Rare earth martensitic stainless steel and preparation method thereof

Abstract

The invention belongs to the technical field of martensitic stainless steel and heat treatment thereof, and particularly relates to rare earth martensitic stainless steel and a preparation method thereof.The preparation method of the rare earth martensitic stainless steel comprises the following steps that S1, vacuum smelting and casting are conducted, and a casting blank is obtained; s2, the casting blank is subjected to homogenizing annealing, cogging and hot rolling, and then a hot-rolled plate is obtained; s3, determining a first temperature for annealing the hot-rolled plate according to the content of each element in the hot-rolled plate; s4, determining the thickness of a cold-rolled plate obtained by cold rolling the annealed hot-rolled plate according to the contents of the boron element and the rare earth element in the hot-rolled plate; s5, determining a second temperature for quenching the cold-rolled sheet according to the content of alloy elements in the cold-rolled sheet; the corrosion resistance of the steel is improved by designing chemical components and adding a proper amount of rare earth purified molten steel and modified inclusions, and meanwhile, the steel has relatively high strength and plasticity by combining a proper heat treatment process and regulating and controlling the size and the distribution form of carbides.

Description

Technical Field

[0001] This invention belongs to the field of martensitic stainless steel and its heat treatment technology, specifically a rare earth martensitic stainless steel and its preparation method. Background Technology

[0002] Martensitic stainless steel is widely used in the manufacture of components such as cutting tools, valves, turbine blades, and structural frames due to its high hardness, strength, and good corrosion resistance.

[0003] Cr13 martensitic stainless steel, containing 13% wt.% Cr, exhibits good corrosion resistance and is widely used in various industries. However, Cr, as a strong carbide-forming element, readily combines with C to form carbides, and reducing the Cr content in the matrix can affect the corrosion resistance of stainless steel. Furthermore, the quantity, size, and distribution of carbides, as second-phase particles, significantly influence the mechanical properties of stainless steel.

[0004] The prior art discloses a martensitic stainless steel for cutting tools and its preparation method. The specific components, by mass percentage, are: C: 0.2%-0.3%, Cr: 11%-14%, Mn: 0.5%-1.0%, Si: 0.5%-1.0%, Mo: 0.1%-0.4%, V: 0.1%-0.3%, Ni: 0.2%-0.8%, P<0.01%, S<0.01%, with the balance being Fe and unavoidable impurity elements. The martensitic stainless steel of this invention has a yield strength of 1150-1200 MPa, a tensile strength of 1450-1600 MPa, an elongation ≥9.0%, and a hardness of 48-50 HRC. Compared with this invention, its hardness, strength, and plasticity are inferior.

[0005] The prior art discloses a high-strength and high-toughness martensitic stainless steel and its preparation method. The specific components, by mass percentage, are: C: 0.32%-0.35%, Cr: 12.5%-13.5%, Mn: 0.4%-0.6%, Si: 0.4%-0.6%, Mo: 0.1%-0.2%, V: 0.1%-0.15%, Ni: 0.2%-0.3%, P<0.01%, S<0.01%, with the balance being Fe and unavoidable impurity elements. This invention achieves high strength through the synergistic effect of multiple elements and the refinement of grains using hot rolling and cold rolling processes, but its plasticity is poor, with an elongation of ≥8.0%.

[0006] Currently, the corrosion resistance of domestically produced martensitic stainless steel materials is increasingly failing to meet growing demands. Simultaneously, most martensitic stainless steel materials, while possessing high strength, lack sufficient ductility. This makes it difficult to meet the production requirements of martensitic stainless steel products that demand high strength, high ductility, high hardness, and good corrosion resistance. Summary of the Invention

[0007] To address the aforementioned technical problems, the present invention aims to provide a method for preparing rare-earth martensitic stainless steel, comprising the following steps:

[0008] S1. Vacuum smelting and casting to obtain a billet;

[0009] S2. The cast billet is homogenized by annealing, opened, and hot rolled to obtain a hot-rolled plate.

[0010] S3. Determine the first temperature for annealing the hot-rolled plate based on the content of each element in the hot-rolled plate, and anneal the hot-rolled plate using the first temperature;

[0011] S4. Determine the thickness of the cold-rolled plate obtained by cold rolling the annealed hot-rolled plate according to the content of boron and rare earth elements in the hot-rolled plate.

[0012] S5. Determine a second quenching temperature for the cold-rolled sheet based on the content of each element in the cold-rolled sheet, and quench the cold-rolled sheet using the second temperature;

[0013] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel, wherein the rare earth element content of the martensitic stainless steel is 0.003wt.%-0.014wt.% and the boron element content is 0.001wt.%-0.011wt.%.

[0014] As a preferred technical solution for the preparation method of rare earth martensitic stainless steel according to the present invention, the elemental composition of the rare earth martensitic stainless steel is as follows by mass percentage: C: 0.40wt.%-0.60wt.%, Si: 0.25wt.%-0.35wt.%, Mn: 0.20wt.%-0.60wt.%, Nb: 0.04wt.%-0.10wt.%, V: 0.10wt.%-0.30wt.%, Cr: 12.50wt.%-14.00wt.%, Mo: 0.30wt.%-0.80wt.%, Ni: 0.35wt.%-1.00wt.%, B: 0.001wt.%-0.011wt.%, RE: 0.003wt.%-0.014wt.%, with the balance being Fe and unavoidable impurities.

[0015] As a preferred technical solution of the method for preparing rare earth martensitic stainless steel according to the present invention, in step S2, the homogenization annealing temperature is 1100-1200℃, the holding time is 5-10h, the billet temperature is 1000-1200℃, and the forging ratio is 2-6.

[0016] Furthermore, the homogenization annealing temperature is any one of 1100°C, 1150°C, and 1200°C, or a range between two of them.

[0017] Furthermore, the billet temperature is any one or a range between 1000℃, 1050℃, 1100℃, 1150℃, and 1200℃, and the forging ratio is any one or a range between 2, 3, 4, 5, and 6.

[0018] As a preferred technical solution of the method for preparing rare earth martensitic stainless steel according to the present invention, in step S2, the hot rolling temperature is 1000-1200℃ and the rolling reduction is 40%-70%; in step S6, the tempering temperature is 120-300℃ and the holding time is 30-120min, followed by air cooling.

[0019] Furthermore, the hot rolling temperature is any one or a range of two of 1000℃, 1050℃, 1100℃, 1150℃, and 1200℃, and the rolling reduction is any one or a range of two of 40%, 50%, 60%, and 70%.

[0020] As a preferred technical solution of the method for preparing rare earth martensitic stainless steel according to the present invention, in step S2, the homogenization annealing, billet opening and hot rolling are carried out continuously without cooling, and the hot-rolled plate is slowly cooled to room temperature when it is cooled to 900°C.

[0021] As a preferred technical solution for the preparation method of rare earth martensitic stainless steel according to the present invention, wherein, according to the formula: T1=562+305.8w C +13.9w Si +22.7w Mn -8.22w Nb -8.09w V +10.2w Cr -3.2w Mo +18.2w Ni +2263w (RE+B) T1 is calculated; in step S3, the first temperature is T1, the holding time is 1-3 hours, and the cooling method is furnace cooling.

[0022] Among them, w C w Si w Mn w Nb w V w Cr w Mo w Ni w (RE+B)The value is obtained by multiplying the mass fraction of C, Si, Mn, Nb, V, Cr, Mo, Ni, and (RE+B) elements in rare earth martensitic stainless steel by 100.

[0023] As a preferred technical solution of the method for preparing rare earth martensitic stainless steel according to the present invention, wherein, according to the formula: T2=722+262w C +12.6w Si +26.6w Mn +7.6w Nb +6.3w V +14.4w Cr +2.6w Mo +16.2w Ni +5.9w (RE+B) T2 is calculated; in step S5, the second temperature is T2, the holding time is 15-30 min, and the cooling method is oil cooling;

[0024] Among them, w C w Si w Mn w Nb w V w Cr w Mo w Ni w (RE+B) The value is obtained by multiplying the mass fraction of C, Si, Mn, Nb, V, Cr, Mo, Ni, and (RE+B) elements in rare earth martensitic stainless steel by 100.

[0025] As a preferred technical solution of the method for preparing rare earth martensitic stainless steel according to the present invention, step S4 further includes: when the sum of the mass fractions of rare earth elements and boron elements in the rare earth martensitic stainless steel is greater than or equal to 0.009 wt.%, the thickness of the cold-rolled plate is greater than or equal to 6 mm; when the sum of the mass fractions of rare earth elements and boron elements in the rare earth martensitic stainless steel is less than 0.009 wt.%, the thickness of the cold-rolled plate is less than 6 mm.

[0026] This invention provides a rare-earth martensitic stainless steel, which is prepared using the above-described preparation method.

[0027] As a rare-earth martensitic stainless steel according to the present invention, the rare-earth martensitic stainless steel has a Rockwell hardness ≥ 52 HRC, tensile strength ≥ 1600 MPa, yield strength ≥ 1100 MPa, elongation ≥ 9%, self-corrosion potential ≥ -0.37 V, and self-corrosion current density ≤ 1.3 μA·cm. -2 .

[0028] This invention improves the corrosion resistance of molten steel by adding appropriate amounts of rare earth elements through chemical composition design, modifying inclusions, and controlling the size and distribution of carbides through suitable heat treatment processes, thereby giving it both high strength and plasticity. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0030] Figure 1 This is a diagram showing the morphology and distribution of carbides in Example 1 of the present invention;

[0031] Figure 2 This is a stretching curve diagram of Embodiment 1 of the present invention;

[0032] Figure 3 This is a diagram showing the morphology and distribution of carbides in Example 2 of the present invention;

[0033] Figure 4 This is a stretching curve diagram of Embodiment 2 of the present invention;

[0034] Figure 5 This is a diagram showing the morphology and distribution of carbides in Comparative Example 1 of the present invention;

[0035] Figure 6 This is a stretching curve diagram of Comparative Example 1 of the present invention;

[0036] Figure 7 First-principles calculations of density of states plots for Cr13 steel and Cr13RE steel.

[0037] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0038] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0040] Example 1

[0041] The mass percentages of each element in the rare earth martensitic stainless steel used in Example 1 are shown in Table 1.

[0042] Table 1

[0043]

[0044] A method for preparing rare earth martensitic stainless steel:

[0045] S1. Vacuum smelting and casting to obtain a billet;

[0046] S2. The billet is homogenized and annealed, then opened and hot rolled to obtain a hot-rolled plate. The homogenization annealing temperature is 1200℃ and held for 10 hours. The opening temperature is 1200℃ and the forging ratio is 4. The hot rolling temperature is 1200℃ and the rolling reduction is 60%. After cooling to 900℃, the plate is slowly cooled to room temperature to obtain a hot-rolled plate.

[0047] S3. Based on the element content in the hot-rolled plate, determine the first annealing temperature for the hot-rolled plate, and anneal the hot-rolled plate at the first temperature, wherein, according to the formula T1=562+305.8w C +13.9w Si +22.7w Mn -8.22w Nb -8.09w V +10.2w Cr -3.2w Mo +18.2w Ni +2263w (RE+B) The calculated value is T1 = 901.4, the initial temperature is 901.4℃, the temperature is held for 2 hours, and then the furnace is cooled to room temperature;

[0048] S4. Based on the content of boron and rare earth elements in the hot-rolled plate, determine the thickness of the cold-rolled plate obtained by cold rolling the annealed hot-rolled plate, wherein the sum of the mass fractions of rare earth elements and boron elements is 0.008 wt.%, and the thickness of the cold-rolled plate is 4 mm.

[0049] S5. Based on the element content in the cold-rolled sheet, determine the second quenching temperature for the cold-rolled sheet, and quench the cold-rolled sheet at the second temperature, wherein, according to the formula: T2=722+262w C +12.6w Si +26.6w Mn +7.6w Nb +6.3w V +14.4w Cr +2.6w Mo +16.2w Ni +5.9w (RE+B)The calculated value of T2 is 1082.1℃, the second temperature is 1082.1℃, the temperature is held for 20 minutes, and then the oil is cooled to room temperature;

[0050] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel. The tempering temperature is 200℃, the holding time is 1 hour, and then the sheet is air-cooled to room temperature.

[0051] The as-cast samples of the prepared martensitic stainless steel were observed and statistically analyzed, with a thickness of approximately 9.84 mm. 2 The type, quantity, and size of non-metallic inclusions within the field of view. In Example 1, the inclusions were Al2O3, Al2O3-SiO2, Al2O3-MnS, MnO, MnS, Ce-Al-O-Mn, Ce-Al-SO-Mn, Ce-S, Ce-SO, Ce-SO-Mn, Ce-Al-SO, Ce-O-Mn, CeAlO3, and Ce2O3. The number of inclusions was 1737, and the average size was 2.94 μm.

[0052] The electrochemical test results for martensitic stainless steel showed a self-corrosion potential of -0.37V and a self-corrosion current density of 1.15μA·cm. -2 .

[0053] Please see Figure 1 , Figure 1 This is a diagram showing the morphology and distribution of carbides in Example 1 of the present invention. The carbides are uniformly distributed, consisting of large and small spherical carbides; no short rod-shaped carbides were found. The hardness of the martensitic stainless steel is 53.5 HRC.

[0054] Please see Figure 2 , Figure 2 The tensile curve of Embodiment 1 of the present invention shows a tensile strength of 1941 MPa, a yield strength of 1258 MPa, and an elongation of 16.3%.

[0055] Example 2

[0056] The mass percentages of each element in the rare earth martensitic stainless steel used in Example 2 are shown in Table 2.

[0057] Table 2

[0058]

[0059] A method for preparing rare earth martensitic stainless steel:

[0060] S1. Vacuum smelting and casting to obtain a billet;

[0061] S2. The billet is homogenized and annealed, then opened and hot rolled to obtain a hot-rolled plate. The homogenization annealing temperature is 1200℃ and held for 10 hours. The opening temperature is 1200℃ and the forging ratio is 4. The hot rolling temperature is 1200℃ and the rolling reduction is 66%. After cooling to 900℃, the plate is slowly cooled to room temperature to obtain a hot-rolled plate.

[0062] S3. Based on the element content in the hot-rolled plate, determine the first annealing temperature for the hot-rolled plate, and anneal the hot-rolled plate at the first temperature, wherein, according to the formula T1=562+305.8w C +13.9w Si +22.7w Mn -8.22w Nb -8.09w V +10.2w Cr -3.2w Mo +18.2w Ni +2263w (RE+B) The calculated value is T1 = 897.9, the initial temperature is 897.9℃, the furnace is held at this temperature for 2 hours, and then cooled to room temperature.

[0063] S4. Based on the content of boron and rare earth elements in the hot-rolled plate, determine the thickness of the cold-rolled plate obtained by cold rolling the annealed hot-rolled plate, wherein the sum of the mass fractions of rare earth elements and boron elements is 0.014 wt.%, and the thickness of the cold-rolled plate is 8 mm.

[0064] S5. Based on the element content in the cold-rolled sheet, determine the second quenching temperature for the cold-rolled sheet, and quench the cold-rolled sheet at the second temperature, wherein, according to the formula: T2=722+262w C +12.6w Si +26.6w Mn +7.6w Nb +6.3w V +14.4w Cr +2.6w Mo +16.2w Ni +5.9w (RE+B) The calculated value of T2 is 1071.2, the second temperature is 1071.2℃, the temperature is held for 30 minutes, and then the oil is cooled to room temperature;

[0065] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel. The tempering temperature is 130°C, the holding time is 35 minutes, and then the sheet is air-cooled to room temperature.

[0066] The as-cast samples of the prepared martensitic stainless steel were observed and statistically analyzed, with a thickness of approximately 9.84 mm. 2The types, quantities, and sizes of non-metallic inclusions within the field of view: In Example 2, the inclusions were Al2O3, Al2O3-SiO2, Al2O3-MnS, MnO, MnS, Ce-Al-O-Mn, Ce-Al-SO-Mn, Ce-S, Ce-SO, Ce-SO-Mn, Ce-Al-SO, Ce-O-Mn, CeAlO3, and Ce2O3, with a total of 447 inclusions and an average size of 3.14 μm.

[0067] The electrochemical test results for martensitic stainless steel showed a self-corrosion potential of -0.35V and a self-corrosion current density of 1.27μA·cm. -2 .

[0068] Please see Figure 3 , Figure 3 The image shows the morphology and distribution of carbides in Example 2 of this invention. The carbides are uniformly distributed, consisting of large and small spherical carbides, with no short rod-shaped carbides found.

[0069] The hardness of martensitic stainless steel is 52.8 HRC.

[0070] Please see Figure 4 , Figure 4 The tensile curve of Embodiment 2 of the present invention shows a tensile strength of 1792 MPa, a yield strength of 1147 MPa, and an elongation of 13.24%.

[0071] Example 3

[0072] The difference from Example 1 is:

[0073] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel. The tempering temperature is 280℃, the holding time is 60min, and then the sheet is air-cooled to room temperature.

[0074] The as-cast samples of the prepared martensitic stainless steel were observed and statistically analyzed, with a thickness of approximately 9.84 mm. 2 The type, quantity, and size of non-metallic inclusions within the field of view are as follows: In Example 3, the type of inclusions is the same as in Example 1, the number of inclusions is 1688, and the average size is 2.96 μm.

[0075] The electrochemical test results for martensitic stainless steel showed a self-corrosion potential of -0.35V and a self-corrosion current density of 1.17μA·cm. -2 .

[0076] The hardness of martensitic stainless steel is 56.8 HRC, the tensile strength is 2084 MPa, the yield strength is 1625 MPa, and the elongation is 9.2%.

[0077] Example 4

[0078] The difference from Example 1 is:

[0079] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel. The tempering temperature is 280℃, the holding time is 110 min, and then the sheet is air-cooled to room temperature.

[0080] The as-cast samples of the prepared martensitic stainless steel were observed and statistically analyzed, with a thickness of approximately 9.84 mm. 2 The type, quantity, and size of non-metallic inclusions within the field of view. In Example 4, the type of inclusions is the same as in Example 1, the quantity of inclusions is 1711, and the average size is 2.88 μm.

[0081] The electrochemical test results for martensitic stainless steel showed a self-corrosion potential of -0.35V and a self-corrosion current density of 1.22μA·cm. -2 .

[0082] Martensitic stainless steel has a hardness of 55.7 HRC, a tensile strength of 2187 MPa, a yield strength of 1550 MPa, and an elongation of 9.4%.

[0083] Example 5

[0084] The difference from Example 1 is:

[0085] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel. The tempering temperature is 200℃, the holding time is 60min, and then the sheet is air-cooled to room temperature.

[0086] The as-cast samples of the prepared martensitic stainless steel were observed and statistically analyzed, with a thickness of approximately 9.84 mm. 2 The type, quantity, and size of non-metallic inclusions within the field of view. In Example 5, the inclusions are the same as in Example 1, with a quantity of 1694 and an average size of 2.93 μm.

[0087] The electrochemical test results for martensitic stainless steel showed a self-corrosion potential of -0.36V and a self-corrosion current density of 1.25μA·cm. -2 .

[0088] The hardness of martensitic stainless steel is 55.2 HRC, the tensile strength is 2154 MPa, the yield strength is 1436 MPa, and the elongation is 11.2%.

[0089] Comparative Example 1

[0090] The mass percentages of each element in the martensitic stainless steel used in Comparative Example 1 are shown in Table 3.

[0091] Table 3

[0092]

[0093] A method for preparing rare earth martensitic stainless steel:

[0094] S1. Vacuum smelting and casting to obtain a billet;

[0095] S2. The billet is homogenized and annealed, then opened and hot rolled to obtain a hot-rolled plate. The homogenization annealing temperature is 1200℃ and held for 10 hours. The opening temperature is 1200℃ and the forging ratio is 4. The hot rolling temperature is 1200℃ and the rolling reduction is 66%. After cooling to 900℃, the plate is slowly cooled to room temperature to obtain a hot-rolled plate.

[0096] S3. Based on the element content in the hot-rolled plate, determine the first annealing temperature for the hot-rolled plate, and anneal the hot-rolled plate at the first temperature, wherein, according to the formula T1=562+305.8w C +13.9w Si +22.7w Mn -8.22w Nb -8.09w V +10.2w Cr -3.2w Mo +18.2w Ni +2263w (RE+B) The calculated value is T1 = 909.7, the initial temperature is 909.7℃, the temperature is held for 2 hours, and then the furnace is cooled to room temperature;

[0097] S4. Based on the content of boron and rare earth elements in the hot-rolled plate, determine the thickness of the cold-rolled plate obtained by cold rolling the annealed hot-rolled plate, wherein the mass fraction of boron is 0.009 wt.% and the thickness of the cold-rolled plate is 6 mm.

[0098] S5. Based on the element content in the cold-rolled sheet, determine the second quenching temperature for the cold-rolled sheet, and quench the cold-rolled sheet at the second temperature, wherein, according to the formula: T2=722+262w C +12.6w Si +26.6w Mn +7.6w Nb +6.3w V +14.4w Cr +2.6w Mo +16.2w Ni +5.9w (RE+B) The calculated value of T2 is 1088.8, the second temperature is 1088.8℃, the temperature is held for 20 minutes, and then the oil is cooled to room temperature;

[0099] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel. The tempering temperature is 200℃, the holding time is 60 minutes, and then the sheet is air-cooled to room temperature.

[0100] The observed diameter of the as-cast sample is approximately 9.84 mm. 2 The types, quantities, and sizes of non-metallic inclusions within the field of view: In Comparative Example 1, the inclusions are Al2O3, Al2O3-SiO2, Al2O3-MnS, MnO, and MnS, with a total number of 2723 inclusions and an average size of 1.90 μm.

[0101] The electrochemical test results for the sample showed a self-corrosion potential of -0.43V and a self-corrosion current density of 1.38μA·cm. -2 .

[0102] Please see Figure 5 , Figure 5 The diagram shows the morphology and distribution of carbides in Comparative Example 1 of this invention. The carbides are uniformly distributed, consisting of large and small spherical carbides, with no short rod-shaped carbides found.

[0103] The hardness of martensitic stainless steel is 51.4 HRC.

[0104] Please see Figure 6 , Figure 6 The tensile curve of Comparative Example 1 of the present invention shows a tensile strength of 1557 MPa, a yield strength of 1177 MPa, and an elongation of 13.08%.

[0105] Comparative Example 2

[0106] The difference from Example 1 is:

[0107] The composition of rare earth martensitic stainless steel, the mass percentage of each element in the rare earth martensitic stainless steel used in Comparative Example 2 is shown in Table 4.

[0108] Table 4

[0109]

[0110] The first temperature is 929.6℃, and the second temperature is 1078.3℃.

[0111] The sum of the mass fractions of rare earth elements and boron is 0.021 wt.%, and the thickness of the cold-rolled sheet is 8 mm;

[0112] The as-cast samples of the prepared martensitic stainless steel were observed and statistically analyzed, with a thickness of approximately 9.84 mm. 2The types, quantities, and sizes of non-metallic inclusions within the field of view: In Comparative Example 2, the inclusions are Al2O3, Al2O3-SiO2, Al2O3-MnS, MnO, MnS, Ce-Al-O-Mn, Ce-Al-SO-Mn, Ce-S, Ce-SO, Ce-SO-Mn, Ce-Al-SO, Ce-O-Mn, CeAlO3, and Ce2O3. The number of inclusions is 1142, and the average size is 3.32 μm.

[0113] The electrochemical test results for martensitic stainless steel showed a self-corrosion potential of -0.36V and a self-corrosion current density of 1.28μA·cm. -2 .

[0114] The hardness of martensitic stainless steel is 59.2 HRC, the tensile strength is 2200 MPa, the yield strength is 1128 MPa, and the elongation is 7.3%.

[0115] Comparative Example 3

[0116] The difference from Example 1 is:

[0117] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel. The tempering temperature is 100℃, the holding time is 60min, and then the sheet is air-cooled to room temperature.

[0118] The observed diameter of the as-cast sample is approximately 9.84 mm. 2 The types, quantities, and sizes of non-metallic inclusions within the field of view: In Comparative Example 3, the inclusions were Al2O3, Al2O3-SiO2, Al2O3-MnS, MnO, and MnS, with a total number of 2769 inclusions and an average size of 1.88 μm.

[0119] The electrochemical test results for the sample showed a self-corrosion potential of -0.48V and a self-corrosion current density of 1.42μA·cm. -2 .

[0120] The hardness of martensitic stainless steel is 57.5 HRC, the tensile strength is 2017 MPa, the yield strength is 1039 MPa, and the elongation is 7.8%.

[0121] Comparative Example 4

[0122] The mass percentages of each element in the martensitic stainless steel used in Comparative Example 4 are shown in Table 5.

[0123] Table 5

[0124]

[0125] A method for preparing rare earth martensitic stainless steel:

[0126] S1. Vacuum smelting and casting to obtain a billet;

[0127] S2. The billet is homogenized and annealed, then opened and hot-rolled to obtain a hot-rolled plate. The homogenization annealing temperature is 1150℃ and held for 9 hours. The opening temperature is 1200℃ and the forging ratio is 4. The hot rolling temperature is 1100℃ and the rolling reduction is 66%. After cooling to 900℃, the plate is slowly cooled to room temperature to obtain a hot-rolled plate.

[0128] S3. Based on the element content in the hot-rolled plate, determine the first annealing temperature for the hot-rolled plate, and anneal the hot-rolled plate at the first temperature, wherein, according to the formula T1=562+305.8w C +13.9w Si +22.7w Mn -8.22w Nb -8.09w V +10.2w Cr -3.2w Mo +18.2w Ni +2263w (RE+B) The calculated value of T1 is 870.7, the initial temperature is 870.7℃, the temperature is held for 2 hours, and then the furnace is cooled to room temperature.

[0129] S4. Based on the content of boron and rare earth elements in the hot-rolled plate, determine the thickness of the cold-rolled plate obtained by cold rolling the annealed hot-rolled plate, wherein the sum of the mass fractions of rare earth elements and boron elements is 0.005 wt.% and less than 0.009 wt.%, and the thickness of the cold-rolled plate is 4 mm.

[0130] S5. Based on the element content in the cold-rolled sheet, determine the second quenching temperature for the cold-rolled sheet, and quench the cold-rolled sheet at the second temperature, wherein, according to the formula: T2=722+262w C +12.6w Si +26.6w Mn +7.6w Nb +6.3w V +14.4w Cr +2.6w Mo +16.2w Ni +5.9w (RE+B) The calculated T2 = 1063.7, the second temperature is 1063.7℃, hold at this temperature for 20 minutes, and then cool to room temperature with oil.

[0131] S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel. The tempering temperature is 120°C, the holding time is 120 min, and then the sheet is air-cooled to room temperature.

[0132] The observed diameter of the as-cast sample is approximately 9.84 mm. 2 The types, quantities, and sizes of non-metallic inclusions within the field of view: In Comparative Example 4, the inclusions were Al2O3, Al2O3-SiO2, Al2O3-MnS, MnO, and MnS, with a total number of 2846 inclusions and an average size of 1.81 μm.

[0133] The electrochemical test results for the sample showed a self-corrosion potential of -0.49V and a self-corrosion current density of 1.45μA·cm. -2 .

[0134] The hardness of martensitic stainless steel is 50.3 HRC, the tensile strength is 1352 MPa, the yield strength is 1126 MPa, and the elongation is 12.34%.

[0135] Comparative Example 5

[0136] The difference from Example 1 is that the first temperature is 800°C, the thickness of the cold-rolled sheet is 7mm, and the second temperature is 1100°C.

[0137] The observed diameter of the as-cast sample is approximately 9.84 mm. 2 The types, quantities, and sizes of non-metallic inclusions within the field of view are as follows: the inclusion types in Comparative Example 5 are consistent with those in Example 1, the number of inclusions is 2744, and the average size is 1.92 μm.

[0138] The electrochemical test results for the sample showed a self-corrosion potential of -0.35V and a self-corrosion current density of 1.22μA·cm. -2 .

[0139] The hardness of martensitic stainless steel is 50.3 HRC, the tensile strength is 1476 MPa, the yield strength is 832 MPa, and the elongation is 11.4%.

[0140] Comparative Example 6

[0141] The difference from Example 2 is that the first temperature is 890°C, the thickness of the cold-rolled sheet is 5mm, and the second temperature is 1250°C.

[0142] The as-cast samples of the prepared martensitic stainless steel were observed and statistically analyzed, with a thickness of approximately 9.84 mm. 2 The types, quantities, and sizes of non-metallic inclusions within the field of view are as follows: the inclusion types in Comparative Example 6 are the same as those in Example 2, the number of inclusions is 459, and the average size is 3.12 μm.

[0143] The electrochemical test results for martensitic stainless steel showed a self-corrosion potential of -0.34V and a self-corrosion current density of 1.21μA·cm. -2 .

[0144] The hardness of martensitic stainless steel is 48.6 HRC, the tensile strength is 1412 MPa, the yield strength is 737 MPa, and the elongation is 7.63%.

[0145] Compared with Comparative Example 1, Example 1 showed a significant reduction in the number of inclusions Al2O3, Al2O3-SiO2, Al2O3-MnS, MnO, and MnS, while new inclusions Ce-Al-O-Mn, Ce-Al-SO-Mn, Ce-S, Ce-SO, Ce-SO-Mn, Ce-Al-SO, Ce-O-Mn, CeAlO3, and Ce2O3 were added, resulting in a total of 1737 inclusions with an average size of 2.94 μm.

[0146] The main non-metallic inclusions in martensitic stainless steel include MnS, SiO2, Al2O3, and composite inclusions. These inclusions are easily decomposed in corrosive environments, leading to pitting corrosion. The addition of rare earth elements to steel allows them to combine with O and S to form rare earth sulfides and rare earth oxides. Rare earth composite inclusions exhibit lower pitting corrosion susceptibility, and rare earth elements significantly reduce the number of inclusions in the steel. Therefore, rare earth elements increase the corrosion resistance of the steel in corrosive environments.

[0147] The addition of rare earth elements to martensitic stainless steel reduces the density of states at and near the Fermi level in the matrix. A lower density of states at and near the Fermi level indicates better corrosion resistance. During corrosion, a higher density of states at and near the Fermi level increases electron transfer between the anode and cathode; conversely, a higher density of states at and near the Fermi level indicates poorer corrosion resistance. The density of states of Cr13 (without rare earth elements) and Cr13RE (with rare earth elements) martensitic stainless steel are shown below. Figure 7 As shown, rare earth element Ce reduces the density of states at and near the Fermi level of Cr13 steel, thereby enhancing its corrosion resistance.

[0148] The addition of rare earth element Ce results in the precipitation of numerous fine, dispersed rare earth sulfide inclusions during the solidification of molten steel. These inclusions promote heterogeneous nucleation of the δ-Fe / γ-Fe phases during solidification, refining the steel's grain size. During heat treatment, rare earth elements hinder the diffusion of carbon from the retained austenite, improving its stability. The retained austenite, in turn, inhibits crack propagation during steel deformation. Therefore, rare earth elements enhance the mechanical properties of steel by refining the grain size and improving the stability of the retained austenite.

[0149] Hot-rolled annealing can reduce the geometric dislocation density in steel, dissolve inhomogeneous carbides, eliminate residual stress, and reduce the tendency to crack during subsequent cold rolling. Carbides, as non-deformable particles, hinder dislocation movement. During cold rolling, dislocations multiply and easily accumulate around the carbides, increasing the work hardening rate of the steel. Rare earth element Ce tends to segregate at grain boundaries, hindering carbon diffusion during heat treatment and reducing the amount of carbides. With increasing boron (B) content (correspondingly decreasing Ce content) in Cr13 martensitic stainless steel, the amount of carbides after hot rolling increases. Determining an appropriate first annealing temperature for hot-rolled martensitic stainless steel can effectively dissolve inhomogeneous carbides, improve compositional and microstructure uniformity, obtain a uniform initial microstructure, reduce the risk of localized stress concentration during cold rolling, improve its cold workability, and enhance its cold-rolled quality. Furthermore, the addition of rare earth elements increases the solubility of Nb and C in fccFe, effectively reducing the amount of MC carbides in the steel, which is beneficial for subsequent cold rolling after hot-rolled annealing.

[0150] When determining the first temperature, the formula is used based on the content of each element: T1 = 562 + 305.8w C +13.9w Si +22.7w Mn -8.22w Nb -8.09w V +10.2w Cr -3.2w Mo +18.2w Ni +2263w (RE+B) T1 is calculated and is the first temperature. Determining the first temperature using this method ensures the uniform diffusion of alloying elements such as carbon, chromium, rare earth elements, and boron during the annealing holding process. It also dissolves a large amount of carbides precipitated along grain boundaries during hot rolling, reducing microscopic composition segregation and eliminating a large amount of residual stress in the steel. This results in a balanced and uniform microstructure in the annealed rare earth martensitic stainless steel, which is beneficial for subsequent cold rolling and heat treatment. The applicant's research found that if the first temperature is higher than the calculated value, the average grain size of the rare earth martensitic stainless steel increases significantly after annealing, and a large amount of carbides in the matrix dissolves, leading to serious quality defects on the steel surface during subsequent cold rolling. During quenching, carbides easily precipitate at grain boundaries, forming large carbide particles, severely damaging the plasticity and surface quality of the rare earth martensitic stainless steel. If the first temperature is lower than the calculated value, a large amount of processing stress and structural stress exists inside the rare earth martensitic stainless steel after annealing, making it prone to deformation and cracking during subsequent cold rolling and quenching.

[0151] When determining the second temperature, the formula T2 = 722 + 262w is used based on the content of each element. C +12.6w Si +26.6w Mn+7.6w Nb +6.3w V +14.4w Cr +2.6w Mo +16.2w Ni +5.9w (RE+B) T2 is calculated and is the second temperature. Determining the second temperature using this method ensures uniform diffusion of alloying elements such as rare earth elements and boron during heat preservation, allowing the alloying elements to fully dissolve in austenite. The quenched rare-earth martensitic stainless steel exhibits a suitable amount of carbides and fine grain size, resulting in significant solid solution strengthening of alloying elements in the martensitic matrix. This also reduces the impact of element segregation on microstructure and property uniformity, as well as the damage to corrosion resistance caused by uneven chromium distribution. The applicant's research found that if the second temperature is higher than the calculated value, the rare-earth martensitic stainless steel has coarse grains, excessive retained austenite, and surface decarburization, leading to insufficient toughness, inadequate surface hardness, and uneven performance. Conversely, if the second temperature is lower than the calculated value, carbon and alloying elements are not fully dissolved into the matrix during quenching, and some microstructures are not completely transformed into austenite, resulting in insufficient carbon and alloying element content in the matrix. Consequently, the hardness and strength of the martensitic stainless steel fail to meet technical specifications, and some original ferrite remains embedded in the martensite, easily causing stress concentration at the ferrite-martensite interface and impairing plasticity.

[0152] The quantity, size, and distribution of carbides in steel all affect its mechanical properties. During tempering, carbides continuously precipitate and grow. During rolling, dislocation slip occurs inside the carbides, and the carbides are cut by the dislocation slip, which refines the carbides. During annealing, the broken carbides spheroidize. Thus, after quenching and tempering heat treatment, the carbides are smaller and rounder, which improves the strength and plasticity of the steel.

[0153] The heat treatment scheme produces different results for rare-earth martensitic stainless steel and martensitic stainless steel without rare-earth elements. Adding rare-earth element Ce to martensitic stainless steel allows Ce atoms, which segregate at grain boundaries, to hinder the diffusion of carbon atoms, leading to increased Ac1 and Ac3 temperatures and a decreased Ms temperature. Simultaneously, rare-earth Ce can regulate inclusions in martensitic stainless steel, refine its microstructure, and improve the morphology and distribution of carbides. Martensitic stainless steel without rare-earth elements, after heat treatment using the same process, cannot achieve the same properties as rare-earth martensitic stainless steel.

[0154] This invention improves the corrosion resistance of molten steel by adding appropriate amounts of rare earth elements through chemical composition design, modifying inclusions, and controlling the size and distribution of carbides through suitable heat treatment processes, thereby giving it both high strength and plasticity.

[0155] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for preparing rare-earth martensitic stainless steel, characterized in that, Includes the following steps: S1. Vacuum smelting and casting to obtain a billet; S2. The cast billet is homogenized by annealing, opened, and hot rolled to obtain a hot-rolled plate. S3. Determine the first temperature for annealing the hot-rolled plate based on the content of each element in the hot-rolled plate, and anneal the hot-rolled plate using the first temperature; S4. Determine the thickness of the cold-rolled plate obtained by cold rolling the annealed hot-rolled plate according to the content of boron and rare earth elements in the hot-rolled plate. S5. Determine a second quenching temperature for the cold-rolled sheet based on the content of each element in the cold-rolled sheet, and quench the cold-rolled sheet using the second temperature; S6. Temper the quenched cold-rolled sheet to obtain the martensitic stainless steel, wherein the rare earth element content of the martensitic stainless steel is 0.003wt.%-0.014wt.% and the boron element content is 0.001wt.%-0.011wt.%.

2. The method for preparing rare earth martensitic stainless steel according to claim 1, characterized in that, The rare earth martensitic stainless steel has the following elemental composition by mass percentage: C: 0.40wt.%-0.60wt.%, Si: 0.25wt.%-0.35wt.%, Mn: 0.20wt.%-0.60wt.%, Nb: 0.04wt.%-0.10wt.%, V: 0.10wt.%-0.30wt.%, Cr: 12.50wt.%-14.00wt.%, Mo: 0.30wt.%-0.80wt.%, Ni: 0.35wt.%-1.00wt.%, B: 0.001wt.%-0.011wt.%, RE: 0.003wt.%-0.014wt.%, with the balance being Fe and unavoidable impurities.

3. The method for preparing rare earth martensitic stainless steel according to claim 2, characterized in that, In step S2, the homogenization annealing temperature is 1100-1200℃ and the holding time is 5-10h; the billet temperature is 1000-1200℃ and the forging ratio is 2-6.

4. The method for preparing rare earth martensitic stainless steel according to claim 3, characterized in that, In step S2, the hot rolling temperature is 1000-1200℃ and the rolling reduction is 40%-70%; in step S6, the tempering temperature is 120-300℃ and the holding time is 30-120min, followed by air cooling.

5. The method for preparing a rare-earth martensitic stainless steel according to claim 4, characterized in that, In step S2, the homogenization annealing, billet opening, and hot rolling are carried out continuously without cooling. When the hot-rolled plate is cooled to 900°C, it is slowly cooled to room temperature.

6. The method for preparing rare earth martensitic stainless steel according to claim 2, characterized in that, According to the formula: T1 = 562 + 305.8w C +13.9w Si +22.7w Mn -8.22w Nb -8.09w V +10.2w Cr -3.2w Mo +18.2w Ni +2263w (RE+B) T1 is calculated; In step S3, the first temperature is T1, the holding time is 1-3 hours, and the cooling method is furnace cooling.

7. The method for preparing a rare-earth martensitic stainless steel according to claim 6, characterized in that, According to the formula: T2 = 722 + 262w C +12.6w Si +26.6w Mn +7.6w Nb +6.3w V +14.4w Cr +2.6w Mo +16.2w Ni +5.9w (RE+B) T2 is calculated; In step S5, the second temperature is T2, the holding time is 15-30 minutes, and the cooling method is oil cooling.

8. The method for preparing a rare-earth martensitic stainless steel according to claim 2, characterized in that, Step S4 further includes that when the sum of the mass fractions of rare earth elements and boron elements in the rare earth martensitic stainless steel is greater than or equal to 0.009 wt.%, the thickness of the cold-rolled sheet is greater than or equal to 6 mm; when the sum of the mass fractions of rare earth elements and boron elements in the rare earth martensitic stainless steel is less than 0.009 wt.%, the thickness of the cold-rolled sheet is less than 6 mm.

9. A rare-earth martensitic stainless steel, characterized in that, It is prepared by the method for preparing rare earth martensitic stainless steel as described in any one of claims 1-8.

10. A rare-earth martensitic stainless steel according to claim 9, characterized in that, The rare-earth martensitic stainless steel has a Rockwell hardness ≥ 52 HRC, tensile strength ≥ 1600 MPa, yield strength ≥ 1100 MPa, elongation ≥ 9%, self-corrosion potential ≥ -0.37 V, and self-corrosion current density ≤ 1.3 μA·cm. -2 .

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