The high-speed steel composite roll uses high-speed steel with high hardness and good wear resistance as the working layer material of the roll, and alloy ductile iron or cast steel with high strength and good toughness through Centrifugal casting composite as the core material of the roll.
1 Overview of high-speed steel rolls
With the advent of advanced rolling mills and high-efficiency rolling technology, the rapid development of the steel rolling industry has been promoted, and at the same time, the roll manufacturing industry has been promoted to a new technical field. How to improve the service life of the roll to meet the needs of the rolling mill is a new challenge for the roll developers. topic. In recent years, with the efforts of roll developers, the application of high-speed steel materials to roll manufacturing has achieved breakthrough development.
Because high-speed steel has high wear resistance and hardenability, especially its red hardness at high temperatures, high-speed steel is very suitable as a material for manufacturing rolls. First of all, high-speed steel rolls are used in the front section of the hot strip continuous rolling mill for finishing rolling. Later, the finishing rolling section and the roughing stand are also used. Reduce the number of roll changes, increase the use of rolls, reduce inventory, and reduce roll consumption and costs. At the same time, with the improvement of the precision of the rolled piece, it can realize non-scheduled rolling and so on.
Now, high-speed steel rolls are also developing on wire and bar mills, and have a wide range of application fields and application prospects. The structure of the working layer of the roll used in the past is mostly M3C eutectic carbide distributed on the matrix, the structure is coarse and the hardness is low, while the working layer of the high-speed steel roll is generally made of high-carbon and high-vanadium type high-speed steel, and the matrix of the working layer is distributed. High-hardness MC and M6C carbides, and with the increase of vanadium content, the fishbone-like M6C carbides in the as-cast structure of high-speed steel decrease, while the MC carbides increase, and the matrix structure after quenching are tempered martensite and bainite.
2 Development and production process of high-speed steel composite roll
2.1 Determination of the chemical composition of the working layer and core of the role
2.1.1 Determination of the composition of the working layer
(1) Carbon
Simply put, the main characteristic of HSS is that it can be quenched to high hardness and maintain its hardness at relatively high temperatures. Therefore, carbon is its most important constituent element. Part of the carbon in high-speed steel is solid-dissolved in the matrix, and part of it exists in the alloy carbide in a compound state.
In the composition design of high-speed steel rolls, carbon is calculated as follows: 0.3% carbon is required in the matrix, and 1.5% carbon is required for 15vol% alloy carbides. Therefore, the carbon content is selected as 1.6% to 2.0%. As we all know, the conventional carbon content of high-speed steel is only 0.8%, but the carbon content of high-speed steel rolls is close to 2%.
This is because the high-speed steel roll is not like a general high-speed steel tool. First, it is a rolling tool, and here it is a hot-rolled tool; secondly, it is a casting, and it is a large casting. There must be enough carbides in the roll to maintain its high-temperature hardness and reduce the adhesion with the rolled piece; again, the high carbon means the low solidus line, and fluidity is very important for casting. Of course, if the carbon is high, the austenitization temperature cannot be raised, which is a constraint for heat treatment. However, high-speed steel rolls are bulky and do not use a salt furnace for quenching, so the quenching temperature cannot be increased, so the restrictive relationship between heat treatment temperature and carbon content cannot be considered like ordinary high-speed steel.
(2) Vanadium and niobium
When the outer layer of high-speed steel liquid metal solidifies in the model, the carbides of vanadium and niobium are first precipitated.
Generally speaking, more than 95% of vanadium and niobium in as-cast high-speed steel are precipitated in the state of primary carbide (including eutectic carbide). Casting rolls have been making extensive use of primary carbides. Among primary carbides, the morphology of vanadium carbide and niobium carbide is the most favorable, and it is also the most favorable in terms of volume. Of course, both vanadium and niobium are expensive, especially niobium.
However, due to the special consideration of the thermal cracking resistance of the roll, vanadium and niobium are added at the same time, and the total amount of vanadium and niobium is controlled at about 4% to 6%. In this way, in the finished roll, there will be about 7vol% to 9vol% eutectic NbC+VC. Due to the high hardness of vanadium-niobium carbides and their grain refinement, vanadium and niobium will simultaneously contribute to the wear resistance and toughness of high-speed steel rolls.
(3) Tungsten
There are several alloying elements that contribute to the temper resistance or red hardness of HSS.
Historically, tungsten was the element of choice, so contributions from other elements were converted to tungsten equivalents. Tungsten mainly generates Fe4W2C carbides. The eutectic M6C carbide is fishbone in shape, and the quenching temperature of the roll is low, so it is not easy to become dispersed.
In addition, the theoretical density of W2C carbide is as high as 17.2, and under the action of centrifugal force, segregation is prone to occur during casting. Of course, in terms of the volume percentage of carbides, it is not cost-effective to use tungsten.
However, tungsten improves the oxidation resistance of steel, because the high-speed steel roll is heated in an air furnace before quenching, so tungsten is still used as a reserved element in the composition design. Calculated by 2% tungsten, only less than 1vol% carbide is formed.
(4) Molybdenum
The role of molybdenum and tungsten in high-speed steel is similar, but 1% molybdenum can replace 1.8% tungsten, that is, the tungsten equivalent of molybdenum is 1.8. The eutectic Mo2C-type carbides are rod-like or feather-like in appearance and can be easily processed into a dispersed state. The theoretical density of Mo2C is 9.18, which is more favorable than tungsten in terms of the amount and segregation of carbides.
However, if the tungsten is completely replaced by molybdenum, the oxidation, and decarburization will be intensified due to the heating in the air furnace during the heat treatment of the roll. Therefore, it contains 2% tungsten and 5% molybdenum in the composition. That is to say, the tungsten equivalent of tungsten and molybdenum reaches 11 alone.
(5) Chromium
The hardenability of high-speed steel mainly depends on chromium, because more than 80% of chromium is located in the matrix. Chromium controls the carbon balance in the matrix, so high-speed steel has historically contained 4% chromium. In addition, it is also an essential element to ensure oxidation resistance, so the chromium in its composition is higher than 4%.
(6) Nickel
Nickel improves the mechanical properties of steel, increases the hardenability of steel, and is a very good alloying element. But nickel is never used in high-speed steel because it increases the amount of retained austenite. However, long-term experience in processing high-nickel-chromium rolls can reduce the amount of retained austenite caused by nickel in cast rolls. Therefore, in the composition design of high-speed steel rolls, less than 2% nickel is added to further improve the mechanical properties of the rolls.
2.1.2 Determination of core composition
In view of strength, alloy elements are also added to the core of the roll. However, it does not require a lot of alloys. It has the dual functions of alloying elements and heat treatment, which can ensure various strength requirements for the roll core and roll neck from finish rolling to rough rolling.
2.1.3 List of ingredients
According to the specific working conditions (hot rolling mill) and specific manufacturing conditions (centrifugal casting) of the roll, the chemical composition of the working layer and core of the high-speed steel composite roll is determined.
2.2 Determination of production method of high-speed steel composite roll
The so-called high-speed steel composite roll uses high-speed steel with high hardness, good wear resistance, and red hardness as the working layer material of the roll, and uses forged steel, cast steel, ductile iron, or gray cast iron that meets the requirements of the rolling mill as the roll. The core material is a high-performance roll that combines the working layer material and the core material by metallurgical bonding.
There are mainly four production methods for high-speed steel composite rolls.
(1) centrifugal casting method (CF)
The working layer of the roll is made of high-speed steel, and the core of the roll is made of ductile iron or cast steel, and the two different materials are compounded by centrifugal casting.
(2) Continuous casting composite method (CPC)
The basic principle of the CPC method is to pour the high-speed steel liquid metal (roll body part) used as the working layer into the gap between the vertically erected mandrel (cast steel or forged steel) and the annular water-cooled crystallizer. The composite roll castings are withdrawn in an intermittent manner. At present, the CPC method has mixed reviews, but technically, it is still a very good idea.
(3) Electroslag remelting (ESR)
The mandrel that has been manufactured as the roll neck is placed in the concentric annular water-cooled casting mold, and the consumable electrode made of high-speed steel is remelted and continuously welded with the mandrel to form a composite roll. This is one of the more common methods among the various forms of ESR.
(4) Hot isostatic pressing (HIP)
The powder metallurgy composite roll produced by the hot isostatic pressing method is generally made of cast steel or forged steel. Sintered into a composite roll under pressure above 100MPa. This method is only suitable for producing smaller rolls.
Among the above four production methods of high-speed steel composite rolls, although the latter three methods have stable product quality, the investment in equipment is large and the production cost is high; although the first centrifugal casting method is complicated in process operation, it has less equipment investment and low production cost.
In this production method, the high-speed steel molten steel used as the working layer of the roll is first poured into the centrifuge mold with a certain rotation speed, and then special mold slag is added to the inner surface of the working layer to prevent oxidation of the inner surface.
When the outer layer of molten steel has just solidified, stop the rotation, lift the cold type and the roll working layer that has formed the outer shell together to match the sandboxes of the upper and lower roll necks, and then pour the alloy ductile iron molten iron in the core or Molten steel enables complete metallurgical bonding of two materials with different properties. For the pouring of molten iron or steel in the core, on the one hand, it is necessary to ensure full metallurgical bonding with the solidified working layer; on the other hand, it is necessary to minimize the melting of the solidified working layer to prevent the final alloy content of the core from being out of tolerance.
At the same time, special control should be carried out in the composite process to reduce the diffusion of the bonding layer alloy and reduce the strength of the bonding layer.
2.3 Research on the heat treatment process of high-speed steel composite roll
High-speed steel can have high hardness and wear resistance, especially good thermal stability. The main reason is that in addition to containing a large amount of alloying elements that can form carbides, it also depends on the high-temperature quenching of high-speed steel. Usually, the heat treatment of high-speed steel is quenching + tempering.
When heated to a 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 are dissolved in bainite and martensite during quenching, and dispersed carbides are precipitated during tempering, making the steel exhibit a “secondary hardness” that is higher than that during quenching. Therefore, the heating during quenching of high-speed steel can increase the heating temperature as much as possible under the principle of ensuring that the grains do not grow.
However, for high-speed steel centrifugal composite rolls, when the core of the roll is made of alloy ductile iron, the quenching heating temperature is too high and the core material will be deformed or even melted. Therefore, the heat treatment of high-speed steel centrifugal composite rolls must not only ensure the quenching of high-speed steel in the working layer effect but also take into account the core material of the roll.
3 Trial production results
When the quenching temperature is 1150°C and the tempering temperature is 550°C, the high-speed steel has a secondary hardening phenomenon, and the hardness after tempering is 3HS higher than that after quenching. No matter what temperature quenching and tempering are used, the hardness will decrease, especially for tempering at 600°C, the hardness will drop greatly. When quenching at 950°C + tempering at 500°C, although the hardness is also decreasing, the final hardness of 83HS is still achievable.
According to the above test results, there are two heat treatment schemes, the first is quenching at 1150°C + tempering at 550°C; the second is quenching at 950°C + tempering at 500°C.
3.1 Hardness
The hardness of the roll is not only the main index of technical requirements but also an important guarantee for the wear resistance of the roll. Therefore, hardness testing must be done for each roll to ensure the service life of the roll.
3.2 Metallographic structure
The structure of the roll is an important factor related to the performance of roll. For the high-speed steel composite roll, the working layer structure must not only have extremely fine-tempered martensite but also carbides must be evenly distributed. Therefore, it is necessary to carry out a metallographic examination of the working layer of the roll.
However, due to the extremely fine structure of high-speed steel, it cannot be identified with a workpiece microscope on site, and samples must be taken for high-power observation in the laboratory in order to grasp the type and distribution of the matrix structure and carbides. Through scanning electron microscope observation, it is found that there are a large number of MC-type carbides in the structure. This type of MC carbide is in the state of free particles, its size is small, and it is more evenly distributed in the structure, with high hardness (HV3000) and good wear resistance.
3.3 Roll neck strength
The tensile strength of the roll neck is not only an important index of technical requirements but also an important guarantee for the assessment of the roll on the machine. Therefore, sampling inspection must be carried out for each batch of rolls in order to provide the necessary basis for the rolls to be used on the machine. When the roll core material is made of alloy ductile iron, the tensile strength of the roll neck can reach more than 550MPa; when the roll core material is made of cast steel, the roll neck tensile strength can reach 800MPa.
3.4 Roll use
The trial-produced high-speed steel composite roll is used on the finished stand of the hot-rolled narrow-strip steel mill. The amount of steel rolled each time is 2.5 to 3 times higher than that of the original high-nickel-chromium-molybdenum infinite chilled cast iron roll, and the amount of grinding is only the original 1/4, and its comprehensive service life is more than 10 times higher than the original one.
4 Conclusion
High-speed steel is used as the working layer material of the roll, and alloy ductile iron or cast steel is used as the core material of the roll. Two different materials are centrifugally cast to make a high-speed steel composite roll. The roll body has high hardness and good wear resistance. , At the same time, the strength of the core of the roll is also high, and the roll is not easy to break. The practice of using this type of roll for the finished product frame of hot-rolled narrow strip steel shows that the comprehensive service life can be increased by more than 10 times.
