Bethlehem Steel Corp. said yesterday it will lay off another 600 to 700 workers at its Sparrows Point plant, raising the toll of idled employes to more than a quarter of the work force at Bethlehem -- the state's largest single private employer.

Reflecting a slump in the steel industry caused by the decline in the economy, increased pressure from imports and the failing automobile industry, the company said it will shut off the blast furnace and two remaining furnaces operating in the open hearth shop by June 28 and lay off the workers within 10 days.

The additional layoffs will raise to a total of approximately 2,700 the number of workers indefinitely out of work. In addition, another 2,815 workers out of a ormal work force of 18,500 are "scheduled off" for the current work week. Being scheduled off is being laid off for a definite period of time.

"I think it's not the last of it. It's going to get a lot worse." said Dave Wilson, presient of United Steel Workers Local 2609. Wilson said workers with up to 40 years of seniority are being laid off.

Because the layoffs began relatively recently, the laid-off workers have not exhausted unemployment benefits.

The company blamed the shutdown and the layoffs on a decline in steel orders. Finished-steel shipments have declined sharply, in large part because of a lack of demand from the auto industry.

In April 1980, the last month for which figures are available, shipments for the industry were 7.296 million net tons compared with 8.711 million tons in March.

We're aware of it, and we're concerned about it, just as we are with the General Motors layoffs," said Hans Mayer, Maryland's deputy secretary of economic and community development.