EP2818571B1 - Diffusion of aluminium-silicon into a steel sheet web - Google Patents

Description

The invention relates to an apparatus and a method for diffusing aluminum-silicon (Al-Si) into a surface of an Al-Si-coated steel sheet, in which a high-melting aluminum-silicon-iron alloy is formed by the indiffusion.

In the art, in many applications in different industries there is a desire for high strength sheet metal parts with low part weight.

For example, in the automotive industry there is a desire to reduce the fuel consumption of motor vehicles and to reduce CO ₂ emissions, while at the same time increasing occupant safety. There is therefore a rapidly increasing demand for body components with a favorable strength to weight ratio. These components include, in particular, A and B pillars, side impact beams in doors, sills, frame members, bumper catches, floor and roof crossmembers, front and rear side members. In modern motor vehicles, the body shell with a safety cage usually consists of a hardened steel sheet with about 1,500 MPa strength.

This is usually achieved by the process of so-called press-hardening. In this case, a sheet steel part is heated to about 800 - 1000 ° C and then deformed in a cooled tool and quenched. The strength of the component thereby increases up to about three times.

For reasons of process reliability and economy, continuous furnaces for heat treatment have prevailed. The metal parts to be treated are continuously conveyed through the oven. Alternatively, chamber furnaces can also be used in which the metal parts are batchwise placed in a chamber, heated there and then removed again.

In press hardening, a distinction is always made between direct and indirect processes.

In the indirect method, a board is punched out of a steel strip, cold formed and fed the preformed component of the heat treatment. After the heat treatment, the hot component is press fed into the press in an indirectly cooled tool. Subsequently, the components are trimmed again and sandblasted to remove any existing scaling.

In the direct process, a board is also punched out of a steel strip, however, there is no pre-deformation, but the board is fed directly to the furnace. After the heat treatment, the hot board is fed to the press and deformed in an indirectly water-cooled tool and simultaneously press-hardened. Subsequently, the molded components are trimmed again if necessary.

For both methods, so-called roller hearth furnaces have prevailed for reasons of process reliability and economy. As an alternative furnace design, for example, the walking beam furnace may be mentioned, in which the metal parts are transported by lifting bars through the oven. Multi-layer chamber furnaces are also becoming increasingly important.

Since the components are preformed in the indirect process, they must be due to their complex shape promoted on goods carriers through the oven or spent in the oven chamber. Furthermore, continuous furnaces for this process are usually equipped with inlet and outlet locks, since in the indirect process uncoated components must be heat-treated. In order to avoid scaling of the component surface, such a furnace must be operated with inert gas. These inlet and outlet locks serve to prevent the air from entering the oven. Chamber furnaces for this procedure can also be equipped with a lock. However, it is also possible with this type of furnace to exchange the atmosphere in the furnace chamber for each cycle. Continuous furnaces for this process must be equipped with a product carrier return conveyor system to ensure the circulation of product carriers. Ceramic ovens are used in these ovens. Only the entry and exit tables and the goods carrier return conveyor are equipped with metal conveyor rollers.

In continuous furnaces for the direct process, the use of goods carriers is eliminated. Therefore, the construction is somewhat simpler than that of the furnaces for the indirect process. Instead of being transported by means of goods carriers, the boards are placed directly on ceramic conveyor rollers in the direct process and conveyed through the oven. These ovens can be operated with and without inert gas. Again, the oven housing is welded as standard gas-tight. Another advantage of this type is the positive effect of the conveyor roller on the uniform heating of the metal parts to be treated to see: heated by the furnace heating with stationary rollers heated by radiation and heat conduction transported on them and therefore in contact with them in contact metal part , In addition, these ovens are operated with a significantly lower energy consumption, since there are no goods carriers that can cool down on the return transport after the oven flow and must therefore be reheated in the oven in a new run again. The direct method is therefore preferably used with the use of continuous furnaces.

The plates used in vehicle construction should not rust as much as possible. Also, scaling should be avoided during the machining process, since such scaling for further processing, at the latest before the welding or painting process, must be removed consuming and costly. However, since untreated steel sheets would inevitably scale up under the high temperatures required in press-hardening in the presence of oxygen, it is common to use coated sheets and / or to carry out the heat treatment process in the absence of oxygen.

From the German patent DE 103 03 228 B3 is a device for heat treatment of metallic strip in the run, especially for operation under inert gas low density, known. The device has stabilizing stabilizing nozzle systems stabilizing the strip, which effect the heat transfer predominantly with forced convection in the heating part and at least in the first part of the cooling. The band is guided without contact in the roller-bound treatment part. The band profile has, in the treatment region, viewed from above, at least in a partial region a concave curvature. The area of this concave curvature may be in a fluid different from the fluid with which the band is blown in the heating part and at least in the first part of the cooling area. Typically, aluminum-silicon (Al-Si) coated sheets are used for press-hardened components for the automotive industry. The coating prevents the rusting of the sheets, as well as a scaling of the hot sheets on the transfer from the oven to the press. The Al-Si of the coating diffuses when heating the board to hardening temperature in the steel surface and protects the base material against scaling. Recently, boron-alloyed tempering steels such as 22MnB5 (material number 1.5528) or 30MnB5 (material number 1.5531) are being used as the base material.

Al-Si diffusion coatings are in the documents WO 93/23247 and US 5,922,409 disclosed.

A major disadvantage of direct press-hardening in the above-described roller hearth furnaces is that Al-Si coated boards are placed directly on the ceramic conveyor rollers, thereby causing strong thermochemical reactions between the Al-Si coating and the ceramic rollers comes. Another major disadvantage of the described process is the cycle time, as the predominant furnace time is used to melt the Al-Si on the surface and to diffuse into the substrate surface in order to achieve the desired weld, corrosion and paint adhesion properties.

The rollers currently in use in roller hearth furnaces are hollow rollers made of the material sintered mullite (3Al203 • 2Si02) and solid rollers made of fused silica. The fused quartz rolls consist of more than 99% SiO 2 and have an application limit of about 1100 ° C with the disadvantage that they bend at about 700 ° C to 800 ° C by its own weight. Rollers made of sintered mullite can be loaded up to 1350 ° C without causing significant bending. The big advantage of both materials is the high thermal shock resistance. However, both materials have a very high affinity to react with molten aluminum to different aluminum silicate or even silicide compounds. As a result of the Al-Si coating, during the heating to the approximately 930 ° C. required for the diffusion, a molten phase of the coating passes through at approximately 670 ° C. The short-term melt of the coating has proven to be very aggressive on the furnace rollers and destroys them under unfavorable circumstances within a few days.

The object of the invention is to provide a method in which aluminum-silicon can be diffused into a surface of a sheet-steel web and wherein from the thus treated sheet-steel web a form hardened in the press hardening steel sheet component can be produced, wherein the disadvantages described are avoided.

According to the invention, this object is achieved by a method having the features of independent claim 1. Advantageous developments of the method will become apparent from the dependent claims 2 to 8.

• Zunächst wird die Stahlblechbahn einem auf Diffusionstemperatur aufheizbaren
  Ofen zugeführt und anschließend durch den auf Diffusionstemperatur aufgeheizten Ofen berührungslos hindurchgeführt. Dabei wird die Stahlblechbahn
  auf Diffusionstemperatur aufgeheizt, wobei Al-Si in eine Oberfläche der Stahlblechbahn eindiffundiert. Gleichzeitig diffundiert auch Eisen aus dem Stahlblechsubstrat in die Al-Si-Schicht auf der Oberfläche der Stahlblechbahn ein. Es entsteht eine hochschmelzende Aluminium-Silizium-Eisen-Legierung an der Oberfläche der Stahlblechbahn. Anschließend wird die Stahlblechbahn mit einer Geschwindigkeit von weniger als circa 25K/sec abgekühlt, so dass ein Ferrit-/Perlitgefüge entsteht. Dabei entsteht eine behandelte Stahlblechbahn, aus der in einem späteren Prozessschritt ein mittels Presshärteverfahren formgehärtetes Stahlblechbauteil herstellbar ist. Beispielsweise kann in einem Stanzverfahren zunächst eine Stahlblechplatine aus der behandelten weichen Stahlblechbahn geschnitten werden, die dann für das Presshärteverfahren beispielsweise in einem üblichen Rollenherdofen auf Martensitbildungstemperatur aufgeheizt werden kann, ohne dass es zu einer flüssigen Phase des Al-Si und damit zu einer die Rollen des Rollenherdofens schädigenden Reaktion kommt.

The method according to the invention for diffusing Al-Si into a surface of an Al-Si-coated sheet steel web is characterized by the following steps:

• First, the steel sheet is heated to a diffusion temperature
  Furnace supplied and then passed through the heated to diffusion temperature furnace contact. This is the steel sheet
  heated to diffusion temperature, wherein Al-Si in a surface of the Steel sheet diffused. At the same time, iron from the steel sheet substrate also diffuses into the Al-Si layer on the surface of the sheet steel web. The result is a refractory aluminum-silicon-iron alloy on the surface of the sheet steel web. Subsequently, the steel sheet web is cooled at a speed of less than about 25 K / sec to form a ferrite / pearlite structure. This produces a treated sheet steel web, from which a steel sheet component which has been form hardened by means of a press hardening process can be produced in a later process step. For example, in a stamping process, first a sheet steel blank can be cut from the treated soft sheet steel web, which can then be heated to martensite formation temperature for the press hardening process in a conventional roller hearth furnace without causing a liquid phase of the Al-Si and thus the rolls of the Roller hearth furnace damaging reaction comes.

In an advantageous embodiment of the method, Al-Si is diffused into both surfaces of a sheet-steel web coated on both sides with Al-Si.

Advantageously, the steel sheet web is taken directly from a first Stahlblechcoil. The coil form corresponds to the usual delivery form of sheet steel tracks.

Furthermore, it has proved to be advantageous that the steel sheet web after being passed through the furnace and slowly cooling to a temperature at which soft ferrite / pearlite microstructure is formed, is wound up into a second Stahlblechcoil. Due to the winding, the diffusion of the Al-Si from the next process step, for example the punching of boards, can be decoupled so that cycle times do not have to be coordinated. Alternatively, the steel sheet web pretreated in the inventive method can also be further processed immediately, whereby the winding up to a second steel sheet coil can be dispensed with.

In a further advantageous embodiment, the steel sheet web is heated in a first furnace part to diffusion temperature. After reaching the required diffusion time and a possible final annealing to achieve certain desired physical parameters, the steel sheet web is cooled in a second furnace part of the same furnace after the diffusion of Al-Si into a surface of the steel sheet to a temperature at which ferrite / pearlite microstructure forms. The cooling rate is less than 25K / sec. Thus, a later cutting of the individual boards in the stamping process is possible. For better handling, the sheet steel web can then be further cooled quickly to a handling temperature.

In a particularly advantageous embodiment, the steel sheet web is guided on a hot air pad without contact through the oven. In this case, the hot air can also have diffusion temperature, so that Al-Si is diffused into both surfaces of the sheet steel web. On the hot air cushion, the sheet steel web floats without contact through the oven, so that no harmful Reaction of the molten Al-Si with support means, such as rollers or walking beams take place.

In an alternative embodiment, the steel sheet web is passed through the furnace by applying a tensile force. The pulling force can be unwound via the take-off means, for example a driven second reel on which the treated sheet steel web can be wound into a coil in connection with a braked first reel, from which the untreated Al-Si-coated steel sheet web is unwound from a coil. be applied. The steel sheet trajectory follows a rope line through the furnace, for example, between the unwinding from the first reel and the winding point on the second reel depending on the applied tensile force and the distance of the unwinding from the winding point. In this case, the device for producing a hot air cushion can be dispensed with. However, this cable pull method can also be combined with the hot air cushion. This is particularly advantageous if, for example, for reasons of faster passage through the furnace while maintaining constant the diffusion time and a possible final annealing and the slow cooling with a cooling rate of less than 25K / sec to a temperature at which forms ferrite / pearlite, the oven length is longer. With a larger furnace length, the tensile force applied to the sheet steel web must be increased. When combined with the hot air cushion, however, the tensile force can be reduced.

In a further particularly advantageous embodiment, the furnace is arranged substantially vertically. The sheet steel web is advantageously guided from top to bottom through the oven. This feed-through direction has advantages in terms of temperature control, since the first furnace region with the higher diffusion temperature in this way is arranged above the second furnace region with the lower temperature at which a ferritic / pearlitic microstructure forms. But it is also possible to choose the direction of implementation of the steel sheet web from bottom to top.

The inventive device for diffusing Al-Si into a surface of an Al-Si-coated sheet steel web is characterized in that the Device having a furnace, wherein the furnace has a heatable to diffusion temperature first region, wherein the Al-Si-coated steel sheet web is guided through the furnace without contact. From the thus treated sheet steel web a form hardened in the press hardening steel sheet component can be produced.

In an advantageous embodiment, the furnace has a device for producing a hot air cushion, on which the sheet-steel web can be guided without contact through the furnace. In this case, the hot air can also have diffusion temperature, so that Al-Si can be diffused into both surfaces of the sheet steel web. On the hot air cushion, the steel sheet web floats without contact through the furnace, so that no damaging reaction of molten Al-Si to support devices, such as rollers or walking beams, can take place.

In a further advantageous embodiment, the furnace as a device for producing a hot air cushion on a hot air nozzle.

In an alternative embodiment, the furnace comprises means for applying a tensile force to the steel sheet web for non-contact passage of the steel sheet web through the furnace. The steel sheet is held under tension so that it does not sag at least so far that it touches the oven. The cable can also be combined with the hot air cushion. This is particularly advantageous if the oven is too long, so that the sheet steel web would sag too much despite the applied tensile force. In this case, the tensile force can also be reduced in the combination of hot air cushion and cable, so that no or only low voltages are introduced into the sheet steel web.

In a further particularly advantageous embodiment, the furnace is arranged substantially vertically. Here, the Al-Si-coated sheet steel web can be guided without contact from top to bottom through the oven, without the need for a hot air cushion or a cable. Nevertheless, this embodiment can both with the application of a tensile force and / or a Hot air cushion can be combined, wherein the hot air cushion can also be present on both sides of the sheet steel web.

It has furthermore proven to be advantageous if the furnace further has a second furnace region arranged in the feedthrough direction of the steel sheet web behind the first furnace region, wherein the steel sheet web can be cooled to a temperature during passage through the second furnace region at a rate of less than 25 K / sec , in which forms soft ferrite / pearlitic structure. By providing the second furnace area, the steel sheet web can be cooled to such a temperature, wherein the cooling rate of less than 25 K / sec can be reliably maintained. Soft ferrite / pearlite microstructure forms, which makes it possible to cut the individual blanks later in the stamping process.

In an advantageous embodiment, the apparatus further comprises a feeding device for feeding the steel sheet web to the furnace and a take-off device for removing the steel sheet web from the furnace. In this case, a voltage can be applied to the sheet steel web of the feed and the exhaust device, so that it does not sag too much in a substantially horizontal furnace arrangement and the tensile force does not exceed the tensile strength of a rope line.

It has furthermore proven to be advantageous if the feed device has a first reel and the take-off device has a second reel. In this case, a coil can be clamped as a standard delivery form of steel strip on the first reel. The second reel can rewind the pre-treated sheet steel strip as a coil. The second reel can also be omitted if the pretreated steel strip immediately further processed, for example, fed to a punching device to be. In order to minimize diffusible hydrogen formation, the low dew point furnace can be operated from -70 ° C to + 10 ° C, especially from about + 5 ° C to + 10 ° C.

Further advantages, features and expedient developments of the invention will become apparent from the dependent claims and the following description of preferred embodiments with reference to the drawings.

Fig. 1

eine erfindungsgemäße Vorrichtung in Waagerechtausführung

Fig. 2

eine erfindungsgemäße Vorrichtung in Vertikalausführung

From the pictures shows:

Fig. 1

a device according to the invention in horizontal design

Fig. 2

a device according to the invention in vertical design

Fig. 1 shows a device according to the invention in horizontal design. The device has a first reel 210 with a first Stahlblechcoil 310 located thereon. The first steel sheet coil 310 consists of a wound Al-Si coated steel sheet 300 in belt form. By turning the first reel 210 in a clockwise direction, the steel strip 300 is unwound and fed to the furnace 100. In this case, a feed device in addition to the first reel 210 continue to guide rollers (not shown). The furnace 100 has a first furnace region 110 which is heated to a temperature at which the Al-Si of the coating diffuses into the surface of the sheet-steel sheet 300. At the same time, iron diffuses from the steel sheet substrate into the Al-Si. The result is a refractory aluminum-silicon-iron alloy on the surface of the sheet steel web. In this case, the heating of the furnace via the heaters 150 and a hot air cushion 165, which is generated via hot air nozzles 160 under the sheet-steel web. The steel sheet 300 floats on the hot air cushion 165 through the oven 100 without touching it. Other support or guide elements, such as rollers or the like, are not required. As a result, no damaging reaction of molten Al-Si with these support and / or guide elements take place. The heaters 160 are gas burners. But there are also conceivable, for example, electric infrared heaters or hot air heaters. The length of the first furnace area is 300 depending on the speed of the steel sheet 300 so that the steel sheet is heated to the diffusion temperature, for example, 930 ° C to 950 ° C and the required diffusion time remains at this temperature. Also is one possible final glow time in the length measurement of the first furnace area 110 taken into account. In the feedthrough direction of the sheet-steel web, a second furnace region 120 follows the first furnace region 110. The temperature control in the second furnace region 120 and the length of the second furnace region 120 are dimensioned so that the steel sheet web at a cooling rate of less than 25 K / sec in the temperature range of ferrite / Perlite microstructure is cooled so that subsequently a board can be punched out of the sheet steel web.

The second furnace area 120 is followed by a take-off device with a second reel 220. The second reel 220 also rotates in a clockwise direction, whereby the pretreated sheet steel web is rewound to a second coil 320. A take-off device may further include guide rollers (not shown) adjacent to the second reel 220.

Fig. 2 shows a device according to the invention in vertical design. The furnace 100 is designed as a tower in a substantially vertical direction. The steel sheet 300 is passed from top to bottom through the furnace 100. Due to the vertical construction, no measures such as the provision of hot air cushions or cable pull devices are required in order to guide the sheet steel web without contact through the furnace 100. The implementation direction from top to bottom facilitates the temperature control in the furnace, since the cooler second furnace region 120 is below the heated to diffusion temperature first furnace region 110. Since a hot air cushion 165 is not needed, heaters 150 are provided for homogeneously heating both surfaces of the steel strip 300 on both sides of the furnace. These can be carried out as in the case of the horizontal arrangement, for example as a gas burner or as hot air heaters or, for example, electric radiant heaters.

Feed and discharge device for the sheet steel strip 300 are constructed analogously to the horizontal embodiment.

The embodiments shown herein are only examples of the present invention and therefore should not be considered as limiting. Alternative embodiments contemplated by one skilled in the art are equally within the scope of the present invention.

LIST OF REFERENCE NUMBERS

100

oven

110

first furnace area

120

second oven area

150

heater

160

hot air

165

Hot air cushion

210

first reel

220

second reel

300

Sheet steel rail

310

first sheet steel coil

320

second steel coil

Claims (8)

1. A method for diffusing Al-Si into a surface of a sheet steel web (300) coated with Al-Si, wherein a sheet steel component that has been hot-stamped in a hot-stamping process is produced from the treated sheet steel web (300),
   characterized by the steps

   a. introducing the sheet steel web (300) into a furnace (100) that can be heated to a diffusion temperature of 930° to 950 °C;

   b. contactlessly conveying the Al-Si-coated sheet steel web (300) through the furnace (100) which has been heated to the diffusion temperature, thereby heating the sheet steel web (300) to the diffusion temperature and diffusing the Al-Si into a surface of the sheet steel web (300);

   c. cooling the sheet steel web (300), which has Al-Si diffused into a surface, to below the martensite formation temperature at a rate of less than ca. 25 K/sec.
2. The method according to claim 1,
   characterized in that
   the sheet steel web (300) is coated on both sides with Al-Si and that Al-Si is diffused into both surfaces.
3. The method according to claim 1 or 2,
   characterized in that
   the sheet steel web (300) is taken from a first sheet steel coil (310).
4. The method according to one of the preceding claims,
   characterized in that
   after passing through the furnace (100) and cooling to the temperature range of a ferrite/pearlite structure, the sheet steel strip (300) is wound into a second sheet steel coil (320).
5. The method according to one of the preceding claims,
   characterized in that
   the sheet steel web (300) is heated to the diffusion temperature in a first section of the furnace (110) and, after the Al-Si is diffused into a surface of the sheet steel web (300), said web is cooled in a second section (120) of the same furnace to the temperature range of ferrite/pearlite structure at a cooling rate of less than 25 K/sec.
6. The method according to one of the preceding claims,
   characterized in that
   the sheet steel web (300) is conveyed contactlessly through the furnace (100) on a cushion of hot air (165).
7. The method according to one of the preceding claims,
   characterized in that
   the sheet steel web (300) is conveyed through the furnace (100) by the application of a tractive force.
8. The method according to one of the preceding claims,
   characterized in that
   the furnace (100) is arranged substantially vertically and that the sheet steel web (300) is conveyed downwardly through the furnace (100) from above.

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