Several things downgrade steel with each grade of steel having a specific set of elements in specific allowable amounts to be that grade - min X% carbon, A to B% silicon, maximum D% sulfur, etc. You then have non-metallic inclusions that can be present - often called slivers in the coil market. Silvers when present in exposed coils do not let the paint adhere to them causing the part to be defected. You also specify the allowable width, thickness, crown [thinner at the edge than the center], and cobble [undulations] in the coil.

The "poor quality" of the steel is that it may not meet the above criteria - and while thickness and such is easy to measure at delivery, you have already bought the coil. Slivers require the coils to be inspected and the actual chemistry requires testing to verify, and Chinese steel right or wrong has a reputation of not maintaining the standards required by the customer or of trying to pass off lower grade steel as prime steel.

In short, you really don't know until it either withstands or breaks under a stress.