Prevention of slab surface transverse cracking by microstructure control

Abstract

Slab surface transverse cracking is well known to be induced by strain concentration at film-like primary ferrite, i.e. allotriomorphs of ferrite formed along the austenite grain boundaries. In the present study, a new concept for the prevention of transverse cracking by means of microstructure control at continuous casting strand is examined. Three kinds of examinations in charge of each objective were conducted; (a) ingot cooling tests for microstructure control with secondary cooling; (b) hot tensile tests for hot ductility with the microstructure; and (c) continuous casting tests for cracking susceptibility on continuously cast slab. Results obtained are concluded as follows. (1) Slab surface microstructure could be controlled by secondary cooling condition. Surface structure control (SSC) cooling, providing intensive cooling until less than A3 transformation temperature just below mold and subsequently reheated up to 1250 K in secondary cooling, brings film-like ferrite free structure. (2) Hot tensile tests subsequent to in-situ remelting and solidification prove that hot ductility is much improved and ductility trough almost disappeared with that microstructure control. The results also confirm that in-situ remelting of specimen is indispensable on the hot tensile test to evaluate the effect of microstructure on susceptibility to transverse cracking. (3) Continuous casting test confirms that susceptibility to transverse cracking could be alleviated with this microstructure control. (4) The prevention of transverse cracking and microstructure control is a result of uniform fine precipitates dispersion, such as (Ti, Nb)(C, N), according to SSC cooling.