Modern gray cast iron smelting technology

0 Foreword

Cast iron has a long history as a structural material, because it has abrasion resistance, heat resistance, oxidation resistance, corrosion resistance, acid alkali resistance good shock absorption; compared with other alloys, it has a low melting point, good moldability, good processability. The advantage of low cost, so with the improvement of social civilization people's living standards, the application of cast iron will become wider wider.

The development of cast iron is determined by absorbing adopting advanced technology, materials control technology. Strict control of metallurgical factors, reasonable selection of cast iron components, control of metal cooling crystallization, without reducing the carbon equivalent, can produce thin-walled high-strength gray cast iron with high strength, low hardness low stress.

The output of gray cast iron still occupies the first place in iron castings. In order to promote the improvement development of its level, the author puts forward some suggestions on various aspects of controlling metallurgical factors.

1 Selection of raw materials

1.1 Multi-purpose steel scrap

Under the condition of relatively balanced use of recycle material, the amount of new iron is reduced, scrap is used for smelting. The reasons are as follows.

(1) New-born iron (especially high-carbon nascent iron) itself contains coarse graphite flakes. Although it has been remelted, the melting point of graphite cannot be completely melted during the remelting process, it becomes graphite crystalline in the crystallization. The core makes graphite become coarse becomes a weak link in the structure of cast iron. Therefore, the use of new iron is more, the probability of forming coarse graphite is higher.

(2) The amount of scrap steel used is directly related to the strength of the cast iron produced. Dongfeng No. 1 Cast Iron Factory uses a power frequency furnace to melt produce engine block castings.

(3) The use of scrap steel is a measure to increase nitrogen during melting. Nitrogen is an element that promotes pearlite. As the amount of nitrogen increases, graphite becomes shorter, thicker, bald, but excessively high content is likely to produce crack-like pores.

(4) The purity of the composition of the scrap steel is high, the non-metallic inclusions such as sulfur phosphorus are low, but there are trace alloy elements.

(5) Increasing scrap steel can reduce the amount of nascent iron weaken the influence of genetic factors in the origin of nascent iron.

(6) Using scrap steel instead of nascent iron can greatly reduce the price of raw materials.

1.2 Using steelmaking pig iron

(1) Steelmaking pig iron is cheaper than cast iron. Steel-making pig iron has low silicon content dense matrix structure.

(2) Use steel-making pig iron as the charge, add ferrosilicon to the charge add ferrosilicon before the furnace is discharged to control the silicon mass fraction with less impurity content, small white mouth, good molten iron fluidity, small shrinkage, which can produce thin-walled castings , The shrinkage is low, the leakage phenomenon is greatly reduced.

(3) Make steel-making pig iron to melt cast iron, compare under the condition of equivalent composition: the hardness difference is small, the graphite is uniformly distributed in short flakes, the crystal grains are fine, the pearlite is dispersed.

1.3 Low alloying

Many foreign engine cylinder blocks, cylinder heads, other castings have slightly increased costs, but the performance obtained, whether it is casting performance, strength, wear resistance, life, indirect benefits, will win users' favor. These alloying elements are called stabilizing elements are very convenient to add.

2 Reasonable melting operation

2.1 Selection of melting equipment

Melting technology has been improving on saving energy, improving the quality of molten iron, improving the labor environment. The current general trend is to give full play to the high efficiency of the cupola melting heat, use an induction furnace to heat up adjust the composition when conditions permit.

The cupola has developed preheating air supply developed various types of dust collection devices. The life of the furnace lining is also improving. The domestic small furnace of 4-6t/h can also operate continuously for one week. Computers are increasingly widely used in cupolas, which can monitor automatically adjust various parameters.

The cast iron is melted in a power frequency furnace. Due to the long holding time of the molten iron the strong stirring effect of the molten iron, the molten iron is seriously oxidized, reducing the graphite core, increasing the tendency of white mouth, the tendency of shrinkage, the fluidity of the molten iron to be poor. It is economical to operate the melting holding in the same furnace. Generally, units with a monthly output of 700t more use cupola B induction furnaces. With a monthly output of 100-700t units, due to the application of thyristor, the medium high frequency induction furnaces have been developed, the investment cost is reduced, the melting efficiency is significantly improved, the oxidation of molten iron is reduced, the quality of molten iron is significantly improved.

2.2 Use power frequency induction furnace to melt molten iron

The power frequency induction furnace melts the molten iron slowly (3h for a 10t furnace), the molten iron exists for a long time (the high temperature of the cupola is only a few minutes), it has the characteristics of churn, so the oxidation of the molten iron is serious. In order to avoid reduce the disadvantages as much as possible, the following points must be paid attention to in operation.

(1) Adjust the sulfur content in molten iron. Generally controlled in the range of 0.04%~0.08%. Industrial frequency furnaces bring in less sulphur melting, too low sulphur content poor inoculation effect, so pyrite is usually used to increase the sulphur content in the ingredients.

(2) Adjust the amount of carbon. The duration of melting high-temperature molten iron in a power frequency furnace is prolonged the carbon fluctuates greatly. In order to improve the graphite morphology, the carbon content of the batching should be 0.05%~0.1% higher than that of the cupola molten iron to reduce the tendency of white mouth. Graphite rods with broken waste electrodes are often used as recarburizers in operation.

(3) Reasonable feeding order. It is necessary to charge reasonably according to the melting characteristics of the industrial frequency furnace to obtain a good batching result. The order of charging should be: recarburizer! Scrap steel! Rechargeable material! Newborn iron! Ferroalloy. When the amount of nascent iron is less than 10%, it is generally added in the late melting stage, the ferroalloy is added before the furnace is released. The chemical composition of this loading sequence is small easy to control.

(4) Fast melting. When liquid iron is needed for a short period of time, solid furnace charge is used for heat preservation, it should be melted quickly when needed to minimize the loss caused by liquid iron oxidation. In order to melt quickly, try to shorten the melting period to 30min. Now it is developed to increase the power density operate with a frequency doubler furnace (180~540HZ).

When the equipment is difficult to modify, the qualification rate of ingredients should be controlled once, so that the high temperature adjustment time can be shortened.

3 operation before furnace

3.1 Control the tapping temperature

Under the same conditions, the temperature of molten iron has a great influence on the quality, its performance changes as the temperature increases.

When the temperature is low, the change in the structure of pearlite ferrite plays a leading role. When the temperature gradually increases, in addition to the change in the structure, the crystal grains gradually become finer the distance between the pearlite sheets decreases.

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. But 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 E-type graphite. To enhance the beneficial inoculation effect, it is generally appropriate to control it at 1500 degrees Celsius.

3.2 Using thermal analysis techniques

Using the thermal analysis method in front of the furnace, the carbon, silicon, carbon equivalents can be determined in less than 2 minutes, the nucleation supercooling temperature of primary austenite the cooling of the austenite growth period can be learned several inflection points of the cooling curve. The speed, the nucleation temperature of eutectic graphite eutectic cementite, the degree of subcooling of eutectic growth, the time, temperature speed of eutectic growth, can determine the change of cast iron structure the shape of graphite the change of the curve. Sometimes you can use different inoculations to adjust improve immediately, you can observe the changes in the solidification of the molten iron after the inoculation is improved.

Using thermal analysis technology, you can calmly grasp control the properties of cast iron, which is a more effective method.

3.3 Inoculation technology

Pre-furnace inoculation is an essential method for the production of high-grade cast iron. It is an important measure to reduce white mouth, promote the formation of graphite, increase the number of eutectic clusters. When the sulfur content in the molten iron is 0.04%~0.05%, 0.2%~ 0.3% 75SiFe inoculation can produce obvious inoculation effect.

Various inoculants have been developed according to the different components of the original molten iron the different properties of cast iron. Analysis the crystalline state: It is the key to reduce the cross-section sensitivity so that it can produce more nuclei during solidification

3.4 Control the pouring temperature

Control the pouring temperature, on the one hand, is to retain good fillability, more importantly, to improve 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).

If the pouring temperature is too high, shrinkage increases, shrinkage porosity are easily generated. If the temperature is too low, although the mold can be filled, defects such as inclusions pores are likely to occur, the internal structure of the casting increases unevenness.

3.5 Pay attention to the heat preservation of molten iron

In advanced industrial countries, a covered ladle is used to keep it warm, which is beneficial to slag removal the uniformity of molten iron.

It is more difficult to stamp on the bag, but we have developed a slag collector (perlite) with commodity supply, which only has good heat preservation, but also reduces pollution improves working conditions. Adhering to the use of slag collectors is only a heat preservation measure, but also a means to reduce casting inclusions pore waste.

4 Several suggestions for smelting gray cast iron

4.1 The influence control of gas on the properties of cast iron

4.1.1 Oxygen has a certain degree of solubility in cast iron, it varies with the temperature of molten iron the content of carbon silicon. However, more oxygen exists in the crystal grains as oxides becomes inclusions, which worsens the compensation in the crystallization process. Shrinkage also affects the strength of cast iron.

High temperature of molten iron, high degree of oxidation, low temperature of molten iron, insufficient self-deoxidation, fast disappearance of inoculation, easy to appear pinholes, pores, mixed graphite, reduce mechanical properties.

Some people say that inoculation can deoxidize, so cupola melting requires less inoculant than electric arc furnace melting, electric arc furnace uses less than induction furnace. The influence of oxygen on gray cast iron involves a series of factors is a subject to be studied in the future.

4.1.2 Nitrogen is a pearlite stabilizing element, but too high content can easily cause crack-like pores reduce sealing performance. There are many experimental studies on the influence mechanism of nitrogen on cast iron its control measures. Blindly treating nitrogen as an inert gas in cast iron smelting may be wrong.

4.2 Cast iron crystallization process

Nowadays, steel some non-ferrous alloys (such as zinc-aluminum alloy) can obtain superplasticity other special properties by refining the structure, by adding elements, by heat treatment. Through the study of the crystallization process of molten iron, only can we grasp the law of graphite morphology change, but also can control the crystallization process to achieve a new level of cast iron performance.

4.3 Establish the idea of quality first

The concept of quality is to meet the needs of users. The production of castings must only meet the standards required on the drawing, but also meet the requirements of the next process.