Development of new steel materials and weldability and welding materials in China

I. The relationship between the development of the steel industry and welding technology

The main object of welding is all kinds of steel. Therefore, the output of steel, the apparent consumption, the quality of steel and its development trend directly determine the sustainable development of the welding industry and the development direction of welding technology. Since 1996, China's steel output has exceeded 100 million tons, and steel production has increased year after year. Especially since 2001, China's steel output has developed by leaps and bounds. The annual steel output has increased by about 40 million tons. By 2004, China's steel output has exceeded 297 million tons, and the apparent consumption of steel reached 312 million tons. Such large steel production and steel consumption have greatly affected the development of the welding industry. The demand for welding equipment has increased, the amount of welding has increased, which has increased employment opportunities in the welding industry, and has strengthened the welding industry team. It has also promoted the development of welding technology in the direction of quality, efficiency, low cost and automation.
With the improvement of metallurgical technology and controlled rolling and controlled cooling technology, the quality of steel is continuously improved, the tendency of cracking and embrittlement of steel is significantly reduced, and the weldability has been significantly improved. However, some new welding problems have emerged, driving Our welding workers develop new technologies in welding methods, processes, materials, etc., solve new problems, and constantly push forward the development of welding technology.

Second, the development trend of the steel industry

1. Sustainable steel production in China's steel industry and welding industry is one of the important indicators to measure a country's comprehensive economic strength, and is also a pillar industry in China's industrialization process. Since the reform and opening up of China, the sustained and rapid economic growth has spurred the development of the steel industry, so in recent years, China's steel output has grown rapidly. This not only makes China the world's number one steel producer, but also the number one steel consumer (see Table 1). Table 1 China's steel production development list of steel production / billion tons of world ranking steel apparent consumption / billion tons of steel net imports / billion t 1949 0.00158 26 - 1957 0.0535 9 - 1965 0.1223 8 - 1978 0.3178 5 - 1996 1.0025 1 1.0515 0.1598 1997 0.9987 1 1.0847 0.1322 1998 1.0738 1 1.1623 0.1242 1999 1.2102 1 1.3220 0.1486 2000 1.276 1 1.4121 0.1596 2001 1.53 1 1.7020 0.1722 2002 1.9218 1 2.1122 0.2449 2003 2.30 1 2.666 0.3655 2004 2.97 1 3.12 0.138.

In 2004, China's steel output reached 297 million tons. In the first quarter of 2005, steel output increased by 23.7% compared with the same period of last year. According to this development speed, this year's steel output may exceed 350 million tons. As can be seen from Table 1, the steel industry is still developing momentum before 2010, and even after the development reaches equilibrium, it will still maintain a relatively long time in peak production.

According to statistics, China's welding structure steel accounts for about 50% of steel production, and the development of the steel industry has created a great development space for the sustainable development of China's welding industry. Based on this, it is inferred that in the past 10 years, it is still the prime time for the development of the welding industry.

2. Major problems in the development of the steel industry

Although China has become the number one steel country in the world, it is not a steel power. The steel produced in China is mostly low-level ordinary steel. The production of high-efficiency and high-quality steel is small or impossible. The main part of China's steel industry is high consumption and high energy consumption. High pollution and low quality. Although in recent years, various steel mills have invested a lot of money, but due to the market drive, the eyes of many steel mills are still focused on steel production, which is far behind the requirements of the situation in the research and development and production of high value-added steel. Such as high-strength steel, weathering steel, heat-resistant steel, corrosion-resistant steel, low-temperature steel, micro-alloy controlled rolling and cold-steel steel and other professional steel, as well as various plates and sections can not meet the market demand, so many high-quality steel also Need to rely on imports.

This development of the steel industry, which pursues only quantity and neglects quality, is in urgent need of reversal. In 2001 alone, China imported 17.2 million tons of high-quality steel. In 2002, the net import was 24.49 million tons. In 2003, the net imported steel was 36.55 million tons. China's steel imports exceeded the United States, ranking first in the world. The price of high-quality steel is three times that of ordinary steel. If it is not imported or imported, it will cause a significant increase in benefits. In 2004, net imports were about 13.83 million tons, down 62.16% from 2003. That is, 45% of the increase in steel output in 2004 was used to replace imports and expand exports. This is a significant turning point.

The lessons of the world's steel powers are worth learning. In the 1970s, Japan and the European Union's steel production reached 120 million tons, and in the early 1980s it turned back to 100 million tons. The production will be changed from production to quality-based production. In the early 1980s, the transformation will be completed. High-quality steel will hit the US and the world market. On the surface, production will decrease, but the total output value and profits will increase. Moreover, investment has been reduced, energy and resources have been saved, pollution has been reduced, and social and economic benefits have been significantly improved.

China is in the process of building a modern industry. The demand for steel is large and the contradictions are still not sharp. However, due to the blind expansion of low-level steel, it is bound to cause oversupply, product backlog, and even loss. High-quality steel is in short supply and needs to be imported. If the above situation is not reversed in time, after a few years, it will inevitably lead to serious consequences.

In response to the above-mentioned development of the steel industry, the State Council reviewed and approved the "Steel Industry Development Policy" on April 20, 2005, and timely proposed a macro-control policy for the development of China's steel industry. It is pointed out that in accordance with the guiding ideology put forward by the State Council executive meeting, the implementation of the scientific development concept, adherence to the "three priorities" and "a fundamental change", that is, the emphasis on increasing high value-added products, improving quality, can not unilaterally pursue quantitative expansion; In order to improve industrial concentration and strengthen the reorganization and transformation of existing enterprises, we cannot rely solely on paving new booths and launching new projects; we must focus on reducing consumption and improving the competitiveness of enterprises and products, and cannot rely on consuming resources and polluting the environment. We must adhere to the road of new industrialization development, realize the fundamental transformation of China's steel industry from large to strong, and strive to build a steel power with international competitiveness, enhance competitiveness, and achieve sustainable development.

3. Current development trend of China's steel industry The above situation has attracted the attention of China's high-level and some large steel mills, and has invested a large amount of funds for structural adjustment, speeding up technological transformation and new product development. (1) Application of continuous casting or continuous casting and rolling process At present, all major steel mills have adopted continuous casting or continuous casting and rolling processes instead of steel ingot casting, and applied electromagnetic stirring technology, which has caused the segregation and interlayer defects of steel sheets to be basically extinct. . Continuous casting and rolling technology has reached 95% in 2001, far exceeding the world average of 89%, reaching the world's advanced level. (2) Smelting technology has greatly improved the application of advanced steelmaking technologies such as iron pretreatment, compound blowing, and refining outside the furnace, so that W (S, P, O, N, H) and other impurities in the molten steel have been Drastically reduced. At present, the physical level of many low alloy steel Ws is less than 0.005%, and X70 steel used for oil and gas pipelines is supplied with Ws<0.03%. (3) Application of controlled rolling and controlled cooling (TMCP) technology controlled rolling refers to stopping rolling at a lower temperature, and preparing high temperature austenite crystal growth; controlling cooling immediately after rolling, speeding up cooling, avoiding crystal growth It also increases the nucleation rate and produces fine bainite or acicular ferrite with higher toughness, which causes carbon and nitride to be dispersed at a lower temperature, and significantly improves the strength of the steel by refining the grains. toughness. The conventional fine-grain steel has a grain diameter of <100 μm, while the TMCP steel has a grain size of 10 to 50 μm, and the ultra-fine grain steel has a grain size of 0.1 to 10 μm. (4) The development of low-alloy and micro-alloy high-strength steels is currently used in shipbuilding, bridges, steel structure construction, pressure vessels, low-temperature steel, heat-resistant steel, pipeline steel, etc. to "purified, low-carbon, ultra-low-carbon, The development of microalloying and controlled rolling and cooling. Low-alloy (alloy content <5%) and micro-alloy (total amount of microalloying elements <0.2%) high-strength steel is not only China, but also the development direction of the world steel industry. The basic idea is to break the traditional C, Mn, Si-based steel design ideas, use carbon reduction, a variety of trace elements (such as V, Nb, Ti, Cu, Re, etc.) alloying, and through the controlled rolling and cooling process Refine the grain, improve the toughness and ensure the comprehensive mechanical properties. This type of steel has the following characteristics: First, carbon reduction: in order to improve plasticity, toughness and weldability.

Carbon is the most important strengthening element, but it will strongly deteriorate the toughness and weldability. Therefore, the carbon content is strictly controlled in the new steel grades. For example, the Wc in the X70 and X80 steels is only 0.03% to 0.04%, and some even reach the ultra-low carbon level. Second, the microalloying technology: strengthening the steel by adding a small amount of alloying elements such as Ti, V, Nb, Al, Re, etc. to the steel, increasing the strength, changing the structure, refining the grains, and purifying the matrix. Third, high cleanliness: by refining, removing impurities, purifying the matrix, and controlling the mass fraction of W (S, P, O, N, H), this is a research hotspot in the 1990s. The total mass fraction of impurities S, P, O, N and H in steel decreases from W (S+P+O+N+H)<0.025% of ordinary steel to W(S+P+O+N of economical clean steel). +H)<0.012%, and began to study ultra-clean steel with W(S+P+O+N+H)<0.005%. A variety of new steel grades have been developed using the above basic ideas, such as pipeline steel X60, X70, X80, and X100 and X120 have been developed abroad. The current West-East Gas Transmission uses X70 steel with a total length of nearly 4,000 km. (5) Development of a new generation of steel materials In 1998, China launched the “Major Basic Research on Steel Materials” project in the Major Basic Research and Development Program (973), which aims to produce carbon steel that accounts for 60% of China's steel output. The strength and life of low alloy steel and alloy structural steel are doubled. Japan and South Korea also started a 21-century structural steel project in 10 years in 1997. The new generation of steel materials is characterized by ultra-cleanness, ultra-uniformity, and ultra-fine grain, which doubles the strength and life without increasing or even reducing carbon and alloying elements. Ultra-cleanness means that W(S+P+O+N+H)<0.008% in steel; ultra-fine grain means that the grain diameter is between 0.1-10μm; super-uniformity refers to composition, structure and properties. Uniform and emphasize the dominant position of the organization. Through the comprehensive application of the above technology, the toughness and toughness of steel can be greatly improved. In 2000, China's 400MPa grade and 800MPa grade ultra-fine grain steel research has made substantial progress, and 400MPa grade ultra-fine grain steel has begun to be popularized in production. It can be expected that the successful development of this type of steel will revolutionize China's steel production. Third, the weldability transformation caused by the development of steel technology 1. The development of steel weldability The development of alloy structural steel meets the requirements of various aspects of welding structure, such as high strength, high temperature resistance, low temperature resistance and corrosion resistance, etc. Ships, construction machinery, oil pipelines, boilers and pressure vessels, bridges, automobiles, trains and power generation equipment have been widely used. The superior performance of the above-mentioned alloy structural steel is achieved by adjusting the content of carbon and alloying elements in the steel or by appropriate heat treatment. The increase of carbon and alloying elements tends to adversely affect the weldability of the steel. Different weldability problems occur in different steel grades. In alloy structural steels, as the content of carbon and alloying elements increases, it tends to cause brittleness, softening and cracking tendencies of joints. The occurrence of these weldability problems tends to reduce the reliability of the safe operation of the welded structure and cause early damage to the welded structure. In order to continuously improve the weldability of alloy structural steel, from the end of the 1960s, domestically, from the 1980s, smelting began to develop and produce microalloyed controlled-rolled steel with good weldability, and began to study the next generation of ultra-fine grain steel. The emergence of new steel grades has brought about major changes in the weldability of steel. 2. The main characteristics of microalloyed steel micro-alloy controlled rolling and controlled cold steel are high strength, high toughness and easy welding. The steel grade has high toughness due to low carbon content, high cleanliness and grain refinement. The so-called easy welding means that no preheating or only low temperature preheating welding does not produce cracks; the use of large or large heat input welding heat affected zone does not produce embrittlement. The microalloyed controlled rolling steels produced by Baosteel and Wuhan Iron and Steel are mainly pipeline steel, steel for pressure vessels, bridge steel, etc., and have been successfully applied in engineering. Among them, pipeline steel is the first example of microalloying and the most successful application. For pipeline steels, the main weldability problems are also analyzed from weld cracking and HAZ embrittlement problems. (1) Weld crack micro-alloy controlled rolling cold steel has low carbon and impurity content. For example, the carbon content of X70 steel produced by Baosteel and Wuhan Iron and Steel Co., Ltd. is Wc≤0.05%, and the elements such as C, S and P are effectively controlled, so the liquefaction crack during welding And the tendency of crystal cracking is small. However, due to the large forming stress or additional stress during the forming and welding of steel pipes, especially in the case of submerged arc welding with multi-filament heat input, C and S appear due to excessive growth of the weld grains. Local partial segregation of P is also likely to cause crystal cracks.

It is precisely because the pipeline steel has low carbon content, few alloying elements, and low hardening tendency (for example, X70 steel belongs to acicular ferritic steel), so the tendency of cold cracking is small. However, as the strength level increases, the thickness of the plate increases, and there is still a certain tendency of cold cracking. Especially when the pipeline steel is laid on site for circumferential seam welding, because the cellulose electrode with high hydrogen content is often used for bottoming, the heat input is small, the cooling rate is fast, and the deposited metal has high hydrogen content, which will increase the cold. The sensitivity of the crack. The higher the strength, the more prominent the cold row problem (such as pipeline steels such as X80, X100 and X120). (2) The embrittlement of the heat-affected zone in the heat-affected zone is a common problem in the welding of fine-grained steel. The larger the heat input is, the more serious the embrittlement tendency is. The embrittlement problems of HAZ mainly include the coarse-grained zone (CGHAZ) embrittlement, the critical heat-affected zone (ICHAZ) embrittlement, the critical coarse-grained heat-affected zone (IRCGHAZ) embrittlement during multi-layer welding, and the brittleness of the supercritical coarse-grained heat affected zone. (SRCGHAZ), subcritical coarse-grained heat affected zone (SCGHAZ) embrittlement, etc. Among them, embrittlement of CGHAZ, IRCGHAZ, and SCGHAZ is the most important embrittlement area for microalloyed steel welding. In order to prevent embrittlement in the heat-affected zone, the following measures are often taken: first, carbon reduction in composition, control of impurity content, and addition of a small amount of Ni toughening matrix. The second is to suppress the grain growth in the heat-affected zone, add Ti, V, Nb and other refined grain elements to the steel, and inhibit the growth of HAZ grains by forming nitrogen oxides such as TiN, TiO, NbN and VN. The third is to improve the organization of the heat affected zone. By adding a modifier to the steel, the phase deformation nucleation rate is increased and the structure is refined. For example, if fine, uniform dispersion of TiO particles is added to the steel, formation of low-toughness microstructure such as grain boundary ferrite + side slab ferrite can be avoided, and fine acicular ferrite is formed in the austenite crystal. Can significantly improve toughness. Even with large or large heat input welding, no embrittlement occurs. The fourth is to use appropriate welding process parameters. For steels that are sensitive to general overheating, the welding process parameters are adjusted during welding to reduce the high temperature residence time and avoid austenite grain growth. The appropriate t8/5 is used to obtain the toughened structure of HAZ. 3. Weldability of a new generation of steel materials Due to the extremely fine grain of the new generation of steel materials, the serious problems encountered during welding are the toughening of the weld and the grain growth of the heat affected zone. (1) Strengthening and toughening of weld metal The weld metal is mainly strengthened and toughened by alloying to control the structure of the weld. For 400MPa grade fine grain steel, the ideal toughness can be obtained by adjusting the weld microstructure to obtain acicular ferrite. For ultra-fine grain steels of 800 MPa or higher, it is difficult to match the weld metal to the base metal. At present, the welding materials to be developed in China and South Korea that match the ultra-fine grain steel of 800 MPa or higher are non-preheated ultra-low carbon bainite welding materials. (2) Grain growth tendency in the heat-affected zone For ultra-fine grain steel, a serious grain growth tendency tends to occur during welding. This will not only cause embrittlement of the HAZ, but also cause softening of the HAZ. In order to solve this problem, welding methods such as laser welding, ultra-narrow gap GMA welding, pulsed MAG welding, etc., should be used. Fourth, the development of the steel industry to promote the development of the welding materials industry With the acceleration of China's industrialization process and the rapid development of the steel industry, in recent years, China's welding material output has increased at a rate of several hundred thousand tons per year, and the apparent consumption of steel The increase is almost proportional to the increase. At present, China's welding material production accounts for about 40% of the world's total welding consumables. It has truly become the world's largest consumer of welding consumables, but it is not a strong country of welding consumables.

In recent years, the apparent consumption of steel in the future statistics of the development of welding consumables should pay attention to the following points: 1. Further growth of welding consumables, the structure of welding consumables will be further adjusted. Due to the strong growth of steel in recent years, the consumption of welding materials in China is still Will continue to grow. In 2003, China's steel consumption was 266 million tons, an increase of 88% over 2000. The consumption of welding consumables was about 2.05 million tons, an increase of 86% over 2000. In 2004, China's steel and welding consumables increased by 17% over the same period last year. It is estimated that China's steel consumption will reach 350 million tons in the past two years, and as China's steel structure steel consumption increases, the consumption of welding materials will be even greater. It is estimated that in the past five years, the apparent consumption of welding materials in China will reach about 3 million tons. As welding technology moves toward higher efficiency, automation and high quality, the structure of the consumables will change. The proportion of the electrode will decrease and the proportion of the wire will increase.

In recent years, this proportional relationship has undergone significant changes. 2003-2004 welding materials, sub-item statistics, welding consumables, 2003/10,000 t, 2004/10,000 t, relative growth (%), proportion of welding consumables (%), 2003, 2004 Annual welding electrode 150 165 10 73 68.7 Gas shielded solid wire 30 45 50 14.6 18.8 Flux cored wire 4 6 50 2 2.5 Submerged arc welding wire + flux 20 24 20 10 10 Total 205 240 17 Among them, the total wire is 54 75 39 26.34 31.25.

In 1970, developed countries abroad still focused on electrode arc welding. At that time, the proportion of welding rods in the total output of welding consumables was about 70% in the United States and Western Europe, and 87% in Japan. By 1980, the proportion of welding rods in the United States and Western Europe had fallen to 40%, and has now fallen to between 20% and 30%. In 1982, Japan's welding rods accounted for 50% of the total welding consumables, which fell to 24% in 1990 and only 17.4% in 2002. This shows that Japan's electrode arc welding has dropped to 20%, and more than 80% are automatic welding and high-efficiency welding. In the 1980s, the proportion of welding rods in China accounted for 90% of the total amount of welding consumables. In 1999, the welding rods accounted for 85.5%. After entering the 21st century, the proportion of China's welding rods has gradually decreased. It can be seen from Table 2 that in 2003, the welding rods accounted for 73%, and in 2004, the welding rods accounted for 68.7%, and the proportion of welding wires reached 31.25%. The above data shows that in recent years, China's welding consumables are gradually developing in a rational direction. It is estimated that the apparent consumption of steel in 2005 will reach 350 million tons, and the total amount of welding consumables will increase to 2.6 million tons. Among them, the proportion of welding rods will account for about 65%, which is 165-1.7 million tons. The proportion of various welding wires will increase to more than 900,000 tons. 2. Improvement of the quality of existing welding consumables China is not only a large steel producing country and a major steel consumption country, but also the world's largest producer of welding consumables and welding consumables. However, the existing welding consumables, more common welding consumables, less high-quality welding consumables, most of the high-quality welding consumables still need to rely on imports. Therefore, China's welding consumables industry should not only adjust the product structure according to the needs of market development, but also continuously improve the quality and specifications of welding consumables to meet market requirements and improve the ability to compete in the international market. For example, the gas-shielded solid wire should be developed in the direction of low spatter, high performance and multi-species; the flux-cored wire should be developed in the direction of wide current, low dust, low spatter and fast welding; the wire and flux for submerged arc welding should be highly efficient. , multi-variety, high toughness direction development; welding rods, welding wire should be developed with the development of new steel types. 3. Development of microalloy controlled rolling steel welding materials The development of steel metallurgy technology has enabled the low alloy high strength steel to achieve clean, fine crystallization and mechanical properties. This requires that the matching welding materials must also be cleaned. Chemical, fine-grained and toughened. However, since the weld metal cannot be controlled by the controlled rolling and cooling control to achieve "fine crystallization" and "ultrafine grain", the deformation strengthening of the steel rolling cannot occur, and it is difficult to realize the weld and the base material under similar chemical composition. Strong toughness match. Therefore, the commonly used welding processes and welding materials will not be suitable for low-carbon microalloyed controlled rolling steel, and it is not suitable for the new generation of steel materials in the 21st century, which requires major changes in common welding materials and processes. According to domestic and international research, new welding materials suitable for microalloyed fine grain steel and new generation steel grades should be high purity acicular ferrite or low carbon bainite (ULCB) welding materials. This welding material has high toughness, especially ULCB welding materials offer potential possibilities for obtaining high strength, high toughness and high quality weld metal. ULCB is characterized by its inflexibility to the cooling rate, which is only a function of the chemical composition. Therefore, the weld can maintain high toughness over a wide range of heat input and can improve the deposition efficiency. In addition, since ULCB uses ultra-low carbon and has a strong ability to resist hydrogen cracking, it can be preheated without cracking. Obviously, due to the imbalance of welding metallurgy, it is quite difficult to make the cleanliness of the weld metal reach the level of clean steel. However, it can work in the following aspects: (1) Clean welding of raw and auxiliary materials The main body in the raw and auxiliary materials is wire rod or steel strip, which can be purified by the above smelting technology. At present, the cleanliness of wire rod steel produced by WISCO reaches: Ws ≤ 0.005%, Wp ≤ 0.01%, brittle inclusions in steel < 1 level, reaching the international advanced level, can be used for welding of clean steel. At the same time, the content of impurities in other raw and auxiliary materials should be strictly controlled.


(2) In the clean welding process of welding metallurgical reaction, intense chemical metallurgical reaction is carried out. Desulfurization, desulfurization, dephosphorization, denitrification and hydrogen removal by welding metallurgical reaction are commonly used in welding materials. Therefore, in the study of welding materials, the formulation and process parameters should be optimized, and the purification reaction should be carried out as much as possible. 4. The development trend of high-tech new welding materials With the acceleration of industrialization in China and the continuous development and application of new steels, the requirements for welding materials are getting higher and higher, and the quality of existing welding consumables should be continuously improved. New types of welding consumables to meet the needs of modern construction. (1) Develop new welding materials for high-grade pipeline steels (such as X70, X80, X100, X120, etc.), including solid-core welding wire for gas protection and submerged arc, and self-shielded flux-cored welding wire. (2) Develop high-intensity (yield strength ≥ 490 MPa) and thick plate (20-45 mm) large heat input for large-scale oil storage tanks (162 large-scale oil storage tanks with a capacity of 162 to 150,000 cubic meters in the first phase of the national project) Welding rods, solid wire and flux cored wire. (3) Welding materials for a new generation of microalloyed heat-resistant steels (such as T91/P91, T92/P92, T23/P23, etc.). (4) New welding materials matched with the new generation of ultra-fine grain steel of steel materials in the 21st century. (5) Welding materials for ultra-low carbon Bayesian high strength steel (600 to 1500 MPa). Ultra-low carbon Bayesian high-strength steel has the advantages of high strength, high toughness and low welding crack sensitivity. Under certain material processes, preheating may not be preheated, but the weld seam is prone to cracking at this strength level. (6) Improve the quality of existing stainless steel welding materials. For important occasions with high quality requirements, stainless steel welding rods still need to be imported. In addition to the need for domestic welding material manufacturers to improve their technical level, raw material manufacturers (including welding wire and raw and auxiliary materials) should also provide high quality raw and auxiliary materials. At the same time, the welding materials of the duplex stainless steel and the "nickel-nickel" and "nitrogen-containing" stainless steel welding materials should be developed to alleviate the serious shortage of nickel resources in China. V. Conclusion As can be seen from the above, the development of the steel industry presents new opportunities and challenges for the development of welding technology and welding materials. It is predicted that major changes will occur in welding methods and welding materials in the 21st century. Especially in the past 10 years, all large enterprises should "take advantage of the trend", concentrate human, material and technical strength, increase scientific research and development efforts, and combine production, education and research to lay a good foundation for the reform of new welding technology and welding materials in the 21st century.