The chemical composition has a great influence on the mechanical properties and microstructure of bainitic Martian multiphase wear-resistant cast steel. The depressing effect of alloying elements on bainite phase transition temperature (BS) point and martensite phase transition temperature (MS) point directly affects the morphology and properties of bainite structure. Theoretically, the lower the Min point, the more lower the amount of lower bainite in phase transition structure, and the better the strength bremsstrahlung matching of steel. At the same time, Δ ｂｓ／ Δ The larger the MS ratio, the narrower the temperature range of bainite growth and the finer the bainite structure, which plays a role similar to fine grain strengthening and improves the strong ductility of steel. In a word, the strengthening effect of alloy elements in bainitic steel is mainly manifested in reducing bainite, refining ferrite grains and solid solution strengthening. In the composition system of this study, the most important alloy elements are C, Mn, Cr and B.
Carbon is the most significant and cheapest element for interstitial solid solution strengthening, which determines the hardness and initial properties of materials. It is also an effective element for stabilizing austenite. With the increase of carbon content, the strength and hardness of steel increase, but the bremsstrahlung decreases. However, if the carbon content is too low, the hardenability and wear resistance of the steel are poor;
Carbon has become the main contradiction to be solved in the design because of its great influence on CCT diagram, strength, initial property and process performance. Carbon contributes the most to the strength (interstitial solid solution strengthening is the most effective, and the effect of carbide strengthening depends on its existence), but it has the worst effect on bremsslasticity and weldability, and it is also the most powerful element to delay the C curve and lower the MS point. The consideration of this contradiction in the design is: through alloying, carbon can not precipitate carbides in the transformation process, but mainly exists in the matrix in the form of solid solution, which can make the greatest contribution to the strength, so as to reduce the amount of carbon as much as possible on the premise of ensuring ultra-high strength, so as to obtain better initial plasticity without producing a large amount of martensite at a large cooling rate, It is even possible to make it a weldable steel.
Manganese is the strongest element to improve the hardenability of steel. To make the steel have good hardenability, manganese needs not less than 0.8%. It can not only strengthen ferrite by solid solution, but also form weak carbon compounds. It can strengthen cementite, refine pearlite clusters, reduce the distance between pearlite layers, achieve the purpose of refining as cast grains and improve the plastic toughness of steel. Manganese is cheap and plays a significant role in reducing BS point. In addition, bainitic steel can combine hot working forming process and heat treatment process in the production process, eliminating quenching process, which can not only save energy, simplify process and improve production efficiency, but also avoid deformation, cracking, oxidation, decarburization and other heat treatment defects caused by quenching, Therefore, manganese boron bainitic steel will soon enter industrial application.
Manganese strongly reduces the bainite and martensite transformation temperature of steel and promotes the bainite transformation to occur at a lower temperature, which is conducive to obtaining lower bainite, refining microstructure and improving strength. At the same time, manganese has the characteristics of delaying the transformation of high temperature zone than that of bainite. When the content of manganese is 2% – 3%, combined with boron, the bainite transformation zone can be highlighted, and the air-cooled bainitic steel can be obtained in a large cooling rate range. However, the addition of manganese also has an adverse effect, which increases the overheating sensitivity of the steel. In the case of slight overheating, the grain coarsens; In addition, temper brittleness is increased; Improper cooling after smelting and pouring is easy to cause white spots in steel. Generally, the manganese content in bainitic steel is 1.5% – 3.0%.
Chromium is a weak carbide forming element. With the increase of chromium content, a series of carbides such as (Fe, Cr) 3C, (Fe, Cr) 7C3, (Fe, Cr) 23c6 can be formed. Chromium can be dissolved in ferrite and strengthen ferrite. Due to the bi-directional strengthening of chromium (ferrite and cementite), it also contributes greatly to the mechanical properties of steel. With the increase of chromium content, the strength, plasticity and toughness increase. Chromium increases hardenability, and the addition amount is large (greater than Mn, SI). Especially when combined with Ni and Mo, steel with very good hardenability can be obtained.
When a certain amount of chromium is added to the steel, the C curve not only moves to the right, but also the pearlite transformation curve and bainite transformation curve separate and become a double C curve. At the same time, chromium, like manganese, has the characteristics of delaying the transformation of high temperature zone than that of bainite. Due to the improvement of hardenability and solid solution strengthening of chromium, it can improve the strength and hardness of steel under heat treatment. At the same time, when the content is less than 2%, it can also improve the plasticity of steel, which is the unique advantage of chromium and increase the tempering stability of steel.
The effect of boron is similar to that of manganese. A very small amount of boron (such as 0.002%) can greatly delay the high temperature transition zone and flatten the bainite zone. Most bainitic steels contain boron, but boron is an active element with chemical properties. It is not easy to control during melting, and boride is easy to form and segregate at the grain boundary, reducing the toughness of the steel.
The addition of trace boron to bainitic steel can significantly improve the hardenability and toughness. The combined action of boron and Mo makes the isothermal transformation curve from supercooled austenite to ferrite move further to the right, which makes the beginning line of bainite transformation prominent. Trace boron can improve the high temperature strength and creep properties of heat resistant steel. However, boron is an active element with chemical properties, which is not easy to control during smelting, and boride is easy to form and segregate at the grain boundary, reducing the bremsstrahlung of steel. When the boron content is greater than 0.006%, it is easy to precipitate coarse fe23 (CB) 6, which will cause boron embrittlement. Boron can significantly delay ferrite transformation and has little effect on medium temperature transformation. Polygonal ferrite transformation is very fast in medium and low carbon bainitic steel, and it is impossible to obtain the maximum amount of bainite transformation during continuous cooling. Adding about 0.002% boron to the steel can inhibit polygonal ferrite transformation rather than bainite transformation.