What Is Microalloyed Steel?

Micro alloyed steel (micro alloyed steel), which adds trace alloy elements (mainly strong carbide-forming elements, such as Nb, V, Ti, Al, etc.) to the chemical composition of ordinary soft steel and ordinary high strength low alloy steel ) Steel, the addition of alloying elements is not more than 0.20%. After the addition of trace alloy elements, one or more properties of the steel are significantly changed. Typical microalloyed steels are 15MnVN and 06MnNb. Micro-alloyed steel contains one or more micro-alloyed elements, the content of which is between about 0.01% and 0.20%.

The earliest microalloying element that attracted attention was vanadium. In 1916, the United States tested mild steel with 0.12% to 0.20% vanadium. In 1934, a carbon-manganese steel containing 0.10% to 0.18% vanadium was developed. Niobium was added to steel around 1939, and in the United States in the late 1950s, an industrial test of niobium-containing semi-killed steel was carried out. The application of titanium in steel began around 1944. At that time, the United States studied low-carbon manganese copper-titanium steel plates. In 1957, the Federal Republic of Germany published performance data of rolled and normalized steel. In the early 1960s, the Federal Republic of Germany recommended 0.06 Carbon manganese steel with% to 0.15% titanium is used for manufacturing steel and steel plates. In the past, it was difficult to control the titanium content in steel from 0.01% to 0.20%. In the early 1980s, due to the development of ladle injection technology, this problem has been basically solved.

The development of microalloyed steel is closely related to the development of low alloy high strength steel. At the beginning of the 20th century, the design of steel was based on tensile strength, and less consideration was given to the toughness and weldability of steel. Therefore, the carbon content of steel was higher, about 0.3%. After welding is used instead of riveting, the carbon content in steel is reduced. During the Second World War, the brittle fracture accident of the welding tanker made people realize that the carbon content of steel should be further reduced, the manganese-carbon ratio should be higher, and the grain size should be finer to improve the toughness of steel. In order to refine the grains, the initial method was to use AlN. However, this method can only be used in the normalized state. Then, in order to refine the grains and improve the yield strength, vanadium, niobium, and titanium were added to the steel. Since 1959, micro-alloyed steel has attracted more and more attention as a versatile and inexpensive engineering material. ^[1]

In its simplest early form, microalloyed steels were ferritic-pearlite. After the 1980s, the connotation of microalloyed steel has widely included: low-alloy high-strength steel with low pearlite, no pearlite, acicular ferrite, ultra-low carbon bainite and other structures.

The smelting of microalloyed steel is similar to ordinary carbon steel, but in order to improve the yield of microalloyed elements, sufficient deoxidation of the molten steel is required. In addition, microalloyed steels that require high formability of steel must control the shape of inclusions. Add proper amount of calcium or rare earth element by spraying technology.

In general, the rolling process of microalloyed steel is similar to ordinary carbon steel, but when the microalloyed elements are solid-dissolved in austenite, their lattice is distorted to a certain degree, thereby improving the resistance to thermal deformation. Plants with more advanced rolling equipment often use controlled rolling or even controlled rolling-accelerated cooling to produce microalloyed steel, because only in this way can the role of microalloyed elements be maximized.

The carbon content of microalloyed steel is often lower than that of ordinary carbon steel and low-alloy high-strength steel, so it will have a higher work hardening rate, maximum uniform true strain and total elongation at the same yield strength. Good cold formability.

In addition, the low carbon content can also offset the damage to the weldability of steel by microalloying elements, so the weldability of microalloyed steel is better, especially for microalloyed steel with titanium, because titanium nitride particles are still quite high at high temperatures. Stable, can effectively prevent the grain coarsening of the heat affected zone, and make the weldability better.

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The main uses of microalloyed steel are as follows: (1) Structural parts on various vehicles, such as train carriage chassis, train carriages, automobile beams and bumpers, brackets for engines and steering devices. (2) Beams, plates and trusses of bridges. (3) Plates and sections for ships and production platforms. (4) Power generation equipment, such as boiler drums, coolers, economizers and superheaters. (5) Various storage tanks and heat exchangers in the petrochemical industry. (6) Various pipelines, such as oil and gas pipelines, coal pipelines, and water pipes of hydropower stations. (7) mining equipment, such as tanks, dumpers, hydraulic roof support systems. (8) Engineering machinery parts, such as the boom and bucket of the loader.

The economic benefits of applying microalloyed steel vary from product to product. According to the US calculation, if the price per ton of steel is $ 300, the cost of adding microalloying elements increases by $ 5 to $ 10, and the price of microalloy steel can reach $ 312 to $ 315, so steel mills can increase profits. At the same time, mechanical components made of this type of steel can reduce their own weight by 10% to 25%, which can save steel and energy required for transmission, and has greater social benefits.

(1) Transition from single microalloying to composite microalloying. The principle of composite microalloying is to optimize the comprehensive properties of steel by using the dissolution and precipitation behavior of vanadium, niobium, titanium and other carbides and nitrides in austenite and their influence on phase transformation. Composite microalloying will bring some new topics, such as more accurate qualitative and quantitative determination of the precipitated phase, and in the case of kinetic effects, the precipitation order of the precipitated phase should be considered.

(2) From the research of the composition of microalloyed steel to the research of the interaction of the composition and process of microalloyed steel. Because micro-alloyed carbon and nitrides will precipitate in austenite and ferrite, and deformation will change the behavior of precipitation, the cooling rate after deformation will change the amount of precipitation, it is difficult to study the interaction between ingredients and processes. After composite microalloying, the interaction between the two often needs to be predicted by a computer.

(3) Expansion from production plate to bar and forging. In the past, micro-alloyed steel was mainly used for the production of sheet metal. After the middle of the 1970s, research and application of microalloyed steel forgings and bars have made great progress, but the rapid cooling or direct quenching control methods for hot processing of bars and forgings and the cooling rate on the finished products (especially different cross sections (Forgings) performance needs to be further studied. ^[2]