JPH11140598A - High strength stainless steel strip with high spring limit value, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength stainless steel strip having a high spring limit value suitably used for a high efficiency spring material for a metallic gasket, etc. SOLUTION: The steel strip has a composition which contains, by mass, <=0.15% C, <=3.0% Si, <=4.0% Mn, 4.0-7.0% Ni, 12.0-20.0% Cr, <=0.15% N, 0-5.0% Mo, and 0-5.0% Cu and satisfies C+N>=0.10 and in which the value of X in equation X=1305-41.7(Cr+Mo+ Cu)-61.1Ni-33.3Mn-27.8Si-1667(C+N) is regulated to-100 to 200. Further, the steel strip has fine martensite, transformed from austenitic base phase of <=2 μm average grain size, and age precipitates and also has >=1200 N/mm<2> spring limit value. This steel strip can be obtained by subjecting a steel strip containing >=80 vol.% martensite to inverse transformation treatment at 600 to 850 deg.C for 0.1 to 60 min and then to aging treatment at 400 to 550 deg.C for 0.1 to 120 min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength stainless steel strip having a high spring limit and suitable for a metal gasket or various spring materials requiring excellent spring characteristics, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, when stainless steel is applied to a spring material such as a metal gasket, a work hardening type austenitic stainless steel represented by SUS301, SUS630, SU
Precipitation hardening stainless steels such as S631 have been used.

[0003] Work-hardening austenitic stainless steel exhibits an austenitic phase in a solution-treated state.
The purpose is to produce work-induced martensite by subsequent cold rolling to obtain high strength. The strength depends on the amount of cold work and the amount of martensite, but it is difficult to precisely adjust the desired strength with only cold work, and as the cold work rate increases, the anisotropy of the material increases and The toughness also decreases.

[0004] Precipitation hardening type stainless steels are those which add an element contributing to precipitation hardening and are hardened by aging treatment, and SUS630 added with Cu and SUS631 added with Al are typical. The former aims at hardening by aging treatment after solution treatment. The latter is intended to change the metastable austenite phase to a martensite phase by a pretreatment such as cold rolling after the solution treatment, and to precipitate and harden the intermetallic compound Ni ₃ Al by aging treatment thereafter.
Considerable high strength is obtained.

[0005]

It is important for a spring material used for a metal gasket or the like to exhibit a high spring limit value in addition to high strength and high toughness in order to improve the performance of parts. Among the conventional high-strength stainless steels mentioned above,
The precipitation hardening stainless steel avoids the drawbacks of work hardening stainless steel such as difficulty in strength adjustment, anisotropy of properties, and reduced toughness. In particular, it strengthens the strengthening mechanism by intermetallic compound precipitation like SUS631. Considerable high strength is obtained in the alloy system incorporated. However, even with such an alloy-based material, the spring limit value has not reached a level that is sufficiently satisfactory as a spring material such as a metal gasket.

An object of the present invention is to provide a high-strength stainless steel strip having a high spring limit and suitable for spring applications such as metal gaskets.

[0007]

The object of the present invention is to provide, in mass%, C: 0.15% or less, Si: 3.0% or less, Mn: 4.0%.
Hereinafter, Ni: 4.0 to 7.0%, Cr: 12.0 to 20.0%, N: 0.
15% or less, Mo: 0 to 5.0% (including no addition), Cu: 0
5.05.0% (including no addition), C + N ≧ 0.10, and X = 1305-41.7 (Cr + Mo + Cu) -61.1Ni-33.3Mn-27.8Si
A steel strip containing these elements so that the value of X at -1667 (C + N) becomes -100 to 200, and having a chemical composition consisting of a balance of Fe and unavoidable impurities, and having an average grain size of 2 μm or less. A metal structure substantially consisting of fine martensite transformed from a phase and an aging precipitate (claim 1), or fine martensite transformed from an austenite matrix having an average particle diameter of 2 μm or less, containing at least 80% by volume, and the balance being austenite. This is achieved by a high-strength stainless steel strip having a metal structure substantially consisting of aging precipitates (Claim 2) and exhibiting a high spring limit value of 1200 N / mm ² or more.

Here, the lower limit of the contents of Mo and Cu is 0%.
Means that the element is not added. Therefore,
For example, a steel containing neither Mo nor Cu, or a steel containing only one of Mo and Cu in a range of 0.5% or less is also included. Formula, X = 1305−41.7 (Cr + Mo + Cu) −
Cr and M in 61.1Ni-33.3Mn-27.8Si-1667 (C + N)
o, Cu, Ni, Mn, Si, C, and N mean values in which the content of each element is expressed in mass%. "Substantially" in "a metal structure substantially consisting of..." Means that inclusions inevitably contained in production or small amounts (approximately 5% by volume or less) of δ ferrite may be present. Means that. The spring limit value is a value obtained by a "repeated bending test" as shown in JIS G 4313,
The test method is based on JIS H 3130.

Further, in the present invention, a metal structure having the above-mentioned chemical composition and a metal structure substantially consisting of fine martensite transformed from an austenite matrix having an average particle size of 2 μm or less (Claim 3), Alternatively, a steel strip having a metal structure (claim 4) containing 80% by volume or more of fine martensite transformed from an austenite matrix having an average particle size of 2 μm or less and having a balance of 400 to 5%.
Provided is a method for producing a high-strength stainless steel strip having a high spring limit, which is subjected to aging treatment at 50 ° C. for 0.1 to 120 minutes.

As a typical embodiment of the production method, a solution-treated steel strip or a cold-rolled steel sheet having a metal structure containing 80% by volume or more of martensite has the above-described chemical composition. 600-8 for rolled steel strip
Heating was performed at 50 ° C. for 0.1 to 60 minutes to reverse-transform the total amount of martensite into fine austenite having an average particle size of 2 μm or less, and then formed martensite from the fine austenite in the course of cooling to room temperature. Thereafter, there is further provided a method for producing a high-strength stainless steel strip having a high spring limit value, which is further subjected to aging treatment at 400 to 550 ° C for 0.1 to 120 minutes (claim 5).

Further, according to the present invention, there is provided a high-strength stainless steel strip according to claim 1 or 2, wherein the use is for a metal gasket. Similarly, the use is for a metal gasket. , 4 or 5, a method for producing a high-strength stainless steel strip (claim 7).

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the spring limit value of a high-strength stainless steel strip is increased by forming a specific form of fine martensite and forming a precipitate by aging treatment. Hereinafter, matters for specifying the steel strip of the present invention will be described.

C is extremely effective for strengthening martensite, and plays a role of forming a precipitate to increase the spring limit value. Further, it suppresses the formation of δ ferrite at a high temperature as an austenite forming element. In order to sufficiently exhibit such effects, the C content is desirably 0.05% by mass or more. On the other hand, the excessive C content promotes the formation of coarse Cr carbides and causes intergranular corrosion and deterioration of fatigue properties. Therefore, the upper limit of the C content needs to be 0.15% by mass or less. It is more preferable to set the following.

Si is usually used for the purpose of deoxidation,
In the present invention, it is desirable to positively add Si, and it is particularly desirable to contain 0.50% by mass or more of Si, since it effectively acts on solid solution strengthening of martensite. However,
If the Si content exceeds 3.0% by mass, high-temperature cracking is likely to occur, and various problems occur in production.

Mn is contained in an appropriate amount determined mainly by the balance with other elements for the purpose of controlling the stability of the austenite phase. In the component system of the present invention, it is desirable to contain 0.50% by mass or more of Mn to secure austenite stability. However, when the Mn content increases, it becomes difficult to generate martensite, and 4.0%
If the amount is more than mass%, it becomes difficult to induce a sufficient amount of martensite even by cold rolling.

Ni is an element necessary for obtaining an austenite phase at a high temperature. In the present invention, by refining the austenite matrix, the martensite generated from the austenite is refined. For this purpose, it is extremely advantageous that the whole amount of martensite reversely transforms to austenite at a relatively low temperature, that is, lowering the reverse transformation temperature. Ni is an austenite-forming element, and its content can lower the reverse transformation temperature. In order to obtain the effect sufficiently in the present invention, it is necessary to secure a Ni content of 4.0% by mass or more. However,
If the Ni content exceeds 7.0% by mass, it becomes difficult to generate a sufficient amount of martensite in the cooling process after the reverse transformation. Therefore, the Ni content is set in the range of 4.0 to 7.0% by mass.

Cr is an essential element for imparting corrosion resistance. To ensure the corrosion resistance of stainless steel, a Cr content of 12.0 mass% or more is required. However, since Cr is also an element forming ferrite, if too much is added, a large amount of δ ferrite will be formed at high temperatures. To suppress the formation of δ ferrite, austenite-forming elements (C, N, Ni, Mn, Cu, etc.) must be added, but the addition of a large amount of these elements leads to stabilization of austenite at room temperature. However, even with the help of cold working, it becomes difficult to secure the necessary amount of martensite (described later) before the reverse transformation treatment, and as a result, a high spring limit value cannot be obtained. Therefore, in the present invention, the upper limit of the Cr content is set to 20.0% by mass.

N, like C, is an austenite-forming element and an extremely useful element for hardening martensite to increase the strength of a steel strip. It also contributes to the formation of precipitates. However, since a large amount of addition causes blowholes at the time of casting, the N content is set to 0.15% by mass or less.

Mo is an element effective for suppressing the grain growth of the reverse transformed austenite and for finely dispersing the precipitates. Therefore, in order to maximize the effect of the present invention, Mo
Is preferably added positively, and its content is 0.5
It is desirable that the content be not less than mass%. However, when a large amount of Mo is added, δ ferrite is generated at a high temperature. In addition, the hot deformation resistance increases, and the hot workability is impaired. Therefore, the upper limit of the Mo content needs to be suppressed to 5.0% by mass or less.

Cu, like Ni, is an element that lowers the reverse transformation temperature of martensite to austenite. Therefore, in the present invention, it is effective to add Cu actively, and its content is desirably 0.5% by mass or more. However, the addition of a large amount of Cu causes deterioration of hot workability and causes cracks in the steel strip. For this reason, the upper limit of the Cu content must be suppressed to 5.0% by mass or less.

In addition, as long as the object of the present invention is not impaired, as other elements, for example, 0.01 mass% or less of B or 100 ppm or less of Ca for the purpose of improving hot workability, and for improving the age hardening property. Nb of 1.0 mass% or less for the purpose, 1.0 mass%
The following V, 1.0% by mass or less of Zr may be added. However, in the present invention, Ti is not added. The addition of Ti causes the formation of coarse Ti-based inclusions upon melting, which causes the fatigue properties to be significantly reduced. In addition, according to the configuration defined in the present invention, sufficient age hardening can be achieved without relying on Ti. Because it can be obtained.

As described above, C and N form aging precipitates and contribute to an increase in the spring limit value. In the present invention, in order to sufficiently exhibit the effect, the total amount of C + N needs to be 0.10% by mass or more.

The following formula, X = 1305-41.7 (Cr + Mo + Cu) -61.1
Ni-33.3Mn-27.8Si-1667 (C + N) is an index indicating the easiness of formation of martensite, and can be applied to the component alloy specified in the present invention. In order to obtain a high-strength steel strip having a high spring limit, which is the object of the present invention, the X value is -10.
It is extremely important that the chemical composition be in the range of 0 to 200. In steels having an X value lower than -100, it is difficult to generate a sufficient amount of martensite even if cold rolling is performed before the reverse transformation treatment. Generation is inadequate. In addition, the amount of martensite generated in the cooling process after the reverse transformation treatment is insufficient. Therefore, a remarkable improvement in the spring limit cannot be expected with such steel. On the other hand, if the X value is 200
When the temperature exceeds the above range, martensite is sufficiently generated, but martensite is generated at a considerably high temperature in the cooling process of the reverse transformation process, so that tempering occurs during cooling, and the spring limit value is significantly reduced.

The high-strength stainless steel strip of the present invention has a significant feature in its metallographic structure. That is, the average particle size 2μ
The characteristic feature is that fine martensite formed from an austenite matrix having a diameter of m or less occupies a large part and has a metal structure in which an aging precipitate exists. This results in a high spring limit.

The fine martensite may occupy almost 100% by volume excluding the aging precipitate, or may contain 80% by volume or more of fine martensite and the balance substantially consists of austenite and the aging precipitate. It may be a phase organization. Whether or not martensite is derived from an austenite matrix having an average particle size of 2 μm or less can be determined by observing the metal structure with an electron microscope or an optical microscope. This is because the austenite grain boundary of the parent phase remains in the metal structure even after the martensitic transformation.

The aging precipitate is, for example, Cr ₂₃ C ₆ , FeM
o and the like. These precipitates are formed to some extent in the reverse transformation process for forming fine martensite. But that alone is not enough to raise the spring limit. In other words, in the present invention, it is important to perform aging treatment at a relatively low temperature separately from the reverse transformation treatment,
This requires a large amount of precipitates (aged precipitates) generated thereby. Specifically, it is desirable that precipitates are present in the martensite with a precipitation interval of 0.3 μm or less. Here, the precipitation interval is represented by L / (N-1) when N precipitates exist with respect to the scanning distance L (intercept method).

The steel strip of the present invention has a spring limit value of 1200 due to a synergistic effect of “fine martensite transformed from an austenite matrix having an average grain size of 2 μm or less” and “aging precipitate”.
It has excellent spring characteristics of N / mm ² or more. A high-strength steel strip exhibiting such a high spring limit value is suitable for a spring material requiring high performance, such as a metal gasket.

A metal structure mainly composed of "fine martensite transformed from an austenite matrix having an average grain size of 2 μm or less" can be obtained by the following reverse transformation treatment. First, it is necessary that 80% by volume or more of martensite exists in the steel strip subjected to the reverse transformation treatment. If the amount of martensite is smaller than this, the amount of finally generated fine martensite is insufficient, and the spring limit value cannot be sufficiently increased. It does not matter whether the martensite before the reverse transformation treatment is a cooled martensite or a work-induced martensite. That is, when 80% by volume or more of martensite generated in the cooling process of the solution treatment is present, it can be directly subjected to the reverse transformation treatment,
If it is less than 80% by volume, cold rolling is further performed to generate work-induced martensite, and the total martensite amount may be 80% by volume or more.

The reverse transformation is performed at a relatively low temperature.
All martensite in the steel strip is reverse transformed to austenite. The reverse transformed austenite thus formed becomes fine crystal grains. According to our research,
The average particle size is 2μ by heating at 600 ~ 850 ℃ for 0.1 ~ 60 minutes
It is possible to generate fine inverse transformed austenite of m or less. In order to cause the reverse transformation completely at a relatively low temperature, it is necessary to use a steel whose reverse transformation temperature is lowered by adjusting the chemical composition as described above.

Since the reverse transformed austenite thus generated is not stabilized, the martensitic transformation can sufficiently occur in the cooling process of the reverse transformation treatment. As a result, a metal structure mainly composed of fine martensite transformed from an austenite matrix having an average particle size of 2 μm or less can be obtained.

In the present invention, an aging treatment is further performed to generate an "aged precipitate" in an amount sufficient to increase the spring limit value. The aging treatment may be performed at 400 to 550 ° C. for 0.1 to 120 minutes. Considering the stability and cost in actual operation, the condition at 420 to 520 ° C for 1 to 60 minutes is preferable, and the condition at 450 to 500 ° C for 5 to 30 minutes is more preferable.

[0032]

EXAMPLES Table 1 shows the chemical composition of the test material, and the following formula, X
= 1305-41.7 (Cr + Mo + Cu) -61.1Ni-33.3Mn-27.8Si-
1667 (C + N). In Table 1, A ~
I is an invention steel whose chemical composition falls within the range specified in the present invention;
Is a comparative steel. After melting both steels,
After hot rolling → annealing → cold rolling, it was turned into a cold-rolled steel strip having a thickness of 2.0 to 2.5 mm, and then subjected to a solution treatment at 1050 ° C. × 1 minute to be water-cooled. 80% by volume
1m thick including the above martensite (cooled martensite and / or work induced martensite)
m steel strip. These steel strips were subjected to reverse transformation at a temperature in the range of 500 to 50 ° C., air-cooled to room temperature, and then further aged at a temperature in the range of 400 to 550 ° C.

[0033]

[Table 1]

A sample was cut out from each steel strip after the aging treatment, and the microstructure of the cross section was observed, a tensile test was performed at room temperature, and a spring limit value was measured. The metal structure observation was performed using a scanning electron microscope or an optical microscope. The average particle size of the austenite matrix derived from martensite was determined by averaging the particle sizes of 40 to 50 measured by the intercept method. The volume percent of martensite was also determined. Further, the spring limit value was determined by a “repeated bending test” specified in JIS H3130. At that time, the deflection coefficient (Young's modulus) was measured for each test piece. Table 2 shows the results together with the X value, reverse transformation condition, and aging condition.
Shown in

[0035]

[Table 2]

As can be seen from the results in Table 2, the steel strips of Nos. 1 to 15 according to the present invention have a very high spring limit of 1200 N / mm ² or more, and are particularly suitable for spring materials such as metal gaskets. It is a thing. In contrast, the steel strip of the comparative example has a high tensile strength, but has a low spring limit. As described above, the steel strip of the present invention is characterized in that the level of the spring limit value is significantly higher even though the tensile strength level is equal to that of the comparative steel strip. All of the martensite present in the steel strip of the present invention were formed after the reverse transformation treatment.

The steel strips of Comparative Examples Nos. 14 to 25 were all subjected to the aging treatment under appropriate conditions specified in the present invention. However, in No. 14, since the X value was too high as 250, martensite was generated from a considerably high temperature during cooling in the reverse transformation process,
The tempering phenomenon occurs during the cooling process, and the spring limit value is reduced. In Nos. 15 and 16, the Ni content was so low that they could not be sufficiently reverse-transformed into austenite, and the amount of fine reverse-transformed austenite required to generate fine martensite was small. Eventually almost 100
% Fine martensite arising from reverse transformation austenite despite having a 1% martensite structure
No. 5 was 40% and No. 16 was 70%, which did not improve the spring limit. The martensite that had existed before the reverse transformation treatment was transformed from an austenite phase having a particle size of about 50 μm, and the manner of etching was different from the fine martensite generated after the reverse transformation treatment. These can be distinguished by observation with an optical microscope. No.1
Nos. 7 and 18 were those in which the total amount of C + N was less than 0.10% by mass and precipitates were not sufficiently formed and the spring limit was not improved, and Nos. 19 and 20 were martensite because the X value was lower than -100. Was insufficient and the spring limit value did not improve. In addition, No. 21 to 25 perform reverse transformation at 850 ° C.
Since the test was carried out at a higher temperature, the grain size of the reverse transformed austenite was coarsened and the spring limit value was not improved.

For reference, the results of plotting the relationship between the X value and the spring limit based on the data in Table 2 are shown in FIG. 1, and the relationship between the average grain size of the austenite matrix in the martensite phase and the spring limit. 2 is shown in FIG.

Next, aging treatment was performed on steel strips whose chemical composition and reverse transformation treatment conditions satisfy the requirements of the present invention at various temperatures, and the influence of the aging treatment temperature on the spring limit value was examined. Table 3 shows the results.

[0040]

[Table 3]

As can be seen from the results in Table 3, the steel strips (Nos. 26 to 30) having the aging treatment temperature in the range specified in the present invention were compared with the other steel strips (Nos. 31 to 35). It showed extremely high spring limits. For reference, FIG. 3 shows the result of plotting the relationship between the aging temperature and the spring limit value based on the data in Table 3.

As described above, i) having an appropriate chemical composition,
ii) exhibits a very high spring limit value only when it has a proper reverse transformation structure (structure mainly composed of fine martensite) and iii) precipitates are formed under proper aging conditions. It was found that a high-strength stainless steel strip was obtained.

[0043]

According to the present invention, it is possible to provide a high-strength stainless steel strip having a significantly higher spring limit value than before. This steel strip can be suitably used for many spring materials including metal gasket applications, and high-performance spring parts can be manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an X value and a spring limit value.

FIG. 2 is a graph showing a relationship between an average grain size of an austenite matrix in a martensite phase and a spring limit value.

FIG. 3 is a graph showing a relationship between an aging treatment temperature and a spring limit value.

Claims (7)

[Claims] 1. In mass%, C: 0.15% or less, S
i: 3.0% or less, Mn: 4.0% or less, Ni: 4.0 to 7.0%,
Cr: 12.0 to 20.0%, N: 0.15% or less, Mo: 0 to 5.0%
(Including no addition), Cu: 0 to 5.0% (including no addition), C + N ≧ 0.10, and X = 1305-41.7 (Cr + Mo + Cu) -61.1Ni-33.3Mn-27.8Si
A steel strip containing these elements so that the value of X at -1667 (C + N) becomes -100 to 200, and having a chemical composition consisting of a balance of Fe and unavoidable impurities, and having an average grain size of 2 μm or less. A high-strength stainless steel strip having a metal structure substantially consisting of fine martensite transformed from a phase and aging precipitates and exhibiting a high spring limit value of 1200 N / mm ² or more. 2. The metal alloy according to claim 1, wherein the metal structure contains at least 80% by volume of fine martensite transformed from an austenite matrix having an average particle size of 2 μm or less, and the balance substantially consists of austenite and aging precipitates. Strength stainless steel strip. 3. In mass%, C: 0.15% or less, S
i: 3.0% or less, Mn: 4.0% or less, Ni: 4.0 to 7.0%,
Cr: 12.0 to 20.0%, N: 0.15% or less, Mo: 0 to 5.0%
(Including no addition), Cu: 0 to 5.0% (including no addition), C + N ≧ 0.10, and X = 1305-41.7 (Cr + Mo + Cu) -61.1Ni-33.3Mn-27.8Si
-1667 (C + N), which contains these elements so that the value of X becomes -100 to 200, and is transformed from an austenite matrix having a chemical composition consisting of a balance of Fe and unavoidable impurities and having an average particle size of 2 μm or less. A method for producing a high-strength stainless steel strip having a high spring limit value, comprising subjecting a steel strip having a metal structure substantially composed of fine martensite to aging treatment at 400 to 550 ° C for 0.1 to 120 minutes. 4. The metal structure of a steel strip subjected to aging treatment contains at least 80% by volume of fine martensite transformed from an austenite matrix having an average grain size of 2 μm or less, and the balance substantially consists of austenite. Item 4. The method for producing a high-strength stainless steel strip according to item 3. 5. In mass%, C: 0.15% or less, S
i: 3.0% or less, Mn: 4.0% or less, Ni: 4.0 to 7.0%,
Cr: 12.0 to 20.0%, N: 0.15% or less, Mo: 0 to 5.0%
(Including no addition), Cu: 0 to 5.0% (including no addition), C + N ≧ 0.10, and X = 1305-41.7 (Cr + Mo + Cu) -61.1Ni-33.3Mn-27.8Si
These elements are contained so that the value of X in -1667 (C + N) becomes -100 to 200, the chemical composition is composed of the balance of Fe and unavoidable impurities, and contains 80% by volume or more of martensite. 0.1 to 6 at 600 to 850 ° C for a solution-treated steel strip or a steel strip cold-rolled after solution treatment having a metal structure.
Heating for 0 minutes to reverse transform the entire amount of martensite into fine austenite with an average particle size of 2 μm or less,
Then, after forming martensite from the fine austenite in the process of cooling to room temperature, 400-550
A method for producing a high-strength stainless steel strip with a high spring limit, which is subjected to aging treatment at 0.1 ° C for 0.1 to 120 minutes. 6. The high-strength stainless steel strip according to claim 1, wherein the use is for a metal gasket. 7. The method for producing a high-strength stainless steel strip according to claim 3, wherein the use is for a metal gasket.