Method of forming parts from sheet metal

Abstract

A method of forming a part from sheet metal and a part formed by said method. The method comprising the steps of: (a) heating a metal sheet to a temperature T; and (b) forming the sheet into the part between dies while applying cooling means to the sheet, where in step a) the metal sheet is heated at a rate of at least 50° C..Math.s.sup.−1, and temperature T is above a critical forming temperature and does not exceed a critical microstructure change temperature of said metal sheet.

Claims

1. A method of forming a part from a metal sheet, the method comprising the steps of: heating said metal sheet to a temperature T at a rate from 50° C..Math.s.sup.−1 to 300° C..Math.s.sup.−1; said temperature T being above a critical forming temperature and does not exceed a critical microstructure change temperature of said metal sheet; providing a cooling means for cooling said metal sheet; and forming in a first forming step the metal sheet into the part between at least one of a plurality of dies while simultaneously applying said cooling means to the metal sheet in a first cooling step at a rate of at least from 10° C..Math.s.sup.−1 to 300° C..Math.s.sup.−1.

2. The method of claim 1, wherein: said cooling means conducts said first cooling step between 100 to 300° C.

3. The method of claim 1, wherein: said temperature T is from 50 to 600° C.

4. The method of claim 3, wherein: said heating to said temperature T is conducted by at least one of a contact heater, an infra-red heater, an induction heater, and a resistance-heater.

5. The method of claim 4, wherein: said step of forming in said first forming step further comprises a step of: closing said plurality of dies with a force within a critical contact pressure range.

6. The method of claim 5, wherein: said force in said critical contact pressure range is between 15 MPa to 300 MPa.

7. The method of claim 6, wherein: said step of forming further comprises a second step of post-forming where said metal sheet is held between said plurality of dies; said first forming step is conducted with said force between 20 MPa to 50 MPa; and said second step of post-forming is conducted with said force between 50 MPa to 150 MPa.

8. The method of claim 1, wherein: said first cooling step applied during said first forming step is between 10%-20% of a total cooling applied to said metal sheet; conducting a second cooling step of said metal sheet after said first forming step while said metal sheet is between said plurality of dies; and said second cooling step being an in-die quenching and being 80% to 90% of said total cooling applied to said metal sheet.

9. The method of claim 1, wherein: said heating and forming occurs in from 2 to 60 seconds.

10. The method of claim 8, wherein: said forming occurs in from 1 to 3 seconds and said second cooling step occurs in from 1 to 4 seconds.

11. The method of claim 1, wherein: the metal sheet is a material selected from a group consisting of: aluminum, magnesium, titanium, an alloy of aluminum, an alloy of magnesium, and an alloy of titanium.

12. The method of claim 1, wherein: said metal sheet is an alloy of iron; said alloy of iron is steel; said steel is an ultra-high strength steel (UHSS); and said ultra-high strength steel (UHSS) is a martensitic steel.

13. A formed part product formed according to the method of claim 1.

14. A method of forming a part from a metal sheet, the method comprising the steps of: heating said metal sheet to a temperature T at a rate from 50° C..Math.s.sup.−1 to 300° C..Math.s.sup.−1; said temperature T being above a critical forming temperature and does not exceed a temperature which would cause changes to the microstructure of said metal sheet; and forming in a forming step the metal sheet into the part between at least one of a plurality of dies.

15. A method of forming a part from a metal sheet, the method comprising the steps of: Heating said metal sheet to a temperature T at a rate from 50° C..Math.s.sup.−1 to 300° C..Math.s.sup.−1; said temperature T being above a critical forming temperature and does not exceed a critical microstructure change temperature of said metal sheet; providing a cooling means for cooling said metal sheet; and forming in a first forming step the metal sheet into the part between at least one of a plurality of dies while simultaneously applying said cooling means to the metal sheet in a first cooling step at a rate of at least from 10° C..Math.s.sup.−1 to 300° C..Math.s.sup.−1; said temperature T is from 50 to 600° C.; said heating to said temperature T is conducted by at least one of a contact heater, an infra-red heater, an induction heater, and a resistance-heater; said step of forming in said first forming step further comprises a step of: closing said plurality of dies with a force within a critical contact pressure range; said force in said critical contact pressure range is between 15 MPa to 300 MPa; said step of forming further comprises a second step of post-forming where said metal sheet is held between said plurality of dies; said first forming step is conducted with said force between 20 MPa to 50 MPa; and said second step of post-forming is conducted with said force between 50 MPa to 150 MPa.

16. A method of forming a part from a metal sheet, the method comprising the steps of: heating said metal sheet to a temperature T at a rate from 50° C..Math.s.sup.−1 to 300° C..Math.s.sup.−1; said temperature T being above a critical forming temperature and does not exceed a critical microstructure change temperature of said metal sheet; providing a cooling means for cooling said metal sheet; forming in a first forming step the metal sheet into the part between at least one of a plurality of dies while simultaneously applying said cooling means to the metal sheet in a first cooling step at a rate of at least from 10° C..Math.s.sup.−1 to 300° C..Math.s.sup.−1; said first cooling step applied during said first forming step is between 10%-20% of a total cooling applied to said metal sheet; conducting a second cooling step of said metal sheet after said first forming step while said metal sheet is between said plurality of dies; and said second cooling step being an in-die quenching and being 80% to 90% of said total cooling applied to said metal sheet.

17. The method of claim 16 wherein: said forming occurs in from 1 to 3 seconds and said second cooling step occurs in from 1 to 4 seconds.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 provides a schematic of an existing hot stamping method.

(2) FIG. 2 provides a schematic of an existing warm stamping method, with comparisons drawn to existing hot stamping method.

(3) FIG. 3 provides a schematic of a fast warm-stamping method in accordance with the invention, with comparisons drawn to existing hot and warm stamping methods.

(4) FIG. 4 provides a temperature profile for a method in accordance with the invention.

(5) FIG. 5 provides a residual hardness profile as a function of temperature T for a formed martensitic steel part made by a method in accordance with the invention.

(6) FIG. 6 provides a residual hardness profile as a function of heating rate for a formed martensitic steel part made by a method in accordance with the invention.

(7) FIG. 7 provides a residual hardness and elongation profiles as a function of heating rate for a formed martensitic steel part made by a method in accordance with the invention.

(8) FIG. 8 provides a springback profile as a function of temperature T for a U-shaped part formed by a method in accordance with one aspect of the invention.

(9) FIG. 9 provides a residual hardness profile as a function of temperature T for a U-shaped formed martensitic steel part formed by a method in accordance with the invention.

(10) FIG. 10 provides the effects of temperature T on stress-strain relationship for a martensitic steel part formed by a method in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) Reference will now be made in detail to embodiments of the invention. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. For purposes of convenience and clarity only, directional (up/down, etc.) or motional (forward/back, etc.) terms may be used with respect to the drawings. These and similar directional terms should not be construed to limit the scope in any manner. It will also be understood that other embodiments may be utilized without departing from the scope of the present invention, and that the detailed description is not to be taken in a limiting sense, and that elements may be differently positioned, or otherwise noted as in the appended claims without requirements of the written description being required thereto.

(12) Various operations may be described as multiple discrete operations in turn, in a manner that may be helpful in understanding embodiments of the present invention; however, the order of description should not be construed to imply that these operations are order dependent.

(13) As described above, existing hot-stamping and warm-stamping methods are shown schematically in FIGS. 1 and 2 respectively. A very important aspect of existing methods for forming high strength steel is that, before a part is formed, the steel sheet to be formed is heat treated at temperatures sufficiently high, e.g. more than 900° C., for a prolonged period of time (known as soaking time) to enable austenitisation to occur, thereby prompting a phase change to a softer phase of the material (austenite). This aspect of the existing methods is energy intensive, and is known to take approximately 75% of the overall processing time to form the finished formed part.

(14) Another very important aspect of existing methods is that, as the hot stamped part is held in cold dies, the cooling rate should be sufficiently high, e.g. more than 25° C..Math.s.sup.−1 on average, to enable the hardest phase of the material (for example martensite in cases where steel sheets are used) to be formed. In this way, high strength components can be made. Although a critical aspect of existing methods, cooling the hot stamped part until the hardest phase has formed is time consuming.

(15) The method in accordance with this invention provides a faster stamping method by rapidly heating the metal sheet to be formed to a temperature which is below that which would cause changes to the microstructure of said metal sheet. Such rapid heating has been found to have a surprisingly positive effect on ductility and the post-form strength of the finished formed part (as discussed in more detail below), whilst avoiding any changes to the material's microstructure has been found to reduce energy consumption and overall process time due to avoiding the need for any energy intensive and time consuming heating and cooling steps.

(16) The method in accordance with this invention may be applied to sheets of different metals as defined above. An example of the method in accordance with this invention will now be given where the metal sheet is high strength steel.

(17) The new method involves the following steps:

(18) First, a high strength steel sheet (which also may be referred to as a “blank”) is selected and prepared. The preparation of the blank may involve cutting the blank to size when in a cold state and may be followed by ensuring that the initial phase of the high strength steel corresponds to the phase desired after forming. If the initial phase of the high strength steel (before forming) does not correspond to the phase desired in the formed part, then pre-forming treatments can be applied (e.g. heat treatments) before the fast warm stamping method is used.

(19) Secondly, the blank is heated to a temperature T, e.g. between 350-450° C., which is above the critical forming temperature and below the austenitisation temperature of the high strength steel. The heat is applied using a contact heater, which comprises two hot platens which press against the blank from opposing sides, which applies heat at a rate of between 50 to 150° C..Math.s.sup.−1. The exact heating rate and critical forming temperature will vary depending on geometric configuration of the formed part and the material of the sheet being formed.

(20) The heating rate in fast warm stamping may be determined by using a thermo-mechanical simulator such as a Gleeble® 3800 to inspect a metal sheet to be formed to find the minimum required heating rate that maintains the material's microstructure when heated to temperature T and provides a required post-form strength. The cooling rate applied by the cooling means is determined by using a thermo-mechanical simulator such as a Gleeble® 3800 to find the minimum required cooling rate that maintains the material's microstructure. The heating rate is determined when the microstructure of the test-piece does not change remarkably. The critical forming temperature may be determined experimentally using the method discussed above, where ductility is considered as a function of temperature to determine minimum ductility required to form the part.

(21) Thirdly, the warm blank is transferred from the contact heater to a cold die set comprising cold forming tools within a pre-determined period of time to ensure that the temperature of the blank does not fall below the critical forming temperature of the high strength steel. This third step is optional, and may not be required if, for example, the blank is heated in the die set.

(22) Fourthly, once the blank is transferred to the die set comprising cold forming tools (which also may be referred to as a “press”) the blank is formed and cooled. The forming process shapes the blank to the desired shape by holding the blank between dies whilst cooling is applied simultaneously to provide initial first stage cooling to the blank. The forming process uses the dies to apply a fast forming pressure of up to approximately 30 MPa and for approximately 1 or 2 seconds. The initial first stage cooling cools the blank to approximately 10 to 20% towards a final target temperature of approximately 100 to 300° C. After forming, the pressure applied to the dies (and therefore the pressure applied to the formed part) may be changed to more than 30 MPa but below 140 MPa, and cooling is maintained to cool the blank to the final target temperature of between 100 to 300° C. (as mentioned above, cooling may not be required if no changes are made to the microstructure of the metal sheet during heating). The entire forming and cooling (quenching) time during this fourth step is approximately 1 to 4 seconds. As mentioned above, the provision of further cooling once the formed part is removed from the die set is optional.

(23) After the stamping and quenching process is complete, the formed part may be removed from the press for immediate use or for further processing. If the formed part is made from aluminum or alloys thereof, the formed part may be removed from the press before the part is cooled to room temperature and moved to an incubation chamber for further processing where residual heat left in the formed part is used to shorten the artificial aging process.

(24) FIG. 4 shows a temperature profile of a blank undergoing the fast-warm stamping process described above. Referring to FIG. 4, B represents the fast heating step, C represents the transfer period, D represents the stamping and quenching period, and E represents an incubation period. Ac3 shown on FIG. 4 represents the austenitisation temperature of the high strength steel. The transfer period D of the temperature profile shown in FIG. 4 does not show any reduction in temperature, however, in some cases there may be a temperature drop from where the blank is removed from the heater to when the forming begins.

(25) It has been found that by using a fast-warm stamping method in accordance with the present invention, the total time taken from heating a blank to removing a formed part from a press (known as a “cycle time”) is less than 10 seconds (as shown on FIG. 3), compared to existing hot-stamping and warm-stamping processes which have cycle times in excess of 10 minutes, as shown in FIGS. 2 and 3 where total cycle time is 840 seconds.

(26) FIGS. 5 to 8 show data obtained from formed parts produced by the method in accordance with the present invention using a high strength steel sheet.

(27) FIG. 5 shows uniaxial tensile test results where residual hardness (in terms of percentage of hardness of the high strength steel prior to forming) is shown as a function of temperature T with a heating rate of 50° C..Math.s.sup.−1 and with no soaking time. Forced air cooling was applied during the forming to cool the formed part to below 200° C.

(28) FIG. 6 shows residual hardness (in terms of percentage of hardness of the high strength steel prior to forming) varying as a function of heating rate, with temperature T as 450° C. and with no soaking time or cooling of the formed part.

(29) FIG. 7 shows residual hardness (in terms of percentage of hardness of the high strength steel prior to forming) and percentage elongation (ductility), both varying as a function of heating rate, with temperature T as 450° C. and with no soaking time. Forced air cooling was applied to cool the high strength steel to below 200° C. FIG. 7 shows improved post-form ductility and strength with increasing heating rate.

(30) FIG. 8 shows springback exhibited by a beam formed in a U-shape at a function of temperature T.

(31) FIG. 9 shows post-form strength in a U-shape component formed by using fast warm stamping in accordance with the present invention, conducted at different temperatures T.

(32) FIG. 10 shows uniaxial tensile test results of a UHSS conducted at fast warm stamping conditions, in accordance with the present invention.

(33) Having described at least one of the preferred embodiments of the present invention with reference to the accompanying drawings, it will be apparent to those skills that the invention is not limited to those precise embodiments, and that various modifications and variations can be made in the presently disclosed system without departing from the scope or spirit of the invention. Thus, it is intended that the present disclosure cover modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.