Cr-ni-mo-v-nb high-strength steel and preparation process thereof

Abstract

The application belongs to the technical field of metal material heat treatment, and relates to a Cr-Ni-Mo-V-Nb high-strength steel and a preparation process thereof. The Cr-Ni-Mo-V-Nb high-strength steel is obtained in the form of a plate after being hot-rolled at 1150 DEG C. The plate is heated to 860 DEG C and then kept for 2 hours, and then cooled to 760 DEG C in the furnace, and then kept for 2 hours, and then cooled to room temperature in air after being reduced to 400 DEG C. The plate is heated to 880 DEG C, kept for 0.5 hours, and then quenched to room temperature in water. The plate is tempered at 200 DEG C for 4 hours, and then air-cooled to room temperature. The Cr-Ni-Mo-V-Nb high-strength steel has a structure including lath martensite, twin, residual austenite and carbide, the carbide includes needle-shaped epsilon-carbide and spherical MC carbide, and the steel has excellent comprehensive mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, specifically to a Cr-Ni-Mo-V-Nb high-strength steel and its preparation process. Background Technology

[0002] Ultra-high strength steel is developed based on ordinary alloy steel and possesses high yield strength and tensile strength. For many years, researchers have been dedicated to the development of ultra-high strength steel and have developed several research directions, such as strong precipitation hardening steel, ultrafine grain steel, and martensitic aging steel. Medium carbon low alloy steel, with its excellent weldability and low production cost, is widely used in aerospace, engineering machinery, and military fields, such as AISI 4340 steel and 300M steel. Due to the trade-off between strength and toughness, increasing yield strength always comes at the cost of toughness, meaning that steel may experience brittle failure in complex working environments. Therefore, it is necessary to develop a high-strength steel with a better balance of strength and toughness to ensure its performance in engineering applications. Summary of the Invention

[0003] To address the problems existing in the prior art, the main objective of this invention is to propose a Cr-Ni-Mo-V-Nb high-strength steel and its preparation process.

[0004] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution:

[0005] A process for preparing Cr-Ni-Mo-V-Nb high-strength steel includes the following steps:

[0006] S1 and Cr-Ni-Mo-V-Nb high-strength steel are hot-rolled at 1150℃ to obtain plates;

[0007] S2. After heating the board to 860℃, hold it at that temperature for 2 hours, then cool it down to 760℃ in the furnace, continue to hold it at that temperature for 2 hours, and then cool it down to 400℃ in the air to room temperature.

[0008] S3. Heat the plate to 880℃, hold for 0.5 hours, and then water quench to room temperature;

[0009] S4. The sheet material is tempered at 200℃ for 4 hours, and then air-cooled to room temperature.

[0010] As a preferred embodiment of the preparation process of Cr-Ni-Mo-V-Nb high-strength steel according to the present invention, in step S1, the thickness of the hot-rolled steel plate is 10mm.

[0011] As a preferred embodiment of the preparation process of Cr-Ni-Mo-V-Nb high-strength steel according to the present invention, in step S1, the final rolling temperature is 900℃.

[0012] As a preferred embodiment of the preparation process of Cr-Ni-Mo-V-Nb high-strength steel according to the present invention, wherein: in step S1, the composition of Cr-Ni-Mo-V-Nb high-strength steel, by mass percentage, includes: C 0.27%, Si 0.28%, Mn 0.45%, P 0.0028%, S 0.0025%, Cr 1.1%, Ni 1.4%, Mo 0.35%, V 0.15%, Nb 0.05%, with the balance being Fe and unavoidable impurities.

[0013] To solve the above-mentioned technical problems, according to another aspect of the present invention, the present invention provides the following technical solution:

[0014] A Cr-Ni-Mo-V-Nb high-strength steel is prepared using the aforementioned Cr-Ni-Mo-V-Nb high-strength steel preparation process.

[0015] As a preferred embodiment of the Cr-Ni-Mo-V-Nb high-strength steel described in this invention, the microstructure of the Cr-Ni-Mo-V-Nb high-strength steel includes lath martensite, twins, retained austenite and carbides, the carbides include acicular ε-carbides and spherical MC carbides, and the average diameter of the carbides is 14.6±0.1nm.

[0016] As a preferred embodiment of the Cr-Ni-Mo-V-Nb high-strength steel described in this invention, the Cr-Ni-Mo-V-Nb high-strength steel has a yield strength of 1513±14MPa, a tensile strength of 1308±12MPa, an elongation of 8.2±0.1%, and an impact toughness of 40.5±0.2J / cm. 2 Its hardness is 437±1HV.

[0017] The beneficial effects of this invention are as follows:

[0018] This invention proposes a Cr-Ni-Mo-V-Nb high-strength steel and its preparation process. The Cr-Ni-Mo-V-Nb high-strength steel is hot-rolled at 1150℃ to obtain a sheet. The sheet is heated to 860℃ and held for 2 hours, then furnace-cooled to 760℃ and held for another 2 hours. After cooling to 400℃, it is air-cooled to room temperature. The sheet is then heated to 880℃ and held for 0.5 hours, followed by water quenching to room temperature. The sheet is then tempered at 200℃ for 4 hours, and subsequently air-cooled to room temperature. The microstructure of the Cr-Ni-Mo-V-Nb high-strength steel of this invention includes lath martensite, twins, retained austenite, and carbides. The carbides include acicular ε-carbides and spherical MC carbides, exhibiting excellent comprehensive mechanical properties. Attached Figure Description

[0019] 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.

[0020] Figure 1 This is a sampling diagram of the present invention;

[0021] Figure 2 The diagram shows the engineering stress-strain curves of the treated plates in Embodiment 1 and Comparative Examples 1-2 of the present invention.

[0022] Figure 3 The ultimate tensile strength, yield strength, and elongation of the treated plates in Embodiment 1 and Comparative Examples 1-7 of this invention;

[0023] Figure 4 The impact toughness of the treated plates in Embodiment 1 and Comparative Examples 1-7 of the present invention;

[0024] Figure 5 The hardness of the treated plates in Embodiment 1 and Comparative Examples 1-7 of the present invention;

[0025] Figure 6 SEM images of the treated plates of Embodiment 1 and Comparative Examples 1-7 of the present invention;

[0026] Figure 7 These are TEM images of the treated plates from Embodiment 1 and Comparative Examples 1-2 of the present invention.

[0027] Figure 8 The images show the morphology, size, and distribution of carbides on the treated plates of Examples 1 and 1-2 of this invention.

[0028] 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

[0029] 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.

[0030] According to one aspect of the present invention, the present invention provides the following technical solution:

[0031] A process for preparing Cr-Ni-Mo-V-Nb high-strength steel includes the following steps:

[0032] S1 and Cr-Ni-Mo-V-Nb high-strength steel are hot-rolled at 1150℃ to obtain plates;

[0033] S2. After heating the board to 860℃, hold it at that temperature for 2 hours, then cool it down to 760℃ in the furnace, continue to hold it at that temperature for 2 hours, and then cool it down to 400℃ in the air to room temperature.

[0034] S3. Heat the plate to 880℃, hold for 0.5 hours, and then water quench to room temperature;

[0035] S4. The sheet material is tempered at 200℃ for 4 hours, and then air-cooled to room temperature.

[0036] Preferably, in step S1, the thickness of the hot-rolled steel plate is 10 mm, and the final rolling temperature is 900℃.

[0037] Preferably, in step S1, the composition of the Cr-Ni-Mo-V-Nb high-strength steel, by mass percentage, includes: C 0.27%, Si 0.28%, Mn 0.45%, P 0.0028%, S 0.0025%, Cr 1.1%, Ni 1.4%, Mo 0.35%, V 0.15%, Nb 0.05%, with the balance being Fe and unavoidable impurities.

[0038] According to another aspect of the present invention, the present invention provides the following technical solution:

[0039] A Cr-Ni-Mo-V-Nb high-strength steel is prepared using the aforementioned Cr-Ni-Mo-V-Nb high-strength steel preparation process.

[0040] Preferably, the microstructure of the Cr-Ni-Mo-V-Nb high-strength steel includes lath martensite, twins, retained austenite, and carbides. The carbides include acicular ε-carbides and spherical MC carbides, and the average diameter of the carbides is 14.6±0.1 nm.

[0041] Preferably, the Cr-Ni-Mo-V-Nb high-strength steel has a yield strength of 1513±14 MPa, a tensile strength of 1308±12 MPa, an elongation of 8.2±0.1%, and an impact toughness of 40.5±0.2 J / cm. 2 Its hardness is 437±1HV.

[0042] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0043] In the examples and comparative examples, Cr-Ni-Mo-V-Nb high-strength steel was smelted in a 25kg vacuum induction melting furnace to obtain ingots. The composition of the Cr-Ni-Mo-V-Nb high-strength steel, by mass percentage, includes: C 0.27%, Si 0.28%, Mn 0.45%, P 0.0028%, S 0.0025%, Cr 1.1%, Ni 1.4%, Mo 0.35%, V 0.15%, Nb 0.05%, with the balance being Fe and unavoidable impurities.

[0044] Example 1

[0045] This embodiment provides a preparation process for Cr-Ni-Mo-V-Nb high-strength steel, including the following steps:

[0046] S1 and Cr-Ni-Mo-V-Nb high-strength steel are hot-rolled at 1150℃ to obtain plates; the thickness of the steel plate after hot rolling is 10mm, and the final rolling temperature is 900℃.

[0047] S2. After heating the board to 860℃, hold it at that temperature for 2 hours, then cool it down to 760℃ in the furnace, continue to hold it at that temperature for 2 hours, and then cool it down to 400℃ in the air to room temperature.

[0048] S3. Heat the plate to 880℃, hold for 0.5 hours, and then water quench to room temperature;

[0049] S4. The sheet material is tempered at 200℃ for 4 hours, and then air-cooled to room temperature.

[0050] Comparative Example 1

[0051] The difference from Example 1 is that tempering is performed at 350°C in step S4.

[0052] Comparative Example 2

[0053] The difference from Example 1 is that tempering is performed at 500°C in step S4.

[0054] Comparative Example 3

[0055] The difference from Example 1 is that tempering is performed at 150°C in step S4.

[0056] Comparative Example 4

[0057] The difference from Example 1 is that tempering is performed at 250°C in step S4.

[0058] Comparative Example 5

[0059] The difference from Example 1 is that tempering is performed at 300°C in step S4.

[0060] Comparative Example 6

[0061] The difference from Example 1 is that tempering is performed at 400°C in step S4.

[0062] Comparative Example 7

[0063] The difference from Example 1 is that tempering is performed at 450°C in step S4.

[0064] Samples were taken from the treated plates of Example 1 and Comparative Examples 1-7 for analysis. All samples used for microstructure observation and mechanical property testing were cut along the rolling direction, such as... Figure 1 As shown. After grinding with sandpaper and polishing with diamond polishing paste, the sample was etched with a 4% nitric acid alcohol solution. The microstructure and impact fracture morphology of the sample were characterized using a scanning electron microscope (SEM, Hitachi-SU5000) at an accelerating voltage of 20 kV. The fine structure of martensite and the distribution and type of carbides were observed using a transmission electron microscope (TEM, Tecnai G2 F30 S-Twin) at 200 kV. First, the TEM sample was sliced ​​to a thickness of 0.5 mm and ground to a thickness of 50 μm with diamond sandpaper. Then, the slice was punched into a disc with a diameter of 3 mm. Finally, it was further thinned in a solution of 7% perchloric acid and 93% ethanol using a dual-jet polisher at -20℃ to -30℃ and 20V. Tensile tests were conducted using an electronic tensile testing machine (Instron 3382). Dog-bone shaped tensile samples with a gauge length of 25 mm, a width of 6 mm, and a thickness of 2 mm were prepared and tested at a constant rate of 2 mm / min at room temperature. Impact toughness at room temperature was determined using a pendulum impact testing machine (JB-300B). The Charpy V-notch specimen had dimensions of 55 × 10 × 10 mm. 3 Hardness testing was performed on a Vickers hardness tester (HV-5) with a load of 500 gf for 10 seconds. At least ten indentations were tested on each specimen, and the average value was taken as the hardness value. To ensure repeatability, tensile and impact toughness tests were performed on three equivalent specimens, and the results were averaged.

[0065] The engineering stress-strain curves of the plates treated in Example 1 and Comparative Examples 1-2 at tempering temperatures of 200℃, 350℃, and 500℃ are shown below. Figure 2 As shown, the ultimate tensile strength and yield strength obtained after tempering at 200℃ are the highest, at 1513 MPa and 1308 MPa, respectively, with an elongation of 8.2%.

[0066] Figure 3The changes in ultimate tensile strength, yield strength, and elongation of the plates treated in Examples 1 and Comparative Examples 1-7 with different tempering temperatures are shown. When the tempering temperature increases from 150°C to 250°C, the yield strength and ultimate tensile strength first increase and then decrease, reaching a peak after tempering at 200°C. It should be noted that when the tempering temperature increases to 350°C, both ultimate tensile strength and elongation decrease sharply, with the elongation dropping to a minimum of 6.4%. As the tempering temperature further increases to 500°C, the yield strength and tensile strength decrease to their lowest values, but the yield strength remains above 1000 MPa.

[0067] Figure 4 The changes in impact toughness of the plates treated in Example 1 and Comparative Examples 1-7 with tempering temperature are shown. After tempering at 200°C, the impact toughness reaches 40.5 J / cm². 2 When the tempering temperature is increased to 250℃, the impact toughness drops to a minimum of 35.5 J / cm. 2 This indicates that tempering embrittlement has occurred. As the tempering temperature further increases, the impact toughness continues to increase, reaching a maximum value of 65 J / cm. 2 .

[0068] Figure 5 The hardness of the plates treated in Example 1 and Comparative Examples 1-7 varies with tempering temperature. The hardness is highest after tempering at 200°C, at 437 HV, which is 50 HV higher than that after tempering at 150°C. When the tempering temperature is increased to 350°C, the hardness decreases rapidly, which is consistent with the strength trend. With further increases in tempering temperature, the hardness changes slightly.

[0069] In summary, Cr-Ni-Mo-V-Nb steel exhibits an excellent combination of strength and toughness after tempering at 200℃. Its tensile strength reaches 1513 MPa, elongation is 8.2%, and Charpy V-notch impact energy is 40.5 J. Furthermore, temper brittleness was observed after tempering at 250℃. Although high-temperature tempering at 500℃ produced good impact energy (65 J), the strength decreased significantly (1098 MPa). These changes in mechanical properties mainly depend on the evolution of the microstructure at different tempering temperatures.

[0070] Figure 6 The images show SEM images of the plates treated in Example 1 and Comparative Examples 1-7 after being treated at different tempering temperatures. Figure 6 The tempering temperature of (a) is 150℃ (Comparative Example 3). Figure 6 The tempering temperature of (b) is 200°C (Example 1). Figure 6 The tempering temperature of (c) is 250℃ (Comparative Example 4). Figure 6 The tempering temperature of (d) is 300℃ (Comparative Example 5). Figure 6The tempering temperature of (e) is 350℃ (Comparative Example 1). Figure 6 The tempering temperature of (f) is 400℃ (Comparative Example 6). Figure 6 The tempering temperature of (g) is 450℃ (Comparative Example 7). Figure 6 The tempering temperature of (h) is 500℃ (Comparative Example 2). From Figure 6 It can be seen that the matrix is ​​composed entirely of lath martensite, and its composition changes with increasing tempering temperature. From... Figure 6 As can be seen in (a) and (b), the original austenite grains are divided into several martensite groups, each martensite group consisting of interleaved martensite bundles. Figure 6 (The yellow dashed lines in (a) and (b)). When tempered below 200°C, as martensite decomposes, supersaturated carbon precipitates as fine carbides within the laths, as indicated by the yellow arrows. When the tempering temperature exceeds 200°C, in addition to the carbides precipitated at the lath boundaries, more thin-shell carbides precipitate within the laths, as shown by the yellow arrows. Figure 6 As shown in (c) and (d). From Figure 6 As can be seen from (e), (f), and (g), the martensitic laths are significantly coarsened after tempering above 350℃. The original austenite grain boundaries and lath boundaries formed during quenching become preferential nucleation sites for carbides, such as... Figure 6 The yellow arrows in (e), (f), and (g) indicate that the lath boundaries become blurred at 500°C. Figure 6 (The yellow arrow in (h) indicates that) carbides tend to spherize (e.g.) Figure 6 (See illustration in middle (h)).

[0071] The detailed features of martensite were further characterized using transmission electron microscopy, such as... Figure 7 As shown. Figure 7 (a) shows lath martensite after tempering at 200°C (Example 1). Figure 7 Image (b) is a magnified view of the area within the red dashed box in image (a). Figure 7 Image (c) is a dark-field image of the retained austenite. Figure 7 In the middle (d), twinned martensite is present; Figure 7 (e) shows the original austenite grain boundaries after tempering at 350℃ (Comparative Example 1). Figure 7 Image (f) is a dark-field image of the retained austenite. Figure 7 In the middle (g), there is twinned martensite. Figure 7 (h) is a magnified view of the part within the red dashed box in (g);

[0072] Figure 7 In section (i), lath martensite is tempered at 500℃ (Comparative Example 2). Figure 7 Image (j) is a bright-field image of twinned martensite. Figure 7 (k) is the dark-field image of twinned martensite. Figure 7 In the middle (l), the average width of the martensitic lath is represented. Figure 7 The illustration on the left is a selected area electron diffraction pattern. (Example:) Figure 7 As shown by the yellow dashed lines in (a), (e) and (i), lath martensite and proto-austenite grain boundaries with high dislocation density were observed after tempering at 200℃, 350℃ and 500℃. Figure 7 Images (b) and (c) show the bright-field image of martensite and the dark-field image of retained austenite, respectively. Comparison of the bright and dark-field images reveals a small amount of thin-film retained austenite between the laths, exhibiting a certain degree of stability at low tempering temperatures. Figure 7 As can be seen from (d), (h), and (j), twinned martensite appeared on the matrix after tempering at 200℃, 350℃, and 500℃. With increasing tempering temperature, the twinned martensite gradually decomposed, especially at 500℃, transforming into a large amount of carbides, such as... Figure 7 The white arrows in the dark field image (k) indicate this. The average width of the martensite laths tempered at different temperatures was statistically analyzed using Image Pro Plus software, as shown below. Figure 7 As shown in (1).

[0073] The morphology, size, and distribution of carbides were analyzed using transmission electron microscopy, such as... Figure 8 As shown. Figure 8 (a1) and (a3) ​​are two types of carbides precipitated inside the martensitic laths after tempering at 200℃. Selected area electron diffraction (SED) analysis identified the needle-like carbides as ε-carbides, approximately 60 nm long and 7 nm wide, exhibiting

[100] interaction with the martensitic matrix. α ∥

[1120] ε Orientation relationship. It is noteworthy that ε-carbides nucleate along the twin boundaries between laths, forming barriers to dislocation slip within the laths, and ε-carbides also nucleate on dislocations (e.g., Figure 8 (As shown by the red arrow in (a1)). Figure 8 Image (a2) is a high-resolution transmission electron microscopy (HEM) image of the ε-carbide, which clearly shows its structure and atomic arrangement. Furthermore, combined with selected area electron diffraction (SEED) and energy dispersive spectroscopy (EDS) analysis, the spherical carbide was identified as cubic MC, mainly containing V and small amounts of Nb, Cr, Mo, and Ni. When the tempering temperature is increased to 350℃, rod-shaped Fe3C precipitates within the martensitic laths. Figure 8 In the middle (b1), its stability is much higher than that of ε-carbides. Besides Fe3C, Cr-rich spherical M was also detected. 23 The presence of C6. When the tempering temperature is increased to 500℃, spheroidized Fe3C nucleates at the original austenite grain boundaries, and Fe3C exists in the matrix

[100] . α ∥

[110] Fe3C

[111] α ∥

[001] Fe3C Orientation relationship. Furthermore, two types of MC carbides remained stable over a wide tempering temperature range: the larger spherical precipitates were V-rich MC(I) carbides, and the smaller precipitates were Nb-rich MC(II) carbides. Twenty transmission images were selected from each sample, and the average size of the carbides was statistically analyzed. The average diameter of the carbides at different tempering temperatures was 14.6 nm (…). Figure 8 (a4)), 53.2nm ( Figure 8 (b4) and 48.6nm Figure 8 (c4)

[0074] As can be seen from the various embodiments and comparative examples of the present invention, the Cr-Ni-Mo-V-Nb high-strength steel of the present invention is hot-rolled at 1150℃ to obtain a plate. The plate is heated to 860℃ and held for 2 hours, then cooled in the furnace to 760℃ and held for another 2 hours. After being cooled to 400℃, it is air-cooled to room temperature. The plate is then heated to 880℃, held for 0.5 hours, and water-quenched to room temperature. The plate is then tempered at 200℃ for 4 hours, and then air-cooled to room temperature. The microstructure of the Cr-Ni-Mo-V-Nb high-strength steel described in this invention comprises lath martensite, twins, retained austenite, and carbides. The carbides include acicular ε-carbides and spherical MC carbides, exhibiting excellent comprehensive mechanical properties. The yield strength of the Cr-Ni-Mo-V-Nb high-strength steel described in this invention is 1513±14 MPa, the tensile strength is 1308±12 MPa, the elongation is 8.2±0.1%, and the impact toughness is 40.5±0.2 J / cm². 2 The hardness is 438±1HV.

[0075] 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 preparation process for Cr-Ni-Mo-V-Nb high-strength steel, characterized in that, Includes the following steps: S1 and Cr-Ni-Mo-V-Nb high-strength steel are hot-rolled at 1150℃ to obtain plates; the composition of Cr-Ni-Mo-V-Nb high-strength steel, by mass percentage, includes: C 0.27%, Si 0.28%, Mn 0.45%, P 0.0028%, S 0.0025%, Cr 1.1%, Ni 1.4%, Mo 0.35%, V 0.15%, Nb 0.05%, with the balance being Fe and unavoidable impurities; S2. After heating the board to 860℃, hold it at that temperature for 2 hours, then cool it down to 760℃ in the furnace, continue to hold it at that temperature for 2 hours, and then cool it down to 400℃ in the air to room temperature. S3. Heat the plate to 880℃, hold for 0.5 hours, and then water quench to room temperature; S4. The sheet material is tempered at 200℃ for 4 hours, and then air-cooled to room temperature. The microstructure of the Cr-Ni-Mo-V-Nb high-strength steel comprises lath martensite, twins, retained austenite, and carbides. The carbides include acicular ε-carbides and spherical MC-carbides, with an average diameter of 14.6 ± 0.1 nm. The Cr-Ni-Mo-V-Nb high-strength steel exhibits a tensile strength of 1513 ± 14 MPa, a yield strength of 1308 ± 12 MPa, an elongation of 8.2 ± 0.1%, and an impact toughness of 40.5 ± 0.2 J / cm². 2 Its hardness is 437±1HV.

2. The preparation process of Cr-Ni-Mo-V-Nb high-strength steel according to claim 1, characterized in that, In step S1, the thickness of the hot-rolled steel plate is 10mm.

3. The preparation process of Cr-Ni-Mo-V-Nb high-strength steel according to claim 1, characterized in that, In step S1, the final rolling temperature is 900℃.

4. A Cr-Ni-Mo-V-Nb high-strength steel, characterized in that, It is prepared using the preparation process of Cr-Ni-Mo-V-Nb high-strength steel according to any one of claims 1-3.

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