Exhibit 4.13
                          FIRST SUPPLEMENTAL INDENTURE
                           Dated as of August 6, 1999
           THIS FIRST SUPPLEMENTAL INDENTURE to the Indenture referred to below
is dated as of August 6, 1999 (this "First Supplemental Indenture") among AK
STEEL CORPORATION, a Delaware corporation ("AK Steel"), AK STEEL HOLDING
CORPORATION, a Delaware corporation ("Holding") and FIFTH THIRD BANK, as trustee
(the "Trustee").
           AK Steel, Holding and the Trustee are parties to an Indenture, dated
as of February 10, 1999 (the "Indenture"), providing among other things, for the
authentication, delivery and administration of AK Steel's 7-7/8% Senior Notes
due 2009 (the "Notes").
           AK Steel has solicited consents from Holders (as defined below) of
the Notes to certain amendments to the Indenture described in Article II hereof
(the "Proposed Amendments").
           Pursuant to Section 9.02 of the Indenture, the Holders of at least a
majority in principal amount of the outstanding Notes have consented in writing
to the Proposed Amendments
           AK Steel has directed the Trustee to execute and deliver this First
Supplemental Indenture in accordance with the terms of the Indenture.
           In consideration of the foregoing premises, the parties mutually
agree as follows for the benefit of each other and for the equal and ratable
benefit of the Holders of the Notes:
                                    ARTICLE I
                          DEFINITIONS AND EFFECTIVENESS
           Section 1.1 Definitions. Except as otherwise defined herein,
capitalized terms defined in the Indenture are used herein as therein defined.
           Section 1.2 Condition to Effectiveness. The Proposed Amendments will
become effective upon (the "Effective Time") the consummation of the merger
contemplated by the Agreement and Plan of Merger, dated as of May 20, 1999, as
it may be amended, among Holding, AK Steel and Armco Inc.
                                   ARTICLE II
                             AMENDMENTS TO INDENTURE
           Section 2.1 Amendments to Indenture. On and after the Effective Time,
the Indenture shall be amended as follows:
                     (a) Section 1.1 of the Indenture shall be amended to add
           the following definitions in proper alphabetical order:
NY2:\841133\01\$10T01!.DOC\38055.0020
                               (i) "`domestic' means, with respect to any
                     Person, that such Person is organized and existing under
                     the laws of the United States, any State thereof or the
                     District of Columbia."; and
                               (ii) "`Restricted Subsidiary' means any
                     Subsidiary of AK Steel that AK Steel has not designated as
                     a Non-Recourse Subsidiary (or, if AK Steel has so
                     designated such Subsidiary, has thereafter removed such
                     designation) pursuant to Section 4.22(a) hereof. For the
                     avoidance of ambiguity, a Restricted Subsidiary is any
                     Subsidiary other than a Non-Recourse Subsidiary."
                     (b) The definition of "Consolidated Net Income" contained
           in Section 1.1 of the Indenture shall be amended as follows:
                               (i) the word "Restricted" shall be inserted
                     immediately before the first reference to the word
                     "Subsidiary" in clause (a) thereof;
                               (ii) the parenthetical clause "(other than a
                     Non-Recourse Subsidiary)" shall be deleted in clause (a)
                     thereof;
                               (iii) the word "and" at the end of clause (e)
                     thereof shall be deleted;
                               (iv) the text "; and" shall replace the "." at
                     the end of clause (f) thereof; and
                               (v) a new clause (g) shall be inserted after
                     clause (f) thereof as follows:
                                       "(g) solely for purposes of Section 4.7
                     hereof, special charges, costs and other expenses
                     (including restructuring charges and associated investment
                     banking, legal, accounting, printing and related fees and
                     expenses) recorded by Holding, AK Steel or any Restricted
                     Subsidiary (and related tax effects) in connection with the
                     merger of Armco Inc. with and into AK Steel pursuant to an
                     Agreement and Plan of Merger dated as of May 20, 1999, as
                     it may be amended, among Holding, AK Steel and Armco Inc.
                     and any other merger or other business combination
                     transaction involving Holding, AK Steel or any Restricted
                     Subsidiary, to the extent that such charges, costs and
                     other expenses are not permitted under generally accepted
                     accounting principles to be capitalized and amortized over
                     future periods, in each case in respect of which Holding
                     has delivered to the Trustee an Officers' Certificate, made
                     in good faith by responsible financial or accounting
                     Officers of Holding, at the time such special charges,
                     costs and other expenses are recorded, setting forth in
                     reasonable detail such special charges, costs and other
                     expenses.".
                     (c) The definition of "Guarantor Subsidiary" contained in
           Section 1.1 of the Indenture shall be amended in its entirety and
           replaced with the following:
                               "`Guarantor Subsidiary' means (a) any domestic
                     Restricted Subsidiary or (b) any other Restricted
                     Subsidiary that is a Significant Subsidiary of which 80% or
                     more of the total voting power of Equity Interests or other
                     interests (including partnership interests) entitled
                     (without regard to the occurrence of any contingency) to
                     vote in the election of directors, managers or trustees
                     thereof is at the time owned or controlled, directly or
                     indirectly, through one or more intermediaries, or both, by
                     AK Steel, which in each case executes a supplement to this
                     Indenture pursuant to which such Restricted Subsidiary
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                     jointly and severally unconditionally guarantees the due
                     and punctual payment and performance of the Obligations and
                     assumes the other obligations of a Guarantor Subsidiary
                     pursuant to this Indenture, in the manner provided by this
                     Indenture."
                     (d) The definition of "Non-Recourse Subsidiary" in Section
           1.1 of the Indenture shall be amended in its entirety and replaced
           with the following:
                               "`Non-Recourse Subsidiary' means a Subsidiary of
                     AK Steel that is not a Restricted Subsidiary."
                     (e) The definition of "Permitted Credit Facility" or
           "Facilities" in Section 1.1 of the Indenture shall be amended by
           replacing the two references to the word "Guarantor" with the word
           "Restricted" therein.
                     (f) The definition of "Permitted  Investments"  contained
           in Section 1.1 of the Indenture  shall be amended as follows:
                               (i) a ";" shall be inserted at the end of clause
                     (d) thereof;
                               (ii) each of the five references to the word
                     "Guarantor" in clauses (e) and (f) thereof shall be
                     replaced with the word "Restricted";
                               (iii) the word "and" at the end of clause (e)
                     thereof shall be deleted; and
                               (iv) the text "; and" shall be inserted at the
                     end of clause (f) thereof.
                     (g) The definition of "Wholly Owned Guarantor Subsidiary"
           in Section 1.1 of the Indenture shall be amended in its entirety and
           replaced with the following:
                               "`Wholly Owned Guarantor Subsidiary' means any
                     Wholly Owned Subsidiary that is a Restricted Subsidiary
                     (whether or not a Guarantor Subsidiary). For the avoidance
                     of doubt, not all Wholly Owned Guarantor Subsidiaries are
                     required to be Guarantor Subsidiaries."
                     (h) Section 4.7 of the Indenture shall be amended by
           inserting the word "Restricted" immediately before the first
           reference to the word "Subsidiary".
                     (i) Section 4.11 of the Indenture shall be amended by
           inserting the word "Restricted" immediately before the first
           reference to the word "Subsidiary" therein.
                     (j) Section 4.12 of the Indenture shall be amended by
           replacing the word "Guarantor" with the word "Restricted" in clause
           (e) thereof.
                     (k) Section 4.14 of the Indenture shall be amended by
           replacing the word "Guarantor" with the word "Restricted" therein.
                     (l) Section 4.15 of the Indenture shall be amended in its
           entirety and replaced with the following:
                               "SECTION 4.15. Lines of Business. AK Steel shall
                     be permitted to engage in any business, either directly or
                     through any Subsidiary, provided that AK Steel and its
                     Subsidiaries, taken as a whole, remain principally engaged
                     in the same business, or any business reasonably related
                     thereto, in which they were engaged on the date on which
                     the Initial Securities were originally issued."
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                     (m) Section 4.18 of the Indenture shall be amended by
           replacing the word "Guarantor" with the word "Restricted" and by
           replacing each of the two references to the words "any Guarantor
           Subsidiary" with the words "such Restricted Subsidiary" therein.
                     (n) Section 4.19 of the Indenture shall be amended by
           replacing the word "Guarantor" with the word "Restricted" therein.
                     (o) Article 4 of the  Indenture  shall be amended by
           inserting after Section 4.21 thereof a new Section 4.22 as follows:
                               "SECTION 4.22. Designation of Non-Recourse
                     Subsidiaries and Restricted Subsidiaries. (a) AK Steel may
                     designate any of its Subsidiaries (including an existing or
                     newly formed or acquired Subsidiary) as a Non-Recourse
                     Subsidiary if (i) such Subsidiary has total assets of
                     $1,000 or less or (ii) such designation is effective
                     immediately upon such Person becoming a Subsidiary of
                     either AK Steel or any of its Restricted Subsidiaries.
                     Unless so designated as a Non-Recourse Subsidiary, any
                     Person that becomes a Subsidiary of AK Steel shall be
                     classified as a Restricted Subsidiary. Subject to Section
                     4.22(b) hereof, the designation as a Non-Recourse
                     Subsidiary may be removed. The designation of a
                     Non-Recourse Subsidiary or the removal of such designation
                     in compliance with Section 4.22(b) hereof shall be made by
                     the Board of Directors pursuant to a resolution delivered
                     to the Trustee and shall be effective as of the date
                     specified in the applicable resolution, which shall not be
                     prior to the date such resolution is delivered to the
                     Trustee.
                               (b) AK Steel shall not, and shall not permit any
                     of its Restricted Subsidiaries to, take any action or enter
                     into any transaction or series of transactions that would
                     result in a Person becoming a Restricted Subsidiary
                     (whether through an acquisition, the removal of the
                     designation as a Non-Recourse Subsidiary or otherwise)
                     unless, after giving effect to such action, transaction or
                     series of transactions:
                                          (i) on a pro forma basis, AK Steel
                               could issue at least $1.00 of additional Debt
                               pursuant to the Consolidated EBITDA Coverage
                               Ratio as set forth in the first paragraph of
                               Section 4.5 hereof;
                                          (ii) such Restricted Subsidiary could
                               then issue, pursuant to Section 4.6 hereof, all
                               Debt as to which it is obligated at such time;
                                          (iii) no Default or Event of Default
                               would occur or be continuing; and
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                                          (iv) there exist no Liens with respect
                               to the property or assets of such Restricted
                               Subsidiary other than Liens permitted to be
                               incurred under Section 4.12 hereof.
                               (c) AK Steel shall not, and shall not permit any
                     of its Restricted Subsidiaries to, take any action or enter
                     into any transaction or series of transactions that would
                     result in any such Restricted Subsidiary ceasing to be a
                     Subsidiary (other than a merger or consolidation with AK
                     Steel or another Restricted Subsidiary) unless, after
                     giving effect to such action, transaction or series of
                     transactions, either:
                                          (i) (A) neither AK Steel nor any of
                               its Affiliates (other than a Person that is an
                               Affiliate by virtue of its ownership of Equity
                               Interests or control of AK Steel) shall own any
                               Equity Interests of such former Restricted
                               Subsidiary or any successor in interest to the
                               business thereof, and (B) there shall not exist
                               any Debt of such former Restricted Subsidiary or
                               any successor in interest to the business thereof
                               in favor of AK Steel or any of its Restricted
                               Subsidiaries; or
                                          (ii) AK Steel and its Restricted
                               Subsidiaries would be permitted to make a
                               Restricted Payment in the amount of the aggregate
                               Investment (excluding (A) any Investment to the
                               extent of cash or the Fair Market Value of
                               property or assets other than cash received by AK
                               Steel or its Restricted Subsidiary, as the case
                               may be, in respect of or as a repayment of such
                               Investment, and (B) the amount of Debt of such
                               former Restricted Subsidiary received by AK Steel
                               or its Restricted Subsidiaries as part of the
                               consideration for the acquisition of the Equity
                               Interests or assets of such former Restricted
                               Subsidiary), if any, made in such former
                               Restricted Subsidiary after October 1, 1996."
                     (p) Section 5.1 of the Indenture shall be amended by (i)
           inserting the word "Guarantor" immediately before the word
           "Subsidiary" and (ii) deleting the parenthetical clause "(other than
           a Non-Recourse Subsidiary)", in each case, in the last paragraph
           thereof.
                     (q) Clause (a) of Section 10.4 of the Indenture shall be
           amended as follows:
                               (i) the word "Guarantor" shall be inserted
                     immediately before the first reference to the word
                     "Subsidiary" therein;
                               (ii) the parenthetical clause "(other than a
                     Non-Recourse Subsidiary)" shall be deleted; and
                               (iii) the words "become a Guarantor Subsidiary
                     with respect to the Securities by executing and delivering"
                     shall be deleted and replaced with the words "execute and
                     deliver" therein.
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           Section 2.2 Notification to Holders. AK Steel shall notify the
Holders in accordance with Section 9.02 of the Indenture of the execution of
this First Supplemental Indenture. Any failure of AK Steel to give such notice
to all Holders, or any defect therein, shall not impair or affect the validity
of this First Supplemental Indenture.
           Section 2.3 Receipt by Trustee. In accordance with Sections 9.6 and
11.4 of the Indenture, the parties acknowledge that the Trustee has received an
Officers' Certificate and an Opinion of Counsel as conclusive evidence that this
First Supplemental Indenture complies with the applicable requirements of the
Indenture, including that the Proposed Amendments are authorized or permitted by
the Indenture.
                                   ARTICLE III
                                  MISCELLANEOUS
           Section 3.1 Parties. Nothing expressed or mentioned herein is
intended or shall be construed to give any Person, other than the Holders and
the Trustee, any legal or equitable right, remedy or claim under or in respect
of this First Supplemental Indenture or the Indenture or any provision herein or
therein contained.
           Section 3.2 Governing Law. The rights and duties of AK Steel, Holding
and the Trustee under this First Supplemental Indenture shall, pursuant to New
York General Obligations Law Section 5-1401, be governed by the laws of the
State of New York.
           Section 3.3 Separability Clause. In case any provision in this First
Supplemental Indenture shall be invalid, illegal or unenforceable, the validity,
legality and enforceability of the remaining provisions shall not in any way be
affected or impaired thereby.
           Section 3.4 Ratification of Indenture; First Supplemental Indenture
Part of Indenture. Except as expressly supplemented hereby, the Indenture is in
all respects ratified and confirmed and all the terms, conditions, and
provisions thereof shall remain in full force and effect. This First
Supplemental Indenture shall form a part of the Indenture for all purposes, and
every Holder of Notes heretofore or hereafter authenticated and delivered shall
be bound hereby. The Trustee makes no representation or warranty as to the
validity or sufficiency of this First Supplemental Indenture.
           Section 3.5 Multiple Originals. The parties may sign any number of
copies of this First Supplemental Indenture. Each signed copy shall be an
original, but all of them shall represent the same agreement. One signed copy is
enough to prove this First Supplemental Indenture.
           Section 3.6 Headings. The headings of the Articles and Sections of
this First Supplemental Indenture have been inserted for convenience of
reference only, are not intended to be considered a part hereof and shall not
modify or restrict any of the terms or provisions hereof.
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           IN WITNESS WHEREOF, the parties hereto have caused this First
Supplemental Indenture to be duly executed as of the date first written above.
                                AK STEEL CORPORATION
                                By:   /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇▇▇▇▇
                                      ------------------------------------------
                                      Name:       ▇▇▇▇▇ ▇. ▇▇▇▇▇▇▇▇▇
                                      Title:      Vice President, Treasurer and
                                                  Chief Financial Officer
                                AK STEEL HOLDING CORPORATION,
                                 as Guarantor
                                By:   /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇▇▇▇▇
                                      ------------------------------------------
                                      Name:       ▇▇▇▇▇ ▇. ▇▇▇▇▇▇▇▇▇
                                      Title:      Vice President, Treasurer and
                                                  Chief Financial Officer
                                FIFTH THIRD BANK,
                                 as Trustee
                                By:    /s/ ▇▇▇▇▇▇▇ ▇. ▇▇▇▇
                                      ------------------------------------------
                                      Name:       ▇▇▇▇▇▇▇ ▇. ▇▇▇▇
                                      Title:      Assistant Vice President
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