Although cast carbon steel is widely used, it has many shortcomings in performance, such as poor hardenability, and thick-section steel castings cannot be strengthened by quenching and tempering; the operating temperature range is narrow, generally limited to -40~400 ℃; poor wear resistance and corrosion resistance, etc. It cannot meet the various needs of modern industry for steel castings. Therefore, in terms of engineering and structural steel castings, the development of various cast low-alloy steels has made up for the shortcomings of cast carbon steel.
Cast low alloy steel is a steel type composed of one or several alloying elements added to cast carbon steel. The total amount of alloying elements generally does not exceed 5% (mass fraction). In addition, cast low alloy steel also includes trace alloyed cast steel, in which the amount of alloying elements added generally does not exceed 0.15% (mass fraction). At present, the most widely used cast low alloy steel types are the two series of manganese series and chromium series, that is, manganese or chromium is mainly added with alloying elements, and on this basis, other alloying elements are added, such as silicon, aluminum, vanadium, Nickel, etc., to further strengthen or obtain some special performance (such as heat resistance, wear resistance), thereby forming binary, ternary or even more diversified cast low alloy steels. Compared with cast carbon steel, cast low alloy steel has better mechanical properties and usability, can reduce the quality of steel castings, and increase the service life of steel castings. In addition, cast low alloy steel has similar casting properties to cast carbon steel and is more suitable for casting production.
It should be noted that with the needs of industrial development, the quality requirements for steel castings are getting higher and higher, and the requirements for the content of harmful elements phosphorus and sulfur are also getting stricter, such as bolsters and side frames installed on railway vehicles. The cast low alloy steel standards used for steel castings have limited the mass fractions of phosphorus and sulfur to less than 0.030%. Some manufacturers have also set the mass fractions of phosphorus and sulfur in high-quality steel standards to less than 0.020%.
In recent years, cast low-alloy steel has been used to replace cast carbon steel in many mechanical products, which has greatly reduced the weight of the machine and improved the efficiency and service life of the machine. As mechanical products develop towards high performance and low consumption, low alloy steel castings will be more widely used.
The influence of alloying elements on the structure and properties of steel
(1) The influence of alloying elements on ferrite. The atoms of alloying elements are dissolved into α-ferrite by substitution. Since the atomic size and structure of these elements are different from those of iron atoms, they will appear in the crystal lattice. Internal stress changes the lattice constant. The greater the difference in atomic size between iron and alloying elements, the greater the change in lattice constant. Changes in ferrite lattice size will cause changes in ferrite properties, increasing strength and hardness and reducing toughness.
When the solubility of a certain alloying element in ferrite decreases significantly as the temperature decreases, supersaturated partial alloying elements can be precipitated at a temperature lower than the eutectoid temperature, causing a High-stress state to strengthen it.
(2) The influence of alloying elements on pearlite. Most alloying elements shift the point S on the iron-carbon phase diagram to the left, that is, the eutectoid carbon content decreases. Therefore, they can increase the pearlite content in hypoeutectoid steel, making the steel Increased strength. Since nickel and manganese also reduce the eutectoid temperatures Ac₁ and Ar₁ of steel, they can refine the pearlite and also help improve the strength of steel.
(3) Effects of alloying elements on austenite Except for manganese and boron, almost all alloying elements tend to reduce the growth of austenite grains and refine the grains. But their effects are different: non-carbide-forming elements nickel, cobalt, silicon, and copper have a relatively weak effect on grain growth; carbide-forming elements chromium, molybdenum, tungsten, vanadium, and zirconium can significantly refine grains. The reason is that the carbon (nitride) compounds of these elements have different stabilities, and the remaining carbides that cannot be dissolved into austenite can hinder the growth of austenite grains. Therefore, when there is a small amount of undissolved carbides in the steel, the steel still maintains a fine-grained structure even at very high heating temperatures.
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