I am not able to download your picture therefore I cannot see well what it is. But I suggest to do some chemical EDS first to see what elements are present in it and then try to understand what this could be taking into account the process. I hope it helps. Greetings

EDS analysis indicates that Oxygen,Aluminium, Silicon and Fe(96%) are present. So could these be oxides formed on ferrite laths?

What was the welding method?

What is the basic steel composition and what was the welding rod composition. These are basics which we need to know.

the base metal used was St37 and welding electrode was a self-shielded flux cored wire. Also, we used SAW flux for the external protection to compare with the results when there is no such protection. 

We are unable to open your slide. I feel it is slag entrapment and will be be revealed by EDAX Analysis 

This is slag from the welding process. Welders defect in not cleaning properly after first pass.

The specimen is etched? Considering the high magnification used, I think those rods can be oxide formed during  etching procedure.

In dark upper part of image spheroidal oxides are present (most probably slag), in lower part bright edges of fine dimples give "white" effect (slight plastic deformation), especially that you applied 25keV. I recommend to use backscattered electrons contrast.

Are we looking at a fracture surface? If yes I would have thought that these are shear lips from plastic fracture.

These are simply edge effect in electron microscopy. Very thin regions appear much brighter especially a high kV electron beam is used. This has nothing to do with steel composition or the welding method used.

If it is fracture surface, it might be related to shear lips. Or, it's better to check the surface contamination, e.g., oxidation, gold/carbon sputtering to reduce charging effect.

I would also say that these lines are simply edge effects in electron microscopy.