HI Alfred,
the leather is just the vehicle to rivet the plates to. You are correct that it doesn't need to be hardened. This is an easy type of armor to make. Make the plates out of 18 or 16 gage cold rolled steel (I have no idea if the gages are different in Hong Kong). Then layout your cardboard/posterboard pattern and mark it onto the steel with a black marker (I use a sharpie). Now you have to cut it out. The best way is with a beverly shear (a throatless hand operated set of bench mounted scissors for steel), alternatly you could use a power jigsaw with metal cutting blades, or if your hands are tough, a pair of hand shears (this will generally not do 16 but will work for 18). Using a rubber or rahide mallet bend the few plates that need to be curved over a piece of pipe that is held in a vise, tied down between 2 saw horses, etc. Mark and drill your rivet holes in the plates, or if your a lucky guy and have a steel punch, do that instead, or you could go old school and heat them in a forge and use a steel punch on your anvil to create the holes. Now it is time to cut and sew out your leather cover. Make yourself a paper pattern and adjust it to fit your body, when you are happy with this, then and only then cut your leather. If you want to save money, use a heavy canvas instead. For really inexpensive canvas, buy yourself a heavy painters drop cloth. Now lay your plates out on the inside of your canvas or leather cover and mark all the rivet points with a marker. Then using a hand punch for leather punch all the rivet points, if your using canvas, just use a sharp awl to push the holes through the canvas (this causes the least thread breakage for strength). Then rivet the plates to your cover (I like to use a washer on the outside of the canvas/leather cover to help support and spread out the pull of the rivet against the cover. Attach your straps and buckles.
Congradulations you now have your first piece of armor, now all you need is legs, arms, a helmet and gaunlets and you are set for armoured combat.
If you are just wanting something to protect you for padded/wooden weapon sparring, I would go with making a heavy gambeson/padded-jack instead. Cloth armour is a different ball of wax, than a coat of plates, and has it's own inherent challenges. At
arador.com they have a forum discussion group dedicated to cloth armour, I would do some serious reading there if you decide to go with cloth armour instead.
hope this helps.
Brian Hunt
GFS.