Significant influence of carbon and niobium on the precipitation behavior and microstructural evolution and their consequent impact on mechanical properties in microalloyed steels

et al. 2020

Materials

Self Cite

Isothermal treatment affects the microstructural evolution and the precipitation behavior of high-strength low alloy (HSLA) steels. In this regard, thermal simulation of different isothermal treatment temperatures was adopted by using a thermomechanical simulator. The results showed that hardness reached the maximum value at 600 °C holding temperature, which was related to a finer grain structure and granular bainite. The strengthening effect of precipitates was remarkable due to the combination of small particle size and small interparticle spacing. It is presumed that the precipitation started after 600 s at 600 °C. Precipitation strengthening continued to exist, even though coarsening of ferrite grains led to softening phenomena when the specimen was isothermally held at 750 °C, which led to relatively high hardness. The precipitates were fcc (Ti, Nb) (N, C) particles, and belonged to MX-type precipitates. Average size of precipitates increased from 3.14 to 4.83 nm when the specimens were isothermally held between 600 °C and 800 °C. Interparticle spacing of precipitates also increased with increasing isothermal treatment temperatures. These led to a reduction in precipitation strengthening. At the same time the polygonal ferrite content increased and ferrite grain size got larger, such that the hardness decreased continuously.

Section: Introductionmentioning

confidence: 99%

The Impact of Isothermal Treatment on the Microstructural Evolution and the Precipitation Behavior in High Strength Linepipe Steel

Tian

et al. 2020

Materials

Self Cite

“…[ 14–17 ] Most importantly, for high strength steels, precipitate of microalloy element plays an important role in refining and strengthening microstructure. [ 18–23 ] Therefore, Nb and Ti were added compositely to further strengthen the experimental steel.…”

Section: Introductionmentioning

confidence: 99%

Effects of Finish Rolling Temperature and Deformation on Microstructural Evolution and Precipitation Behavior of Non‐quenched and Tempered Carbide‐Free Bainitic Steel

Chen

Yang

et al. 2020

steel research int.

Double compression tests are carried out to investigate the microstructural evolution and precipitation behavior of Nb–Ti microalloyed carbide‐free bainitic steel under different finish rolling conditions. With decreasing finish rolling temperature and increasing finish rolling deformation, the area fraction of lath bainite (LB) and martensite (M) was increased, the width of bainitic ferrite lath was decreased, the dislocation density and the number fraction of high‐angle grain boundaries are raised. Meanwhile, the larger deformation and lower temperature reduce the amount of retained austenite (RA) and refine the precipitated particles, which benefit the performance further. Hence, the phase transformation strengthening, refinement strengthening, dislocation strengthening, and precipitation strengthening synergistically contribute to the increase in hardness with increasing deformation and decreasing temperature.

“…Besides, a trace amount of B inhibits the nucleation of ferrite and promotes the formation of bainite [22,23] in the experimental steel. In addition, Nb improves the strength and toughness by the grain refining and the precipitation strengthening of microalloying carbonitride [24]. Ti is used to refrain from the formation of BN.…”

Section: Introductionmentioning

confidence: 99%

Dynamic continuous cooling transformation, microstructure and mechanical properties of medium-carbon carbide-free bainitic steel

Chen

High Temperature Materials and Processes

2020

The effects of the cooling rate after hot deformation on phase transformation, the microstructure of the designed nonquenched and tempered medium-carbon carbide-free bainitic steel have been investigated during the dynamic continuous cooling process. The results show that with the increase of the cooling rate, the morphology of the carbide-free bainite of the experimental steel evolves from granular bainite to lath bainite. Meanwhile, the hardness increases, and the amount of the retained austenite decreases with the increase of the cooling rate. Besides, the morphology evolution of the retained austenite from block to film is revealed by EBSD. Moreover, 0.5°C/s is considered to be the favorable cooling rate to obtain the best strength–toughness matching. Furthermore, the semi-industrial experimental results proved that the tensile strength, yield strength and Charpy impact energy were 1,298 MPa, 847 MPa and 38 J, respectively.