A heat treatment process for preparing 20CrNiMo ferritic-martensitic dual-phase steel and the dual-phase steel itself.

Abstract

This invention discloses a heat treatment process for preparing 20CrNiMo ferrite-martensitic dual-phase steel and the dual-phase steel itself, belonging to the field of metal heat treatment technology. The process selects hot-rolled 20CrNiMo steel, heats it to the critical zone of 780-820℃ at a rate of 10-50℃ / min and holds it at that temperature for 10-30 min; then performs two-stage cooling: first, air cooling at a rate of 20-30℃ / s to 550-600℃, followed by immediate rapid quenching to room temperature at a rate of ≥50℃ / s; finally, low-temperature tempering at 180-250℃ for 1-3 h followed by air cooling. This invention solves the problem of high time consumption in traditional spheroidizing annealing and can obtain a dual-phase microstructure of polygonal ferrite and island-shaped tempered martensite. The treated steel has a tensile strength of 700-900 MPa, a yield strength ratio as low as 0.55-0.65, excellent plasticity, and improved cold forming performance of this steel grade.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, specifically relating to a method for transforming the original hot-rolled microstructure (mainly pearlite + ferrite) of 20CrNiMo alloy structural steel into a ferrite-martensite dual-phase microstructure through a specific heat treatment process. Background Technology

[0002] 20CrNiMo is a widely used low-carbon alloy structural steel with high hardenability and good comprehensive mechanical properties, often used to manufacture core components such as gears and shafts. However, the microstructure of conventional hot-rolled 20CrNiMo steel is pearlite and ferrite, with high hardness and yield strength. Directly performing cold forming processes such as stamping and cold heading makes it prone to cracking or mold damage. To improve forming, traditional processes typically require a spheroidizing annealing treatment lasting 10-20 hours. This process is not only extremely time-consuming but also energy-intensive, significantly increasing production costs.

[0003] However, with the development of industries such as lightweight automotive structural components, the demand for materials that simultaneously possess good formability, low yield strength ratio, and high tensile strength is increasing. Ferritic-martensitic dual-phase steel (DP steel) possesses these characteristics: low yield strength ratio, high work hardening rate, and good uniform elongation.

[0004] Low-alloy steels containing Ni and Mo are extremely sensitive during cooling. Existing conventional heat treatments for duplex steels, such as the heat treatment of 20CrMo steel mentioned in the literature, often only involve direct water quenching. However, because the presence of Ni in 20CrNiMo alters the stability of the supercooled austenite, conventional cooling easily leads to the precipitation of bainite or incomplete retention of proeutectoid ferrite. Furthermore, sub-temperature quenching of 20CrMo steel often yields lath martensite and strip ferrite.

[0005] Currently studied dual-phase steel preparation processes differ significantly from those for 20CrNiMo, and the resulting microstructures cannot meet the cold forming requirements of 20CrNiMo. For example, process 202111556232.1, designed for high-Si, Mn, and V-containing steels, requires complete austenitization followed by quenching to obtain full martensite and bainite, then heating to the two-phase region, ultimately yielding fine lamellar and lattice-like martensite and lamellar ferrite. Process CN202210440817.5, for 15Mn-20Mn steels, employs a complex process involving repeated heating and cooling along the Ac1 line, ultimately resulting in network martensite and ferrite.

[0006] For low-alloy structural steels like 20CrNiMo, which contain Cr, Ni, and Mo elements, directly applying existing dual-phase steel processes can easily lead to uneven distribution of ferrite and martensite, the appearance of bainitic structures, uncontrollable retained austenite, or performance fluctuations.

[0007] The microstructure of island-shaped martensite uniformly distributed in an island-shaped ferrite matrix exhibits superior strength and plasticity compared to lath or network microstructures. However, existing technologies have not explored how to transform the original hot-rolled state of 20CrNiMo into an island-shaped two-phase structure through heat treatment.

[0008] Therefore, a heat treatment process specifically designed for 20CrNiMo hot-rolled steel is needed to stably obtain a dual-phase microstructure of uniform island-shaped polygonal ferrite and island-shaped tempered martensite, enabling 20CrNiMo steel to possess a lower yield strength ratio and excellent strength and plasticity. Summary of the Invention

[0009] To address the issue of poor matching between the strength and formability of the hot-rolled microstructure of existing 20CrNiMo low-alloy structural steel, this invention provides a heat treatment process that transforms the hot-rolled microstructure of 20CrNiMo low-carbon alloy structural steel into a ferrite-martensite dual-phase microstructure. This process can stably obtain a uniform island-shaped ferrite and martensite dual-phase microstructure, achieving a combination of low yield strength ratio, high tensile strength, and good toughness.

[0010] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0011] A heat treatment process for transforming the microstructure of 20CrNiMo low-carbon alloy structural steel from its hot-rolled state to a ferrite-martensite dual-phase steel microstructure includes the following steps:

[0012] Step 1: Sample Preparation: Select hot-rolled or air-cooled 20CrNiMo steel with a microstructure of pearlite + ferrite. The cross-sectional thickness or diameter of the steel should be controlled between 5mm and 20mm.

[0013] Step 2: Partial austenitization heating in the critical region: Place the above-mentioned steel in a heat treatment furnace and heat it to the range of 780℃ to 820℃ (between Ac1 and Ac3) at a heating rate of 10 to 50℃ / min. Hold at this temperature for 10 to 30 minutes to fully austenitize the original pearlite structure while retaining some proeutectoid ferrite structure, forming a two-phase mixture of ferrite and austenite.

[0014] Step 3: Segmented Controllable Cooling

[0015] The first stage of cooling: Starting from the critical temperature, the steel is rapidly cooled to 550℃~600℃ using a high-volume airflow or high-pressure gas purging method at a cooling rate of 20~30℃ / s. The purpose of this stage is to reduce the re-precipitation of ferrite at the austenite grain boundaries in the early stage of cooling, promote the enrichment of carbon elements in the untransformed austenite, thereby improving the stability of the retained austenite, ensuring the formation of island martensite during subsequent rapid cooling, and avoiding the formation of bainite structure.

[0016] The second stage of cooling: After the steel is cooled to 550℃~600℃, it is immediately cooled to room temperature (≤ 50℃) by water quenching or oil quenching at a critical cooling rate of ≥50℃ / s. This stage quickly passes through the nose region of the C curve to ensure that most of the austenite formed in the critical region does not undergo pearlite or bainite transformation, but directly transforms into martensite.

[0017] Step 4: Low-temperature tempering

[0018] The quenched steel is then subjected to low-temperature tempering at 180℃~250℃ for 1~3 hours, followed by air cooling. This step is used to eliminate quenching internal stress, stabilize the martensitic structure, and without significantly reducing strength.

[0019] The chemical composition of 20CrNiMo steel, by mass percentage, is as follows: C: 0.17–0.23%, Si: 0.17–0.37%, Mn: 0.60–0.95%, Cr: 0.40–0.70%, P: ≤0.030%, S: ≤0.030%, Ni: 0.35–0.75%, Mo: 0.20–0.30%, with the remainder being Fe and trace amounts of other unavoidable impurity elements.

[0020] After the above-mentioned processing, the original hot-rolled microstructure of the 20CrNiMo steel is transformed into a uniform dual-phase microstructure. The microstructure of the 20CrNiMo steel consists of approximately 50%–70% polygonal island-shaped ferrite and approximately 30%–50% island-shaped tempered martensite. The martensite islands are uniformly distributed on the ferrite matrix, and the ratio of the two phases can be precisely controlled by the critical zone heating temperature.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) This invention optimizes the critical zone temperature and segmented cooling process for 20CrNiMo containing Cr, Ni and Mo elements, effectively avoiding the formation of coarse martensite or harmful structures such as bainite, and obtains a typical and uniform island-shaped martensite + ferrite dual-phase structure, avoiding the occurrence of coarse lath or network structures.

[0023] (2) The present invention obtains a uniform island-shaped dual-phase structure and eliminates internal stress by low-temperature tempering. The tensile strength of the steel after treatment can reach 700-900MPa, the yield strength is 400-500MPa, the yield ratio is as low as 0.55-0.65, and the elongation after fracture is 18%-25%. Compared with the original hot-rolled state, it obtains high tensile strength while making the material have uniform elongation properties.

[0024] (3) This method can be directly applied to hot-rolled plates, bars and wires without prior spheroidizing annealing or full annealing, as well as the final quenching and tempering steps, which simplifies the process and reduces energy consumption. It does not require the use of large hot rolling equipment or cold rolling and continuous annealing equipment with controlled rolling and cooling capabilities; only ordinary heat treatment furnaces and quenching equipment with staged controlled cooling capabilities are needed.

[0025] (4) Conventional 20CrNiMo has a narrow application due to its strength limitation, while the dual-phase 20CrNiMo treated by the present invention has a low yield ratio and a high work hardening rate. This dual-phase 20CrNiMo is very suitable for cold forming operations such as stamping and bending, and can be used as a material for complex structural parts, thus broadening the application field of this steel. Attached Figure Description

[0026] Figure 1 The diagram illustrates the temperature-time curve of the heat treatment process of this invention. The horizontal axis represents time, and the vertical axis represents temperature. The curve segments represent: heating section, critical zone holding section, two-stage cooling section, and low-temperature tempering section.

[0027] Figure 2 Typical metallographic images of 20CrNiMo steel in its hot-rolled state before processing (magnified 500 times).

[0028] Figure 3 Typical metallographic image of the dual-phase structure of 20CrNiMo steel after treatment in Example 1 (magnified 500 times); it consists of ferrite + tempered martensite islands.

[0029] Figure 4 Typical metallographic image of the dual-phase structure of 20CrNiMo steel after treatment in Example 2 (magnified 500 times); it consists of ferrite + tempered martensite islands.

[0030] Figure 5 Typical metallographic image of quenched and tempered 20CrNiMo steel after treatment in Comparative Example 1 (magnified 500 times); it is tempered martensite.

[0031] Figure 6 Typical metallographic image of quenched and tempered 20CrNiMo steel after treatment in Comparative Example 2 (magnified 500 times); it consists of quenched martensite plus a small amount of undissolved ferrite.

[0032] Figure 7 Typical metallographic structure of 20CrNiMo steel treated with slow cooling quenching and tempering after Comparative Example 3 (magnified 500 times); it consists of ferrite + pearlite + a small amount of bainite.

[0033] Figure 8 Typical metallographic image (500x magnification) of 20CrNiMo steel treated in Comparative Example 4 after heating, holding, quenching and tempering below AC1 temperature; it consists of ferrite and pearlite. Detailed Implementation

[0034] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and comparative examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the scope of protection of the invention.

[0035] The chemical composition of the 20CrNiMo steel described in this invention, by mass percentage, is as follows: C: 0.21%, Si: 0.24%, Mn: 0.91%, Cr: 0.62%, Ni: 0.47%, Mo: 0.24%, P: 0.012%, S: 0.008%, with the balance being Fe and other unavoidable impurity elements.

[0036] Example 1

[0037] 1. Material: 20CrNiMo hot-rolled round steel, 12mm in diameter, with a microstructure of ferrite + pearlite (F+P). Its typical metallographic morphology is as follows: Figure 2 As shown.

[0038] 2. Process:

[0039] (1) Critical zone heating: The steel is placed in a heat treatment furnace and heated to 800℃ at a rate of 45℃ / min; (2) Holding: The steel is held at 800℃ for 15 minutes;

[0040] (3) First stage cooling: A large volume of nitrogen air is used for purging, and the steel is cooled to 580°C at a first cooling rate of 30°C / s;

[0041] (4) Second stage cooling: When the steel temperature drops to 580°C, it is immediately water quenched and rapidly cooled to room temperature (water temperature about 25°C).

[0042] (5) Low temperature tempering: The quenched steel is placed at 200℃ for 2 hours and then air-cooled after being taken out of the furnace.

[0043] Upon microscopic tissue observation, such as Figure 3 As shown, the area ratio of each phase in the microstructure is 65% island ferrite + 35% island tempered martensite.

[0044] Mechanical testing, conducted according to GB / T228.1, yielded a tensile strength of 785 MPa, a yield strength of 440 MPa, a yield-to-tensile ratio of 0.56, and an elongation at break of 22%.

[0045] Example 2

[0046] 1. Material: 20CrNiMo hot-rolled round steel, 20mm in diameter, original microstructure: ferrite + pearlite (F+P).

[0047] 2. Process:

[0048] (1) Critical zone heating: The steel is placed in a heat treatment furnace and heated to 810℃ at a rate of 20℃ / min;

[0049] (2) Heat preservation: Keep warm at 810℃ for 20 minutes;

[0050] (3) First stage cooling: Purge with compressed air and cool to 560°C at a rate of 25°C / s.

[0051] (4) Second stage cooling: After the steel is cooled to 560°C, it is immediately oil quenched at a second cooling rate of ≥50°C / s (oil temperature about 25°C) and cooled to room temperature.

[0052] (5) Low temperature tempering: The quenched steel is placed at 220℃ for 1.5 hours and then air-cooled after being taken out of the furnace.

[0053] Upon microscopic tissue observation, such as Figure 4 As shown, the final microstructure is approximately 55% island ferrite + 45% island tempered martensite;

[0054] Mechanical tests: tensile strength is 850 MPa, yield strength is 470 MPa, yield ratio is as low as 0.55, and elongation after fracture is 19%.

[0055] Comparative Example 1

[0056] Material preparation: Same as in Example 1, 20CrNiMo hot-rolled round steel.

[0057] The steel is heated to 850℃ for austenitization and held for 15 minutes, then directly oil-quenched to room temperature, and then tempered at 200℃ for 1.5 hours before being air-cooled.

[0058] Comparative Example 1 uses conventional full quenching and low-temperature tempering, and the corresponding metallographic structure is as follows: Figure 5 As shown, the microstructure obtained is tempered martensite with a small amount of bainite. Mechanical property tests revealed a tensile strength of 1215 MPa, a yield strength of 916 MPa, an elongation after fracture of 11%, and a yield-to-tensile ratio as high as 0.75. However, the poor plasticity and high yield strength make it unsuitable for cold forming complex structural components.

[0059] Comparative Example 2

[0060] Material preparation: Same as in Example 1, 20CrNiMo hot-rolled round steel.

[0061] (1) Critical zone heating: The steel is placed in a heat treatment furnace and heated to 800℃ at a rate of 45℃ / min; (2) Holding: The steel is held at 800℃ for 15 minutes;

[0062] (3) After holding at 800℃, do not perform segmented controlled cooling, but directly and rapidly water quench to room temperature;

[0063] Comparative Example 2, without segmented controlled cooling, has the following metallographic structure: Figure 6 As shown, the final microstructure is quenched martensite with a small amount of undissolved ferrite. Mechanical property test results: tensile strength is 1294 MPa, yield strength is 1109 MPa. Elongation after fracture is 7.5%, and the yield strength ratio is as high as 0.86. The strength is too high, the plasticity is too low, and the deformation capacity is poor.

[0064] Comparative Example 3

[0065] Material preparation: Same as in Example 1, 20CrNiMo hot-rolled round steel.

[0066] The steel is heated to 800℃ and held for 15 minutes, then cooled to room temperature at a cooling rate of 5℃ / s (using a weak airflow or air cooling method), and then tempered at 200℃ for 1.5 hours before being removed from the furnace and air-cooled.

[0067] The corresponding metallographic structure is as follows Figure 7 As shown, the microstructure obtained is ferrite + pearlite + a small amount of bainite. Mechanical property tests showed a tensile strength of 597 MPa, a yield strength of 336 MPa, an elongation after fracture of 26%, and a yield-to-tensile ratio of 0.56. It exhibits good plasticity but low strength, limiting its practical application value.

[0068] Comparative Example 4

[0069] Material preparation: Same as in Example 1, 20CrNiMo hot-rolled round steel.

[0070] The steel was heated to 720°C and held for 15 minutes. The subsequent quenching and tempering processes were the same as in Example 1.

[0071] Because the heating and holding temperature did not reach Ac1, the corresponding metallographic structure is as follows: Figure 8 As shown, the microstructure obtained is ferrite + pearlite. Mechanical property testing revealed a tensile strength of 562 MPa, a yield strength of 305 MPa, an elongation after fracture of 28%, and a yield-to-tensile ratio as low as 0.54. It exhibits good plasticity, but its low strength limits its practical application value.

[0072] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. 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. Such 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 heat treatment process for preparing 20CrNiMo ferrite-martensitic dual-phase steel, characterized in that, Includes the following steps: (1) Select 20CrNiMo steel with original microstructure of pearlite and ferrite in hot-rolled state or hot-rolled and air-cooled state; Step 2, critical zone heating: The steel is placed in a heat treatment furnace and heated to 780℃~820℃ at a heating rate of 10~50℃ / min, and held at this temperature for 10~30 minutes; Step 3, segmented controllable cooling: First stage cooling: The steel is cooled from the insulation temperature to 550℃~600℃ by forced air blowing or gas purging at a cooling rate of 20~30℃ / s. Second stage of cooling: After the steel is cooled to the above temperature, it is immediately cooled to room temperature by water quenching or oil quenching at a cooling rate of ≥50℃ / s. Step 4, Low-temperature tempering: Place the steel after the second stage of cooling at a temperature of 180℃~250℃ for 1~3 hours for low-temperature tempering treatment, and then remove it from the furnace and air cool it.

2. The heat treatment process according to claim 1, characterized in that: The cross-sectional thickness or diameter of the 20CrNiMo steel mentioned in step one is controlled between 5mm and 20mm.

3. The heat treatment process according to claim 1, characterized in that: In the first stage of cooling in step three, nitrogen or compressed air is used as the high-pressure gas.

4. The heat treatment process according to claim 1, characterized in that: In the second stage of cooling in step three, cooling to room temperature refers to cooling the steel to a temperature of ≤50℃.

5. A 20CrNiMo ferritic-martensitic dual-phase steel prepared by the heat treatment process described in any one of claims 1 to 4.

6. The 20CrNiMo ferritic-martensitic dual-phase steel according to claim 5, characterized in that: The microstructure of the dual-phase steel comprises 50% to 70% polygonal island ferrite by volume and 30% to 50% island tempered martensite by volume; wherein the island tempered martensite is uniformly distributed on the polygonal island ferrite matrix.

7. The 20CrNiMo ferritic-martensitic dual-phase steel according to claim 5, characterized in that: The tensile strength of the duplex steel is 700-900 MPa, the yield strength is 400-500 MPa, the yield strength ratio is 0.55-0.65, and the elongation after fracture is 18%-25%.

Images