Types and causes of defects in pickling process of stainless steel plate

by yane Yang Sales Manager

Stainless steel plate in pickling, will inevitably encounter some problems, such as due to surface defects of stainless steel plate for pickling, if you do not know the cause of these defects and solutions that will make the same mistake next time. Here is to introduce these defects and causes.

Pickling bubble

The bubble is because acid and acid exposed stainless steel board reacts to generate hydrogen, with the formation mechanism refers to the stainless steel plate and acid to produce chemical reaction, generate some hydrogen atoms, it penetrates into the crystal lattice of stainless steel, and the deformation, deformation after the hydrogen diffusion in metal to more among them, a part of hydrogen atoms and molecules through the stainless steel plate, escaping from the acid liquid, part of the hydrogen atom molecule in the lattice distortion of "micro hole" on the boundary of inclusions and pores or stainless steel, pressure of hydrogen in air holes can reach a large value (tens of MPA), the stainless steel plate produced in the internal stress caused by hydrogen embrittlement.

This will make the stainless steel plate cracking occurred in the cold rolling process of the explosion, the appearance feature is a strip drum bubble, after the breakdown performance of fine black cracks. After rolling, the bubble cracks on the surface of the plate extend and expand, thus reducing the mechanical properties of the products (impact toughness).

The method to avoid pickling bubbles is to adjust the concentration of acid solution, control the temperature of pickling solution and the flat state of strip steel.

Over pickling

If the stainless steel plate to stay too long in the acid solution, the acid solution under the action of the surface of stainless steel plate is slowly becoming rough pits phenomenon over pickling.

The elongation of pickling strip is greatly reduced, and it is easy to break and break in the rolling process, and cause the sticking roll. Even if the strip is rolled, it can not be used as a finished product because its mechanical properties are greatly reduced.

The cause of acid pickling is the continuous operation interruption of the unit, which leads to the loss of continuity of the pickling or the long treatment time due to strip breaking. Prevention method must be close to the whole unit operation coordination, to ensure normal production.

Under acid washing

After pickling, the stainless steel strip is called pickling due to its residual surface which does not wash out the oxide scale. In stainless steel strip pickling, the light will face a dark or shaped in the rolling surface; severe iron oxide can be pressed into a black plate. In addition, the extension of the oxide scale is poor, so uneven extension after rolling will make stainless steel appear wave shape or ladle bending defects. Sometimes the iron oxide sheet is firmly attached to the roll surface, which directly results in the increase of rolling waste.

Produced under pickling is generally because of the uneven thickness of the oxide scale, thick scale part of the longer the pickling time and at the same time Fe0 is decomposed into a difficult dissolved Fe2O3 (Fe3O4); also some stainless steel wave degree and camber is large, in the pickling process, fire bending part or not soaking in acid pickling by leakage. In actual production, under pickling occurs mainly at the end and end of the strip and on both sides of the strip.

Rust

The corrosion of stainless steel plate after pickling is called corrosion. The reason for this defect is that the surface of the stainless steel strip is acid free after pickling, or the stainless steel surface is rusted again after the strip cleaning has not been completely dried.

The surface of stainless steel plate corrosion performance in the dark after rolling, the product will lead to corrosion again in place, thereby reducing the surface quality of finished products, even the stainless steel board scrap. The way to avoid corrosion is to strictly carry out pickling and cleaning procedures, and should be stored in a dry place.

Inclusion

The stainless steel strip is sometimes covered with deeply dotted black spots on the surface after pickling. It is called inclusion. It is because the iron oxide sheet is pressed into the hot rolling. Such defects can not be taken to remove acid washing. When it is cold rolled, black spots expand and extend into black strip, which seriously affects the impact properties of finished stainless steel plate.

Scratch

The stainless steel strip in a new unit operation in the process of scratching, because foreign body hard surface drive roller, a guide roller, or stainless steel strip shaped and folding edge and guide into the line contact, or strip in unwinding process on the folding scraper, which makes the surface draw new scars. In addition, part of the scar appeared in the process of cooling and coiling after hot rolling.

The scratch of stainless steel strip can be divided into upper surface scratch and lower surface scratch. The cold rolled stainless steel plate will form wide and long black strip on the surface of finished stainless steel strip after cold rolling. Stainless steel strip scratch depth exceeds the allowable tolerance of stainless steel strip half, even after rolling can not be eliminated.

The way to avoid scratching is to check the rolling parts and guide plates of the unit regularly and maintain the equipment well.

Indentation

Indentation refers to the indentation of stainless steel strip on the surface of the concave. The reason is in the indentation forming and roll welding without blowing clean welding slag, steel strip is to bottom roller, which pressed on the strip surface marks; hearth roll caused by roll sticking in sliding surface of the strip. The strip surface indentation; hot rolling pressure failure, sudden pressure stop, then lift the pressure under rolling etc.. When the indentation depth exceeds half of the allowable thickness of the strip, the indentation can not be eliminated after cold rolling.

Generally speaking, the defects on the surface of the above stainless steel plate can be avoided and reduced by careful operation, strict implementation of the pickling process system and operation procedures, and timely inspection and maintenance of the equipment.

Source: China Stainless Steel Plate Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)