Abstract: The main reason for the damage of four-high hot strip rolling mill bearings in a steel mill is that they are subjected to excessive radial and axial loads during work. The non-parallelism between the working roller and the supporting roller leads to the relative axial sliding of the contact surface, resulting in a large axial sliding friction force. The rolling parts deviate from the rolling center line in the rolling process, resulting in uneven radial load of each row of bearings, which often leads to the radial load overload of one row of bearings, resulting in the damage of the whole row of bearings. Therefore, it is necessary to study the force analysis of four-high rolling mill bearing, the stability of rolling mill frame, etc., to improve the bearing type, the frame repair to improve the accuracy of equipment, to improve the rolling mill bearing life, reduce abnormal damage.

There are altogether 6 four-high rolling mills in a steel strip mill. The working roller bearings of each rolling mill adopt 2097938 four-row tapered roller bearings, and the supporting roller bearings adopt 6890250 four-row short cylindrical bearings and 1060 deep groove ball thrust bearings. Specification and continue to produce rolling steel grade as well as the mechanical drive, hydraulic system stability tracking, statistical analysis of roller bearing life track, continue to every one of burning bearing inspection analysis of disintegration, found that the work roll bearing damage occurs mainly in 2097938 near the mill operation side roller diameter side of the first column bearing damaged the whole column bearing damage, The bearing of back-up roll 6890250 is mainly four rows of short cylindrical bearing outer ring and rolling body broken.

1 Analysis of roller bearing damage causes

1.1 Work roll 2097938 bearing damage reason analysis

Continue to every one of burning bearing inspection analysis of disintegration, combined with burning bearing life track statistics and rolled steel grade and specifications, and rolling mill equipment, hydraulic system running stability analysis, found that the work roll bearing damage occurs mainly in 2097938 is a column mill operation side fixed end bearing, the drive side free end bearing damage is rare, And from the burning phenomenon, the bearing lubrication is good, but the bearing outer ring and cage are broken, and part of the damaged bearing rolling body is broken. FIG. 1 shows the force analysis diagram of the work roll and the back-up roll of the four-high mill. From the force analysis, the deformation resistance of the rolled parts is mainly transmitted from the work roll to the back-up roll in the rolling process of the strip products, while the axial force of rolling is mainly transmitted from the friction between the rolled parts and the work roll to the work roll. That is to say, the working roll 2097938 bearing is subjected to more axial rolling force in the rolling process, while the supporting roll 6890250 bearing is subjected to more radial rolling force in the rolling process.

From the scene after the break-up of the damaged bearing inspection analysis are consistent with theoretical analysis, when the production of strip steel rolling force large and frame system stability is poor at this time, the axial force of rolling will impact on the work roll fixed end bearing is very big, will cause the work roll bearing damage, which can be concluded that work roll bearing damage mainly is caused by the larger axial force. Of course any four high rolling mill in the rolling axial force can be generated in the process of rolling a steel DaiGangChang mill housing use fixed number of year is long since the launch in 1996, the window frame work roll and backup roll window shape tolerance deviation is larger, larger frame window center and the roll axis is not parallel, there is a big crossing Angle, in the rolling process in figure 1 will produce larger axial force.

In addition, there are some other reasons for the work roll bearing damage, such as rolling parts in the process of rolling away from the rolling center line, will also cause the work roll bearing each column bearing load bias, resulting in a column of bearing overload damage, resulting in the entire bearing damage; For example, in the process of roller assembly, the working roller bearing four row tapered roller and the bearing outer ring clearance adjustment will also cause the bearing offset load damage.

1.2 Analysis of damage causes of backing roller bearings

As can be seen from the previous roll force analysis, the back-up roll mainly bears the radial force during rolling. Due to the limited size of the rack arch in a steel strip mill, the bearing selection of the back-up roll is 6890250 four-row short cylindrical bearing, and the bearing damage phenomenon is basically the rolling body breakage and the bearing outer ring runway breakage. From the damage phenomenon, it can be concluded that the cause of the bearing damage is overload fatigue damage.

2 Improvement measures to reduce roll bearing damage
After the roller is loaded into the frame, due to improper tolerance control of frame window shape and position, wear of work roll and supporting roll bearing seat, low grinding precision of the roller, large deviation of the inside and outside of the mill floor, large deviation of the rolling center line, and inconsistent adjustment of the gap between the inside and outside of the roller, all these will cause the roller axis space crossing. This is shown in Figure 2.

When the roller space crossing is large, the axial force of the work roller bearing is particularly obvious, and the influence on the offset load of the support roller bearing radial force is also large, and there is a gap in the damage of the work roller bearing and the support roller bearing.

According to the current situation of a steel strip mill, in order to reduce the abnormal damage of four-high mill bearings, the following improvement measures should be taken:

(1) Start with improving the stability of the frame: on the one hand, arrange a special person to check the window size of the frame arch of 6 frames in a planned manner during routine maintenance. If the window size deviation is found to be more than 0.8mm, the rolling mill lining plate must be replaced to control the window size. On the other hand, check the balance cylinder of the work roll and the upper supporting roll to reduce leakage and leakage; And check the elevation of the inside and outside floor of the mill, and the elevation difference is controlled within 0.5mm.

(2) Start with improving the bearing housing accuracy and roll assembly quality: test the external size, symmetry, inner hole size and ellipticity of all roll bearing housing every 6 months, and repair or scrap the bearing housing exceeding the tolerance range of the drawing; Each roll assembly shall be assembled according to the specifications and shall be marked and recorded for statistics, which is convenient for tracing and finding the reasons.

(3) Improve the grinding precision of roller, and control the roundness and ellipticity of roller within 0.03mm.

(4) Improve the level of production operation: on the one hand, control the raw material size of rough rolling into four-high mill, reduce the impact force of rolled parts on rollers; On the one hand, it can improve the balance of the down load of the center line and both sides of the four-high rolling mill, and balance the down load of each stand, so as to effectively control the rolling force balance of the rolled pieces.

(5) The roll bearing is improved: the working roll 2097938 bearing rolling body and the inner and outer ring thickness of the bearing are redistributed to improve the ability of bearing axial force; The supporting roll 6890250 bearing is modified to 6890260, and the bearing material Cr15 of the supporting roll is improved to carburizing material.

3 conclusion
Through the implementation of the above improvement measures and tracking the application effect, it is found that the on-line burning rate of rolling mill bearings is reduced from the original 5.7 to 3.5 / ten thousand tons, with a decrease of 38%.