Limits and evolution of needs in the simulation of flat product rolling Eliette Mathey

Presented by Nelson Souto

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Context

2/12

ArcelorMittal – Flat product rolling research:

Hot strip mills

Cold rolling mills

Temper mills

2 complementary approaches for simulation

1) Complex models (finite element or other “heavy” numerical models)

• Understand phenomena during rolling

– improvement of process & solving of problems (only R&D models)

• Models with several interactions (strip models/roll deformation models) – Thermal, mechanical, microstructural, tribological phenomena

• Low constraints on computational time (hours OK)

• Reference models to develop simplified models

2) Simplified models (replace supplier models in plants)

• Models for setup & control – On-line models or R&D offline models to run many times

• Strong constraints on computational time – Just few msec for multiple computations on each strip during rolling campaign

• Regression of complex models (lower dimension, fewer interactions…)

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Introduction Simulation needs in hot rolling

Thickness : 220-260mm Length : 10-15m Width : 800 – 2200 mm Discharging temp: ≈ 1200°C

Width reduction Thickness down to 25-35mm

Thickness : 1.5-25mm Length : 400-1500m Width : 800 – 2200 mm Finishing rolling temp : ≈800-950°C

Cooling down to coiling temp ≈ 550-750°C

Typical tolerances on final product: Thickness : +/- 0.1 mm Crown : 40-60µm at 40mm from edge

3/12

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Introduction Simulation needs in hot rolling

Thickness : 220-260mm Length : 10-15m Width : 800 – 2200 mm Discharging temp: ≈ 1200°C

Width reduction Thickness down to 25-35mm

Thickness : 1.5-25mm Length : 400-1500m Width : 800 – 2200 mm Finishing rolling temp : ≈800-950°C

Cooling down to coiling temp ≈ 550-750°C

Typical tolerances on final product: Thickness : +/- 0.1 mm Crown : 40-60µm at 40mm from edge

Combustion, energy consumption, temp. homogeneity, productivity

Temperature, scale

Force, temperature, shape (microstructure)

Force, temperature, shape, flatness, microstructure

Temperature, flatness, microstructure

4/12

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Lam3: strip model

Very performant unique stationnary formulations

- Visco-Plastic & Elasto-Visco-Plastic material behavior

- Low computation time for simulation of stationnary process

Can use high anisotropic mesh without precision loss

- very well adapted for simulation of very thin strip rolling

User can define friction, strip behavior, microstructure

evolution laws

Tec3: roll deformation model

Very accurate work roll deformation prediction

Almost no computation cost

- no need for 3D roll simulation

y

z

Roll 1 (WR)

Roll 2 (shiftable IR))

Roll 3 (BUR)

Contact line 0

Contact line 1

Contact line 2

f0

f1

f2

Ftot/2 Ftot/2

Fwrb/2 Fwrb/2

Complex models Lam3-Tec3 R&D rolling model

ArcelorMittal is co-owner of LAM3-TEC3 source code with Constellium 5/12

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Surface quality defect:

displacement of slab corner on top & bottom surfaces

during roughing (succession of vertical & horizontal passes)

Simulation of H/V passes to predict corner movement

Optimization of width reduction schedule

Optimization of edger roll shape (flat, groove shape)

After roughing mill, edge line defect up to 40mm product edge

Horizontal pass (H) Vertical pass (V)

Initial corner position

Final corner position

Complex models Lam3 application: Reduction of edge line defect in the roughing mill

6/12

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Complex models Lam3-Tec3 application: Effect of crown & flatness actuators (FM)

Finishing mill (FM):

Strip crown and flatness is consequence of work roll deformation and actuators like

bending forces, roll shape…

Coupled Lam3-Tec3 simulations provide accurate prediction of strip crown and flatness

less strip crown latent flatness

Effect of bending forces

+

-

+

-

Non-homogeneous stress (latent flatness)

1st stand of finishing mill (example)

wavy edges

center buckle

Accurate 3D computation for strip + roll deformation

Only a few minutes CPU

7/12

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Recrystalized fraction

850°C

However some questions remains:

1) How to take into account flatness defect at the entry of the stand

geometrical defect not possible with stationary simulation

stress distribution applied on the entry section will evolve before the roll bite…

2) Chaining of passes: how to take into account interstand phenomena

effect of creep on stress distribution

proper chaining residual stresses between passes

3) Effect of microstructure evolution in the interstand

Recrystalization influences interstand creep rate and might have effect on stress, flatness

and width evolution [M.Zhang, PhD 2014]

Complex models Lam3-Tec3: Difficulties and Limits

8/12

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Limited evolution of Lam3-Tec3 in the last 20 years

Chaining/coupling with flatness models (PhD thesis S. Abdelkhalek, R. Nakhoul, K. Kpogan)

but huge loss of competencies to use these developements

Coupling with microstructure models (Internship)

needs to be updated, adapted to multipass rolling

Parallelisation of Lam3 solver (Internship + Open Engineering)

limited improvement (only for resolution of linear system)

‘‘Aging’’ code

Mainly interfaces developement (Open Engineering)

to ease the use of the model but remains difficult to master

Limited internal ressources & competencies to modify code

Other finite element software used (Abaqus, Forge, etc.)

Complex models Lam3-Tec3: Difficulties and Limits

9/12

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Simplified models Force Prediction

Estimate rolling force in FM for setup, capability estimation…

- Temperature evolution (usually 1D model)

- Roll deformation : Hitchcock model (analytical model)

- Force/Torque/Power estimation: from simple models (Sims) to more elaborate 2D models (Von Karman, Orowan)

- 2 main unknown data:

• Product yield stress acording to temperature Compression trials according to temperature or microstructure model

• Friction coefficient on roll bite still difficult to estimate beforehand (evolution of roll surface, lubrication…)

However tuning and adaptation allow accurate estimations (but measurements needed!)

Temperature/Force/Torque prediction along FM (example)

Before tuning

After tuning on a database of the

same grade

10/12

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Simplified models Threading refusal model

Estimate threading refusal risk for setup based on ½ analytical model

Incremental computation on evolution of the strip kinetic energy

Energy balance between deformation and friction

Increasing friction better threading Decreasing s0 easier threading

Unknow data: Product yeld stress and friction coefficient

Good prediction of risk with back-calculated data

Calculation time only 0.1 sec

but

0

0.05

0.1

0.15

0.2

0.25

0.3

0 10 20 30 40 50 60 70 80 90

Position in campaign

Lim

it f

rictio

n c

oe

ffic

ien

t

Laminés - µ Calculé Laminés - µ Limite Patinages - µ Limite

137B

Back-calculated friction

Limit friction to avoid threading

Coils with slippage at threading

11/12

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Simplified models Difficulties and Limits

On-line models in plants based on regression of complex models

Fitting of linear models based on experimental plans

Lam3 or Tec3 simulations

Very fast computations

just few ms

Model cannot be easily modified when changes are made on the plant or new grade

are produced

adaption will over-compensate or reach unrealistic values

Fast models with phisical basis needed nowadays

strip models for 3D applications (widening, flatness)

friction evolution

12/12

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Conclusions

Complex models

• Finite element modelling remains reference for understanding

• Lam3-Tec3: “relatively fast” stationary model with roll deformation

Simple models (needed)

• With physical background

• Fast computation

• Easy to maintain & roll out in mills

Both models

• Parameters still difficult to estimate beforehand (friction, yield stress)

• Assumptions difficult to validate (temperature distribution, stress distribution…)

13/12

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Thank you for your attention !

Limits and evolution of needs in the

simulation of flat product rolling

Eliette Mathey

(Presented by Nelson Souto)