<SUBMISSION>
<ACCESSION-NUMBER>0001397059-07-000001
<TYPE>3
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20070415
<FILING-DATE>20070424
<DATE-OF-FILING-DATE-CHANGE>20070424
<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>
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>SCHNEIDER BARRY
<CIK>0001397059
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>3
<ACT>34
<FILE-NUMBER>000-21719
<FILM-NUMBER>07785087
</FILING-VALUES>
<BUSINESS-ADDRESS>
<PHONE>317-892-7000
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>8000 N. COUNTY ROAD 225 EAST
<CITY>PITTSBORO
<STATE>IN
<ZIP>46167
</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>2007-04-15</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001397059</rptOwnerCik>
            <rptOwnerName>SCHNEIDER BARRY</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>8000 N. COUNTY ROAD 225 EAST</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>PITTSBORO</rptOwnerCity>
            <rptOwnerState>IN</rptOwnerState>
            <rptOwnerZipCode>46804</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>121</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Employee Stock Option (right to buy)</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>15.62</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2006-05-21</value>
            </exerciseDate>
            <expirationDate>
                <value>2010-11-21</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1922</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Employee Stock Option (right to buy)</value>
                <footnoteId id="F2"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>28.32</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2006-11-21</value>
            </exerciseDate>
            <expirationDate>
                <value>2011-05-21</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1060</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Employee Stock Option (right to buy)</value>
                <footnoteId id="F2"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>30.92</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2007-05-21</value>
            </exerciseDate>
            <expirationDate>
                <value>2011-11-21</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>971</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the Amended and Restated Steel Dynamics, Inc. 1996 Equity Incentive Stock Option Plan approved by stockholders and exempt from Section 16(b) of the Securities and Exchange Act of 1934 pursuant to Rule 16b-3.</footnote>
        <footnote id="F2">Pursuant to Steel Dynamics, Inc.'s 2006 Equity Incentive Plan approved by stockholders and exempt from Section 16(b) of the Securities and Exchange Act of 1934 pursuant to Rule 16b-3.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Barry Schneider</signatureName>
        <signatureDate>2007-04-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
