Boron increases the ductility of the steel in hot rolling => You have less edge cracking and other similar problems in the hot rolling mill. I am not sure about the mechanism, one text book answer is that boron atoms occupy vacancies in the grain boundaries, and this somehow makes the steel more resistant against cracking. 

It is known that boron addition (10-30 ppm) enhances the hardenability in low alloy steels. I do not know its effect on hot or cold rolling. Please see the paper which is related to boron on hot working in a low C steel: - "The hot working characteristics of a boron bearing and a conventional low carbon steel", Waldo Stumpf, Kevin Banks, Materials Science and Engineering A 418 (2006) 86–94.

You may also see: (1) Brownrigg, A., 1973, "Boron in Steel- A Literature Review 1956-1972," J. of Australian Institute of Metals, Vol. 18(3), pp. 124-136. (2) "Principles of the Heat Treatment of Plain Carbon and Low Alloy Steels", by Charlie R. Brooks, ASM International, 1996.

Please try to find:-

http://www.osti.gov/scitech/biblio/4829575

https://www.jstage.jst.go.jp/article/isijinternational1989/34/1/34_1_99/_article

http://rd.springer.com/article/10.1007%2FBF00592607?LI=true#page-1

During hot rolling, B added steels form Boro-carbides which segregates in the austenite grain interfaces and thus retards the austenite decomposition. Its effect mainly depends on the size of the Boro-carbides. If it possess coarse precipitate, it increases the ferrite nucleation rather than retarding the ferrite nucleation. (reason is because of the incoherency of the precipitate with the matrix). Hence size of the precipitate matters a lot.