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1 Overview1

1.1 The Technology of Controlled Rolling and Controlled Cooling2

1.2 R＆D Program of"Super Steels"and"New Generation Steel Materials"7

1.3 The Formation of Ultra-fine Grains and Microstructural Refinement of Steels-Core Technique for the R＆D of New Generation Steel Materials10

1.4 Theory and Technology on Ultra-fine Grains19

1.4.1 The state change and microstructure refinement of austenite during hot deformation19

1.4.2 Deformation induced ferrite transformation20

1.4.2.1 Thermodynamic consideration of deformation induced ferrite transformation21

1.4.2.2 DIFT phase transformation and characters of transformed products23

1.4.3 Deformation induced precipitation and medium temperature phase transformation control26

1.4.4 The influence of nanometer size precipitates on ultra fine grain steel31

1.4.5 Ultragrain refinement of alloy structural steels and the way of increasing the resistance against delayed fracturing35

1.4.6 The development of carbide-free bainite/martensite multiple phase steels38

1.5 Several Key Technologies Concerning the Development of Ultra Fine Grain Steels41

1.5.1 Steel cleanness42

1.5.2 Refinement and homogenization of solidification structure45

1.5.3 Brief introduction of welding technique and economy of ultra fine grain steels47

References51

2 Refinement of Austenitic Microstructure and Its Influence on γ→α Transformation53

2.1 Thermomechanical Control Process and Refinement of Austenitic Microstructure53

2.1.1 Rolling at the austenite-recrystallization temperature region(RARTR)56

2.1.1.1 Metadynamic recrystallization63

2.1.1.2 Static recrystallization65

2.1.2 Rolling in austenite non-recrystallization temperature region70

2.1.3 Rolling at the under-cooled austenite73

2.1.4 Accelerated cooling and microstructural refinement78

2.2 Influence of Austenitic Recrystallization on Subsequently Transformed Grain Size79

2.2.1 Influence of recrystallized or deformed austenite on ferrite transformation79

2.2.1.1 Influence of recrystallized austenite on ferrite transformation79

2.2.1.2 Influence of partially recrystallized austenite on ferrite transformation80

2.2.1.3 Influence of non-recrystallized austenite on ferrite transformation80

2.2.2 Influence ofrecrystallization in the austenite on DIFT81

References82

3 Deformation Induced Ferrite Transformartion86

3.1 Introduction86

3.2 Experimental Confirmation and Study Method of DIFT88

3.2.1 Microstructure observation on the quenched sample88

3.2.2 Mechanical behavior measurement89

3.2.3 Dilatometry measurement90

3.2.4 In-situ X-ray diffraction92

3.3 Thermodynamics of DIFT93

3.3.1 Deformation stored energy93

3.3.2 Transformation driving force95

3.3.3 Ad3 versus deformation stored energy96

3.4 Kinetics of DIFT98

3.4.1 Microstructural evolution and nucleation sites99

3.4.2 Transformation fraction versus strain102

3.4.3 Ferrite grain number and grain size versus transformation fraction104

3.4.4 Theoretical analysis105

3.4.4.1 Ferrite nucleation rate and deformation stored energy105

3.4.4.2 Ferrite grain growth and deformation stored energy106

3.5 Mechanisms of DIFT108

3.6 Factors Influencing DIFT109

3.6.1 Deformation variables110

3.6.1.1 Strain110

3.6.1.2 Deformafion temperature110

3.6.1.3 Strain rate114

3.6.2 Chemicai compositions115

3.6.2.1 Effect of carbon and manganese115

3.6.2.2 Effect of niobium and vanadium117

3.6.3 Prior austenite grain size127

3.7 Applications of DIFT127

3.7.1 Applications of DIFT in plain low carbon steel127

3.7.1.1 Plain low carbon steel rebar128

3.7.1.2 Plain low carbon steel strip129

3.7.2 Applications of DIFT in microalloyed steel131

3.7.2.1 Laboratory trial production of 700MPa grade ultrafine grained steel131

3.7.2.2 Industrial production of high strength Cu-P-Cr-Ni weathering resistance steel132

References134

4 Microstructure Refinement of Steels by TSCR Technology137

4.1 Microstructure Refinement Process and Austenite Recrystallization of Low Carbon Steels Produced by TSCR Technology137

4.1.1 Contrast between TSCR technology and traditional technology137

4.1.2 The refinement process of microstructure during CSP hot continuous rolling140

4.1.2.1 Variation of the grain size in rolling direction141

4.1.2.2 Variation of grain size in rolling plane142

4.1.2.3 Comparison of microstructure in transverse,rolling and surface direction of ZJ330 rolling block workpiece for different passes143

4.1.3 The relationship between texture and the austenite and the ferrite144

4.1.3.1 Texture analysis by EBSD144

4.1.3.2 Orientation analysis by EBSD146

4.1.4 Austenite recrystallization of low carbon steel during continuous hot rolling process147

4.1.4.1 Microstructure evolution model of austenite during hot continuous rolling for low carbon steel148

4.1.4.2 Simulation of microstructure evolution for austenite during hot continuous rolling of low carbon steel150

4.2 Microstructure and Properties of Low Carbon Steel Produced by Thin Slab Casting and Rolling(TSCR)153

4.2.1 Comparison on microstructure and properties of low carbon hot strip with different thermal histories153

4.2.1.1 Production comparison experiments of CSP and traditional technology in producing hot low carbon strip153

4.2.1.2 Comparison and analysis of mechanical properties and microstructure of strips produced by two kinds of process154

4.2.2 Analysis of structure property in low carbon hot strip produced by CSP and traditional process156

4.2.2.1 Comparison of structure and property of low carbon hot strip produced by CSP and traditional process156

4.2.2.2 Analysis of influencing factor on microstructure and properties of hot strip produced by CSP159

4.2.2.3 Microstructure and properties of C-Mn strips with high strength produced by CSP process160

4.3 Mechanism and Precipitation Characteristic of AlN in Low Carbon Steel Produced by Thin Slab Casting and Rolling Technology164

4.3.1 AlN precipitation in low carbon steel of thin slab casting and rolling164

4.3.1.1 Experirnent on precipitation of AlN during heating and rolling164

4.3.1.2 Experiment analysis on precipitation of AlN in hot rolled strip by thin slab casting and rolling165

4.3.2 Precipitation dynamics of AlN166

4.3.2.1 Dynamics model of AlN precipitation167

4.3.2.2 Dynamics condition of AlN and simulation result170

4.3.3 Effect of fine AlN particles on structure and performance173

4.3.3.1 Effect of AlN particles on the precipitation of austenite section173

4.3.3.2 Effect of AlN precipitation during the phase transformation174

4.4 Control on Soft Mechanism of Cold Rolling Thin Slab by Continuous Casting and Rolling175

4.4.1 Requirements of cold-rolled sheet for deep drawing to the property of cold rolling billet and the control methods on soften steel175

4.4.2 Mechanism of adding B micro-alloy into low-carbon steel on grain growth coursing and steel softening176

4.4.2.1 Action of adding B micro-alloy into steel on the coursing of grain growth176

4.4.2.2 Effects of adding B into steel on the precipitation in low carbon steel177

4.4.3 Effects of the hot rolling and cooling technology on the softening of low carbon steel181

4.4.3.1 The effects of finishing temperature on theperformance of SPHC181

4.4.3.2 The effects of finishing reduction on the property of SPHC181

4.4.3.3 The effects of coiling temperature on the property of SPHC182

4.4.3.4 The effects of hot rolled lubrication on the property of SPHC182

4.4.3.5 The effects of cooling methods on the property of SPHC182

4.4.4 The effects of the control method of different softening technology on the formability of cold-rolled sheet 08Al183

4.4.4.1 The composition and technology of hot rolled low carbon steel of CSP184

4.4.4.2 The technologies of cold rolling and annealing184

4.4.4.3 The effects of coiling temperature and total cold-rolled reduction on r-value of steel 08Al with and without B185

4.4.4.4 The effects of coiling temperature and total cold-rolled reduction on the yield strength of08Al steel with and without B186

4.4.4.5 The effects of coiling temperature and the total cold-rolled reduction on elongation percentage188

4.4.4.6 Texture analysis of B free and B added steel188

4.5 Precipitations in the CSP Low Carbon Steels190

4.5.1 Introduction190

4.5.2 Sulfide and oxide dispersive precipitates191

4.5.2.1 Precipitates in slabs and rolling pieces of the low carbon steels192

4.5.2.2 Sulfides in the low carbon steels with varying content of sulfur198

4.5.2.3 Mechanism of the sulfide precipitation in the condition of CSP process202

4.5.2.4 Effects of the sulfide and oxide on formation of other phases214

4.5.2.5 Other nanometer precipitates in the steels218

4.5.3 Carbides and carbonitrides in Ti containing steels221

4.5.3.1 In general feature221

4.5.3.2 Experimental investigation223

Summary227

References228

5 Microstructure Fining Theory of Low-carbon Bainitic Steel235

5.1 Social Needs for Low-carbon Bainitic Steel with a Grade of More than 600MPa235

5.2 Strengthening Mechanism of Low(Ultra-low)Bainitic Steel237

5.3 Primary Characteristics of Several Kinds of Low-carbon Bainitic Steels Developed in China238

5.3.1 CCT curve characteristics of the steels238

5.3.2 Recrystallization curve characteristics during hot-processing240

5.3.2.1 C-Mn steel and Nb or B individually added steel240

5.3.2.2 When alloying elements such as Nb,,B,Cu are combinedly added241

5.3.3 PTT curve characteristics of the steel242

5.4 Theoretical Thought for Furthering Fining the Intermediate-temperature Transformation Microstructures246

5.4.1 Basic key points for intermediate temperature transformation microstructure fining248

5.4.2 Theoretical background for proposing the relaxation-precipitation-controlling transformation(RPC)technology249

5.4.3 Basic ideas of TMCP+RPC technology251

5.5 Ultra-fining Process,Actual Fining Effect and Typical Microstructures254

5.5.1 Selecting composition range of micro-alloying elements fully performing ultra-fining process effect254

5.5.1.1 Principle of composition design254

5.5.1.2 Starting points of composition selection255

5.5.1.3 Strength evaluation255

5.5.2 Typical process of relaxation-precipitation-controlling transformation(RPC)technology257

5.5.3 Typical fining microstructures under RPC process and its comparison with other processes258

5.5.4 Effects of RPC process and composition on microstructure and properties263

5.5.4.1 Effect of relaxing time264

5.5.4.2 Effects of final-rolling temperature265

5.5.4.3 Effects of cooling rate on microstructure and properties266

5.5.5 Strength,plasticity and toughness of the steel from industrial Trial Production of RPC process267

5.6 Study on Fining Process Parameters of Intermediate-temperature Transformed Microstructure Through Thermo-mechanical Simulation269

5.6.1 Microstructure evolution after deformation and relaxation under different temperatures270

5.6.2 Quantitative statistics of bainitebundle size273

5.7 Forming Mechanism of Typical Fining Microstructures276

5.7.1 Two kinds of typical microstructure morphology in samples after RPC process276

5.7.2 Formation and influence of substructure during relaxation278

5.7.3 Induced precipitation in deformed austenite and Its effects(Yuan et al,2004;Yuan et al,2003)280

5.7.4 Formation,Morphology of acicular ferrite and its effect on fining288

5.7.4.1 Morphology characteristics of acicular ferrite288

5.7.4.2 Effect of relaxation on formation of acicular ferrite289

5.8 Study on the Variation of Microstructure and Properties of Fined Steels during Tempering and Its Cause Analysis291

5.8.1 Hardness changes and their difference between the microstructure-fined steel and the quenched and tempered steel with the same compositions291

5.8.2 Microstructure stability in tempering process293

5.8.3 Effect of tempering temperature on mechanical properties of the steel296

5.9 Concluding Note297

References298

6 Microstructure Refining and Strengthening of Martensitic Steel300

Introduction300

6.1 Challenges of High Strength Martensitic Steel301

6.1.1 Delayed fracture301

6.1.2 Fatigue failure304

6.2 Microstructure Refinement in Toughening and Improving DF Property of Martensitic Steels305

6.2.1 Technologies for martensitic microstructure refining305

6.2.2 Effect of microstructure refinement on strength and toughness306

6.2.3 Effect of microstructure refinement on DF resistance310

6.2.3.1 Stress corrosion cracking310

6.2.3.2 Sustained load tensile delayed fracture312

6.2.3.3 Discussion of the dependence of DF resistance on grain size315

6.3 Grain Boundary Strengthening in Improving DF Property of Martensitic Steels318

6.3.1 Reducing segregation of impurities at grain boundaries319

6.3.2 Controlling grain boundary carbide321

6.3.2.1 Increasing tempering temperature321

6.3.2.2 Intercritical quenching322

6.3.2.3 Ausforming process324

6.3.3 Effect of Mo alloying325

6.3.3.1 Mo raising tempering temperature326

6.3.3.2 Mo carbide as hydrogen trap328

6.3.3.3 Mo controlling impurities and strengthening grain boundaries329

6.3.3.4 Influence of Mo content330

6.4 Controlling of Hydrogen Trap in Martensitic Steels to Improve Its DF Resistance331

6.5 Effect of Cleanliness on the Fatigue Performance of High Strength Martensitic Steels335

6.6 New Developed High Strength Martensitic Steels and Their Industrial Application341

References345

7 Carbide-free Bainite/Martensite(CFB/M) Duplex Phase Steel350

7.1 CFB/M Duplex Phase Structure351

7.2 Alloy Design of CFB/M Duplex Phase Steel by Tsinghua University Bainitic Steel R＆D Center352

7.2.1 Alloy design of CFB/M duplex phase steel and its structure352

7.2.2 Effect of cooling rate on CFB/M duplex phase microstructure356

7.2.3 Effect of CFB/M duplex phase microstructure on strength and toughness of the steel357

7.3 Effect of Tempering on Strength and Toughness of CFB/M Duplex Phase Steel359

7.3.1 Effect of CFB/M duplex phase microstructure on the initial temperature of temper embrittleness the first kind361

7.3.2 Effect of CFB/M duplex phase microstructure on yield-tensile ratio of steel367

7.4 Susceptibility to Hydrogen Embrittlement for CFB/M Duplex Phase High Strength Steel369

7.4.1 Effect of hydrogen content on susceptibility to hydrogen embrittlement for CFB/M duplex phase high strength steel370

7.4.2 Effect of heat treatment process on susceptibility to hydrogen embrittlement for CFB/M duplex phase high strength steel374

7.4.2.1 Effect of BU and CFB376

7.4.2.2 Effect of CFB quantity on fracture surface topography378

7.4.3 Influence of microstructure refinement and retained austenite on susceptibility to hydrogen embrittlement for CFB/M steel379

7.5 Stress Corrosion of CFB/M Duplex Phase High Strength Steel385

7.5.1 Stress corrosion cracking property of CFB/M duplex phase high strength steel385

7.5.2 Stress corrosion fracture of CFB/M duplex phase high strength steel388

7.6 Hydrogen in CFB/M Duplex Phase High Strength Steel391

7.6.1 Measure hydrogen diffusion coefficient using double electrolysis cell392

7.6.2 Hydrogen trap in CFB/M duplex phase high strength steel396

7.6.2.1 Bainitic/martensite lath boundary398

7.6.2.2 Retained austenite398

7.7 Mechanism of Resistance to Delayed Fracture of CFB/M Steel402

7.7.1 Relationship between susceptibility to hydrogen embrittlement and hydrogen trap for CFB/M steel402

7.7.2 Relationship between stress corrosion and hydrogen trap in steel403

7.7.3 Crack propagation model of CFB/M duplex phase steel405

7.8 Fatigue Behavior of 1500MPa CFB/M Duplex Phase High Strength Steel405

7.8.1 Fatigue behavior of CFB/M duplex phase steel405

7.8.1.1 Fatigue strength of CFB/M duplex phase steel405

7.8.1.2 Fatigue crack propagation behavior406

7.8.1.3 Fatigue fracture of CFB/M duplex phase high strength steel410

7.8.2 Effect of microstructure characteristics of CFB/M duplex phase steel on fatigue behaviors413

7.8.2.1 Effect of microstructure characteristics on fatigue strength413

7.8.2.2 Effect of microstructure characteristics on △Kth and da/dN416

7.8.3 Effect of retained austenite on fatigue behaviors of CFB/M duplex phase steel419

7.8.3.1 Retained austenite content and cyclical stability419

7.8.3.2 Effect of retained austenite and its cyclical stability on fatigue strength420

7.8.3.3 Effect of retained austenite film on fatigue crack propagation421

7.8.4 Fatigue fracture mechanism of CFB/M duplex phase steel422

7.9 Application Prospect of CFB/M Duplex Phase Steel422

References424

8 Extra Low Sulfur and Non-metallic Inclusions Control for Ultra Fine Grain High Strength Steels431

8.1 Introduction431

8.2 Refining Technology for Extra Low Sulfur Steels432

8.2.1 Hot metal De-S pretreatment433

8.2.2 Reducing[S]pick up in BOF steelmaking435

8.2.3 Desulfurization in secondary refining of liquid steel438

8.2.3.1 Desulfurization during BOF tapping441

8.2.3.2 Desulfurization in ladle furnace refining(LF)443

8.2.3.3 Powder injection desulfurization methods444

8.3 Extra Low Oxygen and Non-metallic Inclusions Control of High Strength Alloying Steels447

8.3.1 Influence of non-metallic inclusions on fatigue property of steel448

8.3.2 Refining and non-metallic inclusion control of extra low oxygen alloy steels450

8.3.3 Deformable non-metallic inclusions for tyre cord and valve spring steels458

8.3.3.1 Deformable non-metallic inclusions459

8.3.3.2 Control of[Al]in liquid steel461

8.3.3.3 Slag control462

8.3.4 Steel with premium cleanliness468

References470

9 Fundamental Study on Homogeneity of Solidification Structure of Steel473

9.1 The Structure of Liquid Fe-C Alloy473

9.1.1 Experimental473

9.1.2 Data analysis474

9.1.3 Medium-range order structure in liquid Fe-C alloy475

9.1.4 Conclusions479

9.2 Observation and Analysis of Heterogeneous Nucleation Phenomena480

9.2.1 Experimental481

9.2.2 Effects of vibration frequency and amplitude482

9.2.3 Effects ofsolid substrate temperature and surface roughness483

9.2.4 Conclusions486

9.3 Homogeneity and Equiaxed Grain Structure of Steels486

9.3.1 Relation of segregation and equiaxed grain structure487

9.3.2 Titanium-based inoculation technology488

9.3.2.1 Precipitation of TiN particles488

9.3.2.2 Competitive precipitation between TiN and Ti2O3489

9.3.2.3 Particles for nucleus ofδ-ferrite dendrites490

9.3.3 Small temperature gradient technology491

9.3.4 Conclusions492

References492

10 Welding of Ultra-Fine Grained Steels494

10.1 Introduction494

10.2 Simulation of Welding of Fine-grained Steel495

10.2.1 Simulation of grain growth in HAZ495

10.2.1.1 The monte carlo model of the HAZ496

10.2.1.2 The EDB model497

10.2.1.3 MC simulation ofgrain growth in HAZ of fine-grained steels498

10.2.1.4 Experimental identification499

10.2.2 Fluid flow in welding pool of ultra fine grain steel501

10.2.2.1 Mathematical model501

10.2.2.2 Numerical method505

10.3 Welding of Fine Grained Carbon Steel Plate509

10.3.1 Laser welding of low carbon steel509

10.3.1.1 Experiment material and equipment509

10.3.1.2 Weld shape and microstructure of welded joints510

10.3.1.3 Mechanical properties of laser welded joint514

10.3.1.4 Conclusions517

10.3.2 Arc welding of fine grained low carbon steel518

10.3.2.1 Experiment material and method518

10.3.2.2 Experiment results and discussion519

10.3.2.3 Conclusions523

10.3.3 Arc welding of fine grained atmospheric corrosion resistant steel524

10.3.3.1 Experiment materials and procedure524

10.3.3.2 Experiment results and discussion525

10.3.3.3 Conclusions527

10.3.4 Welding of 400 MPa grade fine grained rebar528

10.3.4.1 Experiment material and procedure528

10.3.4.2 Experiment results and discussion529

10.3.4.3 Conclusions531

10.4 Welding of Ultra-Fine Structure Bainite Steel532

10.4.1 Development of ultra-low carbon bainitic high strength welding wire532

10.4.1.1 Designing principles of the ULCB welding wires532

10.4.1.2 Compositions and mechanical properties of ULCB wire deposited metals533

10.4.1.3 Optical microstructure of the ULCB deposited metals536

10.4.1.4 Fine microstructure of ULCB wire deposited metals538

10.4.1.5 Conclusions539

10.4.2 Microstructures and properties of the GMAW welded joint of the ultra fine structure bainitic steel540

10.4.2.1 Weld microstructure of ultra fine grained bainitic steel540

10.4.2.2 Mechanical properties of weld metal in ultra-fine grained bainitic steel543

10.4.2.3 Conclusions545

10.4.3 Laser welding of ultra-fine microstructural bainitic steel546

10.4.3.1 Chemical composition and microstructure of base metal546

10.4.3.2 Experimental procedure547

10.4.3.3 Grain size548

10.4.3.4 Microstructure of CGHAZ549

10.4.3.5 Hardness and strength of CGHAZ553

10.4.3.6 Toughness of CGHAZ555

10.4.3.7 Conclusions560

References561

Subject Index567