With the development of the iron and steel industry, the requirements for the surface quality, dimensional accuracy and flatness of the plate are getting higher and higher. In order to further improve the quality grade of plates and continuously reduce production costs, iron and steel enterprises put forward higher requirements for the wear resistance of the post-finishing products, the ability to maintain surface roughness and good anti-accident ability.
Especially after the high-speed steel rolls are widely promoted, the wear resistance of the rolls in the front stage of finishing rolling has been greatly improved. However, because there is no graphite in the structure of high-speed steel, it is difficult to be successfully applied in the post-finishing stage, especially on the finished frame. Therefore, ICDP rolls have become It is a key factor that seriously restricts the improvement of the production rhythm of the rolling mill.
The post-rolling material is in urgent need of a revolutionary upgrade. In response to this problem, Chinese companies have successfully developed high-carbon high-speed steel rolls to replace ICDP rolls. They can be used directly on the machine without adjustments to the rolling mill. They have wear resistance, accident resistance and good surface quality. It can completely replace the traditional ICDP rolls and become the mainstream material for the work rolls at the end of finishing rolling in the future.
1. Proprietary production technology for high-carbon high-speed steel rolls
1.1 Graphite control technology
Graphite is an important factor for ensuring the surface lubrication of the post-finishing rollers of the hot strip tandem rolling mill, improving the quality of the board surface, and improving the accident resistance. Due to the addition of a large number of alloy elements in high-carbon high-speed steel, the material has an increased anti-graphitization tendency. How to ensure the normal precipitation of graphite is the key to the production of high-carbon high-speed steel rolls. Through a large number of basic tests, Sinosteel Xingji has successfully developed a proprietary new technology for outer layer melting and inoculation, which ensures the uniform precipitation of graphite in the working layer of high-carbon high-speed steel rolls. Within the scope of the working layer, the graphite content fluctuates within 0.2%, as shown in Figure 1.
Figure 1: Graphite content change/%
1.2 Carbide control technology
In order to solve the phenomenon of carbide segregation in the working layer of the roll caused by the segregation of carbides of different densities to the outer or inner layer under the action of centrifugal force, Sinosteel Xingji has gone through several scientific researches, through the rationalization of alloy matching and Appropriate addition of trace elements forms composite MC, M2C, M3C, and M6C carbides composed of various alloys that are closer to the liquid phase density, which solves the problem of carbide segregation, and the content and type of carbides in the working layer tend to be consistent, as shown in Figure 2.
Figure 2: Change of carbide content/%
1.3 Organizational control techniques
Due to the solid solution of more alloys in the matrix of high-carbon high-speed steel rolls, the stability of austenite is enhanced. In order to avoid the loss of wear resistance caused by excessive residual austenite and reduce the casting stress, Sinosteel Xingji passed a series of re-optimization practices of tempering temperature, time and heating and cooling speed have explored a perfect multi-stage tempering process and realized the stable control of the working layer structure.
2. Characteristics of high-carbon high-speed steel rolls
2.1 Hardness consistency of the working layer
The content of graphite and carbide in the working layer of high-carbon high-speed steel rolls has little change, the structure control is stable and consistent, and the hardness drop in the working layer is small. The ICDP and high-carbon high-speed steel rolls were produced with the same specifications and sizes, and the anatomical hardness test in the working layer was carried out. The hardness drop of the high-carbon high-speed steel rolls within a radius of 50mm can be controlled to about 2HS, which is significantly improved compared with ICDP rolls. Good hardness drop makes high-carbon high-speed steel rolls more stable in rolling performance throughout their service life.
2.2 Organizational Characteristics
As shown in the 50× metallographic photo in Figure 3, graphite particles are dispersed in the high-carbon high-speed steel roll, and the graphite particles are more uniform and finer than the ICDP roll. On the one hand, graphite particles can effectively relieve the stress at the tip of the crack, reduce the tendency of crack expansion, and improve the anti-accident performance of the roll. On the other hand, graphite particles on the surface of the roll can fall off between the roll and the steel plate during rolling, playing a good role in lubrication Improve surface quality.
Figure 3: Graphite, Carbide and Microstructure of High Carbon High-Speed Steel
High-carbon high-speed steel rolls use M3C-type eutectic carbides as the skeleton, and a large number of MC-type granular carbides are dispersed in the matrix. At the same time, there are also compound carbides such as M2C and M6C that only exist in high-speed steel rolls before finishing rolling. thing. MC, M2C, and M6C carbides with high microhardness are important guarantees for the improvement of roll wear resistance.
In the laboratory, the LECIA-QWIN analysis software is used to measure the area size and quantity of carbides in the high-carbon high-speed steel structure. In the area of 0.35mm2, the measured results are shown in the table. It can be seen that the high-carbon high-speed steel roll of about 93% of the carbide area size is less than 5μm2, the number of carbide particles is large, and the smallest particle area reaches 0.06μm2.
The size of eutectic carbides in high-carbon high-speed steel rolls is smaller than that of ICDP rolls, and the network carbides are disconnected, which reduces the tendency of cracks to expand rapidly along the edge of carbides and matrix, and further improves the accident resistance of rolls.
Table 1: Measurement of carbide size and quantity in high-carbon high-speed steel
| Carbide amount | Less than 5μ m² (%) | Less than 10μ m² (%) | Less than 50μ m² (%) | Minimum area/μ m² | Maximum area/μ m² |
| 7465 | 93.06 | 95.54 | 98.53 | 0.06 | 12097.47 |
There is a martensite structure in the matrix structure of the high-carbon high-speed steel roll. Compared with the bainite matrix of the traditional ICDP roll, the hardness of the matrix is greatly improved, which is close to the level of the high-speed steel in the pre-finish rolling stage. The high-hardness matrix can provide effective support for carbides, and better exert the wear resistance of carbides, and the martensitic matrix has higher strength and stronger wrapping effect on carbide particles, which can avoid particle carbonization during rolling The shedding of objects will affect the performance of wear resistance.
2.3 Wear resistance test
The actual production environment of rolling and sliding friction during steel rolling was simulated in the laboratory, and the relative wear resistance of high-carbon high-speed steel and ICDP rolls were tested under the same material and stroke. The test results showed that high-carbon high-speed steel The relative wear resistance of the roll is more than 2 times that of the ICDP roll.
2.4 Cold and heat fatigue test
In the laboratory, the same heating temperature, heating time, cooling time and fixed cycle times are used to carry out the thermal fatigue test. The crack depth of the final sample shows that the crack depth of the high-carbon high-speed steel is significantly lower than that of the ICDP roll, indicating that the roll is resistant to Accident performance enhancements.
3. Use of high carbon high-speed steel
The 4 high-carbon high-speed steel rolls produced by a Chinese steel mill were used in a domestic steel mill, and they were used normally without any adjustments to the rolling mill.
The average millimetric rolling volume of high-carbon high-speed steel products is 5841.23t/mm, which is 96% higher than the 2976.51 t/mm of ICDP material, which is nearly twice that of ICDP. More importantly, the use of high-carbon high-speed steel solves the long-term problem of roller surface colour difference that has plagued users, and the surface quality is greatly improved.
4. Conclusion
(1) The working layer of high-carbon high-speed steel has high graphite, carbide, and organizational consistency, and the hardness drop of the working layer is small so that the performance of the roll is more stable throughout the service life.
(2) High-carbon high-speed steel roll graphite and granular carbides are uniformly and finely dispersed, carbide types are diversified, and the matrix strength and hardness are higher, making high-carbon high-speed steel rolls have better wear resistance and accident resistance than ICDP rolls properties and surface quality.
3) The high-carbon high-speed steel rolls can be used directly on the machine without adjustment of the rolling mill. The actual rolling shows that the mm rolling volume of the high-carbon high-speed steel rolls has reached nearly twice that of the ICDP rolls, and the surface quality has been greatly improved after being off the machine. High-carbon high-speed steel rolls will surely become the mainstream material for the post-finishing rolls of the hot strip tandem mill in the future.
