Development and integrated application of key technologies for the efficient and green interface between large slab continuous casting and steel rolling
In the production process of micro-alloyed steel, surface quality defects of cast slabs and strips are common in domestic and foreign iron and steel enterprises, even the world’s major advanced iron and steel enterprises are no exception. Long before the establishment of the project, the chamfering mould technology developed by the main member units of the project had made a breakthrough in the defect-free production technology of the micro-alloyed steel strip continuous casting slab surface and has been widely promoted and applied in domestic production enterprises. good economic and social benefits. However, some key technical problems that restrict the efficient production of micro-alloyed steel continuous casting-rolling process still exist. It is mainly manifested in the following aspects:
1) No engineering application technology has been formed in the control of straight cracks or peeling defects on the edge of the plate and strip, which causes about 3% of the trimming loss.
2) Due to the existence of red wire cracks, micro-alloyed steel continuous casting slabs are still in the stage of cold charging or palletizing warm charging below 550 degrees Celsius, which restricts the realization of high-temperature hot charging of microalloyed steel continuous casting slabs.
3) In strip steel production enterprises, the early large-chamfer crystallizer technology met the casting speed requirements of micro-alloyed steel slabs, but the maximum working speed of narrow-section low-carbon steel and ultra-low carbon steel was only 1.5 meters per minute, the longitudinal cracking defect at the billet corner caused by the higher casting machine pulling speed has not been resolved, which limits the application range of the technical achievements of the project in iron and steel enterprises.
4) Although the application of chamfering crystallizer technology has solved the problem of transverse cracks at the corners of continuous casting slabs of micro-alloyed strip steel, the defects of transverse cracks at the corners of wide and thick plates with strong crack sensitivity need to be improved.
5) The high-efficiency and green improvement technology of the casting-rolling interface needs to be further improved.
Relying on the national key research and development plan “Steel process casting-rolling interface material flow and energy flow collaborative optimization and intelligent control technology”, the General Iron and Steel Research Institute, Shougang Group, Hesteel Group Handan Iron and Steel Company, Shanxi Taigang Stainless Steel Co., Ltd., Shanxi Iron and Steel Co., Ltd. Group Laiwu Steel Yinshan Section Steel Co., Ltd., Shougang Jingtang, and Zhongda National Engineering Research Center for Continuous Casting Technology conducted in-depth cooperative research in order to achieve breakthroughs in key technologies for the production of micro-alloyed steel strips and take the lead in building large slabs in China Continuous casting-steel rolling interface efficient, green key technology integrated application demonstration production line, and carry out engineering promotion to iron and steel enterprises, so that the continuous casting billet truly becomes the carrier of material flow, energy flow and information flow, and is directly transported to the next step of rolling. The casting-rolling interface has been completely opened up and smoothed, laying a solid foundation for the high-efficiency, green and high-quality production of the next process.
Problem-solving ideas and technical solutions
In view of the actual situation of high energy consumption, low yield and low production efficiency in the production of micro-alloyed steel strips at home and abroad, this project is based on the typical production process of domestic large-scale iron and steel enterprises and focuses on edge cracking control technology, red Send crack control technology, micro-alloyed steel continuous casting slab surface defect-free production technology and other research work, develop key technology and equipment control technology, realize the integration and industrial application of key technology, and promote it to domestic and foreign metallurgical enterprises.
Main innovative achievements
Innovation point 1: Formation mechanism and control of straight edge cracking of plate and strip
Through thermal simulation and industrial tests, the project team revealed the formation mechanism of the straight-edge cracking of the plate and strip. Strip edge cracks occur after the second pass of the finish rolling process. The temperature at the corner of the rolled piece enters the two-phase region, resulting in inconsistent deformation and folds at the corner. As the rolling process progresses, this fold turns over to Straight cracks formed on the surface of the strip; the straight cracks of the wide and thick plates are caused by the folds caused by the initial widening and rolling.
The development of continuous casting slabs with secondary chamfer shapes can increase the temperature of the corners of the slab and maintain the uniformity of the temperature of the slab. The maximum value of the right angle billet is reduced from 0.32 to 0.04, which greatly reduces the probability of side folding.
For this reason, the project team pioneered the patented technology of billet corner shape optimization equipment in the world. The equipment is placed at the exit of the casting machine. It makes full use of the high temperature of the casting slab and the transmission energy of the casting machine to optimize the shape of the corner of the single-chamfering continuous casting slab online.
As a result of this project, the incidence of edgeless straight cracks has increased from 30% to more than 95%, and the locations of edge straight cracks have all been reduced to within 10 mm from the edge. It can increase the yield of plate and strip by more than 1%.
Innovation point 2: Formation mechanism and control of micro-alloyed steel hot delivery cracks
A breakthrough has been made in the study of the formation mechanism of red steel cracks in micro-alloyed steels. It is proposed that the precipitation and solid solution of the second phase particles along the grain boundary ferrite film are the reasons for grain boundary embrittlement and red steel cracks. In order to suppress the occurrence of hot-carrying cracks and achieve high-temperature direct loading, the project team has developed a dynamic rapid cooling control technology based on billet temperature detection and real-time feedback, and a dual-station billet surface layer rapid cooling technology and equipment, which has improved the overall quality of cast billets. The hot charging ratio and hot charging temperature; at the same time meet the needs of continuous casting efficient production, energy saving and environmental protection.
The results of this project have shortened the time required for casting slabs to enter the heating furnace from 8 hours for stacking and warming to 30-45 minutes, and the average temperature of the cross-section of slabs has increased from 500 degrees Celsius to more than 750 degrees Celsius. Relying on the results of this project, the comprehensive hot charging rate of continuous casting slabs has reached 95%, the direct charging rate has reached more than 85%, and the fuel consumption of heating furnaces has been reduced by 6.9 kg standard coal per ton.
Innovation point 3: micro-alloyed steel surface defect-free and high-speed continuous casting production technology
The project research found that the corner of the chamfered continuous casting slab is an essential high-temperature zone, the slab shell at the exit of the crystallizer has an obvious return temperature, and the length of the return temperature interval is more than 300 mm; with the increase of the casting speed, the return temperature situation When the casting speed increases from 1.2 m/min to 1.8 m/min, the corner stress of the slab will increase by 3 times; the sudden “renewal of the slag film” at the lower part of the crystallizer weakens the support of the slab shell. These are the root causes of longitudinal cracks at the corners of chamfered slabs.
In this regard, the project team developed for the first time the continuously variable cross-section chamfering crystallizer technology, the taper dynamic control technology based on heat flow monitoring, and the combined structure side support foot roll equipment technology, etc., to achieve high casting speed of large chamfered continuous casting slabs Production, effectively avoiding the longitudinal cracks at the corners of the slab and the transverse cracks at the corners of the crack-sensitive wide and thick slabs.
The results of this project have reduced the corner stress of the billet at the outlet of the mould by more than 80%. While meeting the control of transverse cracks at the corners of crack-sensitive steels, the working speed of low-carbon steel has reached 1.7 m/min, the highest 1.8 m/min, and at the same time, the defect-free rate on the surface of the slab reaches more than 99.6%.
Green manufacturing of plates and strips
Integrated application of key processes and equipment technologies
The project integrates key technologies such as strip edge straight crack control, hot delivery crack control, and slab surface defect-free production to the same continuous casting production line. Intelligent control software and technical processes such as wide, quick-change tundish, secondary cooling dynamic control, soft reduction at the end of solidification, etc., built a function-enhanced large slab continuous casting machine and realized the green, The engineering application of high-efficiency manufacturing technology has completely opened up and smoothed the casting-rolling interface, providing a guarantee for iron and steel enterprises to reduce production costs, save energy and reduce emissions.
Intellectual property and patent situation
The development of this project has obtained 33 authorized patents, including 14 invention patents and 19 utility model patents. Among them, there are 10 invention patents in China, 4 international invention patents in the United States and Italy, and 6 utility model patents in Germany and Austria. Obtaining the international patent authorization provides a guarantee for the project’s achievements to open up the international market.
Industrial application and economic benefits
The results of this project have been integrated into the large slab continuous casting production lines of Handan Iron and Steel, Shougang Jingtang, Anshan Iron and Steel, Laiwu Steel and other enterprises, and established industrial application demonstration production lines, which have achieved good results and attracted extensive attention from relevant domestic and foreign enterprises.
In September 2015, the General Institute of Iron and Steel Research cooperated with Handan Iron and Steel Co., Ltd. to carry out key technologies such as the control technology of medium-thick slab edge straight cracks and micro-alloyed steel red feed crack control technology on the large slab continuous casting machine of the third steelmaking plant of Handan Iron and Steel Co., Ltd. Research work on single-body technology, and at the end of 2017, a high-efficiency medium-thick plate integrated with key processes and equipment technologies such as micro-alloyed steel continuous casting slab surface defect-free production technology, edge straight crack control technology, and red feed crack control technology Chemical and green manufacturing demonstration production line.
In June 2017, the General Institute of Iron and Steel Research and Laigang Yinshan Section Steel Co., Ltd. carried out a cooperative research on “Development of Wide and Thick Plate Edge Straight Crack Control Technology”. At the end of the year, a high-efficiency and green manufacturing demonstration production line for wide and thick plates will be built integrating key processes and equipment technologies such as micro-alloyed steel continuous casting slab surface defect-free production technology, edge straight crack control technology, and red feed crack control technology. From 2019 to 2021, the total economic benefits generated by the two companies will be 180.6895 million yuan, an average of 60.2265 million yuan per year.
In addition, the results of the project have begun to be promoted to key domestic production enterprises such as Liuzhou Iron and Steel Co., Ltd., Baotou Steel, Yingkou Medium Plate Factory, and Xinjiang Bayi Iron and Steel Co., Ltd.
Social benefits and promotion prospects
According to statistics, in 2020, the domestic output of medium and thick plates and strips will be 261.77 million tons, of which the output of micro-alloyed steel will be close to 100 million tons. It can be seen that the promotion of the project results has a broad market prospect.
The results of this project can realize the transformation of micro-alloyed steel billets from cold charging to high-temperature rapid cooling and direct charging, thereby reducing heating energy consumption by 16 kg standard coal/ton steel and reducing carbon dioxide emissions by 43.2 kg/ton steel. If the project results are promoted to half of the domestic microalloyed steel market, that is 50 million tons, 800,000 tons of standard coal can be saved each year, and 2.16 million tons of carbon dioxide emissions can be reduced, with significant social and economic benefits.