Journal of Iron and Steel Research(International)最新文献

The effect of solution pH, Cl^– concentration and temperature on the electrochemical corrosion behavior of PH13-8Mo steel in acidic solution was investigated by using the electrochemical tests, scanning electron microscopy and X-ray photoelectron spectroscopy. The PH13-8Mo martensitic precipitation hardened stainless steel is in the passivity state when the pH value is above 3. 0, below which the anodic polarization curves of the steel are actively dissolved. The corrosion current density gradually decreases with increasing the solution pH and decreasing Cl^– concentration and solution temperature. Pits are initiated on the sample surface in the presence of the Cl^– and gradually developed into uniform corrosion with increasing the Cl^– concentrations. Moreover, the corrosion is more serious with an increase in solution temperature.

Microstructural evolution in weld metals was in-situ observed through utilizing a laser scanning confocal microscope at two cooling rates. The specimens with various nickel contents were adopted for the observation. In the specimen with low fraction of Ni (≤2 wt. %), granular bainite microstructure (i. e. broad surface relief) transformation from intragranular nucleation site was in-situ observed, while, lath bainite microstructure originating from grain boundary of austenite was in-situ observed for specimens with high mass percentage of Ni (≥ 4 wt. %). With increasing nickel content, the transformation temperature dropped. The prior austenite grain size was initially depressed and subsequently coarsened dramatically with the addition of Ni. The microstructure difference was ascribed to various nucleation sites and growth direction in the matrix. On account of those observations, not only the chemical component, cooling rate and microstructure were systematically correlated, but also the microstructural evolution was definite.

The influence of fluorine on the structure of CaO-SiO₂-Al₂O₃-Na₂O-CaF₂ continuous-casting-type slag was measured by Raman spectroscopy, and the degree of polymerization of mould flux and the structural behavior of F⁻ in the melt were investigated by classifying and quantifying the structural species of F⁻ ions. The results exhibit that the main structural units of Si-O tetrahedra are Q⁰, Q¹ and Q² and the actual measured number of non-bridging oxygen ions in the [SiO₄]-tetrahedra (denoted by NBO/T) increases from 2. 73 to 3. 44 with increasing the molar ratio of F to (F+O) (denoted by X_F/X_(F+O)) from 0. 06 to 0. 19. It means that the degree of polymerization of melt structure decreases with an increase in X_F/X_(F+O). In addition, most of F⁻ ions were distributed in Si-O tetrahedra and Al-O tetrahedra. With increasing X_F/X_(F+O), the complex structural units Al-O tetrahedra are gradually replaced by discrete structural units AlF^1– because of the breakage of Al-O bonds in Al-O tetrahedra by F⁻ ions, and the Si-O (bridging oxygen) bonds of Si-O tetrahedra are broken to form [SiO_nF_1–n]-tetrahedra by F⁻ ions coordinating with Si⁴⁺.

The formation and propagation of the popular off-corner subsurface cracks in bloom continuous casting were investigated through thermo-mechanical analysis using three coupled thermo-mechanical models. A two-dimensional thermo-elasto-visco-plastic finite element model was developed to predict the mould gap evolution, temperature profiles and deformation behavior of the solidified shell in the mould region. Then, a three-dimensional model was adopted to calculate the shell growth, temperature history and the development of stresses and strains of the shell in the following secondary cooling zones. Finally, another three-dimensional model was used to analyze the stress distributions in the straightening region. The results showed that the off-corner cracks in the shell originated from the mould owing to the tensile strain developed in the crack sensitive regions of the solidification front, and they could be driven deeper by the possible severe surface temperature rebound and the extensive tensile stress in the secondary cooling zone, especially upon the straightening operation of the bloom casting. It is revealed that more homogenous shell temperature and thickness can be obtained through optimization of mould corner radius, casting speed and secondary cooling scheme, which help to decrease stress and strain concentration and therefore prevent the initiation of the cracks.

The possible decomposition of metastable austenite during the partitioning process in the high-end quenching and partitioning (Q&P) steels is somewhat neglected by most researchers. The effects of primary martensite and alloying elements including manganese, cobalt and aluminum on the isothermal decomposition of austenite during typical Q&P process were studied by dilatometry. The transformation kinetics was studied systematically and resulting microstructures were discussed in details. The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries. This effect can be eliminated by a flash heating which recovered dislocations. Co addition significantly promoted the bainite transformation during partitioning while Al and Mn suppressed the isothermal bainite transformation. The bainite transformation played an important role in carbon distribution during partitioning, and hence the amount and stability of austenite upon final quenching. The bainite transformation during partitioning is an important factor in optimizing the microstructure in Q&P steels.

Based on uniaxial tensile and plane strain deformation tests, the effects of strain states on the stability of RA (retained austenite) in medium Mn steels, which were subjected to IA (intercritical annealing) and Q&P (quenching and partitioning) processing, were investigated. The volume fractions of RA before and after deformation were measured at different equivalent strains. The transformation behaviors of RA were also investigated. The stability of RA differed across two different transformation stages at the plane strain state: the stability was much lower in the first stage than in the second stage. For the uniaxial tension strain state, the stability of RA corresponded only to a single transformation stage. The main reason was that there were two types of transformations from RA in the medium Mn steel for the plane strain state. One type was that the martensite originated in the strain-induced stacking faults (SISF). The other type was the strain-induced directly twin martensite at a certain equivalent strain. However, for the uniaxial tension state, only the strain-induced twin martensite was observed. Dislocation lines and dislocation tangles were also observed in specimens deformed at different strain states. In addition, complex microstructures of stacking faults and lath-like phases were observed within a grain at the plane strain state.

The Fe-0. 21C-2. 2Mn-0. 49Si-1. 77 A1 transformation induced plasticity (TRIP)-aided steel was heat treated at various austenitizing temperatures under both TRIP-aided polygonal ferrite type (TPF) and annealed martensite matrix (TAM) processes. The microstructure evolution and their effects on mechanical properties were systematically investigated through the microstructure observation and dilatometric analysis. The microstructure homogeneity is improved in TPF steel heated at a high temperature due to the reduced banded martensite and the increased bainite. Compared with the mechanical properties of the TPF steels, the yield strength and elongation of the TAM steels are much higher, while the tensile strength is lower than that of TPF steels. The stability of intercritical austenite is affected by the heating temperature, and thus the following phase transformation influences the mechanical properties, such as the bainite transformation and the precipitation of polygonal ferrite. Obvious dynamic bainite transformation occurs at TAM850, TAM900 and TAM950. More proportion of polygonal ferrite is found in the sample heated at 950 °C. The bainite transformation beginning at a higher temperature results in the wider bainitic ferrite laths. The more proportion of polygonal ferrite and wide bainitic ferrite laths commonly contribute to the lower strength and better elongation. The uniform microstructure with lath-like morphology and retained austenite with high average carbon content ensures a good mechanical property in TAM850 with the product of strength and elongation of about 28 GPa · %.

A new model on predicting the density of hot-rolled multi-phased medium-Mn steel has been presented on the basis of thermodynamic calculations. This is an integrated model, which includes one for calculating the retained austenite (RA) fraction and the other for volume expansion during the austenite-to-martensite transformation, because both of them are key parameters for calculating the density of steel at ambient temperature. The existing empirical equations for calculating Mm_s temperature and lattice constants of both martensite and austenite have been all reassessed by the XRD measurements on the microstructures of seven hot-rolled medium-Mn steels. Finally, the densities of seven steels were calculated merely from compositions and compared with the measured ones. The difference between them is no more than 1%, suggesting that the presented model should be of good value in designing the low-density steels.

A combined process of hot-deformation plus two-step quenching and partitioning (HDQP) treatment was applied to a low-carbon 20Si2CrNi3MoV steel, and transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers hardness and tension test were used to characterize the microstructure and mechanical properties. More stable retained austenite due to fine microstructures and typical curved micromorphology is obtained, and the newly-treated steel obtains more retained austenite because of the effect of hot deformation. The retained austenite fraction increases and then decreases with the increasing quenching temperature from 200 to 350 °C. The maximum retained austenite fraction (18. 3%) and elongation (15%) are obtained to enhance the ductility.

Mechanical properties of a newly developed microalloyed bainitic steel were investigated after the hot forging, air cooling and tempering process. The microstructure of the as-forged bainitic steel mainly consists of granular bainite and ∼20 vol. % martensite. The fraction of retained austenite remains unchanged until tempering at 200 °C, above which it decreases significantly. The increase of tempering temperature leads to decreases of both ultimate tensile strength and total elongation but decreases of both yield strength and reduction of area. The maximum and minimum values of impact toughness were observed after tempering at around 200 and 400 °C, respectively. These effects are mainly attributed to the decomposition of martensite/austenite constituents and the tempering effects in martensite. The tempering of the forged bainitic steel at around 200 °C results in an excellent combination of strength and toughness, which is comparable to that of the conventional quenched-and-tempered 40Cr steel. Therefore, low-tempering treatment coupled with post-forging residual stress relieving is a feasible method to further improve the mechanical properties of the bainitic forging steel.