JPH10219407A - Ferritic stainless steel excellent in ductility, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively utilize a transformation induced plasticity(TRIP) and to obtain a high ductility by specifying the composition of a stainless steel containing ferritic phase as a principal constituent structure and also specifying respective percentages of retained austenitic phase and martensitic phase. SOLUTION: This steel has a composition which consists of, by mass, <=0.10% C, <=1.0% Si, <=1.5% Mn, 16.0-25.0% Cr, 0.020-0.070% N, and the balance iron and contains, if necessary, <=1.0% Cu, <=3.0% Ni and/or <=1.0% Al, and <=1.0% Mo and in which Cr equivalent, represented by (Cr equivalent)=Cr%+Mo%+1.5 Si%, and Ni equivalent, represented by (Ni equivalent)=Ni%+30C%+30N %+0.5Mn%, are regulated to 17.5-24.5% and 2.0-6.0%, respectively. Further, this steel has a structure which contains 5-20% retained austenitic phase while having ferritic phase as a principal constitutent phase at room temp. and in which the amount of martensitic phase is limited to <=5%. By this method, the ferritic stainless steel having a characteristic of >= about 30% tensile fracture elongation can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel and a method for producing the same, particularly to automobiles, home appliances,
The present invention relates to a ferritic stainless steel having excellent ductility used in a kitchen or a building material and suitable for forming such as overhang forming and a method for producing the same.

[0002]

2. Description of the Related Art Stainless steel used for automobiles, home appliances, kitchens, building materials, and the like is required to have good workability such as overhang forming. A lot of research has been done for that,
For example, as disclosed in Japanese Patent Application Laid-Open No. Hei 3-2355, it has been proposed to achieve the object by purifying steel,
Used in reality. This technology improves workability by reducing interstitial elements such as C and N and facilitating the movement of transition during plastic deformation, but requires a high degree of decarburization and denitrification. Therefore, there is a disadvantage that the manufacturing cost is high.

On the other hand, in austenitic stainless steel specified in SUS304 and low alloy steel containing a retained austenite phase, for example, Journal of the Japan Institute of Metals, Vol.
No. (1998), p. 623 et seq., So-called TRIP (Transformation Induced Plasticity) phenomenon is used to improve ductility. This is because, when plastic working is performed on steel containing an austenitic phase, the austenite phase transforms into a martensitic phase and hardens in the part where plastic strain is applied, so that deformation in that part is suppressed in subsequent processing, and as a result, local strain The use of the phenomenon that hardly occurs, the uniform elongation increases, and the elongation at break is improved.However, in a stainless steel having a ferrite phase as a main structural structure, an austenite phase and a ferrite phase coexist. It has not been put to practical use because it is difficult.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to improve the ductility of ferritic stainless steel by using relatively inexpensive means, not by purifying expensive steel as in the prior art. In particular, the present invention provides a stainless steel having a ferrite phase as a main constituent structure capable of extremely effectively utilizing the TRIP effect at the time of plastic working and a method for producing the same, and a ferritic stainless steel having a tensile elongation at break of 30% or more. It is an object of the present invention to provide a manufacturing method thereof.

[0005]

Means for Solving the Problems The inventors of the present invention have proposed that TRIP can be used in a stainless steel having a ferrite phase as a main constituent structure by limiting the austenite phase to 5 to 20% and the martensite phase to 5% or less. The present invention has been completed by finding that the phenomenon can be effectively used and high ductility can be obtained. Specifically, the composition of a ferritic stainless steel having a ferrite phase as a main constituent phase is expressed by mass ratio of C: 0.10% or less, Si: 1.0%
Hereinafter, Mn: 1.5% or less, Cr: 16.0% or more, 2
5.0% or less, N: 0.020% or more, 0.070% or less, the balance consisting of iron and unavoidable impurities, and Cr
Equivalent: 17.5% or more, 24.5% or less, Ni equivalent:
In addition to satisfying 2.0% or more and 6.0% or less, the retained austenite phase is 5% or more and 20% or less while the ferrite phase is the main constituent phase at room temperature.
% And less than 5% of the martensite phase. Here, Cr equivalent = Cr% + Mo% + 1.5Si%, Ni equivalent = Ni% + 30C% + 30N% + 0.5Mn% are defined.

In the present invention, the composition of the steel further includes one or more of Cu: 1.0% or less, Ni: 3.0% or less, Al: 1.0% or less, and Mo: 1.0% or less. 0
% Or less of one or two or both of them.

Further, the present invention provides a method for producing a ferritic stainless steel as described above, wherein in a final annealing step of a cold-rolled steel sheet obtained by performing hot rolling and cold rolling, a heating temperature is set to a two-phase comprising a ferrite phase and an austenite phase. Temperature Tα and (Tα−
An annealing method is employed in which the temperature is in a temperature range of 100 ° C. and the subsequent cooling rate is 800 ° C., the average cooling rate is 5 ° C./s or more.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the composition of the ferritic stainless steel of the present invention will be described. C: 0.10% or less C is an austenite-forming element, and has a relatively large diffusion coefficient and equilibrium distribution coefficient to the austenite phase, so that it is more equilibrium-partitioned to the austenite phase in a high-temperature two-phase region, and It is effective for securing and stabilizing the phase. Therefore, although it is an essential additive element for stable generation of the austenite phase, if it exceeds 0.10%, a large amount of carbide is precipitated and the corrosion resistance is impaired, so the upper limit is limited to 0.1%.

Si: 1.0% or less Si is effective as a deoxidizing agent, but if it exceeds 1.0%, the steel becomes brittle. Therefore, the upper limit is set to 1.0% and added in a range necessary for deoxidizing. .

Mn: 1.5% or less Mn is an austenite-forming element and is effective for detoxifying S. However, if added excessively, it embrittles the steel. Added.

Cr: not less than 16% and not more than 25% Cr is a ferrite-forming element and a constituent element of a passive film, and is an essential addition element of ferritic stainless steel for ensuring corrosion resistance. In order to exhibit these functions, it is necessary to add 16.0% or more.
If it exceeds 0%, the steel will be embrittled.
Limited to 0%. Note that it is necessary to satisfy the Cr equivalent as described later.

N: not less than 0.020% and not more than 0.070% N is an austenite-forming element and has an extremely large diffusion coefficient and an equilibrium distribution coefficient to the austenite phase. It is more equilibrium-divided into phases and is extremely effective in securing and stabilizing the austenite phase. Therefore, 0.02
Add 0% or more. However, if added excessively, a large amount of nitride precipitates, and the ductility is rather deteriorated.
Within the following range.

The basic composition of the steel according to the present invention contains the above components, with the balance being iron and unavoidable impurities such as P,
S, and the following components can be selectively added. Cu: 1.0% or less, Ni: 3.0% or less Any of these elements is an austenite-forming element and is effective in securing an austenite phase. Therefore, it is preferable to add as appropriate as long as the alloy cost of the steel is not unnecessarily increased. Specifically, each is 1.0% or less,
It is preferable to add one or two kinds in a range of 3.0% or less.

Al: 1.0% or less, Mo: 1.0% or less These elements are ferrite-forming elements and are effective in securing a ferrite phase. Al is also a passivation film forming element and is effective in improving corrosion resistance. On the other hand, Mo
Is effective for improving pitting corrosion resistance. However, if these elements are added excessively, they cause embrittlement of the steel. Therefore, the upper limit is set to 1.0%, and one or two elements are added.
The ferrite-forming element and the austenite-forming element can be used in combination depending on the required properties of the steel.

Cr equivalent: not less than 17.5% and not more than 24.5% In the present invention, it is necessary to leave an appropriate amount of austenite phase and not to form a martensite phase as much as possible as described later. To do so, it is not sufficient to simply limit the composition range as described above.
It should be noted that the balance of the overall steel composition determines the phase fraction that makes up the steel. In particular, attention must be paid to the Cr equivalent and the Ni equivalent. The Cr equivalent is defined by the following: Cr equivalent = Cr% + Mo% + 1.5Si% If this value is less than 17.5%, the austenite phase becomes unstable and the martensite phase is likely to be generated. The organizational structure cannot be obtained. On the other hand, if it exceeds 24.5%, it becomes difficult to secure an austenite phase at a high temperature. Therefore, the Cr equivalent is 1
It is limited to 7.5% or more and 24.5% or less.

The Ni equivalent is defined by Ni% + 30C% + 30N% + 0.5Mn%. If this value is less than 2.0%, it is difficult to secure a high-temperature austenite phase, while 6.0%
If the ratio exceeds the austenite phase fraction, the structure required in the present invention cannot be obtained in any case. Therefore, the Ni equivalent is 2.0% or more and 6.0% or less.

In the present invention, the ferrite phase is used as the main constituent phase, while maintaining excellent ductility due to the TRIP phenomenon. For that purpose, 5 to 20% of the austenite phase must remain in the structure of the steel. When the austenite phase is less than 5%, the effect of increasing the strength in the portion where plastic deformation progresses is small, so that local elongation during plastic deformation cannot be suppressed, and a large uniform elongation characteristic of the TRIP phenomenon is obtained. Can not do. On the other hand, even if the residual austenite phase exceeds 20%, the effect of increasing the elongation by the TRIP phenomenon is saturated and only increases the alloy addition cost, so the upper limit of the retained austenite phase is 20%.
%.

However, even when the austenite phase is 5% or more and 20% or less as described above, the effect of the TRIP phenomenon due to the transformation of the austenite phase to the martensite phase is expected when the martensite phase is present in a large amount. Can not. Therefore, the martensite phase is 5
%. This is particularly noticeable in steel compositions in which austenite remains, and can be solved in production by selecting the following heat treatment conditions. The martensite phase is particularly preferably 3% or less from the above viewpoint.

If the above conditions for the composition and structure are satisfied, uniform elongation due to the TRIP phenomenon can be obtained during plastic working and the object of the present invention can be achieved. It is better to apply. First, when the cold-rolled steel strip is finally annealed, it is heated to a two-phase structure region of ferrite and austenite. In this two-phase region, the higher the temperature, the more N is distributed to the austenite phase, the more the austenite phase is stabilized, and the effect of improving the ductility due to the TRIP phenomenon appears remarkably. Therefore, the heating temperature is based on the temperature Tα at which the transformation from the two-phase region of ferrite and austenite to the high-temperature ferrite single-phase region is completed, and the heating is performed within 100 ° C. from this temperature. That is, the heating temperature range is Tα or lower and Tα−100 ° C. or higher. The above heating temperature is set to the α single phase region (for example, 85 ° C.) where the annealing of ordinary ferritic stainless steel is relatively low.
0 ° C.), which is on the higher temperature side.

The steel sheet heated to the two-phase region under the above conditions is then cooled, and it is important to increase the cooling rate. That is, even if a phase in which many austenite-stabilizing elements such as N are once formed in the two-phase region at a high temperature, if the temperature subsequently decreases, the content (partition ratio) of these elements in the austenite phase decreases. The stability of the austenite phase is reduced, eventually leading to a reduction in the residual austenite phase fraction or excessive formation of the martensite phase. Such adverse effects become remarkable when the cooling rate is lower than 5 ° C./s. Therefore, the cooling rate from the two-phase region is set to 5 ° C./s or more. In this case, in the temperature range where the temperature of the steel sheet is lower than 800 ° C., the influence of the cooling rate no longer appears. Therefore, the control range of the cooling rate is set to the range from the heating temperature to 800 ° C.

[0021]

EXAMPLES Examples of the present invention will be described below together with comparative examples. A steel having the composition shown in Table 1 was melted and made into a slab, and then hot-rolled and cold-rolled to produce a 0.7 mm-thick ferritic stainless steel sheet. Steels A to E are steels satisfying the composition range defined in the present invention, while F to J are steels outside the scope of the present invention. That is, steel F
Is too low in Cr content and Cr equivalent, and steel G is too high in Cr and Cr equivalent. Steel H is a steel with too high N content, while steel I is a steel with too low N content. Steel J contains a large amount of Ni, and is therefore a steel having a too high Ni equivalent.

The ferritic cold-rolled steel sheet having these components was annealed under the conditions shown in Table 2, the amount of retained austenite and the amount of martensite in the annealed steel sheet were measured, and the tensile fracture in the tensile fracture test was performed. The elongation was measured. Tensile rupture test was performed from steel sheet according to JIS Z 2201-
13B tensile test piece (plate-like tensile test piece: sheet thickness 0.7 mm,
(Distance between reference points 50 mm, parallel part width 12.5 mm) was cut out, and a tensile test was performed at room temperature in accordance with JIS Z 2241. Table 2 shows the obtained measurement results together with the annealing conditions.

[0023]

[Table 1]

[0024]

[Table 2]

From these results, those satisfying the composition and microstructure conditions (the amount of retained austenite and the amount of martensite) specified in the present invention show high tensile elongation at break, that is, high ductility. All of the steels that did not satisfy the structural conditions because of the failure to satisfy the above have low tensile elongation at break. Test No. 13 (Steel J) shown in Table 2 has a high Ni content, satisfies the structural conditions such as the amount of retained austenite and the amount of martensite, and has a high tensile elongation at break. Are excluded from the applicable scope.

[0026]

Industrial Applicability According to the present invention, it is possible to produce a stainless steel having extremely high ductility represented by a tensile elongation at break in a tensile test, while being a ferritic stainless steel.
Home appliances, kitchens, building materials, etc., have become extremely easy to produce cold with stainless steel.

Claims (5)

[Claims] 1. A steel composition having a mass ratio of C: 0.10% or less, Si: 1.0% or less, Mn: 1.5% or less, Cr:
16.0% or more and 25.0% or less, N: 0.020% or more and 0.070% or less, the balance being iron and unavoidable impurities, and Cr equivalent: 17.5% or more;
24.5% or less, Ni equivalent: 2.0% or more and 6.0% or less, respectively, and the structure at room temperature has a ferrite phase as a main constituent phase and a residual austenite phase of 5% or more and 20% or less. % Or less, and the martensitic phase is limited to 5% or less. Here, Cr equivalent = Cr% + Mo% + 1.5Si%, Ni equivalent = Ni% + 30C% + 30N% + 0.5Mn% 2. The steel composition has a mass ratio of C: 0.10% or less, Si: 1.0% or less, Mn: 1.5% or less, and Cr:
16.0% or more, 25.0% or less, N: 0.020% or more, 0.070% or less, and Cu: 1.0% or less,
Ni: contains 3.0% or less of one or two kinds, the balance being iron and unavoidable impurities, and Cr equivalent: 1
2. The ferritic stainless steel having excellent ductility according to claim 1, wherein the ferrite stainless steel has a content of 7.5% or more and 24.5% or less, and a Ni equivalent of 2.0% or more and 6.0% or less. 3. A steel composition having a mass ratio of C: 0.10% or less, Si: 1.0% or less, Mn: 1.5% or less, and Cr:
16.0% or more, 25.0% or less, N: 0.020% or more, 0.070% or less, and Al: 1.0% or less,
Mo: 1.0% or less, containing one or two kinds, the balance being iron and unavoidable impurities, and Cr equivalent: 1
2. The ferritic stainless steel having excellent ductility according to claim 1, wherein the ferrite stainless steel has a content of 7.5% or more and 24.5% or less, and a Ni equivalent of 2.0% or more and 6.0% or less. 4. A steel composition having a mass ratio of C: 0.10% or less, Si: 1.0% or less, Mn: 1.5% or less, and Cr:
16.0% or more, 25.0% or less, N: 0.020% or more, 0.070% or less, and Cu: 1.0% or less, N
i: 3.0% or less, one or two of Al and Al:
1.0% or less, Mo: 1.0% or less, containing one or two kinds, the balance consisting of iron and unavoidable impurities, and Cr equivalent: 17.5% or more and 24.5% or less; The ferritic stainless steel excellent in ductility according to claim 1, wherein the Ni equivalent: 2.0% or more and 6.0% or less are respectively satisfied. 5. In a final annealing step for a cold rolled sheet obtained by performing hot rolling and cold rolling, a temperature Tα at which transformation from a two-phase region consisting of a ferrite phase and an austenite phase to a high-temperature ferrite single phase is completed. The ductility according to any one of claims 1 to 4, wherein the material is heated to a temperature range of (Tα-100) ° C, and then cooled at an average cooling rate up to 800 ° C of 5 ° C / s or more. Excellent method for producing ferritic stainless steel.