As far as I know, a steam press is used for ironing fabrics. A steam box can be used to heat wood to make it more pliable. Then the lumber is bent into shapes. Normally you want riven lumber, split not sawn from the log, and it should have even straight grain without knots or other imperfections. If there is significant difference between the thickness and width, like a 1x6, the board normally cups across the grain when steamed. You may end up with a half pipe propeller. When a board comes out of the steam box, you have a few seconds to get it clamped up into a pre-made form. Often a backing strap of thin steel is used to keep the ends compressed on tight bends. To flatten a board, you would have to make a very strong flat platform and have a means of applying even and heavy pressure to force the board flat in the form. Problem being, the boards may crack and break and go back they way they were. It's not a method that is used for flattening boards.
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