The dynamic coefficient of thermal expansion for steel Q235 during cooling was investigated using experimental measurements and model calculations. The coefficient of thermal expansion under 1400°С was measured with a thermal dilatometer. The coefficient was derived from a segregation model of two-phase zone for of temperature range of 1400°С to zero-strength temperature (ZST). The obtained results reveal that the coefficient of thermal expansion shows two jumps in the temperature zone of the peritectic reaction and the eutectoid reaction. The anti-expansion phenomenon caused by the eutectoid reaction in the cooling process corresponds to the low-temperature crack-sensitive area. Therefore, the slab straightening process should be avoided in this temperature range during continuous casting.