"Over years, the companies agreed on important aspects of stainless steel prices," Federal Cartel Office President Andreas Mundt said in a statement on Thursday.

The Cartel Office was alerted to the price-fixing by Austrian steelmaker Voestalpine, which said in a separate statement that it had discovered the violations during routine internal audits.

Cartel Office investigators searched several offices and homes in November 2015 and found evidence that steel companies had since 2004 been agreeing on how to calculate scrap and alloy surcharges.

They also coordinated increases in the base price of engineering steel and exchanged information about their order books, their customers' inventories, production outages and planned price increases, the Cartel Office said.

The six companies being fined are ArcelorMittal Commercial Long Deutschland GmbH, a German division of ArcelorMittal, Doerrenberg Edelstahl GmbH, Kind & Co. Edelstahlwerke GmbH & Co.KG, Saarstahl AG, Schmidt + Clemens GmbH + Co. KG and Zapp Precision Metals GmbH.

The Cartel Office said it was still investigating four further companies, which it did not name. It also did not name the individuals it was fining.

The industry association that was involved in the price-fixing was the Edelstahl-Vereinigung e.V., which has since been disbanded, the Cartel Office said.

Voestalpine will not face a fine because it blew the whistle on the cartel.

The Austrian company said the employees who were mainly involved no longer worked for the group. No members of its management board were involved in the violations, nor were they aware of them, it said.

($1 = 0.8567 euros)

(Reporting by Caroline Copley and Maria Sheahan; Editing by Victoria Bryan and Jane Merriman)