This article introduces the large-diameter steel pipe manufacturing process, steel pipe performance requirements and technological advancements in upstream processes.
Large-diameter steel pipes are formed and welded from thick plates and hot-rolled plates. The manufacturing method is first classified according to the welding method. The submerged arc welding steel pipe with high efficiency and high reliability is called saw steel pipe. The large-diameter steel pipe discussed in this article mainly targets this kind of saw steel pipe. Saw steel pipe is divided into longitudinally welded submerged arc welded straight seam steel pipe l-saw and spiral welded submerged arc welded spiral steel pipe h-saw. L-saw steel pipes have UO forming method suitable for mass production, bending roller method and press machine tool method suitable for large varieties and small quantities.
The jcoe forming method is a method between the uo forming method and the press machine tool method. Whether there is a diameter expansion process at the end of the pressure stage has a great impact on the performance of the steel pipe. The presence or absence of this process is also one of the classification methods. h-saw is equivalent to a spiral welded pipe. It is formed by forming a rolled steel plate into a spiral shape and then welding the butt joints. In addition to the traditional method of forming, positioning welding and then sawing, in order to obtain high productivity and high-quality steel pipes, the two-step spiral welding method of positioning welding and then sawing in other production lines can also be used.
The raw materials of Uo steel pipes usually use thick plates, and sometimes sheared hot-rolled coils are also used. Methods for weld bevel processing of steel plates include: pre-bending (use a C-shaped press to bend the edge of the steel plate to close to the curvature of the product); U-forming (use a U-shaped press to press the steel plate into a U-shape); o-forming ( Cold-formed into an O-shape using an O-shaped press). After processing, the weld is positioned and welded using methods such as gmaw (gas metal arc welding: gas metal arc welding). After welding the weld with saw from the inside and outside, the diameter is expanded by about 1%. While improving the roundness, the residual stress in the weld is eliminated. This method is generally called UOE forming method. In order to measure the integrity of the steel pipe, 1% plastic deformation without cracking is used as a measurement index.
The spiral welded pipe rolling mill is composed of uncoiler, forming machine, internal and external welding machine, ultrasonic flaw detector, flying shear and other equipment. Hot-rolled strip coils are generally used as raw materials, and hot-rolled steel plates produced by Steckel mills are also used. Usually, the inner surface is welded first, and then the outer surface is welded after rotating 0.5 or 1.5 times. Submerged arc welding is generally used for welding. The spiral welding pipe making method starts from continuous forming to welding, and then is cut to a specified length by flying shearing or plasma cutting using gas cutting or plasma cutting. Then, processes such as end face processing, non-destructive testing, secondary processing and appearance dimensional inspection are carried out.
The use of pipelines to transport crude oil and natural gas, especially to ensure the safety of gas pipelines, requires pipeline steel pipes to have high technical characteristics. Natural gas is basically transported by pipelines, and long-distance transportation can reduce the transportation cost of liquefied natural gas (LNG). When the transportation volume is 10bcm/a, the transportation distance that makes the LNG cost more reasonable is 3000km. When the transportation volume is 25bcm/a, the reasonable value is 5000km. Pipeline transportation increases the cost advantage if the volume of gas transported is increased. In addition, if high-pressure transportation is adopted, for example, the inlet pressure of the pipeline is increased from the original 10mpa to 14mpa, the transportation distance of the pipeline can be lengthened. Large-diameter pipeline steel pipes used for such pipelines are required to have the following technical characteristics.
In order to increase the gas delivery volume, the inner diameter of the pipeline can be expanded under the same delivery gas pressure, or the delivery gas pressure can be increased under the same pipeline inner diameter. In order to control pipeline construction costs, high-pressure gas transportation is often used. Onshore pipelines generally use 10mpa. The design pressure of the main pipeline of the Second West-East Gas Pipeline is 12mpa, and the design pressure of the Alaska Pipeline Project is 15mpa. Since it is difficult to install an air compression station in the middle of the submarine pipeline steel pipe, it is transported at a higher pressure. Onshore pipeline steel pipes are also planned to increase the delivery pressure, but the maintenance of peripheral machines such as air compressors, reducing the energy consumption of air compressor operation and ensuring safety are also important.
If the circumferential stress σ of the steel pipe is represented by an approximate thin-walled cylinder, then σ = pid/2t (pi: internal pressure; d: diameter; t: wall thickness). It can be seen that increasing the stress on the total wall thickness will increase the internal stress. pressure. In this way, there are two options of increasing the wall thickness or improving the strength, so the pipeline steel pipe is required to be thicker-walled and high-strength. For the same diameter, increasing strength can reduce pipe wall thickness. Regardless of the condition, increasing strength also reduces the weight of the steel pipe per unit length. Even if the weight reduction rate is not large, the cost of steel can generally be reduced, thereby reducing the transportation costs of steel pipes, the excavation of laying trenches, and the cost of circumferential welding.
In this case, high-strength line steel pipes were developed and applied. The representative pipeline steel pipe standard is the 2007 version of ISO 3183, which adds x90, x100, and x120 to the original x80 and below.
In recent years, the demand for x80 grade steel pipes has increased dramatically. In order to reduce the cost of pipeline construction, on-site circumferential welding has changed from manual welding to gmaw automatic welding, which has improved the welding efficiency and low-temperature cracking of high-strength steel welding is no longer a problem.
As the high-strength trend develops, the most fundamental problem of strength determination arises. According to the grade regulations of pipeline steel pipes, the parameter indicating the ability to withstand internal pressure is circumferential yield strength (c-ys), but it is difficult to measure the c-ys of steel pipes. The correct measurement of circumferential strength includes the expansion ring test, but it is not suitable for large-scale measurements. As a small-scale test, the material sheared in an arc shape is generally prepared into a flat full-thickness specimen for strength measurement. The strength change of flat specimens below level x65 is small, but for levels above x80, the work hardening of the material becomes smaller, the Bauschinger effect of flat specimens is obvious, and there is a problem that the ys measured with flat specimens is lower than the actual ys.
In addition, flat tensile specimens are not used for x80 and above, but processable round bar specimens are mostly used. Round bar specimens are recognized by all standards. However, the value for the round bar specimen only represents a part of the wall thickness direction, and it must be recognized that it is somewhat different from the value for the full wall thickness (ts is low). In the previous API standards, the definition of ys for oil well pipes and pipeline steel pipes was 0.5% light load yield strength. For example, x120 is the 0.65% light load yield strength; x100 is the 0.60% light load yield strength close to ys. In ISO 3183, a yield strength of 0.2% displacement is used for grades x90 and above. However, in the Canadian standard CSA, the yield strength of x100 is 0.5% light load yield strength, which is slightly lower than the yield strength value of 0.2% displacement.
With the development of the polar regions, lower low temperature toughness is required. The toughness guarantee temperature of pipelines laid in northern Canada is generally -5°C, but some polar onshore pipelines require -60°C. Moreover, when the pipe ruptures, the temperature decrease caused by the thermal expansion of the gas when it is ejected should also be considered.
The low-temperature toughness of gas pipelines should consider the high-speed ductile fracture of crack occurrence and propagation. The possibility of cracks arising from welding defects at circumferential welding sites in the field is extremely high. Therefore, the toughness of the weld metal and welding heat affected zone (haz) of the weld is specified. In ISO 3183, it is required that the welding metal and haz of the welding seam parts above x80 should reach a V-notch impact value of above 40j. In recent years, the dnv-os-f101 specification requires more than 45j. In the past, the fracture mechanical property value ctod of crack occurrence characteristics was rarely used to evaluate the weld toughness of pipeline steel pipes, but now there is an increasing trend.
In addition, the coarse-grained haz formed by inner surface welding is also welded on the outer surface, so the toughness of the irog-haz part that undergoes reheating in the two-phase area is low, and it is difficult to improve the toughness of this part. However, UOE steel pipes are expanded in the plastic area, and UST flaw detection is used in this part, and there are no cracks of about 1mm or more. Generally speaking, when conducting safety evaluation on fracture mechanics calculations based on CTod ≥ 0.15mm specified by DNV-OS-F101 and other regulations, the required value is often much larger than the required value. So try shallow notch ctod test and sent test. In addition, considering the restraint stress, it is considered that the equivalent ctod evaluation method is also effective.
Even if a gas pipeline ruptures, it is difficult to reduce the internal pressure, so if a crack occurs, the instability will expand. The necessary condition for stopping this crack is that the crack propagation surface first becomes the main body of ductile damage, and the crack expansion speed slowly decreases and is slower than the decompression speed. Therefore, the ductile section ratio of DWT is required to be above 85%. Generally, Barthel’s two-dimensional curve method is used to calculate the required Charpy impact value. It can be said that the propagation energy of dwtt or pre-cracked dwtt energy is more suitable than the Charpy impact value. It is difficult to predict the Charpy impact value of high-strength steel pipes. In recent years, attempts have been made to use CTOA for evaluation. However, sometimes it is difficult for high-strength pipeline steel pipes to stop crack propagation on their own. In this case, crack arresters are used at certain intervals.
Pipes must withstand design internal pressures equivalent to 72%, 80%, etc. of the minimum yield strength (smys). Therefore, originally only elastic deformation was considered to specify the circumferential strength. However, when designing pipelines, the bending of steel pipes that occurs during S-lay (S-lay method for submarine pipelines), stratigraphic changes caused by earthquakes and seasonal stratigraphic changes in discontinuous permafrost zones, as well as the occurrence of pipeline changes, should be taken into consideration. Plastic deformation, etc. These properties have a greater impact on the longitudinal strength properties of steel pipes than the circumferential strength. The deformation value of the steel pipe body due to buckling due to bending and compression is large.
The buckling that occurs on the compression side during bending deformation is first greatly affected by the steel pipe diameter/wall thickness ratio. If d/t is small, the buckling deformation limit (compression deformation limit) will be large. At the same d/t, reducing the yield ratio (y/t), increasing the work hardening coefficient (n value) and uniform elongation (uel) can increase the compression deformation limit. The relationship between the tensile strain limit and the mechanical properties of steel pipes is not clear, but the longitudinal ys of the steel pipe is lower than the ys of the circumferential welded metal. In order to achieve this purpose, the standard lower limit value of longitudinal ys (l-ys) is sometimes set lower than the standard lower limit value of circumferential c-ys. Deformation performance and low-temperature toughness have become research topics for high-strength pipes.
Pipeline steel pipes are coated with anti-corrosion coatings on the outside, especially the epoxy resin coating (FBE) that has been commonly used in recent years. Cold forming of steel pipes produces strain aging, the stress-strain curve changes, and the strength increases. In some cases, it is required that the strength properties before and after coating meet the required values without yield extension. It has been reported that if yield extension occurs, the compression deformation limit becomes smaller when the internal pressure decreases.
When laying pipelines in deep sea, steel pipes may be crushed by water pressure. If the water depth exceeds 2000m, crushing pressure becomes the first design element. When high-pressure operation is not performed (no internal pressure), no crushing will occur (safety rate 1.41), which means that the pipeline steel pipe is required to have high-pressure crushing properties. Since bending stress reduces the crushing value, the influence of bending stress on it should also be paid attention to during design. The crush value is greatly affected by d/t, so in order to prevent the steel pipe from collapsing, use a steel pipe with a small d/t. Low d/t means that the steel pipe diameter (d) becomes smaller and the transportation capacity decreases; if the steel pipe wall thickness (t) becomes larger, thick-walled steel pipes are used. Therefore, deep-sea projects require ultra-thick-walled steel pipes.
In recent years, deep-sea pipelines with a water depth of more than 2,000m have been constructed in the Black Sea (the deepest depth is 2,150m) and the Mediterranean Sea (the deepest depth is 2,160m), and there are plans to build a fourth Mediterranean pipeline that passes through the deep sea (the deepest depth is 2,800m).
Crude oil and natural gas often contain hydrogen sulfide (h2s). If steel is exposed to a humid hydrogen sulfide environment (acidic environment), a large amount of hydrogen will invade into the steel, causing various forms of hydrogen embrittlement. The representative damage form of pipeline steel pipes is hydrogen-induced cracking (hic). Japan has developed a method of immersing steel in artificial seawater (ph=5) saturated with H2S gas, which is called the bp test. Later, this hic test has been adopted as the NACE standard TM0284 (formulated in 1984). However, in this solution, cu is easily added to steel to form sulfides. Although hydrogen intrusion is suppressed, there is a problem that crack sensitivity cannot be accurately evaluated.
Afterwards, the H2S saturated solution with a low pH value of 0.5% ch3cooh+5% nacl (general nace solution, initial pH value is 2.7, and rises to 4.0 at the end of the test) used in the evaluation of the oil well pipe sulfide stress corrosion test was used for the hic test. The latter is now called a solution and the former is called b solution and is recorded in the revised version of TM0284. In recent years, most require the use of a solution for hic testing, requiring clr (crack length ratio) ≤ 0.15%.
Acidic environments are corrosive environments, so pipes made of acid-resistant pipeline steel are generally used. When using inhibitors, it can cope with the highest corrosion rate of 0.1-0.5mm/a. Therefore, a steel pipe thicker than the wall thickness calculated based on the design pressure is used. Even if gas containing H2S is transported, it will not corrode or cause hydrogen intrusion if it is dehydrated. If it is just to deal with the failure of dehydration equipment, high-strength steel pipes can be used regardless of the amount of corrosion. Under such conditions, acid-resistant x80 grade steel pipes can also be used.
2.1.6 High corrosion resistance
When the water content is high, the carbon dioxide partial pressure is high, and the oil content that inhibits corrosion is small, corrosion-resistant materials such as 13cr, duplex stainless steel, and nickel-based alloys are used according to the environment. Since nickel-based alloys are expensive, only the corrosion-resistant alloy layer is used on the inner surface of the steel pipe, and composite steel pipes of low-alloy steel pipes are mostly used on the surface of the pressure-resistant material. The press machine tool method is widely used to form steel pipes that are rolled and clad, welded pipe seams and heat-treated steel pipes, and steel pipes with corrosion-resistant inner pipes mechanically inserted into the outer pipes. There are also reports on using UOE technology to develop high-nickel alloy composite steel pipes. In addition, 13cr uoe steel pipe has also been developed.
Non-destructive testing of large-diameter pipeline steel pipes after on-site circumferential welding uses automatic ust. In order to install the guide device of the machine, there are an increasing number of cases where we are required to cut the pipe seam welds at the ends of the inner and outer pipes in the factory, and it is necessary to realize cutting automation and efficiency.
Domestic spiral welded pipes in Japan are mainly used in civil construction, water pipes and other fields. In particular, steel pipe piles and steel pipe sheet piles account for a large part. Many products require accessory processing and coating that can help increase the added value of the product. By combining steel pipes with superior tensile strength and concrete with good compressive strength, the mechanical properties of structural parts are improved. In order to improve the bonding strength between steel pipes and concrete, it is required to use reticulated hot-rolled spiral welded pipes. The coating requires polyethylene and polyurethane coatings to prevent corrosion.
In order to achieve high toughness and acid resistance of pipeline steel pipes, liquid steel with high purity and cleanliness is required. In order to suppress the occurrence of hic, the generation of mns should be suppressed, so vacuum degassing method and powder spraying technology are used to produce low-sulfur steel. In the mid-1980s, various steel companies established technologies to control sulfur content below 10 ppm, and established ca addition technology to modify mns into cas.
Reducing the center segregation of continuous casting slabs is very important to suppress hic. Studies have shown that technologies such as shortening the roller spacing and lightly pressing the end of slab solidification can be used to reduce center segregation. The vertical section of the continuous casting machine has a great influence on the floating inclusions.
TMCP technology is a technology developed simultaneously with the manufacturing of advanced pipeline steel pipes. Accelerated cooling after controlled rolling has been industrialized since the 1980s. In the high-strength process from x60 to x65 and x70 grade pipeline steel, the utilization rate of accelerated cooling of acid-resistant steel and pipeline steel has increased significantly. Subsequently, second-generation accelerated cooling equipment aimed at further rapid cooling and uniform cooling was introduced, such as super-olac (1998, Fushan plant) and clc-μ (2006, Junjin plant). The goal of controlled cooling is to produce thick steel plates with uniform materials. There are also online heat treatment equipment (HOP) with induction heating devices behind accelerated cooling for rapid heating. It is an application example of diversified TMCP conditions.
The main structure of x60 and x65 grade pipeline steel produced by accelerated cooling is composed of austenite transformed ferrite. However, the ratio of the second phase in low carbon steel is low, so the main structure of ferrite cannot be strengthened. Therefore, pipeline steel above grade x80 should be suitable for Bainite structure. From the perspective of weldability, high-strength pipeline steel pipes are made from molten steel with a C content of 0.03%-0.08%, and low carbon bainite steel is used. Semi-quantitative relationship between the 50% phase transformation temperature and tensile strength of these steels. For example, grade Grade x100 is a steel with a C content of 0.06% upper bainite + granular bainite that undergoes phase transformation at 550-600°C to gain strength. Grade x80 is a granular bainite steel that undergoes phase transformation at 600-650°C to gain strength.
The carbon in the lower bainitic steel suitable for x120 grade pipeline steel pipes has a great influence on the strength. When the slab is heated, it causes abnormal phase transformation and sometimes turns into coarse austenite grains. The low-carbon bainite structure is easy to achieve high strength, and the Charpy low-temperature toughness generally measured is good, but it has shortcomings such as sometimes low dwtt performance and small work hardening. In order to make up for these shortcomings, there are also cases where the ma component (martensite-austenite) and polygonal ferrite are introduced into the bainite main structure. In addition, refining the flat austenite grain thickness can also improve low-temperature toughness.
As the ma ratio increases, the y/t (yield strength/tensile strength) of the steel plate decreases. Low carbon steel with high hardenability generates a small amount of MA during accelerated cooling. When using hop, if it is reheated during the phase transformation, carbon will diffuse into the untransformed austenite phase, and the MA ratio will increase. Of course, if the steel plate is formed by the uoe process, due to the change in cold working strength, the y/t of the steel plate is low, and the y/t of the steel pipe is not necessarily low as well. The c direction is the main deformation direction. Depending on the type of steel, the amplitude of each change is also different. As the proportion of ferrite increases, the dwtt toughness section ratio increases. When the steel plate is cooled to a low temperature region by accelerated cooling, changes in the ferrite ratio have little effect on the tensile strength. This is because if the ferrite ratio is increased, carbon is concentrated into the surrounding austenite phase and becomes a high-strength low-temperature phase transformation structure.
The pipeline steel pipes of spiral welded pipes have large diameters, so most of them are thick-walled steel pipes, and the advantages of high strength are not easy to show. In recent years, in order to produce 20mm ultra-thick hot-rolled strip coils, rolling mills have been equipped with enhanced coiler capabilities and enhanced water-cooling equipment. However, compared with thick plate rolling, low-temperature controlled rolling in the hot rolling process has more restrictions. For example, it is difficult to increase the cooling capacity due to fast rolling speed, and it is difficult to coil when cooled to low temperature.
In addition, HTP (high temperature processing) steel with 0.08%-0.11% NB added to 0.04% C steel has also been developed and has been mass produced. As the amount of nb added increases, the cvn energy decreases and the intensity increases, but reaches saturation at about 0.1%. Therefore, it is speculated that 0.1%nb is the limit.
