Microstructure simulation of rapidly solidified ASP30 high-speed steel particles by gas atomization

In this study, the microstructure evolution of rapidly solidified ASP30 high-speed steel particles was predicted using a simulation method based on the cellular automaton-finite element （CAFE） model. The dendritic growth kinetics, in view of the characteristics of ASP30 steel, were calculated and combined with macro heat transfer calculations by user-defined functions （UDFs） to simulate the microstructure of gas-atomized particles. The relationship among particle diameter, undercooling, and the convection heat transfer coefficient was also inves- tigated to provide cooling conditions for simulations. The simulated results indicated that a columnar grain microstructure was observed in small particles, whereas an equiaxed microstructure was observed in large particles. In addition, the morphologies and microstructures of gas-atomized ASP30 steel particles were also investigated experimentally using scanning electron microscopy （SEM）. The experimental re- suits showed that four major types ofmicrostructures were formed： dendritic, equiaxed, mixed, and multi-droplet microstructures. The simu- lated results and the available experimental data are in good agreement.

Gene cloning,protein expression,and enzymatic characterization of a double-stranded RNA degrading enzyme in Apolygus lucorum

RNA interference(RNAi)is a powerful tool that post-transcriptionally silences target genes in eukaryotic cells.However,silencing efficacy varies greatly among different insect species.Recently,we met with little success when attempting to knock down genes in the mirid bug Apolygus lucorum via dsRNA injection.The disappearance of double-stranded RNA(dsRNA)could be a potential factor that restricts RNAi efficiency.Here,we found that dsRNA can be degraded in midgut fluids,and a dsRNase of A.lucorum(AldsRNase)was identified and characterized.Sequence alignment indicated that its 6 key amino acid residues and the Mg2+-binding site were similar to those of other insects’dsRNases.The signal peptide and endonuclease non-specific domain shared high sequence identity with the brown-winged green stinkbug Plautia stali dsRNase.AldsRNase showed high salivary gland and midgut expression and was continuously expressed through the whole life cycle,with peaks at the 4th instar ecdysis in the whole body.The purified AldsRNase protein obtained by heterologously expressed can rapidly degrade dsRNA.When comparing the substrate specificity of AldsRNase,3 specific substrates(dsRNA,small interfering RNA,and dsDNA)were all degraded,and the most efficient degradation is dsRNA.Subsequently,immunofluorescence revealed that AldsRNase was expressed in the cytoplasm of midgut cells.Through cloning and functional study of AldsRNase,the enzyme activity and substrate specificity of the recombinant protein,as well as the subcellular localization of nuclease,the reason for the disappearance of dsRNA was explained,which was useful in improving RNAi efficiency in A.lucorum and related species.

Modeling effect of cooling conditions on solidification process during thermal cycle of rollers in twin-roll strip casting

In the twin-roll strip casting process,molten steel solidifies by losing heat through its interface with the casting rollers.The heat extraction along this interface has an effect on the quality of the strips and should be affected by coating,rolls’material,and cooling water flow rate.It is necessary to understand the effect of these casting parameters on the solidification structure of twin-roll strip casting.A three-dimensional computational domain is set up to simulate the solidification process of molten steel and heat exchange between steel strip/air,coating,rolls,and cooling water in the channel of roll sleeves.The effect of the cooling water intensity and flow intensity of molten steel in the pool on the solidification structures is studied during the thermal cycle of rolls in the twin-roll strip casting.These predicted results are helpful to optimize casting parameters and improve the strip quality in the twin-roll strip casting process.

Numerical simulations of solidification and hot tearing for continuous casting of duplex stainless steel

Hot tearing is one of the major defects in continuous casting of steels, which severely limits the productivity of steelmaking processes. To further understand the defect, the problem of hot tearing in duplex stainless steel produced by a vertical continuous caster was investigated. A three-dimensional heat transfer and elastic-plastic model was developed based on the realistic roller layout in continuous slab casting, using ProCAST software. According to the hot tearing indicator criterion, the influence of operating parameters on the hot tearing susceptibility was evaluated. The results show that the surface temperature distribution is not sensitive to the superheat. The center of wide surface shell at the mold exit is the thinnest, and the thickness is about 10.52 mm at the superheat temperature of 40 -C. The hot tearing mainly concentrates on the slab solidification front and near the narrow face. However, corner cracks are prone to appear near the corner. With the increase in casting speed and the decrease in the cooling intensity in the secondary cooling zone, the solidification end point is rushed, which leads to the position of hot tearing lowering accordingly.

Experimental Investigation on Viscosity of CaO-MgO(-Al_2O_3)-SiO_2 Slags and Solid-liquid Mixtures

The viscosities for the selected CaO-MgO-SiO2 and CaO-MgO Al2 O3-SiO2 slags were measured by rota- ting cylinder method in a wide temperature range from 1608 K to 1 823 K. The effects of temperature, slag basicity and Al2 O3 content were studied. The results indicated that the viscosity decreased with increasing the slag basicity at the same temperature. And the viscosities of the slag initially increased linearly with increasing Al2 O3 content in the slags and then decreased as the Al2 O3 content in the slags continued to increase. Al2 O3 addition has a more signifi- cant effect on the viscosity than slag basicity in the selected slags. Based on the experimental data and the fraction of solid phases calculated by the thermodynamic software, the effect of solid phases on slag viscosity was evaluated. The results indicated that the relationship between the relative viscosity and the volume fraction of solid phases esti- mated from the slag composition can be reasonable by allowing the inverse maximum fraction of solid particles to vary.

First principles application for mechanical properties of Ti-doped W particles enhanced U matrix composite

The stability, bonding, work of adhesion and electronic structure of the U/W interface with and without Ti were investigated by first principles to explore the me- chanical properties of W particles enhanced U-Ti alloy matrix composite as a construction material. The calculated results indicate that the preferable orientation of the U/W interfacial structure is （001）U/（110）w crystallographic plane, Ti atoms originating from U slab are prone to diffuse into W slab through the interface, and additional Ti in U matrix is the stronger adhesion to W, with an ideal work of adhesion of 6.93 J.m-2 for U-Ti/W interface, relative to the value of 6.72 J.m-2 for clean U/W interface. The stronger adhesion performance is due to the increase in valence electron hybridization for U-Ti/W compared with U/W interface, as evidenced by the characteristic of the local density of states for the interfacial atoms.

Effects of Thermal Gradients and Rotational Flows on Grain Growth in 22 t Steel Ingots

Heavy ingots are widely used in many industrial fields. The coarse grains formed during the process of in- got solidification influence the properties and fracture behaviors of the final products. The coarse grain growth was simulated under different thermal gradients. A 30Cr2Ni4MoV steel ingot was melted in a cubic crucible with dimen-sions of 15 cm×10 cm×23 cm, and the cooling conditions on each side of the crucible were controlled by different thermal curves. The influences of thermal gradients and rotational flows on grain growth in heavy steel ingots were then investigated both numerically and experimentally. The results showed that when the amplitude of the rotation angle was 60°, the metal was solidified under a reciprocating horizontal rotational condition when the angular velocity was 10 （°）/s or 20 （°）/s. As the thermal gradient increased, the lengths of the primary columnar grains in- creased, and the diameters of equiaxed grains decreased. When the direction of flow rotation was perpendicular to the direction of grain growth, the columnar grain zone was nearly eliminated, and the average diameter of equiaxed grains was 0.5 mm.

Thermodynamic Behaviors of Nb and P in Dephosphorization of Niobium-bearing Hot Metal with BaO-based Slag

The Bayan Obo iron ore contains valuable metallic elements such as niobium and rare earth elements. However, developing appropriate metallurgical processes to achieve effective dephosphorization of Nb-bearing hot metal meanwhile retaining Nb in the steel products remains a challenge. We carried out a thermodynamic study on dephosphorization of Nbbearing hot metal and investigated the effects of the initial [C] content and the dephosphorization temperature on oxidation behaviors of [Nb] and [P]. In particular, we focused on thermodynamic analysis of dephosphorization of Nb-bearing hot metal with highly basic BaO-based slag. The results showed that the highly basic BaO-based slag system is more suitable for dephosphorization under oxidative conditions, and increasing the [C] content favors dephosphorization of Nb-bearing hot metal without oxidizing [Nb]. Moreover, the values of equilibrium oxygen activity for oxidation reactions of [Nb] and [P] are increased as the dephosphorization temperature increases. Considering the hot metal containing 0.02% of [Nb] in Baotou Steel, thermodynamic calculations indicated that the content of [P] can be reduced to 0.02%–0.05% and [Nb] remains unoxidized when dephosphorization occurs at the cconditions of T=1573–1673 K, [C]=4.0%, [Nb]=0.02%, aP2O5=10-24, aNb2O5=10-10.

Mechanical properties of U-0.95 mass fraction of Ti alloy quenching and aging treatment:a first principles study

First principles plane wave pseudopotential method was executed to calculate the mechanical properties with respect to the uranium-0.95 mass fraction of titanium （U-0.95 mass fraction of Ti） alloy for quenching and aging, including the elastic modulus, the value of shear modulus to bulk modulus （G/B） and the ideal tensile strength. The further research has also been done about the crack mechanism through Griffith rupture energy. These results show that the elastic moduli are 195.1 GPa for quenching orthorhombic ~ phase and 201.8 GPa for aging formed Guinier-Preston （G.P） zones, while G/B values are 0.67 and 0.56, respectively. With the phase change of uranium-titanium （U-Ti） alloy via the quenching treatment, the ideal tensile strength is diverse and distinct with dif- ferent crystal orientations of the anisotropic ~ phase. Comparison of quenching and short time aging treatment, both of the strength and toughness trend to improve slightly. Further analysis about electronic density of states （DOS） in the electronic scale indicates that the strength increases continuously while toughness decreases with the aging proceeding. The equilibrium structure appears in overaging process, as a result of decomposition of metastable quenching 7 phase. Thereby the strength and toughness trend to decrease slightly. Finally, the ideal fracture energies of G.P zones and overaging structure are obtained within the framework of Griffith fracture theory, which are 4.67 J/m2 and 3.83 J/m2, respectively. These results theoretically demonstrate strengthening effect of quenching and aging heat treatment on U-Ti alloy.

Physical simulation of mixing on a C-H2 smelting reduction reactor with different tracer feeding positions

With the growing demand for energy saving,emission reduction,and green metallurgy,we had designed a new C-H2 smelting reduction reactor.In order to solve the key problem that the heat transfer efficiency from high temperature oxidation zone in upper region to low temperature reduction zone in lower region is low in traditional metallurgical reduction reactor,a water simulation was adopted to optimize the mean residence time and to improve the transmission efficiency within the reactor.According to the modified Froude similarity,a water model experimental reactor with a ratio of 1:1 to the prototype was constructed.In the prototype,the feed port was used to feed preheated ore and flux.In order to simulate the effect of different feeding positions of the tracer on the mixing behavior in the molten pool,four points of tracer feeding position were arranged for a systematic study.At the same time,based on double-row side nozzle with thick slag layer in a C-H2 smelting reduction reactor,nine influencing factors,including relative angle between upper and lower side nozzles,were studied.The experimental results showed that the tracer feeding position had a great influence on the mean residence time,and the relative angle also had a great influence on tracer feeding position.Finally,through comprehensive analysis,the optimal condition parameters were obtained under different tracer feeding positions.These results provide valuable help for the design and optimization of the C-H2 smelting reduction reactor.

High speed steel produced by spray forming

In this paper, ASP2030 （A30） high speed steel （HSS） was produced by spray forming and the microstructure was studied by optical microscopy （OM）, scanning electron microscopy （SEM）, transmission elec- tron microscopy （TEM）, electron probe microanalysis （EPMA） and X-ray diffraction （XRD）. The spray formed A30 （SF A30） steel exhibited a very uniform and fine microstructure consisting of martensite, retained austenite and uniformly distributed network carbides. Microstructure refining can be explained in terms of the rapid solidification of spray forming. M2C, MC and M6C type carbides were found in the as-sprayed A30 HSS by XRD and TEM. A uniform distribution of carbides was obtained after forging and annealing. The microstructure properties of SF A30 steel indicate that spray forming can be considered as a cost-effective route for the production of A30 steels and other highly alloyed steels.

Interface reaction of high-strength low-alloy steel with Al-43.4Zn-1.6Si(wt.%)metallic coating

The microstructure,elemental distribution,phase composition,and thickness of intermetallic layers between high-strength low-alloy steel(H420)/mild carbon steel(DC51)and Al–43.4Zn–1.6Si(wt.%)(galvalume,GL)alloy were comparatively investigated.The experimental results reveal that the interfacial reaction layer was composed of Fe2Al5,Fe4Al13,and Al8Fe2Si intermetallic compounds.Moreover,the growth curves of the Fe2Al5 and Fe4Al13 intermetallic layers fit the parabolic law well,and the total thickness of the intermetallic layers of H420+GL was almost the same as that of DC51+GL.However,the thickness of the Fe2Al5 layer in H420+GL was thinner than that in DC51+GL.In addition,first-principle calculations were performed to explore the effect of Mn on the growth of the Fe2Al5 intermetallic phase,and the results indicate that Mn substitution in Fe2Al5 removes electronic charge from the Al atoms,thus decreasing the thickness of the Fe2Al5 interface layer.

Development of a computational tool for materials design

An integrated modeling tool coupling thermo- dynamic calculation and kinetic simulation of multicom- ponent alloys is developed under the framework of integrated computational materials engineering. On the basis of PandatTM software for multicomponent phase diagram calculation, the new tool is designed in an inte- grated workspace and is targeted to understand the com- position-processing-structure-property relationships of multicomponent systems. In particular, the phase diagram calculation module is used to understand the phase stability under the given conditions. The calculated phase equilib- rium information, such as phase composition and chemical driving force, provides input for the kinetic simulation. In this paper, the design of the modeling tool will be pre- sented and the calculation examples from the different modules will also be demonstrated.