New Challenges in Microalloyed Steel Welding Research
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Hot-rolled slab products are mainly used in shipbuilding, building structures, bridges, boilers and pressure vessels, conveying pipelines, offshore platforms, construction machinery and other important occasions, so the quality and weldability requirements of products are also getting higher and higher. This requirement is reflected in the following aspects:
(1) Maximally meet the requirements for strength and toughness;
(2) Excellent service performance, including high and low temperature resistance, fatigue resistance, and medium corrosion resistance;
(3) Good welding performance, including wide welding process adaptability, high crack resistance, suitable for large-line energy welding;
(4) Compliance with various laws and regulations;
Among them, the requirement of weldability is one of the most important contents, and it is also the key technology that steel producers are trying to solve. Microalloying is at the heart of steel product improvement and improved weldability.
Progress in smelting and rolling technology
1 Pure steel technology
Microalloying is inseparable from the overall progress of metallurgical technology. The use of refining technologies such as hot metal pretreatment, converter steelmaking, ladle refining, vacuum refining, etc., makes the content of impurity elements such as S and P in steel much lower than the previous low carbon steel. And low alloy steel.
Taking actual performance as an example, the current pure steel smelting technology can reach the following levels:
[P+S+N+O+H] ≤80ppm
P≤20ppm, S≤5ppm, N≤20ppm, O≤10ppm, H≤1.0ppm
With the sharp decrease of impurity elements, the incidence of crystal cracks is greatly reduced, which is no longer the focus of attention and research.
Due to the structural design requirements during use, the plate thickness direction performance of the steel sheet cannot be ignored. Eliminating the perfection of the intermediate segregation technology of the continuous casting billet, greatly reducing the degree of segregation and improving the Z-direction performance of the thick plate.
2 Control rolling and controlled cooling
The development of controlled cooling technology has promoted the advancement of rolling technology, effectively combining controlled rolling and controlled cooling. The combined results make the steel composition simpler and the overall performance of the steel plate further improved. At the same time, with the continuous maturity of automation control technology in rolling steel applications, it is conditional to produce high quality and high precision products. The microalloying technology combines controlled rolling and controlled cooling to achieve deformation heat treatment under controlled conditions. It has the combined effects of deformation strengthening, precipitation strengthening and phase transformation strengthening. It can be better than alloying method, normalizing treatment and quenching and tempering treatment. The plasticity, low temperature toughness, high strength, and more importantly, due to the reduction of carbon equivalent Ceq and crack sensitivity index Pcm, the welding performance is greatly improved, and gradually develops from weldable to easy welding.
Currently, hot rolled thick steel plate manufacturing is widely used for controlled rolling (TM) with normalizing rolling, controlled rolling (also known as CR, which is divided into two-stage rolling and three-stage rolling); and the controlled cooling process has accelerated cooling ( ACC) and direct quenching (DQ); combined rolling and controlled cooling processes form the TMCP process.
Japan has used the TMCP process to produce structural steels with a yield strength of 570 MPa for bridges, pressure vessels and pipelines, and to ensure that 75 mm steel plates are welded without preheating or cracking. At the same time, structural steel with a yield strength of 690 MPa is being developed using the same process.
Advantages of new microalloyed steel welding
Using TMCP technology, a variety of high-strength welded structural steels have been developed abroad. The more typical steel types are:
(1) Steel produced by Nippon Steel with a yield strength of 420 MPa (in accordance with API 2WGr.60), thickness 40-70 mm, post-weld heat treatment Akv (-40 °C) 280 J, and FATT reaching -90 ° C ~ -100 ° C, Akv150J Used for offshore platforms; and subsequently developed steels for offshore platforms with an oxide dispersion distribution with a yield strength of 500 MPa.
(2) Kawasaki Steel and Kobe Steel developed a yield strength of 570 MPa steel, welding heat input up to 200 KJ/cm (4 times that of conventional steel), use at -20 °C, welding is not preheated, no crater crack No hardening, thickness up to 75mm, compared with SM570 has obvious advantages, used for bridge construction, and no need to paint.
(3) Nippon Steel uses HTUFF-Super HAZ Toughness Technology with Fine Microstructure Imparted by Fine Particles to develop 490MPa, 520MPa, 590MPa series of seismic steel for construction, maximum thickness of 100mm, welding heat The input can reach 1000KJ/cm and the local embrittlement is weakened.
(4) Finland adopts TMCP + ACC technology to produce NVE360, NVE400, NVE500 for icebreakers, NVE400 and NVE500 for icebreakers. The CVE of NVE500 is only 0.40%.
From the above examples, it can be found that the development of modern rolling technology brings about the low cost of steel enterprises (less alloy addition), and more importantly, provides more direct benefits to downstream users of steel production.
1 low crack sensitivity
The factors affecting the weld crack sensitivity of steel grades come from hardened structure, diffused hydrogen, restraint, carbon equivalent Ceq and crack sensitivity index Pcm determine the tendency of hardening. The application of microalloying technology and controlled rolling technology makes the design of steel components simple, the total amount of alloys is reduced, especially the reduction of carbon content (ultra-low carbon), the proportion of high temperature and medium temperature transition structure is increased, and the precipitation phase is Influence, nucleation increases, grain refinement. Formation of acicular ferrite and low carbon bainite-based structures, thus achieving high strength and toughness. The simplification of the composition provides a guarantee for improving the cold crack resistance of the steel grade.
2 High heat affected zone toughness
The toughness of the weld heat affected zone is the most important problem of the new microalloyed steel. This problem is accompanied by efficient welding technology. In order to improve welding efficiency, submerged arc automatic welding, gas electric welding (monofilament, multifilament, fuser), electroslag welding (KES, SES) are widely used, and the damage to the toughness of the welded heat affected zone increases with the increase of welding line energy. The more obvious. Welding researchers and steel designers have to find techniques that can effectively prevent HAZ grain coarsening.
After years of research, metallurgical workers in various countries first discovered the effective effect of titanium nitride and put it into practice, and achieved good results. Even more interesting is that Japanese researchers have found that titanium oxide has higher temperature stability than titanium nitride, and is more effective in pinning grain boundaries and preventing grain growth (ie, HTUFF technology). The energy of the welded wire is greatly increased.
Problems in welding research
China started late in the application of rolling technology such as TMCP. At present, the production capacity and technical level of domestic rolling mills lag far behind those of Japan and other countries. As the welding technology essential for its application, there is also a lack of research. The history and experience of foreign development are worth learning. . Combine the work I have done to briefly describe the following ideas.
1 welding material
The improvement of steel grade strength and toughness is the first thing that needs to be solved is the matching problem of welding materials. These supporting welding materials include welding wire (solid, core), flux, welding rod and special welding consumables, especially high-efficiency welding materials. Therefore, the following varieties will become the focus of future research and development.
(1) Develop a welding wire (especially a flux-cored wire) suitable for large-line energy welding to meet gas-electric vertical welding and electroslag welding;
(2) Due to the decrease of the crack resistance of the steel plate, the crack is transferred to the weld metal, and it is the key to develop a welding material with good crack resistance;
(3) High-strength solid gas shielded welding wire, requiring low splash and high performance;
(4) Sintered flux matched with high-speed multi-wire submerged arc automatic welding;
(5) The welding wire required for special service performance requires corrosion resistance, fire resistance and earthquake resistance respectively;
(6) special fuser electrodes;
More importantly, the domestic production of welding materials with relatively complete varieties (including solid wire, flux cored wire, flux, electrode) and stable quality.
2 welding standard
1) Welding materials
The welding material standard reflects the development level of welding technology to a certain extent. China's existing welding consumables standards, with solid gas shielded welding wire and flux cored wire as an example, there are "carbon steel for gas shielded welding, low alloy steel welding wire" (GB/T8110), "corroded wire for carbon steel" (GB/ T 10045) and "Fluid-cored wire for low-alloy steel" (GB/T17493); and Japan's classification of welding material standards is more suitable for users' choice, "carbon steel and high-strength steel MAG welding with solid wire" (JISZ3312) , "Solid steel wire for CO2 gas shielded welding for weathering steel" (JIS Z3315), "Solid core wire for MAG welding of molybdenum and chrome molybdenum steel" (JIS Z3317), "Solid core wire for welding of low temperature steel MAG" (JIS Z3325) , "Gas-steel, high-strength steel and low-temperature steel gas shielded welding and self-shielded arc welding flux cored wire" (JIS Z3313) "molybdenum and chrome molybdenum steel MAG welding flux cored wire" (JIS Z3318) "gas electric vertical welding core Welding wire (JIS Z3319) and "QS Z3320 for weathering steel CO2 welding" (JIS Z3320), a total of eight, Japan is currently the country with the most complete welding material standards.
2) Welding procedures
At present, China's welding procedures are mainly divided into industries, such as JB/T4709 "Steel Pressure Vessel Welding Procedures" and GB50094 "Spherical Tank Construction and Acceptance Specifications". In Japan and Germany, WES and SEW have developed welding rules for steel grades (or steel grades), such as WES3001 "High-strength steel welding" and SEW088 "Welding rules for weldable fine-grained structural steels". The German Welding Society also recommends The selection of welding materials for each steel grade is extremely convenient for manufacturing users.
3 Promotion of new technology
The advantages of microalloyed steel are obvious, and the problems of cold cracking and heat-affected zone embrittlement are solved. Since the post-weld cooling rate is lower than the cooling rate during rolling cooling due to large-line energy welding, softening occurs in HAZ, and conventional alloy strengthening Compared to softening, it becomes a new problem. Therefore, reasonable welding process measures are still the research content of welding workers, including the control of welding line energy, the matching of welding material strength and base metal.
With the advancement of welding technology, high-efficiency multi-wire submerged arc automatic welding, multi-wire gas shielded welding, cold wire welding and narrow gap welding have become the most practical welding methods. The main purpose is to improve efficiency and control welding heat influence. The high temperature residence time of the zone and the accelerated cooling rate.
The concept of precision manufacturing, the new welding method is accepted by the industry, among which the laser welding technology develops most rapidly, firstly applied to the automotive industry on a large scale, especially the introduction of Tailor Blank Welding. Welding has been promoted. Recently, laser hybrid arc welding has been started in China for the research of thick plates (mainly marine steel plates), which will provide a new way for the application of microalloyed steel.
The application of microalloying technology in the control of cold control technology provides sufficient conditions for the development of high-strength steel, improves the weldability of steel, and especially reduces the sensitivity of cold cracks, which simplifies the welding process to a certain extent. The manufacturing cost of the welding process is small. At the same time, microalloying also improves the overall performance of the weld heat affected zone, and the welding line energy is improved, enabling efficient welding. However, there are still many problems to be solved, especially high-efficiency, high-performance welding materials.