JPH0445245A - High heat resistant alpha and gamma finely mixed cast steel - Google Patents

Abstract

PURPOSE:To obtain a high heat resistant alpha and gamma finely mixed cast steel having the low coefficient of thermal expansion and excellent in high temp. strength by specifying a compsn. constituted of C, Si, Mn, Cr, Ni, Nb, P, S and Fe and regulating a fine ferrite precipitated phase. CONSTITUTION:This is a high heat resistant alpha and gamma finely mixed cast steel having a compsn. contg., by weight, 0.20 to 0.40% C, 1.5 to 2.5% Si, <=1.0% Mn, 13 to 17% Cr, 4.5 to 8.5% Ni, 0.30 to 0.70% Nb, <=0.05% P, <=0.10% S and the balance Fe with impurity elements and having a structure in which 40 to 50vol.% fine ferritic phase with the intervals of 0.1 to 1.0mum is precipitated into austenitic grains. The steel has the low coefficient of thermal expansion and small deformation as well as excellent in high temp. strength. The above structure can be obtd. by rising the temp. of a cast steel as cast to the A3 point or above to perfectly convert its phase into an austenitic one, thereafter subjecting it to furnace cooling to the A1 point or below and perfectly decomposing partially precipitated pearlite into ferrite.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a stainless steel heat-resistant cast steel.

[Prior Art] Stainless steel cast steel has excellent corrosion resistance and heat resistance, so it is widely used as heat-resistant cast steel. Among these stainless steel heat-resistant cast steels, 3
It has a ferrite structure containing 0% or less Cr and 7% or less Ni. Therefore, although its high temperature strength is relatively low, it has excellent oxidation resistance and corrosion resistance against S. As the Cr-Fe system, for example, JIS, S containing 28% Cr
CH2 (ASTM: HC) has excellent oxidation resistance and corrosion resistance against S up to about 1100°C.

In addition, Cr-Ni-Fe system has 18 to 23% Cr, 8 to 2
It contains 2% Ni and has a partially or completely austenitic structure, and has higher high temperature strength and toughness than the Cr-Fe system. It can withstand larger loads and temperature changes, and can be used in oxidizing and reducing atmospheres containing S. For example, 25
%Cr-20%Ni JIS:SCH14(AST
M:HK) has the highest strength at 1038°C or higher, and 11
Widely used as a structural material up to 50°C.

[Problems to be Solved by the Invention] The Cr-Fe-based ferritic heat-resistant cast steel has 21 to
The coefficient of thermal expansion at 1093℃ is 13.91/'CXl
0-6 and has a low coefficient of thermal expansion, which has the advantage of little deformation, but has the disadvantage of poor strength at high temperatures. Therefore, it is possible to improve the high-temperature strength by high-alloying this ferritic heat-resistant cast steel, but high-alloying reduces the toughness at room temperature.

On the other hand, although Cr-Ni-Fe-based austenitic heat-resistant cast steel has excellent high-temperature strength, it
The coefficient of thermal expansion is 18.21/'C×104, which is about 30% higher than that of the ferritic heat-resistant cast steel, and it has the disadvantage of being easily deformed.

However, in recent years, due to the demand for improved fuel efficiency and power performance of automobile engines, it is necessary to make exhaust system parts that require heat resistance thinner, so heat resistant parts with low thermal expansion and excellent high temperature strength are required. It is hoped that cast steel will emerge.

The present invention was made to solve the above-mentioned problems of stainless steel heat-resistant cast steel, and aims to provide a heat-resistant cast steel that has a low coefficient of thermal expansion, little deformation, and excellent high-temperature strength. .

[Means for Solving the Problems] The high heat resistant α, γ fine mixed cast steel of the present invention has a weight ratio of
C: 0.20-0.40%, Si: 1.5-2°5%,
Mn: 1.0% or less, Cr: 13-17%, Ni; 4.
5-8.5%, Nb; 0.30-0.70%, P; 0.
Contains 0.05% or less, S; 0.10% or less, and the balance is Fe.
and impurity elements, and 0.
Fine ferrite phase with an interval of 1 to 1.0 μm at a volume ratio of 4
The gist is to have a structure with 0 to 50% precipitation.

The inventor came up with the idea of creating a two-phase mixed structure of an austenite phase and a ferrite phase in order to improve high-temperature strength and ensure a low coefficient of thermal expansion. Therefore, Cr; 13-17%, N
i; 4.5 to 8.5% to form a mixed structure of austenite and ferrite, and Nb, 0.30%.
-0.70% was included to ensure high temperature strength. Also,
We obtained new knowledge that the shape of the ferrite phase precipitated within austenite grains has an important relationship with high-temperature strength and coefficient of thermal expansion, and completed the present invention by regulating the shape of the ferrite phase through heat treatment.

In the steel of the present invention, the desired high-temperature strength and low coefficient of thermal expansion can be achieved by precipitating a fine ferrite phase with a spacing of 0.1 to 1.0μ in the austenite grains at a volume ratio of 40 to 50% through heat treatment. It will be done. In order to precipitate 40 to 50% by volume of fine ferrite phases with an interval of 0°1 to 1.0 μm within the austenite grains by heat treatment, A
After holding at one or more temperatures, for example, 1000 to 1100°C, for 2 to 3 hours to completely transform into an austenite phase, it is furnace cooled to a temperature below the A1 point, for example, 550 to 650°C.

Furthermore, this temperature is maintained for 4 to 5 hours to completely decompose the partially precipitated pearlite into ferrite, resulting in a two-phase mixed structure of austenite and ferrite.

[Effects] In the steel of the present invention, Cr is 13 to 17% and Ni is 4.5 to 8%.
Since the content was 5%, the high temperature strength was improved and a low thermal elongation of 11 could be ensured. In addition, Nb is 0.30 to 0
．． Since the content was 70%, the high temperature strength could be further improved. In addition, 0.1 to 1.0
40-50% volume fraction of fine ferrite phase with μm spacing
By precipitating it, we were able to ensure high-temperature strength comparable to that of austenitic heat-resistant cast steel, and to have a coefficient of thermal expansion close to that of ferritic heat-resistant cast steel.

Next, the reason for limiting the component composition in the present invention will be explained.

C・0.20-0.40% C is an element necessary to improve the strength characteristics and castability of steel, and in order to obtain the above effects, it must be at least 0.2%.
It is necessary to contain the above. However, C is originally an austenite stabilizing element, and if the content exceeds 0.4%, it becomes difficult to form ferrite and a two-phase mixed structure cannot be obtained, so the upper limit was set at 0.4%.

Si 1.5-25% Si is an element that is effective as a deoxidizing agent and improves castability and oxidation resistance.

If the content is less than 1.5%, the above-mentioned improvement effect cannot be obtained, so the lower limit is set to 1.5%. However, if the content exceeds 2.5%, the toughness decreases. Further, since Si is a ferrite stabilizing element and causes excessive precipitation of ferrite, the upper limit was set at 2.5%.

Mn: 1.0% or less Mo acts as a deoxidizing and desulfurizing agent and is effective in stabilizing austenite. However, if the content exceeds 1.0%, the toughness deteriorates, so the upper limit was set at 1.0%.

P: 0.05% or less, S, 0.10% or less Both P and S are harmful elements in stainless steel, and when contained in large amounts, the mechanical properties deteriorate, so the upper limit for P is 0.05%. %, and for S, the upper limit was limited to 0.910%.

Cr: 13-17% Cr is a basic component of stainless steel and is extremely effective for oxidation resistance, but if it is less than 13%, the effect is not sufficient, and since it is a ferrite stabilizing element, it is difficult to achieve the target. Considering that the ferrite content should not exceed 17%, the Cr content was set at 13 to 17%.

Ni: 4.5 to 8.5% Ni is an element that interacts with Cr to improve oxidation resistance and stabilize austenite. Since the above effect is not sufficient if the content is less than 4.5%, the lower limit was set at 4.5%. Moreover, if the content exceeds 8.5%, the ferrite phase disappears, the workability deteriorates, and the cost increases, so the upper limit was set at 8.5%.

Nb: 0.30-0.70% Nb is a strong carbide-forming element and significantly improves mechanical properties, especially strength. In order to obtain the above effect, it is necessary to contain at least 0.3% or more. However, when added in large amounts, it stabilizes ferrite and destabilizes austenite.2. Please set the upper limit to 0.70%.

In the steel of the present invention, the distance between fine ferrite phases precipitated within austenite grains is reduced to 0°1 to 1.0 μm by heat treatment.
The reason for using steel is that if the spacing between the ferrite phases is less than 0.1 μm, the coefficient of thermal expansion will not decrease sufficiently.
, 0 μm, the desired high temperature strength cannot be obtained. In addition, the reason why the volume fraction of the ferrite phase is set to 40 to 50% is because if the ferrite phase is less than 40%, the coefficient of thermal expansion will be too large, and if it exceeds 50%, the high temperature strength will be too low. It is.

[Example] Examples of the present invention will be explained while comparing them with comparative examples to clarify the effects of the present invention.

Materials of the present invention and comparative materials having the chemical components shown in Table 1 were melted. In addition, comparative material 1 is a ferritic heat-resistant cast steel corresponding to JIS: 5CH2, and comparative material 2 is ASTM:
This is an austenitic heat-resistant cast steel corresponding to HK40, and Comparative Material 3 is a ferritic-austenitic heat-resistant cast steel corresponding to ASTM/HE.

(The following is a blank space) In the invention material, the metal structure in the as-cast state was a state in which ferrite was scattered in austenite base. The tensile strength at 950°C and the thermal expansion coefficient at 20 to 900°C of the invention material before heat treatment were measured. Subsequently, this invention material was heat treated (at 1050°C for 2.5
As a result of holding for an hour → furnace cooling to 600°C and holding for 4.5 hours → air cooling, fine ferrite was precipitated on the austenite base, as shown in the micrograph showing the metal structure of the invented material in Figure 1. tissue was obtained. The spacing between the fine ferrite phases was 0.5 μm, and the volume fraction of ferrite was 45%. Regarding the invented material after this heat treatment, 95
The tensile strength at 0°C and the coefficient of thermal expansion from 20 to 900°C were measured.

Comparative material 1 was aged at 760°C for 24 hours and then furnace cooled, and the tensile strength at 950°C and the p! ! , l! The 5 tension coefficient was measured. Further, for Comparative Material 2 and Comparative Material 3, the tensile strength at 950°C and the coefficient of thermal expansion at 20 to 900°C were measured in an unrusted state.

The results obtained are summarized in Table 2.

Table 2 As is known from Table 2, Comparative Material 1 is a ferritic heat-resistant cast steel, so its coefficient of thermal expansion is low, but its high temperature strength is low at 2.0 kgf/-Peng 2. Comparative Material 2 is ausbinite. Since it is a heat-resistant cast steel, the high temperature strength is 10.2K.
Although the gf/am2 is high, the thermal expansion coefficient is 17.5 x 1
Comparative material 3, which has a large length of 0-'1/'C, has a mixed structure of ferrite and ausinite, but since ferrite is not mixed finely, its high temperature strength is lower than that of the inventive material, and its thermal expansion is lower. The coefficient is as large as 19°OX 10-'1/'C.

On the other hand, in the as-cast state, the material of the present invention has ferrite scattered in the ausinite base, so its high temperature strength is as low as 6.5 kgf/s+m2, and its coefficient of thermal expansion is also 17.
．． 0x10-1/'C, which is similar to austenite grains, but the heat-treated one has a two-phase mixed structure with fine ferrite precipitated on an ausinite base, so the high temperature strength is 9.1 kgf/am'', Thermal expansion coefficient is 15.
5 x 10-'1/'C, and it was confirmed that the steel of the present invention has excellent high-temperature strength and a low coefficient of thermal expansion.

[Effect of the invention] As explained above, the high heat-resistant α, γ fine mixed cast steel of the present invention contains 13 to 17% Cr and 4.5 to 8.5% Ni, so it has improved high-temperature strength. However, by ensuring a low coefficient of thermal expansion and containing 0.30 to 0.70% Nb, high-temperature strength is further improved, and 0.3% Nb is contained within the austenite grains.
Fine ferrite phase with a ring spacing of 1 to 1.0μ, with a volume fraction of 4
Since the precipitation was 0 to 50%, it was possible to ensure high-temperature strength comparable to that of austenitic heat-resistant cast steel, and to bring the coefficient of thermal expansion close to that of ferritic heat-resistant cast steel. As a result, we can provide heat-resistant cast steel with low thermal expansion and excellent high-temperature strength, making it possible to reduce the thickness of exhaust system parts that require heat resistance in automobile engines, which is extremely useful for improving fuel efficiency and power performance. be.

[Brief explanation of drawings]

FIG. 1 is a micrograph showing the metal structure of the steel of the present invention. Patent applicant: Toyota Motor Corporation

Claims (1)

[Claims] (1) Weight ratio: C: 0.20-0.40%, Si: 1
．． 5-2.5%, Mn; 1.0% or less, Cr; 13-1
7%, Ni; 4.5-8.5%, Nb; 0.30-0.
70%, P: 0.05% or less, S: 0.10% or less, the remainder consists of Fe and impurity elements, and a fine ferrite phase with an interval of 0.1 to 1.0 μm is formed within the austenite grains. A highly heat-resistant alpha, gamma micro-mixed cast steel characterized by having a structure with a precipitated structure of 40 to 50%.