INFLUENCE OF SULFUR ON AIN PRECIPITATION DURING COOLING AFTER SOLIDIFICATION AND RESULTING HOT SHORTNESS IN LOW-CARBON STEEL

The influence of sulfur on AlN precipitation during cooling after soli dification and, hot ductility was investigated in low carbon aluminium killed steels by means of a Gleeble tester together with chemical ana lysis and analyitical TEM. 1) The precipitation kinetics of ALN is ver y slow in austenite, but it is promoted with the addition of sulfur. 2 ) In the steels bearing sulfur, MnS precipitates first in austenite gr ain boundaries followed by the ALN precipitation, which preferentially takes place on MnS. The following orientation relationship was found between the two precipitates: (111 $($) over bar$$)(MnS)//(0001)(AlN), [110](MnS)//[2110 ($) over bar ($) over bar](AlN) and [121](MnS)//[10 10 $$($) over bar](AlN). 3) This coherent heterogeneous precipitation was inferred to stem from the fact that the condition for the heteroge neous nucleation, which holds when the contribution of interfacial ene rgy is large, was satisfied due to the relatively low interfacial ener gy of ALN/MnS as compared with that of gamma-Fe/MnS. 4) The embrittlem ent of slabs during CC-DR (Continuous Casting-Direct Rolling) becomes understandable when the present result of AlN precipitation is superim posed in the convetional time-temperature embrittle curve. Namely, hot ductility becomes poor under the condition of the complex precipitati on of MnS and AlN.