<PAGE>


                                 Exhibit 4.20
                       NINTH AMENDMENT TO RECEIVABLE AND
                         INVENTORY FINANCING AGREEMENT

     THIS NINTH AMENDMENT TO RECEIVABLE AND INVENTORY FINANCING AGREEMENT (this
"Amendment") is made and entered into as of the 31st day of July, 1999, among
SHEFFIELD STEEL CORPORATION, f/k/a HMK INDUSTRIES OF OKLAHOMA, INC., successor
by merger to SHEFFIELD STEEL CORPORATION-SAND SPRINGS, f/k/a SHEFFIELD STEEL
CORPORATION and SHEFFIELD STEEL CORPORATION - JOLIET, ("Sheffield"), WADDELL'S
REBAR FABRICATORS, INC., a Missouri corporation ("Waddell"), WELLINGTON
INDUSTRIES, INC., an Oklahoma corporation ("Wellington"; Sheffield, Waddell and
Wellington are sometimes referred to individually as a "Company" and
collectively as the "Companies"), and BANK OF AMERICA, N.A., a national banking
association, formerly NationsBank, N.A., also formerly known as NationsBank,
N.A. (South) and also formerly known as NationsBank of Georgia, N.A. (the
"Lender").

                             W I T N E S S E T H :
                             - - - - - - - - - -

     WHEREAS, heretofore, the Companies and the Lender are parties to that
certain Receivable and Inventory Financing Agreement, dated as of January 16,
1992 (hereinafter, as previously amended, the "Agreement"), pursuant to which
the Lender agreed to provide certain financial accommodations on the terms and
conditions stated therein; and

     WHEREAS, the Companies and the Lender desire to amend the Agreement as set
forth herein.

     NOW, THEREFORE, in consideration of the foregoing premises, and other good
and valuable consideration, the receipt and legal sufficiency of which are
hereby acknowledged, the parties hereto hereby agree as follows:

     1.   All capitalized terms used herein and not otherwise expressly defined
herein shall have the respective meanings given to such terms in the Agreement.

     2.   The Agreement is amended as follows:

     (a)  Add the following new definition to Section 1.1 in the appropriate
alphabetical order:

          "Fixed Charge Coverage Ratio" shall mean, as of the date of
     determination, in each case on a consolidated basis for the Companies and
     their consolidated Subsidiaries, the ratio of (a) EBITDA, minus dividends
     and other distributions with respect to the capital stock of Sheffield,
     minus taxes paid in cash, in each case for the twelve fiscal month period
     most recently ended, to (b) Interest Expense, plus payments made or
     scheduled to be made on long term debt and Capitalized Lease Obligations,
     in each case for the twelve fiscal month period most recently ended.

     (b)  Delete the definitions of "Applicable Margin" and "Loans" from Section
1.1 and replace them with the following:

          "Applicable Margin" shall mean an amount determined in accordance with
     Annex I attached hereto based upon the Fixed Charge Coverage Ratio as of
     the date of determination. Adjustments to the Applicable Margin shall be
     effective as of the first day of the calendar month after the Lender's
     receipt of the Companies' financial statements as of the last day of each
     fiscal month (commencing with the financial statements for the fiscal month
     ending on or about July 31, 1999) during each fiscal year in conformance
     with Section 8.3(a), together with the officer's certificate described in
     Section 8.3(a) setting forth the calculations necessary to determine the
     ratio referred to above. The calculations of the Applicable Margin made at
     fiscal year end shall be subject to adjustment upon the Lender's receipt of
     the Companies' audited financial statements in conformance with Section
     8.3(b). In the event the Companies fail to timely provide the financial
     statements and certificates referred to above, and without prejudice to any
     additional rights under Section 11, the maximum Applicable Margin shall
     apply to all Eurodollar Rate Loans and Prime Rate Loans until the first day
     of the calendar month after the Lender's receipt of such financial
     statements and certificates, unless the Lender has agreed to extend the
     time for delivery of such financial statements.


<PAGE>


          "Loans" shall mean the Loan or Loans in the original principal amount
     of up to US Fifty Million Dollars (US$50,000,000.00) made by the Lender to
     the Companies pursuant to and in accordance with the terms of this
     Agreement.

     (c)  Delete the first sentence of Section 2.1 and replace it with the
following:

          2.1  THE LOANS.  The Lender agrees to make loans to the Companies, and
     the Companies agree to borrow from the Lender, upon the request of any
     Company from time to time up to (i) 80% of the face value of the Companies'
     Eligible Receivables, plus (ii) 60% of the lower of the fair market value
     or cost of the Companies' Eligible Inventory, less (iii) such reserves as
     the Lender may in its absolute discretion establish at any time that an
     Event of Default exists; provided, however, that the total amount of all
     Loans outstanding at any one time under the terms of this Agreement shall
     not at any time exceed US Fifty Million Dollars (US$50,000,000.00), and the
     total amount of all Loans outstanding at any one time under this Agreement
     against Eligible Inventory shall not exceed US Thirty-Four Million Dollars
     (US$34,000,000.00).

     (d)  Delete the first sentence of Section 2.2(d) and replace it with the
following:

          (d)  Certain Fees.  As additional consideration for the credit
     facility established in Section 2.1 hereof, the Companies agree to pay the
     Lender a facility fee payable on the first day of each month equal to 0.5%
     per annum of the average unused portion of the facility, less
     $10,000,000.00, during the prior month; provided, from and after the date
     on which the amount of Obligations outstanding first exceeds
     $30,000,000.00, such facility fee will be equal to 0.5% per annum of the
     average unused portion of the facility during the prior month.

     (e)  Delete the first sentence of Section 2.6 and replace it with the
following:

          2.6  EFFECTIVE DATE AND TERMINATION.  This Agreement shall be
     effective on January 16, 1992 and shall continue in full force and effect
     until November 1, 2002, and from year to year thereafter unless terminated
     on any such anniversary date by either the Lender or Sheffield, on behalf
     of the Companies, giving to the other not less than sixty (60) days' prior
     written notice.

     (f)  Delete Section 8.13 and replace it with the following:

          8.13 MINIMUM FIXED CHARGE COVERAGE RATIO.  The Companies and their
     consolidated Subsidiaries shall maintain at all times a Fixed Charge
     Coverage Ratio, measured as of any date of determination for the 12-month
     period ending on such date, of at least 1.0 to1.

     (g)  Delete Section 9.16 and replace it with the following:

          9.16 MINIMUM AVAILABILITY.  The Companies shall not permit
     Availability to be less than the amount that corresponds to the then
     current Fixed Charge Coverage Ratio as set forth in the matrix below:

          Fixed Charge Coverage Ratio                   Minimum Availability
          ---------------------------                   --------------------
          Less than 1.20                                $5,000,000.00
          1.20 or greater, but less than 1.40           $2,500,000.00
          1.40 or greater, but less than 1.60           $1,500,000.00
          Greater than or equal to 1.60                 $1,000,000.00

     (h)  Add the following new Section 9.18:

          9.18 CAPITAL EXPENDITURES.  From and after the earlier to occur of (a)
     the date on which Availability first becomes less than $7,500,000, and (b)
     the date on which the outstanding Obligations first become greater than
     $30,000,000, the Companies shall not make or incur Capital Expenditures, in
     the aggregate, for the then current fiscal year in excess of the greater of
     (y) $4,000,000.00 and (z) (i) $8,000,000.00, multiplied by (ii) the number
     of fiscal months remaining in such fiscal year, and (iii) divided by
     twelve. From and after such fiscal year, the Companies shall not

<PAGE>


     make or incur Capital Expenditures in excess of $8,000.000.00 in the
     aggregate in any fiscal year thereafter.

     (i)  Delete the existing ANNEX I and replace it with the following:

                                    ANNEX I

                          Performance Pricing Matrix
                          --------------------------

               =====================================================
               FIXED CHARGE             Prime Rate      Eurodollar
               RATIO                    Margin          Rate Margin
               =====================================================
               1.60 or greater            0.00%          2.25%
               -----------------------------------------------------
               1.40 or greater, but       0.00%          2.50%
               less than 1.60
               -----------------------------------------------------
               1.20 or greater, but       0.00%          2.75%
               less than 1.40
               -----------------------------------------------------
               Less than 1.20             0.25%          3.00%
               =====================================================


     3.   Borrowing Base Certificate. The Companies shall deliver to the Lender
          --------------------------
a borrowing base certificate setting forth the amount of Loans available to be
borrowed in accordance with Section 2.1 of the Agreement, as amended by this
Amendment, together with such other information as the Lender may reasonably
request, for each week by the third Business Day of the following week.

     4.   Restatement of Representations and Warranties.  The Companies hereby
          ---------------------------------------------
reaffirm each and every representation and warranty heretofore made or deemed to
be made by them under or in connection with the execution and delivery of the
Agreement and the documents executed in connection therewith (including, without
limitation, those representations and warranties set forth in Section 7 of the
Agreement) as fully as though such representations and warranties had been made
on the date hereof and with specific reference to this Amendment.

     5.   No Default.  To induce the Lender to enter into this Amendment, each
          ----------
Company hereby, as of the date hereof, and after giving effect to the terms
hereof, (i) represents and warrants that there exists no Event of Default (or
any event which, with the giving of notice or the passage of time, or both,
would constitute an Event of Default), and (ii) agrees that there exists no
right of offset, defense, counterclaim, claim or objection in favor of any
Company as against the Lender arising out of or with respect to any of the
Obligations.

     6.   Effect of Amendment.  Except as expressly set forth herein, the
          -------------------
Agreement and documents executed in connection therewith shall be and remain in
full force and effect and shall constitute the legal, valid, binding and
enforceable obligations of the Companies to the Lender and each Company hereby
restates, ratifies and reaffirms each and every term and condition set forth in
the Agreement and documents executed in connection therewith effective as of the
date hereof.

     7.   Counterparts.  This Amendment may be executed in any number of
          ------------
counterparts and by different parties hereto in separate counterparts, each of
which, when so executed and delivered, shall be deemed to be an original and all
of which counterparts, taken together, shall constitute but one and the same
instrument.

     8.   Successors and Assigns.  This Amendment shall be binding upon and
          ----------------------
inure to the benefit of the successors and permitted assigns of the parties
hereto.

     9.   Section References.  Section titles and references used in this
          ------------------
Amendment shall be without substantive meaning or content of any kind whatsoever
and are not a part of the agreements among the parties hereto evidenced hereby.


<PAGE>


     10.  Costs, Expenses, Taxes and Fees.  The Companies agree to pay, on the
          -------------------------------
date hereof, a fee of $25,000 for the structuring and approval of this Amendment
(which fee shall be fully earned on such date and shall not be subject to refund
or rebate), and on demand all costs and expenses of the Lender in connection
with the preparation, execution, delivery and enforcement of this Amendment and
any other transactions contemplated hereby, including, without limitation, the
fees and out-of-pocket expenses of legal counsel to the Lender.  Such fees are
fees for services and are not, nor shall they be deemed to be, interest or a
charge for the use of money.

     11.  Further Assurances.  Each Company agrees to take such further action
          ------------------
as the Lender shall reasonably request in connection herewith to evidence the
amendments herein contained to the Agreement.

     12.  Governing Law.  This Amendment shall be governed by, and construed in
          -------------
accordance with, the laws of the State of Georgia.


<PAGE>


     IN WITNESS WHEREOF, the Companies and the Lender have caused this Amendment
to be duly executed under seal, all as of the date first above written.

                         SHEFFIELD STEEL CORPORATION, F/K/A HMK INDUSTRIES OF
                         OKLAHOMA, INC., SUCCESSOR BY MERGER TO SHEFFIELD STEEL
                         CORPORATION-SAND SPRINGS, F/K/A SHEFFIELD STEEL
                         CORPORATION and
                         SHEFFIELD STEEL CORPORATION - JOLIET


                         By: /s/ Stephen R. Johnson - Vice President and CFO
                            ----------------------------------------------------
                                                                         (Title)
                         Attest: /s/ Cheryl Kaiser
                                ------------------------------------------------

                                              [CORPORATE SEAL]


                         WADDELL'S REBAR FABRICATORS, INC.


                         By: /s/ Stephen R. Johnson - Treasurer
                            ----------------------------------------------------
                                                                         (Title)
                         Attest: /s/ Cheryl Kaiser
                                ------------------------------------------------

                                              [CORPORATE SEAL]


                         WELLINGTON INDUSTRIES, INC.


                         By: /s/ Stephen R. Johnson - Treasurer
                            ----------------------------------------------------
                                                                         (Title)
                         Attest: /s/ Cheryl Kasier
                                ------------------------------------------------

                                              [CORPORATE SEAL]


                         BANK OF AMERICA, N.A., F/K/A NATIONSBANK, N.A., F/K/A
                         NATIONSBANK, N.A. (SOUTH), F/K/A NATIONSBANK OF
                         GEORGIA, N.A.


                         By: /s/ Stuart A. Hall - Vice President
                            ----------------------------------------------------
                                                                         (Title)