Seamless steel pipe production process and selection of rolling mill rolls
2014-09-09
The production process of seamless steel pipes and the introduction of the material and hardness selection of several main rolling tools are as follows: 1 Seamless Steel Pipe Production Process The production of seamless steel pipes mainly goes through the following processes: pipe blank - cutting - heating - perforation - rolling - sizing (reducing) - straightening - cutting head and tail - inspection - packaging - warehousing. According to different rolling methods, seamless steel pipe mills can be divided into automatic rolling mills, continuous rolling mills (MPM two-roll continuous rolling mill, PQF three-roll continuous rolling mill), Assel rolling mill, Accu rolling mill, periodic rolling mill, etc. The newly installed mills are mostly MPM continuous rolling mills and PQF three-roll continuous rolling mills, such as Tianjin Steel Pipe Company's Φ460, Φ258 PQF continuous rolling mill, Hengyang Steel Pipe's Φ273 MPM continuous rolling mill, Panzhihua Steel Chengdu Seamless's Φ340 MPM continuous rolling mill, Φ159 PQM continuous rolling mill, Anshan's Φ170 PQF continuous rolling mill, etc. 2 Perforating Machine The perforation of pipe blanks is the most important deformation process in the production of hot-rolled seamless steel pipes. Its task is to pierce solid pipe blanks into hollow tubes. Based on the structure of the perforating machine and the deformation characteristics during the perforation process, existing perforation methods can be divided into: inclined roll perforation, push roll perforation, and pressure perforation. Among these methods, inclined roll perforation is the most widely used. The shapes of rolls in inclined roll perforating machines include roller type, mushroom type (conical), and disc type. Roller-type perforating machines are used for small mills while large units use mushroom-type rolls. Disc-type perforating machines are less commonly used. Regardless of the shape of the rolls, to ensure that the pipe blank bites in and achieves successful perforation during processing, there are components such as a piercing cone (roll entrance cone), a reduction cone (roll exit cone), and a roll forming zone (the transitional section between entrance cone and exit cone). Most rolls in perforating machines adopt a combination structure of roll shafts and sleeves; both use direct interference fit or key interference fit. This way, when a roll is scrapped, the shaft can still be reused to reduce tool costs. The material for small perforating machine shafts is often selected from 45 or 40Cr with heat treatment. Large perforating machine shafts mostly use 42CrMo with heat treatment. Currently, manufacturers have various selections for roll materials; some choose forged high-quality carbon structural steels like 45 or 55 while others opt for alloy cast steels or forged steels like 50Mn, 65Mn, 70Mn, 70Mn2Mo, or 60CrMnMo; hardness requirements also vary with some requiring HB180-220 while others require HB220-260. To enhance bite performance, large perforating rolls generally have a mesh pattern with a depth of 0.8 to 1.2mm processed on their entrance cones. Based on an overview of material selections and usage situations from domestic and foreign manufacturers, I believe that using cast medium carbon alloy steels like 50Mn or 60CrMnMo for rolls in perforating machines is more suitable due to their good strength and wear resistance as well as their applicability for various types of steel pipes. A hardness control range around HB200-240 is reasonable. 3 Rolling Mill The hot-rolling process for hollow tubes is a major deformation step in producing hot-rolled seamless steel pipes; its function is to make the wall thickness of hollow tubes close to or reach that of finished pipes while eliminating longitudinal wall thickness unevenness caused during piercing; additionally it can improve surface quality inside and outside rough pipes while controlling outer diameter and roundness. 1. Automatic Rolling Mill: Automatic rolling mills are two-roll non-reversible longitudinal mills characterized by having a pair of high-speed counter-rotating return rolls set behind working rolls which serve to send previously rolled hollow tubes back to the front stage for second-time processing; therefore its production efficiency is relatively low with most domestic units having been modified or eliminated. The material for these rolls typically consists of unlimited cold-hard cast iron or ductile iron with hardness above HSD50. 2. Two-Roll Continuous Rolling Mill: Continuous rolling mills are more advanced steel pipe mills where pierced hollow tubes are fitted onto long mandrels which then undergo sequential processing through multiple stands arranged so that adjacent stands' roll gaps stagger by 90°. The materials used for continuous rolls are mostly nickel-chromium-molybdenum unlimited cold-hard ductile iron (also known as pearlitic ductile iron) or nickel-molybdenum unlimited cold-hard ductile iron (also known as acicular ductile iron), with hardness HSD50-65 in hole sections. Some foreign-designed first stands use forged steel or graphite steel materials to further enhance strength. 3. Three-Roll Continuous Rolling Mill: The three-roll adjustable core rod continuous pipe mill (PQF) was developed by Italy's INNSE company to overcome inherent limitations found in two-roll continuous pipe mills; it divides a full circle hole shape into three parts which reduces speed differences at each roll hole bottom compared to edges resulting in higher precision rolled steel pipes. Small PQF mills have integral design for their continuous rolls using nickel-chromium-molybdenum unlimited cold-hard ductile iron with hardness controlled around HSD55-65 being more suitable; large PQF mills feature assembly-type designs where rolls utilize wear-resistant nickel-chromium-molybdenum unlimited cold-hard ductile iron while shafts employ high-strength forged steel materials treated thermally allowing reuse which significantly saves costs—this structure has been successfully implemented at Tianjin Steel Pipe Company. 4 Assel Rolling Mill: Assel tube forming uses three-roll inclined milling consisting of three active rollers surrounding a core rod forming an annular closed hole shape. The body sections of these rollers consist of an entrance cone section , shoulder section , leveling area ,and exit cone section made from materials such as forged steel grade4Cr5MoSiV1 , semi-steel ,or alloyed ductile cast iron etc., with heat treatment hardness controlled at HSD65—75 for forged rollers , HSD50—60 for semi-steel rollers ,and HSD55—65 for alloyed ductile cast iron rollers (this type has been designed primarily for imported units with few successful applications domestically). It is the last hot deformation process of the production of hot-rolled seamless steel pipes, mainly aimed at eliminating the uneven outer diameter caused during the rolling process of previous steps, in order to improve the outer diameter accuracy and roundness of the hot-rolled finished pipes. The processes of sizing, reducing, and tension reduction are continuous rolling processes for hollow bodies without a core rod. Currently, advanced sizing (reducing) machines in China use three-roll mills with 120° equal division. After adopting this type of sizing (reducing) machine, the surface quality of the pipe material is good, and there are many specifications for finished pipe materials. The rolls and shafts of the sizing (reducing) machine adopt interference fit or key connection, with interference fit being more commonly used. The rolls are made from nickel-chromium-molybdenum infinite cold-hard ductile iron, and the blanks are produced by centrifugal casting to ensure a dense outer layer structure with a hardness between HSD55-68. When rolling steel pipes with specifications less than Φ219, the hardness of the rolls can be selected between HSD60-68 to ensure consistent hardness in hole shapes and good wear resistance; when rolling steel pipes with specifications above Φ219, due to larger roll hole dimensions and greater drop differences, to ensure that wear at the bottom and edges of hole shapes does not differ too much, the hardness of the rolls can be selected between HSD55-61. Conclusion Various rolls are important tools and consumables in seamless steel pipe production. Proper selection of material and hardness can extend service life, reduce downtime for roll changes, and lower production costs. Roll manufacturing companies should continuously research new technologies and materials, while steel pipe manufacturers should boldly promote and apply these new technologies to further improve the level of seamless steel pipe production in our country.
