Nina Fonstein

Effects of Nb on Microstructural Evolution and Mechanical Properties of Low-Carbon Cold-Rolled Dual-Phase Steels

The effects of Nb additions, up to 0.06%, on the microstructure and properties of dual-phase (DP) steels containing 0.08% C and 2.2% Mn, were evaluated after hot rolling at different coiling temperatures, and after intercritical annealing of cold-rolled steels. During hot rolling, Nb precipitation was promoted by higher coiling temperature. During annealing of cold-rolled steels, microstructural characterization revealed that recovery and recrystallization processes overlapped with austenite formation. The addition of Nb increased the ferrite recrystallization start temperature, but had no significant effect on the start temperature of austenite formation during heating. Nb additions accelerated austenite formation once the transformation started, and also resulted in the formation of a finer and more homogeneous microstructure after annealing. In comparison with the steel without Nb, the ultimate tensile strength increased with the addition of up to 0.04% Nb, whereas addition of 0.06% Nb did not further increase the strength level of cold-rolled DP steels. Hot rolling at lower coiling temperatures improved both the strength and the ductility of the Nb-containing DP steels after annealing.

Factors that Determine the Level of the Yield Strength and the Return of the Yield-Point Elongation in Low-Alloy Ferrite-Martensite Steels

The results of laboratory investigations of dual-phase steels with different contents of carbon and alloying elements after the controlled cooling from the two-phase field and the final low-temperature tempering are presented. It is shown that the ratio of the yield strength to the tensile strength of dual-phase steels, just as the return of the yield-point elongation, depends on the volume fraction of martensite, temperature of the martensite transformation of the austenite component, quenching stresses, concentration of carbon in ferrite, and the temperature of the final tempering.

Candidates for Third-Generation Steels: Q&P Processed Steels

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.

The Effect of Chemical Composition on Formation of Ferrite–Martensite Structures and Mechanical Properties of Dual-Phase Steels

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.

Candidates for the Third Generation: Medium Mn Steels

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.

Effect of Cr Additions on Ferrite Recrystallization and Austenite Formation in Dual-Phase Steels Heat Treated in the Intercritical Temperature Range

An experimental investigation was conducted using laboratory-processed low carbon Mn-Mo-Nb-Ti-B dual phase steel with additions of 0.2Cr or 0.6%Cr. This study investigates the effect of Cr additions on ferrite recrystallization and austenite formation during continuous annealing in the intercritical temperature range. It was found that moderate additions of Cr (0.2%) accelerates the ferrite recrystallization (nucleation and growth) resulting in coarse ferrite grains. An increase in Cr content up to 0.6% also accelerates the ferrite recrystallization which in addition to the presence of Cr-rich carbides facilitates the austenitization predominantly through the nucleation process resulted in finer austenite grains. Thereby, the higher Cr content increases the volume of homogenized and refined martensite in the final quenched structure.

Effect of Structure on Mechanical Properties of Dual-Phase Steels

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.

Candidates to AHSS of Third Generation: Steels with Carbide-Free Bainite

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.

Martensitic Sheet Steels

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.

Austenitic Steels with TWIP Effect

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.