Microalloyed steel

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

The production and mechanical properties of ultrafine ferrite

Abstract: A new thermomechanical process has been developed to produce ultrafine (1 μm) equiaxed ferrite grains in hot rolled steel strip. This process is remarkably simple and is applicable to a wide range of steel chemistries, including low and high carbon and microalloyed steels. Strips are reheated to produce a coarse austenite grain size, then rolled in a single pass at or just above the austenite to ferrite transformation temperature. It is suggested that the observed refinement is due to strain induced transformation from austenite to ferrite. The requirements for this appear to be high strain induced by shear in the strip surface layers, and thermal gradients created by heavy quenching of the strip surface by the work rolls. The yield strength was markedly higher than conventionally processed strip, although there was little work hardening even though total elongation of over 20% was achieved.

Influence of TiN Inclusions on the Cleavage Fracture Behavior of Low-Carbon Microalloyed Steels

Abstract: Toughness is a major concern for low-carbon microalloyed steels. In this work, the impact fracture behavior of two low-carbon Ti-V microalloyed steels was investigated in order to better understand the role of TiN inclusions in the toughness of the steels. The steels had similar chemical compositions and were manufactured by the same rolling process. However, there was an obvious difference in the ductile brittle transition temperature (DBTT) in the Charpy V-notch (CVN) impact tests of the two steels; one (steel 1) possessing a DBTT below −20 °C, while the DBTT of the other (steel 2) was above 15 °C. Scanning electron microscopy (SEM) fractography revealed that there were TiN inclusions at the cleavage fracture initiation sites on the fracture surfaces of steel 2 at both low and room temperatures. It is shown that the TiN inclusions had nucleated on Al2O3 particles and that they had pre-existing interior flaws. A high density of TiN inclusions was found in steel 2, but there was a much lower density in steel 1. Analysis indicates that these inclusions were responsible for the shift of DBTT to a higher temperature in steel 2. A mechanism is proposed for understanding the effect of the size and density of TiN inclusions on the fracture behavior, and the cleavage fracture initiation process is analyzed in terms of the distribution and development of stresses ahead of the notch tip during fracture at both low and room temperatures.

Evolution of microstructure and precipitation state during thermomechanical processing of a X80 microalloyed steel

Abstract: A series of anisothermal hot torsion tests were carried out to simulate hot rolling on a high-strength low-carbon CMnNbMoTi microalloyed steel corresponding to an industrial X80 grade for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling, which was also studied by optical microscopy and SEM on samples quenched from several temperatures. On the other hand, particles precipitated at different temperatures during rolling were analyzed by means of TEM using the carbon extraction replica technique and their size distribution and mean size were determined, as well as their morphology, nature and chemical composition. The effect of rolling temperature and austenite strengthening obtained at the end of thermomechanical processing on final microstructure and precipitation state was studied. Austenite strengthening was characterized by means of the parameter known as accumulated stress (Δ σ ). It was found that ferrite grains are finer and more equiaxed when the austenite is more severely deformed during finishing (higher values of Δ σ ) but lower values of Δ σ generate a higher density of acicular structures after cooling, which should improve the balance of mechanical properties. The increase in strength associated to acicular ferrite compared to polygonal ferrite is revealed by the higher values of Vickers microhardness measured on samples corresponding to low Δ σ . On the other hand, (Ti, Nb)-rich carbonitrides can be found from reheating and their size keeps a constant value near 20–30 nm during thermomechanical processing. A second population of much finer (Nb, Mo)-rich carbonitrides whose size is close to 5 nm forms from lower temperatures, near 1000 °C. The accomplishment of two different levels of Δ σ at the end of hot rolling schedule does not seem to introduce major differences in precipitation state before final cooling.

Microstructural evolution and mechanical properties of high strength microalloyed steels: Ultra Fast Cooling (UFC) versus Accelerated Cooling (ACC)

Abstract: We describe here the microstructural evolution and mechanical properties of high strength microalloyed steels processed using different cooling trajectory. Pilot-scale studies demonstrated that high strength of ∼700 MPa can be obtained in a microalloyed steel using ultra fast cooling (UFC) positioned at the exit of hot rolling mill, while the yield strength obtained via the conventional thermo-mechanical controlled processing (TMCP) with accelerated cooling (ACC) is ∼100 MPa less. The underlying reason is that ultra fast cooling positioned immediately after hot rolling enhances strengthening associated with precipitation and grain refinement. Theoretical calculations and experiments indicated that grain refinement and precipitation in TMCP with in-front UFC led to strength increment of ∼49 and 54 MPa, respectively over the conventional TMCP with ACC process. Furthermore, the microstructural characterization indicated that the density of high angle grain boundaries was increased and the average size of precipitates was reduced from ∼34 nm to ∼10 nm, when the cooling pattern is changed from ACC to UFC. The theoretical estimate also indicated that when the cooling profile is changed from the conventional ACC to UFC+ACC, and to UFC, a higher degree of precipitation is responsible for increase in strength in UFC processed hot rolled microalloyed steels.