Excel displays gridlines by default. If you don't want to show gridlines –
1. Go to View Tab
2. Un-check Gridlines box.
This is specific to sheet. Hence, if you want want to do for entire workbook, you will have to
1. Go to File > Options > Advanced
2. Select your workbook in "Display options for this worksheet". By Default, it would show your current worksheet.
3. Un-check "Show gridlines"
