Method for Delaying of Cooling and Hardening of Desired Zones of a Sheet During a Hot Metal Stamping Process
It is a die type which reduces the heat transfer from hot sheet to dies, cutting/piercing tools, and blank holder in necessary zones in the way process conditions require and can increase it in die hardening by heat treatment effect, contrary to the approach used for stamping of hot workpieces between cold dies. This die type has the capability of forming the sheet and making forming process by heat transfer in necessary zones simultaneously according to the properties of workpiece.
1 ) A method of blanking sheet before a hot stamping process characterized in that,
direct resistive heating of a metallic blank by applying electrical current to said blank,
blanking said work piece at an elevated temperature by cutting certain peripheral zones between direct resistive heating and hot forming steps,
obtaining an appropriate blank form facilitating forming,
forming said work piece at an elevated temperature.
2 ) A method according to claim 1 , further comprising,
placing upper ( 8 ) and lower ( 9 ) cutting tools outside of upper ( 11 ) and lower ( 12 ) dies respectively
moving down one press table towards other one,
shearing at least one zone at the periphery of the blank sheet ( 13 ) between said upper ( 8 ) and lower ( 9 ) cutting tools during movement of said press table,
forming said trimmed sheet at an elevated temperature.
3 ) A method of improving die quench quality in sheet forming and die quenching process of a heat treatable metallic blank performed at an elevated temperature characterized in that,
using die(s) ( 16 , 24 ) including cooling channels ( 25 ),
heating said metallic sheet ( 27 ) up to a temperature level required for heat treatment,
forming and/or fixing said sheet ( 27 ) between said dies with cooling channels at an elevated temperature,
holding formed sheet under compression of the dies ( 16 , 24 ),
sweeping of sheet surface by the cooling agent which passes through these channels ( 25 ) one side of which is sheet surface
increasing the cooling rate of sheet in the die,
quenching formed part and completing the heat treatment.
4 ) A method of providing a predetermined strain rate in a hot forming process of metallic sheet material characterized in that,
grooving channels ( 4 , 5 ) on the contact surface of blank holder members ( 1 ) to sheet ( 3 ),
applying a certain contact pressure rate to the sheet by said blank holders,
keeping said contact pressure below a certain rate so that most of the sheet surface ( 39 is avoided from contacting with inner surfaces of said grooves of contact member(s),
holding said sheet between lower edges ( 4 ) of upper blank holder member(s), and upper edges ( 5 ) of lower blank holder member(s),
reducing heat loss at the sheet ( 3 ) periphery by reducing contact interface,
reducing strength increase resulting from temperature decrease at the periphery of forming zone,
providing a predetermined strain during material flow towards the inner forming zone.
5 ) A method according to claim 4 , further comprising;
employing separate contact inserts ( 1 ) contacting with hot blank,
manufacturing said contact inserts ( 1 ) which made of a material with lower thermal conductivity than that of material of blank holder body ( 2 ).
6 ) A method of piercing a hot formed sheet after forming characterized in that,
employing moveable ledge parts ( 33 ) around the piercing zones which placed inside at least one die ( 31 ) and can go in and out of die ( 31 ),
reducing contact interface between hot sheet and cold tools at the piercing zone by means of pushing hot sheet out of the die surface,
forming of the hot sheet by compressing between dies,
preventing piercing zone from early contacting to die surface and from hardening by cooling during the hot stamping;
piercing by entering of the piercing edges ( 34 ) into piercing holes ( 36 ).
7 ) A method of piercing of a hot formed sheet characterized in that,
employing notch ( 41 ) and ledge ( 42 ) structures around piercing edges ( 40 ) which will be placed in the die ( 44 ) and female piercing holes ( 45 ) over the opposite die ( 46 );
piercing by entering of the piercing edges ( 40 ) into piercing holes ( 45 ),
taking back of some parts of sheet from piercing hole surrounding by the notches ( 41 ) and ledge ( 42 ) structures around the piercing zone through standing on each other with compressed sheet between them,
preventing the cooling and shriveling sheet enclosing piercing edges ( 40 ) from squeezing the piercing tool ( 40 ) by sending cut parts of the sheet away from surrounding of the piercing edges ( 40 )
ensuring the piercing tool ( 40 ) to exit easily
8 ) A method of trimming of a hot formed sheet part made of a heat treatable metallic material before completing heat treatment at the shearing zone characterized in that,
employing separate and replaceable cutting tools ( 29 ) placed outside of forming dies ( 16 , 24 ),
employing a movement mechanism for shearing action of said cutting tools ( 29 ) acted by pressured fluid piston(s),
forming a metallic blank sheet ( 27 ) at an elevated temperature,
keeping formed sheet between the dies ( 16 , 24 ) to obtain dimensional stability
acting said cutting tool(s) ( 29 ) by said pressured fluid piston(s),
trimming excess portions of the formed part.
9 ) A method as claimed in claim 8 further comprises of,
employing notches ( 26 ) at the cutting tools at the trimming and/or piercing zones of the sheet,
avoiding early cooling at the trimming and/or piercing zone ( 19 ) resulted from conductive heat transfer between hot sheet and cold tools at said zones,
avoiding strength increase before shearing operation at the trimming and/or piercing zones.