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Feb 11 (Reuters) - U.S. stock index futures slipped on Tuesday as investors awaited a testimony from Federal Reserve Chair Jerome Powell for insight on tariffs and their impact on inflation in the world's biggest economy.

Most megacap and growth stocks ticked lower, while some steelmakers extended gains from the previous session, when President Donald Trump had substantially raised tariffs on steel and aluminum imports.

Inflation data, due on Wednesday, also dominated market focus ahead of Fed Chair Powell's semiannual monetary policy testimony before the Senate Banking, Housing and Urban Affairs Committee on Tuesday.

"The Fed has so far, rightly, avoided commenting on the possible implications of the trade and immigration policies of the new administration, but the emergence of new details on tariff measures since last week offers a platform for Powell to add more specifics around the thought process," Societe Generale analysts wrote in a morning note.

Comments from three other Fed heads, including New York's John Williams, are on the radar throughout the day.

On Wednesday, the January consumer price index $(CPI.UK)$ reading will be released at 8:30 a.m. ET, with Powell set to testify before the House Financial Services Committee later on.

Traders expect at least one 25-basis-point interest-rate cut from the Fed this year, and a 60% chance of another reduction of the same magnitude, according to LSEG data.

Shares of some steelmakers and aluminum producers rose in premarket trading, with Cleveland-Cliffs CLF.N and U.S. Steel X.N rising 2.2% and 1.6%, respectively, a day after Trump raised tariffs on steel and aluminum imports to a flat 25% "without exceptions or exemptions". The measures are set to take effect on March 4.

Aluminum Century Aluminum CENX.O also gained 2.5%.

Trump also said he would follow Monday's action with announcements about reciprocal tariffs on all countries that impose duties on U.S. goods over the next two days.

All three main indexes closed higher on Monday, as investors bought back into AI-linked shares, with Nvidia NVDA.O closing almost 3% higher.

On Tuesday, most megacap and growth stocks edged lower. Amazon.com AMZN.O and Meta Platforms META.O were down about 0.4% each.

At 04:55 a.m. ET, Dow E-minis 1YMcv1 were down 103 points, or 0.23%, S&P 500 E-minis EScv1 were down 19.75 points, or 0.32%, and Nasdaq 100 E-minis NQcv1 were down 95.5 points, or 0.44%.

Coca-Cola KO.N, industrial conglomerate DuPont DD.N, financial information company S&P Global SPGI.N and health insurer Humana HUM.N are some of the prominent companies reporting earnings before the bell.

With more than half of the S&P 500 .SPX companies done with reporting their quarterly numbers, 76.6% have beaten earnings expectations of analysts, per data complied by LSEG.

Among other early movers, Phillips 66 PSX.N gained 5.6% after Reuters reported that activist Elliott Investment Management had built a stake of over $2.5 billion in the oil refiner.

(Reporting by Shashwat Chauhan in Bengaluru; Editing by Devika Syamnath)

((Shashwat.Chauhan@thomsonreuters.com;))