Edge Cracking in Cold Rolling DP Steel — PatSnap Eureka
Edge Cracking in Cold Rolling of High-Strength Dual-Phase Steel
Edge cracking is the foremost yield-limiting defect in cold rolling of dual-phase steel at 780–1200 MPa. This landscape maps 37 patent and literature records spanning 2001–2024, covering hot-rolling thermal control, pickling-line optimisation, post-rolling heat treatment, and stamping process adaptation for automotive applications.
Two Linked Problem Domains Drive Edge Cracking Innovation
Edge cracking in cold-rolled dual-phase steel arises from a convergence of metallurgical and process factors. The retrieved dataset, spanning 2001 to 2024, covers two interlinked problem domains: strip-level edge cracking during the cold rolling and pickling-rolling sequence — causing strip breakage, yield loss, and production interruption — and part-level edge cracking during subsequent automotive stamping, at shear-cut or trimmed edges that propagate during flanging, hole expansion, or stretch-flanging operations.
Both domains share a common root cause chain: brittle or heterogeneous microstructure at the strip edge leads to elevated crack nucleation sensitivity and crack propagation under tensile stress during deformation. As tensile strength targets rise beyond 980 MPa toward 1200 MPa and above, the problem intensifies due to reduced edge ductility, brittle microstructure formation, and sensitivity to temperature non-uniformity during hot rolling and coiling. PatSnap Analytics enables systematic landscape mapping across these interlinked domains.
Approximately 70% of patent records in this dataset were filed after 2018, indicating the field is in an active engineering maturation phase rather than fundamental discovery. The dataset spans publication dates from 1952 through September 2024, with a clear cluster of activity from 2017 onward. Key Chinese steelmakers including WIPO-registered assignees have filed prolifically since 2013 on process-specific fixes for grades up to 1200 MPa.
Four Engineering Clusters Address Edge Cracking at Different Process Stages
Innovation in this dataset organises into four distinct clusters, each targeting a different point in the production chain from slab preparation through to automotive stamping.
Hot-Rolling Thermal Process Control for Edge Microstructure Homogenisation
The dominant cluster in the dataset, with at least 7 patent filings. Interventions include slab slow cooling at 5–10°C/h, intermediate bar edge heating to ≥1000°C, runout table cooling masking at groups 2–5, coiling temperature targeting at 550–610°C, and post-coiling airtight insulation within 30 minutes of unloading maintaining ≥450°C for less than 20 hours. Angang Steel Co., Ltd. filed dedicated patents for grades ≥1200 MPa in 2022 and 2024. Learn more at PatSnap Analytics.
≥7 patent filings · Ansteel, Baosteel, ShougangCold-Rolling and Pickling Line Parameter Optimisation
Once the hot-rolled coil arrives at the cold mill, several parameters governing the pickling-rolling sequence are manipulated: limiting total cold-rolling reduction to 45–50% for high-hardenability grades, reducing inter-stand tension between stands 4 and 5 to 10–30 kN additional, reducing pickling leveler extension rate to 50% of standard, and maintaining emulsion iron powder content below 400 ppm. Handan Steel reported qualification rate improvement from 95.3% to 98.6% after correction. PatSnap Chemicals covers related formulation IP.
Handan Steel, Shougang, North China UniversityPost-Rolling Heat Treatment for Edge Formability Recovery
Rather than attacking edge cracking at the rolling stage, this cluster restores edge ductility through controlled annealing. Approaches include intercritical annealing at 725–840°C, Tata Steel’s two-step quench from 875–930°C then intercritical anneal at 725–790°C, and Nisshin Steel’s intermediate annealing cycles at 600–800°C with cold rolling reduction capped at 85% per cycle. Tata Steel Limited holds two active IN patents (2021, 2023) covering the dual quench-anneal pathway — a differentiated approach not duplicated by Chinese filers in this dataset.
Tata Steel IN 2021 & 2023 · Nisshin Steel EP · Baosteel USStamping Process Adaptation and Edge Quality Management
This cluster addresses edge cracking as a stamping process failure. Two-stage shear-cutting processes were shown to increase hole expansion ratio by approximately 100% for thick high-strength steel suspension components. Slug ironing converts tensile residual stress at the sheared edge to compressive stress, preventing hydrogen-induced delayed cracking in 1.5 GPa sheets. Ansteel’s 2023 CN patent introduced a principal strain evaluation method comparing maximum principal strain in the edge deformation zone to the material’s ultimate elongation force state as a go/no-go formability criterion. Research data is indexed by ISO standard 16630 for hole expansion ratio.
WISCO 2024, FAW Toyota 2021–2022, Ansteel 2023Patent Filing Activity and Production Yield Impact
Two data views from the retrieved dataset: filing distribution by technology cluster, and production yield improvement after cold-mill process correction.
Patent Filings by Technology Cluster
Hot-rolling thermal control dominates the dataset with at least 7 filings; cold-rolling, heat treatment, and stamping clusters each contribute 5–6 records.
Production Yield: Before vs. After Process Correction
Cold-mill process correction improved yield rate from 87.6% to 92.3% and qualification rate from 95.3% to 98.6%, as reported in Chinese steelmaker patent filings.
Where in the Process Chain Does Each Intervention Act?
Edge cracking control requires coordinated interventions from slab preparation through to stamping die design. The three-stage view below maps the primary levers at each production step.
China Dominates Filing Volume; Western Assignees Hold Differentiated Positions
| Assignee | Jurisdiction | Filing Period | Focus Area | Status |
|---|---|---|---|---|
| Angang Steel Co., Ltd. (Ansteel) | CN | 2013, 2016, 2018, 2019, 2022, 2024 | Hot-rolling process control for ≥1200 MPa DP; stamping evaluation methods | Active |
| Handan Iron & Steel / HBIS Handan Branch | CN | 2020, 2021 | Acid-rolling edge cracking at 780 MPa+; 94% defect reduction | Active |
| Shougang Jingtang United Iron & Steel | CN | 2019, 2021, 2023 | Cold-rolling edge crack control; 980 MPa DP cold line breakage | Active |
| Baoshan Iron & Steel Co., Ltd. (Baosteel) | US | 2021, 2024 | Post-coiling thermal insulation; cold-rolled/zinc-plated DP >980 MPa | Active |
| Tata Steel Limited | IN | 2021, 2023 | Two-step quench + intercritical anneal for edge formability; DP steel | Active |
| JFE Steel Corporation | EP | 2019 | High-strength cold-rolled DP for automotive formability and crashworthiness | Active |
Five Strategic Signals for R&D and IP Teams
Derived from emerging directions and strategic implications in the retrieved patent and literature dataset.
Hot-Rolling Thermal Management is the Highest-Leverage Lever
Process patents from Chinese steelmakers consistently demonstrate that edge microstructure heterogeneity is set during the hot rolling → coiling → slow cooling sequence, not primarily during cold rolling itself. R&D investment should prioritise runout table edge-masking hardware, coil insulation logistics, and transfer bar edge heating rather than cold-mill parameter tuning alone.
The 1200 MPa Barrier Represents a Clear Technology Whitespace
In this dataset, only Ansteel has filed patents explicitly targeting DP steel at ≥1200 MPa for cold-rolling edge crack reduction. Producers planning to commercialise this grade class face significant process development risk and IP freedom-to-operate gaps may be limited in China; in Western jurisdictions, the space appears largely open.
Tata Steel’s Two-Step Heat Treatment is an Underexplored Differentiation Path
The quench + intercritical anneal approach addresses edge formability at the microstructural level rather than through process logistics, and is currently claimed only under IN jurisdiction — leaving US, EP, and other jurisdictions potentially available for parallel IP development.
Where Edge Cracking Control Matters Most in Automotive Manufacturing
The retrieved dataset maps edge cracking innovation to four primary automotive application contexts, each with distinct severity and failure mode profiles.
Automotive Body-in-White Structural Parts
The overwhelming application context across this dataset is automotive lightweighting. Cold-rolled DP steel at 780–1200 MPa is applied to door reinforcements, B-pillars, bumper beams, seat side panels, floor cross-members, and other safety-critical structural components. Edge cracking in stamped parts during flanging and hole expansion is reported as a primary rejection cause. FAW Toyota’s CN patents (2021, 2022) specifically address thick (>1 mm) high-strength steel stampings for safety members. See PatSnap Life Sciences for adjacent material science coverage.
780–1200 MPa · B-pillar, door reinforcement, bumper beamAutomotive Chassis and Suspension Components
Retrieved literature on two-stage shear-cutting and hole expansion ratio improvement explicitly targets suspension components, where flanging operations on holes in high-strength steel brackets are prone to edge cracking. JFE Steel Corporation’s EP patent (2019) targets structural and suspension parts for the automotive industry. Two-stage shear-cutting processes were shown to increase hole expansion ratio by approximately 100% for thick high-strength steel suspension components. Data on shear-affected zone behaviour in DP steels is tracked by research groups at American Iron and Steel Institute.
JFE Steel EP 2019 · Two-stage shear cutting · ~100% HER improvementWheel Rims and Structural Sections (Hot-Rolled DP)
JSW Steel Limited (IN, 2022) targets wheel rim and disk applications with hot-rolled DP steel at ≥780 MPa tensile strength and hole expansion ratio greater than 70%, demonstrating that edge formability requirements extend into hot-rolled product families as well. This signals that edge cracking control is not limited to cold-rolled sheet but applies across the full DP steel product range used in automotive structural applications. Standards for wheel rim steel are tracked by SAE International.
JSW Steel IN 2022 · ≥780 MPa · HER >70%Cold Mill Production Line Yield Optimisation
A secondary but critical application domain is the cold mill production operation itself: edge cracking causing strip breakage in the acid-rolling tandem mill is a production reliability problem distinct from part-level stamping failure. Multiple Chinese steel plant patents address this operational domain, with reported improvement in yield rate from 87.6% to 92.3% and qualification rate from 95.3% to 98.6% after process correction. PatSnap customer case studies document similar yield improvement outcomes in steel and materials manufacturing.
Yield 87.6% → 92.3% · Qualification 95.3% → 98.6%Edge Cracking in Cold Rolling of Dual-Phase Steel — Key Questions Answered
Edge cracking arises from brittle or heterogeneous microstructure at the strip edge — martensite islands, bainite, or embrittled grain boundaries — formed during non-uniform cooling after hot rolling and coiling. This elevated crack nucleation sensitivity leads to crack propagation under tensile stress during cold rolling deformation.
Edge cracking frequency for 980 DP and 1180 MS grades before intervention can reach 40%, dropping to approximately 2.4% after targeted process correction — a 94% reduction reported by Handan Iron & Steel Group in CN patent filings.
The dominant approach is hot-rolling thermal process control: blocking cooling water at runout table groups 2–5 to reduce edge temperature drop, targeting coiling temperature at 550–610°C, using U-shaped cooling profiles, and applying post-coiling insulation to maintain edge microstructure homogeneity.
Key adjustments include limiting cold-rolling total reduction to 45–50% for high-hardenability grades, reducing inter-stand tension between stands 4 and 5 to 10–30 kN additional, reducing pickling leveler extension rate to 50% of standard, and maintaining emulsion iron powder content below 400 ppm.
The hole expansion ratio (HER) per ISO 16630 is the industry-standard measure of edge stretchability in stamping. However, retrieved literature highlights its limitations — DIC-based alternatives such as Half-Specimen Dome Tests and KWI tests provide more accurate strain data for simulation of high-strength steel edge cracking.
As automotive steel grades push beyond 1200 MPa, the distinction between cold-forming edge cracks and hydrogen-induced delayed cracks blurs. Wuhan Iron and Steel Co., Ltd.’s 2024 CN patent explicitly links stamping edge cracking to hydrogen-induced delayed cracking, signaling that combined material and process solutions are required at this grade level.
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