WO2012107110A1

WO2012107110A1 - Furnace

Info

Publication number: WO2012107110A1
Application number: PCT/EP2011/053096
Authority: WO
Inventors: Rolf-Josef Schwartz
Original assignee: Schwartz, Eva
Priority date: 2011-02-10
Filing date: 2011-03-02
Publication date: 2012-08-16
Also published as: ES2528001T3; EP2487268B1; EP2487268A1

Abstract

The invention relates to furnace, comprising a device and a method generating a reducing atmosphere for annealing furnaces. The furnace according to the invention for heating up sheet steel parts to the austenitic temperature between 850°C and 950°C [850°F and 1742°F] has an interior and a furnace roof as well as a device that is installed vertically in the furnace roof in such a way that one part of the device projects into the furnace opposite from the z-direction, while the other part projects out of the furnace, whereby, on the part projecting out of the furnace, the device has a connection piece through which a nitrogen-oxygen mixture can be fed in as well as another connection piece through which coal granules can be fed in, and there is a gas-permeable floor at the bottom of the part of the device that projects into the furnace and that is located opposite from the z-direction. Here, the gas-permeable floor of the device can capture the coal granules that were fed in, a process in which the floor remains gas-permeable. The device projects so far towards the interior of the furnace that a temperature of at least 750°C [1382°F] is established inside the device at the level of the gas-permeable floor when the temperature in the interior of the furnace is at least 850°C [1562°F]. Consequently, during operation of the furnace, the temperature inside the device at the level of the gas-permeable floor is at or above the self-ignition temperature of the coal granules.

Description

Furnace

Description The invention relates to a furnace, comprising a device and a method for generating a reducing atmosphere for annealing furnaces.

In many application cases in a wide array of sectors, the state of the art calls for high-strength metal parts that are lightweight. For instance, the automotive indus- try strives towards developing vehicles that consume as little fuel as possible, a goal that can be achieved, among other things, by reducing the weight of the vehicles. On the other hand, vehicle parts have to comply with increasingly stricter safety requirements. For this reason, the structural steel used for the car body has to display high strength while also being lightweight. This is normally achieved by employing the so-called press hardening process, in which sheet steel is first heated up to the austenitic temperature between 850°C and 950°C [850°F and 1742°F], subsequently placed into a press tool, quickly formed and rapidly quenched to the martensitic temperature of about 250°C [482°F] by the water-cooled tool. This results in a hard, high-strength martensitic structure that exhibits a strength of about 1500 MPa.

When steel is heated up to the temperatures needed for austenite formation, metal oxide is created in the presence of oxygen in free or chemically bound form since the reactivity is increased by the oxygen.

Moreover, such structures often pose problems with so-called hydrogen embrit- tlement, in which hydrogen is incorporated into the martensitic structure and can cause the formation of cracks at a later point in time. Many mechanisms have not yet been investigated in this context. However, is seems evident that temperature, time and dew point in the furnace all play a significant role.

It is a known procedure for furnaces to have an inert-gas atmosphere for such applications. Here, the furnaces are operated either with pure nitrogen having a dew point of approximately -60°C [-76° F] or with a mixture of nitrogen and natural gas, or else with an exothermic gas or endothermic gas produced in the system. Pure nitrogen has a dew point of approximately -60°C [-76°F]. A process operated with pure nitrogen, however, does not have any reserves either against oxygen entrained into the furnace together with the feedstock or against entrained moisture.

As disclosed, for example, in German patent DE 103 47 312 B3, exothermic gas is produced from a hydrocarbon, for instance, natural gas, and air. This inert gas is produced in exothermic gas generators. For this purpose, the premixed gas stream consisting of natural gas/air is fed into a combustion chamber, where it is reacted. This is an exothermal reaction that generates thermal energy in excess. The very moist exothermic gas thus generated is cooled off to room temperature and fed into a dryer. The dew point of the dried gas is then approximately -30°C [- 22°F].

German patent DE 103 47 312 B3 also describes the production of endothermic gas. It is carried out in endothermic gas generators. Towards this end, the premixed natural gas/air mixture is fed into a heated retort with a catalyst filling, where it is reacted. The catalyst retort has to be heated up and the gas mixture that flows through the retort has to be brought to the reaction temperature in order to react on the surface of the catalyst. This is an endothermic reaction, that is to say, part of the heating enthalpy and all of the reaction enthalpy have to be fed into the system. The generated endothermic gas is cooled down to room temperature and is then ready for use. The dew point of the endothermic gas that is employed as inert gas during the heat treatment of iron materials is within the range from -10°C to +5°C [14°F to 41 °F]. This "pure" endothermic gas is diluted (mixed) with nitrogen and subsequently fed into the furnace. In these inert-gas mixtures containing 1 % to 5% CO, the strong dilution with nitrogen causes the dew point to drop to values of -20°C to -30°C [-4°F to -22°F] so that there is no need to use an additional dryer, as is the case with the generation of exothermic gas. Moreover, all of the familiar inert gases that have a reducing effect contain hydrogen to varying degrees, which likewise has a detrimental impact on the tendency towards hydrogen embrittlement. Particularly in the automotive sector, there is an ever-increasing demand for a hydrogen-free reducing annealing atmosphere.

Before this backdrop, it is the objective of invention to put forward a furnace in which a hydrogen-free reducing atmosphere can be provided in the simplest and most cost-effective manner possible.

According to the invention, this objective is achieved by a furnace having the features of the independent Claim 1 . Advantageous refinements of the invention ensue from the subordinate Claims 2 to 1 1.

Another objective of the invention is to put forward a method for heating up sheet steel parts to the austenitic temperature between 850°C and 950°C [850°F and 1742°F] in a hydrogen-free reducing atmosphere. This objective is achieved by a method according to Claim 12. Advantageous embodiments of the method ensue from the subordinate Claims 13 to 15.

The furnace according to the invention for heating up sheet steel parts to the austenitic temperature between 850°C and 950°C [850°F and 1742°F] comprises an interior and a furnace roof as well as a device that is installed vertically in the furnace roof in such a way that one part of the device projects into the furnace opposite from the z-direction, while the other part projects out of the furnace, whereby, on the part projecting out of the furnace, the device has a connection piece through which a nitrogen-oxygen mixture can be fed in as well as another connection piece through which coal granules can be fed in, and there is a gas- permeable floor at the bottom of the part of the device that projects into the furnace and that is opposite from the z-direction. Here, the gas-permeable floor of the device can capture the coal granules that were fed in, a process in which the floor remains gas-permeable. The device projects so far towards the interior of the furnace that a temperature of at least 750°C [1382°F] is established inside the device at the level of the gas-permeable floor when the temperature in the interior of the furnace is at least 850°C [1562°F]. Consequently, during operation of the furnace, the temperature inside the device at the level of the gas- permeable floor is at or above the self-ignition temperature of the coal granules.

In a preferred embodiment, the device is made at least partially of silicon carbide, whereby advantageously the section of the device made of silicon carbide is the part of the device located inside the furnace. This protects it against the risk of carburization.

Preferably, the connection piece through which the nitrogen-oxygen mixture is fed into the furnace, and/or the connection piece through which the coal granules are fed into the furnace is located in the z-direction at the top of the device, in other words, in the cold zone above the furnace roof.

In an especially advantageous embodiment, coal granules are continuously fed into the device, whereby the coal granule flow rate amounts to approximately 1.5 kg per hour. A typical continuous annealing furnace with a steel throughput rate of about 5 tons per hour consumes about 20 to 100 m³/h of inert gas which, at a corresponding flow of a nitrogen-oxygen mixture, can be obtained from the above-mentioned 1 .5 kg/h of coal granules. The inert gas here can consist of 2% to 5% by volume of carbon monoxide in order to yield annealed sheet steel parts that are free of scaling.

In another advantageous embodiment, the device has an end piece that faces the interior of the furnace and that has a gas outlet opening. Preferably, this gas outlet opening is dimensioned in such a way that the gas outflow speed is between 20 m/s and 50 m/s. Consequently, the furnace atmosphere is circulated very thoroughly by the resultant pulse.

It has proven to be particularly advantageous for the end piece to be shaped like a funnel. In an especially advantageous embodiment, the furnace is configured as a continuous furnace, whereby it has a conveying means on which the sheet steel part to be heated up can be conveyed through the furnace. The method according to the invention for heating up sheet steel parts to the aus- tenitic temperature between 850°C and 950°C [850°F and 1742°F] is characterized in that a hydrogen-free reducing atmosphere is generated in the furnace where the sheet steel parts are heated up in that coal granules are continuously conveyed into a device that at least partially has a sieve floor and that projects partially into the furnace, and a nitrogen-oxygen mixture is fed in from above onto the coal granules present in the device, so that the coal granules burn under the effect of the heat inside the furnace, forming carbon monoxide that flows into the interior of the furnace through the gas-permeable floor. It has proven to be advantageous for the coal granules to be conveyed into the device at a flow rate of approximately 1 .5 kg/hr, and for the outflow speed of the carbon monoxide out of the device and into the interior of the furnace to be 20 m/s to 50 m/s. In this context, weight measuring instruments or volume- throughput measuring devices can be employed to meter in the components nitrogen, oxygen and coal granules.

Other advantages, special features and practical refinements of the invention ensue from the subordinate claims and from the presentation below of a preferred embodiment making reference to the figures.

These figures show the following:

Figure 1 - a continuous annealing furnace with a device for providing a hydrogen-free reducing atmosphere;

Figure 2 - a device for providing a hydrogen-free reducing atmosphere, in an enlarged depiction. Figure 1 shows a continuous annealing furnace 10 that is heated up by means of the heating element 1 1. The furnace 10 has a roller belt 30 as the conveying means with which a sheet steel part 20 that is to be heated up can be conveyed into and through the furnace. The furnace has a front door 12 that opens upwards in the z-direction when a sheet steel part 20 approaches. The sheet steel part 20 is conveyed through the furnace 10 horizontally on the roller belt 30 in the x- direction. A rear door 13 opens to allow the material to exit the furnace 10. A temperature of 850°C to 950°C [850°F to 1742°F] prevails in the interior 5 of the furnace to austenize the sheet steel part 20. In order to generate a hydrogen-free reducing atmosphere, a device 1 projects partially through the furnace roof 2 into the interior 5 of the furnace 10 opposite from the z-direction. At the upper end in the z-direction that projects out of the furnace 10, the device 1 has a connection piece 7 through which a nitrogen-oxygen mixture can be fed into the device 1. Moreover, at the same end, the device 1 also has a connection piece 8 through which coal granules 4 can be fed into the device 1 . The device 1 is located so far into the interior 5 of the furnace 10 that the temperature in the interior of the device 1 at the end that is opposite from the z-direction is at least 750°C [1382°F] when the furnace has been heated up. Here, the coal granules 4 fall onto a gas- permeable floor 3 inside the device 1 , where they immediately burn due to the temperatures that prevail there during operation, which are above the self-ignition temperature of the coal granules 4 of 750°C [1382°F]. A combustion gas containing carbon monoxide and nitrogen is formed in this process and it flows through the gas-permeable floor 3 - which remains gas-permeable in spite of its being charged with coal granules 4 - into the space of the device 1 that is below the gas-permeable floor 3 and that is delimited with respect to the interior 5 of the furnace 1 by a funnel-shaped end piece 9. The end of this space located opposite from the z-direction, which is formed by the funnel-shaped end piece 9 and by the gas-permeable floor 3, has a gas outlet opening 6 through which the combustion gas containing carbon monoxide flows into the interior 5 of the furnace 10. The gas outlet opening is dimensioned in such a way that, when the device 1 is charged with coal granules at a throughput rate of approximately 1 .5 kg/hr and at a corresponding flow of nitrogen-oxygen mixture fed in through the connection piece 7, the gas outflow speed is between 20 m/s and 50 m/s. This creates a pulse in the interior 5 of the furnace by means of which the furnace atmosphere is circulated in such a way that the carbon monoxide concentration throughout the interior 5 of the furnace 10 is sufficient to prevent scaling on the one hand, and hydrogen embrittlement of the sheet steel part 20 on the other hand. Weight measuring instruments or volume-throughput measuring devices can be employed to meter in the components nitrogen, oxygen and coal granules.

A continuous furnace with a throughput rate of approximately 5 tons per hour typically consumes about 50 to 100 m³/hr of inert gas. Under normal conditions, 2% to 5% carbon monoxide is sufficient to operate scale-free. The toxicity is comparable to that of other reducing inert gases such as, for instance, endothermic gas with approximately 15% carbon monoxide or exothermic gas with approximately 7% carbon monoxide. Work with such inert gases is regulated in EN 746 and actual experience has been available for many years in hundreds of furnace installations.

Figure 2 shows the device for providing a hydrogen-free reducing atmosphere in an enlarged depiction. The coal granules 4 can reach the interior of the device 1 via the connection piece 8. For this purpose, a conveying screw 30 can be used in order to convey the coal granules 4 horizontally. The nitrogen-oxygen mixture is fed into the device 1 from above through the vertical connection piece 7. The coal granules fall due to gravity out of the horizontal connection piece 8 into the device 1 , a process in which they are carried along by the nitrogen-oxygen mixture. In the device 1 , the prevailing temperature ranges from room temperature at the connection pieces 7 and 8 up to at least 750°C [1382°F] directly above the gas-permeable floor 3 when the furnace is in operation and the temperature in the interior of the furnace is at least 850°C [1562°F]. The device does not have to be heated separately, but rather, it obtains the heat it needs from the heated interior of the furnace. As soon as the coal granules 4 have fallen so far downward in the vicinity of the device 1 that the temperature is at least 750°C [1382°F], the coal granules burn while forming carbon monoxide, and the combustion gases flow through the gas-permeable floor 3 and through the gas outlet opening 6 into the interior 5 of the furnace 10. The device 1 is made of silicon carbide in order to prevent its carburization. It goes without saying that the device 1 can also be made of other materials. A furnace 10 having a device 1 is described in the embodiments. By the same token, a furnace 10 can also have several devices 1 , especially when the furnace 10 is larger and the throughput rate of sheet steel parts 20 is greater.

List of reference numerals:

1 device, retort

2 furnace roof

3 gas-permeable floor

4 coal granules

5 interior

6 gas outlet opening

7 connection piece for feeding in the nitrogen-oxygen mixture

8 connection piece for feeding in the coal granules

9 end piece

10 furnace

1 1 heating element

12 front door

13 back door

20 sheet steel part

30 conveying means, roller belt

40 conveying screw

Claims

Patent Claims

1 . An furnace (10) for heating up sheet steel parts to the austenitic temperature between 850°C and 950°C [850°F and 1742°F], comprising an interior (5) and a furnace roof (2),

characterized in that

a device (1 ) is installed vertically in the furnace roof (2) in such a way that one part of the device (1 ) projects into the furnace (10) opposite from the z- direction, while the other part projects out of the furnace (10), and on the part projecting out of the furnace (10), the device (1 ) has a connection piece (7) through which a nitrogen-oxygen mixture can be fed in and a connection piece (8) through which coal granules (4) can be fed in, and there is a gas-permeable floor (3) at the bottom of the part of the device (1 ) that projects into the furnace (10) and that is opposite from the z-direction.

2. The furnace (10) according to Claim 1 ,

characterized in that

the gas-permeable floor (3) of the device (1 ) can capture the coal granules (4) that were fed in, a process in which the floor (3) remains gas- permeable.

3. The furnace (10) according to Claim 2,

characterized in that

the device (1 ) projects so far towards the interior (5) of the furnace (10) that a temperature of at least 750°C [1382°F] is established inside the device (1 ) at the level of the gas-permeable floor (3) when the temperature in the interior (5) of the furnace (10) is at least 850°C [1562°F].

4. The furnace (10) according to one of the preceding claims,

characterized in that

the device (1 ) is made at least partially of silicon carbide.

5. The furnace (10) according to Claim 4,

characterized in that the part of the device (1 ) that is made of silicon carbide is inside the furnace.

The furnace (10) according to one of the preceding claims,

characterized in that

the connection piece (7) through which the nitrogen-oxygen mixture is fed into the device (1 ) is located in the z-direction at the top of the device (1 ).

The furnace (10) according to one of the preceding claims,

characterized in that

coal granules can be continuously fed into the device (1 ) at a flow rate of approximately 1 .5 kg per hour.

The furnace (10) according to one of the preceding claims,

characterized in that

the device (1 ) is delimited with respect to the interior (5) of the furnace (10) by an end piece (9) that has a gas outlet opening (6).

The furnace (10) according to Claim 8,

characterized in that

the gas outlet opening (6) is dimensioned in such a way that the gas outflow speed is between 20 m/s and 50 m/s.

The furnace (10) according to Claim 8 or 9,

characterized in that

the end piece (9) is shaped like a funnel.

The furnace (10) according to one of the preceding claims,

characterized in that

the furnace (10) has a conveying means (30) on which the sheet steel part to be heated up can be conveyed through the furnace (10).

12. A method for heating up sheet steel parts to the austenitic temperature between 850°C and 950°C [850°F and 1742°F], characterized in that

a hydrogen-free reducing atmosphere is generated in the furnace (10) in which the sheet steel parts are heated up in that coal granules (4) are continuously conveyed into a device (1 ) that at least partially has a sieve floor (3) and that projects partially into the furnace (10), and a nitrogen-oxygen mixture is fed in from above onto the coal granules (4) present in the device (1 ), so that the coal granules (4) burn under the effect of the heat in the furnace interior (9), forming carbon monoxide that flows into the interior (5) of the furnace (10) through the gas-permeable floor (3).

The method according to Claim 12,

characterized in that

the coal granules (4) are conveyed into the device (1 ) at a flow rate of approximately 1 .5 kg/hr, and the outflow speed of the carbon monoxide out of the device and into the interior (5) of the furnace (10) is 20 m/s to 50 m/s.

The method according to Claim 12 or 13,

characterized in that

weight measuring instruments are employed to meter in the components nitrogen, oxygen and coal granules (4).

The method according to Claim 12 or 13,

characterized in that

volume-throughput measuring devices is employed to meter

components nitrogen, oxygen and coal granules (4).