Strength-Toughness Optimization And Effect Of Niobium Microalloying Of DP1180 Steel For Automotive Applicaitons

To meet the requirements of green development and energy saving,the demand for automotive steel which combines excellent strength and elongation increases.Dual phase steel(DP)plays the leading role in automotive steels due to its simple composition,low cost and excellent performance,but the strength of traditional DP steel is relatively low.Aiming at this problem,based on Fe-0.18C-2.6Mn-2Si(wt.%)system,high strength dual-phase steels with tensile strength(?1180MPa)and elongation(?16%)were studied by Nb microalloying.In view of this background,the following aspects were studied.1)By the addition of element Nb,steels with different Nb contents were obtained:Al,A2(0.027%)and A3(0.061%),then,the phase transition points of the three steels were measured by thermal expansion instrument.The results indicates with the increase of Nb content,the temperature of Ac1 changes slightly,but the temperature of Ac3 increases obviously;when Nb content is 0.027%and 0.061%,the temperature of Ac3 increased by 8?and 36? respectively.While the austenite hardenability is increased by Nb addition,which makes the Ms point increases.2)Phase content of austenite and ferrite at 730?,750?,780? and 820? was determined accurately by means of thermal dilatometer experiment and lever theorem.After continuous annealing,microstructure consists of ferrite and martensite,the product of strength and plasticity(PSE)performs poor.Therefore,aging is introduced in the later stage of continuous annealing process in order to realize the aging strengthening of Nb element;and promote the transformation behavior of austenite to bainite during aging process,aims at reduce martensite phase content.Finally the multi-structure contains ferrite,martensite,bainite and retained austenite were obtained.The results shows that the material after continuous annealing at 820? and extra aging performs well mechanical properties.Sample aging at 300? with strength of 1310 MPa and elongation of 16%,aging at 350? with lower strength of 1050MPa and maximum PSE of 24.98GPa%,the strength,elongation and PSE of aging steel at 400? are 1150MPa,21%and 24.15GPa%respectively.3)Different initial microstructures of samples were obtained by adjusting the ways of cooling(such as quenching or coiling)after hot-rolling and different technics of heat treatment(HR-Q?HR-Q&T-CR?HR-Coil-CR).Hereditary phenomena was discussed after continuous annealing process based on different initial microstructure.The results indicate that after HR-Coil-CR heat treatment,zonal ferrite can be still observed in microstructure and the pearlite was replaced by martensite causing the matrix was less uniformity and poor mechanical properties.Compared with HR-Coil-CR,HR-Q&T-CR heating treatment homogenizes the structure and martensite was dispersed at the ferrite grain boundary.Such combination of uniform ferrite and martensite makes a lower tensile strength but a higher ductility(compared with a treatment method).Increasing the Continuous annealing temperature gradually eliminate the difference of microstructure after different heat treatment.Especially,when continuous annealing temperature reaches 820?,both microstructure and mechanical properties of material after the above two heat treatment are less distinguishing and structure inheritance basically disappeared.4)Research about the microalloying effect of Nb element,it is found that grain refinement can be achieved by adding Nb to hot-rolled and cold-rolled samples.For the cold rolled sample,the precipitation of Nb-rich phase which caused by tempering at 600? before cold rolling,can't be dissolved during continuous annealing,which makes the elongation of DP steel is significantly deteriorated,with strength slightly increasing.For hot-rolled steel,at relatively low aging temperature,the addition of Nb delays bainite transformation during isothermal process and improves strength on the premise of guaranteeing material elongation.After continuous annealing at 820?,the PSE of micro-Nb element material aged at 350? is the highest,which is about 25.2GPa%.The average strength and elongation of the material are 1220MPa and 18%respectively,which meet the property requirements.