Fundamental Investigations And Design In High Strength And High Ductility Low Density Lightweight Steels

With rapid developments of industry,steels are also facing challenges such as safety,environment,resource,energy,cost and so on.In this circumstance,lightweight becomes an important development trend in the future development of industry.Lightening means that the vehicle mass should be decreased on the premise of security restriction in order to reduce fuel consumptions and low exhaust emissions.Since steels account for about 60 percent of the total vehicle's body weight,it is an effective way to achieve lightweight using thinner steel sheet with higher strength instead of common steel.A new class steel has been developed by adding Al to the medium manganese steels,and through optimization of the alloy composition design and heat treatment processes,obtaining low density,optimal high strength and high ductility combination.With a great potential and a promising prospect,high strength and high ductility low density steels may be widely used in automotive structures.Both the conventional high strength steels(HSS)and the 1st generation advanced high strength steels(AHSS)cannot meet the entire requirements of mechanical performance for automobile steel in the future,and the cost of the 2nd generation AHSS,such as austenitic stainless steel and twinning induced plasticity(TWIP)steel,is quite higher mainly because of the high level of alloy elements.However,the research on high strength and high ductility low density steels for automobile aopplications has just started,and there are many theoretical problems to be solved.On the basis of above background and reasons,the mechanical properties and the microstructural evolution and mechanical properties for high strength and high ductility low density medium Mn steels during annealing heat treatments were investigated.Fe-Mn-Al-C-Nb and Fe-Mn-Al-C-Ni-Nb Q&P steel and medium Mn steel were designed.To decode the experimental results for high strength and high ductility low density steels can provide us the understanding of the structure-propertie relationship of high strength and high ductility low density steels,and also provide some methods to optimize the microstructures and improve mechanical properties for high strength and high ductility low density steels.The chief original work and results are as follows:(1)Different heat treatments were carried out for two typical mediun manganese Fe-Mn-Al-C low density steels,microstructure and mechanical properties of these steels during heat treatment processes were studied.Based on the concept of Q-T&P,controlling the Mf temperature below room temperature,sufficient retained austenite was received at room temperature.The former alloy contains 20?21vol.%retained austenite with mainly film-like morphology,while the later consists of 3.5?9.6 vol.%retained austenite with blocky shape.And the content of retained austenite changed significantly with the increase of partitioning temperature and partitioning time.The retained austenite content of the first alloy increases with the temperature and the time of partitioning,and the later is opposite.With the increase of carbon and manganese content,the microstructure of alloy steel changed,and the martensite type changed from dislocation martensite to twin martensite.(2)The volume of retained austenite is direct proportion to product of tensile strength and elongation.We gained plenty of retained austenite benefiting from different heat treating.However,the carbon content in austenite decreases gradually,once the carbide is formed.Based on that,we designed the composition and routes of heating treatment.The steel was annealed at high temperature to partition more Mn and C to austenite and then it was annealing above the carbide dissolved temperature.Combined the effect of Mn and C,much more austenite was retained at room temperature.(3)The as-hot-rolled steels annealed in the range of 750?800? and had similar or better mechanical properties than those of low-alloy and medium Mn TRIP steels.urthermore,less cold-rolling work or annealing time was required in the present work.1 alloy teel exhibited combinations of UTS of 875?1000MPa and TE of 46?61%.2 alloy steel demonstrated a high strength range of 875?1100MPa and a large elongation range of 14?52%.(4)The different annealing process on the microstructure evolution and mechanical properties was studied.Annealing at low temperature 650?,carbide was still exist and got higher strength and lower plasticity.Annealing at meidium temperature 700?,carbide was mostly dissolved,and the experimental steel has a high elongation and good strength-ductility balance as 53152MPa%.Annealing at 800?,the strength and toughness of steel are worsely matched,because of grain roughening.(5)Through the study of the strain hardening behavior of samples annealed from different temperature,it was found that the deformation of ferrite initially suppressed the TRIP effect,thus,TRIP effect occurred at a higher strain,leading to superior ductility.(6)Discontinuous TRIP effect was first observed and proposed by studing the strain hardening behavior of as-hot-rolled 1 alloy.There are two important factors for the discontinuous TRIP effect.First,the martensitic transformation leads to a volume expansion,which leads to plastic deformation of the ?-ferrite and the intercritical ferrite(IF)laths,and consequently introduces relaxation and transfer of localized stress.Second,austenite with different degree of stability is responsible for the TRIP effect that occurs discontinuously only when a critical stress is attained.Austenite with different degree of stability result form its different morphology and composition.