Abstract: Tin free-cutting steel is a new environmentally friendly free-cutting steel that is expected to replace lead free-cutting steel. The existing form of tin in steel is an important factor affecting the cutting performance of steel. In this paper，metallographic microscopy (OM)，scanning electron microscopy (SEM)，electron microprobe (EPMA) and other characterization methods were used to analyze the state of tin in high sulfur free-cutting steel and the effect of tin content on the cutting performance. It was found that no significant tin precipitation phase was observed in the tin-containing steel. Tin was soluted in the solid steel matrix，and mainly tended to be soluted in the ferrite. Tin distorted the lattice of the ferrite. The cutting tests showed that when the mass fraction of tin changed from 0 to 1 600×10^-6，the C-type chip ratio increased from 60.19 % to 90.29 % at the cutting speed of 560 r/min. Tin content had a significant enhancement effect on the formation of C-type chip. In cutting the same sample，the proportion of C-type chip was also the largest at cutting speed 560 r/min compared to that at other two speeds，which indicated that the cutting performance was better at this speed. In addition，the addition of tin led to a reduction in surface roughness and better surface quality of the machined sample. The Vickers hardness also increased from 118.2 HV to 143.1 HV with the addition of tin mass fraction from 0 to 1 800×10^-6 by microhardness test. The solid solution of Sn in the steel increased the hardness of the soft steel matrix，which led to a significant improvement of the cutting performance of the steel.

Key words: Sn, free-cutting steel, 1215 steel, sulfide, microhardness