| Comparing with quenched and tempered steel,the non-quenched and tempered steels eliminate the quenching and tempering process,which can save the energy,reduce the production cost and shorten the production cycle.So the non-quenched and tempered steel has been widely used in the auto parts manufacturing and machinery manufacturing industry.27MnCr5 steel has the processing characteristics of non-quenched and tempered steel and it contains MnS inclusions.Not only do the steel improve cutting property by these inclusions,but also promotes nucleation of intra-granular ferrite.And microstructure and properties of the steel can be controlled by forging and cooling process.In this paper,the microstructure evolution of 27MnCr5 steel in forging and cooling process is studied systematically,which provides a theoretical basis for microstructure controlling in the hot forming process of parts.Firstly,effects of MnS inclusions on microstructure of the steel is deeply studied,and the fundamental reasons and solutions for the formation of banded microstructure are clarified.Secondly,the dynamic recrystallization behavior of the steel is studied systematically,and the recrystallization characteristics of this steel in process of hot die forging are simulated and analyzed.Finally,continuous cooling phase transition of steel after deformation is emphatically investigated.obtaining dynamic CCT curve of this steel.And the cooling condition after forging was defined.Through this study,the main achievements are as follows:Obvious segregation of alloy element Cr and Mn existed in this non-quenched and tempered steel,which is the fundamental reason for banded microstructure of the forming parts.Aggregation of large size MnS inclusions can inhibit the diffusion of alloy elements,which make the band microstructure more difficult to eliminate.After homogenizing annealing at high temperature,the segregation of alloy elements in steel is eliminated effectively.The size of MnS inclusions is smaller and its distribution is uniform,which eliminate the inhibition effect of large size inclusions on the diffusion of alloy elements.Moreover,the small size MnS inclusions can promote the nucleation of ferrite in steel.Finally,the uniform distribution of ferrite and pearlite microstructure was obtained.Flow stress and dynamic recrystallization are both strongly influenced by temperature,the flow stress decreases and the volume fraction of dynamic recrystallization increases with increasing deformation temperature.When the deformation temperature rises to 1100 ~oC,the grain coarsening is obvious,leading to a large average grain size.Based on these results,a constitutive relation and a dynamic recrystallization model of this steel was established.Deform is used for the numerical simulation of dynamic recrystallization,choosing the best thermal deformation parameters.The effect of continuous cooling conditions on microstructure of 27MnCr5 steel after hot deformation is defined.The temperature of ferrite and pearlite formation is600-700 ~oC.When cooling rate is below 1 ~oC/s.the final microstructure is mainly composed of ferrite and pearlite.When cooling rate is between 1 to 3 ~oC/s,the final microstructure is mainly composed of bainite.When cooling rate is more than 20 ~oC/s,the final microstructure is mainly composed of martensitic.With the increase of cooling rate,the hardness is also increasing.The hardness of the sample is stable while martensite is the main structure.On the basis of experimental research,the test of controlling forging and cooling was carried out.The temperature of hot die forging was selected at 950 ~oC,and the cooling mode of the work-pieces was controlled by a slow cooling rate after forging.After examination,the microstructure of the parts has been obviously improved.The ferrite-pearlite microstructure is more homogeneous and fine,and the percentage of ferrite increases obviously.Finally,the goal of microstructure controlling in the forging process has been achieved. |
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