HIGH-Ni ALLOY STEEL

Abstract

To provide a high-Ni alloy steel having excellent hot workability in the temperature range of 900°C to 1000°C.SOLUTION: There is provided a high-Ni alloy steel containing 0.001% to 0.15% of C, 0.10% to 1.00% of Si, 0.10% to 1.50% of Mn, 0.030% or less of P, 0.0015% or less of S, 30.0% to 50.0% of Ni, 20.0% to 30.0% of Cr, 0.01% to 0.80% of Mo, 0.01% to 0.50% of Cu, 0.020% to 0.300% of Al, 0.12% to 0.30 of N, V:0.01% o 0.50% of V, 0.01% to 3.00% of W, 0.30% to 0.90% of Nb, 0.0001% to 0.0050 of Ca, 0.010% or less of Mg, 0.0030% or less of B, 0.20% or less of Ti and the balance of iron and impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a high Ni alloy steel. [Background technology]

[0002] High Ni alloy steel containing 10% by mass or more of Ni and Cr each has high strength and high corrosion resistance, but tends to have poor hot workability in the temperature range of 900° C. to 1000° C. Therefore, for example, when hot rolling a slab to produce a thick or thin plate of high Ni alloy steel, so-called scabs, in which the material is partially peeled off thinly, may occur on the surface of the steel plate.

[0003] Patent Document 1 describes the following: C: 0.03 to 0.08%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.20%, P: 0.001 to 0.040%, S: 0.0001 to 0.0030%, Ni: 35.5 to 45.5%, Cr: 23.5 to 26.0%, Mo: 0.30 to 1.50%, Cu: 0.01 to 0.30%, Al: 0.010 to 0.150%, Ti: 0.10% or less, B: 0.0005 to 0.0050%, Co: 0.02 to 0.30%, Nb: 0.4 and N: 0.15-0.30%, with the balance being Fe and impurities, the 0.2% proof stress is 400 MPa or less, the elongation is 40.0% or more, the 0.2% proof stress in a high temperature tensile test at 1100°F is 140 MPa or more, the elongation is 50.0% or more, the average grain size is -3 to 6, and one or more of NbN, NbC, TiN, and TiC having a major axis of 0.5 to 5 μm are present in an average of 20 to 180 particles / mm in total in any cross section. 2 In addition, if one or more of NbN, NbC, TiN, or TiC with a major axis of more than 5 μm are contained, the total number of these elements must be an average of 5.0 particles / mm 2 The following Fe-Cr-Ni based alloys are described:

[0004] The Fe-Cr-Ni alloy described in Patent Document 1 achieves both excellent yield strength and elongation at room temperature and at high temperatures, but no study has been conducted on its hot workability in the temperature range of 900°C to 1000°C. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7174192 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high Ni alloy steel that is excellent in hot workability in the temperature range of 900°C to 1000°C. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention employs the following configuration. [1] In mass %, C: 0.001%~0.15%, Si: 0.10% to 1.00%, Mn: 0.10% to 1.50%, P: 0.030% or less, S: 0.0015% or less, Ni: 30.0%~50.0% Cr: 20.0%~30.0%, Mo: 0.01% to 0.80%, Cu: 0.01% to 0.50% Al: 0.020%~0.300%, N: 0.12%~0.30%, V: 0.01%~0.50%, W: 0.01%~3.00%, Nb: 0.30%~0.90%, Ca: 0.0001%~0.0050%, Mg: 0.010% or less, B: 0.0030% or less, Ti: 0.20% or less, Remainder: High Ni alloy steel consisting of iron and impurities. [2] The high Ni alloy steel according to [1], further satisfying the following formula (1): Mggb=27Mg-160B-0.06Ti+0.5≦0.015 …(1) In the formula (1), Mggb is the grain boundary segregation parameter of Mg, and Mg, B, and Ti are the contents (mass%) of each element. [3] The high Ni alloy steel according to [1] or [2], further containing, by mass%, one or more of the following Group A, Group B, or Group C: [Group A] One or more elements selected from the group consisting of Ta: 0.001 to 0.50%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.05%, and Hf: 0.001 to 0.10%. [Group B] La, Ce, Nd and Pr, one or more of which in total is 0.001 to 0.050%. [Group C] Zn, Pb and Bi, any one or more of which total not more than 0.0010%. [Effects of the Invention]

[0008] According to the present invention, a high Ni alloy steel having excellent hot workability can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present inventors have conducted various studies to improve the hot workability of high-Ni alloy steels containing large amounts of Cr and Ni. Generally, in austenitic stainless steels, strong deoxidation is performed to prevent sulfides such as CaS and MgS, which are removed from molten iron and transferred to slag during the refining process, from reacting with oxygen in the molten iron and becoming resulfurized. During this process, the oxygen content in the molten steel decreases, and MgO in the slag is reduced, inevitably increasing the Mg concentration in the molten steel. However, the increase in Mg concentration does not affect the hot workability. On the other hand, the present inventors have newly discovered that when strong deoxidation is performed on high-Ni alloy steels containing 30% or more Ni, the amount of soluble Mg decreases, causing high concentrations of Mg, similar to S, to segregate at grain boundaries, inducing grain boundary sliding, reducing grain boundary strength, and deteriorating hot workability at 900 to 1000°C.

[0010] Generally, the addition of B is considered effective in improving the hot workability of steel. However, in Cr-containing steel, the addition of B reduces the corrosion resistance and oxidation resistance, so there is a limit to how much the addition of B can improve the hot workability.

[0011] As a result of extensive research, the inventors have found that by reducing the amount of B added as much as possible and incorporating Ti, B and Ti are co-precipitated at the grain boundaries, which enables B and Ti to be segregated at the grain boundaries in preference to Mg and S, and that the addition of a small amount of B improves hot workability.

[0012] Furthermore, the present inventors have found that hot workability can be further improved by adjusting the contents of Mg, B, and Ti in a well-balanced manner.

[0013] Hereinafter, a high Ni alloy steel according to an embodiment of the present invention will be described. The high Ni alloy steel of this embodiment contains, in mass %, C: 0.001% to 0.15%, Si: 0.10% to 1.00%, Mn: 0.10% to 1.50%, P: 0.030% or less, S: 0.0015% or less, Ni: 30.0% to 50.0%, Cr: 20.0% to 30.0%, Mo: 0.01% to 0.80%, Cu: 0.01% to 0.50%, and Al: 0. It is characterized by containing 0.020% to 0.300%, N: 0.12% to 0.30%, V: 0.01% to 0.50%, W: 0.01% to 3.00%, Nb: 0.30% to 0.90%, Ca: 0.0001% to 0.0050%, Mg: 0.010% or less, B: 0.0030% or less, Ti: 0.20% or less, and the remainder being iron and impurities. Moreover, it is preferable that the high Ni alloy steel of this embodiment further satisfies the following formula (1). Mggb=27Mg-160B-0.06Ti+0.5≦0.015 …(1) In the formula (1), Mggb is the grain boundary segregation parameter of Mg, and Mg, B, and Ti are the contents (mass%) of each element.

[0014] The present invention will be described in detail below. First, the reasons for limiting the component elements of the high Ni alloy steel according to the present invention will be described. Note that the notation of % in the chemical composition means % by mass unless otherwise specified.

[0015] <C:0.001%~0.15%> C is added to ensure strength. However, adding a large amount of C deteriorates workability and increases Cr carbides, which reduces corrosion resistance, so the upper limit is set to 0.15%. When high-process forming is performed, the upper limit is preferably set to 0.10%, more preferably 0.07% or 0.03%. However, since excessive reduction of C increases the cost of refining, the lower limit is set to 0.001%. The lower limit may be 0.010% or more, or 0.020% or more.

[0016] <Si:0.10%~1.00%> Si is an element added as a deoxidizing element during refining and is also an element that improves oxidation resistance and high-temperature strength. Reducing the excessive Si content leads to an increase in refining cost and a decrease in oxidation resistance, so the lower limit is set at 0.10% or more. However, adding a large amount may harden the material and there is a risk of deterioration in workability, so the upper limit is set at 1.00% or less. When performing highly processed forming, it is preferable to set the upper limit at 0.40% or less.

[0017] <Mn: 0.10% - 1.50%> Mn has the effect of increasing the stability of the austenite phase and improving heat resistance. However, if the Mn content is too high, hot workability deteriorates and oxidation resistance at high temperatures significantly deteriorates. Therefore, the Mn content is set at 1.50% or less. On the other hand, Mn is mixed in from raw material scrap etc., and it is necessary to reduce the use of scrap to significantly reduce the Mn content. Reducing the use of scrap leads to an increase in cost. Therefore, the Mn content is set at 0.10% or more.

[0018] <P: 0.030% or less> P is an element harmful to hot workability and toughness. P is an element inevitably mixed in from raw materials and has the effect of increasing solidification cracking susceptibility, so the upper limit is set at 0.030% or less. From the perspective of improving workability, it is preferably 0.025% or less. Since it is difficult to remove P during refining, the lower limit of the P content may be 0.001% or more, or even 0.002% or more.

[0019] <S: 0.0015% or less> S is an element harmful to hot workability and corrosion resistance. In particular, when a large amount of S is contained, it segregates at the grain boundaries and weakens the grain boundary bonding force, thus reducing hot workability. S is an impurity contained in raw materials, but the upper limit is set at 0.0015% or less. The lower the content, the better the hot workability and corrosion resistance, so it is preferably 0.0010% or less. However, reducing the excessive S content increases the desulfurization load and the refining cost, so the lower limit may be 0.0001% or more.

[0020] <Ni:30.0%~50.0%> Ni is a powerful austenite-stabilizing element and is an extremely important element for stabilizing the austenite parent phase and ensuring corrosion resistance and heat resistance, so the lower limit is set to 30.0% or more. From the viewpoints of manufacturing stability and heat resistance, it is preferable to set it to 35.0% or more. On the other hand, an excessive increase in Ni content not only increases alloy costs but also reduces hot workability, so the upper limit is set to 50.0% or less. From the viewpoint of hot workability, it is preferable to set the upper limit to 45.0% or less or 39.0% or less.

[0021] <Cr:20.0%~30.0%> In the present invention, Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If the Cr content is less than 20.0%, these effects are not achieved, so the Cr content is set to 20.0% or more. From the viewpoint of ensuring oxidation resistance and corrosion resistance, the lower limit is preferably set to 22.0% or more or 23.0% or more. On the other hand, since adding a large amount of Cr reduces workability and toughness, the upper limit is set to 30.0% or less. From the viewpoint of manufacturing stability, the Cr content is preferably set to 28.0% or less or 27.0% or less, and more preferably 25.0% or less.

[0022] <Mo:0.01%~0.80%> Mo is an element that increases strength. Therefore, the Mo content is set to 0.01% or more. The Mo content may be 0.08% or more. On the other hand, Mo is an expensive element, and in the steel of the present invention, the upper limit is set to 0.80% or less from the viewpoint of suppressing the alloy cost of the steel. Furthermore, excessive addition of Mo increases the deformation resistance during hot working, which may increase the rolling load and cause casting cracks. The preferred upper limit is 0.70% or less or 0.45% or less.

[0023] <Cu:0.01~0.50%> Cu is an element that dissolves in the austenite matrix and has the effect of increasing high-temperature strength. Therefore, the Cu content is preferably 0.01% or more. On the other hand, if the Cu content is excessive, edge cracks may occur during hot rolling. Therefore, the Cu content is set to 0.50% or less. Preferably, it is 0.30% or less or 0.10% or less.

[0024] <Al:0.020%~0.300%> Al is an element that constitutes the intermetallic compound γ', which contributes to precipitation strengthening. Compared to Ti and Nb, Al improves heat resistance without deteriorating hot workability, so it is added in an amount of 0.020% or more. On the other hand, excessive Al content lowers the melting point of the alloy, making hot working at high temperatures difficult. Therefore, the upper limit is set to 0.300% or less. From the viewpoint of ensuring sufficient heat resistance, the upper limit is preferably set to 0.200% or less or 0.100% or less.

[0025] <N:0.12%~0.30%> Since N may form nitrides and reduce workability, the upper limit is set to 0.30%. When the degree of workability is severe, the content is preferably set to 0.25% or 0.20%. However, since excessive reductions increase refining costs, the lower limit is set to 0.12% or more. From the viewpoint of manufacturing stability, the lower limit may be set to 0.15% or more.

[0026] <V:0.01~0.50%> V is an element that contributes to improving high-temperature strength through solid-solution strengthening. To achieve this effect, the V content is set to 0.01% or more, preferably 0.02% or more. On the other hand, V combines with C and N to form carbides and nitrides. If the V content is too high, coarse carbides and nitrides are formed, deteriorating the workability of the material. Therefore, even if V is contained, the V content is set to 0.50% or less, preferably 0.30% or less.

[0027] <W:0.01~3.00%> W is an element that improves high-temperature strength as a solid-solution strengthening element. Therefore, the W content is set to 0.01% or more. Preferably, it is 0.10% or more or 0.20% or more. On the other hand, when a large amount of W is contained, in addition to an increase in alloy cost, the deformation resistance during hot working increases, making it difficult to perform hot rolling to a predetermined plate thickness. Therefore, the W content is set to 3.00% or less. Preferably, it is 2.50% or less or 2.00% or less.

[0028] <Nb: 0.30% - 0.90%> Nb is contained to ensure high-temperature strength. The Nb content is set to 0.30% or more. Preferably, it is 0.40% or more. On the other hand, Nb is an element that lowers the melting point of the alloy and makes hot working at high temperatures difficult. Therefore, the Nb content is set to 0.90% or less. Preferably, it is 0.70% or less, more preferably 0.50% or less.

[0029] <Ca: 0.0001% - 0.0050%> Ca is an element that has the effect of improving hot workability. When hot workability is improved, the manufacturing cost can be reduced. To obtain this effect, the Ca content is set to 0.0001% or more. On the other hand, when the Ca content becomes large, troubles such as clogging of the molten metal nozzle during casting occur, making manufacturing extremely difficult. Therefore, the Ca content is set to 0.0050% or less. Preferably, it is 0.0040% or less.

[0030] <Mg: 0.010% or less> Mg is an element that has the effect of improving hot workability. When hot workability is improved, the manufacturing cost can be reduced. The Mg content is preferably set to 0.0001% or more or 0.0005% or more. On the other hand, when the Mg content becomes large, troubles such as clogging of the molten metal nozzle during casting occur, making manufacturing extremely difficult. Furthermore, Mg is an element that tends to segregate at the grain boundaries of the steel when the Ni content in the steel is high. When Mg segregates at the grain boundaries, it reduces the grain boundary bonding force like S, thus reducing hot workability. Therefore, the Mg content is set to 0.010% or less. Preferably, it is 0.005% or less.

[0031] <B: Less than 0.0030%> B is an element that segregates at the grain boundaries, forms an intermetallic compound between Fe and B at the grain boundaries, and strengthens the grain boundaries, thereby suppressing slip at the grain boundaries and contributing to an improvement in high-temperature strength. Further, B has a tendency to segregate at the grain boundaries preferentially to Mg and S, and has the effect of reducing the segregation of Mg and S. Therefore, the B content is preferably 0.0001% or more. On the other hand, when a large amount of B is contained, the precipitation of borides becomes remarkable, and the corrosion resistance and hot workability are significantly deteriorated. Therefore, the B content is set to 0.0030% or less. Preferably, it is 0.0020% or less.

[0032] <Ti: 0.20% or less> Ti is an element that constitutes the intermetallic compound γ' that contributes to precipitation strengthening, and is an important element for ensuring heat resistance together with Ni and Al. Further, Ti co-precipitates at the grain boundaries together with B, and further reduces the segregation of Mg and S that causes grain boundary embrittlement. Therefore, the Ti content is preferably 0.001% or more. On the other hand, excessive addition of Ti causes deterioration of hot workability and an increase in rolling load, so the Ti content is set to 0.20% or less. Preferably, it is 0.15% or less or 0.10% or less, and more preferably 0.05% or less.

[0033] Furthermore, the high-Ni alloy steel of the present embodiment preferably contains any one or two or more of the following Group A, Group B, or Group C in mass%.

[0034] [Group A] Any one or two or more of Ta: 0.001 to 0.50%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.05%, Hf: 0.001 to 0.10%.

[0035] [Group B] A total of 0.001 to 0.050% of any one or two or more of La, Ce, Nd, and Pr.

[0036] [Group C] A total of 0.0010% or less of any one or two or more of Zn, Pb, and Bi.

[0037] About [Group A] <Ta:0.001~0.50%> Like V and Nb, Ta improves the high-temperature strength of steel, particularly high-temperature creep strength, by finely precipitating carbides and nitrides at operating temperatures. Therefore, Ta can be added in an amount of 0.001% or more, preferably 0.01% or more, or 0.10% or more, as needed. Like V, the combined addition of Ta and Nb increases the amount of nitride aging precipitation in the operating temperature range compared to the addition of Nb alone. However, like V and Nb, excessive Ta addition increases susceptibility to region II embrittlement, adversely affecting hot workability. Therefore, the upper limit of Ta in this chemical composition is set to 0.50% or less. The preferred upper limit is 0.40% or less.

[0038] <Co:0.01~1.00%> Co is an element effective in improving the high-temperature structural stability and corrosion resistance of the alloy, and to improve these properties, it may be contained in an amount of 0.01% or more, preferably 0.02% or more, and more preferably 0.10% or more. However, since Co is an expensive element, if it is contained in an amount exceeding 1.00%, the effect commensurate with the cost will not be obtained, so the upper limit is set at 1.00%. The preferred upper limit of Co is 0.80% or less, and more preferably 0.50% or less.

[0039] <Sn:0.001~0.05%> Sn is an element that improves the corrosion resistance and high-temperature creep strength of steel when contained in an amount of 0.001% or more, preferably 0.005% or more, and can be added as needed. However, since Sn content exceeding 0.05% reduces hot workability, the upper limit is set to 0.05%.

[0040] <Hf:0.001~0.10%> Hf can be contained as necessary, as its content of 0.001% or more, preferably 0.005% or more or 0.050% or more, has the effect of fixing P and S, thereby improving the solidification cracking susceptibility, hot workability, and high-temperature oxidation resistance of steel. On the other hand, a large amount exceeding 0.10% forms coarse nitrides, which has an adverse effect on productivity. Therefore, the upper limit of the Hf content is set to 0.10% or less.

[0041] Regarding Group B <La + Ce + Nd + Pr: 0.001 - 0.050%> La, Ce, Nd, and Pr, when contained in a total amount of 0.001% or more, preferably 0.005% or more, can fix P and S, thereby improving the oxidation resistance and hot workability of steel. On the other hand, when the total amount of these elements exceeds 0.050%, coarse oxides and nitrides are generated, significantly impairing productivity such as nozzle clogging during refining and an increase in surface defects. Therefore, the upper limit of the content is set to 0.050% or less in total for these elements. As for the addition method of these elements, there are methods such as addition with each metal or alloy, and addition with mischmetal.

[0042] Regarding Group C <Zn + Pb + Bi: 0.0010% or less> Zn, Pb, and Bi also significantly reduce the hot workability in austenite single-phase alloys, so it is necessary to strictly define the upper limit. The total of Pb, Zn, and Bi is set to 0.0010% or less. Preferably, Pb ≤ 0.0010%, Zn ≤ 0.0010%, and Bi ≤ 0.0010%.

[0043] Regarding Formula (1) The high-Ni alloy steel of this embodiment preferably satisfies the above chemical composition and also satisfies the following Formula (1).

[0044] Mggb = 27Mg - 160B - 0.06Ti + 0.5 ≤ 0.015 …(1) However, in Formula (1), Mggb is the grain boundary segregation parameter of Mg, and Mg, B, and Ti are the contents (mass%) of each element, respectively.

[0045] In this embodiment, both Mg and B improve hot workability. However, if the amount of B is excessive relative to the amount of Mg, boride, a eutectic structure of Fe, Cr, and B, may precipitate, reducing corrosion resistance and oxidation resistance. Furthermore, Ti enhances the effect of adding B, but if added in excess relative to Mg and B, it promotes the precipitation of Ti(C,N), which can lead to nozzle clogging during casting. Therefore, in this embodiment, it is preferable that the contents of Mg, B, and Ti satisfy the relationship of the above formula (1). By satisfying the above formula (1), castability can be further improved, and corrosion resistance and oxidation resistance can also be improved.

[0046] The high Ni alloy steel according to this embodiment may contain the above elements (essential elements), with the remainder being Fe and impurities. Impurities refer to components that are unintentionally contained in the steel sheet from raw materials or other manufacturing steps during the manufacturing process. However, in order to improve various properties, the following elements may be contained in place of part of Fe. In order to reduce alloy costs, it is not necessary to intentionally add these optional elements to the steel, so the lower limit of the content of each of these optional elements is 0%.

[0047] Next, a method for producing the high Ni alloy steel of this embodiment will be described. For example, raw materials are melted in a vacuum melting furnace to produce molten steel having the above-mentioned chemical composition, and this molten steel is then, for example, continuously cast to obtain a slab made of the high Ni alloy steel of this embodiment. Furthermore, the obtained slab may be hot-rolled to adjust the plate thickness. Furthermore, annealing may be performed after hot rolling.

[0048] Furthermore, the shape of the cast piece obtained by casting is not limited to a slab, but may be a bloom or a billet. Furthermore, casting is not limited to continuous casting, but may be performed by pouring molten steel into a mold of a predetermined shape. The high Ni alloy steel may be in the form of a slab, bloom, or billet, or may be in the form of a steel plate, steel bar, or the like. [Example]

[0049] Examples of the present invention will be described below, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0050] High-Ni alloy steels having the chemical compositions shown in Tables 1 to 3 were melted in a vacuum and cast to produce rectangular ingots. Round bar-shaped test pieces with a diameter of 8 mm and a length of 80 mm were cut out from the ingots. The longitudinal direction of the test pieces was aligned with the direction from the bottom of the mold toward the surface of the molten steel during casting. The test pieces were then heat-treated at 1220°C for 60 minutes.

[0051] Next, the test specimens were subjected to a thermostatic test to evaluate their hot workability. The test specimens were heated from room temperature to 1220°C over 60 seconds, with the soaking zone length set to 15 mm, and then held there for 60 seconds. The specimens were then cooled to 900°C at an average cooling rate of 20°C / s, held there for 60 seconds, and then subjected to a tensile test at 900°C. The crosshead speed in the tensile test was 20 mm / s. Other conditions conformed to JIS Z 2241:2011. The reduction of area (RA), which is the ratio of the original cross-sectional area of ​​the test specimen to the cross-sectional area of ​​its smallest part after the test, was then measured. Test specimens with an RA of 60.0% or greater were deemed to have acceptable hot workability. The results are shown in Table 3. The evaluation criteria in Table 3 were as follows: ◎ and ◯ indicate acceptable results.

[0052] ◎: RA is over 63.0% 〇: RA is 60.0-63.0% ×: RA is less than 60.0%

[0053] As shown in Tables 1 to 3, invention examples M1 to M26 satisfied the chemical compositions of the present invention and had good hot workability. On the other hand, Comparative Examples M27 to M35 did not satisfy the chemical composition of the present invention and were inferior in hot workability.

[0054] [Table 1]

[0055]

Table 2

[0056]

Table 3

Claims

1. In mass%, C: 0.001% to 0.15%, Si: 0.10% to 1.00%, Mn: 0.10% to 1.50%, P: 0.030% or less, S: 0.0015% or less, Ni: 30.0% to 50.0%, Cr: 20.0% to 30.0%, Mo: 0.01% to 0.80%, Cu: 0.01% to 0.50% Al: 0.020% to 0.300%, N: 0.12% to 0.30%, V: 0.01% to 0.50%, W: 0.01% to 3.00%, Nb: 0.30% to 0.90%, Ca: 0.0001% to 0.0050%, Mg: 0.010% or less, B: 0.0030% or less, Ti: 0.20% or less, Remainder: High Ni alloy steel consisting of iron and impurities.

2. The high Ni alloy steel according to claim 1, further satisfying the following formula (1): Mggb=27Mg-160B-0.06Ti+0.5≦0.015...(1) In the formula (1), Mggb is the grain boundary segregation parameter of Mg, and Mg, B, and Ti are the contents (mass%) of each element.

3. The high Ni alloy steel according to claim 1 or 2, further comprising, in mass %, one or more of the following Group A, Group B, or Group C: [Group A] One or more elements selected from the group consisting of Ta: 0.001 to 0.50%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.05%, and Hf: 0.001 to 0.10%. [Group B] One or more of La, Ce, Nd and Pr in total of 0.001 to 0.050%. [Group C] Zn, Pb and Bi, any one or more of which is 0.0010% or less in total.

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