Study Of Structure Ultrafinement Of Micro-alloyed Steel Based On Deformation Enhanced Transformation

Deformation enhanced ferrite transformation (DEFT) and intragranular nucleation mechanism of alloying elements were used to study the microstructure evolution law of low carbon microalloyed steel by single-pass and multi-pass thermal simulation experiments. The grain refining mechanism of DEFT and effect of nano precipitates, deformation parameter, chemical composition on ultra-fine grained structure were systematically discussed to explore a structure ultrafining process of micro-alloyed steel which adapts rolling capacity of recent hot tandem mills in our country.The research shows that driving force of transformation was increased substantially by combining action of over-cooling temperature and deformation, as a result, high nucleation rate was obtained. The increment of transformation product depends on continuously nucleation. The ultra-fine structure of microalloyed steel was obtained by the combining effect of DEFT, in situ ferrite dynamic recrystallization, intragranular nucleation and secondary nucleation of precipitated phase, acicular ferrite, fragment of austenite grain and nanoparticle's pinning effect on substructure. As a result of the pinning effect on grain boundary, sub-boundary and dislocation cell wall, it's hard for deformation enhanced transformation ferrite grain to coarsen but occur in situ dynamic recrystallization even if continuously proceed deformation on it. Consequently the stability of ultrafine structure of micro-alloyed steel is ensured.The research results also shows that MnS nano-particle, which is traditionally a harmful inclusion, can provide nucleating center for V(C,N) and intragranular ferrite (IGF), thus refine ferrite grain remarkably. The relationship between the size of nano-particle and ferrite grain is remarkably linear: The size of nanophase is less than 30nm when the volume percent of ultra fine ferrite is higher than 80% and grain size of ferrite is less than 4μm. The effects of V, Nb on DEFT behave in two aspects: V, Nb solutionized in steel confine DEFT; on the other hand, V(C,N) and Nb(C,N) precipitated phase from action of deformation process provide more nucleating center for IGF, meanwhile, coherency relationship between V(C,N), Nb(C,N) and matrix disappears gradually, then new hetero-interface appears. These interface become nucleation site for new IGF phase which generated in subsequent deformation and cooling, thus promote nucleation of IGF. So the optimum refine grain effect was achieved by taking advantages of V, Nb and avoiding disadvantages of them.Based on the above research, a kind of micro-alloyed steel plate with ultra-fine grain structure and tensile strength about 750MPa was developed by using exiting rolling equipment in our country. The structure parameters of ultra-fine grain steel were predicted by using genetic neural networks. Compared with the experimental value, it turned out that prediction accuracy of volume percent of ultra fine ferrite is above 91%, and prediction accuracy of the grain size of ultra-fine grain ferrite is above 90%.