* US trade panel approved imported paper, salt probes

* 45 lawmakers urge Obama to back their currency bill

* China blasts US duties in record steel pipe case (Adds detail on currency bill, background, reaction)

By Doug Palmer

WASHINGTON, Nov 6 (Reuters) - The United States pushed ahead on Friday with two new investigations into charges of unfair trade practices by China, but rejected a third case one week ahead of President Barack Obama's trip to Asia.

U.S. lawmakers also asked Obama to renew his support for legislation targeting China's exchange rate practices.

The U.S. International Trade Commission approved probes into imports of glossy magazine-quality paper from both China and Indonesia totaling hundreds of millions of dollars, as well as certain salts from China that are used in cleaning products, food additives and fertilizer.

The votes came one day after the U.S. Commerce Department slapped preliminary anti-dumping duties on some $2.6 billion worth of steel pipe from China used in the oil and gas sector. Those were in addition to preliminary countervailing duties set on the pipe in September to offset Chinese subsidies.

The steel pipe case is biggest U.S. trade action against China to date and Beijing quickly denounced the new duties.