ClosedXML

using(var wb = new XLWorkbook()) {
    var ws = wb.Worksheets.Add("MySheet");
    // セル結合
    ws.Range(1,1,1,10).Merge();
    ws.Cell(1,1).Value = "ClosedXML Test";
    // 文字配置
    ws.Cell(1,1).Style.Alignment.SetHorizontal(XLAlignmentHorizontalValues.Center);

    for(int i=1; i <= 10; i++) {
        for(int j=1; j <= 10; j++) {
            ws.Cell(i+2,j).Value = i*j;
        }
        var xrng = ws.Range(i+2,1,i+2,10);
        if ((i%2) == 0) {
            // セル背景色
            xrng.Style.Fill.SetPatternType(XLFillPatternValues.Solid);
            xrng.Style.Fill.SetBackgroundColor(XLColor.AirForceBlue);
        }
    }

    for(int i=0; i < 10; i++) {
        // Excel関数
        char c = (char)('A'+i);
        ws.Cell(13,i+1).SetFormulaA1($"=SUM({c}3:{c}12)");
    }

    // 罫線
    var rng = ws.Range(3,1,13,10);
    rng.Style.Border.SetOutsideBorder(XLBorderStyleValues.Thin);
    rng.Style.Border.SetInsideBorder(XLBorderStyleValues.Thin);

    // 幅自動調整
    ws.Columns(1,10).AdjustToContents();

	// 保存
    wb.SaveAs(filename);
}

public async Task<IActionResult> OnPostAsync() {
	・・・
	// ダウンロード
	using(MemoryStream stm = new MemoryStream()) {
	    wb.SaveAs(stm);
	    await stm.FlushAsync();
	    byte[] buf = stm.ToArray();
	    return File(buf,"application/octet-stream",FileName);
	}
}