HOT CRACKING DURING SOLIDIFICATION OF STE EL - EFFECT OF CARBON, SULFUR AND PHOSPHORUS

Segregated internal cracks in continuous casting are dependent for the ir formation on both technological parameters of casting and physical- chemistry properties of steels. In this study, we focussed our attenti on upon the relations between solidification and hot cracking phenomen a. Solidification development in series of carbon manganese steels was studied using quenched metal samples which had undergone solidificati on in a positive thermal gradient. Three series of carbon manganese st eels with respectively variable sulphur, phosphorus and carbon content s were studied. Sulphur and phosphorus segregations in the liquid are important at the end of solidification. For instance, the ratio of max imum concentration of phosphorus to minimum concentration is around 16 . This value is very large compared with the one's measured for mangan ese (1.9) and silicon (1.4). Hot cracking interval, deduced from solid ification study, increase with carbon, sulphur and phosphorus contents . Fracture criteria for different steel grades were determined by bend ing test performed on ingots (300 kg) during solidification. Sulphur c ontent appears to have a stronger effect on cracking threshold drop. E xploitation of a model concerning thermomechanical behaviour of produc t during solidification permitted to obtain billet stress charts. The obtained results verify that segregated internal cracks in billets are found preferably near the angles. The presence of sulphur and phospho rus enhances the width of the brittleness region inside the mushy zone .