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Keywords:

  • precipitation behavior;
  • precipitation strengthening;
  • transformation;
  • transmission electron microscopy (TEM);
  • ultra-high strength steel

Abstract

In order to precisely control the nano-scale (Nb,Ti)C precipitate in hot-rolled 780 MPa Nb–Ti microalloying C–Mn steel, isothermal precipitation behavior of nano-scale (Nb,Ti)C precipitate in the ultra-high strength steel was investigated by the thermal simulation experiments. The results indicated that defects of deformed supercooled austenite became the preferential nucleation sites of nano-scale (Nb,Ti)C precipitate and ferrite, so there was a competition mechanism for austenitic defects between ferritic transformation and precipitate nucleation. Bainitic transformation could effectively freeze austenitic defects, and additional defects are formed because of volume expansion in bainitic transformation process, so bainitic transformation could promote precipitate nucleation. However, precipitate was impacted by both nucleation driving force and atom diffusibility, so the peak temperature of nano-scale (Nb,Ti)C precipitate was 550°C. On the basis of the above theoretical results, hot rolling experiments results showed that when the coiling temperature was 550°C, the yield strength and tensile strength were 710 and 790 MPa, respectively, and the microstructure of hot-rolled steels was mainly bainitic ferrite, and a large number of <10 nm nano-scale (Nb,Ti)C precipitates were obtained. Precipitation strengthening contribution to reached 325 MPa.