In the final product bundle of steel warehouse, double sampling for re-inspection is conducted. The steel cooling bed in the rolling mill production line automatically bundles and packs the steel, with each bundle weighing approximately Zt. The steel is packed in bundles and maintained at a temperature ranging from 20°C to 300°C. Once stored indoors, after seven days, the batch is double sampled in a steel library where the steel has cooled down to room temperature. Upon re-examining the mechanical properties, it was observed that the area reduction significantly improved, and the hydrogen content decreased substantially. The average calculated area shrinkage was 28% higher than the initial test, while the hydrogen content dropped by 47%. This indicates that during the stacking and cooling process on the cold bed, the hydrogen content in the steel can be effectively reduced.
The face-centered cubic structure is not as tightly packed as the body-centered cubic, resulting in a larger octahedral gap. Therefore, hydrogen solubility in austenite is higher than in ferrite. After the phase transformation completes, hydrogen diffuses into the austenite with higher solubility and tends to concentrate in the center. White spots form between 150°C and 200°C as the steel cools down to room temperature. When the steel reaches the finishing platform, its temperature ranges from 200°C to 300°C. Although the core contains some hydrogen, no white spots are formed at this stage. However, if the sample is placed separately and rapidly cooled to room temperature, excessive hydrogen may aggregate under internal stress, forming a sawtooth crack known as a white spot.
When the steel is bundled at 200°C–300°C and stacked in an indoor warehouse, it cools gradually to room temperature. This slow cooling reduces internal stress significantly, making it difficult for excess hydrogen to form white spots. Additionally, the ferrite in the steel has a body-centered cubic (BCC) structure, which is less densely packed compared to face-centered cubic (FCC). At temperatures around 20°C to 30°C, long-term insulation allows atoms to diffuse out more easily, reducing hydrogen content. As a result, when double sampling and re-testing the finished steel products, the area shrinkage rate increases considerably, and the hydrogen content also drops significantly.
Once a white spot forms, it results in a sawtooth crack, which is considered an irreversible defect. This method cannot eliminate such defects. The company's steelmaking process does not include a vacuum degassing device. Hydrogen content in the 6osiZMn steel, as analyzed from samples taken in the crystallizer, ranged between 4.0 and 6.5 ppm. During the rolling process, the billet remains in a high-temperature austenite (γ) state. As the billet is rolled and its size decreases, the middle part spreads outward. However, due to the face-centered cubic (FCC) structure of austenite, the close-packed layers are tightly arranged, and the octahedral gaps are large, making it difficult for hydrogen to escape.
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