In-situ high-temperature EBSD study of austenite reversion from martensite, bainite and pearlite in a high-strength steel

Abstract

The austenite (γ) reversely transformed from lath martensite (LM), lath bainite (LB), granular bainite (GB) and pearlite+ferrite (P+F) in a high-strength steel was studied at high temperatures using in-situ electron backscatter diffraction (EBSD). The memory effect of initial γ significantly affects the nucleation of the reverted γ in LM and GB structures, while a weak influence on that of LB and P+F structures. This results in a significant difference in γ grain size after complete austenitization, with the first two obtaining larger γ grains while the latter two are relatively small. Crystallographic analysis revealed that the reverted γ with acicular morphology (γA), most of which maintained the same orientation with the prior γ, dominated the reaustenitization behavior of LM and GB structures through preferential nucleation within γ grains and coalesced growth modes. Although globular reverted γ (γG) with random orientation or large deviation from the prior γ can nucleate at the grain boundaries or within the grains, it is difficult for it to grow and play a role in segmenting and refining the prior γ due to the inhibition of γA coalescing. For LB and P+F structures, the nucleation rate of intragranular γG increases with increasing temperature, and always shows a random orientation. These γG grains can coarsen simultaneously with the intergranular γG, ultimately playing a role in jointly dividing and refining the final γ grains. Research also found that the differences in the effects of four different microstructures on reverted γ nucleation are closely related to the variant selection of the matrix structure, as well as the content and size of cementite (θ). High density of block boundaries induced by weakening of variant selection and many fine θ formed in the lath are the key to promoting LB structure to obtain more intragranular γG formation, as well as the important role of the large-sized θ in P+F structure.