Nippon Steel, the world's fourth-biggest steelmaker, already owns 45 pct of Siam United, which makes cold-rolled sheet steel, with annual capacity of 1 million tonnes. In 2010 Siam United had sales of 19 billion baht.

Nippon Steel said in April it would invest $300 million to build a steel galvanising production line in Thailand to supply automotive sheet steel by 2013.

Siam Cement's divestment is part of its plan to focus on core businesses. It said in a statement it would book a gain of 690 million baht from the sale in the fourth quarter.

At the midsession break, Siam Cement shares were up 0.86 percent, while the broad market was 0.28 percent higher.

Copyright Reuters, 2010