(1) after hot rolling, non-metallic inclusions (mainly sulphides and oxides, as well as silicates) inside the steel are pressed into thin sections, resulting in stratification (interlayer). Delamination greatly worsens the tensile properties of steel along the thickness direction and may lead to interlaminar tears during weld contraction. Local strain induced by weld contraction often reaches several times of yield point strain and is much larger than that caused by load.
(2) residual stress caused by non-uniform cooling. Residual stress is internal self-phase equilibrium stress without external force. Hot rolled steel sections of various sections all have such residual stress. Although the residual stress is self-phase equilibrium, it has some influence on the performance of steel members under the action of external forces. Such as deformation, stability, fatigue, and other aspects may be adversely affected.
(3) hot rolling cannot precisely control the mechanical properties of products, and the structure and performance of hot rolled products cannot be uniform. Its intensity index is lower than that of cold-hardened products and higher than that of completely annealed products; The plasticity index is higher than that of cold hardened products and lower than that of fully annealed products.
(4) the thickness and size of hot rolled products are difficult to control and the control accuracy is relatively poor; The Ra value of hot rolled products was generally 0.5-1.5 mm than that of cold rolled products. Therefore, hot rolled products are generally used as blank for cold rolling.