JP3939568B2 - Nonmagnetic stainless steel with excellent workability - Google Patents

Description

【００２０】
６０％冷間圧延材及び焼鈍酸洗材それぞれから試験片を切り出し、次の試験によって透磁率及び加工硬化指数ｎを測定した。
〔透磁率の測定〕
切り出し端面の歪による影響を除去するため、１０ｃｍ角に切り出した試験片を電解研磨し、直径５ｍｍの円板形状にした。この円板状試験片を用い磁気天秤により印加磁場−磁化曲線上での磁場１．８ｋＯｅにおける傾きを測定することにより透磁率を求めた。
〔加工硬化指数ｎの測定〕
ＪＩＳ１３Ｂ号試験片を作製し、引張り速度４０ｍｍ／分（歪速度１．３×１０^-2／秒）の引張試験で応力歪曲線を求めた。公称歪３０％，４０％に相当する真歪０．２５，０．３４（それぞれ、ε₀，ε₁）における真応力δ₀，δ₁を求め、次式に従って加工硬化指数ｎ値を算出した。
ｎ値＝ln(δ₁／δ₀)／ln(ε₁／ε₀)
【００２１】
６０％冷間圧延材の透磁率及び焼鈍酸洗材の加工硬化指数ｎを表２に示す。鋼No.１〜５及び鋼No.７〜１１は、本発明で規定した非磁性ステンレス鋼の成分設計条件を満足しており、６０％冷間圧延材の透磁率が０.０１以下，焼鈍酸洗材の加工硬化指数ｎが０．４５以上であった。他方、鋼No.１３〜１７は、Ｎｉ_eq≧１７．０，ＳＦＥ≦２０．０，Ｎｉ_bal≧０の何れかを満足しておらず、０.０１の透磁率は得られなかった。また鋼No.１８，１９は、ＳＦＥが高すぎたため、加工硬化指数ｎも０．４５未満の低い値を示した。
【００２２】
６０％冷間圧延材の透磁率をＮｉ当量Ｎｉ_eqで整理したところ、図１にみられるようにＮｉ_eq≧１７．０で０．０１以下の透磁率が得られることが判った。ただし、鋼No.１６，１７は、Ｎｉ_eq≧１７．０であるにも拘らず０.０１を超える透磁率が示された。この高い透磁率は、Ｎｉバランス値Ｎｉ_balがマイナスであるためにδフェライト相が生成したことが原因である。
また、積層欠陥エネルギーの生成指数ＳＦＥと加工硬化指数ｎとの関係を調査した結果，図２にみられるようにＳＦＥ≦２０．０のときｎ≧０．４５になっており、積層欠陥エネルギーの生成指数ＳＦＥを２０．０以下にすることによって良好な加工性が得られることが確認できた。
更に、非磁性で良好な加工性を示した鋼No.１〜１２について、（Ｃ＋Ｎ）量と硬さとの関係を調査した。図３の調査結果にみられるように、（Ｃ＋Ｎ）≦０.０６質量％以下でビッカース硬さが１２０ＨＶ以下の軟質ステンレス鋼が得られた。
【００２３】

【００２４】
【発明の効果】
以上に説明したように、本発明の非磁性ステンレス鋼は、Ｎｉ当量をＮｉ_eq≧１７．０以上，積層欠陥エネルギーの生成指数をＳＦＥ≦２０．０，Ｎｉバランス値をＮｉ_bal≧０とする成分設計を採用しているため、６０％冷間圧延材にあっても透磁率が０．０１以下と優れた非磁性を維持し、加工誘起変態塑性を利用しなくても良好な加工性を呈する。したがって、従来のＳＵＳ３０５等の安定オーステナイトステンレス鋼ではなし得なかった成形加工が可能で、ＳＵＳ３０４を加工した後で非磁性化するための焼鈍も省略でき、非磁性が要求される電気部品，機械部品，ケーシング等の材料として広範な分野で使用される。
【図面の簡単な説明】
【図１】 Ｎｉ当量Ｎｉ_eqが６０％冷間圧延材の透磁率に及ぼす影響を示したグラフ
【図２】 積層欠陥エネルギーの生成指数ＳＦＥと加工硬化指数ｎとの関係を示すグラフ
【図３】 （Ｃ＋Ｎ）量に応じて硬さが変わることを表したグラフ[0001]
[Industrial application fields]
The present invention relates to stainless steel that exhibits excellent workability and maintains non-magnetism after processing.
[0002]
[Prior art]
Cr-Ni austenitic stainless steel represented by SUS304 exhibits good workability and corrosion resistance, and becomes a nonmagnetic austenitic structure in the annealed state. Therefore, as a nonmagnetic steel plate, as an electrical component, mechanical component, casing, etc. Used in a wide range of applications.
In SUS304 stainless steel, the austenite phase is metastable, so martensitic transformation occurs during the forming process. Generation of work-induced martensite exhibits high ductility based on work-induced transformation plasticity, but the work piece becomes magnetic. Therefore, in applications where non-magnetism is required, a method of erasing magnetism by annealing a metastable steel such as SUS304 that has been formed is employed. Further, by using a material such as SUS305 or SUS316 which is stable in austenite phase and hardly undergoes martensite transformation, non-magnetism is maintained after molding.
[0003]
[Problems to be solved by the invention]
In the method of processing a metastable steel such as SUS304 as a raw material and subsequently annealing the processed product, the processing cost becomes high. That is, since many processed products have a hollow shape, voids increase during annealing, and heat treatment efficiency is significantly reduced. Further, when heat treatment is performed in an air atmosphere, a pickling process for removing scales generated by the heat treatment is required. As a method of avoiding pickling, there is a heat treatment in a non-oxidizing atmosphere, but since an expensive hydrogen gas atmosphere furnace or the like is used, the processing cost is significantly increased.
On the other hand, stable stainless steels such as SUS305 and SUS316 have lower ductility than metastable stainless steels such as SUS304. The low ductility of stable stainless steel is due to the lack of work-induced transformation plasticity effects, and is often not applicable to severe forming processes that require stretchability.
[0004]
[Means for Solving the Problems]
The present invention has been devised to solve such a problem. By adopting a component design in which Ni equivalent, stacking fault energy generation index, and Ni balance amount satisfy specific conditions, workability is improved. An object is to provide an austenitic stainless steel that is excellent and maintains non-magnetism after processing.
[0005]
In order to achieve the object, the nonmagnetic stainless steel of the present invention has C: 0.10% by mass or less, Si: 1.0-6.0% by mass, Mn: 0.2-9% by mass, Ni: 9 -18 mass%, Cr: 12-20 mass%, Mo: 3 mass% or less, Cu: 3 mass% or less, N: 0.10 mass% or less, with the balance of Fe and inevitable impurities, calculated as Ni equivalent Ni _eq is 17.0 or more wherein the laminated product index SFE of fault energy 20.0, wherein the Ni balance value Ni _bal are components adjusted to be 0 or more .
Ni _eq = Ni + Cu + 0.6Mn + 9.69 (C + N) + 0.18Cr-0.11Si ²
SFE = 2.2Ni-1.1Cr-13Si-1.2Mn + 6Cu + 32
Ni _bal = Ni + 30 (C + N) + 0.5Mn + 0.3Cu-1.1 (Cr + 1.5Si + Mo) +8.2
[0006]
In order to ensure good workability, the work hardening index n, which is the gradient of nominal strain 30% and 40% in the true stress-logarithmic elongation strain curve obtained by the tensile test, is 0.45 or more, and C + N is 0. It is preferable that the Vickers hardness of the annealed material is 120 HV or less at 0.06 mass% or less. Further, when the magnetic permeability when cold rolled at a rolling reduction of 60% is 0.01 or less, non-magnetism is maintained even after various processing.
This nonmagnetic stainless steel further has V: 1.0% by mass or less, one or more of Ti, Nb, and Zr: 1.0% by mass or less, B: 0.03% by mass or less, Ca: 0.00%. 1 type (s) or 2 or more types selected from 03 mass% or less can be included.
[0007]
[Action]
In the non-magnetic stainless steel of the present invention, by adopting a component design in which Ni equivalent Ni _eq , stacking fault energy generation index SFE, and Ni balance value Ni _bal are maintained in appropriate ranges, it is excellent in workability and after processing. Non-magnetic is maintained.
[0008]
Ni equivalent Ni _eq is an index representing the contribution of each alloy component to the stability of the austenite phase. Ni equivalent Ni defined as Ni _eq = Ni + Cu + 0.6Mn + 9.69 (C + N) + 0.18Cr-0.11Si ² _{When eq} is 17.0 or more, non-magnetism after processing is secured. In general, in a stainless steel in which the austenite phase is metastable, the austenite phase is transformed into a martensite phase (processing induced transformation) with the introduction of processing strain. Since the produced martensite phase is magnetic, the resulting stainless steel is magnetic.
[0009]
Therefore, by adopting a component design in which the transformation from the austenite phase to the martensite phase is difficult to occur, the magnetization due to the formation of the martensite phase is suppressed. Therefore, the austenite phase is stabilized by indexing the influence of each alloy component on the degree of difficulty in forming the work-induced martensite (α ′) phase. As a result of various investigations and examinations on the influence of each alloy component on the stability of the austenite phase in this component system, when Ni equivalent Ni _eq is 17.0 or more, even when rolled at a reduction ratio of 60%, the permeability is 0.01 or less. It was found that non-magnetism can be obtained. A magnetic permeability of 0.01 or less after rolling at a rolling reduction of 50% is suitable as an index indicating the non-magnetism of stainless steel subjected to various processes.
[0010]
In the austenitic phase with the fcc structure, stacking faults are more likely to occur and work hardening is easier than with the bcc structure. However, a large amount of austenite generating elements are required to ensure nonmagnetic properties. In this respect, in the conventional alloy design based on non-magnetism, work hardening is unavoidable because work hardening is suppressed. On the other hand, in the present invention, an alloy design that easily causes stacking faults is adopted in order to increase work hardening while maintaining non-magnetism. Specifically, the component design that reduces the stacking fault generation energy can improve workability while maintaining non-magnetism.
The present inventors investigated and examined the influence of each alloy component on the stacking fault formation energy from such a viewpoint. As a result, when the production index SFE of stacking fault energy defined as SFE = 2.2Ni-1.1Cr-13Si-1.2Mn + 6Cu + 32 is adjusted to 20.0 or less, excellent workability is obtained without using work-induced transformation plasticity. Was found to be expressed.
[0011]
Austenitic stainless steel has a different tendency to produce a δ ferrite phase during solidification depending on the balance of constituent elements. Although the δ ferrite phase decreases through the manufacturing history of hot rolling, annealing, cold rolling, annealing, etc., it is difficult to completely eliminate the δ ferrite phase. When the magnetic δ ferrite phase remains on the cold-rolled annealed plate, magnetism is exhibited even in the annealed state. The present inventors conducted various investigations and studies on the influence of alloy components on the formation of a δ ferrite phase that inhibits non-magnetism. As a result, the Ni balance value Ni _bal defined by Ni _bal = Ni + 30 (C + N) + 0.5Mn + 0.3Cu−1.1 (Cr + 1.5Si + Mo) +8.2 is an effective index, and the austenite-generating element and ferrite-forming element It was clarified that no δ ferrite phase was formed by setting the Ni balance value Ni _bal to 0 or more.
[0012]
The work hardening index n, which is a gradient between the nominal strain of 30% and 40% in the true stress-logarithmic elongation strain curve determined by the tensile test, is an index that has not been considered in the conventional nonmagnetic stainless steel. From the survey and research results, it was found to correlate with the overhanging property. When the work hardening index n is 0.45 or more, uniform dispersion of strain introduced by the processing is promoted, and the overhang workability is improved. As a result, defects such as necking in practical press molding that also includes deep drawing elements are suppressed, and workability is greatly improved.
[0013]
Ni eq Ni _eq ≧ 17.0, generates index SFE ≦ 20.0 stacking fault energy, so as to satisfy the Ni balance value Ni _bal ≧ 0, the alloy components are selected in the following ranges.
C, N: 0.10% by mass or less are all effective alloy components for stabilizing the austenite phase and suppressing the formation of the δ ferrite phase. However, if contained in a large amount, the austenite phase becomes hard due to solid solution strengthening and the workability is improved. descend. Therefore, the upper limit of the C and N content is set to 0.10% by mass. In particular, in applications where severe workability such as shape freezing is required, it is effective to soften the annealed material to a hardness of 120 HV or less by regulating the total content of C + N to 0.06% by mass or less. is there.
Si: 1.0-6.0 mass%
It is an alloy component added as a deoxidizer at the time of melting, and stacking faults are easily generated as the Si content increases, and work hardening and ductility are improved. Such an effect becomes remarkable when the Si content is 1.0 mass% or more. However, when an excessive amount of Si exceeding 6.0% by mass is contained, not only does the austenite phase harden due to solid solution strengthening, but the workability deteriorates, and it becomes easy to be magnetized due to processing.
[0014]
Mn: 0.2-9 mass%
It is an alloy component that greatly contributes to the improvement of work hardening by maintaining non-magnetism, suppressing the formation of δ ferrite phase, and reducing stacking fault energy. However, addition of an excessive amount of Mn exceeding 9% by mass leads to an increase in inclusions, which causes a deterioration in corrosion resistance and workability.
Ni: 9 to 18% by mass
It is an essential alloy component for austenitic stainless steel, and 9% by mass or more of Ni is necessary to generate an austenitic phase. Ni has a greater effect of suppressing magnetization and δ ferrite phase formation as the content increases. However, the addition of an excessive amount of Ni increases the stacking fault generation energy and decreases work hardening and ductility, so that workability is impaired. And since it is an expensive element, it also becomes a cause which raises steel material cost. For this reason, the upper limit of the Ni content was set to 18% by mass.
[0015]
Cr: 12-20 mass%
It is an alloy component necessary for obtaining the corrosion resistance required for stainless steel, and the corrosion resistance is remarkably improved at 12% by mass or more. In addition, stacking faults are easily generated as the Cr content increases, so that workability is improved and magnetization after processing is suppressed. However, the addition of an excessive amount of Cr exceeding 20% by mass decreases the workability by hardening the material.
Mo: 3% by mass or less Mo is an alloy component effective for improving the corrosion resistance. However, if it is contained in an excessive amount, magnetism appears due to the formation of δ ferrite, and the workability is lowered, so the upper limit of the Mo content is 3% by mass. Set to.
[0016]
Cu: 3% by mass or less Cu is an alloy component exhibiting an action of suppressing magnetization and δ ferrite phase after processing. However, excessive addition of Cu increases the stacking fault formation energy and lowers the workability, so the upper limit of the Cu content was set to 3% by mass.
V: 1.0 mass% or less One, two or more of Ti, Nb, Zr: 1.0 mass% or less (total)
It is an alloy component added as necessary, and has the effect of fixing solid solution strengthening elements such as C and N to suppress the hardening of stainless steel and improve the workability. The effect of addition of each element is saturated at V: 1.0% by mass and / or Ti, Nb, Zr: 1.0% by mass, and even if added more than that, an effect commensurate with the increase cannot be expected.
[0017]
B: 0.03 mass% or less It is an alloy component added as necessary, improves hot workability, and is effective in preventing cracking during hot rolling. However, excessive addition of B will decrease the hot workability, so when adding B, the upper limit is set to 0.03% by mass.
Ca: 0.03% by mass or less Ca is an alloy component added as necessary, and exhibits the effect of improving hot workability, but the Ca addition effect is saturated at 0.03% by mass.
[0018]
【Example】
Various stainless steels having the compositions shown in Table 1 were melted and hot rolled steel sheets having a thickness of 4 mm were manufactured by hot rolling at an extraction temperature of 1230 ° C. The hot-rolled steel sheet was annealed at 1150 ° C. × 1 soaking, cold-rolled to 1.5 mm after pickling, further annealed at 1050 ° C. × 1 soaking, and pickled. A 60% cold-rolled material was obtained by cold-rolling the annealed pickling material to a thickness of 0.6 mm. Some of the cold-rolled materials were further pickled after annealing at 1050 ° C. × soaking for 1 minute (annealed pickled material).
[0019]

[0020]
A test piece was cut out from each of the 60% cold rolled material and the annealed pickling material, and the magnetic permeability and work hardening index n were measured by the following test.
[Measurement of permeability]
In order to remove the influence due to the distortion of the cut end face, the test piece cut into a 10 cm square was electropolished into a disk shape having a diameter of 5 mm. The magnetic permeability was determined by measuring the slope at a magnetic field of 1.8 kOe on the applied magnetic field-magnetization curve using a magnetic balance using this disk-shaped test piece.
[Measurement of work hardening index n]
A JIS 13B test piece was prepared, and a stress-strain curve was obtained by a tensile test at a tensile rate of 40 mm / min (strain rate 1.3 × 10 ⁻² / sec). True stresses δ ₀ and δ ₁ at true strains of 0.25 and 0.34 (ε ₀ and ε ₁ ) corresponding to nominal strains of 30% and 40%, respectively, were obtained, and a work hardening index n value was calculated according to the following equation. .
n value = ln (δ ₁ / δ ₀ ) / ln (ε ₁ / ε ₀ )
[0021]
Table 2 shows the magnetic permeability of the 60% cold-rolled material and the work hardening index n of the annealed pickling material. Steel Nos. 1 to 5 and Nos. 7 to 11 satisfy the component design conditions of the nonmagnetic stainless steel defined in the present invention, and the magnetic permeability of 60% cold-rolled material is 0.01 or less, and annealing. The work hardening index n of the pickling material was 0.45 or more. On the other hand, Steel Nos. 13 to 17 did not satisfy any of Ni _eq ≧ 17.0, SFE ≦ 20.0, and Ni _bal ≧ 0, and a magnetic permeability of 0.01 was not obtained. Steel Nos. 18 and 19 also had a low work hardening index n of less than 0.45 because SFE was too high.
[0022]
When the magnetic permeability of the 60% cold-rolled material was arranged by Ni equivalent Ni _eq , it was found that a magnetic permeability of 0.01 or less was obtained when Ni _eq ≧ 17.0 as seen in FIG. However, the steels Nos. 16 and 17 showed a magnetic permeability exceeding 0.01 in spite of Ni _eq ≧ 17.0. This high magnetic permeability is due to the formation of the δ ferrite phase because the Ni balance value Ni _bal is negative.
Further, as a result of investigating the relationship between the generation index SFE of stacking fault energy and the work hardening index n, as shown in FIG. 2, n ≧ 0.45 when SFE ≦ 20.0. It was confirmed that good workability was obtained by setting the generation index SFE to 20.0 or less.
Furthermore, the relationship between the (C + N) amount and the hardness was investigated for steel Nos. 1 to 12 which were nonmagnetic and showed good workability. As can be seen from the investigation results in FIG. 3, a soft stainless steel having (C + N) ≦ 0.06 mass% and a Vickers hardness of 120 HV or less was obtained.
[0023]

[0024]
【The invention's effect】
As described above, non-magnetic stainless steel of the present invention, Ni eq and Ni _eq ≧ 17.0 or higher, SFE ≦ 20.0 the production index of stacking fault energy, the Ni balance value and Ni _bal ≧ 0 Since the component design is adopted, even in 60% cold-rolled material, the magnetic permeability is 0.01 or less, maintaining excellent non-magnetism, and good workability can be obtained without using work-induced transformation plasticity. Present. Therefore, it is possible to perform forming processing that could not be achieved with conventional stable austenitic stainless steel such as SUS305, and annealing for demagnetization after processing SUS304 can be omitted. , Used in a wide range of fields as casing materials.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of Ni equivalent Ni _eq on the magnetic permeability of a 60% cold-rolled material. FIG. 2 is a graph showing the relationship between stacking fault energy generation index SFE and work hardening index n. ] (C + N) A graph showing the change in hardness according to the amount

Claims (5)

C: 0.10 mass% or less, Si: 1.0-6.0 mass%, Mn: 0.2-9 mass%, Ni: 9-18 mass%, Cr: 12-20 mass%, Mo: 3 Inclusive of Cu: 3% by mass or less, Cu: 3% by mass or less, N: 0.10% by mass or less, with the composition of the balance Fe and inevitable impurities, Ni equivalent Ni _eq calculated by the following formula is 17.0 or more, lamination A nonmagnetic stainless steel excellent in workability, characterized in that the component adjustment is made so that the generation index SFE of defect energy is 20.0 or less and the Ni balance value Ni _bal is 0 or more.
Ni _eq = Ni + Cu + 0.6Mn + 9.69 (C + N) + 0.18Cr-0.11Si ²
SFE = 2.2Ni-1.1Cr-13Si-1.2Mn + 6Cu + 32
Ni _bal = Ni + 30 (C + N) + 0.5Mn + 0.3Cu-1.1 (Cr + 1.5Si + Mo) +8.2   The nonmagnetic stainless steel according to claim 1, wherein a work hardening index n, which is a gradient of nominal strain 30% and 40% in a true stress-logarithmic elongation strain curve obtained by a tensile test, is 0.45 or more.   The nonmagnetic stainless steel according to claim 1, wherein the magnetic permeability when cold rolled at a rolling reduction of 60% is 0.01 or less.   The nonmagnetic stainless steel according to claim 1, wherein C + N is 0.06% by mass or less, and the Vickers hardness of the annealed material is 120HV or less. Furthermore, V: 1.0% by mass or less, one or more of Ti, Nb, and Zr: 1.0% by mass or less, B: 0.03% by mass or less, Ca : 0.03% by mass or less Furthermore , the nonmagnetic stainless steel in any one of Claims 1-4 containing 1 type or 2 types or more.

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