Study On Microstructures And Properties Of Hot Rolled Thin-Gauge DP590 Dual Phase Steel

Dual phase steel is widely used in automobile industry due to its excellent mechanical properties.At present,cold rolled dual phase steel is the main type of thin-gauge dual phase steel used in automobile industry,and few hot rolled thin-gauge dual phase steels are used.Compared with cold rolled thin-gauge dual phase steel,the difficulty of hot rolling producing thin-gauge dual phase steel is control of strip shape,surface quality and mechanical properties.In this thesis,the low cost C-Si-Mn-Cr DP590 dual phase steel is taken as the research object,and the development of hot rolled thin-gauge DP590 dual phase steel based on ESP line is taken as the target.The effects of austenite deformation behavior,austenite continuous cooling transformation behavior and cooling process parameters on the microstructure and properties of the experimental steel were studied.The hot rolled thin-gauge DP590 dual phase steel was developed by hot rolling experiment in laboratory.The main work and results are as follows:(1)The dynamic recrystallization behavior and deformation resistance of experimental steel were studied by thermal simulation experiments.The results showed that the type of true stress-true strain curves for the experimental steel were dynamic recrystallization,and the dynamic recrystallization activation energy was 237.442kJ/mol,when the strain rates were 0.05s-1and 0.1s-1,and the deformation temperatures were 900?1050?.The dynamic recrystallization model of experimental steel was established and the dynamic recrystallization region diagram was drawn.When the strain rates were 3?20s-1 and the deformation temperatures were 850?1000?,the type of the true stress-true strain curves for experimental steel were dynamic recovery,the deformation resistance model of experimental steel was established,and the accuracy of the model was high.(2)The effects of deformation temperature and deformation process on the microstructures of the experimental steel were studied by thermal simulation experiments.The results showed that austenite grain and ferrite grain formed by subsequent cooling were refined with the decrease of deformation temperatures.Compared conventional deformation process and ESP deformation process,before the second deformation,austenite grain size of ESP deformation process was larger than that of conventional deformation process,the variation of ferrite grain formed by subsequent cooling was consistent with that of austenite grain,and deformation resistance of ESP deformation process was lower than that of conventional deformation process during the second deformation.(3)The dynamic CCT curves of conventional deformation process and ESP deformation process for the experimental steel was drawn through the thermal simulation experiments.The results showed that,with the increase of the cooling rates,the grain size and volume fraction of the ferrite were reduced,the phase transformation of ESP deformation process was slow,the martensite phase region was appeared in ESP deformation process,and at the same cooling rates,ferrite grain size of ESP deformation process was larger than that of conventional deformation process,and ferrite volume fraction of ESP deformation process was less than that of conventional deformation process.(4)The effect of cooling process parameters on the microstructures of experimental steel under the condition of ESP deformation process was studied by thermal simulation experiments.The results showed that grain size and volume fraction of ferrite were decreased with the increase of cooling rates and martensite changed from island to block.When the total cooling time was constant,the holding time had a great effect on the microstructure of the experimental steel,ferrite grain size and volume fraction of the experimental steel were increased with the increase of the holding time,and martensite changed from block to island.With the decrease of holding temperature,ferrite grain size was increased first and then remained unchanged,and ferrite volume fraction was gradually increased.When the holding temperatures were low,pearlite and bainite appeared in the microstructures of experimental steel.With the increase of holding time,both the grain size and volume fraction of ferrite were increased,and the martensite changed from block to island,and grain size and volume fraction of ferrite were gradually decreased with increase of last stage cooling rates.(5)Hot rolling experiments of simulating conventional rolling process and simulating ESP rolling process were carried out in laboratory.The cooling mode was air+quenching two stage cooling mode.The results showed that with the decrease of quenching temperatures,ferrite volume fraction was increased and martensite volume fraction was decreased,martensite changed from large block to small island,and the yield strength and tensile strength of experiment steel were decreased,and when quenching temperature was low,pearlite appeared.At the same quenching temperatures,compared with conventional rolling process,ferrite grain size was larger,ferrite volume fraction was fewer,martensite volume fraction was more,strength and elongation were higher in ESP rolling process.There are obvious martensite banded microstructures in experiment steel of conventional rolling process.Pearlite appeared in experiment steel of ESP rolling process when quenched at 560?,it is quenched at 600?,pearlite appeared in experiment steel of conventional rolling process.