Development of seamless steel pipe for 20MnV hydraulic cylinder
In recent years, the global construction machinery industry has shown a stable development trend. With the rapid development of various major infrastructure construction in China, especially the development of the engineering construction business represented by “One Belt One Road”, the traditional market is growing steadily, and the demand for domestic construction machinery is growing steadily, while the technical indicators of energy consumption, strength and stability of construction machinery have raised higher requirements. As an important part of construction machinery, hydraulic cylinder has extremely high requirements on cushioning, high load, high frequency and stability. As the most important raw material of hydraulic cylinder, cold drawn seamless steel pipe for high strength cylinder directly determines the accuracy, strength, life and other important parameters of hydraulic cylinder. Therefore, it is necessary to develop cold-drawn seamless steel tubes for hydraulic cylinders with high strength, precision, energy saving and light weight. 20MnV seamless steel tubes for hydraulic cylinder barrels are the most important raw material for high strength hydraulic cylinders, which can be widely used in industries such as engineering machinery, metallurgical machinery, plastic machinery, petrochemical machinery, light industry machinery, aerospace, ships and marine engineering equipment [1,2,3].
1. Technical requirements and industrial production scheme
1.1 Technical requirements
The development of seamless tubes for hydraulic cylinder barrels is based on the requirements of a user, and the dimensional tolerances and mechanical properties need to meet the following requirements: outer diameter accuracy is required to be controlled within plus or minus 1%, wall thickness is required to be controlled within plus or minus 10%, and the out-of-roundness and wall thickness unevenness of the tubes are required to be no more than 85% of the outer diameter and wall thickness tolerances respectively; -20℃ longitudinal low temperature impact value is not less than 27J, tensile strength is not less than 560MPa; yield Strength not less than 400MPa, elongation after break not less than 17%, grain size grade not less than 5.
1.2 Hydraulic cylinder steel characteristics
As an important part of construction machinery, hydraulic cylinder has high requirements in cushioning, high load, high frequency and stability. Seamless steel pipe for high strength cylinder, as the most important raw material for hydraulic cylinder, directly determines the accuracy, strength, life and other important parameters of hydraulic cylinder. Therefore, the comprehensive performance of seamless steel tubes for hydraulic cylinders requires high strength and the best low temperature impact performance, and its composition and process design need to take into account the economy.
1.3 Industrial production solutions
Industrial production requires reasonable design of alloy composition and strict control of continuous casting and hot rolling process to ensure the comprehensive mechanical properties and surface quality of the trial steel pipe.
1.3.1 Chemical composition optimization
According to the comprehensive performance requirements of the product to ferritic and pearlite based metallographic organization design and V, Cr as the main alloying elements of the composition design, to determine the test steel economic and reasonable alloying element ratios and organizational composition, grain refinement and precipitation strengthening to further refine the grain and control the organizational form, significantly improve the low-temperature impact work and ensure the rationality and stability of the production process, to ensure that the test steel pipe Comprehensive mechanical properties. Chemical composition as shown in Table 1.
Table.1 Hydraulic cylinder steel chemical composition (mass fraction)
The chemical composition of hydraulic cylinder steel designed in Table 1, the P element is prone to macroscopic segregation, its content is controlled at 0.018% or less, and the Si content is appropriately increased to accelerate the solidification speed of the cast billet, which can reduce the segregation of Si elements.
1.3.2 Process flow
Blast furnace molten iron → iron pretreatment → converter smelting → LF furnace refining → VD vacuum treatment → continuous casting → casting billet slow cooling → storage → continuous casting billet → sawing → ring furnace heating → perforation → continuous rolling → de-tubing → fixing (Zhang reduction) diameter → cold bed → sawing → straightening → leakage magnetic flaw detection → ultrasonic flaw detection → inspection → length measurement, weighing, spraying standard → packaging into storage → factory
1.3.3 Continuous casting process
LF+VD” is used to ensure the cleanliness of the cast billet. The final slag alkalinity of converter smelting is not less than 3.0, must do a good job of end carbon content control, not over-oxidation and ultra-low carbon out of steel, converter end control target: ω (C) ≥ 0.06%, ω (P) ≤ 0.010%, steel temperature in 1610 ~ 1630 ℃, the whole process according to the requirements of LF refining blowing argon, control the intensity of blowing argon, keep the steel creep and not bare steel. In the middle and late stages of refining to add ferrochrome. After vacuum treatment, the soft blowing time should not be less than 13min, at the same time, attention should be paid to the weak stirring effect, not bare steel, must do a good job of removing non-metallic inclusions in the molten steel.
During the continuous casting process, the whole process of casting is protected, and the pulling speed is strictly controlled according to the corresponding specifications to ensure constant pulling speed casting. The superheat of continuous casting molten steel is controlled in the range of 15~35℃ to increase the equiaxial crystal ratio. The crystallizer electromagnetic stirring and end electromagnetic stirring process is used to inhibit the flow of impurity-rich steel to the center at the end of solidification to reduce central segregation.
1.3.4 Hot rolling process
Strictly control the atmosphere in the furnace during the heating process of the cast billet, follow the prescribed heating regime, and monitor the temperature and time of each section of the furnace during the heating process to reduce the expansion of thermal stress on the core or outer surface defects of the billet and prevent cracks in the subsequent production. Ensure that the pipe billet is heated evenly in the heating furnace to prevent shading, impermeable heating, overheating or overburning. According to the design of the alloy composition in the steel quality plan and the actual production of billet specifications, determine to control the casting billet out of the furnace temperature at 1260 ~ 1290 ℃. The smoothness of pushing steel bite in, piercing, holding rolls holding tubes and throwing steel is strictly controlled during piercing. Rough rolling in the austenite recrystallization zone, in order to fully refine the original austenite grains, try to achieve a large amount of rolling under pressure, in the equipment parameters allow the conditions, increase the total deformation of the rough rolling stage. In the unrecrystallized area of the finishing stage of deformation, the appropriate increase in the first few passes of the finishing rolled down to ensure the uniformity of steel wall thickness direction, and in the last 3-4 passes of pipe shape control.
1.3.5 Normalizing process
The design of the normalizing process system is based on the previous composition design, as well as the empirical formula for AC3 temperature (AC3 = 910-320 × ω (C) – 14 × ω (Ni) – 12 × ω (Cu) – 10 × ω (Mn) + 5 × ω (Cr) + 7 × ω (W) + 14 × ω (Mo) + 5 × ω (V) + 18 × ω (Si)), AC3 theoretical value of 830 ℃ In order to ensure the quality of the outer surface of the steel pipe and prevent surface defects such as pockmarked surface, the pressure of high pressure water descaling is not less than 12MPa after normalizing, and the straightening force should be strictly controlled to ensure the steel pipe The straightening force should be strictly controlled to ensure the requirements of the curvature of the steel pipe, and there should be no correction of concave, oval, cracking and so on.
2. Industrial trial production
2.1 Quality of continuous casting billet
This trial casting billet is a Φ350mm continuous casting round billet, and its internal quality and surface quality are inspected and analyzed. According to GB/T226-2015  requirements to take the continuous casting billet specimen and ensure the suitable roughness of the specimen inspection surface after grinding, according to the standard hot acid corrosion method to erode the taken low times specimen, the resulting cast billet low times organization is shown in Figure 1.
Figure.1 Seamless steel tube billet for hydraulic cylinder barrel low times organization
According to the standard YB/T4003-1997 , the low times organization after erosion is evaluated, the center is loose 0.5, the center is cracked 1, intermediate cracks, shrinkage, subcutaneous cracks, bubbles, non-metallic inclusions are not. It can be seen that the internal quality control of this trial continuous casting billet is good.
2.2 CCT curve
From the trial steel tube rolling state specimens to analyze the continuous cooling conditions under the law of subcooling austenite transformation, the CCT curve results are shown in Figure 2, Figure 2, the cooling rate curve from left to right is 110 ℃/s, 50 ℃/s, 30 ℃/s, 20 ℃/s, 10 ℃/s, 7 ℃/s, 5 ℃/s, 2 ℃/s, 1 ℃/s, 0.5 ℃/s.
Figure.2 CCT curve of the trial steel pipe
The CCT curve from Figure 2 can be seen in the trial steel pipe AC3 temperature of 813 ℃, because of the presence of vanadium and chromium and other alloying elements, the normalizing temperature needs to be increased by 30 ~ 50 ℃ on the basis of AC3, industrial production design process normalizing temperature of 890 ℃, fully meet the requirements. The average cooling speed of the material is 0.6℃/s, and the cooling curve should be between the curves of 1℃/s and 0.5℃/s in Figure 2.
2.3 Metallographic organization
Intercepted metallographic specimens from the trial steel pipe, the specimens were ground and polished and then corroded by 4% nitric acid alcohol solution, and the tissue morphology was observed under an optical microscope, and the grain size rating was 8.5 according to GB/T6394-2017  standard requirements, as shown in Figure 3.
Figure.3 Metallographic organization of the trial steel pipe
It can be seen from Figure 3: the steel pipe organization are ferrite + pearlite organization, confirming the room temperature organization results of the material in Figure 2. Ferrite grains are smaller, and the pearlite band organization is completely eliminated, which helps to improve its impact toughness.
2.4 Mechanical properties
The mechanical properties of the test steel pipe were tested and analyzed. In accordance with GB/T228-2010  and GB/T229-2007  standard requirements, mechanical properties testing, the results are shown in Table 2.
Table.2 Mechanical properties of the test steel pipe
|Outer diameter / mm||Wall thickness / mm||Yield strength / MPa||Tensile strength / MPa||Elongation /%||Impact value / J|
|User requirements||User requirements||≥400||≥560||≥17.0||≥27|
Table 2 shows that the properties of the test steel pipe meet the design requirements, yield strength of 482 ~ 525MPa, greater than 400MPa requirements, tensile strength of 655 ~ 708MPa, greater than 560MPa requirements, elongation of 19% ~ 28%, greater than 17%, the strong plasticity match reasonable. The longitudinal low temperature impact value of -20℃ is 100～178J, more than 27J, with excellent low temperature impact toughness.
- (1) Through the analysis of the performance requirements and process characteristics of seamless steel tubes for hydraulic cylinder barrels, the design of V, Cr-based alloy composition, and the development of the corresponding smelting, rolling, heat treatment process control measures, the successful development of seamless steel tubes for hydraulic cylinder barrels with excellent overall performance.
- (2) The yield strength of the trial steel pipe is more than 480MPa, tensile strength is more than 550MPa, longitudinal low temperature impact value of -20℃ is more than 100J, the surface quality and mechanical properties of the product are up to the user’s requirements, the strength and plasticity match reasonably, the impact toughness is excellent, and has been stable batch supply.
Authors: Jiang Hailong, Chen Lin, Zhang Xueying, Mi Yongfeng, Yao Xiaole, Liu Yurong, Guo Zhitao, Song Jiangbo
Source: China Forged Cylinders Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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