First, in this day and age, it is good for us to be thoughtful about how we phrase things. "Liberating" people from Zoroastrianism is rude. Zoroastrianism, like most world religions, has good things and bad things. In any case, it is good to respect people's faith and heritage, especially since Zoroastrianism continues to be practiced today.
Second, describing the conquest of lands by the Muslim Empire as "liberation" can come across the wrong way.
It is true that 'Umar ibn al-Khattab ordered an invasion of the Sassanian Empire which led to the conquest of what is present-day Iran. So, it is more correct to say that he annexed Iran to the Arab-Muslim Empire, and this was one of the factors that led Iran to become a Muslim region.
That being said, military annexation of a region is not the same thing as converting a people to Islam, or even introducing them to Islam. Actual conversion to Islam in conquered lands happened over a span of time. Conversely, Islam spread in many areas, such as Indonesia, where the initial Muslim Empire had no power. There were good and bad points about the conquests of the early Muslim Empire.
Sometimes - in my experience - some of our Sunni brothers and sisters glorify those conquests, perhaps out of respect for the first 3 caliphs, without taking into consideration that they were in fact military conquests which were not dissimilar to the expansion of other empires. Like other military expansions they brought some good and and some difficulty, some people in these areas were (according to what we can glean from historical records) fine with being under Muslim rule or even welcomed it, and some resisted it. Basically it's good to take a more intermediate position, neither to say "Islam spread by the sword and those Muslims are violent!" nor to say "Muslims liberated all the other regions from their former faiths." (In fact, that comes across as quite violent.)
As for who introduced Iran to Islam, why not begin with Salman al-Farsi? And, second, what about the letter that the Prophet (S) sent to the Persian king, inviting him to Islam?