Use of a weldable low-chromium ferritic cast steel, having excellent high-temperature strength

Abstract: This invention relates to low-Cr ferritic cast steels having excellent high-temperature strength, weldability, oxidation resistance and high-temperature corrosion resistance. These low-Cr ferritic cast steels consist essentially of, on a weight percentage basis, 0.03 to 0.12% C, 0.03 to 0.7% Si, 0.02 to 1% Mn, up to 0.3% Co, up to 0.025% P, up to 0.015% S, 0.8 to 3% Cr, 0.01 to 1% Ni, 0.01 to 0.5% V, 0.1 to 3% W, 0.01 to 0.2% Nb, 0.001 to 0.05% Al, 0.0001 to 0.02% B, 0.001 to 0.05% N, up to 0.03% O, 0.0005 to 0.05% Mg, and the balance being iron and incidental impurities, provided that the Mg content satisfies the following inequality as expressed on a weight percentage basis: (Mg content) > (24/32)(S content) + (24/16)[(O content) - (8/9)(Al content)] As a result, they exhibit excellent weldability and markedly improved high-temperature strength.

Description: BACKGROUND OF THE INVENTION Field of the invention This invention relates to the use as cast materials of low-Cr ferritic cast steels which have excellent high-temperature strength, weldability, oxidation resistance and high-temperature corrosion resistance, especially for use in a high-temperature environment at or above 450°C, in the fields of boilers, nuclear power industry, chemical industry and the like. Description of the related art Materials for use as heat-resistant and pressure-tight members in various types of equipment in the fields of boilers, nuclear power industry, chemical industry and the like include austenitic steels, high-Cr ferritic steels having a Cr content of 9 to 12%, low-Cr ferritic steels having a Cr content of 3.5% or less (e.g., 2·1/4Cr-1Mo steel), and carbon steel. These materials are suitably selected according to the service temperature, pressure and atmosphere for the particular member and with consideration for economic efficiency. Among others, high-Cr ferritic steels having a Cr content of 9 to 12% and low-Cr ferritic steels having a Cr content of 3.5% or less have been extensively investigated with respect to alloy systems containing various trace elements. As a result, ferritic steels having high-temperature strength equal to or greater than that of austenitic steels have been developed. However, most of them are intended to be used after being worked by forging, rolling or the like, and there are very few materials (such as cast steels) which are used without requiring forging and rolling. The reason for this seems to be that it has been difficult to develop a material having excellent overall performance from the viewpoint of high-temperature strength, weldability, impact resistance, economic efficiency and the like. As compared with forged steels, cast steels have the advantage that they can be easily formed into articles of complicated shapes without requiring a forging step and hence involve a less working cost. With the recent progress of casting techniques, the reliability of cast steels which was apprehended in the past has made a marked improvement. Accordingly, there is a need for an inexpensive cast steel having excellent high-temperature strength and weldability. As described above, the existing Cr-containing ferritic cast steels have the following problems: (1) Low-Cr ferritic cast steels tend to develop a material deterioration due to the production of porosity and high-temperature cracking, especially in thick-walled members. (2) Their high-temperature creep strength at 450°C or above is low. (3) They have poor impact resistance. (4) They require preheating prior to welding. EP-A-0505732, JP-A-02 217439, JP-A-02 217438 and EP-A-0560375 all describe low-chromium ferritic steels. However, these references are silent on the inter relationship of magnesium, oxygen, sulphur and aluminium and furthermore, all require post-casting mechanical working such as by forging or rolling to provide adequate properties. Accordingly, an object of the present invention is to provide low-Cr ferritic cast steels which develops no casting defect even in thick-walled members, show a marked improvement in high-temperature strength (particularly high-temperature creep strength) at 450°C or above as compared with conventional cast steels, have performance equal to or higher than that of the existing forged steels with respect to toughness and weldability, and achieve high economic efficiency. SUMMARY OF THE INVENTION The present inventors have made an effort to solve the above-described problems on the basis of the fundamental conceptions that (1) internal defects should be minimised even in thick-walled cast steels, (2) creep strength at 450°C or above should be improved as a result of precipitation hardening by V and Nb and solid solution strengthening by W, Mo and Cu, and (3) weldability should be improved by controlling the contents of C, Mn and B. As a result, the following facts have been found. Low-Cr ferritic cast steels most probably tend to suffer from the macrosegregation of S, and this tendency becomes more pronounced in large ingots and weakly deoxidized materials. Even if sufficient deoxidation is effected, porosity tends to be concentrated in the parts where the macrosegregation of S occurs. Consequently, the macrosegregation of S also needs to be suppressed for the purpose of minimizing material deterioration due to porosity. Moreover, the macrosegregation of S causes the following problems: (1) the promotion of high-temperature cracking, for example, during welding, (2) a reduction in oxidation resistance and high-temperature corrosion resistance due to the destabilisation of Cr 2 O 3 film, and (3) a reduction in grain boundary strength. Accordingly, the present inventors have investigated various methods for suppressing the segregation of S in low-Cr ferritic cast steels, and have discovered the following solution. S can be stabilised by effecting sufficient deoxidation with Al and, at the same time, adding Mg having a strong affinity for S. Thus, the macrosegregation and microsegregation of S can be markedly suppressed. As a result, internal defects and high-temperature cracking during welding which are caused by the segregation of S can be minimised. Besides Mg, Ca and rare earth elements are also effective for the stabilisation of S. However, in the low-Cr ferritic cast steels of the present invention which are used at high temperatures, it is also important to secure the stability of scale at high temperatures. Since Mg also has the effect of stabilising scale of, e.g, Cr 2 O 3 , it is desirable to add Mg for the purpose of stabilising S. When Mg is added, its effect is governed by the balance between the Mg content and the S, O and Al contents. Accordingly, the Mg content must satisfy the following inequality: (Mg content) > (24/32) (S content) + (24/16) [(O content) - (8/9) (Al content)] That is, Mg has not only the effect of stabilising S in the form of MgS, but also the effect of stabilising scales in itself. As described above, the present inventors have completed the present invention on the basis of the synergistic effect of a measure for suppressing the segregation of S and an optimisation of the contents of other alloying elements. That is, the present invention relates to the use according to claim 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The action of various components in the low-Cr ferritic cast steels used in the present invention, and the reasons for the selection of their content ranges are described below. In the following description, all percentages are by weight. C combines with Cr, Fe, W, V and Nb and with optionally added Mo and Cu to form carbides and thereby contributes to the improvement of high-temperature strength. At the same time, C itself acts as an austenite-stabilizing element to stabilize the structure. If its content is less than 0.03%, the precipitation of carbides will be insufficient to achieve adequate high-temperature strength. If its content is greater than 0.12%, excessive amounts of carbides will precipitate, resulting in marked hardening of the steel. Accordingly, the proper content of C is in the range of 0.03 to 0.12%. In this range, lower C contents provide better weldability. Consequently, the content of C should preferably be in the range of 0.05 to 0.08%. Si is an element which acts as a deoxidizer and improves steam oxidation resistance. If its content is greater than 0.7%, Si will cause a marked reduction in toughness and will be detrimental to creep strength. If its content is less than 0.03%, the melt flowability during casting will become poor. Accordingly, the content of Si should be in the range of 0.03 to 0.7% by weight. Where greater importance is attached to creep strength than to melt flowability, the content of Si should preferably be in the range of 0.03 to 0.30% by weight. Mn has desulfurizing and deoxidizing effects, and is effective in stabilizing the structure. If its content is less than 0.02%, no sufficient effect will be produced. If its content is greater than 1%, Mn will harden the steel and enhance sensitivity to temper embrittlement. When the content of S is particularly low, the content of Mn may be reduced. Accordingly, the content of Mn should be in the range of 0.02 to 1%. When the content of S is particularly low, the content of Mn may be in the range of 0.02 to 0.30%. Depending on the history of melting, Co may be present as a steel impurity in an amount of up to 0.3%. However, Co will exert no appreciable harmful effect at a content of up to 0.3%. Accordingly, the content of Co as an inevitable impurity should be up to 0.3%. Thus, Co need not be positively added during compositional adjustment. Both P and S are elements which are detrimental to toughness. Since even a very slight amount of S destabilizes grain boundaries and Cr 2 O 3 scale film and thereby causes a reduction in high-temperature strength and toughness, its content should preferably be as low as possible within the aforesaid limit. Accordingly, the contents of P and S as inevitable impurities should be up to 0.025% and up to 0.015%, respectively. Cr is an element which is indispensable from the viewpoint of the oxidation resistance and high-temperature corrosion resistance of low-alloy steels. If its content is less than 0.8%, Cr will fail to produce sufficient oxidation resistance and high-temperature corrosion resistance. On the other hand, Cr added in an amount of greater than 3% will detract from strength and toughness. Accordingly, the content of Cr should be in the range of 0.8 to 3% by weight. Ni is an austenite-stabilizing element and contributes to the improvement of toughness. However, if its content is less than 0.01%, no sufficient effect will be produced. If its content is greater than 1%, Ni will detract from high-temperature creep strength. Moreover, the addition of large amounts of Ni is also disadvantageous from an economic point of view. Accordingly, the content of Ni should be in the range of 0.01 to 1% by weight. V combines with C and N to form a fine precipitate comprising V(C,N) and the like. This precipitate contributes greatly to the improvement of long-time creep strength at high temperatures. However, if its content is less than 0.01%, no sufficient effect will be produced. If its content is greater than 0.5%, the precipitation of V(C,N) will become excessive and, on the contrary, detract from creep strength and toughness. Accordingly, the proper content of V is in the range of 0.01 to 0.5%. W acts as a solid solution strengthening and fine carbide precipitation strengthening element and is effective for the improvement of creep strength. Although Mo has a similar effect, W has a lower diffusion rate in Fe and is hence more excellent in the high-temperature stability of its fine carbide which contributes to the improvement of creep strength. When added in combination with Mo, W brings about a greater improvement in strength, particularly in high-temperature creep strength, than when added alone. If its content is less than 0.1%, no effect will be produced, and if its content is greater than 3%, W will harden the steel and detract from its toughness. Accordingly, the content of W should be in the range of 0.1 to 3%. In this range, the content of W should preferably be in the range of 1.0 to 2.0%. Nb, like V, combines with C and N to form Nb(C,N) and thereby contributes to the improvement of creep strength. In particular, Nb shows a marked strength-improving effect at relatively low temperatures of 600°C or below. If its content is less than 0.01%, the above-described effect will not be produced. If its content is greater than 0.2%, Nb will harden the steel significantly and detract from its toughness and weldability. Accordingly, the content of Nb should suitably be in the range of 0.01 to 0.2%. In order to achieve a satisfactory combination of weldability and creep strength, the content of Nb should desirably be in the range of 0.03 to 0.15%. Al is an indispensable deoxidizing element and forms a carbonitride. Moreover, Al also has the effect of making the structure finer. If its content is less than 0.001%, no effect will be produced, and if its content is greater than 0.05% by weight, Al will detract from creep strength and workability. Accordingly, the content of Al should be in the range of 0.001 to 0.05% by weight. The addition of a very slight amount of B has the effect of dispersing and stabilizing carbides and thereby contributes to the improvement of long-time creep strength. If its content is less than 0.0001%, no sufficient effect will be produced, and if its content is greater than 0.02%, B will detract from workability. Accordingly, B should be added so as to give a B content in the range of 0.0001 to 0.02%. In this range, the addition of B is also effective for the improvement of hardenability. Consequently, it is necessary from the viewpoint of structure control to regulate the amount of B added as required. N is necessary for the formation of carbonitrides by combination with V and Nb. If its content is less than 0.001%, no effect will be produced. However, as its content becomes higher, N in solid solution will increase and the nitrides will become coarse, resulting in a reduction in creep strength. Moreover, if its content is greater than 0.05%, N may be responsible for the formation of blow-holes during casting. Accordingly, the content of N should be in the range of 0.001 to 0.05%. O increases casting defects such as pipe flaws and blow-holes, and also exerts an adverse influence on toughness and hot workability. Accordingly, the content of O should be up to 0.03% and preferably up to 0.02%. Mg is an element which stabilizes S and is effective for the suppression of porosity resulting from the segregation of S during casting, the suppression of weld defects, and the strengthening of grain boundaries. Moreover, Mg is also an important element which stabilizes Cr 2 O 3 film and, in the case of Cu addition as will be described later, Cu-O film. However, if its content is less than 0.0005% or does not satisfy the following inequality as expressed on a weight percentage basis: (Mg content) > (24/32)(S content) + (24/16)[(O content) - (8/9)(Al content)], the desired effect will not be produced. On the other hand, even if Mg is added in an amount greater than 0.05%, its effect will become saturated. Accordingly, the content of Mg should be in the range of 0.0005 to 0.05% and, at the same time, should satisfy the following inequality: (Mg content) > (24/32)(S content) + (24/16)[(O content) - (8/9)(Al content)] The above inequality means that it is necessary to secure a certain amount of Mg which is not fixed by S or O but exists in solid solution as a free metal. This inequality has been formulated by considering the atomic weights of Mg, S, O and Al to be 24, 32, 16 and 27, respectively. Ca, Ti, Zr, Y, La, Ce and Ta combine with P, O and S that are impurities. In order to control the morphology of the resulting precipitates (or inclusions), one of more of these elements are added in very small amounts. The addition of 0.01% of each element makes it possible to free the steel of such impurities as P, O and S, and thereby improve its strength and toughness. This is particularly effective for the improvement of creep strength. However, if the content of each element is greater than 0.2%, the resulting inclusion will increase and, on the contrary, detract from toughness. Accordingly, the content of each of these elements should be in the range of 0.01 to 0.2%. Mo, like W, is effective for the improvement of creep strength. However, Mo need not necessarily be added to the steels of the present invention which contain a large amount of W. Nevertheless, Mo produces a strength-improving effect when added in combination with W, and is also effective for the improvement of toughness when added in small amounts. If the content of Mo is less than 0.01%, the above-described effects will not be produced. If its content is greater than 3%, intermetallic compounds will precipitate at high temperatures, resulting in not only a reduction in toughness but also the loss of its effect on strength. Accordingly, when Mo is added, its content should be in the range of 0.01 to 3%. Cu not only improves the strength of the steel owing to solid solution strengthening and precipitation hardening, but also contributes to the improvement of oxidation resistance. Moreover, Cu changes the structure into martensite or bainite and is hence effective for the improvement of toughness. However, the addition of excessive amounts of Cu will harden the steel to an undue extent. When Cu is added to the steels of the present invention which need not be worked by forging, rolling or the like, the content of Cu should be up to 2.5% and its lower limit is 0.1%. Example 30 kg each of steels having the respective chemical compositions shown in Tables 1 were melted in a vacuum melting furnace, cast into the form of Y-type test pieces, and then cooled slowly. Steels A and B are typical conventional cast steel materials which have chemical compositions corresponding to SCPH 21 and SCPH 32, respectively, of JIS (Japanese Industrial Standards). Steels C and D have chemical compositions corresponding to those of heat-resisting steels for small-diameter pipes which are used in boilers and the like. Steels E to M are comparative steels in which the contents of some alloy components are modified so as to be outside the scope of the present invention. Steels 1 to 24 are steels used in accordance with the present invention. As a conventional heat treatment, steels A to D were normalized by heating at 950°C for 2 hours and air cooling, and then tempered by heating at 730°C for 2 hours and air cooling. Steels E to M and the inventive steels 1 to 24 were normalized by heating at 1,050°C for 2 hours and air cooling, and then tempered by heating at 770°C for 1.5 hours and air cooling. With respect to each steel, the presence or absence of internal defects was examined by performing a dye-check test on sections corresponding to 1/4 and 1/2 of the thickness of the ingot. In comparative steel N having a Mg content outside the scope of the present invention, defects were observed in both sections corresponding to 1/4 and 1/2 of the thickness of the ingot. Moreover, its creep resistance and weldability were also insufficient. On the other hand, no internal defect was observed in the cast steels used according to the present invention. In order to compare mechanical properties, room temperature tension tests, Charpy impact tests and creep rupture tests were performed on the comparative steels and steels used in the present invention. Moreover, y-type weld cracking tests were performed in order to evaluate weldability. For use in the room temperature tension tests and the creep rupture tests, test pieces having a diameter of 6 mm and a gage length of 30 mm were cut out from the bottom of the Y-type test pieces in a direction perpendicular to the direction of solidification. The tension tests were performed at room temperature. In the creep rupture tests, long-time rupture tests were performed at 500°C, 550°C, 600°C and 650°C for a period of time up to about 10,000 hours, and the 600°C x 10,000 hour creep rupture strength was determined. The Charpy impact tests were performed according to JIS Z2202. That is, using No. 4 test pieces, the impact value at 0°C was measured three times, and the average of the three impact values was obtained. The y-type weld cracking tests were performed according to JIS Z3158 by using a plate thickness of 20 mm and without preheating (i.e., at 20°C). The weldability was evaluated in terms of longitudinal section cracking rate. The test results thus obtained are shown in Table 2. In the tension tests, the steels used in the present invention exhibit a tensile strength in the range of 600 to 700 MPa and an elongation of 20% or greater. With respect to 600°C x 10,000 hour creep rupture strength which indicates high-temperature strength, the comparative steels including conventional steels have a value of at most 84 MPa. In contrast, the steels used in the present invention have a value of 130 MPa or greater, indicating a marked improvement in high-temperature strength by a factor of more than 1.5 times. Among them, steels 4 and 5 containing Mo have a higher creep rupture strength than steels 1-3, and steel 11 additionally containing Cu shows a further increase in creep rupture strength. Steels 16-24, which contain one or more of Ca, Ti, Zr, Y, La, Ce, Ta and Mg, shows no reduction in creep rupture strength and hence have excellent high-temperature strength, even in the presence of relatively large amounts of impurities such as P and S. Of the comparative steels, even those having the most excellent impact resistance exhibit an impact value of 126 J/cm 2 or less. In contrast, the steels used in the present invention exhibit an impact value of 176 J/cm 2 or greater, indicating that they have excellent toughness at low temperatures. The y-type weld cracking tests have revealed that the occurrence of full cracking or partial cracking was observed in all of the comparative steels, but the steels used in the present invention undergo no cracking even at 20°C. Thus, it can be seen that the steels used in the present invention have very excellent weldability and their preheating during welding may be omitted. Test results Designation Room temperature tension test 600°C x 10,000 h creep rupture strength (MPa) Impact value (0°C) in Charpy impact test (J/cm 2 ) y-type weld cracking test Casting defects Tensile strength 0.2% veld strength Elongation (MPa) (MPa) (%) Comparative cast steels A 479 321 38 35 29 Δ xx B 559 372 33 70 34 Δ xx C 545 368 26 75 48 Δ xx D 568 380 24 73 56 Δ xx E 658 510 32 80 53 x xx F 652 478 29 78 35 x xx G 661 465 25 70 68 x xx H 721 498 19 80 80 x xx I 694 503 20 71 119 x xx J 668 480 21 77 126 Δ xx K 671 474 24 81 27 Δ xx L 764 567 17 84 97 x xx M 758 543 18 84 31 x xx N 632 503 32 101 48 Δ xx cast steels used in the invention 1 648 518 28 130 198 O OO 2 657 526 29 131 191 O OO 3 641 513 28 135 222 O OO 4 633 506 26 159 231 O OO 5 642 514 30 168 205 O OO 6 672 538 28 145 237 O OO 7 623 498 31 144 241 O OO 8 602 482 28 151 208 O OO 9 604 483 26 149 197 O OO 10 612 490 29 159 223 O OO 11 666 533 27 178 242 O OO 12 633 506 28 160 220 O OO 13 641 513 26 161 189 O OO 14 652 522 25 151 189 O OO 15 644 515 24 167 180 O OO 16 655 524 26 164 207 O OO 17 639 511 23 166 210 O OO 18 672 538 21 156 176 O OO 19 654 523 23 162 178 O OO 20 634 507 25 147 257 O OO 21 655 524 24 156 231 O OO 22 639 511 27 167 289 O OO 23 653 522 26 169 246 O OO 24 647 518 26 157 264 O OO x: full cracking Δ: partial cracking O: no cracking xx: casting defects were observed OO: no casting defect was observed The low-Cr ferritic steels used in the present invention are materials which show a marked improvement in high-temperature strength over conventional low-Cr ferritic steels and also have excellent impact resistance and weldability. Consequently, the steels used in the present invention having such excellent properties may be substituted for forged steels in parts which have conventionally required the use of forged steels, resulting in a reduction in cost and an increase in reliability. The steels used in the present invention can be widely used for cast steel articles of various shapes which are used as heat-resistant and pressure-tight members in the industrial fields of boilers, chemical industry, nuclear power industry and the like.

Claims: Use of a low-Cr ferritic cast steel consisting of, on a weight percentage basis, 0.03 to 0.12% C, 0.03 to 0.7% Si, 0.02 to 1% Mn, up to 0.3% Co, up to 0.025% P, up to 0.015% S, 0.8 to 3% Cr, 0.01 to 1% Ni, 0.01 to 0.5% V, 0.1 to 3% W, 0.01 to 0.2% Nb, 0.001 to 0.05% Al, 0.0001 to 0.02% B, 0.001 to 0.05% N, up to 0.03% O, 0.0005 to 0.05% Mg, an optional addition of 0.01 to 0.2% by weight of one or more elements selected from the group consisting of Ca, Ti, Zr, Y, La, Ce and Ta; an optional addition of 0.01 to 3% by weight of Mo; an optional addition of 0.1 to 2.5% by weight of Cu; and, the balance being iron and incidental impurities, the cast steel wherein the Mg content satisfies the following inequality as expressed on a weight percentage basis: (Mg content) > (24/32)(S content) + (24/16) [(O content) - (8/9) (Al content)] in the as-cast condition for cast steel articles.

Inventors: IGARASHI MASAAKI, KOMAI NOBUYOSHI, MASUYAMA FUJIMITSU, MIYATA KAORI, SAWARAGI YOSHIATSU, YOKOYAMA TOMOMITSU, IGARASHI, MASAAKI, KOMAI, NOBUYOSHI, MASUYAMA, FUJIMITSU, MIYATA, KAORI, SAWARAGI, YOSHIATSU, YOKOYAMA, TOMOMITSU, IGARASHI, MASAAKI, C/O SUMITOMO METAL IND. LTD., Komai, Nobuyoshi, Mitsubishi Heavy Ind. Ltd., Masuyama, Fujimitsu, Mitsubishi Heavy Ind. Ltd., MIYATA, KAORI, C/O SUMITOMO METAL IND. LTD., Sawaragi, Yoshiatsu, c/o Sumitomo Metal Ind. Ltd., Yokoyama, Tomomitsu, Mitsubishi Heavy Ind. Ltd.
Applicants: MITSUBISHI HEAVY IND LTD, SUMITOMO METAL IND, MITSUBISHI HEAVY INDUSTRIES, LTD., SUMITOMO METAL INDUSTRIES, LTD.
Application year: 1997
Main classification: C22C38
IPC codes: C22C38