| More and more attention has been paid to steel castings because of the rationality and practicability of the design.Compared with hot-rolled steel members,the casting process of steel castings is easy to be involved in gases,impurities,etc,and the lack of rolling process of rolling mill leads to the number of defects in steel casting far more than in hot-rolled steel.The existence of defects destroys the continuity of the material,reduces the effective cross-section area of the component,and results in the decline of the mechanical properties of the local component and the whole structure.Because the steel casting always works with defects,this problem restricts the further development and application of steel casting materials,components and structures.Only by fully understanding the influence of defects on the mechanical properties of steel castings and structural systems can fully reflected the advantages of steel castings in design and application.The macro defects in steel castings are easy to be observed and found,and there are many researches related to the macro defects,and regulations on the detection and repair of macro defects have been formulated.Therefore,this paper focuses on the micro/meso defects in the cast steel,which mainly are the meso porosities with an equivalent diameter greater than 62.23μm.Firstly,the chemical composition,microstructure and mechanical properties of cast steel were studied;then,based on X-ray tomography,the characteristics(quantity,size,shape and distribution law)of the meso defects in the cast steel were statistically analyzed,and 3D finite element models with the same size and similar morphology as the real meso defects were established to explore the influence of the meso defects on the mechanical properties of the material;finally,the GTN damage model was improved and the improved model was used to check the bearing capacity of steel castings.The main research contents and results are as follows:1.Study on chemical composition,microstructure and mechanical properties of cast steel.In order to master the characteristics of cast steel more comprehensively and accurately,the chemical composition,microstructure and macro mechanical properties of cast steel G20Mn5 N commonly used in engineering,were observed and tested,and compared with ordinary hot-rolled steel Q345 and high-strength hot-rolled Steel Q460 in detail.The chemical composition of three kinds of steel was analyzed by direct reading spectrometer;the microstructure of three kinds of steel was observed by optical microscope;the tensile fracture of three kinds of steel was analyzed by scanning electron microscope;the mechanical properties of G20Mn5 N cast steel were measured by3 D digital image correlation(DIC)method,and compared with the mechanical properties of Q345hot-rolled steel and Q460 hot-rolled steel tested in published literature.The results show that the largest difference between the content of cast steel and hot-rolled steel is Ni.Because the content of Ni in G20Mn5 N cast steel is higher than that in Q345 hot-rolled steel and Q460 hot-rolled steel,the metallographic structure of cast steel is more uniform and smaller.The microstructure further affects the macro mechanical properties of the cast steel,which mainly shows that the cast steel has better toughness.2.Study on the quantity,size,morphology and distribution of meso defects in cast steel.Based on3 D X-ray tomography technology,the meso defects in G20Mn5 N cast steel,Q345 hot-rolled steel and Q460 hot-rolled steel were observed,and the similarities and differences in the quantity,size,morphology of the meso defects between the cast steel and the hot-rolled steel were analyzed.Then,according to the characteristics of defects,the meso defects of cast steel were classified,and the distribution law of meso defects was counted.The analysis results show that the quantity of meso defects in cast steel is about 5 times of that in hot-rolled steel,and the volume of the meso defects is larger and the morphology is more irregular in cast steel.Meso defects in cast steel can be divided into gas pore,gas-shrinkage pore,and shrinkage pore.The equivalent diameters of meso defects follow a three-parameter log-normal distribution.The distribution of various defects in cast steel is not directly related to their quantity,but is determined by the cause of defects.The coalescence tendency between defects is greatly affected by the quantity of defects,and the more the quantity,the greater the probability of coalescence.3.Study on the influence of meso defects on the mechanical properties of cast steel.Based on 3D X-ray tomography data of G20Mn5 N cast steel,3D finite element models with the same size and similar morphology as the real meso defects were established.Then,RVE models considered periodic boundary conditions were established to study the relationship between the characteristics of meso defects and the macro mechanical properties of cast steel.The research results show that it cannot simplify or idealize the meso defects with complex morphology into spheres,which will cause large analysis errors.The nolume and morphology of the meso defects have an effect on stress concentration and elastic modulus of cast steel.The larger the defect volume and the more irregular the defect morphology,the more serious stress concentration and the greater reduction of the elastic modulus caused by defects.The critical strain value is the boundary point of defect growth rate from slow to fast.The larger the initial defect volume fraction is,the smaller the corresponding critical strain is.However,the influence of defect morphology on critical strain is not significant.4.Study on the evolution process of meso defects in cast steel under load.Based on the interrupted ex-situ X-ray tomography test,the deformation and failure mechanisms of cast steel were revealed from three stages of defect,nucleation,growth and coalescence.Due to the inhomogeneity of the microstructure of cast steel,with the continuous accumulation of matrix strain,new defects continue to occur.At the same time,the volume of defects is growthing.When the size and quantity of defects have evolved to a certain extent,they will coalesce with each other to form cracks.The results show that the nucleation,growth,and coalescence behavior of defects in cast steel are complementary and not completely independent,so the analysis of one evolutionary stage of defects should not be performed independently of other stages.The central crack propagates in the direction perpendicular to the load under the action of the axial tensile load.Due to the influence of the maximum shear stress and the minimum necking cross section,the cracks actually propagate alternately along two 45° planes.5.Research on meso damage constitutive model of cast steel and check the bearing capacity of steel castings.An improved GTN damage model is obtained by coupling the Ramberg-Osgood elastoplastic hardening criterion,the Thomason plastic limit load model,and the GTN damage model.The improved GTN damage model effectively solves the problems of material hardening and defects coalescence.Then the parameters of improved GTN damage model were determined by combining the X-ray tomography experiment and the inversion method,which overcomes the limitations of the single calibration method.Finally,based on the constitutive model without considering damage and the constitutive model with considering damage,the bearing capacity of steel castings was checked and calculated.The results show that whether or not damage is taken into account has little effect on the serviceability limit state,but it has a greater impact on the ultimate bearing capacity of steel castings. |
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