1.What are the "residual oil" and "residual iron" on the surface of cold-rolled coils? Why are they tested for?
Residual oil refers to the oily substances remaining on the surface of the strip steel after the cold rolling process, such as rolling oil and leveling fluid. Residual iron refers to tiny iron powders or filings adhering to the surface of the strip steel due to friction and wear between the rolls during rolling.
The significance of detecting these two types of residues lies in:
Affecting coating quality: Residual iron will accumulate and oxidize during subsequent annealing, leading to "missed galvanizing" or poor coating adhesion during hot-dip galvanizing.
Affecting surface appearance: Excessive residual oil will form carbide deposits during annealing, causing surface color differences or "oil spots" defects.
Contaminating the annealing furnace atmosphere: After residual oil volatilizes, it may increase the hydrocarbon content in the furnace, interfering with dew point control.
Basis for process control: The test results can guide the optimization and adjustment of the rolling process and emulsion system.
2.How is residual oil content tested? What are some common methods?
**Gravimetric Method (Weighing Method):** A strip steel sample of known area is immersed in an organic solvent (such as petroleum ether or carbon tetrachloride). After evaporating and drying the solvent containing the residual oil, the mass of the residue is weighed, and the residue content (mg/m²) is obtained by dividing by the sample area. This method is simple to operate and is one of the industry standard methods, with an extraction rate of over 94.98%.
**Ultraviolet Spectrophotometry:** This method utilizes the characteristic absorption of oils at specific wavelengths (e.g., 2930 cm⁻¹, 2960 cm⁻¹) to calculate the oil concentration by measuring the absorbance. This method is rapid, sensitive, and suitable for batch testing.
**Infrared Spectrophotometry:** This method measures the absorbance of oils in three wavelength bands (2930 cm⁻¹, 2960 cm⁻¹, 3030 cm⁻¹), offering higher accuracy and better resistance to interference compared to the ultraviolet method.
**Reflectance Method:** This method indirectly assesses cleanliness by measuring the light reflectance of the strip steel surface; higher reflectance indicates less residual oil.
3.How is residual iron content tested? What is the testing principle?
Residual iron detection also employs a "sampling + dissolution + quantification" technical approach, with spectrophotometry being the industry standard method. A typical procedure is as follows:
Sampling: Using a cotton ball or filter paper soaked in solvent, wipe a known area of the steel strip surface to transfer the residual iron onto the sampling medium.
Dissolution: Place the sampled cotton ball in an acidic solution (such as hydrochloric acid) to dissolve the iron powder into iron ions (Fe²⁺/Fe³⁺).
Volume Adjustment and Color Development: Add a colorimetric reagent (such as o-phenanthroline) to form a colored complex with the iron ions.
Spectrophotometric Determination: Measure the absorbance at a specific wavelength (approximately 510 nm), calculate the iron concentration based on the standard curve, and then convert it to residual iron content per unit area (mg/m²).
This method achieves an extraction rate of over 98.67%, and the detection range is generally 1 mg/m² to 400 mg/m² (single-sided).
4.Are there any unified industry standards for the testing of residual oil and residual iron?
The main contents of this standard include:
Scope of application: Steel plates and strips before annealing after cold rolling.
Residual oil determination: The standard specifies a gravimetric method, with extraction using an organic solvent followed by weighing for calculation.
Residual iron determination: The standard specifies a spectrophotometric method, using acid dissolution-colorimetric quantification.
Detection range: Residual oil 50 mg/m²～500 mg/m² (single-sided), residual iron 1 mg/m²～400 mg/m² (single-sided).
5.Are there any rapid and portable testing methods available on the production site?
Portable Reflectivity Testing Tool: This tool uses transparent tape to collect residue from the strip surface. A built-in reflectivity measuring device measures the attenuation of light reflection from the tape, indirectly reflecting the cleanliness level. This tool is simple to operate, portable, and can quickly determine whether the strip surface meets cleanliness requirements.
Water Drop Method (Contact Angle Method): Water droplets are placed on the strip surface, and the spreading pattern of the water droplets is observed. Highly clean surfaces are hydrophilic, and the water droplets spread quickly; surfaces with more residual oil are hydrophobic, and the water droplets form spherical shapes.
Wiping Method (White Cloth/Paper Wiping): The strip surface is vigorously wiped with a clean white cloth or filter paper. The degree of dirt on the wiping cloth is visually assessed for qualitative judgment. While not precise, this method is quick and suitable for on-site inspections.
Fluorescence Method: A fluorescent tracer is added to the rolling oil. The strip surface is irradiated with ultraviolet light. The residual oil fluoresces under ultraviolet light, allowing for a direct visual assessment of the residual oil distribution.