When you read the nc file, the nc reader should automatically convert the data into kelvin already so you don't need to convert again. Please try and let us know if you have further questions.
I didn't "convert". I just read the variable sea_surface_temperature, and the attributes (scale_factor and add_offset) of it, and then use the formula
(SST = sea_surface_temperatur * scale_factor + add_offset) to calculate SST.
All available elements of the matrix sea_surface_temperature are between 200K to 300K. The scale_factor is 0.005. The add_offset is 273.15K.
So the result SST can only be about 274K. And I use MATLAB to read it.
I think the scale_factor should be about 0.5, then the result can be reasonable. Do I miss some steps before getting SST?