Steel Armor isn't your normal Armor, instead of just slightly making iron Armor better it makes the Armor weak but very blast resistant like real life.

Steel Helmet:
To make it basically is the same recipe as iron but with two coal blocks underneath the bottom two iron pieces. It also has a glass pane in the middle. Diagram below.

###
#X#
0 0
X = Glass Pane

# = Iron Ingot

0 = Coal Block

Steel Chestplate:
The recipe is the same as the iron chestplate but with coal blocks replacing the two middle pieces.
Diagram below.

# #
#0#
#0#
# = Iron Ingot

0 = Coal Block

Steel Leggings:

The recipe is the same as the iron leggings but with a coal block replacing the top middle iron ingot.
Diagram below.

#0#
# #
# #
# = Iron Ingot

0 = Coal Block

Steel Boots:

The recipe is the same as the iron boots but with coal blocks replacing the bottom iron ingots.
Diagram below.

# #
0 0
# = Iron Ingot

0 = Coal Block