Hi Sandeep,
Just after your activity step for manager approval, create activity step type with custom task for sending a email to HR manager. use method SENDTASKDESCRIPTION of Busines object SELFITEM. SENDTASKDESCRIPTION is used for sending emails. Write appropriate work item subject (subject of your email ) and task description (email body). Bind 'Addressstring' container of your task with workflow container having email address of HR manager. Bind 'TypID' container of your task to 'U' (for internet email address) from workflow container.
Regards,
Ibrahim Ansari