High-strength steel, heat treatment method for improving low-temperature toughness of high-strength steel and application of high-strength steel

Abstract

The invention belongs to the field of metal material heat treatment, and particularly discloses high-strength steel, a heat treatment method for improving the low-temperature toughness of the high-strength steel and application. The heat treatment method comprises the following steps: firstly, quenching treatment is carried out, then water cooling is carried out to 50-100 DEG C, heat preservation is carried out for a period of time, and then air cooling is carried out to room temperature; and tempering and air cooling are matched, so that the toughness of the steel plate is remarkably improved. The tensile strength of the prepared steel reaches 900 MPa or above, high strength and good low-temperature toughness can be considered at the same time, and the steel is more suitable for long-term service in complex environments such as the ocean.

Description

Technical Field

[0001] The present invention belongs to the field of heat treatment of metal materials, and in particular relates to a high-strength steel material and a heat treatment method and application thereof for improving the low-temperature toughness thereof. Background Art

[0002] In recent years, with the acceleration of ocean research and exploration, the application of marine engineering steel has continued to expand into harsh and cold regions such as the deep sea and polar regions. The diverse and complex marine service environment places increasing demands on marine engineering steel for high strength, high-temperature toughness, and other properties. Existing 40-80mm thick marine engineering steel plates generally suffer from insufficient toughness during production. This is primarily due to limitations in the rolling process and chemical composition, resulting in the formation of high-density dislocation martensite, which results in extremely poor toughness. Currently, quenching and tempering heat treatment processes are widely used in the production of marine engineering steel, typically incorporating normalizing and multi-step heat treatments to enhance strength and toughness. However, existing processes suffer from poor toughness enhancement and complex operational procedures. In particular, for steels with strength grades of 900 MPa and above, insufficient low-temperature toughness persists after heat treatment, manifested in low low-temperature impact strength and elongation, making them difficult to meet practical application requirements. Therefore, significantly improving the low-temperature toughness of high-strength marine engineering steel plates remains a major challenge in marine steel research. Summary of the Invention

[0003] In response to the problems of low-temperature impact energy and low elongation at break of marine engineering steel involved in the above-mentioned prior art, the present invention will provide a high-strength steel and a heat treatment method and application for improving its low-temperature toughness.

[0004] To achieve the above objectives, the following technical solutions are specifically included:

[0005] In one aspect, the present invention provides a method for heat treating steel, comprising the following steps:

[0006] (1) a steel plate is sequentially subjected to quenching treatment, water cooling to 50-100° C., holding the temperature, and air cooling; the steel plate comprises the following elemental compositions by mass percentage: C 0.05%-0.15%, Si 0.3%-0.5%, Mn 0.8%-1.5%, Ni 2%-3.5%, Mo 0.3%-0.42%, Cr 0.1%-0.2%, Ti 0.005%-0.02%, V 0.01%-0.08%, Nb 0.04%-0.06%, B 0.001%-0.002%, Al 0.05%-0.07%, Cu 0.1%-0.2%, and the balance is Fe and unavoidable impurities;

[0007] (2) After tempering and air cooling, steel is obtained.

[0008] The heat treatment method of the present invention first performs a quenching treatment above the austenitizing temperature. To prevent excessive grain growth, the steel is then water-quenched, rapidly cooled to 50-100°C, briefly held at that temperature, and then air-cooled to room temperature. The principle behind this rapid cooling to 50-100°C and short holding period is that, after dwelling at a temperature below the martensite transformation start point of the steel, both the martensite transformation start and end temperatures of the retained austenite are significantly increased. Holding at 50-100°C helps increase the transformation driving force of the retained austenite, thereby significantly raising its martensite transformation start and end temperatures. This process forms high-density dislocation martensite and bainite structures, as well as partially stabilized retained austenite. Subsequently, the steel is heated, tempered, and held at this temperature, resulting in a microstructure composed primarily of martensite, lower bainite, ferrite, and stable retained austenite. The grains are uniform and fine, effectively improving the material's strength and toughness. At the same time, tempering causes a large amount of carbides to precipitate, enhancing precipitation strengthening. The precipitated carbides pin dislocations, effectively hindering dislocation movement, enhancing dislocation strengthening, and increasing yield strength. Furthermore, the precipitation of carbides softens the matrix, and the precipitated small-sized spherical carbides increase crack perturbations and hinder crack propagation, ultimately significantly enhancing the material's impact toughness. The improvement in impact toughness is also related to the proportion of high-angle grain boundaries. High-angle grain boundaries can hinder crack propagation or change the direction of crack propagation. The large number of martensite grain boundaries generated during quenching, as well as the newly generated lower bainite, ferrite, and refined retained austenite grain boundaries during tempering, further enhance impact toughness. Therefore, the heat treatment method of the present invention can significantly improve the low-temperature toughness of the steel plate, and in particular, can significantly improve the elongation after fracture and the low-temperature impact energy while maintaining the strength of the steel.

[0009] Preferably, in step (1), the steel plate comprises the following elemental composition by mass percentage: C 0.08% to 0.10%, Si 0.37% to 0.43%, Mn 1.1% to 1.2%, Ni 2.9% to 3%, Mo 0.32% to 0.37%, Cr 0.13% to 0.16%, Ti 0.008% to 0.014%, V 0.03% to 0.038%, Nb 0.045% to 0.055%, B 0.0013% to 0.0017%, Al 0.055% to 0.065%, Cu 0.13% to 0.15%, with the remainder being Fe and unavoidable impurities. The steel plate of the present invention using the above composition is more suitable for use in marine environments.

[0010] Preferably, in step (1), the P content in the steel plate is ≤100 ppm, and the S content is ≤40 ppm.

[0011] Preferably, in step (1), the temperature of the quenching treatment is greater than or equal to the austenitization start temperature Ac1 of the steel.

[0012] Further preferably, in step (1), the temperature of the quenching treatment is 860-910° C., and the time of the quenching treatment is 30-60 minutes.

[0013] The steel plate of the present invention is a steel plate composed of specific elements and is adapted to marine engineering environments. Within the above-mentioned temperature and time range, the degree of austenitization is better, which is beneficial to the subsequent phase transformation process and is more conducive to improving the low-temperature toughness of the steel plate with specific composition of the present invention.

[0014] Preferably, in step (1), the thickness of the steel plate is 40-80 mm.

[0015] The thickness of the steel plate is 40-80 mm, which is a thickness commonly used in marine engineering structures. The use of the above steel plate thickness range is also more compatible with the heat treatment method of the present invention.

[0016] Preferably, in step (1), the insulation time is 30 to 60 minutes.

[0017] Rapid cooling to 50-100°C and short-term holding within the above time range is more conducive to increasing the martensitic transformation start temperature and end temperature of the retained austenite, thereby obtaining a lath martensite structure and retained austenite structure with a higher dislocation density.

[0018] Preferably, in step (2), the tempering temperature is 650-700° C., and the tempering time is 30-120 min.

[0019] Heating the steel to a suitable temperature for tempering and keeping it warm for a relatively long time is more conducive to obtaining a microstructure composed mainly of martensite, lower bainite, ferrite and stable retained austenite, with uniform and fine grains, so that a large amount of carbides precipitate, effectively improving the strength and toughness of the material; at the same time, it is more conducive to tempering to enhance the precipitation strengthening effect, and the precipitated carbides will pin dislocations, effectively hinder dislocation movement, enhance dislocation strengthening, and increase yield strength.

[0020] On the other hand, the present invention provides a steel material obtained by the heat treatment method of the steel material, which has a yield strength of 600-800 MPa, a tensile strength of 900-1000 MPa, an elongation after fracture of 20-30%, and a yield strength ratio of 0.7-0.8.

[0021] Preferably, the average impact energy of the steel at -40°C is 100-160J.

[0022] The heat treatment method of the present invention significantly improves the defects of poor low-temperature toughness and high yield strength ratio of steel, especially can make the steel after heat treatment reach the tensile strength level of 900MPa, while taking into account the advantages of low yield strength ratio and good low-temperature toughness.

[0023] In addition, the present invention also provides an application of the steel in the preparation of marine engineering structural devices. The carbon equivalent of the steel of the present invention satisfies: CET≤0.35%, P cm ≤0.28%, among which CET and P cm The calculation formulas are as follows: CET=C+(Mn+Mo) / 10+(Cu+Cr) / 20+Ni / 40, P cm =C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B, CET and P cm The calculation formula of each element represents its mass percentage in the steel. The steel of the present invention meets the requirements of GB / T712-2022 standard for ship and marine engineering structure steel, and belongs to 900MPa grade low yield ratio ultra-high strength marine steel, which is very suitable for use in marine engineering structures. In addition, the steel P cm ≤0.28%, good welding effect when used in marine steel, more in line with actual needs.

[0024] Compared to existing technologies, the present invention offers the following advantages: The heat treatment method of the present invention first performs a quenching treatment, followed by water cooling to 50-100°C for a period of time, and then air cooling to room temperature. This combined with tempering and air cooling significantly improves the toughness of the steel plate, achieving a tensile strength exceeding 900 MPa, achieving both high strength and good low-temperature toughness. Furthermore, the heat treatment method of the present invention is simple, produces stable products, is low-cost, and is readily available for widespread adoption. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The figure is a process flow chart of the heat treatment method of hot steel according to the present invention.

[0026] Figure 2 The microstructure of the steel in Example 1 of the present invention is martensite, lower bainite, ferrite and retained austenite with fine grains.

[0027] Figure 3 This is the microstructure morphology of the steel of Example 2 of the present invention, which is martensite, lower bainite, ferrite and retained austenite structure, and the grains are coarser than those of Example 1.

[0028] Figure 4 The microstructure of the steel in Example 3 of the present invention is martensite, lower bainite, ferrite and retained austenite, and the grains are coarser than those in Example 1 and Example 2.

[0029] Figure 5The microstructure of the steel in Comparative Example 1 of the present invention is mainly martensite. Compared with the grains of the steels in Examples 1 to 3, the grain size of Comparative Example 1 is the coarsest. DETAILED DESCRIPTION

[0030] To better illustrate the objectives, technical solutions, and advantages of the present invention, the present invention will be further described below with reference to specific examples. The experimental methods used in the examples and / or comparative examples are conventional methods unless otherwise specified; the materials and reagents used are commercially available unless otherwise specified.

[0031] Example 1

[0032] The marine engineering steel plate of this embodiment has a thickness of 40 mm and includes the following chemical compositions and their mass percentages: C 0.08%, Si 0.37%, Mn 1.10%, Ni 2.90%, Mo 0.32%, Cr 0.13%, Ti 0.008%, V 0.04%, Nb 0.045%, B 0.0013%, Al 0.055%, Cu 0.13%, P ≤ 100 ppm, S ≤ 40 ppm, CET = 0.31%, and the balance is Fe and unavoidable impurities.

[0033] The process flow chart of heat treatment of steel plate in this embodiment is shown in the attached figure. Figure 1 As shown, the specific steps include:

[0034] (1) The above-mentioned marine engineering steel plate was heated to 910°C for quenching and kept at this temperature for 30 minutes, then water-cooled to 100°C for 50 minutes, and then air-cooled to room temperature;

[0035] (2) Heating to 680°C in a furnace for tempering, keeping the temperature for 60 minutes, and air cooling to room temperature to obtain steel.

[0036] Example 2

[0037] The thickness of the marine engineering steel plate in this embodiment is 60 mm, and the chemical composition and its mass percentage are as follows: C 0.10%, Si 0.39%, Mn 1.2%, Ni 3.0%, Mo 0.35%, Cr 0.14%, Ti 0.009%, V 0.038%, Nb 0.049%, B 0.0015%, Al 0.06%, Cu 0.15%, P ≤ 100 ppm, S ≤ 40 ppm, CET = 0.34%, P cm ≤0.28%, the balance is Fe and unavoidable impurities;

[0038] The process flow chart of heat treatment of steel plate in this embodiment is shown in the attached figure. Figure 1 As shown, the specific steps include:

[0039] (1) The above-mentioned marine engineering steel plate was heated to 860°C for quenching, kept at this temperature for 60 minutes, water-cooled to 80°C and kept at this temperature for 30 minutes, and then air-cooled to room temperature;

[0040] (2) Heating to 660°C in a furnace for tempering, keeping the temperature for 120 minutes, and air cooling to room temperature to obtain steel.

[0041] Example 3

[0042] The thickness of the marine engineering steel plate in this embodiment is 80 mm, and the chemical composition and its mass percentage are as follows: C 0.10%, Si 0.43%, Mn 1.20%, Ni 2.90%, Mo 0.37%, Cr 0.16%, Ti 0.014%, V 0.03%, Nb 0.055%, B 0.0017%, Al 0.065%, Cu 0.15%, P ≤ 100 ppm, S ≤ 40 ppm, CET = 0.35%, P cm ≤0.28%, the balance is Fe and unavoidable impurities;

[0043] The process flow chart of heat treatment of steel plate in this embodiment is shown in the attached figure. Figure 1 As shown, the specific steps include:

[0044] (1) The above-mentioned marine engineering steel plate was heated to 900°C for quenching, kept at this temperature for 60 minutes, water-cooled to 50°C and kept at this temperature for 40 minutes, and then air-cooled to room temperature;

[0045] (2) Heating to 680°C in a furnace for tempering, keeping the temperature for 120 minutes, and air cooling to room temperature to obtain steel.

[0046] Comparative Example 1

[0047] The difference between this comparative example and Example 1 is that the steel plate in this comparative example has not been heat treated and is a rolled steel plate for marine engineering.

[0048] Comparative Example 2

[0049] The difference between this comparative example and Example 1 is that the quenching temperature in step (1) of this comparative example is 1100°C.

[0050] Comparative Example 3

[0051] The difference between this comparative example and Example 1 is that in step (1) of this comparative example, the steel is not water-cooled to 100° C. and kept warm for 50 minutes, but is directly water-cooled to room temperature.

[0052] Comparative Example 4

[0053] The difference between this comparative example and Example 1 is that the tempering temperature in step (2) of this comparative example is 550°C.

[0054] The steel materials of the above embodiments and comparative examples were subjected to performance tests:

[0055] (1) The steel is subjected to metallographic testing. The metallographic test results are as shown in the attached Figure 2-5 shown.

[0056] (2) The yield strength, tensile strength, elongation after fracture, yield strength ratio and -40℃ impact energy of the steel prepared above were tested with reference to GB / T 228.1-2021 and GB / T229-2020:

[0057] Table 1

[0058]

[0059] Table 2

[0060] Sample Yield strength / MPa Tensile strength / MPa Elongation after break / % Yield-to-strength ratio -40℃ impact energy / J Example 1 710 925 23 0.77 159 Example 2 683 910 21 0.75 137 Example 3 666 900 20 0.74 119 Comparative Example 1 756 967 11 0.78 3 Comparative Example 2 587 723 15 0.81 47 Comparative Example 3 751 974 16 0.77 87 Comparative Example 4 771 950 13 0.81 73

[0061] As can be seen from Examples 1-3, the steel of the present invention has a yield strength of 600-800 MPa, a tensile strength of 900-1000 MPa, an elongation after fracture of 20-30%, and a yield strength ratio of 0.7-0.8 after heat treatment.

[0062] By comparing Example 1 with Comparative Example 1, Figure 2 and 5 It was observed that before heat treatment, the steel was primarily martensite with coarse grains. After heat treatment, the steel structure consisted of lath martensite, lower bainite, ferrite, and retained austenite, with significantly smaller grain sizes. Combined with Table 2, it can be seen that the heat treatment method of the present invention significantly improves the room-temperature and low-temperature toughness of the steel by regulating the metallographic structure and refining the grain size.

[0063] By the attached Figure 2-4 As can be seen from Table 2, although the three steels of Examples 1-3 all have lath martensite, lower bainite, ferrite and retained austenite structures, it can be seen that the elemental composition, quenching treatment and tempering conditions of the steel will affect the grain size, metallographic distribution and mechanical properties inside the steel. Under the conditions of Example 1, the steel obtained in Example 1 has smaller grains and better toughness.

[0064] It can be seen from Example 1 and Comparative Examples 2-4 that when the quenching temperature in Comparative Example 2 is too high, the austenite grains in the steel will be coarse, resulting in a coarse martensite structure after heat treatment, which reduces the strength and toughness of the material. Therefore, when the temperature of the quenching treatment of the steel of the present invention is selected to be 860-910°C, the strength and low-temperature toughness are better; in Comparative Example 3, the steel is directly and quickly cooled to room temperature without being kept warm at 50-100°C, which will cause part of the austenite to be directly transformed into acicular martensite or other structures, rather than lath martensite and bainite with high-density dislocations, which seriously reduces the toughness; in Comparative Example 4, the tempering temperature is too low, and it cannot play an effective precipitation strengthening role, and has little effect on improving the low-temperature toughness of the steel; if the tempering temperature is too high, close to or exceeding the phase transformation temperature (A c3 ), long-term heat preservation in this temperature range will lead to rapid coarsening of the grains, and the second phase generated during the tempering process will gather and grow at the grain boundaries. The coarse matrix grains and the second phase gathered at the grain boundaries will seriously deteriorate the strength and toughness of the material. Therefore, after the quenching treatment, the tempering temperature range of the steel of the present invention is selected to be 650-700℃, and the strength and low-temperature toughness of the steel are better.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for heat treatment of steel, characterized in that: The steps include: (1) a steel plate is sequentially subjected to quenching treatment, water cooling to 50-100° C., heat preservation, and air cooling; the steel plate comprises the following elemental components in percentage by weight: C 0.05%-0.15%, Si 0.3%-0.5%, Mn 0.8%-1.5%, Ni 2%-3.5%, Mo 0.3%-0.42%, Cr 0.1%-0.2%, Ti 0.005%-0.02%, V 0.01%-0.08%, Nb 0.04%-0.06%, B 0.001%-0.002%, Al 0.05%-0.07%, Cu 0.1%-0.2%, and the balance is Fe and unavoidable impurities; (2) After tempering and air cooling, steel is obtained.

2. The heat treatment method for steel according to claim 1, characterized in that: In step (1), the temperature of the quenching treatment is 860-910° C., and the time of the quenching treatment is 30-60 min.

3. The heat treatment method for steel according to claim 1, characterized in that: In step (2), the tempering temperature is 650-700° C., and the tempering time is 30-120 min.

4. The heat treatment method for steel according to claim 1, characterized in that: In step (1), the thickness of the steel plate is 40-80 mm.

5. The heat treatment method for steel according to claim 1, characterized in that: In step (1), in the steel plate, the P content is ≤100 ppm, and the S content is ≤40 ppm.

6. The heat treatment method for steel according to claim 1, characterized in that: In step (1), the insulation time is 30 to 60 minutes.

7. A steel material obtained by the heat treatment method for steel material according to any one of claims 1 to 6.

8. The steel material according to claim 7, characterized in that: The yield strength is 600-800MPa, the tensile strength is 900-1000MPa, the elongation after break is 20-30%, and the yield strength ratio is 0.7-0.

8.

9. The steel material according to claim 7, characterized in that: The average impact energy of the steel at -40°C is 100-160J.

10. Use of the steel material according to any one of claims 7 to 9 in the preparation of marine engineering structural devices.

Images