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Herein, a new kind of overaging strategy: two steps of overaging for tailor rolled blank of dual phase steel (DP-TRB) was investigated. The results indicate that two steps of the overaging process is a useful way to control the mechanical properties of DP-TRB. In the premise of satisfying the requirement for the strength of DP590 grade, the total elongation can be significantly increased (3~7% in most cases). Due to the different degrees of ferrite recrystallization (differences of densities of dislocation) among the thicknesses, the obvious changes of mechanical properties among thicknesses are found. The thicknesses zones of 1.0~1.4 mm show lower strength, while the zones of 1.6~1.8 mm present higher strength. Otherwise, the high density of dislocations in samples of 1.6~1.8 mm provide more locations for Cottrell atmospheres, yield plateau occur easier. The zones with different thicknesses for one DP-TRB show two kinds of yield behaviors (continuous yield and non-continuous yield) simultaneously. The subtle C diffusion control by two step overaging leads to the quite different pinning effect of Cottrell atmospheres. Thus, the pinning effect occurs in a gradual way, and a transition state of yield behavior, which combines the characteristic of smooth curve in continuous yield and the plateau in non-continuous yield, is found.

The long production time required for large-scale parts fabricated by laser powder bed fusion (LPBF) tends to induce cracks, distortions, and overheating problems. In this work, to address these challenges, we explored and established a suitable strategy for producing large AlSi10Mg components. The platform temperatures to prevent cracks and distortions were firstly determined. Then, the in situ aging behavior was investigated for samples under various platform temperatures and holding times. Our results revealed that platform temperatures of 150°C and 200°C can effectively prevent cracks and minimize distortions. Besides, using 150°C, samples can reach peak hardness with a holding time less than 13 h. In comparison, those samples produced with a holding time longer than 13 h at 150°C and 200°C show obvious over-aging responses and thus lower hardness. However, such a hardness impoverishment can be recovered by using a T6 post-process heat-treatment.

18% Ni maraging steels are based on the Fe-Ni-Co-Mo system with low-carbon content. They display an excellent combination of high strength and high toughness. However, they suffer from a low fatigue ratio, the ratio decreasing monotonically with increasing strength. Considering prospective applications for these steels involving fatigue loading, attempts were made by researchers to improve their fatigue life. The studies suggest that fatigue strengths higher than those realized in peak aged condition can be obtained through controlled overaging with a small amount of reversed austenite playing a critical role. Corrosion fatigue in different environments is a serious problem with these steels. Double aging seems to reduce the susceptibility to corrosion fatigue under high humidity conditions. Inclusion content has a strong influence on the fatigue life, inclusion size and type playing an important role. Surface treatments such as shot peening, laser peening, and nitriding were found to improve the fatigue life; however, it is important to optimize the process parameters. This paper attempts a critical review of studies reported in the published literature aiming to improve the fatigue life of 18% Ni maraging steels.

Continuous annealing simulation is used in studying the influence of continuous annealing process parameters on the microstructure and mechanical properties of a GPa-grade C-Si-Mn-Cr-Mo dual-phase steel. The experimental results indicate that increasing soaking time increases the volume fraction of martensite and the size of martensite islands, as well as tensile strength (TS) and yield strength (YS) while decreasing plasticity. As the steel slowly cools to a lower temperature prior to final quenching, TS and YS decrease, whereas elongation increases. The decrease in martensite content is due to the partial decomposition of austenite into ferrite during long slow cooling before quenching. As overaging temperature increases because of the tempering of martensite and aging of ferrite, TS decreases and YS increases. Work hardening analysis shows that in the initial stage of deformation, low overaging temperatures enhance work hardening ability.

JSW Steel has been producing a number of high strength dual phase steels for automotive applications. These steels are processed through the state-of-the-art continuous annealing lines. The cold rolled steel sheets are passed through different zones including preheating, heating, soaking, slow cooling, rapid cooling, overaging and final cooling in a very short time. The strengthening mechanisms in these processes such as phase transformation, precipitation etc. are a strong function of line speed and strip temperatures. Minor change in any of these parameters causes significant variations in mechanical properties. Optimization of these process parameters becomes challenging due to the associated costs involved in plant scale experimentation. The same can be effectively carried out on a lab scale using thermo-mechanical simulator “Gleeble”. Processing routes for the different annealing cycles can be converted into time-temperature plots and programmed in Gleeble for lab scale continuous annealing line (CAL) simulations. In the present work, an offline simulation methodology was established for CAL process through Gleeble. The mechanical properties in Gleeble simulated samples were found to be in close agreement with the actual plant trials. Furthermore, the established methodology was successfully utilized to investigate the effects of different CAL parameters on the mechanical properties of a high strength dual phase steel grade. Consequently, on the basis of physical simulations, the optimum processing conditions were identified to achieve desired mechanical properties during plant processing.

The effects of alloying elements, Cr and Si, and the conditions of hot-dip galvanizing on the aging behaviors and their related tensile and torsional properties in cold drawn hyper-eutectoid steel wires were investigated. The Cr and Si additions were found to increase tensile strength, but decrease the limit of drawing strain without delamination in cold drawn steel wires. The Cr and Si additions encouraged the suppression of age hardening and age softening during annealing. The increased amount of deformation in cold drawn steel wires made age hardening occur at the lower annealing temperature and shorter annealing time, and expanded the delamination region to high temperature during annealing. The additions of Cr and Si did not show the noticeable improvement of tensile strength without delamination in cold drawn steel wires, but increased the upper limit of tensile strength without delamination in cold drawn and annealed steel wires.

Cold-rolled advanced high-strength steel sheets have become the material of choice for the automotive industry because of their unique attributes of high strength and balanced mechanical properties. High-hydrogen gas jet cooling and water quenching are the most commonly used ultrafast cooling technologies for producing martensite-containing high-strength steel sheets. The water quenching technology ensures the fastest industrial cooling rate of 1 000 K/s;therefore,it has the highest potential with respect to saving alloys. In this study,the water quenching of a C-Mn-containing steel sheet is simulated during continuous annealing to investigate the effect of water quenching and tempering parameters on is mechanical properties. The results reveal that at low quenching temperatures,the strength of the steel sheet decreases as the soaking temperature increases. However, at high quenching temperatures,a high soaking temperature corresponds to increased strength after quenching,regardless of whether the material was austenitized in the single austenite zone or the inter-critical zone. Therefore, a high quenching temperature always results in a high strength and a high yield ratio after quenching. Low-temperature overaging % tempering) considerably influences the yield strength and yield ratio, and the extent of this influence is correlated with the soaking temperature.

In order to optimize the production process, improve the production efficiency and accelerate the development and application of the domestic dual-phase steel, the effects of heat treatment process parameters on microstructure and mechanical properties of DP440 cold rolled strip were studied by the CAS-300 simulated continuous annealing equipment. When the heating rate increased from 5 to 100 ℃/s, both the tensile strength and yield strength increased because of the decreased grain size. When the intercritical annealing temperature increased from 780 to 850 ℃, the martensite content decreased so that the tensile strength decreased first, then increased, and the yield strength increased. When the rapid cooling rate increased from 25 to 100 ℃/s, because the martensite content increased, the tensile strength increased, while the yield strength decreased. When the overaging temperature increased from 260 to 400 ℃, the tensile strength decreased, while the yield strength increased. When the overaging time increased from 240 to 480 s, the tensile strength decreased a little, while the yield strength increased a little.

First principles plane wave pseudopotential method was executed to calculate the mechanical properties with respect to the uranium-0.95 mass fraction of titanium (U-0.95 mass fraction of Ti) alloy for quenching and aging, including the elastic modulus, the value of shear modulus to bulk modulus ( G / B ) and the ideal tensile strength. The further research has also been done about the crack mechanism through Griffith rupture energy. These results show that the elastic moduli are 195.1 GPa for quenching orthorhombic α ´ phase and 201.8 GPa for aging formed Guinier-Preston (G.P) zones, while G / B values are 0.67 and 0.56, respectively. With the phase change of uranium-titanium (U-Ti) alloy via the quenching treatment, the ideal tensile strength is diverse and distinct with different crystal orientations of the anisotropic α ´ phase. Comparison of quenching and short time aging treatment, both of the strength and toughness trend to improve slightly. Further analysis about electronic density of states (DOS) in the electronic scale indicates that the strength increases continuously while toughness decreases with the aging proceeding. The equilibrium structure appears in overaging process, as a result of decomposition of metastable quenching α ´ phase. Thereby the strength and toughness trend to decrease slightly. Finally, the ideal fracture energies of G.P zones and overaging structure are obtained within the framework of Griffith fracture theory, which are 4.67 J/m 2 and 3.83 J/m 2 , respectively. These results theoretically demonstrate strengthening effect of quenching and aging heat treatment on U-Ti alloy.

Precipitation strengthening of Cu-3Ti-1Cd alloy has been studied using hardness and tensile tests, electrical resistivity measurements, and transmission electron microscopy. The alloy exhibited a hardness of 117 Hv in solution-treated (ST) condition and attained a peak hardness of 288 Hv after aging at 450 °C for 72 h. Electrical conductivity increased from 7%IACS (International Annealed Copper Standard) in ST condition to 13%IACS on aging at 450 °C for 16 h. The alloy exhibited yield strength (YS) of 643 MPa and ultimate tensile strength (UTS) of 785 MPa in peak-aged (PA) condition. Strengthening in Cu-3Ti-1Cd alloy in PA condition is attributed to solid solution strengthening effect of cadmium (Cd) as well as fine scale precipitation of metastable and coherent β′-Cu4Ti phase. On overaging at 450 or 500 °C, the alloy showed a decrease in hardness as a result of formation of equilibrium precipitate β-Cu3Ti as continuous precipitation within the matrix and as discontinuous precipitation at the grain boundaries. While the tensile properties are better, the electrical conductivity of Cu-3Ti-1Cd alloy is less than that of binary Cu-2.7Ti alloy. The strengthening mechanism is the same in both binary and ternary alloys of Cu-Ti, i.e., precipitation of metastable and coherent β′-Cu4Ti phase.