Stainless steel is one of the important inventions of the 20th century. After nearly a hundred years of research and development, a series of more than 300 grades of steel has been formed. In the special steel system, stainless steel has unique properties and a wide range of applications, which can not be replaced by other special steels. Stainless steel can almost cover any other special steels.
1 The evolution of austenitic steel
In developed countries, about 70% of the stainless steel consumed each year is austenitic stainless steel. Although my country’s consumption level is not high, the consumption of austenitic stainless steel has reached about 65% of the total consumption. Therefore, to see the development trend of stainless steel grades, we must first look at the trend of austenitic stainless steel.
Early researchers have discovered that carbon is the main cause of corrosion damage to the grain boundary of austenitic stainless steel. Limited to the level of metallurgical equipment at that time, it is difficult to control carbon below 0.03%. Finally, they came up with adding Ti and Nb to steel. It preferentially reacts with carbon to generate TiC and NbC, and the method of fixing the carbon prevents the precipitation of carbon at the grain boundary to generate Cr23C6, causing intergranular corrosion. Due to the high cost of Nb, until the mid-1970s, Ti-containing stabilized steel 1Cr18Ni9Ti still dominated stainless steel.
The 1Cr18Ni9Ti molten steel is viscous, and the surface quality of the continuous casting billet is difficult to pass. With die casting, the surface quality of the steel ingot is not good, so it must be peeled and ground, and the yield rate is very low. The finished steel contains TiN inclusions, has low purity, poor surface polishing performance, and many broken wires in the drawing. By the end of the 1960s, breakthroughs had been made in stainless steel smelting technology. AOD and VOD methods were widely used to make steel, reducing the carbon in stainless steel and no longer apologizing. Qunopiao? ⒘ ship pirates and weapons to swallow their own system? Tun a few Zheng? I stabilized steel is gradually replaced by low-carbon and ultra-low-carbon steel. In the 1970s, the United States, Japan and other countries have eliminated 1Cr18Ni9Ti from the standard. Although 0Cr19Ni11Ti (321) was retained, its output only accounted for 0.7 to 1.5% of the total. The transition from titanium-containing stabilized steel to low carbon was successfully completed. And the transition of ultra-low carbon steel.
The production and application of stainless steel in my country are relatively lagging. Although the national standard GB1220-84 “Stainless Steel Rods” was promulgated in 1984, 1Cr18Ni9Ti was listed as not recommended, but the dominant position of 1Cr18Ni9Ti has not changed. Until 1995, with the development of the national economy, especially the intervention of joint ventures, the domestic market gradually integrated with the international market. In just 5-6 years, my country’s austenitic stainless steel has completed the transition from titanium-containing stabilized steel to low-carbon And the transition of ultra-low carbon steel. At present, except for a few traditional industries that still use 1Cr18Ni9Ti, 304 (0Cr19Ni9) and 316 (0Cr17Ni12Mo) have become the leading brands of stainless steel.
2 Replace carbon with nitrogen and develop nitrogen-containing stainless steel
In austenitic stainless steel, nitrogen and carbon have many common characteristics, such as increasing the stability of austenite and effectively improving the cold working strength of steel. Increasing the carbon content will reduce the intergranular corrosion resistance of stainless steel. The affinity of nitrogen and chromium is smaller than that of carbon and chromium. Austenitic steel rarely sees Cr2N precipitation. Therefore, adding a proper amount of nitrogen can improve the strength and oxidation resistance of steel without reducing the intergranular corrosion resistance of stainless steel. Substituting nitrogen for carbon and developing nitrogen-containing stainless steel has become a hot topic.