Steel Partners Bids \$700 Mln for Gencorp  CHICAGO (Reuters) - U.S. investment fund Steel Partners II  on Thursday offered to buy 90 percent of Gencorp Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=GY.N target=/stocks/quickinfo/fullquote"&gt;GY.N&lt;/A&gt; it  does not already own for about \$700 million, driving shares to  highs not seen in more than five years.