JPH1180906A - High strength stainless steel strip increased in yield stress, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength stainless steel strip increased in yield stress and suitable for steel belt and spring material. SOLUTION: The steel strip has a composition which consists of, by mass, <=0.15% C, <=3.0% Si, <=4.0% Mn, 4.0-7.0% Ni, 12.0-20.0% Gr, <=0.15% N, 0-5.0% Mo, 0-5.0% Cu, and the balance Fe with inevitable impurities 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, and further, this steel strip has a metallic structure composed essentially of fine martensite transformed from austenitic base phase of <=2 μm average grain size. This steel strip can be obtained by applying heating treatment to a steel strip containing >=80 vol.% martensite at 600-850 deg.C for 0.1-60 min to subject the whole quantity of the martensite to inverse transformation into fine austenite of <=2 μm average grain size.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel belt and a spring material which require a high plastic deformation initiation stress.
The present invention relates to a high-strength stainless steel strip having a high yield stress and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, when stainless steel is applied to a steel belt, a spring material, or the like, work-hardening austenitic stainless steel represented by SUS301, SUS630, SUS6
Precipitation hardening stainless steels such as 31 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. On the other hand, the latter is intended to change the metastable austenite phase into a martensite phase by a pretreatment such as cold rolling after solution treatment, and then to precipitate an intermetallic compound Ni ₃ Al by aging treatment to thereby achieve hardening. Yes, very high strength can be obtained.

[0005]

Although steel belts and spring materials are required to have high strength and high toughness, it is also important that the yield stress is high. Due to its properties, steel belts and spring materials are used in a state where a high stress is applied within an elastic limit. A use in which the applied stress may slightly exceed the elastic range is not allowed. Therefore, it can be said that a material having a high yield stress, that is, a material having a high proof stress is a material that can exhibit higher reliability in steel belt and spring applications.

[0006] Among the above-mentioned high-strength stainless steels used for these applications, precipitation hardening stainless steels are difficult to adjust the strength, which is a disadvantage of work hardening stainless steel.
Problems such as anisotropy in properties and reduction in toughness are avoided, and particularly high strength is obtained in an alloy system such as SUS631 which incorporates a strengthening mechanism by intermetallic compound precipitation. However, even with such alloys,
In steel belt and spring applications, their yield stress levels are not yet fully satisfactory. The present invention
In view of the current situation, an object of the present invention is to provide a stainless steel having a high yield stress, which is suitable for a steel belt or a spring.

[0007]

In order to achieve the above object, according to the first aspect of the present invention, in terms of mass%, C: 0.15% or less; Si: 3.0% or less; Mn: 4.0% or less;
7.0%, Cr: 12.0 to 20.0%, N: 0.15% or less, Mo: 0
-5.0% (including no addition), Cu: 0-5.0% (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 the balance being Fe and unavoidable impurities. It is a high-strength stainless steel strip having a metal structure substantially composed of transformed fine martensite and having increased yield stress.

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) -6
Cr, M in 1.1Ni-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 the "metal structure substantially consisting of .." means "precipitates, inclusions inevitably contained in the production, and a small amount (approximately 5% by volume or less) of delta ferrite." Means that it may be.

[0009] The invention of claim 2 is particularly directed to the above steel.
Mo: 0.5 to 5.0%, Cu: 0.5 to 5.0%, containing one or two of them.

According to a third aspect of the present invention, there is provided a steel strip according to the first or second aspect, wherein the metal structure comprises 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 comprises austenite. It has been changed to a metallic structure that becomes more realistic.

[0011] The invention of claim 4 is the invention of claim 1, 2, or 3.
In particular, the steel strip is specified to have a 0.01% proof stress: 850 N / mm ² or more and a 0.2% proof stress: 950 N / mm ² or more.

The invention of claim 5 defines a method for producing these steel strips, and is a steel strip having a chemical composition in the above specified range, wherein 80% by volume or more of martensite exists. The solution-treated steel strip or the steel strip cold-rolled after the solution treatment is heated at 600 to 850 ° C for 0.1 to 60 minutes to reverse transform all the martensite into fine austenite with an average grain size of 2 μm or less. And then producing martensite from the fine austenite in the course of cooling to room temperature to obtain a metal structure having fine martensite.

According to a sixth aspect of the present invention, the steel strip of the first, second, third or fourth aspect is specified for a steel belt.

According to a seventh aspect of the present invention, the method for manufacturing a steel strip according to the fifth aspect is defined as a method for manufacturing a steel strip for a steel belt.

[0015]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, the yield stress (proof stress) of a high-strength stainless steel strip is increased by forming fine martensite of a specific form and combining the effect of precipitates. Hereinafter, matters for specifying the steel strip of the present invention will be described.

C is very effective in strengthening martensite, and plays a role of increasing the yield stress by forming precipitates in the reverse transformation temperature range from martensite to austenite. 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.

For the purpose of mainly controlling the stability of the austenite phase, Mn is contained in an appropriate amount determined by the balance with other elements. 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 Mn content exceeds the 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 element essential for imparting corrosion resistance. In order to impart corrosion resistance as stainless steel, a Cr content of 12.0% by 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. Even with the help of cold working, it is difficult to secure the required amount of martensite (described later) before the reverse transformation treatment, and it is impossible to obtain a high yield stress. Therefore, in the present invention, C
The upper limit of the r content was 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 reverse transformed austenite and for finely dispersing 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 from 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% by 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 precipitates at the time of heating in the reverse transformation process and contribute to the increase in yield stress. 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 yield stress aimed at by the present invention, the X value is -100.
It is very important to have a chemical composition in the range of ~ 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, improvement in yield stress cannot be expected with such steel. On the other hand, when the X value is higher than 200, martensite is sufficiently formed, but martensite is generated at a considerably high temperature in the cooling process of the reverse transformation process, so that a tempering phenomenon occurs during cooling, and the yield stress is remarkably increased. Will drop.

The steel strip of the present invention is characterized in that it has a metal structure mainly composed of fine martensite formed from an austenite matrix having an average grain size of 2 μm or less. Almost 100% by volume of the metal structure may be occupied by such fine martensite, or a multi-phase structure containing fine martensite of 80% by volume or more and substantially consisting of austenite, and Is also good. As demonstrated in Examples described later, it has been revealed by detailed studies by the present inventors that the yield stress is significantly increased by realizing such a fine martensite-based structure. Conventionally, there is no example in which such a metal structure is specified to improve the yield stress of high-strength stainless steel. Note that 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.

In addition to the fine martensite as described above, the presence of precipitates also contributes to the improvement in yield strength. For example, Cr
Precipitates such as ₂₃ C ₆ , FeMo and the like are mentioned, and it is desirable that these exist in the martensite at a precipitation interval of 0.5 μm or less. These precipitates can be formed by subjecting a steel having the chemical composition specified in the present invention to a reverse transformation treatment at a relatively low temperature.

The yield stress can be specifically evaluated by the value of "proof stress". Usually 0.2% proof stress (= 0.2%
(Nominal stress in which the plastic strain remains as a history) is often used, but in the case of steel belts and spring materials, the stress value at the elastic limit becomes important, so it is not preferable to determine the yield stress based on only 0.2% proof stress. Therefore, in order to represent a stress value as close to the elastic limit as possible, a 0.01% proof stress was measured here. 0.01% proof stress: 850N / mm ² or more and 0.2% proof stress: 9
A high-strength stainless steel strip exhibiting a high yield stress of 50 N / mm ² or more has characteristics very suitable for a steel belt and a spring material.

Next, a method of manufacturing a high-strength stainless steel strip having a high yield stress as described above will be described.

First, it is necessary that 80% by volume or more of martensite is present in the steel strip at room temperature before heating in the reverse transformation treatment. If the amount of martensite is smaller than this, the yield of finally desired fine martensite is insufficient, and the yield stress 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. In other words, when 80% by volume or more of the martensite generated in the cooling process of the solution treatment is present, it can be directly subjected to the reverse transformation treatment, and when the martensite after the solution treatment is less than 80% by volume. In addition, cold rolling may be further performed to generate work-induced martensite, and the total martensite amount may be 80% by volume or more.

Then, all the martensite in the steel strip is reverse transformed into austenite at a relatively low temperature. The reverse transformed austenite thus formed becomes fine crystal grains. According to the study of the present inventors, 600-850 ° C., 0.1-6
By heating for 0 minutes, it is possible to generate fine inverse transformed austenite having an average particle size of 2 μm 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.

The austenite matrix having an average particle size of 2 μm or less can be obtained by the reverse transformation treatment at a relatively low temperature as described above. Since the reverse transformation temperature is lowered by strict regulation of the chemical composition of the steel used, even when the reverse transformation treatment temperature is lowered, the reverse transformation to austenite can be completely performed. Since this austenite is not stabilized, the martensitic transformation can sufficiently occur in the cooling process after the reverse transformation treatment. In the present invention, fine martensite is sufficiently generated from the austenite matrix having an average particle size of 2 μm or less in this way, and a metal structure with an increased yield stress is obtained.

[0034]

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 strips were subjected to a reverse transformation at a temperature in the range of 500 to 500 ° C. and then air-cooled to room temperature.

[0035]

[Table 1]

A sample was cut out from each of the steel strips thus obtained, and the metal structure of the cross section was observed and a tensile test was performed at room temperature. 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 martensite volume% was also determined. Further, 0.01% proof stress, 0.2% proof stress, and tensile strength were determined from the stress-strain curve of the tensile test. The results are shown in Table 2 together with the X values and the reverse transformation conditions.

[0037]

[Table 2]

As can be seen from the results in Table 2, the steel strip according to the present invention has a very high yield stress of 0.01% proof stress of 850 N / mm ² or more and a 0.2% proof stress of 950 N / mm ² or more, and has a high steel belt. It is especially suitable for spring materials. On the other hand, in the steel strip of the comparative example, a high value was obtained for the tensile strength, but both the 0.01% proof stress and the 0.2% proof stress were low. As described above, the steel strip of the present invention is characterized in that the level of the yield stress is significantly higher even though the level of the tensile strength 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.

In the steel strip of Comparative Example No. 14, the X value was too high as 250, so that martensite was generated from a very high temperature during cooling in the reverse transformation treatment, and the phenomenon of tempering occurred in the cooling process and yield stress was reduced. It has fallen. In Nos. 15 and 16, the Ni content was low, so that the reverse transformation to austenite could not be sufficiently completed, and the amount of fine inverse transformed austenite required to generate fine martensite was small. Despite having almost 100% martensite structure, the amount of fine martensite generated from reverse transformation austenite was as small as 40% in No. 15 and 70% in No. 16, and as a result, yield stress did not improve. Things. The martensite that was present before the reverse transformation treatment was transformed from the 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. Can be distinguished by optical microscope observation. Nos. 17 and 18 were those in which the total amount of C + N was less than 0.10% by mass, so that precipitates were not sufficiently formed during the reverse transformation treatment and the yield stress was not improved, and Nos. 19 and 20 had an X value of -100. Because of the lower value, the generation of martensite was insufficient and the yield stress did not improve. In Nos. 21 to 25, since the reverse transformation treatment was performed at a temperature higher than 850 ° C., the grain size of the reverse transformation austenite was coarsened and the yield stress was not improved.

For reference, the X value and 0.
Figure 1 shows the result of plotting the relationship between 1% proof stress and 0.2% proof stress.
FIG. 2 shows the results of plotting the relationship between the average grain size of the austenite matrix and the 0.1% proof stress and 0.2% proof stress in the martensite phase.

[0041]

According to the present invention, it is possible to provide a high-strength stainless steel strip in which the yield stress is drastically improved by combining the optimization of the chemical composition and the thermomechanical treatment under specific conditions. . This steel strip has characteristics very suitable for a steel belt and a spring material. If this steel strip is used, a high-performance and highly-reliable steel belt or spring, which has never existed before, can be manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an X value and a proof stress.

FIG. 2 is a graph showing the relationship between the average grain size of the austenite matrix in the martensite phase and proof stress.

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 the balance being Fe and unavoidable impurities. A high-strength stainless steel strip having an enhanced yield stress and a metal structure substantially consisting of transformed fine martensite. 2. 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: 0.5 to
5.0%, Cu: contains one or two of 0.5 to 5.0%, 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 the balance being Fe and unavoidable impurities. A high-strength stainless steel strip having an enhanced yield stress and a metal structure substantially consisting of transformed fine martensite. 3. The metal structure contains 80% by volume or more 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.
The high-strength stainless steel strip according to claim 1. 4. A 0.01% proof stress: 850 N / mm ² or more, and 0.2%
% Proof stress: high strength stainless steel strip according to claim 1, 2 or 3 with a 950 N / mm ² or more high yield stress. 5. A solution-treated steel strip in which 80% by volume or more of martensite is present or a steel strip cold-rolled after solution treatment is heated at 600 to 850 ° C. for 0.1 to 60 minutes to form the martensite. Reverse transformation of the total amount to fine austenite having an average particle size of 2 μm or less, and then forming martensite from the fine austenite in the course of cooling to room temperature to obtain a metal structure having fine martensite. 5. The method for producing a high-strength stainless steel strip according to 2, 3, or 4. 6. The high-strength stainless steel strip according to claim 1, wherein the steel strip is for a steel belt. 7. The steel strip for a steel belt.
2. The method for producing a high-strength stainless steel strip according to item 1.