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

Segregated internal cracks in continuous casting are dependent for their formation on both technological parameters of casting and physical-chemistry properties of steels. In this study, we focussed our attention upon the relations between solidification and hot cracking phenomena. Solidification development in series of carbon manganese steels was studied using quenched metal samples which had undergone solidification in a positive thermal gradient. Three series of carbon manganese steels with respectively variable sulphur, phosphorus and carbon contents were studied. Sulphur and phosphorus segregations in the liquid are important at the end of solidification. For instance, the ratio of maximum concentration of phosphorus to minimum concentration is around 16. This value is very large compared with the one's measured for manganese (1.9) and silicon (1.4). Hot cracking interval, deduced from solidification study, increase with carbon, sulphur and phosphorus contents. Fracture criteria for different steel grades were determined by bending test performed on ingots (300 kg) during solidification. Sulphur content appears to have a stronger effect on cracking threshold drop. Exploitation of a model concerning thermomechanical behaviour of product 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 phosphorus enhances the width of the brittleness region inside the mushy zone.