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Monday, July 4, 2022

Performance Requirements and Development Direction of Steel for Rolls

Different kinds of metal materials, rolling methods and rolling mills have different requirements for roll characteristics. Therefore, there are many kinds of rollers. The rolling mill is also different due to the different shapes of metal materials (plates, tubes, bars, wires) after forming. Some rolling mills are equipped with different types of rolls, and different types of rolls are used in rolling. The following mainly describes the work roll used by the hot-rolled steel sheet mill and in direct contact with the rolled piece.

1. Roll Manufacturing Method

Rolls for steel rolling production can be manufactured by forging, casting, powder sintering and other methods. Hot rolling work rolls are mainly cast rolls. In the past, the roller was not a composite structure, but a single material. In order to make the roll high-performance, the roll is developed into a composite structure roll with the high degree of freedom of outer material. The casting methods of the composite roll are mainly the centrifugal casting method and the continuous casting forming composite method.

The inner layer of the centrifugal casting roll is ductile iron, and the inner layer of the continuous casting forming composite roll is cast steel. Cold rolling work rolls are mainly forged rolls. Cast and forged rolls are made of various materials. Adopt the corresponding roll manufacturing method according to the material. After the roll is rolled, the surface properties of the roll change due to wear and tear. At this time, the deteriorated layer on the roll surface can be removed by grinding and other methods before it can be used again.

2. Performance Requirements of Hot Strip Mill Roll

The hot strip mill is divided into the rough rolling section and the finish rolling section. The finish rolling section is divided into the front section and the rear section. Because hot rolling is carried out at a high temperature, the roll is required to have wear resistance at high temperatures. Therefore, the main material used for the roll is the casting material containing a large number of carbides. The rolling load and heat load of the roll in the roughing section are large, so the roll material should be a casting material containing a large number of carbides.

However, starting from finish rolling, the material of each stand roll can be changed to the material with less carbide content. The cast roll using carbide is generally a roll with a composite structure. The outer layer of the roll is made of hard material, so the outer layer of the roll has wear resistance and orange peel resistance. The inner layer of the roll is made of strong and tough materials, so that the roll will not break due to torque and screw-down force.

The rough rolling section has a large reduction rate, large rolling load, and high rolling temperature, so the rough roll uses materials with high strength and good heat resistance. The finishing roll is made of materials with good wear resistance and orange peel resistance. In addition, the rollers used in the finishing section should also be made of accident-resistant materials. That is, when a rolling accident occurs in the post-finishing section, the roll will not produce deep cracks and cause damage.

3. Development Direction of Roll Steel

The front roll of the finishing mill emphasizes wear resistance and orange peel resistance. In the past, high Cr cast iron rolls were used, but high-speed steel rolls have been used since the 1990s. At present, high-speed steel rolls are basically used in the front section of the finishing mill in Japan. At present, the use of high-speed steel roll is expanding, but in different countries and regions, the use rate of high-speed steel roll is different. High Cr cast iron is mainly used for various purposes as cold working wear-resistant cast iron. The carbon content of high-speed tool steel for cutting tools is about 1%, while the carbon content of high-speed steel for casting rollers is more, about 2%.

The roll material of roughing section is high-speed steel, and high hardness carbides are also used to obtain wear resistance. Therefore, the carbon content is also increased. The addition of Mo and W makes the matrix of steel produce secondary hardening during tempering, and improves the heat resistance and wear resistance. This method is the same as that of high-speed tool steel. The rolls in the finishing section is required to have good wear resistance and orange peel resistance. In the past, high alloy steel was used.

The wear resistance of high-speed steel is 3-5 times higher than that of high Cr cast iron and high alloy steel, so high-speed steel is also used for the roll at the end of finish rolling. However, high-speed steel was not used in the last finishing mill because of the roll crack damage caused by the rolling accident in the last finishing mill. The crack damage degree and accident resistance of high-speed steel roll is not as good as high-alloy roll.

In order to improve the wear resistance of high alloy roll, V and other alloy elements are added to steel, and high hardness carbide MC is used to improve the wear resistance. This kind of roll is an improved high alloy roll. The C content of high alloy roll and improved high alloy roll is higher, about 3%. Therefore, the carbides in improved high alloy roll, except MC, are mainly cementite.

In addition, graphite with an area ratio of several percent in the structure of the improved high alloy roll can lubricate the thermal bonding of steel plates in the event of rolling accidents. In addition, about 4% Ni is added to the improved high alloy roll steel to form bainite or martensite in the matrix and improve the wear resistance of the matrix. The rolling efficiency is improved by using high-speed steel in the front section of finish rolling. However, there is a problem of low rolling efficiency when using improved high alloy rolls in the rear section. At present, the development of high alloy rolls with improved performance is underway.

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Tuesday, June 28, 2022

Flying Shear of Rolling Mill

The shears used for transverse shearing of moving rolled pieces are called flying shears. With the development of continuous steel plate mills, section steel mills and billet mills and the improvement of flying shear productivity, flying shear is more and more widely used.

There are many types of flying shears, which are widely used: disc flying shears, double drum flying shears, crank rotary and pendulum flying shears, etc.

1. Disc-type Flying Shear

This kind of flying shear is generally used in the small rolling locomotive rooms. It is installed in front of the cooling bed to roughly cut the rolled pieces so that the rolled pieces entering the cooling bed will not be too long; before installation in the finishing mill, the rolled piece shall be cut to ensure the smooth-rolling process of the finishing mill. The flying shear is composed of two or more pairs of disc-shaped blades, and the axis of the disc is equal to the moving speed of the steel.

When the flying shear is in the original position, the steel advances along the inlet duct to the left of the flying shear. When the steel acts on the flag switch or optical waveguide, the inlet conduit and the steel deflect to the right, and the steel enters the middle of the two discs for shearing. After the lower blade descends, the guide tube returns the steel to the original left position, and then the lower blade rises again. The disadvantage of this kind of flying shear is that the cut is inclined, but it has little effect on the rough cutting of rolled pieces before cutting or cooling beds. Due to its reliable operation, simple structure, and shearing speed of more than 10m/s, the shear has been widely used in small rolling locomotives.

2. Double Drum Flying Shears

Double drum flying shears are widely used to shear steel sections and plates in motion. Their working principles are as follows:

On the two rotating rollers, two blades are fixed radially. When the rolled piece moving along the roller table passes through the middle of the two rollers, it is sheared by the two blades that meet. The circumferential speed of the blade shall be slightly greater than the moving speed of the rolled piece, otherwise, the rolled piece will bend at the inlet during shearing.

The disadvantage of this kind of flying shear is that the section is not smooth when cutting thick rolled pieces (it has too much influence on cutting thin rolled pieces); When cutting wide steel plate, the shear force is large. Therefore, this kind of flying shear is used to shear small sections and thin plates of high-speed rolled pieces.

3. Crank Rotary Flying Shear

Its shearing mechanism is composed of a four-bar linkage mechanism, and the shearing edge moves in an approximate plane in the shearing area and is perpendicular to the surface of the rolling piece. Therefore, the cut section of the rolled piece is relatively straight. The shearing mechanism of the flying shear is composed of a tool holder, an eccentric sleeve, and a swing rod. The tool holder is in the shape of a lever, one end of which is fixed on the eccentric sleeve, and the other end is connected with a swing rod. The swing fulcrum of the swing rod is hinged to the column.

When the eccentric sleeve (crank) rotates, the tool holder moves in translation, and the blade fixed on the tool holder can be perpendicular or approximately perpendicular to the rolling piece. When shearing steel plates, an oblique blade can be used to reduce the shearing force. The disadvantage of this flying shear is that the structure is complex, the dynamic load characteristics of the shearing mechanism are poor, and the movement speed of the blade cannot be too fast. Generally used for shearing thick steel plates or billets.

4. Pendulum Flying Shear

The pendulum flying shear is sometimes used in the cross-cutting unit of the continuous steel plate rolling locomotive. The blade of the flying shear also makes translational movement, and the quality of cutting plates is good. The upper tool rest is fixed on the swing frame. The swing frame is supported by the eccentricity of the main shaft. There are two pairs of eccentricities on the main shaft, of which the other pair is connected with the lower tool rest through the connecting rod, and the lower tool rest can slide into the chute of the swing frame. Due to the difference of 180 ° between the two eccentric positions on the main shaft, when the main shaft rotates, the upper tool rest descends with the frame, while the lower tool rest rises to complete the shearing action. However, it can only shear stationary rolled pieces.

In order to shear the moving rolled piece, it is necessary to make the swing frame swing back and forth. The lower part of the swing frame is hinged with an eccentric rod, and the eccentric wheel is installed on the rear axle. The rear axle is connected to the main shaft through pinion, rack, and synchronizing disc. When the spindle rotates, the frame can swing back and forth with the spindle as the center by synchronizing the disc, rack, pinion, and the eccentric connecting rod on the rear axle. At this time, the tool rest moves in translation to realize the cutting work of the pendulum flying shear.

The role of shears in the rolling mill is not the key position, but it plays a vital role in the accuracy and shape of finished products. Different types of rolling mills should select the corresponding flying shears, so that the whole production line can achieve the target size and accuracy.

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Tuesday, June 7, 2022

Cold Rolling Process and Characteristics of Forged Steel

The so-called cold rolling of forged steel is not simply "cold rolling". Annealing, pickling, grinding, etc. are required before cold rolling, and leveling, straightening, shearing, stacking, etc. are required after cold rolling, so it is actually the whole process from the hot rolling coil to cold rolling finished products.

Due to the characteristics of cold rolling of forged steel and special requirements for product quality, the cold rolling production process has the following characteristics:

1. Cold rolling of forged steel is a kind of high alloy steel, which has great rolling deformation resistance. In order to carry out high-efficiency and high-precision rolling, the mill with high rigidity should be used, and the multi-roll cold rolling mill is generally used.

2. When the strip steel is cold rolled on the reversing mill, the head and tail parts wound on the coiler cannot be pressed down and are cut off as scrap. In order to change this situation and improve the yield, both ends of the strip steel should be welded before rolling; In addition, if the weight of the hot-rolled coil is too small, in order to improve the rolling efficiency and yield, the steel coil needs to be welded in advance; On the continuous annealing and pickling unit, the head and tail connections of the steel strip also need to be welded due to the continuous operation. Therefore, welding is an indispensable link in the cold rolling of forged steel. However, the cold rolling welding of forged steel is different from that of ordinary steel, which is much more difficult to weld; In particular, some welds need to be under pressure, and the requirements for welding quality are much stricter. Therefore, the special welding process is also a feature of the cold-rolled strip production of forged steel.

3. During the cold rolling production of forged steel, the raw materials (hot-rolled coil) shall be annealed, the intermediate annealing shall be conducted during the cold rolling process, and the final products shall be annealed. Therefore, annealing is an important link in production. However, there are many kinds of cold-rolled forged steel. The properties of various steels are different. The purpose, methods and requirements of heat treatment are different from those of general steel. There is a unique process system.

4. Cold rolling of forged steel is a kind of high-grade steel product. The requirements for surface quality are very strict. Not only metallurgical defects caused by the previous process, but also obvious defects caused by the cold rolling process are not allowed. Therefore, a series of elimination and protection measures have been taken in the production process. For example, in order to eliminate the hot-rolled scale, the hot-rolled strip steel should be shot-peened and pickled; In order to eliminate the defects caused by billets and after cold rolling and heat treatment, the strip steel often needs to be polished on the grinding unit; In order to ensure the surface quality after cold rolling, there are very strict requirements for roll grinding; In order to prevent scratches in the production process, the steel coils of each unit are required to be rolled tightly and neatly, and many units before and after cold rolling must pad process paper between the layers of the steel coils。

In addition, some special protective measures have been taken for operations and equipment that are prone to scratch; In order to obtain a good and uniform surface gloss, the finished product should be pickled after annealing, and the bright plate with special requirements should be annealed in the protective atmosphere; In order to protect the surface of finished products, some products have to be coated on the surface. In a word, the production of cold rolling of forged steel is an exquisite process, which is incomparable with other steel grades.

5. The finishing of cold rolling of forged steel is also different from that of ordinary steel. There are some special requirements. For example, the flatness process should not only improve the shape of the plate, but also use rollers with a high finish (except for hemp panels) to produce a smooth 2B steel plate. For ferritic steel, the formability of the steel plate should be improved by controlling the flattening reduction (or elongation), which is actually a kind of quenching and tempering rolling. Cold rolling of forged steel must be straightened with a strong straightener because the steel is hard and has strict requirements on flatness, and products of different thicknesses often need different straightening methods. Cold-rolled products of forged steel are usually delivered in two ways: coil forming and plate cutting, and some products need to be delivered in strips. Therefore, the modern cold rolling mill of forged steel should be equipped with longitudinal and transverse cutting units respectively.

5. The production of cold rolling stainless steel is a joint operation of multiple units. Process balance and process management are very important. A strong production scheduling system is necessary to ensure continuous high-quality production. In addition, the equipment of a cold-rolled stainless steel plant is relatively complex. There are many units and many high-quality, precision and advanced equipment, involving the technology of machinery, electricity, instrument automation, thermal engineering and other aspects. Doing well in equipment management is the fundamental guarantee to improving the production level and product quality.

Cold rolling process of forged steel

The difference between the production process of forged steel cold rolled strip and that of the general cold-rolled strip is that the forged steel cold rolled strip must be annealed before cold rolling and the surface of the strip must be kept clean at any time in the production process, so as to improve the yield and corrosion resistance.

The production process of forged steel cold rolled strip is as follows: hot rolled strip (blank) - annealing, alkali and pickling - Inspection and grinding - cold rolling - annealing, alkali and pickling - leveling - polishing - shearing - Inspection and classification - Packaging - warehousing.

Ferritic and martensitic steels have a long annealing time to facilitate recrystallization and dissolution of carbides. It is usually annealed in a bell-type furnace at an annealing temperature of about 800 ℃ for 2 ~ 6h. Ferritic steel shall be rapidly cooled in air to prevent embrittlement. Rapid cooling of martensitic steel is not allowed to avoid excessive internal stress and hardening crack. Austenitic steel is heated at 1000 ~ 1100 ℃ in a continuous furnace and cooled rapidly in water or air.

The annealed strip steel shall be shot blasted to break the iron oxide scale on the surface, and then enter the pickling tank after brushing to completely remove the iron oxide scale and passivate the surface. Generally, nitric acid or sulfuric acid is used for pickling. The temperature of the nitric acid solution is 20 ~ 55 ℃, and that of the sulfuric acid solution is 50 ~ 70 ℃.

The annealing and pickling of cold-rolled strips of forged steel are generally carried out in the annealing and pickling line.

After the pickling strip steel is inspected, the surface defects are repaired on the belt grinder, and then sent to the cold rolling mill for rolling. Cold rolling of forged steel is a kind of hard to deform steel, which is easy to produce work hardening during cold rolling, especially during multi-pass low compressibility rolling. Forged steel cold-rolled strip is generally rolled on four high mill and multi high mills, such as partial eight high mills (MKW mill) and twenty high mills. For austenitic steel that is easy to roll, the compression ratio of each pass shall not exceed 25%, and the total compression ratio of each rolling pass shall not exceed 75%. For martensitic steel with high carbon content, the compression ratio of each pass is 15%, and the total compression ratio of each rolling pass is not more than 50%.

After the completion of one rolling pass, it needs to be subject to intermediate annealing and pickling to eliminate cold rolling work hardening (recrystallization annealing). For austenitic steel, the heating temperature is 1050 ~ 1080 ℃. For Ferritic and martensitic steels, the heating temperature is about 800 ℃. Quenching in water, air or steam after heating. The iron scale produced during heat treatment is oxidized and loosened in the salt bath furnace to facilitate acid pickling.

After pickling, the surface of the strip steel shall be inspected and the defects shall be removed from the grinding machine. The finished cold rolled strip of forged steel is subject to bright annealing, that is, the last recrystallization annealing in a non-oxidizing atmosphere. Decomposed ammonia is usually used as protective gas.

After bright annealing, grinding and polishing are generally not carried out to maintain the obtained surface roughness of the strip steel. The purpose of flatness is to improve the surface quality of strip steel, improve the shape of strip steel and eliminate the yield platform. The compression ratio during leveling is generally not more than 2%. In order to obtain a very smooth strip surface, the roll should be polished frequently during the leveling process.

In order to improve the corrosion resistance of forged steel cold rolled strip, some special kinds of forged steel cold rolled strips must be ground and polished. Generally, wet grinding (oil and emulsion) is used to prevent burning spots or cracks caused by the poor thermal conductivity of cold-rolled forged steel. The polishing process consists of polishing and wiping. Generally, the emulsion is used as a polishing agent. In order to prevent the surface of the strip steel from being scratched, the surface of all rollers in contact with the strip steel in the whole process must be very smooth, or the rubber-coated roller shall be used. During coiling, the paper tape must be padded between each layer. For the collection of finished products, the paper shall be padded between each layer to protect the surface from mutual scratch.

If you are interested in cold rolling mill machine or relevant machine, please mail at marketing2@hanrm.com or kerrirollingmill@gmail.com.

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Wednesday, June 1, 2022

Application And Development of High- speed Steel Roll

The application of cast high-speed steel roll in steel rolling production began in 1988, when it was used on a hot strip mill in Japan. Subsequently, the United States and Europe began to introduce the technology of casting high-speed steel rolls. The cast high-speed steel roll was initially used in the front section of the finishing mill of the hot strip mill to replace the high chromium cast iron roll. Later, the cast high-speed steel roll was also used in the finishing and roughing sections, wire rod, bar and cold-rolled strip mills, and satisfactory results were obtained.

The cast high-speed steel composite roll produced by Nippon Steel Corporation with CPC method has high wear resistance. After using the cast high-speed steel rolling rod, the steel passing per millimeter is significantly increased, the number of roll changes is significantly reduced, and the roll grinding amount is reduced. The main reason is that high-speed steel rolls contain high hardness carbides such as MC and M6C. In addition, the matrix structure of high-speed steel rolling rod is composed of stable and high hardness martensite and good hardenability and hardenability, which makes the hardness drop of high-speed steel roll significantly smaller than that of other rolls. These characteristics can ensure that the roll has good wear resistance during the whole working period.

In order to improve the production efficiency of the steel tube mill and reduce the roll consumption, the use of high-speed steel rolls in the steel tube mill is also quite successful. High-speed steel rolls have excellent wear resistance and resistance to surface orange peel defects. Their service life is 6-10 times longer than that of traditional rolls, and there is no steel sticking. Since the trial use of cast high-speed steel rolls in 1993, the Canadian Dofasco company has continuously increased the proportion of use. At present, cast high-speed steel rolls have been used on F2, F3, and F4 racks. After using the high-speed steel roll, the roll consumption is significantly reduced, and the rolling mill operation rate is significantly improved. The average steel passing capacity of F4 stand is increased from 360 t /h in june1992 to 490 t/h in november1994, and the strip surface quality is improved by 20%.

The semi-high speed steel composite roll produced by Hitachi has also achieved good results in rolling mills. Compared with the conventional forged steel roll containing 5.0%cr and 10.0%cr, the semi-high speed steel roll has less wear and a shallow fatigue layer, so the roll consumption is significantly reduced. For the production of the same amount of cold-rolled strip, the roll consumption is only 21% of that of 5.0%cr steel. Recently, the 2050 hot rolling mill of the Vanderbijlpark plant in South Africa used the new hss7 high-speed steel roll developed by Marichal ketin company in Belgium. The steel rolling volume per mm was increased to 12000t, while the ordinary high chromium cast iron roll was only 5000 t. After using high-speed steel rolls, the surface quality of the hot-rolled strip is greatly improved, and the problem of strip quality degradation caused by the iron oxide scale is reduced by 2/3. Using high-speed steel rolls has good economic benefits.

Baosteel 2050 hot strip roughing mill adopts semi high-speed steel roll, which has excellent hot crack resistance and wears resistance, and its service life is more than twice that of the original high chromium cast iron roll; When used in the front stand of finish rolling, its wear resistance is three times that of high chromium cast iron roll, and effectively overcomes the defect of spot strip. It can roll high surface quality strips, prolong the roll change cycle, and improve the production rate of the mill. The high-speed steel roll has been used in the front section of rough rolling and finish rolling of 158opc mill, and it also has good results. Hansteel uses high-speed steel rolls for F2, F3 stands and sometimes F1 stands of CSP production line. It mainly rolls SS400, SPHC, spa2h and other steel grades. The corresponding thickness ranges are 1.8-6 mm and 1 5-3 mm,3-6 mm。 According to the statistics, the millimeter rolling volume is 10500 T, 7800 T and 8200 t respectively, and the average millimeter rolling volume reaches 8900 T, which is 2-3 times that of ordinary high chromium iron rolls. The surface quality of the products is significantly improved; The roll can be used continuously for 2-3 times in each cycle. The roll grinding can be reduced by 1.6 times and the grinding loss can be reduced by 0.4 mm. Under normal conditions, the consumption of each roll can be reduced by about 25 RNM.

The general iron and Steel Research Institute selects high-carbon high-speed steel as the working layer, alloy nodular cast iron as the roll center, and the centrifugal casting method is used to manufacture high-speed steel composite rolling rod, which has the characteristics of high working layer hardness and uniform roll body hardness. The comprehensive life of the rolling mill for the hot-rolled narrow strip is 10.5 times longer than that of the high-alloy cast iron roll. Although the hardness (HRC 65) and wear resistance of the high-speed steel roll ring are not as good as that of the cemented carbide roll ring, the heat cracking resistance is much better than that of the cemented carbide roll ring. Its single groove rolling capacity can reach about 6500 T, which is nearly 1 time higher than that of the cemented carbide roll ring. The total rolling capacity of a pair of high-speed steel roll rings and a pair of cemented carbide roll rings is roughly the same. The benefit of replacing the cemented carbide roll ring with a high-speed steel roll ring is very obvious. In addition, due to the high rolling volume of the single groove of a high-speed steel roll ring, the time of roll change and groove change can be reduced and the operation rate can be improved; The weight of a single piece is much lower than that of cemented carbide roller ring, which can reduce the labor intensity of workers; The refitting of the roll ring only needs to be processed on an ordinary lathe, which reduces the processing cost.

However, when using the high-speed steel roll, it should be noted that the high-speed steel roll is easy to form a temperature difference on the roll due to its poor heat dissipation, so as to reduce the service life of the roll. It is necessary to strengthen the cooling capacity of the roll and keep the roll surface temperature below 50 ℃. In addition, high-speed steel has low toughness and accident resistance. It is suitable for hot rolling mills with stable equipment conditions. In case of any abnormality during the rolling mill operation, it is necessary to stop the mill for inspection at any time to avoid serious roll breakage accidents. When using a high-speed steel roll, because the working roll has high hardness, the relative support roll also needs to be changed from the original cast steel to forged steel.

The grinding properties of high-speed steel are quite different from those of traditional rolls. The equipment, grinding materials and management of the roll grinding workshop should be improved in order to give full play to the special properties of high-speed steel rolls. In addition, it is also important to work to find the cracks in the use of high-speed steel rolls as early as possible and prevent crack propagation. As the compressive residual stress of high-speed steel roll is larger than that of ordinary roll, cracks are not easy to be found. It is necessary to carry out online eddy current flaw detection, ultrasonic flaw detection or magnetic particle flaw detection on the roll that has been removed from the rack on-site, and timely repair and clean the cracks found.

As a professional rolling mill manufacturer with 26 years of experience, we can supply rebar rolling mills, wire rod rolling mills, roughing rolling mills, intermediate rolling mills, finished rolling mills, short stress rolling mills, strip steel rolling mills, hot rolling mill, cold rolling mill, continuous casting machine, continuous caster, electric arc furnace, submerged arc furnace, ladle refining furnace, blast furnace, induction furnace, flying shear, cold shear, chopping shear, gripping shear, cooling bed, pinch roll, rebar tying machine, mill roll, Cardan shaft, and related spare parts.

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