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2022 Vol. 57 No. 10
Published: 2022-10-15
Technical Reviews
Raw Material and Ironmaking
Steelmaking
Metal Forming
Materials
Environmental Protection and Energy
Technical Reviews
1
ZHU Rong, WEI Guang-sheng, ZHANG Hong-jin
Research and prospect of EAF steelmaking with near-zero carbon emissions
Carbon emissions from China's iron and steel industry account for 16% of the country's total emissions. Reducing carbon emissions from the iron and steel industry is an important starting point for the Chinese government to realize its commitment of "carbon peaking and carbon neutrality". In the context of carbon neutrality,the global steel industry is accelerating technological innovation to reduce carbon emissions. This paper puts forward a new steelmaking process of electric arc furnace(EAF) steelmaking with near-zero carbon emissions,and points out that technological innovation should be carried out from three aspects: near zero carbon emissions in energy source,smelting process and raw material production,so as to realize near zero carbon emissions in steelmaking process. In the aspect of near zero carbon emissions energy source,it is proposed to use mixed utilization of solar energy,wind energy and valley power,effective storage and release of energy,micro smart grid and optimized power supply to realize the direct and efficient utilization of green energy. In the aspect of near zero carbon emissions in smelting process,the key technologies such as hydrogen burner,carbon free foaming agent,CO
2
-Ar bottom blowing for denitrification,submerged O
2
-CaO injection for dephosphorization and system energy efficiency evaluation are proposed to realize non-carbon smelting. In the aspect of near zero carbon emissions in raw material production,it is proposed to use green hydrogen direct reduction ironmaking and related supporting technologies,green electric plasma hot blast furnace and carbon capture process to reduce carbon emissions in the production process of raw and auxiliary materials. The energy input in EAF steelmaking process, carbon dioxide emissions from smelting and raw materials production are calculated. Combined with the analysis of carbon reduction capacity of the above key technologies,it is calculated and predicted that the ultimate limit carbon emissions of the new process can be reduced to 64 kg/t. Therefore,it is necessary to carry out research on the EAF steelmaking with near-zero carbon emissions and accelerate its industrial application,which would promote the innovative development and green low-carbon development of China's iron and steel industry.
2022 Vol. 57 (10): 1-9 [
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10
HAN Xiu-li, LIU Ying-ying, LIU Lei, YAN Xiao-peng, LIU Zi-yao
Review on research progress of properties and application of titanium-containing continuous casting mold flux
With the advancement of iron and steel technology and the needs of social development,iron and steel enterprises are developing and producing various types of high-quality special steel grades with high strength,good toughness and strong corrosion resistance. How to effectively control the stability of the continuous casting production process for special steel grades is a common problem faced by the current mold flux development. Designing and developing reasonable chemical composition of mold slag,stabilizing its physical and chemical properties,and ensuring good slab quality of special steel grades are hot issues concerned by researchers in the field of iron and steel metallurgy. To this end,relevant researchers have carried out a large number of basic theoretical and applied researches on mold slag,and achieved certain results. Most scholars believe that adding a certain amount of TiO
2
to the traditional mold slag can improve the physical and chemical properties of the mold slag,improve the ability to adsorb inclusions, prevent the steel-slag interface reaction,and reduce the occurrence of slab quality problems. When the TiO
2
content in the mold slag is in the range of 4% to 8%,the alkalinity is controlled between 1.1 and 1.3,which can effectively improve the physical and chemical properties of the mold slag such as melting temperature,viscosity and heat flux density,and can also promote the crystallization of the slag film. It can basically meet the requirements of special steel grades such as high-titanium steel and high-alumina steel;the perovskite in the titanium-containing mold slag film can replace the gun spar in the traditional mold slag film,which has a great effect on the heat transfer of the mold slag. It plays a decisive role;after the titanium-containing mold slag absorbs the inclusions in the molten steel,the physical properties of the mold slag remain stable,and can prevent the "fishing" on the surface of the slab caused by the interface reaction between the titanium in the special molten steel and the mold slag. The occurrence of titanium-containing mold slag has been initially explored and applied in some special steel grades and staged results have been achieved. About the influence mechanism of perovskite and other minerals in the slag film on the heat transfer of titanium-containing mold powder,how to better coordinate the contradiction between the basic performance of mold powder and the lubricating heat transfer mechanism,and realize the continuous connection of titanium-containing mold powder. The wide application in the casting process to meet the high-quality,high-efficiency and stable production needs of the continuous casting process is still the main subject of further research by metallurgical workers.
2022 Vol. 57 (10): 10-18 [
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19
ZHONG Hong-gang, WANG Ren-jie, ZHAI Qi-jie
Experimental research methods and applications of solidification processes in metallurgical industry
The metal solidification in metallurgical production is extremely complex. Besides the non-equilibrium self-organization phenomenon of the solidification process of multi-component alloys,it is also influenced by various factors,such as geometry,heterogeneous substrates,physical fields and so on,making it very difficult to study. Since solidification process has a significant impact on the quality of the final product,it is of great value to study it under production conditions,to investigate the evolution of structures and defects,and then to optimize the process parameters. While,experimental studies are the basis and key to investigate the phenomena,to find the laws,and to verify the numerical models. We review the common experimental research methods of metal solidification process under production conditions,focusing on physical simulation,thermal simulation and industrial experiment. The physical simulation,including transparent substance simulation,water simulation and low melting point metal simulation,has advantages in observing solidification phenomena and revealing mechanisms,but cannot directly reflect the phenomena and principles of actual metals in metallurgical solidification process. The thermal simulation method directly adopts the target metal as the experimental material and conducts experimental simulation research based on the similarity of the temperature field,which can directly reflect the metallurgical solidification process. It is a method with low cost and closer to the actual situation. Industrial experiments are an indispensable research tool to validate models and new technologies,but they are also characterized by high costs,difficulties in implementation,and long lead times. In general,several experimental research methods have their own characteristics and fields of application. Combining and developing different experimental methods is the future trend of metallurgical solidification process research.
2022 Vol. 57 (10): 19-29 [
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30
YAN Feng, HUANG Xiao-ming, GUO Rong-xin, FENG Ming-jie, ZHANG Shuai, LI Hao
Research status of improving volume stability of steel slag by pretreatment
The volume restlessness of steel slag is one of the main factors restricted its application in road materials. The volume stability pretreatment of steel slag is the key to effective utilization of steel slag, and also is the necessary condition to improve the utilization rate of steel slag in the direction of road materials. In view of volume restfulness for steel slag, the research progress and present situation of volume restfulness of steel slag at home and abroad are summarized from four aspects, that are, physical property of steel slag, factors causing volume restfulness of steel slag, determination methods of volume restfulness for steel slag and pretreatment measures for improving volume restfulness of steel slag. The results show that performance of steel slag is good, but the mineral composition is complex. The main factors causing the volume restlessness of steel slag are free calcium oxide and free magnesium oxide, which can make steel slag expand and crack under certain conditions and restrict the application of steel slag. Ethylene glycol-EDTA-TG differential thermal analysis and EDTA titration are more accurate and common methods for the determination of free calcium oxide and free magnesium oxide in steel slag. Most of domestic pretreatment improvement measures are aimed at the free calcium oxide in steel slag, including natural aging, steam curing, the surface modification treatment, chemical treatment and carbonization processing. Among them, surface modification and chemical treatment for pretreatment of steel slag measures are hot spots in recent years, while carbonization can cut steel slag expansive rate around 70% to 90%, which has better effect compared with other pretreatment measures, but its equipment requirements and costs are higher. Foreign countries mainly adopt natural aging method to improve the volume stability of steel slag. Therefore, the application on a large scale for pretreatment of steel slag needs to have social, economic, environmental and other benefits at the same time, which requires researchers to carry more in-depth research and discussion. Finally, suggestions and prospects for the future development of steel slag resource utilization in road materials are put forward based on previous studies.
2022 Vol. 57 (10): 30-42 [
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Raw Material and Ironmaking
43
LIU Qi-hang, WANG Di, ZHAO Xiao-wei, YANG Shuang-ping, HU Qu
Multiscale characterization and application of microstructural evolution of blast furnace coke
Coke is an important raw material for blast furnace smelting, and the multi-scale characterization of its microstructural evolution plays a crucial role in the rational evaluation of coke quality and blast furnace operation. During carbon loss reaction, different degrees of degradation gradients generate inside the porous structure of coke, which have a very important impact on coke behavior, but the current coke evaluation system fail to take this factor into account. The ratio of the reaction rate constant
k
rea
to the diffusion coefficient
D
eff
(
k
rea
/
D
eff
) was proposed to characterize the effect of the carbon loss reaction on the deterioration of the coke's spatial structure, it could provide a basis for further accurate characterization and prediction of the degradation gradient inside the coke, thus optimizing the coke quality. The matrix reaction characteristics and pore structure evolution characteristics of coke with different particle size were studied. The composition and pore structure parameters of different carbon loss cokes were analyzed by Fourier transform infrared spectroscopy, optical microscope, SEM-EDS and BET specific surface area pore size distribution analyzer. The results show that with the deepening of coke's carbon loss, minerals gradually precipitate on the surface of coke, and play a catalytic role in the process of carbon loss, resulting in a decrease in activation energy
E
a
and an increase in
k
rea
. At this point, the micropores in coke expand and merge into mesopores and macropores, the diffusion path of CO
2
molecule decreases, the diffusion activation energy
E
D
decreases, and
D
eff
increases gradually. In the middle and late period of carbon loss reaction, the active components in coke are consumed and a large amount of ash in coke is precipitated, which increases the activation energy
E
a
and decreases the
k
rea
. Moreover, the number of macropores in coke is further increased, and the tortuous degree of porous structure is greatly reduced, which leads to the decrease of diffusion activation energy
E
D
and the increase of
D
eff
. Sudying the ratio of coke
k
rea
and
D
eff
shows that
k
rea
/
D
eff
decreases rapidly with the increase of carbon loss reaction. And the larger the
k
rea
/
D
eff
is, the larger the deterioration gradient inside the coke is, and the coke powder is easily generated from the coke surface, which is not conducive to the actual production of the blast furnace. Therefore, on the basis of reducing the coke dissolution loss rate or
CSR
, controlling
k
rea
/
D
eff
within a reasonable range is an effective measure to further improve the coke quality.
2022 Vol. 57 (10): 43-54 [
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55
ZHEN Chang-liang, CHENG Cui-hua, ZHANG Qiao-rong, ZHAO Kai
NO
x
formation behavior analysis and control of oxygen coal combustion at front end of tuyere
Taking pulverized coal combustion process at the front end of blast furnace tuyere as the research object, the formation rules of gas composition and NO
x
under different constraints were analyzed by thermodynamic calculation systematically. The effects of different operating parameters such as temperature, ratio of coal species, pulverized coal composition, oxygen-enriched and coal injection on NO
x
formation behavior were discussed, and several directional advices to reduce NO
x
formation were proposed. The results show that there are three ways to generate NO
x
by pulverized coal combustion at the front end of blast furnace tuyere, thermal, prompt and fuel. Temperature is an important factor affecting the formation of thermal NO
x
, when the temperature is higher than 2 000 ℃, the NO
x
production at the front end of blast furnace tuyere increases by more than 30% for each 100 ℃ increase. Within the calculation temperature range (2 000-2 400 ℃), NO
x
production increases from 4 056 mg/m
3
to 12 942 mg/m
3
, which is far more than the traditional coal-fired boiler. The ratio of bituminous coal increases from 0 to 50%, NO
x
production increases from 4 152 mg/m
3
to 7 486 mg/m
3
, and the production content of NO
x
increases by 80%. The ratio of bituminous coal increased from 0 to 50%, and production content of NO
x
increased by 80%. When the coal ratio is 80 kg/t, NO
x
production still reaches 18 006 mg/m
3
even without rich oxygen. NO
x
generation is significantly reduced by 68% when the oxygen supply for 1 mol carbon in pulverized coal decreased from 2.0 mol to 1.2 mol. Therefore, source control of NO
x
generation can be realized by adjusting the theoretical combustion temperature, reducing the ratio of bituminous coal, using low volatile coal, reasonably matching the level of oxygen-enriched and coal injection. In addition, from the perspective of blast furnace NOx emission, the content level of NO
x
in the top gas is also significantly affected by the internal reduction of the blast furnace: under normal smelting conditions, the generated NO
x
can be fully reduced in blast furnace, and the NO
x
content of the top gas of the blast furnace is usually below 50 mg/m
3
, but attention should be paid to the NO
x
emission level of blast furnace gas and downstream gas users, when it is in the special operating mode such as low stock line, hanging, blowing-off, blowing-in, and so on.
2022 Vol. 57 (10): 55-63 [
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Steelmaking
64
WANG Kang-hao, JIANG Min, LI Kai-lun, WANG Xin-hua
Formation and evolution of inclusions in GCr15 bearing steel produced by process of BOF-LF-RH-CC
To study the evolution of inclusions in GCr15 bearing steel, the GCr15 bearing steel produced by BOF-LF-RH-CC process was sampled in various stages in a steel factory. The evolution behavior of inclusions in various processes was systematically analyzed by ASPEX scanning electron microscopy and thermodynamic calculation. The result indicates that due to the strong deoxidation of aluminum after the converter, at the beginning of LF refining, the inclusions in molten steel consists of MgO-Al
2
O
3
binary system and CaO-MgO-Al
2
O
3
ternary system with high Al
2
O
3
(
w
(Al
2
O
3
)=84%); the account of MgO-Al
2
O
3
binary system and CaO-MgO-Al
2
O
3
ternary system are 74% and 26% at the end of LF refining. At this moment, the inclusions in the liquid steel have a size distribution from 1 to 6 μm account for 87%. During refining from LF to RH, the total number of inclusions decreased from 198 pcs/(20 mm
2
) at the end of ladle furnace refining to 103 pcs/(20 mm
2
) at the end of RH vacuum degassing, a drop of 48%. Among them, MgO-Al
2
O
3
binary system were mainly generated during LF refining, and then removed during RH refining, specifically, the number increased from 88 pcs/(20 mm
2
) at the beginning of LF refining to 139 pcs/(20 mm
2
) at the end of LF refining, and after the end of RH soft blowing, it was reduced to 4 pcs/(20 mm
2
); CaO-MgO-Al
2
O
3
ternary system were mainly generated during RH refining, and their number increased from 49 pcs/(20 mm
2
) at the end of ladle furnace refining to 108 pcs/(20 mm
2
) at the end of RH soft blowing. This shows that RH vacuum refining has a better effect on inclusion removal. Thermodynamic calculations show that the contents of Al
s
and Mg in the liquid steel are in the MgO-Al
2
O
3
inclusion formation region during the secondary refining process, indicating that MgO-Al
2
O
3
inclusions are easier to form; When
w
([Mg])in the liquid steel is 0.000 3% and
w
([Ca]) is greater than 0.000 25%, the thermodynamic conditions for the transformation of MgO-Al
2
O
3
inclusions into CaO-MgO-Al
2
O
3
inclusions are satisfied. When
w
([Al
s
]) is 0.022%,
w
([Ca])is controlled between 0.000 25% and 0.007 00%,which is more conducive to the formation of liquefied calcium aluminate. During the test,
w
([Ca]) in the liquid steel was about 0.000 1%-0.000 4%, and the inclusions were mostly converted into CaO-MgO-Al
2
O
3
ternary system.
2022 Vol. 57 (10): 64-72 [
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73
CHANG Li-zhong, XU Tao, SU Yun-long, ZHANG Long-fei, ZHU Chun-li, SHI Xiao-fang
Changes of cleanliness and carbide during vacuum preparation of stainless bearing steel
In order to improve the cleanliness and refine the carbide structure of G102Cr18Mo high carbon stainless bearing steel, the process route of vacuum induction melting(VIM), vacuum consumable arc remelting(VAR) and forging with large forging ratio was adopted to research the influence of vacuum treatment and large forging ratio on chemical composition, gas content, inclusions distribution, secondary dendrite spacing and carbide size. The results show that during VIM process, the deoxidation ability of carbon decreases greatly with the increase of aluminum content. Even 0.003% aluminum has an obvious obstacle function to deoxidation of carbon. The oxygen content is greatly reduced due to the improvement of thermodynamic conditions such as high vacuum degree, high remelting temperature and reaction kinetic conditions during VAR process. After the primary VAR, the mass percent of oxygen decreases to 0.000 57% from 0.001 49%, reduced 61.7%, and after the secondary VAR, the mass percent of oxygen decreases to 0.000 50%. During VIM and VAR, the composition of inclusions changes little, the main inclusions are Al-Si inclusions, followed by Al
2
O
3
inclusions, MnS inclusions, Mg-Al-Ca and Mg/Ca-Al inclusions. The inclusions of diameter over 20 μm are completely removed, there are only a few inclusions with a diameter of 10-20 μm and the inclusions are mainly composed of fine inclusions with diameter less than 5 μm after double vacuum smelting. The cleanliness of G102Cr18Mo steel is greatly improved. It is found that the morphology of secondary dendrite changes little from edge to core on the cross section of VAR ingot. The secondary dendrite spacing increases slowly from the edge to the core, and varies from 85 to 95 μm. Such fine dendrite structure benefits from low remelting speed. The VAR ingot is forged repeatedly and finally forged into a round rod with a diameter of 40 mm. Through metallographic examination, it is found that the maximum size of carbide particles is no more than 20 μm and the average size is 15 μm. No aggregated carbides are found. Low VAR speed and forging with large forging ratio are the key to refinement of carbides.
2022 Vol. 57 (10): 73-83 [
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84
TIAN Yu-feng, LI Guang-qiang, XIAO Yong-li, LIU Yu
Effect of residual CO
2
on dissolution of lime in converter slag
In the process of converter steelmaking, the rapid dissolution of lime is of great significance for efficient dephosphorization of converter. The calcium silicate product layer formed at the slag/lime interface in the process of lime dissolution is considered to be the key factor hindering lime dissolution. Two kinds of partially calcined limestone with different CO
2
content were prepared. The dissolution behavior of partially calcined limestone in the primary slag of converter was studied by immersion method, and compared with that of pure lime and limestone. The results show that the mass transfer coefficient of CaO in the liquid slag during the dissolution of limestone is 2.1 times that of lime, and the mass transfer coefficient of partially calcined limestone with 10% residual CO
2
is as high as 6.7 times that of limestone. When the mass fraction of CO
2
is in the range of 0-43.5%, the dissolution rate of lime firstly increases and then decreases. The 2CaO·SiO
2
layer formed in the process of lime dissolution seriously hinders the diffusion of FeO
x
, so it slows down the dissolution rate of lime. Significantly different from lime, CO
2
produced by limestone decomposition can destroy the 2CaO·SiO
2
layer and internal structure, which is conducive to the penetration of slag. This is also applicable to partially calcined limestone with residual CO
2
. In the process of preparing pure lime, in order to ensure the complete calcination of the lime core, it is very easy to cause over-burning on the outer surface of lime, and the preparation of partially calcined limestone can solve the problem of over-burning on the surface. Furthermore, compared with limestone, the temperature drop of slag near the surface of partially calcined limestone is relatively lower because the surface is lime shell, which can avoid the stagnation stage at the initial stage of dissolution. When the surplus heat of converter is limited, the lime replacement ratio of partially calcined limestone is higher than that of limestone, which depends on the CO
2
residue in partially calcined limestone.
2022 Vol. 57 (10): 84-90 [
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91
SUN Cong-lei, CAI Lai-qiang, WANG Xu-dong, YAO Man
Investigation on thermo-mechanical behavior of continuous casting slab based on element-free Galerkin method
Compared with the numerical computation method based on grid discretization,the meshless method avoids the problems of complex topology structure,element numbering and information transfer by replacing the grid with discrete nodes,and shows great flexibility in simulation and analysis of movement interface of phase transition,large deformation and dynamic crack propagation. A field variable approximation function is constructed based on the moving least squares method. According to the heat transfer characteristics of continuous casting slab during the initial solidification in mold,the control equation of thermo behavior of continuous casting slab based on element-free Galerkin method are constructed and derived. Then combined with the weak form of Galerkin integral and the variation principle,the thermal elastoplastic model for two-dimensional cross-section slab is established. Taking the heat flux calculated inversely from the measured temperatures of copper plate as boundary conditions,the non-uniform heat transfer,stress and strain of initial solidified shell inside mold are simulated and analyzed. The results show that in the width direction of mold,thermo-mechanical behavior shows significant non-uniform characteristics. The stress-strain increases rapidly within 200 mm from the meniscus,and then the growth rate of stress-strain slows down. At 550 mm from the meniscus,the equivalent stress-strain appear maximum values,which are 15.97 MPa and 0.76%. In the width direction,due to the influence of two-dimensional cooling,the temperature of the corner decreases rapidly and the phenomenon of stress concentration appears. However,there is a low valley of stress-strain in the adjacent deflection corner,which is different from the corner in temperature,stress and strain. The results verify the feasibility and applicability of the meshless method to calculate the non-uniform heat transfer and mechanical behavior for continuous casting process,and provide a basis and reference for the application of the meshless method to the research of formation and propagation of slab cracks.
2022 Vol. 57 (10): 91-100 [
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101
MA Fan, LIU Qing, ZHANG Jiang-shan, WANG Chao, SUN Jian-kun, LI Ming
Influence of jet characteristics nozzle on penetration behavior to vapor film in secondary cooling zones of continuous casting
The surface temperature of the slab in the second cooling zone of continuous casting is usually higher than 900 ℃. At this temperature,the spraying droplets will not wet the surface of the slab when they contact the high-temperature slab,but only form a vapor film on it,which prevents the subsequent heat transfer between the droplets and the slab surface. In view of the above problems,a nozzle jet simulation calculation model is established based on the flat water nozzle in the secondary cooling zone of continuous casting in a domestic steel plant. The model is verified by theoretical calculation and laboratory experiments. The flow field distribution in the free jet area of the nozzle is studied by numerical simulation and the evolution law of the jet droplet size is measured by the continuous casting nozzle cooling detection system. Combined with the numerical simulation results and laboratory measurement results,the variation law of the depth of the vapor film on the surface of the billet by the jet droplets impacted by the nozzle under different working conditions is quantitatively analyzed. The results show that the maximum jet velocity of the nozzle is at the nozzle outlet. The jet can maintain a large jet velocity near the nozzle outlet. With the increase of water volume,the distance of the jet to maintain a high jet velocity also increases. The axial velocity of the whole jet accounts for more than 80%. When the amount of water is larger, the particle size of jet droplets becomes smaller. As the distance from the nozzle outlet increases,the droplet size at the center of the jet increases gradually and reaches the maximum value. When the water flow rate is 9 and 12 L/min,respectively,the droplet size is basically the same,indicating that the increase of the cooling water does not affect the droplet size distribution when the water flow rate increases to a certain amount. Under different initial flow rates,the depth of droplet penetration through the steam film on the billet surface increases firstly and then decreases slightly with the increase of spray distance. When the spray distance is between 100-200 mm,the penetration depth of droplet is the largest,which indicates that the spray cooling effect is the best when the nozzle spray height is within this range.
2022 Vol. 57 (10): 101-109 [
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110
ZHANG Wei-yang, CHENG Shu-sen
Mechanism of air aspiration and control model of argon flow volume for submerged entry nozzle
The submerged entry nozzle and its connection with the upper nozzle are easy to inhale air in continuous casting. Air aspiration leads to the oxidation of molten steel,nodule at the nozzle and serious continuous casting accidents. Argon injection inside the nozzle to prevent air aspiration,improper control of injecting argon is easy to cause defects in slab. Based on Bernoulli's theorem and law of conservation of mass,a velocity-mass model from the tundish to the mold is established,and the mechanism of air aspiration by the above components is discussed. Firstly,the influences of nozzle diameter,nozzle immersion depth,tundish level, slab width and casting speed on the ratio(
A
A
/
A
P
) of cross-sectional area at nozzle inlet to cross-sectional area at nozzle outlet are analyzed in both ideal and pressure loss cases. Then,the nozzle structure is optimized and the control model of argon flow is established. In order to prevent air aspiration into the submerged entry nozzle,the diameter of the nozzle should be reduced,the difference between the liquid level in the tundish and the nozzle immersion depth should be decreased,the casting velocity and the width of the cast slab should be increased. Among them,the nozzle diameter has the greatest influence to
A
A
/
A
P
. When the nozzle diameter increased by 10%,
A
A
/
A
P
increased from 2.15 to 2.62. The effect of slab width and casting speed was slightly lower than that of nozzle diameter. When the slab width and casting speed increased by 10%,
A
A
/
A
P
decreased from 2.15 to 1.96. The liquid level and immersion depth of tundish have the least influence on it. Based on the research results,the nozzle structure is optimized in accordance with the flow beams of liquid steel. Combined with the volume of vacuum area in the nozzle,the control model of argon blowing quantity is determined,which keeps the slightly positive pressure in the nozzle. The results provide a theoretical basis for reducing or even eliminating the air aspiration and controlling the argon flow rate,which is of great significance to the production of high-quality steel and energy saving and consumption reduction.
2022 Vol. 57 (10): 110-119 [
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120
WU Yu-han, CHEN Wen, YANG Xin, HE Zhi-jun
Erosion behavior of low carbon mold flux on submerged nozzle refractory
In order to achieve the goal of "double carbon", domestic iron and steel enterprises are currently committed to developing high-performance low-carbon and ultra-low carbon mold powder. Low carbon or ultra-low carbon mold flux has appropriate physical and chemical properties,which is the basic condition to ensure efficient and stable continuous casting production. However,there is little research on the wetting and erosion behavior of low carbon mold flux on submerged nozzle materials. In this paper,the wetting and erosion behavior between medium and low carbon mold flux and ZrO
2
-C nozzle refractory were studied through high temperature wetting experiment. Combined with microstructure analysis,the erosion mechanism of low carbon mold flux on nozzle slag line material was clarified. The results show that the wettability of low-carbon mold flux and ZrO
2
-C nozzle refractory is better,and compared with the traditional medium carbon mold flux,the contact angle between low-carbon mold flux and ZrO
2
-C material is smaller and the mold flux spreads faster on the nozzle surface at the same temperature range. According to the microstructure analysis,the erosion degree of low-carbon protective slag on ZrO
2
-C material of nozzle slag line is serious, and the erosion depth is large. Due to the influence of carbon content,the wettability between low-carbon mold flux and ZrO
2
-C material of nozzle slag line is better,which provides favorable kinetic conditions for the dissolution and penetration of low-carbon mold flux into the nozzle. In addition,compared with medium carbon slag,the carbon concentration difference between low-carbon mold flux and nozzle material is larger,resulting in greater diffusion driving force of carbon atoms,which affects the wetting,dissolution and chemical reaction of the interface between the two phases. In addition, the industrial test results show that the necking of the slag line of the immersed nozzle is more serious when the low-carbon mold powder is used during the continuous casting production process. The results of this study can provide certain guidance and reference for the continuous casting production.
2022 Vol. 57 (10): 120-126 [
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127
JI Qing-tao, YU Jie, NING Jing, LIANG Jian-xiong, YANG Zhi-yong, LIU Zhen-bao
Numerical simulation of vacuum arc remelting process of USS122G ingot
Ultra high strength stainless steel with its ultra high strength and good toughness and excellent corrosion resistance is widely used in aviation,aerospace and other fields. Vacuum arc remelting(VAR) is the main production technology of ultra-high strength stainless steel ingot,which can remove harmful impurities and improve element segregation in steel. In this paper,the vacuum arc remelting process of a high strength stainless steel USS122G was studied. Through the process simulation optimization software (Melf-Flow),the macroscopic heat transfer,mass transfer and flow phenomena of the VAR process were simulated,and the two-dimensional axisymmetric mathematical model of the VAR process of USS122G alloy was established.The temperature field and morphology of molten pool under different cooling rates are predicted. The evolution of magnetic field,temperature field and flow field under specific cooling rates and the macrosegregation of elements with or without helium cooling are emphatically analyzed. In order to validate the agreement between the model and experiment results,a full-scale USS122G ingot has been prepared. The results show that with the increase of melting rate,the morphology of molten pool changes from flat to semi-circular,and finally to deep U-shape,the molten pool depth becomes deeper gradually. The melting rate of 4.5 kg/min is determined as the actual melting rate,molten pool morphology has a narrow mushy zone,under the melting speed,molten pool morphology presents the arc shape,and the input power of the vacuum self-consuming furnace is low. The flow field simulation results show that the flow direction of the fluid is downward along the edge and upward in the middle,and it moves clockwise on the right side of the ingot. The simulated pool depth is 132 mm after reaching steady state,which is consistent with the measured results. The Cr and C elements have positive segregation,and the segregation degree of the elements in the ingot cooled by helium is small. From 1/2
R
away from the ingot axis to the edge,the measured element distribution is in good agreement with the simulation results. The research results provide reliable data support for the steady industrial production of steel.
2022 Vol. 57 (10): 127-138 [
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Metal Forming
139
LI Wei-gang, XU Kang, LI Jin-ling, ZHAO Yun-tao
Research and application of surface defect recognition algorithm for hot rolled strip
In the production process of hot continuous rolling, different types of defects will appear on the surface of the strip,which will adversely affect the strip performance and cause quality objection in serious cases. The strip surface inspection system currently used online often needs to manually adjust the defect image library,and the adjustment of typical defect images of some categories will affect the detection accuracy of other types. This paper uses the deep learning method to design a lightweight residual network LDS-ResNet14,which reduces the number of layers and width of the original residual network ResNet18,and replaces the ordinary convolution with a depthwise separable convolution. At the same time,the knowledge distillation (KD) method is used to transfer the knowledge of the large residual network ResNet50,and a loss function that mixes KL divergence and cross entropy is proposed to better transfer the knowledge to LDS-ResNet14,the accuracy and generalization ability of the model are improved while compressing the model. The experimental verification shows that the average recognition accuracy of the network LDS-KD-ResNet14 proposed in this paper is 99.16% for the 8 types of strip surface defects of the WISCO CSP hot strip mill,which is 0.67% higher than that of ResNet18,and the calculation amount is only 12.1% of the original. The actual field application shows that for the four types of defects of WISCO CSP,namely folding,oil pollution,inclusion and pitting,the model defect detection rate reaches 96.43%,the defect recognition rate reaches 94.10%,and the detection speed of a single image is 16.5 ms,to meet the actual production requirements.
2022 Vol. 57 (10): 139-147 [
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Materials
148
ZHAO Wei, HUANG Jin, XU Guo-hua, WANG Lei
Effect of heat treatment on microstructure and mechanical properties of super alloy GH3128 weld joint
With the increasing demand of electricity and awareness of environmental protection,nuclear is gradually replacing fossil. As the core component of ULTRA-high temperature gas cooled reactor (UHTR),intermediate heat exchanger (IHX) has also attracted extensive attention. The superalloy GH3128 is expected to become candidate material for ultra-high temperature gas cooled reactor intermediate heat exchanger (UHTRIHX) due to well weldability,excellent mechanical properties,superior thermal stability and outstanding creep resistance. The welding process is necessary for heat exchanger for production and manufacture. Due to the high degree of heat input concentration and the speedy cooling rate welding. During the subsequent high-temperature service process,brittle phase precipitation and recrystallization occur in this zone,which affects the mechanical properties and long-term service life. Therefore,the GTAW welding of superalloy GH3128 sheet was conducted,the microstructure evolution,residual stress and mechanical properties of weld joint was investigated. The results show that under the optimization of welding parameters,solid-solution plate joint shows the highest tensile strength and plasticity,both room temperature and high temperature tensile fracture position were the base metal. Residual stress in the HAZ was exist without post weld heat treatment (PWHT). The hardens,and the residual stress induces μ phase precipitate in the during high temperature deformation,the precipitation of μ phase deteriorates the creep rupture properties of weld joint. Post-weld heat treatment eliminated residual stress of the joint,precipitation of μ phases was decrease during high temperature deformation,and improved the creep rupture time of weld joint. Under the Temperature of 1 200 ℃,the residual stress can provide activation energy for recrystallization,recrystallization occured in the heat affected zone of the joint during the post-weld heat treatment process. The hardness decreases,and the plastic deformation was ununiform,induced the room temperature and 950 ℃ tensile fracture position was welded joints. After different post-welding heat treatment of the solid solution plate specimen,the EBSD results showed that after the joint heat treated at 1 100 ℃×10 min,the residual stress and strain disappeared,and with the temperature raised to 1 140 ℃,the heat affected zone began to recrystallize.
2022 Vol. 57 (10): 148-157 [
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158
TAO Xue-ru, GENG Xin, JIANG Zhou-hua, LI Yang, PENG Lei-zhen
Effect of element B on microstructure and mechanical properties of FB2 steel
In order to study the effect of B content on the microstructure and mechanical properties of FB2 steel,five groups of FB2 steels with different B contents were immersed and corroded for 90 s with Veller's reagent (1 g picric acid,5 mL hydrochloric acid and 100 mL anhydrous ethanol),and the morphology was observed and analyzed. Then,The M
23
C
6
carbide in the matrix was observed and analyzed by SEM-EDS,and the size of precipitated phase was analyzed by Image-Pro Plus software. Then,the AG-XPLUS 100 kN universal testing machine produced by Shimadzu Manufacturing Institute of Japan was used to conduct room temperature tensile test and room temperature impact test on 5 groups of FB2 steel with different B content. Finally,the microstructure and mechanical properties of five groups of FB2 steels with different B contents were analyzed. The results show that the B element in FB2 steel can inhibit the growth coarsening of M
23
C
6
type carbides,but B element is easy to form BN inclusions with N element. With the increase of B content,the average size of M
23
C
6
carbide decreases,and the unit number and average size of BN inclusions show an increasing trend. The tensile properties of FB2 steel at room temperature increase first and then decrease with the increase of B content,and the tensile properties of FB2 steel reach the maximum at 0.010% B mass percent. When the mass percent of B increases from 0 to 0.010%,the tensile properties of FB2 steel increase at room temperature because the addition of B prevents the coarsening of M
23
C
6
carbide,and thus improves the strength of FB2 steel. When the mass percent of B increases from 0.010% to 0.030%,the tensile property of FB2 steel decreases at room temperature because the BN inclusion formed by the combination of B element and N element causes stress concentration,and the large size BN inclusions induce the formation of holes,which reduces the properties of FB2 steel. The impact energy of FB2 steel decreases with the increase of B content,and plummets when the B mass percent is 0.020% and 0.030%,which is caused by the unit number of BN inclusions,the increase of average diameter,and the precipitation of large size BN and debris.
2022 Vol. 57 (10): 158-169 [
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170
ZHANG Ning-fei, CUI Zhi-qiang, WANG Jie, HOU Qing-yu, HUANG Zhen-yi
Tensile mechanical behavior and strengthening-toughening mechanism of silicon-nickel low density steel
In order to analyze the tensile deformation behavior of silicon-nickel alloyed austenite base low density steel at medium temperature,instron electronic tensile testing machine was used to analyze the tensile deformation behavior of Fe-28.64Mn-8.99Al-1.68Si-1.39 Ni-1.0C(Mn29Al9Si2Ni). The mechanical behavior of the low-density steel was studied by temperature tensile tests at 23-300 ℃. The strengthening and toughening mechanism of the steel was studied by SEM,TEM and thermodynamic calculation. The results showed that with the increase of strain,the stress-strain curve of warm tension mainly included several processes,such as elastic deformation,uniform plastic deformation and fracture,and there was no obvious yield phenomenon. With the increase of temperature,the strength of the steel decreases gradually,and the plasticity(elongation to fracture)increases first and then decreases and then increases. At 200 ℃,the plastic trough appears. At this time, the stress-strain curve and the strain hardening rate curve of the steel have obvious sawtooth characteristics,and the strain hardening rate has little change with the increase of strain. However,the zigzag characteristics of stress-strain curve and strain hardening rate curve at other temperatures are not obvious,and the strain hardening rate decreases gently with the increase of strain. The main strengthening and toughening mechanisms of the steel at 23-300 ℃ are κ-carbide strengthening,strain strengthening, twin-induced plasticity and dynamic strain aging strengthening. Low temperature dislocation mobility is poor for the promotion of twinning induced effect,Ni elements and Si elements twin inhibition,temperature increase twin inhibitory effect and temperature variations and promoting or inhibitory effect on dynamic strain aging is the steel in 23,100 and 300 ℃ obvious twinning induced plasticity and obvious dynamic strain aging under 200 ℃. The main cause of reinforcement. Dynamic strain aging strengthening is the main reason for the plastic trough of the steel at 200 ℃.
2022 Vol. 57 (10): 170-177 [
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178
ZHU Kang-feng, MA Heng, SONG Xin-li, JIA Juan, LIANG Xiao-kai, SUN Xin-jun
Cause analysis of low temperature toughness fluctuation of 550 MPa steel plate for offshore engineering
The impact energy of 550 MPa Marine engineering steel fluctuates greatly at low temperature. In order to further explore the cause of low temperature toughness fluctuation,impact tests were carried out on the test steel at different temperatures (-100 ℃-room temperature). The impact fracture,microstructure,second phase and inclusions were analyzed with optical microscope,scanning electron microscope and transmission electron microscope. The reason of low temperature toughness fluctuation was analyzed and discussed by thermodynamics calculation. The results show that the strength of the tested steel meets the requirements of grade,the impact absorption energy decreases with decreasing temperature,and the toughness and brittleness transition temperature is about -50 ℃. At -60 ℃,the impact energy fluctuates greatly,and an extremely low value of 18 J appears. The fracture is quasi-cleavage fracture,the shear section rate is 8%,and there are compound inclusions of Ti,Nb(C,N) and MnS at the crack source. At the same temperature,the shear section of the test steel with impact energy of 122 J is 34%,and there are obvious dimples in the fracture. The microstructure of the test steel is mainly tempered bainite and very little ferrite. There are high density dislocations in bainite slat. There are (Fe,Cr)
3
C alloy cementite and a small amount of NbC and Cu-rich precipitates on grain boundary. The test steel mainly has small Angle grain boundary,and large Angle grain boundary is relatively low. There are a small amount of (Ca,Al,Mg,Mn,S) inclusions in the matrix,which are nearly round and polygonal in size between 1-3 μm,accounting for 85.87% of the total amount detected. The highest proportion of cas-oxide-MNS inclusions was 31.2%. Thermodynamic calculation results show that TiN precipitates earlier than MnS during solidification. Many factors,such as coarse precipitated phase of grain boundary and ingrain,inclusions,high proportion of small Angle grain boundary and immobile dislocation of plugging,have adverse effects on the impact toughness at low temperature. The existence of large titanium-containing precipitated phase is the key reason for the large fluctuation of impact toughness.
2022 Vol. 57 (10): 178-187 [
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Environmental Protection and Energy
188
HAN Jing, DU Bin
Analysis method of influence of iron steel ratio change on comprehensive energy consumption per ton of steel
In the context of carbon peak and neutralization,increasing the proportion of scrap consumption and reducing the iron/steel ratio can directly affect the decline of comprehensive energy consumption per ton of steel in iron and steel enterprises. Reducing the iron/steel ratio is an effective way to improve energy efficiency,save energy and reduce carbon in China's iron and steel enterprises based on integrated iron and steel route production,which has become a research hotspot in the iron and steel industry. The main ways and corresponding technical measures to reduce the iron/steel ratio,such as reducing the heat loss of molten iron,increasing the temperature of scrap entering the furnace and reducing the tapping temperature of converter,are briefly analyzed from the point of view of heat balance. In order to completely and quantitatively analyze the impact of the change of iron/steel ratio on the comprehensive energy consumption per ton of steel in iron and steel enterprises,based on the system energy saving in iron and steel enterprises,this paper adds the impact of the recovery of residual energy per ton of steel caused by the change of iron/steel ratio on the comprehensive energy consumption per ton of steel on the basis of the change of iron/steel ratio,and puts forward the quantitative analysis method of the impact of the change of iron/steel ratio on the energy consumption of integrated iron and steel route enterprises. Finally,taking the actual data of several iron and steel enterprises as an example,the quantitative analysis is carried out. Under the condition of this case,the iron/steel ratio is reduced from 0.950 in the benchmark period to 0.790 in the statistical period. The changes of the steel ratio coefficient in the pre iron process,the energy consumption in the steelmaking process and the recovery of residual energy per ton of steel affect the comprehensive energy consumption per ton of steel by -71.282 kgce/t,-1.000 kgce/t and 9.687 kgce/t respectively,and the total comprehensive energy consumption per ton of steel is reduced by 62.595 kgce/t. The quantitative analysis of the case shows that the comprehensive energy consumption per ton of steel decreases by 3.912 kgce/t for every 0.01 reduction of iron/steel ratio. The decline of iron/steel ratio can significantly improve the performance of comprehensive energy consumption per ton of steel in integrated iron and steel route enterprises.
2022 Vol. 57 (10): 188-194 [
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