JP3470660B2 - Chromium stainless steel material for spring and multi-layered structure for spring and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromium-based stainless steel having excellent spring characteristics, workability and corrosion resistance.

[0002]

2. Description of the Related Art Conventionally, as a stainless steel strip or a steel plate for springs (simply referred to as "steel plate"), for example, JIS-G4 is used.
According to 313, SUS301 as austenite type
-CSP and SUS304-CSP, SUS420J2-CSP as a martensite system, and SUS631-CSP as a precipitation hardening system.

[0003] Austenitic spring steel sheets are used to increase the strength and to obtain spring characteristics by work hardening by cold rolling, and have three to four hardness levels obtained by changing the cold rolling rate. It is prescribed. The above steel plate is made of material
The product is shipped from the car in a cold rolled state and processed into a desired shape by a processing manufacturer. After processing, aging heat treatment is often performed for the purpose of improving spring characteristics.

Austenitic spring steel plates are excellent steel plates for springs because they have good corrosion resistance and workability. However, since it contains a large amount of expensive Ni, there is a problem that the steel material cost is high.
Further, when manufacturing an ultra-thin steel sheet having a thickness of 0.3 mm or less, there is a problem that a rolling load during cold rolling is high and it is difficult to obtain a steel sheet having a good shape.

Since the martensitic spring steel plate has a chemical composition of chromium, it is less expensive than the austenitic spring steel plate. Since the martensitic spring steel plate is difficult to work in a high strength state after quenching or quenching-tempering, it is usually shipped from the steel sheet manufacturer in an annealed state. This spring steel plate is processed into various shapes by a processing maker, and then subjected to quenching-tempering treatment to increase strength and obtain spring characteristics. However, since quenching-tempering is required after processing, there is a problem that the cost of the final product becomes high. Further, since the Cr content is as low as 12 to 14%, there is a problem that corrosion resistance may be insufficient.

The precipitation hardened spring steel plates are cold rolled from the material manufacturer in the same manner as the austenitic spring steel plates except for SUS631-CSP-0 which is shipped after solution heat treatment. Are shipped and processed into a desired shape by a processing maker and then subjected to precipitation hardening heat treatment. Precipitation-hardening spring steel plates are expensive because they contain a large amount of Ni, and since work hardening is large, the rolling load during cold rolling is high. For example, when manufacturing ultra-thin steel plates of 0.3 mm or less, There are problems such as difficulty in obtaining a steel sheet having a good shape.

As a manufacturing method for improving the performance of a chromium-based stainless steel sheet, Japanese Patent Laid-Open No. 63-169330 discloses a chemical composition in which the mass% is Cr: 10 to 20%, and C: 0.
15% or less, N: 0.12% or less, and (C + N):
Ac 1 point or more that satisfies the relationship of 0.02 to 0.20% 1
Disclosed is a method for producing a high-strength dual-phase chromium stainless steel strip having excellent ductility by heating to a two-phase region temperature of 100 ° C. or lower and then rapidly cooling to have a mixed structure of ferrite and martensite.

According to the above-mentioned publication, high strength and workability can be obtained by heating the steel to a two-phase region and then quenching it so that the steel structure has a multi-phase structure composed of a soft ferrite phase and a hard martensite phase. It is a manufacturing method that has both. In Japanese Patent Laid-Open No. 3-56621, Cr: 10 to 2 in mass%.
0%, C: 0.01 to 0.15%, Ni, Mn or C
A cold-rolled steel sheet containing 0.1 to 4% of one or more of u is subjected to a multiphase heat treatment to form a crystal structure of the mixed structure of ferrite and martensite, and then temper rolling is performed if necessary. Then, a method for producing a high-strength stainless steel plate for spring by performing an aging treatment is disclosed.
Japanese Patent Laid-Open No. 8-19519 discloses that C: 0.
01 to 0.15%, Cr: 10 to 20%, and one or more of Ni, Mn or Cu in total of 0.3 to
5.0% included, martensite 30-90% by volume, the balance ferrite, and by setting the reduction ratio of cold rolling according to the target hardness, the reaction width is small at HV 300 or more and the in-plane anisotropy is small. Disclosed is a method for producing a dual-phase stainless steel sheet having spring properties with low properties.

[0009]

[Patent Document 1] Japanese Patent Application Laid-Open No. 3-56621
In order to obtain high spring characteristics equivalent to those of the H specification material of the austenitic spring steel plate by the manufacturing method disclosed in the publication,
In addition to increasing the Ni content to increase the ratio of the martensite phase generated during the multi-phase heat treatment, C and / or N
It is necessary to increase the content to obtain a high-strength martensite phase.

However, if the ratio of the martensite phase is increased and the strength of the martensite phase is increased, the hot workability may be deteriorated and defects such as edge cracks may occur during hot rolling. There is also a problem that the cost of steel increases by increasing the amount of Ni. Further, these stainless steels have a drawback that the corrosion resistance is deteriorated due to the sensitization phenomenon during the duplex heat treatment, and in order to avoid this, there is a problem that it is costly to modify the equipment for controlling the cooling rate at high speed.

In a stainless steel having a dual-phase structure of ferrite and martensite, the austenite forming element content is increased to increase the martensite phase ratio in order to obtain high strength. If it is too large, the hot workability of the steel is impaired and hot rolling becomes difficult. There is also a problem that if the C and N contents are increased in order to increase the amount of martensite, the corrosion resistance of steel will be impaired.

[0012] In order to avoid such an adverse effect, in Japanese Patent Laid-Open No. 8-319519, the C content of the steel is limited to a low level, and the reduction of the strength level during the multi-phase heat treatment is carried out by cold working according to the target strength. By setting the rolling rate, desired strength and corrosion resistance are obtained. However, in the method disclosed herein, the ductility of the steel sheet decreases due to cold rolling,
The problem that workability deteriorates arises. As described above, the steels disclosed so far have the workability, spring characteristics,
The properties required for spring steel such as corrosion resistance and economy were not comprehensively satisfied.

The object of the present invention is to solve these problems,
It has workability (for example, bendability) equivalent to that of H-spec material of austenitic spring steel sheet and spring characteristics exceeding it, and is superior in corrosion resistance to martensitic spring steel, for example SU.
It is to provide an inexpensive chromium-based stainless steel having corrosion resistance equal to or higher than that of S430 steel (16% Cr), and an inexpensive and easy manufacturing method thereof.

[0014]

Means for Solving the Problems The present inventors have aimed to improve various performances of chromium-based stainless steel as described above.
As a result of various studies such as the influence of the crystal structure of steel, the following new findings were obtained.

The amount of deformation when the steel is processed by bending or the like is larger on the surface (surface layer portion) than on the inside (inner layer portion) of the steel. By allowing the retained austenite phase (γ phase) to exist in the surface layer of steel and utilizing the effect of improving the deformability associated with the work-induced transformation of the γ phase that occurs during processing, surface cracks (micro Suppress the occurrence of cracks)
The processing performance of steel can be improved. Therefore, by forming the surface layer portion of the steel into a mixed structure containing a martensite phase and a retained austenite phase, high strength corresponding to the amount of the martensite phase and excellent workability due to the inclusion of the retained austenite phase are obtained. It is possible to obtain steel having both

Further, in the steel containing the retained austenite phase in the surface layer portion, it is expected that the strength increase due to the work-induced transformation and the improvement of the spring characteristics corresponding to the strength increase can be expected. Furthermore, by providing the surface layer with the mixed structure containing the retained austenite phase, it is possible to suppress the sensitization phenomenon conventionally observed in the ferritic stainless steel and the ferrite-martensite dual phase structural steel. Therefore, it exhibits excellent corrosion resistance. This is an o-ray with a large absorption capacity for C, N, etc.
It is speculated that the arrangement of the stenite phase absorbs C and N, which cause the sensitization phenomenon during welding and heat treatment, and prevents precipitation at grain boundaries. C in mass% is 0.
Chromium-based stainless steel containing 01% or more and 16% or more of Cr can generate a retained austenite phase in the surface layer portion of the steel when brightly annealed and cooled in an N-containing atmosphere in a temperature range of Ac1 point or more. . It is considered that this is because the austenite phase becomes stable by absorbing nitrogen in the surface layer of steel during bright annealing.

The present invention has been completed based on these newly obtained findings, and the gist thereof is as follows (1)-
The chromium-based stainless steel having excellent spring properties described in (3) and the manufacturing method thereof described in (4). It

[0018] (1) Table layer portion martensite phase and residual O - consists austenite phases containing mixed structure, the inner layer portion is characterized by comprising a two-phase mixed structure consisting of ferrite phase and martensite phase spring use multi-layer tissue chrome stainless steel material.

(2) Steel composition in mass% C: 0.01 to
0.10%, Cr: spring multilayer structure chromium stainless steel material according to claim 1, characterized in that one containing 16 to 20%.

(3) N content in the surface layer is 0.1 to 0.5
Spring multilayer structure chromium stainless steel material according to claim 1 or 2, wherein the% by mass.

(4) C: 0.01 to 0.10.
%, Cr: 16 to 20%, a step of producing a steel slab containing it, hot rolling it, a step of annealing and pickling the obtained hot rolled steel material, annealed and pickled hot rolled steel material Cold-rolling process, the cold-rolled steel material obtained, hydrogen: 50
In an atmosphere of volume% or more, nitrogen: 20 volume% or more and less than 50 volume%, and dew point: −40 ° C. or less, heating to a ferrite + austenite two-phase temperature range of Ac1 transformation point or more and cooling.
On the other hand, martensite phase and retained austenite are present in the surface layer.
Any method for producing a spring multilayer structure chromium stainless steel material according to the above, wherein (1) to (3) further comprising the step of applying the bright annealing of a mixed tissue containing phase.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be specifically described below. In addition, the% display of the chemical composition described below means mass%.

A. Chemical composition Cr: Cr is an element necessary for forming a ferrite phase and ensuring corrosion resistance. Further, when bright annealing is performed in a temperature range of Ac1 point or higher, nitrogen supplied from the atmospheric gas is absorbed to stabilize the austenite phase of the steel surface layer portion, and a residual austenite phase is generated after cooling. Is effective for.

In order to generate a retained austenite phase in the steel surface layer and to secure the corrosion resistance equal to or higher than that of SUS430 steel, Cr is preferably contained in an amount of 16% or more. On the other hand, if Cr is contained excessively, not only the cost of the steel material becomes expensive, but also the temperature at which the austenite phase is formed (Ac
1 point) becomes excessively high, and due to insufficient high-temperature strength of steel, there are cases in which there is a problem in operation such as difficulty in strip passing during bright annealing. To avoid this, the Cr content is preferably 20
% Or less. It is more preferably 18% or less.

C: C is an austenite forming element,
In addition, it greatly affects the martensite hardening ability. In order to obtain spring characteristics exceeding the H-spec material of the austenitic spring steel plate, 0.01% or more of C is preferably contained.
If the C content is excessively increased, the proportion of the martensite phase becomes too high, the hardness of the martensite phase also increases, and the hot workability and the workability of the product deteriorate. Furthermore, Ac
When bright annealing is performed in a temperature range of 1 point or more, a sensitization phenomenon is likely to occur and corrosion resistance deteriorates. In order to avoid these disadvantages, the C content is preferably 0.10% or less.

N: N has a typical austenite forming action together with C, and is an element effective for improving spring characteristics. However, it is difficult to add a large amount of N at the time of manufacturing steel by a normal method, and the steel containing a large amount of N does not have good hot workability and causes a cause of ear crack defect during hot rolling. Becomes Therefore, the N content in the chromium-based stainless steel specified by the present invention does not have to be particularly limited in the stage before the bright annealing, and the content of about 0.01 to 0.04% obtained by the usual manufacturing method. Good.

In the present invention, bright annealing is performed in an atmosphere containing nitrogen in a temperature range of Ac 1 point or higher, and N is applied to the steel surface layer portion.
To absorb. As a result, the austenite phase is stabilized, and the crystal structure of the surface layer portion after cooling becomes a mixed structure containing the retained austenite phase. The N content in the surface layer of the steel sheet after bright annealing is preferably 0.1% or more in order to contain the retained austenite phase necessary for improving the spring characteristics. More preferably, it is 0.2% or more. On the other hand, in order to increase the N content, there arises a production problem such as a longer heat treatment time, so the upper limit is preferably 0.50%. More preferably 0.40%
It is the following.

Ni, Mn, Cu: All of these are austenite forming elements and are effective elements for controlling the amount and hardness of the martensite phase after annealing. Also,
By including these elements, the (C + N) content can be reduced, so that the martensite phase can be made soft. Therefore, it is suitable for improving the workability of steel. Furthermore, these elements are Ac1
It has the effect of lowering the point, and has the effect of promoting nitriding by bright annealing at high temperature.

Therefore, although not essential, these elements may be contained in order to obtain the above effects. When they are contained, it is preferable to contain each of them by 0.3% or more. On the other hand, if Ni or Cu is excessively contained, not only the economical efficiency is impaired but also the nitrogen absorption capacity during the bright annealing is reduced, so that the upper limit of each content should be 2.0%. . Mn has a function of increasing the nitrogen absorption capacity during bright annealing, but if it is contained in excess, it has a function of impairing economic efficiency and lowering corrosion resistance. Therefore, even if it is contained, its upper limit is made 2.0%. Good.

Nb: Nb is a ferrite-forming element, and suppresses the sensitization phenomenon that occurs during the cooling process when bright annealing is performed in a temperature range of Ac 1 point or higher, and further, the ferrite phase and the austenite phase (cooling Later, it becomes a martensite phase and a retained austenite phase), and has a function of increasing the strength without lowering the workability. Therefore, it is not essential, but may be contained in order to obtain the above effect. When it is contained, 0.01% or more is preferably contained. 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 preferably 0.1% even when it is contained. Mo:
Mo is not an essential element, but since it is a ferrite-forming element and has the effect of significantly improving corrosion resistance,
Even when the Cr content is low, the target corrosion resistance can be obtained by including Mo. When it is contained, 0.1% or more is preferably contained. However, Mo is expensive, and if it is contained in excess, it impairs economy. Therefore, even if it is contained, the upper limit is preferably 3.0%.

V: Although it is not an essential element, it may be contained because it is an element effective for obtaining strength. When it is contained, 0.05% or more is preferably contained.
However, the above-mentioned effect is saturated when it exceeds 0.3%, so even if it is contained, it is preferable to make it 0.3% or less.

Rare earth element: Normally, it is not contained, but it may be contained because it has the function of improving the oxidation resistance of steel. However, if the total content exceeds 0.1%, the effect is saturated and the cost becomes high. Therefore, even if the total content is 0.1%, it is preferable to be 0.1% or less.

Si: Si is an element effective as a deoxidizing agent for steel, and since it also has the effect of increasing strength, it may be contained. However, if it is contained excessively, the toughness of the steel is impaired, so even if it is contained, the upper limit is preferably 2.0%. Al: Al is an element effective as a deoxidizing agent for steel and may be contained. However, since Al forms a nitride, if Al is contained in excess, it has the effect of reducing the amount of solid solution nitrogen during bright annealing. Therefore, even if it is contained, the upper limit is preferably 0.05%. The balance is Fe and inevitable impurities.

B. Metal Structure The steel of the present invention has a mixed structure containing a martensite phase and a retained austenite phase in the surface layer part, and a ferrite phase and a martensite phase in the inner layer part.
The steel has a phase-mixed structure.

In the martensite phase, the strength and hardness of the steel are increased, and the aging heat treatment is performed to precipitate the solid solution elements (C, N), thereby increasing the elastic proportional limit of the steel and improving the spring characteristics. Is obtained. In order to obtain this effect, the ratio of the martensite phase is preferably 40% by volume or more. 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. Therefore, the martensite ratio of the surface layer portion is preferably 95% by volume or less. It is more preferably 90% or less.

The retained austenite phase is softer and richer in workability than the martensite phase, and at the same time, it 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 hardened steel. Further, by aging heat treatment of the tough structure obtained by the work-induced transformation to precipitate solid solution elements, the elastic proportional limit of the steel can be increased and the spring characteristics can be improved. Further, by arranging an austenite phase having a large absorbing ability such as C and N in the surface layer portion, it is possible to absorb C and N which cause the sensitization phenomenon and improve the corrosion resistance during welding and heat treatment. .

To obtain these effects, the ratio of the austenite phase in the surface layer portion is preferably 5% by volume or more. More preferably, it is 10% by volume or more.

The surface layer portion may contain a ferrite phase other than the above two phases. The inclusion of the ferrite phase in the surface layer is not essential, but the presence of the ferrite phase has a workability improving effect. However, if the ratio of the ferrite phase becomes high, the spring characteristics are impaired. Therefore, even if the ferrite phase is contained, the upper limit is preferably 10% by volume or less.

The volume ratios of the martensite phase, the retained austenite phase and the ferrite phase in the surface layer portion are such that the sum of them does not exceed 100%. The ratio of the metal structure in the present invention may be approximated by the area% on the metal structure observation surface instead of the volume%.

If the surface layer of the steel has a mixed structure containing a martensite phase and a retained austenite phase, workability, spring characteristics and corrosion resistance can be improved. In order to obtain a more effective effect, it is desirable that the thickness of the surface layer portion be 3% or more of the thickness of steel (the diameter of the wire or bar steel). If it is a steel plate, it is 3% or more of the thickness of the steel plate on each of its front and back surfaces. It is more preferably at least 5%.

As the thickness of the surface layer portion becomes thicker, the spring characteristic improving action saturates, and if it is made too thick, the workability of steel may be impaired. Therefore, the thickness of the surface layer is
It is preferably 20% or less of the thickness of the steel (the diameter of the wire rod or bar steel). It is more preferably 15% 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 in order to secure workability and strength. The reason for this is that the amount of work deformation due to bending and the like is small in the inner layer portion of steel, and even if there is a retained austenite phase, strength improvement due to work-induced transformation cannot be expected.

The ferrite phase ratio of the inner layer portion is preferably 30% by volume or more in order to secure the workability of steel. It is more preferably 50% by volume or more. In order to secure the strength of steel, it is desirable that the ratio of the martensite phase is 20% by volume or more. It is more preferably 30% by volume or more. The volume ratio of the ferrite phase and the martensite phase in the inner layer portion is such that the sum of these two phases does not exceed 100% by volume.

C. Manufacturing Method A preferred method for manufacturing the steel of the present invention will be described by taking the case where the product shape is a steel plate as an example.

A steel slab adjusted to the chemical composition range described in the item a is melted in a known method, for example, a converter or an electric furnace, and then subjected to vacuum degassing to obtain a continuous casting method or a steel ingot. After that, the slab is manufactured by a method such as slab rolling.
The obtained slab is hot rolled by a known method to produce a hot rolled steel sheet. The scale of the surface of the hot rolled steel sheet is removed by a known method such as annealing and pickling according to a conventional method.

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

After the final cold rolling, hydrogen: 50
In an atmosphere of volume% or more, nitrogen: 20 volume% or more and less than 50 volume%, and dew point: −40 ° C. or less, heating to a two-phase temperature range of ferrite + austenite having an Ac1 transformation point or more,
Then, bright annealing for rapid cooling is performed.

The mixed structure containing the retained austenite phase in the surface layer portion of the steel is obtained by increasing the stability of the austenite phase by allowing the austenite phase to absorb nitrogen atoms during bright annealing. The hydrogen concentration of the atmosphere is 50% by volume in order to quickly absorb nitrogen from the annealing atmosphere into the steel surface layer.
The above is preferable. When an oxide film is formed on the surface of steel, absorption of nitrogen from the atmosphere is hindered. However, by keeping the hydrogen concentration in the atmosphere within the above range and keeping the dew point low, the formation of the oxide film can be suppressed. It is more preferably 70% by volume or more. The thickness of the oxide film should be less than 100Å.

In order to promote absorption of nitrogen into the steel surface layer,
The nitrogen concentration in the atmosphere is preferably 20% by volume or more. It is more preferably 25% by volume or more.

If the dew point of the atmosphere is high, the thickness of the steel material exceeds 100Å and a dense oxide film is formed, and the absorption of nitrogen to the surface layer does not proceed, so the dew point of the atmospheric gas is -40 ° C. The following control is preferable. More preferably, it is −45 ° C. or lower. The atmosphere gas may include an inert gas such as Ar gas that does not cause surface oxidation and a catalyst such as NH ₃ that accelerates the nitriding reaction.

The bright annealing temperature is set to Ac1 point or higher. Ordinary chromium-based stainless steel is annealed in a temperature range of less than Ac1 point. When bright annealing is performed at less than the Ac1 point, the oxide film (Cr ₂ O ₃ ) that is thermodynamically formed on the surface of the steel material is stable, and since the amount of solid solution nitrogen in the ferrite phase is small, the steel from the nitrogen gas phase can Absorption of nitrogen to the surface layer does not occur.

On the other hand, annealing is performed in a temperature range exceeding Ac1 point.
And the stability of the oxide thermodynamically decreases,
In a low dew point atmosphere with low tension, Cr on the steel surface₂
O₃Is reduced to Cr₂O₃Layer thickness is less than 100Å
Become In the above temperature range, Cr ₂O₃Nitrogen in steel and steel
The diffusion rate of elementary atoms is fast, and the amount of solid solution nitrogen in steel is also large.
By the synergistic effect of such things, absorption of nitrogen into the steel surface layer is promoted.
Be done. On the other hand, if the holding temperature exceeds 1100 ° C, the high temperature of steel
The strength may be reduced and the annealing work may be hindered.
Therefore, the holding temperature is preferably 1100 ° C. or lower.

The holding time at the bright annealing temperature is not particularly limited, and may be in the range of 10 to 60 seconds which is the processing time of the continuous annealing which is usually performed. The cooling rate after bright annealing does not have to be particularly limited, and may be in the range of 10 to 40 ° C./sec which is usually performed in a continuous annealing furnace. The thickness of the surface layer portion of the steel of the present invention may be adjusted by the holding time at the bright annealing temperature, but it is more preferable to adjust the holding temperature. The higher the holding temperature, the faster the absorption rate of nitrogen, but the range of 950 ° C to 1050 ° C is preferable.

The steel sheet which has been subjected to the bright annealing can be used as it is as a spring steel, but it may be subjected to a heat treatment such as an aging heat treatment for the purpose of improving the spring characteristics.

In the above description, the shape of the multi-layered chromium-based stainless steel for springs of the present invention was described as a steel plate, but the shape of the steel of the present invention does not need to be limited to a steel plate, and other shapes such as wire rods, bar steel and tubular shapes can be used. Even in the form, the effect of the present invention is sufficiently exerted.

[0055]

EXAMPLE Chromium-based stainless steel slabs having various chemical compositions shown in Table 1 were heated to 1200 ° C. and finished at a temperature of 9
Hot rolling was completed at 50 ° C. to obtain a hot rolled steel sheet having a thickness of 3.2 mm.

[0056]

[Table 1] These hot-rolled steel sheets were annealed at 800 ° C., shot-blasted, pickled with hydrofluoric acid and descaled, and then cold-rolled with an intermediate annealing to obtain a thickness of 0. Two
A 5 mm cold rolled steel sheet was used, and bright annealing was further performed under the conditions described below. The annealing atmosphere is 25% by volume of nitrogen,
A mixed gas consisting of 75% by volume of hydrogen and having a dew point controlled at −40 ° C. or lower was used. Annealing temperature is 850 to 1050 ° C
The heating rate was 20 ° C./second on average, the soaking holding time was in the range of 10 to 40 seconds, and the cooling rate after soaking was 15 to 30 ° C./second on average.

As a comparative example, the cold-rolled steel sheets of Steel 1 and Steel 3 had the same annealing cycle as above, but had a dew point of +50.
Oxidative annealing was performed in an atmosphere of nitrogen gas that had been humidified to ℃. Further, as a comparative example, a commercially available
Stenite spring steel plate SUS304 (H-spec material), martensite spring steel plate 420J2 (hardened / tempered material), and ferritic stainless steel plate SUS43
0 was prepared. The metal structures of these steel sheets and the hardness, spring characteristics, workability and corrosion resistance after the aging heat treatment of soaking at 450 ° C. for 30 minutes were evaluated by the following methods.
Before subjecting some steel plates to tensile strain in the range of 3 to 8% before subjecting them to aging heat treatment, the effect of work strain on spring properties and corrosion resistance was also investigated.

The thickness of the surface layer is S in the cross section of the corroded test piece.
It was measured by EM observation. The ratio of the martensite phase in the surface layer portion was measured by observing a sample polished and corroded by a conventional method with a microscope. The ratio of the retained austenite phase was determined by measuring the integrated intensity of α-Fe and γ-Fe by the X-ray diffraction method,
The integral intensity value of e / (integral intensity value of α-Fe + integral intensity value of γ-Fe) × 100. The balance was the ratio of the ferrite phase. The volume ratios of the martensite phase and the ferrite phase in the inner layer structure were measured by observing with a microscope as in the surface layer part. The nitrogen content of the surface layer is a dispersive crystal L dedicated to nitrogen measurement.
It was quantified by an EPMA device having AD (artificial multilayer film). Table 2 shows the metal structure of each steel together with the bright annealing conditions.

[0059]

[Table 2] The hardness was measured by the Vickers hardness test method under the condition of a load of 1 kg. For the spring characteristics, the test pieces in the rolling direction (L direction) and the rolling orthogonal direction (T direction) were used, and the spring limit value (Kb) and spring fatigue limit specified in JIS-H3732 were measured by a plane bending tester. . Kb is defined as the maximum surface stress that causes permanent deformation equivalent to elastic deformation when the maximum surface stress due to bending is 36.25 GPa. For the spring fatigue limit, the maximum stress at which the test piece did not break was measured with an upper limit of 10 ⁷ times in a repeated bending test with a constant amplitude of 30 Hz. As for workability, a V-bending test specified in JIS-Z2248 was performed on the test pieces in the L direction and the T direction, and the ratio (R / t) of the minimum bend radius at which bending work was possible to the steel plate thickness was measured.

Corrosion resistance is measured by adjusting the Cl concentration to 1%.
After being immersed in the NaCl aqueous solution of 45 ° C. of No. 4 for 100 hours,
The state of rust generation was visually observed, and the case of having rust resistance equal to or higher than that of SUS430 was judged to be acceptable (◯). Furthermore, in order to examine the details of the sensitization phenomenon, the crystal grain boundaries of the surface layer portion were observed using a transmission electron microscope, and the presence or absence of grain boundary precipitation of Cr carbonitride was confirmed.

Table 3 shows the performance measurement results of each steel obtained.

[0062]

[Table 3] As shown in Table 2, reference numerals 1A, 1B, 1C, 2A, 3A
Each of the steel sheets of 5A and 5A contains a martensite phase and a retained austenite phase in the surface layer portion, and the inner layer portion has a two-phase mixed structure of a ferrite phase and a martensite phase.
Reference numerals 1D, 3B and 4A all have no austenite phase in the surface layer portion.

As shown in Table 3, trial numbers 1 to 6, 8 to 10,
Steel sheets 13 and 14 are both SUS3, which is the target.
No. 04 (Trial No. 15) showed a spring limit value equal to or higher than that of No. 04 (trial No. 15), and excellent properties exceeding the spring fatigue limit were obtained.
Workability is equivalent to SUS304 (Trial No. 15), corrosion resistance is S
The performance was equal to or higher than that of US430 (Trial No. 17). Furthermore, no grain boundary precipitation of chromium carbonitride was observed in the electron microscope observation results.

In Trial No. 7, Trial No. 11 and Trial No. 12, any one of the spring characteristics, bendability and corrosion resistance was inferior. Trial No. 7 even though I could not satisfy all
In the same way as the above, the steel of residual austenite trial No. 12 is Cr on the surface
Since the content was as small as 14%, it was slightly inferior to SUS430, but the performance was almost the same as SUS420J2. Although SUS420J2 shown in trial number 16 had good spring characteristics, it did not have the desired workability and corrosion resistance. Although the SUS430 shown in the trial number 17 had good workability and corrosion resistance, the desired spring characteristics could not be obtained.

[0065]

INDUSTRIAL APPLICABILITY The steel of the present invention is an inexpensive chromium-based stainless steel, but has workability equivalent to that of an austenitic spring steel and extremely excellent spring characteristics exceeding it. Its corrosion resistance has excellent performance equal to or higher than that of SUS430 steel. Further, the chrome-based stainless steel of the present invention can be easily manufactured without causing an increase in manufacturing cost.

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Claims (4)

(57) [Claims] 1. A residual Table layer portion and the martensite phase O - consists austenite phases containing mixed structure, spring inner layer portion is characterized by comprising a two-phase mixed structure consisting of ferrite phase and martensite phase multilayer structure chromium stainless steel material. 2. A steel composition having a mass% of C: 0.01 to 0.1.
0%, Cr: spring multilayer structure chromium stainless steel material according to claim 1, characterized in that one containing 16 to 20%. Wherein the surface portion according to claim 1 or 2 spring multilayer structure chromium stainless steel material according to the N content is characterized in that 0.1 to 0.5% by weight. 4. C: 0.01 to 0.10% by mass%, C
r: a step of producing a steel slab containing 16 to 20% and hot rolling the slab, a step of annealing the obtained hot rolled steel material and pickling, and a step of cooling the hot rolled steel material annealed and pickled. Hot rolling step, the obtained cold rolled steel material, hydrogen: 50 volume% or more, nitrogen: 20 volume% or more and less than 50 volume%, dew point: -4
In an atmosphere of 0 ° C. or lower, heating is performed in a ferrite + austenite two-phase temperature range of Ac1 transformation point or higher and cooling is performed.
Contains martensite phase and retained austenite phase in layer
Claims 1-3 noise method for manufacturing a spring multilayer structure chromium stainless steel material according to either characterized by having a step of performing bright annealing in which a mixed tissue.