ES2383964T3 - Multi-chamber continuous passage furnace with protective gas operation and procedure for oxide-free heating of galvanized work pieces - Google Patents

Abstract

In a method for heating a galvanized workpiece (20) by passage on a transporter (50) through a run-through furnace (10) with a heater (60), the workpiece is passed through successive chamber regions (11-14), each with a protective gas supply point (31-34), the gas flowing against the direction of motion of the workpiece. Convective circulation of the gas through the overall furnace is inhibited by guiding systems (71-73) between the chambers : Independent claims are included for: (1) the corresponding furnace; and (2) a method for press-hardening workpieces, where the workpiece is heated by the present process before introduction into the press.

Description

Multi-chamber continuous passage furnace with protective gas operation and procedure for oxide-free heating of galvanized work pieces

The invention relates to a process for heating a galvanized workpiece, in which the workpiece is conducted by means of a means of transport through several consecutive chamber zones of a continuous passage furnace and is heated in the by means of a heating means, a mixture of protective gas is fed into the chamber areas of the continuous passage furnace through respective feeding points.

The invention also relates to a continuous passage oven for carrying out the process.

In the field of the automobile industry there is an effort to develop vehicles with the lowest possible fuel consumption. A common means of reducing fuel consumption is based on the reduction of vehicle weight in this regard. However, to correspond to increasing safety requirements, used structural body steels must have, with a reduced weight, greater strength. This is usually achieved through the process of so-called press tempering. In this case, a sheet metal part is heated to approximately 800-1000 ° C and then shaped into a cooled tool and cooled sharply. The resistance of the piece is increased in this way up to approximately three times.

Galvanized steel sheets are also preferably used in vehicle construction, since they have adequate corrosion properties. Press hardening of galvanized steel sheets is not possible satisfactorily so far with the corresponding procedures and furnaces. When the metal surfaces of galvanized sheet steel are heated in a continuous-pass furnace, a metal oxide is formed in the presence of oxygen in free or chemically bonded form, since the reactivity is increased by oxygen. In this way the workpiece is encascarilla and since the metal oxide has a specific weight essentially less than the metal, it is detached from the base material. In this way the electrolytic protection property of zinc on the base material is damaged.

To protect against this scale formation it is known, for example, to cover the sheet to be heated on both sides with an Al-Si alloy. This metal coating is alloyed on one side on the steel surface and on the other hand it forms a dense layer of Al-Si oxide, which protects the base material against an additional shell formation. However, this coating is difficult to form before heating, as well as after tempering in the press it is no longer protected by galvanization.

Other coatings with the so-called NANO particles of the NANOX company or with a zinc-aluminum alloy represent other alternatives. Although no protective gas is necessary with the use of a zinc-aluminum alloy coating, however, the coating is very expensive and after the tempering in the press, no galvanically active protective layer is formed.

An additional known solution is the use of uncoated steel sheet, in which, however, the atmosphere of oxygen-containing air is exchanged for a protective gas atmosphere (for example, endogas). On the other hand, also with the use of a protective gas to heat the work piece, after the tempering in the press, the shell must be removed by sandblasting, which has been generated during the transfer to the press.

If a workpiece is heated in a protective gas atmosphere, internal or external endogas generators are used for a conventional oven. The known gas generators provide for example the conduction of the gas mixture through a bed of nickel catalyst at high temperature. In the case of an external gas generator, the gas generated with it must, however, be cooled for additional transport to the furnace and in this case it loses through the formation of carbon chains in reduction potential.

Internal inbred generators are known for example from the German patent document DE 196 21 036 C2. The document describes the use of a bed of nickel-based catalyst, which is incorporated into the furnace space. The catalyst bed serves in this case to fractionate the hydrocarbon-air mixtures fed with an additional heating unit. This catalyst bed must be heated to approximately 900 to 1100 ° C, to be reactive.

Continuous passage furnaces with protective gas atmosphere usually carry the disadvantage that the atmosphere is contaminated due to a convection that is generated during production inside the oven continuously by moisture and oxygen carried with the merchandise from the surface of the the merchandise. Convection is caused by the work pieces still cold at the beginning of the oven, since they cool the atmosphere and a thermal generated from it generates a large circulation of protective gas throughout the oven installation, which causes mixing unwanted oxidized gases entrained on the inlet side in the critical end zone of the oven.

The article “ANNEALING HOT-ROLLED SHEETS IN AN ATMOSPHERE OF NITROGEN WITH NATURAL GAS ADDITIONS” by I.M. MIF, Yu.M. Brunzel and N.G. Ryabova, published in "STEEL IN TRANSLATION" in January 1993, unveils a continuous three-chamber heat treatment furnace, with a protective gas stream

that runs against the direction of movement of the work pieces.

German patent document DE 197 19 203 C2 discloses a sintering process for pressed moldings based on iron powder, in which a conduction of protective gas in the furnace is provided. The operation of this known sintering furnace cannot, however, be applied to the heating and tempering of galvanized steel sheets.

Therefore, it is an object of the invention to provide a process with the aid of which galvanized workpieces in particular of hardenable steel sheet can be heated, to be tempered in the press below, without losing part of the good cold deformability and high corrosion resistance

In this regard, the process must both reduce the oxides already present on the metal, as well as avoid a new formation of oxide and also reduce the consumption of protective gas.

Likewise, it is an object of the invention to provide an oven for carrying out the process.

According to the invention, this objective is solved by a method with the characteristics of independent claim 1. Advantageous refinements of the process result from dependent claims 2-6 and the object of claim 7 completes the invention around a process for tempering work pieces in press, which were previously heated with the process according to the invention. The objective is further solved by means of a continuous step furnace according to claim 8. Advantageous embodiments of this furnace result from dependent claims 9-12.

The invention comprises a method according to the preamble of claim 1, wherein guiding systems between the chamber zones cause a total current of the protective gas mixture against the direction of passage of the workpiece through the oven of continuous passage, the desired gas stream being favored by a slightly oblique position of the entire continuous passage furnace, in which an anterior end of the continuous passage furnace is higher than a rear end and because the transport means conducts the workpiece by means of the guide systems, the guide systems being separation walls with in each case an opening, and avoiding a circulation by convection of protective gas through the entire continuous passage furnace.

In a particularly preferred embodiment of the invention, a mixture of protective gas is generated by partial combustion of a hydrocarbon-air mixture in a noble metal catalyst. The heat necessary for partial combustion is generated by the fractionation process in the catalyst. Partial combustion in the noble metal catalyst takes place in this respect for example from about 700 ° C.

Preferably, the composition of a mixture of protective gas fed into a chamber zone of the continuous passage furnace is selected as a function of the temperature of the workpiece in the respective chamber zone so that a galvanization of the workpiece does not oxidize . The flow rate of the protective gas mixture through the oven is preferably in this case greater than the backscatter rate.

Also included by the invention is a method for tempering a workpiece in a press in a press, in which the workpiece was heated before being placed in the press with the method according to the invention.

Also, the invention comprises a continuous passage furnace according to the preamble of claim 8, in which guide systems are arranged between the chamber areas to cause a flow of protective gas against a movement of the workpiece and for avoid convection of protective gas through the entire oven, the guide systems being separating walls with in each case an opening, through which the transport means runs and presenting the continuous passage oven a slightly positioned position oblique, in which an anterior end of the continuous passage furnace is higher than a posterior end.

In addition to the chamber zones, protective gas guide systems are arranged, which prevent the formation of a large circulation by convection of the protective gas mixture throughout the entire oven installation. In a particularly preferred embodiment of the invention in the case of the guide systems, these are separation walls with in each case an opening, through which the means for transporting the furnace runs. In the continuous step furnace, a protective gas stream is also generated against the direction of passage of the workpiece.

The speed of the protective gas stream through the continuous pass furnace is preferably adjusted so that it is greater than the backscatter rate. In addition, the oven can be conveniently maintained thermostatically at a temperature that is above the predetermined heating temperature of the workpiece.

The process according to the invention and the associated continuous passage furnace have the advantage that a protective gas is conducted through the furnace such that in each section of the furnace the protective gas mixture is offered

correct that adjusts to the temperature of the merchandise. To this end, the endogas generated in the catalyst bed in the low temperature furnace wall inside the continuous passage furnace is controlled in a controlled manner by means of constructions that prevent a large circulation by convection through the entire oven installation. Rather, the protective gas is conducted so that the ratio of the reacting components is always maintained with respect to the temperature in the reducing zone. The use of a noble metal catalyst allows in this respect the generation of endogas already starting at temperatures of 700 ° C, a noble metal catalyst being innocuous against, for example, a nickel catalyst.

The invention therefore departs from the continuous passage furnaces in which the protector is generated outside the oven and fed into the oven space. It also departs from nickel retorts heated in the furnace itself and the different procedures for coating galvanized metal construction parts, to make a protective gas unnecessary.

An advantage of the invention with respect to conventional methods for preventing a shell formation of galvanized steel construction parts is based on the protective gas atmosphere always adjusted to the temperature of the workpiece. Although the only supply of protective gas at several points in the furnace space would also create the desired atmosphere precisely at these feeding points, instead due to a convection generated during production inside the oven the atmosphere would be continuously contaminated by the dragged moisture and the oxygen dragged with the merchandise from the merchandise surface.

The reason for this is the workpiece still cold in the oven entrance area. The workpiece also cools the protective gas atmosphere in this area, so that it becomes heavier specifically than the atmosphere during the rest of the oven. In this way, the gas with its highest specific weight falls down and displaces the hottest and best qualified atmosphere in the rest of the oven. This increases in the outlet area upwards and thus generates in the oven a circulation of protective gas caused by the thermal, which causes an unwanted mixing of the oxidized gases entrained on the input side in the last part, critical heat of the oven.

This worsening of the quality of the protective gas atmosphere in the relevant rear area of the furnace could be compensated to some extent by an economically disadvantageous increase in the amount of protective gas, however, the invention advantageously solves this problem by means of guidance systems. inside the oven of the oven, which prevent a circulation of protective gas throughout the oven. By means of the separation walls used as guiding systems between the individual chamber zones of the oven, the formation of a large circulation of protective gas throughout the entire oven installation is prevented. If necessary, only small gas circulations appear within the chamber areas. However, the remaining protective gas stream through the openings in the separation walls cannot generate any gas circulation with which the protective gas can reach the back of the oven with low quality.

The use of a noble metal catalyst, which can generate protective gas from a combustion temperature of approximately 700 ° C, also has the advantage that it is less expensive with respect to the usual catalyst beds and more economical due to low consumption of energy The temperature necessary for the combustion of gases in the noble metal catalyst can be achieved by the fractionation process in the catalyst, while conventional nickel catalysts require for example a temperature of at least 1000 ° C, which can only be achieved by a contribution of additional energy. Also in practice it has been found that in the range of a catalyst at approximately 1000 ° C it is difficult to perform or even the temperature regulation of an oven at 930 ° C cannot be performed.

Other advantages, particularities and convenient improvements of the invention result from the dependent claims and the following description of a preferred embodiment by means of the figures.

The figures show:

1 shows a schematic representation of a preferred embodiment of the continuous passage oven according to the invention; Y

Figure 2 a cross-sectional view of the continuous passage furnace according to Figure 1;

3 shows a section of the furnace wall of a continuous passage furnace with an inert protective gas generator;

Figure 4 a diagram with reduction curves of different metals in gas mixtures; Y

Figure 5 a diagram for the ratio of air mixture to methane to generate different mixtures of protective gas.

A particularly preferred embodiment of the continuous passage furnace according to the invention is schematically shown in Figure 1. The continuous passage oven 10 normally comprises a longitudinally extending housing with an inlet opening and an outlet opening, through which they can

Work pieces that are going to get hot in the oven. The oven also comprises at least two areas separated from each other, in which in each case protective gas is fed. These zones are configured in the form of cameras. In the exemplary embodiment shown in Figure 1, the oven comprises four chamber zones 11, 12, 13 and 14.

The chambers are separated from each other by guide systems 71, 72 and 73, the guide systems serving the controlled conduction of protective gas through the furnace. In the case of guide systems, these are preferably separation walls with an opening, through which a workpiece can be driven. To prevent a circulation of protective gas through the entire interior of the oven, the opening in the separation wall is as small as possible, however it must be of sufficient size to be able to transport work pieces that are to be heated in the oven with possibly different sizes and shapes on the means of transport through the oven.

The continuous passage furnace also has a means of transport 50, with which a workpiece 20 is transported through the oven. In the case of this conveyor means, for example, it is a roller hearth. A workpiece 20 is represented for this purpose in FIG. 1 by way of example as an arched construction piece, which is placed on the roller floor 50, to be heated in the oven to a predetermined temperature. The transport means 50 passes through the oven with the work piece, running through the inlet opening, the openings in the separation walls and the outlet opening. The workpiece can be transported in this respect directly on the transport device or indirectly with the aid of workpiece supports.

The direction of movement of the conveyor means with the workpiece is indicated in figure 1 with a large arrow. On the contrary, the flow of protective gas is represented in Figure 1 with small arrows and runs according to the invention against the movement of the work pieces. This flow of protective gas is caused by the guidance systems inside the oven. The desired gas stream can also be favored by a slightly oblique position of the entire oven installation, in which the front end of the oven is higher than the rear end. In this way the heated protective gas mixture flows from the end of the oven upwards and therefore to the front end of the oven. In the case of an oven length of 20 m, it has proved advantageous, for example, an elevation of the previous oven section of approximately 5 cm. The flow of protective gas against the movement of the work pieces can also be favored by an orientation of the feeding points for the protective gas. In this case, the respective gas outlets are adjusted so that a directed current of the protective gas is produced.

The speed of the protective gas flow is preferably greater than the speed with which the backscattering takes place. Thus, although the quality of the protective gas at the beginning of the oven is minimal, however this is not harmful, since it affects workpieces with low temperature, which were just introduced into the oven. These workpieces demand little in terms of the quality of the protective gas, while the fully heated workpieces at the end of the continuous pass furnace require a higher quality of protective gas and this can be guaranteed in particular by means of the guidance systems within the oven.

In the case of a workpiece 20 to be heated, it is often a molded piece of galvanized steel sheet. However, otherwise molded work pieces of other metals may also be heated. The process according to the invention is particularly suitable for heating steel sheet work pieces for press-fit body parts in car construction.

To heat the workpiece, the oven 10 comprises a heating unit 60. The heating elements used for this are found in the embodiment shown in Figure 1 in the upper part of the oven chambers, so that the part Work heats up from above. The heating elements may, however, also be arranged below or on both sides of the work pieces. Heating can take place, for example, electrically through resistors or by means of burners that run on fuel. After a predetermined residence time in the hot zone of the oven, each of the work pieces introduced therein is brought to the predetermined temperature, which rises for example for some steels at 930-980 ° C.

After a predetermined period of time, each workpiece is removed from the hot zone and can then be shaped into a press as well as tempered. The dam process can be performed with procedures and presses generally known to the expert. In this case it is advantageous that the transfer from the furnace to the press takes place quickly, so that an unwanted oxidation of zinc in the surrounding air does not take place.

The oven preferably comprises in each chamber zone 11, 12, 13 and 14 in each case a feeding point 31, 32, 33 and 34, to feed a mixture of protective gas. A feed point comprises a metal catalyst, which is preferably incorporated into the deepest point of the oven. From Figure 2, a cross-section through the continuous passage furnace according to Figure 1 can be seen schematically. A workpiece 20 is transported on a transport means 50 through the oven 10 and is heated in this respect by means heating 60 arranged above the conveyor means. The catalyst 40 of

a feeding point is incorporated in the oven wall 15.

An exemplary embodiment for the incorporation of a catalyst in the furnace wall to generate a mixture of protective gas is represented in Figure 3. It is preferably a noble metal catalyst that is incorporated in the furnace wall so that it can feed with gas from outside. Normally a pipe system is connected, for example, for natural gas and air, with which a given mixing ratio can be adjusted.

The protective gas is generated for example by partial combustion of combustion gases rich in hydrocarbons such as natural gas or propane. The heat for this combustion generates the catalyst fractionation process, the process being stable at the comparatively low temperature level of approximately 800 ° C. The noble metal catalyst can preferably convert already at temperatures from 700 ° C hydrocarbon-air mixtures into strongly reducing endogas and is harmless to health with respect to a conventional nickel catalyst. Likewise, the half-life of a noble metal catalyst is longer than for example that of a nickel catalyst.

The generated protective gas is essentially composed of nitrogen, hydrogen, carbon monoxide and other gases. To ensure a reducing atmosphere, the ratio of individual gases must be below the reduction curve for Zn / ZnO, which is represented in Figure 4 in a diagram. The diagram shows the reduction curves for different metals based on the relationships of the partial pressures of individual gases in the atmosphere with respect to temperature. The position of the reduction curve for zinc therefore depends on the temperature of the workpiece inside the continuous pass furnace. Since the temperature of the merchandise when passing through the oven increases continuously, the optimum protective gas mixture throughout the oven is also variable. Preferably, therefore, a different protective gas mixture is fed into each chamber area through a feed point.

If a workpiece is transported through a continuous passage oven 10, it increases during the heating in the individual chamber areas 11, 12, 13 and 14 for example the temperatures indicated in Figure 1 of 500, 700 , 800 and 980 ° C. Therefore, in the last chamber area the workpiece has the highest temperature and has a merchandise temperature of approximately 980 ° C. In the case of this merchandise temperature, it can be seen from the diagram in Figure 4, that for the annealing of oxide-free zinc, a ratio of the partial pressures H2 / H2O of more than 80 and CO / CO2 of more is necessary of 90. If this atmosphere, for example, takes place through a partial combustion of natural gas with 90% methane (CH4) or propane (C3H8) with air, it must be determined at which air / methane ratio these partial pressure ratios are maintained. in the generated protective gas. This determination is possible by means of the diagram of Figure 5, in which curves are represented for the percentages in the combustion air of H2, H2O, CO and CO2 in% by volume with respect to the ratio of air to methane in the fuel mixture

As indicated in the diagram of Figure 5, a ratio of partial pressures H2 / H2O of approximately 80 and a ratio of partial pressures CO / CO2 of approximately 90 in the gas mixture generated for example with an air ratio is achieved with respect to methane in the fuel mixture of about 2.4. In this case, the curves for wet gases (f) and H2O in the form of steam (D) are used in each case. In this case, approximately 39% by volume of H2 and 0.45% by volume of H2O are found in the protective gas atmosphere, while approximately 21% by volume of CO and 25% by volume of CO2 are found. .

At a merchandise temperature of approximately 980 ° C, natural gas and air are then introduced into the last zone 14 of the continuous passage furnace in the ratio of approximately 2.4 in the noble metal catalyst 40 and partially burned. Since the workpiece adopts in this area the highest temperature and in this case there is therefore the greatest risk of an unwanted reduction, the optimum protective gas mixture is introduced through the feed point 34 in the area, to avoid a scale formation of the zinc layer. In the previous chamber areas, the optimum protective gas atmosphere is also determined analogously for the temperatures of merchandise present in order to avoid a reduction of zinc on the workpiece and the necessary mixing ratio of air with respect to methane .

In this case, it has proved convenient to reduce the ratio of air to methane at feed points 31, 32, 33 and 34 when the workpiece passes through the furnace, to provide in each case an atmosphere of protective gas that prevents a reduction of zinc on the work piece. In the range of approximately 980 ° C at the end of the furnace, therefore, the lower ratio of air to methane is adjusted. For the previous zones, the atmosphere of protective gas is little demanded, since the temperature of merchandise is lower in them. Therefore, they can be supplied with protective gas with a higher percentage of air, which leads to a reduction in fuel costs. However, it is also possible to feed a mixture of protective gas with a percentage of oxygen in all furnace chamber zones, as is really necessary only for the last zone 14. While this increases the costs for the protective gas, however in this way the risk of a scale formation can be further reduced.

In this way the protective gas is generated and fed according to the needs in the separate sections 11, 12, 13 and 14 of the continuous passage furnace. In this case, the different requirements of the metal and its temperature are taken into account. Likewise, by means of the installations inside the oven, the formation of a protective gas circulation is avoided, which could conduct protective gas with an oxygen percentage that is too high in the critical rear oven zone.

5 List of reference numbers:

10 continuous step oven 11,12,13,14 partial zone of the continuous passage furnace, chamber, chamber zone

15 oven wall 20 work piece

10 31.32.33.34 feed point for mixing of protective gas 40 catalyst 50 transport medium, transport medium, roller floor

60 heating medium, heating unit 71.72.73 guide system

Claims (11)

1. Procedure for heating a galvanized workpiece (20), in which the workpiece (20) is conducted by means of a transport means (50) through several consecutive chamber zones (11; 12; 13 ; 14) of a continuous passage oven (10) and is heated therein by means of heating (60), feeding in the chamber areas (11; 12; 13; 14) of the continuous passage oven (10) through respective feed points (31; 32; 33; 34) a mixture of protective gas, characterized because guidance systems (71; 72; 73) between the chamber zones (11; 12; 13; 14) cause a total current of the protective gas mixture against the direction of passage of the workpiece (20) a through the continuous passage oven (10), the desired gas stream being favored by a slightly oblique position of the entire continuous passage oven (10), in which an anterior end of the continuous passage oven (10) is higher that a rear end and because the means of transport (50) drives the workpiece (20) through the guide systems (71; 72; 73), the guide systems (71; 72; 73) being separation walls with in each case an opening and preventing a circulation by convection of protective gas through the entire continuous passage furnace. 2. Method according to claim 1, characterized because the compositions of the mixtures of protective gas introduced through the respective feeding points (31; 32; 33; 34) in the chamber areas (11; 12; 13; 14) differ, presenting the mixture of protected gas fed in the last chamber zone (14) the lowest percentage of oxygen. 3. Method according to one or both of claims 1 and 2, characterized because A mixture of protective gas is generated by partial combustion of a hydrocarbon-air mixture in a noble metal catalyst (40) in the furnace wall (15) of the continuous passage furnace (10), generating the heat necessary for combustion partial by the fractionation process in the catalyst (40). 4. Method according to claim 3, characterized because Partial combustion in the noble metal catalyst (40) takes place at temperatures from approximately 700 ° C. 5. Method according to one or more of claims 1 to 4, characterized because the composition of a mixture of protective gas fed into an area (11; 12; 13; 14) of the continuous passage furnace (10) is selected based on the temperature of the workpiece (20) in the respective zone (11; 12; 13; 14) so that a galvanization of the workpiece (20) does not oxidize. 6. Method according to one or more of claims 1 to 5, characterized because The flow rate of the protective gas mixture through the furnace (10) is greater than the backscatter rate. 7. Procedure for tempering in the press a work piece in a press, characterized because The workpiece before being placed in the press was heated by a method according to one or more of claims 1 to 6. 8. Continuous step furnace for heating a galvanized workpiece (20), comprising a means of transport (50) to drive the workpiece (20) through several chamber zones (11; 12; 13; 14 ) of the continuous passage furnace (10) and a heating means (60) to heat the workpiece when passing through the oven (10), being provided in each of the chamber areas (11; 12; 13; 14 ) at least one feed point (31; 32; 33; 34) to feed a mixture of protective gas, characterized because guide systems (71; 72; 73) are arranged between the chamber areas (11; 12; 13; 14) to cause a flow of protective gas against a movement of the workpiece and to prevent convection circulation of protective gas throughout the furnace (10), the guide systems (71; 72; 73) being separation walls with in each case an opening, through which the transport means (50) runs and presenting the continuous step oven (10) a slightly oblique position, in which an anterior end of the continuous passage furnace (10) is higher than a posterior end. 9. Continuous passage oven according to claim 8, characterized because the compositions of the mixtures of protective gas introduced through the respective feeding points (31; 32; 33; 34) in the chamber areas (11; 12; 13; 14) differ, presenting the mixture of protected gas fed in the last chamber zone (14) the lowest percentage of oxygen. 10. Continuous passage furnace according to one or more of claims 8 and 9, characterized because In the furnace wall (15) of the continuous passage furnace (10) at least one noble metal catalyst (40) is provided, which generates a protective gas by partial combustion of a hydrocarbon-air mixture, generating the necessary heat for partial combustion through the fractionation process in the catalyst (40). 11. Continuous passage oven according to one or more of claims 8 to 10, characterized because The speed of the protective gas stream through the continuous pass furnace (10) is greater than the backscatter rate. 10 12. Continuous step furnace according to one or more of claims 8 to 11, characterized because The oven can be thermostatically maintained at a temperature that is above the predetermined heating temperature of the workpiece (20).   rich, poor