Were there any Chinese expeditions to explore the world? If not, why not?

Laslo Montgomery's excellent podcast The China History Podcast has loads on foreign relations even a 3 part series on Zheng He

He traveled to parts of Western Asia and Africa starting in 1405. Unfortunately there is no transcript of the podcast but it goes into detail as to his travels. You can also search google for Zheng He and there is loads to be found on him.

Well, firstly, the initial global-scale contact with China over the oceans was by the Arabs, not the Europeans. That's why many countries in southeast Asia are Muslim and have Arabic ruling dynasties. When the Europeans finally arrived that had to fight the Arabs first to establish bases.

Long-range exploration is very expensive and the kind of ships required need to very tough and resilient. Both European and Arab ships used ribbed construction and wrought iron nails and bolts. This gave them the strength to sail long distances in rough seas. Chinese junks were constructed with wood-only mortise-and-tenon technology which was not strong enough for extended ocean journeys. The Romans used mortise and tenon technology also and were unable to undertake long ocean voyages for exactly the same reason.

A secondary issue, neglected even by experts, is the role of steel in ship maintenance. An important innovation by both Arabs and Age-of-Exploration sailors was the use of a ship's carpenter. Armed with steel tools and wrought iron fittings (such as butterflies, bands and nails) a ship carpenter can repair many of the common breakdowns a ship will experience during a long voyage. More primitive Roman and Chinese expeditions would have to return to a civilized port to make these sorts of repairs. Having advanced on-board tooling was an important factor in allowing ships to stay at sea thousands of mile from home and keep sailing.

I recommend reading Ian Morris' book Why The West Rules - For Now. He discusses this topic in a few chapters.

Although Ming Dynasty China had ships which could cross the Pacific and sail around the entire world, the government ministers chose not to. (The Ming Emperor was 12 years old at the time, so the government mandarins would have been making the decisions, similar to the constitutional monarchies of today although there was no elected Parliament). Sending ships across the world would have put massive amounts of men and money at risk, and what is the upside? All the resources the country needed were either in the territory it controlled or could be acquired through overland trade routes. Sending expeditions to faraway lands was a high risk, low return venture, to borrow an investment phrase. Finally, consider that crossing the Pacific is way longer than crossing the Atlantic.

Contrast this to the Atlantic facing nations of Western Europe. Resources were more contested, given the fact that Europe of the time was fragmented into many nations and they were at war with each other for most of the time. A western European nation state was in control of a smaller amount of land than the imperial Chinese dynasties and could not muster the same resources. Therefore, sending ships across the world had a much higher potential for a big payday. They had to do it, there was no choice. Even living in Britain today, go to the grocery store and look at how much stuff on the shelves comes from other countries. Also, see my earlier comment regarding how the Atlantic is a lot smaller than the Pacific.

Western Europe was the ideal place to sail around the world, it is in the centre of the half of the earth which has the most land. See this Wikipedia article: https://en.wikipedia.org/wiki/Land_and_water_hemispheres

Finally some food for thought: Ian Morris mentions that the monarchies in Britain and the Netherlands were weaker than the Spanish one. This meant that the businessmen and merchants in northwest Europe had more of an incentive to finance overseas trade whereas Spain used seafaring trade voyages to enrich the government.