In order to meet the increasing demand for construction steel, China's metallurgical industry has introduced dozens of high-speed wire rod production lines from abroad in recent years, with the discharge port speed ranging from 63 ~ 100m/s, and the annual output is mostly between 60×10⁴～100×10⁴t. Due to the particularity of high line rolling mill, more stringent requirements are put forward for the bearing. This paper briefly introduces the working characteristics, structural forms, design points and manufacturing keys of high speed wire rolling mill bearings (hereinafter referred to as high speed rolling mill bearings).

1   Performance requirement

1．1　High reliability

The most remarkable feature of the high speed wire rod production line is the high reliability of continuous operation of the rolling mill. Due to the high unit output of the high-speed wire rod production line, the bearing, which is a key component in the rolling mill, is required to have higher reliability than the ordinary rolling mill bearings during the specified point inspection period. If the bearing failure will cause the entire production line to stop production, which is not tolerated by the manufacturer. Therefore, in the design and manufacture of high-line rolling mill bearings, the reliability of continuous operation under high-speed running conditions should be guaranteed.

1．2　Wear-resistant

High line rolling mill bearing wear resistance is strong, can adapt to high-speed rotation of the workplace. Example: a high-speed wire production line, the outlet line speed of 63m/s, the working diameter of the outlet mill roll is 330 mm, the working speed of the bearing is about 3650 r/min, in high speed, heavy load for a long time to run, the bearing wear resistance will be a severe test. This requires that the bearing supplied must have the requirements of wear resistance.

1．3　Long fatigue life

The bearing fatigue life of high line rolling mill is longer. Under normal circumstances, the high-line rolling mill bearings provided by foreign countries are used for three years (because half of the time the roll assembly needs to be unloaded for maintenance and renovation, the actual working time is one and a half years), so the user requires that domestic bearings can be used for at least two years.

1．4　Good impact toughness

The impact toughness of high line rolling mill bearings is better, especially the bearings used in roughing mill, and the impact load is great in the process of rolling continuous casting billet into bar material. Therefore, the bearing of high line rolling mill should not break when working at high speed and high impact.

In summary, the working characteristics of high-line rolling mill bearings are that they can work continuously, stably and reliably at high speed and high impact, and have a long fatigue life.

2　Bearing structure form

There are four row tapered roller bearings and two row cylindrical roller bearings for high line rolling mill. In general, the coarse and medium rolling mill uses four row tapered roller bearings, the finishing mill uses two row cylindrical roller bearings, and the horizontal mill and the plumb mill use the same type of bearings.

Whether four-row tapered roller bearings or double-row cylindrical roller bearings, their basic structure is similar to traditional rolling mill bearings, but the design of each part fully considers the problem of lubrication and heat dissipation, that is, the inner ring, the outer ring, the spacer ring (for four-row tapered roller bearings), the cage, the roller (for double-row cylindrical roller bearings) has many slots, holes, etc. In this way, the lubrication and cooling channel formed by the groove and hole and the bearing seat and the channel on the roll constitute a reliable lubrication and cooling system, coupled with a good sealing device, so that the bearing can operate stably in a high-speed and high-pressure state.

Although the design style of high-line rolling mill bearings provided by various manufacturers is different, they all have one thing in common, that is, the inner ring wall thickness is thicker than that of ordinary bearings of the same specification, and the outer ring is relatively thin.

3　Bearing design method

First of all, according to the operation of each line bearing, service life, failure state and user requirements, appropriate adjustment of the main parameters, so that the design to achieve more satisfactory results.

Secondly, reasonably determine the technical conditions, according to the measured results of similar products, and consider the specific working state of the bearing, put forward reasonable technical requirements.

4　Manufacture of high line rolling mill bearings

In view of the particularity of high line rolling mill, two aspects of work are mainly done in the manufacturing process. The first is the choice of material and heat treatment process, and the second is the roughness control of the rolling surface. Because the inner ring wall is thicker and the outer ring wall is thinner, some manufacturers stipulate in the technical document that the inner ring is made of carburized steel and the outer ring is made of high-carbon chromium steel. Accordingly, the inner ring adopts the carburizing process, and the outer ring adopts the isothermal quenching and tempering process as far as possible, so that the obtained metallurgical structure is lower bainite, and the performance indicators such as the strength, wear resistance, impact toughness and fatigue life of the bearing are taken into account after this treatment, so that the bearing can meet the above-mentioned working requirements. In addition, the hardness of the heat treatment is also different according to the different conditions of the rough, medium and finishing mill, that is, for the rough mill, the speed is relatively low, and the impact load is relatively large, so the hardness of the bearing parts is low; For the finishing mill, the speed is relatively high, and the impact load is relatively small, so the hardness of the bearing parts is high; The hardness of bearings used in medium rolling mill is in the middle.

Through observation and test analysis, the surface roughness values of raceway surface, guard surface and reference end face of rolling surface of high line rolling mill bearings are smaller than those of ordinary rolling mill bearings. To this end, measures are taken in the process to make the surface roughness reach or close to the level of foreign products.

5　Bearing trial situation

Since May 1997, 9 types of high-line rolling mill bearings have been trial-produced or mass-produced for 3 high-speed wire rod production lines in China, most of them are in good condition, and no operation failure has led to the shutdown of the whole line. Among them, in May 1997, it was the first batch of trial-produced middle mill bearings (four row tapered roller bearings) installed on the plumb mill of a high line in Southwest China (plumb mill works worse than the horizontal mill), except that one set of bearings had been running for a period of time and the outer ring was found to be broken during the normal unloading inspection. In the remaining three sets, no failure conditions such as wear, fatigue and rupture were found by June 2000.

6　Suggestion

Further development and application of high line rolling mill bearings, so that its performance is stable at a high level, should be carried out in the following aspects.

(1) According to the literature, on the basis of test and research, new rolling mill bearing steel with excellent performance has been smelted, including carburized bearing steel and high-carbon chromium bearing steel.

(2) On the basis of analysis, research and comparison of similar products, determine the main parameters according to the use of bearings and do a good job in product design. In addition, the structural changes of the parts, such as the layout of the slots and the shape of the spacer rings, can not be ignored.

(3) To carry out technological research to make up for the reality of relatively poor material performance and relatively backward equipment capacity. The emphasis is on heat treatment and grinding to ensure the effective carburizing layer depth and hardness uniformity, the impact toughness of high-carbon chromium steel parts and the shape and position tolerance and surface roughness of parts within a certain precision range.

(4) Develop specifications for detailed installation, commissioning, maintenance and spot inspection instructions; Send technical personnel to the user, explain the precautions for the use of bearings for the field technicians and workers, answer the questions raised by the field personnel, and do a good job in after-sales service. The user needs to cooperate well with the bearing manufacturer.