* Expectations of slower winter demand amid steel output curbs
* S&P cuts China's credit rating on rising debt risks
* Steel, iron ore, coking coal set for biggest weekly drop in months
By Manolo Serapio Jr
MANILA, Sept 22 (Reuters) - Chinese prices of steel and its raw materials iron ore and coking coal slid to multi-week lows on Friday and were set to post their steepest weekly losses in months, pressured by slower demand and S&P's downgrade of China's credit rating.
A selloff in steel futures spilled over to prices of its raw materials, pulling back further after this year's sharp rally largely fueled by China's supply-side reform aimed at cutting overcapacity.
The most-active rebar contract on the Shanghai Futures Exchange SRBcv1 was down 2.9 percent at 3,614 yuan ($549) a tonne by 0217 GMT, after earlier hitting its weakest since Aug. 3 at 3,594 yuan.
The construction steel product has fallen more than 5 percent this week, on course for its biggest such drop since late March.
The most-traded iron ore contract on the Dalian Commodity Exchange DCIOcv1 fell as far as 464.50 yuan per tonne, the lowest since July 17. It has lost 7.5 percent so far this week, its biggest such drop since early May.
"Demand for iron ore is expected to weaken in winter because steel factories will cut production and iron ore supply will rise as global miners produce more during the last quarter of the year," said a trader in Jinan in China's eastern Shandong province.
China has ordered mills in its main producing areas including the northern Hebei province to reduce output by up to half during winter to improve air quality.
Iron ore for delivery to China's Qingdao port .IO62-CNO=MB tumbled 5.1 percent to $66.09 a tonne on Thursday, the lowest since July 14, according to Metal Bulletin. The spot benchmark has fallen 8.4 percent this week, the most since early May.
S&P Global Ratings' downgrade of China's long-term sovereign credit rating on Thursday also soured investor sentiment towards commodity futures, with base metals lead, nickel and zinc sold off as well.
The downgrade by S&P which cited increasing risks from China's rapid build-up of debt, comes less than a month ahead of the twice-a-decade Communist Party Congress. coal and coke futures were not spared from the selloff, and Zhang Min, analyst with China Sublime information group, said S&P's move had further weakened sentiment towards both commodities already hit by softer demand.
"Weak steel prices and high inventory of coal supplies as well as steel products point to lukewarm demand," said Zhang.
Coking coal in Dalian DJMcv1 was last down 3.7 percent at 1,225 yuan a tonne and coke DCJcv1 slipped 3.6 percent to 2,070. Coking coal has lost nearly 10 percent this week, and coke has dropped 11 percent, the steepest for both since April.
($1 = 6.5837 Chinese yuan)