The life of the roll mainly depends on the internal performance and working force of the rolling mill rolls, and the internal performance includes strength and hardness. 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 also determines the service life of the roll to a certain extent. The hardness requirements of the roll can be met through reasonable material selection and heat treatment. The traditional roll material selection and heat treatment process are summarized, and the development of roll material and heat treatment process is prospected.
Traditional cold rolling mill rolls materials and their heat treatment methods
During the working process, the cold rolls have to bear a lot of rolling pressure, coupled with problems such as weld seams, inclusions, and edge cracks of the rolled pieces, it is easy to cause instantaneous high temperature. The work rolls are scrapped due to strong thermal shock, cracks, roll sticking or even peeling off. Therefore, cold rolled rolls must have the ability to resist cracking and peeling caused by bending, torsion, and shear stress, and must also have high wear resistance, contact fatigue strength, fracture toughness, and thermal shock strength.
The materials generally used in international cold rolling work rolls are GCr15, 9Cr2, 9Cr, 9CrV, 9Cr2W, 9Cr2Mo, 60CrMoV, 80CrNi3W, 8CrMoV, 86CrMoV7, Mo3A, etc.
In the 1950s and 1960s, most of the rolled pieces in this period were carbon structural steels with low strength and hardness, so the rolls generally used 1.5% to 2% Cr forged steel. The final heat treatment of this type of steel usually adopts quenching and low temperature tempering. Common quenching methods include induction surface hardening and overall heating quenching. Its main task is to consider how to improve the wear resistance and peeling resistance of the roll, and increase the depth of the hardened layer, so as to ensure that the surface structure of the roll is as uniform as possible, and improve the stability of the metal structure of the roll surface.
Since the 1970s, with the improvement of the alloying degree of the rolled piece and the wide application of high-strength low-alloy structural steel (HSLA), the strength and hardness of the rolled piece have also increased, and the strength and hardness of the roll material have also been raised. In order to meet higher requirements, Cr-Mo type or Cr-Mo-V type steel work rolls with a chromium content of about 2% are generally used in the world, such as 9Cr2Mo, 9Cr2MoV and 86CrMoV7 that have been used in my country, 9X2MΦ in Russia, and 86Cr2MoV7 in West Germany. , Japan’s MC2 and so on. The degree of alloying of this kind of material is low. After the final heat treatment, the depth of the hardened layer is generally 12-15mm (radius), which can only meet the general requirements, and the tendency of spalling and cracking is serious during use, and the rolling life is low.
By improving the heat treatment method, that is, re-quenching 1 or 2 times, the hardened layer of this type of roll is improved, but each re-quenching not only requires a certain amount of heat treatment costs, but also causes the diameter of the roll to lose about 5mm. At the same time, the roll is easily deformed after multiple heat treatments, and it is difficult to meet the shape and position tolerance requirements of the high-precision roll. Therefore, the development of cold rolls with deep quenching and hardening layers can not only greatly reduce the consumption of cold rolls, reduce the number of re-quenching rolls during use, and prolong the life of rolls, it has significant economic benefits.
In order to reduce the consumption of re-quenching, improve the depth of hardened layer, contact fatigue strength and toughness of the roll, and prolong its service life. From the late 1970s to the mid-1980s, the international community began to study the use of deep hardened layer cold-rolled work roll steel with a chromium content of 3% to 5%. The 3% chromium cold rolling roll does not need re-quenching, and the effective hardening layer depth can reach 25-30mm, and the 5% Cr cold rolling roll has an effective hardening layer depth of 40mm, and its wear resistance and accident resistance performance are also significantly improved. At this stage, China manufactured and trial-produced 9Cr3MoV steel, and some manufacturers in other countries have also developed and promoted deep hardened layer cold rolling rolls, such as 3.25%Cr steel and 5%Cr steel in the United States, KantocRP53, FH13, MnMC3 and MC5 etc. These steels are all made of high-carbon and high-alloy materials, which have good hardness and wear resistance, but the surface of the hardened roll is brittle, the contact fatigue life is low, and the quality is unstable.
In order to increase the depth of the hardened layer and the contact fatigue life, reduce the brittleness and overheating sensitivity of the hardened layer, and also to meet the further requirements of the rolled parts for the mechanical properties and performance of the cold-rolled work rolls, since the middle and late 1980s, The international roll production plant has optimized the chemical composition of 5%Cr cold roll steel, mainly by increasing the content of molybdenum and vanadium or adding elements such as titanium and nickel to the 5%Cr steel.
In the 5%Cr steel roll with about 0.1% titanium added, titanium is finely precipitated in the matrix in the form of carbonitride (TiCN). After friction loss, TiCN falls off and scratches are formed on the surface of the roll, regenerating moderate roughness. In the actual operation of the tinplate rolling mill, the advantage of small roughness reduction is effectively utilized, and high-speed rolling can be performed from the initial rolling stage.
In the final heat treatment, the quenching and heating of the roll steel is limited to the extent that the carbon content in the austenite does not exceed 0.6%, followed by cooling as intensely as possible, so that a deep hardened layer can be obtained. At this time, the hardened layer structure of the roll contains about 4% carbide and about 10% retained austenite in addition to cryptoneedle martensite (mainly lath). The surface hardness of the roll (including the influence of residual compressive stress) is about HS (D) 95-99. Finally, adjust the surface hardness of the roll to the specified value by low-temperature tempering. The more sufficient the low-temperature tempering, the better the toughness and the higher the thermal cracking resistance when the hardness is low. The increase of molybdenum and vanadium content leads to more retained austenite in the steel after quenching, and most of them transform into new martensite after tempering. This helps to increase roll hardness, enhance wear resistance and reduce wear surface roughness.
Selection of Traditional Hot Rolling Mill Roll Material and Heat Treatment Process
Hot rolling rolls often work in a high-temperature environment of 700°C to 800°C. They are in contact with hot steel billets and need to withstand strong rolling force. Subject to thermal fatigue with large temperature changes. This requires that the hot roll material must have high hardenability, low thermal expansion coefficient, high thermal conductivity, high high temperature yield strength and high oxidation resistance.
China once used forged steel rolls and infinitely chilled cast iron rolls. In addition to ordinary chilled cast iron, there are also low-nickel-chromium-molybdenum, medium-nickel-chromium-molybdenum, and high-nickel-chromium-molybdenum cast iron materials. The high-grade chilled cast iron material is high-nickel-chromium-molybdenum chilled cast iron. The disadvantage of this type of material roll is low hardness and poor wear resistance. Later, nodular graphite composite cast iron rolls were adopted. Relatively speaking, the service life was increased several times, and they are still in use today. Other countries generally use semi-steel and high-hardness special semi-steel materials, which are very effective in overcoming surface roughness and anti-wear.
In order to improve the surface wear resistance of the hot roll, the material of the hot roll is continuously improved, and its basic development process is from chilled cast iron to high chromium cast iron to semi-high-speed steel and high-speed steel.
The chemical composition of high chromium cast iron rolls is: 2.0%~4.0%C, 10%~30%Cr, 0.15%~1.6%Ni, 0.3%~2.9%Mo. Its essence is a high-alloy white iron with high wear resistance, the chromium content is generally 10% to 15%, and its carbide is mainly M7C3 type. Different from the continuous M8C carbide of white cast iron, it not only has good wear resistance, but also has high hardness (HV can reach 1800), and the matrix is austenite and martensite. Therefore, it has a good combination of hardness and toughness. The actual rolling production shows that high-chromium cast iron rolls have better thermal cracking resistance, because a dense and tough chromium oxide film is formed on the surface of the rolls, which can reduce the number and depth of hot cracks. Therefore, high chromium cast iron rolls were widely used in finish rolling front stands in the 1980s. At present, high-chromium cast iron composite rolls have been widely used as hot strip (steel) continuous rolling mills, rough rolling and finishing rolling front-end work rolls, and wide medium and heavy plates. Rough rolling and finishing work rolls and small section steel and bar mill finishing rolls, etc.
There are two forms of heat treatment for high chromium cast iron rolls, one is subcritical heat treatment below the critical transformation temperature, and the other is high temperature heat treatment above the critical point A3. The pearlite matrix of the high chromium roll surface material is expected to have an extremely fine lamellar spacing, and there are a large number of dispersed secondary carbides on the matrix, and the residual austenite and residual stress are required to be as low as possible. Therefore, the latter form of heat treatment is generally used, specifically normalizing and tempering.
The application of high-speed steel as a hot roll material began in Japan in 1988, and was also developed in the United States and Europe in the early 1990s. China began to develop and use high-speed steel rolls in the late 1990s. Generally, the composition of high speed steel is 1%~2%C, 0%~5%Co, 0%~5%Nb, 3%~10%Cr, 2%~7%Mo, 2%~7%V, 1% ~5%W. Because there are a large number of alloying elements such as W and V that can form strong carbides, the final microstructure contains about 10% to 15% carbides with extremely high hardness and high temperature stability. Therefore, it can maintain high strength and hardness when working at high temperature. The hardness of the working layer is high, which can reach 80-85HS, and has good wear resistance and thermal crack resistance. There is no thermal crack on the surface of the roll, and there is generally no peeling phenomenon.
In recent years, the use of semi-high-speed steel rolls in hot-rolled thin plate roughing stands abroad has also been successful. Its wear resistance is twice that of high-chromium steel rolls, and its bite performance and thermal fatigue resistance are good. Therefore, it is an ideal choice for hot-rolled thin plate roughing stand and wire bar middle stand roll. The chemical composition range of semi-high speed steel is: 1.5%~2.5%C, 0.5%~1.5%Si, 0.4%~1.0%Mn, 1.0%~6.0%Cr, 0.1%~4.0%Mo, 0.1%~3.0%V , 0.1%~4.0%W.
The heat treatment method of high-speed steel hot rolling rolls generally adopts quenching and tempering. When heated to high temperature, the secondary carbides in the steel are fully dissolved, and the primary eutectic carbides are partially dissolved. The carbon and alloying elements contained in these carbides dissolve into the austenite, increasing the content of carbon and alloying elements in the austenite. They dissolve in bainite and martensite during quenching, and precipitate dispersed carbides during tempering, making the steel exhibit a secondary hardness higher than that during quenching. Therefore, in order to increase the hardness of the matrix, the quenching temperature should be increased. At the same time, in order to prevent the appearance of massive carbides in the matrix, the quenching temperature should be reduced as much as possible. Generally, the optimum quenching temperature is determined to be 1050 ° C ~ 1150 ° C, and the tempering temperature is 550℃～600℃.
In order to ensure that the matrix contains a large number of dispersed spherical MC carbides, the V content should be increased, but V should not be too high, because V will reduce the hardenability and form coarse primary carbides during solidification，the austenite cannot be completely dissolved during quenching, thereby reducing the fracture toughness, and at the same time reducing the surface roughness of the roll.
Development trend of rolling mill roll material and heat treatment process
The development direction of cold rolling will be to ensure a certain toughness while further increasing the strength, hardness and depth of hardened layer. Large-scale cold-rolled 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% to 10% Cr and higher chromium has been used in actual production, but the increase of Cr content will lead to poor toughness, so 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, thereby reducing the fracture of the roll and reducing its fracture sensitivity.
In addition, with the further improvement of forging manufacturing technology, high-chromium steel work rolls will be more used in large-scale cold rolling mills. 5%Cr and its vanadium-containing improved steel are widely used in large backup roll forgings, and large forged steel backup rolls with high chromium content have entered the practical stage. Large-scale cold-rolled work rolls are required to be forged with electroslag remelting ingots, while steel for large-scale backup roll forgings is widely produced by ladle refining and vacuum degassing smelting process, and the purity of molten steel reaches a high level.
The hot roll works under the action of alternating high temperature and force, and its surface is repeatedly rubbed, which will cause strong wear. Therefore, the development of the hot roll is mainly to further improve its wear resistance. In actual rolling production, the hot rolls treated with surface
quenching and carburizing can no longer meet the requirements for high wear resistance, but the cost of the overall high-speed steel or hard alloy rolls is extremely high, and the core material of the rolls will be cause waste. Therefore, the production of rolls urgently needs surface treatment, and the surface of the rolls is clad with cemented carbide or ceramic materials as the working surface of the rolls. Surface chrome plating, flame spraying, plasma spraying, and laser texturing are tool surface alloy strengthening techniques that will be further used to improve roll performance.
In short, reasonable material selection and high-quality manufacturing of rolls with appropriate heat treatment methods can save a lot of roll materials, reduce the cost of rolling steel production, and improve the quality and output of rolls. Therefore, attention should be paid to the new trend of roll material selection, starting from the actual conditions of steel rolling, developing new materials for rolls, and improving the manufacturing quality of rolls.