Super-strong dislocation-structured high-carbon martensite steel

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.

Enhanced Strength and Ductility of 2.4 GPa Hot‐Stamped Steel with an Extremely Thin Oxide Layer

“…Although the strength and the ductility of hot‐stamped steels were influenced by twin martensite, twin martensite cannot be entirely avoided. [ 39 ]…”

A Novel Heat Treatment Strategy Based on Quenching and Carbides Preprecipitation and Subsequent Critical Quenching to Improve the Thermal Fatigue Performance of AISI H13 Tool Steel

“…[ 37 ] Small and uniformly distributed carbides in the matrix further improve the strength and hardness of the alloy through the Orowan dislocation strengthening mechanism. [ 38 ] Compared with the CQT‐treated samples (hardness of HRC), the impact toughness of the QPQT samples increased by 34.8% under the condition of a slightly increasing hardness of HRC. It can be concluded that in addition to the reduction of carbide size and uniform distribution, the refinement of grain size also plays an important role in improving the mechanical properties of the test steel after QPQT treatment.…”

Advances in research on deformation and recrystallization for the development of high-functional steels

“…This orientation has a high Taylor factor, which leads to plastic instability, as in the case of rolling under DSA condition, which results in the formation of shear bands. Recent investigations into the microstructure within shear bands by means of a transmission electron microscope (TEM) and EBSD have revealed that crystal rotation occurs locally from (111) to (110)[001] along TD// [1][2][3][4][5][6][7][8][9][10]. This is due to the introduction of immense shear which results in very fine equi-axed cells comprised of locally rearranged dislocations with rather low density [12,14,16].…”

“…Ultra-steel research, which aimed to achieve an average ferrite (α) grain size of 1 μm [3], and research into bulk nano-metallic materials, which aimed to obtain an average grain size of less than 1 μm [4], are typical of the research currently being performed in Japan. Recently it has been reported that high carbon steel consisting of fine martensite (αʹ) transformed from fine austenite (γ) with a grain size of~4 μm exhibits excellent ductility and fracture toughness balanced with extremely high strength greater than 2.4 GPa [5]. Moreover, Torizuka recently proposed the 3rd generation of high strength-good performance ultra-steel with a fine martensitic structure consisting of a single variant transformed from very fine austenite (γ) grains with~2 μm in diameter [6].…”