| With the continuous increase of the amount of multi-phase high strength steels and the thinning of the steel plate thickness,both the inhomogeneity and the difference of mechanical properties of multiphase microstructure have a more prominent influence on the sheet forming performance,and they has become the key problem that restricts the quality of sheet metal forming.To improve of the quality of sheet forming and describe the complex mechanical properties of multiphase materials precisely,it is of great theoretical significance and engineering application value to study of the characteristics and mechanical properties of the microstructure.In this paper,two typical dual phase high strength steels DP600 and DP800 were employed to investigate the inhomogeneous mechanical properties of microstructure.The main work includes the inhomogeneous distribution of microstructure,the inhomogeneous deformation of microstructure,the inhomogeneous hardness distribution of microstructure and the inhomogeneous hardening properties of microstructure,respectively.Firstly,offline metallographic observations had been performed on two kinds of materials to study the inhomogeneous characteristics of grain size,grain shape,grain distribution and martensite content in microstructure and to establish corresponding feature parameters of microstructure inhomogeneous distribution.Secondly,microstructure images with different strains were obtained by using online metallographic observations during uniaxial static In-situ tensile tests.Both the inhomogeneity of microstructure deformation and the effect of various feature parameters of microstructure inhomogeneous distribution had been investigated.Thirdly,the inhomogeneous hardness distribution in microstructure was analyzed by using nano-indentaion tests.Based on the feature parameters of microstructure inhomogeneous distribution,their effects on microstructure hardness distribution were analyzed.Finally,the proposed feature parameters of inhomogeneous microstructure was considered to be incorporated into a typical dislocation model of individual phase to establish the dislocation-based inhomogeneous hardening model of individual phase and the corresponding inhomogeneous mixed hardening model of the material.Using two proposed model,macro and micro hardening properties of DP600 and DP800 steels were predicted to reveal the inhomogeneous hardening characteristics of microstructure and to reveal the influence mechanism of inhomogeneous hardening.The main conclusions of this paper are summarized as follows:(1)The inhomogeneous distribution of microstructureThe non-uniform distribution characteristics of microstructures are mainly manifested in four aspects:the inhomogeneous distribution of grain size,inhomogeneous distribution of grain shape coefficient,diversity of martensite distribution and inhomogeneous distribution of martensite content,respectively.For DP600 and DP800 steels,both the distribution of their grain size and grain shape coefficient conforms to lognormal distribution.The grain shape of DP600 steel is more irregular than that of DP800 steel and the grain size of DP600 steel is more inhomogeneous than that of DP800 steel.Four feature parameters of inhomogeneous distribution of microstructure were proposed to describe the in-homogeneity of microstructure and they are grain diameter d(the diameter of equal-area or equal-perimeter circular grain),grain shape coefficient?(??1),martensite proximity coefficient p of ferrite(0?p?1)and distribution coefficient?of martensite content,respectively.The irregular degree of the grain shape and the inhomogeneity of martensite content are positively correlated with coefficients?and?.The proportion of F-M boundary on a ferrite grain is linear related to martensite proximity coefficient p.Various parameters are related to each other.In general,with increasing the martensite content,p increases,?decreases and the inhomogeneity of grain size distribution is weakened.(2)The inhomogeneous deformation of microstructureMicrostructure deformation is inhomogeneous for different locations,different constituent phases and different grains of the same phase.The deformation inhomogeneity of microstructure is related to the position of the fracture.The deformation in location near fracture position is more inhomogeneous than that far away from fracture position.The measured results indicate that the typical characteristics of ferrite deformation are the inhomogeneous linear deformation and the obvious change in grain shape.However,the inhomogeneous angular deformation and the significant change in location are the typical characteristics of martensite deformation.The inhomogeneous distribution of microstructure has an important impact on microstructure deformation.Deformation of ferrite grain is positive related to both its grain size d_f and martensite proximity coefficient p.Moreover,Deformability of ferrite grain is influenced by the distribution mode of martensite.The dispersed distribution of martensite can significantly enhance the deformation ability of ferrite grains.On the contrary,the accumulation of martensitic grains decreases the deformation ability of ferrite grains.(3)The inhomogeneous hardness distribution of microstructureThe hardness distribution of microstructure can be described as ferrite hardness zone and martensite hardness zone.Hardness distribution in ferrite hardness zone is significantly inhomogeneous because of the important impact of martensite.According to the influence of martensite,a parameter l of indentation location was proposed and according to which ferrite hardness zone is further divided into three regions referring to hardness zone directly affected by martensite(0<l<1?m),hardness zone indirectly affected by martensite(1<l<2?m)and hardness zone of parent ferrite(l>2?m),respectively.In general,H_f first obviously decreases with increment of parameter l and then fluctuates in a certain range.The negative relationship between H_f and l reveals the influence of martensite on ferrite hardness decreases with the increase of l.The inhomogeneous hardness distribution of microstructure was closely affected by various feature parameters of inhomogeneous distribution of microstructure.The effect of grain shape and martensite grain size would become more apparent when d_m decreases and(or)?increases.The average hardness of ferrite grain would increase when martensite proximity coefficient p increases.Compared with the accumulation of martensitic grains,ferrite hardness is more easily affected by the isolated,netted and chained martensite the dispersed/netted/chained distribution of martensite(4)The inhomogeneous hardening of microstructureBoth a developed dislocation-based inhomogeneous hardening model of individual phase and a mixed hardening model of DP steels were proposed by incorporating those proposed microstructure feature parameters into a typical dislocation-based hardening model of individual phase,the macro/micro deformation model and the mixed hardening model of the material,respectively.The inhomogeneous hardening behaviors of ferrite,martensite and the material were predicted using two proposed models.The predicted hardening properties of DP600 and DP800 steels were in good agreement with the experimental results.It is found that the inhomogeneous strengthening properties of microstructure are mainly manifested by the inhomogeneity of the strengthening properties of ferrite.Comparing with martensite,the hardening behavior of ferrite is more easily influenced by the feature parameter of inhomogeneous distribution of microstructure.Moreover,the martensite proximity coefficient p has the greatest impact on ferrite hardening behavior among all the proposed feature parameters of inhomogeneous microstructure.It was found that both grain size d and grain shape coefficient?are negative related to the dislocation density and stress of individual phase,but the martensite proximity coefficient p is positive related to the dislocation density and stress of individual phase. |
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