JPS5928560A - Stainless steel excellent in surface properties and corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain stainless steel excellent in corrosion resistance and generating no gold dust flaw, in stainless steel with a specific composition, by limiting P content. CONSTITUTION:In stainless steel containing 0.12% or less C and 10-25% Cr, the content of P is limited to 0.02% or less. It is recognized that minute spills resulting in gold dust flaw in a cold rolled steel plate manufacturing process of stainless steel is generated because the protruded part of the base material or the edge part of grain boundary falls down to overspread the steel sheet surface. However, in addn. to this phenomenon, a crack is generated in the part of surface grain boundary at the time of cold rolling and the edge part thereof falls down to ovespread the steel surface at the next or succeeding rolling path to form the minute defect flaw which, in turn, results in the gold dust flaw. This crack of the grain boundary is grain boundary breakage generated due to the brittleness directly under a roll at the time of cold rolling, and the brittleness of the grain boundary is related to P concentrated in the grain boundary. Therefore, by reducing the content of P, objective stainless steel is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to stainless steel which has excellent corrosion resistance and does not generate gold dust flaws.

Cold-rolled stainless steel sheets have excellent corrosion resistance, and as a result, the beauty of the surface remains intact for a long time.
It is also used for highly decorative applications such as automobile parts and architectural interior parts.

Surface finishes for applications that require such beautiful surfaces include buffing mirror finishing, mechanical polishing (A3-8 finishes), electrolytic polishing, and mixed gas H2 and N2 after cold rolling. A BA treatment finish, which is bright annealed in a non-oxidizing gas such as or CO gas, is selected.

Mechanical polishing and electrolytic polishing are suitable for relatively small stones and complex shapes, but they have the drawback of being expensive and not suitable for mass production. On the other hand, BA finishing is widely used because it has high productivity and is suitable for mass production. So-called gold dust flaws, which appear sparkling to the naked eye, may occur due to the vinyl film attached to the surface, and the beauty of the surface may be significantly impaired.

In addition to the BA finish, minute scratches can also occur on cold-rolled steel sheets with a 2B finish (skin pass threading after pickling) and 2D finish (as-pickled) without applying a vinyl film for surface protection. Because it is easy to become a starting point for rusting, the corrosion resistance of the finished product was poor.

Conventionally, due to this drawback, after finishing such as BA treatment, it was necessary to perform a polishing finish according to the purpose. Until now, gold dust defects and fine dents that cause corrosion resistance deterioration have been caused by minute irregularities on the surface of the material before cold rolling, intergranular corrosion caused by pickling for descaling, or by the intergranular corrosion of the rolls during rolling. It has been thought that the unevenness caused by the striations is one of the reasons for this.

On the other hand, with regard to intergranular corrosion, there is no problem with hot-rolled steel sheets that have been subjected to normal annealing because they have almost no susceptibility to intergranular corrosion. Regarding unevenness on the surface of the material, a method of limiting the surface roughness of the material before cold rolling (Special Publication Publication No. 49-166)
Regarding unevenness caused by roll marks during rolling, a method of limiting the roughness of the roll (Japanese Patent Publication No. 1983-1988) has also been proposed.
41801) has been proposed. In addition to this, a method has been proposed (Japanese Unexamined Patent Publication No. 72728/1983) in which the amount of pickling and dissolution is increased based on the assumption that there is a defective part in the surface layer, although the cause is unknown. However, even if these measures were taken, pooled dust defects could not always be completely eliminated.

The present inventors tracked and observed changes in the bottom surface of cold-rolled steel sheets for each rolling arm in the manufacturing process of cold-rolled steel sheets for stainless steel, and as a result, they found a different cause of gold dust flaws than those described above, and based on this, they invented the present invention. achieved.

It has been recognized that the minute sludge defects that lead to gold dust defects are caused by the above-mentioned convex parts of the material or the edges of intergranular corrosion falling down to cover them. The causes include the unevenness of the material mentioned above. However, the present inventors have discovered that cracks also occur in part of the grain boundaries on the surface during cold rolling, and the edges of the cracks are covered during the next and subsequent rolling passes. It has been found that the metal falls down and becomes minute scratches, leading to gold dust scratches.

FIG. 1 is a scanning electron micrograph of the entrance surface of a conventional hot-rolled annealed pickled coil of SUS 430 steel after it has been reduced by 1 mm at a reduction rate of 17 inches. The parts with severe minute convexities and convexities can be seen in the four parts of the shot shot that was removed by pickling and descaling, and the etch bits caused by pickling and the bottom lines caused by rolling deformation overlap. The relatively flat part was restrained by the rolls during rolling, and roll marks can be seen. Open cracks are intergranular cracks that have been newly discovered to be the cause of gold dust defects.

Furthermore, it was found that this grain boundary crack is a grain boundary fracture that occurs brittlely right under the roll during cold rolling, and that the weakness of the grain boundary is due to P concentrated in the grain boundary.

Based on the above knowledge, the present invention was completed by studying the reduction of P in the base material in order to reduce the concentration of P concentrated in the grain boundaries. Figure 2 shows various stainless steel materials immediately before cold rolling. A test piece with a width of 5rrra, 1 length, G 50 tm% and a thickness of 3 corners was cut out from the steel, and subjected to impact bending using a Charpy impact tester at room temperature.The intergranular crack score of the bent surface was sorted by the P content in the steel. It is a diagram. The grain boundary cracking score is A when the bent surface is observed with a scanning electron microscope at 400x magnification, and there are no cracked grain boundaries at all.
The width is 0ctn, that is, 0.255w+, and the length is 1.25 tns" since the entire width of the sample is observed 5 strips.) 91 pieces are B, and 2 or more and less than 10 pieces are 0.
110 or more pieces were determined as D. As the P content in the pot decreases, the score for intergranular cracking improves. P content is 0.
When the content is less than 0.08%, the grain boundary cracking score becomes 8 or more, and when it becomes less than 0.015%, it becomes A. Since gold dust defects usually have a grain boundary cracking rating of 3 or higher, which is considered to be no problem in practice, gold dust defects can be reduced by setting the P content to 0.02% or less, preferably 0.015% or less. There is no practical problem.

Next, the reasons for limiting the components of the present invention will be described.

For C, the upper limit was set at 0.12% because stainless steels that undergo BA finishing, 2B, and 2D finishing are low C materials and there is no high carbon steel that requires quenching.

Regarding Cr, since the object of the invention is stainless steel for BA, 2B, and 2D finishing, the lower limit is 10%, which is the minimum Cr1l required for stainless steel, and BA
, 2B, the upper limit was set at 25 inches, which is the limit value that eliminates the use of 2D finishing.

P has a practical problem of brittle intergranular cracking during cold rolling lEp
The upper limit was set at 0.02%, which is the level where there is no.

The present invention can be applied to any type of stainless steel for BA, 2B, and 2D finishing. For example, not only normal SUS 430 steel but also 5US 410S steel, SUS 304GM, Ti, Nb or Mo
, Cu-added stainless steel. Next, further explanation will be given using examples.

Stainless steel containing the components shown in Table 1 was hot rolled on three mornings, annealed under the rough annealing conditions shown in Table 1, and then pickled, cold rolled to a depth of 0.5 m, and bright annealed. After application, gold dust flaws and corrosion resistance were evaluated. Gold dust scratches were evaluated by applying a vinyl adhesive tape to the surface and then peeling it off in accordance with the method shown in Japanese Patent Publication No. 49-16698. Items with no gold dust scratches were given a score of 01 test. A score of 10 was given to the level at which intergranular corrosion occurred and gold dust defects were generated on the entire surface as a result, and the score was divided into 10 equal parts for evaluation.

The results are shown in Table 2, and it can be seen that the steel of the present invention has a rating of 3 or less, and has excellent surface quality with few gold dust scratches.

As shown above, the invention has industrial advantages such as reducing the P content and preventing the occurrence of gold dust flaws, minute sagging, and corrosion resistance deterioration, eliminating the need for a+ polishing after BA. Very large.

Table 2 ** Salt water spray test 96 hours according to JIS-22371
Displayed in Reitei/Goo-ni

[Brief explanation of the drawing]

FIG. 1 is a scanning electron micrograph of the surface of a 5US430% hot-rolled annealed coil after one pass reduction at 17%. FIG. 2 is a diagram showing the influence of P on the intergranular crack occurrence score when a hot rolled annealed stainless steel plate is subjected to impact milling using a Charpy impact tester. Figure 1/θμ

Claims (1)

[Claims] A stainless steel with excellent surface texture and corrosion resistance, characterized in that it contains 0.12% or less of CO, 25% or less of Cr2O, and 0.02% or less of Pt.