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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
<FISCAL-YEAR-END>1231
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<BUSINESS-ADDRESS>
<STREET1>7030 POINTE INVERNESS WAY
<STREET2>SUITE 310
<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
<PHONE>2194593553
</BUSINESS-ADDRESS>
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<STREET1>7030 POINTE INVERNERSS WAY
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<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
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<REPORTING-OWNER>
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<CONFORMED-NAME>MILLETT MARK D
<CIK>0001195949
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<ACT>34
<FILE-NUMBER>000-21719
<FILM-NUMBER>03683321
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<STREET1>6714 POINTE INVERNESS WAY
<STREET2>SUITE 200
<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
</MAIL-ADDRESS>
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<DESCRIPTION>PRIMARY DOCUMENT
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<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

<documentType>4</documentType><periodOfReport>2003-05-05</periodOfReport>
<issuer><issuerCik>0001022671</issuerCik>
        <issuerName>STEEL DYNAMICS INC</issuerName>
<issuerTradingSymbol>STLD</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId><rptOwnerCik>0001195949</rptOwnerCik>
            <rptOwnerName>MILLETT MARK D</rptOwnerName>
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        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
<reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Vice President</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-05-05</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>S</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>50000</value></transactionShares><transactionValue><value>12.27</value></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>1044282</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><footnotes/><ownerSignature><signatureName>Mark D. Millett</signatureName><signatureDate>2003-05-05</signatureDate></ownerSignature></ownershipDocument>

</XML>
</TEXT>
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