Automotive Steels
eBook - ePub

Automotive Steels

Design, Metallurgy, Processing and Applications

Radhakanta Rana, Shiv Brat Singh

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eBook - ePub

Automotive Steels

Design, Metallurgy, Processing and Applications

Radhakanta Rana, Shiv Brat Singh

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About This Book

Automotive Steels: Design, Metallurgy, Processing and Applications explores the design, processing, metallurgy, and applications of automotive steels. While some sheet steels are produced routinely in high volume today, there have been significant advances in the use of steel in the automotive industry.

This book presents these metallurgical and application aspects in a way that is not available in the current literature. The editors have assembled an international team of experts who discuss recent developments and future prospects for automotive steels, compiling essential reading for both academic and industrial metallurgists, automotive design engineers, and postgraduate students attending courses on the metallurgy of automotive materials.

  • Presents recent developments on the design, metallurgy, processing, and applications of automotive steels
  • Discusses automotive steels that are currently in the early stages of research, such as low-density and high modulus steels that are driving future development
  • Covers traditional steels, advanced high strength steels, elevated Mn steels and ferrous composite materials

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1

Design of auto body

Materials perspective

J.R. Fekete1 and J.N. Hall2, 1National Institute of Standards and Technology, Boulder, CO, United States, 2Steel Market Development Institute, Southfield, MI, United States

Abstract

The automobile has been under constant development since the late 1800s. The many classes of materials used to make today’s vehicles are similar to the ones of the first production vehicles. These materials have also developed through the past 100+ years. Many historical events impacted the development of automobile design and material grades. The most notable events led to regulations around safety and fuel economy. This chapter will review the history of automotive materials and how these regulations have influenced changes in these materials. The focus will be on body, chassis, and suspension applications with a comparison of the dominant material, steel, to competing materials, aluminum, magnesium, and polymer composites.

Keywords

Steel; high-strength steel; advanced high-strength steel; applications; automotive; sheet steel; Third Generation Advanced High-Strength Steel; mass reduction; light-weighting

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 resulting 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 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 regulatory framework for monitoring and reducing emissions of air pollutants, and created 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 implementation 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 initiatives 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.
image

Figure 1.1 History of vehicle curb weight, CAFE mileage requirements and actual CAFE performance for the U.S. fleet [11].
This focus on mass reduction led to demonstrations of the improvement in structural efficiency made possible when the strength-to-weight ratio of the materials 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 different concepts were investigated. At this time, ingot casting and rolling were still the most widely used processes for producing slabs. The so-called “rimmed” 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 processes 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 “LĂŒders lines,” an objectionable surface condition, 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.
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 requirements were not as difficult. Initial development of dual phase (DP) steels also occurred during this time [7,8]. These materials were processed to produce microstructures 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 capabilities 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 critical 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 ...

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