Help with For block commands--command/batch file parsing

Here is my working test:

set line=abc1234
set line2=%line:~0,3%
echo %line2%
pause

The expected output is:  abc, and it is.

Here is the goal:

FOR /F "tokens=*" %%i IN (d:\rt.txt) DO (
  set line=%%i
  set line2=%line:~0,3%
  echo %line2%
)
pause

When %%i is ==========================, the expected output is:  ===

Instead, the output is:

D:\>(
set line===========================
 set line2=~0,3
 echo
)
ECHO is on.

How do I get the expected output?

(Eventually, I am going to replace Echo with an If statement, but I want to make sure it is working first.)