Seamless steel pipe production process and selection of mill rollers

The production process of seamless steel tubes and the selection of materials and hardness of several major roll tools are described as follows:

1 seamless steel pipe production process

Seamless steel pipe production mainly through the following several processes; tube blank - cutting - heating - piercing - rolling tube - set (reduction) diameter - straightening - cutting head and tail - inspection - packaging - Storage. According to different rolling methods, seamless steel tube rolling mills can be divided into automatic rolling tube units, continuous rolling tube units (MPM two-roll continuous rolling tube units, PQF three-roll continuous rolling tube units), Assel roll units, Accu tube mills , cycle rolling mills, etc., the new rolling mills are mostly MPM tandem mills, PQF three-roll mills, such as the Φ460, Φ258 PQF tandem mills of Tianjin Pipe Company, and the Φ273 MPM tandem mill of Hengyang Steel Pipes. , Panzhihua Iron and Steel Chengdu seamless Φ340 MPM continuous rolling mill unit, Φ159 PQM continuous mill unit, Anshan Φ170 PQF continuous mill unit.

2 Punching machine

Pipe blank perforation is the most important deformation process in the production of hot-rolled seamless steel tubes. Its task is to pierce and roll the solid tube into a hollow capillary tube. According to the perforator structure and the deformation characteristics of the perforation process, the existing perforation methods can be divided into: oblique rolling perforation, push rolling perforation and pressure perforation, and the oblique rolling perforation is the most widely used, the shape of the roll of the diagonal rolling puncher Roller type, bacteria type (cone type) and disc type are three. Roller puncher rolls are used in small rolling mills, while large unit punchers use bacterial rolls. Disk puncher application is less. Regardless of the roll shape, in order to ensure that the tube bite and perforation process is achieved, there are perforated cones (roller inlet cones), milling cones (roller outlet cones) and roll rolling belts (excessive portions between inlet cone and outlet cone). )composition.

The punching machine rolls mostly use the combined structure of the roller shaft and the roller sleeve, and the two use the direct interference combination or the keying interference combination. In this way, when the roll is scrapped, the roll shaft can be reused to reduce the tool cost. The material of the small puncher roller shaft is made of 45 or 40Cr, and quenched and tempered. Large-scale puncher roller shafts are mostly made of 42CrMo and tempered. Roller material is currently selected by more manufacturers than others, and some manufacturers choose to forge 45, 55 and other high-quality carbon structural steel. Some manufacturers use 50Mn, 65Mn, 70Mn, 70Mn2Mo, 60CrMnMo alloy cast steel or forged steel; hardness range requirements are also inconsistent, and some require HB180-220, and some require HB220-260. In order to increase biting, large perforated rolls are generally machined with a mesh pattern with a depth of 0.8 to 1.2 mm on the entrance cone.

Comprehensive domestic and foreign manufacturers of material selection and use, the author believes that: the punching machine roll selection cast 50Mn, 60CrMnMo and other medium-carbon alloy steel material is more appropriate, one has a good strength and wear resistance, and the second is suitable for rolling each Kind of material steel pipe. Hardness control in HB200-240 is more reasonable.

3 Tube Rolling Machine

Capillary rolling is the main deformation process for the production of hot-rolled seamless steel tubes. Its role is to make the thickness of the capillary tube close to or reach the wall thickness of the finished tube, and to eliminate the uneven wall thickness in the perforation process of the capillary tube. The internal and external surface quality of the waste pipe is controlled, and the outer diameter and roundness of the waste pipe are controlled. The hollow capillary is rolled into a waste tube close to the finished product size.

1. Automatic Tube Rolling Machine: The automatic tube rolling mill is a two-roll non-reversible vertical rolling mill, which is characterized by a pair of high-speed counter-rotating and rewinding rolls behind the work rolls. The function is to send the once-rolled tubes to the front desk. The second rolling was carried out, so its production efficiency was low, and most of the domestic units had been modified or eliminated. Roller material is mostly infinite chilled cast iron, ductile iron or chilled ductile iron, hardness above HSD50.

2. Two-roll tandem rolling mill: The tandem rolling mill is a relatively advanced steel tube rolling mill. It is a perforated capillary tube that is sleeved on a long core rod, arranged in multiple racks in sequence, and adjacent rack roll gaps are misaligned by 90°. The continuous pipe mill performs rolling.

The material of the continuous roll is mostly nickel-chromium-molybdenum infinite cold hard nodular cast iron (also known as pearlitic ductile iron), nickel-molybdenum infinite cold hardened ductile iron (also called needle-shaped ductile iron), and the hole portion hardness is HSD50-65. The first roll of foreign designed roll is made of forged steel or graphite steel to further increase the strength of the roll.

3. Three-roll mill unit: The three-roll adjustable mandrel mandrel mill (PQF) was developed by INNSE, Italy, to overcome the inherent limitations of the two-roll mill, due to its 360° hole. The type is divided into three parts, which reduces the linear velocity gap between the bottom and the edge of each roll pass, so the precision of the rolled steel tube is high.

Compact PQF rolling mill rollers are monolithic and use nickel-chromium-molybdenum infinite cold hard nodular cast iron. Hardness control is appropriate at HSD 55-65. Larger PQF rolling mills are equipped with continuous rollers, ie the rollers are wear-resistant. It is good for nickel-chromium-molybdenum infinite cold hard nodular cast iron, and the shaft is made of high-strength forged steel material, and after quenching and tempering, it can be reused, which greatly saves the cost. This structure has been successfully used in Tianjin Pipe Company.

4. Assel tube mill: Assel tube is three-roller oblique rolling. It consists of three active rolls and a mandrel to form a closed closed hole.

Roller rolls are composed of 4Cr5MoSiV1 forged steel, cast semi-steel or alloy spheroidal graphite cast iron for the entrance cone, roll shoulder, leveling zone and exit cone. Heat treatment hardness: forged steel roll is controlled at HSD65-75, semi-steel roll is controlled at HSD50 -60, alloy ductile iron roll control in HSD55-65 (this type of material for the design of the imported roller, the successful application of less domestic examples).

4 Steel pipe sizing and reduction

It is the last waste heat deformation process in the production of hot-rolled seamless steel tubes. Its main function is to eliminate the difference in outer diameter caused by the previous process rolling process, so as to improve the outer diameter accuracy and roundness of the hot-rolled product tubes. The process of sizing, reducing and tension reducing of steel pipe is a continuous rolling process without hollow mandrel. At present, the domestic advanced (reducing) diameter machines adopt three-roller 120° equal division rolls. After adopting this type of fixed (reducing) diameter machine, the pipe surface quality is good, and the finished pipe material has more specifications.

Fixed (reduced) diameter rollers and shafts are fitted with interference fits or keys. Currently, interference fits are used. The roll adopts nickel-chromium-molybdenum infinite cold-hard spheroidal graphite cast iron. The blank adopts centrifugal casting to produce blanks, which ensures the denseness of the outer layer and the hardness is between HSD 55-68. When rolling the steel pipe below Φ219, the hardness of the roller can be selected between HSD60-68 to ensure that the hardness of the hole type is consistent and the wear resistance is good; when rolling the steel pipe with Φ219 or above, the size of the roll hole is Larger, larger drop, in order to ensure that the bottom of the hole and the wear of the edge is not too different, the hardness of the roller can be selected between HSD55-61.

Conclusion

All kinds of rolls are important tools and consumables in the production of seamless steel tubes. The proper choice of materials and hardness can prolong the service life, reduce the down time of roll change, and reduce the production cost. Roller manufacturers must constantly research new technologies and new materials. Steel pipe manufacturers must dare to promote and apply these new technologies to further improve the level of seamless steel pipe production in China.