JPS6233746A - Heat-resistant cast steel - Google Patents

Abstract

PURPOSE:To provide castability and workability which are well-balanced with heat resistance and durability characteristics, by selectively adding Nb, Ta, V, Mo and W to a steel containing specific amounts of C, Si, Mn, P, S and Cr and by carrying out casting and then annealing treatment. CONSTITUTION:The steel consisting of, by weight, 0.3-2.0% C, 2.0-4.5% Si, <=1.0% Mn, <=0.05% P, <=0.05% S, 5.0-10.0% Cr, 1.0-6.0% of >=1 kind among Nb, Ta and V, 0.2-1.0% Mo and/or W and the balance essentially Fe is refined. The molten steel is cast and subjected to ordinary annealing treatment so as to decompose a pearlite structure precipitating in as-cast condition to spheroidize carbides. This steel combines castability, workability and low-cost characteristic with resistance to heat, heat checks and oxidation, so that it can be suitably used for material for exhaust-system parts of vehicle engines and the like.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to heat-resistant cast steel, and more specifically, it has performance and durability characteristics such as excellent heat resistance (high temperature strength), heat cracking resistance, oxidation resistance, etc. It is a heat-resistant cast steel that can be suitably applied to exhaust system parts in vehicle engines because it has production characteristics such as excellent castability 9 machinability and low cost.

[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 system parts such as exhaust manifolds for automobile engines, Tahif/%1 housings for turbochargers, pre-combustion chambers for diesel engines, and parts for exhaust gas purification devices are used under high-temperature conditions, so heat cracking resistance and oxidation resistance are required. Since the requirements for properties such as properties are particularly severe, conventionally heat-resistant cast irons such as high-Si cast iron, Niresist cast iron, and AI cast iron, and in special cases, expensive high-alloy heat-resistant cast steels such as ferritic or austenitic heat-resistant cast steels have been used. He had been hired.

[Problem that the invention seeks to solve]

In view of the current state of the conventional technology as described above, the problem to be solved by the present invention is that high-Si cast iron, which is used as a material for exhaust system parts in conventional automobile engines,
Heat-resistant cast irons such as Niresist cast iron and AI cast iron have good productivity due to their excellent castability and machinability, but their performance and durability characteristics such as heat resistance (high temperature strength), heat cracking resistance, oxidation resistance, etc. Heat resistance (high temperature strength) and heat cracking resistance at high temperatures of 800°C or higher.

It cannot be used for parts with strict requirements for oxidation resistance, etc., and high-alloy heat-resistant cast steel has poor performance such as heat resistance (high-temperature strength), heat cracking resistance, and oxidation resistance even at high temperatures of 800°C or higher.・Although it has excellent durability, it has poor castability and is prone to casting defects such as "sinkage cavities" and "poor hot water circulation" during casting. 1. Productivity is poor due to poor machinability, etc. Therefore, as a heat-resistant component, it has production characteristics such as excellent castability, machinability, and low cost, as well as performance and durability characteristics such as excellent heat resistance (high temperature strength), heat cracking resistance, and oxidation resistance. There was a strong desire to develop a heat-resistant casting material that has both of these characteristics in a well-balanced manner.

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. The objective is to create a heat-resistant cast steel that has a well-balanced combination of production characteristics such as these, and performance and durability characteristics such as heat resistance (high-temperature strength), heat cracking resistance, and oxidation resistance that are comparable to conventional high-alloy heat-resistant cast steel.

[Means for solving problems]

In view of such problems in the conventional technology, the present invention has a solution to solve the problems in the conventional technology by using a weight ratio of C: 0.3 to 2.0% and Si: 2.0%. ~
4.5%, Mn: 1.0% or less, P: 0°05% or less,
S, 0.05% or less, Cr; 5.0 to 10.0%, N
At least one of b, Ta, and V from 1.0 to 6.0
% and at least one of Mo and W in an amount of 0.2 to 1.0%, with the remainder substantially consisting of Fe, and is characterized by being annealed after casting. Made of heat-resistant cast steel.

Note that the annealing treatment after casting of the heat-resistant cast steel of the present invention includes furnace cooling to 680 to 750°C after austenitization treatment of 900 to 900 b.
680-b A normal annealing process of cooling is sufficient.

[Effect]

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

In the present invention, by using the above-mentioned structure as a means for solving the problems of the conventional technology, impurities such as P and S can be kept low in terms of composition in heat-resistant cast steel, and C
, S1. Cr and Nb.

By adding at least one of Ta and V and at least one of Mo and W in a well-balanced manner, heat resistance (high temperature strength), heat cracking resistance, and machinability are improved, making it possible to improve the heat resistance of conventional heat-resistant cast iron. In addition to having excellent production characteristics such as comparable castability 2 machinability and low cost, it has properties such as heat resistance (high temperature strength), heat cracking resistance, and oxidation resistance that are close to those of conventional high-alloy heat-resistant cast steel.・The range is within which durability characteristics can be imparted, and in addition, the base structure is made into a ferrite structure through annealing treatment after casting, and the Cr concentration in the base ferrite structure is increased by decomposing primary carbides. Therefore, the heat-resistant cast steel of the present invention has production characteristics such as castability, machinability, and low cost comparable to conventional heat-resistant cast iron, and heat resistance (high-temperature strength) comparable to conventional high-alloy heat-resistant cast steel. Performance such as heat crack resistance, oxidation resistance, etc.
This makes it possible to create heat-resistant cast steel that has a well-balanced combination of durability and durability.

Hereinafter, the reason for setting the range of the amount of each alloying element added to the heat-resistant cast steel of the present invention will be explained.

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

First, C is effective because it improves the strength properties of the heat-resistant cast steel of the present invention and improves the fluidity (castability) of the molten metal, but if it is less than 0.3%, the effect of improving castability is not sufficient; On the other hand, if added in excess of 2.0%, Si
From a balance with the amount, it is determined to be 0.3 to 2.0% because it not only promotes graphitization of carbon and deteriorates the strength characteristics of heat-resistant cast steel, but also deteriorates oxidation resistance.

In addition, Si is effective not only as a deoxidizer in the heat-resistant cast steel of the present invention but also to improve oxidation resistance, but if it is less than 2.0%, the effect of improving oxidation resistance is insufficient. However, if it is added in excess of 4.5%, (1) the balance with C (carbon equivalent) will coarsen primary carbides and deteriorate the machinability of heat-resistant cast steel.

■ The Si concentration in the ferrite base structure becomes excessive,
It reduces the toughness of heat-resistant cast steel and worsens productivity.

For these reasons, the content was set at 2.0 to 4.5%.

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 base structure like the material of the present invention, but it is effective as a deoxidizing agent like Si. Also, since it is effective as an alloying element that improves "metal flowability" during casting and improves productivity, it is desirable to contain it in a range of 1.0% or less.

Furthermore, if both P and S are added in excess of 0.05%, thermal cracks are likely to occur due to repeated heating and cooling.
Since these are harmful impurities, they were all set at 0.05% or less.

In addition, Cr is effective because it improves oxidation resistance and increases the eutectoid transformation temperature to improve heat resistance properties (high temperature strength, heat crack resistance, etc.), but if it is less than 5.0%, these properties deteriorate. Since the improvement effect is not sufficient and adding more than 10.0% causes coarsening of Cr primary carbides and significantly deteriorates machinability, the content was set at 5.0 to 10.0%.

In addition, Nb, Ta, and V are particularly important alloying elements in the material of the present invention, and they all have similar effects, and are effective in increasing the eutectoid transformation temperature and improving the heat resistance properties as described above. Not only this, but also because carbides are formed preferentially over Cr during casting, the formation of primary Cr carbides that deteriorate machinability is suppressed, and the precipitation of secondary Cr carbides at high temperatures is also suppressed. It is also effective in improving oxidation resistance, but if it is less than 1.0%, the effect of improving these properties is not sufficient. On the other hand, if it is added in excess of 6.0%, Nbl Ta and V will not be dissolved. Since it segregates and remains in the matrix structure and tends to reduce the toughness of heat-resistant cast steel, it is set at 1.0 to 6.0%.

In addition, Mo and W are also particularly important alloying elements in the material of the present invention, and both have similar effects.They dissolve in solid solution in the matrix structure and improve the heat resistance (high temperature strength) of heat-resistant cast steel. It is not only effective because it increases the eutectoid transformation temperature and improves heat resistance properties like Nb, Ta, V, etc., but also deteriorates machinability because it forms carbides preferentially over Cr during casting. It is effective in improving oxidation resistance because it suppresses the formation of primary Cr carbides and also suppresses the precipitation of secondary Cr carbides at high temperatures.
If it is less than 2%, the effect of improving these properties is not sufficient;
On the other hand, if it is added in excess of 1.0%, it will coarsen the primary carbide and deteriorate machinability.
%.

Note that annealing treatment after casting is not only necessary to improve the machinability of heat-resistant cast steel, but also to decompose the pearlite structure that precipitates in the as-cast state and make carbides spheroidal. is also valid.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail based on the accompanying drawings and tables.

In order to evaluate the heat cracking resistance and oxidation resistance of the present invention material,
20 types of the present invention materials A-T as shown in Table 1, and
Three types of comparative materials (1) to (2) were manufactured by casting.

In addition, when casting each test material, the Fe-3i (75% by weight)
After deoxidizing treatment at
Molten metal was poured at a temperature of 450°C or higher and cast into a Y block shape of JIs standard No. A.

Next, each test material in the form of a Y-prosock-shaped cast rough shape that was cast as described above was heated and held at 950°C for 1 hour in an annealing furnace, and then cooled to 750°C. An annealing process was performed in which the sample was held at that temperature for 3 hours and allowed to cool.

Table 1 Table 1 (Continued from previous page) In Table 1, P and S in the invention material A-T and the comparative material ■ are both 0.01% or less, and in the comparative material ■ P is 0.02%.

S is 0.022%, and P in comparative material ■ is 0°0
23%, and S is 0.021%.

In addition, comparative material ■ and comparative material ■ have a temperature of 0°033, respectively.
% and 0.035% Mg.

Moreover, in Table 1, the present invention materials A-E are C251. M
These are test materials in which Nb and MO are added to the basic composition consisting of n, P, S, and Cr, and the present invention materials F to J are test materials in which ■ and MO are added to the above-mentioned basic composition. −0 to material
is a sample material in which Nb and W are added to the above-mentioned basic composition, and the present invention materials P to T are test materials in which (1) and W are added to the above-mentioned basic composition.

In addition, comparative materials ■ and ■ in Table 1 are heat-resistant cast irons conventionally used in heat-resistant parts for automobiles, comparative material ■ is what is called high-Si cast iron, and comparative material ■ is austenite-based cast iron. Comparative material (1) is austenitic heat-resistant cast steel (equivalent to JIS standard 5CH21).

Evaluation tests for heat cracking resistance and oxidation resistance were conducted using each sample material having the composition shown in Table 1, which was manufactured by casting as described above.

First, using an electro-hydraulic servo type thermal fatigue testing machine,
A heat cracking resistance evaluation test was conducted for each of the above-mentioned test materials.

In addition, in the heat crack resistance evaluation test, the gauge distance was 15fl.
Using a round bar test piece with a gage diameter of φLow, one cycle was conducted with a lower limit temperature of 200°C and an upper limit temperature of 900°C, with the thermal expansion and elongation due to heating of the test piece mechanically restrained. Thermal fatigue failure was caused by repeated heating and cooling cycles for 10 minutes.

Note that the restraint rate (%) is a value calculated by the following formula.

Then, the heating and cooling cycle applied to each test piece was fixed to the above conditions, and the restraint rate (%) of each test piece was changed, and the restraint rate (%) of each specimen was repeated until failure due to thermal fatigue. The heat cracking resistance (
It thermal fatigue strength) was evaluated.

The results of the heat crack resistance evaluation test conducted in this way are
It is shown in FIGS. 1 to 4.

As is clear from Figs. 1 to 4, the present invention material A-T has a significantly superior thermal fatigue life (heat cracking resistance ).

Next, a plate-shaped test piece with a shape of 3 QmX 2 QmX 5 mm was manufactured using each of the cast specimens having the above-mentioned composition, and an oxidation test was performed by holding it in the atmosphere at 900°C for 100 hours. carried out.

In addition, as a method for evaluating the test results in this oxidation test, the test piece after the oxidation test is subjected to shot blasting to remove oxide scale, and the weight change per unit area before and after the oxidation test (oxidation loss i mg/Cm 2)
Evaluated by.

The results of the oxidation test as described above are shown in FIG.

As is clear from Fig. 5, the present invention materials A-T have significantly superior oxidation resistance compared to the conventional materials (comparative materials) - ■, and are also austenitic heat-resistant cast steels. Even when compared with a certain comparative material (■), it is understood that it has the same or higher oxidation resistance.

Next, 3. using the present invention material and comparative material. An exhaust manifold for an Oj' gasoline engine was manufactured, mounted on the engine, and subjected to a severe on-board engine durability test to evaluate the heat cracking resistance of the exhaust manifold.

Table 2 shows the chemical components of each exhaust manifold sample used in this on-board severe durability test.

Note that the comparative product used in the heat crack durability test for this exhaust manifold in Table 2 is an exhaust manifold manufactured using a high-Si spheroidal graphite cast iron material.

In addition, the products of the present invention ■ to ■ are all 950°C x 1 hour + 7
An exhaust manifold that had been annealed at 50°C for 1 hour was used, and an as-cast exhaust manifold was used for the comparative product.Table 2 In addition, in Table 2, P and S is 0.01% or less in all cases, and P in the comparative product
is 0.025%, and S is 0.022%.

Moreover, the comparative product contains 0.035% Mg.

The severe on-board engine durability test was conducted under full load conditions with a maximum rotational speed of 5,600 rpm for up to 900 cycles of cold and hot cycles. evaluated.

As a result of evaluating the heat cracking resistance of the exhaust manifold through a severe on-board engine durability test,
In the exhaust manifolds manufactured using the inventive material (inventive products ■ to ■), no thermal cracks were observed until the end of the 900-cycle test, whereas the comparative product made of high-Si spheroidal graphite cast iron In the exhaust manifold manufactured using this material, thermal cracks penetrating the wall thickness were observed to occur after 386 cycles.

As is clear from the results of the above-mentioned engine stand severe durability test, the exhaust manifold manufactured using the material of the present invention (
It has been confirmed that the products of the present invention (■ to ■) have significantly superior thermal cracking durability as an exhaust manifold that is subject to severe heat loads, compared to exhaust manifolds manufactured using high-Si spheroidal graphite cast iron, which is a comparative material. We were able to.

Next, a precombustion chamber for a 2.41 diesel turbo engine was manufactured using the present invention material and a comparative material, and the heat cracking resistance of the precombustion chamber was evaluated by a severe on-board engine durability test.

Table 3 shows the chemical components of each sample of the diesel engine pre-combustion chamber used in this engine bench severe durability test.

The comparative product used in the heat-resistant crack durability test as a pre-combustion chamber for a diesel engine in Table 3 is a pre-combustion chamber manufactured from austenitic heat-resistant cast steel (equivalent to JIS standard 5CH21).

In addition, the products of the present invention ■ to ■ are all 950°C x 1 hour +
A pre-combustion chamber that had been annealed at 750° C. for 1 hour was used, and a comparative product used an as-cast pre-combustion chamber.

Table 3 In addition, in Table 1, P and S in the present invention products ① to ② and the comparative product are all 0.01% or less.

The conditions of the severe on-board engine durability test were as follows: The maximum rotational speed was 440 Orpm, and a cold/hot cycle durability test was conducted at full load for up to 900 cycles. evaluated.

As a result of evaluating the heat cracking resistance of the pre-combustion chamber through a severe on-board engine durability test, it was found that the pre-combustion chambers manufactured from the materials of the present invention (products of the present invention ■ to ■) were subjected to a test of 900 cycles. No thermal cracks were observed until the completion of the combustion process, whereas in the pre-combustion chamber manufactured using austenitic heat-resistant casting &I (equivalent to JIS standard 5CH21), which is a comparison product, a length of 211 mm was observed in the nozzle hole after 600 cycles. Some degree of thermal cracking was observed.

As is clear from the results of the above-mentioned engine stand severe durability test, the pre-combustion chambers manufactured from the materials of the present invention (products of the present invention ■ to ■
) is austenitic heat-resistant cast 11 (JI
Compared to a pre-combustion chamber manufactured according to S standard 5CH21 (equivalent to S standard 5CH21), we were able to confirm that it has excellent thermal cracking resistance even when used in the pre-combustion chamber of a diesel engine with severe heat loads. .

〔Effect of the invention〕

As is clear from the above, according to the heat-resistant cast steel of the present invention, the compositional adjustment of the heat-resistant cast steel and the implementation of annealing treatment after casting result in castability, machinability, and comparable properties to conventional heat-resistant cast iron. To create a heat-resistant cast steel that has a well-balanced combination of production characteristics such as low cost, and performance and durability characteristics such as heat resistance (high temperature strength), heat cracking resistance, and oxidation resistance that are comparable to conventional high-alloy heat-resistant cast steel. Therefore, from the viewpoint of price and performance, it has the advantage that it can be suitably used as a material for exhaust system parts in automobile engines.

[Brief explanation of the drawing]

FIGS. 1 to 4 are graphs showing the results of evaluation tests of heat cracking resistance of the present invention material and comparative material. FIG. 5 is a graph showing the oxidation test results of the present invention material and comparative material. Applicant: Toyota Motor Corporation Ishi 1tlH-'s car story ((times), Figure 1 gLtljτ゛'s light times) Figure 3

Claims (1)

[Claims] 1. Weight ratio: C: 0.3-2.0%, Si: 2.0
% to 4.5%, Mn: 1.0% or less, P: 0.05% or less, S: 0.05% or less, Cr: 5.0 to 10.0%, among Nb, Ta, and V. At least one type from 1.0 to 6
．． Contains 0% and at least one of Mo and W
1. A heat-resistant cast steel, characterized in that it contains 0.2 to 1.0% of Fe, the remainder substantially consists of Fe, and is annealed after casting.