R&D & Manufacturing
Quality control from formulation to finished product
Steel pickling lines
Steel pickling prepares raw material surfaces before stamping, welding and bonding. This reference covers principles, functions, line types, process flow, equipment, parameters, controls, safety and environmental protection.
1. Principles
Hot rolling creates hard, brittle FeO, Fe₃O₄ and Fe₂O₃ scale. If retained, it can cause stamping pits, welding porosity and poor coating adhesion. Pickling removes scale and contamination before further processing. Chemical dissolution is assisted by mechanical scale breaking through tension leveling or shot blasting. Hydrochloric acid is the common medium because it reacts faster and produces more soluble iron salts than sulfuric acid.
2. Core functions
Remove scale; adjust coil weight and trim cracked edges; improve strip flatness through leveling; apply rust-preventive oil for storage and transport.
Line types and development
| Type | Features | Annual capacity | Typical speed |
|---|---|---|---|
| Push-pull / semi-continuous | Simple, low investment, flexible steel grades, easy maintenance | 100,000–600,000 t | ≤120 m/min |
| Continuous | Welded coils and accumulators; continuous process, high output | 600,000–1,000,000 t | Entry up to 800 m/min |
Push-pull lines occupy less space and accommodate material changes at medium capacity. Continuous lines weld coils together and use inlet/outlet accumulators so the process section runs without stopping. Technology evolved from deep tanks (1960s, about 1,100 mm and 240 seconds), through shallow tanks (1980s, about 600 mm), to shallow turbulent tanks (mainstream since the 2000s, about 150 seconds and roughly 30% lower acid consumption). Traditional immersion lines have largely been replaced.
4. Three-stage process
Entry: coil loading → uncoiling → nine-roll leveling → end shearing → welding → entry accumulator.
Process: mechanical descaling → 3–5 gradient acid tanks with turbulence → cascade counterflow rinsing → hot-air drying → mist treatment.
Exit: tensioning → edge trimming → exit accumulator → oiling → recoiling → unloading/packing. Accumulators maintain continuous operation during welding and coil changes; advanced lines may also incorporate skin passing.
Main equipment
| Section | Equipment |
|---|---|
| Entry | Coil supply, uncoiler, shear, welder/stitcher, descaler, strip accumulator |
| Process | Pickling tanks, spray cooling, hot-water rinsing, dryer, mist purification |
| Exit | Accumulator, shears, edge trimmer, local punching, oiler, recoiler, coil unloader |
Tank length depends on strip speed and scale removal demand and limits line capacity. Tanks typically use FRP or PPH, with granite or andesite supports, modular flanged construction and sealed lids for vapor recovery. Collection/storage tanks recycle condensate. Level control replenishes solution, and mist is routed to neutralization.
Key process parameters
| Parameter | Typical range |
|---|---|
| Medium | Hydrochloric acid / sulfuric acid |
| Acid concentration | 180–220 g/L |
| Acid temperature | 75–90°C |
| Treatment time | 90–180 s, thickness dependent |
| Strip thickness | Approx. 1.2–6.0 mm; push-pull typically 1.2–4.5 mm |
| Strip width | Approx. 250–1530 mm |
| Coil weight | Up to 30 t |
| Final rinse conductivity | <50 μS/cm |
| Drying temperature | 120 ±5°C |
| Residual surface iron | ≤30 mg/m² |
The table gives common industry ranges. Actual settings depend on acid type, tank configuration, speed and capacity. Follow the line technical agreement and operating procedures.
7. Controls, safety and environment
Synchronized variable-frequency drives stabilize speed and tension, with electric strip steering and interlocked acid concentration, temperature and strip speed control. Modern two-stage mist treatment can recover over 95%, with typical emission values below 15 mg/m³. Acid regeneration, counterflow rinsing and water recycling reduce acid and fresh water consumption; mechanical descaling can further reduce demand. Strong acid and moving machinery require strict shutdown, power isolation, controlled access and appropriate protective equipment.