The Characteristics of Refractory Steel and Its Development and Application in Petroleum and Other Fields

Because the strength of structural steel decreases sharply at high temperature, it must be sprayed with fire-resistant coating or fire-resistant materials for composite use. The strength of refractory steel can be reduced by no more than 1/3 of the room temperature at high temperature, and maintain good high temperature strength. This article mainly summarizes the development history of refracto-ry steel in China and other countries, analyzes the material selection basis and strengthening mecha-nism of refractory steel, and finds that by combining Mo element solid solution strengthening, phase transformation strengthening, and Nb, the strength guarantee of refractory steel at high temper-atures and V element precipitation strengthening are obtained. Designers can calculate the service temperature of steel structure through the initial stress and design strength of components to select refractory steel. Finally, the application status of refractory steel and its application prospect in the field of oil rescue are summarized and prospected.


Introduction
In China, structural steel is mainly divided into carbon structural steel and high-strength low-alloy steel.Structural steel is widely used in the construction machinery and construction industry due to its good processing performance, high reliability, and high degree of industrialization.Structural steel has poor fire resistance, and its yield strength typically decreases significantly at high temperatures.In order to improve the fire resistance of structural steel, measures such as spraying refractory coatings or composite wrapping of non-metallic refractory materials are often used, which not only increases costs but also increases construction difficulty.In view of this, the development of refractory steel has attracted the attention of scholars.The definition of fire-resistant steel is that at a temperature of 600 ℃, the yield strength is not less than 2/3 of the yield strength at room temperature, and other properties such as room temperature mechanical properties, weldability, and processability are required to be basically consistent with the corresponding specifications of ordinary structural steel [1][2][3] .This article provides a detailed introduction to the characteristics and development applications of refractory steel, especially the application prospects of refractory steel technology in the field of petroleum emergency equipment in China.

Foreign Development
In 1987, the Ministry of Construction of Japan first stipulated the standard for fire-resistant steel in the "Code for Comprehensive Fire Protection Design of Buildings": the strength should be maintained at 2/3 of the strength at room temperature during a fire, which provided a clear technical goal for uncoated fire-resistant steel.D Iron Works, a leading Japanese heavy steel plate production enterprise, has added alloy elements such as Nb, V, Ti to the pipes, greatly improving the performance of the steel and laying an important foundation for the development of refractory steel.Through comprehensive consideration of the tensile test results and the room temperature characteristics and economy of structural steel, Japanese researchers have further specified the ideal heat resistance temperature of refractory steel as 600 ℃, and believe that the duration of fire is 3 hours, thus determining that the basic composition system of refractory steel is the Mo-Nb system.In 1988, refractory steel was successfully applied in the Second Nippon Steel Building in Japan [4] .

Progress in China
Major steel mills in China have also continued to invest in research and production of refractory steel.The refractory steel developed by Shanghai Baosteel Group has properties comparable to those of Japanese FR steel at high temperatures, meeting the requirements for reducing the yield strength of refractory steel at high temperatures.It can last for 2.5 hours at 1080℃ without yielding.In 2001, Zhongfu Building Project, as the first high-rise building in Shanghai to use a fully steel structure design, adopted fire-resistant steel produced by Shanghai Baosteel Group, which was also the first batch production of fire-resistant steel in China [5] .
The composition design concept of Ma Steel's refractory steel adopts a microalloy strengthening method based on low-carbon C-Mn steel.First, the research team conducted research on Mn Mo Nb alloy steel, Mo containing steel, low Mo Nb Ti alloy steel, Mo Nb Cr alloy steel, and then conducted Exploratory research on Mo-V-Ti alloy steel, Mo-Cr-V-Ti alloy steel, and Nb-V-Ti-Re microalloyed steel, finally determined the composition of refractory steel, and successfully developed refractory H-shaped steel.It basically overcomes the weakness of ordinary building structural steel such as Q235 and Q345 grade steel, which quickly softens at high temperatures, reduces the usage rate of refractory coatings in engineering applications, and plays an important role in environmental protection.It not only has good room temperature performance, high temperature performance, and welding performance, but also has excellent high-temperature creep resistance.Its product has been successfully applied to the civil steel structure residential project of Zhongfu Garden in Shanghai [5 ， 6] .
Ansteel's research on refractory steel began in 2000 and currently has mastered the production technology of refractory steel at the levels of 400MPa and 490MPa.The 490 grade refractory steel developed by it has been successfully industrialized and applied to the construction of a factory building in a company in Anshan.
According to the existing standards and test methods in China, the fire resistance limit test results of Ansteel's self-produced refractory steel for building use show that, the yield strength of Ansteel's refractory steel is not less than 2/3 of the room temperature value within 30 to 150 minutes of insulation at a high temperature of 600 ℃; Reaching the fire resistance limit of National Building Class 4 beams in an uncoated state; Thin coating reaches the fire resistance limit of Level 3 beams; The thick coating reaches the fire resistance limit of Level 1 beams, and the thickness of the thick coating material for fire-resistant steel is reduced by 1/2 compared to ordinary Q345 steel [7] .The welding performance is good, fully meeting the requirements of the national standard, and it passed the national appraisal in August 2003.
The composition design and mechanical properties comparison of refractory steel developed in China and Japan are shown in Tables 1 and 2. There is no essential difference between domestic refractory steel and Japanese refractory steel, both of which are molybdenum niobium series refractory steel.However, the production of refractory steel in China is still just beginning.Basically in the trial production stage, there are issues such as high production costs and unstable performance.This requires various manufacturers to collaborate with universities and research institutions to carry out basic research on refractory steel, such as research on refractory mechanism, brittle transformation characteristics, observation and analysis of microstructure, measurement of continuous cooling transformation curve, and metallographic analysis, in order to solve this series of problems.

Design and Material Selection of Refractory Steel
Jin Tao [12] provided a calculation method for the critical temperature of fire-resistant steel in the fire-resistant design of fire-resistant steel.The critical temperature refers to the temperature at which the fire-resistant steel is subjected to fire under the effective load of fire and reaches the ultimate bearing capacity state.For general structural steel,the commonly used calculation formula is Formula 1.Where 1 represents the critical temperature of the steel structure, 2 represents the initial stress of the component, and 3 represents the design strength of the component Due to the significant decrease in yield strength of steel when Ts>550 ℃, when using equation ( 1), when Ts>550 ℃, Ts is taken as 550 ℃.Obviously, equation ( 1) is not applicable for calculating the critical temperature of refractory steel.Through extensive experimental calculations, it can be concluded that when 550 ℃<Ts<900 ℃,The relationship between the effective yield strength of refractory steel and the effective yield strength  , the formula for calculating the critical temperature of refractory steel is: By using equation ( 2), designers can calculate the service temperature of the steel structure based on the initial stress and design strength of the components, making it easier for designers to select refractory steel materials.

Structure and Properties of Refractory Steel
At present, fire-resistant steel mainly adds Mo, Nb, V, Ti, Cu and other microalloying elements on the basis of C-Mn Si Cr alloying.Based on the composition and structure design concept of alloy elements "hot rolling solution, controlled phase transformation, precipitation of nano second phase strengthening in case of fire", it adopts air cooling after hot rolling, water cooling after hot rolling, controlled cooling after hot rolling, hot rolling direct quenching+critical heat treatment processes to form ferrite, bainite, martensite The composite structure of residual austenite and the "soft" and "hard" phases precipitated from the nano second phase.Among them, the main factors affecting the fire resistance of refractory steel are as follows: (1) Alloying element: Mo is considered to be a necessary alloying element in the design of refractory steel.With the increase of its content, the fire resistance increases significantly.The mechanism of Mo to improve the fire resistance mainly includes: solution strengthening, precipitation strengthening, reducing the aging rate of precipitates to refine the second phase precipitation and promoting the bainite transformation.On the one hand, microalloying elements Nb, V and Ti can promote the formation of bainite in steel, reduce the formation temperature of bainite and refine the structure of bainite under the hot rolling solution state; After being heated by fire, nano second phase precipitation is formed, which can hinder the migration of the interface, inhibit tissue coarsening, interact with dislocations, and achieve precipitation strengthening effect.(2) Microstructure: bainite can improve the high temperature strength of refractory steel.The yield strength at 600 ℃ increases with the increase of bainite content.It is generally believed that the hard phase bainite structure can use the ferrite/bainite interface to prevent the yield deformation of soft phase ferrite and improve the high temperature yield strength of steel.It should be noted that the morphology of bainite is also a key factor affecting its fire resistance.
Mn series bainitic steel is one of the two major bainitic steel series in the world.Compared with Mo series bainitic steel, it has the advantages of low cost, air cooling and self hardening, and simple process.After nearly 50 years of development, several series have been formed.Among them, Mn series grain boundary imitated ferrite/granular bainite multiphase steel has high strength, high plasticity, high toughness, and low yield ratio.It has good application prospects in mechanical structure steel, construction steel and other fields.
According to the classical strengthening theory of multiphase structure, Fang Hongsheng [8] of Tsinghua University, based on the ferrite/pearlite multiphase steel, proposed to replace the pearlite in the ferrite/pearlite steel with granular bainite (GB) to improve the strength of the steel, and used a moderate amount of semi continuous non reticulated grain boundary type ferrite FGBA to replace the massive proeutectoid ferrite as the ductile phase to improve the plastic toughness.The model of this grain boundary like ferrite/granular bainite multiphase structure and typical SEM structure morphology are shown in Figure 1.Xu Pingguang [9] and others designed low carbon bainite series of~0.12wt.%C-Mn Si Cr based on the design idea of grain boundary imitated ferrite/granular bainite multiphase structure.The grain boundary imitated ferrite/granular bainite multiphase structure can be obtained under air cooling conditions.Research has shown that in crack propagation, grain boundary ferrite can passivate the crack tip and hinder crack propagation; Due to the fact that FGBA and GB are not always connected, cracks repeatedly traverse the FGBA and GB tissues during the propagation process, constantly changing the propagation direction, and thus absorbing a large amount of impact energy, increasing toughness.
In order to further improve the toughness of grain boundary imitated ferrite/granular bainite multiphase steel, Wang Jianping and Wang Yongwei [10] studied the effect of austenite deformation on the microstructure and impact toughness of this multiphase steel.The research shows that austenite pre deformation can significantly refine the size of FGBA, and the volume fraction of FGBA decreases, but the content of intragranular ferrite (IF) increases.At the same time, the size of granular bainite Flat noodles beam, bainite Flat noodles and M/A island are refined.The strength and toughness of grain boundary imitated ferrite/granular bainite multiphase steel have been greatly improved by austenite pre deformation.On the basis of this research, the controlled rolling process is combined with the grain boundary imitated ferrite/granular bainite multiphase steel, and the plate with good strength and toughness is obtained.
Feng Chun [11] reduced the carbon content to~0.08wt.% on the basis of~0.12wt.%C-Mn Si Cr low-carbon bainite series, and designed~0.08wt.%C-Mn Si Cr Nb low-carbon bainite steel through Nb microalloying.The research shows that the addition of 0.02wt.%Nb can increase the hardenability of bainite, promote the transformation of granular bainite, and significantly refine the size of FGBA, M/A island and bainite Flat noodles.After air cooling after rolling, excellent properties with tensile strength of 937MPa, yield strength of 650MPa, and room temperature impact AKV of 80J are obtained.In order to improve the welding performance of this series of low-carbon bainitic steels, Feng Chun designed a low-carbon bainitic steel of~0.08wt.%C-Mn Si Cr Nb system on the basis of~0.08wt.%C-Mn Si Cr Nb steel by removing the added Cr element and adopting the controlled cooling technology.Through the controlled rolling and cooling technology, the excellent properties of tensile strength 929MPa, yield strength 741MPa and room temperature impact AKV 151J were obtained.
Refractory steel is different from heat-resistant steel in that it is used in a short-term flame burning environment.Therefore, in the composition design process, the main consideration is its high-temperature yield strength, without overly considering its long-term creep performance.As shown in Table 3, refractory steel has achieved good high-temperature performance, and its high-temperature strengthening essence includes microstructure changes, solid solution strengthening effect of alloy elements, and precipitation strengthening effect of the second phase.
Shen Junchang [13] showed that the microstructure of Mo Nb refractory steel is a mixed structure of ferrite, MA structure, and a small amount of pearlite.The solid solution strengthening effect of Mo in the steel, the decomposition of MA structure at high temperature to form stable alloy cementite, and the significant increase of MC and Mo2C precipitates at around 600 ℃ are important reasons for the good high-temperature performance of refractory steel.Liu Zhiyong. [13]believe that good structural stability at high temperatures is beneficial for improving the high-temperature performance of refractory steel.The calculation results using thermo-calc thermodynamic software show that alloy elements such as Mn, Mo, Cr, V can be melted in large amounts in the cementite, forming alloy cementite, which has a significant impact on preventing high-temperature tempering microstructure coarsening and maintaining good high-temperature performance.This research result is also consistent with Rikio Chi jiiwa's conclusion that Mo is prone to forming a segregation layer around the NbC second phase particles, which makes it difficult for the NbC second phase to aggregate and grow at high temperatures, thereby improving the high-temperature yield strength [14] .He Jing [15] conducted a detailed study on the effects of Mo and V elements on refractory steel, and believed that Mo, which is dissolved in ferrite, plays a role in solution strengthening.The MoC particles precipitated in the steel play a role in precipitation strengthening.It is an effective alloy element for improving the yield strength of refractory steel at room temperature and high temperature.V mainly improves high-temperature yield strength through precipitation strengthening.The composite addition of Mo and V is an effective method to improve the high-temperature yield strength.The precipitation strengthening effect of MC and Mo2C, which significantly increases the precipitation phase and maintains small size, effectively improves the high-temperature mechanical properties of refractory steel

Application of Refractory Steel 4.1 Construction field
In recent years, the research and production of fire-resistant steel in China has developed rapidly.Some steel enterprises have successfully developed fire-resistant steel and fire-resistant weathering steel, which have been applied in some steel structural engineering.The main types of refractory steel are plate and H-shaped steel, as well as a small amount of pipe materials.The maximum thickness of the plate is 100mm, and the maximum wall thickness of H-shaped steel is 40mm.At present, advanced foreign countries have developed a series of refractory steel varieties, such as 400MPa grade, 490MPa grade, 520MPa grade, etc., not only forming a strength series, but also developing refractory steel with various other functions, such as corrosion-resistant refractory steel, easily welded refractory steel, seismic resistant refractory steel, etc [16] .
Ma Steel collaborated with the General Research Institute of Iron and Steel to develop a Mo Nb series 490MPa fire-resistant H-beam product using advanced hot-rolled H-beam production lines.It was successfully used in the Fucheng project of Shanghai's first all steel structure high-rise residential building group, using it as a steel beam for residential buildings.Ma Steel supplied H300mm to the project in total × 150mm and H400mm × More than 1400 tons of fire-resistant steel with 150mm2 specifications [3,17] .
The refractory weathering steel B490RNQ produced by Baosteel has excellent fire resistance performance.At present, Baosteel can provide two brands of refractory weathering steel: B400RNQ (mechanical properties at room temperature are equivalent to Q235B) and B490RNQ (mechanical properties at room temperature are equivalent to Q345B).In 2001, over 5000 tons of steel were successfully used in the Shanghai Zhongfu Project, marking the first production and use of this steel in China.In addition, more than 100 tons of B490RNQ fire-resistant weathering steel were supplied to the Baosteel Building of the Great Wall Station in Antarctica [18] .
Ansteel has been developing refractory steel since 2000, and currently has the production technology of 400MPa and 490MPa series refractory steel.Ansteel has put into industrial production the Cr Mo Nb-V series 490MPa high-performance refractory steel.Industrial produced refractory steel plates have been provided to Anshan Yongbao Steel Products Co., Ltd. for welding into H-beams for the construction of the factory building of Anshan Dongfang Color Plate Co [19] .

The Application Prospects of Oil Emergency Equipment in China
In the process of oil and gas extraction, when a fire accident occurs, mechanical devices such as hydraulic sandblasting cutting equipment, obstacle cleaning equipment, and wellhead reset equipment are required to complete the rescue work of the accident well.The operating condition of emergency rescue equipment is a short-term flame burning environment, and rescue equipment needs to complete rescue operations in the fire for about 4 hours.At present, China's oil rescue equipment adopts a mode of using fire-resistant materials wrapped in structural steel and operating under water mist spraying conditions.However, considering that future emergency rescue operations will have high wellhead pressure levels in extreme environments, resulting in insufficient rescue equipment capabilities, it is necessary to select and design high-temperature resistant materials in the future to prevent the failure of wellhead cutting and resetting devices during operation time in extreme environments.Refractory steel is one of the good alternative materials for structural components of petroleum rescue equipment.Feng Chun and others from China Petroleum Engineering Materials Research Institute have achieved good results in simulating and reproducing the petroleum rescue process through the trial production of 420MPa and 460MPa grade high-performance refractory steel.In the future, the application of fire-resistant steel in the field of oil emergency equipment can further improve the safety guarantee of oil extraction.

Conclusion
Refractory steel has overcome the disadvantage of sharp decline in strength of structural steel in hightemperature environments.With the investment of major production and research institutions in China in recent years, the strengthening mechanism of refractory steel has been continuously clarified, and production technology and evaluation methods have been continuously improved.In the future, refractory steel will be increasingly used.In addition, with the continuous development of refractory steel technology and the continuous decrease in production costs, refractory steel will be applied in more fields besides the construction field.
design strength f of the steel is used instead of 20 Y

Table 2 .
Mechanical Properties of Refractory Steel.

Table 3 .
Strengthening Mechanism of Refractory Steel