The support rolls in the four-high rolling mill are used to support deflection of work rolls with small diameters. The roll body and the roll neck are subject to large loads. Therefore, the roll body rigidity, wear resistance and contact fatigue strength, and the toughness of the roll neck are supported. And bending fatigue strength requirements are high. When selecting the support rolls, it is necessary to comprehensively consider the manufacturing process and the use process of the support rolls, make reasonable use of the metallurgical characteristics of the metal forged steel support rolls to make every link of production and use, and obtain the best use of the support rolls.
The support roll in the four-high rolling mill is used to support the deflection of the work roll with a small diameter. It has a long service life, and the roll body has been in extreme contact with the work roll for a long time. At the same time, the roll neck has been subjected to alternating bending stress for a long time. The requirements for the roll body stiffness, abrasion resistance, contact fatigue strength, and the toughness and bending fatigue strength of the roll neck are high. At present, the support rolls of hot-strip steel rolling mills are mainly made of cast steel and forged steel. Although the cost of cast steel support rolls is lower, the strength and ductility of forged steel support rolls are higher, and the fatigue life is longer, which has become the mainstream of advanced rolling mills. Configuration. Tangsteel’s hot-rolled department has two strip production lines, one is an 1810mm short-flow ultra-thin tropical UTSP production line, and the other is a 1700mm traditional production line. The two-line support rollers are made of Cr5 alloy forged steel and a very small number of cast support rollers.
Characteristics of forged backup rolls
The requirements for the use of the back-up rolls are: on the premise of maintaining surface roughness and roll shape, to ensure the complete use of the back-up rolls during the rolling cycle, and to maximize the quality and output of the rolled material. The back-up rolls are required to have good strength and toughness, high hardness and wear resistance, high resistance to contact fatigue and spalling, sufficient depth of hardened layer and transition layer, and good metallurgical quality, so comprehensive consideration must be given to hardness and toughness Balance.
Factors affecting the use of support rolls include the type of rolling mill, process method, use of stand, use cycle, grinding process system, etc. When selecting the support rolls, in addition to meeting the basic technical requirements, the specifications, materials, hardness, and other parameters must be determined after comprehensively considering the rolling mill characteristics, operating conditions, and the grinding process and equipment of the rolls. Support roller with the best performance.
Impact of manufacturing process on support roller use
Composition and performance
Forged steel support rolls have good mechanical properties. First, strictly control the key parameters of the smelting, machining castings, and forging processes to reduce the chance of excessive defects such as solidification, post-aggregation inclusions, and subcutaneous bubbles. Secondly, forging with a forging hammer or a press can effectively eliminate component segregation and microstructure defects in the as-cast structure, while further refining the grains and increasing the hardening depth.
In order to adapt to the manufacturing characteristics of the rolls during use, when signing a technical agreement with the manufacturer, the user requires that the upper feeder (casting gate) be marked, and the lower feeder be marked B. When the support roller is assembled with bearings, the B end is assembled For the driving side, the T end is assembled as the non-driving side to minimize the impact of the manufacturing process on the use. The risk of roll neck breakage is reduced.
The forging ratio is a method of expressing the degree of deformation of the forging during forging. The size of the forging ratio reflects the degree of influence of forging on the structure and mechanical properties of the forging. The forging ratio is too small, the dendrite segregation generated during casting is difficult to be broken, and band-like coarse crystals and local coarse crystals, as well as uneven structure. This phenomenon has certain heredity and stubbornness. It is difficult to change the forging ratio during the subsequent heat treatment process. On the one hand, it will cause unnecessary waste. On the other hand, the high temperature alloy has high heat sensitivity and plasticity. Low, large deformation resistance, can cause cracks in forgings, resulting in the generation of scrap products. The forging ratio in the forging process of the back-up roll is generally more than 3. If the forging ratio is insufficient, defects such as shrinkage, looseness and coarse crystals left in the back-up roll will most likely form crack sources and cause peeling or roll-breaking accidents.
Formulate a reasonable heat treatment process, improve the uniformity of the surface hardness of the roll body, control the number of tempering through the detection of residual stress, and avoid excessive residual stress. Heat treatment types are generally divided into two categories: preliminary heat treatment, quenching and tempering heat treatment. Some forged steel rolls require only preliminary heat treatment to meet performance requirements. The low hardness of the forged steel support roller will affect the abrasion resistance and increase the use cost of the support roller. If the unevenness of hardness is poor, and the hardness of the roll body exceeds 3HSD or even more, it will affect the overall performance of the support roll, cause uneven wear, affect the quality of the plate, and even a vicious accident.
Low support roller strength affects the use effect
If the overall strength of the forged support roller is low, there will be obvious non-wear resistance, rapid decrease in hardness and other phenomena in the late period of use, and serious failure will occur in severe cases.
In 2014, there was a support roller break at the end of the 1810mm line. The fracture is very neat, and there are small steps with uniform thickness at the edge of the port. This is a typical malignant accident in the later period of use of the support roller caused by low strength. This support roller has low hardness at the later stage of use, and the average value after grinding is 67.3HSD. In addition, it is not resistant to wear during the use of the machine. The amount of steel used in a cycle is about 150,000 tons, and the wear of the support is generally 0.8 to 1.1mm. Another form of failure is due to the high gas content in the support rolls, mainly due to the high content of gaseous hydrogen, which reduces the overall strength of the rolls. In severe cases, hydrogen fractures can form. This situation generally occurs in the late manufacturing and early use stages of the support rolls.
Grinding and flaw management
When the rolled product passes through the rolling mill, the bite force of the roll is finally transmitted to the support roll. The force acting on the surface treatment will cause deformation or micro plastic deformation, and the wear of the roll will inevitably increase. At the same time, the rolling material resists deformation and causes surface hardness to increase. This process is commonly known as cold work hardening. Extend the service life; the degree of cold hardening increases, and to a certain extent, the material exceeds its limit, and cracks occur. After the support roller is removed from the machine, all visible defects such as wear marks and cracks on the roller body need to be ground away. The hardened layer should be completely eliminated. After grinding, the eddy current flaw detection and surface ultrasonic flaw detector are used to check the surface quality of the roller body to ensure that the roller body does not have any surface and subsurface damage exceeding the specified value. After grinding, hardness testing shall be carried out. The hardness drop shall be within 3HSD. If the difference is large, you need to appropriately reduce the machine cycle or re-grind.
The production process and use and maintenance of forged steel back-up rolls directly affect the use effect. Only the production and use of each link must be well controlled. The batching, smelting, ingot, forging, heat treatment and machining of the manufacturing process must be strictly controlled In the process of use, the use status of the rolls must be closely monitored, and the metallurgical characteristics of the forged steel back-up rolls can be fully utilized to fully utilize the use performance of the back-up rolls.