JP6816832B2 - High-strength thick steel sheet for cryogenic temperature and its manufacturing method - Google Patents

Description

The present invention relates to a high-strength thick steel sheet for cryogenic temperature, and in particular, has excellent cryogenic toughness and cold workability, and can be suitably used for applications such as a tank for storing liquefied natural gas (LNG). Regarding thick steel sheets. The present invention also relates to a method for producing a high-strength thick steel sheet for extremely low temperatures.

A high level of safety is always required for LNG storage tanks. Therefore, the low-temperature steel sheet used for the tank body is required to have excellent toughness at a temperature at which LNG becomes liquid (about -162 ° C.). Further, in the manufacture of a tank, since strict processing such as molding into a cylindrical tube is performed, the steel plate used is also required to have cold bending workability. Therefore, as a low-temperature steel sheet used for the tank body of an LNG storage tank, a 9% Ni steel sheet having excellent low-temperature toughness has been widely used.

However, since Ni is an expensive alloying element, from the viewpoint of cost reduction, there is a demand for the development of a low temperature steel sheet having a Ni content of less than 9% and toughness equal to or higher than that of a 9% Ni steel sheet. There is.

Normally, when the Ni content of a low-temperature steel sheet is reduced, austenite becomes unstable in a low-temperature region, so that low-temperature toughness decreases, and it becomes difficult to ensure the high degree of safety required for an LNG storage tank. To solve this problem, various techniques for improving steel sheet characteristics such as low temperature toughness have been proposed for low temperature steel sheets having a reduced Ni content.

For example, Patent Documents 1 to 5 propose steel sheets having a Ni content of 7% and having a low temperature toughness equivalent to that of 9% Ni steel.

Patent Document 1 proposes a technique for combining low-temperature high-pressure rolling, two-phase region heat treatment, and quenching and tempering treatment. In the above technique, the retained austenite structure is controlled and stabilized by introducing strain into untransformed austenite to lower the Mf point.

Patent Documents 2 and 3 disclose techniques for securing the amount of retained austenite and refining the particle size by controlling the heating temperature and heating time of the slab and suppressing excessive slab heating.

Patent Document 4 discloses a technique for reducing non-uniformity of alloying elements and uniformly and finely dispersing retained austenite by subjecting a slab to a plurality of heat treatments and further performing a two-phase region heat treatment. Has been done.

Patent Document 5 discloses a technique for obtaining a tempered martensite structure in which retained austenite is finely dispersed by controlling the cumulative reduction rate of the unrecrystallized region and the recrystallized region and defining the quenching and tempering conditions. ..

International Publication No. 2007/034576 Japanese Unexamined Patent Publication No. 2011-219849 Japanese Unexamined Patent Publication No. 2011-241419 International Publication No. 2012/005330 JP-A-2015-86403

In order to improve the cryogenic toughness and cold workability, it is effective to generate a large amount of retained austenite. In the conventional process, quenching is performed after heating in the two-phase region to generate austenite, but since most of the austenite is transformed into martensite during quenching, it is not possible to obtain sufficient stable retained austenite. could not.

Therefore, in the techniques proposed in Patent Documents 1 to 5, tempering heat treatment is performed after quenching in order to generate and stabilize retained austenite. However, in the steel sheet obtained by the techniques proposed in Patent Documents 1 to 5, the amount of retained austenite after the subzero treatment at -196 ° C. is only 11% by volume at the highest, and a large amount of stable retained austenite can be obtained. It is not possible. Therefore, it cannot be said that the cold workability is sufficient.

In view of such circumstances, it is an object of the present invention to provide a low temperature steel sheet having a Ni content of less than 9%, toughness equal to or higher than that of a 9% Ni steel sheet, and excellent cold workability. And.

The present inventors conducted diligent research in order to achieve the above problems, and obtained the following findings.

(1) In order to reduce Ni to less than 9% and obtain cryogenic toughness equivalent to 9% Ni steel and excellent cold workability, residual austenite after subzero treatment at -196 ° C. The volume fraction may be controlled to be more than 11% and 20% or less.

(2) In order to obtain the above-mentioned stable retained austenite structure, a hot-rolled steel sheet having martensite or a martensite and bainite structure formed is heated to a two-phase temperature range at an average cooling rate of 3 ° C./s or more. It may be cooled to 250 to 500 ° C. and then tempered.

(3) According to the above treatment, austenite-stabilizing elements such as C, Ni, and Mn are concentrated in austenite during two-phase region heating, and C is further concentrated in austenite by tempering treatment. Alloy elements can be distributed. In particular, the process of the present invention in which cooling is stopped halfway at a temperature of 250 to 500 ° C. and then tempered is performed, and C is austenite at a lower temperature than the conventional process of cooling (quenching) to 200 ° C. or lower and then tempering. Can be distributed to. Therefore, the above process is effective in stabilizing austenite as compared with the conventional quenching and tempering process, and a large amount of stable retained austenite can be obtained.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

1. 1. By mass%
C: 0.02 to 0.12%,
Si: 0.01-0.30%,
Mn: 0.50 to 2.00%,
Ni: 5.5-8.5%,
P: 0.005% or less,
S: 0.003% or less, and N: 0.0015 to 0.0065%,
The balance has a component composition consisting of Fe and unavoidable impurities.
The microstructure at the 1/4 thickness position
(1) Tempering martensite or a matrix consisting of tempered martensite and bainite,
(2) Consists of retained austenite dispersed in the matrix.
The volume fraction of retained austenite at the 1/4 plate thickness position is more than 11%, 20% or less, and
High-strength thickness for ultra-low temperature, where the volume fraction of retained austenite at 1/4 of the plate thickness is more than 11% and 20% or less after subzero treatment in liquid nitrogen at -196 ° C for 15 minutes. Steel plate.

2. 2. The component composition is further increased by mass%.
Al: 0.01 to 0.10%,
Mo: 0.05 to 0.50%,
Cr: 1.00% or less,
Cu: 0.40% or less,
Nb: 0.05% or less,
The high-strength thick steel sheet for cryogenic temperature according to 1 above, which contains 1 or 2 or more selected from the group consisting of V: 0.05% or less and Ti: 0.03% or less.

3. 3. The component composition is further increased by mass%.
Ca: 0.007% or less,
The high-strength thick steel sheet for cryogenic temperature according to 1 or 2 above, which contains 1 or 2 or more selected from the group consisting of REM: 0.010% or less and Mg: 0.070% or less.

4. A steel material having the component composition according to any one of 1 to 3 above is heated to a heating temperature of 900 ° C. or higher and 1200 ° C. or lower.
The heated steel material is hot-rolled to form a hot-rolled steel sheet.
The hot-rolled steel sheet has an average cooling rate of 1 ° C./s or more in a temperature range of 550 ° C. or lower and 300 ° C. or higher at a temperature at a plate thickness of 1/4 position, and a cooling stop temperature of 300 ° C. at a temperature at a plate thickness of 1/4 position. Apply the following first accelerated cooling,
The hot-rolled steel sheet after the first accelerated cooling is subjected to two-phase region heating that heats the hot-rolled steel sheet to a heating temperature of 1 point or more and less than 3 points of Ac at the temperature at the plate thickness 1/4 position.
The hot-rolled steel sheet after heating in the two-phase region has an average cooling rate of 3 ° C./s or more at the temperature at the plate thickness 1/4 position and a cooling stop temperature of 500 ° C. or less at the temperature at the plate thickness 1/4 position 250 ° C. After applying the second accelerated cooling as described above,
The hot-rolled steel sheet after the second accelerated cooling is air-cooled to 200 ° C. or lower.
The hot-rolled steel sheet after air cooling is subjected to a tempering treatment in which the tempering temperature is 500 ° C. or higher and 650 ° C. or lower at the temperature of 1/2 of the plate thickness, and the hot-rolled steel sheet has the microstructure described in 1 above. A method for manufacturing a high-tensile thick steel sheet for extremely low temperatures, which is a high-tensile thick steel sheet.

5. Further, after the hot rolling and prior to the first accelerated cooling,
The hot-rolled steel sheet is air-cooled to an air-cooling stop temperature of 300 ° C. or lower.
The method for producing a high-strength thick steel sheet for ultra-low temperature, which comprises reheating the air-cooled hot-rolled steel sheet to a reheating temperature of 3 points or more and 1000 ° C. or less.

According to the present invention, although the Ni content is reduced to 5.5 to 8.5%, it has low temperature toughness equal to or higher than that of 9% Ni steel and is also excellent in cold workability. High-strength thick steel sheets for extremely low temperatures can be obtained. This cryogenic high-strength thick steel sheet can be extremely suitably used for applications such as LNG storage tanks. Therefore, the present invention contributes to the improvement of safety of steel structures such as LNG storage tanks, and exerts a remarkable effect in industry.

Hereinafter, embodiments of the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited thereto.

[Ingredient composition]
The high-tensile steel sheet for ultra-low temperature of the present invention (hereinafter, may be simply referred to as "steel plate" or "thick steel sheet") and the steel material used for producing the high-strength thick steel sheet for ultra-low temperature have the above-mentioned composition. .. Hereinafter, each component contained in the component composition will be described. Unless otherwise specified, "%" as a unit of the content of the component in the present specification means "mass%".

C: 0.02 to 0.12%
C is an element having an effect of improving the strength of the steel sheet. In addition, C is an important element for obtaining a desired retained austenite volume fraction. In order to obtain these effects, the C content is set to 0.02% or more, preferably 0.04% or more. On the other hand, when the C content exceeds 0.12%, the low temperature toughness of the steel sheet decreases. Therefore, the C content is 0.12% or less, preferably 0.08% or less.

Si: 0.01 to 0.30%
Si is an element that contributes to improving the strength of the steel sheet, and is also an element that acts as an antacid. In order to exhibit these effects, the Si content is 0.01% or more. On the other hand, if the Si content is excessively high, the toughness decreases. Therefore, the Si content is 0.30% or less, preferably 0.10% or less.

Mn: 0.50 to 2.00%
Mn is an element that enhances the hardenability of steel and contributes to increasing the strength of steel sheets. When the Mn content is less than 0.50%, the hardenability of the steel is lowered, and not only the strength of the steel sheet but also the low temperature toughness is lowered. Therefore, the Mn content is 0.50% or more, preferably 0.60% or more. On the other hand, when the Mn content exceeds 2.00%, the effect of improving the strength of the steel sheet is saturated and the low temperature toughness is rather lowered. Therefore, the Mn content is 2.00% or less, preferably 0.95% or less.

Ni: 5.5-8.5%
Ni is an extremely effective element for improving the low temperature toughness of steel sheets. However, since Ni is an expensive element, the cost of the steel sheet rises as its content increases. Therefore, the Ni content is set to 8.5% or less. On the other hand, when the Ni content is less than 5.5%, stable retained austenite cannot be obtained at a low temperature, and as a result, the low temperature toughness of the steel sheet is lowered. Therefore, the Ni content is set to 5.5% or more.

P: 0.005% or less P is an unavoidable impurity and is a harmful element that adversely affects the low temperature toughness of the steel sheet. For example, it is preferable to reduce the P content as much as possible in order to obtain a sound base material and welded joint when the steel plate is welded to form a welded structure. Therefore, the P content is set to 0.005% or less. On the other hand, 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 permissible to contain it as an unavoidable impurity. However, since excessive reduction causes an increase in cost, the P content is preferably 0.001% or more.

S: 0.003% or less
Like P, S is an unavoidable impurity and is a harmful element that adversely affects the low temperature toughness of the steel sheet. For example, it is preferable to reduce the S content as much as possible in order to obtain a sound base material and welded joint when the steel plate is welded to form a welded structure. Therefore, the S content is set to 0.003% or less. On the other hand, the lower the S content, the better, so the lower limit is not particularly limited and may be 0%, but even in that case, it is permissible to contain it as an unavoidable impurity. However, since excessive reduction causes an increase in cost, the S content is preferably 0.0001% or more.

N: 0.0015 to 0.0065%
N is an element that forms a precipitate in steel, and by forming AlN, it contributes to the refinement of the base metal. In order to obtain the above effect, the N content is set to 0.0015% or more. On the other hand, when the N content exceeds 0.0065%, the toughness of the base metal and the heat-affected zone of welding decreases when the steel plate is welded to form a welded structure. Therefore, the N content is set to 0.0065% or less.

The component composition in one embodiment of the present invention may consist of the above elements, and the balance consisting of Fe and unavoidable impurities.

Further, in another embodiment of the present invention, one or more of the above-mentioned component compositions arbitrarily selected from the group consisting of Al, Mo, Cr, Cu, Nb, V, and Ti are described below. Can be further contained in quantity.

Al: 0.01 to 0.10%
Al is an element contained in the deoxidizer. If the Al content is less than 0.01%, the effect as an antacid is poor. Therefore, when Al is contained, the Al content is 0.01% or more, preferably 0.02% or more. On the other hand, if the Al content exceeds 0.10%, the cleanliness of the steel is impaired. Therefore, the Al content is 0.10% or less, preferably 0.05% or less.

Mo: 0.05 to 0.50%
Mo is an element that can improve the strength of a steel sheet without impairing low temperature toughness. When Mo is added, the Mo content is set to 0.05% or more, preferably more than 0.10% in order to obtain the above effect. On the other hand, when the Mo content exceeds 0.50%, the low temperature toughness decreases. Therefore, the Mo content is 0.50% or less, preferably 0.30% or less.

Cr: 1.00% or less Cr is an element having the same effect as Mo, but when the Cr content exceeds 1.00%, the low temperature toughness of the steel sheet decreases. Therefore, when Cr is added, the Cr content is set to 1.00% or less, preferably less than 0.20%. On the other hand, the lower limit of the Cr content is not particularly limited, but from the viewpoint of enhancing the above effect, the Cr content is preferably 0.01% or more.

Cu: 0.40% or less Cu is an element having the effect of increasing the strength of the steel sheet by improving the hardenability. However, when the Cu content exceeds 0.40%, the low temperature toughness of the steel sheet is lowered and the properties of the steel (slab) surface after casting are deteriorated. Therefore, when Cu is added, the Cu content is 0.40% or less, preferably 0.30% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but from the viewpoint of enhancing the above effect, the Cu content is preferably 0.10% or more.

Nb: 0.05% or less Nb is an effective element that enhances the strength of the steel sheet by strengthening precipitation. However, if the Nb content becomes excessively high, the low temperature toughness of the steel sheet decreases. Therefore, when Nb is added, the Nb content is set to 0.05% or less, preferably 0.03% or less. On the other hand, the lower limit of the Nb content is not particularly limited, but from the viewpoint of enhancing the above effect, the Nb content is preferably 0.010% or more.

V: 0.05% or less V is an effective element for increasing the strength of the steel sheet by precipitation strengthening, like Nb. However, if the V content becomes excessively high, the low temperature toughness of the steel sheet decreases. Therefore, when V is added, the V content is set to 0.05% or less, preferably 0.04% or less. On the other hand, the lower limit of the V content is not particularly limited, but from the viewpoint of enhancing the above effect, the V content is preferably 0.010% or more.

Ti: 0.03% or less Ti is an element having the effect of increasing the toughness of the welded portion without deteriorating the mechanical properties of the base metal when the steel plate is welded to form a welded structure. Therefore, Ti can be optionally contained in the range of 0.03% or less. On the other hand, the lower limit of the Ti content is not particularly limited, but from the viewpoint of enhancing the above effect, the Ti content is preferably 0.001% or more.

Further, in another embodiment of the present invention, the above-mentioned component composition can optionally further contain 1 or 2 or more selected from the group consisting of Ca, REM, and Mg in the amounts described below. ..

Ca: 0.007% or less Ca is an element having an effect of improving the low temperature toughness of the steel sheet by controlling the morphology of inclusions in the steel. However, excess Ca impairs the cleanliness of the steel. Therefore, when Ca is added, the Ca content is set to 0.007% or less, preferably 0.004% or less. On the other hand, the lower limit of the Ca content is not particularly limited, but from the viewpoint of enhancing the above effect, it is preferably 0.0005% or more.

REM: 0.010% or less REM (rare earth metal) is an element having an effect of improving the low temperature toughness of a steel sheet by controlling the morphology of inclusions in steel, like Ca. However, excessive REM impairs the cleanliness of the steel. Therefore, when REM is added, the REM content is 0.010% or less, preferably 0.008% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of enhancing the above effects, the REM content is preferably 0.0005% or more.

Mg: 0.070% or less Mg, like Ca and REM, is an element that has the effect of improving the low temperature toughness of steel sheets by controlling the morphology of inclusions in steel. However, excess Mg impairs the cleanliness of the steel. Therefore, when Mg is added, the Mg content is 0.070% or less, preferably 0.004% or less. On the other hand, the lower limit of the Mg content is not particularly limited, but from the viewpoint of enhancing the above effect, the Mg content is preferably 0.0005% or more.

[Micro tissue]
In the high-strength thick steel sheet for ultra-low temperature of the present invention, in order to ensure cold workability and ultra-low temperature toughness, the microstructure at the plate thickness 1/4 position is dispersed in (1) matrix and (2) the matrix. It consists of retained austenite.

The matrix consists of (A) tempered martensite or (B) tempered martensite and tempered bainite. If the matrix does not meet the above conditions, one or both of a tensile strength of 700 MPa or more and a desired low temperature toughness cannot be obtained.

(Amount of retained austenite before subzero treatment)
The cryogenic high-strength thick steel sheet of the present invention has a volume fraction of retained austenite of more than 11% and 20% or less at a plate thickness of 1/4 of the cryogenic high-tensile steel sheet. If the volume fraction is 11% or less, the desired cold workability cannot be obtained. On the other hand, if the volume fraction exceeds 20%, the desired strength cannot be secured under the condition that the Ni content is 5.5 to 8.5%.

(Amount of retained austenite after subzero treatment)
Further, the cryogenic high-strength thick steel sheet of the present invention has a volume fraction of retained austenite of more than 11% and 20% at the position of 1/4 of the plate thickness after the cryogenic high-tensile steel sheet is subjected to subzero treatment. It is as follows. Here, the sub-zero treatment is performed by holding the steel sheet in liquid nitrogen at -196 ° C. for 15 minutes. If the volume fraction is 11% or less, the desired cold workability cannot be obtained. The volume fraction of retained austenite after the subzero treatment is preferably 12.5% or more. On the other hand, if the volume fraction exceeds 20%, the desired strength cannot be secured under the condition that the Ni content is 5.5 to 8.5%.

Further, the amount of reduction of retained austenite when the subzero treatment is performed under the above conditions is preferably less than 0.5% in terms of volume fraction. Here, the amount of decrease refers to the difference between the volume fraction of retained austenite before the subzero treatment and the volume fraction of the retained austenite after the subzero treatment.

[Plate thickness]
The thickness of the high-strength thick steel sheet for ultra-low temperature of the present invention is not particularly limited and may be any thickness, but it is preferably 6 mm or more and 50 mm or less.

[Mechanical characteristics]
(Tensile strength)
The lower limit of the tensile strength (TS) of the high-tensile thick steel sheet for ultra-low temperature of the present invention is not particularly limited and can be any value, but the tensile strength is preferably 700 MPa or more, preferably 720 MPa or more. It is more preferable that the pressure is 740 MPa or more. On the other hand, the upper limit of the tensile strength is not particularly limited and can be any value, but the tensile strength is preferably 930 MPa or less, and more preferably 900 MPa or less. The tensile strength can be measured by the method described in Examples.

(Toughness)
The toughness of the high-strength thick steel sheet for ultra-low temperature of the present invention is not particularly limited and can be any value, but the Charpy absorption energy (vE _{-196 ° C.} ) at _{-196 ° C.} is preferably 150 J or more. It is more preferably 180 J or more, further preferably 200 J or more, and most preferably 240 J or more. Further, the upper limit of the Charpy absorption energy is not particularly limited, but may be 350 J or less, and may be 280 J or less. The Charpy absorbed energy can be measured by the method described in the examples.

(Cold workability)
The cold workability of the high-strength thick steel sheet for ultra-low temperature of the present invention is not particularly limited, but the brittle fracture surface ratio in the strain aging Charpy test at a strain of 3% and a test temperature of -196 ° C. is 2% or less. It is preferable, and it is more preferable that it is 0%. The brittle fracture surface ratio can be regarded as an index of cold workability. The brittle fracture surface ratio can be evaluated by the method described in Examples.

[Production method]
Next, a method for manufacturing a high-strength thick steel sheet for cryogenic temperature according to an embodiment 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 plate thickness (1/2 position of the plate thickness). The temperature at the center of the plate thickness can be obtained by heat transfer calculation from the surface temperature of the steel plate measured by the radiation thermometer.

In one embodiment of the present invention, by sequentially performing the following steps (1) to (7), a high-strength thick steel sheet for cryogenic temperature having the above-mentioned microstructure can be produced.
(1) Heating of steel material (2) Hot rolling (3) First accelerated cooling (4) Two-phase region heating (5) Second accelerated cooling (6) Air cooling (7) Tempering

(1) Heating of Steel Material First, the steel material having the above-mentioned composition is heated to a heating temperature of 900 ° C. or higher and 1200 ° C. or lower. The method for producing the steel material is not particularly limited, but for example, it can be produced by melting and casting molten steel having the above composition by a conventional method. The melting can be carried out by any method such as a converter, an electric furnace, and an induction furnace. Further, the casting is preferably performed by a continuous casting method from the viewpoint of productivity, but it can also be performed by an ingot-decomposition rolling method. As the steel material, for example, a steel slab can be used.

The heating may be performed after the steel material obtained by a method such as casting is once cooled, or the obtained steel material may be directly subjected to the heating without being cooled.

Heating temperature: 900-1200 ° C
If the heating temperature is less than 900 ° C., the deformation resistance of the steel material is high, so that the load on the rolling mill in hot rolling increases, and hot rolling becomes difficult. Therefore, the heating temperature is set to 900 ° C. or higher. On the other hand, when the heating temperature is higher than 1200 ° C., the oxidation of the steel becomes remarkable, the loss due to the oxidation increases, and the yield decreases. Therefore, the heating temperature is set to 1200 ° C. or lower.

(2) Hot Rolling After the above heating, the heated steel material is hot-rolled to obtain a hot-rolled steel sheet. 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) First Accelerated Cooling After the hot rolling, the hot-rolled steel sheet is subjected to a first accelerated cooling having an average cooling rate of 1 ° C./s or more and a cooling stop temperature of 300 ° C. or less. The hot-rolled steel sheet is hardened by the first accelerated cooling to form a martensite and bainite structure.

Average cooling rate: 1 ° C./s or more In the first accelerated cooling, it is desirable that the average cooling rate in the temperature range of 550 ° C. or lower and 300 ° C. or higher at the temperature at the 1/4 position of the plate thickness is less than 1 ° C./s. The metamorphic structure of is not obtained, and the strength cannot be obtained. Therefore, the average cooling rate is set to 1 ° 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 becomes difficult to control the temperature at each position in the steel sheet, and the material varies in the plate width direction and the rolling direction. It will be easier to get out. As a result, material properties such as tensile properties vary. Therefore, the average cooling rate is preferably 200 ° C./s or less.

Cooling stop temperature: 300 ° C or less The cooling stop temperature is 300 ° C or less at the position where the plate thickness is 1/4. If the cooling stop temperature is higher than 300 ° C., the transformation at the time of quenching becomes insufficient, so that the desired strength cannot be obtained.

The first accelerated cooling can be performed by any method without particular limitation. For example, one or both of air cooling and water cooling can be used. As the water cooling, any cooling method using water (for example, spray cooling, mist cooling, laminar cooling, etc.) can be used.

(4) Two-phase region heating Next, the cooled hot-rolled steel sheet is heated to a heating temperature of 1 point or more and less than 3 points of Ac at the temperature at the plate thickness 1/4 position (two-phase region heating). By performing the two-phase region heating, most of the structure of the hot-rolled steel sheet is reverse-transformed from bainite and martensite to obtain a mixed structure of austenite in which C, Ni and Mn are concentrated.

Heating temperature: Ac 1 point or more and less than Ac 3 points When the heating temperature is less than Ac 1 point, the above-mentioned reverse transformation austenite is hardly obtained, and a desired microstructure cannot be obtained by the subsequent second accelerated cooling. As a result, the desired strength cannot be obtained in the finally obtained thick steel sheet. On the other hand, when the heating temperature is Ac3 or higher, bainite and martensite are all reverse-transformed to austenite, and C, Ni, and Mn are averaged over the entire structure, so that a desired microstructure cannot be obtained. .. As a result, the desired cold workability cannot be obtained.

The Ac1 point and the Ac3 point can be obtained by the following equations (1) and (2).
Ac1 (℃) = 750.8 --26.6C + 17.6Si --11.6Mn --22.9Cu --23Ni + 24.1Cr + 22.5Mo- 39.7V --5.7Ti + 232.4Nb --169.4Al… (1)
Ac3 (℃) = 937.2 --436.5C + 56Si --19.7Mn --16.3Cu --26.6Ni --4.9Cr + 38.1Mo + 124.8V + 136.3Ti --19.1Nb + 198.4Al… (2)
However, the element symbol in the above formulas (1) and (2) represents the content (mass%) of each element, and is set to 0 when the element is not contained.

For the two-phase region heating, 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. A general heat treatment furnace can be used for heating the furnace without particular limitation.

In the two-phase region heating, the next second acceleration cooling may be started immediately after reaching the heating temperature, but the next second acceleration may be started after the heating temperature is maintained for an arbitrary time. Cooling may be started. When holding at the heating temperature, the holding time is not particularly limited, but is preferably 5 minutes or more.

(5) Second Accelerated Cooling Next, the hot-rolled steel sheet after heating in the two-phase region is subjected to a second accelerated cooling having an average cooling rate of 3 ° C./s or more and a cooling stop temperature of 500 ° C. or less and 250 ° C. or more. Give. The hot-rolled steel sheet is hardened by the second accelerated cooling to obtain a hardened structure in which retained austenite is dispersed in a matrix composed of martensite or martensite and bainite.

Average cooling rate: 3 ° C./s or more If the average cooling rate at the temperature at the plate thickness 1/4 position in the second accelerated cooling is less than 3 ° C./s, the desired hardened structure cannot be obtained. The strength of the finally obtained thick steel sheet is reduced. Therefore, the average cooling rate is set to 3 ° C./s or higher. 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 becomes difficult to control the temperature at each position in the steel sheet, and the material varies in the plate width direction and the rolling direction. It will be easier to get out. As a result, material properties such as tensile properties vary. Therefore, the average cooling rate is preferably 200 ° C./s or less. Here, the average cooling rate refers to the average cooling rate from the start of accelerated cooling to the stop of accelerated cooling in the second accelerated cooling step.

Cooling stop temperature: 250-500 ° C
The cooling stop temperature in the second accelerated cooling is 250 ° C. or higher and 500 ° C. or lower at the position where the plate thickness is 1/4. By stopping the accelerated cooling at a temperature of 250 ° C. or higher and 500 ° C. or lower, and then air-cooling, C can be concentrated in untransformed austenite and the austenite can be stabilized. If the accelerated cooling stop temperature is less than 250 ° C., untransformed austenite is transformed into martensite, and the desired amount of retained austenite cannot be obtained. On the other hand, when the accelerated cooling stop temperature is higher than 500 ° C., the distribution of C to the austenite phase becomes insufficient, the amount of retained austenite finally obtained decreases, and the desired toughness cannot be obtained.

The second accelerated cooling can be performed by any method without particular limitation. For example, one or both of air cooling and water cooling can be used. As the water cooling, any cooling method using water (for example, spray cooling, mist cooling, laminar cooling, etc.) can be used.

(6) Air cooling Next, the hot-rolled steel sheet after the second acceleration cooling is air-cooled to 200 ° C. or lower (air cooling after acceleration cooling). The cooling rate in the air cooling is not particularly limited, but the average cooling rate is preferably less than 1 ° C./s.

(7) Tempering treatment Next, a tempering treatment is performed. By the tempering treatment, retained austenite can be stabilized and bainite and martensite structures can be tempered to improve toughness.

Baking temperature: 500-650 ° C
The tempering temperature in the tempering treatment is 500 ° C. or higher and 650 ° C. or lower at the position where the plate thickness is 1/2. If the tempering temperature is less than 500 ° C., the retained austenite is not sufficiently stabilized, and the toughness improvement by tempering the bainite and martensite structures is also insufficient. Therefore, the tempering temperature is set to 500 ° C. or higher. On the other hand, when the tempering temperature exceeds 650 ° C., the stability of retained austenite is rather lowered, and the desired low temperature toughness cannot be obtained. Therefore, the tempering temperature is set to 650 ° C. or lower.

The method for producing a high-strength thick steel sheet for ultra-low temperature in another embodiment of the present invention further optionally follows the following steps (A) and (B) after the hot rolling and prior to the first accelerated cooling. It can be performed.
(A) Air cooling (B) Reheating

(A) Air cooling The hot-rolled steel sheet after hot rolling is air-cooled to an air-cooling stop temperature of 300 ° C. or lower (air-cooled after hot rolling). In the present embodiment, an austenite structure finely divided by phase transformation in the next reheat treatment is obtained. Therefore, in this air cooling step, the microstructure of the steel plate is once made into a martensite + bainite structure by cooling to an air cooling stop temperature of 300 ° C. or lower.

(B) Reheating Next, the air-cooled hot-rolled steel sheet is reheated to a reheating temperature of Ac 3 points to 1000 ° C. prior to the next first accelerated cooling. By the reheating, the ferrite structure of the hot-rolled steel sheet is reverse-transformed to austenite, and the reverse-transformed austenite is transformed into martensite and bainite by the next first accelerated cooling.

Reheating temperature: Ac 3 points to 1000 ° C
The reheating temperature in the reheating is Ac 3 points or more and 1000 ° C. or less. By reheating to the reheating temperature, the structure of the hot-rolled steel sheet can be made into a uniform and finely divided austenite structure. If the reheating temperature is less than 3 points of Ac, the ferrite structure remains on the finally obtained thick steel sheet, and the desired strength cannot be obtained. Further, if the reheating temperature is higher than 1000 ° C., the operating load becomes large and the austenite becomes coarse, so that the desired toughness cannot be obtained.

For the reheating, any heating method can be used as long as the reheating temperature can be controlled as described above. An example of the heating method is furnace heating. A general heat treatment furnace can be used for heating the furnace without particular limitation.

A high-tensile thick steel sheet for cryogenic temperature was manufactured by the procedure described below, and the characteristics of the obtained high-strength thick steel sheet for cryogenic temperature were evaluated.

First, molten steel having the composition shown in Table 1 was melted in a converter to produce a steel slab (thickness: 250 mm) as a steel material by a continuous casting method. Table 1 also shows the Ac1 points (° C.) obtained by the above-mentioned equation (1) and the Ac3 points (° C.) obtained by the equation (2).

Next, the obtained steel slab was heated to the heating temperature shown in Table 2 and hot-rolled to obtain a hot-rolled steel sheet having a plate thickness shown in Table 2. Next, the hot-rolled steel sheet was subjected to the first accelerated cooling. The average cooling rate and the cooling stop temperature in the first accelerated cooling are as shown in Table 2. In some examples, after the hot rolling, air cooling and reheating were performed under the conditions shown in Table 2 prior to the first accelerated cooling.

The hot-rolled steel sheet after the first accelerated cooling was subjected to two-phase region heating at the heating temperatures shown in Table 2. After the two-phase region heating, the hot-rolled steel sheet was subjected to a second accelerated cooling. The average cooling rate and the cooling stop temperature in the second accelerated cooling are as shown in Table 2. After the second accelerated cooling, air cooling was performed to 200 ° C. or lower, and then tempering was performed. The tempering temperature in the tempering was as shown in Table 2.

A heat treatment furnace was used for heating in each of the above steps.

Next, the microstructure, the amount of retained austenite after subzero treatment, the mechanical properties, and the cold workability were evaluated for each of the obtained thick steel sheets. The evaluation was carried out by the method described below.

(Micro tissue)
From each thick steel plate, a test piece for microstructure observation was taken so that the plate thickness 1/4 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. Then, after performing nital corrosion, an image of the tissue was taken by observing with a scanning electron microscope at a magnification of 400 times. The resulting images were analyzed to identify microstructures.

The steel sheet obtained as described above was obtained from Comparative Example No. Except for 6, all had lath-like microstructures, and the microstructure was tempered martensite, or a mixed structure of tempered martensite and tempered bainite structure. In addition, the obtained steel sheet had a volume fraction of retained austenite of 0%. Except for 5, it had a microstructure with a structure in which retained austenite was dispersed in the matrix.

-Retained austenite volume fraction Ten X-ray diffraction test pieces were collected from the thickness 1/4 position of the thick steel sheet in parallel with the plate surface, and five of them were subjected to subzero treatment. The subzero treatment was carried out by holding the test piece in liquid nitrogen at -196 ° C. for 15 minutes. Five of each of the test pieces without sub-zero treatment and the sub-zero treatment were subjected to X-ray diffraction by grinding and chemically polishing the test pieces so that the plate thickness 1/4 position was the measurement surface. Obtain the diffraction intensities of the α-Fe (200) and (211) planes and the (200), (220) and (311) planes of γ-Fe appearing in the symmetrically reflected X-ray diffraction pattern, and determine the volume fraction of γ-Fe. The calculation was performed, and the average value of each of the five test pieces was calculated and used as the volume fraction of retained austenite.

(Mechanical characteristics)
A JIS No. 4 tensile test piece was taken from the position of 1/2 of the thickness of the thick steel plate. Using the above 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 thick steel sheet.

Further, a V-notch test piece was taken from the position of 1/4 of the thickness of the thick steel plate in accordance with JIS Z 2202. Using the V-notch test piece, a Charpy impact test was carried out in accordance with JIS Z 2242 to determine the Charpy absorption energy (vE _{-196 ° C} ) at _{-196 ° C.} The Charpy absorption energy can be regarded as an index of the toughness of a thick steel sheet at an extremely low temperature.

(Cold workability)
A tensile test piece was taken from the thick steel plate so that the rolling direction of the thick steel plate was the tensile direction. Next, after applying a strain of 3% to the tensile test piece, it was subjected to an aging treatment at 250 ° C. for 1 hour. A V-notch test piece was collected from the tensile test piece after the aging treatment in accordance with JIS Z 2202. Using the V-notch test piece, a Charpy impact test was carried out in accordance with JIS Z 2242 to determine the brittle fracture surface ratio at -196 ° C. The brittle fracture surface ratio can be regarded as an index of cold workability of a thick steel sheet.

The evaluation results are shown in Table 3. Table 3 also shows the amount of decrease in retained austenite due to subzero treatment. Here, the amount of decrease in retained austenite is a value obtained by subtracting the volume fraction of retained austenite after the subzero treatment from the volume fraction of retained austenite before the subzero treatment.

As can be seen from this result, all the thick steel sheets satisfying the conditions of the present invention have a tensile strength of 700 MPa or more, vE _{-196 ° C} : 150 J or more, and a brittle fracture surface ratio of 2% or less, which are excellent. It had both mechanical properties and cold workability. When the Charpy absorption energy vE _{-196 ° C.} is 150 J or more, it can be said that the toughness is equal to or higher than that of the 9% Ni steel sheet. On the other hand, the thick steel sheet that does not satisfy the conditions of the present invention is inferior in at least one of the strength, toughness, and cold workability.

Claims (5)

By mass%
C: 0.02 to 0.12%,
Si: 0.01-0.30%,
Mn: 0.50 to 2.00%,
Ni: 5.5-8.5%,
P: 0.005% or less,
S: 0.003% or less, and N: 0.0015 to 0.0065%,
The balance has a component composition consisting of Fe and unavoidable impurities.
The microstructure at the 1/4 thickness position
(1) Tempering martensite or matrix consisting of tempered martensite and tempered bainite,
(2) Consists of retained austenite dispersed in the matrix.
The volume fraction of retained austenite at the 1/4 plate thickness position is more than 11%, 20% or less, and
High-strength thickness for ultra-low temperature, where the volume fraction of retained austenite at 1/4 of the plate thickness is more than 11% and 20% or less after subzero treatment in liquid nitrogen at -196 ° C for 15 minutes. Steel plate. The component composition is further increased by mass%.
Al: 0.01 to 0.10%,
Mo: 0.05 to 0.50%,
Cr: 1.00% or less,
Cu: 0.40% or less,
Nb: 0.05% or less,
Contains 1 or 2 or more selected from the group consisting of V: 0.05% or less and Ti: 0.03% or less .
The high-strength thick steel sheet for cryogenic temperature according to claim 1, wherein the amount of reduction of retained austenite when the subzero treatment is applied is less than 0.5% in volume fraction . The component composition is further increased by mass%.
Ca: 0.007% or less,
The high-strength thick steel sheet for cryogenic temperature according to claim 1 or 2, which contains 1 or 2 or more selected from the group consisting of REM: 0.010% or less and Mg: 0.070% or less. A steel material having the component composition according to any one of claims 1 to 3 is heated to a heating temperature of 900 ° C. or higher and 1200 ° C. or lower.
The heated steel material is hot-rolled to form a hot-rolled steel sheet.
The hot-rolled steel sheet has an average cooling rate of 1 ° C./s or more in a temperature range of 550 ° C. or lower and 300 ° C. or higher at a temperature at a plate thickness of 1/4 position, and a cooling stop temperature of 300 ° C. at a temperature at a plate thickness of 1/4 position. Apply the following first accelerated cooling,
The hot-rolled steel sheet after the first accelerated cooling is subjected to two-phase region heating that heats the hot-rolled steel sheet to a heating temperature of 1 point or more and less than 3 points of Ac at the temperature at the plate thickness 1/4 position.
On the hot-rolled steel sheet after heating in the two-phase region, the average cooling rate at the temperature at the plate thickness 1/4 position is 3 ° C / s or more and 200 ° C / s or less , and the cooling stop temperature is the temperature at the plate thickness 1/4 position. A second accelerated cooling of 500 ° C or lower and 250 ° C or higher is applied.
The hot-rolled steel sheet after the second accelerated cooling is air-cooled to 200 ° C. or lower.
The hot-rolled steel sheet after air cooling is subjected to a tempering treatment in which the tempering temperature is 500 ° C. or higher and 650 ° C. or lower at a temperature of 1/2 of the plate thickness, and the extremely low temperature having the microstructure according to claim 1 is obtained. A method for manufacturing a high-strength thick steel sheet for ultra-low temperature, which is a high-strength thick steel sheet for ultra-low temperature. Further, after the hot rolling and prior to the first accelerated cooling,
The hot-rolled steel sheet is air-cooled to an air-cooling stop temperature of 300 ° C. or lower.
The method for producing a high-strength thick steel sheet for ultra-low temperature according to claim 4, wherein the air-cooled hot-rolled steel sheet is reheated to a reheating temperature of 3 points or more and 1000 ° C. or less.