Hot-Rolling and a Subsequent Direct-Quenching Process Enable Superior High-Cycle Fatigue Resistance in Ultra-High Strength Low Alloy Steels.

Min-Seok Baek,Jinhee Ham,Tae-Won Park,Kee-Ahn Lee,Young-Kyun Kim

doi:10.3390/ma13204651

Min-Seok Baek, Jinhee Ham + Show 3 more

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https://doi.org/10.3390/ma13204651

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Abstract

The current study investigated the effect of hot rolling reduction rate of ultra-high strength low alloy steel manufactured via the direct quenching process on microstructure, tensile and high-cycle fatigue properties of the alloy. In order to control the reduction rate of ultra-high strength steels (UHSSs) differently, the steels were produced with two different thicknesses, 6 mm (46.2%—reduction rate, A) and 15 mm (11.5%—reduction rate, B). Then, the two alloys were directly quenched under the same conditions. Both the UHSSs showed martensite in the near surface region and auto-tempered martensite and bainite in the center region. Tensile results showed that alloy A with higher fraction of finer martensite had higher yield strength by about 180 MPa (1523 MPa) than alloy B. The alloy A was also found to possess a higher tensile strength (~2.1 GPa) than alloy B. In addition, alloy A had higher strength than B, and the elongation of A was about 4% higher than that of alloy B. High-cycle fatigue results showed that the fatigue limits of alloys A and B were 1125 MPa and 1025 MPa, respectively. This means that alloy A is excellent not only in strength but also high-cycle fatigue resistance. Based on the above results, the correlation between the microstructure and deformation behaviors were also discussed.

Highlights

With the advancements in the automobile and aviation industries, the demand for extremely high performance steels is rising
The present study has investigated the effect of hot-rolling and subsequent direct-quenching on the microstructure, tensile and high-cycle fatigue properties of ultra-high strength steels (UHSSs)
For the UHSSs produced via the DQ process, the microstructure of each material generally varies reason, the near surface region (NSR) and center region (CR) were separated to observe the microstructures, and the results are shown in according to the rolling and heat treatment conditions (final rolling temperature (FRT))

Summary

Introduction

With the advancements in the automobile and aviation industries, the demand for extremely high performance steels is rising. Diverse new steel materials are being developed to meet such requirements, and out of these materials, the ultra-high-strength steel (UHSS) is expected to be a useful structural material in the aviation and automobile industry due to its high strength-ductility, superior toughness, and appropriate formability [1,2,3,4,5,6]. UHSS, with bainite and tempered martensite structures, is generally processed via a quenching & tempering (Q&T) heat treatment [10]. In this process, depending upon the Q&T heat treatment conditions, the phase fracture varies and the desired strength and elongation can be properly controlled. Diverse methods have been presented to overcome such weaknesses, and among them, the direct-quenching (DQ) gained extensive attention because it can remarkably reduce the procedural steps

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Discussion

Conclusion