Influence of Chromium Content on the Microstructure and Mechanical Properties of Thermomechanically Hot-Rolled Low-Carbon Bainitic Steels Containing Niobium

Mohammed Ali,Jaakko Hannula,David Porter,Tun Nyo,Antti Kaijalainen,Jukka Kömi

doi:10.3390/app10010344

Mohammed Ali, Jaakko Hannula + Show 4 more

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https://doi.org/10.3390/app10010344

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Abstract

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.

Highlights

Controlled processing (TMCP) followed by direct quenching is considered the most powerful and effective processing route to obtain the best combination of the yield strength, good ductility, low-temperature toughness with a good hardenability in the low-carbon microalloyed steels [1]
The finish rolling temperature (FRT) was controlled to be about 880 ◦ C, which was followed by direct quenching in a water tank with a cooling rate (CR) of about 40–50 ◦ C/s as measured in the centers of the specimens
Increasing the Cr content from 2.5 wt.% to 4 wt.% led to an increase in Ac1 by about 35 °C, otherwise it has no effect on the critical transformation temperatures

Summary

Introduction

Controlled processing (TMCP) followed by direct quenching is considered the most powerful and effective processing route to obtain the best combination of the yield strength, good ductility, low-temperature toughness with a good hardenability in the low-carbon microalloyed steels [1]. The final microstructure of the hot-rolled steel is considered the key factor for getting. Sci. 2020, 10, 344 the targeted combination of the mechanical properties and it is controlled by the quenching process following the TMCP. The microstructures and mechanical properties of low-carbon microalloyed steel can be improved significantly by designing a suitable accelerated cooling/direct quenching process, following the hot deformation schedule [2]. Depending on the chemical composition of the steel and the cooling path, the accelerated cooling rate may promote the formation of bainite with or without the formation of ferrite

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