Analysis and structural optimization of backup rolls breakage

Abstract: The cause of the backup roll breakage accident in a cold rolling mill was analyzed, and an optimization plan was proposed. Finite element software was used to analyze and calculate the optimization plan. After the implementation of the optimization plan, no roll breakage accident occurred again.

Keywords: backup rolls; structural optimization; finite element

The production capacity of cold-rolled strip steel is an important indicator of the development level of a country’s steel industry. With the continuous development of the steel industry, large and medium-sized steel companies mostly use continuous pickling-rolling combined units to produce cold-rolled strip steel, while small companies still mostly use single-stand reversible cold rolling units.

A cold rolling mill currently has a 1050 mm single-stand cold rolling unit with good production status. In order to adapt to market demand, the rolling mill was modified to broaden the product range. After the modification, it has the ability to produce steel plates with a width of 1200 mm. After the unit was renovated and upgraded, it began to produce 1200 mm wide steel plates, but backup roller breakage accidents occurred frequently during the production process.

To this end, in response to the needs of the enterprise, the author analyzed the roller breakage accident and proposed an optimization plan.

Basic parameters of rolling mill

The main equipment of the 1050 mm single-stand reversible cold rolling mill includes a coiling car, a starting machine, a front coiler, a front unloading car, a six-high rolling mill,

Hydraulic shears behind the machine, coilers behind the machine, unloading trolleys behind the machine and main transmission devices of the rolling mill, etc. It produces strip steel with a thickness of 0.16~1.4 mm and a width of 950~1200 mm; product materials include carbon structural steel, high-quality carbon structural steel, low alloy steel, etc.; representative grades include: Q215, 08Al, SPCC, SPCD, SPCE , IF; production capacity 220,000 t/a; blank thickness: 3~3.5 mm, width: 950~1 200 mm; maximum material strength σs ≤ 590 N/mm2, maximum σb ≤ 900 N/mm2. The maximum rolling force is 12500kN, and the maximum rolling speed is 850 m/mm.

Analysis of causes of broken rolls

According to on-site understanding, there are no specific working conditions for roll breakage accidents. Sometimes the roll breakage accident occurs during rolling, and sometimes the roll breakage accident occurs just after the rolls are pressed against each other. The location of broken rollers is relatively fixed, and they are all located at a small groove in the roll neck (see Figure 1, Figure 2). It is initially determined that the stress concentration at the “groove” is too large.

Figure 1 Picture of broken roller on site

Figure 2 Structural diagram of the current structure support roller

Structural analysis of backup rolls

The length of the back-up roller before modification is 1040 mm (see Figure 3). Since the user adheres to the principle of making full use of the original parts during the transformation, the shape and size of other structures should be kept unchanged as much as possible while the roll body is lengthened, and the overall length of the roll should be kept unchanged to the maximum extent. Finally, the length of the support roller body was lengthened from 1040 mm to 1200 mm, and the total length was increased from 3336 mm to 3356 mm. In addition, the original “round corner + bevel” transition method at the junction of the roll neck and the roll body is canceled and modified to a direct transition of the fillet (see Figure 4 and Figure 5)

Figure 3 Structural diagram of the backup rolls before modification

Figure 4 Detailed view of the backup rolls transition area before modification

Figure 5 Detailed view of the transition area of the backup rolls in the current structure

It can be seen that the “groove” is a stress concentration area, and stress concentration will occur after the rolling force is applied. However, no roll breakage accidents occurred in the original structure during use. After the modification, part of the material at the junction of the roll neck and the roll body was removed, which reduced the bending resistance of the roll neck, while the “groove” structure was still retained. The risk of stress concentration intensifies and the potential for accidents increases.

Back-up roller assembly analysis

The author reviewed the modified backup roller assembly based on the assembly drawing of the backup roller before modification and the backup roller parts drawing after modification by the user (see Figure 6). Except for the back-up roller, back-up roller bearing seat and shoulder ring, all parts in the picture are original parts and are executed according to the original assembly technical requirements.

Figure 6 Assembly diagram of the current structure support roller

During the review process, the author found that during the assembly process of the backup roller, due to structural limitations, the rounded corners of the inner ring of the four-row cylindrical bearing were in contact with the chamfer of the backup roller and could not fit closely with the shoulder ring (see Figure 7). As a result, the inner ring and outer ring of the four-row cylindrical bearing cannot be fixed in the axial direction. Since the inner ring and shoulder ring of the bearing are assembled with the support roller by thermal assembly, they cannot move relative to the support roller. In this way, a gap will be formed between the inner ring and the shoulder ring of the four-row cylindrical bearing, and the position will almost coincide with the “groove” position of the backup roller. During the production process, the stress here will be further increased under the action of rolling force.

Figure 7 Enlarged view of backup roller assembly

Optimization plan

Through the above analysis, the author proposes the following two optimization solutions (see Figure 8).

Figure 8 Back-up roller optimization plan

In the two optimization plans, the roll length is the same. In order to meet the transformation requirements, it is recommended to cancel the original design “groove” structure and change it to a rounded R10 transition. Only the transition area at the junction of the roll neck and the roll body is different (see Figure 8 ( b), (c)). The transition area of Scheme 1 adopts R30 large arc transition; the transition area of Scheme 2 adopts R40 arc and 25. Bevel-R20 arc comprehensive transition.

Finite element analysis and calculation

This article uses the large-scale engineering analysis software Ansys/Workbench to analyze and calculate four options: the user’s original design, the user’s modified design, our optimized design 1 and optimized design 2.

Build the model

(1) Establish a three-dimensional solid model

According to the structure and load characteristics of the support roller, the author takes a quarter of the support roller for modeling. In order to make the applied loads and constraints closer to the actual working conditions, the inner ring of the four-row cylindrical bearing and the back-up roller are integrated, and the solid model of the intermediate roller and the back-up roller bearing seat is established (see Figure 9).

Figure 9 3D solid model

(2) Mesh division

The mesh was divided using spatial hexahedral elements and was refined in areas where stress concentration may occur. A total of 1,674,559 nodes and 460,071 elements were obtained (see Figure 10). This model is a nonlinear analysis model of multiple contact bodies.

Figure 10 Mesh division of the original design scheme

(3) Boundary conditions

1) Establish contact between the intermediate roller and the back-up roller, and between the back-up roller and the bearing seat.

2) Apply symmetry constraints on the symmetry plane. Vertically fixed constraints are applied below the intermediate roller.

3) Apply a quarter rolling force of 312.5 t to the bearing seat.

The author gives the boundary conditions of the original design scheme (see Figure 11). Since the boundary conditions are the same, the boundary conditions of the other three schemes are not shown.

Figure 11 Boundary conditions of the original design scheme

After the boundary conditions are added, the model is basically established and meets the calculation conditions.

Analysis and calculation

After calculation and analysis of the original design structure, a von-mises stress distribution cloud diagram was obtained. The maximum stress was 358.57 MPa, appearing at the “groove” (see Figure 12).

Figure 12 Stress cloud diagram of the original structure

After calculation and analysis of the existing structure, a von-mises stress distribution cloud diagram was obtained. The maximum stress was 377.67 MPa, appearing at the “groove” (see Figure 13).

Figure 13 Stress cloud diagram of existing structure

After calculation and analysis of the optimized design scheme 1, the von-mises stress distribution cloud diagram was obtained. The maximum stress was 325.3 MPa, which appeared in the transition area at the junction of the roll neck and the roll body (see Figure 14). After calculation and analysis of the optimized design scheme 2, the von-mises stress distribution cloud diagram was obtained. The maximum stress was 287.85MPa, which appeared in the transition area at the junction of the roll neck and the roll body (see Figure 15).

The material selected for the back-up roller is YB-65, and the yield strength σs = 650 MPa, from which the maximum stress and safety factor of the mandrel are obtained (see Table 1). It can be seen that after the user’s modification, the safety factor of the backup roller was greatly reduced, which increased the risk of using the backup roller; while the optimal design 2 has a higher structural safety factor than the optimal design 1, and the end user determined the optimal design 2 as the final solution.

Figure 14 Stress cloud diagram of optimized design solution 1

Figure 15 Stress cloud diagram of optimized design option 2

Table 1 Maximum stress generated by the mandrel under different loads

planMaximum stress (MPa)Safety factor
original design358.571.81
existing structure377.671.72
Optimized design 1325.32
Optimized design 2287.852.26

Conclusion

This optimization plan has been put into actual production and application, and no roller breakage accidents have occurred again.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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