The growth rate of the shell thickness in the funnel mold for ultrahigh-speed continuous casting of steel has a significant effect on the shell surface quality. By using the node temperature inheritance algorithm, a three-dimensional transient thermal conductivity model of the liquid steel solidification inside the mold was established, and the effects of casting temperature and speed on the heat transfer behavior of the thin slab were investigated. The results show that with an increase in the casting speed from 4.0 to 6.0 m·min ⁻¹ , the maximum thickness of the shell at mold exit was reduced from 26 to 12.8 mm, and the maximum temperature of the slab surface at mold exit increased from 1,210 to 1,305°C. However, the increase of casting temperature from 1,550 to 1,560°C had little effect on the surface temperature and thickness of the slab shell.

Surface cracks are easily produced after friction between continuous casting billets and copper layers in mold cavity, but the formation mechanism is not clear. Based on a steel-based hot-dip copper plating experiment, this study simulated the action behavior of copper adhering to the surface of a continuous casting billet after mold wear and systematically analyzed the formation mechanism of cracks caused by copper infiltration on the surface of the continuous casting billet. It is shown that when the copper liquid adheres to the surface of the slab, in addition to the diffusion of Cu in the steel, Fe is also dissolved in the copper liquid, accelerating the solidification of the copper liquid on the surface of the slab and forming a stable fusion combination between copper and steel. At the same time, due to the enrichment of the Fe-C phase and a large number of vacancies at the grain boundary, the grain boundary becomes the dominant area of copper–steel fusion bonding. For a continuous casting process in which the temperature is kept higher than 900 ℃, Cu’s solubility is high and the diffusion coefficient is very low in Fe, which makes it very difficult for Cu accumulated in the grain boundary to diffuse into the steel matrix during the continuous casting process, resulting in a grain boundary with a greatly weakened strength becoming the origin of cracks in the bending and straightening deformation of the billet.

Reasonable control of the content of niobium (Nb) in a steel slab is of great significance to improve the strength of the slab and the regularity of precipitation of precipitates. In this study, the high-temperature strength and precipitates of the same steel with four different Nb contents (A: 0.006%, B: 0.031%, C: 0.050%, D: 0.065%) were tested and analyzed. The results show that when the Nb content is 0.031%, the tensile strength and yield strength of the steel reach the ideal state; the two-phase particles that precipitated in the Q355GJ steel after heat treatment are mainly square- and star-shaped (Ti, Nb) (C, N) composite precipitates, and their amount increases with the increase in Nb content. Most of the two-phase particles (84%) precipitated in the steel with the Nb content of 0.031% are smaller than 80 nm, and the continuous increase in the Nb content cannot increase the precipitation amount of the two-phase particles in the steel, but can increase the size of the two-phase particles.

Reasonable control of the niobium (Nb) content in steel slabs is of great significance for improving the surface quality of the slabs. In this study, a Gleeble-3500 thermal simulation testing machine was used to test the high-temperature mechanical properties of steel slabs with four different Nb contents (A: 0.006%, B: 0.031%, C: 0.050%, D: 0.065%) and to analyze the effect of Nb on the high-temperature performance of the steel slabs. Optical microscopy and transmission electron microscopy (TEM) were used to study the microstructure of the casting slabs with different Nb contents and the precipitation in the samples after heat treatment at different temperatures. The results of the study show that the microstructure of the cast slab is mainly composed of ferrite and pearlite. The distribution of each of the two structures was found to become more uniform and a significant improvement in yield strength was observed with increasing Nb content. After heat treatment, the precipitates in the high-rise structural steel were mainly square and star shaped Nb(C,N) precipitates. Increasing the Nb content was found to increase the amount of precipitates in steel. Beyond a threshold, increasing the Nb content did not increase the amount of precipitates. However, the size of the precipitates increased.

In this study, a multi-gradient microstructure was introduced into medium-Mn steel through torsion following heat treatment at different annealing temperatures, and through investigations on the mechanical properties under two annealing temperatures, it has also been revealed that different annealing temperatures before torsion affect the stability of austenite after torsion, thereby leading to distinct variations in mechanical performance. The yield strengths of the studied steel after annealing at 600 °C and 620 °C were 762 MPa and 673 MPa, with total elongation of 47.4% and 44.1%, respectively. After 90° torsion, the yield strength of experimental steels increased to 834 MPa and 808 MPa, while the elongation decreased to 21.6% and 29.5%, respectively. The gradient distributions from the center to the edge were observed for the austenite volume fraction, average grain size, martensite volume fraction, GND density, and hardness. The comparative analysis of the two annealing temperatures indicates that the larger grain size in the 620—annealed sample leads to its lower yield strength, while its higher austenite volume fraction and moderate stability promote a more sustained TRIP effect during deformation, contributing to its enhanced elongation. This multi-gradient microstructure is responsible for the yield strength improvements of 72 MPa and 135 MPa in the torsioned samples annealed at 600 °C and 620 °C, respectively.