Papers by Keyword: Strip Rolling

Abstract: Weld line fracture of butt-welded strips in stainless steel continuous rolling has been studied by numerical simulation using ForgeNxT®. The simulation plan includes weld line geometry as well as weld metal constitutive model parameters. The damage criterion used is the non-dimensional Latham & Cockroft function near the singular points of the weld line. Results are confronted to qualitative observations of fracture frequency and fracture initiation loci on the rolling line. Not surprisingly, the protrusion height of the weld line with respect to the strip top surface is found to be a major factor of risk. A second one pertains to the nature of the alloys used: due to the mushroom-like weld line cross-section geometry, damage at the top surface junction of the base metal and the weld metal becomes critical when the weld metal is harder than the base metal. Finally, on the rolling line investigated, prior to rolling properly speaking, the strip goes through a scale-breaking unit followed by acid pickling to eliminate oxides; the multiple, reverse plastic bending applied there contributes significantly to ductile damage, completing the explanation of why the fracture initiation locus is always found to be the same on the rolling line.

Abstract: Torsion simulations were carried out of both plate (long interpass times) and strip (short interpass times) rolling. Both isothermal and continuous cooling conditions were employed. The dynamic transformation of austenite to ferrite was observed under all conditions and at all temperatures within the austenite phase field. About 8 to 10 volume percent ferrite was formed in a given pass, leading to about 50 - 70 % ferrite at the end of selected simulations. During the interpass intervals, some retransformation to austenite took place, the amount of which increased with holding time and temperature and decreased with the addition of alloying elements. It is shown that the driving force for the transformation is the softening associated with the replacement of work-hardened austenite grains by the softer alpha phase. The implications with respect to rolling load (i.e. mean flow stress) are also discussed.

Abstract: The paper focuses on experimental and numerical investigations of own designed, casted and then hot rolled martensitic steel. In order to understand the effect of the thermo-mechanical rolling parameters on the formation of the microstructure and resulting mechanical properties many tests and experiments have been performed. Martensitic steels offer satisfied balance of cost production, light weight and mechanical properties. These characteristics are achieved by using low alloy steel as a basis, and strict control of rolling conditions, strain rate, cooling rate and coiling temperature.

Abstract: Due to some specific features (high corrosion resistance and long-life durability in particular) as well as interesting aesthetic appeal, zinc-titanium strips are widely used as roof coverings, facade panels and rain gutters. The Zn-Ti strips are manufactured either in the hot rolling lines or in the reverse rolling mills at elevated temperatures. A problem of determination the average flow stress of zinc-titanium strip in the latter mentioned technology and in the actual rolling conditions is discussed in the paper. The analysis is based on the results of plastometric compression tests, FEM simulations and industrial practice.

Abstract: High temperature flow curves were evaluated on two Nb steels in both compression and torsion and at a series of temperatures and strain rates. The critical strains for the initiation of dynamic transformation (DT) were determined by the double differentiation method. These are shown to be distinctly lower than those associated with dynamic recrystallization (DRX). It is also evident that the compression critical strains for both DT and DRX are lower than the equivalent torsion critical strains. Mean flow stresses (MFSs) were calculated by integration from the flow curves. When plotted against inverse temperature, stress drops were observed about 30 degrees above the Ae₃. These drops are shown to be caused by the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance fine Widmanstatten plates.

Abstract: In the present paper actual demands for modern simulation strategies for hot rolling are discussed. The main focus of the discussion is on material flow simulation for hot rolling and the computation of material inhomogeneities. An overview on simulation techniques for material flow and microstructure evolution is given. Approaches for new simulation strategies which give fast results with a high modeling depth are discussed. As a result of actual investigations a fast model for material flow is presented.

Abstract: 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 %.

Abstract: In order to carry out automatic transformation to the two-roll reversible hot-rolling mill of a aluminum plate production factory, firstly a series of mathematical models of aluminum plate hot-rolling parameters are established, then a new optimization algorithm which is suitable for aluminum plate rolling production combined with the various constraints in the rolling process is proposed, and rolling schedule optimization software system is developed. Finally, by measuring the process parameters in rolling production site and applying the optimized rolling schedule to the rolling production, many test data are obtained. The analysis of test results and evaluation of the practical production show that the mathematical models established have high accuracy compared with the old rolling schedule. The optimization schedule can not only ensure the production quality, but also has higher efficiency and less energy consumption.

Abstract: Rigid-plastic finite element analysis (RPFEA) is an efficient and practical method to calculate rolling parameters in the strip rolling process. To solve the system of simulations equations involved in the RPFEA, a numerous of numerical methods, including the standard Newton-Raphson method, the modified Newton-Raphson method, and etc., have been proposed by different researchers. However, the computational time of the existed numerical methods can not meet the requirement of the online application. By tracking the computational time consumption for the main components in the standard Newton-Raphson method used in finite element analysis, it was found that linear search of damping factor occupies the most of the computational time. Thus, more efforts should be put on the linear search of damping factor to speed up the solving procedure, so that the online application of RPFEA is possible. In this paper, an improved trust-region method is developed to speed up the solving procedure, in which the Hessian matrix is forced to positive definite so as to improve the condition number of matrix. The numerical experiments are carried out to compare the proposed method with the standard Newton-Raphson method based on the practical data collected from a steel company in China. The numerical results demonstrate that the computational time of the proposed method outperforms that of the standard Newton-Raphson method and can meet the requirement of online application. Meanwhile the computational values of rolling force obtained by the proposed method are in good agreement with experimental values, which verifies the validity and stability of the proposed method.

Abstract: 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.