Abstract
High-carbon martensite steels (with C > 0.5 wt.%) are very hard but at the same time as brittle as glass in as-quenched or low-temperature-tempered state. Such extreme brittleness, originating from a twin microstructure, has rendered these steels almost useless in martensite state. Therefore, for more than a century it has been a common knowledge that high-carbon martensitic steels are intrinsically brittle and thus are not expected to find any application in harsh loading conditions. Here we report that these brittle steels can be transformed into super-strong ones exhibiting a combination of ultrahigh strength and significant toughness, through a simple grain-refinement treatment, which refines the grain size to ~4 μm. As a result, an ultra-high tensile strength of 2.4~2.6 GPa, a significant elongation of 4~10% and a good fracture toughness (K1C) of 23.5~29.6 MPa m1/2 were obtained in high-carbon martensitic steels with 0.61–0.65 wt.% C. These properties are comparable with those of “the king of super-high-strength steels”—maraging steels, but achieved at merely 1/30~1/50 of the price. The drastic enhancement in mechanical properties is found to arise from a transition from the conventional twin microstructure to a dislocation one by grain refinement. Our finding may provide a new route to manufacturing super-strong steels in a simple and economic way.
Highlights
Materials with a combination of ultrahigh strength and high toughness are perpetually desired by human society[1, 2], which are critical for energy efficient, lightweight structures and vehicles such as cars, airplanes and rockets
It should be noted that when having a normal grain (NG) size of 15 μm, the NG HULA-60 steel is still very brittle (Fig. 1a), in agreement with the common wisdom
Tempering at 500 °C can increase its fracture elongation to 12.1% but its strength drastically drops to 1289 MPa
Summary
Materials with a combination of ultrahigh strength and high toughness are perpetually desired by human society[1, 2], which are critical for energy efficient, lightweight structures and vehicles such as cars, airplanes and rockets. Over the past decades several notable inexpensive ultrahigh-strength steels based on interstitial carbon, the strongest strengthening but at the same time strongly embrittling element, have been developed, such as high-strength low alloying steels (HSLA)[3, 5], and nano-bainite steel[6,7,8,9]. Their strength has reached the level of medium-grade maraging steel C250, they are yet to achieve the strength level of high-grade maraging steel C350 with comparable ductility. We further show that this remarkable change of the properties arises from a drastic change from twinned martensite to dislocation martensite
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