Rock drill steel rolling in fully automatic bar mill

Abstract

The manufacture of rock drill steel is a special technology in bar production. The rock drill bar is a thick walled tube and two main manufacturing lines, the cast in method and the drilling method, exist. From the quality point of view the drilling method is superior, and for production of modern high quality rock drills it is the only applied method. Rock drills are produced in four different groups of steel grade. The main groups are 1C–1Cr steel, similar to a bearing steel, 0·5C–1·5Si spring steel, 0·25C–3Cr tool steel, and a 0·20C–1·4Cr–3Ni tool steel. Bar tolerances are close for both outer dimensions and hole, and owing to the need to drill a hole in the billets, the billet length is limited, and rock drill steel rolling does not normally suit modern high production bar mills where the billet length is 6–12 m. A possible solution is to build a special mill for rock drill steel rolling, but in most cases rock drills form a very small part of the production volume in the mill. In that case the mill must be able to roll short billets for rock drills and full length billets for solid bar. By designing the furnace with separate walking beam systems it is possible to heat one row of full length billets and two or even three rows of short rock drill billets. The automation of the mill must be designed to suit the short rock drill billets but it must be borne in mind that the full length billets must be rolled without limitations due to optimisation of rock drill bar rolling. A good solution is a 2 stand 3 high roughing mill, with tilting tables, followed by a continuous intermediate and finishing line. The roll pass design for rolling rock drills should preferably start with gothic diamonds and move on to a conventional diamond sequence, while solid bar can be rolled by a round–oval or square–oval sequence in the intermediate stage. When rolling rock drills, ovals and even breakdown squares should be avoided if possible. The ovalising effect of square breakdown sections makes the shaping of hexagons and rounds with a centrally placed round hole limited, and the possibility of adjusting skew holes by minor gap changes in the 3 high roughing stand is reduced by the introduction of ovals in the pass sequence. In the final shaping of hexagons, the pass reductions should be kept at a safe level above the critical strain, and it is important that the square hole is rounded up correctly in the preparatory and leader passes. The shaping of rounds must be done in a way to achieve smooth surfaces of the finished bar. This is preferably done by using a double radius leader oval.