Mechanism Of Microstructural Evolution During Of TMCP And Mechanical Properties Of V-B Microalloyed Steel

With the rapid development of modern industry,not only the automotive steel production in the steel industry has been increasing,but also the requirements for the automotive steel have been increasing.From the requirements of people on energy consumption,environmental protection and safety,there are urgent needs of improving of the performance,extending of service life and reduce of production costs of automotive steel.It becomes not only the important way for energy conservation,resource conservation,environmental protection and lightweight of automobile,but also a new trends of development in the automotive steel to realize high-strengthize of the steel.The purpose of this paper is to develop low-cost high-performance automotive steel by combining the V-B composite micro-alloying with TMCP,to investigate the change of microstructure in the TMCP process and to determine the optimum TMCP process,according to the research project of Panzhihua Iron and Steel Group Co.Ltd.By the thermal simulation experiments,the austenite high temperature deformation behavior of V-B microalloyed steels,the bainite transformation behavior in the cooling process after rolling and the isothermal precipitation behavior of the vanadium carbonitride in the bainitic ferrite region were investigated.And based on these results,the hot rolling experiments were carried out and the effects of the rolling process and cooling process on the microstructure and performance of the V-B microalloyed high-strength automobile steel were invesgated.Major researches and innovative achievements in this paper are as follows:(1)Using thermal simulation technology and the microstructure observations,the austenitic transformation behavior in the continuous cooling process was studied,the effects of boron and vanadium on the phase transformation were analyzed and the static CCT and the dynamic CCT curves of the tested steels under continuous cooling conditions were determined.The effects of boron on the bainite transformation related to vanadium content in tested steels.When vanadium content was less than 0.086%,the effect of boron on the bainitic transformation was obvious.And under the cooling rate of 0.5?/s,a large amount of bainite was formed in tested steel.When vanadium content is 0.175%,the inhibition effect of B on ferrite transformation was disappeared and a large amount of ferrite was formed.When vanadium content was less than 0.086%,the effect of vanadium content on bainitic transformation behavior was not obvious and the full bainitic microstructure was also obtained under the cooling rate of 2?/s.When vanadium content was 0.175%,because coarse particle of vanadium were precipitated in the austenite and these act as additional nucleation sites for ferrite.So ferrite transformation was promoted and bainite transformation was suppressed.Under the deformation and non-deformation condition,the hardness of V-B microalloyed steel was higher than that non-boron vanadium-microalloyed steel after the phase transformation.(2)Using single and double pass compression experiment,austenitic high temperature deformation behavior was studied and the influence of B,V and deformation parameter on dynamic recrystallization behavior.The dynamic and static recrystallization activation energy of tested steel was calculated and coefficients of' the deformation resistance model were regressed.The results show that because solid solution boron and borides oppositely affect dynamic recrystallization behavior,dynamic recrystallization activation energy and peak stress were decreased when adding boron in steel,thus dynamic recrystallization activation energy and peak stress were decreased,and boron promote dynamic recrystallization of tested steel to some extent.With the increase of vanadium content,dynamic recrystallization activation energy and peak stress were increased and dynamic recrystallization of tested steel was inhibited to some extent.(3)Using the thermodynamic analysis on formation of boron nitride in austenite,it was proposed for improving the effectiveness of boron on bainite transformation.The influence of boron on the stability of austenite was related to the solid solubility of boron in the steel,it should be added Ti which of binding force with N is stronger than boron to improve solid solubility of boron in boron steel.Vanadium content in tested steel will not affect solid solubility of boron,only related to added titanium content in the steel and the content was 3.4-3.8 times of nitrogen content in steel.(4)By thermodynamic analysis on carbonitride precipitation in the austenitic cooling transformation process and thermal simulation experiments of isothermal precipitation,the precipitation behavior of vanadium carbonitride was studied in the bainitic ferrite area.The main of nucleation mechanism of V(C,N)was nucleation on the dislocation line in the range of 350?700?.The relationship between nucleation rate and precipitation temperature showed reverse C-curves,and the maximum nucleation temperature is roughly between 550?600?.The relationship between relative precipitation start time and end time showed C-curves,and the fastest precipitate temperature was roughly between 600?675?.Bainite or bainite+ferrite microstructure was obtained in 500?600? isothermal process,the precipitation volume fraction was statistically calculated under different test temperature and holding time.The precipitation amounts were largest under the condition of the same holding time and 600?.With the increase of vanadium content in steel,quantity of the fine precipitates was increased.(5)Using thermal simulation experiment,effects of the hot rolling parameters and vanadium content in tested steels on the microstructure and performance were studied.Based on these,the test rolling was carried out and optimal TMCP process system was determined.With the decrease of coiling temperature,not only bainite content was increased and phase transformation strengthening was increased,but also the amount of vanadium precipitates was decreased and was not conducive to enhance effect of precipitation strengthening of vanadium.Best coiling temperature with vanadium content was different.Compared to the coiling temperature,finish rolling temperature and cooling rate had little effect on the microstructure and performance of tested steel.When vanadium content of tested steel was 0.086%,using the phase transformation strengthening and precipitation strengthening,it could be obtained yield strength 605MPa,tensile strength 723MPa and elongation 18.5%.The optimal TMCP process was as follows:Finishing rolling temperature is 840?860 ?,coiling temperature is 540?560?,cooling rate is 25 to 39 ?/s.(6)Through experiment of cold bending property and low temperature impact property,confirmed the boron vanadium micro-alloy steel had excellent bending performance and good impact resistance.