Abstract
The microstructural evolution and properties for varied austempering routes are investigated in a cold-rolled bainitic steel. Special attention is given to the effect of retained austenite (RA) in terms of its fraction, carbon concentration, and morphology resulting from different austempering routes on mechanical properties and stretch flangeability. Bimodal sized bainitic laths are provided, and the carbon concentration of RA maintains the highest value through the two-step austempering. Total elongation (TEL) is remarkably enhanced for the two-step austempering, deviating from the exponential relationship between tensile strength (TS) and TEL as maintained by the one-step austempering. Considering the two plateaus of the strain-hardening exponent, it is considered that the hierarchical stability of RA is provided by the two-step austempering, leading to the postponed necking point so as to improve the uniform elongation. Two-step austempering could provide more complete bainitic transformation as well as more stable film-like RA, supplying a promising way to improve the combination of strength, ductility, and stretch flangeability.
Highlights
The application of high-strength steel is one of the most effective ways to reduce car body weight while ensuring the required safety
We have studied the effect of austempering treatment in terms of temperature and holding time on the microstructure, mechanical properties, and stretch flangeability
2a,e), which were formed from untransformed austenite during lath morphology
Summary
The application of high-strength steel is one of the most effective ways to reduce car body weight while ensuring the required safety. The multiphased steels with a combination of varied phases, such as ferrite, martensite/bainite, and retained austenite (RA). It is well accepted that the bainitic lath size, which is determined by the transformation temperature, strongly affects the strength. Sandvik et al [1] found that strength can be obviously increased by refining the bainitic lath size through decreasing the austempering temperature. Bhadeshia et al [2,3] developed an ultra-high strength bainitic steel with an excellent balance of strength and ductility (tensile strength higher than 2000 MPa and total elongation higher than 20%) by refining the bainitic lath to tens of nanometers through a low-temperature austempering treatment. Remarkable ductility is provided due to a considerable fraction of RA by means of the transformation of metastable austenite into martensite, i.e., the transformation-induced plasticity (TRIP) effect
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