For the purpose of developing the as-hot-rolled dual phase sheet steel, hot rolling conditions and subsequent cooling pattern were investigated by using 1%Si-1.5%Mn-1.0%Cr steels with carbon contents of 0.05%C and 0.08% and 0.05%C-1.0%Si-1.5%Mn steel.
In order to study the exact transformation behavior immediately after finish-rolling, hot-rolled strips of these steels were stopped on the runout table and water-cooled and/or air-cooled to room temperature.
This unique experiment exhibited that the optimum cooling pattern consisted of three stages;
1) rapid cooling to a suitable temperature (700°C) to increase ferrite nucleation sites,
2) holding at the temperature (700°C) to enhance ferrite transformation, thereby stabilizing untransformed austenite due to C and N concentration, and
3) rapid cooling to a coiling temperature to avoid the formation of pearlite and bainite.
Experimental rolling was performed at the production mill. The steels obtained showed dual phase structure with excellent tensile properties Lin terms of low yield to tensile strength ratio and high ductility.
