with leap 15.0 installation all is ready in pam files to open gnome keyring but not with sddm.

you must only install "gnome-keyring" rpm.

you must choose same password for gnome keyring as for user

you must add ",sddm" at the end of "session optional pam_gnome_keyring.so auto_start only_if=gdm,gdm-password,lxdm,lightdm,mdm"
in /ect/pam.d/common-session-pc file

you must have

session optional pam_systemd.so
session required pam_limits.so
session required pam_unix.so try_first_pass
session optional pam_umask.so
session optional pam_gnome_keyring.so auto_start only_if=gdm,gdm-password,lxdm,lightdm,mdm,sddm
session optional pam_kwallet5.so
session optional pam_env.so

perhaps after the first use of the gnome keyring you will need to restart leap according to open automatically gnome keyring

enjoy