements can be contained alone or in combination. Note that the content of REM means the total content of these elements.

Mg: 0.070% or less Mg, like Ca and REM, is an element that has the effect of improving the cryogenic toughness of the steel sheet by controlling the form of inclusions in the steel. However, excessive Mg impairs the cleanliness of the steel and lowers its Charpy toughness. Therefore, when adding Mg, the Mg content is preferably 0.070% or less. More preferably, it is 0.004% or less. On the other hand, the lower limit of the Mg content is not particularly limited, but 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 retained austenite amount (hereinafter also referred to as residual γ amount) at a depth of 1/4 of the plate thickness t (hereinafter also referred to as 1/4t) from the surface of the steel plate in the thickness direction. It has a tissue that is less than 1.7% by volume. As a typical position, when the amount of residual γ at the above position is 1.7% or more, it can be seen that a relatively large amount of unstable γ is generated, especially in the concentrated region of the Mn segregation band, resulting in brittle cracking. is more likely to occur. From the viewpoint of improving brittle crack generation suppression performance, the amount of residual γ at the above position is preferably as small as possible, preferably 1.5% or less, more preferably 1.0% or less, even more preferably 0.5% or less, and 0.5% or less. It is more preferably 1% or less, and even more preferably 0%.
Note that the amount of residual γ can be measured by the method described in Examples described later, and may be measured on either the front surface or the back surface of the steel plate.

Further, the structure of the steel plate is preferably a structure mainly composed of martensite and bainite, specifically, the total area ratio of martensite and bainite is preferably 98.3% or more, and 99.0% or more. More preferably, it is 100%. This is because, as described above, if the structure is mainly composed of martensite and/or bainite, sufficient strength can be easily obtained while ensuring excellent cryogenic toughness. Note that the ratio of martensite to bainite can be set arbitrarily without any problem. Moreover, the types of structures other than martensite and bainite are not particularly limited.

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

[Mechanical properties]
(Tensile strength)
Although the lower limit of the tensile strength of the steel plate does not need to be particularly limited, it is preferable that the lower limit is 690 MPa. More preferably, it is 720 MPa or more. On the other hand, there is no need to particularly limit the upper limit of the tensile strength, but it is preferable that the upper limit is 930 MPa. More preferably, it is 900 MPa or less.
Incidentally, the tensile strength can be measured by the method described in Examples described later.

(cryogenic toughness)
The toughness value of the steel plate requires that the Charpy absorbed energy (vE _-196° C) at -196°C is 200 J or more in a Charpy impact test using a full-size test piece. On the other hand, it is preferably 350J or less, more preferably 280J or less. Furthermore, in the Charpy impact test using a half-size test piece, vE _-196°C needs to be 100 J or more. On the other hand, it is preferably less than 200J, more preferably 150J or less.
Note that the cryogenic toughness can be measured by the method described in Examples described later.

(Brittle crack generation suppression performance)
As for the brittle crack generation suppression performance of a steel plate, it is preferable that the maximum load point be reached without a sudden load drop in the CTOD (Crack Tip Opening Displacement) test, as described in the Examples below.

[Production method]
Next, a method for manufacturing a steel plate according to the present invention will be explained. The manufacturing method of the present invention uses a steel material having the above-mentioned predetermined composition; cumulative reduction rate and final rolling end temperature in hot rolling; average cooling rate and cooling stop temperature in quenching; and temperature range in tempering. This is a method of obtaining a steel plate that satisfies the above-mentioned predetermined amount of residual γ by controlling the amount to a predetermined condition.
In the following description, unless otherwise specified, temperature refers to the temperature at the center of the plate thickness. The temperature at the center of the plate thickness can be determined, for example, by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer.
As an example, the steel plate of the present invention can be suitably manufactured by sequentially performing the following steps (1) to (4).
(1) Heating of steel material (2) Hot rolling (3) Quenching (accelerated cooling)
(4) Tempering

(1) Heating the steel material First, the steel material needs to have the above-mentioned composition. The steel material is preferably heated to a temperature of 900°C or higher and 1200°C or lower. Although the method for manufacturing the steel material is not particularly limited, it can be manufactured, for example, by melting and casting molten steel having the above-mentioned composition by a conventional method. Melting can be performed by any method such as a converter, electric furnace, induction furnace, etc. In addition, casting is preferably carried out by a continuous casting method from the viewpoint of productivity, but it can also be carried out by an ingot-forming-decomposition rolling method. As the steel material, for example, a steel slab can be used.
Here, the steel material may be heated after once cooling the steel material obtained by a method such as casting, or the obtained steel material may be directly heated 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, so the load on the rolling mill during subsequent hot rolling increases, making it difficult to perform hot rolling. Therefore, the heating temperature of the steel material is preferably 900°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1200° C., the oxidation of the steel becomes significant, and the loss due to removing the oxide film due to oxidation increases, resulting in a decrease in yield. Therefore, the heating temperature of the steel material is preferably 1200°C or less.

(2) Hot rolling [Reduction rate: Cumulative reduction rate of 870°C or less is 15% or more and 75% or less]
In hot rolling, if the cumulative reduction ratio in the austenite non-recrystallization temperature range of 870° C. or lower is less than 15%, the structure will not be refined sufficiently and the toughness will decrease. In addition, strength and brittle crack generation suppression performance are also reduced. On the other hand, if the cumulative rolling reduction exceeds 75%, rolling at the final rolling end temperature described later becomes difficult. Therefore, the cumulative rolling reduction rate below 870°C is set to 15 to 75%. The cumulative reduction rate is preferably 30% or more, preferably 70% or less, and more preferably 30 to 70%.
[Final rolling end temperature: Steel plate surface temperature of 700°C or more and 830°C or less]
In hot rolling, if the final rolling end temperature (finishing temperature) is less than 700° C. at the surface temperature of the steel sheet, separation due to texture tends to occur, resulting in a decrease in toughness. In addition, the brittle crack generation suppression performance also decreases. On the other hand, when the final rolling end temperature exceeds 830° C. in terms of the surface temperature of the steel sheet, it becomes difficult to sufficiently reduce the unrecrystallized region, making it impossible to obtain a fine structure and reducing the toughness. In addition, the brittle crack generation suppression performance also decreases.
Note that the thickness of the hot rolled sheet obtained after hot rolling (corresponding to the final thickness of the 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)
[Average cooling rate: 1/4 t, 3°C/s or more in the temperature range of 600 to 300°C]
The hot rolled sheet after hot rolling is directly quenched. In this direct hardening, it is important that the average cooling rate in the temperature range of 600° C. or lower and 300° C. or higher at the 1/4 t position of the steel plate is 3° C./s or higher.

That is, in direct quenching, if the above-mentioned average cooling rate is less than 3° C./s, it is difficult to obtain a desired transformed structure, and it is difficult to obtain sufficient strength and toughness. Furthermore, it is also difficult to obtain sufficient brittle crack generation suppression performance. The average cooling rate is preferably 5°C/s or more, more preferably 10°C/s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but if the average cooling rate is higher than 200°C/s, it will be difficult to control the temperature at each position within the steel plate, resulting in variations in material quality in the width direction and rolling direction. It becomes easier. As a result, variations in material properties such as tensile properties and toughness tend to occur. Therefore, the average cooling rate is preferably 200° C./s or less.

[Cooling stop temperature: 300℃ or less at 1/4t]
Further, in direct quenching, if the cooling stop temperature is higher than 300° C. at 1/4 t, unstable γ is likely to be generated and remain, and as a result, brittle crack generation suppression performance cannot be improved. Moreover, toughness also decreases. Therefore, the cooling stop temperature is set to 300° C. or less at 1/4 t. The cooling stop temperature is preferably 250°C or lower, more preferably 200°C or lower. By directly performing accelerated cooling on the hot rolled sheet after hot rolling under such conditions, the hot rolled sheet can be quenched well.

The cooling treatment in hardening can be performed by any method without particular limitation as long as the above-mentioned conditions are satisfied. For example, one or both of air cooling and water cooling can be used. As water cooling, any cooling method using water (for example, spray cooling, mist cooling, laminar cooling, etc.) can be used.

(4) Tempering [Tempering temperature: 550°C or higher and less than Ac ₁ transformation point]
Next, the hot-rolled sheet after quenching is tempered. The tempering temperature is 550° C. or higher and lower than the Ac ₁ transformation point. If the tempering temperature is less than 550°C, the tempering will be insufficient and the Charpy toughness will decrease. In addition, when the tempering temperature exceeds the Ac ₁ transformation point, the ability to suppress brittle crack generation decreases due to a decrease in strength and the generation of unstable γ.

Note that the Ac ₁ transformation point can be determined by the following equation (1).
A _C1 transformation 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)
However, the element symbol in the above formula (1) represents the content (mass%) of each element, and is set to 0 when the element is not contained.

For heating in the tempering step, any heating method can be used as long as the heating temperature can be controlled as described above. An example of the heating method is furnace heating. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

Note that after reaching the tempering temperature, arbitrary cooling may be started after holding at the tempering temperature for an arbitrary period of time. When holding at the tempering temperature, the holding time is not particularly limited, but is preferably 5 minutes or more.

A steel plate was manufactured according to the procedure described below, and its 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) as a steel material was manufactured by a continuous casting method. Note that Table 1 also shows the A _C1 transformation point (° C.) determined by the above-mentioned formula (1).

Next, the obtained steel slabs were heated and hot-rolled according to the conditions shown in Table 2 to produce hot-rolled plates having various plate thicknesses (final plate thicknesses). Further, the obtained hot rolled sheet was quenched and tempered according to the conditions shown in Table 2 to obtain a steel sheet.

Next, for each of the obtained steel plates, the microstructure (total area ratio of martensite + bainite), residual γ amount, tensile strength (TS), Charpy absorbed energy at -196°C (vE _-196°C ), and The brittle crack generation suppression performance by CTOD test was evaluated according to the following method. The evaluation results are also listed in Table 2.

[Microstructure]
A test piece for microstructure observation was taken from each steel plate so that the observation position was at 1/4 t. This test piece was buried in resin so that the cross section perpendicular to the rolling direction served as the observation surface, and mirror-polished. Next, after performing nital corrosion, images of the tissue were taken by observing with a scanning electron microscope at magnifications of 2,000 and 10,000 times. The resulting images were analyzed to identify the microstructure.

[Residual γ amount at 1/4t]
Five test pieces for X-ray diffraction were taken parallel to the plate surface from the 1/4 t position of each steel plate, and after 30 minutes immersion in liquid nitrogen at -196°C (deep cooling treatment), the 1/4 t position was measured. The specimen was ground and chemically polished so as to have a flat surface, and then subjected to X-ray diffraction. The diffraction intensities of the (200) and (211) planes of α-Fe and the (200), (220) and (311) planes of γ-Fe that appear in the symmetric reflection X-ray diffraction pattern are determined, and the volume fraction of γ-Fe is calculated. The average value of the five test pieces was calculated and used as the residual γ amount (volume ratio).
Note that the residual γ amount "-" shown in Table 2 means that the residual γ amount is 0% by volume.

(Tensile strength)
A JIS No. 4 tensile test piece was taken from the 1/4 t position of the steel plate. Using this tensile test piece, a tensile test was conducted 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, it was considered to be high strength and passed.

(Cryogenic toughness)
A V-notch test piece was taken from a 1/4 t position of the steel plate in accordance with the regulations of JIS Z 2202. Using this V-notch test piece, a Charpy impact test was conducted in accordance with the provisions of JIS Z 2242, and the Charpy absorbed energy at -196°C (vE _-196°C ) was determined. This Charpy absorbed energy can be considered as an index of the cryogenic toughness of the steel sheet. In the Charpy impact test, three test pieces were taken in the rolling direction for each steel plate and measured. Table 2 shows the average values of the measurement results. In this Charpy impact test, if a full size test piece is used and the impact is 200 J or more, and if a half size test piece is used and the impact is 100 J or more, it is evaluated as having excellent cryogenic toughness and passed. And so. Note that in this example, No. A half-size test piece was used only in the case of No. 41, and a full-size test piece was used in the other cases.

(Brittle crack generation suppression performance)
In accordance with ISO 12135, a 10 x 10 x 55 mm test piece is taken from the surface of the steel plate if the plate thickness is more than 11 mm and less than 20 mm, or from the 1/4t position if the plate thickness is 20 mm or more, and When the plate thickness was 11 mm or less, a test piece of plate thickness x plate thickness x 55 mm was taken, and a CTOD test was conducted in accordance with ISO 12135. The test temperature was -165°C. In the CTOD test, a specimen that reached the maximum load point without visually confirming unstable fracture (brittle fracture) of the steel plate due to the occurrence of brittle cracks was considered to have passed. Note that it is possible to unambiguously determine whether or not each test piece reached the maximum load point without confirming brittle fracture by referring to the above standards.

Claims (5)

In mass%,
C: 0.01-0.15%,
Si: 0.01-0.50%,
Mn: 0.05-0.40%,
Ni: 6.0% or more and less than 7.0%,
Cr: 0.01-1.00%,
Mo: 0.01-0.50%,
P: 0.030% or less,
S: 0.0050% or less,
N: 0.0010~0.0080% and Al: 0.008~0.100%
, with the remainder consisting of Fe and unavoidable impurities,
The total area ratio of martensite and bainite is 100.0%,
The amount of retained austenite at a depth of 1/4 of the plate thickness in the thickness direction from the surface of the steel plate is less than 1.7% in terms of volume percentage,
A steel plate whose Charpy absorbed energy at -196°C in a Charpy impact test in accordance with JIS Z 2242 is 200 J or more when a full-size test piece is used and 100 J or more when a half-size test piece is used. The component 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, containing one or more selected from Ti: 0.03% or less and B: 0.0030% or less. The component composition further includes, in mass%,
Ca: 0.007% or less,
The steel plate according to claim 1 or 2, containing one or more selected from REM: 0.010% or less and Mg: 0.070% or less. A steel material having the composition according to claim 1 or 2 is subjected to hot rolling 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 steel plate surface temperature. After forming a hot-rolled sheet, the hot-rolled sheet is subjected to an average cooling rate in a temperature range of 600°C or lower and 300°C or higher at a temperature at a depth of 1/4 of the sheet thickness in the thickness direction from the surface of the hot-rolled sheet. is 3°C/s or more and the cooling stop temperature is 300°C or less, and then tempered to a temperature range of 550°C or more and less than the Ac1 transformation point.The total area ratio of martensite and bainite is 100. .0%, the amount of retained austenite at a depth of 1/4 of the thickness in the thickness direction from the surface of the steel plate is less than 1.7% in terms of volume percentage, and the Charpy impact test in accordance with JIS Z 2242 A method for producing a steel plate, wherein the Charpy absorbed energy at -196°C is 200 J or more when a full-size test piece is used and 100 J or more when a half-size test piece is used. A steel material having the composition according to claim 3 is subjected to hot rolling 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. as a steel plate surface temperature. After being made into a plate, the hot-rolled plate has an average cooling rate of 3 in a temperature range of 600°C or lower and 300°C or higher at a temperature at a depth of 1/4 of the plate thickness in the thickness direction from the surface of the hot-rolled plate. Direct quenching is performed at ℃/s or higher and the cooling stop temperature is 300℃ or lower, and then tempering is performed to a temperature range of 550℃ or higher and lower than the Ac1 transformation point.The total area ratio of martensite and bainite is 100.0. %, and the amount of retained austenite at a depth of 1/4 of the plate thickness in the thickness direction from the surface of the steel plate is less than 1.7% in terms of volume percentage, as determined by the Charpy impact test in accordance with JIS Z 2242. A method for manufacturing a steel plate, wherein the Charpy absorbed energy at 196° C. is 200 J or more when a full-size test piece is used, and 100 J or more when a half-size test piece is used.