Effects of cooling on the microstructure and mechanical properties of B-added low carbon HSLA steels

Effects of cooling rate and cooling finish temperature (CFT) on the microstructure and mechanical properties have been investigated in B-added low carbon (0.05wt%C) HSLA steels aiming for the use of linepipes. At first, continuous cooling transformation (CCT) diagram of the present steel has been established by using thermo-mechanical process simulation in a Gleeble system at various cooling rates from 0.1 degrees C/s to 50 degrees C/s, followed by microstructure identification based on metallographic observations and micro-hardness measurement. The microstructure identified in the CCT experiments could be classified as granular bainite, acicular ferrite, bainitic ferrite and lath martensite as increasing the cooling rate. The microstructure as well as mechanical property also depended upon cooling finish temperature. As cooling finish temperature increased, the fraction of acicular ferrite increased without a decrease of hardness value. This was due to the softening retardation effects caused by good carbide forming elements of V, Cr, and Mo. In addition, lower bainite microstructure was newly developed when the accelerated cooling stopped at temperature below 450 degrees C. The present simulation results have been well compatible to pilot plant scale rolling. That is, the increasing cooling finish temperature could result in enhanced upper shelf energy without a decrease of tensile strength, which was attributed to the decrease of dislocation density in addition to the softening retardation effects of V, Cr and Mo.