Austenitic-ferritic stainless steel with excellent formability

Abstract

An austenitic-ferritic stainless steel having a low Ni content and a high N content is provided. A stainless steel which contains, in mass %, 0.2 % or less of C, 4 % or less of Si, 12 % or less of Mn, 0.1 % or less of P, 0.03 % or less of S, 15 to 35 % of Cr, 3 % or less of Ni, and 0.05 to 0.6 % of N, and is mainly composed of an austenite phase and a ferrite phase, wherein the proportion of said austenite phase is 10 to 85 volume %. The above austenitic-ferritic stainless steel exhibits good formability and high punch stretch formability, and is excellent in the resistance to crevice corrosion, to the corrosion of a welded zone and to intergranular corrosion. The above austenitic-ferritic stainless steel wherein the austenite phase contains C and N in a total amount of 0.16 to 2 mass % exhibits further improved formability.

Description

Technical field [0001] The invention relates to an austenitic-ferrite (Stainless steel with austenite and ferrite (two-phase) structure) containing low Ni and high N. Background technique [0002] As a material with good corrosion resistance, stainless steel is widely used in auto parts, building materials, kitchen equipment and other fields. In automobile wheel caps and the like, materials that have both high punch stretchability and crevice corrosion resistance are required. According to the structure of the steel, stainless steel is generally classified into the following four types: austenitic stainless steel, ferritic stainless steel, austenitic-ferritic stainless steel, and martensite Martensitic stainless steel. Among them, austenitic stainless steel represented by SUS304 and SUS301 (JIS (Japanese Industrial Standard)) has good corrosion resistance and good workability, so it is most widely used. As stainless steel plates for automobile wheel covers, austenitic stainless s...

AI Technical Summary

Problems solved by technology

[0003] However, although austenitic stainless steel has higher workability than other stainless steels, it has a problem of high price because it contains a large amount of expensive Ni.

[0004] In addition, when austenitic stainless steel is processed to the vicinity of the forming limit, it is easy to produce aging cracks (seasoned crack), or the susceptibility to stress corrosion cracking (Stress Corrosion Cracking: SCC for short), so when it is suitable for fuel Problems may arise when placing boxes and other parts that require extremely high safety

In addition, although martensitic stainless steel has good strength, it has poor ductility, stretch formability and corrosion resistance, so it is not suitable for stamping.

[0005] In addition, austenitic stainless steel represented by SUS301 has also been pointed out to have the following problems: in the bay area due to floating salt, in the snowfall area due to snow melting salt, especially in the gap between the wheel and the cover (gap), etc., Problems such as corrosion may occur, and its corrosion resistance is insufficient

Therefore, when the cooling rate after solution annealing is slow, N precipitates as chromium nitrides, and there is so-called "sensibility" (sensibility, which is caused by the formation of chromium carbides and chromium nitrides at grain boundaries) that deteriorates corrosion resistance. Deterioration of corrosion resistance, hereinafter referred to as sensitization) problem

[0015] Specifically, when the final annealed sheet with a thickness of 1.5 mm or more was air-cooled, it was found that the cooling rate of the material was slow, so it was sensitized during cooling, and the corrosion resistance was not sufficient

[0016] In addition, for materials with a final plate thickness of less than 1.5 mm, there is also a problem of sensitization during annealing of the hot-rolled sheet as an intermediate process.

That is, the final annealed sheet less than 1.5 mm is manufactured by hot rolling, hot rolled sheet annealing, pickling descaling, cold rolling, and final annealing after steelmaking and casting. When the material is sensitized during air cooling after a thickness of 1.5-7mm), the crystal grain boundaries are preferentially etched during the subsequent pickling, and the preferential etching groove does not disappear even during cold rolling, so there is a surface of the final annealed sheet The problem that the trait will be significantly deteriorated

[0018] On the other hand, the method disclosed in Yasuyuki Katada's "Pressure-type ESR method による high-concentration element steel production", Ferrum, Vol.7 (2002) p. Large-scale equipment for pressure melting, and electrodes for melting raw materials must be prepared in advance, and there are many factors that cause cost increases in operation

Further, only by substituting Ni for N, a material having both stretch formability and crevice corrosion resistance cannot be obtained.

[0019] And, in the method disclosed in "NIKEL-FREE DUPLEX STAINLESSSTEELS, Scripta Materialia Vol.40, No.1, pp.123-129, 1999" by J.Wang et al., in order to save Ni, add Mn: 10% by mass at the same time , N: A large amount of Mn and N of 0.35 to 0.45% by mass, so hot workability is insufficient, and cracks and flaws are likely to occur during hot working

Although the cost of the alloy is low, there are factors that increase the cost such as surface chipping and steel chipping

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