KR102219893B1 - Austemperedductile iron hooks and their manufacturing methods - Google Patents

Abstract

An embodiment of the present invention provides a method for manufacturing austempered nodular cast iron hook. The Autempered nodular cast iron hook manufacturing method includes carbon (C) 3.3 wt% to 3.7 wt%, silicon (Si) 2.5 wt% to 2.9 wt%, and magnesium (Mg) 0.02 wt% to 0.05 wt%, and the balance Charging the molten metal composed of iron (Fe) and unavoidable impurities into the shell lamination mold, austenizing the molten metal charged to the shell lamination mold, and performing austempering of the austenitized molten metal in a salt bath It may include the step of. Further, when manufacturing an austempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having high tensile strength through optimal composition and austempering.

Description

Autempered ductile iron hooks and their manufacturing methods {Austemperedductile iron hooks and their manufacturing methods}

The present invention relates to a method of manufacturing a nodular cast iron hook, and more particularly, to a method of manufacturing a nodular cast iron hook with improved mechanical properties through aus tempering.

Automobile towing hooks are required to have high tensile strength due to their application characteristics. Nodular graphite cast iron is an iron-based alloy with a high carbon content, and is a cast iron that improves strength, hardness, ductility and toughness by spheroidizing molten metal and crystallizing spheroidal graphite during solidification. In the kitchen state, globular graphite is mainly dispersed in a base where ferrite and pearlite are mixed. As the volume fraction of pearlite increases, the strength and hardness increase, and the ductility and toughness decrease. As described above, the mechanical properties of ductile cast iron vary according to the matrix structure, and the mechanical properties in the kitchen state vary depending on the chemical composition. Therefore, there is a need to develop a hook for traction for a vehicle using nodular cast iron.

Korean Patent Registration No. 10-1020174

The technical problem to be achieved by the present invention is to provide an austempered nodular cast iron hook having excellent tensile strength.

The technical problem to be achieved by the present invention is not limited to the technical problems mentioned above, and other technical problems that are not mentioned can be clearly understood by those of ordinary skill in the technical field to which the present invention belongs from the following description. There will be.

In order to achieve the above technical problem, an embodiment of the present invention provides a method of manufacturing an Austempered nodular cast iron hook.

In an embodiment of the present invention, the method of manufacturing the Autempered nodular cast iron hook is from 3.3 wt% to 3.7 wt% of carbon (C), 2.5 wt% to 2.9 wt% of silicon (Si), and 0.02 wt% of magnesium (Mg) Charging a molten metal containing 0.05 wt% and the balance consisting of iron (Fe) and inevitable impurities into a shell lamination mold, austenizing the molten metal charged to the shell lamination mold, and the austenitizing molten metal It may include the step of performing austempering in a salt bath.

At this time, the austenitizing step is characterized in that it is carried out at a temperature of 800 °C to 1000 °C.

In this case, the austenitizing step is performed for 1 to 3 hours.

At this time, the step of performing the aus tempering is characterized in that it is performed at a temperature of 250 °C to 450 °C.

In this case, the step of performing the aus tempering is characterized in that it is performed for 30 to 90 minutes.

At this time, it is characterized in that the graphite spheroidization rate of the Autempered nodular cast iron hook is 90% or more.

In order to achieve the above technical problem, an embodiment of the present invention provides an Ostempered nodular cast iron hook.

In an embodiment of the present invention, the austempered nodular cast iron hook is characterized in that it is manufactured by the method of manufacturing the austempered nodular cast iron hook.

At this time, the tensile strength of the austempered nodular cast iron hook is characterized in that ^{800N / mm 2 or more.}

At this time, the yield strength of the austempered nodular cast iron hook is characterized in that ^{750N/mm 2 or more.}

At this time, the elongation of the Oustempered nodular cast iron hook is characterized in that 9% or more.

According to an embodiment of the present invention, it is possible to provide a hook having excellent mechanical properties through shell lamination casting.

According to an embodiment of the present invention, it is possible to provide a hook having a high productivity and recovery rate through the shell lamination casting.

According to an embodiment of the present invention, it is possible to provide a hook having high strength and low weight characteristics through aus tempering.

According to an embodiment of the present invention, it is possible to provide a hook having high tensile strength through optimum composition and austempering.

The effects of the present invention are not limited to the above effects, and should be understood to include all effects that can be inferred from the configuration of the invention described in the detailed description or claims of the present invention.

1 is a flow chart showing a method of manufacturing an Ostempered nodular cast iron hook according to an embodiment of the present invention.
2 is a cross-sectional view showing the structure of a KS B 08014 standard specimen and a sub-size specimen.
3 is a microstructure SEM photograph of a shell lamination test cast product.
Figure 4 is a photograph of a prototype Oustempered nodular cast iron hook according to an embodiment of the present invention.

Hereinafter, the present invention will be described with reference to the accompanying drawings. However, the present invention may be implemented in various different forms, and therefore is not limited to the embodiments described herein. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and similar reference numerals are assigned to similar parts throughout the specification.

Throughout the specification, when a part is said to be "connected (connected, contacted, bonded)" with another part, it is not only "directly connected", but also "indirectly connected" with another member in the middle. "Including the case. In addition, when a part "includes" a certain component, it means that other components may be further provided, rather than excluding other components unless specifically stated to the contrary.

The terms used in the present specification are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as "comprise" or "have" are intended to designate the presence of features, numbers, steps, actions, components, parts, or a combination thereof described in the specification, but one or more other features. It is to be understood that the presence or addition of elements or numbers, steps, actions, components, parts, or combinations thereof, does not preclude in advance.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

A method of manufacturing an austempered nodular cast iron hook according to an embodiment of the present invention will be described.

1 is a flow chart showing a method of manufacturing an Ostempered nodular cast iron hook according to an embodiment of the present invention.

Referring to FIG. 1, the method of manufacturing the Autempered nodular cast iron hook is carbon (C) 3.3 wt% to 3.7 wt%, silicon (Si) 2.5 wt% to 2.9 wt%, and magnesium (Mg) 0.02 wt% to 0.05 wt. Including% and the balance being charged with iron (Fe) and unavoidable impurities into a shell-laminated mold (S100), austenitizing the molten metal charged to the shell-laminated mold (S200), and the austenite It may include a step (S300) of performing austempering of the molten metal treated in the salt bath.

At this time, the content of the carbon (C) is characterized in that 3.3wt% to 3.7wt% based on the total weight.

In this case, when the carbon content is less than 3.3 wt%, thermal conductivity and machinability may be lowered due to spheroidization of graphite due to the low carbon content.

In this case, when the carbon content exceeds 3.7 wt%, brittleness may increase due to the high carbon content, thereby deteriorating durability.

At this time, the silicon (Si) content is characterized in that 2.5 wt% to 2.9 wt%.

In this case, when the silicon content is less than 2.5 wt%, durability may decrease due to an increase in shrinkage during cooling due to a low silicon content.

In this case, when the silicon content exceeds 2.9 wt%, graphitization may be excessively promoted due to the high silicon content.

At this time, the magnesium is used as a spheroidizing agent.

In this case, when the magnesium content is less than 0.02 wt%, the spheroidization rate of graphite cast iron may be lower than 90% due to the low magnesium content. In addition, the mechanical properties of the hook may be deteriorated due to the low spheroidization rate.

In this case, when the magnesium content is greater than 0.05wt%, spheroidization may occur excessively due to the high magnesium content, and castability may be deteriorated.

At this time, the shell laminated mold is characterized in that the mold for casting the Autempered nodular cast iron hook.

At this time, the austenitizing step (S200) is characterized in that it is performed at a temperature of 800 °C to 1000 °C.

In this case, when the austenitizing step (S200) is performed at a temperature of less than 800°C, the austenitization may be insufficient.

In this case, when the austenitizing step (S200) is performed at a temperature of more than 1000°C, the austenitization may be insufficient due to the high temperature.

At this time, the austenitizing step (S200) is characterized in that it is performed for 1 to 3 hours.

In this case, if the austenitizing step (S200) is performed for less than 1 hour, the austenitization of the steel structure may be incomplete and the toughness of the hook, which is the final cast, may decrease.

In this case, when the austenitizing step (S200) is performed for more than 3 hours, economic problems may occur due to heat treatment more than necessary.

In this case, the step of performing the aus tempering (S300) is characterized in that it is performed at a temperature of 250 °C to 450 °C.

At this time, when performing the austempering step (S300) is performed at a temperature of less than 250 °C, cracks may occur due to rapid cooling.

At this time, when performing the austempering step (S300) is performed at a temperature higher than 450°C, a microstructure other than bainite may be formed due to the high temperature.

At this time, the step of performing the aus tempering (S300) is characterized in that 30 to 90 minutes.

In this case, if the step of performing the austempering (S300) is performed for less than 30 minutes, the generation of the ausferitized structure may be insufficient, so that the toughness or strength of the cast product may be reduced.

In this case, when the step of performing the aus tempering (S300) is performed for more than 90 minutes, an economic problem may occur due to more than necessary heat treatment.

At this time, it is characterized in that the graphite spheroidization rate of the Autempered nodular cast iron hook is 90% or more.

At this time, when the graphite spheroidization rate of the Autempered nodular cast iron hook is less than 90%, mechanical properties may be deteriorated due to the low spheroidization rate.

In this case, after the step of performing the ostempering (S300), a step (not shown) of performing secondary ostempering may be further included.

In this case, the step of performing the secondary austempering (not shown) may be performed to stabilize the austenite structure formed in the austenitizing step (S200).

At this time, the step (not shown) of performing the secondary austempering is characterized in that it is performed at a temperature of 150 °C to 350 °C.

In this case, the step (not shown) of performing the second aus tempering may be performed for 30 to 120 minutes.

The shell stack casting method is characterized in that a plate-shaped shell mold is manufactured using a shell mold, and the shell mold plates are stacked to assemble a whole mold set. At this time, the number of template plates used per set of molds is not limited, but the lower part of the template is a space for the upper mold, and the upper part is a space for the lower mold. When casting through the shell stack casting method, a high product recovery rate and productivity can be secured, as well as a product having excellent mechanical properties compared to a general sand-casting product.

The primary elements that influence the control of the shape of graphite by the chemical composition of nodular cast iron are carbon, silicon and manganese, and magnesium is mainly added as a spheroidizing element. This mainly affects the matrix structure and heat treatment reaction of the cast product. On the other hand, silicon, as a graphitization accelerating element, plays a role in reducing the carbon solubility of austenite and lowering the vacancy temperature. It also greatly influences the ultimate ratio of pearlite and ferrite, the hardness of ferrite, and the ductile-brittle transition behavior.

In general, ductile graphite cast iron can be heat treated to control the microstructure and mechanical properties, and the heat treatment method is different depending on the purpose. At this time, in order to increase the heat treatment reactivity, several alloying elements such as copper, molybdenum, and tin may be added. Such an alloying element plays a role in delaying the decomposition of austenite into pearlite during cooling after the ductile graphite cast iron is austenitized. At this time, the heat treatment of the ductile cast iron may be performed to increase strength and hardness or increase toughness and ductility by removing carbides and making the matrix structure uniform. In particular, in order to increase strength, ductility, and toughness at the same time, an austempering heat treatment may be performed.

The austempering heat treatment is a heat treatment for removing carbides and obtaining an ausferite matrix structure having strength, ductility, and toughness at the same time. The cast iron material having the ausferite matrix structure is referred to as austempered ductilecast iron (ADI). Autempered nodular cast iron has superior properties such as strength, toughness and abrasion resistance compared to general nodular cast iron. In addition, the Autempered ductile cast iron may have a strength of twice or more than that of ordinary ductile graphite cast iron having high ductility and toughness. This property and the weight of parts can be reduced when manufactured with Autempered nodular cast iron material.

Therefore, when manufacturing an Autempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having excellent mechanical properties through shell lamination casting.

In addition, when manufacturing an Autempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having a high productivity and recovery rate through the shell lamination casting.

Further, when manufacturing an Ostempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having high strength and low weight characteristics through Ostempering.

Further, when manufacturing an austempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having high tensile strength through optimal composition and austempering.

Example 1

A shell-laminated mold was prepared in order to manufacture an Autempered nodular cast iron hook according to an embodiment of the present invention.

The shell-laminated mold was manufactured to have a width, length, and thickness of 330mm, 300mm, and 35mm, respectively, and a hole having a diameter of 20mm was installed on both sides of the cast specimen to serve as a pressing metal during assembly. A hole with a diameter of 40 mm was drilled in a position of 100 mm from the right vertical side to the left at a position of 150 mm in the center of the vertical length, and a hole was made in the injection cup and a core adhesive was used to attach it to the mold. After shaping the upper and lower plates, the shells were laminated up to five layers, and the shell laminated side was spread out using a core adhesive. Next, the mold was cured using a torch. In order to minimize the change in thickness caused by the mold separation force due to pressure during injection of the molten metal, it was fixed using a heavy weight.

Example 2

A molten metal was prepared in order to manufacture an Autempered nodular cast iron hook according to an embodiment of the present invention.

First, a casting ship and steel scrap were charged in a high frequency induction melting furnace to obtain a molten metal. Next, 5wt% Fe-Si was added to the molten metal in the furnace after spheroidization treatment to obtain the silicon content of the final cast product. Then, when the temperature of the molten metal reached about 1450 °C, in order to change the residual magnesium content of the final cast product, a 5.0 wt% Mg-Fe-Si alloy was spheroidized into the molten metal in the furnace by plunging and sandwiching. Next, slag was removed from the surface of the molten metal using a slag and tapped on a ladle. At this time, post-inoculation treatment was performed using a 2.0wt% Ba-Fe-Si alloy to control the silicon content. Next, the temperature of the post-inoculated molten metal was kept constant at 1350°C to 1370°C and injected into the shell lamination mold. At this time, the molten metal was injected using a small ladle to obtain a specimen for chemical composition analysis. Next, after the injected molten metal was completely solidified, it was cooled to room temperature and then desagnized to obtain a test cast.

At this time, the chemical compositions of the charge, additives and molten metal treatment agent used in Example 2 are shown in Table 1.

[Table 1]

Experimental Example 1

In order to confirm the chemical composition of the test casting, the surface of the coating specimen for chemical composition analysis was polished 100 times using a polishing paper and then analyzed using an emission spectrometer.

First, KS B 08014 standard specimens and sub-size specimens were cut, and the cut surface was roughened by a conventional method, and then finely polished to a 1㎛ grade with diamond slurry. Next, the graphite structure of the specimen before corrosion and the matrix structure after corrosion with 3.0% nitalum solution were observed with an optical microscope. Then, the spheroidization rate and the volume fraction of pearlite in the matrix were measured using an image analysis device. In all cases, 5 positions were selected and measured, and the average of the three values excluding the maximum and minimum values was obtained.

At this time, the structure of the KS B 08014 standard specimen and the sub-size specimen is shown in FIG. 2. In this case, (a) of FIG. 2 is a cross-sectional view of a KS B 08014 standard specimen, and (b) is a cross-sectional view of a sub-size specimen.

The results are shown in Table 2 and FIG. 3 below.

[Table 2]

Experimental Example 2

The test was performed using a universal material tester using a kitchen condition and heat-treated tensile specimen.

First, in the case of a standard specimen, it is assembled with a crosshead speed of 2.0mm per minute, and in the case of a sub-sized tensile specimen, it is assembled with an STS 316L stainless steel jig and mounted on a universal testing machine. I did. The cross-sectional diameter and gage distance before and after the tensile test were measured using a projector to calculate yield, tensile strength, and elongation values, and the average value was obtained by testing three times for each condition. In addition, specimens having a height of 16 mm and 7.0 mm were cut and polished from the center of the cross-sectional contraction portion of the test casting with the remaining cross-sectional contractions of 12.5 mm and 6.25 mm for each condition, and a Brinell hardness test was performed after polishing. At this time, the test was performed 5 times to obtain the average value of the remaining values excluding the maximum and minimum values.

The results are shown in Tables 3 to 4 below.

Table 3 is a table showing the microstructure and mechanical properties according to the cross-sectional size of the sub-size test piece.

[Table 3]

Table 4 is a table showing the microstructure and mechanical properties according to the cross-sectional size of a standard test piece.

[Table 4]

Referring to Tables 2 to 4 and FIG. 4, as the content of residual magnesium increased, the spheroidization rate, the number of graphite particles, the volume fraction of pearlite, strength, hardness and elongation increased. This is because the spheroidization rate increases as the residual magnesium content increases. In addition, it can be seen that as the spheroidization rate of graphite acting as a bond in the metal matrix increases, the stress concentration in the matrix in front of the graphite decreases, thereby increasing the mechanical properties. On the other hand, it can be seen that the spheroidization rate, the number of graphite particles, and the volume fraction of pearlite decrease as the carbon content increases. Accordingly, mechanical properties such as strength and hardness decreased. On the other hand, it is judged that the formation of eutectic graphite was promoted by increasing the temperature of the equilibrium reaction, austenite-graphite, as the silicon content increased. Accordingly, the spheroidization rate and the number of graphite particles increased, while the volume fraction of pearlite decreased, resulting in an increase in elongation and a decrease in strength. Therefore, in the optimal composition of the nodular cast iron hook according to the embodiment of the present invention, the carbon content may be 3.30wt% to 3.50wt%. At this time, in the optimal composition of the nodular cast iron hook according to an embodiment of the present invention, the silicon content may be 2.50wt% to 2.70wt%. At this time, in the optimal composition of the nodular cast iron hook according to an embodiment of the present invention, the residual magnesium content may be 0.04wt% to 0.05wt%.

Example 3

In order to find out the effect of the aus tempering according to the embodiment of the present invention, the test cast product cast in Example 2 was subjected to an aus tempering treatment.

First, the solidified test piece after melting and casting, that is, the test piece in a kitchen state, was austenitized for 2 hours at a temperature of 900°C using a box furnace in which the heating element of 3.1 kW capacity was kanthal. At this time, for the austempering treatment, the heating element with a capacity of 7.8 kW is a nichrome wire and a low-temperature mixed salt of 45:55 in weight ratio of sodium nitrate and potassium nitrate is used for 30 and 60 minutes in a salt bath maintained at 350°C During the austempering treatment. Next, the microstructure was observed and mechanical properties were tested using the heat-treated test piece. Table 5 shows the results.

Table 5 is a table showing mechanical properties after austempering heat treatment.

[Table 5]

Referring to Table 5, it can be seen that the yield strength, tensile strength, elongation and Brinell hardness are all significantly enhanced when the aus tempering is performed compared to the results shown in Tables 3 and 4 in which the austempering is not performed. In addition, when the austempering treatment was performed at a temperature of 350 °C, it was found that the mechanical condition of 60 minutes was better than 30 minutes.

Example 4

A prototype hook for towing a vehicle according to an embodiment of the present invention was manufactured.

First, a molten metal was prepared using an induction melting furnace capable of obtaining 2.5 tons of molten metal upon one dissolution. At this time, the target composition was set to 3.5wt%C-2.7wt%Si. Next, a 5% Mg-Fe-Si alloy was used as a spheroidizing agent to spheroidize, and the molten metal was injected into a shell lamination mold. Next, a basic molten metal was obtained by dissolving steel scrap iron having a carbon content of 1.5 wt% or less and pig iron for casting 4.34 wt%. At this time, a recarburizing agent, ferrosilicon, and silicon carbide were added to adjust the carbon and silicon content of the basic molten metal. Next, tapping was performed using a ladle of 300kg capacity. At this time, the tapping temperature was 1500 °C. During tapping, spheroidization was performed by a sandwich method, and a 5% Mg-Fe-Si alloy was added to obtain a residual magnesium content of 0.040wt%. Next, the ladle was subjected to post inoculation by removing slag from the surface of the molten metal and increasing the silicon content by 0.4wt%.

Next, a molten metal was injected into the shell-laminated mold at a temperature of about 1400 °C. At this time, the molten metal was injected into the mold to obtain a coating specimen for chemical composition analysis using a small ladle. Finally, after complete coagulation took place, it was desalited at room temperature. The characteristics of the Autempered nodular cast iron hooks according to the embodiment of the present invention were evaluated using the above-described analytical coating specimen and are shown in Table 6.

Figure 4 is a photograph of a prototype of austempered nodular cast iron hook according to an embodiment of the present invention.

Table 6 is a table showing the microstructure and mechanical properties of the hook prototype according to an embodiment of the present invention.

[Table 6]

Referring to Table 6, it can be seen that the spheroidization rate of the hook prototype according to the embodiment of the present invention is 90% or more, and has a high volume fraction of pearlite. In addition, when comparing before and after the austempering treatment, it can be seen that the hardness of the hook prototype was significantly increased when the austempering was performed.

Therefore, when manufacturing an Autempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having excellent mechanical properties through shell lamination casting.

In addition, when manufacturing an Autempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having a high productivity and recovery rate through the shell lamination casting.

Further, when manufacturing an Ostempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having high strength and low weight characteristics through Ostempering.

Further, when manufacturing an austempered nodular cast iron hook according to an embodiment of the present invention, it is possible to manufacture a hook having high tensile strength through optimal composition and austempering.

An Ostempered nodular cast iron hook according to an embodiment of the present invention will be described.

In an embodiment of the present invention, the austempered nodular cast iron hook is characterized in that it is manufactured by the method of manufacturing the austempered nodular cast iron hook.

At this time, the tensile strength of the austempered nodular cast iron hook is characterized in that ^{800N / mm 2 or more.}

At this time, the yield strength of the austempered nodular cast iron hook is characterized in that ^{750N/mm 2 or more.}

At this time, the elongation of the Oustempered nodular cast iron hook is characterized in that 9% or more.

Ostempered nodular cast iron hook according to an embodiment of the present invention can provide a hook having excellent mechanical properties through the shell lamination casting.

In addition, the Ostempered nodular cast iron hook according to an embodiment of the present invention can provide a hook having a high productivity and recovery rate through shell lamination casting.

Further, the Ostempered nodular cast iron hook according to an embodiment of the present invention may provide a hook having high strength and low weight characteristics through austempering.

Furthermore, the Ostempered nodular cast iron hook according to an embodiment of the present invention can provide a hook having high tensile strength through optimum composition and austempering.

The above description of the present invention is for illustrative purposes only, and those of ordinary skill in the art to which the present invention pertains will be able to understand that it can be easily modified into other specific forms without changing the technical spirit or essential features of the present invention. will be. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not limiting. For example, each component described as a single type may be implemented in a distributed manner, and similarly, components described as being distributed may also be implemented in a combined form.

The scope of the present invention is indicated by the claims to be described later, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be interpreted as being included in the scope of the present invention.

Claims (10)

A molten metal containing 3.3 wt% to 3.5 wt% of carbon, 2.5 wt% to 2.7 wt% of silicon (Si), and 0.04 wt% to 0.05 wt% of magnesium (Mg), and the balance consisting of iron (Fe) and unavoidable impurities Charging the shell laminated mold;
Austenizing the molten metal charged into the shell-laminated mold; And
It characterized in that it comprises the step of performing austempering the austenitized molten metal in a salt bath,
Ostempered nodular cast iron hook manufacturing method, characterized in that the graphite spheroidization rate is 90% or more. The method of claim 1,
The step of the austenitizing treatment is performed at a temperature of 800 °C to 1000 °C. Austempered nodular cast iron hook manufacturing method. The method of claim 2,
The step of the austenitizing treatment is a method of manufacturing an austenitized nodular cast iron hook, characterized in that performed for 1 to 3 hours. The method of claim 1,
The austempered nodular cast iron hook manufacturing method, characterized in that the step of performing the austempering is performed at a temperature of 250 °C to 450 °C. The method of claim 4,
The austempered nodular cast iron hook manufacturing method, characterized in that the step of performing the austempering is performed for 30 to 90 minutes. delete Austempered nodular cast iron hook, characterized in that produced by the method of manufacturing austempered nodular cast iron hook of claim 1. The method of claim 7,
Ostempered nodular cast iron hook, characterized in that the tensile strength of the austempered nodular cast iron hook is 800N/mm ^{2 or more.} The method of claim 7,
Ostempered nodular cast iron hook, characterized in that the yield strength of the austempered nodular cast iron hook is 750N / mm ^{2 or more.} The method of claim 7,
Ostempered nodular cast iron hook, characterized in that the elongation of the austempered nodular cast iron hook is 9% or more.

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