I have told about this in Naval Programs, but there is slightly wrong information in here.The crude steel supplied from Erdemir was transformed into HY-100 by Miilux Oy and delivered for the prototype production of the MILDEN-National Submarine Project.
Millux does not make conventional steel into HY steel, millux will be processing the HY steel acquired from Eregli demir or Kardemir. Both companies have individually developed a formula.
Also, it is a test section production and not prototype production. Golcuk Shipyard will go through an upgrade to accomodate new rigs for larger diameter hulls. So the test section could even be a scaled version of Milden's pressure hull section.