Abstract: Effect of a small amount addition of 0.30%Ni on continuous cooling transformation (CCT) behavior and hardenability of a novel Si-Mn-Cr-B type ultrahigh strength spring steel was explored by using thermal dilatometer and end-quenching test, and their CCT curves of undercooled austenite were obtained combined with metallurgical and Vickers hardness tests. The results show that adding 0.30%Ni has considerable effects on the CCT curves of the experimental steel, that is, the phase transformation temperatures decrease at different cooling rates, and the bainite and martensite transformations are promoted. The starting cooling rate for martensite transformation decreases from about 0.78 ℃/s to about 0.48 ℃/s, but the critical cooling rate of obtaining full martensite is barely changed. The hardness test results show that the hardness at different cooling rates of the Ni added steel is higher than that of the Ni-free steel, and their hardness difference exhibits a sharp increase-decrease trend with cooling rate in the range of 0.28-4.40 ℃/s, and adding Ni has almost no effect on the hardness of full martensite. The result of the hardenability tests show that adding Ni element significantly improves the hardenability of the experimental steel, making the declining trend of the hardenability curve gentler, and the maximum dimension of the steel with full martensite structure increases from 30 mm to 40 mm.

Key words: ultrahigh strength spring steel, Ni, CCT curves, hardenability, microstructure, hardness