| As the representative of the third generation of high strength automotive steel,high strength Q&P steel has both high strength and excellent plasticity,which has attracted great attention from researchers.However,the selective oxidation of alloying elements in Q&P steel during galvanizing will reduce the wettability of the steel plate surface,thus deteriorating the platability of the steel plate,which greatly limits the application of high strength Q&P steel.In addition,the over-aging insulation period in the galvanizing process of high strength Q&P steel has a great influence on the properties of Q&P steel,these problems are urgent to be solved.Therefore,according to the characteristics of annealing process of high strength Q&P steel and the difficulties of hot dip galvanizing process,this paper studies the best continuous galvanizing Q&P process system which can not only obtain better comprehensive mechanical properties and excellent galvanizing quality,but also take into account both Q&P treatment and galvanizing process.On this basis,the influence of typical galvanizing process parameters(hydrogen content,dew point temperature and annealing parameters)on the plating ability of high strength Q&P steel was studied,and the specific influence mechanism of each parameter was clarified,and the following conclusions were drawn:When the hydrogen content increases from 5%to 25%in the annealing furnace,the oxygen partial pressure in the annealing furnace decreases while the atmosphere reducibility is significantly improved,the combined effect results in the selective oxidation trend of alloying elements on the surface of the steel plate is significantly reduced,the compactness of the inhibition layer is increased,the wettability of the steel plate surface is enhanced,and the leakage of plating is reduced.The content of Si-Mn oxide increases,the content of Si-rich oxide decreases,and the morphology of each oxide changes accordingly.The surface oxides of the steel plate under 5%H2 atmosphere show a large film Si-rich oxides,which are embedded with many ellipsoidal Si-Mn oxides;When the hydrogen content increases to 15%,the surface oxides change into spindle-rod like Si-rich oxides,and small ellipsoidal Si-Mn oxides are grown attached to the rods;In the atmosphere of 25%H2,the oxides grow in two ways:the fine ellipsoidal Si-Mn oxides grow together or attach to the large ellipsoidal Si-Mn oxides.When the dew point temperature is adjusted in the atmosphere with a certain hydrogen content,the galvanized wettability of the Q&P steel surface is best at a dew point of-10℃,.Under these circumstances,the selective oxidation behavior of Si and Mn on the surface is reduced,and the inhibition layer has the best compactness.The surface oxides show that some small ellipsoidal Si-Mn oxides are distributed discretely or grow attached to the large ellipsoidal Si-Mn oxides,and a small part of the surface oxides show the phenomenon of oxide aggregation.However,at-30℃ and 0℃ dew point,the surface oxidation is more serious,showing multiple splits,the inhibition layer has more exposed areas,the surface wettability of the steel plate is relatively poor,and the macroscopic leakage is slightly more.At the dew point temperature of-30℃,the surface oxides show a state of large film branches with Si-rich oxides and a large amount of Si-Mn oxides between the branches;Some areas on the surface of 0℃ showed branch-like Si-rich oxides,and a small amount of ellipsoidal Si-Mn oxides distributed in it.Appropriate adjustment of annealing parameters will also change the coating properties of Q&P steel.The increasing of annealing time(100s→300s)and annealing temperature(730℃→770℃)will cause the enrichment of Mn and Si elements on the surface of the steel plate to increase,which leads to a serious surface oxidation,a decreasing of the compactness on the inhibition layer.The change of the plating property caused by the extension of the annealing time is more obvious.With the extension of annealing time,the surface oxides change from the state of irregular shape oxides combined with a small amount of ellipsoidal Si-Mn oxides to the state of twig rich Si-rich oxides combined with a small amount of ellipsoidal Si-Mn oxides.However,the increasing of the annealing temperature will increase the partial pressure of oxygen in the atmosphere,speed up the nucleation rate of oxides,and the formed Si-rich oxides branches will bond in some areas.Based on the range of galvanizing parameters and the method of Q&P steel partitioning in the bainite transformation phase zone,a new type of continuous annealing hot-dip galvanizing process system for Q&P steel is proposed and the bainite transformation phase zone is studied.The effect of temperature and holding time on the properties of high-strength Q&P steels and the mechanism of action during temperature distribution.The results show that the retained austenite content decreased gradually with the extension of the partitioning time at the bainite region temperature of 440℃ and 480℃,while the carbon content in the retained austenite and elongation of the steel decreased gradually.The product of strength and elongation reaches the maximum 20.001 GPa%after partitioning at the temperature of 440 ℃for 10s.The retained austenite is mainly composed of bulk austenite in the ferrite remaining after annealing in part of the austenite zone,undecomposed bulk austenite after partitioning at the ferrite-martensite interface,and film-like austenite between martensite.Some of the retained austenite is decomposed from the same original austenite and has the same orientation.The large amount of bainite formed in the early stage of partitioning is due to the carbon diffusion of martensite into the retained austenite during the partitioning and the combined effect of carbon emission from generated bainite to untransformed austenite during partitioning,while the slow decline in carbon content of retained austenite in the later stage resulted in better stability of retained austenite,and reduced the rate of bainite transformationThe dip corrosion tests were carried out on the galvanized Q&P steel obtained under different dew point conditions.Meanwhile,the macroscopic corrosion evaluation of the zinc layer obtained by optimum continuous hot dip galvanizing annealing process for high strength Q&P steel was carried out and its corrosion resistance mechanism was revealed.The corrosion resistance of the coating of high strength Q&P steel formed at-10℃ dew point is better than that at-30℃ and 0℃ dew point.The compactness of the substrate and the coating is greater than that at-30℃ and 0℃ dew point.Meanwhile,there are few cracks at the boundary of zinc particles on the surface of the coating formed under the condition of-10 dew point.During the corrosion process,it is difficult for the corrosive liquid to flow from the gap between the zinc layer/substrate and the gap between the surface zinc structure,which avoids the zinc layer from falling off,resulting in the best corrosion resistance.The macro-corrosion loss curve of galvanized Q&P steel obtained by optimum continuous hot dip galvanizing annealing process follows a power exponential function,and the power index N is 0.754.The corrosion products of galvanized Q&P steel are very protective to the substrate,And the whole corrosion process is a deceleration process.the main corrosion products are Zn5(OH)8C12·H2O,Zn5(CO3)2(OH)6 and Zn(OH)2,the content of Zn5(OH)8Cl2·H2O gradually increases in the later corrosion period,resulting in a slow corrosion rate.The unstable corrosion product film Zn5(CO3)2(OH)6 generated in the early stage of galvanized Q&P steel is decomposed in the middle of corrosion,resulting in a slight increase in the initial corrosion current density and a decline in the middle stage.In the late stage of corrosion,most of the dense corrosion products Zn5(OH)8Cl2·H2O cover the surface of the steel plate,so that the corrosion rate drops sharply,the current density decreases and the corrosion resistance increases. |
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