From Iron Cometh Strength

Chaos Space Marines of Iron Warriors legion (formerly IVth Legiones Astartes). Generally Iron Warriors specialize in siegebreaking operations and attrition warfare. Typically they are described as stoic and near-emotionless troopers who consider maniacal laughter or other signs of Warp taint beneath them and viewing Warp as merely a weapon. While they are stereotyped as "Ranged Warfare Legion", Iron Warriors are close combatants on par with World Eaters (aka "Close Combat Legion").
Unlike most of other Chaos forces, Iron Warriors' battlecry is not a yell at the top of all three lungs; it is a monotone chant of "Iron Within, Iron Without".