Commonly Used Roll Materials and Selection Requirements

Rolls are important parts of rolling mills. They use the pressure generated by a pair or a set of rolls to roll steel. It mainly bears the effects of dynamic and static loads, wear and temperature changes during rolling. Rolls can be divided into cast iron rolls, cast steel rolls, forged steel rolls, special rolls, etc. according to their materials. The main performance indicators of the roll are the strength of the core of the roll and the toughness and wear resistance of the working layer (surface layer).

Commonly used roll materials include alloy forged steel, alloy cast steel and cast iron:

(1) Alloy forged steel. The alloy forged steel used for rolls has been stipulated in my country’s national standards. The steel for hot-rolling rolls and cold-rolling rolls is listed in the GB/T 13314-1991 standard. Steels for hot rolling rolls include 55Mn2, 55Cr, 60CrMnMo, 60SiMnMo, etc. Steels for cold rolling rolls include 9Cr, 9Cr2, 9CrV, 9Cr2W, 9Cr2Mo, 60CrMoV, 80CrN13W, 8CrMoV, etc.

(2) Alloy cast steel. There are not many types of alloy cast steel used for rolls, and there is no uniform standard. With the development of electroslag remelting technology, the quality of alloy cast steel is gradually improving, and alloy cast steel rolls will be widely used in the future.

(3) Cast iron. Cast iron can be divided into ordinary cast iron, alloy cast iron and ductile iron. When casting rolls, different casting molds can be used to obtain cast iron rolls of different hardness. Therefore, there are semi-chilled rolls, chilled rolls and infinitely chilled rolls:

1) Semi-chilled rolls. There is no obvious white layer on the surface of the roll, and the hardness of the roll surface is HS≥50.

2) Chilled rolls. There is an obvious white mouth layer on the surface, a gray mouth layer in the center and a pit layer in the middle. The hardness of the roller surface is HS≥60.

3) Infinite cold hard roll. The surface is a white mouth layer, but there is no obvious boundary between the white mouth layer and the gray mouth layer, and the roller surface hardness is HS≥65.

Cast iron rolls have high hardness, smooth surface, wear resistance, simple manufacturing process and low price. The disadvantage is that the strength is lower than that of steel rolls. Only the ductile iron roll has better strength.

Material requirements for rolls:

(1) Wear resistance. Wear is the most common form of roll damage, which affects the surface accuracy and dimensional accuracy of the rolled pieces. Because the wear mechanism of rolled piece and roll in roll gap is more complex, there is no wear resistance index which can represent the wear resistance of roller, so the wear resistance of roll can only be judged indirectly by composition and hardness.

The selection of roll hardness is based on the rolled piece, rolling mill and rolling conditions, as well as the cost. Sometimes, it is more important to consider operating habits and roller experience. The higher the hardness, the worse the toughness, which must be considered comprehensively. The hardness of the roll is generally measured by Shore hardness, because the Shore hardness machine is portable and can be used on the roll, leaving only a small indentation on the roll.

(2) Toughness index. The toughness index will be selected according to the main damage of roll surface. The toughness indexes that can be selected include yield strength, impact toughness, cold and hot fatigue, fracture toughness, contact fatigue strength, etc. According to the specific use conditions of the roll and the main damage mechanism, generally only one of them needs to be selected as the roll toughness index.

In the process of casting, heat treatment, surfacing welding and cutting, the roller may leave residual stress due to inconsistent deformation. The residual stress and the working stress are superimposed during use, which together cause different damage phenomena.

(3) Core strength. The working layer and the core of the roll have different strength requirements. Generally speaking, the roll strength should be the core strength to resist bending and torsion stress and avoid roll breakage.

The working layer of the roll sometimes directly requires strength. For example, the compressive strength can represent the ability of the roll surface to resist fracturing. Because the test cost of toughness index is very high, sometimes the strength is used to roughly indicate the ability of the roll surface to resist various cracks.