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The effects of the second phase on the tensile properties of a high‐Mn high‐Al austenitic lightweight steel processed by thin‐strip casting and subsequently aged at 600 °C for different times are studied. Depending on the aging time, the second phase includes short‐range ordered (SRO) phase, nanosized κ phase, and/or microsized κ phase. At aging time of 1 min, the 2 nm SRO phase precipitates and evolves into 6 nm intragranular κ phase when the time is 60 min, and microsized intergranular κ phases exist at austenite grain boundaries at aging time of 480 min. The precipitation of SRO phase increases the yield strength from 470 to 610 MPa, and the elongation only decreases from 55.9% to 52.1%, with ductile fracture for 380 MJ m−3. Although the precipitation of intragranular κ phase increases the yield strength to 910 MPa, but elongation decreases to 33.4%, there is a mixture of ductile and brittle fracture for 316 MJ m−3. The presence of intergranular κ phase increases the yield strength to 960 MPa, but reduces the elongation to only 3.6% with brittle fracture for 35 MJ m−3. All the second phases reduce strain hardening rate, and SRO has smaller effect. [ABSTRACT FROM AUTHOR]

With the rapid development of low-carbon high strength steel, higher requirements are put forward for the matching welding consumables. The deposited metals with 0.62–2.32% Cu addition was prepared by tungsten inert gas welding via metal cored wire. The effect of Cu element on microstructure and mechanical properties of deposited metals were investigated. The multiphase microstructure of deposited metals consists of bainite, martensite, residual austenite, and martensite-austenite constituents. It is found that Cu decreases the start temperature of martensite (Ms) and enlarges the temperature range of bainite from 372 K to 416 K, improving the formation of bainite. With the increase of Cu content, the fraction of martensite decreases and the shape of M-A constituents changes from strip into granular. There are BCC and FCC Cu precipitates in deposited metals. The diameter of Cu precipitates is 14–28 nm, and the volume fraction of it increases with the increase of Cu content. Meanwhile, the deposited metals with 1.79% Cu can achieve a 10% enhancement in strength (yield strength, 873–961 MPa, ultimate tensile strength, 1173–1286 MPa) at little expense of impact toughness (64.56–56.39 J at −20 °C). Cu precipitation can effectively improve the strength of the deposited metals, but it degrades toughness because of lower crack initiation energy. The deposited metal with 1.79% Cu addition shows an excellent strength-toughness balance. [ABSTRACT FROM AUTHOR]

The quenching-partitioning-tempering (QPT) multi-step heat treatment process was employed to enhance the strength-toughness balance of low-carbon ultrastrong steel based on the controllable transformation-induced plasticity (TRIP) effect and co-precipitation strengthening. The aging embrittlement of ultrastrong steel, with an impact toughness of only 5.8 J at peak strength, has been effectively addressed through the QPT process. The sufficient reversed austenite (RA) can be obtained by controlling the partition temperature to 650 °C, followed by aging at 550 °C to fully re-precipitate nanoparticles. The volume fraction of RA in QPT650 steel reached 16.4 %, and the number density of nanoparticles increased dramatically, resulting in a high yield strength of 1233 MPa and excellent impact toughness of 60.4 J. The coarsening and re-precipitation of nanoparticles were clarified, and the strengthening increments of QPT650 steel reaches 584.6 MPa, and the substantial enhancement of Orowan strengthening is responsible for the high yield strength. The excellent impact toughness of QPT650 steel is attributed to the obvious TRIP effect of RA that consumes a large amount of crack propagation energy, and a small number of sheared nanoparticles provide limited micro-cracks, resulting in a strong crack resistance. • The strength-toughness strategy of the controllable co-precipitation strengthening and TRIP effect was proposed. • The sufficient reversed austenite of 16.4 % and high-density nanoparticles were obtained through the QPT process. • The QPT650 steel simultaneously achieves a high yield strength of 1233 MPa and excellent impact toughness of 60.4 J. • The enhancement of Orowan strengthening and TRIP effect are responsible for excellent strength-toughness. [ABSTRACT FROM AUTHOR]

The multi-pass deposited metals were prepared by metal-cored wire with low (2.5 wt%) and high (4.0 wt%) Ni to research the effect of Ni on the bainite/martensite transformation. Results showed that deposited metals exhibited a multiphase structure comprised of bainite, martensite and residual austenite, which is not only explained from SEM/TEM, but also identified and quantified each phase from crystallographic structure through XRD and EBSD. With Ni content increasing, the fraction of martensite increases from 37% to 41%, and that of bainite decreases from 61% to 55% accordingly because 4% Ni element narrows the temperature range of the bainite transformation ~20 °C. The 7.8% residual austenite exhibited block and sheet in the deposited metal with low Ni, while the fraction of residual austenite was 3.26% as a film with high Ni, caused by different transformation mechanisms of bainite and martensite. The tensile strengths of deposited metals were 1042 ± 10 MPa (2.5% Ni) and 1040 ± 5 MPa (4% Ni), respectively. The yield strength of deposited metals with high Ni was 685 ± 18 MPa, which was higher than low Ni due to the high fraction of martensite. The impact values of deposited metals with high Ni content decreased because the volume fraction of bainite and residual austenite and area fraction of large-angle grain boundary were lower. [ABSTRACT FROM AUTHOR]

Abstract Simulated inter-critical heat affected zones (ICHAZ) of a large thickness bainitic steel were prepared with three different peak temperatures (T p) of 750 °C, 780 °C and 800 °C. The microstructure and crystallographic feature were investigated. Toughness of simulated ICHAZ specimens was assessed using instrumental Charpy impact test at − 40 °C. It was found that the volume fraction of newly formed bainite increased and the effective grain size decreased with the increase in inter-critical T p. The toughness decreased slightly with the T p of 750 °C compared with the base metal (BM), while the toughness improved sharply when the T p exceeded 780 °C. The network-like martensite was primarily responsible for the low crack initiation energy with the T p of 750 °C. In contrast, the fine bainite formed with the T p above 780 °C was effective in increasing the crack nucleation resistance, resulting in the high initiation energy. The matrix softening as well as the grain refinement played important roles in improving the crack propagation energy. Furthermore, it was confirmed that the presence of a multi-phase microstructure can lead to the scattering of toughness. [ABSTRACT FROM AUTHOR]

Abstract The inter-critical heat-affected zone (ICHAZ) and fine grain heat-affected zone (FGHAZ) of a 690 MPa grade rack plate steel were simulated by Gleeble3500 simulator. The toughening mechanism of ICHAZ was clarified by analyzing microstructural characteristics and instrumental Charpy impact tests. The study showed that the ICHAZ had higher toughness than FGHAZ. And the microstructure in FGHAZ was martensite, while bainitic microstructure was obtained in ICHAZ. Both the prior austenite grain size and effective grain size of ICHAZ were smaller than that of FGHAZ, which played an important role in improving the toughness of ICHAZ. It was found that the extensive plastic deformation prior fracture in ICHAZ could consume large energy during crack initiation and propagation. The large plastic deformation capacity of ICHAZ was related to the large fraction of large-angle boundaries and low dislocation density, as well as the homogeneous distribution of local strain and geometrically necessary dislocations. Highlights • Relationship between toughness and microstructure was confirmed. • The toughness of ICHAZ is higher than that of FGHAZ. • Fine bainitic microstructure in ICHAZ plays an important role on improving toughness. • The homogeneous distribution of dislocation and local strain facilitate the plastic deformation prior fracture. [ABSTRACT FROM AUTHOR]

To obtain a better strength-toughness balance of martensite-austenite dual phase deposited metals, an austenite reversed treatment (ART) with short holding time (namely S-ART), combining with the advantage of quenching & partitioning (Q&P) is proposed. The dual phase deposited metal is heated to the intercritical region (620 °C) for austenite reversion with a short holding time of 0.5 h. Compared to the conventional ART (C-ART) with the holding time of 2.0 h, S-ART provides significant improvement in yield strength, elongation and impact toughness. Due to the stronger partitioning effect of carbon, the carbon concentration of the retained austenite (RA) during S-ART is higher than that of C-ART and as-welded. Therefore, the stability of the RA in S-ART state increases dramatically and induces sustained transformation induced plasticity (TRIP) effect, which contributes to the enhancement of yield strength, ductility and toughness. Moreover, after S-ART, the volume fraction of RA increases and the dislocation density in martensite decreases significantly, further improving the ductility and toughness. [ABSTRACT FROM AUTHOR]

The recrystallization behavior of a high-Mn high-Al lightweight steel (Fe-28.4Mn-8.3Al-1.27 C (wt%)) with 50% cold rolling deformation is investigated at the annealing temperature of 800 °C, 900 °C and 1000 °C. The detailed microstructure evolution is characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The partial recrystallized bimodal austenite grains and κ-phase (Fe, Mn) 3 AlC form at 800 °C, while fully recrystallized austenite grains without κ-phase appear at 900 °C and 1000 °C. With the increase in annealing temperature, the increased frequency of annealing twin boundaries reduces the average austenitic grain size and the dispersion of the austenite grain size effectively. The effect of austenite grain size on the tensile properties is discussed. The strain hardening behavior is also investigated by Hollomon analysis and C-J analysis, and the later one is better to explain the strain hardening behavior in different four stages. The recrystallization behavior significantly improves the tensile toughness from 59 MJ/m 3 to 436 MJ/m 3 at the expense of tensile strength decrease from 1460 MPa to 890 MPa, due to the homogenous fine austenite grains and high frequency of high misorientation angle. The steel with fully recrystallized austenite grains of 10.0 µm annealed at 900 °C exhibits the optimum mechanical properties with excellent tensile strength of 965 MPa, ductility of 48.3%, and good toughness of 400 MJ/m 3 . [ABSTRACT FROM AUTHOR]