**4.3 Complex Phase (CP) steels**

Complex Phase (CP) steels belong to the group of steels with usually very high ultimate tensile strength (UTS ≈ 800 MPa or even greater). CP steels generally have chemical composition and microstructure similar to TRIP steels, but it contains some quantities of other elements, e.g., Nb, Ti, and V. These additional elements enhance the precipitation strengthening effect. CP steels typically do not have retained austenite, but contain more hard phases like martensite and bainite within the ferrite/bainite matrix.

The mechanical properties of CP steels may be characterized by continuous yielding and high uniform elongation. CP steels with the bainitic matrix have excellent formability. It is primarily due to the relatively small difference between the hardness of bainite and martensite. In CP steels, the bainitic ferrite is strengthened by high density of dislocations (dislocation density is above ρ > 1012/cm2 ) together with fine dispersion of martensitic second phase and carbo-nitrides or carbides. This bainite microstructure of CP steels exhibits better strain hardening and strain capacity than that for fully bainitic microstructure. In its microstructure, the martensite and bainitic ferrite phases are separated by a third phase of intermediate strength.

### **4.4 Martensitic (MS) steel**

Martensitic steels (MS) have mostly martensitic microstructure with some small amounts of ferrite and bainite. These steels have the highest strength but lowest formability. Martensitic steels, currently available with strengths of 900–1800 MPa, are used for body parts where deformation may be limited [26].

Producing MS steels, the austenite is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. MS steels may be characterized by martensitic matrix containing small amounts of ferrite and/or bainite. Within the group of multiphase steels, MS steels have the highest tensile strength level. Martensitic steels show the highest ultimate strength in final products, up to 1800 MPa or even higher [27]. Their concept is based upon well-established rules with respect to chemical composition and processing technology. In order to improve ductility and provide adequate formability even at extremely high strength values, MS steels are often subjected to post-quench tempering.

Additional carbon in MS steels increases the hardenability and contributes to further strengthening the martensite. Further elements (like manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel) are used in various combinations to further increase hardenability. Microstructure of martensitic steels is mainly composed of lath martensite, which is developed by the transformation of austenite during quenching after hot rolling or annealing. Martensitic steels are very hard to form, so they typically are roll formed or press hardened (hot stamped): it will be detailed in the next section where the Press Hardening Steels (PHS) will be described.

### **4.5 Press Hardening Steels (PHS)**

Among the Advanced High Strength Steels, Press Hardening Steels (PHS) form a unique group: these are mostly different kinds of boron-alloyed manganese steels

**111**

**Figure 8.**

*Temperature vs process time for hot press forming of PHS.*

*Development of Lightweight Steels for Automotive Applications*

hot forming of Press Hardening Steels will be analyzed.

and gain wide application to produce high strength structural body elements (e.g., A- and B-pillars, etc.). Press Hardening Steels are widely used in car body manufacturing in hot forming processes. There are several grades of Press Hardening Steels; among them, the 22MnB5 alloy is regarded as the basic type of PHS steels. Here, the

Hot forming of steels is a complex forming and tempering operation: it is often termed as hot press forming or press hardening of steels, too. The full austenitization of the material is regarded as the first step in hot press forming. Forming is performed in this state when the material has good formability; then the part is cooled down rapidly in the tool applying the critical cooling rate, hence resulting in

The usual temperature–time diagram for hot press forming is shown in **Figure 8**. Through the above-described combination of heating, holding, forming, and rapid cooling, very complex parts can be produced with excellent strength properties [28]. There are various process variants in hot press forming: among them, the socalled direct and indirect hot forming may be regarded as the basic ones. In direct or single-stage hot forming, the blank sheet is directly austenitized, then transferred to the stamping tool, and cooled down rapidly in the forming tool providing excellent strength properties [29]. In indirect or often termed as two-stage hot press forming, the initial blank is formed in cold state, and then either a hot forming is used to produce the complex parts or just a calibration process occurs in hot forming condition. The austenitization and the subsequent quenching are the inherent parts

of this process chain, too, to provide the required high strength properties. There are further process variants besides these two basic ones: the final microstructure, as well as the mechanical properties of the part, can be effectively controlled by the holding temperature and the controlled cooling process. These process variants may be derived either by altering the holding temperature or by changing the cooling rate. Depending on the holding temperature, two further process variants can be proposed: full austenitization is the basic alternative, i.e., when the holding temperature is selected in the homogeneous γ-zone. A further process variant depending on the holding temperature is derived if the holding

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

*4.5.1 The hot forming process of PHS steels*

martensitic microstructure.

and gain wide application to produce high strength structural body elements (e.g., A- and B-pillars, etc.). Press Hardening Steels are widely used in car body manufacturing in hot forming processes. There are several grades of Press Hardening Steels; among them, the 22MnB5 alloy is regarded as the basic type of PHS steels. Here, the hot forming of Press Hardening Steels will be analyzed.

#### *4.5.1 The hot forming process of PHS steels*

*Engineering Steels and High Entropy-Alloys*

**4.3 Complex Phase (CP) steels**

the ferrite/bainite matrix.

**4.4 Martensitic (MS) steel**

post-quench tempering.

**4.5 Press Hardening Steels (PHS)**

strength.

grain size, and resulting in a granular type morphology.

properties by enhancing the carbon content and dislocation density, decreasing the

Complex Phase (CP) steels belong to the group of steels with usually very high ultimate tensile strength (UTS ≈ 800 MPa or even greater). CP steels generally have chemical composition and microstructure similar to TRIP steels, but it contains some quantities of other elements, e.g., Nb, Ti, and V. These additional elements enhance the precipitation strengthening effect. CP steels typically do not have retained austenite, but contain more hard phases like martensite and bainite within

The mechanical properties of CP steels may be characterized by continuous yielding and high uniform elongation. CP steels with the bainitic matrix have excellent formability. It is primarily due to the relatively small difference between the hardness of bainite and martensite. In CP steels, the bainitic ferrite is strengthened

with fine dispersion of martensitic second phase and carbo-nitrides or carbides. This bainite microstructure of CP steels exhibits better strain hardening and strain capacity than that for fully bainitic microstructure. In its microstructure, the martensite and bainitic ferrite phases are separated by a third phase of intermediate

Martensitic steels (MS) have mostly martensitic microstructure with some small

Producing MS steels, the austenite is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. MS steels may be characterized by martensitic matrix containing small amounts of ferrite and/or bainite. Within the group of multiphase steels, MS steels have the highest tensile strength level. Martensitic steels show the highest ultimate strength in final products, up to 1800 MPa or even higher [27]. Their concept is based upon well-established rules with respect to chemical composition and processing technology. In order to improve ductility and provide adequate formability even at extremely high strength values, MS steels are often subjected to

Additional carbon in MS steels increases the hardenability and contributes to further strengthening the martensite. Further elements (like manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel) are used in various

combinations to further increase hardenability. Microstructure of martensitic steels is mainly composed of lath martensite, which is developed by the transformation of austenite during quenching after hot rolling or annealing. Martensitic steels are very hard to form, so they typically are roll formed or press hardened (hot stamped): it will be detailed in the next section where the Press Hardening Steels (PHS) will be

Among the Advanced High Strength Steels, Press Hardening Steels (PHS) form a unique group: these are mostly different kinds of boron-alloyed manganese steels

amounts of ferrite and bainite. These steels have the highest strength but lowest formability. Martensitic steels, currently available with strengths of 900–1800 MPa,

are used for body parts where deformation may be limited [26].

) together

by high density of dislocations (dislocation density is above ρ > 1012/cm2

**110**

described.

Hot forming of steels is a complex forming and tempering operation: it is often termed as hot press forming or press hardening of steels, too. The full austenitization of the material is regarded as the first step in hot press forming. Forming is performed in this state when the material has good formability; then the part is cooled down rapidly in the tool applying the critical cooling rate, hence resulting in martensitic microstructure.

The usual temperature–time diagram for hot press forming is shown in **Figure 8**. Through the above-described combination of heating, holding, forming, and rapid cooling, very complex parts can be produced with excellent strength properties [28]. There are various process variants in hot press forming: among them, the socalled direct and indirect hot forming may be regarded as the basic ones. In direct or single-stage hot forming, the blank sheet is directly austenitized, then transferred to the stamping tool, and cooled down rapidly in the forming tool providing excellent strength properties [29]. In indirect or often termed as two-stage hot press forming, the initial blank is formed in cold state, and then either a hot forming is used to produce the complex parts or just a calibration process occurs in hot forming condition. The austenitization and the subsequent quenching are the inherent parts of this process chain, too, to provide the required high strength properties.

There are further process variants besides these two basic ones: the final microstructure, as well as the mechanical properties of the part, can be effectively controlled by the holding temperature and the controlled cooling process. These process variants may be derived either by altering the holding temperature or by changing the cooling rate. Depending on the holding temperature, two further process variants can be proposed: full austenitization is the basic alternative, i.e., when the holding temperature is selected in the homogeneous γ-zone. A further process variant depending on the holding temperature is derived if the holding

**Figure 8.** *Temperature vs process time for hot press forming of PHS.*

temperature is in the (α + γ) intercritical zone (i.e., between the A1 and the A3 temperature). In this case, there is no full austenitization; the starting microstructure contains, besides austenite, ferrite, too. Obviously, just the austenite content can be transformed into martensite, and the final microstructure after the hot forming and cooling process is completed has a certain amount of ferrite, too. Obviously, it results in lower strength than the full austenitization; however, it also leads to a certain amount of ductility leading to better toughness properties, as well.

Further process variants can be also derived by changing the cooling rate after the forming process. If the cooling rate is higher than the upper critical one, the final microstructure is martensite; when the cooling rate is lower than the upper critical one, besides martensite, bainite can be also found in the microstructure. However, it also results in somewhat lower strength depending on the quantity of bainite; however, it also results in the increase of toughness that may be advantageous, for example, increasing the crashworthiness of the part due to the better energy absorption properties of bainite [30].

It is essential that the forming could be finished above the Ms temperature: at this stage, these material grades still have suitable formability. After forming, the component is cooled down together with the tool: this cooling should provide the critical cooling rate to get high strength of martensitic microstructure. By this process, springback is eliminated, and very strong components can be formed to complex geometries.

Typical press hardened steels (PHS) have tensile strength of 1500–2000 MPa. In the last decades, they are already extensively used in safety and crash-resistant car body components. New-generation PHS are expected to have higher strength even above 2000 MPa. However, it should be noted that these PHS grades are used where only very small deformation is allowed. These steels have been adopted for use in many parts, including, for example, sill structures, or A- and B-pillar reinforcements. Recently, many floor panels also are made by hot forming to save weight.

#### **4.6 Twinning-induced plasticity (TWIP) steels**

TWIP steels belong to the second generation of AHSS and are based on the potential mechanism of obtaining a superior balance of tensile strength and elongation using the TWIP effect. The name of this steel is originated from this characteristic deformation mode, i.e., the twinning. The twinning causes high value of the instantaneous hardening rate (n-value) as the microstructure becomes finer and finer. The resultant twin boundaries serve as grain boundaries and strengthen the steel (**Figure 9**).

TWIP steels have high manganese content (Mn = 17–24%) that causes the steel to be fully austenitic even at room temperatures. TWIP steels are normally composed of Fe, Mn, or Ni (15–35%), Si (1–3%), and Al (1–3%) [31]. These steels exhibit outstanding tensile strength-ductility combination (e.g., a TWIP steel with tensile strengths above 1000 MPa may possess 50–60% ductility) [32]. The n-value may increase to a value of 0.4 that may result in 50–60% uniform elongation. The tensile strength may be even higher than 1500 MPa [33].

In TWIP steels, the strain hardening is strongly dependent on the stacking fault energy (SFE). This parameter controls the deformation behavior of the steel. Alloying elements generally decrease SFE leading to enhanced twinning behavior during deformation and hence lead to improved ductility. It is also known that SFE < 20 mJ/m2 causes austenite to martensite conversion and by this results in the TRIP effect. For pure twinning, SFE is desired to be greater than 20 mJ/m<sup>2</sup> . Aluminum is added to steel to raise SFE, to retard the TRIP effect and to result in pure twinning.

**113**

properties can be calculated.

*Development of Lightweight Steels for Automotive Applications*

TWIP steels show superior mechanical performance, but this category is not practically viable for industrial applications because of its limitations: poor productivity and high production costs. The main production route of TWIP steels includes homogenizing above the upper critical temperature for a long period and quenching to room temperature [34]. TWIP steels can be also produced by homogenizing, followed by deformation at a temperature above the upper critical one, with subsequent quenching to room temperature. Deformation at higher temperature provides fine grain size and high volume fraction of twins. The finer the grain structure, the

*Schematic view and micrograph of TWIP steel microstructure. Left: Schematic view of TWIP steel* 

Two types of twins are observed in the TWIP steels: (a) annealing twins caused by heat treatment and (b) deformation twins caused by deformation. The yield stresses of coarse-grained TWIP steels usually result in less than 400 MPa strength, which restricts the use of TWIP steels in the automotive sector, particularly for those parts that are supposed to be active during a crash. Many attempts are reported in the literature to increase the yield strength of TWIP steels. These attempts include, for example, grain size refinement by using V, Ti, and Nb as alloying elements to enhance precipitation of carbides, cold rolling followed by anneal-

As it was already discussed at the Classification of AHSS Developments (Section

However, it is also obvious that potential production requires a systematic design methodology to identify the possible combinations of microstructural constituents,

One of the possibilities to apply a systematic design methodology is the application of a simplified composite model [36] considering various combinations of multiphase (ferrite, austenite, bainite, and martensite) materials. With the variations of phase fractions in the hypothetical microstructure, the predicted mechanical

3.1), the main target in developing the third-generation AHSS is to achieve the properties in the range between the first- and second-generation AHSS with less alloying elements, hence, with less expensive processing that are suitable for early commercialization. The range of third-generation AHSS (3GAHSS) development maybe clearly identified on the diagram of tensile strength vs total elongation in between the first- and second-generation AHSS regions as shown in **Figure 2**.

more twinning occurs that improves ductility and strength.

*microstructure. Right: Micrograph of a TWIP steel in annealed condition.*

**5. Recent results and future trends in AHSS development**

ing treatment, and partial recrystallization [35].

which may lead to the required mechanical properties.

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

**Figure 9.**

**Figure 9.**

*Engineering Steels and High Entropy-Alloys*

energy absorption properties of bainite [30].

**4.6 Twinning-induced plasticity (TWIP) steels**

tensile strength may be even higher than 1500 MPa [33].

complex geometries.

ing to save weight.

temperature is in the (α + γ) intercritical zone (i.e., between the A1 and the A3 temperature). In this case, there is no full austenitization; the starting microstructure contains, besides austenite, ferrite, too. Obviously, just the austenite content can be transformed into martensite, and the final microstructure after the hot forming and cooling process is completed has a certain amount of ferrite, too. Obviously, it results in lower strength than the full austenitization; however, it also leads to a

Further process variants can be also derived by changing the cooling rate after the forming process. If the cooling rate is higher than the upper critical one, the final microstructure is martensite; when the cooling rate is lower than the upper critical one, besides martensite, bainite can be also found in the microstructure. However, it also results in somewhat lower strength depending on the quantity of bainite; however, it also results in the increase of toughness that may be advantageous, for example, increasing the crashworthiness of the part due to the better

It is essential that the forming could be finished above the Ms temperature: at this stage, these material grades still have suitable formability. After forming, the component is cooled down together with the tool: this cooling should provide the critical cooling rate to get high strength of martensitic microstructure. By this process, springback is eliminated, and very strong components can be formed to

Typical press hardened steels (PHS) have tensile strength of 1500–2000 MPa. In the last decades, they are already extensively used in safety and crash-resistant car body components. New-generation PHS are expected to have higher strength even above 2000 MPa. However, it should be noted that these PHS grades are used where only very small deformation is allowed. These steels have been adopted for use in many parts, including, for example, sill structures, or A- and B-pillar reinforcements. Recently, many floor panels also are made by hot form-

TWIP steels belong to the second generation of AHSS and are based on the potential mechanism of obtaining a superior balance of tensile strength and elongation using the TWIP effect. The name of this steel is originated from this characteristic deformation mode, i.e., the twinning. The twinning causes high value of the instantaneous hardening rate (n-value) as the microstructure becomes finer and finer. The resultant twin boundaries serve as grain boundaries and strengthen the steel (**Figure 9**). TWIP steels have high manganese content (Mn = 17–24%) that causes the steel to be fully austenitic even at room temperatures. TWIP steels are normally composed of Fe, Mn, or Ni (15–35%), Si (1–3%), and Al (1–3%) [31]. These steels exhibit outstanding tensile strength-ductility combination (e.g., a TWIP steel with tensile strengths above 1000 MPa may possess 50–60% ductility) [32]. The n-value may increase to a value of 0.4 that may result in 50–60% uniform elongation. The

In TWIP steels, the strain hardening is strongly dependent on the stacking fault energy (SFE). This parameter controls the deformation behavior of the steel. Alloying elements generally decrease SFE leading to enhanced twinning behavior during deformation and hence lead to improved ductility. It is also known that

the TRIP effect. For pure twinning, SFE is desired to be greater than 20 mJ/m<sup>2</sup>

Aluminum is added to steel to raise SFE, to retard the TRIP effect and to result in

causes austenite to martensite conversion and by this results in

.

certain amount of ductility leading to better toughness properties, as well.

**112**

SFE < 20 mJ/m2

pure twinning.

*Schematic view and micrograph of TWIP steel microstructure. Left: Schematic view of TWIP steel microstructure. Right: Micrograph of a TWIP steel in annealed condition.*

TWIP steels show superior mechanical performance, but this category is not practically viable for industrial applications because of its limitations: poor productivity and high production costs. The main production route of TWIP steels includes homogenizing above the upper critical temperature for a long period and quenching to room temperature [34]. TWIP steels can be also produced by homogenizing, followed by deformation at a temperature above the upper critical one, with subsequent quenching to room temperature. Deformation at higher temperature provides fine grain size and high volume fraction of twins. The finer the grain structure, the more twinning occurs that improves ductility and strength.

Two types of twins are observed in the TWIP steels: (a) annealing twins caused by heat treatment and (b) deformation twins caused by deformation. The yield stresses of coarse-grained TWIP steels usually result in less than 400 MPa strength, which restricts the use of TWIP steels in the automotive sector, particularly for those parts that are supposed to be active during a crash. Many attempts are reported in the literature to increase the yield strength of TWIP steels. These attempts include, for example, grain size refinement by using V, Ti, and Nb as alloying elements to enhance precipitation of carbides, cold rolling followed by annealing treatment, and partial recrystallization [35].
