EP3512969A1

EP3512969A1 - Flexible heat treatment installation for metallic strip of a horizontal construction

Info

Publication number: EP3512969A1
Application number: EP17781402.7A
Authority: EP
Inventors: Bernhard Engl; Andreas Köppe
Original assignee: SMS Group GmbH
Current assignee: SMS Group GmbH
Priority date: 2016-09-13
Filing date: 2017-09-15
Publication date: 2019-07-24
Also published as: WO2018050857A1; DE102016011047A1

Abstract

1. Title. Flexible heat treatment installation for metallic strip of a horizontal construction. 2. Abstract. 2.1 To achieve the best possible properties when continuously annealing and hot-dip galvanizing Al-killed deep-drawing and TRIP steels, in-line postheat treatments are carried out with relatively long treatment times. However, this has the serious disadvantage of adversely affecting the conditions for optimum production of IF and dual-phase steels with respect to their properties and cost-effectiveness. The invention is intended to make it possible to adapt the treatment times in a horizontal postheat treatment unit specifically to the requirements of the type of steel treated in each case. 2.2 The solution to the problem addressed is a device in a postheat treatment chamber (1) of a continuous strip installation. A supporting structure (2) is designed in such a way that in it a front roller arrangement can be moved horizontally forwards or backwards on at least one looper car (3). Acting together with a rear fixed roller arrangement (4), a strip can be looped through the treatment chamber, loops of different lengths, and consequently different treatment times, being achieved by moving the front arrangement forwards or backwards. 2.3 The invention makes it possible that, when strip is running through a flexible postheat treatment zone of a horizontal arrangement, the run-through times are adapted optimally to the requirements of the type of steel treated in each case.

Description

Flexible heat treatment plant for metallic strip in horizontal construction

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 suitable for carrying out a

Heat post-treatment process after annealing and cooling is determined. In the heat post-treatment plant, the strip is treated at different, predetermined temperatures and times. The tape is guided in loops so that it is deflected to roles that are stored in an outlet-side stationary arrangement and an inlet-side arrangement on at least one loop carriage, wherein the horizontal distance between the two arrangements by methods of / the

Loop carriage is changeable.

State of the art

Durchlaufqlühen

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 / Verlag 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, performed. This is followed by a heat treatment in the form of a

Obsolescence. 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. As typical process parameters are

Temperatures around 400 ° C and times called 150 s. To improve the

As a result, aging can also be associated with falling temperature, e.g. to 350 ° C or 300 ° C, are performed.

Hot-dip galvanizing

The conventional continuous production of galvanized steel sheet (Sendzimir type) is described extensively in the literature, e.g. 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 in a reducing

Atmosphere heated and held to perform a 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 a plant type is a hot dip galvanizing line from ArcelorMittal (see www.arcelormittal-ehst.com). The plant also includes 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) that includes 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 in the form of an overage after the hot dip galvanizing, a problem due to a poor coating adhesion 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 pointed out in EP 0072874.

Tape storage technology

For plant-technical execution of continuous belt plants tape storage 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.

According to DE 298 22 325 vertical tape storage, which are also known as loop tower, Schiingenturm or Looper used. Also known as tape storage are those of horizontal type (e.g., DE 2144515) which use looping carts. The loop carts are arranged horizontally movable. They can be driven by means of a cable pull and a winch drum (DE 10 201 1 01 1 666, whereby synchronous driving of all loop carriages is ensured, see DE 3636652. In order to avoid an excessive sagging of the steel strip, the strip must be supported Swing gates (DE 19838780) or support car (EP 0773074) used.

Heating Technique For the introduction of heat for the above-mentioned additional heat treatments, basically all known methods of heating of strip come into question.

Banderwärmungsöfen are designed as vertical ovens or as horizontal ovens. The rollers used for tape guide are permanently installed in their position, ie not movable, such as in a tape storage described above. Used steel grades

Steel plates 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 deterioration of the mechanical properties over time

Properties, such as increasing the yield strength, lowering the elongation and

Yield point elongation.

Usually, galvanized cold rolled steel sheets which are the above

Meet material properties requirements, essentially produced by the following two manufacturing processes.

The first method uses elements for strongly bonding carbon and nitrogen, such as titanium and / or niobium, and enables the production of galvanized steel sheet having excellent formability and free from

Strain age. These are the so-called IF steels. However, this method requires the application of costly measures

Microalloying with titanium and / or niobium 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. Remedy can

Heat treatments in the form of overaging in the temperature range of 300 to 460 ° C are performed. Thus, to improve the properties of soft deep-drawing steels in a hot dip galvanizing according to DE 40 38 186 proposed to choose the temperature profile between annealing and galvanizing so that first as fast as possible cooling to a holding temperature just above the zinc bath temperature is performed and the passage through the further part of the plant until the immersion of the strip in the zinc bath is used so that a short

Holding process can be performed. This improved the mechanical properties. However, for one Over-aging treatment to achieve sufficient aging resistance for a deep-drawing steel DX53 D according to DIN EN 10346 times of more than 35 s in a range above 400 ° C to ensure the required properties required. To promote overaging, it is proposed in EP 0406619 to provide cooling before

Galvanizing even 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 in principle with the same annealing cycles, i. Temperature-time courses, as produced in a hot-dip galvanizing plant.

There is a distinct difference to hot dip galvanizing for the lower cost, low carbon AI killed steels. 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 used both in the

Hot dip galvanizing process as well as produced by continuous annealing. In the manufacture of TRIP steels, a holding process is performed after annealing and gas cooling. According to EP 2 738 276, such a holding operation is carried out by holding the steel sheet after cooling at a temperature between 500 and 350 ° C for 30 to 1000 seconds. One occurs

Bainite transformation on and the retained austenite is stabilized. If the temperature is kept below 350 ^° C, the bainite transformation takes a long time. For TRIP steel are according to the prevailing knowledge holding times for the bainitische

Conversion of 80 to 220 seconds (EP 1 889 935, WO 2012/153016, US

2014/0120371, Ehrhardt, B. et al. / Steel Grips 2 (2004) 4) for the safe achievement of sufficient properties.

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. Also there is

Treatment process for the production of higher-strength steels with relatively good formability, such as TRIP and dual-phase steels.

However, a traditional hot dip galvanizing line is not the best option to produce AI-tempered deep-drawing steels and TRIP steels because it does not normally contain a post heat treatment zone and therefore does not allow a hold phase. More suitable is a galvanizing plant with a heat post-treatment zone for

Over aging of deep drawn steel or an isothermal one

bainitic transformation of TRIP steel. At a typical belt speed, long cycle times are required for such post-heat treatments. As discussed above, there are suggested solutions to the problem

insufficient holding times in a hot-dip galvanizing line is to use a number of multiple ribbon blanks, optionally with additional reduced ribbon speed, after annealing and cooling in a post-heat treatment zone.

The best prerequisite for a post-heat treatment offers 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 makes subsequent galvanizing on an electrolytic

Galvanizing plant required. Since the rollers are arranged rigidly 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 post-treatment zone or

Overaging zone, e.g. to DE 199 36 151. A long cycle time of

For example, 150 seconds means a reduction in the mean

Cooling speed to around 2 ° C / s. According to the state of knowledge require

Dual phase steels but even at elevated alloying levels one more

Cooling rate of at least 20 ° C / s 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 the continuous annealing or hot-dip galvanizing of strip after annealing and cooling in a heat post-treatment unit

Treatment times and treatment temperatures in a wide range targeted change. The invention is intended in particular to enable the

Treatment times in the post-heat treatment to the requirements of each treated steel type are optimally adapted.

Features of the invention

The present invention relates to a device in a heat treatment chamber for performing a heat post-treatment operation after annealing and cooling in a belt-passing plant, see Figure 1. In the image, with (2) the support structure for tape guide through the

Heat treatment plant referred to, in which the invention is used. In the heat treatment chamber (1), a number of vertically arranged

Band loops formed by the tape is guided after entering the treatment part to the rear, is deflected around the first rear roller and then led forward. This process repeats itself as often as there are loops in the

Treatment part gives. At the exit the band is led out of the treatment area.

The arrangement of the front loop carriage (3) is structurally designed so that it can be moved horizontally forward or backward. In cooperation with an arrangement of fixed deflection rollers (4), a metal band can be guided in loops through the treatment chamber, wherein by treating the front arrangement of loop carriage forward or rear different treatment or

Throughput times can be achieved in the treatment part, see pictures 2 and 3

In a further development of the invention, the device can also be designed so that only a part of the front loop carriage on a horizontally movable

Arrangement is arranged. The advantage of this solution lies in the lower technical effort for the horizontal shift roller and can be used, for example, when the product mix of a plant is quantitatively strong on the side of the steel types that make a heat treatment with a holding zone required. Then only one has to Part and not the total number of front rollers are moved to the front position, because the horizontally immovable roles already occupy the position that must be taken to realize a post-heat treatment. However, this solution also allows the realization of shorter holding times when the horizontally moving rollers are moved to the front position, and the

Belt speed may be increased to optimal values to further reduce the hold 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. At 800 ° C. preferably heat treated at 350 to 500 ° C.

Temperatures above 500 ° C are for an overaging treatment of

Deep-drawing steel is too high, because then too high levels of dissolved carbon must be expected after completion of the overaging treatment with a correspondingly high residual carbon content and high aging susceptibility.

If the temperatures for the overaging treatment are too low, e.g. Below 300 ° C, then it is to be expected that no further dissolved carbon content separates out and / or the precipitates are too fine with the result of a

Deterioration of properties.

For the desired bainite formation in the case of TRIP steel, a temperature range between 300 and 500 ° C is to be selected. If the temperatures are higher or lower, the conversion of retained austenite does not lead to bainite and thus not to the

targeted TRIP properties.

For the band heating as well as for the heat insulation in the

Heat aftertreatment plant offer the various methods familiar to the expert. Preferably, the heating takes place by means of an electrical

Heating process. The technologies of an electrical indirect

Resistance heating or inductive heating are 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 minimum RT, preferably 350 to 500 ° C, e.g. in a gas stream is carried out, 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 Merkmai is that the treatment chamber is made gas-tight and that the dew point of the annealing gas can be variably selected in one

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. To ensure good gas tightness it is recommended to move the support devices in coordination with the movement of the loop carts across structurally smallest possible sealed bushings. For the same reason that is used for Schutzbegasung

Conduit system via an external coupling by structurally smallest possible

sealed feedthroughs are led into the treatment chamber.

Explanation of the examples

Table 1 shows typical prior art plant data which were used in the examples in Table 2, unless the

According to the invention and according marked data were used. When Belt speed was chosen at 200 m / min, a typical value according to the prior art for continuous annealing.

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. A sling consists of two passports. The pass length was chosen to be 25 m (CGL) or 30 m (CAL) for the present case, but could be longer or shorter.

The example of a conventional hot-dip galvanizing plant CGL, but not belonging to the invention example S 1 shows the results in the event that no

Treatment part to perform an additional heat treatment was installed, d. 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 both in deep drawn steel and in TRIP steel, see Tables 3 and 4. The examples also not belonging to the invention S 1 a and b show the results in the event that in a conventional system, a post-heat treatment was installed with a choice of several slings. The compared to the example S 1 significantly increased throughput times of 38 and 83 seconds contribute significantly to the improvement of the mechanical properties of thermoforming steel or TRIP steel, s. Tables 3 and 4. This is particularly evident in an improvement in the elongation at break values. However, such long lead times in the

Heat post-treatment to a deterioration of the manufacturing conditions for IF steel and dual-phase steel in a hot-dip galvanizing plant. Since IF and dual phase steels do not require such a treatment, this measure means a

Reduction in production output resulting from the increase in total throughput time, s. Table 2.

The examples of the invention E 1 a and b show the results in a hot dip galvanizing in the event that in a choice of several loops the

Additional treatment is performed using the movable front arrangement of loop carriage according to the invention, and this in the forward position This results in a throughput time of 38 or 83 seconds, as in Examples S 1 a and b, whereby in this case as well, the lead time significantly increased compared to Example S 1 leads to the same improvement in the mechanical properties of deep-drawing steel (see Table 3) and TRIP steel (see Table 4).

The example E 1 c, which is also part of the invention, shows the results for the case in which no specific additional treatment is carried out by moving the front loop carriage to the front position, see Figure 3. This results in a short cycle time of 12 seconds (see FIG. Table 2). Even with the new concept with the shortened cycle time, the belt speed can be increased if necessary for the purpose of increasing performance.

This can be used with IF steels and dual-phase steels, as they do not require any additional treatment to improve the properties, as mentioned.

Examples S 2 a and E 2 b refer to treatments in one

Continuous annealing system CAL.

The non-invention example S 2 a shows the results in the case where in a conventional system, a prior art over-aging furnace with rigid vertical roller assembly was installed. The long lead time of approx. 135 seconds at a temperature of 400 ° C is known to contribute significantly to the improvement of the mechanical properties of thermoforming steel (see, for example, Umformtechnik Plastomechanik und Werkstoffkunde / Verlag Stahleisen Springer Verlag / 1993) and TRIP steel (see for example WO2012 / 53016). This condition is also achieved when according to the invention the front arrangement of looping carriage is moved as in Figure 2 in the front position. However, such long lead times lead to a corresponding deterioration of the manufacturing conditions for IF steel and

Dual-phase steel. As IF and dual phase steels as mentioned no such

Require additional treatment, this measure leads to a reduction in production capacity in a continuous annealing. In addition, in dual-phase steel to compensate for the negative impact of the long cycle time to achieve sufficient Duaiphasenstahleigenschaften an increase in the alloy content required.

The example E 2 b belonging to the invention shows the results in the case where a horizontally movable system for the front rollers is used for passing through the belt through the overaging furnace, and the front rollers are moved to the front position, s. Figure 3. In this way, no specific additional treatment is carried out. This results in a short cycle time (see Table 2).

Even with the new concept with the shorter lead time, the

Belt speed can be increased if necessary for an increase in performance. This can be used with IF steels and dual-phase steels, as they do not require any additional treatment to improve the properties, as mentioned.

Advantage of the invention

The advantage of the invention is that when passing through a flexible

Heat treatment zone with horizontal design, the treatment times can be selectively changed in a wide range. This will make it possible for the

Treatment times optimally adapted to the requirements of each treated steel type.

Claims

claims

1 . Horizontal plant for carrying out a post-heat treatment of a

metallic strip in a continuous process at different temperatures, wherein the band is guided by deflection of rollers in the form of loops through the treatment zone, characterized in that the system is equipped with a support structure for receiving an arrangement of one or more looping car, which is horizontal to move to set different tape transit times.

2. Heat treatment plant according to claim 1, characterized in that the system is designed so that not all, but only a selectable part of the front rollers is horizontally movable.

3. Heat treatment plant according to claim 1 to 2, characterized in that the system is installed in a continuous hot-dip galvanizing plant for strip.

4. Heat Treatment plant according to claim 1 to 2, characterized in that the system is installed in a continuous continuous annealing system.

5. A method for heat post-treatment with a device according to claim 1 to 2, characterized in that are selected for the heat treatment times of zero to 250 seconds.

6. A method for heat post-treatment with a device according to claim 1 to 2, characterized in that are selected for the heat treatment temperatures between RT and 800 ° C.

7. The method according to claim 6, characterized in that for the

Heat treatment temperatures between 300 and 500 ° C can be selected.

8. A method for heat treatment of a metal strip with a device according to claim 1 to 2, characterized in that the treatment for

Overaging of thermoforming steel is used.

9. A method for post heat treatment 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.

10. A method for heat 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 front rollers in the rearmost position.