Modification treatment of high-speed steel rolls and other cast steel rolls

After rare earth elements are added to the semi-steel roll, the rare earth elements react with harmful impurities such as sulfur and oxygen in the steel to generate high melting point rare earth sulfur oxide compounds. Most of these solid phase particles float into the slag, purifying the molten steel. A small part of the particles that do not have time to emerge become non-spontaneous cores, which refine the eutectic group and achieve the purpose of metamorphosis. At the same time, rare earth elements also improve the shape and distribution of inclusions, thereby improving the performance of semi-steel rolls. In addition, rare earth elements are dissolved in trace amounts in steel and iron. Since their atomic radius is larger than that of iron atoms, they distort the crystal lattice during solid solution, thereby increasing the strength of semi-steel rolls.

After rare earth elements are added to 60CrMnMo roll steel, the rare earth elements improve their morphology and distribution by purifying and modifying inclusions, delaying the development of cracks, and thereby improving the thermal fatigue properties of 60CrMnMo roll steel.

70Mn large cast steel rolls are very popular among users because of their low price. However, during the manufacturing and use of the roll, the roll broke repeatedly, threatening equipment and personal safety. Analysis shows that one of the reasons for roller breakage is insufficient plasticity of the material; secondly, the crack resistance performance is reduced due to the severe segregation of chemical components and the influence of harmful elements such as sulfur, phosphorus, and aluminium. By using rare earth elements to purify the molten steel and vanadium-titanium modification treatment, the plasticity of the 70Mn roll is improved and micro-cracks are reduced. Practical applications show that rare earth and vanadium-titanium elements improve the cracking resistance and rolling ability of the roll, and effectively overcome the roll breakage phenomenon.

 

Deterioration treatment of high-speed steel rolls

 

Modification treatment of high-speed steel rolls

Research has found that adding different modifiers to high-carbon, high-vanadium high-speed steel rolls will have a certain degree of influence on their structures. When 0.3% rare earth is added to the modification, the distribution of carbides in the structure is not significantly improved, but it can promote the disconnection and pelletization of network carbides during the heating process. When adding 0.1% magnesium for modification, the eutectic carbides in the structure are significantly reduced, and the carbide network is refined. Adding 1.0% titanium or adding RE-Ti-Mg composite modifier has little effect on the type transformation of carbides, but can significantly refine the grains and eutectic carbide network. After adding the RE-Ti-Mg composite modifier, the eutectic carbide network in the structure is basically eliminated.

Song Yanpei and others studied the effect of rare earth composite modifiers on the structure and properties of high-speed steel rolls containing 1.8% C, 5.0% Cr, 4.0% Mo, 8.0% W and 2.0% V. Before modification, the carbides were in the form of needle sheets and continuous networks distributed in grain boundaries, after modification treatment, the carbide shape changes from needle-like and continuous network to discontinuous network and granular shape.

The reason why rare earths can change the shape of carbides is, on the one hand, because during the solidification process, rare earths can be concentrated around these high-melting point composite carbides to prevent the carbides from growing along the grain boundaries and refine the carbides. On the other hand, during the heat treatment process, rare earths are enriched at the grain boundaries, which reduces the grain boundary energy, making it difficult for carbides to nucleate on the grain boundaries, thereby preventing carbides from precipitating and growing along the grain boundaries, thus improving the carbonization.

In addition, the study also found that the inclusions in unmodified high-carbon high-speed steel are mainly type II sulfides, plus a small amount of type I and type III inclusions, and these inclusions are concentrated and distributed at the grain boundaries. After metamorphism, mainly type I spherical inclusions, with a few lumpy inclusions, are small in size, evenly distributed, and increased in number, eliminating Type II sulfide inclusions clustered and distributed along the grain boundaries. The reason is that rare earths have a strong affinity with sulfur or oxygen. They form high-melting-point composite inclusions with sulfur or oxygen in steel. These inclusions form and grow in the liquid state and can serve as crystallization cores, making the liquid steel adhere to the crystallization of inclusions, thereby preventing the growth of inclusions, making the inclusions small, round, and evenly distributed within the grain, eliminating the harm to steel caused by type II sulfide inclusions clustered and distributed along the grain boundaries.

After the high-carbon high-speed steel roll is modified, its mechanical properties are improved. Compared with unmodified steel, the impact toughness increased by 73.6% without changing the hardness. Pan et al. studied the effect of K/Na modification treatment on the microstructure of high-speed steel rolls. With the K/Na As the addition amount increases, the carbides transform from a continuous network distribution to a broken network distribution and an isolated distribution, and the carbides are obviously refined and distributed uniformly.

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