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

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

However, it is also obvious that potential production requires a systematic design methodology to identify the possible combinations of microstructural constituents, which may lead to the required mechanical properties.

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 properties can be calculated.

Another possibility to use a systematic design methodology is the application of the Integrated Computational Materials Engineering (ICME). It provides a framework for utilizing computational multiscale material development driven by multidisciplinary engineering design, analysis, and performance requirements [37]. This concept is initiated and supported by the National Research Council in the USA [38]. The ICME model can be used to guide both the material selection and the design optimization. Alternatively, it can be also used to new material development to get the best-suited macroscopic properties for a given structural application, through the determination of chemical composition and microstructural characteristics in a "reverse engineering" approach. In the automotive industry, the potential of the ICME method for vehicle lightweighting was recognized by the United States Department of Energy (DOE), too, which funded the project "Integrated Computational Materials Engineering Approach to Development of Lightweight Third Generation Advanced High Strength Steel (3GAHSS) Vehicle Assembly [39]. The abovementioned ICME approach was implemented in this project in two ways. First, the ICME principles were applied in the development of a material modeling tool set by combining material models at different length scales. Second, the Combined Constraints Crystal Plasticity (CCCP) model was implemented as a microscale constitutive model [40]. In this project, two targets were set by the United States Department of Energy: one is the 1200 MPa strength with 30% total elongation (which means C = 1200 × 30 = 36,000 MPa × %), and the other one is 1500 MPa strength with 25% total elongation (which means C = 1500 × 25 = 37,500 MPa × %) [41].

Similar projects were initiated by other steel companies and research institutes in the world. Among others, ArcelorMittal announced systematic developments of third-generation steels [42]. 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). In the next sections, some results and representatives of these 3G developments will be introduced.

## **5.1 Medium manganese steels**

The development of austenitic steel grades with high alloying contents of manganese (15–30%) was already applied during the development of second-generation AHSS. It resulted in outstanding mechanical properties (high strength with excellent elongation), which made it attractive for the automotive industry. These high strength and ductility grades were based on the austenitic single-phase concept. Their deformation mechanisms were mainly the twinning-induced plasticity (TWIP). Additionally, it was also discovered that combining specific proportions of TWIP and TRIP mechanisms allows precise control of strength and ductility [43].

In recent steel developments, it was experienced that a further deformation mechanism is provoked when different alloying concepts are used. Microbandinduced plasticity (MBIP) is one of these newly discovered mechanisms, which localizes the deformation within arrays of precipitates and, thus, retarding the onset of mechanical instability and supporting homogenous yielding. Beside the outstanding mechanical properties, the steels offer processing challenges compared to low carbon steels; however, they are very expensive due to the high alloy additions required to produce austenitic microstructure.

However, these high manganese content steels initiated the development of another new group of steels belonging to the third-generation AHSS grades, namely, the medium manganese steels. The microstructure of these steels consists of a high strength phase (e.g. nano/ultrafine-grained ferrite, martensite, or bainite)

**115**

*Development of Lightweight Steels for Automotive Applications*

combined with a further phase or constituent that provides substantial ductility and work hardening (e.g. austenite). The carbide-free bainite (CFB) or ultrafine lamellar bainite (ULB) is another possible concept. By choosing the alloying concept and the cooling condition, it is possible to suppress the carbide formation and, thus, to produce a very fine lamellar bainitic structure with austenite films between the bainite leaves. This concept provides very high strength steels right above 1 GPa

Quenched and partitioned (Q&P) steels are the result of the recent developments of third-generation AHSS steels. The elaboration of Q&P steels is partly based on the knowledge of duplex stainless steels and the quenching and partitioning process [44], as well as on the properties of medium manganese steels [45]. The Q&P steels usually contain carbon, manganese, silicon, nickel, and molybdenum alloying elements. The amount of alloying elements can be around 4%, which is much lower than that of in the second-generation AHSS. During heat treatment of Q&P steel, quenching is interrupted and is reheated for partitioning. With this reheating process, a unique microstructure is created containing 5–12% stable

Baosteel was one of the first companies to apply Q&P steels, initially with 980 MPa and later 1180 MPa strength [46]. It was demonstrated that a B-pillar reinforcement could be cold-formed using Q&P 1180. Auto/Steel Partnership (A/SP) also has tested Q&P 980 using GM's B-pillar die, proving that this steel

Recently, Q&P steels were developed up to 2100 MPa tensile strength with 9% uniform elongation and about 13% total elongation. The elongation level of this

Q&P steels are a series of C-Si-Mn, C-Si-Mn-Al, or other similar compositions that are processed by the quenching and partitioning (Q&P) heat treatment. Q&P steels possess an excellent combination of strength and ductility with a final microstructure of ferrite (in the case of partial austenitization), martensite and retained austenite. This microstructure makes them suitable to use in the automotive industry as new-generation AHSS. They are suitable for cold stamping of various structures and safety parts having complicated shape to improve fuel economy and

It is possible to change the amount of retained austenite at room temperature and its stability with alloying elements as carbon, manganese, nickel, etc. based on the knowledge gained by duplex stainless steels. However, it affects the cost and may be detrimental concerning the welding properties. The third generation of AHSS grades were developed to overcome these disadvantages; few of the good examples are those third-generation AHSS that are based partly on the quenching and partitioning process (Q&P steels) and on the properties of medium manganese steels. In this case, the composition of steel is not adequate for keeping the retained austenite at room temperature, but annealing, cooling, and thermal processes are optimized to change the austenite's composition and decrease its Ms temperature. For medium-Mn steels, where a relatively larger manganese amount (typically between 5 and 8 wt. %) is characteristic, the thermal treatment is slightly simplified. The intercritical annealing provides a chance to form austenite and to increase its carbon and manganese content; then the steel is cooled down to

has better formability and is less prone to edge cracking than DP 980.

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

and with remarkable formability.

*5.2.1 Metallurgy of Q&P steels*

promoting passenger safety.

**5.2 Quenched and partitioned (Q&P) steels**

retained austenite, 20–40% ferrite, and 50–80% martensite.

steel is comparable to DP 980, which is a cold-formable grade.

*Engineering Steels and High Entropy-Alloys*

C = 1500 × 25 = 37,500 MPa × %) [41].

**5.1 Medium manganese steels**

tives of these 3G developments will be introduced.

tions required to produce austenitic microstructure.

Another possibility to use a systematic design methodology is the application of the Integrated Computational Materials Engineering (ICME). It provides a framework for utilizing computational multiscale material development driven by multidisciplinary engineering design, analysis, and performance requirements [37]. This concept is initiated and supported by the National Research Council in the USA [38]. The ICME model can be used to guide both the material selection and the design optimization. Alternatively, it can be also used to new material development to get the best-suited macroscopic properties for a given structural application, through the determination of chemical composition and microstructural characteristics in a "reverse engineering" approach. In the automotive industry, the potential of the ICME method for vehicle lightweighting was recognized by the United States Department of Energy (DOE), too, which funded the project "Integrated Computational Materials Engineering Approach to Development of Lightweight Third Generation Advanced High Strength Steel (3GAHSS) Vehicle Assembly [39]. The abovementioned ICME approach was implemented in this project in two ways. First, the ICME principles were applied in the development of a material modeling tool set by combining material models at different length scales. Second, the Combined Constraints Crystal Plasticity (CCCP) model was implemented as a microscale constitutive model [40]. In this project, two targets were set by the United States Department of Energy: one is the 1200 MPa strength with 30% total elongation (which means C = 1200 × 30 = 36,000 MPa × %), and the other one is 1500 MPa strength with 25% total elongation (which means

Similar projects were initiated by other steel companies and research institutes in the world. Among others, ArcelorMittal announced systematic developments of third-generation steels [42]. 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). In the next sections, some results and representa-

The development of austenitic steel grades with high alloying contents of manganese (15–30%) was already applied during the development of second-generation AHSS. It resulted in outstanding mechanical properties (high strength with excellent elongation), which made it attractive for the automotive industry. These high strength and ductility grades were based on the austenitic single-phase concept. Their deformation mechanisms were mainly the twinning-induced plasticity (TWIP). Additionally, it was also discovered that combining specific proportions of TWIP and TRIP mechanisms allows precise control of strength and ductility [43]. In recent steel developments, it was experienced that a further deformation mechanism is provoked when different alloying concepts are used. Microbandinduced plasticity (MBIP) is one of these newly discovered mechanisms, which localizes the deformation within arrays of precipitates and, thus, retarding the onset of mechanical instability and supporting homogenous yielding. Beside the outstanding mechanical properties, the steels offer processing challenges compared to low carbon steels; however, they are very expensive due to the high alloy addi-

However, these high manganese content steels initiated the development of another new group of steels belonging to the third-generation AHSS grades, namely, the medium manganese steels. The microstructure of these steels consists of a high strength phase (e.g. nano/ultrafine-grained ferrite, martensite, or bainite)

**114**

combined with a further phase or constituent that provides substantial ductility and work hardening (e.g. austenite). The carbide-free bainite (CFB) or ultrafine lamellar bainite (ULB) is another possible concept. By choosing the alloying concept and the cooling condition, it is possible to suppress the carbide formation and, thus, to produce a very fine lamellar bainitic structure with austenite films between the bainite leaves. This concept provides very high strength steels right above 1 GPa and with remarkable formability.
