Initial Oxidation of a 9%CrMo‐ and a 12%CrMoV‐Steel

For the oxidation and high temperature corrosion resistance of 9‐12% Cr steels, the rapid formation of a dense protective Cr‐rich oxide scale is important. The initial oxidation processes were investigated on a 9%CrMo‐steel and a 12%CrMoV‐steel. Auger spectroscopic studies in the first hours showed a non‐uniform oxidation and effects of surface segregation and microstructure. Grain boundaries and martensite laths provide sufficient Cr to the surface for chromia formation, in other areas formation of SiO₂ may prevent Fe‐oxide growth. For up to 100 hours the oxidation kinetics is largely controlled by diffusion of the selectively oxidized elements Cr, Mn and Si in the steel, only gradually the transition to control by diffusion in the oxide occurs. Thickness and composition of the scale, and also the diffusion profiles in the steel were determined after 10 and 100 hours oxidation by SNMS, revealing important effects of surface treatments, such as electropolishing, polishing, grinding or sandblasting. Strong surface deformation favours the formation of Cr‐rich scales and of flat Cr‐depletion minima. Also the effects of different oxidation atmospheres, H₂‐H₂0, N₂‐H₂‐H₂0 and N₂‐O₂ were investigated and described. The kinetics of the oxide growth and the possibilities to obtain protective scales by pre‐oxidation are discussed.     

Microstructual Evolution of a Nb‐Microalloyed Advanced High Strength Steel Treated by Quenching‐Partitioning‐Tempering Process

The recently developed “quenching and partitioning” heat treatment and “quenching‐partitioning‐tempering” heat treatment are novel processing technologies, which are designed for achieving advanced high strength steels (AHSS) with combination of high strength and adequate ductility. Containing adequate amount of austenite phase is an important characteristic of the above steel, and the partitioning treatment is a key step in Q&P or Q‐P‐T process during which the austenite phase is enriched with carbon and achieves thermal stability. However, the microstructural evolution of the steel during the partitioning process is rather complicated. In present study, evolution of complex microstructure in a low carbon steel containing Nb during the Q‐P‐T process has been studied in detail. The microstructural evolution of the steel was investigated in terms of X‐ray diffraction, scanning electron microscope and transmission electron microscope. The experimental results show that the Nb‐microalloyed steel demonstrates a complex multiphase microstructure which consists of lath martensite with high dislocation density, retained austenite, alloy carbide, transition carbide, and a few twin martensite after the Q‐P‐T process. The experimental results can be helpful for the design of Q‐P‐T heat treatment and for the control of mechanical properties of Q‐P‐T steel.     

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.     

Composition Control of CaO‐MgO‐Al2O3‐SiO2 Inclusions in Tire Cord Steel ‐ a Thermodynamic Analysis

The effect of oxide component content on the low melting point zone (LMP) in the CaO‐MgO‐Al₂O₃‐SiO₂ system has been analysed using FactSage software. The contents of dissolved elements [Si], [Mg], [O] and [Al] in liquid steel in equilibrium with the LMP inclusions in the CaO‐MgO‐Al₂O₃‐SiO₂ system have been calculated. The results show that the CaO‐MgO‐Al₂O₃‐SiO₂ system has the largest LMP zone (below 1400°C) when the Al₂O₃ content is 20% or the MgO content is 10%. The LMP zone becomes wide with the increase in CaO content (within the range of 0~30 mass%) and the decrease in SiO₂ (from 25 to 5 mass%). To obtain the LMP (below 1400°C) inclusions, the [Mg], [Al] and [O] contents must be controlled within the range of 0.2~2 ppm, 1.0~2.0 ppm and 60~100 ppm, respectively.     

Finite‐Element Simulations of the Intercritical Deformation of a Low‐Carbon Steel

A mathematical model has been developed to describe the intercritical deformation of a low‐carbon steel under compression. It applies the finite‐element method in a 2D axially symmetric computation domain. By means of a special algorithm different types of microstructures to be used by the deformation model can be constructed, as far as ferrite and austenite phase fractions and distributions are concerned. Calculated results on the basis of constructed microstructures deviate significantly from mixture‐rule predictions. Compression curves exhibit a decreasing strength with increasing connected areas of either phase. The corresponding strain ratios demonstrate the difference in strain behaviour between the ferrite and the austenite phase. In the case of larger connected areas the contribution of the soft ferrite phase to the total strain is much more pronounced than that of the hard austenite phase.     

Investigation of the Gas‐Liquid Flow in a Stopper‐Rod Controlled SEN

The behaviour of two‐phase gas‐liquid flows in a stopper‐rod controlled submerged entry nozzle (SEN) is investigated in water model experiments. The observed two‐phase flow patterns can be classified into either bubble coring or bubbly slug. The scaling of the two‐phase flows by means of similarity parameters is discussed. In the experiments, it is found that the liquid flow rates depend strongly on the two‐phase flow patterns. Additionally, the influence of swirl on the flow patterns is investigated in detail. It is shown that swirl has a marked impact on the transition from bubble coring to bubbly slug. Finally, an estimation of the two‐phase argon‐steel flow patterns in industrialscale SEN flows is given.     

Ferritic rolling with additional annealing to produce a deep‐drawable ultra‐thin‐gauge hot strip

In order to produce ultra‐thin hot strips by ferritic rolling, that exhibit a good deep‐drawability, finishing temperatures must be reduced, so that an additional recrystallization annealing of hot strips after coiling becomes necessary. In the present work laboratory investigations were carried out on a commercial IF and ELC steel by using the hot deformation simulator Wumsi. By means of texture measurement and the calculation of r‐values the process parameters of the ferritic rolling and of a subsequent annealing were optimized. Additionally to a conventional annealing of cold coils a new energy saving processing route with direct charging of warm coils was tested by the laboratory simulation. Particularly good deep‐drawabilty were achieved by this technology.     

Development of a hot‐rolled Nb‐bearing Si‐TRIP steel with excellent fatigue behaviour for automotive applications

A hot‐rolled TRIP steel with a tensile strength level above 800 MPa and an excellent fatigue behaviour has been developed with the purpose of satisfying the increasing demands of the automotive industry. Nb was added to the steel to guarantee a stable process and to make the production of the steel possible in a conventional rolling mill. The formation of red scale and the surface quality problems that arise from it have been studied. Additionally, hot torsion tests have been performed in order to validate a new route that could avoid the formation of this red scale allowing the application of the steel for exposed parts (e.g. wheel sector).     

Processing of Copper‐Containing Steel via Strip Casting ‐ a Laboratory Evaluation

Copper in recycled steel made from scrap is well known to create problems of hot shortness during rolling. The present study was carried out to ascertain how the situation could be improved when using strip casting in conjunction with direct hot rolling. This has included steels having copper contents up to 2.5 wt.% and in some cases also tin levels up to 0.1 wt.%. Laboratory simulations have been carried out to simulate the process conditions from the outlet of the strip caster through hot rolling, and the resulting materials have been examined with regard to their hot cracking behaviour and microstructural condition. Mechanical properties have also been measured on samples having different simulated coiling temperatures after hot deformation. Conditions can be established to avoid hot shortness even in steel containing high copper contents, due to the short time that is available for oxidation. Depending on the steel composition and coiling temperature, it is possible to achieve very significant strengthening due to precipitation of copper‐rich particles.     

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.