连铸工艺参数对20CrMnTi齿轮钢150 mm x 150 mm铸坯凝固组织影响数值模拟和生产应用

为改善20CrMnTi钢小方坯凝固组织,基于ProCAST软件中的CAFE模型,对其凝固组织进行数值模拟,研究了不同钢水过热度、铸坯拉速、二冷比水量对凝固组织的影响。模拟结果表明,降低钢水过热度、提高铸坯拉速、降低二冷比水量均可达到增大铸坯等轴晶率和细化晶粒的目的,其中过热度对其影响最大。过热度每降低10℃,等轴晶率平均增加3.7%;拉速每增加0.1 m/min,铸坯等轴晶率平均增加1.8%;比水量每降低0.1 L/kg,铸坯等轴晶率平均增加1.65%。生产应用表明,钢水过热度30℃时,当拉速由原2.2 m/min降低至2.1 m/min,二冷比水量由0.6 L/kg提高至0.7 L/kg,铸坯中心疏松明显减少。     

Solidification Structure of Continuous Casting Large Round Billets under Mold Electromagnetic Stirring

The solidification structure of a continuous casting large round billet was analyzed by a cellular-automaton-finite-element coupling model using the ProCAST software.The actual and simulated solidification structures were compared under mold electromagnetic stirring (MEMS)conditions (current of 300 A and frequency of 3 Hz).There-after,the solidification structures of the large round billet were investigated under different superheats,casting speeds,and secondary cooling intensities.Finally,the effect of the MEMS current on the solidification structures was obtained under fixed superheat,casting speed,secondary cooling intensity,and MEMS frequency.The model accurately simulated the actual solidification structures of any steel,regardless of its size and the parameters used in the continuous casting process.The ratio of the central equiaxed grain zone was found to increase with decreasing su-perheat,increasing casting speed,decreasing secondary cooling intensity,and increasing MEMS current.The grain size obviously decreased with decreasing superheat and increasing MEMS current but was less sensitive to the casting speed and secondary cooling intensity.     

Temperature Differences in the Mold of a Continuous-Casting Machine with a New Cooling System

Attention focuses on the processes in the mold of a continuous-casting machine when using a patented new cooling system. In particular, the temperature differences in the steel billet and in the wall over the mold height are of interest in modeling the casting processes, because those differences affect the quality of the billet produced. A literature review covers research on the slag-forming mixture, which affects the heat flux from the billet to the mold. Non-Russian authors highlight mild cooling of the mold in selecting the slagforming mixture. Improvement of billet cooling in the mold permits improvement in the surface quality of the slab, extension of mold life, and increase in productivity. According to numerous authors, that may be accomplished by mathematical modeling of the process. The mold cooling depends directly on the convective motion of liquid steel in the mold, a topic addressed by many non-Russian authors. Researchers have considered systems in which the heat pipes in the cooling system of the mold are based on porous material, with water and air as the working fluid, and those in which liquid droplets on nanostructured superhydrophilic surfaces are evaporated. Mold cooling at steel casting rates higher than 7 m/min, accompanied by increase in the heat-flux density, is of great importance, as reflected by the number of studies published. The relations between the basic process parameters are determined by means of Rayleigh dimensionality theory. The basic parameter selected is the temperature difference in the metal mold wall, which depends on the casting rate (the time that the billet is in the mold), the properties of the steel (specific heat, thermal diffusivity), the thermal conductivity of the mold wall, and the temperature difference in the cast steel. In determining the exponents in the dimensionless relations, the available experimental data regarding the dependence of the heatflux density on the casting rate and the parameter of the steel are taken into account. On the basis of the ratio Δt_me/t_me obtained (where Δt_me is the mean temperature difference over the wall thickness and t_me is the mean wall temperature) for molds with the existing and new (patented) cooling systems, the temperature difference in the steel billet may be determined. For the two cooling systems compared, Δt_me1 = 450°C and Δt_me2 = 231°C. Consequently, Δt_me1/Δt_me2 = 1.95. The smaller temperature difference Δtme2 indicates milder cooling of the mold with the new cooling system.     

连铸参数对高碳钢小方坯二次枝晶间距的影响

通过研究高碳钢小方坯连铸参数对铸坯二次枝晶臂间距的影响发现:采用结晶器电磁搅拌、加大二冷比水量,二次枝晶臂间距减小;过热度升高、拉速增大,二次枝晶臂间距增大.无M-EMS的铸坯接近中心的区域,仍然是枝晶臂发达的柱状晶组织;采用结晶器电磁搅拌,促使铸坯中心区域形成粒状等轴晶组织;随搅拌电流增大,二次枝晶间距明显减小.随二次枝晶臂间距增大,渗透率增加,造成铸坯中心碳偏析加重.     

板坯连铸工艺参数对取向硅钢铸坯中心等轴晶率的影响

通过对取向硅钢铸坯的低倍检验,研究了中间包钢水过热度、电磁搅拌电流、二冷水强度、拉速及断面尺寸对铸坯中心等轴晶率的影响。结果表明,试验条件下,中间包钢水过热度控制在14~18 ℃范围内时,因过热度不同引起的中心等轴晶率的变化较小,变化范围仅为0%~3%;电磁搅拌电流和铸坯断面尺寸对中心等轴晶率的影响较为明显,随搅拌电流的增大,中心等轴晶率呈增加的趋势,而当铸坯断面尺寸增加时其呈减小的趋势;二冷水强度和拉速对中心等轴晶率的影响相对较小,随着二冷水强度的增大,中心等轴晶率呈减小的趋势,而当拉速提高时其呈增加的趋势。     

Experimental investigation on solidification of GCr15 bearing steel by the simulated continuous casting

ABSTRACT

An experimental apparatus for simulation of continuous casting process of GCr15 bearing steel billet is established. With the apparatus, the billets of diameter 140?mm are casted in various superheats and cooling conditions. The solidification macrostructure, dendrite morphology, segregation and carbide are investigated. It is shown that melting superheats and cooling conditions remarkably influence the microstructure and solute segregation. It is found that the secondary dendrite arm spacing of the steel increases with the increase of the superheat temperature, and with decrease of the cooling rate. Lower superheat with higher cooling rate promotes the refining of the microstructure. Refining equiaxed grains structure in the centre of the billet leads to lower segregation of carbon. Furthermore, with increasing cooling rates, the spacing of the pearlite laminar is refined and the precipitation of proeutectic carbides is suppressed.     

微观组织参数及工艺条件对430不锈钢凝固显微结构的影响

在多元合金CAFE模型的基础上,分析了微观组织参数（形核密度、高斯分解参数、Gibbs-Thomson系数等）与430不锈钢凝固过程中晶粒形貌的复杂关系,以及过热度与冷却强度等工艺参数对凝固组织的影响.研究发现,晶粒尺寸和柱状晶向等轴晶转变不仅与体最大形核过冷度有关,也受体形核密度的影响.高斯分解参数和Gibbs-Thomson系数增大时,一次枝晶间距减小,等轴晶范围增大;但当它们增加至一定范围后,其对显微结构的影响逐渐变得不明显.过热度或冷却强度增大时,等轴晶范围减小,但一次枝晶间距的变化不明显.      

Effects of Process Parameters on Solidification Structure of A390 Aluminum Alloy Hollow Billet

The effects of process parameters on the solidification structure of A390 aluminum alloy hollow billets prepared by direct-chill casting were investigated. The decrease of casting temperature deteriorated the homogeneity and increased the size of primary Si particles in the hollow billet. Although the average size of primary Si particles was not obviously affected by the increase of casting speed, the thickness of Si-depleted layer at the inner wall increased with the higher casting speed. The tensile strength of A390 alloy is a function of the percentage of coarse Si particles (larger than 35 μm) and the average size of primary Si particles. Higher and more stable tensile strength can be received in the hollow billet with the casting temperature of 1050 K (777 °C), because the fine and uniformly distributed primary Si particles were obtained in the hollow billet.     

Effects of Growth Rates and Compositions on Dendrite Arm Spacings in Directionally Solidified Al-Zn Alloys

Dendritic spacing can affect microsegregation profiles and also the formation of secondary phases within interdendritic regions, which influences the mechanical properties of cast structures. To understand dendritic spacings, it is important to understand the effects of growth rate and composition on primary dendrite arm spacing (λ ₁) and secondary dendrite arm spacing (λ ₂). In this study, aluminum alloys with concentrations of (1, 3, and 5 wt pct) Zn were directionally solidified upwards using a Bridgman-type directional solidification apparatus under a constant temperature gradient (10.3 K/mm), resulting in a wide range of growth rates (8.3–165.0 μm/s). Microstructural parameters, λ ₁ and λ ₂ were measured and expressed as functions of growth rate and composition using a linear regression analysis method. The values of λ ₁ and λ ₂ decreased with increasing growth rates. However, the values of λ ₁ increased with increasing concentration of Zn in the Al-Zn alloy, but the values of λ ₂ decreased systematically with an increased Zn concentration. In addition, a transition from a cellular to a dendritic structure was observed at a relatively low growth rate (16.5 μm/s) in this study of binary alloys. The experimental results were compared with predictive theoretical models as well as experimental works for dendritic spacing.     

Determination of the heat-transfer coefficient between the AK7ch (A356) alloy casting and no-bake mold

The heat-transfer coefficient h between a cylindrical cast made of AK7ch (A356) aluminum alloy and a no-bake mold based on a furan binder is determined via minimizing the error function, which reflects the difference between the experimental and calculated temperatures in the mold during pouring, solidification, and cooling. The heat-transfer coefficient is h _L = 900 W/(m² K) above the liquidus temperature (617°C) and h _S = 600 W/(m² K) below the alloy solidus temperature (556°C). The variation in the heat-transfer coefficient in ranges h _L = 900–1200 W/(m² K) (above the alloy liquidus temperature) and h _S = 500–900 W/(m² K) (below the solidus temperature) barely affects the error function, which remains at ~22°C. It is shown that it is admissible to use a simplified approach when constant h = 500 W/(m² K) is specified, which leads to an error of 23.8°C. By the example of cylindrical casting, it is experimentally confirmed that the heat-transfer coefficient varies over the casting height according to the difference in the metallostatic pressure, which affects the casting solid skin during its solidification; this leads to a closer contact of metal and mold at the casting bottom.     

降低SWRH82B铸坯中心偏析的探索与实践

通过大量的现场取样,结合实验室研究手段,系统的研究钢水过热度、拉速、二次冷却强度等工艺参数对高碳连铸坯中心偏析的影响,从而得出最佳的生产参数控制范围,改善swRH82B高碳钢连铸坯的中心偏析,提高铸坯质量和加工性能。     

Microstructure,Texture, and Mechanical Properties of <Emphasis Type="Italic">β</Emphasis> Solution-Treated and Aged Metastable <Emphasis Type="Italic">β</Emphasis> Titanium Alloy,Ti-5Al-5Mo-5V-3Cr

The current study describes the aging characteristics and mechanical properties of a metastable β titanium alloy Ti-5Al-5Mo-5V-3Cr. The aged microstructures consist of fine α-phase precipitates (lath morphology) in equiaxed β grains. The sizes of the α-phase precipitates increase with the increasing aging temperature. The β ST WQ and 823 K (550 °C)-aged material exhibits maximum hardness due to precipitation hardening. The low- and high-temperature aging conditions result in strong c-type basal and prismatic textures in the α-phase, respectively. The β-phase of the alloy aged at low temperature reveals the presence of texture with moderate intensity. In contrast, high-temperature-aged material exhibits very strong β-phase texture. The strengths of the alloy under β ST WQ- and 923 K (650 °C)-aged conditions are the maximum and minimum along TD and RD, while the ductility values are the maximum and minimum along the RD and TD direction samples, respectively. The flow curves follow typical Holloman equation along three sample directions, and the work hardening rate curves display two distinctive regimes, namely, stage I and stage II. The yield locus plots of the β ST WQ and aged materials exhibit the presence of anisotropy.     

Elevated-Temperature Mechanical Behavior of As-Cast and Wrought Ti-6Al-4V-1B

This work studied the effect of processing on the elevated-temperature [728 K (455 °C)] fatigue deformation behavior of Ti-6Al-4V-1B for maximum applied stresses between 300 to 700 MPa (R = 0.1, 5 Hz). The alloy was evaluated in the as-cast form as well as in three wrought forms: cast-and-extruded, powder metallurgy (PM) rolled, and PM extruded. Processing caused significant differences in the microstructure, which in turn impacted the fatigue properties. The PM-extruded material exhibited a fine equiaxed α + β microstructure and the greatest fatigue resistance among all the studied materials. The β-phase field extrusion followed by cooling resulted in a strong α-phase texture in which the basal plane was predominately oriented perpendicular to the extrusion axis. The TiB whiskers were also aligned in the extrusion direction. The α-phase texture in the extrusions resulted in tensile-strength anisotropy. The tensile strength in the transverse orientation was lower than that in the longitudinal orientation, but the strength in the transverse orientation remained greater than that for the as-cast Ti-6Al-4V. The ratcheting behavior during fatigue is also discussed.     

On Shear Testing of Single Crystal Ni-Base Superalloys

Shear testing can contribute to a better understanding of the plastic deformation of Ni-base superalloy single crystals. In the present study, shear testing is discussed with special emphasis placed on its strengths and weaknesses. Key mechanical and microstructural results which were obtained for the high-temperature (T?≈?1000 °C) and low-stress (τ?≈?200 MPa) creep regime are briefly reviewed. New 3D stereo STEM images of dislocation substructures which form during shear creep deformation in this regime are presented. It is then shown which new aspects need to be considered when performing double shear creep testing at lower temperatures (T?<?800 °C) and higher stresses (τ?>?600 MPa). In this creep regime, the macroscopic crystallographic [11?2](111) shear system deforms significantly faster than the [01?1](111) system. This represents direct mechanical evidence for a new planar fault nucleation scenario, which was recently suggested (Wu et al. in Acta Mater 144:642–655, 2018). The double shear creep specimen geometry inspired a micro-mechanical in-situ shear test specimen. Moreover, the in-situ SEM shear specimen can be FIB micro-machined from prior dendritic and interdendritic regions. Dendritic regions, which have a lower γ′ volume fraction, show a lower critical resolved shear stress.     

82B硬线钢碳硫偏析的影响因素及合理工艺参数研究

以现场试验和实验室分析相结合的方法,对不同连铸工艺条件下生产的82B铸坯的碳硫偏析行为进行了系统研究。结果表明:过热度、拉速和末搅拌三个影响铸坯偏析的因素所起的作用随铸坯内部位置和偏析元素种类的不同而不同。针对中心碳硫偏析,拉速的影响大于其它两个因素。在拉速提高的情况下,适当增加二冷强度可以有效地减轻铸坯的碳硫偏析特别是中心部位的碳硫偏析。中间包的水力学模拟表明适宜的拉速为1.8 m/min。     

Microstructure,Texture, and Mechanical Behavior of As-cast Ni-Fe-W Matrix Alloy

This article describes the tensile properties, flow, and work-hardening behavior of an experimental alloy 53Ni-29Fe-18W in as-cast condition. The microstructure of the alloy 53Ni-29Fe-18W displays single phase (fcc) in as-cast condition along with typical dendritic features. The bulk texture of the as-cast alloy reveals the triclinic sample symmetry and characteristic nature of coarse-grained materials. The alloy exhibits maximum strength (σ_YS and σ_UTS) values along the transverse direction. The elongation values are maximum and minimum along the transverse and longitudinal directions, respectively. Tensile fracture surfaces of both the longitudinal and transverse samples display complete ductile fracture features. Two types of slip lines, namely, planar and intersecting, are observed in deformed specimens and the density of slip lines increases with increasing the amount of deformation. The alloy displays moderate in-plane anisotropy (A_IP) and reasonably low anisotropic index (δ) values, respectively. The instantaneous or work-hardening rate curves portray three typical stages (I through III) along both the longitudinal and transverse directions. The alloy exhibits dislocation-controlled strain hardening during tensile testing, and slip is the predominant deformation mechanism.     

Coupling Effects of Melt Treatment and Ultrasonic Treatment on Solidifying Microstructure and Mechanical Performance of Ti44Al6Nb1Cr Alloy

The coupling effects of melt treatment and ultrasonic treatment on the solidifying microstructure and mechanical performance of Ti44Al6Nb1Cr alloy are investigated. During melt treatment, a low superheat degree is beneficial for microstructure refinement, with the lamellar colony size decreasing from 512 to 243 μm, while a low cooling rate leads to the microstructure coarsening as the lamellar colony size enlarges from 458 to 615 μm. After coupling with ultrasonic treatment, under moderate superheat degree and cooling rate, the original coarse lamellar colony size is significantly refined to 56 and 38 μm, the compressive strength is improved by 60.71 and 47.89 pct, and the compressive strain is enlarged by 80.19 and 112.33 pct, respectively. It is found that the ultrasonic refining efficiency is dominated by the melt temperature, and there is an optimum temperature range near the crystallization temperature: a too-high temperature leads to the remelting of crystal nuclei, impairing the refining efficiency, whereas a too-low temperature results in high viscosity, hindering the ultrasonic effects. Under ultrasonic treatment, the melt supercooling is increased, leading to an extended constitutional supercooling region, which will enlarge the crystal nucleation; the solute enrichment is enhanced, forming a quasi-steady state with a higher solution concentration gradient, which improves the crystal growth velocity.     

连铸工艺参数对SWRH82B钢160 mm x 160 mm坯中心碳偏析的影响

试验研究了结晶器电磁搅拌频率、拉速、过热度及二冷强度对SWRH82B连铸小方坯中心碳偏析的影响。研究结果表明,结晶器搅拌电流为300 A时,低电磁搅拌频率下铸坯中心碳偏析情况较好;拉速为1.8 m/min时,提高二冷比水量有利于改善中心碳偏析,但二冷比水量过高会加剧偏析;拉速为1.8 m/min时,二冷比水量为0.75 L/kg较为合适,拉速为1.9 m/min时,二冷比水量为0.8 L/kg是比较合适的;当过热度在20～30℃时,过热度对铸坯中心碳偏析的影响不大。     

SWRH82B铸坯凝固质量控制

针对安钢SWRH82B钢生产的工艺现状,研究了拉速、过热度、二冷配水等工艺参数对于铸坯中心缩孔、疏松和偏析等内部质量的影响。在实验室进行了铸坯凝固组织的研究,测量了二次枝晶间距,进一步了解连铸工艺参数对铸坯凝固组织的影响。     

12Cr1MoVG?210mm连铸坯表面裂纹分析与控制

分析出12Cr1MoVG?210 mm铸坯表面裂纹为应力裂纹,其直接原因是受热应力和组织应力影响,根本原因是不合理的连铸工艺制度。避免裂纹措施包括控制浇注过热度20~30℃,拉速1.5~1.7 m/min,使用更适合包晶钢生产的保护渣;优化二冷配水,比水量由0.30 L/kg增加到0.48 L/kg,并增加3区和4区冷却水比例;铸坯冷床下线后堆放缓冷。