Tata Steel is cutting 900 jobs around the UK, including almost 600 jobs at plants in south Wales.
Most of the Welsh losses are management and administrative posts in Port Talbot, while sites in Crosskeys in Caerphilly county, and one near Tredegar, Blaenau Gwent, will close.
Other job cuts include 155 in Yorkshire, 120 in the West Midlands and 30 on Teesside.
However, 120 jobs will created at a new hot strip rolling mill at Llanwern.
Shift levels at the company's Rotherham and Hartlepool plants will be reduced to match production to lower demand for bar products and pipelines.
Tata, which employs 19,000 people in its UK steel business, also announced that it would restart one of two blast furnaces at Port Talbot early next year as part of a £250m investment programme.
This will also lead to the restarting of a hot strip mill at Tata's site in Llanwern.
The company confirmed that the net effect in Wales of its announcement would be 584 fewer jobs.
Karl Kohler, chief executive of Tata Steel's European operations, said: "Today's proposals are part of a strategy to transform ourselves into an all-weather steel producer, capable of succeeding in difficult economic conditions.
"These restructuring proposals will help make our business more successful and sustainable, but the job losses are regrettable and I know this will be a difficult and unsettling time for the employees and their families affected.
"We will be working with our trade unions and government at a national and local level to ensure we provide them with as much assistance and support as possible.
"In addition, our subsidiary UK Steel Enterprise will be looking at how it can provide more support to local steel communities and stimulate new jobs following today's announcement.
"We will strengthen this work with a further £650,000 to help them create new jobs in affected areas.
"UK Steel Enterprise has teams in all the affected locations who, for almost four decades, have helped to regenerate local economies and create 70,000 new jobs in the UK.
"We will do everything we can to reduce the impact of the proposals on employees and, where possible, we will look to achieve job losses through voluntary redundancies."