Because high speed steel has good wear resistance and hardenability, especially the red hardness at high temperature, it has become an ideal material for manufacturing bar mill rolls. The working layer of acicular bainite ductile iron roll used in the past is acicular bainite, and Fe3C carbides are distributed on the matrix. Fe3C not only has low hardness (hv840 ~ 1100), but also has coarse structure; The working layer of high-speed steel roll contains a large number of tungsten, chromium, molybdenum, vanadium and other alloy elements. The carbides are high hardness MC type (hv2000 ~ 3000) and M6C type (hv1500 ~ 1800), and the matrix structure is tempered martensite + bainite. Compared with acicular bainite ductile iron, the matrix structure has higher hardness, finer structure and better wear resistance!
Title：Application of high speed steel roll in continuous bar mill
Keyword：roll, high speed steel roll, continuous bar mill
1.Machining performance requirements of high speed steel roll
Due to the high hardness and wear resistance of high-speed steel roll, reasonable machining tools and turning parameters must be used in roll machining. During turning, the cutter head can be cubic boron nitride (CBN) or composite crystal synthetic diamond (PCD) or yg6a and YD05 cemented carbide turning tools. The turning speed is 5 ~ 20m / min, the feed rate is 0.2 ~ 0.3mm/r, and the feed depth is 0.2 ~ 1.0mm. The turning speed is inversely proportional to the feed depth. However, when turning with PCD turning tool, cooling shall be carried out, and the coolant shall be applied before entering the notch. When rolling ribbed bars, the ribbed bars need to be milled on the rolling groove. The cutter can use CBN or YD05 milling cutter or Swedish Sandvik cutter.
Due to the high hardness of high-speed steel roll and great processing difficulty, the processing time is 3 ~ 4 times that of alloy cast iron roll.
2.Cooling conditions and requirements of high speed steel roll
In the rolling process, the purpose of cooling the roll is three: first, to prevent the roll from overheating and eventually leading to spalling; The second is to prevent the roll surface temperature from being too high, resulting in the change of roll surface structure, which affects the roll wear resistance and the surface quality of rolled products; The third is to prolong the time of thermal fatigue crack formation and prevent the propagation of thermal fatigue crack. Roll heating
It mainly comes from the contact with the rolled piece, and the deformation of the rolled piece also has a certain influence on the heating of the roll. It is generally believed that the temperature in the contact area between the rolled piece and the roll is the average of the rolled piece temperature and the roll temperature. Therefore, the instantaneous temperature of the roll surface in contact with the rolled piece can reach about 500C. Especially with the introduction of endless rolling technology, the requirements of roll bearing conditions are more stringent. How to make the roll surface temperature cool quickly is very important. The ideal cooling system should have annular distribution of nozzles and be able to spray continuous and sufficient water flow to the rolling groove. Among these nozzles, the main nozzle shall be located near the outlet guide. The water ejected from the main nozzle is 30% of the total water on the whole rolling groove, and the injection angle is 20 ° ~ 30 °. The function of the main nozzle is to ensure that the cooling water is sprayed quickly to the part just separated from the rolled piece in the rolling groove. The water pressure shall be controlled at 0.4 ~ 0.6MPa. The water volume of each tank shall not be less than 500L / min. the water flow shall not be scattered. The cooling water shall be precipitated to reduce the content of solid particles as much as possible.
Although there are great differences in the cooling mode, cooling water volume, water pressure and nozzle angle of the roll water cooling device of different rolling mills, reducing the roll surface temperature, maintaining the roll cooling with uniform temperature distribution and ensuring the cooling effect of the roll surface are very important to improve the service life of the roll and improve the surface quality of rolled products. When the cooling mode is similar to the water pressure, increasing the cooling water volume of the roll is of great significance to reduce the roll temperature and reduce the thermal crack on the roll surface
Significant effect. Because high-speed steel is highly sensitive to cracks at high temperature, the final cooling effect should be based on the roll surface temperature, usually in the rolling mill, regardless of the control of water pressure, water volume and nozzle angle. When the rolling is stopped for 20 ~ 30min, the roll temperature should not exceed 55C.
3.Maintenance of high speed steel roll
Due to the high brittleness and poor accident resistance of high-speed steel materials, management should be strengthened to ensure the normal operation of the rolling mill and reduce the damage to the roll caused by rolling accidents. In case of steel jamming or steel stacking, the upper roll shall be lifted quickly and the roll shall be cooled continuously. When the temperature of rolled piece is reduced, the water can be stopped for disposal; If the high-speed steel roll has been running under the condition of anhydrous cooling for a period of time due to an accident, the roll must be removed, cooled slowly, reground and put into use. Otherwise, serious consequences will occur if cooling water is poured onto the hot rolling roll. During the rolling process, oxide film will be formed on the surface of channel hole of high-speed steel roll. The oxide film starts to be orange or light blue, and will become dark blue or purple black if it continues to be used. When the oxide film thickens to a certain extent or has scratches from the outside, the oxide film is easy to fall off, and meteor spots will be formed on the surface of channel hole. In serious cases, the roll surface will be roughened or even pitted., The factors leading to the groove change of high speed steel roll are the roughening of the groove and the appearance of small thermal cracks on the surface of the groove. It is difficult for hot cracks to appear on the roll surface due to thermal fatigue during the use of rolling groove
However, if the hot crack is still used after it appears, or the crack is not completely polished during grinding, the crack will continue to expand, lose meat, or even fracture during further use, and finally the roll will be scrapped in advance.
In short, the key to the service results of high-speed steel rolls lies in the service conditions and maintenance of rolls.
4.Application effect of high speed steel roll,
High speed steel rolls are used for rolling 28, 30 φ 35mm round steel and 18mm ribbed steel finished frame. When rolling $28mm round steel, one groove rolled three shifts, with a total of The steel rolling capacity is 3000t. After stopping rolling, observe that the whole slot hole of the upper roll is very smooth, the measured slot hole wear is only 0.2mm, and there are pits at the bottom of the lower roll slot. It is analyzed that it is caused by the poor cooling effect of the lower roll, but there is no crack. The ideal rolling volume should be about 2500t. The output per shift is increased from 700T to 1000t. It can be seen that the use of high speed steel roll will greatly improve the operation rate of rolling mill. When rolling 480 and 435mm round steel, the contract quantity of steel is only
1400 and 1300T, so it will be shut down after the rolling tonnage is completed. The measured wear of the rolling groove is only 0.1 ~ 0.2mm, and the knife marks still exist when processing the pass. The surface of the groove is smooth and there is no thermal crack. It is expected that each groove can be rolled for about 1000t. When the needle bainite ductile iron roll is used to roll b0mm round steel, the amount of steel rolled in a single groove is about 350t. It can be seen that when the high-speed steel roll is used to roll the finished frame of round copper, the amount of steel rolled in single groove is higher than that of acicular bainite
The rolling rent of nodular iron is increased by 7 times: at the same time, due to less groove wear. Therefore, when the roller is repaired, the heavy vehicle quantity is only 1 / 2 of the original, so the service cycle of the roller will be increased. When rolling c18mm band assisted reinforcement, one group of groove is rolled continuously for two shifts, with a total rolling amount of 2000 and a single groove rolling amount of 100. After the roll is off the machine, the groove hole surface is smooth, the wear amount is very small and there is no peeling phenomenon; When the needle bainite ductile iron roll is used to roll (18mm ribbed reinforcement), the maximum single groove rolling amount is 2501. It can be seen that when the high-speed steel roll is used to roll the finished frame with auxiliary reinforcement, the single groove rolling amount can be increased by 4 times compared with the needle bainite ductile iron roll.
As a new type of contemporary roll, high speed steel composite roll will be widely used in bar rolling mill in a few years. In the practice of applying this roll, the following advantages are fully shown:
(1) It can increase the rolling quantity of single groove. When it is used to roll the finished round steel frame, the rolling quantity of single groove is more than 6 times higher than that of acicular bainite ductile iron roll; When it is used to roll the finished frame of ribbed reinforcement, the steel rolling quantity of each single groove is 4 times higher than that of acicular bainite ductile iron roll, and the price is only 2.5 times that of the original roll.
(2) The surface quality of rolled products is significantly improved, which increases the market competitiveness of products.
(3) The roll changing times are reduced, the operation rate of the rolling mill is improved, and the output is increased.
(4) The roller consumption cost is reduced and the production cost is reduced.
(5) So that the steel rolling plant can increase production, reduce consumption and increase efficiency.