Corner crack mechanism of boron-containing LCAK steel slabs

摘要

At Baoshan Iron & Steel Co., Ltd., corner cracks of boron containing LCAK steel slabs had caused remarkable quality loss and mass flow disorder. With the help of fractography and thermodynamics analysis,the embrittlement mechanism of this steel grade was studied and the results are as follows: ① The transformation from γ to α starts at the austenite grain boundaries and a layer of thin ferrite film gradually forms around the austenite grains. Strain concentration will preferentially start inside the ferrite phase when the stress accumulates to a certain level.② The coarse BN particles acceleratedly precipitated at the γ/α interfaces further deteriorate the ductility of the ferrite film,and brittleness results in strain concentration and microvoid coalescence inside the ferrite film.Therefore the austenite grain boundaries are prone to intergranular failure.③ The stoichiometry among Al,N and B is a very important factor influencing the hot ductility of this steel grade.By controling the B-to-N atomic ratio to above 1,all N can be fixed by B instead of Al.Thus coarse-grained steel is available and fewer grain boundaries and higher ductility can reduce the risk of corner cracks.④ By adjusting the B-to-N atomic ratio,Baoshan Iron & Steel Co. ,Ltd.successfully reduced the number of cracks to nearly one tenth of that in the past and the hot tensile tests confirmed remarkable improvement in the hot ductility of this steel.