JP2002097554A - Bilayer structured cr-based stainless steel and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Cr based stainless steel with high strength and adequate workability, and a manufacturing method therefor. SOLUTION: The bilayer structured Cr-based stainless steel is characterized by that an outer layer part consists of a mixed structure containing a martensite phase and a retained austenite phase, and an internal layer part substantially consists of a single-phase of martensite structure. The steel comprises 0.05-0.20% C, and 10-16% Cr by mass%, and the outer layer part preferably comprises 0.05-0.5% nitrogen. The method for manufacturing the steel preferably comprises uniformly heating the steel with the above chemical component in nitrogen including atmosphere of hydrogen >=10% by volume, 10% or more and 90% or less nitrogen by volume, and a dew point <=-30 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a Cr-based stainless steel having high strength and good workability, and a method for producing the same. The Cr-based stainless steel according to the present invention is suitable as a material of a member to be provided to a spring, a spring component, an electronic device requiring a spring characteristic, and a machine component in general after being processed into various shapes. In particular, it is optimal as a material for an engine gasket that requires high fatigue strength after processing.

[0002]

2. Description of the Related Art An engine gasket is a seal component inserted into a gap between a cylinder head and a block in order to seal combustion gas, cooling water and lubricating oil. A stack of two or three stainless steel sheets having holes corresponding to (shape, number) is usually used. Its basic structure is such that an annular convex portion (bead) is formed at a position corresponding to the periphery of the combustion chamber by press working or the like, and the above-mentioned gap generated at the time of combustion is sealed by the repulsive force of the bead. Things.

[0003] Materials used for gaskets for engines are required to have high strength and good workability. For this reason,
Austenitic stainless steel sheets represented by SUS301 and SUS304 have been widely used in the past. The above steel sheet is obtained by increasing the strength by the martensite phase generated by the work-induced transformation to obtain the spring property. The steel sheet is shipped from a material maker in a cold-rolled state, and processed into a desired shape by a processing maker. After processing, aging treatment is often performed for the purpose of improving spring characteristics.

[0004] Austenitic stainless steel sheets are excellent steel sheets having high strength and good workability. However, since it contains a large amount of expensive Ni, there is a problem that steel material costs are high. In addition, when an ultra-thin steel sheet having a thickness of 0.3 mm or less is manufactured, there is also a problem that a rolling load at the time of cold rolling increases and it is difficult to obtain a steel sheet having a good shape.

As an attempt to solve the above problems, Japanese Patent Application Laid-Open No. 7-278758 discloses that the chemical composition is
Cr: 11-18%, C: 0.1-0.5%, N: 0.
A stainless steel for an engine gasket (martensitic stainless steel) containing 01 to 0.2% and containing almost no Ni and a method for producing the same are disclosed.

The stainless steel for an engine gasket disclosed in the above publication is manufactured by performing a quenching heat treatment from a high temperature stable austenite phase region and a tempering heat treatment at 150 to 500 ° C. before or after forming. Tempered martensite having a Vickers hardness of 400 or more and 550 or less.

[0007]

In recent years, cylinder heads and cylinder blocks of engines have been made thinner in order to meet user needs for higher output and lighter weight engines. As a result, the rigidity of the engine decreases, and the displacement of the gap between the cylinder head and the cylinder block during combustion tends to increase. As a result, the engine gasket is repeatedly subjected to a larger stress than before. ing. Therefore, higher fatigue strength has been required for materials applied to engine gaskets.

Generally, fatigue strength can be improved by increasing the strength (hardness) of a material. However, the strength of the material and the workability are in opposition to each other, and the workability deteriorates with the increase in strength. Therefore, it is difficult to improve the fatigue strength due to the workability restriction of securing a predetermined member shape. May be. Also, even if the workpiece can be processed into a predetermined shape, surface defects such as microcracks are introduced into the surface layer of the member during processing, and thereafter the stress is concentrated on the surface defects due to repeated stress, resulting in fatigue failure. May be reached. That is, a material that does not have good workability even with high strength is inferior in fatigue strength after working.

For example, the stainless steel for an engine gasket disclosed in Japanese Patent Application Laid-Open No. 7-278758, as shown in FIG. It is presumed that the fatigue strength after processing was reduced due to the phenomenon described above.

[0010] Therefore, in order to obtain good fatigue strength even after working, it is necessary to provide the material with high strength and good workability. In order to apply Cr-based stainless steel to members that require strength, it is necessary to further improve the strength and workability of Cr-based stainless steel.

[0011] In addition, steel having a martensitic single phase structure such as stainless steel for an engine gasket disclosed in JP-A-7-278758 is generally difficult to form in a high strength state after quenching heat treatment. In many cases, tempering heat treatment is required before forming, and the cost of the final product may increase. In addition, there is a problem that carbonitride precipitates due to the tempering heat treatment and corrosion resistance is impaired.

In view of the above problems, the present invention has good workability (for example, bendability) while having high strength (for example, over 550 in Vickers hardness), and has good fatigue strength after working. An object of the present invention is to provide a Cr-based stainless steel and a method for producing the same.

[0013]

Means for Solving the Problems The inventors of the present invention have conducted various studies on the effects of the crystal structure of steel in order to improve the above-described various performances of Cr-based stainless steel, and have obtained the following new results. Knowledge was obtained.

(A) The amount of deformation when a process such as bending is performed on steel is greater than the surface (hereinafter referred to as “steel inner layer” or “inner layer”) of the steel (hereinafter referred to as “steel inner layer”). Surface layer portion ”or“ surface layer portion ”). The presence of a retained austenite phase in the surface layer of the steel allows the workability of the steel to be improved by utilizing the effect of improving the deformability accompanying the work-induced transformation of the retained austenite phase that occurs during processing, thereby improving the fatigue fracture after processing. Of the surface cracks (microcracks), which are the starting points of the cracks, can be suppressed. Furthermore, a tough structure can be formed in the surface layer by the transformation induced transformation of the retained austenite phase. By these actions, the fatigue strength after processing can be improved.

Therefore, by forming the steel surface layer with a mixed structure containing a martensite phase and a retained austenite phase and the inner layer with a martensite single phase structure, high strength and good workability can be provided. Thus, a steel having good fatigue strength after working is obtained.

(B) In mass%, C: 0.05% or more.
C containing 20% or less, Cr: 10% or more and 16% or less
r-type stainless steel is soaked in a nitrogen-containing atmosphere to form an austenitic single phase, and the nitrogen in the nitrogen-containing atmosphere is absorbed by a surface layer of the steel, and then cooled at a cooling rate of 1 ° C./sec or more. By performing the heat treatment, a residual austenite phase can be generated in the surface layer portion of the steel. This is considered to be because the austenite phase is stabilized by absorbing nitrogen into the surface layer of the steel during the soaking process.

The “surface layer” used in this specification
The term refers to a high nitrogen concentration region near the steel surface formed by, for example, nitrogen absorbed from the atmosphere diffused inside the steel, and is generally a term relative to the inner layer portion and includes the surface of the steel material. Refers to the area. In the case exemplified above, the thickness of the surface layer portion can be determined by measuring the profile of the nitrogen concentration from the surface of the steel with an EPMA device, or by performing SEM observation after corroding the cross section, The structure of the surface layer is determined by the structure of the high nitrogen concentration region, and the structure of the inner layer is determined by the structure of the low nitrogen concentration region inside the steel. Here, the high nitrogen concentration region is a region in which 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. This is a region where the nitrogen concentration is lower than the region.

The "multilayer structure" is, for example, a mixed structure in which the steel surface layer contains a martensite phase and a retained austenite phase and the inner layer has a martensite single phase structure, as described above. Generally, it refers to an organization in which the structure of the surface layer and the structure of the inner layer are different. The “multilayer heat treatment” is, for example, as described above, in mass%, C: 0.0
A Cr-based stainless steel containing 5% or more and 0.20% or less and Cr: 10% or more and 16% or less is soaked in a nitrogen-containing atmosphere to form an austenitic single phase, and the nitrogen in the nitrogen-containing atmosphere is changed to a steel surface layer. Means a heat treatment for cooling at a cooling rate of 1 ° C./second or more, and generally a heat treatment for forming a multilayer structure.

The present invention has been completed based on these newly obtained findings, and the gist of the present invention is as follows:
A method for producing a multi-layer Cr-based stainless steel according to (3) and a multi-layer Cr-based stainless steel according to (4) and (5).

(1) A multi-layer Cr-based stainless steel characterized in that the surface layer portion has a mixed structure containing a martensite phase and a retained austenite phase, and the inner layer portion substantially has a martensite single-phase structure. .

(2) In mass%, C: 0.05-0.20
%, Cr: 10 to 16%. The multi-layer structure Cr-based stainless steel according to the above item (1), characterized in that: (3) The multilayer Cr-based stainless steel according to the above (1) or (2), wherein the nitrogen content of the surface layer is 0.05 to 0.5% by mass.

(4) In mass%, C: 0.05 to 0.20
%, Cr: 10 to 16% Cr-based stainless steel is soaked in a nitrogen-containing atmosphere to form an austenitic single phase, and the nitrogen in the nitrogen-containing atmosphere is absorbed into the steel surface layer, and then 1 ° C./sec. A method for producing a multi-layer Cr-based stainless steel, comprising performing a multi-layer heat treatment for cooling at the above cooling rate.

(5) The nitrogen-containing atmosphere is hydrogen: 10
The method for producing a multi-layer Cr-based stainless steel according to the above item (4), wherein the Cr-based stainless steel has a content of not less than 10% by volume and less than 90% by volume and a dew point of not more than -30 ° C.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below. In addition, the percentage display of the chemical composition described below means mass%.

A. Chemical composition Cr: Cr is an element necessary for ensuring corrosion resistance. Furthermore, when soaking is carried out in a low dew point nitrogen-containing atmosphere, it is effective to absorb nitrogen in the atmosphere to stabilize the austenite phase in the steel surface layer and to generate a residual austenite phase after cooling.

An austenite phase is formed on the steel surface layer,
To ensure corrosion resistance, the Cr content is desirably 10
% Or more. On the other hand, when Cr is contained excessively, not only does the steel material cost increase, but also a ferrite phase is formed in the steel inner layer, and stable fatigue properties may not be obtained. To avoid this, it is desirable to use Cr
The content is 16% or less. More preferably, it is 12% or more and 14% or less.

C: C is a typical austenite-forming element and has a large effect on the martensite hardening ability. When producing spring steel, the C content is desirably 0.05% or more in order to obtain spring characteristics superior to those of austenitic spring steel. If the C content is excessively increased, the hardness of the martensite phase increases, and the hot workability and the workability of the product decrease. Furthermore, in the cooling process of the multilayer heat treatment, a sensitization phenomenon is likely to occur, and the corrosion resistance may deteriorate. To avoid these inconveniences, C
The content is desirably 0.20% or less.

N: Like C, N is a typical austenite-forming element, and is an element effective for improving the fatigue strength and for generating a residual austenite phase in the steel surface layer. However, it is difficult to add a large amount of N at the time of smelting steel by a normal method, and the steel containing a large amount of N has poor hot workability, and causes surface cracks such as edge cracks during hot rolling. It may cause flaws. Therefore, there is no particular limitation on the stage before the multilayer heat treatment, and the content may be about 0.001 to 0.04% obtained by an ordinary manufacturing method.

In the method for producing a multi-layer Cr-based stainless steel according to the present invention, N is absorbed in the surface layer of the steel by performing soaking in a nitrogen-containing low dew point atmosphere. Thus, the austenite phase is stabilized, and the metal structure of the steel surface layer after cooling is a mixed structure containing a martensite phase and a residual austenite phase.

The N content in the surface layer of the steel after the heat treatment for multi-layering is desirably 0.05% or more so that retained austenite necessary for ensuring workability is present in the surface layer of the steel. More preferably, it is 0.10% or more. Although the upper limit of the N content in the steel surface layer after the multilayer heat treatment is not particularly limited, it may be necessary to lower the dew point of the nitrogen-containing atmosphere to -50 ° C or lower to increase the N content. Since it is industrially difficult to control the soaking atmosphere to have a low dew point, the upper limit is preferably set to 0.50%.

Ti: Ti has an effect of suppressing the coarsening of crystal grains in the heat treatment for forming a multilayer structure and further suppressing the sensitization phenomenon that occurs during the cooling process. Therefore, in order to obtain the above-mentioned effects, it is preferable that the content is 0.001% or more. On the other hand,
Excessive addition of Ti not only impairs economic efficiency, but also causes C and N in the steel to be fixed, resulting in a decrease in strength.
Even when it is contained, the content is preferably 0.02% or less.

Ni, Mn, Cu: These are all austenite forming elements and are effective elements for adjusting the amount and hardness of the martensite phase after soaking. Also,
By containing these elements, the (C + N) content can be reduced, so that the martensite phase can be made soft. Therefore, it is suitable for improving workability of steel.

Therefore, these elements are not essential elements, but may be contained in order to obtain the above effects. When it is contained in steel, it is better to contain each in an amount of 0.3% or more. On the other hand, if Ni or Cu is contained excessively, it not only impairs the economic efficiency but also has a function of lowering the nitrogen absorbing ability at the time of the multilayer heat treatment. Is good. M
n has the effect of increasing the nitrogen absorption capacity during the heat treatment for forming a multilayer structure. However, if it is contained excessively, it has the effect of impairing the economic efficiency and lowering the corrosion resistance. It is good to do.

Nb: Nb suppresses the sensitization phenomenon that occurs in the cooling process after the heat treatment for forming a multilayer structure, and further forms a solid solution in an austenite phase (which becomes a martensite phase and a residual austenite phase after cooling). It has the effect of increasing the strength without significantly reducing the properties. Therefore, although it is not an essential element, it may be contained in order to obtain the above effects. When it is contained, it is preferably contained at 0.01% or more. On the other hand, if Nb is excessively contained, C and N in the steel are fixed and cause a reduction in strength. Therefore, even when Nb is contained, the content is preferably 0.1% or less.

Mo: Mo is not an essential element, but has an effect of significantly improving the corrosion resistance. Therefore, even when the Cr content is small, the desired corrosion resistance can be obtained by adding Mo. When it is contained, it is preferred to contain 0.1% or more. However, Mo is expensive and its economical efficiency is impaired if Mo is contained excessively. Therefore, even when Mo is contained, the upper limit is preferably set to 2.0%.

V: V is not an essential element, but may be contained because it is an effective element for improving the strength. When it is contained, it is preferable to contain 0.05% or more. However, the above effect saturates when it exceeds 0.3%, so it is preferable to make the content 0.3% or less even when it is contained.

Si: Si is an element effective as a deoxidizing agent for steel and also has an effect of increasing the strength, so that Si may be contained. However, if it is contained excessively, the toughness of the steel is impaired.
It is good to make it 0%.

Al: Al is an element effective as a deoxidizing agent for steel and may be contained. However, since Al forms a nitride, if it is contained excessively, it has an effect of reducing the amount of dissolved nitrogen during the heat treatment for forming a multilayer structure. Therefore, even when it is contained, its upper limit is preferably set to 0.05%.

Rare earth element: Usually not contained, but may be contained since it has the effect of improving the oxidation resistance of steel. However, if the total content exceeds 0.1%, the effect is saturated and the cost increases. Therefore, the content is preferably set to 0.1% or less.

The balance is Fe and inevitable impurities. b. Metal Structure The steel of the present invention has a surface layer having a mixed structure containing a martensite phase and a retained austenite phase, and an inner layer having a martensite single phase structure.

The martensite phase increases the hardness and the fatigue strength of the steel by increasing the strength and hardness of the steel, and by aging to precipitate solid solution elements (C, N). Has an action.

For example, an austenitic spring steel plate SU
In order to obtain a fatigue strength exceeding S301L-CSP (H specification material), the surface hardness needs to be Hv450 or more.
Therefore, it is preferable that the ratio of the martensite phase in the surface layer is 60% by volume or more. More preferably 70% by volume
That is all. On the other hand, when the ratio of the martensite phase is increased to more than 95% by volume, the surface hardness becomes Hv700 or more, the ductility of the steel decreases, and the workability is impaired. Therefore, the upper limit of the surface hardness is preferably Hv700, and the martensite ratio of the surface layer is preferably 95% by volume or less.

The retained austenite phase is softer and more workable than the martensite phase, and also has the effect of undergoing work-induced transformation when subjected to working to make the structure extremely tough. It also has the effect of increasing the toughness of the steel. Further, by aging the tough structure obtained by the work-induced transformation to precipitate the solid solution element by aging, the effect of increasing the hardness of the steel and further improving the fatigue strength can be obtained. In order to obtain these effects, the ratio of the retained austenite phase in the surface layer is preferably set to 5% by volume or more.

In the surface layer portion, there may be a ferrite phase which is inevitably mixed due to segregation of the material, in addition to the above two phases, as long as the properties of the steel are not adversely affected. The volume ratio of the martensite phase, retained austenite phase, and ferrite phase in the surface layer portion is a range in which the sum of these does not exceed 100%. In addition, 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%.

When the surface layer of the steel has a mixed structure containing a martensite phase and a retained austenite phase, workability and fatigue strength after the processing can be improved. In order to obtain a more effective effect, the thickness of the surface layer portion is desirably set to 3% or more of the thickness of the steel (in the case of a wire or a steel bar, the diameter thereof). In the case of a steel plate, the thickness is 3% or more of the thickness of the steel plate on each of the front and back surfaces. In the case of a steel plate applied to an engine gasket, the thickness is preferably 0.01 mm or more.

Although the fatigue strength is improved as the thickness of the surface layer is increased, if the thickness is excessively large, the time required for the heat treatment for forming multiple layers becomes longer, and the productivity may be lowered. Therefore, the thickness of the surface layer portion is preferably 30% or less of the thickness of the steel (in the case of a wire or a steel bar, the diameter thereof).

The metal structure of the inner layer is substantially a martensite single phase structure. The reason is that in the inner layer of steel, the amount of deformation due to bending or the like is small, and even if there is a retained austenite phase, improvement in strength due to deformation induced transformation cannot be expected. Further, in order to generate a residual austenite phase in the inner layer portion, the time required for the multi-layer heat treatment becomes longer, which may cause a decrease in productivity.

In the chemical composition defined by the present invention, when the ferrite phase substantially exists in the inner layer, it becomes difficult to make the hardness of the inner layer Hv 350 or more. For example, in order to obtain a fatigue strength exceeding the austenitic spring steel plate SUS301L-CSP (H specification material), the hardness of the inner layer portion needs to be Hv350 or more. Therefore, the metal structure of the inner layer portion is substantially a martensite single phase structure.

[0049] The "substantially martensite single phase structure" means, in addition to the martensite phase, a ferrite phase which is inevitably mixed due to segregation of the material within a range which does not adversely affect the properties of steel. Is a meaning including the case where exists.

C. Manufacturing Method A preferred manufacturing method of the multi-layer Cr-based stainless steel of the present invention will be described by taking a case where a product is a cold-rolled steel sheet 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 a vacuum degassing treatment to obtain a continuous casting method or a steel ingot. After that, a 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. This hot-rolled steel sheet is annealed according to a conventional method, and scale on the surface is removed by a known method such as pickling.

Thereafter, cold rolling is performed by a known method to produce a cold rolled steel sheet. The cold rolling may be a plurality of times of cold rolling including intermediate annealing, or may be a cold rolling not including intermediate annealing. The dimensions of the cold-rolled steel sheet are not particularly limited, and the thickness is usually used (for example, 0.1 mm).
２．０2.0 mm).

After the final cold rolling, the cold-rolled steel sheet is soaked in a nitrogen-containing atmosphere to absorb nitrogen in the atmosphere, and then subjected to a multilayer heat treatment for cooling. The mixed structure containing the martensite phase and the retained austenite phase in the surface layer of the steel increases the stability of the austenite phase by absorbing nitrogen in the austenite phase in the soaking process of the multilayer heat treatment as described above. can get.

The above-mentioned nitrogen-containing atmosphere is desirably set as follows in order to efficiently absorb nitrogen into the steel material in the multilayer heat treatment. The hydrogen concentration in the nitrogen-containing atmosphere is preferably set to 10% by volume or more. When an oxide film is formed on the steel surface, absorption of nitrogen from a nitrogen-containing atmosphere is hindered.However, the hydrogen concentration in the atmosphere is in the above range, and the formation of an oxide film can be suppressed by lowering the dew point. it can. More preferably 50% by volume
That is all. The thickness of the oxide film is preferably less than 100 °.

The nitrogen concentration in the nitrogen-containing atmosphere is 10
It is preferable to be at least volume%. More preferably 20
% By volume or more. If the nitrogen-containing atmosphere has a high dew point, a dense oxide film having a thickness exceeding 100 ° is formed on the surface of the steel material, and the nitrogen absorption efficiency of the steel material is reduced, so that the nitrogen-containing atmosphere has a dew point of -30 ° C or less. Is preferred. More desirably, it is -40 ° C or lower.

The nitrogen-containing atmosphere may contain an inert gas such as Ar gas which does not oxidize the surface of the steel material, or a catalyst such as NH ₃ which promotes the nitriding reaction.

The surface temperature of the steel material during the soaking process of the heat treatment for forming a multilayer (hereinafter also referred to as “soaking temperature”) is 900.
It is desirable that the temperature be equal to or higher than ° C. The soaking temperature can be measured by disposing a radiation thermometer in a heating furnace for soaking. 90% soaking temperature in low dew point atmosphere with low oxygen potential
If soaking is performed at 0 ° C. or higher, the oxide film on the surface of the steel material is reduced, so that the oxide film on the surface of the steel material can be thinned to less than 100 °. Further, in the above temperature range, the nitrogen atoms in the oxide film and the steel are diffused at a high rate, and the amount of nitrogen dissolved in the steel is increased. On the other hand, if the soaking temperature exceeds 1200 ° C., the high-temperature strength of the steel material decreases, which may hinder the soaking operation. Therefore, the soaking temperature is preferably 1200 ° C. or less.

It is desirable that the time of the soaking process of the above-mentioned heat treatment for forming multiple layers (hereinafter also referred to as “soaking time”) is 5 seconds or more. If the soaking time is less than 5 seconds, the thickness of the surface layer required for obtaining the desired workability may not be obtained. The upper limit of the soaking time is not particularly limited, but when performing the multi-layer heat treatment in the continuous heat treatment furnace, in order to suppress the occurrence of the sensitization phenomenon due to the decrease in productivity and the cooling rate after the soaking. It should be less than 3 minutes. When performing a multi-layer heat treatment in a continuous heat treatment furnace, the soaking time is 10 seconds or more in order to raise the surface temperature of the steel material in the soaking process to 900 ° C. or more by the heating capacity of a normal heat treatment furnace. It is good to

The thickness of the surface layer of the steel of the present invention may be controlled by adjusting the soaking time, but it is more preferable to control the soaking temperature. The higher the soaking temperature is, the faster the nitrogen absorption rate is, but preferably in the range of 1000C to 1150C.

After the above soaking, cooling is performed in a temperature range of 900 to 500 ° C. at a cooling rate of 1 ° C./sec or more in order to suppress the occurrence of the sensitization phenomenon. If the cooling rate is less than 1 ° C / sec,
In some cases, the occurrence of the sensitization phenomenon in the cooling process of the multilayer heat treatment cannot be sufficiently suppressed. Although the upper limit of the cooling rate is not particularly limited, it is practically difficult to increase the cooling rate to more than 1000 ° C./sec.

The steel sheet after the above-mentioned multi-layer heat treatment can be used as it is as spring steel. When used as spring steel, heat treatment such as aging heat treatment may be further performed for the purpose of improving spring characteristics.

In the above description, the case where the shape of the multi-layer structure Cr-based stainless steel of the present invention is a steel plate has been described. However, the shape of the steel of the present invention does not need to be limited to a steel plate. The effects of the present invention are sufficiently exhibited even in other forms such as a tubular shape.

[0063]

EXAMPLES Example 1 A cold rolled steel sheet of 0.25 mm thick Cr-based stainless steel having the chemical composition shown in Table 1 was placed in a nitrogen-containing low dew point atmosphere (hydrogen: 75 vol%, nitrogen: 25% by volume, dew point: -45 ° C.), a soaking temperature of 1100 ° C., a soaking time of 1 minute, and then a multilayer heat treatment for cooling at a cooling rate of 20 ° C./sec. The surface layer of the steel sheet was made to absorb 0.2% by mass of nitrogen, the surface layer was a mixed structure of 85% by volume of martensite phase and 15% by volume of a retained austenite phase, and the inner layer was a single phase structure of martensite. A multilayer structure Cr-based stainless steel sheet was manufactured. Here, the structure of the surface layer portion was determined based on that of a region having a depth of 15 μm from the steel sheet surface.

[0064]

[Table 1] As a comparison material, commercially available austenitic spring steel sheet SUS
301L-CSP (H specification material) and martensitic spring steel plate SUS420J2-CSP (hardened / tempered material) were prepared.

Samples were taken from the above-described Cr-based stainless steel sheet having a multi-layer structure and a comparative material, and the fatigue properties after bead working and the effect of aging treatment on hardness were investigated. The fatigue characteristics were as follows: strip-shaped test specimens in the rolling direction (L direction) having a width of 10 mm and a length of 40 mm were sampled from the above-mentioned steel sheet, and a beaded test specimen was prepared. The number of times of repeated bending until a predetermined bending stress was applied to breakage was determined. Here, the upper limit of the number of times of repeated bending was 10 ⁶ times. The aging conditions are as follows: aging temperature: 300 to 600 ° C., aging time: 10 minutes,
The surface hardness and the hardness of the inner layer portion were measured under a load of 1 kg by a Vickers hardness test method.

FIG. 1 is a perspective view showing the shape of a test material subjected to a fatigue test. FIG. 2 is a graph showing fatigue characteristics after bead processing. As shown in the figure, the multi-layered Cr-based stainless steel sheet exhibited excellent fatigue properties exceeding SUS301-CSP. This is due to the improvement in deformability due to the work-induced transformation of the retained austenite phase in the surface layer,
It is considered that the occurrence of surface cracks (microcracks), which are the starting points of fatigue fracture after metal working, was suppressed, and the fatigue strength was improved by the tough structure obtained by the work-induced transformation. 6 is a graph showing the relationship between the temperature and the aging treatment temperature.

As shown in the figure, the multi-layer structure Cr-based stainless steel sheet was SUS301L-C before and after the aging treatment.
Shows higher surface hardness than SP (H specification material) and SUS420J2-CSP,
And the aging treatment at 550 ° C. increased the surface hardness to Hv650. This is because the solid solution strengthening of martensite by nitrogen dissolved in the surface layer of the steel sheet in a supersaturated manner by the multi-layer heat treatment and the solid solution elements (C,
It is considered that the surface hardness was improved by the fine aging precipitation of N). (Example 2) A slab of Cr-based stainless steel having various chemical compositions shown in Table 2 was heated to 1180 ° C, hot rolling was completed at a finishing temperature of 900 ° C, and a hot roll having a thickness of 3.2 mm was obtained. A rolled steel sheet was obtained.

[0068]

[Table 2] After annealing these hot-rolled steel sheets at 780 ° C,
After shot blasting and nitric hydrofluoric acid washing and descaling, cold rolling with intermediate annealing is performed to give a thickness of 0.25
mm cold-rolled steel sheet and subjected to a multi-layer heat treatment under the following conditions. Some of them were further aged.

The multi-layer heat treatment is performed by using a continuous bright annealing furnace, and the soaking atmosphere is nitrogen: 25% by volume, hydrogen: 7%.
5% by volume, dew point: a mixed gas controlled to -40 ° C or lower was used. The soaking temperature is 850-1150 ° C., the soaking time is in the range of 10-60 seconds, and the cooling rate after soaking is 5-4.
0 ° C./sec.

For comparison, the conditions other than the dew point of the soaking atmosphere in the multilayer heat treatment were the same as above, and the dew point was +5.
The heat treatment at 0 ° C. was also performed using a continuous annealing pickling furnace.

Further, as comparative materials, commercially available austenitic spring steel plates SUS301L-CSP (H specification material),
Martensitic spring steel plate SUS420J2-CSP
(Hardened / tempered material) was prepared.

Test pieces were taken from these steel sheets, and the metal structure, Vickers hardness, bending workability, and fatigue strength were evaluated by the following methods. Furthermore, the fatigue strength was also measured for the beaded test piece shown in FIG. In addition, the aging treatment was performed at an aging treatment temperature of 350 ° C. and an aging treatment time of 10 minutes. When bead processing was performed on the test piece, the aging treatment was performed after the bead processing for the purpose of improving spring fatigue characteristics.

The ratio of the retained austenite phase in the surface layer is
The integrated intensity of α-Fe and γ-Fe was measured by the X-ray diffraction method, and the integrated intensity value of γ-Fe / (the integrated intensity value of α-Fe +
(Integrated intensity value of γ-Fe) × 100. The ratio of the martensite phase to the ferrite phase in the surface layer portion was measured by microscopic observation of a sample obtained by polishing and corroding a steel sheet surface by a conventional method.

The volume ratio of the martensite phase and the ferrite phase in the inner layer was determined by measuring the cross section of the test piece which had been corroded by a conventional method.
M and microscopic observation. The nitrogen content in the surface layer portion was determined by an EPMA apparatus having a spectral crystal LAD (artificial multilayer film) dedicated to nitrogen measurement. The thickness of the surface layer portion was determined by measuring a nitrogen concentration profile from the steel sheet surface toward the center of the sheet thickness.

Table 3 shows the metallographic structure of each steel sheet together with the conditions for heat treatment for forming multiple layers.

[0076]

[Table 3] The hardness was measured by a Vickers hardness test method under a load of 1 kg.

The fatigue strength was measured using a test piece in a direction perpendicular to the rolling direction (L direction) and a direction perpendicular to the rolling direction (T direction) using a repetitive plane bending tester. Fatigue strength was 10 ⁷ times in the repetition bending test of constant amplitude 30Hz and maximum stress test piece did not lead to rupture as the upper limit.

The bending workability was determined by performing a V bending test specified in JIS-Z2248 on test pieces in the L direction and the T direction, and calculating the ratio (R / t) of the minimum bending radius to the thickness of the steel sheet that can be bent. It was measured.

Table 4 shows the test results of each steel sheet.

[0080]

[Table 4] As shown in Table 3, each of the steel sheets 1A, 2A and 3A contains a martensite phase and a retained austenite phase in a surface layer portion, and has a martensite single phase structure in an inner layer portion. Each of the steel sheets 1B, 2B and 3B has no residual austenite phase in the surface layer.

As shown in Table 4, test numbers 1, 2, 5, 8,
Each of the steel sheets 9 and 10 has a surface hardness of Hv450.
The hardness of the super and inner layers was more than Hv350, and exhibited a fatigue strength exceeding that of SUS301L-CSP (H specification material) as a comparative steel. The bending workability was higher than SUS420J2-CSP, and had the same performance as SUS301L-CSP (H specification material).

In Test Nos. 3, 4, 6, 7, 11 and 12, any one of the fatigue strength and bending workability was inferior. Test Nos. 3 and 4 have workability, but are inferior in spring fatigue properties because the surface hardness is less than Hv450. Test Nos. 6 and 7 have a surface hardness of more than Hv450, but have poor bending workability, and thus have poor fatigue strength. Trial number 11
Nos. And 12 are a two-phase mixed structure of ferrite and martensite, and although good workability is obtained, the fatigue strength is inferior because the surface hardness is less than Hv450.

[0083]

The Cr-based stainless steel of the present invention has good workability (for example, bendability) while having high strength (for example, over 550 in Vickers hardness), and has good fatigue strength even after working. Have.

Further, the Cr-based stainless steel of the present invention can be produced without increasing the production cost or lowering the productivity.
It can be easily manufactured.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a shape of a test material subjected to a fatigue test.

FIG. 2 is a graph showing fatigue characteristics after bead processing.

FIG. 3 is a graph showing a relationship between hardness and aging treatment temperature.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuhiko Adachi 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Inside Sumitomo Metal Industries, Ltd. (72) Inventor Kazuyoshi Fujisawa 2--12 Minatomachi, Joetsu-shi, Niigata No. 1 Sumitomo Metal Naoetsu, Ltd. (72) Inventor Kenichi Goshokubo 2-1-1 Minatomachi, Joetsu-shi, Niigata Prefecture Sumitomo Metal Naoetsu, 72-72 Inventor Masahiro Aoki 2-cho, Minatocho, Joetsu-shi, Niigata No. 12 No. 1 F-term (reference) 4K028 AA02 AB01 AC04 AC07 AC08 4K037 EA05 EA06 EA12 EA13 EA15 EA17 EA18 EA19 EA20 EA27 EA31 EA32 EB11 EB14 EC01 GA07

Claims (5)

[Claims] 1. A multi-layer Cr-based stainless steel, wherein a surface layer portion has a mixed structure containing a martensite phase and a retained austenite phase, and an inner layer portion substantially has a martensite single phase structure. 2. C: 0.05 to 0.20% by mass%,
The multi-layered Cr-based stainless steel according to claim 1, wherein Cr: 10 to 16% is contained. 3. The nitrogen content of the surface layer portion is 0.05 to 0.05.
The multilayer Cr-based stainless steel according to claim 1 or 2, wherein the content is 0.50% by mass. 4. C: 0.05 to 0.20% by mass%
Cr: A Cr-based stainless steel containing 10 to 16% is soaked in a nitrogen-containing atmosphere to form an austenitic single phase, and after the nitrogen in the nitrogen-containing atmosphere is absorbed by the surface layer of the steel, the temperature is increased to 1 ° C./sec or more. A method for producing a multi-layer Cr-based stainless steel, comprising performing a multi-layer heat treatment for cooling at a cooling rate. 5. The nitrogen-containing atmosphere contains hydrogen: 10% by volume or more, nitrogen: 10% by volume or more and less than 90% by volume, and has a dew point of −30 ° C. or less. Of producing a multi-layered Cr-based stainless steel.