Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

Enhanced mechanical properties by retained austenite in medium–carbon Si-rich microalloyed steel treated by quenching–tempering, austempering and austempering–tempering processes

Abstract: The microstructure evolution and mechanical properties of a designed 0.56C-1.48Si-0.70Mn-0.71Cr-0.148V-0.011Nb (wt.%) steel subjected to three heat treatments including quenching–tempering, austempering, and austempering–tempering have been investigated. In the quenching–tempering sample, the microstructure of the steel consisted of tempered martensite and a small amount of retained austenite (~8 vol%) with 1.3 wt% C. In the austempering sample, the microstructure contained martensite/bainite laths and adequate amount of retained austenite (~15 vol%) with 1.33 wt% C. Subsequent tempering treatment promoted MC carbides to form and retained austenite to decompose, respectively. Resulting in decrease in the retained austenite amount (~10 vol%) and C concentration (~1.24 wt%). The carbon distribution indicated the stability of the C-rich area along prior austenite grain boundary is higher than that between the martensite/bainite laths. As a result, the present steel shown an ultrahigh ultimate tensile strength (>2200 MPa) and an excellent total elongation (~15%) after austempering treatment. Also, the impact toughness increases from ~12.5 J (quenching–tempering) to ~16 J (austempering) and ~17.5 J (austempering–tempering). As a result of transformation-induced plasticity (TRIP) effect, the austempering sample had the highest values of strain hardening rate and strain hardening exponent in the uniform strain stage and necking stage, thus it obtained a maximum uniform true strain of ~0.087. In addition, the crack source analysis of the three samples all shown ductile dimpled fracture while crack propagation presented as a quasi-cleavage fracture.

Influence of Strain Rate on Hot Ductility of a V-Microalloyed Steel Slab

Abstract: The hot ductility and malleability of a vanadium-microalloyed steel is investigated by means of tensile and compression tests at temperatures ranging from 700 to 850°C and strain rates of 3 × 10−4 to 0.3 s−1. The deformation tests are performed after austenitization and cooling to test temperature. The so-called second ductility minimum is located around 750°C for all strain rates except for the highest one, where no ductility trough is observed. Ductility steadily increases with strain rate at a given temperature, and the fracture mode progressively changes from intergranular to transgranular. In the region of minimum ductility, intergranular cracking occurs at low strain rates by void nucleation, growth and coalescence within thin layers of deformation induced ferrite covering the austenite grain boundaries. Cracking is favoured by V(C,N) precipitation associated with the γ/α phase transformation. Ductility remains low above the temperature of minimum ductility, where no apparent ferrite formation is observed (790 °C). Void formation takes place as a result of grain boundary sliding in combination with matrix and grain boundary precipitation. These voids are able to grow and link up forming intergranular cracks. Ductility increases with strain rate mainly due to the short time available for precipitation as well as for intergranular void growth and coalescence.

Particle-Size-Grouping Model of Precipitation Kinetics in Microalloyed Steels

Abstract: The formation, growth, and size distribution of precipitates greatly affects the microstructure and properties of microalloyed steels. Computational particle-size-grouping (PSG) kinetic models based on population balances are developed to simulate precipitate particle growth resulting from collision and diffusion mechanisms. First, the generalized PSG method for collision is explained clearly and verified. Then, a new PSG method is proposed to model diffusion-controlled precipitate nucleation, growth, and coarsening with complete mass conservation and no fitting parameters. Compared with the original population-balance models, this PSG method saves significant computation and preserves enough accuracy to model a realistic range of particle sizes. Finally, the new PSG method is combined with an equilibrium phase fraction model for plain carbon steels and is applied to simulate the precipitated fraction of aluminum nitride and the size distribution of niobium carbide during isothermal aging processes. Good matches are found with experimental measurements, suggesting that the new PSG method offers a promising framework for the future development of realistic models of precipitation.

Modeling of the hot deformation behavior of boron microalloyed steels under uniaxial hot-compression conditions

Abstract: The present study shows that the hot deformation behavior of boron microalloyed steels can be quantitatively described by constitutive equations. These equations take into account both dynamic recovery and recrystallization phenomena. They have been fitted using experimental data taken from hot compression tests of four boron microalloyed steels in order to determine their characteristic parameters. The tests were carried out over a wide range of temperatures (950, 1000, 1050 and 11008C) and strain rates (10 –3 , 10 –2 and 10 –1 s –1 ). The analysis of the characteristic parameters of the constitutive equations describing the hot flow behavior of these steels shows that boron additions play a major role in softening mechanisms rather than on hardening. A quantification of the boron effect is also presented. The experimental data were compared with the predictions of the proposed model and an excellent agreement between measured and predicted values for all boron microalloyed steels over a wide range of temperatures and strain rates was obtained.