POSCO warned its second-half profit would fall by 30 per cent after posting its second-best ever quarterly profit on Tuesday, as steel prices crumble on weak Chinese demand. Asian steelmakers face a tough outlook as steep price declines in China force many to slash output and the market could slide into deeper oversupply as Beijing tightens policy and moves to curb exports. Signs of weakness in global auto sales will cast a shadow over the second-half profit growth of major Asian producers such as Nippon Steel and Posco. But the steel major is the most profitable among Asian steelmakers, thanks to two price hikes.