The microstructure and properties of high-strength structural steel for fruit auxiliary picking equipment

The microstructure and mechanical properties of high-strength structural steel used in the fruit-picking equipment were studied by using a metallographic microscope, a tensile tester, and an impact toughness tester. The results revealed that the structure of the experimental steels was a mixed microstructure of ferrite and pearlite, with an average grain size of ferrite of 12 μm. The yield strength was between 515-540 MPa and the tensile strength was 635-645 MPa. The impact toughness at room temperature reached over 200 J. The physical properties of the studied steels completely fulfilled the requirements of the high-strength structural steel for fruit auxiliary picking equipment. The new method cut down the tempering heat treatment process and decreased production costs.


Introduction
The application of the fruit auxiliary picking equipment in the agricultural and rural fruit planting industry has become the focus of research both at home and abroad.The fruit auxiliary picking equipment is agricultural machinery with a rotating mechanical arm device.The mechanical arm of this equipment has to bear a huge load, therefore, it requires the use of high-strength structural steel with a yield strength of 460 MPa and a tensile strength of over 630 MPa [1][2][3] .This kind of steel for fruit auxiliary picking equipment is usually prepared by incorporating ultra-low carbon content (0.04%-0.07%wt.) and is produced by using the thermo-mechanical rolling process (TMCP) combined with the tempering heat treatment process to improve the quality of the produced steel [4] .However, this production process has certain drawbacks, such as a long process duration and a high cost.In this context, a novel type of high-strength structural steel for use in the fruit auxiliary picking equipment was designed in the present study with a medium carbon content of 0.16%-0.18%wt [5][6] .The chemical composition, production manufacturing technique, structure, and physical properties of the steel of fruit auxiliary picking equipment with different thickness specifications (8, 10, 12, and 14 mm) were investigated.Certain micro-alloy elements, such as Nb, V, and Ti, were added to refine the grain during the smelting process, which also improved the quality of the steel.In the production process, the advanced rolling process (CR) and cooling process(CC) were adopted.The produced steel plate could be used directly after rolling without the tempering treatment, which cuts down the tempering heat treatment process and reduces the production cost.The present study would serve as a reference basis for the production of such kinds of steel.

Experimental materials and methods
The molten iron pretreatment, converter steelmaking, LF furnace refining, RH furnace vacuum degassing, dynamic soft reduction, and electromagnetic stirring technology were employed to control the production of the cc slab during the test steel smelting.The slab is 200 mm in thickness, and its chemical composition is shown in Table 1.The steel samples with different thicknesses of 8, 10, 12, and 14 mm were rolled by using different rolling procedures, then subjected to accelerated cooling, cooling bed cooling, trimming process, and quality inspection, and finally were stored until use.The flowchart for the production process is presented in Figure 1.The rolling manufacturing process was roughing and finishing stages: rough rolling and finishing rolling.The heating temperature of rough rolling was controlled at 1, 240°C.The heat preservation was performed for 180 min.After the descaling process, rough rolling was conducted at 980-1, 040°C.After the completion of rough rolling, the temperature control stage commenced.After the completion of temperature control, the finishing rolling was conducted at 960-830°C.After the completion of finishing rolling, the accelerated cooling process was performed by using the relevant device and water for cooling.The cooling was performed at a rate of 20-25°C until the final cooling temperature of 600°C was reached.Subsequently, the natural air cooling was performed on a cooling bed to reach the temperature of 300°C, then cooled to room temperature in the air.The entire rolling process is illustrated in Figure 2.

Steel making
Steel rolling

Microstructure and morphology of test steels
The structure of the test steels is depicted in Figure 3.As visible in the figure, the metallographic microstructure of the test steel comprised black pearlite and white ferrite [7][8] .The black pearlite was distributed between the white ferrite in strips.The ferrite grain size of 8 mm in thick test steels was relatively small, and the average ferrite grain size was 12 m.With increase in the thickness of the experimental steel, the ferrite grain size also increased, while the ferrite grain level decreased.The statistical analysis was performed next.As depicted in Figure 3(a), the ferrite grain size reached Grade 14 for a thickness of 8 mm, Grade 13.5 for a thickness of 10 mm, and Grade 13 for thicknesses of 12 mm and 14 mm.

Mechanical properties of test steel
The yield strength of the steel was predicted and estimated by Formula (1) [9][10] provided below.
The calculated A  was approximately 43.1 MPa.
In the formula, D  denotes the dislocation strengthening, and the yield stress was then estimated through the general formula for calculating dislocation density, which is provided below: In the case of a cubic crystal,  is 0.5, while for a closely arranged hexagonal crystal,  is 1.1.

The calculated D
 was approximately 123.2 MPa.
In the formula,   denotes the precipitation strengthening and was estimated by using Formula (4) provided below: After the above estimation, the yield strength was determined to be approximately 525 MPa, which was equivalent to the measured value presented in Table 2.
The determined mechanical properties are shown in Table 2.The yield strength was between 515 and 540 MPa, and the tensile strength was between 635 and 645 MPa.The steel fulfilled all requirements for the steel used in the fruit auxiliary picking equipment, exhibiting excellent impact toughness, with the impact energy value reaching over 200 J.

Conclusions
(1) The structure of the experimental steel comprises a mixture of ferrite and pearlite.The grain size of ferrite is between Grades 13 and 14, and the average grain size of ferrite is 12 m.
(2) The yield strength of the experimental steel is between 515 and 540 MPa, and the tensile strength is between 635 and 645 MPa.The steel exhibits excellent impact toughness at room temperature, with the impact energy value reaching over 200 J.
(3) The tested steel is designed with medium carbon content and produced by rough rolling and finishing the rolling process.The physics properties of the studied steel completely fulfill the requirements of the high-strength structural steel for fruit auxiliary picking equipment.The new method cuts down the tempering heat treatment process and decreases production costs.

Figure 3 .
Figure 3. Microstructure and morphology of test steel.

1 
the dislocation density  ＝1.5  10 8 mm 2 , the shear modulus  ＝ 8  10 4 MPa, Burgess vector b ＝2.510 -7 mm, the particle diameter d = 20 nm, and the precipitation spacing l = 250 nm.The calculated value   was approximately 99.6 MPa.At IN  ＝ 9.4  10 4  f, the value of f was 10 -3 , and the calculated value IN  was approximately 94.3 MPa.When the contribution of grain refinement was considered, k = 20 N/mm -3/2 was used, and the average grain size was d  12 m.The calculated value 2 kd was then approximately 115.2 MPa.

Table 2 .
Mechanical properties of experimental steel.