JPS60197841A - Spheroidal graphite cast iron - Google Patents

Abstract

PURPOSE:To improve heat treatability by incorporating specific weight % of carbon, manganese, molybdenum, copper, etc. into a titled cast iron and cooling the cast iron from the austenitization temp. down to bainitization temp. at a cooling rate at which pearlite does not precipitate. CONSTITUTION:A spherical graphite cast iron consisting essentially, by weight %, of 3-4% carbon, 1.5-3% silicon and 0.005-0.2% graphite spheroidizing treating agent is incorporated therein with 0.3-0.8% manganese, 0.3-2% copper, 0.2-2.5% molybdenum, and if necessary, <=0.3% Ni and is cooled from the austenitization temp. down to the bainitization temp. at the cooling temp. at which pearlite does not precipitate. The manganese, molybdenum, etc. improves the stability of the supercooled austenite, by which the heat treatability is improved. Since Ni is not always contained, the inexpensive spheroidal graphite cast iron having excllent strength and toughness is obtd.

Description

[Detailed description of the invention] (+1 field of industrial use) This invention is applicable to various automobile parts and tractor parts.

Used as material for parts such as crankshafts, rocker arms, casings, piston rings, drums, forks, metal fittings, etc. in ship parts, steel manufacturing machinery parts, etc., especially for austempering of thick-walled products, austempering without a salt bath, or This invention relates to spheroidal graphite cast iron for austempering that utilizes upper bainite and has excellent heat treatability and can be applied when the cooling rate from austenitizing 1 kAla to the bainitizing temperature cannot be made sufficiently fast.

(Prior art) In the past, austempered spheroidal graphite I4 iron in particular has been attracting attention as an inexpensive and lightweight material that can replace steel. This is due to the fact that it has a significantly higher level of elongation and strength than the spheroidal graphite cast iron. For example, Gies
Serei 65 (1978), 4, 73. The forged gears are made from austenitic spheroidal graphite cast iron in order to achieve the following: ■Cost reduction due to lower material costs, processing and heat treatment costs, ■Weight reduction due to density differences, and ■Lower noise due to high vibration damping ability. There have been reports of success in replacing it with gears.

However, in reality, despite the strong desire to apply austempering to thick-walled parts and austempering using upper bainite, spheroidal graphite cast iron with a normal composition originally has a high Austemper heat treatment has been unable to provide excellent properties, and the range of application of Austemper heat treatment has been extremely narrow. This is because in austempering based on the above requirements, the cooling rate from the austenitizing temperature to the bainitizing temperature is slow, and a large amount of pearlite is precipitated from the properly cooled austenite during cooling. A salt bath is generally used for such austempering heat treatment, but the same is true when austempering heat treatment by air cooling is desired to avoid the disadvantages of a 1M bath such as danger, environmental deterioration, and poor workability. It is difficult to achieve this purpose with spheroidal graphite cast iron except for small parts.

In other words, in spheroidal graphite cast iron with a normal composition, the stability of austenite is insufficient, so if the cooling rate from the austenitizing temperature to the bainitizing temperature cannot be made fast enough, normal austempering cannot occur. However, there was a drawback that excellent properties could not be obtained.

Therefore, as a conventional way of thinking to solve the above problem, for example, Japanese Patent Application Laid-open No. 50-127823, Japanese Patent Application Laid-Open No. 54-13
As disclosed in Japanese Patent Publication No. 3420 or Japanese Patent Publication No. 47-19496, spheroidal graphite cast iron containing a large amount of molybdenum, nickel, etc. to stabilize ausnite is used. In this way, since the austenite is stabilized, even if the process from the austenitization temperature to the bainitization temperature is performed by air cooling with an extremely slow cooling rate, normal austenite can be produced without precipitating pearlite. In addition, it has the advantage that the generation of distortion can be reduced due to the slow cooling rate.

However, such high-alloy spheroidal graphite cast iron: ■ Contains both molybdenum and nickel in large quantities, making it an extremely expensive material. ■ Containing a large amount of nickel slows the progress of bainite formation. However, in order to obtain excellent mechanical properties, an extremely long bainitic treatment is required.

In addition, as other spheroidal graphite cast iron, we have improved heat treatability in a composition range that ensures mechanical properties by controlling the stars of manganese and copper, and suppressed the presence of expensive molybdenum and nickel as much as possible. The inventors have already proposed this. This spheroidal graphite cast iron is: - low cost due to low nickel and molybdenum content; - high nickel content,
It has the characteristic that the bainitization time is short because of the low amount of bainite, and it is an extremely useful material that can apply the excellent properties of austempered spheroidal graphite cast iron to austempering of thick-walled parts and austempering that does not use a salt bath. be. However, even this spheroidal graphite cast iron has a limit in its heat treatability, and there has been a desire to develop a material that can be austempered even in thicker and larger parts.

(Purpose of the Invention) This invention was made as a result of a detailed study of the influence of molybdenum on materials in which the amounts of manganese and copper have been adjusted to appropriate values, focusing on such conventional problems. The purpose of the present invention is to provide an inexpensive austempered spheroidal graphite cast iron that can be applied to austempering of fairly thick parts and austenite without using a salt bath while keeping the time +1Jf short, and can be used to obtain strong cast parts. .

(Structure of the Invention) The spheroidal graphite cast iron according to the present invention has a carbon content of 3 to 3% by weight.
4%, silicon = 1.5-3%, graphite spheroidizing agent: 0
．． 005~o, 2% as a basic component, other manganese 20.3~0.8%, copper: 0.3~2%, molybdenum 20
．． 2 to 2.5%, including 0.3% or less of nickel if necessary, the remainder being substantially iron, and in particular cooled from the austenitizing temperature to the bainitizing temperature at a cooling rate that does not precipitate a pearlite structure. It is a feature.

Next, the composition range (weight%) of the spheroidal graphite cast iron according to this invention
) will be explained below.

Carbon (C): 3 to 4% Carbon is the main alloying element of cast iron, but if it is less than 3%, it deteriorates castability, so it is not preferable, and if it exceeds 4%, primary graphite will crystallize due to coexistence with silicon. The content was set in the range of 3 to 4%, since the content would become easy and impair mechanical properties.

Silicon (Si): 1.5-3% Silicon is a major alloying element in cast iron, along with carbon.
If it is less than 5%, it will impair graphitization and castability, making it undesirable for tl-f, and if it exceeds 3%, it will impair mechanical properties, so 1.5
The range was set at ~3%.

Graphite spheroidizing agent: 0.005-0.2% Graphite spheroidizing agent is added to spheroidize graphite during casting. In this case, the amount of residual graphite spheroidization treatment agent that can properly spheroidize graphite and do not deteriorate mechanical properties is 0.00.
It is in the range of 5-0.2%. Although magnesium is preferable as the graphite spheroidizing agent, it is also possible to use cerium (Ce), calcium (Ca), etc. in addition to magnesium as the graphite spheroidizing agent.

The above-mentioned range of components of carbon, silicon, and graphite nodularizing treatment agent was selected as a composition that falls within the range of general spheroidal graphite cast iron, and as long as it falls within this range, even other compositions are substantially There is little impact.

Manganese (Mn): 0.3 to 0.8% Manganese is an effective element for increasing the stability of properly cooled austenite in spheroidal graphite cast iron, and in order to obtain this effect, 0.3%
or more.

However, too much will reduce the strength and toughness, so 0
．． It was set to 8% or less.

Copper (Cu): 0.3 to 2% Copper is an element effective in improving heat treatability, and in order to achieve this effect, it is contained in an amount of 0.3% or more. However, if the addition amount increases, it becomes difficult to make the graphite spheroidized, and if the addition amount becomes too large, it becomes impossible to avoid a decrease in strength and impact value, so the addition amount was set at 2% or less.

Molybdenum (Mo): 0.2-2.5% Molybdenum is an effective element for increasing the stability of properly cooled austenite, so in order to fully obtain this effect, 0.2%
or more.

However, since it is an expensive element, it was set at 2.5% or less.

Nickel (Ni): 0.3% or less Nickel is an effective element for increasing the stability of properly cooled austenite, so it can be added as necessary, but if it exceeds 0.3%, its effect will be reduced. perlite weird 1 le i
It has a stronger force on bainite transformation than on steel, requires longer bainite transformation to obtain the same mechanical properties, and is also an expensive element, so it should be kept at 0.3% or less. It is better.

Furthermore, the above-mentioned manganese, molybdenum, and copper.

When considering the influence of nickel on the incubation time (t i ) of pearlite nose in the TTT diagram, 3
It is more desirable to set the range such that (Mn+Mo)+2 (Cu+Ni)≧7.

This spheroidal graphite cast iron produced by IJJ has the above composition and is cooled from the austenitizing temperature to the bainitizing temperature at a cooling rate that does not precipitate pearlite. By stipulating the above and adjusting the molybdenum content h1, it can be applied to austempering of fairly thick parts and austempering without using a salt bath, while keeping the bainitization time short without deteriorating the stability of austenite. It is a high-performance spheroidal graphite cast iron that can be used to produce strong cast parts at low cost.

(Example) In this example, the influence of molybdenum on the thermal stability of properly cooled austenite of spheroidal graphite cast iron containing copper and manganese was investigated. The basic composition of the cast iron used here is, in weight percent, 3.6% carbon, 2.8% silicon, and 0.5% phosphorus.
02%, Io 0.007%, Magnesium 0.04
%belongs to. The basic composition of the sample material used in this example was selected as a composition that falls within the range of general spheroidal graphite cast iron, and as long as it falls within this range, the experimental results will be valid even if other compositions are used. There is no real impact. In this example, magnesium was mainly used as the graphite spheroidizing agent, but in addition, calcium (Ca) or cerium (Ce) may also be used if the purpose of graphite spheroidization is sufficiently achieved. I can't help it.

Then, for the above composition, 1.5% copper, 0.5%
Based on spheroidal graphite cast iron composition containing % manganese, 0~
Heat treatability and mechanical properties were evaluated by adding 2.5% molybdenum. In addition, cast iron containing 0.3% nickel, 0.8% manganese and 2% copper is used as the upper limit of high alloying in the above range, and 0.3% is the lower limit of low alloying.
of manganese and 0.3% copper, and add 1
% of molybdenum was also investigated.

On the other hand, the stability (heat treatability) of properly cooled austenite was evaluated by measuring a TTT diagram using a transformation expansion (contraction) measuring device. In addition, the mechanical properties were confirmed by performing a static strength test using a JIS No. 14-A φ7 test piece and an impact test using a JIS No. A test piece. The austempering heat treatment in this case was carried out by austenitizing at 900°C for 4 hours and bainizing at 250°C for 2 hours, during which cooling was performed using a fluidized bed furnace. These results are shown in the accompanying figures and tables.

The attached ζj diagram shows that molybdenum is manganese (0.3%).

0.5%, 0.8%) and copper (0.3%, 1.5%, 2%
) is the result of considering the influence on the thermal stability of appropriately cooled austenite in spheroidal graphite cast iron. The vertical axis of the attached diagram is 900°OX TTT after 15m1n austenitization
The incubation time of the pearlite nose appearing in the diagram (t i
).

Moreover, the horizontal axis shows the content of molybdenum. Here, ti
The larger the value of the austenite, the more stable the austenite is, and it does not precipitate pearlite even when slowly cooled, allowing normal austempering heat treatment, so it can be said that it is a material with excellent heat treatability.

As shown in the 1.5Cu-o and 5Mn lines in the diagram,
It can be seen that the effect becomes more pronounced when molybdenum is contained in an amount of 0.2% or more, and the stability of austenite is significantly improved as the amount of molybdenum increases. Also, 2Cu-0, 8Mn-0, 3N i and 0.3C
The effect of adding morisothene was also confirmed in u-0,3Mn with 1% Mo added. In this way, 0.
3-2% copper, 0.3-0.8% manganese and θ~
The heat treatability improvement effect of molybdenum on spheroidal graphite cast iron containing 0.3% nickel starts from 0.2%.
The heat treatability of this kind of cast iron can be improved very effectively by applying more force than that.

Next, the evaluation results of the mechanical properties of spheroidal graphite cast iron having the composition shown in the table are also shown in the table.

As shown in the table, as Mo increases from 20% for No.l to 48% for No.5(7)MO2, the value of ti, which indicates the pearlite nose position in the TTT diagram, becomes more pronounced. , the increase in Mo stabilizes austenite, making it difficult for pearlite to precipitate even when kept cold.
It is clear that heat processability can be significantly improved. In addition, in the case of No. 6, in which MO is 1.01% and Ni is added to it by 0.29%, it is possible to improve the stability of austenite more than that with the same amount of MO@. , No. 7, with Ni addition as well as Mo
By roughly increasing 5, the heat treatability can be further improved.

On the other hand, in the case of NO48, which does not contain Mo, and N099, which contains too little MO, the heat treatability is not good, and M
No, 10 with too much n, No, 1 with too much Cul,
It is clear that No. 12, No. 13, which have too much Ni9, have poor mechanical properties. In addition, in the case of high Ni, the strength and impact value are considered to decrease due to a delay in the progress of bainite formation.

Furthermore, in the high nickel material shown in Comparative Example No. 12,
The incubation time of bainitic metamorphism I at 250°C is approximately 2
000 sec, whereas No. 000 sec of the present invention example
In 6 and 7, the A1a is less than 1ooOsec, and the pearlite nose time is extended (improvement of heat treatability).
Nevertheless, it has also become clear that the cast iron according to the invention requires less time to form into bainite.

Next, in terms of weight percent, ■3.6% carbon, 2.6% silicon, 0.8% manganese, 2% copper, MOo 2%, ■3.6 carbon
%, silicon 2.6%, mankan 0.8%, copper 2%, Mo
0.3%, Ni 0.1%, ■ Carbon 3.6%, Silicon 2.
6%, mankan 0.8%, copper 2%, M o 0, 3%
Diameter 60IIIffl made of three types of spheroidal graphite cast iron,
A round bar with a length of 180 II 1 m was subjected to austempering heat treatment including austenitizing treatment at 850°C for 4 hours and bainitizing treatment at 350°C for 2 hours. Note that cooling from the austenitizing temperature to the bainitizing temperature was performed using a fluidized bed furnace. At this time, the fluidized medium of the fluidized bed furnace is
80 mesh alumina was used, the gas was N2 gas, the flow rate was 230Q/win, and the temperature was 250°C. Next, when the austempered round bar was cut and the metallographic structure was examined using a microscope, no pearlite structure appeared even in the center, confirming that complete austenitization was achieved.

(Effects of the Invention) As explained above, the spheroidal graphite cast iron according to the present invention has a carbon content of 3 to 4% and a silicon content of 1.5 to 3% by weight.
, graphite spheroidization treatment agent 70, 0.3 to 0.8% of mankanji to spheroidal graphite cast iron whose basic component is 0.05 to 0.2%.
, copper = 0.3-2%, molybdenum di 0.2-2.5%,
It contains Ni: 0.3% or less as necessary, and is cooled from the austenitizing temperature to the bainitizing temperature at a cooling rate that does not precipitate pearlite, ensuring mechanical properties and basic heat treatability. By adding molybdenum to the material, heat treatability can be further improved, and for thick parts and austenitization without using a salt bath, the heat treatability can be improved by adding molybdenum to the material. It is possible to provide a high level of strength and toughness, and since it does not necessarily contain nickel, the time required for bainiticization is shortened, and at the same time it is inexpensive, and the application of austenba spheroidal graphite cast iron with high strength and high toughness. This dramatically expands the scope.

[Brief explanation of the drawing]

The attached figure is a graph showing the results of investigating the influence of MO stress on the vertical nose position of the TTT diagram. Patent applicant Yutaka Oshio, patent attorney representing Nissan Motor Co., Ltd.

Claims (3)

[Claims] (1) In weight%, carbon: 3-4%, silicon = 1.5-3
%, manganese: 0.3-0.8%, copper 0.3-2%,
Molybdenum = 0.2-2.5%, graphite spheroidizing agent: 0
．． 0.005 to 0.2%, the balance being substantially iron, and is characterized by being cooled from the austenitizing temperature to the bainitic temperature at a cooling rate at which pearlite does not precipitate. . (2) The spheroidal graphite cast iron according to claim (1), containing nickel: 0.3% or less. (3) Spheroidal graphite cast iron according to claim (1) or tfTj (2), wherein 3 (Mn+Mo)+2 (Cu+Ni)≧7.