Process for producing thin strip of Cr-stainless steel having high toughness

#In recent years, a technology of casting a thin cast strip having a strip thickness of 10 mm or less directly from a steel melt was developed and tested on an industrial scale. This new technology provides a process of producing a cold-rolled thin sheet product, in which a hot rolling step is either simplified or omitted, and thereby attracts considerable attention and is expected from the view point of saving energy and cost.

#Processes of producing a thin sheet, including the above process, will hereinafter be referred to as "STC processes" (Strip Casting Process). In contrast, processes of producing a cold-rolled thin sheet product which includes the steps of continuous-casting a 100 mm or more thick steel slab, hot-rolling the slab to a several millimeters thick hot-rolled strip, and cold-rolling the hot-rolled strip, will be referred to as a conventional process.

#The present invention relates to a process of producing a thin cast strip having a high toughness, particularly a thin cast strip of a Cr-stainless steel containing Nb, Ti, Al, etc., by an STC process.

#FIG. 1 is a graph showing the relationship between the hot rolling conditions of the cast strip and the cast strip toughness.

#FIG. 2 is a graph showing the relationship between the heat treatment conditions after hot rolling and the cast strip toughness.

#FIG. 3 is a graph showing the relationship between the heat treatment conditions after hot rolling and the cast strip toughness.

#Various Cr-stainless steels having the chemical compositions shown in Table 1 within the claimed range of the present invention were melted to provide 10-ton melts, which were then cast to thin cast strips having a thickness of 3 mm in a water-cooled twin-drum caster. The cast strips were hot-rolled in the temperature range of from 1150.degree. to 950.degree. C. at different reductions in thickness of from 5 to 50%, were held, or slowly cooled, for 5 sec in the temperature range of from 1150.degree. to 950.degree. C., and were coiled in the form of a thin strip.

#For comparison, Cr-stainless steels having the chemical compositions shown in Table 1 as comparative examples were also cast in a similar manner. The cast strips were hot-rolled, heat-treated after the hot rolling, and coiled under the respective conditions, at least one of which was outside the claimed range, to produce thin strips.

#As can be seen from Table 2, the thin strips produced by the present inventive process had a high toughness of 2 kgf-m/cm.sup.2 or greater at 0.degree. C. whereas the thin strips produced by the comparative process had too low a toughness of less than 2 kgf-m/cm.sup.2 to carry out the subsequent step of cold rolling.