Compression technique of solidification end in continuous casting
1 Research Background
Extra-thick plate, large size/bar products are widely used in major projects and major equipment in important fields of ocean engineering, energy and power, national defense and military industry, which has important strategic significance and huge economic value. At present, there are more than 60 large-scale continuous casting production lines in China. However, as the bad section of continuous casting continues to widen and thicken, its internal cooling conditions deteriorate significantly, and the segregation, porosity and shrinkage hole solidification defects become more and more serious. Traditional low superheat casting and electromagnetic stirring technologies are not enough to solve this problem, which greatly restricts the improvement of product yield, production efficiency and service stability.
In view of this, the high efficiency continuous casting research team of Northeastern University based on the major needs of the country and the industry, in collaboration with domestic enterprises and design institutes, from the aspects of theory, process, equipment, etc., has developed a continuous, dynamic key process and equipment technology suitable for China's "front-line prolific", that is, the implementation of large deformation compression at the end of casting failure and after complete solidification. Make full use of the temperature difference of internal heat and external cold up to 500℃ to achieve efficient transfer of the reduction amount to the core of the casting billet, so as to fully improve the effect of segregation loosening and closing the solidification shrinkage hole, so as to fundamentally solve the technical problems of mass production of high-end ultra-thick plate/large-size bar by continuous casting billet.
2. The key technology and equipment of the solidification end pressing
2.1 "Where to press, how much to press" to press the core process
The deformation characteristics of the billet in the process of compaction are quite different from those of conventional continuous casting and rolling. With the increase of the reduction amount, the deformation, solidification heat transfer, microstructure growth and solute segregation of the billet become more and more complicated.
In order to solve the above problems, the deformation law of continuous casting failure under the comprehensive action of roll pressure, billet pulling force and thermal stress and the solute transport behavior law in the two-phase zone were systematically revealed. A new mechanism was proposed to inhibit the accelerated enrichment of solute under large deformation pressure at the end of solidification of continuous casting. It is found that the single pass critical reduction should be no less than 8.4mm, and the multi-pass reduction rate should be no less than 4mm/m to significantly improve the closure of the shrinkage hole in the core of the slab. It is clear that the temperature difference between inside and outside is the most critical factor affecting the transfer efficiency of the reduction amount to the core of the billet, and the best reduction area to ensure the efficient penetration of the reduction amount to the core of the billet is established within the solid phase ratio of 0.5 to 5.0m after the billet is completely solidified, which accurately answers the key process core question of "where to press and how much to press".
2.2 Stable implementation of large deformation reduction of the core equipment
Compared with the single-roll press-down equipment with fixed installation position, the sector section is continuously press-down on the continuous surface, and can be subject to press-down action within its press-down range, and can prevent the rebound deformation of the cast failure. However, the current press-down capacity of the sector section of the conventional continuous casting billet is far from the requirement of stable implementation of large deformation press-down, thus affecting the effective implementation of the press-down technology at the solidification end.
To solve the above problems, an enhanced compact segment of wide and thick plate continuous casting was developed and applied, which has the strongest depressingability in the world. The crushing force was increased by 4 times compared with the conventional segment. For the first time, the depressingability of a single segment was 18mm and the total depressingability of multiple segments was 40mm at the solidified end and after complete solidification of wide and thick plate continuous casting billet. Developed and formed the gradient curvature convex roll which greatly reduces the pressure resistance and efficiently squeezes the bad core of the die casting. The pressure reduction per roll is increased by 3 times, and the life of the casting roll is increased by more than 5 times. On this basis, the self-monitoring system of sector segment and double feedback hydraulic control system are developed, which realizes the high-precision control of roll gap and pressure under heavy load and strong impact, and provides important equipment support for the "true detection" of solidification process based on pressure reduction feedback.
2.3 High precision online calibration technology of solidification end position and morphology
In the process of continuous casting, the position of the solidifying end of the cast blank changes with the change of the process parameters such as composition, drawing speed and superheat, so the accurate prediction of the position and shape of the solidifying end is the first prerequisite to determine the rational process.
To solve the above problems, a high-precision online detection technology based on the combination of soft measurement and true detection was developed and formed. The technology accurately established the thermophysical parameter distribution of the cast fault section in the whole solidification process based on the evolution law of the solute distribution, breaking through the technical limitations of the existing soft measurement method (heat transfer calculation) which used the whole whole section to fix the thermophysical parameter. Based on the measured thickness of solidified billet shell and thermal/mechanical analysis, an accurate relationship of "pressure - reduction amount - thickness of billet shell" was established to solve the problem that the drift of solidified end could not be accurately predicted due to factors such as deformation under pressure and solute migration. Based on the accurate prediction of the position of the solidification end and the formation characteristics of center segregation and porosity in the solidification process of continuous casting billet, a targeted process solution was proposed, that is, the two-stage continuous heavy pressure process according to the solidification process was divided, and finally the uniformity and density of casting failure were improved synchronously.
2.4 Risk prediction technology of crack initiation and propagation during the process of compression
Although the solidification end compression technology can effectively improve the segregation, center porosity, shrinkage and other defects, it increases the risk of crack occurrence to a certain extent. If the segregation, porosity and shrinkage can be effectively improved at the same time, the crack defects can be effectively controlled, and the quality of large section continuous casting billet can be further improved and the production of high-quality steel can be continuously promoted.
To solve this problem, a new method for determining the critical strain of intermediate crack initiation and corner crack propagation is proposed. The critical strain of surface crack propagation is determined by high temperature tensile test. Based on the thermal/mechanical coupling model of the whole process of continuous casting, the recrystallization process under coupled heavy pressure can narrow the surface structure and improve the thermoplastic factor, and the risk of crack initiation and propagation during the process under heavy pressure can be quantitatively predicted. The risk prevention and control of intermediate crack initiation in the medium and low solid phase zone is clarified when the slab compression rate is not more than 3mm/m and the single roll compression amount of the generous slab is not more than 5mm. The risk control criterion of surface crack propagation with a single pass reduction of no more than 15 mm ensures the safe and stable implementation under heavy pressure.
3 Application Effect
3.1 Construction of Bentang special steel billet continuous casting production line
In the upgrading process of Bensteel special steel, Northeastern University and MCC South jointly designed two billet continuous casting machines, of which 430mmx510mm section large square continuous casting machine (No. 1 machine) is the first independent design of China using flat roller continuous solidification end of the heavy pressure of the billet continuous casting machine, its single frame maximum reduction of 15mm, the total reduction of up to 40mm. The two billet continuous casting machines of Bensteel Special steel have adopted the process control system of cutting down and dynamic secondary cooling of billet solidification end independently developed by Northeastern University. It has the functions of high precision solidification process prediction considering the influence of solute and temperature evolution on thermophysical parameters, one-key inverse calculation of secondary cooling water table based on the target temperature of the exit of each secondary cooling zone, equipment monitoring diagnosis based on the real-time data of casting machine operation data and casting billet quality determination. After the casting machine is put into operation, it runs smoothly, and the proportion of the casting center loose and the segregation is not more than 0.5 grade is more than 95%, which is in the international leading level, and has formed the φ150mm large-size high-end bearing bar supply capacity, the production of large-size bearing steel, gear steel and other excellent quality, has been successfully applied to wind power generation, high-speed rail cars and other major equipment. It has effectively promoted the structural adjustment and upgrading of the special steel products of Bensteel.
3.2 Xiangsteel 450mm thick extra-thick slab continuous casting machine process and control system overall upgrade
For the 450mm thick slab continuous casting machine (No. 10 machine) of Xianggang, the solidification end pressing and secondary cooling process and the overall upgrade of the control system were carried out. System design and development of different section specifications, steel grades related to solidification end compression and cooling process, the application of digital twin-based process control system, the existing manual/static control to dynamic control, to achieve the whole pouring process automatic operation and casting machine equipment status monitoring and evaluation. A series of problems such as abnormal locking of joints, failure of tail to adjust the reduction strategy, abnormal alarm of tension straightening current, and thickness fluctuation of casting billet are solved in the unsteady casting process, and the application rate of the reduction process is greatly improved. At present, the process control system has been stably put into use in multiple sections such as 450mm and 350mm, and the quality of the production billet has increased significantly. The proportion of center segregation class C 1.5 in low alloy, offshore ships, buildings, Bridges, wind power, pressure vessels and other varieties is not less than 90%, and the proportion of center segregation class C 1.0 in high alloy steel such as die steel and wear-resistant steel is not less than 80%. It solves the serious bottleneck of center segregation and porosity of high-end large-section continuous casting billet, basically eliminates the middle crack defect of low-alloy steel thick slab casting billet, and achieves 98.5% pass rate of inspection of thick slab over 100mm. Our products have been successfully used in LNG/VLGC carriers, large container ships, YAMAL project in Russia, ZAWTIKAL Project in Thailand and other major projects.
3.3 Jiyuan Iron and Steel medium/square billet continuous casting machine
Jiyuan Iron and Steel is an important special steel production base in the Central Plains of China. In order to further improve the internal quality of the billet, the patent implementation transformation was carried out in 5 billet continuous casting machines of Jiyuan Iron and Steel, among which the solidification end pressing technology was implemented in the large square billet continuous casting machines (No. 1, No. 5, No. 6). The central carbon segregation ratio of gear steel and bearing steel continuous casting billet with 400mmx500mm section (No. 5 machine) increased from 50% to 90%, and the pass rate of central carbon segregation of gear steel continuous casting billet with 320mmx360mm section (No. 6 machine) increased from 66% to 92%. In addition, the key process simulation system of continuous casting based on digital twins was constructed to realize the rapid simulation and simulation design of key processes such as secondary cooling and compression of all continuous casting production lines. The collaborative use of secondary cooling and electromagnetic stirring processes realized that the center porosity of all casting lines was no more than 1.5, the center shrinkage hole was no more than 1.5, and the middle crack was no more than 1.0. Help enterprises upgrade and adjust the product structure. At present, the high-end bars produced are widely used in equipment manufacturing, rail transit, new energy and other industries.
3.4 Ansteel Bayuquan Steel Branch 300 mm thick slab continuous casting machine
The technology was applied in 300mm thick and wide thick slab continuous casting machine (No. 3 machine) of Ansteel Bayuquan Company. On the basis of fully considering the non-uniform solidification characteristics of wide and thick slabs and the continuous compression characteristics of the sector segment, the gradient curvature convex roll (FIG. 1) suitable for the non-uniform solidification characteristics of wide and thick slabs is designed and applied for the first time. On the basis of not upgrading the main equipment of the casting machine such as hydraulic, mechanical and transmission, etc., The breakthrough of 7.4mm reduction in a single stage (no more than 3mm before implementation) was achieved, which greatly improved the segregation and porosity defects at the solidification end. After the technology is fully put into use, the center segregation grade C rate of high-end thick plate products such as pipeline steel and die steel is increased from less than 50% to 75%, and the center porosity is not more than 0.5 grade to 100%, eliminating the intermediate crack defects, and greatly improving the quality and yield of casting billet and rolled material (Figure 2). The production of large-wall thickness pipeline steel is of excellent quality, which can meet the needs of complex service environment, and has been applied to major national projects such as the China-Russia East Route and West-east gas transmission, effectively enhancing the core competitiveness of Angang products.
15358968703