Captain America has no way to reduce Iron Man's HP but can potentially slam or stun him
Using karma for a power stunt or taking advantage of the fact that Captain America has talents in all forms of Martial Arts, he is capable of slamming or stunning Iron Man, which offers some limited ways to "hurt" him. However, let's look at Cap's options for actually dealing damage.
Captain America has three main combat options: a blunt attack with his fists, a blunt attack with a weapon, or throwing his shield. (This doesn't go into talents, which would make it easier to hit with his attacks but wouldn't affect the damage they deal)
Blunt attack with fists
When fighting with bare fists, a character can inflict
points of damage up to his Strength rank number on an
opponent. That many Health points are subtracted from
the opponent's total. If anyone reaches 0 Health, he is
knocked out (see Getting Hurt).
Captain America's Strength is Remarkable (30). With Iron Man's armor providing Amazing protection from physical damage, he can't be hurt by any amount of damage that doesn't exceed 50, so Cap's fist attack won't affect him.
Blunt attack with weapon
A character using a blunt weapon does more damage;
he gains a + 1 CS, but only for figuring damage. For
example, Spider-Man, with Incredible Strength, inflicts 40
points of damage when he hits someone. If he hits someone with an engine block, he gets a +1 CS on damage,
so he inflicts Amazing damage (50 points) instead. The
material strength of the blunt weapon must be equal to or
greater than the attacking hero's Strength.
Let's assume Cap picks up a blunt weapon which is materially greater than Remarkable, and attacks with that. He can column shift into Incredible, which deals up to 40 damage. Iron Man's Amazing armor will still absorb the whole lot.
Throwing his shield
Cap's shield is a blunt throwing object (this is specifically listed as an example in the rules):
The blunt throwing attack inflicts damage equal to either the
Strength of the thrower or the material strength of the
item being thrown, whichever is less. The effect of a
thrown blunt weapon can always be reduced to a lesser
color or can inflict fewer points of Health damage.
Cap's Strength is Remarkable, as we've already covered, so this attack falls prey to the same pitfall as the other two. Of the shield itself:
It can do up to Incredible (40) blunt damage when wielded by Captain America based on his current strength, and Remarkable (30) edged damage depending on how he chooses to strike with it.
Unfortunately, despite the shield being materially sufficient to get a higher column on damage, Cap's Strength limits it, so you can't even damage him with the shield on the basis of it being Vibranium.