1. Pre-Annealing Preparation: Strip Handling & Surface Cleaning
Uncoiling & Welding: Individual cold-rolled coils are uncoiled, and the leading/trailing ends of adjacent strips are welded together (via laser or resistance welding). This creates a continuous steel strip (up to 2,000 meters long, per Summary 1) to avoid interruptions in the continuous line.
Electrolytic Cleaning: The welded strip passes through an electrolytic cleaning tank (part of entry-side equipment, Summary 1) to remove residual oils, oxides, or contaminants from the cold rolling process. Clean surfaces prevent uneven heating or surface defects (e.g., scaling) during annealing, which is critical for maintaining Q355GNH's weathering resistance (contaminants can disrupt the formation of a uniform protective rust layer later).
Entry Looping: The cleaned strip is fed into an entry looper-a system of tension-controlled rollers that stores excess strip. This compensates for speed differences between uncoiling, welding, and furnace entry, ensuring steady tension (200–700 N/mm², typical for structural steels) to avoid strip warping.
2. Core Annealing Stages: Heating → Soaking → Controlled Cooling
(1) Heating Zone: Rapid, Uniform Temperature Rise
Objective: Heat the strip to the recrystallization temperature range (745–755°C for Q355GNH, per Summary 2) to activate the movement of iron atoms, breaking up tangled dislocations caused by cold rolling (the main source of work hardening).
Key Parameters:
Temperature: Strictly controlled at 745–755°C (15–25°C above Q355GNH's Ac1 temperature, ~730°C). Exceeding 760°C risks partial austenitization (Summary 2), leading to excessive grain growth and reduced strength; temperatures below 740°C result in incomplete recrystallization (hardness remains too high).
Heating Rate: Rapid (10–15°C/s) via radiant heating or induction heating, ensuring the strip's surface and core reach the target temperature simultaneously (critical for thick strips, e.g., 2–6mm for Q355GNH).
Atmosphere Control: The furnace is filled with protective gas (95% nitrogen + 5% hydrogen) to prevent oxidation. Oxidation would deplete surface alloying elements (Cu, Cr) and damage Q355GNH's weathering performance.
(2) Soaking Zone: Dwell for Microstructural Homogenization
Objective: Maintain the target temperature (745–755°C) to complete recrystallization, dissolve fine precipitates (e.g., carbides) into the steel matrix, and ensure uniform grain size.
Key Parameters:
Dwell Time: 1–3 minutes (controlled by strip travel speed: 200–500 m/min for Q355GNH strips of 2–4mm thickness, per Summary 1). For thicker strips (e.g., 5–6mm), speed is reduced to 200–300 m/min to extend soaking time to 3–4 minutes.
Microstructural Change: Cold-rolled deformed grains are fully transformed into fine, equiaxed ferrite grains (5–10 μm in size), eliminating work hardening. Precipitates (e.g., Fe3C) dissolve to ensure subsequent controlled cooling can form uniform, weathering-friendly precipitates.
(3) Cooling Zone: Multi-Stage Controlled Cooling
First Cooling (Rapid Cooling):
Target: Cool the strip from 745–755°C to 370–410°C at a rate of 20–30°C/s (via water spray or gas jet cooling). This suppresses coarse precipitate formation and locks in a supersaturated solid solution of alloying elements.
Overaging Stage (Isothermal Holding):
Target: Hold at 390–410°C for 2–4 minutes to induce controlled reprecipitation of fine, dispersed precipitates (e.g., Cu-rich phases, Cr carbides). These precipitates enhance both strength (yield strength ≥355 MPa) and weathering resistance (the precipitates promote a dense, stable rust layer).
Final Cooling:
Target: Cool the strip from 390–410°C to room temperature at a slow rate (5–10°C/s) via air cooling or water mist cooling. Slow cooling avoids thermal stress and ensures dimensional stability (flatness ≤2mm/m for Q355GNH strips).
3. Post-Annealing Finishing: Temper Rolling & Recoiling
Temper Rolling (Skin Pass Rolling): A light cold rolling process with an elongation rate of 0.2–0.6% (Summary 1, 5). This corrects minor flatness defects (e.g., edge wave), improves surface smoothness (Ra ≤1.6 μm), and slightly increases yield strength (by 5–10 MPa) without sacrificing ductility (elongation remains ≥24% for Q355GNH).
Shearing & Recoiling: The continuous strip is cut into fixed-length coils (per customer orders, e.g., 1–5 tons per coil) via flying shears. The coils are then recoiled with tension control to prevent loose winding, ready for subsequent processing (e.g., slitting, coating) or delivery.
