Due to their strength and ductility, high-strength low-alloy (HSLA) steels play a crucial role in a range of applications, from automotive to structural uses. However, a challenge for HSLA producers is that, within one coil, ultimate tensile strengths can vary. Here, Thermo Fisher Scientific outlines how microalloyed steel precipitate was characterized with various electron microscopy techniques.
Using two samples of hot rolled, heat-treated coils, each with different ultimate tensile strengths, the study explored the ways in which microalloy additions affect material properties. The samples were taken from the same coil, which had been produced by Tata Steel Port Talbot Works, UK using a blast furnace, oxygen steelmaking and slab casting process.
The production process of the HSLA coil involved reheating slabs to around 1,200°C, before hot rolling them to bring their material thickness down to about 3mm. Then, to control the grain size, water cooling was applied as the steel changed structure from austenitic to ferritic.
By exploring precipitate characterization, an imaging study set out to understand why the head and middle of the same microalloyed steel coil had different ultimate tensile strengths, at 688 MPa and 608 MPa respectively.
Achieving stronger steels
Austenite grain size was a key area of focus in the study. As a finer grain size is needed to improve the strength of steel, it is an integral factor when producing HSLAs.
A fine grain size can be achieved by adding small amounts of elements such as niobium, titanium and vanadium during the hot rolling process. Carbides, nitrides or carbonitride precipitates form in reaction to microalloy additions, but if these nanosized precipitates coarsen, they become less able to reduce the austenite grain growth. As such, it is an aspect of steel production that needs to be carefully controlled and measured.
The evaluation process
Looking at the relationship between precipitate distribution and ultimate tensile strength, researchers undertook an imaging study of the coil samples. This involved using transmission electron microscopy (TEM) analysis to observe networks of dislocations and coarse precipitates in the samples.
To prepare the lamellae sample, researchers used the Thermo Scientific™ Helios™ 5 DualBeam, which is a focused ion beam scanning electron microscope (FIB-SEM).
TEM analysis was then performed using the Thermo Scientific™ Talos™ F200X G2 TEM. And, through a combination of scanning transmission electron microscopy (STEM) and energy dispersive x-ray spectroscopy (EDS) imaging, the samples’ small precipitates were studied.
After the manual TEM imaging, overnight scans allowed wide areas to be characterized with the Thermo Scientific Automated Precipitate Workflow (APW). This involved using tools such as Maps and Velox Software, allowing the research team to collect tile-by-tile STEM images and EDS maps across the sample using the Thermo Scientific Avizo2D Software.
Findings from the study
Within both samples, niobium carbide precipitates were analyzed in terms of size and number density.
Interestingly, the sample from the coil head was found to have a higher density of precipitates than the one from the middle of the coil. Not only this, but the average size of the precipitates was also smaller in the head of the coil compared to the middle. This combined leads to the coil head having a higher ultimate tensile strength.
Understanding why the two samples from the same coil had different ultimate tensile strengths is an important breakthrough. This research guided Tata Steel’s selection of hot rolling cooling patterns to ensure the desired properties could be achieved, as determined by these precipitates.
With applications including car suspensions, bridges, railway parts and more, HSLAs are used in many critical cases where reliability is essential. From the advancements in material development that are made possible with this study, processes of producing and selecting microalloyed steel products can be fine-tuned, with added confidence that the required levels of strength and ductility will be achieved.
To find out more about Thermo Fisher Scientific’s latest research, visit their website.
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