Influence of Boron and Nitrogen Contents on Strength and Toughness of Controlled-rolled and Acceleratedly-Cooled Low Carbon Steel

Abstract

An influence of boron and nitrogen contents on the strength and toughness of 0.03%C-1.50%Mn-0.15%Mo-0.07%V-0.04%Nb-0.02%Ti-B steel, finish-rolled at 720°C, then acceleratedly-cooled after heating at 1 050°C was studied.
The strength and toughness can be estimated by the content of calcurated free-boron. The microstructure changes from coarse ferrite to fine ferrite, or from fine ferrite to bainite by increasing of free-boron content, or cooling rate after controlled-rolling, consequently the tensile strength increases. The toughness increases by the change to fine ferrite, but decreases by the change to upper bainite. The decrease of toughness is caused by increase of a unit crack path. Optimum free boron content to obtain both high strength and high toughness is less than 7 ppm.