A Study Of The Annealing And The Hot-dip Galvanizing Process Of 980MPa DP Steel For Automobile

The thinning and weight reduction of automotive panels is one of the important measures for vehicle weight reduction.To reduce the weight and weight while ensuring the safety of automobiles,high-strength steels for vehicles have become a development trend.At present,the most widely used advanced high-strength steels in automobiles are DP steels.The extensive use of DP steels greatly reduces the weight of vehicles,saves energy,and reduces emissions.Steel materials are highly corrosive and can be extended by hot dip galvanizing.Hot-dip galvanizing is currently the most widely used and most economical method for galvanizing,but due to the relatively high alloying elements in advanced high-strength steels,some alloying elements preferentially oxidize with oxygen in the annealing atmosphere during annealing.Deterioration of the wettability of the zinc liquid affects the final galvanizing effect.In this paper,two kinds of dual-phase steels with different compositions(low-carbon low-silicon steel,high carbon high silicon steel)are used as the research object.Firstly,980MPa grade DP steels for automotive use are tested through static phase change test,hot coiling process research,cold rolling annealing process optimization and continuous hot dip galvanizing.The experimental study,and the comparison of its organizational structure and mechanical properties,the following main conclusions are obtained:(1)For the low-carbon low-silicon steel,AI temperature is about 634?,A3 temperature about 815?,the cementite dissolution temperature about 660?,and martensite transformation temperature is about 434?;For the high carbon high silicon steel,A1 temperature is about 662?,The temperature of A3 about 796?,the dissolution temperature of cementite about 700?,and the martensite transformation temperature is about 391?.From the static CCT curve,AC1 temperature of low carbon low silicon steel is 656?,AC3 temperature is 774?;AC1 temperature of high carbon high silicon steel is 674?,and AC3 temperature is 789?.(2)The phase changer was used to simulate the hot-rolling process of the experimental steel and hot-rolling coiling experiments were performed.Experiments show that when the coiling temperature is 680?,the mechanical properties of the hot-rolled and cold-rolled sheets of high-carbon high-silicon steel are all optimal.When the continuous annealing temperature is 800?,the mechanical properties are good;the slow cooling termination temperatures of 650?,690?,and 730? are studied.Among them,the performance at 690? is good,the yield strength is about 1117MPa and the tensile strength is about 1381 MPa,the elongation about 8.4%;set the 480? and 500? two sets of rapid termination temperature,indicating that the rapid termination temperature is better at 480?.(3)The pre-quenching process was increased prior to annealing in the critical zone.The results show that the mechanical properties of low carbon low silicon steels such as tensile strength and elongation after the pre-quenching process are greatly improved.Observing the tissue under OM and SEM,it was found that after the pre-quenching process,the banded tissue was eliminated and the tissue distribution was more uniform.Considering that this study is mainly aimed at 980MPa grade DP steels,the final choice of annealing at 780? and salt bath over-aging time is 30s.At this time,the yield strength is about 566MPa,the tensile strength is about 1099MPa,and the elongation is about 16%.(4)High-carbon high-silicon steels were annealed and galvanized under certain dew point conditions.The surface oxides were examined by SEM and EDS.It was found that their presence will affect the wettability of the steel plate,leading to defects in the plating defects.The cross-section of the galvanized layer EPMA study shows that in the coating/substrate interface contains aluminum and iron elements,indicating that the iron and aluminum inhibition layer.The study of the morphology of the inhibition layer shows that the hot dip galvanizing effect is better when the suppression layer particles are denser and the distribution is uniform.