Markus Daamen

Microstructure Analysis of High-Manganese TWIP Steels Produced via Strip Casting

Steels with manganese contents of more than 20% offer a new and favourable combination of material properties like high strength and high ductility. These extraordinary mechanical properties are based on the TWIP effect, which depends on the Stacking Fault Energy (SFE). But there are still problems in the conventional production of high-manganese steels, which prevents their widespread use. Both in casting and subsequent hot rolling difficulties occur, with the consequence that the production is very expensive. One alternative production process of high-manganese steels is strip casting, which basic feasibility was shown in earlier work. Strip casting allows the casting and rolling of hot strip in one combined process. In this way hot strip with a thickness of less than 3 mm could be produced. Characteristic for the strip cast material is the as-cast structure with a fine dendritic structure, which shows pronounced microsegregations with a short wavelength. The pronounced microsegregations can have an impact on the local chemical composition and thus on the dominating forming mechanisms that occur. In this work therefore the microsegregations of strip cast material are investigated by means of electron probe microanalysis (EPMA) measurement. Besides the local element distribution, also the presence and composition of non-metallic inclusions are analysed. Especially oxides from the casting process and sulfides from the raw material are expected. Furthermore, different annealing processes for the elimination of the dendritic as-cast structure are examined. In these experiments the temperatures were varied in the range from 900 to 1150°C at annealing times from several minutes to a few hours.

Numerical and Experimental Investigation of a Modified Twin Roll Strip Casting Process for the Production of Clad Steel Strips

A process for the production of clad steel strips by means of vertical twin-roll strip casting is under investigation at the Institute of Metal Forming (IBF). This production concept is based on the introduction of a solid strip in the casting pool in order to join it with the solidifying melt. The advantages of this concept are a shorter process chain and a higher energy efficiency compared to the existing cladding methods. The contact time and the thickness ratio between solid strip and solidifying melt, their relative position and the alloy combination are fundamental parameters to be considered in the design of this cladding process. A numerical model is used to predict the temperature increase in the solid strip and the solidification of the melt for the material combinations 1.4301/C75 and C75/Cu, and for two process configurations. Carbon steel and copper strips were introduced in the melting pool during casting experiments. Both carbon steel and copper strips could be clad without melting when positioned on the casting roll surface. This indicates that the surface temperature of the copper strip did not rise over 1084°C. Both the copper and the carbon steel strips show partial melting when introduced in a central position.

Advances in the Twin-Roll Strip Casting of Strip with Profiled Cross Section

Direct thin strip casting is an economically end energetically smart process for the production of steel strip. In a single process step, liquid steel can be cast and directly rolled to hot strip in thicknesses ranging from one to four millimeters. With the use of specifically profiled casting rolls it is possible to produce strip with optimized cross-sections, allowing this process to compete with tailor welded and tailor rolled blanks for the production of a class of products already widely applied in industry. Numerical and experimental studies proved the feasibility of this concept and additional simulations were used to optimize the profile to be used for the experiments. A thickness variation of one millimeter from the edge to the center could be successfully achieved. However, the dimensional precision and the roughness distribution along the cross section of the produced strip were not satisfactory. Additional profiles were applied for the experimental analysis leading to better roughness distribution and geometrical accuracy. In order to further improve the uniformity of properties along the profiled section it is necessary to increase the homogeneity of the microstructure. The coating and surface preparation of the casting rolls play a very important role in the strip casting process as they strongly affect the solidification behavior. This observation lead to the idea of selectively coating the casting rolls, applying a less conductive layer on the areas where the casted profile is thinner. Thus, a more homogeneous solidification front can be obtained. The effect of a locally modified casting roll coating on the solidification is numerically investigated and the results applied for the selection of the coating parameters to be used for the experiments.

Vertical Twin-Roll Strip Casting of Steel

Vertical strip casting enables the near-net-shape production of hot rolled steel strip in a compact process. In less than half a second, the liquid steel melt solidifies in contact with the casting rolls to a one to five millimetres thin strip. Since the strip casting process combines casting and rolling in one single process, slab re-heating and numerous hot rolling steps can be eliminated. Thus, an efficient, economical and environmental friendly production of steel is possible. Today, strip casting of steel is already used in a few industrial steel plants. Recent developments allow for the production and further processing of advanced high strength steels and steels with a nanocrystalline structure by strip casting. Regarding the process design, research activities focus on casting of tailored strips with defined thickness distribution across the strip length or width. Thus, strip casting can be used for the production of tailored hot strips in a considerable shortened process.

Tailored Strips by Welding, Strip Profile Rolling and Twin Roll Casting

Conventional strips can be converted into tailored strips by further processing such as rolling and welding. Tailored strips have a thickness or thickness distribution which is designed according to the expected loads. A new approach for the production of tailored strips is the twin roll casting of profiled strips. This technology combines the advantages of direct strip casting and the production of steel strip with an optimized cross section. In this paper the achievable process limits regarding the geometry of tailored strips with varying thickness in the cross section made by strip profile rolling, twin roll casting and welding are discussed and compared. Furthermore, experiments to demonstrate the suitability of twin roll casting to produce tailored strips made of AISI 304 stainless steel are treated. A selected tailored strip geometry of 150 x 1.5 mm2 (width x thickness) with a difference in strip thickness of 33% over the width was cast.