Evaluation of Mechanical Properties and Microstructures of Direct‐Quenched and Direct‐Quenched and Tempered Microalloyed Ultrahigh‐Strength Steels

Abstract

Herein the effects of molybdenum and niobium on the microstructures and mechanical properties of laboratory‐rolled and direct‐quenched and direct‐quenched and tempered steels are revealed. The microstructures are martensitic with yield strength of 766–1119 MPa in direct‐quenched condition and 632–1011 MPa in direct‐quenched and tempered condition. Mo and Nb additions lead to a fine martensitic microstructure that imparts a good combination of strength and toughness. Steel with 0.5 wt% molybdenum has a high yield strength of 1119 MPa combined with low 28 J transition temperature of −95 °C in direct‐quenched condition. Molybdenum and niobium increase the strength significantly during tempering due to enhanced solute drag and precipitation hardening. Addition of 0.25 wt% molybdenum increases yield strength from 632 to 813 MPa after tempering. However, the combination of niobium and molybdenum results in even greater increase in yield strength during tempering compared to the nonalloyed version, producing almost 400 MPa increase in yield strength. Transmission electron microscopy reveals that only niobium forms stable precipitates during tempering, indicating that molybdenum largely remains in solution. X‐ray diffraction analysis elucidates that molybdenum and molybdenum–niobium alloying prevents annihilation of dislocations, leading to the presence of high densities of dislocations after tempering.

Improvement of Mechanical Properties and Wear Resistance of Direct‐Quenched Wear‐Resistant Steel by Deformed Austenite

“…The possible reason for the precipitation of carbides is that during the DQ process, the larger steel plates were prone to residual heat during quenching, resulting in the self‐tempering phenomenon. [ 23 ] In the RQ steel, no significant carbide precipitation was found.…”

The Effect of Direct Quenching on the Microstructure and Mechanical Properties of NiCrMo and Cu-Bearing High-Strength Steels

“…However, the newly developed direct quenching (DQ) process offers an effective way to produce high-strength steels through controlled rolling, online quenching, and tempering treatment. The DQ process reduces the cost by avoiding the reheating step and allows for reducing costly alloying elements by achieving mechanical properties with leaner chemistry [ 12 , 13 , 14 ]. Wang [ 15 ] introduced the application of the DQ process in TRIP steel, a type of advanced high-strength steel (AHSS) in the automobile industry, and superior tensile strength (880 MPa) and total elongation (28%) were obtained.…”

Effect of Mo and Mo+Nb Additions on the Phase Transformation and Microstructure of a Developed Low-Carbon CrNiMnB Ultrahigh-Strength Steels with a Preceding Hot Deformation

“…The usefulness of the combined addition of Nb and Mo on steels with yield strengths of 1000 MPa or greater is, however, still poorly understood. Recently, Hannula et al [10], illustrated the greater effect of Mo+Nb on the precipitation kinetics and the final mechanical properties.…”