JP3503959B2 - High-strength stainless steel excellent in toughness and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to members and parts which are required to have high strength and toughness as well as corrosion resistance, such as leaf springs, coil springs, and blade plates for producing Si single crystal wafers (I
The present invention relates to stainless steel most suitable for materials such as metal gaskets that compose engines such as D blades) and automobiles, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, when manufacturing the above-mentioned members and parts from stainless steel, martensitic stainless steel, work hardening type stainless steel or precipitation hardening type stainless steel has been used.

[0003] Martensitic stainless steel is hardened by rapidly cooling from a high-temperature austenitic state and undergoing martensitic transformation, and steel types such as SUS410 and SUS420J2 correspond to this. High strength and toughness are obtained for these steels by tempering treatment of quenching and tempering. However, if the product is extremely thin, it will be deformed due to thermal strain during quenching, so it will be difficult to create the desired shape.

Therefore, work-hardening austenitic stainless steel is usually used for such applications. These are typified by SUS301 and SUS304, which exhibit an austenite phase in the solution heat treated state and generate work-induced martensite in the subsequent cold working to obtain high strength. Its strength depends on the amount of cold working and martensite, but it is very difficult to adjust the strength only by cold working, and if the cold rolling rate is increased significantly, the anisotropy of the material increases, Toughness decreases.

Precipitation hardening type stainless steel is one in which an element having a high precipitation hardening ability is added to be hardened by aging treatment, and SUS630 containing Cu and SUS631 containing Al are typical.

Among these, SUS630 is a martensite single phase after solution treatment, and is hardened by aging treatment, and its tensile strength is about 1400 N / mm ² at most.

On the other hand, SUS631 is a metastable austenite phase after solution treatment, and a part of austenite is changed to a martensite phase by a pretreatment such as cold working, and then an aging treatment is performed to form an intermetallic compound Ni ₃ Al is precipitated and hardened. In other words, it is intended to obtain high strength by utilizing the hardening by the process-induced martensite and the age hardening together, and a considerably high strength can be obtained. For example, by positively generating the work-induced martensite phase, it is possible to increase the tensile strength up to 1800 N / mm ² in this system.

Utilizing the strength increase due to such aging treatment, stainless steel having higher strength than the above-mentioned steel types has been developed. For example, JP 61-295356 A and JP 4-2
No. 02643 discloses that a metastable austenite system to which Cu and Si are added together is subjected to appropriate cold working to obtain a two-phase structure of a work-induced martensite and an austenite phase, and a tensile strength of 2000 N / mm after aging treatment. ² , stainless steel having Vickers hardness of 580 is described. However, since the optimum aging temperature range of these steels is extremely narrow, it is very difficult to obtain a strength level of this level stably from the viewpoint of manufacturability.

[0009]

[Problems to be Solved by the Invention] When attempting to increase the strength by using work hardening and age hardening in austenitic stainless steel, it is necessary to incorporate a large amount of work strain as the microstructure before aging and to make fine precipitates of aging precipitates. Precipitation cannot be expected. Therefore, it is necessary to increase the cold rolling rate, but there is a problem that the toughness is significantly impaired when the cold rolling rate is increased. In addition, if the product is an ultra-thin material, the shape of the product will be impaired, and the strength will decrease due to deviation from the prescribed aging temperature.

In order to remove as much as possible the deterioration of toughness due to the cold-rolling during strong working, aging at a high temperature is desirable, and if a material with a small decrease in hardness even with high-temperature aging, that is, a material with a high tempering resistance is produced, it will have high strength and high toughness. It is optimal as a material for spring materials, ID blade plates, metal gaskets, etc. The present invention aims at the development of stainless steel that satisfies this use application.

[0011]

According to the present invention, the mass% is
In, C: 0.10% or less, Si: 1.0% to 3.0%, Mn:
2.0% or less, Ni: 4.0% to 9.0%, Cr: 12.0% to 18.0% ,
In addition to containing Mo: 1.0% to 5.0%, N: 0.15% or less ,
Containing C and N so as to satisfy the 且 one C + N ≧ 0.10% relationship to provide a high strength stainless steel balance excellent toughness of Fe and manufacturing inevitably mixed impurities.

According to the present invention, a stainless steel having a chemical composition value within the above range and exhibiting a metastable austenite phase in a solution-treated state is sufficient for forming 30 to 80% of work-induced martensite. It has a tensile strength of 1800 N / mm ² or more and excellent toughness, which consists of a step of cold working at a high cold working rate and a step of short-term aging at 500 to 600 ° C for 0.5 to 5 minutes. A method of manufacturing high strength stainless steel is provided.

[0013]

[Operation] When stainless steel that exhibits a metastable austenite phase in the solution heat treatment state is used to generate work-induced martensite by cold working (hereinafter referred to as cold rolling) and further precipitation hardening by aging treatment, The strain introduced by cold rolling becomes the nucleation site of precipitates in the early stage of aging treatment, and effectively acts on the strength increase due to fine precipitation.

However, although the excessive strain eventually contributes to the increase in strength, the accompanying decrease in toughness poses a problem.
The present invention is intended to solve this problem from the viewpoint of the composition design of steel, and in order to prevent the deterioration of toughness due to high strength as much as possible, rolling strain introduced in cold rolling (obstructing toughness is impaired). The excessive strain) can be reduced by the aging treatment in the next step.

In the steel of the present invention, the main hardening elements due to aging are Si , C and N. S i fixes the strain (dislocation) introduced during rolling, and C and N contribute to hardening as precipitates. The strain due to this rolling decreases with increasing aging temperature, but this change is very sensitive to temperature increase.

The present inventors have found that when Mo is added, Mo-based precipitates are formed in a high temperature range, and the strength reduction due to the precipitates other than Mo-based that are overaged in this temperature range can be canceled out and high strength can be expressed. Found out. Moreover, since Mo atoms have a slow diffusion rate, they have an action (drug effect) of suppressing the recovery of the structure (dislocation disappearance) in the temperature range of 500 to 600 ° C. where Mo-based precipitates are formed.

From the above, in the case of the steel of the present invention,
Even if the aging treatment is performed at a high temperature, the contribution of the work hardening to the strength can be maintained at the same level as in the case of the conventional low temperature aging. In addition, since aging treatment can be performed at high temperature, it has an advantageous effect on the improvement of toughness. In addition, the allowable range of aging treatment temperature is expanded and manufacturability (strength is less dependent on aging temperature. (It can be made relatively rough and the processing time will be shortened)
This also has an advantageous effect.

The action of each component in the steel of the present invention and the reason for limiting the content range thereof are summarized as follows.

C is an austenite-forming element, which acts very effectively in suppressing δ-ferrite produced at high temperature and strengthening the martensite phase induced by cold working, but since the steel of the present invention has a high Si content, The solid solubility limit of has decreased.
Therefore, when C is increased, Cr carbide is precipitated at the grain boundary, which causes intergranular corrosion resistance and toughness reduction. For this reason, C is set to 0.10% or less.

Si is usually used for deoxidation at the time of steel making, but when it is added for this purpose, it is generally 1.0 as shown in work hardening type stainless steels SUS301 and 304.
% Or less. However, in the case of the steel of the present invention, Si is set higher than this. This significantly promotes the formation of the martensite phase during cold working, and at the same time, serves to harden the martensite phase induced by this working by strain aging, and also forms a solid solution in the remaining austenite phase. It serves to cure this. Therefore, it has the effect of increasing the strength after cold working.

[0021] While achieve the various functions and effects of Si in the steel of the present invention as this, the effect as the conventional steels 1.0%
It is small below, and if it exceeds 3.0%, hot cracking tends to be induced, which causes various problems in manufacturing. Therefore, the content range is set to more than 1.0% and 3.0% or less.

Mn is an element that controls the stability of the austenite phase, and its utilization is carried out in balance with other elements. In the steel of the present invention, if this content is high, the ductility decreases, Various problems arise in use. For this reason 2.
It was set to 0% or less.

Ni is an essential element for obtaining an austenite phase at high temperature and room temperature, but in the case of the steel of the present invention, it must be a metastable austenite phase at room temperature and induce a martensite phase by cold working. I won't. Inventive steel
When it is lower than 4.0%, a large amount of δ-ferrite phase is formed at high temperature, and a martensite phase is formed during the cooling process up to room temperature, so that it cannot exist as an austenite single phase.
On the other hand, if it exceeds 9.0%, the martensite phase is less likely to be induced by cold working, so the range is 4.0 to 9.0%.

Cr is an essential component for corrosion resistance. At least 12.0% Cr is required to provide the intended corrosion resistance. However, since Cr is also a ferrite-forming element, if it is made too high, a large amount of δ-ferrite phase will be formed at high temperatures. Therefore, in order to suppress the δ-ferrite phase, it is necessary to add austenite forming elements (C, N, Ni, Mn, Cu, etc.), but excessive addition of these elements leads to stabilization of austenite at room temperature and The work-induced martensite phase is not formed by hot working, which makes it impossible to obtain high strength after aging treatment. Therefore, the upper limit of Cr is set to 18.0%.

[0025]

Mo improves corrosion resistance, acts very effectively in suppressing the release of strain during cold working accompanying aging treatment, and further produces precipitates that contribute to strength during aging treatment. This requires a Mo of at least 1.0%. However, addition of a large amount brings about formation of δ-ferrite at high temperature, so the upper limit was made 5%.

The action of Mo is as described above, but it is summarized in the following two points. (1) Mo-based precipitates are formed in a high temperature range of 500 to 600 ° C. Therefore, the decrease in strength due to the precipitates other than the Mo-based alloy that are overaged in this temperature range is canceled out. (2) Mo atoms have a slow diffusion rate and have an action (drug effect) of suppressing the recovery of the structure (dislocation disappearance) particularly in the temperature range of 500 to 600 ° C. where Mo-based precipitates are formed. Therefore, even if aging treatment is performed at a higher temperature than before, the contribution of work hardening to strength can be maintained to the same level as before.

Due to the effects of (1) and (2), the allowable range of aging temperature is expanded, manufacturing is facilitated, and aging treatment at high temperature is possible, so that toughness is improved.

N is an austenite-forming element and an extremely effective element for hardening an austenite phase and a martensite phase. However, since a large amount of addition causes blowholes during casting, it is set to 0.15% or less. .

Although C and N have almost the same actions, in the case of the steel of the present invention, 0.10% or more in total of N + C is required to harden the austenite phase and martensite phase to the intended extent. To do.

In the production of the steel of the present invention, after melting, hot rolling or further cold rolling is performed, and then the structure is adjusted to a metastable austenite phase by solution treatment.

After the solution treatment, cold rolling is performed. Due to the processing strain introduced at this time, part of the metastable austenite phase transforms into the martensite phase. In order to obtain high strength after aging treatment, it is necessary to generate a certain amount of martensite at this stage. The strength of the aged material increases as the cold working ratio increases, but the toughness decreases significantly. Therefore, considering the required balance between strength and toughness, the amount of work-induced martensite is 30 to 80% (volume%).
It is desirable to set within the range.

That is, the work-induced martensite needs to be 30% or more in order to promote the strain aging due to Si during the aging for increasing the strength of the material itself. However, from the viewpoint of toughness, residual (untransformed) austenite is also necessary, and it is necessary to suppress the work-induced martensite to 80% or less.

[0034] In the aging treatment performed after the cold rolling, considering continuously be Tsuban the heat treatment furnace to cold rolled steel strip, also
In order to obtain a sufficient effect of the aforementioned M o, it may be carried out for a short time aging treatment 0.5-5 minutes at a temperature range of 500 to 600 ° C.. The hardness reaches a peak near 500 ° C, and at temperatures higher than 500 ° C, the steel of the present invention containing an appropriate amount of Mo exhibits a remarkable effect of suppressing softening. When heated above 650 ° C, part of the work-induced martensite phase undergoes reverse transformation to the austenite phase, resulting in a decrease in strength. As for the soaking time, if 0.5 minutes or less, a sufficient aging effect cannot be expected, and it is inconvenient for soaking to take 5 minutes or more in consideration of manufacturability.

The effects of the present invention will be specifically described below with reference to examples.

[0036]

[Examples] Table 1 shows the chemical component values (mass%) of the test materials. In the table, M1 to M6 are steels of the present invention, and a to c are comparative steels. All steels were melted in a vacuum melting furnace, forged and hot-rolled to form hot-rolled steel strips with a thickness of 4.0 mm. This hot rolled material is 1050
After carrying out the solution treatment of holding at 1 ° C for 1 minute, water cooling treatment was performed.

Each of the obtained solution treated materials was cold-rolled at the cold-rolling rate shown in Table 2. As a result, the amount of processing-induced martensite (volume%) displayed was generated. Table 2 shows the Vickers hardness of each cold rolled material.

Each cold rolled material was subjected to an aging treatment at 600 ° C. for 1 minute. The hardness, tensile strength and notch strength of the obtained aged material are also shown in Table 2. The notch strength is a value obtained by performing a tensile test using the test piece shown in FIG. 3 and dividing the breaking load by the initial cross-sectional area of the notch portion of the test piece. This value means the stress required for crack propagation, and the higher the value, the better the toughness.

[0039]

[Table 1]

[0040]

[Table 2]

Further, each cold rolled material was
After aging treatment for a minute, the change in hardness depending on the aging temperature was examined.
The results are shown in Fig. 1. Fig. 1 shows the invention steel M as a typical example.
2, showing reference steel M5 and comparative steel c,
All show a hardness peak near 500 ° C. 500 ° C
At higher temperatures, the comparative steel shows a sharp decrease in hardness, whereas the steel of the present invention suppresses this phenomenon.

This tendency remarkably appears at 600 ° C. (× 1
The hardness change ΔHV (hardness of aging material-hardness of cold rolling material) between the hardness of the aging material at that temperature and the hardness of the cold rolling material before aging treatment was arranged by the amount of Mo. It is FIG.

As shown in FIG. 2, the hardness change ΔHV remarkably increases as the amount of addition of Mo increases.
That is, precipitation hardening due to Mo-based precipitates occurs due to aging at high temperature. This means that in the conventional steels that used work hardening and precipitation hardening, the tendency of decreasing hardness that occurs when aging treatment at high temperature (see comparative example in Fig. 1) occurs, but in the case of the present invention steel, it occurs during high temperature aging. It is shown that it was suppressed by the precipitation hardening due to the Mo-based precipitates. The action effect of Mo becomes remarkable when the amount of Mo is 1% or more.
Up to about%, it appears strongly depending on its content.

As a result, as shown in Table 2, the aging material of the present invention steel containing Mo has higher hardness and higher tensile strength than those of the comparative steel. What is more remarkable is that the notch strength exceeds the value of tensile strength even at such a high strength level, and the toughness is excellent. That is, stainless steel that has both high toughness and high strength was obtained by aging treatment at high temperature for a short time.

[0045]

As described above, the steel of the present invention has not only higher strength than the conventional work hardening type stainless steel or precipitation hardening type stainless steel but also high toughness. Therefore, the application can be expanded to materials such as leaf springs, coil springs, blade plates for producing Si single crystal wafers, and metal gaskets constituting engines for automobiles, which are required to have high strength and toughness at the same time as corrosion resistance.

The stainless steel of the present invention is excellent in strength-short time aging treatment characteristics, and in particular, addition of Mo expands the optimum aging temperature range for high strength, which is very advantageous in manufacturing.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between aging treatment temperature and Vickers hardness.

FIG. 2 is a diagram showing the effect of Mo amount on the hardness difference ΔHV (hardness of aging material−hardness of cold rolling material) between the aging material at 600 ° C. (× 1 minute) and the cold rolling material before aging treatment.

FIG. 3 is a view showing the geometrical dimensions of a test piece used for measuring notch tensile strength.

Continued front page    (72) Inventor Hiroki Tomimura               4976 Nomura-Minami-cho, Shinnanyo-shi, Yamaguchi Nisshin               Steelmaking Laboratory, Steelmaking Co., Ltd.                (56) References JP-A-63-317628 (JP, A)                 JP-A-4-191352 (JP, A)

Claims (2)

(57) [Claims] 1. In mass%, C: 0.10% or less, Si: 1.0% to 3.0%, Mn: 2.0% or less, Ni: 4.0% to 9.0%, Cr: 12.0% to 18.0%, Mo: 1.0% to 5.0%, N: 0.15% or less, C and N are included so as to satisfy the relation of C + N ≧ 0.10%, and the balance is Fe and impurities that are inevitably mixed in during manufacture. Stainless steel. 2. In mass%, C: 0.10% or less, S
i: 1.0% to 3.0%, Mn: 2.0% or less, Ni: 4.0% to 9.0
%, Cr: 12.0% to 18.0%, Mo: 1.0% to 5.0%, N: 0.
A stainless steel containing 15% or less and containing C and N so as to satisfy the relationship of C + N ≧ 0.10%, with the balance being Fe and impurities inevitably mixed in during manufacturing, in a solution heat treated state. A stainless steel exhibiting a metastable austenite phase was cold-worked at a cold-working rate sufficient to produce 30 to 80% work-induced martensite, and then 500
A method for producing a high-strength stainless steel having a tensile strength of 1800 N / mm ² or more and excellent toughness, which comprises performing a short-term aging treatment at 600 ° C for 0.5 to 5 minutes.