The trial production process primarily involves the smelting and pouring stages. A key aspect is ensuring that the solution pool is cleaned and properly filled at 0.90%. During the oxidation period, high-temperature decarburization (at 1580°C) must be no less than 0.30% to effectively remove gases and inclusions. Pre-deoxidation is carried out by inserting Al at 0.5 kg/Pt, followed by reduction using the C powder white slag method. The composition before tapping is adjusted to a carbon content of C0.
Macroscopically coarse-grained 20Cr1Mo1VNbTiB forging material often exhibits a coarse grain structure, mainly due to excessive heating cycles and elevated processing temperatures. To address this, the rolled material uses a fired material approach to eliminate the macroscopic coarseness. After forming nine steel ingots, cracks were observed, reaching depths of up to 20 mm, with only one crack per ingot. Metallographic analysis revealed that the crack tips were not expanded or passivated, with a decarburization depth of 0.11 mm. Some red steel ingots also showed cracks on one side before rolling, indicating that the cracking originated from longitudinal cracking in the ingots. This issue may be attributed to improper annealing or excessive soaking temperature, which requires further investigation.
The effect of tempering temperature on mechanical properties such as Rb, Rs, D5, and W was analyzed. Within the range of 400–600°C, Rb and Rs increase with rising temperature, but significantly decrease above 600°C. D5 remains relatively stable (14–15%) between 400–650°C, then increases sharply beyond 650°C. W shows a slow rise as temperature increases. Below 400°C, tempering results in decreased Rb and Rs, while D5 increases. At 300°C, the metallographic structure indicates a reduction in internal stress, contributing to lower strength.
For the heat treatment of 1020°C oil-cooled + 300°C for 6 hours air-cooled, Rb and Rs increased within the 400–600°C range, primarily due to secondary hardening of Cr, Mo, and Nb elements. Secondary hardening occurs when alloy carbides precipitate along dislocation lines in martensite, maintaining coherence with the parent phase, thereby increasing hardness. According to data <4>, molybdenum-containing steels above 500°C exhibit significant secondary hardening, with vanadium showing a pronounced effect, peaking after 5 hours of tempering at 600°C. This explains the increase in Rb and Rs between 400–600°C, mainly due to the secondary hardening of Mo, V, and Nb.
After 550°C tempering for 6 hours, visible alloy carbides precipitated clearly. In the case of 1020°C oil-cooled + 550°C for 6 hours air-cooled, and 1020°C oil-cooled + 700°C for 6 hours air-cooled, the microstructure evolved as the granular carbides aggregated and grew. Needle-like ferrite disappeared, and carbides were distributed on a polygonal ferrite matrix, forming tempered sorbite, which offers excellent strength, ductility, elasticity, and toughness. After 700°C tempering for 6 hours, needle-shaped ferrite was no longer visible, and carbides had grown larger. When tempered between 400–650°C, D5 remained more stable, though it approached the edge of technical specifications. Therefore, for comprehensive mechanical properties, a tempering temperature around 700°C is considered ideal.
In conclusion, the trial steel meets the required technical specifications. The selected heat treatment process—1020°C oil cooling + 700–720°C for 4–6 hours air cooling—produced the best overall performance. One challenge was the low Nb recovery rate, likely due to the high Nb content in NbFe, which raised the melting point too high to fully melt. Effective degassing and deoxidation before adding boron (B) are crucial for successful B recycling.
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