KR101442400B1 - Thick steel plate excellent in ultra low temperature toughness - Google Patents

Abstract

The object of the present invention is to provide a Ni steel having an Ni content of about 5.0 to 7.5% and having excellent cryogenic toughness at -196 DEG C or lower (particularly, cryogenic toughness in the C direction) and a brittle fracture rate & Strength steel sheet having a strength exceeding 690 MPa, which is capable of realizing a high strength of steel sheet.
The post-steel sheet according to the present invention comprises a predetermined steel component, and the retained austenite phase present at -196 DEG C has a volume fraction of 2.0% to 12.0% and an average circle equivalent of inclusions having a circle- And a diameter of 3.5 m or less.

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 (Corresponding to the L treatment described in Non-Patent Document 1) is repeated once or twice or more, and then tempered at a temperature not higher than the Ac _forming point. 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 region where brittle fracture occurs, the energy absorbed by the steel material to the fracture tends to be remarkably reduced, and the fracture progresses easily. Therefore, in the technique for improving the cryogenic temperature toughness, improvement in the general Charpy impact value (vE _-196 ) However, it is also an extremely important requirement to set the brittle fracture ratio to 10% or less. However, as described above, a technique for satisfying the above requirements of the brittle fracture surface ratio in 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 remaining amount is iron and inevitable impurities, the residual austenite phase present at -196 캜 in a volume fraction of 2.0% to 12.0%, and the inclusions having a circle equivalent diameter of more than 2.0 탆 The average circle equivalent diameter of the particles is 3.5 mu m or less.

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 1.0% 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 Ni steel having a Ni content of about 5.0 to 7.5% (Particularly, the cryogenic toughness in the C direction) is excellent, and a brittle fracture surface ratio at -196 캜 is ≤10% (preferably, a brittle fracture surface ratio at -233 캜 is ≤50%). Could.

In the Ni steel having a Ni content of about 5.0 to 7.5%, in order to further improve the cryogenic temperature toughness in the C direction, the characteristic portion of the steel sheet according to the present invention is characterized in that (A) the retained austenite phase (The residual γ phase) is controlled to 2.0% to 12.0% (volume fraction) (preferably controlled to 4.0% to 12.0% (volume fraction)),

(B) coarse inclusions having a circle-equivalent diameter exceeding 2.0 占 퐉 (hereinafter, simply referred to as coarse inclusions, sometimes abbreviated as N1) have an average circle equivalent diameter of not more than 3.5 占 퐉. 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 studies to provide a steel sheet excellent in cryogenic temperature toughness of -196 DEG C or less in Ni steel having a Ni content of 7.5% or less. Specifically, in the present invention, it is preferable that the cemented carbide satisfies all the characteristics of a brittle fracture surface ratio ≤10% at -196 캜 in the C direction, a tensile strength TS of 690 MPa, and a yield strength YS of 590 MPa, From the viewpoint of providing a steel sheet, 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 ~ A _c3 (L treatment) in a temperature range of not less than the A _c1 transformation point is recommended, and it is taught that the cryogenic temperature toughness is improved. However, according to the examination results of 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-mentioned target level referred to in the present invention (-196 10 < / RTI >%) can not be realized.

Therefore, as a result of further investigations, it has been found that, in order to obtain a steel sheet having excellent cryogenic toughness desired, it is indispensable to add requirements to the steel sheet in the steel sheet and its manufacturing method while basically following the above-described technique. Specifically, in (a) a steel sheet having a residual γ-phase at -196 ° C. in a range of 2.0% to 12.0% (volume fraction), a circular equivalent diameter of 2.0 μm It is effective to make the average circle equivalent diameter N1 of the coarse inclusions exceeding 3.5 .mu.m. (2) In order to produce such a steel sheet, the amount of dissolved oxygen (free O amount) And the control of A _{c1 to} A _c3 (L treatment) → tempering treatment in a predetermined temperature range, the control of a single layer in the molten steel step is effective, the holding time t1 from the addition of Al to the start of casting is set to 15 minutes or more, It is effective to control the cooling time (t2) at 1450 to 1500 DEG C to 300 seconds or less.

(C) By controlling the residual gamma -phase present at -196 ° C. in the range of 4.0% to 12.0% (volume fraction) in (a) above, the brittle wavefront ratio can be reduced to 50 % Or less. (D) In order to produce such a steel sheet, it is preferable that A _{c1 to} A _c3 (L treatment) between the heat treatment and the heat treatment is effective for a predetermined time, and the present invention has been completed.

In the present specification, " excellent in cryogenic temperature toughness " means a value obtained by measuring the vE _-196 and the brittle fracture surface ratio in the Charpy impact absorption test in the C direction (plate width direction) by the method described in the column of the later- , The brittle fracture rate at -196 캜 is? 10%. 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 %. ≪ / RTI >

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%) And the balance amount is iron and inevitable impurities, the retained austenite phase present at -196 캜 in a volume fraction of 2.0% to 12.0%, and the mean circle equivalent diameter of inclusions having a circle equivalent diameter of more than 2.0 탆 is 3.5 Mu m or less.

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 preferable 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, 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 coarse inclusions increase, 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, 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, and 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 preferable lower limit of the amount of Ni is at least 5.2%, more preferably at least 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 even more 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 preferred upper limit of the amount of N is 0.006% or less, and 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 1.0% (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 1.0% or less. A more preferable upper limit of the Cu amount is 0.8% or less, and 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 tends to be excessively improved, and the desired low-temperature toughness can not be secured. Therefore, the upper limit of the Cr content is preferably 1.20% or less (more preferably 1.1% Or less, still more preferably 0.5% or less), and the upper limit of the Mo content is 1.0% or less (more preferably 0.8% or less, and still more 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 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%, still more preferably not more than 0.02%), the preferable upper limit of the amount of N 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 upper limit of the amount of B is preferably 0.0050% or less (more preferably 0.0030% or less, still more preferably 0.0020% Or less).

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 addition of O, decreases the oxygen concentration in the steel and reduces coarse inclusions. 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 coarse inclusion is increased and the cryogenic temperature toughness is lowered. Therefore, the preferred upper limit of the amount of Ca is 0.0030% or less (more preferably 0.0025% or less) and 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 steel component of the present invention has been described above.

The post-steel sheet of the present invention satisfies 2.0 to 12.0% (preferably 4.0 to 12.0%) of the residual gamma-phase present at -196 DEG C as a volume fraction.

It is known that the residual? Existing at -196 占 폚 contributes to the improvement of the cryogenic temperature toughness. To effectively exhibit such action, the volume fraction of the residual? Phase occupying all tissues present at -196 占 폚 is 2.0% or more. However, since the residual y is relatively soft as compared with the matrix phase and the residual y amount becomes excessive, YS can not secure a predetermined value, so the upper limit is set to 12.0% (see No. 39 in Table 2 hereinafter) 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 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 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. It would be. 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 satisfies the average circle-equivalent diameter N1 of inclusions (coarse inclusions) having a circle-equivalent diameter of more than 2.0 占 퐉 of 3.5 占 퐉 or less. In contrast to the above-mentioned prior art, the steel sheet of the present invention has the greatest feature that the coarse inclusions are miniaturized to 3.5 탆 or less.

Here, the " circle-equivalent diameter " refers to the diameter of the circle assumed to be equal in area, considering the size of the inclusion.

That is, according to the examination results of the inventors of the present invention, coarse inclusions having a circle-equivalent diameter exceeding 2.0 탆 have origins of brittle fracture, and when the average size (average circle equivalent diameter N 1) of the coarse inclusions is large, It has been found that even if the volume fraction of the residual? Phase is controlled within the above range, the desired extremely low temperature toughness can not be realized (see Nos. 33 to 35 and 45 to 49 in Table 2 below). The average circle equivalent diameter of N1 is preferably as small as possible, preferably 3.2 m or less, and more preferably 3.0 m or less. In the present invention, the inclusions having a circle-equivalent diameter of more than 2.0 占 퐉 are present in an amount of about 10 to 100 / mm2.

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 2.0 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 (average circle equivalent diameter), not by the type 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 .

In addition, from the viewpoint of inclusion control, a similar technique is disclosed in the aforementioned Patent Document 3. However, the direction of inclusion control differs greatly from that of the present invention. That is, in Patent Document 3, attention is focused on Mg, 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, regardless of the kind thereof, coarse inclusions, which serve as a starting point of brittle fracture, reduce the toughness, and the control methods of inclusions are completely different from each other. Therefore, although the present invention does not control fine inclusions at all, 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.

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) and (B).

(O2) of not more than 100 ppm before Al addition and a holding time (t1) from Al addition to the start of casting for not less than 15 minutes at 1450 to 1500 占 폚 at the time of casting in the step (A) ) To 300 seconds or less. With the method (A), the average circle-equivalent diameter of the above-mentioned coarse inclusions is miniaturized to 3.5 탆 or less.

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

In the relation of the above-mentioned prior art, the most characteristic feature is that t1 and t2 are specifically controlled in the method (A).

Hereinafter, each process will be described in detail.

(For the solvent process)

In the present invention, based on the viewpoint that the Al-based inclusions are coarsened by aggregation and coalescence, and coarse inclusions that are the origin of brittle fracture are easy to form, the addition of Al Special attention is paid to the method.

First, in adding Al as a de-oxidizing material to the molten steel, the free oxygen amount (the amount of dissolved oxygen, sometimes referred to as [O] amount) before the Al addition is controlled to 100 ppm or less. If the amount of [O] exceeds 100 ppm, the size of the inclusions produced at the time of Al addition becomes large and N1 can not be properly controlled, and the desired extremely low temperature toughness can not be realized (see No.33 in Table 2 below) . 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 refinement of coarse 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, 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.

Next, after adding Al to the molten steel, the holding time t1 from the addition of Al to the start of casting is set to 15 minutes or more. As a result, the coarse inclusion is floated and separated and removed. Conventionally, casting has been started within a maximum of 13 minutes at the same time as or after Al addition, but when t1 is less than 15 minutes, the effect of removing coarse inclusions is not effectively exerted and coarse inclusions are not miniaturized , And it was proved that the desired cryogenic toughness was not exerted (see No.44 and No.55 of Table 2 below). From the above viewpoint, t1 is preferably longer, preferably 18 minutes or more, and more preferably 20 minutes or more. The upper limit of t1 is not particularly limited in view of the above, but the maintenance for a long time leads to an increase in the production cost, so that it is preferably 180 minutes or less, more preferably 150 minutes or less.

Then, casting is started. The temperature range for casting is generally 1650 占 폚 or less. 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 it is refined. If t2 exceeds 300 seconds, secondary inclusions are generated in a complex manner using the inclusions as nuclei, so that the size of the coarse inclusions increases, and the desired extremely low temperature toughness is not exhibited (No.35, No.56 ). 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 and the component concentration in the molten steel progresses, Is a temperature region in which the growth of the semiconductor layer is promoted.

Further, 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 DEG C compared to the temperature of the center portion. 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 t1 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 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, after the slab is heated at about 1100 DEG C for about 1 to 4 hours, And the like.

After hot rolling, the steel sheet is heated to a temperature range (TL) of A _{c1 to} A _c3 , held and then 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, when A _{c1 is} less than or equal to A _c3 , the residual γ phase at -196 ° C can not be sufficiently ensured (see No.36 and 37 in Table 2 below). 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 concentration of the alloy element in the γ phase does not sufficiently progress, while when the time exceeds 50 minutes, the α phase is annealed and the strength is lowered. 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 described above (30 minutes or less).

Then, after cooling to room temperature, tempering is performed. 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. 40 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.41 (T3 is a high example) and No.54 (t3 is a short example)), which are listed later in Table 2). 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.42 of Table 2 to be described later).

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 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).

A _c1 point

= 723-10.7 x [Mn] -16.9 x [Ni] + 29.1 x [Si] + 16.9 x [Cr] + 290 x [As] + 6.38 x [W]

A _c3 point

= 910-203 x [C] ^1/2 -15.2 x [Ni] + 44.7 x [Si] + 104 x [V] + 31.5 x [Mo] + 13.1 x [W]

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 Table 2, [O] is the amount of dissolved oxygen (ppm) before Al addition, t1 is time (minutes) from the addition of Al to the start of casting, and t2 is the cooling time (seconds) at 1500 to 1450 ° C . The cooling at 1500 to 1450 占 폚 was controlled to be the cooling time by air cooling or water cooling.

Next, the ingot was heated to 1100 캜, 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 cooled to obtain a post-steel plate having a thickness of 25 mm. The thus obtained steel sheet was heated to the temperature shown in Table 2 (TL in Table 2), and then heated and maintained for 5 to 60 minutes (see tL in Table 2), and then cooled 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.

With respect to the steel sheet obtained as described above, the average circle-equivalent diameter N1 of inclusions having a circle equivalent diameter of more than 2.0 占 퐉, the amount of residual γ phase (volume fraction) existing at -196 占 폚, the tensile property 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 circle-equivalent diameter N1 of inclusions having a circle-equivalent diameter exceeding 2.0 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 to calculate the circle equivalent diameter (diameter) of the inclusions having a circle equivalent diameter (diameter) exceeding 2.0 袖 m, Respectively.

(2) Measurement of the amount of residual γ phase (volume fraction) 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 Z2241 were taken parallel to the C direction from the t / 4 position of each steel sheet and subjected to a tensile test by the method described in ZIS Z2241 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 surface ratios in the C direction 10%) could be provided.

In contrast, Nos. 33 to 42 and 54 to 56 in Table 2B are Comparative Examples which do not satisfy at least the requirements of the present invention because they do not satisfy at least any of the preferable manufacturing conditions of the present invention, .

Specifically, in No. 33, No. 33 of Table 1B, which satisfies the requirements of the present invention, is used as the component in the steel, but since the amount of dissolved oxygen [O] before addition of Al is large, this is an example in which coarse inclusions are not refined . As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness at -196 deg. C could not be realized.

No.34 is an example in which the No. 34 of Table 1B having a large amount of C is used and the time t1 until the start of casting after the addition of Al is short is an example. On the other hand, in No. 55, 5 > of Table 1B, which satisfies the above condition, is used. However, t1 is a short example. In any case, since t1 is short, coarse inclusions are not miniaturized. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

No.35 is an example in which the No. 35 of Table 1B having a large amount of P is used and the cooling time (t2) of 1500 to 1450 DEG C at the time of casting is long. On the other hand, No.56 of Table 1B that satisfies the requirements of the present invention is used, but t2 is a long example. In any case, since t2 is long, coarse inclusions are not miniaturized. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

In No. 36, No. 36 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 below the two-phase region temperature TL. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

No. 37 is an example in which the remaining amount of? Is insufficient because No. 37 of Table 1B having a large amount of Si is used and the temperature is exceeded beyond the two-phase region temperature (TL). As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

In No. 38, although the No. 38 in Table 1B that satisfies 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.

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 long, to be. As a result, the yield strength YS was lowered and the desired base material strength could not be secured.

No. 40, No. 40 of Table 1B, which satisfies the requirements of the present invention, is used as the component in the steel, but the residual γ amount is insufficient because the tempering temperature T3 is low. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

No. 41 is an example in which remaining No. of γ is insufficient because No. 41 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 also increased, and the desired extremely low temperature toughness could not be realized.

In No. 42, No. 42 of Table 1B, which satisfies the requirements of the present invention, was used for the components in the steel, but this is an example in which the residual? Amount is increased because the tempering time t3 is long. As a result, the yield strength YS was lowered and the desired base material strength could not be secured.

In No. 54, the No. 54 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 tempering time t3 was short. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

Nos. 43 to 53 are comparative examples prepared by the method of the present invention using only those components which are out of the steel.

Specifically, No. 43 is an example in which the residual? Amount is insufficient because No. 43 of Table 1B having a small amount of Mn is used. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

No. 44 is an example using No. 44 of Table 1B with 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. 45 is an example in which the coarse inclusion is not refined and the residual? Amount is insufficient because the No. 45 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 also increased, and the desired extremely low temperature toughness could not be realized. TS also decreased.

No. 46 is an example in which coarse inclusions are not refined because No. 46 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 wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

No.47 is an example in which the coarse inclusions are not refined because the No. 47 of Table 1B having a large amount of Cu and Ca as selective components is used. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

No.48 is an example in which the coarse inclusions are not refined because the No. 48 of Table 1B in which the amount of Cr and the amount of Zr being selected are large is used. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

No. 49 is an example in which coarse inclusions are not refined because No. 49 of Table 1B, which contains a large amount of Nb and REM as selective components, is used. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

In No. 50, since No. 50 of Table 1B, which contains a large amount of Mo as a selective component, was used, the brittle wavefront ratio increased, and desired low temperature toughness could not be realized.

In No. 51, since No. 51 in Table 1B having 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. 52, since No. 52 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. 53, because No. 53 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 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"

The results are shown in Table 3.

[Table 3]

In Table 3, Nos. 3, 4, 6, 13, 15, 19 and 23 show examples in which the heating time tL at the two-phase region temperature is controlled to 15 minutes or longer (see Table 2A) Of 4.0% or more. 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 (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 retained austenite phase present at -196 캜 is 2.0% to 12.0% by volume,
Wherein the average circle equivalent diameter of inclusions having a circle equivalent diameter of more than 2.0 占 퐉 is 3.5 占 퐉 or less. The coated steel sheet according to claim 1, wherein the retained austenite phase present at -196 캜 has a volume fraction of 4.0% to 12.0%. The backsheet according to any one of claims 1 to 3, wherein the backsheet further comprises at least one of the following groups (a) to (e) as other elements.
(a) Cu: not more than 1.0% (not including 0%),
(b) at least one selected from the group consisting of Cr: not more than 1.20% (not including 0%) and Mo: not more than 1.0% (not including 0%),
(c) Ti: 0.025% or less (excluding 0%), Nb: 0.100% or less (excluding 0%), and V: 0.50% or less One or more,
(d) B: 0.0050% or less (not including 0%),
(e) 0.0030% or less Ca (not including 0%), REM: 0.0050% or less (not including 0%), and Zr: 0.005% or less At least one