Katie Addison-Snart, 36, was on her way to drop her son off at nursery when she was struck in the forehead by the razor-sharp steel square.
She lost control of hr vehicle and ploughed into the central reservation before spinning back across the carriageway and coming to a halt on the hard shoulder.
Her four-year-old son, Hugo, manage to escape unscathed after the metal narrowly missed him in the passenger seat.
Other motorists were able to dial 999 following the freak accident, which happened at 8.30am on July 11.
Paramedics rushed to the A1 near Grantham, and Katie was taken to the Queen’s Medical Centre in Nottingham with multiple head injuries.
She underwent a five-hour operation where surgeons put metal plates in her skull and she needed 40 stitches in her head.
The grammar school teacher, from Great Ponton, suffered a fractured skull, smashed eye socket and cheek bone.
Katie’s husband Robert, 29, a council officer, said: ‘Despite having a serious head injury and being in shock, she was awake and talking which calmed me down enough to drive to hospital.
‘A woman travelling alongside said that she saw something metal falling from a lorry travelling one car ahead of Katie.
‘It hit the grill of the car in front before flicking up and crashing through Katie’s windscreen.
‘The metal plate hit her between the eyes and she momentarily lost her vision and hit the central reservation.’
He added: ‘The lorry didn’t stop and at this stage we’d like to think that it was because the driver was unaware about what had just happened.
‘It is still early days. Katie is still in the major trauma unit at hospital.
‘She can’t open her right eye at the moment but doctors think that it is because she is still swollen from surgery.
Doctors told Robert that it was touch for a while and he added: ‘It is a miracle that she is still here.’
Lincolnshire Police are appealing for witnesses and the lorry driver to come forward.