JPS6176612A - Manufacture of high strength spheroidal graphite cast iron - Google Patents

Abstract

PURPOSE:To manufacture the titled cast iron inexpensively, by cooling rapidly after austenitizing a base material having a specified compsn. composed of C, Si, Mn, Mg and Fe, etc. under a suitable condition, next, austemper treating said material in salt bath furnace. CONSTITUTION:The base material of rod shape, etc. composed of 3.0-4.5wt% C, 1.5-3.5% Si, <=1.5% Mn, 0.02-0.1% Mg and the balance Fe with impurity is cast from molten metal, heated to 800-950 deg.C for 0.5-3hr and austenitized. Successively, under the state in which heat is remaining inside the base material, the surface thereof is cooled rapidly to a prescribed temp. of <=about 210 deg.C by using medium such as water or oil to make the vicinity of said surface martensitic. Thereafter, said material is held at 210-500 deg.C for >=20min in salt bath furnace to perform austemper treatment composed of isothermal transformation condition for making inside thereof bainitic, and the titled cast iron is obtd. easily.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing high-strength spheroidal graphite cast iron.

(Prior Art) As a method for improving mechanical properties such as strength and toughness of spheroidal graphite cast iron, a method of austenitizing the x512 graphite cast iron to form bainitic M iron is generally known. However, in the conventional austempering method, when cooling a heated base material of spheroidal graphite cast iron, a problem arises in that pearlite structure is generated in the base material when the cooling rate is slow. As a countermeasure to this, alloys such as molybdenum, nickel, and copper are further added to the above-mentioned base material.6 If the diameter of the rod-shaped base material manufactured from spheroidal graphite cast iron is 30 nmφ or more, the influence of pearlite structure formation will be large. , especially 70mm
At φ or more, it was difficult to obtain bainitic cast iron without adding the above alloy.

(Problems to be Solved by the Invention) As described above, in the conventional manufacturing method of high-strength spheroidal graphite cast iron, when the thickness of the spheroidal graphite cast iron base material is thick, it is necessary to add an alloy of expensive metals such as molybdenum. Therefore, the price is high, and if the above-mentioned alloy is not added, only thin walled products can be manufactured, which has been a problem.

The present invention is intended to solve the above-mentioned problems in the prior art, and its purpose is to produce thick spheroidal graphite P without adding expensive alloys! The object of the present invention is to provide a method for increasing the strength of an iron base material by transforming it into bainite.

(Means for Solving the Problems) That is, the method for manufacturing high-strength spheroidal graphite cast iron of the present invention includes carbon 3.0 to 4.5%, silicon 1.5 to 3.5%,
Manganese 1.5% or less, magnesium 0.02-0.1
%, the balance consists of iron and impurities, 800 to 950
The austenitizing condition is heated at ℃ for 5 to 3 hours, and then the surface of the base material is rapidly cooled to a predetermined temperature or lower using a medium such as water or oil while the preheating remains inside the base material. After turning the vicinity of the base material surface into martensite, it was heated in a salt bath furnace for 21 hours.
The process is characterized by austempering conditions consisting of isothermal transformation conditions in which the inside of the base material is turned into bainite by holding the base material at 0 to 500°C for 20 minutes or more.

Carbon and silicon, which are components of the base material, work together to produce good castings during casting and cooling of spheroidal graphite cast iron, and the general content is 3.0 to 4.5% carbon by weight; Silicon is 1.5-3.5%.

Manganese affects the heat treatment of spheroidal graphite cast iron and promotes the formation of bainite in the cast iron, but if the content is too high, there is a risk of carbide formation, and the time required for bainization is increased, so the content is is preferably 1.5% or less by weight.

Magnesium is well known as an alloy component that produces spheroidal graphite, but if it is less than 0.02% by weight, a sufficient effect cannot be obtained, and it is not necessary to contain more than 0.1%.

Condition: Section A is performed by treating the base material containing the above-mentioned components under austempering conditions consisting of austenitization conditions and isothermal transformation conditions.

Under austenitizing conditions, the heating temperature is 800 °C
At temperatures below 950°C, a long time is required for austenitization, and on the other hand, at temperatures above 950°C, thermal energy is wasted.

Similarly, under the austenitizing conditions, the heating time was 0.
If the time is shorter than 5 hours, the base material will not become uniform austenite, and on the other hand, if it is more than 3 hours, the austenite crystal grains will become coarse, making it impossible to obtain stable retained austenite under isothermal transformation conditions, and the thermal energy will be too low. It will be wasted.

Therefore, the conditions for austenitizing are to heat the base material containing the specified components at a temperature of 800 to 950°C with a temperature of 0.5 to 3
Preferably, the mixture is heated for a period of time.

The austenitized base material is then rapidly cooled using a medium such as water or oil to turn the surface into martensite. At this time, the surface temperature is preferably 200 to 500°C or lower.

Subsequently, the substrate whose surface has been rapidly cooled is immersed in molten salt in a salt bath furnace while the substrate is sufficiently preheated inside. Most preferably, the substrate is placed in a salt bath furnace at an internal temperature of between 200 and 500°C.

Under isothermal transformation conditions, the temperature at which martensitic transformation of normal spheroidal graphite cast iron starts (Ms point) is around 200°C, so the lower limit temperature is preferably 210°C. On the other hand, if the temperature exceeds 500°C, the Pinkers hardness will be insufficient and mechanical properties such as strength will not improve.

Further, the immersion time in the molten salt is preferably 20 minutes or more.

Therefore, as isothermal transformation conditions, the base material after austenitizing treatment is heated to a temperature of 210 to 500'C in a salt bath furnace for 20 minutes.
It is preferable to hold the temperature for at least a minute.

Due to the austenitization described above, the surface of the base material becomes martensite and the inside becomes bainite. Martensite is very hard, so if more than 3A1m from the surface is martensite, problems will arise in product processing.

Therefore, it is preferable to use martensite for about 2 mm from the surface of the base material, and bainite for the inside.

('i!Example) The present invention will be explained in further detail in the following Examples. Note that the present invention is not limited to the following examples.

- A rod-shaped base material with a diameter of 70 II 1 mφ was cast from a molten metal containing 3.65% carbon, 2.6% silicon, 0.8% Manocan, 0.045% magnesium, and the balance iron and impurities in terms of K-film ratio. Next, this was heated at 900°C for 1 hour to turn it into austenite, and the surface was rapidly cooled to 210°C or lower using water while the preheating remained inside to turn it into martensite. Next, this base material was placed in a salt bath furnace and held at 360''C for 2 hours to turn the inside of the base material into bainite, thereby producing high-strength spheroidal graphite cast iron. In figure 0 showing the cooling curve,
1 is a cooling curve near the surface, which becomes a martensitic structure by passing through the Ms point. 2 is the internal cooling curve, and by rapid cooling, the material undergoes isothermal transformation into a haynite structure without becoming pearlite. FIGS. 2A and 2B are micrographs of the bainitic cast iron structure in high-strength spheroidal graphite cast iron obtained by the method of the present invention, in which 3 is spheroidal graphite and 4 is the base material.

31A shows a cooling curve in a conventional austempering process for the same base material as in the example. When the base material is rapidly cooled to the target temperature, the cooling curve 3 passes through the Bs point and the Bf point, and the bainite transformation is completed. However, in order to obtain such a cooling curve, it is necessary to reduce the thickness of the base material. As another method, it is necessary to add an alloy such as molybdenum or nickel to move the bainite nose to the long-term side. Cooling curve 3 shows normal isothermal transformation, but when the thickness of the base material is thick, in the conventional method, the inside of the base material is cooled as shown in cooling curve 4, so it becomes pearlite and has sufficient strength. I couldn't. FIG. 4 shows a micrograph of bainitic cast iron obtained by conventional austempering treatment.

As is clear from a comparison between Figure 2 CB) and Figure 4, the high-strength spheroidal graphite cast iron obtained by the method of the present invention has a denser bainitic structure than that obtained by the conventional austempering method. The tensile strength was improved by about 5 to 10%, and the fatigue strength was improved by about 2 to 8%.

(Effects of the Invention) As described above, the method for producing high-strength spheroidal graphite cast iron of the present invention is to austemper a base material containing each component in a predetermined ratio under predetermined conditions to martensite the vicinity of the surface of the base material, and Because it converts into bainite, thick, high-strength spheroidal graphite cast iron can be easily produced using conventional equipment without adding expensive alloys containing molybdenum or the like.

As a result, materials with excellent mechanical properties in various sizes and shapes can be supplied at lower cost than before, allowing them to be widely used in structural materials and mechanical parts.

This is effective in improving these qualities.

[Brief explanation of the drawing]

Figure 1 is a cooling curve diagram of the base material during austempering treatment in the method for producing high-strength spheroidal graphite cast iron of the present invention. Figure 2 is a micrograph of bainitic cast iron MlH in the high-strength spheroidal graphite cast iron obtained by the method of the present invention. , FIG. 3 is a cooling curve diagram of a base material in a conventional austempering treatment of spheroidal graphite cast iron, and FIG. 4 is a micrograph of a bainitic cast iron structure obtained by a conventional method. In the figure.

Claims (1)

[Claims] (1) Carbon 3.0-4.5%, silicon 1.5-3% by weight
．． 5%, manganese 1.5% or less, magnesium 0.02
A base material consisting of ~0.1%, the balance iron and impurities,
The austenitizing condition is heating at ~950°C for 0.5 to 3 hours, and then the surface of the base material is rapidly cooled to a predetermined temperature or lower using a medium such as water or oil while the preheat remains inside the base material. After turning the vicinity of the surface of the base material into martensite, it is treated under austempering treatment conditions consisting of isothermal transformation conditions in which the inside of the base material is turned into bainite by holding it at 210 to 500 ° C. for 20 minutes or more in a salt bath furnace. A method for manufacturing high-strength spheroidal graphite cast iron.