<PAGE>   1





                                                              Exhibit 10.1c(1)




                     FIRST AMENDMENT TO CREDIT AGREEMENT


           THIS FIRST AMENDMENT TO CREDIT AGREEMENT (this "Amendment"), dated
as of March 15, 2001, by and among STEEL 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 certain
Arrangers entered into a Credit Agreement, dated as of May 5, 2000 (the
"Original Agreement"), pursuant to which the Lenders have agreed to extend
credit to the Borrower;

           WHEREAS, the Borrower and the Required Lenders (as defined in the
Original Agreement) desire to amend the Original Agreement to make certain
changes therein;

           WHEREAS, capitalized terms not otherwise defined herein shall have
the meanings assigned thereto in the Original 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. Amendment. Section 6.04 of the Original Agreement is
hereby amended by deleting the period at the end of paragraph (f) thereof, by
inserting a semicolon in lieu of such period and adding, at the end of Section
6.04, a new paragraph (g) to read as follows:

                               (g) the guaranty by the Borrower of not more
                    than $8,250,000 principal amount of Indebtedness of IDI
                    owing to lenders under the Credit Agreement, dated as of
                    December 31, 1997, as amended from time to time, among
                    IDI, certain Lenders and Mellon Bank, N. A., as Issuing
                    Bank and Agent.

<PAGE>   2

           Section 2.  Directions to Agent.  By execution of this Amendment,
the Required Lenders hereby direct the Agent to execute and deliver this
Amendment.

           Section 3.  Miscellaneous.  (a) This Amendment shall become
effective upon execution and delivery hereof by the Required Lenders, the
Borrower and the Agent

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

           (c) 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.

           (d) 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.


<PAGE>   3


           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.


                                        STEEL DYNAMICS, INC.



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



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


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



                                        KREDITANSTALT FUR WIEDERAUFBAU



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


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



                                        THE HUNTINGTON NATIONAL BANK



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