Microstructures And Mechinacal Properties Of A Novel High Strength Medium-Mn Steels Combined With Ductility

To constructing lightweight automobiles solving the problem fuel efficiency and CO2 reduction,advanced high strength steel is still preferred for the balance of high ductility to absorb energy to loading for passenger safety and cost.But several of new lightweight materials has been used in automobiles.They are lighter but still more expensive recently than steel.To deal with the challenge of new materials,AHSS acted as the traditional main material in automobiles has to reach higher strength and higher toughness.The research of AHSS has gone through three generation,and Q&P and Medium Mn steel are typical representative of the third generation automobile steel.In the Q&P process,the quenching temperature is the most critical factor affecting the final microstructure and properties,since the final microstructure after partitioning is the same with that at the quenching temperature at which the initial martensite and austenite fractions are determined.However,uniform and accurate temperature control is not easy for large-scale or complex-shaped products in industry.In this research,a new type steel design based on medium Mn steel was designed,and grained a certain amount of retained austenite in room temperature.To solve the problem of cold rolling of this kind of steels,microstructure evolution and work hardening behaviors after different softening annealing process were researched.Annealing temperature on the microstructure evolution and properties after cold rolling was researched finally.The main original works of this paper are present as follows:(1)Based on the concept of Q-T&P,controlling the Mf temperature below room temperature,sufficient retained austenite was received at room temperature.We successfully developed an original steel.The product of strength and elongation achieved 40GPa%,whose strength reached 1890MPa and the total elongation reached 21.5%.This novel design can be manufactured by a simpler process,which applies to for all kinds of industrial production,including ultra-high strength automotive steel,armor steel and ultra-high strength steel for mechanical engineering.(2)A process solving the problem of cold rolling of this kind of steel was proposed by softening annealing.The static CCT curve was determined,and martensite transformation was the one and only transformation even on a very slow cooling rate of 0.1 K/s.We concluded it was difficult to decrease the hardness by controlling cooing rate.(3)Different heat treatments were carried out to soften the steel and the microstructure evolution was researched.It is important to design proper heat treatment to received better mechanical property.The volume of retained austenite first increased and then decreased,with the increasing of annealing temperature.And the volume of retained austenite decreased when annealing time was extend form 5 hours to 10 hours.(4)The behavior and related mechanisms of work hardening during uniaxial tensile deformation was studied.In combination with strain hardening behavior and microstructure characteristics,tensile deformation process was divided into four stages.In deformation stageⅠ,work hardening exponent decreases continually,ferrite began to yield.In deformation stageⅡ,work hardening exponent holds invariantly,and mechanical twinning is generated in the edge of some retained austenite.In deformation stage Ⅲ,work hardening exponent increases invariantly,and austenite transforms to martensite by routes of γ→ε→α’ and γ→α’.In deformation stage Ⅳ,work hardening exponent begins to decrease gradually.(5)The volume of retained austenite is direct proportion to product of tensile strength and elongation(PSE).We gained plenty of retained austenite benefiting from different heat treating.The content of manganese in austenite is higher in lower temperature,and the content of carbon in austenite has same trend to that of manganese.However,the carbon content in austenite decreases gradually,once the carbide is formed.Based on that,we designed the routes of heating treatment.The steel was annealed at low temperature to partition more Mn to austenite before cold rolling,and then it was annealing above the carbide dissolved temperature to partition C to austenite.Combined the effect of Mn and C,much more austenite was retained at room temperature.(6)The different annealing process on the microstructure evolution and mechanical properties was studied.Annealing at low temperature 630℃,carbide was still exist and got less retained austenite.Annealing at high temperature 690℃,carbide was completely dissolved,and more austenite was retained.However,the austenite had less stability and transformed to martensite during the cooling process,resulting in the decrease of its yield strength.Annealing at 670℃,the strength and toughness of steel are well matched,and the PSE is close to 50 GPa%.