Along with wet-rice agriculture, bronze, and iron were all introduced almost at the same time – at the beginning of the Yayoi Period (iron was introduced just slightly earlier than bronze).
Bronze and iron tools and weapons in the early days were either imported from the Korean continent in finished form or cast from imported pig iron. Finished metal products or the raw materials were brought over in ships to the Japanese archipelago – metalworking skills and techniques were advanced — clearly not an indigenous innovation.
The earliest bronze swords and spearheads had probably been introduced by the 1st century B.C. An arrowhead from the 2nd century or 3rd century B.C. was discovered at Imakura, Fukushima prefecture … the oldest bronze artefact in Japan.
Bronze casting required highly trained specialists and production centers had to be supervised to control quality and distribution of bronze products. Iron, on the other hand, could be made in backyard furnaces by just about anybody with a little training.
So bronze and iron came to have these patterns of use: Bronze objects were upper-class symbols of status as well as weapons of war; Iron implements were lower-class tools for manual labour and farming.
Bronze objects included bronze arrowheads and other weapons, Chinese and northeast Asian mirrors, dagger-swords, spearheads, halberds, sword ornaments, shuriken -shaped ornaments for shields, bracelets, coins, vessels and dotaku and bataku horse bells.
But within the next hundred years, the Yayoi people learned how to produce their own products locally. The first products were of good quality not technically inferior to those found in Korea so it is believed that immigrants were directly involved at first. Sandstone molds for weapons and bracelets have been found at several sites in North Kyushu. As the locals got better in their techniques, their forms changed too. Swords, daggers and halberds became longer, wider and thinner.
Early bronze bells came from Korea, locally made in Japan ones were initially poorly made with many technical flaws. By the 2nd century, the quality of local bronze products had improved and by the 2nd century and were able to make good bells with pictorial or geometric patterns in sharp, linear relief. By the third century, the bells became thin-walled, taller and slenderer.
Bell foundries were located in the Osaka-Nara area (evidenced by a mold found in Ibaraki city in Osaka prefecture) within 30 kilometres of a sandstone source. Stone and clay fragments of molds for small bells have been found in Fukuoka and Saga prefectures.
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Further reading:
Iron implements and agricultural tools
Heritage of Japan 
japan had been importing iron ore from china since around 600 ad , because of the lack of high quality ore ( sand iron they were used to )