Study On Microstructural Evolution And Mechanical Properties Of Medium Manganese Steels With Superior Strength And Ductility

There is a growing interest in advanced lightweight automotive steels with excellent combinations of strength and ductility, such that the demand for energy conservation and environmental protection is met. The conventional steels characterized by PSE, i.e. the product of ultimate tensile strength (UTS) and total elongation (TE) of15±10GPa%, such as interstitial free steels and dual phase steels, were categorized as first generation automotive steels, whereas the high alloyed steels with PSE of60±10GPa%, such as austenitic steels were classified as second generation automotive steels. These two classes of steels have attractive limited use in the automotive industry because of inherent disadvantages, for instance, the PSE of the first generation steels is low, while the second generation steels are too expensive. Recently, a number of studies were directed toward medium Mn-content (4-12%) steels, which are considered as promising third generation automotive steels with PSE>30GPa%.Microstructural evolution and mechanical properties of TRIP steels containing8/11(wt.%) Mn have been investigated. It was reported that mechanical properties of TRIP steels are largely depending on the amount and stability of austenite. The optimized quenching and tempering (Q&T) treatment produced a large amount of austenite that was characterized by significant TRIP effect during tensile testing. Furthermore, the factors related to austenite stability, such as grain size, morphology, chemical elements, etc, were thoroughly studied. The results obtained are summarized as following:(1) The as-hot-rolled steels quenched in the range of750-800℃and then tempered had a similar or better mechanical properties than those of low-alloy and medium Mn TRIP steels. Furthermore, less cold-rolling work or annealing time was required in the present work.8Mn steel exhibited combinations of UTS of810-1000MPa and TE of32-39%.11Mn steel demonstrated a high strength range of880～1100MPa and a large elongation range of 34-40%. Superior combinations of strength of960-1160MPa and ductility of28-40%were achieved on11Mn-Nb steel.(2) After tempering at200℃, the ductility of the as-hot-rolled sample quenched at800℃is improved significantly, and there is no decrease in the strength. It was found that the improvement of ductility was attributed to carbon diffused from δ-ferrite to austenite during tempering, which enhances the stability of austenite, leading to superior ductility. For the samples quenched in the range of850～900℃, tempering enhances the ductility significantly, but meanwhile, it deteriorates the strength. It is generally considered that the formation of tempered martensiteduring tempering decreases the internal stress of the samples.(3) The quenched as-cold-rolled steels had excellent mechanical properties.8Mn cold-rollded steel quenched from730℃exhibited combination of UTS of873MPa and TE of57%.11Mn steel quenched from750℃demonstrated a high strength range of998MPa and a large elongation range of67%. Excellent combination of strength of979MPa and ductility of63%was achieved on11Mn-Nb steel. The obtained mechanical properties of the11Mn and11Mn-Nb steels are superior to a number of previously studied medium Mn-content TRIP steels.(4) Discontinuous TRIP effect was first observed and proposed by studing the strain hardening behavior of as-hot-rolled11Mn steel. There are two important factors for the discontinuous TRIP effect. First, the martensitic transformation leads to a volume expansion, which leads to plastic deformation of the δ-ferrite and the intercritical ferrite (IF) laths, and consequently introduces relaxation and transfer of localized stress. Second, austenite with different degree of stability is responsible for the TRIP effect that occurs discontinuously only when a critical stress is attained. Moreover, it was found that the division of block austenite to laminar ones with different thickness and length induced by IF is responsible for varying degree of stability of austenite.(5) Comparing the microstructure of samples prior to and after tensile testing by EBSD, it was indicated that the morphology and orientation of austenite grain were the important factors in determining the mechanical stability; nonetheless, the morphology played a more significant role than orientation.(6) Through the study of the strain hardening behavior of samples quenched from different temperature, it was found that the deformation of ferrite initially suppressed the TRIP effect, thus, TRIP effect occurred at a higher strain, leading to superior ductility. Furthermore, the experimental results are consistent with the strain hardening behavior predicted by Crussard-Jaoul (C-J) approach.(7) Based on the study of the as-cold-rollded11Mn steel, it was found that the serrated strain-hardening behavior in stage3, because of discontinuous TRIP effect, was a consequence of non-uniform distribution of Mn that led to different degree of austenite stability. Moreover, it was found that prolonging annealing time makes distribution of Mn uniform, while increase of annealing temperature has an opposite effect. The critical factor that governed the stability of austenite in the cold rolled steel was grain size.