What Is High Strength Steel

Traditional high-strength steels are mostly strengthened through solid solution, precipitation and grain refinement, while advanced high-strength steels (AHSS) refer to steels that are strengthened through phase transformation. and/or retained austenite, mainly including dual phase (DP) steels, transformation induced plasticity (TRIP) steels, martensitic (M) steels, complex phase (CP) steels, hot forming (HF) steels and Twin induced plasticity (TWIP) steel.

The strength and plasticity of advanced high-strength steel is better than that of ordinary high-strength steel, and it has both high strength and good formability, especially the high work hardening index, which is conducive to improving the energy absorption during the collision process, which ensures weight reduction and at the same time. Security is very beneficial.

The strength of AHSS is between 500MPa and 1500MPa, and it has good energy absorption. It plays a very important role in reducing the weight of automobiles and improving safety. It has been widely used in the automobile industry, mainly used in automobile structural parts, safety parts and Reinforcing parts such as A/B/C pillars, door sills, front and rear bumpers, door anti-collision beams, beams, longitudinal beams, seat rails and other parts; DP steel was first mass-produced by Sweden SSAB Steel Plate Co., Ltd. in 1983.
Development and Research Progress of Advanced High Strength Steel

All high-speed steels are produced by controlling the cooling rate of the austenite phase or austenite plus ferrite phase, either by hot grinding on the peripheral surface (such as hot rolled products) or by local cooling in a continuous annealing furnace (Continuous Annealed or Hot Dip Coated Products).

Martensitic steels are produced by rapid quenching that causes most of the austenite to transform into the martensite phase. Ferritic and martensitic dual-phase steels are produced by controlling the cooling rate so that the austenite phase (as found in hot rolled steels) or the ferrite+martensite dual phase (as seen in continuous annealing and hot dip coating) steel) some of the retained austenite is transformed into ferrite before rapid cooling into martensite.

TRIP steels generally need to be maintained at moderate isothermal conditions to produce bainite. The higher silicon carbon content makes the TRIP steel contain too much retained austenite in the final microstructure. Multiphase steels also follow a similar cooling pattern, but in this case, the adjustment of the chemical elements produces very little retained austenite and forms fine precipitations to strengthen the martensite and bainite phases.

High-strength steel for automobiles is divided into hot-rolled, cold-rolled and hot-dip galvanized products, and its technological characteristics are all strengthened through phase transformation. In addition, there is a kind of ultra-high-strength steel quenched and hardened by hot stamping forming molds, which has been widely used in the automobile manufacturing industry in Europe.

As safety and fuel economy needs grow, the automotive industry is increasingly demanding high-strength, lightweight materials. Driven by the lightweight of automobiles, the proportion of aluminum alloys, magnesium alloys, plastics and other components used in automobiles has increased year by year, and the dominant position of steel in automobile materials has also been threatened. In order to improve the safety of automobiles and meet the challenges from other materials, the development of steel materials is currently focused on high-strength steels.

-1- Duplex Steel

Dual-phase steel is obtained from low-carbon steel or low-carbon microalloyed steel by heat treatment or controlled rolling and controlled cooling in the two-phase region, and its microstructure is mainly ferrite and martensite. Ordinary high-strength steels refine the grains through controlled rolling, and strengthen the matrix through the precipitation of carbonitrides of microalloying elements, while dual-phase steels are dispersed in pure ferrite grain boundaries or grains with relatively hard particles. martensitic phase, so its strength and toughness are well coordinated.

The strength of the dual-phase steel is mainly determined by the proportion of the hard martensite phase, which varies from 5 to 30 . The tensile mechanical properties are characterized by:

①The stress-strain curve is smooth and arched, and there is no yield point extension;

② has a high work hardening rate, especially the initial work hardening rate;

③ Low yield strength and high tensile strength, the formed components have high crush resistance, impact absorption energy and high fatigue strength;

④ Large uniform elongation and total elongation. Dual-phase steel is ideal for high strength and good formability for automobiles.
-2- Transformation-induced plasticity steel

The transformation-induced plasticity steel refers to the steel with multiphase structure in the steel. These phases are usually ferrite, bainite, retained austenite and martensite.
During the deformation process, the transformation of the stable retained austenite to martensite causes phase transformation strengthening and plastic growth. For this reason, the retained austenite must have sufficient stability to achieve a gradual transformation.
On the one hand, it strengthens the matrix On the other hand, it improves the uniform elongation and achieves the target of simultaneous increase in strength and plasticity. The performance range of TRIP steel is: yield strength 340-860 MPa, tensile strength 610-1080 MPa, elongation 22%-37%.

In recent years, TRIP steel has developed rapidly. TRIP steel is mainly used to make automobile fenders, chassis parts, wheel rims and door impact beams. In addition, TRIP steel sheets can be used as substrates for hot-dip galvanizing and Zn—Ni electro-galvanizing to produce galvanized sheets with high strength, high plasticity, high drawing bulging and high corrosion resistance.
South Korea’s Posco has successfully developed TRIP steels of 800MPa and 1000MPa grades.
The formability of the steel plate is very good, and it can be processed into complex shapes of automobile parts. Currently, they are working on developing TRIP steel in the 1200MPa grade. In Japan, Mitsubishi Motors has cooperated with Nippon Steel, Sumitomo Metal and Kobe Steel to develop TRIP high-strength steel sheets for automotive chassis parts. More than 80 types of TRIP steel sheets for chassis parts have been manufactured in its new models.

Many research results show that TRIP steel with high silicon content has better ductility and tensile strength than low alloy high strength steel, and its composition series are: C—Mn—Si—N—V, C—Mn—Si -Ti and Si-Nb, etc. However, high silicon content will lead to the red oxide scale on the surface of the strip and the deterioration of hot-dip galvanizing performance.
In recent years, some researchers have begun to focus on partially replacing silicon with other elements (such as aluminum, phosphorus, etc.) to reduce the silicon content in steel, improve coating properties, and improve the coating performance by adding elements such as niobium, vanadium, titanium, and molybdenum. Strength of TRIP steel.
-3- Complex phase steel

The structure of multiphase steel is similar to that of TRIP steel, and its main structure is fine ferrite and a high proportion of hard phases (martensite, bainite), containing elements such as niobium and titanium.
Through the composite action of martensite, bainite and precipitation strengthening, the strength of CP steel can reach 800~1000MPa, with high absorption energy and hole expansion performance, especially suitable for automobile door bumpers, bumpers and B Safety parts such as uprights.
Depending on alloy composition design, micro-alloying, controlled rolling and controlled cooling technology and continuous annealing technology, hot-rolled and cold-rolled high-strength strips can obtain different microstructures, such as ferrite + bainite dual-phase structure, ferrite + Martensite dual-phase structure, ferrite + bainite + retained austenite multi-phase structure and martensite structure, the strength of steel can be increased from 500MPa to more than 1000MPa, and even can reach 1200MPa.

Practice has shown that due to the higher content of microalloying elements in the steel, the deformation resistance during controlled rolling in the non-recrystallized zone increases, resulting in a larger rolling mill load. In the process of controlled rolling and controlled cooling, titanium element is very sensitive to heating temperature and coiling temperature. Fluctuations in slab heating temperature and post-rolling coiling temperature easily lead to very significant fluctuations in coil properties such as yield strength and tensile strength.

For cold-rolled high-strength structural steels, ferrite + bainite + martensite complex structures with different volume ratios can be obtained through the complex heat treatment process during the continuous annealing process.
This cold-rolled multiphase steel has good comprehensive mechanical properties, and has higher toughness and plasticity under the same strength as conventional quenched martensitic steel, so it has a broad application market in the automotive industry.
-4- Martensitic steel

The production of martensitic steel is through rapid quenching of high-temperature austenite structure to transform into lath martensite structure, which can be realized by hot rolling, cold rolling, continuous annealing or annealing after forming, and its maximum strength can reach 1600MPa, which is the current The highest strength grade of commercial high-strength steel sheets.
Therefore, when producing plate-shaped products, due to the limitation of formability, only parts with simple shapes can be produced by roll forming or stamping, which are mainly used for parts such as door bumpers with low forming requirements to replace tubular parts. manufacturing cost.

Hot stamping steel (MnB steel) is a Nippon Steel method that achieves high formability and extremely high strength by quenching after hot forming. The specific hot forming method is: heating the steel plate (880–950°C), stamping (quenching treatment in the die of the stamping machine), shot blasting (removing iron oxide scale), and finished product (1500MPa).
The entire hot stamping process takes 15 to 25 seconds.
In order to solve the problem that iron oxide scale is easily formed during hot working of steel plate, it is generally necessary to perform aluminizing treatment on the surface of ultra-high-strength steel plate. Ultra-high-strength MnB steel plates are mainly used to make anti-collision parts.
-5- Twin induced plasticity steel

Twin-induced plasticity steel: the second generation of advanced high-strength automotive steel, whose room temperature microstructure is single-phase austenite. Most austenitic steels, such as austenitic stainless steels and high-manganese steels, have low-to-moderate stacking fault energies and thus tend to form extensive stacking faults, twinning, and plane dislocation structures.
When C or Al and Si are added to high manganese steel, a wide range of mechanical twinning can be found. When w(Mn) reaches 25%, w(Al)>3%, and w(Si) is between 2% and 3%, there is a large area of ​​mechanical twins in the steel. The same situation occurs when the carbon is very low time. These steels have very high ductility, up to 80%.

They introduced twin induced plasticity steels to name these steel grades, referred to as TWIP steels. The excellent mechanical properties of TWIP steel come from twinning-induced plasticity, and the role of twinning in deformation is completely different from the traditional concept. It is generally believed that in materials with relatively low crystal structure symmetry and relatively few slip systems, twinning occurs in some stress concentrations when the deformation rate is large, or when the force is applied in the case of unfavorable slip orientation.

Face-centered cubic metals are not prone to twinning, and mechanical twinning can only be formed at extremely low temperatures. Since the amount of deformation generated by twinning is small, it only plays the role of adjusting the orientation when slip is difficult, so that the slip can be be able to continue. But in TWIP steel, it can be formed in face-centered cubic austenite when the deformation temperature is -70~400 ℃, and the deformation rate can be as low as 10-4/s.
During the deformation process, twins are formed in the high-strain region, and the twin boundaries prevent the slippage in this region, which promotes the slippage of other lower-strain regions. At present, France, China and other countries have started the production of TWIP steel.
Technology development. Although TWIP has excellent mechanical properties, the problems of smelting, continuous casting, delay, fracture, notch sensitivity and coatability of the steel are technical difficulties that hinder the large-scale application of this steel in the automotive industry. .

Currently, steel mills and research institutes are working on a new generation of TWIP steel, FeMnA1 steel, also known as TRIPLEX steel. FeMnAl steel does not exhibit TRIP and TWIP effects. During deformation, dislocations slip to form shear bands, resulting in high plasticity, that is, shear band-induced plastic SIP effects. So far, its application in automobiles has been widely recognized.
-6- Hardened distribution steel

In recent years, J. g. Speer et al. proposed a new process – quenching and partitioning. This process can be used to produce carbon-rich retained austenitic grades, known as Q&P steels. This process mechanism is based on a new knowledge and understanding of the diffusion law of carbon in the martensite/austenite mixed structure. Q&P steel belongs to the third generation of AHSS, and the mechanical properties that can be achieved are in the range of tensile strength of 800 to 1500 and elongation of 15% to 40%.
First, the matrix is rapidly cooled to the quenching temperature (TQ) between M and Mf after being held at the austenite zone or critical zone temperature (TA) for a period of time, and isothermal for a short time to generate an appropriate amount of martensite, and then heated to the partition temperature (T) and treat for a period of time to ensure the completion of the carbon-rich process of retained austenite.

Although the thermodynamic mechanism of martensite formation in the Q&P process and the traditional Q&T process is the same, the evolution mechanism and final composition of the microstructure are completely different. In the Q&T process, when tempered martensite is formed, part of the carbon is consumed by the formation of cementite, and the retained austenite is decomposed. However, the Q&P process intentionally inhibits the precipitation of Fe-C compounds and stabilizes the retained austenite without being decomposed. Therefore, effectively inhibiting the precipitation of compounds is the key to this process.
Development Trend of Advanced High Strength Steel

Iron and steel product manufacturers are faced with more stringent quality requirements for existing products from users, which requires accelerating the development of new steel materials to ensure that new product manufacturing processes that meet user needs must be reliable and inexpensive.
Another development idea for automotive materials is to reduce the mass density of steel on the premise of retaining the advantages of steel itself, that is, strength, toughness, machinability, life, noise reduction and recyclability. One of these methods is to add light metal alloying elements such as Al and Si to the steel. These steels have been developed at an early stage with higher strength, lower bulk density and improved corrosion resistance, and so far have great potential for development, with further potential for weight reduction.

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