That's is called anti-aliasing, and is pixel dependant, so no matter how high 'sample' you use, if your image is not big enough, then the engine won't have enough 'pixel density' to refine your line work.
Having said that, if you are using mental ray, there are many ways to enhance your antialiasing. samples is a confusing word here, but they are not the same.
Did you increase your quality? not only the sample maximum and minimum but also the quality.
Also, you need to adjust the noise threshold accordingly, it doesn't matter how high samples you are using if the noise threshold is too bold then the sampler won't work. This is the same in Mental Ray or V Ray.
Last but not least, the easiest way to get lines more visible is to just render larger. 5K pixel images should render nicely, you can try larger if you fancy.
of course, the larger you render the more careful you need to be with your samples, do not go 10240 min if you are rendering 8K, it will just kill your machine, besides is just ridiculous.
On mental Ray, I will go default, and only increase the quality level and reducing the noise threshold as need it, then render at 5K and see how it goes.