Method And Device for Energy-Efficient Hot Forming

Abstract

The invention relates to a method for hot forming of steel parts, in particular blanks or semi-finished products of steel, in which the steel parts are heated in a furnace for at least partial austenitisation to a temperature above the Ac1 temperature and hot formed in a forming tool. The object of providing a method for hot forming of steel parts which allows an energy-efficient performance of the hot forming while at the same time reducing the cycle times for producing a hot formed component, is achieved in that the waste heat from the furnace is used for further heat treatment steps of the steel part before and/or after the at least partial austenitisation of the steel part.

Description

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation of PCT/EP2010/060052, filed Jul. 13, 2010, which claims priority to German Application No. 102009026251.2, filed Jul. 24, 2009, the entire teachings and disclosure of which are incorporated herein by reference thereto.

FIELD OF THE INVENTION

The invention relates to a method for hot forming of steel parts, in particular blanks or semi-finished products of steel, in which the steel parts are heated in a furnace for at least partial austenitisation to a temperature above the Ac1 temperature and hot formed in a forming tool. In addition the invention relates to a device for performing the method with a furnace for heating the steel parts to above the Ac1 temperature in order to form an at least partially austenitic structure, a forming tool for hot forming of the steel parts and means for transporting the steel parts from the furnace to the forming tool.

BACKGROUND OF THE INVENTION

Steel parts within the meaning of the present invention are for example blanks or semi-finished products provided for hot forming. The semi-finished products may for example be in the form of cut-outs or pre-formed blanks. In order to perform the hot forming the steel parts provided for hot forming must initially be heated to a temperature above the Ac1 temperature point, so that at least a partial austenitisation of the structure of the steel part takes place. Preferably, however, the steel parts are fully austenitised. To this end they are usually heated to a temperature above the Ac3 temperature point, at which the formation of the austenitic structure is complete and thus the steel part exists with a fully austenitic structure.

The steel part is then placed in a forming tool and hot formed at a temperature above the Ac1 temperature point. By means of the forming tool a so-called “press hardening” takes place during which because of the rapid cooling the austenitic structure of the formed steel part is converted into a martensitic structure. This is what gives hot formed steel parts their enormous strength. In order to prevent scaling during heat treatment of the steel parts and at the same time provide corrosion protection, the steel parts provided for hot forming are preferably coated with an AlSi coating. Finally, the press hardening can be followed by a further work step in which the formed steel parts undergo a further heat treatment, for example tempering, in order to again change the characteristics of the steel part in certain areas or as a whole, for example to increase its ductility.

Hot forming is on the one hand a very energy-intensive process, since the steel parts have to be heated for a plurality of minutes to a temperature above the Ac1 temperature point or must be heated to above the Ac3 temperature point and remain at this temperature for a plurality of minutes. Further energy in the form of heat is then required by the forming tool and in any further heat treatment step. This is also the case with any preheating that takes place prior to heating of the steel part to a temperature above the Ac1 temperature point. It has transpired that the energy costs, in particular for the mass production of such steel parts, have a negative effect on the economic efficiency of the method. It has also been found that the cycle times for producing the hot formed steel part are in need of improvement. From German patent specification DE 10 2005 033 042 B3 it is known to preheat a semi-finished product having a metallic coating provided for press hardening by using the waste heat from the cooling process of the semi-finished product heated to at least above the Ac1 temperature. It has, however, transpired that the residual quantities of heat emanating from the semi-finished product cooling process are insufficient for a significant reduction in energy costs.

The object of the present invention, therefore, is to provide a method for hot forming of steel parts, which allows an energy-efficient performance of hot forming and at the same time reduces the cycle times for producing a hot formed component. In addition a device should be proposed for performing the method according to the invention.

SUMMARY OF THE INVENTION

For a generic method the above object is achieved by a first teaching of the present invention in that the waste heat from the furnace is used for further heat treatment steps of the steel part before and/or after the at least partial austenitisation of the steel part.

It has transpired that the furnaces used for performing the at least partial austenitisation produce enormous amounts of waste heat which to date has remained unused. According to the invention the waste heat from the furnace is now used for further heat treatment steps on the steel part before and/or after the at least partial austenitisation of the steel part, allowing significant energy savings to be made.

If the steel part is preheated prior to the at least partial austenitisation using the waste heat from the furnace, the cycle time for the hot forming as a whole can be reduced, since the components in the furnace for at least partial austenitisation because of their higher entry temperature reach the desired temperature more quickly without significantly more energy being used for the process. The quantities of waste heat from the furnace for at least partial austenitisation of the steel part are so great that quantities of heat that are also necessary after heat treatment can be provided by the waste heat from the furnace. This allows the energy efficiency of the method according to the invention to be increased further.

According to a first embodiment of the method according to the invention a continuous furnace, preferably a roller hearth furnace is used for at least partial austenitisation of the steel part. A continuous furnace or a roller hearth furnace offers a simple option for performing the at least partial austenitisation of the steel part in a continuous process and incorporating this in a corresponding process line.

The quantities of waste heat which can be used in upstream and downstream heat treatments, are increased in that the furnace for at least partial austenitisation of the steel part is at least partially heated by gas and the waste heat from the combustion exhaust gases from the furnace is used. These so-called radiant furnaces can in a very short space of time heat the component to the desired temperature above the Ac1 temperature point or also to above the Ac3 temperature point, so that the cycle times for the at least partial austenitisation of the steel part are relatively short. Furthermore the waste heat from the combustion gases can be simply used in that for example by means of a heat exchanger the quantity of energy is removed from the gas or the waste gas is used directly for further heat treatments. It is conceivable to further equip the furnace heated with gas with means for inductive heat transfer and/or means of radiated transmission of the heat.

If in a preheating furnace using the waste heat from the furnace for at least partial austenitisation of the steel part the steel parts, in particular metal-coated, preferably AlSi-coated steel parts are heated or pre-alloyed, the cycle times that are necessary for passing through the furnace for at least partial austenitisation of the steel part can be reduced and at the same time energy saved. This is because on the one hand the already heated steel part requires less quantities of energy in order to be heated to the desired temperature. On the other hand, for example through pre-alloying of a metal-coated, preferably an AlSi-coated, steel part, a relatively higher iron proportion in the AlSi coating is achieved, which leads to a roughening of the surface layer, so that radiant heat in the furnace can be used considerably more efficiently for at least partial austenitisation of the steel part. The reflection characteristics of the pre-alloyed metal-coated, preferably AlSi-coated, steel part, are considerably improved.

A further increase in the energy efficiency of the method according to the invention can be achieved in that the forming tool for hot forming is at least partially heated with the waste heat from the furnace for at least partial austenitisation of the steel part. At the correspondingly heated points, for example, another structure in the steel part can be partially set, so that varying component characteristics in the formed steel part can be provided.

Optionally following forming of the steel part a further heat treatment takes place, which according to a further configuration of the method according to the invention is advantageously carried out using the waste heat from the furnace for at least partial austenitisation of the steel part.

A typical heat treatment following forming is for example tempering, for example partial tempering of the formed, annealed steel part, in order to for example change the component characteristics on a localised basis.

In order to further increase the efficiency of use of the waste heat, the heat treatment steps on the steel part are performed before and/or after the at least partial austenitisation of the steel part by forced convection. In a forced convection the heat transfer is considerably increased, since the steel parts to be heated or for example parts of the tool have a hot gas jet passed over them so that an intensive contact between hot gas particles and the part to be heated takes place.

Preferably the steel parts are heated in the preheating furnace to a temperature of 200° C. to Ac1 temperature, preferably to 250° C. to 700° C. Uncoated steel parts then require even less quantities of energy for heating above the Ac1 temperature in the furnace for at least partial austenitisation. Furthermore, coated components can for example be pre-alloyed with an AlSi coating in this temperature range with a short dwell time, in order to achieve in particular a shorter heating time in the furnace for at least partial austenitisation of the steel part.

According to a second teaching of the present invention the object illustrated above for a device is achieved in that means are provided for using the waste heat from the furnace before and/or after at least partial austenitisation of the steel part.

The means for using the waste heat from the furnace for at least partial austenitisation of the steel parts are manifold and can take different forms. As a rule they comprise pipelines and fan devices, which can be used for the use and transport of such hot gases. In addition heat exchangers can be provided which transfer the heat from the combustion exhaust gases from the furnace to a further heat conduction medium. As a result of the use of these means the energy efficiency of the method according to the invention for hot forming of steel parts is considerably improved and in addition the cycle times for producing hot formed steel parts is reduced.

Preferably therefore means are provided, which are used for heating a preheating furnace, the forming tool and/or a tempering furnace. In this way the utilisation of the waste heat from the furnace for at least partial austenitisation of the steel part is used, in order to operate the essential processes from preheating to tempering of the steel part in an energy efficient manner. The heating of the preheating furnace in particular, however, leads to a considerable increase in energy efficiency since here large quantities of energy are required.

According to a next configuration of the device according to the invention a continuous furnace, preferably a roller hearth furnace is provided for at least partial austenitisation of the steel parts. As already explained, by using a continuous furnace, preferably a roller hearth furnace a simple integration of the heating of the steel part to above the Ac1 temperature in a process chain for producing hot formed steel parts is possible.

According to a simple configuration of the device according to the invention as means for heating the preheating furnace, the forming tool and/or the tempering furnace gas pipes and/or fans for guiding the combustion exhaust gases from the furnace for the at least partial austenitisation of the steel parts to the individual devices are provided. As already explained, heat exchangers can also be used if the combustion gases are not used directly.

BRIEF DESCRIPTION OF THE DRAWINGS

There are now a plurality of possibilities for configuring and developing the device according to the invention. To this end reference is made on the one hand to the dependent claims of claims 1 and 9 and to the description of an exemplary embodiment in conjunction with the drawing.

FIG. 1 shows in a schematic view an exemplary embodiment of a device for performing the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The exemplary embodiment shown in FIG. 1 of a method according to the invention comprises a furnace 1 for at least partial austenitisation of a steel part 2. The steel part 2, in this case a flat blank of a temperable steel, is in the exemplary embodiment according to FIG. 1 heated to a temperature of at least 850° C. and maintained for approximately 2 to 15 minutes at this temperature in the furnace 1, which takes the form of a roller hearth furnace. Through the temperature of 850° C. and higher a complete austenitisation of the blank 2 is achieved. Preferably the blank consists of a type 22MnB5 steel.

The furnace for at least partial austenitisation of the blank 2 is heated by gas. Corresponding means 3 for heating the furnace are shown schematically in FIG. 1. In addition the transport rollers 4 typical of a roller hearth furnace are likewise shown in FIG. 1. The combustion exhaust gases from the roller hearth furnace 1 are extracted via the medium supply lines 5, 6, 7 for further heat treatments from the roller hearth furnace 1. For introducing the hot gases into the preheating furnace 8, the forming tool 9 or the tempering furnace 10 are also provided with additional fans 5a, 6a and 7a.

The medium supply line 5 carries the hot combustion exhaust gases to a preheating furnace 8, in which the blank 2 is heated to a temperature of more than 200° C. to Ac1 temperature, preferably to a temperature of between 250° C. and 700° C. The dwell time in the preheating furnace is approximately 3 to 20 minutes, preferably however 5 to 10 minutes. For transporting the blanks into the preheating furnace rollers 4 are likewise shown. In the preheating furnace the blank 2 as already described is heated to a relatively high temperature. If the blank 2 is made with an AlSi coating, then at corresponding temperatures, which for example may be between 500 and 700° C., these can be pre-alloyed in the preheating furnace and in this respect obtain a roughened surface structure, making the heating of the blank 2 for example in the roller hearth furnace 1 easier. Via the heating medium supply line 5 the temperature of the preheating furnace can be adjusted by means of control means that are not shown.

By using the waste heat from the roller hearth furnace 1 for, by way of example, preheating the blank 2 in the preheating furnace 8, a considerable energy saving is made, since the high temperatures which are necessary for pre-alloying of the coated blanks, can be provided in a simple manner. Following austenitisation in the roller hearth furnace 1 the blank 2 by means of a handling system, not shown, is placed at an as high as possible temperature in a forming tool 9 where it is hot formed, preferably press hardened. Blanks 2 with an AlSi-coating can on the one hand be heated without scaling to austenitisation temperature and likewise hot formed or press hardened in the forming tool 9.

The medium supply line 7 is used for tempering the forming tool at specific positions using the waste heat from the roller hearth furnace 1. At the heated or tempered areas of the forming tool another structure, for example a more ductile structure, of the formed steel part 11 can be specifically created, whereas in the other areas of the formed steel part 11 a martensitic structure is created by a rapid cooling. The formed steel part 12 can optionally undergo heat treatment again in a tempering furnace 10. The tempering furnace 10 can also use the energy of the waste gases from the roller hearth furnace 1, in order to perform the tempering process, for example in partial sections of the annealed steel part 12.

The energy saving which is achieved by using the waste heat from the roller hearth furnace 1 is considerable. The transport between the individual stations, in particular the preheating furnace 8, the roller hearth furnace 1, the forming tool 9 and the tempering furnace 10 can be ensured by handling systems that are not shown. A rollerway 4, as illustrated in the present exemplary embodiment, can alternatively be replaced by handling systems. The result of this is that with the method according to the invention or with the device according to the invention, as shown in FIG. 1, a considerable reduction in cycle times for producing a hot formed steel part 11, 12 with a simultaneous reduction in energy consumption can be ensured.

Claims

1. Method for hot forming of steel parts, in particular blanks or semi-finished products of steel, in which the steel parts are heated in a furnace for at least partial austenitisation to a temperature above the AC1 temperature and hot formed in a forming tool, comprising using waste heat from the furnace in a heat treatment step for the steel parts occurring at least one of before and/or after the at least partial austenitisation of the steel parts.

2. Method according to claim 1, wherein a continuous furnace is used for the at least partial austenitisation of the steel parts.

3. Method according to claim 1, wherein the furnace for at least partial austenitisation of the steel parts is at least partially heated by gas and the waste heat from the combustion exhaust gases from the furnace is used for the heat treatment step.

4. Method according to claim 1, wherein using the waste heat from the furnace in a heat treatment step is achieved in a preheating furnace to pre-alloy the steel parts.

5. Method according to claim 1, wherein the forming tool for hot forming is at least partially heated with the waste heat from the furnace for at least partial austenitisation of the steel part.

6. Method according to claim 1, wherein using the waste heat from the furnace in a heat treatment step includes tempering of the formed steel parts.

7. Method according to claim 1, wherein the heat treatment step on the steel parts are performed by forced convection.

8. Method according to claim 1, wherein the steel parts are heated in the preheating furnace to a temperature of 200° C. to Ac1 temperature.

9. Device for performing a method according to claim 1, comprising:

a furnace for heating steel parts to above the Ac1 temperature in order to form an at least partially austenitic structure;

a forming tool for hot forming of the steel parts;

means for transporting the steel parts from the furnace to the forming tool;

means for using the waste heat from the furnace in a heat treatment step in at least one of before and/or after the at least partial austenitisation of the steel parts.

10. Device according to claim 9 wherein the means using the waste heat are provided for heating at least one of a preheating furnace, the forming tool, and/or a tempering furnace.

11. Device according to claim 9, wherein the furnace is a continuous furnace.

12. Device according to claim 10, wherein as means for heating the preheating furnace, the forming tool, and the tempering furnace, at least one of medium supply lines and fans are provided for guiding the combustion exhaust gases from the furnace for the at least partial austenitisation.

13. Method according to claim 2, wherein the continuous furnace is a roller hearth furnace and is used for the at least partial austenitisation of the steel parts.

14. Method according to claim 1, wherein the steel parts are AlSi-coated steel parts.

15. Method according to claim 1, wherein the steel parts are heated in the preheating furnace to a temperature of about 250° C. to about 700° C.

16. Device according to claim 11, wherein the continuous furnace is a roller hearth furnace.