Be very careful with trying to fit an equivalent grade. You need to evaluate both chemical composition and mechanical properties and ultimate end use of the product. In some cases there is no "accepted" equivalent grade, you evaluate yourself and decide.
Thanks to each of you, the links were very helpful. We quoted a substitute material of 1018 A709 gr36, which our customer accepted. I'm checking that the estimator did not go too far off base in suggesting this material.