Application of high-speed steel rolls in finishing stand of bar mill

Abstract: This article introduces the problems, experiences and effects of using high-speed steel rolls in the finished product rack of the bar factory of Wuhan Iron and Steel Group Echeng Iron and Steel Co., Ltd. The main improvement is to increase the diameter of the cooling elbow and increase the number of nozzles, and after using high-speed steel rolls, the average single-hole steel passing capacity of Φ18 and Φ25mm ribbed steel bars reached 716.35 and 1028.47t respectively.

With the advancement of steel rolling technology and the improvement of equipment level, the bar production line has moved towards continuous, automated and efficient production, and its requirements for the wear resistance and thermal stability of the rolls are also getting higher and higher. In the past, various types of Ductile iron rolls could no longer meet the production requirements of high output, high operating rate, and low labour intensity. In addition to the special requirements for the hardness and toughness of the rolls in the finished frame of the bar line, the pre-slitting and slitting pass for rolling small-sized hot-rolled ribbed steel bars using slitting technology is equally important, because the cutting technology has strict requirements on the stability of the pre-cut and divided material types. Therefore, the application of high wear-resistant and long-life rolls is an important means to improve the production efficiency of bar production lines. At present, this type of roll mainly includes centrifugally cast high-speed steel composite rolls, forged high-speed steel roll ring combination rolls, tungsten carbide roll ring combination rolls and high alloy steel roll ring combination rolls.

 

Production status of Hubei Steel Bar Factory

 

The bar factory has two workshops. The first workshop is an old production line put into operation in 1995. The designed annual output is 300,000 tons, which has reached 680,000 tons currently. The main rolling specifications are Φ12~Φ22mm hot-rolled ribbed steel bars, Φ14~ For Φ20mm round steel, Φ12mm steel bars are rolled in three cuts, and Φ14~Φ18 steel bars are rolled in two cuts. The roll model of the finishing rolling unit is Φ360mm*600mm. The second workshop is a more advanced rolling line put into operation in 2003, with a designed annual output of 500,000 tons, which has now reached 700,000 tons. The main rolling specifications are Φ20~Φ36mm hot-rolled ribbed steel bars, and Φ22~Φ150mm round steel, among which Φ20~Φ22mm steel bars are rolled in two sections, and the roll model of the finishing rolling unit is Φ370mm*650mm.

At that time, the Esteel bar production line still generally used traditional cast iron rolls. The finishing rolling unit is generally used centrifugal cast high-nickel-chromium needle-shaped bainite ductile iron rolls. The single-groove steel passing capacity of the finished rolls was based on the rolling specifications and processes. Conditions vary, generally only 150~350t/trough. This means that a bar production line with an output of about 1,000t per shift (8 hours) needs to change the rolling groove 3 to 5 times per shift, resulting in a long cumulative roll change time, frequent rolling mill adjustments, and many production accidents, thus reducing the operation rate of the entire production line. Decline, technical and economic indicators such as yield rate are poor, product size fluctuates greatly, and comprehensive production costs increase.

The main performance indicators of the rolls in bar rolling mills are core strength and working layer wear resistance. When selecting rolls, in addition to considering the economy and matching the roll replacement cycle and downtime, the more important thing is to reasonably select rolls with different performance characteristics based on the pass shape differences of each unit, the deformation characteristics of the rolling stock, and product accuracy requirements.

The bar finishing mill has many rolling varieties, complex pass shapes, uneven deformation distribution, high rolling speed, and large changes in rolling force. Therefore, the performance requirements of the rolls are mainly wear resistance and toughness, taking into account thermal crack resistance and strength. Due to the influence of transverse ribs, the finished holes of ribbed steel bars are easily prone to thermal cracks during use. Due to the requirements for stable product quality and high production pressure, Egang Bar Factory began to trial high-speed steel in the second half of 2009. For rolls, the Esteel Bar Factory conducted trials on two specifications of Φ18 and Φ25mm in the K1 holes of the two workshops.

 

High-speed steel roll trial process and results

 

Since high-speed steel rolls have poorer thermal crack resistance than cast iron rollers, during the first trial rolling of Φ18mm ribbed steel bars (cut into two), the amount of steel passing through a single hole was less than 400t, mainly at the two ends of the transverse bars on the surface of the steel bars. There are obvious micro-cracks, and the lettering is easy to wear. According to the analysis, there are two reasons: one is that there is a problem with the roll turning, and the other is that the cooling water is not enough, and the cooling water of the high-speed steel roll needs to be modified.

The main problems in roll turning are that the feed amount is too large and the engraving depth is slightly shallow when milling transverse ribs. The feed amount and engraving depth are 0.1mm and 1.2mm respectively. After being changed to: 0.05mm and 1.3mm respectively, the phenomenon of micro cracks is eliminated.

In order to ensure the amount of water, the cooling elbow requires sufficient water. For this reason, the diameter of the cooling elbow is increased. In addition, there is a pause time for the cooling water to be sprayed onto the roll to fully cool it instead of spraying it onto the roll. It will splash out immediately, and the water pressure requirement cannot be too high, so while increasing the diameter of the elbow, increase the diameter of the cooling nozzles and increase the number of nozzles. After several attempts, the optimal values for elbow parameter design were found. The cooling elbow before and after modification is shown in Figure 1.

 

Cooling elbow
Figure 1: Cooling elbow of K1 hole roll
a) Before transformation     b) After transformation

 

Through the above improvements, the average steel passing through a single hole of Φ18mm ribbed steel bars reached 716.35t. On this basis, the cooling bend of the second workshop was also modified. The average single hole passing steel passing through Φ25mm ribbed steel bars reached 716.35t in the first trial. 1028.47t, which greatly increases the steel passing capacity of a single groove of high-speed steel rolls and improves production efficiency.

After the trial of high-speed steel rolls in the two workshops of Egang Bar Factory, the economic value of high-speed steel rolls was confirmed. To greatly increase the production capacity of the rolling line, it was decided to promote the use of high-speed steel rolls in three-slit and two-slit slitting rolls and pre-slit rolls, finished front holes of ribbed steel bars, and finished round steel rolls.

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