Exploration of thermal damage cracks on work rolls of hot continuous rolling high chromium iron

Various types of cracks will occur in hot rolling work rolls during the entire life cycle of production and use. In order to ensure the production safety of rolling mills and rolls, these cracks, especially mechanical impact cracks and manufacturing defect cracks, must be removed according to production process requirements. However, rolls are the main spare parts for the steel rolling process. A large number of roll abnormalities will inevitably lead to an increase in steel rolling costs. It will also affect the turnover efficiency and roll preparation efficiency of the rolls, affecting production stability and safety. In order to solve the above problems, the experimental strip of high chromium iron work rolls with thermal damage cracks in the front section of finishing rolling was used on the machine.

There are three main types of cracks generated during the use of hot rolling work rolls, namely mechanical cracks, fatigue cracks, and thermal damage cracks (caused by overheating of stuck steel, etc.). A certain hot rolling line has a high accident rate in the front section of finishing rolling (F1~F4), mainly including F1 slipping and jamming, F1~F4 steel pile-up, etc. This results in a high loss of high-chromium iron work rolls in the front stage of finishing rolling, averaging more than 60 mm per month, of which thermal damage crack losses account for about 80%. According to the production process requirements, these cracks must be removed to ensure the operational safety of the rolling mill and rolls. However, in order to reduce the cost of steel rolling (roller consumption cost and grinding cost) and improve the grinding efficiency and turnover efficiency of the rolls, it is necessary to explore the mechanism of using the work rolls with hot cracks on the finishing rolling high-chromium iron material.

Stress analysis of work rolls in the front section of finishing rolling

In addition to rolling force, the finish rolling work roll also experiences friction force, micro-tension, thermal stress, etc. during the rolling process. However, friction, micro-tension, and thermal stress are much smaller than the rolling force. Therefore, when analyzing the stress of the finishing rolling work roll, the role of the rolling force is mainly considered. As shown in Figure 1, when the roller surface A is in contact with the strip, it is approximately considered that the resultant force of the positive pressure and friction force there is equivalent to the rolling force. The maximum rolling force of the finishing rolling front-end stand of the hot tandem rolling line is 35 MN. Generally, the rolling force is 15~20 MN during normal production.

Figure 1 Schematic diagram of stress during the rolling process of finishing rolling work rolls

Analysis of critical conditions for thermal damage crack propagation of high-chromium iron work rolls during operation

This article explores the use of thermal damage cracks on high-chromium iron work rolls in the early stage of finishing rolling. For this purpose, the concept of material fracture criteria is quoted. Starting from the elastic mechanics equation or the elastic-plastic mechanics equation, the crack is regarded as a condition, the stress, strain and displacement field at the crack tip are considered, and the critical condition for crack expansion – fracture criterion is established.

According to the shape of cracks in components, they are generally divided into three types: penetrating cracks, surface cracks, and buried cracks, as shown in Figure 2:

Figure 2 Schematic diagram of different crack shapes: (a) penetration crack; (b) surface crack; (c) buried crack

Thermal damage cracks of high-chromium iron work rolls in the early stage of finishing rolling are generally surface cracks. For this reason, we mainly study the fracture toughness of surface cracks. In the laboratory, fracture toughness experiments were conducted on rolls made of high-chromium iron. The intensity of stress at each point near the crack tip is related to the quantity of σ√πa. That is, the stress at each point near the crack tip does not increase or decrease in proportion to the tensile stress σ on the component, but increases or decreases in proportion to σ√πa. σ√πa is called the stress intensity factor and is recorded as KI. The unit is MPa⋅m1/2MPa⋅m1/2, KI =σ√πa, where a is the crack depth.

As the load increases, the stress intensity factor KI also gradually increases. Experimental results show that when it reaches a certain critical value KIc, the crack will propagate unstablely, causing the component to break. KIc is called short crack toughness. That is, if the fracture strength factor KI is lower than KIc, crack propagation will not occur in the component.

According to Engineer Fumio Tanaka of Mizushima Steel Plant of Kawasaki Steel in Japan, the fracture mechanics criterion was used to determine the use limit of the roll, KI=1.12σ√πa⩽KIc. Theoretically calculated from this formula, cracks with a depth less than 10.59 mm can be used normally under the action of a rolling force of 34 MN. Therefore, the high-chromium iron work rolls in the front section of finishing rolling of the rolling line can be used normally on the machine.

Computer experiments and analysis

Characterization of thermal damage crack morphology and eddy current characteristics

After the work roll of high chromium iron in the finish rolling is jammed, a thermal damage crack zone will generally be generated in the steel passing area. The shape of the cracks is generally grid-like (Figure 3), with a depth of 3~10 mm, and the roller surface is abnormally obvious after being removed from the machine. After the roll is ground, eddy current testing will be performed. It will be found that the detection results include crack value bands and soft point value bands corresponding to the cracks on the roll body, along the axial direction of the roll (Figure 4). The thresholds of eddy current crack value and soft point value are 0.2 and 0.4 respectively, and anything exceeding is considered abnormal.

Figure 3 Hot crack morphology on stuck steel roller surface

Figure 4 Hot crack morphology on stuck steel roller surface

Detection and guarantee work for experiments with cracks

The eddy current testing results feed back the changes in thermal cracks. The eddy current crack value and soft point value of the hot crack zone generally change little or continue to decrease, indicating that the crack is gradually weakening. At the same time, combined with observing the shape of the crack zone (the width of the hot crack zone in the axial and circumferential directions becomes narrower, the crack boundary becomes thinner, etc.), it is determined whether the roll can be continuously used on the machine.

Use angle probes to detect and analyze crack depth changes. After each grinding of a work roll with thermal cracks, an ultrasonic oblique probe can be used to detect the depth of the cracks. If the depth detected by the oblique probe gradually decreases, it indicates that the crack is gradually eliminated.

Use dual element straight probes to detect and analyze thermal crack growth. The dual-element straight probe detection is the most important, and its main function is to check whether the crack is expanding at an angle. According to the characteristics of longitudinal waves, if the crack is perpendicular to the cross section of the roll surface, the dual element probe cannot detect it and can only detect the reflected wave of the bonding layer. When a crack propagates, the crack wave detected by the twin crystals shows a “wave walking” phenomenon, gradually deepening from shallow to shallow.

Experimental work and results

At the beginning of the experiment, the rolls with shallower cracks were first used as the experimental objects. After the cracks were eliminated normally, rolls with deeper cracks were gradually tested, and subsequent experiments were carried out on rolls with a diameter close to scrap. As a result, all eight experimental rolls were used until the cracks were eliminated normally, or the cracks were normal and scrapped, which shows that the high-speed steel work rolls in the front section of finishing rolling can be used safely with hot cracks. Detailed experimental data are shown in Table 1.

Table 1 Experimental conditions of hot cracks on high-chromium iron work rolls in the front section of finishing rolling

Roll numberFirst machine diameter with crack/mmTheoretical scrap diameter/mmNumber of machine runs with cracksAngle probe detects crack depth/mmUse crack working layer/mmCrack formresult
1748.34670.004not detected2.48Hot cracks in stuck steelCracks are eliminated normally
2748.85670.005not detected2.75Hot cracks in stuck steelCracks are eliminated normally
3702.67670.00164.057.05Hot cracks in stuck steelCracks are eliminated normally
4745.85670.00186.5511.58Hot cracks in stuck steelCracks are eliminated normally
5731.60670.00288.4814.01Hot cracks in stuck steelCracks are eliminated normally
6735.93670.00256.4112.71Hot cracks in stuck steelCracks are eliminated normally
7678.93670.00165.508.93Hot cracks in stuck steelNormal scrap
8680.42670.00259.5010.42Hot cracks in stuck steelNormal scrap

Then modify the use system to include the rolls with thermal cracks on the machine into normal use, and the accident rolls with cracks on the machine, the roll replacement cycle and grinding amount are the same as normal rolls. After the implementation of the system, the hot crack grinding loss of high-chromium iron is reduced by more than 40 mm per month, and a large amount of costs such as grinding wheels, cutting fluids, and manual grinding are saved. It alleviates the situation of heavy investment in work rolls, large capital occupation and tight turnover in the front stage of on-site finishing rolling.

Conclusion

Theoretical calculations and experimental verification show that the high-chromium iron work rolls in the front section of finishing rolling can be used on the machine with thermal damage cracks, and the roll changing cycle and grinding amount can be consistent with normal rolls. The crack growth rate is smaller than the normal diameter reduction of the roll, and the thermal damage cracks gradually disappear as the roll is used. At the use site, the ultrasonic dual-element probe cannot detect the existence of cracks, that is, the direction of the cracks is consistent with the direction of the ultrasonic waves and perpendicular to the cut surface of the roll surface.

The high-chromium iron work rolls with thermal damage cracks in the front stage of finishing rolling are put on the machine, which not only reduces the abnormal loss of high-chromium iron work rolls, but also saves a lot of costs such as grinding wheels, cutting fluids, and manual grinding. It also alleviates the situation of heavy input of work rolls and tight turnover in the front stage of on-site finishing rolling, which is of great significance.

 

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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