JP3289980B2 - Method for producing high-strength austenitic stainless steel with excellent toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a member or a component requiring high strength and toughness as well as corrosion resistance, such as a leaf spring, a coil spring, and a blade plate (I) for producing a Si single crystal wafer.
The present invention relates to a method for producing stainless steel, which is most suitable for a material such as a metal gasket constituting an engine of an automobile or the like.

[0002]

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

[0003] Martensitic stainless steel is hardened by rapidly cooling from a high-temperature austenite state and transforming it into martensite, and corresponds to steel types such as SUS410 and SUS420J2. These steels can obtain high strength and toughness by quenching-tempering heat treatment. However, if the product is extremely thin, it will be deformed by thermal strain during the quenching process, and it is difficult to produce a product having a desired shape.

[0004] For this reason, work-hardening austenitic stainless steel is usually used for such applications. These are typified by SUS301 and SUS304, which exhibit an austenitic phase in the solution treatment state, and form a work-induced martensite in the subsequent cold working to obtain high strength. The strength depends on the amount of cold work and the amount of martensite, but it is very difficult to adjust the strength only by cold work, and if the cold rolling reduction is significantly increased, the anisotropy of the material increases, The toughness decreases.

[0005] Precipitation hardening type stainless steels are hardened by aging treatment by adding an element having high precipitation hardening ability, and SUS630 to which Cu is added and SUS631 to which Al is added are typical.

[0006] Of these, SUS630 is solution heat treatment after the martensitic single phase, which was cured by the subsequent aging treatment, the tensile strength is at most 1400 N / mm ² approximately.

[0007] 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 pretreatment such as cold working, and then aging treatment is performed to obtain an intermetallic compound Ni _3. Al is precipitated and hardened. That is, high strength is intended to be obtained by utilizing both the hardening by the work-induced martensite and the age hardening, and a considerably high strength can be obtained. For example, by actively forming a work-induced martensite phase, it is possible to increase the tensile strength of this system to 1800 N / mm ² .

[0008] Utilizing the strength increase due to such aging treatment, a stainless steel having higher strength than the above-mentioned steel types has been developed. For example, JP-A-61-295356 and JP-A-4-14-1
No. 47946 discloses that a metastable austenitic system containing Si is subjected to appropriate cold working to form a two-phase structure of a work-induced martensite and an austenite phase, and a tensile strength of 2000 N / mm ² , Vickers A stainless steel with a hardness of 580 is described. However, toughness is reduced by strong working before aging and the presence of a large amount of martensite phase.

[0009]

When attempting to increase the strength of austenitic stainless steel using age hardening or work hardening, the toughness is significantly impaired due to the high cold rolling ratio, and the product is made of extremely thin material. If it is, its shape is further impaired.

If a high-strength austenitic stainless steel with excellent toughness can be manufactured, this material is most suitable as a material for a spring material, an ID blade plate, and a metal gasket that require high strength and high toughness. The present invention seeks to provide a method for producing stainless steel that satisfies this intended use.

[0011]

According to the present invention, mass%
C: 0.10% or less, Si: more than 1.0% to 4.0%,
Mn: 2.0% or less, Ni: 4.0% to 10.0%, Cr: 12.0% to 1
8.0%, N: 0.15% or less have free, addition, C + with N ≧ 0.10%, and Md (N) = 580-520C-2Si -16Mn-16Cr-23Ni-3
The value of Md (N) according to the formula of N-26Cu-10Mo is 20-7.
0, these elements are contained so that the range is
e and stainless steel consisting of impurities which are inevitably mixed in the production and exhibiting a metastable austenite phase in a solution-treated state, at a temperature of −20 ° C.
Transforming a part of the austenite phase to work-induced martensite by cold working at a low temperature of 30 ° C. and a reduction of 30 to 70%, and then subjecting to aging at 300 to 650 ° C. for 0.1 to 90 minutes A method for producing a high-strength austenitic stainless steel having excellent toughness, comprising:

[0012]

In the steel of the present invention, the main hardening elements by aging are Si , C and N. Of these S i contribute to curing by fixing the strain introduced during rolling, C and N contributes to curing as a precipitate. In addition, all the alloying elements act in the direction of suppressing the transformation from austenite to work-induced martensite, and by adjusting the amounts of these components, it is possible to generate work-induced martensite suitable for deformation.

The conventional work hardening type stainless steel has a component composition such that martensite is easily formed by cold rolling, and the austenite phase is relatively unstable during processing. According to the present invention, the composition of the austenite is adjusted to increase the stability to processing, and then processed (rolled).
At the low temperature side to induce a required amount of martensite. The structure after aging (remaining austenite +
(Martensite). To be precise, it is a two-phase mixed structure of a retained austenite phase and a tempered work-induced martensite.

In the two-phase mixed structure of the aged material, each component is adjusted so that the value of Md (N) specified in the present invention falls within an appropriate range, and the rolling temperature and the rolling reduction in cold rolling are adjusted. When properly controlled, the toughness can be improved while increasing the strength of the aging material. The fact is as proved in the examples described later, but the principle of improving the toughness of the aged material of the present invention can be considered as follows.

When the retained austenite of the aging material is represented by γ and the work-induced martensite is represented by α ′, the soft γ phase and the hard α ′ phase (actually, the α ′ phase tempered by the aging treatment) When deformation stress is applied to the mixed aging material, the soft γ-phase is deformed and the stress is concentrated, causing micro cracks to occur. The distortion concentrates and an α ′ phase is newly generated. For this reason, the generated α 'phase prevents the propagation of cracks, and when an external stress is applied, the stress concentrates on another γ phase and new cracks are observed. A phase forms.

At this time, the easiness of α ′ phase formation is Md
There is a correlation with the value of (N).
For this reason, the crack propagates in γ, progresses to a large crack, and eventually breaks. That is, the toughness is reduced, and molding such as bending cannot be performed satisfactorily. On the other hand, if Md (N) is too large, the α 'phase is immediately formed, making it difficult to deform. If this is forcibly deformed, it will develop into a crack, and in this case, the toughness will also decrease. That is, even if Md (N) is too large, it is not possible to prevent the propagation of cracks due to the transformation of γ → α ′. The definition of Md (N) in the range of 20 to 70 specified in the present invention has a close relationship with the toughness behavior of such an aged material.

As described above, the present invention is intended to improve the toughness by positively utilizing the transformation behavior of the retained austenite phase softer than α ′, and to improve the stability of the austenitic steel to processing. In order to induce a constant amount of α ', a rolling method with a lower rolling temperature was introduced to achieve both good strength and toughness at the same time. Here, it is desirable that the certain amount of α ′ is an amount in the range of 20 vol.% To 80 vol.%.

Next, the outline of the action of each component specified in the present invention and the reason for limiting the content will be described individually.

C is an austenite-forming element, which has an extremely effective effect on suppressing δ-ferrite formed at high temperatures and strengthening the martensitic phase induced by cold working. Has a lower solid solubility limit.
For this reason, when C is made high, Cr carbide precipitates at the grain boundaries, which causes intergranular corrosion resistance and a decrease in toughness. For these reasons, C is set to 0.10% or less.

Although Si is usually used for deoxidation, when it is added for this purpose, it is generally 1.0% or less as seen in SUS301 and 304 of work hardening stainless steel. However, the steel used in the present invention has higher Si. As a result, the martensitic phase induced by the cold working is hardened, and at the same time, it is dissolved in the residual austenite phase to harden it, thereby increasing the strength after the cold working . Although Si as this have various effects, the effect is small at 1.0% or less as in the conventional steels,
If it exceeds 4.0%, hot cracking tends to be induced during hot deformation, and various problems arise in production. Therefore, the component range
More than 1.0% and less than 4.0%.

Mn is an element that controls the stability of the austenite phase, and its use is carried out in balance with other elements. Problems arise. Therefore, the content is set to 2.0% or less.

Ni is an essential element for obtaining an austenite phase at high temperature and room temperature. In the present invention, Ni must be a metastable austenite phase at room temperature and a martensite phase must be induced by cold working. If the Ni content is lower than 4.0%, a large amount of δ ferrite phase is formed at high temperatures,
In addition, a martensitic phase is formed during the cooling process to room temperature and cannot be present as an austenite single phase.
%, The martensitic phase is less likely to be induced by cold working, so the component range was set to 4.0 to 10.0%.

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 too high, a large amount of δ ferrite phase will be formed at high temperatures. Therefore δ austenite forming element for ferrite phase suppression (C, N, Ni, etc. M n) must be added, excessive addition of these elements results in stabilization of austenite at room temperature, cold No work-induced martensite phase is formed by working, and it becomes impossible to obtain high strength after aging treatment. Therefore, the upper limit of Cr is 1
8.0%.

[0024]

[0025]

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

C and N exhibit the same hardening action as each other, and in order to exhibit their ability sufficiently, the total amount of C + N is 0.3%.
Must be at least 10%.

Regarding the Md (N) value. As explained earlier,
In the present invention, the austenite phase remaining after aging is transformed into a martensite phase during deformation (bending during forming into a final product, etc.). Md (N), which is a degree of stability to the processing of the above, is limited to the range of 20 to 70. The fact that Md (N) is in the range of 20 to 70 plays an important role in combination with the rolling temperature in achieving the object of the present invention.

When Md (N) is 20 or less, it is necessary to cold-roll at a low temperature, which is industrially very difficult, in order to form a martensitic phase which contributes to strength by working, and toughness of the aged material. Austenite → martensitic transformation that works effectively for improvement does not occur sufficiently. On the other hand, when Md (N) is 70
Beyond, it is necessary to increase the rolling temperature in order to obtain a suitable martensite phase by cold working, and to improve the toughness of the final product, the martensite phase is too fast for deformation. This is inconvenient because it is generated.

In the present invention, it is necessary to adjust the microstructure to a metastable austenite phase by solution treatment after the steel is melted and then subjected to hot rolling or further cold rolling.

By performing cold rolling after the solution treatment, a part of the metastable austenite phase is transformed into a martensite phase due to the processing strain at that time. To obtain high strength after aging treatment, a certain amount of martensite is required in this cold rolling stage. The strength of the aged material increases as the cold working ratio increases, but the toughness decreases significantly. Therefore, considering the balance between required strength and toughness,
It is desirable to set it to 30 to 70%.

At this time, the rolling temperature should be set to 70 ° C. or less. This is because sufficient strength cannot be obtained when rolling beyond 70 ° C, even with the same rolling ratio and the same amount of martensite, as compared to rolling at lower temperatures. Since the rolling at a rolling temperature of -20 ° C or less is very difficult, the rolling temperature during cold rolling was set to -20 to 70 ° C. By controlling the rolling temperature and the rolling reduction by the cold rolling, the amount of the work-induced martensite phase to be formed is preferably adjusted to a range of 20 vol.% Or more and 80 vol.% Or less.

In the aging treatment performed after the cold rolling, 300 to 650
It is desirable to heat for 0.1 to 90 minutes in a temperature range of ° C. Three
When the temperature is lower than 00 ° C, the strength increase due to aging does not appear sufficiently.
When heated above 0 ° C, a part of the work-induced martensite phase is transformed back into an austenite phase, resulting in a decrease in strength. If the soaking time is less than 0.1 minute, sufficient aging effect cannot be expected, and it is inconvenient to take more than 90 minutes for soaking in consideration of manufacturability. Within this condition range, there is a correlation between aging temperature and aging time. This relationship is shown in FIG. Actually, the aging treatment may be performed at a temperature and time within the range of the hatched area in FIG.

[0034]

EXAMPLES Table 1 shows the chemical component values (% by mass) and Md (N) of the test materials. T5 in the table is the inventive steel, a to d
Is a comparative steel. All steels were melted in a vacuum melting furnace,
A hot-rolled steel strip with a thickness of 4.0 mm was formed by forging and hot rolling. This hot-rolled material was subjected to a solution treatment of holding at 1050 ° C. for 1 minute, and then subjected to a water cooling treatment. After that, cold rolling was performed to produce work-induced martensite at a rolling reduction of 30 to 70%.

[0035]

[Table 1]

The amount of work-induced martensite at that time depends on the chemical composition and the rolling temperature. Md (NT), which is obtained by adding the rolling temperature term to the austenitic stability index Md (N) in consideration of the influence of chemical components, is set as in the following equation.
FIG. 1 shows the relationship between T) and the amount of work-induced martensite arranged for each of the reduction ratios (working ratios) of 30%, 50%, and 67%.

Md (NT) = Md (N) -0.6 (T-70) T: Rolling temperature (° C.) Md (N) = 580-520C-2Si-16Mn-16Cr-23Ni-3
00N-26Cu-10Mo

As can be seen from the results shown in FIG. 1, there is a good correspondence between the Md (NT) value at each working ratio and the amount of work-induced martensite. The rolling temperature for obtaining the amount of martensite is determined.

The rolling temperature was selected so that martensite was formed at a rolling reduction of 50% at 30, 50 and 70 vol%, each steel was cold-rolled, and each cold-rolled material was aged at 530 ° C. for 1 minute to obtain a steel. The tensile strength and Erichsen rupture stress of each aged material thus obtained were measured, and the results are shown in Table 2. Here, the Erichsen rupture stress is a crack initiation stress (a value obtained by dividing a crack initiation load by a plate thickness and a punch diameter) in an Erichsen test, and is an effective means for evaluating the toughness of a thin plate material.

[0040]

[Table 2]

As can be seen from the results in Table 2, those according to the method of the present invention simultaneously satisfied a tensile strength of 1600 N / mm ² or more and an Erichsen breaking stress of 1000 N / mm ² or more, and were excellent in both strength and toughness. While the material was obtained, the comparative method was not satisfactory at the same time.

The Erichsen fracture stress tends to decrease as the amount of martensite increases, but when compared with the same amount of martensite, a steel having an Md (N) value within the range of the present invention has a good value. Is shown. As an example, FIG. 2 shows the Erichsen rupture stress of the aging material when the amount of martensite is set to 50%, arranged by Md (N).

As can be seen from FIG. 2, when the Md (N) value is in the range of 20 to 70, the Erichsen rupture stress shows a good value. It can be seen that it has decreased. This can be considered as follows.

Md (N) is an index of the stability of the austenite phase with respect to working. If this value is small, it is difficult to form work-induced martensite during deformation, and conversely, if it is large, it means that it is easy to form. I do. Since the test material of the present invention is a metastable austenite phase, austenite → martensitic transformation occurs during deformation (during the Erichsen test), and the likelihood of the transformation is determined by the toughness of the material (Erichsen fracture stress). ). In other words, in steels with Md (N) less than 20, the transformation from austenite phase to martensite phase does not occur during deformation, and conversely, in steels with Md (N) over 70, the formation of martensite phase does not occur. It is thought that such a decrease in toughness was caused because the material was too fast.

FIG. 3 shows that the amount of work-induced martensite was reduced to about 50% by changing the rolling temperature with the cold rolling reduction being 50%.
% And about 70%, the tensile strength and Md of the aged material (530 ° C x 1 minute)
It shows the relationship of (N).

As can be seen from FIG. 3, the steel having a lower Md (N) has a higher tensile strength, although the rolling reduction and the amount of martensite are almost constant. In other words, in order to obtain the same martensite content, it is necessary to lower the rolling temperature for steels with lower Md (N). Represents.

In Table 2, T4, T
In the case of 5, 5c and d, the rolling temperature exceeded 70 ° C. As is clear from comparison with these examples and the present invention examples having the same martensite content, those of these comparative examples have low tensile strength of the aged material, low rupture stress by Erichsen test, and high strength. High toughness cannot be achieved at the same time.

As described above, the rolling temperature has an important meaning in achieving the object of the present invention. From the viewpoint of metallurgical factors, the stacking fault energy of the austenite phase itself decreased due to the decrease in rolling temperature, and the dislocation morphology in the austenite phase introduced by cold working changed.
This is considered to be a large resistance in tensile deformation, which eventually increases the tensile strength.

[0049]

As described above, according to the method of the present invention, stainless steel not only achieves higher strength but also has higher toughness than the conventional work hardening stainless steel or precipitation hardening stainless steel. was gotten. This is because the amount of martensite that contributes to strength was adjusted by the cold rolling temperature, and the stability of austenite remaining after aging treatment, which affects toughness, to processing was changed.

Accordingly, the high-strength austenitic stainless steel sheet of the present invention can be used for plate springs, coil springs, blade plates for producing Si single crystal wafers, and metal gaskets for engines of automobiles and the like, which are required to have high strength and toughness as well as corrosion resistance. It has an effect not found in conventional materials as a raw material, and can expand the use of stainless steel.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the value of Md (NT) and the amount of martensite induced at each cold working ratio.

FIG. 2 is a diagram showing the relationship between the value of Md (N), the aging material, and the Erichsen fracture stress when the cold rolling reduction and the amount of work-induced martensite are constant.

FIG. 3 is a diagram showing the relationship between the value of Md (N) and the tensile strength of an aged material when the cold rolling reduction and the amount of work-induced martensite are constant.

FIG. 4 is a diagram showing the relationship between the aging temperature and the aging time of the aging treatment performed in the present invention.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-317628 (JP, A) JP-A-2-225647 (JP, A) JP-A-2-156046 (JP, A) JP-A-4- 191352 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C21D 9/46-9/48 C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. In mass%, C: 0.10% or less, Si: more than 1.0% to 4.0%, Mn: 2.0% or less, Ni: 4.0% to 10.0%, Cr: 12.0% to 18.0%, N: 0.15% And Md (N) is in the range of 20 to 70 according to the formula of C + N ≧ 0.10% and Md (N) = 580-520C-2Si-16Mn-16Cr-23Ni-300N-26Cu-10Mo. The stainless steel containing these elements so as to fall within the range of Fe, the balance consisting of Fe and impurities unavoidably mixed in during production, and exhibiting a metastable austenite phase in a solution-treated state is defined as minus. A part of the austenite phase is transformed into work-induced martensite by cold working at a low temperature of 20 ° C. to 70 ° C. and a rolling reduction of 30 to 70%,
Next, a method for producing a high-strength austenitic stainless steel having excellent toughness, which comprises subjecting to an aging treatment at 300 to 650 ° C. for 0.1 to 90 minutes. 2. The method according to claim 1, wherein the amount of work-induced martensite generated in the cold working is adjusted to a range of 20 to 80% by controlling the rolling temperature and the rolling reduction in the cold working. 3. The stainless steel after aging treatment has a tensile strength.
The process according to claim 1 or 2 in 1600 N / mm ² or more and and Eriksen breaking stress 1000 N / mm ² or more.