JP6361402B2 - Duplex stainless steel for spring and method for producing the same - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a duplex stainless steel for springs having an excellent balance between strength and formability, and a method for producing the same, and in particular, with the recent progress in miniaturization and weight reduction of products, stainless steel which has high strength and is a material. The present invention relates to a duplex stainless steel for springs suitably used for products having complex and precise shapes formed by forming steel sheets and components thereof, and a method for producing the same.

  “Product” as used herein refers to automobiles, home appliances, personal computers, mobile phones, etc., and “parts” refers to components that are built in and used within these products. means. Examples of the “parts” include gaskets used for automobile engines, home appliances, personal computers, mobile phone casings, springs incorporated under buttons, and the like.

  Due to the recent progress in the miniaturization and weight reduction of products and parts that respond to environmental problems and the effective use of valuable resources, the steel sheets used as the materials for these products and parts have also been made thinner, and the rigidity has been reduced due to the thinner sheets. Many of them require high strength to cope. In addition, the shapes of products and parts are complicated and precise, and the accuracy is continuing to increase. For this reason, the steel plate used as a raw material also requires better formability.

  However, in general, in metal materials, a decrease in formability due to an increase in strength is inevitable, and a trade-off relationship is established between them. For this reason, there is a growing demand for a material having particularly excellent formability while maintaining high strength.

  Stainless steel is generally characterized by its excellent corrosion resistance, but has been widely used as a material for spring parts. Specifically, the transformation from the austenite (A) matrix to the hard martensite (M) phase (work-induced martensite transformation) is caused by cold working, and high strength can be obtained relatively easily and in a wide range. Metastable austenitic stainless steels represented by adjustable SUS301 and SUS304 have been mainly used.

  Metastable austenitic stainless steel has excellent formability due to high elongation of the austenitic matrix, and is hardened by transformation of the deformed part into the martensite phase as described above, with soft undeformed parts preferentially. The entire material is uniformly deformed (TRIP effect) by being deformed to show excellent moldability.

  From these characteristics, SUS301 and SUS304 are classified as spring stainless steel strips according to the JIS standard (JIS-G-4313), and the mechanical properties are also defined.

  However, the large work hardening of metastable austenitic stainless steel has many fluctuation factors, and there are many cases where the expected properties cannot be obtained with the target product thickness. In addition, there has been a problem that the rolling load becomes large during the production, especially during rolling, when the thin plate and the high strength corresponding to the recent reduction in size and weight of the spring parts are produced. Further, the metastable austenitic stainless steel has a problem that it is expensive because it contains a large amount of Ni which may be classified as a rare element.

  On the other hand, martensitic stainless steels such as SUS403, SUS410, and SUS420 that are transformed into a hard martensite phase as an intermediate phase by heat treatment (quenching) to obtain high strength are also applied to the spring component material. Some steels use the same phase as a raw material and a multiphase structure with a ferrite (F) phase.

  These martensitic stainless steels contain little Ni, which is an expensive and rare alloy element, and are less expensive than the metastable austenitic stainless steel described above. These martensitic stainless steels are disclosed in Patent Document 1 (high-strength dual-phase stainless steel and method for producing the same), Patent Document 2 (manufacturing method for high-strength dual-phase stainless steel strip or steel plate), and Patent Document 3 (steel belt). Double phase stainless steel strip), patent document 4 (stainless steel for gasket), patent document 5 (stainless steel for engine gasket and its manufacturing method), patent document 6 (high strength dual phase structure chromium stainless steel plate for punching and its Manufacturing method), patent document 7 (stainless steel material for metal gasket excellent in sag resistance and its manufacturing method), patent document 8 (high strength martensitic stainless steel for metal gasket excellent in high temperature sag resistance), patent Reference 9 (High-strength stainless steel for metal gaskets with excellent fatigue characteristics and high temperature resistance and its manufacturing method), Patent Document 10 (high-strength dual phase stainless steel plate and a manufacturing method having high elasticity), the like Patent Document 11 (method for producing a high strength stainless steel sheet excellent in ductility), is disclosed.

Japanese Patent No. 3363590 Japanese Patent No. 36002201 Japanese Patent No. 4252893 Japanese Patent No. 4353060 JP 07-278758 A JP 09-263912 A JP 2000-256802 A JP 2002-317251 A JP 2002-332543 A Japanese Patent Laid-Open No. 2003-089851 JP 2004-323960 A

  However, even in these stainless steels, adjustment to a predetermined strength is difficult, and adjustment has become more difficult as the strength increases.

  In addition, the shape of recent products and parts has become complicated and precise, and the steel sheet that is the above-mentioned material is required to have more stable and excellent formability while maintaining high precision. For this reason, while maintaining high strength, there is an increasing demand for inexpensive stainless steel having even better formability.

  INDUSTRIAL APPLICABILITY As described above, the present invention can be used as a material for products that are reduced in size and weight and components thereof, and is a stainless steel plate for springs that has an excellent balance between strength and formability (hereinafter simply referred to as a stainless steel plate for springs). It is an object of the present invention to provide a stainless steel plate for springs that can be obtained stably with excellent formability by using inexpensive stainless steel mainly composed of Cr.

  That is, an object of the present invention is to provide a spring stainless steel excellent in the balance between strength and formability and a method for producing the same in a broad sense. Specifically, in particular, the recent downsizing and light weight of products. Stainless steel for springs, which is suitable for products and components that have complex and precise shapes by forming high-strength stainless steel sheets as they are made, and providing manufacturing methods thereof It is to be.

  In a broad sense, according to the present invention, low-cost and high-performance stainless steel for springs can be industrially stably supplied to meet the recent downsizing and weight reduction of products and components that respond to environmental problems. The purpose is to promote effective use of resources.

  As a result of intensive studies in order to solve the above-mentioned problems, the present inventor can obtain the knowledges a to d listed below, and based on these knowledge a to d, a number of small castings whose components are adjusted. After conducting a trial production with a lump and through a number of trial productions with an actual machine, it was found that mass production was possible with the actual machine, and the present invention was completed.

  (A) In order to obtain an excellent balance between high strength and formability, and to obtain particularly excellent formability stably, the work transformation induction state to the hard martensite (M) phase and the accompanying TRIP effect are compatible. A stable austenite (A) phase is very effective. In the present invention, by using an inexpensive stainless steel mainly composed of Cr and having a mixed structure with a soft ferrite (F) phase, the balance between high strength and formability is excellent, and particularly excellent formability is stable. And get.

  (B) In order to utilize the metastable austenite phase having a large solid solubility limit of C and N most effectively, the most powerful austenite stabilization and inexpensive solid solution of C and N, which are strengthening elements, are used for the adjustment. .

  (C) The ferrite (F) phase has a small solid solubility limit of C and N, is soft, and can share deformation with the metastable austenite phase. Furthermore, the diffusion rate of C and N is large, and rapid diffusion is expected.

  (D) In the general manufacturing method in which the mixed structure of the metastable austenite phase and the ferrite phase described above is performed by combining hot working on the slab and then performing cold working and subsequent heat treatment at least once. It can be obtained by absorbing and diffusing C and N by heating in a high temperature two-phase region and then partitioning and precipitating by heating at a relatively low temperature.

The present invention is listed below.
(1) By mass%, C: 0.1 to 0.4%, Si: 2.0% or less, Mn: 0.1 to 6.0%, Cr: 10.0 to 28.0% or less, N : 0.17% or less, with the balance being a chemical composition consisting of Fe and impurities, the C + N solid solution amount of the austenite phase is 0.5% or more, and the austenite phase ratio is 10 to 90 by area ratio. Of which Cr is a carbide of Cr and Fe having a metastable state of at least 5% by area, the balance being a ferrite phase or a ferrite phase and a martensite phase, and a cross-sectional equivalent circle diameter of 10 μm or less. Or a duplex stainless steel for springs, characterized by having a metal structure in which a fine compound of nitride exists.

  However, the ratio of the austenite phase in the metastable state is the ratio at which the austenite phase decreased before and after this cold rolling after cold rolling with a sheet thickness reduction rate of 20%.

  (2) The chemical composition further includes one or two kinds selected from the group consisting of Ni: 2.0% or less and Cu: 2.0% or less in mass%. Duplex stainless steel for springs.

  (3) The chemical composition may be one or more selected from the group consisting of Nb: 0.5% or less, V: 0.5% or less, and Ti: 0.5% or less in terms of mass%. And a metal structure in which a fine compound of Nb, V, Ti carbides and / or nitrides having a cross-sectional equivalent circle diameter of 10 μm or less is present, for a spring described in (1) or (2) Duplex stainless steel.

(4) For springs described in any one of items (1) to (3) by combining hot and cold processing and heat treatment after the processing at least once This is a method for producing duplex stainless steel, and after the final cold working to reduce the thickness of the product, it is heated to a temperature range from Ac ₁ transformation point to Ac ₃ transformation point for 10 seconds or more. Then, after absorbing and diffusing one or both of nitrogen and carbon, it is cooled to a predetermined temperature of 200 to 500 ° C. at a cooling rate of 1 ° C./second or more, kept at the predetermined temperature for 10 seconds or more, and then room temperature. A method for producing a duplex stainless steel for springs, characterized by cooling to a minimum.

That is, using the Cr-based stainless steel having the chemical composition described in any one of the items (1) to (3), general hot and cold processing and subsequent heat treatment are combined one or more times. When manufacturing a stainless steel plate, after reducing the thickness to the product plate thickness, heat it to the temperature range of Ac ₁ transformation point to Ac ₃ transformation point for 10 seconds or more to absorb nitrogen or carbon, or nitrogen and carbon from the surface. And then cooled to a predetermined temperature of 200 ° C. or more and 500 ° C. or less at a rate of 1 ° C./second or more, kept at this predetermined temperature for 10 seconds or more, preferably 10 seconds or more and 600 seconds or less, and then to room temperature. By cooling, the spring duplex stainless steel according to the present invention according to any one of (1) to (3) is manufactured.

  The present invention provides a stainless steel plate for springs that can be used as a material for products that are promoted to be reduced in size and weight and components thereof, and has an excellent balance between strength and formability. There is provided a stainless steel plate for a spring that can stably obtain excellent formability using a special stainless steel.

  That is, according to the present invention, in a broad sense, a stainless steel for springs having an excellent balance between strength and formability (elongation) and a method for producing the same are provided. Along with the progress, low-priced duplex stainless steel for high-strength spring parts that has high strength and is suitably used for products and components that have complex and precise shapes by forming on stainless steel plates as raw materials. It can be supplied industrially stably. As a result, it is possible to cope with recent environmental problems and further promote the effective use of resources by reducing the size and weight.

FIG. 1 is an explanatory diagram showing an excerpt of a material manufacturing process in the example.

1. Hereinafter, the chemical composition and metal structure of the spring duplex stainless steel according to the present invention will be described.

(1-1) Chemical composition [C: 0.1 to 0.4%]
C is an inexpensive and most powerful austenite stabilizing element and interstitial solid solution strengthening element, and when Nb, V and Ti are added, a compound is formed (precipitated) and crystal grain growth is caused. It is also an effective element to suppress. For this reason, it is an element unavoidable in the duplex stainless steel for springs, and the lower limit of the content is 0.1%. However, if it is contained in a large amount, a coarse compound is formed in the middle of the production, and various properties may be deteriorated due to its remaining. For this reason, the upper limit of the C content is set to 0.4%. Preferably, it is 0.12% or more and 0.38% or less.

[Si: 2.0% or less]
Si is an effective solid solution strengthening alloy element after the interstitial solid solution strengthening element and is a ferrite stabilizing element, and is contained in consideration of the balance with the austenite stabilizing element. On the other hand, Si is also used as a deoxidizer at the time of melting, and excessive inclusion forms coarse inclusions, which may deteriorate various properties. For this reason, Si content is 2.0% or less, Preferably it is 1.8% or less.

[Mn: 0.1 to 6.0%]
Mn is an austenite stabilizing element and expands a two-phase region composed of an austenite phase and a ferrite phase at a high temperature to a lower temperature and facilitates the adjustment of the metal structure. At the same time, Mn suppresses grain growth by the austenite phase and ferrite phase acting as obstacles to each other, contributing to higher performance. Furthermore, Mn is thought to increase the solid solution amount of C and N. For this reason, Mn is an element necessary for the duplex stainless steel for springs according to the present invention, and the Mn content is 0.1% or more in order to achieve the above-described effects. However, excessive inclusion of Mn forms a coarse compound and deteriorates various properties. For this reason, Mn content is 6.0% or less, Preferably it is 5.6% or less.

[Cr: 10.0 to 28.0% or less]
Cr is one of the basic elements of stainless steel, and is contained in an amount of 10.0% or more in order to obtain the basic corrosion resistance. However, it is also a ferrite stabilizing element, and is contained in consideration of the balance with the austenite stabilizing element. Moreover, when Cr is contained excessively, a required strength cannot be obtained, and formability is also deteriorated due to the formation of a coarse compound. For this reason, the Cr content is 28.0% or less. The Cr content is preferably 10.2% or more, and more preferably 26.0% or less.

[N: 0.17% or less]
N is a strong austenite stabilizing element and interstitial solid solution strengthening element after C. Further, when Nb, V, or Ti is added, it is an effective element that forms (precipitates) a compound and suppresses crystal grain growth. However, when N is contained excessively, the hot workability is remarkably deteriorated. Therefore, the N content is 0.17%. Preferably, it is 0.15% or less. Moreover, in order to acquire said effect, it is preferable that content of N is 0.01% or more.
The spring duplex stainless steel according to the present invention may contain the elements described below as optional additional elements as required.

[One or two selected from the group consisting of Ni: 2.0% or less and Cu: 2.0% or less]
Ni and Cu are austenite stabilizing elements, and may be contained in order to expand the two-phase region composed of an austenite phase and a ferrite phase at a high temperature and facilitate the adjustment of the metal structure. However, since both Ni and Cu are expensive elements and are added as additive elements for supplementing the effect of inexpensive Mn, both contents are 2.0% or less, preferably 1. 8% or less.

[One or more selected from the group consisting of Nb: 0.5% or less, V: 0.5% or less, and Ti: 0.5% or less]
Nb, V, and Ti may all be included to form a compound with C and N and suppress the growth of crystal grains by their pinning effect. For this reason, each content of Nb, V, and Ti is 0.5% or less, preferably 0.4% or less.

The balance other than the above is Fe and impurities. The impurity here means an element that can be contained as long as the effects of the present invention can be exerted. In addition to the above components, elements added from an industrial aspect, for example, Ca, Al used as a deoxidizer during melting Alternatively, REM (rare earth metal) and B which is expected to improve hot workability may be contained in a total amount of 0.3% by mass or less as necessary. When using scrap as a raw material, you may contain 0.4 mass% or less of Mo as an impurity. In addition, impurities in a normal melting method may be included.
Moreover, the said chemical composition means the average composition in the whole board thickness.

(1-2) Metal structure [(C + N) solid solution amount in austenite phase: 0.5% or more]
The solid solution amount of C + N in the austenite phase is 0.5% or more because it is the most powerful austenite stabilizing element and interstitial solid solution strengthening element made of inexpensive stainless steel mainly composed of Cr. C and N are effectively utilized and distributed at a high concentration during holding at a low temperature from a high-temperature two-phase region, so that the austenite phase remains to room temperature, and as a part thereof, a metastable state with excellent characteristics is obtained. It is because it is obtained. The (C + N) solid solution amount of the austenite phase is preferably 0.52% or more.
In order to suppress precipitation of coarse compounds, the (C + N) solid solution amount of the austenite phase is preferably 1.0% or less.
The (C + N) solid solution amount of the austenite phase is more preferably 0.54% or more and 0.9% or less, which is considered to provide a metastable austenite phase.

[The ratio of the austenite phase is 10 to 90% in area ratio, of which at least 5 area% is metastable, the balance is composed of ferrite phase or ferrite phase and martensite phase, and fine compounds are dispersed. ]
The ratio of the austenite phase is 10% or more and 90% or less in area ratio, of which at least 5% or more is a metastable state, and the balance is a ferrite phase or a metal structure composed of a ferrite phase and a martensite phase. This is because stainless steel, the main component of which is Cr, is used as a raw material, and the balance between the target strength and formability (elongation) is excellent, and particularly excellent formability is obtained. Specifically, it is effective to have a metal structure mainly composed of a metastable austenite phase excellent in strength and formability, and the balance being a soft ferrite phase, with the metastable austenite phase being 50% or more and the balance being a ferrite phase. The goal is an organization consisting of

  However, when a metastable austenite phase is obtained by adjusting the C and N by heat treatment using stainless steel as a main component of Cr, the remaining of the stable austenite phase containing more of them, the martensite phase generated when there are few, The generation of ferrite phase is unavoidable.

In order to obtain the above-mentioned target structure, after reducing the thickness to the product plate thickness, it is heated to a temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point and held in a two-phase region composed of austenite and ferrite. It absorbs one or both of carbon and mainly diffuses into the austenite phase.

  Further, after cooling to a predetermined temperature of 200 to 500 ° C. at a rate of 1 ° C./second or more and holding at the predetermined temperature for 10 seconds or more, the retained austenite phase and the C of the martensite phase generated from the austenite phase , N is precipitated as a compound, the stability of the retained austenite phase is lowered, the martensite phase is softened, and the ferrite phase is decomposed. That is, in order to obtain a target structure in which the metastable austenite phase is 50% or more and the balance is the ferrite phase, the ratio of the metastable austenite phase and the ferrite phase is increased.

  As a result, the proportion of the austenite phase including the metastable state is 90% or less, and the balance is mainly composed of a soft ferrite phase that shares deformation. The metastable austenite phase needs to be at least 5% or more, and is preferably more than half of the austenite phase. Moreover, it is good also as a metal structure containing the martensite inevitably formed.

  In addition, there is no difference in the structure of the metastable state and the austenite phase of a stable state, and discrimination | determination is difficult. The only difference is whether the structure is changed or maintained by cold working or the like. For this reason, in the present invention, the proportion of the metastable austenite phase is calculated from the amount of decrease before and after cold working. Specifically, the stainless steel sheet is subjected to cold rolling with a sheet thickness reduction rate of 20%, and the ratio of the austenite (A) phase decreased before and after this cold rolling is defined as the ratio of the metastable A phase.

  According to the present invention, during the cold working, the metastable austenite (A) phase is transformed into a hard martensite (M) phase, and the soft undeformed portion is preferentially deformed so that the entire material is uniformly deformed. Excellent workability can be obtained by the TRIP effect.

Furthermore, during the heat treatment, in addition to the compounds with Cr and Fe as the main alloy elements, the compounds with Nb, V and Ti added as compound forming elements are also precipitated during the adjustment of C and N. These compounds have the effect of refining the crystal grains, but on the other hand, they are harder than the martensite phase, and if they are coarse, they become the starting point of destruction and the workability of the material deteriorates. Here, the duplex stainless steel for springs according to the present invention is a metal structure having a fine compound, and this fine compound means a compound having a cross-sectional equivalent circle diameter of 10 μm or less. However, the amount of the fine compound is preferably 100 pieces / mm ² or less because if the number of equivalent circle diameters of 1 μm or more is too large, the effect of refining crystal grains decreases.

2. Manufacturing method Next, the reason for limitation of the manufacturing method of the duplex stainless steel for springs concerning this invention is demonstrated.

[After the final cold working to reduce the thickness of the product, heat it to the temperature range from Ac ₁ transformation point to Ac ₃ transformation point for 10 seconds or more to absorb one or both of nitrogen and carbon from the surface. Let it diffuse]
In a general manufacturing method in which hot and cold processing and heat treatment after the processing are combined one or more times, after performing the final cold rolling to reduce the product sheet thickness, Ac ₁ heating for 10 seconds or more to a temperature range of less than the transformation point Ac ₃ transformation point. By this heating, C or N, or C and N are absorbed and diffused from the surface.

The reason why the material is heated to a temperature range not lower than the Ac ₁ transformation point at which transformation starts to the austenite phase during heating and not higher than the Ac ₃ transformation point at which the transformation is completed is to make the material a two-phase structure of an austenite phase and a ferrite phase. .

The Ac ₁ transformation point and the Ac ₃ transformation point both depend on the chemical composition of the material, but in the present invention, they are about 750 ° C. or lower and 1000 ° C. or lower (preferably 950 ° C. or lower).

The reason for heating to the temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point for 10 seconds or more is to absorb and diffuse C or N or C and N from the surface. It is preferable to heat for 60 seconds or more.

  C and N diffuse further to the inside through the ferrite phase having a high diffusion rate, and dissolve in the austenite phase having a large solid solubility limit. It is desirable that the amount of ferrite phase solid solution is small. When C and N absorption and diffusion are used, the C and N concentrations are generally high in the vicinity of the plate surface and decrease with approaching the center of the plate thickness direction. More metastable austenite phases are formed in the vicinity, and moldability is improved.

[Cool at a cooling rate of 1 ° C./second or more to a predetermined temperature of 200 to 500 ° C., hold at the predetermined temperature for 10 seconds or more, and then cool to room temperature]
In addition, in order to adjust the amount of C and N distributed between the austenite phase and the ferrite phase and the amount of solid solution due to compound precipitation, the temperature is maintained at a predetermined temperature of 200 ° C. to 500 ° C. for 10 seconds or longer during cooling. Preferably, hold for 60 seconds or more.

  The cooling rate during this period is set to 1 ° C./second or more in order to avoid precipitation of coarse compounds. The cooling rate is preferably 3 ° C./second or more.

  Subsequent cooling to room temperature maintains a metastable austenite phase without undergoing processing-induced transformation, so a moderate cooling rate may be used, preferably 1 ° C./second or less.

In addition, the atmosphere of the heat treatment performed after the final cold rolling may be an atmosphere that can absorb N and C when heated and held at a high temperature from the Ac ₁ transformation point to the Ac ₃ transformation point. For example, an atmosphere using a gas such as ammonia or methane, as well as a simple gas of N gas, a commonly used mixed gas of N and reducing H, is exemplified. However, during heat treatment, the absorption of N and C is suppressed by the formation of the oxide film. For this reason, it is desirable that the dew point of the atmospheric gas is low, for example, −40 ° C. or lower, preferably −45 ° C. or lower.

Next, the present invention will be described more specifically with reference to examples.
Table 1 shows the composition of the test material, and FIG.

  The sample was processed into a plate thickness of 4.0 mm by hot rolling and heat treatment from an actual machine melted material and a small ingot at the laboratory level. Thereafter, cold rolling and heat treatment shown in FIG. 1 were repeated using laboratory level equipment to produce a 0.3 mm stainless steel plate corresponding to the product plate thickness.

And the various characteristics of the stainless steel plate were investigated by the investigation method listed below.
(I) Metal structure The ratio of the austenite phase (A phase) and the amount of (C + N) solid solution in the austenite phase were measured for the thin plate after the final thermal treatment using an X-ray diffractometer. The ratio of the austenite phase (A phase) is determined by measuring the integrated intensity of each diffraction peak, and by the following equation (1), the ratio of the sum of the integrated intensity of the austenite phase (A phase) peak to the sum of the integrated intensity of all peaks. Calculated. It is considered that the solid solution amount of the interstitial solid solution element (C + N) is effective in stabilizing and strengthening austenite from the change in the lattice constant of the austenite phase (A phase) according to the following empirical formula (2). The amount of solid solution was calculated.

Usually, the content of (C + N) is obtained by chemical analysis or gas analysis, but in this example, the austenite phase including metastable state is strengthened by solid solution of interstitial solid solution strengthening elements C and N. Therefore, X-ray diffraction was used to calculate the amount of solid solution, excluding the precipitated portion.
That is, all the (C + N) contents are the values of chemical analysis, and, except for the precipitated carbonitride, the amount of solid solution that can act effectively for strengthening and maintain the metastable austenite state has been clarified.

Austenite phase (A phase) ratio (%) = 100 × (sum of A phase peak integrated intensity / sum of all peak integrated intensity) (1)
(C + N) Solid solution amount (% by mass) = {Lattice constant of phase A (Å) −3.595} /0.035
(2)
In addition, by simulating the processing of the product and its components, the sheet was cold-rolled with a thickness reduction rate of 20%, and the rate of reduction of the austenite phase (A phase) was taken as the rate of metastable A phase. . For example, in No. 1 (example of the present invention) in Table 2 described later, the ratio of the austenite phase after heat treatment is 24 area%, and the austenite phase is subjected to cold rolling at a sheet thickness reduction rate of 20%. The proportion is reduced to 12 area%. That is, 12 area% of 24 area% before cold rolling is a metastable austenite phase, and the remaining 12 area% is a stable austenite phase. In Table 2, No. 2 (example of the present invention) has an austenite phase after heat treatment of 26 area%, of which 15 area% is a metastable austenite phase and 11 area% is a stable austenite phase. In the present invention, basically, this is in proportion to the proportion of this metastable austenite phase, and both high strength and excellent workability are achieved.

  Furthermore, after embedding, polishing and etching the cross section, the compound was observed using an optical microscope, and the maximum equivalent circle diameter of 1 μm or more and the number of compounds were measured.

(Ii) Hardness: The plate surface was measured at 9.8 N using a Vickers hardness meter.
(Iii) Tensile properties: JIS-13B test pieces collected in a direction parallel to the rolling direction were measured using an Instron type tester, and the elongation was measured.
(Iv) Bendability: Strip specimens taken in a direction parallel to the rolling direction were measured using a right-angle bending die with a bending radius of 1.0 mm and a hydraulic press tester, and the presence or absence of cracks was investigated after processing. . In the evaluation, the case where there was no crack was evaluated as ○, and the presence of crack was evaluated as ×. In addition, although it was not cracked, the wrinkles and the unevenness | corrugation produced, and what was not able to bend about the invention materials 1-22 was evaluated by (triangle | delta).

Table 2 summarizes various properties of the stainless steel plate after the final heat treatment. In Table 2, those satisfying the conditions defined in the present invention were designated as invention materials, and others were designated as comparative materials.
Regarding conditions other than those shown in Table 2, heat treatment other than high temperature holding was also performed in the same atmosphere as high temperature heating. The dew point of the atmosphere was −40 ° C. The cooling rate after holding at a low temperature was 0.5 ° C./second.

  Inventive materials 1 to 22 shown in Table 2 have an austenite phase (A phase) ratio of 15% or more, of which 8% or more are in a metastable state, and the balance is ferrite phase (F phase) or It showed a multiphase structure consisting of a ferrite phase (F phase) and a martensite phase (M phase).

  Inventive materials 1 to 22 exhibited a value exceeding 250 in hardness HV1 and a value exceeding 10% in elongation. In particular, the relationship between the excellent elongation and hardness required for the spring parts was exhibited, and the product value thereof was 5000 or more, which was generally high. Furthermore, the inventive materials 1 to 22 also had excellent bendability.

  On the other hand, like the comparative materials 23 to 27, even if the chemical components defined in the present invention are satisfied, if the manufacturing conditions do not match, the metal structure defined in the present invention cannot be obtained, and the hardness and elongation The product did not reach 5000, and the bendability was unsatisfactory with some wrinkles and irregularities as indicated by Δ.

  In addition, the comparative materials 28 to 33 that do not satisfy the chemical components defined in the present invention were similarly unsatisfactory.

Claims (4)

In mass%, C: 0.1 to 0.4%, Si: 2.0% or less, Mn: 0.1 to 6.0%, Cr: 10.0 to 28.0% or less, N: 0.0. 17% or less, the balance has a chemical composition consisting of Fe and impurities, and the C + N solid solution amount of the austenite phase is 0.5% or more, and the austenite phase ratio is 10 to 90% by area ratio, among them, at least 5 area% or more metastable state, with the balance gaff ferrite phase and martensite phase, C r, the carbide and / or nitride of Fe, a maximum equivalent circle diameter is 10μm or less in the cross-section have a metal structure is greater than the hardness HV1 250, elongation more than 10%, and a spring for multi-phase product of the elongation and the hardness HV1 is characterized der Rukoto 5000 or Stainless steel.
However, the ratio of the austenite phase in the metastable state is the ratio at which the austenite phase decreased before and after this cold rolling after cold rolling with a sheet thickness reduction rate of 20%.   2. The compound for spring according to claim 1, wherein the chemical composition further includes one or two kinds selected from the group consisting of Ni: 2.0% or less and Cu: 2.0% or less in terms of mass%. Phase stainless steel.   The chemical composition further includes one or more selected from the group consisting of Nb: 0.5% or less, V: 0.5% or less, and Ti: 0.5% or less in mass%. 3. The duplex stainless steel for springs according to claim 1, further comprising a metal structure in which a fine compound of Nb, V, Ti carbide and / or nitride having a circle equivalent diameter of 10 μm or less is present. The duplex stainless steel for springs according to any one of claims 1 to 3 is obtained by performing one or more combinations of hot and cold processing and heat treatment after the processing. After the final cold working to reduce the thickness of the product to a product plate thickness, it is heated for 10 seconds or more in a temperature range from Ac ₁ transformation point to Ac ₃ transformation point, and from the surface, nitrogen and carbon One or both of them are absorbed and diffused, then cooled to a predetermined temperature of 200 to 500 ° C. at a cooling rate of 1 ° C./second or more, held at the predetermined temperature for 10 seconds or more, and then cooled to room temperature. A method for producing a spring duplex duplex stainless steel.

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