阅读说明：本技术 一种船用低速柴油机排气阀阀杆的制造方法 (Manufacturing method of exhaust valve rod of marine low-speed diesel engine ) 是由 吴慕华 于 2020-11-30 设计创作，主要内容包括：本发明公开了一种船用低速柴油机排气阀阀杆的制造方法,包括选取坯料、锻造开坯、第二尺寸圆棒锻造、变截面圆棒的制造、模锻排气阀阀杆毛坯、固溶处理、机加工并检测达到需求尺寸、阀杆检测并贴标,先采用自由锻方式对坯料进行预锻,确保锻坯的变形量并保证锻坯组织均匀致密,通过反复锻打获得均匀晶粒度,避免出现局部混晶；再通过两镦两拔锻造形成阀杆杆身和端部锤形的粗坯,最后将粗坯压入到模具中使阀面成型。方法工艺简单、减少材料,提高了阀杆的稳定性和使用寿命,同时降低生产成本。(The invention discloses a manufacturing method of a marine low-speed diesel engine exhaust valve rod, which comprises the steps of selecting a blank, forging and cogging, forging a second-size round rod, manufacturing a variable-section round rod, forging an exhaust valve rod blank, carrying out solution treatment, machining and detecting to reach the required size, detecting the valve rod and labeling, pre-forging the blank by adopting a free forging mode, ensuring the deformation of a forging blank and ensuring the uniform and compact structure of the forging blank, obtaining uniform grain size by repeated forging, and avoiding local mixed crystals; and then, forging the two forging heads and the two drawing heads to form a hammer-shaped rough blank of the rod body and the end part of the valve rod, and finally pressing the rough blank into a die to form the valve surface. The method has simple process, reduces materials, improves the stability of the valve rod, prolongs the service life of the valve rod and reduces the production cost.)

1. A manufacturing method of a valve rod of an exhaust valve of a marine low-speed diesel engine is characterized by comprising the following steps:

s1, selecting a blank, and selecting an alloy ingot meeting the requirement of the base material of the valve rod of the corresponding engine exhaust valve;

s2, forging and cogging, namely, performing two-upsetting two-drawing multi-fire cogging forging on an alloy ingot to a round bar with a first size by using a fast forging machine, fully grinding the surface of the round bar with the first size, performing surface quality inspection, and entering the next procedure after the round bar is qualified;

s3, forging the round bar with the second size, namely forging the round bar with the first size to the round bar with the second size by using a radial forging machine or a fast forging machine, cutting the end face of the free end of the round bar with the second size flat, and turning to the working procedure of forging the round bar with the variable cross section;

s4, manufacturing a variable cross-section round rod, namely forging the second-size round rod to a third-size variable cross-section round rod comprising a valve seat diameter rod section with the size of D1, a seat back diameter-changing rod section with the size of D2 and a valve rod diameter rod section with the size of D3 by adopting a diameter forging machine or a quick forging machine;

s5, forging the exhaust valve rod blank, and forging the exhaust valve rod blank on a hydraulic machine by adopting an open type forming die;

s6, performing solid solution treatment, namely putting the exhaust valve rod die forging blank in an environment with solid solution temperature for solid solution treatment, and performing oil cooling or water cooling after high-temperature solid solution treatment;

s7, machining and detecting to reach the required size, firstly roughly machining the blank of the die forging piece of the exhaust valve rod to the fourth size and reach the required smooth finish, carrying out precipitation hardening heat treatment on the rough machined part of the valve rod, then air-cooling, and then finely machining to obtain the final required size of the exhaust valve rod;

and S8, detecting the valve rod, carrying out ultrasonic flaw detection and damage detection on the valve rod after finish machining, and making a detection report and a qualification certificate corresponding to the valve rod.

2. The manufacturing method according to claim 1, characterized in that: in step S1, a nickel-based alloy ingot of Nimonic 80A is selected as the blank, and the diameter is 510mm, so as to meet the quality requirement of the marine diesel engine valve rod.

3. The manufacturing method according to claim 1, characterized in that: in step S2, a 2000-ton or 4000-ton rapid forging machine is used to pre-forge and press the gold ingot, so as to ensure the deformation of the forged blank and ensure the uniform and compact structure of the forged blank, and the uniform grain size is obtained by repeated forging, thereby avoiding local mixed crystals; and then, cogging and forging the alloy ingot with the diameter of 510mm to a round rod with the first size of 340mm through two-upsetting two-drawing multi-firing times.

4. The manufacturing method according to claim 1 or 3, characterized in that: in the forging and cogging of the step S2, the highest heating temperature of the alloy ingot and the return heating temperature of the steel billet after each hot forging are both controlled at 1160 +/-14 ℃, the heat preservation time of the alloy ingot heating is not less than 4 hours, and the heat preservation time of the return heating of the steel billet is not less than 1.5 hours; and fully grinding the surface of the round bar with the first size, and carrying out surface quality inspection.

5. The manufacturing method according to claim 1, characterized in that: in the step S3, a 1300-ton radial forging machine, a 2000-ton fast forging machine or a 4000-ton fast forging machine is used to radially forge a round bar with a first size of 340mm to a round bar with a second size of 230mm, and the round bar is held at 1140 ℃ for more than 2 hours, and the maximum elongation coefficient of a single pass is controlled to be 1.4.

6. The manufacturing method according to claim 1, characterized in that: in the step S4 of manufacturing the variable cross-section round bar, cutting the end surface of the free end of the round bar with the diameter of 230mm and the second size flat, then adopting a 1300-ton radial forging machine, a 2000-ton rapid forging machine or a 4000-ton rapid forging machine to radially forge the round bar to the variable cross-section round bar, wherein the highest heating temperature is controlled at 1020 +/-14 ℃, the heat preservation time is more than or equal to 1 hour, and the round bar is taken out of the furnace, coated with a layer of heat insulation cotton on the surface and then returned to the furnace to be heated for more than 0.5 hour from the end surface of the free; forging a round bar with the diameter of 110mm multiplied by 1570mm from the upper diameter of a heated round bar section with the diameter of 230mm coated with the heat insulation cotton by using a clamp, flattening the end surface of the free end of the round bar with the diameter of 110mm, cutting a test sample, and cutting a variable cross-section round bar blank for the valve rod of the blanking die forging exhaust valve to a fixed length according to the shape and the size requirements of the variable cross-section round bar; and continuously producing the variable-section round rods on the remaining round rods with the diameter of 230mm according to requirements until the complete variable-section round rods with the third size are obtained.

7. The manufacturing method according to claim 1, characterized in that: in the step S5 of die forging the valve rod of the exhaust valve, the die forging method is specifically that open die forging is adopted, the material of the die is 5CrNiMo hot-work die steel, the upper die of the combined die for die forging is a flat die, the size and the shape of the lower die cavity are matched with that of the valve rod disc of the exhaust valve, an axial round hole with taper is processed at the middle part of the center of the lower die cavity, the minimum diameter of the round hole is larger than the diameter of the valve rod radial rod section with the size of D3, the lower die is arranged on a plurality of supporting die modules, the center of the supporting die module is provided with an axial round die hole with the same diameter, the diameter of the round die hole is matched with the minimum diameter of the axial round die hole at the middle part of the lower die cavity, and the length of the.

8. The manufacturing method according to claim 1 or 7, characterized in that: in the die forging exhaust valve rod of the step S5, the die forging process is that a 2500-ton hydraulic press is adopted for die forging, a die is preheated within the temperature range of 400-450 ℃, and after the die is installed, gas is ignited through a flame ring to continuously heat an upper die and a lower die; keeping the blank at 1020 +/-14 ℃ for more than 2 hours; the blank is insulated by using the adhesive heat insulation cotton.

9. The manufacturing method according to claim 1, characterized in that: in step S6, performing solution treatment on the die forging blank in a shaft furnace at the temperature range of 1020-1040 ℃, wherein the temperature fluctuation does not exceed +/-14 ℃, and monitoring the equipment condition and the process state.

10. The manufacturing method according to claim 1, characterized in that: in step S7, the surface smoothness of the valve rod of the exhaust valve with the fourth size after rough machining is Ra3.2, precipitation hardening heat treatment is carried out in a chamber type heating furnace, the treatment temperature is 700 ℃, the temperature fluctuation is less than or equal to +/-10 ℃, heat preservation is carried out for 16 hours, air cooling is carried out, and finish machining is carried out.

Technical Field

The invention relates to a manufacturing technology of engine parts, in particular to a manufacturing method of a valve rod of an exhaust valve of a marine low-speed diesel engine.

Background

The valve rod of the exhaust valve of the marine low-speed diesel engine plays a role in opening and closing the combustion chamber in the combustion chamber, and bears the influences of mechanical stress caused by sudden change of high-temperature and high-pressure fuel gas, high-speed scouring of corrosive gas, abrasion generated by relative motion with the guide sleeve and the like during working. Therefore, the exhaust valve stem material must satisfy the performance requirements of sufficient high-temperature strength and hot hardness, excellent high-temperature creep resistance and high-temperature corrosion resistance, higher wear resistance and impact resistance, good high-temperature oxidation resistance and thermal conductivity, smaller thermal expansion coefficient and the like. The valve rod stacking surface is in contact with the sealing surface of the valve seat to open and close the combustion chamber, is in direct contact with high-temperature and high-pressure fuel oil, and is subjected to the effects of scouring and ablation of high-temperature and high-pressure fuel gas, so that the bottom surface of the air valve must have good high-temperature mechanical performance and uniform fine-grained structure, and the grain size should not be coarser than 4 grades. In the forging and forming process of the valve rod, the bottom surface of the air valve and the rod body are in transition arc positions, and the phenomenon of coarse grains is easy to occur due to small deformation amount, so that the valve rod is early failed.

The exhaust valve rod is usually made of nickel-based alloy materials, and is subjected to solid solution treatment after integral forging forming and is subjected to finish machining to a specified size. The nickel-based alloy is adopted for manufacturing, has good high-temperature mechanical property and high-temperature corrosion resistance, but has poor high-temperature deformation capability, is difficult to obtain large deformation in the heating forging process, is easy to generate the phenomena of nonuniform textures such as mixed crystals and the like after forging, and can not meet the technical requirements due to poor texture.

In the prior art, a nickel-based alloy round rod is heated and then axially extruded by an electric upsetting machine to form a head cup-shaped structure, and then the head cup-shaped structure is placed in a pressing machine die to be extruded and molded, so that the equipment investment cost is high, the process is complex, and the manufacturing difficulty is high. In addition, the extrusion deformation is difficult to control in the electric upsetting process, the heating and deformation time is long, and the manufacturing cost is increased.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a manufacturing method of a marine low-speed diesel engine exhaust valve rod, which can solve the problems of uneven structure, complex process, time and labor waste and high cost in the prior forging technology.

The purpose of the invention is as follows: the invention aims to manufacture a valve rod meeting design requirements, ensure the normal and reliable operation of a diesel engine and reduce the production cost, and the specific scheme is as follows.

The technical scheme is as follows: firstly, pre-forging a blank in a free forging mode, ensuring the deformation of a forging blank and ensuring the uniform and compact structure of the forging blank, and obtaining uniform grain size through repeated forging to avoid local mixed crystals; and then, forging the two forging heads and the two drawing heads to form a hammer-shaped rough blank of the rod body and the end part of the valve rod, and finally pressing the rough blank into a die to form the valve surface. The method specifically comprises the following steps.

S1, selecting an alloy ingot meeting the material requirement of the exhaust valve rod substrate, wherein the alloy ingot is Nimonic 80A (N80A for short) nickel-based alloy.

S2, selecting phi 510 alloy ingot, adopting 2000 ton (or 4000 ton) fast forging machine to upset and draw and forge the alloy ingot to phi 340 round bar (the linear dimension measurement unit of this document is mm, the same below).

And S3, adopting a 1300-ton radial forging machine (or more than or equal to 2000-ton rapid forging machine) to radially forge the phi 340 round rod to the phi 230 round rod, cutting the end surface of the free end of the phi 230 round rod, and turning to the step of forging the round rod with the variable cross section.

And S4, adopting a 1300-ton radial forging machine (or more than or equal to 2000-ton quick forging machine) to radially forge the phi 230 round rod to a third-size variable-section round rod comprising a valve seat radial rod section with the size of D1, a seat back variable-diameter rod section with the size of D2 and a valve rod radial rod section with the size of D3.

And S5, adopting open type die forging, and die forging the exhaust valve rod blank on a 2500-ton hydraulic press.

S6, carrying out solution treatment on the die forging blank at the temperature of 1020-1040 ℃, and then carrying out ultrasonic detection on the die forging blank of the exhaust valve rod subjected to solution treatment.

And S7, performing ultrasonic detection after rough machining on the surface of the exhaust valve rod blank.

And S8, performing precipitation hardening heat treatment on the exhaust valve rod at 700 ℃, preserving heat for 16 hours, and then air-cooling to finish the preparation of the exhaust valve rod blank die forging subjected to complete heat treatment in the step.

The invention has the beneficial effects that:

1. the exhaust valve rod manufactured by the method has good high-temperature mechanical property and high-temperature corrosion resistance under severe working conditions of high-temperature and high-pressure fuel oil of a diesel engine, corrosive waste gas, abrasion impact of relative movement with a contact part and the like, and meets the corresponding technical specification requirements.

2. The valve rod of the exhaust valve manufactured by the invention can be completed in only one forging device, the equipment investment is less, and the equipment cost and the site cost can be saved.

3. The size of the hammer shape is determined by adopting accurate calculation and combining with experimental data, the thickness of the hammer shape is designed to be 1.3-1.5 times of the thickness of the bottom surface of the air valve, and the material cost can be saved.

4. The forging and forming speed of the bottom surface of the air valve is high, the initial forging and final forging temperatures are easy to control, and the rough blank is ensured to have enough deformation capacity in the forging and forming process.

Drawings

FIG. 1 is a drawing showing the shape and size of a round bar subjected to radial forging to a first size in a pre-forging step;

FIG. 2 is a dimension view of a third dimension variable cross-section round bar;

FIG. 3 is a schematic view of the form of the tire mold;

FIG. 4 is a rough shape and size drawing of the exhaust valve stem;

fig. 5 is a flow chart of a manufacturing process of the exhaust valve stem.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.

The manufacturing method and the flow of the valve rod of the exhaust valve of the marine low-speed diesel engine are shown in figure 5. As shown in FIG. 5, the manufacturing method of the exhaust valve rod of the present invention comprises the steps of firstly forging and cogging an alloy ingot, and upsetting and drawing the alloy ingot into a round rod with a diameter of 340mm by a free forging method, wherein the shape and the size are shown in FIG. 1. Further, the phi 340 round bar is radially forged to the phi 230 round bar, referring to fig. 2, and then the phi 230 round bar is radially forged to the variable cross-section round bar after the cross section of the free end of the phi 230 round bar is cut flat. Further, the variable cross-section round rod is placed in a die to be forged, the overall dimension of the valve rod forging is obtained, and the schematic shape diagram of the die is shown in figure 3. Further, carrying out solution treatment on the exhaust valve rod blank at the temperature of 1020-1040 ℃, carrying out further rough machining after ultrasonic detection, wherein the shape and the size of the rough machined valve rod are shown in figure 4, and carrying out precipitation hardening treatment after the rough machining is finished.

The dimensions in fig. 1-4 are, for the specific dimensions, D1: phi 215mm to phi 230 mm; d3: phi 105mm to phi 110 mm; l1: 315 mm-335 mm; l2: 160 mm-170 mm; l3: not less than 1370 mm. The concrete is selected according to actual needs. In the following examples, for specific dimensions, D1: phi 230 mm; d3: phi 110 mm.

Referring to fig. 5, the method for manufacturing the exhaust valve rod of the marine low-speed diesel engine comprises the following steps.

And S1, selecting a blank, and selecting an alloy ingot meeting the requirement of the base material of the valve rod of the exhaust valve of the corresponding engine.

Wherein, the blank is a Nimonic 80A nickel-based alloy ingot with the diameter of phi 510mm so as to meet the quality requirement of the valve rod of the marine diesel engine. When the blank is determined, the volume of the blank is obtained by using the drawing software for shaping assistance and performing inverse calculation so as to save materials. In the specific embodiment, a solid model of an exhaust valve stem blank (shown in figure 5) is established according to a ratio of 1:1 by adopting computer 3D drawing software (such as Unigraphics NX, SolidWorks, CATIA and the like), and the volume V of the blank is accurately calculated to be about 24245.47cm³The material density p parameter (Nimonic 80A p is 8.15 g/cm)³) The mass m of the blank is 197.60 kg. The size of the hammer-shaped blank is determined according to the calculated data of the volume V and the mass m, the hammer-shaped thickness is designed to be 1.3-1.5 times of the thickness of the bottom surface of the air valve, and the material cost can be saved.

And S2, forging and cogging, namely, performing two-upsetting two-drawing multi-fire cogging forging on the alloy ingot to a first-size round rod by using a fast forging machine, fully grinding the surface of the first-size round rod, performing surface quality inspection, and entering the next procedure after the surface is qualified.

Specifically, a 2000-ton or 4000-ton quick forging machine is adopted for pre-forging and pressing a gold ingot, so that the deformation of a forging stock is ensured, the uniform and compact structure of the forging stock is ensured, uniform grain size is obtained through repeated forging, and local mixed crystals are avoided; and then, cogging and forging the alloy ingot with the diameter of 510mm to a round rod with the first size of 340mm through two-upsetting two-drawing multi-firing times.

Further, in the forging and cogging, the highest heating temperature of the alloy ingot and the return heating temperature of the steel billet after each hot forging are both controlled at 1160 +/-14 ℃, the heat preservation time of the alloy ingot heating is not less than 4 hours, and the heat preservation time of the return heating of the steel billet is not less than 1.5 hours; and fully grinding the surface of the round bar with the first size, and carrying out surface quality inspection.

And S3, forging the round bar with the second size, forging the round bar with the first size to the round bar with the second size by adopting a radial forging machine or a fast forging machine, cutting the end face of the free end of the round bar with the second size flat, and turning to the working procedure of forging the round bar with the variable cross section.

Furthermore, in the forging of the round bar with the second size, a 1300-ton radial forging machine, a 2000-ton fast forging machine or a 4000-ton fast forging machine is adopted to radially forge the round bar with the first size and the diameter of 340mm to the round bar with the second size and the diameter of 230mm for a plurality of times, the temperature is kept for more than 2 hours at the temperature of 1140 ℃, and the maximum elongation coefficient of a single pass is controlled to be 1.4.

And S4, manufacturing the variable cross-section round rod, and forging the second size round rod to a third size variable cross-section round rod comprising a valve seat diameter rod section with the size of D1, a seat back diameter-changing rod section with the size of D2 and a valve rod diameter rod section with the size of D3 by adopting a diameter forging machine or a quick forging machine.

Specifically, after the end face of the free end of a round bar with the diameter of 230mm and the second size is cut flat, a 1300-ton radial forging machine, a 2000-ton fast forging machine or a 4000-ton fast forging machine is adopted to radially forge the round bar to a variable cross section, wherein the highest heating temperature is controlled to be 1020 +/-14 ℃, the heat preservation time is more than or equal to 1 hour, the round bar is taken out of a furnace, and the round bar is returned to the furnace and heated for more than 0.5 hour after the end face of the free end is coated with a layer of heat insulation cotton; forging a round bar with the diameter of 110mm multiplied by 1570mm from the upper diameter of a heated round bar section with the diameter of 230mm coated with the heat insulation cotton by using a clamp, flattening the end surface of the free end of the round bar with the diameter of 110mm, cutting a test sample, and cutting a variable cross-section round bar blank for the valve rod of the blanking die forging exhaust valve to a fixed length according to the shape and the size requirements of the variable cross-section round bar; and continuously producing the variable-section round rods on the remaining round rods with the diameter of 230mm according to requirements until the complete variable-section round rods with the third size are obtained.

And S5, forging the exhaust valve rod blank, and forging the exhaust valve rod blank on a hydraulic machine by using an open type forming die.

Die forging mode: in the die forging exhaust valve stem, open die forging is adopted, the material of a die is 5CrNiMo hot-working die steel, the upper die of a combined die for die forging is a flat die, the size and the shape of a lower die cavity are matched with the size and the shape of an exhaust valve stem disc, an axial round hole with a taper is processed at the middle part of the center of the lower die cavity, the minimum diameter of the round hole is larger than the diameter of a valve stem radial rod section with the size of D3, the lower die is arranged on a plurality of supporting die modules, an axial round die hole with the same diameter is formed in the center of each supporting die module, the diameter of the round die hole is matched with the minimum diameter of the axial round die hole in the middle part of the lower die cavity, and the length of the axial round die hole. Referring to fig. 4, the die cavity has a clearance of 3mm from the blank.

And (3) die forging process: adopting a 2500-ton hydraulic press for die forging, preheating a die within the temperature range of 400-450 ℃, and igniting gas through a flame ring to continuously heat an upper die and a lower die after the die is installed; keeping the blank at 1020 +/-14 ℃ for more than 2 hours; the blank is insulated by using the adhesive heat insulation cotton. The concrete measures comprise. Firstly, after the blank reaches the temperature, the D1 diameter and the deformation part of the transition region are coated with the adhesive heat insulation cotton, and the temperature is maintained for more than or equal to 0.5 hour after the blank is put back into the furnace; secondly, when the blank is heated, heat insulation cotton is adopted to carry out heat insulation shielding on the D2 diameter round rod part where the die forging variable cross-section forged rod is not deformed, and the temperature rise of the D2 diameter round rod part is weakened; thirdly, when die forging is carried out, the blank is heated by igniting fuel gas through a flame ring, the hot working window is controlled to be not more than 60 ℃ in an intensified mode, and the finish forging temperature is controlled to be not less than 950 ℃.

Wherein the molding is carried out at an initial pressing speed of 50mm/min and then pressure holding is carried out.

S6, performing solid solution treatment, namely putting the exhaust valve rod die forging blank in an environment with solid solution temperature for solid solution treatment, and performing oil cooling or water cooling after high-temperature solid solution treatment.

Specifically, the blank of the die forging is subjected to solution treatment in a well type furnace at the temperature range of 1020-1040 ℃ to prevent the valve rod blank from bending during the solution treatment, the temperature fluctuation does not exceed +/-14 ℃, and the equipment condition and the process state are monitored.

S7, machining and detecting to reach the required size, firstly roughly machining the blank of the exhaust valve rod die forging to the fourth size and reach the required smooth finish, carrying out precipitation hardening heat treatment on the rough valve rod workpiece, then air-cooling, and then finely machining to obtain the final required size of the exhaust valve rod

Specifically, the surface smoothness of the exhaust valve rod of the fourth size after rough machining is Ra3.2, precipitation hardening heat treatment is carried out in a chamber type heating furnace, the treatment temperature is 700 ℃, the temperature fluctuation is less than or equal to +/-10 ℃, heat preservation is carried out for 16 hours, then air cooling is carried out, and finish machining is carried out.

And S8, detecting the valve rod, carrying out ultrasonic flaw detection and damage detection on the valve rod after finish machining, and making a detection report and a qualification certificate corresponding to the valve rod.

The ultrasonic flaw detection and the destructive detection of the exhaust valve rod die forging blank subjected to complete heat treatment in the process flow comprise chemical components, metallographic structures and mechanical properties, and subsequent factory reports and qualified certificates.

The exhaust valve rod manufactured by the invention has good high-temperature mechanical property and high-temperature corrosion resistance, can meet the corresponding technical requirements under severe working conditions of high-temperature and high-pressure fuel oil, corrosive waste gas, abrasion and impact of relative movement with a contact part and the like of a diesel engine, and ensures the safe and reliable operation of the diesel engine. In addition, the manufacturing of the exhaust valve rod can be completed in only one forging and pressing device, the device investment is low, the adopted forging process is determined and implemented under the mutual verification of theoretical calculation and experimental data, and the uniform and fine material structure, high strength and high rigidity can be ensured. By means of easy control of the initial forging and final forging temperature, the rough blank is ensured to have enough deformation capacity in the forging and pressing forming process, so that the problem of poor deformation capacity of the nickel-based alloy at high temperature is solved, and an ideal exhaust valve rod blank is manufactured.

The specific size and shape designed in the method can be adjusted according to the actual requirement of the engine, and the method is applicable to the field of engines for engineering machinery such as marine diesel engines, nuclear power, mines, automobiles and the like.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.