Cylinder block cylinder head are parts of automobiles with large size, complex shape high technical requirements. The quality of cylinder block cylinder head castings comprehensively reflects the technological level, management level personnel quality of the foundry. At present, automobile design strives to reduce unit power weight, reduce fuel consumption, improve economic efficiency. Therefore, castings such as cylinder blocks cylinder heads strive to reduce wall thickness weight under the premise of ensuring strength rigidity. At the same time, a series of requirements are put forward for this kind of castings in terms of dimensional accuracy, surface roughness, cavity cleanliness, material properties, internal quality, material uniformity dimensional stability [1]. For example, materials are required: sufficient strength, rigidity compactness; materials are required to be lightweight have the highest possible power ratio; accurate shapes high dimensional accuracy; smooth clean inner outer surfaces of castings; good processing properties.

1 Raw material quality control

The quality of raw materials depends on the content of trace elements in the raw materials. The trace elements in cast iron mainly come pig iron scrap steel. High-quality pig iron must be used for pig iron, carbon steel must be used for scrap steel, alloy steel is strictly prohibited. Due to the large amount of pig iron added, the quality of pig iron has an important impact on the quality of castings. The best pig iron in our country comes the Benxi vein. However, due to limited resources, some other ores are added to the pig iron produced now, the content of certain trace elements is much higher than before. Because the accumulation of trace elements to a certain extent will cause the cast iron to produce bad graphite structure affect the matrix structure, sometimes the effect is fatal, so it is necessary to understand the characteristics of various trace elements. The influence of trace elements can be divided into three major aspects. One is the influence on graphite, such as lead, which will seriously deteriorate the graphite morphology; the other is the influence on the matrix structure, such as vanadium will promote the production of a large amount of sorbite deteriorate the cutting performance; 3. It is the impact on inclusions, such as titanium can form titanium compound particles, which has an impact on cutting performance.

2Smelting of high-strength gray cast iron

2.1 Selection of smelting method

Due to the different ways of obtaining carbon in molten iron, it will have a fundamental impact on the shape distribution of graphite. Obtaining high-quality graphite is the most critical technology for producing high-quality gray cast iron. All technological measures should focus on this goal. The main gap between the domestically produced gray cast iron material foreign countries is the form of graphite. There are many deep-seated reasons for this gap. The most fundamental reason is the different ways of obtaining carbon in the molten iron. There are two ways to obtain carbon, one is to obtain more pig iron; the other is to obtain hot metal carburizing. These two methods have a huge impact on the performance of gray cast iron, that is, the same chemical composition of molten iron, the performance will differ by 1 to 2 grades. If the carbon in the molten iron is obtained by carburizing, the graphite has a good shape, uniform distribution, moderate size; if the carbon in the molten iron is obtained by adding more pig iron, the shape of the graphite is poor, some coarse graphite is often produced. Said pig iron heredity.

2.2 Melting temperature control

Under the same conditions, the temperature of the molten iron has a great influence on the quality. The increase in the temperature of the molten iron mainly benefits the change in the purity of the molten iron also changes the organization structure. However, it is that the higher the temperature, the better, the temperature is too high, the burning loss is also serious. The high concentration of C Si will also reduce the micro-area, which is easy to form dendritic framework special-shaped graphite. Therefore, the molten iron should retain partially supersaturated oxygen, which is beneficial. The strengthening of the inoculation effect is generally controlled at 1500～1550℃.

3 Hot metal pouring temperature cooling rate

To control the pouring temperature, on the one hand, it retains good fillability, more importantly, it improves the uniformity of the molten iron in the mold, so as to reduce the cooling rate, reduce the degree of subcooling, try to make the molten iron cooling in the mold consistent, so as to achieve The requirement of uniform structure uniform hardness is also an important measure to improve the quality coefficient of cast iron (that is, the ratio of relative strength to relative hardness). The pouring temperature of the cylinder block cylinder head parts is specified as 1380～1440℃ (rapid thermocouple temperature measurement). If the pouring temperature is too high, shrinkage increases, shrinkage shrinkage are easy to occur. If the temperature is too low, it is easy to produce defects such as inclusions pores. , Increase the unevenness of the internal structure of the casting.

4 Alloying of high-strength gray cast iron

The main function of alloying is to improve the strength hardness of the casting material. But alloying also has an effect on the morphology of graphite, many people ignore this effect. Copper is the most commonly added alloying element in the production of gray cast iron. The main reason is that copper has a low melting point, is easy to melt, has a good alloying effect, does increase the white mouth tendency of molten iron. Therefore, many people think that adding more copper will cause harm. But this view is wrong. The appropriate amount of copper added is 0.2% to 0.4%. Adding a large amount of copper, adding tin chromium at the same time is harmful to the cutting performance, it will promote a large amount of sorbite structure in the matrix structure. Titanium has a strong affinity with nitrogen carbon in hot metal, it is easy to form titanium nitride titanium carbide particles. A certain amount of titanium nitride titanium carbide particles are dispersed in the matrix, which can improve the wear resistance of the cylinder block, but titanium Excessive amount of material is unfavorable to processing performance, especially for fine boring honing, which is the most harmful, so it must be restricted. To control the titanium content, it is generally to control the titanium content at the lower limit of the process requirements, that is, 0.03% to 0.05%. Too much titanium content will cause the graphite to be too cold, it is easy to produce a large amount of E-type supercooled graphite, so the titanium content must be controlled at the lower limit, effective incubation measures should be taken to eliminate the influence of supercooled graphite. The burning loss of titanium is serious. When ferro-titanium is directly added to the molten iron, the absorption rate of titanium is unstable the composition is difficult to control, so it should be made into an intermediate alloy added to the molten iron.

5 Inoculation technology of high-strength gray cast iron

In recent years, our country has developed a variety of inoculants based on 75SiFe to meet the requirements of different types of castings, especially thin-walled high-strength gray iron castings:

(1) Inoculant containing barium. Barium has a significant effect on improving the wall thickness sensitivity of castings improving the ability to resist recession, that is, it has long-term effect.

(2) Inoculants containing rare earths. A small amount of rare earth can improve the inoculation effect, significantly reduce the tendency of white mouth, slow down the rate of decline. However, the amount added should be too large, otherwise the white mouth will increase. If it is used in combination with calcium barium, its whitening tendency will be reduced, the supercooled graphite will be reduced. It has a long-term function is a good inoculant for the production of thin-walled high-strength gray cast iron.

(3) Inoculants containing strontium. It can reduce shrinkage cavities porosity in thick sections, thin sections can significantly eliminate white mouths slow down.

(4) Inoculant containing zirconium. The addition of zirconium can improve the resistance to recession, which is beneficial to eliminate the pores in the casting reduce the tendency of white mouth.

Different inoculants have different characteristics applications. For high-strength gray cast iron parts with uneven wall thickness, such as cylinder blocks cylinder heads, traditional inoculants can no longer meet the requirements. These composite inoculants have considerable effects in overcoming section sensitivity, improving graphite types, increasing strength.

6 Conclusion

Casting is an important foundation of the manufacturing industry. Gray iron castings occupies a considerable proportion of mechanical products. Looking forward to the 21st century, the foundry industry will experience rapid development. Carrying out multidisciplinary comprehensive research actively promoting high-strength, thin-walled cast iron technology is an important part of achieving green intensive casting. Among the five major factors that have a significant impact on the quality of cast iron materials, cast iron smelting technology, raw material quality, mold cooling rate have the most fundamental impact on cast iron materials. If the gap in material is caused by these factors, it is sometimes impossible to make up for it through alloying technology inoculation technology. Therefore, in the casting production of cylinder blocks, cylinder heads other parts, it is necessary to improve perfect the control of smelting technology the quality of raw materials. At the same time, attention must be paid to the research development of new alloy elements, new inoculants inoculation technology.