KR20130116202A - Thick steel plate excellent in ultra low temperature toughness - Google Patents

Abstract

PURPOSE: A thick steel plate with excellent extreme low temperature toughness is provided to implement the percentage of brittle fracture which is <=10% by having excellent extreme low temperature toughness at -196°C. CONSTITUTION: A thick steel plate with excellent extreme low temperature toughness is composed of, in wt%: C: 0.02-0.10; Si: 0.40 or less (excluding 0); Mn: 0.50-2.0 (excluding 0); P: 0.007 or less (excluding 0); S: 0.007% or less (excluding 0); Al: 0.005-0.050; Ni: 5.0-7.5; N: 0.010% or less (excluding 0); and the remainder Fe and inevitable impurities. The thick steel plate has the remainder austenite phase which exists at -196°C, and satisfies the volume percentage (V) of 2.0-12.0%. When the number density of an inclusion with the diameter of over 1.0 um per circular phase which exists on the thick steel plate is N, N is N<=200 unit/mm^2.

Description

Thick sheet with excellent cryogenic toughness {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 thick steel plate for LNG tanks used for the storage tank of liquefied natural gas (LNG) requires high toughness which can endure the cryogenic temperature of -196 degreeC 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.

The technique similar to the said nonpatent literature 1 is described in patent document 1 and patent document 2. 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. In addition, these methods do not consider the strength and there is no description of the brittle fracture rate.

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, although the thing showing the high toughness of a rolling direction (L direction) is described, the toughness value of a 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, further improvement of the cryogenic toughness (improvement of cryogenic toughness in the C direction) under a high strength with a high base material strength (in detail, tensile strength TS> 690 MPa and yield strength YS> 590 MPa) is strongly demanded.

In addition, the literature mentioned above has not examined brittle fracture rate. The brittle fracture rate represents the rate of brittle fracture occurring when a load is applied in the Charpy impact test. At the site where brittle fracture has occurred, the energy absorbed by the steel material until the fracture is significantly reduced and the fracture easily proceeds. In particular, in order to suppress the breakdown at cryogenic temperatures, it appears in the general Charpy impact test. Reducing the brittle fracture rate at low level (10% or less) is an extremely important requirement. However, as the strength is higher, brittle fracture is more likely to occur, and therefore, it is generally difficult to realize brittle fracture rate ≤ 10% under high base material strength as described above. Therefore, in the high strength thick steel plate with high base material strength, the technique provided with both of these 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% &Lt; / RTI &gt; &lt; RTI ID = 0.0 &gt; 10%. &Lt; / RTI &gt;

The thick steel sheet which is excellent in the cryogenic toughness which concerns on this invention which could solve the said subject is mass%, C: 0.02-0.10%, Si: 0.40% or less (not containing 0%), Mn: 0.50-2.0%, P: 0.007% or less (0% not included), S: 0.007% or less (0% not included), Al: 0.005 to 0.050%, Ni: 5.0 to 7.5%, N: 0.010% or less (0% ), The remainder being a thick steel sheet containing iron and unavoidable impurities, and the volume fraction (V) of the residual austenite phase present at -196 ° C satisfies 2.0% to 12.0%, and is also present in the steel sheet. When the number density of inclusions exceeding 1.0 micrometer in diameter is set to N, N <= 200 pieces / mm <2> and A value represented by following formula (1) satisfy | fill 11.5 or less.

&Quot; (1) &quot;

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 Cu: 1.00% or less (does not contain 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 &gt;

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 in a low Ni steel in which the Ni content is reduced to about 5.0 to 7.5%, even when the base material strength is high (specifically, the tensile strength TS> 690 MPa, the yield strength YS> 590 MPa), it is -196 degrees C or less. Has excellent cryogenic toughness (particularly cryogenic toughness in the C direction), and specifically, in the Charpy impact absorption test in the C direction, the brittle fracture rate ≤10% at -196 ° C (preferably at -233 ° C). It was possible to provide a high strength thick steel sheet that satisfies the brittle fracture rate of 50%).

MEANS TO SOLVE THE PROBLEM In the high strength thick steel plate which Ni content reduces to 7.5% or less and satisfy | fills tensile strength TS> 690 Mpa, and yield strength YS> 590 Mpa, the Charpy impact value of C direction is -196 degreeC. In order to provide the cryogenic toughness improvement technique which satisfy | fills brittle fracture rate <10%, it examined. As a result, (a) the volume fraction (V) on the residual austenite (residual γ) present at -196 ° C is controlled to 2.0 to 12.0% (preferably, 4.0% to 12.0% (volume fraction)). At the same time, (b) the number density (N) of the inclusions is 200 pieces / mm 2 or less with respect to inclusions (hereinafter, simply referred to as inclusions) having a diameter larger than 1.0 μm, which promotes the progress of brittle fracture. When the A value represented by the following formula (1) was reduced to 11.5 or less, the desired object was achieved, and the present invention was completed.

&Quot; (1) &quot;

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.

MEANS TO SOLVE THE PROBLEM The present inventors repeated examination in order to provide the thick steel plate excellent in cryogenic toughness of -196 degreeC in Ni steel whose Ni content is 7.5% or less. Specifically, the present invention provides a high-strength thick steel sheet excellent in cryogenic toughness that satisfies all properties of brittle fracture rate ≤ 10% in the C direction, tensile strength TS> 690 MPa, and 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. In consideration of the manufacturing method as a whole, in the molten steel step, the amount of dissolved oxygen before the addition of deoxidation element is controlled, Al is finally added to the molten steel for casting, and the α-γ two-phase coexistence region (A _{c1) is} cast. After the heat treatment (L treatment) in the range of ˜A _c3 , a method of tempering at a temperature below the A _c1 transformation point is recommended, and it is taught that cryogenic toughness is improved thereby. However, according to the examination results of the present inventors, the cryogenic toughness in the L direction is improved by the above method, but the cryogenic toughness in the C direction is not sufficient, and the aforementioned target level (the brittleness in the C direction) mentioned in the present invention. It has been found that the wavefront ratio ≤ 10%) cannot be realized.

Therefore, as a result of further studies, in order to obtain a thick steel sheet having excellent cryogenic toughness, it has been found that it is indispensable to add further requirements in the thick steel sheet and its manufacturing method while basically following the above-described technique. Specifically, in the (i) thick steel sheet, in addition to having the volume fraction V of the residual γ-phase at -196 ° C in the range of V = 2.0 to 12.0%, it has been found to promote the progress of brittle fracture. Attention is given to inclusions having a circular equivalent of more than 1.0 μm in diameter, and the number density N of the inclusions is reduced to N ≦ 200 / mm 2, while the number density N of the inclusions (pieces / mm 2) is present at −196 ° C. It is effective to reduce the A value represented by the formula (1), which is a relation of the volume fraction V (%) of the residual γ phase, to A value ≤ 11.5, (ii) in order to manufacture such a thick steel sheet, in the molten steel step Heat treatment between the control of the dissolved oxygen amount (pre O amount) before the Al addition, the control of the slab heating temperature (T2) in the hot rolling step, and the A _{c1 to} A _c3 after the 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) In (a), by controlling the residual γ phase present at -196 ° C to 4.0% to 12.0% (volume fraction), the brittle fracture rate is 50% even at -233 ° C, which is lower temperature. The thing which can be maintained at the following favorable levels, (D) In order to manufacture such a thick steel plate, A _c1 -A _c3 after hot rolling. In the heat treatment (L process) in between, it discovered that holding | maintenance of predetermined time is effective, and completed this invention.

In the present specification, &quot; excellent in extremely low temperature toughness &quot; 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 &gt;% &lt; / RTI &gt; In the examples described later, the brittle fracture rate in the L direction (rolling direction) is not measured. However, if the brittle fracture rate in the C direction is 10% or less, the brittle fracture rate in the L direction is necessarily 10% or less. It is based on the rule of thumb that

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

Moreover, in this invention, it aims at the high strength thick steel plate which satisfy | fills tensile strength TS> 690 Mpa and yield strength YS> 590 Mpa.

Hereinafter, the thick steel plate of this invention is demonstrated in detail.

As described above, the thick steel sheet of the present invention is, in mass%, C: 0.02 to 0.10%, Si: 0.40% or less (not containing 0%), Mn: 0.50 to 2.0%, P: 0.007% or less (0 %), S: 0.007% or less (0% not included), Al: 0.005 to 0.050%, Ni: 5.0 to 7.5%, N: 0.010% or less (not including 0%) The remainder is a thick steel sheet containing iron and unavoidable impurities, and the volume fraction (V) of the retained austenite phase present at -196 ° C satisfies 2.0% to 12.0%, and more than 1.0 µm of the original equivalent diameter present in the steel sheet. When the number density of the inclusions is set to N, N? 200 pieces / mm 2, and the A value represented by the following equation (1) satisfies 11.5 or less.

&Quot; (1) &quot;

First, the component in steel is demonstrated.

C: 0.02 to 0.10%

C is an element essential for securing strength and retained austenite. In order to exert such an effect effectively, the lower limit of the amount of C is made 0.02% or more. The minimum with preferable C amount is 0.03% or more, More preferably, it is 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 upper limit with preferable C amount is 0.08% or less, More preferably, it is 0.06% or less.

Si: 0.40% or less (does not contain 0%)

Si is an element useful as a deoxidation 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 with preferable Si amount is 0.35% or less, More preferably, it is 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 exhibit such an effect effectively, the minimum of Mn amount is made into 0.50%. The minimum with preferable Mn amount is 0.6% or more, More preferably, it is 0.7% or more. However, when excessively added, tempering embrittlement is caused and desired cryogenic toughness cannot be ensured, so the upper limit thereof is made 2.0% or less. The upper limit with preferable Mn amount is 1.5% or less, More preferably, it is 1.3% or less.

P: 0.007% or less (does not include 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 upper limit with preferable P amount is 0.005% or less. The smaller the amount of P, the better the less, but it is difficult to make the amount of P 0% industrially.

S: 0.007% or less (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 upper limit with preferable S amount 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 deoxidation element. When the Al content is insufficient, the oxygen concentration in the steel increases, and the number density of inclusions having a circular equivalent diameter of more than 1.0 µm increases, so the lower limit thereof is made 0.005% or more. The minimum with preferable Al amount is 0.010% or more, More preferably, it is 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 upper limit with preferable Al amount is 0.045% or less, More preferably, it is 0.04% or less.

Ni: 5.0 to 7.5%

Ni is an essential element for securing residual austenite (residual γ) useful for improving cryogenic toughness. In order to exhibit such an effect effectively, the lower limit of the amount of Ni is made 5.0% or more. The minimum with preferable amount of Ni is 5.2% or more, More preferably, it is 5.4% or more. 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 upper limit with preferable Ni amount is 7.0% or less, More preferably, it is 6.5% or less, More preferably, it is 6.0% or less.

N: 0.010% or less (does not contain 0%)

Since N reduces cryogenic toughness by strain aging, the upper limit thereof is made 0.010% or less. The upper limit with preferable N amount is 0.006% or less, More preferably, it is 0.004% or less.

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 can be contained for the purpose of providing further characteristics.

Cu: 1.00% or less (does not contain 0%)

Cu is a gamma stabilizing element and is an element that contributes to an increase in the amount of residual gamma. In order to exhibit such an effect effectively, it is preferable to contain Cu 0.05% or more. However, when excessively added, an excessive improvement in strength is caused and a desired cryogenic toughness effect is not obtained. Therefore, the upper limit thereof is preferably 1.00% or less. The upper limit with more preferable amount of Cu is 0.8% or less, More preferably, it is 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%)

Cr and Mo are both strength improving elements. These elements may be added independently and may use two types together. In order to exhibit the said effect effectively, it is preferable to make Cr amount 0.05% or more and Mo amount 0.01% or more. However, when excessively added, an excessive improvement in strength is caused, and the desired cryogenic toughness cannot be secured. Therefore, the upper limit of the Cr content is preferably 1.20% or less (more preferably 1.1% or less, even more preferably 0.9% or less). More preferably, it is 0.5% or less), and the upper limit with preferable Mo amount is 1.0% or less (more preferably 0.8% or less, 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 are all elements which precipitate as carbonitride and raise strength. These elements may be added alone, or two or more of them may be used in combination. In order to exhibit the said effect effectively, it is preferable to make Ti amount 0.005% or more, Nb amount 0.005% or more, and V amount 0.005% or more. However, when excessively added, an excessive improvement in strength is caused, and the desired cryogenic toughness cannot be secured. Therefore, the upper limit of Ti amount is preferably 0.025% or less (more preferably 0.018% or less, more preferably 0.015%). Or less), the upper limit of the amount of Nb is preferably 0.100% or less (more preferably 0.05% or less, even more preferably 0.02% or less), and the upper limit of the amount of V is preferably 0.50% or less (more preferably 0.3% or less), More preferably, it is 0.2% or less).

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

B is an element which contributes to strength improvement by hardenability improvement. In order to exhibit the said effect effectively, it is preferable to make B amount 0.0005% or more. However, when excessively added, an excessive improvement in strength is caused, and desired cryogenic toughness cannot be secured. Therefore, the upper limit of the amount of B is preferably 0.0050% or less (more preferably 0.0030% or less, even 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 deoxidation elements, and by addition, oxygen concentration in steel falls, and the number density of the inclusions whose original equivalent diameter exceeds 1.0 micrometer decreases. 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, when excessively added, the number density of the inclusions increases, and thus the cryogenic toughness decreases, so that the upper limit of the amount of Ca is preferably 0.0030% or less (more preferably 0.0025% or less), and the upper limit of the amount of REM is preferably 0.0050%. The upper limit of the Zr amount is preferably 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, 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.

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

In addition, in the thick steel sheet of the present invention, the volume fraction V of the residual γ phase present at -196 ° C satisfies 2.0% to 12.0% (preferably 4.0 to 12.0%).

It is known that residual (gamma) which exists in -196 degreeC contributes to the improvement of cryogenic toughness. In order to exert such an effect effectively, the volume fraction V of the residual γ phase in all the tissues present at −196 ° C. is made 2.0% or more. However, the residual γ is relatively soft compared to the matrix phase, and when the amount of residual γ becomes excessive, YS cannot secure a predetermined value, so the upper limit thereof is 12.0% (No. 43 in Table 2B described later). Reference). With respect to the volume fraction V of the residual γ phase, the lower limit is preferably 4.0% or more, more preferably 6.0% or more, a preferable upper limit is 11.5% or less, and a more preferable upper limit is 11.0% or less.

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 addition, in the thick steel plate of this invention, control of the volume fraction V of the residual (gamma) phase is important among the structures which exist at -196 degreeC, and is not limited at all about other structures other than residual (gamma), and exists normally in a thick steel plate. All you have to do is. As structures other than the residual (gamma), carbides, such as bainite, martensite, and cementite, are mentioned, for example.

In addition, in the thick steel sheet of the present invention, the number density N of the inclusions satisfies N ≤ 200 pieces / mm 2, and A is represented by the following formula 1 with respect to inclusions having a diameter larger than 1.0 μm in the steel sheet. The value satisfies 11.5 or less.

&Quot; (1) &quot;

Here, the "circle equivalent diameter" refers to the size of the inclusions, and calculates the diameter of the circle assumed to be equal in area.

Here, in the present invention, attention is paid to inclusions having a circular equivalent diameter of more than 1.0 µm because it has been found that the inclusions promote the progress of brittle fracture. That is, in order to improve the brittle fracture rate at cryogenic temperature while securing a predetermined high strength, it is necessary to reduce inclusions that promote brittle fracture, but according to the results of the present inventors, when the number density N of the inclusions increases, For example, even if the volume fraction V of the residual γ-phase at -196 ° C was controlled in the above range, it was found that desired cryogenic toughness could not be realized (No. 33, 35, 36, 47 to 50 in Table 2B to be described later). Reference). The number density N of the said inclusions is so good that it is good, Preferably it is 150 pieces / mm <2> or less, More preferably, it is 120 pieces / mm <2> or less. In addition, in this invention, the average size (average circular equivalent diameter) of the inclusion more than 1.0 micrometer in circular equivalent diameter is generally 2.0 micrometers or less.

The inclusions can be measured by the method described in Examples described later. Here, the kind of "inclusion | inclusion" in the inclusion more than 1.0 micrometer in circular equivalent diameter is not specifically limited in this invention. The occurrence of brittle fracture is because the size (circular equivalent diameter) of the inclusion affects the most, not the kind of inclusion. Examples of the inclusions include single particles such as oxides, sulfides, nitrides, and oxynitrides, composites of two or more of these single particles, or composite particles of these single particles and other elements bonded together. Can be.

In addition, from the viewpoint of inclusion control, a similar technique is disclosed in the aforementioned Patent Document 3, but the direction of inclusion control differs greatly from the present invention. That is, in the said patent document 3, it focuses especially on Mg, and disperse | distributes coarse austenite grain at high temperature, and improves toughness by disperse | distributing many fine Mg containing oxide particles whose size is 2 micrometers or less, In the present invention, regardless of the kind, the inclusions that reduce the toughness by starting from brittle fracture or ductile fracture are reduced, and the control methods of the inclusions are completely different. Moreover, according to the preferable manufacturing method of this invention mentioned later, the fine inclusion of a round equivalent diameter of 2.0 micrometers or less exists about 100-1000 pieces / mm <2> in general. 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.

In addition, in the present invention, it is necessary not only to control the absolute value of the number density N of the inclusions, but also to satisfy the A value represented by the above expression (1) with A value ≤ 11.5.

Here, the value A is calculated from the relationship between the number density N of the inclusions and the volume fraction V on the residual austenite (residual γ) present at -196 ° C, as shown in the above formula (1). In order to suppress the brittle fracture rate to 10% or less, it is necessary to reduce the inclusions that promote brittle fracture, ensure the residual γ effective for promoting ductile fracture, and then control the form of both properly. , The contribution ratios of both to the brittle fracture rate in the cryogenic region are experimentally obtained and derived based on a number of basic experiments. Incidentally, the inclusion of π in the above formula (1) indicates that brittle fracture is encouraged as the amount of hard inclusions on the growth surface of the fracture crack in the Charpy test increases, and thus, as an influence parameter for the brittle fracture, This is because the empirical formula was derived based on the hypothesis that the area ratio of π x radius ² becomes important. As shown in Examples to be described later, in addition to the volume fraction V of the residual γ phase and the number density N of the inclusions, by controlling the A value to 11.5 or less, the cryogenic temperature at -196 ° C in a predetermined high strength thick steel sheet is finally achieved. Toughness, especially the brittle fracture rate in the Charpy impact absorption test, can achieve the desired high level. On the other hand, when the said A value exceeded 11.5, brittle wavefront ratio 10% was not able to be ensured. It is so good that the said A value is small, Preferably it is 11.0 or less, More preferably, it is 10.0 or less. In addition, although the minimum of A value is not specifically limited from the said viewpoint, Considering the balance with the implementable range of the volume fraction V of the residual (gamma) phase, the number density N of the said interference | inclusion, etc., it is preferable that it is generally 2.5 or more.

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

The characteristic part of the manufacturing method which concerns on this invention is in following (A)-(C).

In the (A) molten steel step, the free oxygen amount [O] before Al addition is 100 ppm or less, and the cooling time t2 at 1450-1500 degreeC at the time of casting is controlled to 300 second or less. By the method of said (A), especially the number density N of the interference | inclusion mentioned above can be reduced to a predetermined range.

(B) In a hot rolling process, the heating temperature T2 before rolling is controlled to 1120 degreeC or more. By the method of said (B), especially the number density N of the interference | inclusion mentioned above is reduced to 200 pieces / mm <2> or less.

(C) After hot rolling, after heating and holding in the temperature range of A _c1 -A _c3 point, it is water-cooled, and after tempering for 10 to 60 minutes continuously at the temperature range of 520 degreeC-A _c1 point, it is air-cooled or Cool water. By the method of the above (C), the volume fraction of the residual γ phase especially present at -196 ° C is appropriately controlled.

In addition, A value prescribed | regulated by this invention controls the said A value suitably by controlling said (A)-(C) because of the parameter which concerns both the number density of the inclusions mentioned above, and the volume fraction of residual (gamma). It can be controlled in a predetermined range.

Speaking of the relationship of the prior art mentioned above, the biggest characteristic is that of t2 was especially controlled among the methods of said (A).

Hereinafter, each process will be described in detail.

(About a solvent process)

In the present invention, it is noted that the addition method of Al is particularly noted. Incidentally, inclusions having a diameter larger than 1.0 μm of the original equivalent to be controlled in the present invention are mainly based on Al-based inclusions generated in the molten metal. This is because the secondary inclusions are formed at the time of cooling in combination, but the Al-based inclusions tend to coarsen by aggregation and coalescence, and the number density of the inclusions increases.

First, in adding Al which is a deoxidation material in molten steel, the amount of free oxygen (may be abbreviated as dissolved oxygen amount and [O] amount) before Al addition is controlled to 100 ppm or less. This is because when the amount of [O] exceeds 100 ppm, the size of inclusions generated at the time of Al addition increases, the number density of inclusions larger than 1.0 μm in diameter is increased, and the desired cryogenic toughness cannot be realized (see Table 2B described later). See No. 33). The smaller the amount of [O], the better. Preferably it is 80 ppm or less, More preferably, it is 50 ppm or less. In addition, the minimum of [O] amount is not specifically limited from a viewpoint of reducing the number density of the said interference | inclusion.

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. In addition to the above elements, in the case of adding deoxidizing materials such as Ti, Ca, REM, and Zr as optional components, the amount of [O] can also be controlled by these additions.

In order to control Al inclusions, it is important to control the amount of [O] before Al addition, and the order of addition of Al and other deoxidation elements does not matter. 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. Moreover, it is preferable to add the said selective component, such as Ti, in molten steel after addition of Al.

Next, casting is started. Although the temperature range at the time of casting is generally 1650 degrees C or less, in this invention, it is especially important to control the cooling time (t2) in the temperature range of 1450-1500 degreeC to 300 second or less, and by this, 1.0 micrometer of original equivalent diameters It has been found that the number density of excess inclusions is properly controlled. When t2 exceeds 300 seconds, the inclusion of the inclusion as a nucleus facilitates the complex production of secondary inclusions, which increases the number density of inclusions larger than 1.0 μm in diameter or increases the A value, thereby exhibiting the desired cryogenic toughness. (See Nos. 34 and 35 in Table 2B to be described later). 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.

Moreover, in this invention, what focused on the temperature range of 1450-1500 degreeC especially among the temperature range at the time of casting, the said temperature range solidifies at the time of casting, and the component concentration to molten steel advances, This is because the temperature is promoted in the temperature range.

In addition, the said temperature range of 1450-1500 degreeC means the temperature of the center part of slab thickness. The slab thickness is generally 150 to 250 mm, and the surface temperature tends to be about 200 to 1000 ° C. lower than the temperature at 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 addition, in this invention, what is necessary is just to control the cooling time t2 in the temperature range of 1450-1500 degreeC to 300 second or less, and does not limit the means. 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 addition, in this invention, the cooling method with respect to the temperature range at the time of casting other than the said temperature range is not limited at all, A conventional method (air cooling or water cooling) can be employ | adopted.

After casting as described above, hot rolling is applied to heat treatment.

In a hot rolling process, it is preferable to make heating temperature T2 before hot rolling into 1120 degreeC or more. As a result, relatively unstable sulfides disappear from the secondary inclusions produced in a complex, and the inclusion size is reduced, so that the number density of inclusions with a diameter larger than 1.0 μm is reduced. In particular, in the present invention, since the amount of S in the steel is small (that is, the amount of S that contributes to the amount of sulfide produced), considering that the influence of the heating temperature before hot rolling on the amount of sulfide is further increased, the heating temperature T2 before the rolling is increased. It is necessary to manage strictly, and to control higher than general temperature range (about 1100 degreeC vicinity). If T2 is less than 1120 ° C, the number density of inclusions to be controlled increases, so that the desired cryogenic toughness is not exhibited (see No. 36 in Table 2B to be described later). From the above point of view, the higher T2 is, the better, preferably 1140 ° C or higher, and more preferably 1160 ° C or higher. However, when T2 becomes too high, since manufacturing cost increases, it is preferable to control the upper limit to 1180 degrees C or less generally.

In addition, it is preferable to make heating time in the heating temperature T2 before hot rolling into the range of 1 to 4 hours in general.

Processes other than the above (finish rolling, rolling reduction, etc.) are not specifically limited, The method normally used may be employ | adopted so that predetermined | prescribed plate | board thickness may be obtained.

After hot rolling, it heats and maintains in the temperature range TL of the point A _c1 -A _c3 , hold | maintains, and then water-cools. This treatment corresponds to the L treatment described in the above-described prior art, whereby the residual γ stably present at -196 ° C can be ensured in 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. If it is less than A _c1 point or more than A _c3 point, as a result, the residual (gamma) phase in -196 degreeC cannot fully be secured (refer No. 37, 38 of Table 2B which is mentioned later). Preferable heating temperature is 660-710 degreeC in general.

It is preferable to make heating time (holding time, tL) in the said two phase area temperature into 10 to 50 minutes in general. In less than 10 minutes, the alloying element concentration to (gamma) phase does not fully advance, whereas in more than 50 minutes, (alpha) phase is annealed and strength falls. Preferable heating time is 15 to 30 minutes in general. The upper limit of a preferable 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. In addition, the upper limit of preferable heating time is the same as the above (30 minutes or less).

Subsequently, after cooling to room temperature, it tempers. Tempering is performed for 10 to 60 minutes (t3) in the temperature range T3 of 520 degreeC-A _c1 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. If tempering temperature T3 is lower than 520 degreeC, the quasi-stable residual gamma phase produced | generated during 2-phase coexistence area | region maintenance will decompose into an alpha phase and a cementite phase, and it will become impossible to fully ensure the residual gamma phase in -196 degreeC. 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 concentration in the quasi-stable residual γ phase does not sufficiently proceed to secure a desired amount of residual γ at -196 ° C. (Refer to No. 42 (Example of high T3) and No. 55 (Example of short t3) in Table 2 described later). Moreover, when tempering time t3 exceeds 60 minutes, residual (gamma) phase in -196 degreeC will generate | occur | produce excessively, and predetermined | prescribed intensity | strength will become impossible (refer No. 43 of Table 2 mentioned later).

Preferable tempering treatment conditions are tempering temperature T3: 570-620 degreeC, and tempering time t3: 15 minutes or more and 45 minutes or less (more preferably 35 minutes or less, still more preferably 25 minutes or less).

After tempering as described above, it is cooled to room temperature. The cooling method is not specifically limited, Any of air cooling or water cooling may be sufficient.

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 said formula, [] means the density | concentration (mass%) of the alloying element in steel materials. In addition, since As and W are not contained as a component in steel in this invention, [As] and [W] are computed as 0% in the said 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 addition, in this Example, the time t1 from Al addition to the start of casting was all about 10 minutes (not shown in the table). In addition, in Table 2, [O] is the dissolved oxygen amount (ppm) before t Al addition, and t2 is the cooling time (second) of 1500-1450 degreeC at the time of casting. Cooling of 1500-1450 degreeC was controlled so that it might become said cooling time by air cooling or water cooling.

Next, after heating the said ingot to various temperature T2 as shown in Table 2, it rolls to plate | board thickness 75mm at the temperature of 830 degreeC or more, and rolls it to the final rolling temperature of 780 degreeC, and then cools it to board | board thickness. A thick steel plate of 25 mm was obtained. The steel sheet thus obtained 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), followed by water cooling to room temperature. Then, as shown in Table 2, after performing a tempering process (T3 = tempering temperature, t3 = tempering time), air cooling or water cooling was performed to room temperature.

With respect to the thick steel sheet thus obtained, the number density N (pieces / mm 2) of inclusions having a diameter larger than 1.0 μm in original equivalents, the volume fraction (%) of residual γ phase present at −196 ° C., and the tensile properties as follows. (Tensile strength TS, yield strength YS) and cryogenic toughness (brittle fracture rate in C direction at -196 ° C or -233 ° C) were evaluated.

(1) Measurement of number density N of inclusions with a diameter larger than 1.0 µm

The t / 4 position (t: plate | board thickness) of the said steel plate was mirror-polished, and four-field photography was performed at 400 times using the optical microscope. In addition, the area per one field of view is 0.04 mm 2, and the total area of 4 fields is 0.15 mm 2. About the inclusions observed in these four fields, image analysis was carried out by "Media-Pro Plus" manufactured by Media Cybernetics, and the number density N (pieces / mm &lt; 2 &gt;) of inclusions having a diameter (diameter) exceeding the original equivalent diameter (diameter) was calculated. The average value was calculated.

(2) Measurement of 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). Next, the peaks of the lattice planes of (110), (200), (211) and (220) of the ferrite phase and the peaks of the lattice planes of (111), (200), (220) and (311) of the residual? Phases. On the basis of the integral intensity ratios of the respective peaks, the volume fractions of (111), (200), (220), and (311) of the residual γ phases are respectively calculated, and their average values are obtained, and this is determined as "the residual γ Volume fraction (%) ”.

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

From the t / 4 position of each steel plate, the No. 4 test piece of JIS Z 2241 was extract | collected in parallel to the C direction, the tension test was done by the method of ZIS Z 2241, and tensile strength TS and yield strength YS were measured. 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 rate in 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. Of the two average values calculated in this manner, the average value of the side having the lower characteristic (that is, the higher brittle fracture rate) was adopted, and the value of 10% or less was evaluated as being excellent in cryogenic toughness in this example. .

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 consider as follows.

First, Nos. 1 to 32 of Table 2A are examples of satisfying all the requirements of the present invention, and even if the base material strength is high, the cryogenic toughness (in detail, the average value of the brittle fracture rate in the C direction) at -196 ° C. 10%) was able to provide an excellent thick steel sheet.

On the other hand, since No. 33-43 and 55 of Table 2B do not satisfy | fill any one of the preferable manufacturing conditions of this invention at least, it is a comparative example which does not satisfy the requirements of this invention, and the desired characteristic was not acquired.

Specifically, No. 33 uses No. 33 in Table 1B, which satisfies the requirements of the present invention, but since the dissolved oxygen amount [O] is large before Al addition, the number density N of the inclusions increases. Yes. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 34 uses No. 34 in Table 1B, which satisfies the requirements of the present invention, but since the cooling time (t2) of 1500 to 1450 ° C is long at the time of casting, A value is predetermined. This is an example that exceeds the range. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 35 uses No. 35 of Table 1B having a large amount of P, and has a long cooling time (t 2) of 1500 to 1450 ° C. at the time of casting, so that the number density N of the inclusions is increased. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 36 uses No. 36 in Table 1B having a large amount of C, and because the heating temperature T2 before hot rolling is low, the number density N of the inclusions increases, and the A value exceeds a predetermined range. One example. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 37 uses No. 37 in Table 1B, which satisfies the requirements of the present invention, but is heated to a temperature below the two-phase region temperature TL. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 38 is an example in which the amount of residual γ is insufficient because No. 38 in Table 1B having a large amount of Si is used and the heating is performed at a temperature exceeding the two-phase region temperature TL. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 39 uses No. 39 in Table 1B which satisfies the requirements of the present invention as a component in steel. to be. As a result, the brittle wavefront ratio also increased, and the desired extremely low temperature toughness could not be realized.

No. 40 uses No. 40 in Table 1B, which satisfies the requirements of the present invention, but the amount of residual γ is increased because the heating holding time tL in the two-phase region temperature TL is long. to be. As a result, yield strength YS and tensile strength TS fell, and the desired base material strength could not be secured.

No. 41 uses No. 41 in Table 1B, which satisfies the requirements of the present invention, but since the tempering temperature T3 is low, No. 41 is an example in which the residual γ amount is insufficient. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 42 uses No. 42 of Table 1B having a large amount of Mn, and because the tempering temperature T3 is high, the amount of residual γ is insufficient. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 43 uses No. 43 in Table 1B, which satisfies the requirements of the present invention, but the amount of residual γ 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.

No. 55 uses No. 55 in Table 1B, which satisfies the requirements of the present invention, but the temper time t3 is short. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

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

Specifically, No. 44 is an example in which the residual γ amount is insufficient because No. 44 in Table 1B having a small Mn amount is used. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No.45 is an example using No.45 of Table 1B with many S amounts. 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 increases because the amount of C is small, the amount of Al is large, and the amount of Ni is small. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized. TS also decreased.

No. 47 is an example in which the number density N of the inclusions increased and the A value exceeded a predetermined range because No. 47 in Table 1B having a small amount of Al and a large amount of N was used. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 48 is an example in which the number density N of the inclusions increased and the A value exceeded a predetermined range because No. 48 in Table 1B having a large amount of Cu and Ca as optional components was used. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 49 is an example in which the number density N of the inclusions increased because No. 49 in Table 1B having a large amount of Cr and Zr as optional components was used. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

No. 50 uses No. 50 in Table 1B having a large amount of Nb and REM as optional components, so that the number density N of the inclusions increased and the A value exceeded a predetermined range. As a result, the brittle fracture rate increased, and the desired cryogenic toughness could not be realized.

Since No. 51 used No. 51 of Table 1B having a large amount of Mo as a selective component, the brittle fracture rate was increased, and the desired cryogenic toughness could not be realized.

No. 52 used No. 52 in Table 1B having a large amount of Ti as a selective component, so that the brittle fracture rate increased, and thus the desired cryogenic toughness could not be realized.

No. 53 used No. 53 in Table 1B having a large amount of V as a selective component, so that the brittle fracture rate increased, and thus the desired cryogenic toughness could not be realized.

No. 54 used No. 54 in Table 1B having a large amount of B as a selective component, so that the brittle fracture rate increased, and thus the desired cryogenic toughness could not be realized.

Example 2

In the present Example, the brittle fracture rate in -233 degreeC was evaluated about the some data (all this invention example) used for the said Example 1.

Specifically, three No. of specimens are collected from the t / 4 position and the W / 4 position with respect to No. (No. of Table 3 corresponds to No. of Table 1 and Table 2 above) described in Table 3. And the Charpy impact test at -233 ° C by the method described below to evaluate the average value of the brittle fracture rate. In the present Example, it was evaluated that the said brittle fracture rate <= 50% was excellent in the brittle fracture rate in -233 degreeC.

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

These results are shown in Table 3.

[Table 3]

Nos. 3, 4, 6, 15, 19, and 24 in Table 3 are all examples in which the heating time (tL) in the two-phase region temperature is controlled to 15 minutes or more (see Table 2A). % Or more could be ensured. 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 mass%,
C: 0.02 to 0.10%
Si: 0.40% or less (not including 0%),
Mn: 0.50 to 2.0%
P: 0.007% or less (not including 0%),
S: 0.007% or less (not including 0%),
Al: 0.005 to 0.050%
Ni: 5.0 to 7.5%,
N: 0.010% or less (does not contain 0%)
And the remainder being a thick steel sheet containing iron and unavoidable impurities,
The volume fraction (V) of residual austenite phase present at -196 ° C satisfies 2.0% to 12.0%, and
When the number density of inclusions having a diameter greater than 1.0 μm in the steel sheet is N,
N≤200 pieces / mm 2, and also
A thick steel sheet excellent in cryogenic toughness, wherein A value represented by the following formula (1) satisfies less than 11.5.
&Quot; (1) &quot;

The thick steel sheet having excellent cryogenic toughness according to claim 1, wherein the thick steel sheet further contains at least one of the following groups (a) to (e) as another element.
(a) Cu: 1.00% or less (does not contain 0%),
(b) at least one member selected from the group consisting of Cr: 1.20% or less (does not contain 0%), and Mo: 1.0% or less (does not contain 0%),
(c) selected from the group consisting of Ti: 0.025% or less (does not contain 0%), Nb: 0.100% or less (does not contain 0%), and V: 0.50% or less (does not contain 0%) At least one,
(d) B: 0.0050% or less (not including 0%),
(e) selected from the group consisting of Ca: 0.0030% or less (without 0%), REM: 0.0050% or less (without 0%), and Zr: 0.005% or less (without 0%) At least one