Microalloyed steel

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

Formation of Widmanstätten Ferrite in a 0.036% Nb Low Carbon Steel at Temperatures Above the Ae3

Abstract: The formation of Widmanstatten ferrite was investigated in a 0.036% Nb microalloyed steel at temperatures above the Ae3 using optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Such strain-induced ferrite appears to form in two consecutives stages: (i) stage I, first observed at strains below 0.5, leads to the presence of Widmanstatten ferrite plates only about 200 nm wide, (ii) stage II, observed at strains >0.5, involves the coalescence of the Widmanstatten plates into grains. Thus the microstructures formed by dynamic transformation are composed of fresh Widmanstatten plates (stage 1) and polygonal grains (stage 2). Over the experimental temperature range of 836–896°C, the ultrafine ferrite plates and grains have areas below 2 µm2 and are difficult to detect using optical microscopy.

Thermomechanically Hot Rolled High and Ultra High Strength Steel Grades - Processing, Properties and Application

Abstract: In all application fields for hot rolled strip products for direct processing, e.g. construction and engineering but also crane and truck industry, there is a strong customers demand for grades with increased strength levels and well balanced formability. At voestalpine Stahl GmbH these requirements were met with the development of the high strength microalloyed steel grade ALFORM700M and the ultra high strength steel grade ALFORM900M with a minimum yield strength (YS) of 700 MPa and 900 MPa, respectively. In the present paper investigations on the steel grades ALFORM700M and ALFORM900M are introduced. To explain the evolution of the obtained complex microstructures consisting of bainitic ferrite, bainite and martensite alloy design and industrial production process is discussed on the base of dilatometric experiments and TEM investigations. The formation of precipitates is studied by using a numerical model, chemical methods and mechanical testing after heat treatment. Mechanical and mechanic-technological properties of the two steel grades are compared. Furthermore, some processing aspects as weldability an bending behaviour are highlighted. Finally, some typical applications for this high and ultra high strength steel grades are presented.

Effect of duplex ferrite grain size distribution on local fracture stresses of Nb-microalloyed steels

Abstract: Two commercial thermomechanically controlled rolled (TMCR) microalloyed steel plates have been used to investigate the relationship between the duplex ferrite grain size distribution and local fracture stresses. Statistical analyses of the grain size distributions were performed for the fine and coarse ferrite grains in the two steel plates. Microhardness values were measured for each grain size region and it was found that the fine grain areas have significantly higher microhardness values than the coarse grain areas. Tensile and blunt-notch slow bend tests were carried out over a range of temperatures on samples from the two commercial TMCR steel plates. The local fracture stress (σF) values were calculated and the results show that the σF values are almost independent of temperature. The presence of a mixed grain size distribution results in significant scatter in the local fracture stresses of the steels. The distribution of fracture stress values can be correlated to the coarse grain size distribution in the steels examined.

Effect of zirconium additions on austenite grain coarsening of C-Mn and microalloy steels

Abstract: The effect of zirconium carbonitrides on austenite grain coarsening behaviour in controlled rolled Al–killed Zr and Zr–Nb microalloyed HSLA steels with various Zr/N ratios (2.8–22) has been studied, and compared with those of Ti-bearing carbonitrides in Ti–Nb and Ti–Nb–Zr steels. TEM observations and PEELS analysis showed that, in the Zr and Zr–Nb steels, with hypo Zr additions(Zr/N=2.8–6.3), the Zr-bearing particles were large ZrN-rich carbonitrides with sizes from ≈100 nm to several microns and of irregular shape, while with hyper Zr additions(Zr/N=15–22), which were far from stoichiometry (Zr/N=6.5), fine spherical particles of Zr-rich Zr carbonitrides(10–100 nm) were formed together with many more large ZrC-rich carbonitrides. Moreover, only in the steel with the lowest Zr/N ratios, i.e. 2.34 for the 0.011 Zr steel, and 2.75 for the 0.022Zr–Nb steels, was a significant fraction of AlN formed. Austenite grain coarsening occurred around 1050–1100°C in all the hypo Zr and Zr–Nb steels because very few small carbonitrides(