Heat-resistant cold rolled ferritic stainless steel sheet, hot rolled ferritic stainless steel sheet for cold rolling raw material, and methods for producing same

Abstract

Provided is a heat-resistant cold rolled ferritic stainless steel sheet containing, in terms of mass %, 0.02% or less of C, 0.1% to 1.0% of Si, greater than 0.6% to 1.5% of Mn, 0.01% to 0.05% of P, 0.0001% to 0.0100% of S, 13.0% to 20.0% of Cr, 0.1% to 3.0% of Mo, 0.005% to 0.20% of Ti, 0.3% to 1.0% of Nb, 0.0002% to 0.0050% of B, 0.005% to 0.50% of Al, and 0.02% or less of N, with the balance being Fe and inevitable impurities, in which {111}-oriented grains are present at an area ratio of 20% or greater in a region from a surface layer to t / 4 (t is a sheet thickness), {111}-oriented grains are present at an area ratio of 40% or greater in a region from t / 4 to t / 2, and {011}-oriented grains are present at an area ratio of 15% or less in the entire region in a thickness direction.

Description

TECHNICAL FIELD[0001]The present invention relates to a heat-resistant cold rolled ferritic stainless steel sheet which is appropriate for use especially in exhaust system members of vehicles and the like requiring a high temperature strength and oxidation resistance and has excellent workability, a hot rolled ferritic stainless steel sheet for a material for cold rolling (cold rolling raw material), and methods for producing the same.[0002]The present application claims priority on Japanese Patent Application No. 2012-081998 filed on Mar. 30, 2012, the content of which is incorporated herein by reference.BACKGROUND ART[0003]Heat-resistant steel containing Cr is used in exhaust system members such as exhaust manifolds and mufflers of vehicles, since the members require a high temperature strength and oxidation resistance. Since these exhaust system members may be manufactured through press working from a steel sheet or through various formings after pipe working of a steel sheet, co...

AI Technical Summary

Benefits of technology

The invention is about a way to make a heat-resistant cold rolled ferritic stainless steel sheet with a high r-value. By specifying the composition of the steel and optimizing the hot rolling and cold rolling processes, as well as controlling the microstructures in the sheet, a high r-value can be achieved. Specifically, if the coiling temperature and annealing temperature of the hot rolled sheet are set, and the steel microstructure before the cold rolling process is set to be non-recrystallized with a {111} texture, a lot of crystal grains with the {111} direction can be generated, which improve the r-value in the subsequent cold rolling and annealing processes. This results in a recrystallized microstructure that improves workability.

Problems solved by technology

However, in the case where the amounts of added elements are increased, the workability of a raw material steel sheet is decreased in a simple manufacturing method, and thus press forming may not be performed on members having a complicated shape.

However, since a relatively thick cold rolled steel sheet (approximately 1.5 mm to 2.5 mm) is used as a raw material for the above-described exhaust system members, there is a problem in that a sufficient cold rolling reduction ratio cannot be secured in the current manufacturing process in which the raw material thickness is regulated to a certain degree when performing cold rolling.

Patent Document 1 discloses component adjustment to improve workability of conventional heat-resistant ferritic stainless steel sheets, but with this, a problem occurs such as press cracking in thick materials having a relatively low cold rolling reduction ratio.

However, with this, sufficient workability may not be obtained according to influences of other elements (C, N, Cr, Mo, and the like) involved especially in Nb-based precipitates.

Patent Document 3 discloses a method of subjecting a hot rolled sheet to aging for 1 hour or longer, but in this case, there is a disadvantage in that the industrial manufacturing efficiency is greatly reduced.

However, since the r-value is not determined only with the crystal orientation of the center layer in a sheet thickness direction of the product, sufficient workability may not be obtained.

In addition, since a heating temperature of a slab in hot rolling is in a range of 1,000° C. to 1,150° C. which is low, there is a problem such as surface scratches.

However, in the case where only the r-value in the direction at an angle of 45° is increased, press formability is not satisfied, and in the case where annealing of a hot rolled sheet is omitted, surface defects which are called ridging cause a problem in press working and there is a problem in manufacturability such as surface scratches.

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