**4. Main types and properties of Advanced High Strength Steels**

AHSS are complex, sophisticated materials, with carefully selected chemical compositions and multiphase microstructures, achieved by precisely controlled heating and cooling processes. Various strengthening mechanisms are applied to get significantly increased strength, better formability, improved toughness, and fatigue properties to meet the various requirements that are defined for automotive body structures [13].

The group of AHSS materials includes Dual Phase (DP), Complex-Phase (CP), Ferritic-Bainitic (FB), Martensitic (MS), Transformation-Induced Plasticity (TRIP), Hot-Formed Press Hardened (HF, or PHS), Twinning-Induced Plasticity (TWIP) and some austenitic stainless steels (AUST SS) with high manganese content. These first- and second-generation AHSS grades due to their unique mechanical properties are well qualified to meet many of the functional and performance requirements in automobiles. From these generations of AHSS, DP and TRIP steels are excellent in the crash zones due to their high-energy absorption [14], while structural elements of the passenger compartment can be made from extremely high strength steels, such as martensitic and boron-alloyed Press Hardened Steels (PHS), and hence, resulting in improved safety performance.

Recently, there is an increased research interest for the development of the third generation of AHSS. These steels usually apply special alloying and thermomechanical processing to provide improved strength-ductility combinations compared to the present first- and second-generation AHSS grades but at lower costs. There are several good examples for these, e.g., in the USA, a program sponsored by the Department of Energy made available the development of 1200 MPa steels with threefold improvements in ductility [15]. New generation of Advanced High Strength Steel (AHSS) grades contains significant alloying and multiple phases. The multiple phases provide increased strength and ductility not attainable with single-phase steels, such as the high strength, low alloyed (HSLA) grades. In the next sections, these AHSS will be discussed.

### **4.1 Dual-Phase (DP) steels**

As we could see from the historical analysis, Dual-Phase steels have a dominant role in the last 40 years of automotive industry; therefore, we start the overview of Advanced High Strength Steels with this group.

The development of Dual-Phase (DP) steels was right at the beginning of the new age of steel development following the conventional high strength steel era. Current commercially available and widely applied AHSS steels have evolved from significant early work on Dual-Phase steels in the late 1970s and early 1980s. Dual-Phase steels are one of the more widely applied Advanced High Strength Steels in todays' car making industry. This is mainly due to their better strength and formability parameter combination than the conventional high strength steels like HSLA steels. DP steels possess high specific strength, good initial work hardening rate, continuous yielding behavior, and superior ductility compared to conventional steel grades. These properties make them particularly suitable for body structures, closures, fuel tanks, etc. in vehicles [16].

**107**

**Figure 5.**

*Processing routes for producing DP steels.*

**Figure 4.**

*islands in ferrite matrix.*

*Development of Lightweight Steels for Automotive Applications*

Dual-Phase (DP) steels generally consist of ferrite matrix containing mainly hard martensite or in some cases bainite second phase as islands as shown in **Figure 4**. It is very characteristic that the ferrite phase is generally continuous providing excellent ductility. During forming, strain is concentrated in the lower strength ferrite phase surrounding the martensite islands providing unique work

There are various commonly used processing routes for producing DP steels. One of the methods (Route A in **Figure 5**) involves rapid cooling from the intercritical temperature to room temperature directly. The resulting microstructure comprises ferrite and martensite [17]. Higher intercritical temperatures, for the same soaking period, result in larger amounts of martensite with increased tensile strength and decreased percentage elongation [18]. It is reported by several papers [19] that the increase in martensite fraction in DP steels promotes crack initiation and thus results in worse ductility. Therefore, martensite fraction should be kept in

Another method for processing of DP steels (Route B in **Figure 5**) applies first slow cooling from the austenitic region to the desired ferrite transformation temperature, followed by quenching to room temperature for transforming the

*Schematic view and real micrograph of a DP steel. Left: Schematic view of a microstructure of a DP steel containing martensite islands in ferrite matrix. Right: Micrograph of a DP 690 steel containing martensite* 

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

hardening rate that is experienced in DP steels.

*4.1.1 Processing of DP steels*

the range of 10–40%.

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

Dual-Phase (DP) steels generally consist of ferrite matrix containing mainly hard martensite or in some cases bainite second phase as islands as shown in **Figure 4**. It is very characteristic that the ferrite phase is generally continuous providing excellent ductility. During forming, strain is concentrated in the lower strength ferrite phase surrounding the martensite islands providing unique work hardening rate that is experienced in DP steels.
