The firm, formerly British Steel, has appointed Carmargue to the £100,000 account.
Corus marketing and branding manager Nick Forwood said the Biss Lancaster relationship 'came to a natural end'.
He insisted the incumbent had been 'completely aware' of a pitch for the business and Biss Lancaster was one of four agencies to contest the account.
However, Biss Lancaster corporate and B2B MD Belinda Taylor said she had 'been unaware that was the situation'.
A source close to Biss Lancaster said Corus had difficulty providing a business plan for its 14 construction divisions. 'The only way you can be proactive is by understanding the near and medium-term goals the business has,' the source added.
Forwood conceded the plethora of Corus's construction businesses could make planning PR difficult but said Carmargue had until October to establish a media strategy.