What Is Vanadium Steel?

Vanadium steel is an alloy steel that uses vanadium as the main alloying element or plays an important role. The role of vanadium in steel is to enhance hardenability and carbides, and it can withstand high temperatures, has a strong secondary hardening effect, and has a significant effect on improving hardness. It can refine the grains and stabilize the structure. It is widely used in alloy tool steel. Strictly speaking there are only two grades of vanadium steel: V, 8V. Vanadium steel is a tungsten alloy steel, which is often added to the alloy steel at the same time as tungsten, and has the same function as tungsten, such as WCrV, 8W2CrV, W18Cr4V, W9Cr4V2. Vanadium is also added to chrome alloy tool steel, such as 8CrV, CrV, 9CrV, 20CrV. Vanadium is also added to heat-resistant stainless steel, such as: Cr11MoV, CrSiMov.

The V, Ti, Nb microalloying technology developed in the 1960s has been widely used worldwide due to its significant technical and economic advantages. The development of microalloying technology has greatly promoted the progress of the steel industry. Some people call it one of the most outstanding achievements in physical metallurgy in the steel industry in the 20th century. After more than half a century of research and development, micro-alloying technology, including its alloy design principles, production processes, and application fields, has been greatly developed and improved. Micro-alloyed steel has also developed into an indispensable class of structural applications. steel. Among the three microalloying elements V, Ti, and Nb, it is generally believed that V mainly improves the strength of steel through precipitation strengthening.

The results show that in order to give full play to the precipitation strengthening effect of V, it is necessary to increase N in V-containing steels. At present, the viewpoint of N as an important alloying element in V microalloyed steel has been widely accepted. In-depth research in recent years also confirms that the use of V-N microalloying can not only give full play to the precipitation strengthening effect of V, but also effectively refine the ferrite grains by promoting intra-crystalline ferrite nucleation. V can produce significant precipitation strengthening in low carbon bainite. These new achievements have changed people's traditional understanding and expanded the application field of V microalloying technology.

China has abundant V and Ti resources.

V was first used in tool steel. At the end of the 19th century, Professor Arnold of the University of Sheffield in the United Kingdom studied the alloying effect of V in various steels and found that V carbides have high hardness and play a key role in high temperature stability, laying the foundation for V's application in the field of tool steel . At the beginning of the 20th century, research found that V alloying can greatly increase the strength of carbon steel, especially under the conditions of quenching and tempering, the performance improvement is more obvious, which promotes the application of V in engineering steel. Some other important applications of V alloyed steel are mainly concentrated in high-temperature power station steel, rail steel, and cast iron developed before the 1970s. At the same time, V is widely used in some special steels such as tool steel, heat-resistant steel and various military steels.

Effect of N on V (C, N) precipitation

Increasing N in V-containing steel increases the precipitation temperature of vanadium carbonitride and increases its driving force for precipitation. With the increase of N content, the carbon and nitrogen components in the precipitate phase changed significantly. In the case of low N, the precipitated phase is mainly vanadium carbide. As the N content increases, it gradually transforms into a precipitated phase mainly composed of vanadium nitride. When the N mass fraction in the steel is increased to 0.02A, it is the precipitated VN or nitrogen-rich V (C, N) in the entire precipitation temperature range. Due to the stronger affinity between N and V, the addition of N increases the driving force for V (C, N) precipitation and promotes the precipitation of V (C, N).

There are obvious differences in the phase distribution of V in high and low N steels. In low-N V steels, nearly 60 V are solid-dissolved in the matrix, and only about 35 V are precipitated as V (C, N); in high-N VN steels, the opposite is true, and 70% of V is V (C , N) form, and only 20 of V remain in solution in the matrix. This result shows that in the absence of N in the steel, most of the V does not fully exert its precipitation strengthening effect, which can be said to be wasted; increasing N causes the V in the steel that was in the solid solution state to change to the V in the precipitation state. Give full play to the precipitation strengthening effect of V.

After increasing N in V-containing steel, not only the number of precipitated phases increased exponentially, but also the particle size of the precipitated phases was more finely dispersed, so the effect of precipitation strengthening was also significantly enhanced.

Precipitation strengthening

Because N optimizes the precipitation of V in steel, its precipitation strengthening effect is significantly improved. In various C content steels, the precipitation strengthening effect of V (C, N) increases linearly with the increase of N content, and the maximum strength increase can reach 300 MPa. For every 0.00 mass fraction of N in V-containing steel, the strength can be increased by more than 6 MPa.

To obtain a strength increase of 150 MPa in a steel with a N mass fraction of 0.005, it is necessary to add V with a mass fraction of about 0.1; when the N mass fraction in the steel is increased to 0.01, the same strength increase is obtained. The required V content can be reduced to the level of 0.07 V; if the N mass fraction in steel is further increased to 0.015%, the vanadium mass fraction required to obtain the same strength increase can be reduced to the level of 0.05, The required N content in the steel with N mass fraction of 0.005 9/6 is reduced by half. It can be seen that increasing N can obviously save V consumption and significantly reduce production costs.

V (C, N) precipitation kinetics in VN steel

Refinement of ferrite grains using intragranular ferrite (IGF) technology has become an important means of grain refinement. The results show that the VN and TiN particles in steel are favorable nucleation sites for IGF. Due to the increase of N content in VN steel, the driving force for V (C, N) precipitation was increased, V (C, N) precipitation in austenite was promoted, and favorable conditions were created for IGF nucleation.

After increasing N in steel, the kinetic conditions of V (C, N) precipitation in austenite are greatly improved. At the nose temperature of 850 870 , the precipitation time is greatly shortened, and the mass fraction is 0.2 in C steel. The reduction of time to less than 10 S creates favorable conditions for the precipitation of V (C, N) in austenite. However, the precipitation of V (C, N) in austenite in V steel with low N is very slow, which usually takes several hours, which is difficult to occur in the actual production process.

V (C, N) particles precipitated in austenite are relatively coarse, and can be precipitated on MnS inclusions, and can also form individual V (C, N) particles. The size range is about 60-120nm. This relatively coarse V (C, N) precipitation contributes relatively little to precipitation strengthening, but it can play the role of nucleation core of ferrite in the grain and significantly refine the ferrite grains of steel.

V (C, N) Intracrystalline Ferrite Nucleation

V (C, N) particles precipitated in the austenite of VN steel are the effective core positions of intragranular ferrite nucleation. In VN steel, whether it is V (C, N) particles precipitated on MnS inclusions, or V (C, N) particles precipitated separately, they all play the core of ferrite nucleation in the crystal during the ferrite transformation. effect.

By using the technique of forming intragranular ferrite on V (C, N) particles, the strength level of grain refining effect steel in VN microalloyed steel is obviously obtained. The precipitation of V in bainite can offset the strength loss caused by carbon reduction, so that the same high strength level is obtained in the steel with lower c content, and the toughness and plasticity of the steel are improved. The latest research points out that increasing N in steel helps to promote the precipitation of V (C, N) in bainite, and further improves the strength of bainite steel. Research on the application of V in bainite steel is still in its infancy, and its mechanism of action needs to be further studied ^[3]

High-strength reinforcement

Rebar is the most consumed type of steel products in China, accounting for about one-fifth of China's steel output. In 2011, China's steel output reached 154 million tons. In order to meet the needs of the rapid development of the construction industry, while expanding the production of steel bars, the upgrading of steel bars has been accelerated. In 2000, the output of China's Grade III steel bars was only 260,000 tons, accounting for 1 of the total steel bar output. By 2011, China's output of Grade 3 steel bars had reached about 70 million tons, which was close to half of the total steel bars. The rapid growth of high-strength steel bars has strongly promoted the application of V micro-alloying technology in the Chinese steel industry.

The production of long products such as steel bars is fast, and the rolling temperature is high, usually above 1 000 ° C. Its process characteristics determine that the alloy design of steel bars is suitable for V micro-alloying technology. On the basis of 20MnSi rebar, by adding an appropriate amount of V or VN, the performance requirements of 400MPa and 500MPa high-strength rebar can be met. As mentioned above, N is a very effective alloy element in V-containing steel. By making full use of cheap nitrogen elements, the strengthening effect of V steel can be significantly improved, and the purpose of saving alloy consumption and reducing costs can be achieved. VN alloy composition optimized design, combined with the control of the cooling process, at the same strength level, the required V content in VN steel bars is significantly lower than in V steel.

The mass fraction of V in VN microalloyed 400MPa high-strength steel can be reduced to a level of 0.02 to 0.04. Compared with V-Fe microalloyed steel, the V content is reduced by half. The VN microalloying process has become the main process route for high-strength building reinforcement in China. Large-scale industrial production experience proves that VN rebar has stable and high performance, and its strength fluctuation range can be stably controlled within 75 MPa, which meets the requirements of first-class earthquake resistance.

Non-quenched and tempered steel

Adding a small amount of microalloying element V to medium carbon steel, relying on the precipitation of fine vanadium carbonitride, strengthens the ferrite-pearlite structure, thereby achieving the strength level required by traditional quenched and tempered steel. This is a basic principle of non-quenched and tempered steel alloy design.

Non-quenched and tempered steels developed in various countries have adopted V micro-alloying technology. According to different strength levels, the vanadium content (mass fraction) in non-quenched and tempered steel is generally 0.06 to 0.20.

In order to effectively exert the precipitation strengthening effect of V, it is necessary to increase N in non-quenched and tempered steel. The research results show that increasing N to 0.0015 to 0.020 in non-quenched and tempered steel is very beneficial to improve the properties of steel. N plays three main roles in non-quenched and tempered steel:

1) Promote the precipitation of V and improve the precipitation strengthening effect;

2) refine grains;

3) Improve the stability of TiN.

The grain refining effect includes two aspects: one is due to the precipitation of V (C, N) during the phase transition, which hinders the growth of ferrite grains; the other important reason is that due to VN or V ( C, N) precipitates in the austenite and promotes the formation of intragranular ferrite (IGF).

The role of s in non-quenched and tempered steel is significant. Properly increasing the S content can improve the cutting performance of non-quenched and tempered steel on the one hand, and on the other hand, the formed MnS particles can be used as the core of induced precipitation of V. The combined effect of S and V is closely related to the existence form of V in non-quenched and tempered steel. When the V and N content in steel is low, the temperature of nucleation and precipitation of V on MnS inclusions decreases, and the number of precipitates decreases. At the same time, the precipitates are C-rich V (C, N) particles with a larger lattice constant a. . In theory, in order to maintain a coherent relationship with the ferrite structure, N-rich V (C, N) particles with a smaller lattice constant a have a smaller mismatch with the BCC structure and are more conducive to ferrite shape. nuclear. Therefore, while maintaining the S and V content, increasing the N content in non-quenched and tempered steel can significantly improve the regulation effect of intragranular ferrite. Furuhara et al. Observed that under the conditions of different sizes of MnS, the MnS + V (C, N) composite precipitates promote the nucleation ability of intragranular ferrite significantly higher than the precipitates of MnS or MnS + VC, which has a better Inducing nucleation.

TiN is effective in controlling the growth of austenite grains. TiN technology is widely used in high toughness non-quenched and tempered steels. In order to give full play to the fine grain refinement of TiN, it is necessary to control the content of Ti and N in the steel close to the ideal chemical ratio (3.42: 1), and to accelerate the solidification rate of the molten steel, so that the volume fraction of TiN particles precipitated in the steel reaches the highest The smallest size. The use of TiN technology in non-quenched and tempered steel usually only requires micro-titanium treatment, and the Ti addition amount is from 0.010 to 0.015. The test results have proved that N plays a beneficial role in improving the effect of TiN pinning austenite grain boundaries. When the N content level in the steel exceeds the ideal Ti / N ratio, the effect of TiN pinning the grain boundaries is more effective. Increasing N in steel reduces the dissolution of TiN particles in high-temperature austenite, hinders particle growth, and thus improves the stability of TiN particles.

Thin slab continuous casting and rolling high strength strip steel

The thin slab continuous casting and rolling process is very different from the traditional hot-rolled strip process. First of all, due to its near-net shape and rapid solidification characteristics, the thin slab continuous casting and rolling process cannot be used to produce steel (C mass fraction 0.07 to 0.15) in the peritectic zone, and this composition range is exactly Is a typical composition of traditional HSLA steel. In order to meet the requirements of process conditions, high-strength steels produced by thin slab continuous casting and rolling technologies are mostly designed with low carbon content (C mass fraction is less than 0.07). Secondly, the traditional high-strength hot-rolled strip steel mainly uses Nb microalloying technology. Through controlled rolling and cooling of Nb-containing steel, grain strength and precipitation strengthening are used to increase the strength of the steel. However, for the thin slab continuous casting and rolling process, Nb-containing steel has caused production difficulties due to the problem of slab cracks, and this problem has not yet been solved well.

In addition, the international thin slab continuous casting and rolling production line mainly uses the electric furnace process to smelt. The higher N mass fraction (0.008 to 0.0010) in the electric furnace steel not only exacerbates the tendency of the Nb-containing steel continuous casting slab to form transverse cracks, but also weakens due to the precipitation of Nb (C, N) in the austenite. The effect of Nb's grain refining and the strengthening effect of Nb are reduced. In view of the above characteristics of the thin slab continuous casting and rolling process, the principle of its alloy design must be adjusted accordingly. The development of VN microalloying technology has opened an effective way for the development of high-strength thin slab continuous casting and rolling products. At present, a series of HSLA steels developed internationally for the thin slab continuous casting and rolling process use the VN microalloying technology route.

Low-carbon (<0.07) and VN microalloyed alloy design technology routes are adopted for thin slab continuous casting and rolling high-strength steels with a yield strength of 350 to 550 MPa. For lower strength steels (350-450 MPa), the use of VN alloy systems can meet the requirements. For high-strength steel at 550 MPa, a small amount of Nb is added on the basis of VN microalloying, which further refines the ferrite grains without damaging the thermoplasticity, and significantly improves the strength of the steel ^[2]

V is an important alloying element in special steel varieties such as high-speed steel, alloy tool steel, and high-temperature power station steel. This is also the earliest application of V in steel. It has a long history of over 100 years. The technology applied by V in this field is mature and its application is also very wide. With the continuous development of China's special steel industry, V will maintain a growing trend in this application field.

V microalloyed high-strength low-alloy steel is currently the largest field of V application in steel, and its application level is a symbol of the development level of a country's steel variety structure. In recent years, China has made significant progress in the research, development, and production of V-containing high-strength low-alloy steels. V micro-alloying technology has been used in high-strength X65 / X70 pipeline steel, N80 seamless oil well pipes, high-strength grade III steel bars, and high-strength. H-beam and angle steel, non-quenched and tempered steel, rail steel and other products have been widely used. However, compared with the international advanced level, there is still a large gap in the consumption intensity of V in Chinese steel varieties. At present, the consumption intensity of V in China's steel products is less than 30 g / t, which is about half of the world average, and it is far from the advanced level (80-90 g / t) of western industrialized countries. It can also be seen that there is huge development space and broad application prospects for the promotion and application of V alloying technology in the Chinese steel industry.

Recent research advances in V microalloying technology also provide impetus for the development of V-containing steels. For example, the research of VN microalloying technology makes N an economical and effective alloying element in steels containing V. By fully utilizing the role of cheap N elements, the precipitation of V in steel is optimized and the fine grain strengthening of V is enhanced. And precipitation strengthening, significantly improve the strength of the steel, save the amount of V, and significantly reduce the cost of steel. At present, the research results of VN microalloying technology have been widely recognized by people. They have been successfully applied in China's high-strength steel bars, non-quenched and tempered steels, high-strength seamless steel pipes, and high-strength large-angle steels, which have effectively promoted China's V-micro Development of alloyed steel.

The combination of V (C, N) intragranular ferrite nucleation technology and recrystallization control rolling technology forms a new generation of TMCP process. While exerting the traditional precipitation strengthening effect of V-containing steel, it also relies on VN in austenite The nucleation of the intragranular ferrite from the precipitation effectively refines the ferrite grains. This technology has been well applied in the development of high-strength thick-section H-shaped steel and high-strength thick steel plates. The combination of VN microalloying and V-Nb composite microalloying alloy design and thin slab continuous casting and rolling process technology has changed people's understanding of traditional strip steel products, and developed high strength ultrafine grains with good toughness and matching The new ferritic-pearlite structured strip steel has a yield strength level of 550 to 650 MPa. China is currently the country with the most equipment for thin slab continuous casting and rolling production lines in the world, with a production capacity of more than 30 million tons. The successful application of VN microalloying technology in high-strength thin slab continuous casting and rolling strip products has important guiding significance for the development of similar product structures in China. In addition, some other new research results, including the strengthening effect of V in bainite steel, the application of V in TRIP steel and BH steel, the effect of V absorption of H to improve the delayed fracture resistance of steel, etc. Helps expand the application of V in steel ^[2]

The improvement and rapid development of China's manufacturing industry have put forward higher requirements for steel varieties. In order to meet China's ever-changing industrialization needs, the requirements for large-scale and ultra-large-scale industrial equipment and structures are increasingly urgent, and the service environment is becoming more and more severe. In the case of the same grade of steel, it is inevitable to continuously increase the thickness or diameter of steel products to meet the safety requirements of equipment or structures. This not only brings a rapid increase in material procurement and manufacturing costs, but also brings difficulties in design and processing. At the same time, the risks of using equipment and structures have increased dramatically. Therefore, in order to meet the huge demand of China's industrialization process and the needs of the steel industry's own industrial structure upgrade, the upgrading of steel varieties is still one of China's important strategic goals in the next 5 to 10 years. Reasonable use of V (micro) alloying technology and economic advantages has always been one of the important technical ideas for upgrading steel products. The structural adjustment of China's steel varieties provides broad prospects for the development of V-containing steels ^[3] .