Lightweight steel door for vehicle and method for manufacturing the same

A method for manufacturing a vehicle door assembly comprising an inner panel in the form of a substantially rectangular perimeter frame portion having a continuous perimeter integrally formed by upper and lower horizontal beams interconnected by front and rear upright beams, said method comprising the steps of:

• providing front and rear flat U-shaped steel sheets of a first steel material, each of said left and right U-shaped steel sheets comprising a cross portion and a pair of spaced apart leg portions extending from said cross portion;
• providing top and bottom flat steel sheets of a second steel material;
• non-detachably connecting each of said leg portions of each of said U-shaped steel sheets to a respective one of said top and bottom steel sheets so as to form a flat door perimeter frame blank;
• and
• stamping said door perimeter frame blank into said perimeter frame portion of said inner panel;
• said first steel material of said front and rear flat U-shaped steel sheets being different from said second steel material of said top and bottom flat steel sheets;
• said frame portion comprising opposite front and rear U-shaped side frame members non-detachably attached to each other by upper and lower intermediate frame members so as to form said continuous perimeter;
• said side frame members made of said first steel material and said intermediate frame members made of said second steel material.