Steel was a material that had been in use for many centuries. Even in the 21st century, steel continued to be used for certain purposes.
Humans once made tools out of stone. It is for that reason that there is a period of time called the Stone Age. Around 5000 BC, humans learned how to extract metals from rocks. The first was copper. There was a problem. Copper could not hold an edge. These tools had to be pounded. Around 3600 BC, copper was alloyed with tin to make bronze. Because bronze could hold an edge, it was useful for weapons. The most well-known Bronze Age civilizations were Egypt and Mesopotamia. Many different empires existed in these areas over the course of history. Toward the end of the Bronze Age, a civilization in Anatolia, the Hittites, became the first civilization to use iron-based weapons. They were so well-known in the Fertile Crescent that the Hittites were even mentioned in the Bible. Iron became the most common material in weapons throughout the Classical Age. After the Middle Ages, it became the most common material in its carbonized form of steel. In the 1850s, the Bessemer process was invented making steel cheaper. Controlling heat made steel stronger. It became used in about everything in a city. Even buildings. Steel would continue to be use for years.
Tech Level: 7-10
When 9/11 occurred, the fires that resulted in the Twin Towers were so hot it caused the steel to break resulting in the towers collapsing. Even with 9/11, One World Trade Center was still constructed with steel. All skyscrapers after 9/11 were still constructed with steel. Even Burj Khalifa in Dubai was built with steel. Steel was still being used in skyscraper construction after 9/11. With mini steel mills being used instead of conventional ones, steel could be reused for anything. However, as lighter and stronger materials replaced it in the 21st century, steel began to be used less and less. This did not diminish its use in any way. Steel would continue to be used for years.