The chemical composition of ZG35Cr24Ni7SiN material of heat-resistant steel casting is as follows:
| Brand | C | Mn | Si | Cr | Ni | Mo | P | S |
| ZG35Cr24Ni7SiN | 0.3~0.4 | 0.8~1.5 | 1.3~2.0 | 23~25.5 | 7~8.5 | (0.2~0.28) | ≤0.04 | ≤0.03 |
ZG35Cr24Ni7SiN, a kind of heat-resistant steel for iron-based austenitic lining plate, requires a single austenite structure, high lasting strength at high temperature, no phase transformation stress during heating and cooling, no micro battery effect and high corrosion resistance.
| Brand | Minimum yield strength | Minimum tensile strength | Minimum elongation |
| ZG35Cr24Ni7SiN | 340 | 540 | 12 |
Carbon is an element that forms austenite and steel base strength. In order to stabilize austenite and improve fluidity, The carbon content should be increased to a higher level than that of stainless steel, but the high carbon content will lead to the decline of corrosion resistance, weldability and crack sensitivity, which will increase the hardenability of the lining plate. The impact resistance will decline and the lining plate is easy to crack. Generally, it is better to control the carbon content between 0.3% ~ 0.4%, which should not be reduced to less than 0.25%, because the low carbon content will reduce the strength and hardness of the matrix, The high temperature strength is low.
The amount of manganese content is the factor affecting the matrix strength, and Mn expands γ In the phase region, infinite solid solution is formed, which has a strong solid solution strengthening effect on ferrite. It is a weak carbide forming element, which enters the cementite to replace some iron atoms to form alloy cementite. Manganese sulfide with high melting point formed with sulfur can prevent thermal embrittlement caused by ferrous sulfide. The effect of improving the hardenability of steel is strong, but it has an adverse tendency to increase grain coarsening and tempering brittleness. Manganese affects the impact toughness of steel. With the increase of manganese content, the strength properties and impact toughness are improved. Manganese is an element that promotes the rapid growth of austenite dendrite. High manganese content reduces the thermal conductivity of metal and makes it easier to obtain transgranular structure. Therefore, the content of manganese should be controlled within the standard range.
The silicon content is also a factor affecting the strength of the matrix. If the silicon content is higher than 0.8%, it is considered as an alloy additive because it greatly increases the tempering resistance of castings. Si reduction γ Phase region, formation γ Phase circle; In α Iron and γ The maximum solubility in iron is 18.5% and 2.15% respectively, and no carbide is formed. Silicon can improve the hardenability and re ignition resistance of steel, and is beneficial to the comprehensive mechanical properties of steel, especially the elastic limit. It can also enhance the corrosion resistance of steel under natural conditions. When the content is high, it is unfavorable to the weldability of steel and easy to lead to cold embrittlement, and graphitization is easy to occur during tempering. If the silicon content is low, the toughness is poor. Therefore, the content of silicon should be controlled within the standard range.
The quality of oxide film is determined by the control of chromium content. Chromium forms a dense Cr2O3 film in steel prior to iron. Due to the compactness and passivation of this film, it plays a good role in isolating and protecting the oxidation of the inner layer of steel. Generally, the Cr content of steel with service temperature above 1100 ℃ shall be greater than 26% ~ 28%. Heat resistant steels such as ZG35Cr24Ni7SiN for iron-based austenitic lining plates require a single austenite structure, high endurance strength at high temperature, no phase transformation stress during heating and cooling, no micro battery effect and high corrosion resistance. Therefore, the chromium content should not be less than 23%.
Nickel is the main element to form and stabilize austenite, which is second only to carbon and nitrogen, but its content can be much higher than carbon and nitrogen, so the effect is relatively significant, but its price is expensive and must be used reasonably. Generally, the control of nickel content must comply with the requirements of forming a single austenite structure. In order to reduce the cost and price, some carbon and nitrogen can be used to replace nickel, and 1% carbon and nitrogen can replace 20 ~ 30% nickel, which can stabilize austenite. The solubility of nitrogen in steel varies greatly with the state of steel. Its solubility in liquid steel is much greater than that in solid steel. In the process of steel crystallization, excess nitrogen is easy to precipitate and form bubbles. Nitrogen forms Cr2N with chromium and MNN with manganese in steel. These are stable compounds. Therefore, the content of N must be controlled according to the content of Cr and Mn. Generally, the solubility of nitrogen in steel is: n = (Cr + Mn) / 100. If the total amount of Cr and Mn is 25%, The solubility of nitrogen in steel can reach 0.25%.
The nitrogen dissolved in the steel can stabilize the austenite phase, expand the austenite phase zone (R), prevent the austenite crystal growth at high temperature, play the role of solid solution strengthening and aging precipitation strengthening, and improve the high-temperature strength of the steel. Nitride particles dispersed and precipitated in grain boundaries and grains can inhibit the creep deformation of steel at high temperature, so it can improve the creep rupture strength of steel.
Zg35cr24ni7si steel often has residual δ Ferrite is present, δ The existence of ferrite will reduce the oxidation resistance and creep resistance of steel. In addition, when the nickel equivalent and chromium equivalent are not matched properly, it will often form part δ Ferrite. Therefore, while increasing the chromium content to improve the high-temperature oxidation performance and high-temperature corrosion resistance, the nickel content must be increased at the same time, and the contents of C, N and Mn should be increased accordingly, otherwise there will be two phases. After the nitrogen content is increased, forced solid solution measures should be taken.
In order to refine the grain and purify the grain boundary, some mixed rare earth elements (such as californium cerium ferroalloy) can be added. The rare earth elements exist between the alloy and the oxide film, so that they have good adhesion, the surface oxide film is not easy to fall off, the oxidation and decarburization of the grain boundary almost disappear, and the hot cracking tendency of the steel is also improved.
Phosphorus (P) or sulfur (s) are harmful impurities in heat-resistant steel and should be removed as far as possible. P causes serious segregation in steel, forms cold brittleness and is easy to produce cold crack. S and many elements can form sulfide, and iron can form FES. When the liquid steel solidifies, it finally precipitates along the grain boundary to form continuous or discontinuous network structure. The melting points of these sulfides are low, which is easy to cause melting and falling or thermal cracking of the lining plate during heating or cooling.