Steel Desulphurisation and Sulphur Bonding in Solidified Ladle Slags

It was investigated in which form sulphur is bonded in ladle slags. For this purpose, the mineralogical phases of the ladle slags were qualitatively and quantitatively evaluated by means of a microprobe. This slag characterisation allowed reconstructing the bonding of sulphur into the phases of the ladle slag. Moreover, the reaction process with regard to the ionic theory as well as the structural formula for the mechanism of sulphur bonding were discussed. The bonding of sulphur into calcium aluminate slags is determined by the sulphide ion content in the slag as well as by the ratio of (CaO)/(Al₂O₃). Calcium sulphide separates only if the slag contains a large number of sulphide ions. When Ca₁₂Al₁₄O₃₃ is separated the sulphide ion is successively integrated into the lattice. The crystal lattice of Ca₁₂Al₁₄O₃₃ is characterised by an open, three‐dimensional anionic network composed of aluminium‐oxygen tetrahedrons linking the edges. In this network, large interstitial spaces are formed, which may be filled with calcium ions on the one hand and statistically with a free oxygen or sulphide ion on the other hand.     

Characteristics analysis of inclusion of 60Si2Mn–Cr spring steel via experiments and thermodynamic calculations

A plant trial of the production of 60Si2Mn–Cr spring steel using silicon–manganese combined with aluminium to deoxidise was performed, and the characteristics of inclusions during ladle furnace refining, calcium treatment and in billets were investigated by scanning electron microscope–energy dispersive spectroscopy and thermodynamic calculations. The formation mechanisms of oxide and CaS inclusions are discussed. The experimental observation and thermodynamic analysis showed that calcium treatment cannot entirely modify large-size MgO·Al₂O₃ spinel inclusions into homogeneous CaO–MgO–Al₂O₃ inclusions, but formed a liquid xCaO·yAl₂O₃ layer on its surface. When the Al content was 0.05 mass%, [Mg], [Ca] and [O] in molten steel could be controlled at 2.7～5 ppm, 2.5～8 ppm and 4.1～5.2 ppm, respectively, to achieve inclusions in the low melting point region. A large amount of CaS was generated in the present process due to a higher sulphur concentration in the molten steel and an excessive amount of Ca–Si wire. To avoid/reduce its formation, the sulphur concentration should be controlled to below 70 ppm.     

LF精炼工艺对36MnVNS4含硫非调质钢硫含量和夹杂物形貌的影响

试验的36MnVNS4含硫非调质钢（/%:0.36C,0.66Si,1.00Mn,0.010P,0.045S,0.26V,0.0110N）的冶炼工艺流程为铁水+废钢-70 t EBT EAF-LF-方坯连铸-轧制。研究了LF 19.82%Al2O3,（CaO）/（SiO₂）=2.64和14.63%Al2O3,（CaO）/（SiO₂）=2.15两种渣系精炼对软吹后钢中氧含量,喂S线后S的收得率以及钢中夹杂物成分和形貌的影响。结果表明,高碱度白渣精炼工艺有利于钢中氧含量的降低,但不利于钢中硫含量的稳定;精炼渣碱度（CaO）/（SiO₂）由2.64降低至2.15时,有利于钢中硫含量的稳定控制,硫的回收率由35%提高至75%;两种精炼工艺下钢中的夹杂物分布、形貌和组成基本相同。通过钢包钙处理,长条状MnS夹杂转变为球状复合夹杂。     

Inclusion evolution after calcium addition in Al-killed steel with different sulphur content

The inclusion evolutions after calcium treatment in Al-killed steel with different sulphur content were determined by chemical experiments and thermodynamics calculations. The results show that the inclusions are calcium aluminate with a little amount of calcium sulphide in low content sulphur steel after calcium treatment. As the sulphur content in steel increases, the typical inclusions generated in steel are layered or homogeneous irregular oxysulphides. A number of pure sulphides can be observed in higher content sulphur steel, and the morphology of sulphides with different CaS/MnS mass ratio has different shape. Thermodynamics calculations of Al–Ca–O–S–Mn system inclusions in steel at 1873?K and during solidification process were comprehensively conducted, considering all types of inclusions. The experimental results are in good agreement with thermodynamic calculations, which can predict the formation of the inclusions in Al-killed steel with different sulphur content.     

Optimisation of ladle slag composition by application of sulphide capacity model

Abstract

The sulphur distribution ratio for ladle slags at Ovako Steel AB has been determined by calculating the sulphide capacity using the Royal Institute of Technology (KTH) model and the alumina activity via an empirical expression suggested by Ohta and Suito, and by using the well established theories for sulphur refining. A parameter study was carried out based on plant data from a previous study on desulphurisation. The effect of carbon and aluminium in the steel, the temperature of the steel, and alumina and lime in the slag on the sulphur distribution ratio was investigated. It was shown that the Al₂ O₃ /CaO ratio had the largest influence on the conditions studied, and that when it increased the sulphur distribution ratio decreased. Based on these results, new plant trials were carried out, in which the alumina content in the slag was changed. The calculated sulphur distribution ratios for these trials were found to be in good agreement with experimentally determined sulphur distribution ratios. It is concluded that the present approach can be used to optimise multicomponent ladle slags with respect to sulphur refining.     

Quantitative study of modification of sulphide inclusions by calcium and its effect on the impact toughness of a resulfurised alloy steel

In this paper, the mechanisms of the shape control of sulphide inclusions by calcium in a resulfurised alloy steel and the quantitative effect of the shape factor, the size and the quantity of sulphide inclusions on the transverse impact toughness are investigated. There are three different types of sulphide inclusions in the calcium-treated resulfurised alloy steel, 58CrNiMnMoVSCa, namely elongated MnS, oval (Mn,Ca)S with and without any calcium aluminate nucleus. The proportion of the former is reduced while that of the two latter raised as the ratio of Ca/S in steel increases. The mechanisms of the shape control of sulphide inclusions by calcium are mainly due to its solid solution hardening effect. Besides, the calcium aluminate, existing as the nucleus of the oval (Mn,Ca)S, restricts the hot deformation of the sulphide inclusions and as a result, enhances further the shape control. Based on the solid solution effect of calcium on sulphide inclusions, the quantitative relationship between the shape factor Λ of sulphide inclusions and the ratio of Ca/S in steel at a hot forging reduction of 85 is developed. Finally, the quantitative relationship between the transverse impact toughness and the parameters of sulphide inclusions, namely the shape factor (Λ), the size (L) and the quantity (N) is also developed.     

Y20CaRE易切削钢的研究

在50kg的真空感应炉中用硅钙合金和稀土金属处理了含硫的Q235钢,研制了一种新型的环保型易切削钢,并进行了切削和力学性能的测试.借助于扫描电镜和能谱分析技术研究了钢中夹杂物.其测试结果表明,适量加入钙和稀土可明显改善钢中的硫化物夹杂、有效提高钢的易切削性能,且对钢材力学性能无不利影响.     

X70管线钢钙处理研究

在X70管线钢LF精炼后期分别以100 m/min和200 m/min速度向钢中喂入300~500 m硅钙线或添加硅钙钡铁,对钢中夹杂物进行变性处理。喂钙后钢样、结晶器钢样和铸坯样中的CaTot和夹杂物分析表明:钙处理后钢中钙含量在不断下降;夹杂物的组成、形貌和尺寸也在不断变化。根据钢中夹杂物的变化、夹杂物对高级别管线钢性能和连铸顺行的影响,提出了高级别管线钢生产中夹杂物变性控制策略。     

Ladle deoxidation,desulphurisation and inclusions in steel –Part 2: Observations in practice

Practical observations on the application of oxygen sensors, argon stirring, mould additions for rimstabilized steel. Discussion of ladle desulphurisation. Deformability of oxide inclusions, control of blowhole formation. Suppression of the precipitation of MnS by absorbing the dissolved sulphur with minute calcium aluminate particles entrained in liquid steel containing less than 0.002% S, and by rare earth treatments before casting steel containing 0.002 to 0.006% S. Aspects of steelmaking and casting clean steel, particularly high-strength, low-alloy steels.     

Ladle deoxidation,desulphurisation and inclusions in steel – Part 1: Fundamentals

Critical review and reassessment of technology on the fundamental aspects of ladle deoxidation, desulphurisation and inclusions in steel. Selected equilibrium data on the solubility products of simple and complex oxides, sulphides and oxysulphides in liquid steel. Equilibrium data for sulphur distribution between calcium aluminate slags and liquid steel deoxidised with aluminium. Capability of lime-saturated calcium aluminate of desulphurising aluminium-killed steel to levels below 0.001% S. Mechanism of dissolution of metallic additives in liquid steel. Nucleation, growth and flotation of reaction products.     

钙处理对成品无取向硅钢夹杂物特性的影响

通过取样检测结合热力学计算,分析了钙处理对成品无取向硅钢中夹杂物特征及硫化物夹杂的析出机制的影响。结果表明,钢中尺寸大于3μm的有害夹杂物主要是AlN、MgO-SiO2、CaO-Al2O3-SiO2类复合夹杂物及其与MgS、MnS、CaS的复合析出物。钙处理钢中没有检测到单独的Al2O3、SiO2及铝酸钙类夹杂物。钙处理钢中形成的液态3CaO·Al2O3、MgO·SiO2和Al2O3夹杂物被精炼渣吸收,改性去除了钢中大尺寸Al2O3夹杂物。钙处理钢中尺寸大于3μm的氧化物夹杂主要是含CaO和(或)CaS的Al2O3-SiO2类夹杂。硫化物在MgO-SiO2类氧化物表面的析出有利于其形貌趋于规则。钢中不同形貌的AlN夹杂物呈多尺度分布,钙处理对大尺寸AlN的析出特性影响不大。氧硫化物及其与AlN复合析出并定向长大的过程,与其晶体结构有关。氧化物夹杂的硫容量决定了其与硫复合的难易程度。钙处理钢中CaS在氧化物表面呈局部包裹析出和局部吸附析出。     

低碳铝镇静钢中间包浇注过程夹杂物的行为

 通过对BOF-Ar站-CC炼钢流程生产低碳铝镇静钢的中间包不同浇注时间取样及正常坯的取样,采用氧氮化学分析、光学显微镜以及扫描电镜-能谱(SEM+EDS)等多种方法研究了中间包浇注过程夹杂物特征的变化。结果表明：每炉钢包开浇时与浇注末期,钢中T[O]含量均高于浇注中期的T[O]含量,这是由于换包过程中钢水被二次氧化;中间包钢水及正常坯中的夹杂物,按照其形貌与成分可以分为以下3类：Al2O3基夹杂物,MnS基夹杂物,来自中间包覆盖剂或者钢包下渣所卷入的外来夹杂物。中间包及铸坯中的夹杂物主要以1~4 μm的Al2O3为主,同时在铸坯中发现了大量的MnS夹杂物,使铸坯中夹杂物的数量密度升高。当钢液中硫含量较高时,铸坯中气泡+Al2O3类型的夹杂物增加。在当前的工艺条件下,交换钢包之后的开浇阶段与浇注末期,钢水的二次氧化对铸坯的洁净度产生重要影响,同时应合理控制钢中的硫含量,减少铸坯中气泡+Al2O3类型的夹杂物,避免钢液在凝固过程中析出大量的MnS夹杂物。     

Minimisation of calcium additions to low carbon steel grades

Abstract

This investigation aimed to determine the in plant feasibility of decreasing, to an amount close to the minimum literature value, the calcium addition to liquid steel for prevention of alumina buildup during continuous casting. Six plant trials were carried out at calcium additions of 0·14 kg/t of steel (reduced from the original 0·19 kg/t), added to the second ladle of a two or three ladle sequence. Total oxygen samples were taken at the ladle furnace and tundish to determine total oxygen and nitrogen contents of the steel. The total oxygen content at the ladle furnace varied between 19 and 26 ppm, with a slight degree of reoxidation between the ladle furnace and the caster. Alumina clogging was successfully prevented by the addition of 0·14 kg calcium/t of steel during the first five trials. During the sixth trial the submerged entry nozzle (SEN) failed and, although the stopper behaved as if clogging occurred, this behaviour was caused by the poor perfomance of the SEN rather than actual clogging. Microanalysis of inclusions in steel samples revealed a distribution in degrees of modification between different inclusions, and the formation of a substantial amount of CaS (which is taken to indicate overmodification, based on equilibrium calculations). However, the CaS is mostly associated with at least partly liquefied oxide inclusions, which is likely to reduce the potential clogging effect of solid CaS.     

钙处理钢中大型球状及棒状夹杂的成因

 大型夹杂物对钢材的加工性能、力学性能和耐腐蚀性能等产生十分有害的影响。用电解萃取法研究了钙处理钢中大型球状/棒状夹杂物的性质,通过对大型球状/棒状夹杂物形貌的扫描电镜观察和元素成分能谱分析,指出钢中的大型球状/棒状夹杂起源于呈团簇状的铝脱氧产物Al₂O₃。大量小颗粒Al₂O₃夹杂组成尺寸较大的夹杂团簇,在钢包内复杂流场作用下形成球状或棒状。钢液在钙处理过程中,变性充分的夹杂物形成了低熔点的铝酸钙,在钢液凝固后形成致密的球状夹杂物;变性不充分的夹杂外形仍然保留Al₂O₃夹杂颗粒形貌。钙处理使Al₂O₃夹杂变性所需的w([Ca])/w([Al])主要受钢液中硫质量分数影响。铝酸钙对钢液中的硫有较强的吸收溶解能力,在浇铸过程中,随着钢液温度下降,铝酸钙吸收的硫以CaS夹杂形式从基体中饱和析出。     

改善套管钢冶炼过程浸入式水口堵塞

 通过金相分析、扫描电镜、能谱分析等手段对国内某钢厂生产石油套管钢的水口堵塞物进行了分析,对冶炼过程Al2O3夹杂的控制问题从热力学角度进行了研究。热力学计算结果表明,当钢中的 [Al]s含量为0.025%,若钙处理时钢中钙含量高于22×10-6,硫含量低于0.008%,可较容易地将Al2O3夹杂变性为低熔点易上浮的C12A7。在热力学计算基础上,通过强化钢水钙处理,钢中钙含量提高至25×10-6～40×10-6,夹杂物变性上浮容易,连浇炉数由3～4炉提高至大于8炉。     

Deep steel desulphurisation technology in ladle furnace at KSC

Abstract

At the Mizushima Works of Kawasaki Steel Corporation, a new deep desulphurisation technology to produce ultralow sulphur steels has been developed to replace the conventional process that comprised ladle furnace treatment followed by flux injection. In the new process, the use of a double plug gas injection system in the ladle furnace to promote desulphurisation, and optimisation of the ladle slag composition by thermodynamic calculations to maximise sulphide capacity, have enabled the flux injection stage to be omitted and shortened the desulphurisation time. As a result of these modifications, 35min after treatment start the sulphur content can be reduced to 5·3 ppm. On average, the time from tapping at converter until teeming start at caster has been reduced by 20·8min enabling production schedules to be synchronised and the maximum number of sequential casting heats to be increased from 3 to 10. There have also been economic benefits: the total steelmaking cost of ultralow sulphur steel has been reduced by 25·5%.     

含硫-铝钢GS35C-A硫化钙夹杂热力学分析和工艺改进

针对试验GS35C-A钢（/%:0.34～0.35C,0.20～0.22Si,0.69Mn,0.013～0.016P,0.026～0.045S,0.027～0.046A1,0.000 8～0.001 9Ca）夹杂物变性进行了热力学计算,并对工艺1[BOF-LF（硫合金化）-RH-钙处理-CC]和优化工艺2[BOF-LF-RH-钙处理-硫合金化-CC]对应的钢中夹杂物形貌及数量进行了研究。结果表明:（1）随着钢液中[S]的升高,生成CaS所需的平衡钙含量降低,钙处理变性生成低熔点夹杂物时钢液中的[Al]逐渐降低;（2）优化工艺2后RH离站Ca、S含量分别从优化前（工艺1）的0.001 9%和0.045%降至0.000 8%和0.026%,平均连浇炉数由原2.5炉提高至8炉;（3）优化前（工艺1）中间包典型夹杂物组分为硫化钙包裹的镁铝尖晶石,优化后（工艺2）中间包夹杂物为铝酸钙;RH离站、中间包钢中每mm²夹杂物个数分别从优化前9.09和6.23降至5.06和3.76。     

Effect of Calcium Treatment on Non-Metallic Inclusions in Ultra-Low Oxygen Steel Refined by High Basicity High Al2O3 Slag

 The influence of calcium treatment on non-metallic inclusions had been studied when control technology of refining top slag in ladle furnace was used in ultra-low oxygen steelmaking. A sufficient amount aluminium was added to experimental heats for final deoxidizing during BOF tapping, and the refining top slag with strong reducibility, high basicity and high Al2O3 in ladle furnace was used to produce ultra-low oxygen steel and the transformation of non-metallic inclusions in molten steel was compared by calcium treatment and no calcium treatment. The results show that the transformation of Al2O3→MgO·Al2O3 spinel→CaO-MgO-Al2O3 complex inclusions has been completed for aluminum deoxidation products and calcium treatment to molten steel is unnecessary when using the control technology of ladle furnace refining top slag to produce ultra-low oxygen steel, and the complex inclusions are liquid at the temperature of steelmaking and easily removable to obtain very high cleanliness steel by flotation. Furthermore, the problems of nozzle clogging in casting operations do not happen and the remaining oxide inclusions in steel are the relatively lower melting point complex inclusions.     

Model of Desulphurization of Steel Melts in Ladles – Deoxidation and Slag Formers Calculation

The authors present their own model applicable for effective desulphurization of steel by ladle treatment. The model is based on a thermodynamic approach (equilibrium data) and technological data (correction factors). It consists of two parts. In the first part the authors present the formulae derived for estimation of the additions of deoxidation agents which ensure sufficiently low oxygen level prior to the desulphurization process, while the other part gives a qualitative and quantitative selection of slag formers to achieve low final levels of sulphur in steel. For the deoxidation process two variants were considered: (i) with Al only (for low silicon steels) or (ii) Al‐Si (for silicon steels) as deoxidizers. For the desulphurization process three variants were assumed as to ladle slag composition: (i) slag consisting of a fraction of furnace slag, lime addition and deoxidation products, (ii) slag made of synthetic CaO–Al₂O₃ and (iii) slag based on lime and fluorspar. The model formulae for desulphurization were derived using the sulphide capacity concept which relies on the optical basicity. In addition, rough estimates of the slag liquidus temperatures are given. A numeric example of the model application and the model algorithm (appendix) are enclosed.