2022-06-13
At present, the greenhouse greenhouse commonly used galvanized round steel pipe material quality varies, many specifications, and no detailed selection of reference. Selected Ф25mm × 1.5mm, Ф25mm × 1.8mm, Ф25mm × 2.0mm, Ф32mm × 1.5mm, Ф32mm × 1.8mm, Ф32mm × 2.0mm, Ф42mm × 2.0mm, Ф48mm × 2.0mm 8 kinds of agricultural hot-plated round steel pipe, through the tensile test, flattening test, hardness test, comparative study of wall thickness The results show that under the same treatment conditions, the wall thickness, diameter, high temperature and high humidity environment (water bath and hot air aging treatment) and other factors on the mechanical properties of the application of galvanized steel pipe. The results show that under the same treatment conditions, the same diameter, the wall thickness of the steel pipe increases, the tensile strength decreases and the elongation at break increases, and as the wall thickness increases, the high temperature and high humidity accelerates the decrease in tensile strength and the increase in elongation at break. Under the same treatment conditions, the same wall thickness, tube diameter increases, the tensile strength decreases, the elongation at break increases, and high temperature and high humidity effect on the tensile strength is small, the effect on the elongation at break is obvious. Ф25mm × 1.5mm steel tube tensile strength is the largest, up to 558MPa, elongation at break is small, 11.5%; Ф48mm × 2.0mm steel tube tensile strength is the smallest 37.6MPa, elongation at break 6MPa,elongation at break is larger, 27.5%. The larger the wall thickness and diameter, the better the compressive properties, Ф25mm × 1.5mm, Ф25mm × 1.8mm wall thickness and diameter are smaller, easy to crack. Under the same treatment conditions, with the increase in wall thickness and diameter, the Vickers hardness decreases, and with the increase in wall thickness, the effect of high temperature and high humidity on the steel pipe hardness decreases. Ф25mm × 1.5mm steel pipe hardness is the largest, up to 210HV; Ф48mm × 2.0mm hardness is smaller, 127HV. Therefore, this study from the mechanical properties of the steel pipe performance analysis, can be used for greenhouse greenhouse Steel pipe material selection and use to provide reference.
China's facility agriculture is developing rapidly, and has built a considerable scale of steel-frame greenhouses and solar greenhouse agricultural facilities.Compared with the previous structures such as bamboo and wood skeleton, welded steel and mixed clay, galvanized steel pipes are widely used because of their high structural strength, good corrosion resistance, low metal consumption, easy installation and construction, and low production price.However, some quality problems were found in the development process, which caused great losses. Quality problems are mainly reflected in 2 aspects: on the one hand, the structure of greenhouses and greenhouses is unreasonable and weak against natural disasters such as wind, rain and snow; on the other hand, the quality of steel pipe materials, their mechanical properties, corrosion resistance, etc. are not qualified, shortening the service life.
Most of the current studies focus on the structural design and load calculation of greenhouses and greenhouses, but there are few studies on the quality of the steel pipe material itself. Due to the structural and environmental characteristics of greenhouse, the steel tube material is mainly affected by high temperature aging and water corrosion, thus weakening the mechanical properties. Therefore, in this study, galvanized steel tubes with different wall thicknesses and diameters were selected to simulate the high temperature and high humidity environment that has the greatest impact on the mechanical properties of steel tubes in greenhouse, and were subjected to water bath treatment and hot air aging treatment. The effects of wall thickness, diameter, high temperature and humidity on the tensile strength, elongation at break, compressive properties, and hardness of galvanized steel tubes were compared and analyzed through tensile, flattening, and hardness tests, and the reasons for the effects were theoretically discussed to provide reference for the selection and use of steel tube materials in greenhouses.
1 Materials and methods
1.1 Test subjects
For the test greenhouse greenhouse commonly used galvanized steel pipe for hot-dip galvanized welded steel pipe, tropical cold-rolled, from the Jiangsu Provincial Academy of Agricultural Sciences, five continuous greenhouse construction site, a total of eight commonly used round steel pipe, different sizes are as follows: outer diameter Ф25mm, wall thickness of 1.5, 1.8, 2.0mm; Ф32mm, wall thickness of 1.5, 1.8, 2.0mm; Ф42mm, wall thickness of 2.0m m; Ф48mm, wall thickness 2.0mm.
1.2 Test method
1.2.1 Tensile test In accordance with GB/T228.1-2010 standard, the steel pipe cut into lengths of 220mm specimens, each size of steel pipe to take 9 specimens, divided into three groups, each group of three parallel specimens, respectively, untreated, water bath treatment and hot air aging treatment, using the CMT5305 microcomputer-controlled electronic universal testing machine, the maximum test force of 300kN. Test force measurement range 0.2% to 100%, the relative error ±1.0%, stretching speed 6mm/min, tensile test, take the average value as the final tensile strength and elongation at break.
1.2.2 Crush test In accordance with GB/T246-2007 standard, the steel pipe cut into 100mm length specimens, each size of the steel pipe to take 9 specimens, divided into 3 groups, each group of 3 parallel specimens, were not treated, water bath treatment and hot air aging treatment, using SHT4106 microcomputer-controlled electro-hydraulic servo universal test machine, the maximum load of 1000kN, the test force measurement range of 1% ~ 100%, the relative error ± 1%, the pressure plate speed of 6mm / min, the flattening test, observe the cracking.
1.2.3 Hardness test In accordance with GB/T4340.1-2009 standard, the steel pipe cut into length 10mm specimens, each size of the steel pipe to take 9 specimens, divided into 3 groups, each group of 3 parallel specimens, respectively, untreated, water bath treatment and hot air aging treatment, using VH-50AC Vickers hardness tester, measuring range 8 ~ 2967HV, test force 98.0N, hardness test, take the average value as the final hardness value. ～ VH-50AC Vickers hardness tester, measuring range 8 ~ 2967HV, test force 98.0N, hardness test, take the average value as the final hardness value.
1.2.4 Specimen processing Water bath treatment: simulate the high temperature and high humidity environment in farmland (the highest temperature in the shed is generally 60℃), refer to GB10703-1989 and GB 6458-1986, put the specimen into HH-4 digital display thermostatic water bath, 60℃ water bath for 72h, for galvanized steel pipe Water bath treatment, then remove and dry the surface water drops, room temperature for mechanical properties test. Hot air aging treatment: simulate farmland high temperature environment (the highest temperature in the shed is generally 60 ℃), according to GB/T18244-2000, the specimen into DHG-9070 blast drying oven, 60 ℃ blast drying 168h, hot air aging treatment of galvanized steel pipe, and then removed, room temperature for mechanical properties test. Mechanical properties test at room temperature.
2 Results and Analysis
2.1 Analysis of the variation of tensile properties of galvanized steel pipes
When the wall thickness of Ф25mm steel pipe increased from 1.5mm to 1.8mm, the tensile strength of untreated only decreased by 2MPa, the tensile strength of hot air aging treatment only decreased by 10 MPa, while the tensile strength of steel pipe treated with water bath decreased more to 67MPa; when the wall thickness of Ф25mm steel pipe increased from 1.8mm to 2.0mm, the tensile strength of untreated, water bath and hot air aging treatment were greatly reduced by 119, 80 and 106MPa. Ф32mm steel pipe wall thickness increased from 1.5mm to 1.8mm, and then increased to 2.0mm, the tensile strength of untreated steel pipe were gradually reduced by 24, 39MPa, while the water bath and hot air aging treatment of steel pipe tensile strength changes are very close, has been maintained at 50MPa or less. When Ф25mm steel wall thickness from 1.5mm to 1.8mm, the overall elongation at break of untreated and water bath treatment remains unchanged, the elongation at break of hot air aging treatment increased by 3 percentage points; Ф25mm steel wall thickness from 1.8mm to 2.0mm, the elongation at break of untreated, water bath, hot air aging treatment of steel pipe increased by 4 percentage points, 8 percentage points, 8.5 percentage points, respectively. Ф32mm steel pipe wall thickness from 1.5mm to 1.8mm, and then increased to 2.0mm, untreated steel pipe elongation at break were increased by 9.5 percentage points, 7.0 percentage points, water bath treatment were increased by 8.5 percentage points, hot air aging treatment were increased by 6.5 percentage points. This may be due to the increase in temperature, if at the same time the role of water, corrosion, the greater the wall thickness, material uniformity is reduced, the greater the expansion of the steel tube metal, making the steel tube toughness increased, resulting in reduced tensile strength of the steel tube, elongation at break increased.
Different diameters of the steel pipe, wall thickness 2mm, the most obvious pattern of change, the same treatment conditions, the diameter of 25, 32, 42, 48mm steel pipe tensile strength decline is very similar, are gradually reduced by about 20MPa. But the change in elongation at break varies greatly, wall thickness of 2mm, diameter from 25mm to 32mm, not treated, water bath treatment, hot air aging treatment of steel pipe elongation at break were increased by 10.5 percentage points, 9 percentage points, 3 percentage points; Ф32mm, the diameter continues to increase, the elongation at break increase is not more than 2 percentage points. Wall thickness is certain, the larger the diameter, the greater the amount of metal material, but the uniformity is reduced, making the tensile strength is reduced, the elongation at break increased. The above analysis shows that the wall thickness and diameter of the steel pipe size is the main factor affecting the tensile performance of the steel pipe, while high temperature and high humidity will accelerate the weakening of the tensile properties of the steel pipe.
2.2 Characterization of the variation of the flattening performance of galvanized steel pipes
Only the tubes with a diameter of 25 mm and wall thicknesses of 1.5 mm and 1.8 mm were cracked, while the rest were crack-free. The results show that the increase in wall thickness and diameter helps to improve the compressive properties of the tube. This may be due to the sum of the tensile stresses on the tube is greater than the tensile strength of the metal, then the specimen cracking. Cracks in thin-walled tubes appear on the horizontal bus of the tensile stress, and cracks in thick-walled tubes appear on the vertical bus of the tensile stress. Wall thickness and diameter are small, flattening test, steel pipe by the role of transverse stress is larger, easy to develop transverse deformation, the most horizontal tensile stress, therefore, Ф25mm steel pipe is easy to crack. Steel pipe flattening test schematic as shown in Figure 1. Point A is the compressive stress, the nearby metal is compressed, point a is the tensile stress, the nearby metal is stretched, point B is the tensile stress, the metal is stretched, point b is the compressive stress, the metal is compressed. When the sum of the tensile stresses at point B or point a is greater than the tensile strength of the metal, cracks appear on the specimen. The tensile stress in the B-b direction is the highest in thin-walled tubes, and cracks appear on the bus at point B, which is prone to cracking.
The Vickers hardness test results of different steel tubes are shown in Table 1. Ф32mm steel pipe wall thickness from 1.5mm to 1.8mm, untreated, water bath treatment, hot air aging treatment of steel pipe hardness were reduced by 33, 49, 44HV; wall thickness of 1.8mm and 2.0mm hardness is very close, untreated steel pipe hardness reduced by 7HV, while water bath treatment and hot air aging treatment of steel pipe hardness were only reduced by 1.8mm and 2.0mm hardness. This is probably due to the fact that the greater the wall thickness, the less uniformity of the material in the rolling process, and the same forces, the easier the indenter presses into the metal, the greater the deformation of the metal, the larger the indentation, and therefore the decrease in hardness value.
Under different diameters, the hardness of the steel pipe with a wall thickness of 2 mm changes most obviously, and under the same treatment conditions, the hardness of the steel pipe with diameters of 25 mm and 32 mm is very close, both around 140 HV. With the increase in diameter, Ф42mm and Ф48mm hardness in about 130HV. This may be due to the larger diameter, the more ductile the metal, the more obvious deformation, the indentation is also larger, so the hardness value is smaller [19]. The above analysis shows that the wall thickness is the main factor affecting the Vickers hardness of the steel pipe, the diameter has less impact, high temperature and high humidity effect on the hardness of the steel pipe has a certain impact.
3 Conclusion
The same diameter, the same treatment conditions, the tube wall thickness increases, the tensile strength decreases, elongation at break increases, and with the increase in wall thickness, high temperature and humidity accelerates the reduction in tensile strength, and the more obvious effect on the increase in elongation at break. The same wall thickness, the same treatment conditions, the tube diameter increases, the tensile strength decreases, the elongation at break increases, and the effect of high temperature and humidity on the tensile strength is small, the effect on the elongation at break is obvious. Ф25mm, wall thickness 1.5mm untreated steel tube tensile strength is the largest, up to 558MPa, the elongation at break is small, 11.5%; Ф48mm, wall thickness 2.0mm untreated The tensile strength of Ф48mm, wall thickness 2.0mm untreated steel pipe is the smallest, 376MPa, elongation at break is larger, can reach 27.5%.
Wall thickness and diameter are smaller, the flattening test, the steel pipe is subject to greater lateral stress, lateral deformation, therefore, Ф25mm, wall thickness 1.5mm and 1.8mm of steel pipe is prone to cracking, the greater the wall thickness and diameter, the better the compressive performance.
The Vickers hardness decreases when the wall thickness of the tube increases under the same diameter and treatment conditions, and the effect of high temperature and humidity on the hardness of the tube decreases as the wall thickness increases. The same wall thickness, the same treatment conditions, the tube diameter increases, the Vickers hardness decreases, but the reduction does not exceed 12 HV. wall thickness is the main factor affecting the Vickers hardness of the tube, the diameter has a smaller impact, the effect of high temperature and humidity on the tube hardness has a certain impact. Ф25mm, wall thickness 1.5mm untreated tube hardness is the largest, 210 HV; Ф48mm, wall thickness 2.0mm untreated tube hardness is smaller, 127 HV. Steel tube hardness is smaller, 127 HV.