| In order to meet the needs of automobile lightweight,improving corrosion as well as reducing production costs,at present,dual phase(DP)steel is being developed in the direction of high strength,zinc plating and fine crystallization.Si plays a very important role in dual phase steel,however,Si is easy to be oxidized during annealing,which deteriorates the surface properties and coating ability.Considering that Al has the similar function to Si in DP steel,according to the composition design idea of "the substitution of Al for Si",coupled with the effect of Nb of grain refinement and precipitation strengthening,a new composition of low silicon Al-containing and niobium micro-alloying was designed based on an industrial DP780.Cold rolling continuous annealing process,hot-dip galvanizing continuous annealing process and the selective oxidation behavior of the tested steel were studied systematically.The main works are present as follows:(1)The composition design and basic theory research of tested steel were studied.Firstly,according to the composition design idea of "the substitution of Al for Si”,on the basis of industrial DP780 composition,Al was added to replace Si to retard the selective oxidalization of Si and Nb was added as well to obtain precipitation strengthening and fine-grain strengthening to make up the loss of strength.Fully considered the balance of various elements and combined with Thermo-Calc software,a new composition of low silicon Al-containing and niobium micro-alloying was designed,which was composited of 0.11C-1.83Mn-0.062Si0.37Cr-0.17Mo-0.04Nb.Then,the continuous cooling transformation behavior of tested steel was studied,and the static CCT curve of tested steel was plotted.The formation of austenite in the critical region was studied.The kinetics curves of austenite formation and kinetics equation of austenite isothermal transformation were obtained,providing guidance for the development of subsequent annealing parameters.(2)The continuous annealing process of cold-rolled dual phase steel was simulated on the multi-function continuous annealing testing machine.The interactional relationship and control principle among annealing parameters,microstructure and properties were given.Firstly,the change rules of morphology and volume fraction of martensite under different annealing temperature,slow cooling temperature,over-aging temperature and over-aging time were explored.Then,the relationship among annealing parameters,miscrostructure and properties was established,and the plan of microstructure-properties control was proposed.Finally,the optimum annealing parameters of the tested steel were obtained,i.e,the annealing temperature was 780?,the slow cooling temperature was 640?,the over-aging temperature was 300?,and the over-aging time was 300s.The optimized mechanical property could be obtained at the optimum annealing parameters.The tensile strength,elongation,and product of strong and elongation reached 986MPa,18.68%and 18418 MPa.%,respectively.(3)The continuous annealing processes of hot-dip galvanizing DP steel were investigated,including initial microstructure,heating rate and annealing temperature.A low silicon Al-containing high strength dual phase steel was developed,which was suitable for galvanizing.With the initial microstructure changed from ferrite+pearlite(F+P)to ferrite+martensite(F+M)and M,uniform and refined microstructure was obtained after annealing,and the product of strong and elongation was increased from 15090MPa·%to 18024MPa·%.With the increasing of heating rate,the grain size was obviously refined,and Martensite volume fraction increased with obvious banded structure.The comprehensive mechanical properties were optimal when the heating rate was 100?/s.The tensile strength,elongation,and product of strong and elongation were 980MPa,19.20%and 18816MPa-%,respectively.With the increasing of annealing temperature,the volume fraction of austenite increased,meanwhile the permeability decreased.When the annealing temperature reached 810?,some bainite appeard.The comprehensive mechanical properties were optimal when the annealing temperature was 780 ?,the tensile strength,elongation,and product of strong and elongation were 992MPa,18.24%and 18094MPa·%,respectively.(4)Based on the thermodynamic calculation,numerical calculation of Wagner model and simulated annealing experiments,the selective oxidation behavior of tested steel was systematically studied.Firstly,the thermodynamic calculation of selective oxidation of the tested steel was carried out by using Thermo-Calc software,and the influences of different annealing temperature,hydrogen ratio and dew point on the selective oxidation were obtained theoretically.With the increasing of annealing temperature and hydrogen proportion,and the decreasing of dew point,the selective oxides of alloying elements in tested steel decreased.The dew point was the most affected,followed by the annealing temperature and hydrogen ratio.Then,the oxidation type of alloying elements and the critical molar concentrations of different alloying elements under different annealing conditions were obtained by using Wagner model.The critical molar concentration of Al is much larger than that of Mn and Si,so Al was more prone to internal oxidation than Mn and Si.The oxidation mode of alloying elements was changed from external oxidation to internal oxidation by increasing the dew point.The change occurred at different dew points of different alloying elements,which was mainly related to annealing condition,alloying element type and content.Last,simulation experiments were carried out under different dew point,which had the greatest effect on the selective oxidation in the theoretical calculation.The oxides were less both in low dew point and high dew point.It was concluded that the oxygen potential is the root cause of selective oxidation.The transition from external oxidation to internal oxidation occurred when the dew point increased,which was basically the same as the calculation results of Wagner model. |
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