JPH0461047B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing parts made of spheroidal graphite cast iron. (Prior art) Generally, as shown in Japanese Patent Application Laid-Open No. 55-94459, after austenitizing a spheroidal graphite cast iron material by heating and holding it at a solid solution formation temperature (830 to 1000°C) for a certain period of time, for example, It is well known that the so-called austempering treatment, in which the material is kept at a constant temperature of 220 to 420 degrees Celsius for a predetermined period of time, greatly improves its toughness, and this toughness is used to produce strong automotive parts such as steering nuts and connecting rods. It is being used. (Problem to be Solved by the Invention) By the way, when the above-described austempering treatment is applied to a thick spheroidal graphite cast iron part, the cooling rate of the central part of the spheroidal graphite cast iron part is extremely slow compared to the surface part. Therefore, there is a problem in that a pearlite structure is precipitated in the center, and the required bainite structure is not generated, making it impossible to obtain the desired toughness. Therefore, after austenitizing the large-walled spheroidal graphite cast iron parts mentioned above, the normal constant temperature treatment temperature (220
A bainite structure can be obtained in the center of a spheroidal graphite cast iron part by rapidly cooling it at a temperature lower than 420°C, for example, in a low-temperature salt furnace at 170°C. However, the surface of spheroidal graphite cast iron parts is cooled to 170℃, which is equivalent to the temperature of a low-temperature salt furnace.
The problem is that the temperature of this surface region drops below the Ms point (the temperature at which martensitic transformation begins, usually 210°C), and a martensitic structure is generated, making it impossible to obtain the desired toughness. For this reason, the present invention changes the composition of the above-mentioned spheroidal graphite cast iron from that of normal ones and substantially lowers the Ms point, so that even in thick spheroidal graphite cast iron parts, The purpose is to generate a uniform bainite structure without forming a martensitic structure on the surface, and to obtain a tough spheroidal graphite cast iron part. (Means for Solving the Problems) In order to achieve this object, the present invention provides a method for manufacturing spheroidal graphite cast iron parts.
, 2.0-4.0 wt% Si, and 0.02-0.1 wt%
Mg, 0.5-2.0 wt% Mn, 0.5-3.0 wt%
Spheroidal graphite cast iron having a composition of at least one of Ni or 0.3 to 2.0 wt. Rapidly cool to a temperature of 210℃,
Thereafter, an austempering treatment is performed by heating and holding at a temperature of 220 to 420°C for 5 minutes or more. (Function) With the above configuration, in the present invention, spheroidal graphite cast iron is heated and held at a temperature of 830 to 1000°C for less than 5 hours to austenitize, and then rapidly cooled to a temperature of 170 to 210°C. However, the center part is not susceptible to being cooled and transformed into a pearlite structure even in a short period of time, so it can be easily austempered at low cost, and the structure can be made into a uniform bainite structure throughout. This makes it possible to efficiently manufacture tough spheroidal graphite cast iron parts. Also, at that time, the casting material contains alloying elements.
Mn and Ni (or Cu) are contained in a predetermined composition ratio, and this combination of Mn and Ni (Cu)
The Ms point is lower than that of normal spheroidal graphite cast iron, reaching a temperature value lower than 170℃. For this reason,
After the above austenitization, the casting material is heated to 170~210℃.
Even when rapidly cooled to a temperature of , there is only a slight risk that the matrix will transform into a martensitic structure, and the mechanical properties (toughness) will not be impaired. (Example) Hereinafter, an example of the present invention will be described based on the drawings. The spheroidal graphite cast iron parts produced according to the embodiments of the present invention contain 3.0 to 4.0% by weight of C and 2.0 to 4.0% by weight of Si.
and 0.02 to 0.1% by weight of Mg, 0.5 to 2.0% by weight of Mn, and at least one of 0.5 to 3.0% by weight of Ni or 0.3 to 2.0% by weight of Cu, and the balance is substantially Fe. The base structure is formed as a bainite structure, and it is used as strong parts such as automotive parts such as steering knuckles and cooking rods. Here, to explain the reason for limiting the blending ratio of each of the alloying elements mentioned above, if the blending ratio of C is less than 3.0% by weight, the cementite structure due to the formation of Fe ₃ C will occur due to the inclusion of Mn as another alloying element. is easily formed and chilled, making processing difficult and causing casting defects such as shrinkage cavities. on the other hand,
If it exceeds 4% by weight, defects such as carbon flotation and dross will occur, so 3.0~
It is set in the range of 4.0% by weight. Furthermore, if the blending ratio of Si is less than 2.0% by weight, it tends to chill as in the case of C above, and the castability deteriorates, while if it exceeds 4% by weight, casting defects such as dross may occur. However, since the austenitizing temperature becomes high and uneconomical, the content is set in the range of 2.0 to 4.0% by weight. Furthermore, Mg is an element necessary to promote the production of spheroidal graphite, and its blending ratio is set in the range of 0.02 to 0.1% by weight, as in the case of other ordinary spheroidal graphite cast iron materials. Regarding the blending ratio of Mn, in normal spheroidal graphite cast iron used as as-cast, it is set to 0.3% by weight or less in order to suppress the tendency to form carbides and increase ductility.
It is said that it is good to set the Mn/S ratio to about 10 to 30. However, in the present invention, Mn is
It is an essential element for lowering the Ms point, and 0.5% by weight or more is required to obtain the desired effect. On the other hand, if it exceeds 2.0% by weight, chill will be generated in the as-cast state, making processing difficult, and after austempering treatment, white and locally hard so-called bright areas will occur, impairing mechanical properties. Therefore, it is set in the range of 0.5 to 2.0% by weight. Furthermore, the addition of Ni usually has the effect of delaying bainite transformation and suppressing the formation of pearlite transformation, but in the present invention, the main effect is to lower the Ms point without intending to delay bainite transformation. That's what I'm looking forward to. And this
If the blending ratio of Ni is less than 0.5% by weight, the above effects will be weak, and if it exceeds 3.0% by weight, a large amount of austenite structure will remain, which will not only weaken the strength at room temperature and impair mechanical properties. , which is undesirable because it causes an increase in product cost. Yotsute 0.5
It is set in the range of ~3.0% by weight. On the other hand, Cu has the effect of lowering the Ms point as in the case of Ni, but its effect is approximately half that of Ni. However, while Ni is expensive, Cu is cheap, and Cu also has the effect of greatly suppressing the above-mentioned chilling tendency caused by Mn. In this case, the blending ratio of Cu is preferably in the range of 0.3 to 2.0 wt%, because if it is less than 0.3 wt%, the above effect will not be obtained, and if it exceeds 2.0 wt%, the effect will be saturated and the spheroidization of graphite will be inhibited. It is. In addition, the above C, Si, Mg, Mn and Ni (or
Cu) is an essential element for the spheroidal graphite cast iron parts according to the present invention, and the effects of the present invention can be effectively obtained if this essential component is present. However, in addition to this, the remaining Fe
There is no problem even if Cr and B are added as supplements, but by appropriately limiting their blending ratio,
It can contribute to the decline of Ms point to some extent.
In that case, the blending ratio of Cr is 0.1% by weight.
If it is less than
If it exceeds 0.5% by weight, a plate-like cementite structure will crystallize during casting, resulting in so-called auto-ironization, making machining difficult. Therefore, it is preferably set in the range of 0.1 to 0.5% by weight. Regarding the blending ratio of B, if it is less than 0.01% by weight, it will not have the effect of lowering the Ms point as in the case of Cr, whereas if it exceeds 0.05% by weight, it will turn into self-pigment, so 0.01 to 0.05% by weight. It is preferable to set it within the range of . An example of a method for manufacturing such a spheroidal graphite cast iron component will be described. First, a spheroidal graphite cast iron material having the above-mentioned composition is cast by a normal casting process, and then processed to a spheroidal graphite cast iron material having a composition as described above.
The base structure is austenitized by heating and maintaining the base structure at a temperature of 50° C. for 5 hours or less. In this case, the heating temperature is limited to a range of 830 to 1000°C because 830°C is the minimum temperature required to turn the base structure into austenite, and if it exceeds 1000°C, the crystal grains will become coarse and the heating This is due to the high cost. In addition, the heating holding time varies depending on the wall thickness of the part, but if it exceeds 5 hours, crystal grains will become coarse and decarburization will occur. It is preferable to set the heating time to a range of 5 minutes to 5 hours, taking into account austenitization. After such austenitization, the casting material is put into a refrigerant such as liquid nitrogen or a low-temperature salt furnace and rapidly cooled to a temperature of 170 to 210°C. At that time, the above casting material contains Mn as an alloying element.
and Ni (or Cu).
Due to the combination of Mn and Ni (Cu), the Ms point is substantially lower than that of ordinary spheroidal graphite cast iron to 170
The temperature value is lower than ℃. Therefore, even if the quenching temperature after austenitization is set at 170 to 210°C, there is only a slight risk that the matrix will transform into a martensitic structure, and the mechanical properties (toughness) will not be impaired. After that, the rapidly cooled cast iron material is heated and held at a constant temperature of 220 to 420°C for 5 minutes or more in an electric furnace or fluidized bed furnace, so-called austempering treatment, and then air cooled, and the heat treatment is completed. Thus, the spheroidal graphite cast iron part of the present invention is obtained. In this case, the heating temperature for the austempering treatment is preferably in the range of 220 to 420°C, since the effect of the austempering treatment will be insufficient if it is less than 220°C, while a pearlite structure will be formed if it exceeds 420°C. Further, the heating holding time is set to 5 minutes or more since a heating holding time of less than 5 minutes is insufficient for the base structure to transform into a bainite structure. Therefore, since the spheroidal graphite cast iron parts obtained in this way are rapidly cooled using a low-temperature refrigerant,
As shown in Fig. 1, even in the case of a thick-walled part, the center part is not subject to cooling and transforming into a pearlite structure even in a short time compared to the conventional case, which is indicated by the broken line in the figure. There is no need to add Mo, which has the effect of shifting the pearlite transformation start time of the isothermal transformation curve to the longer side, as is normally done, and therefore thick-walled parts can be austempered easily and inexpensively. can do. In addition, since the austenitization is rapidly cooled, the structure can be made into a uniform bainite structure throughout, and a tough spheroidal graphite cast iron part can be efficiently produced. Furthermore, since the cooling temperature can be lowered by lowering the Ms point of spheroidal graphite cast iron, inexpensive oil, liquid nitrogen, etc. In addition, after quenching, constant temperature treatment (temperature-raising austempering treatment) can be performed using ordinary inexpensive electric furnaces, etc., thereby reducing equipment costs and running costs for heat treatment. Spheroidal graphite cast iron parts as strong parts can be obtained at low cost. Next, to explain a specific example, 150 kg each of the inventive material and the conventional material (FCD45 according to JIS standard) having the compositions shown in Table 1 below were melted in a high frequency furnace and cast into a Y-shaped block. And this 2
A plurality of tensile test pieces were formed from each type of Y-shaped block.

[Table] Next, these specimens were held in a heating furnace at a temperature of 900℃ for 2 hours while preventing decarburization to austenite, and then placed in a low-temperature salt furnace and heated at 150℃.
Rapidly cooled by holding at four different temperatures in the range ~190°C for 2 minutes (completely cooled in less than 1 minute), then held at a temperature of 350°C for 2 hours in a high-temperature salt oven. After austempering treatment, temperature-raising austempering treatment was performed by air cooling. In addition to such temperature-raising austempering treatment, direct austempering is performed in which the austenitized test piece is directly placed in a high-temperature salt furnace and austempered under the same conditions as the temperature-raising austempering treatment described above. treatment (conventional heat treatment method). These test pieces were subjected to a tensile test, and the test results are shown in Table 2 below. Furthermore, from among the test results, the relationship between elongation and cooling temperature (temperature of the low-temperature salt furnace) is shown in FIG. Further, FIG. 3 shows a microscopic photograph of one of the above tensile test pieces that was rapidly cooled at a temperature of 190°C.

[Table] From the micrograph shown in Fig. 3, the conventional spheroidal graphite cast iron material (Fig. 3b) shows the formation of a tempered martensitic structure, which is presumed to be epsilon carbide, whereas the present invention It can be seen that the spheroidal graphite cast iron material (Fig. 3a) is formed into a uniform acicular bainite structure. In addition, from Table 2 and Figure 2 above, the elongation of the material of the present invention is equivalent to that of the high-cost heat treatment method of direct austempering, but it is higher than that of the conventional material, especially when the cooling temperature is 170 to 210℃. It can be seen that sometimes a good constant value is obtained. Therefore, from these test results,
It can be seen that the inventive example has higher toughness than the conventional example and can be manufactured at a lower cost. (Effects of the Invention) As explained above, according to the method for manufacturing spheroidal graphite cast iron parts of the present invention, Mn is contained as one component of the composition.
After austenitizing spheroidal graphite cast iron with a composition containing at least one of Ni or Cu, it is rapidly cooled to a temperature lower than the austempering temperature, and then heated austempering treatment is performed, so the Ms point is lower than that of the conventional method. It is possible to easily obtain spheroidal graphite cast iron parts with excellent toughness and uniform bainite structure without precipitating pearlite structure even in large parts, making it suitable for use in various types of strong parts such as automobile strong parts. It is possible to manufacture spheroidal graphite cast iron parts that are optimal for use efficiently and at low cost.

[Brief explanation of the drawing]

The drawings show examples of the present invention; FIG. 1 is a graph showing a cooling curve due to heat treatment, FIG. 2 is a graph showing the relationship between cooling temperature and elongation after heat treatment, and FIGS. 3 a and b are graphs showing examples of the present invention. 1 is a micrograph showing the crystal structure of spheroidal graphite cast iron materials of Example and Conventional Example.

Claims (1)

[Claims] 1 C: 3.0 to 4.0 weight%, Si: 2.0 to 4.0 weight%
, Mg: 0.02 to 0.1% by weight, Mn: 0.5 to 2.0% by weight, and at least one of Ni: 0.5 to 3.0% by weight or Cu: 0.3 to 2.0% by weight, and the remainder is substantially Fe. Spheroidal graphite cast iron with a composition of 830~
170-210℃ after heating and holding at 1000℃ for less than 5 hours
1. A method for manufacturing a spheroidal graphite cast iron part having toughness, which comprises rapidly cooling the parts to 220 to 420°C, and then performing an austempering treatment by heating and holding the parts at 220 to 420°C for 5 minutes or more.