The service life of a roll primarily depends on its internal properties and the mechanical forces it encounters during operation. Key internal properties include strength and hardness. Sufficient strength is achieved mainly through appropriate material selection, while hardness—generally referring to the working surface—determines wear resistance and significantly influences service life. Target hardness levels can be attained through rational material choice and heat treatment. This article summarizes traditional roll materials and heat treatment methods and discusses future trends in roll material and process development.
Traditional Materials and Heat Treatment for Cold Rolling Mill Rolls
Cold rolls are subjected to high rolling pressures and transient high temperatures caused by welds, inclusions, and edge cracks in the strip, which can lead to thermal shock, cracking, roll sticking, or spalling. Hence, cold rolls must exhibit high resistance to cracking and spalling under bending, torsion, and shear stresses, along with high wear resistance, contact fatigue strength, fracture toughness, and thermal shock resistance.
Internationally common cold roll materials include GCr15, 9Cr, 9Cr2, 9CrV, 9Cr2W, 9Cr2Mo, 60CrMoV, 80CrNi3W, 8CrMoV, 86CrMoV7, and Mo3A.
In the 1950s–60s, rolled materials were primarily low-strength carbon structural steels, so rolls were typically made of 1.5%–2% Cr forged steel. Final heat treatment usually involved quenching and low-temperature tempering, applied via induction surface hardening or overall heating. The main goals were improving wear and spalling resistance, increasing hardened layer depth, ensuring uniform microstructure, and enhancing structural stability.
By the 1970s, as stronger, harder high-strength low-alloy (HSLA) steels became common, roll material requirements also increased. Commonly used materials included 2% Cr-Mo or Cr-Mo-V steels such as 9Cr2Mo, 9Cr2MoV, 86CrMoV7 (China), 9X2MΦ (Russia), 86Cr2MoV7 (West Germany), and MC2 (Japan). These low-alloy steels typically achieved a hardened depth of only 12–15 mm (radius) after final heat treatment, which often led to spalling, cracking, and short service life.
Re-quenching (1–2 times) could improve hardened depth, but this increased costs, reduced roll diameter by ~5 mm per treatment, and caused shape inaccuracies. Developing rolls with deeper hardened layers can greatly reduce consumption, minimize re-quenching, extend service life, and offer economic benefits.
From the late 1970s to mid-1980s, 3%–5% Cr steels were adopted internationally for deeper hardened layers (25–30 mm for 3% Cr, ~40 mm for 5% Cr), along with better wear and accident resistance. China developed 9Cr3MoV steel, while other countries introduced materials like 3.25%Cr, 5%Cr (U.S.), KantocRP53, FH13, MnMC3, and MC5. Although these high-carbon, high-alloy steels offer good hardness and wear resistance, they often suffer from surface brittleness, low contact fatigue life, and unstable quality.
To further improve hardened depth, contact fatigue life, and reduce brittleness and overheating sensitivity, international manufacturers began optimizing 5% Cr steel compositions from the mid-1980s onward, mainly by increasing Mo and V or adding Ti and Ni.
Adding ~0.1% Ti promotes fine TiCN precipitates that enhance surface roughness retention, beneficial in high-speed tinplate rolling. In heat treatment, quenching is controlled to limit austenite carbon content to ≤0.6%, followed by intensive cooling to achieve a deep hardened layer containing ~4% carbides, ~10% retained austenite, and cryptoneedle/lath martensite. Surface hardness reaches ~HS 95–99. Low-temperature tempering adjusts final hardness; more thorough tempering improves toughness and thermal cracking resistance. Higher Mo and V increase retained austenite, which transforms to new martensite during tempering, further enhancing hardness, wear resistance, and surface finish.
Material Selection and Heat Treatment for Hot Rolling Mill Rolls
Hot rolls work at 700°C–800°C, under high rolling force and thermal fatigue. They require high hardenability, low thermal expansion, high thermal conductivity, high-temperature yield strength, and oxidation resistance.
China previously used forged steel and indefinite chilled iron rolls (low-/mid-/high-Ni-Cr-Mo types). High-Ni-Cr-Mo chilled iron offers higher performance but still has limited hardness and wear resistance. Nodular graphite composite iron rolls improved service life and remain in use. Other countries often use semi-steel or high-hardness special semi-steel for better wear resistance and surface quality.
To enhance wear resistance, hot roll materials evolved from chilled iron to high-chromium iron, then to semi-high-speed steel (SHS) and high-speed steel (HSS).
High-chromium iron typically contains 2.0%–4.0% C, 10%–30% Cr, 0.15%–1.6% Ni, and 0.3%–2.9% Mo. With 10%–15% Cr, it forms M₇C₃ carbides (HV ~1800) in an austenite/martensite matrix, offering a good hardness-toughness balance. A protective chromium oxide layer reduces thermal cracking. These rolls are widely used in roughing and early finishing stands of hot strip mills, plate mills, and section/barbar mills.
Heat treatment involves either subcritical or above-A₃ temperature processing. Normalizing and tempering are commonly used to refine pearlite spacing, disperse secondary carbides, and minimize retained austenite and stress.
HSS rolls emerged in Japan (1988) and later in the U.S. and Europe (early 1990s), with China adopting them in the late 1990s. Typical composition: 1%–2% C, 0%–5% Co, 0%–5% Nb, 3%–10% Cr, 2%–7% Mo, 2%–7% V, 1%–5% W. The structure contains 10%–15% hard, stable carbides, offering high hot hardness (80–85 HS), excellent wear and thermal crack resistance, and minimal spalling.
SHS rolls are also used successfully in hot strip roughing and bar intermediate stands. Their wear resistance is twice that of high-chromium steel, with good bite and thermal fatigue resistance. Typical composition: 1.5%–2.5% C, 0.5%–1.5% Si, 0.4%–1.0% Mn, 1.0%–6.0% Cr, 0.1%–4.0% Mo, 0.1%–3.0% V, 0.1%–4.0% W.
HSS rolls are heat treated by quenching (1050°C–1150°C) and tempering (550°C–600°C) to dissolve secondary carbides, enhance austenite alloy content, and precipitate hardening carbides. V content is optimized to form MC carbides without reducing hardenability or toughness.
Development Trends in Roll Materials and Heat Treatment
Future cold rolls will focus on higher strength, hardness, and hardened depth without compromising toughness. Large cold work rolls will use improved 5% Cr steels with V, Mo, and Ni. Higher Cr (e.g., 8%–10%) improves hardenability but may reduce toughness, requiring balanced C/Cr content and lower quenching temperatures to minimize fracture sensitivity.
With advances in forging, high-chromium steel work rolls will see broader use in large cold mills. 5% Cr and V-modified steels are common in large backup rolls, with high-Cr forged backup rolls entering practical application. Large cold work rolls are increasingly made from electroslag remelting (ESR) ingots, while ladle refining and vacuum degassing ensure high steel purity for backup rolls.
Hot rolls endure alternating thermal-mechanical stress and severe wear. Future development will emphasize higher wear resistance. While surface quenching and carburizing are insufficient, full HSS or carbide rolls are costly and wasteful. Cladding with carbide or ceramic surfaces, along with chrome plating, thermal spraying, plasma spraying, and laser texturing, will be key for surface enhancement.
In summary, optimal material selection, high-quality manufacturing, and suitable heat treatment can significantly save roll materials, reduce production costs, and improve product quality and output. Continuous attention to new material trends and tailored development based on rolling conditions are essential for advancing roll technology.
