High pressure water descaling process
Date: 2025/3/12
Category: Metallurgical encyclopedia terms
Views: 16
Importance of high pressure descaling process
Iron is oxidized at high temperatures, forming a dense layer of scale (phosphorus skin) on its surface. If this layer of oxide can not be removed before steel rolling, they will be pressed into the strip surface during rolling, affecting its surface quality. Residual scales will also accelerate the wear of the roll and reduce the service life of the roll. If the strip needs pickling, the residual iron scales will increase the difficulty of pickling and also increase the acid consumption. Therefore, the iron scales on the surface of the billet must be removed before steel rolling. The method of using the mechanical impact force of high pressure water to remove the oxide scales (high pressure water descaling) is the most popular and effective method at present.
High pressure decaling process
In the descaling system, the high pressure water produced by the high pressure pump enters the descaling nozzle. Under the action of the nozzle, the high pressure water forms a fan of water with a large impact, which is sprayed onto the surface of the billet (or intermediate billet). Under the action of this high pressure fan-shaped water jet beam, the iron scale undergoes the process of being cut, quenched, stripped from the base material, and washed away from the surface of the billet (or intermediate billet), thus cleaning the iron scales.
When high pressure water hits the billet surface through the nozzle, the following changes occur:
a. The fan-shaped surface formed by the water flow is like a sharp blade, cutting the dense iron sheet and forming cracks. It can be seen that the thin fan surface has a greater impact;
b. Through the crack, the high pressure water encountered high temperature base metal and rapid vaporization and evaporation occurs, forming a similar effect of blasting and stripping the iron scales from base metal;
c. The iron scale shrinks after being impacted by water, resulting in transverse shear force, which strips the iron scales from the base material.
d. The scouring effect of water jet with forward Angle will wash away the loose iron scales.
High pressure water descaling process
Water of external introduction is the clear circulation water. Water quality can reach the requirements of the descaling pump water (inlet water pressure is at 0.25 ~ 0.30MPa), from outside to the high level water tank, and then pressurized by the high pressure descaling pump into the high pressure water piping system. Due to the material of the billet and the descaling level is different, the amount of water and pressure required for billet descaling is not the same. In order to ensure the effect of billet descaling, the frequency converter is used to adjust the amount of water and pressure of descaling.
Basic principle of high pressure descaling
After the billet comes out of the reheating furnace, the iron scales covering its surface cools rapidly, and the iron scales generated in the furnace shows net-like cracks. Under the high-pressure water jet, the surface of the iron scales has rapid cooling locally, resulting in a large contraction, so that the iron scale cracks expand, and some is warping. Through the impact of high pressure water flow, in the cracks, the dynamic pressure of high pressure water is changed into the static pressure of the fluid and put into the bottom of the iron scale, so that the iron scale peeled off from the surface of the billet, to achieve the purpose of removing the iron scale. In order to improve the quality of steel, it is very important to install a set of high pressure water descaling system on the rolling line. Hot rolled iron scale production: at high temperatures, the billet surface and the oxygen in the air react, generating oxides. In the oxide, ferrous oxide has the lowest oxygen content and is in the innermost layer of the oxide layer, i.e. the layer closest to iron. When the temperature is lower than 570, ferrous oxide is in an unstable state, the content of ferrous oxide in the iron scale increases with the increase of the temperature of the surface of the billet, when the temperature of the steel is more than 700, the content of ferrous oxide in the iron scale is about 95%. Ferrous oxide has a lower melting point compared to others in the iron scale, and generally melts at 1370 ~ 1425. The melting of the ferrous oxide layer tends to accelerate the rate of generation of iron scale, increasing the penetration into the grain boundaries, thus causing quality problems on the surface of the steel, as well as increasing energy consumption and reducing the yield strength of steel.
