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.

中文翻译：

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低碳钢高硬度表层的组织特征

表面硬化可提高低碳钢的强度，而不会影响其核心的韧性。在这项研究中，我们专注于低碳钢表层（0–200μm）的微观结构，在那里我们发现了出乎意料的高硬度。我们确认在硬质表层中不仅存在上贝氏体和针状铁素体，还存在板条马氏体。在0–50μm的区域内，由于高冷却速率（约50–100 K / s），形成了板条马氏体和B ₁上贝氏体的混合组织。在50–200μm的范围内，针状铁素体和B ₂的混合微观结构上贝氏体形成。马氏体的平均纳米硬度高达9.87±0.51 GPa，这相当于碳含量为20倍的钢所报告的水平。平均宽度为128 nm的超细板条被认为是高纳米硬度的关键原因。铁素体的平均纳米硬度远低于马氏体：上贝氏体为4.18±0.39 GPa，针状铁素体为2.93±0.30 GPa。同样，马氏体（2378±123 MPa）的屈服强度比上贝氏体（1007±94 MPa）或针状铁素体（706±72 MPa）高得多。马氏体的高屈服强度值使表面层具有出色的耐磨性，达到了在其他含碳量相似的钢中不进行额外热处理就无法实现的程度。