A gas lance was used to reignite the furnace in a special ceremony. The move will mark the resumption of steelmaking on Teesside following the mothballing of the plant in February 2010.
Around 1,700 workers are now employed on the site and the figure will increase to 1,800, similar to the numbers on site under the previous ownership.
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Mr Win Viriyaprapaikit, president of SSI, donned a white hard hat while attending the ceremony in Redcar.
He said: "This is a very proud day for us in SSI and for me personally. I believe that the investment we have made at Teesside will result in a very successful business which will benefit all of the stakeholders involved, including the local community and employees for generations to come."
Long tradition of steelmaking
Mr Phil Dryden, chief executive of SSI UK, said: "I would like to pay tribute to all those who have made this day possible, including the stakeholders for their investment and the people who have worked so hard on the restart project. We now look forward with confidence to resuming the long tradition of steelmaking on Teesside and establishing SSI UK as world class steelmaker."
Watch video Workers return as steelworks furnace is relit.
Watch video Steelmaking returns to Teesside after two years.