KR20200140907A - High tensile strength thick steel sheet for cryogenic use and its manufacturing method - Google Patents

Abstract

It is an object of the present invention to provide 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. The microstructure at the position of 1/4 thickness of the plate having a predetermined composition composition is (1) a matrix composed of tempered martensite or tempered martensite and bainite, and (2) residual dispersed in the matrix. After a sub-zero treatment consisting of austenite and retaining austenite in a volume ratio of more than 11% and 20% or less at a position of 1/4 thickness of the plate, and holding in liquid nitrogen at -196°C for 15 minutes And, the volume ratio of retained austenite at the position of 1/4 of the sheet thickness is more than 11% and not more than 20%, a high tensile strength thick steel sheet for cryogenic use.

Description

High tensile strength thick steel sheet for cryogenic use and its manufacturing method

The present invention relates to a high-strength thick steel sheet for cryogenic temperatures, and in particular, to a high-strength thick steel sheet for cryogenic temperatures, which has excellent cryogenic toughness and cold workability, and can be preferably used in applications such as tanks for storing liquefied natural gas (LNG). . In addition, the present invention relates to a method of manufacturing the cryogenic high-tensile steel sheet.

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

However, since Ni is an expensive alloying element, from the viewpoint of cost reduction, 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 is desired.

Generally, when the Ni content of the low-temperature steel sheet is reduced, austenite becomes unstable in the low-temperature region, so that low-temperature toughness decreases, and it becomes difficult to secure the high level of safety required for the LNG storage tank. In response to this problem, in a low-temperature steel sheet having a reduced Ni content, various techniques for improving steel sheet properties such as low-temperature toughness have been proposed.

For example, in Patent Documents 1 to 5, a steel sheet having a low-temperature toughness equivalent to that of Ni steel from 7% to 9% Ni has been proposed.

In Patent Document 1, a technique of combining low-temperature depressurization rolling, two-phase heat treatment, and quenching tempering treatment is proposed. In the above technique, by introducing a strain into untransformed austenite to lower the Mf point, the residual austenite structure is controlled and stabilized.

Patent Documents 2 and 3 disclose a technique for securing the amount of retained austenite and miniaturizing the particle size by controlling the heating temperature and heating time of the slab to suppress excessive slab heating.

Patent Document 4 discloses a technique for reducing the non-uniformity of alloying elements by subjecting the slab to a plurality of heat treatments and further performing two-phase reverse heat treatment to disperse residual austenite in a large amount and uniformly and finely.

Patent Literature 5 discloses a technique for obtaining a tempered martensite structure in which residual austenite is finely dispersed by controlling the cumulative reduction ratio of the unrecrystallized region and the recrystallized region and stipulating the quenching tempering conditions.

International Publication No. 2007/034576 Japanese Patent Application Publication No. 2011-219849 Japanese Unexamined Patent Publication No. 2011-241419 International Publication No. 2012/005330 Japanese Unexamined Patent Publication No. 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, in order to generate austenite, a quenching treatment was performed after two-phase heating, but since most of the austenite was transformed into martensite during quenching, stable retained austenite could not be sufficiently obtained. .

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 technology proposed in Patent Documents 1 to 5, the amount of retained austenite after treatment with the subzero at -196°C is only 11% by volume, and stable retained austenite cannot be obtained in large amounts. . Therefore, it cannot be said that cold workability is sufficient.

In view of such circumstances, an object of the present invention is to provide 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.

In order to achieve the said subject, the inventors of the present invention conducted extensive research and obtained the following knowledge.

(1) After reducing Ni to less than 9%, in order to obtain cryogenic toughness equivalent to 9% Ni steel and excellent cold workability, the volume ratio of retained austenite after treatment with subzero at -196°C exceeds 11%. , It may be controlled to 20% or less.

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

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

The present invention has been completed based on the above findings, and the summary 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 to 0.0065% is contained,

The balance has a component composition consisting of Fe and unavoidable impurities,

The microstructure at the position of 1/4 thickness of the plate,

(1) a matrix consisting of tempered martensite or tempered martensite and bainite, and

(2) consisting of residual austenite dispersed in the matrix,

The volume fraction of retained austenite at the position of 1/4 of the plate thickness is more than 11% and not more than 20%, and

The volume ratio of retained austenite at the position of 1/4 of the plate thickness after performing the subzero treatment held in liquid nitrogen at -196°C for 15 minutes is more than 11% and not more than 20%, a high tensile strength thick steel sheet for cryogenic use.

2. The component composition is further 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,

V: 0.05% or less, and

Ti: 0.03% or less

The cryogenic high-tensile strength thick steel sheet according to 1 above, containing 1 or 2 or more selected from the group consisting of.

3. The component composition is further by mass%,

Ca: 0.007% or less,

REM: 0.010% or less, and

Mg: 0.070% or less

The cryogenic high tensile strength thick steel sheet according to 1 or 2 above, containing 1 or 2 or more selected from the group consisting of.

4. The steel material having the component composition according to any one of the above 1 to 3 is heated at a heating temperature of 900°C or more and 1200°C or less,

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 less and 300°C or more at a temperature at a position of 1/4 thickness of the sheet, and a cooling stop temperature of 1°C/s or more, First accelerated cooling is performed at a temperature of 300° C. or less,

The hot-rolled steel sheet after the first accelerated cooling is heated to a heating temperature of not less than the Ac1 point and less than the Ac3 point at a temperature at a position of 1/4 of the plate thickness,

In the hot-rolled steel sheet after 2 phase-inverse heating, the average cooling rate at the temperature at the position of 1/4 plate thickness is 3°C/s or more, and the cooling stop temperature is 500°C or less at the temperature at the position of 1/4 plate thickness 250 Performing a second accelerated cooling of not less than °C,

Air cooling the hot-rolled steel sheet after the second accelerated cooling to 200°C or less,

The hot-rolled steel sheet after air cooling is subjected to a tempering treatment in which the tempering temperature is 500°C or more and 650°C or less at the temperature at the position of 1/2 of the plate thickness to obtain a cryogenic high-tensile strength thick steel sheet having the microstructure described in 1 above, Method of manufacturing high-tensile steel plate for cryogenic use.

5. Additionally, after the hot rolling, before the first accelerated cooling,

Air cooling the hot-rolled steel sheet to an air cooling stop temperature of 300 °C or less,

The method for producing a cryogenic high-tensile steel sheet according to 4 above, wherein the air-cooled hot-rolled steel sheet is reheated to a reheating temperature of not less than Ac3 and not more than 1000°C.

According to the present invention, although the Ni content is reduced to 5.5 to 8.5%, it is possible to obtain a high tensile strength thick steel sheet for cryogenic use, which has a low-temperature toughness equal to or higher than that of 9% Ni steel and excellent in cold workability. This ultra-low-temperature high-tensile steel plate can be very suitably used for applications such as tanks for storing LNG. Therefore, the present invention contributes to improving the safety of steel structures such as LNG storage tanks, and exhibits a special effect in the industry.

Hereinafter, embodiments of the present invention will be specifically described. In addition, 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 cryogenic use of the present invention (hereinafter, simply referred to as "steel sheet" or "thick steel sheet" in some cases), and the steel material used in the manufacture of the high-strength steel sheet for cryogenic use have the above-described component composition. Hereinafter, each component included in the component composition will be described. In addition, "%" as a unit of the content of a component in this specification means "mass%" unless otherwise stated.

C: 0.02 to 0.12%

C is an element having an effect of improving the strength of the steel sheet. Moreover, C is also an important element in obtaining a desired retained austenite volume ratio. In order to obtain these effects, the C content is made 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 set to 0.12% or less, preferably 0.08% or less.

Si: 0.01 to 0.30%

Si is an element that contributes to the improvement of the strength of the steel sheet, and is also an element having an action as a deoxidizing agent. In order to express these effects, the Si content is made 0.01% or more. On the other hand, when the Si content becomes excessively high, the toughness decreases. Therefore, the Si content is set to 0.30% or less, preferably 0.10% or less.

Mn: 0.50 to 2.00%

Mn is an element contributing to increase of the strength of the steel sheet by enhancing the etchability of steel. When the Mn content is less than 0.50%, the hardening property of the steel decreases, and not only the strength of the steel sheet but also the low-temperature toughness decreases. Therefore, the Mn content is made 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 on the other hand, the low-temperature toughness decreases. Therefore, the Mn content is 2.00% or less, preferably 0.95% or less.

Ni: 5.5 to 8.5%

Ni is an element very effective in improving the low-temperature toughness of a steel sheet. However, since Ni is an expensive element, the cost of the steel sheet increases 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 decreases. Therefore, the Ni content is 5.5% or more.

P: 0.005% or less

P is an inevitable impurity and is a harmful element that adversely affects the low-temperature toughness of the steel sheet. For example, in order to obtain a sound base material and a weld joint when a steel plate is welded to form a welded structure, it is preferable to reduce the P content as much as possible. Therefore, the P content is set to 0.005% or less. On the other hand, since the lower the P content is, the better, the lower limit is not particularly limited and may be 0%. Even in that case, it is allowed to contain it as an inevitable 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 inevitable impurity and is a harmful element that adversely affects the low-temperature toughness of the steel sheet. For example, in order to obtain a sound base material and a weld joint when a steel plate is welded to form a welded structure, it is preferable to reduce the S content as much as possible. Therefore, the S content is set to 0.003% or less. On the other hand, since the lower the S content is, the better, the lower limit is not particularly limited and may be 0%. Even in that case, it is allowed to contain it as an unavoidable impurity. However, since excessive reduction causes cost increase, 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 by forming AlN, it contributes to fine graining of the base material. In order to obtain the above effect, the N content is made 0.0015% or more. On the other hand, when the N content exceeds 0.0065%, when the steel sheet is welded to form a welded structure, the toughness of the base metal and the welded heat affected zone decreases. Therefore, the N content is set to 0.0065% or less.

In one embodiment of the present invention, the component composition may be composed of the element and the balance Fe and inevitable impurities.

In addition, in another embodiment of the present invention, the component composition is optionally 1 or 2 or more selected from the group consisting of Al, Mo, Cr, Cu, Nb, V, and Ti in the amount described below. It may contain additionally.

Al: 0.01 to 0.10%

Al is an element contained in the deoxidizing agent. When the Al content is less than 0.01%, the effect as a deoxidizer is insufficient. Therefore, in the case of containing Al, the Al content is set to 0.01% or more, preferably 0.02% or more. On the other hand, when the Al content exceeds 0.10%, the cleanliness of 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 capable of improving the strength of a steel sheet without impairing the low-temperature toughness. In the case of adding Mo, in order to obtain the above effect, the Mo content is made 0.05% or more, preferably more than 0.10%. 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 adding Cr, 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 made 0.01% or more.

Cu: 0.40% or less

Cu is an element having an effect of increasing the strength of the steel sheet by improving the quenchability. However, when the Cu content exceeds 0.40%, in addition to lowering the low-temperature toughness of the steel sheet, the properties of the surface of the steel (slab) after casting deteriorate. Therefore, when Cu is added, the Cu content is made 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 to increase the strength of the steel sheet by precipitation strengthening. However, when 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 be 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, it is preferable to set the Nb content to 0.010% or more.

V: 0.05% or less

Like Nb, V is an effective element that increases the strength of the steel sheet by precipitation strengthening. However, when 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 be 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 the welded portion without deteriorating the mechanical properties of the base metal when a steel sheet is welded to form a welded structure. Therefore, it can optionally contain Ti 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 component composition may optionally further contain 1 or 2 or more selected from the group consisting of Ca, REM, and Mg in an amount described below.

Ca: 0.007% or less

Ca is an element having an effect of improving the low-temperature toughness of a steel sheet by controlling the shape of inclusions in the steel. However, when Ca becomes excessive, cleanliness of steel is impaired. Therefore, when adding Ca, the Ca content is made 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), like Ca, is an element having an effect of improving the low-temperature toughness of a steel sheet by controlling the shape of inclusions in the steel. However, when REM becomes excessive, the cleanliness of the steel is impaired. Therefore, when REM is added, the REM content is made 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

Like Ca and REM, Mg is an element that has an effect of improving the low-temperature toughness of a steel sheet by controlling the shape of inclusions in the steel. However, when Mg becomes excessive, cleanliness of the steel is impaired. Therefore, when Mg is added, 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.

[Micro organization]

In order to secure cold workability and cryogenic toughness, the high-tensile steel sheet for cryogenic use of the present invention has a microstructure at a position of 1/4 of the sheet thickness, using (1) a matrix and (2) residual austenite dispersed in the matrix. Done.

The matrix is composed of (A) tempered martensite, or (B) tempered martensite and tempered bainite. When the matrix does not satisfy the above conditions, one or both of the tensile strength of 700 MPa or more and the desired low-temperature toughness cannot be obtained.

(Amount of retained austenite before subzero treatment)

In the high tensile strength thick steel sheet for cryogenic use of the present invention, the volume fraction of retained austenite at a position of 1/4 of the sheet thickness of the high tensile strength steel sheet for cryogenic use is more than 11% and not more than 20%. When the volume ratio is 11% or less, desired cold workability cannot be obtained. On the other hand, when the volume ratio exceeds 20%, the desired strength cannot be secured under the condition of the Ni content of 5.5 to 8.5%.

(Amount of retained austenite after subzero treatment)

In addition, in the high-tensile steel sheet for cryogenic use of the present invention, the volume ratio of retained austenite at the position of 1/4 of the sheet thickness after subzero treatment is applied to the high-tensile strength steel sheet for cryogenic use is more than 11% and 20% or less. Here, the sub-zero treatment is performed by holding the steel sheet in liquid nitrogen at -196°C for 15 minutes. When the volume ratio is 11% or less, desired cold workability cannot be obtained. It is preferable that the volume fraction of retained austenite after the subzero treatment is 12.5% or more. On the other hand, when the volume ratio exceeds 20%, the desired strength cannot be secured under the condition of the Ni content of 5.5 to 8.5%.

In addition, it is preferable that the reduction amount of retained austenite when subjected to the subzero treatment under the above conditions is less than 0.5% by volume. Here, the reduction amount is assumed to represent the difference between the volume ratio of retained austenite before subzero treatment and the volume ratio of retained austenite after subzero treatment.

[Plate thickness]

The sheet thickness of the high tensile strength thick steel sheet for cryogenic use 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 properties]

(The tensile strength)

The lower limit of the tensile strength (TS) of the high tensile strength thick steel sheet for cryogenic use of the present invention is not particularly limited and can be any value, but the tensile strength is preferably 700 MPa or more, more preferably 720 MPa or more. And it is more preferable to set it as 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. In addition, the tensile strength can be measured by the method described in Examples.

(tenacity)

The toughness of the high tensile strength thick steel sheet for cryogenic use of the present invention is not particularly limited and can be any value, but it is preferable that the Charpy absorbed energy (vE _-196°C ) at _-196°C is 150 J or more, and 180 It is more preferable to set it as J or more, it is still more preferable to set it as 200 J or more, and it is most preferable to set it as 240 J or more. Moreover, although it does not specifically limit also about the upper limit of the said Charpy absorbed energy, 350 J or less may be sufficient, and 280 J or less may be sufficient. In addition, the Charpy absorbed energy can be measured by the method described in Examples.

(Cold workability)

The cold workability of the high tensile strength thick steel sheet for cryogenic use of the present invention is not particularly limited, but deformation imparting: 3%, test temperature: It is preferable that the brittle fracture ratio in the strain aging Charpy test at -196°C is 2% or less, and 0 It is more preferable that it is %. The brittle fracture ratio can be regarded as an index of cold workability. In addition, the brittle fracture ratio can be evaluated by the method described in Examples.

[Manufacturing method]

Next, a method of manufacturing a high tensile strength thick steel sheet for cryogenic use in one embodiment of the present invention will be described. In addition, in the following description, unless otherwise specified, the temperature shall indicate the temperature at the center of the plate thickness (the position of the plate thickness 1/2). The temperature at the center of the plate thickness can be obtained by calculating the heat transfer from the surface temperature of the steel plate measured with a radiation thermometer.

In one embodiment of the present invention, by sequentially performing the steps of the following (1) to (7), it is possible to manufacture a high tensile strength thick steel sheet for cryogenic use having the above-described microstructure.

(1) heating of steel material

(2) hot rolling

(3) first accelerated cooling

(4) 2 phase heating

(5) second accelerated cooling

(6) air cooling

(7) Tempering treatment

(1) heating of steel material

First, a steel material having the above-described component composition is heated at a heating temperature of 900°C or more and 1200°C or less. The method for producing the steel material is not particularly limited, but it can be produced, for example, by melting and casting molten steel having the above composition by a conventional method. The solvent can be carried out by any method such as a converter, an electric furnace, and an induction furnace. Moreover, although it is preferable to carry out the said casting by a continuous casting method from a viewpoint of productivity, it can also carry out by the ingot-decomposition rolling method. As the steel material, for example, a steel slab may be used.

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

Heating temperature: 900 ~ 1200 ℃

If the heating temperature is less than 900° C., since the deformation resistance of the steel material is high, the load on the rolling mill in hot rolling increases, and it becomes 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 as a result, loss due to oxidation increases, the yield decreases. Therefore, the heating temperature is set to 1200°C or less.

(2) hot rolling

After the heating, the heated steel material is hot-rolled to obtain a hot-rolled steel sheet. The final sheet thickness of the hot-rolled steel sheet is not particularly limited, but it is preferably 6 mm or more and 50 mm or less, as described above.

(3) first accelerated cooling

After the hot rolling, the hot-rolled steel sheet is subjected to first accelerated cooling with 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 quenched by the first accelerated cooling to form a martensite and bainite structure.

Average cooling rate: 1 ℃/s or more

In the first accelerated cooling, if the average cooling rate in a temperature range of 550° C. or less and 300° C. or more at a temperature at a position of 1/4 of the plate thickness is less than 1° C./s, a desired transformation structure cannot be obtained, Can't get Therefore, the average cooling rate is set at 1°C/s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but when the average cooling rate is higher than 200°C/s, temperature control at each position in the steel sheet becomes difficult, resulting in material variations in the width direction and rolling direction. It becomes easy to do. And as a result, variations in material properties such as tensile properties occur. Therefore, it is preferable that the average cooling rate is 200°C/s or less.

Cooling stop temperature: 300 ℃ or less

The cooling stop temperature is set to 300°C or less at the temperature at the position of 1/4 of the plate thickness. If the cooling stop temperature is higher than 300°C, the transformation during quenching becomes insufficient, and therefore the desired strength cannot be obtained.

The first accelerated cooling is not particularly limited and can be performed by any method. For example, one or both of air cooling and water cooling may be used. As the water cooling, any cooling method using water (for example, spray cooling, mist cooling, lamina cooling, etc.) can be used.

(4) 2 phase heating

Subsequently, the cooled hot-rolled steel sheet is heated to a heating temperature of not less than the Ac1 point and less than the Ac3 point at a temperature at a position of 1/4 of the plate thickness (2 phase reverse heating). By performing the above two-phase reverse 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: more than Ac1 point, less than Ac3 point

When the heating temperature is less than the Ac1 point, the reverse transformation austenite is hardly obtained, and the desired microstructure cannot be obtained by the subsequent second accelerated cooling. And 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 equal to or higher than the Ac3 point, both bainite and martensite are reversely transformed to become austenite, and C, Ni, and Mn are averaged over the entire structure. And as a result, desired cold workability cannot be obtained.

In addition, the Ac1 point and the Ac3 point can be obtained by the following (1) equations and (2) equations.

Ac1 (℃) = 750.8-26.6 C + 17.6 Si-11.6 Mn-22.9 Cu-23 Ni + 24.1 Cr + 22.5 Mo-39.7 V-5.7 Ti + 232.4 Nb-169.4 Al… (One)

Ac3 (℃) = 937.2-436.5 C + 56 Si-19.7 Mn-16.3 Cu-26.6 Ni-4.9 Cr + 38.1 Mo + 124.8 V + 136.3 Ti-19.1 Nb + 198.4 Al. (2)

However, the element symbol in the above formulas (1) and (2) represents the content (% by mass) of each element, and is 0 when the element is not contained.

Any heating method can be used for the two-phase heating as long as the heating temperature can be controlled as described above. Furnace heating is mentioned as an example of a heating method. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

In the second phase heating, the next second accelerated cooling may be started immediately after reaching the heating temperature, but the next second accelerated cooling may be started after holding at the heating temperature for an arbitrary time. In the case of holding at the heating temperature, the holding time is not particularly limited, but is preferably 5 minutes or longer.

(5) second accelerated cooling

Subsequently, the hot-rolled steel sheet after the second phase heating is subjected to 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. The hot-rolled steel sheet is quenched by the second accelerated cooling to obtain a quenched structure in which residual austenite is dispersed in martensite or a matrix composed of martensite and bainite.

Average cooling rate: 3 ℃/s or more

In the second accelerated cooling, if the average cooling rate at the temperature at the position of 1/4 of the plate thickness is less than 3°C/s, the desired quenching structure cannot be obtained, and the strength of the finally obtained thick steel sheet is not Is lowered. 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 when the average cooling rate is higher than 200°C/s, temperature control at each position in the steel sheet becomes difficult, resulting in material variation in the width direction and rolling direction. It becomes easy to do. And as a result, variations in material properties such as tensile properties occur. Therefore, it is preferable that the average cooling rate is 200°C/s or less. Here, the average cooling rate is assumed to represent the average cooling rate during the period from the start of the accelerated cooling to the stop of the accelerated cooling in the second accelerated cooling step.

Cooling stop temperature: 250 ~ 500 ℃

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 position of 1/4 of the plate thickness. Accelerated cooling is stopped at a temperature of 250° C. or higher and 500° C. or lower, followed by air cooling, whereby C can be concentrated with untransformed austenite to stabilize austenite. When the accelerated cooling stop temperature is less than 250°C, untransformed austenite is transformed into martensite, and a 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 is not particularly limited and can be performed by any method. For example, one or both of air cooling and water cooling may be used. As the water cooling, any cooling method using water (for example, spray cooling, mist cooling, lamina 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-cooled 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 treatment

Subsequently, a tempering treatment is performed. By the tempering treatment, the retained austenite is stabilized and the bainite and martensite structures are tempered to improve toughness.

Tempering temperature: 500 ∼ 650 ℃

The tempering temperature in the tempering treatment is 500°C or more and 650°C or less at the temperature at the position of 1/2 of the plate thickness. When the tempering temperature is less than 500°C, the retained austenite is not sufficiently stabilized, and the improvement of toughness due to tempering of 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 rather decreases, and desired low-temperature toughness cannot be obtained. Therefore, the tempering temperature is set to 650°C or less.

In another embodiment of the present invention, the method of manufacturing a high tensile strength thick steel sheet for cryogenic use may further optionally perform the following steps (A) and (B) after the hot rolling and prior to the first accelerated cooling. have.

(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 less (air-cooled after hot rolling). In this embodiment, a fine-grained austenite structure is obtained by phase transformation in the next reheating treatment. Therefore, in this air cooling step, by cooling to an air cooling stop temperature of 300°C or less, the microstructure of the steel sheet is once made into a martensite + bainite structure.

(B) reheating

Next, the air-cooled hot-rolled steel sheet is reheated to a reheating temperature of from Ac3 point 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 not less than the Ac3 point and not more than 1000°C. By reheating to the above reheating temperature, the structure of the hot-rolled steel sheet can be made into a uniform and fine-grained austenite structure. When the reheating temperature is less than the Ac3 point, a ferrite structure remains in the finally obtained thick steel sheet, and the desired strength cannot be obtained. Further, when the reheating temperature is higher than 1000°C, in addition to increasing the operating load, 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. Furnace heating is mentioned as an example of a heating method. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

Example

In the procedure described below, a high tensile strength thick steel sheet for cryogenic use was produced, and the characteristics of the obtained high tensile strength steel sheet for cryogenic use were evaluated.

First, molten steel having the component composition shown in Table 1 was melted with a converter, and a steel slab (thickness: 250 mm) as a steel material was produced by a continuous casting method. In addition, the Ac1 point (°C) determined by the above-described (1) equation and the Ac3 point (°C) determined by the (2) equation are also listed in Table 1.

Next, the obtained steel slab was heated to the heating temperature shown in Table 2, hot-rolled, and a hot-rolled steel sheet having the thickness shown in Table 2 was obtained. Next, the first accelerated 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. Further, in some examples, after the hot rolling, before the first accelerated cooling, air cooling and reheating were performed under the conditions shown in Table 2.

The hot-rolled steel sheet after the first accelerated cooling was subjected to two-phase reverse heating at the heating temperature shown in Table 2. After the second phase 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 2nd accelerated cooling, air cooling to 200 degreeC or less was performed, and then tempering was performed. The tempering temperature in the tempering was as shown in Table 2.

In addition, for heating in each of the above steps, a heat treatment furnace was used.

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

(Micro organization)

From each thick steel plate, a test piece for microstructure observation was taken so that the position of 1/4 of the plate thickness became the observation position. The test piece was buried in a resin so that a cross section perpendicular to the rolling direction became an observation surface, and mirror polished. Subsequently, after nital corrosion was performed, it was observed with a scanning electron microscope with a magnification of 400 times, and an image of the tissue was taken. The obtained image was analyzed and the microstructure was identified.

In addition, the steel sheet obtained as described above, except for Comparative Example No. 6, all have lath-shaped microstructures, and the microstructures are tempered martensite, or tempered martensite and tempered bainite structure It was a mixed organization of. Moreover, the obtained steel sheet had a microstructure of a structure in which residual austenite was dispersed in a matrix, except for Comparative Example No. 5 in which the volume ratio of retained austenite was 0%.

· Retained austenite volume fraction

Ten X-ray diffraction specimens were taken parallel to the plate surface from the position of 1/4 of the plate thickness of the thick steel plate, and 5 of them were subjected to sub-zero treatment. The subzero treatment was performed by holding the test piece in liquid nitrogen at -196°C for 15 minutes. Each of the five specimens not treated with the sub-zero and treated with the sub-zero was subjected to grinding and chemical polishing on the specimen so that the position of 1/4 of the plate thickness was the measurement surface, and subjected to X-ray diffraction. The diffraction intensity of the (200), (211) planes of α-Fe and the (200), (220), (311) planes of γ-Fe appearing in the symmetrical reflection X-ray diffraction pattern is calculated, It calculated, the average value in each of five test pieces was calculated|required, and it was set as the volume fraction of retained austenite.

(Mechanical properties)

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

Further, a V-notch test piece was taken from the position of 1/4 of the plate thickness of the thick steel plate in accordance with the regulation of JIS Z 2202. Using the V-notch test piece, a Charpy impact test was performed in accordance with the regulations of JIS Z 2242, and the Charpy absorbed energy (vE _-196°C ) at _{-196°C was} determined. The Charpy absorbed energy can be regarded as an index of toughness of a thick steel sheet at cryogenic temperatures.

(Cold workability)

From the thick steel sheet, a tensile test piece was taken so that the rolling direction of the thick steel sheet was the tensile direction. Subsequently, after 3% strain was applied to the tensile test piece, an aging treatment was performed at 250°C for 1 hour. A V-notch test piece was taken from the tensile test piece after the aging treatment in accordance with the regulations of JIS Z 2202. Using the V-notch test piece, a Charpy impact test was performed in accordance with the regulations of JIS Z 2242, and the brittle fracture surface rate at -196°C was determined. The brittle fracture rate can be seen as an index of cold workability of a thick steel sheet.

Table 3 shows the evaluation results. In addition, in Table 3, the amount of reduction of retained austenite by sub-zero treatment is also indicated. Here, the reduction amount of 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 of 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 ratio of 2% or less, and excellent It had both mechanical properties and cold workability. If the Charpy absorption energy vE _-196°C is 150 J or more, it can be said that it has a toughness equal to or higher than that of a 9% Ni steel sheet. On the other hand, the thick steel sheet which does not satisfy the conditions of the present invention is inferior in at least one of strength, toughness, and cold workability.

Claims (5)

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 to 0.0065% is contained,
The balance has a component composition consisting of Fe and inevitable impurities,
The microstructure at the position of 1/4 thickness of the plate,
(1) a matrix consisting of tempered martensite or tempered martensite and tempered bainite, and
(2) consisting of residual austenite dispersed in the matrix,
The volume fraction of retained austenite at the position of 1/4 of the plate thickness is more than 11% and not more than 20%, and
The volume ratio of retained austenite at the position of 1/4 of the plate thickness after performing the subzero treatment held in liquid nitrogen at -196°C for 15 minutes is more than 11% and not more than 20%, a high tensile strength thick steel sheet for cryogenic use. The method of claim 1,
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,
V: 0.05% or less, and
Ti: 0.03% or less
Containing 1 or 2 or more selected from the group consisting of, cryogenic high-tensile strength thick steel sheet. The method according to claim 1 or 2,
The component composition is further, in mass%,
Ca: 0.007% or less,
REM: 0.010% or less, and
Mg: 0.070% or less
Containing 1 or 2 or more selected from the group consisting of, cryogenic high-tensile strength thick steel sheet. The steel material having the component composition according to any one of claims 1 to 3 is heated at a heating temperature of 900°C or more and 1200°C or less,
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 less and 300°C or more at a temperature at a position of 1/4 of the sheet thickness, and a cooling stop temperature of 1°C/s or more, at a position of 1/4 of the sheet thickness. First accelerated cooling is performed at a temperature of 300° C. or less,
The hot-rolled steel sheet after the first accelerated cooling is heated to a heating temperature of not less than the Ac1 point and less than the Ac3 point at a temperature at a position of 1/4 of the plate thickness,
In the hot-rolled steel sheet after 2 phase-inverse heating, the average cooling rate at the temperature at the position of 1/4 plate thickness is 3°C/s or more, and the cooling stop temperature is 500°C or less at the temperature at the position of 1/4 plate thickness 250 Performing a second accelerated cooling of not less than °C,
Air cooling the hot-rolled steel sheet after the second accelerated cooling to 200°C or less,
The hot-rolled steel sheet after air cooling is subjected to a tempering treatment in which the tempering temperature is 500°C or more and 650°C or less at the temperature at the position of 1/2 of the plate thickness to obtain a cryogenic high-strength thick steel sheet having the microstructure described in claim 1. , Method of manufacturing a high-tensile steel sheet for cryogenic temperatures. The method of claim 4,
In addition, 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 less,
A method for producing a high tensile strength thick steel sheet for cryogenic use in which the air-cooled hot-rolled steel sheet is reheated to a reheating temperature of not less than Ac3 and not more than 1000°C.