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Quality Aspects of Hydrogen Annealead Steel Strip_e

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Metallurgical Plant and Technology

November 2007 Original verffentlicht in MPT 6/2007

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INTERNATIONAL SPECIAL PRINT

SONDERDRUCK

Quality aspects of hydrogen annealed steel strip

Furnaces and Services for Metals.

Reprint from Metallurgical Plant and Technology 6 (2007), pages 54162 2007, Verlag Stahleisen GmbH, Dsseldorf

Quality aspects of hydrogen annealed steel stripIn the past, the bell-type annealing process has often been seen as the sole cause of stickers although there are a number of different types of stickers caused by many different factors. This article discusses the various types of stickers, the mechanisms that produce them and the precautions which can be taken to prevent their occurrence. Throughout the world, there are companies which suffer losses of up to 10% as a result of stickers. Leading rolling mills which have focussed on the problem of stickers and the processes that produce them have been able to reduce the average share of stickers to less than 1%. Stickers have therefore lost the prominence among surface defects that they had in the 1990s.

Figure 1. Modern batch annealing plant supplied by LOI Thermprocess GmbH

IntroductionSheet steels annealed in state-of-theart Bell-Type Annealing Furnaces (BAF) fulfil highest quality requirements. The mechanical properties have been optimised in respect of drawability, for low yield strength, high elongation and high Lankford values. With the same importance the BAF has to ensure best surface quality without any defects. Since soft grades tend to be very sticker sensitive the avoidance of sticker marks is a major issue of quality aspects.

anisms. However, a number of different types of stickers have been observed. In some cases, the processes which create these defects have been identified. Ridge stickers are sharply limited defects over the width of the strip. These stickers are mainly caused by strip profile anomalies (such as ridges), which can cause high radial pressures within the windings. Spot stickers are limited to localized spots on the strip. The main cause of these stickers is the local application of high pressures (by coil tongs, for example) and the unintentional winding of foreign objects into the coil, which become apparent when the coil is unwound. Edge stickers, as the name implies, occur at the edge of the strip. They are sharply limited flaws on the strip edge caused by relative movement between the windings of the coil and convector plates or charge carriers as a result of differences between the heating and cooling rates of the coil and these parts. Damage caused by coil handling, such

Classification of stickers"Sticker" is an overall term for the plastic deformations or flow and kink marks already apparent on steel strip on the temper mill during the uncoiling of the windings of the coil before it reaches the rolling gap. Stickers are produced by the pressure welding of bare metal surfaces. It has not yet been decisively established whether this welding is caused by diffusion welding processes between two surfaces, sintering processes or other adhesion mech-

Dr.-Ing. Peter Wendt, Director of Sales; Dipl.-Ing. Frank Maschler, Senior Process Engineer; Dr.-Ing. Peizhong Wang, Senior Sales Manager, LOI Thermprocess GmbH LOI Italimpianti, Essen, Germany Contact: www.loi-italimpianti.com E-mail: [email protected]

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MPT International 6/2007

Heat treatmentas positioning on a surface which is not level, may also cause edge stickers. Edge stickers can be reduced by using convector plates with a higher manganese content, which are harder. The design of the convector plates [1] and charge carriers is also important. The use of intermediate convector plates with a carrying area of 50% or more and machined intermediate convector plates can help to minimize edge stickers on thin strip (thickness less than 0.7 mm). General stickers are irregular kinked lines which take on a sickle-shaped appearance with increasing width (50 to 400 mm wide) and are mainly distributed over a large area in the centre of the strip, with reference to its width. They are also referred to as flowing material shapes or Lders lines (figure 2). Because of the crown of the strip and the resulting higher pressure on the strip centre, the flaw is positioned in a large area at the centre of the strip width in the case of strips with an optimum symmetrical profile. If the crown of the hot strip, which determines the profile of the cold strip, is too high or too sharp, the result may be a narrow but very pronounced track of stickers along the centre of the strip width. Strips with profiles which are too low or too flat may have very wide sticker areas or sticker tracks in the outer thirds of the strip width. In the case of strips which have a slightly wedge-shaped, asymmetrical cross section, the stickers will be located away from the central position at the highest part. The creation of general stickers is affected by a wide variety of process parameters, some of which have contrary effects. The causes of stickers and possible countermeasures are considered below.

Figure 2. Sticker marks

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Coils with stickers, %

24 20 16 12 8 4 0

Influence of yield stressCritical Not critical

0 120-149 150-179 180-209 210-239 240-269 >270

Yield stress, N/mm2400 350

Yield stress, N/mm2

300 250 200 150 100 50 0 Soft quality High-strength quality IF quality Separation force

Figure 3. Effect of the yield strength on the sensitivity to sticker formation

General stickers and their causesStickers are created on the temper mill when coils annealed in a bell-type plant are uncoiled and the force required to open or separate the coil windings is in excess of the yield strength of the annealed material. In other words, general stickers depend both on the stress applied to the coil during unwinding and on the yield

strength of the annealed material. As a result, "welds", between the windings of the strip which can be separated by applying force below the material yield strength do not result in stickers. Investigations have shown that the formation of sticker marks is largely determined by the actual value of the yield strength and by the behaviour of the material at stress approaching the yield strength (figure 3). An evaluation of experience obtained with stickers at a German cold rolling mill over a period of several years shows that: - sticker frequencies are higher with soft grades (yield strength < 200 N/mm2),

- no or virtually no stickers occur with higher-strength grades (yield strength > 220 N/mm2), - no or virtually no stickers occur with soft IF steels because of the lack of a precisely defined yield strength. The effective local force acting on the strip during uncoiling in the hatched area of the above diagram must therefore be 120 - 220 N/mm2 (figure 3). This force results from the separation force and the additional effects of uncoiling speed and geometry. Higher separation forces are caused by strong adhesion between the strip windings resulting in the "welding" of tightly packed strip surfaces. These adhesion forces are affected by condiMPT International 6/2007

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Heat treatmenttions during the annealing process (temperature, pressure and annealing time). The causes of these welding phenomena were comprehensively investigated for the first time by Pawelski et al. [2]. To date, knowledge of the causes of stickers in Europe is still based on this report, published in 1989. The effects of pressure on maximum separation force are considerably increased at higher temperatures. At temperatures up to about 600 C, practically no welding occurs, irrespective of the tension on the strip. This conclusion is confirmed by practical experience, although temperatures up to a maximum of 620C are generally regarded as uncritical with regard to stickers. In general, the higher the annealing temperature, the less time is required for welding the samples. strength and micro-alloyed steels are considered to be less susceptible to stickers. Strip profile. The profile of the coldrolled strip reflects that of the hotrolled strip. A steel strip with a profile optimized to avoid stickers has a crown of 40-60 m. Significantly lower crowns (flatter profiles) lead to increased sticker rates in the edge or central sections as a result of higher pressures. Asymmetric (for example, wedge-shaped) cross sections and cross section anomalies such as ridges also result in higher pressures and produce more intensive welding between the windings of a coil during the batch annealing process. Coiling tension. Coiling tension is a key factor in determining the stress in the coil and therefore also the sticker rate. Minimal coiling tension (near to the collapsing limit) helps to reduce overall compressive stress during the annealing process. Lower limits to the coiling force are set by the need to ensure safe handling during removal from the coiler and transfer to the belltype annealing furnace, the need for a stable coil and the avoidance of scratches caused by slippage during annealing and dressing. Leading cold rolling mills are in a position to reduce the coiling tension applied to a minimum (~ 22 N/mm2). The coiling tension must be optimized for the individual coil dimensions, widths and thicknesses. Strip roughness and topography. The roughness of the strip surface has considerable impact in minimizing welding and reducing the separation force required. Pawelski et al. [2] have already investigated these effects. The results of their experiments are shown in figure 3. The separation forces determined for roughened strip samples in the temperature range from 670 to 700C were only about half the corresponding figure for smooth sheet which had not be