EFFECT OF TEMPFORMING TEMPERATURE ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A HIGH-STRENGTH LOW-CARBON STEEL

The effect of tempforming temperature on the microstructure and mechanical properties, especially impact toughness, of the S700MC-type high-strength low-carbon steel was investigated. Tempforming at temperatures of 923 and 973 K was characterized by the development of ultrafine-grained layered microstructure consisting of grains highly elongated in the rolling direction with the transverse grain size of 530 and 920 nm, respectively. This microstructure improved significant the mechanical properties including the toughness in a wide range of test temperatures. The steel tempformed at 923K exhibited the ultimate tensile strength of 1110 MPa and the impact toughness, KCV, above 450 J/cm(2) at a temperature of 293 K, and KCV of 109 J/cm(2) at a liquid nitrogen temperature for the impact direction perpendicular to the rolling plane. An increase in the toughness was achieved by absorbing the impact energy during the separation of steel in the direction perpendicular to the direction of impact.