| Ultrahigh strength steels exhibit extreme high strength with good ductility and toughness,and are widely used in the area of heavy machinery,advanced mould manufacturing,high speed railway,aerospace,national defense equipment and automobile.With the increasing development of industry and pressure on resources and environment in recent years,the development of inexpensive ultrahigh strength steel with higher mechanical properties has been the most important task of the steel industry.It is well known that the microstructural refinement is one of the effective way to improve the strength and toughness of materials.It was found that the medium temperature thermo-mechanical treatment(MTTMT)can effectively refine the austenite grain and produce fine structured martensite after transformation,thus improving the strength of martensitic steel.Therefore,the MT-TMT process becomes a new approach for making the ultrahigh strength steel.However,most of the studies about the MT-TMT by now focused on the medium carbon high alloyed steel,maraging steel and Fe-Ni alloy.And studies about the medium carbon low alloy steel(alloy concentration is below 6 wt.%)which is processed by MT-TMT process with large deformation ratio(>85%)were rarely reported.Moreover,the microstructural evolution and strengthening mechanism of the MT-TMT processed steel with large deformation ratio are still not well understood.Therefore,in this study,the medium carbon low alloy steel was processed with heavy warm rolling to produce ultrafine-grained ultrahigh strength steel,and the effects of warm rolling and alloy element on the microstructure and mechanical properties of steels were systemically studied,finally the strengthening-toughnening mechanism of the steel was discussed.The main conclusions are described as follows:In this paper,the medium carbon low alloy AISI4140 steel were firstly chosen as the experimental steel,and the steel was heavily warm rolled to produce an ultrafine grained ferrite-martensite dual phase steel.The effect of warm rolling on the microstructure and mechanical properties was systemically studied.It was found that the warm rolling promoted the formation of ultrafine grained ferrite in the steel.As the deformation ratio increased,the volume fraction of ferrite increased and that of martensite decreased,resulting in the decreased strength and the increased ductility in the steel.When the deformation ratio increased,the ferrite grains gradually transformed from equiaxed to lamellar morphology.The AISI4140 steel processed by 60% warm rolling exhibited the highest strength,and the mirohardness,yield strength,ultimate tensile strength and total elongation of the steel was HV654,1570 MPa,2370 MPa and 2% respectively.The AISI4140 steel processed by 85% warm rolling exhibited the optimum mechanical properties,and the respective yield strength,ultimate tensile strength and total elongation was 1080 MPa,2022 MPa and 7.9%.Both the grain refinement strengthening and martensite transformation strengthening were the strengthening mechanisms of ultrafine grained ferrite-martensite dual phase steels,but the strength of steels mainly depended on the relative volume fraction of ferrite and martensite.In this study,0.5%,1%,2%,4% and 8% Ni were added into the AISI4140 steel,and then heavy warm rolled with a reduction ratio of 85%.The effect of Ni on the microstructure and mechanical properties of heavy warm rolled steel was analysed.It was found that the addition of Ni can increase the stability of undercooled austenite and inhibit the ferrite transformation.When the Ni concentration of the steel was in the range of 0.5%~2%,with the increasing concentration of Ni,the volume fraction of soft phases such as ferrite and austenite decreased gradually while that of martensite increased,and the strength of steel increased.When the Ni concentration reached 2%,the warm rolled steel was mainly composed of martensite and a small amount of retained austenite.With a further increase of Ni concentration,the volume fraction of retained austenite increased while that of martensite decreased,and the strength of steel decreased,which showed that the excess addition of alloy element would decrease the strength of steel.The AISI4140+2Ni steel processed by 85% warm rolling exhibited the highest strength,and the mirohardness,yield strength,ultimate tensile strength and total elongation of the steel was HV695,2054.3 MPa,2701 MPa and 8.3% respectively.A medium carbon low alloy steel with chemical composition of Fe-0.4C-1.65Si-1.1Mn-1.8Ni-0.85Cr-0.4Mo-0.08 V was designed based on the chemical composition of AISI4140+2Ni steel.Utrafine grained martensitic steel was produced by heavy warm rolling.It was found that the martensite structure gradually transformed from equiaxed to ultrafine lamellar morphology.Through the results of 3D-APT,the C segregated and formed cluster in the steel,which showed that warm rolling process would not promote the precipitation of carbides and inhibit the formation of twin martensite.The amount of twin martensite increased with increasing deformation ratio.When the deformation ratio reached 85%,a large number of ultrafine dispersed twin martensite were formed in the warm rolled steel.As the deformation ratio increased,the volume fraction of retained austenite showed an increasing trend as well.The increase of volume fraction of twin martensite and retained austenite in the steel was mainly attributed to the austenite grain refinement and the increased dislocation density after warm rolling.It was found that the warm rolling could effectively improve the strength of the steel.The 30% warm rolled steel exhibited a mirohardness of HV663,a yield strength of 1400 MPa,together with an ultimate tensile strength of 2390 MPa and a total elongation of 10.3%.For the 90% warm rolled steel,the mirohardness,yield strength,ultimate tensile strength and total elongation was HV728,2018 MPa,2920 MPa and 6.8% respectively.The dislocation strengthening,grain refinement strengthening and twin strengthening were the strengthening mechanisms of the warm rolled martensitic steels,and the dislocation strengthening was the dominate mechanism contributing to the increased strength.The increase of ductility of warm rolled martensitic steel was mainly attributed to the refinement of twin martensite plate,increased amount of retained austenite and the formation of ultrafine laminated structure.It was suggested that the formation of ultrafine dispersed twin martensite could effectively relieve the stress concentration and consequently reduced the adverse effect of twin martensite on the ductility.The ultrafine laminated structure could effectively impede the propagation of transverse cracks in the steel and consequently improved the ductility.The 90% warm rolled ultrafine grained martensitic steel was treated with different temperature tempering,and the effect of tempering on the microstructure and mechanical properties of steels was investigated.When the 90% warm rolled steel was tempered at 200?~600?,the martenstie in the steel was recovered.As the tempering temperature increased,the substructures such as lath and twin gradually disappeared while the dislocation density decreased and the amount of carbide increased.As the temperature reached 700?,the ferrite recrystallization occurred in the warm rolled martensitic steel and ultrafine grained ferrites with an average grain size of 720 nm was formed.When the tempering temperature increased,the ultimate tensile strength of the warm rolled martensitic steel decreased continuously.The decreased dislocation density and the reduced solid solution strengthening of C were the main reasons for the strength decrement of warm rolled steel.When compared to the direct quenched steel,it was found that the wam rolling promoted the martensite recovery and ferrite recrystallization in the steel.Therefore,the 700? tempered warm rolled steel exhibited both higher strength and ductility than 700? tempered direct quenched steel. |
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