The roughing mill work rolls of many hot rolling mills in the world are faced with the problem of contact fatigue in the bonding and bonding zone before reaching the scrap diameter. This phenomenon is caused by factors such as rolling steel with high resistance to deformation, long rolling cycle, increased reduction in order to increase the capacity of the rolling mill and reduce the number of rolling passes, and insufficient maintenance of support rolls or work rolls. To help users optimize the use of rolls, a numerical tool was developed to optimize roll geometry. This article will review the influence of different rolling mill rolls geometries on roll stress, and how optimized geometries can reduce roll contact fatigue damage (roll crown and chamfer design, work roll shell thickness and material design, rolling cycle, etc.).
Title: How to optimize the roughing rolling mill roll to prevent early contact fatigue damage
Solve the problem :
- How to optimize the use of rolling mill rolls
- Increase the production capacity of the rough rolling mill
High-chromium steel rolls were developed in Europe in the early 1980s, with the purpose of replacing roll materials with poor performance. Such as: semi-steel rolls, centrifugal cast infinite chilled cast iron (ICDP) or forged steel rolls. Since then, this steel grade has been introduced into the roughing stands of most existing hot strip rolling mills and early thin slab continuous casting and rolling finishing mills.
The increasing requirements for roughing mills in terms of cost and performance ratio include: higher output, higher product quality and higher safety. This stimulated roll manufacturers to develop new roll steel grades for roughing mill stands in the early 1990s. Namely: semi-high speed steel (semi-high speed steel). Semi-high speed steel has been rapidly and widely used, especially in Western Europe. However, some applications such as the rolling of stainless steel and special steel require further improvements in rolls to overcome some of the shortcomings of semi-high speed steel. In the late 1990s, a special high-speed steel (HSS) for roughing mills was developed to meet this new challenge-Figure 1 shows the history of the development of work rolls for strip roughing mills.
After the introduction of semi-high-speed steel rolls, the life of roughing mill work rolls has been greatly improved, and the standard process of 7-pass reduction has been changed to 5-pass reduction, thereby increasing the production capacity of the reversible roughing mill. The excellent ability of semi-high-speed steel rolls to bite into the rolled piece caused this process change.
In the late 1980s, for semi-continuous rolling mills, high-chromium steel rolls rolled 20,000 to 40,000 tons of steel plates under standard rolling conditions; for fully continuous rolling mills, the production cycle was about 80,000 to 100,000 tons. At present, semi-continuous rolling mills use semi-high-speed steel rolls. The amount of steel plate rolled during the rolling cycle ranges from 65,000 to 120,000 tons, and the production volume of full continuous rolling mills can reach 20 tons. It is easy to understand that the rolling volume of each pair of rolls in one cycle has doubled or even tripled compared to the past.
Table 1 illustrates the current status of most hot strip mills in Europe in terms of roll types and rolling volume in one cycle. This table summarizes the data of strip rolled by carbon steel and stainless steel mills, excluding medium plate mills and coil mills. This table shows the type of rolling mill, the type of work roll used for the roughing mill, the total amount of rolled strips in the standard cycle, and whether the rolls have fatigue problems.
(1. The table illustrates the current status of most hot strip mills in Europe in terms of roll types and rolling volume in one cycle. 2.This table summarizes the data of strip rolled by carbon steel and stainless steel mills, excluding medium plate mills and coil mills. 3.The table shows the type of rolling mill, the type of work roll used for the roughing mill, the total amount of rolled strips in the standard cycle and whether the roll has fatigue problems. )
(1. The above figure illustrates three different types of roughing mills encountered in Europe. 2.Different types of rough rolling mills: semi-continuous rolling mill (a), 3/4 continuous rolling mill (b) and full continuous rolling mill (c))
The core material of the rolls of the new generation of roughing mills remains basically unchanged, and the interface properties of the working layer shells of the rolls have not changed significantly. As the rolling load of the rolling mill rolls increases (the rolling cycle becomes longer and the rolling procedure is reduced from 7 passes to 5 passes), the fatigue phenomenon becomes more and more obvious. These fatigue problems are observed on work rolls of roughing mills, and sometimes lead to premature failure of the shell-core interface (bonding zone) of the roll. This type of failure mainly occurs in the semi-continuous or 3/4 process arrangement of the rolling mill, so far this phenomenon has not been found on the full continuous rolling mill.
In Europe, 8 out of 15 reversible roughing mills are facing fatigue problems. This number is increasing every year. This is a realistic problem that users and roll manufacturers must face. A current standard practice increases the scrap diameter of the roll and increases the scrap thickness of the working layer to 25mm or more (related to the roll radius) to prevent fatigue problems in the bond bond zone of the roll.
To this day, fatigue problems in rough rolling mills still lead to premature failure or damage of work rolls and backup rolls. In many cases, the fatigue problem can be solved by the chamfer of the work roll, the crown curve of the roll body, the correct design of the outer layer of the work roll or the current rolling volume.
For this reason, CRM has developed a special modeling tool.
Solve the problem of premature failure or damage of work rolls and backup rolls:
Development of digital tools for predicting the stress of work rolls and backup rolls
(Figure 3 Model overview: Close-up observation of the fine mesh of the contact area of the support roll (BUR) (upper) and work roll (WR) (lower) (a) and the work roll WR (b))
The tool is fully parameterized to quickly adapt to each rolling mill configuration. Its purpose is to flexibly adapt to the roll user site, taking into account the material properties and roll design, and is also actually used by the rolling mill.
In this way, it is possible to describe the current mechanical load of the roll and the solution proposed to improve the use of the roll to the roll user.
For work rolls and support rolls, the following data is required:
- Roller diameter.
- The length of the roll body.
- Chamfer and edge geometry graphics.
- The crown and shape of the roll body.
- Wear curve.
- Mechanical properties (used in the shell, junction area and core).
- The thickness of the outer shell of the work roll.
- Fatigue resistance (only for core materials).
The required rolling parameters are:
- Rolling pressure.
- The average, minimum and maximum width of the slab.
- Rolling cycle.
They are related to the geometry of the roll and rolling parameters, and have an important influence on the accuracy of the model. (Material properties, mechanical properties and fatigue resistance, working layer thickness of work rolls) are published by the roll manufacturer database. The mechanical properties of the model are based on the experimental measurement of the compressive yield stress of the material, which conforms to the theoretical work hardening law. Since the crack initiation and propagation law is not considered in the model, the ultimate tensile strength of the material is not considered. The use of this model requires manual work to verify whether the maximum stress reached is consistent with the ultimate strength of the material.
((A) Mechanical properties (compression properties) of the shell and core of the support roll BUR and work roll WR; (b) Wöhler of the work roll core material. Figure 4a shows the work roll shell and the work roll core. The typical compression characteristics of the roll material including the part and the backup roll.)
The model also takes into account the fatigue resistance of the core, which is the weakest part of the work roll. In the interpretation of the results, the bonding area characteristics are considered to be between the outer shell and the core layer, from a safety point of view, but it is classified as the core layer material.
In order to avoid readers’ misunderstanding, this article will define some terms related to roll diameter and shell thickness. In fact, the life of the roll that users are concerned about, the remaining shell thickness of the scrap diameter that the roll manufacturer is concerned about, and the total shell thickness that the modeler is concerned about, they may use different words. Figure 5a shows a cross section:
Figure 5 Definition of diameter and shell thickness: D1: outer diameter, D2: scrap diameter, D3: core diameter (a); 1: total shell thickness, 2: usable shell thickness, 3: residual shell at scrap diameter Body thickness (b)
- D1 is the outer diameter of the roll.
- D2 is the diameter of the roll when it is scrapped, that is, rolls larger than this diameter can be used in the rolling mill and have the same performance as when delivered. Below this diameter, the performance of the roll cannot be guaranteed.
- D3 is the diameter of the core material.
Figure 5b explains:
- The total shell thickness of the working layer of the roll body, which can be related to the total shell thickness when the roll is delivered. If the roll has been used and processed and ground, it refers to the remaining total shell thickness.
- Available roll body working layer shell thickness, which is a parameter of interest to roll users.
- The thickness of the remaining working layer shell at the scrap diameter is lower than this thickness, the performance of the roll is no longer guaranteed. Until the end of the 1990s, the standard practice was to specify the remaining shell thickness to be 15mm, but it was usually increased to 30mm thick or more to prevent fatigue problems in related rolling mills.
The influence of the thickness of the working layer of the roll and the roll shape on the stress
According to industrial production data and rolling mill conditions, the influence of working layer shell thickness on work roll surface and bonding zone stress under several conditions is calculated.
This type of rolling mill uses two straight chamfers on the back-up rolls (Figure 6a). The wear of the work roll and the support roll in the center of the roll body reached 1750 μm and 1000 μm, respectively (see Figure 6b).
It should be noted that when the thickness of the remaining shell reaches 30mm (15mm higher than the specified thickness of the remaining 15mm shell), start to observe the fatigue damage of the work roll in the bonding area. The initial shell thickness of the working layer is 70mm . This means that work rolls with a residual shell thickness of less than 30 mm can continue to be rolled. For this reason, the stress distribution of the total thickness of the three shells is calculated:
- The thickness of the working layer shell is 60mm; after the roll has been used for several cycles, the stress condition in the shell is simulated (compared with the total thickness of the shell when the new roll is delivered, the thickness minus 10mm).
- 30mm thickness of the working layer shell: simulate the stress of the total thickness of the shell, and start to observe the fatigue damage of the bonding area.
- 15mm thickness of the working layer shell: The stress condition of the thickness of the front residual shell at the scrap diameter is simulated in this rolling mill.
The influence of roll shape on stress was studied under three conditions: wear-free straight roll body; worn back-up roll BUR and work roll WR after grinding; worn back-up roll BUR and work roll WR (Table 3).
In state 4, the support roll is worn, the work roll is not worn, the working layer thickness of the work roll is 60mm, and the simplified code is BWWW60.
The results show the evolution of Von Mises stress along the width of the roll body. Take the state of the work roll and support roll after grinding with the working layer shell thickness of 60mm as the basic reference state. The Von Mises stress in this state is set to 100%, and all other conditions are compared with the reference state. Figure 7 shows the stress evolution of the work roll surface under these seven states. The final conclusion is:
- The maximum stress is located at 175 to 200mm from the edge of the roller body, which corresponds to the end of the chamfer of the support roller.
- When the support roll is worn, the contact between the work roll WR and the support roll is limited to the 400 mm wide area on both sides of the work roll. In the case of a small contact area, compared to the reference case (backup roll and work roll after grinding), the stress in this area is higher, reaching 220%, while the reference case is 150%.
- The thickness of the work roll shell has a limited effect on the surface stress.
- When the working layer shell thickness of the work roll reaches 15mm (scrap diameter thickness), an increase in the stress in the center of the work roll can be observed. This is caused by the higher bending of the work roll. The thickness of the working layer of the work roll has little effect on the stress. This effect is compensated by the wear of the support roll, so the work roll and the support roll are in contact at the center of the roll body.
- When the support roll is worn, the stress on the edge of the work roll WR increases by 400% compared to the reference state.
- Work roll wear has a limited impact on stress compared to support roll wear (400 -500%).
- The thickness of the working layer of the work roll is reduced from 60mm to 30mm and 15mm, respectively. Under wear conditions, the stress in the bonding zone increases from 500% to 850% and 1200%.
As long as the total thickness of the working layer shell of the work roll is maintained above 30 mm, no fatigue damage is observed in the bonding area through ultrasonic testing (UT) at the customer site. Therefore, it can be concluded that the maximum Von Mises stress in the bonding area must be Keep it below 850%.
Modify the chamfer of the support roll
Based on this modification, the chamfer of the support roller is changed from a straight line to a two rounded configuration (see the diagrams in Figures 9a and 9b). Three situations are simulated:
- After grinding the support roll BUR and work roll WR, the working layer shell thickness of the work roll is 60mm.
- Worn support rolls and work rolls after grinding, the working layer shell thickness of the work roll is 30mm.
- Worn support rolls and work rolls after grinding, the working layer shell thickness of the work roll is 15mm.
The change of chamfer design has little effect on surface stress. However, in the bonding area, the modification of the chamfering process has an effect on the working layer thickness of 30mm. Compared with 750% when using a straight chamfer, the stress in the bonding area is limited to 600%.
The model clearly explains the location and cause of fatigue damage, and the influence of the chamfer of the support roll on the stress. The complete study also included evaluating the maximum allowable wear of the work rolls and support rolls to reach the scrapped diameter of the work rolls, and the remaining shell thickness of the remaining work rolls was 15mm. Complete research enables roll manufacturers and users to find the best combination state.