Effect Of Annealing Process On The Surface Structure And Galvanizability Of Automobile High Strength Steel

Medium manganese steel and Q&P steel were expected to be ideal steels for lightweight of automobile.However,the alloy composition in the matrix was relatively high,and the formation of alloy oxide would adversely affect galvanizability during the continuous hot-dip galvanizing process.The effect of temperature,dew point,the content of hydrogen on the surface structure and galvanizability of the experimental steel were studied by IWATANI SURTEC galvanizing simulator.Moreover,the effect of pre-oxidation process was also investigated.The main research results were as follows:?1?For the medium Mn steel,with the increase of temperature,the galvanizability of the experimental steel was decreased during the annealing process;when the dew point increased from-50?to-10?,the galvanizability of the experimental steel was significantly deteriorated;when the dew point reached 10?,the galvanizability could be improved.The higher the content of hydrogen,the better of galvanizability at the same of annealing temperature and dew point.For the Q&P steel,the change of annealing temperature had little effect on the galvanizability,and the galvanizability could be improved by the increase of dew point.?2?The effect of annealing process condition on the formation of surface oxide could be comprehensively evaluated as the change of oxygen potential pressure.For the medium Mn steel,with the increase of oxygen partial pressure,the amount of surface oxide would increase,which hindered the continuity of the inhibition layer and damaged the galvanizability.Combined with the bending test,it was shown that in the process of hot-dip galvanizing of medium manganese steel,when the annealing temperature was lower,its galvanizability and mechanical properties were better,while when the oxygen partial pressure was lower,the galvanizability would be better.Therefore,when the annealing temperature was 650?and the dew point was-50?,the zinc layer had the best quality.?3?The selective oxidation behavior of alloy elements in Q&P experimental steel was studied by thermodynamic calculation.It was shown that at the condition of10^-24?10^-19atm oxygen partial pressure,the surface oxides of steel plate were mainly composed of Mn₂SiO₄ and SiO₂.SiO₂ was difficult to be reduced by Al in the zinc solution,which reduced the wettability of the zinc solution on the surface of the steel plate and damaged the galvanizability;When the oxygen partial pressure increased to10^-19 atm,SiO₂ was not a thermodynamic stable phase,so at the condition of 870?annealing temperature and 30?dew point,Q&P steel had good galvanizability.?4?The alloy elements in the experimental steel after pre-oxidation process could be enriched to the surface by adopting high dew point and increasing oxygen partial pressure in the heating stage,thus forming a thin oxide layer.Then the reduced iron layer precipitated on the surface of the Q&P experimental steel by adopting low dew point at annealing process and its galvanizability would be improved.However,for medium Mn experimental steel,due to the small amount of iron oxide formation,it was impossible to form a complete reduced iron layer.The area of SiO₂ in the oxide layer was more likely to crack under loading,and the crack was parallel to the surface,which was easy to cause the surface failure.