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Strengthening and toughening method of steel: strengthening and toughening of second phase particles

The second phase particle is an important method of strengthening and toughening steel, which can take into account the strength, toughness and other service properties of steel at the same time. With the improvement of steel strength requirements in recent years, researchers have developed a large number of new steel materials. The strengthening and toughening of second phase particles also plays an important role in these new steels. But at the same time, in these new steel materials, the strengthening and toughening of second phase particles also faces many challenges and opportunities: the characterization and regulation of nano scale particles, the interaction between second phase particles and multiphase microstructure, and the optimization of service properties of steel, etc. Through different steels such as automobile lightweight steel, ultra-high strength maraging steel, nuclear reactor steel and high modulus steel, the effects of second phase particles on the strength, toughness, weldability, formability, resistance to hydrogen induced delayed fracture, creep and radiation damage of new steel materials are summarized. Combined with the unique microstructure, preparation process The mechanism of strengthening and toughening of second phase particles and the problems to be solved are analyzed, in order to provide reference for the further research and development in the field of strengthening and toughening of second phase particles in steel.

Strengthening mechanism of second phase particles in steel

The second phase particles in steel can hinder the deformation processes such as dislocation movement and twinning and realize strengthening. Taking the blocking effect on dislocation movement as an example, the second phase particle enhancement mechanism is mainly divided into shear mechanism and Orowan mechanism.
When the second phase particles are deformable particles, the strengthening mechanism is the cutting mechanism, that is, the dislocation can cut the particles and deform them with the matrix. In the shear mechanism, the interaction between dislocations and second phase particles is very complex, including chemical strengthening, stacking fault strengthening, modulus strengthening, coherent strengthening, ordered strengthening and so on. The larger the radius or volume fraction of the second phase particles, the more obvious the enhancement of the shear mechanism.

When the second phase particles are not deformable, the enhancement mechanism is Orowan mechanism (also known as bypass mechanism, as shown in Figure 1). During the movement of dislocations, dislocations are blocked by particles and bend, resulting in reverse stress τ= T/BR, B is the magnitude of the Berger vector, and R is the radius of curvature of the dislocation line. When r = λ/2 (λ Is the particle spacing), the reverse stress is the largest, and the critical shear stress is τ c=λ/2bλ. In practical materials, the particle spacing is often reduced by reducing the particle size or increasing the particle volume fraction λ, So as to obtain better strengthening effect. After the dislocation bypasses the second phase particles, a dislocation ring will be formed to surround the second phase particles, further hindering the movement of other dislocations on the slip plane, as shown in Fig. 1b.

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Fig. 1 Orowan mechanism of particle enhancement in the second phase: (a) the dislocation bends before bypassing the particle; (b) After the dislocation bypasses the particle, a dislocation ring is formed
The second phase particles usually have the effect of grain refinement. According to hall page formula, the smaller the grain radius, the higher the strength of the material. In addition, fine grain strengthening generally does not reduce the toughness of materials. Many steels containing second phase particles use this strengthening mechanism.

Application of second phase particle strengthening and toughening in new steel

Steel for automobile lightweight

Low alloy high strength steel

Low alloy high strength steel (HSLA steel) is a low carbon steel with high strength obtained by precipitation strengthening and grain refinement produced by microalloying elements such as Nb, Ti and V. HSLA steel plays an important role in vehicle weight reduction and energy saving. It is still widely used in vehicle chassis, body reinforcement and other occasions.
The strengthening of HSLA steel mainly comes from the carbon nitride precipitation formed by microalloying elements. The strengthening mechanism includes precipitation strengthening mainly based on Orowan mechanism and grain refinement during thermomechanical rolling. Specifically, during thermomechanical rolling, carbon and nitrogen compounds can inhibit the growth and recrystallization of austenite grains, accumulate more ferrite nucleation positions such as deformation bands and dislocations in deformed austenite, and refine ferrite grains. After final rolling, the solid solution microalloyed elements in HSLA steel will continue to precipitate during cooling and coiling. The precipitation methods include precipitation during Austenite Cooling, interphase precipitation at austenite ferrite interface, uniform precipitation in ferrite and so on. The size and spacing of precipitates formed by interphase precipitation (Fig.2) are usually very small, and the precipitation strengthening effect is excellent, which is one of the research hotspots in recent years. It is found that in order to obtain the best interphase precipitation strengthening effect, the driving force of carbide formation should be increased and the moving speed of austenite ferrite interface should be reduced.
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Fig.2 fine dispersed carbides precipitated between phases:

            (a) tem photos;                    (b) High resolution TEM photos
Although Nb, Ti and V can play the role of fine grain strengthening and precipitation strengthening. However, due to the different solubility in austenite, the main roles of these elements and their precipitates in HSLA steel are also different. For example, the nitride of Ti is mainly used to avoid grain coarsening during reheating of austenite, the carbon nitride of Nb is mainly used to control the temperature range of non recrystallized zone and A3 transition temperature, and the carbon nitride of V is mainly used to produce precipitation strengthening effect. In actual production, better strengthening effect can be obtained by combining the advantages of different microalloyed elements (such as fine grain strengthening of Nb and precipitation strengthening of V). However, some studies have pointed out that the addition of Ti will consume a lot of elements such as Nb, V and N, form large particles with weak strengthening effect, and have an adverse iMpact on the precipitation strengthening effect of Nb and V.
The second phase particles in HSLA steel also have an important relationship with weldability and low temperature toughness. Firstly, because the second phase particles improve the strength of HSLA steel, HSLA steel can appropriately adopt low carbon content and total alloy element content to prevent the formation of brittle phase, so as to ensure welding performance and low-temperature toughness. At the same time, precipitates such as titanium nitride can become the nucleation point of acicular ferrite in the crystal. These fine acicular ferrite can reduce the effective grain size and improve the weldability and low temperature toughness of HSLA steel.

Advanced high strength steel

With the continuous improvement of automobile lightweight requirements, researchers began to explore advanced high strength steel (AHSS steel) with higher strength. Among them, the first generation AHSS steel (such as duplex steel) has been widely used in the automobile manufacturing industry, and the research and development of the second and third generation AHSS steel is still a hot spot in recent years.
Dual phase steel (DP steel) belongs to the first generation of AHSS steel. Its microstructure is mainly composed of ferrite and martensite. Martensite ensures high tensile strength and ferrite ensures good toughness. At present, the strength of DP steel produced commercially is up to 1180Mpa, and the elongation remains above 5%, which can be used for bumper and other parts of vehicles.
The strengthening and toughening of the second phase particles in DP steel mainly depends on microalloyed elements such as Nb and V. It is reported that the Nb content of most dual phase steels at home and abroad is about 0.03%, and the V content is usually no more than 0.1%. By optimizing the heat treatment process, on the one hand, most dispersed carbonitride particles can be precipitated in ferrite to improve the yield strength of ferrite in microstructure, so as to improve the yield strength of materials; On the other hand, the precipitation of a small amount of relatively large carbonitride particles in martensite can reduce the carbon content of martensite and improve the toughness of martensite. At the same time, it can also reduce the strength difference between ferrite and martensite and reduce the strain disharmony at the interface, so as to reduce the risk of micro cracks at the ferrite martensite interface and improve the toughness of DP steel. In addition, there is often a mismatch between the reaming performance and high elongation in DP steel, which is mainly caused by the strain disharmony between martensite and ferrite. The above second phase particle strengthening and toughening mechanism can effectively reduce the strain disharmony and improve the reaming performance of DP steel.
Twin induced plasticity steel (TWIP Steel) in the second generation AHSS steel has both high tensile strength (up to 1000MPa) and high elongation (up to 50%), which is expected to realize the room temperature forming of high-strength complex auto parts. By adding a certain amount of C and Mn elements (Mn content (mass fraction, the same below) is usually 15% – 25%), TWIP Steel has stable austenite structure at room temperature. During deformation, a large number of twins are formed in austenite, which hinders dislocation slip and produces dynamic hall page effect, so that TWIP Steel has very high work hardening rate. Nevertheless, TWIP Steel has some disadvantages, such as low yield strength and high risk of hydrogen induced delayed fracture, which limits its practical application.
The second phase precipitated particle strengthening can effectively improve the yield strength of TWIP Steel. It should be noted that the Mn content in TWIP Steel is very high, which may change the properties of carbon nitrides, such as increasing the solubility of vanadium carbide in austenite, so it is difficult to determine the best carbide forming elements and heat treatment process in TWIP Steel. Scott et al. Found that Nb, V and other elements in TWIP Steel are mainly strengthened by Orowan mechanism, and their contributions to yield strength are different (Fig.3). It can be seen from the figure that when the alloy element content is less than 0.1%, the strengthening effect of Ti is the best, but if the Ti content continues to increase, large vanadium carbide inclusions will be formed in the steel, and the strengthening effect will reach saturation, about 150MPa. Among the three elements, V element can increase the yield strength by about 250Mpa, with the largest increase. In addition, due to the formation of a large number of twins and stacking faults in TWIP Steel during deformation, the interaction between these defects and carbides is also worthy of further study. Yen et al. Studied TWIP Steel Containing v4c3 precipitates and found that incomplete dislocations and deformed twins can bypass carbides in a way similar to Orowan mechanism. At the same time, the resistance of carbides to twin movement will increase with the decrease of twin thickness.
Due to the high strength and elongation of TWIP Steel, the residual stress of cold-formed parts is very large, so the risk of hydrogen induced delayed fracture of TWIP Steel parts is often great. The addition of Al can alleviate the risk of delayed fracture of TWIP Steel, but it often reduces the yield strength of TWIP Steel. The results show that vanadium carbide and other second phase particles can be used as effective hydrogen traps in TWIP Steel, which is expected to improve the yield strength of TWIP Steel and reduce the risk of delayed fracture. However, some studies have found that once there are large titanium nitride particles in TWIP Steel Containing Ti, the hydrogen induced delayed fracture resistance of the material will decrease.
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Fig.3 Effect of different alloying elements on yield strength of TWIP Steel
Quenched and proportioned steel (Q & P steel) is the third generation AHSS steel, which combines the characteristics of low alloy composition, high strength and high ductility. In the process of heat treatment, Q & P steel needs to be quenched (q) to between the Martensite Start transformation temperature (MS) and the transformation end temperature (MF), and then maintained at or above this temperature for distribution (P). In the distribution process, carbon atoms diffuse from supersaturated martensite to untransformed austenite, which improves the stability of austenite. Therefore, after cooling to room temperature, Generally, 10% – 20% of metastable retained austenite can be retained in Q & P steel. During deformation, these retained austenite gradually transforms into martensite, resulting in transformation induced plasticity (trip effect), so as to realize the combination of high strength and high elongation.

Maraging steel

Maraging steel is mainly strengthened by precipitation of nano intermetallic compounds. Similar ultra-high strength steels also include secondary strengthening steel strengthened by alloy carbide, low alloy steel strengthened by transition carbide and ferrite steel strengthened by nano precipitation. This section mainly introduces maraging steel.
Maraging steel has the characteristics of high alloy content and very low carbon content. Its main strengthening sources are carbon free/ultra-low carbon martensite high dislocation density and nano intermetallic precipitates in the matrix. Its toughness mainly comes from its carbon free/ultra-low carbon martensitic matrix. It is worth noting that in high carbon martensite, carbon atoms cause serious distortion of martensite lattice, resulting in a large reduction of dislocation slip system. Dislocations are difficult to produce and slide at the crack tip of high carbon martensite, resulting in brittle fracture. Different from high-carbon martensite, the dislocation slip system of carbon free/ultra-low-carbon martensite is not much different from that of ordinary ferrite, but its dislocation density is higher than that of ordinary ferrite and its dislocation proliferation ability is not strong, resulting in lower work hardening rate and uniform elongation than ferrite. However, dislocations can occur and move at the crack tip of carbon free/low-carbon martensite, resulting in crack passivation and improving toughness. Therefore, while having high strength, maraging steel also takes into account high toughness, which plays an important role in key occasions such as aircraft landing gear.
The heat treatment process of maraging steel mainly includes solution treatment and aging treatment. In the process of high temperature solution treatment, all precipitates are dissolved in austenite. In the process of rapid quenching, the parent austenite shears to form lath martensite with high dislocation density. In the aging treatment, these high-density dislocations can be used as nucleation points to promote the formation of high-density nano precipitates. In a typical 18Ni Maraging steel, the size and density of the main nano precipitates Ni3Ti and Ni3Mo particles can be changed by adjusting the alloy content of CO, Mo and Ti, and a maraging steel with yield strength ranging from 1400 to 2400Mpa can be obtained. Among them, the main role of CO is to reduce the solubility of Mo in the matrix, promote the precipitation of ni3mo nanoparticles and fully improve the strength of maraging steel.
However, maraging steel also has some disadvantages. On the one hand, nano precipitates such as Ni3Ti and ni3mo form semi coherent or non coherent interfaces with martensitic matrix, so they are more inclined to non-uniform nucleation at defects such as dislocation and grain boundary, and the optimization of size, density and distribution is limited; On the other hand, in order to ensure the density of nano precipitates during heterogeneous nucleation, a high content of CO must be added to maraging steel, which makes the cost of maraging steel very expensive.
In recent years, the research of new Co free maraging steel has made good progress. Through the design idea of minimizing the lattice mismatch of precipitates, the team of Professor Lv Zhaoping of Beijing University of science and technology developed a Co free maraging steel with low price, tensile strength of 2.2GPa and elongation of 8.2% (Fig.4). This new steel is mainly strengthened by Ni (al, Fe) coherent nano precipitates. Through the adjustment of alloy composition, the lattice mismatch between the precipitates and martensite matrix is only 0.03% ± 0.04%. It can form nuclei uniformly in martensite, and realize the nano precipitation strengthening with extremely high density (1024m-3), very small size (2.7nm) and uniform distribution, with excellent strengthening effect. At the same time, because the alloy no longer relies on Ni3Ti and ni3mo strengthening, cheap Al is used to replace expensive CO and Ti elements, and its production cost is greatly reduced.
In addition, eutectoid nano strengthening is also expected to be used to improve the non-uniform nucleation and high cost of maraging steel. At the same time, a variety of coherent precipitates, such as BCC Cu, B2 NiAl and l21-ni2alti, are used to obtain nano precipitates with high density, small size and uniform distribution. Different precipitates will also interact to make the material have better mechanical properties. Taking Cu/NiAl eutectoid nano strengthened steel as an example, its tensile strength can reach 1.9gpa, elongation can reach 10% and area shrinkage can reach 40%. With the change of alloy composition, the eutectoid mechanism of Cu/NiAl will also change: when the Cu content is high, Cu rich nano precipitates will be formed in the matrix first. In the growth process, Ni and Al elements will be biased to the interface between the precipitates and the matrix, so as to inhibit the growth of Cu rich nano precipitates. At the same time, NiAl nano precipitates will form uneven nuclei at the interface; When the Cu content is low, NiAl nano precipitates are formed first. During the growth of precipitates, the Cu in the precipitates will also be biased to the interface between the precipitates and the matrix, and Cu nano precipitates are formed at the interface. By adjusting the alloy composition, precipitates with a size less than 5nm can be obtained in Cu/NiAl eutectoid strengthened steel.
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Fig.4 new Co free maraging steel: (a) stress-strain curve (b) high resolution TEM photos of coherent nano precipitates

Nuclear reactor steel

In response to global climate change, the development of nuclear power plants has attracted increasing attention, and nuclear power safety has always been the focus of public attention. When nuclear reactors are in service, structural materials need to withstand very harsh conditions such as high temperature, high stress, high radiation and chemical corrosion environment. Reliable structural materials play a vital role in nuclear power safety. At present, steel plays an important role in structural materials for nuclear power. At the same time, it is found that the precipitated particles of the second phase in steel are expected to not only improve the creep resistance of the material, but also enhance the radiation damage resistance of the material. This chapter will mainly introduce the nano structured ferritic alloy (NFA alloy) further developed on the basis of oxide dispersion strengthened ferritic steel (ODS steel).
The main feature of ODS steel is to use Y2O3 particles to improve the creep and radiation damage resistance of the steel at the same time. However, due to the limitation of solubility, the traditional smelting technology can not introduce y element into steel. Even after the introduction of Y, the Y atom in steel is difficult to form oxide and can not play the expected role. In the 1980s, Fisher et al. Found that ODS steel with nano precipitates can be obtained by mechanical alloying (MA) and thermosetting treatment of Y2O3 powder and ferroalloy rich powder. This ODS steel has high quasi-static strength, high creep strength and high radiation damage resistance at the same time. At present, mechanical alloying is the main preparation method of ODS steel (alloy with NFA). Recent studies have found that during mechanical alloying, Y2O3 can be dissolved in iron rich alloy powder by high-energy ball milling, and fine oxide nanoclusters can be precipitated in the subsequent thermosetting treatment. CoMpared with the larger oxide particles in traditional ODS steel, such oxide nanoclusters have more obvious enhancement effect on creep strength and radiation damage resistance. By adjusting the alloy composition and process parameters, very high-density oxide nano clusters can be obtained. In order to distinguish from traditional ODS steel, this kind of steel is called NFA alloy. At present, in addition to Y2O3, a certain amount of Ti will be added to NFA alloy to reduce the size of oxide nanoclusters.
The strengthening mechanism of oxide nanoclusters on the creep resistance of NFA alloy mainly includes the following aspects. Firstly, the oxide nanoclusters in NFA alloy have very high thermal stability. For example, the density of oxide nanoclusters in 14ywt alloy has not changed significantly after being stored at 1200 ℃ for 24 hours. Second, at high temperature, these fine, high-density and stable oxide nanoclusters can hinder the slip and climb of dislocations, slow down the recovery process of dislocations and inhibit the dislocation creep mechanism. Thirdly, a large number of dispersed oxide nanoclusters at the grain boundary can also improve the diffusion creep strength of NFA alloy.
The radiation of neutrons or charged particles will cause radiation hardening/softening, radiation brittleness, radiation swelling and creep life reduction, resulting in the deterioration of steel properties. Figure 5 coMpares the radiation damage mechanism in NFA alloy and tempered martensitic steel. The radiation damage resistance of tempered martensitic steel is poor. Firstly, radiation will form a large number of self interstitial atomic dislocation rings and fine precipitates in the microstructure, resulting in radiation hardening and radiation brittleness; Radiation can also form holes in microstructure, resulting in radiation swelling; In addition, he produced by radiation will accumulate at the grain boundary, promote the formation of creep holes and reduce the creep life of the material. In contrast, there are a large number of fine and stable oxide nanoclusters in NFA alloy, which can be used as annihilation sites of radiation defects and effectively inhibit radiation swelling; At the same time, oxide nanoclusters ensure that the steel still has high dislocation density in the environment of high temperature and high radiation. High density nanoclusters and dislocations can disperse he into small bubbles to avoid intergranular fracture caused by he aggregation at the grain boundary.
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Fig.5 neutron radiation damage mechanism of NFA alloy (a) and tempered martensitic steel (TMS) (b)

High modulus steel

In order to realize the lightweight of steel structures, it is necessary to reduce the thickness of steel structures. However, the stiffness of structural members is very sensitive to their thickness. In order to avoid catastrophic buckling caused by geometric instability, the young’s modulus of steel needs to be further improved. Conventional steel strengthening and toughness methods, including solution strengthening, precipitation strengthening and grain boundary strengthening, can not effectively improve the overall Young’s modulus of the material. Nowadays, the only effective way to enhance the young’s modulus of steel is to add a ceramic as a reinforcing phase to the steel. The introduced method can be introduced from the outside (ex situ), such as powder metallurgy, or in-situ formed by chemical reaction inside the steel. The ceramic reinforced steel prepared in this way, also known as high modulus steel, has many advantages that traditional metal materials lack, such as high modulus, low density, high hardness and good wear resistance. At the same time, the steel itself as the matrix also makes it have reliable plasticity and elongation. Therefore, high modulus steel can be used not only for engine or brake components requiring heat resistance and wear resistance, such as pistons and brake discs, but also as materials for high strength and high modulus components, such as chassis, crankshaft, connecting rod, etc. The selection of ceramic types is mainly based on whether they have properties such as high strength, high modulus, low density, high wear resistance and high melting point. Therefore, common ceramic materials are mainly compounds of C, O and B, including tic, TiB2, SiC, WC, SiO2, Al2O3, NBC, etc. In addition, the selected ceramics can not react with the steel itself to form a new structure, because the structure is likely to affect the mechanical properties of the composite.
The strengthening mechanism of high modulus steel can be divided into direct strengthening and indirect strengthening. The former is mainly realized by load transfer, that is, the load borne by the whole material will be transferred from the matrix through the steel/ceramic interface to ceramics with higher strength. In this way, the reinforcing phase shares most of the applied load, so that the material as a whole has higher strength. The efficiency of direct strengthening is not only affected by the volume fraction or morphology of the reinforcing phase (higher aspect ratio is usually better than spherical particles), but also mainly depends on the strength of the interface, because the interface acts as a medium for the transfer of load from the matrix to ceramic particles. According to the analysis, the traditional powder metallurgy method will not only lead to the formation of non coherent interface (weak), but also lead to the formation of pores at the interface, which will seriously affect the overall strength of the interface. The high modulus steel prepared in this way will take the lead in interfacial debonding and lead to the failure of direct strengthening mode. In contrast, a new reinforcing phase can be formed in situ through eutectic reaction during casting solidification. The in-situ high modulus steel (such as iron-based TiB2 reinforced steel) prepared based on this has higher interface strength (coherent interface) and load transfer efficiency, and its tensile properties will be better than the same (composition, microstructure and morphology) materials prepared by powder metallurgy.
The indirect strengthening is caused by the difference of thermal expansion coefficient between the two phases. Because the coefficient of thermal expansion of steel is usually much larger than that of ceramics, the matrix will shrink to a greater extent during cooling (such as quenching and annealing), resulting in extrusion of ceramics. Due to the difference of modulus between matrix and ceramic, the deformation caused by matrix cooling can not be effectively transmitted to ceramic, resulting in obvious deformation gradient at the two-phase interface (one side of matrix), which will inevitably lead to the formation of geometrically necessary dislocations and new residual stresses. In other words, the direct reason for the indirect strengthening of high modulus steel is the geometrically necessary dislocation at the interface. It is found that the yield strength of ceramic reinforcement is significantly higher in compression than in tension, which proves the existence of residual stress. However, coMpared with direct strengthening, indirect strengthening is more difficult to analyze quantitatively, mainly because the uneven distribution of geometric necessary dislocations and its contribution to the overall strength of materials are not mature.

Conclusion

As an effective method to improve strength and toughness at the same time, the strengthening and toughening of second phase particles plays a vital role in all kinds of steels. In new steels, higher performance requirements and complex microstructure also bring more space and challenges to the strengthening and toughening of second phase particles. Taking automotive steel as an example, many new steels have been mass produced in steel mills, but they can not be popularized in the automotive industry because of the limitations of welding performance, low-temperature toughness and hydrogen induced delayed fracture resistance. The strengthening and toughening of second phase particles can not only ensure the strength and toughness of new steel, but also provide an effective method to improve these practical service properties. At the same time, the research topics involved in new steel, such as multiphase precipitates, the interaction between hydrogen and second phase particles, nano eutectoid strengthening, also put forward higher requirements for researchers. In the future, with the continuous development of atomic scale experimental characterization technology and simulation technology and the deepening understanding of the strengthening and toughening of second phase particles, second phase particles are expected to give better play to their strengthening and toughening role and promote more new steels to practical application.

Source: Network Arrangement – China Butt Weld Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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