| Popis: |
The hot tensile test has been used for a variety of steels, to investigate the influence of such factors as inclusions, precipitation, phase transformation and grain size on hot ductility. Tests were performed in the temperature range 700-1000 °C and at a strain .4 _2 1 rates of (10 —10 S ), generally to simulate the conditions experienced during the straightening operation in the continuous casting process in which transverse cracks propagate. Two zones of reduced ductility can be identified. The Y-a transformation zone which has been shown to produce a significant ductility trough due to strain concentration in the softer ferrite films surrounding the V grains, cating voiding around he MnS inclusions which link to give intergranular failure, and factors which alter the A3 temperature, such as carbon content, have been shown to produce a change in the temperature at which the ductility trough occurs. The second zone of reduced hot ductility was observed in the austenite region, due to the retardation of dynamic recrystallisation associated with the presence of fine carbides, nitrides and/or inclusions precipitates at the austenite grain boundaries, which allows intergranular cracks to develop. The depth and width of this ductility trough is primarily dependent on the size and amount of precipitates present. The influence of MnS inclusions in reducing hot ductility has been noted above and below the Ae3 temperature. Above the Ae3, inclusions act in a similar way to fine precipitation preventing dynamic recrystallisation and below the Ae3, they offer more sites for micro-voiding to occur. Although grain size refinement improves ductility, it does so as long as similarity in precipitate volume fraction and size exist. Improved hot ductility has been achieved by adding Ca to the steel which reduces the amount of l9nS inclusions precipitated during cooling from solution temperature at the austenite grain boundaries. |
