Abstract
Warm rolling of medium Mn steel is a promising technology to meet the needs of vehicle weight reduction and energy conservation while ensuring the safety for passengers. In this study, a novel Nb-Mo microalloyed 6.6Mn steel was subjected to a series of warm rolling tests at temperatures ranging from 630 to 690 oC, based on the thermodynamic-based prediction of inter-critical annealing using a Thermo-Calc software. As a comparison, a conventional multi-stage thermo- mechanical process involving hot rolling, cold rolling and inter-critical annealing was carried out as well. The optimal rolling temperature parameters for warm rolling was explored through tensile tests and the detailed microstructural characterization. The experimental results show that the microstructure and mechanical characteristics strongly depended on the warm rolling temperature. A better combination of UTS and TE products (~32.0 GPa·%) was achieved in the 660oC-warm-rolled specimen, which is much higher than 12.3 GPa·% for the hot formed 22MnB5 steel and is also comparable to 38.3 GPa·% for the annealed cold-rolled specimens.
Highlights
In the past few years, the automotive industry has been searching for high/ultrahigh strength steels with good ductility to reduce the weight of vehicles while increasing its crashworthiness 1, 2
It was found that rolling temperature had a strong effect on the final microstructures, which can be compared by their substructure development as well as martensitic transformation to austenite
As the rolling temperature rose to 660 oC, the deformed ferrite with sharp grain boundaries (Fig. 1b) was observed, which was mainly attributed to the continuous absorption of dislocations into the low-angle boundaries
Summary
In the past few years, the automotive industry has been searching for high/ultrahigh strength steels with good ductility to reduce the weight of vehicles while increasing its crashworthiness 1, 2. Recent research mainly focused on improving the ductility of hot formed steels by either introducing a post-hot forming tempering treatment for 22MnB5 or developing an alternative class of hot formed steels such as a medium Mn steel 4 In the latter case, UTS and TE in the ranges of 1330-1448 MPa and 16.7~25.3% can be achieved after applying hot forming (HF) cycles at 700 oC due to an additional TRIP effect and grain ultra-refinement of constituent phases (ferrite, austenite and martensite). UTS and TE in the ranges of 1330-1448 MPa and 16.7~25.3% can be achieved after applying hot forming (HF) cycles at 700 oC due to an additional TRIP effect and grain ultra-refinement of constituent phases (ferrite, austenite and martensite) These new processing routes may offer a new opportunity for practical production of ultrahigh strength medium Mn steel, in view of the heavy springback during cold rolling 5-8. The potential of warm rolling of medium Mn steel has not yet been researched in detail so far
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