JP3142224B2 - Ferritic heat-resistant cast steel and diesel engine pre-combustion chamber member using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant ferritic cast steel excellent in heat crack resistance and a preheating furnace member for a diesel engine using the same.

[0002]

2. Description of the Related Art A preheating furnace type diesel engine has a preheating chamber 4 in a cylinder head 2 as shown in FIG. 1, for example. The pre-combustion chamber 4 is provided with a chamber 3, which is a pre-combustion chamber member having an injection port 5, a fuel injection nozzle 6, and a glow plug 7. The air compressed by the piston 1 flows into the pre-combustion chamber through the injection port 5 of the chamber 3 to generate a high-pressure vortex. In this state, fuel is injected from the fuel injection nozzle 6 and ignited. . By the explosion due to the ignition, high pressure is applied to the upper surface of the piston 1 through the nozzle 5 to push down the piston 1. Therefore chamber 3
Results in a very severe heat load. Especially in recent years, due to the high rotation speed accompanying the generalization of the mounting of the supercharger by the turbocharger or the rise of the combustion temperature due to the exhaust gas regulation, the vicinity of the injection port of the chamber may partially exceed 900 ° C. Materials with excellent heat resistance and heat crack resistance are required.

Conventionally, such a chamber is made by a lost wax precision casting method, a machining process, or a forging method. Due to its required characteristics, the material is martensitic heat-resistant steel such as SUH616 and austenitic heat-resistant steel such as SUS310S and LCN155. Alternatively, a Ni-based super heat-resistant alloy has been used. Recently, as described in Japanese Patent Application Laid-Open No. 63-47346, a composite material in which the periphery of an injection port having a high thermal load is formed of a Ni-base super heat-resistant alloy, and the other portions are formed of a martensitic heat-resistant steel, The use of is also considered.

Attempts have also been made to use ferritic heat-resistant cast steel as the above-mentioned chamber material. For example,
In Japanese Patent Publication No. 46-18845, C0.05-0.4
%, Si 0.5-1.0%, Mn 0.2-1.0%, C
r20-23%, Mo0.5-2.5%, W0.5-
A material with 3.5%, Nb 0.5 to 3.5% balance Fe is known. This heat-resistant ferritic cast steel has excellent machinability and castability, and is effective as a material having high heat crack resistance.

[0005]

The chamber of the preheating furnace needs to have excellent heat resistance as described above.
Since heating and cooling are constantly repeated, a heat crack may occur in a thin portion around the injection port, which may lead to a leakage of hot gas and damage to the cylinder head. To minimize this, heat-resistant austenitic steels and Ni-base super heat-resistant alloys with excellent hot strength are used, but because of their large coefficient of thermal expansion and low thermal conductivity, they can be used during heating and cooling. However, there was a problem that sufficient internal heat stress could not be obtained and sufficient heat crack resistance could not be obtained due to extreme heat storage. Martensitic heat-resistant steel has low oxidation resistance and α → γ
It is difficult to exhibit sufficient performance due to expansion and contraction at the transformation point.

The above-mentioned composite material has both the heat resistance and oxidation resistance of a Ni-base superalloy and the high thermal conductivity of martensitic stainless steel, and has excellent heat crack resistance. There is a problem. In addition, ceramic materials have problems in practical use because of their toughness and poor workability. Further, in the conventional ferritic heat-resistant steel, the crystal grain size around the injection hole is large and the crystal grain boundary is weak, so that there is a tendency to promote the propagation of heat cracks. SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a new heat-resistant ferritic cast steel and a diesel engine pre-combustion chamber having improved machinability and inexpensive ferritic heat-resistant cast steel, which has improved oxidation resistance, high-temperature strength and heat crack resistance. An object is to provide a member.

[0007]

The present inventor has focused on a heat-resistant ferritic cast steel having high heat crack resistance. In order to further improve the heat crack resistance, the steel has both high temperature strength and oxidation resistance. , Nb, and Cr were found to be achievable by optimizing the amounts, and the present invention was reached. That is, in the present invention, C: 0.15 to 0.3% or less by weight,
i: 2.0% or less, Mn: 1.0% or less, Cr: 15 to
25%, Nb: more than 2.0% and 3.0% or less, balance Fe
And Nb% / C% = 10
No. 15 is a heat-resistant ferritic cast steel in which Nb is dissolved in a ferrite matrix. Also preferably, Ni is contained in an amount of 0.1 to 5.0%. Further, the content of Mo and / or W may be 3.0% or less.

Further, the present invention is a preheating furnace member for a diesel engine, which is formed of the above-mentioned heat-resistant ferritic cast steel, and preferably has a crystal grain size on the side of the injection port which is smaller than a crystal grain size inside the member. Smaller than

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One of the most significant features of the present invention is that the amounts of C, Nb and Cr in a heat-resistant ferritic cast steel are optimized. The basic technology is that first, C exceeds 0.15 to 0.3% by weight and Nb exceeds 2.0%.
The content of Nb is set to be as high as 0% by weight, and the amount of Nb is significantly increased more than the stoichiometric compound of Nb and C, that is, Nb / C = 7.73, which is a weight ratio of Nb / C required for formation of NbC.
If b / C is set to 10 or more and the amount of Nb that does not become carbide is sufficiently secured, remarkable refinement of crystal grains can be achieved,
In addition, they have found that the ferrite matrix can be strengthened.

If C is less than 0.15%, the improvement of heat crack resistance by strengthening the grain boundary by the compound of Nb and C is not sufficient, and the flowability of the molten metal is deteriorated, and the castability is deteriorated. Was set to 0.15%. C is 0.3
If it exceeds 0%, a martensite structure is formed, and oxidation resistance and
In addition to deteriorating corrosion resistance and workability, in an alloy composition containing a large amount of Nb, the amount of NbC generated becomes too large, and castability is deteriorated.

As described above, Nb is one of the most important elements for ensuring the heat crack resistance of the present invention.
In the present invention, the amount of Nb required to produce a compound of NbC, that is, Nb is remarkably increased (Nb / C: 10 to 15) than Nb / C = 7.73 by weight ratio. . Thus, in the present invention, Nb is dissolved in a ferrite matrix to increase the high-temperature strength and the thermal fatigue strength, and to improve the heat crack resistance. Further, as described above, the present invention has been found based on a new finding that if Nb is added in a remarkably large amount to C, crystal grains can be remarkably refined. Therefore, Nb is an important element also in securing heat crack resistance due to the refinement of crystal grains.

When the Nb content is 2% or less, the above-mentioned effects do not appear remarkably. Even if the Nb content exceeds 3%, no further effect is obtained and the castability is deteriorated.
% Or less. The reason why the Nb / C amount is defined as 10 to 15 is that when Nb / C is less than 10, the effect of excess Nb on strengthening the ferrite matrix and the effect of refining crystal grains are small. Nb / C is Nb
When C is increased, most of C bonds with Nb which is more active than Cr, so that formation of coarse carbides of Cr can be suppressed, and there is also an effect of preventing embrittlement. On the other hand, if the Nb / C amount is 1
If added in excess of 5, the toughness will be degraded, so it is set to less than 15.

[0013] Cr is an element that determines the high-temperature oxidation resistance and acts on the stabilization of the ferrite structure. For this purpose, a minimum of 15% is required, and even if it exceeds 25%, the oxidation resistance hardly changes, and a σ phase is formed at a high temperature to become brittle. Therefore, the content was set to 15 to 25%. Preferably, it is 17 to 23%.

[0014] Si improves the strength and oxidation resistance, and also has an improvement in castability and a deoxidizing effect. However, if it is too much, it causes a decrease in toughness and forms a σ phase at a high temperature. Therefore, the content of Si is set to 2% or less. Mn is a deoxidizing element for Si as well, and also improves the fluidity of the molten metal. However, if the content is too large, the toughness is reduced. Mo and W are elements that have the same effect as elements that strengthen the ferrite matrix and increase high-temperature strength, and can be added in an amount of 3% or less. Excessive addition of Mo and W generates coarse eutectic carbides and deteriorates machinability, so that the content was set to 3% or less.

Ni is an austenite phase forming element, and is preferably added in an amount of 0.1% or more for improving toughness. However, if it exceeds 5%, the ferrite phase becomes unstable and the oxidation resistance deteriorates. Especially C
When r exceeds 20%, 2 to 5% is desirable for improving toughness.

By using the above-mentioned heat-resistant ferritic cast steel of the present invention as a preheating chamber member for a diesel engine, a long life of the preburning member can be achieved. Preferably, it is preferable that the cooling rate on the side of the nozzle is increased so that the crystal grain size of the nozzle is smaller than the crystal grain size inside the member. More preferably, by forming a microstructure having a crystal grain size of 0.05 mm or less within a range of 5 mm from the injection port, propagation of heat cracks can be significantly suppressed.

[0017]

EXAMPLE A precision ferritic heat-resistant cast steel having the composition shown in Table 1 was produced from a molten alloy by precision casting in the form of a tensile strength test piece, an oxidation resistance test piece and a chamber shape which is a precombustion chamber member for a diesel engine shown in FIG. . In Table 1, Samples 1 to 6 are obtained by fixing the amounts of other elements and increasing or decreasing the amount of Nb. Samples 7 to 10 have an increased C content. Samples 11 to 14 were obtained by increasing or decreasing the amount of Cr. Samples 15 to 18 were obtained by increasing or decreasing the amount of Ni.

[0018]

[Table 1]

Each of the obtained samples was measured for tensile strength and elongation at room temperature and 900 ° C. using a tensile tester. In addition, a round bar test piece with a diameter of 10 mm and a length of 10 mm was prepared and 900
° C., and held for 200 hours at 1100 ° C., was evaluated the oxidation resistance by determining the weight change per unit area before and after the removal to the oxidation test the oxide scale by performing cooling after sandblasting (mg / cm ^2). The sample cast into a product shape was evaluated for heat check resistance by a heat cycle shown in FIG. 2 in which the nozzle 5 was heated with a propane gas burner and then cooled by water spray. heating,
The heat cycle of cooling is 190 seconds per cycle, for a total of 3
After the 00 cycle, the portion shown in FIG. 3 was cut, and after obtaining the cross section shown in FIG. 4, the maximum heat crack length generated around the injection port was measured. Table 2 summarizes the average crystal grain size in the vicinity of the nozzle (5 mm from the nozzle surface) for each material. FIG. 5 is a photograph of the metal structure around the nozzle according to the present invention. FIG. 5 shows 1.A.
It is a cross-sectional structure photograph of the periphery of a three times injection hole. As shown in FIG. 5, it can be seen that the diesel engine pre-combustion chamber member obtained by using the heat-resistant cast steel of the present invention has a finer crystal grain size on the nozzle side than on the inside.

[0020]

[Table 2]

As shown in Table 2, the amount of Nb is small,
Samples 1 and 2 of Comparative Examples having a low value of / C are within the specified range of the present invention, and have larger crystal grain sizes and inferior heat crack resistance than Samples 3 to 5 in which a large amount of nB is dissolved in a ferrite matrix. I have. On the other hand, Sample 6 having a large Nb content and a high Nb / C value is inferior in heat crack resistance as compared with Samples 3 to 5 of the present invention. Further, Sample 7 has a low Nb / C, and Sample 10 has a too low C, so that the crystal grain size is large and the heat crack resistance is inferior to the sample of the present invention. Sample 11 having a low Cr had significantly poor oxidation resistance, and Sample 1 having a high Cr
No. 4 is inferior in toughness. Sample 1 with low Ni
5 is slightly inferior in toughness as compared with Sample 17 having a high Ni,
Is too high, the heat crack resistance of Sample 18 of Comparative Example is inferior. As shown in Table 2, the present invention provides C,
By optimizing the values of Nb and Cr, the heat crack resistance is extremely high. In particular, good castability due to high C, high productivity and high machinability due to the ferritic properties of diesel engine reserve It turns out that it is excellent as a combustion chamber member. In addition, Samples 19 and 20 of the present invention to which Mo or W was added had a higher tensile strength at 900 ° C. than, for example, Sample 4 of the present invention in which the amounts of Cr and Nb were close to each other. It turns out that it is effective in improving high temperature strength.

[0022]

Industrial Applicability The present invention can improve the heat crack resistance, which is a disadvantage of a low cost, high machinability heat resistant ferritic cast steel, and can be used for a pre-combustion chamber member of a diesel engine having a high thermal load. It has become possible.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view around a diesel engine preheating chamber.

FIG. 2 is a diagram showing a heat cycle of a heat check test.

FIG. 3 is a view showing a cut surface of a chamber for evaluating a heat crack after a heat check test.

FIG. 4 is a view showing a state of a heat crack.

FIG. 5 is a photograph of a cross-sectional metal structure of a diesel engine pre-combustion chamber member of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piston 2 Cylinder head 3 Chamber 4 Pre-combustion chamber 5 Injection port 6 Fuel injection nozzle

Claims (4)

(57) [Claims] 1. C: 0.15 to 0.3% by weight, S:
i: 2.0% or less, Mn: 1.0% or less, Cr: 15 to
25%, Nb: more than 2.0% and 3.0% or less, Ni:
A heat-resistant, ferritic cast steel comprising 0.1 to 5.0%, the balance being Fe and inevitable impurities, wherein Nb is solid-dissolved in a ferrite matrix with Nb% / C%: more than 10 and 15 or less. 2. The heat-resistant ferritic cast steel according to claim 1, wherein a part of Fe is replaced by one or both of Mo and W in a weight ratio of 3% by weight or less. 3. A pre-combustion chamber member for a diesel engine, which is formed of the heat-resistant ferritic cast steel according to claim 1. 4. The pre-combustion chamber member for a diesel engine according to claim 3, wherein the crystal grain size at the injection port side is smaller than the crystal grain size inside the member.