The mechanical properties of 70 mm x 1 250 mm slab of steel SPHC (% : 0.02C
0.18Mn
0.03Si
0.04Als) at 600 ~ 1 350 ℃ have been studied by Gleeble thermal simulation machine and the fracture of tensile specimen is analyzed by scanning electron microscope and energy dispersive X-ray spectrometer. Results show that the I brittle zone and the III brittle zone of thin slab of steel SPHC are respectively at 1200 °C ~ solidus and at 600 ~ 850 ℃
the plasticity of thin slab is best at 850 〜1200 °C ; the specimen at temperature in the III brittle zone is fractured along grain boundary ; and the inclusions in grain boundary and lamellar ferrite formed in γ to α phase transfomiation lead to brittleness of grain boundary and decrease of plasticity of material in the III brittle zone.