Advanced steelmaking process (CHAPTER THREE)

Recycling technology innovation

1. Hot desulphurization slag recycling process

The hot cycle method of desulfurization slag is a method of direct reuse of the steel slag (desulfurization slag) after desulphurization and refining at high temperature during the pretreatment of the next hot metal. The desulphurization slag after treatment is in the state of condensed particles, but the temperature drops afterwards, causing the condensed particles to collapse and form a new unreacted surface. Repeated use of desulphurization slag,

The desulphurization ability of the slag can be maximized by using the unreacted surface effectively. At the same time, due to the reuse of the slag in the high temperature state, the heat released during the slag discharge can be recovered, which is conducive to saving resources and reducing the environmental load.

2. Process for the return and utilization of liquid decarbonized slag

Process flow of hot slag return to dephosphorization furnace of converter: After adding scrap steel to the dephosphorization furnace and adding hot metal, the dephosphorization blowing process begins. After the end of the blowing process, the dephosphorization slag with higher P2O5 mass fraction (ω(P2O5)=6%-8%) is dumped, and the half steel (ω[P]semi≈0.03%) is poured into the half ladle, and the half steel is added into the decarburization furnace by blowing oxygen and decarburization smelting. After the end of the decarburization furnace smelting, the liquid steel from the steel to the ladle into refining and smelting, P2O5 mass fraction of low decarburization slag (ω(P2O5)≈1.5%) into the slag tank, liquid slag through the slag tank into the dephosphorization furnace, began to splash slag operation, liquid slag splash dry, began to add scrap steel and hot metal, into the next converter double operation.

3. Ladle casting slag, liquid steel return utilization process

After the hot metal is blown in the dephosphorization furnace, the half steel is poured into the half ladle, and the RH/LF/CAS ladle is poured from the ladle to the half ladle, each half ladle receives 3-4 furnace castings, and then the castings and refining slag are poured into the decarburization furnace through the half ladle.

Hot refining slag and cast residue return after the industrial production of semi-ladle is put into operation, the cast residue of ladle can be safely recycled in liquid form. Ladle molten steel is directly poured into the semi-ladle to solve the problem of heavy scrap steel; Retaining steel in ladle can improve the quality of high-grade steel. Steel slag with high alkalinity can be recycled; The cast surplus can be returned to the converter at high temperature to achieve the purpose of energy saving.

 

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Advanced steelmaking process (CHAPTER TWO)

Off-furnace refining technology

The refining process is to optimize the quality of molten steel by degassing, deoxidation, desulphurization, removal of inclusions, fine-tuning of alloy composition and precision of temperature. Through secondary refining process, the quality of molten steel can be improved and the special requirements of different steel grades can be met. Therefore, refining is a core part of the construction and production sequence of modern steel mills.

Refining is a necessary means for steel mills to develop new varieties, enhance market competitiveness, and a buffer for production rhythm. It plays an irreplaceable role in the control of production rhythm and the guarantee of product quality.

The refining process configuration is divided into three categories: LF furnace process, CAS process, RH process.

At present, the refining process is mainly divided into the following four categories:

1) LF refining process: For low-oxygen and low-sulfur steel, such as low-alloy steel and low-grade pipeline steel, it needs to be processed by LF ladle refining furnace and sent to continuous casting machine.

2) RH refining process: For low carbon steel, ultra-low carbon steel and steel with high gas content control requirements, it is necessary to be sent to continuous casting after RH treatment.

3) LF+RH dual treatment process: For some steel with special quality requirements, such as high grade pipeline steel and high strength steel, it is necessary to be double treated by LF ladle furnace and RH vacuum degassing device.

4) CAS refining process: as a parallel or alternative process route for LF refining, CAS can complete most of the functions of LF in addition to desulfurization.

Slab continuous casting technology

The ladle containing the refined molten steel is transported to the rotary table, and after the rotary table is turned to the pouring position, the molten steel is injected into the tundish, and the tundish is then distributed to the various crystallizers by the water outlet. The mold is one of the core equipment of the continuous casting machine, which makes the slab form and solidify and crystallize quickly. The pull straightening machine and the crystallization vibration device work together to pull out the slab in the mold, cool it by the fan section, and cut the slab into a certain length by the flame.

Continuous casting is the process of turning high temperature liquid steel into solid slab.

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Advanced steelmaking process (CHAPTER ONE)

 The difference between steel and pig iron:

The most fundamental difference between iron and steel is the carbon content, in theory, the carbon content of steel is less than 2.11%, its melting point is 1450-1500℃, and the melting point of pig iron is 1100-1200℃. In steel, with the increase of carbon content, its strength and hardness increase, while the plasticity and impact toughness decrease.

Why steel must be made: Pig iron is not widely available. High carbon content: no austenite at high temperature; Poor performance: hard and brittle, poor toughness, poor welding performance, can not be processed; Many impurities: S, P, high content of inclusions.

Definition of steelmaking: use oxidation method to remove impurities in pig iron and scrap steel, add appropriate alloying elements, so that it becomes a steel with high strength, toughness or other special properties, this process is called "steelmaking".

The "three removal" process of steelmaking is equipped with 4 300t KR desulfurization stations, 2 300t dephosphorization converter and 3 300t decarburization converter. Dephosphorization station and decarbonization station adopt "2+3" double-height span arrangement, which is convenient for "semi-steel" hot metal transfer; Refining is equipped with 2 300t double-station RH furnaces, 2 300t CAS furnaces and 1 300t LF furnace; Four double-flow slab continuous casting machines are used for continuous casting. Process features: The use of advanced "one can to the end", "all three" technology, 100% refining process of molten steel, casting machine high pulling speed, to create an efficient and fast-paced clean steel production platform.

Converter steelmaking technology

Mix molten iron with scrap steel, pour it into the converter and then blow oxygen, the furnace temperature rises to about 1600℃, the reaction in the furnace is very violent, like a volcanic eruption, the carbon and the main impurities are quickly burned off (manganese and silicon in the molten iron are oxidized, and the carbon in the molten iron is also oxidized into carbon dioxide). The whole process is only about 30 minutes, and no longer add any fuel, you can chain a furnace of steel, and even "negative energy steel" with this method of steel, the quality can be comparable to the open furnace steel, the time required is only 1/10 of the open furnace, the efficiency is very high. Converter steelmaking has become the main process of modern steel production. The quality of the product was further improved after the addition of off-furnace refining.

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Heat treatment stress of steel and its influence (Chapter Three)

3. The influence of residual compressive stress on the workpiece

The reason why carburized surface strengthening is widely used as a method to improve the fatigue strength of steel parts. On the one hand, because it can effectively increase the strength and hardness of the workpiece surface, improve the wear resistance of the workpiece, on the other hand, carburizing can effectively improve the stress distribution of the workpiece, obtain a large residual compressive stress on the surface layer of the workpiece, and improve the fatigue strength of the workpiece. If the isothermal quenching is carried out after carburizing, the residual compressive stress of the surface layer will be increased, and the fatigue strength will be further improved.

Isothermal quenching has higher surface residual compressive stress than the usual quenching low temperature tempering process. The surface residual compressive stress is higher than that of low temperature tempering after isothermal quenching even after low temperature tempering. Therefore, it can be concluded that the surface residual compressive stress obtained by isothermal quenching after carburizing is higher than the usual low temperature tempering of carburizing quenching. From the point of view of the beneficial influence of the residual compressive stress on the fatigue resistance of the surface layer, the isothermal quenching process of carburizing is an effective method to improve the fatigue strength of carburizing parts.

Why can carburizing and quenching process obtain surface residual compressive stress?

There are two main reasons for this:

One reason is that the specific volume of the surface high-carbon martensite is larger than that of the low carbon martensite in the core, and the volume expansion of the surface surface is large after quenching, while the volume expansion of the low carbon martensite in the heart is small, which restricts the free expansion of the surface surface and causes the stress state of the surface compression core.

Another more important reason is that the beginning transition temperature (Ms) of the transition from high-carbon supercooled austenite to martensite is lower than the beginning temperature (Ms) of the transition from supercooled austenite to martensite with low carbon content in the core. That is to say, during the quenching process, it is often the heart that first produces martensitic transformation causing the volume expansion of the heart and is strengthened, and the surface is not cooled to its corresponding martensitic beginning transition point (Ms), so it is still in the supercooled austenite state with good plasticity, and will not seriously suppress the volume expansion of the martensitic transformation of the heart.

With the continuous decrease of quenching cooling temperature, the surface temperature drops below the (Ms) point, and martensitic transformation occurs in the surface layer, causing the expansion of the surface volume. However, at this time, the heart has already been transformed into martensite and strengthened, so the heart will play a great role in suppressing the volume expansion of the surface surface, so that the surface surface can obtain residual compressive stress. When the isothermal quenching is carried out after carburizing, when the isothermal temperature is above the martensitic beginning transition temperature (Ms) of the carburizing layer, the appropriate temperature isothermal quenching below the martensitic beginning transition temperature (Ms) point of the core is more than continuous cooling quenching to ensure the sequential characteristics of this transformation (that is, to ensure that the surface martensitic transition only occurs in the cooling process after isothermal. 

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Heat treatment stress of steel and its influence (Chapter Two)

 

2 Influence of heat treatment stress on quenching crack of steel

Factors that exist in different parts of the quenched part that can cause stress concentration (including metallurgical defects) have a promoting effect on the generation of quenched cracks, but only in the tensile stress field (especially under the maximum tensile stress) will be manifested, if there is no cracking promotion effect in the compressive stress field.

The quenching cooling rate is an important factor that can affect the quenching quality and determine the residual stress, and also a factor that can give an important and even decisive influence on the quenching crack. In order to achieve the purpose of quenching, it is usually necessary to accelerate the cooling rate of the parts in the high temperature section, and make it exceed the critical quenching cooling rate of steel in order to obtain martensitic structure.

As far as residual stress is concerned, this can reduce the tensile stress on the surface of the workpiece and achieve the purpose of restraining the longitudinal crack by increasing the thermal stress value that counteracts the effect of organizational stress. The effect will increase with the speed of high temperature cooling. Moreover, in the case of quenching, the larger the cross-section size of the workpiece, although the actual cooling rate is slower, the risk of cracking is greater. All this is due to the fact that the thermal stress of this kind of steel slows down with the increase of the size of the actual cooling rate, the thermal stress decreases, the organizational stress increases with the increase of the size, and finally the formation of the tensile stress based on the organizational stress on the surface of the workpiece.

For this type of steel, only longitudinal cracks can be formed in high-hardenability steel hardened under normal conditions. The reliable principle to avoid quenching cracking is to try to minimize the anischronicity of the martensite transition inside and outside the section. Slow cooling in the martensitic transition zone alone is not sufficient to prevent the formation of longitudinal cracks. Under normal circumstances, it can only produce arc cracks in non-hardenable parts, although the overall rapid cooling is the necessary formation conditions, but its real cause of formation is not in the rapid cooling (including martensitic transition zone) itself, but the local location of the quenching part (determined by the geometric structure), the cooling rate in the high temperature critical temperature zone is significantly slowed down, and therefore not caused by hardening.

The transverse break and longitudinal split produced in the large non-hardenable parts are caused by the residual tensile stress with thermal stress as the main component acting on the center of the quenched part, and at the center of the section of the quenched part at the end of the quenched part, the crack is first formed and expanded from the inside out. In order to avoid such cracks, the water-oil dual-liquid quenching process is often used. In this process, rapid cooling in the high temperature section is implemented only to ensure that the outer metal gets martensitic structure, which is harmful from the point of view of internal stress. Secondly, the purpose of slow cooling at the later stage of cooling is not to reduce the expansion rate and the stress value of the martensitic phase transition, but to minimize the temperature difference of the section and the shrinkage rate of the metal in the center of the section, so as to reduce the stress value and ultimately inhibit the quenching crack.

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Heat treatment stress of steel and its influence (Chapter One)

 The residual force of steel heat treatment refers to the final residual stress of the workpiece after heat treatment, which has a very important influence on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it causes the deformation of the workpiece, and when it exceeds the strength limit of the material, it will crack the workpiece, which is its harmful side and should be reduced and eliminated.

However, under certain conditions, controlling the stress to make it reasonably distributed can improve the mechanical properties and service life of the parts, and turn harmful into beneficial.

Analyzing the distribution and variation of stress in the process of heat treatment of steel and making it reasonably distributed has far-reaching practical significance for improving product quality.

1. Heat treatment stress formation principle of steel

During the heating and cooling process of the workpiece, due to the inconsistent temperature difference between the cooling speed and time of the surface layer and the heart, it will lead to uneven volume expansion and contraction and produce stress, that is, thermal stress. Under the action of thermal stress, because the surface temperature is lower than the core, the contraction is greater than the core and the core is strained. When the cooling ends, the surface compression core is strained because the final cooling volume contraction of the core cannot be carried out freely. That is, under the action of thermal stress, the surface of the workpiece is finally compressed and the heart is strained.

This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. The faster the cooling rate, the higher the carbon content and alloy composition, the larger the non-uniform plastic deformation under the action of thermal stress during the cooling process, and the larger the residual stress.

On the other hand, due to the change of the structure of steel in the heat treatment process, that is, when the austenite is transformed into martensite, the increase of the specific volume will be accompanied by the expansion of the volume of the workpiece, and the phase change of each part of the workpiece will result in inconsistent volume growth and organizational stress. The end result of the structural stress change is the surface tensile stress and the core compressive stress, which is exactly the opposite of the thermal stress. The size of the structure stress is related to the cooling rate, shape and chemical composition of the workpiece in the martensitic phase transformation zone.

Practice has proved that any workpiece in the heat treatment process, as long as there is a phase change, thermal stress and organizational stress will occur. But the thermal stress has been generated before the organizational transformation, and the organizational stress is generated during the organizational transformation process, in the entire cooling process, the result of the combined effect of thermal stress and organizational stress is the actual stress in the workpiece.

The result of the combined action of these two stresses is very complex, affected by many factors, such as composition, shape, heat treatment process and so on. In terms of its development process, there are only two types, namely thermal stress and organizational stress, which cancel each other when the direction of action is opposite, and overlap each other when the direction of action is the same. Whether they cancel each other or overlap each other, the two stresses should have a dominant factor, and the effect of thermal stress when it dominates is that the workpiece is strained and the surface is compressed. The effect of the predominant structural stress is that the compression surface of the workpiece core is strained.

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Four ways of strengthening steel metal materials(Chapter Three)

Third, fine crystal strengthening

01. Definitions

With the decrease of grain size, the strength and hardness of the steel material increase, and the plasticity and toughness are also improved, which is called fine crystal strengthening.

02. Mechanism

The principle lies in the retarding effect of grain boundary on dislocation slip. For polycrystals, dislocation movement must overcome the resistance of grain boundaries, which is due to the different orientation of dislocations on both sides of the grain boundary, so in a grain, the slip dislocation can not directly cross the grain boundary into the adjacent grain, only after a large number of dislocations are packed at the grain boundary to cause stress concentration, in order to stimulate the movement of the adjacent grain has a fault to produce slip. So the finer the grain, the higher the strength of the material.

03. Rules

The finer the grain size, the larger the grain boundary area, according to the Hall-Page formula

The smaller the average diameter d of the grain, the higher the yield strength σs of the material

04. Refining grain method

(1) During the crystallization process, the grains can be refined by increasing the degree of supercooling, metamorphic treatment, vibration and agitation to increase the nucleation rate;

② For cold deformed metals, grains can be refined by controlling deformation degree and annealing temperature;

③ The grains can be refined by normalizing and annealing heat treatment methods;

④ Alloying elements can be added to the steel to form a new phase to inhibit grain growth.

           

Fourth, the second phase of strengthening

01. Definitions

There is also one or more other phases in the metal matrix, the presence of these phases to improve the strength of the metal. Due to the different processes for obtaining the second phase, the second phase strengthening is divided into: ① precipitation strengthening: the second phase is obtained by phase change heat treatment; ② dispersion strengthening: the second phase is obtained by powder sintering or internal oxidation.

02. Mechanism

The dislocation encounters the second phase in the process of movement and needs to bypass or cut through the second phase, which hinders the movement of the dislocation and makes the strength of the material increase.

03. Examples

The strength of steel is improved by the presence of cementite in steel.

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Four ways of strengthening steel metal materials (Chapter Two)

 Second, solid solution strengthening

01. Definitions

With the increase of solute atomic content, the strength and hardness of the solid solution increase, and the plasticity and toughness decrease.

02. Mechanism

(1) The dissolution of solute atoms distorts the crystal lattice of the solid solution and hinders the dislocation moving on the glide plane.

(2) The Coriolis air mass formed by solute atoms on the dislocation line acts as a pinning effect on the dislocation, increasing the resistance of the dislocation movement.

(3) The segregation of solute atoms in the layer fault region hinders the movement of the extended dislocation. All the factors that hinder the movement of the dislocation and increase the resistance of the dislocation to move can increase the strength.

03. Regularity

① In the range of solid solution solubility, the greater the mass fraction of alloying elements, the greater the strengthening effect

② The larger the size difference between solute atoms and solvent atoms, the more significant the strengthening effect.

③ The strengthening effect of solute elements forming interstitial solid solutions is greater than that of elements forming replacement solid solutions

④ the larger the difference in valence electron number between solute atom and solvent atom, the greater the strengthening effect.

 

04. Methods

Alloying, that is, adding alloying elements.

05. Examples

The strength of copper-nickel alloys is greater than that of pure copper and nickel metals.

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Four ways of strengthening steel metal materials (Chapter One)

 1. Deformation strengthening (or strain strengthening, work hardening)

01. Definitions

After the yield of the steel material, with the increase of the degree of deformation, the strength and hardness of the material increase, and the plasticity and toughness decline.

02. Mechanism

With the progress of plastic deformation, the dislocation density continues to increase, so the mutual delivery of dislocation in motion is intensified, resulting in fixed step cutting, dislocation entanglement and other obstacles, so that the dislocation movement resistance increases, resulting in increased deformation resistance, causing difficulties to continue plastic deformation, thereby improving the strength of the metal

Rule: The degree of deformation increases, the strength and hardness of the material increase, the plasticity and toughness decrease, and the dislocation density continues to increase

The strength is proportional to the half power of ρ of the dislocation density. The larger the Bergdahl vector b of the dislocation, the more significant the strengthening effect is.

03. Methods

Cold deformation, such as cold pressing, rolling, shot peening, etc.

04. Examples

Cold-drawn steel wire can multiply its strength.

05. The practical significance of deformation strengthening (advantages and disadvantages)

(1) Advantages:

Deformation strengthening is an effective way to strengthen the metal, for some materials that can not be strengthened by heat treatment, the strength of the material can be improved by deformation strengthening, which can increase the strength exponentially.

②It is an important factor in the processing and forming of some workpieces or semi-finished products, so that the uniform deformation of the metal, so that the forming of the workpiece or semi-finished products is possible, such as cold drawn steel wire, stamping of parts.

(3) Deformation strengthening can also improve the safety of parts or components in the process of use, some parts of the stress concentration or overload phenomenon, so that there is plastic deformation, due to work hardening to stop the deformation of the overload part to improve safety.

(2) Disadvantages:

Deformation strengthening also brings trouble to the production and use of materials, deformation increases the strength and reduces the plasticity, and it is difficult to continue deformation, requiring more power consumption.

In order to allow the material to continue to deform, recrystallization annealing is required in the middle, so that the material can continue to deform without cracking, increasing the production cost.

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Common defects and countermeasures of steel heat treatment (Chapter Five)

 Fifth, the mechanical properties of non-ferrous metal alloys are not qualified

The most widely used non-ferrous metals in industry are aluminum, copper, magnesium, titanium and their alloys. The heat treatment principle of non-ferrous metals and steel is the same, but it has its own characteristics. For example, eutectoid transformation plays an important role in the heat treatment of steel, but it is rarely encountered in non-ferrous metals; Martensitic transformation is the main means of strengthening steel materials, but except for a few copper alloys and titanium alloys, other non-ferrous metals generally cannot be strengthened by martensitic transformation. The common heat treatment processes of non-ferrous metals are homogenization annealing, recrystallization annealing, stress relief annealing, solution treatment and aging treatment. Solution aging is the most commonly used and important heat treatment strengthening process for non-ferrous metals.

Non-ferrous metal heat treatment should pay special attention to the following problems:

1) Non-ferrous metals are active and require strict heating environment. For example, the heating environment of titanium alloys should generally be vacuum or micro-oxidation atmosphere; In order to avoid oxidation, magnesium alloys are often heated in a protective atmosphere of sulfur dioxide or carbon dioxide. In order to avoid hydrogen embrittlement, copper needs to be heat treated in a neutral or weak oxidizing atmosphere.

2) In order to achieve the maximum solution effect, the solution temperature of many non-ferrous metal alloys is close to the temperature of the solid phase line, in order to prevent overheating and overburning, the furnace temperature and heating and holding time must be strictly controlled.

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Common defects and countermeasures of steel heat treatment (Chapter Four)

 Fourth, the persistent creep performance is not qualified

In power plants, chemical plants, boilers and aircraft engines, some parts need to operate at high temperatures for a long time, for these parts, excessive creep deformation and creep fracture is one of the common failure modes. Creep limit, lasting strength and lasting plasticity are the main high temperature mechanical properties of materials. The creep limit represents the resistance of steel material to creep deformation under the action of high temperature and long-term load. The durable strength is the resistance index to evaluate the resistance of a material to creep fracture, while the capacity of a material to withstand creep deformation is expressed by the durable plasticity. The deformation and fracture of materials at high temperature are not only affected by temperature and external force, but also closely related to the composition and microstructure of materials. The early failure of high-temperature components caused by improper heat treatment and poor organization that the high temperature mechanical properties of materials cannot meet the service requirements should be regarded as a heat treatment defect and be prevented. 

4.1 Heat treatment and persistent creep properties of superalloys

Aviation superalloys include nickel based superalloys, iron based superalloys and cobalt based superalloys. Nickel-based superalloy refers to the alloy with a mass fraction of nickel > 50%; Iron based superalloys are actually iron-nickel based alloys, and the mass fraction of nickel can be roughly divided into 25%, 35%~40% and 45% grades; Cobalt-based superalloy is rarely used in China.

Superalloys are complex alloying systems, and most of them use solid solution strengthening, second phase strengthening, grain boundary strengthening and comprehensive strengthening to obtain the desired properties.

The main task of heat treatment of super alloy steel is to adjust the process parameters according to the service conditions of the workpiece, restrain the harmful phase precipitation and change the number, shape, size and distribution of the beneficial phase in order to obtain the desired performance.

The substandard persistent creep strength of super alloy steel is often caused by low solution treatment temperature and improper aging process, and too high solution treatment temperature will cause the decrease of room temperature strength and the decrease of durable plasticity. The properties of superalloys can be controlled within a wide range by adjusting the heat treatment process. Optimizing the heat treatment process according to the workpiece service conditions is particularly important for superalloys.

4.2 High temperature creep brittleness

Under the action of long-term stress at high temperature, the elongation and section shrinkage of heat-resistant steels and alloys are greatly reduced, which often leads to brittle fracture, which is called high temperature creep brittleness. This brittleness is measured by the plasticity at the lowest point of the curve of the relationship between δ and experimental time at the time of creep fracture. Creep embrittlement is caused by the change of the internal structure of the material under the action of high temperature and long-term load, which occurs in both the metal and austenitic steel of the body-centered cubic lattice.

The way to reduce the creep brittleness is to reduce the intragrystalline strength to balance the intragrystalline strength with the grain boundary strength. Strengthen grain boundary or reduce the influence of weakening factors of grain boundary. The results show that the addition of trace elements such as B, B+Ti and B-Nb to the low-alloy heat-resistant steel can increase the permanent plasticity of the steel by forming fine TiC near the grain boundary or changing the morphology of carbides on the grain boundary.

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