Abstract
The article deals with FEM analysis, with mathematical modeling of structure evolution and with prediction of mechanical properties during hot rolling of seamless steel tubes. There is a brief description of stretch-reducing mill in the introduction. It is a forming unit that reduces diameter of the tube semi-finished product and at the same time changes the thickness of the wall without inner tool. In this unit the tubes is achieving the final dimensions and after cooling process also the final mechanical properties. Mathematical model of stretch-reducing mill of Železiarne Podbrezová, Inc. was created in Deform 3D software. Calibration sequence for rolling tube semi finished product diameter 144 mm and wall thickness 4,7 mm to diameter 88,9 mm and wall thickness 5 mm was chosen for creation of mathematical model of stretch-reducing mill. This calibration sequence consists of eleven rolling stands. Numerical simulation was created on mathematical model. By using the numerical simulation the values of thermo-mechanical parameters for each stand were gained. Time dependencies of strain, strain rate and rolling force were made from the obtained values. The next part presents mathematical model describing the tube production for low carbon steels grade St52 on analyzed calibration sequence. Model is valid for range of rolling start temperatures 870–960°C and start rolling velocity 1,23 m·s-1. Model includes: − calculation of deformation temperature on each rolling stand in dependency on heating temperature, − calculation of kinetics of static recrystallization in conditions of continual cooling, − calculation of diameter of austenite grain after each deformation, − calculation of Ar3 and Ar1 temperatures in dependency on chemical composition, austenite grain size, amount of residual deformation and cooling rate, − calculation of austenite grain size after cooling to Ar3 temperature, − calculation of ferrite grain size in dependency on chemical composition, austenite grain size, amount of residual deformation and cooling rate, − calculation of structural fractions (ferrite, pearlite, bainite) in dependency on chemical composition, austenite grain size and cooling rate, − calculation of yield and tensile strength in dependency of structural fractions and ferrite grain size, − estimation of ductility for normalization conditions in dependency on chemical composition. Numerical simulation and process condition statistic data were used for calculation of material temperature both during tube pass through stretch-reducing mill and during cooling. Values of strain and strain rate from numerical simulation were substituted into the mathematical models. The calculated values were compared with experimental ones resulting from mechanical tests of industry rolled tubes of various chemical compositions. Good agreement of structural and mechanical properties was achieved for all chemical compositions and treatment conditions. The conclusion deals with the possibilities of increasing the accuracy of presented mathematical model as well as the possibilities of extension dimension assortment and areas of applications of mathematical modeling of seamless steel tubes production.