martensite

Or are they rolling it so severely that the austenite is mostly transformed into martensite?

Do you know if it affects it's anti-rust properties after you work it / make it martensite?

But when it's cold worked, it turns into martensite, which is harder and magnetic.

These opposite properties are now known to arise from different crystalline phases, or allotropes, of (carbon-alloyed) iron, in particular martensite and austenite.

I wonder if this process results in direct production of martensite, or if the parts still need heat treatment.

All the complexities of, say, martensite grain boundaries and what-not are implicit in how we use steel to reinforce concrete.

But note that slight working like bending it a bit will only turn a small portion of it into martensite, most of it will remain austenitic.

Interesting that the martensite transform naturally only happens on the surface (which as you say is actually desirable) due to the rapid cooling it requires, I didn't know that.

There are four major phases that play roles even in the commonest carbon steel (ferrite, cementite, austenite, and martensite), plus others that can form at times like graphite, which plays an important role in cast irons.

These cooling rates are only able to be achieved a little ways into the bulk of the material, so you usually end up with a hardened case made of martensite, and a softer more ductile core that is usually pearlite (layers of ferrite [pure iron] and cementite [iron carbide]) that form due to the slower cooling in the core.

To express that quantity in more familiar units, That's 758 MPa From Wikipedia article on Nickel-Titanium: "A great deal of pressure can be produced by preventing the reversion of deformed martensite to austenite—from 35,000 psi (240 MPa) to, in many cases, more than 100,000 psi (690 MPa).

Martensite may not literally be a hole in the metal, but it has many of the same properties.

Martensite (α’) phase and a mixture of austenite (γ) and δ-ferrite (δ) phases were pointed out in the microstructure.

Martensite requires extremely fast cooling rates (on the order of 100s of degrees/second), which is why most carbon steels are quenched to harden them.