Application of high-speed steel rolls in bar slitting and rolling

1. Introduction

The Φ260mm unit of Anyang Iron and Steel Group Company is one of the earliest semi-continuous bar production lines put into operation in my country. It introduces key technologies and equipment from the Danieli Company of Italy. Its varieties and specifications are 12~20mm round steel and Φ12~32mm ribbed steel bars. The original design production capacity was 200,000 tons per year. After continuous strengthening of management, technological progress, innovation and tapping potential, it now has a production capacity of 900,000 tons, which is more than three times the design capacity. In the past two years, with the continuous improvement of output and major technical indicators, the problem of multiple roll and slot changes and long time for slit-size finishing rolling units has become more and more prominent, which has seriously affected the further improvement of operation rate and output and other indicators. For this reason, high-speed steel rolls have been used in the production of Φ12, Φ14, and Φ16 mm hot-rolled ribbed steel bars since April 2008.

 

2. Characteristics and existing problems of bar split rolling

 

2.1 Characteristics of bar split rolling

 

There is a big difference between the split rolling process and the single-line production process, which is mainly reflected in the following two aspects:

 

(1) The reduction rate at the wedge tip of the pre-cut pass is particularly large and the size is small. The wedge tip arc at the pre-cutting point is generally designed to be R1.4~R2.0mm. Therefore, from a design perspective, the rolling groove is required to have both high wear resistance and a certain degree of toughness, otherwise, wedges will easily occur. The tip is excessively worn or the wedge tip is broken, causing process failure.

(2) The stable and high output of slit rolling depends on the material size, speed adjustment, and sensitivity of the looper reaction. Among them, the stability of the material shape is the most critical. Therefore, the technical requirements for the rolls in the slitting process are stricter than those in the single-line production process.

 

2.2 Problems in bar split rolling

 

With the continuous improvement of unit output and operating rate, higher requirements have been put forward for the single-groove rolling capacity of the rolls. However, cast iron rolls are increasingly unable to meet the current production needs, especially in the pre-cutting passes. The improvement of the effective operating rate of the rolling mill and the physical quality of the product has obviously restricted the effect, which is mainly reflected in the following four aspects:

(1) The rolling groove life is short, and roll and groove changes are frequent, which has a great impact on the operating rate. For example, the rolling volume of the pre-slit pass is about 400t, and each change of the rolling groove takes 10 minutes. The operation rate is affected by about 3% due to simple slot changes throughout the day.

(2) The physical quality of steel is unstable, and it is difficult to adjust the rolling groove after use. Due to the low hardness of the rolling groove, the wedge tip of the pre-cut hole wears very quickly, resulting in an unstable material shape and uneven distribution. This is reflected in the finished steel material, that is, large dimensional changes between each line and in the length direction of a single branch. The impact on the finished product becomes more severe in the later stages, so defective products are prone to occur and it is difficult to achieve stable negative deviation rolling.

(3) After the rollers are used, the weight of the vehicle will be large and the roller consumption will be high. Due to the different wear characteristics of the hole patterns in each pass, in order to ensure the quality after repair, it is necessary to re-roll the machine by 6~10mm each time, and the roller consumption is relatively high.

(4) High labour intensity and poor environment. In order to ensure the physical quality of the finished product, the finishing rolling unit must be adjusted every 10 minutes. In addition, due to the high temperature and humidity around the rolling mill, it has also caused great harm to the health of employees.

 

3. Application of high-speed steel rolls

 

High-speed steel rolls are high-carbon high-speed steel composite rolls. The working layer of the roll is made of high-carbon high-speed steel, and the core material of the roll is made of ductile iron, graphite steel or forged steel. The two different materials are composited by centrifugal casting or CPC technology. Become. High-speed steel rolls have high carbon content and high alloying elements such as vanadium, chromium, tungsten, molybdenum, and niobium. The carbide types in the structure are mainly MC type and M2C type. The carbides have high hardness and good wear resistance. , the surface hardness of the roll body is HSD80~90, which is on average HSD20″ higher than the CrMo infinitely chilled cast iron rolls cast by conventional methods. The hardenability is good, and the hardness from the surface of the roll body to the inside of the working layer hardly drops, thus ensuring that the roll is It has equally good wear resistance. However, the material has a small thermal conductivity, high brittleness, and poor impact resistance. Therefore, it cannot be chilled or heated, and it cannot collide with other objects. Otherwise, it is easy to cause the groove to fall off or even break the roll.

 

3.1 Processing of high-speed steel rolls

 

In the application of high-speed steel rolls, many problems have been encountered in processing. After exploration, some solutions have been found:

 

(1) Tool selection.

 

The roller material has high hardness and the tool wear resistance is poor. The tool surface needs to be replaced frequently during processing, resulting in rough-machined hole shapes of different depths, which brings greater difficulties to finishing. If the tool wears too fast, it is easy to cause chipping or tool handle failure, resulting in the scrapping of the groove. During the trial process, although YG6A carbide, ceramic and other materials were used for cutting tools, there was no obvious improvement.

After a comprehensive comparison of cutting tools made of various materials, it was decided to use PCBN (cubic boron nitride) cutting tools. After testing, the efficiency increased by 2 to 3 times.

(2) Tool problems seriously restrict the improvement of processing efficiency. In order to adapt to the tool, the cutting amount during rough turning can only be selected as follows: spindle speed 12r/min, cutting depth 0.5mm, and feed rate 1~3mm/min. During finishing turning, the spindle speed is 14r/min, the cutting depth is 0.1mm, and the feed rate is 2~4mm/min.

(3) When replacing the blade, because the tool is not processed in place after it becomes blunt, and the deformation in the Knife phenomenon.

(4) The entire processing technology should be divided into three processes: rough turning, semi-finishing turning and finishing turning.

(5) During the rough turning cycle, attention should be paid to the feed speed during feed.

(6) There is an irregular hole pattern problem during finishing turning, that is, the entry point at the bottom of the hole pattern is shallow and the retraction point is slightly deeper. Analyzing the reasons, first, there is a gap in the X-axis of the machine tool. Second, the rigidity of the machine tool is insufficient, resulting in elastic deformation. The third is that the tool wears out and becomes dull. The measures taken for this are: making a table to compensate for the gap, replacing the blade, and adopting forward and reverse procedures to deal with the problem of elastic deformation caused by insufficient rigidity of the machine tool.

 

3.2 Cooling of high-speed steel roll grooves

 

The water pressure, water volume and cooling position of the rolling groove cooling water are some of the key factors affecting the rolling amount, especially for high-alloy rolls.

During the rolling process, cooling the rolls has three purposes:

1) Prevent the roll from overheating and ultimately causing peeling.

2) Prevent the roll surface temperature from being too high, causing changes in the roll surface structure and affecting the roll wear resistance and rolled material surface quality.

3) Extend the formation time of thermal fatigue cracks and prevent the expansion of thermal fatigue cracks. In order to obtain good results from high-speed steel rolls, the cooling requirements for the rolling groove are as follows:

(1) The cooling water pressure is 0.8~1.0MPa and shall not be lower than 0.6MPa. Cooling water must be filtered and must not contain coarse particle impurities.
Avoid clogging cooling water pipes and nozzles. It is recommended that the cooling water of the finished rack be introduced from the through-water cooling water pipe.

(2) The cooling water nozzle should be positioned as close as possible to the exit of the rolling groove. The entrance of the rolling groove does not need to be installed. The spray angle of the nozzle is 20~30° to ensure that the cooling water is quickly sprayed to the part of the rolling groove that has just separated from the rolled piece. Import guide The cooling water pipe must not wash away the root of the rolling groove (it can easily cause pitted defects).

(3) The arc length of the cooling water pipe is approximately one-third of the outer diameter of the roll, as shown in Figure 1. If there is no curved pipe, a straight pipe can be used instead.

 

(4) Each arc-shaped pipe matches a water inlet pipe. Do not match one water inlet pipe to multiple arc-shaped pipes. The diameter of the cooling water pipe must be greater than Φ25.4mm.

 

4. Application effects

 

4.1 Increase in single groove rolling volume.

 

The comparison of the results of using high-speed steel rolls and cast iron rolls in hot-rolled ribbed steel bars K and pre-cut holes of various specifications is shown in Table 1.

 

Table 1: Comparison of average single groove rolling amount between high-speed steel rolls and cast iron rolls

Ribbed steel bar specifications/mmAverage single groove rolling volume/tImprovement rate/%
High-speed steel rollCast iron roll
φ12 two-slitting rolling1924640201
φ12 three-slitting rolling144680081
φ141830840118
φ162231960132

 

4.2 Economic and technical indicators

 

After the high-speed steel rolls went online, compared with 2008, the effective operating rate increased by an average of 3.8%, the yield rate increased by an average of 0.31%, and the negative margin rate increased by an average of 0.27%. The number of accidents in the slitting process and the impact time of the accidents have increased on average at a high level. A 20% reduction can create an annual benefit of more than 2 million yuan.

It also produces a series of indirect benefits:

(1) Reduces employee labour intensity and improves work enthusiasm.

(2) Material type control is precise and finished product size control is more stable.

(3) The increase in single-groove rolling volume can reduce the number of spare rolls, save space resources, and reduce intermediate costs such as hoisting, installation, and swing.

(4) The frequency of human-machine combination is reduced, and safety is more guaranteed.

 

5. Conclusion

 

More than a year of use has shown that the application of high-speed steel rolls in bar slitting and rolling production has a good cost performance, can greatly shorten the roll replacement time, effectively improve the calendar operation rate, and its high wear resistance has It is conducive to stable and high-yield production, and product quality is better guaranteed. Therefore, high-speed steel rolls have broad market space and application prospects in online bar rolling mills.

 

Article source: “Steel Rolling”

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