JPH04325658A - Heat resistant cast steel - Google Patents

Abstract

PURPOSE:To offer a ferritic stainless heat resistant cast steel excellent in high temp. strength, high in cold ductility and capable of casting parts with a complicated shape. CONSTITUTION:The compsn. of a heat resistant cast steel is regulated to a one constituted of, by weight, 0.06 to 0.2% C, <0.4% Si, <=1.0% Mn, <=0.04% P, <=0.04% S, 15 to 22% Cr, 0.01 to 2.0% Nb, 0.01 to 0.l% N and the balance Fe with inevitable impurities. This ferritic stainless cast steel is subjected to normalizing treatment after casting. In addition to the above alloy compsn., this heat resistant cast steel can be mixed with at least one kind of element among Ni, Mo and W in the range of 0.1 to 2.0% Ni, 0.1 to 1.0% Mo and 0.1 to 2.0% W.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to heat-resistant cast steel, and more particularly to heat-resistant cast steel having excellent high-temperature strength and room-temperature ductility.

0002

[Prior art] Automotive engine exhaust manifold,
Exhaust system parts such as turbocharger housings, diesel engine preheating chambers, and exhaust gas purification system parts are used under harsh conditions at high temperatures, so the materials for these exhaust system parts are generally: Heat-resistant cast irons such as high-Si spheroidal graphite cast iron, Niresist cast iron, and Al cast iron are used.

0003

[Problem to be Solved by the Invention] However, as the exhaust gas temperature has been rising due to the recent improvements in engine performance and combustion efficiency, conventional heat-resistant cast iron has poor heat resistance and oxidation resistance. There is a question of sufficiency. To solve this problem, stainless steel cast steel is sometimes used instead of heat-resistant cast iron, and among these stainless steel cast steels, ferritic stainless cast steel, which has a small coefficient of thermal expansion, is generally used from the viewpoint of thermal fatigue properties. It is true. However, ferritic stainless cast steel has relatively low high-temperature strength and tends to be deformed due to lack of strength due to thermal fatigue, so it cannot be said that its thermal fatigue properties are sufficient. Also,
Ferritic stainless steel cast steel has poor castability compared to austenitic stainless steel cast steel, and there is a problem in that it is particularly difficult to cast complex-shaped parts such as automobile exhaust system parts. The present invention was made in order to solve these problems, and its purpose is to provide a heat-resistant ferritic stainless cast steel that has excellent high-temperature strength, high ductility at room temperature, and enables the casting of complex-shaped parts. do.

0004

[Means for Solving the Problems] A heat-resistant cast steel according to the first aspect of the present invention for achieving the above object has C:
0.06-0.2%, Si: 0.4%
less than, Mn: 1.0% or less,
P: 0.04% or less, S: 0.04% or less,
Cr: 15~22%, Nb: 0.01~
2.0%, N: 0.01 to 0.1%, the balance being Fe and unavoidable impurities, and is characterized by being normalized after casting. The heat-resistant cast steel according to the second invention of the present invention includes, in addition to the alloy composition of the heat-resistant cast steel according to the first invention, Ni, M
At least one element of o and W is in the range of Ni: 0.1 to 1.0%, Mo: 0.1 to 1.0%, W: 0.1 to 2.0% in weight%. It is characterized by being added.

The reason why the alloy composition of the heat-resistant cast steel in the first invention is limited as described above is as follows. C is the most effective element for increasing strength at room temperature and high temperature, and also improves ductility at room temperature. Therefore, in order to ensure strength and improve ductility, C is at least 0.0
6% or more is required. Since the amount of deformation caused by local thermal stress due to α-γ phase transformation increases as C increases, C is set to 0.20% or less in view of the relationship with Cr. Si has a great effect of improving the flow of the metal during casting and improving the castability, but
As the ductility at room temperature decreases significantly due to the increase in Si,
In order to ensure ductility at room temperature, the Si content was set to less than 0.4%. In particular, when a vacuum casting method such as a vacuum anti-gravity casting method is used, it is possible to cast exhaust system parts and the like that have complex shapes even with a low Si content.

[0006] Mn improves oxidation resistance. But M
Excessive addition of n tends to generate MnS, which causes a decrease in ductility, so Mn was set to 1% or less. P is an inevitable element as an impurity, but it has oxidation resistance,
Since it deteriorates thermal fatigue resistance, it is set at 0.04% or less. S easily forms MnS by combining with Mn, which causes a decrease in ductility, so it was set to 0.04% or less. Cr is an element that ensures oxidation resistance and suppresses deformation caused by local thermal stress due to α-γ phase transformation, and at least 15% or more is required to exhibit this effect. Further, if a large amount of Cr is added, it promotes the formation of σ phase at high temperatures and causes embrittlement, so the Cr content is set to 22% or less.

[0007] Nb increases high-temperature strength through the formation of NbC, and also suppresses deformation caused by local thermal stress due to α-γ phase transformation. Furthermore, Nb suppresses the formation of Cr carbides and improves oxidation resistance. In order to obtain these effects, it is necessary to add Nb in an amount of 0.1% or more. On the contrary, N
Addition of a large amount of b reduces ductility due to coarsening of NbC, so Nb was set at 2.0% or less. N has the effect of improving high-temperature strength, and in order to obtain this effect, 0
．． 0.1% or more is required. Addition of a large amount of N causes Cr2
Since excessive precipitation of N causes a decrease in ductility, N is 0.
．． 1% or less.

The chemical composition limitations of the heat-resistant cast steel according to the second invention are as follows. At least 0.1% or more of Ni is required to improve toughness. However, an increase in the amount of Ni makes it easier to generate an austenite phase, so the Ni content was set to 1.0% or less in order to stabilize the phyllite phase and suppress deformation caused by local thermal stress due to α-γ phase transformation. Mo has the effect of stabilizing the ferrite phase and suppressing deformation caused by local thermal stress due to α-γ phase transformation, and in order to obtain this effect, 0.1%
The above additions are necessary. Excessive addition of Mo tends to generate coarse carbides, so in order to prevent a decrease in ductility, the Mo content is set to 1.0% or less. W improves high temperature strength through solid solution strengthening. For this purpose, W is required to be 0.1% or more,
Since excessive addition of W causes a decrease in ductility, it is limited to 2.0% or less.

[0009]

[Examples] Examples of the present invention will be described below. The chemical composition of the ferritic heat-resistant cast steel used in the present invention is shown in Table 1.

0010

[Table 1]

Test pieces of ferritic heat-resistant cast steel having the composition shown in Table 1 were prepared as follows. First, the alloy was melted in a high-frequency induction furnace and cast into a JISA No. test piece. Thereafter, the temperature was maintained at 780° C., cooled in air, and each test piece was collected. Each test piece was subjected to (1) a room temperature tensile test, (2) a high temperature tensile test, and (2) a thermal fatigue test. (2) Regarding the room temperature tensile test, tensile strength, 0.2% proof stress and elongation were measured. ■
The high-temperature tensile test was conducted at a temperature of 900°C, and the tensile strength, 0.2% proof stress, and elongation were measured. ■For the thermal fatigue test, a disk-shaped test piece with an outer diameter of 45 mm was heated to 150°C.
After repeating the cycle of exposing the specimen for 3 minutes in a fluidized bed furnace at a temperature of 900° C. for 3 minutes, the total length of the cracks generated on the circumference of the specimen was measured. The results are shown in Table 2.

0012

[Table 2]

As shown in Table 2, Comparative Example 1 has high C,
In a thermal fatigue test of a high-Si material, deformation occurred, which appeared to be caused by local thermal stress due to α-γ phase transformation. Comparative example 2 is
It had a high Si content, and its elongation at room temperature was extremely small. Comparative Example 3 had a low C content, had low high-temperature strength, and suffered from deformation in the thermal fatigue test. Examples 1 to 7 had good tensile strength, 0.2% yield strength, and elongation at room temperature. It was also found that thermal fatigue strength was high in thermal fatigue tests. When an exhaust manifold for a gasoline engine was cast using a special vacuum casting method using the heat-resistant cast steel having the composition range of the present invention described above, the casting yield was high with good flowability and no casting defects such as shrinkage cavities. was able to secure over 70%. This confirmed that productivity was excellent.

[0014]

[Effects of the Invention] As explained above, the heat-resistant cast steel of the present invention not only has excellent high-temperature strength, which is inherent to heat-resistant cast steel, but also has good ductility at room temperature. It has the advantage of being easy to process, and in particular, it is possible to cast parts with complex shapes. Further, by taking advantage of the characteristics of heat-resistant cast steel, it has the advantage of having extremely good high-temperature strength and thermal fatigue strength.

Claims (2)

[Claims] Claim 1: C: 0.06 to 0.2% in weight%;
Si: less than 0.4%, Mn: 1.0% or less, P: 0.04% or less, S:
0.04% or less, Cr:15
~22%, Nb: 0.01~2.0%, N
: 0.01 to 0.1%, the balance being Fe and unavoidable impurities, which is a ferritic stainless steel cast steel, characterized in that it has been subjected to a normalizing treatment after casting. 2. In addition to the alloy composition of the heat-resistant cast steel according to claim 1, at least one element of Ni, Mo, and W is contained in weight percent, Ni: 0.1 to 1.0%, Mo: 0. 2. The heat-resistant cast steel according to claim 1, wherein W: 0.1-2.0%.