We will describe a rigorous perturbative quantization of the B-twisted topological sigma-model via AKSZ-formalism on derived mapping space around constant maps. We show that the first Chern-class of the target manifold is the obstruction to the quantization, and the factorization algebra of observables on Calabi-Yau target give the expected topological correlation functions in the B-model. This is joint work with Qin Li.