JP3122515B2 - Method for producing ferritic stainless steel with excellent press formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel excellent in press formability, and particularly to severe press forming of a cold-rolled steel sheet, such as a part for automobiles or home electric appliances, which is deep drawn. The present invention relates to a manufacturing method for providing a ferritic stainless steel capable of forming a portion processed by the method.

[0002]

2. Description of the Related Art Ferritic stainless steel is expensive N
Since it contains almost no i, it is relatively cheaper than austenitic stainless steel, and has a certain degree of corrosion resistance, so that it is widely used for building materials, automobile parts, kitchenware and the like. However, in view of its press formability, austenitic stainless steel such as SUS3
In general, it is inferior to that of steel 04 and the like, and it is necessary to use expensive austenitic stainless steel in parts where it is necessary to ensure corrosion resistance.

[0003] With regard to the press formability of ferritic stainless steel, it has been required to improve properties such as drawability, stretchability, and deep drawability, and many studies have been made to improve these properties. For example, deep drawability by a combination of reduction of C and N in steel and addition of a carbonitride forming element such as Nb or Ti (average r value─rolling direction, direction perpendicular to rolling direction, direction at 45 ° to rolling direction) (Refer to the average value of the r-values of the respective drawability) in Japanese Patent Publication No. 51-2969.
No. 4, JP-A-51-14811, etc.),
Deep drawability by addition of Al (JP-B-51-44888), addition of B (JP-B-44-736), etc.
There are improvements from materials such as ridging improvements.

[0004] Improvements have also been made in terms of manufacturing methods. For example, the slab heating temperature before hot rolling is set to 1130.
The average r value is improved and the ridging is reduced by lowering the temperature so that the temperature does not exceed ℃ (Japanese Patent Application Laid-Open No. 58-71356), or the strength is reduced by accelerating the precipitation of coarse TiN—Nb (C, N). Japanese Unexamined Patent Publication (Kokai) No. 3-247722), a slab heating temperature of 1000 ° C. or more and 1200 ° C. or less, a finish hot rolling start temperature of 900 ° C. or more, and a winding temperature of 65 ° C.
A production method (JP-A-58-71356) has been proposed in which the average r value is improved by setting the temperature to 0 ° C. or lower.

[0005]

However, these techniques seek to improve the deep drawability (average r value) or soften the steel sheet, and another important factor in press formability. Sufficient studies have not been made from the viewpoint of improving the stretch formability, that is, the elongation of the material in the biaxial direction. In recent years, for example, in automobile exhaust system parts, the conversion from conventional ordinary steel or low Cr stainless steel to high Cr stainless steel has been progressing in order to achieve high performance and long life. Material conversion from austenitic stainless steel to ferritic stainless steel is also actively pursued to reduce costs,
In these cases, troubles such as cracking of the material are liable to occur in press molding with a conventional processing machine, and it is necessary to change the shape of the mold or reconsider the pressing conditions, which may raise the cost. In other words, it is clear that no matter how much the average r-value is improved and softened, this alone does not lead to an improvement in press formability. Even if high Cr stainless steel is used, the elongation of the material, that is, the stretch formability is maintained. Need to be improved.

An object of the present invention is to provide a method for producing a ferritic stainless steel having good stretch formability and improved press formability.

[0007]

Means for Solving the Problems Accordingly, the present inventors have:
In order to improve the press formability of ferritic stainless steel, in particular, the stretch formability, we conducted intensive studies on both the component composition and the production means. At the same time as adding a trace amount, in terms of production means, after holding the slab at a relatively high temperature, by hot rolling with a planetary mill hot rolling mill,
The inventors have found that the above objects can be achieved, and have reached the present invention.

That is, the present invention has achieved the object by the following means. (1) C: 0.015 wt% or less, Si: 0.10 w
t% or less, Mn: 0.50 wt% or less, Cr: 10 to 2
5.0 wt%, Al: 0.05 wt% or less, N: 0.0
20 wt% or less, S: 0.010 wt% or less, and Ti: 8 × {(C wt%) + (N wt%)} or more and 0.5
wt% or less, the balance being substantially composed of Fe and unavoidable impurities, after heating and holding at 1120 to 1180 ° C, the slab is subjected to 90 in one pass by a planetary mill hot rolling mill.
A method for producing a ferritic stainless steel excellent in press formability, wherein hot rolling is performed at a rolling reduction of at least%. (2) The slab further contains Ni: 1.0 wt% or less, Mo: 2.0 wt% or less, Cu: 1.0 wt% or less, Nb: 0.50 wt% or less, W: 0.50 wt% or less, and V: The method for producing a ferritic stainless steel according to the above item (1), wherein one or more kinds of the ferritic stainless steels are contained at 0.50 wt% or less.

[0009]

Next, the means for producing the ferritic stainless steel according to the present invention will be described in detail, together with the operation thereof and the reasons for limitation. The present inventors have found that ferrite stainless steel does not reduce the average r value and has a strength (proof stress).
It has been found that, by increasing the work hardening coefficient of the material, that is, the n value, within a range where the is not extremely increased, the stretch formability is improved. Accordingly, as a result of intensive studies on the conditions under which such a phenomenon is obtained, a slab containing Ti added as a stabilizing element for fixing C and N in steel was obtained from 1120 to 1240.
After heating and holding at 1180 ° C., it was found that it was necessary to perform hot rolling with a planetary mill hot rolling mill.

FIG. 1 shows the results obtained by adding 0.25 wt% of Ti.
A finished cold rolled sheet (3.0 mm thickness) of a hot rolled sheet (5 mm thickness, reduction rate 97%) in which a slab (145 mm thickness) of 7Cr stainless steel is reduced by 90% or more in one pass with a planetary mill hot rolling mill. 4 is a graph showing the influence of the slab heating temperature on the n value, strength (proof stress), and elongation at break. According to this figure, the n value and the proof stress increase with an increase in the slab heating temperature, but there is a stagnation area between about 1120 ° C. and 1180 ° C., and the elongation at break shows the maximum area in this range. When the temperature reaches 1180 ° C. or more, the n value
Although the proof stress increases again, the elongation at break decreases rather because the increase in strength is remarkable.

Regarding the mechanism that causes such a phenomenon,
Although metallurgy has not yet been fully elucidated and its elucidation is not directly related to the realization of the present invention, the present inventors have presumed that TiN or TiC However, when the slab heating temperature is low, it is dispersed as coarse precipitates, and thus the n value is low. However, since the decrease in the proof stress is not so large, the elongation does not increase so much. Then, it is thought that since the coarse precipitates and the fine precipitates are mixed, the balance between the n value and the strength is maintained, and the elongation indicates the maximum region.

FIG. 2 shows the effect of the slab heating temperature on the n-value, proof stress and breaking elongation when hot rolling is performed under the same conditions in a 6-stand tandem hot rolling mill by the method of gradually rolling down the slab. The effect of in this case,
There was no stagnation area in the n value and proof stress, and there was no slab heating temperature at which the elongation showed the maximum area. According to the study results based on FIGS. 1 and 2 described above, according to the present invention, hot rolling of a slab is performed by a planetary mill hot rolling mill at 90% in one pass.
The slab heating temperature is set to 1120 to
It is limited to 1180 ° C.

Next, the reasons for limiting the component composition of the ferritic stainless steel according to the present invention, together with its action, will be described. First, from the viewpoint of improving formability and corrosion resistance, the lower the amount of C, the better. However, since there is an industrial limit, the upper limit of C is limited to 0.015 wt% or less. Si
Is the most important element in the alloy design of the present invention.
This Si is usually used as a deoxidizing agent, but it has been known that the workability is deteriorated when the residual amount is large. However, the present inventors have found that by reducing the Si content as much as possible, specifically, by strictly suppressing the content to 0.10 wt% or less, the formability, particularly the stretch formability, is significantly improved. Preferably 0.05wt
% Or less is good.

The amount of Si will be described in more detail. First, the present inventors consider that Cr: 17 to 18 wt.
%, Ti: a substantially constant value of 0.20 to 0.25 wt%, other components also substantially constant within the limited range of the present invention, and a component in which Si is changed within a range of 0 to 0.30 wt%. After producing a slab of the composition into a hot-rolled sheet with the above-mentioned planetary mill hot rolling mill, a finish cold-rolled, annealed steel sheet was obtained as a sample, and the stretch formability was investigated with a hydraulic bulge tester. As shown in FIG. 3, when the Si content exceeds about 0.05 wt%, the bulge height starts to decrease, and when it exceeds 0.10 wt%, it sharply decreases. For this reason, in order to improve moldability,
Although the upper limit of the Si content is specified as 0.10 wt%, it is preferably 0.05 wt% or less.

If the content of Mn exceeds 0.50% by weight, the moldability decreases. For this reason, the content of Mn is limited to 0.50 wt% or less. Cr must have a content of at least 10 wt% or more in order to secure the corrosion resistance unique to stainless steel. However, if it exceeds 25.0 wt%, the hardness increases and the formability deteriorates. For this reason C
The content of r was limited to the range of 10 to 25.0 wt%.
Al is used as a deoxidizing agent at the time of refining steel. However, when Al exceeding 0.05 wt% is present, coarse Al oxides are formed in the steel, and particularly when this Al oxide precipitates on the steel sheet surface. , Could lead to serious defects.
For this reason, in the present invention, the content of Al is set to 0.1.
It was decided to regulate to a trace amount of not more than 05 wt%. Since S forms MnS in combination with Mn in steel and deteriorates corrosion resistance, it is desirable that S is as low as possible. In the present invention, the content of S is limited to 0.010 wt% or less.

Further, Ti fixes C and N in steel,
Of course, the addition of an appropriate amount of Ti is effective in improving the formability as described above, in addition to improving the corrosion resistance. Therefore, in order to obtain a desired effect on these effects, at least 8 × ｛(Cwt%)
+ (Nwt%)} or more is required.
If the content exceeds 0 wt%, the effect is saturated, and conversely, the hardness increases. For this reason, the content of Ti is 8 × {(C wt%) + (N wt%)}｝ 0.50 w
Limited to t%. N is desirably as low as possible from the viewpoint of moldability and corrosion resistance as in the case of C. In the present invention, the content of N is limited to 0.020% by weight or less.

Ni is added to improve the corrosion resistance. However, if it is contained in an amount of 1.0 wt%, the hardness increases, the formability is deteriorated, and the economic efficiency of the steel material is impaired. For this reason,
The content of Ni was limited to 1.0 wt% or less. Mo is
Although it is an extremely effective element for improving corrosion resistance, an increase in its content promotes an increase in hardness and formation of a sigma phase, thereby deteriorating formability. In addition, the addition of a large amount of expensive Mo impairs the economic efficiency of steel. For this reason, the content of Mo is set to 2.0 w
t% or less. Cu is an element effective for improving the corrosion resistance, but the hardness increases and the formability deteriorates with an increase in the content thereof.
It was limited to 0 wt% or less. Nb, V, and W are effective in improving corrosion resistance, and Nb is particularly effective in fixing C and N in steel. However, if the amount is too large, the hardness increases and the formability is deteriorated. Therefore, for these reasons, the upper limits thereof are each specified to be 0.50 wt% or less.

[0018]

The present invention will be described below in detail with reference to examples. However, the present invention is not limited to only this embodiment. Example 1 A slab having a thickness of 145 mm and made of a ferritic stainless steel having a component composition shown in Table 1 and having a thickness of 1000 to 125 was used.
After heating for 1 hour at a temperature of 0 ° C., hot rolling was performed in a single pass to a thickness of 3.0 mm with a planetary mill hot rolling mill to form a hot rolled sheet, then hot rolled sheet annealing, 0.7 mm thick Cold rolling and annealing were performed to obtain a cold rolled sheet sample. Subsequently, a tensile test and a hydraulic basil test were performed on each sample. Table 2 shows the results.

According to Table 2, sample No. 1 having the component range of the present invention and the slab heating temperature was used. Sample Nos. 1 to 9 are comparative samples Nos. 1 and 2 both having elongation at break and hydraulic bulge height.
10 to 14 show excellent characteristics. Further, since there is no decrease in the average r value and no sharp increase in the strength, the present invention, that is, a slab in which Si is controlled to 0.05 wt% or less is heated to 1120 to 1180 ° C., and then a planetary mill hot rolling mill is used. By rolling at a rolling reduction of 90% or more in one pass, the formability of ferritic stainless steel, in particular, the stretch formability was improved.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

According to the present invention, C and N are reduced, and a small amount of a stabilizing element Ti is added, and at the same time, Si is reduced as much as possible. After maintaining the temperature range, hot rolling at a rolling reduction of 90% or more at a time with a planetary mill hot rolling mill can produce a ferritic stainless steel with improved press formability, especially stretch formability. it can. Ferritic stainless steel obtained by the present invention is a material for which corrosion resistance is required, such as architectural materials, automobile parts, kitchen supplies, etc.
Suitable for parts.

[Brief description of the drawings]

FIG. 1 n value, strength (strength) of a finished cold-rolled hot-rolled sheet obtained by hot rolling a 17Cr-ferritic stainless steel slab with a planetary mill hot rolling mill in one pass at a rolling reduction of 90% or more. FIG. 7 is a diagram showing the relationship between the elongation at break and the slab heating temperature.

FIG. 2 shows the n-value, strength (proof stress), elongation at break, and slab heating temperature of a finished cold-rolled hot-rolled sheet in which a slab of 17Cr-ferritic stainless steel is hot-rolled so as to be gradually reduced by a hot rolling mill. FIG.

FIG. 3 is a graph showing the relationship between the Si content in steel of a cold-rolled annealed sheet and the hydraulic bulge height.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/50 C22C 38/50 (72) Inventor Yutaka Kobayashi 4-2 Kojimacho, Kawasaki-ku, Kawasaki-shi, Kanagawa Nippon Yakin Kogyo Co., Ltd. (56) References JP-A-3-53025 (JP, A) JP-A-62-10219 (JP, A) JP-A-59-193705 (JP, A) JP-A-57-88905 (JP, A) (58) Fields surveyed (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/02-8/04 B21B 3/02 C22C 38/00-38/60

Claims (2)

(57) [Claims] C: 0.015 wt% or less, Si: 0.1% or less.
10 wt% or less, Mn: 0.50 wt% or less, Cr: 1
0 to 25.0 wt%, Al: 0.05 wt% or less, N:
0.020 wt% or less, S: 0.010 wt% or less,
Further, a slab containing Ti: 8 × {(Cwt%) + (Nwt%)} or more and 0.5 wt% or less, and the balance substantially consisting of Fe and unavoidable impurities, is heated to 1120 to 1180 ° C.
And hot rolling at a rolling reduction of 90% or more in one pass with a planetary mill hot rolling mill after heating and holding the ferrite stainless steel excellent in press formability. 2. The slab further comprises Ni: 1.0 wt.
%, Mo: 2.0 wt% or less, Cu: 1.0 wt%
Below, Nb: 0.50 wt% or less, W: 0.50 wt%
The method for producing ferritic stainless steel according to claim 1, wherein one or more of V: 0.50 wt% or less are contained.