Influence of alloying elements on the thermal contraction of peritectic steels during initial solidification

Abstract

A modified SSCT (submerged split chill tensile) test was used to measure the contraction forces occurring during the solidification of steels with a carbon content of between 0·05 and 0·2 wt-% at constant Mn (1·55%) and Si (0·3%) contents. Further test series were performed with varying Si, V, Cr, Mn, Ni and Nb contents at a constant C content of 0·1%. Any cracks generated were detected, counted and measured. The total crack length allows conclusions to be drawn about the influence of alloying elements on crack susceptibility. The measured results match very well with results calculated from hot tearing criteria. Besides the steel composition, the superheat of the melt proved to be the most important factor in crack formation caused by contraction.     

Modification of oxide inclusions in calcium-treated Al-killed high sulphur steels

A study has been carried out to understand the modification of alumina inclusions in Al-killed high sulphur steel with calcium treatment. For calcium treatment to be effective, a general practice is to desulphurise the steel to prevent the formation of solid CaS inclusions that are harmful to steel quality and final properties. To avoid this additional desulphurising step, the authors developed a new approach of calcium treatment of steel at an industrial scale. This approach involves treating the liquid steel with calcium treatment at low aluminium levels which enables formation of liquid calcium aluminate inclusions (C12A7) in the melt and then adding remaining amount of required aluminium. Based on this principle, Al-S diagram has been developed and calcium treatment has been modified accordingly. The inclusion transformation and morphology were studied using scanning electron microscope /energy dispersive spectroscopy analysis and. activity of CaS was calculated.     

帘线钢凝固过程夹杂物生成热力学及工业实践

 非金属夹杂物是影响帘线钢拉拔性能的重要因素之一,为了研究帘线钢中夹杂物的生成及转变机理,使用ASPEX自动扫描电镜观察分析了帘线钢工业生产过程中不同碱度条件下从钢液到铸坯中非金属夹杂物的转变现象,并使用FactSage7.0热力学计算软件对非金属夹杂物的转变机理进行了讨论。在高碱度条件下,钢液中非金属夹杂物主要类型为低熔点的CaO-SiO₂-Al₂O₃-MnO,铸坯中非金属夹杂物的CaO和MnO含量有所降低,同时SiO₂含量有所增加。在低碱度炉次中,钢液中非金属夹杂物主要为较高熔点的SiO₂-MnO-CaO类型,Al₂O₃含量较低。连铸坯中非金属夹杂物的SiO₂含量与钢液相比有所增加,同时MnO含量降低。热力学计算结果表明,帘线钢凝固和冷却过程中的非金属夹杂物转变由夹杂物自身的相转变和析出、非金属夹杂物和钢液间的化学反应以及溶解氧和钢基体化学成分的反应3方面原因造成。热力学计算结果较好地解释了帘线钢工业生产中钢液和铸坯中非金属夹杂物成分和形貌的转变,为帘线钢中非金属夹杂物的控制提供参考。     

重轨钢中MnS析出热力学和动力学分析

 采用无水有机溶液电解法分离提取重轨钢中的MnS夹杂物,采用扫描电镜观察铸坯内和钢轨中MnS夹杂物的三维形貌,并结合能谱仪分析其成分。铸坯被轧制成钢轨后,相应的MnS夹杂物都沿着轧制方向被轧制成长条状。基于热力学和动力学模型,分析重轨钢中MnS夹杂物析出行为以及在钢液凝固过程中锰元素和硫元素偏析的程度。热力学分析表明,MnS夹杂物在凝固末期凝固分数为0.94时开始析出,其析出量由初始[w([Mn])]和初始[w([S])]决定,且在凝固过程受到冷却速率的影响,对比发现,热力学的计算析出结果与Thermo-Calc和FactSage6.4的计算结果有较好的一致性;动力学分析表明,在钢液凝固过程增加冷却速率,凝固析出的MnS颗粒尺寸将减小。通过调整钢中[w([Mn])]和[w([S])]以及改变冷却速率,可以控制MnS的析出时机和形态,减小其对钢性能的有害影响。     

GCr15轴承钢脱氧过程中非金属夹杂物生成热力学及工业实践

通过热力学软件FactSage 7.0和工业实践,对1 873 K下GCr15轴承钢脱氧过程中非金属夹杂物生成热力学进行研究。计算结果表明,当轴承钢中的w(Mg)>0.4×10^-6时,钢中夹杂物由Al₂O₃转变为MgO·Al₂O₃;当钢中的w(Mg)>10×10^-6时,钢中夹杂物主要为MgO。当轴承钢中w(Al)>100×10^-6、w(Ca)>0.1×10^-6时,钢中开始生成固态CaO·6Al₂O₃和CaO·2Al₂O₃夹杂物;当钢中w(Ca)>2×10^-6时,钢中生成的夹杂物为液态钙铝酸盐;当钢中w(Ca)>13×10^-6时,钢中开始生成固态CaO夹杂物。工业实践检测和热力学计算结果基本吻合,此外,研究发现纯铁液的脱氧热力学与轴承钢差异较大,因此...     

Formation and evolution of macro inclusions in IF steels during continuous casting

The evolution of macro inclusions during continuous casting was investigated by large-area inclusion characterization using ASPEX and analysis of nozzle blockage deposits. Six kinds of inclusions over 5?μm were observed in samples taken from the tundish and the slabs: single alumina particles, alumina dendrites, refractory-related alumina, alumina associated with bubbles, alumina clusters and Al–Ti complex oxides. By examining the morphology of the nozzle blockage deposit, it was concluded that the refractory-related alumina in the slabs came from the decarburization layer washed away by the steel stream. Some of the alumina clusters that came from the nozzle blockage deposit dislodged by the steel flow, were formed by the agglomeration and sintering of 5–20?μm inclusions that were carried over from the tundish. Two kinds of Al–Ti oxides were found in the tundish, and their evolution mechanisms during the casting process were proposed.     

Nucleation and growth kinetics of inclusions during liquid steel deoxidation

Abstract

The phenomenon of solid inclusion nucleation and growth during the liquid steel deoxidation process is studied numerically. It is shown that the diffusion of atoms, the Ostwald ripening effect, and Brownian movement together control the growth of small size particles. It appears that this stage of initial precipitation lasts for only about 10 s. When the particle diameter exceeds a threshold of ~1 μm, the growth process becomes dominated by particle collisions and agglomeration. The efficiency of these mechanisms depends on the particle size distribution as produced during growth. Comprehensive evaluations are still impeded by the limited capacity of computers at present.     

脱氧工艺对车轮钢氧化物夹杂控制的影响

从脱氧材料的选择、加入时间和加入方式以及脱氧产物排除方法试验了不同脱氧工艺对车轮钢中氧化物夹杂控制的影响,试验结果表明:采用出钢预脱氧+LF一次性加Al+钙处理工艺,能有效降低钢中B类夹杂,可完全满足车轮钢对B类夹杂物的技术条件要求。     

吹氩钢脱氧铝消耗及脱氧工艺对夹杂物的影响

 由250 t转炉冶炼吹氩钢的生产数据分析,发现转炉脱氧工艺对脱氧铝消耗的影响大。出钢全脱氧工艺比半脱氧工艺的脱氧铝单耗高0.28 kg/t、脱氧铝利用率低8.3%;随转炉温度的上升或氧质量分数增加,脱氧铝单耗逐步增加,同时,转炉温度的上升会引起氧质量分数的增加。通过对SPHC板坯在两种脱氧工艺下的夹杂物分析,发现：宏观上两种脱氧工艺的全氧控制水平相当,约为0.002 0%;半脱氧工艺的氮质量分数为0.002 0%,全脱氧工艺时为0.002 5%。微观上,夹杂物主要为Al2O3、Al2O3-MnS、氧化物和[MnS/MnS-CuxS;]全脱氧与半脱氧工艺相比,Al2O3夹杂物平均尺寸小0.6 μm、Al2O3夹杂物比例低约25%、Al2O3-MnS夹杂物比例高约20%、硫化物比例低约5%。Al2O3夹杂物尺寸的减小易于Al2O3-MnS夹杂物的生成。     

超低碳IF钢脱氧合金化过程中夹杂物的行为

 为深入了解超低碳IF钢在脱氧合金化过程中夹杂物的行为,用高温电阻炉开展试验模拟实际生产中铝脱氧钛合金化过程,通过密集取样,详细研究了该过程中夹杂物的转变。研究发现,加铝前,钢中夹杂物主要为球形的FeO_x;加铝后,最先生成浅灰色的球形Al₂O₃,然后向椭球形或单体块状Al₂O₃转变,随后迅速聚合形成不规则状Al₂O₃,最终聚合成簇群状Al₂O₃,整个过程大约在加铝后2 min内完成;加钛后,钢液中形成3种类型的Al-Ti复合类夹杂物,但在加钛大约4 min后便会转化为稳定的Al₂O₃相。整个脱氧合金化过程中,氧含量和夹杂物的量呈下降的趋势,钢液的洁净度逐渐提高。     

高品质弹簧钢精炼过程氧化物夹杂的控制

研究了采用LF-VD-CC精益窄窗口控制工艺路线时,合适的脱氧、造渣、合金及夹杂物控制等技术对钢水洁净度的影响。结果表明,采用本工艺能够生产出全氧质量分数低于0.000 8%、Ti质量分数低于0.001 8%、Al_s质量分数低于0.002 2%的高品质弹簧钢;将精炼顶渣中Al₂O₃质量分数控制在不超过5%时,能够有效实现氧化物夹杂的低熔点化和塑性化;精炼过程中,熔点进入1 600 ℃等温线以内的氧化物夹杂在不断长大,并且通过上浮去除,而残留于钢中的夹杂物不仅熔点低、变形性能好,而且尺寸细小,对疲劳性能的影响较小。     

Deoxidation with silicon and the control of oxide inclusions in electrical steels

The oxygen solubility in Fe-Si melts in equilibrium with SiO₂ at 1873 K has been determined in a concentration range of 0.1–70 wt % Si. Model alloys are melted in quartz crucibles in an argon atmosphere. The oxygen content in analytical samples is determined by the inert-gas reducing-fusion method after careful sample preparation. The results obtained have been processed using a thermodynamic model that can calculate the oxygen activity and solubility in Fe-Si melts up to 100 wt % Si. The effects of the heating rate and the silicon content on the carbon concentration in carbonyl iron and Fe-Si alloys are studied using the inert-gas reducing-fusion method in the temperature range 1673–2373 K. Oriented electrical steels are investigated using fractional gas analysis. The main forms of oxygen in these steels are found to be silicates, Al₂O₃, and MgAl₂O₄.     

Adverse effect of high purity atmosphere on sintering of manganese steels

Abstract

The Fe–(2, 3)Mn and Fe–(2, 3)Mn–0·4C specimens were sintered in the dilatometer tests for 30 min at 1150°C in an H₂–H₂ O atmosphere with a dew point of –30 and –70°C. The Mn addition at sintering of the specimens with a dew point of –30°C resulted in higher swelling with a maximum at 1150°C and processing at a dew point of –70°C resulted in minimal differences in swelling of the specimens in dependence on the Mn addition. Processing in the high purity atmosphere (–70°C) causes a loss of Mn in the form of Mn vapour transported away by the flowing atmosphere needed for alloying of the Fe matrix. On the contrary, at sintering in a low purity atmosphere (–30°C) the reaction of Mn vapour with oxygen of high partial pressure occurs near the surface of the specimens. Sintering of Mn steels requires the low purity atmosphere in contrast to the thermodynamic requirements. The sintering of Mn steels occurred under the effect of Mn vapour formed by sublimation regardless the thermodynamics for Mn–O system.     

The effects of MO and W on solidification of high speed steels

The effects of systematic variations in Mo content, W content, and the Mo:W ratio upon the freezing process and as-cast carbide morphology of high speed steels were studied for four series of alloys encompassing the nominal composition ranges of AISI type M2 (6 W-5 Mo-4Cr-2V-0.85C) and MIO (0W-8Mo-4Cr-2 V-0.85C) high speed steels. Thermal analysis, metallographic examination, and quantitative metallography were used to characterize these effects. The Hquidus, peritectic, and eutectic reactions were similarly influenced by molybdenum and tungsten, the peritectic temperature being strongly depressed by additions of either element. The types of carbides found in the as-cast structures did not vary, but the amount of feathery eutectic carbide (a layered structure of MC and M₆C) was directly relatedto the total Mo plus W content. The amount of isolated vanadium-rich MC type carbide was seen to increase as the amount of feathery eutectic decreased, and also varied with the Mo:W ratio.     

Formation of CaS on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO inclusions during solidification of steels

In this article, the effect of CaS formation on the evolution of Al₂O₃-CaO inclusions in low-carbon Al-killed and Ca-treated steel during the solidification process is investigated through high-temperature confocal scanning laser microscopy (CSLM). The inclusions started as mostly liquid-globular inclusions that did not agglomerate with each other on the melt surface but during solidification were seen to change shape into an irregular morphology. The shape change was found to be due to the reaction between the Al₂O₃-CaO inclusions with the dissolved S and Al in the melt, resulting in the formation of dense CaS shells around the inclusions. The melt composition during solidification, estimated from the observed solid δ-front advance rate, was compared to the thermodynamic limit for CaS precipitation. The observed growth rate of the CaS shell was found to initially increase with decreasing temperature because of the higher, solid δ-front advance rates at lower temperatures, which results in higher rates of S and Al partitioning. Once CaS had precipitated, the inclusions were found to form agglomerates on the melt surface because of fluid flow, initially, and later, the capillary depression.     

Evolution of the distribution function of the composition of oxide inclusions for the growth model of a disperse system formed during deoxidation

The growth model for nuclei of oxide inclusions growing during the process of steel deoxidation is proposed. The evolution of the inclusion composition has been analysed by solving the continuity equation for the probability-density function f(x,t) (where x is the oxide mole fraction). The calculations were carried out for model parameters and for steel deoxidation by aluminium or silicon-manganese. The result obtained cannot be verified due to the lack of any functional dependence relating chemical composition to inclusion radius.     

Variations in the average size and quantity of nonmetallic inclusions during oxidation and deoxidation of steel

An investigation has been carried out to study the effects of the degree of oxidation or deoxidation of liquid steel on the average size, number, and quantity of nonmetallic inclusions. During deoxidation the variations are strongly influenced also by the nature of the deoxidizing elements and may, therefore, be used to study the individual characteristics of the elements. Some calculations of the oxygen and inclusion contents of the bath are made on the simple assumption that all inclusions are spherical and have the same radius at any given instant. It is shown that such calculations may be used to obtain reasonably reliable estimates.     

Equilibrium and growth characteristics of hydrogen-induced intergranular cracking in phosphorus-doped and high purity steels

By means of fracture mechanics analyses, acoustic emission techniques, and fracture surface analyses by scanning Auger microscopy and X-rays, it was determined how segregated phosphorus, yield strength and grain size affect the equilibrium and growth characteristics of hydrogen-induced intergranular cracking in high strength steels. The effect of yield strength on the threshold stress intensity was found to be greater than those of phosphorus segregation and grain size. The intergranular phosphorus segregation greatly accelerated the growth rate of hydrogen-induced intergranular cracking and caused a large number of acoustic signals to be emitted during the crack growth. The crack growth rate increased in a steel with segregated phosphorus and slightly decreased in high purity steels, where only occasional acoustic emissions were measured during the cracking process, by increasing grain size. Fracture surface analyses indicated more featureless intergranular fracture facets and higher levels of residual strain in the lattice adjacent to the fracture surface in the phosphorus-doped steel than in the high purity steels. These results suggest that while in steels with segregated impurities the macrocrack tends to grow by discretely rapid formation of intergranular microcracking which gives rise to dislocation emissions at the growing crack tip, in high purity steels slow growth of intergranular microcracking proceeds which is accompanied by either the absence or substantial mitigation of dislocation generation at the crack tip.     

高纯氧化镥中痕量铁的富集及测定研究

采用吡咯二硫代氨基甲酸铵（APDC）沉淀富集高纯氧化镥中痕量的铁,再将沉淀溶于甲基异丁基酮（MIBK）溶液中,分离富集后的溶液用石墨原子吸收法直接测定.当盐酸酸度控制在0.4 mol/L,加2%APDC溶液2 mL,沉淀放置时间为15 min,选取石墨炉原子吸收光谱仪的灰化温度1100℃,原子化温度2300℃时,铁的测定效果最佳.铅、镍、铬、钴、铜等基体元素对铁的测定基本无干扰.该方法具有较好的精密度和准确度,相对标准偏差为4.5%,加标回收率在94.0%～108.0%之间.     

Slag formation during high temperature interactions between refractories containing SiO2 and iron melts with oxygen

Abstract

Refractory–metal interactions and associated reactions are important during casting as they can lead to the formation of inclusions. High temperature interactions between aluminosilicate refractories and molten steel were studied in the present investigation using the sessile drop method. Changes in the contact angles between molten iron with well defined oxygen levels and substrates of silica, mullite (3Al₂O₃.2SiO₂) as well as commercial refractories, were measured in the dynamic mode. In all cases, the reaction led to slag formation. The experiments were performed under isothermal conditions at 1823 and 1873 K. A known and constant oxygen partial pressure was imposed above the iron drop, through appropriate CO–CO₂–Ar mixtures. For all substrates, the contact angles started decreasing due to lowering of the surface tension of iron, as oxygen was imposed onto the system. When a critical level of oxygen was reached, slag started forming at the drop/substrate interface and at this stage, the contact angle dropped suddenly. Subsequent behaviour of silica and mullite was observed to be different, with SiO₂ showing a non-wetting behaviour. In the case of SiO₂ substrate, there were also deep erosions along the periphery of the drops and there was slag accumulation at the sides of the metal drop, probably due to Marangoni flow and/or SiO_(g) formation. SEM and EDS analysis of the substrates revealed that the formed slag was fayalite. The thermodynamic criteria for slag formation and possible mechanisms of the reaction are discussed in the light of the experiments.