Rudolf Kawalla

Advanced high strength steels for automotive industry

The aim of this paper is to present the basic concepts of advanced high strength steels (AHSS) for use in the automobile industry, including chemical composition design, microstructure and mechanical properties development during thermomechanical processing, production technology characterisation, potential applications and performance in service. AHSS steels are considered to be the major materials for future applications in this production sector. As opposed to the cold formable single phase deep-drawable grades, the mechanical properties of AHSS steels are controlled by many factors, including: phase composition and distribution in the overall microstructure, volume fraction, size and morphology of phase constituents, as well as stability of metastable constituents. The main feature of these steels is that they do not permit to rely on the well-established traditional microstructure-properties relationships. Therefore, many different alloy concepts and alternative processing routes are still under development by different steel producers for comparable steel grades.

Influence of asymmetric hot rolling on microstructure and rolling force with austenitic steel

In order to establish a new processing technique of producing steels with fine grain, asymmetric hot rolling and symmetric rolling were performed. The asymmetric condition was introduced by applying mismatched roll diameters. The diameter ratios between big and small rolls were 1.00, 1.05 and 1.11. The rolling temperatures were between 900 °C and 1100 °C. The thickness reduction of workpiece was set at 15%, 30% and 60%. The results showed that asymmetric rolling produced higher volume fraction of recrystallization grain and smaller average grain size at the center layer of rolled sample than symmetric rolling. The diameter ratio of 1.05 tended to generate the highest recrystallization level and the smallest average grain size. With high temperature and low thickness reduction value, the grain growth was obvious. The conventional dynamic recrystallization mechanism was prevailing, while accompanied by twinning. At low temperature of 900 °C, even at thickness reduction of 60%, symmetric rolling cannot initiate dynamic recrystallization as asymmetric rolling did. The asymmetric rolling force was smaller than that of symmetric rolling, except at 900 °C and thickness reduction of 60%. The rolling force also increased with descending temperature and climbing thickness reduction.

Magnesium semi-finished products for vehicle construction

At the Institute of Metal Forming, basic research-oriented and application oriented, two research foci on magnesium are pursued. One focus is on the production of semi-finished products for magnesium sheets and strip production by twin-roll-casting, the other focus is on the production of long products as wire, intended for the fabrication of joining elements. Magnesium sheets produced by twin-roll-casting and subsequent rolling offer very good processing properties and are partly superior to conventionally produced sheets. Advantages result from a shortened process chain whose technology leads to a favourable initial condition for strip production. Additionally, economical advantages due to the few process steps clearly exist. Wire production for the production of joining elements is based on a specifically developed calibration which causes advantageous stress and deformation conditions during caliber rolling. Properties of both product groups of magnesium alloy AZ21, AZ31, AZ91 and WE43 are described comprehensively. Their application for the production of parts and components is shown on examples.

Properties of Magnesium Strips Produced by Twin-Roll-Casting and Hot Rolling

The combination of Twin-Roll-Casting (TRC) and subsequent rolling constitutes the most promising process chain for producing magnesium strips economically. Fast solidification (10 times faster than continuous casting) combined with partial deformation lead to a fine primary microstructure as well as less shrinking holes, pores, segregations and brittle precipitations which all together has a very positive effect on forming behavior of the initial material and quality of the final product. The paper elaborates on metallurgical processes in consideration of microstructure and texture results and on the obtained mechanical properties of TRC magnesium strips and finished strips. In addition, the influence of twin-roll-cast and rolling conditions on the mechanical properties will be discussed. The investigation has also been expanded to possible heat treatments and their influence. Concluding remarks will be made on results of rolling trail which were carried out on an industrial scale rolling mill, revealing that the production of hot rolled thin sheets of magnesium alloy AZ31 is possible with a very promising combination of strength and ductility.

High Manganese TWIP Steel - Technological Plasticity and Selected Properties

Over the last few years national as well as international research centres conducting research on the development of high-manganese steels. Some of these materials belong to the group of AHS steels, are characterized by the twinning induced plasticity (TWIP) effect which is a new type of steel possessing together with high strength a great plastic elongation, and an ideal uniform work hardening behavior. It is therefore a good candidate for deep drawing applications in the automobile and railway industry. The article presents the results of researches of TWIP-type austenitic steel in case of determination some of the more important parameters for continuous casting simulation process and the results of tests regarding the influence of strain parameters on sensitivity to plastic forming and deformation strengthening. It has been shown that the researched steel reaches a zero plasticity temperature at 1250°C. The deformation tests indicate its good workability of hot processing within the temperature range of 1100 ÷ 800°C. The relation between yield stress and strain during the hot deformation is typical for the presence of dynamic recrystallization processes. The tested steel has good formability and high mechanical properties, especially when being deformed at a high strain rate. Analysis of the substructure of researched steel was indicate presence of mechanical twinning.

Effect of Different Finish-Rolling Parameters on the Microstructure and Mechanical Properties of Twin-Roll-Cast (TRC) AZ31 Strips

The microstructure and the resulting mechanical properties of Twin Roll-Cast (TRC) AZ31 strips have been investigated after rolled from thickness of 5.2 mm to 1.00 mm and 1.25 mm, respectively. Twin-Roll-Casting (TRC) was used to produce AZ31 strips with a near-net final thickness directly from the liquefied material. The two-stage rolling experiments were carried out on the four high reversing mill at the Institute of Metal Forming in Freiberg. The first stage was two roughing passes followed by intermediate annealing. The second stage was subsequent finish rolling with 1 to 3 rolling passes. The influence of the finish-rolling on the properties of the final strip was investigated, including the variations of rolling pass and pass reduction. The TRC strip exhibits a heterogeneous microstructure with random texture due to a deformed structure combined with partial cast columnar and equiaxed grains. Significant grain refinement was achieved using high deformation degree per pass (> 30 %). Increasing rolling passes to 3 during finish-rolling reduces the strain per pass and also leads to a temperature drop so that incomplete dynamic recrystallization after the final rolling pass occured, leading to a more coarse and heterogeneous microstructure. It was found, that a 2-pass finish rolling provides the optimum strategy for the material properties as well as the process stability. Due to homogeneous fine grained microstructure, the 2-pass rolled strip showes high mechanical properties with low anisotropy. This includes yield point of 234 MPa, tensile strength of 285 MPa and total elongations of 25 %.

Modelling the Temperature Evolution During Hot Reversing Strip Rolling of Magnesium Alloys

The paper proposes the approach for the thermal through process modelling of the strip hot rolling chain of magnesium alloys. This strip hot rolling chain comprises the coil reheating after the Twin-Roll Casting (TRC) process, storage or transport operations and reversing hot rolling. The modelling of reversing rolling is implemented in connection with the simultaneous un-/coiling process of a coil. The numerical calculation is based on an object-oriented FEM tool kit written in MATLAB™ and is carried out in three spatial dimensions and time.

Evolution and Interaction of the Microstructure and Texture at the Different Processing Steps for Ferritic Nonoriented Electrical Steels

The fabrication process of nonoriented electrical steels comprises casting, hot rolling, cold rolling, and final annealing. The development of new technologies for the fabrication of hot band offers new possibilities. In this paper, we will shortly describe the similarities and differences with respect to the evolution of microstructure (grain structure) and texture along the conventional processing route and thin strip casting. We will point out the most relevant features at the different processing steps, which are important for the optimum texture and microstructure of the finally processed material. Thereby, we will regard ferritic FeSi steels, where no homogenization of the microstructure appears due to the austenite-ferrite phase transformation.

Effect of the Interdependence of Cold Rolling Strategies and Subsequent Punching on Magnetic Properties of NO Steel Sheets

Nowadays, optimization of non-oriented (NO) electrical steels toward lower iron-loss, improved, and isotropic magnetizability is critical to the improvement of rotating electrical machines. The whole production process chain adjusts the microstructure evolution, e.g., grain size and crystallographic texture, determining the magnetic properties. In particular, the interdependence of raw material properties and the resulting mechanical stress distribution during final assembly, e.g., punching, leading to magnetic property deterioration is crucial for the optimization of NO steel properties of rotating machines. This paper studies the effect of different cold rolling strategies, annealing treatments, and sheet metal blanking (punching) regarding microstructure evolution, magnetic properties, and deterioration.

Effect of Inter-Metallic Phases on the Bonding Strength and Forming Properties of Al/Mg Sandwiched Composite

Composite sheet of Twin Roll Cast (TRC) AZ31-Mg alloy and industrial pure Al-1050 was fabricated as Al/Mg/Al with a hot roll bonding process. As the diffusion zone at the interface between the layers plays a crucial role in the formation of bonding strength and formability of clad sheets. It is important to describe the processes of inter-diffusion layer generation in order to assess the abilities of the laminate composites for further processing. Thus, the aim of this study was to investigate the development of the bonding strength, microstructure and mechanical properties of the bonding interface directly after roll bonding and additionally post-annealing processes. Microstructure characterization techniques such as optical and scanning electron microscopy including energy dispersive X-ray analysis (EDX) were applied to investigate the bonding area. No inter-metallic phases were present directly after the roll bonding process. The creation and growth of Al2Mg3 and Al12Mg17 phases were apparent after annealing at temperatures of 250 to 400 °C at different times ranging from five to 120 minutes. The results prove that the growth rate of inter-metallic phases increases considerably with an increase in the annealing temperature. The micro-hardness of the interface-area was also investigated. It was observed that the two inter-metallic phases were significantly harder than the substrate Mg and Al. In order to examine the influence of the inter-metallic phases on the resulting bonding strength after the annealing process, the shear bonding strength test has been conducted for different samples that were annealed at different heat treatment conditions. The results indicate that the optimum annealing temperature is 200 °C leading to a maximum bonding strength. Moreover, the mechanical properties of the composite after roll bonding and post annealing were determined by means of room temperature tensile test. The fracture mechanism after tensile test was also discussed.Keywords:Rollbonding,Al/Mghotrollbonding,TwinrollcastAZ31,bondingstrength