以SUS321不锈钢为实验材料，探究了300~4 K连续冷却过程中，磁场作用下晶粒尺寸对变温马氏体相变行为的影响规律及作用机制。结果表明，相同的晶粒尺寸下，马氏体开始转变温度、最终转变量均随磁场强度的增加而增加。在相同的强磁场强度下，随着晶粒尺寸的增加，存在最为明显的促进马氏体生成的临界晶粒尺寸，加速整个连续冷却过程中的马氏体相变。组织观察表明：磁场作用能有效促进温度诱发ε-马氏体相变的形核质点的形成，进而提高连续冷却过程中α'-马氏体相变的形核质点数量，促进α'-马氏体相变。从相变机理方面进一步完善了前人提出的磁场加速马氏体相变的表象研究结果。此外，通过对组织形貌和晶体学特征分析，揭示磁场作用虽然促进温度诱发ε-马氏体相变的形核质点的形成，但是对γ→ε-马氏体变体选择影响不敏感，而晶粒尺寸对其敏感性较高。在磁场作用下，随着晶粒尺寸增大，ε变体的各向异性逐渐转变为各向同性，各项同性的ε-马氏体的形核质点长大过程中加速硬碰撞，使得晶粒尺寸较大时，马氏体相变受到抑制。ε变体进一步相变时，ε→α'-马氏体变体选择对晶粒尺寸和磁场敏感性均不高。

Extrinsic (magnetic fields) and intrinsic (austenite grain sizes) factors can effectively control the martensitic transformation. Until now, research has mainly focused on the separate effects of magnetic fields and austenite grain sizes on the kinetics of the martensitic transformation. Systematic studies considering the coupling effects of magnetic fields and austenite grain sizes on the temperature at which martensite is formed (M_s), the final volume fraction of the transformed martensite, and the kinetics of the martensitic transformation during continuous cooling are still lacking. Furthermore, no study has yet been reported on the mechanism underlying how magnetic fields and austenite grain sizes affect the martensitic transformation. In this study, SUS321 stainless steel is used to investigate the effect of grain size on the kinetics and mechanisms of the martensitic transformation during continuous cooling from 300 K to 4 K under various magnetic fields by using the physical property measurement system (PPMS). The results show that at a constant grain size, the M_s temperature and the final amount of martensite increase as a function of the magnetic field magnitude. Under the same magnetic field, a critical austenite grain size exists, which obviously accelerates the martensitic transformation during cooling. Detailed microstructural characterizations also show that the external magnetic field effectively promotes the formation of ε nucleation sites, which consequently enhances the nucleation rate of α′-martensite and its transformation during further cooling. These findings provide mechanistic insights into the previously found phenomenological results. Additionally, in-depth crystallographic analyses also demonstrate that although the magnetic field promotes ε nucleation, the variant selection during the γ→ε transformation is insensitive to the magnetic field magnitude, unlike the austenite grain size. Under the same magnetic field, the increase in the austenite grain size results in more ε variants during cooling. The collision of similar ε variants restricts the growth of martensite laths and retards the martensitic transformation in coarse-grained austenite. The variant selection of the final transformation ε→α′ is insensitive to the magnetic field magnitude and the austenite grain size.