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The behavior of MnS, a significant non-metallic inclusion in steel with low sulfur content, is poorly understood, in particular, how the MnS inclusion becomes a favorable secondary phase. To clarify how the sulfur content in DH36 structural steel affects the behaviors of MnS, and the steel properties, samples containing 20-380 ppm sulfur were melted, and the ingots were rolled in a semi-industrial single-roll mill. According to experiments using optical microscopy (OM), scanning electron microscopy (SEM), an automatic inclusion analysis system, and transmission electron microscopy (TEM), the morphology of MnS inclusions changed from thread-shaped to spherical-or spindle-shaped with decreasing sulfur content, and their size and number also decreased. The steel with 20 ppm S contained much more nanoscale MnS, whose pinning effect created a fine grain size in the steel. The reason is that the temperatures of maximum nucleation rate and fastest precipitation of MnS in this sample (900 • C and 920 • C, respectively) are much lower than the soaking temperature. Compared to samples with 70-380 ppm sulfur, the sample with 20 ppm sulfur content demonstrated the highest yield strength, tensile strength, and impact energy. Finally, the industrially produced DH36 samples were analyzed, and the steel with the lower S content had a higher stability and yield strength, in agreement with our laboratory results.

Stress intensity factor (SIF) is one of three important parameters in classical linear elastic fracture mechanics (LEFM). The evaluation of SIFs is of great significance in the field of engineering structural and material damage assessment, such as aerospace engineering and automobile industry, etc. In this paper, the SIFs of a central straight crack plate, a slanted single-edge cracked plate under end shearing, the offset double-edge cracks rectangular plate, a branched crack in an infinite plate and a crucifix crack in a square plate under bi-axial tension are extracted by using the p-version finite element method (P-FEM) and contour integral method (CIM). The above single- and multiple-crack problems were investigated, numerical results were compared and analyzed with results using other numerical methods in the literature such as the numerical manifold method (NMM), improved approach using the finite element method, particular weight function method and exponential matrix method (EMM). The effectiveness and accuracy of the present method are verified.

In this paper, the effects of a super-gravity field with multi-rotational speeds on the grain refinement and tensile properties of as-cast H13 steel were investigated systematically. The experimental results showed that compared to the single-rotational speed (conventional) super-gravity field, the as-cast grains of H13 steel can be significantly refined in a multi-rotational speed (speed increased in stages) super-gravity field. In the conventional super-gravity field, with the decrease in rotational radius, the secondary dendrite arm spacing (SDAS) and the prior austenite grain size (PAGS) increase, and the maximum values of SDAS and PAGS are 90 and 55 µm, respectively, while in multi-speed super-gravity fields, at the range of increasing rotational speeds, SDAS and PAGS decrease as the rotational radius decreases. In the three-rotational speed super-gravity field, the maximum values of SDAS and PAGS are 80 µm and 50 µm. In the five-rotational speed super-gravity field, the maximum values of SDAS and PAGS are reduced to 58 µm and 34 µm. Accordingly, both the tensile strength and the plasticity are enhanced when increasing the number of rotational speeds in the super-gravity field, especially for the inner position of the super-gravity sample. The ultimate tensile strengths at outer, middle, and inner positions of H13 steel solidified in the conventional super-gravity field are 1445 MPa, 1378 MPa, and 1023 MPa, corresponding to elongations of 2, 1.5, and 0.5%, respectively, while in the five-rotational speed super-gravity field, they are 1408, 1443, and 1453 MPa, corresponding to elongations of 1.8, 3.9, and 2.2%, respectively. The mechanism for the grain refinement is that multi-speed super-gravity can reduce the critical nucleation work of austenite and the tangential force produced by increasing the rotational speed break dendrites at the solidification front, refining the solidified structure.