1.What are the core control objectives?
Performance meets standards: Achieves the required yield strength, tensile strength, and elongation (e.g., soft, semi-hard, fully hard state).
Uniform microstructure: Complete recrystallization process with uniform grain size.
Excellent strip shape: Maintains or improves the straightness of the strip during heat treatment.
Perfect surface: Free from oxidation, scratches, adhesion, and oil contamination.
2.What is the function of tension control?
Strip shape control: Appropriate tension can stretch the strip steel, improving or eliminating three-dimensional shape defects such as waviness and edge ripples.
Stable operation: Ensures stable operation of the strip steel in the furnace, preventing deviation and vibration.
Affects performance: Excessive tension can cause "creep" in the strip steel at high temperatures or hinder recrystallization, resulting in abnormally high strength (especially yield strength); insufficient tension leads to poor strip shape.
3.What are the principles for setting this?
**Inlet Section (After Cleaning):** Low tension is used, primarily for stable strip threading.
**Heating Section:** Medium to low tension is used. Because the strip strength is lowest at this stage (in the recovery phase), high tension can easily lead to narrowing or even breakage. Tension at this stage plays a crucial role in improving the incoming strip shape.
**Soaking/Heating Section:** Extremely low or "zero tension" is used. This is a critical stage for recrystallization and grain growth, requiring stress release to allow the material to soften sufficiently. High tension inhibits recrystallization, resulting in higher product strength and hardness.
**Slow Cooling and Over-Aging Section:** Low to medium tension is used, primarily for stabilizing the strip shape.
**Outlet Section (After Cooling):** The strip strength has recovered, allowing for higher tension, which is beneficial for final strip shape control.
4.What are the effects of temperature profile control?
Heating Rate: The rate affects the recrystallization nucleation rate. For low-carbon steel, a faster rate is acceptable; for high-strength steel or IF steel, control is necessary to prevent uneven microstructure.
Peak Temperature (Soaking Temperature): The most critical parameter. It determines the degree of recrystallization and grain size.
Too low: Insufficient recrystallization, uneven properties, high strength.
Too high: Coarse grains, deteriorated properties, increased risk of surface oxidation.
Holding Time: Ensures uniform temperature across the strip cross-section and completes recrystallization. Determined by furnace length and process speed.
Cooling Rate and Path:
Slow Cooling: Used to control carbide precipitation.
Rapid Cooling: For high-strength steel or duplex steel, rapid cooling to the over-aging temperature is required to fix the dissolved carbon or obtain martensite.
Over-aging Temperature and Time: Crucial for low-carbon aluminum killed steel, etc., allowing the dissolved carbon to fully precipitate, eliminating aging brittleness, and improving formability.
5.What are the effects of process speed on furnace atmosphere?
Process Speed: The heat treatment time is determined jointly with the furnace chief. Speed, tension, and temperature must be synchronized.
Furnace Atmosphere:
Protective Gas: Typically a mixture of H₂ and N₂ (e.g., 5% H₂ + 95% N₂). H₂ has reducing properties, preventing oxidation and maintaining a bright surface.
Dew Point Control: Strictly control the atmosphere dew point (typically < -30°C) to prevent strip oxidation or nitriding.
Furnace Pressure Control: Maintain a slight positive pressure (e.g., tens of Pascals) to prevent air infiltration.