In this paper, the characteristics of inclusions, microstructures and mechanical properties in non-quenched and tempered steel with Zr and Al addition were investigated to compare the effect of ZrO2 ...and Al2O3 particles on the precipitation behavior of MnS and the formation of intragranular ferrite. The results show that oxides, sulfides and oxy-sulfides in Zr addition steel are all finer than those in Al addition steel. As a result, the volume fraction of intragranular polygonal ferrite (IPF) induced by tiny MnS + V(C, N) particles in Zr addition steel has increased to 14.9%, resulting in higher transverse plasticity. Low lattice mismatch between ZrO2 and MnS inclusions plays an important role in refinement of MnS inclusions. Besides, the single ZrO2 particle, which is not wrapped by MnS inclusions, would induce formation of IPF further due to the small mismatch with α-Fe.
Nowadays, under the context of severe resource shortage, it is urgent to develop non‐quenched and tempered steel. Herein, a new method of combining rare‐earth cerium (Ce) and magnesium (Mg) to treat ...the 49MnVS3 steel with low oxygen and rather high sulfur is proposed. The inclusions modification, microstructure evolution, and mechanical properties change of 49MnVS3 steel with different Ce and Mg content composite treatment are investigated by optical microscope (OM), scanning electron microscope (SEM)–energy‐dispersive spectrometer (EDS), electron probe microanalyzer (EPMA), and tensile and impact tests. The results indicate that the small spherical Ce–Mn–Mg–O–S composite inclusions replace the Al2O3 wrapped MnS composite inclusions and elongated MnS inclusions after Ce and Mg joint treatment. After Ce and Mg composite treatment, the proportion of ferrite is significantly improved. Meanwhile, the interlamellar spacing of pearlite is refined. Attributed to modified inclusions and optimized microstructure, the strength and impact toughness are gradually enhanced. In contrast to Ce–Mg free steel, the impact energy of 4# steel with 0.0122 wt% Ce and 0.0012 wt% Mg addition is increased by 33%. In the current study, the optimal mass fractions of Ce and Mg are 0.0122 and 0.0012 wt%, respectively.
A novel method of combining rare‐earth cerium and magnesium to treat the 49MnVS3 steel with low oxygen and fair sulfur was proposed. After Ce and Mg treatment, attributed to modified inclusions and optimized microstructure, the steel achieves the optimum comprehensive mechanical properties with the proper amount of Ce and Mg.
Distribution and morphology of MnS inclusions in as-cast ingots and as-forged bars of two Zr-bearing resulfurized non-quenched and tempered (NQT) steels have been performed. In the low Zr-bearing ...steel (0.001 wt%), MnS inclusions, which are teardrop-shaped or rod-like in two-dimensional (2D) morphology and dendritic or skeletal in three-dimensional (3D) morphology, are mainly distributed and segregated at the grain boundaries. While in the high Zr-bearing steel (0.0066 wt%), MnS inclusions are spherical or angular in both 2D and 3D observation and the distribution is more uniform than those in low zirconium steel. The calculated results by Thermo-Calc software show that the content of oxygen is not the direct factor that influences the morphology and distribution of MnS inclusions in medium-sulfur low-oxygen NQT steels. ZrO2 particles are ideal partcles for generation of spherical type I MnS inclusions owing to their strong nucleation capability and large amounts. However, the Zr and Al contents should be controlled cautiously to avoid generating large-sized agminated complex oxides, which are not easy to float and be removed owing to their high density. Otherwise, the ideal particles ZrO2 would decrease sharply in number and fail in offering sufficient heterogeneous nuclei for type I MnS inclusions. Besides, high proportion of complex MnS inclusions would decrease the supersaturation when pure MnS inclusions begin to precipitate, suppressing the generation of dendritic type II MnS inclusions.
Quenching and tempering (QT) are common procedures in manufacturing high strength steel for building applications to achieve structural, economic and architectural benefits. However, the tempered ...martensitic microstructure of QT steel can change dramatically when exposing to high temperatures, leading to significant deterioration in mechanical properties. This paper investigates the post-fire mechanical properties of two types of QT steel with nominal yield stresses of 690 MPa and 1070 MPa, respectively. The main test variables include the exposure temperature (100–1200 °C), heat soak time (0.5–4 h), and cooling method (cooled in furnace, air or water). It is found that the heat soak time has obvious influence on the residual mechanical properties of QT steel, when the exposure temperature is above 500 °C. The cooling method, however, only severely affects the residual properties at a temperature above the phase change temperature (around 724 °C). It is also found that QT steel with a higher yield stress experiences more severe strength deterioration after exposure to a temperature of over 300 °C. When the exposure temperature is higher than 500 °C, however, the residual properties of the two types of steel become similar. Based on the test data in this paper and the literature, empirical equations are proposed to predict the residual elastic modulus, yield stress, ultimate strength, and ultimate strain of QT steel after exposure to elevated temperatures.
•Heat soak time affects residual mechanical properties at 500 °C or above.•Cooling method affects residual properties at a temperature above 700 °C.•Empirical equations are proposed to predict residual mechanical properties.
In this study, three kinds of non-quenched and tempered forged steels with V, V–Nb and V–Nb–Ti microalloying elements were designed. The microstructure and low–temperature impact fracture behavior of ...the three steels were comprehensively compared and analyzed. The results showed that the addition of Nb and Ti elements can significantly refine the grains and form precipitated particles to improve the strength of the steel. The fracture behavior of the three steels types at a low temperature of −28 °C was dominated by brittle cleavage fractures. V–Nb–Ti steel exhibited the worst low–temperature impact toughness. The SEM analysis result showed that coarse (V, Nb, Ti) (C, N) particles present in the steel acted as local cleavage initiation sites. No cleavage fracture caused by (V, Nb) (C, N) particles occurred in the V–Nb steel. The results showed that the high impact energy (70.4 J) at room temperature and low ductile–brittle transition temperature (DBTT, −20 °C) of the V–Nb steel were mainly attributable to the grain refinement of Nb. The micron–sized coarse (V, Nb, Ti) (C, N) particles were the main factor responsible for the low impact energy (35.5 J) at room temperature and high DBTT (−7 °C) of V–Nb–Ti steel and can act as the local cleavage initiation sites to dominate the cleavage fracture at low temperature and decrease the impact toughness.
The strength of high-strength steel butt-welded joints with a similar steel grade may be quite different due to their discrepancies in the processing methods of high-strength steel. The process ...methods mainly include quenching and tempering (QT) and thermo-mechanically controlled process (TMCP). These differences in the strength of high-strength steel butt-welded joints with different processing methods of steel have not been quantitatively determined and considered in several design codes, which may lead to the unsafe design for the high-strength steel butt-welded joints. This research demonstrated a comparative and quantitative study on the strength of double-V butt-welded joints made of QT550, QT690, TMCP 550, and TMCP690. The true stress-strain behavior of various zones within these welded joints, including the hardened heat affected zone (HHAZ), softened HAZ (SHAZ), weld metal (WM), and base metal (BM), were investigated based on the test and simulation results of the round notched specimens and the flat grooved specimens in tension. The developed true stress-strain models were validated against the test results of high-strength steel butt-welded joints. The strength of QT and TMCP high-strength steel butt-welded joints were then numerically analyzed considering the effect of different heat inputs of 1.0, 1.5 and 1.9 kJ/mm. The results show that strength ratios of SHAZ to BM of butt-welded joints made of TMCP steel were lower than those made of QT steel when the heat input was <1.9 kJ/mm. The strength of each zone in the TMCP high-strength steel butt-welded joint may not necessarily decrease with increasing heat inputs.
•The material properties of the heat-affected zone of butt-welded joints made of QT and TMCP steel were determined.•A comparative study of the true stress-strain relationship of each zone of the butt welds of QT and TMCP steels was performed.•The effect of the heat input during welding on the strength of butt welds of QT and TMCP steels was studied.•The discrepancy in the strength of high-strength steel butt-welded joints due to the different processing methods of steel was quantitatively determined.
Repeated welding caused by welding defects in the weld joint will result in changes in the mechanical properties of the weld joint, especially in the Heat Affected Zone (HAZ). Significant changes ...will occur when welding on steel materials that have undergone special treatment during the manufacturing process, such as quenched and tempered S690Q steel. S690Q steel is a structural steel with high yield strength with quenched and tempered conditions. The research was conducted by doing repeated welding as a simulation of the repair process using the GMAW process. Macro observations as well as tensile, bending, hardness, toughness tests were carried out on cross sections of welded joints. From the results of all the tests carried out the results meet the requirements of the BS EN ISO 15614-1 standard, but the results of the hardness test can identify significant changes in mechanical properties in the HAZ, especially in the bottom or root pass and hot pass areas. The average hardness value decreased from 334.07 HV at the top (face) to 209.55 HV and 198.88 HV at the bottom (hot pass and root pass) on the left side of the HAZ. While on the right side of the HAZ, the hardness value is 337.40 HV to 254.34 HV and 208.64 HV. Keywords: HAZ, S690Q, Structural Steel, Quenched and Tempered Steel
A novel quantitative characterization method of MnS distribution was proposed based on the data from automatic inclusions analysis software to evaluate their effects on intragranular polygonal ...ferrite (IPF) formation in medium carbon non-quenched and tempered steels. The results show that agminated MnS inclusions in the steel are less effective to promote the IPF formation and lots of inclusions are actually wasteful because their space distances are too small to act as nucleation sites individually, even their number is larger than that in the steel with plentiful uniformly distributed MnS inclusions. Due to the difference of observation method, three-dimensional dendritic MnS inclusions would be revealed as several agminated small-sized ones in two-dimensional observation. In order to avoid misunderstanding, the characteristics of the nearest space distances between inclusions in widely used two-dimensional observation could be applied to recognize and define these agminated MnS inclusions to more accurately evaluate their effects on IPF formation.
The thermal hysteresis in fusion welding causes serious deterioration of welds of medium to high-carbon steels, so the development of an effective alternative welding process are expected. Friction ...Stir Welding (FSW) is considered to be an effective alternative. FSW is a solid-state joining process in atmosphere, which reduces the risks associated with melting and solidification of metals. Another advantage is the in-process flexible controllability of heat input by controlling welding parameters. From this perspective, the authors are engaged in a series of studies to elucidate the characteristics of friction stir welded joints for medium- to high-carbon steels, including high-strength tempered steel.This report describes the results of applying friction stir welding to 1.4 GPa-grade tempered JIS-S55C steel plates. Five types of joints with different welding parameters were obtained by varying the joining parameters e.g. tool rotation speed or welding speed. The temperature of the FSW tool and material interface during friction stir welding was measured using a thermal imaging camera. The microstructure of the friction stir welded butt joint was evaluated by optical microscopy and FE-SEM / EBSD. The mechanical properties of the welds were evaluated by Vickers hardness test and tensile test, and DIC analysis was applied to analyze the details of local deformation during the tensile test. The effects of joining parameters on microstructure, microstructure of welds and mechanical properties of welds were examined in detail by properly conducting FE-SEM micro-observations, EBSD measurements.
The thermal hysteresis in fusion welding causes serious deterioration of welds of medium to high-carbon steels, so the development of an effective alternative welding process are expected. Friction ...Stir Welding (FSW) is considered to be an effective alternative. FSW is a solid-state joining process in atmosphere, which reduces the risks associated with melting and solidification of metals. Another advantage is the in-process flexible controllability of heat input by controlling welding parameters. From this perspective, the authors are engaged in a series of studies to elucidate the characteristics of friction stir welded joints for medium- to high-carbon steels, including high-strength tempered steel.This report describes the results of applying friction stir welding to 1.4 GPa-grade tempered JIS-S55C steel plates. Five types of joints with different welding parameters were obtained by varying the joining parameters e.g. tool rotation speed or welding speed. The temperature of the FSW tool and material interface during friction stir welding was measured using a thermal imaging camera. The microstructure of the friction stir welded butt joint was evaluated by optical microscopy and FE-SEM / EBSD. The mechanical properties of the welds were evaluated by Vickers hardness test and tensile test, and DIC analysis was applied to analyze the details of local deformation during the tensile test. The effects of joining parameters on microstructure, microstructure of welds and mechanical properties of welds were examined in detail by properly conducting FE-SEM micro-observations, EBSD measurements.
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