Ferritic stainless steel sheet with excellent thermal fatigue properties, and automotive exhaust-gas path member

Abstract

Disclosed is a ferritic stainless steel sheet with excellent thermal fatigue properties, including, by mass %, 0.03% or less of C, 1.0% or less of Si, 1.5% or less of Mn, 0.6% or less of Ni, 10˜20% of Cr, 0.05˜0.30% of Ti, 0.51˜0.65% of Nb, 0˜less than 0.10% of Mo, 0.8˜2.0% of Cu, 0˜0.10% of Al, 0.0005˜0.02% of B, 0˜0.20% of V, and 0.03% or less of N, with the balance being Fe and inevitable impurities, having a composition satisfying the following equations (1) and (2) and having a structure in which ε-Cu phase grains each having a long diameter of 0.5 μm or more are present in a density of 10 or less per 25 μm2: Nb−8(C+N)≧0 . . . (1), 10 Si+20 Mo+30 Cu+20(Ti+V)+160 Nb−(Mn+Ni)≧100 . . . (2). The ferritic stainless steel sheet, having a relatively inexpensive component composition, has excellent thermal fatigue properties, and is suitable for use in an automotive exhaust-gas path member, including an exhaust manifold, a catalyst converter, a front pipe, or a center pipe.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a stainless steel sheet with excellent thermal fatigue properties, and to an automotive exhaust-gas path member using the same, and in particular, to a member that constitutes the upstream part of the exhaust-gas path and is heated to temperatures of 700° C. or higher upon the use of automobiles.[0003]2. Description of the Related Art[0004]Among automotive exhaust-gas path members, an upstream member, which is heated to 700° C. or higher during use thereof, is required to have, first of all, heat resistance in a range of high temperatures exceeding 700° C. In this regard, with the main goal of increasing high-temperature strength in the high-temperature range of 700˜1000° C., characteristic materials therefor have been developed. Further, the automotive exhaust-gas path member undergoes repeated heat cycles, including heating to the high-temperature range and cooling to room temperature,...

AI Technical Summary

Benefits of technology

[0011]The present invention aims to provide a ferritic stainless steel sheet for use in the upstream member of an automotive exhaust-gas path, through the development of a technique for stably improving thermal fatigue properties, having a strong correlation with the durability of the member, using relatively inexpensive component designs while fundamentally imparting high-temperature strength, forming workability, and low-temperature toughness.

[0013]As a result of further investigation by the present inventors, to increase the durability and reliability of the upstream member of the automotive exhaust-gas path, a stable improvement in thermal fatigue properties has been found to be very favorable, after taking everything into consideration. Fundamental durability (high-temperature strength and oxidation resistance) of the high-temperature range exceeding 700° C. reaches a considerably high level at present, due to technical advancement of the prior art. More enhancements in such properties need the addition of specific elements and so on, leading to high costs. Further, durability (thermal fatigue properties), which allows the automobiles to endure heat cycles of heating and cooling upon the use thereof, should be greatly improved. In the current situation, it is considered that the improvement of this durability is very effective in increasing the durability and reliability of the upstream member of the automotive exhaust-gas path.

[0014]Leading to the present invention, intensive and thorough research into a ferritic stainless steel sheet containing Cu, carried out by the present inventors, resulted in the finding that the form of the Cu precipitate (here, referred to as “ε-Cu phase”) is controlled to exist in any predetermined state in the step before the steel sheet is applied to actual use as the exhaust-gas member, and thereby thermal fatigue properties may be stably improved under repeated cycles of subsequent heating and cooling.

[0018]According to the present invention, the thermal fatigue properties of the ferritic stainless steel sheet may be considerably improved without the use of expensive elements, including Mo. This steel sheet has a reasonable component design, which obviates the need for an excessive increase in high-temperature strength in the high-temperature range exceeding 700° C., and exhibits excellent durability in end uses, in which heating to the high-temperature range exceeding 700° C. and cooling to room temperature are repeated. The steel sheet, which is provided in the form of a steel plate, may be subjected to bending or welding to form a pipe, and has good low-temperature toughness. Thus, the ferritic stainless steel sheet is suitable for use in the automotive exhaust-gas path member, in particular, in the upstream member, which is heated to temperatures exceeding 700° C. The automotive exhaust-gas path member using such a steel plate may realize both low material costs and increased durability and reliability.

Problems solved by technology

As mentioned above, however, to increase the high-temperature strength in the wide temperature range from 500° C. to 1000° C. merely depends on component designs in which specific elements are added in large amounts, undesirably resulting in increased material costs.

Moreover, forming workability is poor.

However, the present inventors have conducted intensive investigation and thus discovered that, even if ferritic stainless steel simply containing a predetermined amount of Cu is annealed, thermal fatigue properties necessary for the upstream member of the automotive exhaust-gas path, which undergoes repeated heating to temperatures exceeding 700° C. and cooling to room temperature, may not always be repeatedly exhibited.