NSMutableAttributedString *str = [[NSMutableAttributedString alloc] initWithString:@"开票说明"];
//文字范围
NSRange strRange = {0,[str length]};
//设置下划线
[str addAttribute:NSUnderlineStyleAttributeName value:[NSNumber numberWithInteger:NSUnderlineStyleSingle] range:strRange];
//设置颜色
[str addAttribute:NSForegroundColorAttributeName value:[UIColor orangeColor] range:strRange];

[kaiPiaoBtn setAttributedTitle:str forState:UIControlStateNormal];