Cleaners at the Tata steelworks in Port Talbot struck over pay on Monday and Tuesday of this week.
The 70 cleaners, members of the Community union, are employed at the plant by contractor OCS Group.
They walked out after a resounding vote for strikes.
Most are paid the minimum wage, and the workforce has never been on strike before.
The union is demanding they are paid in line with other contractors at the site.
It says it has been in pay negotiations since June last year with no results.
Community campaign manager Susan Lewis said, “Our members at OCS are the most poorly paid on the whole of the Tata Steel site, and they perform some of the toughest and dirtiest jobs.
“They work hard and all they are asking for is a fair deal.”
Community says OCS is “resorting to union-busting tactics”, including bringing in managers and other staff not usually employed on the site as cover.
The workers are considering further strikes.
Private contractor OCS—“One Complete Solution”—was the firm employing cleaners at nearby Neath Port Talbot hospital when they struck in 2004 and again in 2007.
In 2007 the firm slashed cleaners’ working hours. When asked how it could justify asking workers to do the same amount of cleaning in less time, the company said it had “issued new mops”.
OCS was also the contractor employing cleaners on Eurostar trains when they struck in 2008.