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Roll material and heat treatment process
Release Date:
2024-04-29
The life of the roll mainly depends on the internal performance and working force of the roll, and the internal performance includes strength and hardness. In order to make the roll have sufficient strength, it is mainly considered from the roll material; hardness usually refers to the hardness of the working surface of the roll, which determines the wear resistance of the roll and the service life of the roll to an appropriate extent. Reasonable material selection and heat treatment can meet the hardness requirements of the roll.
The life of the roll mainly depends on the internal performance and working force of the roll, and the internal performance includes strength and hardness. In order to make the roll have sufficient strength, it is mainly considered from the roll material; hardness usually refers to the hardness of the working surface of the roll, which determines the wear resistance of the roll and the service life of the roll to an appropriate extent. Reasonable material selection and heat treatment can meet the hardness requirements of the roll.
Cold roll in the working process to bear a lot of rolling pressure, coupled with the welding, inclusions, edge cracks and other problems, easy to lead to instant high temperature, so that the work roll by strong thermal shock caused by cracks, stick roll and even spalling and scrap. Therefore, the cold roll should have the ability to resist cracking and spalling caused by bending, torsion and shear stress, as well as high wear resistance, contact fatigue strength, fracture toughness and thermal shock strength.
From the late 1970 s to the mid-1980s, domestic and foreign research began to use chromium content in 3% to 5% of the deep-hardened cold-rolled work roll steel. The 3% chromium cold roll does not need to be re-quenched, and the depth of the effective hardening layer can reach 25 to 30mm, and the depth of the effective hardening layer of the 5% Cr cold roll can reach 40mm, and its wear resistance and accident resistance are also significantly improved. At this stage, the domestic trial-produced 9Cr3MoV steel, some foreign manufacturers have also developed and promoted the deep hardening layer cold roll, such as the United States 3.25%Cr steel and 5% Cr steel, Japan's KantocRP53, FH13, MnMC3 and MC5 and so on. These steels are made of high carbon and high alloy materials, which have good hardness and wear resistance, but the surface of the roll is brittle, the contact fatigue life is low, and the quality is unstable.
In order to improve the depth and contact fatigue life of the hardened layer, reduce the brittleness and overheating sensitivity of the hardened layer, and at the same time meet the further requirements of rolled pieces on the mechanical properties and service properties of cold rolling work rolls, foreign roll manufacturers have optimized the chemical composition of 5% Cr cold roll steel since the middle and late 1980 s, mainly to increase the content of molybdenum and vanadium in 5% Cr steel or to add titanium, nickel and other elements. In the 5% Cr steel roll with about 0.1 titanium, titanium is finely precipitated in the matrix in the form of carbon nitride compound (TiCN). After friction loss, TiCN falls off and scratches are formed on the surface of the roll to regenerate the moderate roughness. In the actual operation of the tinplate mill, the advantage of small roughness reduction is effectively used, and high-speed rolling can be achieved from the early stage of rolling.
During the final heat treatment, the quenching and heating of the roll steel is limited to a carbon content of no more than 0.6 percent in the austenite, and then cooled as strongly as possible, so that a deeper hardened layer can be obtained. At this time, the hardened layer of the roll, in addition to the hidden needle martensite (mainly lath), there are about 4% of the carbide and about 10% of the residual Auschus. The surface hardness of the roll (including the influence of residual compressive stress) is about HS(D)95~99.
Finally, the surface hardness of the roll is adjusted to a specified value by low-temperature tempering. The more fully the low-temperature tempering, the better the toughness when the hardness is low, and the higher the thermal cracking resistance. The increase of molybdenum and vanadium content leads to more residual austenite in the steel after quenching, and most of them are transformed into new martensite after tempering, which helps to improve the hardness of the roll, enhance the wear resistance and reduce the roughness of the wear surface.
The development direction of cold roll will be to ensure toughness while further improving strength hardness and hardening layer depth. Large cold rolling work rolls will generally be made of improved 5% Cr steel containing vanadium, milling, nickel and other elements. In order to improve the hardenability of the material, the content of Cr will be further increased. For example, forged steel with 8% ~ 10% Cr and higher chromium has been used in actual production, but the increase of Cr content will lead to poor toughness. Therefore, it is necessary to properly balance the content of C and Cr and quench at a lower temperature to obtain the required hardness of the cold roll, thus reducing the fracture of the roll and its fracture sensitivity. In addition, with the further improvement of forging manufacturing technology, high chromium steel work rolls will be more used in large cold rolling mills. 5% Cr and its improved steel containing vanadium are widely used in large support roll forgings, and large forged steel support rolls with high chromium content have entered the practical stage. Large cold-rolled work rolls require the use of electroslag remelting ingot forging, and large support roll forgings steel is widely used ladle refining and vacuum degassing smelting process production, the purity of molten steel are up to a high level.
In short, reasonable material selection and the use of appropriate heat treatment methods to manufacture high-quality rolls can save a lot of roll materials, reduce the production cost of rolling steel, and improve the quality and output of the rolls. Therefore, attention should be paid to the new trend of roll material selection, starting from the actual conditions of rolling. Develop new materials for rolls and improve the manufacturing quality of rolls.
Key words:
Weldments
Iron castings
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