WO2019239761A1

WO2019239761A1 - Cryogenic high-tensile thick steel sheet and method for producing same

Info

Publication number: WO2019239761A1
Application number: PCT/JP2019/018968
Authority: WO
Inventors: 佐藤　祐也; 善明村上; 聡伊木
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2018-06-12
Filing date: 2019-05-13
Publication date: 2019-12-19
Also published as: JPWO2019239761A1; CN112236539A; CN112236539B; JP6816832B2; KR102388436B1; KR20200140907A

Abstract

The present invention addresses the problem of providing a cryogenic steel sheet having a Ni content of less than 9% and also having both of toughness at the same level or a higher level as or than that of a 9%-Ni steel sheet and excellent cold workability. A cryogenic high-tensile thick steel sheet having a specified ingredient component, wherein the micro-structure at a position of one-fourth the thickness of the steel sheet comprises (1) a matrix composed of tempered martensite or a matric composed of tempered martensite and bainite and (2) retained austenite dispersed in the matrix, the content ratio of the retained austenite at a position of one-fourth the thickness of the steel sheet is more than 11% by volume and equal to or less than 20% by volume, and the content ratio of the retained austenite at a position of one-fourth the thickness of the steel sheet becomes more than 11% by volume and equal to or less than 20% by volume after the steel sheet is subjected to such a subzero treatment that the steel sheet is retained for 15 minutes in liquid nitrogen at -196°C.

Description

Cryogenic high-strength thick steel plate and method for producing the same

The present invention relates to a high-tensile steel sheet for cryogenic use, and in particular, it is excellent in cryogenic toughness and cold workability, and can be suitably used for applications such as LNG storage tanks. It relates to a thick steel plate. Moreover, this invention relates to the manufacturing method of the said high-tensile steel plate for cryogenics.

LNG tanks are always required to have a high level of safety. 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 tanks, since severe processing is performed as in the case of forming into a cylindrical tube, cold bending workability is also required for the steel sheet used. Therefore, as a low-temperature steel plate used for the tank body of the LNG storage tank, conventionally, a 9% Ni steel plate having excellent low-temperature toughness has been widely used.

However, since Ni is an expensive alloy 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 a toughness equal to or higher than that of a 9% Ni steel sheet. Yes.

Usually, 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 is lowered, and it is difficult to ensure a high level of safety required for an LNG storage tank. In response to this problem, various techniques have been proposed for improving steel sheet properties such as low temperature toughness in low temperature steel sheets with a reduced Ni content.

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

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

Patent Documents 2 and 3 disclose techniques for securing the amount of retained austenite and reducing 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 alloy elements and dispersing residual austenite in a large amount and uniformly and finely by subjecting the slab to thermal processing multiple times and further performing two-phase region heat treatment. Has been.

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

International Publication No. 2007/034576 JP 2011-219849 A JP 2011-241419 A International Publication No. 2012/005330 JP2015-86403A

In order to improve cryogenic toughness and cold workability, it is effective to produce a large amount of retained austenite. In the conventional process, a quenching treatment is performed after two-phase heating to generate austenite. However, since most of the austenite is transformed into martensite at the time of quenching, it is possible to sufficiently obtain 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 subzero treatment at −196 ° C. is only 11% by volume, and a large amount of stable retained austenite is obtained. It is not possible. Therefore, it cannot be said that the cold workability is sufficient.

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

The inventors of the present invention conducted intensive research to achieve the above-mentioned problems, and obtained the following knowledge.

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

(2) In order to obtain the above-mentioned stable retained austenite structure, the hot-rolled steel sheet in which martensite or martensite and bainite structures are generated is heated to a two-phase temperature range, and the average cooling rate is 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 to austenite during two-phase heating, and further C is concentrated to austenite by tempering. Alloy elements can be distributed. In particular, the process of the present invention in which the tempering treatment is performed after the cooling is stopped at a temperature of 250 to 500 ° C., the C is austenite at a lower temperature than the conventional process of tempering after cooling (quenching) to 200 ° C. or less. Can be distributed. Therefore, the above process is more effective for stabilizing austenite than the conventional quenching and tempering step, 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. % By mass
C: 0.02 to 0.12%,
Si: 0.01 to 0.30%,
Mn: 0.50 to 2.00%,
Ni: 5.5 to 8.5%,
P: 0.005% or less,
S: 0.003% or less, and N: 0.0015-0.0065%,
The balance has a component composition consisting of Fe and inevitable impurities,
The microstructure at the 1/4 thickness position is
(1) tempered martensite or a matrix composed of tempered martensite and bainite;
(2) consisting of residual austenite dispersed in the matrix,
The volume fraction of retained austenite at a thickness of 1/4 position is more than 11% and not more than 20%, and
A high tensile thickness for cryogenic temperatures in which the volume ratio of retained austenite at a 1/4 position of the plate thickness is more than 11% and not more than 20% after sub-zero treatment in liquid nitrogen at −196 ° C. for 15 minutes. steel sheet.

2. The component composition is further in 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-tensile steel plate for cryogenic temperature according to 1 above, containing 1 or 2 or more selected from the group consisting of V: 0.05% or less and Ti: 0.03% or less.

3. The component composition is further in mass%,
Ca: 0.007% or less,
The high-tensile steel plate for cryogenic use 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). The steel material having the component composition according to any one of the above 1-3 is heated to a heating temperature of 900 ° C. or higher and 1200 ° C. or lower,
Hot-rolling the heated steel material into 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 thickness of 1/4 position, and a cooling stop temperature of 300 ° C. at a temperature of 1/4 position. The following first accelerated cooling is applied,
The hot-rolled steel sheet after the first accelerated cooling is subjected to two-phase region heating that is heated to a heating temperature of Ac1 point or more and less than Ac3 point at a temperature at a thickness of 1/4 position,
In the hot-rolled steel sheet after the two-phase region heating, the average cooling rate at the temperature at the 1/4 position of the sheet thickness is 3 ° C./s or more, and the cooling stop temperature is 500 ° C. or less at the temperature at the 1/4 position of the sheet thickness 250 ° C. The second accelerated cooling is applied,
The hot rolled steel sheet after the second accelerated cooling is air-cooled to 200 ° C. or less,
The air-cooled hot-rolled steel sheet is subjected to a tempering treatment in which the tempering temperature is 500 ° C. or higher and 650 ° C. or lower at a position at a thickness of 1/2, and for the cryogenic temperature having the microstructure described in 1 above A method for producing a high-tensile steel plate for cryogenic use, which is a high-tensile steel plate.

5. Furthermore, after the hot rolling, prior to the first accelerated cooling,
Air-cooling the hot-rolled steel sheet to an air-cooling stop temperature of 300 ° C. or lower,
5. The method for producing a high-tensile steel plate for cryogenic use according to 4 above, wherein the air-cooled hot-rolled steel sheet is reheated to a reheating temperature of Ac3 or higher and 1000 ° C or lower.

According to the present invention, although the Ni content is reduced to 5.5 to 8.5%, it has low temperature toughness equivalent to or better than 9% Ni steel, and also has excellent cold workability. In addition, a high-tensile steel plate for cryogenic temperatures can be obtained. This high-tensile steel plate for cryogenic use can be used very suitably for applications such as LNG storage tanks. Therefore, this invention contributes to the safety | security improvement of steel structures, such as a tank for LNG storage, and has an industrial remarkable effect.

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 to this.

[Ingredient composition]
The steel material used in the production of the cryogenic high-tensile thick steel plate (hereinafter sometimes simply referred to as “steel plate” or “thick steel plate”) and the cryogenic high-tensile thick steel plate has the above-described component composition. . Hereinafter, each component contained in the component composition will be described. Unless otherwise specified, “%” as a unit of content of components in the present specification means “% by mass”.

C: 0.02 to 0.12%
C is an element having an effect of improving the strength of the steel plate. C is also an important element in obtaining a desired retained austenite volume fraction. In order to obtain these effects, the C content is 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 is lowered. 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 having an action as a deoxidizer. In order to express these effects, the Si content is 0.01% or more. On the other hand, when 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 high strength of the steel sheet. 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, if the Mn content exceeds 2.00%, the effect of improving the strength of the steel sheet is saturated and, on the other hand, the low temperature toughness is lowered. Therefore, the Mn content is 2.00% or less, preferably 0.95% or less.

Ni: 5.5 to 8.5%
Ni is an extremely effective element for improving the low temperature toughness of the steel sheet. However, since Ni is an expensive element, the steel sheet cost increases as its content increases. Therefore, the Ni content is set to 8.5% or less. On the other hand, if the Ni content is less than 5.5%, stable austenite that is stable at low temperatures cannot be obtained, and as a result, the low temperature toughness of the steel sheet decreases. 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 metal and a welded joint when a steel plate is welded to obtain a welded structure. Therefore, the P content is 0.005% or less. On the other hand, since the lower the P content, the better. Therefore, the lower limit is not particularly limited and may be 0%, but in that case, inclusion as an unavoidable impurity is permitted. However, since excessive reduction causes an increase in cost, the P content is preferably 0.001% or more.

S: 0.003% or less
S, like 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 S content as much as possible in order to obtain a sound base metal and a welded joint when a steel plate is welded to obtain 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. Therefore, the lower limit is not particularly limited and may be 0%, but in that case, inclusion as an unavoidable impurity is permitted. However, excessive reduction causes an increase in cost, so the S content is preferably 0.0001% or more.

N: 0.0015 to 0.0065%
N is an element that forms precipitates in steel, and contributes to the refinement of the base material by forming AlN. In order to acquire the said effect, N content shall be 0.0015% or more. On the other hand, when the N content exceeds 0.0065%, the toughness of the base material and the weld heat affected zone is lowered when the steel plate is welded to form a welded structure. Therefore, the N content is 0.0065% or less.

The component composition in an embodiment of the present invention may be composed of the above elements, with the balance being Fe and inevitable impurities.

In another embodiment of the present invention, one or more selected from the group consisting of Al, Mo, Cr, Cu, Nb, V, and Ti is arbitrarily described below. It can be further contained in an amount.

Al: 0.01 to 0.10%
Al is an element contained in the deoxidizer. When the Al content is less than 0.01%, the effect as a deoxidizer 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 the steel sheet without impairing the low temperature toughness. When adding Mo, in order to acquire the said effect, Mo content shall be 0.05% or more, Preferably it exceeds 0.10%. On the other hand, if 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 adding Cr, the Cr content is 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 an effect of increasing the steel sheet strength by improving the hardenability. However, if the Cu content exceeds 0.40%, the low-temperature toughness of the steel sheet decreases, and the properties of the steel (slab) surface after casting deteriorate. Therefore, when adding Cu, the Cu content is set to 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 effects, the Cu content is preferably set to 0.10% or more.

Nb: 0.05% or less Nb is an effective element that increases the strength of the steel sheet by precipitation strengthening. However, when the Nb content is excessively high, the low temperature toughness of the steel sheet is lowered. 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 set to 0.010% or more.

V: 0.05% or less V, like Nb, is an effective element that increases the steel sheet strength by precipitation strengthening. However, when the V content is excessively high, the low temperature toughness of the steel sheet is lowered. Therefore, when V is added, the V content is 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 set to 0.010% or more.

Ti: 0.03% or less Ti is an element having an effect of increasing the toughness of a welded part without deteriorating the mechanical properties of the base metal when a steel plate is welded to form a welded structure. Therefore, Ti can be arbitrarily 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.

Moreover, in other embodiment of this invention, the said component composition can further contain further 1 or 2 or more selected from the group which consists of Ca, REM, and Mg in the quantity 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 form of inclusions in the steel. However, if Ca is excessive, the cleanliness of the steel is impaired. 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 is preferably 0.0005% or more from the viewpoint of enhancing the above effect.

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 form of inclusions in the steel, like Ca. However, when REM becomes excessive, the cleanliness of the steel is impaired. Therefore, when REM is added, the REM content is set to 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 effect, the REM content is preferably 0.0005% or more.

Mg: 0.070% or less Mg, like Ca and REM, is an element having an effect of improving the low temperature toughness of the steel sheet by controlling the form of inclusions in the steel. However, if Mg is excessive, the cleanliness of the steel is impaired. Therefore, when adding Mg, the Mg content is set to 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.

[Microstructure]
In the high-tensile steel plate for cryogenic use according to the present invention, in order to ensure cold workability and cryogenic toughness, the microstructure at the 1/4 thickness position is dispersed in (1) matrix and (2) the matrix. And retained austenite.

The matrix is composed of (A) tempered martensite or (B) tempered martensite and tempered bainite. If the matrix does not satisfy 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)
In the high-tensile steel plate for cryogenic temperatures of the present invention, the volume ratio of retained austenite at the 1/4 thickness position of the cryogenic high-tensile steel plate is more than 11% and not more than 20%. If the volume ratio is 11% or less, desired cold workability cannot be obtained. On the other hand, if the volume ratio exceeds 20%, the desired strength cannot be ensured under the condition that the Ni content is 5.5 to 8.5%.

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

In addition, it is preferable that the amount of decrease in retained austenite when the sub-zero treatment is performed under the above conditions is less than 0.5% in volume ratio. Here, the amount of decrease refers to the difference between the volume fraction of retained austenite before subzero treatment and the volume fraction of retained austenite after subzero treatment.

[Thickness]
The thickness of the high-tensile steel plate for cryogenic use of the present invention is not particularly limited, and can be any thickness, but is preferably 6 mm or more and 50 mm or less.

[Mechanical properties]
(Tensile strength)
The lower limit of the tensile strength (TS) of the high-tensile steel plate for cryogenic temperatures of the present invention is not particularly limited and can be any value, but the tensile strength is preferably 700 MPa or more, and 720 MPa or more. It is more preferable to set it to 740 MPa or more. On the other hand, the upper limit of the tensile strength is not particularly limited and may be any value, but the tensile strength is preferably 930 MPa or less, and more preferably 900 MPa or less. In addition, the said tensile strength can be measured by the method described in the Example.

(Toughness)
The toughness of the high-temperature steel sheet for cryogenic use of the present invention is not particularly limited and can be any value, but the Charpy absorbed energy (vE-196 _{° C.} ) at _{−196 ° C.} is preferably 150 J or more, More preferably, it is 180 J or more, more preferably 200 J or more, and most preferably 240 J or more. Further, the upper limit of the Charpy absorbed energy is not particularly limited, but may be 350 J or less or 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-temperature steel sheet for cryogenic use of the present invention is not particularly limited, but the brittle fracture surface ratio in a strain-aged Charpy test at 3% strain and a test temperature of -196 ° C is 2% or less. Preferably, it is more preferably 0%. The brittle fracture surface ratio can be regarded as an index of cold workability. In addition, the said brittle fracture surface rate can be evaluated by the method described in the Example.

[Production method]
Next, the manufacturing method of the high-tensile steel plate for cryogenic temperature in one Embodiment of this invention is demonstrated. In the following description, unless otherwise specified, the temperature refers to the temperature at the center of the plate thickness (plate thickness 1/2 position). 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 with a radiation thermometer.

In one embodiment of the present invention, a high-tensile steel plate for cryogenic temperature having the above-described microstructure can be manufactured by sequentially performing the following steps (1) to (7).
(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 treatment

(1) Heating of steel material First, the steel material which has the component composition mentioned above is heated to the heating temperature of 900 degreeC or more and 1200 degrees C or less. Although the manufacturing method of the said steel raw material is not specifically limited, For example, it can manufacture by melting and casting the molten steel which has the above-mentioned composition by a conventional method. The melting can be performed by an arbitrary method such as a converter, electric furnace, induction furnace or the like. The casting is preferably performed by a continuous casting method from the viewpoint of productivity, but can also be performed by an ingot-making / decomposing rolling method. As the steel material, for example, a steel slab can be used.

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

Heating temperature: 900-1200 ° C
If the heating temperature is less than 900 ° C., the deformation resistance of the steel material is high, so the load on the rolling mill in hot rolling increases, making it difficult to perform hot rolling. 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 and the loss due to the oxidation increases, resulting in a decrease in yield. Therefore, the heating temperature is set to 1200 ° C. or lower.

(2) Hot rolling After the said heating, the heated steel raw material is hot-rolled to make a hot-rolled steel sheet. The final thickness of the hot-rolled steel sheet is not particularly limited, but is preferably 6 mm or more and 50 mm or less as described above.

(3) 1st accelerated cooling After the said hot rolling, the 1st accelerated cooling whose average cooling rate is 1 degree-C / s or more and whose cooling stop temperature is 300 degrees C or less is given to the said hot-rolled steel plate. The hot-rolled steel sheet is quenched by the first accelerated cooling, and becomes a martensite and bainite structure.

Average cooling rate: 1 ° C./s or more In the first accelerated cooling, if the average cooling rate in the temperature range of 550 ° C. or less and 300 ° C. or more is less than 1 ° C./s, the temperature at the 1/4 thickness position is desired This transformation structure cannot be 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 there is a variation in material in the sheet width direction and the rolling direction. It becomes easy to come out. As a result, variations in material properties such as tensile properties occur. Therefore, the average cooling rate is preferably 200 ° C./s or less.

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

The first accelerated cooling can be performed by any method without any 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 not lower than Ac1 point and lower than Ac3 point at a temperature at the 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 converted into a mixed structure of bainite and austenite in which C, Ni, and Mn are concentrated by reverse transformation from martensite.

Heating temperature: not less than Ac1 point and less than Ac3 point When the heating temperature is less than Ac1 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 plate. On the other hand, when the heating temperature is Ac3 point or higher, bainite and martensite are all reversely transformed into 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.

In addition, Ac1 point and Ac3 point can be calculated | required by the following (1) Formula and (2) Formula.
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 (° C) = 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 symbols in the above formulas (1) and (2) represent the content (% by mass) of each element, and 0 when the element is not contained.

Any heating method can be used for the two-phase region heating 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.

In the two-phase region heating, the next second accelerated cooling may be started immediately after reaching the heating temperature, but the second second acceleration is performed after holding the heating temperature 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 second accelerated cooling in which the average cooling rate is 3 ° C./s or more and the cooling stop temperature is 500 ° C. or less and 250 ° C. or more is applied to the hot-rolled steel sheet after the two-phase region heating. Apply. The hot rolled steel sheet is quenched by the second accelerated cooling to obtain a quenched 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 In the second accelerated cooling, if the average cooling rate at the temperature at the plate thickness 1/4 position is less than 3 ° C./s, a desired quenched structure cannot be obtained, The strength of the thick steel plate finally obtained decreases. Therefore, the average cooling rate is set to 3 ° 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 there is a variation in material in the sheet width direction and the rolling direction. It becomes easy to come out. As a result, variations in material properties such as tensile properties occur. Therefore, the average cooling rate is preferably 200 ° C./s or less. In addition, the said average cooling rate shall point out the average cooling rate between the acceleration cooling start in the 2nd acceleration cooling process to an acceleration cooling stop here.

Cooling stop temperature: 250-500 ° C
The cooling stop temperature in the second accelerated cooling is set to 250 ° C. or more and 500 ° C. or less at the temperature at the plate thickness ¼ position. By accelerating cooling at a temperature of 250 ° C. or more and 500 ° C. or less and then air cooling, C can be concentrated to untransformed austenite and austenite can be stabilized. If the accelerated cooling stop temperature is less than 250 ° C., untransformed austenite is transformed into martensite, and a desired retained austenite amount cannot be obtained. On the other hand, if the accelerated cooling stop temperature is higher than 500 ° C., the distribution of C to the austenite phase becomes insufficient, the amount of residual austenite finally obtained decreases, and the desired toughness cannot be obtained.

The second accelerated cooling can be performed by any method without any 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 accelerated cooling is air-cooled to 200 ° C. or less (air cooling after accelerated 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 process Next, a tempering process is performed. By the tempering treatment, the retained austenite can be stabilized and the bainite and martensite structure can be tempered to improve toughness.

Tempering temperature: 500-650 ° C
The tempering temperature in the tempering treatment is set to 500 ° C. or more and 650 ° C. or less at a temperature at a position of 1/2 the plate thickness. When the tempering temperature is less than 500 ° C., the retained austenite is not sufficiently stabilized, and the toughness improvement by tempering of the bainite and martensite structure is insufficient. Therefore, the tempering temperature is set to 500 ° C. or higher. On the other hand, if the tempering temperature exceeds 650 ° C., the stability of retained austenite is lowered, and the desired low-temperature toughness cannot be obtained. Therefore, the said tempering temperature shall be 650 degrees C or less.

In another embodiment of the present invention, the method for producing a cryogenic high-tensile steel plate further optionally includes 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 the hot rolling is air-cooled to an air-cooling stop temperature of 300 ° C. or less (air-cooling after hot rolling). In this embodiment, an austenite structure refined by the phase transformation in the next reheating treatment is obtained. Therefore, in this air cooling step, the microstructure of the steel sheet is once changed to 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 Ac3 to 1000 ° C. prior to the next first accelerated cooling. By the reheating, the ferrite structure of the hot-rolled steel sheet is reversely transformed into austenite, and the reversely transformed austenite is transformed into martensite and bainite by the following first accelerated cooling.

Reheating temperature: Ac3 point ~ 1000 ℃
The reheating temperature in the reheating is set to Ac3 point or higher and 1000 ° C or lower. By reheating up to the reheating temperature, the structure of the hot-rolled steel sheet can be made into a uniform and refined austenite structure. If the reheating temperature is less than Ac3 point, the ferrite structure remains in the finally obtained thick steel sheet, and the desired strength cannot be obtained. On the other hand, when the reheating temperature is higher than 1000 ° C., the operation load becomes large, and austenite coarsens, so that desired toughness cannot be obtained.

Any heating method can be used for the reheating as long as the reheating 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.

A high-tensile steel plate for cryogenic temperatures was manufactured according to the procedure described below, and the characteristics of the obtained high-tensile steel plate for cryogenic temperatures were evaluated.

First, molten steel having the composition shown in Table 1 was melted in a converter, and a steel slab (thickness: 250 mm) as a steel material was manufactured by a continuous casting method. The Ac1 point (° C.) obtained from the above-described equation (1) and the Ac3 point (° C.) obtained from the equation (2) are also shown in Table 1.

Next, the obtained steel slab was heated to the heating temperature shown in Table 2 and hot-rolled to obtain a hot-rolled steel plate having the thickness shown in Table 2. Next, first hot cooling was performed on the hot-rolled steel sheet. The average cooling rate and cooling stop temperature in the first accelerated cooling were 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 two-phase region heating at the heating temperature shown in Table 2 was performed on the hot rolled steel sheet after the first accelerated cooling. After the two-phase region heating, the hot rolled steel sheet was subjected to second accelerated cooling. The average cooling rate and cooling stop temperature in the second accelerated cooling were as shown in Table 2. After the second accelerated cooling, air cooling to 200 ° C. or lower was performed, and then tempering was performed. The tempering temperature in the tempering was as shown in Table 2.

In addition, the heat processing furnace was used for the heating in each said process.

Next, for each of the obtained thick steel plates, the microstructure, the amount of retained austenite after sub-zero treatment, mechanical properties, and cold workability were evaluated. The evaluation was performed by the method described below.

(Micro structure)
From each thick steel plate, a test piece for observing the microstructure was taken so that the ¼ position of the plate thickness becomes the observation position. The test piece was embedded in resin so that a cross section perpendicular to the rolling direction was an observation surface, and mirror-polished. Next, after performing the nital corrosion, an image of the tissue was taken by observation with a scanning electron microscope having a magnification of 400 times. The obtained image was analyzed to identify the microstructure.

In addition, the steel plate obtained as described above is comparative example No. Except 6, all had lath-like micro, and the microstructure was tempered martensite or a mixed structure of tempered martensite and tempered bainite structure. Moreover, the obtained steel plate was comparative example No. in which the volume fraction of retained austenite was 0%. Except for 5, the microstructure had a structure in which retained austenite was dispersed in the matrix.

-Residual austenite volume fraction Ten X-ray diffraction test pieces were collected in parallel with the plate surface from the 1/4 thickness position of the thick steel plate, and five of the test pieces were subjected to sub-zero treatment. Sub-zero treatment was performed by holding the test piece in liquid nitrogen at -196 ° C for 15 minutes. Each of the five test pieces without subzero treatment and the subzero treatment was subjected to X-ray diffraction by grinding and chemical polishing the test piece so that the plate thickness ¼ position was the measurement surface. Diffraction intensities of (200), (211) planes of α-Fe and (200), (220), (311) planes of γ-Fe appearing in the symmetrical reflection X-ray diffraction pattern are obtained, and the volume fraction of γ-Fe is calculated. The average value in each of the five test pieces was calculated and used as the volume fraction of retained austenite.

(Mechanical properties)
A JIS No. 4 tensile test piece was taken from the position of 1/2 the thickness of the thick steel plate. Using the tensile test piece, a tensile test was performed in accordance with the provisions of JIS Z 2241 to evaluate the tensile strength (TS) of the thick steel plate.

In addition, a V-notch test piece was taken from the position of the thickness ¼ of the thick steel plate according to JIS Z 2202. Using the V-notch test piece, a Charpy impact test was performed in accordance with JIS Z 2242, and Charpy absorbed energy (vE-196 _{° C} ) at _{-196 ° C} was obtained. The Charpy absorbed energy can be regarded as an index of toughness at a very low temperature of a thick steel plate.

(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, an aging treatment was performed at 250 ° C. for 1 hour. A V-notch test piece was sampled 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 performed in accordance with JIS Z 2242, and the brittle fracture surface ratio at -196 ° C. was determined. The brittle fracture surface ratio can be regarded as an index of cold workability of the thick steel plate.

Evaluation results are shown in Table 3. Table 3 also shows the amount of decrease in retained austenite due to the sub-zero treatment. Here, the amount of decrease in retained austenite is a value obtained by subtracting the residual austenite volume ratio after subzero treatment from the residual austenite volume ratio before subzero treatment.

As can be seen from the results, all the thick steel plates that satisfy the conditions of the present invention have excellent tensile strength: 700 MPa or more, vE-196 _{° C .} : 150 J or more, and a brittle fracture surface ratio of 2% or less. Combined mechanical properties and cold workability. If the Charpy absorbed energy vE-196 _{° C.} is 150 J or more, it can be said that the toughness is equal to or higher than that of a 9% Ni steel plate. On the other hand, a thick steel plate that does not satisfy the conditions of the present invention has inferior at least one characteristic among strength, toughness, and cold workability.

Claims

% By mass
C: 0.02 to 0.12%,
Si: 0.01 to 0.30%,
Mn: 0.50 to 2.00%,
Ni: 5.5 to 8.5%,
P: 0.005% or less,
S: 0.003% or less, and N: 0.0015-0.0065%,
The balance has a component composition consisting of Fe and inevitable impurities,
The microstructure at the 1/4 thickness position is
(1) Tempered martensite or a matrix composed of tempered martensite and tempered bainite;
(2) consisting of residual austenite dispersed in the matrix,
The volume fraction of retained austenite at a thickness of 1/4 position is more than 11% and not more than 20%, and
A high tensile thickness for cryogenic temperatures in which the volume ratio of retained austenite at a 1/4 position of the plate thickness is more than 11% and not more than 20% after sub-zero treatment in liquid nitrogen at −196 ° C. for 15 minutes. steel sheet.
The component composition is further in 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-tensile steel plate for cryogenic temperature according to claim 1, containing one or more selected from the group consisting of V: 0.05% or less and Ti: 0.03% or less.
The component composition is further in mass%,
Ca: 0.007% or less,
The high-tensile steel plate for cryogenic temperatures 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 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,
Hot-rolling the heated steel material into 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 thickness of 1/4 position, and a cooling stop temperature of 300 ° C. at a temperature of 1/4 position. The following first accelerated cooling is applied,
The hot-rolled steel sheet after the first accelerated cooling is subjected to two-phase region heating that is heated to a heating temperature of Ac1 point or more and less than Ac3 point at a temperature at a thickness of 1/4 position,
In the hot-rolled steel sheet after the two-phase region heating, the average cooling rate at the temperature at the 1/4 position of the sheet thickness is 3 ° C./s or more, and the cooling stop temperature is 500 ° C. or less at the temperature at the 1/4 position of the sheet thickness 250 ° C. The second accelerated cooling is applied,
The hot rolled steel sheet after the second accelerated cooling is air-cooled to 200 ° C. or less,
The ultra-low temperature having the microstructure according to claim 1, wherein 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 at the position of the thickness 1/2. A method for producing a high-tensile thick steel plate for cryogenic use, which is a high-tensile thick steel plate for industrial use.
Furthermore, after the hot rolling, prior to the first accelerated cooling,
Air-cooling the hot-rolled steel sheet to an air-cooling stop temperature of 300 ° C. or lower,
The manufacturing method of the high-tensile steel plate for cryogenic temperature of Claim 4 which reheats the said hot-rolled steel plate cooled by air to the reheating temperature of Ac3 point or more and 1000 degrees C or less.