Q:What are the advantages of cold-rolled galvanized surface quality?
A:Cold-rolled sheet:
Before rolling, the scale is removed by pickling, and rolling oil is used for lubrication during rolling. The surface roughness is low (Ra≤1.6μm), and there is no scale indentation defect. It can be directly used for appearance parts (such as refrigerator side panels and automobile outer panels).
Hot-rolled sheet:
During hot rolling, scale is easily pressed into the surface, forming defects such as "pitting" and "roller marks". The surface roughness is high (Ra≥5μm), and additional sandblasting or pickling treatment is required for galvanizing, which increases the cost.
Q:What are the advantages of coating adhesion?
A:1. Stronger surface activity of substrate
Cold-rolled sheet is pickled and electrolytically cleaned before galvanizing to completely remove oil and oxide film. The surface is in an active metal state and reacts more fully with zinc liquid to form a 3-10μm thick Fe-Zn alloy layer. When the coating bonding strength is tested by the cross-cut method, the peeling area is less than 5%.
The hot-rolled sheet has ferrous oxide that has not been completely removed on the surface, which hinders the formation of the alloy layer during galvanizing. The thickness of the alloy layer is only 1-3μm. The peeling area of the cross-cut test may be greater than 10%, and even "plating bubbling" may occur.
2. Higher density of coating
The surface of cold-rolled sheet is flat, the zinc liquid spreads evenly during galvanizing, and the coating thickness deviation is ≤±5μm; the surface of hot-rolled sheet is uneven, and the coating is easy to accumulate in the depression or thin in the protrusion, resulting in uneven local corrosion resistance.
Q:What are the advantages of processing performance?
A:1. Suitable for complex forming processing
Cold-rolled galvanized sheet has moderate hardness and elongation ≥25%. It can withstand complex deformations such as deep drawing, flanging, and hole expansion, and the coating is not easy to crack or fall off.
Hot-rolled galvanized sheet has high hardness due to work hardening during hot rolling, and the elongation is ≤15%. It is easy to crack the substrate or peel off the coating during processing.
2. Better welding performance
The surface of cold-rolled sheet is clean and oil-free, the arc is stable during welding, and the weld penetration is uniform. It is suitable for precision welding such as spot welding and laser welding; the surface of hot-rolled sheet has residual rolling oil and oxide film, which is easy to produce pores or splashes during welding, and requires additional cleaning process.
Q:What are the advantages of corrosion resistance and service life?
A:1. Stronger protection performance at the same coating thickness
The alloy layer of cold-rolled galvanized sheet is denser, which can effectively prevent the corrosive medium from penetrating into the steel base.
If hot-rolled sheet needs to achieve the same corrosion resistance, the coating thickness needs to be increased to more than 50μm, resulting in an increase in material cost of about 30%.
2. Better long-term service stability
After 10 years of outdoor exposure, the coating thickness retention rate of cold-rolled galvanized sheet is greater than 85%; the coating retention rate of hot-rolled galvanized sheet is less than 70% due to its rough surface, which is easy to accumulate moisture and pollutants, and anti-corrosion maintenance is required in advance.
Q:What is the difference between cost and efficiency?
A:1. Lower overall cost
Although the price of cold-rolled substrate is about 20% higher than that of hot-rolled substrate, cold-rolled galvanized sheet can achieve high protection through thin coating, reduce zinc consumption by 30%, and improve the yield of subsequent processing, and the overall cost is reduced by 10-15%.
2. Higher production efficiency
Cold-rolled galvanizing usually adopts continuous hot-dip galvanizing production line with a speed of 80-120 meters/minute, while hot-rolled galvanizing has poor substrate flatness, and the production line speed is only 40-60 meters/minute, and the production capacity is reduced by 50%.