Numsa members employed by Evraz Highveld Steel and Vanadium Limited in eMalahleni (formerly Witbank) have gone on strike over retrenchments, the union said on Tuesday.
"The decision by Evraz Highveld to retrench workers indicates their intentions of reproducing racialised poverty and consigning Black African workers into deeper economic misery," said National Union of Metalworkers spokesman Castro Ngobese.
He said 4000 workers began the protected strike on Monday.
Numsa believed retrenchments should not be considered; rather, executives and managers should cut their own pay to save jobs.
"The union's doors are open to engage with Evraz Highveld's ruling oligarchy with the sole intention of stopping possible retrenchments, in order to avert the strike action," said Ngobese.
Highveld Steel spokeswoman Cathie Lewis said on Monday the company informed unions on May 4 that it planned retrenchments as part of its fixed cost reduction programme.
So far there had been no retrenchments, but the company was in the process of shutting down the plant.
She could not be reached for comment on Tuesday.
Deon Reynecke, who heads the Solidarity trade union's metal and engineering sector, said on Monday the union was not on strike and was still negotiating with management about the retrenchments.
He said Solidarity had asked management to give assurances about the safety of its workers, after striking Numsa workers reportedly stopped a coal truck and tipped some of its load onto the road accessing the steelworks.