Heat-resistant, austenite cast steel and exhaust equipment member made thereof

Abstract: The invention relates to a heat-resistant, austenite cast steel having a composition consisting essentially, by weight, of: C: 0.15-0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0-20.0 %, Cr: 15.0-30.0 %, W: 2.0-6.0 %, Nb: 0.2-1.0 %, and B: 0.001-0.01 %, the balance consisting of Fe and inevitable impurities, and to exhaust equipment members made of that heat-resistant, austenite cast steel. This steel is of excellent high-temperature strength and room ductility, and can be produced according to known methods at low cost.

Description: BACKGROUND OF THE INVENTION The present invention relates to a heat-resistant cast steel suitable for exhaust equipment members for automobiles, etc., and more particularly to a heat-resistant austenite cast steel having an excellent high-temperature strength, particularly at 900 °C or higher, and to exhaust equipment members made of such a heat-resistant cast steel. Conventional heat-resistant cast iron and heat-resistant cast steel have compositions shown in Table 1 as Comparative Examples. In exhaust equipment members such as exhaust manifolds, turbine housings, etc. for automobiles, heat-resistant cast iron such as high-Si spheroidal graphite cast iron, NI-RESIST cast iron (Ni-Cr-Cu austenite cast iron), heat-resistant cast steel such as ferritic cast steel, etc. shown in Table 1 are employed because their operating conditions are extremely severe at high temperatures. Further, attempts have been made to propose various heat-resistant austenite cast steels. For instance, JP-A-61-87852 discloses a heat-resistant austenite cast steel consisting essentially of C, Si, Mn, N, Ni, Cr, V, Nb, Ti, B, W and Fe showing improved creep strength and yield strength. In addition, JP-A-61-177352 discloses a heat-resistant austenite cast steel consisting essentially of C, Si, Mn, Cr, Ni, Al, Ti, B, Nb and Fe having improved high-temperature and room-temperature properties by choosing particular oxygen content and cleaning rate. JP-A-57-8183 discloses a heat-resistant austenite cast steel having improved high-temperature strength, without suffering from the decrease in high-temperature oxidation resistance by increasing the carbon content of the heat-resistant austenite cast steel made of an Fe-Ni-Cr alloy and by adding Nb and Co. Among these conventional heat-resistant cast irons and heat-resistant cast steels, for instance, the high-Si spheroidal graphite cast iron is relatively good in room-temperature strength, but it is poor in high-temperature strength and oxidation resistance. The NI-RESIST cast iron is relatively good in high-temperature strength up to 900 °C, but it is poor in durability at 900 °C or higher. Also, it is expensive because of the high Ni content. Heat-resistant ferritic cast steel is extremely poor in high-temperature strength at 900 °C or higher. Since the heat-resistant austenite cast steel disclosed in JP-A-61-87852 has a relatively low C content of 0.15 weight % or less, the resulting cast steel shows an insufficient high-temperature strength at 900 °C or higher. In addition, since it contains 0.002-0.5 weight % of Ti, harmful non-metallic inclusions may be formed by melting under normal atmosphere. In addition, since the heat-resistant austenite cast steel disclosed in JP-A-61-177352 contains a large amount of Ni, cracks may occur when used in an atmosphere containing sulfur (S) at a high temperature. Further, since the heat-resistant austenite cast steel disclosed in JP-A-57-8183 has a high carbon (C) content, it may become brittle when operated at a high temperature for a long period of time. OBJECT AND SUMMARY OF THE INVENTION Accordingly, the object of the present invention is to provide a heat-resistant austenite cast steel having an excellent high-temperature strength, which can be produced at a low cost, thereby solving the above problems inherent in the conventional heat-resistant cast iron and heat-resistant cast steels. Furthermore, exhaust equipment members made of such heat-resistant cast steel are to be provided. The above object is achieved according to claim 1. The dependent claims relate to prefered embodiments. As a result of intense research in view of the above object, the inventors have found that by adding proper amounts of W, Nb and B and optionally Mo and/or Co to the Ni-Cr base austenite cast steel, the high-temperature strength of the cast steel can be improved. The present invention has been completed based upon this finding. Thus, the heat-resistant austenite cast steel according to the present invention has the following composition, by weight: C: 0.20-0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0-20.0 %, Cr: 15.0-30.0 %, W: 2.0-6.0 %, Nb: 0.2-1.0 %, and B: 0.001-0.01 %, and optionally Mo: 0.2-1.0 % and/or Co: 20.0 % or less, the balance being Fe plus incidental impurities. The heat-resistant austenite cast steel according to a first embodiment of the present invention has the following composition, by weight: C: 0.20-0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0-20.0 %, Cr: 15.0-30.0 %, W: 2.0-6.0 %, Nb: 0.2-1.0 %, B: 0.001-0.01 %, and Mo: 0.2-1.0 %, the balance being Fe and incidental impurities. The heat-resistant austenite cast steel according to a second embodiment of the present invention has the following composition, by weight: C: 0.20-0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0-20.0 %, Cr: 15.0-30.0 %, W: 2.0-6.0 %, Nb: 0.2-1.0 %, B: 0.001-0.01 %, and Co: 20.0 % or less, the balance being Fe and incidental impurities. The heat-resistant austenite cast steel according to a third embodiment of the present invention has the following composition, by weight: C: 0.20-0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0-20.0 %, Cr: 15.0-30.0 %, W: 2.0-6.0 %, Nb: 0.2-1.0 %, B: 0.001-0.01 %, Mo: 0.2-1.0 %, and Co: 20.0 % or less, the balance being Fe and incidental impurities. The exhaust equipment member according to the present invention is made of any one of the above heat-resistant austenite cast steels. DETAILED DESCRIPTION OF THE INVENTION The present invention will be explained in detail below. Due to the addition of 2.0-6.0 % of W, 0.2-1.0 % of Nb and 0.001-0.1 % of B, by weight and, optionally, proper amounts of Mo and Co alone or in combination, the resulting heat-resistant austenite cast steel shows an excellent high-temperature strength. The reasons for restricting the composition range of each alloy element in the heat-resistant austenite cast steel of the present invention will be explained below. In the heat-resistant austenite cast steel of the present invention, C, Si, Mn, Ni, Cr, W, Nb and B are indispensable alloy elements. (1) C (carbon): 0.20-0.60 % C has the function of improving the fluidity and castability of a melt and also partly dissolves into the matrix phase, thereby exhibiting a solution strengthening function. Besides, it forms primary carbides, thereby improving the high-temperature strength. To exhibit such functions effectively, the amount of C should be 0.20 % or more. On the other hand, when the amount of C exceeds 0.60 %, secondary carbides are excessively precipitated, leading to a poor toughness. Accordingly, the amount of C is 0.20-0.60 %. The preferred amount of C is 0.20-0.50 %. (2) Si (silicon): 2.0 % or less Si has the function of a deoxidizer and is also effective for improving the oxidation resistance. However, when it is excessively added, the austenite structure of the cast steel become unstable, leading to a poor high-temperature strength. Accordingly, the amount of Si should be 2.0 % or less. The preferred amount of Si is 0.50-1.50 %. (3) Mn (manganese): 1.0 % or less Mn is effective like Si as a deoxidizer for the melt. However, when it is excessively added, its oxidation resistance is deteriorated. Accordingly, the amount of Mn is 1.0 % or less. The preferred amount of Mn is 0.30-0.80 %. (4) Ni (nickel): 8.0-20.0 % Ni is an element effective for forming and stabilizing the austenite structure of the heat-resistant cast steel of the present invention, together with Co and Cr, thereby improving the high-temperature strength. Particularly, to have a good high-temperature strength at 900 °C or higher, the amount of Ni should be 8.0 % or more. As the amount of Ni increases, such effects increase. However, when it exceeds 20.0 %, the effects are levelled off. This means that an amount of Ni exceeding 20.0 % is economically disadvantageous. Accordingly, the amount of Ni is 8.0-20.0 %. The preferred amount of Ni is 8.0-15.0 %. (5) Cr (chromium): 15.0-30.0 % Cr is an element capable of austenizing the cast steel structure when it coexists with Ni and Co, improving high-temperature strength and oxidation resistance. It also forms carbides, thereby further improving the high-temperature strength. To exhibit effectively such effects at a high temperature of 900 °C or higher, the amount of Cr should be 15.0 % or more. On the other hand, when it exceeds 30.0 %, secondary carbides are excessively precipitated and a brittle δ-phase, etc. are also precipitated, resulting in an extreme brittleness. Accordingly, the amount of Cr should be 15.0-30.0 %. The preferred amount of Cr is 15.0-25.0 %. (6) W (tungsten): 2.0-6.0 % W has the function of improving the high-temperature strength. To exhibit such an effect effectively, the amount of W should be 2.0% or more. However, if it is excessively added, the oxidation resistance is deteriorated. Thus, the upper limit of W is 6.0 %. Accordingly, the amount of W is 2.0-6.0 %. The preferred amount of W is 2.0-4.0 %. (7) Nb (niobium): 0.2-1.0 % Nb forms fine carbides when combined with C, increasing the high-temperature strength. Also, by suppressing the formation of the Cr carbides, it functions to improve the oxidation resistance. For such purposes, the amount of Nb should be 0.2 % or more. However, if it is excessively added, the toughness of the resulting austenite cast steel is deteriorated. Accordingly, the upper limit of Nb is 1.0 %. Therefore, the amount of Nb should be 0.2-1.0 %. The preferred amount of Nb is 0.2-0.8 %. (8) B (boron): 0.001-0.01 % B has the function of strengthening the crystal grain boundaries of the cast steel and making carbides in the grain boundaries finer and further deterring the agglomeration and growth of such carbides, thereby improving the high-temperature strength and toughness of the heat-resistant austenite cast steel. Accordingly, the amount of B is desirably 0.001 % or more. However, if it is excessively added, borides are precipitated, leading to a poor high-temperature strength. Thus, the upper limit of B is 0.01 %. Therefore, the amount of B is 0.001-0.01 %. The preferred amount of B is 0.001-0.007 %. In the preferred embodiments, Mo and Co may be added alone or in combination together with the above indispensable elements. (9) Mo (molybdenum): 0.2-1.0 % Mo has functions which are similar to those of W. However, by addition of Mo alone, smaller effects are achieved than in cases where W is used alone.Accordingly, to have synergistic effects with W, the amount of Mo should be 0.2-1.0 %. The preferred amount of Mo is 0.3-0.8 %. (10) Co (cobalt): 20.0 % or less Co is an element effective like Ni for stabilizing the austenite structure, thereby improving the high-temperature strength. Particularly when added together with Ni, the austenite structure is further stabilized. Also, in an operating atmosphere containing S, Ni tends to form a low-melting point sulfide. Accordingly, Co is more preferable. When the total amount of Ni + Co exceeds 30 %, no further improvement is achieved, leading to an economical disadvantage. Accordingly, the total amount of Ni + Co should be 8.0-30.0 %. However, Co contents exceeding 20.0 % would provide no further improvement, also leading to an economical disadvantage. Accordingly, the amount of Co should be 8.0-20.0 %. The preferred amount of Co is 3.0-15.0 %. The heat-resistant, austenite cast steel of the present invention is particularly suitable for thin parts such as exhaust equipment members, exhaust manifolds, turbine housings, etc., particularly for automobile engines, which should be durable without occurence of cracks under conditions of repeated heating-cooling cycles. The present invention will be explained in detail by way of the following Examples. Examples 1-19, and Comparative Examples 1-5 With respect to heat-resistant austenite cast steels having compositions shown in Table 1, Y-block test pieces (No. B according to JIS) were prepared by casting. Incidentally, the casting was conducted by melting the steel under normal atmosphere in a 100-kg high-frequency furnace, removing the resulting melt from the furnace while it is at a temperature of 1550 °C or higher, and pouring it into a mold at about 1500 °C or higher. The heat-resistant austenite cast steels of the present invention (Examples 1-19) showed good fluidity at casting, thereby avoiding cast defects such as voids. Next, test pieces (Y-blocks) of Examples 1-19 and Comparative Examples 3, 4 and 5 were subjected to a heat treatment comprising heating them at 1000 °C for 2 hours and then cooling them in the air. On the other hand, the test piece of Comparative Example 1 was used the an as-cast state for the tests. The test piece of Comparative Example 2 was subjected to a heat treatment comprising heating it at 800 °C for 2 hours in a furnace and cooling it in the air. Incidentally, the test pieces of Comparative Examples 1-5 in Table 1 are those used for heat-resistant parts such as turbo charger housings, exhaust manifolds, etc. for automobiles. The test piece of Comparative Example 1 is high-Si spheroidal graphite cast iron. The test piece of Comparative Example 2 is a CB-30 according to the ACI (Alloy Casting Institute) standards. The test pieces of Comparative Examples 3 and 4 are D2 and D5S of NI-RESIST cast iron. The test piece of Comparative Example 5 is a conventional heat-resistant austenite cast steel SCH-12 according to JIS. Next, with respect to each cast test piece, the following evaluation tests were conducted. (1) Tensile test at room temperature Conducted on a rod test piece having a gauge distance of 50 mm and a gauge diameter of 14 mm (No. 4 test piece according to JIS). (2) Tensile test at a high temperature Conducted on a flanged test piece having a gauge distance of 50 mm and a gauge diameter of 10 mm at temperatures of 900 °C and 1050 °C, respectively. (3) Thermal fatigue test Using a rod test piece having a gauge distance of 20 mm and a gauge diameter of 10 mm, a heating-cooling cycle was repeated to cause thermal fatigue failure in a state where expansion and shrinkage due to heating and cooling were completely restrained mechanically, under the following conditions: Lowest temperature: 150 °C. Highest temperature: 1000 °C. Duration of 1 cycle: 12 min each. Incidentally, an electro-hydraulic servo-type thermal fatigue test machine was used for the test. (4) Oxidation test A rod test piece having a diameter of 10 mm and a length of 20 mm was kept in the air at 1000 °C for 200 hours, and its oxide scale was removed by a shot blasting treatment to measure the weight variation per unit surface area. By calculating oxidation weight loss (mg/cm²) after the oxidation test, the oxidation resistance was evaluated. The results of the tensile test at room temperature are shown in Table 2, the results of the tensile test at high temperature are shown in Table 3, and the results of the thermal fatigue test and the oxidation test are shown in Table 4. Tests at Room Temperature Example No. 0.2 % Offset Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Hardness (H B ) 1 250 595 26 170 2 300 555 11 179 3 280 510 7 201 4 265 555 13 179 5 275 560 12 187 6 275 590 19 179 7 300 565 11 197 8 285 540 12 183 9 300 555 11 192 10 255 565 14 179 11 325 540 4 223 12 280 600 14 197 13 325 525 4 217 14 335 540 4 217 15 315 540 10 201 16 290 540 6 217 17 320 545 5 223 18 305 540 7 201 19 305 535 9 201 Comparative Example No. 1 510 640 11 217 2 540 760 4 240 3 190 455 16 179 4 255 485 9 163 5 250 560 20 170 Example No. Thermal Fatigue Life (Cycles) Weight Loss by Oxidation (mg/mm²) 1 88 25 2 92 30 3 115 15 4 105 18 5 102 18 6 120 35 7 135 40 8 105 50 9 110 50 10 152 26 11 145 35 12 160 30 13 175 35 14 185 18 15 180 23 16 150 28 17 195 15 18 165 20 19 177 22 Comparative Example No. 1 - - 2 10 105 3 56 765 4 85 55 5 80 85 As is clear from Tables 2-4, the test pieces of Examples 1-19 are comparable to or even superior to those of Comparative Examples 3 and 4 (NI-RESIST D2 and D5S) with respect to the properties at room temperature, and particularly superior with respect to the high-temperature strength at 900 °C or higher. In addition, the test pieces of Examples 1-19 are superior to that of Comparative Example 5 (SCH12) with respect to the high-temperature strength at 1000 °C. Also, as shown in Table 2, the test pieces of Examples 1-19 show relatively low hardness (H B ) of 170-223. This means that they are excellent in machinability. Next, an exhaust manifold (thickness: 2.5-3.4 mm) and a turbine housing (thickness: 2.7-4.1 mm) were produced by casting the heat-resistant austenite cast steel of Examples 5, 15 and 19. All of the resulting heat-resistant cast steel parts were free from casting defects. These cast parts were machined to evaluate their cuttability. As a result, no problem was found in any cast parts. Next, the exhaust manifold and the turbine housing were mounted to a high-performance, straight-type, four-cylinder, 2 -ℓ gasoline engine (test machine) to conduct a durability test. The test was conducted by repeating 500 heating-cooling (Go-Stop) cycles each consisting of a continuous full-load operation at 6000 rpm (14 minutes), idling (1 minute), complete stop (14 minutes) and idling (1 minute) in this order. The exhaust gas temperature under full load was 1050 °C at the inlet of the turbo charger housing. Under these conditions, the highest surface temperature of the exhaust manifold was about 980 °C in a pipe-gathering portion thereof, and the highest surface temperature of the turbo charger housing was about 1020 °C in a waist gate portion thereof. As a result of the evaluation test, no gas leak and thermal cracking were observed. It was thus confirmed that the exhaust manifold and the turbine housing made of the heat-resistant austenite cast steel of the present invention had excellent durability and reliability. As described above in detail, the heat-resistant austenite casting steel of the present invention has an excellent high-temperature strength, particularly at 900 °C or higher, without deterioration of the room-temperature ductility, and it can be produced at low cost. The heat-resistant austenite cast steel of the present invention is particularly suitable for exhaust equipment members for engines, etc., such as exhaust manifolds, turbine housings, etc. The exhaust equipment members made of such heat-resistant austenite cast steel according to the present invention have excellent high-temperature strength, thereby showing extremely good durability.

Claims: Heat-resistant austenite cast steel having the following composition, by weight: C: 0.20 - 0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0 - 20.0 %, Cr: 15.0 - 30.0 %, W: 2.0 - 6.0 %, Nb: 0.2 - 1.0 %, and B: 0.001 - 0.01 %, and optionally Mo: 0.2 - 1.0 % and/or Co: 20.0 % or less, the balance being Fe plus incidental impurities. Heat-resistant austenite cast steel according to claim 1, having the following composition, by weight: C: 0.20 - 0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0 - 20.0 %, Cr: 15.0 - 30.0 %, W: 2.0 - 6.0 %, Nb: 0.2 - 1.0 %, B: 0.001 - 0.01 %, and Mo: 0.2 - 1.0 %, the balance being Fe plus incidental impurities. Heat resistant austenite cast steel according to claim 1, having the following composition, by weight: C: 0.20 - 0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0 - 20.0 %, Cr: 15.0 - 30.0 %, W: 2.0 - 6.0 %, Nb: 0.2 - 1.0 %, B: 0.001 - 0.01 %, and Co: 20.0 or less, the balance being Fe plus incidental impurities. Heat resistant austenite cast steel according to claim 1, having the following composition, by weight: C: 0.20 - 0.60 %, Si: 2.0 % or less, Mn: 1.0 % or less, Ni: 8.0 - 20.0 %, Cr: 15.0 - 30.0 %, W: 2.0 - 6.0 %, Nb: 0.2 - 1.0 %, B: 0.001 - 0.01 %, Mo: 0.2 - 1.0 %, and Co: 20.0 % or less, the balance being Fe plus incidental impurities. Heat-resistant austenite cast steel according to claims 1 to 4, characterized by one or more of the following composition features: C: 0.20 - 0.50 % (by weight), Si: 0.50 - 1.50 %, Mn: 0.30 - 0.80 %, Ni: 8.0 - 15.0 %, Cr: 15.0 - 25.0 %, W: 2.0 - 4.0 %, Nb: 0.2 - 0.8 %, B: 0.001 - 0.007 %, Mo: 0.3 - 0.8 %, Co: 3.0 - 15.0 %. Exhaust equipment member made of a heat-resistant austenite cast steel according to one of claims 1 to 5. Exhaust equipment member according to claim 6, characterized in that it is an exhaust manifold. Exhaust equipment member according to claim 6, characterized in that it is a turbine housing.

Inventors: FUJITA TOSHIO, TAKAHASHI NORIO, FUJITA, TOSHIO, TAKAHASHI, NORIO
Applicants: HITACHI METALS LTD, HITACHI METALS, LTD.
Application year: 1991
Main classification: C22C38
IPC codes: C22C38, F16N7