Production process of stainless steel seamless pipe

Posted by zora li on April 12th, 2023

Production process of stainless steel seamless pipe

With the development of the world economy, the requirements for corrosion resistance of stainless steel are gradually increasing, and the use of ultra-low carbon stainless steel is becoming more and more popular; in addition, due to the shortage of nickel resources and the high price of nickel, the rapid development of 400 series stainless steel, duplex stainless steel and super stainless steel Therefore, the smelting of stainless steel should adopt the combination of two-step method and three-step method. The two-step method is mainly used in the production of ordinary carbon-containing stainless steel; the three-step method is mainly used in the production of ultra-low carbon and nitrogen stainless steel, so as to improve the smelting quality of stainless steel.

1 Seamless steel pipe production technology

(1) Improve the level of stainless steel smelting and adopt continuous casting tube billet
At present, stainless steel has basically adopted continuous casting, and continuous casting machines are mainly vertical and arc. Compared with the traditional steel ingot-rolled billet, the continuous casting billet of stainless steel has better quality, and the metal yield is increased by 10% to 15%, which not only saves energy but also reduces production costs.
During the pouring process of stainless steel, some alloying elements are easily oxidized and easily produce oxide inclusions; in addition, the viscosity of molten steel is high, which can easily cause nozzle blockage. Since the solidification form of stainless steel is relatively complicated, the thermal conductivity of molten steel is poor, and the continuous casting slab is prone to cracks, so the requirements for the cooling and casting speed of the continuous casting slab are strict; A series of new technologies such as stirring and liquid level automatic control have greatly improved the quality of continuous casting slabs. For some difficult-to-cast steel types, such as titanium-containing stainless steel, single-phase austenitic stainless steel and martensitic stainless steel, it can also be poured smoothly, expanding the range of steel types for stainless steel continuous casting slabs.

(2) The production of stainless steel seamless pipe mainly adopts hot extrusion process
With the development of production technology, although some hot-rolled steel pipe units can also produce stainless steel pipes, the extrusion unit is the main unit that produces stainless steel seamless pipes in the world. There are more than 60 sets of extrusion units in the world, except for a few for profile extrusion, 70% of the units are used for the production of steel pipes, and the main varieties are stainless steel pipes. Compared with rolling (longitudinal rolling and skew rolling) production methods, the extrusion method is characterized by three-dimensional compressive stress during metal deformation. In this optimal stress state, for stainless steel pipes with many steel grades and high deformation resistance, better deformation, inner and outer surface quality and metallographic structure can be obtained. The improvement of the vertical hydraulic piercing process equipment can make the wall thickness deviation accuracy of the extruded blank pipe reach 5%~7%. Extrusion machine tools and dies are simple, easy to manufacture and replace, and are suitable for the production of small batches and multi-specification stainless steel pipes. The extrusion unit can also produce steel pipes with a wide range of specifications. A 50Mn (5000t) extrusion unit can produce steel pipes with a specification of Φ25~245mm, which is much more adaptable to market demand than other pipe rolling units.

The biggest advantage of using the extrusion method to produce stainless steel pipes is that continuous casting billets can be used directly as raw materials, the product quality is stable, and the variety can be changed flexibly. It can directly produce hot-extruded finished pipes, and can also produce stainless steel pipes with various special-shaped sections. However, the disadvantage of producing steel pipes by extrusion is that the yield is low. In order to improve the finished product rate, foreign production enterprises have improved the production process. For high-demand varieties, the production process of pre-drilling, hydraulic punching machine reaming and extrusion into tubes is generally used. For cold-worked raw material pipes with general requirements, different production processes can be adopted according to specific conditions: the solid blanks for small-sized steel pipes are directly pierced and extruded into pipes on a vertical hydraulic piercing machine; Type hydraulic piercing machine for reaming and extruding into pipes; after pre-drilling larger holes, the blanks used for large-scale steel pipes are directly entered into the extruder to be extruded into pipes. Obviously, producing finished pipes by extrusion and providing raw pipe materials for cold processing is a relatively economical and practical production method widely used in foreign countries for hot-processing stainless steel pipes.
With the adoption of high-speed, automatic, large-tonnage extrusion machines, and the development of related technologies such as glass lubricants, non-oxidative heating, high-strength and high-toughness hot-working molds, stainless steel seamless pipes are produced by hot extrusion, especially The production of hard-to-deform stainless steel pipes and high-alloy steel pipes has developed rapidly. However, the production method of using a single two-roller cross-rolling perforation to provide blank tube material for cold processing has basically no longer been used abroad.

(3) The cold-processed stainless steel pipe adopts the production process of mainly cold rolling and supplementary by cold drawing

50%~80% of stainless steel pipes need to be made into finished products through cold processing. Therefore, cold processing equipment has been greatly developed. There are basically three kinds of cold processing processes, namely cold drawing process, cold rolling process, and cold rolling-cold drawing combined process. Most of the cold processing of foreign stainless steel pipes adopts the combined process of cold rolling and cold drawing, and the main processing technology is cold rolling and cold drawing is supplemented.
Modern cold rolling mills can realize large wall reduction and large diameter reduction of stainless steel pipes, and 80% of the deformation can be completed on cold rolling mills. Using cold rolling to fix the wall, supplemented by cold drawing to change the steel pipe specification and control the outer diameter, can meet the production requirements of different specifications and varieties. The advantages of the cold-rolling-cold-drawing joint production process are: the quality of the steel pipe is good, the wall thickness accuracy and surface quality of the cold-rolled steel pipe are high, the outer diameter accuracy of the cold-drawn steel pipe is high; the cold processing cycle is short, reducing the intermediate degreasing, heat treatment, Shrinking, straightening and other processes and metal consumption save energy; large-scale waste pipe raw materials can be used to produce small-size steel pipes, simplifying the types of raw material specifications.

At present, the world's cold rolling and cold drawing process technology and equipment level have made great progress. Cold rolling mills are developing towards high speed, long stroke, ring pass and high precision. The characteristics of this pipe rolling mill are as follows: ?A vertical balance mechanism of inertial force and inertial torque is adopted, and the number of round trips of the pipe rolling mill is increased; The uniformity is improved and the feed rate is increased; ?Using long tube blanks, the length of blank tube raw materials can be increased to 12~15m, which improves the utilization rate of the tube mill, and the effective utilization coefficient of rolling is above 0.80; ?Uses curved mandrel and The best parabolic surface shape, the metal deformation is reasonable, and the service life of the tool and mold is improved.

The above features greatly improve the production capacity of the cold rolling mill and the dimensional accuracy of the products produced. The structure of cold-drawn pipe machines for producing stainless steel pipes is still dominated by chain-type cold-drawn pipe machines, and there are also a few hydraulic cold-drawn pipe machines. Multi-line, high-speed, and fully automatic are the development directions of cold-drawn tube machines. In addition, there is a new technology of warm rolling, which is to heat the stainless steel pipe material before cold rolling to 150~400°C through an induction heating device, and then roll it. The warm rolling technology has the advantages of small rolling force and large deformation at one time. The 0Cr18Ni9Ti stainless steel pipe is warm rolled 2~3 times without intermediate annealing, and the total deformation can reach 98%.

2 Development of stainless steel welded pipe production technology

At present, the stainless steel welding methods used in industry mainly include tungsten argon arc welding (TIG), high frequency welding, plasma welding and laser welding. Several welding methods have their own characteristics, but the most used ones are argon arc welding and high frequency welding.

(1) Argon arc welding technology

Stainless steel welded pipe requires deep penetration, no oxide inclusions, and the heat-affected zone is as small as possible. Tungsten inert gas shielded argon arc welding has good adaptability, high welding quality and good penetration performance. Its products have been widely used in the fields of chemical industry, nuclear industry and food industry. The disadvantage of argon arc welding is that the welding speed is not high. In order to increase the welding speed, some foreign enterprises have researched and developed various methods. Among them, there is a welding method developed from a single electrode single welding torch to a multi-electrode multi-torch welding method. In the 1970s, Germany first adopted multi-welding torches arranged in a straight line along the direction of the weld to form a long heat flow distribution and significantly increase the welding speed; For argon arc welding with three-electrode torch, the thickness of the welded steel pipe is S≥2mm, the welding speed is 3~4 times higher than that of single torch, and the welding quality is also improved. The combination of argon arc welding and plasma welding can weld steel pipes with larger wall thickness. In addition, using 5% to 10% hydrogen in argon, and then using a high-frequency pulse welding power supply, can also increase the welding speed. Multi-torch argon arc welding is suitable for welding austenitic and ferritic stainless steel pipes.

(2) High frequency welding technology

High-frequency welding has been used in the production of carbon steel welded pipes for more than 40 years. It is still a relatively new technology for welding stainless steel pipes. The economy of its production makes its products more widely used in architectural decoration, household appliances and mechanical structures, etc. industry. The power of high-frequency welding is relatively large, and it can achieve high welding speed for steel pipes of different materials, outer diameters, and wall thicknesses. Compared with argon arc welding, its welding speed is more than 10 times the highest speed of argon arc welding, so the production of stainless steel pipes for general purposes has higher productivity. Because of the high speed of high-frequency welding, it is difficult to remove the burrs in the welded pipe. At present, high-frequency welded stainless steel pipes cannot be accepted by the chemical and nuclear industries, and the above reasons are also one of them. High-frequency welding can weld various types of austenitic stainless steel pipes. At the same time, due to the development of new steel types and the progress of forming welding methods, ferritic stainless steel AISI409 and other steel types can be welded.

(3) Laser welding technology

Laser welding technology is the latest welding technology. The weld seam of the welded steel pipe is very small, the welding quality is high, the welding speed is fast, and it is suitable for welding high-quality stainless steel pipes. Germany, Japan and other countries have adopted this technology (some domestic enterprises also using this technique). Using laser welding technology can produce high-standard, high-quality, high-value-added stainless steel welded pipes, and the effect is very good.

(4) Combined welding technology

Various welding methods of stainless steel welded pipes have their own advantages and disadvantages. How to develop strengths and avoid weaknesses, combine several welding methods to form a new welding process, and meet people's requirements for the quality and production efficiency of stainless steel welded pipes are the new trends in the development of stainless steel welded pipe technology. After several years of exploration and research, the combination welding process has made progress, and Japan, France and other countries have used certain combination welding technology in the production of stainless steel welded pipes. Combined welding methods include: argon arc welding + plasma welding, high frequency welding + plasma welding, high frequency preheating + three torch argon arc welding, high frequency preheating + plasma welding + argon arc welding, etc. Combined welding can significantly increase the welding speed. High-frequency preheating is used to weld the steel pipe, and the weld seam quality of the welded pipe is equivalent to that of conventional argon arc welding and plasma welding. The combined welding operation of high-frequency preheating is simple, and the whole welding system is easy to realize automation. This combination is easy to connect with the existing high-frequency welding equipment, and the investment cost is low and the benefit is good.

3 Adopt protective gas bright continuous heat treatment furnace

For the heat treatment of stainless steel pipes, non-oxidation continuous heat treatment furnaces with protective gas are generally used abroad for intermediate heat treatment and finished product heat treatment. The steel pipe can obtain a bright surface without oxidation, thus canceling the traditional pickling process. The use of this heat treatment process not only improves the quality of the steel pipe, but also avoids the pollution of the pickling discharge to the environment. According to the current development trend of the world, bright continuous heat treatment furnaces are basically divided into three types.

(1) Roller hearth bright heat treatment furnace

This type of furnace is suitable for heat treatment of large-scale and large-volume steel pipes, with an hourly output of more than 1.0t. The protective gases that can be used are high-purity hydrogen, decomposed ammonia and other protective gases. It can be equipped with a convection cooling system to cool the steel pipe faster.

(2) Mesh belt type bright heat treatment furnace This type of furnace is suitable for small-

diameter thin-wall precision steel pipes, with an hourly output of about 0.3~1.0t, steel pipe lengths up to 40m, and coiled capillary tubes; it can also be equipped with convection cooling systems Do a quick chill. Gas fuel or electric heating can be used, and various protective gases are suitable. The steel pipe after heat treatment has no scratches and good brightness.

(3) Muffle bright heat treatment furnace

The steel pipe is placed on a continuous bracket and heated in the muffle tube, which can process high-quality small-diameter thick-walled pipe at a lower cost, and the hourly output is above 0.3t. Protective gas can be used economically, and the heat source for heating can be gas, oil or electricity. Different heat treatment furnace types and process systems should be selected according to different steel types of martensite, ferrite, austenite and duplex stainless steel.

4 Equipped with modern finishing equipment to strengthen quality control

The important part of the quality control of stainless steel pipes is the finishing process and its equipment settings, and non-destructive testing is an indispensable means of quality inspection. The advancement of non-destructive testing technology has led more manufacturers to use eddy current and ultrasonic combined flaw detection units, and to equip the flaw detection units with laser diameter measuring and ultrasonic thickness measuring devices. Before the steel pipe leaves the factory, the production plant checks the quality from multiple links. This combined flaw detection unit has an advanced processing system, and the measured signals are all digitized and stored by computer processing, with high automation and production efficiency. High-precision straightening machines, cold-state pushing and spinning machines, multi-head (6-8 heads) polishing machines, full-length marking devices, etc., are all high-efficiency stainless steel finishing equipment widely used in the world at present.