(Recasts, adds details and background on takeover)

CARACAS, April 27 (Reuters) - Venezuelan President Hugo Chavez on Sunday threatened to expropriate steelmaker Ternium (TX.N: Quote, Profile, Research) Sidor if the company cannot reach an agreement over its nationalization "in the coming hours."

The announcement comes only a day after Venezuela said it would offer Ternium less than $800 million for its 60 percent stake in the company, less than a quarter of the $4 billion Chavez said the company wants for its holdings.

The aggressive stance could complicate Chavez's effort to take over the nation's largest steelmaker without creating a conflict with the allied government of Argentina, home of the Ternium steel conglomerate that controls Sidor.

"I will not pay $4 billion for that company," Chavez said, during his weekly Sunday broadcast.

"If they do not want to reach an agreement with us, on Tuesday I will sign an expropriation decree and take immediate control of the company."

Venezuela said on Sunday its preliminary valuation of Ternium's holdings was $800 million, though it would subtract from this amount any pending debts -- which could include a $200 million debt to state energy company PDVSA.

"I do not want to harm them ... but I just had to laugh when I read they wanted $4 billion. I said that cannot be, they must be crazy," Chavez said.

He ordered the nation's vice-president, who has overseen much of the Sidor talks, to meet with the company on Monday to seek an agreement.