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The mechanical properties and microstructure evolution of a one step quenching and partitioning steel containing bainite/martensite/retained austenite mixed microstructure was studied though static and dynamic tensile tests (strain rates ranging from10-3 s-1 to 5×102 s-1 ).Scanning electron microscopy (SEM) and electron-back-scattered diffraction (EBSD) were applied to describe the microstructure evolution near the fracture. XRD characterization shows the volume fraction of retained austenite decrease exponentially with the strain rates increased.EBSD phase maps reveal that the first type of retained austenite is less sensitive to strain rate than the second type.

In this work, a new type of δ-TRIP steel was designed. The fatigue limit and S-N curve were measured by the fatigue testing machine. The fatigue fracture was observed by the scanning electron microscope (SEM), and influencing factors of fatigue crack initiation were analyzed. The results show that the fatigue limit of δ-TRIP steel is 361MPa, and fatigue cracks are initiated on the surface of the sample. In the initial stage of fatigue crack initiation, the crack growth rate of the specimen is relatively slow under the action of stress. As the crack continues to expand, the magnitude and direction of the stress on the crack tip change and the stress situation becomes more complicated. The crack begins to branch and grows in different directions The metastable retained austenite in δ-TRIP steel undergoes phase transformation under the action of external force, which will absorb a lot of energy. At the same time, there are many large angle grain boundaries in the steel hindering the crack propagation. These two characteristics make the δ-TRIP steel has excellent fatigue performance.

A salt spray test was used to evaluate the corrosion resistance of pickling high-strength steel JSH440W to phosphating film on its surface, and scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to analyze the micro-morphology and element content of the surface defects area of the sample after salt spray test. The results show that the enrichment of Si, Mn, and O elements on the surface of pickling high-strength steel JSH440W badly influences the film-forming performance of phosphating film. The enrichment of Si, Mn, and O elements on the strip steel surface will slow down the film-forming speed of the phosphating film and make the thickness of the phosphating film thinner, which will lead to the reduction of corrosion resistance in this area, and it is easy to cause pitting corrosion behavior, to cause the paint film on the sample surface to fall off. Reducing the enrichment of Si, Mn, and O elements on the surface of acid-washed high-strength steel and increasing the roughness uniformity of strip steel is beneficial to improving the quality of phosphating film and the coating property of pickling high-strength steel.

A hot-dip galvanizing simulator studied the effects of soaking temperature and rapid cooling end temperature on the microstructure and mechanical properties of 980MPa grade hot galvanizing dual-phase steel. The results show that at the end of a rapid cooling temperature of 290°C, with the increase of soaking temperature, the tensile strength of the test steel increases, the yield strength decreases at first and then increases, and the elongation decreases as a whole. The microstructure is composed of ferrite and martensite. At the soaking temperature of 800°C, the strength decreases, and the elongation increases with the increase of the end temperature of rapid cooling. When the end temperature of rapid cooling is lower than 320°C, the martensite tempering is not apparent, and there are many high-density dislocations in the ferrite near the neighbor. When the cooling end temperature is 350°C, some martensites are tempered. The twins in martensite are widened, carbides are unevenly precipitated along the twin planes, and the dislocation density decreases. When the soaking temperature is between 800°C and 820°C, the end temperature of rapid cooling is between 260°C and 290°C. Annealing and zinc plating are beneficial to obtain dual-phase steel with excellent mechanical properties.

The abrasive particles in the working emulsion were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The wettability and lubricating properties of different emulsion samples were tested by MRS-10E four-ball friction and wear testing machine. The results show that the average particle size of the abrasive particles in the working emulsion is 2.72 μm, the smallest particle size of the abrasive particles is 0.36μm, and the largest size of the abrasive particles is 6.57μm. Furthermore, the abrasive particles can increase the internal friction of emulsions, which leads to the increasing wetting Angle. The abrasive particles cause the lubrication performance of working emulsion to decrease, which eventually leads to larger wear scar diameter and the worse morphology of wear scar on the tested ball.

In this research, the four-ball tribometer was examined to evaluate the tribological behavior of environment-friendly nano-MoS2 water-based rolling liquid. This experimental method is used to measure the maximum non-seizure load (PB) and the coefficient of friction (μ). The newly introduced parameter ω can be combined with PB and μ to evaluate the tribological performances of the rolling liquid comprehensively. The rolling lubrication performance of nano-MoS2 water-base lubricant was investigated using a single stand 4-high reversing rolling mill. The surface morphologies and rolling textures of the samples were measured by laser scanning confocal microscope after cold rolling. The results show that when the concentration of nano-MoS2 is 0.4wt.%, the water-based rolling liquid has excellent tribological and lubricating properties. Meanwhile, compared with the surface quality of the rolled strip under other lubrication conditions, the surface quality of the cold-rolled strip using 0.4wt% nano-MoS2 rolling liquid as the lubrication condition is smooth without visible surface defects.

The effect of dew points (− 50, − 10 and + 10 °C) on the galvanizing properties of a high-manganese twinning-induced plasticity (TWIP) steel was studied. Scanning electron microscopy (SEM), glow discharge optical emission spectrometry (GDOES) and X-ray photoelectron spectroscopy (XPS) were used for microscopic observation and qualitative analysis of the interfacial layer between the steel surface and the zinc layer after hot-dip galvanizing. SEM analysis results show that three different morphologies of metallic oxides are formed on the interfacial layer under the different dew points. GDOES results show that Al is present in the molten zinc, reacting with Fe on the steel surface to form Fe2Al5, which can increase the galvanizing properties. XPS results show that the valence states of Mn in the interfacial alloy layer are Mn2+ and Mn4+, and the valence states of Fe are Fe0, Fe2+ and Fe3+. The experimental results show that the hot-dip galvanizing performance is the best at − 10 °C and the formation of Mn and Fe intermetallic oxides has a bad effect on hot-dip galvanizing behavior of the high-manganese TWIP steel. The types of the formed surface oxides (MnO, Mn3O4, Mn2O3, FeO, and Fe2MnO4) on the surface of the steel sheet are confirmed. It can obtain the best hot-dip galvanizing performance of the high-manganese TWIP steel by controlling the dew point from − 10 to − 5 °C.

BACKGROUND: Pancreatic cancer (PC) can be considered a representative cancer type of the human body. As demonstrated by some studies, microRNA (miR)-499 is dysregulated in various cancer types including PC, for which chemotherapy involving 5-fluorouracil (5-FU) has long been considered the first-line therapy. However, there are complex and comprehensive mechanisms related to 5-FU, which have not been fully elucidated. This study thus aimed to examine the molecular mechanisms of 5-FU resistance through miR-499a-5p in PC. METHODS: The expression of miR-499a-5p in PC was measured using quantitative polymerase chain reaction (PCR). MiR-499a-5p was examined in-vivo for its effects on the malignant phenotypes of PC cells. RESULTS: The results of the present study demonstrated miR-499a-5p to be upregulated in PC and 5-FU resistant PC tissues. According to in vitro assays in PC cells (PANC1/FR), miR-499a-5p was found to affect adenosine triphosphate (ATP) binding cassette subfamily B member 1 (P-gp), ATP binding cassette subfamily C member 1 (MRP1), and ATP binding cassette subfamily G member 2 (BCRP), thereby facilitating 5-FU resistance in PC cells. Functions assays indicated that suppressed miR-499a-5p expression inhibited the proliferation and migration of cells but facilitated apoptosis in the PC cell line; by contrast, miR-499a-5p overexpression triggered the inverse phenotypic changes of cells. Concerning the mechanisms involved, miR-499a-5p increased PI3K/Akt signaling by targeting phosphatase and tensin homolog (PTEN). CONCLUSIONS: Taken together, these findings demonstrate that miR-499a-5p can be potentially applied to PC therapy.

To meet the demand of the 10% weight reduction goal for automotive steel, the microstructure and mechanical properties of Fe–18Mn–Al–C steel with different carbon and aluminum contents were investigated under different solid solution treatments, and the deformation mechanisms of the experimental steels were elucidated. Aided by thermodynamic calculation, transmission electron microscopy (TEM) and in situ scanning electron microscope (SEM) analysis, it was shown that for the 18Mn–1.5Al experimental steel with about 20 mJ/m2 stacking fault energy (SFE), the twinning-induced plasticity (TWIP) effect always dominated in this steel after different solid solution treatments under tensile deformation. With the 7 wt% aluminum addition, the SFE of austenite was affected by temperature and the range of SFE was between 60 and 65 mJ/m2. The existence of δ-ferrite obviously inhibited the TWIP effect. With the increase in the solution treatment temperature, δ-ferrite gradually transformed into the austenite, and the n-value remained low and stable in a large strain range, which were caused by the local hardening during the tensile deformation. Due to the difference in the deformability of the austenite and δ-ferrite structure as well as the inconsistent extension of the slip band, the micro-cracks were easily initiated in the 18Mn–7Al experimental steel, then, it exhibited lower plasticity.

There are many blackening defects on the steel surface after the pickling of the hot-rolled strip. The microstructure and roughness of the picked steel surface were analyzed using SEM and LEXT OLS4000 laser scanning confocal microscope, XPS, and surface roughness measure. The experiment result shows that the blackening defects of the hot-rolled pickled steel sheet surface are aging of anti-rust oil film to the strip steel surface. In order to improve the surface quality of the hot rolling pickled sheet, the anti-rust oil coating process is improved, the thickness and uniformity of the oil film are controlled, and the management in the course of packing, storage and transportation are strengthened.

In view of the chromatic aberration defects on the surface of hot-rolled strip A510L after pickling, the microscopic morphology features of different positions of the pickled strip surface were compared and analyzed by using laser confocal microscope (CLSM), and the generation mechanism of chromatic aberration defect was studied by using high-resolution scanning electron microscope and energy dispersive spectrometer (EDS). The results show that the main cause of surface chromatic aberration defects on the surface of pickling plate A510L is closely related to the interface state between hot-rolled raw material substrate and iron oxide scale, and there is an enrichment area of Fe2SiO4/FeO binary eutectic near the hot-rolled raw material substrate and iron oxide scale, and the proportion of Fe2SiO4 is higher as it approaches the strip steel matrix, and Fe2SiO4/FeO has a strong pinning effect, which leads to the adhesion of iron oxide scale. The existence of Fe2SiO4/FeO changes the structure of carbon steel iron oxide scale, which makes the bonding force between iron oxide scale and strip steel matrix increase the difficulty of pickling process, and finally presents the surface chromatic aberration defects after pickling.

In this work, a novel type of δ-TRIP steel was designed, and the content and stability of retained austenite in δ-TRIP specimens under different annealing processes were detected and studied, respectively. The volume fraction of austenite was determined by X-ray diffraction (XRD). The microstructure and mechanical properties were analyzed systematically. The results show that a complex microstructure composed of three phases (ferrite, bainite and retained austenite) was obtained in the δ-TRIP steel. With the increasing of annealing temperature, both retained austenite and bainite content in the specimen increased, while the carbon content in retained austenite decreased, leading to a poor stability for retained austenite. Both tensile and yield strength improved with the increasing of annealing temperature, while the elongation reduced. The feature of retained austenite led to an excellent combination of ductility and strength, which was better than traditional TRIP steel.

In this paper, a new type of automotive 1500 MPa grade hot-formed steel without boron but containing niobium was subjected to thermoforming experiments. The phase transition point and Continuous Cooling Transformation (CCT) curve of the hot-formed steel were measured by thermal dilatometer, and then the best austenitizing parameters was determined. The microstructure of the cold-rolled sheet and the hot-formed steel sheet were observed by electron microscopy. The microstructure of the steel sheet after hot forming was studied by X-ray diffraction (XRD) method to determine whether the microstructure after hot forming had residual austenite. The influence of residence conditions on its mechanical properties was studied. The experimental results has shown that the microstructure of the original cold-rolled sheet is mainly composed of ferrite and pearlite. After thermoforming, the basic microstructure are martensite and a small amount of ferrite; When the hot forming parameters is that 900 °C of the heating temperature, 3 min of the holding time, 8 s of the residence time, quenching temperature is the room temperature, the new 1500 MPa grade hot formed steel has the best mechanical properties that the tensile strength is 1519 MPa, the yield strength is 1060 MPa, the yield ratio is 0.73, and the elongation reaches 10.52%. The result shows that the new 1500 MPa grade hot formed steel could obtain excellent mechanical properties through a reasonable process under the premise of ensuring hardenability.

The effects of the microalloying element niobium (Nb) on the microstructure and mechanical properties of the hot stamped steel 38MnB5 were investigated. The impact of Nb addition on the microstructure was studied through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The experimental results indicated that the microstructures of the steel containing Nb were finer than those of the steel without Nb. Moreover, Nb mainly presented as a second-phase particle in 38MnB5 steel, and the particles included Nb carbonitrides. In addition, the tensile strength and elongation of the hot rolled and hot stamped steels were also measured, and they demonstrated that the appropriate addition of Nb was beneficial to the mechanical properties of 38MnB5. Under the same conditions, the tensile strength of 38MnB5Nb was higher than that of 38MnB5, which increased from 2011 to 2179 MPa. The yield strength also increased from 1316 to 1476 MPa, and the elongation increased from 5.92% to 6.64%. Overall, Nb had a positive effect on the performance of the hot stamped steel.

The effect of different content of Nb on the microstructure, mechanical properties and hydrogen trapping of press hardening steel (PHS) 22MnB5 was investigated. The function of Nb addition on the microstructure was studied by SEM, TEM and XRD, etc. which indicated that the microstructure of the steels containing Nb are finer than that of steel without Nb, what's more, the lath martensite is narrower with the increase of Nb content. Certainly, the tensile strength and elongation of hot-rolled steels, hot stamping steels also were measured. To understand the influence of adsorbed hydrogen on the mechanical property of PHS, Slow Strain Rate Test (SSRT) of smooth specimen was conducted. The results of precipitation behavior and its impact on hydrogen induced cracking manifested that hydrogen embrittlement susceptibility decreased obviously with the addition of Nb content due to reduced hydrogen diffusivity.

Manganese-rich austenitic twinning-induced plasticity (TWIP) steels with high strength and superior ductility have received much attention in the past two decades. Tremendous efforts have been made to explore their unusual hardening behaviour which includes contributions from twins, dislocations, grain boundaries and solid solution. Nevertheless, the individual hardening effects of twins, dislocations, grain boundaries and solid solution on the high strength of TWIP steels are still unclear. In the present work, the flow stress of a TWIP steel was experimentally decomposed into the respective contributions of twins, dislocations, grain boundaries and solid solution. For the forest hardening, synchrotron X-ray diffraction experiments with line profile analysis were carried out to measure the dislocation density. It is found that the yield stress of the present TWIP steel is controlled by solid solution and grain boundary hardening, which contribute to 238.3 and 238.5 MPa, respectively. After yielding, the work-hardening rate is dominated by dislocation multiplication which accounts for up to 922 MPa at a true strain of 0.4, equal to about 60% of the flow stress. In comparison, twins contribute to only 118 MPa at the same true strain, equal to about 8% of the flow stress. In other words, twins have minor effect on the flow stress, in contrast to the current understandings in the literature.

The surface/subsurface characterization of chromium nitride (CrN)/silicon nitride (Si3N4) sliding contact was carried out, to comprehend how the composition migration and structural transformation across the dynamically-formed tribo-interface evolve with the varying temperature. Superior wear and friction performance is achieved for the chromium nitride (CrN)/silicon nitride (Si3N4) contact at 800 °C. The tribo-interface demonstrates two structural characteristics, a superficially disordered nano-scale grained tribofilm and an underlying deformed coarse-grained layer, being temperature-dependent. The unique multilayered structure and oxides segregation at the sliding interface are responsible for the adaptive capability of the rubbing contact during their interaction with oxygen/air at 800 °C. The computational analysis based on the Density Functional theory (DFT) was further implemented to confirm the tribochemical interaction of predominant oxide heterogeneities.

Microstructure and tensile properties of the laser welded joint of Fe–18.8Mn–0.6C TWIP steel were investigated in this research. The microstructure of fusion zone (FZ) was characterized by means of X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). TEM and in-situ SEM observation were employed to investigate the microstructural evolution and strengthening mechanism of FZ during deformation. The welded joint with a fully austenitic structure was obtained by the laser welding. The granular divorced eutectic phases (Fe, Mn)3C and inclusions formed in the interdendritic regions during the solidification of FZ. The fully austenitic structure and coarse dendrite grains were responsible for the fracture at the weld seam. The FZ exhibited a good combination of strength (e.g. tensile strength up to 1000 MPa) and ductility (e.g. total elongation up to 73%). The microstructural evolution revealed that dislocation slip was the main deformation mechanism at low strains of FZ, while at relatively high strains, mechanical twinning was the domain deformation mechanism and played an important role in improving the strength and ductility as well as the work-hardening effect of FZ. Keywords: Laser welding, Microsegregation, Shrinkage porosities, Mechanical twins, Microstructural evolution

Thermodynamics of martensite transformation in low alloy TRIP steel was evaluated. It follows from the calculation that ɛ phase might be formed at above room temperature in the low Mn steel. Tested steel was therefore prepared according to the estimation result. High volume fraction of retained austenite (above 20%) containing 1.37%C was obtained in the steel after continuous annealing treatment. Tensile test showed that the ultimate strength of the steel was 1248 MPa, elongation rate 25%, the product of strength and elongation rate 31200 MPa%, reaching the grade of the third generation automobile steel. XRD detection indicated that after tensile test there existed α′ martensite while TEM revealed that ɛ martensite also existed in the steel. Strengthening-toughening of the steel was thus raised due to the effect of transformation induced plasticity of two kinds of martensite. In addition, the phase stability of carbonitrates containing V was estimated and the complex micro-alloying effect was applied to the steel to control the size of austenite (about 1 μm) and enhance the stability of austenite.

Based on cold rolled TRIP 780 and HSLA 340 steel sheet, the stamping of U-beam and its springback was studied experimentally and numerically. Collision analysis model of closed hat section beam was established on the basis of simulation analysis of U-beam stamping and springback. Mapping was used to transfer results of thickness, stress and strain from U-beam forming part to impact structural part. Collision processes of closed hat section were analyzed under various operating conditions considering the influence of forming. The results show that thickness variation, residual stress, and work hardening of the material after forming affect collision processes directly. The current paper provides reference for improving precision of simulation for vehicle collision.

The stretch-bend springback of dual phase steel DP590 was studied experimentally and numerically in this paper. The aim of this work is to investigate the effects of elastic modulus degradation on springback prediction. Two anisotropic yield function yld89 and yld2000 were utilized along with isotropic hardening law. The appropriate material parameters characterizing the elastic modulus decrease were identified by response surface modeling. The current work showed that the accuracy of springback simulation improved when considering elastic modulus decreased with plastic strain and yld2000 can predict springback more accurately considering blank orientation in stretch bending.

A more accurate estimation of stress-strain relationships for martensite and ferrite was developed, and the modified law of mixture was used to investigate the stress-strain partitioning of constituent phases in dual phase (DP) steels with two different martensite volume fractions. The results show that there exist great differences in the stress-strain contribution of martensite and ferrite to DP steel. The stress-strain partitioning coefficient is not constant in the whole strain range, but decreases with increasing the true strain of DP steel. The softening effect caused by the dilution of carbon concentration in martensite with the increase of martensite volume fraction has great influence on the strain contribution of martensite. The strain ratio of ferrite to martensite almost linearly increases with increasing the true strain of DP steel when the martensite volume fraction is 22%, because martensite always keeps elastic. But the strain ratio of ferrite to martensite varies indistinctively with the further increase in true strain of DP steel above 0.034 when the martensite volume fraction is 50%, because plastic deformation happens in martensite. The stress ratio of martensite to ferrite decreases monotonously with increasing the true strain of DP steel whether the martensite volume fraction is 22% or 50%.

As an industry-university cooperative project, an experimental research was conducted to develop a new cold-rolled transformation-induced plasticity (TRIP) steel with a tensile strength of 780 MPa and an elongation of 25% under the conditions that some processing parameters were preestablished according to the actual conditions of factory production lines. The optimal heat treatment conditions for obtaining excellent strength-ductility balance, specifically in intercritical annealing temperature and isothermal treatment temperature, were investigated by means of thermal simulation tests. Ultimately the desirable mechanical properties were attained successfully, and the microstructure and retained austenite stability were studied by optical microscopy (OM) and electron back-scattered diffraction (EBSD). It has been discovered that the sizes of retained austenite grains are generally proportional to the sizes of carbon-clear space (ferrite and bainite) around them, and there is marked selectivity in their transition sequences under stress-strain.

A cold rolled dual phase (DP) steel with the C-Si-Mn alloy system was trial-produced in the laboratory, utilizing a Glee-ble-3800 thermal simulator. The effects of continuous annealing parameters on the mechanical properties and microstructures of the DP steel were investigated by mechanical testing and microstructure observation. The results show that soaking between 760 and 820℃ for more than 80 s, rapid cooling at the rate of more than 30℃/s from the quenching temperature between 620 and 680℃, and overaging lower than 300℃ are beneficial for the mechanical properties of DP steels. An appropriate proportion of the two phases is one of the key factors for the favorable properties of DP steels. If the volume fraction of martensite and, thereby, free dislocations are deficient, the tensile strength and n value of DP steels will decrease, whereas, the yield strength will increase. But if the volume fraction of martensite is excessive to make it become a dominant phase, the yield and tensile strength will increase, whereas, the elongation will decrease obviously. When rapid cooling rate is not fast enough, pearlite or cementite will appear, which will degrade the mechanical properties. Even though martensite is sufficient, if it is decomposed in high temperature tempering, the properties will be unsatisfied.

The bake hardening (BH) behavior of transformation-induced plasticity (TRIP) and dual-phase (DP) steels after different prestrains was studied. The experimental results indicate that TRIP steel exhibits good BH ability while DP steel does not, and prestrain displays a strong effect on the BH values of both steels. The comparison of microstructures of the two steels showed that the hard second phase in the matrix might be harmful to the BH ability. For deformed specimens, baking resulted in a loss of uniform elongation, but there was no obvious decrease in uniform elongation for unprestrained specimens.

Formation of austenite strongly influences the microstructures and mechanical properties of dual phase steels. In present work, austenization process during intercritical annealing was studied in a Fe-C-Mn steel using Gleeble-1500 thermal simulator and quantitative microscopy. The experimental results show that austenite formation is separated into three different stages: (i) growth of high carbon austenite into pearlite rapidly until pearlite dissolution is completed; (ii) slower growth of austenite into ferrite; (iii) very slow equilibration between ferrite and austenite. The thermodynamic and kinetic analyses show that growth of austenite into ferrite is controlled by carbon diffusion in austenite in the primary stage and manganese diffusion in ferrite in the subsequent stage because diffusion coefficient of Mn in ferrite is several orders of magnitude smaller than that of C in austenite. The slow final equilibration between ferrite and austenite is obtained by manganese diffusion through the austenite. Based on the analysis, one dimensional diffusion model of intercritical austenization was developed and solved using finite volume method on the assumption that solute flux was local balance at interface, and the kinetics calculated was compared with experimental results. Simulated results indicate that growth of austenite reaches paraequilibrium in about one second, but remains thousands of seconds to reach final equilibrium. Simulated concentration profiles show that manganese atoms transferred from ferrite congregate in austenite near phase interface, which is consistent with the experimental phenomenon.

A new type of transformation induced plasticity (TRIP) steel with not only high strength and high ductility but also superior welding and galvanizing properties was designed and developed recently. Low carbon and low silicon content were preliminarily selected with the aim of meeting the requirements of superior quality in both welding and galvanizing. Phosphorus was chosen as one of the alloying elements, because it could reduce carbon activity in cementite and increase the stability of austenite. In addition, the possibility of phosphorus segregating at grain boundary was also discussed by thermodynamics as well as kinetics. Phase diagram was estimated at high temperature and the composition of the steel was then selected in the hyperperitectic range to avoid problems, which might occur in sheet steel continuous casting. Phase diagram in the inter-critical temperature was estimated for the steel to obtain the starting temperature of fast cooling. For understanding the minimum rate of fast cooling, pearlite growth kinetics was calculated with self-developed diffusion coefficients of elements in grain boundary. Overaging temperature was determined through the calculation of To temperature by both equilibrium and para-equilibrium assumptions, which was different from the current determination, which is only based on an equilibrium estimation.