A method of heat treatment of a low carbon bainitic steel

Abstract

The application discloses a heat treatment method of low-carbon bainite steel and belongs to the technical field of metallurgy. The heat treatment method comprises normalizing accelerated cooling, reheating quenching and tempering accelerated cooling processes. In the normalizing accelerated cooling process, the heating temperature of the steel plate is 910-930 DEG C, the heating coefficient is 2.5-3 min / mm, and after heat preservation, the steel plate is taken out of the furnace and water-cooled at a cooling speed of greater than or equal to 15 DEG C / s to 350-450 DEG C and then air-cooled to room temperature. In the tempering accelerated cooling process, the heating temperature is 650-680 DEG C, the heating coefficient is 3.5-5 min / mm, and after heat preservation and taking the steel plate out of the furnace, the steel plate is accelerated cooled to room temperature at a cooling speed of greater than or equal to 10 DEG C / s. The application improves the original structure of the as-rolled steel plate, fully refines the grains, uniformly distributes the alloy elements, improves the segregation and internal defects of the steel plate, suppresses the tempering brittleness and improves the impact toughness of the steel plate.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a heat treatment method for low-carbon bainitic steel. Background Technology

[0002] Low-carbon bainitic steel, after proper quenching and tempering, can achieve high strength, high toughness, and excellent weldability. It is mainly used in industries such as cryogenic pressure vessels, engineering machinery, shipbuilding and marine engineering, and bridges. Brittle fracture is not allowed during service, and the toughness requirements are very strict.

[0003] Currently, high-grade low-carbon bainitic steels successfully used both domestically and internationally are all produced by quenching and tempering to eliminate internal stress and stabilize the microstructure, thereby achieving good strength and impact toughness. However, low-carbon bainitic steels with a thickness of 42mm or more, after conventional quenching and tempering, exhibit problems such as coarse grains and temper brittleness, resulting in an average impact energy of less than 100J at -40℃. Temper brittleness in bainitic steel typically occurs during slow cooling after medium-high temperature tempering or during prolonged service at medium temperatures. The temper brittleness of bainitic steel severely hinders the widespread application of thick-gauge low-carbon bainitic steels. Summary of the Invention

[0004] This invention provides a heat treatment method for low-carbon bainitic steel, which improves the original microstructure of rolled steel plate, fully refines grains, homogenizes alloying elements, improves segregation and internal defects of steel plate, suppresses temper brittleness, and enhances the impact toughness of steel plate.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A heat treatment method for low-carbon bainitic steel, the heat treatment method comprising normalizing accelerated cooling, reheating quenching, and tempering accelerated cooling processes; in the normalizing accelerated cooling process, the steel plate is heated to 910-930℃ with a heating coefficient of 2.5-3 min / mm, and after holding at the temperature, it is water-cooled to 350-450℃ at a cooling rate of ≥15℃ / s and then air-cooled to room temperature; in the tempering accelerated cooling process, the heating temperature is 650-680℃ with a heating coefficient of 3.5-5 min / mm, and after holding at the temperature, it is accelerated cooled to room temperature at a cooling rate of ≥10℃ / s.

[0006] In the reheating and quenching process described in this invention, the steel plate is heated to 890-900℃ with a heating coefficient of 2.5-3 min / mm, and then cooled to room temperature after being taken out of the furnace.

[0007] The holding time in the normalizing accelerated cooling process described in this invention is 10 to 40 minutes.

[0008] The holding time in the reheating and quenching process described in this invention is 20 to 30 minutes.

[0009] The holding time in the tempering accelerated cooling process described in this invention is 40-50 minutes.

[0010] The chemical composition and mass percentage of the steel plate described in this invention are as follows: C: 0.07-0.09%, Si: 0.20-0.40%, Mn: 1.0-1.4%, P≤0.008%, S≤0.0020%, Ni: 0.4-1.0%, Cr: 0.5-0.8%, Mo: 0.4-0.5%, Ti: 0.012-0.016%, V: 0.03-0.05%, Nb: 0.020-0.030%, Al: 0.03-0.05%, B: 0.0012-0.0014%, with the balance being Fe and unavoidable impurities.

[0011] The steel plate of this invention has a thickness of 42-64 mm.

[0012] The metallographic structure of the steel plate described in this invention is bainite.

[0013] The steel plate of this invention has a yield strength ≥733MPa, tensile strength ≥792MPa, elongation ≥20%, and average transverse impact energy ≥289J at -40℃.

[0014] The beneficial effects of adopting the above technical solution are as follows: 1. The present invention performs accelerated cooling with normalizing before reheating and quenching, which improves the original microstructure of the rolled steel plate, fully refines the grains, homogenizes alloying elements, and improves segregation and internal defects of the steel plate; after water cooling to 350-450℃ and then air cooling to room temperature, a bainitic microstructure is obtained after normalizing, further refining the grains. 2. The present invention uses an accelerated cooling process after tempering to prevent the precipitation of large carbide particles during the tempering cooling process, suppressing temper brittleness and improving the impact toughness of the steel plate. Attached Figure Description

[0015] Figure 1 The image shows the metallographic structure of the low-carbon bainitic steel plate from Example 1. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to specific embodiments.

[0017] Examples 1-8

[0018] A heat treatment method for low-carbon bainitic steel includes normalizing and accelerated cooling, reheating and quenching, and tempering and accelerated cooling. The specific process steps are as follows:

[0019] (1) Normalizing accelerated cooling: The steel plate is heated to 910~930℃, the heating coefficient is 2.5~3min / mm, and after holding for 10~40min, it is taken out of the furnace and water-cooled to 350~450℃ at a cooling rate of ≥15℃ / s, and then air-cooled to room temperature.

[0020] (2) Reheating and quenching: The steel plate is heated to 890-900℃ with a heating coefficient of 2.5-3 min / mm. After holding for 20-30 min, it is taken out of the furnace and water-cooled to room temperature.

[0021] (3) Tempering to accelerate cooling: The heating temperature is 650~680℃, the heating coefficient is 3.5~5min / mm, and the temperature is held for 40~50min. After taking it out of the furnace, it is accelerated to room temperature at a cooling rate of ≥10℃ / s.

[0022] The production process parameters for each embodiment are shown in Table 1, the chemical composition and mass percentage of the steel plate are shown in Table 2, and the specifications and properties of the steel plate after heat treatment are shown in Table 3.

[0023] Figure 1 The image shown is a metallographic diagram of the low-carbon bainitic steel plate from Example 1. The accompanying drawings for the other examples are similar and are therefore omitted.

[0024] Table 1 Production process parameters for each embodiment

[0025]

[0026] Table 2. Chemical composition and mass percentage (%) of steel plates in each embodiment.

[0027]

[0028] The remaining components in Table 2 are Fe and unavoidable impurities.

[0029] Table 3 Mechanical properties of steel plates after heat treatment in each embodiment

[0030]

[0031] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A heat treatment method for low-carbon bainitic steel, characterized in that, The heat treatment method includes normalizing and accelerated cooling, reheating and quenching, and tempering and accelerated cooling processes. In the normalizing accelerated cooling process, the steel plate is heated to 910-930℃ with a heating coefficient of 2.5-3 min / mm. After holding at the temperature, it is taken out of the furnace and water-cooled to 350-450℃ at a cooling rate of ≥15℃ / s, and then air-cooled to room temperature. The tempering and accelerated cooling process involves heating at 650–680°C with a heating coefficient of 3.5–5 min / mm, followed by accelerated cooling to room temperature at a rate of ≥10°C / s after the furnace is removed from the heat. In the reheating and quenching process, the steel plate is heated to 890-900℃ with a heating coefficient of 2.5-3 min / mm, and then cooled to room temperature after being taken out of the furnace. The chemical composition and mass percentage of the steel plate are as follows: C: 0.07-0.09%, Si: 0.20-0.40%, Mn: 1.0-1.4%, P≤0.008%, S≤0.0020%, Ni: 0.4-1.0%, Cr: 0.5-0.8%, Mo: 0.4-0.5%, Ti: 0.012-0.016%, V: 0.03-0.05%, Nb: 0.020-0.030%, Al: 0.03-0.05%, B: 0.0012-0.0014%, with the balance being Fe and unavoidable impurities; The thickness of the steel plate is 42-64 mm.

2. The heat treatment method for low-carbon bainitic steel according to claim 1, characterized in that, The holding time in the normalizing accelerated cooling process is 10 to 40 minutes.

3. The heat treatment method for low-carbon bainitic steel according to claim 1, characterized in that, The holding time in the reheating and quenching process is 20 to 30 minutes.

4. The heat treatment method for low-carbon bainitic steel according to claim 1, characterized in that, The holding time in the tempering accelerated cooling process is 40-50 minutes.

5. A heat treatment method for low-carbon bainitic steel according to any one of claims 1-4, characterized in that, The metallographic structure of the steel plate is bainite.

6. A heat treatment method for low-carbon bainitic steel according to any one of claims 1-4, characterized in that, The steel plate has a yield strength ≥733MPa, tensile strength ≥792MPa, elongation ≥20%, and average transverse impact energy ≥289J at -40℃.

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