Abstract:
A heat-resistant, austenitic 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%, B: 0.001-0.01%, and the balance being Fe and inevitable impurities is disclosed. The austenitic cast steel of the invention is ideally suited for use in exhaust equipment members.

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 an exhaust equipment member 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, Japanese Patent Laid-Open No. 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, Japanese Patent Laid-Open No. 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. Japanese Patent Publication No. 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 high Ni content. Heat-resistant, ferritic cast steel is extremely poor in high-temperature strength at 900° C. or higher. 
     Because the heat-resistant, austenite cast steel disclosed in Japanese Patent Laid-Open No. 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, because it contains 0.002-0.5 weight % of Ti, harmful non-metallic inclusions may be formed by melting in the atmosphere. 
     In addition, because the heat-resistant, austenite cast steel disclosed in Japanese Patent Laid-Open No. 61-177352 contains a large amount of Ni, it may suffer from cracks when used in an atmosphere containing sulfur (S) at a high temperature. 
     Further, because the heat-resistant, austenite cast steel disclosed in Japanese Patent Publication No. 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, an object of the present invention is to provide a heat-resistant, austenitic cast steel having 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 steel. 
     Another object of the present invention is to provide an exhaust equipment member made of such heat-resistant cast steel. 
     As a result of intense research in view of the above objects, 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 austenitic 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, austenitic cast steel according to a first embodiment of the present invention has a composition consisting essentially, by weight, of: 
     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 
     the balance being Fe and inevitable impurities. 
     The heat-resistant, austenitic cast steel according to a second embodiment of the present invention has a composition consisting essentially, by weight, of: 
     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 
     Fe and inevitable impurities: balance. 
     The heat-resistant, austenitic cast steel according to a third embodiment of the present invention has a composition consisting essentially, by weight, of: 
     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%, 
     Co: 20.0% or less, and 
     the balance being Fe and inevitable impurities. 
     The heat-resistant, austenitic cast steel according to a fourth embodiment of the present invention has a composition consisting essentially, by weight, of: 
     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%, 
     Co: 20.0% or less, and 
     the balance being Fe and inevitable impurities. 
     The exhaust equipment member according to the present invention is made of any one of the above heat-resistant, austenitic cast steels. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be explained in detail below. 
     By adding 2.0-6.0% of W, 0.2-1.0% of Nb and 0.001-0.1% of B by weight and, if necessary, proper amounts of Mo and Co alone or in combination, the resulting heat-resistant, austenitic cast steel shows an excellent high-temperature strength. 
     The reasons for restricting the composition range of each alloy element in the heat-resistant, austenitic cast steel of the present invention will be explained below. 
     In the heat-resistant, austenitic 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 a function of improving the fluidity and castability of a melt and also partly dissolves into a matrix phase, thereby exhibiting solution strengthening function. Besides, it forms primary carbides, thereby improving a 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 is a deoxidizer and also is effective for improving oxidation resistance. However, when it is excessively added, the austenite structure of the cast steel become unstable, leading to 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, 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 an austenite structure of the heat-resistant cast steel of the present invention, together with Co and Cr, thereby improving 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 level off. This means that the 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 a 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 austenitic 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 a 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, austenitic cast steel. Accordingly, the amount of B is desirably 0.001% or more. However, if it is excessively added, borides are precipitated, leading to 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 the addition of Mo alone, less effects are obtainable than 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 an 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 exceeding 20.0% would provide no further improvement, 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%. 
     Such heat-resistant, austenitic cast steel of the present invention is particularly suitable for thin parts such as exhaust equipment members, exhaust manifolds, turbine housings, etc. for automobile engines which should be durable without suffering from cracks under 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, austenitic 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 in the atmosphere in a 100-kg high-frequency furnace, removing the resulting melt from the furnace while it was at a temperature of 1550° C. or higher, and pouring it into a mold at about 1500° C. or higher. The heat-resistant, austenitic cast steels of the present invention (Examples 1-19) showed good fluidity during casting, thereby generating no cast defects such as voids. 
     
                       TABLE 1______________________________________         Additive Component (Weight %)______________________________________           C      Si      Mn   Ni    Cr______________________________________Example No.1               0.19   1.04    0.51 9.78  20.632               0.29   0.96    0.55 10.14 16.503               0.28   1.05    0.49 15.09 28.204               0.30   1.01    0.59 15.05 25.315               0.29   0.99    0.47 18.44 21.476               0.29   1.02    0.47 9.86  19.337               0.31   1.01    0.51 9.79  18.828               0.30   0.87    0.54 10.80 19.789               0.31   1.05    0.48 10.43 19.8510              0.29   1.03    0.52 9.97  20.0211              0.49   1.00    0.49 9.97  19.5812              0.28   1.06    0.49 9.74  19.2813              0.48   1.06    0.50 9.93  20.2814              0.41   1.00    0.50 9.96  20.2115              0.43   0.97    0.51 9.05  20.5216              0.38   0.92    0.46 9.26  19.5617              0.37   0.97    0.49 10.09 19.2618              0.32   0.98    0.53 10.70 20.6219              0.27   0.96    0.49 9.89  20.17Comparative Example No.1               3.33   4.04    0.35 --    --2               0.28   1.05    0.44 --    17.93               2.77   2.12    0.88 21.10 2.444               1.89   5.32    0.41 34.50 2.355               0.21   1.24    0.50 9.1   18.8______________________________________           W      Nb      B    Mo    Co______________________________________Example No.1               2.02   0.28    0.002                               --    --2               2.50   0.32    0.003                               --    --3               3.01   0.31    0.004                               --    --4               3.07   0.29    0.004                               --    --5               3.02   0.32    0.008                               --    --6               2.93   0.28    0.004                               --    --7               2.89   0.48    0.003                               --    --8               2.02   0.31    0.003                               0.49  --9               2.03   0.52    0.004                               0.52  --10              2.86   0.94    0.003                               --    --11              3.09   0.98    0.003                               --    --12              4.88   0.48    0.003                               --    --13              5.03   0.48    0.003                               --    --14              3.05   0.50    0.003                               --    --15              3.02   0.44    0.003                               --    4.5016              2.04   0.42    0.004                               0.55  9.3117              2.94   0.47    0.004                               --    18.7418              3.00   0.51    0.004                               --    10.3919              2.89   0.47    0.003                               --    17.66Comparative Example No.1               --     --      --   0.62  --2               --     --      --   --    --3               --     --      --   --    --4               --     --      --   --    --5               --     --      --   --    --______________________________________ 
    
     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 cooling them in the air. On the other hand, the test piece of Comparative Example 1 was used in the 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 a Room Temperature 
     Conducted on a rod test piece having a gauge length 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 length 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 length 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. 
     Each cycle: 12 minutes. 
     Incidentally, an electric-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 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/cm 2 ) 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 thermal fatigue test and the oxidation test results are shown in Table 4. 
     
                       TABLE 2______________________________________  at Room Temperature  0.2% Offset            Tensile  Yield Strength            Strength Elongation                               Hardness  (MPa)     (MPa)    (%)       (H.sub.B)______________________________________Example No.1        250         595      26      1702        300         555      11      1793        280         510      7       2014        265         555      13      1795        275         560      12      1876        275         590      19      1797        300         565      11      1978        285         540      12      1839        300         555      11      19210       255         565      14      17911       325         540      4       22312       280         600      14      19713       325         525      4       21714       335         540      4       21715       315         540      10      20116       290         540      6       21717       320         545      5       22318       305         540      7       20119       305         535      9       201ComparativeExample No.1        510         640      11      2172        540         760      4       2403        190         455      16      1794        255         485      9       1635        250         560      20      170______________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________        at 900° C.  at 1050° C.        0.2% Offset                Tensile    0.2% Offset                                   Tensile        Yield Strength                Strength                     Elongation                           Yield Strength                                   Strength                                        Elongation        (MPa)   (MPa)                     (%)   (MPa)   (MPa)                                        (%)__________________________________________________________________________Example No.1            65      120  36    33      59   382            66      129  32    36      65   363            84      172  27    35      77   274            80      153  42    42      75   375            84      151  28    44      77   336            82      145  34    37      65   387            88      155  25    46      75   348            81      140  34    40      69   429            85      150  31    43      72   3610           77      139  29    37      68   3411           97      173  22    62      101  3012           77      146  32    40      74   3413           94      177  28    50      97   3014           103     206  32    60      96   2715           90      150  38    53      88   4016           97      167  27    56      91   3017           108     186  31    54      89   3518           97      166  28    46      77   3019           98      166  36    49      82   38Comparative Example No.1            20      40   33    --      --   --2            25      42   58    15      28   1033            41      64   27    22      36   364            48      73   29    25      45   225            65      128  93    30      50   100__________________________________________________________________________ 
    
     
                       TABLE 4______________________________________         Thermal Fatigue                    Weight Loss         Life       by Oxidation         (Cycle)    (mg/mm.sup.2)______________________________________Example No.1               88           252               92           303               115          154               105          185               102          186               120          357               135          408               105          509               110          5010              152          2611              145          3512              160          3013              175          3514              185          1815              180          2316              150          2817              195          1518              165          2019              177          22Comparative Example No.1               --           --2               10           1053               56           7654               85           555               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 of 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, austenitic 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, 2000-cc 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 a full load was 1050° C. at the inlet of the turbo charger housing. Under this condition, 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 leaks or thermal cracking were observed. It was thus confirmed that the exhaust manifold and the turbine housing made of the heat-resistant, austenitic cast steel of the present invention had excellent durability and reliability. 
     As described above in detail, the heat-resistant austenitic cast steel of the present invention has excellent high-temperature strength, particularly at 900° C. or higher, without deteriorating its room-temperature ductility, and it can be produced at a low cost. Such heat-resistant, austenitic 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.