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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 Cx86BHMnIo5lVqiREF4Ra9bP3yEDI93XpxycecqSr3YJ/Ld9E+c4ouTwQmcuBlEF
 7GvaC+LlScItwRF15Pr1mw==

<SEC-DOCUMENT>0001195948-09-000005.txt : 20091124
<SEC-HEADER>0001195948-09-000005.hdr.sgml : 20091124
<ACCEPTANCE-DATETIME>20091124150757
ACCESSION NUMBER:		0001195948-09-000005
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20091121
FILED AS OF DATE:		20091124
DATE AS OF CHANGE:		20091124

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			BUSSE KEITH E
		CENTRAL INDEX KEY:			0001195948

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-21719
		FILM NUMBER:		091204643

	MAIL ADDRESS:	
		STREET 1:		7575 W. JEFFERSON BLVD.
		CITY:			FORT WAYNE
		STATE:			IN
		ZIP:			46804

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			STEEL DYNAMICS INC
		CENTRAL INDEX KEY:			0001022671
		STANDARD INDUSTRIAL CLASSIFICATION:	STEEL WORKS, BLAST FURNACES  ROLLING MILLS (COKE OVENS) [3312]
		IRS NUMBER:				351929476
		STATE OF INCORPORATION:			IN
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		7575 W JEFFERSON BLVD
		CITY:			FORT WAYNE
		STATE:			IN
		ZIP:			46804
		BUSINESS PHONE:		260 459 3553

	MAIL ADDRESS:	
		STREET 1:		7575 W JEFFERSON BLVD
		CITY:			FORT WAYNE
		STATE:			IN
		ZIP:			46804
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-11-21</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001195948</rptOwnerCik>
            <rptOwnerName>BUSSE KEITH E</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>7575 W. JEFFERSON BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>FORT WAYNE</rptOwnerCity>
            <rptOwnerState>IN</rptOwnerState>
            <rptOwnerZipCode>46804</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chairman and CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Employee Stock Option (right to buy)</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>16.15</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-11-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3096</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2010-05-21</value>
            </exerciseDate>
            <expirationDate>
                <value>2014-11-21</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3096</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3096</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Formula stock option grant, but provided by order of the Compensation Committee and Board, for purposes of this grant date, at one-half of the otherwise applicabgle dollar amount per grantee, pursuant to the 2006 Equity Incentive Plan and exempt from Section 16(b) pursuant to Rule 16b-3.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Keith E. Busse</signatureName>
        <signatureDate>2009-11-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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