In this paper, the effects of different tempering temperatures on a recently developed ultrahigh-strength steel with 0.4 wt.% carbon content were studied. The steel is designed to be used in press-hardening for different wear applications, which require high surface hardness (650 HV/58 HRC). Hot-rolled steel sheet from a hot strip mill was austenitized, water quenched and subjected to 2-h tempering at different temperatures ranging from 150 °C to 400 °C. Mechanical properties, microstructure, dislocation densities, and fracture surfaces of the steels were characterized. Tensile strength greater than 2200 MPa and hardness above 650 HV/58 HRC were measured for the as-quenched variant. Tempering decreased the tensile strength and hardness, but yield strength increased with low-temperature tempering (150 °C and 200 °C). Charpy-V impact toughness improved with low-temperature tempering, but tempered martensite embrittlement at 300 °C and 400 °C decreased the impact toughness at −40 °C. Dislocation densities as estimated using X-ray diffraction showed a linear decrease with increasing tempering temperature. Retained austenite was present in the water quenched and low-temperature tempered samples, but no retained austenite was found in samples subjected to tempering at 300 °C or higher. The substantial changes in the microstructure of the steels caused by the tempering are discussed.

The effect of chromium content in the range of 1 wt.%–4 wt.% on the microstructure and mechanical properties of controlled-rolled and direct-quenched 12 mm thick low-carbon (0.04 wt.%) steel plates containing 0.06 wt.% Nb has been studied. In these microalloyed 700 MPa grade steels, the aim was to achieve a robust bainitic microstructure with a yield strength of 700 MPa combined with good tensile ductility and impact toughness. Continuous cooling transformation diagrams of deformed and non-deformed austenite were recorded to study the effect of Cr and hot deformation on the transformation behavior of the investigated steels. Depending on the cooling rate, the microstructures consist of one or more of the following microstructural constituents: bainitic ferrite, granular bainite, polygonal ferrite, and pearlite. The fraction of bainitic ferrite decreases with decreasing cooling rate, giving an increasing fraction of granular bainite and polygonal ferrite and a reduction in the hardness of the transformation products. Polygonal ferrite formation depends mainly on the Cr content and the cooling rate. In both deformed and non-deformed austenite, increasing the Cr content enhances the hardenability and refines the final microstructure, shifting the ferrite start curve to lower cooling rates. Preceding austenite deformation promotes the formation of polygonal ferrite at lower cooling rates, which leads to a decrease in hardness. In hot-rolled and direct-quenched plates, decreasing the Cr content promotes the formation of polygonal ferrite leading to an increase in the impact toughness and elongation but also a loss of yield strength.

The direct quenching process is an energy- and resource-efficient process for making high-strength structural steels with good toughness, weldability, and bendability. This paper presents the results of an investigation into the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory rolled and direct-quenched 11 mm thick steel plates containing 0.16 wt.% C. Three of the studied compositions were niobium-free, having molybdenum contents of 0 wt.%, 0.25 wt.%, and 0.5 wt.%. In addition, a composition containing 0.25 wt.% molybdenum and 0.04 wt.% niobium was studied. Prior to direct quenching, finish rolling temperatures (FRTs) of about 800 °C and 900 °C were used to obtain different levels of austenite pancaking. The final direct-quenched microstructures were martensitic and yield strengths varied in the range of 766–1119 MPa. Mo and Nb additions led to a refined martensitic microstructure that resulted in a good combination of strength and toughness. Furthermore, Mo and Nb alloying significantly reduced the amount of strain-induced ferrite in the microstructure at lower FRTs (800 °C). The steel with 0.5 wt.% Mo exhibited a high yield strength of 1119 MPa combined with very low 28 J transition temperature of −95 °C in the as-quenched condition. Improved mechanical properties of Mo and Mo–Nb steels can be attributed to the improved boron protection. Also, the crystallographic texture of the investigated steels showed that Nb and Nb–Mo alloying increased the amount of {112} and {554} texture components. The 0Mo steel also contained the texture components of {110} and {011}, which can be considered to be detrimental for impact toughness properties.