Steel cord includes a first group and a second group. The second group is helically twisted around the first group with a cord twisting step. The first group includes a first number of first steel filaments. The first number ranges between three and eight. The second group comprises a second number of second steel filaments. The second number is equal to or greater than the first number. The first filaments having a twist step greater than 300 mm. At least one of the second filaments is polygonally performed in order to allow rubber penetration.