Data fabrication at Kobe Steel continued at its aluminium and steel unit after an internal investigation discovered the misconduct, the Nikkei newspaper reported on Friday.
Japan's No.3 steelmaker will hold a news conference on Friday afternoon to announce details of its misconduct, Nikkei said, without citing sources.
A Kobe Steel spokesman said the company is checking on the Nikkei report and that the company may hold a news conference later on Friday.
On Thursday, several Japanese automakers announced there are no safety problems with aluminium parts from Kobe Steel.
Checks are continuing at hundreds of companies involved in complex supply chains spanning the globe.