Tungsten carbide spraying in steel production!

       Continuous casting process roll tension roll, furnace roll,

 

Thousands of rolls are used in the production and processing of steel. The wear resistance and corrosion resistance of the roll are mainly determined by the surface of the roll, and the surface quality of the roll has an important influence on the surface finish and quality of the steel. Therefore, a lot of paint is used to protect the surface of the roller

 

Different coating solutions are required in different application environments. The following mainly introduces the technical services developed to meet the requirements of tension roll surface roughness and to meet the high temperature wear and corrosion challenges of furnace rolls.

 

6.1 Tensioner

 

Tensioning rollers are used to control the tension of the steel strip and are mainly used in continuous pickling lines, annealing lines and galvanizing lines. In order to control the tension of the strip, the surface of the roll must have sufficient roughness to prevent slippage between the roll and the steel plate, and the roughness of the roll must not give the surface of the roll a significant surface texture. In addition, the surface of the roll must be wear resistant in order to maintain this suitable surface roughness over a long service life.

 

In the past, chrome plating was often used. Due to the excellent wear resistance of HVSP tungsten carbide spray coatings, hard chromium coatings have been replaced by tungsten carbide coatings. The average roughness of the coating during chrome plating is Ra = 6 μm. This roughness ensures that sufficient friction is maintained between strip and roll without affecting the surface quality of the strip. The surface roughness after HVOF spraying is usually greater than that of the chrome coating, and further grinding and processing techniques are required to ensure the roughness of the sprayed coating to avoid various damages during use.

 

Tension rollers coated with tungsten carbide and cobalt have a significantly longer lifespan than conventional chrome-plated rollers. Industrial tests on a continuous annealing line showed that the Ra of the chrome-plated rolls decreased from 6 μm to 2 μm after 8 months, while the roughness of the blast sprayed tungsten carbide-cobalt coating decreased from 6 μm to 3 μm only after 36 months . Use for several months. . The guide rollers were sprayed with a tungsten carbide-cobalt coating compared to the chrome-plated rollers and had similar test results.

6.2 Furnace roll

 

Rolling most cold rolled steel requires heat treatment on a continuous annealing line to increase the ductility of the steel sheet. The steel plate is passed through various sections of the heat treatment furnace in a horizontal or vertical direction, as shown in Figure 4. In a horizontal continuous annealing line, the diameter of the rolls supporting the steel sheet is usually 100-150 mm; in a vertical continuous annealing line, in order to avoid high surface stress of the steel sheet, the diameter of the rolls is 900-1600 mm. The furnace consists of a rapid heating zone, a holding zone, a cooling zone and an aging zone. The heating atmosphere reduces N2-5%H2, and the temperature is determined by the composition of the steel and the thickness of the plate. The annealing temperature of most cold rolled steel is 730~830℃. Special heat treatment can reach 1200 ℃.

Rolls must be able to withstand corrosion under high temperature reducing conditions and be able to withstand multiple thermal cycles without failure. The surface is wear-resistant and maintains the required minimum surface roughness to avoid slipping or squeezing of the steel sheet on the roll surface. The control of strip motion is becoming more and more important due to increased production efficiency and faster strip running speeds. Finally, the roll must be resistant to surface agglomeration due to adhesion and accumulation of foreign particles/oxides. When the surface of the roll is smooth, the average load on the surface of the roll is relatively low. However, due to the formation of deposits, the local load on the roll surface increases, causing dents and scratches on the surface of the thermally softened steel strip. These flaws will follow. Difficult to remove during leveling. Defects such as grooves and wear marks on the roll surface promote the formation of deposits and affect the surface quality of the steel sheet.

 

6.2.1 Buildup on furnace rolls.

 

The adhesive on the roll surface is formed by two mechanisms, one is the sliding mechanism between the roll and the steel plate, and the other is the reaction mechanism between the steel plate and the oxides on the roll surface.

 

Slip build-up will occur due to a mismatch in the surface speeds of the strip and roll. This mechanism exists in both horizontal and vertical continuous annealing lines, and is more common in horizontal annealing lines. Debris from the steel sheet can stay on or stick to the roll surface, causing build-up. Once the buildup is formed, the size of the adhering buildup will increase rapidly at the high load line due to the speed mismatch between the steel plate and the rollers.

 

Many reaction processes in production will result in buildup on the roll surface. Iron oxides on the surface of the steel sheet are reduced in the furnace atmosphere, and these compounds together with the iron powder cause scaling on the surface of the rolls. The sodium hydroxide left over from the cleaning process will react with the oxides on the plate to form low melting point oxides that adhere to the surface of the drum.

 

The most serious problem with build-up and sticking is the processing of high-strength steels. High strength steel contains manganese and silicon. These elements will be selectively oxidized during the annealing process. These oxides are transferred from the sheet to the roll surface and react with Al2O3 or Cr2O3 on the roll surface to form a highly brittle spinel, thereby enhancing the strength of the roll. The occurrence of surface sticking leads to the deterioration of the surface condition of the rolls, and to the deterioration of the quality of the steel sheet.

 

6.2.2 Furnace roll coating of steel with low manganese content

 

Low manganese steel rolls are usually coated with chromium carbide cermet. These coatings can peel off due to thermal cycling after high temperature or prolonged use. Internal compressive stress caused by volume expansion of the coating due to internal oxidative phase changes, warping and peeling of the coating during cooling. This phenomenon occurs with mixed Cr3C2/NiCr powders. The use of agglomerated coating Cr3C2-NiCr powder changes the microstructure of the coating and improves the reaction of the coating at high temperature. The use of agglomerated powders as a raw material will improve the particle build-up resistance of chromium carbide coatings prepared by plasma spraying. The build-up of roll material basically occurs on nichrome. Finely structured nichrome and coated powder coatings help increase its corrosion resistance.

In a working environment of 1100°C, oxide ceramics or boride cermets are usually used. Typical materials include MCrAlY + Al2O3, NiCrAlY + 50% ZrB/CrB/TiB, CoCrAlY + CrB2/Y2O3. The superalloy is oxidized to form a thin oxide layer, which improves the build-up resistance of the coating. The cermet coating and furnace roll material form a diffusion bond during pre-oxidation or use, which significantly increases the bond strength of the coating and minimizes peeling during thermal cycling.

 

At higher temperatures, such as when annealing stainless steel sheets, it is necessary to use an oxide ceramic layer on the bottom of the alloy. Typical coatings include Cr2O3-Al2O3, ZrO2SiO2, TiO2-Al2O3, Cr2O3, SiO2 + ZrO2 + stable oxides and ZrO2 + (Y2O3 or CeO2). Thermal expansion mismatch between coating and bottom roll material requires extensive use of MCrAlY bond coats to cause cracking and debonding. Due to the high surface loading of the rolls, creep occurs at temperatures below the theoretical value.