A METHOD OF HYDROFORMING A BLANK

Abstract: A blank with closed profile is made of hardenable sheet steel and is hydroformed when hot. The tool is cooled and the blank is heated to a hardening temperature and formed so fast that it will not have time to harden before it contacts the tool and becomes rapidly cooled by the tool and by the liquid fluid so that it hardens. In this simple way, a product of high strength steel can be achieved which has a closed profile and complicated form. The product can be, for example, a bow-formed bumper beam.

Description: A method of hydroforming a blank This invention relates to a method of hydroforming a blank with a closed profile in a partible tool by supplying a pressure fluid into the blank. This well known technique is limited substantially to steels with a good formability. Steel with a high yield strength has too limited elongation and can therefore only be used when the forming is light. It is, however, conventional to hydroform in several steps and to anneal between the forming steps when greater forming is desired than the steel itself permits. It is an object of the invention to receive a high strength product in a fast and simple way. According to the invention, the tool is maintained cooled, a blank that is hardenable is heated to a hardening temperature, placed in the tool and pressurised so that it is formed into contact with the tool, and kept in contact with the tool and the pressure fluid long enough to permit it to harden before it is removed from the tool, the forming being so fast that the blank will not have time to harden by the contact with the pressure fluid. The hardenable blank can be made of boron alloyed sheet steel. The pressure fluid can be a gas but it is preferably a liquid, for example water. It can also be an oil. It can be an advantage to have the liquid pre-heated since its initial cooling effect will then be reduced. In this way, the liquid can be used to control the structure of the product. The liquid can be pre-heated to a temperature above the martensite forming temperature M. and the resulting structure will be mainly bainite. The liquid can also be held at a lower temperature so that the structure will be a mixture of bainite and martensite. At still a lower temperature of the liquid, there will be a mixture of bainite and martensite and the martensite will be annealed during the cooling. The suitable temperature varies with the analysis of the steel and with the desired properties of the product. By simple testing, the temperatures of the tool and fluid that result in a product with desired properties can be found easily. A temperature of the pressure liquid below about 200° C can be expected to have only a minor effect on the structure when the thickness of the sheet steel is 1 -1,5 mm, but it will reduce the cooling during the forming as compared with a liquid at room temperature. The temperature of the pressure liquid can thus be from room temperature up to about 500' C, but a temperature of 50° - 200° C can often be advantageous if a mainly martensitic structure is desired. Since the hardening reduces the formability, the fluid flow should be so large a complete forming or almost complete forming takes place before the blank hardens. This means that the forming should usually take place in less than 2 seconds. The air in the blank can be evacuated before the blank is pressurised, but the air could also remain during the forming. If the air is to remain, one should see to it that the air is left in places where the reduced cooling will not be harmful. The two tool parts (the passive tool) can be cooled by a liquid that circulates in channels in the tools. With the described method, bumper beams with closed profile, pillars for doors and other parts for a vehicle can be made in accordance with the invention. Suitably, tube formed blanks can be used which are made of welded sheet steel of a hardenable steel quality, for example boron steel. The thickness of the sheet steel can be freely selected so that the finished product will have desired thickness of for example 1 - 4 mm. A blank can consist of several steel qualities and it may have different sheet thickness in different parts. In this way, a product with a closed profile and complicated form may be formed in high strength steel and it will have narrow tolerances in form and size.

Claims: Claims 1 A method of hydroforming a blank with a closed profile in a partible tool by supplying a pressure fluid into to blank, characterised in that the tool is maintained cooled, a blank that is hardenable is heated to a hardening temperature, placed in the tool and pressurised so that it is formed into contact with the tool, and kept in contact with the tool and the pressure fluid long enough to permit it to harden before it is removed from the tool, the forming being so fast that the blank will not have time to harden by the contact with the pressure fluid. 2 A method according to claim 1, characterised in that the pressure fluid is a liquid. 3 A method according to claim 2, characterised in that the flow of liquid is large enough to permit for the forming to be carried out in less than 2 seconds. 4 A method according to any one of the preceding claims, characterised in that the liquid is preheated to a temperature that results in the desired hardening structure.

Inventors: LUNDSTROEM ERLAND, LUNDSTROEM, ERLAND
Applicants: LUNDSTROEM ERLAND, SSAB HARDTECH AB, LUNDSTROEM, ERLAND
Application year: 1998
Main classification: C21D1
IPC codes: C21D1