Open Access Research Article

The Metallurgy of Ultra High-Strength (Giga Strength) Ferritic Hot Band Steels

Bing Ma1,2, Yingjie Wu1, Mingjian Hua1,3, Juha Uusitalo4 and Anthony J DeArdo1,4*

1Department of Mechanical Engineering and Materials Science, University of Pittsburgh, USA

2Global Solar Energy, USA

3Sichuan University-Pittsburgh Institute (SCUPI), China

4Department of Mechanical Engineering, University of Oulu, Finland

Corresponding Author

Received Date: July 22, 2021;  Published Date: September 07, 2021

Abstract

As the result of the need to increase fuel economy, reduce CO2 emissions and improve passenger safety, the usage of advanced high strength steels in the automotive industry has increased rapidly in the recent years. These steels are multiphase in nature, i.e., dual-phase, complex-phase, transformation induced plasticity or quench and partitioning steels, and evolve from cold rolled and inter critically annealed initial microstructures. They often exhibit low yield strength, high ratios of work hardening and high levels of ultimate tensile strength, all while maintaining reasonable values of uniform and total elongations. One drawback to these steels is their rather low level of sheared edge ductility, of which hole expansion ratio (HER) is a typical representative. The literature in HER studies has shown that single phase monolithic microstructures exhibit high HER values, especially for ferritic or bainitic microstructures. In an effort to achieve both a high strength and HER values, it has been suggested that a finegrained polygonal ferrite microstructure, further strengthened by large amounts of fine microalloyed carbides or nitrides, as might be expected in a hot rolled and coiled steel, might satisfy both goals of high strength and reasonable hole expansion performance. The goal of this current study is to critically test this hypothesis. In this current study, the relationships existing among the hot mill processing, as-coiled microstructure and mechanical properties of one highly microalloyed steel hot rolled to 3 mm and then coiled were investigated. The major alloying elements used in this steel were Mo, Ti, and V. Discrete processing parameters, i.e., finish rolling temperatures (FRT) and coiling temperatures (CT), were also applied. It was found that the FRT had only a very minor influence on either microstructures or mechanical properties. However, the CTs strongly influenced both the microstructures and mechanical properties. It was further found that despite the high level of microalloying, the strength appeared to be controlled mainly by the dislocation density of non-polygonal ferrite observed at all CTs, with a minor-to-moderate contribution from the microalloyed precipitates. With falling CT, the microstructure in the matrix was observed to change from mainly polygonal ferrite to quasi-polygonal ferrite to granular bainite and upper bainite, accompanied by the formation of martensite/austenite (M/A) constituents at the lower coiling temperatures. The strength appeared to be increased by the dislocations originating by the shear component of the displacive phase transformation, and by fine (Ti, Mo)C precipitates, both formed during the coiling process. This contrasts to the polygonal ferrite formed at high transformation temperature that are reconstructive in nature and controlled by long range diffusion. Strengths reaching values as high as 1166 MPa in yield strength and 1225 MPa in tensile strength were observed in specimens after coiling at 610°C, and the steels still had a reasonable total elongation of around 20%. However, the hole expansion ratios of these steel conditions were rather low in this study, especially for those steel conditions with higher strength. Several factors appeared to contribute to the poor hole expansion ratios found in these steel conditions: the presence of coarse TiN inclusions, a large amount of strengthening precipitates and a high dislocation density as well as the presence of M/A constituents.

Keywords: Ferritic hot band steels; Thermo-mechanical controlled processing; Precipitation hardening; Dislocation strengthening; TiN inclusions; Hole expansion

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