1. Structural analysis and technical requirements of automobile steering knuckle
The automobile steering knuckle has complex curved surface, large cross-section variation, large filling height during tail forming, and it is not easy to fill, and there are many bends, which makes forming difficult. At the same time, the internal quality requirements of the product are high, and shrinkage porosity is not allowed. The common structure of the product is the claw structure and claw structure with irregular space, as shown in Figure. Due to the uneven wall thickness, there will be multiple hot spots, which can be divided into main hot spots, secondary hot spots and isolated small hot spots. In actual production, if the treatment is improper, most of the final shrinkage porosity will be distributed on the main heat joint, so the production of nodular cast iron is difficult.
The material grade of nodular cast iron parts of automobile steering knuckle is QT450-10, which should meet the standard of GB/T 1348-2009. Its main technical requirements are that the weight of a single piece is 15~20 kg, the chemical composition (mass fraction, the same below) is 3.6%~3.7% C, 1.7%~1.8% Si, Mn ≤ 0.3%, P ≤ 0.05%, S ≤ 0.02%, 0.03%~0.045% Mg residue, 0.02%~0.03% RE residue; The mechanical property is required to be tensile strength σ b> 450 MPa, elongation δ> 10%, Brinell hardness 160~210 HB; The metallographic structure requires that the matrix structure is ferrite+pearlite, in which ferrite>85%, spheroidization grade 1~3, graphite ball size 5~8, and nodular cast iron are not allowed to have cracks, cold shuts, shrinkage and porosity.
2. Production process
30% grade I pig iron, 40% scrap steel, 30% recycled material, graphitized carburetor, 0.2% silicon carbide pre-treatment agent are used, and the furnace temperature is controlled at 1550 ℃ without heat preservation. The spheroidizing process adopts the wire feeding process, and the wire feeding time is 60 s. Each furnace is 1 t, the casting ladle is three 350 kg ladle, the amount of inoculant added in the inverted ladle is 0.2%, and then 0.1% ordinary 75 ferrosilicon inoculant is used for stream inoculation. The casting temperature is controlled at 1 380~1 420 ℃, the undercooling is controlled within 3 ℃, and the total casting time is 10 minutes. High-quality raw materials, reasonable outlet temperature, good inoculation, and silicon carbide supplementary core are the guarantee of controlling the undercooling, and also the core risk point of the production technology of the no-feeder and no-shrinkage steering knuckle. Its principle is to greatly increase the nucleation core of graphite, thus enhancing the feeding capacity of graphite self-expansion.
3. Test method
The chemical composition was analyzed by Dongyi DF200 full-spectrum direct-reading spectrometer, and the thermal analysis curve was determined by Tianjin Thabrons NSP1303 thermal analyzer. The metallographic structure of the alloy was photographed by MA series inverted metallographic microscope of Shenzhen Maside Equipment Co., Ltd., and then the tensile property was tested by Shandong Lingchuang microcomputer controlled hydraulic universal testing machine (WES-300B), and the hardness was tested by using the digital Brinell hardness tester (HBRVD-187.5) produced by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences.