Many machines around us are made of steel. However, this does not mean that the metallic element iron is used. All kinds of steel materials we see everyday are not pure iron with 100% purity, but smelted after adding various alloy elements. Of course, the most used steel material is carbon steel added with an appropriate amount of carbon, which is generally expressed as steel. Today, let’s talk about how “steel” and “iron” are made.
Steelmaking begins with ironmaking. The raw material of iron and steel is iron ore, Including hematite (Fe2O3) and magnetite (Fe3O4). Either way, it exists in the form of iron oxide. For this reason, coke needs to be added as heat source and reducing agent in the smelting process. At the same time, limestone needs to be added to remove impurities, so that the three can be burned in the furnace for smelting. It becomes pig iron. This smelting process is carried out in the blast furnace. The blast furnace has a cylindrical shell made of steel , the lining is made of refractory materials, such as fire-resistant bricks. The height of large blast furnace can reach more than 100 meters. Ventilation holes and opening and closing devices are set at the top of the blast furnace to eliminate the gas in the furnace, and environmental protection measures are taken to prevent the emission of toxic gas.
In addition, there is a raw material input port called hopper at the upper part of the blast furnace, and an externally installed inclined elevator lifts raw materials such as iron ore to the raw material input port.
Pig iron smelted in blast furnace cannot be used directly because it is very hard and brittle. Therefore, it needs to be transported to the converter for oxygen blowing smelting in the following process, or sent to the electric furnace to react with scrap steel, and then made into steel through the steelmaking process.
After the steelmaking process is completed, the molten steel flows into the mold to make ingots, or into the continuous casting and rolling process to roll into plates or rods. Ingots are semi-finished products that need to be reheated to make plates or bars. On the other hand, because continuous casting and rolling process is a manufacturing process that effectively uses the waste heat of heated materials, it has been rapidly popularized from the perspective of improving production efficiency and saving energy.
Carbon steel is an alloy of iron and carbon. The carbon content of this steel is usually between 0.02% and 2%. Carbon steel usually contains small amounts of elements such as silicon, manganese, phosphorus and sulfur. The functions of each element are as follows: carbon can improve the strength and hardness of metal materials, silicon can remove oxygen from molten steel, and manganese can remove sulfur.
In addition, phosphorus and sulfur are impure substances, and the content in steel should be as low as possible. For this purpose, the content limits of phosphorus and sulfur are specified. In addition, due to different carbon content, carbon steel can be divided into low carbon steel, medium carbon steel and high carbon steel; Due to different hardness, it can be divided into very mild steel, low carbon steel and hard steel.
Carbon steel with a carbon content of 0.02% or less is generally referred to as pure iron. When it comes to the mechanical properties of pure iron, it is superior in terms of electromagnetic properties. It is often used to make cores and armatures for DC appliances and motor components.
JIS standard specifies the use of different steel materials and the main terms related to quality. Here, we introduce several typical steel materials.
Rolled steel for general structures is often used as steel for general structures, such as buildings, bridges, ships, railway vehicles and iron towers. Rolled steel for general structure includes steel plate, strip steel, flat steel, strip steel and section steel. According to the English name of structural steel, these materials are called SS materials. Four varieties SS330, SS400, ss490 and ss540 are specified in JIS standard.
The number 400 in SS400 is the lower limit to ensure the tensile strength of the material, which means that the minimum tensile strength of SS400 is not less than 400N/mm².
The structural materials made into mechanical parts not only support static loads, but also, like gears and shafts, are often used in occasions where they are subjected to long-term load and movement. The typical material used for mechanical parts is carbon steel for mechanical structure. The number marked between S and C indicates the carbon content. For example, the material S45C means that the carbon content in the metal is 0.45%.
Compared with SS material, S-C material stipulates the chemical composition of the material and is a more reliable material. Secondly, S-C material is a material suitable for finishing, suitable for forging, cutting, stamping and other processing and heat treatment in the manufacturing process.
For reference, the chemical composition (unit: %) of S45C is introduced as follows.
In addition, heat treatment is an operation method to heat and cool the steel at an appropriate temperature to improve the mechanical properties of the steel. There are types of heat treatment such as quenching, tempering, annealing and normalizing. Using these heat treatment methods can improve the hardness, strength and toughness of steel.
In order to improve the performance of steel, in addition to the five main elements, steel containing one or more than two elements is called alloy steel. The main alloy components include chromium to improve metal quenching performance and corrosion resistance, molybdenum, vanadium, and tungsten to improve friction resistance.
Compared with SS materials, structural alloy steels with improved tensile strength and flexibility are called alloy steels for mechanical structure, and they are classified into strong and tough steels and high-strength steels.
There are chromium steel, chromium-molybdenum steel, nickel-chromium steel, nickel-chromium-molybdenum steel, manganese steel, manganese-chromium steel, etc., no matter which kind of steel has improved tensile strength and toughness through heat treatment. In addition, the alloy steel that guarantees the quenching performance is called H steel (the Japanese alloy steel grade is followed by H), which is a steel grade that guarantees the upper, lower or range of the hardness of the material at a certain distance from the quenching end. High-toughness steel is widely used as a manufacturing material for parts such as gears, bolts, nuts, and axles.
High-strength steel is an alloy steel that has added nickel, silicon, and manganese in addition to carbon to strengthen its tensile strength. Sometimes omitted in English, High Tensile Steel is called high-grade steel (HTS), and the tensile strength of general high-strength steel is between 490 and 790MPa. Mainly used in construction, bridges, ships, vehicles, other structures, pressure vessels or cold-rolled steel plates for automobiles, etc., with a wide range of applications.
Stainless steel is an alloy steel containing a single element of chromium or two elements of chromium and nickel for the purpose of improving corrosion resistance and hardness. The JIS standard stipulates that the code name of stainless steel is SUS (usually SUS is read as SARS), such as SUS304 or SUS430, and the number after SUS indicates its brand.
Generally, alloy steel with a chromium content of about 10.5% or more can be called stainless steel. A thin oxide film is formed on the surface of the steel through chromium to improve the corrosion resistance of the steel.
The so-called excellent corrosion resistance and not easy to rust means that there is no need to electroplate or paint the surface of the product. 13Cr steel containing 13% chromium is used in situations where the hardness is higher than the anti-rust performance, such as bolts, nuts, knives, forks, tableware, etc. that require high hardness. 18Cr containing 18% chromium is used in situations with high requirements for rust resistance, such as cooking machinery, auto parts, chemical devices, etc. 18Cr-8Ni, which contains 18% chromium and 8% nickel, not only has good corrosion resistance, but also has good tensile strength. It is used in the manufacture of structural parts for automobiles, railways, etc., and is widely used as a building material.
Cast iron is an iron alloy with a carbon content in the range of 2.14% to 6.67%. Because it has a lower melting point than carbon steel, it is used as a casting material. In general, compared with carbon steel, cast iron has low elongation, high hardness, and high brittleness. It is not suitable for use as a structural material, but it has advantages in hardness and wear resistance.
Gray cast iron is a general cast iron without special alloying elements, usually called cast iron. It is denoted as FC in the JIS standard, and six varieties of FC100 to FC350 are specified.
Here, the number after the symbol FC is the same as the SS material, which is the lowest guaranteed value for the tensile strength of the material. Comparing this value with SS400, you will find that the tensile strength of cast iron is lower.
If you observe the section of gray cast iron with a microscope, you can see the shape of the graphite dispersed in the gray cast iron material. Because the graphite in the material is crescent-shaped, it is also called flake graphite cast iron. Another way of saying the name of gray cast iron is that its name is not only derived from the mouse gray color of cast iron, but also includes the graphite inside which looks like a mouse.
The crescent-shaped graphite in cast iron acts as a lubricant. In addition, it has good thermal conductivity and easily emits frictional heat, so gray cast iron has superior wear resistance. In addition, because gray cast iron has an excellent ability to absorb vibrations, the use of gray cast iron has always started in the manufacture of machine tool beds, and has been gradually applied to the manufacture of automobile cylinders and brake-related parts. The application range is very wide .
Observing the sharp point of the crescent-shaped graphite seen in gray cast iron, we can think that this is a defect, which is the cause of the brittleness of gray cast iron. For this reason, it is improved. Spherical graphite cast iron is to improve the shape of graphite into a spherical shape, so that it can withstand a certain degree of plastic deformation. Spherical graphite cast iron is recorded as FCD in the JIS standard, and seven varieties of FCD370 ~ FCD800 are specified. In addition, spheroidal graphite cast iron is sometimes called malleable cast iron.
Spherical graphite cast iron has lower vibration absorption capacity than gray cast iron, but because of its superior strength and toughness, it is widely used in the manufacture of auto parts, mechanical equipment parts, steel pipe joints, valves, etc.
In addition, there are alloy cast irons containing nickel, chromium, manganese, silicon and other elements in cast iron.