1.How to screen raw materials for cold rolling?
Raw Material Selection: Select the appropriate grade of hot-rolled steel coil based on the final cold-rolled product requirements (such as thickness, strength, and material). For example, low-carbon hot-rolled steel coil is used for ordinary cold-rolled steel sheets (such as SPCC), while hot-rolled coil containing alloying elements such as manganese and silicon is used for high-strength cold-rolled steel.
Steel Coil Inspection: Manual or machine inspection is performed to check the hot-rolled coil's surface (for signs of severe scale, cracks, or scratches), coil shape (for flatness, absence of convolutions, or loose coils), and dimensions (thickness deviation must be within the tolerances of the cold-rolling equipment. Typically, the hot-rolled billet thickness is 2-6mm, and the target thickness after cold rolling is 0.1-3mm).
Steel Coil Butt Welding (Optional): If the length of a single coil is insufficient, multiple hot-rolled coils are welded end-to-end to form a continuous billet. This avoids frequent cold-rolling stops for coil changes and improves production efficiency.
2.What are the key steps in pickling?
Pickling: Removes scale from the coil.
After uncoiling, the coil enters a "scale breaker" (or "stretching and leveling machine"): Mechanical stretching and bending create cracks in the scale, facilitating subsequent acid penetration and improving pickling efficiency.
Pickling tank immersion/spraying: Based on the coil thickness and scale thickness, the pickling time (usually 1-5 minutes), acid concentration (10%-20% for hydrochloric acid, 5%-15% for sulfuric acid), and temperature (room temperature for hydrochloric acid pickling, 50-80°C for sulfuric acid pickling) are controlled to prevent excessive acid corrosion of the steel substrate.
Rinsing: After pickling, residual acid and salts on the steel surface require spraying with clean water (or weak alkaline water) to prevent secondary rusting.
3.How to remove oil and moisture?
If rolling oil residue (residue of cooling lubricant during hot rolling) is present on the surface of hot-rolled steel coils, it should be removed by alkaline cleaning (using sodium hydroxide solution) before or after pickling, followed by a hot water rinse.
The steel plate surface, which contains moisture after rinsing, should be dried using hot air drying (using high-temperature hot air) or mechanical squeezing to prevent residual moisture and rust.
4.How does annealing work?
If the hardness of a hot-rolled steel coil is too high (e.g., tensile strength exceeds 600MPa) due to rapid cooling after hot rolling, direct cold rolling may cause cracking. In this case, "annealing" is required first: the steel coil is heated to 600-800°C in a protective atmosphere (e.g., nitrogen) or a vacuum environment, kept at this temperature for a period of time, and then slowly cooled to stabilize the internal grains of the steel, reduce the hardness (e.g., reduce the hardness from HB 150 to below HB 100), and improve the plasticity to facilitate subsequent cold rolling.
5.Is cold rolling a core process?
If the hardness of a hot-rolled steel coil is too high (e.g., tensile strength exceeds 600MPa) due to rapid cooling after hot rolling, direct cold rolling may cause cracking. In this case, "annealing" is required first: the steel coil is heated to 600-800°C in a protective atmosphere (e.g., nitrogen) or a vacuum environment, kept at this temperature for a period of time, and then slowly cooled to stabilize the internal grains of the steel, reduce the hardness (e.g., reduce the hardness from HB 150 to below HB 100), and improve the plasticity to facilitate subsequent cold rolling.