Influence of thermal mechanical treatments on the mechanical properties of a finely spheroidized eutectoid-composition steel

Abstract

Thermal mechanical treatments were performed on a plain carbon steel of eutectoid composition to improve ductility and strength of the fine spheroidized structure. After warm rolling at 873 K to spheroidized the original lamellar structure, the material consisted of cementite particles 0.1–0.5 μm in size situated on grain boundaries. The grain size was about 0.5 μm and the room temperature yield strength was 165 k.s.i. (1140 MPa) with 1% elongation. Low temperature annealing at 773 K for 100 h increased the ductility to 10% uniform elongation while the yield stress dropped to 125 k.s.i. (862 MPa). This reduction in flow stress resulted from annihilation of dislocation substructure and was not primarily due to particle coarsening or grain growth. Up to 50% of the ferrite matrix was converted to low carbon martensite by heat treating for short times at 1023 K in a salt bath and water quenching. Colonies (3–8 μm in size) of low carbon martensite plus cementite particles formed. The material with 50% martensite had a yield stress of 156 k.s.i. (1080 MPa) and strain hardened to 210 k.s.i. (1450 MPa) with 4% uniform elongation. We believe that the ideal structure where fine spheroidzied particles are distributed in a uniform matrix of 100% low carbon martensite could be achieved with low alloyed steels.