Second phase precipitation in ultrafine-grained ferrite steel

Abstract

Grain size refinement is one of the most efficient strengthening mechanisms applied to modern High-Strength Low-Alloy steels (HSLA) because yield strength and toughness are both improved. This paper discusses the distribution of carbides by using transmission electron microscopy (TEM) in a low-carbon steel with ultrafine grained (UFG) ferrite. Fine cementite particles were formed during water quenching due to the auto-tempering of highly distorted martensite. Other fine particles observed under the same condition were nucleated due to the presence of carbide formers such as niobium, titanium and vanadium. TEM analysis showed that cementite particles underwent Ostwald ripening during warm rolling but they were still able to inhibit ferrite grain growth, which was maintained 1µm size approximately.

Keywords

• ultrafine-grained steel
• thermomechanical processing
• carbide
• ferrite
• microstructure
• transmission electron microscopy
• electron diffraction