Magic Technology

Plate armor is a staple of the typical fantasy world.

That's not quite right.

STEEL plate armor is a staple of the typical fantasy world.

There's a disconnect though.

Most fantasy settings are based on societies that haven't advanced far enough to have developed the blast furnace, and until you've got a blast furnace, you really can't make steel in sufficient quantities to make large plates.

Before that you're either pounding and folding bloomery iron or, if you're lucky, reshaping a crucible pig.

It's just not economical to make a steel plate from even crucible steel because the quantities are so small. 

Now that I'm thinking on it...  Several of the polearms we take for granted don't really show up until plate is common.  Their predecessors will be iron.

In GURPS the blast furnace is one of the items that marks that you've made it to TL4.

TL3 is the norm for fantasy settings.

But magic...

Is there a combination of spells that can replace the blast furnace?

The key for making steel with magic is knowing what steel is. 

The Heat spell is sufficient to get to the temperatures that the crucible requires, but it's not really economical either because of the fatigue costs for the mage.

You could get your iron and charcoal mixture heated to the needed 2,100° in just 102 minutes, and spend 204 fatigue to get there at the normal casting rate of 20° per minute.  You can heat it at 60° a minute for triple the cost and get to heat in 34 minutes for the same 204 fatigue.

That's much faster than building the crucible furnace, let alone running it over night.

That's also the endurance of 20 average people working themselves to collapse; and one very tired person.

There's way to store up this energy for use all at once, but...  The simple way is to have 20 mages working in a chain.  That makes steel at TL3 with magic a lot more expensive than bronze which is readily made with conventional means from expensive materials.

Create Earth then Earth to Stone at double cost can be used to create metals from thin nothing, but just simple metals like iron or bronze.  This might have a significant effect on the price of bronze because you won't need to find, process and ship tin around, let alone the much cheaper copper.

I am not sure if spells like Shape Metal can get you the composition and crystalline structure needed for weapons and armor.  Shape metal will certainly revolutionize the making of high-end mail.  In effect you can have welded rings without actually welding them.  It's up to the GM if each ring needs a separate casting (that'd be cost prohibitive).  But I could see a mage starting with a block of iron and, in effect, forming the wire and rings as he goes around a pattern and using the spell to cut off each, now continuous and homogeneous, ring as they go.

Riveted mail is a high-labor item already and a mage is as much a master tradesman, so the pricing might end up being similar.