Method for determining and optimizing CCT and TTT curves of high-hardenability die steel

Abstract

The invention provides a method for determining and optimizing CCT and TTT curves of high-hardenability die steel, which comprises the following steps of: cutting the high-hardenability die steel into a plurality of blocks by electric spark, preprocessing to obtain a prefabricated part, vacuumizing, heating, compressing and cooling to obtain an expansion curve, and performing data processing to obtain a phase change curve; and then the same high-hardenability die steel is taken to be subjected to electric spark cutting into a plurality of blocks, pretreatment is conducted on the blocks, after prefabricated parts are obtained, heat treatment is conducted on the prefabricated parts, structure characterization is conducted on the prefabricated parts, a curve measured before is revised, and the phase variable of a sample obtained after heat treatment is calculated through hardness and structure verification. The method disclosed by the invention is small in experimental quantity, low in cost, short in development period, high in adaptability to different materials and strong in replicability. The finally optimized curve determination method greatly improves the accuracy of the material curve, and can accurately guide the development of subsequent heat treatment experiments.

Description

Technical Field

[0001] The invention belongs to the field of metal heat treatment and tissue phase transformation, and in particular relates to a method for measuring and optimizing CCT and TTT curves of high-hardenability die steel. Background Art

[0002] With the continuous development of China's manufacturing industry, the requirements for mold materials are gradually increasing. As mold sizes continue to grow, the dimensional effects of these oversized molds require excellent hardenability, a suitable heat treatment process, and precise phase transformation curve analysis. However, due to the high hardenability of the material, traditional curve determination takes a long time, at least a month, and the readiness is low, which greatly slows down the development of high-hardenability mold steel.

[0003] Therefore, there is an urgent need for a method that can effectively guide the determination of the phase transformation curve of high hardenability die steel. Summary of the Invention

[0004] The present invention aims to overcome the above-mentioned shortcomings and provide a method for optimizing the measurement of the CCT and TTT curves of high-hardenability die steel. The present invention uses the phase transformation laws of the material and plots the continuous cooling transformation (CCT) curve and the isothermal transformation (TTT) curve accordingly. The curve measurement method of the present invention has a small experimental amount, low cost, a short development cycle, high adaptability to different materials, and strong reproducibility. The final optimized phase transformation curve greatly improves the material heat treatment yield and microstructure uniformity, improves the overall performance of the material, and can effectively guide the phase transformation curve measurement of high-hardenability die steel.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] A method for measuring and optimizing the CCT and TTT curves of a high hardenability die steel comprises the following steps:

[0007] (1) High hardenability die steel is sequentially electrospark cut into multiple pieces, which are pretreated separately. After the preforms are obtained, vacuuming, heating, insulation and cooling are sequentially performed to obtain expansion data. The expansion data are plotted into expansion curves and data processing is performed to obtain the measured CCT and TTT curves;

[0008] (2) The same high hardenability die steel raw material in step (1) is sequentially electrospark cut into multiple pieces, which are pretreated separately. After the prefabricated parts are obtained, they are heat treated and characterized for microstructure, and the curve measured in step (1) is revised; the phase variable of the heat-treated sample is calculated by hardness and microstructure verification.

[0009] In the above steps, the expansion curve measurement experiment is performed in a Gleeble-3500 thermal simulator, which undergoes vacuuming, heating, holding, and cooling. The specific source of the Gleeble-3500 thermal simulator is not particularly limited in the present invention; any commercially available product familiar to those skilled in the art can be used. The heat treatment can be performed in a small box-type furnace.

[0010] In the present invention, the dimensions of the high hardenability die steel raw material are independently preferably: diameter 6-8 mm, length 75-90 mm, more preferably: diameter 6 mm, length 75 mm.

[0011] Preferably, the high-hardenability die steel is pretreated before measurement. The pretreatment method includes initially grinding the die steel using a surface grinder, and then sequentially polishing the surface with 200#, 600#, 800#, and 1000# sandpaper to obtain a prefabricated part. This pretreatment of the sample removes impurities from the surface of the compressed sample, while also smoothing the galvanic welding surface and preventing the influence of impurities on the weld point, thereby improving the accuracy of data collection and the stability of the experiment.

[0012] Preferably, the CCT curve is measured using a Gleeble-3500 thermal simulation tester, with a heating rate of 1-2°C / s, a holding temperature of 1080°C, and a holding time of 5 minutes. The vacuum degree of the vacuum pump is 1×10 -3 ~1×10 -4 pa.

[0013] Preferably, the cooling rates of the CCT expansion curve are 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20°C / s, respectively, when cooled to room temperature;

[0014] Preferably, the cooling and holding temperatures of the TTT expansion curve are 700, 650, 600, 550, 500, 450, 400, 350, 300, 250, and 200°C, respectively.

[0015] Preferably, the size of the heat treatment sample is 20*20*20 mm, and the experiment is carried out in a small box furnace.

[0016] In the present invention, the TTT curve of the heat treatment furnace is that the heating rate of furnace No. 1 is 10℃ / min, the holding temperature is 1080℃, and the holding time is 15 minutes; the heating rate of furnace No. 2 is 10℃ / min, and the holding temperatures are 700, 650, 600, 550, 500, 450, 400, 350, 300, 250, and 200℃, respectively. A double-furnace experiment is adopted (after holding at 1080℃ for 15 minutes in furnace No. 1, transfer to furnace No. 2 with the set holding temperature).

[0017] Preferably, in the No. 2 furnace, the pearlite transformation zone is kept warm for 72 hours, and the bainite transformation zone is kept warm for 36 hours.

[0018] Preferably, the cooling rates of the CCT curve of the heat treatment furnace are 0.05, 0.04, 0.03, 0.02, and 0.01°C / s respectively when cooled to room temperature.

[0019] The present invention performs tissue characterization and hardness measurement on heat-treated samples to calculate phase variables.

[0020] In the present invention, the phase change curve measured by Gleeble-3500 is combined with the results of the heat treatment furnace to obtain accurate CCT and TTT curves.

[0021] The technical solution of the present invention is an optimized phase transformation curve measurement method. This method is particularly suitable for measuring phase transformations in high-hardenability mold steels that are not susceptible to phase transformations, with a measurement cycle of no more than 14 days. Conventional methods are only applicable to ordinary mold steels, with a measurement cycle of approximately one month and difficulty measuring phase transformations in high-hardenability mold steels.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] This paper optimizes the measurement of the CCT and TTT curves of high-hardenability die steel by combining physical simulation and heat treatment experiments. This method requires minimal experimental effort, is low-cost, has a short development cycle, is highly adaptable to different materials, and is highly reproducible. The resulting optimized phase transformation curve significantly improves the material's heat treatment yield and microstructure uniformity, enhancing its overall performance.

[0024] This method replaces traditional curve development methods by combining expansion curve measurement with heat treatment experiments. This method effectively reveals the phase transformation characteristics of materials, enables a quantitative description of microstructural evolution, optimizes the phase transformation curve determination method for high-hardenability die steel, and significantly reduces the cycle and cost of phase transformation curve development. The research results have important academic contributions and engineering value for promoting the development of the die manufacturing industry and achieving the efficient development of high-hardenability die steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the invention with reference to the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the invention.

[0026] Figure 1 This is a diagram of the phase change curve measurement testing machine and heat treatment furnace of the present invention.

[0027] Figure 2 Schematic diagram of the sample size for expansion curve measurement of the present invention.

[0028] Figure 3 This is an expansion curve diagram obtained by the method of Example 1 of the present invention.

[0029] In the figure, (a) is the expansion curve at a cooling rate of 10°C / s; (b) is the expansion curve at a cooling rate of 20°C / s.

[0030] Figure 4 This is the metallographic structure diagram of the bainite phase transformation zone obtained by the method of the embodiment of the present invention.

[0031] Figure 5 This is the metallographic structure diagram of the pearlite transformation zone obtained by the method of the embodiment of the present invention.

[0032] Figure 6 The hardness and microstructural transformation curves obtained by the method of the embodiment of the present invention are shown in the figure: (a) pearlite transformation under different heat treatment parameters; (b) hardness curves under different heat treatment parameters.

[0033] Figure 7 The CCT and TTT phase diagrams measured by the method of the embodiment of the present invention are shown in the figure. In the figure, (a) the revised CCT curve; (b) the revised TTT curve DETAILED DESCRIPTION

[0034] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided to make the present invention more thorough and complete and to fully convey the scope of the present invention to those skilled in the art.

[0035] In the following examples, the expansion curve measurement experiment was carried out in a Gleeble-3500 thermal simulator, and the heat treatment was performed using a small box furnace.

[0036] The high hardenability die steel described in the following examples was purchased from Tiangong Aihe Special Steel Co., Ltd. The specific composition is shown in Table 1.

[0037] Table 1

[0038]

[0039] Example 1

[0040] A method for measuring and optimizing the CCT and TTT curves of a high hardenability die steel comprises the following steps:

[0041] (1) The high hardenability die steel was sequentially electrospark cut into multiple specimens with a size of φ6*75mm. After pretreatment before measurement, the multiple preforms were vacuumed to a vacuum degree of 1×10 -3 ~1×10-4 pa, heating to 1080℃, heating rate is 1℃ / s, keeping the temperature for 5min after reaching the specified temperature, cooling down, and obtaining expansion data. The expansion data is plotted into expansion curve (see Figure 3 ) is imported into the drawing software and the measured CCT and TTT curves are obtained by the intercept method, that is, the phase change curve (see Figure 7 ); The pretreatment method is to grind the sample with a surface grinder, and then polish the surface with 200#, 600#, 800# and 1000# sandpaper.

[0042] The CCT expansion curve was measured using a Gleeble-3500 thermal simulation tester. The test conditions for the sample were as follows: a heating rate of 1°C / s, a holding temperature of 1080°C, a holding time of 5 min, and a compression vacuum of 1×10 -3 ~1×10 -4 pa, resistance heating by thermocouple wire; cooling rates of CCT expansion curves are 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 °C / s to room temperature;

[0043] The TTT expansion curve test specimen size is Φ6*75mm. The Gleeble-3500 thermal simulation tester is also used for the test. The heating rate is 1°C / s, the holding temperature is 1080°C, the holding time is 5 minutes, and the cooling rate is 20°C / s to the holding temperature for 24 hours.

[0044] The TTT expansion curves when cooled to the holding temperature are 700, 650, 600, 550, 500, 450, 400, 350, 300, 250, and 200 °C, respectively.

[0045] (2) The same piece of high-hardenability die steel material in step (1) was electrospark cut into multiple specimens with a size of 20*20*20 mm. The specimens were pre-treated before measurement, and the obtained preforms were heat-treated and characterized respectively, and the curve measured in step (1) was revised. The pre-treatment method was to grind the specimens with a surface grinder, and then polish the surface with 200#, 600#, 800# and 1000# sandpaper in sequence.

[0046] In the heat treatment method, a dual-furnace experiment was used. A small box furnace was used with a heating rate of 10°C / min. The preforms were kept at 1080°C in Furnace No. 1 for 15 minutes and then transferred to Furnace No. 2, which had a set holding temperature. Specifically, the holding temperature of Furnace No. 1 was 1080°C, and the holding temperatures of Furnace No. 2 were 700, 650, 600, 550, 500, 450, 400, 350, 300, 250, and 200°C, respectively. The pearlite transformation zone was kept at this temperature for 72 hours, and the bainite transformation zone was kept at this temperature for 36 hours.

[0047] In the heat treatment method, the CCT expansion curves of multiple samples were measured at cooling rates of 0.05, 0.04, 0.03, 0.02, and 0.01°C / s, respectively, when cooled to room temperature.

[0048] The heat treated samples were characterized and hardness was measured. The pearlite transformation amount under different heat treatment parameters was obtained (see Figure 6 -a) and hardness curves of different heat treatment parameters (see Figure 6 -b). Through the metallographic organization diagram (see Figure 5 ), count the area proportions of different phases, and calculate the phase variables.

[0049] Depend on Figure 4 It can be seen that the metallographic structure diagram after different holding times in the bainite transformation temperature range is Figure 4 (a) 360℃ for 2h, Figure 4 (b) 360℃ for 8h, Figure 4 (c) 340℃ for 1.4h, Figure 4 (d) 340℃ for 26.7h, Figure 4 (e) 310℃ for 1.9h, Figure 4 (f) 310℃ for 7.5h, Figure 4 (g) 250℃ for 8.5h, Figure 4 (h) Keep warm at 250℃ for 18h.

[0050] Depend on Figure 5 It can be seen that the metallographic structure diagram after different holding times in the pearlite transformation temperature range is Figure 5 (a) 640℃ for 12h, Figure 5 (b) 640℃ for 24h, Figure 5 (c) 640℃ for 32h, Figure 5 (d) 640℃ for 48h, Figure 5 (e) 640℃ for 72h. Figure 6 It can be seen that the area of the metallographic structure of different heat treatment parameters is calculated, the pearlite phase variable is calculated, the material hardness is measured, and the Figure 6 (a) Pearlite transformation amount and time relationship diagram Figure 6 (b) Graph showing the relationship between hardness and time.

[0051] Depend on Figure 7 It can be seen that this is the revised CCT and TTT curve. Compared with the conventional measurement method, this method can greatly save experimental measurement time and cost for measuring high hardenability materials.

Claims

1. A method for measuring and optimizing the CCT and TTT curves of a high hardenability die steel, comprising the following steps: (1) High hardenability die steel is sequentially electrospark cut into multiple pieces, which are pretreated separately. After the preforms are obtained, vacuuming, heating, insulation and cooling are sequentially performed to obtain expansion data. The expansion data are plotted into expansion curves and data processing is performed to obtain the measured CCT and TTT curves; (2) The same high hardenability die steel raw material in step (1) is electrospark cut into multiple pieces in sequence, and each piece is pretreated separately. After the preforms are obtained, they are heat treated and characterized respectively, and the curve measured in step (1) is revised.

2. The method according to claim 1, characterized in that The pretreatment method in step (1) is: firstly grind the high hardenability die steel with a surface grinder, and then grind the surface with 200#, 600#, 800# and 1000# sandpaper in sequence to obtain a prefabricated part.

3. The preparation method according to claim 1, characterized in that The vacuum degree of the vacuum in step (1) is 1×10 -3 ~1×10 -4 pa, the heating rate is 1-2℃ / s, after heating to 1080℃, keep warm for 5 minutes, and then cool.

4. The preparation method according to claim 1, characterized in that In the step (1), the heating rate used for the CCT curve measurement is 1°C / s, the holding temperature is 1080°C, and the holding time is 5 minutes; the heating rate used for the TTT curve measurement is 1°C / s, the holding temperature is 1080°C, and the holding time is 5 minutes.

5. The preparation method according to claim 1, characterized in that In the CCT curve heat treatment experiment in step (2), the heating rate is 10°C / min, the holding temperature is 1080°C, and the holding time is 15 minutes; in the TTT curve heat treatment experiment, the heating rate is 10°C / min, the holding temperature is 1080°C, and the holding time is 15 minutes, and the furnace is quickly changed to the experimental temperature for holding.

6. The preparation method according to claim 1, characterized in that In the step (1), the cooling rates of the CCT expansion curve are 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20°C / s, respectively, and the temperature is lowered to room temperature; the cooling rate of the TTT expansion curve is 20°C / s, and the temperature is lowered to 700, 650, 600, 550, 500, 450, 400, 350, 300, 250, and 200°C, respectively, and the insulation time is 24 hours.

7. The preparation method according to claim 1, characterized in that The heating rate used in the heat treatment in step (2) is 10°C / min.

8. The preparation method according to claim 1, characterized in that The heat treatment in step (2) adopts a double furnace experiment, wherein the heat treatment is carried out in furnace No. 1 at 1080° C. for 15 minutes and then transferred to furnace No. 2 with a set heat preservation temperature.

9. The preparation method according to claim 8, characterized in that In the step (2), the insulation temperature of furnace No. 1 is 1080°C, and the insulation temperatures of furnace No. 2 are 700, 650, 600, 550, 500, 450, 400, 350, 300, 250, and 200°C respectively.

10. The preparation method according to claim 1, characterized in that The cooling rates of the CCT curves measured by the heat treatment in step (2) are 0.05, 0.04, 0.03, 0.02, and 0.01°C / s, respectively, when cooled to room temperature.

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