Hydrogen embrittlement-resistant annealing process for bainitic steel

Abstract

The present invention provides a hydrogen embrittlement-resistant annealing process for bainitic steel, comprising: a first stage: cooling a steel plate, which has been forged at 920-950°C, to 330-350°C, and maintaining the temperature for 1.5 h; a second stage: heating the steel plate to 630-650°C, and maintaining the temperature for 1 h; a third stage: reheating the steel plate to 900-920°C, and maintaining the temperature for 1.5 h; a fourth stage: re-cooling the steel plate to 330-350°C, and maintaining the temperature for 1.5 h; a fifth stage: heating the steel plate to 650-670°C, and maintaining the temperature for 1 h; and a sixth stage: slowly cooling the steel plate to 180-200°C, and then carrying out air cooling on same to room temperature. The multi-stage repeated annealing treatment process can effectively improve the mechanical properties of bainitic steel, such as tensile strength, elongation, reduction of area, and impact toughness, and can effectively reduce the hydrogen embrittlement sensitivity of bainitic steel.

Description

An annealing process for hydrogen embrittlement resistance of bainitic steel Technical Field

[0001] This invention relates to stainless steel annealing processes, and more particularly to a hydrogen embrittlement annealing process for bainitic steel. Background Technology

[0002] Bainitic steel is a type of steel whose matrix microstructure is bainitic in its service state. This classification is based on the microstructure of the normalized state. Heated to 900℃ and then cooled in air, its microstructure contains a significant amount of bainite. Bainitic steel possesses excellent mechanical properties and wear resistance, and is widely used in automobiles, machinery manufacturing, aerospace, shipbuilding, petrochemical high-pressure pipelines, and pressure vessels.

[0003] The high strength, wear resistance, toughness, and weldability of bainitic steel are related to the depth of its heat treatment process. Heat treatment of bainitic steel typically includes heating, holding, and cooling. Heating causes a phase transformation in the bainitic steel's microstructure, holding transforms the microstructure into bainite, and cooling stabilizes the bainitic microstructure. For example, normalizing, quenching, tempering, and annealing are all different heat treatment methods for bainitic steel.

[0004] Patent CN106521107B discloses a novel spheroidizing annealing method for low-carbon bainitic steel. The method involves first heating the steel to a relatively low temperature (T3) and holding it there, allowing the retained austenite to fully transform into ferrite and carbides. Then, the steel is heated to a higher temperature (T4) and held there, causing carbide particles to precipitate and gradually grow into spheroids, ultimately resulting in a spheroidized microstructure of spheroidized carbides and ferrite. The steel is then air-cooled or slowly cooled to room temperature. The T3 temperature ranges from 350℃ to 550℃ or from 570℃ to 660℃, with a holding time of 3-8 hours in the low-temperature section. The T4 temperature ranges from 670℃ to 770℃, with a holding time of 8-20 hours in the high-temperature section. The heating rate between T3 and T4 is unlimited. This spheroidizing annealing method is suitable for low-carbon bainitic steel with a carbon content ≤0.3wt%, effectively improving the spheroidizing effect. After annealing, the pearlite content is above 90%, reaching a maximum of 100%. However, bainitic steel treated with this annealing method has a high sensitivity to hydrogen embrittlement. Therefore, it is necessary to improve the annealing process of bainitic steel to enhance its resistance to hydrogen embrittlement. Summary of the Invention

[0005] The purpose of this invention is to improve the hydrogen embrittlement susceptibility of bainitic steel, and to propose a hydrogen embrittlement annealing process for bainitic steel.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] An annealing process for hydrogen embrittlement resistant bainitic steel involves melting and casting into a billet according to a predetermined composition, heating the billet at 1200℃ for 1.5-2 hours, rolling it to a final rolling temperature of 920-950℃, and finally rolling it into a steel plate with a thickness of 6mm. The steel plate is then transferred to an annealing furnace for annealing treatment, including the following stages:

[0008] First stage: The steel plate is cooled from 920-950℃ to 330-350℃ in the annealing furnace and held at 330-350℃ for 1.5 hours;

[0009] Second stage: The steel plate is heated from 330-350℃ to 630-650℃ in the annealing furnace and held at 630-650℃ for 1 hour;

[0010] The third stage: The steel plate is heated again from 630-650℃ to 900-920℃ in the annealing furnace and held at 900-920℃ for 1.5 hours;

[0011] Fourth stage: The steel plate is cooled again from 900-920℃ to 330-350℃ in the annealing furnace and held at 330-350℃ for 1.5 hours;

[0012] Fifth stage: The steel plate is heated from 330-350℃ to 650-670℃ in the annealing furnace and held at 650-670℃ for 1 hour;

[0013] Stage 6: The steel plate is slowly cooled from 650-670℃ to 180-200℃ in the annealing furnace, and then the steel plate is taken out of the annealing furnace and slowly air-cooled to room temperature.

[0014] Preferably, the cooling rate of the steel plate in the first stage and the fourth stage is less than 20°C / h.

[0015] Preferably, the heating rate of the steel plate in the second stage and the fifth stage is 38-40℃ / s.

[0016] Preferably, the heating rate of the steel plate in the third stage is 45-48℃ / s.

[0017] Preferably, the steel plate in the sixth stage is slowly cooled at a cooling rate not exceeding 15°C / h.

[0018] Preferably, the bainitic steel, by mass percentage, comprises: C: 0.25-0.28%; Mn: 1.9-2.2%; Si: 1.7-1.8%; Cr: 1.5-1.6%; Ni: 1.35-0.4%; Al: 0.18-0.22%; S: 0.014-0.016%; and at least one of Mo, W, and Nb, wherein Mo: 0.2-0.4%, W: 0.8-1.0%, and Nb: 0.02-0.06%; the balance being Fe and other unavoidable impurities.

[0019] Preferably, the bainitic steel, by mass percentage, comprises: C: 0.25-0.28%; Mn: 1.9-2.2%; Si: 1.7-1.8%; Cr: 1.5-1.6%; Ni: 1.35-0.4%; Al: 0.18-0.22%; S: 0.014-0.016%; W: 1.0%; Nb: 0.02-0.06%; with the balance being Fe and other unavoidable impurities.

[0020] Preferably, the unavoidable impurities contain P ≤ 0.02%.

[0021] Preferably, the bainitic steel has a tensile strength ≥1700MPa, elongation ≥18%, reduction of area ≥55%, and impact toughness ≥100J / cm2.

[0022] Compared with existing technologies, this invention adopts a multi-stage repeated annealing process to improve the diffusion coefficient of hydrogen in the multi-stage process, which is conducive to the escape of hydrogen and reduces the amount of hydrogen dissolved in steel. This can effectively improve the mechanical properties of bainitic steel, such as tensile strength, elongation, reduction of area, and impact toughness, while effectively reducing the hydrogen embrittlement sensitivity of bainitic steel. Attached Figure Description

[0023] Figure 1 shows the microstructure of a steel plate subjected to hydrogen embrittlement annealing process for bainitic steel.

[0024] Figure 2 shows the precipitates of steel plates subjected to hydrogen embrittlement annealing process for bainitic steel. Detailed Implementation

[0025] To provide a further understanding of the purpose, structure, features, and functions of the present invention, detailed descriptions are provided below with reference to specific embodiments.

[0026] Referring to Figures 1 and 2, a hydrogen embrittlement annealing process for bainitic steel is described. The steel is melted according to a set composition and cast into a billet. The billet is heated at 1200℃ for 1.5-2 hours and then rolled to a final rolling temperature of 920-950℃, ultimately producing a steel plate with a thickness of 6mm. The steel plate is then transferred to an annealing furnace for annealing treatment, including the following stages:

[0027] First stage: The steel plate is cooled from 920-950℃ to 330-350℃ in the annealing furnace and held at 330-350℃ for 1.5 hours;

[0028] Second stage: The steel plate is heated from 330-350℃ to 630-650℃ in the annealing furnace and held at 630-650℃ for 1 hour;

[0029] The third stage: The steel plate is heated again from 630-650℃ to 900-920℃ in the annealing furnace and held at 900-920℃ for 1.5 hours;

[0030] Fourth stage: The steel plate is cooled again from 900-920℃ to 330-350℃ in the annealing furnace and held at 330-350℃ for 1.5 hours;

[0031] Fifth stage: The steel plate is heated from 330-350℃ to 650-670℃ in the annealing furnace and held at 650-670℃ for 1 hour;

[0032] Stage 6: The steel plate is slowly cooled from 650-670℃ to 180-200℃ in the annealing furnace, and then the steel plate is taken out of the annealing furnace and slowly air-cooled to room temperature.

[0033] The annealing process of this invention is applicable to low-carbon bainitic steel. The bainitic steel annealed using this process has the following composition by mass percentage: C: 0.25-0.28%; Mn: 1.9-2.2%; Si: 1.7-1.8%; Cr: 1.5-1.6%; Ni: 1.35-0.4%; Al: 0.18-0.22%; S: 0.014-0.016%; and also contains at least one of Mo, W, and Nb, wherein Mo: 0.2-0.4%, W: 0.8-1.0%, and Nb: 0.02-0.06%; the balance being Fe and other unavoidable impurities.

[0034] The annealing process of this invention is for furnace cold annealing of hot-rolled steel billets, and involves two similar annealing operations in the annealing furnace. The first round of annealing consists of the first to third stages, and the second round of annealing consists of the fourth to sixth stages. In both rounds, the temperature is first reduced from over 900°C to about 350°C, and then raised to about 650°C. The difference is that in the first round of annealing, the third stage starts from about 650°C and then rises back to over 900°C for the second round of annealing, while in the second round of annealing, the sixth stage starts from about 650°C and is first furnace cooled to about 200°C, and then air cooled to room temperature.

[0035] Bainite, being an unstable microstructure, undergoes a transformation towards equilibrium during annealing, resulting in bainitic decomposition. During this decomposition, dissolved carbon from the supersaturated iron matrix precipitates as carbides. As the holding time increases, the bainite continues to decompose, leading to the formation of bulk ferrite within the steel plate. However, excessively long holding times will gradually decrease the strength of the steel plate.

[0036] In the first and fourth stages, the temperature of the steel plate in the annealing furnace decreases, with the largest decrease among all stages, reaching a lower supercooling temperature. This allows for a more complete and rapid transformation of austenite into bainite. As the temperature decreases, the solubility of hydrogen in the steel also decreases sharply. Hydrogen has very low solubility in α-Fe, and it has a larger diffusion coefficient in α-Fe than in γ-Fe; the difference becomes greater at lower temperatures. In these two stages, austenite fully transforms into bainite, and due to the lower temperature, the residual austenite content is lower. Dehydrogenation of the steel plate in a ferrite state effectively reduces the dissolved hydrogen content in the steel, thus effectively reducing the hydrogen embrittlement sensitivity of bainitic steel.

[0037] In a preferred embodiment, the steel plate in the first stage can be cooled from 950°C to 350°C in an annealing furnace.

[0038] In a preferred embodiment, the steel plate in the fourth stage can be cooled from 920°C to 320°C in an annealing furnace.

[0039] In the second and fifth stages, the temperature is slightly increased to around 650℃ for two-phase annealing, and held at that temperature for 1 hour. When the annealing temperature exceeds 650℃, the yield strength and tensile strength gradually increase as the annealing temperature decreases; conversely, when the annealing temperature is below 650℃, the yield strength and tensile strength gradually decrease as the annealing temperature continues to decrease; the yield strength and tensile strength of the steel plate are optimal at around 650℃.

[0040] In a preferred embodiment, the steel plate in the second stage can be heated to 650°C in an annealing furnace and held for 1 hour.

[0041] In a preferred embodiment, the steel plate in the fourth stage can be heated to 670°C in an annealing furnace and held for 1 hour.

[0042] This invention employs a staged, repeated annealing process, performing two similar annealing operations, followed by a third stage where the temperature is raised again to 900-920℃ for re-annealing. First, the temperature is drastically reduced to a supercooled temperature (around 350℃ in this invention), allowing austenite to transform more fully and rapidly into bainite, reducing the residual austenite content and enabling dehydrogenation of the steel plate in a ferrite state. This effectively reduces the dissolved hydrogen content in the steel, thereby significantly lowering the hydrogen embrittlement sensitivity of bainitic steel. Next, the temperature is slightly raised to around 650℃ and held in the two-phase region for 1 hour. Under these conditions, the steel plate maintains a state where bainite decomposition is weak and Nb precipitation strengthening is dominant, effectively improving the tensile strength, elongation, impact toughness, and other mechanical properties of bainitic steel, while simultaneously reducing its hydrogen embrittlement sensitivity.

[0043] Furthermore, this invention causes elements such as Nb added to the steel to precipitate repeatedly and diffusely in the form of carbides, which plays a role in precipitation strengthening, increases the strength of the steel plate, and effectively compensates for the strength reduction caused by the decomposition of bainite during annealing.

[0044] In some embodiments, the heating rate of the steel plate in the second and fifth stages is 38-40℃ / s.

[0045] In some embodiments, the heating rate of the steel plate in the third stage is 45-48℃ / s.

[0046] In one embodiment, the bainitic steel, by mass percentage, comprises: C: 0.25-0.28%; Mn: 1.9-2.2%; Si: 1.7-1.8%; Cr: 1.5-1.6%; Ni: 1.35-0.4%; Al: 0.18-0.22%; S: 0.014-0.016%; W: 1.0%; Nb: 0.02-0.06%; with the balance being Fe and other unavoidable impurities. After annealing using the method of this invention, the W-containing bainitic steel exhibits lower hydrogen embrittlement sensitivity compared to Mo-containing and Nb-containing bainitic steels, and also demonstrates better surface properties such as elongation after fracture and impact toughness. The unavoidable impurities include P ≤ 0.02%.

[0047] The bainitic steel treated by the annealing process of this invention has a tensile strength ≥1700MPa, elongation ≥18%, reduction of area ≥55%, and impact toughness ≥100J / cm². 2 .

[0048] In the description of this invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0049] The present invention has been described in the above-described embodiments; however, these embodiments are merely examples for implementing the present invention. It must be noted that the disclosed embodiments do not limit the scope of the present invention. Conversely, any modifications and refinements made without departing from the spirit and scope of the present invention are within the scope of patent protection of the present invention.

Claims

1. A hydrogen embrittlement annealing process for bainitic steel, characterized in that, The steel is melted and cast into a billet according to the set composition. The billet is heated at 1200℃ for 1.5-2 hours and then rolled. The final rolling temperature is 920-950℃, and finally rolled into a steel plate with a thickness of 6mm. The steel plate is transferred to an annealing furnace for annealing treatment, which includes the following stages: First stage: The steel plate is cooled from 920-950℃ to 330-350℃ in the annealing furnace and held at 330-350℃ for 1.5 hours; Second stage: The steel plate is heated from 330-350℃ to 630-650℃ in the annealing furnace and held at 630-650℃ for 1 hour; The third stage: The steel plate is heated again from 630-650℃ to 900-920℃ in the annealing furnace and held at 900-920℃ for 1.5 hours; Fourth stage: The steel plate is cooled again from 900-920℃ to 330-350℃ in the annealing furnace and held at 330-350℃ for 1.5 hours; Fifth stage: The steel plate is heated from 330-350℃ to 650-670℃ in the annealing furnace and held at 650-670℃ for 1 hour; Stage 6: The steel plate is slowly cooled from 650-670℃ to 180-200℃ in the annealing furnace, and then the steel plate is taken out of the annealing furnace and slowly air-cooled to room temperature.

2. The hydrogen embrittlement annealing process for bainitic steel as described in claim 1, characterized in that, The cooling rate of the steel plate in the first stage and the fourth stage is less than 20°C / h.

3. The hydrogen embrittlement annealing process for bainitic steel as described in claim 1, characterized in that, The heating rate of the steel plate in the second and fifth stages is 38-40℃ / s.

4. The hydrogen embrittlement annealing process for bainitic steel as described in claim 1, characterized in that, The heating rate of the steel plate in the third stage is 45-48℃ / s.

5. The hydrogen embrittlement annealing process for bainitic steel as described in claim 1, characterized in that, The steel plate in the sixth stage is slowly cooled at a cooling rate not exceeding 15°C / h.

6. The hydrogen embrittlement annealing process for bainitic steel as described in claim 1, characterized in that, The bainitic steel, by mass percentage, comprises: C: 0.25-0.28%; Mn: 1.9-2.2%; Si: 1.7-1.8%; Cr: 1.5-1.6%; Ni: 1.35-0.4%; Al: 0.18-0.22%; S: 0.014-0.016%; and at least one of Mo, W, and Nb, wherein Mo: 0.2-0.4%, W: 0.8-1.0%, and Nb: 0.02-0.06%; the balance being Fe and other unavoidable impurities.

7. The hydrogen embrittlement annealing process for bainitic steel as described in claim 6, characterized in that, The bainitic steel, by mass percentage, comprises: C: 0.25-0.28%; Mn: 1.9-2.2%; Si: 1.7-1.8%; Cr: 1.5-1.6%; Ni: 1.35-0.4%; Al: 0.18-0.22%; S: 0.014-0.016%. W: 1.0%, Nb: 0.02-0.06%; balance is Fe and other unavoidable impurities.

8. The hydrogen embrittlement annealing process for bainitic steel as described in claim 6, characterized in that: Unavoidable impurities include P ≤ 0.02%.

9. The hydrogen embrittlement annealing process for bainitic steel as described in claim 6, characterized in that: The bainitic steel has a tensile strength ≥1700MPa, elongation ≥18%, reduction of area ≥55%, and impact toughness ≥100J / cm². 2 .

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