DE102015001438A1 - Flexible heat treatment plant for metallic strip - Google Patents

Abstract

Continuous annealing and hot-dip galvanizing of heat-treated deep-drawn and TRIP steels are performed in-line after-treatment with relatively long treatment times to achieve the best possible properties. However, this is associated with a serious disadvantage, because the conditions for an optimal production of IF and dual-phase steels are deteriorated in terms of their properties and efficiency. The invention is intended to make it possible to adapt the treatment times in a heat post-treatment unit specifically to the requirements of the particular type of steel being treated. The solution of the problem is a device in a heat aftertreatment chamber (1) a belt pass line. In this case, a supporting structure (2) is designed so that an upper roller bridge (3) can be moved vertically up or down in it. In cooperation with a lower roller bridge (4) a band can be guided in loops through the treatment chamber, whereby different treatment times are achieved by moving the upper roller bridge up or down. The invention makes it possible, when passing through strip in a flexible heat post-treatment zone, to optimally adapt the throughput times to the requirements of the respectively treated steel type.

Description

Subject of the invention

The present invention relates to a treatment part in a continuous annealing or hot-dip galvanizing plant for strip, which is intended for carrying out a heat post-treatment process after annealing and cooling. In the heat post-treatment plant, the strip is treated at different, predetermined temperatures and times. The tape is guided in loops (in the literature also referred to as loops) so that it is deflected to rollers which are mounted on an upper and a lower horizontal roller bridge, wherein the vertical distance of the roller bridges is variable by moving the upper roller bridge.

State of the art

Continuous annealing

Continuous annealing is a thermal treatment applied to steel strips after cold rolling. It is described in detail in the known literature (eg DE 21 59 597 . DE 24 01 381 . ASM Handbook / Vol. 4 Heat Treating / ASM Int. 1991 . Umformtechnik Plastomechanik und Werkstoffkunde / Publisher Stahleisen Springer Verlag / 1993 ). Thereafter, the continuous annealing process consists of passing the steel strip through an oven where it is subjected to controlled heating and cooling. The belt runs in the form of successive vertical loops, also called loops, and passes through various treatment steps. First, the strip is subjected to an annealing process for recrystallization consisting of preheating, heating and holding at maximum temperature. Thereafter, a cooling process, usually divided into a slow and a subsequent rapid cooling, made. This is followed by heat post-treatment in the form of overaging. Then the final cooling and the dressing take place. As a particularly characteristic part of the installation, continuous annealing plants behind the annealing part and a cooling stage contain an aging furnace (also referred to as overaging, annealing or tempering) in which the proportion of dissolved carbon is to be reduced to low values. Typical process parameters are temperatures around 400 ° C and times around 150 s. To improve the result, the aging can also with decreasing temperature, eg. B. at 350 ° C or 300 ° C, are performed.

Hot-dip galvanizing

The conventional continuous production of galvanized steel sheet (Sendzimir type) is described in detail in the literature, for. B. Manual Hot Dip Galvanizing / Ed. Maaß, P. u. P. Peißker / 2008 Wiley-VCH Verlag / Weinheim , It usually consists of the following steps: Before galvanizing, the steel strip is heated and held in a reducing atmosphere to perform recrystallization. Thereafter, the steel strip is cooled in a gas cooler, immersed in the zinc bath and, if necessary, subjected to an alloying treatment to then obtain the final galvanized sheet product after further cooling and final temper rolling. Recently, plants have become known which allow additional heat treatment between the gas cooling and the galvanizing process. An example of such an installation type is a hot-dip galvanizing line from ArcelorMittal (see www.arcelormittal-ehst.com ). The plant also contains a six-loop equalization zone called "overaging" between the gas cooling zone and the zinc pot. Another example of such an additional heat treatment is a plant (SMS SIEMAG Symposium India 2012), which contains a compensation zone between the gas cooling and the zinc pot, which has two loops and is called "Bainitic Demixion for TRIP Steel". On the other hand, if a steel strip is subjected to a post heat treatment after aging by hot dip galvanizing, a problem due to poor coating adhesion may occur ( EP 0276457 ). However, if the treatment is performed before the hot dipping, there is no danger of the deterioration of the zinc layer adhesion as in the EP 0072874 is exposed.

Tape storage technology

For the plant technical execution of continuous belt plants are after the DE 298 22 325 vertical tape storage, also known as loop tower, loop tower or looper, used to maintain a continuous process speed. Specific operations requiring such a plant technique include, for example, changing coils and then welding the beginning of the strip of the new coil to the strip end of the preceding coil. Another example is the roll change in the skin pass mill. The tape storage have the task of producing and providing large tape lengths. This happens because in stationary lower roller bridge the upper roller bridge is moved up the support frame, so that the stored between the roller bridges strip length increases. The movable roller bridge is preferably connected via chains to the drive. The problem that the roller bridges undergo an unintentional change in position as a result of changes in length of the chain drives is, according to the DE 298 22 325 achieved in that the changes in length of the chain drives are compensated by a chain tensioning device.

Autogenous

For heat input for the above-mentioned additional heat treatments in principle, all known methods of heating tape in question. Banderwärmungsöfen are nowadays usually designed as vertical ovens. The upper and lower rollers used for the tape guide are firmly installed in position, d. H. not vertically movable, such as in a tape storage as described above.

Used steel grades

Steel sheets made of deep-drawing steels produced in the hot-dip galvanizing process must have excellent formability and should as far as possible show no deformation-aging. Deformation aging is caused by carbon and nitrogen in solid solution and causes surface defects in the form of so-called "flow patterns" and leads to time-dependent deterioration of the mechanical properties, such as increasing the yield strength, lowering the elongation and yield stress. Usually, galvanized cold rolled steel sheets which satisfy the above requirements of material properties are mainly produced by the following two manufacturing methods. The first method uses elements for strong bonding of carbon and nitrogen, such as titanium and / or niobium, and enables the production of galvanized steel sheet having excellent formability and free from deformation aging. These are the so-called IF steels. However, this method requires the use of expensive measures of micro-alloying with titanium and / or Mob and the use of vacuum degassing of molten steel to set the required very low carbon contents. Another method uses less expensive, low carbon AI killed steel as the starting material. In a conventional continuous galvanizing plant, the steel sheets of this material contain a considerable amount of solid solution carbon which causes strain aging. As a remedy, post-heat treatments in the form of overaging in the temperature range from 300 to 460 ° C can be carried out. Thus, to improve the properties of soft deep-drawing steels in a hot-dip galvanizing after DE 40 38 186 proposed to choose the temperature profile between the annealing and the galvanizing so that first as fast as possible cooling to a holding temperature just above the zinc bath temperature is carried out, and the passage through the other part of the plant is used until immersion of the tape in the zinc bath so that a short hold can be performed. This improved the mechanical properties. However, for an overaging treatment to obtain sufficient aging resistance for a deep drawing steel, DX53 D is after DIN EN 10346 Times of over 35 s in a range above 400 ° C for the safe achievement of the required properties required. To promote an aging population is in the EP 0406619 proposed to make the cooling even before galvanizing to significantly lower holding temperatures and then reheat the tape to the zinc bath temperature.

In continuous annealing with normally subsequent electrolytic galvanizing IF-steels are basically the same annealing cycles, ie. H. Temperature-time courses, as produced in a hot-dip galvanizing plant. The lower cost low carbon AI killed steels have a characteristic difference to hot dip galvanizing. Due to the possibility of prolonged obsolescence in a continuous annealing plant aging and therefore higher quality grades can be produced.

Even higher-strength steels should have good formability. A comparatively high formability / strength ratio is shown by the so-called Advanced High Strength Steels AHSS, where dual-phase and TRIP steels play a dominant role. A two-phase structure consisting of ferrite and martensite is typical of the microstructure of dual-phase steel. The TRIP steels make use of the transformation induced plasticity caused by transformation of retained austenite to martensite during forming. High-strength steels, in particular the AHHS steels, are produced both in the hot-dip galvanizing process and in the continuous annealing process. In the manufacture of TRIP steels, a holding process is performed after annealing and gas cooling. After EP 2 738 276 is carried out such a holding operation by the steel sheet after cooling at a temperature between 500 and 350 ° C for 30 to 1000 seconds. Bainite transformation occurs and the retained austenite is stabilized. If the temperature is kept below 350 ° C, the bainite transformation takes a long time. For TRIP steel, according to the prevailing knowledge, holding times for the bainitic transformation of 80 to 220 seconds ( EP 1 889 935 . WO 2012/153016 . US 2014/0120371 . Ehrhardt, B. et al./Steel Grips 2 (2004) 4 ) to achieve sufficient properties required.

Critical evaluation of the prior art

As stated above, for continuous flow annealing and continuous hot dip galvanizing, metallurgical treatment processes have been developed to produce soft steels which are very suitable for deep drawing. There are also treatment methods for producing higher-strength steels with comparatively good formability, such as TRIP and dual-phase steels. However, a conventional hot dip galvanizing line is not the best option to produce Al-killed deep drawing steels and TRIP steels because it does not normally contain a post heat treatment zone and therefore does not allow a holding phase. More suitable is a galvanizing plant with a heat post-treatment zone for the overaging of deep-drawing steel or an isothermal bainitic transformation of TRIP steel. At a typical belt speed, long cycle times are required for such post-heat treatments. As discussed above, solutions to the problem of insufficient hold times in a hot dip galvanizing line are to use a number of multiple belt loops, optionally with additional reduced belt speed, after annealing and cooling in a post heat treatment zone. The best prerequisite for a post-heat treatment is the continuous annealing process with a comparatively high number of belt loops and a correspondingly long treatment time in an overaging oven. However, this option requires subsequent galvanizing on an electrolytic galvanizing line.

Since the upper and lower rollers are rigidly arranged in a conventional heat treatment unit, the time for running through this system is fixed at a given belt speed. There is no way to shorten the turnaround time. Such an inflexible heat treatment unit has a serious disadvantage. It significantly worsens the conditions for optimum production of IF and dual-phase steels in terms of their properties and cost-effectiveness. Namely, a dual-phase steel requires the shortest possible cycle times at the lowest possible temperatures in an existing heat post-treatment zone or overaging zone, eg. B. after DE 199 36 151 , A long cycle time of 150 seconds, for example, means a reduction in the average cooling rate to around 2 ° C / s. According to the state of knowledge, however, dual phase steels require a cooling rate of at least 20 ° C / s, even at elevated alloy contents, to ensure the required transformation of retained austenite into martensite. In addition, undesirable tempering effects in martensite may occur after further passage through the overaging zone. Also, for IF steels, as mentioned above, post heat treatment is not required at all, and the passage of such steels in a post heat treatment zone without the possibility of reducing the holding time means unnecessary extension of the entire process time.

The desired goal of optimally producing dual-phase steels and IF steels in the same system configuration for a design for a heat post-treatment zone optimized for AI-tempered deep-drawing steels and TRIP steels has not yet been achieved. To overcome this problem, the present invention has been made.

Object of the invention

The aim of the invention is to change the treatment times and treatment temperatures in a wide range during continuous annealing or hot-dip galvanizing of strip after annealing and cooling in a heat post-treatment unit. The invention is intended to make it possible in particular for the treatment times in the postheat treatment to be optimally adapted to the requirements of the particular type of steel being treated.

Features of the invention

The present invention relates to a device in a heat treatment chamber for carrying out a heat aftertreatment process after annealing and cooling in a belt-passing plant, see Figure 1. In the image, with ( 2 ) designates the support structure for tape guide through the heat treatment plant, in which the invention is used. In the heat treatment chamber ( 1 ) becomes one Number of vertically arranged belt loops formed by the fact that the tape is fed after entering the treatment part on the first lower pulley up, is deflected around the first upper roller and then guided downwards. This process repeats itself as often as there are loops in the treatment part. At the exit, the belt is led out of the treatment section via the last lower deflection roller. The upper roller bridge ( 3 ) is designed so that it can move vertically up or down. In cooperation with a lower roller bridge ( 4 ), a metal band can be looped through the treatment chamber, whereby by moving the upper roller bridge up or down different treatment or throughput times can be achieved in the treatment part, see Figures 2 and 3.

In a further development of the invention, the device can also be designed so that only a part of the upper rollers is arranged on a vertically movable roller bridge. The advantage of this solution lies in the lower technical complexity for the vertical roller displacement and z. B. be used when the product mix of a plant is quantitatively strong on the side of the steel types that make a heat after treatment with a holding zone required. Then only a part and not the total number of upper rollers must be moved to the upper position, because the vertically immovable roles yes already occupy the upper position, which must be taken to realize a heat post-treatment. However, this solution also allows the realization of shorter hold times, when the vertically movable rollers are moved to the lower position, and the belt speed is optionally increased to optimum values for further reduction of the holding time.

During the passage through the heat aftertreatment chamber, the strip is heated by heaters at different, predetermined temperatures less than the annealing temperature, z. B. at 800 ° C, preferably at 350 to 500 ° C heat treated.

Temperatures above 500 ° C are too high for an overaging treatment of deep-drawing steel, since then must be expected after completion of the overaging treatment with excessive levels of dissolved carbon with a correspondingly high residual carbon content and high aging susceptibility. If the temperatures for the overaging treatment are too low, e.g. B. below 300 ° C, then it is to be expected that no further dissolved carbon precipitates and / or the precipitates are too fine, resulting in a deterioration of the properties. For the desired bainite formation in the case of TRIP steel, a temperature range between 300 and 500 ° C is to be selected. When the temperatures are higher or lower, the conversion of retained austenite does not result in bainite and hence in the TRIP properties sought.

For the band heating as well as for the thermal insulation in the heat post-treatment plant, the various methods familiar to the expert are available. Preferably, the heating takes place by means of an electrical heating method. The technologies of electrical indirect resistance heating or inductive heating can be used. Alternatively, the heating can also be carried out with a heat transfer heating gas in the circulation operation with an external heating. The heat transfer heating gas used is preferably a mixture of nitrogen and hydrogen, which is also used as protective gas. In addition to the furnace walls mounted electrical heating elements can help to increase the heating power.

In an extension of the invention, the process can also be designed so that in the treatment chamber according to the invention immediately after the arrival of the tape first cooling to a minimum RT, preferably 350 to 500 ° C, z. B. in a gas stream, is performed, and then the band for the purpose of heat treatment to a higher temperature, preferably max. 500 ° C, reheated and held there for a while.

Another feature is that the dew point of the annealing gas can be variably selected in a range between -60 ° C and ambient temperature. Thus, the furnace atmosphere resulting from the dew point and the associated moisture content can be adapted to the requirements of the surface.

Explanation of the examples

Table 1 shows typical plant data according to the prior art which were used in the examples in Table 2, if not according to the invention and accordingly marked data were used. The belt speed of 200 m / min was chosen as a typical prior art value for continuous annealing equipment.

The following sizes were selected for the geometrical design of the postheat treatment unit: Roll diameter: 1200 mm, this value being typical for furnace rolls in continuous lines according to the prior art. Loop height: 18 m, which is typical for stoves or loops in prior art continuous lines.

The example S1, which is valid for a conventional hot-dip galvanizing plant CGL but does not belong to the invention, shows the results for the case in which no treatment part was installed for carrying out an additional heat treatment, ie. H. that the tape is enforced in the direct pass without additional loops. The resulting short cycle time does not lead to optimal mechanical properties in both deep-drawing steel and TRIP steel, see Tables 3 and 4. The examples S1a and b, which are not part of the invention, show the results for the case where in a conventional plant a post-heat treatment part was installed with a choice of three or seven loops. The significantly increased throughput times of 33 and 80 seconds, respectively, compared to Example S1, contribute significantly to the improvement of the mechanical properties of deep-drawing steel or TRIP steel, cf. Tables 3 and 4. This is particularly evident in an improvement in the elongation at break values. However, such long throughput times in the postheat treatment lead to a deterioration of the production conditions for IF steel and dual phase steel in a hot-dip galvanizing plant. As it is known that IF and dual phase steels do not require such a treatment, this measure means a reduction in the production output resulting from the increase in the total cycle time, s. Table 2.

The examples E1a and b belonging to the invention show the results in a hot-dip galvanizing in the case where, with a choice of three or seven loops corresponding to six or fourteen passes, the additional treatment is carried out using the movable upper roller bridge according to the invention, and the upper roller bridge This results in a throughput time of 33 or 80 seconds, as in Examples S1a and b, whereby in this case too, the significantly increased throughput time compared with Example S1 leads to the same improvement in the mechanical properties of Deep drawing steel (see Table 3) and TRIP steel (see Table 4) contributes. The example E1c, which also belongs to the invention, shows the results for the case in which no targeted additional treatment is carried out by moving the upper roller bridge into the lower position, see Figure 3. This results in a short cycle time of 7 seconds (see Table 2). The shortened lead time leads to a corresponding increase in performance in a hot-dip galvanizing plant. This can be used in IF steels and dual-phase steels, as they do not require any additional treatment to improve the properties, as mentioned.

Examples S2a and E2b relate to treatments in a continuous annealing CAL. The non-invention Example S2a shows the results in the case where a prior art over-aging furnace with a rigid roller assembly was installed in a conventional plant. The long lead time of approx. 140 seconds at a temperature of 400 ° C is known to contribute significantly to the improvement of the mechanical properties of deep-drawing steel (see eg. Umformtechnik Plastomechanik und Werkstoffkunde / Publisher Stahleisen Springer Verlag / 1993 ) and TRIP steel (see eg WO2012 / 153016 ). This condition is also achieved when according to the invention, the upper rollers are moved as shown in Figure 2 in the upper position. However, such long cycle times lead to a corresponding deterioration of the manufacturing conditions for IF steel and dual-phase steel. Since IF and dual-phase steels require no such additional treatment as mentioned, this measure leads to a reduction of the production capacity in a continuous annealing plant. In addition, in dual-phase steel, to compensate for the negative impact of the long cycle time to achieve sufficient dual-phase steel properties, an increase in alloy content is required. The example E2b according to the invention shows the results in the case where a vertically movable upper roller system is used to pass through the over-aging furnace and the upper rollers are moved to the lower position, see FIG. Figure 3. In this way, no specific additional treatment is carried out. This results in a short cycle time (see Table 2). The shortened cycle time leads to a corresponding increase in performance in a continuous annealing system. This can be used in IF steels and dual-phase steels, as they do not require any additional treatment to improve the properties, as mentioned.

Advantage of the invention

Verfahren Haltezeit 467°C s Dehngrenze Rp02 MPa Streckgrenze Re MPa Bruchdehnung A80% nach 1 Monat nach 1 Monat CGL Standard 0 267 277 29 22–24 CGL mit Haltezeit 35 245 255 33 32 Tabelle 3 mech. Werte von Tiefziehstahl nach einer Feuerverzinkung CGL Quelle: DE 4038186 Verfahren Haltezeit 467°C s Dehngrenze Rp02 MPa Zugfestigkeit Rm MPa Bruchdehnung A80% CGL ohne Haltezeit 0 400 837 22 CGL mit Haltezeit 100 530 820 27 Tabelle 4 mech. Werte von TRIP-Stahl nach einer Feuerverzinkung CGL Quelle: Ehrhardt, B. et al., Steel Grips 2 (2004) 4, S. 247–255 The advantage of the invention is that the treatment times can be varied in a wide range when passing through a flexible heat post-treatment zone. This makes it possible to optimally adapt the treatment times to the requirements of the type of steel being treated. investment Belt speed. Track length Throughput time Additional treatment (bainitic transformation or aging) m / min m s Time s Temp. ° C CGL 200 800 240 until 100 465 CAL 200 2000 600 150 400 Table 1 Typical plant data according to the state of the art CAL: Cont. Continuous conveyor system, CGL: Cont. galvanizing

method Holding time 467 ° C s Yield point Rp02 MPa Yield strength Re MPa Elongation at break A80% after 1 month after 1 month CGL standard 0 267 277 29 22-24 CGL with hold time 35 245 255 33 32 Table 3 mech. Values of deep drawn steel after hot dip galvanizing CGL Source: DE 4038186 method Holding time 467 ° C s Yield point Rp02 MPa Tensile strength Rm MPa Elongation at break A80% CGL without hold time 0 400 837 22 CGL with hold time 100 530 820 27 Table 4 mech. Values of TRIP steel after hot dip galvanizing CGL Source: Ehrhardt, B. et al., Steel Grips 2 (2004) 4, pp. 247-255

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 2159597 [0002]
• DE 2401381 [0002]
• EP 0276457 [0003]
• EP 0072874 [0003]
• DE 29822325 [0004, 0004]
• DE 4038186 [0006, 0025]
• EP 0406619 [0006]
• EP 2738276 [0008]
• EP 1889935 [0008]
• WO 2012/153016 [0008, 0024]
• US 2014/0120371 [0008]
• DE 19936151 [0010]

Cited non-patent literature

• ASM Handbook / Vol. 4 Heat Treating / ASM Int. 1991 [0002]
• Umformtechnik Plastomechanik und Werkstoffkunde / Verlag Stahleisen Springer Verlag / 1993 [0002]
• Manual Hot Dip Galvanizing / Ed. Maaß, P. u. P. Peißker / 2008 Wiley-VCH Verlag / Weinheim [0003]
• www.arcelormittal-ehst.com [0003]
• DIN EN 10346 [0006]
• Ehrhardt, B. et al. / Steel Grips 2 (2004) 4 [0008]
• Umformtechnik Plastomechanik und Werkstoffkunde / Publisher Stahleisen Springer Verlag / 1993 [0024]
• Ehrhardt, B. et al., Steel Grips 2 (2004) 4, pp. 247-255 [0025]

Claims (10)

Vertical plant for carrying out a heat aftertreatment of a metallic strip in a continuous process at different temperatures, wherein the strip is guided by deflection of rollers in the form of loops through the treatment zone, characterized in that the plant is equipped with a support structure for receiving an upper roller bridge, which can be moved vertically to set different tape transit times. Heat treatment plant according to claim 1, characterized in that the plant is designed so that not all, but only a selectable part of the upper rollers is arranged on a vertically movable roller bridge. Heat treatment plant according to claim 1 to 2, characterized in that the plant is installed in a continuous hot-dip galvanizing plant for strip. Heat treatment plant according to claim 1 to 2, characterized in that the system is installed in a continuous continuous annealing system. A process for heat after treatment with a device according to claim 1 to 2, characterized in that are selected for the heat treatment times from zero to 250 seconds. A method for heat after treatment with a device according to claim 1 to 2, characterized in that for the heat treatment temperatures between RT and 800 ° C are selected. A method according to claim 6, characterized in that temperatures of between 300 and 500 ° C are selected for the heat treatment. A method for post-treatment of heat of a metal strip with a device according to claim 1 to 2, characterized in that the treatment is used for overaging of thermoforming steel. Process for the post-treatment of heat of a metal strip with a device according to claim 1 to 2, characterized in that the treatment is used for the bainitic transformation of TRIP steel. A method for heat post-treatment of a metal strip with a device according to claim 1 to 2, characterized in that the device is used to set the shortest possible throughput time in the post heat treatment by moving the upper rollers in the lowest position.

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