| With the car ownership continues to break through a new high level,followed by the problems of road,parking,traffic safety and fuel tensions and those have become increasingly prominent.Therefore,the weight loss,energy saving,small size,safety and environmental protection and other issues catch more concern.And ultra-high strength steel with ultra-high strength and good plasticity has been favored by the automotive industry.The annealing process is very important in the production of automobile sheet,and the core technology of the annealing process is the fast cooling technology.If the cooling rate in the continuous annealing process of high strength and ultra-high strength cold rolled steel can not meet the requirements of the production process,the mechanical properties of the products will not be well.Therefore,it is important to improve the cooling rate of the ultra-fast cooling section,and to improve the performance of the products,meet the requirements of users and improve the competitiveness of the market.In this paper,the main characteristics of high strength cold rolled steel in chemical composition are low carbon and low alloy.The main alloying elements are Mn.Si,and the Nb element is added according to the requirements of production process and strength.The composition design,melting and forging of high strength steel were carried out in the laboratory,and the multi-pass control rolling and control cooling were carried out in the laboratory of the two rolls hot rolling mill.After pickling,the hot rolling plate was rolled in the laboratory of four roller cold rolling mill,and the final microstructure of the high strength steel was polygonal ferrite and martensite.Through the study of the annealing process parameters,the experimental results showed that the tensile strength and yield strength of the steel increased first and then decreased with the increase of annealing temperature.With the aging temperature increasing,the tensile strength of the steel was gradually decreased,and when the aging temperature was higher,the tensile strength of the steel decreased more quickly.The yield strength increased gradually with the increasing of aging temperature.With the increase of ultra-fast cooling rate,tensile strength and yield strength increased significantly and with the increasing of annealing temperature,yield strength increased more obviously.At the lower cooling rate,the change of elongation was not obvious with the increase of the annealing temperature,and the trend of the elongation increased first and then decreased,but the elongation decreased obviously when the cooling rated increased to 1000℃/s.In this paper,the strengthening mechanism of the experimental steel was studied using No.1 and No.2 experimental steels.The main strengthening methods of the low alloy ultra-high strength steel were solid solution strengthening,fine grain strengthening,phase transformation strengthening and dislocation strengthening.The solid solution strengthening increment of two kinds of components in the experiment was 107MPa and 123MPa respectively for No.l and No.2 experimental steels.For the No.l experimental steel,the fine grain strengthening increment corresponding to the ultra-fast cooling rate of steel were 234MPa,254MPa,265MPa and 271 MPa,respectively.For the No.2 experimental steel,the fine grain strengthening increment corresponding to the ultra-fast cooling rate of steel were 241 MPa,254MPa,271 MPa and 278MPa.respectively.For the No.2 experimental steel,the increase of the dislocation strengthening in the steel under different cooling rate was 24MPa,81 MPa,122MPa and 275MPa.Through the comprehensive analysis and comparison of the theoretical value and actual value of steel,there was a gap about 40MPa.So the strengthening mechanism in addition to increase the strength of the experimental steel alone,there would also be an interaction between the strengthening mechanism,that was,the existence of overlap and strengthening.On the aspect of deformation mechanism,1000MPa grade cold rolled high strength steel was studied.Mechanical properties of nine strain rates from 10-4S-1 to 1000s-1 were studied by means of tensile test.It could be found that the strength of 1000MPa grade cold rolled high strength steel did not change significantly under quasi-static condition,and the elongation remained unchanged.However,in the condition of middle and high strain rate,the strength and elongation increased,which showed the apparent strain rate sensitivity.The deformation characteristics of 1000MPa grade cold rolled high strength steel were studied by using Hollomon equation,the modified C-J analysis method and the dynamic factor.It could be found that there were two-stage deformation characteristics.The first stage was the ferrite plastic deformation,and the second stage was the martensite plastic deformation,while the capacity of strain hardening in stage one was stronger than that in stage two.The micro deformation mechanism of 1000MPa grade cold rolled high strength steel was studied by transmission electron microscope.After dynamic tension,the internal structure of dual phase steel has changed obviously.Ferrite appeared in a large number of dislocations,which mainly accumulated in martensite/ferrite grain boundaries.And ferrite itself was broken.There was no obvious increase in the dislocation in the martensite,but it would be extended along the tensile direction.Subsequently,associating with microstructure characterization and deformation behavior of 1000MPa grade cold rolled high strength steel,the Johnson-Cook model and the Zerilli-Armstrong model were modified accordingly,and the constitutive models of 1000MPa grade cold rolled high strength steel under different strain rates were obtained.Through the comparison of the coefficient of determination,it was found that two kinds of models had good fitting degree under quasi-static condition.But at high strain rate,the modified Z-A model could better respond to the plastic deformation characteristics of cold rolled high strength steel. |
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