-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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<SEC-DOCUMENT>0000276889-06-000051.txt : 20060316
<SEC-HEADER>0000276889-06-000051.hdr.sgml : 20060316
<ACCEPTANCE-DATETIME>20060316141649
ACCESSION NUMBER:		0000276889-06-000051
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20060315
FILED AS OF DATE:		20060316
DATE AS OF CHANGE:		20060316

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:			MURPHY TERRY M
		CENTRAL INDEX KEY:			0001015235

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-05725
		FILM NUMBER:		06691226

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		6167792200

	MAIL ADDRESS:	
		STREET 1:		ONE MADISON AVENUE
		CITY:			CADILLAC
		STATE:			MI
		ZIP:			49601
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
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<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

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    <periodOfReport>2006-03-15</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

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

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            <rptOwnerName>MURPHY TERRY M</rptOwnerName>
        </reportingOwnerId>
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            <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>Senior VP-Finance and CFO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

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            <transactionAmounts>
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                </transactionShares>
                <transactionPricePerShare>
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                <transactionPricePerShare>
                    <value>64.04</value>
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                <transactionAcquiredDisposedCode>
                    <value>D</value>
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            <postTransactionAmounts>
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                    <value>20718.8675</value>
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        <nonDerivativeTransaction>
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            <transactionDate>
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                <transactionFormType>4</transactionFormType>
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            <transactionTimeliness>
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                </transactionShares>
                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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            <transactionTimeliness>
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                <value></value>
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                <directOrIndirectOwnership>
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        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes></footnotes>

    <ownerSignature>
        <signatureName>John J. Mannion, Power of Attorney</signatureName>
        <signatureDate>2006-03-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----