With the improvement of performance requirements of automobile parts forparts, its material brand and performance exceed the national standards of our country. The impact absorption energy of Ferritic Ductile Iron with ferrite matrix of QT450-10 in current national standards is less than 3J at -40 C, which makes it difficult to meet the requirements of low-temperature impact performance of automobiles traveling in high-cold areas.For example, the tensile strength of steering castings for chassis security of automobiles reaches 450MPa and 5J respectively, which have not been clearly identified in national standards.Therefore, the ductile iron material with low temperature, high toughness and high strength has great development value. It is the focus of many enterprises and scientific research institutions and will also be one of the future development trends of low temperature ductile iron.
Chemical Composition Design
(1) Carbon content is a graphitized element, which can improve the degree of graphitization of ductile iron.When carbon content is high, the amount of precipitated graphite is large, the spherical diameter is small and the roundness is increased, which is conducive to reducing the low temperature TOUGHNESS-BRITTLE transition temperature of the sphere and improving the low temperature impact toughness, but will cause graphite floating and ball explosion.Therefore, the carbon content (mass fraction, the same below) should be controlled from 3.6% to 3.8%.
(2) Silicon content strongly promotes graphitization and is conducive to the increase of ferrite volume fraction. At the same time, solution-strengthening ferrite matrix improves the tensile strength, yield strength and hardness of the casting. However, silicon has a significant influence on the low-temperature impact toughness of ductile iron. When its content exceeds 2.5%, the ductile-brittle transition temperature increases by 0.1%, and the ductile-brittle transition temperature increases by 10-15 C. The ductile iron with too much silicon content increases.Iron, even though all ferrite matrix structures can be obtained, is difficult to use under low temperature impact load. Therefore, the silicon content of low temperature impact ductile iron should be 1.9%~2.3%.
(3) Manganese content Mn is an element to promote whitening. When ductile iron solidifies, it increases the tendency to form white and carbide, strongly promotes the formation of pearlite and cementite, which seriously impairs the impact toughness of low temperature ductile iron.Mn is easy to segregate between grains, produce intergranular compound and reduce impact value.For high toughness low temperature ductile iron, the impact toughness of low temperature ductile iron will be seriously impaired if the Mn content is increased by 0.1% and the ductile-brittle transition temperature is increased by 10-12 C. Therefore, the manganese content should be controlled below 0.3%.
(4) Nickel content Ni can decrease austenite transition temperature and ductile-brittle transition temperature of ductile iron, which has the effect of stabilizing and refining pearlite structure, but has the least tendency to strengthen the matrix and promote pearlite.Adding nickel to low temperature and high toughness ductile iron can strengthen ferrite matrix and improve strength and hardness.After Ni addition, annealing treatment is generally required to decompose the pearlite. At the same time, annealing can reduce the brittle transition temperature of ductile iron and increase the energy absorbed by impact at low temperature.Therefore, the amount of Ni added is 0.3%~0.6%.
Based on the above factors, the chemical composition and metallographic structure requirements of low temperature high toughness QT450-10 were designed.
(1) In order to explore the influence of Si and Ni content on the structure and properties of QT450-10, an orthogonal test was designed.
(2) Test process parameters metal smelting: Medium frequency induction furnace neutral lining smelting; raw material proportion: high purity pig iron 45%-60%, furnace charge 25%-40%, the rest is scrap steel; spheroidizing agent and incubator: spheroidizing agent uses rare-earth magnesium ferrosilicon FeSiMg8RE5, incubator uses common incubator (75SiFe 4-10) or long-acting incubator (SiBaCa FYJ-3-10) and common incubator (75SiFe SiFe 3-10).4-10) Combined use; Temperature of molten iron: outlet temperature 1490-1520 C, pouring temperature 1370-1420 C.
According to GB/T 1348-2009 Standard of Ductile Iron Castings, universal tensile tester, Y-type single cast block are used to process the sample shape according to the national standard, V-type block is sampled in bulk and processed according to the national standard, impact energy absorption at low temperature (-40 C) is tested by impact tester, and metallographic examination is carried out according to the metallographic examination standard of GB/T 9441-209 ductile iron.
(3) Heat treatment process When as-cast matrix structure contains more than or equal to 3% cementite and more than or equal to 1% phosphorus eutectic, high-temperature graphitization two-stage annealing is generally used (see Figure 1); when as-cast matrix structure is composed of graphite, ferrite and pearlite, low-temperature graphitization annealing is generally used (see Figure 2).The purpose of heat treatment is to keep the pearlite content at about 10%, eliminate carbides, phosphorus eutectics, etc., and to achieve the mechanical properties of the material with tensile strength (> 450MPa) and impact energy (> 5J) at -40 C.