Standard & Poor’s Ratings Services lowered its credit rating on J.C. Penney Co. further into junk status, saying the department store chain’s performance remains weak. The ratings agency said it cut its corporate credit rating on the company to ‘‘B+’’ from ‘‘BB-.’’ The junk rating is four notches below investment grade. The move marks the second downgrade from S&P since mid-May. Under a new CEO, former Apple Inc. executive Ron Johnson, J.C. Penney is overhauling its business from its pricing to the brands it carries. But concerns are growing about the company’s ability to turn business around. J.C. Penney posted a bigger-than-­expected loss and a 20 percent drop in revenue in the first quarter as shoppers were turned off by the new pricing plan.