BE1021749B9 - THICK PLATE OF STEEL HAS EXCELLENT TENACITY AT ULTRA LOW TEMPERATURE - Google Patents

Abstract

Thick, high-strength steel plate of greater than 690 MPa with excellent low-temperature toughness at -196 ° C or lower, capable of achieving a brittle fracture surface ratio of -196 ° C 10 <10% Ni steel with a Ni content of 5.0 - 7.5%. The steel plate satisfies the fact that the residual austenitic phase present at -196 ° C is 2.0-12.0% in terms of volume fraction, and that the average circle diameter of inclusions having more than 2, 0 pm of circle equivalent diameter is at most 3.5 pm.

Description

Thick steel plate with excellent toughness at ultra low temperature

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a thick steel plate with excellent toughness at ultra-low temperature and more specifically relates to a thick steel plate with excellent toughness at ultra-low temperature equal to or lower than -196 ° (in particular the tenacity in the direction of the width of the plate (direction C)) even when the Ni content is reduced to approximately 5.0 - 7.5%. Below, thick steel plates for liquefied natural gas (LNG) (typically, storage tank, transport vessel, and the like) exposed to the ultra-low temperature described above will be mainly described, but the The thick steel plate of the present invention is not limited thereto and is applicable to thick steel plates generally used for applications exposed to the ultra-low temperature of -196 ° C or less. 2. Description of Related Art [0002]

In a thick LNG tank steel plate, used for a liquefied natural gas (LNG) storage tank, high toughness that can withstand the ultra-low temperature of -196 ° C or lower is required in addition to high resistance. So far, approximately 9% thick steel plates containing Ni (9% Ni steel) have been used as thick steel plates for this purpose but, as the cost of Ni has increased these In recent years, the development of thick steel plates with excellent toughness at ultra low temperature even with a low Ni content of less than 9% has made progress.

[0003]

For example, the non-patent literature 1 (Yano et al., "The influence of α-γ two-phase coexisting heat treatment region exerted on low temperature toughness of 6% Ni steel", Tetu-To-Hagane (Iron and Steel) , 1973, vol 6, pp. 752-763), describes the influence exerted by a heat treatment of a coexistence region of two α-γ phases on the low temperature fracture toughness of 6% Ni steel. More specifically, it describes that, by subjecting the coexistence region of two α-γ phases (between Ad-AC3) (treatment L) to a thermal treatment before exerting a treatment of income, it is possible to confer a tenacity on an ultra-low temperature of -196 ° C equal to or greater than that of 9% Ni steel which has been subjected to tempering and ordinary tempering treatment; this heat treatment also improves the toughness of a direction specimen C (width direction of the plate); these effects result from the presence of residual austenite which is in large quantity, fine and stable even under an impact load at ultra low temperature and the like. But, according to the method, although the ultra-low temperature toughness in the rolling direction (L direction) is excellent, the ultra-low temperature toughness in the direction of the plate width (C direction) tends to be lower than that in the direction L. There is also no description of the ratio of brittle fracture surface.

[0004]

Technologies similar to the non-patent literature 1 are described in JP-A Nos. S49-135813 and JP-A No. S51-13308. Of these, JP-A No. S49-135813 discloses a process wherein 4.0-10% Ni-containing steel with an austenite grain size and the like controlled within a predetermined range is rolled. when heated and is then heated between Aci-Ac3, then a cooling treatment (equivalent to the treatment L described in the non-patent literature 1) is repeated once or twice, and the income is then carried out at a transformation point temperature Aci or less. JP-A No. S51-13308 also discloses a process in which 4.0-10% Ni-containing steel with AIN size before hot rolling brought to 1 μm or less is subjected to a heat treatment. similar to that of JP-A No. S49-135813 (treatment [.- ► income treatment). It is assumed that the impact values at -196 ° C (vE-i96) described in these processes are believed to be probably those in the L direction, but the C-direction toughness value is unclear. In these methods, the resistance is also not taken into consideration and there is no description relating to the brittle fracture surface ratio.

[0005]

Non-patent literature 2 (Furuya et al., "Development of 6% Ni Steel for LNG Tank", CAMP-ISIJ, Vol 23 (2010), 1322) also describes the development of 6% Ni steel. for LNG tank that combines L treatment (two-phase region quench treatment) and TMCP. According to the literature, although it is described that the toughness in the rolling direction (L direction) has a high value, there is no description of the toughness value in the direction of the width of the plate ( direction C).

[0006] JP-A No. 2001-123245 discloses a high-strength high-tenacity steel whose toughness is excellent in the weld section with 570 MPa or more and which contains Ni at 0.3-10% and Mg in a predetermined amount with Mg oxide particles of a suitably dispersed predetermined grain size. JP-A No. 2001-123245 discloses that the heated austenite grain size is refined by controlling the Mg-containing oxide and the base metal toughness and the heat-affected weld zone (ZAC) improves. ; and that for this purpose, the amount of O (oxygen) before adding deoxidizing elements and the order of adding Mg and other deoxidizing elements are important, and the molten steel with a quantity of dissolved oxygen of 0.001 - 0.02% is added with Mg, Ti and Al at the same time and is then cast to obtain a billet, or when adding Mg, Ti and Al, Al is added last, and molten steel is then cast to obtain a billet. In an example of JP-A No. 2001-123245, a toughness value in the C direction (breaking surface transition temperature vTrs) is described. Although the property of the 9% Ni steel is excellent (fracture surface transition temperature vTrs <-196 ° C), the property of the Ni steel close to 5% is -140 ° C, and there is still room for improvement.

SUMMARY OF THE INVENTION

[0009]

As described above, so far, in Ni steel having a Ni content of approximately 5.0 - 7.5%, excellent technologies in terms of ultra-low temperature toughness at -196 ° These have been proposed, but the ultra-low temperature tenacity in the C direction has not been sufficiently studied. In particular, further improvements in ultra-low temperature toughness when base metal strength is high (more specifically, tensile strength TS> 690 MPa, yield strength YS> 590 MPa) (improvement of Ultra-low temperature tenacity in the D direction) have been in great demand.

[0010]

In addition, the ratio of brittle fracture surface has never been studied in the literature described above. The brittle fracture surface ratio is a brittle fracture rate when a load is applied in the Charpy impact test. In a section where the brittle fracture occurred, the energy absorbed by the steel over time until the break occurs becomes extremely small and the breakage progresses easily and, therefore, in a technology to improve the Ultra-low temperature toughness, an extremely important condition is to achieve that the brittle fracture surface ratio is 10% or less in addition to improving the commonly used Charpy shock value (vE.ig6). However, a technology which satisfies the condition of the brittle fracture surface ratio for a thick high strength steel plate with a high base metal strength as described above has not yet been proposed.

[0011]

The present invention has been developed in view of these circumstances and its purpose is to provide a thick, high strength steel plate with excellent ultra low temperature toughness (especially ultra-low temperature toughness in the C direction). ) at -196 ° C and capable of achieving a brittle fracture surface ratio <10% in Ni steel having a Ni content of approximately 5.0 - 7.5%.

[0012]

A thick steel plate with excellent ultra low temperature toughness in connection with the present invention that could solve the problems described above is a thick steel plate containing in mass%, C: 0.02-0 , 10%, Si: 0.40% or less (not including 0%), Mn: 0.50 - 2.0%, P: 0.007% or less (not including 0%), S: 0.007% or less ( not including 0%), Al: 0.005 - 0.050%, Ni: 5.0 - 7.5% and N: 0.010% or less (not including 0%), the remainder comprising iron and unavoidable impurities, wherein the phase residual austenitic present at -196 ° C represents 2.0-12.0% in terms of volume fraction, and the average circle diameter of the inclusions having a circle equivalent diameter of more than 2.0 μm is 3.5 μm or less.

[0013]

In one embodiment of the present invention, the thick steel plate satisfies the condition that the residual austenitic phase at -196 ° C is 4.0-12.0% in terms of volume fraction.

[0014]

In one embodiment of the present invention, the thick steel plate contains Cu: 1.0% or less (not included 0%).

[0015]

In one embodiment of the present invention, the thick steel plate further contains one or more elements selected from a group consisting of Cr: 1.20% or less (not including 0%) and Mo: 1.0 % or less (not including 0%).

[0016]

In one embodiment of the present invention, the thick steel plate further contains one or more members selected from a group consisting of Ti: 0.025% or less (not including 0%), Nb: 0.100% or less ( not including 0%) and V: 0.50% or less (not including 0%).

[0017]

In one embodiment of the present invention, the thick steel plate contains B: 0.0050% or less (not including 0%).

[0018]

In one embodiment of the present invention, the thick steel plate further contains one or more elements selected from a group consisting of Ca: 0.0030% or less (not including 0%), REM: 0.0050 % or less (not including 0%) and Zr: 0.005% or less (not including 0%).

[0019]

According to the present invention, in Ni steel having a Ni content of approximately 5.0 - 7.5%, a thick, high strength steel plate could be provided which has excellent ultra low temperature toughness. at -196 ° C or lower (especially ultra-low temperature toughness in the C direction) and satisfies the brittle fracture area ratio at -196 ° C <10% (preferably at the brittle fracture surface ratio to 233 ° C <50%) even when the strength of the base metal is high (more specifically, tensile strength TS> 690 MPa, yield strength YS> 590 MPa).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020]

Features of the thick steel plate in connection with the present invention are, in Ni steel having a Ni content of approximately 5.0 - 7.5%, to further improve the toughness to ultra low temperature in the C direction, (A) control the residual austenitic phase (residual phase y) present at -196 ° C to 2.0% - 12.0% (volume fraction) (preferably, to control this same phase at 4.0-12.0% (volume fraction)), and (B) the average circle diameter of coarse inclusions with a circle equivalent diameter of more than 2.0 μm (hereinafter referred to as "coarse inclusions"). "and can be abbreviated as N1) be refined to 3.5 pm or less. In particular, a notable feature in relation to the conventional technologies described above is the latter (B).

[0021]

The manner in which the present invention has been realized will be described below.

[0022]

The present inventors have conducted intensive studies to provide a thick steel plate with excellent ultra-low temperature toughness of -196 ° C or less in Ni steel having a Ni content of 7.5% or less. More specifically, in the present invention, from the point of view of providing a high strength high strength steel plate with excellent ultra low temperature toughness which satisfies all the characteristics of the brittle fracture surface ratio to - 196 ° C <10%, tensile strength TS> 690 MPa and yield strength YS> 590 MPa in the C direction, processes taught in the literature described in the prior art were studied.

[0023]

In literature, it is taught that it is important to stabilize the residual austenite (residual γ) present at -196 ° C in order to improve the ultra-low temperature toughness of the 5% Ni steel. It is also taught that when the manufacturing process is taken into consideration as a whole, it is recommended a process in which the amount of oxygen in solution before adding deoxidizing elements is controlled in a melting step of the the casting is carried out in such a way that Al is added last in the molten steel, the heat treatment (treatment L) in the region where two α-γ phases coexist (between ACi-AC3) is carried out, and the treatment The income is then run at a transformation point temperature of Ac 1 or less, and the ultra low temperature toughness is thereby improved. However, according to the result of the studies by the present inventors, it has been found that by said process, although ultra-low temperature toughness in the L direction has been improved, the ultra-low temperature toughness in the C direction was not sufficient, and the target level targeted in the present invention (the brittle fracture area ratio at -196 ° C in the C direction <10%) could not be reached.

[0024]

As a result, other studies have been done. As a result, it was found that in order to obtain a thick steel plate with excellent ultra low temperature toughness, it was essential to add other requirements with regard to the thick plate of and manufacturing process for this purpose while basically following the technologies described above. More specifically, it has been found that: (A) it is effective that the average circle equivalent diameter (N1) of coarse inclusions having a circle equivalent diameter of greater than 2.0 μm which become a brittle fracture starting point is refined to 3.5 μm and in addition the residual γ phase at -196 ° C is present in a range of 2.0 -12.0% (volume fraction) in a thick steel plate, and (B ) to manufacture such a thick steel plate, it is effective to control the amount of oxygen in solution (free amount of O) before adding Al to the steel melting step and to perform a subsequent control at the melting step of the steel in addition to the heat treatment between Ad-A (treatment L) after hot rolling - tempering in a predetermined temperature range, and it is efficient to have a holding time (t1) from the addition of Al to the beginning of the casting of 15 min or more and from check the cooling time (t2) at 1450-1500 ° C in casting at 300 s. or less.

[0025]

In addition, the following has been found and the present invention has been completed: (C) by controlling the residual γ phase present at -196 ° C to 4.0-12.0% (volume fraction) in (A) ci above, the brittle fracture surface ratio can also be maintained at an excellent level of 50% or less even at the lowest temperature of -233 ° C, and (D) to manufacture such a thick steel plate, it is effective that it be maintained for a predetermined time in the heat treatment between Aci-Ac3 (L-treatment) after hot rolling.

[0026]

In this specification, "excellent ultra-low temperature toughness" means satisfying the brittle fracture surface ratio at -196 ° C <10% when VE-ig6 and the brittle fracture surface ratio in the test. Charpy shock absorption in the direction C (width direction of the plate) is measured by a method described in a column of an example described below. Although the ratio of brittle fracture surface in the L direction (rolling direction) was not measured in the example described below, it is based on empirical knowledge that the brittle fracture surface ratio in the L direction inevitably becomes 10% or less when the ratio of brittle fracture surface in the C direction is 10% or less.

[0027]

In this specification, "thick steel plate" means a steel plate having a thickness of approximately 6 - 50 mm.

[0028]

In the present invention, a thick high strength steel plate satisfying the tensile strength TS> 690 MPa at YS yield strength> 590 MPa is also a goal.

The thick steel plate of the present invention will be described in detail below.

[0030]

As described above, the thick steel plate of the present invention is a thick steel plate containing, in mass%, C: 0.02-0.10%, Si: 0.40% or less (not including 0%), Mn: 0.50 - 2.0%, P: 0.007% or less (not including 0%), S: 0.007% or less (not including 0%), Al: 0.005-0.050% , Ni: 5.0-7.5% and N: 0.010% or less (not including 0%), the remainder comprising iron and unavoidable impurities, wherein the residual austenitic phase at -196 ° C represents 2 0-12.0% in terms of volume fraction, and the average circle diameter of the inclusions having a circle equivalent diameter of more than 2.0 μm is 3.5 μm or less.

[0031]

The composition of the steel will be described first.

C: C: 0.02 - 0.10% C is an essential element to obtain the resistance and residual austenite. In order to perform such an action effectively, the lower limit of the amount of C is set to 0.02% or more. The lower limit of the amount of C should preferably be 0.03% or more, more preferably 0.04% or more. However, when C is added in excess, the ultra-low temperature toughness deteriorates due to an excessive increase in resistance and therefore the upper limit of C is set to 0.10%. The upper limit of the amount of C should preferably be 0.08% or less, more preferably 0.06% or less.

[0033]

If: 0.40% or less (not including 0%)

Si is a useful element as a deoxidizing material. However, when Si is added in excess, the formation of an island-shaped hard martensite phase is favored, the ultra-low temperature toughness deteriorates and, therefore, the upper limit of Si is set to 0.40. % or less. The upper limit of the amount of Si should preferably be 0.35% or less, more preferably 0.20% or less.

[0034]

Mn: 0.50 - 2.0%

Mn is a stabilizing element austenite (γ) and is a contributing element to increase the amount γ residual. In order to exert such action effectively, the lower limit of the amount of Mn is set at 0.50%. The lower limit of the amount of Mn should preferably be 0.6% or more, more preferably 0.7% or more. However, when Mn is added in excess, an income-related embrittlement occurs, the desired ultra-low temperature toughness can not be obtained and, therefore, the upper limit of Mn is set at 2.0% or less. The upper limit of the amount of Mn should preferably be 1.5% or less, more preferably 1.3% or less.

P: 0.007% or less (not included 0%) P is an impurity element that becomes a cause of intragranular fracture, and in order to obtain the desired toughness at ultra low temperature, the upper limit of P is established at 0.007% or less. The upper limit of the amount of P is preferably 0.005% or less. Although the amount of P should be as small as possible, it is difficult to achieve a 0% P level at the industrial level.

S: 0.007% or less (not included 0%)

Similar to P described above, S is also an impurity element which becomes a cause of intragranular fracture and, in order to obtain the desired toughness at ultra low temperature, the upper limit of S is set to 0.007% or less. As shown in an example described below, as the amount of S increases, the ratio of brittle fracture surface increases, and the desired toughness at ultra low temperature (the brittle fracture surface ratio at -196 ° C <10% ) can not be obtained. The upper limit of the amount of S is preferably 0.005% or less. Although the amount of S should be as small as possible, it is difficult to arrive at a quantity of S of 0% at the industrial level.

[0037]

Al: 0.005 - 0.050%

Al is a deoxidizing element. When the Al content is insufficient, the oxygen content of the steel increases, the coarse inclusions increase and therefore the lower limit of Al is set to 0.005% or more. The lower limit of the amount of Al is preferably 0.010% or more, more preferably 0.015% or more. However, when Al is added in excess, conglomeration and integration of inclusions are favored, the size of the inclusions also becomes larger and, therefore, the upper limit of AI is set to 0.050% or less. The upper limit of the amount of Al should preferably be 0.045% or less, more preferably 0.04% or less.

[0038]

Ni: 5.0 - 7.5%

Ni is an indispensable element for obtaining residual (residual) austenite which is useful for improving the ultra low temperature toughness. In order to perform such an action effectively, the lower limit of the amount of Ni is set at 5.0% or higher. The lower limit of the amount of Ni should preferably be 5.2% or more, more preferably 5.4% or more. However, when Ni is added in excess, the cost of the material increases and, therefore, the upper limit of Ni is set at 7.5% or less. The upper limit of the amount of Ni should preferably be 7.0% or less, more preferably 6.5% or less, more preferably 6.0% or less.

N: N: 0.010% or less (not included 0%)

Since N deteriorates the ultra low temperature toughness by stress aging, the upper limit of N is set to 0.010% or less. The upper limit of the amount of N should preferably be 0.006% or less, more preferably 0.004% or less.

The thick steel plate of the present invention comprises the compositions described above as base compositions and the remainder is iron and inevitable impurities.

[0041]

The present invention may contain the following selective compositions for the purpose of imparting additional characteristics.

[0042]

Cu: 1.0% or less (not including 0%)

Cu is a stabilizing element γ and is an element contributing to increasing the residual quantity γ. In order to exert such action effectively, the Cu content should preferably be 0.05% or more. However, when Cu is added in excess, the resistance increases excessively, the desired ultra low temperature toughness can not be obtained and therefore the upper limit of Cu should preferably be 1.0% or less. The upper limit of the amount of Cu should preferably be 0.8% or less, more preferably 0.7% or less.

[0043]

One or more elements selected from a group consisting of Cr: 1.20% or less (not including 0%) and Mo: 1.0% or less (not including 0%)

Cr and Mo are both elements that improve resistance. These elements can be added alone, and both elements can be used in combination. In order to exert the action effectively, it is preferred to set the amount of Cr to 0.05 or more and the amount of Mo to 0.01% or more. However, when they are added in excess, the resistance increases excessively, the desired toughness at ultra low temperature can not be obtained and, therefore, the upper limit of the amount of Cr should preferably be 1.20% or less (better still 1.1% or less, better still 0.9% or less, and even better 0.5% or less) and the upper limit of the amount of MB should preferably be 1.0% or less (better still 0.8% or less, better still 0.6% or less).

[0044]

One or more elements selected from a group consisting of Ti: 0.025% or less (not including 0%), Nb: 0.100% or less (not including 0%) and V: 0.50% or less (not including 0%)

Ti, Nb and V are all elements precipitating as carbonitride and improving the resistance. These elements can be added alone, and two or more elements can be used in combination. In order to exert the action effectively, it is preferable to set the amount of Ti to 0.005% or more, the amount of Nb to 0.005% and the amount of V to 0.005% or more. However, when they are added in excess, the resistance increases excessively, the desired toughness at ultra low temperature can not be obtained and, therefore, the upper limit of the amount of Ti should preferably be 0.025% or less (Better still 0.018% or less, more preferably 0.015% or less), the upper limit of the amount of Nb should preferably be 0.100% or less (more preferably 0.05% or less, more preferably 0%). , 02% or less) and the upper limit of the amount of V should preferably be 0.50% or less (more preferably 0.3% or less, more preferably 0.2% or less).

B: 0.0050% or less (not included 0%) B is an element contributing to improving the strength by improving the quenchability. In order to perform the action effectively, it is preferable to set the amount of B at 0.0005% or higher. However, when B is added in excess, the resistance increases excessively, the desired toughness at ultra low temperature can not be obtained and, therefore, the upper limit of the amount of B should preferably be 0.0050% or less (more preferably 0.0030% or less, more preferably 0.0020% or less).

[0046]

One or more elements selected from a group consisting of Ca: 0.0030% or less (not including 0%), REM (rare earth element): 0.0050% or less (not including 0%) and Zr: 0.005% or less (not including 0%)

Ca, REM and Zr are all deoxidizing elements. Their addition reduces the oxygen content of the steel and reduces coarse inclusions. These elements can be added alone, and both elements can be used in combination. In order to perform the actions effectively, it is preferable to set the amount of Ca to 0.0005% or more, the amount of REM (when REM described hereinafter is contained alone, the quantity is the content alone, and when two or more types are contained, the quantity is the total quantity thereof, and hereinafter also for the amount of REM) at 0.0005% or more and the amount of Zr at 0.0005% or more. However, when they are added in excess, the coarse inclusions on the contrary increase, the ultra-low temperature toughness deteriorates and, therefore, the upper limit of the amount of Ca should preferably be 0.0030% or less (better 0.0025% or less), the upper limit of the amount of EMR should preferably be 0.0050% or less (more preferably 0.0040% or less) and the upper limit of the amount of Zr should be preferably, 0.005% or less (more preferably 0.0040% or less).

[0047]

In this specification, REM (rare earth element) is a group of lanthanide elements (15 elements of La having the atomic number 57 at Lu having the atomic number 71 in the periodic table) plus Sc (scandium) and Y (yttrium), and they can be used alone or two or more elements can be used in combination. The rare earth elements are preferably Ce and La. The form of REM addition is not particularly limited. REM can be added in the form of a mischmetal containing mainly Ce and La (eg Ce: approximately 70%, La: approximately 20 - 30%, or can be added otherwise as a single body of Ce, La and the like.

[0048]

The composition of the steel of the present invention has been described above.

The thick steel plate of the present invention further satisfies 2.0-12.0% (preferably 4.0-12.0%) of the residual phase y present at -196 ° C. terms of volume fraction.

[0050]

It is known that the residual y-phase present at -196 ° C. contributes to improving the ultra-low temperature toughness. In order to exert such action effectively, the volume fraction of the residual phase y relative to the total structure present at -196 ° C is set at 2.0% or higher. However, the residual phase y is comparatively softer than a matrix phase, a predetermined value of YS can not be obtained when the residual phase y becomes excessive and, therefore, the upper limit thereof is set to 12. 0% (see No. 39 in Table 2 below). With respect to the volume fraction of the residual phase y, the lower limit should preferably be 4.0% or more, more preferably 6.0% or more, and the upper limit should preferably be 11.5 % or less, better still 11.0% or less.

[0051]

By controlling the volume fraction of the residual γ phase relative to the total structure present at -196 ° C to 4.0% or higher, the brittle fracture surface ratio can be maintained at an excellent level of 50% or less at -233 ° C which is below -196 ° C described above. A more preferable lower limit when such an effect is to be exerted is 6.0% or more, and the preferable upper limit is the same as above.

[0052]

In addition, in the thick steel plate of the present invention, the control of the volume fraction of the residual γ phase is important with respect to the structure present at -196 ° C. and the structure other than the residual γ phase. is limited in any way and may be those usually present in thick steel plates. For example, there may be mentioned as a structure other than the residual γ phase, bainite, martensite, cementite and the like.

The thick steel plate of the present invention also satisfies 3.5 μm or less of the average circle diameter N1 of inclusions having an equivalent circle diameter of more than 2.0 μm (coarse inclusions). When compared to the prior art described above, the most distinctive feature of the thick steel plate of the present invention is that the coarse inclusions are refined to 3.5 μm or less.

[0054]

Here, the "diameter equivalent circle" is the diameter obtained as that of a supposed circle so that, by observing the size of the inclusion, the areas of the inclusion and the circle become equal to each other .

That is to say that, according to the result of the study by the present inventors, it was found that coarse inclusions having more than 2 pm diameter circle equivalent become the starting point of brittle fracture and when the average size (mean circle diameter N1) of the coarse inclusions became large, the desired toughness at ultra low temperature could not be obtained when the volume fraction of the residual phase y at -196 ° C was controlled. in the range described above (see Nos. 33-35, 45-49 of Table 2 below). The average circle diameter of N 1 above should preferably be as small as possible and should preferably be 3.2 μm or less, more preferably 3.0 μm or less. Also in the present invention, inclusions having more than 2.0 pm circle equivalent diameter are present at approximately 10-100 / mm 2.

[0056]

Inclusions can be measured by a method described in the example below. Here, the type of "inclusions" in the inclusions having more than 2.0 pm of circle equivalent diameter is not particularly limited in the present invention. The reason for this is that the occurrence of brittle fracture is more strongly influenced not by the type of inclusions but by the size (average circle diameter) of the inclusions. As regards the type of inclusions, mention may be made in addition to individual particles such as oxides, nitrides, oxynitrides and the like, for example, a complex obtained by combining two or more types of these individual particles, or complex particles. obtained by uniting these individual particles and other elements and the like.

[0057]

In addition, from the sole point of view of inclusion control, similar technology has been disclosed in JP-A No. 2001-123245, but the inclusion control direction is very different from that of the present invention. That is, in JP-A No. 2001-123245, Mg in particular is monitored, and the magnification of the austenite grains at high temperature is suppressed and the toughness is improved by dispersing a large number of fine particles of Mg-containing oxide having a size of 2 μm or less, while in the present invention coarse inclusions which become the starting point of brittle fracture and deteriorate toughness are reduced regardless of their type, and both are totally different from each other with respect to the inclusion control method. Therefore, in the present invention, although the fine inclusions are not controlled in any way, according to a preferred manufacturing method of the present invention described below, fine inclusions having 2.0 μm or less of circle equivalent diameter are present at approximately 100-1000 / mm2. Also, when they are limited to Mg-containing oxides among the fine inclusions having 2.0 pm or less of circle equivalent diameter, they are hardly present in the present invention.

[0058]

A method of manufacturing the thick steel plate of the present invention will now be described.

[0059]

The manufacturing method in connection with the present invention is characterized in (A) and (B) below. (A) During the steel melting step, the amount of oxygen [O] free before Al addition is controlled to 100 ppm or less, the hold time (t1) from the addition of Al until the beginning of the casting is controlled at 15 min or more, and the cooling time (t2) at 1450-4500 ° C in casting is controlled at 300 s or less. The average circle diameter of the coarse inclusions particularly described above is refined to 3.5 μm or less by the method (A). (B) After hot rolling, the steel plate is heated and maintained in the temperature range of AC1-AC3 points and is then subjected to tempering for 10-60 min in the temperature range of 520 ° C. Aci point. The volume fraction of the residual y-phase present at -196 ° is suitably controlled in particular by the method (B).

[0060]

Compared with the prior art described above, in the above process (A), the most distinct characteristic is to control t1 and t2 in particular.

[0061]

The respective steps will be described in detail below.

(Melting step)

In the present invention, based on the view that Al-based inclusions are magnified by conglomeration and integration and are likely to form coarse inclusions that become the starting point for brittle fracture, special considerations are given to method of adding Al in order not to form such gross Al-based inclusions.

[0063]

First, by adding Al which is a deoxidizing material in molten steel, the amount of free oxygen (amount of oxygen in solution, can be abbreviated as amount [O]) before the addition of Al is controlled at 100 ppm or less. The reason for this is that, when the amount [O] exceeds 100 ppm, the size of the inclusions formed by adding Al grows, N1 can not be appropriately controlled and the desired ultra low temperature toughness can not be obtained ( see No. 33 in Table 2 below). The amount [O] should preferably be as small as possible and should preferably be 80 ppm or less, more preferably 50 ppm or less. The lower limit of the quantity [O] is not particularly limited from the point of view of the refinement of coarse inclusions.

[0064]

As a method for controlling the amount [O] as described above, there may be mentioned for example a deoxidation process by adding deoxidizing elements Mn, Si in molten steel. When deoxidizing materials such as Ti, Ca, REM, Zr and the like are added as selective compositions other than the elements described above, the amount [O] can also be controlled by adding them.

[0065]

In order to control Al-based inclusions, controlling the amount [O] before adding Al is important, and the order of adding Al and other deoxidants is not the issue. However, when Al is added in a state where the amount [O] is high, the temperature of the molten steel increases due to an oxidation reaction which is dangerous in service and, therefore, it is preferable to add Si and Mn before Al. It is also preferable to add the selective compositions such as Ti and the like in the molten steel after adding Al.

[0066]

Then, the hold time (t1) after adding Al to the molten steel until the start of casting is set to 15 min or more. Thus, coarse inclusions are separated by flotation and are eliminated. Also conventionally, the casting was started at the same time as the addition of Al or within 13 min after the addition of Al, but it was found that when t1 was less than 15 min, The effect of the elimination of coarse inclusions was not effected effectively, the coarse inclusions were not refined and, therefore, the desired toughness at ultra-low temperature was not achieved (see No. 34 and 55 in Table 2 below). From the above point of view, t1 should preferably be as long as possible, and should preferably be 18 min or longer, more preferably 20 min or more. The upper limit of t1 is not particularly limited from the above point of view, but since maintaining for a long time implies an increase in production costs, t1 should preferably be 180 min or less, better still 150 min or less.

[0067]

Then the casting is started. Although the casting temperature range is generally 1650 ° C or lower, according to the present invention, it has been found that it is important to control in particular the cooling time (t2) in the temperature range of 1450.degree. 1500 ° C to 300 s. or less, and coarse inclusions were thus appropriately refined. When t2 exceeds 300 sec., Secondary inclusions are formed in a composite manner with inclusions becoming nuclei, the size of coarse inclusions growing and the desired toughness at ultra low temperature is not exerted (see No. 35 and No. 56). of Table 2 below). From the above point of view, t 2 should preferably be as short as possible and should preferably be 290 s. or less, better still 280 s. or less. The lower limit of t2 is not particularly limited from the point of view above.

[0068]

In the present invention also, the reason why the temperature range of 1450-1500 ° C is monitored particularly in the casting temperature range, is that the temperature range is a temperature range where the growth of inclusions is favored by the progression of casting solidification and the progression of the concentration of the molten acid composition.

[0069]

The temperature range of 1450-1500 ° C also means the temperature of the central part of the thickness of the slab. The thickness of the slab is generally 150-250 mm and the surface temperature tends to be less than the core temperature of approximately 200-1000 ° C. Since the variation of the temperature difference of the surface temperature is great, the temperature in the central part (near the thickness tx1 / 2) where the variation is small is the object. The temperature of the central portion of the slab thickness can be measured by inserting a thermocouple into a mold.

[0070]

In the present invention also, the cooling time (t1) in the temperature range of 1450-1500 ° C only must be controlled at 300 s. or less, and the method for this purpose is not limited. For example, cooling may be performed at a constant rate in the temperature range at an average cooling rate of approximately 0.17 ° C / sec. or less so that the cooling time in the temperature range will be 300 s. or less, or the cooling can be run at different rates so that the cooling time in the temperature range will be 300 s. or less.

[0071]

Also in the present invention, the cooling method for the casting temperature range other than the temperature range described above is in no way limited, and an ordinary method (air cooling or water cooling) can be employee.

[0072]

After running the casting as described above, hot rolling is performed, and the steel plate is subjected to a heat treatment.

[0073]

Here, the hot rolling step is not particularly limited and a commonly used method may be employed to obtain a predetermined plate thickness, but, more specifically, after the slab has been heated for 1 4 hours at approximately 1100 ° C, the temperature (final rolling), the milling rate and the like can be adjusted.

[0074]

After hot rolling, the steel plate is heated to the temperature range of the points AC1-AC3 (TL), is maintained and is then cooled with water. These treatments are equivalent to the treatment L described in the prior art described above, and the residual γ phase stably at -196 ° C can thus be obtained by a predetermined range.

[0075]

More specifically, the steel plate is heated to the temperature of the two-phase region (ferrite (α) -γ) points Aci-AC3 (TL). By heating the steel plate to the temperature range, alloying elements such as Ni and the like are concentrated at the formed γ phase, and a quasi-stable residual γ phase exhibits almost stably at room temperature. obtained. Therefore, below the Ad point or above the Azz point, the residual γ phase at -196 ° C can not be sufficiently obtained (see # 36 and # 37 in Table 2 below). The preferred heating temperature is approximately 660-710 ° C.

[0076]

The heating time (holding time, TL) at the temperature of the two-phase region should preferably be in the range of 10-50 min. When it is less than 10 min., The concentration of the alloying elements of the γ phase does not progress sufficiently, whereas when it is more than 50 min., The a phase is annealed and the resistance deteriorates. The upper limit of the preferential heating time is 30 min.

[0077]

In addition, setting the heating time to 15 min. or more, the volume fraction of the residual γ-phase at -196 ° C obtained is 4.0% or more, and as a result, excellent toughness is obtained even at the even lower temperature with the rupture surface ratio fragile at -233 ° C which is 50% or less. Better yet, the lower limit when such an effect is to be exercised is 5.0% or more. The upper limit of the preferred heating time is also the same as above (30 min or less).

[0078]

Then, after cooling with water at room temperature, the income treatment is performed. The tempering is performed for 10-60 min (t3) in the 520 ° C-point Ac1 (T3) temperature range. Thus, C is concentrated in the residual quasi-stable γ phase during the income and the stability of the quasi-stable residual γ phase increases, and consequently, the residual γ phase stably present even at -196 ° C is obtained. When the tempering temperature T3 is less than 520 ° C., the quasi-stable residual phase γ formed while the two-phase coexistence region is maintained is disintegrated in phase a and cementite phase and the residual γ phase at -196 ° C. can be sufficiently obtained (see No. 40 in Table 2 below). On the other hand, when the temperature of income T3 exceeds the point Aci or the time of income is less than 10 min., The concentration of C in the residual stable γ phase does not progress sufficiently and the desired amount of γ phase residual at -196 ° C can not be obtained (see No. 41 (the case where T3 is high) and No. 54 (the case where t3 is short) of Table 2 below). In addition, when the recovery time t3 exceeds 60 min., The residual γ phase at -196 ° C is excessively formed and the predetermined resistance can not be obtained (see No. 42 of Table 2 below).

[0079]

The preferential income treatment condition is, T3 income temperature: 570-620 ° C, t3 revenue time: 15 min. or more and 45 min. or less (better still 35 min or less, even better 25 min or less).

[0080]

After the income treatment has been performed as described above, the cooling is carried out to room temperature. The cooling process is not particularly limited, and either air cooling or water cooling can be employed.

[0081]

In this specification, the Aci point and the Ac3 point are calculated on the basis of the expressions below (from "KouzaGendai-No Kinzoku-Gaku (Lecture: Contemporary Metallurgy), material part 4, Tekkou-Zairyou" (Iron and Steel Material). ), The Japan Institute of Metals).

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

Point Ac3 = 910-203x [C] 1 / 2-15,2x [Ni] + 44,7x [Si] + 104x [V] + 31,5x [Mo] + 13,1x [W] where [] means the content (% by weight) of alloying elements of steel. In the present invention also, since As and W are not included in the composition of the steel, in the expressions, the calculation is done with [As] and [W] being 0%.

[Examples] [0082]

Although the present invention is explained in more detail below by specific reference to examples, the present invention is not limited by the examples below and can also be implemented with modifications added in the scope adaptable to goals described above and below and any of these should be included in the technical scope of the present invention.

[0083]

Example 1

Sample steels of the componential compositions shown in Table 1 (the remainder: iron and unavoidable impurities, the unit is% by weight) were melted under the melting conditions shown in Table 2 using a vacuum melting furnace ( 150 kg, VIF) and were cast and 150 mm x 150 mm x 600 mm ingots were then manufactured by hot forging. In the present example, REM was used with approximately 50% Ce-containing methanol and approximately 25%. In addition, the order of addition of deoxidizing elements, when the selective compositions were not included, Si, Mn (added simultaneously) - »Al; while when the selective compositions of Ti, REM, Zr, Ca were included, Si, Mn (added simultaneously) - »Al -» Ti - »REM, Zr, Ca (added simultaneously). In addition, in Table 2, [O] is the amount of oxygen in solution (ppm) before adding Al, t1 is the time (min) from the addition of AI to the beginning of casting, and t2 is the cooling time (s) at 1500-1450 ° C in casting. Cooling at 1500-1450 ° C was performed by air cooling or water cooling and was controlled in such a way that the cooling time was as described above.

[0084]

Then, after heating to 1100 ° C, the ingot was rolled to a 75 mm plate thickness at the temperature of 830 ° C or above temperature, was rolled at 780 ° C to the final rolling temperature, a. it was then cooled with water, and a thick 25 mm thick steel plate was thus obtained. The steel plate thus obtained was heated to the temperature shown in Table 2 (TL in Table 2), then heated and held for 5-60 min. (see TL of Table 2), and was then cooled with water to room temperature. Then, after the income treatment (T3 = temperature of income, t3 = time of income) was performed as shown in Table 2, the air cooling or water cooling was performed up to room temperature.

[0085]

With regard to the thick steel plate thus obtained, the average circle diameter N1 of the inclusions having more than 2.0 pm of circle equivalent diameter, the quantity (volume fraction) of the residual phase y is -196 °. C, tensile properties (tensile strength TS, yield strength YS), and ultra low temperature toughness (the ratio of brittle fracture surface in the C direction to -196 ° C or -233 ° C) have been evaluated as described below.

(1) Measurement of the average circle diameter N1 of inclusions having more than 2.0 pm diameter circle equivalent

The t / 4 position (t: plate thickness) of the steel plate was polished as a mirror and 4 fields of view were photographed at 400x magnification using an optical microscope. The area per field of vision was 0.04 mm2 and the total area of the 4 fields of view was 0.15 mm2. The inclusions observed in these 4 fields of view were analyzed by "Image-Pro Plus" produced by Media Cybernetics, Inc., the equivalent diameter circle (diameter) of inclusions having more than 2.0 pm diameter circle (diameter) ) was calculated and the average value of it was calculated.

[0087]

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

A 10 mm x 10 mm x 55 mm specimen was taken from the t / 4 position of each steel plate, held for 5 min. at a temperature of liquid nitrogen (-196 ° C) and was then measured by X-ray diffraction on a small two-dimensional part by an X-ray diffraction apparatus (RINT-RAPID II) made by Rigaku Corporation. Then, with respect to respective lattice plane peaks of (110), (211), (220) of the ferritic phase and respective lattice plane peaks (111), (200), (220), (311) of the residual y phase, the volume fractions of (111), (200), (220), (311) of the residual y phase were respectively calculated on the basis of the integrated intensity ratio of the respective peaks, and their value average was obtained, the average value of which was made the "volume fraction of the residual phase y".

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

Specimen No. 4 of JIS Z 2241 was taken parallel to the C direction of the t / 4 position of each steel plate, the tensile test was performed by a method described in JIS Z 2241, and the resistance TS tensile strength and YS yield strength were measured. In the present example, those with TS> 690 MPa and YS> 590 MPa were rated as excellent in terms of base metal strength.

(4) Measurement of the ultra low temperature toughness (Fracture failure surface ratio in the C direction) 3 pieces of the Charpy impact test specimens (V notch specimen of JIS Z 2242) were taken in parallel to direction C of position T / 4 (: plate thickness) and position W / 4 (W: plate width) as well as position t / 4 and position W / 2 of each steel plate, the ratio Fracture brittle area (%) at -196 ° C was measured by the method described in JIS Z 2242, and the average value of each was calculated. Of the two values thus calculated, the lower average value of the property (which is large in the brittle fracture area ratio) was used, and one with 10% or less of this value was rated as excellent in terms of toughness at ultra low temperature in the present example.

[0090]

These results have been shown side by side in Table 2. For reference, the Aci point and the Ac3 point have also been shown in Table 1 and Table 2.

[0091] [Table 1A]

The following study is possible from Table 2.

[0096]

First, Nos. 1-32 of Table 2A are examples which satisfy all the requirements of the present invention and the thick steel plate with excellent ultra low temperature toughness (more specifically, the average value of the ratio of brittle fracture surface in the C <10% direction) at -196 ° C even when the strength of the base metal was high could be provided.

On the other hand, Nos. 33-42, 54-56 of Table 2B do not satisfy at least one of the preferred production conditions of the present invention and are therefore references which do not meet the requirements of the present invention. the present invention and the desired properties could not be obtained.

[0098]

More specifically, No. 33 is an example in which the steel composition of No. 33 of Table 1B that met the conditions of the present invention was used but as the amount [O] which was the amount of oxygen. in solution before adding Al was large, coarse inclusions were not refined. As a result, the brittle fracture area ratio also increased, and the desired ultra low temperature toughness could not be achieved at -196 ° C.

[0099]

No. 34 is an example in which the composition of the steel of No. 34 of Table 1B, the amount of C of which was high, was used and the time (t1) after the addition of Al to the beginning of the casting was short; while No. 55 is an example in which the steel composition of No. 55 of Table 1B that met the requirements of the present invention was used, but the time t1 was short. In both cases, since t1 was short, coarse inclusions were not refined. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0100]

No. 35 is an example in which the composition of the steel of No. 35 of Table 1B whose amount P was high was used and the cooling time (t2) of 1500-1450 ° C in casting was long. ; while No. 56 is an example in which the steel composition of No. 56 of Table 1B satisfying the requirements of the present invention was used, but t2 described above was long. In both cases, since t2 was long, coarse inclusions were not refined. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0101]

No. 36 is an example in which the steel composition of No. 36 of Table 1B satisfying the requirements of the present invention was used but, as it was heated to a temperature below the temperature of the two phases (TL), the amount of residual γ was insufficient. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0102]

No. 37 is an example in which the composition of the steel of No. 37 of Table 1B whose Si amount was high was used and the heating was carried out at a temperature above the temperature of the region of two. phases (TL) and therefore the amount of γ was insufficient. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0103]

No. 38 is an example in which the steel composition of No. 38 of Table 1B satisfying the requirements of the present invention was used but, as the heating holding time (tL) at the temperature of the region in two phases was short, the amount of residual γ was insufficient. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0104]

No. 39 is an example in which the steel composition of No. 39 of Table 1B satisfying the requirements of the present invention was used but, as the heating holding time (tL) at the temperature of the region in two phases was long, the amount of residual γ increased. As a result, the yield strength YS deteriorated and the desired toughness at ultra low temperature could not be obtained.

[0105]

No. 40 is an example in which the steel composition of No. 40 of Table 1B satisfying the requirements of the present invention was used but, since the tempering temperature (T3) was low, the amount of γ was insufficient. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0106]

No. 41 is an example in which the composition of the steel of No. 41 of Table 1B whose amount of Mn was high was used and the tempering temperature (T3) was high and therefore the amount of γ a been insufficient. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0107]

No. 42 is an example in which the composition of the steel of No. 42 of Table 1B satisfying the requirements of the present invention was used but, as the time of return (t3) was long, the amount of γ residual has increased. As a result, the yield strength YS deteriorated and the desired toughness at ultra low temperature could not be obtained.

[0108]

No. 54 is an example in which the steel composition of No. 54 of Table 1B satisfying the requirements of the present invention was used but, since the time of return (t3) was short, the amount of γ residual was insufficient. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0109]

Nos. 43-53 are references made by the process of the present invention using one in which only the steel composition differs.

[0110]

More specifically, No. 43 is an example in which the amount of residual γ was insufficient because the steel composition of No. 43 of Table 1B whose amount of Mn was less was used. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0111]

No. 44 is an example in which the composition of steel of No. 44 of Table 1B whose amount of S was high was used. As a result, the brittle fracture area ratio increased and the desired ultra low temperature toughness could not be obtained.

[0112]

No. 45 is an example in which the composition of the steel of No. 45 of Table 1B in which the amount of C was lower, the amount of Al was high and the amount of Ni was less was used, and therefore , coarse inclusions have not been refined and the amount of it has been insufficient. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved. In addition, TS has also deteriorated.

[0113]

No. 46 is an example in which the composition of the steel of No. 46 of Table 1B whose Am quantity was lower and the amount of N was high was used and, therefore, the coarse inclusions were not been refined. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0114]

No. 47 is an example in which the composition of the steel of No. 47 of Table 1B in which the amounts of Cu and Ca, which were the selective compositions, were high was used and, therefore, the coarse inclusions have not been refined. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0115]

No. 48 is an example in which the composition of the steel of No. 48 of Table 1B whose amounts of Cr and Zr, which were the selective compositions, were high was used and, therefore, the coarse inclusions have not been refined. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0116]

No. 49 is an example in which the composition of the steel of No. 49 of Table 1B whose amounts of Nb and REM, which were the selective compositions, were high was used and, therefore, the coarse inclusions have not been refined. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.

[0117]

In No. 50, as the composition of the steel of No. 50 of Table 1B in which the amount of Mo, which was the selective composition, was high was used, the ratio of brittle fracture area increased and toughness increased. desired at ultra low temperature could not be obtained.

[0118]

In No. 51, as the composition of the steel of No. 51 of Table 1B in which the amount of Ti, which was the selective composition, was high was used, the ratio of brittle fracture area increased and toughness increased. desired at ultra low temperature could not be obtained.

[0119]

In No. 52, as the steel composition of No. 52 of Table 1B whose V amount, which was the selective composition, was high was used, the brittle fracture surface ratio increased and the toughness increased. desired at ultra low temperature could not be obtained.

[0120]

In No. 53, as the composition of the steel of No. 53 of Table 1B, of which the amount of B, which was the selective composition, was high was used, the ratio of brittle fracture area increased and toughness increased. desired at ultra low temperature could not be obtained.

[0121]

Example 2

In the present example, with respect to a portion of the data used in Example 1 (all of which are examples of the present invention), the brittle fracture area ratio at -233 ° C was evaluated. .

[0122]

More specifically, for the purposes described in Table 3 (No. in Table 3 corresponds to No. in Table 1 and Table 2), 3 pieces of specimen were taken from position t / 4 and position W / 4, the Charpy shock test at -233 ° C was performed by a method described below, and the average value of the brittle fracture area ratio was evaluated. In the present example, one in which the brittle fracture surface ratio <50% has been rated excellent for the brittle fracture area ratio at -233 ° C. "Kouatu-Gasu" (High Pressure Gas) , flight. 24, p. 181, "Ultra low temperature impact test of austenite-based cast stainless steel" [0123]

These results are shown in Table 3.

[0124] [Table 3]

[0125]

All Nos. 3, 4, 6, 13, 15, 19 and 23 of Table 3 are examples in which the heating time (tL) at the temperature of the two-phase region was checked at 15 min. or more (see Table 2A) and the residual γ phase could be ensured by 4.0% or more. As a result, not only was the brittle fracture area ratio at -196 ° C but also the same ratio at -233 ° C that was lower than -196 ° C was excellent and very excellent ultra low temperature toughness was achieved. could be obtained.

Claims (3)

1. Thick steel plate with excellent low temperature toughness containing in% by weight: C: C: 0.02-0.10%; If: 0.40% or less (not including 0%); Mn: 0.50 - 2.0%; P: 0.007% or less (not including 0%); S: 0.007% or less (not including 0%); Al: 0.005-0.05%; Ni: 5.0 - 7.5%; and N: 0.010% or less (not including 0%); the remainder comprising iron and unavoidable impurities, wherein the residual austenitic phase at -196 ° C is 2.0 - 12.0% in terms of volume fraction, inclusions having more than 2.0 pm circle equivalent diameter are present at 10-100 / mm2, and the average circle diameter of the inclusions having more than 2.0 pm circle equivalent diameter is 3.5 pm or less. The thick steel plate according to claim 1, wherein the residual austenitic phase at -196 ° C is 4.0-12.0% in terms of volume fraction. The thick steel plate according to claim 1 or 2 further containing, as other elements, at least one group of groups (a) - (e) below: (a) Cu: 1.0% or less ( not including 0%); (b) one or more elements selected from a group consisting of Cr: 1.20% or less (not including 0%) and Mo: 1.0% or less (not including 0%); (c) one or more elements selected from a group consisting of Ti: 0.025% or less (not including 0%), Nb: 0.100% or less (not including 0%) and V: 0.50% or less (not included 0%); (d) B: 0.0050% or less (not including 0%); (e) one or more elements selected from a group consisting of Ca: 0.0030% or less (not including 0%), REM: 0.0050% or less (not including 0%) and Zr: 0.005% or less ( not including 0%).