Heretofore, low alloy steels such as 0.5% Mo steel, 0.5% Cr -- 0.5% Mo steel, 1% Cr -- 0.5% Mo steel, 1.25% Cr -- 0.5% Mo steel and 2.25% Cr -- 1% Mo steel have been known to be usable at moderately high temperatures not exceeding 600.degree.C., and are suitably employed in accordance with the purpose of use. It is particularly well known that the addition of Cr is effective for the improvement of heat resistance at higher temperatures; the higher the temperature to which steels are subjected, the higher the Cr content.
However, since thin steel sheet made from said conventional steel materials is inferior in its cold formability at an ambient temperature and is not capable of withstanding intense cold rollings, it is difficult to manufacture products in complicated shapes by what is commonly known as the press working method.
A skin casing for a boiler (a coating made of thin steel sheet which is used to keep the portion of a boiler exposed to high temperature air-tight), and an expansion joint for a duct or the like are both used at considerably high temperatures. Steel sheets to be shaped into such members are treated by severe cold forming operations, i.e., deep drawing operations or folding a steel sheet along certain radii of curvature. Therefore, it becomes necessary, in case steel sheets incapable of withstanding such cold forming are employed, to make each constituent part separately, using steel materials suitable for the extent of working required, and assemble the parts thus prepared. This inevitably results in a cost increase.
Steel sheets made from conventional heat-resisting low alloy steel are not provided with good cold formability sufficient for such working as mentioned above. It is, therefore, desirable to find a method for producing steel sheet which has excellent heat-resistance and good cold formability.
In contrast to the aluminum-killed steels of this invention, rimmed steels are known. As explained by Rinesch in U.S. Pat. No. 3,488,187, rimmed steels containing a small amount of chromium have a characteristic of lessening strain aging. Rimmed steels, containing tungsten as an essential component, are described, for example, by Nagashima et al in U.S. Pat. No. 3,642,468.
As explained in the "Handbook of Iron and Steel Stocks," The Iron and Steel Institute of Japan and The Metal Institute of Japan, section 12.1 at page 505:
"Since proper rimming action is required for a rimmed steel ingot, its chemical composition is restricted; ordinarily C content is defined as less than 0.30% and Mn content as less than 0.50%. It is impossible to make the rimmed steel contain a large amount of a strong deoxidizing agent, such as Al or Si."
In section 12.2 at page 515, killed steels are noted, viz.:
"In order to maintain the characteristic features of a killed steel ingot, it is necessary that it contain more than 0.25% of Si and more than 0.020% of acid soluble Al in the steel ingot."
The distinctions between rimmed and killed steels are also provided in the text: "The Making, Shaping and Treating of Steel;" United States Steel; 1957; Seventh Edition, at pages 396, 397.
Killed steels are described in U.S. Pat. No. 3,496,032, and a core killed steel the outside of which is a rimmed steel and the inside of which is a killed steel is described in U.S. Pat. No. 3,556,866.