Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-01-14
2004-05-11
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
Reexamination Certificate
active
06733601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of the Related Art
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 industrial fields. 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.
Several attempts have been made to improve the formability of ferritic stainless steels. Among these, Japanese Unexamined Patent Publication No. 3-264652 proposes optimization of manufacturing conditions of ferritic stainless steels containing Nb and Ti in order to obtain an aggregation structure of 5 or more in X-ray intensity ratio (222)/(200) and to improve the formability.
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. On the other hand, ternesheets, i.e. soft steel sheets provided with plating containing lead, have been widely used as the material for automobile fuel tanks. However, regulations on the use of lead are becoming stricter from an environmental point of view and substitutes for the ternesheets have been developed. The substitutes developed have the following problems. 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. Use of austenitic stainless steels which can be used without lining have also been attempted. 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.
In such a situation, enormous advantages such as improvement of the global environment can be achieved if these materials can be substituted by ferritic stainless steels which are recyclable.
Since the r-value of ternesheets is approximately 2.0, ferritic stainless steels must attain an r-value of 2.0 or more for them to replace the ternesheets. Ferritic stainless steels must also have long-term corrosion resistance to deteriorated gasoline containing organic acids such as formic acid and acetic acid which are formed in the ambient environment in order for the ferritic stainless steels to be applied to fuel components such as automobile fuels tanks and pipes. 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. Here, rough surfaces include the orange peel condition caused by rough crystal grains and the presence of corrugations aligned in the rolling direction (L direction) as a result of cold rolling thereby rendering undulating surfaces in the sheet width direction.
OBJECTS OF THE INVENTION
In view of the above, a first object of the invention is to provide a ferritic stainless steel exhibiting enhanced deep-drawability which is suitable for application to automobile fuel tanks and pipes by improving the r-value to 2.0 or more and provide a method for making the same.
In particular, an object of the invention is to provide a ferritic stainless steel exhibiting an average r-value as the parameter of deep-drawability of 2.0 or more, preferably about 2.2 more, having a crystal grain size number in the finished annealed sheet as the parameter of the surface-roughness of about 6.0 or more, and developing no red rust after corrosion resistance testing using deteriorated gasoline containing 800 ppm of formic acid at 50° C. for 5,000 hours.
The average r-value is defined as the average plastic strain ratio according to Japanese Industrial Standard (JIS) Z 2254 calculated using the equation below:
r
=(
r
0
+2
r
45
+r
90
)/4
wherein,
r
0
denotes a plastic strain ratio measured using a test piece sampled in parallel to the rolling direction of the sheet;
r
45
denotes a plastic strain ratio measured using a test piece sampled at 45° to the rolling direction of the sheet; and
r
90
denotes a plastic strain ratio measured using a test piece which is sampled at 90° to the rolling direction of the sheet.
Another object of the invention is to solve the problems conventionally experienced during forming the ferritic stainless steel sheets into fuel tanks and pipes of severe shapes and during a process such as pressing which requires omission of application of vinyl lubricant or oil.
SUMMARY OF THE INVENTION
Based on our research, we found that application of a lubricant coat containing acrylic resin as the primary component on the surface of the steel sheet at an amount within a predetermined range improves the sliding property during press forming and reduce the dynamic friction coefficient between the ferrite stainless steel and pressing dies. Thus, “galling” can be prevented and products of further complicated shapes can be manufactured.
In order to attain the above-described objects, we conducted extensive research on improvement of the corrosion resistance with deteriorated gasoline, deep drawability, and surface roughness after processing required for applying ferritic stainless steels to automobile fuel components. We found that the corrosion resistance with deteriorated gasoline can be effectively improved by including about 0.5 mass percent (hereinafter, simply referred to as %) of Mo, controlling the sum Cr+3.3Mo (pitting index) to not less than about 18%, and inhibiting the rough surface after processing. We also found that the disadvantages of including large amounts of Mo, i.e., degradation in deep drawability and generation of rough surfaces, can be overcome by performing cold rolling at least twice with an intermediate annealing process therebetween and by optimizing the manufacturing conditions such as crystal grain sizes during cold rolling. Moreover, we found that the dynamic friction coefficient between ferritic stainless steel sheets and dies can be reduced by coating the steel sheet surface with a lubricant coat to improve sliding properties during forming. Thus, the ferritic stainless steel sheets can be formed into products having more complex shapes.
Baba Yukihiro
Fukuda Kunio
Furukimi Osamu
Muraki Mineo
Ozaki Yoshihiro
JFE Steel Corporation
Piper Rudnick LLP
Yee Deborah
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