Effect of Delamination and Grain Refinement on Fracture Energy of Ultrafine-Grained Steel Determined Using an Instrumented Charpy Impact Test.

Improving the balance of strength and toughness in structural materials is an ongoing challenge. Delamination and grain refinement are some of the methods used to do this. In this paper, two different steels, 0.15% C-0.3% Si-1.5% Mn-Fe and 0.4% C-2% Si-1% Cr-1% Mo-Fe (mass %), were prepared. Two steel bars with an ultrafine elongated grain (UFEG) structure were fabricated via multipass warm caliber rolling. The UFEG steels were characterized by a strong <110>//rolling-direction fiber texture. The transverse grain size, dt, was 1.0 µm for the low-carbon steel and 0.26 µm for the medium-carbon steel. For comparison, conventional heat-treated steels were also fabricated. An instrumented Charpy impact test was performed, and the impact load (P) and deflection (u) during the test were recorded. The P-u relations at the test temperature at which delamination fracture occurred exhibited a unique curve. Delamination effectively enhances the low-temperature toughness, and this was characterized by a plateau region of constant load in the P-u curve. Assuming no delamination, two routes in the P-u curves, the ductile route and the brittle route, were proposed. The results showed that the proposed methods can be predicted by an energy curve for ultrafine grained steels. Delamination is a more effective method of enhancing toughness for ultra-high-strength steels.

Dynamic balance ability in single-leg drop jump landing in junior female athletes after shin splints-assessment from a preventive perspective.

[Purpose] To develop assessment/rehabilitation indices for prevention of chronic and recurrent shin splint injuries among sport athletes, we analyzed the plantar center of pressure trajectories after drop-jump landing on one leg, and compared the foot function on the injured and healthy sides. [Participants and Methods] The participants were 10 female athletes who received consultation at our facility due to shin splints of the lower leg. The exercise task was the "single-leg drop jump landing test", in which the participants maintained a static posture on one leg for 8 s after landing. Using the collected data, the peak value of the vertical component of the floor-reaction force at the landing and the rectangular areas at 20-200 ms and 1-5 s after landing were calculated and compared between the healthy and affected sides. [Results] The peak value of the vertical component and the rectangular area at 20-200-ms were significantly larger on the affected side. However, the value for the 1-5-s rectangular area was significantly larger on the healthy side. [Conclusion] The feedforward function may have been reduced on the affected side in comparison with that on the healthy side, and the reduction in dynamic balance early after landing may have increased the influence of the non-vertical component. The 1-5-s rectangular area was smaller on the affected side than that on the healthy side, suggesting that the feedback function excessively worked on the affected side and caused immobility by excessively locking the joint in single-leg balance and reducing body sway.

Plastic Instability in Medium-Carbon Tempered Martensite Steel.

Inhomogeneous plastic deformation damages the surface quality of a product in the metal forming process. Therefore, it is necessary to investigate the plastic instability of a metal. Tempered martensite is a common microstructure of medium-carbon steel. Plastic instability (Lüders phenomenon, Portevin-Le Châtelier phenomenon) in this phase was investigated by a uniaxial tension test performed at room temperature. The formation and propagation of a plastic band were analyzed via two-dimensional digital image correlation, and the strain and strain-rate fields were experimentally evaluated. The results obtained are as follows: (1) there was no clear yield plateau on the stress-strain curve; (2) Lüders phenomenon was present, but the Portevin-Le Châtelier phenomenon was not found; (3) in the Lüders deformation process, local strain distribution in tempered martensite is more complicated than that in ferrite.

Ductile-to-Brittle Transition and Brittle Fracture Stress of Ultrafine-Grained Low-Carbon Steel.

Ductile-to-brittle transition (DBT) temperature and brittle fracture stress, σF, are important toughness criteria for structural materials. In this paper, low-carbon steels with an ultrafine elongated grain (UFEG) structure (transverse grain size 1.2 µm) and with two ferrite (α)-pearlite structure with grain sizes 10 µm and 18 µm were prepared. The UFEG steel was fabricated using multipass warm biaxial rolling. The tensile tests with a cylindrical specimen and three-point bending tests with a single-edge-notched specimen were performed at -196 °C. The local stress near the notch was quantitatively calculated via finite element analysis (FEA). The σF for each sample was quantified based on the experimental results and FEA. The relationship between σF and dα in the wide range of 1.0 µm to 138 µm was plotted, including data from past literature. Finally, the conditions of grain size and temperature that cause DBT fracture in low-carbon steel were shown via the stress-d-1/2 map. The results quantitatively showed the superiority of α grain size for brittle fracture.

Grain-to-Grain Interaction Effect in Polycrystalline Plain Low-Carbon Steel within Elastic Deformation Region.

A grain is surrounded by grains with different crystal orientations in polycrystalline plain low-carbon steel. The grain is constrained by its adjacent grains in the tension process. The interaction of the grain with the adjacent grains was investigated within the elastic deformation region. The following results have been obtained: (1) the Young's modulus of a grain without consideration of grain-to-grain interaction is denoted as the inherent Young's modulus; when the inherent Young's modulus of a grain is equal to the Young's modulus of the bulk material, there is almost no interaction between the grain and its adjacent grains; when a grain has a great difference between its inherent Young's modulus and the Young's modulus of the bulk material, its grain-to-grain interactions increase significantly; (2) the grain-to-grain interaction is mainly caused by the difference in the inherent Young's modulus between the grain and its adjacent grains; the misorientation angle between the grain and its adjacent grains has almost no effect on the grain-to-grain interaction.

[Radiofrequency ablation therapy combined with hepatectomy for liver metastasis of colorectal cancer].

Radiofrequency ablation (RFA) therapy combined with hepatectomy was performed in 5 patients with synchronous liver metastases of colorectal cancer. RFA of liver metastases was performed using a Cool-tip electrode (Radionics; Burlington, MA, USA). The ablation time used in each session varied according to the tumor size and intraoperative impedance. In 2 patients, hepatectomy and resection of the colorectal primary lesion were performed synchronously. In patients with multiple liver metastases, relative curative resection was performed using the complementary RFA. In the other 3 patients, synchronous hepatectomy was considered difficult and systematic chemotherapy was performed after resection of the colorectal primary lesion. After systematic chemotherapy, the range of hepatectomy was restricted for liver injury, but relative curative resection was performed using RFA therapy. Computed tomography performed after hepatectomy showed that the region that underwent RFA appeared necrotic with a safety margin. The average observation period was 25 months (maximal survival period, 50 months) and 3 of the patients are alive.

Shape effect of ultrafine-grained structure on static fracture toughness in low-alloy steel.

A 0.4C-2Si-1Cr-1Mo steel with an ultrafine elongated grain (UFEG) structure and an ultrafine equiaxed grain (UFG) structure was fabricated by multipass caliber rolling at 773 K and subsequent annealing at 973 K. A static three-point bending test was conducted at ambient temperature and at 77 K. The strength-toughness balance of the developed steels was markedly better than that of conventionally quenched and tempered steel with a martensitic structure. In particular, the static fracture toughness of the UFEG steel, having a yield strength of 1.86 GPa at ambient temperature, was improved by more than 40 times compared with conventional steel having a yield strength of 1.51 GPa. Furthermore, even at 77 K, the fracture toughness of the UFEG steel was about eight times higher than that of the conventional and UFG steels, despite the high strength of the UFEG steel (2.26 GPa). The UFG steel exhibited brittle fracture behavior at 77 K, as did the conventional steel, and no dimple structure was observed on the fracture surface. Therefore, it is difficult to improve the low-temperature toughness of the UFG steel by grain refinement only. The shape of crystal grains plays an important role in delamination toughening, as do their refinement and orientation.

Inverse temperature dependence of toughness in an ultrafine grain-structure steel.

Materials are typically ductile at higher temperatures and become brittle at lower temperatures. In contrast to the typical ductile-to-brittle transition behavior of body-centered cubic (bcc) steels, we observed an inverse temperature dependence of toughness in an ultrahigh-strength bcc steel with an ultrafine elongated ferrite grain structure that was processed by a thermomechanical treatment without the addition of a large amount of an alloying element. The enhanced toughness is attributed to a delamination that was a result of crack branching on the aligned {100} cleavage planes in the bundles of the ultrafine elongated ferrite grains strengthened by nanometer-sized carbides. In the temperature range from 60 degrees to -60 degrees C, the yield strength was greater, leading to the enhancement of the toughness.