<SUBMISSION>
<ACCESSION-NUMBER>0001229452-06-000003
<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>CRAWFORD THOMAS J
<CIK>0001229452
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>3
<ACT>34
<FILE-NUMBER>000-21719
<FILM-NUMBER>06764749
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>P O BOX 13948
<CITY>ROANOKE
<STATE>VA
<ZIP>24038-3948
</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>0001229452</rptOwnerCik>
            <rptOwnerName>CRAWFORD THOMAS J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>P.O. BOX 13948</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ROANOKE</rptOwnerCity>
            <rptOwnerState>VA</rptOwnerState>
            <rptOwnerZipCode>24038-3948</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>16010</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Stock option - right to buy</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>11.42</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2006-04-11</value>
            </exerciseDate>
            <expirationDate>
                <value>2007-12-04</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4433</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock option - right to buy</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>14.02</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2006-04-11</value>
            </exerciseDate>
            <expirationDate>
                <value>2009-02-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4433</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The 16,010 shares of Steel Dynamics (SDI) common stock were received by Mr. Crawford as part of the merger consideration, in exchange for 40,025 shares of Roanoke Electric Steel Corporation (&quot;Roanoke&quot;) common stock formerly held by Mr. Crawford, regarding the merger of Roanoke with and into a wholly-owned subsidiary of SDI, effective April 11, 2006. Mr. Crawford'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. Crawford'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>
        <footnote id="F2">The stock option to purchase 4,433 shares of common stock of SDI at $11.42 per share, expiration December 4, 2007, and the option to purchase 4,433 shares at $14.02 per share, expiration February 2, 2009, were received by Mr. Crawford, incident to the merger of Roanoke with and into a wholly-owned subsidiary of SDI, effective April 11, 2006. Mr. Crawford's former Roanoke options, for 6,000 shares each, were assumed by and exchanged for stock options of SDI, as required by the merger agreement. Mr. Crawford's disposition of his Roanoke stock options was approved in advance by the board of directors of Roanoke, and the issuance of SDI stock options (and the underlying SDI shares) in exchange was likewise approved in advance by SDI's Board in accordance with the exemption from Section 16(b) described by SEC Rule 16b-3(d), as interpreted by the SEC in Skadden, Arps, Slate, Meagher &amp; Flom LLP (January 12, 1999).</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>T. Joe Crawford</signatureName>
        <signatureDate>2006-04-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
