Pickling of scale formed on hot rolled strip of carbon steel
Pickling of scale formed on hot rolled strip of carbon steel
During the hot rolling or heat treatment of steel, oxygen from the atmosphere reacts with the surface iron to form a crust that is made up of oxides of iron. This crust is known as scale and need to be removed before steel is further processed in cold rolling mill. Non removal of scale will have the following detrimental effects.
- Scale not only give bad appearance to the product but also accelerate corrosion
- During cold rolling of the strip scale patches affects the reduction with the possibility of the skidding of rolls. Effective scale removal is essential for the success of not only for cold rolling but also of subsequent annealing and coating operations.
- During cold rolling and annealing the scale will produce a dirty surface and cause the rusting of the strip
- During coating of the strip, presence of scale causes poor to total adhesion failure.
Fig 1 shows hot rolled strip surface as well as pickled surface.
Fig 1 hot rolled surface and pickled surface
Scale and its origin
The normal scale found on hot rolled strip is blue/grey in colour and covers the entire strip surface. This scale is generated during rolling in the last stands of the finishing mill, across the run out table (ROT) and during cooling of the coil. It is composed of three well defined layers of iron oxides. Adjacent to the steel is the thickest layer consisting of wustite having an approximate composition of FeO. The intermediate layer consists of magnetite (Fe3O4) while the outermost layer is hematite (Fe2O3). The thickness of these layers will depend on the temperature of the strip at the exit of the finishing mill, temperature of the coiling and the availability of oxygen at the strip surface. The layer at the surface of the strip is richest in oxygen and constitutes 0.5 % to 2 % of scale thicness. The layer at the metal surface is richest in iron and constitutes about 85 % of the scale thickness. The intermediate layer of scale constitutes around 12 % to 14.5 % of scale thickness. Beside the normal blue/grey scale on the hot rolled strip there are three other forms of rolled in scale as given below. Pickling process does not remove rolled in scale.
Various methods used for scale removal include abrasive blasting, brushing, mechanical descaling and pickling etc. The preferred method of scale removal for hot rolled strip is by pickling. The pickling process of hot rolled strip is a part of finishing process by which scale is removed from the surface of steel by dissolution in an acid before hot strip is cold rolled. It is a chemical process and is either done in a solution of sulphuric acid (H2SO4) or hydrochloric acid (HCl). The chemical reactions in case of sulphuric acid pickling is as follows
FeO + H2SO4 = FeSO4 + H2O
Fe + H2SO4 = FeSO4 + H2
The mechanism of pickling by sulphuric acid is explained in the following lines. Acid finds its way to the steel surface through the cracks in the mill scale and dissolves the surface iron. The process forms hydrogen bubbles. This hydrogen looses the scale. The dissolving of scale in the acid is a slow process and hence it falls down in the bath and slowly gets dissolved. Both the reactions of pickling are exothermic reactions but does not compensate for the heat loss associated with the heating of the cold strip and heat losses to the surrounding atmosphere and hence bath heating is needed. The heating is done by steam. Since the pickling is carried through attack of acid on steel the chances of over pickling is high in the process. The product of pickling is ferrous sulphate which is green in colour and produced during regeneration of sulphuric acid. H2SO4 acid pickling is dependent on concentration and temperature of the acid. The pickling rate goes up proportionately as the concentration of the acid is increased from 0 % to 25 %. Above 25 % the increase in the pickling rate is slow. The effect of temperature on the pickling rate is that the pickling rate is doubled for each rise of 6-8 deg C between temperatures of 25 Deg C and 95 deg C.
HCl acid pickling is a chemical process which uses an HCl acid solution to dissolve the scale from the surface of the hot rolled strip without any significant attack on the steel strip. In the case of pickling by hydrochloric acid the following chemical reaction takes place
Fe2O3 + Fe + 6 HCl = 3 FeCl2 + 3 H2O
Fe3O4 + Fe + 8 HCl = 4 FeCl2 + 4H2O
FeO + 2 HCl = FeCl2 + H2O
Fe + 2 HCl = FeCl2 + H2O
In the pickling with the HCl acid normally a chemical inhibitor is used to reduce the attack of the acid on the base metal. Pickling with HCl acid started in 1964 and slowly many picking installations have switched over to HCl pickling. The comparison of two types of pickling is shown in Table 1.
|Tab 1 Comparison of pickling by HCl and H2SO4 acids|
|H2SO4 acid||HCl acid|
|Weight of output strip||tons||0.99||0.98|
|Make up acid||Kg|
|H2SO4 ( 25 % solution )||85|
|HCl (17 % solution )||140|
|Sulphate in spent acid||Kg||18|
|Chloride in spent acid||Kg||22|
HCl acid pickling has the following advantages over H2SO4 acid pickling
- The pickling process is faster and cleaner
- Pickling cost is lower
- Better utilization of the acid
- Lower consumption of steam
- More uniform product quality
- Reduced quantity of waste pickle liquor
The heart of any pickling line is its acid baths which generally consists of four tanks in a row and contains the pickling acid. The strip passes through the bath section at speeds 100 m/min to 200 m/min and then rinsed in rinse tanks with water sprays. The strip then is air dried leaving the air dryer with a dull silver structure. Large strip accumulators also called loopers are installed both in the entry and exit ends of the acid tanks to keep the strip moving through the pickle and rinse tanks at constant speed when the entry and exit ends are stopped for change of coils. This is important not only from productivity point of view but also to avoid stains that may occur when the strip may occur when the strip stops between the acid tanks and air dryer.
The continuous pickling line needs the coils to be joined together, head to tail. To achieve this, hydraulic shears at the entry end cut a section of the strip from each end of the coil, squaring up the ends of the coil and removing damaged outer wraps of the coil. To expedite the preparation of each coil, the head end is sheared shortly after the strip is cut, before it is even charged onto the line. The head of the next coil to be charged is butted up against the tail of the last coil, and high voltage (and current) is applied across the seam, melting the two ends. The two strips are then forced together (upset) hydraulically, joining them together with what is called a ‘butt-weld’. Cutting tools immediately after the welder trim the flash that is forced out of the seam during the upset. The soundness of the weld is very important since strip breaks on the line needs rethreading which is time consuming.
At the exit end of the line the edges of product are trimmed by rotary shear ‘knives’ when required, resulting in a more uniform width and edge condition. Typically, shearing the edges at the Pickle Line removes around 30 – 50 mm of ‘side-trim’ from the width. Oil is applied to the surface of the strip just before it is recoiled. The oiler applies oil with sulfur additives to cold rolled sheet products to improve the cleanliness of the final product. Galvanized products typically are not oiled after pickling. The pickled steel is then recoiled.
Types of pickling lines
Basically there are three types of pickling lines. They are described below.
- Push and pull type – These types of lines are used normally for small and medium production capacities. In these lines preferred thickness of hot strip is more than 1.5 mm. In these type of lines the strip is neither welded or stitched but is pushed or pulled through the line strip by strip. These lines are more flexible in production planning. Schematic of this line is shown in Fig 2.
Fig 2 Schematics of push and pull pickling line
- Semi continuous lines – These are also suitable for small or medium sized production capacities. These lines are suitable for thin and ultra thin strip thicknesses. In these lines the strips with thicknesses less than 3 mm are connected by a welder or stitcher to form an endless strip at the entry section. This makes it possible for thin strips to be pulled through the line as in the continuous pickling therefore avoiding a threading process. Schematics of this line is in Fig 3
Fig 3 Schematics of semi continuous pickling line
- Continuous lines – These lines are meant for medium to high capacities and also for thin to medium strip thicknesses. In these lines the individual coils are welded together by a welding machine to form an endless strip. The use of loopers/accumulators allows a constant and continuous mode of operation in the process section of the pickling line. Schematics of continuous pickling line is at Fig 4
Fig. 4 Schematics of continuous pickling line