Briefly expounds on the current development status of the stainless steel pipe industry, including the output and distribution of stainless steel pipes, and the innovation and development of enterprises.
This paper focuses on the application status of stainless steel pipes in the field of water pipes, including the production status, global application and popularization, the application of ferritic stainless steel in water pipe material selection, and the current problems in the stainless steel water pipe industry.
Keywords: stainless steel pipe, water pipe, ferritic stainless steel, material selection.
The production process of stainless steel seamless steel pipe is completed by piercing, cold working, heat treatment, pickling and inspection.
Perforated blank pipe is the first deformation process in the seamless steel pipe production process.
There are many methods for the production of waste pipes: two-roll or three-roll cross-rolling piercing, hydraulic punching, push-rolling piercing, extrusion machine extrusion.
At present, the methods of cross-rolling two-roll perforation and extruder extrusion are mainly used. Cold working includes cold rolling, cold drawing, cold extrusion and spinning, etc. The cold working process generally adopts a combination of rolling and drawing. At present, most domestic heat treatment adopts roller hearth furnace and box furnace, and TISCO adopts hydrogen protection solution treatment furnace.
There are continuous production lines (ERW) and discontinuous production lines (UOE, JCOE) for stainless steel welded pipes.
The ERW unit is to divide the steel coil into the width of the steel pipe by the slitting machine, and then undergo W forming, welding, weld grinding, sizing, weld rolling, hydrogen protection induction heating treatment and sawing, flaw detection and hydraulic test (underwater air pressure), pickling is completed offline.
Welding includes hydrogen arc welding, plasma welding and plasma plus argon arc welding. Welding guns include single gun, double gun and three guns. Generally, laser welding or submerged arc welding is not recommended. It is not correct to use high frequency welding for industrial stainless steel welded pipes. practice.
UOE or JCOE is to cut the steel plate according to the expanded size of the steel pipe, and then first arc the two edges through the press according to the pipe diameter R, then U-shaped by the press, and O-shaped welded by the joint machine, after O-formed, pipe expansion, The production process of welded pipes including heat treatment, shaping, straightening, inspection and warehousing.
It has been proved by engineering practice at home and abroad that it is the best comprehensive performance, environmental protection and energy-saving pipe among water supply pipes. Thin-walled stainless steel pipes are the future trend of pipe use and development. Now it can be predicted that building water supply pipes will eventually return to the era of metal pipes.
The application of stainless steel pipes started from the direct drinking water system, and was first used in star hotels and high-end residences.
In recent years, the field of application has been continuously expanded, such as
From the 1950s to the 1970s, the main materials of water supply pipes in Japan were plastic and steel-plastic composite pipes, which caused serious water leakage, with the leakage rate as high as 40% to 45% at one time, and the water pollution problem was obvious.
The Tokyo Water Supply Bureau spent 10 years doing research and testing, and found that 60.2% of the water leakage was caused by the insufficient strength of the water pipe material and the impact of external forces.
The Japan Water Association suggested improving water pipe materials and connection methods. In 1980, Japan formulated a series of standards for industrial stainless steel pipes and their connections. In 1999, the Tokyo Water Supply Bureau adopted 316 stainless steel bellows as a standard material, and stainless steel used as water supply pipes for tap water and water supply pipes in buildings entered the track of standardization.
According to statistics from the Tokyo Waterworks Department, the market penetration rate of stainless steel rose from 11% in 1982 to over 90% in 2000. At present, the penetration rate of stainless steel water supply pipes in Tokyo, Japan is nearly 100%, and all residential areas are equipped with stainless steel pipes.
From 1993 to 2005, the market penetration rate of stainless steel water pipes in Seoul, South Korea increased from 32% to nearly 90%.
Optional materials for stainless steel water pipes include 304, 304L, 316, 316L, 445J2, etc. When selecting materials, mainly refer to the concentration of chloride ions in the water. Refer to Table 3 for material selection based on the concentration of chloride ion corrosion resistance. When selecting materials for water delivery pipes, materials that are resistant to corrosion at higher chloride ion concentrations are generally selected. For salty tide pipes, materials with resistance to chloride ion concentration above 600×10-6 should be selected.
In 2020, there are about 70 domestic water supply companies that are using or plan to start using stainless steel water pipes in the near future, and the purchase amount is expected to be 1.3 billion yuan. The consumption of stainless steel water pipes in 2020 is about 250,000 tons.
Ferritic stainless steel is superior to austenitic stainless steel in resistance to pitting corrosion, crevice corrosion and stress corrosion cracking. The difference between ferritic stainless steel and austenitic stainless steel is only in performance and use. Water supply and drainage systems are more suitable for ferritic stainless steel pipes. Ferritic stainless steel mainly has the following characteristics:
According to the performance characteristics of ferrite, the grades of ferritic stainless steel recommended for use in home improvement water supply pipes mainly include: 00Cr18Mo2Ti, 00Cr22Mo1Nb (Ti), 00Cr22Mo2Ti (Nb), 019Cr24Mo 2NbTi, now focus on chemical composition, mechanical properties, resistance Corrosion performance, welding performance medium, etc. introduce the performance of various steel types, which is convenient for comparison with austenitic stainless steel such as 304 (0Cr18Ni9), 316 (0Cr17Ni12-Mo2) steel, etc. It is more reasonable to choose ferritic stainless steel.
The chemical composition of high-purity 00Cr18Mo2Ti steel is shown in the figure below.
The mechanical properties of high-purity 00Cr18Mo2Ti steel at room temperature are shown in the figure below.
The pitting corrosion resistance of 00Cr18Mo2Ti is generally equivalent to that of 0Cr17Ni12Mo2. The pitting test is carried out in 5% FeCl3 +0.05 mol/L HCl solution. The relationship between the corrosion rate and the solution temperature is shown in the figure below, and its pitting potential is equivalent to that of 0Cr17Ni12Mo2.
Stress corrosion resistance
The stress corrosion resistance of 00Cr18Mo2Ti is better than that of 0Cr17Ni12Mo2 stainless steel. The stress corrosion resistance of 00Cr18-Mo2Ti in boiling 42% MgCl2 solution is shown in the table below.
00Cr18Mo2Ti can be welded by the conventional method of welding stainless steel. The corrosion resistance of the welded state is equivalent to that of the base metal, and it is suitable for domestic water supply pipes.
00Cr22Mo1Nb (Ti) is a ferritic stainless steel with a Cr content of not less than 21% and a Mo content of about 1%. Its corrosion resistance is between that of 0Cr18Ni9 and 0Cr17Ni12Mo2 steels; Inexpensive material to replace 316L (00Cr17Ni 14Mo2) stainless steel in aqueous solution.
The chemical composition of 00Cr22Mo1Nb (Ti) steel is shown in the table below.
The room temperature mechanical properties of 00Cr22Mo1Nb (Ti) annealed steel sheet (850 °C for 5 min, air cooling) are shown in Table 8. The strength and plasticity of the steel are high, the elongation is above 40%, and the anisotropy is not obvious. In addition, it also has good impact toughness at room temperature, good cold formability, and no obvious anisotropy.
Electrochemical samples were taken from a 4 mm thick hot-rolled steel plate that was air-cooled at 870 °C for 10 minutes, and the pitting potential was measured. The pitting potential of Cr22Ti ferritic stainless steel with different molybdenum content in 3.5% NaCl solution is shown in the figure below Show.
In 10% FeCl3·6H2O solution, the pitting corrosion resistance (corrosion rate) of 00Cr22Mo1Nb(Ti), 00Cr22Mo1.5Ti, 00Cr22Mo2.5Ti and 00Cr17Ni12Mo2 steel is shown in the figure below. The stress corrosion cracking test was carried out in boiling CaCl2 and boiling 26% NaCl solutions using O-type samples of 2 mm×10 mm×120 mm (heated at 870 °C for 10 min, air cooling). The results showed that 00Cr22Mo1Nb (Ti) has excellent Stress corrosion cracking resistance. The stress corrosion cracking resistance of 00Cr22Mo1Ti, 00Cr22Mo2Ti, 0Cr17Ni12Mo2 steel in chloride solution is shown in the figure below.
00Cr22Mo1Nb (Ti) cold-rolled annealed steel plate has good welding performance, and can be welded by the common method of welding stainless steel. It does not need to be preheated before welding. The mechanical properties after welding are shown in Table 10. The elongation is not lower than that of the base metal. Other properties are the same as The parent material is similar. 00Cr22Mo1Nb (Ti) steel is suitable for domestic water supply pipes.
00Cr22Mo2Ti (Nb) is a steel grade with better pitting corrosion resistance and stress corrosion resistance than 0Cr17Ni12Mo2, and its annealed state is almost immune to stress corrosion in aqueous chloride solution.
00Cr22Mo2Ti (Nb) can be single-stabilized by Nb and Ti, or double-stabilized by Nb+Ti. The steel has the best toughness when it is stabilized by Nb, and the toughness of the steel is the worst when it is stabilized by Ti. The toughness of the steel is in the middle during Nb+Ti double stabilization, and Ti is beneficial to the toughness after welding. Therefore, Nb+Ti double stabilization measures are usually used to ensure the toughness of the steel and prevent the post-weld toughness from being significantly reduced.
00Cr22Mo2Ti (Nb) cold-rolled annealed sheet has excellent cold forming properties.
The corrosion resistance of 00Cr22Mo2Ti (Nb) steel is much better than that of 0Cr18Ni9 steel and significantly better than that of 0Cr17Ni12Mo2 steel in the accelerated spray test (spray in seawater at room temperature → dry at 60 °C for 30 min → wet at 50 °C, 100% RH for 30 min) ,As shown below.
These data show that the performance of 00Cr22Mo2Ti (Nb) steel is significantly better than that of 0Cr17Ni12Mo2 steel, and its crevice corrosion resistance is also better than that of 00Cr17Ni14Mo2 steel. .
The mechanical properties of TIG (Tungsten Inert Gas Welding, non-metal inert gas shielded arc welding) welded joints are close to those of the base metal (Table 12). 00Cr22Mo2Ti (Nb) steel is especially suitable for home improvement water supply pipes, and when the Cr content is higher than 22% and the Mo content is not less than 2%, it can also be used for floor heating pipes.
019Cr24Mo2NbTi steel is an ultra-low carbon, nitrogen-rich molybdenum-containing ferritic stainless steel stabilized by Ti. It has excellent resistance to pitting corrosion and crevice corrosion in chloride-containing media; it has good strength, toughness and Weldability; good wear resistance, wear-resistant iron performance loss is 1/3 of titanium, 1/22 of brass.
019Cr24Mo2NbTi steel is a newly developed steel type in my country, and it is mainly used in urban water medium environments with chloride ion content below 5 000 mg/L.
019Cr24Mo2NbTi thin-wall stainless steel straight seam welded steel pipe can replace copper B10 seamless pipe and 00Cr17Ni14Mo2, 00Cr19Ni13Mo3 (317L) austenitic stainless steel pipe in the environment with high chloride ion content.
The chemical composition requirements of 019Cr24Mo2NbTi steel are shown in the figure below.
The mechanical properties of 019Cr24Mo2NbTi steel are shown in the table below.
The measurement results of the pitting corrosion resistance potential of 019Cr24Mo2NbTi steel are shown in Table 15. Under standard conditions, the pitting corrosion resistance of 019Cr24Mo2NbTi steel is much higher than that of 00Cr17Ni14Mo2 and 00Cr19Ni13Mo3, and there is no significant difference with 2205 steel. The higher the pitting corrosion potential, the better the pitting corrosion resistance.
019Cr24Mo2NbTi cold-rolled annealed sheet has good weldability. It adopts conventional TIG welding for welding stainless steel. There is no need for preheating before welding and no heat treatment after welding.
Stainless steel water pipes in the current market are divided into stainless steel seamless pipes and stainless steel welded pipes. The market price of stainless steel seamless pipe is higher than that of welded pipe, and its use advantage is more obvious. Stainless steel seamless pipe and stainless steel welded pipe are compared in the table below.
At present, the stainless steel water pipes on the market are mainly connected by double clamping fittings. In addition, there are also some self-developed connection forms with reliable connections, such as convex fittings.
The application and promotion of stainless steel pipes and quality assurance are inseparable from standardization, but judging from the existing standards in the stainless steel water pipe industry, there is a lack of special stainless steel pipe standards for water supply and drainage. Due to the lack of product standards for this range, the problem is that the products of stainless steel pipe manufacturers have poor interchangeability.
This has affected the application of stainless steel water pipes to a certain extent, and has become a “bottleneck” in the development and application of stainless steel water pipes. It is urgent to formulate national standards covering the main connection mode products for conveying water medium, but this is related to the production molds of various enterprises, and there is certain resistance. And who will take the lead in formulating the standard will inevitably involve the interests of all parties.
First of all, it is manifested in the diversification of connection methods. Due to the material characteristics of stainless steel pipes, the connection methods are diverse.
Secondly, some stainless steel water pipe manufacturers have thinned the wall of stainless steel water pipes in order to reduce costs. This further exacerbates the diversification of stainless steel water pipe connection methods.
Third, the problem of stainless steel water pipe fittings. Stress corrosion will occur when stainless steel bolts and stainless steel nuts are tightened, causing the bolts and nuts to corrode together and cannot be disassembled. Thus, two solutions emerged:
The use of thin-walled stainless steel pipes in the final customer is a systematic project, which requires supporting pressing tools and system installation procedures to ensure installation quality.
However, the existing professional installers in the market are slightly scarce compared with the market demand.
Although there are many manufacturers of stainless steel water pipes, the quality of the products is uneven, and most of them are small enterprises with low-quality control levels, cutting corners in production, and reducing key processes such as non-destructive testing and heat treatment.
These problems caused corrosion and leakage of the product during use, which affected the reputation of thin-walled stainless steel pipes in the market. That is the lack of systematic and mature product certification.
After nearly 20 years of exploration and development, China’s stainless steel water pipe industry has already possessed the production technology service ability to adapt to the rapid growth of market demand. At present, the main production enterprises in the country are concentrated in Zhejiang, Jiangsu, Guangdong, Sichuan, Shandong and other places.
All enterprises basically have the production capacity of pipe fittings. Due to the large number of types, specifications and series of pipe fittings, the large investment in production equipment and the large number of workers, many enterprises cannot complete the complete independent matching of pipe materials, and there are mutual procurement and matching among enterprises; In addition, some production enterprises do not directly enter the terminal market, but rely entirely on the calendering processing and production cost advantages of raw material plates to carry out OEM production for other enterprises.
According to statistics from the Water Supply and Drainage Equipment Branch of China Construction Metal Structure Association, the demand for raw materials in China’s stainless steel water pipe industry will be about 200,000 tons in 2020, and it will grow at a rate of about 10% every year. The industry has a bright future. In recent years, China’s stainless steel pipe industry has developed rapidly. The number of thin-walled stainless steel pipe manufacturers has increased from more than 100 in 2018 to nearly 250 in 2020. New entrants have the following situations:
In the early marketing period, due to the small number of participating companies, although the market demand is small, the operating profit margin is high.
With the hot market in the past two years, leading enterprises have obtained sufficient orders by virtue of their performance, brand influence, channels and other advantages, and can maintain full production and operation. However, the profit margin has dropped significantly, new entrants are facing insufficient orders, and the operating rate is not high. The comprehensive capacity utilization rate of the entire industry is around 50%.
Through research on the procurement price of stainless steel pipes in water supply companies, it is estimated that the gross profit margin of stainless steel pipes of production enterprises is about 24%, the gross profit margin of stainless steel pipe fittings is about 60%, and the gross profit margin of the entire product is about 30%.
Among the raw material steel belt sources of production enterprises, steel trading enterprises process and distribute 40%, direct procurement and self-processing account for 10%, and calendering processing and distribution account for 50%.
The market share of new entrants of stainless steel pipes is low, and the pressure of competition is high. At the same time, there are the following problems:
Designing target materials for the use environment is an inevitable trend in the development of stainless steel pipes. For example:
It is an inevitable requirement for the development of stainless steel pipes to develop new materials in response to different environments to meet the needs of various industries.
Mainly address the following environmental needs:
It is the future development direction of stainless steel pipes in my country to develop steel types from the source to meet the performance and high purity requirements of stainless steel pipes in different environments.
Innovative technology, energy saving and emission reduction to achieve green enterprises and green products are the main theme of the development of stainless steel pipes.
Process innovation refers to the production and commercialization of existing or researched and developed new technologies, and the realization of major innovations will bring about systematic changes in the industry economy.
“Digital intelligent factory”, “process intelligence service platform”, “intelligent safety management”, and “computer-aided process design” are all management innovation platforms.
Through intelligent management, the production cost management of the enterprise, the physical quality of the product, the salary level of the staff and the safety production will enter a higher level. It has promoted domestic stainless steel pipe enterprises to gradually move towards an intelligent era of unmanned chemical factories and intelligent control of quality, intelligent management of processes, no casualties, and scheduled delivery of goods.
National and local environmental protection policies are becoming increasingly stringent, and the zero discharge of waste liquid, waste water, solid waste and foreign matter and NO2 waste gas produced by the pickling process of stainless steel pipes is an inevitable trend in future development.
The development of technologies such as microfiltration, electrodialysis film forming technology and limit crystallization have provided technical feasibility for the surface treatment of stainless steel pipes to achieve zero emissions.
At the same time, the application of zero-emission technology is not only a means of ensuring environmental protection and achieving social benefits in the production of stainless steel pipes, but also the recycling and reuse of waste liquid, wastewater, and sludge has actual economic benefits.
The extended processing chain of enterprise products must advance and retreat with the demand side.
For example, when a company produces 445J2 ferrite large-scale reducing tubes, it starts with extended processing and provides services for supporting tubes for air conditioners, so that the sales of this product increase greatly with the sales of air conditioners. This is one of the future development directions.
As a popular application field in recent years, stainless steel water pipes have developed rapidly and are gradually playing an important role in improving residents’ healthy water use.
It is believed that the stainless steel pipe industry will further develop in the direction of specialization and industrialization in terms of continuous innovation, green development and innovative management in the future.