Function ExcelToCsv ($File) {
    $Excel = New-Object -ComObject Excel.Application
    $wb = $Excel.Workbooks.Open($File)

    $x = $File | Select-Object Directory, BaseName
    $n = [System.IO.Path]::Combine($x.Directory, (($x.BaseName, 'csv') -join "."))

    foreach ($ws in $wb.Worksheets) {
        $ws.SaveAs($n, 6)
    }
    $Excel.Quit()
}

Get-ChildItem C:\Junk\*.xlsx |
    ForEach-Object{
        ExcelToCsv -File $_
    }