液相线保温法低合金钢半固态组织的形成机理

Nucleation Mechanism of Semi-Solid Microstructure in Low Alloy Steel Prepared by Liquidus Holding

采用液相线保温法制备了低合金钢ZG25MnCrNiMo半固态熔体，获得了晶粒细小、圆整、均匀的半固态组织．研究了该组织的形成过程和形成机理，掌握了保温温度和保温时间对组织形态以及晶粒尺寸的影响规律．结果表明，在液相线温度1510℃保温时，熔体内易形成大量准固相原子团簇并发展为游离晶均匀分布在熔体内，它们能在凝固时显著细化、匀化晶粒，使水淬组织细小、均匀．在空冷条件下，这种细化机制仍起作用，使液相线温度保温后的空冷组织比常规铸造浇铸温度下的空冷组织显著细小．由于中频感应炉的电磁搅拌作用，在液相线短时保温就可孕育出相当数目的游离晶，并能在随后的保温过程中维持游离晶的形态和数目．在保温5～50min的时间范围内，液相线温度下不同保温时间的水淬组织没有显著变化，短时保温与长时保温的效果相当。

The semi-solid slurry of low alloy steel was prepared by liquidus holding, in which fine, spherical and homogeneous microstructure was found. The nucleation mechanism of the microstructure and the influence of holding time, holding temperature and heat means on the microstructure were studied. Results showed that after holding at liquidus（1 510℃ ） a quantity of free nucleus which could refine and homogenize grains were formed in water quenched samples. Furthermore, the grain size of samples after liquidus holding was the smallest of dry quenched samples. In order to forming the free nucleus and keeping them, a shorter holding time （5 min） was needed because of electromagnetic stirring brought by medium-frequency induction. The microstructure of water quenched samples were not significant difference after holding 5 to 50 min at liquidus.