The basic production process of iron and steel is to first obtain iron ore and coking coal and then to convert them into pig iron in an ironmaking blast furnace; the next step is to use pig iron as raw material, and use different steelmaking furnaces to smelt steel into steel; The shape of a steel ingot or continuous cast billet is then sent to a rolling mill for rolling or forging to finally become available in various shapes of steel.
First, iron ore mining and processing
From the day of the birth of the earth, the iron deposits were very unevenly distributed throughout the world.
Iron is present in the natural state in the form of a compound, and is particularly present in the state of iron oxide. In theory, any ore containing iron or iron compounds can be called iron ore. However, industrially or commercially, iron ore not only contains iron components, but also must have a relatively high iron content before it can be used. There are two kinds of hematite and magnetite which have good smelting performance and utilization value.
Dark red or brown is hematite. Its main component is iron oxide Fe2O3 with a specific gravity of about 5.26, most of which contain Fe below 70%, and O with more than 30%. It is the most important iron ore species.
The magnetite is dark gray or metallic luster. Its main component is Fe 3 O 4 Fe3O4 with a specific gravity of about 5.15. The theoretical maximum Fe content is up to 72.4%, O is at least 27.6%, and it is magnetic. Iron ore species.
Some iron ore is buried shallow and open-pit mining is used. It is like digging a large pit for the Earth to open a skylight, and the mining cost is relatively low. Some of the deeper iron ore can only be mined in underground mine roadways, similar to the methods used to extract underground coal.
However, there are not many ores with more than 66% iron in the world. Many iron ores have low “grades” (that is, iron content), which may be between 30% and 50%. The stone composition in the ore is too high. Is not directly used for ironmaking. Therefore, in order to increase the iron content of ore raw materials, it is necessary to use mechanical equipment to remove some of the stone impurities and further enrich the iron-containing components. This is to carry out “crushing” and “concentration”, in which the beneficiation links must be heavily used.
A small-scale hematite site can process 2,000 to 3,000 tons of iron-containing ore concentrates of about 60% per day. Iron ore concentrates are in powder form, commonly known as iron ore fines, with high iron content of 62-66%.
Such smelting can not be directly put into the blast furnace, just as people only burn briquette in the heating furnace, they must be artificially “kneaded” into blocks to increase the permeability and reducibility. The internationally accepted method is secondary processing and manufacturing of “sintered ore” and “pellet ore”. These two types of “artificial block ore” increase the compressive strength at the same time, which is beneficial to blast furnace smelting.
The fuel added to the iron concentrate raw material, after high-temperature ignition, burns a lot of heat to make the mineral in the material layer melt. With the combustion layer down and the cold air passing, the molten liquid phase generated at 1000-1100°C is Upon cooling and recrystallization, solidified into a massive sintered ore having a mesh structure.
The concentrate powder mixing ingredients to “ball”, and then also on the machine roasting, its specifications are not as large as sinter. Sinters and pellets are ores that can be added to blast furnaces for smelting.
Second, coal coke
More than 95% of the world’s steel production is still using the coke ironmaking method invented by the Darby British 300 years ago. Therefore, ironmaking requires coke, mainly when it is used for fuel, and coke is also a reducing agent. Without it, it cannot. Iron is displaced from the iron oxide.
Coke is not a mineral, but it is to be refined with certain types of coal. The general ratio is 25-30% for fat coal and 30-35% for coking coal, and then it is charged in a coke oven for 12-24 hours. , forming a hard porous coke.
The appearance of coke is somewhat similar to that of coal, but its calorific value is very high. It is purer than coal, almost pure carbon, and weighs more than half as much as coal because most of the impurities are removed.
Third, blast furnace ironmaking
Blast furnace ironmaking is to iron ore and fuel (coke has a dual role, as a fuel, two as reducing agent), ingredients such as limestone, melting in the blast furnace, so that it occurs in the reduction reaction at high temperature, restore from the iron oxide Basically iron-based, part-carbon “pig iron”, that is, molten iron. The molten iron is poured into the ladle and transported to the steel plant or cast iron. If the molten iron is not sent directly to the steelmaking, it can be cast into pig iron, stored or sold on the market. However, because of its carbon content exceeding 2%, the pig iron blocks are brittle and break when they fall. Direct cast iron can be used to cast a variety of cast iron products, such as diesel crankshafts and cast iron pipes.
Four, ironmaking into steel
The fundamental difference between the nature of iron and steel is the carbon content, and carbon content below 2% is the real “steel”. The so-called “steelmaking” is to decarburize pig iron and transform it into steel during the high temperature smelting process. The commonly used steelmaking equipment is a converter or an electric furnace.
Pour hot metal and scrap into oxygen converter
The raw materials for converter steelmaking include 85% of hot metal and 10-15% of scrap steel, and then they are blown into oxygen for combustion, without adding any fuel, and relying on the physical heat of hot molten iron to transform the steel within a short time. The electric furnace steelmaking relies on external energy (electric energy) to heat and melt the scrap steel and pig iron. It does not use molten iron to make steel.
Fifth, casting billets
At present, in addition to the production of special steels and large steel castings, a small amount of cast steel ingots are also required for forging. In general, domestic and foreign mass production of ordinary steel basically abolishes the old process of casting steel ingots—blanking—rolling. The method of casting molten steel into billets and rolling them is called “continuous casting.”
If you do not wait for the billets to cool down, do not drop on the way, and go directly to the rolling mill, you can produce the steel products you need. If the billet is cooled halfway and stored on the floor, the billet can be sold as a commodity. A small number of companies are more advanced and can adopt advanced “near-end” continuous casting methods to directly cast molten steel into very thin steel strips or shaped steel billets to reduce the processing of steel rolling mills, saving energy and increasing profits.
Six, billet rolled into wood
There are many kinds of common steel products. There are various types of steels, rebars, steel plates, steel strips, steel pipes, etc., some of which are hot-worked and some are cold-worked. Therefore, equipment for rolling steel materials is used, which is called “rolling mills”. “.
We will briefly describe two examples of hot-rolled bars (round bars) and hot-rolled steel coils.
Under rolling of the rolling mill, the billets are coarser and narrower, getting closer and closer to the final diameter of the product, and are sent to a bar cooling bed for cooling. Bars are mostly used for machining mechanical structural parts.
If pattern rolls are used on the last bar finishing mill, rebar can be produced, which is called “rebar” for building structural timber.
Hot coil rolling mills are used to produce rolled steel. Hot coils are rolled out first, and then cold rolled coils with a higher degree of smoothness are further processed by cold rolling and further processed into coated coils. This kind of thin steel has no breakage in the middle, and it is very advantageous to make cuts and cuts on the steel plate as in the case of clothing, and to make complex products such as various household appliances steel plates and car shells.
Seven, forged steel
According to different needs, some steels with special properties or special shapes cannot be processed by rolling mills that produce large quantities of products, but they must be produced in small batches or even only one or two pieces. Therefore, it is necessary to cast a number of steel ingots after steelmaking, and then after heating, special-shaped special steels are formed on special processing equipment such as die forging machines and hydraulic presses. In particular, it should be noted that more than half of this type of steel is not produced in steel companies, and their production sites are large and medium-sized mechanical factories.
For example, we forged large-scale wind turbine main shafts to illustrate the production of forged steel products.
Forged blanks (steel ingots) are heated and released
Manufacture of large shafts with 4000 tons of forging hydraulic presses
We have seen how the “really refined” process has led to the birth of steel products.
Source: China Stainless Steel Pipelines Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)