Ferritic stainless steel sheet and method for manufacturing the same

Abstract

Provided are a ferritic stainless steel sheet and a method for manufacturing the steel sheet. The ferritic stainless steel sheet according to the present invention has a chemical composition containing, by mass %, C: 0.005% to 0.025%, Si: 0.02% to 0.50%, Mn: 0.55% to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001% to 0.10%, Cr: 15.5% to 18.0%, Ni: 0.1% to 1.0%, N: 0.005% to 0.025%, and the balance being Fe and inevitable impurities, an elongation after fracture of 28% or more, an average r-value of 0.75 or more, and a minimum value of maximum logarithmic strain at the forming limit, which is, determined on the basis of a forming limit diagram (FLD), of 0.15 or more.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This is the U.S. National Phase application of PCT / JP2015 / 003339, filed Jul. 2, 2015, the disclosure of this application being incorporated herein by reference in its entirety for all purposes.TECHNICAL FIELD OF THE INVENTION[0002]The present invention relates to a ferritic stainless steel sheet having sufficient corrosion resistance and being excellent in formability and ridging resistance and a method for manufacturing the steel sheet.BACKGROUND OF THE INVENTION[0003]A ferritic stainless steel sheet is more economical than austenitic stainless steel, which contains a large amount of expensive Ni. Since a SUS430-type stainless steel sheet (containing 16 mass % to 18 mass % of Cr) is an especially economical ferritic stainless steel, it is used for various applications such as building materials, transporting instruments, home electrical appliances, kitchen appliances, and automobile parts, and its range of applications is being further e...

AI Technical Summary

Benefits of technology

The present invention provides a ferritic stainless steel sheet that has good corrosion resistance, formability, and resistance to ridging (which is cold-bending without a spring). This is achieved by performing annealing on the hot-rolled sheet to form a mixed-grain microstructure composed of ferrites containing a large number of carbonitrides and a small number of carbonitrides, followed by further annealing on the cold-rolled sheet to form a ferrite single-phase microstructure.

Problems solved by technology

In the case where an excessively large amount of ridging is generated after forming has been performed, since a polishing process is necessary in order to remove the surface asperity, there is a problem of an increase in the manufacturing costs of a formed product.

However, when the present inventors attempted to manufacture a ventilation hood by using press working which requires mainly bulge forming capability, it was not possible to obtain a specified shape, which means that it was not possible to achieve such a high level of bulge forming capability as to be expected from the elongation after fracture.

Since such box annealing takes about one week including heating and cooling processes, there is a problem of a decrease in productivity.

In addition, since a technique for decreasing the amount of solid solution N by adding an expensive transition metal element, that is, V, is used, there is also a problem of an increase in manufacturing costs.

Moreover, since hot-rolled-sheet annealing is performed by using a box annealing method in a temperature range in which a ferrite single phase is formed, most ferrite colonies are retained without being broken, which also results in a problem of a significant deterioration in ridging resistance.

However, in the case where a box annealing furnace is used, there is a problem of low productivity as in the case of Patent Literature 1 described above.

In addition, when the present inventors attempted to manufacture parts by using the steel according to Patent Literature 2 and press working which involves mainly bulge forming as in the case of Patent Literature 1, it was not possible to obtain a specified shape, which means that it was not possible to achieve such a high level of bulge forming capability as to be expected from elongation after fracture.

Moreover, generally, in the case of ferritic stainless steel such as that according to Patent Literature 2, since a colony, which is a group of crystal grains having similar crystal orientations, is formed when casting or hot rolling is performed, it is not possiblesto sufficiently break the colony of a ferrite phase in the case where hot-rolled-sheet annealing is performed in a temperature range in which a ferrite single phase is formed.

Therefore, the colony is retained in the elongated state in the rolling direction of cold rolling which is performed after hot-rolled-sheet annealing, which results in a problem of significant ridging occurring after forming has been performed.

However, when the present inventors manufactured a steel sheet by using the method according to Patent Literature 3 and attempted to manufacture a ventilation hood by using a method which involves mainly bulge forming, there were some cases where it was not possible to obtain a specified shape due to cracking occurring during press working, which clarifies that there may be a case where it is not possible to achieve such a high level of bulge forming capability as to be expected from its elongation after fracture.

As described above, a technique for manufacturing a SUS430-type stainless steel sheet having sufficient corrosion resistance and being excellent in formability and ridging resistance has not yet been established.