Advanced High Strength Sheet Steels

This chapter describes briefly the history of evolution of requirements of automotive steels motivated by growing requirements of car safety, reduction in fuel consumption, and therefore in car weight, and environment protection from CO₂ emission. Starting from 70s, the evolution of automotive steels included new and breakthrough development of steels grades with strength from initial ~350 to 2000 MPa. A few generations of developed steels are considered, starting from dual phase, TRIP steels, complex phase, and martensitic steels, which are combined by the term of AHSS (advanced high-strength steels) to TWIP steels and steels of so-called third generation.

This chapter considers the main features of heat treatment that allows for obtaining dual-phase steels including enrichment of austenite by carbon in the intercritical temperature range and transformations in cooling affected by preexisting austenite–ferrite interfaces. Individual steps of the processing, including formation of austenite phase, its growth, decomposition, and final tempering of ferrite–martensite mixture, are described. The role of heating rate and initial microstructure before cold rolling is highlighted. The phase transformation as a base of manufacturing of as-hot-rolled dual phase is presented.

The chapter contains detailed theoretical analysis and experimental data on relationship of structure parameters of ferrite–martensite steels (volume fraction and hardness of martensite, ferrite grain size, and structure morphology) and various mechanical properties as yield and tensile strength, strain hardening, elongation, and reduction of area. Experimentally shown redistribution of strain depending on martensite strength explains limitation in application of the law of mixture. Various fracture characteristics including resistance to crack initiation and propagation, as well as resistance to fatigue hydrogen embrittlement, are discussed.

This chapter describes various effects of steel composition on processes during heating in the intercritical temperature range, including kinetics austenitization, recrystallization of initial structure, and morphology of the formed austenite–ferrite mixture. Detailed consideration of effects of alloying elements on transformation of austenite in cooling includes influence of steel composition on ferrite solid solution, precipitation hardening, and martensite start temperature with some example of complicated overlapping with changes in carbon content in austenite. Effects of chemical composition on tensile properties, behavior at aging, and tempering are summarized.

The phenomenon of Transformation-Induced Plasticity (TRIP effect) and metallurgical concept of low-alloyed TRIP steels are presented. Optimal heat treatment including roles of initial structure, annealing temperature, and parameters of isothermal bainite reaction is discussed. The relationship between microstructure and mechanical properties of TRIP steels including the strength, ductility, strain hardening, and baking hardenability is considered focusing on the importance of austenite stability, minimizing fresh martensite, and microstructure refinement. Consideration of various effects of alloying elements separate the role of ferrite-stabilizing elements preventing carbide formation and hence facilitating the enrichment of austenite by carbon and elements affecting austenite hardenability and kinetics of bainite reaction. Effects of microalloying elements on structure refinement and the balance of strength and ductility are presented. Fracture features of TRIP steels and in particular, high energy absorption, fatigue behavior, and resistance to hydrogen embrittlement are described.

Complex phase (CP) concept is justified based on the improvement of stretch flangeability (hole expansion) due to replacement of martensite, at least partially, by bainite. Relationship between microstructure and mechanical properties of CP steels is presented as well as recommended for processing parameters to obtain bainite fraction, keeping in mind effects of chemical composition on phase transformations. Effects of alloying/microalloying on mechanical properties of complex phase steels are discussed.

Sheet martensitic steels for automotive application are presented including as-annealed martensitic grades, as-hot-rolled grades, and grades where martensitic structure is obtained after quenching in cooled dies (press-hardened martensitic steels). New developments of ultrahigh strength as-annealed and press-hardened steels with tensile strength up to 2000 MPa are included. Factors affecting susceptibility of martensitic steels to delayed fracture are discussed, and ways of significant improvement of resistance to hydrogen embrittlement are presented.

Steels with carbide-free bainite containing high carbon austenite at minimal fractions of ferrite and martensite are considered as a real candidate to meet requirements of steels of third generation. The chapter presents the main concept of their processing and the fundamentals of relationship of structure and mechanical properties. Effects of steel composition on kinetics of bainite transformation are discussed as well as factors determining the strength of this group of steels. Conditions ensuring the best combination of strength and formability including high hole expansion values are discussed.

Medium (4–10 %) Mn steel is considered as one of the candidates that can meet requirements of steels of third generation. This chapter presents the main factors affecting the combination of tensile properties of med MN steels: parameters of annealing, amount and stability of austenite, and Mn content. Additional alloying/microalloying effects are considered. Due to critical role of stability of retained austenite, the influence of various parameters is discussed including effect of annealing time, carbon and Mn content, as well as effect of grain size.

The fundamentals of quenching and partitioning process are discussed including various points of view and existing different approaches. Evolution of structure is considered including effect of processing parameters (quenching temperature, temperature, and duration of partitioning) and contribution of bainite reaction. The role of retained austenite stability and possible impacts of various factors are presented. The chapter includes the discussion of relationship between microstructure and the main properties of Q&P steels such as the combination of strength and ductility, strain hardening, and hole extension, as well as appropriate influence of steel composition. Modern modifications of Q&P thermal cycle are presented.

High-Mn austenitic steels with twinning-induced plasticity demonstrate the highest combination of strength and elongation although their commercialization delayed. This chapter contains fundamentals of TWIP phenomenon, features of deformation mechanism, and strain hardening, as well as the role of grain size, alloying, and microalloying. Impacts of temperature of testing and strain rate are presented. Propensity to delayed fracture and found ways to suppress it are discussed.