Steel for military industry

Basic categories of military steel:

General term for alloy structural steel used to manufacture the main components of weapons such as guns, cannons, tanks and tactical missiles Conventional weapons steel (steels for conventional weapons) is a general term for alloy structural steel used to manufacture the main components of weapons such as guns, cannons, tanks and tactical missiles. Modern conventional weapons have the characteristics of strong firepower and good flexibility and mobility. The technical requirements for gun and cannon steel depend on their firing parameters (such as the pressure of the cannon, the initial velocity of the shell, the firing speed, the range, etc.) and the shell blasting parameters (such as the effective number of fragments, the fragmentation speed, etc.), while the steel for tanks and tactical missiles depends on the weight of the weapon and the time required to move from one place to another. The steel used to design and manufacture the main components of various conventional weapons must meet these technical requirements.

Conventional weapons steel mainly includes thick-walled large-caliber gun barrel steel, shell body steel,

homogeneous armor steel and missile engine shell steel. Thick-walled large-caliber gun barrel steel

When shooting, the gun barrel is subjected to very complex stresses. Due to the shooting parameters (such as the initial velocity of the shell can reach 500-3000m/s), the high temperature, high pressure, high speed gunpowder gas on the tube wall and the extrusion of the shell on the tube wall, high tangential stress is formed; and the surface of the barrel is subjected to alternating rapid heating (the temperature is raised to above 710℃ at a heating rate of 10℃/s) and rapid cooling (10℃/s), which causes the steel structure to undergo repeated phase transformations of austenite and martensite, forming a large organizational stress and thermal gradient. The harsh service conditions put forward high requirements on the steel used for the gun barrel, so the gun steel should have the following properties: high transverse proportional limit σp or high yield strength σ0.1, no permanent deformation during shooting; high transverse room temperature and low temperature toughness of -40℃, no brittle fracture during shooting; low crack growth rate, high number of cyclic fatigue, so as to have a long service life. In order to prevent the barrel steel from softening and expanding under shooting conditions, it should also have high high temperature strength..

In order to ensure the good comprehensive performance of steel for large-caliber gun barrels, nickel-chromium-molybdenum-vanadium series are generally adopted in alloy design in various countries around the world (see Table 1). In order to ensure the hardenability of steel and improve the low-temperature toughness of steel, a higher nickel is added, usually 3.0% to 3.5%. However, it can be seen from Table 1 that the alloy design of 155mm barrel steel in the United States is different from that of 175mm barrel steel. The carbon content of 155mm barrel steel is reduced by about 0.05%, which is conducive to the improvement of toughness; the chromium content is reduced by about 0.6%, which is conducive to the refinement of grains and the improvement of toughness; the vanadium content is increased by about 0.10%, which is not conducive to toughness. Thick-walled large-caliber gun barrel steel has high transverse proportional limit or high yield strength. And good low-temperature toughness (see Table 2). To ensure these properties, in the heat treatment process, the US 175mm barrel steel and the corresponding Chinese barrel steel both use a

Steel for shells In order to ensure that the shells have great lethality, the steel for shells usually uses high-strength, low-toughness steel. In alloy design, high carbon, high manganese, high silicon and other brittle elements are usually used to increase the number of carbides in the steel, increase the tendency of temper brittleness, and coarsen the austenite grains to ensure that the shell steel has a high fragmentation rate, thereby increasing the lethality. The chemical composition of the shell steel is shown in Table 3, and the mechanical properties are shown in Table 4. Homogeneous armor steel In war, armor steel will be impacted by various projectiles of different distances, calibers, and speeds, and bombarded by explosives. It will instantly bear huge kinetic energy, fragments, shock waves, and energy-gathering effects, causing the armor steel to undergo plastic deformation, rupture, and even partial melting or gasification under high temperature, high pressure, and high speed. Therefore, armor steel is required to have good anti-ballistic performance. The anti-ballistic performance of armor steel mainly refers to its ability to resist the penetration, impact, and collapse of projectiles. The anti-penetration ability refers to the maximum kinetic energy bullet impact speed or the ability to resist a certain standard armor-piercing projectile under the conditions of a certain armor thickness and projectile impact angle. It is generally believed that it increases with the increase of the hardness and elastic modulus of the armor material. The impact resistance refers to the ability of the armor not to crack or collapse under the high-speed impact of the projectile. It is related to the toughness and strength of the material. Therefore, armor steel is required to have good anti-penetration, anti-impact, and anti-collapse capabilities, that is, armor steel should have high strength and good toughness to improve the anti-ballistic performance of the steel. In the process of manufacturing armored vehicles, armor steel is also required to have good cold and hot processing properties and welding properties. The chemical composition and mechanical properties of typical homogeneous armor steel are listed in Tables 5 and 6.

Steel for missile engine casing In order to reduce the weight of the missile body, the casing steel is required to have a high specific strength to increase the thrust of the rocket; to prevent low stress damage, the casing steel is required to have high fracture toughness; to reduce manufacturing costs, the casing steel is required to have good processing and welding properties.

The alloy design of missile engine casing steel usually adopts medium carbon nickel, chromium, molybdenum, and vanadium series (see Table 7). Carbon can increase the strength of steel, but it is harmful to the plasticity and toughness of steel. Under the premise of ensuring strength, its content should be as low as possible, generally 0.3% to 0.4%; to ensure the hardenability of steel, appropriate amounts of nickel, chromium and molybdenum should be added; because chromium can generate Cr7C3 carbide in steel, the temperature of maximum resistance (or secondary hardening temperature) is reduced. To prevent softening, the chromium content in steel should be limited. Mechanical properties are shown in Table 8. As can be seen from Table 8, during the heat treatment process, medium-temperature tempering can be used for steels with high silicon and molybdenum content, and low-temperature tempering can be used for steels with low silicon and molybdenum content. Smelting process

The steels used to manufacture the main components of modern conventional weapons are basically high-strength steels or ultra-high-strength steels. Both types of steel are prone to low-temperature (-40°C) embrittlement and notch embrittlement. Improving the purity of steel, that is, reducing the sulfur and phosphorus content in steel, and reducing the non-metallic inclusions and gas content in steel, is an effective way to reduce these two embrittlement tendencies and improve the toughness of steel. The development of structural material metallurgical technology has always been advancing in solving the contradiction of "high strength and low toughness". The emergence of special metallurgy has adapted to this requirement. The steel used for the barrel of the US 175mm cannon is smelted using a vacuum carbon deoxidation process.

In the 1950s, the alkaline open-hearth double-linked method was used to smelt cannon barrels; in the 1970s, the electroslag remelting method was used to smelt howitzer barrels; in the mid-1980s, the alkaline electric furnace plus powder spraying treatment plus electroslag remelting method was used to smelt cannon barrels; tank barrels were smelted using an alkaline electric furnace plus powder spraying treatment plus vacuum degassing process. Ultra-high strength steel is usually smelted by a double vacuum smelting method of a vacuum induction furnace plus a vacuum white consumption furnace. Homogeneous armor steel has recently been smelted using an electric furnace plus a ladle refining furnace, so that the oxygen content in the steel reaches 20×10, the hydrogen content is 1.5×10, and the nitrogen content is 56×10. Development trend With the development of modern metallurgical technology, conventional weapon steel has developed rapidly. The service conditions of thick-walled large-caliber artillery barrel steel are very harsh. It must withstand high speed, high temperature, high pressure, rapid cooling and heating, repeated alternating loads, etc. Therefore, the barrel steel must have a high proportional limit Lang or high yield strength and good impact toughness. To achieve this goal, thick-walled large-caliber artillery barrel steel is developing in the direction of pursuing good comprehensive performance. There are two main developments. One is represented by the United States. The development of high-strength barrel steel, the highest yield strength σ0.1 has reached 1240~1310MPa, but the impact toughness of this steel is relatively low, only 8.3J; the second is represented by China, which attaches importance to the development of steel with good strength and toughness matching, and appropriately reduces the yield strength to 980~1120MPa. By reducing the carbon content in the steel and increasing the nickel content, the toughness of the barrel steel is improved, good comprehensive performance is obtained, and the service life of the barrel steel is extended. The latest development of homogeneous armor steel is mainly reflected in the following aspects. In terms of alloy design ideas, countries around the world generally adopt multi-element and small-amount alloy design methods to develop medium- and low-carbon low-alloy armor steels, which not only improves the process performance of steel, but also reduces the production and processing costs of steel; in terms of steel smelting technology, high-purification smelting technology is generally adopted to significantly reduce the sulfur and phosphorus in steel, such as refining outside the furnace, jet metallurgy and electroslag remelting, among which electroslag remelting is the most widely used. The armor steel smelted by the United States and the former Soviet Union using electroslag remelting can improve its ability to resist armor-piercing projectiles by 30% to 40%.

And it has been included in the military standards of their respective countries; in terms of rolling technology, controlled rolling and controlled cooling technology are generally used to produce armor steel, which further reduces the welding carbon equivalent of steel, improves the welding processability of steel, and reduces production costs; in terms of armor types, composite armor is generally used. As an important component of composite armor, homogeneous armor steel is developing in the direction of medium and thin plates, generally with a thickness of less than 35mm and a maximum thickness of no more than 50mm. In order to improve the protection level of armor, the United States has successfully developed depleted uranium composite armor, which has been used on the US M1A1 main battle tank. It has a high level of protection, with an armor penetration greater than 600mm and an armor penetration greater than 130mm, becoming one of the most advanced composite armors in the world. Therefore, actively carrying out research on depleted uranium armor materials is another new direction for the development of armor steel. The steel used for artillery shells is currently developing towards high strength (tensile strength greater than 1000-1400MPa), high fragmentation rate and low toughness to continuously improve the lethality and killing area of ​​artillery shells. The low toughness and high brittleness of projectile steel can greatly increase the effective fragmentation rate, thereby increasing the lethality. However, the low toughness of projectile steel is also limited, and at least it must meet the requirements of the projectile in manufacturing, transportation, storage and safe use. Therefore, the reasonable matching of high strength, high fragmentation rate and low toughness of projectile steel is also one of the key research directions.