Development of Precipitation Strengthened Steel with Adequate Stretch Flangeability

An experimental steel of the composition (in wt.%) 0.04C-0.81Mn-0.38Si-0.15Ti-0.01S-0.013P-0.043Al was hot rolled into 4 mm plates at three different temperatures of 1100, 1000, and 900 °C. The yield strengths of these plates were in the range of 434-484 MPa while the ultimate tensile strength varied from 508 to 586 MPa. Elongation values ranged from 13.0 to 17.8%. Hole expansion ratios (λ) varied from 23 to 30.7%. In particular, the plate rolled at 1000 °C showed a yield strength of 484 MPa, an ultimate tensile strength of 586 MPa, a total elongation of ~15%, and a hole expansion ratio of ~23%. Transmission electron microscopy showed the presence of fine precipitates of titanium carbosulfide (~10-50 nm). Therefore, maximum precipitation strengthening was obtained in the plate that was hot rolled into a thickness of 4 mm at 1000 °C.