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.

中文翻译：

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高强度钢中马氏体、贝氏体和珠光体对奥氏体回归的原位高温 EBSD 研究


采用原位电子背散射衍射 （EBSD） 在高温下研究了由板条马氏体 （LM）、板条贝氏体 （LB）、颗粒贝氏体 （GB） 和珠光体 + 铁素体 （P+F） 逆向转化的高强度钢中的奥氏体 （γ）。初始γ的记忆效应显著影响 LM 和 GB 结构中回归γ的成核，而对 LB 和 P+F 结构的影响较弱。这导致完全奥氏体化后γ晶粒尺寸存在显着差异，前两者获得较大的γ晶粒，而后两者相对较小。晶体学分析表明，具有针状形态 （γA） 的还原γ，其中大多数与前γ保持相同的取向，通过γ晶粒内的优先成核和聚结生长模式主导了 LM 和 GB 结构的再奥氏体化行为。虽然具有随机取向或与前γ较大偏差的球状回归γ （γG） 可以在晶界或晶粒内成核，但由于抑制 γA 聚结，它很难生长并在分割和细化前γ中发挥作用。对于 LB 和 P+F 结构，晶内 γG 的成核速率随着温度的升高而增加，并且始终呈现随机取向。这些 γG 晶粒可以与晶间 γG 同时粗化，最终在共同分γ细化最终晶粒中发挥作用。研究还发现，四种不同微观结构对还原γ成核影响的差异与基质结构的变体选择以及渗碳体 （θ） 的含量和大小密切相关。 变体选择减弱诱导的高密度块边界和板条中形成的许多细小 θ 是促进 LB 结构获得更多晶内 γG 形成的关键，也是大尺寸 θ 在 P+F 结构中的重要作用。