Oracle SUBSTRB equivalent in SQL SERVER

I need to convert oracle to sql server. In a procdure, I need to use the substrb function to cut a string(Japanese) by bytes and then insert it into a table. Does sql server have some equivalent function? I wrote a function that returns CAST(SUBSTRING(CAST(@L_CHAR AS VARBINARY(MAX)), @L_POSITION, @L_BYTE) AS VARCHAR(MAX)) but the processing speed seems slow. Can someone help me improve?

It will help if you can downgrade the VARBINARY(MAX) and the VARCHAR(MAX). If you know the max length of the paramater. Your performance will improve if you can use VARCHAR(256) for example.