ATI美股招股说明书(2026-06-04版)

$450,000,000 5.875% Senior Notes due 2033 We are offering $450,000,000 aggregate principal amount of our 5.875% Senior Notes due 2033 (the “notes”). The notes will mature on June15,2033. Interest will accrue from June8, 2026, and the first interest payment date will be December15, 2026. We may redeem the notes in whole or in part at any time at the applicable redemption prices set forth under “Description of Notes — OptionalRedemption.” We must offer to repurchase the notes upon the occurrence of a change of control triggering event at the price described in this prospectussupplement in “Description of Notes — Purchase of Notes upon a Change of Control Repurchase Event.” The notes will be our senior unsecured obligations, ranking equally in right of payment with all of our existing and future senior unsecuredindebtedness and senior to our future subordinated indebtedness. The notes will be effectively subordinated to our existing and future securedindebtedness to the extent of the value of the assets securing that indebtedness and to the existing and future indebtedness and other liabilities of oursubsidiaries. We conduct a significant portion of our business through our subsidiaries. None of our subsidiaries will guarantee the notes. You should read this prospectus supplement and the accompanying prospectus carefully before you invest in our notes. Investing in our notesinvolves a high degree of risk. See “Risk Factors” beginning on page S-13 for a discussion of certain risks that you should consider in connection withan investment in the notes. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these notes or passed uponthe adequacy or accuracy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense. (1)Plus accrued interest, if any, from June8, 2026. The notes will not be listed on any securities exchange or automated quotation system. We expect that delivery of the notes will be made to investors in book-entry form through The Depository Trust Company for the accounts of itsparticipants, including Clearstream Banking,société anonyme, and Euroclear Banking, S.A./N.V., on or about June8, 2026. BofA SecuritiesMUFG CitigroupHSBC TABLE OF CONTENTS Prospectus Supplement About This Prospectus SupplementWhere You Can Find More InformationForward-Looking StatementsSummaryRisk FactorsUse of ProceedsCapitalizationDescription of NotesMaterial U.S. Federal Income Tax ConsiderationsUnderwritingLegal MattersExperts Prospectus About this ProspectusWhere You Can Find More InformationSummaryRisk FactorsForward-Looking StatementsUse of ProceedsDescription of Debt SecuritiesDescription of Other SecuritiesDescription of Capital StockPlan of DistributionLegal MattersExperts ABOUT THIS PROSPECTUS SUPPLEMENT This document is in two parts. The first part is this prospectus supplement, which describes certain matters relating to us and this offering. Thesecond part, the accompanying prospectus dated February23, 2024, gives more general information about securities, including debt securities, we mayoffer from time to time, some of which may not apply to the notes offered by this prospectus supplement and the accompanying prospectus. Forinformation about the notes, see “Description of Notes” in this prospectus supplement and “Description of Debt Securities” in the accompanyingprospectus. We are responsible for the information contained and incorporated by reference in this prospectus supplement and the accompanying prospectusand in any related free-writing prospectus we prepare or authorize. We have not authorized anyone to give you any other information, and we take noresponsibility for any other information that others may give you. We are not, and the underwriters are not, making an offer of these notes in anyjurisdiction where the offer or sale is not permitted. You should not assume that the information contained in this prospectus supplement, theaccompanying prospectus or the documents incorporated by reference in this prospectus supplement or the accompanying prospectus is accurate as ofany date other than their respective dates. Our business, financial condition, results of operations and prospects may have changed since those dates. Before you invest in the notes, you should carefully read this prospectus supplement and the accompanying prospectus. You should also read thedocuments we have referred you to under “Where You Can Find More Information” for information about us. The shelf registration statement describedin the accompanying prospectus, including the exhibits thereto, can be read at the Securities and Exchange Commission’s (the “SEC”) website asdescribed under “Where You Can Find More Information.” If the information set forth in this prospectus supplement varies in any way from the information set forth in the accompanying prospectus, youshould rely on the informatio