JP2003171744A - Chromium based stainless steel with double layer structure, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide chromium based stainless steel which is provided with properties equal to or above those of expensive high Ni-austenitic stainless steel (SUS304-CSP) for a spring having excellent strength and rust resistance, and to provide a production method therefor. <P>SOLUTION: The stainless steel contains 0.01 to 0.15% C, 16 to 20% Cr, and 1.5 to 3.0% Cu. A surface layer part consists of a mixed structure containing a martensitic phase and a retained austenitic phase. An internal layer part consists of a mixed structure containing a ferritic phase and a martensitic phase, or substantially consists of a martensitic single phase structure. The maximum grain diameter of unsolidified Cu grains in the surface layer part is ≤0.5 μm. The content of nitrogen in the surface layer part is 0.03 to 0.5 mass%. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a chromium-based stainless steel material and a method for manufacturing the same. In particular, the present invention relates to a chromium-based stainless steel material having excellent rust resistance and spring characteristics, and a method for manufacturing the same.

[0002]

2. Description of the Related Art SUS is used for chromium-based stainless steel for springs.
420J2-CPS martensitic stainless steel is specified (JIS-G4313). However, there is a problem that the rust resistance is insufficient because the Cr content is as low as 12 to 14%.

Even in the past, for example, Japanese Patent Laid-Open No. 3-5662
Japanese Patent Publication No. 1 proposes a chrome-based stainless steel which has an excellent spring property and a Cr content close to 20% by weight. However, it utilizes a mixed structure of a ferrite phase and a martensite phase.

On the other hand, in Japanese Patent Laid-Open No. 2001-140041,
The applicant of the present application has proposed a multi-layer structure having a surface layer portion and an inner layer portion as spring steel. This is a mixed structure in which the surface layer of the steel material contains a martensite phase and a retained austenite phase,
The inner layer is a multi-layered chromium-based stainless steel having a mixed structure of ferrite phase and martensite phase.

[0005]

Recently, however, automobile parts manufacturers have been studying cost reduction by omitting the painting process, and accordingly, more severe rust resistance has been required for stainless steel materials. For example, chrome-based stainless steel for springs is used for a diaphragm of an automobile horn. However, the rusting caused by salt sprayed to prevent road surface freezing in cold regions such as North America and Northern Europe, and the corrosion caused by a drop in pH due to chloride ion concentration in the gap, SUS430
Even with (general purpose 16Cr steel), there are cases where there is not enough resistance. Generally, these rust resistance is improved by increasing the amounts of Cr and Mo. However, these elements are expensive and are ferrite-forming elements. Therefore, there is a problem that the addition of the same element raises the material cost and lowers the material strength required for the spring material.

An object of the present invention is to provide an inexpensive chromium-based stainless steel material applicable to an environment requiring more severe rust resistance and a method for producing the same, in view of the recent trends in users. .

Specifically, the object of the present invention is to provide a property (corrosion resistance) equivalent to or higher than that of an expensive austenitic stainless steel for springs (SUS304-CSP) containing a large amount of Ni and excellent in strength and rust resistance. , A spring property and workability) and a method for producing the same.

[0008]

Means for Solving the Problems The inventors of the present invention have made various studies in order to solve the above problems, and as a result,
In a multi-layer structure chromium-based stainless steel material provided with a surface layer portion and an inner layer portion proposed in 2001-140041,
Unexpectedly, if the required amount of Cu is dissolved in the surface layer,
We obtained the finding that the effect of suppressing the progress of corrosion in a NaCl environment is extremely large, and conducted further studies to find that the effect of 1.5-3.0%
In the chromium-containing stainless steel containing Cu, by finding that the maximum particle size of the undissolved Cu particles in the surface layer is 0.5 μm or less, it was found that the rust resistance of the multi-layer structure chromium-based stainless steel material is significantly improved, The present invention has been completed.

In the above-mentioned Japanese Patent Laid-Open No. 3-56621, Cr: 10 to 20% by weight, C: 0.01 to 0.15% by weight, Ni, Mn.
Or 0.1 to 4.0% by weight of one or more of Cu
The steel composition to be contained is disclosed, but Cu added in that case has an effect equivalent to that of Ni and Mn, and is added to obtain a ferrite + austenite two-phase structure as an austenite forming element at high temperature. Therefore, there is no disclosure or suggestion of improvement in corrosion resistance or rust resistance.

An example of adding Cu to chromium-based stainless steel is also found in Japanese Unexamined Patent Publication No. 10-237597. Cu added in this case: 0.4 to 5 wt% is a Cu-rich phase. This is because the steel is provided with antibacterial properties.

On the contrary, these conventional examples have a mixed structure of a ferrite phase and a martensite phase (multiphase structure), and it is feared that the corrosion resistance is deteriorated by such a sensitization phenomenon during the heat treatment for multiphase formation. Will be

Therefore, the present invention is as follows. (1)% by mass, C: 0.01 to 0.15%, Cr: 16 to 20%, Cu:
Contains 1.5 to 3.0%, consisting of a multilayer structure of the surface layer portion and the inner layer portion, the surface layer portion is composed of a mixed structure containing a martensite phase and a retained austenite phase, the inner layer portion is a ferrite phase and a martensite phase A multi-layered chromium-based stainless steel material comprising a mixed structure or a substantially single-phase martensite single-phase structure containing, and having a maximum particle diameter of 0.5 μm or less of undissolved Cu particles in the surface layer portion.

(2) The nitrogen content in the surface layer is 0.03
The multi-layered chromium-based stainless steel material according to (1) above, which is ˜0.5 mass%. (3)% by mass, C: 0.01 to 0.15%, Cr: 16 to 20%, Cu:
Chromium-based stainless steel containing 1.5 to 3.0% was soaked in a nitrogen-containing atmosphere to a soaking temperature T defined by the following formula (1), and nitrogen in the nitrogen-containing atmosphere was absorbed by the surface layer of the steel. After that, a method for producing a multi-layer structure chromium-based stainless steel material, which comprises performing a multi-layer heat treatment for cooling at a cooling rate of 1 ° C./second or more.

T (° C.) ≧ 93Cu (mass%) + 760 (1) (4) The nitrogen-containing atmosphere contains hydrogen: 10 vol% or more, nitrogen: 5 vol% or more, and dew point: −30 The method for producing a multi-layer structure chromium-based stainless steel material according to the above (3), which has a temperature of ℃ or less.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described more specifically with reference to the accompanying drawings. In the present specification,
Unless otherwise specified, the chemical composition, that is, “%” indicating the steel composition means “mass%”.

FIG. 1 is a graph showing typical experimental results for rust resistance, which is a graph showing the corrosion weight loss after half immersion in an aqueous solution containing 45 ° C.-1.5% NaCl (pH hydrochloric acid adjustment). The steel material of the present invention is obtained by adding 2% Cu to a SUS430 (16% Cr steel) material, and is subjected to a multilayer heat treatment at a soaking temperature at which Cu theoretically completely forms a solid solution, and the surface layer portion is substantially The solid solution of 2% Cu is rapidly cooled before the experiment. The maximum particle size of undissolved Cu was 0.1 μm. The steel material of the present invention has a significantly suppressed progress of corrosion in an acidic environment (pH 2-1) as compared with the SUS430 steel, and in a wide range of neutral to acidic environments,
It was found that SUS304-CSP (18% Cr-8% Ni steel) shows excellent rust resistance equivalent to or better than that. Although the above-mentioned mechanism in the present invention has not been clarified yet, it is presumed as follows at present.

That is, in an aqueous solution containing NaCl, the solid solution Cu in the surface layer forms a CuCl film having a passivation effect,
This is considered to have suppressed the progress of corrosion. 16% Cr
In the steel, the Cr film shifts from the passivated state to the active dissolved state at the boundary of pH 2. However, in the steel material of the present invention,
It is speculated that the dissolution of the base material (Fe elution) was suppressed by the interfacial reaction between Cu and Cl ions in the active dissolution state of the Cr film.

The reason for limiting the steel composition in the present invention is as follows. Cr: Cr is a ferrite-forming element and is an essential element for ensuring rust resistance. In order to secure the rust resistance targeted by the present invention, the amount of Cr is set to 16% or more. On the other hand, an increase in Cr content causes an increase in steel cost and a decrease in material strength, so the upper limit is 20%. It is preferably 18% or less.

C: C is a typical austenite forming element and has a great influence on the martensite hardening ability. In order to obtain the material strength required for the spring material, the C content should be 0.01% or more. On the other hand, an increase in the amount of C causes deterioration in hot workability and product workability, so the lower limit is made 0.15%.

N: N is a typical austenite forming element like C, and is an element effective for improving spring fatigue strength. However, it is difficult to add a large amount of N by an ordinary melting method, and a steel containing a large amount of N has poor hot workability and causes surface defects such as edge cracks during hot rolling. . Therefore, the amount of N is 0.01 to 0, which is obtained by an ordinary melting method.
04% is fine. ． Cu: Cu is an austenite forming element and is an element effective in adjusting the amount and hardness of the martensite phase. Furthermore, it is an essential element for obtaining the rust resistance targeted by the present invention. The lower limit of the content is 1.5% to obtain the target rust resistance. On the other hand, excessive addition impairs the hot workability of steel, so the upper limit is 3.0%. Preferably 2.5%
Below.

Ti: Ti is an element that forms ferrite, and is an element that is effective for refining crystal grains. Therefore, although it is not an essential element, it may be contained. In that case, the content should be 0.003% or more. On the other hand, if Ti is contained excessively, not only the economical efficiency is impaired, but also C and N in the steel are fixed to cause a decrease in strength, so the upper limit is made 0.03%.

Nb: Nb is a ferrite-forming element, and also has an action of fixing C and N to suppress the sensitization phenomenon caused by the heat treatment for multilayering. Therefore, although not an essential element,
It may be contained. In that case, the content is 0.005%
That is all. On the other hand, when Nb is excessively contained, C in steel,
Since the N element is fixed and causes a decrease in strength, the upper limit is 0.1%.

Mo: Mo is a ferrite-forming element and has the effect of significantly improving rust resistance. Therefore, although it is not an essential element, it may be contained. In that case, the content shall be 0.1% or more. However, Mo is expensive, and if it is contained in an excessive amount, it impairs economic efficiency, and the upper limit is made 1.0% in order to reduce the strength required for the spring material.

Ni and Mn: All of these are austenite forming elements and are effective elements for adjusting the amount and hardness of the martensite phase. Therefore, although it is not an essential element from such a viewpoint, it may be contained. In that case, the content of each should be 0.3% or more. On the other hand, Ni
Is excessively contained, it impairs economic efficiency, so the upper limit is
1.0% If Mn is contained excessively, it has the effect of lowering rust resistance, so its upper limit is made 1.0%.

V: V is an effective element for obtaining strength. Therefore, although it is not an essential element, it may be contained. In that case, the content should be 0.05% or more. But,
If it exceeds 0.3%, the effect will be saturated, so the upper limit is 0.3.
%.

Si: Si is an element used as a deoxidizer for steel. However, if it is contained excessively, the toughness of the steel is impaired, so its upper limit is made 1.0%. Al: Al is an element effective as a deoxidizer for steel. But,
Since Al forms a nitride and reduces the workability, the upper limit of the content is 0.05%.

Rare earth element: It may be contained because it has the effect of improving the oxidation resistance of steel. But in total amount
If the content exceeds 0.1%, the effect will be saturated and the cost will increase, so the content should be 0.1% or less.

The balance is Fe and inevitable impurities. Summarizing these, C: 0.01 to 0.15%, Cr: 16 to 20%, Cu:
The present invention is not particularly limited to this as long as it contains 1.5 to 3.0%, but the steel composition according to the present invention can be described as follows.

C: 0.01 to 0.15%, Cr: 16 to 20%, Cu: 1.5 to
3.0% or, if desired, any of the following compositions in addition to any of the following compositions: As a ferrite-forming element, Ti: 0.003 to 0.03%, Nb:
At least one austenite forming element selected from the group consisting of 0.005 to 0.1% and Mo: 0.1 to 1.0%, Ni: 0.3 to 1.0% and / or Mn: 0.3 to 1.0%, V: 0.05 to 0.3%, Si : 1.0% or less, Al: 0.05% or less, rare earth element: 0.1% or less.

In such an embodiment, the balance of the steel composition may be Fe and inevitable impurities. The steel material of the present invention is composed of a mixed structure containing a martensite phase and a retained austenite phase in the surface layer portion, and the maximum particle diameter of undissolved Cu particles in the metal structure of the surface layer portion is 0.5 μm or less, and the inner layer portion is ferrite. A two-phase mixed structure composed of a phase and a martensite phase or a martensite single-phase structure.

The presence of the martensite phase has the effect of increasing the hardness and elastic proportionality limit of the steel and improving the spring characteristics. To obtain this effect, the ratio of martensite phase is 40
It is preferable that the content is at least vol%. It is more preferably 50% by volume or more. On the other hand, if the ratio of the martensite phase is excessively high, the ductility of the steel decreases and the workability is impaired, so the martensite ratio of the surface layer portion is preferably 95% by volume or less.

The retained austenite phase is softer and richer in workability than the martensite phase, and has a function of causing work-induced transformation when processed to make the structure extremely tough.
It also has the effect of increasing the toughness of the steel material after the heat treatment for multilayering.
Further, by arranging an austenite phase having a high absorbing ability such as C and N in the surface layer portion, C and N which cause a sensitization phenomenon are absorbed to suppress deterioration of rust resistance caused by the heat treatment for multilayering. can do. In order to obtain these effects, the ratio of the retained austenite phase in the surface layer portion is preferably 3% by volume or more. More preferably 5% by volume
That is all.

In the surface layer portion, in addition to the above-mentioned two phases, there is no problem even if a ferrite phase to be mixed is present as long as the characteristics of the steel are not adversely affected. The ferrite phase causes deterioration in spring characteristics and rust resistance due to heat treatment for multilayering, and therefore, even when mixed, it is desirable that the content thereof be 10% by volume or less.
It is more preferably 5% by volume or less.

Surface layer part involved in corrosion (surface-subcutaneous 0.01
mm) is for improving rust resistance in a low pH environment.
The maximum particle size of undissolved Cu particles is 0.5 μm or less. Cu particles hinder the formation of a passive film on the surface of stainless steel and reduce rust resistance. When the maximum particle size of the undissolved Cu particles exceeds 0.5 μm, the chrome-based stainless steel to which the present invention is applied has remarkably deteriorated rust resistance in a NaCl environment of low pH. Therefore, in order to obtain the effect of improving the rust resistance by solid solution of Cu, the maximum particle size of undissolved Cu particles is set to 0.5 μm or less.
It is more preferably 0.1 μm or less. Of course, Cu may be in a completely solid solution state, that is, the maximum particle size of undissolved Cu particles may be 0 μm.

The surface layer has a mixed structure containing a retained austenite phase in addition to the martensite phase, and the precipitation amount of the second phase mainly composed of Cu is less than 0.2% by volume.
By solid solution of Cu of 5 to 3.0%, not only spring property and workability but also rust resistance can be remarkably improved. The thickness of the surface layer is 5 μm or more in order to obtain the above-mentioned effective effects.
More preferably, it is 10 μm or more. If it exceeds 15 μm, the productivity of the multilayer heat treatment is hindered and the above properties may be adversely affected. Therefore, the thickness of the surface layer portion is preferably 15 μm or less.

The metal structure of the inner layer portion is a two-phase mixed structure composed of a ferrite phase and a martensite phase or a substantially martensite single phase structure. The reason is that the amount of work deformation due to bending work is small in the inner layer portion of the steel, and the strength improvement due to work-induced transformation cannot be expected even if there is a retained austenite phase.

The inclusion of the ferrite phase in the inner layer is not essential, but the presence of the ferrite phase has the effect of improving workability. However, if the ratio of the ferrite phase increases, the strength decreases and the spring characteristics, especially the spring fatigue characteristics are impaired, so
Even when the ferrite phase is contained, the upper limit is preferably 90% by volume.

As used herein, the term "surface layer portion" means, for example, a high nitrogen concentration region near the steel surface formed by diffusion of nitrogen absorbed from the atmosphere inside the steel, and generally, It is a relative term to the inner layer portion and refers to a region including the surface of the steel material. And, in the case exemplified above, the thickness of the surface layer portion can be obtained by measuring the nitrogen concentration profile from the surface of the steel with an EPMA device, or by observing the cross section after SEM observation, etc. The structure of the surface layer part is determined by the structure of the high nitrogen concentration region, and the structure of the inner layer part is determined by the structure of the low nitrogen concentration region inside the steel. Here, the high nitrogen concentration region is a region where the nitrogen concentration is increased by the multilayer heat treatment with respect to the nitrogen concentration of the heat-treated material before the multilayer heat treatment, and the low nitrogen concentration region is the high nitrogen concentration region. It is a region where the nitrogen concentration is lower than the region.

More specifically, the metallographic structure of the surface layer and the diameters of the undissolved Cu particles are determined by the structure observed by surface polishing, as can be seen from the description of the examples described later, and in that region. The diameter of the undissolved Cu particles may be determined by the major axis of the undissolved Cu particles that is extracted by extracting the Ti mesh and observing it with a transmission electron microscope at a magnification of 2000 times.

The "multilayer structure" is, for example, as described above, a mixed structure in which the steel surface layer contains a martensite phase and a retained austenite phase, and the inner layer contains a ferrite phase and a martensite phase. It refers to a mixed structure or a structure that is substantially a martensite single-phase structure, and generally refers to a structure in which the surface layer portion structure and the inner layer portion structure are different.

"Multilayer heat treatment" means, for example, mass%
And, C: 0.01% or more and 0.15% or less, Cr: 16% or more and 20% or less, and Cu: 1.5% or more and 3.0% or less are uniformly soaked in a nitrogen-containing atmosphere, and at least the surface layer portion is austenite. This is a heat treatment in which a single phase is used, the nitrogen in the nitrogen-containing atmosphere is absorbed by the steel surface layer portion, and then the steel surface layer portion is cooled at a cooling rate of 1 ° C./sec.

The term "substantially martensitic single-phase structure" means that, in addition to the martensite phase, a ferrite phase mixed due to segregation of the raw material exists within a range that does not adversely affect the characteristics of the steel. It is meant to include cases.

The form of the steel material according to the present invention is typically a cold-rolled steel sheet or a hot-rolled steel sheet, but it is not limited to this, and includes foils, wire rods, bar steels, pipes and the like. Is. In short, the form is not particularly limited as long as it is a steel material having a multilayer structure.

The method for producing a chromium-based stainless steel material of the present invention will be described by taking the case where the "steel material" is a cold rolled steel sheet as an example. A known method for slab of steel having the above-mentioned steel composition, for example, melting steel in a converter or an electric furnace, performing vacuum degassing treatment, and continuously casting or ingot-forming slab rolling Etc. to produce slabs. The obtained slab is hot rolled by a known method to produce a hot rolled steel sheet. This hot rolled steel sheet is annealed according to a conventional method, and the surface scale is removed by a known method such as pickling.

After that, the steel sheet is manufactured by cold rolling by a known method. The cold rolling may be performed multiple times of cold rolling including intermediate annealing, or may be cold rolling that does not include intermediate annealing. The dimension of the cold-rolled steel sheet is not particularly limited, and may be a commonly used thickness (for example, 0.1 to 2.0 mm).

After the final cold rolling, a multilayer heat treatment is performed in a nitrogen-containing atmosphere at a soaking temperature at which the solubility of Cu completely dissolves Cu in steel. The upper limit of the soaking temperature of the multilayer heat treatment is 1200 ° C in order to secure the high temperature strength of the steel necessary for passing the steel in the continuous annealing line. The solubility of Cu in stainless steel increases with the increase of the soaking temperature of the multilayer heat treatment. The soaking temperature of the multilayer heat treatment is related by the Cu content as follows.

T (° C.) ≧ 93 × Cu (mass%) + 760, T: Soaking temperature Here, the soaking temperature is a temperature aimed at the complete solid solution of Cu. Cooling after soaking is performed at 1 ° C./sec or more in order to suppress the occurrence of a sensitization phenomenon and the precipitation of coarse Cu particles. If the cooling rate is less than 1 ° C / sec, the rust resistance deteriorates due to the sensitization phenomenon and the precipitation of coarse Cu particles. It is preferably 5 ° C./second or more. On the other hand, since it is practically difficult to set the cooling rate to over 1000 ° C / sec, the upper limit is set to 1000 ° C / sec or less. It is preferably 50 ° C./sec or less, more preferably 25 ° C./sec or less.

The above nitrogen-containing atmosphere is preferably set as follows in order to control the metallographic structure of the steel material surface layer in the heat treatment for multilayering. The hydrogen concentration in the nitrogen-containing atmosphere is preferably 10% by volume or more. If the hydrogen concentration in the atmosphere is less than 10% by volume, a thick oxide film (> 500Å) is likely to be formed on the surface of the steel material, impairing the aesthetic appearance of the surface and making it difficult to control the nitrogen absorption amount. More preferably
It is 50% by volume or more.

The nitrogen concentration in the nitrogen-containing atmosphere is set to 5% by volume or more in order to control the amount of nitrogen absorbed by the steel surface layer. If the nitrogen concentration in the atmosphere is less than 5% by volume, a multilayered structure cannot be obtained.

The dew point of the above nitrogen-containing atmosphere is -30 ° C or lower, more preferably -40 ° C or lower. If the dew point exceeds -30 ° C, a thick oxide film easily forms on the steel surface,
The appearance of the surface is impaired, and it becomes difficult to control the nitrogen absorption amount.

Here, in the steel material according to the present invention, the diameter of the undissolved Cu particles can be adjusted, for example, by changing the soaking temperature of the multilayer heat treatment. The term "corrosion resistance" as used in the present specification is one characteristic generally included in the corrosion resistance, but in the case of the present invention, it is the characteristic evaluated by the corrosion weight loss in the NaCl environment.

Next, the function and effect of the present invention will be described more specifically with reference to Examples.

[0053]

[Examples] Table 1 shows the steel composition, that is, the chemical composition, of the test steel in this example. A chromium-based stainless steel continuous casting slab having these steel compositions is heated to 1150 to 1200 ° C, hot rolling is completed at a finishing temperature of 900 to 950 ° C, and a thickness of 3.2 mm is obtained.
Hot rolled steel strip. These hot-rolled steel strips are annealed at 750 to 830 ° C, then shot-blasted, pickled with hydrofluoric acid and descaled, and then cold-rolled with an intermediate anneal to give a thickness of 0.25. A steel sheet having a thickness of mm was further subjected to a multilayer heat treatment under the conditions described below.

[0054]

[Table 1]

For the multilayer heat treatment, a continuous bright annealing furnace is used, and the soaking atmosphere is 5 to 25% by volume of nitrogen and 95 to 75% by volume of hydrogen.
And the atmosphere dew point was -40 ° C or lower. The steel surface temperature during soaking was 850-1050 ℃. The soaking time was in the range of 5 to 45 seconds, and the cooling rate after soaking was 10 to 25 ° C / second. For comparison, a gas having a mixing ratio other than the above was used as the soaking atmosphere.

In this example, the diameter of the undissolved Cu particles was adjusted by changing the soaking temperature of the multilayer heat treatment.
As comparative steels, commercially available austenitic spring stainless steels SUS301-CSP (3 / 4H specification material) and SUS304-CSP (H specification material) were prepared.

The ratio of the martensite phase in the surface layer portion was measured by observing the polished and corroded sample surface with a microscope by a conventional method. The ratio of the retained austenite phase was obtained by measuring the integrated intensity of α-Fe and γ-Fe by X-ray diffraction on the sample surface and calculating the ratio of the integrated intensity. The balance was the ratio of the ferrite phase.

The volume ratios of the martensite phase and the ferrite phase in the inner layer portion were determined by observing the cross section of the sample polished and corroded by a conventional method with a microscope. The nitrogen content in the surface layer portion was quantified in the cross section of the test piece mirror-polished by an EPMA apparatus having a dispersive crystal LAD (artificial multilayer film) dedicated to measuring the nitrogen content. In addition, γ obtained from the measured values of these nitrogen contents and X-ray diffraction
The lattice constant of -Fe was regressed to obtain the following relational expression.

Nitrogen content in surface layer [mass%] = (γ-Fe
Lattice constant [Å] −3.592) /0.0394 The nitrogen content in the surface layer portion can also be simply calculated by the above formula using the lattice constant of γ-Fe obtained from X-ray diffraction.

The undissolved Cu particles in the surface layer were obtained by buffing the surface of the steel sheet and then using 10% acetylacetone-1 on the corroded sample surface.
It was immersed in a% TMAC-methanol solution and extracted into a Ti mesh. The Cu particles extracted in the Ti mesh were observed with a transmission electron microscope at × 2000, the maximum value of the major axis of the observed Cu particles was measured, and this was taken as the maximum particle diameter of the undissolved Cu particles. Cu
The chemical composition of the particles was confirmed by EDX elemental analysis.

The surface hardness was measured by the Vickers hardness test method defined in JIS-Z2244 under a load of 9.8 N. The spring fatigue limit was measured by a repeated plate bending tester using test pieces in the rolling direction (L direction) and the rolling vertical direction (T direction). The spring fatigue limit was the maximum stress at which the test piece did not break, with a maximum of 10 ⁷ times in the repeated flat plate bending test with a constant amplitude of 30 Hz.

The rust resistance was evaluated by visually observing the rusting condition after immersing in a 45 ° C.-1.5% NaCl aqueous solution (neutral, pH 1: hydrochloric acid adjusted) for 100 hours and a half. I decided. When stains were confirmed, it was regarded as equivalent (△) to SUS304.

Table 2 shows these results and evaluation.
And summarized in Table 3.

[0064]

[Table 2]

[0065]

[Table 3]

In Table 2, reference characters A1, B1, B2, C1, D1,
D2 is an essential element, C: 0.01 to 0.15% by mass,
Cr: 16 to 20% by mass, Cu: 1.5 to 3.0% by mass, the surface layer part has a mixed structure containing a martensite phase and a retained austenite phase, and undissolved Cu in the metal structure of the surface layer part.
It is a multi-layered chromium-based stainless steel sheet with a maximum particle size of 0.5 μm or less.

On the other hand, reference numerals A2, B3 and D3 are metallic structures containing no retained austenite phase. Also, the symbols A2 and D3
Is a steel sheet in which the maximum particle size of undissolved Cu particles exceeds 0.5 μm. Codes El and F1 are steel sheets that do not contain the essential element Cu or the content thereof is less than 1.5%.

In Table 3, symbols Al, Bl, B2, Cl, D1 and D2
Has a spring fatigue limit (≧ 600 N / mm ² ) and bending workability (R / t ≦ 2) equivalent to the comparative steels 301-CSP (3 / 4H) and 304-CSP (H). -Has excellent rust resistance equivalent to or better than -CSP (H).

Reference numerals A2, B3, D3, El and F1 are target 3
Corrosion resistance equal to or higher than that of 04-CSP (H) was not obtained.

[0070]

EFFECTS OF THE INVENTION According to the present invention, it is possible to obtain excellent rust resistance equivalent to or better than SUS304 steel (18Cr-8Ni) in an inexpensive chromium-based stainless steel material for springs containing almost no Ni. In particular, it is most suitable as a diaphragm for automobile horns for the purpose of omitting painting.

[Brief description of drawings]

FIG. 1 is a graph showing rust resistance of a steel material according to the present invention.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kazuhiko Adachi             4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture             Sumitomo Metal Industries, Ltd. (72) Inventor Masahiro Aoki             2-12-1 Minatomachi, Joetsu City, Niigata Prefecture Stock Association             Sumitomo Metals Naoetsu (72) Inventor Kenichi Goshokubo             2-12-1 Minatomachi, Joetsu City, Niigata Prefecture Stock Association             Sumitomo Metals Naoetsu F-term (reference) 4K028 AA02 AB01 AC07 AC08 CC02                       CD02 CE02

Claims (4)

[Claims] 1. In mass%, C: 0.01 to 0.15%, Cr: 16 to
20%, Cu: 1.5-3.0%, consisting of a multi-layered structure of a surface layer and an inner layer, a surface layer of a mixed structure containing a martensite phase and a retained austenite phase, and an inner layer of a ferrite phase. And a martensite single-phase structure or a mixed structure containing a martensite phase,
The maximum particle size of undissolved Cu particles in the surface layer is 0.5 μ
A multi-layered chromium-based stainless steel material characterized by having a thickness of m or less. 2. The nitrogen content in the surface layer is 0.03 to
The multi-layer structure chromium-based stainless steel material according to claim 1, which is 0.5% by mass. 3. In mass%, C: 0.01 to 0.15%, Cr: 16 to
A chromium-based stainless steel material containing 20% and Cu: 1.5 to 3.0% is soaked in a nitrogen-containing atmosphere to a soaking temperature T defined by the following formula (1), and the nitrogen in the nitrogen-containing atmosphere is used as the surface layer of the steel material. A method for producing a multi-layered chromium-based stainless steel material, which comprises performing a heat treatment for multi-layering, which is absorbed at a part and then cooled at a cooling rate of 1 ° C / sec or more. T (° C) ≥ 93Cu (mass%) +760 (1) 4. The nitrogen-containing atmosphere is hydrogen: 10% by volume
As described above, the method for producing a multi-layer structure chromium-based stainless steel material according to claim 3, wherein the content of nitrogen is 5% by volume or more and the dew point is −30 ° C. or less.