Formation of austenite in 1.5 pct Mn steels
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recent years the energy crisis has changed the trend of materials used in the automotive industry. The energy crisis has caused a revolution in the American automotive industry which was forced to embark on a program to reduce the weight of the vehicles produced. This reduction must be accomplished while still maintaining the specified collision properties. The high steels in current use in the automotive industry are the high strength, low alloy (HSLA) or microalloyed (MA) steels. These steels exhibit a ferrite-pearlite microstructure and derive their interesting combination of properties from a fine grain size, precipitation strengthening, and inclusion shape control. A principal requirement of any steel used for structural purposes in vehicles is that it must be able to survive rather substantial forming strains, especially those encountered in stretch-forming operations. A comparison of the properties of MA steels with conventional carbon steels reveals that the MA steels are, in general, of higher strength and of lower ductility, This relatively low ductility has kept MA steels from being even more extensively utilized by the automotive industry. Recent metallurgical developments have led to marked improvement in the ductility of MA steels. It has been shown, for example that when the ferritepearlite microstructure of the conventional MA steel is changed to a ferrite-martensite microstructure, the ductility of the steel can be substantially improved. Steels that exhibit ferrite-martensite microstructures have been labeled "dual-phase" steels, and these steels have demonstrated that the two-phase microstructure promotes continuous yielding with a rapid rate of work hardening and improved elongation when compared to their ferrite-pearlite counterparts. Dual-phase steels are produced by either of two methods. One is where the MA steel hot band is given an intercritical anneal and is then quickly cooled. 1,2The intercritical anneal leads to the formation of islands of austenite which transform to martensite or some other C. I. GARCIA and A. J. DEARDO are both with Department of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, PA. Manuscript submitted May 13, 1980. METALLURGICALTRANSACTIONSA
low temperature transformation product during the rapid cooling. The second method develops the dualphase microstructure in the hot band? In this case, the hardenability of the steel and the cooling rate are matched such that the ferrite forms during the cooling period between hot rolling and coiling while the martensite forms during the coiling. Both approaches can lead to dual-phase microstructures and properties. The first step in producing a dual-phase microstructure by intercritical annealing is the formation of austenite in the intercritical region of the phase diagram. The formation of austenite in low alloy steels has been the subject of several investigations;4-~ however, there has been little systematic work done on the formation of austenite in low carbon steels that have been heate
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