austenite

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

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

If you anneal it by heating it and letting it cool, it turns back into austenite.

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

Here's a list of some of the various grains in steel: Pearlite, Cementite, Bainite, Austenite (there are many more).

Martensite forms by rapidly cooling austenite without giving the atoms the ability to re-organize into their preferred structure (ferrite, pearlite, or cementite depending on the carbon concentration).

As I understand it, nickel is there to stabilize austenite, which has some potentially nice mechanical properties but is non-magnetic and not great at conducting heat.

There are many important crystalline phase transitions in metallurgy that are more similar to dunting; the most important of many examples is probably the ferrite-to-austenite transition in steels at 700–1400°.

Austenite is one of the high temperature crystal structures of iron, and is not usually seen at room temperature except in certain non-heat-treatable stainless steels, and in highly alloyed steels some small amounts of retained austenite remains that doesn't get the chance to transform due to low cooling rates and suppressed M_f temperatures.

In this study, potential nucleation sites for B2 during annealing of wrought sheet steel include (1) grain boundaries or edges of recrystallized austenite crystals and (2) deformation shear bands, which are common in hot- or cold-worked low-density steel.

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.

To expand the stability domain of B2 above the recrystallization temperature (normally, 800–900 °C) of deformed austenite, the alloying recipe of an austenitic low-density steel was modified by adding 5 weight per cent nickel (Ni), which is one of the most effective elements for forming B2 with aluminium.

When you go from high carbon steel to very high carbon steel, you get additionally "left over austenite".

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, Ferrite Austenite, Cementite, etc.