High-toughness low-carbon bainite steel as well as preparation method and application thereof

Abstract

The invention discloses high-toughness low-carbon bainite steel and a preparation method and application thereof, and belongs to the technical field of metal heat treatment, and the high-toughness low-carbon bainite steel comprises the following chemical components: 0.2-0.3% of C, 1.50-1.70% of Si, 0.3-0.6% of Al, 0.1-0.3% of Mo, 1.80-2.1% of Mn, 0.75-0.95% of Cr, 0.01-0.03% of Ni and inevitable impurities. The preparation method comprises the following steps that steel is subjected to heat preservation heat treatment and then sequentially subjected to two-stage isothermal quenching heat treatment, two times of subzero treatment and tempering treatment, and the high-toughness low-carbon bainite steel is obtained. According to the preparation method disclosed by the invention, the low-carbon bainite steel is ensured to have good comprehensive mechanical properties on the basis of a preparation process combined with structure regulation and optimization. And in addition, a complex process is avoided, the production efficiency is improved, and rapid and large-batch production of the low-carbon bainite steel is facilitated.

Description

Technical Field

[0001] This invention belongs to the field of metal heat treatment technology, and particularly relates to a high-toughness, low-carbon bainitic steel, its preparation method, and its application. Background Technology

[0002] Traditional high-strength steels (such as quenched and tempered martensitic steel) primarily achieve high strength by increasing carbon content and alloying elements. However, this leads to significant performance bottlenecks: insufficient toughness and brittleness risk, deteriorated weldability, complex manufacturing processes, and high energy consumption. To address these challenges, particularly in bridge engineering, offshore platforms, large hydraulic and electrical power steel pipes, building structures in cold regions, and heavy equipment manufacturing, the market urgently needs a new generation of high-performance structural steel that combines ultra-high strength, excellent low-temperature toughness, good weldability, and low manufacturing costs.

[0003] In the prior art, publication number CN108642403A proposes a 780MPa grade ultra-high strength Fe-Mn-Al-C lightweight cast steel and its preparation method. This method improves the quantity and morphology of k-type carbides by controlling the weight percentage of Mn and Al. The resulting lightweight cast steel has an austenitic-ferrite dual-phase structure, containing k-type carbides, V, Nb, and rare earth carbides. Carbide precipitation can pin grain boundaries, refine grains, and improve deformation resistance, while the retained austenite undergoes deformation-induced martensitic transformation during service, resulting in high strength and toughness. However, this technical solution relies excessively on carbon, and the heat treatment process is complex and requires a large amount of energy. Publication number CN 101565800 A discloses an HT780 steel plate and its manufacturing method, while publication number CN118222911 A discloses a high fracture toughness steel plate and its production method. Although the above-mentioned prior art, based on online heat treatment technology for ultra-low carbon steel, has successfully achieved an excellent combination of strength and toughness, its complex process inevitably leads to increased costs. Therefore, exploring the applicability of this technical path in steels with higher carbon content is not only an important technical issue but also an urgent strategic research. Thus, the preparation of high-toughness low-carbon bainitic steel represents a significant shift in high-strength steel from relying on carbon elements and complex heat treatment to relying on a technical path of ultra-purification, ultra-fine graining, and precise phase transformation control, and is a concentrated manifestation of the progress in materials science and metallurgical technology. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a high-toughness, low-carbon bainitic steel, its preparation method, and its applications.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A high-toughness, low-carbon bainitic steel, with the following chemical composition by mass percentage:

[0007] C 0.2-0.3%, Si 1.50-1.70%, Al 0.3-0.6%, Mo 0.1-0.3%, Mn 1.80-2.1%, Cr 0.75-0.95% and Ni 0.01-0.03%, as well as unavoidable impurities.

[0008] Preferably, the mass percentage ratio of C, Si, Al, Mo, Mn, Cr and Ni is: 2.3≤a≤13.4, a=10ln(4Al+6Mo+2Cr+20Ni-5C)-1.2(Si+Mn).

[0009] Beneficial Effects: Based on a deep understanding of the mechanism of action and economic efficiency of alloying elements, this invention scientifically designs the composition of high-toughness, low-carbon bainitic steel, guided by the principle of low-cost alloying. By precisely controlling the content of key elements, a balance is achieved between the material's excellent toughness and its low-cost preparation, significantly improving the strength and toughness of the steel. While reasonably controlling the chemical composition range of each element, the proportion of alloying elements is controlled by adjusting the relationship "2.3≤a≤13.4, a=10ln(4Al+6Mo+2Cr+20Ni-5C)-1.2(Si+Mn)", and the relative addition amounts of some key elements are clearly defined. Specifically, by controlling the parameter a (an index reflecting the comprehensive content and proportion of alloying elements) between 2.3 and 13.4, the steel achieves high strength while avoiding problems such as high alloy costs and component segregation caused by excessively high a values, and also preventing adverse effects on strength and toughness caused by excessively low a values. Therefore, based on the design of low-cost alloying elements and performance regulation, by controlling the alloying elements and regulating the microstructure of high-toughness steel, a microstructure of high-toughness steel with a multiphase structure of bainitic ferrite + M / A islands + cryogenic carbides was obtained, and good toughness was obtained while achieving high strength.

[0010] A method for preparing high-toughness, low-carbon bainitic steel includes the following steps:

[0011] After heat treatment, the steel is subjected to two-stage isothermal quenching heat treatment, two deep cryogenic treatments, and tempering treatment to obtain the high-toughness low-carbon bainitic steel.

[0012] Beneficial Effects: This invention employs a two-stage heat treatment process to refine the microstructure through two isothermal quenching processes, resulting in a refined matrix microstructure for superior performance. Subsequent deep cryogenic treatments further shrink the crystal lattice, stimulating atomic activity and creating favorable conditions for precipitation. Precise deep cryogenic temperatures ensure uniform distribution of precipitates within the material. Finally, low-temperature tempering removes the quenching stress caused by deep cryogenic treatment. These fine precipitates effectively refine the fracture path of the material, enhancing its toughness and ductility, ultimately resulting in superior comprehensive mechanical properties.

[0013] Preferably, the temperature of the heat treatment is 920~960℃ and the time is 25~40min.

[0014] Preferably, the two-stage isothermal quenching heat treatment includes a first-stage isothermal quenching heat treatment and a second-stage isothermal quenching heat treatment.

[0015] Preferably, the first-stage isothermal quenching heat treatment includes the following steps:

[0016] Cool to 420-450℃ at a rate of 15-35℃ / s and hold for 80-100 minutes.

[0017] Preferably, the second-stage isothermal quenching heat treatment includes the following steps:

[0018] Cool to 390-410℃ at a rate of 30-35℃ / s, hold for 60-80 minutes, and finally cool to 20-30℃ at a rate of 10-15℃ / s.

[0019] Preferably, the cryogenic treatment includes the following steps:

[0020] Cool to -70 to -80°C at a cooling rate of 40 to 60°C / s, hold for 10 to 15 minutes, and then heat up to 20 to 30°C at a rate of 10 to 15°C / s.

[0021] Two cryogenic treatments are performed after the above cryogenic treatment process is completed, and then the same cryogenic treatment is repeated once.

[0022] Preferably, the tempering process involves heating to 300-350°C at a heating rate of 15-25°C / s, holding at that temperature for 50-70 minutes, and then cooling at a rate of 10-15°C / s.

[0023] Compared with the prior art, the present invention has the following advantages and technical effects:

[0024] This invention, based on microstructure regulation combined with two-stage isothermal quenching heat treatment, cryogenic treatment, and tempering, yields a multiphase structure of nano-bainite + M / A islands + cryogenic carbides, improving the strength and toughness of the steel and ensuring excellent comprehensive mechanical properties of low-carbon bainitic steel. Furthermore, this invention shortens the preparation process of high-toughness low-carbon bainitic steel by controlling the two-stage isothermal process, cooling rate control, and the coupled regulation of cryogenic holding temperature and time, avoiding complex processes, improving production efficiency, and facilitating rapid and mass production of low-carbon bainitic steel. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0026] Figure 1 TEM image of the high-toughness, low-carbon bainitic steel prepared in Example 1;

[0027] Figure 2 SEM image of the high-toughness, low-carbon bainitic steel prepared in Example 2;

[0028] Figure 3 The image shows a SEM image of the high-toughness, low-carbon bainitic steel prepared in Example 3.

[0029] Figure 4 The image shows a TEM image of the high-toughness, low-carbon bainitic steel prepared in Comparative Example 1. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0032] Unless otherwise specified, all raw materials used in the embodiments of this invention were purchased through commercial channels;

[0033] The steel to be processed is a steel ingot prepared by a commissioned steel mill according to its chemical composition.

[0034] Unless otherwise specified, room temperature or normal temperature in the embodiments of the present invention refers to 25±3℃.

[0035] Example 1

[0036] A method for preparing high-toughness, low-carbon bainitic steel includes the following steps:

[0037] (1) Prepare the steel to be treated. The composition of the steel to be treated by mass percentage is C 0.21%, Si 1.55%, Al 0.35%, Cr 0.80%, Mo 0.12%, Mn 1.85% and Ni 0.01%, with the balance being Fe and unavoidable impurities. a=6.46. The steel to be treated is kept at 920℃ for 30min.

[0038] (2) After the heat preservation is completed, the steel is subjected to the first stage of isothermal quenching heat treatment: cooled to 450°C at a cooling rate of 15°C / s and held for 80 min. Then, the second stage of isothermal quenching heat treatment is carried out: cooled to 410°C at a cooling rate of 25°C / s and held for 60 min. Then cooled to 20°C at a cooling rate of 10°C / s to complete the double-stage isothermal quenching heat treatment.

[0039] (3) The steel after double isothermal quenching heat treatment is subjected to a first deep cryogenic treatment: cooled to -80℃ at a cooling rate of 40℃ / s and held for 15min, and then heated to 20℃ at 10℃ / s; then subjected to a second deep cryogenic treatment: cooled to -80℃ at a cooling rate of 40℃ / s and held for 15min, and finally heated to 20℃ at 10℃ / s to complete the two deep cryogenic treatments.

[0040] (4) The steel that has undergone cryogenic treatment is heated to 300°C at a heating rate of 15°C / s, held for 60 min, and finally cooled to 20°C at a rate of 10°C / s to obtain high-toughness low-carbon bainitic steel.

[0041] Example 2

[0042] A method for preparing high-toughness, low-carbon bainitic steel includes the following steps:

[0043] (1) Prepare the steel to be treated, wherein, by mass percentage, the composition of the steel to be treated is C 0.24%, Si 1.60%, Al 0.42%, Cr 0.85%, Mo 0.22%, Mn 1.92% and Ni 0.02%, with the balance being Fe and unavoidable impurities, a=9.39, and hold the steel to be treated at 940℃ for 30min.

[0044] (2) After the heat preservation is completed, the steel is subjected to the first stage of isothermal quenching heat treatment: cooled to 440°C at a cooling rate of 15°C / s and held for 90 min, and then subjected to the second stage of isothermal quenching heat treatment: cooled to 400°C at a cooling rate of 25°C / s and held for 70 min, and then cooled to 20°C at a cooling rate of 10°C / s to complete the double-stage isothermal quenching heat treatment.

[0045] (3) The steel after double isothermal quenching heat treatment is subjected to a first deep cryogenic treatment: cooled to -75℃ at a cooling rate of 45℃ / s and held for 12min, and then heated to 20℃ at 12℃ / s; then subjected to a second deep cryogenic treatment: cooled to -75℃ at a cooling rate of 45℃ / s and held for 12min, and finally heated to 20℃ at 12℃ / s to complete the two deep cryogenic treatments.

[0046] (4) The steel that has undergone cryogenic treatment is heated to 320°C at a heating rate of 15°C / s, held for 60 min, and finally cooled to 20°C at a rate of 15°C / s to obtain high-toughness low-carbon bainitic steel.

[0047] Example 3

[0048] A method for preparing high-toughness, low-carbon bainitic steel includes the following steps:

[0049] (1) Prepare the steel to be treated. The composition of the steel to be treated by mass percentage is C 0.30%, Si 1.70%, Al 0.58%, Cr 0.92%, Mo 0.28%, Mn 2.1% and Ni 0.03%, with the balance being Fe and unavoidable impurities. a=11.41. The steel to be treated is kept at 960℃ for 40min.

[0050] (2) After the heat preservation is completed, the steel is subjected to the first stage of isothermal quenching heat treatment: cooled to 420°C at a cooling rate of 35°C / s and held for 100 min, and then subjected to the second stage of isothermal quenching heat treatment: cooled to 390°C at a cooling rate of 30°C / s and held for 80 min, and then cooled to 20°C at a cooling rate of 10°C / s to complete the double-stage isothermal quenching heat treatment.

[0051] (3) The steel after double-stage isothermal quenching heat treatment is subjected to a first deep cryogenic treatment: cooled to -70℃ at a cooling rate of 60℃ / s and held for 12 minutes, and then heated to 20℃ at 15℃ / s; then subjected to a second deep cryogenic treatment: cooled to -70℃ at a cooling rate of 60℃ / s and held for 12 minutes, and finally heated to 20℃ at 15℃ / s to complete the two deep cryogenic treatments.

[0052] (4) The steel that has undergone cryogenic treatment is heated to 320°C at a heating rate of 15°C / s, held for 60 min, and finally cooled to 20°C at a rate of 15°C / s to obtain high-toughness low-carbon bainitic steel.

[0053] Comparative Example 1

[0054] A method for preparing high-toughness, low-carbon bainitic steel includes the following steps:

[0055] (1) Prepare the steel to be treated. The composition of the steel to be treated by mass percentage is C 0.21%, Si 1.55%, Al 0.35%, Cr 0.80%, Mo 0.12%, Mn 1.85% and Ni 0.01%, with the balance being Fe and unavoidable impurities. a=6.46. The steel to be treated is kept at 920℃ for 30min.

[0056] (2) After the heat preservation is completed, the steel is subjected to isothermal quenching heat treatment: cool to 410°C at a cooling rate of 15°C / s and hold for 60 minutes, then cool to 20°C at a cooling rate of 10°C / s to complete the isothermal quenching heat treatment.

[0057] (3) The steel that has undergone isothermal quenching heat treatment is heated to 300°C at a heating rate of 15°C / s, held for 60 min, and finally cooled to 20°C at a rate of 10°C / s to obtain high toughness low carbon bainitic steel.

[0058] Comparative Example 2

[0059] A method for preparing high-toughness, low-carbon bainitic steel includes the following steps:

[0060] (1) Prepare the steel to be treated, wherein, by mass percentage, the composition of the steel to be treated is C 0.24%, Si 1.60%, Al 0.42%, Cr 0.85%, Mo 0.22%, Mn 1.92% and Ni 0.02%, with the balance being Fe and unavoidable impurities, a=9.39, and hold the steel to be treated at 940℃ for 30min.

[0061] (2) After the heat preservation is completed, the steel is subjected to isothermal quenching heat treatment: cool to 400°C at a cooling rate of 15°C / s and hold for 70 minutes, then cool to 20°C at a cooling rate of 10°C / s to complete the isothermal quenching heat treatment.

[0062] (3) Deep cryogenic treatment of steel after isothermal quenching: cool to -75℃ at a cooling rate of 45℃ / s and hold for 12 minutes, then heat to 20℃ at a rate of 12℃ / s to complete the deep cryogenic treatment.

[0063] (4) The steel that has undergone cryogenic treatment is heated to 320°C at a heating rate of 15°C / s, held for 60 min, and finally cooled to 20°C at a rate of 15°C / s to obtain high-toughness low-carbon bainitic steel.

[0064] Comparative Example 3

[0065] A method for preparing high-toughness, low-carbon bainitic steel includes the following steps:

[0066] (1) Prepare the steel to be treated, wherein, by mass percentage, the composition of the steel to be treated is C 0.50%, Si 1.50%, Al 0.3%, Cr 0.8%, Mo 0.1%, Mn 2.1% and Ni 0.03%, with the balance being Fe and unavoidable impurities, a=-0.5, and hold the steel to be treated at 960℃ for 40min.

[0067] (2) After the heat preservation is completed, the steel is subjected to the first stage of isothermal quenching heat treatment: cooled to 420°C at a cooling rate of 35°C / s and held for 100 min, and then subjected to the second stage of isothermal quenching heat treatment: cooled to 390°C at a cooling rate of 30°C / s and held for 80 min, and then cooled to 20°C at a cooling rate of 10°C / s to complete the double-stage isothermal quenching heat treatment.

[0068] (3) The steel after double-stage isothermal quenching heat treatment is subjected to a first deep cryogenic treatment: cooled to -70℃ at a cooling rate of 60℃ / s and held for 12 minutes, and then heated to 20℃ at 15℃ / s; then subjected to a second deep cryogenic treatment: cooled to -70℃ at a cooling rate of 60℃ / s and held for 12 minutes, and finally heated to 20℃ at 15℃ / s to complete the two deep cryogenic treatments.

[0069] (4) The steel that has undergone cryogenic treatment is heated to 320°C at a heating rate of 15°C / s, held for 60 min, and finally cooled to 20°C at a rate of 15°C / s to obtain high-toughness low-carbon bainitic steel.

[0070] Technical effects:

[0071] 1. Performance Characterization

[0072] Figure 1 The image shows a TEM image of the high-toughness, low-carbon bainitic steel prepared in Example 1. It can be seen that the microstructure of the high-toughness, low-carbon bainitic steel prepared in Example 1 consists of nanobainite + M / A islands + cryogenic carbides.

[0073] Figure 2 The image shows a SEM image of the high-toughness, low-carbon bainitic steel prepared in Example 2. It can be seen that the microstructure of the high-toughness, low-carbon bainitic steel prepared in Example 2 consists of nanobainite + M / A islands + cryogenic carbides.

[0074] Figure 3 The image shows a SEM image of the high-toughness, low-carbon bainitic steel prepared in Example 3. It can be seen that the microstructure of the high-toughness, low-carbon bainitic steel prepared in Example 3 consists of nanobainite + M / A islands + cryogenic carbides.

[0075] Figure 4 The image shows a TEM image of the high-toughness, low-carbon bainitic steel prepared in Comparative Example 1. The microstructure of the prepared high-toughness, low-carbon bainitic steel is nanobainite + M / A islands.

[0076] 2. Mechanical properties

[0077] The mechanical properties of the high-toughness, low-carbon bainitic steels obtained in Examples 1-3 and Comparative Examples 1-3 were tested.

[0078] Among them, the hardness of high-toughness low-carbon bainitic steel was tested according to GB / T 230.1-2018 standard;

[0079] The impact toughness of high-toughness low-carbon bainitic steel at -70~-80℃ was tested according to GB / T 229-2020 standard.

[0080] The results are shown in Table 1:

[0081] Table 1

[0082] Hardness (HRC) <![CDATA[Impact toughness (J / cm 2 )]]> Example 1 44.1 62.5 (two deep-cooling cycles to -80℃) Example 2 45.3 66.2 (two deep freezes to -75℃) Example 3 46.6 68.4 (two deep freezes to -70℃) Comparative Example 1 39.4 45.5 (room temperature) Comparative Example 2 42.3 53.2 (single cryogenic exposure to -75℃) Comparative Example 3 49.1 48.4 (two deep freezes to -70℃)

[0083] Comparative Examples 1-3 of this invention are schemes obtained by changing the relevant preparation process parameters of Examples 1-3, respectively. As can be seen from the above data, the performance parameters of Comparative Examples 1-3 changed significantly after the preparation process parameters were changed. Specifically, compared with Example 1, Comparative Example 1 changed the heat treatment method after heat preservation of the steel, including only one isothermal quenching heat treatment and excluding two deep cryogenic treatments. Figure 1 It can be seen that the steel obtained in Comparative Example 1 has a coarsened microstructure and does not produce cryogenic carbides, thus increasing hardness and significantly reducing toughness. Compared with Example 2, Comparative Example 2 changed the heat treatment method after heat treatment of the steel, including only one isothermal quenching heat treatment and only one cryogenic treatment, according to... Figure 2 It can be seen that the steel obtained in Comparative Example 2 has a coarsened microstructure, less ferrite lattice shrinkage, and produces a small amount of cryogenic carbides. Furthermore, compared to Example 2, the strength and toughness of the steel obtained in Comparative Example 2 are significantly reduced. Compared to Example 3, Comparative Example 3 alters the chemical composition of the steel to be treated, increasing the C content and decreasing the Al, Cr, and Mo elements. Ultimately, compared to Example 3, the steel obtained in Comparative Example 3 exhibits increased brittleness, a slight increase in ductility and toughness, and although the hardness increases significantly, the toughness decreases significantly. Therefore, it can be concluded that the chemical composition of the steel to be treated in this invention cannot be obtained through conventional substitution using existing technologies.

[0084] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A high-toughness, low-carbon bainitic steel, characterized in that, Its chemical composition, by mass percentage, is as follows: C 0.2-0.3%, Si 1.50-1.70%, Al 0.3-0.6%, Mo 0.1-0.3%, Mn 1.80-2.1%, Cr 0.75-0.95% and Ni 0.01-0.03%, as well as unavoidable impurities.

2. The high-toughness, low-carbon bainitic steel according to claim 1, characterized in that, The mass percentage ratio of C, Si, Al, Mo, Mn, Cr and Ni is: 2.3≤a≤13.4, a=10ln(4Al+6Mo+2Cr+20Ni-5C)-1.2(Si+Mn).

3. A method for preparing high-toughness, low-carbon bainitic steel as described in claim 1 or 2, characterized in that, Includes the following steps: After heat treatment, the steel is subjected to two-stage isothermal quenching heat treatment, two deep cryogenic treatments, and tempering treatment to obtain the high-toughness low-carbon bainitic steel.

4. The method for preparing a high-toughness, low-carbon bainitic steel according to claim 3, characterized in that, The heat treatment temperature is 920~960℃ and the time is 25~40min.

5. The method for preparing a high-toughness, low-carbon bainitic steel according to claim 3, characterized in that, The two-stage isothermal quenching heat treatment includes a first-stage isothermal quenching heat treatment and a second-stage isothermal quenching heat treatment.

6. The method for preparing a high-toughness, low-carbon bainitic steel according to claim 5, characterized in that, The first-stage isothermal quenching heat treatment includes the following steps: Cool to 420-450℃ at a rate of 15-35℃ / s and hold for 80-100 minutes.

7. The method for preparing a high-toughness, low-carbon bainitic steel according to claim 5, characterized in that, The second-stage isothermal quenching heat treatment includes the following steps: Cool to 390-410℃ at a rate of 30-35℃ / s, hold for 60-80 minutes, and finally cool to 20-30℃ at a rate of 10-15℃ / s.

8. The method for preparing a high-toughness, low-carbon bainitic steel according to claim 3, characterized in that, The cryogenic treatment includes the following steps: Cool to -70 to -80°C at a cooling rate of 40 to 60°C / s, hold for 10 to 15 minutes, and then heat to 20 to 30°C at a rate of 10 to 15°C / s.

9. The method for preparing a high-toughness, low-carbon bainitic steel according to claim 3, characterized in that, The tempering process involves heating to 300-350°C at a heating rate of 15-25°C / s, holding at that temperature for 50-70 minutes, and then cooling at a rate of 10-15°C / s.

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