Improvement in the rolling of special shapes

Abstract

The Omutninsk Metallurgical Plant has collaborated with the rolling laboratory at the Dneprodzerzhinsk Industrial Institute to improve the pass system on a mill which rolls billets for the production of shapes measuring 36 x 20 mm in cross section. The steel 45 shapes are made in small lots (25-30 tons) and are shipped in cold-worked state with a surface finish corresponding to precision class B (GOST 1051-73). To achieve the required surface quality and dimensional accuracy, the technology provides for hot rolling of a 38 x 22 mm billet and its subsequent drawing to the finished shape. On a semicontinuous 300/280 mill which includes a continuous four-stand 300 group and a open three-stand 280 group the hot-roUed shape is obtained from a 52-mm-square billet in seven passes: f ive passes on a smooth roll and two traverse edging passes. Here, the draft is 1.18. To reduce the number of passes, eliminate manual labor, and increase dimensional accuracy, we designed a pass system which uses a four-rol l finishing pass. This makes it possible to obtain a shape of the correct geometry in the minimum number of passes, regulate the grooves in two mutually perpendicular directions, and create a stress state which is close to cubic compression. The partings of the passes are vertical, while the gap size is 0.5 mm. In the leader pass, the corner sections have been blunted by levels with an angle of 45 ~ in order to prevent overfilling of the finishing pass. Here, the diagonals of the leader pass are equal to the diagnosis of the finishing pass, with allowance for the blunting of the corners. The rough shape is obtained by reducing the billet on the smooth roll and in a box pass. The draft in this system is 1.34. The pass system was tested in the continuous four-stand 300 group of the 300/280 mill at the Omutninsk combine, the last stand of the mill having been replaced by a four-rol l stand designed by the Dneprodzerzhinsk Industrial Institute. The four-rol l pass was created by two driven horizontal rolls 315 mm in diameter and two undriven vertical rolls 135 mm in diameter. The dimensions of the shape were within the tolerance. The fluctuation in the dimensions of ihe hot-rolled shape was connected with inaccuracies in the dimensions of the initial semifinished product, the temperature difference, and the change in the tension of the product over its length. After drawing of the hot-rolled shape, the finished product corresponded to the requisite dimensions. Compared to the existing technology, the new rolling technology makes it possible to obtain the specified shape in fewer passes while eliminating manual labor during the actual rolling operation.