The flowsheet of 236 mm x 1 350 mm casting slab of tested steel CR1030/1300MS (/% : 0. 13C

0. 26Si

L53Mn

0.011P

0.002S

0.31Cr

0.047Als

0.029Nb

0.032Ti

0.002 5B

0.003 2N) is 260 t BOF-LF-RH-CC process. With using Gleeble3500 thermal simulation test machine the thermoplasticity curves of test slab at 650 ~ 1 300 °C has been measured and the structure of high temperature tensile specimen fracture is analyzed. The test results obtain that the two brittle zones in test steel are melting point 〜1 120 °C and 800 〜650 °C

and the better plasticity occurs at 1 120 ~ 800 °C

in this zone the reduction of area of steel is more than 85%. The fracture of tensile of steel at 900 °C. is transgranular toughness fracture and at 700 °C it is toughness and along crystal fracture. The structure near fracture of tensile of steel at 1 250 °C and 900 °C is martensite and with increasing temperature the martensite lath is coarser. The basic reason to form crack is that the proeutectoid ferrite occurs in original austenite grain boundary at 750 °C.Straightening at 1 120 ~ 800 °C is available to avoid the casting bloom forming crack.