Technical Article

The selection of work roll material should consider the following aspects:

The selection of work roll material should consider the following aspects:

(1) The thickness of the slab is large, and the roll must have good biting properties.

(2) The slab temperature is high, the rolling speed is slow, and the contact time between the rolled piece and the roll is long. The roll must have good thermal crack resistance and thermal fatigue resistance.

(3) The diameter of the work roll is large (Φ1210/1110mm), the length of the roll body is large (5050mm), the rolling force it bears is high, and the main motor drives the work roll to drive. The roll is required to have high fracture resistance, and the roll body and roll neck must have high strength.

(4) The high rolling temperature also requires the rolls to have high temperature wear resistance.

(5) Since the rough rolling and finishing rolling are completed in the same stand, it is necessary to consider the large thickness and small width of the rolled piece and the slip between the rolled piece and the roll. Also we should consider when finishing rolling, the rolled piece is wide and long, and the roll area is large. At the same time, the single-stand four-high rolling mill must adopt controlled rolling and controlled cooling technology when rolling low-alloy special steel and high-strength steel, usually cross rolling, with low rolling temperature and high rolling force. The roll is required to have good wear resistance, good thermal crack resistance, good surface roughness resistance, high strength, and low heat sensitivity.

Traditional finishing mills with four-high often have a roughing mill (two-high, three-high or four-high) in front. The work rolls for rough rolling usually use high nickel-chromium infinite chilled cast iron rolls. In recent years, high-chromium cast iron rolls have been used. This kind of roll has a higher carbide content, low friction coefficient, high hardness and excellent wear resistance. However, it is very sensitive to the influence of heat, easy to slip, and deep thermal cracks formed in accidents such as stuck steel.

The working conditions of the finishing mill rolls are relatively good, and the thermal influence and mechanical impact are also small. When choosing the material, the wear resistance and surface roughness resistance are mainly considered to ensure the surface quality of the steel plate. The rough rolling and finishing rolling of the 5m wide and heavy plate four-high rolling mill are all done by the same rolls. After years of practice, and taking into account the high prices of other newly developed products such as high-speed steel rolls, the work rolls of the 5m wide and heavy plate mill adopt high nickel-chromium infinite chilled cast iron rolls.

This kind of roll is mostly made of vertical centrifugal composite casting. Due to the effect of the outer layer of nickel, chromium, molybdenum and other alloying elements, the pearlite transformation is delayed, so the structure in the as-cast state is bainite + martensite paralyzed + carbide + short flake graphite. The structure after heat treatment is bainite + a small amount of martensite paradox + carbide + short flake graphite. The core structure of the roll is bainite + flake graphite or pearlite + a small amount of carbide (+ bull’s eye ferrite) + spherical graphite. The typical composition range is shown in the table:

Typical composition range of high nickel-chromium infinite chilled cast iron roll:%

Its main performance characteristics:

a. The matrix structure is controlled as bainite, with high strength and good toughness.

b. Although the wear-resistant quality point carbide is still M3C type, but (Fe. Cr) 3C type is increased, the hardness is high, and the wear resistance is good.

c. The matrix structure contains a certain amount of graphite, which has good thermal conductivity and high heat resistance. The self-lubricating effect of graphite can ensure the surface quality of the rolled material and prevent sticking to the roll.

d. The graphite content in the outer layer gradually increases from the outside to the inside, and there is a certain hardness drop in the working layer.

e. The heart is made of gray cast iron with high toughness, or ductile iron with high strength. Based on the trend of increasing rolling force, at present, high-strength ductile iron is mainly used for the core of high-nickel-chromium cast iron composite rolls.