Methods of melting metal
Converter melting of stainless steel is carried out with bottom air blast (Bessemer and Thomas methods), as well as with oxygen blowing from above and below. Oxygen in the air oxidizes the impurities, causing the cast iron to turn into steel. The heat released during oxidation heats the melt to a temperature of 1600 °C.
1. Bessemer method
Converter acid lining designed by an English engineer. Bessemer in the middle of the 19th century, makes it possible to melt cast iron enriched with silicon with a minimum content of phosphorus and sulfur. The lining of the Bessemer converter is sour — dinas brick. Cast iron (0.7−1.2% Si) heated to 1250−1300 °C is poured into the converter and blown with air. Oxidation of silicon, carbon and manganese occurs, and acidic slag is formed from oxides. After that, the purge is completed. The metal is poured into the ladle, while deoxidizing it. Because the slag is acidic, sulfur and phosphorus are not removed.
2. Thomas method
The Thomas method was proposed by S. Thomas at the end of the 19th century for the processing of cast iron enriched with phosphorus. Internal. The lining of the Thomas converter is the main one (resin-dolomite). An air box is attached to the converter casing from below. The blast through the tuyeres (nozzles) enters the converter. Lime is loaded into the converter, which forms the main slag. Then cast iron is poured (0.3 — 0.5% Si, 1.6 — 2.0% P; <0.08% S;), heated to 1250 ° C and blown with air. Silicon, carbon and manganese are oxidized a. sulfur and phosphorus are removed into the resulting slag. The purge is completed when the phosphorus content in the metal decreases by 30 times.
These methods have a common disadvantage — the saturation of steel with nitrogen, which enters the melt from the air during metal blowing. Therefore, Thomas and Bessemer and steel are more brittle and prone to aging. Therefore, at present, these methods have been superseded by BOF melting with top and bottom blowing.
3. Oxygen converter
A converter with a main lining and oxygen purge through a water-cooled tuyere is used. The following charge materials are used:
- liquid pig iron;
- scrap metal;
- slag-forming (lime, feldspar, iron ore, bauxites).
Before melting, scrap metal (scrap) is loaded into the converter and cast iron heated to 1300−1400°C is poured. After that, a water-cooled tuyere is introduced into the converter and oxygen is supplied through it. Lime, bauxite, and iron ore are poured into the converter with the start of blowing. Oxygen causes circulation and mixing of metal with slag, oxidizes manganese, silicon, carbon. Intense oxidation heats the mixture. Phosphorus binds FeO and CaO If phosphorus is more than 0.15%, to remove it, it is necessary to drain the slag and introduce a new one. The removal of sulfur into the slag takes place during the entire melting. The oxygen supply is terminated when the carbon content in the metal corresponds to the specified value. After that, deoxidizers and alloying additives are introduced into the ladle. The duration of melting in a converter with a capacity of 50 — 350 tons is less than an hour.
4. Melting in a bottom blown converter.
Steel scrap is loaded into the converter and liquid iron is poured. During pouring, the converter lies almost horizontally, so that liquid iron does not flood the lances, through which nitrogen or air is blown at this time, powdered lime is blown in, sometimes with the addition of fluorspar. Then turn on oxygen, and the converter is transferred to a vertical position. Purging oxidizes silicon, carbon, manganese. A slag is formed into which phosphorus and sulfur pass. The purge is completed at a given carbon content in the metal. Decarburization proceeds more intensively due to the mixing of the bath and more complete assimilation of oxygen.
Production of stainless and heat-resistant steel by remelting. Oxygen smelting
Today, the production of austenitic stainless and heat resistant steels is based on the oxygen purge remelting method. This ensures optimal dissolution of the alloying elements contained in the mixture. It should be taken into account that in arc furnaces, melting without oxidation leads to carburization of the metal by electrodes, which requires a minimum initial carbon content. This limits the use of high-alloyed wastes and forces the inclusion of low-carbon and phosphorus- and sulfur-free soft iron into the charge composition, which increases the cost of the final product.
Oxygen smelting eliminates the use of high-chromium waste, since with a similar chromium and carbon affinity for oxygen, it is difficult to oxidize carbon without simultaneously oxidizing chromium. But the affinity for oxygen of carbon and chromium changes differently with temperature: with increasing temperature, the affinity of chromium for oxygen decreases, and the affinity of carbon increases. Therefore, the higher the temperature, the more actively carbon is oxidized and removed with furnace gas at a constant chromium content.
The role of oxygen
The use of gaseous oxygen makes it possible to effectively raise the temperature of the metal and oxidize carbon. It is known from practice that for the usual melting temperature of steel it is possible to achieve 0.1% carbon content only if there is less than 3% chromium in the metal. But if t° 1800 °C, then the target carbon concentration is achieved with a chromium content of 15%. Oxidation of carbon with gaseous oxygen at an elevated temperature makes it possible to achieve its optimal concentration in the chemical composition of steel and use an unlimited amount of high-chromium waste in filling. An increased carbon content in the charge does not prevent the production of low-carbon metal. And the chromium content in the charge will determine only the percentage of carbon in the finished steel and the melting temperature at the end of blowing.
Filling filling. Chrome, nickel, molybdenum
To obtain 13−14% Cr, the charge can contain up to 70% steel waste. To protect the chromium from oxidation and increase the bath temperature more quickly during blowing, it is important that the charge contains 0.7−0.9% Si. The missing amount of silicon is introduced in the form of ferrosilicon, silicochrome, or waste silicon steel grades. The missing part of the molybdenum is included in the filling in the form of ferromolybdenum. And the required amount of nickel is introduced into the filling in the form of nickel alloys, metallic nickel, or nickel oxide.
silicon and phosphorus
Transformer steel contains 3−4% Si, so it is desirable to introduce 20−25% waste of transformer steel into the charge. This introduces the required amount of silicon into the filling, which makes it possible to refuse the use of more expensive silicon alloys. In addition, transformer steel contains very little phosphorus, which guarantees an optimal phosphorus composition. In the absence of transformer steel waste, it is necessary to introduce waste of low-phosphorus carbon steels into the filling to dilute phosphorus.
Carbon
For good degassing of the metal during boiling, the charge must contribute at least 0.3% carbon. If necessary, an electrode blast or coke is used for carburizing. In addition, lime is added to the hearth (0.5−1.0%) and sometimes to reduce chromium waste — 0.5% chromium ore
Purging
Blowing the bath with oxygen begins after melting ¾ of the charge. This speeds up melting and reduces power consumption. Earlier purge is associated with a large waste of chromium. The purge is carried out on the included furnace to accelerate the heating of the bath. Due to the heat of the exothermic oxidation reactions of silicon, chromium and the temperature of the bath increases rapidly. At a certain temperature, the oxidation of carbon begins — a curling flame appears from the furnace. After that, the furnace is turned off, a sample is taken for chemical analysis, and the bath is continued to be blown until the required carbon content is obtained.
Temperature
The temperature of the bath can exceed 1800 °C during blowdown. This high temperature destroys the durability of the furnace lining. Therefore, after the end of the purge, the bath is quickly deoxidized with ferromanganese and the calculated amount of red-hot ferrochrome is added. For faster cooling of the metal, sometimes up to 5% of pure waste steel of the grade being smelted is additionally added.
Slag
By the end of blowing, the slag usually contains up to 30% chromium and manganese oxides, up to 20% silicon, iron and calcium oxides, up to 10% manganese and aluminum oxides. The high content of refractory chromium oxides makes the slag heterogeneous. To restore chromium from slag, it is treated with silicochromium or ferrosilicon powder. Deoxidation with silicochromium is preferable, since the steel is alloyed with chromium. If additional lime is not given during deoxidation into slag, then acidic slag with a pH of 0.6−0.8 will form in the furnace. The accumulation of silicon oxide in the slag slows down the reduction of chromium by silicon.
Oxygen
You can bind oxygen in chromium oxides with lime additives. Ca contributes to the reduction of chromium by silicon. But lime additives increase the amount of slag, which, even with a low content of chromium oxides in slag, can cause large losses of chromium with slag. Therefore, it is not advisable for the pH of the slag to be more than 1.5−1.6.
Test results
After smelting and metal processing, various types of mechanical tests are carried out to determine the quality of the resulting product and its compliance with the requirements of domestic and international standards. Thus, the results of tensile testing of metals make it possible to determine their elastic limit. This parameter refers to the mechanical characteristics and is a very important element in the calculations for the construction of structures of various types of complexity. The scope of its application depends on how high the tensile strength of the metal is.
Hardness
Another type of control of the resulting steel is the methods of testing metals for hardness by pressing the «indenter». The results obtained during the study are attributed to the physical characteristics of steel and, as a rule, Rockwell or Brinell scales are used.
Rental Information
In addition, there are chemical tests of metals designed to determine the composition of the alloy in terms of the quantity and quality of the elements present. As well as cyclic tests of metals based on the application of different loads to establish their endurance. Additionally, general information about metal rolling suggests that, in addition to the above methods for studying the characteristics of smelted metal, tests are used in production for:
impact strength;
deep drawing;
·compression;
·creep;
fracture.
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