| The development and application of high strength steel,as an significant way of automobile lightweight,has always been concerned and favored by all the major automobile manufacturers at home and at abroad.In terms of the processes of advanced high strength steel for the third generation of automobiles,the development of the quenching-partitioning process and the transformation of its achievements have a broad prospect of research and application.Based on the organic combination of martensite and retained austenite in Q&P steel,the high-strength martensite will resist the deformation of the steel plate when the car is impacted by the external force.And the residual austenite,which subjected to the external forces transformed into induced martensite for the TRIP effect,delaying the generation of necking and absorbing the external impact energy.As a result,Q&P steels,which have high strength and elongation can withstand greater magnitude of impact force than the same thickness of the traditional automobile steels.Therefore,using the Q&P steel can achieve the purpose of automobile lightweight while ensuring the safety of the vehicle through the thinning of the steel plate.The influence of C,Mn elements,the two most important austenite stabilizing elements in Q&P steel,on the properties of the steel after the partitioning process is obvious.Therefore,the research on its role in regulating the microstructure and mechanical properties of Q&P steel still needs to be further.For this,the microstructure,the content of carbon in austenite,the partitioning behavior of C,Mn elements and their distribution and the mechanical properties of C-Mn-Si steel after comprehensive diffusion process were observed and analyzed in this paper by optical microscope,scanning electron microscopy,electron probe X-ray microanalyzer,X-raydiffractometer and universal tensile tester.And the results of the following five aspects are obtained:(1)The relationship curve between the partitioning time of Q&P process and the fitting factor was established.The effect of fitting factor on the calculation of carbon content in retained austenite at room temperature is obvious.And the theoretical model of the carbon content of retained austenite and key parameters was established.More than 60% of the carbon content of retained austenite comes from the partitioning process of Q&P,ultimately.And the change of carbon content has a decisive influence on the stability of austenite.(2)The aggregation phenomenon of carbon and manganese elements are shown in the primary and secondary C-Mn comprehensive diffusion and D-Q-P processes.And the partition trend of the two elements was basically the same with the characteristics of "Mn-rich areas are rich in C and Mn-poor areas are also poor in C".The differences of C and Mn distribution in the secondary manganese partitioning process,which with the best partitioning effect is the most obvious.This shows that the design of manganese partitioning process is effective and contributes greatly to the improvement of mechanical properties of C-Mn-Si steel.In addiction,a dual-phase Mn diffusion model is established.(3)The microstructures of the primary Mn partitioning process,the D-Q-P process and the traditional Q&P process steel are made up of lath martensite,ferrite and film-like retained austenite.The grain boundary in primary Mn partitioning and D-Q-P process is more obvious.And the martensite laths are distributed in parallel in each grain,which has a great influence on the tensile strength of C-Mn-Si steel.In primary Mn partitioning process,carbon elements have undergone insufficient diffusion to excessive diffusion while the carbon partition time from 10 s to 50 s,resulting the parallelism of martensite laths decreased,and the degree of diversification became deeper.Finally,martensite lath appeared short disordered and blurry boundaries.The increased temperature of the secondary Mn partitioning process makes martensite laths diversification.(4)Within the test parameters,the tensile strength of primary manganesepartitioning steel is lower than that of the traditional Q&P steel.And the elongation and the product of strength and elongation are just the opposite,and the peak of the product of strength and elongation curve moves forward.When Mn partitioning at820℃ for 20 min,the mechanical properties are the best with the carbon partitioning at 240℃ for 20 s.The effect of the second manganese partitioning temperature on the tensile strength was obvious.Under the second manganese partitioning temperature of890℃,the product of strength and elongation appeared obviously low trough when the carbon was diffused for 30 s.When the second manganese partitioning began for7 min,the excessive diffusion of manganese began to occur.The tensile strength of D-Q-P steel is more than 875 MPa,the elongation can reache up to 24%,and the product of strength and elongation can be up to 21GPa·%.The mechanical properties of D-Q-P are much higher than those of the primary and secondary Mn partitioning steel and the traditional Q&P steel.(5)Tensile fracture of the four kinds of process steel are distributed dimples with different diameters and depths,showing obvious ductile fracture characteristics.The dimple size of D-Q-P process steel is the most uniform,and dimples exhibit equiaxed features.The average diameter and depth of the dimples are obviously larger than the traditional Q&P process steel,and the maximum strain value is much larger than those of the primary and secondary C-Mn partitioning processes and the traditional Q&P process steels.It’s a kind of heat treatment process which can produce steels with excellent comprehensive mechanical properties. |
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