WO2005045082A1

WO2005045082A1 - AUSTENITIC HIGH Mn STAINLESS STEEL EXCELLENT IN WORKABILITY

Info

Publication number: WO2005045082A1
Application number: PCT/JP2004/016057
Authority: WO
Inventors: Masaharu Hatano; Eiichiro Ishimaru; Akihiko Takahashi
Original assignee: Nippon Steel & Sumikin Stainless Steel Corporation
Priority date: 2003-11-07
Filing date: 2004-10-22
Publication date: 2005-05-19
Also published as: KR20060099388A; TWI268960B; KR101177540B1; JP2005154890A; TW200521251A; JP4498847B2

Abstract

An austenitic high Mn stainless steel excellent in wokability, characterized in that it has a chemical composition, in mass %, C + N: 0.03 to 0.15 %, Si: 0.1 to 1 %, Mn: 3 to 15 %, Cr: 10 to 16 %, Ni: 1 to 6 %, Cu: 0.3 to 3 %, Mo: : 0.3 to 3 %, and the balance: Fe and inevitable impurities, and the chemical composition is so designed that Md30, which is an indicator for the stability degree of austenite, and SFE, which is an indicator for the formation of stacking fault energy, satisfy the following formulae: -10 < Md30 < 30, and 40 < SFE < 80, Md30 (°C): 497-462(C+N)-9.2Si-8.1Mn-13.7Cr-20(Ni+Cu)-18.5Mo SFE (mJ/m2): 6.2Ni+18.6Cu+0.7Cr+3.2Mn+9.3Mo-53. The above austenitic high Mn stainless steel is improved in workability and retains non-magnetism also after being worked.

Description

Description Austenitic high-Mn stainless steel with excellent workability Technical field

The austenite of the present invention is soft, has low work hardenability, has excellent workability such as cold working or deep drawing at a high working rate, and maintains non-magnetism even after working. Related to high-Mn stainless steel. Background art

Austenitic stainless steels are classified into 300 series (SUS304, SUS316, SUS301, etc.) and 200 series (SUS201, SUS202, etc.) specified in JIS SG4305.

300 based Osutenai preparative stainless steel is, Mn is 2.0 mass% or less, N i is 6: contains degree 15 mass ⁰ I. Ni-based austenitic stainless steel represented by SUS304 has good workability and excellent corrosion resistance, but has the disadvantage of high raw material cost because it contains a large amount of expensive Ni. . In addition, SUS304 has a problem in that the austenite phase is metastable, so that martensitic transformation occurs during molding, and the processed product becomes magnetic. .,

On the other hand, 200 type austenitic stainless steel is a high Mn stainless steel in which Ni is replaced with Mn, and has high strength and non-magnetic properties because it contains a large amount of C and N. It is also less expensive than Ni-based austenitic stainless steel. However, high-Mn stainless steels, such as SUS201 and SUS202, have higher strength and higher work hardenability than the 300 series in the annealed state, and therefore have high press formability such as cold workability and deep drawing. To There is an inferior problem.

Regarding means to improve the workability of austenitic stainless steel,

Many studies have been made on the 300 series including Mn of less than 3% and Ni of 6% or more. For example, as disclosed in Japanese Patent No. 3039838, Japanese Patent No. 339 8258, Japanese Patent No. 3398260, Japanese Patent Application Laid-Open No. 10-102210 and Japanese Patent Application Laid-Open No. 10-121207, press moldability and the like are disclosed. It is known that the addition of Cu works effectively to improve the workability of steel.

On the other hand, 200 series austenitic stainless steel containing 5.5% or less of 1 ^ and 5.5% or more of Mn is a high-strength steel for electronic equipment, wires for bicycle spokes, nails for construction and construction materials, etc. Mainly applied to members that require non-magnetism. For this reason, many studies have been made on the high Mn stainless steel for further improvement of high strength non-magnetization. For example, Japanese Patent No. 2618151 and Japanese Patent Application Laid-Open No. Hei 6-235048 disclose that high strength and non-magnetization include a small amount of Nb, Mo, and P by suppressing an increase in Mn and Cr in conjunction with increasing N. It is disclosed that the addition works effectively.

In order to improve the corrosion resistance of low Ni austenitic stainless steel, Japanese Patent Application Laid-Open Nos. 11-92885 and 2000-34546 disclose reducing the impurity elements of Ca, B and S. Is disclosed to be effective. These low-Ni austenitic stainless steels contain a large amount of N exceeding 0.1%, and have high strength (0.2% power resistance) similar to the above-mentioned high-Mn stainless steel, and have problems in workability. is there. On the other hand, Japanese Patent Application Laid-Open No. 2004-143576 and British Patent No. 2359095 disclose a low Ni austenitic stainless steel with a reduced Cr content in order to improve workability in addition to corrosion resistance. It is disclosed and its mechanical properties are relatively close to SUS304. These low Ni austenitic stainless steels suppress the generation of ferrite at high temperatures to ensure hot workability. -Although austenite stability was adjusted, no study was performed on austenite stability at room temperature. That is, in these low Ni austenitic stainless steels, there is a concern that work-induced martensite is generated by cold working such as press forming, and the processed product becomes magnetic or cracks are generated. .

As described above, high-Mn stainless steel with a reduced Ni content is not intended to improve workability to be applied to press forming applications where Ni-based stainless steel represented by SUS304 is used. In other words, at present, high-Mn stainless steel that has excellent workability equal to or higher than that of SUS304 and realizes non-magnetism even after processing has not yet emerged. Disclosure of the invention

The present invention has been devised to improve the workability of the high-Mn stainless steel described above, and includes C + N, an austenitic stability index Md30 value (° C), a stacking fault energy generation index SFE By designing the component so that (mj / m ² ) satisfies the specified conditions, austenitic stainless steel that has workability equal to or higher than SUS304 and maintains non-magnetism even after processing is obtained. The purpose is to provide. The gist of the present invention is as follows.

(1) The high Mn stainless steel of the present invention has a mass of ⁰ /. C + N: 0.03 to 0.15%, Si: 0.1 to 1%, Mn: 3 to 15%, Cr: 10 to 16%, Ni: 1 to 6%, Cu: 0.3 to 3%, Mo: 0.3 to It consists of 3%, the balance being Fe and unavoidable impurities, and is characterized by its component design so that the austenitic stability index Md30 value and the stacking fault energy generation index SFE satisfy the following.

-10 <Md30 <30, 40 <SFE <80

Md30 (° C): 497-462 (C + N) -9.2Si-8.lMn-13.7Cr-20 ( Ni + Cu)-18.5Mo

SFE (mj / m ² ): 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn + 9.3Mo-53

(2) The high Mn stainless steel, for hot workability and oxidation resistance improve, the REM can and 0.001 mass ^_0/0 whatever child.

(3) In order to ensure the workability of press forming such as cold working or deep drawing at a high working rate, C + N must be 0.15% by mass or less, the 0.2% proof stress required in the tensile test should be less than 300MPa, The work hardening index n, which is the slope of the nominal strain of 25% and 35% in the true stress logarithmic elongation strain curve, is 0.45 or less. When the magnetic permeability (μ) at the time of cold rolling at a rolling reduction of 60% is 1.05 or less, non-magnetism is maintained even after various types of processing.

As described above, the high Mn stainless steel of the present invention has a 0.2% proof stress because it adopts a composition design of C + N: 0.03 to 0.15%, -10 <Md30 <30, 40 and SFE 80. It is soft with less than 300 MPa, has low work hardening properties, has excellent workability such as cold working or deep drawing working at a high working rate, and maintains non-magnetism even after working. Therefore, it is possible to carry out forming work that cannot be performed with conventional high-Mn stainless steel, and it is used for press forming where Ni-based stainless steel represented by SUS304 is used. Further, annealing for demagnetization after processing SUS304 can be omitted, so that it can be applied in a wide range of fields as a material for forming processing requiring nonmagnetic properties. Brief Description of Drawings

Figure 1 is a graph showing the effect of Md30 on the elongation of steel.

Fig. 2 is a graph showing the effect of the Md30 value on the magnetic permeability of a 60% cold-rolled material.

Figure 3 is a graph showing the relationship between SFE and work hardening index n. BEST MODE FOR CARRYING OUT THE INVENTION

High Mn stainless steel of the present invention, C + N, austenite stability index Md30 value C), generates an index SFE of stacking fault energy (mj / m ²⁾ and the adopted child components designed to satisfy the appropriate range As a result, it has workability equal to or higher than that of SUS304, and maintains non-magnetism even after processing. Hereinafter, the function and effect of the composition design of the high Mn stainless steel of the present invention and the reason for limiting the same will be described.

• C + N: 0.15% or less

C and N are effective elements for stabilizing the austenite phase and suppressing the formation of the δ ferrite phase. On the other hand, these elements increase the 0.2% resistance of steel by solid solution strengthening and lower the workability. Therefore, the upper limit of C + N is set to 0.15%. Since Ν has a large effect of increasing the power resistance by 0.2% compared to C, Ν is preferably designed to be lower than C. For applications requiring press forming such as cold working and deep drawing at a high working rate, which is the objective of the present invention, design C + N to 0.15% or less (high C). Therefore, it is effective to soften the steel 0.2% resistance to less than 300MPa.

However, when C + N is less than 0.03%, not only is it difficult to demagnetize the processed product, but it also imposes a burden on steelmaking costs for reducing C and N. Therefore, the lower limit of C + N is set to 0.03%. A preferred range is from 0.08 to 0.12%.

• Austenitic stability index: Md30 value (° C)

Metastable austenitic stainless steel undergoes martensitic transformation by plastic deformation even at temperatures above the Ms point. The maximum temperature at which a transformation point occurs during processing is called the Md value. That is, the Md value is an index indicating the degree of stability of austenite. The temperature at which 50% martensite occurs when 30% strain is applied by tensile deformation is referred to as the Md30 value. Md30 = 497-462 (C + N)-9.2Si-8.1Mn-13.7Cr-20 (Ni + Cu)-18.5Mo The Md30 value (° C) defined for the high Mn stainless steel of the present invention It has been found that the workability and the non-magnetism aimed at by the present invention are ensured by designing the temperature in the range of 10 ° C to 30 ° C.

If the Md30 value is lower than -10 ° C, the elongation of the steel material is reduced (by 50%) due to the high austenite stability, and workability is impaired. On the other hand, when the Md30 value exceeds 30 ° C, the elongation of the steel material improves due to the formation of the work-induced martensite (α 'phase), but the formed α, phase has magnetism, and the processed product becomes less magnetic. Take on. When the Md30 value is -10 to 30 ° C, the high-Mn stainless steel of the present invention can improve the workability of the steel material while maintaining the non-magnetic properties of the processed product.

• Stacking fault energy generation index: SFE (mj / m ² )

Compared with ordinary steel with bcc structure, austenitic stainless steel with fee structure has larger work hardening because stacking faults are easily generated. In the present invention, in order to enable press forming such as cold working or deep drawing at a high working rate, a component design is adopted which allows easy dislocation cross-slip where stacking faults are hardly generated.

In recent years, stainless steel sheets are often manufactured by cold-working products with complex shapes. In such a case, it is necessary to obtain a large degree of workability by repeatedly softening a steel material with high work hardening while softening it with an intermediate annealing step in the middle of working. For steel materials with low work hardening, the intermediate annealing step can be omitted and the product can be processed, greatly contributing to a reduction in product cost. The present inventors have studied the effects of components on stacking fault energy (SFE) from such a viewpoint. As a result, when the SFE defined as SFE (mj / m ² ): 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn + 9.3Mo-53 is adjusted to a high range of 40 to 80, the present invention aims to provide an excellent object. Workability was developed. When the SFE is less than 40, high-Mn stainless steel tends to generate stacking faults and has a large work hardening, so that the workability targeted by the present invention cannot be obtained. At this time, the work hardening index n value (the slope of nominal strain of 25% and 35% in the true stress logarithmic elongation strain curve) obtained in the tensile test exceeds 0.45. On the other hand, when the SFE exceeds 80, the work hardening is small and the n value is less than 0.3. At this time, there is a problem that overhang workability is reduced in practical press forming. Therefore, in the present invention, it is preferable that the n value determined by the tensile test is in the range of 0.3 to 0.45. Soft, low work hardening, non-magnetic steel material that satisfies the Md30 value and SFE of the present invention is a non-magnetic steel that is a problem in Ni-based austenitic stainless steels such as SUS304. And excellent in deep drawability over multiple steps without causing cracks. In other words, in SUS304, the austenite phase is metastable, so that martensite transformation occurs during processing, and the flange portion becomes too hard during deep drawing, and a residual crack increases due to an increase in residual stress.

C + N of the present invention: 0.03 to 0.15%, Md30 value: —10 to 30. C, SFE: 40 to 80 high Mn stainless steel is adjusted to (mj / m ²⁾ 0.2% yield strength is cold working by work hardening property small Ku high processing rate soft than 300MPa possible non Magnetic stainless steel. Hereinafter, other alloy elements except C and N of the present invention are selected in the following ranges.

-Si: 0.1 ~ 1%

Si is effective as a deoxidizing agent at the time of smelting, and 0.1% or more is added to obtain the effect. It is more preferably at least 0.3%. Si is an element that promotes work hardening by strengthening solid solution and lowering SFE. Therefore, the upper limit is 1% or less in order to obtain the 0.2% heat resistance of less than 300 MPa and the work hardening index n value of less than 0.45 of the present invention. Preferably it is less than 0.2-0.7%.

-n: 3-15% In addition to being used as a deoxidizer during smelting, Mn works effectively with non-magnetic retention and austenite-forming elements as an alternative to Ni. In the present invention, Mn is added in an amount of 3% or more to obtain these effects. More preferably, it is 5% or more. On the other hand, there is a problem that the addition of Mn increases the amount of S-based inclusions and impairs corrosion resistance and workability. Therefore, the upper limit is 15%. Preferably it is 10% or less.

• Cr: 10 or more: 16%

Cr is an alloying element necessary for obtaining the corrosion resistance required for stainless steel, and is preferably required to be 10% or more. It is more preferably at least 12%. On the other hand, Cr is an element that promotes work hardening by reducing solid solution strengthening and SFE. Therefore, the upper limit is 16% or less in order to obtain the 0.2% heat resistance of less than 300 MPa and the work hardening index n value of less than 0.45 of the present invention. Preferably it is 15% or less.

• Ni: 1-6%

Ni is an expensive element, and more than 6% of 300 series austenitic stainless steel raises the cost of raw materials. Therefore, Ni is less than 6%. It is preferably at most 5%. Ni is an element necessary for austenitic stainless steel and is an effective element for ensuring non-magnetism and ductility after cold working. Therefore, the lower limit is 1%.

• Cu: 0.3-3%

Cu is an alloy element effective for lowering the Md30 value defined in the present invention and increasing SFE to improve workability. In the present invention, in order to obtain these effects, the lower limit of Cu is set to 0.3% or more. Preferably, it is 1% or more. However, excessive addition of Cu has the problem of inducing Cu contamination and hot embrittlement during steelmaking. In addition, the SFE is excessively increased, resulting in deterioration of workability. Therefore, the upper limit of Cu is set to 3% or less.

• Mo: 0.3-3% It is an element effective for improving corrosion resistance. Further, it is an element effective for lowering the Md30 value defined in the present invention and increasing SFE to improve workability. In order to ensure the corrosion resistance and workability of the high Mn stainless steel of the present invention, the lower limit of Mo is set to 0.3% or more. However, when Mo is contained in excess, magnetism is generated by the formation of δ ferrite, and the strength is increased by solid solution strengthening. Therefore, the upper limit of Mo should be 3% or less.

• REM: 0.001 to 0.2%

It is an element added as needed and has the effect of improving hot workability and oxidation resistance. To obtain these effects, add 0.001% or more. However, the effect of adding REM saturates at 0.2%, and adding more than 0.2% hardens the steel material and reduces workability. Therefore, the upper limit is preferably 0.2%. Example

Stainless steel having the chemical composition shown in Table 1 was melted and hot-rolled at a heating temperature of 1200 ° C to produce a hot-rolled steel sheet with a thickness of 4.0 mm. Hot-rolled steel sheet is annealed at 1120 ° C and soaking time of 2 minutes, cold-rolled to 1.5mm thickness after pickling, further intermediate-annealed at 1060 ° C and soaking time of 2 minutes, and after pickling A cold-rolled steel sheet with a thickness of 0.7 mm was subjected to final annealing at 1060 ° C and soaking time of 1 minute (annealed pickling material). A 60% cold-rolled material was obtained by cold rolling the intermediate annealed pickled material to a sheet thickness of 0.6 mm.

A JIS13B tensile test piece was cut out from the annealed pickling material, and 0.2% power resistance, tensile strength, elongation, and work hardening index n were measured by a tensile test. The work hardening index n is obtained by calculating the true stress δ ₂₅ , δ ₃₅ at the true strains _{ε 25} , ε ₃₅ corresponding to the nominal strains of 25% and 35%, and calculating the work hardening index η value according to the following equation: η value = ln Ε ₃₅ / ε ₂₅ j / In (δ ₃₅ / δ ₂₅ ) A Φ96mm disk (blank) is cut out from the annealed pickling material and subjected to a 5-stage deep-drawing test with a punch diameter φ48 → φ44 → φ40 → φ35 → φ30mm. (Punch diameter) was investigated.

A test piece was cut out from a 60% cold-rolled material, and the magnetic permeability was determined by measuring the attractive force due to the magnetization by measuring the slope at a magnetic field of 5000 gauj3 on an applied magnetic field-magnetization curve using a magnetic balance.

Table 2 shows the 0.2% heat resistance, tensile strength, elongation, n-value, and magnetic permeability (μ) of the 60% cold-rolled material of the annealed pickling material. Steel No .:! To 6 satisfy the component design conditions for high-Mn stainless steel specified in the present invention, and have mechanical properties with a 0.2% proof stress equivalent to 304 less than 300 MPa and an elongation of 50% or more. The work hardening index n was less than 304 with a work hardening index n of 0.3 to 0.45, the work hardening was small, and the magnetic permeability μ of the 60% cold-rolled material was non-magnetic with a magnetic permeability μ of 1.05 or less. In addition, this steel did not undergo any cracking due to multi-stage deep drawing, and the cracking limit drawing ratio was much higher than SUS304, which was 3.2 or more. In steel Nos. 7 to 14, the workability and non-magnetism of the steel material targeted by the present invention cannot be obtained because the amount of C + N, the Md30 value and / or the SFE are out of the conditions specified by the present invention. It is a thing. Steel No. 15 is SUS304 which is a comparison of workability. Steels 16 to 29 do not satisfy the component range specified by the present invention, and the target workability and non-magnetism of the steel material could not be obtained.

As a result of investigating the relationship between 0.2% proof stress and components, a regression equation expressed by the following equation was obtained, and it was confirmed that the 0.2% proof stress could be softened to less than 300 MPa by reducing the amount of C + N. 0.2% resistance [NZmm ² ] = 875 * (C + N) + 3.87Mn- 1.48Ni- 3.53Cu + 8.58Cr + 19.7

Figures 1 and 2 show the results of an examination of the effect of the austenitic stability index Md30 on the elongation and permeability of steel. As shown in Fig. 1 and Fig. 2, by controlling to -10 It was confirmed that the target growth of Akira was 50% or more and μ: 1.05 or less.

In addition, as a result of examining the relationship between the stacking fault energy generation index SFE and the work hardening index η, as shown in FIG. Was obtained.

table 1

* It shows that the high workability and non-magnetism of the high Mn stainless steel targeted by the present invention are not achieved. Md30 = 497-462 (C + N) -9.2Si-8.1Μη-13.7Cr-20 (Ni + Cu)-18.5Mo

SFE [mJ / m ² ] = 6.2N1 + 18.6Cu + 0.7Cr + 3.2Mn + 9.3Mo-53> 40

0.2% resistance [N / mm ² ] = 875 * (C + N) + 3.87Mn- 1.48N1-3.53Cu + 8.58Cr + 19.7 (regression formula)

Those outside the scope of the invention

Industrial applicability

The high-Mn stainless steel of the present invention can be formed by conventional high-Mn stainless copper, and can be used for press forming applications where Ni-based stainless steel represented by SUS304 is used. . In particular, it is most suitable for multi-step deep drawing applications in which time cracking is a problem with SUS304. Furthermore, annealing for demagnetization after processing of SUS304 can be omitted, so that it can be applied in a wide range of fields as a material for molding that requires nonmagnetism.

Claims

1. Mass ⁰ /. C + N: 0.03 to 0.15%, Si: 0.1 to 1%, Mn: 3 to 15%, Cr: 10 to 16%, Ni: 1 to 6%, Cu: 0.3 to 3%, Mo: 0.3 to Austenitic steel with excellent workability, characterized by 3%, the balance being Fe and unavoidable impurities, and having an austenitic stability index Md30 value and a stacking fault energy generation index SFE satisfying the following: High Mn stainless steel.

— 10 Md30, 30, 40 SFE, 80

Md30 (° C): 497-462 (C + N) — 9.2S i _ 8. ΙΜη — 13.7Cr — 20 (Ni + Cu)-18.5Mo

Enclosure

SFE (mj / m ² ): 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn + 9.3Mo-53

2. The austenitic high-Mn stainless steel excellent in workability according to claim 1, characterized in that REM contains 0.001 to 0.2% by mass%.

3.Excellent workability according to claims 1 and 2, wherein 0.2% Kaka resistance is less than S300MPa, the heat hardening index n is a gradient of 25% and 35% of nominal strain, 0.30 to 0.45, and the elongation is 50% or more. Austenitic high Mn stainless steel.