Forged rolls can be divided into forged steel hot rolls and forged steel cold rolls.
Forged steel hot rolls are mainly used for hot rolling blanking and section steel rough rolls.
Forged steel cold rolls are widely used as cold-rolled work rolls.
Because cold-rolled sheets require high quality, especially cold-rolled sheets are widely used in light industry, automobiles, construction and railway industries. To produce high-quality cold-rolled sheets, high-quality cold-rolled work rolls must be guaranteed. This places high demands on the performance of cold-rolled work rolls.
- The surface of the roll body must have high and uniform hardness to ensure the dimensional accuracy and good surface quality of the cold-rolled strip or steel plate.
- The roll body has a certain depth of hardened layer;
- The roll body has a high ability to resist accidents.
In order to achieve these properties, the original structure of the roll material is very strict. One is the pure metallurgical quality; the other is the uniformity of the organization.
High metallurgical quality means that the roll steel ingot has dense crystallization, no serious segregation, no white spots are allowed, and the remaining non-metallic inclusion content in the steel is as low as possible.
The structure of the roll in use requires that the carbide particle size be as small as possible (0.5-2 microns) in a dispersed distribution. At the same time, a certain depth of the hardened layer is guaranteed. The increase of the depth of the hardened layer can reduce the number of times of re-quenching of the roll during use, and greatly reduce the cost of the roll.
Rolling mill rolls Design:
1. Material of forged roll
In order to ensure the necessary wear resistance of cold rolled work rolls, the hardness of the roll surface is required to reach 780-900V PN. To meet this hardness requirement, a new heat treatment quenching and tempering process has been developed. Through this process, the surface layer of the roll is quenched and tempered, so that a martensitic structure can be obtained at a depth of 25 to 50 mm on the surface layer of the roll. The roll material used by the company contains high-purity steel with C0.8% and CrZ-5%. In fact, the rolls used to roll the foil require ultra-pure steel, produced by electroslag remelting in the company’s specialized production plant.
2. Heat treatment of forged rolls
The forging billet is provided by a professional forging factory, using open die forging or precise GFM forging. Forged billets are machined to positive tolerance dimensions prior to heat treatment and then finished to final product dimensions after heat treatment. First, heat treatment is carried out to dissolve the carbon into the matrix and make the remaining compounds spheroidize.
Conditioning heat treatment is carried out in a gas combustion furnace, and each heat treatment furnace can hold 10 rolls. The total processing time is 10 days from the time when the cold rolls are loaded into the furnace. After conditioning heat treatment, the roll is reheated to about 900 °C for austenitization,
Water spray quenching is then carried out. Two addition methods can be used:
Quenching of mill rolls
(1) Conventional method. The method is with a new 5-zone vertical gas burner. The rolls are first preheated in a preheating furnace, which uses the exhaust gas from the austenitizing furnace to heat the rolls. The austenitizing furnace uses calculations to regulate its high-velocity gas burners. After the roll is austenitized, the surface of the roll is sprayed with 12~5000 liters/min of water in a vertical quenching water tank, and the water is allowed to flow through a drilled hole passing through the roll axis. The whole heat treatment takes about 12 hours .
(2) Induction heating The company applies a new type of double induction coil furnace, which can harden a roll within three hours. The surface of the roll is gradually heated to the austenitizing temperature through an induction coil. The roll is clamped vertically and slowly descends through the induction coil. The working rate of the main induction coil is 50 Hz, and when the roll passes through the induction coil, the area about 50 mm deep on the surface of the roll is heated to 900 °C
Due to the different types and uses of the rollers, the time of self-flaw detection is about 1/4~1/2 shorter than that of manual flaw detection. The operating frequency of the second induction coil is 5 H, which is used to compensate the cooling of the roll surface. Immediately below the induction coil is a quenching ring, which can spray pressurized water on the roll, and the whole operation process takes about 3 hours. After the roll is quenched, it must be tempered at a temperature of 180~200 ℃ for 24 hours. About 10% of the rolls need to be completely transformed to ensure isotropic consistency. Before tempering, the rolls were placed in a cryogenic tank of liquid nitrogen for 2 to 4 h to partially transform the retained austenite to martensite.
After the heat treatment is complete, the rolls undergo final machining and grinding.
Compared with the conventional quenching method, induction hardening shows many special advantages except the cost.
In fact, the fuel cost per roll has dropped somewhat in the new conventional furnace. Which method to choose depends on the final performance requirements, that is, the residual stress of heat treatment and the depth of hardened layer achieved.