High-toughness and high-thermal-stability hot work die steel as well as preparation method and application thereof

Abstract

The invention discloses hot work die steel with high toughness and high thermal stability as well as a preparation method and application thereof, and belongs to the technical field of die steel, the hot work die steel comprises the following chemical components in percentage by mass: 0.22%-0.32% of C, 0.10%-0.30% of Si, 0.30%-0.50% of Mn, less than or equal to 0.015% of P, less than or equal to 0.003% of S, 2.80%-3.50% of Mo, 4.70%-5.20% of Cr, 0.30%-0.60% of V, 0.03%-0.05% of N and the balance of Fe and inevitable impurity elements. By means of the method, high-density forming of large-size die steel can be achieved, 3D printing additive manufacturing forming can be achieved, the die steel material which is small in grain size, excellent in toughness and high in heat stability is obtained, and premature initiation and expansion of cracks are avoided. The problem that common hot work die steel is prone to cracking due to thermal fatigue cracks in the service process is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of die steel, and particularly relates to a hot work die steel with high toughness and high thermal stability, a preparation method thereof, and an application thereof. Background Art

[0002] Dies are indispensable basic process equipment in industrial production. They play a key role in various forming processes such as injection molding, blow molding, and die casting, helping to achieve mass production and standardization of products. Their structural design ensures that products meet strict industrial standards. Dies are in a harsh service environment. They are not only often threatened by strong impact loads to the structural integrity, but also due to repeated heating and cooling, the internal temperature field changes violently, and thermal expansion and contraction cause complex stresses, leading to the initiation and propagation of cracks, reducing the die life, and even causing sudden failure, resulting in economic losses and production stagnation.

[0003] The material properties are crucial for the thermal crack resistance of hot work die steel. High thermal stability enables the die to maintain stable structural and mechanical properties at high temperatures, delaying the appearance of thermal fatigue cracks; good thermal conductivity can avoid local overheating and reduce thermal stress; a low coefficient of thermal expansion reduces thermal stress at the root; high toughness can prevent cracks from propagating into the die interior and extend the service life.

[0004] For example, H13 steel, as the most widely used martensitic hot work die steel, is widely used in key fields such as hot forging dies, hot extrusion dies, and die casting dies due to its comprehensive performance advantages and can meet the requirements of different working conditions. However, H13 steel has the defects of poor tempering softening resistance and thermal conductivity. Insufficient tempering softening resistance causes the hardness and strength of the die to decrease during high-temperature service, affecting the forming accuracy and dimensional stability; poor thermal conductivity makes it difficult to balance the temperature field during rapid heating and cooling, intensifies thermal stress, and is prone to generating thermal fatigue cracks. Facing complex and harsh working conditions, H13 steel dies are difficult to serve stably for a long time, which limits its expansion in the high-end industrial manufacturing field. Summary of the Invention

[0005] The purpose of the present invention is to provide a hot work die steel with high toughness and high thermal stability, a preparation method thereof, and an application thereof, so as to solve the problems of low thermal stability and low toughness of hot work die steel.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] The first aspect of the present application provides a hot work die steel with high toughness and high thermal stability. The chemical composition of the hot work die steel with high toughness and high thermal stability in mass percentage is as follows: C: 0.22% - 0.32%, Si: 0.10% - 0.30%, Mn: 0.30% - 0.50%, P ≤ 0.015%, S ≤ 0.003%, Mo: 2.80% - 3.50%, Cr: 4.70% - 5.20%, V: 0.30% - 0.60%, N: 0.03% - 0.05%, and the balance is Fe and inevitable impurity elements.

[0008] In some possible implementation manners, among them: the sum of the mass percentages of C and N is between 0.28% and 0.35%; N makes up for the strength loss caused by the reduction of C content, as well as the influence on strength and impact toughness, and further studies and analyzes the influence of titanium nitride inclusions on toughness, as well as the solubility of nitrogen and the precipitation law of supersaturated nitrogen;

[0009] Among them: the ratio of the mass percentage of V to (C + N) is between 1.3 and 1.5. It ensures a certain secondary hardening effect, and at the same time avoids excessive precipitation of large-sized carbides, resulting in reduced toughness.

[0010] Among them: the mass percentage of P is between 0% and 0.010%. At this time, the toughness of the hot work die steel will be more excellent.

[0011] The second aspect of the present application provides a preparation method of a hot work die steel with high toughness and high thermal stability, including the following steps:

[0012] The first step: preparing hot work die steel powder, canned cladding, and hot isostatic pressing forming;

[0013] The second step: refinement treatment and spheroidizing annealing to obtain the hot work die steel;

[0014] The refinement treatment is: heating to 1020 - 1050 °C and holding for 3 - 5 h, with a heating rate ≤ 100 °C / h, and then alternately cooling with water and air until the temperature of the hot work die steel is between 200 - 300 °C;

[0015] The spheroidizing annealing process is: heating to 880 ± 5 °C and holding for 6 - 10 h for 5 - 7 h, then cooling at a rate ≤ 20 °C / h to 730 ± 5 °C, holding for 15 - 20 h, and then cooling at a rate ≤ 20 °C / h to 500 °C and taking it out of the furnace for air cooling.

[0016] Within the range of the above conditions, it is possible to increase the supercooling degree during the transformation of austenite to pearlite, reduce the nucleation work, significantly increase the nucleation rate, and reduce the grain size

[0017] In some possible embodiments, the hot work die steel has a hardness ≥ 45 HRC, a yield strength ≥ 1350 MPa, a tensile strength ≥ 1600 MPa, an elongation after fracture ≥ 12%, and a notch-free impact toughness ≥ 550 J / cm 2 , and after holding at 550 °C for 100 h, the hardness ≥ 43 HRC.

[0018] In some possible embodiments, preparing the hot work die steel powder includes the following steps:

[0019] Step S1: Melting a low-phosphorus raw material, with the composition requirements: C ≥ 1.0%, P ≤ 0.015%, Mo: 2.8% - 3.5%, Cr ≤ 0.30%, and the balance being Fe and unavoidable impurities;

[0020] Step S2: Dephosphorization: Transport the molten steel melted in Step S1 to an AOD furnace, add high-alkalinity slag materials, then raise the temperature for 5 - 10 min to reach 1550 - 1570 °C, blow for 20 min for dephosphorization, after completion, remove the dephosphorization slag, take a sample for composition analysis. If the phosphorus content is greater than 0.010%, then re-add high-alkalinity slag materials for blowing. After qualified dephosphorization, P ≤ 0.010%, and add high-chromium ferrochrome to make the Cr content reach 4.7% - 5.20%;

[0021] Step S3: Decarburization: Add slag materials and then raise the temperature to ≥ 1650 °C, blow for 10 min for decarburization, after completion, remove the oxidized slag, take a sample for composition analysis. If the carbon content is greater than 0.15%, then re-add slag materials for blowing until the carbon content ≤ 0.15%;

[0022] Step S4: Adjusting the composition: Add slag materials and ferrosilicon, blow for 5 - 10 min for reduction, after completion, remove the reduction slag, add alloys and slag materials, with the requirements: C ≤ 0.15%, Si ≤ 0.10%, P ≤ 0.010%, S ≤ 0.010%, Cr: 4.80% - 5.20%, Mo: 2.80% - 3.50%;

[0023] Step S5: Deoxidation and desulfurization: Transport the molten steel from Step S4 to an LF refining furnace, then connect argon, adjust the argon pressure to 0.2 - 0.4 MPa, raise the temperature for 10 - 15 min to ≥ 1580 °C, add carbon powder and silicon powder for deoxidation, and add lime and fluorite to adjust the furnace slag components. After completing desulfurization and raising the temperature to ≥ 1650 °C, remove the furnace slag, transport the molten steel to a VD furnace for vacuum degassing, and after completion, transport the ladle to the atomization powder-making process;

[0024] Step S6: Atomization powder-making: The atomization powder-making process heats the molten steel to 1610 - 1620 °C and uses high-pressure nitrogen for gas atomization powder-making. During the powder-making process, the nitrogen pressure is maintained above 19 bar.

[0025] In some possible embodiments, the canned envelope includes loading die steel alloy powder into the envelope, connecting a hose to the envelope and then performing a vacuum treatment, and performing welding and sealing after maintaining at a vacuum degree of 1.0×10 -3 Pa for more than 4 h.

[0026] In some possible embodiments, the hot isostatic pressing includes performing a three-stage heat preservation process;

[0027] In the first stage, the pressure is first increased to a furnace pressure of 20 - 35 MPa through a hot isostatic pressing pressure system, and then the temperature is increased at a heating rate of 10 °C / min to 700 - 950 °C. While heating, the pressure is continuously increased to 120 - 145 MPa and maintained for 2 - 4 h;

[0028] In the second stage, the temperature and pressure are continuously increased simultaneously to 1120 - 1165 °C and 140 - 170 MPa, and heat is preserved for 0.5 - 4 h;

[0029] In the third stage, the temperature is continuously increased to 1160 - 1190 °C, the pressure is increased to 170 - 190 MPa, and heat is preserved for 0.5 - 4 h.

[0030] The third aspect of the present application provides an application of a high-toughness and high-thermal-stability hot work die steel, which is applied to 3D printing additive manufacturing.

[0031] Step 1: Screen the above-mentioned hot work die steel powder through a powder sieve to obtain a particle size suitable for 3D printing additive manufacturing;

[0032] Step 2: Add the screened powder into a 3D printing device, level it with a scraper, and fill it with an inert protective gas to discharge air or perform vacuum pumping;

[0033] Step 3: Use the 3D printing device to read the printing data and start the 3D printing device for printing;

[0034] Step 4: Perform stress relief annealing heat treatment on the printed part. The heat treatment temperature is between 650 °C and 850 °C, and it is cooled in the furnace to 500 °C - 600 °C and then air-cooled.

[0035] In some possible embodiments, the flowability of the hot work die steel powder is < 25 s / 50 g, the oxygen content is < 500 ppm, and the loose packing density is > 3.3 g / cm 3 .

[0036] In some possible embodiments, the hardness of the 3D printing additive is ≥ 45 HRC, the yield strength is ≥ 1350 MPa, the tensile strength is ≥ 1600 MPa, the elongation after fracture is ≥ 12%, the Charpy V-notch impact energy is ≥ 35 J, and the hardness is ≥ 43 HRC after heat preservation at 550 °C for 100 h.

[0037] The beneficial effects of the present invention:

[0038] The present invention provides a hot work die steel with high toughness and high thermal stability. Through theoretical calculations, repeated experiments, and summary and analysis verification, a scientific alloy composition range is obtained, solving the problem that ordinary hot work die steels are prone to thermal fatigue cracks and cracking during service. The mass percentages of the alloy components of the hot work die steel are determined as follows: C: 0.22% - 0.32%, Si: 0.10% - 0.30%, Mn: 0.30% - 0.50%, P ≤ 0.015%, S ≤ 0.003%, Mo: 2.80% - 3.50%, Cr: 4.70% - 5.20%, V: 0.30% - 0.60%, N: 0.03% - 0.05%, and the balance is Fe and inevitable impurity elements;

[0039] For the hot work die steel with the above alloy components, the present invention also provides corresponding preparation process parameters, which can not only achieve high-density forming of die steels with larger sizes (diameter 400 mm × length 1200 mm), but also achieve 3D printing additive manufacturing forming, obtaining die steel materials with fine grains, excellent toughness, and high thermal stability, avoiding premature initiation and propagation of cracks. The preparation process includes corresponding heat treatment processes, achieving high toughness and high thermal stability (maintaining a hardness of 43 HRC at 550 °C for 100 h) of the hot work die steel, effectively improving the thermal fatigue resistance, and being very suitable for application scenarios with high requirements for die toughness and thermal stability, such as hot extrusion and die casting. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention will be further described below with reference to the accompanying drawings.

[0041] Figure 1 It is the solidification path of the die steel powder composition in Example 1 designed by means of the JMatPro thermodynamics calculation software.

[0042] Figure 2 It is the ultrasonic flaw detection test result of the hot work die steel in Example 2.

[0043] Figure 3 It is the metallographic photo (structure GA1) of the hot work die steel in Example 2.

[0044] Figure 4 It is the metallographic photo (segregation SB1) of the hot work die steel in Example 2.

[0045] Figure 5 It is the CCT curve (continuous cooling transformation curve of supercooled austenite) and TTT curve (isothermal transformation curve of supercooled austenite) of the hot work die steel composition designed by means of the JMatPro thermodynamics calculation software.

[0046] Figure 6 It is the ultrasonic flaw detection test result of the hot work die steel in Example 3.

[0047] Figure 7 Metallographic photograph of the hot work die steel in Example 3 (structure GA1).

[0048] Figure 8 Metallographic photograph of the hot work die steel in Example 3 (segregation SA1).

[0049] Figure 9 Fracture morphology of the hot work die steel in Example 3.

[0050] Figure 10 Hardness change curve of the hot work die steel in Example 3 during holding at 550 °C for 100 h.

[0051] Figure 11 Actual CCT curve of the hot work die steel in Example 3 obtained by using the LINSEIS DIL L78Rita QDT thermal expansion deformation / phase transformation tester of LINSEIS, Germany.

[0052] Figure 12 Morphology of the impact specimen of the formed part of the hot work die steel in Example 4.

[0053] Figure 13 Morphology of the tensile specimen of the formed part of the hot work die steel in Example 4.

[0054] Figure 14 Hardness change curve of the H13 die steel in Comparative Example 3 during holding at 550 °C for 100 h.

[0055] Figure 15 Crack photographs of the hot work die steel in Example 3 and the H13 die steel in Comparative Example 3 after the thermal fatigue experiment. Detailed implementation manners

[0056] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0057] Example 1

[0058] This embodiment provides a die steel powder, the composition of which needs to meet the following chemical composition mass percentages: C: 0.22% - 0.32%, Si: 0.10% - 0.30%, Mn: 0.30% - 0.50%, P ≤ 0.015%, S ≤ 0.003%, Mo: 2.80% - 3.50%, Cr: 4.70% - 5.20%, V: 0.30% - 0.60%, N: 0.03% - 0.05%, and the balance is Fe and inevitable impurity elements;

[0059] Among them: the sum of the mass percentages of C and N is between 0.28% and 0.35%;

[0060] Among them: the ratio of the mass percentage of V to (C + N) is between 1.3 and 1.5.

[0061] The mass percentage of phosphorus P is between 0% and 0.010%.

[0062] Figure 1 It is the solidification path of the die steel composition designed by means of JMatPro thermodynamic calculation software. This die steel powder is used as the initial powder in the subsequent embodiments, and the manufacturing method includes the following steps:

[0063] Step S1: Melting low-phosphorus raw materials, with the composition requirements: C ≥ 1.0%, P ≤ 0.015%, Mo: 2.8% - 3.5%, Cr ≤ 0.30%, and the balance is Fe and inevitable impurities;

[0064] Step S2: Dephosphorization: Transport the molten steel melted in step S1 to an AOD furnace (argon-oxygen refining furnace), add high-alkalinity slag materials and then raise the temperature for 5 - 10 minutes until the temperature reaches 1550 - 1570 °C, blow for 20 minutes for dephosphorization, and after completion, remove the dephosphorization slag, sample and analyze the composition. If the phosphorus content is greater than 0.010%, then re-add high-alkalinity slag materials for blowing. After the dephosphorization is qualified (P ≤ 0.010%), add high-chromium ferrochrome to make the Cr content reach the range of 4.7% - 5.20%;

[0065] Step S3: Decarburization: Add slag materials and then raise the temperature to ≥ 1650 °C, blow for 10 minutes for decarburization, and after completion, remove the oxidation slag, sample and analyze the composition. If the carbon content is greater than 0.15%, then re-add slag materials for blowing until the carbon content ≤ 0.15%;

[0066] Step S4: Adjust the composition: Add slag materials and ferrosilicon, blow for 5 - 10 minutes for reduction, and after completion, remove the reduction slag, add alloys and slag materials, adjust the furnace slag to white or light green, sample and analyze the composition, with the requirements: C ≤ 0.15%, Si ≤ 0.10%, P ≤ 0.010%, S ≤ 0.010%, Cr: 4.80% - 5.20%, Mo: 2.80% - 3.50%;

[0067] Step S5, deoxidation and desulfurization: Transfer the molten steel from Step S4 to the LF refining furnace, connect argon gas, adjust the argon gas pressure to 0.2 - 0.4 MPa, heat up for 10 - 15 min until the temperature ≥ 1580 °C, add carbon powder and silicon powder for deoxidation, and add lime and fluorite to adjust the slag components to complete the desulfurization work. Keep the slag white for more than 15 min, take a sample for composition analysis, which meets the requirements of the die steel powder composition in this embodiment. Subsequently, after heating up to a temperature ≥ 1650 °C, remove the slag, transfer the molten steel to the VD furnace for vacuum degassing, and after completion, transfer the ladle to the atomization powder-making process;

[0068] Step S6, atomization powder-making: In the atomization powder-making process, heat the molten steel to 1610 - 1620 °C, install a nozzle with a diameter of 5.5 - 6.2 mm, and use high-pressure nitrogen for gas atomization powder-making. During the powder-making process, the nitrogen pressure is maintained above 19 bar;

[0069] To prevent the metal droplets from rapidly solidifying and blocking the nozzle during the gas atomization process, no aluminum-containing slag materials are added throughout the smelting process. Finally, an alloy powder of high-toughness and high-thermal-stability hot-work die steel (denoted as die steel powder) is obtained, and its chemical composition is detected. The chemical composition (mass percentage) results of the die steel powder prepared in this embodiment are shown in Table 1.

[0070] Table 1

[0071]

[0072] Example 2

[0073] This embodiment provides a high-toughness and high-thermal-stability hot-work die steel, and the manufacturing method includes the following steps:

[0074] Step S1, put the die steel powder in Example 1 into the sleeve. The sleeve material is Q235 carbon steel, and the sleeve size is φ510×1403 mm. After connecting the sleeve to the hose, perform vacuum treatment, and keep it for more than 4 h under a vacuum degree of 1.0×10 -3 Pa and then carry out welding and sealing;

[0075] Step S2, perform hot isostatic pressing on the welded and sealed sleeve to obtain a circular hot-work die steel with a size of φ438×1250 mm. During the hot isostatic pressing process, a three-stage heat preservation process and a staged cooling are carried out; in the first stage, first pressurize through the hot isostatic pressing pressure system to a pressure of 30 MPa in the furnace chamber, then heat up at a heating rate of 10 °C / min to 800 °C, and continue to pressurize to 130 MPa while heating up, and keep it for 2 h. In the second stage, continue to heat up and pressurize simultaneously to 1140 °C and 165 MPa, and keep it for 3 h. In the third stage, continue to heat up to 1180 °C, pressurize to 175 MPa, and keep it for 3 h.

[0076] The hot work die steel prepared in Example 2 was subjected to ultrasonic flaw detection inspection (in accordance with the ultrasonic testing method for steel forgings GB / T6402-2008), and the results are as Figure 2 shown; and its chemical composition, gas content, density, metallographic structure, segregation, heat treatment hardness and impact toughness were detected. The density was measured by the Archimedes drainage method, and the result was 7.818 g / cm 3 . The remaining test results are shown in Table 2, Table 3, Figure 3 , Figure 4 shown. The annealing structure and segregation were evaluated according to the North American Die Casting Association NADCA#207 standard atlas, and the impact toughness was subjected to a transverse impact test in accordance with "ASTM A370 Methods and Definitions for Mechanical Property Testing of Steel Products".

[0077] It can be seen from Figure 2 that the hot work die steel in Example 2 has a typical coarse grain waveform (weed wave). This is due to the slow cooling rate of the hot work die steel after hot isostatic pressing, resulting in coarse grains, which have a greater impact on the absorption and scattering of ultrasonic waves during detection and cause attenuation.

[0078] The chemical composition and gas content (mass percentage) of the hot work die steel in Example 2 are shown in Table 2:

[0079] Table 2

[0080]

[0081] The impact toughness of the hot work die steel in Example 2 is shown in Table 3:

[0082] Table 3

[0083]

[0084] It should be noted that the size of the impact specimen in Table 3 is 7×10×55 mm, without a notch, the heat treatment process is 1030 °C × 30 min, oil cooling, the specimens in Group A are tempered at 595 °C 3 times, 2 h each time; the specimens in Group B are tempered at 590 °C 3 times, 2 h each time.

[0085] It can be seen from Figure 3 , Figure 4 that the annealing structure of the hot work die steel in Example 2 is GA1 and the segregation is SB1.

[0086] Example 3

[0087] This example provides a high-toughness and high-thermal-stability hot work die steel, and the manufacturing method includes the following steps:

[0088] Step S1: Refine the hot work die steel in Example 2. The specific refinement process is as follows: Heat it to 1030°C and hold for 4 hours with a heating rate of 80°C / h, then cool it alternately in water and air until the actual temperature of the hot work die steel reaches 230 - 240°C;

[0089] Step S2: Perform spheroidizing annealing on the refined hot work die steel. The annealing process is as follows: Heat it to 880°C in 7 hours and hold for 10 hours, then cool it to 730°C at an average rate of 15°C / h and hold for 20 hours. Subsequently, cool it to 500°C at an average rate of 15°C / h and then take it out of the furnace and cool it in air.

[0090] Figure 5 The CCT curve (continuous cooling transformation curve of supercooled austenite) and TTT curve (isothermal transformation curve of supercooled austenite) of the die steel composition obtained by designing Example 3 with the aid of JMatPro thermodynamic calculation software.

[0091] Perform ultrasonic flaw detection on the hot work die steel in Example 3 (in accordance with the ultrasonic testing method for steel forgings GB / T6402 - 2008), and the results are as Figure 6 shown; and detect its chemical composition, gas content, density, annealing structure, segregation, heat treatment hardness, and impact toughness. The density is measured by the Archimedes drainage method, and the result is 7.820 g / cm 3 , and the remaining test results are shown in Table 4, Table 5, Figure 7 , Figure 8 shown. The annealing structure and segregation are evaluated according to the North American Die Casting Association NADCA#207 standard atlas, and the impact toughness is subjected to a transverse impact test in accordance with "ASTM A370 Methods and Definitions for Mechanical Property Testing of Steel Products".

[0092] From Figure 2 and Figure 6 it can be seen that the "weed - like wave" of the hot work die steel in Example 2 is a low - amplitude, randomly distributed chaotic echo generated by the scattering of ultrasonic waves inside the material due to coarse grains, presenting as a dense small - amplitude waveform similar to a grassland on the flaw detector display screen. The hot work die steel in Example 3 eliminates this defect through refinement treatment. This is because the cooling rate during the refinement treatment of the steel ingot is relatively fast, resulting in an increase in the supercooling degree during the transformation of austenite to pearlite, a decrease in the nucleation work, a significant increase in the nucleation rate, and a reduction in the grain size.

[0093] The chemical composition and gas content (mass percentage) of the hot work die steel in Example 3 are shown in Table 4:

[0094] Table 4

[0095]

[0096] The impact toughness of the hot work die steel in Example 3 is shown in Table 5:

[0097] Table 5

[0098]

[0099] It should be noted that the size of the impact specimen in Table 5 is 7×10×55 mm, without a notch. The heat treatment process is 1030 °C × 30 min, oil cooling. The specimens in Group C are tempered at 595 °C for 3 times, 2 h each time; the specimens in Group D are tempered at 590 °C for 3 times, 2 h each time.

[0100] Compared with the hot work die steel in Example 2, the impact toughness of the hot work die steel in Example 3 has been significantly improved.

[0101] From Figure 7 and Figure 8 it can be seen that the annealing structure of the hot work die steel in Example 3 is GA1, and the segregation is SA1.

[0102] Compared with the hot work die steel in Example 2, the degree of segregation has been improved. This is because during the heating of the refinement treatment, the elements in the hot work die steel are fully diffused, and the undissolved carbides are partially dissolved during high-temperature austenitization. During the air cooling process, the carbides precipitate in a finer and more dispersed form.

[0103] According to GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", the tensile properties of the hot work die steel in Example 3 were tested, and the results are shown in Table 6.

[0104] Table 6

[0105]

[0106] It should be noted that the heat treatment process of the specimens in Table 6 is 1030 °C × 30 min, oil cooling, and tempered at 590 °C for 3 times, 2 h each time.

[0107] The fracture morphology of the hot work die steel in Example 3 is as Figure 9 shown.

[0108] From Figure 9 the uniformly distributed fine and dense dimples, it can be seen that the material has undergone plastic deformation before fracture. The deep and large dimples are typical high-toughness fracture characteristics, indicating that the die steel of the present invention has high energy absorption and good toughness.

[0109] The hot stability of the hot work die steel in Example 3 was tested. The specific process was as follows: Ten specimens with dimensions of 20×20×15 mm were heated to 1030 °C and held for 30 min, then immediately oil quenched after being taken out of the furnace, cooled to room temperature, and then heated to 520 °C and held for 2 h and tempered three times. The hardness value after tempering was measured. Subsequently, these 10 specimens were heated to 550 °C and held for 100 h. One specimen was taken out every 10 h, air cooled to room temperature, and its hardness value was measured. The hot stability data results of the hot work die steel in Example 3 are shown in Table 7, and the hardness change curve is as Figure 10 shown.

[0110] Table 7

[0111]

[0112] The CCT curve of the hot work die steel in Example 3 was tested using a German LINSEIS DIL L78Rita QDT thermal expansion deformation / phase transformation tester. The results are as Figure 11 shown, and the pearlite transformation nose temperature is approximate to the simulation result of the JMatPro thermodynamic calculation software.

[0113] Example 4

[0114] A manufacturing method for a high-toughness and high-hot-stability hot work die steel forming part provided in this example includes the following steps:

[0115] Step S1: Take the die steel powder in Example 1, and screen out the particle size suitable for 3D printing additive manufacturing through a powder sieve. Check the particle size distribution, fluidity, oxygen content, and loose bulk density of the powder. The results are shown in Table 8.

[0116] Step S2: Add a printing substrate to the 3D printing device and level it. The distance between the leveled substrate and the doctor blade is less than 50 μm;

[0117] Step S3: Select an appropriate amount of the sieved powder and add it to the 3D printing device, level it with a doctor blade, and fill it with an inert protective gas to discharge air (laser printing device) or evacuate (electron beam printing device);

[0118] Step S4: Import the three-dimensional model of the product to be printed into three-dimensional slicing processing software, and select an appropriate layer thickness, select a layer thickness of 50 μm. Two-dimensionalize the three-dimensional data into slices, extract the contour of each layer, select an appropriate filling method, line spacing, etc. to allocate the filling path, match the path with the power and scanning speed, generate a print data format that can be recognized by the 3D printing device, and import this data into the 3D printing device;

[0119] Step S5: Use a 3D printing device to read the printing data, start the 3D printing device for printing. After printing is completed, take out the substrate, and cut the printed part from the substrate using wire cutting or a band saw;

[0120] Step S6: Perform stress-relieving annealing heat treatment on the printed part. The heat treatment temperature is between 750 °C, cool in the furnace to 500 °C and then air-cool.

[0121] The particle size distribution, apparent density, fluidity, and oxygen content of the sieved die steel powder in Example 4 are shown in Table 8:

[0122] Table 8

[0123]

[0124] Detect the chemical composition, gas content, density, hardness, and impact toughness of the high-toughness and high-thermal-stability hot work die steel formed parts in Example 4. The density is measured using the Archimedes drainage method, and the result is 7.814 g / cm 3 , and the impact toughness is subjected to a transverse impact test in accordance with "ASTM A370 Methods and Definitions for Mechanical Property Testing of Steel Products". The chemical composition and gas content (mass percentage) of the hot work die steel formed parts in Example 4 are shown in Table 9.

[0125] Table 9

[0126]

[0127]

[0128] The Charpy V-notch impact energy of the hot work die steel formed parts in Example 4 is shown in Table 10:

[0129] Table 10

[0130]

[0131] The morphology of the impact specimens of the hot work die steel formed parts in Example 4 is as Figure 12 shown.

[0132] In accordance with GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", perform tensile property testing on the die steel formed parts in Example 4. The tensile property results of the die steel formed parts are shown in Table 11.

[0133] Table 11

[0134]

[0135] The morphology of the tensile specimens of the hot work die steel formed parts in Example 4 is as Figure 13 shown.

[0136] The hot stability of the die steel formed parts in Example 4 was tested. The specific process was as follows: 10 specimens with dimensions of 20×20×15 mm were heated to 1030 °C and held for 30 min, then immediately oil quenched after being taken out of the furnace, cooled to room temperature, and then heated to 520 °C and held for 2 h and tempered 3 times. The hardness value after tempering was detected. Subsequently, these 10 specimens were heated to 550 °C and held for 100 h. One specimen was taken out every 10 h, air cooled to room temperature, and its hardness value was tested. The hot stability data of the die steel formed parts and the hardness results are shown in Table 12.

[0137] Table 12

[0138]

[0139]

[0140] It can be seen from the result analysis that Mo and V dissolved in the matrix combine with C to precipitate fine and dispersed carbides, which have good high-temperature stability and are not prone to coarsening during service, and can effectively improve the high-temperature mechanical properties and tempering stability of the steel.

[0141] Comparative Example 1

[0142] The chemical composition mass percentages of the hot work die steel in this comparative example were as follows: C: 0.32%, Si: 0.1% - 0.3%, Mn: 0.30% - 0.50%, P ≤ 0.010%, S ≤ 0.003%, Mo: 2.80% - 3.50%, Cr: 4.70% - 5.20%, V: 0.43%, N: 0.04%, and the balance was Fe and unavoidable impurity elements;

[0143] Among them: the sum of the mass percentages of C and N was 0.36%;

[0144] Among them: the ratio of the mass percentage of V to (C + N) was 1.2.

[0145] The die steel powder of this comparative example was prepared according to the steps in Example 1, and the hot work die steel was obtained according to the steps in Example 3, and its chemical composition, gas content, density, heat treatment hardness and impact toughness were detected. The density was measured by the Archimedes drainage method, and the result was 7.822 g / cm 3 , and the rest of the detection results are shown in Table 13 and Table 14. The impact toughness was tested by a transverse impact test according to "ASTM A370 Methods and Definitions for Mechanical Property Testing of Steel Products".

[0146] The chemical composition and gas content (mass percentage) results of the hot work die steel in Comparative Example 1 are shown in Table 13:

[0147] Table 13

[0148]

[0149] The impact toughness results of the hot work die steel in Comparative Example 1 are shown in Table 14 as follows:

[0150] Table 14

[0151]

[0152] It should be noted that the size of the impact specimen in Table 14 is 7×10×55 mm, without a notch. The heat treatment process is 1030 °C × 30 min, oil cooling, tempering at 595 °C for 3 times, 2 h each time. Compared with the hot work die steel in Example 3, the impact toughness of the die steel in Comparative Example 1 decreased significantly.

[0153] Comparative Example 2

[0154] The chemical composition mass percentages of the hot work die steel in this comparative example are as follows: C: 0.23%, Si: 0.1% - 0.3%, Mn: 0.30% - 0.50%, P ≤ 0.010%, S ≤ 0.003%, Mo: 2.80% - 3.50%, Cr: 4.70% - 5.20%, V: 0.43%, N: 0.04%, and the balance is Fe and inevitable impurity elements;

[0155] Among them: the sum of the mass percentages of C and N is 0.27%;

[0156] Among them: the ratio of the mass percentage of V to (C + N) is 1.59.

[0157] The die steel powder of this comparative example was prepared according to the steps in Example 1, and the hot work die steel was obtained according to the steps in Example 3. Then, its chemical composition, gas content, density, heat treatment hardness, and impact toughness were detected. The density was measured by the Archimedes drainage method, and the result was 7.826 g / cm 3 , and the remaining test results are shown in Table 15 and Table 16. The impact toughness was tested by a transverse impact test in accordance with "ASTM A370 Methods and Definitions for Mechanical Property Tests of Steel Products". The chemical composition and gas content (mass percentage) results of the die steel in Comparative Example 2 are shown in Table 15:

[0158] Table 15

[0159]

[0160] The impact toughness of the die steel in Comparative Example 2 is shown in Table 16:

[0161] Table 16

[0162]

[0163] It should be noted that the size of the impact specimen in Table 16 is 7×10×55 mm, without a notch. The heat treatment process is 1030 °C × 30 min, oil quenching, and tempering at 595 °C for 3 times, 2 h each time.

[0164] Compared with the hot work die steel in Example 3, the impact toughness of the die steel in Comparative Example 2 decreased significantly.

[0165] Through Comparative Example 1, Comparative Example 2, and a large number of thermodynamic calculations and experiments, it is shown that the sum of the mass percentages of C and N is between 0.28% and 0.35%, and the ratio of V to the mass percentage of (C + N) is between 1.3 and 1.5, which can ensure a certain secondary hardening effect and avoid excessive precipitation of large-size carbides. Like carbon atoms, nitrogen atoms occupy interstitial positions, and their atomic radius is slightly smaller than that of carbon and has a higher solubility in austenite. Nitrogen forms nano-scale nitrides VN with vanadium (V) to produce a precipitation strengthening effect, and hinders dislocation movement through the Orowan mechanism. Replacing carbon with nitrogen reduces free carbon, thereby reducing the brittle tendency of grain boundary carbides.

[0166] Comparative Example 3

[0167] In this comparative example, H13 die steel is selected.

[0168] Figure 14 is the hardness change curve of H13 die steel during holding at 550 °C for 100 h.

[0169] Compared with Figure 10 the thermal stability curve of the hot work die steel in Example 3, it can be seen that during the entire holding process at 550 °C, the hardness of the hot work die steel in Example 3 is overall better than that of H13 die steel, which indicates that the high-toughness and high-thermal-stability hot work die steel proposed by the present invention has very good thermal stability. This is because the hot work die steel of the present invention has a higher Mo content. During the heat treatment process, Mo and C combine to precipitate fine and dispersed M2C-type carbides, which have good thermal stability and are not prone to coarsening during service in a high-temperature environment, and can effectively improve the high-temperature mechanical properties of the steel. At the same time, the nano-scale nitride VN formed by nitrogen and vanadium is more stable than the carbide, and its growth rate is lower than that of the carbide, which can also improve the creep strength and high-temperature creep rupture strength of the steel and delay material softening.

[0170] The thermal fatigue resistance of the hot work die steel in Example 3 was tested separately, and compared with the thermal fatigue performance of the comparative material H13 die steel. The self-constrained thermal fatigue test method was used to qualitatively analyze the thermal fatigue resistance of the hot work die steel. The self-constrained thermal fatigue test method locally cyclically heats and cools the specimen, and uses the uneven alternating stress-strain generated on the specimen surface due to rapid cooling and heating to cause thermal fatigue damage to the material. Although this method cannot directly measure the values of stress-strain, it can effectively simulate the actual working conditions of the hot work die steel. The specimens were heated and cooled through an induction heating device and a cooling device respectively. The heating temperature was 600 °C, and the heating time was ≤3 s. After reaching the temperature, it was quickly sprayed and cooled to room temperature with circulating water, and the cooling time was 5 s. After the specimens were continuously heated and cooled 200, 500, 1000, and 2000 times, they were cleaned with 10% HCl and the oxide on the specimen surface was removed. The crack morphology on the specimen surface was observed through a scanning electron microscope, and then wire cutting was used to cut radially in the middle of the fatigue crack where the cracks were dense, and the cross-section crack depth was observed with a scanning electron microscope after mechanical polishing.

[0171] The compositions of the hot work die steel in Example 3 and the comparative material H13 die steel are shown in Table 17:

[0172] Table 17

[0173] C Mo Cr V Mn Si N H13 0.40 1.48 5.05 0.99 0.42 1.00 -- Example 3 0.29 3.18 4.87 0.43 0.36 0.18 0.04

[0174] Figure 15 The crack depths of the hot work die steel in Example 3 and the comparative material H13 die steel after the thermal fatigue experiment are shown. It can be seen that the high-toughness and high-thermal-stability hot work die steel in Example 3 has excellent thermal fatigue resistance, and the extension depth of its thermal fatigue cracks is much smaller than that of the thermal fatigue cracks of the comparative material H13 die steel. It is not difficult to conclude from the above analysis that this is related to the excellent toughness and thermal stability of the high-toughness and high-thermal-stability hot work die steel.

[0175] Comparative Example 4

[0176] In this comparative example, the Swedish ASSAB Dievar die steel was selected.

[0177] The compositions of the Swedish ASSAB Dievar die steel and the forming parts of the hot work die steel in Example 4 are shown in Table 18:

[0178] Table 18

[0179] C Mo Cr V Mn Si N Dievar 0.35 2.30 5.00 0.60 0.50 0.20 -- Example 4 0.28 3.18 4.88 0.43 0.37 0.19 0.04

[0180] After forming according to the steps of Example 4, the Charpy V-notch impact energy was detected in accordance with "Methods and Definitions for Mechanical Property Testing of Steel Products ASTM A370", and the tensile property testing was carried out in accordance with GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature". The tensile properties and impact energy of the hot work die steel formed parts and the comparative material Dievar die steel in Example 4 are shown in Table 19 as follows:

[0181] Table 19

[0182]

[0183] By comparison, the toughness of the high-toughness and high-thermal-stability hot work die steel proposed by the present invention is superior to that of the comparative material Dievar. This is because replacing carbon with nitrogen reduces free carbon while ensuring strength, thereby reducing the brittle tendency of grain boundary carbides and avoiding excessive precipitation of large-sized carbides.

[0184] The role of carbon C in the alloy composition of the hot work die steel of the present invention: C is the main strengthening element in the material, which directly affects the strength, plasticity, and toughness of the steel, and can effectively improve the hardenability of the material and the hardness of martensite after quenching. At the same time, C can combine with solute elements such as Cr, Mo, and V to form carbides, improving the thermal stability and wear resistance of the material. However, too high a C content will form large-sized irregular carbides, increasing the brittleness of the material and reducing the service life. Therefore, the C content of the hot work die steel designed in the present invention is controlled at 0.22% - 0.35%, which can not only avoid cracking during the 3D printing preparation process but also ensure that the die has sufficient strength and toughness.

[0185] Molybdenum Mo can dissolve in the matrix, improving the hardenability of the steel and its comprehensive mechanical properties. During the heat treatment process, Mo can combine with C to precipitate fine and dispersed M2C-type carbides, which have good thermal stability and are not prone to coarsening during service in a high-temperature environment, effectively improving the high-temperature mechanical properties of the steel.

[0186] Manganese Mn and silicon Si are deoxidizers that must be added during the steelmaking process to remove oxygen dissolved in the molten steel. Mn and Si have a solid solution strengthening effect, which is beneficial to improving the strength and hardness of the steel. Mn also has the function of desulfurization, that is, combining with sulfur in the molten steel to form high-melting-point MnS, thereby largely eliminating the harmful effects of sulfur in the steel. Therefore, considering the oxygen content of the original steel powder, the inevitable residues during the steelmaking process, and the strength of the alloy, the lower limit of the Mn content in the present invention is set at 0.3%, and the lower limit of the Si content is set at 0.1%. However, when the contents of Mn and Si are relatively high, inclusions will be formed, damaging the toughness of the steel. Therefore, through experiments, the upper limit of the Mn content in the present invention is set at 0.5%, and the upper limit of the Si content is set at 0.3%.

[0187] Although phosphorus (P) can increase the strength and hardness of steel, it significantly reduces plasticity and impact toughness. Especially at low temperatures, it makes the steel significantly brittle, and this phenomenon is called "cold brittleness". Cold brittleness deteriorates the cold working and weldability of steel. The higher the phosphorus content, the greater the cold brittleness. Therefore, the phosphorus content in steel is strictly controlled. The Japanese company Daido Special Steel reduced the P and S in SKD61 (whose composition is similar to that of hot work die steel H13) from 0.03% to 0.01%, and the impact toughness doubled; Hitachi Metals reduced the P in SKD61 from 0.03% to 0.001%, and the impact toughness increased from 40 J / cm 2 to 130 J / cm 2 . Preferably, the upper limit of the P content in the present invention is set to 0.010%, ensuring the toughness of the material.

[0188] Like carbon, nitrogen (N) can dissolve in iron to form an interstitial solid solution. The solution strengthening effect of nitrogen is stronger than that of C because nitrogen atoms are smaller and easier to incorporate into the lattice to increase strength. At the same time, nitrides (such as CrN and VN) formed by nitrogen and elements such as chromium, aluminum, vanadium, and titanium are more stable than carbides. The growth rate of nitrides is lower than that of carbides, which can improve the creep strength and high-temperature creep rupture strength of steel, delay material softening, and delay the appearance time of thermal fatigue cracks. After nitrogen replaces a part of carbon, the precipitation of carbides is reduced, the risk of brittle fracture is lowered, toughness is improved, and the risk of intergranular corrosion can also be reduced because carbon is likely to form carbides at grain boundaries, leading to corrosion sensitivity, while nitrogen can reduce this situation. In addition, nitrides can act as hydrogen traps, delay hydrogen diffusion, and reduce hydrogen embrittlement sensitivity. Therefore, considering both strength and toughness, the N content in the present invention is set to 0.03% - 0.05%. Preferably, the sum of the mass percentages of C and N is between 0.28% and 0.35%.

[0189] Vanadium (V) has a very strong affinity with carbon, nitrogen, and oxygen, forming corresponding stable compounds. Vanadium mainly exists in the form of carbides in steel. Its main function is to refine the structure and grains of steel, and reduce the strength and toughness of steel. Vanadium can increase the tempering stability of steel and produce a secondary hardening effect. Therefore, considering both tempering stability and the matching of strength and toughness, the V content in the present invention is set to 0.3% - 0.6%. Preferably, the ratio of the mass percentage of V to (C + N) is between 1.3 and 1.5.

[0190] It should be noted that in this text, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variant thereof are intended to cover non-exclusive inclusion, such that a process, method, article or device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or device.

[0191] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A hot working die steel with high toughness and high thermal stability, characterized in that: The chemical composition mass percentage of the hot working die steel is: C: 0.22% to 0.32%, Si: 0.10% to 0.30%, Mn: 0.30% to 0.50%, P≤0.015%, S≤0.003%, Mo: 2.80% to 3.50%, Cr: 4.70% to 5.20%, V: 0.30% to 0.60%, N: 0.03% to 0.05%, and the remainder is Fe and unavoidable impurity elements.

2. The high-toughness and high-thermal-stability hot-working die steel according to claim 1, characterized in that: The sum of the mass percentages of C and N is between 0.28% and 0.35%; The ratio of the mass percentage of V to (C+N) is between 1.3 and 1.5; The mass percentage of P is between 0% and 0.010%.

3. A method for preparing a hot working die steel with high toughness and high thermal stability as claimed in claim 1 or 2, characterized in that: The steps include: The first step is to prepare hot working die steel powder, can package and hot isostatic pressing; The second step is refinement treatment and spheroidizing annealing to obtain hot working die steel; The refinement treatment is as follows: heating to 1020-1050°C for 3-5h, with a heating rate of ≤100°C / h, and then cooling alternately with water and air until the temperature of the hot working die steel is between 200-300°C; The spheroidizing annealing process is: heating to 880±5℃ for 5-7h and keeping it for 6-10h, then cooling to 730±5℃ at a rate of ≤20℃ / h, keeping it for 15-20h, and then cooling to 500℃ at a rate of ≤20℃ / h and air cooling out of the furnace.

4. The method for preparing a hot working die steel with high toughness and high thermal stability according to claim 3, characterized in that: The hot working die steel has a hardness of ≥45HRC, a yield strength of ≥1350MPa, a tensile strength of ≥1600MPa, an elongation after fracture of ≥12%, and an unnotched impact toughness of ≥550J / cm 2 , hardness ≥43HRC after keeping at 550℃ for 100h.

5. The method for preparing a hot working die steel with high toughness and high thermal stability according to claim 3, characterized in that: The preparation of hot working die steel powder includes the following steps: Step S1, smelting low-phosphorus raw materials, composition requirements: C ≥ 1.0%, P ≤ 0.015%, Mo: 2.8% to 3.5%, Cr ≤ 0.30%, the balance is Fe and unavoidable impurities; Step S2, dephosphorization: transport the molten steel in step S1 to an AOD furnace, add high-basicity slag and heat it for 5-10 minutes to make the temperature reach 1550-1570°C, blow it for 20 minutes to dephosphorize, and after the dephosphorization is completed, remove the dephosphorized slag, take samples and analyze the composition. If the phosphorus content is greater than 0.010%, add high-basicity slag again for blowing. After the dephosphorization is qualified, P≤0.010%, and add high-chromium ferrochrome to make the Cr content reach 4.7% to 5.20%; Step S3, decarburization: after adding slag, raise the temperature to ≥1650°C, blow for 10 minutes to decarburize, remove the oxidized slag after completion, take samples and analyze the components, if the carbon content is greater than 0.15%, add slag again and blow until the carbon content is ≤0.15%; Step S4, adjusting the composition: adding slag and ferrosilicon, blowing for 5-10 minutes for reduction, after which the reduced slag is removed, and alloy and slag are added, with the requirements of: C≤0.15%, Si≤0.10%, P≤0.010%, S≤0.010%, Cr: 4.80%-5.20%, Mo: 2.80%-3.50%; Step S5, deoxidation and desulfurization: transport the molten steel in step S4 to the LF refining furnace and connect argon gas, adjust the argon pressure to 0.2-0.4MPa, heat up for 10-15min, make the temperature ≥1580℃, add carbon powder and silicon powder for deoxidation, add lime and fluorite to adjust the slag components, and after desulfurization, heat up to a temperature ≥1650℃, remove the slag, transport the molten steel to the VD furnace for vacuum degassing, and after completion, transport the ladle to the atomization powder making process; Step S6, atomization powder making: In the atomization powder making process, the molten steel is heated to 1610-1620° C., and high-pressure nitrogen is used for gas atomization powder making. During the powder making process, the nitrogen pressure is maintained above 19 bar.

6. The method for preparing a hot working die steel with high toughness and high thermal stability according to claim 3, characterized in that: Canned package: put the mold steel alloy powder into the package, connect the package to the hose and then evacuate it. - 3 After maintaining the vacuum degree of Pa for more than 4 hours, welding and sealing are performed.

7. The method for preparing a hot working die steel with high toughness and high thermal stability according to claim 3, characterized in that: Hot isostatic pressing involves a three-stage holding process; In the first stage, the pressure in the furnace is pressurized to 20-35MPa by the hot isostatic pressing system, and then the temperature is increased to 700-950℃ at a heating rate of 10℃ / min. While the temperature is increased, the pressure is continued to be increased to 120-145MPa and maintained for 2-4h. In the second stage, the temperature and pressure are raised to 1120-1165℃, 140-170MPa, and kept at this temperature for 0.5-4h; In the third stage, the temperature continues to rise to 1160-1190°C, the pressure is increased to 170-190MPa, and the temperature is kept at this temperature for 0.5-4h.

8. An application of the high toughness and high thermal stability hot working die steel as claimed in claim 1 or 2, characterized in that: Applied to the preparation of 3D printing additive materials, including the following steps: Step 1: Use a powder sifter to sift the hot working die steel powder into a particle size suitable for 3D printing additive manufacturing; Step 2: Add the sieved powder into the 3D printing equipment, flatten it with a scraper, fill it with inert protective gas, exhaust the air or evacuate it; Step 3: Use the 3D printing device to read the printing data and start the 3D printing device to print; Step 4: Perform stress relief annealing heat treatment on the printed parts at a temperature between 650℃-850℃, furnace cool to 500℃-600℃ and then air cool.

9. The use of a hot working die steel with high toughness and high thermal stability according to claim 8, characterized in that: The hot working die steel powder has a fluidity of <25s / 50g, an oxygen content of <500ppm, and a bulk density of >3.3g / cm 3 .

10. The use of a hot working die steel with high toughness and high thermal stability according to claim 8, characterized in that: 3D printing additive material hardness ≥45HRC, yield strength ≥1350MPa, tensile strength ≥1600MPa, elongation after fracture ≥12%, Charpy V-notch impact energy ≥35J, hardness ≥43HRC after keeping at 550℃ for 100h.

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