Rate splitting multiple access (RSMA) is promising to achieve high spectral efficiency with a higher flexibility relative to non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA). In this paper, a novel RSMA-based secure transmission scheme with artificial noise (AN) and an adaptive beamforming is developed with the aim of maximizing the secrecy sum rate (SSR) of the considered system subject to specific constraints. The joint optimization of the power allocation between useful messages and AN signals, rate splitting between the two legitimate receivers, and the two mastering parameters of the beamforming design is tackled. To solve such a non-convex problem, we first analytically reveal some properties of the solution and then focus on an asymptotic scenario with a sufficiently large number of transmit antennas to derive closed-form expressions for the optimal power allocation coefficients. This enables us to develop an efficient method to identify the optimal rate splitting and beamforming parameters. Our examinations demonstrate that the proposed RSMA-based scheme outperforms two benchmark schemes in terms of achieving a higher SSR and the achievable performance gain is exceptional when the number of transmit antennas is small.