Research On Processing And Microstructure And Properties Of Large Deformation Eutectoid Flat Wire Produced By Continuous Rolling

In this paper, the as-drawn65Mn and60steel wires (the drawn true strain wererespectively up to2.08and2.26) were used to produce flat wires through three-passcold continuous rolling process at room temperature. And then, to acquire high strengthand toughness, the rolled steel wires were successively optimized in annealing atdifferent temperatures and oil quench tempering process. The main research includesthe microstructure and mechanical properties evolution of as-drawn65Mn steel wiresrolled by different processes, the microstructure and mechanical properties evolution ofrolled as-drawn65Mn steel wires after annealing at different temperatures, themicrostructure and mechanical properties evolution of as-rolled60flat steel wires andthose wires treated by annealing and oil quench tempering at different temperatures. Themicrostructure evolution and solution phenomenon and mechanism of cementite inas-drawn65Mn during the rolling process were discussed. The precipitation regularityof carbide existed in the rolled as-drawn65Mn steel wires in the process of annealingwere analyzed. The contrast of the precipitation regularity of carbon atom in the rolledas-drawn60steel wires after oil quench tempering process and the whole annealingwere discussed.The results show that:(1) the65Mn and60steel wire processed by cold-drawing(true strain was greater than2.0) had outstanding capacity of plastic deformation. Theround steel wires with the diameter of Φ=2.29mm and Φ=2.29mm were chosen toproduce the high strength flat wires with the sizes of0.44×5.6mm and0.8×3.4mm,respectively. The flat wires were unbroken during the whole rolling processing.(2)The changes about tensile strength and hardness of as-drawn65Mn and60steel wires rolledby different processes were almost the same. It showed that when the accumulatedrolling deformation degree was at the range of30%~68%, the tensile strength andhardness increased slowly, according with the linear growth law. However, the tensilestrength and hardness increased rapidly when the accumulated rolling deformationdegree was greater than68%, which fitted the law of quadratic parabola.(3)The tensilestrength and hardness of as-drawn65Mn and60steel wires which had been rolled bylarge deformation and annealed at different temperatures had the same transformationlaw. It showed that the tensile strength continued to rise with the increase of annealingtemperature below200℃and got the maximum value at200℃. However, the hardnessdid not change too much and the elongate was too inferior during this process. Andwhen the annealing temperature was greater than200℃, with the increase of annealingtemperature, the tensile strength and hardness decreased continually. What’s more, theplasticity was terrible at the temperature scope of400℃~480℃.(4)The quenchingtemperature and tempering temperature that could meet the excellent mechanicalproperty requirements of oil tempered flat60steel wires rolled by large deformationwere at the range of860℃~900℃and500℃~550℃, respectively.(5)During the wholerolling processes, the changes of cementite showed that it turned with ferrite to therolling direction at the stage of lower accumulate strain. The crooked cementite wasbroken, and then kept on deformation and turned to the rolling direction. Almost allorientation of cementite which surrounded by ferrite were parallel to the rollingdirection, and were refined overly at the stage of higher accumulate strain. Finally, thecementite was decomposed largely.(6)The main mechanism of cementitedecomposition during the whole rolling processing was as follow：a small quantity ofdecomposition of cementite was caused by the adsorption effect of dislocation on thecarbon atom at the stage of lower strain. And the abundant of decomposition ofcementite happened at the stage of higher strain. That was because the extreme brokencementite couldn’t steadily exist under the effect of Gibbs-Thomson, which wasas-named interfacial energy mechanism.(7)The dissolved carbon atom was not steadyextremely and would separate out at sub-boundary of ferrite layer with the increase ofannealing temperature. Through the SEM, it could be seen that carbon atoms would gather and grow up at300℃and above. The precipitate of carbon atom could hindergrain boundary migration, and also the temperature of recrystallization was delayed to600℃.(8)Through the comparison of the process of annealing and oil quenchtempering, it was more superior that using large deformation with short time annealingprocess to produce flat wires than oil quench tempering, under the circumstance ofroughly same hardness and similar plasticity in the actual production.