1. Raw material preparation: steelmaking and continuous casting
1. Steelmaking process
Furnace charge ratio: According to the material of the steel plate (such as Q235 carbon steel, 304 stainless steel, 5160 spring steel), iron ore, scrap steel, and alloy elements (chromium, nickel, manganese, etc.) are put into the electric arc furnace or converter for smelting in proportion.
Example: When producing 304 stainless steel plate, the chromium content needs to be controlled at 18%-20%, nickel at 8%-10%, and carbon content ≤0.08%.
Refining treatment: Remove impurities (sulfur, phosphorus) through refining outside the furnace (such as LF furnace, VD furnace), adjust the uniformity of composition, and improve the toughness and corrosion resistance of the steel plate.
2. Continuous casting
The molten steel is injected into the continuous casting machine crystallizer through the tundish, cooled and solidified into a rectangular billet (thickness 50-250mm, width 600-2200mm) or slab as the raw material for rolling steel plate.
The continuous casting billet needs to be flame cleaned or mechanically ground to remove the surface oxide scale to avoid subsequent rolling defects.
2. Rolling process: hot rolling and cold rolling
(I) Hot rolling steel plate process (applicable to medium and thick plates and rough-processed plates)
Heating and descaling
The continuous casting billet is heated to 1100-1250℃ in the heating furnace to soften the billet;
The surface oxide scale is removed by high-pressure water descaling (water pressure 10-20MPa) to prevent it from being pressed into the surface of the steel plate during rolling.
Rough rolling and finishing rolling
Rough rolling stage: The billet thickness is rolled from 200mm to 20-50mm by a reversible rolling mill (such as a two-roller or four-roller rolling mill) for initial shaping;
Finishing rolling stage: The thickness is rolled to the target size (accuracy ±0.1mm) by multiple passes of a continuous rolling mill (such as a hot continuous rolling unit), and the plate shape is controlled (flatness ≤5mm/m).
Controlled rolling and controlled cooling
Controlled rolling: by controlling the rolling temperature and deformation, the grain size is refined and the strength of the steel plate is improved (such as high-strength low-alloy steel Q355B);
Controlled cooling: spray water or air cooling after rolling, adjust the cooling speed according to the type of steel (such as wear-resistant plate NM400 requires rapid cooling), and optimize the mechanical properties.
Shearing and sizing
The hot straightening machine eliminates the bending of the steel plate, and then uses a flying shear or cold shearing to a fixed length (such as 6m×2m), and the edge is ground to remove burrs.
(II) Cold-rolled steel plate process (applicable to thin plates and high-precision plates)
Pretreatment of hot-rolled coils
The hot-rolled coils must first be softened by annealing (temperature 600-750℃), and then the oxide scale is removed by pickling (hydrochloric acid or mixed acid), and the surface roughness is controlled at Ra1.6-3.2μm.
Cold rolling
Using a multi-roller cold rolling mill (such as the Sendzimir rolling mill), the thickness is rolled from 2mm to 0.1-3mm through small roller extrusion, with an accuracy of ±0.02mm and a surface finish of Ra≤0.8μm.
During the cold rolling process, emulsion cooling and lubrication are required to prevent the rollers from sticking to the steel.
Annealing and leveling
Recrystallization annealing: After cold rolling, the steel plate is hardened (work hardening) and needs to be heated to 650-850℃ in an annealing furnace to restore plasticity (such as deep drawing performance of cold-rolled plates for automobiles);
Leveling rolling: Through slight rolling with a 1-2% reduction rate, the yield platform is eliminated and the surface flatness and glossiness are improved.
Surface finishing
Electrolytic cleaning removes oil stains, electrostatic oiling is applied to prevent rust, and finally cut into target sizes (such as 1.2m×2.4m for home appliance plates).
3. Special processes and surface treatment
1. Coating and coated steel plates
Galvanized steel plates: through hot-dip galvanizing (zinc liquid temperature 450℃) or electroplating, the coating thickness is 5-80μm, which improves corrosion resistance and is used for building roofs and automobile chassis;
Tin-plated steel plates: electroplated tin layer 0.1-0.5μm, used for food cans (non-toxic, corrosion-resistant);
Color-coated steel plates: roll-coated organic coatings (such as polyester, fluorocarbon paint), with various colors, used for building curtain walls and home appliance shells.
2. Special forming process
Medium and thick plate cutting: Steel plates with a thickness of > 40mm need to be flame cut or plasma cut, and the edges are heat treated after cutting to eliminate stress;
Pattern plate rolling: diamond and lentil-shaped patterns are directly rolled out through notched rollers to increase friction, which is used for anti-slip floors and stair treads.
3. Heat treatment strengthening
Quenching + tempering: such as spring steel plate 60Si2Mn, quenching temperature 860℃, tempering 480℃, improve elastic limit;
Annealing softening: tool steel plate T10 is spheroidized annealed to reduce hardness and facilitate cutting.
IV. Quality control and inspection
Mechanical property test
Tensile test: Determine yield strength, tensile strength, elongation (such as Q235 steel plate yield strength ≥235MPa);
Impact test: Detect low temperature toughness (such as bridge steel requires -20℃ impact energy ≥34J).
Surface and internal defect detection
Surface flaw detection: visual inspection, fluorescent flaw detection (detect cracks, holes);
Internal flaw detection: Ultrasonic testing (UT) to check interlayers and pores, suitable for thick plates (>20mm);
Plate shape detection: Laser leveling instrument measures waviness and sickle bend to ensure flatness.
Chemical composition analysis
The spectrometer detects the alloy element content (such as the deviation of Cr and Ni elements in stainless steel plates ≤ 0.5%), which complies with GB/T 3280, ASTM A240 and other standards.
5. Comparison of typical steel plate types and processes
Steel plate type Process characteristics Thickness range Application scenarios Representative products
Hot-rolled carbon steel plate High strength, low cost, rough surface 2-200mm Structural steel (bridges, steel frames) Q345B hot-rolled plate
Cold-rolled low-carbon steel plate High precision, smooth surface, good deep drawing performance 0.1-3mm Automobile panels, home appliance shells SPCC cold-rolled plate
Hot-dip galvanized steel plate Zinc layer rust-proof, strong corrosion resistance 0.5-4mm Building roofs, ventilation ducts DX51D+Z galvanized plate
Stainless steel cold-rolled plate Excellent corrosion resistance, the surface can be polished to a mirror finish 0.3-6mm Medical equipment, food equipment 316L cold-rolled plate
High-strength wear-resistant steel plate Quenching + tempering strengthening, hardness HB350-500 4-80mm Mining machinery, engineering machinery NM500 Wear-resistant plate
VI. Frontier technology and development trends
Thin-gauge rolling: The thickness of cold-rolled plate exceeds 0.05mm (such as ultra-thin steel plates for mobile phone shielding covers), which requires nano-level precision control;
Digital rolling: Optimize rolling parameters through AI algorithms and monitor plate shape in real time (such as intelligent straightening systems for medium and thick plates);
Green process: Electric arc furnace short-process steelmaking (using scrap steel) reduces carbon emissions and meets carbon neutrality requirements.