NbV microalloying high-toughness die-casting die steel with high Ni content, production method and heat treatment process

Abstract

According to the high-Ni-content NbV microalloyed high-toughness die-casting die steel, the production method and the heat treatment process, alloy Cr, Mo, Ni, Nb and V are added, Nb / N + V / Nb is controlled to be larger than or equal to 30 and smaller than or equal to 35, and Nb / N + V / Nb is controlled to be larger than or equal to 30 and smaller than or equal to 35. The stacking fault energy is further improved by means of solid solution of V in lattice gaps, meanwhile, through the three-upsetting and three-drawing forging process, strict control over the homogenization temperature and the spheroidizing annealing process and the pretreatment and stepped quenching and tempering process, under the hardness of 45 HRC, the impact toughness is larger than or equal to 45 J, crystal grains are refined, the structure is uniform, and the average size of the crystal grains is smaller than or equal to 22 microns; moreover, the product is low in cost, uniform in structure and free of network carbide, and the toughness of the product is improved.

Description

Technical Field

[0001] This invention belongs to the field of mold steel technology, specifically relating to high-Ni-content NbV microalloyed high-toughness die-casting mold steel, its production method, and heat treatment process. Background Technology

[0002] Die casting steel represents advanced industrial technology. With the rapid development of new energy vehicles, integrated die casting parts are becoming increasingly common, leading to more stringent requirements for die casting mold steel. Furthermore, die casting mold steel operates at temperatures exceeding 600℃, and the production process is fast, with the mold cavity surface subjected to high-temperature molten metal erosion and significant stress, creating a harsh operating environment. Therefore, NADCA#207-2022 imposes strict requirements on the toughness of die casting mold steel, and impact toughness is a key indicator of its quality. To improve impact toughness, the forging process and alloy composition design can be optimized, achieving toughness improvement through grain refinement.

[0003] Chinese patent CN119243046A, published on January 3, 2025, discloses a novel high-strength, high-toughness hot work die steel and its preparation method. This patented technology achieves an impact energy of 32-38J by adding 0.25-0.48% Nb, while simultaneously controlling the Cr content at 2.35-2.89%. However, the excessively high Nb content leads to high costs, and the difficulty in controlling liquid carbide precipitation negatively impacts performance.

[0004] Therefore, it is necessary to develop a mold steel with high toughness to reduce costs while improving toughness. Summary of the Invention

[0005] The purpose of this invention is to provide a high-Ni-content NbV microalloyed high-toughness die-casting mold steel and its production method. This involves adding alloys Cr, Mo, Ni, Nb, and V, while controlling the chemical composition to 30 ≤ Nb / N+V / Nb ≤ 35. Simultaneously, the forging process needs to be optimized to control the carbide morphology. Furthermore, the Nb content is 0.035-0.045%, resulting in low cost, uniform microstructure, and the absence of network carbides, thus improving the product's toughness.

[0006] Another objective of this invention is to provide a heat treatment process for high-Ni-content NbV microalloyed high-toughness die-casting mold steel, and to design a matching heat treatment process that can ensure uniform microstructure, absence of network carbides, and achieve the best match between microstructure and properties.

[0007] The specific technical solution of this invention is as follows:

[0008] High-Ni content NbV microalloyed high-toughness die-casting mold steel, comprising the following mass percentage components:

[0009] C: 0.30-0.35%, Si: 0.20-0.50%, Mn: 0.80-1.20%, Cr: 5.5-6.0%, Mo: 2.0-2.5%, Ni: 1.6-2.0%, Nb: 0.035-0.045%, V: 1.0-1.3%, Al: 0.030-0.040%, P: ≤0.015%, S: ≤0.003%, TO: ≤10ppm, [H]: ≤1.0ppm, [N]: 70-100ppm, with the remainder being Fe and unavoidable impurity elements.

[0010] The composition of the high-Ni-content NbV microalloyed high-toughness die-casting mold steel also satisfies: 30≤Nb / N+V / Nb≤35, where each element symbol represents its content ×100, i.e., the value before % can be substituted into the calculation.

[0011] The present invention provides a method for producing high-Ni-content NbV microalloyed high-toughness die-casting mold steel, which includes smelting, electroslag remelting, forging and spheroidizing annealing.

[0012] The smelting process involves melting the steel in a medium-frequency induction furnace or an electric furnace according to the above chemical composition ratio, with a smelting temperature greater than 1450°C, and then casting it into steel ingots. The steel ingots are then air-cooled and used to make electric shock rods for electroslag furnaces.

[0013] The electroslag remelting process involves secondary refining in an electroslag furnace, followed by casting into steel ingots.

[0014] The forging process involves: firstly, high-temperature homogenization is performed by holding the temperature at 1200-1220℃ for 12±2 hours for homogenization, followed by air cooling. This invention performs a high-temperature homogenization treatment before conventional heating forging to uniformly dissolve the secondary phase. After cooling, the temperature is raised to the forging heating temperature of 1180℃-1200℃ and held for 9-11 hours for forging. The forging process adopts a three-dimensional three-pump three-drawing process. After forging, the temperature is water-cooled to room temperature.

[0015] The spheroidizing annealing is carried out at 860-880℃, with a spheroidizing holding time of not less than 10h, a cooling rate of ≤15℃ / s, and air cooling at 300℃.

[0016] This invention improves the spheroidization rate and quantity of carbides through the above-mentioned forging process, without increasing the size of the carbides.

[0017] The present invention provides a heat treatment process for high-Ni-content NbV microalloyed high-toughness die-casting mold steel. This invention employs a high Ni content and utilizes normalizing treatment to increase the solid solubility of Ni. Ni solid solution increases the stacking fault energy at grain boundaries, preventing carbide precipitation. The process involves first performing normalizing pretreatment, followed by air cooling, and then a stepped heating process; next, oil quenching, followed by three high-temperature tempering processes.

[0018] The normalizing pretreatment is performed at a temperature of 1050±10℃, and the temperature is maintained for 2-3 hours before being air-cooled to room temperature.

[0019] The stepped heating process is as follows: first, the temperature is raised to 650±10℃ and held for 2-3 hours; then, the temperature is raised to 940±10℃ and held for 2-3 hours; and then, the temperature is raised to 1030±10℃ and held for 1-2 hours.

[0020] The oil quenching refers to oil quenching and cooling to room temperature;

[0021] The three high-temperature tempering processes are as follows: the first tempering is performed at 590℃±10℃ for 2-3 hours, the second tempering is performed at 620℃±10℃ for 2-3 hours, and the third tempering is performed at 590℃±10℃ for 2-3 hours. The tempering temperature is adjusted according to the hardness, and the final hardness is adjusted to 45HRC.

[0022] Impact toughness was tested after the above heat treatment process. The heat treatment process designed in this invention can ensure uniform microstructure and absence of network carbides.

[0023] The high-Ni-content NbV microalloyed high-toughness die-casting mold steel is rated according to the NADCA#207 standard. The annealed microstructure is spheroidized and the rating is not lower than AS3. The quenched microstructure is martensite + carbides + a small amount of retained austenite. The area content of retained austenite is <3% and the quenched microstructure rating is not lower than HS3. After quenching and tempering heat treatment, the impact energy at 45HRC can reach ≥45J and the average grain size is ≤22μm.

[0024] The design concept of this invention is as follows:

[0025] C: C is the most basic and effective strengthening element in steel, ensuring that mold steel has a certain hardenability and matrix hardness. Carbon is one of the main strengthening elements in steel, which can improve the strength and hardness of steel and ensure the hardness requirements of mold steel during use. At the same time, carbon is dissolved in martensite, and the carbon content in the matrix martensite is relatively high. During use, the hardness of mold steel is not easy to decrease. However, if the carbon content is too high, it will easily cause the mold steel to be more brittle, and the internal stress after heat treatment will be greater, which will easily cause heat treatment cracking. Therefore, the carbon content should be kept at 0.30-0.35%.

[0026] Si: Si is a deoxidizer and also enhances the strength and hardness of steel through solid solution strengthening. It can also improve the hardenability of gear steel. The Si content should not be less than 0.20%. However, excessive silicon increases the activity of carbon, promoting decarburization and graphitization of steel during rolling and heat treatment, making the carburized layer prone to oxidation. At the same time, increasing the Si content can improve strength but results in a significant loss of toughness. Therefore, the Si content should not exceed 0.50%. The Si content should be controlled between 0.20% and 0.50%.

[0027] Mn: Mn can expand the austenite phase region and stabilize the austenite structure, improving the hardenability of steel and increasing the hardness and strength of ferrite and austenite in steel. Simultaneously, Mn can improve the stability of the austenite structure, significantly enhancing the hardenability of steel. However, excessive Mn will reduce the plasticity of steel, and the toughness will deteriorate during hot rolling. The Mn content should be controlled between 0.80-1.20%.

[0028] Cr: Cr can improve the hardenability and strength of steel. Cr combines with carbon in steel to form carbides. Die casting mold steel will be repeatedly subjected to thermal cycling during use and needs to have a certain degree of thermal stability. Excessive Cr is prone to enrichment at grain boundaries, which will reduce thermal stability. Therefore, the Cr content should not be higher than 5.5%. Thus, the Cr content is controlled between 5.5% and 6.0%.

[0029] Mo (Mo) significantly improves the hardenability of steel and prevents temper brittleness and overheating tendency. Furthermore, the appropriate combination of Mo and Cr elements in this invention significantly improves hardenability and tempering resistance, and Mo also refines grain size. Another major function of Mo is to improve tempering resistance; however, if the Mo content is too low, these effects are limited, while excessive Mo content promotes the formation of grain boundary ferrite films, which is detrimental to the hot plasticity of steel, increases the tendency for reheat cracking, and increases costs. Therefore, the Mo content is controlled at 2.0-2.5%.

[0030] Ni: Ni can effectively improve the core toughness of steel, lower the ductile-brittle transition temperature, improve low-temperature impact performance, and enhance the fatigue strength of steel materials. Another role of Ni in this invention is to increase stacking fault energy, thereby increasing the dislocation's ability to cross the potential barrier and improving impact toughness. However, Ni is relatively expensive, and excessively high Ni content can reduce machinability after hot working. Therefore, the Ni content is controlled within 1.60-2.0%.

[0031] Al: Al is an effective deoxidizer and can form AlN to refine grains. When the Al content is below 0.030%, its effect is not obvious, and when it is above 0.040%, it is easy to form coarse inclusions, which deteriorates the properties of steel. Therefore, it is necessary to adjust the timing of Al addition during the steelmaking process to ensure that the Al content is controlled between 0.030-0.040%.

[0032] Nb: Nb forms a composite precipitate of carbides and nitrides in steel. The solid solution temperature of this precipitate can reach above 1200℃. It plays a role in refining the average grain size during high-temperature forging and ultrafine treatment. However, excessive Nb content can easily lead to the formation of liquid carbides, causing quenching cracks during heat treatment. Therefore, the Nb content is controlled at 0.035-0.045%.

[0033] V: V plays a role in refining grains and improving wear resistance in steel. In die casting mold steel, it can effectively control the degree of melting loss. However, excessive V content has a significant impact on impact toughness, reduces the mold steel's resistance to internal stress, and easily leads to quenching cracks. At the same time, this invention utilizes the interaction between V and Nb to reduce the probability of Nb carbides from liquid precipitation. Therefore, V is controlled at 1.0-1.3%.

[0034] P and S: Sulfur easily forms MnS inclusions with manganese in steel, causing hot brittleness; P is an element with a strong tendency to segregate, increasing cold brittleness of steel, reducing plasticity, and is detrimental to the uniformity of product structure and properties. P should be controlled ≤0.015%, and S ≤0.003%.

[0035] TO and [H]: TO forms oxide inclusions in steel, so TO should be controlled to ≤10ppm; [H] forms white spots in steel, which seriously affects product performance, so [H] should be controlled to ≤1.0ppm.

[0036] [N]: It can form compounds with Nb, V, and Al, refining the grain size. A reasonable Nb / [N] ratio has a significant effect on grain refinement, while excessive [N] can lead to continuous casting defects such as bubbles. Simultaneously, V and Nb nitrides form a composite second phase, which has a higher solution temperature and stronger erosion resistance. Therefore, the [N] content should be controlled between 70-100 ppm, with 30 ≤ Nb / N + V / Nb ≤ 35.

[0037] Compared with existing technologies, the steel of this invention, through alloy composition design and reasonable production process control, proposes a micro-alloyed high-end die-casting mold steel product. Grain refinement is achieved through Nb and V nitrides, while Ni increases stacking fault energy to improve toughness. V solid solution in the interstitial spaces further enhances stacking fault energy. Simultaneously, a three-forging process with strict control of homogenization temperature and spheroidizing annealing, along with pretreatment and stepped tempering, results in an impact toughness ≥45J at 45HRC hardness, refined grains, uniform microstructure, and an average grain size ≤22μm. Furthermore, the product of this invention is not only low-cost but also has a uniform microstructure, free of network carbides, thus improving toughness. Attached Figure Description

[0038] Figure 1 The annealed microstructure of Example 1 is characterized by a uniform distribution of spherical or granular carbides on a ferrite matrix, without any network carbides.

[0039] Figure 2 The annealed microstructure of Example 2 is characterized by a uniform distribution of spherical or granular carbides on a ferrite matrix, without any network carbides.

[0040] Figure 3 The annealed microstructure of Example 3 is characterized by a uniform distribution of spherical or granular carbides on a ferrite matrix, without any network carbides.

[0041] Figure 4 The annealed microstructure of Example 4 is characterized by a uniform distribution of spherical or granular carbides on a ferrite matrix, without any network carbides.

[0042] Figure 5 The annealed microstructure of Example 5 is characterized by a uniform distribution of spherical or granular carbides on a ferrite matrix, without any network carbides.

[0043] Figure 6 The annealed microstructure is that of Comparative Example 1; the microstructure consists of spherical or granular carbides distributed on a ferrite matrix, with a small amount of network carbides.

[0044] Figure 7 The annealed microstructure is that of Comparative Example 2; the microstructure consists of spherical or granular carbides distributed on a ferrite matrix, with a small amount of network carbides.

[0045] Figure 8 This is a grain diagram of Example 1; the grain size is 12 μm.

[0046] Figure 9 This is a grain diagram of Example 2; the grain size is 15 μm.

[0047] Figure 10 This is a grain diagram of Example 3; the grain size is 16 μm.

[0048] Figure 11 This is a grain diagram of Example 4; the grain size is 17 μm.

[0049] Figure 12 This is a grain diagram of Example 5; the grain size is 15 μm.

[0050] Figure 13 The grain diagram is for Comparative Example 1; the grain size is 35 μm.

[0051] Figure 14 This is a grain diagram of Comparative Example 2; the grain size is 31 μm.

[0052] Figure 15 The diagram shows the quenched microstructure of Example 1; the microstructure is mainly martensite, with a small amount of dispersed carbides and retained austenite, and the retained austenite content is 2.2%.

[0053] Figure 16 The diagram shows the quenched microstructure of Example 2; the microstructure is mainly martensite, with a small amount of dispersed carbides and retained austenite, and the retained austenite content is 2.4%.

[0054] Figure 17 The diagram shows the quenched microstructure of Example 3; the microstructure is mainly martensite, with a small amount of dispersed carbides and retained austenite, and the retained austenite content is 2.3%.

[0055] Figure 18 The diagram shows the quenched microstructure of Example 4; the microstructure is mainly martensite, with a small amount of dispersed carbides and retained austenite, and the retained austenite content is 2.1%.

[0056] Figure 19 The diagram shows the quenched microstructure of Example 5; the microstructure is mainly martensite, with a small amount of dispersed carbides and retained austenite, and the retained austenite content is 2.5%.

[0057] Figure 20 The diagram shows the quenched microstructure of Comparative Example 1; the microstructure is mainly martensite with grain boundary carbides and retained austenite, and the retained austenite content is 5.5%.

[0058] Figure 21 The image shows the quenched microstructure of Comparative Example 2. The microstructure is mainly martensite with grain boundary carbides and retained austenite, with a retained austenite content of 6.5%. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all 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.

[0060] Examples 1-5

[0061] High Ni content NbV microalloyed high toughness die casting mold steel, comprising the following mass percentage composition as shown in Table 1, with the remainder not shown in Table 1 being Fe and unavoidable impurity elements.

[0062] Comparative Example 1 - Comparative Example 2

[0063] Die casting mold steel comprises the following composition by mass percentage as shown in Table 1. The remainder not shown in Table 1 is Fe and unavoidable impurity elements.

[0064] Table 1. Chemical composition (wt%) of the embodiments and comparative examples of the present invention.

[0065]

[0066] The production methods of die-casting mold steel in each embodiment and comparative example include smelting, electroslag remelting, forging, and spheroidizing annealing. Specifically, the steel is smelted in a medium-frequency furnace using the composition shown in Table 1, at a smelting temperature greater than 1450℃, and then cast into steel ingots. The steel ingots are air-cooled and then used to make electric shock rods for the electroslag furnace. Secondary refining is performed in the electroslag furnace. After refining into steel ingots, they are homogenized by holding at 1200-1220℃ for 12 hours. After cooling, they are heated to a forging temperature of 1180-1200℃ for forging using a three-axis, three-upsetting, three-drawing process. After forging, the steel is water-cooled to room temperature, air-cooled, and then spheroidized annealed at 860-880℃ for a holding time of not less than 10 hours at a cooling rate ≤15℃ / s. The steel is then cooled to 300℃ and air-cooled. Specific production parameters for each embodiment and comparative example are shown in Table 2.

[0067] Table 2 Production forging process parameters for each embodiment and comparative example

[0068]

[0069] The heat treatment process for die-casting mold steel in each embodiment and comparative example is as follows: First, normalizing pretreatment is performed at a temperature of 1050±10℃, held for 2-3 hours, and then air-cooled to room temperature. Next, tempering heat treatment is performed using a stepped heating method: first, the temperature is raised to 650±10℃ and held for 2-3 hours; then, the temperature is raised to 940±10℃ and held for 2-3 hours; then, the temperature is raised to a homogenization temperature of 1030±10℃ and held for 1-2 hours; finally, oil quenching is performed to cool to room temperature. After oil quenching, a three-stage high-temperature tempering process is used: the first stage is held at 590℃±10℃ for 2-3 hours; the second stage is held at 620℃±10℃ for 2-3 hours; and the third stage is held at 590℃±10℃ for 2-3 hours. The tempering temperature is adjusted according to the hardness, and the final hardness is adjusted to 45HRC. The heat treatment parameters for each embodiment and comparative example are shown in Table 3. Comparative example 1 has no normalizing pretreatment process, while comparative example 2 is directly heated to 1020℃ after normalizing and then oil quenched without step heating. The impact toughness is tested after tempering.

[0070] Table 3 Heat treatment parameters for each embodiment and comparative example

[0071]

[0072] The microstructure and properties of the die-casting mold steel produced in each embodiment and comparative example (NADCA#207-2022) are shown in Table 4.

[0073] Table 4. Microstructure and properties of die-casting mold steels in each embodiment and comparative example.

[0074]

[0075] This invention presents a micro-alloyed high-end die-casting mold steel product through alloy composition design and rational production process control. Grain refinement is achieved through Nb and V nitrides, while Ni increases stacking fault energy to improve toughness. V solidification in the interstitial spaces further enhances stacking fault energy. Simultaneously, a three-stage forging process with strict control over homogenization temperature and spheroidizing annealing improves carbide spheroidization rate and quantity without increasing carbide size. Pretreatment and stepped tempering processes result in an impact toughness ≥45J at 45HRC, with refined grains, uniform microstructure, and no network carbides. Rated according to NADCA#207 standards, the annealed microstructure is spheroidized with a rating of at least AS3, and the quenched microstructure consists of martensite + carbides + a small amount of retained austenite, with a retained austenite area content <3% and a rating of at least HS3. After tempering, the impact toughness at 45HRC reaches ≥45J, and the average grain size is ≤22μm.

[0076] Comparative Example 1 has a low Mn content, no Nb added, and is produced using a two-tower, two-draw process. The spheroidizing annealing temperature is low. Comparative Example 1 does not have a normalizing pretreatment process, resulting in an annealed microstructure rating of AS9, a quenched microstructure rating of HS7, a grain size of 35μm, and an impact toughness of only 18J at 45HRC.

[0077] Comparative Example 2 had a low Mn content, and the contents of Cr, Ni and Nb did not meet the requirements of this invention. V was not added, and the N content was low. During heat treatment, normalizing was followed by direct heating to 1020℃ for oil quenching without step heating. This resulted in an annealed microstructure rating of AS9 and a quenched microstructure rating of HS9. The grain size reached 31μm, and the impact toughness at 45HRC was only 15J.

[0078] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A high-Ni-content NbV microalloyed high-toughness die-casting mold steel, characterized in that the Ni-content NbV microalloyed high-toughness die-casting mold steel comprises the following components by mass percentage: C: 0.30-0.35%, Si: 0.20-0.50%, Mn: 0.80-1.20%, Cr: 5.5-6.0%, Mo: 2.0-2.5%, Ni: 1.6-2.0%, Nb: 0.035-0.045%, V: 1.0-1.3%, Al: 0.030-0.040%, P: ≤0.015%, S: ≤0.003%, TO: ≤10ppm, [H]: ≤1.0ppm, [N]: 70-100ppm, the remainder being Fe and unavoidable impurity elements.

2. The high-Ni-content NbV microalloyed high-toughness die-casting mold steel according to claim 1, characterized in that, The composition of the high-Ni-content NbV microalloyed high-toughness die-casting mold steel also satisfies: 30≤Nb / N+V / Nb≤35.

3. The high-Ni-content NbV microalloyed high-toughness die-casting mold steel according to claim 1 or 2, characterized in that, Annealed microstructure is spheroidized with a rating of not less than AS3. Quenched microstructure consists of martensite, carbides, and a small amount of retained austenite, with a retained austenite area content of less than 3% and a rating of not less than HS3. After tempering heat treatment, the impact energy at 45HRC can reach ≥45J, and the average grain size is ≤22μm.

4. A method for producing high-Ni-content NbV microalloyed high-toughness die-casting mold steel according to any one of claims 1-3, characterized in that, The production method includes smelting, electroslag remelting, forging, and spheroidizing annealing.

5. The production method according to claim 4, characterized in that, The forging process involves: firstly, high-temperature homogenization is performed by holding the temperature at 1200-1220℃ for 12±2 hours; after homogenization, the mixture is air-cooled; after cooling, it is heated to the forging heating temperature of 1180℃-1200℃ and held for 9-11 hours for forging; the forging process adopts a three-way three-pronged forging and three-pulling process; and after forging, it is water-cooled to room temperature.

6. The production method according to claim 4, characterized in that, The spheroidizing annealing is carried out at 860-880℃ with a cooling rate of ≤15℃ / s, and then air-cooled to 300℃.

7. A heat treatment process for high-Ni-content NbV microalloyed high-toughness die-casting mold steel according to any one of claims 1-3, characterized in that, The heat treatment process includes: first, normalizing pretreatment, followed by air cooling and then stepped heating; then oil quenching, followed by three high-temperature tempering processes.

8. The heat treatment process according to claim 7, characterized in that, The normalizing pretreatment is performed at a temperature of 1050±10℃, and the temperature is maintained for 2-3 hours before being air-cooled to room temperature.

9. The heat treatment process according to claim 7, characterized in that, The stepwise heating process is as follows: first, the temperature is raised to 650±10℃ and held for 2-3 hours, then the temperature is raised to 940±10℃ and held for 2-3 hours, and then the temperature is raised to 1030±10℃ and held for 1-2 hours.

10. The heat treatment process according to claim 7, characterized in that, The three high-temperature tempering processes are as follows: the first tempering is performed at 590℃±10℃ for 2-3 hours, the second tempering is performed at 620℃±10℃ for 2-3 hours, and the third tempering is performed at 590℃±10℃ for 2-3 hours.

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