Picklingability of hot-rolled stainless steel strips

In the iron oxide scale, the richtenite structure only exists on the surface close to the stainless steel plate, and the Fe3O4 and Fe2O3 in the outer layer of the iron scale are difficult to dissolve in the acid solution. However, because there are cracks and pores in the iron layer, especially after scale breaking or tension straightening, it is easier for the acid solution to reach the surface of the stainless steel strip and the richtenite layer through these cracks and pores. Dissolution will reduce the adhesion between the iron sheet and the stainless steel strip, and under the action of hydrogen formed during the reaction between the acid solution and the stainless steel strip iron, the iron oxide scale will fall off the matrix and sink to the bottom of the acid tank. At the same time, the insoluble Fe3O4 and Fe2O3 are also reduced to easily soluble FeO to separate the iron oxide scale from the surface of the strip.
Another important factor affecting the pickling properties of stainless steel strips is the density of the iron sheet. Fultensite has large pores, and the Fe2O3 layer and Fe3O4 layer are dense. They will block all the pores in other oxide layers in the iron sheet, thus preventing the penetration of acid. Although some cracks will form in the stainless steel strip during the cooling process, it cannot ensure that the acid penetrates deep into the iron oxide scale. Especially for hot-rolled strip steel produced by modern rolling mills, the thickness of the iron sheet is very stable and the density is very high. Therefore, in order to enhance the pickling ability of the oxide iron scale, it is still necessary to use scale breaking equipment to increase cracks.
During pickling, the iron oxide scale on the tail surface of the stainless steel strip is easier to wash off. This is because the rolling temperature at the tail end of the stainless steel strip is usually 30-50°C lower than that in the middle and the head, and is intensively cooled by the water falling from the coiler during coiling. Therefore, the iron sheet at the tail end of the stainless steel strip is formed. The process ends earlier, the iron oxide scale is thinner, and FeO has no time to transform.
What is difficult to pickle is the iron oxide scale on the head of the stainless steel strip. This is because the formation process of the iron oxide scale on the head of the stainless steel strip ends slower than that on the tail, resulting in thickening of the iron oxide scale. In addition, the slow cooling of the iron oxide scale will promote the decomposition of FeO into Fe3O4 or Fe2O3, which is also one of the reasons why it is difficult to wash.
It will also form during pickling of stainless steel strips. Unwashed black edges will appear on the edges of stainless steel strips. This is because the iron oxide scale cools more slowly on the middle edge of the length of the stainless steel strips, and the turnover of oxygen The path to the surface of the stainless steel strip is relatively smooth, which causes the Fe2O3 layer in the iron oxide scale to increase.