真空电渣重熔炉发展前景的探讨

真空电渣重熔炉是在真空电弧重熔炉和气体保护电渣炉的基础上发展起来的一种冶炼高性能合金的特殊真空重熔冶金装备。真空电渣重熔炉冶炼的钢种和合金有优质合金钢,高温合金,精密合金,耐蚀合金,铝、钛、银等合金。介绍了真空电渣重熔炉的工作原理,冶金特点,比较了真空电弧重熔、普通电渣重熔和真空电渣重熔的钢材的冶炼成本、冶金质量和性能,并讨论了真空电渣重熔炉的应用前景。     

Predicting the Effect of Pouring Temperature on the Crystallite Density,Remelting, and Crystal Growth Kinetics in the Solidification of Aluminum Alloys

In the present work, we developed an analytical model to describe the effect of pouring temperature on the crystallite density, remelting, growth kinetics, and the resultant final grain size for aluminum (Al)-based alloys synthesized using gravity casting. The model predicts that there are three regimes of pouring temperature/grain size-related behavior: (i) at low superheats, grain size is small and relatively constant; (ii) at intermediate levels of superheat, there appears to be a transitional behavior where grain size increases in a rapid, non-linear fashion; and (iii) at high superheats, grain size increases linearly with increasing temperature. This general pattern is expected to be shifted upward as distance from the bottom of the casting increases, which is likely a result of the slower cooling rates and/or longer solidification times with increasing distance from the bottom of the casting. To validate the model, a set of experiments has been conducted using Al-Cu and Al-Si alloys (i.e., Al-3.0 wt pct Cu, Al-4.5 wt pct Cu, and Al-A356.2 alloys), and the experimental measurements showed consistent results with theoretical predictions.     

特种冶金生产流程的发展趋势

特种冶金是生产高端特殊钢和特种合金的主要手段,代表了一个国家的工业化发展水平,一直是国内外冶金技术竞争的制高点.本文首先回顾和总结了传统特种冶金技术的种类和主要生产流程;其次,概述了典型特殊钢品种的特种冶金生产流程;再次,对典型特钢企业的特种冶金生产流程进行了介绍;最后,对特种冶金生产流程的未来发展趋势作出预测.新技术将进一步促进特种冶金生产流程的变革与发展.     

电渣重熔满足大型锻件的要求

 与锭模浇注出低密度、带缩孔铸锭相比,高性能铸锭材料的生产趋于用重熔设备来实现,铸锭的力学性能也会更均匀。水冷铜结晶器中重熔的铸锭具有较高的材料产出量,从而使具有相同最终外形的铸锭其单锭的质量更小,密度更接近于锻造的材料。因此,后续的锻造工艺不需要高锻造比来生产高密度铸锭。在100t以下的铸锭通常可以在固定锭模中生产,并配以外部底加热裝置,以避免可能的铸锭底部开裂;50t以上的铸锭采用高度复杂的拉锭系统配以多电极交换,在短的环形坩埚中重熔技术才是最经济的。大型的带有电极交换技术的电渣重熔炉,为生产超过钢厂中最大单个钢包浇铸质量的大尺寸铸锭提供了可能性。介绍了生产145t重铸锭的炉型设计。对于容易出现开裂的高度复杂的合金,可以通过应用隔热绝缘来生产。     

Surface Oxidation of the High-Strength Steels Electrodeposited with Cu or Fe and the Resultant Defect Formation in Their Coating during the Following Galvanizing and Galvannealing Processes

This study examined the surface oxidation of high-strength steels electrodeposited with Cu or Fe and the resultant defect formation in their coating during the following galvanizing and galvannealing processes. The high-strength steels were coated with an Cu or Fe layer by the electroplating method. Then, the coated steels were annealed in a reducing atmosphere, dipped in a molten zinc, and finally transformed into galvannealed steels through the galvannealing process. The formation of Si and Mn oxides on the surface of the high-strength steel was effectively suppressed, and the density of surface defects on the galvanized steel was significantly reduced by the pre-electrodeposition of Cu and Fe. This effect was more prominent for the steels electrodeposited at higher cathodic current densities. The finer electrodeposit layer formed at higher cathodic current density on the steels enabled the suppression of partial surface oxidation by Mn or Si and better wetting of Zn on the surface of the steels in the following galvanizing process. Furthermore, the pre-electrodeposited steels exhibited a smoother surface without surface cracks after the galvannealing process compared with the untreated steel. The diffusion of Fe and Zn in the Zn coating layer in the pre-electrodeposited steels appears to occur more uniformly during the galvannealing process due to the low density of surface defects induced by oxides.     

Microstructural evolution of austenite under conditions simulating thin slab casting and hot direct rolling

Abstract

The hot direct rolling (HDR) of thin slabs introduces some new microstructural phenomena with respect to conventional hot rolling of steels. This paper aims to investigate the microstructural changes of as cast austenite under these conditions. Current laboratory techniques for HDR simulation require a freshly cast slab for every experiment and a perfect link between casting and hot deformation. The present work adopted a new approach; the C–Mn steel is substituted by austenitic Fe–30Ni alloy, Conventional reheating before rolling replaces the direct link. The experimental ingot casting of Fe–30Ni alloy resulted in a solidification structure in good agreement with that of thin slabs of C–Mn steels. From metallographic observations, a mixed softening process and a strong grain refinement and homogenisation characterise the microstructural changes during HDR simulation. The microstructural behaviour and the grain refinement measured for the Fe–30Ni alloy is closely comparable with that predicted for C–Mn steels for the same conditions. The steel substitution appears to constitute a suitable and advantageous experimental approach for HDR simulation.     

A Study on Hot Tearing Susceptibility of Al–Cu,Al–Mg,and Al–Zn alloys

The present investigation deals with the hot tearing susceptibility of A206, A518, and A713 alloys. The hot tearing tests of the mentioned alloys were conducted at three different pouring temperatures using sand mold casting. Metallic cores designed to facilitate constrained radial contraction of the aforementioned alloys were used for casting. Macroscopic cracks were found in all the samples except in A518 alloy. It was observed that pouring temperatural and grain size have significant effect on crack susceptibility. Among the investigated alloys, A713 was found to be extremely prone to hot tearing. The microstructure characteristics of the alloys were studied using optical and scanning electron microscopy. Relationships between the pouring temperature, grain size and crack lengths of the alloys were also established.     

Thermodynamic modeling as a strategy for casting highly alloyed steels

 Oxides are encountered in steel plant operations, such as in steel making and continuous casting - in the form of fluxes, slags and refractories. Further, oxidation of some readily oxidizable elements often takes place - leading to certain difficulties in the process. The formation / presence of certain oxides can lead to problems such as clogging of Submerged Entry Nozzle during continuous casting. Difficulties associated with the oxides of Aluminium and Chromium have been reported in the literature. Strategies based on thermodynamic calculations can be used to overcome the problems associated with such oxides. Approaches to enable sustained casting of highly alloyed steels have been taken up in the present study. One of the strategies to avoid the oxidation of Chromium is to add small amounts of other elements (subject to other constraints), which do not cause subsequent problems. The problem has been studied using the Thermo-CalcR software, with related databases; and, the results obtained for different process conditions / generic compositions have been presented in this paper.     

Surface Oxidation of Steel Powder

Powder metallurgical production of fully dense steel such as high‐speed steel, tool steel and stainless steel is of large industrial importance. The process route is as follows: melting – inert gas atomization – encapsulation – hot isostatic pressing and subsequently often also hot forming. Final products comprise near‐net‐shape components, billets and semi‐finished parts e.g. rods. The mechanical and functional properties of the PM steels are often superior to those of conventional manufactured steels over casting + forging with the same chemical composition. The properties of the PM‐steels are highly dependant on their oxide contents. There are three types of oxides in PM‐steels: exogenous and endogenous slag inclusions and oxides originating from surface oxidation of the atomized particles. The negative effect from the inclusions on the properties of the PM‐steels is dependant on their origin. The exogenous inclusions are normally highly detrimental while the endogenous inclusions are harmless. Surface oxides are harmful when present in larger amounts. A special experimental technique was developed to study surface oxidation of powders. Large emphasis was also focused on sampling to enable that the oxygen determination was representative of the bulk powder. The results thus obtained gave most important information on when and how the atomized powder was oxidized during the process. Furthermore, a new methodology was developed based on the experimental results. It is now used world‐wide for quality control of atomized steel powders. This method enables the distinction between exogenous + endogenous inclusions and surface oxides.     

Modern small-capacity unit for the vacuum-oxygen refining of steel

A vacuum-oxygen refining unit has been installed in the pouring and furnace bays of electric steelmaking shop No. 2 at the Zlatoust Metallurgical Plant. The shop specializes in the production and casting of stainless, chromium-nickel, and complex-alloyed steels, alloyed high-speed and tool steels, and nickel alloys. The unit can be used to perform both vacuum degassing and vacuum oxygen refining.     

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To investigate the segregation and remelting properties of B- bearing steel, segregation of B in SS400 B- bearing steel was studied through original position analyzer for metal(OPA), secondary ion mass spectrometry(SIMS), micro- segregation model, and the retrograde melting phenomenon on cracks during casting of B- bearing steels was observed by confocal scanning laser microscope (CSLM). The micro- segregation model shows that when solidified fraction is 0. 96, the segregation level of B is beyond 16. The OPA result indicates that the maximum segregation levels of B come from 4 samples are 32. 37, 18. 01, 18. 40 and 12. 96. Model calculations and experimental results show a strong segregation of B in steel, it also shows that although the mass fraction of B in SS400 B- bearing steel is only 0. 0013%, but since B segregates in steel, mass fraction of B on local region may exceed 0. 015%. The analysis of SIMS shows that the serious segregation of element B occurs in the crack area. The results of CSLM studies show that the remelting phenomenon are found around surface crack in slab when the temperatures are 1008. 4??1150. 9??, 1294. 8??1304. 9??, 1325. 8??1342. 8??, 1379. 8??1420. 6??.     

Equilibrium Model of Precipitation in Microalloyed Steels

The formation of precipitates during thermal processing of microalloyed steels greatly influences their mechanical properties. Precipitation behavior varies with steel composition and temperature history and can lead to beneficial grain refinement or detrimental transverse surface cracks. This work presents an efficient computational model of equilibrium precipitation of oxides, sulfides, nitrides, and carbides in steels, based on satisfying solubility limits including Wagner interaction between elements, mutual solubility between precipitates, and mass conservation of alloying elements. The model predicts the compositions and amounts of stable precipitates for multicomponent microalloyed steels in liquid, ferrite, and austenite phases at any temperature. The model is first validated by comparing with analytical solutions of simple cases, predictions using the commercial package JMat-PRO, and previous experimental observations. Then it is applied to track the evolution of precipitate amounts during continuous casting of two commercial steels (1004 LCAK and 1006Nb HSLA) at two different casting speeds. This model is easy to modify to incorporate other precipitates, or new thermodynamic data, and is a useful tool for equilibrium precipitation analysis.     

High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying and Adding Nitrided Ferroalloys

 A simple and feasible method for the production of high nitrogen austenitic stainless steels involves nitrogen gas alloying and adding nitrided ferroalloys under normal atmospheric conditions. Alloying by nitrogen gas bubbling in Fe Cr Mn Mo series alloys was carried out in MoSi2 resistance furnace and air induction furnace under normal atmospheric conditions. The results showed that nitrogen alloying could be accelerated by increasing nitrogen gas flow rate, prolonging residence time of bubbles, increasing gas/molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. Nitrogen content of 069% in 18Cr18Mn was obtained using air induction furnace by bubbling of nitrogen gas from porous plug. In addition, the nickel free, high nitrogen austenitic stainless steels with sound and compact macrostructure had been produced in the laboratory using vacuum induction furnace and electroslag remelting furnace under nitrogen atmosphere by the addition of nitrided alloy with the maximum nitrogen content of 081%. Pores were observed in the ingots obtained by melting and casting in vacuum induction furnace with the addition of nitrided ferroalloys and under nitrogen atmosphere. After electroslag remelting of the cast ingots, they were all sound and were free of pores. The yield of nitrogen increased with the decrease of melting rate in the ESR process. Due to electroslag remelting under nitrogen atmosphere and the consequential addition of aluminum as deoxidizer to the slag, the loss of manganese decreased obviously. There existed mainly irregular Al2O3 inclusions and MnS inclusions in ESR ingots, and the size of most of the inclusions was less than 5 μm. After homogenization of the hot rolled plate at 1 150 ℃×1 h followed by water quenching, the microstructure consisted of homogeneous austenite.     

Surface Cracks in Wire Rod Rolling

Surface defects in wire rod and bar rolling are common and well‐known to mill people. Nowadays, surface defects are not accepted on high‐alloyed steel wire rods. The steel making, casting and rolling processes give rise to defects. Also, the final handling of the wire and bar can destroy the surface. In this work, artificial V‐shaped cracks in the longitudinal direction were investigated for different reduction series. The false round‐oval series are known as a series for high quality steels and are usually better than square–oval series. Experiments confirmed that in the false round‐oval sequences a surface crack in the groove bottom may open up during rolling at the same time as its depth is reduced, which is a beneficial situation. Surface cracks found at 45° to the rolling direction, at groove “corners” and on free surfaces will be closed or reduced in depth. The closing of cracks is detrimental since the cracks usually hide rolled‐in oxides beneath the bar surface. The experiments showed that for the subsequent oval–false round sequence the visible crack at the groove bottom will be closed and become shallower. The cracks at 45° and on the free sides will also be closed, but deeper causing a serious surface defect. An FE‐analysis was carried out, explaining the experimental results. Flat oval grooves are better than round ovals and false rounds are superior to square for opening and decreasing the depth of a longitudinal crack. It is difficult to eliminate a surface defect constituting a closed crack.     

Effects of Mold Powder Chemistry on Shell Growth,Momentum and Heat Transfer in a Billet Mold

Effects of mold powder chemistry on shell growth and thinning have been studied using Computer Fluid Dynamic techniques under conditions of constant casting speed and steel superheat for a peritectic steel in a billet caster. Two mold powders were considered; a basic powder suitable for peritectic steels and an acid powder not recommendable for this steel in order to emphasize the importance of chemistry on shell stability. Numerical results indicate a strong interaction between powder composition and steel flow‐heat transfer phenomena. The acid powder creates recirculating flows at both sides of the entry jet that transport sensible heat to the shell inducing its remelting and thinning leading, eventually, to a strand breakout. Meanwhile, the basic powder induces a single recirculating flow in the internal radius side of the mold without severe shell thinning. A colder meniscus is predicted using the acid powder which is in agreement with the casting practice experience. Powder infiltration of the basic powder in between the mold hot wall and the strand provides a powder shell with a macroscopically smooth surface while the acid powder yields irregular infiltration. Buoyancy forces along the mold working height and mold curvature play a fundamental role on the generation of the recirculating flows. Interaction between powder chemistry and fluid flow‐heat transfer are two‐way coupled phenomena that must be considered for powder design purposes.     

Identification of Defect Prone Peritectic Steel Grades by Analyzing High-Temperature Phase Transformations

Continuous casting of peritectic steels is often difficult and critical; bad surface quality, cracks, and even breakouts may occur. The initial solidification of peritectic steels within the mold leads to formation of surface depressions and uneven shell growth. As commercial steels are always multicomponent alloys, the influence also of the alloying elements besides carbon on the peritectic phase transition needs to be taken into account. Information on the solidification sequence and phase diagrams for initial solidification are lacking especially for new steel grades, like high-alloyed TRIP-steels with high Mn, Si, and particularly high Al contents. Based on a comprehensive method development, the current study shows that differential scanning calorimeter measurements allow a clear prediction if an alloy is peritectic (i.e., critical to cast). In order to confirm these results, thermo-optical analyses with a high-temperature laser-scanning-confocal-microscope are performed to observe the phase transformations in situ up to the melting point.     

Thermodynamic Modeling as a Strategy for Casting High Alloy Steels

Strategies based on thermodynamic calculations can be used to overcome the problems associated with oxides encountered in steel plant operations, which can lead to certain difficulties in the process such as clogging of submerged entry nozzle during continuous casting. Approaches to producing high alloy steels by continuous casting have been taken. One of the strategies to avoid the oxidation of chromium is to add a small amount of other elements (subject to other constraints), which do not cause subsequent problems. The problem has been studied using the Thermo-CalcR software, with related databases; and the results obtained for different process conditions or generic com-positions have been presented.     

Reversible temper embrittlement of rotor steels

The susceptibility to temper embrittlement of eight different rotor steels has been studied in terms of the effects of composition, of cooling rate from tempering temperature, of isothermal aging, of steel-making practice and of strength level and tempering temperature. The Ni Cr Mo V steels tested showed increasing susceptibility to temper embrittlement with increasing nickel content. The normally marked susceptibility of a high phosphorus 3 pct Cr Mo steel was eliminated by the removal of manganese. Embrittlement in a 3 pct Ni Cr Mo V steel was caused by the equilibrium segregation of solute atoms to the prior austenite grain boundaries. Two Cr Mo V steels tested were not susceptible to temper embrittlement. Electroslag remelting and refining had very little effect on the susceptibility of the steels tested. Strength level and tempering temperature had no effect on the degree of embrittlement of the 3 pct Ni Cr Mo V disc steel. The possibilities of remedial action include an adjustment of the post tempering cooling rate, to optimize the conflicting interests of minimum temper embrittlement and adequate stress relief, and the production of very low manganese rotor steels.     

粉末冶金锰钢研究新进展

铁基粉末冶金产业在继续开发与扩大使用非传统粉末冶金合金化元素.Mo、Ni与Cu的价格、环境与可回收性一直在推动这种扩大使用.Mn是铸锻钢的价格比较低廉且有效的合金化元素;不过,过去氧的敏感性限制了Mn在粉末冶金钢中的应用,现在氧分压低的氮-氢烧结气氛可允许使用Mn.Mn与适量Mo的组合可使粉末冶金钢合金的力学性能接近F...