static lv_style_t style_btn;
  lv_style_init(&style_btn);
  lv_style_set_radius(&style_btn, LV_STATE_DEFAULT, 0); //设置圆角

lv_obj_t* btn2 = lv_btn_create(lv_scr_act(), NULL); /* 创建按钮 */
  lv_obj_set_size(btn2, 100,30);

  lv_obj_set_pos(btn2, 100, 60);

lv_obj_add_style(btn2, LV_BTN_PART_MAIN, &style_btn);