<PAGE>   1
                                                                EXHIBIT 10.1b(7)

                     SEVENTH AMENDMENT TO CREDIT AGREEMENT


       THIS SEVENTH AMENDMENT TO CREDIT AGREEMENT (this "Amendment"), dated as
of March 15, 2001, by and among IRON DYNAMICS, INC., an Indiana corporation (the
"Borrower"), the lenders listed on the signature pages hereof and MELLON BANK,
N.A., a national banking association, as agent for the Lenders under the Credit
Agreement referred to below (the "Agent").

                                    RECITALS:

         WHEREAS the Borrower, certain Lenders, the Agent and Mellon Bank, N.A.,
as Issuing Bank, entered into a Credit Agreement, dated as of December 31, 1997,
as amended by the Amendment and Waiver, dated as of June 10, 1998, the Second
Amendment to Credit Agreement, dated as of March 15, 1999, the Third Amendment
and Waiver to Credit Agreement, dated as of June 30, 1999, the Fourth Amendment
to Credit Agreement, dated as of December 21, 1999, the Fifth Amendment and
Waiver to Credit Agreement, dated as of March 29, 2000 and the Sixth Amendment
to Credit Agreement, dated as of November 30, 2000 (as so amended, the "Credit
Agreement"), pursuant to which the Lenders have agreed to extend credit to the
Borrower;

         WHEREAS, the Borrower has requested the Lenders to effect certain
amendments to the Credit Agreement and the Lenders are willing to do so to the
extent provided herein;

         WHEREAS, capitalized terms not otherwise defined herein shall have the
meanings assigned thereto in the Credit Agreement.

         NOW, THEREFORE, the parties hereto, in consideration of their mutual
covenants and agreements hereinafter set forth and intending to be legally bound
hereby agree as follows:

         SECTION 1. AMENDMENTS TO CREDIT AGREEMENT.

         1.01.    Amendments with respect to Final Acceptance. It is the
intention of the parties that the Credit Agreement be amended so that the
"Preliminary Acceptance" and "Final Acceptance" refer to the same set of events
and conditions, even though both terms will continue to appear in the Credit
Agreement, and that certain other changes be made with respect to final
acceptance of the Project. Accordingly, the following amendments are made:





<PAGE>   2

         (a)      The definitions of the terms "Final Acceptance", "Final
Acceptance Date", "Preliminary Acceptance", "Preliminary Acceptance Date" and
"Project Monitor" appearing in Section 1.01 of the Credit Agreement are amended
to read, respectively, as follows:

                           "Final Acceptance" shall mean the achievement by the
                  Project of the production of DRI pellets from the rotary
                  hearth furnace at the rate of 55 tonnes per hour for a period
                  of thirty consecutive days, the production of hot briquetted
                  iron from the briquetters (using DRI pellets from the rotary
                  hearth furnace) at the rate of 55 tonnes per hour for a period
                  of three consecutive days and the production of liquid pig
                  iron from the submerged arc furnace at the rate of 51 tonnes
                  per hour for a period of thirty consecutive days, in each case
                  of a quality which meets the requirements of Schedule B to the
                  Offtake Agreement (as amended as contemplated by the Seventh
                  Amendment to this Agreement) and which are the subject of a
                  certificate, in form reasonably satisfactory to the Agent,
                  from an authorized officer of SDI stating (i) that such
                  quality meets such requirements, (ii) that the Project has
                  been Completed in a good and workmanlike manner and (iii) that
                  all requirements for or conditions to Final Acceptance have
                  occurred.

                           "Final Acceptance Date" shall mean the earliest date
                  on which Final Acceptance shall have occurred and construction
                  of the Project shall have been Completed.

                           "Preliminary Acceptance" shall mean Final Acceptance.

                           "Preliminary Acceptance Date" shall mean the Final
                  Acceptance Date.

                           "Project Monitor" shall mean an engineering,
                  construction or consulting firm appointed by the Agent or the
                  Required Lenders to act as Project Monitor hereunder (and the
                  Agent or the Required Lenders shall have the right, in their
                  respective discretion (subject, in the case of the Agent, to
                  any contrary direction of the Required Lenders) to so appoint
                  such a firm). If no such firm has been so appointed and is
                  serving as Project


<PAGE>   3


                  Monitor at the time of the satisfaction of the other
                  requirements listed in the definition of the term "Complete",
                  the condition of delivery of a certificate of the Project
                  Monitor included in the definition of the term "Complete"
                  shall be deemed to be satisfied notwithstanding the failure of
                  such report to be delivered. If no firm has been so appointed
                  at the time the Borrower is required to provide a certificate
                  or report of the Project Monitor under Section 5.01(p) hereof,
                  such requirement shall be deemed to have been waived. If no
                  Project Monitor has been so appointed at a time relevant to
                  determinations under Section 6.19 hereof, references in such
                  Section to the Project Monitor shall be deemed to be
                  references to the Required Lenders.


         (b)      Section 3.26 of the Credit Agreement is hereby amended by
deleting the phrase "in order to achieve the Preliminary Acceptance Date on or
before March 31, 2001" appearing therein and inserting in lieu thereof the
phrase "in order to achieve the Final Acceptance Date on or before December 31,
2002".

         (c)      Section 5.01(j)(viii) of the Credit Agreement is hereby
amended by deleting the phrase "in order to achieve the Preliminary Acceptance
Date by March 31, 2001 or the Final Acceptance Date by September 30, 2001"
appearing therein and inserting in lieu thereof the phrase "in order to achieve
the Final Acceptance Date by December 31, 2002".

         (d)      Section 5.14(a) of the Credit Agreement is hereby amended by
deleting the second sentence thereof and inserting in lieu thereof as a new
second sentence the following:

                  The Borrower (i) will cause the Project to be designed and
                  constructed in a workmanlike manner and to industry standards
                  so as to produce direct reduced iron, hot briquetted iron and
                  pig iron which meet the quality requirements of the SDI
                  Offtake Agreement, with designed capacities at least as great
                  as those contemplated by the projections provided by the
                  Borrower to the Lenders most recently prior to March 15, 2001,
                  (ii) will cause the Final Acceptance Date to occur on or
                  before December 31, 2002, (iii) will cause the Project to be
                  Completed, free and clear of all Liens (and claims of Liens)
                  for material


<PAGE>   4


                  supplied or work performed in connection therewith, on or
                  before December 31, 2002, and will provide to the Agent on the
                  Final Acceptance Date a certificate to such effect signed by a
                  Responsible Officer of the Borrower and (iv) will assure that
                  on December 31, 2001, the Project is free and clear of all
                  Liens (and claims of Liens) for material supplied or work
                  performed in connection therewith and will provide to the
                  Agent on such date a certificate to such effect signed by a
                  Responsible Officer of the Borrower.

         (e)      Section 6.19 of the Credit Agreement is hereby amended by
deleting the phrase "do not involve any delay in the Preliminary Acceptance Date
beyond March 31, 2001 or the Final Acceptance Date beyond September 30, 2001"
appearing therein and inserting in lieu thereof the phrase "do not involve any
delay in the Final Acceptance Date beyond December 31, 2002."

         (f)      Section 7.01(q) of the Credit Agreement is hereby amended to
read as follows:

                           (q) The Final Acceptance Date does not occur on or
                  before December 31, 2002.

         (g)      Section 7.01(v) of the Credit Agreement is hereby amended by
deleting the phrase "that the Preliminary Acceptance Date will occur on or
before March 31, 2001 or that the Final Acceptance Date will occur on or before
September 30, 2001" appearing therein and inserting in lieu thereof the phrase
"that the Final Acceptance Date will occur on or before December 31, 2002".

         1.02.    Amendments with Respect to Commencement of Borrowing Base
Requirement.

         (a)      Section 2.01(a) of the Credit Agreement is hereby amended by
deleting the words "Following the Preliminary Acceptance Date" appearing at the
beginning of the second sentence thereof and inserting in lieu of such words the
words: "From and after October 31, 2001".

         (b)      Section 2.10(a) of the Credit Agreement, relating to mandatory
prepayments, is hereby amended by deleting the words "If on any date any
Borrowing Base Certificate is required to be furnished" appearing at the
beginning of the first sentence thereof and inserting in lieu of such words the
words: "If on any date after October 30, 2001 on which any Borrowing Base
Certificate is required to be furnished".


<PAGE>   5

                  1.03.    Amendments Relating to Term Loan Amortization
Schedule.

                  (a)      Section 2.03(e) of the Credit Agreement is hereby
amended to read as follows:

                                    (e)      Scheduled Amortization; Maturity.
                           Subject to Section 2.03(f) hereof, the Term Loans
                           shall be due and payable in installments on the dates
                           and in the amounts corresponding to the percentages
                           as set forth below:


<TABLE>
<CAPTION>
                                                                         Amount of Term Loans
                                    Date                                   Due and Payable
                                    ----                                   ---------------
                              <S>                                             <C>
                               May 30, 2001                                     $4,125,000
                               November 30, 2001                                $4,125,000
                               May 30, 2002                                     $4,125,000
                               November 30, 2002                                $4,125,000
                               May 30, 2003                                     $4,125,000
                               November 30, 2003                                $4,125,000
                               May 30, 2004                                     $5,500,000
                               November 30, 2004                                $5,500,000
                               May 30, 2005                                     $6,875,000
                               November 30, 2005                                $9,625,000
</TABLE>



                           To the extent not due and payable earlier, the Term
                           Loans shall be due and payable on the Term Loan
                           Maturity Date.

                  (b)      Section 2.03 of the Credit Agreement is hereby
further amended by adding at the end thereof a new subsection 2.03(f), to read
as follows:

                                    (f)      Election to Postpone 2001
                           Installments. The Borrower may elect to postpone,
                           until November 30, 2005, the scheduled due date of
                           either or both of the installments of principal of
                           the Term Loans stated in Section 2.03(e) to be due on
                           May 30, 2001 and November 30, 2001. The Borrower will
                           be deemed to have made such election with respect to
                           an installment if it fails to pay such installment on
                           its originally scheduled due date. If such election
                           is made with respect to an installment, the amount of
                           such installment shall be automatically added to the
                           amount of the final installment of principal

<PAGE>   6


                           of the Term Loans due and payable (if not earlier due
                           and payable by acceleration, mandatory prepayment or
                           otherwise) on November 30, 2005.

                  (c)      Section 1.01 of the Credit Agreement is hereby
amended by adding thereto, in appropriate alphabetical sequence, the following
definition:

                                    "SDI Limited Guaranty" shall mean the
                           Guaranty and Suretyship Agreement, dated on or about
                           the date of the Seventh Amendment to this Agreement
                           and in substantially the form set forth on Exhibit B
                           to such Amendment, executed and delivered by SDI to
                           the Lenders and the Agent.

                  (d)      Section 1.01 of the Credit Agreement is hereby
further amended by adding to the definition of the term "Loan Documents"
appearing therein, immediately after the phrase "the Notes," appearing therein,
the phrase "the SDI Limited Guaranty,".


                  1.04.    Amendment to Reporting Requirements. Section 5.01(c)
of the Credit Agreement is hereby amended by adding at the end thereof a new
sentence to read as follows:

                           Each such monthly report will be accompanied by a
                           financial and operating report for such month,
                           certified by a Responsible Officer of the Borrower,
                           in substantially the form set forth on Exhibit C to
                           the Seventh Amendment to this Agreement.


                  1.05.    Amendment to Definition of "Loan Parties".

                  (a)      The definition of the term "Loan Parties" appearing
in Section 1.01 of the Credit Agreement is hereby amended to read as follows:

                                    "Loan Parties" shall mean the Borrower, SDI
                           and each Subsidiary of the Borrower (it being
                           understood that, without the consent of the Required
                           Lenders, the Borrower may have no subsidiaries) and
                           "Loan Party" shall mean any one of them; provided,
                           however, that SDI shall not be a "Loan Party" for
                           purposes of the following provisions of this
                           Agreement: the definition of the term "Future Project
                           Agreement" in Section 1.01; Sections 2.08(b) and
                           2.10(d); Sections


<PAGE>   7


                           3.08, 3.10, 3.14, 3.15, 3.16, 3.20, 3.21 and 3.24(b);
                           Section 4.01; Article V (other than Sections 5.01(j),
                           5.01(l), 5.04 and 5.05); and Article VI. The fact
                           that certain provisions of this Agreement may refer
                           both to SDI and to "Loan Party", shall not be
                           interpreted to imply that SDI is not a "Loan Party"
                           for purposes of any particular provision of this
                           Agreement.

                  (b)      The parties intend to conform the level of
judgements, attachments and the like relating to SDI that would constitute
Events of Default under the Credit Agreement to the level existing in SDI's
credit facilities. Accordingly, each of Sections 7.01(g) and 7.01(h) of the
Credit Agreement is hereby amended by inserting therein, immediately following
the figure "$100,000" appearing therein, the parenthetical phrase "(or, in the
case of SDI, $1,000,000)".

                  1.06.    Amendments with respect to Financial Covenants.

                  (a)      The definition of the term "Financial Covenant Date"
appearing in Section 1.01 of the Credit Agreement is amended by deleting the
date "June 30, 2001" appearing therein and inserting in lieu thereof the date
"September 30, 2001".

                  (b)      The definition of the term "Adjusted Fixed Charges"
appearing in Section 1.01 of the Credit Agreement is hereby amended by deleting
the phrase "sum of (a) Interest Expense for such period and" appearing therein
and inserting in lieu thereof the following: "sum of (a) the difference between
(i) Interest Expense for such period minus (ii) interest expense attributable to
accrued but unpaid interest on Indebtedness owing to SDI contemplated by Section
6.03(h) hereof and".

                  (c)      The definition of the term "Adjusted Fixed Charges"
appearing in Section 1.01 of the Credit Agreement is hereby further amended by
adding at the end thereof the following sentence:

                           In calculating "Adjusted Fixed Charges", interest
                           accrued during a period on subordinated Indebtedness
                           of the Borrower to SDI which is paid in a subsequent
                           period shall be deemed to be interest expense for
                           such subsequent period (notwithstanding the different
                           treatment of such interest expense for purposes of
                           the last sentence of Section 6.03(h) hereof).

                  (d)      The definition of the term "Measurement Period"
appearing in Section 1.01 of the Credit Agreement is hereby


<PAGE>   8


amended by deleting the phrase "for purposes of Section 6.01," appearing therein
and inserting in lieu thereof the phrase "for purposes of Section 6.01(b),".

                  (e)      Section 6.01(b) of the Credit Agreement is hereby
amended to read as follows:

                                    (b)      Fixed Charge Coverage Ratio. As of
                           the last day of each fiscal quarter commencing with
                           the fiscal quarter ending September 30, 2001 (each
                           such last day of the fiscal quarter being called a
                           "test day"), the Fixed Charge Coverage Ratio for the
                           Measurement Period ending on such test day shall not
                           be less than 1.0 to 1. For purposes of this Section
                           6.01(b), "Measurement Period" shall mean (i) the
                           fiscal quarter ending September 30, 2001, (ii) the
                           two consecutive fiscal quarters ending December 31,
                           2001, (iii) the three consecutive fiscal quarters
                           ending March 31, 2002, (iv) the four consecutive
                           fiscal quarters ending June 30, 2002 and (v) each
                           succeeding period of four consecutive fiscal
                           quarters.

                  (f)      Section 6.01(d) of the Credit Agreement is hereby
amended to read as follows:

                                    (d)      Funded Indebtedness to EBITDA. The
                           ratio of (x) Funded Indebtedness minus the amount of
                           Funded Indebtedness of the Borrower to SDI
                           outstanding in accordance with Section 6.03(h)
                           hereof, minus the amount on deposit in the Debt
                           Service Reserve Account to (y) EBITDA for each
                           Measurement Period ending on or after December 31,
                           2002 shall be not greater than 3.0 to 1 on the last
                           day of each such Measurement Period.

                  1.07.    Amendment with respect to Dividends. Section 6.06 of
the Credit Agreement is hereby amended by deleting paragraph (a) thereof and
inserting "Intentionally Deleted" in lieu thereof. Paragraph (b) thereof remains
unchanged.

                  1.08.    Amendment with respect to Indebtedness of the
Borrower to SDI. Section 6.03(h) of the Credit Agreement is hereby amended (i)
by deleting clause (ii) of the first sentence of Section 6.03(h) and inserting,
as a new clause (ii) of such sentence, the words "(ii) no Event of Default or
Potential Default shall have occurred and be continuing or shall exist", and
(ii) by deleting the last sentence of Section 6.03(h) and


<PAGE>   9


inserting in lieu thereof, as a new last sentence, the following:

                           Any other provision hereof or of the SDI
                           Subordination Agreement or of the SDI Loan
                           Documentation referred to therein to the contrary
                           notwithstanding, no payment of interest on the
                           principal amount of such Indebtedness owing to SDI
                           under this Section 6.03(h) (i) shall be paid by the
                           Borrower at any time after the Borrower shall have
                           elected, or shall be deemed to have elected, to
                           postpone one installment (or both installments) of
                           principal of the Term Loans originally stated to be
                           due in 2001 pursuant to Section 2.03(f) hereof; (ii)
                           shall be paid by the Borrower for any day unless such
                           day (x) is January 1, 2002 or a later date, (y) is a
                           day which is later than the date on which the fee
                           described in Section 2.08(c) of this Agreement has
                           been paid and (z) is a day which occurred during a
                           completed fiscal quarter with respect to which the
                           Borrower was in compliance with the Fixed Charge
                           Coverage Ratio requirement contained in Section
                           6.01(b) hereof (such Ratio to be calculated, for
                           purposes of this Section 6.03(h) only, as if such
                           interest proposed to be paid had been paid on the
                           date for which it was accrued and not on the actual
                           proposed payment date), (iii) shall be paid by the
                           Borrower for any day in 2002 in an aggregate amount
                           (regardless of when paid) exceeding one-half of the
                           interest accruing for such day or (iv) shall be paid
                           by the Borrower if, after such payment, the amount on
                           deposit in the Debt Service Reserve Account is less
                           than $2,000,000 or if the Borrower shall not have
                           furnished a Borrowing Base Certificate as of the
                           Business Day immediately preceding the date proposed
                           for such payment or shall not have made the
                           prepayment of Loans, if any, or furnished the cash
                           collateral, if any, required by Section 2.10 hereof
                           arising from the furnishing of such Borrowing Base
                           Certificate.

                  1.09.    Amendment with respect to Change of Control. The
definition of the term "Change of Control" appearing in Section 1.01 of the
Credit Agreement is hereby amended to read as follows:

                                    "Change of Control" shall mean that at any
                           time SDI shall fail to own (beneficially and of

<PAGE>   10


                           record) 100% of the equity securities of all classes
                           of Borrower free and clear, except for liens in favor
                           of certain lenders to SDI, of any security interest,
                           lien, pledge or claim or the failure of SDI to
                           maintain effective management control of Borrower,
                           including without limitation failure to retain the
                           right to elect at least a majority of the board of
                           directors of Borrower.

                  1.10.    Amendment with respect to Debt Service Reserve
Account. Section 5.17 of the Credit Agreement is hereby amended by deleting the
figure "$4,500,000" in each of the two places it appears therein and inserting
in lieu thereof the figure "$2,000,000".

                  1.11.    Amendment with respect to Applicable Margin and
Related Amendments.

                  (a)      Clause (i) of Section 2.06(b) of the Credit Agreement
is hereby amended to read as follows:

                                    (i)      The "Applicable Margin" for every
                           day for both the Revolving Credit Loans and the Term
                           Loans, as applicable, shall be 0.25% for the Base
                           Rate Option and 2.0% for the Euro-Rate Option.

                  (b)      Section 1.01 is hereby amended by deleting the
respective definitions of the terms "Level 0 Day", "Level I Day", "Level II
Day", "Level III Day", "Level IV Day", "Level V Day" and "Replacement Revolver
Facility".

                  (c)      Section 2.01(e) of the Credit Agreement is hereby
deleted and Section 2.01(f) of the Credit Agreement is hereby renumbered as
Section 2.01(e).


                  1.12.    Amendment to Add Provision for Seventh Amendment Fee.
Section 2.08 of the Credit Agreement is hereby amended by adding thereto a new
Section 2.08(c) to read as follows:

                                    (c)      Seventh Amendment Fee. By execution
                           and delivery of the Seventh Amendment to this
                           Agreement the Lenders have earned, and the Borrower
                           agrees to pay to the Lenders an amendment fee in the
                           aggregate amount of $156,250.00, payable to the
                           Lenders ratably in accordance with the respective
                           principal amounts of Loans held by the respective
                           Lenders (or their predecessors) on March 15, 2001.
                           Such fee shall


<PAGE>   11


                           be payable on the earlier to occur of (i) March 31,
                           2003 and (ii) the day before the day on which
                           interest is first paid by the Borrower to SDI on the
                           Borrower's Indebtedness owing to SDI. No fee under
                           this Section 2.08(c) shall be payable if all Loans
                           have been indefeasibly paid in full, all Commitments
                           have been terminated and no such interest has been
                           paid to SDI, in each case prior to March 31, 2003.


         SECTION 2. CONSENT TO MODIFICATION OF TAX SHARING AGREEMENT. The
Lenders hereby consent to the modification of the Tax Sharing Agreement
described on or in the form of Exhibit D to this Amendment.

         SECTION 3. CERTAIN MATTERS WITH RESPECT TO SECTION 2.01(d). The
Borrower hereby requests the amortization of the amount outstanding under the
Revolving Credit Notes on the Revolving Credit Maturity Date as contemplated by
Section 2.01(d) of the Credit Agreement. The Lenders hereby agree that such
request shall be deemed to have been made within the time period required by
such Section.

         SECTION 4. DIRECTIONS TO AGENT. The Lenders hereby direct the Agent to
execute and deliver this Amendment.

         SECTION 5. EFFECTIVENESS. The terms of this Amendment hereof shall
become effective, as of the date of this Amendment, upon the execution and
delivery hereof by the Borrower, the Agent and the Lenders and the receipt by
the Agent of the following:

                           (i)     an amendment (in form and substance
                           satisfactory to the Agent) to the commitment letter
                           of SDI, dated December 23, 1999 and addressed to the
                           Borrower, increasing the amount of SDI's commitment
                           thereunder from $55,000,000 to $75,000,000 (with
                           appropriate adjustment to reflect the conversion of
                           $40,000,000 of such debt to equity) and extending the
                           last date for borrowing thereunder from December 30,
                           2000 to December 31, 2002, executed and delivered by
                           SDI;

                           (ii)    an amendment to the SDI Offtake Agreement
                           executed and delivered by SDI and the Borrower, in
                           the form set forth as Exhibit A to this Amendment;


<PAGE>   12


                           (iii)   a Guaranty and Suretyship Agreement executed
                           and delivered by SDI in the form set forth as Exhibit
                           B to this Amendment; and

                           (iv)    such certificates of the Borrower and SDI (as
                           to, among other things, corporation action,
                           incumbency, accuracy of representations and
                           warranties and absence of default), legal opinions
                           and other documents with respect to the transactions
                           contemplated by this Amendment as the Agent or any
                           Lender may request.



         SECTION 6. MISCELLANEOUS. (a) The Credit Agreement, as amended or
modified by this Amendment, is in all respects ratified, approved and confirmed
and shall, as so amended and modified, remain in full force and effect. From and
after the date hereof, all references to the "Agreement" in the Credit Agreement
and in the other Loan Documents shall be deemed to be references to the Credit
Agreement as amended and modified by this Amendment.

         (b)      This Amendment shall be deemed to be a contract under the laws
of the State of New York and for all purposes shall be governed by and construed
and enforced in accordance with the laws of said State.

         (c)      This Amendment may be executed in any number of counterparts
and by the different parties hereto on separate counterparts, each of which,
when so executed, shall be deemed an original, but all such counterparts shall
constitute but one and the same instrument.



         IN WITNESS WHEREOF, the parties hereto, by their officers thereunto
duly authorized, have executed and delivered this Amendment as of the date first
above written.

                                      IRON DYNAMICS, INC.



                                      By
                                        ---------------------------------------
                                      Title:


<PAGE>   13

                                      MELLON BANK, N.A., as Lender,
                                        as Issuing Bank and as Agent


                                      By
                                        ---------------------------------------
                                      Title:



                                      KREDITANSTALT FUR WIEDERAUFBAU



                                      By
                                        ---------------------------------------
                                      Title:

                                      By
                                        ---------------------------------------
                                      Title:



                                      COMERICA BANK



                                      By
                                        ---------------------------------------
                                      Title:



                                      NATIONAL CITY BANK, INDIANA



                                      By
                                        ---------------------------------------
                                      Title:



                                      LASALLE BANK NATIONAL
                                          ASSOCIATION



                                      By
                                        ---------------------------------------
                                      Title: