What is a stainless steel bar
Stainless steel bars are made of stainless steel ingots by hot rolling or forging.
The production of stainless steel bar is rising with the development of stainless steel. As the application scope of stainless steel bar is more and more extensive, such as the foundation of high-rise buildings in cold areas, isolation network beside highways, household articles, etc., the hot rolling production of stainless steel bar has been greatly developed. 30% of stainless steel products in China and only 8% – 10% in foreign countries. With the rapid development of cutting-edge technologies such as petroleum, chemical industry, energy and atomic energy, aerospace and marine development, higher comprehensive performance requirements are put forward for stainless steel, which requires not only good corrosion resistance, but also high strength, high temperature and high pressure resistance, radiation protection and low temperature resistance, so as to further develop the varieties and types of stainless steel.
Classification of stainless steel bars
Stainless steel bars are mainly divided into 2 series, 3 series and 4 series.
201 stainless steel bar: it belongs to chromium nickel manganese austenitic stainless steel with low magnetism.
202 stainless steel bar: it belongs to chromium nickel manganese austenitic stainless steel, and its performance is better than 201 stainless steel.
316L stainless steel bar: 316 stainless steel contains molybdenum and low carbon. Its pitting corrosion resistance in marine and chemical industry environment is much better than 304 stainless steel（ 316L low carbon, 316N nitrogen high strength, 316F stainless steel with high sulfur content, easy to cut stainless steel.
304L stainless steel bar: as a low-carbon 304 steel, in general, the corrosion resistance is similar to 304, but after welding or stress relief, it has excellent grain boundary corrosion resistance and can maintain good corrosion resistance without heat treatment.
304 stainless steel bar: it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties, good hot workability such as stamping and bending, and no heat treatment hardening phenomenon. Uses: tableware, cabinets, boilers, auto parts, medical appliances, building materials, food industry (service temperature – 196 ° C-700 ° C).
310 stainless steel bar: the main features are: high temperature resistance, generally used in boiler and automobile exhaust pipe. Other properties are general.
303 stainless steel bar: it is easier to cut than 304 by adding a small amount of sulfur and phosphorus. Other properties are similar to 304.
302 stainless steel bar: 302 stainless steel bar is widely used in auto parts, aviation, aerospace hardware tools and chemical industry. The details are as follows: handicrafts, bearings, sliding flowers, medical instruments, electrical appliances, etc. Characteristics: 302 stainless steel ball belongs to austenitic steel, which is close to 304, but the hardness of 302 is higher, HRC ≤ 28, and has good anti rust and anti-corrosion properties.
301 stainless steel bar: good ductility, used for forming products. It can also be hardened rapidly by machining. Good weldability. The wear resistance and fatigue strength are better than 304 stainless steel.
302 stainless steel ball belongs to austenitic steel: it is close to 304, but the hardness of 302 is higher, HRC ≤ 28, and has good rust and corrosion resistance.
408 – good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.
409 – cheap model (British and American), usually used as automobile exhaust pipe, belonging to ferritic stainless steel (chromium steel).
410 stainless steel bar: it belongs to martensite (high-strength chromium steel), with good wear resistance and poor corrosion resistance.
416 – adding sulfur improves the processability of the material.
420 stainless steel bar: “cutting tool grade” martensitic steel, similar to the early stainless steel such as Brinell high chromium steel. It is also used for surgical tools, which can be made bright.
430 stainless steel bar: ferritic stainless steel, used for decoration, such as automobile accessories. Good formability, but poor temperature resistance and corrosion resistance.
440 – high strength cutting tool steel, with slightly higher carbon content, can obtain higher yield strength after appropriate heat treatment, and the hardness can reach 58hrc. It belongs to hard stainless steel. A common application example is “razor blade”. There are three common models: 440A, 440b, 440C, and 440f (easy to process).
Production process of stainless steel bar
Process flow of bar production line: billet acceptance → heating → rolling → double length shearing → cooling → shearing → inspection → packaging → metering → warehousing.
Small bar is produced by small rolling mill. The main types of small rolling mill are continuous, semi continuous and horizontal. At present, most of the newly-built and in use small continuous mills in the world. Nowadays, the popular steel bar rolling mills include general high-speed rolling steel bar rolling mill and 4 cutting high-yield steel bar rolling mill. The billet used in continuous small rolling mill is generally continuous casting billet, and its side length is generally 130 ~ 160mm, but also 180mm × 180mm, the length is generally about 6 ~ 12M, and the single weight of blank is 1.5 ~ 3T. The rolling lines are mostly arranged in the form of horizontal interchange, so as to realize the torsion free rolling of the whole line. The number of stands shall be determined according to the principle of rolling one pass for one stand. Most rolling mills are even pass combinations. For small rolling mills with 18, 20, 22 or even 24 billet specifications and finished product sizes, 18 are the mainstream. Adjustable speed, micro tension and tension-free rolling are the obvious characteristics of modern fully continuous small mill. Some stands of roughing and intermediate rolling are controlled by micro tension, and some stands of intermediate rolling and finishing rolling unit are controlled by non tension to ensure the dimensional accuracy of products. Continuous rolling mill is generally equipped with 6 ~ 10 loopers, and even up to 12 loopers.
Bar rolling is the most easily realized variety of all rolled products, which can be realized in many ways. Bars can be produced from three roll horizontal type, to torsional double type, and from various semi continuous type to fully continuous type, but their output, dimensional accuracy, yield and qualified rate are very different. The stiffness of three high mill is low, and the fluctuation of heating temperature will inevitably lead to serious product size fluctuation. In addition, the slow speed of horizontal type and long rolling time will increase the temperature difference between the head and tail of rolled piece, which is easy to be inconsistent in size, uneven performance, low output, great quality fluctuation and very low quality rate. The full continuous rolling mill generally adopts horizontal and vertical alternation, with no torsion of rolled pieces, less accidents and high output, which can realize large-scale professional production and organization performance control. At the same time, the rolling mill adopts high stiffness and high degree of control automation, which greatly improves the dimensional accuracy and qualified rate, especially the yield, and reduces the waste of return steelmaking. At present, new processes such as step heating furnace, high-pressure water descaling, low-temperature rolling and endless rolling are mostly used in bar rolling. Rough rolling and medium rolling are developed to adapt to large billets and improve rolling accuracy. Finishing mill is mainly to improve accuracy and speed.
Compared with the hot rolling of plain carbon steel, the rolling technology and process know-how of stainless steel are mainly reflected in the inspection and cleaning of ingots, heating method, roll pass design, rolling temperature control and on-line heat treatment of products.
Inspection and cleaning of ingot
The cleaning line includes shot blasting, infrared surface inspection, ultrasonic flaw detection and grinding machine. With the improvement of continuous casting level, if continuous casting can produce defect free billet, billet cleaning line can not be added.
Austenitic stainless steel has stable structure during heating and cannot be strengthened by quenching. This kind of steel has good combination of strength and toughness, excellent low-temperature toughness, non-magnetic, good processing, forming and welding properties, but it is easy to produce work hardening. At the same time, this kind of steel has low thermal conductivity and excellent plasticity at low temperature. Therefore, the heating speed can be faster than that of ferritic stainless steel and slightly lower than that of ordinary carbon steel.
Ferritic stainless steel does not undergo phase transformation when heated, and generally cannot be strengthened by heat treatment. This kind of steel has three kinds of brittleness transformation, namely 475 ℃ brittleness, A-phase precipitation brittleness and brittleness caused by grain growth. Quench after annealing is often used to obtain good properties. Oxidation resistance of high Cr steel at high temperature; Insensitive to stress corrosion; The strength of steel is higher than that of austenitic stainless steel; The toughness increases with the decrease of C and Ni content; Strong magnetism; Poor welding performance. This kind of steel has good hot workability, but the plasticity of ferrite is very low at low temperature. In addition, the residual stress generated during billet (ingot) cooling is consistent with the thermal stress generated during heating (because there is no phase change during heating and cooling) and can be superimposed with each other, so it is easy to produce hot cracking. Therefore, billets (ingots) should be heated slowly at low temperature. The charging temperature of ingot shall not be greater than 800 ℃, and the billet shall not be greater than 850 ℃. When the Cr content is greater than 16%, the as cast structure is very coarse, which is easy to produce coarse-grained structure. The grains broken by thermal processing tend to grow strongly when the temperature is greater than 950 ℃. Because there is no phase transformation during heating and cooling, the grown grains cannot be changed by heat treatment. At the same time, this kind of steel is ferrite with body centered cubic lattice, and the recrystallization temperature is low, The recrystallization speed is high, the plasticity of the steel after recrystallization is also good, and the deformation resistance during hot processing is small. In order to obtain the required fine grain structure, it is generally used to deform at a lower temperature and control the deformation at this temperature. The heating temperature is generally about 950 ℃ ~ 1000 ℃.
Roll pass design
During the production of stainless steel bars, the roll pass generally adopts the elliptical round pass system. During the pass design, it should be considered that the pass has strong adaptability, so as to reduce the replacement pass and the restart of the rolling mill as much as possible, that is, the pass can adapt to a variety of products, and allow a large gap adjustment of the pass, so as to minimize the requirements for the change of the pass of the pre finishing rolling mill in the whole product range.
Rolling temperature control
During stainless steel rolling, because its deformation resistance is quite sensitive to temperature change, especially during rough rolling, due to low rolling speed, the temperature rise caused by deformation work is not enough to compensate the temperature drop of the rolled piece itself, resulting in large temperature difference between the head and tail, which will have an adverse impact on the product tolerance, surface defects and internal defects on the rolled piece, and affect the uniformity of final product performance. In order to solve the above problems, after rough rolling, the heated billet enters the oil (or gas) holding furnace or induction reheating furnace located between rough rolling and medium rolling, and then enters the medium rolling unit for rolling after temperature homogenization. In order to control the excessive temperature rise of rolled pieces in the process of finishing and pre finishing rolling, water cooling devices (water tanks) are generally set behind the two groups of rolling mills and between the racks of finishing rolling units. Therefore, the reasonable control of grain size can be realized in order to improve the technical performance of the final product.
Heat treatment of stainless steel bars
In the past, the heat treatment of stainless steel bars was carried out offline. With the development of science and the deepening of rolling process research, modern stainless steel heat treatment is also carried out online. When producing bars, austenitic and ferritic stainless steel can be air cooled or heap cooled after rolling because it is not easy to produce cold crack and self spot, or a water cooling device is set before flying shear to realize waste heat quenching; During the production of martensitic stainless steel, it is easy to produce cold cracks and cannot be cooled through water, so it directly enters the cooling bed. The structure of the cooling bed is different from that for the production of ordinary carbon steel. One way is to adopt the improved step rack cooling bed, such as the cooling bed of teledyneaiivac plant in the United States designed by Danieli company in Italy and put into operation in 1989, which extends into a groove at the high temperature side, The tank can be filled with water to submerge the cooling bed in the water, so that the austenitic stainless steel can be water quenched, while the varieties without water quenching can directly enter the cooling bed. The cooling bed can also be equipped with an insulating cover to delay the cooling of the rolled piece. When the insulating cover is covered for delayed cooling, the cooling rate is half of the natural cooling rate, Lower cooling rate is very important to ensure the delayed brittle crack of martensitic stainless steel; Another method is: half of the cooling bed is designed as a chain type, and the other half is an ordinary rack type cooling bed. The roller table is equipped with a thermal insulation cover. When producing martensitic stainless steel, the flying shear cuts the rolled piece into a double scale or fixed scale. If it is a double scale, it is quickly pulled into the thermal insulation cover through the chain cooling bed, cut into a fixed scale in the cover, and then sent to the thermal insulation pit. The fixed scale is directly pulled into the thermal insulation pit for slow cooling.
Due to the lack of slow cooling and annealing facilities, the rolled bar is directly exposed to the air environment for cooling, resulting in the following six problems:
- ① surface cracks on the surface of the bar and crystal crack defects in the internal structure;
- ② Waste of energy and long annealing cycle;
- ③ The production logistics organization is not smooth, the products are frequently transported on site, which increases the transportation cost, the labor intensity of post workers is high, and the bars are damaged in the transportation process;
- ④ Due to many product defects, it needs to be inspected one by one, the inspection work is heavy, and the inspection cost is increased;
- ⑤ Long product supply cycle;
- ⑥ Low product yield and many customer quality objections.
Quality control of bar production
Due to the wide range of uses of rods, the market also has a variety of quality requirements for them. According to different uses, they also pay more attention to mechanical strength, cold working performance, hot working performance, free cutting performance and wear resistance. The general requirement is to improve the internal quality. According to the type of deep processing, the material itself should have appropriate performance, so as to reduce the deep processing process and improve the service performance of the final product.
Building materials are required to maintain a certain elongation under high yield strength. Threaded steel bars used as building materials are mainly to ensure chemical composition and good weldability. They are required to have uniform and stable physical properties, which are conducive to cold bending and have certain corrosion resistance. As building materials, improving the dimensional accuracy and uniformity of mechanical properties can save a lot of steel. Similarly, for machining shafts, turning can be reduced and the cost can be reduced.
Mechanical parts are required to have good machining performance. After machining, heat treatment such as quenching, normalizing or carburizing shall be carried out to ensure the mechanical performance in use. Some products also need surface treatment such as plating, painting and coating.
After controlled rolling and cooling, the service performance of bar can also be improved, which can be realized through process control in the rolling process.
Production standard for stainless steel bars
In terms of standards for stainless steel bar rolling, American, British, German, French, Russian, Japanese and international standards are relatively advanced, among which American Standard dimensional tolerance is the most stringent. The latest national standards for hot rolled stainless steel profiles are: ASTM A276 Standard Specification for stainless steel and heat resistant steel bars and profiles; ASTM A484/A484m General requirements for stainless and heat resistant steel bars, billets and forgings; German DIN17440 Technical delivery conditions for stainless steel sheet, hot rolled strip, wire rod, drawn wire rod, steel rod, forging and billet; Japanese JIS64304 stainless steel rod. In the early 1980s, in combination with the relevant standards of the United States, Japan, Germany, the former Soviet Union and the international organization for Standardization (ISO), China formulated the national standard GB1220-92 for stainless steel bars and GB4356-84 for stainless steel wire rods with reference to the Japanese JIS Stainless steel bar standard. At the same time, with reference to foreign standards, China formulated the national standard gb4356-84 for stainless steel wire rods, which improved the steel series and adopted the United States Japan and other international general grades, and some stainless steel grades in China correspond to American standard grades, as shown in table L. At the same time, the commonly used grades in China are retained to make them basically consistent with the stainless steel grades in developed countries such as the United States and more universal. Compared with developed countries, the gap of the standard itself has narrowed a lot, but the surface quality and dimensional tolerance are poor, and the gap between the physical level is large.
Application of stainless steel bar
Stainless steel bars are mainly used for aircraft engine blades, casings, fasteners, combustion chambers, disks, shafts, outer walls of combustion chambers, liquid oxygen kerosene engines, bullet bodies, steam bottles, propulsion devices, etc. It is widely used in aerospace, aviation, atomic energy, nuclear energy, petroleum, chemical industry, food, environmental protection, marine development, boiler heat exchanger and other fields. Bars are widely used in construction, machinery, automobile, shipbuilding and other industrial fields, of which 70% are used for construction, and the rest are used for various shafts, bolts, nuts, anchor chains, springs and other materials. Therefore, the development of iron and steel industry is of great significance. China is a developing country, housing still needs a lot of development, and the demand for construction steel will be very high for a long time. In addition, with the improvement of people’s living standards, the demand for automobile steel will be more and more. In addition to the direct application of threaded reinforcement in construction, a considerable part is processed into various shaft parts.
Development of stainless steel bar production
Rod and wire production has a history of more than 200 years. Although the proportion of plate and strip products increases rapidly, its production technology is becoming more and more perfect, and the production cost is significantly reduced, rod and wire products still occupy a unique and irreplaceable position. It is for this reason that the development level of its production technology is developing rapidly with each passing day.
The past 20 years have witnessed the rapid development of section steel production technology in China. 20 years ago, China’s profile steel equipment level and production technology fell behind the international advanced level for about 30 years, but today, its equipment level is generally close to the international advanced level. China’s section steel production technology has gone through about 50 years of development in 20 years.
In the 1950s, China’s steel output was very low and the variety of steel bars produced was limited. State owned iron and steel enterprises could only produce No. 3 plain round steel bars (grade I steel bars) and No. 5 deformed steel bars, and the standard values of yield strength were 235275mpa respectively; In the 1960s, two kinds of low alloy ribbed bars, 16mnsi (later changed to 20MnSi, also known as grade II reinforcement) and 25mnsi (also known as class reinforcement), were developed. In fact, 20MnSi reinforcement was successfully developed and mass produced, while the output of 25mnsi reinforcement was limited, and the standard value of yield strength of the two was 335375mpa respectively; At the same time, 44mn2si ribbed reinforcement (also known as grade n reinforcement) is developed and put into production, which is mainly used for prestressed reinforcement after heat treatment or cold drawing.
In the 1950s, China was at the climax of national economic construction, and the contradiction between supply and demand of reinforcement was more prominent. In order to develop cold working production, cold work hardening of reinforcement was achieved by cold rolling, cold drawing, cold torsion or cold drawing of low-strength reinforcement, so as to obtain higher yield strength at the expense of the plasticity of the original reinforcement. Cold drawn steel wire is mainly produced by fine diameter wire rod, which is not only used as prestressed reinforcement in small and medium-sized prestressed concrete members, but also partially used as stress reinforcement and structural reinforcement in some reinforced concrete members; In addition, the cold drawn thick ribbed reinforcement is used as the prestressed reinforcement in large and medium-sized prestressed concrete members, and part of it is used as the reinforcement in reinforced concrete members.
In the 1980s, the output and specifications of small-size reinforcement could not meet the needs of engineering construction. In view of this situation, domestic small and medium-sized enterprises began to introduce or self-made cold-rolled ribbed reinforcement equipment, and the number and output of rolling mills reached a considerable scale. In addition, the cold-rolled twisted steel bar with small specification and rectangular section is also produced for reinforcement in reinforced concrete floor slab, which fills the vacancy that the original hot-rolled ribbed steel bar has no small specification, and plays an auxiliary role in supporting the steel bar specifications required for engineering construction.
There are two kinds of new generation hot-rolled ribbed steel bars developed in the 1980s and officially put into production in the 1990s: one is HRB400 steel bar based on microalloyed elements (V, Ti, Nb), and the other is RRB400 (kl400) steel bar produced by waste heat treatment process (including steel bar produced according to British BS standard), which has updated the chemical composition of Chinese steel bars; The waste heat treatment reinforcement can be used for both export and domestic projects. In the same period, a considerable number of low relaxation high-strength steel strand production lines were introduced, which made this high-quality prestressed reinforcement widely used in highway, housing, railway and other engineering fields, and promoted the development of prestressed concrete structure.
From the 1980s to 1990s, China introduced small-scale continuous rolling technology from abroad. After transplantation, digestion and redevelopment, the production technology of small-scale continuous rolling has been developed to the extreme. The output of foreign small-scale rolling mills with an output of only 300000 ~ 400000 T/year can be increased to 800000 ~ 1 million T/year in China, or even higher.
The three main factors to promote the comprehensive improvement of China’s small mill technology are non torsion rolling, the use of short stress line rolling mill and the comprehensive promotion of slitting rolling technology. The increase of unit yield of small rolling mill promotes its better cooperation with high-efficiency converter continuous casting, which is conducive to the adoption of hot delivery and hot charging process, further saving energy, reducing consumption and reducing production cost.
At present, the production status of small and bar in China is as follows: by 2004, China’s small and bar production has three first in the world, namely, the number of rolling mills, annual production capacity and its ratio to the total steel (the number of continuous rolling mills has exceeded 100, the annual production capacity is about 60 million T, and the rolling capacity of continuous rolling mills can reach 70% of the total production). At present, the overall equipment and production technology of small and bar mills in China have reached the international advanced level, and have the following characteristics:
1. Most of the newly-built rolling mills are 18, divided into rough, medium and finish rolling units, with 6 in each group, arranged alternately horizontally and vertically, so as to realize torsion free continuous rolling. Stepping heating furnace and full digital DC drive system are adopted. The blank is 150mm × 150mm continuous casting billet, 10 ~ 12m long. The product specification is Ф 10~ Ф 40, Ф Products below 20 adopt slitting rolling.
2. The products are mainly ribbed reinforcement. There are only Hangzhou Iron and steel company, Shijiazhuang Iron and steel company and Xingtai Iron and steel company that do not produce ribbed steel bars.
3. Endless rolling and slitting rolling technologies beneficial to increasing output are used and popularized more in China than in industrial developed countries. Such as endless rolling technology, by 2004, Tangshan Iron and steel company introduced the technology of Danieli company in Italy; The second high-speed wire mill of Xingtai Iron and steel company introduced the technology of NKK company in Japan; Tonghua Iron and steel company has developed the endless rolling technology of rod and wire. It is worth noting that there is no good application example of this technology in steel producing countries such as the United States, Japan, Germany and Russia. There is only one test plant in Japan for the endless rolling technology of rod and wire of NKK company in Japan. For the 3-slitting and 4-SLITTING rolling technology of bar, domestic enterprises have high enthusiasm for research, promotion and use, and also have very mature technology, but there are still foreign companies specialized in promoting this technology in China.
4. The use and promotion of technologies that are beneficial to improving product quality and meeting user needs and affect output in China are lower than those in advanced countries in the world. These technologies include:
- (1) Free gauge reducing sizing technology of bar. It can provide users with products of any specification and adapt to the market demand of multi variety and small batch.
- (2) High precision bar rolling technology.
- (3) On line controlled cooling technology for alloy steel and low alloy steel bars.
- (4) Market development of multi steel grades and small batch bars.
- (5) Market development of special small timber.
There are more than ten thousand kinds of section steel rolled in industrial advanced countries, while there are only thousands of hot-rolled sections in China. The biggest gap is the special small wood. These profiles are mostly used in automobiles, textiles, motors, agriculture and so on. Compared with 20 years ago, China’s work in this field has made little progress. The above characteristics show that China’s small and bar production is still in the stage of strong market demand. The benefits of steel production plants mainly depend on output.
From the development history of China’s hot-rolled ribbed steel bars, it can be seen that the production level of China’s hot-rolled ribbed steel bars has been continuously improved, and ordinary steel bars have developed from low carbon steel and low alloy steel to microalloyed steel; The development of prestressed reinforcement from low strength and large relaxation to high strength and low relaxation steel strand and steel wire; At the same time, the development trend of cold-worked steel bars is that cold-drawn steel wires and cold-drawn steel bars are widely used to be eliminated. As thin diameter cold-rolled steel bars and cold-rolled twisted steel bars, they will still be a supplement to ordinary steel bars. Their existence and development depend on the quality, price and after-sales service of their products, They will compete with fine-diameter hot-rolled ribbed bars through the market mechanism.
Hot rolled ribbed steel bar is widely used in civil buildings, high-rise buildings and key projects: airport, port, expressway, bridge, power plant and so on. It is a very important steel. Profile steel production plays an important role in the production of steel rolling workshop. According to incomplete statistics, at present, the profile steel produced in China accounts for about 50% of the total steel production. Therefore, mastering the profile steel production theory and process is important to improve the quality and accuracy of profile steel products, develop new varieties, new processes and new equipment, and improve production automation and computer control technology, It has great practical significance.
Source: China Stainless Steel Bars Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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