The constant demand for increasing the strength without ductility loss and production cost encourages industrial and academic societies to propose novel heat treatment processing of commercial steel grades. To improve the mechanical properties of commercial spring steel, a novel quenching and partitioning (Q&P) processing was designed to deliver a complex and desirable nanostructured multicomponent microstructure by controlling the carbon partitioning kinetics. Furthermore, the partitioning of excessive carbon from saturated martensite into untransformed austenite enhances the formation of transition carbides during tempering between 130 and 280 °C. Electron microscopy confirmed a complex multicomponent structure containing BCC tempered lath combined with retained austenite and nanocarbides particles within the tempered laths. Such multicomponent lath-type structure obtained by designed Q&P heat treatment on commercial carbon-silicon spring steel revealed localized mechanical resistance varying from 4.92 GPa for the QP-220-375-400 to 8.22 GPa for the QP-220-325-400 samples determined by nanoindentation test. Moreover, the tensile test showed high ultimate tensile strength and a yield strength up to 1400 MPa and 975 MPa, respectively, in the QP-220-375-400 sample due to a set of complex multicomponent lath-type refined structures designed by Q&P coupled with bainitic transformation, with good strain to fracture (∼0.12%). 

Development of a Complex Multicomponent Microstructure on Commercial Carbon-Silicon Grade Steel by Governing the Phase Transformation Mechanisms to Design Novel Quenching and Partitioning Processing

Constitutional supercooling and corresponding microstructure transition triggered by high magnetic field gradient during directional solidification of Al-Fe eutectic alloy

A new short-time procedure for fatigue life evaluation based on the linear damage accumulation by Palmgren-Miner

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.

https://www.mdpi.com/2075-4701/12/10/1657/pdf?version=1665569410

Strength–Ductility Matching Mechanism for Multi-Phase Microstructure Control of High-Ductility Ship Plate Steel

The microstructure and mechanical properties of a Nb–V microalloyed steel produced by strip casting in as-coiled and aged conditions were investigated using scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, transmission electron microscopy, atom probe tomography and tensile tests. Tensile test results show that, after ageing at 650 °C for 5 min, accompanying an increase of yield strength from 698 ± 3.6 MPa to 758 ± 7.9 MPa, the elongation increases from 7.1 ± 0.4% to 12.6 ± 0.4% simultaneously. Microstructural analysis shows that both the as-coiled and aged specimens possess a microstructure predominantly comprising a mixture of acicular ferrite and bainitic ferrite. Hardness tests for specimens aged at different temperatures show very similar peak hardness values but higher ageing temperature requires less time for peak ageing. Ageing treatment leads to a decrease of dislocation density, which is responsible for the enhancement of elongation for aged specimens, while Nb–N-rich and Nb–V–N-rich clusters develop successively during ageing, which contributes to the increase of yielding strength after ageing. The strengthening contributions from solid solution, dislocations, grain boundaries and clusters were calculated, and the results indicated that dislocations and grain boundaries are the dominant strengthening mechanisms for the current strip casting steels. 

Bijan Khezri

Papers

Morphological and crystallographic features of granular and lath-like bainite in a low carbon microalloyed steel