Vanadium steel is an alloy steel with vanadium as the main alloying element or playing an important role.The role of vanadium in steel is to enhance hardenability and carbide, and it can withstand high temperature, has a strong secondary hardening effect, and has a significant role in improving hardness.It can refine grains and stabilize structure.It is widely used in alloy tool steel.Strictly speaking, there are only two grades of vanadium steel: V and 8V.Vanadium steel belongs to tungsten alloy steel, which is often added to alloy steel together with tungsten, and has the same function as tungsten, such as WCrV, 8W2CrV,W18Cr4V，W9Cr4V2。In chrome systemAlloy tool steelVanadium is also added, such as 8CrV,CrV，9CrV，20CrV。Vanadium is also added to heat-resistant stainless steel, such as Cr11MoV and CrSiMov.
The V, Ti, Nb microalloying technology developed in the 1960s has been widely used worldwide due to its remarkable technical and economic advantages.The development of microalloying technology has greatly promoted the progress of iron and steel industry. Some people call it one of the most outstanding achievements in physical metallurgy in the field of iron and steel industry in the 20th century.After more than half a century of research and development, micro alloying technology, including its alloy design principle, production process, application fields, has been greatly developed and improved,Microalloyed steelIt has also developed into an indispensable type of structural 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 steel containing V.At present, N has been widely accepted as VMicroalloyed steelAn important alloy element in the view of.In recent years, in-depth research has also confirmed that V-N microalloying can not only give full play to the precipitation strengthening effect of V, but also effectively refine by promoting the nucleation of intragranular ferriteFerrite grain。V can produce obvious precipitation strengthening effect in low-carbon bainite.These new achievements have changed people's traditional understanding and expanded the application field of V microalloying technology.
China has rich V and Ti resources, in order to promote China's V-MicroalloyOn the basis of reviewing the development history of V steel, this paper introduces the latest research achievements of V microalloying technology at home and abroad in recent years and its application in high-strength steel barsNon quenched and tempered steel, thin slab continuous casting and rolling strip steel, high strength thick wall H-beam and other products[1]。
Development history
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V was first used inTool and die steelMedium.At the end of the 19th century,university of sheffield Professor Arnold studied the alloying effect of V in various steels and found that the carbide of V has high hardness and plays a key role in high temperature stability, laying the foundation for the application of V in the field of tool steel.At the beginning of the 20th century, research found that V alloying can significantly improve the strength of carbon steel, especially under the quenching and tempering process conditions, the performance improvement is more obvious, promoting the application of V in engineering steel.Other important applications of V alloy 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 and die steels, heat-resistant steels and various military steels.
V-containing microalloyed steel
The largest application of V in steel is high strength low-alloy structural steel (HSLA steel), also known as "microalloyed steel".The development of microalloyed steel began in the late 1950s.With the extensive application of welded structures after World War II, the adverse effects of C on the toughness and weldability of welded structures have become prominent, and the means to improve the strength of steel by increasing C has been limited.At the same time, it is found that grain refinement can simultaneously improve the strength and toughness of materials.Therefore, this new view strongly stimulates the development of new processes and new steel grades.At the same time, it is recognized that the precipitation strengthening of micro alloying elements can replace the strengthening effect of C, and improve the weldability.In the early 1960s, Beth lehem Iron and Steel Company of the United States developed a series of V-N steels based on C-Mn steels. The upper limits of C and Mn mass fractions were 0.22 and 1.25, respectively, and the yield strength reached 320~460MPa. The products were supplied as hot rolled, and the specifications included all products of plates, strips, and sections.
VAN80 steel is a V microalloyed strip steel developed by Jones and Laughlin in the United States at the early stage (around 1975). This steel was first produced by online controlled accelerated cooling process. Grain refinement and precipitation strengthening were increased by using microalloy precipitation, and its yield strength reached 560MPa.With the development of controlled rolling and controlled cooling technology, a new controlled rolling process called recrystallization controlled rolling was developed around 1980.The process adopts v_Ti microalloying design. By repeatedly recrystallizing the austenite after each pass of deformation, the traditional low temperature controlled rolling method (NbMicroalloyed steel）The grain refining effect can be achieved.This process can adopt higher finishing rolling temperature, so it requires lower rolling force of the rolling mill, which can not only improve productivity, but also realize controlled rolling production on the rolling mill with weaker rolling force.The thin slab continuous casting and rolling (TSRC) process developed in the 1990s promoted the application of V microalloying technology in high-strength strip products.With V/v-N microalloying, a series of high-strength strip steel products with yield strength of 350~700MPa have been developed under the TSRC process.
In recent years, the research of V microalloying technology has made some new developments.In high N steel containing V, the precipitation of VN in austenite promotes the nucleation of intragranular ferrite and effectively refinesFerrite grainDimensions.Based on this research achievement, the third generation of TMCP process is formed by combining VN intragranular ferrite nucleation technology (IGF) with recrystallization controlled rolling (RCR) process.This new process not only gives play to the traditional advantages of precipitation strengthening of V in V-N steel, but also uses VN to promote the nucleation of intragranular ferrite to achieve the effect of grain refinement, giving full play to the advantages of grain refinement and precipitation strengthening of microalloyed steel. It has been successfully applied to some steel products that are difficult to achieve low temperature controlled rolling, such as thick wall steel, high-strength thick plate, etc.The research results in Sweden show that the precipitation of V in low-carbon bainite results in precipitation strengthening.In the 21st century, the research achievements and popularization and application of low-cost V-N microalloyed high-strength steel bars in China have strongly promoted the development of V-N microalloyed high-strength steel bars in ChinaMicroalloyed steelProduction and application.At present, the output of 400 MPa high strength steel bars in China has exceeded 30 million tons, and it is precisely to vigorously develop 500 MPa high strength steel bars, which shows the broad application prospect of vanadium in China's steel production.
Including V tool and die steel
V isTool and die steelIt is the earliest application field of V in steel and has a history of more than 100 years.V can effectively improve the hardness, wear resistance and thermal stability of products in tool and die steels, and has been widely used in industrial production.In Germany, the consumption of V in tool steel and high-speed steel accounts for about 1/3 of the total consumption of vanadium.In the standards of alloy tool steel in the world, V is generally an essential alloy element, and the mass fraction of V in steel generally fluctuates in the range of 0.1~3.
ChineseAlloy die steel(including cold work, hot workPlastic mould steel）In the product, V containingDie steelAccounting for 55% of die steel output.In ChinaHigh speed tool steelIn the standard (GB/T9943-2008), all 19 steel grades contain V, and the V mass fraction is usually 1~3. In a few high-speed steels with special requirements, the V mass fraction reaches 5.
Heat resistant steel with V
The high-temperature precipitation of vanadium carbonitride significantly improves the high-temperature rupture strength of steel. Therefore, V is widely used in the field of heat resistant steel for power plants.Most heat-resistant steel alloy systems are added with V, and the amount (mass fraction) is generally 0.15~0.40.
Military steel with V
V is also an important alloy element of various military steel, which is widely used in the key materials of ships, armor, aviation and other fields.
V alloying technology is applied to steel for ships, armor steel, aircraft landing gear, missiles androcket engineShell steel and other military steel fields have played an important role[2]。
effect
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Effect of N on V (C, N) Precipitation
The precipitation temperature of VCN is increased by increasing N in V-bearing steel, and the driving force of precipitation is increased.With the increase of N content, the carbon and nitrogen components in the precipitates change significantly.In the case of low N, the precipitates are mainly vanadium carbide, and with the increase of N content, the precipitates are gradually transformed into vanadium nitride based precipitates.When the mass fraction of N in steel increases to 0.02A, VN or nitrogen rich V (C, N) precipitates throughout the precipitation temperature range. Because of the stronger affinity between N and V, the addition of N increases the driving force of V (C, N) precipitation and promotes the precipitation of V (C, N).
The phase distribution of V in high and low N steels is obviously different.In low N V steel, nearly 60% V is dissolved in the matrix, and only about 35% V is precipitated in the form of V (C, N);In V-N steel with high N content, however, the opposite is true. 70% of V precipitates in the form of V (C, N), and only 20% of V is dissolved in the matrix.This result shows that most of V does not give full play to its precipitation strengthening function when the steel lacks N, which can be said to be a waste;After adding N, the V in the steel in the solution state was transformed into V in the precipitation state, which fully played the role of V in precipitation strengthening.
With the increase of N in the steel containing V, not only the number of precipitates is doubled, but also the particle size of precipitates is finer and more dispersed, so the effect of precipitation strengthening is also significantly enhanced.
Precipitation strengthening
Because N optimizes the precipitation of V in steel, the 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 increment can reach 300 MPa.The strength of steel containing V can be increased by more than 6MPa by adding 0.001 N mass fraction.
In the steel with a mass fraction of 0.005 N, to obtain a strength increment of 150MPa, it is necessary to add V with a mass fraction of about 0.1;When the mass fraction of N in steel increases to 0.01, the V content required to obtain the same strength increment can be reduced to 0.07 V;If the N mass fraction in the steel is further increased to 0.015%, the vanadium mass fraction required to obtain the same strength increment can be reduced to 0.05, which is half of the N content required in the 0.005 9/6 steel.It can be seen that increasing N can significantly save V consumption and significantly reduce production costs.
Grain refinement
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Precipitation Kinetics of V (C, N) in V-N Steel
Refine with intragranular ferrite (IGF) technologyFerrite grainIt has become an important means of grain refinement.The results show that 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 of V (C, N) precipitation is increased, which promotes the precipitation of V (C, N) in austenite and creates favorable conditions for the nucleation of IGF.
After increasing N in steel, the dynamic conditions of V (C, N) precipitation in austenite are greatly improved. At the nose point temperature of 850~870 ℃, the precipitation time is greatly shortened, and the precipitation time in 0.2C steel is reduced to less than 10S, creating favorable conditions for V (C, N) precipitation in austenite.The precipitation of V (C, N) in austenite of low N steel is very slow, which usually takes several hours, and it is difficult to occur in the actual production process.
The V (C, N) particles precipitated from austenite are relatively coarse, which can be precipitated on MnS inclusions, and can also form separate V (C, N) particles with a size range of about 60~120nm.This kind of relatively coarse V (C, N) precipitation contributes little to precipitation strengthening, but can play the role of nucleation core of intragranular ferrite and significantly refine theFerrite grain。
V (C, N) intragranular ferrite nucleation
The precipitated V (C, N) particles in austenite of V-N steel are the effective core location of intragranular ferrite nucleation.Both V (C, N) particles precipitated from MnS inclusions and V (C, N) particles precipitated separately in V-N steel play a central role in the nucleation of intragranular ferrite during ferrite transformation.
V-NMicroalloyed steelThe strength level of the steel with obvious grain refinement effect was obtained.The precipitation of V in bainite can offset the strength loss caused by carbon reduction, thus obtaining the same high strength level in the steel with lower c content and improving the toughness and plasticity of the steel.The latest research points out that increasing N in steel helps to promote the precipitation of V (C, N) in bainite, further improvingBainitic steelStrength.The research on the application of V in bainitic steel is still in its infancy, and its mechanism needs further research[3]。
Application examples
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High strength reinforcement
Rebar is the largest consumption of steel products in China, accounting for about 1/5 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, it has accelerated the upgrading of steel bars.In 2000, the output of Grade III steel bars in China was only 260000 tons, accounting for 1% of the total output of steel bars;By 2011, the output of grade 3 reinforcement in China had reached about 70 million tons, nearly half of the total reinforcement.The rapid growth of high-strength steel bars has strongly promoted the application of V microalloying technology in China's steel industry.
The production speed of long products such as rebar is fast and the rolling temperature is high, usually above 1000 ℃. Its process characteristics determine that the alloy design of rebar is suitable for V microalloying technology.On the basis of 20MnSi reinforcement, the performance requirements of 400MPa and 500MPa high-strength reinforcement can be met by adding appropriate amount of V or V-N.As mentioned above, N is a very effective alloying element in steel containing V. By making full use of cheap nitrogen, the strengthening effect of V steel can be significantly improved, and the purpose of saving alloy consumption and reducing cost can be achieved.With the optimized design of V-N alloy composition and the control of cooling process, under the same strength level, the V content in V-N reinforcement is significantly lower than that in V steel.
The mass fraction of V in V-N microalloyed 400MPa high-strength steel bars can be reduced to 0.02~0.04, and the V content is reduced by half compared with V-Fe microalloyed steel bars.V-N microalloying process has become the main process route of high-strength construction reinforcement in China.Mass industrial production experience has proved that V-N reinforcement has stable and high performance, and its strength fluctuation range can be stably controlled within 75MPa, meeting the requirements of Grade I earthquake resistance.
Non quenched and tempered steel
A small amount of microalloying element V is added to medium carbon steel, and the ferrite pearlite structure is strengthened by the precipitation of fine vanadium carbonitride, so as to reach the strength level required by traditional quenching and tempering steel.This isNon quenched and tempered steelA basic principle of alloy design.
Non quenched and tempered steels developed in various countries have adopted V microalloying technology.According to different strength grades, the addition amount (mass fraction) of vanadium in non quenched and tempered steel is generally 0.06-0.20.
In order to exert the precipitation strengthening effect of V effectively, it is necessary to increase N in non quenched and tempered steel.The results show that increasing N to 0.015 ～ 0.020 in non quenched and tempered steel is very beneficial to improving the properties of steel.N mainly plays three roles in non quenched and tempered steel:
1) Promote the precipitation of V and improve the precipitation strengthening effect;
2) Grain refinement;
3) Improve the stability of TiN.
Grain refinement includes two aspects: one is that the precipitation of V (C, N) during phase transformation preventsFerrite grainGrowing up;Another important reason is that the precipitation of VN or V (C, N) in austenite in high N_V steel promotes the formation of intragranular ferrite (IGF).
S inNon quenched and tempered steelThe role of is significant.Properly increasing S content can improve the cutting performance of non quenched and tempered steel on the one hand, and the MnS particles formed can be used as the induced precipitation core of V.The composite effect of S and V is closely related to the existing form of V in non quenched and tempered steel.When the content of V and N in steel is low, the nucleation and precipitation temperature of V on MnS inclusions decrease, and the precipitation amount decreases. At the same time, the precipitation is C rich V (C, N) particles with larger lattice constant a.In theory, in order to maintain the coherent relationship with the ferrite structure, the N rich V (C, N) particles with smaller lattice constant a have less mismatch with the BCC structure, which is more conducive to ferrite nucleation.Therefore, increasing N content in non quenched and tempered steel while maintaining S and V content can significantly improve the control effect of intragranular ferrite.Furuhara et al. observed that under the condition of different sizes of MnS, MnS+V (C, N) composite precipitates have a significantly higher ability to promote the nucleation of intragranular ferrite than MnS or MnS+VC precipitates, and have an ideal ability to induce nucleation.
TiN pair controlAustenite grainThe effective role of TiN technology in high toughnessNon quenched and tempered steelIt is widely used in.In order to give full play to the role of TiN in grain refinement, it is necessary to control the content of Ti and N in the steel to be close to the ideal chemical ratio (3.42:1), and speed up the solidification rate of liquid steel, so that the volume fraction of TiN particles precipitated in the steel reaches the highest and the size is the smallest.The TiN technology used in non quenched and tempered steel usually requires only micro titanium treatment, and the amount of Ti added is 0.010~0.015.The test results have proved that N plays a beneficial role in improving the effect of TiN pinning austenite grain boundaries.When the content of N in steel exceeds the ideal ratio of Ti/N, the effect of TiN pinning grain boundaries is more effective.Increasing N in steel reduces the dissolution of TiN particles in high temperature austenite, hinders the growth of particles, thus improving the stability of TiN particles.
High strength strip steel produced by thin slab continuous casting and rolling
Thin Slab Continuous Casting and Rolling Process and TraditionHot rolled stripThe processes are very different.Firstly, due to the near net shape and rapid solidification characteristics of thin slab continuous casting and rolling process, the peritectic steel (C mass fraction 0.07~0.15) can not be produced by this process, and this composition range is just the typical composition of traditional HSLA steel.In order to meet the requirements of process conditions, most high-strength steels produced by thin slab casting and rolling technology are designed with low carbon content (C mass fraction is less than 0.07).Secondly, the traditional high strength hot rolled strip mainly adopts Nb microalloying technology, and the strength of steel is improved by grain refinement and precipitation strengthening through controlled rolling and cooling of Nb containing steel.However, for thin slab continuous casting and rolling process, Nb containing steel has difficulties in production due to slab crack, which has not been solved well up to now.
In addition, the thin slab continuous casting and rolling production line in the world mainly adopts the electric furnace process for smelting.The higher N mass fraction (0.008~0.010) in EAF steel not only increases the tendency of transverse crack formation in continuous casting billet of Nb containing steel, but also weakens the grain refining effect of Nb and the strengthening effect of Nb due to the precipitation of Nb (C, N) in austenite.In view of the above characteristics of thin slab continuous casting and rolling process, the principle of alloy design must be adjusted accordingly.The development of V-N microalloying technology has opened up 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 thin slab continuous casting and rolling process adopt V-N microalloying technology.
Low carbon (<0.07) and V-N microalloying alloy design technology route are adopted for thin slab continuous casting and rolling high strength steel with yield strength of 350 ～ 550MPa.For steel with lower strength (350~450MPa), V-N alloy system can meet the requirements.For 550MPa high strength steel, trace Nb is added on the basis of V-N microalloying, which can further refine without damaging the thermoplasticFerrite grain, significantly improving the strength of steel[2]。
expectation
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V is an important alloying element in special steel varieties such as high-speed steel, alloy tool and die steel, high temperature power station steel, etc. This is also the earliest application field of V in steel, which has a long history of 100 years.V has a mature technology and a wide range of applications in this field.With the continuous development of China's special steel industry, V will keep growing in this application field.
V microalloyed high strength low alloy steel is the largest application field of V in steel at present, and its application level is a sign to measure the development level of steel variety structure in a country/region.In recent years, significant progress has been made in the research, development, production and application of high strength low-alloy steel containing V in China. V microalloying technology has been applied to high strength X65/X70 pipeline steel, N80 seamless oil well pipe, high strength grade III reinforcement, high strength H-shaped steel and angle steelNon quenched and tempered steel, rail steel and other series products have been widely used.However, compared with the international advanced level, there is still a big gap in the consumption intensity of V in China's steel varieties.At present, the consumption intensity of V in Chinese steel products is less than 30g/t, about half of the world average level, and the gap with the advanced level (80~90g/t) of western industrial developed countries is even greater.It can also be seen that there is huge development space and broad application prospect for V alloying technology to be popularized and applied in China's iron and steel industry.
The latest research progress in V microalloying technology also provides impetus for the development of V containing steel.For example, the research on V-N microalloying technology has made N become an economic and effective alloying element in steel containing V. By giving full play to the role of cheap N, the precipitation of V in steel has been optimized, the role of V in grain refinement strengthening and precipitation strengthening has been enhanced, the strength of steel has been significantly improved, the amount of V has been reduced, and the cost of steel has been significantly reduced.At present, the research results of V-N microalloying technology have been widely recognizedNon quenched and tempered steelHigh strength seamless steel pipe, high strength large angle steel and other varieties have been successfully applied, which has effectively promoted China's VMicroalloyed steelDevelopment of.
The combination of V (C, N) intragranular ferrite nucleation technology and recrystallization controlled rolling technology has formed a new generation of TMCP technology. While giving play to the traditional precipitation strengthening role of V-containing steel, it effectively refined the size of the steel by relying on the nucleation role of intragranular ferrite played by VN precipitation in austeniteFerrite grain。This technology has been well applied in the development of high strength thick section H-shaped steel, high strength thick steel plate and other varieties.The combination of V-N microalloying and V-Nb composite microalloying alloy design with thin slab continuous casting and rolling technology has changed people's understanding of traditional strip steel products, and developed a new strip steel product with high strength ultra-fine grain ferrite pearlite structure with good strength toughness matching, and the yield strength level reaches 550 ～ 650MPa.At present, China is 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 V-N microalloying technology in high-strength thin slab continuous casting and rolling strip products has important guiding significance for the development of similar product structure in China.In addition, other new research achievements, including the strengthening effect of V in bainitic steel, the application of V in TRIP steel and BH steel, and the H absorption effect of V to improve the delayed fracture resistance of steel, will also help to expand the application field of V in steel[2]。
summary
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The improvement and rapid development of China's manufacturing industry have put forward higher requirements for steel varieties.In order to meet the ever-changing needs of industrialization in China, the requirements for large-scale and super large-scale industrial equipment and structures are increasingly urgent, and the service environment is increasingly harsh.Under the condition that the steel variety and grade remain unchanged, it is necessary to continuously increase the thickness or diameter of steel products to meet the safety requirements of equipment or structures.This not only brings rapid increase in material procurement and manufacturing costs, but also brings difficulties in design and processing. At the same time, the use risk of equipment and structures also increases sharply.Therefore, in order to meet the huge demand of China's industrialization process and the needs of the steel industry's own industrial structure upgrading, the upgrading of steel varieties is still one of China's important strategic goals in the next 5 to 10 years.Rational utilization of technological and economic advantages of V (micro) alloying has always been one of the important technical ideas for upgrading steel products.The adjustment of steel variety structure in China provides broad prospects for the development of V-containing steel[3]。
