<SUBMISSION>
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<REPORTING-OWNER>
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<CONFORMED-NAME>HIDAKA NAOKI
<CIK>0001195955
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<FILM-NUMBER>03718446
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<MAIL-ADDRESS>
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<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
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<ISSUER>
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<CONFORMED-NAME>STEEL DYNAMICS INC
<CIK>0001022671
<ASSIGNED-SIC>3312
<IRS-NUMBER>351929476
<STATE-OF-INCORPORATION>IN
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<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
<PHONE>2194593553
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<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
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<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-05-23</periodOfReport>
    <issuer>
        <issuerCik>0001022671</issuerCik>
        <issuerName>STEEL DYNAMICS INC</issuerName>
        <issuerTradingSymbol>STLD</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001195955</rptOwnerCik>
            <rptOwnerName>HIDAKA NAOKI</rptOwnerName>
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            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
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<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><derivativeSecurity><securityTitle><value>Non-Employee Director Stock Option (right to buy)</value><footnoteId id="F1"/></securityTitle><conversionOrExercisePrice><value>12.76</value></conversionOrExercisePrice><transactionDate><value>2003-05-21</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>1176</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>2003-11-21</value></exerciseDate><expirationDate><value>2008-05-21</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>1176</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>1176</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">Pursuant to Non-Employee Director Stock Option Plan approved by stockholders and exempt from Section 16(b) of the Securities and Exchange Act of 1934 pursuant to Rule 16b-3.</footnote></footnotes><ownerSignature><signatureName>Naoki Hidaka</signatureName><signatureDate>2003-05-23</signatureDate></ownerSignature></ownershipDocument>

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