Heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings

Abstract

The present invention discloses a heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings, which is normalizing - primary quenching - secondary quenching - tempering. By adopting the heat treatment process of the present invention, all indexes of the mechanical properties of the forgings meet the acceptance requirements, solving the problem that the yield strength of 12CrNi5MoV steel forgings and the fibrous rate of the impact fracture in the -20 °C impact test cannot meet the acceptance requirements simultaneously. It is applicable to 12CrNi5MoV steel forgings with an effective thickness ≤ 350 mm, and the process is simple and highly controllable.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgical heat treatment, and particularly relates to a heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings. Background Art

[0002] With the rapid development and wide application of the shipbuilding industry, the research on shipbuilding materials has been gradually deepened and improved. Due to its good hardenability, the 12CrNi5MoV steel forging can obtain ultra-high strength and excellent low-temperature impact toughness after heat treatment, with excellent comprehensive mechanical properties and superior welding performance. It is an ideal material for manufacturing ultra-high strength structural parts of ships.

[0003] When the 12CrNi5MoV steel forging is applied to the manufacture of ultra-high strength structural parts of ships, the acceptance requirements for its mechanical property indicators are as follows: the yield strength R eL is 785 MPa to 925 MPa, the tensile strength R m is not required, the elongation A ≥ 16%, and the reduction of area Z ≥ 45%; in the -20°C impact test, the average impact absorption energy A KV of each group of three specimens ≥ 80 J, any single value ≥ 73 J, and the fiber rate FA of the impact fracture surface ≥ 90% (as shown in Table 1). Among them, the tensile test is carried out according to the provisions of GB / T228.1-2010 (Metallic materials - Tensile testing at ambient temperature), and the tensile specimen is a R4 circular cross-section proportional specimen cut after 1 / 2 of the maximum thickness of the forging and prepared according to the provisions of GB / T228.1-2010; the impact test is carried out according to the provisions of GB / T229-2020 (Metallic materials - Charpy pendulum impact test), and the impact specimen is a standard Charpy V-notch impact specimen cut after 1 / 2 of the maximum thickness of the forging and prepared according to the provisions of GB / T229-2020. The evaluation of the fiber rate of the impact fracture surface is carried out according to the provisions of GB / T12778-2008 (Metallic materials - Determination of Charpy impact fracture surface).

[0004] Table 1 Acceptance requirements for mechanical property indicators of 12CrNi5MoV steel forgings

[0005]

[0006] During the development process of applying the 12CrNi5MoV steel forging to ultra-high strength structural parts of ships, the acceptance of mechanical properties is a huge challenge. Due to the particularly high requirements for its comprehensive mechanical property indicators, the yield strength and the fiber rate of the impact fracture surface cannot meet the acceptance requirements simultaneously. It is necessary to study and design an optimized heat treatment process to improve the internal structure of the forging and enhance its comprehensive mechanical properties so that all mechanical property indicators meet the acceptance requirements.

[0007] The "Manufacturing Method of 12CrNi5MoV Low-carbon High-strength Alloy Steel Shaft Forgings" is disclosed in Publication No. CN109468447A (Application No. CN201811268019). The heat treatment process adopts the method of double quenching with water cooling through + tempering. The properties of the 12CrNi5MoV low-carbon high-strength alloy steel shaft forgings manufactured by this invention can reach the following acceptance standards:

[0008] R eL is 785 MPA to 925 MPA, A ≥ 16%, Z ≥ 45%, A KV (-20 °C) ≥ 80 J, which can meet the manufacturing requirements of ultra-high-strength parts of ships. However, the effective thickness of the forgings to which it applies is not mentioned, nor is the evaluation of the fibrous rate of the impact fracture of the forgings recorded. Comparative experiment three of this invention shows that its mechanical property acceptance is only applicable to 12CrNi5MoV forged steel parts with an effective thickness not exceeding 150 mm, and the fibrous rate of the impact fracture cannot meet the acceptance requirements of this invention (see Table 5).

[0009] The comparative document "Research on the Microstructure and Mechanical Properties of 12CrNi5MoV Steel Forgings with Ultra-thick Wall" (Volume 39, Issue 21 of "Hot Working Technology", pages 31 - 33, publication date November 30, 2010) introduced the heat treatment process of 12CrNi5MoV steel forgings with a wall thickness of 230 mm. Through normalizing + quenching and tempering, mechanical property data meeting the acceptance standards of this research were obtained. However, this document also did not provide the evaluation of the fibrous rate of the impact fracture of the forgings. Comparative experiment four of this invention shows that its mechanical property acceptance is only applicable to 12CrNi5MoV forged steel parts with an effective thickness not exceeding 250 mm, and the fibrous rate of the impact fracture cannot meet the acceptance requirements of this invention (see Table 6). Summary of the Invention

[0010] The purpose of this invention is to overcome the deficiencies of the prior art and provide a heat treatment process with simple process, which can effectively improve the internal structure of the forgings, refine the grains, and improve the comprehensive mechanical properties of 12CrNi5MoV steel forgings.

[0011] To achieve the above purpose, this invention provides a heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings. By weight percentage, the chemical composition of the 12CrNi5MoV steel forgings includes C: 0.10% - 0.13%, Si: 0.20% - 0.40%, Mn: 0.30% - 0.60%, P ≤ 0.015%, S ≤ 0.010%, Cr: 0.90% - 1.20%, Ni: 4.50% - 5.00%, Mo: 0.40% - 0.60%, V: 0.03% - 0.08%, Cu ≤ 0.25%;

[0012] The heat treatment process is as follows:

[0013] 1) Normalizing: First, heat the steel forging to 600°C - 650°C and hold for 2 - 3 hours, then raise the temperature to 820°C - 860°C for equalizing temperature and hold again. After taking it out of the furnace, use a blower for air cooling to room temperature;

[0014] 2) First quenching: Heat the normalized steel forging to 600°C - 650°C and hold for 2 - 3 hours, then raise the temperature to 930°C - 970°C for equalizing temperature and hold again. After taking it out of the furnace, water cool it to below 250°C;

[0015] 3) Second quenching: Heat the steel forging after the first quenching to 600°C - 650°C and hold for 2 - 3 hours, then raise the temperature to 820°C - 860°C for equalizing temperature and hold again. After taking it out of the furnace, water cool it to below 250°C;

[0016] 4) Tempering: Heat the forging after the second quenching to 300°C - 350°C and hold for 2 - 3 hours, then raise the temperature to 580°C - 600°C for equalizing temperature and hold again. After taking it out of the furnace, water cool it to below 250°C.

[0017] Further, in the step 1), raise the temperature to 830°C - 840°C.

[0018] Further, in the step 2), raise the temperature to 950°C - 960°C.

[0019] Further, in the step 3), raise the temperature to 840°C - 850°C.

[0020] Further, in the steps 1), 2) and 3), the holding time for the second time is calculated as 0.8 - 1.2 h / 100 mm, and the equalizing temperature time is 1 / 3 - 1 / 2 of the holding time.

[0021] Further, in the step 4), the holding time for the second time is calculated as 1.5 - 2.5 h / 100 mm, and the equalizing temperature time is 1 / 3 - 1 / 2 of the holding time.

[0022] Further, the effective thickness of the steel forging is not more than 350 mm.

[0023] For the 12CrNi5MoV steel forging heat - treated by the present invention, the comprehensive mechanical properties are significantly improved: the yield strength R eL in the tensile test is 785 MPa - 925 MPa, the tensile strength R m ≥920 MPa (not subject to acceptance requirements), the elongation A≥16%, the reduction of area Z≥45%; in the - 20°C impact test, the impact absorption energy A KV of each group of three specimens is ≥80 J, and any single value ≥73 J, and the fibrous rate of the impact fracture surface FA≥90%.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: By adopting the heat treatment process of the present invention, all indexes of the mechanical properties of the forgings meet the acceptance requirements, solving the problem that the yield strength of 12CrNi5MoV steel forgings and the fibrous rate of the impact fracture in the -20°C impact test cannot meet the acceptance requirements simultaneously. It is applicable to 12CrNi5MoV steel forgings with an effective thickness ≤ 350 mm, and the process is simple and highly controllable. Detailed implementation manners

[0025] The following further elaborates on the present invention in conjunction with specific embodiments for a clearer understanding of the present invention, but they do not limit the present invention.

[0026] Embodiment 1

[0027] Heat treat a 12CrNi5MoV steel forging with an effective thickness of 100 mm. Its chemical composition (%, wt, melting analysis) is as follows: C: 0.13%, Si: 0.28%, Mn: 0.54%, P: 0.007%, S: 0.005%, Cr: 1.02%, Ni: 4.82%, Mo: 0.42%, V: 0.05%, Cu: 0.20%.

[0028] Adopt a heat treatment process of normalizing + double quenching + tempering.

[0029] 1) Normalizing: First, heat the forging to 600°C - 650°C and hold for 2 hours, then raise the temperature to 820°C for equalizing for 0.5 hours and then hold for 1 hour. After taking it out of the furnace, use a blower for air cooling to room temperature;

[0030] 2) First quenching: Heat the normalized forging to 600°C - 650°C and hold for 2 hours, then raise the temperature to 930°C for equalizing for 0.5 hours and then hold for 1 hour. After taking it out of the furnace, water cool it to below 250°C;

[0031] 3) Second quenching: Heat the forging after the first quenching to 600°C - 650°C and hold for 2 hours, then raise the temperature to 820°C for equalizing for 0.5 hours and then hold for 1 hour. After taking it out of the furnace, water cool it to below 250°C;

[0032] 4) Tempering: Heat the forging after the second quenching to 300°C - 350°C and hold for 2 hours, then raise the temperature to 580°C for equalizing for 1 hour and then hold for 2 hours again. After taking it out of the furnace, water cool it to below 250°C.

[0033] The test results of the mechanical properties of the 12CrNi5MoV steel forging heat treated in this embodiment are as follows: The yield strength R of the tensile test eL = 897 MPa, the tensile strength R m= 949 MPa, elongation A = 20%, reduction of area Z = 70%; the impact absorption energy A of three specimens in the same group during the -20 °C impact test KV was 187 J, 191 J, and 174 J respectively, and the fibrous rate of the impact fracture was 100%.

[0034] Example 2

[0035] Heat-treat a 12CrNi5MoV steel forging with an effective thickness of 150 mm. Its chemical composition (%, wt, melting analysis) is as follows: C: 0.11%, Si: 0.23%, Mn: 0.47%, P: 0.007%, S: 0.007%, Cr: 1.06%, Ni: 4.65%, Mo: 0.51%, V: 0.05%, Cu: 0.13%.

[0036] Adopt a heat treatment process of normalizing + double quenching + tempering.

[0037] 1) Normalizing: First, heat the forging to 600 °C - 650 °C and hold for 2 hours, then raise the temperature to 840 °C for equalizing for 0.75 hours and then hold for 1.5 hours. After taking it out of the furnace, use a blower for air cooling to room temperature;

[0038] 2) First quenching: Heat the normalized forging to 600 °C - 650 °C and hold for 2 hours, then raise the temperature to 940 °C for equalizing for 0.75 hours and then hold for 1.5 hours. After taking it out of the furnace, water-cool it to below 250 °C;

[0039] 3) Second quenching: Heat the forging after the first quenching to 600 °C - 650 °C and hold for 2 hours, then raise the temperature to 830 °C for equalizing for 0.75 hours and then hold for 1.5 hours. After taking it out of the furnace, water-cool it to below 250 °C;

[0040] 4) Tempering: Heat the forging after the second quenching to 300 °C - 350 °C and hold for 2 hours, then raise the temperature to 590 °C for equalizing for 1.5 hours and then hold for 3 hours. After taking it out of the furnace, water-cool it to below 250 °C.

[0041] The mechanical property test results of the 12CrNi5MoV steel forging heat-treated in this example are as follows: the yield strength R of the tensile test eL = 882 MPa, the tensile strength R m = 944 MPa, elongation A = 19%, reduction of area Z = 68%; the impact absorption energy A of three specimens in the same group during the -20 °C impact test KV was 181 J, 188 J, and 196 J respectively, and the fibrous rates of the impact fracture were 90%, 100%, and 100% respectively.

[0042] Example 3

[0043] Heat-treat 12CrNi5MoV steel forgings with an effective thickness of 200 mm. Its chemical composition (%, wt, melting analysis) is as follows: C: 0.13%, Si: 0.29%, Mn: 0.54%, P: 0.007%, S: 0.005%, Cr: 1.07%, Ni: 4.82%, Mo: 0.42%, V: 0.05%, Cu: 0.20%.

[0044] Adopt a heat treatment process of normalizing + double quenching + tempering.

[0045] 1) Normalizing: First, heat the forgings to 600°C - 650°C and hold for 2 hours, then raise the temperature to 850°C for uniform temperature for 1 hour and then hold for 2 hours. After taking out of the furnace, use a blower for air cooling to room temperature;

[0046] 2) First quenching: Heat the forgings after normalizing to 600°C - 650°C and hold for 2 hours, then raise the temperature to 930°C for uniform temperature for 1 hour and then hold for 2 hours. After taking out of the furnace, water cool to below 250°C;

[0047] 3) Second quenching: Heat the forgings after the first quenching to 600°C - 650°C and hold for 2 hours, then raise the temperature to 850°C for uniform temperature for 1 hour and then hold for 2 hours. After taking out of the furnace, water cool to below 250°C;

[0048] 4) Tempering: Heat the forgings after the second quenching to 300°C - 350°C and hold for 2 hours, then raise the temperature to 590°C for uniform temperature for 2 hours and then hold for 4 hours. After taking out of the furnace, water cool to below 250°C.

[0049] The mechanical property test results of the 12CrNi5MoV steel forgings heat-treated in this example are as follows: For the tensile test, the yield strength R el = 870 MPa, the tensile strength R m = 927 MPa, the elongation A = 20%, and the reduction of area Z = 70%; In the -20°C impact test, the impact absorption work A KV of the three specimens in the same group are 187 J, 178 J, and 177 J respectively, and the fibrous rates of the impact fracture surfaces are 100%, 100%, and 90% respectively.

[0050] Example 4

[0051] Heat-treat 12CrNi5MoV steel forgings with an effective thickness of 250 mm. Its chemical composition (%, wt, melting analysis) is as follows: C: 0.12%, Si: 0.29%, Mn: 0.52%, P: 0.007%, S: 0.005%, Cr: 1.06%, Ni: 4.87%, Mo: 0.40%, V: 0.05%, Cu: 0.20%.

[0052] The heat treatment process of normalizing + double quenching + tempering is adopted.

[0053] 1) Normalizing: First, heat the forging to 600°C - 650°C and hold for 3 hours, then raise the temperature to 850°C for homogenization for 1.25 hours and then hold for 2.5 hours. After taking it out of the furnace, use a blower for air cooling to room temperature;

[0054] 2) First quenching: Heat the forging after normalizing to 600°C - 650°C and hold for 3 hours, then raise the temperature to 950°C for homogenization for 1.25 hours and then hold for 2.5 hours. After taking it out of the furnace, water cool it to below 250°C;

[0055] 3) Second quenching: Heat the forging after the first quenching to 600°C - 650°C and hold for 3 hours, then raise the temperature to 840°C for homogenization for 1.25 hours and then hold for 2.5 hours. After taking it out of the furnace, water cool it to below 250°C;

[0056] 4) Tempering: Heat the forging after the second quenching to 300°C - 350°C and hold for 3 hours, then raise the temperature to 590°C for homogenization for 2.5 hours and then hold for 5 hours. After taking it out of the furnace, water cool it to below 250°C.

[0057] The mechanical property test results of the 12CrNi5MoV steel forging in this embodiment of heat treatment are as follows: For the tensile test, the yield strength R eL = 871 MPa, the tensile strength R m = 931 MPa, the elongation A = 19%, and the reduction of area Z = 70%; In the -20°C impact test, the impact absorption energies A KV of the three specimens in the same group are 195 J, 196 J, and 188 J respectively, and the fiber ratios of the impact fracture surfaces are 100%, 100%, and 90% respectively.

[0058] Example 5

[0059] For the heat treatment of the 12CrNi5MoV steel forging with an effective thickness of 300 mm, its chemical composition (%, wt, melting analysis) is as follows: C: 0.11%, Si: 0.27%, Mn: 0.54%, P: 0.007%, S: 0.005%, Cr: 1.07%, Ni: 4.68%, Mo: 0.40%, V: 0.05%, Cu: 0.21%.

[0060] The heat treatment process of normalizing + double quenching + tempering is adopted.

[0061] 1) Normalizing: First, heat the forging to 600°C - 650°C and hold for 3 hours, then raise the temperature to 850°C for homogenization for 1.5 hours and then hold for 3 hours. After taking it out of the furnace, use a blower for air cooling to room temperature;

[0062] 2) Primary quenching: Heat the forging after normalizing to 600°C - 650°C and hold for 3 hours, then raise the temperature to 960°C for uniform temperature for 1.5 hours and then hold for 3 hours, and water-cool to below 250°C after taking out of the furnace;

[0063] 3) Secondary quenching: Heat the forging after primary quenching to 600°C - 650°C and hold for 3 hours, then raise the temperature to 860°C for uniform temperature for 1.5 hours and then hold for 3 hours, and water-cool to below 250°C after taking out of the furnace;

[0064] 4) Tempering: Heat the forging after secondary quenching to 300°C - 350°C and hold for 3 hours, then raise the temperature to 600°C for uniform temperature for 3 hours and then hold for 6 hours, and water-cool to below 250°C after taking out of the furnace.

[0065] The mechanical property test results of the 12CrNi5MoV steel forging heat-treated in this example are as follows: For the tensile test, the yield strength R eL = 854 MPa, the tensile strength R m = 920 MPa, the elongation A = 18%, and the reduction of area Z = 65%; In the -20°C impact test, the impact absorption energies A KV of the three specimens in the same group are 155 J, 153 J, and 147 J respectively, and the fiber ratios of the impact fracture surfaces are 100%, 90%, and 90% respectively.

[0066] Example 6

[0067] Heat-treat the 12CrNi5MoV steel forging with an effective thickness of 350 mm, and its chemical composition (%, wt, melting analysis) is as follows: C: 0.13%, Si: 0.25%, Mn: 0.55%, P: 0.006%, S: 0.007%, Cr: 1.20%, Ni: 4.88%, Mo: 0.42%, V: 0.05%, Cu: 0.23%.

[0068] Adopt the heat treatment process of normalizing + double quenching + tempering.

[0069] 1) Normalizing: First heat the forging to 600°C - 650°C and hold for 3 hours, then raise the temperature to 860°C for uniform temperature for 1.75 hours and then hold for 3.5 hours, and cool to room temperature by blowing air with a blower after taking out of the furnace;

[0070] 2) Primary quenching: Heat the forging after normalizing to 600°C - 650°C and hold for 3 hours, then raise the temperature to 970°C for uniform temperature for 1.75 hours and then hold for 3.5 hours, and water-cool to below 250°C after taking out of the furnace;

[0071] 3) Secondary quenching: Heat the forgings after primary quenching to 600°C - 650°C and hold for 3 hours, then raise the temperature to 860°C for uniform temperature for 1.75 hours and then hold for 3.5 hours. After taking out of the furnace, water-cool to below 250°C;

[0072] 4) Tempering: Heat the forgings after secondary quenching to 300°C - 350°C and hold for 3 hours, then raise the temperature to 600°C for uniform temperature for 3.5 hours and then hold for 7 hours. After taking out of the furnace, water-cool to below 250°C.

[0073] The mechanical property test results of the 12CrNi5MoV steel forgings heat-treated in this example are as follows: The yield strength R eL of the tensile test = 858 MPa, the tensile strength R m = 922 MPa, the elongation A = 20%, and the reduction of area Z = 69%; In the -20°C impact test, the impact absorption energy A KV of the three specimens in the same group are 157 J, 159 J, and 159 J respectively, and the fiber ratios of the impact fracture surfaces are 90%, 100%, and 100% respectively.

[0074] Comparative Example 1

[0075] Cancel Step 1), and the rest is the same as Example 6.

[0076] For the heat treatment of 12CrNi5MoV steel forgings with an effective thickness of 350 mm, its chemical composition (%, wt, melting analysis) is as follows: C: 0.13%, Si: 0.25%, Mn: 0.55%, P: 0.006%, S: 0.007%, Cr: 1.20%, Ni: 4.88%, Mo: 0.42%, V: 0.05%, Cu: 0.23%.

[0077] Adopt the heat treatment process of double quenching + tempering.

[0078] 1) Primary quenching: First, heat the forgings to 600°C - 650°C and hold for 3 hours, then raise the temperature to 970°C for uniform temperature for 1.75 hours and then hold for 3.5 hours. After taking out of the furnace, water-cool to below 250°C;

[0079] 2) Secondary quenching: Heat the forgings after primary quenching to 600°C - 650°C and hold for 3 hours, then raise the temperature to 860°C for uniform temperature for 1.75 hours and then hold for 3.5 hours. After taking out of the furnace, water-cool to below 250°C;

[0080] 3) Tempering: Heat the forgings after secondary quenching to 300°C - 350°C and hold for 3 hours, then raise the temperature to 600°C for uniform temperature for 3.5 hours and then hold for 7 hours. After taking out of the furnace, water-cool to below 250°C.

[0081] The mechanical property test results of the 12CrNi5MoV steel forging heat-treated in Comparative Example 1 are as follows: the yield strength R of the tensile test eL = 780 MPa, the tensile strength R m = 905 MPa, the elongation A = 16%, and the reduction of area Z = 56%; in the -20 °C impact test, the impact absorption energies A KV of the three specimens in the same group are 96 J, 101 J, and 99 J respectively, and the fibrous rates of the impact fracture surfaces are all 40%.

[0082] Comparative Example 2

[0083] Step 2) is cancelled, and the rest is the same as in Example 6.

[0084] The 12CrNi5MoV steel forging is heat-treated. The effective thickness is 350 mm, and its chemical composition (%, wt, melting analysis) is as follows: C: 0.13%, Si: 0.25%, Mn: 0.55%, P: 0.006%, S: 0.007%, Cr: 1.20%, Ni: 4.88%, Mo: 0.42%, V: 0.05%, Cu: 0.23%.

[0085] The heat treatment process of normalizing + quenching + tempering is adopted.

[0086] 1) Normalizing: First, heat the forging to 600 °C - 650 °C and hold for 3 hours, then raise the temperature to 860 °C, hold for 1.75 hours for equalizing temperature, and then hold for 3.5 hours. After taking it out of the furnace, use a blower for air cooling to room temperature;

[0087] 2) Quenching: Heat the forging after normalizing to 600 °C - 650 °C and hold for 3 hours, then raise the temperature to 860 °C, hold for 1.75 hours for equalizing temperature, and then hold for 3.5 hours. After taking it out of the furnace, cool it in water to below 250 °C;

[0088] 3) Tempering: Heat the forging after quenching to 300 °C - 350 °C and hold for 3 hours, then raise the temperature to 600 °C, hold for 3.5 hours for equalizing temperature, and then hold for 7 hours. After taking it out of the furnace, cool it in water to below 250 °C.

[0089] The mechanical property test results of the 12CrNi5MoV steel forging heat-treated in Comparative Example 2 are as follows: the yield strength R of the tensile test eL = 836 MPa, the tensile strength R m = 901 MPa, the elongation A = 17%, and the reduction of area Z = 50%; in the -20 °C impact test, the impact absorption energies A KV of the three specimens in the same group are 111 J, 85 J, and 90 J respectively, and the fibrous rates of the impact fracture surfaces are 60%, 50%, and 50% respectively.

[0090] Comparative Example 3

[0091] Cancel step 3), and the rest is the same as in Example 6.

[0092] Heat-treat a 12CrNi5MoV steel forging with an effective thickness of 350 mm. Its chemical composition (%, wt, melting analysis) is as follows: C: 0.13%, Si: 0.25%, Mn: 0.55%, P: 0.006%, S: 0.007%, Cr: 1.20%, Ni: 4.88%, Mo: 0.42%, V: 0.05%, Cu: 0.23%.

[0093] Adopt a heat treatment process of normalizing + quenching + tempering.

[0094] 1) Normalizing: First, heat the forging to 600°C - 650°C and hold for 3 hours, then raise the temperature to 860°C for equalizing for 1.75 hours and then hold for 3.5 hours. After taking it out of the furnace, use a blower for air cooling to room temperature;

[0095] 2) Quenching: Heat the normalized forging to 600°C - 650°C and hold for 3 hours, then raise the temperature to 970°C for equalizing for 1.75 hours and then hold for 3.5 hours. After taking it out of the furnace, water-cool it to below 250°C;

[0096] 3) Tempering: Heat the quenched forging to 300°C - 350°C and hold for 3 hours, then raise the temperature to 600°C for equalizing for 3.5 hours and then hold for 7 hours. After taking it out of the furnace, water-cool it to below 250°C.

[0097] The mechanical property test results of the 12CrNi5MoV steel forging heat-treated in this Comparative Example 3 are as follows: For the tensile test, the yield strength R eL = 747 MPa, the tensile strength R m = 825 MPa, the elongation A = 17%, and the reduction of area Z = 51%; In the -20°C impact test, the impact absorption energy A KV of the three specimens in the same group are 85 J, 85 J, and 90 J respectively, and the fibrous rate of the impact fracture is 50%.

[0098] Comparative Example 4

[0099] Swap steps 1) and 2), that is, the original step 2) becomes step 1), and the original step 1) becomes step 2), and the rest is the same as in Example 6.

[0100] Heat-treat a 12CrNi5MoV steel forging with an effective thickness of 350 mm. Its chemical composition (%, wt, melting analysis) is as follows: C: 0.13%, Si: 0.25%, Mn: 0.55%, P: 0.006%, S: 0.007%, Cr: 1.20%, Ni: 4.88%, Mo: 0.42%, V: 0.05%, Cu: 0.23%.

[0101] Adopt the heat treatment process of normalizing + double quenching + tempering.

[0102] 1) Normalizing: First, heat the forging to 600°C - 650°C and hold for 3 hours, then raise the temperature to 860°C for uniform temperature for 1.75 hours and then hold for 3.5 hours. After taking it out of the furnace, use a blower for air cooling to room temperature;

[0103] 2) First quenching: Heat the forging after normalizing to 600°C - 650°C and hold for 3 hours, then raise the temperature to 860°C for uniform temperature for 1.75 hours and then hold for 3.5 hours. After taking it out of the furnace, water cool it to below 250°C;

[0104] 3) Second quenching: Heat the forging after the first quenching to 600°C - 650°C and hold for 3 hours, then raise the temperature to 970°C for uniform temperature for 1.75 hours and then hold for 3.5 hours. After taking it out of the furnace, water cool it to below 250°C;

[0105] 4) Tempering: Heat the forging after the second quenching to 300°C - 350°C and hold for 3 hours, then raise the temperature to 600°C for uniform temperature for 3.5 hours and then hold for 7 hours. After taking it out of the furnace, water cool it to below 250°C.

[0106] The mechanical property test results of the 12CrNi5MoV steel forging in this Comparative Example 4 are as follows: The yield strength R eL = 758 MPa, the tensile strength R m = 849 MPa, the elongation A = 16%, and the reduction of area Z = 47%; In the -20°C impact test, the impact absorption work A KV of the three specimens in the same group are 163 J, 158 J, and 160 J respectively, and the fiber ratio of the impact fracture is 90%.

[0107] The mechanical property test results of the 12CrNi5MoV steel forgings in Example 6 and Comparative Examples 1 to 4 are shown in Table 2. All the mechanical property indexes of the forgings in Example 6 meet the acceptance requirements; the yield strength and the fiber ratio of the impact fracture of the forgings in Comparative Examples 1 and 3 are unqualified, and the other mechanical property indexes meet the acceptance requirements; the fiber ratio of the impact fracture of the forgings in Comparative Example 2 is unqualified, and the other mechanical property indexes meet the acceptance requirements; the yield strength of the forgings in Comparative Example 4 is unqualified, and the other mechanical property indexes meet the acceptance requirements.

[0108] Table 2 Mechanical Property Test Results of Forgings in Examples and Comparative Examples

[0109]

[0110]

[0111] Comparative Experiment 1

[0112] The original grain size of the forgings described in each example (i.e., the grain size after forging and before final heat treatment) and the grain size of the forgings after each step during the process execution in the examples and comparative examples were measured. The specific situation is shown in Table 3 below. The grain size of the forgings in Table 3 was inspected in accordance with the provisions of GB / T6394 2017 (Determination method for average grain size of metals). Among them, the original grain size specimens, normalized grain size specimens, and the first quenched grain size specimens of the forgings in the examples were cut at 1 / 2 effective thickness of the forgings, while the grain size specimens after the second quenching + tempering were cut from the tensile specimens or impact specimens after the mechanical property tests of the examples and comparative examples.

[0113] Table 3 Grain size of forgings before and after each step in examples and comparative examples

[0114]

[0115] As can be seen from Table 3, after heat treatment of the forgings in each example using the process method of the present invention, the grain size of the forgings was significantly refined. Compared with the grain size of the forgings in Example 6, there was an obvious gap in the grain size of the forgings in Comparative Examples 1 to 4.

[0116] In summary, after heat treatment of 12CrNi5MoV steel forgings with an effective thickness not exceeding 350 mm using the process method of the present invention, the grain refinement was significant, providing a tissue preparation for improving the comprehensive mechanical properties of the forgings, and all the mechanical property indexes of the forgings met the acceptance requirements.

[0117] Comparative experiment 2

[0118] 12CrNi5MoV steel forgings with the same chemical composition and the same effective thickness as those in Examples 1 to 6 were all heat-treated using the preferred process parameters in the present invention (the temperature for normalizing and then raising the temperature was selected as 840 °C, the temperature for the first quenching and then raising the temperature was selected as 960 °C, the temperature for the second quenching and then raising the temperature was selected as 850 °C, and the temperature for tempering and then raising the temperature was selected as 590 °C), and were respectively denoted as Preferred Examples 1 to 6. The mechanical property results of the obtained forgings are as shown in Table 4 below.

[0119] Table 4 Test results of mechanical properties of forgings in preferred examples

[0120]

[0121] As can be seen from Table 4, the adoption of the process parameters within the preferred temperature range further optimized and improved the comprehensive mechanical properties of 12CrNi5MoV steel forgings. Not only did the mechanical property indexes such as the yield strength fully meet the acceptance requirements, but also the fiber rate of the impact fracture surface all reached 100%.

[0122] Comparative experiment 3

[0123] Forgings of 12CrNi5MoV steel with the same chemical composition and the same effective thickness as those in Examples 1 to 6 were respectively heat-treated (double quenching + tempering) using the manufacturing method of 12CrNi5MoV low-carbon high-strength alloy steel shaft forgings disclosed in Publication No. CN109468447A (Application No. CN201811268019), and were respectively denoted as Comparative Experiment 3 Example 1 to Comparative Experiment 3 Example 6. The double quenching temperatures were 910°C, 920°C, 930°C, 940°C, 950°C, and 960°C respectively, and the tempering temperature was 630°C. The mechanical property test results of the obtained forgings are shown in Table 5 below.

[0124] Table 5 Mechanical Property Test Results of Forgings in Comparative Experiment Three

[0125]

[0126] As can be seen from Table 5, after heat-treatment using the manufacturing method of 12CrNi5MoV low-carbon high-strength alloy steel shaft forgings disclosed in Publication No. CN109468447A (Application No. CN201811268019), for 12CrNi5MoV steel forgings with an effective thickness ≤ 150 mm, except for the unqualified fiber rate of the impact fracture surface, the other mechanical property indexes can meet the acceptance requirements of the present invention; for 12CrNi5MoV steel forgings with an effective thickness of 200 mm to 350 mm, both the yield strength and the fiber rate of the impact fracture surface are unqualified, and in the -20°C impact test of 12CrNi5MoV steel forgings with effective thicknesses of 250 mm and 350 mm (i.e., the forgings in Comparative Experiment 3 Example 4 and Comparative Experiment 3 Example 6), the impact absorption energy of each group of three specimens cannot fully meet the acceptance requirements of the present invention.

[0127] Comparative Experiment 4

[0128] Forgings of 12CrNi5MoV steel with the same chemical composition and the same effective thickness as those in Examples 1 to 6 were all heat-treated by normalizing (using 860°C) + quenching and tempering (the process parameters refer to Comparative Document 2: the quenching temperature is 900°C, and the tempering temperature is 605°C), and were respectively denoted as Comparative Experiment 4 Example 1 to Comparative Experiment 4 Example 6. The mechanical property test results of the obtained forgings are as follows in Table 6.

[0129] Table 6 Mechanical Property Test Results of Forgings in Comparative Experiment 1

[0130]

[0131]

[0132] As can be seen from Table 6, after heat treatment with normalizing (at 860°C) + quenching and tempering (the process parameters refer to Comparative Document 2: quenching temperature is 900°C, tempering temperature is 605°C), for 12CrNi5MoV steel forgings with an effective thickness ≤ 250 mm, the tensile properties and the impact absorption energy of each group of three specimens in the -20°C impact test can meet the acceptance requirements of the present invention, but the fibrous rate of the impact fracture is unqualified; for 12CrNi5MoV steel forgings with an effective thickness of 300 mm - 350 mm, both the yield strength and the fibrous rate of the impact fracture cannot meet the acceptance requirements of the present invention.

Claims

1. A heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings, characterized in that: For the 12CrNi5MoV steel forging, by weight percentage, the chemical composition includes C: 0.10% - 0.13%, Si: 0.20% - 0.40%, Mn: 0.30% - 0.60%, P ≤ 0.015%, S ≤ 0.010%, Cr: 0.90% - 1.20%, Ni: 4.50% - 5.00%, Mo: 0.40% - 0.60%, V: 0.03% - 0.08%, Cu ≤ 0.25%; the effective thickness of the steel forging is 100 - 350 mm; The heat treatment process is as follows: 1) Normalizing: First, heat the steel forging to 600°C - 650°C and hold for 2 - 3 hours, then raise the temperature to 820°C - 860°C for soaking and hold again. After taking it out of the furnace, use a blower for air cooling to room temperature; 2) First quenching: Heat the steel forging after normalizing to 600°C - 650°C and hold for 2 - 3 hours, then raise the temperature to 930°C - 970°C for soaking and hold again. After taking it out of the furnace, water cool it to below 250°C; 3) Second quenching: Heat the steel forging after the first quenching to 600°C - 650°C and hold for 2 - 3 hours, then raise the temperature to 820°C - 860°C for soaking and hold again. After taking it out of the furnace, water cool it to below 250°C; 4) Tempering: Heat the forging after the second quenching to 300°C - 350°C and hold for 2 - 3 hours, then raise the temperature to 580°C - 600°C for soaking and hold again. After taking it out of the furnace, water cool it to below 250°C.

2. The heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings according to claim 1, characterized in that: In step 1), the temperature is raised to 830°C - 840°C.

3. The heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings according to claim 1, characterized in that: In step 2), the temperature is raised to 950°C - 960°C.

4. The heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings according to claim 1, characterized in that: In step 3), the temperature is raised to 840°C - 850°C.

5. The heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings according to claim 1, characterized in that: In steps 1), 2) and 3), the holding time for the second holding is calculated as 0.8 - 1.2 h / 100 mm, and the soaking time is 1 / 3 - 1 / 2 of the holding time.

6. The heat treatment process for improving the comprehensive mechanical properties of 12CrNi5MoV steel forgings according to claim 1, characterized in that: In step 4), the holding time for the second holding is calculated as 1.5 - 2.5 h / 100 mm, and the soaking time is 1 / 3 - 1 / 2 of the holding time.

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