A heat treatment method for improving hydrogen embrittlement resistance of coated hot-formed steel

Abstract

The application provides a heat treatment method for improving the hydrogen embrittlement resistance of a coated hot-formed steel, and the heat treatment method comprises the following steps: high-temperature annealing of a pre-coated steel plate at 600-690 DEG C; hot stamping forming; step tempering; the method can effectively reduce the hydrogen content in the coated hot-formed steel with a tensile strength of more than 1000 MPa, and the hydrogen content of the coated hot-formed steel after the heat treatment process of the application can be reduced to less than or equal to 0.65 ppm.

Description

Technical Field

[0001] This invention belongs to the field of hot-formed steel technology, and specifically relates to a heat treatment method for improving the hydrogen embrittlement resistance of coated hot-formed steel. Background Technology

[0002] Against the backdrop of achieving carbon peaking and carbon neutrality, and protecting the ecological environment, lightweighting of automobiles has become an irreversible trend. "High strength and thinner profile" is the most effective way to achieve overall vehicle lightweighting. Therefore, hot-formed steel, with its advantages of ultra-high strength and easy high-temperature forming, has emerged, and many well-known domestic and international automakers are vying for the technological high ground in the application of hot-formed steel in automotive manufacturing. Hot-formed steel products include cold and hot-formed bare sheet hot-formed steel, aluminum-silicon coated hot-formed steel, and zinc-based coated hot-formed steel. Among them, aluminum-silicon coated hot-formed steel and zinc-based coated hot-formed steel are widely used due to their excellent corrosion resistance. It is also worth mentioning that aluminum-silicon coated hot-formed steel also has excellent resistance to high-temperature oxidation.

[0003] Hot-formed steel has a fully martensitic microstructure and very low elongation. Furthermore, engineering applications show that ultra-high-strength steels (especially martensitic steels) with a tensile strength of 1000 MPa or higher exhibit significantly reduced mechanical properties and hydrogen embrittlement when the hydrogen content exceeds 2.0 ppm. The probability of hydrogen embrittlement increases significantly as the tensile strength of hot-formed steel increases from 1500 MPa to 1800 MPa or 2000 MPa, especially when the steel is machined into perforated parts, where its resistance to hydrogen embrittlement decreases more rapidly.

[0004] In addition to the steelmaking process increasing the hydrogen content in steel, the pickling process of hot-rolled pickled bare sheet hot-formed steel also increases the hydrogen content. The electrolytic cleaning process in the production line entrance area of ​​cold-rolled bare sheet hot-formed steel increases the hydrogen content. The electrolytic cleaning process in the production line entrance area and the oxidation-reduction process in the central area (with H2 content of 5.0-10.0% in the furnace) of aluminum-silicon coated hot-formed steel and zinc-based coated hot-formed steel also increase the hydrogen content. The storage, logistics, and re-storage processes from the production end to the customer end also increase the hydrogen content in steel.

[0005] Existing technologies disclose methods for hydrogen removal during steelmaking processes, such as adding Nb and V alloying elements to the chemical composition of high-strength steel alloys to form "hydrogen traps" in the matrix to fix migrating hydrogen. However, the hydrogen atoms fixed in this way are still unstable. Moreover, storage, logistics, and re-storage (including electrolytic cleaning and degreasing before use) during the production-to-consumer process increase the hydrogen content in the steel. Furthermore, when steel is processed into perforated parts, the presence of stress and acidic media significantly and irreversibly reduces the steel's resistance to hydrogen embrittlement. Therefore, for ultra-high-strength steels (especially martensitic steels) with a strength of 1000 MPa or higher, complete and effective hydrogen removal during the finished product manufacturing process can effectively improve the hydrogen embrittlement resistance of hot-formed steel, thereby ensuring the safe service performance of automotive parts made from hot-formed steel. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a heat treatment method for improving the hydrogen embrittlement resistance of coated hot-formed steel. This method can effectively reduce the hydrogen content in coated hot-formed steel with a tensile strength of 1000 MPa or higher, reducing the hydrogen content to below 0.65 ppm.

[0007] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0008] A heat treatment method for improving the hydrogen embrittlement resistance of coated hot-formed steel, the heat treatment method comprising the following steps:

[0009] (1) The coated steel sheet is annealed at a high temperature of 600-690℃. If the annealing temperature is lower than 600℃, the hydrogen in the steel cannot be effectively removed, and a pre-alloyed transition layer cannot be formed at the interface between the steel substrate and the coating to prevent hydrogen from the external environment from penetrating into the steel substrate. The effect of refining the grains is not obvious. If the annealing temperature is higher than 690℃, the grains will be coarse.

[0010] (2) Hot stamping forming;

[0011] (3) Step tempering: If one-time tempering is used, the strength of the steel will be significantly reduced when the tempering temperature is too high, and the hydrogen in the steel cannot be effectively removed when the tempering temperature is too low. Therefore, step tempering will not significantly reduce the strength of the steel, but can effectively remove the hydrogen in the steel and improve the plasticity and toughness of the steel.

[0012] In step (1), the holding time for high-temperature annealing is 25–80 min. If the holding time is less than 25 min, hydrogen cannot be effectively removed, and the grains cannot be refined. If the holding time is greater than 80 min, the grains will be coarse. After high-temperature annealing, the steel is air-cooled to room temperature. This process can refine the grains, improve the strength and toughness of the steel, prepare the microstructure for the hot stamping process, and effectively remove the hydrogen added to the steel in various processes at the steel production end.

[0013] In step (2), the hot stamping forming step specifically includes: first heating the coated steel plate to the austenitizing temperature range of 860-950℃ and holding it at that temperature, then transferring it to the mold for pressure holding and quenching, with a quenching and pressure holding time of 15-25s.

[0014] In step (2), the hot stamping forming step specifically includes: first heating the coated steel sheet to the austenitizing temperature range of 860-950℃ and holding it for 3.5-15.0 min, then transferring it to a mold for pressure quenching, with a transfer time of 5-10 s, a quenching cooling rate of 35-65℃ / s, and a quenching pressure holding time of 15-25 s. In step (3), the stepped tempering specifically includes the following steps: first heating to 260-350℃ and holding it, then air cooling; then heating to 190-250℃ and holding it, then air cooling.

[0015] In step (3), the stepped tempering specifically includes the following steps: first, heat to 260-350℃ and hold for 5-10 minutes, then air cool to room temperature; then heat to 190-250℃ and hold for 10-15 minutes, then air cool to room temperature.

[0016] The coated steel sheet is produced industrially through a process of molten iron treatment → converter smelting → alloy fine-tuning station → LF furnace refining → RH furnace refining → continuous casting steelmaking → hot rolling → pickling and cold rolling → continuous annealing → hot-dip galvanizing → finishing → tension leveling.

[0017] In step (1), the grade of the coated steel sheet is any one of HC800 / 1000HS+AS, HC950 / 1300HS+AS, HC1000 / 1500HS+AS, HC1100 / 1700HS+AS, and HC1200 / 1800HS+AS.

[0018] The hydrogen content of the coated hot-formed steel can be reduced to below 0.65 ppm after being treated by the heat treatment process of the present invention.

[0019] The heat treatment method provided by this invention reduces the amount of hydrogen added to the coated steel sheet from the production end to the customer end by performing high-temperature annealing and stepped tempering treatment on the coated steel sheet before and after hot stamping, thereby improving the hydrogen embrittlement resistance, ductility and toughness and safe service performance of the coated hot-formed steel. Attached Figure Description

[0020] Figure 1 The matrix structure of the coated steel sheet with grade HC1200 / 1800HS+AS in Example 5. Detailed Implementation

[0021] The present invention will now be described in detail with reference to the embodiments.

[0022] Example 1

[0023] (1) High temperature annealing: The coated steel sheet with grade HC800 / 1000HS+AS was heated to 615℃ and held for 35.0 min for high temperature annealing, and then air cooled to room temperature;

[0024] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 860℃ and hold for 3.5min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 5.0s, the quenching cooling rate is 35.0℃ / s, and the quenching holding time is 15.0s.

[0025] (3) Step tempering: The hot stamped steel parts are first heated to 265℃ and held for 5.0 min, and then air cooled to room temperature; then heated to 195℃ and held for 10.0 min, and then air cooled to room temperature.

[0026] Example 2

[0027] (1) High temperature annealing: The coated steel sheet with grade HC950 / 1300HS+AS was heated to 635℃ and held for 45.0 min for high temperature annealing, and then air cooled to room temperature;

[0028] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 880℃ and hold for 5.0 min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 6.0 s, the quenching cooling rate is 50.0℃ / s, and the quenching holding time is 16.5 s.

[0029] (3) Step tempering: The hot stamped steel parts are first heated to 295℃ and held for 6.0 min, and then air cooled to room temperature; then heated to 210℃ and held for 11.5 min, and then air cooled to room temperature.

[0030] Example 3

[0031] (1) High temperature annealing: The coated steel sheet with grade HC1000 / 1500HS+AS is heated to 650℃ and held for 55.0 min for high temperature annealing, and then air-cooled to room temperature;

[0032] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 900℃ and hold for 6.5min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 8.0s, the quenching cooling rate is 55.0℃ / s, and the quenching holding time is 19.0s.

[0033] (3) Step tempering: The hot stamped steel parts are first heated to 315℃ and held for 8.0 min, and then air cooled to room temperature; then heated to 225℃ and held for 12.5 min, and then air cooled to room temperature.

[0034] Example 4

[0035] (1) High temperature annealing: The coated steel sheet with grade HC1100 / 1700HS+AS was heated to 665℃ and held for 65.0 min for high temperature annealing, and then air cooled to room temperature;

[0036] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 925℃ and hold for 8.0 min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 9.0 s, the quenching cooling rate is 60.0℃ / s, and the quenching holding time is 21.0 s.

[0037] (3) Step tempering: The hot stamped steel parts are first heated to 335℃ and held for 9.0 min, and then air cooled to room temperature; then heated to 235℃ and held for 13.5 min, and then air cooled to room temperature.

[0038] Example 5

[0039] (1) High temperature annealing: The coated steel sheet with grade HC1200 / 1800HS+AS is heated to 690℃ and held for 80.0min for high temperature annealing, and then air-cooled to room temperature;

[0040] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 950℃ and hold for 9.0 min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 10.0 s, the quenching cooling rate is 65.0℃ / s, and the quenching holding time is 25.0 s.

[0041] (3) Step tempering: The hot stamped steel parts are first heated to 350℃ and held for 10.0 min, and then air cooled to room temperature; then heated to 250℃ and held for 15.0 min, and then air cooled to room temperature.

[0042] Comparative Example 1

[0043] (1) Annealing: The coated steel sheet with grade HC800 / 1000HS+AS is heated to 500℃ and held for 35.0min for annealing, and then air-cooled to room temperature;

[0044] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 860℃ and hold for 3.5min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 5.0s, the quenching cooling rate is 35.0℃ / s, and the quenching holding time is 15.0s.

[0045] (3) Tempering: The hot-stamped steel parts are first heated to 265℃ and held for 5.0 min, then air-cooled to room temperature; then heated to 195℃ and held for 10.0 min, then air-cooled to room temperature.

[0046] Comparative Example 2

[0047] (1) Annealing: The coated steel sheet with grade HC800 / 1000HS+AS is heated to 615℃ and held for 35.0 min for annealing, and then air-cooled to room temperature;

[0048] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 860℃ and hold for 3.5min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 5.0s, the quenching cooling rate is 35.0℃ / s, and the quenching holding time is 15.0s.

[0049] (3) Tempering: Heat the hot-stamped steel parts to 250°C and hold for 25.0 min, then air cool to room temperature.

[0050] Comparative Example 3

[0051] (1) Annealing: The coated steel sheet with grade HC800 / 1000HS+AS is heated to 615℃ and held for 35.0 min for annealing, and then air-cooled to room temperature;

[0052] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 860℃ and hold for 3.5min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 5.0s, the quenching cooling rate is 35.0℃ / s, and the quenching holding time is 15.0s.

[0053] (3) Tempering: The hot-stamped steel parts are first heated to 390℃ and held for 9.0 min, then air-cooled to room temperature; then heated to 280℃ and held for 10.0 min, then air-cooled to room temperature.

[0054] Comparative Example 4

[0055] (1) Annealing: The coated steel sheet with grade HC1200 / 1800HS+AS is heated to 500℃ and held for 80.0min for annealing, and then air-cooled to room temperature;

[0056] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 950℃ and hold for 9.0 min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 10.0 s, the quenching cooling rate is 65.0℃ / s, and the quenching holding time is 25.0 s.

[0057] (3) Tempering: The hot-stamped steel parts are first heated to 350℃ and held for 10.0 min, then air-cooled to room temperature; then heated to 250℃ and held for 15.0 min, then air-cooled to room temperature.

[0058] Comparative Example 5

[0059] (1) Annealing: The coated steel sheet with grade HC1200 / 1800HS+AS is heated to 690℃ and held for 80.0min for annealing, and then air-cooled to room temperature;

[0060] (2) Hot stamping: First, heat the coated steel sheet after high temperature annealing to 950℃ and hold for 9.0 min. Then transfer it to the mold for pressure holding and quenching. The transfer time is 10.0 s, the quenching cooling rate is 65.0℃ / s, and the quenching holding time is 25.0 s.

[0061] (3) Tempering: Heat the hot-stamped steel parts to 250°C and hold for 25.0 min, then air cool to room temperature.

[0062] The properties of the coated steel sheets before high-temperature annealing in each of the above embodiments and comparative examples are shown in Table 1.

[0063] Table 1

[0064]

[0065] The properties of the coated hot-formed steel after heat treatment according to the various embodiments and comparative examples are shown in Table 2.

[0066] Table 2

[0067]

[0068]

[0069] The results above show that the technical solution of the present invention has good adaptability. The hydrogen content of the hot-formed steel in the embodiment is reduced to below 0.65 ppm. At the same time, the plasticity and toughness (elongation and ultimate cold bending angle) of the hot-formed steel are significantly improved.

[0070] The above detailed description of a heat treatment method for improving the hydrogen embrittlement resistance of coated hot-formed steel with reference to the embodiments is illustrative rather than limiting. Several embodiments may be listed within the defined scope. All technical changes and modifications that fall within the overall concept of this invention should be within the protection scope of this invention.

Claims

1. A heat treatment method for improving the hydrogen embrittlement resistance of coated hot-formed steel, characterized in that, The heat treatment method includes the following steps: (1) The coated steel sheet is annealed at a high temperature of 600-690℃; (2) Hot stamping forming; (3) Step tempering; In step (1), the holding time for high-temperature annealing is 25 to 80 minutes, and after high-temperature annealing, the temperature is air-cooled to room temperature; In step (3), the stepped tempering specifically includes the following steps: first, heating to 260-350℃ and holding, then air cooling; then heating to 190-250℃ and holding, then air cooling.

2. The heat treatment method according to claim 1, characterized in that, In step (2), the hot stamping forming step specifically includes: first heating the coated steel plate to the austenitizing temperature of 860-950℃ and holding it at that temperature, then transferring it to the mold for pressure holding and quenching, with a quenching and pressure holding time of 15-25s.

3. The heat treatment method according to claim 1 or 2, characterized in that, In step (2), the hot stamping forming step specifically includes: first heating the coated steel plate to the austenitizing temperature of 860-950℃ and holding it for 3.5-15.0 min, then transferring it to the mold for pressure holding and quenching, the transfer time is 5-10 s, the quenching cooling rate is 35-65℃ / s, and the quenching pressure holding time is 15-25 s.

4. The heat treatment method according to claim 1, characterized in that, In step (3), the stepped tempering specifically includes the following steps: first, heat to 260-350℃ and hold for 5-10 minutes, then air cool to room temperature; then heat to 190-250℃ and hold for 10-15 minutes, then air cool to room temperature.

5. The heat treatment method according to claim 1 or 2, characterized in that, The coated steel sheet is produced through a process of molten iron treatment → converter smelting → alloy fine-tuning station → LF furnace refining → RH furnace refining → continuous casting steelmaking → hot rolling → pickling and cold rolling → continuous annealing → hot-dip galvanizing → finishing → tension leveling.

6. The heat treatment method according to claim 1, characterized in that, In step (1), the grade of the coated steel sheet is any one of HC800 / 1000HS+AS, HC950 / 1300HS+AS, HC1000 / 1500HS+AS, HC1100 / 1700HS+AS, and HC1200 / 1800HS+AS.

7. The heat treatment method according to claim 1, characterized in that, The hydrogen content of the coated hot-formed steel can be reduced to below 0.65 ppm after being treated by the heat treatment process of the present invention.

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