An Effective Way to Reduce Roll Consumption——Repairing Pinch Rolls Using Surface Welding Technology

Technical requirements for pinch rollers

(1) High-temperature resistance

The temperature of the pinch roller is about 550-650°C; the surface of the pinch roller is required to be resistant to high temperature and alternating heat and cold.

(2) Anti-stress

The maximum pressure setting value of the pinch roller is about 40kN; it is required that the roller surface of the pinch roller can withstand the maximum pressure setting value during online production and run for no less than 10 cycles (60,000 tons/cycle).

(3) Impact resistance

When the strip bites into the roll gap of the pinch roll, the instantaneous impact force is very large, and the roll surface of the pinch roll is required to withstand the instantaneous impact of 500-600kN without damage.

(4) Wear resistance

It is required that after the amount of steel passing through the pinch roll reaches 600,000 t, the wear amount in the diameter direction of the roll surface is less than or equal to 35mm.

(5) Corrosion resistance

The pinch roller surface is required to be resistant to repeated erosion and cooling by circulating water with high alkalinity.

(6) Do a dynamic balance test

After the pinch roller is manufactured, a dynamic balance test must be done. The balance accuracy grade is required to be G6, and a dynamic balance test report must be submitted.

Determination of repair process plan

1) Structure and failure mechanism of pinch rollers

The pinch roller is one of the important components of the coiling machine at the rear of the CSP production line. Its function is to bite into the strip at the beginning of coiling, so that the head of the strip is bent down and smoothly enters the coiling barrel along the direction of the guide plate. Establishes the tension required for the coiling operation. The upper pinch roll is a hollow roll welded structure, and the lower roll is solid; the upper and lower pinch rolls are both hard and wear-resistant surfaces, and the cooling method is external water cooling. Hot continuous rolling pinch rolls are subjected to high-speed impact and tail-flicking blows of steel strips above 400°C, and the relative speed difference between rolls and steel strips occurs from time to time. Therefore, the damage and failure behaviors of pinch rolls are mainly wear, steel sticking and local small pieces. flaking. Among them, the sticking steel is the adhesive wear caused by the relative speed difference.

2) Comparison and selection of surfacing repair process methods

It is extremely important to choose a scientific surfacing method, mainly considering several factors: the quality requirements of the pinch roller, the metallurgical properties of the base metal, the formation of the wear-resistant surfacing alloy layer, the process operation skills and the repair cost.

(1) Use wear-resistant alloy electrodes for manual surfacing. The production time is long, the heat preservation is not good, and pores and slag inclusions are prone to occur. With the increase of surfacing thickness, there are more electrode heads, waste and cost. (2) The use of open arc welding for surfacing welding is not only costly, but also the general hardness and wear resistance of the open arc welding wire after welding are low, which makes it difficult to ensure the service life of the pinch roller.

(3) Submerged arc surfacing with tubular flux-cored welding wire is adopted, and the composition of the flux core of the welding wire can be adjusted according to the technical requirements of the roll surface. According to the above analysis, the tubular flux-cored welding wire is finally used, and the process method of submerged arc surfacing is adopted.

3) Comparison and selection of surfacing repair materials

The pinch roller surfacing material should be selected with high hardness and good plasticity and toughness, good abrasive wear resistance, and the roll surface is easy to form a denser oxide film with good adhesion. In foreign countries, most of such surfacing materials are used. For Cr-Mo type low alloy steel and Cr13 type martensitic-ferritic stainless steel, the hardness of the surfacing layer is generally controlled at 53HRC+2HRC. The research and comparison of the relevant information of surfacing rolls at home and abroad are as follows: If you choose the special welding wire for pinch rolls produced by Kobe Steel in Japan or Mackay in the United States, the price of the flux is high, and the requirements for various supporting equipment are relatively high, and the process methods are strict. , If you choose to develop flux-cored wire with well-known domestic welding consumable manufacturers, it is not only cheap, but also suitable for operation with existing domestic equipment, and the process method is simple and easy to implement. The material was repeatedly studied with Shanghai welding material manufacturers, and the composition and reasonable proportion of the welding wire flux were finally determined after many tests.

4) Determine the repair process route

According to the welding materials and process methods, the final process route is: roughing of pinch rollers – ultrasonic flaw detection – preheating – tubular wire surfacing – post-weld heat treatment – finishing – surfacing metal surface hardness inspection – ultrasonic flaw detection – dynamic balance experiment – Installed for use.

Surfacing repair process

1) Preparation of the roller surface

All oxides and fatigue metal layers (cold deformations on the edges) on the surface of the rolls must be removed by appropriate methods to ensure a good surface for surfacing. Improper preparation will cause porosity, high dilution rate, and coating with The bonding strength of the matrix is ​​poor. The mechanical cutting method is used to remove the fatigue layer and defects formed during the use of the roller, and the processing requirements are adjusted according to the on-site wear conditions until all the fatigue layers and local defects are removed.

2) Flaw detection

Carry out comprehensive coloring and ultrasonic flaw detection on the roller, and take corresponding measures according to the test report.

(1) Ultrasonic and flaw detection are performed on the roll neck and roll body before the roll is welded and machined to judge the repair value of the roll.

(2) Carry out flaw detection on the roll surface after turning, and check whether the fatigue layer, cracks and other defects have been completely removed, (this process may be repeated), until the state of the roll before repair meets the relevant standards for flaw detection

(3) Carry out flaw detection on the surfacing layer for defects such as cracks, slag inclusions and pores; re-inspect the roll neck to check whether the roll neck position is affected during the surfacing process of the roll surface, and issue a report. The product is delivered to the user unit together.

3) Heat treatment

In view of the needs of preheating, intermediate and final heat treatment of pinch rollers in the surfacing process, the heat treatment furnace should meet the requirements of large-scale roller heat treatment processes in terms of ensuring furnace volume, heat treatment power and temperature control.

(1) Preheating

Roller preheating is a very important factor to protect the metal from cracking and deformation. Proper preheating can make the roll fully expand before welding, so that after welding, the deformation and stress of the welding layer and the substrate are minimized. The preheating temperature is 400℃, and the temperature is kept for 2h.

(2) Interlayer temperature control

In the surfacing process, the rollers are heated and kept warm by oxyacetylene flame or electric heating plate, and the interlayer temperature is controlled within the range of 280-300°C.

(3) Tempering after welding

The roll body is heated to 560 ℃ after surfacing welding, and after holding for a period of time, it is cooled to room temperature with the furnace. After tempering, the internal stress caused by welding is eliminated.

(4) Submerged arc surfacing welding

In order to restore the size of the roll body and the corresponding mechanical properties, surfacing welding is the key process that directly affects the material, size, shape and intrinsic quality of the roll in the entire pinch roll repair process. By using two MZ-1000 submerged arc welding machines and large Welding is carried out by welding tires, and the continuous welding can last for 7 to 8 days (24 hours per day), saving 2 to 2.5t of surfacing materials. In the process of surfacing welding, on-site inspection work should be strengthened. Through the use of measuring tools, instruments and meters, various factors affecting the welding quality, such as temperature, voltage, current, wire feeding speed and roller rotation speed, should be grasped at all times. Through effective control and strict implementation of process parameters, the two submerged arc welding machines used at the same time are under almost the same working parameters to ensure the uniform performance of the welding layer. By accurately calculating the amount of material, the same batch of welding wire from a certain manufacturer is used on the same roller to ensure material uniformity and achieve better welding results.

4) Welding parameters

Voltage: 28~34V; Current: 500~600A; Welding wire extension length: 28~31mm; Roller linear speed: 500~700mm/min; Leading distance: 50mm Interlayer temperature: 280~300℃; r/s; Roller overlap rate: 50%.

Quality inspection

(1) Check the diameter of the roll body at six points on the roll body, which conforms to the size of the drawing.

(2) Test the hardness of the roller surface according to the four bus bars A, B, C and D. The angle between the adjacent bus bars is 90°, and the hardness values ​​are measured at 10 points on each bus bar. The hardness values ​​are all greater than 51HRC and less than 54HRC.

(3) After the roll body is inspected according to the standard by the qualified inspection and inspection unit, it is confirmed that there are no excessive defects and crack defects in the surfacing layer of the roll surface.

(4) According to the requirements of the drawings, ask a professional manufacturer to do a weight-loss method dynamic balance test on the pinch roller, reaching the G16 level.

Effect

The roll consumption is greatly reduced, the output, operation rate and product quality of the rolling mill are improved, the standard before the use of imported rolls is reached, and the expected purpose before the repair of the pinch roll is realized.

Economic Benefit Analysis

With the popularization and application of surfacing welding technology, in order to reduce costs, various manufacturers tend to replace the original parts with repair parts. The overall surfacing repair of pinch rollers has the advantage of spare parts efficiency, and the repair price is generally lower than the price of new products. 25% to 35%,

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