More than a quarter of a million South African metal and engineering workers have downed tools across the country.
The 2010 World Cup has increased demand for steel in South Africa
Trade unions said workers had walked out in a protest over pay and a skills shortage in the country.
The indefinite strike is expected to bring production at more than 9,000 companies to a standstill.
The walkout comes just two weeks after the end of a month-long strike by public sector workers which closed most of the country's schools and hospitals.
The civil servant strike is believed to have been the biggest industrial action since the end of apartheid in 1994.
The wave of strikes sweeping the country has exposed the sharp division between the ruling African National Congress and the labour movement which has accused the government of promoting big business at the expense of poor South Africans.
Metal workers and engineers staged the latest walkout after negotiations over pay broke down last week.
Staff have been demanding a 10% increase in salaries while employers have offered a raise of between 7.3% and 7.8%.
The strike by members of the National Union of Metal Workers of South Africa and Solidarity will affect thousands of firms, including Bell Equipment and Anglo American-owned Scaw Metals.
However, the country's two biggest steel producers - Mittal Steel South Africa and Highbveld Steel & Vanadium - said they had not been affected as they negotiate their worker wage deals separately.
The action could mean delays for the country's plans to build and refurbish a number of venues as it prepares to host the 2010 World Cup football tournament.
Construction in the run-up to the competition has led to a huge spike in demand for steel.