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By designing the orthogonal experiment of resistance spot welding process, the range of spot welding process parameters of Q & P980 galvanized high-strength steel was determined, meanwhile the microstructure characterization and the mechanical properties analysis of the welding joint were carried out. The results show that the microstructure of nugget zone is mainly staggered lath martensite, whereas the microstructure of HAZ is composed of lath martensite, residual austenite and ferrite. The maximum average width of martensite lath in incompletely quenched zone is 4.86 μm.The microhardness test exhibits that the hardness value of the welding joint shows a "W"-shaped symmetrical distribution. The peak hardness appears in the fine-grain zone, which is 559HV, while the minimum hardness appears in the incompletely quenched zone, which is 338HV. The incompletely quenched zone shows obvious softening phenomenon. The room temperature tensile tests of the welding joints are carried out, and the peak value of maximum tension-shear load is 27.92 kN. The fracture morphology shows typical dimples, which belongs to ductile fracture.Since the melting point of zinc is lower than that of the steel matrix, the zinc layer is prior to melting and penetrates downward along the grain boundary of the matrix at the welding joints, and then the liquid metal embrittlement cracks can be observed obviously.

The aim of the present work is to evaluate the effect of ultrafast heating on the microstructure and mechanical properties of hot forming steel. The initial microstructure utilized in this study was a cold-rolled microstructure, and the test steel was heated to full austenitization at a rate of 200 °C/s, followed by water quenching. It was observed that the ultrafast heating process significantly refines both the prior austenite grains and martensite laths while inheriting high-density dislocations from the initial cold-rolled microstructure. Consequently, the coupling mechanism between dislocation strengthening and grain refinement strengthening remarkably enhanced both the yield strength and ultimate tensile strength of the test steel. Eventually, the yield strength of the hot forming steel reached 1524 MPa, along with an ultimate tensile strength of 2221 MPa and uniform elongation of 5.2%. [ABSTRACT FROM AUTHOR]

Friction stir processing (FSP) manufacturing technology was used to fabricate medium Mn advanced high-strength steel in this study. The mechanical properties and microstructure of the steel fabricated using FSP were investigated. The steel obtained a total elongation of 35.1% and a tensile strength of 1034.6 MPa, which is about 59% higher than that of the steel without FSP. After FSP, a gradient structure occurs along the thickness direction. Specifically, across the thickness direction from the base material zone to the transition zone and finally to the stirring zone, both the grain size and austenite fraction decrease while the dislocation density increases, which results from the simultaneous effect of severe plastic deformation and recrystallization during FSP. Due to the gradient structure, an obvious difference in the strain across the thickness direction of the steel occurs during the deformation process, resulting in significant hetero-deformation-induced (HDI) strengthening. The deformation mechanism analysis reveals that HDI strengthening and dislocation strengthening are the main factors in the improvement in the strength–ductility balance. The obtained knowledge sheds light on the process of fabricating medium Mn steels with excellent properties using FSP manufacturing technology. [ABSTRACT FROM AUTHOR]

Generally, the development of ship plate steels is mainly concerned with the improvement of strength and toughness, such as F32 and F36. Due to the strength–ductility trade-off, it is difficult to combine excellent ductility with strength improvement, resulting in a poor deformation ability of the traditional ship plate steels during collision. In the present study, a series of high-ductility ship plate steels with property gradients were obtained by multi-phase microstructure control. The strength–ductility matching mechanism was analyzed. Meanwhile, the roles of M/A islands and lamellar pearlites in plastic deformation were also revealed. The results show that the microstructure of "quasi-polygonal ferrite + granular bainite + M/A islands + fewer lamellar pearlites" has the best strength–ductility match. The excellent ductility is mainly dependent on dispersive kernel average misorientation, recrystallized grains without distortion, and soft grains. In addition, the longer branch crack can effectively relieve the stress concentration at the tip of the main crack. Compared with lamellar pearlites, the dispersed M/A island grains have a higher strength contribution and more stable γ-fibers, which is beneficial to delay the appearance of internal micro-voids and micro-cracks. However, the lamellar pearlites can coordinate deformation only when the orientation of thinner lamellae exceeds two. [ABSTRACT FROM AUTHOR]

In order to produce a more isotropic Al-Mg-Si alloy sheet in the T4 temper, pre-annealed AA6016 hot rolled strips were cold rolled by conventional rolling, transverse rolling and an alternative rolling schedule. In this work, a weak and random texture was obtained in the alternative rolling sample, which was treated by solution treatment, as well as a fine and homogeneous recrystallized grain structure that reduced the earing percentage from 8.2% to 2.7%. An earing test was used to characterize the planar anisotropy of the AA6016 alloy. The crystallographic texture and microstructure of the AA6016 aluminum sheet were analyzed by X-ray diffraction and electron back scatter diffraction (EBSD) techniques. The earing percentage was reduced and eight-fold ears were produced in unidirectional rolled samples with strong β-fiber textures. Conversely, a higher earing percentage and four-fold ears were obtained for the alternative rolled sample, which was characterized by an appropriate combination of the Cube {001}<100> and stabilized BND {011}<3 2 2> [(φ1,¢,φ2) = (42°,45°,0°)] orientations. The main reason for the distinct earing profiles was the grain spatial distribution of textures formed during the cold rolling process. The dynamic recovery of the AA6016 alloy was promoted and both the nucleation and subsequent growth of the Cube orientation were suppressed by alternative rolling. [ABSTRACT FROM AUTHOR]

The effect of La addition on primary α-Al and the eutectic Si phase of Al-7Si alloy is investigated systematically in this work. The results indicate that La addition causes a multi-refining efficiency on the microstructure of Al-7Si alloy, including refinement of α-Al grains and secondary dendrite arm spacing as well as eutectic Si particles. The grain size, secondary dendrite arm spacing and area of eutectic Si particles are decreased by 26.8%, 7.7% and 26.7%, respectively, with the addition of 0.1 wt.% La. It is also found that La-rich phases of Al2Si2La form and distribute in the vicinity of the eutectic Si phase. The crystal structure and lattice parameter of Al2Si2La phase are determined to be hexagonal (a = b = 0.405 nm, c = 6.944 nm) based on the TEM analysis results. The multi-refinement effects are mainly attributed to the increased constitutional undercooling caused by the low solubility of La in Al alloy and the growth-restricting factor caused by the Al2Si2La phase. [ABSTRACT FROM AUTHOR]

The dissimilar welded joints of Q&P980 and Q&P1180 steels with and without paint-baking heat treatment were investigated and their microstructures were characterized using confocal laser scanning microscope (CLSM), electron backscatter diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM) in order to understand the change of mechanical performances of the welded joints. The two Q&P steels have slight differences in compositions, but the obviously varying fraction of tempered martensite and thus the difference in the tensile strength. Prior to paint-baking heat treatment, the dissimilar welds exhibit a lower martensite fraction of the intercritical heat-affected zone (ICHAZ) in the Q&P980 side compared to the Q&P1180 side, and it is not affected by the paint-baking treatment. After the treatment, the occurrence of carbides and the coarsening of martensite in the whole welded joint, as well as the decomposition of retained austenite in the ICHAZ, are observed. The paint-baking treatment also causes the decreasing microhardness values of all regions. Especially, the significantly decreasing microhardness in the fusion zone compared to other heat-affected zones results in a partial pull-out failure mode compared to a full pull-out failure mode without paint-baking treatment. The change in failure mode and the softening of the martensitic matrix are found to be responsible for the increase in the peak load and the energy absorption of the dissimilar welded joint of Q&P980 and Q&P1180 steels. • Microstructures of the dissimilar welded joints of Q&P980/Q&P1180 were characterized. • Paint-baking treatment improved the mechanical performance of the dissimilar welded joints. • The failure mode of the dissimilar welded joints changed after the paint-baking treatment. • The significantly decreasing microhardness in the fusion zone was caused by the paint-baking treatment. • The mechanism of increasing the mechanical performance of the dissimilar welded joints was proposed. [ABSTRACT FROM AUTHOR]

The load partitioning between inter-phases and intragranular of austenite, their dislocation density evolution, and mechanical behavior were investigated in two medium Mn steels by in-situ high-energy synchrotron x-ray diffraction during tensile loading. The investigated steels were produced with different hot rolling parameters resulting in different initial microstructures. The initial microstructures were characterized using scanning electron microscope and electron backscatter diffraction analysis. Transmission electron microscope with high angle annular dark field technique was furthermore used to provide the fine microstructural features of deformed specimens in order to supplement the in-situ analysis. Despite the differences in the initial microstructure, the load partitioning from the softer ferrite to the stronger α '-martensite (α ') was observed; while different degrees of load partitioning for intragranular of γ occurred. The sample with a relatively low degree of load partitioning exhibits the intersection of two bundles of stacking faults (SF), one with high density SF, and the other with low density and limited SF, as well as the occurrence of α′ in the intersecting region. The secondary SFs and the relatively intermediate density of primary SFs bundle are characterized in γ for the sample with relatively high degree of load partitioning. The load partitioning between intragranular of γ is found to have a coordination relationship with the dislocation density evolution of γ ; after the load partitioning, dislocation density of γ rapidly rises. The sample with intersection of two bundles SFs and the occurrence of α′ , exhibits the higher transformation rate of γ into ε -martensite/ α ', resulting in the larger work hardening rate. In addition, the early load on α ' is found to contribute to a higher yield strength. These results reveal the importance of assessing both phase-specifc strain/stress and the dislocation density evolution in order to predict the macroscopic mechanical properties of medium Mn steels. [ABSTRACT FROM AUTHOR]

Changes in the mechanical stability of austenite with strain state is a significant aspect that needs to be addressed in the use of medium Mn steel in automobile body construction. New insights are provided in this paper to understand strain-state-dependent austenite stability in a 5 wt% Mn-containing steel based on the analysis of the Schmid factor by electron backscatter diffraction and the calculation of the work done by the applied stress. Four strain states, namely uniaxial tension, simple shear, plane strain, and equibiaxial stretch, were considered and digital image correlation was used to capture the strain field during the entire deformation process. Results show that the mechanical stability of austenite decreases when strain state changes from uniaxial tension to plane strain, and further decreases in the case of equibiaxial stretch due to the increasing work done by the applied stress and the increasing Schmid factor. The austenite acquires the highest mechanical stability when it is deformed at the simple shear state, which corresponds to the lowest work done by the applied stress. The obtained knowledge promotes the basic understanding for the comprehensive application of medium Mn steels in the automobile industry. [ABSTRACT FROM AUTHOR]