This invention relates to a cold-rolled and annealed steel strip, preferably having a metallic coating thereon to impart corrosion protection to such strip, and exhibiting a high degree of creep resistance.
The automotive industry has in recent years sought to improve the performance of automobiles by decreasing the amount of gasoline consumed by automobiles. One aspect of its program of improved performance was to reduce such consumption through a reduction in weight of the automobiles. To compensate for the reduction in weight, by the use of thinner parts, for instance, it was and is necessary to use higher strength materials. By way of example, thinner higher strength low alloy steels are now being substituted for low-carbon, cold-rolled steel. However, since the demands on an automotive component vary due to the component's exposure to high temperatures and/or corrosive conditions, the search for new materials has become very scientific and quite precise. The search for improved materials for automotive exhaust systems represents one of the most challenging needs in the automotive industry.
The investigation which led to this invention was undertaken with the goal of developing a sheet steel having improved high temperature strength, and when coated with a metallic coating being resistant to oxidation/corrosion when subjected to the cyclic conditions of an automotive exhaust system at temperatures ranging up to 1500.degree. F.
Two steels which have enjoyed some commercial success are Type 409 stainless steel (409SS), and an aluminum coated, titanium-stabilized sheet steel. 409SS, while characterized as a lean stainless steel, i.e. only about 10.5% by wt. chromium, balance essentially iron, it is nevertheless a stainless steel for which a premium is extracted. The titanium-stabilized sheet steel lacked sufficient deformation resistance at elevated temperatures. Even a later innovation on the latter steel by Gupta et al, U.S. Pat. No. 4,398,950, was not sufficient to meet the demands herein stated.
It was not until the present invention that titanium, phosphorus and carbon were combined in critical proportions and quantities in steel, and that the role of Ti-P-C concentrations were understood in providing microstructural stability and creep resistance to such steel. This discovery will be described in detail in the specification which follows.