Hello,

Try this should remove the border:

this.UltraChart1.Border.Color = Color.Transparent;

this.UltraChart1.CompositeChart.ChartAreas[0].Border.Color = Color.Transparent;

Please keep in mind , UltraChart is no longer supported , please migrate to UltraDataChart. Here is the help document for the reference:

https://es.infragistics.com/help/winforms/datachart-datachart