InStr also supports a comparison argument, though you have to also provide the optional starting position argument if it is specified, so it doesn't end up any shorter than forcing case before the compare.
For i = 2 To 70000
If Cells(i, 3).Value = "Raw-material feed" Then
If InStr(1,Cells(i, 1), "A", vbTextCompare) > 0 Or InStr(1, Cells(i, 1), "B", vbTextCompare) > 0 Then
Cells(i, 9).Value = "AB"
End If
End If
Next i