Effect of microstructure on the impact toughness transition temperature of direct-quenched steels

et al. 2019

Metals

Self Cite

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}<131> and {554}<225> texture components. The 0Mo steel also contained the texture components of {110}<110> and {011}<100>, which can be considered to be detrimental for impact toughness properties.

Section: Correlation Between Mechanical Properties and Microstructural Featuresmentioning

confidence: 99%

Mechanical Properties of Direct-Quenched Ultra-High-Strength Steel Alloyed with Molybdenum and Niobium

Hannula

Porter

et al. 2019

Metals

Self Cite

“…Previous studies have shown that the impact transition temperature is controlled by the largest grains in the grain size distribution, where grain size refers to the ECD of grains with grain boundary misorientations > 15°, for example d90%. 20,21 By alloying with Nb-Mo, a pancaked austenite structure was achieved after hot rolling, which then produced a finer and more uniform prior austenite structure after re-austenitization, presumably due to the presence of more nucleation sites for austenite grains. This led to a finer martensitic structure with higher strength and better impact toughness.…”

Section: Mechanical Properties After Hot Rolling Direct Quenching Anmentioning

confidence: 99%

Evaluation of Mechanical Properties and Microstructures of Molybdenum and Niobium Microalloyed Thermomechanically Rolled High-Strength Press Hardening Steel

Hannula

Porter

et al. 2019

JOM

Self Cite

This article studied the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory control rolled steels based on grade 22MnB5. The constructed continuous cooling transformation diagrams revealed that an addition of Mo significantly increased the hardenability. Especially in the case of austenite compressed below its recrystallization temperature, an Mo addition delayed ferrite and bainite formation, and it increased martensite hardness. Laboratory hot-rolling experiments with a finish rolling temperature of 850°C produced a flattened pancaked austenite. After hot rolling and direct quenching, samples were rapidly reaustenitized at 900°C followed by water quenching to simulate an actual press hardening process. Especially in the case of Nb-Mo steel, a strongly pancaked austenitic structure was achieved after hot rolling, which led to a fine, uniform prior austenite grain structure after re-austenitization and quenching. The Nb-Mo steel had a tensile strength > 1500 MPa and $ 11% total elongation combined with good impact toughness, which can be considered excellent for this type of press hardening steel.

“…On the refining effect of B removal on grain size, Gao et al [44] made similar findings and proposed that austenite grain size increases with increased boron content due to the formation of BN, which reduces the amount of grain refining AlN. Pallaspuro et al [43] showed the importance of grain refinement and a correlation between toughness and martensite content for martensitic-bainitic high-strength steels, where an almost linear decrease in toughness with increasing martensite content was found. However, the results of the current study concern martensitic-ferritic steels instead of martensitic-bainitic steels.…”

Section: Accepted Manuscriptmentioning

confidence: 83%

Direct-quenched and tempered low-C high-strength structural steel: The role of chemical composition on microstructure and mechanical properties

Saastamoinen

Materials Science and Engineering: A

Nyo

et al. 2019

Self Cite

The direct quenching of low-carbon steels after thermomechanical processing on hot strip mills is able to produce both strong and tough coiled plate without the need for subsequent tempering. The process is energy and time efficient with relatively low emissions when compared to conventional reheating, quenching and tempering. For some applications, however, it is desirable to combine direct quenching with tempering, and, bearing in mind the form of the semi-finished product, it is of interest to study the effect of tempering whole coils in a bell furnace. Here, the effects of boron, carbon, titanium, vanadium and tempering temperature on the microstructure, crystallography and mechanical properties of direct-quenched steels has been studied with the aid of simulated bell furnace heating and cooling cycles. All steels contained (in wt.%) 0.2Si-1Mn-1Cr-0.65Mo-0.03Al, while there were two levels of C (0.095 / 0.140), V (0 / 0.08), Ti (0 / 0.025) and B (0 / 0.0015). Tempering was performed with peak temperatures at 180 and 570 °C. The paper reveals several possible alloying and processing routes to strong and tough low-C steel. Carbon controls the strength and toughness, while titanium and boron affects the grain size of coarsest grains (d 90% ), Vanadium has a strong effect on strength retention during tempering at 570 °C: an addition of 0.08 wt.% vanadium increases yield strength by 70 MPa and ultimate tensile strength by 100 MPa. The removal of boron from the steel is shown to have a huge impact not only on the microstructure but also on the impact toughness.