The definition of “ultra-high-strength steel” is changing relative to the degree of technological progress required by the times. Generally speaking, alloy steel with a yield strength above 1370 MPa (140 kgf/mm) and a tensile strength above 1620 MPa (165 kgf/mm) is called ultra-high strength steel.
According to its alloying degree and microstructure, it is divided into low alloy medium carbon martensitic strengthened ultra-high strength steel, medium alloy medium carbon secondary precipitation hardening ultra high strength steel, and high alloy medium carbon Ni-Co type ultra high strength steel. , ultra-low carbon martensitic age-hardening ultra-high-strength steel, semi-austenitic precipitation-hardening stainless steel, etc.

High alloy
High-alloy medium carbon Ni-Co (9Ni-4Co-××) type ultra-high strength steel is developed on the basis of 9% Ni type low-temperature steel with high toughness and low brittle transition temperature. The purpose of adding cobalt to 9% Ni steel is to increase the Ms (martensite transformation) temperature of the steel and reduce the residual austenite in the steel. At the same time, cobalt plays a solid solution strengthening role in nickel steel and is also obtained by adding cobalt. The self-tempering properties of steel give this type of steel excellent welding properties. Carbon plays a strengthening role in this type of steel. The steel also contains small amounts of chromium and molybdenum to produce a dispersion strengthening effect when tempered. The main brands are HP9-4-25, HP9-4-30, HP9-4-45 and modified AF1410 (0.16%C-10%Ni-14%Co-1%Mo-2%Cr-0.05%V) wait. This type of steel has high comprehensive mechanical properties. It has good stress corrosion resistance, good process performance and welding performance, and is widely used in aviation, aerospace, submarine shells and other products.

medium alloy
Medium alloy medium carbon secondary precipitation hardening ultra-high strength steel is moved from 5%Cr type mold steel. Because it has high strength, satisfactory plasticity and toughness in the high-temperature tempering state, good heat resistance, and stable structure, it is used in aircraft landing gear, rocket casings, etc. Typical steel grades are H11 and H13. Its main components are: C 0.32%–0.45%; Cr 4.75%–5.5%; Mo 1.1%–1.75%; Si 0.8%–1.2%.

low alloy
Low-alloy medium-carbon martensite-strengthened ultra-high-strength steel (MART) is developed on the basis of low-alloy quenched and tempered steel. The total alloying elements generally do not exceed 6%. The main grades include traditional nickel-chromium-molybdenum quenched and tempered steel 4340 (40CrNiMo), nickel-chromium-molybdenum-vanadium steel D6AC (45 CrNiMoV) with a carbon content of 0.45%, and chromium-manganese silicon-nickel steel (30CrMnSiNi2A) with a carbon content of 0.30%. Based on 4340 steel 300M steel (43CrNiSiMoV) developed by adding silicon (1.6%) and vanadium (0.1%), as well as nickel-free silicon-manganese-molybdenum-vanadium or silicon-manganese-chromium-molybdenum-vanadium, etc. Vacuum melting reduces the content of impurity elements in steel and improves the transverse plasticity and toughness of steel. Due to the low content of alloy elements in steel, low cost and simple production process, it is widely used in aircraft girders, landing gear, engine shafts, high-strength bolts, etc. Solid rocket motor casings and chemical high-pressure vessels, etc.