MARAGING STEEL

Abstract

Disclosed is a mar aging steel containing, in combination in mass percent, C in a content from greater than 0% to 0.02%, Mn in a content from greater than 0% to 0.3%, Si in a content from greater than 0% to 0.3%, Ni in a content of 10% to 13%, Mo in a content of 0.5% to 3.5%, Co in a content of 9% to 12%, Cr in a content of 1.5% to 4.5%, Ti in a content of 1.5% to 4.5%, and Al in a content of 0.01% to 0.2%, where the total content of Mo and Ti is 5.0 mass percent or less, and the ratio ([Mo]/[Ti]) of the Mo content [Mo] to the Ti content [Ti] is 1.0 or less, with the remainder consisting of iron and inevitable impurities.

Claims

1. A maraging steel comprising, in mass percent: C in a content from greater than 0% to 0.02%; Mn in a content from greater than 0% to 0.3%; Si in a content from greater than 0% to 0.3%; Ni in a content of 10% to 13%; Mo in a content of 0.5% to 3.5%; Co in a content of 9% to 12%; Cr in a content of 1.5% to 4.5%; Ti in a content of 1.5% to 4.5%; and Al in a content of 0.01% to 0.2%, a total content of Ti and Mo being 5.0% or less, a ratio ([Mo]/[Ti]) of the Mo content [Mo] to the Ti content [Ti] being 1.0 or less, with the remainder consisting of iron and inevitable impurities.

2. The maraging steel according to claim 1, wherein the maraging steel has: a phosphorus (P) content from greater than 0% to 0.01%; a nitrogen (N) content from greater than 0% to 0.01%; and a sulfur (S) content from greater than 0% to 0.01%, where P, N, and S are present in the inevitable impurities.

3. The maraging steel according to claim 1, wherein the maraging steel has a surface hardness in terms of Vickers hardness of 400 Hv or more.

4. The maraging steel according to claim 2, wherein the maraging steel has a surface hardness in terms of Vickers hardness of 400 Hv or more.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The inventors of the present invention made investigations from various different angles so as to actually provide a maraging steel which features compatibility between high-temperature strength and room-temperature toughness. In particular, to achieve the high-temperature strength, the inventors made intensive investigations on how the chemical composition and the microstructure state determined by aging heat treatment after quenching affect the room-temperature toughness.

[0016] In regular maraging steels, precipitates to perform precipitation strengthening are generally intermetallic compounds mainly containing Mo. Assume that a maraging steel has such a chemical composition as to tend to form such intermetallic compounds. In this maraging steel, a Laves phase including Fe.sub.2Mo, which is a binary intermetallic compound, tends to form upon aging heat treatment of the maraging steel. The maraging steel, when containing a larger amount of the Laves phase, tends to readily have lower toughness. In particular, the aging heat treatment of materials to form rotors, which are large-sized members, is performed at a high temperature for a long time, and thereby causes the compound to be readily formed in a large amount, and this lowers the toughness.

[0017] The inventors then hit on an idea that conversion of the intermetallic compounds mainly containing Mo into such intermetallic compounds as not to adversely affect toughness may actually provide good toughness without occurrence of the above-mentioned problems. After further investigations, the inventors found such a chemical composition as to form intermetallic compounds mainly including Ti, such as Ni.sub.3Ti intermetallic compound. The present invention has been made on the basis of these findings.

[0018] A maraging steel having the chemical composition specified in the present invention, when subjected to an aging heat treatment under predetermined conditions, has a microstructure in which finely divided martensite is dispersed in a ferritic phase in which the Ni.sub.3Ti intermetallic compound is precipitated. This maraging steel offers such properties as to offer a surface hardness in terms of Vickers hardness of 400 Hv or more.

[0019] As apparent Am the above-mentioned concept, appropriate settings of, among the chemical composition, in particular the Mo and Ti contents and the relationship between them are important in the maraging steel according to the present invention. Conversion into the precipitates of intermetallic compounds as above requires appropriate settings of not only the contents of Mo and Ti and the total contents of them, but also the ratio ([Mo]/[Ti]) of the Mo content [Mo] to the Ti content [Ti]. Reasons for the settings of these factors are as follows.

[0020] Mo: 0.5% to 3.5%, Ti: 1.5% to 4.5%

[0021] Molybdenum (Mo) and titanium (Ti) form precipitates of various intermetallic compounds mainly containing these elements and are useful for higher strength and better toughness of the steel. To offer these advantageous effects effectively, the steel is controlled to contain Mo in a content of 0.5% or more and Ti in a content of 1.5% or more, and preferably contains Mo in a content of 1.0% or more and Ti in a content of 2.0% or more.

[0022] However, the steel, if having an excessively high Mo content, may suffer from the formation of a larger amount of FeMo, which adversely affects toughness. To eliminate or minimize this, the Mo content is controlled to 3.5% or less, preferably 3.0% or less, and more preferably 2.5% or less. The steel, if having an excessively high Ti content, may suffer from insufficient room-temperature durability. To eliminate or minimize this, the Ti content is controlled to 4.5% or less, preferably 4.0% or less, and more preferably 3.5% or less.

[0023] Total Content of Mo and Ti: 5.0% or less, Ratio ([Mo]/[Ti]): 1.0 or less

[0024] In addition to the settings of the Mo and Ti contents as above, providing of intermetallic compounds formed in the steel mainly including not Mo, but Ti requires the control of the total content of Mo and Ti to 5.0% or less, and the control of the ratio ([Mo]/[Ti]) to 1.0 or less.

[0025] Increase or decrease of the total content of Mo and Ti causes toughness and high-temperature strength to vary in a trade-off manner. To keep toughness and high-temperature strength in balance, the total content of Mo and Ti is controlled to 5.0% or less. The steel, if having a total content of Mo and Ti of greater than 5.0%, has satisfactory high-temperature strength, but fails to surely have toughness, because of excessive amounts of precipitated various intermetallic compounds. The total content is preferably 4.0% or less, and more preferably 3.0% or less. The total content in to terms of lower limit is inevitably 2.0% or more on the basis of the contents of the respective elements, but is preferably 2.2% or more.

[0026] In contrast, the steel, if having a ratio ([Mo]/[Ti]) (namely, mass ratio) of the Mo content [Mo] to the Ti content [Ti] of greater than 1.0, fails to surely have toughness, because of a larger proportion of the Laves phase. The ratio ([Mo]/[Ti]) is preferably 0.8 or less, and more preferably 0.6 or less. The ratio ([Mo]/[Ti]) in terms of lower limit is 0.11 or more on the basis of the respective contents, but is preferably 0.2 or more, and more preferably 0.3 or more.

[0027] The settings of the total content of Mo and Ti and the ratio ([Mo]/[Ti]) of the Mo content [Mo] to the Ti content [Ti] within the predetermined ranges allows the steel to have toughness and high-temperature strength both at satisfactory levels. However, the aging heat treatment, if performed at an excessively high temperature and/or for an excessively long time, may fail to give sufficient high-temperature strength. To eliminate or minimize this, the temperature and time conditions in the aging are preferably controlled so as to allow the steel to have a surface Vickers hardness of 400 Hv or more, as mentioned below.

[0028] In the maraging steel according to the present invention, at least Mo and Ti are to be controlled as mentioned above, but, in addition to these elements, elements such as C, Mn, Si, Ni, Co, Cr, and Al are to be controlled within appropriate ranges. Reasons for the settings on these elements are as follows.

[0029] C: from greater than 0% to 0.02%

[0030] Carbon (C) forms carbides in a high-temperature environment to allow the steel to have high-temperature strength and high-temperature creep strength at higher levels. However, the carbon content should be minimized so as to maximize the precipitation of intermetallic compounds mainly containing Ti. The steel, if having an excessively high carbon content of greater than 0.02%, may contrarily have lower toughness because of formation of TiC in a larger amount. The carbon content in terms of upper limit is preferably 0.015% or less, and more preferably 0.010% or less. The carbon content in terms of lower limit is preferably 0.001% or more, and more preferably 0.005% or more, so as to allow carbon to offer basic actions.

[0031] Mn: from greater than 0% to 0.3%

[0032] Manganese (Mn) has a deoxidation action in molten steel. The element offers the advantageous effect more with an increasing content of the element. To offer the advantageous effect effectively, the Mn content is preferably controlled to 0.005% or more. The Mn content in terms of lower limit is more preferably 0.010% or more, and furthermore preferably 0.015% or more. However, the steel, if having an excessively high Mn content of greater than 0.3%, may fail to include the martensitic phase after quenching, due to increased stability of the austenitic phase. The Mn content in terms of upper limit is preferably 0.2% or less, and more preferably 0.1% or less.

[0033] Si: from greater than 0% to 0.3%

[0034] Silicon (Si) has a deoxidation action in molten steel, as with Mn. This element, even when present in a trace amount, effectively allows the steel to have better oxidation resistance. To offer these advantageous effects effectively, the Si content is preferably controlled to 0.005% or more. The Si content in terms of lower limit is preferably 0.010% or more, and furthermore preferably 0.015% or more. However, the steel, if having an excessively high Si content, may suffer from impaired ductility because of excessive work hardening. To eliminate or minimize this, the Si content is controlled to 0.3% or less. The Si content in tams of upper limit is preferably 0.2% or less, and more preferably 0.1% or less.

[0035] Ni: 10% to 13%

[0036] Nickel (Ni) is an austenitic phase-stabilizing element which is necessary for austenitization of the microstructure in heating before quenching. This element also allows Ti to be precipitated as the Ni.sub.3Ti intermetallic compound and thereby allows the steel to have more satisfactory high-temperature strength. To offer these advantageous effects, the Ni content is contained to 10% or more. The Ni content is preferably 10.5% or more, and more preferably 11.0% or more. However, the steel, if having an excessively high Ni content of greater than 13%, may cause higher cost and may cause austenite to remain after quenching. The Ni content in terms of upper limit is preferably 12.5% or less, and more preferably 12.0% or less.

[0037] Co: 9% to 12%

[0038] Cobalt (Co) is dissolved as a solute in the steel to offer solid-solution strengthening. To offer the advantageous effect, the Co content is controlled 9% or more. The Co content in terms of lower limit is preferably 9.5% or more, and more preferably 10.0% or more. However, the steel, if having an excessively high Co content, may cause higher cost and may have impaired ductility due to excessively increased strength. To eliminate or minimize these, the Co content in terms of upper limit is controlled to 12% or less, and is preferably 11.5% or less, and more preferably 11.0% or less.

[0039] Cr: 1.5% to 4.5%

[0040] Chromium (CO is necessary for better oxidation resistance of the maraging steel. To offer good oxidation resistance, the Cr content is controlled to 1.5% or more. The Cr content in terms of lower limit is preferably 2.0% or more, and more preferably 2.5% or more. However, the steel, if having an excessively high Cr content, may be embrittled due to the formation of o phases in a high-temperature environment in which the steel is used as a product. To eliminate or minimize this, the Cr content in terms of upper limit is controlled to 4.5% or less, and is preferably 4.0% or less, and more preferably 3.5% or less.

[0041] Al: 0.01% to 0.2%

[0042] Aluminum (Al) has a deoxidation action in molten steel, as with Mn. To offer the advantageous effect, the Al content is controlled to 0.01% or more. The Al content in terms of lower limit is preferably 0.02% or more, and more preferably 0.03% or more. However, the steel, if having an excessively high Al content, may suffer from formation of coarse inclusions derived from Al. To eliminate or minimize this, the Al content is controlled to 0.2% or less, and is preferably 0.1% or less, and more preferably 0.05% or less.

[0043] The chemical composition specified in the present invention is as described above, with the remainder being iron and inevitable impurities. Of the inevitable impurities, P, N, and S are preferably decreased to levels as mentioned below. The impurities excluding P, N, and S may include low-melting point impurity metals derived from scrap raw materials, such as Sn, Pb, Sb, As, and Zn. These elements, however, lower grain-boundary strength during hot working and in use in a high-temperature environment and are desirably minimized in content.

[0044] P: from greater than 0% to 0.01%

[0045] Phosphorus (P) is an inevitably-contaminated impurity, and causes the steel to have lower weldability with an increasing content thereof. From this viewpoint, phosphorus is preferably minimized, and the phosphorus content is controlled to preferably 0.01% or less, more preferably 0.005% or less, and furthermore preferably 0.001% or less.

[0046] N: from greater than 0% to 0.01%

[0047] Nitrogen (N) is also an inevitably-contaminated impurity, fixes Ti as nitrides, and lowers the amounts of formed intermetallic compounds that contribute to higher strength, where Ti is contained as an essential element in the steel according to the present invention. From this viewpoint, nitrogen is preferably minimized, and the nitrogen content is controlled to preferably 0.01% or less, more preferably 0.005% or less, and furthermore preferably 0.001% or less.

[0048] S: from greater than 0% to 0.01%

[0049] Sulfur (S) is also an inevitably-contaminated impurity and impairs hot workability necessary typically for forging, with an increasing content thereof. From this viewpoint, sulfur is preferably minimized, and the sulfur content is controlled to preferably 0.01% or less, more preferably 0.005% or less, and furthermore preferably 0.001% or less.

[0050] The maraging steel according to the present invention has a chemical composition as mentioned above. The steel having the chemical composition can be easily obtained by adjusting proportions of raw materials as appropriate via melting. Ingots obtained by ingot making may be subjected to homogenization or soaking (hereinafter also referred to “soaking treatment”) as needed subjected to hot working to adjust its shape, and then subjected to an appropriate quenching heat treatment and a subsequent aging heat treatment.

[0051] When the ingots are those obtained by ingot making, the soaking treatment eliminates or minimizes solidifying segregation of the ingots, by holding the ingots in a temperature range of typically from 1250° C. to 1300° C. for about 10 hours. The hot working may be performed while heating the work at a temperature of about 1000° C. or higher.

[0052] The steel obtained by subjecting an ingot to the soaking treatment and hot working is subjected to quenching so as to form a martensitic phase. The heating temperature in quenching, namely, the heating temperature before cooling is controlled within such a temperature range that the entire steel becomes an austenitic phase and that precipitates undergo solutionization. The steel according to the present invention having the chemical composition as above is preferably subjected to quenching performed at a heating temperature of 900° C. or higher, more preferably 950° C. or higher, and furthermore preferably 1000° C. or higher. However, quenching, if performed at an excessively high heating temperature, may cause the austenitic phase to coarsen, and this may impede the formation of finely divided martensite. From this viewpoint, the heating temperature in quenching is controlled to preferably 1150° C. or lower, more preferably 1100° C. or lower, and furthermore preferably 1050° C. or lower.

[0053] Cooling in quenching is preferably performed via air cooling or water cooling. Cooling in a temperature range down to 80° C., which is lower than the martensitic transformation start temperature Ms, is preferably performed at a cooling rate of 5° C./hr or more. The cooling rate in this temperature range is more preferably 10° C./hr or more, and furthermore preferably 20° C./hr or more. However, the cooling rate has a ceiling with respect to such large-sized steels and is about 100° C./hr or less.

[0054] The steel, in which the martensitic phase is formed in the above manner, has very high strength, but has low ductility and toughness, and thus requires an aging heat treatment so as to adjust balance between strength and toughness, where the aging heat treatment corresponds to a tempering heat treatment.

[0055] The aging heat treatment is performed in such a temperature range as not to increase the austenitic phase, namely, at a temperature lower than the Ac.sub.3 transformation temperature. For the managing steel having the chemical composition as above, the upper limit temperature is 675° C. Accordingly, the temperature and holding time of the aging heat treatment are controlled in a temperature range lower than 675° C. so that the steel has a surface Vickers hardness of 400 Hv or more.

[0056] The aging heat treatment is not limited in temperature and holding time, except for the temperature upper limit. However, the aging heat treatment, typically when performed at a set temperature of 650° C., can stably give a sufficient hardness when performed for a holding time of 3 hours or shorter. To allow the aging heat treatment to proceed effectively at that temperature, the holding time is preferably at least one hour or longer, and is more preferably 1.5 hours or longer.

[0057] The present invention will be illustrated in further detail on operation and advantageous effects thereof, with reference to several examples below. It should be noted, however, that the examples are by no means intended to limit the scope of the present invention; and that various modifications and changes in design without deviating from the spirit and scope of the present invention described herein all fall within the technical scope of the present invention.

Examples

[0058] Steels A to I having chemical compositions given in Table 1 were heated and melted using a vacuum induction furnace, cast into 20-kg ingots, subjected to a soaking treatment at 1280° C. for 12 hours, and further subjected to hot forging to be processed into steels having a size of 60 mm in width by 15 mm thickness by L in length.

TABLE-US-00001 TABLE 1 Chemical composition* (in mass percent) Steel C Si Mn P S Ni Cr Co Mo Ti Al N A 0.009 0.018 0.009 0.005 0.001 11.9 3.1 9.8 1.9 2.0 0.07 0.001 B 0.006 0.008 0.010 0.004 0.001 12.0 3.1 9.8 1.0 2.0 0.09 0.001 C 0.015 0.056 0.130 0.008 0.001 11.3 2.2 11.3 2.0 2.8 0.05 0.008 D 0.012 0.182 0.094 0.003 0.001 11.6 2.5 10.3 0.8 2.3 0.06 0.003 E 0.008 0.087 0.209 0.009 0.001 12.1 2.7 10.8 2.4 2.5 0.04 0.002 F 0.014 0.116 0.165 0.004 0.002 10.8 2.9 9.9 1.9 2.6 0.05 0.005 G 0.011 0.143 0.055 0.005 0.001 11.0 2.8 10.1 3.0 1.7 0.04 0.003 H 0.013 0.221 0.245 0.007 0.001 10.5 3.4 11.0 1.6 4.2 0.02 0.008 I 0.003 0.014 0.012 0.002 0.001 12.1 3.0 10.3 5.0 2.0 0.10 0.002 *Remainder: iron and inevitable impurities excluding P, S. and N

[0059] The obtained steels were heated at 1000° C. for 15 minutes, subjected to quenching via water-immersion cooling, and each subjected to an aging heat treatment in a temperature range of from 650° C. to 700° C. for a time range of from 2 to 30 hours, under one of four conditions (a), (b), (c), and (d) as follows.

[0060] Aging Heat Treatment Conditions

[0061] (a) At a temperature of 650° C. for a holding time of 3 hours

[0062] (b) At a temperature of 650° C. for a holding time of 30 hours

[0063] (c) At a temperature of 700° C. for a holding time of 30 hours

[0064] (d) At a temperature of 650° C. for a holding time of 2 hours

[0065] Table 2 presents the steel type and the aging heat treatment condition each employed in Tests Nos. 1 to 12, together with the total content of Mo and Ti, and the ratio ([Mo]/[Ti]).

TABLE-US-00002 TABLE 2 Total content (in Aging heat Test mass percent) of Ratio treatment number Steel Mo and Ti ([Mo]/[Ti]) condition 1 A 3.9 0.95 (a) 2 B 3.0 0.50 (a) 3 B 3.0 0.50 (d) 4 C 4.8 0.71 (a) 5 D 3.1 0.35 (a) 6 E 4.9 0.96 (a) 7 F 4.5 0.73 (a) 8 G 4.7 1.76 (a) 9 H 5.8 0.38 (a) 10 I 7.0 2.50 (a) 11 I 7.0 2.50 (b) 12 I 7.0 2.50 (c)

[0066] From the above-prepared steels, flanged round bar test specimens each including a gauge portion of 6 mm in diameter by 30 mm in length were prepared, subjected to high-temperature tensile tests at 500° C. in accordance with the method prescribed in Japanese Industrial Standard (JIS) G 0567:2012, to determine a 0.2% yield strength as a high-temperature strength. A sample, when having a 0.2% yield strength as measured of 750 MPa or more, is judged to surely have excellent high-temperature strength

[0067] From the above-prepared steels, full-size 2-mmV notch Charpy test specimens in conformity with JIS Z 2242:2005 were prepared, subjected to Charpy impact tests to measure Charpy impact values at 0° C., on the basis of which toughness was evaluated. The present invention is to improve toughness at room temperature of about 25° C. A sample, when having good toughness at 0° C., can be judged to also have good toughness at room temperature. On the basis of these, toughness was evaluated at 0° C. A sample, when having a Charpy impact value as measured of 10.0 J/cm.sup.2 or more, can be judged to offer more excellent toughness as compared with conventional managing steels. The Charpy impact value is preferably 15.0 J/cm.sup.2 or more, and more preferably 17.0 J/cm.sup.2 or more.

[0068] The above-prepared steels, namely, steels after the aging heat treatment, were subjected to mirror-like finishing via mechanical polishing, followed by measurements of surface Vickers hardness at a load of 500 g. A sample steel, when having a surface Vickers hardness of 400 Hv or more, can be judged to have excellent surface hardness.

[0069] Evaluation results on the high-temperature strength, Charpy impact value, and Vickers hardness are presented in Table 3.

TABLE-US-00003 TABLE 3 Test High-temperature Charpy impact Vickers hardness number strength (MPa) value (J/cm.sup.2) (Hv) 1 835 12.3 443 2 781 17.6 430 3 847 15.8 458 4 856 10.2 486 5 801 21.9 441 6 859 11.4 479 7 848 13.6 468 8 833 6.8 456 9 892 8.2 497 10 940 5.9 520 11 783 5.8 462 12 736 9.7 386

[0070] These results give considerations as follows. Samples of Tests Nos. 1 to 7 are examples which meet all conditions specified in the present invention and are found to offer excellent high-temperature strength and to have better toughness. These samples are also found to have sufficiently high steel surface hardness after the aging heat treatment.

[0071] In contrast, samples of Tests Nos. 8 to 12 are comparative examples which do not meet one or more of the conditions specified in the present invention and offer at least one of high-temperature strength, toughness, and surface hardness at poor level.

[0072] Specifically, the sample of Test No. 8 is a sample using Steel G, which has a ratio ([Mo]/[Ti]) of the Mo content to the Ti content of out of the range specified in the present invention. This sample offers lower toughness even it has undergone an aging heat treatment under appropriate conditions.

[0073] The sample of Test No. 9 is a sample using Steel H, which has a total content of Mo and Ti of out of the range specified in the present invention. This sample offers lower toughness even it has undergone an aging heat treatment under appropriate conditions.

[0074] The sample of Test No. 10 is a sample using Steel I, which has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out of the ranges specified in the present invention. This sample offers lower toughness even it has undergone an aging heat treatment under appropriate conditions.

[0075] The sample of Test No. 11 is a sample using Steel I, which has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out of the ranges specified in the present invention. In addition, this sample has undergone an aging heat treatment for an excessively long holding time. In this sample, the aging heat treatment condition causes the sample to have lower toughness, although it does not so much affect the high-temperature strength and the surface hardness.

[0076] The sample of Test No. 12 is a sample using Steel I, which has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out of the ranges specified in the present invention. In addition, this sample has undergone an aging heat treatment at an excessively high temperature for an excessively long holding time. This sample offers lower toughness, is in the state of over-aging, and has a high-temperature strength and a surface hardness not meeting the predetermined conditions (criteria).