A method for regulating microstructure uniformity of hot work die steel

Abstract

The present application belongs to the technical field of steel smelting and hot working, and particularly relates to a microstructure uniformity regulation method for hot work die steel, comprising: obtaining the hot work die steel through initial smelting by an electric furnace, LF refining purification, VD refining degassing and impurity removal; the present application adopts special smelting, forging and heat treatment processes, so that the prepared product has excellent microstructure uniformity, no banded structure and no mixed crystal structure in the finished product structure, and the carbide is uniformly distributed in a dispersion state; according to the annealing structure rating, annealing structure banded segregation rating and heat treatment structure rating diagrams of the North American Die Casting Association NADCA #207-2016 standard, AS1 level, SA1 level and HS1 level can be respectively reached. The hot work die steel of the present application has excellent wear resistance and impact toughness. The die prepared from the material has a longer service life and can better adapt to the demand of modern industrial production for high-end dies.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel smelting and hot working, and particularly relates to a microstructure uniformity regulation method of hot work die steel. BACKGROUND

[0002] Hot work die steel refers to alloy tool steel suitable for manufacturing dies for hot deformation of metal, such as hot forging dies, hot extrusion dies, die casting dies, hot heading dies and the like. The development of many countries' key construction projects and strategic emerging industries, such as new energy, ocean engineering, biological engineering, medical treatment, transportation and the like, needs a large number of hot work dies to support, which also greatly increases the demand for hot work die steel.

[0003] A large number of studies show that the microstructure non-uniformity of steel is the main reason for poor mechanical properties of steel and eventually leads to early failure of the die. The author also detects and analyzes a large number of short-life failure dies, and summarizes the failure process as follows: due to the microstructure non-uniformity of steel, specifically the non-uniformity of banded structure, mixed crystal structure and carbide size and distribution, the impact toughness of the steel is greatly reduced. When the die is repeatedly subjected to thermal stress and mechanical stress during service, stress concentration is formed in the alloy element aggregation zone, and then micro-cracks are generated at the weak links of the die contact surface. Generally speaking, the cracks can be repaired. However, due to the low impact toughness of the die steel caused by the non-uniformity of the microstructure, when the crack is formed, the die does not have enough toughness to stop the crack propagation. When the stress on the die exceeds the fracture strength of the die, the crack penetrates the die, causing the short life of the die.

[0004] At present, in order to improve the microstructure uniformity of steel, high-temperature long-time homogenization heat treatment is usually used to eliminate microsegregation, so as to improve the uniformity of the microstructure. This method relies on high temperature and long time for element diffusion, and has the following disadvantages: a large amount of energy is consumed, and at the same time, overheating or overburning of the microstructure is easy to occur. Moreover, this method only improves the microsegregation, and has relatively good improvement effect on banded structure and mixed crystal structure, but has no substantial effect on the improvement of carbide, especially in controlling the size and distribution of carbide.

[0005] CN 110317934B discloses a heat treatment process for improving the annealing microstructure uniformity of H13 steel, which avoids the chain-like distribution of carbide along the grain boundary, and significantly improves the annealing microstructure uniformity and isotropic performance of the material.

[0006] The application improves the annealing microstructure uniformity of H13 steel by adjusting the heat treatment process, but the application only aims at the distribution of carbide of H13 steel, and does not improve the banded crystal structure, mixed crystal structure and carbide size problem, so the microstructure uniformity is not improved.

[0007] CN 114273574 B discloses a forging method for controlling the uniformity of microstructure in large-size easily segregated austenitic stainless steel bars. The resulting forged bars have good microstructure uniformity, eliminate fine grain bands and banded carbides, and can effectively improve the microstructure of subsequent deformed products.

[0008] The forging method of this invention produces forged bars with good microstructure uniformity, eliminates fine grain bands and banded carbides, and can effectively improve the microstructure of subsequent deformed products. However, it has a limited range of steel grades and poor applicability; improvements to the forging process alone cannot achieve the goal of controlling microstructure uniformity. Summary of the Invention

[0009] To address the above problems, this invention proposes a method for controlling the uniformity of the microstructure of hot work die steel, comprising:

[0010] High-purity molten steel is obtained through primary refining in an electric furnace, refining and purification in an LF (ladle refining furnace), and refining, degassing and impurity removal in a VD (vacuum refining furnace). The molten steel is then cast into electrode ingots. In the casting process, electrode ingots are cast using a double-layer bottom plate in a staged casting method, with a casting temperature of 1480-1500℃.

[0011] The electrode ingot is subjected to electroslag remelting to obtain an electroslag ingot;

[0012] The electroslag ingot is heated and pre-deformed to obtain a pre-deformed billet;

[0013] The pre-deformed billet is homogenized at high temperature and then forged to obtain a forged billet;

[0014] The forging billet is subjected to annealing heat treatment to obtain hot work die steel;

[0015] The chemical composition of the hot work die steel, by mass percentage, includes: C: 0.40%–0.50%; Si: ≤0.50%; Mn: ≤0.50%; Cr: 5.00%–5.50%; V: 0.80%–1.20%; Mo: 0.50%–0.80%; with the balance being Fe and unavoidable impurities.

[0016] The further preferred technical solution is as follows: the double-layer bottom plate staged casting includes two stages of casting. The casting speed for the first 30 seconds of the ingot body is 200-250 kg / s, the casting speed for the middle and later stages of the ingot body is 120-150 kg / s, and the casting speed for the riser is 200-250 kg / s.

[0017] The further preferred technical solution is as follows: In electroslag remelting, an argon protective atmosphere is used for electroslag remelting, and the slag system is a ternary slag system of 65% CaF2-25% Al2O3-10% MgO.

[0018] The further preferred technical solution is: in the electroslag remelting under an argon protective atmosphere, the voltage is 45V~60V and the current is 7000A~8500A.

[0019] A further preferred technical solution is: in electroslag remelting under an argon protective atmosphere, the melting rate is 0.5 to 0.7 times the diameter of the crystallizer.

[0020] A further preferred technical solution is to heat the electroslag ingot and perform pre-deformation treatment, including: heating the electroslag ingot at high temperature, transferring the heated electroslag ingot to a forging machine for unidirectional elongation along the length direction, with a reduction of 20-40%, to obtain a pre-deformed billet.

[0021] A further preferred technical solution is: high-temperature homogenization and forging of the pre-deformed billet, including:

[0022] The pre-deformed billet is heated at high temperature;

[0023] The heated pre-deformed billet is transferred to a forging machine for multi-directional forging. It is compressed along the diameter direction of the billet and drawn to the billet size along the length direction, and then the final forging is performed.

[0024] A forging blank is obtained.

[0025] A further preferred technical solution is as follows: the high-temperature heating is performed at a temperature of 1280±20℃, and the heat preservation time is 5 to 10 hours.

[0026] The further preferred technical solution is: final forging temperature ≥ 850℃.

[0027] A further preferred technical solution is to perform annealing heat treatment on the forging billet, including:

[0028] The forging billet is heated to 1050-1080℃ in the furnace and held for 2-4 hours.

[0029] Then cool down to 880–910℃ and keep warm for 15–20 hours;

[0030] After the heat preservation is completed, the furnace is cooled to ≤300℃, and then air-cooled after being removed from the furnace.

[0031] The beneficial effects of this invention are:

[0032] First, the present invention employs special smelting, forging, and heat treatment processes, resulting in products with excellent microstructure uniformity. The finished product has no banded structure or mixed crystal structure, and the carbides are dispersed and uniformly distributed. According to the North American Die Casting Association (NADCA)#207-2016 standard annealing structure rating, annealing structure banded segregation rating, and heat treatment structure rating chart, it can reach AS1, SA1, and HS1 grades, respectively.

[0033] Secondly, the hot work die steel of this invention possesses excellent wear resistance and impact toughness. Dies made from this material have a longer service life and can better meet the demands of modern industrial production for high-end dies.

[0034] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description, claims and drawings. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 A process flow diagram according to an embodiment of the present invention is shown;

[0037] Figure 2 The metallographic structure of Embodiment 1 of the present invention is shown;

[0038] Figure 3 The metallographic structure of Comparative Example 1 is shown. Detailed Implementation

[0039] 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 with reference to the accompanying drawings. 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.

[0040] Example

[0041] The alloy composition of hot work die steel, by weight percentage, includes C: 0.40%–0.50%; Si: ≤0.50%; Mn: ≤0.50%; Cr: 5.00%–5.50%; V: 0.80%–1.20%; Mo: 0.50%–0.80%; with the balance being Fe and unavoidable impurities.

[0042] The preparation steps of hot work die steel are as follows: Figure 1 As shown:

[0043] Step S1: The electrode ingot is cast using ingot casting. During ingot casting, a double-layer bottom plate is used for staged casting. The casting temperature is 1480-1500℃. The casting speed for the ingot body in the first 30 seconds is 200-250 kg / s. The casting speed for the ingot body in the middle and later stages is 120-150 kg / s. The riser casting speed is 200-250 kg / s.

[0044] Step S2: The electrode ingot obtained in step S1 is subjected to electroslag remelting treatment using a ternary slag system of 65% CaF2-25% Al2O3-10% MgO, with the voltage controlled between 45 and 60V, the current controlled between 7000 and 13000A, and the melting rate controlled at 0.5 to 0.7 times the diameter of the crystallizer, to obtain an electroslag ingot.

[0045] Step S3: The electroslag ingot obtained in step S2 is subjected to pre-deformation treatment. The heating temperature is 1260±20℃, and the holding time is 2-5h. The pre-deformation forging method is unidirectional elongation along the length direction, with a radial reduction of 20-40%, to obtain a forging billet.

[0046] Step S4: The forging billet obtained in Step S3 undergoes high-temperature homogenization and forging treatment. The heating temperature for high-temperature homogenization is 1250±20℃, and the holding time is 5-10 hours. The forging method is multi-directional forging, where the billet is compressed along its diameter direction (X and Y directions) and drawn along its length direction (Z axis), with repeated upsetting and drawing passes until the finished billet dimensions are met. The final forging temperature is ≥850℃. A forging billet is obtained.

[0047] Step S5: The forging billet obtained in step S4 is heated in the furnace at a temperature of 1050-1080℃ and held for 2-4 hours. After holding, the temperature is lowered to 880-910℃ and held for 15-20 hours. After holding, the billet is cooled in the furnace to ≤300℃ and then air-cooled to obtain the finished hot work die steel.

[0048] Table 1 shows the chemical composition of the hot work die steels in Examples 1-5 and Comparative Examples 1-2;

[0049] Table 2-6 shows the process parameters for hot work die steels in Examples 1-5 and Comparative Examples 1-2, where D is the diameter of the electroslag crystallizer;

[0050] Table 7 shows the performance tests and service life of the hot work die steels in Examples 1-5 and Comparative Examples 1-2.

[0051] Table 1. Chemical composition (mass percentage) of each example and comparative example.

[0052] Serial number C Si Mn Cr Mo V P S 1 0.43 0.28 0.34 5.12 0.61 0.93 0.001 0.0006 2 0.47 0.31 0.37 5.31 0.58 1.01 0.0012 0.0005 3 0.41 0.26 0.4 5.07 0.72 1.1 0.001 0.0008 4 0.43 0.34 0.41 5.26 0.68 0.87 0.0014 0.0004 5 0.45 0.41 0.38 5.41 0.59 1.14 0.0008 0.0006 Comparative Example 1 0.42 0.31 0.41 5.15 0.54 0.83 0.001 0.0005 Comparative Example 2 0.48 0.29 0.37 5.37 0.7 1.02 0.0015 0.0007

[0053] Table 2. Electrode ingot production process parameters for each embodiment and comparative example.

[0054]

[0055]

[0056] Table 3. Production process parameters for electroslag ingots in each embodiment and comparative example.

[0057]

[0058] Table 4. Production process parameters of pre-deformed blanks for each embodiment and comparative example.

[0059]

[0060]

[0061] Table 5. Production process parameters of forged blanks for each embodiment and comparative example.

[0062]

[0063] Table 6. Production process parameters for post-forging heat treatment of each embodiment and comparative example.

[0064]

[0065]

[0066] Table 7 Performance tests and mold lifespan of each embodiment and comparative example.

[0067]

[0068] As shown in Table 7, Examples 1-5 provided by this invention significantly improve yield strength, wear resistance, elongation, and average service life. This invention utilizes a double-layer bottom plate for rapid cooling, supplemented by a "high-temperature slow casting" method, which accelerates the solidification of molten steel and reduces segregation. Unidirectional elongation along the length direction with a suitable reduction and appropriate temperature fully breaks down columnar crystals, forming a fine, fully recrystallized structure. High-temperature heating and holding treatment of the pre-deformed billet accelerates element diffusion and eliminates micro-segregation. Multi-directional forging and repeated upsetting and elongation of the pre-deformed billet avoids the generation of longitudinal banded structures and transverse pattern segregation. Figure 2 and Figure 3 As shown, the finished product prepared in Example 1 of the present invention, compared with Comparative Example 1, has no banded structure, no mixed crystal structure, and the carbides are dispersed and uniformly distributed, resulting in superior performance.

[0069] It should be noted that the terms "first," "second," etc., used in this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this application described herein. In this application, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "middle," "vertical," "horizontal," "lateral," "longitudinal," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings.

[0070] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling the uniformity of microstructure in hot work die steel, characterized in that, include: High-purity molten steel is obtained through electric furnace primary refining, LF refining and purification, and VD refining degassing and impurity removal. The molten steel is then cast into electrode ingots. In the casting process, electrode ingots are cast using a double-layer bottom plate stage casting method, and the casting temperature is 1480~1500℃. The electrode ingot is subjected to electroslag remelting to obtain an electroslag ingot; The electroslag ingot is heated and pre-deformed to obtain a pre-deformed billet; The pre-deformed billet is homogenized at high temperature and then forged to obtain a forged billet; The forging billet is subjected to annealing heat treatment to obtain hot work die steel; The chemical composition of the hot work die steel, by mass percentage, comprises: C: 0.40%–0.50%; Si: ≤0.50%; Mn: ≤0.50%; Cr: 5.00%–5.50%; V: 0.80%–1.20%; Mo: 0.50%–0.80%; with the balance being Fe and unavoidable impurities. The double-layer bottom plate staged casting includes two stages of casting: the casting speed for the first 30 seconds of the ingot body is 200~250 kg / s, the casting speed for the middle and later stages of the ingot body is 120~150 kg / s, and the casting speed for the riser is 200~250 kg / s. Heating and pre-deforming the electroslag ingot includes: heating the electroslag ingot at high temperature, transferring the heated electroslag ingot to a forging machine for unidirectional elongation along the length direction, with a reduction of 20-40%, to obtain a pre-deformed billet. The high-temperature homogenization and forging of the pre-deformed billet includes: heating the pre-deformed billet at a high temperature of 1280±20℃ and holding it for 5 to 10 hours; transferring the heated pre-deformed billet to a forging machine for multi-directional forging, compressing it along the diameter direction and drawing it along the length direction to the billet size, and then performing final forging at a temperature ≥850℃ to obtain a forged billet; The annealing heat treatment of the forging billet includes: heating the forging billet in the furnace to 1050-1080℃ and holding it for 2-4 hours; then cooling it to 880-910℃ and holding it for 15-20 hours; after the holding period, cooling it in the furnace to ≤300℃ and then air-cooling it after it is removed from the furnace.

2. The method for controlling the uniformity of microstructure in hot work die steel according to claim 1, characterized in that, In the electroslag remelting process, an argon protective atmosphere is used for electroslag remelting, and the slag system is a ternary slag system of 65% CaF2~25% Al2O3~10% MgO.

3. The method for controlling the uniformity of microstructure in hot work die steel according to claim 2, characterized in that, In the argon-protected atmosphere electroslag remelting, the voltage is 45V to 60V and the current is 7000A to 8500A.

4. The method for controlling the uniformity of microstructure in hot work die steel according to claim 3, characterized in that, In the argon-protected atmosphere electroslag remelting, the melting rate is 0.5 to 0.7 times the diameter of the crystallizer.

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