Bethlehem Steel Corp. last week announced it is embarking on a major revitalization of its steel-making facilities that will cost about $750 million over the next five years, one of the largest modernization projects in the company's history.

Several projects will begin immediately, including modernization of the plate mills at the Sparrows Point plant in Baltimore. Still in the engineering stage are the installationof a continuous slab cster and modernization of the 68-inch, hot-strip mill, both at Sparrows Point. With 18,000 workers, Bethlehem's Sparrows Point plant is Maryland's largest private employer.

The program is the result of a series of long- and short-range studies conducted by Bethlehem and designed to make the nation's second-largest steel producer more competitive. It will be paid for with company funds and supplemented by borrowing as needed.

"Our objective is to be the best steel producer in terms of productivity, quality and service," said Donald H. Trautlein, Bethlehem's chirman and chief executive officer.

Bethlehem plans to install two other continuous casters -- one at Steelton, Pa., and one at Burns Harbor, Ind. -- that will increase yields of semi-finished steel products. Trautlein predicts the installation of this new technology will mean Bethlehem's share of continuously cast steel "will rise dramatically."

"All of these projects . . . are at the forefront of steelmaking technology," said Trautlein. "Their completion will result in greatly increased efficiency and productivity, substantial energy savings, better product quality and improved customer service."

Trautlein expressesd confidence that the Reagan administration "is genuinely committed to conscientious administration and enforcement of the nation's trade laws. . . We can compete on equal terms with fairly traded steel from other nations. In fact, as we make headway in our modernization efforts, we'll be even more competitive."

The Commerce Department recently launched an investigation of possible unfair trading practices by European and Asian steel producers.

Bethlehem's board of directors already has authorized $115 million for the modernization effort. Funds will be directed toward the steel maker's strongest products and facilities, with the weaker ones being phased out, the company said.

Despite claims by officials of the local steelworkers' union that the modernization at Sparrows Point could eliminate thousands of jobs, a Bethlehem spokesman said the program should have little effect on employment levels at the Sparrows Point and other plants.