Ten years ago, due to the constraints of production scale and technological development, the domestic continuous cold-rolled stainless steel production line has been using traditional argon arc welding machines, but the argon arc welding machine has welding defects, which seriously affects the production capacity of the cold-rolled stainless steel continuous production line. China Metallurgical South Engineering Technology Co., Ltd. developed the first domestic stainless steel continuous rolling mill laser welding machine, which has been successfully applied in many large stainless steel cold rolling plants at home and abroad, which has effectively promoted the development of my country’s laser welding machine high-end equipment manufacturing industry.
With the advancement of science and technology and the development of the national economy, stainless steel is widely used in chemical, food, medicine, aviation, petroleum, metallurgy, printing and dyeing, automotive and other industries due to its corrosion resistance and appearance decoration. At present, the following continuous production lines are mainly used to produce cold-rolled stainless steel: continuous pickling line (CPL), continuous rolling line (TCM), continuous annealing and pickling line (HAPL/APL), continuous rolling annealing and pickling line (HRAPL/DRAPL) ) Etc. to improve production efficiency and reduce production costs. Before 2010, for stainless steel with a thickness greater than 0.8mm, the welding machines on the above-mentioned continuous production lines used argon arc welding, but they all had defects such as undercuts, pores and cracks, which seriously affected the production line’s capacity and yield. Laser welding has been widely used in cold-rolled stainless steel continuous production lines in recent years due to its small heat-affected zone, high welding speed, and low strip breakage rate through the rolling mill. It has almost completely replaced argon arc welding.
Stainless steel welding process
- Comparison of welding processes
The comparison of the main welding processes of stainless steel is shown in Table 1. Through analysis and comparison, it can be known that laser welding or laser-arc hybrid welding has significant advantages in welding quality, efficiency and stability.
- Stainless steel laser welding process
Compared with ferritic stainless steel and martensitic stainless steel, austenitic stainless steel has better welding performance and can adapt to fusion welding. The welded joint has good plasticity and toughness in the welded state. Laser welding has high speed and low heat input, making it suitable for welding austenitic stainless steel. Austenitic stainless steel uses laser welding or laser-arc welding. When austenitic welding materials are used, preheating and post-weld heat treatment are not required.
For ferritic stainless steel, the laser welded seam structure is coarse ferrite structure or a small amount of martensite structure is formed at the ferrite grain boundary. Because there is no or very little austenite at high temperature, the grains grow violently when cooling in the solidification temperature range, so the coarse grain embrittlement phenomenon is more serious. Ferritic stainless steel uses a laser-arc welding process. When austenitic welding materials are used, preheating is generally not required, and heat treatment is required after welding. By using austenitic stainless steel filler metal and adjusting process parameters, a large amount of austenite can be obtained in ferritic stainless steel welds. At high temperatures, the presence of austenite will strictly limit the growth of ferrite grains, thereby improving weld toughness. In addition, the post-weld heat treatment changes the distribution of precipitates in the fusion zone and the heat-affected zone and reduces the welding stress. At the same time, tempering a small amount of martensite in the weld can further improve the weld toughness.
For martensitic stainless steel, in general thermo-mechanical processing conditions, even the cooling rate in still air is sufficient to undergo allotropic transformation, and a large amount of martensite is formed from austenite. Therefore, laser welding produces brittle welds dominated by martensite, which will significantly reduce weld ductility and toughness. Martensitic stainless steel uses laser-arc welding technology. When using austenitic welding materials, preheating is required before welding, and it cannot be annealed immediately after welding. After cooling to a certain temperature, the weld is heat treated to make the weld free from its brittleness Area, thereby improving the plasticity of the weld.
Characteristics of Laser Welding Technology of MCC Southern
The first laser welding machine developed by MCC South Engineering Technology Co., Ltd. (hereinafter referred to as MCC South) in 2013 was successfully applied to the six-stand stainless steel continuous rolling mill of Qingshan Group, and it was also the first domestic laser welding machine applied on the domestic stainless steel continuous production line. , Can adapt to welding of different types of stainless steel, its main technical characteristics are as follows.
1) Using solid-state laser instead of gas laser, laser transmission through optical fiber, low cost of welding machine use and maintenance.
2) The laser welding machine has welding modes such as laser welding and laser-arc hybrid welding, which maximizes the reliability and adaptability of the welding machine.
3) A number of key equipment technologies are used to improve the accuracy and stability of the whole machine, while being convenient for use and maintenance.
MCC Southern Laser Welding Machine Application
The laser welder developed by MCC South has advanced welding technology, stable weld quality, low operating cost, simple maintenance and low equipment investment. Up to now, it has been successfully applied to a total of 15 large steel companies such as Baosteel, Taiyuan Iron and Steel, Tsingshan Group, Maanshan Iron and Steel, and Jiuquan Iron and Steel, including 5 carbon steel laser welding machines, 3 silicon steel laser welding machines, and 7 stainless steel laser welding machines , In the field of high magnetic induction silicon steel and martensitic stainless steel materials, the localized market share of laser welding machines is 100%.
- The existing argon arc welding machine is transformed into a laser welding machine
The Rolling Annealing and Pickling Line (DRAPL) of Baosteel Desheng Stainless Steel Co., Ltd. imported an argon arc welder from an American company. Due to the low strength of the argon arc welding seam and the excessively high residual height, the strip breakage rate during over-weld rolling is high. , Rolls are easily scratched, etc., during production, the rolling force is reduced or the roll gap is opened to empty, so that the thickness of the strip before and after the welding line after the continuous rolling mill is out of tolerance, and the machine composition rate is reduced. The imported argon arc welding machine uses rolling shears to cut strip steel, and the gap between the single-side shear blades of the roller shears is as high as 0.15mm, which cannot meet the requirements of laser welding. If the roller shears are replaced with high-precision double-cut shears, the base and the welding machine body equipment on the base must be removed, which requires a large investment and a long construction period. According to the current status of the welding machine’s equipment, MCC Nanfang has analyzed and studied the laser welding process plan, conducted a large number of welding tests and rolling tests to verify the feasibility of the transformation plan, and finally determined the use of laser-arc hybrid welding process (see Figure 1 ), to retain the welding machine’s rolling shear and other main equipment, and not to make a major change to the existing argon arc welding machine.
The weld strength of the modified welder is greater than 85% of the base metal, and the weld reinforcement is less than 10% of the base metal thickness. The weld is directly rolled by a four-stand 18-high continuous rolling mill without damaging the rolls and rubber lining rollers. , The material ratio of the machine can reach over 99.6%, which greatly improves the production efficiency of the unit.
- Chromium steel continuous pickling line laser welding machine
The chromium steel continuous pickling unit (CPL) is the first domestically produced unit of JISCO Stainless Steel Branch, which specializes in the production of ferritic and martensitic stainless steel, with a strip thickness of 2-6mm, a width of 750-1600mm, and an annual output of 400,000 units. Ton. MCC South aims to use conventional welding processes (argon arc welding or pure laser welding) to weld ferritic and martensitic stainless steels in the heat-affected zone of welds with poor toughness, serious weld embrittlement, and welds prone to fracture, etc. , The laser-arc hybrid welding process is used on this welding machine (see Figure 2).
Since the welding machine was put into production, it has strong welding adaptability, reliable welding seam quality, low strip-breaking rate, and the equipment has been running stably. The overall performance of the welding machine has obviously exceeded that of imported welding machines on other production lines of JISCO Stainless Steel Branch. It solved the long-standing bottleneck problem of poor weld performance in the martensitic stainless steel produced by JISCO, and won praise from users.
During the functional assessment, the ultimate welding ability of the laser welder was also tested. Due to the extremely thick head and tail of the hot-rolled raw material, the welder welded ferritic stainless steel 430 with a thickness of 7.3mm and martensitic stainless steel 30Cr13 with a thickness of 6.5mm. These specifications are beyond the design range, and the welds are uniform and full. And passed the bending test. This fully reflects that the MCC Southern Laser Welding Machine has completely reached and exceeded the design capability in welding steel grades and thickness specifications.
- Continuous rolling annealing and pickling line laser welding machine
Tsingshan Group India CHROMENI Stainless Steel Cold Rolling Phase I Project Five-stand 18-high rolling annealing and pickling line (DRAPL) has a total length of 700m and a designed annual output of 700,000 tons. The unit includes: uncoiling section, five-stand 18-high continuous Rolling mill, alkaline washing section, annealing section, pickling section, leveling section, inspection section, coiling section. The exit speed of the continuous rolling mill is max.300m/min, and the tension is max.550kN. The laser welding machine is installed on the entrance side, and the welding seam has to undergo high-tension rolling, high-temperature annealing, and mixed acid pickling. Therefore, the reliability of the welding seam is extremely high. If the welding seam breaks on the production line, it will seriously affect the production line. Capacity. This unit produces austenitic stainless steel with a strip thickness of 2-4.5mm and a width of 1000-1300mm. The laser welding machine of MCC South is equipped with a 6kW disc laser and adopts the welding process of laser welding. After the welding machine is put into production, it runs stably and reliably. The welding seam smoothly passes through the online five-stand 18-high continuous rolling mill, and the strip steel yield rate of the production line reaches 99.8% or more.