EP1111071A2

EP1111071A2 - Method of manufacturing a compacted vermicular graphite cast iron for engine block

Info

Publication number: EP1111071A2
Application number: EP00123556A
Authority: EP
Inventors: Hyoun Soo Park; Hak Jin Kim; Jong Moon Kim; Seung Cheal Jung
Original assignee: Hyundai Motor Co
Current assignee: Hyundai Motor Co
Priority date: 1999-12-22
Filing date: 2000-10-27
Publication date: 2001-06-27
Also published as: JP2001181779A; KR20010063311A

Abstract

This invention relates to a process of manufacturing a compacted vermicular graphite cast iron for engine block and more particularly, the process of manufacturing a compacted vermicular graphite cast iron for engine block prepared in a manner such that to improve the shortcomings of the conventional gray cast iron and aluminum, the shape of graphite is optimized by adding a certain amount of magnesium to the cast iron, followed by the casting process of a high-strength graphite at a specific range of temperature. According to this invention, the compacted vermicular graphite cast iron for engine block can provide excellent combination of properties such as mechanical properties, better output and lighter weight, while ensuring a higher dimensional stability based on a high strength and rigidity, including improved noise-periodic damping effect.

Description

Field of the Invention

Description of the Prior Art

In parallel with the current market demand for more sophisticated performance and larger output of various industrial engines including automobile engines, their operational conditions become severe. Hitherto, a gray iron is being mainly used for manufacturing for industrial engines and in some cases aluminum is additionally employed for minimizing the weight of engines.
With a continuous shape and arrangement of graphite, the gray cast iron has better properties in terms of a thermal conductivity, a damping capacity and a good processibility but its sharp corners may result in generating cracks.
These cracks are continuously extended along with the surface of graphite and the gray cast iron is finally broken due to the formation of a cleavage plane, thus leading directly to a low strength and brittleness of the conventional gray cast iron.
Under such circumstances, there is an urgent need for improving such drawbacks that the conventional gray cast iron has faced, so as to meet the technical, environmental and economical conditions.

SUMMARY OF THE INVENTION

Therefore, an object of this invention is to provide a compacted vermicular graphite cast iron for engine block prepared in a manner such that the shape of graphite is optimized by adding a certain amount of magnesium to a chemical composition of engine containing iron as a main material instead of the conventional gray cast iron, together with other materials (e.g., carbon, silicon, manganese, phosphorus, sulfur, chromium, molybdenum, copper, and tin), followed by the casting process of a high-strength graphite at a specific range of temperature. According to this invention, the compacted vermicular graphite cast iron for engine block can provide excellent mechanical properties such as tensile strength, yield strength, elastic modulus, and rigidity, as well as improved noise-periodic damping effect, and better performance and durability of engine.

Brief Description of the Drawings

Fig. 1a is a graph showing the profile of temperature in the molten metal depending upon the cooling time in order to obtain an optimal compacted vermicular graphite cast iron of this invention;
Fig. 1b is a graph showing the contents of magnesium depending upon the cooling time in order to obtain an optimal compacted vermicular graphite cast iron of this invention.

Detailed Description of the Invention

This invention is characterized by a process of manufacturing a compacted vermicular graphite cast iron for engine block containing iron as a main material, together with other materials (e.g., carbon, silicon, manganese, phosphorus, sulfur, chromium, molybdenum, copper, and tin), wherein the molten metal with the temperature of 1500-1560°C containing 0.020∼0.028 wt.% of magnesium to the chemical composition is added to a mold, followed by further addition of magnesium with the contents of 0.01∼0.017 wt.%.
This invention is explained in detail as set forth hereunder.
This invention relates to a process of establishing the contents of magnesium and temperature range using the compacted vermicular graphite cast iron having better mechanical properties and quality than the conventional gray cast iron.
The compacted vermicular graphite cast iron is characterized by a fine structure, which is in the middle size of gray cast iron and spherical graphite cast iron. The crack sensibility of the compacted vermicular graphite cast iron becomes reduced due to the fact that the graphite particles have a round and irregularly arranged vermicular shape and in addition to that, it has a relatively wide scope of elasticity, thanks much to isolated graphite particles within the basic structure. Further, a higher portion of carbon equivalence in the compacted vermicular graphite cast iron makes it possible to increase the contents of graphite which can absorb and weaken the wavelengths, thus ensuring the improved noise-periodic damping effect.
This invention is explained in more detail based on the following manufacturing process of the compacted vermicular graphite cast iron.
The desired compacted vermicular graphite cast iron is prepared by screening the contents of magnesium added to control the shape and distribution of graphite, after thorough understanding of the chemical composition of the original molten metal prior to its initial injection into a mold.
After a certain amount of specimens is collected from the molten metal, a thermal analysis is performed by a probe and after tapping, the initial and final profiles of the molten metal in a mold are measured according to the changes in temperature and a consumption amount of magnesium. Then, the decision on further addition of magnesium should be immediately made to prepare the specimens.
In particular, according to this invention, 0.020∼0.028 wt.% of magnesium is added to the above chemical composition to prepare an original molten metal, followed by tapping and injection into the mold. The molten metal is completely injected into the mold within a required time of 13∼17 minutes on average under the following conditions: the initial range of temperature of the molten metal: 1500∼1560°C (melting point of pure iron: 1538±3°C) and cooling rate: 7∼16°C/min. It is preferred that as a result of amendment according to the profiles of the molten metal, the temperature of the molten metal is in the range of 1350∼1420°C and the contents of magnesium is in the range of 0.01∼0.017 wt.% (fading rate of the magnesium: 0.001 wt.%/min), when the molten metal is completely injected into a mold.
Fig. 1a is a graph showing the profile of temperature in the molten metal depending upon the cooling time in order to obtain an optimal compacted vermicular graphite cast iron of this invention. Fig. 1b is a graph showing the contents of magnesium depending upon the cooling time in order to obtain an optimal compacted vermicular graphite cast iron of this invention. When the molten metal is completely injected into a mold, the excessive amount of residual magnesium (corresponding to area "A") may lead to the formation of a spherical graphite cast iron due to a higher portion of cast iron. On the other hand, the insufficient amount of residual magnesium (corresponding to area "C") may lead to the formation of a gray iron in which D-type graphite in ASTM is crystallized. Even if the different cast conditions are responsible for changes in temperature, time or tapping amount until the molten metal is completely injected into the mold, a stable compacted vermicular graphite cast iron can be prepared by controlling the temperature profiles and amount of magnesium on the basis of the hatched area "B" in Figs. 1a and 2b.
During the slow cooling of the molten metal, the primary-phase austenite is crystallized at the initial period of solidification, and transformed to ferrite and D-type graphite at the final period of solidification, since large amount of magnesium are reduced below the critical level. To comply with this matter, it is preferred that in proportion to the crystallization amount of the crystallized austenite, the amount of magnesium is further added to a mold and then, the casting process is completed prior to the formation of gray cast iron. To prepare an ideal compacted vermicular graphite cast iron, it is preferred that the molten metal in a mold is extricated within 1 hour after injection for the prevention of slow cooling, so as to ensure the homogeneous distribution of graphite in the fine structure and a low ferrite fraction.
Since the compacted vermicular graphite cast iron of this invention, so prepared, has better physical properties and higher dimensional stability based on a high strength and rigidity, it can be effectively used for the manufacture of industrial engines including automobiles engines.
This invention is explained in more detail based on the following examples but is not limited by these examples.

Comparative Example

A gray cast iron was prepared in the same manner as the conventional method that magnesium was not added to cast iron.

Example

The compacted vermicular graphite cast iron was prepared in the chemical composition and contents specified in the following table 1.

Composition (wt.%) C Si Mn P S Cr Mo Cu Sn Mg Fe

Original molten metal 3.77 1.73 0.4 <002 0.03 0.04 0.64 0.05 - Rest

Final product 3.63 2.21 0.38 0.02 0.03 0.60 0.08 0.01 Rest

Experimental Example 1

The compacted vermicular graphite cast iron was prepared in a manner such that 0.020∼0.028 wt.% of magnesium was added to a cast including the above chemical composition (table 1), followed by the tapping to inject it into a mold; then, the molten metal was completely injected into the mold within a required time of 13∼17 minutes on average under the following conditions and released out of the mold: the initial range of temperature of the molten metal: 1500∼1560°C (melting point of pure iron: 1538±3°C) and cooling rate: 7∼16 °C /min.
Figs. 1a and 1b showed the changes in temperature of the molten metal and the contents of magnesium depending on the profiles of time ranging from revision after tapping to the mold completion.
When the molten metal was completely injected into a mold, it was noted that its injection completion temperature was in the range of 1350∼1420°C, being equivalent to 0.01∼0.017 wt.% of magnesium (fading rate: 0.001 wt.%/min).
The following table 2 showed the basic material used for the manufacture of an engine block using the CV graphite iron and the average value of the tapping amount.

Original molten metal (by wt. part) TDCR ZL#80 Cover material

800kg 5 ∼ 5.5kg 1.7 ∼ 2kg 4kg

Experimental Example 2

i) Mechanical property Test

As shown in the following table 3, the mechanical properties of both the specimens prepared from Example and Comparative example were measured.

Classification	Example	Comparative example
Shape of graphite	Vermicular	Flake
Tensile strength (kg/mm²)	40 ∼ 50	20 ∼ 25
Yield strength (kg/mm²)	30 ∼ 40	15 ∼ 20
Elastic modulus (GPa)	150	100
Fatigue limit (Mpa)	225	95
Hardness (Hb)	250	200
Elongation (%)	2	<1

As shown in the above table 2, the fine structure of graphite was observed by a scanning electron microscope. The following items were observed by a common method: tensile strength, yield strength, elastic modulus and elongation. The impact test was performed using Charpy impact machine, while the hardness test was performed using Brinell hardness machine.
The mechanical property test showed that the specimens prepared from Example was superior to those from comparative example.

ii) Processibility Test

The processibility test for the compacted vermicular graphite cast irons and gray cast irons, so prepared from Experimental example 1, was performed under the following milling conditions:
Cutting speed: 135m/min, 2.69mm/rev, 0.19mm/tooth
Rotary speed: 268rev/min
Feed: 720 mm/min
Cutting depth: 2.0mm
Coolant: None (dry milling)
Number of specimens: 24 pieces
Cutting-processed length: 2.500mm/piece
Total length: 60,000mm
Under the same processing conditions, the gray cast irons were under excessive cutting load and heat was severe after the 10^th specimen. By contrast, the compacted vermicular graphite cast irons were under less cutting load than the gray cast irons and heat was generated after the 15^th specimen. As a result, it was revealed that the milling processibility of the compacted vermicular graphite cast irons was better than that of gray cast irons. It was also reported that during the intermittent processing, the ideal length shape of chips is 2∼3 curls in length. While the length shape of the gray cast irons was less than 1 curl, the compacted vermicular graphite cast irons was more than 2 curls in length.

iii) Noise-periodic Damping Test

When the noise-periodic damping test was performed, an engine block prepared from the compacted vermicular graphite cast iron of Experimental Example 1 demonstrated the improved noise-periodic damping effect of maximum 2 dB, compared to the gray cast iron.

iv) Durability Test

When an engine block prepared by the compacted vermicular graphite cast iron of Experimental Example 1 was operated for 220 hours and disassembled for its durability performance, there was no breakage, abrasion or deformation from the compacted vermicular graphite cast iron, while an engine block prepared by the conventional gray iron showed that its abrasion and breakage were being in progress.
From the racing durability test under severe conditions increasing toque up to more than 2 times, an engine block prepared by the compacted vermicular graphite cast iron showed that its output was increased from 138 HP to 349 HP, while its torque was increased from 18.3 kg · m to 42.3kg · m.
According to this invention, the compacted vermicular graphite cast iron for engine block, so prepared by changing the contents of magnesium, can provide excellent combination of properties such as mechanical properties, better output and lighter weight, while ensuring a higher dimensional stability based on a high strength and rigidity, including improved noise-periodic damping effect.

Claims

A process of manufacturing a compacted vermicular graphite cast iron for engine block comprising iron as a main material, together with other materials (e.g., carbon, silicon, manganese, phosphorus, sulfur, chromium, molybdenum, copper, and tin), wherein the molten metal containing 0.020∼0.028 wt.% of magnesium to the chemical composition is added to a mold, followed by further addition of magnesium with the contents of 0.01∼0.017 wt.%.