MUMBAI: Tata Steel plans to challenge the validity of its UK unit's trade union 's call for an indefinite strike from July 8 over its move to close certain parts of the operations on the Port Talbot site.The company wants to close two blast furnaces in Port Talbot and switch to lower carbon electric arc furnaces. The closure will lead to axing of 2,800 jobs and the switch to electric arc furnaces - a proposal backed by 500 million pounds of govt money - will help Tata Steel to turn around its loss-making UK operations.