Over the past 20 years, the world’s stainless steel production has seen great development, process technology and equipment have greatly improved, and foreign stainless steel production is characterized by the development of new technology and new equipment to increase production, expand the variety, and the use of automatic control and nondestructive testing to improve and ensure product quality. Therefore, renovation and expansion in recent years in the stainless steel seamless pipe plant, have led way to many effective new technologies and new high efficiency equipment.
China’s stainless steel production after 40 years of development, particularly in the last 20 years, has made great progress; the yield, quality and variety continue to increase and improve the quality of products to an internationally recognized, advanced level. However, China’s stainless steel tube plant technology, equipment, and product quality still have a long way to go. There is still a need for more technological innovation to improve in order to meet the requirements of our national economy.
1. World stainless steel production technology trends
2.1 "Three steps" steelmaking and continuous casting process
The "three steps" steelmaking and continuous casting production of stainless steel tube technology improves quality, reduces costs and creates the better working conditions. Some of the world’s major steel plants who continued to use the "two steps" method conducted sufficient research, and adopted the "three steps" of the new stainless steel smelting process. Most professional stainless steel plants in Japan have adopted the "three-step" production process. Germany Mannesmann Mark developed a metallurgical technology company dedicated to "three steps" of the process equipment, which includes an ultra-high power electric furnace unit, 1 MRP-L converter and a platform VOD devices. The process uses electric furnace melting, MRP-L converter plus rapid de-carbonization oxygen lance, VOD ultimate deep vacuum decarburization furnace. The basic starting point of this process is the combination of AOD and VOD processes and to develop their own advantages and overcome the AOD refractory consumption of argon gas and a large and shortcomings of the processing time to achieve low consumption, shorten time and reduce costs purposes. Of course, the furnace of AOD’s "two steps" smelting of stainless steel is also feasible, more than 60% of the steel production industry continues to use the "two steps" process of production.
Foreign casting of stainless steel tube is vertical and curved in a continuous casting machine, but also uses a horizontal continuous casting machine. The stainless steel billet and steel ingot of a conventional rolled billet compared to the metal recovery rate increased from 10% to 15%, tube quality is better and saves energy, and reduces production costs. Some stainless steel in the continuous casting process is easily oxidized alloying elements, which makes it easy to produce oxide inclusions; In addition, the viscosity of molten steel makes it quite susceptible for nozzle clogging. As more complex forms of solidification of stainless steel, poor thermal conductivity of molten steel makes it quite easy for the billet to crack, so the cooling rate of casting requirements are more stringent.
2.2 Hot-extrusion process
The hot extrusion process is the world’s current primary pipe production process in the stainless steel industry. As technology advances, some types of hot-rolled stainless steel tube units can be produced, but the extrusion unit is the world’s major producer of stainless steel seamless pipe unit. Currently, there are 60 sets of extrusion units, except for a few for extrusion, 70% of the units are used for the production of steel Extrusion and rolling (longitudinal rolling, cross rolling) production methods are characterized by the extrusion of metal deformation processes under the stress test. In this best stress state, deformation resistance for the large stainless steel tube inside and outside will have a better surface quality and microstructure. The vertical hydraulic equipment through the piercing process improvement, will provide an extrusion wall thickness deviation of 5% to 7% accuracy. It is easy to manufacture, and replace, but it is meant for the production of small quantities and specifications of the stainless steel tube.
The extrusion unit can also produce a wide range of steel pipes, 1 set of 50MN of the extrusion unit can produce 25-254mm steel pipe, and will produce far more than the other rolling method. The stainless steel pipe extrusion production process can be used directly as raw material, the product quality is excellent, and replacement of varieties is flexible, it can manufacture pipe extrusion products, but it is also able to produce all kinds of profiled stainless steel tubes. Clearly, the production of finished pipe by extrusion and cold working requires waste management for the material. Currently, thermal processing is currently used widely in stainless steel production as it is a more economical method; with the high-speed, automation, extrusion machine with large tonnage, along with glass, lubricants, non-oxidative heating, high strength and toughness of hot dye, and other related technologies, with the production of stainless steel seamless pipe extrusion, in particular the difficult production and deformation of stainless steel, the high alloy steel pipe has been rapidly developed. With a single two-roll piercing, the cold method of providing material shortage management, is generally not used by foreign countries.
2.3 Dominated by cold rolling, cold-draw supplemented
Cold stainless steel tube tends to adopt a cold rolling based, cold-draw supplemented by the joint production process, cold rolling mill, cold-drawing machines use high-speed, high precision, long stroking, multi-line direction. The Stainless steel tube is 50% to 80% completed within the cold process. Therefore, the development of cold process equipment has been given more attention. There are basically three types of cold working processes, the drawing process, cold rolling, and the cold drawing of a joint process. In most cases, the cold-rolled joint of a drawing process, and cold rolling are supplemented by the processing method. The modern cold rolling mill is expected to achieve a reduced diameter, and reduced wall volume, an 80% rolling reduction has already been completed in the cold rolling mill. The cold wall use has been supplemented by drawing specifications and control changes in diameter to meet the requirements of different varieties and specifications.
The drawing process of a joint production of cold rolled advantages are: good quality steel pipe, cold rolled steel pipe wall thickness accuracy and surface quality with high precision, cold-drawn to ensure the pipe diameter; cold period is short, to reduce intermediate de-greasing, heat treatment, necking, Jiao Direct processes, energy conservation, reduced metal consumption; shortage can be used to produce small diameter steel pipe tube, simplified types of raw material specifications.
Currently, the world’s cold-rolled, cold-drawn process technology and equipment level has been greatly developed. Cold rolling forward speed, long travel, ring larvae L-type, high-precision direction, this mill is characterized by:
1) Inertia force and inertia torque with vertical balance mechanism, the number of mills has increased;
2) The ring groove, the length of deformation zone of rolling mill longer than the 70% short stroke, improve the uniformity of rolling, sending has increased;
3) Using a long tube, pipe shortage of raw materials increased length l2 ~ 15m, can produce long steel pipe, improve mill efficiency, effective utilization coefficient of rolling over up to 80%;
4) Mandrel and the best use of parabolic curved groove, metal deformation and reasonable, tool can be used for a long period of time.
These characteristics make cold rolling mill production capacity and greatly enhance the production of stainless steel products, precision cold drawn tube of the machine structure is still the mainly used drawing machine, a smaller amount continue to use the hydraulic drawing machine. The multi-line, high-speed, automatic drawing machine is the current direction of development.
2.4 A variety of welding technology
Welding process technology is expected to accelerate the development of stainless steel pipe production, the application of a variety of welding methods will further improve the welding quality and productivity. At present, the industrial application of stainless steel welding are Tungsten Inert Gas (TIG), high frequency welding, plasma welding and laser welding and so on. Characteristics of several different welding methods are improving.