Hot stamping process is widely used in the manufacture of the high strength automotive steel, mainly including the stamping and quenching process of the hot-formed steel. In the hot stamping process, the steel is heated above the critical austenitizing temperature, and then it is rapidly stamped in the mold and the quenching phase transition occurs at the same time. The quenching operation in the hot stamping process has a significant influence on the phase transition and mechanical properties of the hot-stamping steel. A proper quenching technique is quite important to control the microstructure and properties of an ultra-high strength hot-stamping steel. In this paper, considering the factors of the austenitizing temperature, the austenitizing time and the cooling rate, a coupled model on the thermal homogenization and phase transition from austenite to martensite in quenching process was established for production of ultra-high strength hot-stamping steel. The temperature variation, the austenite decomposition and martensite formation during quenching process was simulated. At the same time, the microstructure and the properties of the ultra-high strength hot-stamping steel after quenching at different austenitizing temperature were experimental studied. The results show that under the conditions of low cooling rate, the final quenching microstructure of the ultra-high strength hot-stamping steel includes martensite, residual austenite, bainite and ferrite. With the increase of the cooling rate, bainite and ferrite gradually disappear. While austenitizing at 930 °C, the tensile strength, yield strength, elongation and strength-ductility product of the hot-stamping steel are 1770.1 MPa, 1128.2 MPa, 6.72% and 11.9 GPa%, respectively.

Clarifying the influence of Nb and V microalloying on the ultra-high strength hot stamping steel (UHSHSS) and exploring appropriate process parameters are the basis for effectively regulating properties of the final product. In this study, the effects of different austenitizing temperatures and holding times on the phase transitions, grain sizes and mechanical properties of 22MnB5NbV with Nb and V alloyed are studied by using JMatPro thermodynamic calculations and experiments. By comparing with 22MnB5 without Nb and V alloyed, the effects of Nb and V elements on quenching microstructures, grain sizes and mechanical properties of UHSHSS are analyzed. The suitable austenitizing process parameters of 22MnB5NbV have been obtained. The results show that the grain size of Nb-V-alloyed UHSHSS grows with the increase in the austenitizing temperature and holding time. The 22MnB5NbV steel can be completely austenitized while the austenitizing temperatures ≥870 °C and holding time ≥3 min. Combined with the actual production process, the best austenitizing temperature and holding time are 930 °C and 3 min. Under these conditions, the average grain size is 7.7 μm, the tensile strength, elongation and strength-ductility product are 1570.8 MPa, 6.6% and 10.4 GPa·%, respectively. With the addition of Nb and V elements, the nanoscale precipitates lead to the refinement of the quenched structure and grain size, and the comprehensive properties of UHSHSS have been effectively promoted, in which the elongation and strong-plastic products are increased by ~0.6% and ~1.2 GPa·%, respectively.

Pellet ore is an important raw material for blast furnace ironmaking, and its reduction and expansion performance directly affects the smooth operation and smelting indicators of the blast furnace. This paper quantitatively studied the effects of magnesium minerals such as dolomite and serpentinite on the pellet-forming performance, the microstructure after roasting, compressive strength, and the reduction expansion performance of Baiyun Ebo iron concentrate. The optimal ratio of dolomite and serpentinite to add was determined when preparing pellets using Baiyun Ebo iron concentrate powder. The results showed that the drop strength and compressive strength of the green pellet after adding serpentine were relatively higher than those after adding dolomite, indicating that controlling the MgO content in the green pellet at 2.5% using serpentine was beneficial for improving the drop strength and compressive strength. Under the condition of adding dolomite, when the MgO content was 2.5%, the compressive strength of the roasted pellet was the highest, which was 2192.6 N, and the volume expansion rate was 12.32%. Under the condition of adding serpentine, when the MgO content was 2.5%, the compressive strength of the roasted pellet was 2622.2 N, and the volume expansion rate was 9.71%. Compared with dolomite as a magnesium additive, when the reduction expansion rate of Baiyun Ebo iron concentrate was controlled within 20%, serpentine only needed to have a MgO content of about 1.5% in the pellets, while dolomite needed to have a MgO content of about 2.5%. Therefore, under the condition that the MgO contents of dolomite and serpentine were equivalent, the amount of serpentine used was lower.

The occurrence form and size distribution of TiN and sulfide inclusions in 20CrMnTi gear steel directly affect the mechanical and service properties of the steel. Mastering efficient modification methods for TiN and sulfide inclusions in steel is of great significance. In this article, the differential behaviors of rare earth La and Ce on TiN and sulfide inclusions modification of 20CrMnTi gear steel are studied. The results show that the addition of rare earths La and Ce can modify the large TiN and sulfide inclusions into smaller composite inclusions. When rare earth La element (Abbreviated as RE La) is added to steel, the TiN and sulfide inclusions in steel are modified to small size LaAlO ₃ -TiN, LaAlO ₃ -MnS-TiN composite inclusions. When rare earth Ce element (abbreviated as RE Ce) is added to steel, the TiN and sulfide inclusions in steel are modified to small size CeAlO ₃ -TiN and CeAlO ₃ -MnS-TiN composite inclusions. The effect of RE Ce on TiN modification is better than that of RE La. When RE Ce is added, the average size of TiN inclusion in steel is 2.77 μm, and when La is added, the average size of TiN inclusion in steel is 3.24 μm. The modification effect of RE La on sulfide inclusion in steel is better than that of RE Ce. When La is added, the average size of sulfide inclusions in steel is 1.72 μm. When Ce is added alone, the average size of sulfide inclusions in steel is 2.08 μm.

Ludwigite is usually introduced into iron ore concentrate as an additive to enhance the quality of pellets. It is widely accepted that the addition of B₂O₃ can facilitate the growth of hematite grains and enhance the compressive strength of pellets. However, there is currently no consensus regarding the influence of B₂O₃ on the stability of hematite reduction process. This study employed X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy-energy-dispersive X-ray spectroscopy and Raman spectroscopy to analyse the impact of B₂O₃ contents (0–8 wt.%) and roasting times (10–40 min) on the stability of magnetite oxidation process and hematite reduction process. The results indicate that the growth and crystallisation of hematite are facilitated with the increase of B₂O₃ content and roasting time. The addition of B₂O₃ induced crystal defects in Fe₂O_3. However, the properties in question exhibited a downward trend when B₂O₃ content exceeds 6 wt.%. Furthermore, the reduction degree exhibited a notable decline with an increase of B₂O₃ content. Fe₂O₃ particles are tightly enveloped by a liquid phase rich in boron element, hindering their reduction. Nonetheless, the discovery of the effect of B₂O₃ on hematite reduction process has contributed to the application of ludwigite in pellets.