Hydrogen diffusion annealing method for pre-hardened plastic die steel

Abstract

According to the hydrogen diffusion annealing method for the pre-hardened plastic die steel, a cooling scheme of natural air cooling, blast air cooling or blast air cooling and natural air cooling is selected according to forging materials of different specifications, the cooling time and the final cooling temperature are limited according to the forging materials of different specifications, and the core part of the pre-hardened plastic die steel can be sufficiently supercooled; an isothermal heat preservation process at a subcritical temperature is added after high-temperature normalizing, the occurrence of structure transformation is ensured, conditions are created for effective diffusion of hydrogen, the subsequent hydrogen diffusion section can be completed only by controlling the temperature within a narrower range and preserving heat once, the hydrogen diffusion efficiency is improved, the working procedures are simplified, and the production cost is reduced. The white point defect caused by the fact that the hydrogen concentration of the local area is higher than the critical hydrogen concentration of white point formation is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogen-expanded annealing, and particularly relates to a method for hydrogen-expanded annealing of pre-hardened plastic mold steel. Background Art

[0002] Flakes are short and discontinuous internal cracks in ferrous metals. Their lengths are mostly between 1 and 12 mm. Once detected by ultrasonic testing, they will cause the steel to be scrapped or downgraded, resulting in very serious quality losses. It is generally believed that flakes are formed during the cooling process after hot pressure processing of steel under the combined action of tissue stress and changes in hydrogen solubility, and their formation temperature is lower than 200°C. Although the average hydrogen content of molten steel can be as low as 1×10 -4 % after vacuum treatment, pouring under atmospheric pressure generally increases the hydrogen content of the steel by (0.2 - 0.8)×10 -4 %. At the same time, as the size and cross-section of forged steel increase, the corresponding ingot size also increases. During the solidification process of the ingot, liquid segregation and fractional crystallization will inevitably occur, increasing the non-uniformity of the internal gas content and resulting in a higher hydrogen concentration in local areas than the critical hydrogen concentration for flake formation. Therefore, a large number of flake defects still occur in large steel ingots produced after vacuum treatment of molten steel after forging.

[0003] Pre-hardened plastic mold steel is generally medium-carbon Cr-Mo, Cr-Mn-Mo, or Cr-Mn-Ni-Mo steel. Its chemical composition range (by mass percentage) is C: 0.23 - 0.40%, Si: 0.20 - 0.50%, Mn: 0.8 - 1.60%, P ≤ 0.030%, S ≤ 0.010%, Cr: 1.20 - 2.10%, Ni: 0 - 1.2%, Mo: 0.15 - 0.68%, V: 0 - 0.15%, and the balance is Fe and other trace elements. Its process flow is successively smelting, refining, vacuum degassing, casting large steel ingots of 20 tons to 41.5 tons under atmospheric pressure, heating, forging, cooling, hydrogen-expanded annealing, and pre-hardening treatment to produce rectangular or square large-section and large-size forgings with a thickness range of 200 - 1200 mm, a width range of 900 - 1650 mm, and a length range of 1500 - 6000 mm.

[0004] According to the composition range of pre-hardened plastic mold steel, there are tissue transformations under conventional cooling conditions (slow cooling, air cooling, liquid cooling), belonging to pearlite, bainite, or even martensite steel, which are steel grades with high flake sensitivity. Therefore, for medium-carbon Cr-Mo, Cr-Mn-Mo, and Cr-Mn-Ni-Mo steel with large cross-section sizes formed by forging large steel ingots, in addition to controlling the low original hydrogen content, using the diffusibility of hydrogen under specific temperature conditions for hydrogen homogenization is the key anti-flake technical means.

[0005] To prevent white spot defects, the prior art mostly adopts the method of slow cooling or air cooling of ingots (billets) after forging or rolling to the range of 200-400 °C, and then heating up to 850-890 °C for high-temperature normalizing (part of which is an optional process). After normalizing, it is then air cooled or slow cooled to 200-550 °C, and finally heat-preserved one or more times with undulating isothermal heat preservation below the subcritical temperature A1 (generally: 550-700 °C) (see the temperature change curve in Figure 1 ). The technical details disclosed in invention patents such as "An efficient H-expansion process for producing extra-thick plates" disclosed in Chinese Patent No. 201310730255.9; "A periodic dehydrogenation annealing method for large forgings" disclosed in Chinese Patent No. 202011306608.9; "A post-forging dehydrogenation annealing method for forgings" disclosed in Chinese Patent No. 201210591515.4 and other invention patents all belong to the aforementioned prior dehydrogenation annealing methods.

[0006] Therefore, the prior art has poor applicability to forgings with large specification differences during batch production of pre-hardened plastic mold steels (the cross-sectional area of the same batch ranges from 0.18 m 2 to 1.98 m 2 ). Production practice shows that there are still many steel materials with white spot defects after dehydrogenation annealing. Summary of the Invention

[0007] The purpose of the present invention is to provide a dehydrogenation annealing method for pre-hardened plastic mold steel, which overcomes the white spot defects that are prone to occur during the dehydrogenation annealing process of pre-hardened plastic mold steel, simplifies the operation process, and reduces energy consumption.

[0008] To achieve the above purpose, the technical solution of the present invention is:

[0009] A dehydrogenation annealing method for pre-hardened plastic mold steel, comprising the following steps:

[0010] 1) Adopt the cooling methods of natural air cooling, forced air cooling or forced air cooling + natural air cooling to cool the surface temperature of the forged material after forging to 210-330 °C;

[0011] 2) Load the cooled forged material into a heat treatment furnace at 280-320 °C, and then heat it up to 860-900 °C for high-temperature normalizing at a heating rate of 40-70 °C / h. The normalizing time = (1.5-2 / 100) × t, unit h; where t is the maximum thickness of the forged material, unit mm;

[0012] 3) After the forged material after normalizing is taken out of the furnace and cooled, adopt the cooling methods of natural air cooling, forced air cooling or forced air cooling + natural air cooling, and the surface temperature of the cooled forged material is 210-330 °C;

[0013] 4) The forged material after cooling is loaded into a heat treatment furnace at 280 - 320 °C for heat preservation. The heat preservation time = (2.5 - 3 / 100) × t, unit h; where t is the maximum thickness of the forged material, unit mm. After the heat preservation ends, it is heated at a heating rate of 40 - 70 °C / h to 645 - 665 °C for heat preservation. The heat preservation time = (5 - 14 / 100) × t, unit h; where t is the maximum thickness of the forged material, unit mm.

[0014] 5) The forged material is cooled at a cooling rate of ≤30 °C / h to below 200 °C, and then taken out of the furnace and air-cooled to room temperature.

[0015] Furthermore, in steps 1) and 3), specific cooling methods are selected according to the thickness of the forged material:

[0016] When the thickness of the forged material is 200 - 400 mm, it is naturally air-cooled for 7 - 9 hours to cool the surface temperature of the forged material to 260 - 330 °C.

[0017] When the thickness of the forged material is 401 - 600 mm, it is forced-air cooled for 8 - 10 hours to cool the surface temperature of the forged material to 230 - 320 °C.

[0018] When the thickness of the forged material is 601 - 800 mm, it is forced-air cooled for 13 - 15 hours to cool the surface temperature of the forged material to 220 - 290 °C.

[0019] When the thickness of the forged material is 801 - 1200 mm, it is first forced-air cooled for 9 - 10 hours, and then naturally air-cooled for 9 - 10 hours to cool the surface temperature of the forged material to 210 - 280 °C.

[0020] The pre-hardened plastic mold steel described in the present invention is medium-carbon Cr-Mo, Cr-Mn-Mo or Cr-Mn-Ni-Mo steel.

[0021] The medium-carbon Cr-Mo, Cr-Mn-Mo, Cr-Mn-Ni-Mo steels involved in the present invention generally contain relatively more Ni, Mn, Mo elements, resulting in relatively stable supercooled austenite. The surface temperature of the forged plate needs to be supercooled to 210 - 330 °C. Subsequently, it is put into a heat treatment furnace at 280 - 320 °C to ensure that the temperature of the core area of the steel is cooled to within the B transformation temperature range. Then, it is normalized at a high temperature of 860 - 900 °C, and the normalization time is controlled according to the thickness of the forged material to ensure that the pearlite structure is fully transformed and create conditions for the effective diffusion of subsequent hydrogen, avoiding the hydrogen concentration in local areas being higher than the critical hydrogen concentration for the formation of white spots, thereby generating white spots.

[0022] To ensure that the pre-hardened plastic mold steel forging can be fully supercooled during the cooling process, the reasons or principles for adopting different cooling methods, cooling times, and surface temperatures at the end of cooling according to the different thickness specifications of the forging are as follows: On the one hand, the prerequisite for the effective diffusion of hydrogen in steel is to obtain a suitable microstructure with low hydrogen solubility and high diffusion coefficient, and this condition can only be met when the core of the forging is supercooled to the transformation temperature and below for a relatively sufficient microstructure transformation.

[0023] On the other hand, after different specifications of forgings are cooled, there is a temperature gradient between the surface and the core. The larger the thickness, the larger the surface-core temperature difference at the end of cooling, and its temperature gradient ranges from 10°C to 60°C. Correspondingly, for thick specifications, because the surface-core temperature difference is as high as 60°C, in order to ensure that the core reaches the required temperature, the surface temperature must be appropriately lower; while for thin specifications, because the surface-core temperature difference may only be 10°C, the surface temperature must be controlled appropriately higher. Therefore, different requirements must be made for the surface temperature at the end of cooling of forgings with different specifications.

[0024] Finally, the cooling rates of forgings with different thicknesses vary greatly under natural air cooling or forced air cooling conditions. Through experiments and calculations, the cooling rate of the core of a 200-mm-thick forging is about 0.045°C / s, while the cooling rate of a 1200-mm-thick forging is <0.01°C / s, with a difference close to one order of magnitude. Therefore, the cooling rates of forgings with different specifications and thicknesses need to be controlled within different ranges to make the surface-core temperature difference of the forging smaller, so that the core can also be supercooled within the supercooling temperature range of the forging treatment.

[0025] Therefore, different cooling methods, cooling times, and surface temperatures at the end of cooling should be adopted according to the different thicknesses of the forgings to ensure that the core of the forging also reaches the supercooling temperature.

[0026] After high-temperature normalizing, when the temperature is lowered to 280 - 320°C, keep it warm for a period of time according to the thickness of the forging. Mainly by using the principle of isothermal transformation of supercooled austenite, the transformation driving force for solid-state phase transformation of medium-carbon Cr-Mo, Cr-Mn-Mo, and Cr-Mn-Ni-Mo steels in the range of 280 - 320°C (slightly higher than the Ms temperature) is the smallest, which can make the microstructure transformation easier to proceed fully and create the most favorable conditions for subsequent hydrogen diffusion. Normally, or the existing technical solutions do not have this operation.

[0027] Since the present invention ensures the full progress of the steel microstructure transformation in the foregoing steps and creates good conditions for subsequent hydrogen diffusion, the treatment temperature in the subsequent hydrogen diffusion stage can be further optimized to ensure that it can be carried out within a relatively narrow temperature range of 645 - 655°C, obtaining the best hydrogen diffusion efficiency, avoiding the hydrogen concentration in local areas being higher than the critical hydrogen concentration for the formation of white spots, and thus generating white spots.

[0028] Compared with the prior art, the beneficial effects of the present invention:

[0029] The present invention adjusts the cooling control method and time parameters for pre-hardened plastic die steels with different specification group intervals after forging in the existing hydrogen-expanding annealing technology, and adds an isothermal holding process at a subcritical temperature after high-temperature normalizing (see the temperature change curve in Figure 2 ), ensuring the full transformation of the steel structure and creating conditions for the effective diffusion of hydrogen. In the subsequent hydrogen-expanding section, only one heat preservation at a temperature controlled within a narrow range is required to complete the process, improving the hydrogen-expanding efficiency, simplifying the process, and avoiding the hydrogen concentration in local areas being higher than the critical hydrogen concentration for the formation of white spots, thereby preventing white spot defects. The qualified rate of ultrasonic flaw detection of the finished forged materials obtained by the technical solution of the present invention (at the highest quality level according to GB / T 6402) has increased by more than 25% compared with the existing technical solution, basically eliminating the occurrence of white spot defects, and significantly improving the quality level of the forged materials. In the existing technology, after high-temperature normalizing, the process of cooling down to below the subcritical temperature multiple times and then heating up for isothermal holding is adopted, without considering the recovery of the structure after high-temperature normalizing, and the operation is also more cumbersome.

[0030] After forging and high-temperature normalizing, the present invention selects a cooling scheme of natural air cooling, forced air cooling or forced air cooling + natural air cooling according to the forged materials of different specifications, and limits the cooling time and final cooling temperature according to the forged materials of different specifications, which can make the core of the pre-hardened plastic die steel undergo sufficient undercooling and ensure the occurrence of structure transformation. In the existing hydrogen-expanding annealing methods mentioned, only the time multipliers of normalizing heat preservation and subcritical temperature heat preservation are adjusted according to the thickness or equivalent thickness of the finished steel, without considering the control of the cooling time and the surface temperature at the end of cooling after forging or high-temperature normalizing for forged materials of different specifications.

[0031] Moreover, for ultra-large cross-section steel such as pre-hardened plastic die steel, in the slow cooling or air cooling + slow cooling processes adopted after forging or high-temperature normalizing in the existing technology, under such cooling conditions, the average cooling rate of the steel ≤ 15 °C / h (≤ 10 °C / h below 400 °C), and only the cooling process takes a large amount of time, making the entire annealing time too long.

[0032] The present invention uses a cooling scheme of natural air cooling, forced air cooling or forced air cooling + natural air cooling, which can reduce the cooling treatment time after forging and normalizing by at least more than 50%. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the temperature control scheme of the annealing process in the existing technology.

[0034] Figure 2 It is a schematic diagram of the temperature control scheme of the annealing process of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] The present invention will be further described below in conjunction with embodiments.

[0036] Example 1

[0037] Taking a 20-ton atmospheric casting ingot forged from a certain pre-hardened plastic mold steel into a 260 mm (thickness) × 1560 mm (width) × length (as long as possible) as an example, the chemical composition by mass percentage is: C: 0.33%, Si: 0.29%, Mn: 1.26%, P ≤ 0.012%, S ≤ 0.002%, Cr: 1.85%, Ni: 1.08%, Mo: 0.26%, and the balance is Fe and unavoidable impurities; the specific steps of the hydrogen-expansion annealing method are as follows:

[0038] 1) Place the forged rectangular cross-sectioned material with a size of 270 mm (thickness) × 1585 mm (width) × 5000 mm (length) on the padding iron and cool it in a static air environment for 7 hours and 38 minutes. The measured surface end temperature is 294 - 319 °C;

[0039] 2) Load the cooled forged material into a heat treatment furnace at a temperature of 280 °C and raise the temperature (average heating rate ≈ 48.3 °C / h) to 860 °C for high-temperature normalizing in 12 hours. The normalizing time is 4.5 hours;

[0040] 3) After the normalized forged material is taken out of the furnace, place it on the padding iron and cool it in a static air environment for 8 hours and 6 minutes. The measured surface end temperature is 291 - 304 °C;

[0041] 4) Load the forged material cooled in step 3) into a heat treatment furnace set at a temperature of 280 °C for heat preservation. The heat preservation time is 8.5 hours. After the heat preservation ends, raise the temperature to 645 °C in 8.5 hours (average heating rate ≈ 42.94 °C / h) and hold for 17 hours;

[0042] 5) Cool the forged material in the furnace for 31.18 hours (average cooling rate ≈ 14.85 °C / h) until it is cooled below 200 °C, and then take the forged material out of the furnace and air-cool it.

[0043] After the 270 mm (thickness) × 1585 mm (width) × 5000 mm (length) rectangular cross-sectioned forged material undergoes hydrogen-expansion annealing as described above, it is quenched at 860 °C (quenching oil) + tempered at 560 °C, and then ultrasonic flaw detection is carried out according to the GB / T6402 standard. It is qualified in one time, and its quality grade meets quality grade 4 (the highest quality grade).

[0044] Example 2

[0045] Taking the forging of a 41.5-ton atmospheric casting ingot of a certain pre-hardened plastic mold steel into a rectangular section with a thickness of 800 mm, a width of 1360 mm, and a length (as long as possible) as an example, its chemical composition by mass percentage is C: 0.34%, Si: 0.27%, Mn: 1.38%, P ≤ 0.014%, S ≤ 0.003%, Cr: 1.84%, Ni: 0.68%, Mo: 0.29%, and the balance is Fe and inevitable impurities; the specific steps of the hydrogen-expansion annealing method are as follows:

[0046] 1) Place the forged rectangular-section forging with a thickness of 790 mm, a width of 1370 mm, and a length of 4000 mm on the padding iron, and air-cool it by blowing for 13.5 hours. Measure its surface end temperature to be 271.3 - 286.1 °C;

[0047] 2) Load the cooled forging into a heat treatment furnace set at a temperature of 300 °C, and raise the temperature to 890 °C for normalizing at a high temperature in 14.5 hours (average heating rate ≈ 40.69 °C / h), and the normalizing time is 15 hours;

[0048] 3) After the normalized forging is taken out of the furnace, place it on the padding iron, and air-cool it by blowing for 14 hours and 20 minutes. Measure its surface end temperature to be 234.4 - 247.8 °C;

[0049] 4) Load the forging cooled in step 3) into a heat treatment furnace set at a temperature of 300 °C for heat preservation. The heat preservation time is 25 hours. After the heat preservation ends, raise the temperature to 665 °C for heat preservation in 7 hours (average heating rate ≈ 52.14 °C / h), and the heat preservation time is 98 hours;

[0050] 5) Cool the forging in the furnace for 42 hours (average cooling rate ≈ 10.86 °C / h) and then cool it to below 200 °C, and take the forging out of the furnace for air cooling.

[0051] After the above hydrogen-expansion annealing, the rectangular-section forging with a thickness of 790 mm, a width of 1370 mm, and a length of 4000 mm is quenched at 860 °C (water cooling + oil cooling) + tempered at 580 °C, and then ultrasonic flaw detection is carried out according to the GB / T 6402 standard, and it is qualified at one time, and its quality grade meets quality grade 4 (the highest quality grade).

[0052] Example 3

[0053] Taking the forging of a 41.5-ton atmospheric casting ingot of a certain pre-hardened plastic mold steel into a rectangular section with a thickness of 510 mm, a width of 1460 mm, and a length (as long as possible) as an example, the mass percentages of its chemical components are: C: 0.34%, Si: 0.27%, Mn: 1.38%, P ≤ 0.014%, S ≤ 0.003%, Cr: 1.84%, Ni: 0.68%, Mo: 0.29%, and the balance is Fe and inevitable impurities; the specific steps of the hydrogen-expansion annealing method are as follows:

[0054] 6) Place the forged rectangular-section forging with a thickness of 510 mm, a width of 1460 mm, and a length of 4000 mm on the padding iron, and blow air for air cooling for 9 hours and 30 minutes. Measure its surface end temperature to be 235.3 - 246.0 °C;

[0055] 7) Load the cooled forging into a heat treatment furnace set at a temperature of 310 °C, and raise the temperature to 870 °C for normalizing at a high temperature in 9.5 hours (average heating rate ≈ 58.95 °C / h), and the normalizing time is 9 hours;

[0056] 8) After the normalized forging is taken out of the furnace, place it on the padding iron, and blow air for air cooling for 8 hours and 20 minutes. Measure its surface end temperature to be 244.4 - 257.8 °C;

[0057] 9) Load the forging cooled in step 3) into a heat treatment furnace set at a temperature of 310 °C for heat preservation. The heat preservation time is 12 hours. After the heat preservation ends, raise the temperature to 660 °C for heat preservation in 7.5 hours (average heating rate ≈ 46.67 °C / h), and the heat preservation time is 71 hours;

[0058] 10) Cool the forging in the furnace for 21 hours (average cooling rate ≈ 21.91 °C / h) and then cool it below 200 °C, and take the forging out of the furnace for air cooling.

[0059] After the hydrogen-expansion annealing, the rectangular-section forging with a thickness of 810 mm, a width of 1370 mm, and a length of 4000 mm is quenched at 860 °C (water cooling + oil cooling) + tempered at 580 °C, and then ultrasonic flaw detection is carried out according to the GB / T 6402 standard, and it is qualified at one time, and its quality grade meets quality grade 4 (the highest quality grade).

[0060] Example 4

[0061] Taking the forging of a 41.5-ton atmospheric casting ingot of a certain pre-hardened plastic mold steel into a rectangular section with a thickness of 1100 mm, a width of 1270 mm, and a length (as long as possible) as an example, the mass percentages of its chemical components are as follows: C: 0.26%, Si: 0.30%, Mn: 1.45%, P ≤ 0.008%, S ≤ 0.003%, Cr: 1.31%, Ni: 0.97%, Mo: 0.58%, V: 0.10%, and the balance is Fe and unavoidable impurities. The specific steps of the hydrogen-expansion annealing method are as follows:

[0062] 1) Place the forged rectangular-section forging with a thickness of 1120 mm, a width of 1290 mm, and a length of 3100 mm on the padding iron, first blow air for air cooling for 9 hours and 40 minutes, and then cool it for another 9 hours and 36 minutes in a static air environment after stopping the blower. The measured surface end temperature is 265.4 - 279.8 °C;

[0063] 2) Load the cooled forging into a heat treatment furnace set at a temperature of 320 °C, and raise the temperature to 900 °C for high-temperature normalizing in 8.5 hours (average heating rate ≈ 68.24 °C / h), and the normalizing time is 17 hours;

[0064] 3) After the normalized forging is taken out of the furnace and placed on the padding iron, first blow air for air cooling for 9 hours and 34 minutes, and then cool it for another 9 hours and 50 minutes in static air. The measured surface end temperature is 219.3 - 246.7 °C;

[0065] 4) Load the forging processed in step 3) into a heat treatment furnace set at a temperature of 320 °C and stay for 34 hours. After the heat preservation ends, raise the temperature to 650 °C for heat preservation in 8.5 hours (average heating rate ≈ 41.18 °C / h), and the heat preservation time is 132 hours;

[0066] 5) Cool the forging in the furnace for 49 hours (average cooling rate ≈ 9.33 °C / h) and then cool it to below 200 °C, and then take the forging out of the furnace for air cooling.

[0067] After the hydrogen-expansion annealing of the rectangular-section forging with a thickness of 1120 mm, a width of 1290 mm, and a length of 3100 (length), it is quenched at 860 °C (medium cooling) + tempered at 550 °C, and then ultrasonic flaw detection is carried out according to the GB / T 6402 standard, and it is qualified at one time, and its quality grade meets quality grade 4 (the highest quality grade).

[0068] The above Examples 1, 2, 3, and 4 are only examples of the technical solutions of the present invention. The cross-sectional areas of the products produced according to the technical solutions of the present invention are 0.18 m 2 ~1.98 m 2Over 2,000 pieces of medium-carbon Cr-Mo, Cr-Mn-Mo, Cr-Mn-Ni-Mo rectangular or square-section forgings with high white spot sensitivity within the specified range are produced. The finished products obtained are flaw-detected after pre-hardening in accordance with the highest quality grade of the GB / T 6402 standard, and the qualified rate of products without white spot defects reaches 97.99%.

[0069] However, for thousands of forgings produced according to the annealing treatment technical solution of the existing technology, more than 20% of the products are found to have white spot defects after flaw detection.

Claims

1. A method for dehydrogenation annealing of pre-hardened plastic mold steel, characterized in that, It includes the following steps: 1) Adopt a cooling method of natural air cooling, forced air cooling or forced air cooling + natural air cooling to cool the surface temperature of the forged material after forging to 210 - 330 °C; 2) Load the cooled forged material into a heat treatment furnace at 280 - 320 °C, and then heat it up to 860 - 900 °C for normalizing at a heating rate of 40 - 70 °C / h. The normalizing time = (1.5 - 2 / 100) × t, unit h; where t is the maximum thickness of the forged material, unit mm; 3) After the normalized forged material is taken out of the furnace and cooled, adopt a cooling method of natural air cooling, forced air cooling or forced air cooling + natural air cooling. The surface temperature of the cooled forged material is 210 - 330 °C; 4) Load the forged material after the second cooling into a heat treatment furnace at 280 - 320 °C for heat preservation. The heat preservation time = (2.5 - 3 / 100) × t, unit h; where t is the maximum thickness of the forged material, unit mm; After the heat preservation ends, heat it up to 645 - 665 °C for heat preservation at a heating rate of 40 - 70 °C / h. The heat preservation time = (5 - 14 / 100) × t, unit h; where t is the maximum thickness of the forged material, unit mm; 5) Cool the forged material at a cooling rate of ≤ 30 °C / h to below 200 °C, and then take it out of the furnace and air cool it to room temperature.

2. The hydrogen-expansion annealing method for pre-hardened plastic mold steel according to claim 1, characterized in that, In step 1) and step 3), select the specific cooling method according to the thickness of the forged material: When the thickness of the forged material is 200 - 400 mm, cool the surface temperature of the forged material to 260 - 330 °C by natural air cooling for 7 - 9 hours; When the thickness of the forged material is 401 - 600 mm, cool the surface temperature of the forged material to 230 - 320 °C by forced air cooling for 8 - 10 hours; When the thickness of the forged material is 601 - 800 mm, cool the surface temperature of the forged material to 220 - 290 °C by forced air cooling for 13 - 15 hours; When the thickness of the forged material is 801 - 1200 mm, first cool it by forced air cooling for 9 - 10 hours, and then cool it by natural air cooling for 9 - 10 hours to cool the surface temperature of the forged material to 210 - 280 °C.

3. The method for hydrogen-expansion annealing of pre-hardened plastic mold steel according to claim 1 or 2, characterized in that, The pre-hardened plastic mold steel is medium carbon Cr-Mo, Cr-Mn-Mo or Cr-Mn-Ni-Mo steel.

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