Patent Application
20190071756
The present invention relates to a spheroidal graphite cast iron, a cast article and an automobile structure part made thereof, and a method for producing a spheroidal graphite cast iron article.
To improve the mechanical properties, particularly toughness, of spheroidal graphite cast iron articles, it is important to increase crystallized spheroidal graphite, particularly fine spheroidal graphite.
For example, U.S. Pat. No. 5,205,856 discloses a spheroidal graphite cast iron with the average particle size of spheroidal graphite drastically increased and the number of graphite particles increased from 511/mm2 to 1256/mm2 by treatment with a wire inoculant comprising powdery ferrosilicon and powdery magnesium silicide (see
Torjorn Skaland, “A New Method For Chill And Shrinkage Control in Ladle Treated Ductile Iron,” Foundry Trade Journal (UK), 2004, Volume 178 (No. 3620), pp. 396-400 reports the research of spheroidal graphite in disc-shaped spheroidal graphite cast iron products treated with spheroidizing agents containing various rare earth metals, which may be called REMs below. It describes that spheroidization with spheroidizing agents comprising 0.5% of La 5 and 1.0% of La (containing substantially no other RE components such as Ce, etc.), respectively, added to magnesium ferrosilicon containing 45% of FeSi, 6% of Mg, 1% of Ca, and 0.9% of Al, increases the number of graphite particles, improves the sphericity of graphite particles, and provides a non-symmetrical particle size distribution higher in smaller particle sizes. It describes, however, that 5-mm-thick castings obtained by using spheroidizing agents containing 0.5% of La and 1.0% of La, respectively, have hard chill (eutectic cementite), meaning that when structure parts of automobiles having 5-mm-thick portions are formed, they may not have sufficient ductility.
An object of the present invention is to provide a spheroidal graphite cast iron having a higher percentage of fine graphite than in conventional spheroidal graphite cast irons of the above prior art references, etc., thereby having excellent mechanical properties, particularly toughness, a cast article and an automobile structure part of the spheroidal graphite cast iron, and a method for producing a spheroidal graphite cast iron article.
As a result of intensive research in view of the above object, the inventors have found that a spheroidal graphite cast iron containing graphite particles having a particular particle size distribution has excellent mechanical properties, particularly toughness. The present invention has been completed based on such finding.
Thus, the spheroidal graphite cast iron of the present invention meets
N
(5-)≥250,
N
(5-20)
/N
(5-)≥0.6, and
N
(30-)
/N
(5-)≤0.2,
wherein N(5-) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more, N(5-20) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm, and N(30-) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 30 μm or more, among graphite particles observed in an arbitrary cross section of at least 1 mm2.
The spheroidal graphite cast iron of the present invention preferably meets N(2-5)≥100, wherein N(2-5) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 2 μm or more and less than 5 μm.
The spheroidal graphite cast iron of the present invention preferably meets N(5-20)/N(5-)≥0.65.
The spheroidal graphite cast iron of the present invention preferably meets Dmax≥50.4 μm, wherein Dmax represents the maximum equivalent-circle diameter of graphite particles.
The spheroidal graphite cast iron of the present invention preferably meets
−0.15≤[N(5-10)−N(15-20)]/N(5-10)≤0.25,
wherein N(5-10) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 10 μm, and N(15-20) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 15 μm or more and less than 20 μm.
The cast article of the present invention is made of the above spheroidal graphite cast iron.
The cast articles are preferably automobile structure parts.
The method of the present invention for producing a spheroidal graphite cast iron article meeting the following conditions of
N
(5-)≥250,
N
(5-20)
/N
(5-)≥0.6, and
N
(30-)
/N
(5-)≤0.2,
wherein N(5-), N(5-20) and N(30-) represent the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more, the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm, and the number (/mm2) of graphite particles having equivalent-circle diameters of 30 μm or more, respectively, among graphite particles observed in arbitrary cross section of at least 1 mm2; comprising pressing a surface of a melt poured into a gas-permeable casting mold by a gas at a pressure of 1-100 kPa, before the melt starts eutectic solidification, and solidifying the melt while supplying the gas into the casting mold.
The pressure is preferably 10-50 kPa.
The method of the present invention preferably meets
0≤dtpE/dtE≤1,
wherein dtE represents a time period from the start of eutectic solidification of the melt to the completion of the eutectic solidification, and dtpE represents a time period from the start of eutectic solidification of the melt to the completion of the pressing.
Because the spheroidal graphite cast iron of the present invention contains graphite particles with a high percentage of fine graphite and a particular particle size distribution, thereby having excellent mechanical properties, particularly toughness, it is suitable for spheroidal graphite cast iron articles, particularly structure parts for automobiles. The method of the present invention can produce a spheroidal graphite cast iron having excellent mechanical properties, particularly toughness.
[1] Spheroidal Graphite Cast Iron
The spheroidal graphite cast iron of the present invention may have a component composition capable of forming spheroidal graphite cast iron (FCD) of JIS G 5502, austempered spheroidal graphite cast iron of JIS G 5503, spheroidal-graphite, austenitic cast iron of JIS G 5510, etc. For example, it may have a composition comprising by mass 2-4.5% of C, 0.8-6% of Si, and 0.010-0.080% of Mg, the balance being Fe and inevitable impurity elements, which may further contain elements for obtaining desired properties, such as S, P, Mn, Cu, Cr, Ni, Mo, W, etc., in proper amounts.
The spheroidal graphite cast iron of the present invention has spheroidal graphite (graphite particles) having a particle size distribution defined below. Namely, the particle size distribution of graphite particles observed in an arbitrarily cut cross section of at least 1 mm2 meets N(5-)≥250, N(5-20)/N(5-)≥0.6, and N(30-)/N(5-)≤0.2, wherein N(5-) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more, N(5-20) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm, and N(30-) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 30 μm or more.
Thus, the spheroidal graphite cast iron of the present invention has relatively large numbers (250 or more/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more, with a high percentage of relatively fine graphite particles having equivalent-circle diameters of 20 μm or less, and a low percentage of relatively large graphite particles having equivalent-circle diameters of 30 μm or more. With such a structure, the spheroidal graphite cast iron can have excellent mechanical properties, particularly toughness. Particularly in as-cast articles as thick as 40 mm or more, this structure can improve toughness.
The number N(5-) of graphite particles having equivalent-circle diameters of 5 μm or more is preferably 300 or more/mm2. A ratio N(5-20)/N(5-) of the number of graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm to the number of graphite particles having equivalent-circle diameters of 5 μm or more is preferably 0.65 or more, more preferably 0.70 (70%) or more, most preferably 0.75 (75%) or more. A ratio N(30-)/N(5-) of the number of graphite particles having equivalent-circle diameters of 30 μm or more to the number of graphite particles having equivalent-circle diameters of 5 μm or more is preferably 0.15 (15%) or less, more preferably 0.10 (10%) or less.
The spheroidal graphite cast iron of the present invention preferably meets N(2-5)≥100, wherein N(2-5) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 2 μm or more and less than 5 μm. Such a structure having large numbers of extremely fine graphite particles preferably contributes to the improvement of toughness. N(2-5) is more preferably N(2-5)≥150, most preferably N(2-5)≥200.
The spheroidal graphite cast iron of the present invention preferably meets Dmax≥50.4 μm, wherein Dmax represents the maximum equivalent-circle diameter of graphite particles observed in an arbitrarily cut cross section of at least 1 mm2. Even when solidified at such a low cooling speed that the maximum equivalent-circle diameter Dmax of graphite particles becomes 50.4 μm or more, the spheroidal graphite cast iron contains large numbers (250 or more/mm2) of graphite particles, with a high percentage of relatively fine graphite particles having equivalent-circle diameters of 20 μm or less, and a low percentage of large graphite particles having equivalent-circle diameters of 30 μm or more, thereby exhibiting excellent mechanical properties, particularly toughness.
The spheroidal graphite cast iron of the present invention preferably meets−0.15≤[N(5-10)−N(15-20)]/N(5-10)≤0.25, wherein N(5-10) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 10 μm, and N(15-20) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 15 μm or more and less than 20 μm. Namely, another preferred feature of the spheroidal graphite cast iron of the present invention is that in a cut cross section, a ratio of the difference between the number N(5-10) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 10 μm and the number N(15-20) of graphite particles having equivalent-circle diameters of 15 μm or more and less than 20 μm to the number N(5-10) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 10 μm is −0.15 or more and 0.25 or less. This means that when graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm are divided to every 5 μm range, there is a small difference between the number of graphite particles in a small-particle-size range (5 μm or more and less than 10 μm), and the number of graphite particles in a large-particle-size range (15 μm or more and less than 20 μm). Particularly in as-cast articles as thick as 50 mm or more, this structure can improve toughness.
The spheroidal graphite cast iron of the present invention preferably meets d60≤20 μm, wherein d60 (μm) represents an equivalent-circle diameter (60-% particle size) of graphite particles, at which a cumulative number of graphite particles determined by cumulating the number of graphite particles of 5 μm or more toward larger diameters is 60% of the number of graphite particles having equivalent-circle diameters of 5 μm or more. Further, it preferably meets d70≤20 μm, d80≤30 μm, and d90≤35 μm, wherein d70 (μm), d80 (μm), and d90 (μm) represent the equivalent-circle diameters of graphite particles, at which the cumulative numbers of graphite particles are 70%, 80%, and 90%, respectively, of the number of graphite particles having equivalent-circle diameters of 5 μm or more. The conditions expressed by d60≤20 μm, and d80≤30 μm are substantially the same as expressed by N(5-20)/N(5-)≥0.6, and N(30-)/N(5-)≤0.2.
[2] Production Method
The spheroidal graphite cast iron of the present invention can be produced by the following method. Each step in an example of the production methods of the present invention will be explained below.
Melt for Spheroidal Graphite Cast Iron
A melt for the spheroidal graphite cast iron, which is hereinafter referred to simply as “melt,” can be prepared by a known method, for example, by mixing and melting steel scraps, return scraps and various auxiliary materials as raw materials to prepare a molten iron alloy having a desired composition, which is hereinafter called “base melt,” and adding a predetermined amount of a spheroidizing agent containing Mg, etc., for example, an Fe—Si—Mg alloy, to the base melt. The usable spheroidizing agent may contain proper amounts of REM, and if necessary, other minor elements. Spheroidization can be conducted by a widely used sandwiching method, a method of supplying a cored wire containing a spheroidizing agent into a base melt in a ladle, etc.
Inoculation
Inoculation is preferably conducted when pouring the melt into a casting mold, because it increases the number of graphite particles. A usual Fe—Si alloy may be used as an inoculant. The inoculation may be conducted by known methods, such as (a) inoculation in a melt-pouring ladle simultaneously with spheroidization by a sandwiching method, which may be hereinafter called “primary inoculation,” (b) inoculation by adding an inoculant to a pouring flow of a melt, (c) inoculation with an inoculant added in a cavity of a casting mold in advance, etc. The inoculations (b) and (c) may be called “secondary inoculation,” which are conducted after the primary inoculation.
Production Method of Cast Article
Though a cast article of the spheroidal graphite cast iron of the present invention may be produced by known methods such as a gravity casting, etc., it is preferable to use a method of pressing a surface of a melt poured into a gas-permeable casting mold, which may be simply called as “casting mold,” by a gas before the eutectic solidification of the melt starts, and solidifying the melt while supplying the gas into the casting mold, which may be hereinafter called “gas-pressing method.” The gas-pressing method can easily produce spheroidal graphite cast iron having a high percentage of fine graphite particles with the percentage of coarse graphite particles suppressed. The details of the gas-pressing method, one of preferred production methods of the present invention, will be explained below.
The casting mold may be a green sand mold, a shell mold, a self-hardening mold, or other widely used gas-permeable casting molds constituted by sand particles. A casting mold constituted by ceramic particles, metal particles, etc. may also be used, as long as it has necessary gas permeability. Further, even a metal mold with no gas permeability, such as a metal die, can be used as a gas-permeable casting mold, when it is provided with gas-passing holes such as vents, etc. for gas permeability. A casting mold of gypsum, etc. with substantially no gas permeability can also be used as a gas-permeable casting mold, as long as it has sufficient gas permeability by containing a gas-permeable material, or by forming part of the mold by a gas-permeable material.
A gas supplied may be air for cost, but a non-oxidizing gas such as argon, nitrogen, carbon dioxide may be used to prevent the oxidation of the melt. The melt can be pressed by a gas supplied into the casting mold through a gate.
With the melt pressed by a gas, Mg oversaturatingly dissolved in the melt for spheroidization can be suppressed from being released from the melt, increasing Mg compounds such as MgS, MgO, etc., which form nuclei for crystallizing graphite. This method advantageously makes it easier to increase the percentage of fine graphite while decreasing the percentage of coarse graphite, than when the melt is not pressed by a gas. The pressing pressure of a gas is preferably 1-100 kPa. With the pressing pressure of less than 1 kPa, the number of graphite particles is not easily increased. On the other hand, the pressing pressure of more than 100 kPa breaks the casting mold to scatter the melt, undesirable for operation safety. The pressing pressure is more preferably 10-50 kPa, further preferably 20-40 kPa.
The relation between a time period from the start to end of pressing, which may be called “pressing time,” and a eutectic solidification time period inside a cast article to be formed will be explained referring to the figures.
The pressing start time may be in a time period in which a surface temperature of the melt poured into the casting mold, which comes into contact with the pressing gas, is equal to or higher than the eutectic solidification temperature TE. Because a surface temperature of the melt is generally equal to or lower than the melt temperature inside a cast article being formed, and because a melt has higher fluidity at a higher temperature than the eutectic solidification temperature, pressing to the melt surface is started preferably as early as after the completion of pouring. Namely, in
The pressing termination time may be after the eutectic solidification of a cast article being formed starts, and pressing need not be continued until the eutectic solidification of the entire cast article being formed is completed. Namely, in
The eutectic solidification temperature, the eutectic solidification start time, and the eutectic solidification termination time may be measured by a thermocouple placed at a predetermined position in a casting mold in a casting experiment, etc., or determined by solidification analysis by a computer. Because the same articles are mass-produced under substantially the same casting conditions, these parameters of eutectic solidification need not be measured for every article.
Because a blown gas passes through the gas-permeable casting mold and goes outside the casting mold in the course of pressing, the cooling of the casting mold is accelerated, thereby accelerating the solidification of not only a melt surface in direct contact with the blown gas but also a melt portion in contact with the casting mold, so that a solidified shell tends to be rapidly formed in the melt from surface toward inside. In the subsequent solidification of an inner portion of the melt, expanding pressure by the crystallization of spheroidal graphite is directed not outward but inward by an already solidified shell, offsetting the shrinkage of a cooling melt, thereby suppressing the generation of shrinkage cavities. This effect makes it easy to obtain cast articles having high mechanical properties, particularly impact strength.
An arbitrary pressure pattern may be used during pressing, but the gas is preferably supplied to monotonically increase the pressing pressure from the start, thereby suppressing the evaporation of Mg from the melt and cooling the casting mold.
The method of the present invention can produce spheroidal graphite cast iron, in which graphite particles observed in an arbitrary cross section of at least 1 mm2 meet N(5-)≥250, N(5-20)/N(5-)≥0.6, and N(30-)/N(5-)≤0.2, wherein N(5-) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more, N(5-20) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm, and N(30-) represents the number (/mm2) of graphite particles having equivalent-circle diameters of 30 μm or more.
The method of the present invention can also produce spheroidal graphite cast iron meeting 50.4 μm≥Dmax≥15.9 μm, and d60≤10.0 μm, wherein Dmax represents the maximum equivalent-circle diameter of graphite particles, and d60 represents the equivalent-circle diameters (μm) of graphite particles, at which the cumulative number of graphite particles obtained by cumulating the number of graphite particles in the ascending order of equivalent-circle diameters is 60% of the number of graphite particles having equivalent-circle diameters of 5 μm or more.
The present invention will be explained in more detail referring to Examples below, without intention of restriction.
Production by a gravity-casting method and a gas-pressing method according to a preferred embodiment of the present invention will be explained referring to the figures and tables, without intention of restriction.
Melt
Return scraps of spheroidal graphite cast iron, steel scraps, graphite powder, ferrosilicon, ferromanganese, ferrophosphorus, pure copper, and iron sulfide were charged in a predetermined formulation as raw materials into a high-frequency induction furnace, and melted to obtain 100 kg of a base melt. After a melt-pouring ladle having a pocket at the bottom is pre-heated, 1.05% by mass, based on the base melt, of a spheroidizing agent [Fe—Si—Mg alloy containing REM (TDCR-5 available from Toyo Denka Kogyo Co. Ltd.)] was put in the pocket of the melt-pouring ladle, 0.1% by mass, based on the base melt, of a primary inoculant [Fe—Si alloy (Caslon 75H available from Toyo Denka Kogyo Co. Ltd.)] was then put thereon, and 1300 g of punched steel scraps were further put thereon as a covering material. The base melt at 1510° C. was poured from the high-frequency induction furnace into the melt-pouring ladle, to conduct spheroidization by a sandwiching method and primary inoculation simultaneously. 0.20% by mass, as a Si equivalent, of a secondary inoculant [powdery Fe—Si alloy (Stream available from Toyo Denka Kogyo Co. Ltd.)] was then added to the melt in a casting ladle for secondary inoculation. The composition of the melt of Example 1 is shown in Table 1.
(1)Including impurity elements.
Casting Mold
Casting
The casting method used was a combination of a gravity-casting method in an air atmosphere at room temperature and normal pressure outside the casting mold 1, and a gas-pressing method. The above melt M in a volume of filling the product-forming space 6 and the riser 5 was gravity-cast from the above melt-pouring ladle 7 into the cavity 2 at 1365° C. as shown in
Microstructure
A cross section of the as-cast spheroidal graphite cast iron article 100 of Example 1 was etched to observe its microstructure by an optical microscope. An observation portion was indicated by A in
Measurement of Equivalent-Circle Diameter and Number of Particles
The quantitative measurement of spheroidal graphite particles in the spheroidal graphite cast iron was conducted by observing a cross section structure of the spheroidal graphite cast iron by an optical microscope. An arbitrary cross section obtained by cutting near the portion A in
The resultant photograph data were image-treated to determine the number and equivalent-circle diameters of spheroidal graphite. The number (/mm2) of graphite particles (or simply particles) per 1 mm2 was calculated from the results, to obtain a distribution of frequency in equivalent-circle diameter ranges as shown in Table 2. The equivalent-circle diameter ranges were less than 2 μm, 2 μm or more and less than 5 μm, 5 μm or more and less than 10 μm, . . . , 45 μm or more and less than 50 μm (with 5-μm intervals between 5 μm and 50 μm), and 50 μm or more. “A-Image-Kun” available from Asahikasei Engineering Corporation was used as an image analyzer in Example 1 and subsequent Examples and Comparative Examples.
With respect to spheroidal graphite in the spheroidal graphite cast iron of Example 1, the number N of graphite particles, the frequency F of 5 μm or more, the cumulative frequency Cfa of 5 μm or more, and the reverse cumulative frequency Cfb in each equivalent-circle diameter range are shown in Table 2.
(1)D represents an equivalent-circle diameter [x (μm)-y (μm)].
(2)N represents the number of particles.
(3)F represents the frequency of 5 μm or more.
(4)Cfa represents the cumulative frequency of 5 μm or more.
(5)Cfb represents a reverse cumulative frequency.
With respect to spheroidal graphite contained in the spheroidal graphite cast iron of Example 1, the number N(5-) (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more, the number N(5-20) (/mm2) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm, and the number N(30-) (/mm2) of graphite particles having equivalent-circle diameters of 30 μm or more were determined from the particle size distribution shown in Table 2, to calculate a ratio N(5-20)/N(5-) of the number N(5-20) of particles having equivalent-circle diameters of 5 μm or more and less than 20 μm to the number N(5-) of particles having equivalent-circle diameters of 5 μm or more; a ratio N(30-)/N(5-) of the number N(30-) of particles having equivalent-circle diameters of 30 μm or more to the number N(5-) of particles having equivalent-circle diameters of 5 μm or more; and a ratio [N(5-10)−N(15-20)]/N(5-10) of the difference between the number of graphite particles having equivalent-circle diameters of 5 μm or more and less than 10 μm and the number of graphite particles having equivalent-circle diameters of 15 μm or more and less than 20 μm to the number of graphite particles having equivalent-circle diameters of 5 μm or more and less than 10 μm. The value of N(5-20)/N(5-) corresponds to a cumulative frequency Cfa(5-20) of equivalent-circle diameters of m or more and less than 20 μm, and the value of N(30-)/N(5-) corresponds to a reverse cumulative frequency Cfb(30-) of equivalent-circle diameters 30 μm or more. The results are shown in Table 4.
With respect to spheroidal graphite contained in the spheroidal graphite cast iron of Example 1, the cumulative number (/mm2) of graphite particles, which may be called simply cumulative number of particles or Nc, was determined by cumulating the number (/mm2) of graphite particles in the ascending order from the equivalent-circle diameter of 5 μm to a particular equivalent-circle diameter (μm), to obtain a curve indicating the relation between the equivalent-circle diameter (μm) and the cumulative number (/mm2) of graphite particles. Further, a cumulative frequency Cfa at each equivalent-circle diameter was determined with the maximum cumulative number of graphite particles [the number N(5-) of graphite particles having equivalent-circle diameters of 5 μm or more] as 100%, to obtain the relation between the equivalent-circle diameters (μm) and the cumulative frequency (%). The equivalent-circle diameter at a cumulative frequency of n % is expressed by dn, which may be called n-% particle diameter. For example, a 60-% particle size (d60) is the equivalent-circle diameter of graphite particles, at which the cumulative number of graphite particles is 60% of the number of graphite particles having equivalent-circle diameters of 5 μm or more.
Among observed graphite particles having equivalent-circle diameters of 5 μm or more, an equivalent-circle diameter of the minimum graphite particle is expressed by “d0” (also in subsequent Examples and Comparative Examples), and an equivalent-circle diameter of the maximum graphite particle, Dmax, is expressed by “d100.”
The results are shown in Table 3 and
(1)D represents equivalent-circle diameter [x (μm)-y (μm)].
(2)Nc represents the cumulative number of particles.
(3)Cfa represents the cumulative frequency of 5 μm or more.
The broken line in
In Example 1, Dmax (=d100) was 73.2 μm, larger than the equivalent-circle diameter D of 50.4 μm at Cfa=100 in the formula (1). This indicates that graphite particles of d100 were not those grown by the diffusion of graphite in a physical state shown by the formula (1), but those grown in a faster graphite diffusion, for example, in a slow solidification state.
Tensile Test
Test pieces of No. 14A (JIS Z 2241) were cut out of the region B shown in
Charpy Impact Test
Smooth test pieces with no notch (length: 55 mm, height: 10 mm, and width: 10 mm) for a Charpy impact test were cut out of the region B in
Example 2 used only the gravity-casting method without the gas-pressing method, unlike Example 1. Example 2 used the same melt component composition as in Example 1 as shown in Table 1, under the same production conditions as in Example 1 except that the gas-pressing method was not used. The observation method of a microstructure, the methods of measuring the number and sizes of graphite particles, the tensile test method, and the Charpy impact test method were also the same as in Example 1.
With respect to spheroidal graphite in the spheroidal graphite cast iron of Example 2, the measurement results of the number N of particles, the frequency F of 5 μm or more, the cumulative frequency Cfa of 5 μm or more, and the reverse cumulative frequency Cfb are shown in Table 5.
(1)D represents an equivalent-circle diameter [x (μm)-y (μm)].
(2)N represents the number of particles.
(3)F represents the frequency of 5 μm or more.
(4)Cfa represents the cumulative frequency of 5 μm or more.
(5)Cfb represents a reverse cumulative frequency.
With respect to spheroidal graphite contained in the spheroidal graphite cast iron of Example 2, N(5-20)/N(5-), N(30-)/N(5-) and [N(5-10)−N(15-20)]/N(5-10) were determined from the particle size distribution shown in Table 5, in the same manner as in Example 1. The results are shown in Table 7.
As in Example 1, the relation between the equivalent-circle diameter D and the cumulative number Nc of particles, and the relation between the equivalent-circle diameter D and the cumulative frequency Cfa were determined in Example 2. The results are shown in Table 6 and
The comparison of the broken line expressed by the formula (1) with the relation between the equivalent-circle diameter D and Cfa in Example 2 in
Tensile Test, and Charpy Impact Test
Table 8 shows the results of the tensile test (tensile strength, 0.2-% yield strength, and rupture elongation) and the Charpy impact test of the as-cast products of Example 2.
Comparison of Shrinkage Cavities
The comparison of shrinkage cavities observed in the microstructures of Examples 1 and 2 revealed that substantially no shrinkage cavities were observed in Example 1, while a small number of shrinkage cavities (microporosity) were observed in Example 2.
Eutectic Solidification Time Period
Relation between eutectic solidification time period and gas-pressuring termination time
As shown in
Another example produced by using a gas-pressing method in addition to a gravity-casting method according to a preferred embodiment of the present invention will be explained referring to the figures and tables.
Melt
Raw materials were melted in a low-frequency induction furnace in the same manner as in Example 1, to obtain a base melt of 12000 kg. 1.1% by mass of a spheroidizing agent, 0.2% by mass of a primary inoculant, and 11 kg of punched steel scraps, based on the base melt, were then charged in this order into a pocket at the bottom of a melt-pouring ladle, in the same manner as in Example 1, and 1800 kg of the resultant base melt was poured at 1520° C. into the melt-pouring ladle, to conduct spheroidization and primary inoculation by a sandwiching method. The spheroidizing agent and primary inoculant were the same as used in Example 1. When poured into a sprue of a casting mold, 0.1% by mass, as Si equivalent, of a secondary inoculant [powdery Fe—Si alloy inoculant (Stream available from Toyo Denka Kogyo Co. Ltd.)] was added to a target weight of the melt for secondary inoculation. The component compositions of the melts in Example 3 are shown in Table 9.
(1)Plus impurity elements.
Casting Mold
A gas-permeable, green sand mold was used as a casting mold having a cavity for forming an automobile structure part (support beam) shown in
Casting
Casting was conducted in the same manner as in Example 1, using a gravity-casting method and a gas-pressing method. The gravity casting was conducted at 1400° C., and the pressure for pressing a melt surface in the cavity was 35 kPa. With the pressing-starting time tp0=10 s, and the pressing termination time tpf=190 s, the pressing time period dtp was 180 s. With the eutectic solidification start time tEs=35 s, and the eutectic solidification termination time tEf=350 s, the eutectic solidification time period dtE was 315 s, and the pressing time period dtpE after the start of eutectic solidification (=tpf−tEs) was 155 s. Accordingly, a ratio of the pressing time period dtpE after the start of eutectic solidification to the eutectic solidification time period dtE, dtpE/dtE, was 0.492, namely 1/2.0, in Example 2.
Microstructure
The microstructure of the cast article (spheroidal graphite cast iron) of Example 3 was observed in the same manner as in Example 1, to evaluate the particle size distribution of spheroidal graphite in the same manner as in Example 1. The observation was conducted near a thickness-direction center in a portion E (thickness: 30 mm) in
(1)D represents an equivalent-circle diameter [x (μm)-y (μm)].
(2)N represents the number of particles.
(3)F represents the frequency of 5 μm or more.
(4)Cfa represents the cumulative frequency of 5 μm or more.
(5)Cfb represents a reverse cumulative frequency.
With respect to spheroidal graphite contained in the spheroidal graphite cast iron of Example 3, N(5-20)/N(5-), N(30-)/N(5-), and [N(5-10)−N(15-20)]N(5-10) were determined in the same manner as in Example 1 from the particle size distribution shown in Table 10. The results are shown in Table 12.
Further, the relation between the equivalent-circle diameter D and the cumulative number Nc of particles, and the relation between the equivalent-circle diameter D and the cumulative frequency Cfa in Example 3 were determined in the same manner as in Example 1. The results are shown in Table 11 and
Tensile Test and Charpy Impact Test
Table 13 shows the results of the tensile test (tensile strength, 0.2-% yield strength, and rupture elongation) and the Charpy impact test of the as-cast product of Example 3.
The results of Comparative Example 1 using only gravity casting without a gas-pressing method are compared with those of Example 3. Comparative Example 1 used the same production conditions as in Example 3 except for using no gas-pressing method, using the same melt component composition as in Example 3 as shown in Table 9. The methods of measuring the number and sizes of graphite particles, and the tensile test and Charpy impact test methods are the same as in Example 3.
Microstructure
The microstructure of the cast article (spheroidal graphite cast iron) of Comparative Example 1 was observed in the same manner as in Example 3, and the particle size distribution of spheroidal graphite was evaluated in the same manner as in Example 3. The observation point was the same as in Example 3. Its photomicrograph is shown in
(1)D represents an equivalent-circle diameter [x (μm)-y (μm)].
(2)N represents the number of particles.
(3)F represents the frequency of 5 μm or more.
(4)Cfa represents the cumulative frequency of 5 μm or more.
(5)Cfb represents a reverse cumulative frequency.
With respect to spheroidal graphite contained in the spheroidal graphite cast iron of Comparative Example 1, N(5-20)/N(5-), N(30-)/N(5-) and [N(5-10)−N(15-20)]/N(5-10) were determined from the particle size distribution shown in Table 14, in the same manner as in Example 3. The results are shown in Table 16.
Further, the relation between the equivalent-circle diameter D and the cumulative number Nc of particles, and the relation between the equivalent-circle diameter D and the cumulative frequency Cfa in Comparative Example 1 were determined in the same manner as in Example 3. The results are shown in Table 15 and
The comparison of the broken line shown by the formula (1) with the relation between the equivalent-circle diameter D and Cfa in Comparative Example 1 in
Tensile Test and Charpy Impact Test
Table 13 shows the results of a tensile test (tensile strength, 0.2-% yield strength and rupture elongation) and a Charpy impact test of the as-cast product of Comparative Example 1.
Another example using a gravity-casting method and a gas-pressing method according to a preferred embodiment of the present invention will be explained referring to the tables and figures.
Example 4 used a casting mold having a cavity for an automobile structure part (steering knuckle) shown in
(1)Including impurity elements.
With respect to spheroidal graphite in the cast article (spheroidal graphite cast iron) of Example 4, the number N of particles, the measurement results of the frequency F of 5 μm or more, the cumulative frequency Cfa of 5 μm or more, and the reverse cumulative frequency Cfb are shown in Table 18.
(1)D represents an equivalent-circle diameter [x (μm)-y (μm)].
(2)N represents the number of particles.
(3)F represents the frequency of 5 μm or more.
(4)Cfa represents the cumulative frequency of 5 μm or more.
(5)Cfb represents a reverse cumulative frequency.
With respect to spheroidal graphite contained in the spheroidal graphite cast iron of Example 4, N(5-20)/N(5-), N(30-)/N(5-) and [N(5-10)−N(15-20)]/N(5-10) were determined from the particle size distribution shown in Table 18, in the same manner as in Example 1. The results are shown in Table 20.
Further, the relation between the equivalent-circle diameter D and the cumulative number Nc of particles, and the relation between the equivalent-circle diameter D and the cumulative frequency Cfa in Example 4 were determined in the same manner as in Example 1. The results are shown in Table 19 and
The relation between the equivalent-circle diameter D and Cfa in
The spheroidal graphite cast iron of the present invention can be used for various structure parts, and is particularly suitable for structure parts for automobiles because of excellent toughness. For example, it can be used for steering knuckles, crankshafts, support beams, connecting rods, brake bodies, brake brackets, shackles, spring brackets, turbine housings, carriers, differential cases, engine mount brackets, etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-059963 | Mar 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/012066 | 3/24/2017 | WO | 00 |
