High-carbon high-speed steel rolls in the final stage of hot-rolled strip finishing

1. Introduction

ICDP rolls came out in the 1930s and were mainly used as work rolls for hot strip rolling. In the 1950s, they were promoted to work rolls for medium and heavy plate rolling mills. Because it contains more alloy elements, it is generally not produced by integral casting. Due to equipment limitations, early ICDP rolls were produced using reverberatory furnace melting and full flushing composite methods. In the 1970s, Sinosteel Xingji designed and manufactured horizontal centrifuges by himself and put them into use. By applying power frequency induction furnace smelting equipment, the quality of centrifugal composite ICDP rolls was qualitatively improved. In the 1990s, Sinosteel Xingji Machinery developed an improved ICDP roll based on the ordinary ICDP roll. By adding special carbide-forming elements and increasing high microhardness wear-resistant particles in the structure, the roll’s wear resistance and shape were improved. The retention capacity is enhanced, and the comprehensive use effect is increased by more than 10%. The products are widely used in steel companies such as Baosteel, Shougang, Anshan Iron and Steel, Wuhan Iron and Steel, Taiwan’s China Steel, Europe’s Corus, South Korea’s Posco and other steel companies.

With the development of the steel industry, the requirements for plate surface quality, dimensional accuracy, and plate flatness are getting higher and higher. To further improve the quality level of plates and continue to reduce production costs, steel companies have put forward higher demands for wear resistance, surface roughness maintenance and good accident resistance of the products in the final post-finishing rolling process. In particular, high-speed steel rolls are widely promoted After that, the wear resistance of the rolls in the early stage of finishing rolling was greatly improved. However, because there is no graphite in the structure of high-speed steel, it isn’t easy to successfully apply it in the latter stage of finishing rolling, especially on the finished product stand. Therefore, ICDP rolls have become a serious restriction on the improvement of the rolling mill production rhythm—the key factor. The material in the back section of the finish rolling is in urgent need of a revolutionary upgrade. In response to this problem, China Steel Xingji Machinery Co., Ltd. has actively carried out project research in recent years and successfully developed high-carbon high-speed steel rolls to replace ICDP rolls. It can be directly used on the machine without making adjustments to the rolling mill. It also has wear resistance. , accident resistance and good surface quality, it has been able to completely replace the traditional ICDP roll and become the mainstream material for work rolls in the later stages of finishing rolling in the future.

 

2. Proprietary production technology of high-carbon high-speed steel rolls

 

2.1 Graphite control technology

Graphite is an important factor in ensuring surface lubrication of the rollers in the final finishing rolling section of the tropical tandem rolling mill, improving plate surface quality, and improving accident resistance. Due to the addition of a large number of alloying elements, the tendency of anti-graphitization of high-carbon high-speed steel materials increases. How 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 incubation, 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%.

 

2.2 Carbide control technology

In order to solve the problem that under the action of centrifugal force, carbide points of different densities segregate to the outer or inner layer, causing carbide segregation in the working layer of the roll, Sinosteel Xingji Machinery Co., Ltd. has gone through several scientific research and research, and through the rational matching of alloys and Appropriate addition of trace elements form composite MC, M2C, M3C, M6C carbides composed of various alloys that are closer to the liquid phase density, solving the problem of carbide segregation, and the carbide content and type in the working layer tend to be consistent.

 

2.3 Organizational control technology

Because more alloys are solidly dissolved in the matrix of high-carbon high-speed steel rolls, the austenite stability is enhanced. To avoid the wear resistance loss caused by the excessive amount of retained austenite and reduce the casting stress, China Steel Xing Machinery passed through a series of re-optimization practices of tempering temperature, time and temperature rise and fall speed, a perfect multi-stage tempering process has been explored to achieve stable control of the working layer structure.

 

3. Characteristics of high-carbon high-speed steel rolls

 

3.1 Working layer hardness consistency

The content of graphite and carbide in the working layer of high-carbon high-speed steel rolls has small changes, the structure control is stable and consistent, and the hardness gap within the working layer is small. One ICDP and one high-carbon high-speed steel roll were produced with the same specifications and sizes for the hardness anatomy test in the working layer. The hardness drop of the high-carbon high-speed steel roll can be controlled to about 2HS within a radius of 50mm, which is significantly improved compared to the ICDP roll. The good hardness difference improves the rolling performance stability of high-carbon high-speed steel rolls throughout their service life.

 

3.2 Organizational characteristics

As shown in Figure 1: 50× metallographic photo, 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 crack tip stress, reduce the tendency of crack expansion, and improve the accident resistance of the roll. On the other hand, the graphite particles on the surface of the roll can fall off between the roll and the steel plate during rolling, playing a good lubrication effect. Improve surface quality.

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 composite carbides such as M2C and M6C that only exist in high-speed steel rolls in the early stage of finishing rolling. things. MC, M2C, and M6C carbides with high microhardness are important guarantees for improving the wear resistance of rolls.

In the laboratory, LECIA-QWIN analysis software was used to measure the area size and quantity of carbides in the structure of high-carbon high-speed steel. In an area of 0.35mm2, the measured results are shown in Table 1. It can be seen that high-carbon high-speed steel About 93% of the carbide area size of the roll is less than 5 μm2, the number of carbide particles is large, and the minimum particle area reaches 0.06 μm2.

Table 1 Measurement of carbide size and quantity in high carbon high-speed steel

Carbide quantityLess than 5μm2 percentageLess than 10μm2 percentageLess than 50μm2 percentageMinimum area/μm2Maximum area/μm2
746593.0695.5498.530.0612097.47

 

The size of the eutectic carbides of 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 rapidly expand along the edges of the carbides and the matrix, further improving the accident resistance of the rolls.

There is a martensite structure in the matrix structure of high-carbon high-speed steel rolls. Compared with the bainite matrix of traditional ICDP rolls, the matrix hardness is greatly improved and is close to the level of high-speed steel in the early stage of finishing rolling. The high-hardness matrix can provide effective support for carbides and better exert the wear resistance of carbides. The martensitic matrix has higher strength and a stronger wrapping effect on carbide particles, which can avoid particle carbonization during rolling. The shedding of materials affects the wear resistance.

 

3.3 Wear resistance test

The actual production environment of rolling plus sliding friction during steel rolling was simulated in the laboratory. The relative wear resistance of high carbon high-speed steel material and ICDP roll was tested on counter-grinding specimens of the same material and stroke. The test results showed that the high-carbon high-speed steel relative wear resistance of the roller is more than 2 times that of the ICDP roller.

 

3.4 Hot and cold fatigue test

In the laboratory, the same heating temperature, heating time, cooling time and fixed number of cycles were used to conduct hot and cold fatigue tests, and the crack depth of the final sample was detected. The crack depth of the high-carbon high-speed steel was significantly lower than that of the ICDP roll, indicating that the roll Enhanced accident resistance.

 

4. Use of high carbon high-speed steel

 

Four high-carbon high-speed steel rolls produced by Sinosteel Xing Machinery are used in a domestic steel mill. They are used normally without any adjustments to the rolling mill. Comparison with the millimetre rolling volume of the improved high-nickel-chromium material on the machine during the same period shows that the average millimetre rolling volume of high-carbon high-speed steel products is 5841.23t/mm. Compared with 2976.51 t/mm of ICDP material, the millimetre rolling volume has increased by 96%, which is nearly twice that of ICDP. More importantly, the use of high-carbon high-speed steel has solved the problem of roller surface colour difference that has long troubled users and the surface quality has been greatly improved, as shown in Figure 2.

 

High carbon HSS rolls
Figure 2: Comparison of the off-machine surface quality of ICDP rolls (top) and high-carbon high-speed steel rolls (bottom)

 

5. Conclusion

 

(1) The working layer of high-carbon high-speed steel has a high consistency of graphite, carbide, and structure. The hardness difference of the working layer is small, and the performance of the roll is more stable throughout its service life.

(2) The graphite and granular carbides of the high-carbon high-speed steel roll are uniformly and finely dispersed. The types of carbides are diversified and the matrix strength and hardness are higher. The high-carbon high-speed steel roll has better wear resistance and accident resistance than the ICDP roll. properties and surface quality.

(3) The high-carbon high-speed steel roll can be used directly on the machine without any adjustments on the rolling mill. Actual rolling shows that the millimetre rolling volume of the high-carbon high-speed steel roll has reached nearly twice that of the ICDP roll, and the surface quality after removal from the machine has been greatly improved. High-carbon high-speed steel rolls will become the mainstream material for the rolls in the final finishing rolling section of tropical continuous rolling mills in the future.

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