Synergistic effect of austenitizing temperature and hot plastic deformation strain on the precipitation behavior in novel HSLA steel

Examination of thin foils of specimens with various austenitizing conditions by transmission electron microscopy revealed randomly homogeneous precipitation in the ferrite for each experimental condition. Though no interphase precipitation was found in the present study, two types of random precipitation morphologies were identified in the ferrite matrix. One was randomly and homogeneously precipitated carbides of smaller size ( < 10 nm), and the other was randomly precipitated carbides of larger size (1030 nm). Transmission electron microscopy results provided evidence that both types of precipitation carbides could be associated with the supersaturation of microalloying elements in the ferrite and austenite, respectively. A higher austenitizing temperature treatment can lead to more microalloying elements dissolving in the austenite such that many tiny carbides precipitation at the low isothermal holding temperature, which is believed to effectively strengthen the ferrite. Vickers hardness data revealed that, in specimens austenitized at 1200 degrees C and deformed at 900 degrees C with strains of 10% and 30%, the ranges of hardness distribution were 250-360 HV 0.1 and 310-400 HV 0.1, respectively. For specimens austenitized at 1000 degrees C and deformed at 900 degrees C with strains of 10% and 30%, the ranges of hardness distribution were 220-250 HV 0.1 and 220-260 HV 0.1, respectively. Therefore, the average Vickers hardness increased with the austenitizing temperature and deformation strain. However, a wider range of hardness distribution occurred in specimens that underwent treatment at higher austenitizing temperatures. The wider Vickers hardness distribution reflects non-uniform precipitation in each ferrite grain. (C) 2015 Elsevier B.V. All rights reserved.