EP2175041A1

EP2175041A1 - Method of hot forming metal plates

Info

Publication number: EP2175041A1
Application number: EP08425657A
Authority: EP
Inventors: Marco Colosseo; Daniele Bassan; Manuel Lai
Original assignee: Centro Ricerche Fiat SCpA
Current assignee: Centro Ricerche Fiat SCpA
Priority date: 2008-10-09
Filing date: 2008-10-09
Publication date: 2010-04-14

Abstract

Method of hot forming metal plates, in particular steel plates, characterised in that it comprises the following stages: using induction to heat the plate to be formed, forming this plate in a die, cooling the formed plate when the die is in the completely closed condition at the end of forming, with the formed plate still inside it, by means of a flow of liquid nitrogen or a double flow of liquid nitrogen and water, which flows in proximity to the plate through channels in the die, without coming directly into contact with it. The method furthermore provides for using induction to heat the formed plate before subjecting it to a trimming operation.

Description

The present invention has as its subject a method of hot forming metal plates.
The procedure is known in the art of pressing steel plates that envisages heating the plate to a temperature of about 1000°C, forming it in a die, and cooling it inside this die by means of a cooling circuit using water. This hot forming procedure makes it possible to obtain pressed steel plates, even in considerably complex shapes, which are endowed with elevated mechanical resistance. However, the forming process indicated above is afflicted with the great disadvantage of requiring a large expenditure of energy, and of having relatively long cycle times.
The present invention envisages a procedure for hot forming metal plates which is free from the disadvantages indicated above.
In particular, the present invention concerns a method for hot forming having the characteristics of claim 1. The present invention, furthermore, concerns equipment for hot forming metal plates having the characteristics of claim 5. Preferred forms of embodiment of the invention are presented in the dependent claims.
The method according to the present invention is suitable for any plate in metallic material. Excellent results in terms of precision of processing and mechanical strength of the finished article have been obtained with this method especially with steel plates, and specifically with boron steel plates.
Further characteristics and advantages of the invention will be presented in the description which follows.
The method of hot forming according to the present invention envisages first of all a stage of heating the plate to be formed, which is carried out by induction heating means. These means can be of any type suitable for heating the plate to be formed through induction, such as for example an induction furnace. In particular, such means are suitable for creating a low voltage in the plate to be formed, and at the same time a high intensity of current across the plate, by means of which the latter is heated very rapidly. The temperature which the plate must reach is preferably around 1000°C. Induction heating means can also be used, subsequently to the drawing or forming operation, and prior to an operation of trimming the formed plate, for heating the perimeter zones of the plate, which then need to be cut during the trimming, the object being to limit the effort needed and the stresses on the trimming blades, and so prolong their useful life.
The method according to the invention therefore provides for forming the plate in a die, and subsequently cooling it while it is still in the die, by means of a flow of liquid nitrogen or a double flow of liquid nitrogen and water. In particular, an important characteristic of the method according to the present invention consists in the fact that the cooling of the formed plate is performed when the die is in the completely closed condition at the end of forming, with the formed plate still inside it, and is performed by means of a flow of liquid nitrogen or a double flow of liquid nitrogen and water, which flows in proximity to the plate through channels in the die, without coming into contact with it.
In a preferred form of embodiment, the die for forming the plate and subsequently cooling it comprises one or more matrices, defining respective forming surfaces of the die, which include one or more channels passing through the matrices in proximity to the forming surfaces, but without flowing out onto them. The above-mentioned channels are configured for being traversed by a flow of nitrogen generated by means of a recirculating pump communicating with the above-mentioned channels. Further channels of the type indicated above are also formed in the matrices and configured for being traversed by a flow of refrigerated water generated by means of a recirculating pump communicating with the above-mentioned further channels. The combined application, as described above, of a cooling circuit carrying liquid nitrogen, and another circuit carrying water, enables adjustment, over a wide range of values, of the rate of cooling of the formed plate.
Preferably, the nitrogen which is forced through the channels in the die is at a temperature of about -270°C.
In a preferred form of embodiment, the equipment for the execution of the method described here can also include automatic transfer means, capable of picking up the plate that is to be formed (which has been heated) and locating it in the die, and subsequently carrying the pressed plate to a position downstream of the die.
By virtue of the characteristics indicated above, the method of hot forming described here has the advantage of providing considerably reduced cycle times (of around 10 seconds) and of producing pressed plates which exhibit a more homogeneous internal structure, and therefore possessing greater mechanical strength, compared with that of components pressed using traditional methods.
In addition, the method of forming metal plates according to the present invention makes it possible to eliminate the use of anti-oxidant layers with which the plates are coated in order to limit the oxidisation processes favoured during pressing by the high temperatures. This is because the cycle times of the method of forming according to the present invention are so much reduced, and most importantly the induction heating which it provides is so fast, that the plates remain at high temperature for a time that is insufficient for the process of oxidisation to be able to significantly alter the plate being pressed, even if it is lacking any kind of anti-oxidant layer.

Claims

Method of hot forming metal plates, in particular steel plates, characterised in that it comprises the following stages: using induction to heat the plate to be formed, forming this plate in a die, cooling the formed plate when the die is in the completely closed condition at the end of forming, with the formed plate still inside it, by means of a flow of liquid nitrogen or a double flow of liquid nitrogen and water, which flows in proximity to the plate through channels in the die, without coming directly into contact with it.
Method according to claim 1, characterised in that said flow of liquid nitrogen, or said double flow of liquid nitrogen and water, passes through said die.
Method according to claim 1, characterised in that said plates are made of boron steel.
Method according to claim 1, characterised in that it specifies using induction to heat the formed plate before subjecting it to a trimming operation.
Equipment for hot forming metal plates, in particular steel plates, characterised in that it comprises means of using induction to heat the plate to be formed, and a die made up of matrices defining respective surfaces for forming said plate, one or more of such matrices comprising one or more channels passing through the matrices in proximity to said forming surfaces, but without flowing out onto them, wherein means are furthermore provided for recirculating liquid nitrogen, capable of generating a flow of liquid nitrogen through said channels.
Equipment according to claim 5, characterised in that means are furthermore provided for recirculating water, capable of generating a flow of water through further channels in said matrices.
Equipment according to claim 6, characterised in that said recirculating means comprise a pump in fluid communication with said channels, capable of generating a flow of liquid nitrogen through them, and a pump in fluid communication with said further channels, capable of generating a flow of water through them.
Equipment according to claim 5, characterised in that said heating means comprise an induction furnace located upstream of said die, and in that automatic means are provided for transferring the plate from said furnace to said die, and from said die to a position downstream.