Galvannealing of (High‐)Manganese‐Alloyed TRIP‐ and X‐IP®‐Steel

In this study the influence of Mn on galvannealed coatings of 1.7% Mn‐1.5% Al TRIP‐ and 23% Mn X‐IP®‐steels was investigated. It is shown that the external selective oxides like Mn, Al and Si of the TRIP steel which occur after annealing at 800 °C for 60 s at a dew point (DP) of ‐25 °C (5% H₂) hamper the Fe/Zn‐reaction during subsequent galvannealing. Preoxidation was beneficially utilized to increase the surface‐reactivity of the TRIP steel under the same dew point conditions. The influence of Mn on the steel alloy was investigated by using a 23% Mn containing X‐IP®‐steel which was bright annealed at 1100 °C for 60 s at DP ‐50 °C (5% H₂) to obtain a mainly oxide free surface prior to hot dip galvanizing (hdg) and subsequent galvannealing. As well known from the literature Mn alloyed to the liquid zinc melt stabilizes δ‐phase at lower temperatures by participating in the Fe‐Zn‐phase reactions, it was expected that the metallic Mn of the X‐IP®‐steel increases the Fe/Zn‐reactivity in the same manner. The approximation of the effective diffusion coefficient (D_eff(Fe)) during galvannealing was found to be higher than compared to a low alloyed steel reference. Contrary to the expectation no increased Fe/Zn‐reaction was found by microscopic investigations. Residual η‐ and ζ‐phase fractions prove a hampered Fe/Zn‐reaction. As explanation for the observed hampered Fe/Zn‐reaction the lower Fe‐content of the high‐Mn‐alloyed X‐IP®‐steel was suggested as the dominating factor for galvannealing.     

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

Microstructures and Mechanical Properties of High‐Strength Fe‐Mn‐Al‐C Light‐Weight TRIPLEX Steels

High‐strength TRIPLEX light‐weight steels of the generic composition Fe‐xMn‐yAl‐zC contain 18 ‐ 28 % manganese, 9 ‐ 12 % aluminium, and 0.7 ‐ 1.2 % C (in mass %). The microstructure is composed of an austenitic γ‐Fe(Mn, Al, C) solid solution matrix possessing a fine dispersion of nano size κ‐carbides (Fe,Mn)₃ AlC_1‐x and α‐Fe(Al, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation γ_fcc → ?_hcp amounts to ΔG^γ→? = 1757 J/mol and the stacking fault energy was determined to Γ_SF = 110 mJ/m². This indicates that the austenite is very stable and no strain induced ?‐martensite will be formed. Mechanical twinning is almost inhibited during plastic deformation. The TRIPLEX steels exhibit low density of 6.5 to 7 g/cm³ and superior mechanical properties, such as high strength of 700 to 1100 MPa and total elongations up to 60 % and more. The specific energy absorption achieved at high strain rates of 10³ s^?1 is about 0.43 J/mm³. TEM investigations revealed clearly that homogeneous shear band formation accompanied by dislocation glide occurred in deformed tensile samples. The dominant deformation mechanism of these steels is shear band induced plasticity ‐SIP effect‐ sustained by the uniform arrangement of nano size κ‐carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening.     

State‐of‐the‐Art of Friction Stir Processing

The aim of this study is to present an overall view of friction stir processing (FSP), including the method, the state‐of‐the‐art regarding current studies, and possible applications. FSP is a solid‐state thermo‐mechanical processing method. It can be used to produce defect‐free, recrystallized, homogeneous, fine grained microstructures. Structures can be processed at specific locations, through‐section or to a desired depth, or entirely. The benefits obtained with FSP include elimination of casting defects and refinement of microstructures resulting in improved strength, ductility, resistance to corrosion and fatigue, and formability (including high strain rate superplasticity). Alsosurface composites can be produced by FSP.     

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.     

Thermodynamic assessment of CaO‐SiO2‐Al2O3‐MgO‐Cr2O3‐MnO‐FetO slags for refining chromium‐containing steels

The slag system of CaO‐SiO₂‐Al₂O₃‐MgO‐Cr₂O₃‐MnO‐Fe_tO relevant to refining chromium‐containing steels such as bearing steel is thermodynamically assessed at 1873 K. The activity coefficient of Fe_tO shows an initially rapid increment followed by a gradual reduction according to Cr₂O₃ content at a constant basicity, and decreases with increasing slag basicity. γ_MnO is decreased abruptly by increasing Cr₂O₃ content and thereafter, maintains a nearly constant level. From the standpoint of inclusion control, the Cr₂O₃ presence in ladle refining slags is thermodynamically harmful in that it minimizes the inclusion level by inducing the increment of γ_FetO even though Cr₂O₃ exists in extremely small amounts. However, it is beneficial in that it diminishes AI reoxidation by decreasing γ_MnO. The presence of carbon in slag decreases γ_FetO and γ_MnO, which turns out to be favourable for the reduction of Al reoxidation. The thermodynamic equilibria of chromium and manganese are quantified in terms of Fe_tO and Cr₂O₃ content as well as slag basicity by using multiple regression analysis. L_Cr and L_Mn are increased by the presence of Cr₂O₃, indicating a low recovery efficiency of Cr and Mn in the treatment of ferroalloy addition. In determining L_S values, Cr₂O₃ is not so important as the basicity of slags.     

Some Thermophysical Properties of Liquid Cr‐Mn‐Ni‐Steels

In the last years new Cr‐Mn‐Ni‐TRIP/TWIP steels have been developed at the Institute of Iron and Steel Technology, Freiberg University of Mining and Technology. Within the Collaborative Research Center SFB 799, the ZrO₂‐ceramic‐TRIP‐steel composite materials are produced using the infiltration of open foam ceramics with liquid steel and using powder metallurgy with small additions of ceramic powder before sintering. The thermophysical properties of liquid steel play an important role in both production routes. They affect the infiltration efficiency in one process and the produced powder size in the other, and therefore finally determine the composite properties. In this work some of these properties were estimated, as they are not available in literature. The investigated steels contain approximately 16% chrome, 7% manganese and 3% to 9% nickel. The surface tension was estimated using two methods: the drop weight method and the maximum bubble pressure method. In the drop weight method similar conditions at the gas/metal interface exist as during the atomization or the infiltration process, where liquid metal is exposed to high volume of inert gas. In all these cases the evaporation of manganese affects the surface tension. For comparison of results and for estimation of the liquid steel density the maximum bubble pressure method was used where the evaporation of manganese is limited. The wettability on partially MgO‐stabilized ZrO₂ ceramic substrates and its change with contact time was determined using the sessile drop method.     

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.     

In‐Situ Measurement of CO‐ and CO2‐Concentrations in BOF Off‐Gas

Improvement of the efficiency and productivity of steelmaking requires accurate and real‐time process information to precisely adjust the tapping conditions. The response time of BOF process control systems is commonly depending on the time needed for off‐gas sampling and analysis. A FTIR based measurement system was adapted and installed at an industrial steel converter of ArcelorMittal España S.A. in Aviles. An advanced data interpretation routine based on the quantum mechanical fundamentals of remote infrared (IR) spectroscopy was developed to enable precise determination of the {CO}‐ and {CO₂}‐concentrations, resp. their ratio, in the off‐gas. The results of the industrial measurement showed the ability of the FTIR based system for real‐time measurement of temperature, {CO}‐ and {CO₂}‐concentrations in the hot and dusty off‐gas. The influence of dust at the chosen defined optical path lengths in the IR wavelengths region can be neglected, but the collector lens clean has to be kept clean.     

State‐of‐the‐Knowledge on Coating Systems for Hot Stamped Parts

In the present contribution, recent developments of coatings for hot stamped steels are reviewed. The use of bare steel in the initial hot stamping technology is discussed, including the application of lubricant oils which are used as oxidation inhibitors on bare steel surfaces. The aluminized coatings are introduced, focusing on the microstructure evolution of aluminized coatings during the hot stamping process. An analysis of the cracking of the coating, caused by the formation of brittle Fe–Al intermetallic phases and their high temperature deformation, is presented. The development of a ductile aluminide coating formed during the diffusion treatment of an aluminized coating is discussed. This aluminide coating can endure both high temperature oxidation and severe plastic deformation. The recently developed galvanized and galvannealed coatings are also reviewed and the influence of the gas atmosphere during the heating cycle on the coating stability is emphasized. The solutions which have been proposed to avoid liquid Zn‐induced embrittlement are analyzed. The use of Zn–Ni alloy coating, which is characterized by a higher melting temperature, is reviewed. The behavior of sol–gel hybrid coatings on hot stamped steels is discussed. The possible use of the recently developed Al–Zn alloy coatings, dual layer Zn–Al and Zn–Al–Mg coatings is also introduced. The application of Zn–Al–Mg post‐process galvanizing is also discussed. In each case, all available information related to the weldability, paintability, and corrosion resistance of the coating systems is also reported. Finally, the advantages and technical challenges associated with each type of coating are reviewed.     

Iso‐Work‐Rate Weighted‐Taylor Homogenization Scheme for Multiphase Steels Assisted by Transformation‐induced Plasticity Effect

A simple homogenization scheme for multiphase microstructure (composite) is developed. This scheme is based on the classical Taylor (iso‐strain‐rate) averaging scheme. Scalar multipliers are introduced as weighting parameters in order to relax the condition of uniform strain distribution used in the classical Taylor scheme. In the present work, the scalar weighting parameters are determined by satisfying the iso‐work‐rate condition, i.e., work is equally distributed in all constituent phases. In combination with micromechanical models developed for transformation‐induced plasticity (TRIP) effect, the iso‐work‐rate weighted‐Taylor scheme is applied for simulating the effective mechanical behaviour of multiphase TRIP‐assisted steel. The predictions of the iso‐work‐rate weighted‐Taylor scheme are compared with the result of the corresponding simulation with the direct finite‐element method (FEM).     

Spline‐Interpolation and Calculation of Machine Parameters for the Three‐Roll‐Pushbending of Spline‐Contours

Today, bending tasks become more and more complex. Not even constant bending radii are required in the industrial practice. There is a growing demand for bending spline‐contours, too. Such geometries are often produced with Freeform‐Bending procedures like Three‐Roll‐Pushbending. This paper presents a method to interpolate a given spline bending‐contour (by CAD data), in order to calculate its radii distribution, which is needed to determine the machine parameters in certain points for the Three‐Roll‐Pushbending. For the determination of the machine parameters one has to consider the different influences on the bending process. The material springback and the deflection of the bending machine per radius need to be compensated to reach a near net shape bending result. Nevertheless deviations cannot be avoided. To improve the results, a possibility to adjust the pre‐calculated machine parameters is shown. For the investigations tube profiles with constant wall thicknesses were considered. The corresponding plasticity calculations refer to tube cross‐sections. The results were validated by bending a representative spline contour on the bending machine of the Chair of Forming Technology at the University of Siegen.     

Metallurgical Development in Steel‐plant‐internal Multi‐injection Hot Metal Desulphurisation

The use of thermodynamic, statistical, and light and electron‐optical microscopic methods has made it possible to analyse the course of reactions during steel‐plant‐internal hot metal desulphurisation. The theoretical dependence between oxygen and sulphur activity has been used to develop an EMF‐measurement‐based technique for determining the sulphur content as the hot metal is desulphurised. The absence of magnesium sulphide from the collection of phases in the final slag because of the greater stability and secondary formation of calcium sulphide has been explained with the aid of thermo‐chemical calculations.     

New Algorithms for Thermo‐mechanical FE‐Simulation of Quasi‐periodic Processes

With the aid of the finite element method, a lot of mechanical and thermal phenomena in forming processes can be analysed successfully. However, for industrial purposes the necessary computational time for complex processes is often not acceptable. Many researches have attempted to reduce the computational time which is needed to find a solution. In this paper, new algorithms are described which make use of the existing similarity in some forming processes, especially incremental forming process. Incremental forming processes are characterised by very small deformation zones and approximate self similarity. The described algorithms are based on the transformation and interpolation of already computed solutions. The results from transformation and interpolation are verified by means of some physical constraints. If the results pass the latter check, they will be archived as solution. Consequently, at this time step, no iterative computation will be accomplished by the mechanical solver. Then the calculations by the thermo solver are accomplished conventionally in dependence on the archived mechanical results. With the help of these algorithms, the number of iterations will obviously be reduced and the results will be reasonably acceptable. In this paper, all details are described and two forming processes as validation occur to depict the advantages of this method.     

Reoxidation of Ni‐ and Ni‐Fe‐Alloys by Al2O3‐SiO2 Refractory Materials

Reactions at the refractory/melt interface during ingot casting of Ni‐ and Ni‐Fe‐alloys were studied. The casts were performed using different alumino‐silicate bricks as refractory materials. Samples taken from the casting channel before and after casting were investigated using light and scanning electron microscopy with XPS. Thermodynamic calculations were performed with FactSage and the results were compared with the results from industrial tests. After the melt has infiltrated the surface layer of the bricks, refractory corrosion starts with an attack of Mn and Mg of the melt on SiO₂ and Fe₂O₃ of the refractory bonding matrix. Despite the presence of elements with higher oxygen affinity in the melt, low‐melting alumino‐silicate phases are predominantly built by the reaction with Mn and Mg. In a second step this liquid phase either traps non‐metallic inclusions from the melt or, at higher contents of Zr, Ti, Mg, Y etc. in the melt, causes massive reoxidation and inclusion formation. The refractory materials investigated show an increasing trend for reoxidation with an increasing amount of SiO₂ in glassy phases of the refractory bonding matrix. By the use of a refractory material with higher mullite content in the bonding matrix or by use of alumina bricks a strong reoxidation of the melt and intense inclusion formation can be avoided. These observations are also valid for other alloys with higher contents of elements with high affinity to oxygen.