is employed and the metal strip is passed successively in alternating directions through the same roll stand for progressive reduction with the working rolls being brought closer together on subsequent passes because the sensor apparatus 9 utilizes the magnetic properties of the polycrystalline metal to detect microstructure the sensor must be placed in a hot strip mill at a point on the line where the temperature of the steel has dropped below the curie temperature the temperature below which the magnetic properties of the metal return in the metallurgical literature the curie temperature corresponds to a 2 the formerly designated critical temperature at 1414 u00b0 f this temperature does not involve a phase change in the steel microstructure as do a 1 and a 3 critical temperatures but instead marks a gradual heat effect and magnetic change 1414 u00b0 f 1454 u00b0 f ferrite being ferromagnetic below this temperature range and paramagnetic above in a cold rolling mill where the temperature from working the material never exceeds the curie temperature the sensor can be placed at any point on the line also in continuous annealing or coating operations the sensor can be placed at any point on the line other than in the annealing furnace itself or other obviously hostile environments to monitor the polycrystalline microstructure of the steel alloy the sensor portion 9 of the apparatus 11 rides.