Optimizing the annealing of steel 08Yu sheets in rectangular bell-type furnaces

Abstract

In the production of steel 08Yu (GOST 9045-80) of category 1-OSV-A at our combine, stacks of cold-rolled metal are annealed in rectangular gas-fired bell-type furnaces. The furnace stand is not heated from below, and there is no forced circulation of the protective atmosphere. The temperature of the metal is checked by thermocouples installed in the bottom (t b) and top (t t) levels of the charge. The temperature is controlled automatically (by a twoposition controller). The signal for switching of the automatic system comes from the readings of the top thermocouple. When the technology just described is used for annealing (Fig. la), there is a large temperature drop (up to 70~ over the height of the stack. As a result, the physicomechanical properties of the metal over the volume of the 65-ton charge are nonuniform, as well as being unsatisfactory low. To solve these problems, we developed a new regime for the recrystallization annealing of steel 08Yu (Fig. 1 b) which includes four resettings of the control temperature tt: 670-720~ -period I (first pulse); 600-650~ -period II; 670-720"C -period I (first pulse); 600-650"C -period II; 670-720"C -period III (second pulse); 680-660~ -period IV. This regime can be realized by means of any modern devices used for programmed control of production processes. Without lengthening the annealing operation, this regime makes it possible to hold the metal at certain temperatures in the bottom part of the stack. This metal is subjected to intermediate (rl) and final (r 2) holds. The holds improve heating and the physico-mechanical properties of the product. Strictly speaking, the term "hold" is applicable only to the top level of the charge. In the middle and bottom parts of the stack, during holds there is some increase in the temperature of the metal. This increase has to do with heat-transfer conditions inside the charge and its thermal inertia. The duration of period I was changed within the range 5-16 h. An analysis showed that a duration within the range 10-13 h ensures the greatest values for elongation (81o), the coefficient of normal plastic anisotropy (R), and grain size and the lowest value of hardness (HRB). The duration of the pulse in this period should be minimal -0.5-6 h. The maximum reduction in HRB for period I and III (Fig. lb) is seen at a control point t t = 700"C. An increase in the control point to 720"C (particularly in period III) is accompanied by an increase in the hardness and the danger of sticking of the sheets. A pulse duration of 0-1 h in period III will produce a minimal value of HRB, while a pulse duration within the range 7-17 h will result in higher values of HRB, IE, and 810 and lower values of cry. The term "pulse duration" in annealing periods I and III means the time t t is held at 700"C. Having optimum holding parameters in the top part of the charge (period II, t t 600"C, duration 6-7 h) somewhat improves the microstructure, hardness, and Erichsen cup depth IE with a I0-12% reduction in fuel consumption. The best characteristics in the bottom part of the charge are seen with 500-5600C for t b and a holding time of 10 h (annealing period II). A reduction in temperature to 450-550"C leads to an increase in strength properties and a reduction in grain size (to number 9). For sheets measuring 0.8 x 800 x 1510 mm, holding time r 1 is limited to 6-10 h. In annealing sheets up to 1.5 mm thick, holding time is cut to 2-4 h. The duration of the final hold (r~), with a temperature of 640-660"C for t b, should be at least 4 h. Lengthening this hold to 15 h reduces hardness. The overall duration of the two-stage pulse heating regime (38-40 h) is the same as in the continuous annealing practice. Thus, the optimum values of the main parameters for the above-developed annealing regime are as follows: pulse temperature 700"C; holding time at this temperature 10-13 h; duration of the second pulse 0-I 7 h; temperature and duration of the intermediate hold in the least-heated parts of the charge 500-560 and 6-10 h. By varying these parameters, it is possible tO change the physico-mechanical properties of the steel in the required direction. The regime for two-stage pulse annealing of cold-rolled steel 08Yu in bell-type gas-fired rectangular furnaces, without forced circulation of the protective atmosphere or bottom heating of the stand, was optimized in accordance with