Bo Song

In this study, the effect of final cooling temperature on hydrogen induced cracking (HIC) resistance and mechanical properties of Ce-treatment high vanadium X80 pipeline steel has been investigated. The results show that as the final cooling temperature decreased from 880 °C to 560 °C, 460 °C, and 360 °C, the average size, number density, and volume fraction of vanadium precipitates decrease, and the microstructure transforms from polygonal ferrite + degenerate pearlite to acicular ferrite + granular bainite, granular bainite + bainitic ferrite, and bainitic ferrite + martensite, respectively. The changes in the microstructure are the main reason for the variations in the HIC resistance and mechanical properties of tested steels. Selecting a reasonable final cooling temperature (460 °C∼560 °C in this study) to form a microstructure dominated by granular bainite + bainitic ferrite or acicular ferrite + granular bainite can obtain X80 grade pipeline steel that meets the service standards in acidic environments and mechanical property requirements.

To address the central cracking problem in continuous casting slabs of 38CrMoAl steel, high-temperature tensile tests were performed using a Gleeble-3800 thermal simulator to characterize the hot ductility of the steel within the temperature range of 600–1200 °C. The phase transformation behavior was computationally analyzed via the Thermo-Calc software, while the microstructure, fracture morphology, and precipitate characteristics were systematically investigated using a metallographic microscope (MM), a field-emission scanning electron microscope (FE-SEM), and transmission electron microscopy (TEM). Additionally, the effects of different holding times and cooling rates on the microstructure and precipitates of 38CrMoAl steel were also studied. The results show that the third brittle temperature region of 38CrMoAl steel is 645–1009 °C, and the fracture mechanisms can be classified into three types: (I) in the α single-phase region, the thickness of intergranular proeutectoid ferrite increases with rising temperature, leading to reduced hot ductility; (II) in the γ single-phase region, the average size of precipitates increases while the number density decreases with increasing temperature, thereby improving hot ductility; and (III) in the α + γ two-phase region, the precipitation of proeutectoid ferrite promotes crack propagation and the dense distribution of precipitates at grain boundaries causes stress concentration, further deteriorating hot ductility. Heat treatment experiments indicate that the microstructures of the specimen transformed under water cooling, air cooling, and furnace cooling conditions as follows: martensite + proeutectoid ferrite → bainite + ferrite → ferrite. The average size of precipitates first decreased, then increased, and finally decreased again with increasing holding time, while the number density exhibited the opposite trend. Therefore, when the holding time was the same, reducing the cooling rate could increase the average size of the precipitates and decrease their number density, thereby improving the hot ductility of 38CrMoAl steel.

To address the central cracking problem in continuous casting slabs of 38CrMoAl steel, high-temperature tensile tests were performed using a Gleeble-3800 thermal simulator to characterize the hot ductility of the steel within 600 – 1200 °C. Phase transformation behavior was computationally analyzed via Thermo-Calc software, while microstructure, fracture morphology, and precipitate characteristics were systematically investigated using metallographic microscope(MM), field emission scanning electron microscope(FE-SEM), and transmission electron microscopy(TEM). Additionally, the effects of different holding times and cooling rates on the microstructure and precipitates of 38CrMoAl steel were also studied. The results show that the third brittle temperature range of 38CrMoAl steel is 645 – 1009 °C, and the fracture mechanisms can be classified into three types: (I) In the α single-phase region, the thickness of intergranular proeutectoid ferrite increases with rising temperature, leading to reduced hot ductility; (II) In the γ single-phase region, the average size of precipitates increases while the number density decreases with increasing temperature, thereby improving hot ductility; (III) In the α + γ two-phase region, the precipitation of proeutectoid ferrite promotes crack propagation, and the dense distribution of precipitates at grain boundaries causes stress concentration, further deteriorating hot ductility. Heat treatment experiments indicate that under water cooling, air cooling, and furnace cooling conditions, the microstructures of the specimen transform as follows: martensite + proeutectoid ferrite → bainite + ferrite → ferrite; the average size of precipitates first decreases, then increases, and finally decreases again with increasing holding time, while the number density exhibits the opposite trend. Therefore, when the holding time is the same, reducing the cooling rate can increase the average size of precipitates and decrease their number density, thereby improving the hot ductility of 38CrMoAl steel.

The MgO–Al ₂ O ₃ –SiO ₂ –10wt%CaO system is a crucial slag system for high‐carbon ferrochrome (HCFeCr). The slag has high MgO and Al ₂ O ₃ characteristics (22–33 wt%), with a typical liquidus temperature of ≈1650–1750 °C for HCFeCr slag. Understanding the microstructure and thermodynamic properties of this system is critically important for HCFeCr production. This study investigates the effects of w(Al ₂ O ₃ )/w(SiO ₂ ) and w(MgO)/w(Al ₂ O ₃ ) ratios on the microstructure and properties of the system using molecular dynamics simulations combined with FactSage. The results demonstrate that Si ⁴⁺ ions predominantly form [SiO ₄ ] ⁴⁻ tetrahedral structures, while Al ³⁺ ions mainly exist as [AlO ₄ ] ⁵⁻ tetrahedrons. As the w(Al ₂ O ₃ )/w(SiO ₂ ) increases from 0.7 to 1.1, the degree of polymerization (DOP) of the slag increases. At a w(Al ₂ O ₃ )/w(SiO ₂ ) ratio of 1.0, Al ₂ O ₃ exhibits basic characteristics, disrupting the SiOAl structure and slowing the upward trend in polymerization. As the w(MgO)/w(Al ₂ O ₃ ) increases from 0.8 to 1.5, more Mg ²⁺ ions act as network modifiers, promoting the dissociation of three‐coordinated and bridging oxygen into non‐bridging and free oxygen, thereby reducing the DOP. Furthermore, the w(Al ₂ O ₃ )/w(SiO ₂ ) ratio has a greater influence on the system's enthalpy and liquidus temperature than the w(MgO)/w(Al ₂ O ₃ ) ratio.

The desulfurization capacity of top slag in the process of pre-desulfurization of hot metal containing vanadium and titanium was researched. The top slag system of CaO-SiO₂-MgO-Al₂O₃-TiO₂-VO_x that was formed by blast furnace slag and a CaO desulfurization agent reduced the sulfur in hot metal from 0.08 wt.% to 0.02 wt.%. It was found that the resulfurization of the slag happened in the later periods of the desulfurization process. The vanadium–titanium oxides were both acidic in the desulfurization slag. TiO₂ and VO_x reacted with the basic oxides to form CaTiO₃ and MgV₂O₄ at 1623 K, which reduced free CaO and was not conducive to top slag desulfurization. The results of calculation showed that the top slag desulfurization accounted for 15% of the total desulfurization. Using the ionic and molecule coexistence theory of slag structure, it is shown that the desulfurization efficiency could be enhanced by adjusting both the amount of desulfurization agent and the composition of the blast furnace slag before pre-desulfurization.

Coupled with the results of a 2D heat transfer model, a 3D electromagnetic stirring round billet model is developed, which is considered for the difference in the conductivity of solidified shell and molten steel. The electromagnetic field distribution features of the billet and the effect of round billet sizes on the electromagnetic field are investigated. It is found that as the solidified shell conductivity of the Φ600 mm round billet increases from 7.14 × 10⁵ S·m⁻¹ to 1.0 × 10⁶ S·m⁻¹, the magnetic induction intensity decreases and the maximum value of electromagnetic force drops from 7976.26 N·m⁻³ to 5745.32 N·m⁻³. The magnetic induction intensity on the center axis of the stirrer rarely changes in the range of Φ100–Φ200 mm. With the increase in the round billet from Φ300 mm to Φ600 mm, the magnetic induction intensity and the electromagnetic force on the center axis of the stirrer decrease slowly and then significantly. In the range of 2–8 Hz, as the current strength reaches its maximum, the electromagnetic force can be increased by increasing the current frequency for round billets of Φ100–Φ500 mm, while there is an optimal current frequency for round billets larger than Φ600 mm.

The role of vanadium content in hydrogen diffusion behaviour in X80 pipeline steel has been investigated. The results show that the hydrogen diffusion coefficient decreases from 1.54 × 10⁻⁶ cm² s⁻¹ in 1# steel with 0.03 wt-% vanadium to 9.97 × 10⁻⁷ cm² s⁻¹ in 3# steel with 0.14 wt-% vanadium due to the increase in irreversible and reversible traps. With the increase in vanadium content, more vanadium carbides precipitate in tested steels. The length (or number) of the low angle grain boundaries and special grain boundaries increase because of grain refinement, making the number of reversible traps increase. The dynamic condition of diffusible hydrogen in 3# tested steel with the largest number of hydrogen traps is the weakest. To weaken hydrogen-induced cracking dynamic condition, it is recommended to add 0.14 wt-% vanadium to X80 pipeline steel.