Can you please suggest me some sensor part number for the same.

I would try type K thermocouple, not sitting in a housing, only providing a small thermocouple bead. Maybe '397-1573' from RS components, or something similar.
This thermocouple bead is very fast acting by showing very little heat conductivity and heat capacity. Mount this thermocouple as close as possible above the hot pieces. Then, even when the rolls are getting hot, you should be able to clearly distinguish between 'hot piece being there' and 'not being there'.
The thermocouple works more like an IR radiation detector than like a temperature sensor: If the hot piece is coming, then an enormous IR radiation hits the thermocouple, resulting in a fast temperature rise. If the hot piece is away, radiation hitting the thermocouple decreases heaviliy, even if rolls have heated up by the hot piece. If the next hot piece is traversing, again enormeous IR radiation is hitting the thermocouple, again resulting in fast temperature rise.
That radiation detection is the nature of this methode can be seen by the fact, that not absolute temperature results in a detection, but temperature change, means fast temperature change, so fast, that it can only be caused by the traversing of a hot piece.

Ok, I don't know how hot the rolls are becoming in your application, and maybe Raghunathan is right in stating that they can become very hot, but maybe you can find a point, where the rolls are not so hot, where there's a heavy temperature difference between hot piece and rolls.

You can combine K thermocouple with AD595, which was invented for this purpose. Try to use a shielded cable, which does not introduce relevant heat capacity and heat conductiviy to the thermocouple. And: Please do not arranage any electronics in such a way, that it's sitting at this high temperature place! The thermocouple, on the other hand, is allowed to be mounted here, of course.

I don't know, whether this methode will work with your certain application. It's that what I would try, if I were in your situation. Of course, many other methodes can be used, but I think this thermocouple methode would be very cost effective, if it works... It's advantage is, that it really withstands this high temperature.
The methode using this coil, proposed by Erik, also sounds interesting.

Kai