JPH08283915A - Austenitic stainless steel excellent in workability - Google Patents

Abstract

PURPOSE: To stably provide an austenitic stainless steel excellent in workability by simultaneously controlling chemical composition and grain diameter. CONSTITUTION: The austenitic stainless steel excellent in workability can be obtained by providing a composition which contains, by weight, 0.002-0.03% C, <=0.7% Si, 0.5-5.0% Mn, 15.0-20.0% Cr, 7.0-15.0% Ni, 1.0-3.0% Cu, and 0.002-0.07% N and further contains, if necessary, prescribed amounts of Mo, Ti, Zr, Nb, V, Al, and REM and satisfying, with respect to the final crystalline grain diameter (d) (μm), the following relations: 11.18-0.149F<=log(d)<=0.0286F +0.571 and log(d)<=0, where F=40×(C+N)+Si+mn+3×Ni+5×Cu+1.5×(Cr+1.5×Mo)<=90 is satisfied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel excellent in cold workability such as forging and roll forming.

[0002]

2. Description of the Related Art Austenitic stainless steel is used for various tableware and structural members, and most of them are processed by cold forging or roll forming. However, austenitic stainless steel easily causes work hardening, and it is necessary to insert intermediate annealing in the working process to soften the work, resulting in an increase in manufacturing cost.

As a countermeasure against this problem, for example, Japanese Patent No. 1828927 discloses a technique of increasing the amount of Ni to 10 wt% or more and adding Cu of 0.5 wt% or more, and further limiting the C and N contents. There is. In addition, JP-A-4-72038 and JP-A-5-28
Similarly, in 7459, Cu and C are added by adding Cu.
Techniques for limiting the content have been shown.

The cause of work hardening of austenitic stainless steel by cold working is the formation of work-induced martensite and hardening of the austenitic matrix itself.
In the above prior art, (1) the stability of austenite, which is the parent phase, is improved with an increase in Ni content, and the work-induced martensitic transformation is suppressed; (2) work-induced with a decrease in the C and N contents. The strength of martensite is reduced, and (3) the addition of Cu suppresses the accumulation of dislocations and suppresses the hardening of the austenite matrix. That is, the conventional techniques are all countermeasures by controlling the chemical composition.

[0005]

The crystal grain size of austenitic stainless steel slightly changes depending on the chemical composition, hot working conditions and heat treatment conditions. Since the particle size affects mechanical properties and corrosion resistance, it is necessary to control the particle size within an appropriate range in order to stably obtain desired properties. And, as the role of grain size on the workability of austenitic stainless steel, it is known that grain-refining grains make it difficult for the process-induced martensitic transformation to occur (for example, Kiyohiko Nohara et al .: Iron and steel,
Vol. 63 (1977), p. 772), if the crystals are adjusted to the fine grain side, it is considered that stable and superior workability can be obtained.

However, since the strength of the austenite matrix increases in practice due to grain refinement, the workability may rather deteriorate depending on the degree of grain refinement. That is, the crystal grain size must be controlled within an optimum range in consideration of the balance between the influence on the work-induced martensitic transformation and the influence on the strength of the austenite matrix.

Here, it is not so difficult to find the optimum range of the grain size for a certain chemical composition, but in the actual production of austenitic stainless steel, Ni,
There is a change in the contents of Cu, C, N, etc., and the work-induced martensitic transformation amount and the austenite matrix strength change together with this, and the balance between the two changes, so that the desired workability is stably obtained. In order to obtain it, it is necessary to control the chemical composition and the particle size at the same time. Also, in alloy design, it is necessary to consider simultaneous control of chemical composition and grain size.

However, the above-mentioned Japanese Patent No. 18289.
27, JP-A-4-72038, and JP-A-5-
In the conventional technology represented by the technology disclosed in Japanese Patent No. 287459, only the chemical composition is defined, and good workability cannot always be obtained stably. And
A technique for simultaneously controlling the chemical composition and the particle size to improve the workability has not been proposed yet.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an austenitic stainless steel having stable and excellent workability by simultaneously controlling the chemical composition and the grain size.

[0010]

The inventors of the present invention have optimized the balance between work hardening and austenite matrix strength by restricting work hardening by limiting the chemical composition and controlling the crystal grain size. According to the above, it was found that an austenitic stainless steel excellent in cold workability such as forging and roll forming can be obtained.

The present invention has been made on the basis of such findings. Firstly, in% by weight, C: 0.002 to 0.
03%, Si: 0.7% or less, Mn: 0.5 to 5.0
%, Cr: 15.0 to 20.0%, Ni: 7.0 to 1
5.0%, Cu: 1.0 to 3.0%, N: 0.002
Provided is an austenitic stainless steel containing 0.07% and having excellent workability, characterized in that the following relationship is established for the final crystal grain size d (μm).

11.18-0.149 F≤logd≤
0.0286F + 0.571 and logd ≧ 0 However, F = 40 × (C + N) + Si + Mn + 3 × Ni + 5 × C
u + 1.5 × (Cr + 1.5 × Mo) ≦ 90 Secondly, in% by weight, C: 0.002 to 0.03%, S
i: 0.7% or less, Mn: 0.5 to 5.0%, Cr: 1
5.0-20.0%, Ni: 7.0-15.0%, C
u: 1.0 to 3.0%, N: 0.002 to 0.07%, Mo: 3.0% or less, Ti: 0.5%
Below, Zr: 0.5% or less, Nb: 0.5% or less, V:
0.5% or less, Al: 0.05% or less, REM: 0.0
Provided is an austenitic stainless steel having excellent workability, which contains one or more of 1% or less and has the following relationship with respect to the final crystal grain size d (μm). .

11.18-0.149 F≤logd≤
0.0286F + 0.571 and logd ≧ 0 However, F = 40 × (C + N) + Si + Mn + 3 × Ni + 5 × C
u + 1.5 × (Cr + 1.5 × Mo) ≦ 90

[0014]

In order to suppress the work hardening of the austenitic stainless steel due to cold working, (1) work-induced martensitic transformation, and (2) hardening of the austenitic matrix,
It is necessary to suppress the two. Specifically, regarding the work-induced martensitic transformation, by increasing the amounts of Mn, Ni, Cu, etc., the stability of austenite, which is the parent phase, is increased to reduce the transformation amount, and the strengthening elements such as C, N, etc. are restricted. It is necessary to reduce the strength of martensite. For hardening of the austenite matrix, C,
It is necessary to increase the amounts of Ni and Cu, reduce Si and N, increase the stacking fault energy, promote cross-slip of dislocations, and reduce the dislocation integration degree.

Therefore, based on this fact, in weight%,
C: 0.002-0.03%, Si: 0.7% or less, M
n: 0.5 to 5.0%, Cr: 15.0 to 20.0%,
Ni: 7.0-15.0%, Cu: 1.0-3.0%,
The effects of the chemical composition and the crystal grain size on work hardening of the stainless steel having a basic composition of N: 0.002 to 0.07% were investigated. As a result, the final crystal grain size d (μ
Regarding m), it was found that excellent workability can be obtained by satisfying the following formula.

11.18-0.149 F≤logd≤
0.0286F + 0.571 and logd ≧ 0 where F is a parameter indicating the influence of the amount of alloying elements on the work hardening amount, and F = 40 × (C + N) + Si + Mn + 3 × Ni + 5 × C.
u + 1.5 × (Cr + 1.5 × Mo) ≦ 90.

Here, in order to reduce the crystal grain size d of the stainless steel having the chemical composition defined in the present invention to less than 1 μm, strong working and short-time heating are required, and for that purpose, new dedicated capital investment is required. This is costly and therefore not realistic. Therefore, d ≧ 1 μm, that is, lo
gd ≧ 0. Further, considering the cost increase due to the increase of alloying elements, F ≦ 90.

Next, the reason why the range of each alloying element is limited will be described. C suppresses the work hardening of the austenite matrix by increasing the stacking fault energy and at the same time increases the austenite stability to reduce the transformation amount, and further reduces the amount of δ-ferrite in the steel ingot to improve the hot workability. Has the effect of improving. These effects are extremely important, and in order to obtain these effects, it is necessary to contain 0.002% or more. However, when added in excess of 0.03%, the strength of the work-induced martensite is remarkably increased, the strength of the austenite matrix phase is increased by solid solution strengthening, and further the corrosion resistance of the welded portion is deteriorated. Therefore, the content of C is 0.002-0.03%
Range.

Si needs to be added as a deoxidizing agent, but it reduces the stacking fault energy to promote work hardening of the austenite matrix and enhances the strength of the austenite matrix by solid solution strengthening. further,
It increases the amount of δ-ferrite in the steel ingot and deteriorates hot workability. Therefore, in the steelmaking stage, distribute as much as possible into the slag,
The residual amount in steel has a value of 0.
7% or less.

Mn has the effects of increasing the stability of the austenite matrix, reducing the amount of transformation, and reducing the amount of δ-ferrite in the steel ingot to improve hot workability. In order to obtain these effects, it is necessary to contain 0.5% or more. However, if added in excess of 5.0%, the impact properties at low temperatures deteriorate. Therefore,
The Mn content is in the range of 0.5 to 5.0%.

Cr is required to be 15.0% or more in order to secure the minimum corrosion resistance. However, 20.0%
If it is added over the range, it is necessary to add a large amount of Ni in order to secure hot workability, resulting in high cost. Therefore, the content of Cr is set to the range of 15.0 to 20.0%.

Ni suppresses the work hardening of the austenite matrix by increasing the stacking fault energy and at the same time enhances the stability of austenite to reduce the transformation amount and further reduces the amount of δ-ferrite in the steel ingot to reduce the heat. It has an extremely important effect of improving inter-workability. In order to obtain these effects, the content needs to be 7.0% or more, and the larger the addition amount, the more preferable. However, considering economic efficiency, the upper limit is 15.0%. Therefore, N
The content of i is in the range of 7.0 to 15.0%.

Cu suppresses the work hardening of the austenite matrix by increasing the stacking fault energy and at the same time enhances the stability of austenite to reduce the transformation amount, and further reduces the amount of δ-ferrite in the steel ingot to reduce heat. It has an extremely important effect of improving inter-workability. In order to obtain these effects, the content needs to be 1.0% or more, but if added in excess of 3.0%, the hot workability deteriorates significantly. Therefore, the Cu content is 1.0
To 3.0%.

N has an extremely important effect of increasing the stability of austenite and reducing the amount of transformation, and at the same time, reducing the amount of δ-ferrite in the steel ingot to improve the hot workability. In order to obtain these effects, the content needs to be 0.002% or more. However,
When added in excess of 0.07%, the strength of work-induced martensite is significantly increased, the strength of the austenite matrix is increased by solid solution strengthening, and the work hardening of the austenite matrix is promoted by reducing stacking fault energy. To do. Therefore, the content of N is 0.002
The range is up to 0.07%.

The above are the reasons for limiting the basic components, but in the present invention, one or more of Mo, Ti, Zr, Nb, V, Al and REM (representing a rare earth metal) may be contained, The reason for the limitation is as follows.

Mo has the effect of enhancing corrosion resistance,
When added in excess of 3.0%, the hot workability is significantly deteriorated, so when Mo is added, its upper limit is made 3.0%. Ti, Zr, Nb, V and REM have the effect of enhancing hot ductility, but excessive addition increases the strength of the austenite matrix phase, so when these are added, their content should be Ti: 0. 5% or less, Zr: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less, REM:
0.01% or less. Addition of Al enhances the high temperature oxidation resistance, but excessive addition raises the strength of the austenite matrix phase. Therefore, when adding Al, the upper limit is 0.05%.
And

[0027]

Example No. 1 shown in Table 1 Austenitic stainless steel having a chemical composition of 1 to 23 was melted in a vacuum high frequency melting furnace at a rate of 10 kg each, and hot rolled to a thickness of 5 mm, and then 950 to
Solution heat treatment was performed at 1200 ° C. for 1 minute to adjust various crystal grain sizes. Of these, Nos. 1 to 15 are steels of the present invention. 16 to 23 are comparative steels.

After subjecting these test steels to 50% cold rolling, the hardness (Hv10 kg) was measured to evaluate the workability. Table 1 also shows the obtained particle size, F value, whether or not the relational expression of the present invention is satisfied (◯ is satisfied, × is unsatisfactory), and hardness.

[0029]

[Table 1]

No. All of the steels of the present invention Nos. 1 to 15 satisfy the relational expression between the grain size and the F value of the present invention, and therefore Hv350
The following hardness is shown, and intermediate annealing can be omitted. On the other hand, the comparative steel No. Since Nos. 16 to 21 do not satisfy the relational expression between the grain size and the F value of the present invention, all of them have high hardness and require intermediate annealing. In addition, No. Two
In Nos. 2 and 23, the contents of Cu or Si and Mn exceeded the range of the present invention, so that cracking occurred during hot rolling and they could not be used for the subsequent tests.

FIG. 1 shows the results of plotting the F value and the grain size of the test steels. That is, in FIG. 1, the horizontal axis is F.
It is a graph showing these relationships by taking values and taking the particle size on the vertical axis, and it is confirmed that all the steels of the present invention having good workability satisfy the relational expression of the present invention. On the other hand, it is confirmed that the comparative steel having poor workability does not satisfy the relational expression of the present invention.

[0032]

As described above, according to the present invention,
Provided is an austenitic stainless steel that is stable and has excellent workability by controlling the chemical composition and the grain size at the same time. INDUSTRIAL APPLICABILITY According to the present invention, it is possible to stably supply austenitic stainless steel having the same level of workability irrespective of changes in the components, which is very useful industrially.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between F value and grain size in austenitic stainless steel.

Claims (2)

[Claims] 1. C: 0.002-0.03 by weight%.
%, Si: 0.7% or less, Mn: 0.5 to 5.0%, C
r: 15.0 to 20.0%, Ni: 7.0 to 15.0
%, Cu: 1.0 to 3.0%, N: 0.002 to 0.0
An austenitic stainless steel containing 7% and having excellent workability, characterized in that the following relationship is established with respect to the final crystal grain size d (μm). 11.18-0.149F ≦ logd ≦ 0.0286F
+0.571 and logd ≧ 0 However, F = 40 × (C + N) + Si + Mn + 3 × Ni + 5 × C
u + 1.5 × (Cr + 1.5 × Mo) ≦ 90 2. C: 0.002-0.03 by weight%.
%, Si: 0.7% or less, Mn: 0.5 to 5.0%, C
r: 15.0 to 20.0%, Ni: 7.0 to 15.0
%, Cu: 1.0 to 3.0%, N: 0.002 to 0.0
7%, Mo: 3.0% or less, Ti:
0.5% or less, Zr: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less, Al: 0.05% or less, RE
M: Austenitic stainless steel excellent in workability, characterized by containing one or more of 0.01% or less and having the following relationship with respect to the final crystal grain size d (μm). 11.18-0.149F ≦ logd ≦ 0.0286F
+0.571 and logd ≧ 0 However, F = 40 × (C + N) + Si + Mn + 3 × Ni + 5 × C
u + 1.5 × (Cr + 1.5 × Mo) ≦ 90