MIDDLETOWN, Ohio (AP) _ Steel workers today picketed Armco's Middletown Works in the first strike in the 86-year history of the plant, protesting wage and benefit concessions imposed by the company.

''We have accomplished what we set out to do: we shut down the mill,'' said Ray Back, president of the Armco Employees Independent Federation, whose members walked off their jobs at 11 p.m. Friday.

Back said he toured the plant's gates throughout the night, advising about 150 pickets not to impede those who attempt to cross the picket lines.

Armco Inc., the nation's fifth-largest steel manufacturer, confirmed that the pickets were orderly.

Officials of the company and the union said there was no word on when contract talks might resume.

The union said its members voted 3,592 to 247 earlier this week to authorize the strike.

The Middletown Works is Armco's largest plant, employing about 5,300 salaried and union workers. The union says it has 4,187 members.

After a 30-day contract extension expired last week, Armco imposed a 50 cent per hour wage cut and other concessions.

Armco said it needed the cuts so it could remain competitive with other steelmakers and invest in plant improvements.

In return for the concessions, the company offered a profit-sharing plan it said would more than compensate for the loss in pay if the plant remained profitable. Union officials rejected the plan because they said the company could doctor its books to show a lower profit than it actually made.

Average pay under the old contract was $11.48 per hour, Back said.

Union officials say the company's wage and benefit concessions were worth about $5,000 a year for each worker, but Armco put the figure at $2,600.

Don Stokes, 47, was among the first union workers on the picket line.

''Anybody who's got a family is going to worry about the loss of income, but sometimes you have to make a stand,'' said Stokes, a 25-year Armco employee. ''We've been regarded as a scab outfit by a lot of the other steel unions. Maybe now we'll gain some respect.''

At least four trucks loaded with rolled steel left the plant after the picket lines were set up.

''Just make sure it's your last load,'' workers shouted.

Company officials said the plant would remain open, but the union said the plant would close and union members would help supervisors shut its furnaces.

Armco spokesman Terry Ward said 200 supervisors had been ordered to report to work at the plant and to bring enough clothing and food to last them for at least three days.

''We once again are asking those employees willing to join such a strike to think of the severe personal financial risk they are subjecting themselves to. In the meantime, it is our intent to oeprate the Middletown Works and we are encouraging all of our employees to report as normal,'' John Morr, Armco chief negotiator, said in a prepared statement.

Armco is struggling to recover from three years of multimillion-dollar losses. Richard Schaffnit, president of Armco's Eastern Steel Division, sent a letter Thursday to workers saying a strike could force Armco into bankruptcy.