Chris West has done a great service to womankind and mankind by posting a free application to his site which will allow you to save all of the worksheets of multiple Excel files as separate .csv files. See the free download here, of Batch XLS to CSV Converter. This is an html based application. Simply download it from the link on Chris's site, unzip it, and then click on the single file with a .hta file.
The application is quite easy to use. You simply browse to the folder containing your Excel files . . .
In the CSV naming schema you can designate exactly how you want the .csv files to be named. The default setting will show the name of the workbook followed by the worksheet name.
As you can see the application gives you the option to only include specific worksheets. The application may not function with .xlsx files.
This application will be particularly helpful if you use it to prepare Excel files to be processed with the macro which will merge .csv files that was profiled on March 7, 2017.