Nikkei press reported 2015 crude steel market share in Japan. According to it, the top supplier was Nippon Steel & Sumitomo Metal (43%).
Figure: 2015 Crude Steel Market Share in Japan
|Manufacturer||Market share (%)|
|Nippon Steel & Sumitomo Metal||42.7|
Source is Nikkei Sangyo Shimbun newspaper, the article on 2016 July 25th.