Eight main steel pipe rolling process
Following the invention of periodic tube rolling technology, a variety of pipe rolling processes have emerged worldwide. These include Ehrhardt tube rolling, automatic tube rolling, Diescher tube rolling, three-roll tube rolling, hot extrusion tube production, continuous tube rolling, cold drawing (rolling) tube, and hot expanding tube—eight tube rolling technologies in total.
- Ehrhardt Tube Rolling Process: Invented by Ehrhardt in 1891, this process uses hydraulic punching to produce cup-shaped blanks and, in 1899, perfected a method using mandrels to push through a series of gradually decreasing diameter rings to produce seamless steel tubes. Commonly known as the pushing tube process, it is still used for producing extremely large diameter seamless tubes up to 1066 mm in diameter. In the 1980s, Mannesmann developed this into the CPE process, mainly for producing smaller diameter seamless tubes.
- Automatic Tube Rolling Process: After Stiefel obtained a patent for a disc-type piercing machine in 1895, the first automatic tube mill started operation in the USA in 1903, and by 1905, an automatic tube mill with sizing machines was born. Known as the Stiefel tube mill, it became the primary unit for producing seamless steel tubes.
- Diescher Tube Rolling Process and Accu Roll Tube Rolling Process: Sam Diescher introduced rotating guides into two-roll cross rolling mills for extended tube rolling, leading to the creation of the first Diescher tube mill in 1932. Half a century later, the American AS company introduced conical rolls into skew rolling tube mills, obtaining a patent. The world’s first Accu Roll tube mill began production in Yantai, China, in 1989.
- Three-Roll Tube Rolling Process: This is a three-roll cross rolling tube process where the shell is rolled between the mandrel and three rollers. The three-roll tube mill is also known as the Assel tube mill.
- Hot Extrusion Tube Process: Before the Mannesmann brothers invented oblique rolling piercing, there were already patents for producing seamless tubes using hot extrusion technology for aluminum. However, it wasn’t until the successful application of glass lubricants in the 1950s that hot extrusion processes for steel tubes truly took off and rapidly developed.
- Continuous Tube Rolling Process: As early as 1891, the Kellogg steel pipe plant in America had obtained a patent for continuous rolling mills. Due to various technical reasons, this process did not achieve real success until the 1950s, when it developed significantly due to its high capacity, efficiency, and quality, becoming the dominant type of rolling mill for producing seamless tubes.
- Cold Drawing (Rolling) Tube Process: To expand and enhance the varieties and quality of hot-rolled seamless tubes, secondary processing techniques such as cold drawing (rolling) and hot expansion were developed accordingly. Prior to the adoption of hot-rolled seamless tube production methods, cold drawing techniques were already being used for secondary processing of welded tubes. Cold rolling technology, patented by the American Tabl Reducing Company in 1931, employs deformation processes similar to those of periodic rolling mills.The cold drawing (rolling) method can not only produce products with specifications and high quality requirements that are difficult to produce (or uneconomical) or cannot be produced by hot rolling, but also can use the rolled blank instead of hot rolling to produce small size steel tubes and special varieties with high quality (such as bearing tubes
- Hot Expansion Tube Process: This is the primary method for producing large-diameter seamless tubes, available in three forms: roller-type (oblique rolling), pull-expansion, and push-forming. Roller-type expansion can reach diameters up to 1500mm, suitable for mass production; whereas push-forming expansion equipment is simple, requires less investment, has lower output, and is suitable only for small batch production.
Hot-rolled seamless steel pipe production
The current basic process for hot-rolled seamless steel pipes includes: preparation of billets → heating → piercing or punching → reheating and tube rolling → sizing → finishing, inspection, testing (cooling, straightening, cutting heads, sampling, ultrasonic testing, hydrostatic testing, weighing, anti-corrosion treatment, etc.) → packaging and warehousing. Among these, the main hot deformation steps are piercing or punching, tube rolling, and sizing or reducing.
- Piercing primarily transforms solid tube billets into hollow rough tubes;
- tube rolling mainly reduces wall thickness and controls the wall’s uniformity, serving as the primary process for longitudinal deformation.
- Sizing (or diameter reduction) focuses on reducing the outer diameter and controlling it, while also adjusting wall thickness (tension sizing).
Since the tube rolling process is not only crucial for longitudinal deformation but also plays a vital role in controlling wall thickness, surface quality, and overall production efficiency of the unit, people have become accustomed to referring to the tube rolling process by the type of unit used, such as continuous rolling mill units, automatic rolling mill units, precision rolling mill units, etc.
Using hot rolling methods, carbon steel, alloy tubes, high-alloy steels, and some non-ferrous metal seamless tubes can be produced with outer diameters ranging from 16 to 1600 mm and wall thicknesses from 2 to 200 mm. Besides round section tubes, various shaped section tubes and variable section tubes are also available. Hot-rolled seamless tubes, besides being finished products, can also serve as raw materials for cold-drawn or cold-rolled tubes (refer to cold drawing and cold rolling of tube materials).
The basic processes of hot rolling include:
- On a piercing machine, ingots or billets are pierced into thick-walled hollow rough tubes. Methods for piercing billets include two-roll oblique rolling piercing, pressure piercing, push rolling piercing, three-roll oblique rolling piercing, etc.
- On an extension machine, rough tubes are rolled thinner and extended to nearly the finished product’s wall thickness. Methods for extending rough tubes (rolling) include automatic tube mill rolling, continuous tube mill rolling, two-roll and three-roll tube mill oblique rolling (refer to tube material rolling extension), periodic rolling mill rolling, and pushing tube machines.
- Rough tubes are precisely rolled into finished products on finishing mills. The finishing of rough tubes includes processes aimed at improving quality like sizing and tube diameter adjustment, as well as those aimed at expanding product specifications like tube diameter reduction and hot expansion.
Due to the high demands for seamless tube quality and the efficient, rapid development of hot rolling tube mill units, tube finishing techniques and equipment have further developed and modernized. Step-type cooling beds, multi-roll tube straighteners, efficient tube cutting machines and saws, quick head chamfering machines, automatic measuring, weighing, spray printing, and mechanized bundling devices have successively appeared and been widely applied. For higher quality requirements and specialized tubing, further heat treatments (normalizing, annealing, quenching and tempering, etc.) and machining (threading, peeling, polishing, end thickening, etc.) are necessary.
