Demand for cold rolled strip keeps rising as intelligent vehicles and appliances spread. Carmakers and appliance makers keep tightening their quality requirements, and the forged cold work roll sits directly in that chain. To roll thin strip to high precision at production speed, a work roll needs high hardness, haute résistance à l'usure, uniform hardness and a deep hardened layer.
Yet that combination is not a property of the steel alone. It is the output of a cold roll heat treatment sequence, and every stage in that sequence constrains the next. This article therefore follows the sequence stage by stage. It then sets out what rolling accidents do to a roll built this way.
Material grades and the working range
En pratique, domestic forged cold rolls are mainly Cr2, Cr3, Cr5, plus semi-high-speed steel and high-speed steel.
Cr2 and Cr3 are technically mature and relatively low in cost, so they see wide use. Par contre, Cr5 reaches higher hardness and a deeper hardened layer, which raises line productivity, and its cost-performance ratio has won it a large share of the market. Semi-high-speed and high-speed steels are the high-end products. Their alloy content is much higher, their heat treatment differs substantially, and their combined properties improve markedly: haute dureté, haute résistance à l'usure, thermal shock resistance and accident resistance. Only some domestic manufacturers can produce them, and prices run higher.
The figures below therefore apply to cold work rolls of 340 à 530 mm diamètre, which is the common range on domestic cold rolling lines.
The four cold roll heat treatment stages
A cold roll heat treatment route passes through four stages: post-forge heat treatment, overall quenching and tempering, body deep quenching, et tempérer. Some rolls also receive deep cooling to increase hardened depth and reduce retained austenite.
D'abord, post-forge heat treatment covers normalising and spheroidising anneal. It improves the matrix, relieves stress, and improves machinability.
Deuxième, overall quenching and tempering is the preparatory step for body deep quenching. It consists of quenching plus high temperature tempering. After it, the matrix carries a tempered sorbite structure with good strength, dureté, ductility and machinability, which sets up semi-finish machining and the body quench that follows.
Alors, body deep quenching is the single most important stage for service performance. It covers preheating, continuous body quenching, deep cooling and tempering. These steps work together, and together they decide final quality.
Preheat
Preheat follows post-forge heat treatment and rough machining. In that position, it prepares the roll for continuous quench heating.
Preheat temperature is normally 210 °C à 420 °C. Cr2, Cr3 and Cr5 share essentially the same preheat temperature. Semi-high-speed and high-speed steels quench at higher temperature, so their preheat runs correspondingly higher. Entre-temps, soak time varies with roll size, depuis 20 à 30 heures.
Adequate preheat strengthens the quench effect and reduces internal stress. Stress distribution during preheat scales with preheat temperature: raise the temperature and stress rises. Holding preheat lower reduces stress, but it also reduces the heat reservoir available for quenching, which makes the quench heat-up harder.
The setting therefore balances two directions. Keep stress controllable and its later effect small, while still giving the continuous quench a useful thermal base.
Body quenching
Continuous body quenching heats the roll surface inductively and cools it continuously with water. Frequency runs from 50 Hz to above 320 Hz.
Induction heating raises the surface to process temperature and fully austenitises the matrix. Heating temperature ranges from 860 °C à 1120 °C depending on grade: Cr2, Cr3, Cr5, semi-high-speed or high-speed steel. Tableau 1 of the source sets the values per grade.
Temperature control decides structure, and this is the stage in cold roll heat treatment that sets hardened depth. Si la température est trop basse, the matrix does not austenitise fully, which leaves an uneven structure with poor properties after quenching. Inversement, if it is too high, grains coarsen, force, ductility and toughness fall sharply, and overheating can occur. Either direction damages final service performance.
Deep cooling
Suivant, the roll cools to about 100 °C after quenching, before deep cooling begins. Deep cooling runs between −80 °C and −160 °C with an 8-hour hold.
Par conséquent, this step drives retained austenite further toward martensite. Less retained austenite means a more uniform and more stable structure, and body hardness rises. It is the stage that separates a standard cold roll heat treatment from a high-hardness one.
En général, rising chromium content across the grade range from Cr2 through Cr3 and Cr5 to high-speed steel lifts hardness, wear resistance and hardened depth. Body surface hardness reaches 95 HSD to 103 HSD. Acier rapide, with its specifically designed composition, hardens more thoroughly, so its hardened depth and hardness uniformity exceed those of the other forged cold roll types.
Trempe
Alors, tempering follows quenching and deep cooling. It stabilises structure and dimensions, relieves internal stress promptly, and prevents distortion and cracking. It also homogenises the structure and adjusts hardness to target.
Cependant, Cr2, Cr3 and Cr5 rolls temper between 90 °C and 160 °C. High-speed steel rolls, whose alloying raises red hardness substantially, temper as high as 450 °C à 540 °C. Higher temperature tempering precipitates carbon as particles, relieves internal stress more completely, and produces tempered martensite. It further reduces retained austenite and homogenises body structure, so the roll holds high hardness and good wear resistance.
High-speed steel forged cold rolls use two tempers with a deep cooling step between them. The second temper runs twice as long as the first, at the same temperature. Tempering once before deep cooling controls residual stress from quenching, which prevents fracture during deep cooling when combined stress would otherwise be too high.
Rolling accidents and what they do to the roll
A cold work roll works under combined bending, torsion and compression. Production brings slipping, strip breakage, strip deviation and sticking.
D'abord, slipping means relative sliding between strip and roll. The neutral region of the deformation zone gives way entirely to forward or backward slip. Surface quality and output suffer, and strip breakage and pile-up can follow.
Deuxième, strip breakage means the strip tears inside the tandem train. Weld breakage, high-speed breakage, dynamic gauge change breakage and start-up breakage are the common forms. Breakage creates scrap and downtime, cuts output, and damages the roll.
Troisième, deviation means the strip leaves its intended path. It reduces efficiency, harms product quality, and shocks the roll.
De plus, impact events raise local roll surface temperature, typically above 400 °C. Cr2, Cr3 and Cr5 rolls temper at low temperature, so an impact readily opens surface cracks and can cause local spalling, which shortens roll life and cuts line output.
Ainsi, high-speed steel forged cold rolls answer these problems through red hardness, hardenability, thermal shock resistance and temper resistance, combined with high wear resistance, high hardness and good roughness retention. Heat treatment remains the difficult part, because the high quench temperature generates high stress. Fracture and cracking during quenching are real risks, and that is why high-speed steel cold roll production demands high process capability and remains the constraint on volume production.
What the sequence delivers
As tandem mills, single-stand mills and temper mills upgrade, conventional roll materials struggle to meet the demand for high strength, high precision and ultra-thin strip. Entre-temps, semi-high-speed and high-speed steel rolls, with better hardenability and red hardness plus high hardness, résistance à l'usure, temper resistance, thermal shock resistance and roughness retention, are gaining market acceptance.
For a roll buyer, donc, the useful reading of a specification is the cold roll heat treatment behind it. Preheat window, quench temperature by grade, deep cooling range and hold, and temper temperature together produce the 95 HSD to 103 HSD surface and the hardened depth the mill actually needs.
