Microstructure And Properties Control Of 1180 MPa Low-density δ-QP Steels

Lightweight,electric,and intelligent are the three main directions of the development of automotive technology.On the premise of ensuring automobile safety,the lightweight of automobile materials can be realized by using high strength materials to reduce the thickness of materials,and using low-density materials to reduce the weight of components.Low-density high strength steel has become one of the most important directions of automobile steel development in the world.It is worth emphasizing that low-density δ-TRIP steel is listed as one of the vital technical directions of "Made in China 2025".The density of high strength δ-TRIP steel with elongation over 40%can be reduced by 5-8%due to the addition of 3-5 wt%Al element,while the addition of a large number of Al makes the δ-ferrite with meager strength and large grain size,and cannot be eliminated by heat treatment.Therefore,it is difficult to improve the strength of δ-TRIP steel above 980MPa,which has become the bottleneck of the development of this kind of steel.At the same time,the elongation of 1180MPa grade Q&P steel is about 15%,and it has been applied in industry.Compared with δ-TRIP steel,although its strength is much improved,its elongation needs to be further improved to meet the forming requirements of complex parts.Moreover,the process of rapid quenching to 200-300℃ and reheating is difficult to control in commercial production,resulting in a low yield of this kind of steel.The purpose of this study is to develop 1180MPa low-density δ-QP steel with high elongation by combining the advantages of the above two types of steels.At the same time,the heat treatment temperature is changed,and the quenching temperature is reduced to ambient temperature to solve the problem,which is difficult to control the industrial production temperature of traditional Q&P steel.This paper focuses on the alloy composition design of high Al content and low-density δ-QP steel,and makes an in-depth study on its hot rolling process,heat treatment,micro-alloy content,microstructure evolution,strength and toughness mechanism,etc:(1)The process of quenching and partitioning at ambient temperature is realized by intelligent alloy composition design.The critical austenite content and stability are controlled by optimizing the alloy composition and adjusting the critical annealing process.A reasonable martensitic transformation temperature was designed,and the quenching temperature of theδ-QP steel was adjusted to ambient temperature.(2)The δ-ferrite grain is refined.The banded δ-ferrite structure will worsen the mechanical properties of this material.By observing the microstructure evolution of the material in the cooling process,the formation of band ferrite is due to joint action of ferritr phase transformation,growth in Al segregation band and rolling deformation.Using higher finishing rolling temperature and lower reduction can effectively refine the grain of banded ferrite and improve the mechanical properties of the material.Three kinds of experimental steels with different Nb content were designed.The banded δ-ferrite grains were refined by microalloying of Nb.(3)The interface hardening mechanism between ferrite and M&A islands increases the yield strength of δ-QP steel.By reasonably designing the annealing temperature in the critical temperature,the volume expansion effect of critical austenite transformation occurs during quenching,which leads to the formation of high-density dislocations at the phase interface of ferrite.In the subsequent T&P process,the supersaturated carbon in martensite diffuses into the high-density dislocation area at the ferrite interface,which pins the dislocation,improves the ferrite phase strength,and finally leads to the increase of the yield strength of δ-QP steel.(4)High content of retained austenite improves the elongation of δ-QP steel.By adjusting the content and stability of critical austenite,the material retains more austenite after quenching by using lower critical annealing temperature.The TRIP effect occurs in retained austenite during deformation to improve the toughness,and the elongation of 1180MPa steel is more than 20%.