The relationship between microstructure and strength of hot rolled coil products made of high yield strength steels for cold forming

The relationship between rolling ratio and mechanical properties of hot rolled coil products made of high yield strength steels for cold forming (EN 10149-2, S550MC), was investigated. On experiments, slab dimensions were 225 mm*1255 mm* 12000 mm. As rolling conditions, furnace exit temperature of slab was 1240FM (Finishing Mill) entrance temperature was 920°C, FM exit temperature was 820°C, and coiling temperature was 540°C. The chemical compositions of the steel samples were prepared as standard and the sum of Nb, V and Ti allowed to be lower than 0,22%. For the experiments, different rolling ratios were used. The mechanical properties that we tried to reach were, yield strength minimum 550 MPa, tensile strength 600-760 MPa and elongation min 14%. According to EN standard for thicknesses higher than 8 mm the minimum yield strength can be 20 MPa lower. The change of mechanical properties related to rolling ratio differences and microstructures were searched. The study supports the inverse ratio between rolling and the strength both by comparison and statistical analysis.