JPS6233744A - Heat-resistant cast steel - Google Patents

Abstract

PURPOSE:To provide castability and workability which are well-balanced with resistance to heat and oxidation, by subjecting a steel having each specified content of C, Si, Mn, P, S, Cr and B to casting and then annealing treatment. CONSTITUTION:The steel consisting of, by weight, 0.5-2.0% C, 1.5-3.5% Si, <=0.7% Mn, <=0.05% P, <=0.1% S, 5.0-10.0% Cr, 0.1-3.8% B and the balance essentially Fe is refined, which is subjected to deoxidizing treatment, to casting and then to annealing treatment by the ordinary method, by which carbides in a graphite-free ferritic matrix structure are formed into primary carbides crystallizing out in dendritic condition and secondary carbides finely dispersed and precipitated. This steel has durability characteristics such as heat resistance etc., excellent in castability and machinability and capable of inexpensive manufacture, so that it can be suitably used for exhaust system parts of vehicle engines or the like.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to heat-resistant cast steel, and more specifically, to excellent heat resistance (high-temperature strength, particularly creep rupture strength).

It has performance and durability characteristics such as oxidation resistance, as well as excellent castability and machinability, resulting in good productivity and can be manufactured at low cost, making it a popular choice for vehicle engines. This applies to heat-resistant cast steel that can be suitably used for exhaust system parts, etc.

[Conventional technology]

In recent years, in vehicle engines such as gasoline engines or diesel engines, especially automobile engines,
With the increasing demand for improvements in higher output and lower fuel consumption,
Research and development is actively being carried out to improve combustion efficiency.

As a result, in automobile engines that meet such demands, the exhaust gas temperature tends to be significantly higher than that of conventional automobile engines.

In particular, exhaust manifolds for automobile engines and turbine wheels for turbochargers.

Conventionally, high-Si cast iron has been used for exhaust system parts such as turbine housings for turbochargers and pre-combustion chambers for diesel engines, as the requirements for heat resistance (high temperature strength) and oxidation resistance are particularly severe. , Niresist cast iron, AI cast iron, and other heat-resistant cast irons, and in special cases, expensive high-alloy heat-resistant (stainless steel) cast steels such as ferritic or austenitic stainless steel cast steels, Co-based alloys, and N-based alloys.
An i-based alloy was used.

[Problem that the invention seeks to solve]

In view of the current state of the conventional technology as described above, the problem that the present invention attempts to solve is that of high-Si cast iron, Niresist cast iron, AI cast iron, etc., which are used as materials for exhaust system parts of conventional automobile engines. Although heat-resistant cast iron has good productivity characteristics such as excellent castability and machinability, it has poor heat resistance (high-temperature strength, especially creep rupture strength).

Due to inferior performance and durability characteristics such as oxidation resistance,
It cannot be used for parts that have strict requirements for heat resistance at temperatures of 800°C or higher, and high-alloy heat-resistant cast steels such as stainless steel have high heat resistance (high-temperature strength, especially creep rubber) even at temperatures of 800°C or higher. Although it has excellent performance and durability characteristics such as char strength) and oxidation resistance, it has poor castability and may cause "sink cavities" during casting molding.
Easy to cause casting defects such as "poor hot water circulation" 2. Productivity is poor due to poor machinability, etc., and excellent castability as a heat-resistant component 9. Productivity characteristics such as machinability, low cost, etc. There was a strong desire to develop a heat-resistant casting material that has a well-balanced combination of performance and durability characteristics such as , excellent heat resistance (high-temperature strength, especially creep rupture strength), and oxidation resistance. .

Therefore, the technical problem of the present invention is to improve castability, machinability, and low cost comparable to conventional heat-resistant cast iron by adjusting the composition of heat-resistant cast steel and performing annealing treatment after casting. and heat resistance (high temperature strength, especially creep rupture strength) comparable to conventional high-alloy heat-resistant cast steel.

The goal is to create a heat-resistant cast steel that has a well-balanced combination of performance and durability characteristics such as oxidation resistance.

[Means for solving problems]

In view of such problems in the conventional technology, the present invention provides means for solving the problems in the conventional technology by using a weight ratio of Cio: 5 to 2.0%, Si: 1.5% to 3.
．． 5%, Mn; 0.7% or less, p, o.

05% or less, S; 0.1% or less, Cr; 5. O~1
0°0%, B: 0.1-3.8%, the remainder has a composition consisting essentially of Fe, and by annealing after casting, the carbides in the ferrite matrix structure that does not contain graphite are transformed into dendrites. It is made of heat-resistant cast steel characterized by primary carbide crystallized in a shape and granular secondary carbide precipitated in a finely dispersed manner.

In addition, in the heat-resistant cast steel of the present invention, the heat treatment after casting is 9
00~b After tenitizing treatment, furnace cooling to 680~750°C, 68
A normal annealing treatment of 0 to b is sufficient.

[Effect]

Hereinafter, the effects of the present invention will be explained.

In the present invention, by configuring the above-mentioned means for solving the problems of the conventional technology, the heat-resistant cast steel of the present invention can be made heat-resistant due to the composition, especially the balance of B addition and other alloy elements. Characteristics In particular, by improving the creep rupture strength and maintaining productivity characteristics such as castability, machinability, and low cost comparable to conventional heat-resistant cast iron, we can produce high-alloy heat-resistant cast steel such as conventional stainless steel cast steel. Heat resistance close to (
This is a range that can provide performance and durability characteristics such as high-temperature strength (especially creep lubrication strength) and oxidation resistance, and furthermore, the annealing treatment after casting transforms the base structure into a ferrite structure and transforms the base structure into a ferrite structure. The Cr content in the base ferrite structure is increased by decomposition of
It has productivity characteristics such as low cost, and performance and durability characteristics such as heat resistance (high temperature strength, especially creep rupture strength) and oxidation resistance that are comparable to conventional high-alloy heat-resistant cast steel.
This makes it possible to create heat-resistant cast steel with a well-balanced combination of properties.

The reason for limiting the range of the amount of each alloying element added to the heat-resistant cast steel of the present invention will be explained below.

In the following description, the amount of each alloying element added is expressed in percent by weight.

First, C is effective in improving the strength properties and castability of the heat-resistant cast steel of the present invention, but if it is less than 0.5%, the effect of improving these properties is not sufficient;
The addition amount exceeds 0.5% to 2.0% because it promotes graphitization of carbon and also reduces the strength characteristics of heat-resistant cast steel due to the balance with the B content.

In addition, Si is effective not only as a deoxidizing agent in the heat-resistant cast steel of the present invention, but also because it improves castability and oxidation resistance, but if it is less than 1.5%, it has no effect on improving these properties. If it is not sufficient and is added in excess of 3.5%, (1) the balance with C (carbon equivalent) will cause primary carbides to become coarse and deteriorate the machinability of heat-resistant cast steel.

(2) Due to the balance with the B content, the Si content in the ferrite matrix structure becomes excessive, reducing the toughness of heat-resistant cast steel and deteriorating productivity.

For these reasons, the content was set at 1.5 to 3.5%.

Furthermore, since Mn is an element that forms a pearlite structure, it is an alloying element that is not very preferable for heat-resistant cast steel with a ferrite structure as a base structure, such as the material of the present invention.
Similarly, it is effective as a deoxidizing agent, and it is also effective as an alloying element that improves "flowability" during casting and improves productivity, so it is recommended to contain it in a range of 0.7% or less. desirable.

Further, if P is added in an amount exceeding 0.05%, it promotes the formation of pearlite structure in the matrix structure and promotes the crystallization of steadite, so it is desirable that the content is 0.05% or less.

S is usually not a particularly essential alloying element, but when manufacturing parts with strict machinability requirements, increasing the amount of Sil and Mn added to crystallize MnS and machining The content was set at 0.1% or less because it can improve properties.

Also, like Si, Cr is effective because it improves oxidation resistance, but if it is less than 5.0%, the effect of improving oxidation resistance is not sufficient, and if it is added in excess of 10.0%, it will contain B. The content was determined to be 5.0 to 10.0% because the amount of precipitation of high-hardness Cr carbides, Cr borides, etc. increases, which significantly deteriorates machinability.

In addition, B is a particularly important alloying element in the material of the present invention, and is not only particularly effective in improving creep rupture strength through grain boundary strengthening, but also effective in improving grain boundary oxidation resistance of heat-resistant cast steel. However, if it is less than 0.1%, the effect of improving these properties is not sufficient, and if it is added in excess of 3.8%, it increases the hardness of the heat-resistant cast steel and significantly deteriorates the machinability. ~3.8%.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on the accompanying drawings.

In order to evaluate the creep-loveture strength characteristics and oxidation resistance of the inventive materials, three types of inventive materials as shown in Table 1 were used.
~■ and four types of comparative materials ■~■ were manufactured by casting molding.

In addition, during casting, melting is carried out in the atmosphere using a 20 kg high-frequency liquid melting furnace, deoxidized by adding 0.1% metal aluminum, and then tapped at 1600°C or higher.
The hot water was poured at 450°C or higher.

Further, as a mold for casting, a mold for Y block casting according to JIS standard A was used.

Then, a normal annealing treatment was performed on each test material in the state of a cast rough shape that was cast as described above.

Table 1: Comparative materials ■ and ■ in Table 1 are heat-resistant cast irons, and the comparative materials are so-called high-St cast irons, which are spheroidal graphite cast irons with 4.0% by weight of St added. Comparative material (3) is so-called Ni-resist cast iron to which Ni and Cr are added.

Comparative materials (■) and (2) are stainless steel cast steel, comparative material (2) is JIS standard 5C3I (ferritic stainless steel cast steel), and comparative material (■) is JIS standard SC3I3 (austenitic stainless steel cast steel).

Hereinafter, the results of comparative evaluation of creep rupture strength and oxidation resistance of each test material of the present invention material and comparative material will be explained.

First, a creep-loveure test was conducted at 700° C. using test pieces of each sample material having the above-mentioned compositions manufactured by casting.

In addition, the creep loveture test conditions are such that the test temperature is 70
The temperature was fixed at 0° C., and the stress applied to the test piece was varied to measure the time until creep rupture.

The test results are shown in FIG.

In addition, in Figure 1, the vertical axis shows the applied stress (Kg/mm2).
, the time until creep-loveture rupture is shown on a logarithmic scale on the horizontal axis.

As is clear from Fig. 1, the present invention materials (■) and (2) have significantly superior creep rupture strength when compared with the comparative material (■) of Niresist cast iron, and when compared with the comparative materials (■) and (2) of stainless steel cast iron. It is understood that it has an excellent creep rupture strength that is equivalent or higher.

Next, an oxidation test was carried out by holding in the atmosphere at 900° C. for 100 hours using test pieces of each sample material having the above-mentioned composition manufactured by casting.

The measurement results of the oxidation loss of each sample material in the oxidation test are shown in FIG.

As is clear from Fig. 2, the materials of the present invention
It has excellent oxidation resistance equivalent to Standard 5C3I (Ferritic stainless steel cast steel - Comparative material ■) and JIS Standard 5C313 (Austenitic stainless steel cast steel -1-- Comparative material ■) be understood.

Of course, the conventional heat-resistant cast iron, high St cast iron (comparative material ■
) and Niresist cast iron (comparison material ■), it goes without saying that it has much better oxidation resistance.

Next, a 6-cylinder exhaust manifold for a 3000 cc class gasoline engine was manufactured by casting using the material of the present invention using the same casting method as that normally used for casting high-Si cast iron. ．“Pinhole”, “Blowhole”.

It was confirmed that there were no casting defects such as "sand build-up", "poor hot water circulation", and "hot water failure", and that the product had excellent productivity.

As mentioned above, the heat-resistant cast steel of the present invention has productivity characteristics such as castability, machinability, and low cost comparable to conventional heat-resistant cast iron, and heat resistance (high-temperature strength, especially creep resistance) comparable to conventional stainless steel cast steel. It is understood that the material has a well-balanced combination of performance and durability characteristics such as Loveture strength) and oxidation resistance.

〔Effect of the invention〕

As is clear from the above, the heat-resistant cast steel according to the present invention has castability, machinability, and machinability comparable to conventional heat-resistant cast iron by compositional adjustment of the heat-resistant cast steel and implementation of annealing treatment after casting. Productivity characteristics such as low cost,
It has the advantage of being a heat-resistant cast steel that has a well-balanced combination of performance and durability characteristics such as heat resistance (high-temperature strength, especially creep rupture strength) and oxidation resistance comparable to conventional high-alloy heat-resistant cast steel.

[Brief explanation of drawings]

FIG. 1 is a graph showing the results of a creep-loveture test at 700°C for the present invention material and comparative material. FIG. 2 is a graph showing the results of comparing the oxidation resistance at 900°C between the present invention material and the comparative material.

Claims (1)

[Claims] 1. Weight ratio: C: 0.5-2.0%, Si: 1.5
% to 3.5%, Mn; 0.7% or less, P; 0.05% or less, S; 0.1% or less, Cr; 5.0 to 10.0%, B
:0.1 to 3.8%, with the remainder essentially consisting of Fe, and by annealing after casting, the carbides in the ferrite matrix structure, which does not contain graphite, are crystallized in the form of dendrites. A heat-resistant cast steel characterized by having primary carbides and granular secondary carbides finely dispersed and precipitated.