Graphite formation in high-purity cold-rolled carbon steels
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I.
INTRODUCTION
G R A P H I T E is the stable state of carbon in Fe-C alloys. It is usually formed in cast iron and sometimes in hypereutectoid steels held at a high temperature for a long time. In the other steels, however, metastable cementite rather than graphite is a normal state of carbon, because a large volume expansion must be accompanied by the formation of graphite in the ferrite matrix. Graphitization of cementite at subcritical temperature is reported mostly in special carbon steels containing large amounts of silicon, nickel, or aluminumJ ~-51In high-purity Fe-C alloys or in commercial carbon steels, it is believed that graphitization is difficult. Only Higgins and Jeminson t~l have shown graphitization in a simple Fe0.15 pct C alloy. According to their report, graphitization occurred predominantly around inclusions located just beneath the surface when annealed for more than 200 hours at about 600 ~ In the center of the material, it took 1000 hours for the start of graphitization. Harris et al. L21 summarized the previous reports about graphitization at subcritical temperatures, concluding that the graphitization rate is controlled by the diffusion of carbon through the ferrite. Fujihira t3,4] examined the graphitization mechanism in a 0.24 pct C-1.2 pct Si-2 pct Ni steel and showed that graphitization is accelerated by rapid cooling from a ferrite and austenite intercritical temperature or by cold-rolling prior to annealing. A similar effect of cold-rolling on graphitization in a high-Si and -Ni steel has been reported by Sueyoshi and Suenaga. fS1 On the other hand, graphite is occasionally formed on the surface of batch-annealed commercial low-carbon AK sheet steels. This is usually called "graphite stain" and has been a serious problem for sheet steel suppliers, because it impaired paintability and coating adherence of the steels. I6J Inokuti I7'81 examined the effects of alloying elements in the steel and surface pretreatment for the sheets prior to the batch-annealing on the graphite stain. He showed that the addition of carbide-forming elements or those having a tendency to segregate to grain boundaries, in general, are effective in suppressing the graphite formation. ATSUKI O K A M O T O , Senior Research Engineer, is with the Iron and Steel Research Laboratories, Sumitomo Metal Industries Ltd., Amagasaki 660, Japan. Manuscript submitted October 24, 1988. METALLURGICAL TRANSACTIONS A
Okamoto found that extreme reduction of phosphorus and sulfur causes graphitization even in the inside of cold~ rolled low-carbon sheet steels. I9,~~ This paper describes the graphitization process and its effect on the mechanical properties in cold-rolled low-carbon (0.06 pct) and high-carbon (0.5 pct) AK steels.
II.
LOW-CARBON STEEL (0.06 PCT C)
A. Material
Six low-carbon AK steels containing different amounts of P and S were prepared by melting a high-purity electrolytic iron (Atomiron XL, produced by Showa Denko). Chemical compositions of the steels are listed in Table I. The base steel is a 0.05 pct C-0
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