**3.1 Classification of steel developments**

Steel developments—in general and particularly for the automotive industry—may be classified in several different ways. One usual way of classification is done according to the metallurgical designation. These may be grouped into low strength steels (including mild steels, interstitial free (IF) steels), conventional high strength steels like carbon-manganese (C-Mn) steels, bake-hardenable (BH) steels, high strength low alloyed (HSLA) steels, and the newer types of Advanced High Strength Steels (AHSS), e.g., Dual Phase (DP) steels, Transformation Induced Plasticity (TRIP) steels, Twinning Induced lasticity (TWIP) steels, Complex Phase (CP) steels, martensitic (MS) steels. In recent years, new AHSS grades have been developed, for example, Extra Advanced High Strength Steels (X-AHSS) and Ultra Advanced High Strength Steels (U-AHSS), and various types of the so-called thirdgeneration AHSS steels, e.g., TRIP-aided bainitic ferrite (TBF) and Quenching & Partitioning (Q&P) or different types of NanoSteels: all these with the primary aim to provide even higher strength parameters with significantly increased formability.

Another classification introduces various mechanical properties—mainly strength and formability parameters as the Ultimate Tensile Strength (UTS) and Total Elongation (TE). This type of classification is often used together with the designation of development steel generations, as well. In **Figure 2**, the well-known relationship between strength and ductility parameters is shown applying the abovementioned classification method with a graphical representation. From **Figure 2**, it may be also seen that the product of the ultimate tensile strength and the total elongation (UTS × TE) follows a hyperbolic function. The constant (C = UTS × TE) provides further possibility to classify the steel developments by their generation.

In **Figure 2**, the material group shown by gray color includes the conventional mild steels (IF and mild steels) formerly widely applied in Body in White (BiW) production in the automotive industry. The group of conventional high strength steels (colored by light blue) includes bake hardening (BH), isotropic (IS), high strength interstitial free (HS IF), carbon-manganese (C-Mn), and high strength low alloyed (HSLA) steels. Following the conventional high strength steels, an intensive development started in the steel industry in close cooperation with the automotive industry to develop different types of Advanced High Strength Steels (AHSS) to meet the needs of lightweight automotive structures.

The development of first generation of Advanced High Strength Steels for automotive application may be regarded as the first step in this development process. DP steels, Complex Phase (CP) steels, certain types of TRIP steels, and martensitic steels (MART) belong particularly to this group. For these steels, the C constant

**Figure 2.**

*Relationship between ultimate tensile strength (UTS) vs total elongation (TE) for various generations of high strength steels [4].*

defined above can be found between 10,000 and 25,000. The first-generation AHSS (often referred as conventional AHSS) grades have good strength but limited ductility.

However, it is worth mentioning that for these Advanced High Strength Steels, the increase of strength parameters is much more significant than the decrease of the ductility parameters. This is particularly valid for the Dual Phase (DP) steels, Complex Phase steels (CP), Martensitic Complex Phase (MART/CP) steels, and TRIP steels. This is the reason why this group gains wide application in car body production.

The group of steels that can be found around the C = 40,000–60,000 MPa × % may be regarded as the second generation of Advanced High Strength Steels. This group includes the Twinning Induced Plasticity (TWIP) steels and some austenitic Stainless Steels (AUST SS) with high manganese content. These steel grades provide superior combination of strength and ductility. TWIP steel had successfully been trial-produced at POSCO in the early 1990s, but the trial was not extended to commercialization due to limitations in facilities and productivity [4]. Trial productions have also been made at some European steel companies. These attempts demonstrated the outstanding mechanical performance of TWIP steels; however, these trial productions turned to be commercially unsuccessful due to low productivity and high cost [5]. New approaches and further developments are done continuously to reduce these difficulties and make them suitable for automotive parts manufacturing.

The third generation of AHSS is still in the phase of development—though there are already industrial realizations, too. In this development stage, several new concepts have been already proposed. The main target in developing the third generation of AHSS is twofold, i.e., to achieve mechanical properties in the range between the first- and second-generation AHSS shown in the strength-ductility diagram (**Figure 2**) but with less alloying elements and, hence, with less expensive

**105**

**Figure 3.**

*Development of Lightweight Steels for Automotive Applications*

*3.1.1 Development projects for Advanced High Strength Steels*

the GigaPascal range of strength together with increased ductility.

possible the requirements analyzed in Section 2.

production than the second-generation AHSS steels [6]. The microstructure of these steels consists of a high strength phase (e.g., nano/ultrafine-grained ferrite, martensite, or bainite) combined with a further phase or constituent that provides substantial ductility and work hardening (e.g., austenite). With this development concepts, very high strength steels in the GPa range with even though remarkable

The projected changes in the application of Advanced High Strength Steels is given by Matlock et al. [8] for North American vehicle industry (**Figure 3**), but similar trend may be estimated for other geographical regions, e.g., the European Automotive manufacturers and the Far East countries (China, Japan, and Korea). In **Figure 3**, the changes of the absolute content of AHSS applications (in kg) and percentage changes related to the total weight of vehicles are shown. Both changes show an exponential increase with a slightly higher one concerning the absolute

In the last 30–40 years, there were several projects studying and initiating the development of new grades of Advanced High Strength Steels. Most of these projects were initiated by automotive companies, and in most cases various consortiums were established for this purpose. Each of these projects aimed to meet as much as

Among these projects, the Ultralight Steel Automotive Body (ULSAB) satisfied most of the requirements stated for a lightweight automotive structure and proved to be structurally sound, safe, executable, and affordable. Though it was a highly expensive project with the participation of 35 companies representing 18 countries, it could meet the challenges to reduce the weight of steel auto body structures at no additional costs, while maintaining or even improving the overall performance [9]. Further projects followed the ULSAB concept, among them the Ultralight Steel Auto Closures (ULSAC) [10], or the Ultralight Steel Auto Body-Advanced Vehicle Concept (ULSAB-AVC) [11], and the Future Steel Vehicle (FSV) [12]. All these projects led to the further development of Advanced High Strength Steels reaching

In the previous section, we could see the main material development tendencies and their classification that included various kinds of conventional steels as well,

*Projected changes of the absolute amount (kg) and the percentage values for the total weight of vehicles.*

*DOI: http://dx.doi.org/10.5772/intechopen.91024*

formability can be produced [7].

values.

#### *Development of Lightweight Steels for Automotive Applications DOI: http://dx.doi.org/10.5772/intechopen.91024*

*Engineering Steels and High Entropy-Alloys*

defined above can be found between 10,000 and 25,000. The first-generation AHSS (often referred as conventional AHSS) grades have good strength but limited

*Relationship between ultimate tensile strength (UTS) vs total elongation (TE) for various generations of high* 

However, it is worth mentioning that for these Advanced High Strength Steels, the increase of strength parameters is much more significant than the decrease of the ductility parameters. This is particularly valid for the Dual Phase (DP) steels, Complex Phase steels (CP), Martensitic Complex Phase (MART/CP) steels, and TRIP steels. This is the reason why this group gains wide application in car body

The group of steels that can be found around the C = 40,000–60,000 MPa × %

may be regarded as the second generation of Advanced High Strength Steels. This group includes the Twinning Induced Plasticity (TWIP) steels and some austenitic Stainless Steels (AUST SS) with high manganese content. These steel grades provide superior combination of strength and ductility. TWIP steel had successfully been trial-produced at POSCO in the early 1990s, but the trial was not extended to commercialization due to limitations in facilities and productivity [4]. Trial productions have also been made at some European steel companies. These attempts demonstrated the outstanding mechanical performance of TWIP steels; however, these trial productions turned to be commercially unsuccessful due to low productivity and high cost [5]. New approaches and further developments are done continuously to reduce these difficulties and make them suitable for automotive

The third generation of AHSS is still in the phase of development—though there are already industrial realizations, too. In this development stage, several new concepts have been already proposed. The main target in developing the third generation of AHSS is twofold, i.e., to achieve mechanical properties in the range between the first- and second-generation AHSS shown in the strength-ductility diagram (**Figure 2**) but with less alloying elements and, hence, with less expensive

**104**

ductility.

**Figure 2.**

*strength steels [4].*

production.

parts manufacturing.

production than the second-generation AHSS steels [6]. The microstructure of these steels consists of a high strength phase (e.g., nano/ultrafine-grained ferrite, martensite, or bainite) combined with a further phase or constituent that provides substantial ductility and work hardening (e.g., austenite). With this development concepts, very high strength steels in the GPa range with even though remarkable formability can be produced [7].

The projected changes in the application of Advanced High Strength Steels is given by Matlock et al. [8] for North American vehicle industry (**Figure 3**), but similar trend may be estimated for other geographical regions, e.g., the European Automotive manufacturers and the Far East countries (China, Japan, and Korea). In **Figure 3**, the changes of the absolute content of AHSS applications (in kg) and percentage changes related to the total weight of vehicles are shown. Both changes show an exponential increase with a slightly higher one concerning the absolute values.

## *3.1.1 Development projects for Advanced High Strength Steels*

In the last 30–40 years, there were several projects studying and initiating the development of new grades of Advanced High Strength Steels. Most of these projects were initiated by automotive companies, and in most cases various consortiums were established for this purpose. Each of these projects aimed to meet as much as possible the requirements analyzed in Section 2.

Among these projects, the Ultralight Steel Automotive Body (ULSAB) satisfied most of the requirements stated for a lightweight automotive structure and proved to be structurally sound, safe, executable, and affordable. Though it was a highly expensive project with the participation of 35 companies representing 18 countries, it could meet the challenges to reduce the weight of steel auto body structures at no additional costs, while maintaining or even improving the overall performance [9].

Further projects followed the ULSAB concept, among them the Ultralight Steel Auto Closures (ULSAC) [10], or the Ultralight Steel Auto Body-Advanced Vehicle Concept (ULSAB-AVC) [11], and the Future Steel Vehicle (FSV) [12]. All these projects led to the further development of Advanced High Strength Steels reaching the GigaPascal range of strength together with increased ductility.

In the previous section, we could see the main material development tendencies and their classification that included various kinds of conventional steels as well,

**Figure 3.**

*Projected changes of the absolute amount (kg) and the percentage values for the total weight of vehicles.*

which had a prominent role in the history of car making in the last century. In the next sections, we will mainly focus on the main types of Advanced High Strength Steels and their properties.
