Abstract
Surface hardening improves the strength of low-carbon steel without interfering with the toughness of its core. In this study, we focused on the microstructure in the surface layer (0–200 μm) of our low-carbon steel, where we discovered an unexpectedly high level of hardness. We confirmed the presence of not only upper bainite and acicular ferrite but also lath martensite in the hard surface layer. In area of 0–50 μm, a mixed microstructure of lath martensite and B1 upper bainite was formed as a result of high cooling rate (about 50–100 K/s). In area of 50–200 μm, a mixed microstructure of acicular ferrite and B2 upper bainite was formed. The average nanohardness of the martensite was as high as 9.87 ± 0.51 GPa, which was equivalent to the level reported for steel with twenty times the carbon content. The ultrafine laths with an average width of 128 nm was considered to be a key cause of high nanohardness. The average nanohardness of the ferrites was much lower than for martensite: 4.18 ± 0.39 GPa for upper bainite and 2.93 ± 0.30 GPa for acicular ferrite. Yield strength, likewise, was much higher for martensite (2378 ± 123 MPa) than for upper bainite (1007 ± 94 MPa) or acicular ferrite (706 ± 72 MPa). The high yield strength value of martensite gave the surface layer an exceptional resistance to abrasion to a degree that would be unachievable without additional heat treatment in other steels with similar carbon content.
Highlights
Low-carbon steels are widely valued for their high ductility and toughness, but their strength and hardness have remained inferior to other steels [1,2,3]
With a JEM-ARM200cF TEM (JEOL Ltd., Tokyo, Japan), we identified the Selected Area Electron Diffraction (SAED) patterns of each microstructure in each specimen [21]
B1 upper bainite formed prior to lath martensite, and both of them were in 0–50 μm depth
Summary
Low-carbon steels are widely valued for their high ductility and toughness, but their strength and hardness have remained inferior to other steels [1,2,3]. For most applications of low-carbon steels, both hardness and strength, especially surface hardness, are important basic mechanical properties. The hardened surface layer improves the strength of the material while the tough core is retained. Many surface hardening methods have been developed, such as laser hardening, induction hardening, carburizing, and nitriding [4]. All these methods require extra processing, which increases the cost. We produced an as-cast low-carbon steel with a hardened surface layer without introducing extra processing steps such as alloying, rolling, heat treatment, or surface treatment [5]
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