While strength and toughness properties of construction steels are major mechanical properties with respect to the safety assessment of components, increasingly often requirements are defined on the processing properties of materials, in particular cold formability properties. So far, the cold formability of structural steels is characterized in terms of three-point bending tests, which result in very expensive experimental effort and limited understanding of the material behaviour under various stress states. In this paper, the cold formability of a structural steel S355J2 + N is characterized under various stress states in a laboratory scale by a recently proposed hybrid damage mechanics approach. A damage initiation criterion that considers the influence of stress triaxiality and Lode angle is used to describe the onset of material degradation in a microstructural scale in the approach. Subsequent damage evolution is followed to further quantify the accumulation of damage till the final fracture. A series of laboratory tests is designed to calibrate the model parameters as well as to verify the calibration. Good agreement of experimental and numerical force–displacement responses proves the predicative capability of the model. With the laboratory findings, the industrial scale three-point bending test is simulated to predict the forming limit. In addition, the numerical study reveals that the stress state of the critical element in three-point bending test coincides with plane-strain tension, which results in a simpler method to characterize the bendability to reduce the experimental effort.

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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.

This study investigated the influence of pretreatment conditions of helium/oxygen (He/O2) atmospheric pressure plasma, including treatment duration, oxygen flow, and distance between the nozzle and the sample (DBNS), on sizing properties of cotton roving. Results indicate that plasma treatment can effectively improve the surface roughness, static friction coefficient, and the wettability of raw cotton fibers, as well as the absorption ability of cotton rovings for starch size. Consequently, sizing adhesion strength (SAS) and breaking elongation (BE) of the roving sized by starch are greatly influenced by the treatment. They first rise and then slightly drop with the increase of treatment duration or oxygen flow, but decrease with the elongation of the DBNS. Compared with a roving without plasma pretreatment, pretreated rovings can possess 59% and 36% improvement of SAS and BE, respectively, by a chosen plasma treatment condition, i.e. 15 s of treatment duration, 1.5 mm of DBNS, 30/0.3 L/min of He/O2, and 40 W of the power.

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