Automotive steels : design, metallurgy, processing and applications

Automotive Steels

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Automotive Steels

Woodhead Publishing Series in Metals

and Surface Engineering

Design, Metallurgy, Processing

and Applications

Edited by

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Shiv Brat Singh

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British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloging-in-Publication Data

A catalog record for this book is available from the Library of Congress

ISBN: 978-0-08-100638-2 (print)

ISBN: 978-0-08-100653-5 (online)

For information on all Woodhead Publishing visit

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Publisher: Matthew Deans

Acquisition Editor: Gwen Jones

Editorial Project Manager: Charlotte Cockle

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Cover Designer: Christian J. Bilbow

Typeset by MPS Limited, Chennai, India

Contents

List of contributors ix

1 Design of auto body: materials perspective 1

J.R. Fekete and J.N. Hall

1.1 History of steel usage in vehicle body structures and closures 1

1.2 Significant events in history impacting steel application

in vehicle design 1

1.3 Breakdown in vehicle by material mass and application 8

1.4 Improved safety and fuel economy: current regulations 9

1.5 Vehicle energy losses and contribution to fuel economy

through mass reduction 12

1.6 Summary 16

References 16

2 Steels for auto bodies: a general overview 19

J.N. Hall and J.R. Fekete

2.1 Steel grades and design strategy for auto body applications 19

2.2 Steel’s contribution to fuel economy through mass

reduction 25

2.3 Recent body structure & closures production applications 28

2.4 Manufacturing concerns 31

2.5 Future steel technology 35

2.6 Sustainability/life cycle assessment 37

2.7 Summary 43

References 44

3 Formability of auto components 47

E.H. Atzema

3.1 Introduction 47

3.2 Basic concepts 48

3.3 Advanced process analysis 48

3.4 Basic concepts 50

3.5 Advanced process analysis 63

3.6 Forming processes 70

3.7 Formability aspects of different steels 82

3.8 Conclusions 90

Acknowledgments 90

References 90

4 Physical metallurgy of steels: an overview 95

G. Krauss

4.1 Introduction 95

4.2 The iron-carbon phase diagram 96

4.3 Austenite 98

4.4 Ferrite and cementite 99

4.5 Steel microstructure: general considerations 100

4.6 Steel microstructures produced by diffusion:

ferrite, pearlite, and bainite 101

4.7 Diffusionless transformation of austenite: martensite 104

4.8 Transformation diagrams and Jominy End Quench Curves 108

4.9 Summary 110

References 110

5 Deep drawable steels 113

P. Ghosh and R.K. Ray

5.1 Introduction 113

5.2 Aluminum killed (AK) steels 116

5.3 Interstitial free (IF) and interstitial free high strength

(IFHS) steels 127

5.4 Bake hardening (BH) steels 138

5.5 Summary and conclusions 140

References 141

6 High strength low alloyed (HSLA) steels 145

C.I. Garcia

6.1 History and definition 145

6.2 Structure
property relationships: effect of microstructure

on the mechanical properties of HSLA steels 148

6.3 Fundamental metallurgical principles of thermomechanical

processing 150

6.4 Examples of hot and cold rolled HSLA steels used

in the transportation industry 153

6.5 Transformation behavior 155

6.6 Summary 165

References 166

7 Dual-phase steels 169

N. Fonstein

7.1 Introduction 169

7.2 Effect of structure on mechanical properties of dual-phase steels 170

vi Contents

7.3 Obtaining dual-phase steels by transformations of austenite using

controlled cooling from the intercritical region 186

7.4 Obtaining as-rolled dual-phase microstructure by cooling

of deformed austenite 197

7.5 Effects of chemical composition on dual-phase steels 198

7.6 Application of dual-phase steels in modern cars 208

7.7 Summary 208

References 209

8 TRIP aided and complex phase steels 217

K. Sugimoto and M. Mukherjee

8.1 Introduction 217

8.2 Processing route and microstructure 219

8.3 Alloy design 224

8.4 Microstructure modeling 229

8.5 Deformation-induced transformation of retained austenite 231

8.6 Mechanical properties 239

8.7 Press formability 245

8.8 Other mechanical properties 247

8.9 Summary 248

References 249

9 Bake hardening of automotive steels 259

E. Pereloma and I. Timokhina

9.1 Introduction 259

9.2 Mechanisms of bake hardening response 260

9.3 Factors affecting bake hardening response 264

9.4 Bake hardening of multi-phase steels 272

9.5 Modeling 281

9.6 Effect of bake hardening on the performance of automotive steels 282

9.7 Summary 283

References 283

10 Bainitic and quenching and partitioning steels 289

E. De Moor and J.G. Speer

10.1 Introduction 289

10.2 Bainitic steels 289

10.3 Quenching & partitioning 292

10.4 Substitution of silicon by aluminum 294

10.5 Manganese alloying 296

10.6 Carbon alloying 298

10.7 Molybdenum additions 301

10.8 Competing reactions during partitioning 304

10.9 Local formability of bainitic and Q&P steels 308

10.10 Conclusions 312

Contents vii

Acknowledgments 312

Disclaimer 313

References 313

11 High Mn TWIP steel and medium Mn steel 317

B.C. De Cooman

11.1 Introduction 317

11.2 High Mn TWIP steel 320

11.3 Medium Mn TRIP and TWIP 1 TRIP steel 352

11.4 Outlook for high Mn TWIP steel and medium Mn steel 360

11.5 Summary 379

Acknowledgment 379

List of abbreviations 379

References 380

12 Hot formed steels 387

E. Billur

12.1 Introduction 387

12.2 Physical metallurgy of hot forming steels 392

12.3 Hot forming steels 393

12.4 Blank coatings 398

12.5 Typical automotive applications 402

12.6 Summary and future outlook 405

References 406

13 Forging Grade Steels for Automotives 413

O.N. Mohanty

13.1 Introduction 413

13.2 Basic physical metallurgy relevant to hot forging 417

13.3 Evolution of microalloyed forging steels 436

13.4 Steels for automotive forging—the way forward 447

References 448

Index 455

viii Contents

List of contributors

E.H. Atzema Tata Steel, IJmuiden, The Netherlands

E. Billur Billur Metal Form Ltd., Bursa, Turkey; Atılım University, Ankara,

Turkey

B.C. De Cooman Graduate Institute of Ferrous Technology, POSTECH, Pohang,

South Korea

E. De Moor Colorado School of Mines, Golden, CO, United States

J.R. Fekete National Institute of Standards and Technology, Boulder, CO, United

States

N. Fonstein ArcelorMittal Global R&D, East Chicago Labs, United States

C.I. Garcia University of Pittsburgh, Pittsburgh, PA, United States

P. Ghosh Tata Steel, Jamshedpur, India

J.N. Hall Steel Market Development Institute, Southfield, MI, United States

G. Krauss Colorado School of Mines, Golden, CO, United States

O.N. Mohanty RSB Group, Pune, India

M. Mukherjee Tata Steel Ltd., Jamshedpur, Jharkhand, India

E. Pereloma University of Wollongong, Wollongong, NSW, Australia

R.K. Ray Indian Institute of Engineering Science and Technology, Shibpur, West

Bengal, India

J.G. Speer Colorado School of Mines, Golden, CO, United States

K. Sugimoto Shinshu University, Wakasato, Nagano, Japan

I. Timokhina Deakin University, Geelong, VIC, Australia

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1 Design of auto body: materials

perspective

J.R. Fekete1 and J.N. Hall2

1

National Institute of Standards and Technology, Boulder, CO, United States,

2

Steel Market Development Institute, Southfield, MI, United States

1.1 History of steel usage in vehicle body structures

and closures

Steel has been an important material for body construction of motor vehicles in

North America since the early 1900s. At that time, steel competed with aluminum

and wood for predominance in body construction, but by the 1920s it was the

material of choice. Its low cost, coupled with its ability to be pressed into complex

shapes, and easily joined through welding processes, led to this position in the

industry. From these early days, the auto industry depended on secure supplies of

sheet steel, and the steel industry responded by developing a strong capability for

thin, wide steel sheets to support one of its major customers. However, starting in

the 1960s, the automotive industry faced significant new challenges that would

fundamentally change vehicle structural requirements. These challenges included

regulatory demands for safer, cleaner, and more fuel efficient vehicles, as well as

increased competition from new materials entrants in the North American market

and customer demands for higher performance, comfort, and reliability. The

responses to these challenges required the development of new steel products with

higher strength and improved manufacturability.

1.2 Significant events in history impacting steel

application in vehicle design

The 20th century, particularly its second half, was a time of rapid development of

both the steel and auto industries. The amazing improvements in the ability of

people and goods to be moved across great distances resulted in rapid growth of the

transportation industry. This came with a price, though, as injury and deaths result￾ing from accidents skyrocketed, and skies darkened with the emissions of the

expanding numbers of internal combustion engines. At the same time, customers

came to expect an ever increasing level of comfort and speed in their vehicles.

The experience of the United States in the latter half of the 20th century serves as a

Automotive Steels. DOI: http://dx.doi.org/10.1016/B978-0-08-100638-2.00001-8

Copyright © 2017 Elsevier Ltd. All rights reserved.

relevant example of how the steel and auto industries worked together to meet these

emerging needs.

The post-World War II economic expansion in the U.S. resulted in rapid growth

of the automotive industry in the 1950s and 1960s. With this success came increasing

public pressure to improve the safety and environmental performance of this growing

industry. The U.S. government responded to these events through several legislative

actions. The Federal Clean Air Act was passed in 1970. This act established the regu￾latory framework for monitoring and reducing emissions of air pollutants, and cre￾ated the Environmental Protection Agency (EPA), whose mandate included reducing

pollution from motor vehicles. In the same year, the Highway Safety Act was passed,

creating the National Highway and Traffic Safety Administration (NHTSA), charged

with establishing safety requirements for both motor vehicles and the roads on which

they traveled. Examples of these new requirements include implementation of

energy absorbing bumpers, three-point restraint systems, and improved structural

requirements for frontal and side impact energy absorption.

At the same time, the Arab oil embargo of 1973 resulted in disruptions in the

supply of gasoline for motor vehicle usage. The price of gasoline increased

dramatically and became very unstable. One consequence of these events was

increasing demand for smaller, more fuel efficient vehicles. At this time, small cars

constituted a relatively small part of the U.S. market, as the domestic manufacturers

responded to the demand from their customers for larger, more luxurious vehicles.

However, small cars had been exported to the U.S. market for many years by a

number of overseas suppliers (in relatively small numbers). These vehicles included

the Volkswagen Beetle, Honda Civic, and Toyota Corolla. The “gas shocks” helped

boost the demand for these vehicles in the U.S. market, a demand that has increased

over time. These events also resulted in public pressure for political solutions to the

need for improved fuel economy in motor vehicles. The result was the implementa￾tion of CAFE (Corporate Average Fuel Economy) standards by the EPA.

It quickly became clear to automotive engineers that these new regulatory and

consumer demands would necessitate significant vehicle mass reduction. Reducing

mass resulted in higher fuel economy, lower vehicle emissions, and helped

engineers meet new safety requirements. Vehicle downsizing and migration from

body-on-frame (BOF) to body-frame-integral (BFI) structures were two early initia￾tives used to accomplish the mass reduction. Fig. 1.1 demonstrates the dramatic

mass reductions that were accomplished by the domestic automakers, and the

improvement in fuel mileage that followed.

This focus on mass reduction led to demonstrations of the improvement in

structural efficiency made possible when the strength-to-weight ratio of the materi￾als of construction is increased. An example of this work in the late 1970s was the

development of the “Charger XL” by Chrysler Corporation, where application of

both higher strength steel and aluminum resulted in a 286 kg reduction in vehicle

mass with no impact on vehicle quality or performance [1,2]. This work was an

early demonstration of the potential of high-strength steel.

In the early days of automotive high-strength steel development, many differ￾ent concepts were investigated. At this time, ingot casting and rolling were still

the most widely used processes for producing slabs. The so-called “rimmed”

2 Automotive Steels

steels (named for the “rimming” action—the boiling caused by dissolved oxygen

reacting with carbon in the mold to create CO and CO2) were commonly used for

automotive applications because of their superior surface quality, cleanliness, and

ductility. Nitrogen and carbon remained in solid solution in rimmed steel, and

metallurgists could take advantage of this characteristic to increase the strength of

steel parts through strain aging. The strain was induced during the forming pro￾cesses and the subsequent aging occurred during a post forming heat treatment,

which sometimes involved the paint bake cycle. Nitrogen could be added to these

materials to make even higher yield strength grades, up to 500 MPa [3]. These

steels were the precursors to the bake hardenable grades described below.

However, there were two problems with this approach. First, the materials were

susceptible to stretcher strains or “Lu¨ders lines,” an objectionable surface condi￾tion, especially for exposed quality material. Second, and most important, the

industry at this time was moving rapidly toward continuous casting of slabs, a

much more efficient process than the traditional casting of ingots and subsequent

production of slabs through rolling. The continuous casting process requires

“killed” steel, the opposite of “rimmed” steel. Aluminum is added to “kill” the

oxidation of carbon in these steels by replacing the carbon in the oxidation

reaction. It also combines with nitrogen and, to a lesser extent, carbon itself,

removing them from solution. Thus, the strain aging was significantly reduced,

and the high strength levels of rimmed steel could not be reached with killed

steels. There were few applications of strain-aging high strength steel at this

time, and the onset of continuous cast, killed steel quickly ended the use of these

materials in automotive applications.

Figure 1.1 History of vehicle curb weight, CAFE mileage requirements and actual CAFE

performance for the U.S. fleet [11].

Design of auto body: materials perspective 3

So-called “ultra-high strength steels,” with tensile strength levels above

600 MPa, were also in development at this time. These included martensitic steels

[4,5] which were produced in continuous annealing lines, and recovery annealed

steels, which were cold rolled to very high strength levels, then annealed below the

recrystallization temperature to recover enough ductility to survive rudimentary

forming processes [6]. Both of these materials found niches in the marketplace,

mainly in roll-formed parts such as bumpers and beams where formability require￾ments were not as difficult. Initial development of dual phase (DP) steels also

occurred during this time [7,8]. These materials were processed to produce micro￾structures of martensite and/or bainite islands in a ferrite matrix through careful

intercritical annealing and subsequent fast cooling. The potential of these products

was successfully demonstrated, but it was difficult to produce a uniform product

with the available process control technology. Also, the relatively low cooling capa￾bilities of steel processing lines demanded higher alloy contents to achieve the

needed hardenability. This resulted in products that were difficult to weld. It would

be another 20 years before DP steel could be developed into an important structural

material in the automotive industry.

The high-strength steel products that would become most widely used at this

time were the microalloyed high strength, low alloy (HSLA) steels [9]. Automotive

steel makers used a combination of alloying with carbo-nitride formers, such as Nb,

V, Ti, and Zr, and careful thermomechanical processing to produce fine grained,

precipitation strengthened steels. The final products had yield strength levels of

280
550 MPa and relatively high ductility. Additions of rare earth elements such

as Ca or Zr were found to transform sulfide inclusions from long “stringers” to a

more globular morphology, and the resulting improved transverse ductility was crit￾ical to the successful early application of HSLA steels [10]. However, as with the

DP steels, the processing requirements of these products tested the process control

capabilities of steel mills and early versions of these products had much larger

ranges of mechanical properties than the commonly used mild steels. This fact,

along with the reduced formability and higher springback after stamping, made

early applications difficult to produce through stamping. The feedback from the

press shops caused product engineers to slow down their application of high￾strength steel. However, the need for more efficient structures was not going away,

which forced both the automotive and steel industries to improve their processes to

successfully produce parts with these steels and to utilize their capability to reduce

vehicle mass.

The regulatory pressure steadily increased during the decade of the 1980s. The

frontal and side impact requirements conceived and proposed earlier were now fully

implemented. Additional requirements for pole impacts and bumper integrity were

also implemented. As shown in Fig. 1.1, the CAFE requirements for cars steadily

increased from 18 mpg (miles per gallon) at the beginning of the decade to

27.5 mpg by the end. The California Air Resources Board and EPA also continued

to drive reductions in vehicle emissions through regulatory actions.

During the 1980s, the pressure to improve fuel efficiency to reduce weight

caused the majority of car platforms in the United States to convert from BOF to

4 Automotive Steels