JPH04228541A - Ferritic stainless steel excellent in retainability of luster and formability - Google Patents

Abstract

PURPOSE:To obtain a ferritic stainless steel excellent in retainability of luster and formability. CONSTITUTION:This is a ferritic stainless steel contg. <=0.03% C, <=0.03% N, 0.1 to l.0% Si, <=1.0% Mn, <=0.5% Al, >23 to 30% Cr and 0.01 to 3.0% Co, furthermore selectively contg. Nb, Cu, Mo, Ti, V and Zr and the balance Fe with inevitable impurities.

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 having excellent gloss retention and formability.

0002

BACKGROUND OF THE INVENTION Low Cr stainless steel develops red rust, which is a problem before it becomes shiny, and it cannot withstand use. Further improved stainless steel, as in JP-A-58-61220,
Attempts have been made to improve stain resistance by adding Nb and Ti and performing appropriate bright annealing (BA).

0003

[Problem to be solved by the invention] However, even in this case, although the "stain" has become mild, "clouding" occurs if left in the atmosphere or in a certain corrosive environment for a period of time, and the resulting A decrease in gloss could not be avoided. In exterior and interior materials, which require the corrosion resistance and luster unique to stainless steel, such "cloudiness" has become a problem in that it lacks beauty and impairs its luster.

[0004] In addition, although it is unavoidable to some extent to increase Cr and add Mo in order to prevent rust, stains, cloudiness, etc. from impairing the surface gloss, excessive addition significantly impairs the moldability and is not suitable for industrial use. There was a problem that it could not be used.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide a ferritic stainless steel that maintains gloss and has excellent formability.

[0006]

[Means for Solving the Problems] Initially, it was thought that there was a considerable correlation between corrosion resistance and gloss maintenance, but no correlation was found with, for example, pitting corrosion potential.

[0007] Therefore, the inventors have developed a method that has excellent gloss maintenance properties.
With the aim of developing a ferritic stainless steel with excellent formability, we investigated the effects of various trace addition elements on ferritic stainless steel and found that the addition of a trace amount of Co significantly improves gloss retention. At the same time, they discovered that it has the effect of improving moldability, leading to the present invention.

That is, in the present invention, C:0.
03% or less N: 0.03% or less Si: 0.1 to 1.0% Mn: 1.0% or less Al: 0.5% or less Cr: More than 23% to 30% Co: 0.01 to 3.0%, further selectively containing Nb, Cu, Mo, Ti, V
, Zr, with the remainder consisting of Fe and unavoidable impurities, and has excellent gloss retention and formability. The gist of this is to significantly improve gloss retention without inhibiting formability by adding Co to ferritic stainless steel.

[0009]

[Operation] The present invention will be explained in more detail below.

It is a known fact that the addition of Cr and Mo has a significant effect on corrosion resistance, but the effect on "fogging" resistance is small, and on the other hand, the addition of Cr and Mo significantly inhibits moldability, making molding almost impossible. As a result, it was unable to meet the needs of industry.

[0011] Therefore, Cr, Mo or C
In order to improve gloss retention without reducing formability by increasing the addition of u, the present inventors investigated the effects of additive elements on gloss retention and formability using small steel ingots. As a result, it was found that the addition of Co was effective.

Based on these findings, the reasons for limiting the chemical composition of the ferritic stainless steel of the present invention will be described below.

C, N: Since it is preferable to have a small amount of C and N in terms of both corrosion resistance and formability, the upper limit is set to 0.
03%.

[0014]Si: Si is added in an amount of 0.1% or more, especially in bright annealed (hereinafter abbreviated as BA) materials, because an SiOX oxide film is formed on the surface to improve corrosion resistance, and it improves the corrosion resistance of welded parts. is necessary. However, if added too much, it becomes hard, so the upper limit is limited to 1.0%.

Mn: If a large amount of Mn is added, inclusions such as MnS will be formed and the corrosion resistance will be significantly lowered, so the upper limit of Mn is set at 1.0%. Further, there is no problem even if the Mn content is small, so the lower limit is not particularly defined.

Al: If the amount of Al is too large, alumina will precipitate and cause surface flaws, so the upper limit is set to 0.5%.

Co: Co is an effective element in terms of both gloss maintenance and moldability. Since the effect is exhibited when the amount added is 0.01% or more, the lower limit is set at 0.01%. Furthermore, since adding too much would increase the cost and make it difficult to put it into practical use, the upper limit was set at 3.0%.

[0018] The effect of improving formability by adding Cr:Co is as follows:
As shown in FIG. 1, it becomes noticeable when the amount of Cr exceeds 23%, so it is necessary that the amount of Cr added exceeds 23%. Furthermore, since 20 to 23 Cr% or more is required from the viewpoint of corrosion resistance and gloss inhibition due to the occurrence of red rust (FIG. 2), the lower limit was set to exceed 23%. However, since adding a large amount leads to hardening, the upper limit is limited to 30% or less.

Nb: 3% by weight of Nb prevents intergranular corrosion and improves corrosion resistance, and especially when welding is required, it also improves the corrosion resistance of the welded part and does not impede formability in any way.
It is best to add within the range of .

Ti, Zr, V: Ti, Zr, and V may be added for almost the same purpose as Nb, but if the amount is too large, solid solution hardening will occur, and in the case of Ti, the weld zone and B.A.
(Bright annealing) Temper color tends to occur on the surface of the material, so the upper limits are limited to 3% for Ti, 2% for V, and 2% for Zr.

[0021] Mo, Cu: The addition of Mo and Cu has a remarkable effect on suppressing red rust and staining, and in that respect contributes to maintaining gloss, but it has almost no effect on cloudiness, and the addition of a large amount This significantly impedes moldability and is contrary to the spirit of the present invention. However, if it is unavoidably added from the viewpoint of corrosion resistance, the upper limit is 3% for Mo and 3% for Cu.

[0022]

EXAMPLES The present invention will be specifically explained below based on examples.

(Example 1) Various steels having the components shown in Tables 1 and 2 were hot-rolled and cold-rolled to a thickness of 1 mm using vacuum high-frequency melting (30 kg) under known conditions. Made of steel plate. Subsequently finish annealing and polishing to ~00 with emery paper, and finish annealing to BA (950~1100℃, D.P.
Atmospheric exposure test (6 months) and accelerated cycle test (5% NaCl 135°C, SST 240 minutes → dry 60°C, 120 minutes → wet 50°C, 120 minutes) minute is one cycle, 80 cycles), accelerated spray test (H2 SO4
, an aqueous solution adjusted to pH=2 with HCl and NaCl,
SO42-: 500ppm, Cl-1: 500ppm
was sprayed for 24 hours). Evaluation was performed by visually observing the test pieces and by measuring the glossiness Gs (20°) (JIS Z8741) (see Table 3). In addition, regarding moldability,
It was investigated by tensile test and Erichsen and the results are shown in Table 2.

Invention steels 1 to 7 have excellent gloss retention and formability, while comparative steels 9 to 11 have excellent formability but insufficient gloss retention; on the other hand, comparative steel 8 ,
Although No. 12 has excellent gloss maintenance properties, it has poor moldability and does not satisfy both characteristics, making it unsuitable for practical use.

[Table 1]

[Table 2]

[Table 3]

[0025]

[Effects of the Invention] The steel of the present invention, which has excellent gloss maintenance properties, also has good formability, so it can be used in a wide range of applications such as exterior and interior materials for automobile moldings, roofs, etc.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the results of an investigation into the influence of the amount of Co added on tensile properties. In both cases, 0.4Si-0
．． The basic composition is 01Al-0.1Mn steel, and the plate thickness is 1.0
It was done in mm.

FIG. 2 is a diagram showing the results of an investigation into the influence of the amount of Co added on gloss maintenance when an accelerated cycle corrosion test (see Examples) was conducted for one month. ΔGs=Glossiness before test Gs (20°) A value in the range of 0 to 200 for glossiness Gs (20°) before test was defined as good gloss. When cloudiness occurred, ΔGs was 201 or less.

Claims (4)

[Claims] Claim 1: In weight percent, C: 0.03% or less N: 0.03% or less Si: 0.1 to 1.0% Mn: 1.0% or less Al: 0.5% or less Cr: 23 % to 30% or less Co: 0.01 to 3.0%, and the balance consists of Fe and inevitable impurities, and has excellent gloss retention and formability. 2. The ferritic stainless steel according to claim 1, wherein the steel composition further contains Nb: 3% or less. 3. The ferritic stainless steel according to claim 2, wherein the steel composition further contains one or more of Ti: 3% or less, V: 2% or less, and Zr: 2% or less. 4. The ferritic stainless steel according to claim 1, wherein the steel composition further contains one or more of Mo: 3% or less and Cu: 3% or less.