钛脱氧钢中针状铁素体的形成

通过实验研究了钢中加钛脱氧后针状铁素体的形成及夹杂物的变化。对实验钢的化学成分、组织变化、析出相成分、钢中氧含量及夹杂物尺寸进行了分析。实验证明,在钢中加钛处理后,钢中产生了大量的以钛氧化物或钛铝等复合氧化物为核心的针状铁素体,钢的组织明显细化,随着钛加入后孕育时间的延长,钢中夹杂物的尺寸变小,孕育时间存在最佳值。     

Thermodynamics of the oxygen solutions in boron-containing Fe–Co melts

A thermodynamic analysis of the oxygen solutions in boron-containing Fe–Co melts has been performed. The equilibrium constant of reaction between boron and oxygen, which are dissolved in iron–cobalt melts; the activity coefficients at infinite dilution; and the interaction parameters for melts differing in composition have been determined. The oxide phase formed in the Fe–Co melts containing boron and oxygen comprises FeO and CoO along with the B₂O₃ phase. The oxide phase compositions over Fe–Co–B–O melts are calculated. As the cobalt and boron contents in the melts increase, the mole fraction of boron oxide increases; in the case of pure cobalt, it is close to unity. The dependences of the oxygen solubility on the cobalt and boron contents in the melts are calculated. The deoxidizing capacity of boron substantially increases as the cobalt content in a melt increases. The composition dependences of the oxygen solubility in boron-containing Fe–Co melts have a minimum, which shifts to a low boron content as the cobalt content in the melts increases. The boron contents corresponding to the minimum in the oxygen solubility curves and the minimum oxygen concentrations corresponding to the boron contents are determined.     

氧化物冶金在高强度低合金钢生产中的试验研究

通过对氧化物冶金技术在高强度低合金钢生产中的试验研究,探索采用氧化物冶金技术冶炼过程中转炉终点及LF精炼过程钢中的氧含量控制、正确的脱氧方式、脱氧合金元素的加入时机等,通过调节钢液成分,确保形核诱导粒子的生成;同时,利用扫描电镜分析了钢中夹杂物的成分、形貌、尺寸及夹杂物周围的组织形貌,并对试验钢板的性能进行了取样检测。结果表明,试验炉次氧化物冶金的效果良好且试验钢板性能很好。     

Controlling the composition and quantity of oxide inclusions in a chromium steel

The use of a method for measuring the oxygen activity in steels with 2.5 and 9.0% Cr has been studied in order to control the characteristics of oxide inclusions. The composition and number of oxide inclusions can be reliably predicted from data on the oxidation state of the metal and the consumption of deoxidizing agents. The method developed ensures the minimum amount of an oxide phase represented by a certain type of nonmetallic inclusion in the finished metal on the basis of measuring the oxygen activity during the technical process of steelmaking.     

Deoxidizer activity and the composition of the oxide phase in chromium steel

Nonmetallic inclusions in various chromium steels are studied. The contents of silicon and aluminum oxides in the nonmetallic phase are shown to correlate with the activities of these elements in the metal. Relations are obtained to calculate the fraction of the oxide of a deoxidizing element in the nonmetallic inclusions from its activity.     

Manufacture of high-nitrogen corrosion-resistant steel by an aluminothermic method in a high-pressure nitrogen atmosphere

The conditions of aluminothermic synthesis of high-nitrogen Cr-N and Cr-Mn-N steels in a high-pressure nitrogen atmosphere are studied by thermodynamic simulation and metallurgical experiments. Thermodynamic analysis shows that the aluminothermic reduction reactions are incomplete. The most important synthesis parameter is the ratio of the aluminum to the oxygen content in a charge, and its optimum value ensures a compromise between the degree of oxide reduction, the aluminum and oxygen contents in steel (degree of deoxidation), and steel contamination by aluminum nitride. An analysis of experimental heats demonstrates good agreement between the experimental results and the data calculated by a thermodynamic model. As-cast ingots have the structure of nitrogen pearlite, and quenched ingots have an austenitic structure.     

LBE-LF-CSP低碳铝镇静钢脱氧工艺优化实践

针对邯钢三炼钢厂冶炼CSP低碳铝镇静钢时终点氧质量分数较高,原铝锭脱氧工艺铝吸收率低、进站Als不稳定、综合成本较高等问题,通过深入研究、分析及试验,在转炉工序采用钢砂铝脱氧剂代替铝锭优化出钢脱氧 工艺。经过9号、4号转炉1个月的生产实践,取得了良好的实用效果:转炉出钢脱氧铝吸收率及稳定性明显提高, 精炼工序负担减轻,成品Als控制水平明显提高,综合铝制品成本降低了1.13元/t。     

Effect of liquid–solid pouring on the as-cast structure and the distribution of nonmetallic inclusions in a 24.2-t steel 38KhN3MFA ingot

The primary dendrite structure and the distribution of nonmetallic inclusions in large-scale 24.2-t forging ingots of 38KhN3MFA steel cast by a standard technology and with the inoculation of a metallic stream are studied. The dendrite parameter in the inoculated ingot is found to be significantly smaller than that in the usual ingot. Therefore, the solidification process in the inoculated ingot should be faster than that in the usual ingot. This is confirmed by a more homogeneous dendrite structure of the inoculated ingot. The estimation of contamination with nonmetallic inclusions shows that nonmetallic inclusions in the ingots under comparison are of the same type, and among them are oxides, sulfides and oxysulfides. The study of the distribution of nonmetallic inclusions shows that the ingots to be compared are mainly contaminated with oxysulfide nonmetallic inclusions, and the contents of oxide and sulfide nonmetallic inclusions are minimal. The experimental ingot has lower total contamination with nonmetallic inclusions than that of the reference ingot. In this case, the inoculators having formed from a metallic melt stream do not favor an increase in the contamination of steel with nonmetallic (among them are oxide) inclusions. The study of the structure of large metal volumes shows that the inoculation of the stream is accompanied by an increase in the number of metallic droplets, which transform into solid particles in flight and increase the solidification rate when reach a solidifying ingot. This process suppresses the development of segregation phenomena in the cast metal and decreases the chemical and physical heterogeneities.     

Thermodynamics of the oxygen solutions in manganese-containing Fe-Co melts

Thermodynamic analysis of the oxygen solutions in manganese-containing Fe-Co melts has been performed. The equilibrium constants of deoxidation reaction of iron-cobalt melts with manganese, the activity coefficients during infinity dilution, and the interaction parameters in various melts are found. During the deoxidation of manganese-containing Fe-Co melts, the oxide phase contains FeO and CoO along with MnO. The compositions of the oxide phase above Fe-Co-Mn-O melts are calculated. When the cobalt and manganese contents in the melts increase, the mole fraction of manganese oxide increases, and it approaches 1 in the case of pure cobalt. The dependences of the oxygen solubility in the melts on the cobalt and manganese contents are calculated. The deoxidizing capacity of manganese increases substantially with increasing cobalt content in the melt. The curves of oxygen solubility in Fe-Co melts have minima, whose values shift toward low manganese content in a melt. The manganese contents are determined at the minimum points in the oxygen solubility curves, and the corresponding minimum oxygen contents are found.     

Formation of silica and silicates during the solidification of Fe−Si−O alloys

Small (35 g) ingots of Fe?Si?O alloys have been solidified under controlled conditions and the form, distribution, and compositions of the oxides in the cast structures are reported. The composition of a melt was established by charging a preselected amount of silicon, 0.002 < pct Si < 0.7, and equilibrating the melt with the silica crucible at 1550° or 1650°C. The concentration of FeO and the morphology of the oxide particles in the cast structures indicate that during solidification the process of segregation causes the oxygen potential in the interdendritic liquid to increase over that in equilibrium with the silicon content of the liquid. It appears that nuclei for the formation of the oxide phases, which probably are in the liquid prior to the onset of solidification, become surrounded by solid iron and are made ineffective. It is possible at lower silicon levels that homogeneous nucleation may result by the melt becoming sufficiently supersaturated in oxygen to form FeO or an iron-silicate liquid phase.     

Mathematical Modelling of Magnesia Carbon Refractories Reactions with Steel

The production of super clean steel has put in clear evidence the importance of the refractory in influencing the degree on internal cleanliness of steel. In fact chemical species transfer from the refractory to the steel bath. To reduce the steel contamination caused by the refractory it is necessary to know the interaction between the refractory and the steel. A mathematical model of the modifications of the refractory surface in contact with the steel has been developed. The work is focused on magnesia‐carbon refractory (taking into account the presence of reducing agents too, as Al, Si and Mg) and carbon steel. The model calculates the new phases formed when liquid steel and refractory are in contact. The model is based on the following mechanism of reactions: (i) Magnesia reacts with graphite forming gaseous CO and Mg. (ii) The other oxides present in the refractory (mainly CaO and SiO₂ which constitutes a glassy phase) pass into the steel, giving dissolved elements and dispersed oxides. (iii) Gaseous Mg reacts at the interface refractory/steel with oxygen dissolved in the steel forming solid MgO. (iv) The MgO generated and the oxides originating from the glassy phase form the new oxide system. The newly formed phase can potentially be transferred to the bath through the steel flow against the refractory walls. The results of the model simulations have been validated by experimental tests.     

细小异相去除夹杂物的试验研究

提出用脱氧净化剂产生细小气泡去除夹杂物并同时生成低熔点液态复合氧化物吸附夹杂物的控制技术，用SiMo电阻炉进行了热态试验。结果表明，脱氧净化剂在高温下分解生成小气泡和小渣滴，去除了细小夹杂物，处理后钢中的氧化物夹杂物显著降低，全氧的质量分数最低达到19×10^-6。     

含铝钢定氧加铝工艺试验

对含铝钢经预吹氩定氧后，根据钢中氧含量确定加铝量，再补吹氩的铝脱氧合金化工艺进行研究和探索。实践表明该工艺较原根据吹氩后钢水成分微调铝的二次加铝工艺简化了工序操作，缩短了工序时间，提高了酸溶铝命中率，减少铝耗，降低成本，能满足全连铸生产对钢质和生产节奏的要求。     

Deoxidation of high-melting-point metals and alloys in vacuum

Through melting, refractory metals and alloys are becoming important for use as structural materials. Vacuum-melting is considered to be able to provide high-purity ingots because of its purifying effect, particularly the effect of evaporation deoxidation via volatile oxides. Thermodynamic calculation leads us to predict a limited possibility for deoxidizing Ti, Zr, Hf, a good possibility for Mo and W, and an intermediate possibility for V, Nb, and Ta. The sponges of these metals containing oxygen were degassed under high vacuum at temperatures below or over the melting point, and the change in oxygen content was measured as a function of time. The observed rates of purification agreed well with the thermodynamically estimated deoxidizing tendencies. Kinetic analysis of the degassing curves obtained for the Nb-O, Ta-O, Mo-O, and W-0 alloys showed that the rate-determining step was the diffusion of oxygen in the metal or the recombination of oxygen with the base metal atoms and the transfer of the molecules into the vacuum. If carbon was present in the metal, the oxygen liberation was considerably accelerated. However, an excess of oxygen to carbon was necessary for the carbon level to be sufficiently lowered at the same time.     

Thermodynamic Calculation and MnS Solubility of Mn-Ti Oxide Formation in Si-Mn-Ti Deoxidized Steel

Mn-Ti oxides in Si-Mn-Ti deoxidized steels after cooling in the furnace were investigated. The composition and morphology of inclusions were analyzed by using FE-SEM with EDS. Mn-Ti oxides were found to be effective sites to induce intragranular ferrite formation. The thermodynamic calculation was employed to interpret the critical condition for Mn-Ti oxide formation. Mn-Ti oxide formation was controlled not only by Mn and Ti content, but also by total oxygen content in steel. When the Mn and Ti contents were around 1.5% and 0. 005%-0. 01%, respec-tively, Mn-Ti oxide could form as the total oxygen content was 0. 001%-0. 002%. The experimental results were in good agreement with thermodynamic calculation results. Also, MnS solubility was examined in Mn-Ti oxide inclu-sion system. With an increase of MnO content in Mn-Ti oxide, MnS solubility in the oxides increased. MnS precipi-tation benefited from high MnO content in Mn-Ti oxide.     

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.     

Thermodynamic Calculation and MnS Solubility of Mn-Ti Oxide Formation in Si-Mn-Ti Deoxided Steel

 Mn-Ti oxides in Si-Mn-Ti deoxidized steels after cooling in the furnace were investigated. The composition and morphology of inclusions were analyzed by using FE-SEM with EDS. Mn-Ti oxides were found to be effective sites to induce intragranular ferrite formation. The thermodynamic calculation was employed to interpret the critical condition for Mn-Ti oxide formation. Mn-Ti oxide formation is controlled not only by [Mn], [Ti] content, but also by total oxygen in steel. When [Mn] and [Ti] were around 1.5 wt% and 0.005~0.01 wt% respectively, Mn-Ti oxide could form as total oxygen was 0.001~0.002 wt%. The experimental results are in good agreement with thermodynamic calculation results. Also MnS solubility was examined in Mn-Ti oxide inclusion system. With MnO content in Mn-Ti oxide increasing, MnS solubility in these oxides increased. High MnO content had an benefit on MnS precipitation in Mn-Ti oxide.     

Maximum Copper and Tin Contents in LC‐steel Thin Strip ‐ Hot Shortness Model Calculations

For conventional casting processes low copper and tin contents have to be ensured in LC‐steel to avoid hot shortness. It is expected that higher cooling rates, e.g. in thin strip casting, permit higher copper and tin limits. Hot shortness occurs because of selective oxidation of the iron whereby the more noble copper is enriched at the steel‐oxide interface. A liquid metallic copper phase which wets the grain boundaries supports cracking during hot deformation. The enrichment of the liquid copper phase depends on the oxidation temperature: At low temperatures copper is solid, cannot wet the steel surface and is incorporated into the growing oxide layer. At mid temperatures (1083‐1177 °C) the copper phase is liquid, wets the grain boundaries of the steel surface and causes hot shortness. At high temperatures a liquid fayalitic slag is formed in the oxide layer if the steel contains silicon. The fayalitic phase occludes parts of the steel surface and removes copper from the steel surface; then hot shortness is reduced or even avoided. Other mechanisms to remove copper from the steel surface need the presence of Fe₃O₄ and Fe₂O₃ in the oxide layer. These iron oxides are not formed for short oxidation times where linear oxidation takes place. Diffusion of copper into the steel is too slow to reduce hot shortness if copper has an elevated concentration in the steel, e.g. 0.5 wt.‐%. Therefore, only the occlusion mechanism is of importance during linear oxidation. A model is established on the basis of these observations in order to predict an upper copper limit in dependence of the steel strip thickness (cooling behaviour) and the oxygen content in the cooling atmosphere (nitrogen‐oxygen mixture). The model is compared to experimental results from KIMAB which are presented in this issue. It is demonstrated that a copper layer thickness of 0.098 μm at the steel‐oxide interface is sufficient to cause cracks of a depth of more than 0.2 mm. For strip thicknesses below 5 mm a simple approximation can be used to predict the maximum copper content in LC‐steel to avoid hot shortness. For example, thin strip of a thickness of 2 mm will have no cracks (above 0.2 mm) even if 0.7 wt.‐% of copper is contained in the LC‐steel. For atmospheres with a reduced oxygen partial pressure even higher copper contents are possible. Tin is with short oxidation times not a problem concerning hot shortness, as shown by the KIMAB results. This may be explained by the much higher diffusivity of tin in iron compared to copper.     

Computational Thermodynamic Study of Inclusions Formation in the Continuous Casting of SAE 8620 Steel

The general purpose of this work is a thermodynamic study of non‐metallic inclusions in the CC tundish for SAE 8620 steel. Specific purposes are: (1) Obtaining the phases formed and their composition in inclusions by applying computational thermodynamics for SAE 8620 steel; (2) Establishing conditions of steel composition for the improvement of SAE 8620 castability, including the plant validation in terms of the castability index. Thermodynamic studies were performed with the commercial software FactSage and databases. Simulations were carried out by the global chemical composition of SAE 8620 steel in the CC tundish, resulting in both steel composition and non‐metallic inclusions (oxides and sulphides) at the desired temperature. Furthermore, results showed both different solid oxides and liquid phase formation in inclusions by varying calcium content in the steel. Thus, it was possible to determine the inclusion composition as a function of aluminium and calcium content of SAE 8620 steel, and moreover, to establish a range of calcium content in which the inclusions are predominantly formed by liquid phase. In addition, the percentages of liquid and solid phase in inclusions, as well as, oxide compositions could be calculated.     

Effect of the Slag Composition and a Protective Atmosphere on Chemical Reactions and Non‐Metallic Inclusions during Electro‐Slag Remelting of a Hot‐Work Tool Steel

The remelting behavior of the hot‐work tool steel X37CrMoV5‐1 is investigated with several experimental melts on a lab‐scale ESR‐plant. The investigated parameters comprise a variation of the slag compositions and the use of a protective nitrogen atmosphere. Variations of the slag composition include slags with different contents of CaF₂, CaO, and Al₂O₃ as well as a variation of the SiO₂‐content in the slag. The remelted ingots are forged and analyzed regarding their chemical composition. The distribution and composition of the non‐metallic inclusions (NMI) is studied by an automated SEM‐EDX method. Additionally, the chemical composition of the slag after remelting is analyzed. The results show clearly an equilibrium reaction between Si and Al in the steel with SiO₂ and Al₂O₃ in the slag as well as the effect of oxygen in open ESR operation. A protective atmosphere reduces the Si‐losses during remelting, but has no major effect on the number or composition of NMI compared to open remelting. The content of NMI, especially the larger ones, is reduced significantly in all remelting experiments. The majority of the NMI are MA‐spinel type except for the CaO‐free slag, where alumina inclusions prevail. In general, remelting leads to an almost complete removal of sulfides, a reduction of oxisulfides, and a slight increase of oxides.