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Development trend of production technology of stainless steel pipes

One: The development trend of the world’s production technology of stainless steel pipes
2.1 “Three-step method” steelmaking and continuous casting process
The use of the “three-step method” for steelmaking and continuous casting to produce stainless steel pipe blanks has created conditions for improving the quality of stainless steel pipe blanks and reducing costs. Some of the world’s major stainless steel plants have studied and adopted the “three-step method” to smelt stainless steel on the basis of the “two-step method”. At present, most of the professional stainless steel plants in Japan have adopted the “three-step method” production process. Mannesmann Demag Metallurgical Technologies of Germany has developed a process equipment dedicated to the “three-step process”, which includes an ultra-high power electric furnace, an MRP-L converter and a VOD unit. The process is melted in an electric furnace, the MRP-L converter is quickly decarburized with an oxygen lance, and the VOD vacuum furnace is deeply decarburized. The basic starting point of this process is to combine and promote the advantages of both AOD and VOD processes, and overcome the shortcomings of AOD argon and refractory materials with large consumption and long processing time, achieving low consumption, shortening time and reducing cost. purpose. Of course, electric furnace-AOD “two-step method” smelting stainless steel is also feasible. At present, more than 60% of the world’s stainless steel is produced by the “two-step” process. Foreign stainless steel pipe blanks have been basically continuous casting and continuous casting machines with vertical and arc type, and horizontal continuous casting machines are also used. Compared with the traditional ingot billet, the stainless steel continuous casting billet has a metal yield increase of 10% to 15%, the pipe billet quality is better, energy saving and production cost are reduced. In the continuous casting process of stainless steel, some alloying elements are easily oxidized, which is easy to produce oxide inclusions. In addition, the viscosity of molten steel is large, which easily causes the nozzle to block. Due to the complicated solidification shape of stainless steel, the thermal conductivity of molten steel is poor, and the casting blank is prone to cracks. Therefore, the speed of cooling and casting is strict, and each tundish heating, non-oxidation protection casting, electromagnetic stirring, and liquid level automatic control are adopted. A series of new technologies have greatly improved the quality of the slab, enabling some difficult-to-cast steel grades, such as titanium-containing stainless steel, single-phase austenitic stainless steel and martensitic stainless steel, to be cast smoothly, expanding the stainless steel continuous casting steel. Kind.
2.2 Hot extrusion process
The hot extrusion process is the main production process of the world’s current hot-processed stainless steel seamless pipes. With the development of technology, some types of hot-rolled steel pipe units can produce stainless steel pipes, but the extrusion unit is the main production unit of stainless steel seamless pipes in the world. At present, 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. The main type is stainless steel pipes. Compared with the extrusion (rolling, cross-rolling) production method, the extrusion method is characterized by three-direction compressive stress during metal deformation. Under such a good stress state, a good internal and external surface quality and metallographic structure can be obtained for a stainless steel pipe with high deformation resistance. Due to the improvement of the vertical hydraulic perforation process equipment, the wall thickness deviation of the waste pipe after extrusion can be adjusted to an accuracy of 5% to 7%. Extrusion molds are easy to manufacture and replace, making them suitable for small-volume, multi-size stainless steel pipes. The extrusion unit can also produce a wide range of steel pipes. A 50 MN extrusion unit can produce steel pipes of @25-254 mm, which is much more suitable for market demand than other pipe rolling methods. The outstanding advantage of using the extrusion method to produce stainless steel pipes is that the continuous casting slab can be directly used as raw materials, the product quality is stable, the replacement varieties are flexible, and the hot extruded product pipes can be directly produced, and various shaped stainless steel pipes can also be produced. Obviously, the production of finished pipes by extrusion and the provision of waste pipes for cold processing are a more economical method widely used in the production of hot-processed stainless steel pipes abroad, with the adoption of high-speed, automated, large-tonnage extruders, and The development of related technologies such as glass lubricant, non-oxidation heating, high-strength and high-toughness hot work die, and the production of stainless steel seamless steel pipes by hot extrusion, especially the production of difficult-to-deform stainless steel and high-alloy steel pipes have been rapidly developed. The method of providing a waste pipe for cold processing by a single two-roll cross-rolling perforation has been basically not adopted abroad.
2.3 Cold processing is mainly based on cold rolling and cold drawing
Cold-worked stainless steel pipes tend to adopt a combined production process mainly consisting of cold rolling and cold drawing, and cold rolling mills and cold drawing machines are developed in the direction of high speed, high precision, long stroke and multi-line. 50% to 80% of stainless steel pipes are finished by cold working. Therefore, more attention has been paid to the development of cold working equipment. There are basically three types of cold working processes, namely cold drawing process, cold rolling process, cold rolling and cold drawing combined process. Cold processing of stainless steel pipes Most of the foreign countries adopt the cold rolling and cold drawing combined process, and the processing method is mainly cold rolling and cold drawing. Modern cold rolling mills can achieve large reductions and large reductions in wall thickness, and 80% of the rolling deformation is done on cold rolling mills. It adopts cold rolling and wall fixing, supplemented by cold drawing to change specifications and control outer diameter to meet the requirements of different varieties and specifications. The advantages of cold rolling and cold drawing combined process are: good quality of steel pipe, high wall thickness precision and surface quality of cold rolled steel pipe, cold drawing to ensure outer diameter accuracy of steel pipe; short cold working cycle, reducing intermediate degreasing, heat treatment, shrinkage, and correction Straight processes, saving energy and reducing metal consumption; large-diameter pipes can be used to produce small-diameter steel pipes, simplifying the types of raw materials. At present, the world’s cold rolling and cold drawing process technology and equipment level have been greatly developed. The cold rolling mill develops in the direction of high speed, long stroke, ring L type and high precision. The characteristics of this rolling mill are: 1 using inertial force and inertia torque vertical balance mechanism, the number of round trips of the rolling mill is improved; 2 adopting annular hole type, The length of the rolling deformation zone is 70% longer than that of the short stroke mill, the uniformity of rolling deformation is improved, and the delivery volume is increased. 8 The long pipe blank is used, and the length of the raw material of the waste pipe can be increased to 12 to 15 m, which can produce long steel pipes and improve the utilization of the rolling mill. Rate, the effective utilization coefficient of rolling is up to 80% or more; 4 adopting curved mandrel and good parabolic face type, reasonable metal deformation and long tool life. The above characteristics make the production capacity and product precision of cold rolling mill greatly improve the production of stainless steel pipe. The structure of the cold pipe drawing machine is still mainly chain cold drawing machine, and there are also a few hydraulic cold drawing machines. Multi-line, high-speed and fully automatic are the development direction of the current cold drawing machine.
2.4 Multiple welding techniques
Advances in welding process technology have accelerated the development of stainless steel welded pipe production, and the application of various welding methods has further improved welding quality and productivity. At present, the stainless steel welding methods applied in the industry mainly include tungsten argon arc welding (TIG), high frequency welding, plasma welding and laser welding. Several welding methods have their own characteristics, and the most used ones are argon arc welding and high frequency welding:
2.4.1 TIG welding
Stainless steel welded pipe requires deep penetration welding, no oxide inclusions, heat affected zone as small as possible, TIG welding with tungsten inert gas protection has good adaptability, high welding quality and good penetration performance. Its products are in chemical industry. Widely used in the industrial fields such as nuclear industry and food industry. The welding speed is not high. It is the inadequacy of argon arc welding. In order to improve the welding speed, various methods have been developed in foreign countries. Among them, the development of single-electrode single torch to multi-electrode multi-torch has been applied in production. In the 1970s, Germany first used a multi-torch to line up along the direction of the weld to form a long heat flow distribution, and the welding speed was significantly improved. Generally, the argon arc welding of the three-electrode welding torch is adopted. The wall thickness of the welded steel pipe is s ≥ 2 ram, the welding speed is increased by 3 to 4 times than that of the single welding torch, and the welding quality is also improved. The combination of argon arc welding and plasma welding can weld steel pipes of larger wall thickness. In addition, the use of 5% to 10% hydrogen in argon, followed by high-frequency pulse welding power supply, can also increase the welding speed. Multi-torch argon arc welding is suitable for the welding of austenitic and ferritic stainless steel pipes.
2.4.2 High frequency welding
High-frequency welding has been used in carbon steel welded pipe production for more than 40 years, but it is a relatively new technology for welding stainless steel pipes. The economics of its production make its products more widely used in the fields of architectural decoration, household appliances and mechanical structures. High-frequency welding has a large power supply, and it can achieve a higher welding speed for steel pipes of different materials, calibers and wall thicknesses (more than 10 times higher than the high welding speed of argon arc welding). Therefore, high-frequency welding produces general-purpose stainless steel pipes with high productivity because high-frequency welding speed is high, which makes it difficult to remove burrs in the welded pipe. This is also the current high-frequency welding of stainless steel pipes that cannot be accepted by the chemical industry and the nuclear industry. One of the reasons. From the perspective of welding materials, high-frequency welding can weld various types of austenitic stainless steel pipes. At the same time, the development of new steel grades and advances in forming and welding methods, ferritic stainless steel AISI 409 and other steel grades can also be welded.
2.4.3 Combined welding technology
Various welding methods for stainless steel welded pipes have their own advantages and disadvantages. How to improve strengths and avoid weaknesses, combine several welding methods to form a new welding process to meet people’s requirements for stainless steel welded pipe quality and production efficiency, is a new trend in the development of stainless steel welded pipe technology. After several years of exploration and research, the combined welding process has made progress, and Japan, France and other countries have mastered certain welding techniques for the production of stainless steel welded pipe. Combination welding methods are: 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. Combined welding increases the welding speed significantly. For combined welding with high-frequency preheating, the quality of welded steel pipe is equivalent to conventional argon arc welding and plasma welding. The welding operation is simple, and the entire welding system is easy to automate. This combination is easy to connect with existing high-frequency welding equipment. Low investment costs and good returns.
2.5 Bright heat treatment
For the heat treatment of stainless steel pipes, a non-oxidation continuous heat treatment furnace with a protective gas is generally used in foreign countries for medium heat treatment and heat treatment of finished products. The conventional pickling process is eliminated because a bright surface that is free of oxidation is obtained. The use of this heat treatment process not only improves the quality of the steel pipe, but also overcomes the environmental pollution caused by pickling. According to the current trend of world development, bright continuous heat treatment furnaces are basically divided into three types:

  • (1) Roller type bright heat treatment furnace. This type of furnace is suitable for heat treatment of large-size and large-volume steel pipes with an hourly output of 1. Ot above. The shielding gases that can be used are high purity hydrogen, ammonia decomposition and other shielding gases. A convection cooling system can be provided to cool the steel pipe faster.
  • (2) Net belt type bright heat treatment furnace. This type of furnace is suitable for small
  • The thin-walled precision steel pipe has an hourly output of about 0.3 to 1.0 t, and the treated steel pipe can be up to 40 m in length, and can also be processed into a coiled capillary. Equipped with a convection cooling system for fast cooling. Various gas can be used for gas fuel or electric heating. The steel pipe after heat treatment of this furnace type has no scratches and good brightness.
  • (3) Muffle pipe type bright heat treatment furnace The steel pipe is installed on a continuous bracket and heated in the muffle pipe to process high-quality small-diameter thin-walled steel pipe at a low cost, and the hourly output is above 0.3t. The shielding gas can be used economically, and the heating source can be gas, oil or electricity.

2.6 Organic solvent degreasing
The organic solvent degreasing technology has good degreasing effect and high degree of automation. In order to remove the oil stain on the surface of the steel pipe during cold rolling and cold drawing lubrication, the quality of the heat-treated steel pipe is improved and carburization is prevented, and the degreasing effect is good and the technical equipment is advanced. Organic solvent degreasing unit. Commonly used organic solvents are trichloroethylene, tetrachloroethylene, trichloroethane, chlorinated methane, and more commonly used is trichloroethylene. This solvent has good effect, easy recovery and low toxicity. Degreasing is carried out in a sealed chamber. The stainless steel pipe in the basket is taken off in the solvent tank.
Grease, or further condense and degrease in solvent vapor. The entire degreasing process is done automatically by a computer in the sealed chamber. The trichloroethylene degreaser can be regenerated and reused. And monitor the solvent gas content in the use environment to avoid environmental pollution.
2.7 Finishing equipment modernization
Equipped with modern finishing equipment and enhanced quality control is an important part of the production of contemporary stainless steel pipes. The stainless steel pipe finishing section is equipped with a non-destructive testing process, which is an indispensable quality inspection method. The advancement of non-destructive testing technology has enabled more manufacturers to adopt eddy current and ultrasonic combined flaw detection units, and equipped with laser caliper and ultrasonic thickness measuring device. Before the steel pipe leaves the factory, the quality is controlled from multiple links. This combined flaw detection unit has an advanced information processing system, and the measured signals are all digitized, processed and stored by computer, with high degree of automation and high production efficiency. High-precision straightening machine, cold pressing and rotary compression machine, multi-head (6-8 head) polishing machine, full-length marking device, etc., are all widely used stainless steel pipe finishing high-efficiency production equipment in the world.
2.8 Develop new technologies and processes
Actively research and develop new technologies and processes to obtain high-quality, low-cost stainless steel pipe products. Many new processes and new technologies in the field of stainless steel pipe production have promoted the improvement of the quality of steel pipes and the reduction of production costs.
2.8.1 Welding + cold drawing production process
The United States has developed a new process for the production of welded + cold drawn stainless steel pipes. That is, the stainless steel cold belt after slitting is formed into a welded pipe by argon arc welding, and then the finished product is obtained through a cold drawing and finishing process. Due to the high quality of argon arc welding, the performance of the weld is consistent with the substrate. Therefore, the stainless steel pipe produced by this process is 10% to 30% lower than the common process of hot extrusion + cold drawing (rolling). The production process is simplified and the cycle is shortened.
2.8.2 Warm rolling technology
The warm rolling technology was invented by the former Soviet Union. It is obtained by heating the pipe material before cold rolling to 150 to 400 ° C by an induction heating device. This technology has the advantages of small rolling force and large amount of primary deformation. The OCrl8Ni9Ti steel pipe is subjected to two or three times of warm rolling without intermediate annealing, and the total deformation amount can reach 98%.
2.8.3 Ultrasonic vibration cold drawing process
The ultrasonic vibration of the die during cold drawing can increase the reduction rate of each drawing pass, reduce the vibration phenomenon and surface pit of the steel pipe, and improve the surface quality of the steel pipe. This is because the ultrasonic frequency causes the device to vibrate, reducing the friction during drawing. The steel pipe is subjected to the extrusion of the drawing die and the mandrel to reduce the adverse effects of tensile stress.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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