Abstract: Thermal mechanical simulator was used to test the expansion curves of Cr5 series back-up rolls, and the continuous cooling transformation (CCT) curves of supercooled austenite were sketched for the Cr5L and Cr5U samples with lower and upper limit compositions, respectively. Furthermore, the characteristics of microstructural transformation and hardness evolutions were also investigated. The results show that compared with Cr5L sample, Cr5U sample has the lower initial temperature (M_s) and end temperature(M_f) of martensite formation, which are 279 and 153 ℃ respectively. The upper limit composition can improve hardenability of steel, and the critical cooling rate of martensite (M) transformation for Cr5L sample decreases from 1.00 ℃/s to 0.50 ℃/s. The supercooled austenite (A’) of Cr5L and Cr5U samples undergoes pearlite (P), bainite (B) and M transformations during continuous cooling, and the “C” curves of P and B transformation are separated, with the characteristics of double “C” curves. The B transformation in Cr5L and Cr5U samples forms B₁ and B₂ respectively, and the M structure of Cr5U sample contains residual austenite (A_r) in the soft and ductile phase with a mass fraction of about 6%, so the Vickers hardness value of Cr5U sample cooled at a critical cooling rate greater than B is less than that of Cr5L sample.