Abstract: In order to improve the spheroidized structure and cold heading quality of SWRCH35K steel, and to realize on-line annealing control after hot rolling, the phase transformation point of the SWRCH35K cold heading steel was obtained by thermodynamic simulation, and the hot rolled steel was heat treated by isothermal spheroidizing, high temperature tempering and high temperature tempering+isothermal spheroidizing. The results show that there are some long lamellar carbides in the granular pearlite formed after isothermal spheroidizing, and the uneven distribution of carbides is due to the re-precipitation of proeutectoid ferrite heated at the high temperature of the two-phase region, and lamellar pearlite is formed at 790 ℃; the carbides in the lamellar pearlite can become granular form by high temperature tempering, but the distribution of granular carbides is not uniform. The granular pearlite obtained by high temperature tempering and isothermal spheroidizing has good microstructure, uniform distribution of carbides and no obvious long lamellar carbides, so its best heat treatment system is 685 ℃×1.5 h, air cooling+740 ℃×0.5 h, furnace cooling to 685 ℃×3 h, then air cooling.

Key words: SWRCH35K cold heading steel, heat treatment, carbide, spheroidized microstructure