US sheet steel prices are turning lower, sources said Thursday, due to a combination of lackluster demand, import concerns, and a slight weakening of discipline at the mill level.

Platts lowered its hot-rolled coil assessment from $680-690/st to $670-680/st Thursday, and its cold-rolled coil assessment from $800-810/st to $790-800/st as a result.

One large service center buyer said many top-tier mills have short lead times and readily available steel, as well as attractive import options. Faced with modest demand levels, buyers have no motivation to purchase more steel than is necessary.

"You're looking at your inventory and your order book and saying, 'Am I going to run out of steel in two weeks?'" he said. "There's absolutely no reason in the world to buy."

The market has "plenty of supply" and that demand "has improved seasonally, but just seasonally," he added.

A second large service center source added that the first waves of Russian and Korean imports are starting to arrive, biting into domestic prices.

A trader, however, said imports are just a part of the overall downward trend. "The imports are the event looming on the horizon that's going to crack the market in a month or so," he said.

Other buy-side sources blamed overreach and a stumble in discipline at the mill level.

"This market would have crumbled with the way [it was] going four weeks ago," said one service center source. "But the discipline of the mills and them needing profitability has been different."

He also said that he expects mills to close production or refuse to sell below cost if order books continue to be "anemic." Multiple sources confirmed that at least two major mills have lead times as short as two weeks.

A mill source said that it is true that prices are eroding, though the process has been gradual. Cold-rolled and coated products, however, continue to benefit from some tightness in the automotive industry.

"And generally, when the book's strong then pricing is strong," the source said.