Hot Ductility of Nb- and Ti-Bearing Microalloyed Steels and the Influence of Thermal History

Hot Ductility of Nb- and Ti-Bearing Microalloyed Steels and the Influence of Thermal History

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Hot Ductility of Nb- and Ti-Bearing Microalloyed Steels and the InFuence of Thermal History K.R. CARPENTER, R. DIPPENAAR, and C.R. KILLMORE The hot ductility of Nb, Ti, and Nb-Ti containing steels has been studied under direct-cast conditions. A Gleeble 3500 thermomechanical simulator was used to determine hot ductility over the temperature range 1100 ° C to 700 ° C at a low strain rate of 7.5 9 10 À 4 s À 1 . Tensile samples were cooled at two diFerent cooling rates, 100 ° C/min and 200 ° C/min, simulating, respectively, thick and thin slab casting processes. Complex thermal patterns designed to sim- ulate the cooling conditions experienced near the surface of a slab during continuous casting were carried out for the Nb-Ti steel. The Nb-Ti steel had lower ductility than both the Nb and Ti steels. Increasing the cooling rate generally deteriorated ductility. The low recovery of duc- tility at higher temperatures is explained in terms of a low strain rate and ±ne precipitation delaying the onset of dynamic recrystallization. This can promote intergranular cracking as a result of grain boundary sliding in the austenite. At lower temperatures, ductility was further reduced due to the formation of thin ferrite ±lms at the prior austenite grain boundaries. Simulating the thermal history experienced near the surface of thin (90 mm) cast slab improved ductility of the Nb-Ti steel by promoting coarser NbTi(C,N). This exposes a potential flaw in a simpli±ed hot-ductility test: a failure to accurately represent the influence of the thermo- mechanical schedule on precipitation and, hence, hot ductility. DOI: 10.1007/s11661-008-9749-1 Ó The Minerals, Metals & Materials Society and ASM International 2009 I. INTRODUCTION T HE simple hot-tensile test has been found to be useful in assessing the likelihood that steel will develop transverse surface cracks during the straightening oper- ation in the continuous casting process. [ 1 4 ] However, for such a test to have relevance in practice, it is imperative that the thermal history of the surface of a slab that is subjected to straightening be simulated as accurately as possible in the laboratory hot tensile tests. It is therefore more appropriate to use tensile specimens that have been melted in situ (direct cast structures) than specimens that have been reheated to the test temper- ature. [ 1 , 5 , 6 ] It is speci±cally important to use in-situ melting of tensile specimens to evaluate hot ductility of steels containing Ti and S. [ 5 7 ] In these steels, in-situ melting ensures complete resolution of Ti and sul±de particles and provides a segregated, coarse-grained microstructure similar to that observed in continuous casting practice. Commercially, small additions of Ti have been found to reduce the propensity to crack formation during the straightening operation in continuous casting of steel slabs.
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Hot Ductility of Nb- and Ti-Bearing Microalloyed Steels and the Influence of Thermal History

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