JP5973902B2 - Thick steel plate with excellent cryogenic toughness - Google Patents

Description

  The present invention relates to a thick steel plate having excellent cryogenic toughness. Specifically, the present invention provides toughness at extremely low temperatures of −196 ° C. or less [particularly toughness in the plate width direction (C direction)] even when the Ni content is reduced to about 5.0 to 7.5%. Relates to a good thick steel plate. In the following, the explanation will focus on thick steel plates (typically storage tanks, transport ships, etc.) for liquefied natural gas (LNG) exposed to the above-mentioned cryogenic temperatures. The present invention is not intended to be limited, and is applied to all thick steel plates used for applications exposed to extremely low temperatures of −196 ° C. or lower.

  In addition to high strength, a thick steel plate for an LNG tank used in a storage tank for liquefied natural gas (LNG) is required to have high toughness that can withstand an extremely low temperature of -196 ° C. In general, it is known that the addition of Ni improves the hardness-toughness balance particularly at low temperatures. So far, thick steel plates containing about 9% Ni (9% Ni steel) have been used as the thick steel plates used in the above applications. However, since the cost of Ni has increased in recent years, the development of a thick steel plate excellent in cryogenic toughness even with a low Ni content of less than 9% has been underway.

For example, Non-Patent Document 1 describes the effect of heat treatment in the coexistence region of α-γ2 phase on the low temperature toughness of 6% Ni steel. Specifically, by applying heat treatment (L treatment) in the α-γ two-phase coexistence region (between A _{c1 and} A _c3 ) before tempering treatment, Further, it is described that stable retained austenite is generated, and cryogenic toughness at −196 ° C. which is equal to or higher than that of 9% Ni steel which has been subjected to normal quenching and tempering treatment can be secured. However, although the cryogenic toughness in the rolling direction (L direction) is excellent, the cryogenic toughness in the sheet width direction (C direction) generally tends to be inferior to that in the L direction. There is no description of the brittle fracture surface ratio.

A technique similar to that of Non-Patent Document 1 is described in Patent Document 1 and Patent Document 2. Among these, Patent Document 1 discloses that a steel containing 4.0 to 10% Ni and whose austenite grain size is controlled within a predetermined range is hot-rolled and then heated between A _{c1 and} A _c3. A method is described in which a cooling treatment (corresponding to the L treatment described in Non-Patent Document 1 above) is repeated once or twice or more and then _{tempered at} a temperature below the A _c1 transformation point. Patent Document 2 discloses a heat treatment (L treatment → firing) similar to that of Patent Document 1 described above for steel containing 4.0 to 10% Ni and having a size of AlN before hot rolling of 1 μm or less. A method for performing the return processing) is described. The impact value (vE ₋₁₉₆ ) at −196 ° C. described in these methods is presumed to be probably in the L direction, and the toughness value in the C direction is unknown. In these methods, strength is not taken into consideration, and there is no description of the brittle fracture surface ratio.

  Non-Patent Document 2 describes the development of 6% Ni steel for LNG tanks that combines the above-described L treatment (two-phase quenching treatment) and TMCP. According to this document, although it is described that the toughness in the rolling direction (L direction) shows a high value, the toughness value in the sheet width direction (C direction) is not described.

  On the other hand, in Patent Document 3, a steel sheet having a yield strength at room temperature of 590 MPa or more in a Ni steel of more than 5.0% and less than 8.0% is assumed to be as resistant as 9% Ni steel even in a use environment. It describes a Ni-reduced steel plate for low temperature with excellent fracture safety and a method for producing the same. In patent document 3, if the yield point in the low temperature environment which is use temperature can be raised reliably, fracture safety will be improved (that is, high toughness can be obtained in a low temperature environment). In the heating process, the steel ingot is heated at a low temperature in a short time, and in the rolling process, the thickness of the steel ingot at the end of the rough rolling is the product thickness (thickness after finish rolling). The steel sheet is reduced to 3 to 8 times the thickness of the steel sheet. Further, in the examples, rolling is performed from a slab thickness of 300 mm to a finishing thickness of 50 mm or less (mostly to a finishing thickness of less than 50 mm). Thus, by securing a relatively high reduction ratio, the residual γ fraction and the fineness are reduced. Combined with a matrix structure, it achieves low temperature toughness comparable to 9% Ni steel. However, the TS at room temperature of the thick steel plate of Patent Document 3 is 741 MPa at the maximum.

  Moreover, in patent document 3, although it describes about the absorbed energy of C direction, there is no description of a brittle fracture surface rate. Moreover, TS at room temperature in Patent Document 3 is about 741 MPa at the maximum.

JP-A-49-13581 Japanese Patent Laid-Open No. 51-13308 JP 2011-241419 A

Yano et al., “Effect of heat treatment in coexistence of α-γ2 phase on low temperature toughness of 6% Ni steel”, Iron and Steel, 59th (1973) No. 6, p752-763 Furuya et al., “Development of 6% Ni steel for LNG tanks”, CAMP-ISIJ, Vol. 23 (2010), p1322

  As described above, until now, a technique excellent in cryogenic toughness at −196 ° C. in Ni steel having a Ni content of about 5.0 to 7.5% has been proposed, but the cryogenic temperature in the C direction has been proposed. Toughness has not been fully studied. In addition, if it is possible to increase the strength, it is useful in that the design margin can be increased, but a technology that has high strength and excellent cryogenic toughness has not been provided.

In addition, none of the above-mentioned documents has been studied on the brittle fracture surface ratio. The brittle fracture surface ratio indicates the ratio of brittle fracture that occurs when a load is applied in the Charpy impact test. At the site where brittle fracture occurs, the energy absorbed by the steel material until the fracture is significantly reduced, and the fracture proceeds easily. Therefore, in the cryogenic toughness improvement technology, the general-purpose Charpy impact value (vE _-196 ) as well as improving the brittle fracture surface ratio to 10% or less are extremely important requirements. However, a technique that satisfies the above requirement for the brittle fracture surface ratio in a high-strength thick steel plate having a high base metal strength as described above has not yet been proposed.

  The present invention has been made in view of the above circumstances, and its purpose is to achieve cryogenic toughness (especially in the C direction) at -196 ° C in Ni steel having a Ni content of about 5.0 to 7.5%. An object of the present invention is to provide a high-strength thick steel plate that is excellent in low-temperature toughness and can realize a brittle fracture surface ratio ≦ 10%, and a method for producing the same.

The thick steel plate excellent in the cryogenic toughness according to the present invention that can solve the above problems is mass%, C: 0.02 to 0.10%, Si: 0.40% or less (not including 0%). , Mn: 0.50 to 2.0%, P: 0.007% or less (not including 0%), S: 0.007% or less (not including 0%), Al: 0.005 to 0. 0. 050%, Ni: 5.0 to 7.5%, N: 0.010% or less (not including 0%), Cr: 1.20% or less (not including 0%), and Mo : 1.0% or less (excluding 0%), containing at least one element selected from the group consisting of steel and inevitable impurities, and the remainder is a thick steel plate composed of steel components (1) The Di value determined based on the formula is 2.5 or more,
Di value = ([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni] ) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × 1.115 (1)
In formula, [] means content (mass%) of each component in steel,
The residual austenite phase (residual γ) present at −196 ° C. is 2.0 to 12.0% in volume fraction, and
The gist is that the residual γ stabilization parameter determined based on the following formula (2) composed of the components contained in the retained austenite is 3.1 or more.
Residual γ stabilization parameter =
(365 × <C> + 39 × <Mn> + 30 × <Al> + 10 × <Cu> + 17 × <Ni> + 20 × <Cr> + 5 × <Mo> + 35 × <V>) / 100 (2)
In formula, <> means content (mass%) of each component contained in the retained austenite which exists in -196 degreeC.

In a preferred embodiment of the present invention, the volume fraction of the residual γ phase and the residual γ stability calculated based on the following equation (3) composed of the volume fraction of the residual γ phase and the residual γ stabilization parameter: The activation parameter is 40 or less.
Residual γ volume fraction / residual γ stabilization parameter = 10 / (residual γ phase volume fraction × residual γ stabilization parameter) ^1/2 (3)

  In a preferred embodiment of the present invention, the steel sheet further contains Cu: 1.0% or less (excluding 0%).

  In a preferred embodiment of the present invention, the steel sheet further includes Ti: 0.025% or less (excluding 0%), Nb: 0.100% or less (not including 0%), and V: 0.50. % Or less (not including 0%).

  In a preferred embodiment of the present invention, the steel sheet further contains B: 0.0050% or less (excluding 0%).

  In a preferred embodiment of the present invention, the steel sheet is further selected from the group consisting of Ca: 0.0030% or less (excluding 0%) and REM: 0.0050% or less (excluding 0%). Containing at least one kind.

  In a preferred embodiment of the present invention, the steel plate further contains Zr: 0.005% or less (excluding 0%).

A method of manufacturing a steel plate according to the present invention were able to solve the above problems, as well as satisfying the steel in the component according to any of the above, in alpha-.gamma.2 phase coexisting region (between A _c1 to A _c3) The L parameter calculated based on the following formula (4) composed of the temperature in the heat treatment (L treatment) (L treatment temperature) and A _c1 and A _c3 in the steel is 0.25 or more, 0.45 L treatment temperature and steel so that the λ _L parameter calculated on the basis of the following formula (5) composed of the L parameter and the steel component is 7 or less. It is characterized in that a step of adjusting the middle component and a step of performing water cooling to room temperature after L treatment and tempering treatment (T treatment) at a temperature of A _c1 or lower for 10 to 60 minutes.
L parameter = (L treatment temperature−A _c1 ) / (A _c3 −A _c1 ) +0.25 (4)
λ _L parameter = 9.05 × (0.90 × [L parameter] +0.14) × [Mn] + 1.46 × (0.37 × [L parameter] +0.67) × [Cr] −41.5 × (0.26 × [L parameter] +0.79) × [Mo] (5)
In formula, [] means content (mass%) of each component in steel.

  According to the present invention, in Ni steel having a Ni content of about 5.0 to 7.5%, even if the base material strength is high (specifically, tensile strength TS> 741 MPa, yield strength YS> 590 MPa), − Excellent low temperature toughness at 196 ° C. or less (particularly C direction cryogenic toughness), brittle fracture surface ratio at −196 ° C. ≦ 10% (preferably, brittle fracture surface ratio at −233 ° C. ≦ 50% ) Was able to be provided.

  When the Ni content is 7.5% or less and the Charpy impact absorption test in the C direction is performed, the brittle fracture surface ratio at −196 ° C. is 10% or less, the tensile strength TS> 741 MPa, In order to provide a thick steel plate that satisfies the yield strength YS> 590 MPa, investigations were made.

  In particular, the present invention has been studied in consideration of the following points.

  First, regarding the manufacturing method, in the present invention, as in Patent Documents 1 and 3, cryogenic toughness similar to that of 9% Ni steel is achieved without strict management such as cooling after rolling and T treatment. It was assumed that. Specifically, component design is performed in consideration of the case where the rolling reduction as high as that of Patent Document 3 cannot be ensured. For rolling, a rolling reduction of 830 ° C. or higher is approximately 50% or lower, and a rolling reduction of 700 ° C. or higher is approximately 85. It was presupposed that the water cooling after tempering (T treatment) after hot rolling was not performed (that is, air cooling was performed after T treatment). The rolling reduction (%) was calculated by 100 × (thickness before rolling−thickness after rolling) / (thickness before rolling).

  For cryogenic toughness, the evaluation in the C direction, which tends to be more difficult to secure the toughness than in the L direction, was adopted, and from the viewpoint of ensuring toughness, the evaluation was based on the fracture surface ratio rather than the absorbed energy. Further, regarding the tensile strength (TS), in the design of a cryogenic pressure vessel, in consideration of safety, from the viewpoint that TS should be higher if it is within the standard range, in the present invention, TS> 741 MPa is assumed. did.

  Specifically, on the premise of the above manufacturing conditions, in the Charpy impact absorption test in the C direction, the brittle fracture surface ratio ≦ −10% at −196 ° C., the tensile strength TS> 741 MPa, and the yield strength YS> 590 MPa are satisfied. Consideration has been repeated to provide a thick steel plate.

As a result, the residual γ form the following (A), (b), and lambda _L Parameters following (c)], by the following control, in Charpy impact absorption test, not transformed into martensite It was found that a stable residual γ that is plastically deformed can be ensured, and that excellent cryogenic toughness can be obtained.
(A) Components in steel from the viewpoint of plastic deformation without transformation to martensite during impact at cryogenic temperatures and securing stable residual γ useful for improving toughness (to increase the stability of residual γ) The Di value [see the following formula (1)] is controlled by an appropriate balance of the above.
(A) Balance between steel component and temperature (L treatment temperature) in heat treatment (L treatment) in the α-γ two phase coexistence region (between A _{c1 and} A _c3 ) by L parameter [see formula (4) below] After the L treatment, water cooling to room temperature, tempering treatment (T treatment) under predetermined conditions, and air cooling, the volume fraction of residual austenite (residual γ) existing at −196 ° C. is 2 The residual γ stabilization parameter [see the following formula (2)] determined to be 3.1 or more is controlled within the range of 0.0 to 12.0% and is determined by the component in the residual γ existing at −196 ° C. Controlling (preferably, the volume fraction of residual γ and the residual γ stabilization parameter [refer to the following formula (3)] composed of the volume fraction of residual γ and the above-mentioned residual γ stabilization parameter are set to 40 or less. To control).
(C) Control the λ _L parameter determined by the component (Mn, Cr, Mo) and the L processing temperature as shown in the following equation (5).

That is, the thick steel plate of the present invention is mass%, C: 0.02 to 0.10%, Si: 0.40% or less (not including 0%), Mn: 0.50 to 2.0%, P: 0.007% or less (not including 0%), S: 0.007% or less (not including 0%), Al: 0.005 to 0.050%, Ni: 5.0 to 7.5 %, N: 0.010% or less (not including 0%), Cr: 1.20% or less (not including 0%), and Mo: 1.0% or less (not including 0%) ) Is a thick steel plate containing at least one element selected from the group consisting of iron and inevitable impurities, and the Di value determined on the basis of the following formula (1) composed of steel components Is 2.5 or more, and the residual austenite phase (residual γ) existing at −196 ° C. is 2.0 to 12 in terms of volume fraction. 0% and, is characterized in residual γ stabilization parameter determined based on the following equation (2) consists of components contained in the residual austenite is 3.1 or more.
Di value = ([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni] ) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × 1.115 (1)
In formula, [] means content (mass%) of each component in steel,
Residual γ stabilization parameter =
(365 × <C> + 39 × <Mn> + 30 × <Al> + 10 × <Cu> + 17 × <Ni> + 20 × <Cr> + 5 × <Mo> + 35 × <V>) / 100 (2)
In formula, <> means content (mass%) of each component contained in the retained austenite which exists in -196 degreeC.

1. Components in steel First, components in steel will be described.

C: 0.02-0.10%
C is an element essential for securing strength and retained austenite. In order to effectively exhibit such an action, the lower limit of the C amount is set to 0.02% or more. The minimum with the preferable amount of C is 0.03% or more, More preferably, it is 0.04% or more. However, if added excessively, the cryogenic toughness decreases due to an excessive increase in strength, so the upper limit is made 0.10%. The upper limit with preferable C amount is 0.08% or less, More preferably, it is 0.06% or less.

Si: 0.40% or less (excluding 0%)
Si is an element useful as a deoxidizer. However, if added in excess, the formation of a hard island-like martensite phase is promoted and the cryogenic toughness decreases, so the upper limit is made 0.40% or less. The upper limit with the preferable amount of Si is 0.35% or less, More preferably, it is 0.20% or less.

Mn: 0.50 to 2.0%
Mn is an austenite (γ) stabilizing element and is an element contributing to an increase in the amount of residual γ. In order to effectively exhibit such an action, the lower limit of the amount of Mn is set to 0.50%. The minimum with the preferable amount of Mn is 0.6% or more, More preferably, it is 0.7% or more. However, if added excessively, temper embrittlement occurs and the desired cryogenic toughness cannot be secured, so the upper limit is made 2.0% or less. The upper limit with the preferable amount of Mn is 1.5% or less, More preferably, it is 1.3% or less.

P: 0.007% or less (excluding 0%)
P is an impurity element causing grain boundary fracture, and its upper limit is made 0.007% or less in order to secure the desired cryogenic toughness. The upper limit with preferable P amount is 0.005% or less. The smaller the amount of P, the better. However, it is difficult to make the amount of P 0% industrially.

S: 0.007% or less (excluding 0%)
S, like P, is an impurity element causing grain boundary fracture, and its upper limit is made 0.007% or less in order to ensure the desired cryogenic toughness. As shown in the examples described later, when the amount of S increases, the brittle fracture surface ratio increases and the desired cryogenic toughness (the brittle fracture surface ratio at -196 ° C. ≦ 10%) cannot be realized. The upper limit with the preferable amount of S is 0.005% or less. The smaller the amount of S, the better. However, it is difficult to make the amount of S 0% industrially.

Al: 0.005 to 0.050%
Al is an element that promotes desulfurization and fixes nitrogen. If the Al content is insufficient, the concentration of solute sulfur, solute nitrogen, etc. in the steel increases and the cryogenic toughness decreases, so the lower limit is made 0.005% or more. The minimum with the preferable amount of Al is 0.010% or more, More preferably, it is 0.015% or more. However, if added excessively, oxides, nitrides, and the like are coarsened and the cryogenic toughness is also lowered, so the upper limit is made 0.050% or less. The upper limit with the preferable amount of Al is 0.045% or less, More preferably, it is 0.04% or less.

Ni: 5.0-7.5%
Ni is an essential element for securing retained austenite (residual γ) useful for improving cryogenic toughness. In order to effectively exhibit such an action, the lower limit of the Ni amount is set to 5.0% or more. A preferable lower limit of the Ni amount is 5.2% or more, and more preferably 5.4% or more. However, if added excessively, the cost of the raw material is increased, so the upper limit is made 7.5% or less. The upper limit of the Ni content is preferably 7.0% or less, more preferably 6.5% or less, still more preferably 6.2% or less, and even more preferably 6.0% or less.

N: 0.010% or less (excluding 0%)
N lowers the cryogenic toughness by strain aging, so the upper limit is made 0.010% or less. The upper limit with preferable N amount is 0.006% or less, More preferably, it is 0.004% or less.

Cr: at least one selected from the group consisting of 1.20% or less (not including 0%) and Mo: 1.0% or less (not including 0%) Cr and Mo are both strength-enhancing elements. is there. These elements may be added alone or in combination of two kinds. In order to effectively exhibit the above action, the Cr content is 0.05% or more and the Mo content is 0.01% or more. However, if excessively added, the strength is excessively increased and the desired cryogenic toughness cannot be ensured, so the upper limit of the Cr content is 1.20% or less (preferably 1.1% or less, more preferably 0). 0.9% or less, more preferably 0.5% or less) and the upper limit of the Mo amount is 1.0% or less (preferably 0.8% or less, more preferably 0.6% or less).

  The thick steel plate of the present invention contains the above components as basic components, the balance: iron and inevitable impurities.

  In the present invention, the following selective components can be contained for the purpose of imparting further properties.

Cu: 1.0% or less (excluding 0%)
Cu is a γ-stabilizing element and is an element that contributes to an increase in the amount of residual γ. In order to exhibit such an action effectively, it is preferable to contain 0.05% or more of Cu. However, if added excessively, the strength is excessively improved and the desired cryogenic toughness effect cannot be obtained. Therefore, the upper limit is preferably made 1.0% or less. A more preferable upper limit of the amount of Cu is 0.8% or less, and even more preferably 0.7% or less.

Selected from the group consisting of Ti: 0.025% or less (not including 0%), Nb: 0.100% or less (not including 0%), and V: 0.50% or less (not including 0%) At least one of Ti, Nb, and V is an element that precipitates as carbonitride and increases strength. These elements may be added alone or in combination of two or more. In order to effectively exhibit the above action, it is preferable that the Ti amount is 0.005% or more, the Nb amount is 0.005% or more, and the V amount is 0.005% or more. However, if excessively added, the strength is excessively improved, and the desired cryogenic toughness cannot be ensured. Therefore, the preferable upper limit of Ti amount is 0.025% or less (more preferably 0.018% or less, More preferably 0.015% or less), a preferable upper limit of Nb amount is 0.100% or less (more preferably 0.05% or less, further preferably 0.02% or less), and a preferable upper limit of V amount is 0. .50% or less (more preferably 0.3% or less, and still more preferably 0.2% or less).

B: 0.0050% or less (excluding 0%)
B is an element that contributes to improving strength by improving hardenability. In order to effectively exhibit the above action, the B content is preferably 0.0005% or more. However, if added excessively, the strength is excessively improved and the desired cryogenic toughness cannot be ensured, so the preferable upper limit of the B amount is 0.0050% or less (more preferably 0.0030% or less, more preferably Is 0.0020% or less).

Ca: 0.0030% or less (excluding 0%) and REM (rare earth element): at least one selected from the group consisting of 0.0050% or less (excluding 0%) Ca and REM are solid It is an element that fixes dissolved sulfur and renders sulfides harmless. These elements may be added alone or in combination of two or more. If these contents are insufficient, the solid solution sulfur concentration in the steel increases and the toughness decreases, so the Ca content is 0.0005% or more, the REM content (when the REM described below is contained alone) It is a single content, and when it contains two or more kinds, it is the total amount thereof. However, if excessively added, sulfides, oxides, nitrides, etc. become coarse and the toughness is also lowered, so the preferable upper limit of Ca content is 0.0030% or less (more preferably 0.0025% or less), REM A preferable upper limit of the amount is 0.0050% or less (more preferably 0.0040% or less).

  In this specification, REM (rare earth element) means addition of Sc (scandium) and Y (yttrium) to a lanthanoid element (15 elements from La with atomic number 57 to Lu with atomic number 71 in the periodic table). These elements can be used alone or in combination of two or more. Preferred rare earth elements are Ce and La. The addition form of REM is not particularly limited, and may be added in the form of a misch metal mainly containing Ce and La (for example, Ce: about 70%, La: about 20-30%), or Ce, La alone may be added.

Zr: 0.005% or less (excluding 0%)
Zr is an element that fixes nitrogen. If the Zr content is insufficient, the solid solution N concentration in the steel increases and the toughness decreases, so the Zr content is preferably 0.0005% or more. However, if added excessively, oxides, nitrides, etc. become coarse and the toughness also decreases, so the preferable upper limit of the amount of Zr is made 0.005% or less (more preferably 0.0040% or less).

  In the above, the component in steel of this invention was demonstrated.

2. Volume fraction of residual austenite phase (residual γ) Further, in the thick steel plate of the present invention, the residual γ phase present at −196 ° C. is 2.0 to 12.0% (preferably 4.0 to 12). 0.0%).

  In order to improve the cryogenic toughness, it is effective to secure a residual γ that is easily plastically deformed during an impact test at a low temperature. In order to obtain the desired cryogenic toughness, the volume fraction of the residual γ phase in the entire structure existing at −196 ° C. is set to 2.0% or more. However, the residual γ is relatively soft compared to the matrix phase, and if the residual γ amount becomes excessive, YS cannot secure a predetermined value, so the upper limit is made 12.0%. Regarding the volume fraction of the residual γ phase, the preferable lower limit is 4.0% or more, the more preferable lower limit is 6.0% or more, the preferable upper limit is 11.5% or less, and the more preferable upper limit is 11.0% or less. .

  In the thick steel plate of the present invention, the control of the volume fraction of the residual γ phase is important among the structures existing at −196 ° C., and the structure other than the residual γ is not limited at all. Any material that normally exists in thick steel plates may be used. Examples of the structure other than the residual γ include carbides such as bainite, martensite, and cementite.

3. About Di Value Furthermore, in the present invention, the Di value determined based on the following formula (1) composed of steel components satisfies 2.5 or more.
Di value = ([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni] ) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × 1.115 (1)
In formula, [] means content (mass%) of each component in steel.

  The above formula (1) relating to the hardenability Di value is described as the Grossmann formula (Trans. Metall. Soc. AIME, 150 (1942), p. 227). The larger the amount of the alloy element that constitutes the Di value, the easier the firing (the Di value increases) and the finer the structure becomes. Also, the greater the Di value, the higher the strength, and it becomes easier to ensure the desired strength. According to the examination results of the present inventors, there is a correlation between the Di value and the structure size after rolling, and in order to make the structure after rolling fine and secure a desired high strength, the Di value is 2.5. It turned out that it should have done above. In detail, the Di value is such that a fine rolled structure can be obtained even when the rolling reduction of the amorphous region is small, and a sufficient volume fraction of residual γ useful for improving the cryogenic toughness is ensured by the subsequent heat treatment, and the stable residual This parameter is useful as a guideline for securing γ. In addition, the manufacturing conditions described in Patent Document 3 [reduction of reduction rate at low temperature (non-recrystallized region), time limit until the start of cooling, etc.] are relaxed to ensure good characteristics even if the process load is reduced This is an effective parameter.

  In order to effectively exhibit such an action, the Di value is set to 2.5 or more. When the Di value is less than 2.5, a fine structure cannot be sufficiently obtained after rolling, and thus a predetermined amount of residual γ cannot be obtained. Furthermore, since the residual γ stabilization parameters, volume fraction of residual γ and residual γ stabilization parameters described later cannot be controlled to a predetermined level, a stable residual γ structure cannot be obtained, and the desired cryogenic toughness is ensured. Can not. A preferable range of the Di value is 3.0 or more. On the other hand, the upper limit of the Di value is not particularly limited from the above viewpoint, but considering the viewpoint of cost and the like, and the strength standard range of the current LNG tank steel is 830 MPa or less, it is generally 5.0 or less. Preferably there is.

4). Residual γ stabilization parameter Further, in the present invention, the residual γ stabilization parameter determined based on the following equation (2) is controlled to 3.1 or more. Thereby, desired cryogenic toughness is obtained.
Residual γ stabilization parameter =
(365 × <C> + 39 × <Mn> + 30 × <Al> + 10 × <Cu> + 17 × <Ni> + 20 × <Cr> + 5 × <Mo> + 35 × <V>) / 100 (2)
In formula, <> means content (mass%) of each component contained in the retained austenite which exists in -196 degreeC.

  As described above, to improve the cryogenic toughness, it is effective to secure a stable residual γ that undergoes plastic deformation without being transformed into martensite during the impact test. For this purpose, it is conceivable to secure the residual γ fraction before the impact test and to increase the stability of the residual γ so that it can be plastically deformed without being transformed into martensite even when subjected to an impact. In the present invention, from the former viewpoint, the volume fraction of residual γ is defined in the above range. Further, when the experiment was conducted also from the latter viewpoint, the stability of the residual γ existing at −196 ° C. was determined by the component in the residual γ existing at −196 ° C., and the parameter represented by the above formula (2). It was found effective to control. When the amount of Ni is reduced to 7.5% or less as in the present invention, the hardenability generally decreases, so the structure after rolling becomes coarse and the volume fraction of residual γ obtained after heat treatment or the above-mentioned Di value is ensured. In the present invention, these requirements are also appropriately controlled by appropriately controlling the residual γ stabilization parameter determined in consideration of the component balance in the residual γ. This residual γ stabilization parameter is derived with reference to the Ms point equation.

  In order to ensure the desired cryogenic toughness, the lower limit of the residual γ stabilization parameter is set to 3.1 or more. Preferably it is 3.3 or more, More preferably, it is 3.5 or more, More preferably, it is 3.7 or more. The upper limit of the residual γ stabilization parameter is not particularly limited from the viewpoint of improving cryogenic toughness.

5. Residual γ volume fraction / residual γ stabilization parameter Further, in the present invention, for the purpose of securing a further excellent cryogenic toughness, the volume fraction of residual γ and the residual γ calculated based on the following formula (3): It is preferable to control the stabilization parameter to 40 or less.
Residual γ volume fraction / residual γ stabilization parameter = 10 / (residual γ phase volume fraction × residual γ stabilization parameter) ^1/2 (3)

  As shown in the above equation (3), the parameter is composed of a residual γ volume fraction and a residual γ stabilization parameter. The inventors of the present invention have determined the above parameters, considering that the improvement of the cryogenic toughness is due to the plastic deformation during the cryogenic impact test, and that the distribution of residual γ effective for the improvement of the toughness greatly affects. That is, those having a high volume fraction of residual γ and a large residual γ stabilization parameter are those in which the distance between each residual γ is short and finely dispersed, and they are martensite even at low temperatures. Since it is not transformed into a material and is responsible for plastic deformation, it exhibits good cryogenic toughness.

  The volume fraction of residual γ and the residual γ stabilization parameter are preferably 35 or less, and more preferably 30 or less. From the viewpoint of improving the cryogenic toughness, the lower the parameter, the better. The lower limit of the above parameter is not particularly limited in relation to the cryogenic toughness, but is generally 10 or more in consideration of the component system of the present invention.

  Furthermore, as demonstrated in Example 2 to be described later, by controlling the volume fraction of residual γ and the stabilization parameter of residual γ to a more appropriate range, at −233 ° C., which is lower than −196 ° C. described above. However, the brittle fracture surface ratio can be maintained at a favorable level of 50% or less. Specifically, the brittle fracture surface rate at −233 ° C. is reduced to 50% or less by reducing the upper limit of the volume fraction of residual γ and the residual γ stabilization parameter as much as possible (generally, 30 or less). Can do.

  The thick steel plate of the present invention has been described above.

Next, a method for producing the thick steel plate of the present invention will be described. The production method of the present invention satisfies any of the above-mentioned components in the steel, and is the temperature (L treatment temperature) in the heat treatment (L treatment) in the α-γ two phase coexistence region (between A _{c1 and} A _c3 ). And the L parameter calculated based on the following formula (4) composed of A _c1 and A _c3 in the steel is 0.25 or more and 0.45 or less, and the L parameter and the steel A step of adjusting the L treatment temperature and the steel components so that the λ _L parameter calculated based on the following formula (5) composed of the components is 7 or less, and after the L treatment, It is characterized in that after water cooling to room temperature, a tempering process (T process) is performed at a temperature of A _c1 or lower for 10 to 60 minutes.
L parameter = (L treatment temperature−A _c1 ) / (A _c3 −A _c1 ) +0.25 (4)
λ _L parameter = 9.05 × (0.90 × [L parameter] +0.14) × [Mn] + 1.46 × (0.37 × [L parameter] +0.67) × [Cr] −41.5 × (0.26 × [L parameter] +0.79) × [Mo] (5)
In formula, [] means content (mass%) of each component in steel.

  Hereinafter, each process is explained in full detail.

  The production method of the present invention is to produce a thick steel plate that satisfies the above-mentioned requirements by appropriately controlling the rolling step and the subsequent tempering treatment (T treatment), and the steel making step is not particularly limited and is usually used. Can be adopted.

  Hereinafter, the steps after the rolling step characterizing the present invention will be sequentially described in detail.

  First, it is preferable to control the heating temperature to about 900 to 1100 ° C., the FRT (finish rolling temperature) to about 700 to 900 ° C., and the SCT (cooling start temperature) to about 650 to 800 ° C. Here, the SCT is preferably controlled within the above-mentioned range within 60 seconds after finish rolling, whereby a microstructure useful for improving toughness can be obtained after rolling → cooling.

  Next, the temperature range from 800 ° C. to 500 ° C. is cooled at an average cooling rate of about 10 ° C./s or more. In the present invention, the average cooling rate in the above temperature range is particularly controlled in order to obtain a fine structure after cooling. The upper limit is not particularly limited.

  In the present invention, it is preferable to cool at least the above temperature range at an average cooling rate of about 10 ° C./s or more, but the stop temperature at the above average cooling rate is preferably 200 ° C. or less. Thereby, untransformed γ can be reduced, and a fine and uniform structure can be obtained.

After hot rolling, it is heated and maintained at a two-phase region [ferrite (α) -γ] temperature (L treatment temperature) of A _{c1 to} A _c3 , and then cooled with water (L treatment). In the present invention, in order to control the volume fraction of residual γ and the residual γ stabilization parameter (preferably, the volume fraction of residual γ and the residual γ stabilization parameter) within the scope of the present invention, the above equation (4) The L treatment temperature and the components in the steel are appropriately controlled so that the L parameter represented by the formula (1) and the λ _L parameter represented by the above formula (5) fall within a predetermined range.

First, the L treatment temperature after hot rolling is preferably controlled within a range of A _c1 to (A _c1 + A _c3 ) / 2. Thereby, alloy elements such as Ni are concentrated in the generated γ phase, and a part thereof becomes a metastable residual γ phase that exists metastable at room temperature. When the L treatment temperature is less than A _c1 point or more than [(A _c1 + A _c3 ) / 2], the residual γ fraction at −196 ° C. or the stability of residual γ cannot be secured sufficiently as a result (described later) (See No. 29, 30 in Table 2B). A preferable L treatment temperature is approximately 620 to 650 ° C.

In the present specification, the A _c1 point and the A _c3 point are calculated based on the following formulas (“Lecture / Modern Metallographic Materials 4 Steel Materials”, Japan Institute of Metals).
A _c1 point = 723-10.7 × [Mn] −16.9 × [Ni] + 29.1 × [Si] + 16.9 × [Cr] + 290 × [As] + 6.38 × [W]
A _c3 point = 910−203 × [C] ^1/2 −15.2 × [Ni] + 44.7 × [Si] + 104 ×
[V] + 31.5 × [Mo] + − 30 × [Mn] + 11 × [Cr] + 20 × [Cu]
In the above formula, [] means the concentration (mass%) of the alloying element in the steel material. In the present invention, As and W are not included as components in the steel, and in the above formula, [As] and [W] are both calculated as 0%.

  The heating time (holding time) at the two-phase region temperature is preferably about 10 to 50 minutes. If it is less than 10 minutes, the alloy element concentration to the γ phase does not proceed sufficiently, whereas if it exceeds 50 minutes, the α phase is annealed and the strength decreases. The upper limit of the preferred heating time is 30 minutes.

  Furthermore, in the present invention, the L parameter represented by the above formula (4) is set to more than 0.25 and 0.45 or less for each component. The L parameter is a parameter set in order to efficiently use the alloy concentration during the L treatment in order to finally have both the volume fraction of the residual γ and the stability of the residual γ. As shown in the examples described later, when the L parameter is out of the above range, the desired residual γ fraction and / or the stability of the residual γ cannot be sufficiently obtained. Preferably they are 0.28 or more and 0.42 or less, More preferably, they are 0.30 or more and 0.40 or less.

Further, in the present invention, as in the above equation (5), the λ _L parameter determined by the respective contents of Mn, Cr and Mo and the L parameter is controlled to be 7 or less. This λ _L parameter is set to suppress the adverse effect of temper embrittlement that occurs in the concentrated part when P is segregated to the old γ grain boundary during L treatment and Mn and Cr are excessively concentrated. It is. Since it is difficult to directly measure the amount of P segregated at the old γ grain boundary, the λ _L parameter can be regarded as an alternative parameter for the amount of P segregated at the old γ grain boundary. Those having a small segregation of P to the former γ grain boundary have a small λ _L parameter. Preferably it is 0.0 or less, More preferably, it is -10.0 or less. In addition, the lower limit is not particularly limited, but it is preferable to suppress the amount of addition of Mo as much as possible from the viewpoint of cost. In addition, generally considering each content and a preferable range of the L parameter, approximately −30 or more It is preferable that

In particular, it tends to adversely manifestation of trace impurities such as P is at an extremely low temperature range of -196 ° C., for tempering brittleness, when segregation of the old γ grain boundaries of P is large (i.e., lambda _L parameter is greater Case), it is estimated to have an adverse effect on the cryogenic toughness. For example, in Table 1, No. When comparing 1, 2, and 25 (both examples of the present invention), the volume fraction of residual γ and the residual γ stabilization parameter are comparable [No. No. 1, volume fraction of residual γ = 8.0%, residual γ stabilization parameter = 3.7; No. 2, volume fraction of residual γ = 9.4%, residual γ stabilization parameter = 3.8; 25, the volume fraction of residual γ = 7.9%, the residual γ stabilization parameter = 3.7], and the λ _L parameter are −6.8 (No. 1) and −10.9 (No. 2) It is very different from 5.2 (No. 25). Therefore, in the above three examples, the No. with the lowest λ _L parameter. 2 is most excellent in cryogenic toughness.

  Next, after water cooling to room temperature, tempering treatment (T treatment) is performed.

The tempering treatment is performed at a temperature of A _c1 or lower for 10 to 60 minutes. By such low temperature tempering, C is concentrated in the metastable residual γ and the stability of the metastable residual γ phase is increased, so that a residual γ phase that exists stably even at −196 ° C. is obtained. Moreover, a low Ms point can be secured by the low temperature tempering.

  When the tempering temperature exceeds 560 ° C., the metastable residual γ phase generated while maintaining the two-phase coexistence region is decomposed into an α phase and a cementite phase, and a sufficient residual γ phase at −196 ° C. cannot be secured. On the other hand, when the tempering temperature is less than 540 ° C. or the tempering time is less than 10 minutes, the C concentration in the metastable residual γ phase does not proceed sufficiently, and the desired residual γ at −196 ° C. The amount cannot be secured. Moreover, when the tempering time exceeds 60 minutes, the dislocation density of the α phase is excessively reduced, and a predetermined strength (TS and YS) cannot be secured (see No. 33 in Table 2B described later).

  Preferable tempering conditions are tempering temperature: 540 to 560 ° C., tempering time: 15 minutes or more and 45 minutes or less (more preferably 35 minutes or less, further preferably 25 minutes or less).

  After tempering as described above, it is cooled to room temperature. The cooling method after tempering is not water cooling but air cooling. This is because carbon concentrates in the residual γ during air cooling, so that the residual γ stabilization parameter is larger in air cooling than in water cooling.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples, and can be implemented with modifications within a range that can meet the purpose described above and below. They are all included in the technical scope of the present invention.

Example 1
Using a vacuum melting furnace (150 kg VIF), a test steel having the composition shown in Table 1 (the balance: iron and inevitable impurities, the unit is mass%) is melted and cast, and then hot forged to 150 mm × 150 mm. A 600 mm ingot was prepared. In this example, misch metal containing about 50% Ce and about 25% La was used as REM.

  Next, after heating the above ingot to 1100 ° C., it is rolled to a sheet thickness of 75 mm at a temperature of 830 ° C. or higher, the finish rolling temperature (FRT) is 700 ° C., SCT within 60 seconds after FRT is 650 ° C., As a result, the sheet was rolled to a thickness of 25 mm (a rolling reduction of 83%). In addition, the average cooling rate to 800-500 degreeC was 19 degreeC / s, and it cold-rolled to the stop temperature of 200 degrees C or less.

  The steel plate thus obtained was subjected to L treatment at the L treatment temperature shown in Table 2, heated and held for 30 minutes, and then cooled with water. Further, T treatment (tempering) was performed at the temperature (T treatment temperature) and time (T time) shown in Table 2, and then cooled to room temperature.

  About the thick steel plate thus obtained, the amount of residual γ phase (volume fraction) existing at −196 ° C., Di value, residual γ stabilization parameter, tensile properties (tensile strength TS, Yield strength YS) and cryogenic toughness (brittle fracture surface ratio in the C direction at -196 ° C or -233 ° C) were evaluated.

(1) Measurement of amount of residual γ phase (volume fraction) present at −196 ° C. From a t / 4 position of each steel plate, a 10 mm × 10 mm × 55 mm test piece was sampled and liquid nitrogen temperature (−196 ° C.) Was held for 5 minutes, and then X-ray diffraction measurement was performed with a two-dimensional micro part X-ray diffractometer (RINT-RAPIDI value I) manufactured by Rigaku Corporation. Next, the peaks of the lattice planes (110), (200), (211), and (220) of the ferrite phase and the lattices of (111), (200), (220), and (311) of the residual γ phase For the peak of the surface, based on the integrated intensity ratio of each peak, calculate the volume fraction of (111), (200), (220), (311) of the residual γ phase, and obtain the average value of these, Was defined as “volume fraction of residual γ”.

(2) Measurement of residual γ stabilization parameter In order to measure the residual γ stabilization parameter calculated based on the above equation (2), each of the residual γ existing at −196 ° C. constituting the above equation (2) Components, that is, C content <C>, Mn content <Mn>, Al content <Al>, Cu content <Cu>, Ni content <Ni>, Cr content <Cr>, Mo content <Mo>, V content <V > Was measured as follows.

(2-1) Measurement of C content <C> in residual γ existing at −196 ° C. Simultaneously with the measurement of (1) above, the standard material Si was applied to the surface of the test steel, and the angle was corrected with the Si peak. To obtain a precise lattice constant [a ₀ (Å)] of γ-Fe. From the refined lattice constant of γ-Fe and the following components other than C, the amount of C in the residual γ was calculated by back calculation.

(2-2) Measurement of Ni content <Ni> in residual γ existing at −196 ° C. From a t / 4 position of each steel plate, a 10 mm × 10 mm × 55 mm test piece was taken and liquid nitrogen temperature (−196 ° C. ), The Ni concentration was measured under the conditions of an acceleration voltage of 15 kV, an irradiation current of 50 nA, and a minimum beam diameter using an EPMA apparatus manufactured by JEOL Ltd. JXA-8500F. Measurement was performed three times for each sample, and the maximum value was defined as the amount of Ni in the residual γ.

(2-3) Measurement of Al content <Al> in residual γ existing at −196 ° C. Al is assumed to be consumed entirely by oxides or nitrides, and Al in residual γ is 0 (zero) It was.

(2-4) Measurement of Mn content <Mn>, Cu content <Cu>, Cr content <Cr>, Mo content <Mo>, and V content <V> in residual γ existing at −196 ° C. Then, each alloy element concentration <Mn>, <Cu>, <Cr>, <Mo>, <V> after the L treatment → T treatment is the measured Ni amount obtained by the method (2-2). Considering that it is proportional to <Ni>, it was calculated as follows.

The behavior of Ni concentration during each heat treatment of L treatment and T treatment is expressed by the following equation.
(Constant during each heat treatment) x (Driving force of γ reverse transformation) x (Diffusion coefficient of each alloy element)

  Here, (driving force of γ reverse transformation) in the above formula was calculated by commercially available calculation software (thermocalc) based on the temperature during each heat treatment. Further, the (diffusion coefficient of each alloy element) in the above formula was calculated using the value of “Diffusion in Solid Metals and Alloys” based on the temperature and holding time during each heat treatment.

  The (constant during each heat treatment) in the above formula was experimentally determined as follows. According to the above formula, the measured Ni concentration after L treatment → T treatment is {(constant during L treatment) × (driving force of γ reverse transformation) × (diffusion coefficient of Ni during L treatment)} {(Constant at the time of T treatment) × (driving force for γ reverse transformation) × (diffusion coefficient of Ni at the time of L treatment)}. That is, the measured Ni concentration after L treatment → T treatment includes both (constant for L treatment) and (constant for T treatment), and (constant for T treatment) is (constant for L treatment). ) To change the value of the product so that it is closest to the measured Ni concentration after the L treatment → T treatment, the constants [(constants at the L treatment) and ( Constant during T treatment)]. Using the constants thus determined, the alloy element concentrations of <Mn>, <Cu>, <Cr>, <Mo>, and <V> were calculated.

(3) Measurement of tensile properties (tensile strength TS, yield strength YS) From the t / 4 position of each steel plate, No. 4 test piece of JIS Z2241 was taken in parallel to the C direction, and a tensile test was performed by the method described in ZIS Z2241. The tensile strength TS and the yield strength YS were measured. In this example, TS> 740 MPa and YS> 590 MPa were evaluated as having excellent base material strength.

(4) t / 4 position of the measurement the steel cryogenic toughness (brittle fracture surface ratio in the direction C) (t: plate thickness) and W / 4 position (W: plate width), and t / 4 position且one W / 3 position, three Charpy impact test pieces (V-notch test piece of JIS Z 2242) were taken in parallel with the C direction, and the brittle fracture surface rate at -196 ° C. (%) by the method described in JIS Z2242. Were measured and the average value of each was calculated. Of the two average values calculated in this way, the average value that is inferior in characteristics (that is, the brittle fracture surface ratio is large) is adopted, and this value is 10% or less. Then, it evaluated that it was excellent in cryogenic toughness.

  These results are also shown in Table 2.

  From Table 2, it can be considered as follows.

  First, No. 2 in Table 2A. Nos. 1 to 25 are examples that satisfy all the requirements of the present invention. Even if the base material strength is high, the cryogenic toughness at −196 ° C. (specifically, the average value of the brittle fracture surface ratio in the C direction ≦ 10 %), An excellent thick steel plate could be provided.

  On the other hand, No. in Table 2B. Since Nos. 26 to 45 do not satisfy any of the components in steel and the preferable production conditions of the present invention, they are comparative examples that do not satisfy the requirements of the present invention, and the desired characteristics were not obtained.

  First, no. No. 26 is an example in which the Di value does not satisfy the requirements of the present invention, and the desired volume fraction of residual γ cannot be obtained, and the residual γ stabilization parameter also decreases. Furthermore, the volume fraction of residual γ and the residual γ stabilization parameter also exceeded the predetermined range. As a result, the brittle fracture surface ratio also increased, and the desired cryogenic toughness could not be realized at -196 ° C. Moreover, since Di value was low, TS also fell.

  No. No. 27 in Table 1B with a large amount of C. In this example, the cryogenic toughness was lowered.

  No. No. 28 is No. in Table 1B with a large amount of P. In this example, the desired volume fraction of residual γ was not obtained, and the residual γ stabilization parameter was also reduced. Furthermore, the volume fraction of residual γ and the residual γ stabilization parameter also exceeded the predetermined range. As a result, the cryogenic toughness decreased.

  No. No. 29 in Table 1B in which the components in the steel satisfy the requirements of the present invention. 29 is used, but heating is performed at a temperature lower than the two-phase region temperature (L treatment temperature), and the L parameter is low. Therefore, the amount of residual γ was insufficient and the residual γ stabilization parameter was also reduced. Furthermore, the volume fraction of residual γ and the residual γ stabilization parameter also exceeded the predetermined range. As a result, the cryogenic toughness decreased.

No. No. 30 is No. in Table 1B with a large amount of Si. 30 and heating at a temperature exceeding the two-phase region temperature (L treatment temperature), and the L parameter and the λ _L parameter are high. Therefore, the amount of residual γ was insufficient and the residual γ stabilization parameter was also reduced. Furthermore, the volume fraction of residual γ and the residual γ stabilization parameter also exceeded the predetermined range. As a result, the cryogenic toughness decreased.

  No. No. 31 in Table 1B in which the components in the steel satisfy the requirements of the present invention. 31 was used, but because the tempering temperature (T treatment temperature) was low, the amount of residual γ was insufficient and the residual γ stabilization parameter was also lowered. Furthermore, the volume fraction of residual γ and the residual γ stabilization parameter also exceeded the predetermined range. As a result, the cryogenic toughness decreased.

No. No. 32 is No. in Table 1B with a large amount of Mn. 32, and the λ _L parameter is high. As a result, the cryogenic toughness decreased.

  No. No. 33 in Table 1B in which the components in the steel satisfy the requirements of the present invention. Although 33 was used, this was an example in which the tempering time (T time) was long, and the strength (TS and YS) decreased.

  No. No. 34 in Table 1B has a small amount of Mn and a small Di value. In this example, the desired volume fraction of residual γ was not obtained, and the residual γ stabilization parameter was also lowered. Furthermore, the volume fraction of residual γ and the residual γ stabilization parameter also exceeded the predetermined range. As a result, the brittle fracture surface ratio also increased, and the desired cryogenic toughness could not be realized at -196 ° C. Moreover, since Di value was low, TS also fell.

  No. No. 35 in Table 1B with a large amount of S. This is an example using 35. For this reason, the brittle fracture surface ratio increased and the desired cryogenic toughness could not be realized.

  No. No. 36 in Table 1B in which the components in the steel satisfy the requirements of the present invention. Although 36 was used, this is an example where the L parameter is high. As a result, the amount of residual γ was insufficient, and the volume fraction of residual γ and the residual γ stabilization parameter exceeded a predetermined range. As a result, the cryogenic toughness decreased.

  No. No. 37 in Table 1B has a small amount of C, a large amount of Al, and a small amount of Ni. Since 37 was used, the amount of residual γ was insufficient and the residual γ stabilization parameter was also reduced. Furthermore, the volume fraction of residual γ and the residual γ stabilization parameter also exceeded the predetermined range. As a result, the cryogenic toughness decreased. TS also decreased.

  No. No. 38 in Table 1B has a small amount of Al and a large amount of N. Since 38 was used, the cryogenic toughness decreased.

  No. No. 39 in Table 1B, which has a large amount of Cu and Ca as the selection components. Since 39 was used, cryogenic toughness decreased.

  No. No. 40 in Table 1B, which has a large amount of Cr and Zr as selective components. Since 40 was used, the cryogenic toughness decreased.

  No. No. 41 in Table 1B with a large amount of Nb and REM as the selection components. Since 41 was used, the cryogenic toughness decreased.

  No. No. 42 in Table 1B has a large amount of Mo as a selection component and a large Di value. Since 42 was used, the cryogenic toughness decreased.

  No. No. 43 in Table 1B with a large amount of Ti as a selected component. Since 43 was used, the cryogenic toughness decreased.

  No. No. 44 of Table 1B with a large amount of V as a selected component. Since 44 was used, the cryogenic toughness decreased.

  No. No. 45 of Table 1B with a large amount of B as a selected component. Since 45 was used, the cryogenic toughness decreased.

Example 2
In this example, the brittle fracture surface rate at −233 ° C. was evaluated for some data used in Example 1 (both examples of the present invention).

Specifically, No. 1 described in Table 3 is used. (No. in Table 3 corresponds to No. in Table 1 and Table 2 described above) Three test pieces were collected from the t / 4 position and the W / 4 position, and -233 according to the method described below. A Charpy impact test at ℃ was conducted to evaluate the average brittle fracture surface ratio. In this example, the brittle fracture surface ratio ≦ 50% was evaluated as being excellent in the brittle fracture surface ratio at −233 ° C.
"High pressure gas", Vol. 24, page 181, "Cryogenic impact test of austenitic cast stainless steel"

  These results are listed in Table 3.

  No. in Table 3 1 to 3, 6, 8, 9, 14, 18, and 20 all have not only −196 ° C. but also a good brittle fracture surface ratio at a lower temperature of −233 ° C., and extremely excellent cryogenic temperature. Toughness could be achieved. The reason for this may be that the volume fraction of residual γ and the residual γ stabilization parameter are both small (21 or less).

  In contrast, no. Compared with the above example, 10 and 16 had a large volume fraction of residual γ and a residual γ stabilization parameter of about 35, so that the brittle fracture surface ratio at −233 ° C. was large.

  From the above experimental results, in order to obtain a thick steel plate having not only −196 ° C. but also a brittle fracture surface ratio at a lower temperature of −233 ° C., in the above requirements of the present invention, in particular, the volume fraction of residual γ It can be seen that it is effective to make the residual γ stabilization parameter as small as possible.

Claims (8)

% By mass
C: 0.02-0.10%,
Si: 0.40% or less (excluding 0%),
Mn: 0.50 to 2.0%,
P: 0.007% or less (excluding 0%),
S: 0.007% or less (excluding 0%),
Al: 0.005 to 0.050%,
Ni: 5.0 to 7.5%
N: 0.010% or less (excluding 0%)
And containing
Containing at least one element selected from the group consisting of Cr: 1.20% or less (not including 0%) and Mo: 1.0% or less (not including 0%);
The balance is a steel plate with iron and inevitable impurities,
Di value determined on the basis of the following formula (1) composed of components in steel is 2.5 or more,
Di value = ([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni] ) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × 1.115 (1)
In formula, [] means content (mass%) of each component in steel,
The residual austenite phase (residual γ) present at −196 ° C. is 2.0 to 12.0% in volume fraction, and
Der residual γ stabilization parameter 3.1 or more in the component contained in the retained austenite is determined on the basis of the constructed following equation (2) is,
Residual γ stabilization parameter =
(365 × <C> + 39 × <Mn> + 30 × <Al> + 10 × <Cu> + 17 × <Ni> + 20 × <Cr> + 5 × <Mo> + 35 × <V>) / 100 (2)
Wherein <> refers to the content of each component contained in the residual austenite present at -196 ° C. (mass%),
Satisfying the tensile strength TS> 740 MPa and the yield strength YS> 590 MPa,
Charpy impact in parallel to the plate width direction (C direction) from the t / 4 position (t: plate thickness) and W / 4 position (W: plate width), and the t / 4 position and W / 2 position of the thick steel plate. Three test pieces (V-notch test piece of JIS Z 2242) were collected, and the brittle fracture surface ratio (%) at -196 ° C was measured by the method described in JIS Z2242, and the average value of each was calculated. A steel plate excellent in cryogenic toughness, characterized in that the average value of the two brittle fracture surface ratios of the two calculated average values is 10% or less . The volume fraction of residual γ phase and the residual γ stabilization parameter calculated based on the following equation (3) composed of the volume fraction of the residual γ phase and the residual γ stabilization parameter are 40 or less: Item 2. The thick steel plate according to Item 1.
Residual γ volume fraction / residual γ stabilization parameter = 10 / (residual γ phase volume fraction × residual γ stabilization parameter) ^1/2 (3) Furthermore,
The thick steel plate according to claim 1 or 2, containing Cu: 1.0% or less (excluding 0%). Furthermore,
Ti: 0.025% or less (excluding 0%),
Nb: 0.100% or less (not including 0%), and V: 0.50% or less (not including 0%)
The thick steel plate according to any one of claims 1 to 3, comprising at least one selected from the group consisting of: Furthermore,
B: The thick steel plate according to any one of claims 1 to 4, comprising 0.0050% or less (excluding 0%). Furthermore,
Ca: 0.0030% or less (not including 0%), and REM: 0.0050% or less (not including 0%)
The thick steel plate according to any one of claims 1 to 5, comprising at least one selected from the group consisting of: Furthermore,
Zr: 0.005% or less (excluding 0%)
The thick steel plate according to any one of claims 1 to 6, comprising: It is a manufacturing method of the thick steel plate in any one of Claims 1-7,
A step of hot rolling at a rolling reduction of 830 ° C. or more at 50% or less and a rolling reduction of 700 ° C. or more at 85% or less;
Cooling the temperature range from 800 ° C. to 500 ° C. at an average cooling rate of 10 ° C./s or more;
While satisfying the steel component according to any one of claims 1 to 7,
the temperature in the heat treatment in the alpha-.gamma.2 phase coexisting region (A _c1 to A _c3 between) (L process) (L treatment temperature), based on the following formula composed of the A _c1 and A _c3 in the steel (4) L parameter calculated in the above is 0.25 or more and 0.45 or less, and
Adjusting the L treatment temperature and the steel component so that the λ _L parameter calculated based on the following formula (5) composed of the L parameter and the steel component is 7 or less. When,
A step of performing water cooling to room temperature after L treatment and performing tempering treatment (T treatment) at a temperature of 540 ° C. or higher and A _c1 or lower for 10 to 60 minutes;
And a step of air cooling after the tempering treatment in this order.
L parameter = (L treatment temperature−A _c1 ) / (A _c3 −A _c1 ) +0.25 (4)
λ _L parameter = 9.05 × (0.90 × [L parameter] +0.14) × [Mn] + 1.46 × (0.37 × [L parameter] +0.67) × [Cr] −41.5 × (0.26 × [L parameter] +0.79) × [Mo] (5)
In formula, [] means content (mass%) of each component in steel.