Views: 0 Author: Site Editor Publish Time: 2026-08-05 Origin: Site
The core objective of this research report on roll materials for steel mill rolling mills is to systematically study the composition design, manufacturing processes, and performance optimization of roll materials, with emphasis on improving core indicators such as resistance to rolling force, wear resistance, and thermal crack resistance. This aims to extend the service life of rolls under high pressure, high temperature, and high wear conditions, reduce replacement frequency and production costs, ensure rolling process stability, guarantee steel surface quality and dimensional accuracy, and develop dedicated materials for different rolling scenarios. By combining advanced manufacturing technologies and surface treatment processes, we aim to break through the performance bottlenecks of traditional materials, ultimately achieving comprehensive goals of improving rolling efficiency, product quality, and enterprise economic benefits.
Cast iron rolls and cast steel rolls are the two most fundamental and widely used material systems in steel mill rolling mills. Their essential difference originates from carbon content: cast iron has a carbon content of approximately 2.5%–3.5%, with a microstructure characterized by cementite/graphite plus matrix, offering high hardness, high wear resistance, and low cost, but low toughness. Cast steel has a carbon content of approximately 0.4%–2.4% (steel rolls 0.4%–1.4%, semi-steel 1.4%–2.4%), with a microstructure characterized by ferrite/pearlite plus carbides, providing high strength, high toughness, and impact resistance, but with hardness and wear resistance inferior to cast iron.
Core Conclusions at a Glance:
It is not a matter of "which is better" but "which is more suitable": cast iron excels in surface hardness and wear resistance, while cast steel excels in strength and toughness; their service boundaries are determined by rolling conditions.
Roughing/breaking-down heavy impact → prioritize cast steel (and semi-steel) for resistance to roll breakage, impact, and good biting-in.
Finishing/cold rolling with high surface quality → prioritize cast iron for high hardness, wear resistance, and stable roll profile.
Semi-steel (Adamite) serves as a "bridge" between the two: hardness higher than ordinary cast steel, toughness higher than cast iron, and with minimal hardness drop, particularly suitable for deep-pass section steel rolls.
High-chromium and composite approaches are the current mainstream directions: high-chromium cast iron/high-chromium cast steel plus centrifugal composite casting achieve a gradient performance of "wear-resistant outer layer and strong-tough core."
Economics: cast iron has the lowest procurement cost, cast steel is moderate and 30%–50% lower than forged steel; cast steel rolls in roughing stands can achieve 20%–40% higher tonnage per roll than cast iron, so total cost must be assessed over the full life cycle.
Rolls are the core working components in rolling mills that directly contact the workpiece and force it to undergo plastic deformation, withstanding enormous rolling pressure, rolling torque, and dynamic loads. During hot rolling, the workpiece temperature reaches 800–1200°C, and the roll surface undergoes rapid heating and cooling cycles, making operating conditions extremely severe. Therefore, strict requirements are imposed on roll strength, rigidity, toughness, surface hardness, wear resistance, and roll profile stability.
Classification by Hardness
Category | Shore Hardness HS | Typical Application |
Soft rolls | 30–40 | Blooming mills, large section roughing mills |
Semi-hard rolls | 40–60 | Roughing mills for large/medium/small sections and plates |
Hard-faced rolls | 60–85 | Roughing for thin/medium plates, small/medium sections, backup rolls for four-high mills |
Extra-hard rolls | 85–100 | Cold rolling mills |
Classification by Three Major Material Systems
Roll materials are mainly divided into cast iron rolls, cast steel rolls, and forged steel rolls. This report focuses on the comparison of the first two, with forged steel as a reference.
Cast iron rolls: carbon content 2.5%–3.5%, containing graphite (flake/spheroidal) and cementite. Subdivided into chilled cast iron, indefinite chilled cast iron, high-chromium cast iron, and ductile cast iron. Outstanding wear resistance and low cost, but low toughness.
Cast steel rolls: carbon content 0.4%–2.4%, divided into steel rolls (0.4%–1.4%) and semi-steel rolls (1.4%–2.4%). Alloying elements such as Cr, Ni, Mn, Mo, V and special heat treatments can be added to improve performance. High strength and good toughness.
Cast iron rolls have a carbon content of 2.5%–3.5%, much higher than cast steel. Their microstructure consists of three major components: matrix + cementite (carbides) + graphite.
Matrix: austenite, ferrite, pearlite, bainite, martensite – determined by alloying and heat treatment.
Cementite/carbides: provide high hardness and wear resistance, especially M₇C₃-type carbides formed under high-chromium conditions.
Graphite: exists in flake or spheroidal forms. Graphite provides lubrication, friction reduction, vibration damping, and thermal crack resistance, but also weakens strength; spheroidal graphite (ductile iron) has less stress concentration and higher strength.
Sub-Category | Microstructural Features | Hardness HS | Tensile Strength MPa | Typical Applications |
Chilled cast iron | Surface white chilled layer (pure cementite) + mottled transition layer + grey core | 55–85 | 150–220 | Finishing for plates/wire/sections, thin plates |
Indefinite chilled cast iron | Working layer contains fine graphite points, no distinct white/grey boundary, hardness gradually decreases | 55–85 | 150–220 | Medium/finishing for plates/wire/sections, strip finishing |
Ductile cast iron | Spheroidal graphite, pearlitic/acicular bainitic matrix, significantly improved strength | 35–80 | 300–700 | Roughing/medium for sections/wire/tubes, large blooming |
High-chromium cast iron | Cr 12%–22%, M₇C₃ carbides + martensite/bainite matrix | 60–95 | 400–700 | Finishing for small sections/wire, pre-finishing for strip |
Advantages:
High surface hardness (chilled layer/carbides provide excellent wear resistance).
Good compressive strength, suitable for high contact stress.
Thermal crack resistance (graphite absorbs vibration and buffers thermal stress), especially for indefinite chilled and ductile irons.
Low cost, mature casting process, economical raw materials.
Smooth roll surface, favorable for ensuring rolled product surface quality.
Limitations:
Low toughness (tensile strength only 150–700 MPa, weak impact resistance).
Not suitable for heavy impact loads, prone to brittle spalling and roll breakage.
Hardness drop (ordinary chilled rolls show significant hardness decrease from surface to core, limiting deep-pass applications).
High-chromium cast iron has extremely low fracture toughness when carbides form coarse networks, prone to brittle spalling under high-speed impact.
Indefinite chilled cast iron combines a "surface wear-resistant chilled layer" with "good thermal crack resistance," without distinct white/grey layering and with small hardness differential, thus achieving the widest application. High-nickel-chromium indefinite chilled composite cast iron (with Ni, Cr, Mo) is the main material for finishing work rolls in hot strip mills.
Cast steel rolls have carbon content of 0.4%–2.4%, much lower than cast iron, and are divided into two major categories by carbon content:
Category | Carbon Content | Microstructure | Hardness HS | Tensile MPa |
Steel rolls | 0.4%–1.4% | Ferrite + pearlite (low C); pearlite increases with C | 30–70 | 500–1000 |
Semi-steel rolls (Adamite) | 1.4%–2.4% | Pearlite + small amount of carbides | 35–70 | 300–700 |
Graphitic steel rolls | – | Silicon addition + graphitising treatment | 35–60 | 500–900 |
High-chromium cast steel | 0.5%–1.5% | Tempered martensite/bainite + fine chromium carbides | 70–80 | – |
To improve hardness and wear resistance, cast steel rolls are alloyed with Cr, Ni, Mn, Mo, V and subjected to special heat treatments, forming three major alloy systems:
Carbon-manganese steel (C-Mn): C 0.45%–0.80%, Mn 0.60%–1.20%, most economical, hardness HS 35–45, used for roughing stands and billet mills, suitable for toughness-demanding applications.
Chromium-molybdenum steel (Cr-Mo): typical C 0.55%, Cr 2.0%, Mo 0.35%; chromium forms carbides to improve wear resistance, molybdenum suppresses temper brittleness to enhance high-temperature strength; hardness HS 50–65, industrial standard for hot strip intermediate/finishing.
High-chromium cast steel (Hi-Cr Steel): Cr 5%–12% (or 8%–14%), forming a dense M₇C₃ carbide network, hardness HS 65–75, used for demanding finishing applications, but increased carbides reduce fracture toughness, requiring controlled service intervals.
Semi-steel (Adamite) lies between cast steel and cast iron, with carbon content of 1.4%–2.4%. Its most prominent feature is the extremely small internal hardness drop – hardness hardly decreases from the surface to the core, making it particularly suitable for manufacturing deep-pass section rolls (section grooves can be machined deep without premature softening). When silicon is added with graphitising treatment, it obtains high thermal crack resistance and toughness, performing excellently as blooming rolls, large section roughing rolls, and edger rolls for hot strip mills.
Advantages:
High strength and good toughness (tensile 500–1000 MPa, strong impact and roll breakage resistance).
Thermal crack resistance (especially high-carbon steels and graphitic steels).
Good biting-in, suitable for large reductions in breakdown.
Special heat treatment can reach HS 90+, also usable as cold rolling work rolls or backup rolls.
Semi-steel has minimal hardness drop, excellent for deep passes.
Limitations:
Hardness and wear resistance lower than cast iron (ordinary steel rolls).
Higher cost than cast iron (but lower than forged steel).
Requires ladle refining for S and P control, complex heat treatment processes.
As-cast internal defects are relatively more than forged steel; heavy-duty extreme conditions still require forged steel.
Comparison Dimension | Cast Iron Rolls | Cast Steel Rolls |
Carbon content | 2.5%–3.5% | 0.4%–2.4% |
Typical hardness HS | 55–95 | 30–80 |
Tensile strength MPa | 150–700 | 500–1000 |
Toughness / impact resistance | Low | High |
Wear resistance | High | Medium |
Thermal crack resistance | Medium (graphite assisted) | Good |
Thermal shock resistance | Medium | High |
Hardness drop (surface→core) | Large (notable for chilled rolls) | Small (especially semi-steel) |
Relative cost | Low | Medium |
Main applications | Finishing/cold rolling, thin plate, wire finishing | Breakdown/roughing, blooming, backup rolls |
Key mechanical property range comparison:
Indicator | Cast Iron Rolls | Cast Steel Rolls |
Hardness lower limit HS | 55 | 30 |
Hardness upper limit HS | 95 | 80 |
Tensile lower limit MPa | 150 | 500 |
Tensile upper limit MPa | 700 | 1000 |
Chilled casting: using a metal mould (chill) to rapidly cool the surface, forming a white chilled layer (pure cementite), transitioning inward to mottled and grey structures.
Indefinite chilling: controlling cooling rate and composition so that the working layer contains fine graphite points without a distinct white/grey boundary.
Spheroidising / inoculation treatment: ductile iron uses magnesium/rare earth spheroidisers to spheroidise graphite, improving strength.
Centrifugal composite casting: high-chromium/high-nickel-chromium composite rolls have an outer layer of wear-resistant white iron and a core of ductile iron, cast in one pour, achieving "hard outside, tough inside."
Typical production flow requires higher control over internal density and residual stress:
Melting: electric arc furnace / induction furnace melting.
Secondary refining: ladle metallurgy – degassing, desulphurisation, with S < 0.010%, P < 0.015%, reducing hot shortness and improving toughness.
Moulding and pouring: static sand mould / metal mould pouring; composite rolls use centrifugal casting. Large rolls (diameter > 700 mm) often require top risers accounting for 15%–25% of casting weight for feeding.
Controlled cooling and rough machining: controlled cooling in mould for 48–72 hours, rough machining to remove scale and expose defects.
Heat treatment (most critical): multiple stages of normalising / quenching / tempering to regulate pearlite, bainite, martensite structures, achieving hardness-toughness balance. High-chromium cast steel requires multi-stage quenching and tempering.
Finishing and inspection: comprehensive inspection of hardness, ultrasonic testing, etc.
Key process differences:
Cast iron relies on "chilling + graphitisation" to obtain hardness gradients.
Cast steel relies on "refining impurity control + alloying + multi-stage heat treatment" to obtain strength and toughness.
Cast steel requires stricter control of harmful elements such as S and P than cast iron.
Roll failure is the reverse mapping of material selection – different materials correspond to different dominant failure modes.
Failure Mode | Mechanism | Material Tendency |
Wear | Mechanical friction wear + high-temperature softening/melting loss + oxidation/electrochemical corrosion wear | Cast steel (lower hardness) wears more easily; cast iron has wear advantage |
Thermal cracking (network→ring→breakage) | Rapid heating and cooling cycles on hot roll surface, thermal fatigue stress exceeding yield → microcracks → network → ring → deep propagation fracture | Both occur; cast iron graphite buffers but low toughness |
Spalling | Cracks propagate to a certain depth, or coarse carbide network brittle cracking, surface flakes fall off | High-chromium cast iron with coarse carbide network → higher risk of brittle spalling |
Fracture (roll breakage) | Casting defects, structural defects, transgranular cracks in untoughened core, rapidly propagating under alternating stress | Cast iron (low strength) has higher breakage risk; cast steel relatively break-resistant |
Interface delamination | Stress at interface of composite roll outer and inner layers leading to separation | Centrifugal composite rolls (both types involved) |
Dominant failures for cast iron rolls:
Brittle fracture/spalling under heavy impact loads.
High-chromium cast iron carbide network brittle spalling → surface scratching / point defects on strip.
Insufficient hardness in deep passes → rapid wear, loss of roll profile.
Dominant failures for cast steel rolls:
Insufficient hardness → relatively faster wear (acceptable in roughing, uneconomical in finishing).
Thermal fatigue network cracks (in high-temperature zones).
As-cast defects (porosity/segregation) propagation to fracture; therefore heavy-duty extreme conditions still favour forged steel.
Core selection logic: reverse derive material requirements from failure modes – if fracture/spalling dominates, improve toughness, choose cast steel; if wear dominates, improve hardness, choose cast iron; if both are critical, choose semi-steel or composite rolls.
Rolling Conditions | Key Requirements | Recommended Material | Rationale |
Breakdown/blooming (hot rolling 800–1200°C, large reduction, strong impact) | Strength, toughness, impact resistance, biting-in, thermal crack resistance | Cast steel (high-carbon / alloy cast steel) | High strength prevents roll breakage, ensures reduction |
Section roughing (deep passes) | Small hardness drop, wear resistance | Semi-steel (Adamite) / graphitic steel | Minimal hardness drop, deep passes do not soften |
Hot strip roughing front stands / finishing front stands | Wear resistance, thermal crack resistance | High-chromium composite cast iron / high-Ni-Cr indefinite chilled | Carbides for wear + graphite for thermal crack resistance |
Hot strip finishing stands | High hardness, indentation resistance, wear resistance, spalling resistance | High-chromium cast iron / high-chromium cast steel / HSS | Surface quality and life |
Intermediate/finishing (wire, sections, plates) | Surface hardness, wear resistance | Cast iron (indefinite chilled / high-Ni-Cr) | Smooth surface, wear-resistant, low cost |
Cold rolling work rolls | Very high hardness (HS 90+), deep hardened layer, mirror finish | Forged steel (Cr5 series) mainly; specially treated cast steel can substitute | Surface quality difficult for cast rolls to replace |
Backup rolls | High strength, fatigue resistance, rigidity | Cast steel / forged steel | Withstand bending moments, require high strength and toughness |
Service condition matching is the primary principle – avoid "one material fits all stands." For hot rolling heavy impact, prioritise toughness (choose cast steel); for finishing with high surface requirements, prioritise hardness (choose cast iron).
Balance hardness and toughness: one-sided pursuit of high hardness causes sharp drop in toughness, leading to brittle spalling; one-sided pursuit of toughness leads to rapid wear and loss of roll profile.
Emphasise metallurgical quality: control S and P content, reduce porosity, segregation, and inclusions. For heavy-duty applications with many as-cast defects, forged steel is preferred.
Assess total life-cycle cost rather than only purchase price: premium materials deliver higher tonnage per roll and fewer stoppages, making them more economical overall.
Selection decision path:
Hot roughing/breakdown → choose cast steel (consider toughness).
Section deep passes → choose semi-steel.
Intermediate/finishing / strip → choose cast iron (consider hardness and wear resistance).
Cold rolling / mirror finish → choose forged steel.
High-chromium addition is the core path for modern roll performance improvement. Although both high-chromium cast iron and high-chromium cast steel are "high-chromium," their carbon content and carbide morphology are completely different, resulting in significant performance boundary differences.
Parameter | High-Chromium Cast Iron | High-Chromium Cast Steel |
Carbon content % | 2.5–3.5 | 0.80–1.60 |
Chromium content % | 12.0–20.0 | 8.0–14.0 |
Typical matrix | Martensite-bainite + coarse primary chromium carbides | Tempered martensite-bainite + fine dispersed chromium carbides |
Carbide content | Very high | Medium-high |
Shore hardness HS | 70–85 | 60–80 |
Tensile strength MPa | 400–700 | 700–1100 |
Yield strength MPa | 300–500 | 500–900 |
Impact toughness | Medium | High |
Thermal shock resistance | Medium | High |
Wear resistance | Extremely high | High |
Manufacturing | Static / centrifugal casting | Mostly centrifugal casting |
High-chromium cast iron: high carbon forms a large amount of primary chromium carbides, giving extremely high wear resistance and deep hardness penetration, but the coarse carbide network results in low fracture toughness, prone to brittle spalling under high-speed impact. Suitable for wear-dominated, low-impact finishing/pre-finishing.
High-chromium cast steel: low carbon makes carbides fine and dispersed in the steel matrix, providing wear resistance superior to ordinary alloy cast steel, while toughness and thermal shock resistance are higher than high-chromium cast iron. Suitable for roughing/intermediate stands where wear, impact, and thermal fatigue coexist.
Dimension | Cast Iron Rolls | Cast Steel Rolls | Forged Steel Rolls |
Relative procurement cost | Low | Medium | High |
Tonnage per roll in roughing | Baseline | 20%–40% higher than cast iron | Highest |
Relative cost vs. forged steel | – | 30%–50% lower | Baseline |
Roll change frequency | More frequent in finishing (less if wear life long) | Relatively faster wear in roughing, periodic changes | Low (long life) |
Overall economics | Cost-effective for finishing/cold rolling | Cost-effective for roughing/heavy-duty | Irreplaceable for high-end cold rolling |
Key note: low-price rolls save money upfront, but frequent changes and rush repairs cause production losses, making total cost higher. Selection should combine rolled steel grades, annual output, and maintenance intervals, using roll consumption per ton rolled (kg/t) as the core indicator.
Composite gradient structure: centrifugal composite casting achieves a gradient distribution of "high-hardness wear-resistant outer layer, high-toughness impact-resistant inner layer," resolving the inherent conflict between thermodynamics and mechanical properties, becoming mainstream for high-performance rolls.
High alloying: evolution from ordinary alloy cast iron → high-chromium cast iron/cast steel → high-speed steel (HSS). HSS rolls combine red hardness, high-temperature wear resistance, and thermal crack resistance, significantly extending service intervals for hot strip finishing.
Surface residual stress management: precise heat treatment (e.g., sub-zero treatment) introduces residual compressive stress on the roll surface to counteract thermal stress gradients, improving thermal fatigue life.
Cold rolling evolution toward Cr5 series: from traditional Cr2 (about 2% Cr, hardened layer ~10 mm) to Cr5 (5% Cr, hardened layer >30 mm), allowing larger roll diameters and better accident resistance.
Customised selection: shifting from "hardness index" orientation to "performance–condition matching" assessment, customising per stand.
Cast iron rolls and cast steel rolls are not competitive but complementary: cast iron uses "hardness–wear resistance–low cost" to hold the finishing and cold rolling positions for surface quality; cast steel uses "strength–toughness–impact resistance" to serve as the heavy-duty pioneer for breakdown/roughing; semi-steel bridges the hardness drop gap between them.
Engineering recommendations:
Select material per stand: roughing/breakdown → cast steel (high-carbon, alloy cast steel, semi-steel); intermediate/finishing → cast iron (indefinite chilled, high-Ni-Cr); pre-finishing / post-roughing → high-chromium composite; cold rolling → primarily forged steel.
Balance hardness and toughness: avoid one-sided pursuit of high hardness causing brittle spalling; customise hardness range according to maximum rolling force to achieve surface wear resistance and core strength-toughness gradient.
Prioritise composite casting: for key hot rolling stands, use centrifugal composite rolls (outer layer high-chromium/high-Ni-Cr, core ductile iron/cast steel) to combine wear resistance and break resistance.
Strictly control metallurgical quality: S < 0.010%, P < 0.015%, reduce as-cast defects; for heavy-duty extreme conditions, forged steel remains the fallback.
Full life-cycle cost accounting: use roll consumption (kg/t) and downtime losses as comprehensive indicators, not purchase price alone.
Pay attention to the boundary between high-chromium cast iron and cast steel: wear-dominated select high-chromium cast iron; wear plus impact and thermal fatigue select high-chromium cast steel.
One-sentence summary: there is no best roll material, only the material that best matches the service conditions. Anchor selection on rolling conditions, match material type and hardness gradient, strictly control metallurgical and heat treatment quality, and thereby reduce abnormal failures from the source.
