JP3587271B2 - Semi-austenite precipitation hardened stainless steel with excellent cold workability - Google Patents

Description

【００４１】
固溶化処理後の組織の指標については、組織の８５％以上がオーステナイト組織を示すものをγ、逆にマルテンサイト組織を示すものをα’とし、熱処理および圧造を施しても３％以上のδフェライトが存在する場合にδの項目をつけ加えることにした。
【００４２】
材料の熱間加工性については、１１００℃における高温高速引張試験（グリーブル試験）における絞り値が一般的な圧延可能の指標である６０％を超えるものを○、超えないものを×としている。
【００４３】
耐食性については、耐孔食性としてＪＩＳの６％塩化第二鉄溶液の浸漬試験を行い、その腐食減量を示している。
【００４４】
固溶化処理後の組織はＮｉ−ｂａｌ．とＭＡＩにより整理でき、発明鋼が全てγ組織を示すのに際し、比較鋼でＭＡＩが−５以下のものについてはα’（マルテンサイト）組織を示している。加えて、Ｎｉ−ｂａｌ．が−４以下の鋼種についてはδフェライトの生成が認められる。また、Ｎｉ−ｂａｌ．が−４以上の鋼種であってもＴｉ、Ａｌが成分範囲以上に多量添加されているものはδフェライトが生成している。
【００４５】
固溶化処理後の硬さは発明鋼が全て２００ＨＶ以下であるのに対し、ＭＡＩが−５以下の比較鋼においては、マルテンサイト組織となっているため硬さが高くなっている。また、比較鋼でＭＡＩが高い鋼種は５０％までの変形抵抗値が高くなっている。さらに、Ｍｄ３０が−４０以下の比較鋼では加工誘起マルテンサイト変態が十分に起こらないため、５０％の変形抵抗が高くなっている。
【００４６】
冷間加工率と変形抵抗の関係について、本発明鋼の一例と従来鋼との比較を図２により詳しく示す。同じセミオーステナイト型析出硬化ステンレス鋼であるＳＵＳ６３１と比較すると、同じ傾向を示しているが、発明鋼はＣ＋Ｎ量を抑制していることとＣｕを添加していることにより変形抵抗が非常に低くなっていることが分かる。Ｃｕ含有のマルテンサイト型析出硬化ステンレス鋼のＳＵＳ６３０は加工硬化率は低いが、固溶化状態の硬さが高いため低加工率での変形抵抗が高くなっている。しかし、本発明鋼では、ＭＡＩの制御により固溶化状態の硬さを下げていることで低加工率側での変形抵抗が大幅に低下していることに加え、Ｃ＋Ｎ量を抑制しているため、高加工率側の変形抵抗も低下している。また、冷間加工性に優れているＳＵＳＸＭ７は固溶化状態での硬さが低いため、低加工率側では変形抵抗が低くなっているが、加工誘起マルテンサイト変態を起こさないため高加工率側での変形抵抗は、本発明鋼よりも高くなっている。以上より、本発明鋼がＳＵＳＸＭ７と同等以上の冷間加工性を有していることが分かる。
【００４７】
【表２】

【００４８】
さらに、表２より６０％冷間加工を施して析出硬化したときの硬さは、Ｍｄ３０が−４０以下の鋼種においては析出硬化特性が不十分になっており、ドリリグタッピングネジの要求特性である４５０ＨＶを満たしていない。従来鋼であるＳＵＳ３０４やＳＵＳＸＭ７及びＳＵＳ４１０では析出硬化しないため冷間加工ままの硬さとなっているため、これも硬さ不足となっており、高硬度を得ることができない。なお、表２の６０％加工後時効硬さはＨＶで示す。
【００４９】
熱間加工性に関して、発明鋼が良好な熱間加工性を示しているのに対し、Ｎｉ−ｂａｌ．が−４以下の比較鋼ではδフェライトの存在により熱間加工性が劣化している。また、Ｎｉ−ｂａｌ．が−３台でもＢ添加がなされていない鋼種は熱間加工性が劣っている。
【００５０】
耐食性に関しては、発明鋼が低Ｃ化、Ｃｒ、Ｎｉの制御とＭｏの添加により従来鋼のＳＵＳ３０４並の耐食性を有しているのに対し、比較鋼のＣが高い又はＭｏの添加されていない鋼種では耐食性が明らかに劣っている。また従来鋼はオーステナイトステンレス鋼を除いて耐食性が十分でないことも明らかである。
【００５１】
【発明の効果】
以上説明したように、本発明の鋼は、固溶化状態ではオーステナイト組織であり、かつＳＵＳＸＭ７並の良好な冷間加工特性を有し、冷間鍛造後に熱処理を施すことにより加工硬化した基地硬さに析出硬化による硬さが加わり非常に高い強度が得られ、硬さ４５０ＨＶ超を満足し、さらに耐食性に関してもＳＵＳ４１０よりも優れ、耐食用途に汎用されているＳＵＳ３０４並の耐食性を有するセミオーステナイト型析出硬化ステンレス鋼で、耐食性に優れたドリリングタッピングネジやボルト等を冷間鍛造にて製造することができ、かつ熱間加工性も良好であるなど、従来の鋼にない優れた効果を有するものである。
【図面の簡単な説明】
【図１】Ｎｉ−ｂａｌ．とδフェライト量の関係を示す図である。
【図２】冷間加工率と変形抵抗の関係を示す図である。[0001]
[Industrial applications]
The present invention is used for drilling tapping screws manufactured by cold forging, bolts, etc., has good cold working characteristics in a solid solution state, and has high precipitation hardening degree after cold working and high hardness is obtained. And a semi-austenite precipitation hardening stainless steel having good corrosion resistance when used.
[0002]
[Prior art]
Conventional drilling tapping screws may be manufactured by cold forging SUS410 of martensitic stainless steel and subjecting it to quenching and tempering. In this case, the hardness characteristics may not be satisfied as it is, and it is necessary to perform a hardening process such as a nitriding process. Furthermore, since corrosion resistance, especially pitting corrosion resistance, is poor, and rust easily occurs in a saltwater atmosphere such as at a beach, it is necessary to compensate the corrosion resistance by performing plating treatment or the like as a countermeasure. As described above, when SUS410 is used, the number of steps is increased, and the cost is increased.
[0003]
In addition, when SUSXM7, a Cu-containing austenitic stainless steel having both excellent cold workability and excellent corrosion resistance comparable to SUS304, is used, the strength after cold working is insufficient. It is necessary to perform a surface hardening treatment. However, these curing treatments increase the number of steps and deteriorate the corrosion resistance.
[0004]
Further, SUS631, a semi-austenite precipitation-hardened stainless steel, which provides high strength and better corrosion resistance than SUS410 when cold-worked and then subjected to precipitation hardening treatment, may be used. In this case, in order to obtain high strength, it is necessary to perform cold working at a high working ratio. However, the deformation resistance of SUS631 is much higher than that of Cu-containing austenitic stainless steel, and cold working is difficult. In addition, it has better corrosion resistance than SUS410, but is inferior to SUS304, and has a problem in use under severe corrosive environment.
[0005]
As described above, all types of steel have advantages and disadvantages, and there is a problem that each step is increased in order to compensate for the disadvantages. Therefore, although steel types are being developed to satisfy these characteristics, there is no steel type that satisfies all the characteristics at present.
[0006]
[Problems to be solved by the invention]
The problem to be solved by the present invention is an austenite structure in a solid solution state, having a cold workability equivalent to that of SUSXM7 of Cu-containing austenitic stainless steel, and obtaining a high strength by performing a heat treatment after the cold work. In particular, to provide a material that satisfies hardness HV> 450, which is a required characteristic of a drilling tapping screw, and is superior in corrosion resistance to SUS410, and has a corrosion resistance equivalent to that of SUS304, which is widely used for corrosion resistance. It is.
[0007]
[Means for Solving the Problems]
In order to improve the cold workability, it is necessary that the hardness is low in the solution treatment and the work hardening rate thereafter is low.
[0008]
If the structure in the solution treatment state is changed from a martensite structure to an austenitic structure, the hardness can be reduced. It is said that in order to make the structure in the solid solution state austenite, the martensitic transformation start temperature Ms point is usually controlled and the Ms point is set to room temperature or lower. However, it has been found that the Fe-C-Ni-Cr-Mo-Cu component system cannot be controlled by the general equation of the Ms point proposed by Schaeffler et al. Therefore, the present inventors have repeated studies and have derived a control MAI, and have found that the amount of austenite structure at the time of solution treatment can be controlled by this. In addition, it has been found that the hardness of the solution treatment can be further reduced by suppressing the amounts of C and N dissolved in the matrix to be low.
[0009]
In order to lower the work hardening rate during cold working, it is effective to add Cu as seen in Cu-containing austenitic stainless steel. However, it has been found that the Cu-containing low C martensitic stainless steel has a lower work hardening rate than the Cu-containing austenitic stainless steel. However, since Cu-containing low C martensitic stainless steel has a high initial hardness, the initial shape and deformation resistance are inferior to those of Cu-containing austenitic stainless steel.
[0010]
Having obtained the above knowledge, the inventors control the amount of C + N and MAI to lower the hardness of the solution-dissolved structure as an austenite structure, and easily control the austenitic structure by cold working under control of Md30. It has been found that, as for the type of steel having a site structure, the martensitic transformation progresses as the processing proceeds, and the work hardening rate decreases, and cold workability higher than that of Cu-containing austenitic stainless steel can be obtained.
[0011]
Further, the work-induced martensite structure has a high degree of precipitation hardening because Cu, Al, Ti, and Nb are dissolved in a supersaturated form. Therefore, when heat treatment is performed after cold working, precipitation hardening is added to the work hardened base hardness, and very high strength is obtained.
[0012]
It has also been found that this component system has the same corrosion resistance as SUS304, particularly pitting corrosion resistance, by lowering C, controlling the amounts of Cr and Ni, and adding Mo.
[0013]
Therefore, it is clear that, if the above conditions are satisfied, the material has a characteristic capable of solving the problem with the current material for drilling tapping screws.
[0014]
However, in this component system, δ ferrite is generated at the time of solidification, as will be described later, so that a two-phase structure is easily formed and hot workability is inferior. Therefore, for mass production, it is necessary to improve the hot workability by suppressing the amount of δ ferrite. δ ferrite content and Ni-bal. About the relationship of Ni-bal. When -4 or more, it was found that the amount of δ ferrite can be reduced by heat treatment and forging so as not to affect the hot workability, thereby improving the hot workability. In addition, by adding B, the hot workability can be further improved. In particular, Ni-bal. However, it was clarified that if B was not added between -3 and -4, sufficient hot workability could not be obtained even if heat treatment and forging were performed.
[0015]
From these, various characteristics can be satisfied in the following ranges.
That is, according to the first aspect of the present invention, the mass ratio of C: 0.030% or less, Si: 0.5% or less, Mn: 1.0% or less, P: 0.04% Hereafter, S: 0.03% or less, Ni: 6.0 to 9.0%, Cr: 12 to 19%, Mo: 0.5 to 4%, Cu: 2.5 to 5%, Nb: 0. 1 to 0.4%, B: 0.001 to 0.005%, C + N is 0.04% or less, the balance is Fe and unavoidable impurities, and Ni-bal. Is -4 or more, Md30 is -40 or more, MAI is -5 <MAI <40 (MAI: martensite-austenite structure index), and 85% or more has an austenite structure in a solution treatment state. Is a semi-austenite type precipitation hardening stainless steel excellent in cold workability characterized by having a martensitic structure more easily and having excellent precipitation hardening characteristics having a hardness of more than 450 HV .
[0016]
However,
Ni-bal. = Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo) -0.5Si-0.2Nb + 10
Md30 = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.5Mo-68Nb
MAI = 2391 (C + N) + 39.9Si + 47.8Mn + 59.8Cr + 87.7Ni-1653
[0017]
According to the invention of claim 2, in addition to the chemical components in the means of claim 1, one or two selected from Al: 0.5% and Ti: 0.5% or less by mass ratio are contained. The semi-austenitic precipitation hardened stainless steel having excellent cold workability according to the means of claim 1 having high strength.
[0018]
[Action]
The reasons for adding each component of the present invention are shown below.
C: When C is added in excess of 0.030%, the hardness of the martensite structure increases and the cold workability becomes poor. Further, C forms a carbide with Cr and Mo, which are effective elements for corrosion resistance, so that Cr and Mo dissolved in the matrix are reduced to deteriorate the corrosion resistance. In order to suppress these, the C content is set to 0.030% or less, and more desirably 0.020% or less.
[0019]
Si: Si is a very effective element as a deoxidizer, but if it exceeds 0.5%, the corrosion resistance, especially the pitting corrosion resistance, is deteriorated, and the hardness at the time of solution heat treatment is increased to improve the cold workability. Because of deterioration, the upper limit was set to 0.5%.
[0020]
Mn: Mn is an element effective for increasing the austenite tendency in the solution state and improving the strength and toughness. However, if it exceeds 1%, it suppresses martensitic transformation at the time of cold working. Increases the work hardening rate and deformation resistance. In addition, the precipitation hardening characteristics after cold working are impaired. Therefore, the upper limit of Mn is set to 1%.
[0021]
P: P is an effective element for improving the machinability, but if it exceeds 0.04%, adverse effects such as deterioration of hot workability and deterioration of impact toughness are increased. %. More preferably, the upper limit is made 0.03% or less.
[0022]
S: S is a very effective element for improving the machinability, but if it exceeds 0.03%, the corrosion resistance deteriorates, so the upper limit was made 0.03%. More preferably, the upper limit is made 0.02% or less.
[0023]
Ni: Ni is an indispensable element because it has a strong tendency to austenite and has precipitation hardening characteristics. If it is less than 6%, the effect of suppressing the formation of δ ferrite and the effect of suppressing the formation of a martensite structure in a solid solution state to form an austenite structure cannot be sufficiently achieved, so the lower limit was made 6%. However, if it exceeds 9%, the MAI becomes too large to form a stable austenite structure, the work hardening rate and deformation resistance during cold working become high, and the precipitation hardening properties after cold working are lost. %. Further, it is desirably 6% to 8%.
[0024]
Cr: Cr is an indispensable element for stainless steel, and needs to be 12% or more to satisfy the corrosion resistance required for stainless steel. However, the tendency of ferrite formation is strong, and if a large amount is added exceeding 19%, Ni-bal. Is reduced, the amount of δ ferrite increases, and hot workability is impaired, so the upper limit was made 19%. Further, it is desirably 14% to 18%.
[0025]
Mo: Mo is an element effective for improving corrosion resistance, particularly pitting corrosion resistance. In the present component system, when 0.5% or more is added, the lower limit is set to 0.5% because it shows the same corrosion resistance as SUS304. Further, if it exceeds 4%, Ni-bal. Decreases, the amount of δ ferrite increases, impairs hot workability, and the Md30 is lowered to increase the work hardening rate and deformation resistance during cold working, and the precipitation hardening properties after cold working deteriorate. Therefore, the upper limit is set to 4%. Further, it is desirably 0.5% to 2.5%.
[0026]
Cu: If Cu is less than 2.5%, the precipitation hardening property is impaired and the effect of improving cold workability is not sufficient, so the lower limit was made 2.5%. However, if added in excess of 5%, hot workability and toughness deteriorate, so the upper limit was made 5%. Further, it is desirably 2.5% to 4%.
[0027]
Nb: Nb is an indispensable element for fixing C and N and improving the degree of precipitation hardening. However, if the content is less than 0.1%, its effect is small, so the lower limit is set to 0.1%. Further, when added in a large amount exceeding 0.4%, toughness and hot workability are degraded, Md30 is lowered to increase hardness and deformation resistance during cold working, and precipitation hardening characteristics after cold working. Therefore, the upper limit was set to 0.4%.
[0028]
B: B is an element that improves hot workability in a solidified structure state, but B must be contained at 0.001% or more for the effect to be effective. However, the effect is saturated even if 0.005% or more is added, so the upper limit is made 0.005%.
[0029]
C + N: N is a very effective element for improving pitting corrosion resistance, but forms a solid solution in the matrix similarly to C and increases the hardness of the martensite structure. If the total amount of C and N exceeds 0.04%, the hardness in the solution state increases and the cold workability deteriorates. Therefore, the upper limit of C + N is set to 0.04%.
[0030]
Al: Al is very effective as a deoxidizing agent and is an element that increases the degree of precipitation hardening. However, since it is a ferrite-forming element, if added in excess of 0.5%, the amount of δ-ferrite increases, and To impair workability, the upper limit is set to 0.5%.
[0031]
Ti: Ti is an element that fixes C and N and increases the degree of precipitation hardening, but has a strong tendency to form ferrite, and if added over 0.5%, the amount of δ ferrite increases and impairs hot workability. Therefore, the upper limit is set to 0.5%.
[0032]
Ni-bal. : Ni-bal. Is an index effective for estimating the tissue after coagulation. When the calculated value becomes a negative value, a δ ferrite phase is formed. The Ni-bal. Is based on the nickel balance proposed by a renowned Schaeffler, and is obtained by multiple analysis from actual measurement values, and includes Cu and Nb terms contained in the steel of the present invention and not included in the Schaeffler equation.
[0033]
When the amount of δ ferrite increases, hot workability deteriorates. The reason is that, due to the difference in deformation resistance from the matrix, the interface becomes a crack starting point at the time of hot working. In the composition of the steel of the present invention, when the structure has a δ ferrite content of 3% or more during rolling, hot workability is inferior and rolling becomes difficult.
[0034]
FIG. 1 shows Ni-bal. And the relationship between the amount of δ ferrite and δ ferrite. The amount of δ ferrite is Ni-bal. It can be seen that it increased due to the decrease in As shown in FIG. 1, the amount of this δ ferrite is reduced by homogeneous heat treatment and forging. However, Ni-bal. Is less than -4, the δ ferrite does not decrease to 3% or less even if these treatments are performed, and the hot workability deteriorates. Then, Ni-bal. Was set to -4. More preferably, the lower limit is set to -3.
[0035]
Md30: An index for estimating that martensitic transformation occurs due to cold working, and its meaning is the temperature at which 50% martensitic transformation occurs when 30% cold working is performed. The higher the value, the more easily the martensitic transformation occurs by cold working. When Md30 is -40 or more, the amount of martensite transformation increases by cold working, and sufficient precipitation hardening characteristics are obtained. Therefore, the lower limit was set to −40.
[0036]
MAI is an index indicating whether the structure after solution heat treatment becomes austenite or martensite, and is an empirical formula derived as a result of research conducted by the inventors. If this value is 0 or more, the structure becomes almost an austenite structure. When the value is less than 0, a martensitic structure is generated, and as the value decreases, the martensite structure increases.
[0037]
In the present invention steel, lever exceed -5 when subjected to solution heat treatment, mainly of the following hardness 200HV has an austenitic structure, since the good cold working property is obtained, the lower limit -5 Was exceeded . On the other hand, when the value is 40 or more, a stable austenite structure is formed, and even when cold working is performed, martensitic transformation becomes difficult. Therefore, the upper limit was set to less than 40.
[0038]
【Example】
The characteristics of the steel of the present invention will be clarified by examples in comparison with conventional steel and comparative steel. Table 1 shows the chemical components of these test materials and the values of the respective control formulas, wherein 1 to 9 are the present invention steels. Among them, 1 to 5 are invention steels according to claim 1, 6 to 9 are invention steels according to claim 2, 10 to 25 are comparative steels, and 26 to 30 are conventional steels.
[0039]
Table 2 shows the state and hardness of the structure after the solution treatment at 1040 ° C. after the ingot was heat-treated and pressed, and when the solution-treated material was subjected to 20% and 50% cold working. FIG. 3 shows the deformation resistance value, the hardness after the precipitation hardening treatment at 480 ° C. after the 60% cold working, the hot workability of the material, and the corrosion resistance in the precipitation hardened state.
[0040]
[Table 1]

[0041]
Regarding the index of the structure after the solution treatment, γ indicates that 85% or more of the structure shows an austenitic structure, and α ′ indicates that the structure shows a martensite structure, and δ of 3% or more even after heat treatment and forging. When ferrite is present, the item of δ is added.
[0042]
Regarding the hot workability of the material, ○ indicates that the drawing value in a high-temperature high-speed tensile test (Greble test) at 1100 ° C. exceeds 60%, which is a general index of rollability, and × indicates that the drawing value does not exceed 60%.
[0043]
Regarding the corrosion resistance, a immersion test of a JIS 6% ferric chloride solution was performed as pitting corrosion resistance, and the corrosion weight loss is shown.
[0044]
The structure after the solution treatment is Ni-bal. When the invention steels all show a γ structure, the comparative steels having a MAI of −5 or less show an α ′ (martensite) structure. In addition, Ni-bal. -4 ferrite or less, the formation of δ ferrite is recognized. In addition, Ni-bal. However, even if the steel type is -4 or more, δ ferrite is generated when Ti and Al are added in a large amount beyond the component range.
[0045]
The hardness after solution treatment is 200 HV or less for all invention steels, whereas the comparative steel with MAI of -5 or less has a higher hardness due to the martensite structure. In the comparative steel, a steel type having a high MAI has a high deformation resistance value up to 50%. Further, in the comparative steel having Md30 of −40 or less, the deformation-induced martensitic transformation does not sufficiently occur, so that the deformation resistance by 50% is high.
[0046]
FIG. 2 shows a comparison between an example of the steel of the present invention and a conventional steel in detail with respect to the relationship between the cold working rate and the deformation resistance. Compared to the same semi-austenitic precipitation hardening stainless steel, SUS631, it shows the same tendency, but the invention steel has a very low deformation resistance due to the suppressed C + N content and the addition of Cu. You can see that. SUS630, a Cu-containing martensitic precipitation hardening stainless steel, has a low work hardening rate, but has high deformation resistance at a low working rate due to its high hardness in a solid solution state. However, in the steel of the present invention, since the hardness in the solid solution state is reduced by controlling the MAI, the deformation resistance on the low working ratio side is significantly reduced, and the C + N amount is suppressed. Also, the deformation resistance on the high working ratio side is reduced. Further, SUSXM7, which is excellent in cold workability, has a low hardness in a solid solution state, and thus has a low deformation resistance at a low work ratio, but does not cause work-induced martensitic transformation. Is higher than that of the steel of the present invention. From the above, it is understood that the steel of the present invention has cold workability equal to or higher than that of SUSXM7.
[0047]
[Table 2]

[0048]
Further, as shown in Table 2, the hardness when precipitation hardening was performed by performing 60% cold working was inadequate for steel types with Md30 of -40 or less, and the hardness was insufficient for drilling tapping screws. It does not meet a certain 450 HV. Conventional steels such as SUS304, SUSXM7 and SUS410 do not precipitate and harden, and therefore have a hardness as it is cold-worked. Therefore, the hardness is also insufficient, and high hardness cannot be obtained. The aging hardness after 60% processing in Table 2 is indicated by HV.
[0049]
With regard to hot workability, the invention steel shows good hot workability, whereas Ni-bal. However, in the comparative steels having -4 or less, hot workability is deteriorated due to the presence of δ ferrite. In addition, Ni-bal. However, even in the case of -3 units, the steel type without B addition is inferior in hot workability.
[0050]
Regarding the corrosion resistance, the invention steel has the same corrosion resistance as SUS304 of the conventional steel by lowering C, controlling Cr and Ni and adding Mo, whereas the comparative steel has higher C or no addition of Mo. The corrosion resistance of steel grade is clearly inferior. It is also clear that the conventional steels have insufficient corrosion resistance except for austenitic stainless steel.
[0051]
【The invention's effect】
As described above, the steel of the present invention has an austenitic structure in the solution state and has good cold workability comparable to SUSXM7, and has a base hardness work hardened by heat treatment after cold forging. The hardness by precipitation hardening is added to the steel , so that very high strength is obtained, the hardness is more than 450HV, and the corrosion resistance is more excellent than SUS410. It is a hardened stainless steel that has excellent effects that conventional steel does not have, such as drilling tapping screws and bolts with excellent corrosion resistance that can be manufactured by cold forging and good hot workability. is there.
[Brief description of the drawings]
FIG. 1. Ni-bal. FIG. 4 is a diagram showing the relationship between and the amount of δ ferrite.
FIG. 2 is a diagram showing a relationship between a cold working rate and deformation resistance.

Claims (2)

In terms of mass ratio, C: 0.030% or less, Si: 0.5% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.03% or less, Ni: 6.0 To 9.0%, Cr: 12 to 19%, Mo: 0.5 to 4%, Cu: 2.5 to 5%, Nb: 0.1 to 0.4%, B: 0.001 to 0. 005%, C + N is 0.04% or less, the balance is Fe and unavoidable impurities, and Ni-bal. Is -4 or more, Md30 is -40 or more, MAI is -5 <MAI <40 (MAI: martensite-austenite structure index), and 85% or more has an austenite structure in a solution treatment state. A semi-austenite type precipitation hardening stainless steel excellent in cold workability characterized by having a martensitic structure more easily and having excellent precipitation hardening characteristics having a hardness of more than 450 HV. However,
Ni-bal. = Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo) -0.5Si-0.2Nb + 10
Md30 = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.5Mo-68Nb
MAI = 2391 (C + N) + 39.9Si + 47.8Mn + 59.8Cr + 87.7Ni-1653 In addition to the chemical components according to claim 1, one or two kinds selected from Al: 0.5% and Ti: 0.5% or less by mass ratio are included to have high strength. The semi-austenitic precipitation hardened stainless steel according to claim 1 having excellent cold workability.

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