Advanced steels for lightweight automotive structures

C.D. Horvath,     General Motors, USA

Abstract:

High strength steels are commonly used in the automotive industry to reduce mass and improve structural performance. This chapter discusses the history of steel in the manufacture of automobiles, the types of steels currently being used and the new advances in the types of steels that can be used for lightweighting automotive structures. The chapter also reviews the manufacturing and forming aspects of these steels along with some design considerations for the selection of these materials.

Key words

steel

advanced high strength steel

dual phase

trip

complex phase

martensite

boron steel

press hardening

spot welding

adhesive bonding

spot welding

welding

MIG welding

vehicle design

2.1 History of steel in automobiles

The use of steels in the manufacture of automobiles has a rich and varied history. By most accounts, the era of the automobile is considered to have started in the late 19th century. It was in 1885 that the German engineer Karl Benz built what is generally considered to be the first automobile powered by an internal combustion engine. By today's standards, these first internally combustion driven automobiles were little different from the horse-drawn carriages common in the 18th and early 19th centuries. Both were largely constructed from wooden frames and body panels and even used wooden wheels.

Sometime in the late 19th and early 20th centuries, after the advent of sheet metals and the manufacturing processes to form them into complex shapes, the largely wooden structures of early automobiles were gradually being replaced by sheet metals. It was sometime in the early 1900s, that bodies manufactured from a combination of metal sheets of steel and aluminum, although still built largely on wooden frames, were being driven around the streets of North America. With the introduction of the Model T Ford in 1908, the era of mass produced, largely steel automotive bodies was up and running, and the automobile was well on its way to becoming what many consider to be the invention that most significantly impacted the lives of common working people and, just as importantly, led to the development of today's mobile and modern society.

The use of steel virtually dominated automotive body design for the next 100 years. The primary grades used in these first automobiles were mild steels, or as they are more accurately referred to, low carbon steels. For the existing demands of the time, these steels provided the nearly perfect balance of strength, formability, cost and design flexibility that the industry needed to expand throughout the developed regions of the world. It was not until the first oil shock in the early 1970s and the adoption of fuel economy standards in North America that the industry began to seriously look towards higher strength steels to lower the mass of their vehicles and to improve fuel economy. Since that time, the structural designs used for automobiles and the grades of steel used to manufacture them have been in a constant state of evolution in response to not only consumer demands, but also to the complex array of regulations for fuel economy, emissions and crashworthiness that the modern automotive industry must now account for in the designs of their products.

It was common in the early development of the automobile, and even through the late 1930s and early 1940s, that most vehicles were built on frames that carried the body and skin panels - hence it was commonly referred to as body on frame construction or BOF. This type of construction not only allowed automotive designers to easily change the visual styling of the body to meet the ever changing demands of consumers, but also provided an easy and cost effective way to maintain styling differentiation between manufacturers. However, as vehicles became smaller during the 1960s, and especially after the oil shock in the early 1970s, a new type of body costruction became popular. This new design, called body frame integral (BFI), gained favor and started a steady transition to this construction philosophy. Largely unknown to the industry at the time, the increase use of this integrated vehicle structural design would eventually become a key enabler for the future widespread use of high strength steels

In contrast to the BOF designs, this new design was considered to be more mass and fuel efficient, since the body and frame are designed as one system. This is in contrast to the BOF design where the body and frame are independent from each other, resulting in part function redundancy and increased mass. With the increasing focus on mass and fuel economy in the 1970s and 1980s, the more efficient BFI structural designs became more popular and eventually began to dominate automotive designs for passenger cars. In contrast, light trucks and heavy duty vehicles, where high load capacity and durability were considered more important than fuel economy, largely kept the BOF design that was considered superior for the needs of those vehicles.

With the automotive industry's large scale conversion to BFI designs that did not require heavy frames, the stage was set for an increased use of high strength steels. The increased number of stampings used in this design, and the ability to tailor the strength and shape of these panels, gave designers and manufacturing engineers new freedom to adapt part shape and forming technologies to the characteristics of high strength steels. As a result, a new age in automotive steel development would be ushered in to improve product safety and meet the increasing demands of government regulators and consumers for safer, more fuel efficient vehicles: the face of automotive design and engineering would be changed forever.

However, unknown to the automotive industry at the time, the road they were about to embark on was steep and full of potholes. Not surprisingly, the industry's initial attempts at using high strength steels, which largely consisted of high strength low alloy and solid solution strengthened steels, were largely unsuccessful. The relatively low ductility of these steels compared with the low carbon steels in general use at the time presented many challenges that designers and manufacturing engineers were unprepared to handle. The part designs and manufacturing processes used on these initial applications were the same, or at best little changed, from those used with low carbon steels and did not suit the specific needs of high strength steels. Compounding matters, steel companies had not yet perfected their processes to make high strength steels consistently, and the resulting variability in mechanical properties essentially sealed the fate of these first attempts to use them.

Initial attempts to form high strength steels routinely resulted in severe forming cracks and springback that could not be effectively resolved with techniques that were available at the time. The lessons learned during these early experiences were both difficult and costly. Unfortunately, the lessons did not come fast enough to prevent the industry from returning to the low carbon steels that they understood much better and that had contributed to the early successes of the industry. These first attempts at using high strength steels were, by nearly all accounts, a virtual disaster. Even so, the benefits of these steels were recognized across the industry and manufacturers continued to use them in a select number of the most critical structural parts in the automobile. This foresight and perseverance would end up teaching the entire industry valuable lessons that would be used in later years to effectively incorporate these steels into future vehicle designs.

2.2 Types of high strength steels

There are many types of steels used by today's automotive designer to reduce mass and improve energy absorption in crash events. These steels can easily be divided into three general categories that roughly separate materials by their microstructures. The first category consists of both low carbon and conventional types of high strength steels (HSS) that have been the backbone of automotive design for a number of decades. The second category is generally referred to as the first generation of advanced high strength steels (AHSS) and the third category consists of the second generation of AHSS. Each of these categories has a number of different materials that, while different from each other, share some unique physical and metallurgical properties that distinguish them from materials in the other categories. These categories and the materials included in them are shown in Table 2.1.

Table 2.1

Types of automotive steels

Low carbon steels with carbon levels of 0.13% or less have been the most commonly used steels in automotive body panels and structural components for many years. The combination of low cost, excellent formability, weldability, high quality appearance after painting and overall ease of manufacturing has contributed to their widespread use. There are many grades of low carbon steel used in the automotive industry, but the most common types are hot rolled and cold rolled grades with much...