Classification of rolling mill rolls
There are many types of rolls, and their classification methods are not uniform, but they are generally classified according to the following three methods:
Classified by roll use:
1. Primary Roll
Slab blooming roll, billet blooming roll (these two rolls are replaced with the increase of continuous casting ratio)
2. Strip Roll
Thick plate rolls, thin plate rolls, hot-rolled strip rolls, narrow strip rolls, cold-rolled strip rolls
3. Steel Roll
Large steel rolls, small and medium steel rolls, bar rolls, wire rolls, steel pipe rolls
Classified by roll material:
1. cast iron roll
Chilled cast iron rolls, infinite chilled cast iron rolls, semi-chilled cast iron rolls, nodular cast iron rolls, high chromium cast iron rolls
2. cast steel roll
Alloy cast steel rolls, alloy semi-steel rolls, graphite steel rolls, high chromium steel rolls, high speed steel rolls
3. forged steel roll
Alloy forged steel rolls, high chromium forged steel rolls, forged high-speed steel rolls, etc.
4. Special Roll
Tungsten carbide roll, ceramic steel roll
Classified by roll weight:
1. Large rolls, weighing more than 10 tons;
2. Medium-sized rolls, weighing 3-10 tons;
3. Small rolls, weighing less than 3 tons;
Selection of rolling mill rolls material
The roll is an important consumable tool in the steel rolling production process. It directly affects the dimensional accuracy and surface quality of the rolled material. Since the rolling conditions of each rolling mill, the variety and specification of the rolled material are different, the correct design of the roll characteristics and reasonable Proper selection of roll material is very important.
Blooming mill rolls
Blooming mills are mainly divided into slab mills and billet mills. Blooming mills use steel ingots as billets, and the as-cast structure of steel ingots is a coarse dendrite structure, which is prone to segregation. In order to eliminate the casting structure, the high rolling temperature and large deformation are controlled to break the coarse grains. At the same time, the blooming mill has a large reduction and a large rolling torque. Therefore, the thermal crack resistance and strength of the roll are the key to the initial roll.
Before the 1950s, most foreign primary rolls used forged steel rolls. Later, some cast steel rolls were used. Although these rolls have high strength, the main problem is that they are not wear-resistant on the one hand; In the blooming mill rolls, the application of nodular cast iron rolls is considered to be a very effective initiative. In 1958, my country was the first in the world to use nodular iron rolls on the blooming mill, and achieved satisfactory rolling results, making a fruitful contribution to the expansion of the application field of nodular cast iron. In the past, the hardness of the cast steel primary rolling roll was 27-35HSD, but the hardness of the alloy ductile iron roll can reach 43-48HSD, which has good wear resistance and thermal crack resistance, but the disadvantage is that it is easy to slip. In terms of strength, as long as the roll has a good spheroidizing effect and is well heat-treated, the roll strength is sufficient to guarantee.
There are two-roll reversing mill, three-roll Lauter mill and four-roll reversing mill in terms of structure. According to the number of stands, there are single-stand, double-stand and multi-stand semi-continuous rolling or full-continuous rolling.
In a three-roll Lauter-type rolling mill, except the middle roll is a work roll, the upper and lower rolls are both backup rolls and work rolls. It is required that the rolls not only have good wear resistance and heat resistance, but also have good spalling resistance and Fracture resistance. Therefore, the roll should use low nickel chromium molybdenum or medium nickel chromium molybdenum infinitely chilled cast iron roll, and the surface hardness of the roll body is 60-70HSD.
The four-high reversing mill is the most widely used medium and heavy plate mill in modern times. It combines the advantages of the two-high and three-high Lauter mills, which not only reduces the rolling pressure, but also greatly enhances the rigidity of the rolling mill. The selection of rolls in this rolling mill has changed several times. The back-up rolls are usually cast steel rolls with a hardness of 45-52HSD. The use of rollers for this stand is a major innovation. In the 1950s, the work rolls of the finish rolling stand were high-alloy infinite chilled cast iron rolls produced by the full flushing process in foreign countries, with a hardness of 75-82HSD. This material has continued to this day, but it is only in the form of graphite and the state of carbide. It has been improved, and the leveling roll adopts centrifugal composite high-alloy infinite chilled cast iron roll with a hardness of 70-80HSD.
Stacked thin plate rolls
At present, the stacked thin plate rolling mill has been basically eliminated internationally, but there are still dozens of them in my country at this stage. This kind of rolling mill has the following characteristics:
- Irreversible periodic rolling, the idling time of the rolled piece is longer.
- Due to the small thickness and large surface area of the rolled piece, the heat dissipation is fast during the rolling process, coupled with the long empty running time, the temperature of the rolled piece quickly drops to a level that is not suitable for continuous rolling, and it often needs to be returned to the furnace for reheating .
- Due to the limitation of the elastic deformation of the rolling mill, when the thickness of the product is less than 2 mm, it is necessary to carry out multi-sheet stack rolling, and the rolling thickness is generally between 0.28-4.0 mm.
- In order to reduce the cooling of the rolled piece during the rolling process, generally the rolls are not water-cooled, and the working temperature of the rolls is 500-600°C.
- The rolls need to be preheated and ironed before installation and normal steel rolling. The preheating temperature of the rolls is 380-420°C.
- The rolling pressure of the rolling mill is high, generally reaching more than 900 tons, and the maximum pressure can reach as high as 1600 tons.
According to the above characteristics, it can be seen that the working conditions of the rolls of the stacked thin plate mill are quite harsh. It is required that the roll has good thermal conductivity and peeling resistance, and the roll has high impact resistance and fracture resistance. Therefore, this kind of roll is usually made of CrMo infinite chilled nodular composite cast iron roll with low alloy content, and the hardness is 57-63HSD. The most prone problems of the roll are roll breakage and peeling.
Hot-rolled strip steel roll
Hot-rolled strip mills are divided into semi-continuous rolling, 3/4 continuous rolling, full continuous rolling and thin plate continuous rolling. The one consisting of only one reversible rough rolling mill and a group of finishing rolling stands is called semi-continuous rolling. There are several rough rolling mills, one of which is reversible called 3/4 continuous rolling. A tandem rolling mill with several roughing mills and a set of finishing stands is called full tandem rolling. The billet is not opened by the blooming mill, but directly enters the continuous rolling mill from the thin-slab continuous casting billet for rolling, which is called thin-slab continuous rolling.
The two-roller rough roll must have high resistance to twisting, bending strength and good thermal crack resistance, so most of them use integral cast alloy cast steel rolls with a hardness of 43-50HSD or high-strength alloys with a hardness of 55-60HSD Ductile iron rolls. In recent years, some foreign countries have adopted composite cast steel rolls whose outer layer is graphite steel. Good results have been obtained.
In the 3/4 continuous rolling mill and the full continuous rolling mill, because the roughing stand has backup rolls, there is no need to worry about the problem of work roll breakage. The key is the wear and tear of the rolls. In the 1960s and 1970s, the rough rolling stand of the hot strip continuous rolling mill mainly used nickel-chromium-molybdenum infinite chilled composite cast iron rolls or semi-steel rolls with a hardness of 50-55HSD. In the 1980s, Europe began to use high-chromium composite cast iron rolls and achieved rapid promotion. The hardness of the rolls also increased to 70-75HSD. In the 1990s, semi-high-speed steel rolls were successfully applied in the roughing stands of hot-rolled strip mills. Semi-high-speed steel rolls have the following advantages: high wear resistance, good thermal crack resistance, and good spalling resistance sex and bite.
The finishing stand is composed of 5-7 stands, usually called the first stand as F1, the second stand as F2, etc. The finishing stand is a four-high rolling mill, and the back-up rolls are used to prevent the work rolls from deflecting due to rolling force. Back-up rolls generally use cast steel rolls with a hardness of 40-50HSD, and some countries use forged steel rolls with a hardness of 60-70HSD. In short, the selection of backup roll material must have good fatigue strength and fracture resistance. This is because the contact between the backup roll and the work roll is under great pressure, and work hardening occurs in each rolling cycle. If it cannot be removed in time, Will cause peeling.
The work rolls in the front section of finishing rolling (F1-F3) also experienced a revolution in the 1980s. When thick slabs were rolled, a phenomenon called roll sticking appeared, and black oxides would stick to the work rolls. Surface, until it reaches a certain critical thickness, it will start to fall off, and the peeled oxide will cause quality problems in the strip due to the rolling of the rolls. In order to solve this problem, high-chromium composite cast iron rolls with a hardness of 70-80HSD are used in the front section of finish rolling, and good results have been achieved. Of course, not all rolling mills can use this kind of roll. If the cooling water is insufficient or the water quality is not good, serious spalling will occur. When high-chromium composite cast iron rolls have not been widely used, high-speed steel rolls have achieved good results in the front stage of finishing rolling, and their service life is more than three times that of high-chromium cast iron rolls. Now they have basically completely replaced high-chromium cast iron rolls. . The selection of rolls for the same rolling mill is not uniform, and the most suitable rolls should be determined according to the specific usage and operating conditions of the rolling mill.
The work rolls in the last section of finishing rolling (F4-F7) are used in the last few shifts, so the hardness, wear resistance, thermal crack resistance and spalling resistance of the rolls are the main performance requirements. Based on this characteristic requirement, almost all the work rolls in the finishing rolling section of the hot strip continuous rolling mill at home and abroad use high-nickel-chromium-molybdenum infinite chilled composite cast iron rolls. Its hardness range is 75-85HSD. The most important feature of this type of roll is its excellent wear resistance, which ensures high dimensional accuracy and surface finish of the strip. However, with the development of materials, roll developers have achieved remarkable results in the use of high-speed steel materials in the manufacture of rolls. It is not only suitable for the work rolls in the front section of finish rolling, but also for the work rolls in the back section of finish rolling. The roll changing cycle is 3-5 times higher than before, and the hardness of the roll can reach 80-90HSD. At present, not only F1-F3 frames generally use high-speed steel rolls, but also F4 and F5 frames all use high-speed steel rolls. F6 and F7 frames still use improved high-nickel-chromium-molybdenum infinite chilled cast iron rolls with a hardness of 78-83 HSD.
The leveling roll adopts the centrifugal composite gold infinite chilled cast iron roll with a hardness of 70-80HSD.
Hot-rolled narrow-strip steel rolls
Usually people call the strip steel with a width less than 500mm narrow strip steel. The frame form and layout of the narrow strip steel rolling mill are very similar to that of the hot strip continuous rolling mill. Therefore, the selection of rolls is basically similar. Rough rolling stands usually use alloy cast steel rolls with a hardness of 40-50HSD or alloy ductile iron rolls with a hardness of 45-55HSD.
The front section of the finishing rolling unit adopts bainite ductile iron rolls with a hardness of 60-70HSD or medium-high alloy infinitely chilled cast iron rolls; the rear section of the finish rolling unit adopts high-alloy infinitely chilled cast iron rolls with a hardness of 75-85HSD or a hardness of 80-90HSD high-speed steel rolls. It is better to use semi-steel rolls for the back-up rolls.
Cold-rolled strip steel rolls
The cold strip tandem mill consists of three to six stands, which are four-high mills with larger back-up rolls and smaller diameter work rolls. During cold rolling, emulsified oil is used to lubricate the steel strip and also to cool it. Forged steel rolls of 80-95HSD are used for cold rolling work rolls, and high-alloy infinitely chilled cast iron rolls with a hardness of 75-80HSD are used in some rolling mills. The advantage is that the roll shape is easy to control, but the disadvantage is that due to graphite, the strip steel is often used surface contamination. In recent years, the use of semi-high-speed steel rolls and high-chromium forged steel rolls for cold-rolled work rolls is another great breakthrough.
Large section steel roll
Large section steel rolling mills are generally divided into rough rolling, intermediate rolling and finishing rolling, so according to different stands, the selected roll materials should also be different.
The rolls of the rough rolling stand are usually processed with deep grooves, and the rolling temperature is high and the rolling pressure is high, which requires the rolls to have high strength and heat resistance. Therefore, most foreign manufacturers use nickel-chromium alloys with grooves Molybdenum alloy cast steel rolls or graphite steel rolls. In recent years, in order to improve the surface quality of rolled materials, some manufacturers have begun to use alloy semi-steel rolls with a hardness of 45-50HSD. As long as the cooling condition of the roll is improved, the alloy semi-steel roll can achieve the best use effect when the roll temperature is controlled below 60°C during use.
The rolls used in the middle stand must have good thermal crack resistance and wear resistance at the same time, so the rolls of this stand generally use alloy ductile iron rolls with a hardness of 50-60HSD or cast alloy ductile iron rolls with grooves.
The rolling operation of the large section steel finishing stand is an important operation link to ensure the rolling according to the negative tolerance, reduce the allowable deviation, and improve the dimensional accuracy of the large section steel. Therefore, the rolls used in the finishing stands of large section steel mills should have certain strength and excellent wear resistance at the same time. Therefore, the roll is basically made of pearlite ductile iron roll with good wear resistance or high carbon semi-steel roll.
Medium and small steel rolls
Medium and small section steel rolls are generally divided into rough rolling, medium rolling and finishing rolling. The material selection should be determined according to specific conditions such as the type of rolling mill, product specifications and rolling mill shifts.
Rough rolling stands generally have deep grooves, requiring the rolls to have high strength and heat crack resistance. Therefore, most of them use alloy semi-steel rolls or high-strength alloy ductile iron rolls.
The rolls in the middle stand are heavily loaded during the rolling process, and the rolls are required to have good thermal crack resistance and certain wear resistance. Therefore, alloy nodular cast iron rolls or infinitely chilled cast iron rolls are generally used.
The rolling pressure on the rolls of the finishing stand is relatively small, and it is generally not easy to cause roll breakage. However, this stand is the finished product stand or the finished front stand of the product. In order to ensure the dimensional accuracy of the rolled material and ensure that the rolling mill is rolled according to negative tolerances, the wear resistance of the rolls is the primary issue. Therefore, the rolls of the finishing stand usually use pearlitic ductile iron rolls, bainitic ductile iron rolls or nickel-chromium-molybdenum infinite chilled cast iron rolls with good wear resistance, and the hardness of the rolls is 65-75HSD.
The bar rolling mill belongs to the small section steel rolling mill. According to statistics, by the end of 2003, my country had successively introduced and self-made 118 sets of bar continuous rolling mills. Rolling technology, due to the long heating time of the rolls and the long length of the rolled material, the requirements for the rolls are not only good wear resistance and high strength, but also better thermal crack resistance and toughness.
The rolls of the rough rolling stand must firstly have high strength and continuous rolling due to the high cutting force caused by the high rolling pressure of the rolling mill, the deep roll pass, and the large reduction. On the other hand, due to the slow rolling speed of the stand and the long heating time of the rolls, good thermal crack resistance and spalling resistance must be ensured. At the same time, the roll must also have good wear resistance, impact resistance and bite resistance. Clearly, some of these properties are contradictory. Steel rolls have better bite and higher strength than ductile iron rolls, but their wear resistance and thermal crack resistance are not as good as ductile iron rolls. Some manufacturers use semi-steel rolls for rough rolling stands. Although they can meet the requirements of the rolling mill in terms of strength and wear resistance, the thermal cracks of the rolls are very serious. Therefore, at present, the first stand of rough rolling mills of most manufacturers uses ferritic nodular cast iron rolls with a hardness of 45-50HSD, and other stands use pearlitic nodular cast iron rolls with a hardness of 50-55HSD. In fact, the most ideal material for roughing mills should be non-carbide bainite rolls (NCC rolls), with a hardness of 48-55HSD. This type of rolls has high wear resistance and heat resistance due to the absence of network carbides in the structure. The crack expansion ability is strong, but the disadvantage is the high price.
On the one hand, the rolls of the intermediate rolling stand have a deep pass and a large rolling pressure, so the strength of the rolls is still very important. On the other hand, due to the large pass size of the roll, the cooling of the bottom of the groove cannot be fully carried out. Therefore, the thermal crack resistance and impact toughness of the roll cannot be ignored. At the same time, compared with the rough rolling stand, the wear resistance of the rolls of this stand puts forward higher requirements, especially the hardness of the bottom of the groove should be similar to that of the top of the groove. Therefore, the hardness of the middle rolling stand is 60-68HSD. Pearlitic ductile iron rolls are ideal and economical, and the roll surface wear and thermal cracks are better after use. Although the acicular bainite ductile iron has better wear resistance, its thermal crack resistance is lower than that of pearlite. Body iron is slightly worse. The future development trend of middle rolling stand rolls is to use semi-high-speed steel rolls with a hardness of 68-75HSD, whose wear resistance is twice that of acicular bainite ductile iron rolls.
The rolls of the finishing stand should not only ensure the normal operating rate of the rolling mill, reduce the number of roll changes, increase the steel output, but also ensure the surface quality of the steel, that is, more steel and better steel, so the wear resistance of the roll is the key issue . At the same time, due to the rolling of rebar, the toughness and spalling resistance of the roll are still very important, so the acicular bainite ductile iron roll with a hardness of 68-75HSD is generally used for this stand. Because the matrix structure of this roll is acicular bainite, the wear resistance and strength and toughness of the roll are good. In recent years, with the development of materials, high-speed steel rolls and tungsten carbide roll rings have become more and more popular in the steel rolling industry due to their excellent wear resistance and remarkable use effects. Especially for the finished product stand, finished front stand, splitting stand and pre-splitting stand of the bar continuous rolling mill, the effect is better. The wear resistance of high-speed steel rolls is 3-5 times higher than that of bainitic ductile iron rolls, and the wear resistance of tungsten carbide roll rings is 5-10 times higher than that of bainite ductile iron rolls, but the price of high-speed steel rolls is only It is one tenth of the tungsten carbide roller ring, with high price-performance ratio and broad prospects. At the same time, it also shows that the concept of roll selection in rolling mills has also changed. They are no longer keen on low-priced rolls, but value the price-performance ratio of rolls, the improvement of the operating rate of the rolling mill, and the improvement of the quality of rolled products.
High-speed wire rod roll
The high-speed wire rod mill is also divided into rough rolling, intermediate rolling and finishing rolling, and there are 2-4 pre-finishing stands before finishing rolling. The rough rolling unit is usually composed of 4-8 stands, due to the high rolling temperature, large cross-section and deep pass of the rolling mill. Therefore, the rolls used in the roughing stand should have good fracture resistance, thermal crack resistance, wear resistance and good bite performance. Usually the rough rolling stand adopts pearlitic ductile iron rolls with a hardness of 50-60HSD or alloy semi-steel rolls with a hardness of 45-55HSD.
The rolls of the intermediate rolling mill are generally required to have good wear resistance and surface roughness resistance. Therefore, the rolling unit in the high-speed wire rolling mill generally uses pearlitic nodular cast iron rolls or bainitic nodular cast iron rolls with a hardness of 60-70HSD. Some manufacturers also use infinite chilled rolls, which should be determined according to the specific conditions of the rolling mill.
The pre-finishing rolling unit of the high-speed wire rod mill can reduce the rolling accidents of finishing rolling and improve the dimensional accuracy of the finished wire rod. Therefore, the principle of selecting roll materials for the pre-finishing mill should be that the rolls used in this mill can play a role in connecting the past and the future, and the main basis should be to ensure the wear resistance of the rolls. Improving the hardness of the working layer of the roll body, especially increasing the hard wear-resistant phase in the matrix structure of the working layer of the roll body is an effective way to reduce wear. Therefore, in the matrix structure of the working layer of the roll body, increasing the amount of the above-mentioned cemented carbide carbides and controlling their precipitation form is an important guarantee for improving the wear resistance of the pre-finishing roll (or roll ring). Most of the pre-finishing stand rolls are a combined structure, which is made up of a roll ring and a mandrel. At present, the selection of materials for roll rings in foreign countries is very complicated, such as high hardness acicular ductile iron, high chromium cast iron, high vanadium alloy cast iron, high tungsten chromium alloy cast iron, tungsten carbide composite roll rings, etc. Most of the pre-finishing roll rings of China’s high-speed wire mills use high-nickel-chromium-molybdenum infinitely chilled cast iron with an outer hardness of 75-80HSD, and centrifugal composite roll rings with alloy ductile iron inner layers, which have achieved good results. Some manufacturers also use tungsten carbide hard alloy roll rings. Although the wear resistance is good, the strength is low and fractures often occur. However, high-speed steel roll rings are more suitable for pre-finishing of high-speed wire rod mills with three times the wear resistance of high-nickel-chromium-molybdenum infinite chilled cast iron rolls and higher strength than tungsten carbide hard alloy roll rings.
The high-speed wire rod mill finishing unit is an important factor to ensure high-precision rolling of wire rods. Therefore, tungsten carbide roll rings with a hardness of 82-90HSD must be used in the finishing unit of a high-speed wire rod mill. Otherwise, the rolling speed and the wear resistance of the roll ring cannot be guaranteed. So far, there is no material that can replace the tungsten carbide hard alloy roll rings of the finishing mills of high-speed wire rod mills.
Roll material selection for bar continuous rolling mill
Japanese rolling mills have undergone a fundamental change in roll selection from two years ago. Pearlitic ductile iron rolls are generally used for rough rolling stands, semi-high-speed steel rolls for intermediate rolling stands, and high-speed steel rolls for finishing stands. Bar rolls in the United States are still pearlitic ductile iron rolls and bainite ductile iron rolls, and there are even a certain amount of conventional rolls, but they think that after 3-5 years, they will be like Japan, and the selection of rolls will be completely changed.