A brief analysis of the causes of damage to high-speed steel roll grooves

This article introduces the reasons for the cracking of the high-speed steel roll ring and the loss of meat in the groove. By taking technical measures such as increasing the width of the roll ring, increasing the repair volume of the groove, and improving the cooling effect, the production problem has been solved, and the annual production has increased by 15,000t, which has provided the company with an annual production increase of 15,000 tons. Created huge economic benefits.

 

1. Introduction

 

The bar factory of Shandong Shiheng Special Steel Group Co., Ltd. has been testing the use of high-speed steel rolls in K1 finished product runs since early 2006. It is mainly used to roll φ10mm, φ12mm, and φ14mm hot-rolled ribbed steel bars. The roll has a high rolling capacity and long life. , the cost performance is better than rollers made of other materials, but two difficult problems encountered during use are:

First, the roll ring is cracked, which is manifested as the roll ring cracking in the middle of two adjacent rolling grooves and being connected together, making the adjacent grooves no longer usable.

Another situation is that lumps fall off near the centre of the bottom of the rolling groove or meat falls off in filaments along the circumferential direction, causing the rolling groove to be scrapped.

The first type of defect occurs suddenly without warning. If the self-inspection is not in place, it will cause a large number of finished product scraps and cause a steel pile failure in the K1K2 rolling mill. The second type of defect occurs gradually, develops, and can be discovered early. Due to the above two defects, the annual defective product rate reaches an average of 0.02%, and product quality hazards are very high.

 

damage to high-speed steel roll

 

2. Cause analysis

 

During the production process, the loads endured by the rolling groove are very complex. They include normal rolling force and impact load. They are also affected by the high temperature of the rolled piece and cooling water, and there are frequent alternating thermal stresses. Due to the production of hot-rolled ribbed steel bars, the rolling groove must also withstand axial compressive stresses. From a material science point of view, the cracking of the roll ring and the loss of meat at the bottom of the rolling groove should be classified as fatigue failure of the roll. This is mainly due to the fact that the roll ring and rolling groove are subjected to variable loads during the rolling process, which leads to the initiation and expansion of cracks, and ultimately leads to fracture. Based on this theory and combined with production practice, the following analysis is conducted on the cracking of the roll ring and the loss of meat in the rolling groove.

 

2.1 The roller ring is not strong enough

 

Taking the φ12mm hot-rolled ribbed steel bar K1 finished roll as an example, the centre distance of the rolling groove is 19mm and the width of the middle roll ring is 7.9mm, which meets the experience requirements. However, based on the analysis of the cracking of the roll ring and the deformation characteristics of the rolled piece, in extremely complex changes Under load, the fatigue of the roller ring forms cracks and eventually causes the roller ring to break, which is a major cause of failure.

 

2.2 Poor cooling effect

 

During use, the colour of the grooves on both sides of the rolling groove turns blue. Use a hand-held temperature measuring gun to measure the surface temperature above 80°C. Carefully observe both sides of the groove to see the presence of small cracks.

 

2.3 The amount of repair is small, and not all micro-cracks in the rolling groove have been removed.

 

Taking the φ12mm hot-rolled ribbed steel bar as an example, due to insufficient daily supervision, the rolling amount of each groove was controlled by the finished product adjuster, which resulted in inconsistent wear of each groove. During the repair of the rolls, all cracks in individual grooves were not turned. Lose. Due to the existence of micro-cracks, when it is put into use again, the micro-cracks quickly grow and connect, eventually leading to macro-cracks, causing the rolling groove to lose meat.

 

2.4 K2 material width is too large

 

After the ribs are milled, a “notch effect” will be formed around the transverse ribs of the rolling groove, which is where the stress concentration is greater. When the K2 material type width is too large, the K1 reduction amount will be too large, and the deformation resistance and front and rear slippage will increase sharply, causing the metal to flow quickly. Under the repeated action of the rolled piece, filamentous flesh will appear in the rolling groove, or the formation of Blocks drop periodically.

 

2.5 Improper operation

 

Due to improper operation by employees during the production process, the rolling groove was accidentally injured or suffered abnormal external forces. For example, the nose tip of the exit guide is in direct contact with the rolling groove, there are blackheads on the incoming material, or the rolled piece is wrapped around the roller, etc.

 

3. Measures

 

3.1 Increase the width of the roller ring

 

The centre distance of the rolling grooves of φ10mm, φ12mm and φ14mm hot-rolled ribbed steel bars is increased to 19mm, 22mm and 24mm respectively. At the same time, the notch fillet is enlarged. The original fillet was r0.8mm, but now it is increased to r1.0mm~r1.5mm to improve strength and reduce stress concentration.

 

3.2 Improve cooling effect

 

In order to meet production needs and multi-line cutting needs, a new medium-pressure turbid water system was installed with a rated flow of 230m³/h and a lift of 110m. At the same time, the roll cooling device was improved many times to improve the cooling effect. During production, ensure that the cooling water pressure of the roll is above 0.5MPa and that the width of the cooling device exceeds the width of the rolling groove by 2~4cm to ensure that the rolling groove is well cooled. After stopping the steel passing, the surface temperature of the rolling groove should be controlled below 35°C, otherwise it will The amount of water should be adjusted.

 

3.3 Increase the amount of repairs

 

Make sure that all cracks on the edges and bottom of the rolling groove are turned away, and there should be no black skin or small cracks, especially at the groove. The final turning amount of φ10mm, φ12mm, and φ14mm hot-rolled ribbed steel bar finished rolls is adjusted to 4mm, 6mm, and 7mm respectively. If the cracks cannot be completely eliminated in individual rolling grooves, they will be marked as not to be processed again.

 

3.4 Strictly control the width of K2 material

 

Strictly control the ratio of K2 material width to rolling groove inner diameter, no more than 1.7 times. When applying multi-line slitting technology to produce hot-rolled ribbed steel bars, the thickness of the pre-cutting and slitting passes and the size of the ears on both sides must be strictly controlled to prevent rolling marks from forming on the finished roll and causing serious wear of one of the sticks. , while the other branch is a normal phenomenon.

 

3.5 Strict process systems to eliminate improper operations

 

Strict process systems and eliminate improper operations, which mainly include the following points.

a. During production, the rolling mill adjuster is required to strictly install the exit guide device, and the guide device must not be in direct contact with the rolling groove.

b. Strictly control the amount of cooling water at the roll inlet to reduce the quenching of the head of the rolled piece. In addition, the cooling water amount and water pressure are determined according to the ambient temperature and the temperature of the turbid ring water. The measurement after shutdown shall not exceed 30~35℃. When the machine is shut down for a long time in winter and then put back into production, the cooling water of the first three steel rolling troughs is controlled at 50% of the daily level, and then it is opened to normal requirements.

c. Strengthen daily inspections to reduce roll wrapping failures. At the same time, it is stipulated that when rolling the roll, all the scrap products wrapped in the roll must be cooled to room temperature with cooling water before the water is allowed to be stopped for processing scrap steel.

 

4. Effect

 

After the implementation of the above measures, since the end of 2006, the bar factory of Shandong Shiheng Special Steel Group Co., Ltd. has been using high-speed steel material for the finished rolls in the production of hot-rolled ribbed steel bars. Compared with bainite and tungsten carbide roll rings, it has excellent cost performance. Taking the production of φ12mm hot-rolled ribbed steel bars as an example, the average rolling volume in a single groove reaches 350t, exceeding bainite by an average of 230t, and the phenomenon of roll ring collapse and rolling groove meat loss does not occur. Due to the extension of the rolling groove life, a lot of time is saved in changing the roll holes, which increases the calendar operation rate of the bar factory by an average of 2.08% and increases the annual production by 15,000 tons, which is a huge benefit.

 

Source of article: Zhang Zhongfong, Wang Pingji, Yindai, Kong Fanggang, Liu Penggang, “A brief analysis of the causes of damage to high-speed steel roll grooves”

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