Non‐Metallic Inclusions in High‐Manganese‐Alloy Steels

The possibility of applying new high‐strength steels with excellent forming behaviour (TRIP, TWIP and LIP steels) in automotive manufacturing is a significant potential for improvement in the area of reducing weight while simultaneously increasing crash safety. The present work investigates endogenous inclusions in some high‐alloy TRIP and TWIP steels because the most stringent product requirements are tightly related to cleanness. The expected formation of inclusions is discussed based on thermodynamic observations made with ThermoCalc. The solidification conditions were varied in experiments with the so‐called SSCT (submerged split chill tensile) apparatus. Furthermore, different treatment times were set in order to investigate this influence on the inclusions. A catalogue of endogenous inclusions in these new steel grades is currently being created with the help of the automated SEM/EDX inclusion analysis system at voestalpine Stahl GmbH in Linz. Further studies will follow to systematically determine the interactions between steel, slag and refractory materials.     

The Influence of Non‐Metallic Inclusions Physical State on Effectiveness of the Steel Filtration Process

The work presents theoretical fundamentals of the process of refining molten steel from liquid non‐metallic inclusions, using the method of filtration with ceramic filters ‐ the thermodynamic precondition for the absorption of liquid non‐metallic inclusions on the surface of a ceramic filter. The theoretical consideration has been supported by the results of laboratory tests on the filtration of steel which was previously reduced with complex deoxidants giving liquid products of deoxidization. The filtration process of steel melts reduced with complex deoxidants of type Al‐Mn‐Si has proved to be more efficient. The deoxidation products were identified on the filtration surface of the ceramic filters.     

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.     

A Comparison of a Two‐Dimensional and a Three‐Dimensional Method for Inclusion Determinations in Tool Steel

To produce clean commercial tool steel the non‐metallic inclusions characteristics have to be known, since they influence the mechanical properties of steel. In this work, inclusion characteristics in steel samples from plant trials were studied. The samples were collected in the steel plant according the two following methods: (a) 2D investigations of inclusions by a cross sectional method and (b) 3D investigations of inclusions collected on a film filter after electrolytic extraction. More specifically, the chemical composition, morphology, number and size distribution of inclusions in tool steel samples taken from ladle during melt treatment were determined by both methods. In both methods SEM equipped with EDS was used for compositional analysis of inclusions. In addition, in the cross sectional method an automated detection program called ‘INCAFeature’ was used to collect more statistics of non‐metallic inclusions. The composition of inclusions larger than 5 µm was found to contain 49% CaO based on the results from both methods. However, for smaller inclusions it was found that the accuracy of the 2D method was less than that of the 3D method due to the influence of the metal matrix on the results. In addition, it was found that a critical inclusion size of 4 µm could be defined for the 3D method, above which the standard deviation in composition determination was very low.     

Control and Engineering of Non‐metallic Inclusions belonging to xSiO2‐yCaO‐zAl2O3 System in Ca‐treated Al‐killed and Al‐Si‐killed Steel

A model based on the fundamental principles of thermodynamics has been applied to forecast and control the precipitation of non‐metallic inclusions in Ca‐treated Al‐killed or Al‐Si‐killed steels. The engineering of the non‐metallic inclusions takes place during the period just after the tapping from the electric arc furnace until the beginning of the casting period. The construction of the model and its validation have been accomplished through a precise monitoring of the treatment of several steel grades characterized by different oxygen contents after tapping. The oxygen killing of the steel melt with an oxygen content between 750‐1200 ppm is performed by the addition of Si and Al, which produces the formation of pure Al₂O₃ but not always the formation of pure SiO₂. This is a fundamental hypothesis of the model confirmed by experimental observations. The other fundamental aspect is related to the determination of the oxygen activity in the steel bath, which is defined from experimental measurements by an electrochemical concentration cell and through the computation of the equilibrium between the steel bath and the ladle slag. The comparison between the experimental data and the non‐metallic evolution forecast by the model on the basis of the minimization of the oxygen potentials has shown very interesting performances, which makes the model a suitable and very stable tool for industrial application.     

冶金熔渣对不锈钢中非金属夹杂物的影响

利用示踪试验,研究了在304不锈钢生产过程中,钢水接触的各种冶金熔渣对连铸坯中非金属夹杂物的影响。研究发现,AOD钢渣混出时进入钢水中的小渣滴是钢中非金属夹杂物的主要来源之一,而大包顶渣、中间包覆盖剂和结晶器保护渣不会对钢液造成明显污染。     

Characterisation of Dispersed Nano‐Sulphides Precipitated in Low Carbon Steel Solidified under High Rate of Cooling

The aim of this study was to investigate size distribution, chemistry, thermal stability and deformability of nano‐sulphides finer than about 100 nm formed in low carbon steel solidified under high rate of cooling and to assess the effect of the nano‐sulphides on the properties of steel. Laboratory mini‐ingots of low carbon steel containing various amounts of Mn, Cu and S, solidified under conditions simulating the direct strip casting process, were used as experimental material. Nano‐sulphides of MnS, (Mn,Cu)S, CuS, Cu_2‐xS and CuS_2‐x were identified in substantial quantities (volume fraction up to 3 ·10^?4) using TEM techniques. It was shown that interaction between the nano‐sulphides and migrating grain boundaries of austenite occurred during annealing and in the course of thermomechanical processing resulting in grain refinement. Nano‐sulphides of MnS and (Mn,Cu)S type were also identified in significant quantities (volume fraction up to 6 ·10^?4) in the shell and corner areas of commercial continuously cast billet.     

脱氧工艺对硅钢连铸坯非金属夹杂物的影响研究

 研究了Al+FeSi以及FeSi两种不同脱氧工艺对硅钢铸坯中非金属夹杂物的影响。通过合理的试验设计并运用T［O］含量测定、金相分析、SEM及EDS等研究手段,对铸坯中的夹杂物进行了分析,研究表明Al+FeSi复合脱氧不仅比FeSi脱氧具有更强的脱氧能力,且能使铸坯中的夹杂物在尺寸和数量上有明显降低,夹杂物的特征也由Si O Al Mn Ti Fe系变性为以尺寸＜5 μm的Al2O3夹杂为核心、包覆Al Si S Mn Ca Ti Fe O系的复合夹杂。     

Control of Non‐metallic Inclusions by Slag‐metal Reactions for High Strength Alloying Steels

A laboratory study was carried out to investigate non‐metallic inclusions in high strength alloying steel refined by high basicity slag. The results indicated that the inclusions were mainly of the CaO? MgO? Al₂O₃ system, Al₂O₃? MgO and MgO‐based inclusions. The steel/slag reaction time and Al₂O₃ content in slag had a great effect on inclusions characteristics. With the reaction time increasing from 30 to 180 minutes, inclusions experienced a transformation process: from mainly Al₂O₃? MgO system and MgO‐based inclusions to spherical CaO? MgO? Al₂O₃ system inclusions surrounded by a lower melting temperature surface layer of CaO? Al₂O₃. Formation and transformation mechanisms of the inclusions were given based on the results. It was also found that with Al₂O₃ content in slag reduced from 40% to 30%, [Mg] contents in steel melts were increased and MgO in slag reached saturation, which contributed to the formation of more MgO‐based inclusions and a more scattered inclusion composition distribution after 90 min reaction.