Influence of coiling temperature (CT) on the microstructure and mechanical properties of a hot-rolled high-strength steel microalloyed with Ti, Mo and V was elucidated. The precipitation behavior of nano-sized particles was investigated by theoretical calculation and quantitative analysis. And the results revealed that V-enriched (Ti, Mo, V)C precipitated in the ferrite matrix. As the CT decreased, the site fractions of Ti in (Ti, Mo, V)C changed little, the site fractions of Mo increased, and the site fractions of V decreased accordingly. Moreover, the low CT could refine the microstructure and precipitated particles but suppress the precipitation of (Ti, Mo, V)C particles simultaneously, leading to the volume fraction of (Ti, Mo, V)C significantly decreasing, consequently causing an increment of grain refinement strengthening and a reduction in precipitation hardening. When the CT was 600 °C, the steel characterized by fine polygonal ferrite, a small amount of bainite and nano-sized (Ti, Mo, V)C precipitates exhibited the optimum mechanical properties with the ultimate tensile strength of 870 MPa, yield strength of 807 MPa and elongation to fracture of 17%.

Influence of coiling temperature on microstructure and mechanical properties of a hot-rolled high-strength steel microalloyed with Ti, Mo and V

Coherent second phase often exhibits anisotropic morphology with specific orientations with respect to both the second and the matrix phases. As a key feature of microstructure, the morphology of the coherent particles is essential for understanding the second-phase strengthening effect in various industrial alloys. This letter reports anisotropic growth of coherent ferrite from austenite matrix in pure iron based on molecular dynamics simulation. We found that the ferrite grain tends to grow into an elongated plate-like shape, independent of its initial configuration. The final shape of the ferrite is closely related to the misfit between the two phases, with the longest direction and the broad facet of the plate being, respectively, consistent with the best matching direction and the best matching plane calculated via the Burgers vector content (BVC) method. The strain energy calculation in the framework of Eshelby’s inclusion theory verifies that the simulated orientation of the coherent ferrite is energetically favorable. It is anticipated that the BVC method will be applicable in analysis of anisotropic growth and morphology of coherent second phase in other phase transformation systems.

Three-Dimensional Growth of Coherent Ferrite in Austenite: A Molecular Dynamics Study

Preparation of TA15 powder reinforced 45CrNiMoY alloy steel with high mechanical property by pre-laid laser cladding technology

This article reports a new generation of Q460 multi-functional construction structural steel, which has high strength (yield strength larger than 460 MPa), excellent toughness (higher than 110 J/cm2 at − 60 °C), lower yield ratio (lower than 0.8), good fire resistance (yield strength at 600 °C larger than two-thirds of its room-temperature yield strength) and better corrosion resistance. The effects of finish cooling temperature (FCT) on the microstructure and properties were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), emission electron probe micro-analysis (EPMA), electron backscattering diffraction (EBSD), tensile tester, impact tester, periodic immersion cycle acceleration test and electrochemical experiment. The results show that the strength and toughness are simultaneously improved by decreasing the FCT due to more lath-like bainite with large number of dislocations, refined martensite/austenite (M/A) with higher carbon concentration and increased high angle boundaries. In addition, the fire resistance of the newly developed Q460 steel is obviously better than the conventional one, which is mainly due to non-recrystallized lath-like bainite with high dislocation density at elevated temperature. The corrosion resistance of the new Q460 steel is also improved due to the addition of Cu and Cr. 

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Microstructures and Mechanical Properties of a New Multi-functional 460 MPa Grade Construction Structural Steel

Nitrogen and Sulfur in non-oriented electrical steel would form precipitates, which would severely affect its magnetic properties. Precipitates in compact strip production (CSP) process non-oriented electrical steel were investigated using a transmission electron microscope (TEM) and scanning electron microscopy (SEM). The precipitation mechanism and influence on grain growth were analyzed experimentally and theoretically. The results showed that the main particles in steel were AlN, TiN, MnS, Cu2S, and fine oxide inclusions. The spherical or quasi-spherical of MnS and Cu2S were more liable to precipitate along grain boundaries. During the soaking process, the amount of MnS precipitated on the grain boundary was much larger than that of Cu2S. AlN and TiN in cubic shape precipitated inside grains or grain boundaries. Precipitates preferentially nucleated at grain boundaries, and TiN, MnS mainly precipitated during soaking. In the subsequent processes after soaking, AlN and Cu2S would precipitate unceasingly with the decrease in the average size. The distribution density, the volume fraction, and the average size of the precipitates in the annealed sheets were 9.08 × 1013/cm3, 0.06%, and 54.3 nm, respectively. Precipitates with the grain size of 30–500 nm hindered the grain growth, the grains with 100–300 nm played a major role in inhibiting the grain growth, and the grains with the grain size of 70–100 nm took the second place.

Precipitates in Compact Strip Production (CSP) Process Non-Oriented Electrical Steel

Precipitation behavior of (Ti, V, Mo)C and microstructural evolution of the ferritic Ti–V–Mo complex microalloyed steel were investigated through changing coiling temperature (CT). It is demonstrated that the strength of the Ti–V–Mo microalloyed steel can be ascribed to the combination of grain refinement hardening and precipitation hardening. The variation of hardness (from 318 to 415 HV, then to 327 HV) with CT (from 500 to 600–625 °C, then to 700 °C) was attributed to the changes of volume fraction and particle size of (Ti, V, Mo)C precipitates. The optimum CT was considered as 600–625 °C, at which the maximum hardness value (415 HV) can be obtained. It was found that the atomic ratios of Ti, V and Mo in (Ti, V, Mo)C carbides were changed as the CT increased. The precipitates with the size of < 10 nm were the V-rich particles at higher CT of 600 and 650 °C, while the Ti-rich particles were observed at lower CT of 500 and 550 °C. Theoretical calculations indicated that the maximum nucleation rate of (Ti, V, Mo)C in ferrite matrix occurred around 630 °C, which was consistent with the 625 °C obtained from experiment results.

Zheng-Hai Zhu

Papers

Precipitation Behavior and Microstructural Evolution of Ferritic Ti–V–Mo Complex Microalloyed Steel

Kinetics of (Ti, V, Mo)C Precipitated in γ /α Matrix of Ti-V-Mo Complex Microalloyed Steel