-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 Me9b0SiHjwmSgzEoQpPcM3HQiMm8ViS/DbS61p89RRIGTRkU4RXYzGncg/fymBbg
 kUbFBIh9WEStFZhoJ5DICA==

<SEC-DOCUMENT>0000276889-05-000100.txt : 20050420
<SEC-HEADER>0000276889-05-000100.hdr.sgml : 20050420
<ACCEPTANCE-DATETIME>20050420104639
ACCESSION NUMBER:		0000276889-05-000100
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050401
FILED AS OF DATE:		20050420
DATE AS OF CHANGE:		20050420

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			QUANEX CORP
		CENTRAL INDEX KEY:			0000276889
		STANDARD INDUSTRIAL CLASSIFICATION:	ROLLING DRAWING & EXTRUDING OF NONFERROUS METALS [3350]
		IRS NUMBER:				381872178
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1031

	BUSINESS ADDRESS:	
		STREET 1:		1900 W LOOP S STE 1500
		STREET 2:		ATTN BRENT KORB
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77027
		BUSINESS PHONE:		713-877-5328

	MAIL ADDRESS:	
		STREET 1:		1900 WEST LOOP SOUTH
		STREET 2:		SUITE 1500
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77027

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Hammonds Paul A
		CENTRAL INDEX KEY:			0001319419

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-05725
		FILM NUMBER:		05760893

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		713-877-5346

	MAIL ADDRESS:	
		STREET 1:		1900 WEST LOOP SOUTH-SUITE 1500
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77027
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>c0001319419d20050420f4f.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-04-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000276889</issuerCik>
        <issuerName>QUANEX CORP</issuerName>
        <issuerTradingSymbol>NX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001319419</rptOwnerCik>
            <rptOwnerName>Hammonds Paul A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1900 WEST LOOP SOUTH</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1500</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77027</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Vice Pres.-Corp. Development</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2005-04-01</value>
            </transactionDate>
            <deemedExecutionDate>
                <value>2005-04-04</value>
            </deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>8.514</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>54.03</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>439.829</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Shares acquired through Employee Stock Purchase Plan.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Terry M. Murphy, Power of Attorney</signatureName>
        <signatureDate>2005-04-20</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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