The color of heated iron changes predictably (due to black-body radiation) from dull red through orange and yellow to white, and can be a useful indicator of its temperature.
Good quality iron or steel at and above this temperature becomes increasingly malleable and plastic.
[1] When the steel is heated up and worked, the mechanical energy added to the workpiece increases the temperature further.
Manganese sulfide inclusions have a higher melting point and do not concentrate at the grain boundaries.
[3] This is caused by selective oxidation of iron at the grain boundaries, where more noble and softer copper is enriched.