Influence of Temperature on the Quality of Solid and Water-Cooled Mandrels of a 30-80 Screw-Rolling Mill during Rolling of Steel Vessels

The results of pilot industrial experiments on the use of solid and water-cooled mandrels during hot rolling of steel vessels on a three-high 30-80 screw-rolling stand at BF AO NPO Pribor are presented. The factors influencing the formation of external and internal defects in 4Kh5MFS quenched steel piercing mandrels are investigated. During the formation of vessel cavity, the solid mandrel is found to undergo nonuniform heating along its length with a temperature of about 350 degrees C at the base and about 600 degrees C in the nose part. A high temperature (600 degrees C) is shown to cause the formation of a 0.6-mm-thick scale layer and a 0.4-mm-thick decarburized layer in the nose part. Subsequent external water cooling of the mandrel makes it possible to eliminate the influence of temperature on the further growth of a ferritic ring; however, thermal stresses form in this case and induce defects in the form of flaws and cracks. The hardness along the solid mandrel length after piercing 1000 vessels differ by more than 2 times: it is 20 HRC in the nose part and about 40 HRC at the base. A water-cooled mandrel with internal cooling has been designed to form high-quality vessel cavity. The internal cooling of the mandrel is found to decrease the heating temperature of the nose by more than 4 times and that of the base by 2 times. The decrease in the mandrel temperature during the piercing of vessels is shown to eliminate the formation of a decarburized layer and flaws on the mandrel surface almost completely and to stabilize the hardness in a range of 45-50 HRC along the mandrel length.