Picque, Benjamin (2004) Experimental study and numerical simulation of iron oxide scales mechanical behavior in hot rolling. PhD thesis Mécanique Numérique, ENSMP - CEMEF Centre de Mise en Forme des Matériaux, ENSMP.

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Abstract

Hot rolling of steels represents one of the most critical steps to achieve finished products with high surface quality. The increasing productivity added to the rising customer requirements result in more and more severe scheduling rules for the HSM. Strip surface aspect is very important in terms of HSM operation costs and productivity limitation. Among all surface defects, the most crippling comes from the oxide scale formed at the surface of the steel during the hot rolling, at the entry of the finishing mill (last part of the hot strip mill): the secondary scale, mechanical behaviour of which is still poorly known. The secondary scale may fracture under the stresses imposed by the successive rolling passes, and can be embedded in the steel strip surface: this defect is called "rolled-in scale defect". In addition, the extrusion of the subjacent metal inside the oxide cracks induces large local modifications of friction and heat transfer conditions. Consequently, a precise description of oxide scale deformation mechanisms is necessary to better define the boundary conditions in a roll bite and to better understand the initiation mechanisms of rolled-in scale defects. Our scientific objective is then to provide a realistic physical and numerical model to simulate the oxide scale flow in the roll bite and in particular, its damage. After the presentation of the industrial process and the context of this study, the physical and mechanical properties of the oxide scale in the finishing mill are investigated. We introduce the Forge2® finite element software, selected for this study to simulate the oxide scale behaviour in a finishing mill stand. The numerical developments performed to simulate the different kinds of oxide damage are described. Three mechanical tests have been selected to approach the solicitations undergone by the oxide scale at the entry of the roll gap, suspected to be critical for damage: the 4-point hot bending test, the hot tension test and the hot plane strain compression test. A numerical study is performed in parallel. Based on constitutive data obtained from these three mechanical tests, the mechanical description of a rolling stand is sufficient for satisfactory simulation of the industrial process.

Table of content

INTRODUCTION
CHAPTER I. HOT ROLLING PROCESS
I. INTRODUCTION
II. HOT ROLLING PROCESS
III. FINISHING MILL
IV. A STAND
V. THERMO-MECHANICAL DESCRIPTION
V.1. GENERALITIES.
V.2. FRICTION.
V.2.1. Friction and engagement conditions
V.2.2. Friction coefficient
V.3. THERMAL CYCLE IN ROLLING
V.4. HEAT TRANSFER IN A FM STAND AND THEIR CONSEQUENCES
V.4.1. Heat exchanges assessment
V.4.2. The interfacial heat transfer coefficient (IHTC)
V.4.3. Work-rolls degradation.
VI. ORIGIN OF SECONDARY SCALE DEFECTS
VI.1. DEFECTS
VI.2. INITIATION MECHANISMS OF THE ROLLED-IN SCALE DEFECT.
VI.2.1. Scale residues in exit of FSB descaling (Type A)
VI.2.2. Stresses in scale without gap effects (type B)
VI.2.3. Defects related to rolling stresses (Type C)
VI.2.4. Defects generated by roll degradation (Type D)
VI.2.5. Finally
VII. EXAMPLES OF ROLLING
VII.1. INDUSTRIAL ROLLING MILL.
VII.2. PILOT ROLLING MILL.
VIII. CONCLUSION
IX. REFERENCES
CHAPTER II. OXIDE SCALE PROPERTIES
I. INTRODUCTION
II. PHYSICAL PROPERTIES
II.1. OXIDATION GROWTH.
II.1.1. Bibliography
II.1.2. Oxidation kinetics - Scale thickness
II.2. OXIDE SCALE MORPHOLOGY IN THE FINISHING MILL
II.2.1. Adapted thermal cycle and oxidation atmosphere
II.2.2. Preparation and microscopic observations of the oxide layer
II.2.3. Composition using x-rays diffraction analysis (XRD)
II.2.4. Other analyses
II.3. THE TEMPERATURE: A KEYPOINT FOR SURFACE QUALITY
II.3.1. The first parameter: the slab temperature
II.3.2. Thickness of the oxide layer after the secondary descaling
II.3.3. Effects on the acceptable maximum temperatures
II.3.4. The true limit can be on the F2 stand
III. OXIDE SCALE MECHANICAL PROPERTIES
III.1. THIN SCALE COATING.
III.2. MECHANICAL PROPERTIES
III.3. OXIDE SCALE BEHAVIOR
III.4. STRESSES EVALUATION AND DAMAGE CRITERIA
III.4.1. Fracture mechanics theory
III.4.2. Interstand
III.4.3. Roll gap entry
III.4.4. Under the rolls
IV. CONCLUSION
V. REFERENCES
CHAPTER III. NUMERICAL DEVELOPMENTS
I. INTRODUCTION
II. THE EQUATIONS OF MECHANICS.
II.1. MECHANICAL FORMULATION
II.2. CONSTITUTIVE MODELLING
II.3. BOUNDARY CONDITIONS
II.3.1. In the normal direction of an interface between two bodies
II.3.2. In the tangential direction
II.3.3. Standard implementation
II.4. STRONG FORMULATION OF THE MECHANICAL PROBLEM
III. VARIATIONAL FORMULATION
III.1. CONTINUOUS FORM OF THE WEAK FORMULATION
III.2. FINITE ELEMENT SPATIAL DISCRETIZATION
III.3. TEMPORAL DISCRETIZATION
IV. GENERALISATION OF CONTACT MANAGEMENT IN FORGE2®
IV.1. OUR OBJECTIVE
IV.2. A CZM APPROACH
IV.3. EXTENSION OF CONTACT MANAGEMENT IN FORGE2®
IV.3.1. Tangential direction
IV.3.2. Normal direction
IV.3.3. Comparison between our extensions and the CZM model
IV.3.4. Coupling.
IV.4. EXAMPLES
IV.4.1. Interfacial sliding
IV.4.2. Decohesion
IV.4.3. Sliding and decohesion coupling
V. TRANSVERSE CRACKS
V.1. PRE-EXISTING CRACKS
V.2. A MORE REALISTIC MODEL
VI. CONCLUSIONS
VII. REFERENCES
CHAPTER IV. THE 4-POINT HOT BENDING TEST
I. INTRODUCTION
II. BIBLIOGRAPHY
III. THE FOUR POINT HOT BENDING TEST OF IRSID (4-PHBT)
III.1. TEST PREPARATION
III.2. THE HEATING SYSTEM
III.3. WET ATMOSPHERE GENERATOR
III.4. SPECIMENS PREPARATION
III.5. EXPERIMENTAL PROCEDURE
III.6. THE ACOUSTIC EMISSION INSTRUMENTATION.
IV. 4-PHBT RESULTS
IV.1. GENERAL BEHAVIOR OF NON-OXIDIZED SPECIMENS
IV.2. GENERAL BEHAVIOR OF OXIDIZED SPECIMENS
IV.2.1. Damage.
IV.2.2. A granular structure
IV.2.3. Experimental load-deflection curves
IV.3. INFLUENCE OF THE 4-PHBT PARAMETERS ON SCALE DAMAGE
IV.3.1. Temperature influence
IV.3.2. Strain influence
IV.3.3. Steel grade influence
IV.3.4. Scale thickness influence
IV.3.5. Strain rate influence
IV.3.6. Summary of parameters influence
IV.3.7. Crack density depending on parameters
V. THE SIGNIFICANT CONTRIBUTION OF THE AE
V.1. FIRST WORKS
V.2. CRACKS INITIATION
V.3. IDENTIFICATION OF EVENTS
V.4. CONCLUSION
VI. DETERMINATION OF BEHAVIOR LAWS FOR STEEL AND SCALE.
VI.1. THE INVERSE ANALYSIS [APPENDIX4, PIC1]
VI.2. STEEL PARAMETERS IDENTIFICATION.
VI.2.1. Sensitivity study
VI.2.2. Identification of steel mechanical parameters by inverse analysis method
VI.3. OXIDE SCALE CONSTITUTIVE EQUATIONS
VI.3.1. Young's modulus identification.
VI.3.2. Viscoplastic behavior (800°C-900°C)
VI.3.3. Extrapolation to 600°C / 700°C
VII. SIMULATION AND DETERMINATION OF CRITICAL STRESSES
VII.1. DETERMINATION OF CRITICAL STRESSES
VII.2. INFLUENCE OF DEFORMATION
VII.3. TEMPERATURE / STRAIN RATE INFLUENCES
VIII. CONCLUSION
IX. REFERENCES
CHAPTER V. EXTENSION TO OTHER STRAIN AND
STRESS STATES: TENSION AND COMPRESSION
I. HOT TENSILE TEST (HTT)
I.1. A FOCUSED BIBLIOGRAPHIC STUDY
I.2. OUR EXPERIMENTAL DEVICE
I.3. USEFUL DATA
I.4. EXPERIMENTAL RESULTS AND PARAMETERS INFLUENCE
I.4.1. Influence of temperature
I.4.2. Influence of strain
I.4.3. Influence of strain rate
I.4.4. Influence of scale thickness
I.4.5. LC steel and Ex-LC steel.
I.5. LOAD-ELONGATION CURVES
I.6. NUMERICAL SIMULATION OF TENSILE TEST
I.7. CONCLUSION ON HOT TENSILE TEST
II. PLANE STRAIN COMPRESSION TEST (PSCT)
II.1. A HOT ROLLING MODEL.
II.2. THE EXPERIMENTAL DEVICE
II.3. MECHANICAL BEHAVIOR OF THE OXIDE SCALE DURING PSCT
II.4. SIMPLIFIED NUMERICAL SIMULATION OF PSCT
II.4.1. A simplified model.
II.4.2. Parameters influence on extrusion
II.5. EXTRUSION STUDY USING PSCT.
II.6. CONCLUSION ON PSCT.
III. CONCLUSION
IV. REFERENCES
CHAPTER VI. HOT ROLLING SIMULATIONS
I. INTRODUCTION
II. A FOCUSED BIBLIOGRAPHIC STUDY
III. REFINING BOXES.
IV. SIMULATION OF HOT STRIP ROLLING IN A FM STAND
IV.1. THE F2 STAND AS A REFERENCE.
IV.2. LOCATION ON THE STRIP
IV.3. INFLUENCE OF THE TEMPERATURE
IV.3.1. Scale temperature
IV.3.2. Work-rolls temperature
IV.4. INFLUENCE OF REDUCTION
IV.5. INFLUENCE OF SCALE THICKNESS.
IV.6. INFLUENCE OF ROLLING SPEED
IV.7. INFLUENCE OF FRICTION
V. INITIAL DEFECTS
V.1. OVER-OXIDATION.
V.2. COLD OXIDE RESIDUE
V.3. BLISTER DEFECT
VI. CONCLUSIONS
VII. REFERENCES
GENERAL CONCLUSION AND PERSPECTIVES APPENDICES

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