Roll material and heat treatment process

The life of the roll mainly depends on the intrinsic performance and working force of the roll. The intrinsic performance includes strength and hardness. To make the roll have sufficient strength, we mainly consider 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, to a certain extent, also determines the service life of the roll. Through reasonable material selection, the heat treatment method can meet the hardness requirements of the roll.

 

heat treatment of roll

 

Traditional cold roll materials and their heat treatment methods

The cold rolling rolls have to withstand a lot of rolling pressure during the working process. In addition, problems such as welds, inclusions, and edge cracks in the rolled parts can easily lead to instantaneous high temperatures, causing the work rolls to be subject to strong thermal shock, causing cracks, roll sticking, and even peeling. scrapped. Therefore, cold rolling 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 commonly used for cold rolling work rolls at home and abroad are GCr15, 9Cr2, 9Cr, 9CrV, 9Cr2W, 9Cr2Mo, 60CrMoV, 80CrNi3W, 8C rMoV, 86CrMoV7, Mo3A, etc.

In the 1950s and 1960s, most of the rolled products during this period were carbon structural steel, which had 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 uses quenching and low-temperature tempering. Common quenching methods include induction surface quenching and overall heating quenching. Its main task is to consider how to improve the wear resistance and spalling resistance of the roll, increase the depth of the hardened layer, try to ensure a uniform surface structure of the roll and improve the stability of the metal structure on the surface of the roll.

Beginning in the 1970s, with the improvement of alloying of rolled products and the widespread use of high-strength low-alloy structural steel (HSLA), the strength and hardness of rolled products have also increased, and the strength and hardness of roll materials have also been proposed. 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 internationally, such as 9Cr2Mo, 9Cr2MoV and 86CrMoV7 that have been used in my country, and Russia’s 9X2M Φ, West Germany’s 86Cr2MoV7, Japan’s MC2, etc. This type of material has a low degree of alloying. After the final heat treatment, the depth of its hardened layer is generally 12 to 15 mm (radius), which can only meet general requirements. In addition, it has a serious tendency to peel and crack during use, and its rolling life is short. By improving the heat treatment method, that is, re-quenching 1 to 2 times, the hardened layer of this type of roll is increased. However, each re-quenching not only requires a certain heat treatment cost but also causes a loss of about 5mm in diameter of the roll. At the same time, the roll is It is easy to deform after multiple heat treatments, making it difficult to meet the shape and position tolerance requirements of high-precision rolls. Therefore, the development of deep-hardened layer cold rolls can not only greatly reduce the consumption of cold rolls, reduce the number of re-quenchings of the rolls during use, extend the life of the rolls, and have significant economic benefits.

In order to reduce the consumption of heavy quenching, improve the depth of the hardened layer, contact fatigue strength and toughness of the roll, and extend its service life, from the late 1970s to the mid-1980s, research began at home and abroad to use chromium content between 3% and 5%. Deep hardened cold rolled work roll steel. The 3% chromium cold rolled roller does not need to be re-quenched, and the effective hardened layer depth can reach 25~30mm. The effective hardened layer depth of the 5% Cr cold rolled roller can reach 40mm, and its wear resistance and accident resistance are also significantly improved. At this stage, 9Cr3MoV steel was trial-produced in China, and some foreign manufacturers also developed and promoted deep-quenched hard-layer cold rolls, such as 3.25% Cr steel and 5% Cr steel in the United States, and Kantoc RP53, FH13, and MnM in Japan. C3 and MC5, etc. These steels are made of high-carbon and high-alloy materials and have good hardness and wear resistance. However, the hardened surface of the roll is highly brittle, the contact fatigue life is low, and the quality is unstable.

In order to improve the hardened layer depth and contact fatigue life, reduce the brittleness and overheating sensitivity of the hardened layer, and also meet the further requirements for the mechanical properties and service performance of cold-rolled work rolls, since the mid-to-late 1980s, Foreign roll manufacturers have 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 a 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, the TiCN falls off, forming scratches on the surface of the roll, regenerating a moderate roughness. In the actual operation of the tin plate rolling mill, the advantage of small roughness reduction is effectively utilized to enable high-speed rolling from the early stage of rolling.

During the final heat treatment, the quenching and heating of the rolled steel is limited to a carbon content of no more than 0.6% in the austenite, followed by cooling as intensely as possible so that a deep hardened layer can be obtained. At this time, in addition to hidden needle martensite (mainly lath), the hardened layer structure of the roll still contains about 4% carbides and about 10% retained austenite. The surface hardness of the roll (including the effect of residual compressive stress) is approximately HS (D) 95 to 99.

Finally, low-temperature tempering is used to adjust the surface hardness of the roll to the specified value. The more complete the low-temperature tempering, the better the toughness and the higher the resistance to hot cracking when the hardness is low. The increase in molybdenum and vanadium content causes the steel to contain more residual austenite after quenching, and most of it is transformed into new martensite after tempering, which helps to increase the hardness of the roll, enhances wear resistance and reduces the wear surface. Roughness.

Traditional hot roll materials and heat treatment processes

Hot rolling rollers often work in a high-temperature environment of 700°C to 800°C. They are in contact with the hot steel billet and need to withstand strong rolling force. At the same time, the surface has to withstand the strong wear of the rolled material and is repeatedly heated by the hot-rolled material and cooled by cooling water. To withstand the thermal fatigue effects of large temperature changes requires that the hot rolling roll material must have high hardenability, low thermal expansion coefficient, high thermal conductivity, high-temperature yield strength and high oxidation resistance.

In order to improve the surface wear resistance of hot rolls, the materials of hot rolls are constantly being improved. The 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-wear-resistant high-alloy white iron with a chromium content of generally 10% to 15%. Its carbide is mainly M7C3 type, which is different from the continuous M8C type carbide of white cast iron. It not only has It has good wear resistance and high hardness (HV can reach 1800). The matrix is austenite and martensite, so its hardness and toughness are well combined. Actual rolling production shows that high-chromium cast iron rolls have better resistance to hot cracking. The reason is that a dense and tough chromium oxide film is formed on the surface of the roll, which can reduce the number and depth of hot cracks.

Therefore, high chromium cast iron rollers were very widely used in the finishing rolling front frame in the 1980s. At present, high chromium cast iron composite rolls have been widely used as hot strip (steel) continuous rolling mills, roughing and finishing rolling front-end work rolls, wide and medium-thick plates; roughing and finishing rolling work rolls and small section steel and bar mill finishing rolls wait.

There are two forms of heat treatment for high-chromium cast iron rolls, one is subcritical heat treatment below the critical transition 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 extremely fine spacing and a large number of secondary carbides dispersed on the matrix. It is required to have as low residual austenite and residual stress as possible, so the latter one is generally used. A form of heat treatment, specifically normalizing and tempering. The application of high-speed steel as hot roll material was in 1988. The general 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 it has a large number of alloying elements such as W and V that can form strong carbides, its final microstructure contains about 10% to 15% carbides with extremely high hardness and high-temperature stability, so it can maintain a high temperature when working at high temperatures. strength and hardness. The working layer has high hardness, which can reach 80-85HS, and has good wear resistance and thermal crack resistance. There are no thermal cracks on the surface of the roll, and there is generally no peeling off.

In recent years, the use of semi-high-speed steel rolls in hot-rolled thin plate roughing stands has also been successful. Its wear resistance is twice that of high-chromium steel rolls, and it has good bite performance and thermal fatigue resistance. Therefore, it has become the hot-rolled thin plate roughing stand. An ideal choice for the intermediate rolling mill stand rolls of wire and bar products, 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 austenite, increasing the content of carbon and alloying elements in austenite. During quenching, they are solidly dissolved in bainite and martensite, and during tempering, dispersed carbides precipitate, 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 and coarse carbides in the matrix, the quenching temperature should be reduced as much as possible. Generally, the optimal quenching temperature is determined to be 1050℃~1150℃, and the tempering temperature is 550℃~600℃. In order to ensure that the matrix contains a large number of dispersed spherical MC-type carbides, the V content should be increased, but the V should not be too high, because V will reduce the hardenability and generate coarse primary carbides during solidification, which cannot be completely dissolved into the aerosol during quenching. Thereby reducing the fracture toughness and also reducing the surface roughness of the roll.

 

Cold rolling work roll

The development direction of cold rolling rolls will be to further improve the strength, hardness and depth of the hardened layer while ensuring a certain degree of toughness.

Large cold rolling work rolls will generally be made of improved 5% Cr steel containing elements such as vanadium, milling, and nickel. In order to improve the hardenability of the material, the Cr content will be further increased. For example, forged steel with 8% to 10% Cr and higher chromium has begun to be used in actual production, but the increase in Cr content will lead to poor toughness, so It is necessary to properly balance the C and Cr content and quench at a lower temperature to obtain the required cold roll hardness, thereby reducing roll fracture 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 cold rolling work rolls are required to be forged by electro slag remelting ingots, while steel for large backup roll forgings is widely produced using ladle refining and vacuum degassing smelting and casting processes, and the purity of the molten steel reaches a high level.

Hot roll

Hot rolls work under the action of alternating high temperatures and forces, and their surfaces are repeatedly rubbed, which will cause strong wear. Therefore, the development of hot rolls is mainly to further improve their wear resistance.

In actual rolling production, surface quenching and carburizing-strengthened hot rolls cannot meet the requirements for high wear resistance. However, the overall high-speed steel or cemented carbide rolls are extremely expensive, and the core materials of the rolls will cause waste. Therefore, the production of rolls urgently requires surface treatment, where cemented carbide or ceramic materials are clad on the surface of the roll as the working surface layer of the roll. Surface chromium plating, flame spraying, plasma spraying and laser texturing are all tool surface alloy strengthening technologies that will be further used to improve the performance of the rolls.

In short, rational selection of materials and the use of appropriate heat treatment methods to manufacture rolls with high quality can save a large amount of roll materials, reduce steel rolling production costs, and at the same time improve the quality and output of rolls. Therefore, we should pay attention to the new trends in roll material selection, start from the actual conditions of steel rolling, develop new materials for rolls, and improve the manufacturing quality of rolls.

Rolls can be divided into hot rolls and cold rolls according to their working status. They can be divided into work rolls, intermediate rolls and backup rolls according to their functions. They can be divided into forged rolls and cast rolls (chilled cast iron) according to their materials. Generally, the service conditions of rollers are extremely harsh, and they are subjected to high alternating stress, bending stress, contact stress, shear stress and friction during work. It is prone to various failure modes such as wear and peeling.

For hot rolling rolls, cracks are not allowed to appear on the roll surface. Surface crack defects can easily cause stress concentration, accelerate expansion and cause roll failure. Thermal fatigue cracks are mainly caused by periodic alternating thermal stress. In severe cases, crack expansion may cause the roller surface to peel off or even break the roller. The main failure modes of cold rolling rolls include scratches, roll sticking and peeling. The surface of the cold rolling roll body should have high and uniform hardness, and its advantages and disadvantages are reflected in the wear resistance of the working layer of the roll body, that is roughness resistance.

The heat treatment for hot rolling work rolls generally includes post-forging heat treatment and quenching and tempering. The main purpose of post-forging heat treatment is to eliminate post-forging stress, refine grains, and improve cutting performance. Post-forging heat treatment also has a hydrogen expansion effect. The hydrogen expansion time depends on the hydrogen content of the steel ingot. It is generally believed that when [H] ≤ 2×10-4%, the hydrogen expansion treatment can be cancelled. Quenching and Tempering The final heat treatment of hot rolling work rolls is quenching and tempering. The purpose of quenching and tempering is to ensure that the roll surface obtains the specified hardness and mechanical properties and to ensure that the core has sufficient toughness.

The heat treatment of cold rolling work rolls generally includes post-forging heat treatment, quenching tempering and surface quenching.

Post-forging heat treatment

The purpose of post-forging heat treatment is to reduce hardness, eliminate residual stress, and at the same time improve the structure, obtain fine-grained pearlite, and eliminate network carbides. Post-forging heat treatment also has a hydrogen expansion effect. The hydrogen expansion time depends on the hydrogen content of the steel ingot. It is generally believed that when [H] ≤ 2×10-4%, the hydrogen expansion treatment can be cancelled.

Conditioning

The purpose of quenching and tempering is to provide structural preparation for surface quenching so that the roll neck and roll core can obtain a granular pearlite structure with good strength and toughness, and obtain good comprehensive mechanical properties to withstand intense surface quenching.

Surface hardening

Surface quenching gives the work roll a highly hardened layer. Surface quenching methods can be divided into the overall rapid heating quenching method and the continuous induction heating quenching method according to the heating method. At present, the latter is more widely used in production practice.

Among the continuous induction heating and quenching processes, dual-frequency induction quenching is very distinctive. It uses two inductors with different frequencies to match for induction heating. It has obvious advantages in roller surface temperature control and heating layer temperature distribution. Typical dual-frequency quenching equipment uses a power frequency inductor with a large current penetration depth as the main inductor to achieve deep heating. After soaking for a period of time, an intermediate frequency inductor is used to obtain deeper isothermal austenite. The required hardened layer depth is obtained after quenching. The medium frequency power supply is preferably 250Hz, but 500, 1000, 1200 and 2500Hz are also used.

The power of the medium frequency sensor is generally 1/2~1/4 of the power frequency sensor. The distance between the medium frequency sensor and the power frequency sensor is 90~120mm. The rising speed of the sensor is 0.5~0.6mm/s.

The roll should be preheated as a whole in the heating furnace, and the core should be preheated thoroughly. Increasing the preheating temperature is beneficial to increasing the heating depth and shortening the austenitization time. According to the requirements for the depth of the hardened layer of the roll, it can be preheated to 220℃~500℃. After the overall preheating, it is mounted on the quenching machine tool for induction quenching.

During the induction quenching process, after the overall preheated roll is preheated to the specified temperature, the roll is installed on the movable frame. The lower-end surface of the roll body and the lower plane of the power frequency sensor are kept on a horizontal plane. The two sensors are powered from When all the roller body enters normal quenching, its power gradually increases until the quenching reaches full power. Similarly, when the sensor leaves the end surface of the roller body, the power gradually decreases from large to small until it completely leaves the roller body.

After induction heating is completed, in order to reduce quenching stress, appropriate pre-cooling should be performed before spray quenching. Since austenite is relatively stable in the high-temperature stage, there is a 40mm gap between the water sprayer and the lower inductor, and the roller surface temperature drops to about 850°C, which will not affect the quenching hardness.

After the roller body descends into the water sprayer position, low-pressure and large-volume water is continuously sprayed for cooling for 7 to 10 minutes, so that the cooling rate within the depth range of the hardened layer exceeds the critical cooling rate of bainite until it is cooled to below Ms, and then is immersed in water for cooling. The water immersion cooling time is calculated based on 15 minutes of cooling for every 100mm of roll body diameter, and the final cooling temperature is ≤50°C.

The cold rolling work rolls should be tempered in time after quenching. The tempering temperature is determined according to the required surface hardness of the roll body. Generally speaking, the tempering temperature of rolls with a hardness greater than 90HS is 140℃~150℃, and the tempering temperature of rolls with a hardness of 70~85HS is 310℃~330℃. Rolls with a roll body hardness greater than 95HS should be tempered for the second time after finishing turning and rough grinding. The tempering temperature should be 10°C lower than the first time.

At present, in the production of cast iron rolls, processes such as internal stress elimination annealing, graphitization annealing, normalizing and tempering are commonly used to eliminate internal stress annealing. During the cooling process after casting, each part of the cast iron roll undergoes the time required from plasticity to elastic deformation temperature. Different; during the cooling process, the roll undergoes graphitization and its volume changes during various phase changes. These two factors cause great stress from the surface to the center of the roll. Low-temperature annealing can effectively eliminate the internal stress of the roll in a short time. The process parameters such as heating rate, heating temperature, holding time and cooling rate of low-temperature annealing are determined according to the material, size and casting conditions of the roll.

Graphitization annealing

There is often too much free cementite in the structure of ductile iron rolls, which tends to form white spots. In order to obtain higher comprehensive mechanical properties, a graphitization annealing process can be adopted.

Graphitization annealing is divided into high-temperature graphitization annealing and low-temperature graphitization annealing. In order to eliminate a large amount of eutectic cementite or free cementite appearing in the as-cast structure of ductile iron, a heat treatment process of high-temperature graphitization annealing is required. When the free cementite in the original structure is less than 8%, the structure is ferrite + pearlite + graphite or pearlite + graphite. To obtain ductile iron with a ferrite matrix, the heat treatment process of low-temperature graphitization annealing is used. For chilled or infinitely chilled ductile iron rolls, a high-temperature graphitization annealing process is often used.

The purpose of normalizing cast iron rolls is to increase the number and dispersion of pearlite or sorbite structures in the matrix, thereby improving the strength, hardness, and wear resistance of cast iron rolls, and maintaining a certain degree of plasticity and toughness.

Normalizing can be divided into high-temperature austenitizing normalizing and medium-temperature partial austenitizing normalizing according to the heating temperature. The high-temperature austenitizing normalizing heating temperature is in a temperature range above Ac3; the partial austenitizing normalizing heating temperature is in the range of Ac1 to Ac3.

(1) Complete austenitizing and normalizing at high temperatures.

Ductile infinite chilled rolls and ductile semi-chilled rolls can be fully austenitized and normalized at high temperatures. The heating temperature is generally between 900 and 960°C. After the insulation matrix is completely transformed into austenite, it is then air-cooled, air-cooled or spray-cooled out of the furnace to obtain a cast iron roll with a pearlite or sorbate matrix. If the roller diameter is large, the normalizing cooling rate is not enough to suppress the precipitation of secondary cementite, and it is often necessary to increase the primary normalizing to enhance the normalizing effect.

(2) Partial austenitizing and normalizing at medium temperature.

The medium-temperature partial austenitizing normalizing treatment method is to keep the roll at the normalizing temperature (about 800℃~880℃) for a period of time, then air-cool and then temper. Since the austenitization temperature is relatively high, the normalized rolls generally need to be tempered. The heating rate during tempering should be ≤20°C/hour. If the heating rate is too fast, new thermal stress will be generated and the original stress will be superimposed, which may easily cause the roll to break.

 

Article source: Iron and Steel Academy

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