A method for regulating the type and size of carbides in steel

Abstract

The present invention discloses a method for regulating the type and size of carbides in steel, which includes optimizing alloy composition, rolling, and heat treatment steps. By optimizing the alloy composition of 40CrNi2Si2MoVA steel and combining with the heat treatment process, the present invention regulates the type and size of carbides in 40CrNi2Si2MoVA steel, significantly improving the mechanical properties of 40CrNi2Si2MoVA steel and providing useful guidance for the research and development of ultra-high strength steel. Using the method of the present invention, the carbides in the matrix are only M6C, with a size of 50 - 120 nm, a strength of 2000 - 2230 MPa, an elongation of 7.8 - 8.0%, and an original austenite grain size of 7.2 - 8.5 μm.

Description

Technical Field

[0001] The invention relates to the field of materials, and in particular to a method for regulating the type and size of carbides in steel. Background Art

[0002] Ultra-high strength steel is widely used due to its excellent strength and toughness. Due to the trade-off mechanism between strength and plasticity of steel materials, as the strength increases, a certain amount of plasticity will inevitably be lost. In order to improve this problem, domestic and foreign scholars have mainly conducted research from the following aspects: 1. By controlling the purity of raw materials, the purity and uniformity of molten steel can be improved as much as possible, which helps to improve the mechanical properties of steel materials; 2. Through the optimization design of alloy composition, the steel obtains finely dispersed nanocarbides or through solid solution strengthening, a new type of steel with both strength and toughness is developed; 3. Fine grain strengthening technology is the only method that can improve both strength and toughness. Therefore, on the basis of taking into account low cost, a new type of steel with ultra-high strength and toughness is developed through grain refinement technology.

[0003] However, the good strength-toughness matching of steel materials is the result of the combined effect of various strengthening mechanisms. A single strengthening mechanism cannot meet its performance requirements. For example, even if martensitic steel obtains fine original austenite grains, the large size of the precipitated phase in the matrix cannot obtain excellent mechanical properties. 40CrN i2S i2MoVA steel has high strength, good toughness, and is a structural material with good corrosion resistance. It is widely used to manufacture important load-bearing components such as the outer tube and piston rod of the main landing gear. It is a low-alloy ultra-high strength steel that is widely used in the aviation industry. However, in the process of strengthening 40CrN i2Si2MoVA steel, researchers found that with the continuous increase in tensile strength, the toughness of the material has decreased significantly, and it is very sensitive to various surface defects such as cracks, inclusions, welds and defects caused by surface processing. When the material is stressed during use and the plastic deformation is very small, the crack will expand to a critical size and suddenly expand, causing catastrophic damage to the material.

[0004] The martensitic matrix of 40CrNi2Si2MoVA steel is prone to form M7C3 and M 23C6, M6C, MC carbides. In addition, these carbides are structurally unstable and are prone to aggregate, grow, and transform with each other. The stability of these carbides increases gradually from front to back. When 40CrNi2Si2MoVA steel undergoes martensitic transformation, a large number of dislocations are generated, resulting in dislocation entanglement around large carbide particles, causing stress concentration. When 40CrNi2Si2MoVA steel is deformed under external stress, the carbides and the entangled dislocations around them hinder the effective dislocation slip. Coupled with the stress concentration caused by the martensitic transformation, the stress generated exceeds the bonding force between the matrix and M6C, which will generate crack sources around M6C, resulting in the fracture of 40CrNi2Si2MoVA steel and the decrease of plasticity.

[0005] How to improve the toughness and reduce the notch sensitivity of 40CrNi2Si2MoVA steel as much as possible while ensuring its strength, and the size and structure control of carbides in 40CrNi2Si2MoVA steel have become the research focus. Therefore, it is necessary to explore a new method for regulating the size and structure of carbides in 40CrNi2Si2MoVA steel. Summary of the invention

[0006] The purpose of the present invention is to provide a method for regulating the type and size of carbides in steel, especially a method for regulating the type and size of carbides in 40CrNi2S i2MoVA steel. The method can obtain the best carbide type and particle size affecting the mechanical properties of 40CrNi2S i2MoVA steel, and can be used in aerospace, automobile manufacturing, engineering machinery and other fields.

[0007] The technical solution of the present invention is:

[0008] A method for regulating the type and size of carbides in steel comprises the following steps:

[0009] 1) Alloy composition

[0010] The mass percentage of each component of the steel is C: 0.50% ~ 0.54%, Si: 2.0% ~ 2.4%, Mn: 0.14% ~ 0.16%, P: 0.008% ~ 0.01%, S: 0.004% ~ 0.008%, Cr: 0.90% ~ 1.10%, Ni: 2.25% ~ 2.43%, Mo: 0.90% ~ 0.96%, V: 0.1% ~ 0.31%, N: 0.003% ~ 0.004%, O: 0.002%, H ≤ 0.0001%, and the balance is Fe;

[0011] 2) taking the above components, vacuum melting + electroslag remelting to obtain steel ingots;

[0012] 3) Rolling

[0013] The obtained steel ingot is rolled twice to obtain an alloy steel plate with a thickness of 4 to 6 mm;

[0014] 4) Heat treatment

[0015] Heat treatment of alloy steel plate: heating to austenitizing temperature of 880℃~1150℃, holding time of 170s~375s, heating rate of 1.6℃ / s~1.8℃ / s, quenching medium of brine, tempering temperature of 200℃~235℃, holding time of 370s~450s;

[0016] The size of M6C carbides in the regulated steel is 7nm~75nm, and the size of original austenite grains is 7μm~10μm.

[0017] A better technical solution is that the mass percentage of each component of the steel is C: 0.54%, Si: 2.3%, Mn: 0.14%, P: 0.008%~0.01%, S: 0.004%~0.008%, Cr: 1.02%, Ni: 2.41%, Mo: 0.95%, V: 0.29%, N: 0.003%~0.004%, O: 0.002%, H≤0.0001%, and the balance is iron (Fe).

[0018] The method of double hot rolling: the temperature of the first hot rolling is 1100℃~1200℃, and the final rolling temperature is 950℃~1000℃; the temperature of the second hot rolling is 900℃~1160℃, and the final rolling temperature is 880℃~900℃.

[0019] After the first hot rolling, the alloy thickness is 10 mm; after the second hot rolling, the alloy thickness is 4-6 mm.

[0020] A better technical solution is that the tempering temperature in step 4) is 200°C and the holding time is 400s.

[0021] A better technical solution is that the austenitizing temperature in step 4) is 1050°C.

[0022] The present invention optimizes the alloy composition of 40CrNi2Si2MoVA steel, increases the content of Mo, Si and C elements, increases the quenching temperature, shortens the holding time after austenitization, and promotes the carbide of the second hot rolling to be fully precipitated after large deformation during the first rolling, so that only M6C carbide exists in the 40CrNi2Si2MoVA steel, the carbide size is 50-120nm, the original austenite grain size is 7.2-8.5μm, the strength can reach 2000-2230MP, the elongation is 7.8-8.0%, and the material obtains a good strength-toughness match. The mechanical properties of 40CrNi2Si2MoVA steel are significantly improved, providing useful guidance for the research and development of ultra-high strength steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The scanning electron microscope (SEM) images of the invention samples at different quenching temperatures, wherein (a) is the sample with a quenching temperature of 880°C, (b) is the sample with a quenching temperature of 980°C, (c) is the sample with a quenching temperature of 1050°C, and (d) is the sample with a quenching temperature of 1150°C;

[0024] Figure 2 This is a scanning electron microscope (SEM) photo of the control sample at 1050°C quenching temperature;

[0025] Figure 3 Transmission electron microscope (TEM) photos of the invention samples at different quenching temperatures, wherein (a) is the sample with a quenching temperature of 880°C, (b) is the sample with a quenching temperature of 980°C, (c) is the sample with a quenching temperature of 1050°C, and (d) is the sample with a quenching temperature of 1150°C;

[0026] Figure 4 The tensile fracture scanning electron microscope (SEM) photos of the invention samples at different quenching temperatures, wherein (a) is the sample with a quenching temperature of 880°C, (b) is the sample with a quenching temperature of 980°C, (c) is the sample with a quenching temperature of 1050°C, and (d) is the sample with a quenching temperature of 1150°C;

[0027] Figure 5 The scanning electron microscope (SEM) photograph of the tensile fracture of the control sample at 1050℃ quenching temperature;

[0028] Figure 6 The transmission electron microscope (TEM) photos of M6C carbides of the invention samples at different quenching temperatures, wherein (a) is the sample quenched at 880°C, (b) is the sample quenched at 980°C, (c) is the sample quenched at 1050°C, and (d) is the statistical result of carbide particle size at different quenching temperatures in Example 1;

[0029] Figure 7 High-resolution transmission electron microscopy (TEM) images of M6C carbides of the invented samples at different quenching temperatures;

[0030] Figure 8 Energy spectrum (EDS) test results of matrix and M6C carbide of the invention sample at different quenching temperatures, wherein (a) is the EDS analysis result of M6C carbide of the sample quenched at 880°C, (b) is the EDS analysis result of M6C carbide of the sample quenched at 980°C, (c) is the EDS analysis result of M6C carbide of the sample quenched at 1050°C, and (d) is the EDS analysis result of the matrix at 1050°C;

[0031] Fig. 9 Energy dispersive spectrometer (EDS) test results of matrix and M6C carbide of control sample at 980℃ quenching temperature. DETAILED DESCRIPTION

[0032] Table 1 Alloy composition of the embodiment and the comparative example (mass percentage)

[0033] Example C Si Mn Cr Ni Mo V S p N O H Fe 1 0.53 2.2 0.15 1.00 2.40 0.95 0.30 0.008 0.010 0.003 0.002 0.0001 margin 2 0.50 2.4 0.14 0.90 2.30 0.96 0.28 0.005 0.009 0.003 0.002 0.0001 margin 3 0.51 2.3 0.16 0.90 2.28 0.92 0.10 0.006 0.008 0.004 0.002 0.0001 margin 4 0.52 2.0 0.16 1.10 2.37 0.91 0.17 0.004 0.008 0.003 0.002 0.0001 margin 5 0.54 2.1 0.14 0.95 2.43 0.90 0.22 0.007 0.008 0.004 0.002 0.0001 margin Comparison example 0.41 1.65 0.69 0.85 1.86 0.40 0.08 0.001 0.006 0.003 0.002 0.0001 margin

[0034] The control example was produced using conventional 40CrN i2S i2MoV steel alloy (Wang Dahong, Wang Zheng, Zhang Jinzhuo, Yang Junling, Liu Jifei, Sun Xiancheng. Research on vacuum heat treatment process of 40CrN i2S i2MoV A steel [J]. Metal Processing (Hot Working), 2020, (05): 43-45.).

[0035] Take the above components and control the type and size of carbides in steel according to the following method:

[0036] A 200kg medium frequency vacuum smelting furnace (ZGJW0.05-100-2.5) was used for smelting to obtain a 50kg steel ingot (230×150×155mm).

[0037] Rolling steps: The steel ingot is rolled twice: the first rolling temperature is 1100-1200°C, and the final rolling temperature is 990-1000°C; the second rolling temperature is 1080-1100°C, and the final rolling temperature is 880-890°C; the final product thickness after finish rolling is 4-6mm.

[0038] The pass parameters of the first rolling and the second rolling are shown in Table 2 and Table 3 respectively.

[0039] Table 2 First rolling

[0040] path Set roller gap / mm Reduction rate / % Rolling force / KN 1 145.00 6.45 588 2 135.00 6.90 515 3 125.00 7.41 500 4 115.00 8.00 537 5 100.00 13.04 698 6 90.00 10.00 562 7 80.00 11.11 601 8 70.00 12.50 649 9 60.00 14.29 801 10 50.00 16.67 974 11 40.00 20.00 1108 12 30.00 25.00 1416 13 20.00 33.33 2024

[0041] Table 3 Second rolling

[0042] path Set roller gap / mm Reduction rate / % Rolling force / KN 1 10.00 50.00 2385 2 7.00 30.00 3308 3 4～6.00 14.29～20.00 3145

[0043] Heat treatment steps: the hot-rolled plate is heated to an austenitizing temperature of 880°C to 1150°C at 1.7 to 1.75°C / s, kept at this temperature for 250 to 300 seconds, quenched in brine, and then heated to 200 to 220°C at 1.7 to 1.75°C / s, kept at this temperature for 400 to 410 seconds, and then air-cooled.

[0044] Table 4 Process parameters of each embodiment and control example

[0045]

[0046] (1) Material mechanical properties test:

[0047] Tensile properties test:

[0048] Take the ingot of Example 1, and adjust the type and size of carbides in the steel according to the rolling, quenching and tempering steps. Among them, the quenching step adopts 880℃, 980℃, 1050℃ and 1150℃ for quenching test respectively, and the control method of other embodiments is the same as that of the above-mentioned Example 1, and 4 groups of test samples are prepared, each group has 4; take the steel plate after quenching the above-mentioned test sample, and test it. According to the GB / T228.1-2010 standard, sample along the rolling direction of the quenched plate, take four quenched plate samples in each group, and test according to the provisions of the GB / T228.1-2010 standard to obtain the tensile properties with the rolling direction as the longitudinal direction; Table 5 shows the mechanical properties of the samples of each embodiment and the comparative example.

[0049] Table 5 Mechanical properties of the samples after quenching and tempering in each embodiment and the control group

[0050]

[0051]

[0052]

[0053] The tensile strength of the samples obtained by this method is 2227MPa at the highest and 1585MPa at the lowest; the tensile strength of the control group is 1835MPa at the highest. At the same quenching temperature, the mechanical properties of the samples are different due to different alloy compositions. The mechanical properties of 40CrN i2S i2MoVA steel can be greatly improved by using the alloy composition described in this method and following the heat treatment process described in this invention; the difference in tensile properties is the result of the difference in alloy composition; and there is no method for optimizing and controlling the size and shape of M6C carbides through alloy composition and heat treatment process in traditional 40CrN i2S i2MoVA steel. The method described in this invention optimizes the alloy composition and combines it with a suitable heat treatment process to achieve the control of the size and shape of M6C carbides, thereby obtaining good mechanical properties.

[0054] (2) Microstructure analysis of materials:

[0055] The quenched and tempered samples of Example 1 and the control example were taken, and the matrix structure and tensile fracture morphology of the materials were observed by scanning electron microscopy (SEM). Energy dispersive spectrometry (EDS) was used to test the composition of carbide particles in the samples, and transmission electron microscopy (TEM) was used to analyze the type and size of carbides in the samples.

[0056] from Figure 1 It can be seen that at a quenching temperature of 880°C, the carbides distributed on the matrix of the sample of Example 1 are larger in size and larger in number. At this time, the original austenite grains are smaller, with a size of 5.5-6.5 μm. As the quenching temperature increases, the carbide particle size decreases, the number decreases, and the original austenite grains grow. When the quenching temperature increases to 1050°C, the carbide particles in the matrix structure of the sample of Example 1 become finer and the number decreases. The original austenite grain size is 7.2-8.5 μm. When the quenching temperature increases to 1150°C, the carbides in the matrix structure of the sample of Example 1 are completely dissolved, and the original austenite grains grow, with a size of 17.6, where (a) is the sample with a quenching temperature of 880°C, (b) is the sample with a quenching temperature of 980°C, (c) is the sample with a quenching temperature of 1050°C, and (d) is the sample with a quenching temperature of 1150°C.

[0057] The carbides in the matrix of the control sample were completely dissolved at 1050℃ quenching temperature, and the original austenite grain size was relatively large, 12 to 14μm (see Figure 2 ).

[0058] Depend on Figure 3 It can be seen that at a quenching temperature of 880°C, a large number of twins and dislocation cells are distributed around the coarse carbides on the matrix of the sample in Example 1. As the quenching temperature increases, the carbide particle size in Example 1 decreases, and the number of twins and dislocation cells around the carbides decreases. When the quenching temperature increases to 1050°C, the carbides are dispersed, and no twins and dislocation cells are observed. When the quenching temperature increases to 1150°C, the carbides are completely dissolved, the martensite laths are coarse, and similarly no twins and dislocation cells are observed.

[0059] Depend on Figure 4 It can be seen that at the quenching temperature of 880°C, a large number of carbides agglomerated in the pits of the tensile fracture of the sample in Example 1 and were exposed on the fracture. At the quenching temperature of 980°C, carbides agglomerated only at individual positions on the tensile fracture of the sample, and the fracture became smooth. At the quenching temperature of 1050°C, no carbide agglomeration was found on the tensile fracture of the sample, the fracture dimples were small, and secondary dimples existed in the larger dimples. The average diameter of the primary dimples was 0.5-0.9 μm. At the quenching temperature of 1150°C, the tensile fracture dimples of the sample became regular, the dimple diameter became larger, and the depth became shallower.

[0060] The dimples on the tensile fracture of the control sample at 1050℃ quenching temperature are uneven in size and shallow in depth (see Figure 5 ).

[0061] Depend on Figure 6 It can be seen that at the quenching temperature of 880°C, the M6C carbide particles of the sample of Example 1 are larger, more numerous, and have a size of 400-600nm. At the quenching temperature of 980°C, the carbide particles of the sample M6C are smaller, the number is reduced, and the size is 130-145nm. At the quenching temperature of 1050°C, the size and number of the M6C carbide particles of the sample of Example 1 continue to decrease, and the size is 10-25nm.

[0062] Figure 7 TEM analysis of matrix carbides of the sample of Example 1 at a quenching temperature of 1050°C. It can be seen that at a quenching temperature of 1050°C, the carbide particles in the sample of Example 1 are M6C. Figure 6 The analysis results show that the carbide in Example 1 is only M6C.

[0063] Depend on Figure 8 It can be seen that at the quenching temperature of 880°C, the mass percentages of Si, Cr, Mo, Fe and Ni in the carbide particles of the sample M6C of Example 1 are 0.64%, 5.09%, 21.92%, 39.13% and 33.22%, respectively; at the quenching temperature of 980°C, the mass percentages of Si, Cr, Mo, Fe and Ni in the carbide particles of the sample M6C are 4.39%, 1.38%, 19.94%, 69.53% and 3.69%, respectively; at the quenching temperature of 1050°C, the mass percentages of Si, Cr, Mo, Fe and Ni in the carbide particles of the sample M6C are 5.03%, 1.70%, 23.24%, 64.37% and 3.88%, respectively; at the quenching temperature of 1050°C, the mass percentages of Si, Cr, Mo, Fe and Ni in the sample matrix are The mass percentages of element i are 2.11%, 0.78%, 0.97%, 93.73% and 2.15% respectively.

[0064] Fig. 9 Figure 2 is the EDS analysis results of the control sample at 980℃ quenching temperature and matrix. It can be seen that the mass percentages of Si, Cr, Mo, Fe and Ni elements in the M6C carbide particles of the control sample at 980℃ quenching temperature are 27.05%, 10.26%, 1.30%, 11.04% and 47.26%, respectively; the mass percentages of Si, Cr, Mo, Fe and Ni elements in the sample matrix at 980℃ quenching temperature are 2.28%, 2.43%, 0.41%, 90.73% and 1.90%, respectively.

Claims

1. A method for regulating the type and size of carbides in steel, characterized in that: The following steps are involved: 1) Alloy composition The mass percentage of each component of steel is: C: 0.50% ~ 0.54%, Si: 2.0% ~ 2.4%, Mn: 0.14% ~ 0.16%, P: 0.008% ~ 0.01%, S: 0.004% ~ 0.008%, Cr: 0.90% ~ 1.10%, Ni: 2.25% ~ 2.43%, Mo: 0.90% ~ 0.96%, V: 0.17% ~ 0.31%, N: 0.003% ~ 0.004%, O: 0.002%, H ≤ 0.0001%, and the balance is Fe; 2) Take the above components, vacuum melt + electroslag remelt to obtain steel ingots; 3) Rolling The obtained steel ingot is rolled twice to obtain an alloy steel plate with a thickness of 4 to 6 mm. The first rolling is performed in multiple passes. After the first rolling, the alloy thickness is 10 mm. The second rolling is performed in fewer passes than the first rolling. After the second rolling, the alloy thickness is 4 to 6 mm. 4) Heat treatment Heat treatment of alloy steel plate: heating to austenitizing temperature of 880℃~1150℃, holding time of 170s~375s, heating rate of 1.6℃ / s~1.8℃ / s, quenching medium of brine, tempering temperature of 200℃~235℃, holding time of 370s~450s; The size of M6C carbides in the regulated steel is 7nm~75nm, and the size of original austenite grains is 7μm~10μm.

2. The method according to claim 1, characterized in that: The mass percentage of each component of the steel is: C: 0.54%, Si: 2.3%, Mn: 0.14%, P: 0.008%~0.01%, S: 0.004%~0.008%, Cr: 1.02%, Ni: 2.41%, Mo: 0.95%, V: 0.29%, N: 0.003%~0.004%, O: 0.002%, H≤0.0001%, and the balance is iron (Fe).

3. The method according to claim 1, characterized in that The method of double rolling: the temperature of the first rolling is 1100℃~1200℃, and the final rolling temperature is 900℃~1000℃; the temperature of the second rolling is 980℃~1160℃, and the final rolling temperature is 880℃~900℃.

4. The method according to claim 1, characterized in that: The first rolling process has 13 passes, and the second rolling process has 3 passes.

5. The method according to claim 1, characterized in that: The tempering temperature in step 4) is 200°C and the holding time is 400s.

6. The method according to claim 1, characterized in that The austenitizing temperature in step 4) is 1050°C.

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