<SUBMISSION>
<ACCESSION-NUMBER>0001195955-03-000002
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20031215
<FILING-DATE>20031217
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>HIDAKA NAOKI
<CIK>0001195955
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>000-21719
<FILM-NUMBER>031059648
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>6714 POINTE INVERNESS WAY
<STREET2>SUITE 200
<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
</MAIL-ADDRESS>
</REPORTING-OWNER>
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>STEEL DYNAMICS INC
<CIK>0001022671
<ASSIGNED-SIC>3312
<IRS-NUMBER>351929476
<STATE-OF-INCORPORATION>IN
<FISCAL-YEAR-END>1231
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>6714 POINTE INVERNESS WAY
<STREET2>SUITE 200
<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
<PHONE>2604593553
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>6714 POINTE INVERNESS WAY
<STREET2>SUITE 200
<CITY>FORT WAYNE
<STATE>IN
<ZIP>46804
</MAIL-ADDRESS>
</ISSUER>
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<TYPE>4
<SEQUENCE>1
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<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-12-15</periodOfReport>
    <issuer>
        <issuerCik>0001022671</issuerCik>
        <issuerName>STEEL DYNAMICS INC</issuerName>
        <issuerTradingSymbol>STLD</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001195955</rptOwnerCik>
            <rptOwnerName>HIDAKA NAOKI</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TWO PRUDENTIAL PLAZA</rptOwnerStreet1>
            <rptOwnerStreet2>180 NORTH STETSON, SUITE 4600</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-12-15</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>S</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>300000</value><footnoteId id="F1"/></transactionShares><transactionPricePerShare><value>21.28</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>624197</value><footnoteId id="F1"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By Sumitomo Corporation of America</value><footnoteId id="F1"/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><footnotes><footnote id="F1">All shares of common stock are owned beneficially by Sumitomo Corporation of America.  Mr. Hidaka is a senior vice president of Sumitomo Corporation of America.  Mr. Hidaka does not exercise either voting power or investment power with respect to these securities, alone or with anyone else, and Mr. Hidaka specifically disclaims beneficial ownership of these shares.</footnote></footnotes><remarks/><ownerSignature><signatureName>Naoki Hidaka</signatureName><signatureDate>2003-12-16</signatureDate></ownerSignature></ownershipDocument>

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