Optimisation of the Mannesmann Effect by Process Simulation

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Analytical prediction of the central rupture of billets duringpiercing in a cross-roll piercer due to the Mannesmann effectInnovations in metal forming - an international ConferenceSeptember 23-24, 2004 - Brescia, Italy

Analytical prediction of the central ruptureof billets during piercing in a cross-rollpiercer due to the Mannesmann effect

Elisabetta Ceretti, Claudio Giardini, Ferdinando BrisottoUniversità degli Studi di Brescia - Dipartimento di Ingegneria Meccanica

Raul [email protected]

Centro Sviluppo Materiali SpA

Luca [email protected]

Tenaris Dalmine SpA

presented atInnovations in metal forming - an international Conference

September 23-24, 2004 - Brescia, Italy


Contents

• the Rotary Piercing Process– general description– the Mannesmann effect

• FEM simulation of the Mannesmann effect– FE model basics– results validation through industrial trials

• on-line application of the results of the FE model into theprocess control systems at the Tenaris tube rolling plants

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Production steps of seamless steel tubes(hot rolling cycle)

• heating of round billetsup to rollingtemperature

• piercing of the billetinto a hollow shell

• elongation of the shellinto a mother tube

• final rolling of themother tube to the finaldimensions


The Rotary Piercing Process

piercerplug

left roll

right roll

billet

roll-billettouchpoint

position ofthe plug nose

void reductionlength

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The Mannesmann effect

Basic schematization of the forces exerted by the piercer rolls on thebillet in the inlet cone of the piercing mill



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piercerplug

central rupture produced by theMannesmann effect

(as seen on the longitudinal section ofthe semipierced hollow)



central ruptureproduced by the

Mannesmann effecton a billet rolled in

the piercing millwithout plug

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• strongly beneficial to:– shell concentricity (uniform wall thickness on the cross

section)– lifetime of the piercer plugs

• if not controlled, may produce internal defects on thepierced shell, due to oxidation of the internal surface



Pierced shell tail with square end The non-squareness of the tailend of the pierced shell indicateswall thickness eccentricity of the

cross section

square tail end ofthe pierced shell

non- square tail end(eccentric shell)

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Internal defects (as seen on the finished tube) generated by oxidation ofthe broken surface at the billet center, and subsequent rolling by the

piercer plug



Normal wear pattern on thepiercer plug (proper piercer

settings)

Abnormal wear on the nose of thepiercer plug, generated by absentor insufficient centeral rupture on

the billet

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normal wearpattern of worn

piercer plug

worn plug withdeformed shape(central cavity

too large)



It can be concluded that:• the Mannesmann effect should be regarded as a basic

feature of the rotary piercing process• its control is important to assure an acceptable level of

dimensional and surface quality to the rolled product• in consideration of the 3-dimensional complexity of the

piercer machine, it is sometimes difficult to predict the realactual extention of the rupture cone in the billet

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The FEM approach to in-line control of theMannesmann effect

• calculation (with FE model) of the stress and strainconditions in the billet as a function of time during thepiercing process

• determination of the axial position where the materialreaches a treshold value for central rupture (depending onsteel grade, temperature and deformation conditions)

• validation of such theoretical approach with full-scaleexperimental tests (at rolling plant)

• theoretical simulation of possible rolling conditions andextrapolation of the results with artificial neural networktechniques

• implementation in the process control of the rolling plants


The Finite Element modelDescription of the problem

• as previously described, in the rotary tube piercing processthe round bar rotates around its axis, and the fracture islocated on a plane perpendicular to the rotation axis of theround bar

• the influence of the axial stress in the hole formationmechanism has been investigated with a 3D model

• the FEM calculated value of the axial stress generatedduring the process has been found to be constant along thebillet axis and only 10% of the transversal stresses

• it was then decided to approach the problem with a 2-dimensional FE model, neglecting the influence of axialstresses on the hole formation and considering only thestresses acting on the transversal cross section of the billet

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The Finite Element modelCalculation of stress and strain distribution

(step 1) - CAD schematization of the 3-dimensional geometry ofthe piercer mill



(step 2) - determination of the contact points between the billetand the piercer rolls on the void reduction length

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(step 3) - the 3-dimensional geometry is analyzed with a 2-Dapproach by neglecting axial stresses and reproducing the rolls-

material contact with a spiral


The Finite Element modelDetermination of the breaking point

• The axial position where the center of the material breaks isdetermined with the Cockroft-Latham criterion of themaximum cumulated energy:

• The damage index C(x) is calculated along the longitudinalaxis (x) of the billet center

• The starting point of the central crack is then determined asthe point where the calculated damage index C(x) becomesgreater than the characteristic value for the rolled material,at the piercing temperature

∫ ⋅

=

f

dCiε

εσσσ

0

*

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The Finite Element modelDetermination of the breaking point

Damage on the billet cross section for a given piercing condition


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

-450 -400 -350 -300 -250Axial coordinate

CumulateddamageLimit damagefor the material

The Finite Element modelModel validation

Position ofthe nose of

the plug

Position wherethe breakage

occurs

B = Length of the Mannesmann effect

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In-line algorithms for the prediction of theoccurrence of the Mannesmann effect

• FEM simulations and operationalexperience indicate that theoccurrence and extention of theMannesmann effect depends onthe following main factors:– steel grade– piercing temperature– geometry of the contact with

the rolls (spiral), expressedthrough the parameters M, Qand a, where:

M = arctan ( )Q

dQ*α


In-line algorithms for the prediction of theoccurrence of the Mannesmann effect

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Conclusions

• the FE model of the Mannesmann effect has been realizedto exhaustively approach a well-known everyday problemof the tube rolling process

• the results provided by this model have fully matched theevidence of long-term operational experience, and havebeen also verified with specific and dedicated rolling tests

• the implementation of the results of the model in theprocess control of the Tenaris rolling lines will providesupport to the operators of the mills in the decision-takingprocess, with expectations of specific improvements on:– internal surface quality of the pipes– plug life– dimensional quality of the rolled product

Optimisation of the Mannesmann Effect by Process Simulation

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