<SUBMISSION>
<ACCESSION-NUMBER>0001229467-06-000002
<TYPE>3
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20060411
<FILING-DATE>20060418
<DATE-OF-FILING-DATE-CHANGE>20060418
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>STEEL DYNAMICS INC
<CIK>0001022671
<ASSIGNED-SIC>3312
<IRS-NUMBER>351929476
<STATE-OF-INCORPORATION>IN
<FISCAL-YEAR-END>1204
</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>
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>DUKE TIMOTHY R
<CIK>0001229467
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>3
<ACT>34
<FILE-NUMBER>000-21719
<FILM-NUMBER>06764769
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>P O BOX 2547
<CITY>HUNTINGTON
<STATE>WV
<ZIP>25726-2547
</MAIL-ADDRESS>
</REPORTING-OWNER>
<DOCUMENT>
<TYPE>3
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2006-04-11</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001022671</issuerCik>
        <issuerName>STEEL DYNAMICS INC</issuerName>
        <issuerTradingSymbol>STLD</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001229467</rptOwnerCik>
            <rptOwnerName>DUKE TIMOTHY R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>P.O. BOX 2547</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>HUNTINGTON</rptOwnerCity>
            <rptOwnerState>WV</rptOwnerState>
            <rptOwnerZipCode>25726-2547</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Vice President</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>13813</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The 13,813 shares of Steel Dynamics (SDI) common stock were received by Mr. Duke as part of the merger consideration, in exchange for 34,532 shares of Roanoke Electric Steel Corporation (&quot;Roanoke&quot;) common stock formerly held by Mr. Duke, regarding the merger of Roanoke with and into a wholly-owned subsidiary of SDI, effective April 11, 2006. Mr. Duke's disposition of his Roanoke shares in accordance with the terms of the merger agreement was approved in advance of closing by the Roanoke board of directors, and Mr. Duke's acquisition of his SDI shares in exchange therefor, was approved in advance of closing by the board of directors and Compensation Committee of SDI, all in accordance with the requirements of the exemption described by SEC Rule 16b-3(d), as interpreted by the SEC in Skadden, Arps, Slate, Meagher &amp; Flom LLP (January 12, 1999), from the short-swing profits rules of Section 16(b) of the Exchange Act.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Timothy R. Duke</signatureName>
        <signatureDate>2006-04-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
