Aluminum alloy sheet for cold press forming, method of manufacturing the same, and cold press forming method for aluminum alloy sheet

A cold press forming method for an aluminum alloy sheet, comprising:

• providing an Al—Mg—Si based aluminum alloy sheet put into an under-aged state by artificial aging at or below 140° C., or an aging treatment conducted by combining normal-temperature aging with artificial aging at or below 140° C., after a solution treatment and having a 0.2% proof stress of not less than 90 MPa;
• subjecting said aluminum alloy sheet blank to a partial reversion heating treatment;
• and
• cold press forming said aluminum alloy sheet blank with a punch while holding down an end part of said aluminum alloy sheet blank, wherein
• the partial reversion heating treatment comprises the steps of:
• rapidly heating a heated part of said aluminum alloy sheet blank at a temperature rise rate of not less than 50° C/min to a temperature in the range of 180 to 350° C. so as to momentarily dissolve age-precipitates and soften said heated part, while a non-heated part of said aluminum alloy sheet blank is not heated, whereby the strength of said heated part is lowered as compared with the strength of said non-heated part;
• holding said sheet blank at a temperature in said range for a time of not more than 5 minutes including (a time of 0 seconds), and then
• rapidly cooling said heated part to room temperature at a cooling rate of not less than 50° C/min to a temperature of 100° C. or below, where the heated part is the whole part or a smaller-than-whole part of a portion on the outer side of a region of said aluminum alloy sheet blank to be contacted by a punch shoulder part at the time of press forming, and the non-heated part is the other part than said heated part of said aluminum alloy sheet blank, whereby the difference between the tensile strength of said heated part and the 0.2% proof stress of said non-heated part is increased by not less than 20 MPa through said partial reversion heating treatment;
• holding said heated part of said rolled sheet between a die and a wrinkle holding down application;
• and
• cold press forming said rolled sheet by pushing said non-heated part of said rolled sheet with a punch, so that a limit drawing ratio (LDR) of the cold press forming is higher by at least 0.1 than the LDR in a method without the partial reversion heating treatment before the strength of said heated part is returned to the level before said partial reversion heating treatment due to age precipitation during holding at room temperature, so that a limit drawing ratio (LDR) of the cold press forming is higher by at least 0.1 than the LDR in a method without the partial reversion heating treatment.