Perlite with very fine lamellas; in contrast to troostite, it is completely transformed, i.e. no austenite is left which could transform to martensite during further cooling down.

Sorbite is formed at medium cooling rates of approx. 100 to 200 K/s; with higher cooling rates troostite would be formed.

Additional references:
Iron-carbon phase diagram
Pearlite peak
TTT diagram