Research on black spots on the surface of cold-rolled non-oriented silicon steel strip

Black spot defects appeared on the surface of cold-rolled non-oriented silicon steel strips of medium grade produced by a certain steel plant 。 Using SEM and GDS analyzed the microstructure and component distribution of black spots 。 By simulating the cold rolling and cooling process of the steel strip, the formation mechanism of black spots was studied 。 The research results show that the black spots on the non-oriented silicon steel strip of medium grade are 1～2μm thick films, mainly composed of oxides, and containing a small amount of organic matter and emulsion 。 The formation of black spots is due to: During cold rolling, the emulsion 、 iron powder adheres to the surface of the steel strip, and the water in it is higher than 100℃ vaporizes at the rolling temperature, thus corroding the surface of the steel strip; during the cooling process after coiling, the metal atoms on the surface of the steel strip are ionized, causing electrochemical corrosion, and finally forming black spots 。 Based on the reasons for the formation of black spots, effective preventive measures are proposed: Replace the emulsion, reduce impurities mixed into the emulsion, improve the purging effect of the steel strip surface, and shorten the storage time of the steel strip 。

Cold-rolled non-oriented silicon steel strip is a high value-added soft magnetic functional material, widely used in motors 、 compressors 、 generators and other equipment 。 Whether high-performance cold-rolled non-oriented silicon steel strips can be produced reflects the equipment of a steel enterprise 、 technology 、 personnel 、 comprehensive strength in aspects such as control 。 In addition to demagnetization performance, surface quality is also increasingly valued by users of non-oriented silicon steel strips 。 Ensuring the surface quality of non-oriented silicon steel strips has become the key work of silicon steel production enterprises 。 In the process of producing medium-grade non-oriented silicon steel strips in a certain steel plant, when the steel strip is uncoiled after cold rolling on a single-stand reversible rolling mill, many strip-shaped black spots that are difficult to clean are found on the surface of the steel strip, but the non-silicon steel strip materials produced at the same time did not show this defect 。 Relevant reports on the appearance of black spots after cold rolling of non-oriented silicon steel strips have not been found in the literature 。 Therefore, this article uses SEM-EDS (Scanning electron microscope ＆ energy dispersive spectrometer) and GDS (glow discharge atomic emission spectrometer) to analyze the microstructure and component distribution of black spots; the formation mechanism of black spots was studied through simulation experiments; and on this basis, measures to prevent the generation of black spots in cold-rolled medium-grade non-oriented silicon steel strips were proposed 。

Through research, it was found that:

  (1 ) The black spots on the surface of the medium-grade non-oriented silicon steel strip are 1～2μm thick surface film with microcracks, mainly composed of iron oxide, and containing a small amount of residual organic matter and emulsion 。

  (2 ) The main reason for the generation of black spots is: When the steel strip is cold-rolled, the rolling temperature exceeds 100℃ The water vaporizes, penetrates the emulsion oil film, causing the lipids in the oil to decompose into low-carbon chain acids, which react with the iron powder on the surface of the steel strip, thereby corroding the surface of the steel strip, creating the conditions for the formation of black spots or slight black spots; during cooling after coiling, the iron atoms on the surface of the steel strip are further ionized, causing electrochemical corrosion reaction, which aggravates the black spots 。

(3 ) Measures to prevent the generation of black spots on the surface of medium-grade non-oriented silicon steel strips in production should mainly focus on the reasonable use of emulsion, while reducing impurities mixed into the emulsion, improving the purging quality of the steel strip surface, shortening the storage time of the steel strip, to ensure the use effect of the emulsion 。

Source: "Shanghai Metal" Kong Wei et al.