Ni-free and Ni-bearing steels were designed to elucidate the effect of Ni on the free energy of bainitic transformation. Steels with two distinct nano-bainite structures were obtained through conventional one-stage and two-stage isothermal treatments. The effect of work hardening and accompanying strain on martensitic transformation of the two nano-bainite structures was also studied. Microstructure of both the steels consisted of bainitic ferrite and film-like and block-like retained austenite (RA). The total carbon concentration, degree of Fe–C cluster formation, and stability of RA were significantly affected by the free energy of bainitic transformation. The presence of Ni leads to reduction in the free energy of bainitic transformation, refinement of bainite structure, and extension of bainite nucleation regime. The carbon-content in block-like and film-like RA microstructure of Ni-bearing steel was 5.5% and 19.1%, respectively, which was greater than Ni-free steel. The load-displacement behavior indicated superior strain hardenability and higher strain hardening rate (SHR) in Ni-bearing steel. The uniform nanostructure of Ni-bearing steel promoted the formation of stable film-like RA between bainitic ferrite, leading to higher strain hardening rate and stronger transformation-induced plasticity (TRIP) effect at large strain. The Ni-bearing steel was characterized by superior plasticity (13.7%–10.3%) and product of tensile strength and % elongation (25.9 GPa·% to 20.9 GPa·%) in comparision to the Ni-free steel.

设计无镍和含镍钢是为了阐明镍对贝氏体转变自由能的影响。通过常规的一阶段和两阶段等温处理获得具有两种不同的纳米贝氏体结构的钢。还研究了加工硬化和伴随应变对两种纳米贝氏体组织马氏体相变的影响。两种钢的显微组织均由贝氏体铁素体以及薄膜状和块状残余奥氏体（RA）组成。贝氏体转化的自由能显着影响了总碳浓度，Fe-C团簇形成的程度和RA的稳定性。Ni的存在导致贝氏体转变的自由能降低，贝氏体组织的细化以及贝氏体成核机制的扩展。含镍钢的块状和膜状RA显微组织中的碳含量分别为5.5％和19.1％，高于不含镍的钢。载荷-位移行为表明，含镍钢具有较高的应变淬透性和较高的应变淬硬速率（SHR）。含镍钢的均匀纳米结构促进了贝氏体铁素体之间稳定的薄膜状RA的形成，从而导致较高的应变硬化速率和较大应变下的相变诱导塑性（TRIP）效果更强。含镍钢的特点是，与不含镍的钢相比，具有较高的可塑性（13.7％〜10.3％），抗拉强度和延伸率的乘积（25.9 GPa·％至20.9 GPa·％）。载荷-位移行为表明，含镍钢具有较高的应变淬透性和较高的应变淬硬速率（SHR）。含镍钢的均匀纳米结构促进了贝氏体铁素体之间稳定的薄膜状RA的形成，从而导致较高的应变硬化速率和较大应变下的相变诱导塑性（TRIP）效果更强。含镍钢的特点是，与不含镍的钢相比，具有较高的可塑性（13.7％〜10.3％），抗拉强度和延伸率的乘积（25.9 GPa·％至20.9 GPa·％）。载荷-位移行为表明，含镍钢具有较高的应变淬透性和较高的应变淬硬速率（SHR）。含镍钢的均匀纳米结构促进了贝氏体铁素体之间稳定的薄膜状RA的形成，从而导致较高的应变硬化速率和较大应变下的相变诱导塑性（TRIP）效果更强。含镍钢的特点是，与不含镍的钢相比，具有较高的可塑性（13.7％〜10.3％），抗拉强度和延伸率的乘积（25.9 GPa·％至20.9 GPa·％）。在大应变下导致更高的应变硬化速率和更强的相变诱导塑性（TRIP）效果。含镍钢的特点是，与不含镍的钢相比，具有较高的可塑性（13.7％〜10.3％），抗拉强度和延伸率的乘积（25.9 GPa·％至20.9 GPa·％）。在大应变下导致更高的应变硬化速率和更强的相变诱导塑性（TRIP）效果。含镍钢的特点是，与不含镍的钢相比，具有较高的可塑性（13.7％〜10.3％），抗拉强度和延伸率的乘积（25.9 GPa·％至20.9 GPa·％）。