KR20150029754A - Thick steel plate having good ultralow-temperature toughness - Google Patents

Abstract

The steel sheet according to the present invention comprises a predetermined steel component and the average circularity (A) of the inclusions having a circle-equivalent diameter of greater than 1.0 탆 in the steel sheet is 1.8 or less, the volume of retained austenite phase , The fraction (V) is 2.0 to 12.0%, and the B value represented by the following formula (1) is 1.3 or more.

Description

{THICK STEEL PLATE HAVING GOOD ULTRALOW-TEMPERATURE TOUGHNESS}

The present invention relates to a steel sheet excellent in cryogenic temperature toughness and, more particularly, to a steel sheet having excellent toughness at a cryogenic temperature of -196 deg. C or lower (particularly, in the plate width direction (C direction) Toughness] is good. Hereinafter, the steel sheet for liquefied natural gas (LNG) to be exposed at the cryogenic temperature will be mainly described, but the steel sheet of the present invention is not limited thereto. It is applied to the entire steel plate used for applications exposed to cryogenic temperatures below 196 ℃.

The steel sheet used for the LNG tank used in the storage tank of liquefied natural gas (LNG) is required to have high toughness capable of withstanding a cryogenic temperature of -196 DEG C in addition to high strength. Up to now, a post-steel sheet containing about 9% Ni (9% Ni steel) has been used as the post-steel sheet used in the above applications. However, recently, the cost of Ni has increased, The development of a steel sheet having excellent cryogenic toughness is underway.

For example, Non-Patent Document 1 describes the effect of heat treatment in the? -Γ 2 phase coexistence region on the low temperature toughness of 6% Ni steel. Specifically, by applying heat treatment (L treatment) in the? -? 2 phase coexistence region (between Ac1 and Ac3) before the tempering treatment, it is possible to obtain a heat treatment at -196 占 폚, which is equal to or higher than 9% Ni steel subjected to normal quenching- Capable of imparting cryogenic toughness of; This heat treatment is also intended to improve the toughness of the test piece in the C direction (plate width direction); These effects are due to the presence of a stable retained austenite even for a large amount of fine and impact loads at cryogenic temperatures. However, according to the above method, the cryogenic temperature toughness in the rolling direction (L direction) is excellent, but the cryogenic temperature toughness in the plate width direction (C direction) tends to be lower than in the L direction. In addition, there is no description of the brittle fracture ratio.

Patent Literature 1 and Patent Literature 2 describe a technique similar to that of Non-Patent Document 1. Among them, Patent Document 1 discloses a process of hot-rolling a steel containing 4.0 to 10% Ni and controlled in a predetermined range of the austenite grain size, heating it between Ac1 to Ac3, and then cooling Is repeated once or twice or more, and then tempering is carried out at a temperature not higher than the Ac1 transformation point. Patent Document 2 discloses a method of performing a heat treatment (L treatment → tempering treatment) similar to that of Patent Document 1 for a steel containing 4.0 to 10% of Ni and having a size of AlN before hot rolling of 1 m or less . The impact value (vE _{- 196} ) at -196 캜 described in these methods is presumably assumed to be in the L direction, and the toughness value in the C direction is unclear. Further, in these methods, no consideration is given to the strength, and there is no description of the brittle wavefront ratio.

In addition, Non-Patent Document 2 describes the development of 6% Ni steel for LNG tanks in combination with the above-mentioned L treatment (two-phase zone quenching treatment) and TMCP. According to this document, it is described that the toughness in the rolling direction (L direction) is high, but toughness in the plate width direction (C direction) is not described.

Patent Document 3 discloses a high tensile high tensile strength steel having excellent toughness at a weld portion of 570 MPa or higher, containing 0.3 to 10% of Ni and a predetermined amount of Mg and appropriately dispersing Mg-containing oxide particles having a predetermined particle size. Patent Document 3 discloses that the size of the heated austenite is reduced by control of the Mg-containing oxide to improve the toughness of the base material and the weld heat affected zone (HAZ); For this purpose, it is important to add the amounts of O (oxygen) before the deoxidizing element and the order of addition of Mg and other deoxidizing elements, and simultaneously add Mg, Ti and Al to the molten steel having a dissolved oxygen amount of 0.001 to 0.02% Or Al is added last when Mg, Ti or Al is added, and then cast into a steel strip. In the example of Patent Document 3, toughness value (wave-front transition temperature vTrs) in the C direction is described, and the above characteristics of the 9% Ni steel are good (wavefront transition temperature vTrs? The above characteristics of the steel are -140 deg. C, and further improvements are required.

Patent Document 4 also discloses a technique of obtaining a steel sheet having excellent toughness (CTOD characteristics), austenitic property and unstable fracture toughness of a base material and a welded joint by adding 5.0 to 7.5% of Ni and uniformizing the distribution of austenite . However, the evaluation temperature in the CTOD test is a little high at -165 deg. C, so it is not a technique for cryogenic temperature below -196 deg. In addition, even if Patent Document 4 is examined in detail, there is no description of the brittle wavefront ratio in the Charpy impact absorption test. Further, in order to produce the steel sheet described in Patent Document 4, long-time heat treatment at a high temperature of 1250 to 1380 캜 and 8 to 50 hours is required before hot rolling, which is disadvantageous from the viewpoint of production cost.

Japanese Patent Laid-Open No. 49-135813 Japanese Patent Laid-Open No. 51-13308 Japanese Patent Application Laid-Open No. 2001-123245 Japanese Patent No. 4975888

Yano et al., "Influence of heat treatment in the coexistence zone of? -Γ 2 phase on low temperature toughness of 6% Ni steel", Iron and Steel, No. 59 (1973) No. 6, p752-763 Furuya et al., &Quot; Development of 6% Ni steel for LNG tanks ", CAMP-ISI J, Vol. 23 (2010), p1322

As described above, a technique superior in cryogenic temperature toughness at -196 deg. C in Ni steel having a Ni content of about 5.0 to 7.5% has been proposed, but the cryogenic toughness in the C direction has not been fully investigated. In particular, there is a strong demand for improvement of the cryogenic temperature toughness (improvement in cryogenic toughness in the C direction) under high strength at high strength (more specifically, tensile strength TS> 690 MPa, yield strength YS> 590 MPa).

In addition, in the above-mentioned documents, there is no study on the brittle wavefront ratio. The brittle fracture surface ratio is the ratio of the brittle fracture that occurs when a load is applied in the Charpy impact test. In the site where brittle fracture occurs, the energy absorbed into the steel material is remarkably reduced to breakdown, and the fracture progresses easily. Therefore, in order to suppress breakdown particularly at a very low temperature, It is an extremely important requirement to suppress the wavefront ratio to a low level (10% or less). However, as the strength becomes higher, brittle fracture tends to occur, so it is generally difficult to achieve a brittle fracture surface ratio? 10% under the high base material strength as described above. As a result, a technique of providing both of these with a high strength steel sheet having a high base metal strength has not been proposed yet.

The present invention has been made in view of the above circumstances, and its object is to provide a Ni steel having a Ni content of about 5.0 to 7.5% and having excellent cryogenic toughness at -196 캜 (particularly, extremely low temperature toughness in the C direction) High strength steel sheet capable of realizing 10%.

A steel sheet excellent in cryogenic temperature toughness according to the present invention, which can solve the above problems, comprises 0.02 to 0.10% of C, 0.40% or less (not including 0%) of Si, 0.50 to 2.0% of Mn, P: not more than 0.007% (not including 0%), S: not more than 0.007% (not including 0%), Al: 0.005 to 0.050% (A) of the inclusions having a circle-equivalent diameter of more than 1.0 탆 in the steel sheet is 1.8 or less, and is present in the steel sheet at -196 캜 The volume fraction (V) of the retained austenite phase satisfies 2.0 to 12.0%, and the B value represented by the following formula (1) is 1.3 or more.

In a preferred embodiment of the present invention, the steel sheet satisfies 4.0 to 12.0% by volume in terms of the retained austenite phase present at -196 캜.

In a preferred embodiment of the present invention, the steel sheet comprises at least one of Cu: not more than 1.00% (not including 0%), Cr: not more than 1.2% (not including 0%), Mo: Ti: not more than 0.025% (not including 0%), Nb: not more than 0.10% (not including 0%), V: not more than 0.50% (not including 0%), B: 0.0050 (Not including 0%) (not including 0%) Ca: not more than 0.0030% (not including 0%), REM: not more than 0.0050% (not including 0%), Zr: ). ≪ / RTI >

According to the present invention, even if the base metal strength is high (in particular, tensile strength TS> 690 MPa, yield strength YS> 590 MPa) in a low Ni steel reduced in Ni content to 5.0 to 7.5% (Particularly, the cryogenic toughness in the C direction) of the cured product in the C direction, and more specifically, in the Charpy impact absorption test in the C direction, the brittle fracture rate at -196 deg. Brittle fracture rate < = 50%).

The inventors of the present invention have found that in a high strength steel sheet which has a Ni content reduced to 7.5% or less and satisfies a tensile strength TS> 690 MPa and a yield strength YS> 590 MPa, the Charpy impact value in the C- And to provide a cryogenic toughness improving technique that satisfies the brittle fracture rate? 10%. As a result, the volume fraction (V) of the retained austenite (residual?) Present at -196 占 폚 was controlled to 2.0 to 12.0% (preferably 4.0 to 12.0% (volume fraction) (B), the average circularity (A) of the inclusions is reduced to 1.8 or less with respect to the inclusions having a circle equivalent diameter of 1.0 탆 or more (hereinafter sometimes simply referred to as inclusions), which promotes the progress of brittle fracture , And the B value represented by the following formula (1) is controlled to 1.3 or more, the desired object is achieved, and the present invention has been completed.

Particularly, the feature portion to be noted in relation to the above-described prior art is the latter (B). Hereinafter, the process of reaching the present invention will be described.

The inventors of the present invention have conducted extensive investigations to provide a Ni steel having a Ni content of 7.5% or less in order to provide a steel sheet excellent in cryogenic temperature toughness of -196 캜. Specifically, the present invention provides a high strength steel sheet excellent in cryogenic toughness which satisfies all the characteristics of a brittle wavefront ratio ≤10% in the C direction, a tensile strength TS> 690 MPa, and a yield strength YS> 590 MPa First, the method taught in the literature described in the prior art was examined.

This document teaches that it is important to stabilize the residual austenite (residual?) Present at -196 占 폚 for the improvement of the cryogenic toughness of 5% Ni steel. Considering the manufacturing method as a whole, the amount of dissolved oxygen before addition of the deoxidizing element is controlled in the molten steel step, Al is finally added to the molten steel, and the molten steel is cast in the α-γ 2 phase coexistence region Ac1 (L treatment) between the Ac1 transformation point and the Ac1 transformation point, it is recommended that the tempering treatment be performed at a temperature not higher than the Ac1 transformation point, thereby improving the cryogenic toughness. However, according to the results of the studies conducted by the inventors of the present invention, the cryogenic temperature toughness in the L direction is improved by the above method, but the cryogenic temperature toughness in the C direction is not sufficient. 10%) can not be realized.

Therefore, as a result of further investigations, it has been found that it is indispensable to add requirements for a further layer in the steel sheet and its manufacturing method, while basically following the above-mentioned technique in order to obtain a steel sheet having excellent cryogenic toughness . Specifically, it was found that (i) in the post-steel sheet, the volume fraction (V) of the residual? Phase at -196 占 폚 is present in the range of V = 2.0 to 12.0% (A) of the inclusions is reduced to 1.8 or less, and the average roundness (A) of the inclusions and the residual roundness of the inclusions existing at -196 占 폚 It is effective to control the B value represented by the relational expression (1) of the volume fraction (V) (%) of the phase volume phase (V) (%) to the B value? 1.3. (Ii) In addition to the control of the amount of dissolved oxygen (pre-O amount) before addition, the heat treatment between Ac1 and Ac3 after the hot rolling (L treatment), and the tempering treatment in the predetermined temperature range, further control of the molten steel step is effective , The cooling time (t1) at 1450 to 1500 占 폚 at the time of casting is set to 300 seconds (The value at the half position of the slab thickness t), and the cooling time t2 at 1300 to 1200 占 폚 during casting is controlled to be 680 seconds or less Value) is effective.

(C) The residual γ phase present at -196 캜 is controlled to 4.0 to 12.0% (volume fraction) in the above (a), whereby the brittle fracture surface ratio can be reduced to 50% or less (L treatment) between Ac1 and Ac3 after hot rolling, it is found that holding for a predetermined time is effective in the heat treatment (L treatment) between hot rolling and hot rolling, Thereby completing the invention.

In the present specification, " excellent in extremely low temperature toughness " means that when the brittle fracture surface ratio in the Charpy impact absorption test in the C direction (plate width direction) is measured by the method described in the column of the later embodiment, -196 Lt; RTI ID = 0.0 >% < / RTI > In the later embodiments, the brittle wavefront ratio in the L direction (rolling direction) is not measured, but if the brittle wavefront ratio in the C direction is 10% or less, the brittle wavefront ratio in the L direction is also inevitably 10 % Or less.

In the present specification, the term "post-steel plate" means that the thickness of the steel plate is generally 6 to 50 mm.

Further, in the present invention, a high strength steel sheet satisfying a tensile strength TS > 690 MPa and a yield strength YS > 590 MPa is intended.

Hereinafter, the steel sheet of the present invention will be described in detail.

As described above, the steel sheet according to the present invention comprises 0.02 to 0.10% of C, 0.40% or less of Si (not including 0%), 0.50 to 2.0% of Mn, 0.007% or less of P 0.005% or less (excluding 0%), Al: 0.005 to 0.050%, Ni: 5.0 to 7.5%, N: 0.010% or less (not including 0%) And the remaining amount is iron and unavoidable impurities, and the average circularity (A) of the inclusions having a circle equivalent diameter of more than 1.0 탆 in the steel sheet is 1.8 or less and the volume fraction of retained austenite phase present at -196 캜 V) of 2.0 to 12.0%, and the B value represented by the following formula (1) is 1.3 or more.

First, the components in the steel will be described.

C: 0.02 to 0.10%

C is an essential element for securing strength and retained austenite. In order to effectively exhibit such action, the lower limit of the amount of C is 0.02% or more. The lower limit of the C content is preferably 0.03% or more, and more preferably 0.04% or more. However, if it is added in excess, the cryogenic temperature toughness is lowered due to an excessive increase in strength, so the upper limit is set to 0.10% or less. The preferred upper limit of the amount of C is 0.08% or less, more preferably 0.06% or less.

Si: not more than 0.40% (not including 0%)

Si is a useful element as a de-oxidation material. However, if it is added in excess, the formation of a hard island-shaped martensite phase is promoted and the cryogenic temperature toughness is lowered. Therefore, the upper limit is set to 0.40% or less. The upper limit of the Si content is preferably 0.35% or less, and more preferably 0.20% or less.

Mn: 0.50 to 2.0%

Mn also functions as a deacidification material, but it is an austenite (?) Stabilizing element and contributes to an increase in the residual? Amount. In order to effectively exhibit such an effect, the lower limit of the amount of Mn is set to 0.50%. The lower limit of the Mn content is preferably 0.6% or more, and more preferably 0.7% or more. However, if it is added in excess, it causes tempering embrittlement and the desired low-temperature toughness can not be secured. Therefore, the upper limit is set to 2.0% or less. The upper limit of the Mn content is preferably 1.5% or less, and more preferably 1.3% or less.

P: not more than 0.007% (not including 0%)

P is an impurity element which causes grain boundary fracture, and the upper limit of P is set to 0.007% or less in order to secure the desired cryogenic toughness. The preferable upper limit of the P content is 0.005% or less. The smaller the P amount is, the better, but it is difficult to industrially make the P amount to 0%.

S: not more than 0.007% (not including 0%)

S, like P, is an impurity element which causes intergranular fracture, and its upper limit is set to 0.007% or less in order to secure a target cryogenic toughness. As shown in the later examples, when the amount of S is increased, the brittle fracture surface ratio increases, and the target cryogenic toughness (brittle fracture ratio at -196 캜? 10%) can not be realized. The preferable upper limit of the amount of S is 0.005% or less. The smaller the amount of S is, the better, but it is difficult to industrially make the amount of S 0.

Al: 0.005 to 0.050%

Al is a deoxidizing element. If the content of Al is insufficient, the free oxygen concentration in the molten steel rises and secondary inclusions such as oxides or sulfides are mixed on the surface of the inclusions originally present in the molten steel during the casting cooling process, so that the shape of the inclusions is distorted And the average roundness of the inclusions having a circle equivalent diameter of more than 1.0 탆 is increased, so that the lower limit is set to 0.005% or more. The lower limit of the Al content is preferably 0.010% or more, and more preferably 0.015% or more. However, if it is added in excess, aggregation or coalescence of the inclusions is promoted, and the average roundness of the inclusions also becomes large, so the upper limit is set to 0.050% or less. The preferable upper limit of the Al amount is 0.045% or less, more preferably 0.04% or less.

Ni: 5.0 to 7.5%

Ni is an essential element for securing retained austenite (residual?) Useful for improvement of cryogenic toughness. In order to effectively exhibit such an effect, the lower limit of the amount of Ni is 5.0% or more. The lower limit of the amount of Ni is preferably not less than 5.2%, more preferably not less than 5.4%. However, if it is added in excess, the cost of the raw material is increased, so the upper limit is set to 7.5% or less. The preferable upper limit of the amount of Ni is 7.0% or less, more preferably 6.5% or less, and further preferably 6.0% or less.

N: not more than 0.010% (not including 0%)

Since N lowers the cryogenic temperature toughness by deformation aging, its upper limit is made 0.010% or less. The preferable upper limit of the amount of N is 0.006% or less, more preferably 0.004% or less.

The post-steel sheet of the present invention contains the above-mentioned components as basic components, and the remainder is iron and unavoidable impurities.

In the present invention, the following optional components may be contained for the purpose of imparting the characteristics of a further layer.

Cu: not more than 1.00% (not including 0%)

Cu is a? Stabilizing element and is an element contributing to an increase in the residual? Amount. In order to exhibit such an effect effectively, it is preferable that Cu contains 0.05% or more. However, if it is added in an excess amount, the strength is excessively improved, and the desired cryogenic toughness effect can not be obtained. Therefore, the upper limit is preferably 1.00% or less. A more preferable upper limit of the amount of Cu is 0.8% or less, more preferably 0.7% or less.

At least one member selected from the group consisting of Cr: not more than 1.2% (not including 0%) and Mo: not more than 1.00% (excluding 0%)

Both Cr and Mo are strength improving elements. These elements may be added singly or in combination. In order to effectively exhibit the above action, it is preferable that the Cr amount is 0.05% or more and the Mo amount is 0.01% or more. However, if it is added in an excessive amount, the strength is excessively improved, and the desired cryogenic toughness can not be ensured. Therefore, the preferred upper limit of the amount of Cr is 1.2% or less (more preferably 1.1% 0.9% or less, still more preferably 0.5% or less), and the preferred upper limit of the amount of Mo is 1.00% or less (more preferably 0.8% or less, further preferably 0.6% or less).

At least one selected from the group consisting of Ti: not more than 0.025% (not including 0%), Nb: not more than 0.10% (excluding 0%), and V: not more than 0.50%

Ti, Nb and V all precipitate as carbonitride and increase the strength. These elements may be added alone, or two or more of them may be used in combination. In order to effectively exhibit this action, it is preferable that the amount of Ti is 0.005% or more, the amount of Nb is 0.005% or more, and the amount of V is 0.005% or more. However, if it is added in an excessive amount, the strength is excessively improved, and the desired cryogenic toughness can not be secured. Therefore, the preferable upper limit of the amount of Ti is 0.025% or less (more preferably 0.018% or less, (More preferably, not more than 0.05%, and more preferably not more than 0.02%), the upper limit of the amount of Nb is preferably not more than 0.50% (more preferably, not more than 0.3%), Or less, more preferably 0.2% or less).

B: 0.0050% or less (not including 0%)

B is an element contributing to improvement of strength by improvement in quenching property. In order to exhibit the above effect effectively, it is preferable that the amount of B is 0.0005% or more. However, if it is added in an excessive amount, the strength is excessively improved, and the desired cryogenic toughness can not be ensured. Therefore, the preferred upper limit of the amount of B is 0.0050% or less (more preferably 0.0030% or less, 0.0020% or less).

0.0050% or less Ca (not including 0%), 0.0050% or less (excluding 0%), and Zr: 0.0050% or less (excluding 0%) At least one species

Ca, REM and Zr are all deoxidized elements, and the addition of these elements reduces the oxygen concentration in the steel and reduces the amount of oxides, thereby having a good effect on toughness. These elements may be added alone, or two or more of them may be used in combination. In order to effectively exhibit the above-mentioned action, it is preferable that the amount of Ca is 0.0005% or more, the amount of REM (the total amount of REMs described below when they are contained alone, And Zr is preferably 0.0005% or more, and more preferably 0.0005% or more. However, if it is added in an excessive amount, the size of the oxide increases and the cryogenic temperature toughness decreases. Therefore, the preferred upper limit of Ca content is 0.0030% or less (more preferably 0.0025% or less) and the preferable upper limit of REM content is 0.0050% Or less, more preferably 0.0040% or less), and the preferable upper limit of the amount of Zr is 0.0050% or less (more preferably 0.0040% or less).

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

The components in the steel of the present invention have been described above.

Further, the post-steel sheet of the present invention is such that the volume fraction (V) of the residual? Phase present at -196 占 폚 satisfies 2.0 to 12.0% (preferably 4.0 to 12.0%).

It is known that the residual? Existing at -196 占 폚 contributes to the improvement of the cryogenic temperature toughness. To effectively exhibit such an effect, the volume fraction (V) of the residual? Phase occupying the entire structure existing at -196 占 폚 is 2.0% or more. However, since the residual y is relatively soft as compared with the matrix phase, and YS becomes unable to secure a predetermined value when the residual? Amount becomes excessive, the upper limit is set at 12.0% (No.43 of Table 2B, Reference). For the volume fraction (V) of the residual? Phase, the lower limit is preferably not less than 4.0%, more preferably not less than 6.0%, more preferably not more than 11.5%, and still more preferably not more than 11.0%.

Further, by controlling the volume fraction of the residual? Occupied in all tissues present at -196 占 폚 to be 4.0% or more, even at a temperature of -233 占 폚, which is lower than -196 占 폚, the brittle fracture rate can be maintained at a satisfactory level . In order to further exert such effects, a more preferable lower limit is 6.0% or more, and a preferable upper limit is the same as described above.

In the post-steel sheet of the present invention, it is important to control the volume fraction (V) of the residual? Phase in the structure existing at -196 占 폚. The structure other than the residual? Is not limited at all, It may be any that usually exists. Examples of the structure other than the residual? Include carbides such as bainite, martensite, cementite, and the like.

The post-steel sheet of the present invention is characterized in that the inclusions having an average circularity (A) of the inclusions satisfying A? 1.8 and a ratio B Value of 1.3 or more.

Here, the " circle-equivalent diameter " refers to the diameter of the circle assumed to equalize the area of the inclusion in consideration of the size of the inclusion.

In the present invention, the reason why the inclusions having a circle-equivalent diameter exceeding 1.0 占 퐉 is noted is that the inclusions promote the progress of brittle fracture. That is, in order to improve the brittle fracture surface ratio at a cryogenic temperature while securing a predetermined high strength, it is necessary to reduce inclusions that promote brittle fracture. According to the examination results of the present inventors, the average roundness A of the inclusions is It was found that even if the volume fraction (V) of the residual? Phase at -196 占 폚 is controlled to be within the above range, the target cryogenic temperature toughness can not be realized (see No.33, 35, 36 ). The average circularity (A) of the inclusions is preferably as small as possible, preferably not more than 1.7, more preferably not more than 1.5. Most preferably 1. In the present invention, the average size (mean circle equivalent diameter) of inclusions having a circle-equivalent diameter of more than 1.0 占 퐉 is generally 2.0 占 퐉 or less.

The inclusions can be measured by the method described in the later examples. Here, the kind of the " inclusions " in the inclusions having a circle equivalent diameter of more than 1.0 mu m is not particularly limited in the present invention. This is because the occurrence of brittle fracture is most affected by the size of the inclusions (circle equivalent diameter), not by the types of inclusions. As the kind of the inclusions, for example, a composite in which two or more kinds of these single particles are combined, or a composite particle in which these single particles and other elements are combined is used in addition to single particles such as oxides, sulfides, nitrides and oxynitrides .

The mechanism by which the average circularity (A) of coarse inclusions having a circle equivalent diameter of more than 1.0 占 퐉 is 1.8 or less as described in the present invention and the cryogenic temperature toughness is improved while securing a predetermined strength is unclear in detail, Is estimated. Since inclusions generally have a higher hardness than the matrix, stress concentration tends to occur, and as a result, they act as a starting point of brittle fracture in many cases. At this time, as the shape of the inclusions is distorted, the concentration of stress around the inclusions is locally promoted, so that the brittle fracture tends to be more likely to occur. Therefore, when crushed inclusions are reduced (that is, the average roundness A of the inclusions is controlled to be 1.8 or less and controlled to be as close as possible to a full circle (A = 1)), occurrence of stress concentration is avoided, Is improved.

Further, in the present invention, not only the average roundness (A) of the inclusions is controlled but also the B value represented by the above-mentioned formula (1) needs to satisfy the B value? 1.3.

The B value is a parameter for reducing the brittle fracture surface ratio at a cryogenic temperature, and as shown in the above formula (1), the average roundness (A) of the inclusions and the residual roundness of the retained austenite And the volume fraction (V) of the residual?) Phase. Hereinafter, the derivation process of the B value will be described.

It is known that the brittle fracture surface ratio increases as the brittle fracture origin has many points and the resistance to the progress of brittle fracture becomes small. In general, coarse inclusions tend to be a starting point of brittle fracture. However, the present inventors have found that as the roundness of a coarse inclusion becomes larger, in other words, as a crushed shape deviating from a full circle (A = 1) Getting easier; Further, it was found that the greater the residual?, The more resistant to the progress of brittle fracture. On the basis of these findings, the contribution ratio of the two on the brittle fracture surface ratio in the cryogenic temperature region was experimentally determined based on a number of basic experiments. As a result, it was found that the B value represented by the above formula (1) ≪ / RTI > As shown in the later examples, the volume fraction (V) of the residual? Phase and the shape (average roundness) of coarse inclusions having a circle equivalent diameter of more than 1.0 占 퐉 are secured and then the B value is controlled to 1.3 or more Strength and brittle fracture ratios at -196 캜 and -233 캜 are both high.

Is preferably 1.6 or more, more preferably 1.8 or more with respect to the B value. The B value is preferably larger in view of the cryogenic temperature toughness, and the upper limit thereof is not particularly limited. However, as described above, when the volume fraction (V) of the residual? Becomes excessive, YS can not secure a predetermined value, so that the upper limit of the volume fraction V of the residual? Is limited to 12.0% The upper limit of the B value is limited to substantially 5.2 (= 12.0 ^2/3/1 ). (In the calculation formula of the B value, the volume fraction V of the residual? = 12.0% = 1]. In consideration of the balance between strength and toughness, a more preferable value of B is 3.0 or less.

Next, a method for manufacturing the steel sheet of the present invention will be described.

The characteristic parts of the production method according to the present invention are shown in the following (A) to (B).

(O) of not more than 100 ppm before the Al addition in the molten steel step (A), the cooling time t1 at 1450 to 1500 占 폚 at the casting time is not more than 300 seconds And the cooling time t2 at 1300 to 1200 占 폚 during casting is controlled to be 680 seconds or less (value at 1/4 of the thickness t of the slab). With the method (A), the average circularity (A) of the above-mentioned inclusions can be reduced to a predetermined range.

(B) after hot rolling, heating and holding in a temperature range of Ac1 to Ac3, followed by water cooling, followed by tempering for 10 to 60 minutes in a temperature range of 520 deg. C to Ac1, . By the method of (B) above, the volume fraction of the residual? Phase present particularly at -196 占 폚 is appropriately controlled.

The B value defined in the present invention is a parameter related to both the average roundness of the inclusion and the volume fraction of the residual? As described above. Therefore, by appropriately controlling the above-mentioned (A) to (B) Range can be controlled.

With respect to the above-mentioned prior art, there is the greatest feature in the portion of the method (A) in which particularly t1 and t2 are controlled.

Hereinafter, each step will be described in detail.

(For the solvent process)

In the present invention, attention is paid particularly to the method of adding Al. Such inclusions having a circle-equivalent diameter of more than 1.0 mu m to be controlled in the present invention are mainly produced by complex formation of secondary inclusions such as oxides and sulfides starting from Al-based inclusions generated in the molten metal , The Al-based inclusions tend to be coarsened by agglomeration and coalescence, and are liable to be formed into a distorted shape having a large roundness.

First, in adding Al as a de-oxidation material to molten steel, the free oxygen amount (the amount of dissolved oxygen, sometimes abbreviated as [O] amount) before addition of Al is controlled to 100 ppm or less. When the amount of [O] exceeds 100 ppm, Al-based inclusions produced at the time of Al addition increase, and the roundness exceeds the predetermined range (see No.33 of later Table 2B). The amount of [O] is preferably as low as possible, preferably 80 ppm or less, and more preferably 50 ppm or less. The lower limit of the amount of [O] is not particularly limited from the viewpoint of controlling the average roundness of the inclusions.

As a method for controlling the amount of [O] as described above, for example, a method of adding deoxidation elements of Mn and Si to molten steel and deoxidizing them can be mentioned. When a deoxidation material such as Ti, Ca, REM, Zr or the like is added as a selective component in addition to the above elements, the amount of [O] can be controlled by addition of these components.

In order to control Al-based inclusions, it is important to control the amount of [O] before Al addition, and the order of addition of Al and other deoxidizing elements is not critical. However, if Al is added in a state where the amount of [O] is high, the temperature of the molten steel rises due to the oxidation reaction, which is dangerous for operation. Therefore, it is preferable to add Si and Mn prior to Al. It is preferable that the above-mentioned optional components such as Ti are added to molten steel after addition of Al.

Then, casting is started. In the present invention, the cooling time (t1) in the temperature range of 1450 to 1500 占 폚 is controlled to 300 seconds or less, and the cooling time at 1300 to 1200 占 폚 (t2) to be 680 seconds or less, and it has been found that the average circularity of inclusions having a circle-equivalent diameter exceeding 1.0 mu m is controlled appropriately. This will be described in detail below.

First, the cooling time t1 in the temperature range of 1450 to 1500 占 폚 is controlled to 300 seconds or less. If t1 exceeds 300 seconds, complex formation of the secondary inclusions made of the Al-based inclusions is promoted, and the shape of the inclusions having a circle equivalent diameter exceeding 1.0 mu m is distorted to increase the average circularity, decrease the B value So that the targeted cryogenic toughness is not exhibited (see No.44 and 35 in Table 2B, later described). From the above viewpoint, t1 is preferably as short as possible, preferably not more than 290 seconds, more preferably not more than 280 seconds. The lower limit of t1 is not particularly limited in view of the above.

In the present invention, in the temperature range during casting, particularly in the temperature range of 1450 to 1500 占 폚, the solidification progresses during the casting in the temperature range in question, and the concentration of the components in the molten steel progresses, Is a temperature region in which the growth of the semiconductor layer is promoted.

The temperature range of 1450 to 1500 占 폚 means the temperature at the center portion (t / 2) of the slab thickness t. The reason is as follows. As described above, since the oxide-based secondary inclusions are mainly produced in the molten metal, it is necessary to control the cooling time of the molten metal portion. However, since 1450 to 1500 占 폚 is a temperature region in which solidification proceeds, there is a possibility that the cooling time of the molten metal portion can not be accurately measured due to solidification during measurement depending on the temperature measurement position. Therefore, in the present invention, the cooling time at the t / 2 position where the molten metal is present to the lowest temperature was measured. The temperature at the center of the slab thickness can be measured by inserting a thermocouple into the mold.

Next, the cooling time t2 at 1300 to 1200 占 폚 is controlled to be 680 seconds or less. When the t2 exceeds 680 seconds, the complex formation of the secondary inclusions mainly in the Al-based inclusions is promoted, and the average roundness of the inclusions is also increased (see No.36 of Table 2B described later) . From the above viewpoint, the shorter the t2 is, the closer it is to the source, it is useful. The preferable t2 is 650 seconds or less, and more preferably 600 seconds or less. However, when t2 is too short, the cooling load increases. Therefore, in practice, it is generally recommended to set it to 400 seconds or more.

The temperature range of 1300 to 1200 占 폚 means a temperature of 1/4 part (t / 4) of the slab thickness t. The reason is that the cooling time of 1300 to 1200 占 폚 is for controlling secondary inclusions of sulfide based compounds which are mainly produced in solid iron, but since the solidification is almost completed in the temperature region, the measurement of the brittle fracture surface ratio The cooling time at the t / 4 position is measured. The temperature of t / 4 parts of the slab thickness can be measured by inserting a thermocouple into the mold.

In the present invention, the cooling time (t1) in the temperature range of 1450 to 1500 占 폚 and the cooling time (t2) at 1300 to 1200 占 폚 need only be controlled as described above, and the means is not limited thereto. For example, with respect to t1, the temperature range may be cooled at an average cooling rate of about 0.17 deg. C / sec or less so that the cooling time in the temperature range is 300 seconds or less. Alternatively, The cooling may be performed at a different cooling rate so that the time is 300 seconds or less. The same is true for t2.

In the present invention, the cooling method for the temperature range at the time of casting outside the above-mentioned temperature range is not limited at all, and an ordinary method (air cooling or water cooling) can be adopted.

After casting as described above, hot rolling is performed to provide a heat treatment.

Here, the hot rolling step is not particularly limited, and a commonly used method may be employed so as to obtain a predetermined plate thickness. Specifically, the slab is heated at about 1100 캜 for about 1 to 4 hours, Or the like.

After hot rolling, the steel sheet is heated to a temperature range (TL) of Ac1 to Ac3 points, held and water-cooled. This process corresponds to the L process described in the above-described conventional technique, whereby the residual? Stably stably at -196 占 폚 can be secured within a predetermined amount range.

More specifically, it is heated to the two-phase region of Ac1 to Ac3 (ferrite (?) -?) Temperature (TL). By heating in this temperature range, an alloy element such as Ni is concentrated on the generated? Phase to obtain a metastable residual? Phase that exists metastably at room temperature. As a result, when the Ac 1 point is less than Ac 3 or more than Ac 3, the residual γ phase at -196 ° C. can not be sufficiently ensured (see Nos. 37 and 38 of Table 2B). The preferred heating temperature is generally from 660 to 710 占 폚.

The heating time (holding time, tL) at the two-phase region temperature is preferably 10 to 50 minutes. When the time is less than 10 minutes, the concentration of the alloying element in the? Phase does not sufficiently progress, while when the time exceeds 50 minutes, the? Phase is annealed and the strength is lowered. The preferred heating time is approximately 15-30 minutes.

Further, by setting the heating time to 15 minutes or more, the volume fraction of the residual? Phase at -196 占 폚 is secured to 4.0% or more, whereby the brittle fracture surface ratio at -233 占 폚 is 50% or less, Good toughness can be secured even under the above conditions. The upper limit of the preferable heating time is the same as described above (30 minutes or less).

Subsequently, it is water-cooled to room temperature and then tempered. The tempering treatment is performed for 10 to 60 minutes (t3) in a temperature range (T3) of 520 DEG C to _Ac1 point. As a result, at the time of tempering, C is concentrated in the metastable residual?, And the stability of the metastable residual? Phase is increased, so that the residual? Phase stably exists even at -196 占 폚. When the tempering temperature T3 is lower than 520 占 폚, the metastable residual? -Phase formed during the maintenance of the two-phase coexistence region is decomposed into the? -Phase and the cementite phase, so that the residual? -Phase at -196 占 폚 can not be sufficiently secured (See No. 41 in Table 2). On the other hand, when the tempering temperature T3 exceeds the Ac1 point or when the tempering time t3 is less than 10 minutes, the C enrichment in the metastable residual? Phase does not proceed sufficiently and the residual? Amount at -196 deg. [See No.55 (t3 is a short example) of Table 2, which follows). When the tempering time t3 is more than 60 minutes, the residual γ phase at -196 ° C. is excessively generated, so that the predetermined strength can not be ensured (see No. 43 in later column).

The preferred tempering treatment conditions are a tempering temperature T3 of 570 to 620 deg. C and a tempering time t3 of 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 is not particularly limited, and air cooling or water cooling may be used.

In this specification, Ac1 point and Ac3 point are calculated based on the following formula ("Lecture · Modern Metallic Materials 4 Steel Materials", Japan Metal Society).

In the above equation, [] represents the concentration (mass%) of the alloying element in the steel. In the present invention, As and W are not included as components in the steel, so that [As] and [W] are all calculated to be 0% in the above formula.

Example

The present invention will now be described in more detail with reference to the following examples, but it should be understood that the invention is not construed as being limited by the following examples in which it is intended that the invention be construed as being It is included in the technical scope.

Example 1

Using a vacuum melting furnace (150 kgVIF), the steel of the composition shown in Table 1 (the balance amount of iron and inevitable impurities, the unit of mass%) was dissolved in the solvent condition shown in Table 2, An ingot having a size of 150 mm x 150 mm x 600 mm was produced by forging. In this embodiment, mischmetal containing about 50% of Ce and about 25% of La was used as the REM. The order of adding the deoxidizing elements is Si and Mn (simultaneously added) to Al when they do not contain the optional component; On the other hand, when the selective component of Ti, REM, Zr and Ca is included, Si and Mn (simultaneously added)? Al? Ti? REM, Zr and Ca (simultaneously added) are included. In this embodiment, the time from the addition of Al to the start of casting is all about 10 minutes (not shown in the table).

In Table 2, [O] is the amount of dissolved oxygen (ppm) before adding Al, t1 is the cooling time (sec) at 1450 to 1500 占 폚 during casting, t2 is the cooling time at 1300 to 1200 占 폚 )to be. The cooling in each temperature region was controlled to be the cooling time by air cooling or water cooling.

Next, the ingot was heated at 1100 캜 for 1 to 4 hours, then rolled at a temperature of 830 캜 or higher to a plate thickness of 75 mm, rolled at a final rolling temperature of 780 캜, and then water- Steel sheet was obtained. The thus obtained steel sheet was heated to a temperature shown in Table 2 (TL in Table 2), and then heated and maintained for 5 to 60 minutes (see tL in Table 2), followed by water cooling to room temperature. Subsequently, tempering treatment (T3 = tempering temperature, t3 = tempering time) was carried out as shown in Table 2, followed by air cooling or water cooling to room temperature.

The thus obtained steel sheet was subjected to the following tests to determine the average circularity (A) of the inclusions having a circle equivalent diameter of more than 1.0 탆, the volume fraction (%) of the residual γ phase existing at -196 캜 and the tensile properties TS, yield strength YS) and cryogenic toughness (brittle fracture ratio in the C direction at -196 캜 or -233 캜) were evaluated.

(1) Measurement of average roundness (A) of inclusions having a circle-equivalent diameter exceeding 1.0 탆

The t / 4 position (t: plate thickness) of the steel sheet was mirror-polished, and a 4-by-4 picture was taken at 400 times using an optical microscope. In addition, the area per sight field is 0.04 mm 2, and the total area of the four field is 0.15 mm 2. The inclusions observed during these four fields of view were subjected to image analysis by "Image-Pro Plus" manufactured by Media Cybernetics, and circularity of inclusions having a circle equivalent diameter (diameter) exceeding 1.0 μm was calculated based on the following formula, Was defined as the average degree of vacuum (A) of the inclusions. When the shape of the inclusion is a circle, the circularity calculated by the following formula is 1. As the shape of the inclusion is distorted, the roundness value calculated by the following equation becomes larger.

Where L is the circumferential length of the inclusion (unit: mu m), and S is the area of the inclusion (unit: mu m ² ).

Further, in this embodiment, inclusions having a circle-equivalent diameter exceeding 1.0 탆 were observed at about 200 to 300 pieces / mm 2.

(2) Measurement of the volume fraction of residual γ phase present at -196 ° C.

A specimen of 10 mm x 10 mm x 55 mm was taken from the t / 4 position of each steel sheet, held at the liquid nitrogen temperature (-196 DEG C) for 5 minutes, (RINT-RAPIDII). The peaks of the lattice planes of the ferrite phases (110), (200), (211) and (220) and the peaks of the lattice planes of the residual? Phases (111) (200), (220), and (311) on the residual? Phase on the basis of the integral intensity ratio of each peak to calculate the average value of these values, (%) &Quot;.

(3) Measurement of tensile properties (tensile strength TS, yield strength YS)

Fourth test piece of JIS Z2241 was taken parallel to the C direction from the t / 4 position of each steel sheet and tensile strength was measured by the method described in JIS Z2241 to measure tensile strength TS and yield strength YS. In the present embodiment, those having TS > 690 MPa and YS > 590 MPa were evaluated as excellent in base material strength.

(4) Measurement of cryogenic toughness (brittle fracture ratio in the C direction)

Charpy impact test pieces (JISZ) parallel to the C direction from the t / 4 position (t: plate thickness), the W / 4 position (W: plate width), and the t / 4 position and W / 2242 V-notch test piece) were sampled and the brittle fracture ratio (%) at -196 캜 was measured by the method described in JIS Z2242, and the average value of each was calculated. Then, of the two average values calculated in this way, an average value on the side of lowering the characteristics (i.e., a larger brittle fracture surface ratio) was employed and it was evaluated that this value was 10% or less, .

The results thereof are shown in Table 2. For reference, Ac1 point and Ac3 point are listed in Tables 1 and 2.

[Table 1A]

[Table 1B]

[Table 2A]

[Table 2B]

From Table 2, it can be considered as follows.

Nos. 1 to 32 in Table 2A are examples satisfying all the requirements of the present invention. Even if the base material strength is high, the cryogenic toughness at -196 DEG C (specifically, the average value of the brittle fracture ratio in the C direction ≪ = 10%).

In contrast, Nos. 33 to 41, 43, and 55 in Table 2B do not satisfy at least any of the preferred manufacturing conditions of the present invention, and thus are comparative examples that do not satisfy the requirements of the present invention. .

Specifically, in No. 33, No. 33 of Table 1B, which satisfies the requirements of the present invention, was used as the component in the steel, but since the amount of dissolved oxygen [O] before addition of Al was large, the average circularity (A) . As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

In No. 34, although the No. 34 of Table 1B that satisfies the requirements of the present invention was used as the component in the steel, since the cooling time t 1 of 1500 to 1450 캜 at the time of casting was long, This is an example that falls short of the range. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

No.35 is an example in which the average circularity (A) of the inclusions is increased because the No. 35 of Table 1B having a large amount of P is used and the cooling time (t1) of 1500 to 1450 캜 at the time of casting is long . As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

No. 36 is an example in which the No. 36 of Table 1B having a large amount of C is used and the average circularity (A) of the inclusions is increased because the cooling time (t2) at 1300 to 1200 占 폚 during casting is long . As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

In No. 37, No. 37 of Table 1B which satisfies the requirements of the present invention was used as the component in the steel, but the residual γ amount was insufficient because the steel was heated at a temperature lower than the two-phase region temperature TL. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

No. 38 is an example in which the residual amount of? Is insufficient because No. 38 of Table 1B having a large amount of Si is used and heated at a temperature exceeding the two-phase region temperature (TL). As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

In No. 39, although the No. 39 in Table 1B satisfying the requirements of the present invention was used for the components in the steel, since the heating holding time tL in the two-phase region temperature TL is short, to be. As a result, the brittle fracture surface ratio also increased, and the target cryogenic temperature toughness could not be realized.

No. 40, No. 40 of Table 1B, which satisfies the requirements of the present invention, is used as the component in the steel. However, since the heating holding time tL in the two-phase region temperature TL is long, to be. As a result, the yield strength YS and the tensile strength TS were lowered, and the target base material strength could not be secured.

In No. 41, No. 41 of Table 1B which satisfies the requirements of the present invention was used as the component in the steel, but the remaining amount of γ was insufficient because the tempering temperature (T3) was low. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

In No. 43, although the No. 43 in Table 1B satisfying the requirements of the present invention was used for the components in the steel, the residual γ amount was increased because the tempering time t3 was long. As a result, the yield strength YS was lowered and the target base material strength could not be secured.

In No. 55, the steel No. 55 in Table 1B which satisfies the requirements of the present invention is used as the steel component, but the residual γ amount is insufficient because the tempering time t3 is short. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

Nos. 42 and 44 to 54 are comparative examples prepared by the method of the present invention using only those components in the steel which are out of the steel.

Specifically, No. 42 is an example in which the residual? Amount is insufficient because No. 42 of Table 1B having a large amount of Mn is used. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

No.44 is an example in which residual amount of? Is insufficient because No. 44 of Table 1B having a small amount of Mn is used. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

No. 45 is an example using No. 45 of Table 1B, which has a large amount of S. As a result, the brittle fracture surface ratio increases, and the target cryogenic temperature toughness can not be realized.

No. 46 is an example in which the average circularity (A) of the inclusions is increased and the residual? Amount is insufficient because the No. 46 of Table 1B having a small amount of C, a large amount of Al and a small amount of Ni is used. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized. Also, the TS decreased.

No. 47 shows an example in which the average circularity (A) of the inclusions is increased because the No. 47 of Table 1B in which the amount of Al is small and the amount of N is large is used. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

No. 48 is an example using No. 48 of Table 1B, which has a large amount of Cu and Ca as selective components. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

No. 49 is an example using No. 49 of Table 1B, which contains a large amount of Cr and Zr as selective components. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

No. 50 is an example using No. 50 in Table 1B, which has a large amount of Nb and REM as selective components. As a result, the brittle fracture surface ratio increased, and the target cryogenic temperature toughness could not be realized.

In No. 51, since No. 51 in Table 1B having a large amount of Mo as a selective component was used, the brittle wavefront ratio increased, and the target cryogenic temperature toughness could not be realized.

In No. 52, since No. 52 of Table 1B having a large amount of Ti as a selective component was used, the brittle wavefront ratio increased, and the target cryogenic temperature toughness could not be realized.

In No. 53, because No. 53 in Table 1B having a large amount of V as a selective component was used, the brittle wavefront ratio increased, and the target cryogenic temperature toughness could not be realized.

In No. 54, since No. 54 in Table 1B having a large amount of B as a selective component was used, the brittle wavefront ratio increased, and the target cryogenic temperature toughness could not be realized.

Example 2

In this example, the brittle fracture ratio at -233 캜 was evaluated for a part of the data (all examples of the present invention) used in Example 1 above.

Specifically, three test specimens were sampled from the t / 4 position and the W / 4 position with respect to the No. shown in Table 3 (the No. of Table 3 corresponded to the No. of Table 1 and Table 2 described above) And subjected to a Charpy impact test at -233 DEG C by the method described below to evaluate the average value of the brittle fracture surface ratios. In this example, the brittle wavefront ratio of? 50% was evaluated as being excellent in the brittle wavefront ratio at -233 占 폚.

"High Pressure Gas", Volume 24, page 181, "Cryogenic Impact Test of Austenitic Stainless Steel Cast Steel"

These results are shown in Table 3.

[Table 3]

Nos. 3, 4, 6, 15, 19 and 24 in Table 3 are examples in which the heating holding time tL at the two-phase region temperature is controlled to 15 minutes or longer (see Table 2A) 4.0% or more. As a result, the brittle fracture surface ratio at not only -196 deg. C but also at a lower temperature of -233 deg. C was satisfactory, and extremely excellent low temperature toughness was achieved.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention.

The present application is based on Japanese Patent Application (Japanese Patent Application No. 2012-184593) filed on August 23, 2012, the content of which is incorporated herein by reference.

The post-steel sheet of the present invention is excellent in cryogenic temperature toughness, and is particularly useful as a steel material for a storage tank for a liquefied natural gas (LNG) or a conveyor.

Claims (3)

In terms of% by mass,
C: 0.02 to 0.10%
Si: not more than 0.40% (not including 0%),
Mn: 0.50 to 2.0%
P: not more than 0.007% (not including 0%),
S: not more than 0.007% (not including 0%),
Al: 0.005 to 0.050%
Ni: 5.0 to 7.5%
N: not more than 0.010% (not including 0%)
And the balance being iron and unavoidable impurities,
The average circularity (A) of the inclusions having a circle equivalent diameter of more than 1.0 탆 in the steel sheet is 1.8 or less,
The volume fraction (V) of the retained austenite phase present at -196 캜 satisfies 2.0 to 12.0%
Wherein the B value represented by the following formula (1) is 1.3 or more.

The method according to claim 1,
And a residual austenite phase present at -196 캜 in a volume fraction of 4.0 to 12.0%. 3. The method according to claim 1 or 2,
Cu: not more than 1.00% (not including 0%),
Cr: not more than 1.2% (not including 0%),
Mo: not more than 1.00% (not including 0%),
Ti: not more than 0.025% (not including 0%),
Nb: not more than 0.10% (not including 0%),
V: not more than 0.50% (not including 0%),
B: 0.0050% or less (not including 0%)
Ca: not more than 0.0030% (not including 0%),
REM: 0.0050% or less (not including 0%),
Zr: 0.0050% or less (not including 0%)
Wherein the steel sheet further contains at least one member selected from the group consisting of cobalt and iron.