Research On The Strengthening And Toughening Mechanisms Of Ti Microalloyed Hot-rolled High-strength Steel Plates

The lightweight development of marine engineering,ships,automobiles,pipelines,bridges,high-rise buildings,and mechanical engineering equipment and etc.plays an important role in energy and resource conservation and environmental protection.It can be expected that the low-cost high-strength steel will gradually replace the low-strength steel widely used nowadays.In this paper,considering the characteristics of hot rolling process and requirement of low cost and high performance steel plate,the microstructure evolution mechanism,the precipitation rule and the strengthening and toughening mechanism in titanium microalloyed steel were systematically studied,which provides the technical guidance and theoretical basis for developing the titanium microalloyed hot-rolled high-strength steel plate.Main work in this paper is as follows:1.The composition,microstructure,strengthening and toughening design.Based on the performance requirements of low cost and high performance steel plate,a combined strengthening method including microstructure refining,precipitation strengthening,phase transformation strengthening and substructure strengthening was proposed;a multiscale microstructure morphology consist of low carbon bainite matrix,high density sub-structure,dislocation cell structure and nanoscale TiC dispersal phase was designed;a chemical composition of "low carbon,titanium microalloying,Cu-Cr-Ni corrosion resistant system" was established.2.Study on the phase transformation of Ti microalloyed steel.The continuous cooling phase transformation behavior and microstructure for austenite were studied.The results indicated that,with the increase of the cooling rate,the microstructure was gradually changed from the ferrite-dominated high temperature transformation tructures to intermediate temperature transformation structure dominated by granular,acicular and slab bainites.Meanwhile,the yield strength,microhardness and impact toughness show an increasing tendency,and good strength and toughness were obtained when the cooling rate over 10℃/s.3.Study on the recrystallization of Ti microalloyed steel.The recrystallization behavior and microstructure evolution in the process of hot-rolling,as well as the effect of strain-induced precipitation on static recrystallization behavior were studied.The results show that high temperature combined with large deformation facilitates the full dynamic recrystallization and fine austenite grains.The static recrystallization was inhibited by the strain-induced precipitation of Ti microalloyed steel,and the temperature between 950℃ and Ar3 fits for the non-recrystallized controlled rolling.4.Study on the isothermal precipitation of TiC particles in ferrite region.The precipitation,growth and coarsening of TiC in ferrite region were investigated.At the nose temperature of 600℃,the nanoscale TiC precipitates in the ferrite region maintain the Baker-Nutting orientation with the matrix,which significantly increases the steel strength.With the quantitative calculation of precipitation strengthening effect,the isothermal precipitation kinetics curves of TiC in the intermediate temperature region for Ti microalloyed steel can be drawn,thereby a precipitation kinetic equation with high fitting degree can be built.5.Study on microstructure evolution and TiC precipitation.The evolution of microstructure,dislocation and the precipitation of TiC during controlled rolling and cooling process were studied.The results show that the refinement of bainite,the uniform distribution and interaction between the high density cellular dislocation and nanoscale TiC precipitates show a core contribution on strengthening and toughening of Ti microalloyed steel.6.Study on the pilot hot-rolling experiment.Combined with thermal simulation experiment research data,two-stage controlled rolling test was carried out in the pilot plate rolling line.The corresponding relation of “process-precipitation-microstructure-properties" and the strengthening and toughening mechanism of titanium microalloyed steel were comprehensively studied.The results show that the contributions of fine grain,precipitation and dislocation substructure are about 200 MPa respectively.