JP3605874B2 - Heat-resistant cast steel - Google Patents

Description

【００４１】
【表２】

【００４２】
【表３】

【００４３】
本発明の実施例を比較例と比較したとき、常温における引張試験の結果が示す強度および伸びは、劣ってはいるが、その差は著しいものではなく、肝心の高温（９００℃）試験においては、大差ないデータである。９００℃および９５０℃の熱疲労試験においても、割れ長さおよび変形量は比較例にくらべてはるかに小さく、明らかに比較例よりすぐれた熱疲労特性が認められる。
【００４４】
被削性試験の結果を表３にみると、本発明の実施例は工具寿命が少なくとも４０００mm以上であり、比較材の１０５０mmにくらべて、顕著に改善された被削性が認められる。２［％Ｓ］＋［％Ｓｅ］の値が０．５０％以上である実施例３は、工具寿命が１２０００mm以上であり、その条件に加えて多量のＰを含有する実施例１は、工具寿命１５０００mm以上と、とくにすぐれた被削性を示している。ＢおよびＣａの１種または２種を含有させた実施例４および５も、高い被削性を示した。
【００４５】
本発明の好ましい実施態様に従って、Ｖ、ＡｌおよびＴｉの１種または２種以上を含有させることにより、またさらに、Ｍｏ、または、Ｗ、ＮｉおよびＣｏの１種または２種以上を含有させることにより、９００℃での高温強度が増大し、熱疲労試険における変形量が小さくなっており、いっそうの熱疲労特性向上が実証された。
【００４６】
前述したＳおよびＳｅの選択の問題について表３のデータをみると、ＳまたはＰを０．０４％以上含有させる（実施例１〜３）と、９５０℃の熱疲労試験において、変形量が小さい試験片も微小な割れが発生し、熱疲労特性がいくぶん低いことがわかる。一方、Ｓｅの添加量だけ０．０４％以上の高い値にした場合（実施例４）は、９５０℃の熱疲労試験で割れが発生していないことから、Ｓｅ量を増加させても熱疲労特性は低下しないことがわかる。
【００４７】
本発明の耐熱鋳鋼を用いて、ガソリンエンジン用排気マニホールドおよびタービンハウジングを減圧鋳造法により鋳造したところ、湯回り不良や引け巣などの鋳造欠陥の発生はなく、鋳造歩留りも８０％以上を確保することができた。[Industrial applications]
[0001]
The present invention relates to a heat-resistant, ferritic cast steel suitable as a material for automobile exhaust system components, such as an exhaust manifold of an automobile engine, a turbine housing, a front pipe and a joint thereof, an exhaust gas purification device component, and a pre-combustion chamber for a diesel engine. About.
[Prior art]
[0002]
Conventionally, spheroidal graphite cast iron or high Si spheroidal graphite casting with improved heat resistance has been used for exhaust system components such as exhaust manifolds and turbine housings of automobile engines. In some high power engines, since the exhaust gas temperature is high and even high Si spheroidal graphite cast iron does not have sufficient durability, a niresist cast iron, a stainless cast steel, or a welded structure of a stainless steel plate is used.
[0003]
In recent years, as the output of automobile engines has further increased, the demand for purifying automobile exhaust gas has been increasing. In particular, in order to quickly purify exhaust gas generated when the engine is started, it is necessary to quickly set the temperature of the exhaust gas to a temperature at which the exhaust gas purification device operates. For that purpose, it is necessary to reduce as much as possible the amount of heat taken by exhaust system components, such as an exhaust manifold and a turbine housing, on the engine side of the exhaust gas purification device. Accordingly, thinning and weight reduction of these exhaust system components are being promoted, and the above-described welded structures of stainless steel thin plates and thin castings cast by a special casting method have begun to be used.
[0004]
However, in the case of thin castings, the thermal fatigue characteristics and oxidation resistance of conventional spheroidal graphite cast iron are reduced because the thinner the thickness, the higher the thermal stress applied to the parts and the higher the surface temperature. Insufficient and insufficient durability. Therefore, stainless steel castings are being used in some cases.
[0005]
As a cast stainless steel used for such an exhaust system component, a ferritic cast stainless steel having a small coefficient of thermal expansion is preferable from the viewpoint of thermal fatigue characteristics. However, the conventional ferritic stainless steel (for example, CB30 of Alloy Casting Institute standard) has the drawback that the machinability is poor and the machining cost is high as compared with spheroidal graphite cast iron.
[0006]
Conventional ferritic stainless cast steel also has the drawback that it cannot be used above the α-γ transformation temperature because α-γ phase transformation occurs during use, and significant deformation occurs due to local thermal stress generated by transformation. . Therefore, it is desired to raise the α-γ phase transformation temperature or otherwise prevent the α-γ phase transformation from occurring and raise the usable upper limit temperature. Ferritic stainless steels generally have a weak point in that they have low high-temperature strength, and there is also a demand for improving high-temperature strength to improve thermal fatigue properties.
DISCLOSURE OF THE INVENTION
[Problems to be solved by the invention]
[0007]
The present invention has been made in order to meet such a demand, and has as its object to improve the machinability, reduce the cost of machining, and increase the α-γ phase transformation temperature or By providing no α-γ phase transformation, the maximum temperature of use can be increased, and a heat-resistant ferritic cast steel having improved thermal fatigue properties due to an increase in high-temperature strength can be provided. The object of the present invention is to enable manufacture of parts.
[Means for Solving the Problems]
[0008]
The heat-resistant cast steel of the present invention that achieves the above objects is basically composed of, by weight%, C: 0.06 to 0.50%, Si: 4.0% or less, Mn: 3.0% or less, P : 0.50% or less, S: 0.50% or less, Cr: 15 to 22%, Nb: 0.01 to 3.0%, N: 0.01 to 0.15%, and Se: 0.001 to 0.50%, but not more than 2 [% S] + [% Se]: 1.00, with the balance being substantially alloyed with Fe.
[0009]
When it is intended to further improve the high-temperature strength of the heat-resistant cast steel of the present invention, W: 0.1 to 5.0% can be contained in addition to the above basic alloy components.
[0010]
When the heat-resistant cast steel of the present invention is intended to further increase the α-γ transformation temperature, in addition to the above basic alloy components, V: 0.1 to 2.0%, Ti: 0. One or more of 0.01 to 1.0% and Al: 0.01 to 1.0% can be contained.
[0011]
To further improve the high-temperature strength of the heat-resistant cast steel of the present invention, Ni: 0.1 to 1.0%, Mo: 0.1 to 5.0%, Co: 0.1 to 5.0%. Or one or more of these.
[0012]
If further improvement in machinability is desired, one or two of B: 0.005 to 0.10% and Ca: 0.001 to 0.05% can be contained.
[0013]
In order to improve the oxidation resistance of the heat-resistant cast steel of the present invention, it is effective to add REM: 0.001 to 0.50%.
【The invention's effect】
[0014]
The heat-resistant cast steel of the present invention has improved machinability and improved machinability without substantially lowering the high-temperature strength, oxidation resistance and thermal fatigue properties of the known heat-resistant cast steel. . As a result, it has become possible to manufacture inexpensively exhaust system components of a heat-resistant automobile engine.
[0015]
By including one or more of V, Ti and Al according to a preferred embodiment, the heat-resistant cast steel has an increased α-γ transformation temperature, and as a result, has improved high-temperature strength and improved thermal fatigue properties. Is done. By containing W or one or more of Ni, Mo and Co, high-temperature strength can be improved and thermal fatigue properties can be improved. By containing one or two of B and Ca, the machinability can be further improved, and by adding REM, further improvement in oxidation resistance can be expected.
BEST MODE FOR CARRYING OUT THE INVENTION
[0016]
The functions of the essential alloying elements and the optional elements constituting the heat-resistant cast steel of the present invention and the reasons for limiting the composition range will be described below.
[0017]
C: 0.06 to 0.50%
C has the function of improving the fluidity of the molten metal during casting and increasing the high-temperature strength. However, the α-γ phase transformation temperature is lowered by increasing the amount of C, and the usable upper limit temperature is significantly reduced. Therefore, the upper limit is set to 0.50%. When the operating temperature of a casting product increases, the amount of transformation of the α-γ phase increases, the local thermal stress due to the transformation increases, and the amount of deformation increases. It is desirable to avoid the transformation itself. In that case, the C content should be low, at which no α-γ phase transformation occurs, that is, less than 0.06%. However, if the C content is low, the castability is deteriorated, and particularly, casting of a thin casting is difficult. Therefore, the lower limit of the amount of C was set to 0.06%. The problem of poor castability was solved by using a special vacuum casting method, and it was found that a thin cast having a complicated shape could be obtained while avoiding the occurrence of casting defects.
[0018]
Si: 4.0% or less Si is a ferrite stabilizing element and has a function of increasing the α-γ transformation temperature and improving the oxidation resistance and the fluidity of the molten metal. However, the addition of a large amount lowers the ductility and promotes the formation of the σ phase at a high temperature, so the upper limit was set to 4.0%.
[0019]
Mn: 3.0% or less Mn improves oxidation resistance and generates MnS and MnSe to help improve machinability. However, even if added in excess of 3.0%, not only does the effect of improving machinability not increase, but also the ductility decreases, so an upper limit of 3.0% was set.
[0020]
P: 0.50% or less P has an effect of improving the machinability by generating a phosphorus compound. Even if it is added in a large amount, not only this effect is saturated, but also the oxidation resistance and the thermal fatigue characteristics are significantly reduced, so the upper limit is set to 0.50%. When the thermal fatigue property is more important than the machinability, the P content is desirably 0.04% or less.
[0021]
S: 0.50% or less S enhances machinability by generating MnS, as described for Mn. However, even if a large amount is added, further improvement in machinability cannot be expected. , Ductility, oxidation resistance and thermal fatigue properties are significantly reduced, so the upper limit is made 0.50%. When the thermal fatigue property is more important than the machinability, it is desired that the S content be 0.04% or less.
[0022]
Se: 0.001 to 0.50%
Se improves machinability by generating MnSe. In order to obtain this effect, 0.001% or more of Se is required. Addition of 0.50% or more not only saturates the effect of improving machinability but also raises the cost.
[0023]
2 [% S] + [% Se]: 1.00 or less As described above, both S and Se have a function of forming a compound with Mn to improve machinability, and the effect appears. , S: Se = 2: 1. When 2 [% S] + [% Se] exceeds 1.00 (%), the effect of improving machinability is not only increased, but also causes a remarkable decrease in ductility, oxidation resistance and thermal fatigue properties. However, the cost also increases, and this value is set as the upper limit.
[0024]
The addition amounts of S and Se can be selected and determined according to the purpose within the above ranges and limits. That is, S has a large effect of improving the machinability, and the increase in the content due to the increase in the content is small, but the increase in the added amount has a large effect on the deterioration of the thermal fatigue characteristics. On the other hand, Se has a small effect of improving machinability as compared with S, and increases the cost by increasing the addition amount, but hardly decreases the thermal fatigue characteristics due to the increase in the amount. Therefore, when it is desired to improve machinability but prevent cost increase, the amount of S may be increased, and when it is desired to improve machinability and not to lower the thermal fatigue characteristics, the amount of Se may be increased. Should be increased.
[0025]
Cr: 15 to 22%
Cr has an effect of securing oxidation resistance and increasing the α-γ phase transformation temperature. The lower limit of the addition amount at which this effect can be obtained is 15%. The addition of a large amount promotes the formation of the σ phase at a high temperature and causes embrittlement. Therefore, the upper limit is set to 22%.
[0026]
Nb: 0.01 to 3.0%
Nb increases the high-temperature strength by generating stable NbC and increases the α-γ phase transformation. In addition, Nb has a higher tendency to form carbides than Cr and has a function of suppressing the formation of Cr carbides and improving oxidation resistance. Therefore, Nb may be added when it is desired to further raise the upper limit use temperature. In this case, the effect is not exhibited at 0.01% or less, so the lower limit is made 0.01%. However, even if added in an amount of 3.0% or more, not only the effect is not obtained but also the ductility is remarkably reduced, so the upper limit is made 3.0%.
[0027]
N: 0.01 to 0.15%
N has the effect of improving high-temperature strength. This effect cannot be obtained with an N amount of less than 0.01%. On the other hand, addition exceeding 0.15% results in a decrease in ductility due to excessive precipitation of Cr ₂ N, so this value is made the upper limit.
[0028]
W: 0.1-5.0%
W improves high-temperature strength by solid solution strengthening. Therefore, when further improvement in high-temperature strength is desired, it is worth considering addition of W. In the case of adding, the effect does not appear if the addition amount is less than 0.1%. If added in excess of 5.0%, the ductility is significantly reduced.
[0029]
V: 0.1 to 2.0%
V produces a stable VC and has an effect of increasing the α-γ phase transformation temperature, and also has an effect of improving the high-temperature strength. It is recommended to add. In this case, if the addition is less than 0.1%, the effect does not appear. However, adding more than 2.0% is not only meaningless, but also causes a significant decrease in ductility.
[0030]
Ti: 0.01 to 1.0%
Ti has the effect of generating stable TiC and increasing the α-γ transformation temperature, and is also useful for improving the high-temperature strength. It is good to add. If the amount is less than 0.01%, the effect is not exhibited. However, even if added in excess of 1.0%, no further effect can be expected, and the ductility is significantly reduced.
[0031]
Al: 0.01 to 1.0%
Al has the effect of stabilizing ferrite, increasing the α-γ phase transformation temperature, and improving the high-temperature strength. Therefore, like V and Ti, if it is desired to further raise the upper limit of use temperature, it is advisable to add Al. If the amount of addition does not reach 0.01%, the effect of addition cannot be obtained. On the other hand, if it exceeds 1.0%, only the disadvantage of a remarkable decrease in ductility is caused.
[0032]
Ni: 0.1 to 1.0%
Ni also enhances the high-temperature strength by solid solution strengthening, so if it is desired to obtain its benefits, the addition of Ni is significant. If the addition amount is less than 0.1%, the effect is weak, and if the addition amount exceeds 1.0%, the α-γ phase transformation temperature decreases, so that it is selected from the above range.
[0033]
Mo: 0.1 to 5.0%
Mo stabilizes the ferrite phase and increases the α-γ transformation temperature, and also contributes to the improvement of high-temperature strength. Therefore, addition of Mo is recommended for applications in which improvement in high-temperature strength is desired. Additions below the lower limit of 0.1% are poorly effective, while additions outside the upper limit of 5.0% lead to a marked reduction in ductility.
[0034]
Co: 0.1-5.0%
Co enhances high-temperature strength by solid solution strengthening. Therefore, when it is intended to further increase the high-temperature strength, it is worth considering the addition. There is no effect unless it is added at the lower limit of 0.1% or more. Addition exceeding the upper limit of 5.0% increases the α-γ phase transformation temperature.
[0035]
B: 0.005 to 0.10%
B produces BN and improves machinability. When B is added to obtain this effect, at least 0.005% is required. However, even if an excessive amount is added, the effect of improving machinability reaches a plateau, and a remarkable decrease in thermal fatigue properties is observed. Therefore, the addition amount is set to 0.10%.
[0036]
Ca: 0.001 to 0.05%
Ca forms oxides and improves machinability. If Ca is added in anticipation of this effect, the effect is obtained at 0.001% or more, and the effect is saturated, and there is no adverse effect that thermal fatigue characteristics are reduced, and 0.05% or less. The amount added should be chosen.
[0037]
REM: 0.001-0.50%
Since REM has an effect of improving oxidation resistance, it is a useful additive element when it is desired to improve oxidation resistance and increase the maximum use temperature. The amount of addition is preferably selected from a range of 0.001% or more, at which the effect is recognized, and 0.50% or less, which is a limit at which the effect is not saturated and the thermal fatigue property is not deteriorated.
【Example】
[0038]
A heat-resistant cast steel having an alloy composition shown in Table 1 (wt%, balance Fe) was melted in a 50 kg high frequency induction furnace and cast into a JIS-A test piece. After heating each test piece to 750 ° C., it was air-cooled and subjected to a tensile test, a thermal fatigue test, an oxidation test and a machinability test. Table 2 shows the results of the tensile test at room temperature and 900 ° C., and Table 3 shows the results of the thermal fatigue test, the oxidation test, and the machinability test. In these tables, "Comparative Example" is CB30 of the Alloy Casting Institute standard listed for comparison.
[0039]
Each test was performed as follows.
[Thermal fatigue test]
A disk type specimen (diameter: 45 mm, thickness: 7.5 mm) was exposed to a fluidized-bed furnace at 150 ° C. for 3 minutes, and then exposed to a fluidized-bed furnace at 900 ° C. or 950 ° C. for 3 minutes. Then, the total length of cracks generated on the circumference of the test piece and the amount of change in the thickness of the test piece were measured. The length of the crack indicates the likelihood of cracking due to thermal fatigue, and the change in thickness is the likelihood of plastic deformation due to thermal stress caused by heating and cooling and local thermal stress caused by α-γ transformation. Represents
[Machinability test]
When milling with a cemented carbide tip was performed, the cutting length when the corner wear amount of the tip became 200 μm was evaluated as the tool life.
[0040]
[Table 1]

[0041]
[Table 2]

[0042]
[Table 3]

[0043]
When the examples of the present invention were compared with the comparative examples, the strength and elongation indicated by the results of the tensile test at room temperature were inferior, but the differences were not remarkable, and in the essential high temperature (900 ° C.) test. , The data is not much different. Also in the thermal fatigue tests at 900 ° C. and 950 ° C., the crack length and the amount of deformation are much smaller than those of the comparative example, and the thermal fatigue characteristics superior to the comparative example are clearly observed.
[0044]
According to the results of the machinability test, Table 3 shows that the example of the present invention has a tool life of at least 4000 mm or more, and significantly improved machinability compared to 1050 mm of the comparative material. Example 3 in which the value of 2 [% S] + [% Se] is 0.50% or more has a tool life of 12000 mm or more, and Example 1 containing a large amount of P With a life of 15000 mm or more, it shows particularly excellent machinability. Examples 4 and 5 containing one or two of B and Ca also showed high machinability.
[0045]
According to a preferred embodiment of the invention, by including one or more of V, Al and Ti, and furthermore by including one or more of Mo or W, Ni and Co , 900 ° C., the amount of deformation in the thermal fatigue test was reduced, and further improvement in thermal fatigue characteristics was demonstrated.
[0046]
According to the data in Table 3 regarding the above-mentioned problem of selection of S and Se, when S or P is contained at 0.04% or more (Examples 1 to 3), the amount of deformation is small in the 950 ° C. thermal fatigue test. It can be seen that the specimen also cracked slightly and had a somewhat low thermal fatigue property. On the other hand, when the added amount of Se was set to a high value of 0.04% or more (Example 4), cracks did not occur in the thermal fatigue test at 950 ° C. It can be seen that the characteristics do not decrease.
[0047]
When the exhaust manifold and the turbine housing for a gasoline engine were cast by the vacuum casting method using the heat-resistant cast steel of the present invention, no casting defects such as poor runoff and shrinkage cavities were generated, and a casting yield of 80% or more was secured. I was able to.

Claims (6)

By weight%, C: 0.06 to 0.50%, Si: 4.0% or less, Mn: 3.0% or less, P: 0.50% or less, S: 0.50% or less, and Se: 0 0.001 to 0.50%, Cr: 15 to 22%, Nb: 0.01 to 3.0%, and N: 0.01 to 0.15%, provided that 2 [% S] + [ % Se]: a heat-resistant ferritic cast steel having an alloy composition of not more than 1.00, with the balance substantially consisting of Fe, for producing exhaust system parts for automobile engines. The heat-resistant cast steel according to claim 1, further comprising W: 0.1 to 5.0% in addition to the alloy component specified in claim 1. In addition to the alloy components defined in claim 1 or 2, one of V: 0.1 to 2.0%, Ti: 0.01 to 1.0%, and Al: 0.01 to 1.0%. Or the heat-resistant cast steel according to claim 1 or 2 containing at least two kinds. In addition to the alloy components defined in any one of claims 1 to 3, Ni: 0.1 to 1.0%, Mo: 0.1 to 5.0%, and Co: 0.1 to 5.0%. The heat-resistant cast steel according to any one of claims 1 to 3, further comprising one or more of the following. Claims in which one or two of B: 0.005 to 0.10% and Ca: 0.001 to 0.05% are further contained in addition to the alloy components specified in any one of claims 1 to 4. Item 6. The heat-resistant cast steel according to any one of Items 1 to 4. The heat-resistant cast steel according to any one of claims 1 to 5, further comprising REM: 0.001 to 0.50% in addition to the alloy component specified in any one of the claims 1 to 5.