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The existence of angular and hard AlN inclusions would seriously deteriorate the service life of high-nitrogen stainless bearing steels (HNSBSs). In this work, the formation mechanism of AlN inclusion in HNSBSs under as-cast, annealing and austenitizing states was systematically investigated by microstructure observation and thermodynamic, kinetic analyses. The results showed that the concentration product of Al and N could exceed the critical solubility of AlN inclusion at liquidus temperature with the Al content higher than 0.050 wt pct, which led to the formation of AlN inclusions about 1 to 5 μm (equivalent diameter) in liquid steel. Based on the ‘Clyne-Kurz’ model, AlN inclusion could form at the solidifying front due to the enrichment of N in the residual liquid steel with the Al content higher than 0.030 wt pct. Besides, the precipitation of Cr2N and the extremely low diffusion coefficient of Al in α phase restrained the precipitation of AlN during annealing at 1023 K. However, AlN and AlN-MnS composite inclusions less than 0.6 μm could precipitate during austenitizing at 1323 K with the Al content higher than 0.006 wt pct, which was the critical Al content to avoid AlN formation in HNSBSs after melting, solidification, and heat treatment processes.

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240, 360, and 400 ℃. The results show that martensite, bainite, and retained austenite (RA) are the main microstructural phases. The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix, and the transformation of ultrafine bainite into coarse lath bainite and granular bainite, respectively. Thick filmy RA was distributed between bainite laths. The polygonal martensite-austenite islands and blocky RA formed along the grain boundaries. The average carbon concentration in the matrix decreased with the temperature increase, while the impact toughness initially increased and then dropped with temperature. The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps. The microcracks disappeared in the RA on the prior austenite grain boundaries. On the other side, the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns. The polygonal martensite-austenite islands or blocky RA constrained the microcracks. After austempered at 400 ℃, the brittle fracture was dominant, showing river patterns, and the microcracks propagated through the granular bainite without any resistance.

The effect of La on inclusions and fracture toughness of 40CrNi2Si2MoVA steel was investigated via the optical microscope, scanning electron microscope, image software and electronic universal testing machine. The results reveal that the inclusions in steel without La are mainly MnS and Al2O3–MnS, while the inclusions in steels with La primarily contain La–O–S, La–S and other rare earth complex inclusions contain P and As. La–O–S and La–S are formed under the steelmaking temperature and act as the nucleation core of rare earth complex inclusions containing P and As. According to the segregation model, La–O–S–P–As and La–S–P–As are formed through chemical reactions during the solidification stage. As La content in steels increases from 0 to 0.032 mass%, the average spacing of inclusions is gradually increased from 5.28 to 15.91 μm. The volume fraction of inclusions in steels containing less than 0.018 mass% La approaches 0.006%; however, it is significantly improved to 0.058% when La content is increased to 0.032 mass%. With the increase in La content, the fracture toughness is firstly improved from 63.1 to 80.0 MPa m1/2 due to the increase in average spacing of inclusions and then decreases to 69.6 MPa m1/2 owing to the excessive increase in volume fraction of inclusions. The optimal fracture toughness is found in 40CrNi2Si2MoVA steel with 0.018 mass% La.

The effect of the re-heating process on inclusion evolution in Ti-Zr deoxidized low carbon steel was investigated using high-temperature resistance furnace, high-temperature confocal laser scanning microscope, scanning electron microscope with energy-dispersive spectrometer, and electron probe microanalysis with wavelength-dispersive spectrometer. The results indicated that compared with as-cast sample, the entire inclusion characteristics in heated samples were affected by the couple effect of heating temperature and holding time. As temperature and holding time increased, the shape of the Ti-Zr oxide changed from an irregular, curved surface to a smooth, spherical surface owing to the outermost layer being enveloped by liquid inclusion of Si-Mn-Al oxide. Additionally, the Ti-Zr oxide composition distribution changed from homogeneous to heterogeneous during heating in the range of 1100 °C to 1400 °C for 2 hours. Finally, different cooling modes had a significant influence on the inclusion characteristics. Compared with cooling by direct water quenching, the process of initial cooling to 1000 °C with furnace cooling and followed by water quenching could increase the number density of nitrides and sulfides, as well as change the unstable oxide to an effective oxide by precipitation of TiN on the surface.

Precipitation behavior of P550 steel for non-magnetic drill collars was investigated by microstructure characterization as well as thermodynamic calculation. The results demonstrate that the main precipitate formed at 650–900 °C was cellular Cr2N, and its precipitation depended heavily on the aging temperature. The most sensitive precipitation temperature of cellular Cr2N was 750 °C. At 750 °C, the cellular Cr2N exhibited fast-slow precipitation kinetics with the aging time prolonging. The initial precipitation of cellular Cr2N was governed by the short-range intergranular diffusion of Cr. During long-term aging, its growth was controlled by the long-range bulk diffusion of Cr. In addition, cellular Cr2N induced the precipitation of σ phase ahead of the cell after long period of aging. Increasing the nitrogen content resulted in the increment of both the nucleation site and the driving force for the cellular Cr2N, which jointly promoted its precipitation.

Pressurized electroslag remelting (PESR) has been widely used for preparation of high-quality high nitrogen steels. However, desulfurization might be limited by the pressurized-nitrogen atmosphere in the process. In this paper, the desulfurization behaviors of electroslag remelting in air and nitrogen pressure were investigated and further an appropriate method to improve the sulfur removal in PESR was explored. The results show that the ESR in air maintained relatively high and stable desulfurization ability, but the PESR provided worse sulfur removal owing to the lack of sulfur gasifying oxidation under N2 pressure. With the decline of sulfur distribution coefficient (LS) and the enrichment of sulfur in slag during the remelting, the desulfurization in PESR was gradually weakened. The level of gas pressure had little effect on sulfur removal. The Na2O addition in conventional CaO-Al2O3-CaF2 slags significantly increased the activity of desulfurizers and LS, and slightly improved sulfur mass transfer in slags, then alleviated the negative effect brought by the reduction of desulfurization ability in PESR and achieved higher desulfurization degree. In addition, desulfurization in the remelting was mainly achieved at the liquid metal film of electrode tips, which was benefited from the great reaction specific surface area and high initial sulfur content.

The effect of nitrogen on microstructural evolution and tensile properties of transformation-induced plasticity (TRIP) Fe50Mn30Co10Cr10 HEAs was investigated. Nitrogen was fully introduced in solid solution by pressure-induced melting technique. Nitrogen addition turned the TRIP alloy to a twinning-induced plasticity (TWIP) alloy, and simultaneously improved the strength and elongation. For the nitrogen-doped HEA, the high yield strength is mainly resulted from the friction stress via interstitial strengthening effect, and the high ductility is originated from retained high strain-hardening capability via the successive onset of dislocation accumulation and deformation twinning. The strain-hardening behavior and microstructural evolution at specified strains were revealed.

In this study, a comprehensive transient numerical model of electroslag remelting cladding process with dynamic mesh technology is simulated to study the effect of the applied power on the uniformity of melting layer depth along the height of the as-prepared compound roll. The multi-physics fields are solved by the ANSYS Parametric Design Language and Fluent simulation software. The simulation results show that the mandrel absorbs heat (Qmsi) from the slag pool and the melting layer is formed on the mandrel surface. A sufficient metallurgical bonding quality between the mandrel and the clad is confirmed by the close contact of the melting layer with the molten bath of the clad. In addition, the use of high and low power during the early and later stages, respectively, improves the uniformity of the melting layer depth along the height. When high power (235 kW) is applied during the early stage, the height of the compound roll without metallurgical bonding decreases to 52 mm. After the melting layer depth increases to 6 mm along the height, the power decreases to 187 kW. The slag temperature and Qmsi decreases rapidly, and consequently, the melting layer depth initially decreases and then slightly increases along the height. The melting layer depth is acceptable within height of 52 to 260 mm. The change tendency of the melting layer depth along the height of the compound roll obtained by the semi-industrial experiment is in agreement with the simulation results, proving the reliability of the process. Moreover, the results of tensile and Charpy impact tests indicate good metallurgical bonding quality. The process investigated in this paper is expected to be efficient for industrial production of the compound rolls with a uniform melting layer depth.

After applying torsion to cylindrical 304 stainless steel samples, a gradient structure along the radial direction was obtained. It was found that the volume fraction of α′-martensite increased gradually from the center to the surface of samples. The possibility of deformation-induced martensitic transformation was analyzed theoretically, whose conclusions were consistent with the experimental results. It was found that torsional deformation could produce abundant deformation twin and deformation-induced martensite, which could affect the tensile property of steel distinctly. The existence of deformation and martensite during torsion could increase the strength but deteriorate the ductility dramatically during the tensile test, which should be attributed to the weakening of work hardening ability. As a result of severe deformation, both strain-induced and stress-induced martensite have been observed. Also, two types of martensitic transformation mechanisms during torsional deformation were discussed.

Experimental and theoretical studies have been carried out to investigate the effects of slag composition on the MgO·Al2O3 inclusion in ingot during the electroslag remelting (ESR) process with a focus on developing a mass-transfer model to understand the evolution mechanism of MgO·Al2O3 inclusion. H13 die steel was used as the electrode and remelted with two different kinds of slags by using a 30-kg ESR furnace. The inclusion compositions and contents of magnesium, silicon, and aluminum along the axial direction of product ingots were analyzed. On the basis of the unreacted core model as well as the penetration and film theories, the theoretical model developed in this work well elucidates the kinetics of slag–metal-inclusion reactions revealing the mechanism of inclusion evolution during the ESR process. The calculation results obtained from the model agree well with the experimental results. The model indicates that the inclusions of the outer MnS layer, which surrounds the MgO·Al2O3 core in the electrode, are disintegrated and removed during the metal film formation process at the tip of the electrode in the ESR furnace. The more CaO there is in the slag, the higher the aluminum and magnesium in the ingot and the lower the silicon. The concentration of MgO in the MgO·Al2O3 inclusion increases with the increase of CaO/SiO2 in the slag. The aluminum in the electrode has little effect on the MgO·Al2O3 inclusion compositions in the final product ingots.

In this work, the effect of Cu addition on the microstructure and corrosion passivation behavior of Sn alloyed ferritic stainless steel in 3.5 wt.% NaCl solution at 30 °C was investigated by optical microscope (OM), scanning electron microscope (SEM), energy-dispersion spectrum (EDS), potentiodynamic polarization curve and x-ray photoelectron spectroscopy (XPS). The results indicate that Cu addition has certain effect on grain refinement of ferritic stainless steel. Meanwhile, Cu addition has little influence on the cathodic corrosion process of ferritic stainless steel in 3.5 wt.% NaCl solution but shows beneficial effect on enhancing both the corrosion resistance of steel substrate and its passivation behavior. It has been found that the deposition of Cu particles at the bottom of corrosion pits is responsible for the better corrosion resistance and passivation behavior of ferritic stainless steel. Moreover, there is synergistic effect between Sn and Cu on enhancing the corrosion resistance of ferric stainless steel matrix and improving its passivation behavior in NaCl solution.

In order to clarify the influence mechanism of high pressure on inclusion number distribution, the changes in phase transition sequences, key properties, and heat transfer boundary conditions with pressure have been investigated. Furthermore, the inclusion number distribution of H13 die steel ingot under different pressures (0.1, 1, and 2 MPa) has been numerically simulated by the mathematical model verified using the experimental results of cooling rate and the parameter ηn. The changes in key properties and phase transition sequences with pressure were investigated by Thermo-Calc software. With increasing pressure from 0.1 to 1000 MPa, the suppression of ferritic phase (δ) formation and improvement of liquidus/solidus temperature are obvious, but the density, thermal expansion coefficient, specific heat, and latent heat of solidification barely change. Meanwhile, the effect of pressure on heat transfer boundary conditions has been quantitatively revealed with formulas proposed by experimental measurement and numerical calculation: hf,0.1 = 1137.4t−0.23 (for 0.1 MPa), hf,1.0 = 1294.3t−0.23 (for 1 MPa), and hf,2.0 = 1501.6t−0.23 (for 2 MPa). During solidification process, gravity, buoyancy, and drag forces play the key roles in affecting the movement behavior of inclusions. Moreover, their net force drives inclusions to sink downward near the tip of columnar dendrite, and move counterclockwise. With increasing pressure from 0.1 to 2 MPa, the enrichment degrees of inclusions in ingot decrease, and the distribution of inclusion number becomes more uniform due to the stronger escaping ability of inclusions and the weaker inclusion trapping ability of the mushy zone.

While solidification behavior has been investigated in several nickel-based superalloys, few investigations have focused on the meticulous study of solidification behavior in such a highly alloyed and high γ′ volume fraction-containing alloy as GH4151. In this study, several material characterization techniques were employed to identify phase types, illustrate the morphology of precipitates, and detect composition characteristics. In addition, differential scanning calorimeter and isothermal solidification quenching experiments were used to explain the solidification sequence. It has been confirmed that (Nb,Ti)C carbides, η phase, eutectic (γ + γ′), Laves, Ni5Ce, and γ′ phase were found in the as-cast GH4151. The solidification sequence was determined as follows: L → L + γ → L + MC → L + η → L + (γ + γ′) → L + Laves → L+Ni5Ce, and γ → γ′. Combined with electron probe micro-analysis, the results show that Nb, Ti, Mo, and V are significantly enriched in the interdendritic region, which has elemental degrees of segregation such that Nb > Ti > Mo > V, while Al, Co, Cr, W, and Ni are enriched within the dendritic arms, whose degree of segregation is W > Al > Ni > Co > Cr.

Slag–metal reaction experiments in MoSi2 resistance furnace combined with electroslag remelting (ESR) experiments in ESR furnace are used to study the effect of slag on zirconium distribution in ESR ingot by establishing a new mass transfer model of slag–metal reaction. The mass transfer model consists of Al + Al2O3, Si + SiO2, Zr + ZrO2, and Fe + FeO systems based on the penetration and film theories. Both experimental and simulated results show that the returned slag (CaF2:CaO:Al2O3:MgO:ZrO2 = 57:20:16:3:3) combined with extra 4% Al2O3 added into molten slag in the first slag-temperature-rising period can control the zirconium in ESR ingot ranging from 0.35 to 0.40% and improve the homogeneous distribution of zirconium in ESR ingot. The returned slag of Exp.A containing low silica being used in Exp.C can not only contribute to the recycling of returned slag, but also improve the homogeneous distribution of Zr along the height of ESR ingot.

The determination of the local cooling rate has a great significance in optimizing the parameters of electroslag remelting (ESR) and improving the quality of the ingots. An innovative method was proposed for calibrating the local cooling rate of M42 high-speed steel (HSS) in the ESR process. After resolidification at different cooling rates under high-temperature laser confocal microscopy, the carbide network spacing of the specimen was observed using a scanning electron microscope. A functional relationship between the cooling rate and average carbide network spacing was established. The average local cooling rate of the solidification process of the M42 HSS ingot was calibrated. The results show that the higher the cooling rate, the smaller the network spacing of the carbides. For the steel ingot with a diameter of 360 mm, the average local cooling rate was 0.562 °C/s at the surface, 0.057 °C/s at the position of 0.25D (where D is the diameter of the ingot), and 0.046 °C/s at the center of the ingot.

To investigate the interaction mechanism between 95Cr saw-wire steel and different refractories, we conducted laboratory experiments at 1873 K. Five crucible materials (SiO2, Al2O3, MgO·Al2O3, MgO, and MgO-CaO) were used. The results indicate that SiO2, Al2O3, and MgO·Al2O3 are not suitable for smelting low-oxygen, low-[Al]s 95Cr saw-wire steel, mainly because they react with the elements in the molten steel and pollute the steel samples. By contrast, MgO-CaO is an ideal choice to produce 95Cr saw-wire steel. It offers three advantages: (i) It does not decompose by itself at the steelmaking temperature of 1873 K because it exhibits good thermal stability; (ii) [C], [Si], and [Mn] in molten steel cannot react with it to increase the [O] content; and (iii) it not only desulfurizes and dephosphorizes but also removes Al2O3 inclusions from the steel simultaneously. As a result, the contents of the main elements ([C], [Si], [Mn], [Cr], N, T.O (total oxygen)) in the steel are not affected and the content of impurity elements ([Al]s, P, and S) can be perfectly controlled within the target range. Furthermore, the number and size of inclusions in the steel samples decrease sharply when the MgO-CaO crucible is used.

Based on XRD, SEM and EDS analyses, the phases in GH4151 alloy were identified. Differential scanning calorimetry (DSC) experiment and metallographic method were carried out to determine the incipient melting temperature (IMT) of the alloy. The result shows that the IMT of alloy is situated between 1150 and 1160 °C. Subsequently, the dissolution process of Laves phase was carried out, and the dissolution kinetic equations were obtained at different temperatures. And then based on the verification of experiments, the model was confirmed to be credible to predict the fraction of the Laves phase dissolution. Finally, the results of diffusion coefficients indicate that the diffusion of Nb element is a critical factor for homogenization process of GH4151 alloy.

The as-cast M42 high-speed steels containing different contents of cerium (Ce) were manufactured to investigate the effects of Ce on the solidification structures at the micro- and macroscale. The results indicate that the addition of Ce could modify MgO·Al2O3 and MnS into the Ce-containing inclusions. The addition of Ce refined the dendrite structure and eutectic carbides, which can be ascribed to the heterogeneous nucleation of primary austenite on Ce2O2S or Ce2O3 and the increase in both the compositional supercooling at the dendrite forefront and the restriction on the dendrites coarsening. Both the secondary dendrite arm spacing (SADS) and total eutectic carbides content decrease gradually with increasing Ce content. M2C and M6C carbides are the predominant precipitates. The increase of Ce content made the morphology of M2C carbides change from long lamellar or straight-rod morphology into shorter curved-rod or honeycomb morphology because of the overgrowth of eutectic austenite, but it had no significant effect on the morphology of M6C carbides. Ce2O2S and Ce2O3 can serve as the very effective heterogeneous nucleation sites for M6C carbides because of the low lattice disregistry between them, hence Ce addition improved markedly the macroscopic distribution of M6C carbides in the cast ingot and promoted the formation of M6C carbides at the expense of M2C carbides.

In the present study, the comprehensive analyses based on the numerical simulation, in situ observation, and metallographic detection were carried out for the roll core of GCr15/45 carbon steel composite roll manufactured by the new electroslag remelting cladding (ESRC) method. During the ESRC process, the temperature distributions at the different radial and longitudinal positions of the roll core have great changes due to the different degrees of heat conduction from the slag bath, as a result, various microstructure and properties were obtained at the different positions of the roll core. The results illustrated that the ESRC process tended to be stable as the composite height reached a certain value and the high-temperature austenitization process mainly occurred in the radial regions where R > 60 mm, whereas no significant changes occurred at the areas where R ≦ 60 mm. The coarse grains and few Widmanstatten structures with proeutectoid ferrite were generated in the roll core areas near the bimetallic interface, while the fine grains with more proeutectoid ferrite were obtained in the roll core area away from the interface. Therefore, a higher tensile strength and a better plasticity were obtained for the specimens away from the interface.

High-nitrogen (N) austenitic stainless steel (Cr–Mn–N series) is commonly used for non-magnetic drill collars, which exhibits excellent mechanical properties and corrosion resistance. The effects of N content (0.63 to 0.86 wt.%) on the pitting corrosion behavior of the experimental non-magnetic drill collar steel were investigated using the electrochemical tests and immersion tests. Besides, X-ray photoelectron spectroscopy was used to analyze the constitution of the passive film. The results show that with the enhancement of N content from 0.63 to 0.86 wt.%, the metastable pitting corrosion sensitivity of the tested materials in 3.5 wt.% NaCl solution decreased and the pitting corrosion resistance increased. Meanwhile, the corrosion rate in 6 wt.% FeCl3 solution at 30 °C decreased from 10.40 to 4.93 mm/a. On the other hand, nitrogen was concentrated in the form of ammonia (NH4+ and NH3) on the outermost surface of the passive films. The contents of Cr2O3 and Fe2O3 raised in the passive films, together with the content of CrN, at metal/film interface increased as N content increased from 0.63 to 0.86 wt.%, which facilitated protective ability of the passive films, thus contributing to higher pitting corrosion resistance.

Investigating the reaction mechanism between slag and 9CrMoCoB steel is important to develop the proper slag and produce qualified ingots in the electroslag remelting (ESR) process. Equilibrium reaction experiments between molten 9CrMoCoB steel and the slags of 55wt%CaF2-20wt%CaO-3wt%MgO-22wt%Al2O3-xwt%B2O3 (x = 0.0, 0.5, 1.0, 1.5, 2.0, 3.0) were conducted. The reaction mechanisms between molten 9CrMoCoB steel and the slags with different B2O3 contents were deduced based on the composition of the steel and slag samples at different reaction times. Results show that B content in the steel can be controlled within the target range when the B2O3 content is 0.5wt% and the FeO content ranges from 0.18wt% to 0.22wt% in the slag. When the B2O3 content is ≥1wt%, the reaction between Si and B2O3 leads to the increase of the B content of steel. The additions of SiO2 and B2O3 to the slag should accord to the mass ratio of [B]/[Si] in the electrode, and SiO2 addition inhibits the reaction between Si and Al2O3.

A V-Nb microalloyed spring steel was investigated through a series of austempering heat treatments. Optimal properties were obtained after austempering at 240 °C for 2 hours because of the good coordination of multiphase martensitic/bainite, filmy retained austenite and fine carbides. The tensile strength was 2239 MPa and the total elongation was 16.1 pct. With an increase in temperature, the matrix changed from martensite to bainite. The volume fraction of retained austenite decreased from 13.7 pct at 240 °C to 9.6 pct at 280 °C and then increased to a maximum of 19.2 pct at 360 °C. The tensile strength remained above 1500 MPa and the impact energy increased to 30 J. MC carbides formed in martensite lathes and ɛ- or θ-type carbides with acicular nanoscale were precipitated in bainite lathes at a low temperature. MC- and M3C-type precipitated in bainite lathes at high temperature. Samples that were subjected to a low isothermal temperature had a high strain-hardening rate before necking, whereas samples at a high isothermal temperature could exert a TRIP effect for a long time. After stretching to fracture, a hierarchical nanotwinned structure and many shear cells formed, which enhanced the austempered sample ductility and toughness at 360 °C.

The influence of N on the precipitation behavior, associated corrosion, and mechanical properties of S32654 were investigated by microstructural, electrochemical, and mechanical analyses. Increasing the N content results in several alterations: (1) grain refinement, which promotes intergranular precipitation; (2) a linear increase in the driving force for Cr2N and Mo activity, which accelerates the precipitation of intergranular Cr2N and π phase, respectively; (3) a linear decrease in the driving force for σ phase and Cr activity, which suppresses the formation of intragranular σ phase. The total amount of precipitates first decreased and then increased with the N content increasing. Furthermore, the intergranular corrosion susceptibility depended substantially on the total amount of precipitates and also first exhibited a decreasing and then an increasing trend as the N content increased. In addition, aging precipitation caused a considerable decrement in the ultimate tensile strength (UTS) and a remarkable increment in the yield strength (YS). Both the UTS and YS always increased with N content increasing throughout the solution and aging process. Whereas the elongation was considerably sensitive to the aging treatment, it exhibited marginal variation with the N content increasing.

The influence of Cu and Sn on the microstructure, mechanical properties, corrosion resistance, and antibacterial properties of 430L ferritic stainless steel has been investigated by using Thermo-Calc calculations, EPMA (electron probe microanalysis), TEM (transmission electron microscopy), and XPS (X-ray photoelectron spectroscopy). Vichers hardness tests, potentiodynamic polarization, and antibacterial tests were also performed. For a copper content of 1.5 wt.%, the results showed that aging treatments at 600 °C, 700 °C and 800 °C are conducive for the precipitation of an ε-Cu phase in the 430L ferritic stainless steel. It was also found that the ε-Cu phase was composed of almost pure Cu (with a the content of 98.6–99.7 wt.%). Thermo-Calc calculations showed that there was only a very small amount of Fe, Cr, and Mn in the ε-Cu phase. Moreover, the ε-Cu phase was found to substantially coarsen with an extension of the aging time. The size of ε-Cu phase increased from a few nanometers to hundreds of nanometers, and the number of ε-Cu phase decreased gradually. Furthermore, the antibacterial rate of the 2# (430L − 1.5 wt.% Cu) and 3# steel (430L − 1.5% Cu − 0.4 wt.% Sn) samples increased significantly with an extension of the aging treatment time, and the antibacterial rate of the 3# steel was higher than that of the 2# steel sample. The antibacterial rate reached as high as 91.60% and 97.37%, respectively, for an aging time of 79,200 s.

The coupling effect of prior austenite grain size and inclusion characteristics on acicular ferrite (AF) formation was investigated in Ti-Zr deoxidized low carbon steel by utilizing the high temperature confocal laser scanning microscope (HT-CLSM), optical microscope (OM), and scanning electron microscope (SEM) equipped with energy-dispersive spectrometer (EDS). The results indicated that with the target heating temperature increased from 1100 °C to 1350 °C, the average size of prior austenite grain varied from 58.22 to 237.40 μm, and the average grain size increased rapidly when the temperature was above 1200 °C. For inclusion characteristics, different target heating temperatures had no obvious effect on inclusion types, but had a great influence on the average size and number density of each, especially for the intragranular effective inclusions. In addition, as the increase of target heating temperature, the types of microstructure were identical, but both AF volume fraction and AF relative nucleation ability increased first and then decreased. When the target heating temperature of sample was 1250 °C, the AF volume fraction reached the maximum of 49.48 pct. However, the AF relative nucleation ability reached the maximum of 474.5 at the target heating temperature 1200 °C, at this time, the AF volume fraction was 47.92 pct, only 1.56 pct smaller than that at 1250 °C. Therefore, considering the AF volume fraction and AF relative nucleation ability, the optimal target heating temperature for AF formation in this study was 1200 °C, and the corresponding prior austenite grain size was 69.58 μm.

The thermal stability and mechanical properties of China low activation martensitic steel with Zr and Y were investigated via thermal aging at 550 °C for 8000 h. The Laves phase content monotonically increased with thermal aging, and the volume fraction of the Laves phases stabilized in the alloy after 3000 h of thermal aging. The observed degradation in mechanical properties was because of the coarsening of M23C6 carbides and matrix grains during the earlier stages of thermal aging. The precipitation of Laves phases and V3Zr3C particles increased the strength and hardness of the alloy. Grain coarsening was the primary reason for the decrease in impact properties, and the ductile-to-brittle transition temperature increased from −71 to −48 °C after 8000 h of thermal aging.

The effects of normalizing and tempering temperatures on the microstructures and mechanical properties of China Low Activation Martensitic (CLAM) steel with and without yttrium were studied. Based on the optimized traditional heat treatment, two special intermediate heat treatments were studied to improve the mechanical properties of the steels. The AC3 temperature of the CLAM steel was increased by adding yttrium. The optimized traditional heat treatments were 1000 °C × 30 min + 755 °C × 90 min and 1050 °C × 30 min + 755 °C × 90 min for the C1 and C2 alloys, respectively. The intermediate heat treatment results indicated that the precipitation behavior of carbides was effectively controlled. The size of the M23C6 carbides was refined during the two intermediate heat treatments due to the priority precipitation of MX particles. However, coarsening of grains and martensite laths occurred during heat treatment with furnace cooling. The twice-quenched tempering samples had a smaller grain size and martensite lath width than the other samples. The ductile–brittle transition temperature was − 69 and − 103 °C for the C1 and C2 alloys with twice quenching, and the yield strengths were 745 and 760 MPa, respectively. An excellent balance of strength and impact toughness was obtained with the twice-quenching and once tempering heat treatment process.

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.

k]The formation mechanism of acicular ferrite and its microstructural characteristics in 430 ferrite stainless steel with TiC additions were studied by theory and experiment. Using an “edge-to-edge matching” model, a 5.25 mismatch between TiC (FCC structure) and fer-ritic stainless steel (BCC structure) was identified, which met the mismatch requirement for the heterogeneous nucleation of 430 ferritic stainless steel. TiC was found to be an effective nucleation site for the formation of acicular ferrite in a smelting experiment, as analyzed by metallographic examination, Image-Pro Plus 6.0 analysis software, and SEM-EDS. Furthermore, small inclusions in the size of 2–4 μm increased the probability of acicular ferrite nucleation, and the secondary acicular ferrite would grow sympathetically from the initial acicular ferrite to produce multi-dimensional acicular ferrites. Moreover, the addition of TiC can increase the average microstrain and dislocation density of 430 ferrite stainless steel, as calculated by Williamson-Hall (WH) method, which could play some role in strengthening the dislocation.

Since the gas–slag–metal three-phase transfer mechanism of nitrogen is not suitable to account for the nitrogen pickup during pressurized electroslag remelting process, the laboratory experiments have been carried out to study the transfer mechanism of nitrogen using different nitrogen partial pressures and electrode immersion depths. Then, two possible transfer mechanisms of nitrogen have been proposed, and a 2D transient numerical model has been developed to investigate the behavior of nitrogen based on the new proposed mechanisms. The simulated results are compared with the experiment to validate the feasibility of new proposed mechanism. The results show that the nitrogen content in ingot increases with the increasing nitrogen partial pressure and decreasing electrode immersion depth. At the current experimental condition, the electrode immersion depth seems to play a more important role in the nitrogen pickup than the nitrogen partial pressure. Due to the limits of thermodynamic and kinetic conditions, the nitrogen pickup is negligible via the transfer from gas to liquid metal through molten slag whether nitrogen chemically or physically dissolves into slag. The mechanism that nitrogen directly reacts with the partially exposed liquid metal film under the electrode tip due to the fluctuation of gas/slag interface could reasonably account for the experimental results. Furthermore, based on the mechanism, the predicted variation tendency of nitrogen content in ingot is in agreement with the experiment.

Herein, we report a new castable nanostructured alloy (CNA), which was first melted in a vacuum induction furnace to introduce micron- and submicron-sized inclusions, strengthened by the multiscale secondary phases for fusion reactors. The large micron-scale inclusions are removed, whereas submicron-scale inclusions were retained during an electroslag remelting process. The ingot was subjected to heat treatment at 1050 °C for 0.5 h followed by heating at 650 °C for 1.5 h to obtain tempered martensite along with the nanosized (Ti, W) carbides. The improvement in the mechanical properties of the sample treated at 750 °C was attributed to the removal of blocky Y-rich inclusions along with the strengthening of the submicron-sized Y2O3 oxides. The performance of the sample treated at 650 °C was explained by the precipitation of the fine (Ti, W) carbides.

The microstructure evolution and intergranular corrosion (IGC) behavior of high nitrogen martensitic stainless steels (MSSs) by partial replacing C by N were investigated by using microscopy, X-ray diffraction, nitric acid tests and double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests. The results show that the partial replacement of C by N first reduces and then increases the size and content of precipitates in high nitrogen MSSs, and converts the dominant precipitates from M23C6 to M2N, furthermore first improves and then deteriorates the IGC resistance. The high nitrogen MSS containing medium C and N contents provides good combination of mechanical properties and IGC resistance.

The effects of cooling rate on acicular ferrite (AF) nucleation, growth, and inclusion characteristics in Ti-Zr deoxidation steel were studied by utilizing the high temperature confocal laser scanning microscope (HT-CLSM). The results indicated that with the increase of cooling rate, the ferrite start nucleation temperature decreased, and the difference of first nucleation temperature between AF and ferrite side plate (FSP) reduced. When the cooling rate increased to 10.0 °C/s, AF and FSP simultaneously nucleated at 564.5 °C. In addition, the AF actual growth rate rose with the increase of cooling rate and reached 30.13 µm/s at 10.0 °C/s cooling rate. The AF ratio in microstructure increased first and then decreased with the cooling rate increase and was up to the maximum 45.83 pct at 1.0 °C/s cooling rate. For inclusion characteristics, cooling rates had no obvious effect on inclusion types, but had a great influence on inclusions size distribution. With the cooling rate increase, the inclusion average diameter reduced, and diminished to 1.39 µm at 10.0 °C/s cooling rate. Finally, the AF nucleation on the Ti-Zr-Mn-O-S + TiN inclusion could be explained by the low lattice misfit between ferrite and TiN that precipitated on the Ti-Zr-Mn-O-S inclusion surface.

We elucidated the significance of partial substitution of C by N in three high nitrogen Fe-15Cr-1Mo-C-N martensitic stainless steels (MSSs), and particularly its influence on microstructural features and the strength-toughness balance. The results show that partial substitution of C by N avoided the formation of coarse intergranular carbides and caused significant changes in the type of carbonitrides from M23C6 to M23C6+M2N and finally to M2N. Meanwhile, partially replacing C by N first increased and then decreased the effective grain size, the fraction of retained austenite, and the plate martensite, while the variations of the amount of carbonitrides and dislocation density were just the opposite. The microstructural evolutions with varying (C+N) contents played a crucial role in determining the overall properties: the 0.35C-0.37N steel exhibited an improved impact toughness (~ 86.1 J), which was 2 or 3 times higher than those of 0.50C-0.16N and 0.20C-0.54N steels, even at a strength level of over 2 GPa. The contributions of partial substitution of C by N on the strengthening mechanisms of MSSs were also systematically revealed by combining the experimental and theoretical data.