Microstructure-property relations in as-extruded ultrahigh-carbon steels

The microstructure and mechanical properties of as-extruded ultrahigh-carbon steels (UHCSs) are described. Extrusion ratios of 16:1 reduction were used at extrusion temperatures of 900 deg C, 1025 deg C, and 1150 deg C. Air cooling followed the extrusion process. The compositions of the four UHCSs investigated ranged from 1.2-1.8% C and contained either aluminum or silicon and/or chormium as the principal alloying elements. The as-extruded microstructures were predominantly pearlitic. Detailed examination of the microstructures allowed determination of the composition and extrusion temperature at which grain-boundary carbide networks and graphite can be avoided. It is shown that the yield strength of the as-extruded bars, for a given UHCS alloy composition, correlates well with the interlamellar spacing, independent of the extrusion temperature. Ultimate tensile strengths ranged from 1000-1690 MPa, and elongations to failure ranged from 3-12.5%. The present results indicate that single-step mechanical working of UHCSs to achieve fully pearlitic structures, with no postworking heat treatment, can lead to a new class of ultrahigh-strength steels.