(1) After hot rolling, non-metallic inclusions (mainly […]
(1) After hot rolling, non-metallic inclusions (mainly sulfides and oxides, and also silicates) inside the metal are pressed into a sheet, and delamination (sandwich) occurs. The delamination greatly deteriorates the tensile properties of the metal in the thickness direction, and it is possible to cause interlayer tearing when the weld is shrunk. The local strain induced by weld shrinkage often reaches several times the yield point strain and is much larger than the strain caused by the load.
(2) Residual stress caused by uneven cooling. The residual stress is the internal self-phase equilibrium stress without external force. The hot-rolled steel of various sections has such residual stress. The larger the section size of the general steel, the larger the residual stress. Although the residual stress is self-phase balanced, it still has some influence on the performance of the metal under external force. Such as deformation, stability, fatigue and other aspects may have adverse effects.
(3) Hot rolling does not control the mechanical properties required for the product very accurately, and the microstructure and properties of the hot rolled product cannot be uniform. The strength index is lower than that of the cold work hardened product, and higher than the fully annealed product; the plasticity index is higher than the cold work hardened product, and lower than the fully annealed product.
(4) The thickness of the hot rolled product is difficult to control and the control precision is relatively poor; the rough Ra value of the hot rolled product is generally 0.5 to 1.5 μm. Therefore, hot rolled products are generally used as blanks for cold rolling.