1992 Volume 32 Issue 7 Pages 873-881
The critical hot rolling condition which prevents abnormally large grains from forming in the microstructure was investigated in Interstitial Free (IF) steel, using thermomechanical simulation equipment. Abnormal grain structure was produced at finishing temperatures (FT) below 900°C, accompanied by coiling temperature (CT) above 700°C. The said critical FT and CT represent the dynamic transformation temperature (DTT) and the secondary recrystallization temperature (SRT), respectively. The unusually large size grain of >150 μm in diameter is attributed to the high driving force due to the extremely fine grains around itself. These grains are produced through the transformation from unrecrystallized austenite. The austenite recrystallization rate was drastically reduced through the drag effect of dissolved Ti atoms. Unlike typical low carbon steel, a strong retardation of ferrite grain growth due to TiC precipitates resulted in the elimination of the abnormal grain below SRT coiling. Precipitation hardening due to TiC as well as hot deformation hardening was recognized in IF steel. Excessively high hardness might be encountered, if the FT is <DTT and the CT is <SRT.
