查看更多>>摘要：To address the challenge of visualizing internal defects within castings,ultrasonic nondestructive testing technology has been introduced for the detection and characterization of internal defects in castings.Ultrasonic testing is widely utilized for detecting and characterizing internal defects in materials,thanks to its strong penetration ability,wide testing area,and fast scanning speed.However,traditional ultrasonic testing primarily relies on one-dimensional waveforms or two-dimensional images to analyze internal defects in billets,which hinders intuitive characterization of defect quantity,size,spatial distribution,and other relevant information.Consequently,a three-dimensional(3D)layered characterization method of billets internal quality based on scanning acoustic microscope(SAM)is proposed.The method starts with a layered focus scanning of the billet using SAM and pre-processing the obtained sequence of ultrasonic images.Next,the ray casting is employed to reconstruct 3D shape of defects in billets,allowing for characterization of their quality by obtaining characteristic information on defect spatial distributions,quantity,and sizes.To validate the effectiveness of the proposed method,specimens of 42CrMo billets are prepared using five different processes,and the method is employed to evaluate their internal quality.Finally,a comparison between the ultrasonic image and the metallographic image reveals a difference in dimensional accuracy of only 2.94％.The results indicate that the new method enables visualization of internal defect information in billets,serving as a valuable complement to the traditional method of characterizing their internal quality.
查看更多>>摘要：Considering the dynamic variation of roll gap and the transverse distribution of dynamic rolling force along the work roll width direction,the movement and deformation of rolls system,influenced by the coupling of vertical chatter and transverse bending vibration,may cause instability and also bring product defect of thickness difference.Therefore,a rigid-flexible coupling vibration model of the rolls system was presented.The influence of dynamic characteristics on the rolling process stability and strip thickness distribution was investigated.Firstly,assuming the symmetry of upper and lower structures of six-high rolling mill,a transverse bending vibration model of three-beam system under simply supported boundary conditions was established,and a semi-analytical solution method was proposed to deal with this model.Then,considering both variation and change rate of the roll gap,a roll vertical chatter model with structure and process coupled was constructed,and the critical rolling speed for self-excited instability was determined by Routh stability criterion.Furthermore,a rigid-flexible coupling vibration model of the rolls system was built by connecting the vertical chatter model and transverse bending vibration model through the distribution of dynamic rolling force,and the dynamic char-acteristics of rolls system were analyzed.Finally,the strip exit thickness distributions under the stable and unstable rolling process were compared,and the product shape and thickness distribution characteristics were quantitatively evaluated by the crown and maximum longitudinal thickness difference.
查看更多>>摘要：An industrial experiment was conducted at a certain steel plant in China to compare and analyze the effects of Ca treatment and Mg-Ca treatment on inclusions in 45MnVS non-quenched and tempered steel.Through scanning electron microscopy-energy dispersive scanning analysis of the morphology and composition of inclusions,as well as Aspex quantitative analysis of their quantity,type and size,the formation mechanism of MnS-oxide(MnS inclusions with oxide cores)was intensively studied.The influence of sulfide morphology on the impact properties of steel was also analyzed.The results show that the quantity percentage of spindle-shaped sulfides in Ca-treated steel is 19.99％,and that in Mg-Ca-treated steel is 35.38％.Compared with Ca-treated steel,there are more MnS-oxide inclusions in Mg-Ca-treated steel.Controlling the content of Ca and Mg in the oxide core of MnS-oxide inclusion above 10 wt.％and the area ratio below 5 would contribute to the formation of spindle-shaped inclusions after rolling.The mismatch between MnS and oxides decreases with the increase in MgO content in the oxides,which is beneficial to nucleation and precipitation of MnS with this type of oxides as the core.Under the same deformation conditions,the size of sulfide does not affect its aspect ratio.Under the experimental conditions,the inclusion containing a certain amount of MgO can enhance its sulfur capacity,facilitating the formation of composite sulfides.The transverse impact energy of Ca-treated steel is 25.785 J,and that of Mg-Ca-treated steel is 32.119 J.Compared with the traditional Ca-treatment,Mg-Ca treatment can increase the number of spindle-shaped sulfides in the steel,thereby improving the transverse impact toughness of the steel and reducing the anisotropy of the mechanical properties of the material.
查看更多>>摘要：To avoid coarse crystallization of glassy inclusions in Si-Mn deoxidized steel during hot rolling,the effect of MgO on the structure and crystallization behavior of CaO-SiO2-Al2O3 inclusions was investigated.The results showed that the crystallization temperature of the oxide melts decreased with increasing MgO content from 0 to 15.7 wt.％,which suggested that the addition of MgO would increase the temperature range of the crystalline transition.The increase in MgO content could decrease the crystallization activation energy of inclusions.With the increase in MgO content,the relative fractions of QSi0 and QSi1 structure units increased,and those of structure units QSi2,QSi3,and QSi4 decreased,increasing the depolymerization degree of the silicate structure.The crystallization ratio of glassy inclusions in the steel crucible increased from 19.7％to 98.3％with increasing MgO content from 0 to 15.7 wt.％.The addition of MgO improved the crystallization ability of inclusions,because MgO provides free oxygen O2-to destroy the bridging oxygens and form non-bridging oxygens O-,which depolymerizes silicate network structure and simplifies the[SiO4]-tetrahedral structure.In addition,MgO would promote the precipitation of the Mg-containing phases with a high melting point.When the MgO content was increased above 12.1 wt.％,MgO·Al2O3 and 2MgO·SiO2 crystalline phases would precipitate from the inclusions.
查看更多>>摘要：To clarify the deformation behavior of MnS inclusions in a non-quenched and tempered steel at three different positions(edge,1/2 radius and center)in the cross-section of the billet in the course of hot rolling,isothermal compression experiments were performed under the deformation temperature range from 1073 to 1473 K,the reduction rates from 25％to 75％and the strain rates from 0.01 to 10 s-1.The variations of deformability features(i.e.,aspect ratios,size distri-butions,and morphologies)of MnS inclusions with those isothermal compression parameters were revealed.The evalu-ation of the probable maximum aspect ratio of MnS inclusions at the three different positions in the cross-section of the billet after hot rolling was examined using the statistical analysis of extreme values.Results showed that the number densities of MnS inclusions at three different positions(edge,1/2 radius and center)in the cross-section of the steel billet only fluctuated slightly when the deformation parameters varied in the isothermal compression,while the average inclusion aspect ratios in all cases generally have a negative correlation with the deformation temperature and positive correlations with the reduction ratio and the strain rate.Statistical analysis reveals that larger inclusions deform more easily during hot rolling.The effect of rolling temperature on the extreme value of the aspect ratio of inclusions is the smallest,while the effects of initial size,reduction ratio and strain rate are more significant.
查看更多>>摘要：In order to investigate the modification behavior and regularity of inclusions in X80 pipeline steel by Mg-treatment,the comparative industrial experiments of Mg-treatment and Ca-treatment in X80 pipeline steel were carried out.Mg and Ca were added to the steel in the form of cored wire after RH(Ruhrstahl-Hereaeus vacuum degassing)process.After adding Ca-containing cored wire,the inclusions were transformed into CaO-Al2O3 in the steel.With the progress of smelting,the cleanliness of molten steel became worse,and the equivalent diameter of inclusions was at a higher level.Mg-treatment had a good effect on the modification of inclusions.After Mg-containing cored wire was added to the steel,Al2O3 and CaO-Al2O3 in the steel were transformed into MgO,MgO·Al2O3,and CaO-MgO-Al2O3 inclusions,which were basically spherical.The initial precipitated MgO became the core of other inclusions in the steel,which promotes the precipitation of MgO·Al2O3.After Mg-treatment,almost no unmodified calcium aluminate inclusions existed in the hot rolled plate,and the cleanliness of the steel was improved.The effect of Ca and Mg on the transformation of inclusions in pipeline steel was studied by thermodynamic calculation,the result of which is consistent with the experimental results.
查看更多>>摘要：The effect of TiB2 addition on microstructure refinement of the as-cast and reheated A517 steel has been investigated.0.1 wt.％TiB2 addition resulted in a reduction in equiaxed γ grain size from 990±183 to 116±35 pm and an increase in the volume fraction of equiaxed γ grain region from 5％to 67％in the as-cast A517 steel ingots.Microstructure analysis identified TiN particles rather than TiB2.This is attributed to the low thermodynamic stability of TiB2,leading to its decomposition into free Ti and B elements at an elevated temperature.Then,chemical reaction between the free Ti and residual nitrogen in the liquid resulted in the formation of TiN.Hence,it is considered that TiN acted as heterogeneous nucleation sites for the δ-ferrite.This initiated the refinement and columnar to equiaxed transition of δ-dendrites.As a result,the subsequently formed y grains were correspondingly refined.Such microstructure refinement led to improvement of the yield strength and ultimate tensile strength of the as-cast A517 steel.However,the reheating of the as-cast A517 steel resulted in a marginal microstructure refinement in the samples with low TiB2 addition.This is attributed to the limited pinning effect of the coarse TiN particles formed during casting process.Consequently,the tensile properties of the reheated A517 steel remained unaffected by the TiB2 addition.
查看更多>>摘要：The axial single-pass high temperature compression test of a hot-rolled extra-large heavy H-beam steel continuous casting billet(Q420 steel)under different deformation conditions through the Gleeble-1500D thermomechanical simulator was carried out.The modified Johnson-Cook model and the Arrhenius model based on strain-compensated of Q420 steel were established.The latter can more accurately reflect the flow behavior of Q420 steel.For the simulation of thermal com-pression,dynamic recrystallization(DRX)correlation models were developed and imported into DEFORM-3D software.According to simulation results,high temperatures and low strain rates are conducive to DRX.Due to the uneven distribution of equivalent strain and temperature in different parts of the same section,DRX volume fraction and grain size are unevenly distributed.At the center,the DRX volume fraction is the largest and the grain size is the smallest.The upper and lower edges are vice versa,and the left and right edges are centered.Optical microscopy and electron backscatter diffraction characterization methods were used to study the hot compression microstructure under different deformation conditions.As the deformation amount increases,complete DRX is gradually reached.The original austenite grain gradually becomes smaller.Local average misorientation decreases with the progression of DRX but increases with the amount of deformation after completion of DRX.As the temperature increases and the rate decreases,low angle grain boundaries and medium angle grain boundaries gradually decrease,and high angle grain boundaries gradually increase,indicating that dislocation decreases gradually,DRX grains increase gradually,and martensitic multilayer structure is obvious.As austenite grain size increases,the length of martensite lath increases,and the number of martensite blocks decreases.
查看更多>>摘要：The corrosion behavior of an Al0.75CoFeCr1.25Ni high-entropy alloy(HEA)in 0.5 mol/L NaOH solution was investigated using a series of electrochemical and analytical techniques,including X-ray photoelectron spectroscopy,X-ray diffraction,scanning electron microscopy,potentiodynamic polarization measurement and electrochemical impedance spectroscopy.The results showed that the Al0.75CoFeCr1.25Ni HEA exhibited a typical columnar dendritic structure,which is composed of face-centered cubic,body-centered cubic(BCC),and ordered BCC phases(B2 phase).The corrosion resistance of this HEA in 0.5 mol/L NaOH solution is comparable to that of 304 SS,attributed to the change in the composition of the passive film formed on the surface.Although the passive film formed was generally rich in Al,the proportion of Cr2O3 inside it increased with the increasing immersion time,enhancing the stability of the passive film and thus improving the corrosion performance of this HEA in 0.5 mol/L NaOH solution.
查看更多>>摘要：The experiments studying on the components of pure erosion,pure corrosion and their synergism were performed in multiphase systems containing sand particles,NaCl solution and condensate oil.The influence of condensate oil content on damage degree for each component was investigated by designing four different levels(0,5,10 and 20 vol.％).The mechanism about erosion-corrosion was analyzed by measurements of thinning rate,electrochemical experiments,scan-ning electron morphology and white light interferometer.The results showed that the presence of condensate oil enhanced mechanical damage of L360N pipeline steel because of the increase in viscosity.The erosion-corrosion and pure erosion were significantly accelerated with the increase in condensate oil content.However,the condensate oil content had basically no effect on pure corrosion,and the thinning rate for it was much lower than that for pure erosion.Therefore,the pure erosion component dominated the erosion-corrosion damage in such multiphase systems.
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