For strength, plasticity and toughness demand higher machine parts, the need to cast steel. The output of steel castings after cast iron castings, accounts for about 15% of production.
In accordance with chemical composition cast steel can be divided into carbon cast steel and alloy steel casting two kinds big. One of the most widely used in carbon steel casting, accounting for more than 80% of the total output cast steel.
1, carbon steel casting
In general, the melting point of low carbon steel ZG15 higher, casting performance is poor, only used in the manufacture of electrical parts or carburized parts; Medium-carbon steel ZG25 ~ ZG45, is higher than the comprehensive performance of all kinds of cast iron, namely high strength, good plasticity and toughness, suitable for complex shape, so manufacturing requirement for high strength and toughness of parts, such as the train wheels, forging and anvil does a, the roll and high pressure valve and so on, is the most application of carbon steel casting a; ZG55 high carbon steel, low melting point of the casting performance is good, but the medium-carbon steel plasticity and toughness is bad, only a minority of the used in the manufacture of wear parts.
2, alloy cast steel
According to how much of the total amount of alloy element, alloy cast steel can be divided into two low and high alloy steel categories.
1) low alloy steel casting, our country main application of silicon manganese manganese, departments and chromium department, etc. If ZG40Mn, ZG30MnSi1, ZG30Cr1MnSi1, etc. Used to make gear, hydraulic press work cylinder and spare parts, and turbine rotor ZG40Cr1 commonly used to manufacture high strength and high strength axis gear and other important force parts.
2) high alloy cast steel, has wear-resisting, heat resistant or corrosion resistance of special performance. Such as high manganese steel, is a ZGMn13 anti-wear steel, is mainly used to produce in the dry friction work conditions, such as the use of parts of the front wall grab excavators and grab tooth, tractors and tanks; the crawler Chromium nickel and chromium stainless steel ZG1Cr18Ni9 stainless steel ZG1Cr13 and ZGCr28, etc, to nitric acid corrosion resistance is very high, is mainly used to produce chemical, petroleum, chemical fiber and food equipment parts.
Second, cast steel casting process characteristics
The mechanical properties of the cast steel than cast iron, but its high performance than cast iron casting but poor. Because the melting point of cast steel, steel liquid high easy oxidation of molten steel, poor liquidity, shrinkage, the body for big shrinkage 10 to 14% contraction, line for 1.8 ~ 2.5%. To prevent water shortage, cold steel castings produced every, shrinkage and shrinkage, crack and adhering sand, defects, must take than iron complex process measures:
1, because of liquid steel poor liquidity, to prevent cold steel castings produced every and water shortage, cast steel wall thickness not less than 8 mm; Pouring system structure is simple, and to the section size than iron; The dry molded or hot molded; Increasing pouring temperature, generally for 1520 ¡ã ~ 1600 ¡æ, because pouring temperature of molten steel, high heat, remain liquid is long, liquidity can be improved. But the high temperature, can cause of coarse grains, thermal cracks, porosity and adhering sand and other defects. So generally small, thin wall and complex shape of casting, its pouring temperature is about the melting point of steel temperature + 150 ¡æ; Large, thick wall casting pouring temperature higher than 100 ¡æ than its melting point to the left.
2, because the contraction of cast steel cast iron, considerably more than the casting appear to prevent shrinkage and defects, shrinkage in casting process on most used take the mouth and, cold iron and allowance measures, so as to realize the solidification order.
In addition, in order to prevent shrinkage and steel castings produced shrinkage, porosity and crack defects, should make its wall thickness, avoid sharp edges and right structure, in casting on sand with sawdust, add in cores and add coke, the empty heart cores and oil sand core sand mould etc to improve or cores of air permeability and concessions.
(1) the melting point of cast steel high, the corresponding its pouring temperature is high. High temperatures and a molten steel type materials interaction, easily to have adhering sand defects. Therefore, should adopt high refractoriness of artificial quartz sand casting type, and do in molded surface by quartz powder brush or zirconium sand powder system of coating. To reduce gas source, improve liquidity and cast steel type, mostly with dry strength steel castings type or a fast drying, to casting, if use CO2 hardening of the sodium silicate sand.
Three, cast steel heat treatment
Cast steel shall use after heat treatment. Because of as-cast steel castings exist blow hole, crack, shrinkage and shrinkage, coarse grains, organization and uneven residual stress and other casting defect, make the strength of the steel castings and, particularly, plasticity and toughness is reduced greatly.
For refined grain, homogeneous tissue and eliminate stress, cast steel must be is fire or annealing processing. Is the fire of steel, with its mechanical performance is high, after annealing cost is low, so used more often. But because the fire will cause a treatment is big, the internal stress of annealing only applicable to less than 0.35% of the carbon steel castings. Because of low carbon steel castings, plastic good to cracking cooling. To reduce the stress, steel castings are fire in the high temperature should be firing back. The carbon content of 0.35% for quartile, complicated structure and easy to crack cast steel, can only carry on the annealing processing. Cast steel quenching, otherwise not easily craze.
Four, cast steel melting
The general use of cast steel smelting steel mill, arc furnace and induction furnace, etc. The characteristics of an open-hearth furnace is large capacity, available as raw materials, can accurate to scrap the composition and can control steel smelting high-quality steel and low alloy steel, more for smelting high quality requirements, large steel castings with liquid steel.
Electric arc furnace and furnace of three-phase boiler stop convenient operation, can guarantee the composition of liquid steel and quality of the requirements of the burden, not very strict, easy to warm up, and so can refine high-quality steel, alloy steel and senior, is the production of special steel, steel castings molding equipment used.
In addition, the use of industrial frequency or intermediate frequency induction furnace, melting all kinds of advanced alloy steel and can have very low levels of carbon steel. Induction furnace melting speed, the burning of the alloy element of small, energy consumption, and the liquid steel high quality, that is less impurity content, the mingled less, and is suitable for small cast steel workshop used.