Martensitic stainless steel alloy

Abstract

A martensitic stainless steel alloy comprising, in weight percent (wt%): C gt; from 0.50 to 0.60; 0.10 to 0.60% of Si; 0.40 to 0.80% of Mn; 13.50 to 14.50 parts of Cr (chromium); 0 to 1.20 of Ni; 0.80 to 2.50% of Mo; 0.050 to 0.12% of N; 0.10 to 1.50% of Cu; v is 0.10 at most; s is 0.03 at most; p is 0.03 at most; and the balance of Fe and inevitable impurities.

Description

[0001] This application is a divisional application of a Chinese patent application with an application date of June 4, 2020, an application number of 202080041452.8, and an invention title of "Martensitic Stainless Steel Alloy". Technical Field

[0002] The present disclosure relates to a martensitic stainless steel alloy, a stainless steel strip comprising the martensitic stainless steel alloy, and different components made therefrom. Background Art

[0003] Current martensitic stainless steels generally have high performance and good properties, such as high strength and high ductility, making them suitable for different strip applications.

[0004] EP 3031942 discloses a martensitic stainless steel that can be used for a baffle valve. However, this steel is not suitable for high-demand and high-temperature applications because the steel will lose its mechanical strength due to its composition and the manufacturing method used. Therefore, when used, this steel will not have the required mechanical properties, and in addition, it will have a short service life.

[0005] Therefore, there is a need for a martensitic stainless steel alloy that has a combination of good mechanical properties and temperature stability, i.e., has and maintains good mechanical properties in high-demand applications and at high temperatures (temperatures of about 300 °C).

[0006] Therefore, one aspect of the present disclosure is to provide a solution to solve or reduce this problem. Summary of the Invention

[0007] Therefore, the present disclosure relates to a martensitic stainless steel alloy having the following composition in weight percentages (wt%):

[0008] C >0.50 to 0.60;

[0009] Si 0.10 to 0.60;

[0010] Cu >0.4 to 1.50;

[0011] Mn 0.40 to 0.80;

[0012] Cr 13.50 to 14.50;

[0013] Ni 0 to 1.20;

[0014] Mo 0.80 to 2.50;

[0015] N 0.050 to 0.12;

[0016] V max 0.10;

[0017] S maximum 0.03;

[0018] P maximum 0.03;

[0019] The balance is Fe and unavoidable impurities.

[0020] The present disclosure also relates to a component comprising or consisting of a martensitic stainless steel alloy. Additionally, the present disclosure provides a method of manufacturing such a component.

[0021] The present invention is based on the following findings: Components comprising a martensitic stainless steel alloy having a carbon content greater than 0.50 (>0.50) to 0.60 wt% will have improved tensile strength and hardness as well as high ductility, thereby having better fatigue resistance. Additionally, it has been found that the composition of the martensitic stainless steel alloy as defined above or below will provide good temperature stability, whereby the material will be excellent in high-temperature applications. This finding is very surprising because generally such a high carbon content (above 0.50 wt%) will result in both primary carbides and carbide distributions of coarse carbide particles, which will have a negative impact on the mechanical properties.

[0022] Furthermore, in the martensitic stainless steel alloy of the present invention as defined above or below, it has been found that the intentional addition of copper will improve mechanical properties such as strength. Additionally, it has been surprisingly found that the addition of copper will also result in a reduction in the A1 temperature. This will have a positive impact on heat treatment because it will enable the reduction of the temperature used during austenitization during annealing and hardening, which is beneficial from the perspective of energy efficiency and cost.

[0023] Additionally, it has been found that the combination of intentionally added Cu and a large amount of carbon will provide high mechanical strength after heat treatment. Without being bound by any theory, it is believed that this is due to the effect of C increasing the strength of martensite and the effect of Cu providing solid-solution strengthening in austenite and martensite and also providing hardening by forming clusters and precipitates. Thus, due to the high mechanical strength, a higher tempering temperature may be possible after quenching, and therefore the resulting final product will have improved temperature stability.

[0024] Furthermore, an object such as a mechanical component or a bar comprising or consisting of the martensitic stainless steel alloy as defined above or below will have a combination of improved fatigue strength and tensile strength, high hardness, good temperature stability in a high-temperature environment (temperature of about 300 °C), and improved wear resistance. Detailed Description

[0025] The present disclosure relates to a martensitic stainless steel alloy comprising, in weight percentage (wt%):

[0026] C >0.50 to 0.60;

[0027] Si 0.10 to 0.60;

[0028] Mn 0.40 to 0.80;

[0029] Cr 13.50 to 14.50;

[0030] Ni 0 to 1.20;

[0031] Mo 0.80 to 2.50;

[0032] N 0.050 to 0.12;

[0033] Cu >0.4 to 1.50;

[0034] V maximum 0.10;

[0035] S maximum 0.03;

[0036] P maximum 0.03;

[0037] The balance is Fe and unavoidable impurities.

[0038] The martensitic stainless steel alloy of the present invention (also hereinafter referred to as "stainless steel alloy" or "stainless steel") has a microstructure containing martensite, retained austenite, carbides, carbonitrides, and copper precipitates after hardening and tempering. A further feature of the microstructure of the hardened and tempered martensitic stainless steel alloy as defined above or below is the presence of metal carbonitrides; M 23 C 6 and M 7 C 3 carbides; and / or other types of carbides, where M represents one or more metal atoms.

[0039] Compared with conventional martensitic stainless steels, the stainless steel alloy of the present invention will provide an increase in hardness without compromising temperature stability. High-temperature stability is important because it means that the stainless steel alloy can be used in high-temperature applications (about 300 °C).

[0040] It has been found that the suitable hardening temperature for the martensitic stainless steel alloy of the present invention is in the temperature range of 980 °C to 1100 °C, such as 1020 °C to 1060 °C. Depending on the application, a suitable tempering temperature can be found in the range of 200 °C to 500 °C. By performing a tempering step at these temperatures, components comprising or consisting of the stainless steel alloy of the present invention will become temperature stable at high temperatures (about 300 °C). According to one embodiment, the martensitic stainless steel of the present invention can be tempered at a temperature of 400 °C to 450 °C. The resulting material will have a high enough hardness to be used in the desired application.

[0041] The hardening and tempering times can vary depending on the application and the product size. Hardening and tempering are carried out in a furnace.

[0042] According to one embodiment, the martensitic alloy of the present invention contains unavoidable impurities of 0.5 wt% or less, preferably unavoidable impurities of 0.3 wt% or less. The unavoidable impurities may naturally exist in the raw materials or recycled materials used to manufacture the stainless steel alloy. Examples of unavoidable impurities are elements and compounds that are not intentionally added but cannot be completely avoided because they are normally present as impurities. Thus, the unavoidable impurities are present in the alloy at a concentration at which they have only a very limited effect on the final properties. The unavoidable impurities present in the stainless steel alloy may include, for example, one or more of Co, Sn, Ti, Nb, W, Zr, Ta, B, Ce, and O.

[0043] In addition, during the manufacturing process, for example, in the deoxidation step or to improve other properties, small amounts of alloying elements may be added. Examples of such alloying elements are, but not limited to, Al, Mg, and Ca. Depending on the elements used, those skilled in the art will know how much is required. However, according to one embodiment, these elements may be added to the stainless steel alloy at ≤0.02 wt%.

[0044] The alloying elements of the proposed martensitic stainless steel alloy are discussed below. However, their effects mentioned below should not be regarded as restrictive.

[0045] Carbon (C)

[0046] C is important for forming metal carbonitrides; M 23 C 6 and M 7 C 3 carbides; and / or other types of carbides, where M represents one or more metal atoms. C is also important for the hardenability of the steel. However, too high a content of C may combine with other alloying elements, resulting in the formation of large and unwanted primary carbides during the initial manufacturing stage. In addition, a high content of C makes the martensite more brittle and reduces the Ms temperature at which martensite begins to form, and may also increase the amount of retained austenite to an excessive level. Therefore, the maximum C content of the alloy of the present invention is 0.60 wt%, for example 0.58 wt%, for example 0.56 wt%.

[0047] The high carbon content of the alloy of the present invention surprisingly provides a high particle density of carbides and a high particle area fraction. Additionally, surprisingly, the carbides formed are finely dispersed. The presence of smaller-sized and larger-number carbides will improve the mechanical properties. This can have a positive impact on wear resistance. Therefore, the high carbon content is > 0.50 wt%, for example 0.51 wt%, for example 0.52 wt%, for example 0.53 wt%.

[0048] The amount of C in the alloy of the present invention is limited to > 0.50 wt% to 0.60 wt%, preferably 0.51 wt% to 0.56 wt%.

[0049] Copper (Cu)

[0050] In the stainless steel alloy of the present invention, Cu is intentionally added. Cu is an austenite stabilizer, and it has surprisingly been found that in the steel of the present invention, Cu will contribute to the substitutional solid solution strengthening of the steel, thus providing new possibilities for excellent properties. Cu will also form a cluster and / or precipitate, thereby increasing the strength.

[0051] The solubility of Cu in the matrix is greater than 0.4 wt% in the equilibrium state. In the present disclosure, the inventors have found that it is important to have supersaturation of Cu to ensure the maximum substitutional solid solution strengthening of the martensite phase and the retained austenite phase after hardening and tempering. Additionally, supersaturation will enable cluster strengthening and precipitation hardening. Cu will also improve the corrosion resistance of the stainless steel alloy.

[0052] Therefore, the content of Cu is greater than 0.4 wt% to 1.50 wt%, for example 0.50 wt% to 1.50 wt%, for example 0.55 wt% to 1.30 wt%.

[0053] Silicon (Si)

[0054] Si is a ferrite stabilizer and acts as a deoxidizer. Si also increases the carbon activity and contributes to increasing the strength by solid solution strengthening. Excessive content may lead to the formation of unwanted inclusions. Therefore, the amount of Si is limited to 0.10 wt% to 0.60 wt%, for example 0.20 wt% to 0.55 wt%, for example 0.30 wt% to 0.50 wt%.

[0055] Manganese (Mn)

[0056] Mn is an austenite stabilizer and acts as a deoxidizer. Mn increases the solubility of N and improves hot workability. Excessive amounts may contribute to the formation of MnS inclusions in combination with S. Therefore, the amount of Mn is limited to 0.40 wt% to 0.80 wt%, for example 0.50 wt% to 0.80 wt%.

[0057] Chromium (Cr)

[0058] Cr is important for the corrosion resistance of the steel, which is determined by the amount of Cr in the steel matrix. Cr forms carbides (M 23 C 6 、M 7 C 3 、carbonitrides) and increases the solubility of C and N. Cr is a ferrite stabilizer, and excessive amounts may lead to the formation of δ-ferrite. Therefore, the amount of Cr is limited to 13.50 wt% to 14.50 wt%.

[0059] Molybdenum (Mo)

[0060] Mo is a ferrite stabilizer and a strong carbide former. Mo has a positive effect on both the corrosion resistance and hardenability of the steel. Mo also helps to improve ductility. Since Mo is an expensive element, for economic reasons, its content should not be higher than the required amount. Therefore, the amount of Mo is limited to 0.80 wt% to 2.50 wt%, preferably 0.80 wt% to 2.00 wt%, more preferably 0.90 wt% to 1.30 wt%.

[0061] Nitrogen (N)

[0062] N is an austenite stabilizer and increases the strength of the steel by interstitial solid solution strengthening. N helps to increase the hardness of martensite. N forms nitrides and carbonitrides. However, excessive amounts of N will reduce hot workability. Therefore, the amount of N is limited to 0.050 wt% to 0.12 wt%, preferably 0.050 wt% to 0.10 wt%, for example 0.055 wt% to 0.085 wt%.

[0063] Nickel (Ni)

[0064] Ni is an austenite stabilizer and reduces the solubility of C and N. Since Ni is an expensive element, for economic reasons, its content should be kept low, and in the stainless steel alloys of the present invention, Ni is not usually intentionally added. The amount of Ni should be ≤1.20 wt%, preferably ≤0.40 wt%, more preferably ≤0.35 wt%. According to one embodiment, Ni is between 0.15 wt% and 0.35 wt%.

[0065] Vanadium (V)

[0066] V is a strong carbide former and restricts grain growth. As a carbide forming element, V can be present in martensitic alloys and can be intentionally added. It may also be present due to recycled materials, but then it is considered an impurity. The content will also depend on the chromium source. However, too high a content of V may reduce ductility and hardenability and may produce unwanted primary carbides. Therefore, if present in a stainless steel alloy, the amount of V is limited to 0.010 wt% to 0.10 wt%, for example 0.030 wt% to 0.10 wt%.

[0067] Phosphorus (P)

[0068] P causes embrittlement. P is not usually added and should be limited to ≤0.03 wt%.

[0069] Sulfur (S)

[0070] S has a negative impact on hot workability and too high an amount will result in the formation of MnS inclusions. S is not usually added and should be limited to ≤0.03 wt%.

[0071] According to one embodiment, the stainless steel alloy of the present invention comprises any of the above-mentioned alloy elements within any of the above-mentioned ranges. According to another embodiment, the stainless steel alloy of the present invention consists of any of the above-mentioned alloy elements within any of the above-mentioned ranges.

[0072] Thus, the alloys of the present invention and objects made therefrom will have excellent strengthening because the solid solution hardening is maximized by the intentional addition of Cu within the ranges disclosed herein and because precipitation hardening is effected with finely dispersed carbides. In addition, ductility is improved by the microstructure composition.

[0073] Martensitic stainless steel alloys can be suitably manufactured in the form of components such as bars, but they can also be manufactured in the form of wire, rod, bar, tube, etc.

[0074] The martensitic stainless steel alloy of the present invention can be used for different mechanical components, such as valve components for compressors, such as flap valves. The martensitic stainless steel of the present invention is also suitable for other applications where high fatigue strength and / or wear resistance and edge performance are required.

[0075] According to one embodiment, the stainless steel alloy of the present invention can be manufactured accordingly as follows:

[0076] - Melting - The melting process can be carried out by using an EAF - electric arc furnace - followed by an AOD process and an optional final conditioning;

[0077] - Casting - Cast into a blank with a desired shape, such as a blank of 100 mm to 600 mm;

[0078] - Heating - Heat the blank until the material reaches a temperature of 1200 °C to 1350 °C;

[0079] - Rolling - Hot roll the blank into a bar. Depending on the rolling mill used, hot rolling can be carried out multiple times. In this step, if necessary, one or more heat treatment steps can be optionally carried out to obtain the desired bar size.

[0080] - Winding - Wind the bar, and the winding temperature after cooling is about 500 °C to 800 °C

[0081] - Annealing - Anneal the hot-rolled bar at 700 °C to 900 °C for at least 1 hour.

[0082] - Optionally surface treatment

[0083] - Rolling - Cold roll to a final thickness of, for example, 0.040 mm to 3 mm.

[0084] - Optionally annealing - Intermediate annealing may be required at a temperature of about 650 °C to 800 °C for recrystallization.

[0085] - Hardening - Hardening can be carried out in a continuous hardening line with the following steps: austenitization, quenching, additional cooling, tempering, cooling to room temperature and polishing. The speed of the hardening line depends on the thickness or mass flow rate of the material and the size of the furnace, and can be between 100 m / h and 1000 m / h. The lengths of the austenitization furnace and the tempering furnace are approximately the same.

[0086] o The austenitization temperature is between 950 °C and 1100 °C.

[0087] o Quenching should be carried out in such a way that the material temperature rapidly, usually within 2 minutes, reaches below about 500 °C to avoid embrittlement or reduction of corrosion resistance.

[0088] o Optionally, additional cooling is carried out to make the material pass below the Ms temperature and obtain the desired retained austenite level. Depending on the final application, the cooling temperature can be -100 °C to 100 °C, but usually room temperature is applied.

[0089] o Tempering can be set to 250 °C to 500 °C, which depends on the target final tensile strength.

[0090] The present disclosure is further illustrated by the following non-limiting examples.

[0091] Examples

[0092] Example 1

[0093] Multiple alloys are manufactured by melting using a vacuum induction melting furnace (VIM). The elemental composition of the alloys is listed in Table I in weight %. The balance is Fe and unavoidable impurities. When no value is given for a particular element, the amount of that element is below the detection limit. Alloys 1, 2, and 3 are included as comparative examples, while the remaining alloys represent different embodiments of the stainless steel alloys according to the present disclosure. The alloys are manufactured as stainless steel alloys as described below.

[0094]

[0095] Samples in the form of cylindrical test bars are manufactured from these hot melts for testing.

[0096] Therefore, the process flow is as follows:

[0097] Melting the raw materials in a vacuum induction melting furnace (VIM),

[0098] Casting,

[0099] Performing a heat treatment of preheating at 700 °C (30 minutes), followed by 1150 °C (30 minutes) before hot working,

[0100] Annealing (825 °C to 875 °C, 6 hours) and

[0101] Machining the samples;

[0102] Subsequently performing hardening and tempering.

[0103] The specimens are hardened at 1030 °C and 1050 °C, then quenched (to room temperature), and then tempered at 450 °C (for hardening at 1050 °C) and 250 °C and 450 °C (for hardening at 1030 °C) for 2 hours. The results can be seen in Tables IIA and IIB.

[0104] These hardness (HV1) measurements are carried out according to SS-EN ISO 6507. These values are the average of 5 measurements.

[0105]

[0106] As can be seen from Table IIA, the results show an increase in hardness for the two sets of data hardened at 1030 °C. The data show that even with a high tempering temperature, the hardness increases significantly, and due to the addition of Cu, the hardness increases.

[0107] Table IIA also shows that tempering at the higher temperature of 450 °C gives the alloys of the present invention a higher hardness (and thus a higher tensile strength). This means that the alloys of the present invention will have higher performance when used in high-temperature applications.

[0108]

[0109] Table IIB shows that at 1050 HV and 450 °C, the hardness of the alloys of the present invention is higher than that of the comparative alloys. This means that the alloys of the present invention will be suitable for high-temperature applications as they will maintain their high performance.

[0110] Fatigue measurement

[0111]

[0112] To measure the fatigue properties, an alloy, namely alloy 11, having the above composition and a final thickness of 0.305 mm was manufactured and then its fatigue properties were tested using a stepped method with a servo-hydraulic tensile testing machine AMSLER with a 10% preload and operating at approximately 80 Hz resonance. The run out of the test was defined as 5×10 6 cycles. A plurality of samples were manufactured and the samples consisted of a 10 mm waist circumference and a 15 mm length. The method means that the entire cross-section is exposed to the applied stress conditions, thereby testing the limiting factors of the material properties over a larger volume. The samples were flipped to ensure proper edges and high surface residual stresses. The failure probability of the fatigue tests conducted was 50%.

[0113] In Figure 1 the results of the fatigue tests are shown. The relationship R represents the ratio between the fatigue limit and the tensile strength. The standard deviations obtained are represented by the sizes of the respective boxes. As can be seen from the figure, the material of the present invention exhibits a fatigue limit of 1505 MPa, while the reference material (according to EN 1.4031) exhibits a fatigue limit of 1390 MPa.

[0114] Precipitate

[0115]

[0116]

[0117] As can be seen from this table, the alloys of the present disclosure have a particle density greater than 50.

[0118]

[0119] The data in Table V were obtained from SEM images of image processing. Its examples are given in Figure 3 . According to Thermo Calc calculations, the Cu particles of the alloys of the present invention are stable at temperatures below the A1 temperature. The presence of Cu particles in the images indicates that in addition to the maximized solid solution, there will be invisible Cu clusters and invisible finer Cu particles. Both the Cu precipitates and the Cu clusters will contribute to the mechanical properties.

[0120] The thermal stability of some of the alloys in Table III has been evaluated. The results are shown in Figure 2 .

[0121] Figure 2 It is shown that if alloy D is exposed to a temperature higher than that stable during tempering, alloy D will lose its properties. For alloy A, higher hardness and thus higher tensile strength were obtained without compromising thermal stability, which is shown by the hardness being almost unaffected across the temperature range.

Claims

1. A martensitic stainless steel alloy, comprising, by weight percentage (wt%): C > 0.50 to 0.60; Si 0.10 to 0.60; Mn 0.40 to 0.80; Cr 13.50 to 14.50; Ni 0 to 1.20; Mo 0.80 to 2.50; N 0.050 to 0.12; Cu greater than 0.4 to 1.50; V at most 0.10; S at most 0.03; P at most 0.03; the balance being Fe and unavoidable impurities, wherein Cu precipitates and clusters are present in the martensitic stainless steel alloy.

2. The martensitic stainless steel alloy according to claim 1, wherein the content of Si is 0.20 wt% to 0.55 wt%.

3. The martensitic stainless steel alloy according to claim 1 or 2, wherein the content of Mn is 0.50 wt% to 0.80 wt%.

4. The martensitic stainless steel alloy according to claim 1 or 2, wherein the content of Mo is 0.80 wt% to 2.00 wt%.

5. The martensitic stainless steel alloy according to claim 1 or 2, wherein the Ni content ≤ 0.80 wt%.

6. The martensitic stainless steel alloy according to claim 1 or 2, wherein the N content is 0.050 wt% to 0.10 wt%.

7. The martensitic stainless steel alloy according to claim 1 or 2, wherein the V content is 0.030 wt% to 0.10 wt%.

8. The martensitic stainless steel alloy according to claim 1 or 2, wherein the C content is 0.51 wt% to 0.60 wt%.

9. The martensitic stainless steel alloy according to claim 1 or 2, wherein the stainless steel alloy contains 0.50 wt% to 1.5 wt% Cu.

10. A stainless steel object, comprising the martensitic stainless steel alloy according to any one of the preceding claims.

11. The stainless steel object according to claim 10, wherein the object is a bar.

12. The stainless steel object according to claim 10 or 11, wherein the object is cold-rolled, hardened and tempered.

13. The stainless steel object according to claim 12, wherein the microstructure is characterized by the presence of metal carbonitrides; M 23 C 6 and M 7 C 3 carbides; and / or other types of carbides, and wherein M represents one or more metal atoms.

14. The stainless steel object according to claim 12, wherein the microstructure contains Cu precipitates and / or clusters.

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