The National Union of Metalworkers of South Africa said on Tuesday it had gone on strike at ArcelorMittal South Africa (AMSA), accusing the steel company of abusing workers through labour brokers.
Numsa said the strike would affect all AMSA operations in Vanderbijlpark.
"Large numbers of our members are employed through contractors ... but work for AMSA. We are told that the contractors are service providers and not labour brokers," it said.
"Our members have loyally served AMSA while enduring poor wages and intolerable working conditions. Our members employed by the contractors work side by side with workers directly employed by AMSA who receive much higher pay and benefits. This kind of needless exploitation is unacceptable."
The union accused AMSA of circumventing a Constitutional Court ruling last year stipulating that workers brought in through labour brokers must become permanently employed after three months of working full time for the main employer.
"Our key demand is that AMSA must permanently employ all workers presently employed by the contractors who render their services to AMSA," Numsa said.
"We are willing to engage with the management team constructively to resolve this dispute as we would prefer to resolve this amicably."