JPH08188856A - Ferritic heat resistant cast steel and its production - Google Patents

Abstract

PURPOSE: To improve high strength and heat resistance such as thermal fatigue resistance without sacrificing oxidation resistance, machinability, and structural stability for them. CONSTITUTION: A steel, having a composition containing, by weight, 0.05-0.5% C, 1.0-2.0% Si, <0.6% Mn, <0.04% P, <O.04% S, <0.5% Ni, 10-20% Cr, 0.1-1.0% V, 0.5-1.0% Nb, 0.10-0.50% Mo, <0.01% W, 0.01-0.5O% Al, and at least one kind among 0.001-0.5% B, 0.05-0.5% Ti, and 0.1-5.0% Co, is cast and then held at 850-1000 deg.C for 1-5hr. Successively, the cast steel is cooled slowly down to <=700 deg.C to undergo annealing treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic heat-resistant cast steel, and more particularly to a ferritic heat-resistant cast steel suitable for use in an exhaust manifold of an automobile engine exhaust system, a turbine housing, or the like.

[0002]

2. Description of the Related Art In general, high Si spheroidal graphite cast iron, Niresist, etc. have been conventionally used for this kind of exhaust manifold and turbine housing. Materials having more excellent heat resistance have been desired. As a material having excellent heat resistance, a high Ni, high Cr system austenitic heat resistant steel is well known, but these are inferior in castability and machinability, and are not practical in terms of productivity and cost. There was a problem.

Recently, therefore, high Cr ferritic heat-resistant cast steel has attracted attention because it has suitable castability and machinability. For example, in Japanese Patent Laid-Open No. 5-320830, C: 0.05 ~ 0.5, Si: 1.0 ~ 2.0,
Mn: less than 0.6, P: less than 0.04, S: less than 0.04, Ni:
Less than 0.5, Cr: 10 to 20, V: 0.1 to 1.0, Nb: 0.5
~ 1.0, Mo: 0.1 ~ 0.5, W: less than 0.01, Al: 0.01
A ferritic heat resistant cast steel consisting of .about.0.50 is disclosed.

[0004]

However, according to the ferritic heat-resistant cast steel disclosed in the above publication, the heat resistance such as high temperature strength and heat fatigue resistance is still insufficient, and it is sufficient for recent automobile engines. There was a problem that we could not handle it.

The present invention has been made to solve the above-mentioned conventional problems, and an object thereof is to further improve heat resistance and thus to be applied to an exhaust system of an automobile engine. It is to provide a ferritic heat-resistant cast steel having improved heat resistance and a method for manufacturing the same.

[0006]

In order to achieve the above object, the heat-resistant ferritic cast steel according to the present invention has a basic composition of C: 0.05 to 0.5, Si:
1.0 to 2.0, Mn: less than 0.6, P: less than 0.04, S: 0.04
Less, Ni: less than 0.5, Cr: 10-20, V: 0.1-1.0
, Nb: 0.5 to 1.0, Mo: 0.1 to 0.5, W: less than 0.01, Al: 0.01 to 0.50, and B: 0.001 to 0.5, Ti.
: 0.05 to 0.5 and Co: 0.1 to 5.0.

Further, the method for producing a ferritic heat-resistant cast steel according to the present invention comprises:
It is characterized in that it is kept at 850 to 1000 ° C. for 1 to 5 hours, and subsequently subjected to an annealing treatment of gradually cooling to a temperature of 700 ° C. or less.

Here, the reason for limiting the components in the present invention will be explained. C is effective for improving the strength and toughness and improving the fluidity (castability) of the molten metal, but 0.05% by weight (w
Below t)%, their effects are not sufficient, while 0.50wt%
%, It deteriorates the oxidation resistance and lowers the eutectoid transformation temperature, resulting in the precipitation of austenite.
It was set to 5 to 0.50 wt%.

Si improves the oxidation resistance, raises the eutectoid transformation temperature, and is effective as a deoxidizing agent, but 1.0 wt%
If less than 2.0% by weight, the effects are not sufficient, while if over 2.0% by weight, the toughness at low temperature (normal temperature) is deteriorated and the strength at high temperature is reduced, so this was made 1.0 to 2.0% by weight.

Since Mn is an element for forming a pearlite structure, it is not very preferable for the heat-resistant cast steel having a ferrite structure as the matrix as in the present invention, and it lowers the strength, so that it is as low as less than 0.6 wt%. Suppressed.

When P and S are more than 0.04 wt%, the generation of heat cracks (heat cracks) is promoted, so P and S are treated as harmful impurities and are suppressed to less than 0.04 wt%.

[0012] Ni lowers the eutectoid transformation temperature to promote the precipitation of austenite and destabilizes the ferrite structure.

Cr is an extremely useful element because it improves oxidation resistance and raises the eutectoid transformation temperature.
If it is less than 10 wt%, those effects are not sufficient, while 20 wt%
If it exceeds, the toughness at low temperature is lowered, and crystallization of coarse primary carbides is promoted to significantly deteriorate the machinability, so this is set to 10 to 20 wt%.

V significantly increases the eutectoid transformation temperature, forms carbides in preference to Cr, and suppresses the formation of primary carbides of Cr, which deteriorates machinability. Therefore, V is particularly important in the present invention. It is one of the major elements, but 0.1 wt%
If the amount is less than 1.0%, the effects are not sufficient, while if it exceeds 1.0% by weight, the oxidation resistance is deteriorated and the strength at high temperature is lowered, so the content was made 0.1 to 1.0% by weight.

Nb, like V, greatly increases the eutectoid transformation temperature, forms carbides in preference to Cr, and suppresses the formation of primary carbides of Cr which deteriorates machinability.
Moreover, it has the effect of suppressing the precipitation of secondary carbides at high temperatures and improving the oxidation resistance, but if it is less than 0.5 wt%, those effects are not sufficient, while if it exceeds 1.0 wt%, a large amount of carbides are formed. Since the amount of C in the mother phase is remarkably reduced and the strength is lowered, this is set to 0.5 to 1.0 wt%.

Mo has the effect of improving the strength and raising the eutectoid transformation temperature, but if it is less than 0.10 wt%, these effects are not sufficient, while if it exceeds 0.50 wt%, the toughness at low temperatures decreases. In addition, since the oxidation resistance is deteriorated, this is set to 0.10 to 0.50 wt%.

Since W is a harmful element which significantly deteriorates the oxidation resistance, it is treated as an impurity in the present invention in an amount of 0.1.
The value was suppressed to a low value of less than 01wt%.

Al makes the crystal grains finer and prolongs the thermal fatigue life remarkably, but if it is less than 0.01 wt%, these effects are not sufficient, while if it exceeds 0.50 wt%, the toughness at low temperature decreases. This was set to 0.01 to 0.50 wt%.

B is a particularly important element in the present invention because it refines the crystal grains to prolong the thermal fatigue life remarkably, but if it is less than 0.001 wt%, its effect is not sufficient, and
If it exceeds 5% by weight, the toughness at low temperature decreases, so this was made 0.001 to 0.5% by weight.

Ti is an extremely important element like B because it refines the crystal grains and prolongs the thermal fatigue life remarkably, but if it is less than 0.05 wt%, its effect is not sufficient and 0.5 w.
If it exceeds t%, the toughness at low temperature will decrease.
It was set to 0.05 to 0.5 wt%.

Co is a particularly important element in the present invention because it remarkably improves the high temperature strength.
If it is less than 5.0 wt%, the effect is not sufficient, and if it exceeds 5.0 wt%, the toughness decreases, so this was made 0.1 to 5.0 wt%.

[0022]

In the ferritic heat-resistant cast steel configured as described above, Mn, Ni, and W are kept low, while C, S
Since i, Cr, V, Nb, Mo, Al, B, Ti, Co, etc. are contained at a predetermined ratio, oxidation resistance, machinability,
The heat resistance and heat fatigue resistance can be significantly improved without impairing the structural stability. Further, by carrying out an annealing treatment of maintaining a predetermined temperature after casting and then gradually cooling, martensite is decomposed to form a ferrite structure.

[0023]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

As shown in Table 1, C, Si, Ni, Cr,
Materials 1 to 8 of the present invention in which B, Ti, and Co are added individually or in combination to a basic composition containing V, Nb, Mo, and Al (P, S, and W are unavoidable impurities) are cast. 93
After holding it at 0 ° C for 3 hours, it was annealed by furnace cooling (gradual cooling) to 650 ° C, and then, predetermined test pieces were taken from each casting material and subjected to the thermal fatigue test and high temperature tensile test described below. It was submitted to the test. For comparison, as shown in Table 2, Comparative Materials 9 and 10 having the basic composition of the material of the present invention (B, Ti and Co are removed), Comparative Material 11 which is general-purpose high Si spheroidal graphite cast iron, Niresist Comparative material 12 and JIS SCH1 comparative material 13 were cast, and comparative material 11 (high Si spheroidal graphite cast iron) and comparative material 12 (niresist) were left as casting, and comparative material 13 (SCH
Regarding 1), after holding the cast material at 850 ° C. for 2 hours,
It was annealed by furnace cooling to 600 ° C. and subjected to a thermal fatigue test and a high temperature tensile test. Here, the thermal fatigue test is a method of fixing the both ends of a notched test piece having a diameter of 10 mm and a length of 15 mm (restriction rate 100%), and applying a heat cycle from 200 ° C to 950 ° C to obtain the number of repetitions until breakage. The high temperature tensile test was performed at 950.

[0025]

[Table 1]

[0026]

[Table 2]

FIG. 1 shows the result of the thermal fatigue test. From the results shown in the figure, the present invention materials 1 and 2 in which B was added alone to the basic composition, the invention materials 3 and 4 in which Ti was added alone, and the invention material 7 in which B, Ti and Co were added in combination, 8
The thermal fatigue lives of all of the materials are significantly longer than those of the conventional general-purpose comparative materials 11 to 13, and are also longer than those of the comparative materials 9 and 10 which are the basic compositions of the present invention, and thus are significantly excellent in heat fatigue resistance. It became clear. Among the materials of the present invention, the composite addition of B, Ti and Co has a longer thermal fatigue life than the addition of these alone. Also,
As a whole, when each element was added at a low level, the thermal fatigue life tended to be slightly extended compared to when each element was added at a high level, but it is assumed that this was caused by the difference in toughness. To be done.

FIG. 2 shows the results of the high temperature tensile test. From the results shown in the figure, the Co
The tensile strengths of the present invention materials 5 and 6, which are added alone, and the present invention materials 7 and 8 to which B, Ti, and Co are added in addition, are significantly increased as compared with the conventional general-purpose comparative materials 11 to 13, and It was revealed that the strength was higher than that of the comparative materials 9 and 10 which are the basic compositions of the invention, and that the high temperature strength was remarkably excellent. In addition, among the materials of the present invention, those in which each element is added at a higher level as a whole tend to have a higher tensile strength than those in which each element is added at a low level as a whole.

[0029]

As described above in detail, according to the ferritic heat-resistant cast steel according to the present invention, the heat-resistant strength without impairing the oxidation resistance, machinability, structural stability, etc.,
The thermal fatigue resistance can be greatly improved, and sufficient performance can be exerted when applied to, for example, an exhaust system of a high-power engine of an automobile. Further, since the heat resistance is improved, the wall thickness can be reduced, which contributes to the weight reduction.
Further, according to the method for producing a ferritic heat-resistant cast steel according to the present invention, a ferrite structure can be stably obtained, which greatly contributes to ensuring desired heat resistance.

[Brief description of drawings]

FIG. 1 is a graph showing the thermal fatigue resistance of the present ferritic heat-resistant cast steel in comparison with a comparative material.

FIG. 2 is a graph showing high temperature strength of the present ferritic heat resistant cast steel in comparison with a comparative material.

Claims (2)

[Claims] 1. By weight%, C: 0.05 to 0.5, Si:
1.0 to 2.0, Mn: less than 0.6, P: less than 0.04, S: 0.04
Less, Ni: less than 0.5, Cr: 10-20, V: 0.1-1.0
, Nb: 0.5 to 1.0, Mo: 0.1 to 0.5, W: less than 0.01, Al: 0.01 to 0.50, and B: 0.001 to 0.5, Ti.
: 0.05 to 0.5, Co: 0.1 to 5.0, at least one kind of ferritic heat-resistant cast steel. 2. After casting the material having the components according to claim 1, holding the material at 850 to 1000 ° C. for 1 to 5 hours, and subsequently performing an annealing treatment of gradually cooling it to a temperature of 700 ° C. or less. A method for producing a ferritic heat-resistant cast steel, characterized by: