KR101596998B1 - Thick steel plate excellent in ultra low temperature toughness - Google Patents

Abstract

The object of the present invention is to provide a Ni steel which has an Ni content of about 5.0 to 7.5% and is excellent in cryogenic toughness at -196 DEG C or less (particularly in the direction of cryogenic temperature in the C direction), and the brittle fracture rate at -196 DEG C & Strength steel sheet having a strength exceeding 690 MPa, which can realize a high strength steel sheet.
The post-steel sheet according to the present invention comprises a predetermined steel component, wherein the volume fraction (V) of the retained austenite phase present at -196 캜 satisfies 2.0% to 12.0%, and the circle- And the number density of the inclusions exceeding 1.0 占 퐉 is N, the value A satisfies N? 200 pieces / mm2 and the value of A represented by the following formula 1 is 11.5 or less.
&Quot; (1) "

Description

{THICK STEEL PLATE EXCELLENT IN ULTRA LOW TEMPERATURE TOUGHNESS}

The present invention relates to a steel sheet having excellent cryogenic toughness and, more particularly, to a steel sheet having excellent toughness at low temperatures of -196 DEG C or less even when the Ni content is reduced to about 5.0 to 7.5% ) 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 influence of heat treatment in the? -Γ 2 phase coexistence region on the low temperature toughness of 6% Ni steel. Specifically, before the tempering process, the? -? 2 phase coexistence regions (A _{c1 to} A _c3 (L treatment) is applied to the steel sheet to give a cryogenic toughness at -196 deg. C, which is equal to or higher than that of 9% Ni steel subjected to ordinary quenching tempering treatment. This heat treatment is also carried out in the C direction Width direction), and the toughness of the test piece is improved. These effects are due to the presence of a stable retained austenite even for a large amount of fine and impact load at a very low temperature. However, according to the above method, although the cryogenic toughness in the rolling direction (L direction) is excellent, the cryogenic toughness in the plate width direction (C direction) tends to be lower than that 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 hot-rolled steel containing 4.0 to 10% of Ni and having austenite grain size controlled to a predetermined range, and then subjected to hot rolling of A _{c1 to} A _c3 A heating and to repeat the process (corresponding to the L process described in the above Non-Patent Document 1), which is then cooled at least once or twice between then, a method of tempering at a temperature below A _c1 transformation point is described. Patent Document 2 discloses a method of performing a heat treatment (L processing → tempering treatment) similar to that of Patent Document 1 for a steel containing 4.0 to 10% of Ni and 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 supposed 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-strength steel excellent in toughness at a welded portion of 570 MPa or higher, containing 0.3 to 10% of Ni and a predetermined amount of Mg and Mg-containing oxide particles having a predetermined particle size appropriately dispersed . Patent Document 3 discloses that the heating austenite grain size is finely controlled by controlling the Mg-containing oxide and the toughness of the base material and the weld heat affected zone (HAZ) is improved. For this purpose, , The order of addition of Mg and other deoxidizing elements is important, and Mg, Ti and Al are simultaneously added to molten steel having a dissolved oxygen content of 0.001 to 0.02%, and then cast into a steel billet or added with Mg, Ti and Al , Al is finally added, and casting is performed to form a steel piece. In the embodiment of Patent Document 3, the toughness value (wave-surface transition temperature vTrs) in the C direction is described, and the above characteristics of the 9% Ni steel are satisfactory (wavefront transition temperature vTrs? The above characteristics of the steel are -140 deg. C, and further improvements are required.

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

Yano, et al., "Influence of Heat Treatment on α-γ 2 Phase Coexisting Region on Low Temperature Toughness of 6% Ni Steel," Iron and Steel, No. 59 (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, 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 by the steel material to the fracture tends to be remarkably reduced, and the fracture progresses easily. Particularly, in order to suppress fracture at a cryogenic temperature, It is an extremely important requirement to suppress the brittle wavefront ratio to a low level (10% or less). However, as the strength becomes higher, brittle fracture tends to occur. Therefore, it is generally difficult to realize the brittle wavefront 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.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and has an object to provide a Ni steel excellent in cryogenic toughness (in particular, cryogenic toughness in the C direction) at -196 DEG C in a Ni steel having a Ni content of about 5.0 to 7.5% ≪ / RTI > < RTI ID = 0.0 > 10%. ≪ / RTI >

A steel sheet excellent in cryogenic temperature toughness according to the present invention, which can solve the above problems, contains 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%, Ni: 5.0 to 7.5% And the balance amount is iron and inevitable impurities, and the volume fraction (V) of the retained austenite phase existing at -196 캜 satisfies 2.0% to 12.0%, and is present in the steel sheet The number N of the inclusions having a circle-equivalent diameter greater than 1.0 占 퐉 is N, and the value of A represented by the following formula (1) is 11.5 or less.

&Quot; (1) "

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

In a preferred embodiment of the present invention, the steel sheet further contains not more than 1.00% of Cu (not including 0%).

In a preferred embodiment of the present invention, the steel sheet comprises at least one member selected from the group consisting of not more than 1.20% of Cr (not including 0%) and not more than 1.0% of Mo (not including 0%) Lt; / RTI >

In a preferred embodiment of the present invention, the steel sheet comprises 0.025% or less of Ti (not including 0%), 0.100% or less of Nb (not including 0%) and 0.50% or less (Not included), and at least one kind selected from the group consisting of

In a preferred embodiment of the present invention, the steel sheet further contains B: 0.0050% or less (not including 0%).

In a preferred embodiment of the present invention, the steel sheet comprises 0.0030% or less of Ca (not including 0%), 0.0050% or less of REM (not including 0%) and 0.005% or less of Zr (Not included), and at least one kind selected from the group consisting of

According to the present invention, even if the base material strength is high (in particular, tensile strength TS> 690 MPa, yield strength YS> 590 MPa) in a low Ni steel having a Ni content reduced to about 5.0 to 7.5% (Particularly, the cryogenic toughness in the C direction) of the C-direction is particularly excellent, and specifically, in the Charpy impact absorption test in the C direction, the brittle fracture rate at -196 캜 is 10% Of brittle fracture surface ratio? 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, (A) the volume fraction V of the residual austenite (residual?) Present at -196 占 폚 is controlled to 2.0 to 12.0% (preferably controlled to 4.0% to 12.0% (N) of the inclusions is set to 200 pieces / mm 2 or less for inclusions having a circle-equivalent diameter of 1.0 탆 or more (hereinafter sometimes simply referred to as "inclusions") which promote the progress of brittle fracture And that the desired value can be attained by controlling the value of A represented by the following formula (1) to 11.5 or less. Thus, the present invention has been completed.

&Quot; (1) "

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, and Al is finally added to the molten steel to perform casting, and the α-γ 2 phase coexistence region (A _c1 (L treatment) in the temperature range between A _c1 and A _c3 ), it is recommended that the tempering treatment be performed at a temperature not higher than the A _c1 transformation point, thereby improving cryogenic toughness. However, according to the results of the studies conducted by the present inventors, although the cryogenic toughness in the L direction is improved by the above method, the cryogenic toughness in the C direction is not sufficient, and the above- 10%) can not be realized.

Therefore, as a result of further studies, it has been found that, in order to obtain a desired steel sheet with excellent cryogenic toughness, it is indispensable to add requirements to the steel sheet in the steel sheet and its manufacturing method while basically repeating the above-described technique. Specifically, it was found that (i) in the post-steel sheet, the volume fraction V of the residual? Phase at -196 占 폚 exists in the range of V = 2.0 to 12.0%, and further promotes the progress of brittle fracture The number density N of the inclusions is reduced to N? 200 pieces / mm 2, and the number density N (pieces / mm 2) of the inclusions and the existence density of the inclusions existing at -196 占 폚 (A), which is a relational expression of the residual γ phase phase fraction V (%) of the residual γ phase, which is obtained by the following formula (1), to a value A ≦ 11.5. (Ii) (Amount of pre-O) before the Al addition, control of the slab heating temperature T2 in the hot rolling step, and heat treatment between A _{c1 and} A _c3 after hot rolling ) In addition to the tempering treatment in the predetermined temperature range, The control of the effective ground floor, and found the cooling time (t2) of from 1450 to 1500 ℃ during casting that effective to control more than 300 seconds.

(C) By controlling the residual gamma -phase present at -196 ° C. in the range of 4.0% to 12.0% (volume fraction) in the above (a), the brittle fracture surface percentage can be reduced to 50% And (d) in order to produce such a steel sheet, it is preferable to use a steel sheet having a thickness of from A _{c1 to} A _c3 The present invention has been accomplished based on the discovery that the holding for a predetermined time is effective in the heat treatment (L treatment).

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 necessarily 10% It is based on the empirical rule that it becomes.

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, by mass%, 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%) By mass and the balance amount is iron and inevitable impurities, the volume fraction (V) of the retained austenite phase present at -196 캜 satisfies 2.0% to 12.0%, and the circle equivalent diameter in the steel sheet exceeds 1.0 탆 The number N of inclusions of the inclusions is N < 200 pieces / mm < 2 >, and the value of A represented by the following formula 1 is 11.5 or less.

&Quot; (1) "

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 decreases due to an excessive increase in the strength, so the upper limit is set to 0.10%. 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 accelerated to deteriorate the cryogenic toughness. 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 is an austenite (?) Stabilizing element and is an element contributing 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 its upper limit 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 that causes intergranular fracture, and its upper limit is set to 0.007% or less in order to secure a desired cryogenic toughness. As shown in the later examples, when the amount of S is increased, the brittle fracture surface ratio increases, and the desired extremely low temperature 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, 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 oxygen concentration in the steel rises and the number density of inclusions having a circle-equivalent diameter exceeding 1.0 占 퐉 increases, so that the lower limit thereof 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, cohesion or coalescence of inclusions is promoted, which also leads to an increase in the size of the inclusions, so that 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 inevitable 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 excess, 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 1.20% (not including 0%) and Mo: not more than 1.0% (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 low-temperature toughness can not be ensured. Therefore, the upper limit of the Cr content is preferably 1.20% or less (more preferably 1.1% , More preferably 0.5% or less), and the upper limit of the Mo amount is 1.0% 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 0.100% (not including 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 tends to be excessively improved, and the desired low-temperature toughness can not be ensured. Therefore, the upper limit of the Ti content is preferably 0.025% or less (more preferably 0.018% (More preferably not more than 0.05%, more preferably not more than 0.02%), the preferred upper limit of the amount of V is not more than 0.50% (more preferably not more than 0.3% More preferably not more than 0.2%).

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 tends to be excessively improved, and the desired low-temperature toughness can not be secured. Therefore, the preferable upper limit of the amount of B is 0.0050% or less (more preferably 0.0030% or less, still more preferably 0.0020% ).

Ca: not more than 0.0030% (not including 0%), REM (rare earth element): not more than 0.0050% (not including 0%), and Zr: not more than 0.005% At least one species

Ca, REM and Zr are all deoxidized elements, and the oxygen concentration in the steel is lowered by the addition, and the number density of inclusions having a circle-equivalent diameter exceeding 1.0 탆 is reduced. 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 (hereinafter referred to as REM alone, when containing alone and when containing two or more kinds, Is equal to or greater than 0.0005%, and the amount of Zr is equal to or greater than 0.0005%. However, if it is added in excess, the number density of the inclusions increases and the cryogenic temperature toughness decreases. Therefore, the preferred upper limit of the amount of Ca is 0.0030% or less (more preferably 0.0025% or less), the preferable upper limit of the amount of REM is 0.0050% Or less (more preferably 0.0040% or less), and the preferable upper limit of the amount of Zr is 0.005% or less (more preferably 0.0040% or less).

In this specification, REM (rare earth element) refers to Sc (scandium) and Y (yttrium) 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 to 30%) mainly containing Ce and La, La or the like.

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

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

It is known that the residual? Existing at -196 占 폚 contributes to improvement of the cryogenic temperature toughness. To effectively exhibit such a function, the volume fraction V of the residual? Phase occupying all tissues present at -196 占 폚 is set to 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). The preferable lower limit is 4.0% or more, and 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 with respect to the volume fraction V of the residual? Phase.

Further, by controlling the volume fraction of the residual? In all the tissues present at -196 占 폚 to be 4.0% or more, even at -233 占 폚, which is lower than -196 占 폚, the brittle wavefront ratio can be maintained at a satisfactory level . In order to further exert such effects, a more preferred 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, the control of the volume fraction V of the residual? Phase in the structure existing at -196 占 폚 is important, and the structure other than the residual? Is not limited at all, . Examples of the structure other than the residual? Include carbides such as bainite, martensite, cementite, and the like.

Further, the steel sheet of the present invention is characterized in that the number density N of the inclusions satisfies N ≤ 200 pieces / mm < 2 & Value of 11.5 or less.

&Quot; (1) "

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 was noted was that the inclusions promoted 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, when the number density N of the inclusions is increased , The desired cryogenic toughness can not be realized even if the volume fraction V of the residual? Phase at -196 占 폚 is controlled within the above range (see Nos. 33, 35, 36, 47 to 50 in Table 2B Reference). The number density N of the inclusions is preferably as small as possible, preferably 150 pieces / mm 2 or less, and more preferably 120 pieces / mm 2 or less. In the present invention, the average size (average 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 not the kind of inclusions but the size (circle equivalent diameter) of the inclusions the greatest influence. 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 .

In addition, from the viewpoint of inclusion control, similar technology is disclosed in the above-described Patent Document 3, but the direction of inclusion control differs greatly from the present invention. That is, in Patent Document 3, attention is paid to Mg in particular and a large number of fine Mg-containing oxide particles having a size of 2 탆 or less are dispersed to suppress coarsening of austenite at high temperature to improve toughness, In the present invention, inclusions which decrease the toughness as a starting point of brittle fracture or ductile fracture regardless of the kind are reduced, and the control method of the inclusions is completely different between them. Further, according to a preferred manufacturing method of the present invention described later, fine inclusions having a circle-equivalent diameter of 2.0 탆 or less are present in a range of approximately 100 to 1000 pieces / mm 2. Among the fine inclusions having a circle-equivalent diameter of 2.0 占 퐉 or less, there is almost no existence in the present invention as far as it is limited to Mg-containing oxides.

Further, in the present invention, it is necessary not only to control the absolute value N of the number density of the inclusions, but also to ensure that the A value represented by the above-mentioned formula (1) satisfies the A value? 11.5.

Here, the A value is calculated from the relationship between the number density N of the inclusions and the volume fraction V of the residual austenite (residual?) Present at -196 占 폚, as shown in Equation (1) In order to suppress the brittle fracture surface ratio to 10% or less, it is necessary to reduce the inclusions that promote brittle fracture and secure the residual? Effective for promoting soft fracture, , And the contribution rate of both on the brittle fracture surface ratio in the cryogenic temperature region were experimentally determined based on a number of basic experiments. It is considered that the inclusion of π in the above-mentioned formula (1) means that the presence of a large amount of hard inclusions on the progressive surface of the fracture crack in the Charpy test promotes brittle fracture, Is based on the hypothesis that the area ratio (? X radius ² ) of the surface area becomes important. As shown in the later examples, by controlling the A value to 11.5 or less in addition to the volume fraction V of the residual? Phase and the number density N of the inclusions, it is possible to obtain a cryogenic temperature at -196 占 폚 The brittle wavefront ratio in the toughness, particularly the Charpy impact absorption test, can achieve a desired high level. On the other hand, when the A value exceeds 11.5, the brittle fracture rate? 10% could not be secured. The A value is preferably as small as possible, preferably not more than 11.0, more preferably not more than 10.0. Although the lower limit of the A value is not particularly limited in view of the above, it is preferable that the lower limit of the A value is generally 2.5 or more, considering the balance between the volume fraction V of the residual? Phase and the feasible range of the number density N of the inclusions.

Next, a method for manufacturing the steel sheet according to 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 (C).

(O) of 100 ppm or less before the Al addition in the molten steel step (A) and the cooling time (t2) at 1450 to 1500 占 폚 in casting is controlled to be 300 seconds or less. With the method (A), the number density N of the inclusions described above can be reduced to a predetermined range.

(B) In the hot rolling step, the heating temperature (T2) before rolling is controlled to 1120 DEG C or higher. With the method (B), the number density N of the inclusions described above is reduced to 200 pieces / mm 2 or less.

(C) After hot rolling, the steel sheet is heated and held in the temperature range of A _{c1 to} A _c3 , and then water-cooled. Subsequently, the steel sheet is tempered for 10 to 60 minutes at a temperature range of 520 ° C to A _c1 , Cool with water. By the above method (C), the volume fraction of the residual? Phase existing particularly at -196 占 폚 is appropriately controlled.

The A value defined in the present invention can be obtained by suitably controlling the above-mentioned (A) to (C) due to the parameters relating to both the number density of the inclusions and the volume fraction of the residual? It can be controlled to a predetermined range.

Regarding the above-mentioned prior art, the most characteristic feature of the method (A) is that t2 is specifically controlled.

Hereinafter, each process will be described in detail.

(For the solvent process)

Particular attention is paid to the addition method of Al in the present invention. The inclusions having a circle equivalent diameter of more than 1.0 mu m to be controlled in the present invention are mainly composed of Al-based inclusions produced in the molten metal as starting materials, Is generated in a complex manner at the time of cooling. However, the Al-based inclusions are likely to be coarsened by aggregation and coalescence, and the number density of the inclusions is increased.

First, in the addition of Al as the de-oxidizing material to the molten steel, the free oxygen amount (sometimes abbreviated as the amount of dissolved oxygen, [O]) before the addition of Al is controlled to 100 ppm or less. If the amount of [O] exceeds 100 ppm, the inclusions produced at the time of Al addition become large, and the number density of inclusions having a circle equivalent diameter of more than 1.0 탆 increases, and the desired extremely low temperature toughness can not be realized No.33). 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 reducing the number density 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 and Zr is added as a selective component in addition to the above elements, the amount of [O] can be controlled by these additions.

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, when Al is added in a state where the [O] content is high, the temperature of the molten steel rises due to the oxidation reaction, which is dangerous for the 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. The temperature range for casting is generally 1650 占 폚 or less, but in the present invention, it is particularly important to control the cooling time t2 in the temperature range of 1450 to 1500 占 폚 to 300 seconds or less, It has been found that the number density of inclusions in excess is controlled appropriately. If t2 exceeds 300 seconds, complex inclusion of the secondary inclusions is promoted by using the inclusion as nuclei, and the number density of the inclusions having a circle equivalent diameter of more than 1.0 mu m is increased or the value of A is increased, (See Nos. 34 and 35 of Table 2B, later on). From the above viewpoint, t2 is preferably as short as possible, preferably not more than 290 seconds, more preferably not more than 280 seconds. The lower limit of t2 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 of the slab thickness. The slab thickness is generally 150 to 250 mm, and the surface temperature tends to be lowered by about 200 to 1000 ° C compared to the temperature at the center. The surface temperature is the temperature in the center portion (in the vicinity of the thickness t 占 2/2) where the deviation is small because the temperature difference is large. The temperature at the center of the slab thickness can be measured by inserting a thermocouple into the mold.

In the present invention, the cooling time t2 in the temperature range of 1450 to 1500 占 폚 is only required to be controlled to 300 seconds or less, and the means is not limited thereto. For example, the temperature range may be cooled at an average cooling rate of about 0.17 DEG C / second or less at a constant speed so that the cooling time in the temperature range is 300 seconds or less, or the cooling time in the temperature range is 300 seconds Or less at a different cooling rate.

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 a usual method (air cooling or water cooling) can be adopted.

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

In the hot rolling step, the heating temperature (T2) before hot rolling is preferably 1120 DEG C or higher. As a result, relatively unstable sulfides disappear from among the secondary inclusions which are generated in a complex manner, and the inclusions are reduced in size, so that the number density of inclusions having a circle equivalent diameter exceeding 1.0 mu m is reduced. Particularly in the present invention, in consideration of the fact that the influence of the heating temperature before hot rolling on the amount of sulfide formation becomes larger because the amount of S in the steel is small (i.e., the amount of S contributing to the sulfide formation amount is small) It needs to be strictly controlled, and it needs to be controlled higher than a general temperature range (about 1100 ° C). When T2 is less than 1120 占 폚, the number density of inclusions to be controlled increases, and the desired extremely low temperature toughness is not exhibited (see No.36 of Table 2B, later described). From the above viewpoint, T2 is as high as possible, preferably at least 1140 占 폚, and more preferably at least 1,160 占 폚. However, if T 2 is excessively high, the manufacturing cost increases, and therefore, it is preferable to control the upper limit to be generally 1180 ° C. or lower.

It is preferable that the heating time at the heating temperature T2 before hot rolling is within a range of generally 1 to 4 hours.

Other processes (finish rolling, reduction amount, etc.) are not particularly limited, and a commonly used method may be employed so as to obtain a predetermined plate thickness.

After hot rolling, the steel sheet is heated to a temperature range (TL) of A _{c1 to} A _c3 , held and water-cooled. This process corresponds to the L process described in the above-described conventional technique, whereby the residual gamma that stably exists at -196 DEG C can be secured within a predetermined amount range.

More specifically, it is heated to a two-phase region [ferrite (?) -?] Temperature TL of A _{c1 to} A _c3 points. 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, the residual γ phase at -196 ° C. can not be sufficiently ensured when A _{c1 is} less than or more than A _c3 (see Nos. 37 and 38 in 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 in general. When the time is less than 10 minutes, the alloying element is not sufficiently concentrated in the? -Phase, while when it exceeds 50 minutes, the? -Phase is annealed and the strength is lowered. The preferred heating time is generally from 15 to 30 minutes. The upper limit of the preferred heating time is 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, Even in the case of a nonwoven fabric. In order to effectively exhibit such effects, the lower limit is more preferably 5.0% or more. The upper limit of the preferable heating time is the same as the 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 generated 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 A _c1 point or the tempering time t3 is less than 10 minutes, the C enrichment in the metastable residual γ phase does not proceed sufficiently, and the residual? (See No.42 (T3 is a high example) and No.55 (t3 is a short example)), which are listed later in Table 2). If 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 of Table 2 below).

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 either air cooling or water cooling may be employed.

In the present specification, the points A _c1 and A _c3 are calculated based on the following formula ("Lecture: Modern Metallic Materials 4 Steel", Japan Institute of Metals).

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. However, it should be understood that the present invention is not limited to the following examples, but can be carried out by modifying the scope of the present invention. It is included in the technical scope.

Example 1

Using a vacuum melting furnace (150 kg VIF), the steel having the composition shown in Table 1 (residual amount: iron and unavoidable impurities, mass%) was dissolved in the solvent conditions shown in Table 2, , And an ingot having a size of 150 mm x 150 mm x 600 mm was produced by hot forging. In this embodiment, mischmetal containing about 50% of Ce and about 25% of La was used as the REM. The order of addition of the deoxidizing elements is Si, Mn (simultaneously added) to Al when not containing a selective component, and Si, Mn (simultaneously added) when containing a selective component of Ti, REM, Zr, Addition) → Al → Ti → REM, Zr and Ca (simultaneously added). In this embodiment, the time t1 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 dissolved oxygen amount (ppm) before Al addition, and t2 is the cooling time (seconds) at 1500 to 1450 ° C at the time of casting. The cooling at 1500 to 1450 占 폚 was controlled to be the cooling time by air cooling or water cooling.

Next, the above-mentioned ingot was heated to various temperatures T2 as shown in Table 2, then rolled to a plate thickness of 75 mm at a temperature of 830 캜 or higher, rolled at a final rolling temperature of 780 캜 and then water- To obtain a 25 mm thick steel plate. 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 performed as shown in Table 2, and then air cooling or water cooling was performed to room temperature.

The thus obtained steel sheet was subjected to the following procedure to determine the number density N (number / mm 2) of inclusions having a circle equivalent diameter of more than 1.0 탆, the volume fraction (%) of residual γ phase present at -196 캜, (Tensile strength TS, yield strength YS) and cryogenic toughness (brittle fracture ratio in the C direction at -196 캜 or -233 캜) were evaluated.

(1) Number of inclusions having a circle-equivalent diameter exceeding 1.0 탆 Measurement of density N

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 to calculate the number density N (number / mm 2) of inclusions having a circle equivalent diameter (diameter) And the average value was calculated.

(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 a 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) (200), (220), and (311) on the residual? Phase on the basis of the integral intensity ratio of each peak, and calculates the average value of the volume fractions of the remaining? Volume fraction (%) ".

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

Four test specimens of JIS Z 2241 were taken parallel to the C direction from the t / 4 position of each steel sheet and subjected to a tensile test according to the method described in ZIS Z 2241 to measure the tensile strength TS and the yield strength YS. In the present embodiment, it was evaluated that TS> 690 MPa and YS> 590 MPa were excellent in base material strength.

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

Charpy impact test specimens (JIS Z) parallel to the C direction from the t / 4 position (t: plate thickness), W / 4 position (W: plate width), t / 4 position and W / 2242 V-notch test piece) were sampled and the brittle fracture percentage (%) at -196 캜 was measured by the method described in JIS Z 2242, 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, Table 1 and Table 2 list the points A _c1 and A _c3 .

[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 43 and 55 of Table 2B do not satisfy at least any one of the preferable manufacturing conditions of the present invention, and therefore are comparative examples that do not satisfy the requirements of the present invention, and desired characteristics are not obtained.

Specifically, in No. 33, No. 33 of Table 1B, which satisfies the requirements of the present invention, was used as the component in steel, but since the amount of dissolved oxygen [O] before addition of Al was large, the number density N of the inclusions increased Yes. As a result, the brittle wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

In No. 34, the steel No. 36 in Table 1B, which satisfies the requirements of the present invention, was used. However, since the cooling time t 2 of 1500 to 1450 ° C during casting is long, It is an example exceeding the range. As a result, the brittle wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

No. 35 is an example in which the number density N of the inclusions is increased because No. 35 of Table 1B having a large amount of P is used and the cooling time t2 of 1500 to 1450 占 폚 at the time of casting is long. As a result, the brittle wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

In No. 36, since No. 36 of Table 1B having a large amount of C is used and the heating temperature (T2) before hot rolling is low, the number density N of the inclusions increases and the value of A exceeds the predetermined range It is an example. As a result, the brittle wavefront ratio increased, and the desired extremely low 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 to a temperature lower than the two-phase region temperature TL. As a result, the brittle wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

No. 38 is an example in which remaining amount? Is insufficient because No. 38 of Table 1B having a large amount of Si is used and the temperature is higher than the two-phase region temperature (TL). As a result, the brittle wavefront ratio increased, and the desired extremely low 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 wavefront ratio also increased, and the desired extremely low 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 desired 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 wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

No. 42 is an example in which the residual amount of? Is insufficient because No. 42 of Table 1B having a large amount of Mn is used and the tempering temperature T3 is high. As a result, the brittle wavefront ratio increased, and the desired extremely low 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 desired 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 wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

Nos. 44 to 54 are comparative examples produced by the method of the present invention using only those components in the steel that are out of the steel.

Specifically, No. 44 is an example in which the residual? Amount is insufficient because No. 44 in Table 1B having a small amount of Mn is used. As a result, the brittle wavefront ratio increased, and the desired extremely low 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 desired extremely low temperature toughness can not be realized.

No. 46 is an example in which the number density N 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 wavefront ratio increased, and the desired extremely low temperature toughness could not be realized. TS also decreased.

No.47 is an example in which the number density N of the inclusions increases and the value A exceeds a predetermined range because the No. 47 of Table 1B having a small amount of Al and a large amount of N is used. As a result, the brittle wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

No. 48 is an example in which the number density N of the inclusions is increased and the value of A exceeds a predetermined range because No. 48 of Table 1B having a large amount of Cu and Ca as selective components is used. As a result, the brittle wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

No. 49 is an example in which the number density N of the inclusions is increased because the No. 49 of Table 1B in which the amounts of Cr and Zr as the selective components are large is used. As a result, the brittle wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

No. 50 is an example in which the number density N of the inclusions is increased and the value of A exceeds the predetermined range because No. 50 of Table 1B in which the amounts of Nb and REM as selective components are large is used. As a result, the brittle wavefront ratio increased, and the desired extremely low 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 desired extremely low temperature toughness could not be realized.

In No. 52, since No. 52 in Table 1B, which contains a large amount of Ti as a selective component, was used, the brittle wavefront ratio increased, and the desired extremely low temperature toughness could not be realized.

In No. 53, since No. 53 in Table 1B having a large amount of V as a selective component was used, the brittle wavefront ratio increased, and the desired extremely low 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 desired extremely low 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 corresponds to the No. of Table 1 and Table 2 described above) , The Charpy impact test was carried out at -233 캜 by the method described below, and the average value of the brittle fracture surface ratios was evaluated. 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"

The 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 time tL at the two-phase region temperature is controlled to 15 minutes or longer (see Table 2A) %. As a result, the brittle fracture surface ratio at not only -196 캜 but also at a lower temperature of -233 캜 was also good, and extremely excellent low temperature toughness could be achieved.

Claims (2)

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 volume fraction (V) of the retained austenite phase present at -196 캜 satisfies 2.0% to 12.0%
When the number density of inclusions having a circle equivalent diameter of more than 1.0 mu m existing in the steel sheet is N,
N ≤ 200 pieces / mm &
Wherein an A value represented by the following formula (1) is less than 11.5.
&Quot; (1) "

The backsheet according to claim 1, wherein the after-warp steel sheet further comprises at least one of the following groups (a) to (e) as other elements.
(a) Cu: not more than 1.00% (not including 0%),
(b) at least one member selected from the group consisting of Cr: not more than 1.20% (not including 0%), and Mo: not more than 1.0% (excluding 0%
(c) Ti: not more than 0.025% (excluding 0%), Nb: not more than 0.100% (not including 0%), and V: not more than 0.50% At least one,
(d) B: 0.0050% or less (not including 0%),
(e) 0.0030% or less Ca (not including 0%), REM: 0.0050% or less (excluding 0%), and Zr: 0.005% or less At least one species