| Hot-rolled dual-phase steel has been widely used in automotive field for its good comprehensive properties.However,the cost of Cr-Mo dual-phase steel is high in traditional manufacturing process.The problem of parameters controlling,unstable properties and bad surface quailty exists in Si-Mn dual-phase steel.Producing stable quaility hot rolled dual phase steel in low cost has became a main direction of research.This essay aims to develop 650MPa and 700MPa level hot-rolled dual-phase steel with primitive C-Mn-Nb and C-Mn-Ti chemical design respectively and reseach its hole expanding property in further.The main works and results list as follows:(1)Dynamic recrestallizaton behavior of experimental steel was researched through single pass reduaction experiments.It became more diffcult to recrestallize under lower temperature and faster deformation rate.The recrestallizaton activation energy of C-Mn-Nb steel and C-Mn-Ti steel were 411.882kJ/mol and 438.451kJ/mol respective.C-Mn-Ti steel possesses a larger recrestallizaton rolling window and was harder to recrestallize.(2)The curves of static and dynamic continuous cooling transformation were established by the application of thermal dilation method and metallographic analysis.The results showed,C-Mn-Nb steel has no typical pearlite transformation area,C-Mn-Ti steel has larger ferrite transformation area compared with C-Mn-Nb steel.Effects of first stage cooling rate and holding temperature on mirostructure and properties of experiment steels are researched through the thermal simulation experiment.The results showed that the size of ferrite became smaller,the volume of mastensite and the hardness of the samle increased under the increasing cooling rate.When the holding temperature remained 780 ? the microstructure consisted of mastensite+minority ferrite,when it came down to 620 ?,the microstructure appeared acicular ferrite+bainte.(3)Effects of finishing delivery temperature,ultra-fast cooling export temperature and air cooling time on mirostructure and properties of experimental steels was researched through the hot rolled experiment.Additonally,the model of tensile strength of dual phase steel was established and perfected.Prediction of tensile strength of dual phase steel was calculated by the model and Thermo-Calc software.The results matched the actrual value well.The properties of C-Mn-Nb steel can reached 670MPa in tensile strength,0.63 in yield radio and 25.5%in elongation under the parameters settings at 855? in delivery temperature,740? in ultra-fast cooling export temperature and 14s in air cooling time.When the finishing delivery temperature and ultra-fast cooling export temperature remained at 880?,760 ?repectively,adding 17s cooling time,the properties of the experimental steels can reach 730MPa in tensile strength,0.69 in yield radio and 22%in elongation.Enhancing finishing delivery temperature and prolonging air cooling time were better for the precipitation behavior of Nb(C,N),TiC.(4)The influence of the first stage cooling rate on the properties and microstructure in dual phase steel was researched through the hot rolled experiment and 10mm thick dual phase steel was produced.The results showed the properties of C-Mn-Nb steel can reached 710MPa in tensile strength,0.66 in yield radio and 23%in elongation under the parameters settings at 890? in delivery temperature,780? in ultra-fast cooling export temperature and 13s in air cooling time.(5)Microcracks in dual phase steel growed by gathering and combination.The direction of the propagation of the microcracks always lay vertical to the maximum principal stress and turn aroud at part place.Transcrystalline rupture and intergranular fracture exist in experiment steel.Reducing the volume of mastensite,narrowing the hardness gap between mastensite and ferrtite,preventing the band mastensite forming would be good for enhancing the hole expanding property. |
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