'VOLUNTARY' IMPORT RESTRAINT
President Reagan's decision Tuesday to ask other countries to limit their steel shipments to the United States was presented by the White House as upholding the principles of free trade. But the recent history of ''voluntary'' export restraint suggests it is potentially the most protectionist action this Administration has yet taken. Although the White House said the President was rejecting the mandatory quotas sought by American steel producers and the United Steelworkers, the practical effect of Mr. Reagan's decision will be little different. ''We are moving toward globally managed trade in steel, similar to what already exists in textiles,'' commented Robert Z. Lawrence, senior fellow at the Brookings Institution. Barely noticed among documents handed out by the White House Tuesday in connection with the Presidential decision was a 12-page list of all the ''unfair'' trade practices of just about every steel-producing nation.