From Longman Dictionary of Contemporary Englishsteepensteep‧en /ˈstiːpən/ verb [intransitive, transitive] HORIZONTAL#if a slope, road etc steepens, or if something steepens it, it becomes steeper→ See Verb table
Examples from the Corpus
steepen• The valley of the White Kielder Burn steepened again as I trudged north, its curves tightening.• Eventually the rock steepens and turns blocky.• We made it to the Pastukhov rocks without a break and steered left where the slope steepened considerably.• As his dive steepened he felt the speed pick up again; there was still life in the controls, somewhere.• Killion steepened his dive and began picking his targets.• Addition of successively higher odd harmonics steepens the wings and reduces the amplitude of the fluctuations in between.• After a few seconds the attitude should be allowed to steepen up gradually and smoothly until the full climbing angle is reached.• The glider should not be steepened up or levelled out immediately after leaving the ground. 