Strengthening effects in controlled-rolled and quenched low carbon martensites

Four 4 wt.% Mn low carbon steels have been controlled rolled to finish rolling temperatures in the range 950-650°C followed by water quenching. In addition, one 1.5wt.%Mn-0.12wt.%C steel has been quenched at varying rates directly from the austenite in order to assess the effect of dissolved carbon on the strength-grain size relationship in low carbon martensites. Measurements have been made of the martensitic grain size using the mean linear intercept, and these data have been correlated with the strength, ductility and toughness of the alloys. The strength of controlled-rolled and quenched materials was found to be superior to that of the as-quenched product, the former having proof stresses in the range 950-1100 MPa and the latter proof stresses in the range 650-1050 MPa. Solid solution hardening, mainly by carbon, and dislocation strengthening together with a grain size term are thought to be the major factors governing the strength of these materials. Toughness was only measured in the controlled-rolled materials and it is thought that the microcracks formed during quenching together with cementite precipitation may be responsible for the susceptibility to brittle failure in some of these materials.