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<CONFORMED-NAME>NOLAN JOHN W
<CIK>0001195953
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<STATE>IN
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<CONFORMED-NAME>STEEL DYNAMICS INC
<CIK>0001022671
<ASSIGNED-SIC>3312
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    <documentType>4</documentType><periodOfReport>2003-05-22</periodOfReport>
    <issuer>
        <issuerCik>0001022671</issuerCik>
        <issuerName>STEEL DYNAMICS INC</issuerName>
        <issuerTradingSymbol>STLD</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
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            <rptOwnerName>NOLAN JOHN W</rptOwnerName>
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            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
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<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Vice President</officerTitle></reportingOwnerRelationship></reportingOwner><derivativeSecurity><securityTitle><value>Employee 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>3527</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>3527</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>3527</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">Pursuant to the Amended and Restated Steel Dynamics, Inc. 1996 Incentive Stock Option Plan approved by stockholders and exempt from Section 16(b) of the Securities Exchange Act of 1934 pursuant to Rule 16b-3 thereunder.</footnote></footnotes><ownerSignature><signatureName>John W. Nolan</signatureName><signatureDate>2003-05-22</signatureDate></ownerSignature></ownershipDocument>

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