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2022 Vol. 57 No. 2
Published: 2022-02-15
Technical Reviews
Raw Material and Ironmaking
Steelmaking
Metal Forming
Materials
Equipment Technology
Technical Reviews
1
KOU Ming-yin, ZHANG Zhong, ZENG Wang, ZHOU Heng, WU Sheng-li
Research progress on optimization technology and its model of ore-blending for sinter process
With the continuous consumption of iron ore resources, the qualities of iron ores are deteriorating, and a large number of new iron ore appear, making the demand for ore blending optimization of sinter process more urgent. Therefore, the basic characteristics of iron ores at room temperature and high temperature are mainly introduced in this paper. The room temperature characteristics mainly include chemical composition, type of iron ore and particle size distribution. At the same time, high temperature characteristic indicators that are closely related to sintering production are analyzed in this paper, which mainly including the assimilation, liquid phase fluidity, bonding strength, calcium ferrite generation characteristics, crystal joint characteristics, melting characteristic, liquid absorption and consolidation strength of sintered body. Their definitions, meanings and measurement methods are introduced respectively. The development history of ore blending optimization technology is then introduced from the ore blending exploration stage to ore blending based on room temperature characteristics and then to ore blending based on the complementation of high temperature characteristics. The iron ore blending expert system model and its application in actual production are illustrated. It is indicated that the expert system model of ore blending optimization can efficiently guide the ore blending in sinter process, reduce the cost of sintering raw materials, and obtain better sintering production and quality indexes. Furthermore, the paper summarizes the ore blending optimization technology of iron ore sintering, and points out the bottlenecks and shortcomings that still exist, which are including that the standards and index system of high temperature characteristics of iron ore sintering are not yet perfect, there is a lack of quantitative relationship between the basic characteristics of iron ore and the quality of sintered ore, and the accuracy of sintering ore blending expert system is insufficient. The optimization of sintering ore blending is also prospected, it is indicated that the high temperature characteristics and system of iron ore powder sintering still need to be developed and perfected. The optimization and improvement of ore blending expert system for high-temperature characteristics of iron ore, and the automation and intelligence of ore blending optimization for sinter process need to study further.
2022 Vol. 57 (2): 1-11 [
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Raw Material and Ironmaking
12
QIU Hao, QU Li-kai, ZHANG Xue-hong, LIU He-ping
Influence of coking processes on coke quality made by wet coal and dried coal
In order to optimize the coking process and improve the coke quality effectively of coal blending, it is necessary to analyze the influence of coking process and dry cool and wet coal coking on coke strength and reactivity (
CRI
) systematically.Coking experiments of dry coal (moisture less than 3%) and wet coal (moisture more than 6%) were carried out in a 5 kg experimental coke oven under different working conditions. The effects of moisture content, dry base bulk density and charging temperature of coal on coke strength and reactivity were investigated, combined with the comparison of the optical texture and heating characteristics of coke center in coking process of dry coal and wet coal, and the mechanism of influence on coke quality was analyzed, which provides a theoretical reference for optimizing coking process. The results show that when the moisture and the charging temperature keep unchanged, the strength level of coke can be improved effectively by increasing the dry base bulk density of coal. When coking by wet coal, the moisture of which is higher than 6%, excessive moisture content (more than 8%) is not conducive to improve the wear resistance property of coke. However, when coking by dry coal, whose moisture is less than 3%, and when the moisture increases, the coke strength will decrease. If moisture content of coal is maintained at 2% can make the coke reactivity (
CRI
) stay at a higher level. By controlling the heating process to adjust the charging temperature and shorter coking time,the post-reaction strength(
CSR
) of coke could be improved, but the coke reactivity would be reduced.
2022 Vol. 57 (2): 12-18 [
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236
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19
WANG Gui-lin, ZHANG Jian-liang, LIU Zheng-jian, WANG Yao-zu, LIU Fu-cheng, TONG Zhen
Effect of high-sulfur concentrate on properties of sinter
Concentrate is widely used in iron ore sintering process because its higher grade than that of imported ore-rich powder. However, there are few studies about the effect of high-sulfur concentrate on properties of sinter. Therefore, in order to study the effects of high-sulfur concentrate on the properties of sinter and influence of S element in concentrate on sintering process, a series of sinter cup experiments with high-sulfur concentrate concentration of 25%-45% were carried out, and the influences were characterized through microstructure, sintering indices, and metallurgical properties. The experimental results showed that when the concentration of concentrates was 25%, the interface reaction conditions of reduction were poor, and the diffusion of reduction gas were hindered by silicate phase. This resulted in a lower reduction index with 77.80%, the initial softening temperature was 1 200 ℃, and the permeability of cohesive zone deteriorated. The concentrates with 30% increased the sintering utilization index to 1.19 t/(m
2
·h), and improved vertical sintering speed to 22.22 mm/min. The sinter with high-sulfur concentrate concentration of 40% had improved reduction property and deteriorated the low temperature reduction degradation property. When the concentration was 45%, the sintering utilization index was the highest with 1.20 t/(m
2
·h), the reduction performance and reduction degradation property were suitable. On the whole, an appropriate increase in high-sulfur concentrate content was beneficial to improve the reduction performance and softening-melting performance of the sinter, but when the concentrate ratio was 45%, the
S
-value was 281.02 kPa·℃, meaning poor permeability performance of sinter. In the terms of sintering flue gas, when concentration of concentrates was 40%, the CO
2
and NO
x
content of sintering flue gas were high, showing a high oxidizing atmosphere in the sintering process, which decreased the content of low-reducibility minerals such as iron olivine in the sinter and deteriorated the reduction degradation index. The results of flue gas analysis showed that the S element of high-sulfur concentrate basically entered sintering flue gas during sintering process, and the performance of sinter was not deteriorated.
2022 Vol. 57 (2): 19-27 [
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280
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28
HE Xiao-yi, LIU Zhou-li, WU Sheng-li, ZHAO Bin, YANG Fan
Development and application of optimized ore blending platform for blast furnace ironmaking system in Baotou Steel
In order to realize the integration and collaborative analysis of blast furnace ironmaking system from iron ore procurement and blast furnace ore blending optimization, this paper develops a blast furnace ironmaking whole system and whole process optimization ore blending platform for the front ironmaking system. The whole process refers to the whole process from iron ore procurement to blast furnace molten iron output. The whole system refers to the overall system of all blast furnace ironmaking in iron and steel enterprises. The optimized ore blending platform includes four parts, which are database system, single blast furnace optimized ore blending platform, whole system blast furnace optimized ore blending platform and production data acquisition and analysis platform. The platform takes iron ore as the starting point, based on the calculation of blast furnace ironmaking process and material balance, uses planning solution, linear regression, multivariate nonlinear regression, neural network and other algorithms to establish data analysis and calculation model, and forms optimal ore blending decision through statistical analysis of data. The system can realize the integrated optimization of iron ore procurement, raw ore processing, mineral processing, sintering, pellet and blast furnace process, and provides the iron ore procurement and configuration scheme that meets the requirements of blast furnace production and has the lowest cost under specific time and specific conditions. At the same time, the cost of intermediate products and molten iron raw materials in the blast furnace system can be calculated by optimizing ore distribution platform, and the deviation between production cost and planned cost also can be calculated. Managers can adjust and optimize the ore blending scheme according to the deviation of comparative data and the change of market conditions. The application of the platform in Baotou Steel realizes the intelligent, digital and accurate management of iron ore procurement and configuration. Under the premise of meeting the requirements of blast furnace production, the raw material cost of molten iron and the procurement cost of iron ore in Baotou Steel are reduced by optimizing the distribution of platform, which brings considerable economic and social benefits to the enterprise.
2022 Vol. 57 (2): 28-35 [
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36
WANG Zhen, SHEN Feng-man, LIU Chun-cheng, ZHENG Hai-yan, DU Jin-lei, ZHANG Yan
Melting characteristics of blast furnace slag with high Al
2
O
3
In order to understand the melting characteristics of blast furnace slag with high Al
2
O
3
(
w
(Al
2
O
3
)≥15%), the influences of different
w
(MgO)/
w
(Al
2
O
3
), basicity
R
and
w
(Al
2
O
3
) on the melting temperature and heat of high aluminum blast furnace slag were discussed by differential scanning calorimeter (DSC). The experimental results show that the initial melting temperature (
t
onset
) of slag is 1 248-1 291 ℃, the end melting temperature(
t
end
) is 1 432-1 485 ℃, and the heat of melting is 137-211 J/g. When
w
(Al
2
O
3
)=15%, the eutectic reverse reaction occurs at high
w
(MgO)/
w
(Al
2
O
3
), which leads the initial melting temperature of blast furnace slag decreases. However, the end melting temperature of blast furnace slag is almost unchanged because the liquidus temperature does not change. When
w
(Al
2
O
3
)=20%, with the increase of
w
(MgO)/
w
(Al
2
O
3
), magnesia-alumina spinel with high melting point is easily formed in the slag, which leads to the increase of initial melting temperature of slag, meanwhile, the end melting temperature of slag decreases due to the decrease of liquidus temperature. With the increase of
R
, compounds with high melting point are formed in the blast furnace slag, and the liquidus temperature of slag increases, making both the slag initial melting temperature and end melting temperature increase. With the increase of
w
(Al
2
O
3
), the eutectic reverse reaction occurs, so the initial melting temperature of slag decreases. However, with the increase of
w
(Al
2
O
3
), the Al—O bond with higher bond energy in slag increases, and the final melting of slag needs to be realized at a higher temperature, that is, the end melting temperature increases. With the increase of
w
(MgO)/
w
(Al
2
O
3
), basicity
R
and
w
(Al
2
O
3
), the melting heat (Δ
H
) of slag increases. Among them, compounds with high melting point are formed in the slag and the melting heat increases with the increase of
R
. While the
w
(Al
2
O
3
) in slag increases, the Al—O bond in slag increases, and the amount of heat consumed by depolymerization of slag structure also increases. And with the increase of
w
(MgO)/
w
(Al
2
O
3
), the formation of high melting point compounds or the decrease of initial melting temperature is resulting in the increase of melting heat.
2022 Vol. 57 (2): 36-45 [
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Steelmaking
46
WANG Dan, GUO Zhi-hong, HUO Yan-peng, FAN Jian-tong, ZHU Li-guang
Optimization simulation of injection process in 130 t DC electric arc furnace
Under the background of healthy economic development, EAF steelmaking has gradually become the object of vigorous development of major iron and steel enterprises with its characteristics of low pollution, short process and diverse products. It plays an important role in the whole iron and steel production than ever before, and the flow state of EAF molten pool will directly affect the smelting effect of EAF. It is mainly related to the injection intensity and arrangement of oxygen lance. In order to study the change of flow field of electric arc furnace under different working conditions and select the most appropriate process parameters for large electric arc furnace, taking a 130 t electric arc furnace in a plant as the prototype, the side blowing model and three-dimensional full-scale geometric model of electric arc furnace are established. The mixing time, average velocity, turbulent kinetic energy distribution. Based on the volume ratio of dead zone and weak flow zone, the effects of different injection intensity and arrangement on the flow field of 130 t electric arc furnace were systematically studied. The results show that when the arrangement of oxygen lance is C, the stirring effect of oxygen lance on the middle and lower parts of molten pool is significantly enhanced, the mixing time is significantly reduced, and the proportion of dead zone and weak flow zone is reduced, which is 3.8% and 9% respectively compared with the arrangement of A. In the study of oxygen lance injection intensity, it is found that the mixing time of molten pool decreases gradually with the increase of injection intensity, but the reduction range decreases. When the oxygen lance injection intensity is 0.31 m
3
/(t·min), the turbulent kinetic energy of the fluid in the molten pool is the largest. The average velocity of the cross section at 100 and 400 mm below the steel liquid level in the molten pool increases by 42.9% and 31.4% respectively, accelerating the energy transfer in the smelting process.
2022 Vol. 57 (2): 46-53 [
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54
ZHU Ren-lin, LI Jian-li, YU Yue, ZHU Hang-yu
Effect of sulfur content on occurrence state of sulfur in KR desulfurization slag
The CaO component in KR desulfurization slag is an important slagging raw material in converter smelting process. Its reuse in converter smelting can reduce the CaO raw material consumption as well as the accumulation of KR desulfurization slag for metallurgical enterprise, and save the economic cost of smelting. Moreover, 2CaO·SiO
2
(C2S) in the KR desulfurization slag can form stable C2S-C3P(2CaO·SiO
2
-3CaO·P
2
O
5
) solid solution with phosphorus in the slag to improve the stability of phosphorus during converter dephosphorization smelting. Reusing KR desulfurization slag instead of activated lime as converter slag can promote early slagging in the initial stage of converter smelting and improve the dephosphorization efficiency. However,
w
((S)) in the KR desulfurization slag was 1.0%-2.5%, reusing it directly in converter smelting would lead to sulfur increase in molten steel. Therefore, in order to clarify the influence of sulfur content on occurrence state of sulfur in molten slag, and to provide a theoretical basis for subsequent effective removal of sulfur from KR desulfurization slag through oxidizing atmosphere,the solidification process of each phase in the molten slag was simulated by FactSage8.0, the basic phase and micro morphology of molten slag were analyzed and detected by SEM-EDS and XRD, and the average grain area of CaS in the slag sample was analyzed by Image-Proplus 6.0. Thermodynamic calculation showed that the crystallization temperature of CaS in molten slag increased from 1 240 to 1 390 ℃ when
w
((S)) was 0.5%-2.5% in the molten slag. In addition, the content of MeO#1 phase and the viscosity of slag gradually decreased in molten slag with the increase of sulfur content at 1 600 ℃. When
w
((S)) was 0.5%, the sulfur in the molten slag existed in amorphous structure, while
w
((S)) was 1.0%-2.0%, the sulfur mainly existed in the form of CaS. When
w
((S)) was 2.0%-2.5%, the sulfur existed in CaS phase and Ca
11
(SiO
4
)
4
O
2
S phase, and Ca
11
(SiO
4
)
4
O
2
S mainly existed in silicate phase. With the increase of sulfur content, the growth rate and average grain area of CaS in molten slag gradually increased, and the CaS grains gradually evolved from irregular structure to round structure.
2022 Vol. 57 (2): 54-62 [
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63
WANG Zhang-yin, JIANG Min, WANG Xin-hua
Formation and evolution of inclusions in Q345D steel during secondary refining process
The unqualified flaw detection caused by inclusions occurs from time to time when smelting Q345D. In order to further remove and control non-metallic inclusions in steel. The formation and evolution of inclusions in Q345D refined by " LF refining→RH vacuum refining→calcium treatment→soft blowing→continuous casting" was studied through industrial experiments. The process of calcium treatment modified effectively by thermodynamic analysis. The results showed that the inclusions in liquid steel were consisted of Al
2
O
3
when aluminum was used in liquid steel after the converter and at the beginning of LF refining. Slags which have high alkalinity and strong reducibility are used in liquid steel during the LF refining. Liquid steel was strongly stirred during RH vacuum refining. Calcium was wired into the liquid steel after RH vacuum refining. The inclusions in liquid steel were consisted of CaO-MgO-Al
2
O
3
ternary system and MgO-Al
2
O
3
binary system at the end of ladle furnace refining. The percentages of CaO-MgO-Al
2
O
3
ternary system and MgO-Al
2
O
3
binary system were 67% and 33%, respectively. At the end of RH vacuum degassing, inclusions in liquid steel were consisted of CaO-MgO-Al
2
O
3
ternary system, MgO-Al
2
O
3
binary system and CaO-Al
2
O
3
binary system, and their percentages were 49%, 12% and 39%, respectively. After RH vacuum refined, the calcium wire was fed into liquid steel for calcium treatment. Average mass percent of CaS in inclusion was about 27% after calcium treatment. More CaS precipitated on the surface of calcium aluminate to generate an outer layer of CaS-CaO-Al
2
O
3
ternary system. And the cores of CaO-MgO-Al
2
O
3
ternary system were closer to the low-melting zone in the liquid steel. In the afterwards soft blowing and casting, mass percent of CaS in inclusions increased to 42%. In addition, the relationship between the quantitative density and the average size of inclusions during refining is conjugated. Quantitative density of inclusions increased from 2.32 pcs/mm
2
after LF refining to 32.23 pcs/mm
2
during continuous casting process. Average size of inclusions decreased from 2.88 μm after LF refining to 1.74 μm during continuous casting process. Thermodynamic calculations on inclusions at 1 873 K indicate that liquid inclusions can be more effectively modified, when the mass percent of Al and Ca in liquid steel is about 0.04% and in the range of 0.001 2%-0.002 8%, respectively.
2022 Vol. 57 (2): 63-72 [
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73
LUO Teng-fei, WANG Wei-ling, LIU Zong-hui, LUO Sen, ZHU Miao-yong
In-situ observation on solidification of GCr15 bearing steel at cooling rates of continuous casting
The growth of solidification microstructure and the segregation of solute are important factors for the precipitation of carbide from the liquid during continuous casting of GCr15 bearing steel, which are the key to improve the quality of product. Therefore, the present work focused on the continuous casting process of GCr15 bearing bloom with the transverse section of 240 mm×240 mm in a domestic steel plant, and took samples 40, 80 and 120 mm below the bloom surface as research objects. First, a two-dimensional solidification heat transfer model was developed, and the average cooling rate in the mushy zone was obtained combined with temperature measurement of infrared thermal imager. Then, the in-situ observation of their solidification process under continuous casting conditions by means of high temperature laser confocal scanning microscopy (HT-CSLM) was carried out, and the effects of cooling rate on grain growth kinetics, grain size and solute segregation were investigated. The results show that the average cooling rates in mushy zone are 24.70, 17.02 and 18.95 ℃/min at the positions of 40, 80 and 120 mm below bloom surface, respectively. The growth rates of γ-Fe grains equivalent to the solid phase are 1.043, 0.973 and 1.015 μm/s, and the average radius of initial austenite grain after solidification are 148.53±58.41, 168.23±46.47 and 165.3±49.28 μm, respectively. With the decrease of cooling rate, the solidification interface tends to be stable in the solidification, and the grain morphology gradually changes from irregular strip shape to the regular circular shape. Meanwhile, the solidification microstructure becomes coarser, and the solute concentration of C and Cr are more highly distributed at grain boundaries. With the solidification proceeding, different grains contact each other to form grain boundaries. This phenomenon becomes more significant with the decrease of cooling rate, and it is accompanied by the migration of grain boundary.
2022 Vol. 57 (2): 73-84 [
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256
) [
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11356KB] (
459
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Metal Forming
85
YANG Yan-bo, PENG Yan, LIU Yang, LIU Cai-yi, WANG Jin
Dynamic theoretical model of strip hot rolling considering change of roll gap
Accurate rolling model is the key to production of high-quality strips. Currently, the commonly used hot rolling model is only suitable for studying the static rolling process. When faced with dynamic rolling processes such as variable thickness rolling and rolling mill vibration, the model structure lacks integrity since the commonly used static model does not include the roll gap change speed parameters. In order to conduct a comprehensive and in-depth study on dynamic rolling, it is necessary to establish a dynamic rolling model which includes the parameters of roll gap change speed. Based on the Orowan equation, the dynamic velocity field model, the average deformation rate model and the force differential balance equation were established while considering the influence of roll gap change speed on the average deformation rate of strip and the length of deformation zone, and the analytical solution of dynamic rolling force was finally solved. A two-high experimental rolling mill was used to carry out dynamic rolling experiments. Comparing with the results of dynamic hot rolling experiments, the dynamic rolling model had high accuracy, which could provide theoretical guidance for precise preparation of dynamic rolling forces. Through analysis, it is found that when the roll gap change rate is zero, the model in this paper is the same as SIMS model. Change law of various process parameters for the dynamic rolling process in the model above was analyzed. The results show that the strip exit position deviates from the line of two work roll centers during dynamic rolling, and the deviation direction and size are directly affected by the size and direction of roll gap change speed. Under the same rolling schedule, when the roll gap is reduced, the rolling force is greater than that of static rolling, while roll gap is increased, the rolling force is less than that of static rolling. This is because the length of deformation zone and metal deformation rate in the roll gap are affected by size and direction of roll gap change speed. In addition, when the roll gap is reduced, the rolling force will decrease with the increase of the rolling speed.
2022 Vol. 57 (2): 85-93 [
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178
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430
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94
LI Xue-tong, REN Ming-hua, LIU Ya-xing, SUN Ye-xuan, JIA Wen-qiang, TAN Chao-chao
Optimization of five-pass rolling schedule for high strength steel of 5+1 mill
A new type 5+1 tandem cold rolling mill is built in a steel plant, which is mainly used for rolling ultra high strength steel. Its fourth stand uses small roll mill, and the other stands use ordinary mill. In the actual production, the unit is also responsible for rolling other kinds of steel. For avoiding the problems of excess rolling capacity, based on the comprehensive consideration of energy saving and stable rolling, the 5+1 type tandem cold rolling line adopts the five-pass mode for rolling of high strength steel. In order to ensure product quality and rolling stability, a higher requirement is put forward for rolling schedule of unit under the condition of reducing one pass. Considering the outlet shape quality, total pressure reduction, rolling stability and other factors, combined with the characteristics of small roll diameter mill, the units of 5+1 type tandem cold mill under five-pass high strength steel mode were selected. Based on the distribution of outlet profile, the pressing schedule was distributed, and then the pressing schedule was fine-adjusted according to characteristics of two types of rolling mills. The optimization objective of tension system was to ensure the rolling stability of small roll mill. Then the outlet profile of each stand could be adjusted by optimizing the tension, so that it could work together with the results of pressing schedule optimization and complement the rolling stability of the whole stand and the optimization of outlet profile. Thus, the technology of comprehensive optimization setting of rolling schedule for high strength steel in five-pass rolling mode of 5+1 tandem cold mill was developed. Under the condition that a pass of rolling model was reduced, outlet shape of high strength steel was optimized, and the products quality was improved. The slip and vibration trend of small roll diameter unit was decreased, and the overall stability of rolling was ensured at maximum extent, which was greatly improving the economic effects of the whole unit, and had further popularization and application value.
2022 Vol. 57 (2): 94-100 [
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160
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101
WANG Kun-peng, WANG Ying, LIAO Jia-ming, XU Jian-fei, JIANG Min, WANG Xin-hua
Investigation on inclusions and breakages in ultra-deep drawing wire rod
For inclusion control of ultra-deep drawing wire rod (for tire cord, saw wire and diamond wire saw, etc.), three different control ideas of inclusion (low melting point, homogenization and low elastic modulus) of ultra-deep drawing steel were chosen in this paper. Plastic deformation and crush behavior of 4 types typical oxide inclusions (SiO
2
, SiO
2
-Al
2
O
3
-MnO, SiO
2
-CaO-Al
2
O
3
-(MgO) and SiO
2
-CaO-MgO-(Al
2
O
3
)) were studied during hot rolling and drawing in ultra-deep drawing wire rod by scanning electron microscope. The general characteristics of inclusions in twist fracture of tire cord and wet drawing fracture of cutting steel wire and electroplate diamond wire saw busbar were examined and summarized. The results show that the crystalline SiO
2
inclusion cannot deform in the hot rolling process while plastic deformation occurred in glassy SiO
2
inclusion. Both the crystalline and glassy SiO
2
inclusion can be crushed into pieces during the drawing process. SiO
2
-CaO-Al
2
O
3
-(MgO) inclusions with low melting point transformed into long strip shape in hot rolling and also crushed into pieces during the drawing process. The deformation of SiO
2
-Al
2
O
3
-MnO and SiO
2
-CaO-MgO-(Al
2
O
3
) inclusions with low Young′s modulus in hot rolling was not sufficient, but these two inclusions also crushed into pieces in drawing process. Long-term statistical analysis of a large number of fractures shows that the main inclusions causing wire breakage of ultra-deep drawing wire are Al-Mg-(Mn)-O spinel (38.2%), Al-O corundum (32.0%), Al-Si-O mullite (21.9%), a small amount of Mg-Si-(Mn)-O olivine (6.3%) and very few Zr-O inclusions (1.6%). The fracture inclusions have the characteristics of high melting point and high elastic modulus. The sources of these inclusions are mostly related to refined or continuous casting refractories. No inclusion with low melting point and low elastic modulus or high melting point and low elastic modulus has been found on the fracture. “Homogenization” of endogenous inclusions and “low elastic modulus” of external inclusions are the controlling directions of oxide inclusions in ultra-deep drawing wires.
2022 Vol. 57 (2): 101-108 [
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156
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489
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Materials
109
GUO Qin-yu, CHEN Yong-feng, ZHANG Hong-biao, ZUO Xiao-tan, ZHAO Li, ZHANG Ya-bing
Surface quenching technology and microstructure evolution law of 40Cr steel billet
Surface crack,which generates in the process of hot charging,restricts the application of hot charging,and surface quenching technology can effectively improve this problem. This paper takes a 180 mm×180 mm cross-section billet caster as the object,and selects 40Cr steel as quenching experiment steel to carry out off-line quenching experiment research,combined with analysis of microstructure and heat transfer of quenching and self-tempering process by means of optical microscope and numerical simulation technique,the influence of quenching on the surface microstructure evolution of the billet was studied. The results showed that surface quenching can effectively eliminate the film-like ferrite,and form a certain hardening depth. In the quenching time range of 40-100 s,a tempered sorbite layer with the depth of 2-10 mm can form at the surface layer,and film-like ferrite begins to form at the depth of 19-38 mm from the surface. At the same time,with the extension of quenching time,the quenching depth increases significantly. Based on the purpose of quenching,it is proposed to use the depth at which film-like ferrite begins to form as the basis for judging the quenching depth of casting billet after quenching. Combined with heat transfer analysis and types and characteristics of the microstructure after quenching,two online quenching process schemes were proposed. The expected surface microstructure after quenching is refined ferrite with pearlite(weak cooling scheme) and tempered sorbite(strong cooling scheme) respectively. For the weak cooling scheme,the expected hardening depth is 7-13 mm,for the strong cooling scheme,the expected hardening depth can reach 2-10 mm(taking tempered sorbite as the criterion for determining the hardening depth) or 19-38 mm (taking film-like ferrite as the criterion for determining the hardening depth). Based on the off-line quenching experiment combined with heat transfer analysis in this paper,the determined on-line quenching process schemes can make a certain theoretical foundation for the implementation of surface quenching technology at the plant.
2022 Vol. 57 (2): 109-116 [
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249
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381
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117
LI Zhen-tuan, QIN He-yong, TIAN Qiang, ZHANG Wen-yun, ZHAO Guang-pu
Effect of deformation parameters on dynamic recrystallization and γ′-phase of GH4742 superalloy
In order to study the effects of deformation parameters on the dynamic recrystallization and γ′ phase of forged GH4742 superalloy,the true stress-true strain curves of forged GH4742 superalloy at deformation temperature of 1 050-1150 ℃ and the deformation ratio of 30%-70% under the strain rate of 0.1 s
-1
were obtained by single-pass isothermal compression experiments,the variation of true stress-true strain and peak stress under different deformation parameters was analyzed,and meanwhile the micro-substructure and γ′ phase were characterized during the dynamic recrystallization process under different deformation parameters by SEM and EBSD, the geometric dislocation density of the matrix and the proportion of dynamic recrystallization were quantitatively calculated,and the hardness of the matrix under different deformation parameters was measured. The nucleation mechanism of dynamic recrystallization under different deformation parameters was discussed,and the evolution of micro-substructure and γ′ phase was deeply analyzed during the dynamic recrystallization process. The results show that there were a large number of undissolved primary γ′ phase in the matrix at deformation temperature of 1 080 ℃,the proportion of low-angle grain boundaries(LAGs) exceeded 35%,and the proportion of dynamic recrystallization was less than 35%,and the nucleation mechanism was mainly continuous dynamic recrystallization; The size of the primary γ′ phase decreased,and then it re-dissolved progressively in the matrix at deformation temperature of 1 110 ℃,the proportion of LAGs was less than 8%,and the proportion of dynamic recrystallization was more than 75%,and the main nucleation mechanism was discontinuous dynamic recrystallization. With increasing deformation ratio,the size of primary γ′ phase increased and the number density of that decreased,the proportion of LAGs decreased significantly,and the proportion of dynamic recrystallization of the matrix increased. The hardness of the matrix increased significantly with the increase of the deformation ratio at the low deformation temperature,whereas the hardness increased first and then gradually tend to remain unchanged at the high deformation temperature. The dynamic recrystallization of GH4742 alloy had been completed at the deformation temperature of 1 110 ℃ and the deformation ratio of 50%,the microstructure was equiaxed dynamic recrystallized grains,and the matrix hardness was 357HV,which had a good hot forming performance.
2022 Vol. 57 (2): 117-126 [
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127
HAN Rong, LIU Hong-xi, YU Wen-chao, WANG Mao-qiu, SHI Jie
Precipitates and their strengthening in Ti-V-Mo microalloyed 22MnB5 steel
Using warm forming instead of hot forming can avoid the problems of surface oxidation in the process of hot forming,but 22MnB5 steel is commonly used in hot forming,and obvious softening occurs after high temperature tempering. By adding microalloyed elements such as Ti,V and Mo to the steel,fine precipitates and fine grains can be formed in the steel to improve the strength,so this problem can be solved. Therefore,by adding Ti,V and Mo microalloyed elements to 22MnB5 steel,the characteristics and strengthening effect of precipitates in the experimental steel were studied by OM(optical microscope),FE-SEM(field emission scanning electron microscope),EBSD(electron backscattering diffraction),TEM(transmission electron microscope),XRD(X-ray diffraction analysis),EDS(energy dispersive spectrometer) and physicochemical phase analysis. The experimental results show that the microstructure of 22MnB5 steel and Ti-V-Mo microalloyed steel after quenching is typical lath martensite. In addition to carburizing,TiC and a small amount of (V,Mo)C are also precipitated in Ti-V-Mo microalloyed experimental steel. The experimental measurement shows that the tensile strength is more than 90 MPa higher than that of 22MnB5 steel. After tempering at 600 ℃ for 1 h,a large number of nano MC(M=Ti+V+Mo) precipitates with a size of less than 20 nm precipitated in Ti-V-Mo microalloy experimental steel,which refined the effective grain size of martensitic steel to about 1.3 μm. Through theoretical analysis,it is found that the precipitation strengthening effect can reach more than 500 MPa. At the same time,the fine grain strengthening effect caused by grain refinement due to the pinning of (Ti,V,Mo)C precipitates at the grain boundary is more than 70 MPa. Under the combined action of precipitation strengthening and fine grain strengthening,the strength of Ti-V-Mo microalloy experimental steel is more than 600 MPa higher than that of 22MnB5 steel. Therefore,according to the results of this study,Ti-V-Mo microalloy experimental steel improves the softening phenomenon of steel plate during medium temperature forming to a certain extent.
2022 Vol. 57 (2): 127-138 [
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139
WANG Yan, ZHU Li-guang, WU Yao-guang, WANG Chong-jun, LIU Zhi-yuan, HUO Jin-xia
Microstructure characteristics of welding heat affected zone in EH40 shipbuilding steel
In recent years,China′s shipbuilding and ocean engineering manufacturing industries have developed rapidly. High strength, high toughness, weldability and corrosion resistance have become the development direction of steel for shipbuilding and ocean engineering. Welding is a key link in ship hull manufacturing, large heat input welding which improves shipbuilding efficiency has become the first choice of shipbuilding enterprises. However, the increase of welding heat input leads to severe coarsening, local softening and embrittlement of the grains in the heat-affected zone of steel plate welding, and its low-temperature impact toughness deteriorates, which seriously threatens the safe use of hull steel. Oxide metallurgy is a technology that through ultrafine and uniformly distributed oxide inclusions in steel, induces the nucleation and growth of intragranular ferrite during the solidification, hot working of molten steel and to form an interwoven and interlocking structure, which makes the steel have good toughness, high strength and excellent weldability. This article based on the use of EH40 steel produced by oxide metallurgical technology, multi-element microalloy system is used for steel smelting with high heat input welding. The heat affected zone of 35 mm thick high-strength steel plate under the welding energy of 120 kJ/cm was considered, and the average value of impact energy of HAZ at -40 ℃ was 274 J where 2 mm away from the upper surface fusion line. Using metallographic microscope and scanning electron microscope to study the structure characteristics of the welding heat-affected zone, it was shown that, the structure types in EH40 shipbuilding steel included polygonal ferrite, pearlite and granular bainite, the size of various crystal grains mostly was 10-25 μm. Systematically observed the specimen on microstructure of each area for the welding heat affected zone, explored the distribution of organizations in each region. Inclusions are mostly spherical and irregular blocks, through scanning electron microscopy analysis, it is shown that the inclusions which induced IAF in steel are mainly composed of MgO, Al
2
O
3
, TiO
x
and MnS, there is a large amount of acicular ferrite structure in the welding heat-affected zone.
2022 Vol. 57 (2): 139-148 [
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149
MENG Man-ding, AN Tong-bang, WEI Jin-shan, MA Cheng-yong, ZUO Yue
Microstructure and strength-toughness mechanism of 690 MPa grade ULCB deposited metals
When using traditional welding materials for high-strength steels to carry out extensive welding testing of 690 MPa grade copper precipitation strengthening steel,it was still necessary to strictly control the heat input,preheating temperature,and interpass temperature,which makes the excellent performance of copper precipitation strengthening steel and the advantage of saving production cost cannot be brought into full play. The effect of Si,Mn,Ni on the microstructure and mechanical properties of the 690 MPa grade ultra-low carbon bainite (ULCB) deposited metal was investigated by OM,SEM,TEM,in order to provide technical accumulation for domestic production of non-preheating welding materials for copper precipitation strengthening steel. Experimental results showed that the microstructure of deposited metal was primarily comprised of granular bainite (GB),lath bainite (LB),and acicular ferrite (AF). The microstructure of the deposited metal with 0.16% Si and 1.46% Mn was refined, leading to the improvement of impact toughness. However,the Si content was too low to make the width of the bainite ferrite (BF) substructure decreased,resulting in the improvement of toughness was limited. The microstructure of deposited metal with 0.29% Si,1.02% Mn and more Ni content was interlocked,and the lath width of the bainite ferrite decreased,contributing to impact toughness improved significantly. Phase transformation dislocation strengthening was affected by the bainite onset temperature (
B
s
),which was the main reason that affects the strength of ULCB deposited metal. Inclusions were mainly distributed between the bainite ferrite,and a small amount become the nucleation points of the acicular ferrite. Therefore,a greater increase in impact toughness would be realized by reduction in inclusion density and size of the deposited metal.
2022 Vol. 57 (2): 149-156 [
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157
ZHANG Chun-ju, DING Xuan, YANG Ming-qiu, ZHANG Lu-yang, LI Hui
Dynamic mechanical properties and constitutive model of DP980 steel
With the continuous development of automobile lightweight industry, advanced high strength DP steel has been rapidly developed and applied. The simulation of vehicle collision process safety puts forward an urgent demand for dynamic mechanical property data of DP steel. The tension mechanical properties under different strain rates of DP980 steel were investigated using universal tensile testing machine and high-speed tensile testing machine in this paper. The dynamic constitution mathematical model based on JC model was established. The model calculation results were compared with the experimental data. The results indicate that the yield strength and tension strength are relatively stable under the strain rates of 10
-4
-10
-2
s
-1
, which fluctuates in the range of 660-675 and 1 030-1 050 MPa respectively, while increases with the increasing of strain rates under the strains rates of 10
-1
-10
3
s
-1
. When the strain rate increases from 10
-1
to 10
3
s
-1
,the yield strength increases from 690 to 810 MPa,while the tension strength increases from 1 050 to 1 200 MPa. The uniform elongation and breaking elongation are 11% and 16% respectively under the strain rates of 10
-4
-10
-1
s
-1
. The elongation increases first and then decreases with the increasing of strain rates under high strain rates, and reaches the peak value of 25% under the strain rate of 500 s
-1
. DP980 steel has obvious strain rate sensitivity. The simulation results under the strain rates of 10
1
and 10
2
s
-1
based on the dynamic constitutive model have great agreement with the experimental results. Dynamic mechanical property data is the key factor affecting the reliability of vehicle collision simulation results. The dynamic mechanical property evolution law of DP980 steel and the corresponding dynamic JC constitutive model studied in this paper can more conveniently provide accurate data support for automobile plants, shorten the experimental time, reduce the experimental cost, and improve the reliability of simulation results of automobile collision process.
2022 Vol. 57 (2): 157-161 [
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Equipment Technology
162
ZHAO Zi-wei, KONG Fu-lin, TONG Li-ge, YIN Shao-wu, XIE Ya-ru, WANG Li
Analysis of CO
2
emission reduction path and potential of China′s steel industry under the "3060" target
China′s steel production accounts for 56.7% of the world′s total output,CO
2
emissions account for 72.5% of the world's total steel emissions and 15% of national carbon emissions. To achieve the target of "3060",this paper analyzed the current situation of steel industries at home and abroad,as well as their emission reduction path and potential. Further,it discussed the impact of short process,energy structure adjustment,waste energy utilization and carbon tax on emission reduction. In terms of import and export,China′s steel exports in 2020 accounted for 12.8% of the world′s total exports,while accounted for 4.8% of China′s steel production,indicating that China′s steel production was mainly to meet the domestic demand. Increasing the amount of scrap steel imports appropriately can reduce the burden of carbon tax and China′s dependence on iron ore,and enhance the country′s bargaining power in the raw material market. When the proportion of short process rises to 30%,it is expected to reduce carbon emissions by 380 million tons per year,contributing 2.09% to the 2030 target and reducing USD 152 billion of carbon tax. When the EAF(Electric Arc Furnace) process of 70% scrap steel +30% DRI(Direct Reduced Iron) is adopted,a total carbon reduction of 70 million tons can be achieved,contributing 0.39% to 2030 target emissions reduction and achieving carbon tax savings of USD 2.8 billion. Implementation of oxygen blast furnace,hydrogen metallurgy and CCUS(Carbon Capture,Utilization and Storage) technologies can project to achieve carbon reduction of 4.955 billion tons,with a contribution rate of 24.6% to carbon reduction of year 2030 and a carbon tax reduction of USD 198.2 billion. Among them,hydrogen metallurgy has the most significant effect,which can take away 4.263 billion tons of carbon,contributing 20.79% to carbon reduction of year 2030 and USD 170.52 billion carbon tax reduction. And it is followed by oxygen blast furnace,which can take away 342 million tons of carbon,contributing 1.88% to carbon reduction of year 2030,and USD 13.68 billion carbon tax reduction. If all of the high-grade residual energy is effectively utilized,139-140 million tons of carbon reduction is predicted to be achieved,making a contribution of 0.77% to carbon reduction to year 2030 and USD 5.584 billion less of carbon tax. When the utilization rate of low-grade residual energy grows from 30% to 50%,66 million tons of carbon reduction is predict to be achieved,making a contribution of 0.36% to carbon reduction to year 2030 and USD 2.64 billion less of carbon tax. Therefore,to better realize the goal of "3060",China′s steel industry can improve the coverage of short process in the short term,while accelerating the R&D of new carbon reduction technologies such as oxygen blast furnace,hydrogen metallurgy,energy storage,and cascade utilization of residual energy.
2022 Vol. 57 (2): 162-174 [
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175
DAI Meng-bo, LUO Bang-cao, SUN Cai-hong, TIAN Hao, SUN Ye-chang, CHUN Tie-jun
Optimized development of steel slag tailings-GGBS based low-cost backfill material
In order to realize the synergistic utilization of wastes and reduce the backfill cost, this paper took the GGBS-based fine unclassified tailings cemented backfill material as the benchmark, and the flow of backfill slurry and backfill strength as the characteristics to carry on research. For developing of steel slag tailings-GGBS based soft mine backfill material, solid materials ratios were optimized. Hot stuffy steel slag magnetic selection tailings (steel tailings) were used to replace part of the GGBS as cemented material, while desulfurization ash and cement clinker replaced part of the special additive as admixtures. The orthogonal experiments were used to explore the mixing rules, and the influences of cement-sand ratio and the ratio of admixtures to binder were also researched. The micromorphology and XRD maps for backfills of GGBS-based, steel tailings-GGBS based (B1) and steel tailings-slag group with optimum strength admixture(B7) based were analyzed to explore their hydration mechanisms. The results show that with the increase of slag replaced by steel tailings and the decrease of admixture to binder ratio, the backfill strength decreases and the flow of backfill slurry improves. Strength orthogonal experimental results show that the strength is determined by content of steel tailings, and mixing content of desulfurization ash should be more than cement clinker. Under the condition of 1:6 cement-sand ratio and 20% replacement ratio of steel tailings to GGBS, the proportion of admixtures was further adjusted. The optimized admixtures of B7 were that desulfurization ash and cement clinker replaced special additive by 30% and 20%, respectively, the flow was 143 mm, and micromorphology was iron-rich chlorite cemented false cube crystalline wollastonite. The backfill strength was 2.13 MPa in 28 days, which was 0.19 MPa lower than that of GGBS-based group and 0.26 MPa higher than that of B1. The replaced alternative scheme meets the application requirement of C2 strength, which increases flow and reduces backfill costs significantly. The optimal admixture also has strength optimization effect at 1:4 cement-sand ratio but it is worse than 1:6 cement-sand ratio.
2022 Vol. 57 (2): 175-184 [
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