The company said the issue affected shipments from four domestic factories to about 200 customers in the year to August 2017. End users included the troubled Mitsubishi Regional Jet.
The scandal is a blow to Japan’s third-largest steelmaker and highlights wider concerns about inspection and quality control, after a string of scandals from wobbly building pilings at Asahi Kasei to overstated fuel economy at Mitsubishi Motors.
“We deeply regret to have caused this serious incident and apologise from the heart for causing so much concern and inconvenience,” said deputy president Naoto Umehara.
Products affected included 19,300 tonnes of aluminium plate and extrusions; 2,200 tonnes of copper strip and pipe; and 19,400 cast and forged aluminium parts.
Kobe Steel said it was not aware of any safety concerns or actual problems as a result of the shipments. It gave no information on what, if anything, was wrong with the metals it shipped.
The company is in the process of contacting customers and has called in an external law firm to investigate the incident. In addition to steel, aluminium and copper, Kobe Steel is a large maker of construction equipment under the Kobelco brand. It added that the impact on earnings was not yet known.
Mitsubishi Heavy Industries said that aluminium used in its regional passenger jet was among the shipments with falsified data. However, it said there was no safety issue and delivery of the jet — which is currently undergoing certification in the US after a troubled development — remained on schedule.
Corporate Japan trades on its reputation for quality but the country has been hit by a series of scandals related to product inspection in recent years.
Last week, Nissan said it would recall 1.21m vehicles in Japan after it emerged that unauthorised technicians were carrying out checks, and some safety and environmental inspections were not carried out properly.
The news followed a more serious scandal last year where it emerged that Mitsubishi Motors had been cheating on fuel economy tests for more than 25 years. Mitsubishi Motors was forced to turn to Nissan for a capital injection to survive.
An investigation in that case found overworked engineers in a siloed corporation who felt pressured into meeting impractical fuel economy targets.