The scale failure temperature (Tf) during cooling from 1173 K of low carbon steels containing Si of up to 2.1% was assessed by in-situ acoustic emission measurements and analyses including wavelet transform. In general, Tf lowers with an increase in the Si content for steels without S or P, indicating that the scales on higher Si-content steels are more resistant to thermal stress. This tendency becomes larger for higher cooling rate. Contrarily, Tf rises with an increase in the S, P or (S+P) content for 1% Si steels. This means that the scale failure is enhanced by the additives. S segregates at the scale/substrate interface and seems to enhance the partial scale separation. P is incorporated in the (FeO+SiO2) layer on the substrate and forms microspores at the interface to the FeO layer, and thus enhanced the crack initiation by providing the sites for stress concentration. Wavelet transform showed that the scale failure mode is mainly the following; local separation of the scale over a small area takes place first, and then cracking in the scale follows. For steels containing S or/and P scale cracking is the initial failure in many cases, probably because the stress concentration sites are already formed during the scale growth.