Mittal Family Forges \$17.8 Bln Steel Deal  AMSTERDAM/NEW YORK (Reuters) - Steel magnate Lakshmi Mittal  on Monday agreed to buy International Steel Group Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=ISG.N target=/stocks/quickinfo/fullquote"&gt;ISG.N&lt;/A&gt;  for \$4.5 billion and merge it with his other assets, creating  the industry's largest player at a time when steel prices are  soaring.