Low Ni steel alloy with resistance to hydrogen degradation

Abstract

The present invention provides a hydrogen degradation resistant steel alloy containing controlled amounts of Mn and C, along with Al, Cr, Cu, Ni and Si, which has an austenitic microstructure and has relatively high stacking fault energy to avoid the formation of martensite phases that reduce hydrogen resistance.

Description

[Technical field]

[0001] <CROSS REFERENCE TO RELATED APPLICATIONS> This application claims the benefit of U.S. Provisional Patent Application No. 63 / 336,431, filed April 29, 2022, and is incorporated herein by reference.

[0002] The present invention relates to a low-nickel steel alloy having excellent resistance to hydrogen degradation during use. [Background technology]

[0003] <Field of the Invention> Currently, alloys used for high pressure hydrogen storage applications include grade 316L austenitic stainless steel, which nominally contains 18 weight percent Cr and 13 weight percent Ni in addition to iron and several other elements. However, the addition of Cr and Ni is relatively expensive, and lower cost alternative elements would be beneficial for hydrogen applications, such as the use of hydrogen as a fuel for cars and trucks. Summary of the Invention

[0004] <Summary of the Invention> The present invention provides a hydrogen degradation resistant steel alloy containing controlled amounts of Mn and C, and further containing Al, Cr, Cu, Ni and Si, which has an austenitic microstructure and relatively high stacking fault energy, thereby avoiding the formation of martensite phases that reduce hydrogen resistance.

[0005] One aspect of the present invention is to provide a hydrogen degradation resistant steel alloy comprising 15-30 weight percent Mn, 0.15-1 weight percent C, and 0.05-3 weight percent Al, the steel alloy having a microstructure containing at least 99 volume percent austenite with a relative reduction in area of ​​20 percent or less.

[0006] This and other aspects of the invention will become more apparent from the following description. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a graph showing that a fully austenitic steel with desirable stacking fault energy can be achieved by using relatively high amounts of Mn and adding controlled amounts of C, Al, and Ni in accordance with the present invention.

[0008] [Diagram 2] FIG. 2 is a graph showing that the further addition of Al, Cr, Cu, and Ni to the Mn component achieves suitable stacking fault energy in accordance with an embodiment of the present invention.

[0009] [Diagram 3] FIG. 3 is a graph comparing the mechanical properties and hydrogen content of a hydrogen degradation resistant steel alloy according to the present invention with a standard stainless steel alloy, showing ultimate tensile strength. [Figure 4] FIG. 4 is a graph comparing the mechanical properties and hydrogen content of a hydrogen degradation resistant steel alloy according to the present invention with a standard stainless steel alloy, showing yield strength. [Diagram 5] FIG. 5 is a graph comparing the mechanical properties and hydrogen content of a hydrogen degradation resistant steel alloy according to the present invention with a standard stainless steel alloy, showing total elongation. [Figure 6] FIG. 6 is a graph comparing the mechanical properties and hydrogen content of a hydrogen degradation resistant steel alloy according to the present invention with a standard stainless steel alloy, showing the relative reduction in area.

[0010] [Figure 7] 3D plot of various steel alloy compositions with different Mn and C contents showing their effect on the relative reduction in area of ​​the steel alloy. [Figure 8] 3D plot of various steel alloy compositions with different Mn and C contents showing their effect on the relative reduction in area of ​​the steel alloy.

[0011] [Figure 9] FIG. 9 shows photomicrographs of fracture surfaces of a hydrogen-degradation resistant steel alloy of the present invention taken at different locations on the specimen before and after electrochemical hydrogen charging, which show characteristics of ductile fracture. [Figure 10] FIG. 10 shows photomicrographs of fracture surfaces of the hydrogen-degradation resistant steel alloy of the present invention taken at different locations on the specimen before and after electrochemical hydrogen charging, which show ductile fracture characteristics. [Figure 11] FIG. 11 shows photomicrographs of fracture surfaces of a hydrogen-degradation resistant steel alloy of the present invention taken at different locations on the specimen before and after electrochemical hydrogen charging, which show characteristics of ductile fracture. [Figure 12] FIG. 12 shows photomicrographs of fracture surfaces of the hydrogen-degradation resistant steel alloy of the present invention taken at different locations on the specimen before and after electrochemical hydrogen charging, which show characteristics of ductile fracture. [Figure 13] FIG. 13 shows photomicrographs of fracture surfaces of a hydrogen-degradation resistant steel alloy of the present invention taken at different locations on the specimen before and after electrochemical hydrogen charging, which show ductile fracture characteristics. [Figure 14] FIG. 14 shows photomicrographs of fracture surfaces of a hydrogen-degradation resistant steel alloy of the present invention taken at different locations on the specimen before and after electrochemical hydrogen charging, which show ductile fracture characteristics. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The steel alloy of the present invention has a stacking fault energy of, for example, 20 mJ / m 2 The alloy composition can be selected according to the present invention to stabilize austenite and avoid the formation of martensite, which has a relatively high austenite content above 100%, and can be used in hydrogen applications because it can avoid the formation of martensite, which significantly reduces hydrogen resistance.

[0013] The hydrogen resistant steel alloy of the present invention typically comprises at least 15 weight percent Mn, for example at least 18 weight percent, or at least 20 weight percent, or at least 20.5 weight percent, or at least 21 weight percent, or at least 22 weight percent. Mn can comprise up to 30 weight percent, or up to 25 weight percent, or up to 24 weight percent. In certain embodiments, Mn can comprise 15-30 weight percent, or 18-25 weight percent, or 20-24 weight percent. The relatively high Mn content can provide similar qualities to stainless steels, including resistance to hydrogen degradation during use. The inclusion of Mn can provide a fully austenitic steel, which can be resistant to the degrading effects of hydrogen, such as reduced ductility and embrittlement.

[0014] Hydrogen resistant steel alloys typically include at least 0.18 weight percent C, e.g., at least 0.25 weight percent, or at least 0.3 weight percent, or at least 0.4 weight percent. C can include up to 1 weight percent, or up to 0.9 weight percent, or up to 0.8 weight percent, or up to 0.6 weight percent. In certain embodiments, C can include 0.18-1 weight percent, or 0.25-0.9 weight percent, or 0.3-0.8 weight percent, or 0.4-0.6 weight percent.

[0015] Hydrogen resistant steel alloys typically include at least 0.05 weight percent Al, e.g., at least 0.1 weight percent, or at least 0.5 weight percent, or at least 0.8 weight percent, or at least 1.0 weight percent, or at least 1.2 weight percent. Al may include up to 2.5 weight percent, or up to 2.2 weight percent, or up to 2 weight percent, or up to 1.8 weight percent. In certain embodiments, Al includes 0.05-2.5 weight percent, or 0.8-2.2 weight percent, or 1-2 weight percent, or 1.4-1.8 weight percent.

[0016] Hydrogen resistant steel alloys typically include at least 0.5 weight percent Si, e.g., at least 1 weight percent, or at least 2 weight percent, or at least 2.5 weight percent. Si may include up to 4 weight percent, or up to 3.5 weight percent, or up to 3.2 weight percent, or up to 3 weight percent. In certain embodiments, Si includes 1-4 weight percent, or 1.5-3.5 weight percent, or 2-3.2 weight percent, or 2.5-3 weight percent. In certain embodiments, the steel alloy may be substantially free of Si.

[0017] Hydrogen resistant steel alloys typically include at least 0.8 weight percent Ni, such as at least 1 weight percent, or at least 1.2 weight percent. Ni may include up to 2.5 weight percent, or up to 2 weight percent, or up to 1.5 weight percent. In certain embodiments, Ni includes 0.8-2.5 weight percent, or 1-2 weight percent, or 1.2-1.5 weight percent. In certain embodiments, the steel alloy may be substantially free of Ni.

[0018] Hydrogen resistant steel alloys typically include at least 0.2 weight percent Cu, e.g., at least 0.4 weight percent, or at least 0.6 weight percent. Cu may include up to 2 weight percent, or up to 1.5 weight percent, or up to 1.2 weight percent. In certain embodiments, Cu may include 0.2-2 weight percent, or 0.4-1.5 weight percent, or 0.6-1.2 weight percent. In certain embodiments, the steel alloy may be substantially free of Cu.

[0019] Hydrogen resistant steel alloys typically include at least 1 weight percent Cr, e.g., at least 1.5 weight percent, or at least 2 weight percent, or at least 2.2 weight percent. Cr can include up to 3.5 weight percent, or up to 3.2 weight percent, or up to 3 weight percent, or up to 2.8 weight percent. In certain embodiments, Cr can include 1.5-3.5 weight percent, or 2-3.2 weight percent, or 2-3 weight percent, or 2.2-2.8 weight percent. Alternatively, Cr can be less than 1.5 weight percent, or less than 1 weight percent, or less than 0.5 weight percent, or less than 0.2 weight percent. In certain embodiments, the steel alloy can be substantially free of Cr.

[0020] Hydrogen resistant steel alloys typically include at least 0.01 weight percent Ti, such as at least 0.05 weight percent, or at least 0.08 weight percent. Ti may include up to 0.5 weight percent, or up to 0.3 weight percent, or up to 0.2 weight percent. In certain embodiments, Ti may include 0.01-0.5 weight percent, or 0.02-0.3 weight percent, or 0.08-0.2 weight percent. In certain embodiments, the steel alloy may be substantially free of Ti.

[0021] As used herein, the term "substantially free" when referring to alloying additions means that a particular element or material is not intentionally added to the alloy and is present, if at all, only in small amounts as an impurity, e.g., less than 0.05 weight percent, or less than 0.01 weight percent.

[0022] The hydrogen degradation resistant steel alloy has an austenitic microstructure, with the austenite comprising at least 95 volume percent, or at least 98 volume percent, or at least 99 volume percent, or at least 99.5 volume percent. The hydrogen degradation resistant steel alloy may be substantially free of other phases, such as ferrite or martensite, other than austenite. For example, if such phases are present, the amount is less than 1 volume percent, or less than 0.5 volume percent, or less than 0.1 volume percent, or zero volume percent.

[0023] Figures 1 and 2 show the design concept of the present invention. Figure 1 shows that a fully austenitic steel with a target SFE range can be achieved by using a relatively high amount of Mn, e.g., 22 weight percent or 15 weight percent Mn, and adding moderate amounts of C, e.g., 0.45 weight percent C, and adding Al, Cu, and Ni. An alloy with 0.18 weight percent C and 15 weight percent Mn may not meet the SFE design target. Figure 2 further shows that for the carbon and manganese contents considered, the additional addition of Al, Cr, Cu, and Ni brings the SFE into the desired range.

[0024] Samples of each composition listed in Table 1 were melted on a laboratory scale, hot rolled, and prepared for electrochemical charging to generate nascent hydrogen on the sample surface. The electrochemical charging technique was performed by electrochemically charging the test samples in a 20 g / L Na2SO4 solution at 70 °C for 48 hours, with 2 g / L NHSCN added to prevent recombination of the nascent hydrogen. The current density used for the test was 70 A / m 2 During electrochemical charging, the nascent atomic hydrogen diffuses into the test sample.

[0025] The hydrogen resistance of hydrogen-charged samples can be tested by standard tensile testing and comparing ductility to non-hydrogen-charged samples. Reduction in Area (RA) can be used as a measure of ductility. A target Relative Reduction in Area (RRA) of 20% is believed to be competitive with 316L stainless steel. Thus, with RA degradation of 20% or less, the alloys of the present invention are believed to be competitive with 316L stainless steel in terms of hydrogen resistance.

[0026] The following examples are intended to illustrate various aspects of the invention and are not intended to limit the scope of the invention. EXAMPLES

[0027] The actual chemical compositions shown in Table 1 were used to prepare melts in a laboratory vacuum induction furnace. [Table 1]

[0028] Chemical composition was measured by LECO C / N / O / S analyzer or inductively coupled plasma optical emission spectroscopy (ICP-OES). Titanium was added to some of the melts for microalloying to improve yield strength and reduce twinning kinetics. Phosphorus and sulfur were added to each alloy at low levels of 0.015 and 0.005 weight percent phosphorus and sulfur, respectively, to simulate residual phosphorus and sulfur in steel melts. The materials were hot rolled in the laboratory from 7-inch thick ingots to 1.25-inch thick slabs and air cooled. All testing was performed on the hot rolled slabs.

[0029] The samples were measured with a Metis MSAT 30 instrument to determine the percentage of austenite in the material. The results are listed in Table 2 and a comparison is given with the 316L stainless steel material used in this study. A fully austenitic structure is desirable to prevent hydrogen embrittlement. Since the samples were stored in liquid nitrogen to prevent hydrogen desorption, the samples were also tested after 24 hours of storage in liquid nitrogen. No microstructural changes were observed after storage in liquid nitrogen except for a 0.1 volume percent decrease in the percentage of austenite for alloys 4 and 6. The alloy microstructure was almost fully austenitic. [Table 2]

[0030] Round tensile specimens of ASTM E8-22 sample 2, with the specimens in the longitudinal direction, parallel to the rolling direction, were tested before and after electrochemical hydrogen charging according to the ASTM E8-22 standard. Total elongation for all specimens was measured manually because the extensometer measurement range was exceeded during testing. Tensile and hydrogen analysis test specimens were electrochemically charged with 70 A / m in a 20 g / L Na2SC4 solution at 70 °C for 48 h. 2 The specimens were electrochemically charged at a current density of 1000 s. 2 g / L NHSCN was added to prevent recombination of nascent hydrogen. The hydrogen test specimens were selected from the same melt and near the same location as the tensile test specimens to minimize parameters that may affect the amount of hydrogen adsorption, such as grain size. Immediately after electrochemical charging to hydrogen, the tensile test specimens were stored in liquid nitrogen in preparation for tensile testing. There was a delay of 8-12 min for the temperature of the tensile specimens to stabilize to room temperature before testing. The mechanical properties before and after electrochemical charging were compared along with the concentration of hydrogen diffusible from the test specimens, measured at 300 °C with a Bruker Hydrogen Analyzer mass spectrometer.

[0031] The results of mechanical properties and hydrogen concentration before and after electrochemical charging are compared with 316L stainless steel material in Table 4 and Figures 3-6. The higher hydrogen concentration of 316L stainless steel compared with the hot-rolled alloys in this study is believed to be due to the difference in microstructure such as grain size and percentage of ferrite. All mechanical properties showed equal or higher strength and total elongation compared with 316L stainless steel material. The relative reduction in area (RRA) of alloys 2, 3, 4, and 6 was less than 20%, with alloy 3 showing the best results. Alloy 5 had a low carbon content of 0.2 weight percent and manganese content of 16.5 weight percent, and an RRA of 23.2%. When the manganese content was increased to 20.5 weight percent as in alloy 2, the average RRA improved to 11.8%, and when the carbon content was increased to 0.48 weight percent as in alloy 6, the average RRA improved to 14.1%. As carbon and manganese are both increased, to 0.48 weight percent carbon and 24.0 weight percent manganese, the RRA improves further to an average value of 2.6% for alloy 3 and 11.3% for alloy 4.

[0032] Figures 7 and 8 visually show the effect of Mn and C on RRA. The data in Figures 7 and 8 show that the RRA decreases with increasing C and Mn concentrations, indicating the individual contribution of both Mn and C to the RRA. The titanium addition in Alloy 4 compared to Alloy 3 improved the tensile strength and yield strength, but also increased the RRA value. [Table 3]

[0033] Alloy 4 was subjected to Hydrogen Induced Cracking (HIC) testing in accordance with NACE TM0284-2016 and Sulfide Stress Cracking (SSC) testing in accordance with NACE TM0177-2016-Method A. The stress applied during the SSC testing was 85% of the actual yield stress to simulate a higher hydrogen pressure environment. No cracks were observed after each test. Some pitting corrosion was observed during the SSC testing. [Table 4]

[0034] The microstructures of the electrochemically charged tensile fracture surfaces showed ductile fracture characteristics across the fracture surface for both the uncharged and hydrogen-charged specimens. The fracture surface microstructures of Alloy 6 before and after electrochemical hydrogen charging were photographed at the edge (Figs. 9 and 10), quarter (Figs. 11 and 12), and center (Figs. 13 and 14) of the round tensile specimens.

[0035] In this study, low nickel austenitic grades were designed with carbon levels of 0.18-0.5 wt%, manganese levels of 16.5-24.5 wt%, and Al, Cr, Cu, Ni, and Si levels to resist hydrogen embrittlement and have RRA less than 20 wt% after electrochemical charging to introduce new hydrogen atoms. Based on this study and literature review, carbon levels of 0.18-0.6 wt%, manganese levels of 16-30 wt%, chromium levels of 2.0-3.5 wt%, copper levels of 0.6-2 wt%, nickel levels of more than 0.9 wt%, and nickel levels of 1.3 wt% for cost reduction, silicon levels of 2.0-4.0 wt%, and aluminum levels of 0.04-2 wt% are suitable as low-cost low nickel austenitic steels to replace 316L stainless steels in terms of resistance to hydrogen degradation. In this study, low carbon, low Mn grades were more susceptible to hydrogen embrittlement. Titanium may be used to enhance mechanical properties, but it may also increase the material's susceptibility to hydrogen embrittlement.

[0036] As used herein, terms such as "including, comprising," "containing," and the like are open-ended terms and are understood to not exclude the presence of additional elements, materials, phases, or process steps not recited in this application. As used herein, the term "consisting of" is understood to exclude the presence of any unspecified element, material, phase, or process step. As used herein, the term "consisting essentially of" is understood to include the specified elements, materials, phases, or process steps, where applicable, and to include any unspecified elements, materials, phases, or process steps that do not materially affect the basic or novel characteristics of the invention.

[0037] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values ​​set forth in the specific examples are reported as precisely as possible, however, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0038] It should also be understood that all numerical ranges described herein are intended to include all subranges subsumed therein, for example, a range of "1" to "10" is intended to include the subranges between a minimum value of 1 and a maximum value of 10, where the minimum value is 1 or greater than 1 and the maximum value is 10 or less than 10.

[0039] In this application, the use of the singular includes the plural and the plural includes the singular, unless expressly stated otherwise. Also, in this application, the use of "or" means "and / or" unless expressly stated otherwise, even if the word "and / or" is explicitly used in a particular embodiment. In this specification and claims, the articles "a," "an," and "the" include plural referents unless expressly and unambiguously limited to one referent.

[0040] Although particular embodiments of the invention have been described above for purposes of illustration, it will be apparent to those skilled in the art that many changes can be made in the details of the invention without departing from the invention.

Claims

1. A hydrogen degradation resistant steel alloy comprising 15 to 30 weight percent Mn, 0.15 to 1 weight percent C, and 0.05 to 3 weight percent Al, A hydrogen degradation resistant steel alloy having a microstructure containing at least 99 volume percent austenite and a relative reduction in area of ​​no more than 20 percent.

2. 2. The steel alloy of claim 1 , wherein C is greater than 0.2 weight percent.

3. 10. The steel alloy of claim 1 , wherein Mn is greater than 18 weight percent.

4. 2. The steel alloy of claim 1, wherein when Mn is less than 18 weight percent, C is greater than 0.2 weight percent.

5. 2. The steel alloy of claim 1, wherein Mn is greater than 18 weight percent when C is less than 0.3 weight percent.

6. 2. The steel alloy of claim 1 comprising 18 to 25 weight percent Mn and 0.3 to 1 weight percent C.

7. 2. The steel alloy of claim 1 comprising 20 to 24 weight percent Mn and 0.4 to 0.6 weight percent C.

8. 10. The steel alloy of claim 1, further comprising 0.8 to 2.5 weight percent Ni.

9. 10. The steel alloy of claim 1, further comprising at least 0.2 weight percent Cu.

10. 10. The steel alloy of claim 1, further comprising 0.8 to 2.5 weight percent Ni, and 0.2 to 2 weight percent Cu.

11. 11. The steel alloy of claim 10, further comprising at least 0.5 weight percent Si.

12. 11. The steel alloy of claim 10, further comprising at least 1 weight percent Cr.

13. 11. The steel alloy of claim 10, further comprising 0.5 to 4 weight percent Si, and 1 to 3.5 weight percent Cr.

14. 10. The steel alloy of claim 1, further comprising at least 0.02 weight percent Ti.

15. 2. The steel alloy of claim 1 comprising 20 to 24 weight percent Mn and 0.3 to 0.6 weight percent C.

16. 16. The steel alloy of claim 15, further comprising 0.8 to 2.5 weight percent Ni, and 0.2 to 2 weight percent Cu.

17. 17. The steel alloy of claim 16, further comprising 0.5 to 4 weight percent Si, and 1 to 3.5 weight percent Cr.

18. 18. The steel alloy of claim 17, wherein Al comprises 1.4-1.8 weight percent, Ni comprises 1.2-1.5 weight percent, Cu comprises 0.6-1.2 weight percent, Si comprises 2-3.2 weight percent, and Cr comprises 2-3.2 weight percent.

19. 19. The steel alloy of claim 18, further comprising 0.08 to 0.2 weight percent Ti.

20. 10. The steel alloy of claim 1, wherein the microstructure comprises at least 99.5 volume percent austenite.

21. 10. The steel alloy of claim 1, wherein the relative reduction in area is less than 15 percent.

22. 10. The steel alloy of claim 1, wherein the steel alloy has an ultimate tensile strength greater than 700 MPa and a total elongation greater than 50 percent.

23. 10. A method of producing the steel alloy of claim 1 comprising hot rolling the steel alloy to form a slab and cooling the slab.

24. 24. The method of claim 23, further comprising electrochemically charging the steel alloy slab to generate nascent atomic hydrogen.

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