Hot Ductility of Nb- and Ti-Bearing Microalloyed Steels and the Influence of Thermal History
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THE simple hot-tensile test has been found to be useful in assessing the likelihood that steel will develop transverse surface cracks during the straightening operation 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 temperature.[1,5,6] It is specifically 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 sulfide 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
K.R. CARPENTER, Development Metallurgist, formerly with the School of Mechanical, Materials and Mechatronics, University of Wollongong, is with BlueScope Steel, Flat Products. Contact e-mail: [email protected] R. DIPPENAAR, Head of Postgraduate Studies, is with the School of Mechanical, Materials and Mechatronics, University of Wollongong, Wollongong NSW 2522, Australia. C.R. KILLMORE, Product Design Manager, is with BlueScope Steel, Flat Products, Port Kembla NSW 2505, Australia. Manuscript submitted May 6, 2008. Article published online January 21, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
straightening operation in continuous casting of steel slabs.[4,6–8] However, in-situ melted laboratory tensile tests generally show that Ti has little or even an adverse effect on hot ductility and it is alleged that precipitation of fine TiN or Nb-Ti(C,N) particles during laboratory tests accounts for the detrimental effect of Ti on hot ductility.[2,6,9,10] This apparent discrepancy between laboratory and commercial findings could possibly be accounted for by the fact that the laboratory specimen is subjected to a very simple cooling cycle, whereas the surface of a slab experiences a very complex cooling cycle prior to straightening and, hence, a very different precipitation sequence of carbonitrides.[2,6,9,10] Therefore, it is important to simulate as accurately as possible the thermal history of the near-surface region of continuously cast slabs in laboratory tests. The present investigation compares the hot ductility, determined following in-situ melting, of Nb, Ti, and Nb-Ti containing low alloy steels, cooled at rates of 100 C/min and 200 C/min, thus simulating the cooling conditions of conventional and thin slabs, respectively. A major difference between conventional and thin-slab casting is the significantly shorter solidification time (~22.5 minutes for 250-mm-thick slab and 2.5 minutes for 80-mm-thick slab) and faster cooling rates for the t
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