Cause Analysis and Countermeasures of Rough Rolling Slippage

Due to the large reduction in rough rolling of strip hot rolling, unclean descaling, high temperature, and easy formation of oxide scales, all of these lead to low friction coefficients between billets and rolls, so slipping accidents are prone to occur in the rough rolling stage. Since the rough-rolled slab is thick, temperature unevenness is prone to occur on the upper and lower surfaces, so warping generally occurs more often during rough rolling. The slab with serious head warping will hit the insulation cover, affecting the flying shear head, and even affecting the difficulty of strip threading in finish rolling and affecting the rolling rhythm.

 

1. Analysis of slipping causes and preventive measures

The root cause of slippage is the low friction coefficient, which causes relative sliding between the roll and the surface of the billet. Any factor that reduces the coefficient of friction will cause the rolled piece to slip. Due to the large reduction in rough rolling of strip hot rolling, unclean descaling, high temperature, and easy formation of oxide scale, all of these lead to low friction coefficient between billet and roll, so the rolled piece is most likely to slip during the rough rolling stage ACCIDENT. Slight tension rolling between the rough rolling vertical roll mill and the four-high horizontal rolling mill. During odd-numbered rolling, the rolling speed of the horizontal mill is greater than the rolling speed of the vertical rolls. During even-numbered reversible rolling, the vertical rolls do not participate in rolling, and there is no tension between the vertical rolls and horizontal rolls, so slipping generally occurs in odd-numbered passes. passes. Due to a large amount of reduction in the first pass, slipping accidents generally occur in the first pass, so special attention should be paid to the slipping phenomenon in this pass. The main reasons that affect slippage are steel type and rolling process, etc.

 

1.1  the influence of steel type

Oxidized iron scale is easily formed on the surface of some steel grades, which reduces the friction between the surface of the billet and the roll. Such as ship plate, 50B, and other low-alloy steels, such as steels with high elements such as Si, Ni, Nb, etc., these elements are easy to react with an oxidizing gas to form oxides with a low melting point, which melts the iron sheet and increases the viscosity. Some data show that when Si>0.25%, the steel very easy to forms Fe2SiO4 when heated, and it melts above 1175 °C, resulting in part of the oxide scale attached to the surface of the billet after descaling, which will peel off and cause slippage when it is rolled by horizontal rolls. For steel types containing these elements, the descaling process control should be strengthened to reduce the probability of relative sliding between the roll and the substrate caused by the peeling of the oxide scale from the billet substrate.

 

1.2. Influence of the rolling process

As the diameter of the roll increases, the maximum reduction in each pass can be increased, reducing the possibility of slippage. In the case of constant friction coefficient and roll diameter, the maximum reduction must be controlled, and slippage will occur if the maximum reduction exceeds the maximum reduction. However, reducing the reduction and increasing the rolling passes will reduce the rolling efficiency, so choosing reasonable rolling passes is very important. At the same time, if the rolling speed is too high, the coefficient of friction will decrease, and it will also cause slipping. When rolling steel grades that are prone to slipping, low-speed rolling should be used.

Increase the number of rolling passes and reduce the reduction. Reasonably distribute the load, or increase the number of rolling passes, and reduce the single-pass reduction. For example Anyang Iron and Steel 1780 hot continuous rolling line rolling Q235B, incoming material 230mm, slipping is easy when the rolling pass is 5, but the slipping phenomenon disappears after changing to 7 passes.

 

1.3 Other measures to reduce skid

Increase the surface roughness of the roll. Many factories use high-Cr steel rolls for rough rolling. This type of roll has high hardness and wear resistance, but its friction coefficient is smaller than that of ordinary cast iron rolls, so it is easy to slip. Human intervention can be done to increase the roughness, apply scoring and surfacing rolls, and increase the coefficient of friction of the rolled piece. However, the scoring has a harmful effect on the surface quality of the rolled piece, and this method is usually only used in the first few passes of rough rolling.

When the reduction exceeds the allowable value, the end of the rolled piece is processed into a truncated cone to reduce the bite angle. Reduce the rolling speed, increase the friction coefficient, increase the cooling of the descaling water, and reduce the temperature of the slab surface, so that the oxide scale does not melt.

 

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