The mark schemes for the 2015-2018 papers are available from Cambridge Assessment Admissions Testing, bundled in with the past papers (note that the "mark scheme" tab is just for the 2011 STEP 1 mark scheme!). The examiners reports are also well worth reading.

Mark schemes for 2013 and 2014 can be found here.

There is also a link to a google drive page with some miscellaneous mark schemes in the first post of the Student Room STEP Prep Thread 2019.