The plate rolling mill is the largest rolling mill in the steel rolling equipment. The roll is the main part of the rolling mill that directly rolls the rolled piece. During the rolling process, the rolls are in contact with the rolled piece, forcing the rolled piece to undergo plastic deformation. At the same time, the roll is subjected to a huge rolling force, and the stress changes periodically with time due to the rotation of the roll. When rolling medium and thick plates, the deformation temperature range is generally 1150°C-800°C. One side of the roll contacts the hot rolling piece and undergoes severe heat, and the other side is cooled by the water on the roll body to undergo rapid cooling. The working conditions are extremely harsh.
Keyword：Plate Mill Rolls, work rolls, backup rolls
Medium and heavy plate products face a wide range of users, mainly in shipbuilding, bridges, boilers, containers, high-rise buildings, construction machinery, export, oil, natural gas and other fields. Due to the variety of products, the production process is relatively complex and changeable.
The characteristics of the force on the roll of the plate mill:
During the rolling process of medium and heavy plates, the shoulders of the back-up rolls and work rolls of the four-high mill bear relatively large stress. Due to stress fatigue, work hardening is distributed in a saddle shape on the roll generatrix, and the edge is more serious than the middle, which increases the frequency of problems at the roll shoulder.
Main performance requirements for rolls:
> The rolling pressure of medium and thick plates is too high. In recent years, the four-high rolling mills with a diameter of more than 3000mm are newly installed. The rolling pressure is generally 5000-10000t. The main performance requirements of the rolls are shown in the above table. The roll neck puts forward higher strength requirements, generally not less than 400MPa.
> During the reversible rolling of the four-high reversible rolling mill, the repeated start and brake during rolling will form an impact on the roll, which puts forward higher requirements on the impact resistance of the roll.
> Large reduction in one pass, high temperature, and high requirements on the thermal fatigue of the working layer of the roll body. There are few stands, especially the surface quality of the finished product of a single-stand four-high rolling mill requires good roll surface quality and roll shape retention.
> The rolling temperature range of medium and heavy plates is relatively large, generally around 1150-800°C, and both wear-resistant and thermal fatigue properties of the rolls are required.
Use cases of plate mill rolls:
> A 4300×4 single-stand medium and heavy plate production line has a single work roll weight of 51.6 tons. Four high-NiCr infinite chilled cast iron rolls are used. The work roll products of each factory meet the requirements of medium and heavy plate production.
> After the application of work rolls in the 4300×4 single-stand medium and heavy plate production lines in plants A, B, C and D, the average mm rolling volumes are 3508 tons/mm, 3611 tons/mm, 3122 tons/mm, and 4101 tons respectively /mm.
> Two high NiCr infinitely chilled cast iron rolls are used for the work rolls of a 3500×4 single-stand plate mill. The maximum rolling force of the rolling mill is 7000 tons. Millimeter rolling capacity of work rolls is shown in the figure above.
> After the use of work rolls in the 3500×4 single-stand medium and heavy plate production line in E and F factories, the average millimeter rolling volumes are 3160 tons/mm and 3240 tons/mm respectively, both of which are greater than the required 3000 tons/mm.
> In a 3500×4 medium and heavy plate plant, the single weight of the backup rolls of the four-high rolling mill is more than 120 tons, and X forged steel backup rolls and Y composite cast steel backup rolls are used. The material is Cr3, and the millimetre rolling amount is shown in the figure above.
> For X steel back-up rolls, medium and thick plate plants generally replace them once at 60,000 to 80,000 tons, and their mm rolling capacity is 16,200 tons/mm; , its mm rolling capacity is 13700 tons/mm.
Problems in the use of rolls:
> The stability of anti-stripping performance of high NiCr infinitely chilled cast iron rolls for centrifugal casting needs to be further improved, and there are reactions in the 3500×4 and 4300×4 medium and thick plate production lines. At the same time, sufficient attention should be paid to the design and manufacture of the work roll neck strength.
> During the actual use of the support roll in the production of medium and thick plates, cracks and meat loss are common in the roll shoulder. It is related to the force characteristics of the roll during the rolling of the medium and heavy plate mill.
> When rolling plate, the contact area between the edge of the rolled piece, the edge of the work roll and the edge of the backup roll will become a place of stress concentration. With each rotation of the roll, if the maximum residual shear stress is located just below the roll skin, exceeding the compressive strength of the material, a crack will form at that location. Further cycling will propagate the cracks towards the surface where small “crushing” spalling will occur.
Requirements and measures for the use management of plate rolls:
The cooling condition of the rolling mill is very important to the thermal cracking state of the roll. The high temperature of the roll surface will easily cause the thermal stress to rise and cause the roll to fail. In the actual production and use of the medium and heavy plate, it is necessary to ensure that the rolling mill has enough cooling water for the roll body. Generally, under the condition of ensuring the shape of the plate, the temperature of the roll surface is controlled below 70°C.
Regular roll change
> The production of medium and heavy plates generally adopts the production organization plan mode of widening first and then narrowing. In the later stage of rolling, the middle part of the roll is severely worn, forming concavity, and increasing the stress of the roll.
> Cast steel backup rolls are replaced every 50,000 to 60,000 tons of medium and heavy plates, with an average of 55,000 tons. Forged steel rolls are arranged to be replaced every 60,000-80,000 tons of medium and heavy plates rolled, with an average of 70,000 tons. The work rolls are generally replaced every time 4000-5000 tons of medium and heavy plates are rolled. If high-strength steel plates are produced intensively, about 3,500 tons are generally replaced.
Roll grinding regime
Scientific grinding of rolls can not only repair the roll shape of the roll, ensure the dimensional accuracy of the steel plate, but also ensure the safe use of the roll. After the roll is off the machine, the grinding amount of the roll is generally determined through the hardness test of the roll surface. While the fatigue layer is ground away, the fatigue crack should also be ground clean. Roll fatigue cracks are inevitable during use, especially due to the fluctuation of cooling conditions, the fatigue cracks will become deeper and will expand under the action of shear stress, causing spalling. Therefore, the cracks must be ground and cleaned before the roll is put on the machine . The three plants of Shougang are equipped with flaw detection means.
Roll type requirements
In medium and heavy plate rolling, the back-up rolls and work rolls have greater stress on the shoulders. Shougang’s 3500×4 and 4300×4 medium-heavy plate production lines have ground a 200-500mm long unloading zone at the end of the back-up roll distance from the roll body. Good results have been achieved in the stable operation of the equipment.