Effect of Temperature on the Ductility of Continuous Cast Low Carbon Steel Thin Slabs

摘要

The effect of deformation temperature on the fracture ductility of two low-C-Mn CSP-type and two low-C-Mn Conticaster-type thin slabs was evalauted at temperatures in the range 700-100degC. The thin slab samples wee obtained after the unbending section in the casters and the high temperature tensile tests were performed at a cosntant crosshead speed of 0.5 mm/s (initial strain rate of 20~-3 s~-1). The microstructures of the steels within the temperatue range where unbending is carried out, were reproduced in the tests by applying a thermal cycle whereby re-austenitization at 1000degC was accomplished without dissolution of pre-existing second phase carbide, nitride and carbonitride particles. The soaking time at this temperature was selected so that the austenite grain size was approximately equal to that of samples quenched immediately after reaching 1000degC. The limied grwoth of the austenite grains was taken as an indication that dissolution of secondary phase particles did not occur extensively. After soaling at 1000degC. the specimens were cooled at 50degC min~-1 to the testing temperature. It is proposed that this high temperature tensile testing technique reproduces adequately the thermal and the main microstructure features that lead to ductility losses and transverse cracking during continuous casting of low carbon steel thin slabs. The deformation microstructures and the topographic features of the fracture surfaces were characterized by scanning electron microscopy. The general characterstics of the ductility troughs obtained are discussed in terms of microstructures observed on the fracture surfaces. The results show that the fine-grained structure of continuous cast thin slabs enhances crack nucleation and retards crack propagation.