Method of producing a cold-rolled strip with a ferritic structure

Abstract

A method of making a cold strip with a ferritic structure includes casting a molten steel which forms a ferritic structure on cooling into a cast strip, wherein if necessary hot-rolling the cast strip in-line, coiling the hot-rolled strip and in one or more steps cold-rolling to form the cold strip. With such a method, cold strips can be produced, in which the risk of formation of an orange peel appearance or ridging is minimized during a cold forming process. The cast strip is cooled between the casting process and the coiling process from a starting temperature not lower than 1180° C., at a cooling rate of at least 150° C. per second, to a maximum intermediate temperature of 1000° C. and is then held for at least 10 seconds at a maintenance temperature of between 900 and 1000° C.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a National Phase Application of International Application No. PCT / EP2006 / 070223, filed on Dec. 27, 2006, which claims the benefit of and priority to German patent application no. DE 10 2005 063 058.8, filed Dec. 29, 2005. The disclosure of each of the above applications is incorporated herein by reference in their entirety.FIELD OF THE INVENTION[0002]The invention relates to a method for producing a cold strip with a ferritic structure, in which molten steel which forms a ferritic structure on cooling is cast into a cast strip, wherein if necessary the cast strip is hot-rolled in-line, the hot-rolled strip is coiled or wound and in one or more steps is cold-rolled to form the cold strip.BACKGROUND[0003]Due to the high price of nickel, austenitic stainless steel is increasingly being substituted worldwide by ferritic stainless steel, which contains Ni typically only as a production-related companion element. A method of...

AI Technical Summary

Benefits of technology

[0008]In one embodiment, the invention features a method with which a cold-rolled strip can be produced from ferritic stainless steel, in which the risk of formation of orange peel or ridging is minimized during a cold-forming process.

[0012]The effectiveness of the method according to the invention can be enhanced by the combination of strip casting, rapid cooling and holding of the cast strip over a sufficient period of 10 seconds at the minimum at a temperature in the range of 950±50° C., particularly 950±20° C. Surprisingly, it has been shown that neither ridging nor orange peel occurs during cold forming of cold strip which has been produced from cast strip of the kind thermally treated according to the invention, without expensive intermediate annealing between the cold-rolling stages having to be carried out for this.

[0013]Steel alloys used according to the invention initially solidify ferritically during the course of strip casting. When the solidified strip cools down, ferrite then transforms between 1200° C. and 800° C. partly into austenite. The thermodynamic cause lies in the very low solubility, decreasing with the temperature, of carbon in the ferrite. Carbides, which can also absorb carbon, only form below 900° C. On the other hand the solubility of carbon in the austenite is substantially higher.

[0015]The idea on which the invention is based now consists of very rapid cooling to approximately the temperature with the maximum austenite portion (950° C.±50° C., particularly 950° C.±20° C.). In this way, austenite formation at the grain boundaries is minimized, since in this case the diffusion distances covered are not sufficient for carbon and just for the substitutional elements (Cr, Ni, Mn, etc.) during the short cooling period.

[0018]The higher the cooling rate, during high-intensity cooling of the cast strip according to the invention, the more reliably austenite formation is suppressed. In principle therefore highest possible cooling rates are strived for. However, the effects exploited by the invention can reliably occur at cooling rates of 150-250° C. per second.

Problems solved by technology

Regardless which of the ways mentioned above is used to produce the hot strip, the problem with cold-rolled strip made from ferritic stainless steel with a Cr content in the range of 17% is that ridging or orange peel can form in the course of subsequent cold working, particularly deep-drawing.

These expensive annealing steps, however, lead to increased production costs which result in a higher market price of ferritic stainless steel strip compared to equivalent material made from austenitic stainless steel.