the TTT diagram
A phase diagram has temperature and composition on its axes; a TTT diagram swaps composition for time. For one fixed steel composition, it is a map of what austenite turns into if you rapidly drop it to some temperature and hold there, with the horizontal axis being how long you hold (on a log scale) and the vertical axis being the hold temperature.
On the plot you see C-shaped curves: a start line (transformation begins) and a finish line (transformation complete). The curves bulge out to a nose at an intermediate temperature (around 540 degrees C for a plain eutectoid steel) where transformation is fastest, the nucleation-versus-growth compromise. Hold high, just below 727 degrees C, and austenite becomes coarse pearlite (slowly); hold lower, near the nose, and you get fine pearlite; hold in the 250 to 540 degrees C range and you get bainite. Below the martensite-start line (Ms) austenite converts to martensite almost instantly, drawn as a flat horizontal line, not a C.
TTT diagrams are built by quenching thin samples almost instantly to temperature and holding, so strictly they describe isothermal (constant-temperature) holds only. Real parts cool continuously, so for continuous cooling you must use the related CCT diagram, whose curves sit down and to the right of the TTT ones. Each TTT diagram is valid for exactly one steel composition.
Quench a 0.8 percent C steel to 600 degrees C and hold: the TTT diagram says pearlite starts at about 1 second and finishes within a few seconds, giving fine pearlite.
Add time to the phase diagram and you can read off which microstructure forms.
A TTT diagram applies only to isothermal holds and only to the one composition it was measured for; using it to predict a continuous furnace-cool will mislead, which is what CCT diagrams are for.