Ferritic stainless steel sheet with excellent workability and method for making the same

Abstract

A ferritic stainless steel sheet for use in automobile fuel tanks and fuel pipes having smooth surface and resistance to organic acid is provided. The sheet contains, by mass, not more than about 0.1% C, not more than about 1.0 Si, not more than about 1.5% Mn, not more than about 0.06% P, not more than about 0.03% S, about 11% to about 23% Cr, not more than about 2.0% Ni, about 0.5% to about 3.0% Mo, not more than about 1.0% Al, not more than about 0.04% N, at least one of not more than about 0.8% Nb and not more than about 1.0% Ti, and the balance being Fe and unavoidable impurities, satisfying the relationship: 18<=Nb / (C+N)+2Ti / (C+N)<=60, wherein C, N, Nb, and Ti in the relationship represent the C, N, Nb, and Ti contents by mass percent, respectively. A process for making the same is also provided.

Description

[0001] 1. Field of the Invention[0002] This invention relates to ferritic stainless steel sheets having excellent deep-drawability and surface smoothness applicable to home electric appliances, kitchen appliances, construction, and automobile components and to methods for making the same. In particular, the invention relates to a ferritic stainless steel sheet suitable for use in automobile fuel tanks and fuel pipes which are made by high deformation such as deep drawing and pipe expanding, and are highly resistant to organic fuels such as gasoline and methanol which contain organic acids produced in the ambient environment. A method for making the same is also provided.[0003] 2. Description of the Related Art[0004] Ferritic stainless steels which do not contain large amounts of nickel (Ni) are cost effective compared with austenitic stainless steels and are free of stress corrosion cracking (SCC). Due to these advantages, ferritic stainless steels have been used in various industri...

AI Technical Summary

Benefits of technology

"The invention provides a ferritic stainless steel sheet with improved deep-drawability and surface roughness, which is suitable for use in automobile fuel tanks and pipes. The steel sheet has a r-value of at least 2.0 and a crystal grain size number of at least 6.0. The invention also provides a method for making the ferritic stainless steel sheet by controlling the r-value to 2.0 or more and optimizing the manufacturing conditions. The steel sheet has improved sliding properties during forming and can be formed into products with more complex shapes. The invention solves the problem of poor corrosion resistance with deteriorated gasoline and can be coated with a lubricant coat to improve sliding properties during pressing."

Problems solved by technology

However, known ferritic stainless steels exhibit low elongation of approximately 30% and are thereby inferior to austenitic stainless steels, for example, SUS 304, in workability.

Known ferritic stainless steels do not have sufficient workability for high deformation such as deep drawing, and typically, press forming, and are not suitable for mass production.

Because of these problems concerning formability, the use of ferritic stainless steel in various fields such as automobiles, construction, and home electric appliances has been severely limited.

In this technology, however, the r-value is only about 1.8;hence, application to fuel tanks requiring complex forming by deep drawing and to fuel pipes requiring pipe-expansion and bending is difficult.

Moreover, even if applied at all, defect rates are high and mass production is not practical.

Lead-free Al--Si based plating materials are unreliable in terms of weldability and long-term corrosion resistance and the application thereof is thus limited.

Resinous materials have been applied to fuel tanks, but since these materials naturally allow minute amounts of fuel to permeate, the industrial use thereof is inevitably limited under fuel transpiration and recycling regulations.

Although austenitic stainless steels are superior in formability and corrosion resistance to ferritic stainless steels, they are expensive for use in fuel tanks and may suffer from stress corrosion cracking (SCC).

Thus, the use of austenitic stainless steels has not been practical.

However, no investigation has specified suitable compositions for attaining these goals.

As previously described, the r-value of the known ferritic stainless steels is only approximately 2.0 at most, and application of ferritic stainless steels to pressed components requiring extensive deep drawing has not been achieved.

Another problem with ferritic stainless steels is the generation of rough surfaces after pressing by deep drawing.

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