*4.1.1 Processing of DP steels*

*Engineering Steels and High Entropy-Alloys*

Steels and their properties.

body structures [13].

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

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

The group of AHSS materials includes Dual Phase (DP), Complex-Phase (CP),

Recently, there is an increased research interest for the development of the third

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

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,

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

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.

next sections, these AHSS will be discussed.

Advanced High Strength Steels with this group.

closures, fuel tanks, etc. in vehicles [16].

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

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

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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 the range of 10–40%.

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

#### **Figure 4.**

*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 islands in ferrite matrix.*

**Figure 5.** *Processing routes for producing DP steels.*

remaining austenite to martensite [20]. The properties obtained by this method include lower tensile strength and higher ductility than those of gained by the first method (Route A).

The third method for producing DP steels (Route C in **Figure 5**) involves hot rolling of steel, followed by first slow cooling to the intermediate temperature, followed by second cooling at a very fast rate and finally slow cooling (i.e., coil cooling) to room temperature. This method of cooling is known as ultrafast cooling (UFC), and the processing route is referred to as new-generation thermomechanical controlled processing [21]. The properties obtained by Route C are better (as compared to those obtained by Route A and Route B) because higher grain refinement is achieved during rolling.

Several authors have reported that DP steels with ultrafine bainite and fine ferrite-bainite/martensite microstructure with precipitation hardening can achieve good strength without loss of ductility, making this steel category more suitable for third-generation AHSS [5].

## **4.2 Transformation-induced plasticity (TRIP) steels**

Advanced high-strength transformation-induced plasticity (TRIP) steels are well suited for lightweighting car body construction with added advantage to reduce the safety problems. TRIP steels can be found already in the 1st+ generation AHSS as shown in **Figure 2**. One of the main features of TRIP steels that the strain or stress-induced transformation of retained austenite present in the microstructure in a sufficient amount can substantially harden the steel during deformation depending on the processing route and therefore results in a higher ductility [22].

The microstructure of TRIP steels contains retained austenite embedded in a primary matrix of ferrite. **Figure 6a** shows schematic microstructure of TRIP steel, while **Figure 6b** is a micrograph of a typical TRIP steel (TRIP 700).

In addition to a minimum of 5 vol.% of retained austenite, hard phases such as martensite and bainite are present in varying amounts. TRIP steels typically require an isothermal hold at an intermediate temperature, which produces some bainite.

TRIP steels are characterized by a relatively low content of alloying elements. For example, in TRIP 790 steel (UTS ≈ 790 MPa), the total content of alloying elements is about 3.5 wt.%. Thus, the selection of suitable alloying elements and the amount required to produce the intended properties is critical in the alloy design stage. The carbon content in TRIP steels is higher than in DP steels. Carbon

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**Figure 7.**

*Conventional processing route of TRIP steels.*

*Development of Lightweight Steels for Automotive Applications*

is generally kept in the range of 0.20–0.25% because of weldability reasons. The higher carbon content is necessary for stabilizing the retained austenite phase to below ambient temperature. In TRIP steels, austenite stabilizers are present, mainly C, Mn, and/or Ni. These elements assist maintaining the necessary carbon content within the retained austenite. TRIP steels mainly contain multiphase microstructures composed of about 50–55% ferrite, 30–35% bainite, 7–15% retained austenite,

The outstanding combination of ductility and strength in TRIP steels is a result of deformation based on transformation of retained austenite to martensite [23]. This transformation (on deformation) of phases is called the TRIP effect that provides excellent strength and elongation combination together with high impact resistance. These characteristics predestinate TRIP steels as good candidate for the third-generation AHSS, too. Dispersed hard second phases in soft ferrite provide high work hardening rate, as experienced in DP steels, too. Furthermore, in TRIP steels, the retained austenite progressively transforms to martensite with increasing

The main processing of TRIP steels consists of heating the steel to the austenitic zone, cooling down to the intercritical region followed by deformation here, and quick transfer to the bainitic zone with subsequent soaking there, and finally

The deformation in the intercritical region increases the rate of austenite (γ) to ferrite (α) transformation. The remaining austenite is enriched with carbon content, which stabilizes the γ phase. Furthermore, this deformation increases the nucleation rate of bainite but decreases its growth rate that results in small plates of bainite. This part of the T–t cycle also helps to enrich the γ phase with carbon and further increases the stability of γ phase. The stability of retained austenite is enhanced by the high carbon content, and the more carbon in γ phase results in more stability of γ during the TRIP effect, too, since more stable austenite needs more time to transform into martensite; these processes contribute to the increase of the ductility. The austenite to martensite transformation increases the tensile strength of the final microstructure. With this process, an improved strength–ductility combination is achieved [24]. Obviously, this processing route of TRIP steels is more time-consuming. This is because it needs special arrangements to deform the material at high temperature, to hold the specimen in the bainite region, and so on. This limits the use of TRIP steels in industrial applications. Some authors [25] using this route reported that rolling in the intercritical region improves TRIP steel

strain, thereby increasing the work hardening rate at higher strain levels.

quenching to room temperature (as shown in **Figure 7**).

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

and 1–5% martensite.

*4.2.1 Processing of TRIP steels*

#### **Figure 6.**

*Schematic view and micrograph of the microstructure of TRIP steel. (a) Schematic view of the microstructure of a TRIP steel and (b) micrograph of a typical TRIP steel (TRIP 700).*

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

is generally kept in the range of 0.20–0.25% because of weldability reasons. The higher carbon content is necessary for stabilizing the retained austenite phase to below ambient temperature. In TRIP steels, austenite stabilizers are present, mainly C, Mn, and/or Ni. These elements assist maintaining the necessary carbon content within the retained austenite. TRIP steels mainly contain multiphase microstructures composed of about 50–55% ferrite, 30–35% bainite, 7–15% retained austenite, and 1–5% martensite.

The outstanding combination of ductility and strength in TRIP steels is a result of deformation based on transformation of retained austenite to martensite [23]. This transformation (on deformation) of phases is called the TRIP effect that provides excellent strength and elongation combination together with high impact resistance. These characteristics predestinate TRIP steels as good candidate for the third-generation AHSS, too. Dispersed hard second phases in soft ferrite provide high work hardening rate, as experienced in DP steels, too. Furthermore, in TRIP steels, the retained austenite progressively transforms to martensite with increasing strain, thereby increasing the work hardening rate at higher strain levels.
