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Microstructure and mechanical properties in core of a carburizing 20CrNi2MoV bearing steel subjected to cryogenic treatment were investigated. Conventional treatment sample was quenched and tempered at 180 °C for 2 h. Cryogenic treatment samples were quenched, cryogenically treated at − 80 and − 196 °C for 4 h, slowly returned to room temperature and thereafter tempered at 180 °C for 2 h, and finally tempered at 180 °C for 2 h. The scanning electron microscope, electron backscattering diffraction, X-ray diffraction and transmission electron microscope were adopted for microstructure characterization. The results show that cryogenic treatment increases the fraction of high-angle grain boundaries and the precipitation of finely dispersed carbides in the matrix, decreases the volume fraction of inter-lath retained austenite, and hence improves the strength and hardness. Compared with the conventional treatment, the hardness, yield strength and ultimate tensile strength of the steel after cryogenic treatment are increased by 11.7%, 12.6% and 18.3%, respectively, while the impact energy is decreased by 9.8%.

Electrical additive manufacturing can improve manufacturing efficiency and reduce the cost of 16MND5 reactor pressure vessel steel. Impact tests were conducted to compare the impact toughness of 16MND5 steels manufactured by the electrical additive manufacturing and conventional forging, respectively. It is found that the impact toughness of electrical additive manufacturing specimen was slightly higher than that of conventional forging specimen. The characterizations of microstructure show that there were large ferrites and carbides in electrical additive manufacturing specimen. The fracture mechanisms of electrical additive manufacturing specimen were that microvoids or microcracks were prone to nucleate at the large ferrite/bainite interface and large carbide/bainitic ferrite interface, where the stress concentration was high. In addition, the block size and high-angle grain boundaries played a vital role in hindering crack propagation of electrical additive manufacturing specimen, helping to improve the impact energy and leading to a low ductile–brittle transition temperature. The results suggest that the electrical additive manufacturing technology was an effective method to enhance the impact toughness of 16MND5 steel.

The two-pass isothermal hot compression method was used to study the effect of different thermal deformation conditions on static recrystallization behavior in Ni–Cr–Mo series SA508Gr.4N low alloy steel with interval holding time ranging from 1 to 300 s, temperature ranging from 950 to 1150 °C, strain rate ranging from 0.01 to 1 s−1, true strains ranging from 0.1 to 0.2, and initial austenite grain size ranging from 175 to 552 μm. It can be concluded that the static recrystallization volume fraction gradually increases with the increase in the deformation temperature, strain rate, strain and pass interval, and the decrease in the initial grain size, which is mainly due to the increase in the deformation energy storage and dislocations. Moreover, strain-induced grain boundary migration is the nucleation mechanism for static recrystallization of SA508Gr.4N low alloy steel. Based on the stress–strain curve, the predicted value obtained from the established static recrystallization kinetics model is in good consistence with the experimental value, and the static recrystallization thermal activation energy of SA508Gr.4N steel was calculated as 264,225.99 J/mol.

The effects of two alloying elements, i.e., yttrium (Y) and zirconium (Zr), on the inclusion, microstructure, tensile property and impact toughness of reduced activation ferritic/martensitic (RAFM) steel are analyzed. The size and number of particles were determined by optical microscopy, and the particle types were examined by scanning electron microscopy. The size of ~ 20% and ~ 85% of the inclusions lied in the range of 0.5–1.0 μm and 0.5–3.0 μm, respectively. In Y- and Y–Zr-containing specimens, the density of the fine inclusions, with a size less than 0.5 μm, was found to be 1.06 × 1017 and 9.82 × 1016 m−3, respectively. All specimens were normalized at 1323 K for 30 min and tempered at 923 and 1023 K for 90 min, resulting in the formation of tempered martensite with M23C6 carbides and MX precipitates. Zr-containing RAFM steel tempered at 923 K formed M23C6 carbides and nano-sized carbides with Zr, conferring superior strength balance and impact toughness. The yield strength of alloy reached 695 MPa, and a low ductile–brittle transition temperature of 238 K was maintained.

The effect of martensite–austenite (M–A) constituents formed in thermo-mechanical controlled process on impact toughness of 20CrNi2MoV steel was investigated. The variation in fraction, size and morphology of M–A constituent and its effect on toughness under different cooling rates were carried out. The result shows that there was no significant change in the fraction of M–A constituent under different cooling rates, but the distribution and size of M–A constituent were greatly influenced by cooling rate, which consequently influenced toughness. The amount of large blocky M–A constituents decreased from 4.7 to 1.7%, while that of elongated M–A constituents increased from 3.8 to 8.6% with the cooling rate increasing from 7 to 26 °C/s, and the corresponding impact energy decreased from 132 to 84 J. The deterioration of impact toughness could be related to the increase in the elongated M–A constituents. The elongated M–A constituents existing along the prior austenite grain boundaries in samples of 26 °C/s could easily lead to the formation of cleavage crack, which then results in the lower crack initiation energy than that of low cooling rate samples.

Using a Gleeble–1500D thermal–mechanical simulator, the hot-deformation behavior and critical strain in the dynamic recrystallization of SA508Gr.4N steel were investigated by compression tests from 1050 to 1250 °C with strain rates from 0.001 to 0.1 s−1. Stress–strain curves were fitted by a nonlinear fitting method. Based on these tests, the flow stress constitutive equations of the work-hardening dynamical recovery period and dynamical recrystallization period were established for SA508Gr.4N steel. The stress–strain curves of SA508Gr.4N steel predicted by the established models are in a good agreement with the experimental ones. Curves of lnθ − e and − ∂(lnθ)/∂e − e (where θ is the work-hardening rate and e is true strain) were plotted from experimental data. A critical strain (ec) and a peak strain (ep) of dynamic recrystallization were obtained and exhibited a linear relationship, i.e., ec = 0.386ep. The predicted model of ec could be described by the equation of ec = 1.604 × 10−3Z0.127.

Effects of micro-alloying elements and production process on microstructure, mechanical properties and precipitates of 600 MPa grade rebars were studied by using pilot test, metallographic observation, tensile test, thermodynamic calculation and transmission electron microscopy. The results show that the tested steels are composed of ferrite and pearlite, in which the content range of pearlite is 33%–45%. For vanadium micro-alloyed steel, interphase precipitation strengthening effect of V can be promoted and the yield strength of tested steels can be increased with increasing V content and decreasing finishing rolling temperature. The temperature of terminated cooling should be more than 700 °C when the water cooling is used. When niobium is added to the steel, more coarse (Nb, V) C, N precipitates are generated at high temperature, so that the solid solubility of precipitated phases of vanadium is reduced and the precipitation strengthening effect of vanadium is weakened.

The Al0.5 CoCrCuFeNi high-entropy alloy powders with simple face-centered-cubic (FCC) solid solution structure were introduced into the surface layer of a low carbon steel during laser surface alloying. A high performance surface layer with extremely fine martensite as the dominant phase was obtained, resulting in a great improvement in microhardness, wear resistance, and corrosion resistance. The great enhancement of microhardness and wear resistance of the laser alloyed layer is mainly due to the formation of extremely fine martensite hard phase, the solid solution strengthening of the alloying elements in supersaturated α-Fe solid solution, and the existence of size effect and strain effect under rapid solidification. The enhancement of corrosion resistance is due to the alloying of Al, Co, Ni, Cr and Cu in the laser alloyed layer.

Induration process of oxidized pellets involves the oxidation of Fe3O4 and re-crystallization of Fe2O3. The oxidation process of Fe3O4 is significant for pellets to obtain better ambient strength. Thus, the effect of MgO on oxidation process of Fe3O4 was investigated. The unreacted core model was applied to analyze the oxidizing induration process of pellets. The experimental results show that MgO plays a negative role in the oxidation process of Fe3O4. The oxidation rate of Fe3O4 in MgO-fluxed pellets (95.0% Fe3O4 + 5. 0% MgO) is slower than that in standard acid pellets (100% Fe3O4). The relation between oxidation ratio of Fe3O4 and time was calculated based on the unreacted core model for both MgO-fluxed pellets and standard acid pellets. According to verification experiments, the values calculated by model coincide well with the experimental values. Therefore, the unreacted core model could be applied to describe the oxidizing induration process of pellets.

With the development of advanced high strength steel, especially for dual-phase steel, the model algorithm for cooling control after hot rolling has to achieve the targeted coiling temperature control at the location of downcoiler whilst maintaining the cooling path control based on strip microstructure along the whole cooling section. A cooling path control algorithm was proposed for the laminar cooling process as a solution to practical difficulties associated with the realization of the thermal cycle during cooling process. The heat conduction equation coupled with the carbon diffusion equation with moving boundary was employed in order to simulate temperature change and phase transformation kinetics, making it possible to observe the temperature field and the phase fraction of the strip in real time. On this basis, an optimization method was utilized for valve settings to ensure the minimum deviations between the predicted and actual cooling path of the strip, taking into account the constraints of the cooling equipment′s specific capacity, cooling line length, etc. Results showed that the model algorithm was able to achieve the online cooling path control for dual-phase steel.

Microstructural evolution and mechanical properties of cryogenic rolled Fe-36Ni steel were investigated. The annealed Fe-36Ni steel was rolled at cryogenic temperature (123–173 K) with 20%–90% rolling reduction in thickness. The deformation process was accompanied by twinning at cryogenic temperature, and the mean thickness of deformation twins was about 200 nm with 20% rolling reduction. When the rolling reduction was above 40%, twinning was suppressed due to the stress concentration in the tested steel. Deformation microstructure of Fe-36Ni steel consisted of both twin boundaries and dislocations by cryogenic rolling (CR), while it only contained dislocations after rolling at room temperature (RT). The tensile strength of Fe-36Ni steel was improved to 930 MPa after 90% reduction at cryogenic temperature, while the tensile strength after 90% reduction at RT was only 760 MPa. More dislocations could be produced as the nucleation sites of recrystallization during CR process.

The type-1 aluminized (Al-10 mass% Si) coating on hot stamped steel is used in commercial applications. The effect of temperature on microstructure and formability of the Al-10 mass% Si coating was investigated. Hot-dip aluminized 22MnB5 steel was heated at 870, 900, 930, and 1050 °C for 5 min. After heat treatment at different temperatures, the microstructure of Al-10 mass% Si was characterized by field-emission scanning electron microscopy and energy-dispersive spectroscopy. The results show that when the Al-10 mass% Si coating was heated at 870 °C for 5 min, three types of intermetallic phases were formed: two ternary Fe-Si-Al phases (Fe 2 SiAl 7 + Fe 2 Si 2 Al 5 , and Fe 2 SiAl 2 + Fe 2 Si 2 Al 5 ) and one binary Fe-Al phase (FeAl 3 ). The phases in the coating became Fe 2 SiAl 2 + Fe 2 Si 2 Al 5 and FeAl 3 , the binary Fe-Al phase near the interface became Fe 2 Al 5 and the Si-rich layers were decreased with the increase of heating temperature. When the heat treatment was 1050 °C for 5 min, the Si-rich layer disappeared, the coating consisted of FeAl and Fe 3 Al, and the Kirkendall voids were formed. When the heating temperature was increased through 870, 900, and 930 °C, the coating hardness was reduced and cracks were formed in the coating after hot stamping. The formability of the coating with temperature change was confirmed by a hot stamping test. When heated at 1050 °C, the coating was formable but continuous Kirkendall voids were observed.

Based on the rigid-plastic finite element method (FEM), the shear stress field of deformation region for high manganese austenite steel during hot asymmetrical rolling process was analyzed. The influences of rolling parameters, such as the velocity ratio of upper to lower rolls, the initial temperature of workpiece and the reduction rate, on the shear deformation of three nodes in the upper, center and lower layers were discussed. As the rolling parameters change, distinct shear deformation appears in the upper and lower layers, but the shear deformation in the center layer appears only when the velocity ratio is more than 1.00, and the absolute value of the shear stress in this layer is changed with rolling parameters. A mathematical model which reflected the change of the maximal absolute shear stress for the center layer was established, by which the maximal absolute shear stress for the center layer can be easily calculated and the appropriate rolling technology can be designed.