DURING HOT ROLLING RICHARD ANTHONY HARDING M.Met. A thesis submitted for the degree ...

429 Pages·2014·20.32 MB·English

COPY I
TEMPERATURE AND STRUCTURAL CHANGES
DURING HOT ROLLING
BY
RICHARD ANTHONY HARDING M.Met.
A thesis submitted for the degree of Doctor of Philosophy
University of Sheffield
Department of Metallurgy
November 1976.
TEMPERATURE AND STRUCTURAL CHANGES DURING HOT ROLLING
R.A. HARDING
SUMMARY
The 'published literature on strength, structural and
thermal aspects of hot working has been reviewed with particular
reference to their application to the hot rolling of stainless steel.
Mild and stainless steel slabs. have been hot rolled on a
mill instrumentated to measure load, torque and speed. Internal
temperatures have been measured during rolling using embedded
thermocouples whose outputs have been recorded on a U.V. recorder
coupled with an automatic back-off system.
As-rolled stainless steel slabs recrystallised very
~
slowly relative to the rate found by previous workers for simpler
austenitic stainless steels. Additionally, slabs exhibited
retarded recrystallisation at the surface compared with the centre
whereas theoretical analysis predicts the reverse. These
phenomena have been explained by rapid precipitation at temperatures
~950oC and limited electron microscopical evidence has been
obtained to support this. Precipitation also increased the flow
stresses at lower rolling temperatures and masked any effects that
different substructures were expected to have on the hot strength.
A finite difference computer programme has been developed
to predict the temperature changes that occur through a two
dimensional transverse section of a slab during a hot rolling
SChedule. The air cooling and roll contact heat transfer coeffi
cients have been derived by comparison of measured and computed
temperature-time curves. This programme enabled a mean pass
temperature to be derived from measured temperatures which then
resulted in good agreement between hot strengths derived from
rolling and torsion tests on stainless steel.
The programme has been simplified to a one dimenSional
model for temperature changes over central regions of
simu~ating
wide sections during industrial hot rolling. Data have been
presented to enable mean temperatures to be derived from measured
surface temperatures and to illustrate the effects of various
parameters on the heat loss during a pass.
CONTENTS
SUMMARY
CHAPTER 1 BACKGROUND TO THE PRESENT RESEARCH
1 .1 Hot Working 1
1.2 Industrial Hot Rolling 2
CHAPTER 2 STRENGTH DURING HOT WORKING
2.1 Introduction 4
2.2 Hot Workabili ty Tests: 5
2.2.1 Scaled down Tests 5
2.2.2 Mechanical Tests 6
2.3 Stress-Strain Behaviour 7
2.4 Application of Hot Strength Data 11
2.4.1 Introduction 11
2.4.2 Analogy with Plane Strain Compression 12
2.4.3 Friction in Rolling 13
2.4.4 Rolling Theories 14
2.4.4.1 Slip Line Field Theories 21
2.4.5 Deformation of the Rolls 22
2.4.6 Strain Rate during Rolling 23
2.4.7 Effect of Temperature 24
2.4.8 Spread in Hot Rolling 25
2.4.9 Measurement of Load and Torque 26
2.4.10 Comparison Between Theory and Practice 27
2.4.11 Application of Theories to Computer Control 28
CHAPTER 3 STRUCTURE DURING HOT WORKING
3.1 Introduction 30
3.2 Dynamic Restoration Processes 31
3.2.1 Dynamic Recovery
31
3.2.2 Dynamic Recrystallisation 32
3.3 Static Restoration Processes
35
3.3.1 Static Recovery
35
3.3.2 Static Recrystallisation 37
3.3.2.1 Kinetics of Static Recrystallisa tlon 37
3.3.2.2 Nucleation 38
3.3.2.3 Growth 39
3.3.2.4 Effect of Material Variables 40
3.3.2.5 Effect of Initial Grain Size 41
3.3.2.6 Effect of Texture 41
3.3.2.7 Effect of Temperature 42
3.3.2.8 Effect of Strain 43
3.3.2.9 Effect of Strain Rate 45
3.3.2.10 Summary of the Effect of Variables on
Static Recrystallisation 45
3.4 Application to Multipass Deformation Schedules 46
3.4.1 Structural Interactions 46
3.4.2 Structural Dependence of Strength 47
CHAPTER 4 TEMPERATURES DURING HOT WORKING
4.1 Introduction 50
4.2 The Measurement of Temperatures during Rot
Working 50
4.2.1 Surface Temperature Measurements 51
4.2.2 Internal Temperature Measurements 54
4.2.2.1 Effect of Deformation on Thermocouples 55
4.3 Basis of Heat Transfer during Hot Working 56
4.3.1 Laws of Heat Conduction within a body 57
4.3.2 Heat Conduction Between Two Bodies 60
4.3.3 Heat Loss during Air Cooling 62
4.4 Application of Theory to Practice 63
4.4.1 Cooling Outside the Stands 64
4.4.1.1 Air Cooling by Radiation and Convection 64
4.4.1.2 Conduction OutSide Stands 66
4.4.1.3 Water Cooling 67
4.4.2 Heat Transfer during a Pass 68
Heat Produced by Deformation 68
Heat Conduction to the Rolls 71
4.5 Agreement Between Theory and Practice 75
4.6 Application of Mathematical Models 76
4.7 Work in Related Areas 77
CHAPTER 5 EXPERIMENTAL TECHNIQUES
5.1 The Aims of the Present Research 79
5.2 Hot Rolling 80
5.2.1 The Rolling Mill 80
5.2..2 Temperature Measurement during Hot Rolling 81
5.2.3 Rot Rolling Schedules 82.
5.2.3.1 Hot Rolling of Mild Steel 82
5.2.3.2 Rot Rolling of Stainless Steel 83
5.3 Measurement of Temperatures During Air Cooling 86
5.4 Hot Torsion Testing 90
5.5 Metallography 91
5.5.1 Specimen Preparation 91
5.5.2 Quantitative Metallography 93
5.5.2.1 Fraction Recrystallised 93
5.5.2..2 Hardness Measurements 94
5.5.2..3 Grain Size Determination 94
5.6 Electron Probe Microanalysis 94
CHAPTER 6 TEMPERATURE. CALCULATIONS DURING HOT ROLLING
6.1 Introduction 95
6.2 Air Cooling 96
6.2.1 Basis of Model 96
6.2..2 Heat Conduction within a Flat Slab 97
6.2..3 Heat Loss during Air Cooling 99
6.2..4 Surface Temperatures during Air Cooling 101
6.2.5
Stability Criterion during Air Cooling 103
6.2.5.1 Method A 103
6.2.5.2 Method B 105
6.2.5.3 Method A VB Method B 105
6.2.6 Summary of Solution tor Air Cooling 106
6.3 Iiot Rolling' 106
6.3.1 Introduction 106
6.3.2 Subdivision of Matrix Prior to the Pass 107
6.3.3 Stability Criterion During Rolling 108
6.3.4 Definition of Elements to Represent the Roll 108
6.3.4.1 Stability Criterion for Roll 109
6.3.4.2 Dimensions of Elements in Roll 111
6.3.5 Heat Conduction Within the Roll 112
6.3.6 Heat Conduction Between Slab and Roll 114
6.3.6.1 Basis of Model 114
6.3.6.2 Surface Temperature of Roll 115
6.3.6.3 Slab Surface Temperature 118
6.3.6.4 Details of Heat Transfer between Roll and
Slah 119
6.3.6.5 Effect of Simplifying Assumptions 122
6.3.6.6 The Constant C 124
6.3.1 Change in Element Size During a Pass 124
Temperature Rise due to Deformation 126
Summary 128
CHAPTER 7 RESULTS AND DISCUSSION OF PRELIMINARY WORK
7.1. Rot Strength Measurements 129
7.2 Temperature Measurements 1,32
7.3 Discussion of Results 136
CHAPTER 8 RESULTS
8.1 Temperatures during Air Cooling 139
8.1,.1 Qualitative Results 139
8.1.2 Quantitative Results 144
8.2 Temperatures during Hot Rolling 149
8.2.1 Temperature Measurements 149
8.2.2 Temperature Calculations 155
8.3 Results of Structural Studies 159
8.3.1 Qualitative Results 159
8.3.1.1 General Observations 159
8.3.1.2 Structural Changes during Rolling 161
8.3.1.3 Structural Changes during Interpass
Annealing 163
8.3.1.4 Hot Torsion Specimens 164
8.3.1.5 Electron Microscopy 165
8.3.2 Quantitative Results 166
8.3.2.1 Initial Grain Size 166
8.3.2.2 Recrystallisation Kinetics 166
8.3.2.3 Electron Probe Microanalysis 169
8.4 Hot Torsion Tests 169
8.5 Results of Hot Rolling Experiments 173
8.5.1 Strengths during Hot Rolling 173
8.5.2 Deformation during Hot Rolling 178
8.5.3 Dimensional Changes during Hot Rolling
179
CHAPTER 9 DISCUSSION OF RESULTS
Temperature Changes during Hot Rolling
181
Qualitative Results 181
'9.1.2 Quantitative Results
182
9.1.2.1 Interpass Periods
183
9.1.2.2 Roll Contact Period
186
9.1.2.2.1 Heat Input due to Deformation 186
9.1.2.2.2 Heat Loss due to Conduction 189
9.1.2.2.3 Overall Temperature Model 192
9.1. • 2.2.4 Empirical Temperature Model 192
Structural Changes during Hot Working 195
Effect of Second Phase Particles on
Recrystallisation 195
9.2.1.1 Effect of Ferrite 196
9.2.1.2 Effect of Inclusions on Recrystallisation 197
9.2.1.3 Effect of Precipitation on
Recrystallisation 198
9.2.2 Restoration during Hot Working 199
9.2.2.11 Static Restoration during Hot Working 200
9.3 Strength during Hot Working 207
9.3.1 Hot Torsion Tests 207
9.3.2 Hot Rolling Experiments 210
CHAPTER 10 APPLICATION OF MATHEMATICAL MODEL TO INDUSTRIAL
HOT ROLLING
10.1; Introduction 216
to.2 Air Cooling 216
10.3 Roll Contact Period 219
10.3.1 Heat Loss by Conduction 219
1,0.3.1:.1 Effect of Geometry of the Roll Gap 219
1.0.3.1.2- Effect of Initial Roll Tempe ra ture. 220
10.3.1.3 Effect of the Value of C 221
10.3.1.4 Effect of Contact Time 222
1.0.3.1.5 Effect of Thermal Properties 224
10.3.2 Heat Gain due to Deformation 225
1.0.3.3 Application of Computed Data 225
10.3.4 Temperature Gradients within Plate 226
1.0.4 Recalescence following Exi t from Rolls 228
CONCLUSIONS 231
RECOMMENDATIONS FOR FURTHER WORK 235
ACKNOWLEDGEMENTS
REFERENCES
APPENDIX 1: Instrumentation of the Rolling Mill
2: The Measurement of Temperatures During Hot Rolling
3: Details of Mathematical Model.
TABLES 1 to 9.
FIGURES 1 to 138.