A Study of Processing, Microstructure and Mechanical Properties of Ultra-High Strength Microalloyed Steel Hot Band Coils for Automotive Applications

摘要

In the automotive industry, thin gauge high strength steels require not only good tensile ductility, but also good sheared edge ductility or a good hole expansion ratio (HER) value. These properties can be achieved through producing a microstructure consisting of a single-phase ferrite and strengthened by microalloyed precipitates. The objective of this current study was to develop a hot band steel with a ferrite-based microstructure with a tensile strength higher than 1200MPa, total elongation larger than 10%, good HER values and low temperature toughness. The steel being studied in this project has the following nominal composition (wt %): 0.14C, 0.35Mo, 0.163Ti and 0.294V. In this investigation, the relationship between the microstructure and mechanical properties with different finish rolling and coiling temperatures were explored. It was found that the finish rolling temperature did not have an obvious influence on either microstructure or mechanical properties. However, the coiling temperatures strongly affected both microstructure and mechanical properties. Steels with a coiling temperature of 610°C, exhibited predominately polygonal ferrite and a few acicular ferrite grains. The corresponding tensile strength was over 1200MPa, total elongation of about 20%; however, the low temperature toughness and HER were rather low. The fracture surface from broken CVN specimens shows nearly pure brittle fracture. When the coiling temperature was reduced to 530°C, the microstructure appears to be a mixture of granular bainite and coarse quasi-polygonal ferrite grains. Here, the tensile strength drops to under 1000MPa, but the low temperature toughness and HER improve greatly. With a further lower coiling temperature of 450°C, the microstructure is a mixture of granular bainite, quasi-polygonal ferrite and upper bainite. In this case, the tensile strength increases to about 1100MPa, and the steels have intermediate low temperature toughness and HER. Studies showed that the high strength of the steels with the highest coiling temperature were due to the excessively formed fine precipitates at the coiling temperature of 610°; while the strength of steels with lower coiling temperatures originates predominately from dislocation strengthening. Because of the very high percentage of Ti and low amount of N, coarser TiN inclusions were formed in the liquid and in the interdendritic pools separating the dendritic δ ferrite grains. These hyper-stoichiometric TiN particles with a size larger than 3 microns can be observed throughout the steels. The relatively low toughness and HER values can be attributed, at least partially, to the large amount of coarse TiN inclusions found in all steel conditions. In addition, the low values found with 610°C coiling can be partly attributed to excessive precipitation hardening, while the low values found with 450°C coiling is the result of high levels of MA microconstituent. The scientific hypothesis guiding this study is that excessively large amounts of TiN particles are detrimental to toughness and sheared edge ductility (HER) in high strength steels.