1800MPa-grade delayed-cracking-resistant hot-forming aluminum-silicon coated steel plate and manufacturing method thereof

Abstract

The invention discloses a 1800MPa-grade delayed cracking resistant hot forming aluminum-silicon coating steel plate and a manufacturing method thereof. A steel substrate comprises the following chemical components in percentage by weight: 0.23%-0.3% of C, 0.2%-0.3% of Si, 1.0%-1.5% of Mn, 0.005%-0.015% of S, 0.005%-0.016% of P, 0.02%-0.05% of Nb, 0.02%-0.05% of Ti, less than or equal to 0.01% of Als, 0.003%-0.01% of Ca, 0.001%-0.003% of B, 0.12%-0.17% of Cr, 0.08%-0.2% of V, 0.025%-0.035% of O and the balance of Fe and inevitable impurities. The steel contains composite oxides and MnS inclusions, the size of the inclusions is smaller than 1500 nm, and the distribution of the inclusions is 10-40 inclusions / mm < 2 > according to the cross section. The manufacturing method of the steel plate comprises the steps of refining, continuous casting, hot rolling, cold rolling, hot dipping and hot stamping forming. The delayed cracking resistance of the steel plate is further improved by increasing the O content in the steel plate.

Description

Technical Field

[0001] The invention relates to an aluminum-silicon coated steel plate for hot forming with excellent 1800MPa delayed cracking resistance and a manufacturing method thereof, and belongs to the field of hot stamping. Background Art

[0002] Hot pressing technology is currently an ideal and quick way to improve material strength, especially in the automotive field. After the steel plate is heated to the austenitizing temperature, it is kept warm for a certain period of time and quickly stamped into the required part shape in the austenite area, and finally a high-strength part mainly composed of martensite is obtained. However, the oxide layer produced by high-temperature heating cannot meet the subsequent coating process and requires sandblasting and other process treatments, which increases the complexity of the process and brings environmental pressure. The addition of aluminum-silicon coating effectively inhibits the growth of the oxide layer on the surface of the steel plate, and the aluminum-silicon coating only forms a very thin oxide layer through the interdiffusion surface with Fe at high temperature, which does not affect the subsequent painting and other process treatments at all.

[0003] Although this method greatly improves the strength of parts, the residual stress in the finished parts after hot stamping is relatively large. These stresses mainly come from the phase change process of the plate, among which the martensitic shear stress and the residual stress generated during the rapid cooling deformation process are the main ones.

[0004] Among all phases of iron sheet, martensite is the least sensitive to hydrogen embrittlement. When the strength exceeds 1000MPa, the hot-formed sheet contains more than 70vt% martensite, and the martensite content of the sheet above 1500MPa is more than 90vt%. Therefore, the finished parts after hot forming are prone to hydrogen-induced delayed cracking during long-term storage.

[0005] At present, there are many studies on delayed cracking of high-strength steel plates, and the main theories include hydrogen pressure theory and hydrogen-promoted plastic deformation theory. Although there is no single theory that can fully explain this phenomenon, it is generally recognized in the industry that the diffusion, aggregation and synergistic effect of residual stress of H atoms are the main reasons for delayed cracking of high-strength steel plates. Therefore, controlling the diffusion and aggregation of H atoms is an effective way to solve delayed cracking. Studies have shown that dislocations, grain boundaries, vacancies, and precipitated phases can all become effective hydrogen traps. When the grain size is smaller, the more grain boundaries there are, and the small-angle grain boundaries are dominant, the distribution of hydrogen atoms in the grain boundaries is more uniform and less likely to aggregate, and the probability of hydrogen precipitation is smaller. When the size of vacancies and precipitated phases is small, dense dislocation groups are formed around them, which can effectively form hydrogen traps and inhibit the aggregation and precipitation of hydrogen atoms. In the currently commonly used hot-pressed sheets, various carbide precipitation phases are formed by adding Nb, V, and Ti, and the original austenite grains are controlled to obtain smaller grain sizes and more grain boundaries. This can not only inhibit grain growth, but also form hydrogen traps to varying degrees, which can effectively inhibit the diffusion and aggregation of H atoms.

[0006] However, from the actual use, high-strength steel plates, especially hot stamping plates dominated by martensite, still have large residual stress inside. After long-term storage, delayed cracking still occurs under the action of hydrogen evolution reaction. Many delayed cracking occurs after loading, and the potential risk is very high. For 1800MPa grade hot-formed steel plates, the C and Mn element contents are high, the austenitization temperature is reduced, the Ms point is also increased accordingly, the critical quenching temperature is significantly increased, and the martensite content after quenching can be above 95vt%. The steel almost contains ferrite tissue, and ferrite cannot be used to suppress the diffusion rate of H atoms.

[0007] The diffusion coefficient of H atoms in martensite is very high, and the internal stress generated by a large amount of martensite cutting edges is also large. The aggregated H atoms can easily form bubbles at the internal microcracks, causing crack expansion and brittle fracture. Although Nb, V, and Ti microalloys are added to 1800MPa grade hot-formed steel to refine the grains and provide hydrogen traps, the high residual stress still makes the critical point of hydrogen embrittlement low. Studies have shown that the diffusion coefficient of H atoms in austenite is significantly reduced, and thin austenite can even form irreversible traps for H atoms, achieving the effect of fixing H atoms. Therefore, introducing a certain amount of austenite in high-strength and ultra-high-strength steels can effectively improve the delayed cracking resistance. Inclusions are harmful substances and are generally avoided as much as possible. However, tiny inclusions can obviously form nucleation fulcrums and also produce dislocation pinning effects, inhibiting the effective transfer movement of H atoms through dislocations and forming irreversible traps for H atoms. Extremely small inclusion particles even have a hydrogen trap effect similar to that of Nb, Ti, and V carbides, and their size is larger than that of the precipitated phase, causing a more significant degree of lattice change and a better effect in forming hydrogen traps. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide an 1800MPa grade delayed cracking resistant hot-pressed aluminum-silicon coated steel plate and a manufacturing method thereof, by increasing the O content inside the steel plate to increase the number of inclusions inside the steel plate, thereby further improving the delayed cracking resistance of the steel plate.

[0009] To achieve the above object, the technical solution of the present invention is characterized by:

[0010] A 1800MPa grade delayed cracking resistant hot-formed aluminum-silicon coated steel plate, the chemical composition mass percentage of the steel substrate is, C: 0.23-0.3, Si: 0.2-0.3, Mn: 1.0-1.5, S: 0.005-0.015, P: 0.005-0.016, Nb: 0.02-0.05, Ti: 0.02-0.05, Als: <0.01, Ca: 0.003-0.01, B: 0.001-0.003, Cr: 0.12-0.17, V: 0.08-0.2, O: 0.025-0.035, the balance is Fe and unavoidable impurities; the steel contains inclusions of composite oxides and MnS, the size of which is less than 1500nm, and the distribution is 10-40 / mm calculated by the cross section. 2 .

[0011] Furthermore, the inclusions of the composite oxide are Mn-Nb-O(C), Mn-Ti-O(C, Ca), Mn-Nb-O-Al(Si, C, Ca), Mn-VO(C, Ca), and Mn-Ti-O-Al(Si, C, Ca), and the steel contains composite oxides and MnS inclusions of 3 or more kinds.

[0012] Furthermore, the inclusions have a size of 50-800 nm accounting for 60-70%, 801-1200 nm accounting for 25-35%, 1201-1500 nm accounting for <10%, and are approximately circular in shape.

[0013] Furthermore, the volume percentage of retained austenite in the steel plate is 2% to 5%, and the rest is martensite and a small amount of ferrite + cementite.

[0014] Furthermore, when the steel plate is subjected to four-point bending prestressing with a yield strength of 1 times and immersed in a hydrochloric acid solution with a pH of 1, the cracking time of the steel plate is greater than 130 hours.

[0015] The manufacturing method of the present invention for producing a 1800MPa grade delayed cracking resistant hot-formed aluminum-silicon coated steel plate comprises:

[0016] 1. Refining: When the molten steel enters the LF furnace, the temperature is controlled at 1620-1660℃, the total oxygen value is maintained at 400-600ppm, and the temperature out of the LF furnace is 1620-1640℃. During this period, oxygen blowing is used to increase the temperature to ensure that the total oxygen content of the molten steel is maintained at 260-400ppm when it is moved out of the LF furnace;

[0017] 2. Continuous casting: The molten steel enters the tundish, the total oxygen value of the molten steel in the tundish is 260-370ppm, and the temperature of the tundish is maintained at 1510-1530℃;

[0018] 3. Hot rolling: billet heating temperature 1180 ~ 1250 ℃, finishing rolling temperature 1000 ~ 1100 ℃, final rolling temperature 880 ~ 940 ℃, coiling temperature 530 ~ 580 ℃;

[0019] 4. Cold rolling: the pickling speed is controlled at 60-120 m / min; the cold rolling reduction rate is controlled at 40%-65%;

[0020] 5. Hot dip: The continuous annealing and soaking temperature range is 770-810℃. Before the plate is brought into the aluminum pot for treatment, the entering temperature of the plate needs to be controlled at 650-680℃, and the plating solution temperature needs to be controlled at 650-660℃;

[0021] 6. Hot stamping: The aluminum-silicon coated steel plate is heated to 910℃~930℃ and kept warm for 3~6min. After the insulation is completed, the steel plate is quickly air-cooled, and the cooling time is controlled at 8~13s. Then, the steel plate is placed in the mold for stamping and stamping at a cooling rate of >50℃ / s. During stamping, a certain holding time needs to be maintained to ensure that the material is fully formed in the mold. The holding time is 5~8s.

[0022] Preferably, after LF refining, the molten steel is fed with Si-Ca wire, the wire feeding speed is ≥3.5 m / s, and the wire feeding amount is 250-450 m / tank.

[0023] Preferably, the billet drawing speed is 1.0 to 1.5 m / min, and the thickness of the continuous casting billet is 200 to 300 mm.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] This patent is based on the original technology of setting H traps through Nb, Ti, and V precipitation phases. By adding a certain amount of O element, that is, increasing the O content of the finished product, the number of inclusions inside the plate is increased, and the type and size of the inclusions are strictly controlled to avoid affecting the mechanical properties.

[0026] Tiny inclusions can cause a large amount of distortion in the surrounding lattice and produce a large number of dislocation defects. When H atoms fall into the inclusion area, the diffusion coefficient is significantly reduced, making it difficult for them to move, which can effectively avoid the aggregation of H atoms. At the same time, in the hot forming process, a certain amount of retained austenite is retained in the steel plate by controlling the heating temperature, holding time, air cooling time and holding time. The retained austenite is an ideal H atom diffusion barrier, which can greatly slow down the movement and aggregation of H atoms and has a beneficial effect on improving the delayed cracking resistance.

[0027] In addition to the traditional Nb, Ti, and V precipitation phases, hot-formed steel also contains a large number of smaller, nearly circular inclusion particles. The inclusion particles can form effective H traps, inhibit the aggregation and diffusion of H atoms, and improve the delayed cracking performance of the steel plate after hot forming. DETAILED DESCRIPTION

[0028] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The inclusions in the steel of the present invention are shown in Table 1, the chemical composition is shown in Table 2, the steelmaking process parameters, continuous casting process parameters, rolling hot-dip process parameters and hot forming process parameters are shown in Tables 3 to 6, and the measured delayed cracking and rupture time of the steel plate is shown in Table 7.

[0030] The total oxygen content in the hot-formed aluminum-silicon coated steel sheet of the present invention is 250-350ppm; the inclusions contained in the steel are three or more of Mn-Nb-O(C), Mn-Ti-O(C, Ca), Mn-Nb-O-Al(Si, C, Ca), Mn-VO(C, Ca), Mn-Ti-O-Al(Si, C, Ca) composite oxides, and MnS. The inclusion size is 50-800nm ​​accounting for 60-70%, 801-1200nm accounting for 25-35%, and 1201-1500nm accounting for <10%, as shown in Table 1.

[0031] Table 1 Inclusions in hot-formed steel

[0032]

[0033]

[0034] The mass percentage of each element in the substrate of the hot-formed aluminum-silicon coated steel plate is: C: 0.23-0.3, Si: 0.2-0.3, Mn: 1.0-1.5, S: 0.005-0.015, P: 0.005-0.016, Nb: 0.02-0.05, Ti: 0.02-0.05, Als: ≤0.01, Ca: 0.003-0.01, B: 0.001-0.003, Cr: 0.12-0.17, V: 0.08-0.2, O: 0.025-0.035, see Table 2 for details. The volume percentage of retained austenite in the steel plate is 2%-5%, and the rest is martensite and a small amount of ferrite + cementite, see Table 6 for details.

[0035] Table 2 Chemical composition of 1800MPa grade hot-formed steel (wt%)

[0036]

[0037] It can be seen from Tables 1 and 2 that, in addition to the traditional Nb, Ti, and V precipitation phases, the hot-formed steel also contains a large number of smaller, nearly circular inclusion particles. The inclusion particles can form effective H traps, inhibit the aggregation and diffusion of H atoms, and improve the delayed cracking performance of the steel plate after hot forming. From the composition, it can be seen that the O content in the composition design of this patent is significantly higher than that of traditional hot-formed steel, and the Als content is lower, mainly because O in the steel forms inclusion particles.

[0038] A method for manufacturing an 1800MPa grade delayed cracking resistant oxygen-containing hot-formed aluminum-silicon coated steel plate, comprising molten steel refining, continuous casting, hot rolling, cold rolling, hot dip coating and hot stamping forming, specifically:

[0039] Molten steel refining: The temperature of the molten steel entering the LF furnace is 1620-1660℃, the total oxygen value is 400-600ppm, and the temperature of the LF outflow is 1610-1630℃. During this period, oxygen blowing is used to increase the temperature to ensure that the total oxygen content of the molten steel is maintained at 260-400ppm when the LF furnace is outflowed; after LF refining, the molten steel is fed with Si-Ca wire, the wire feeding speed is ≥3.5m / s, and the wire feeding amount is 250-450m / tank. See Table 3 for details.

[0040] Table 3 Steelmaking process parameters of 1800MPa hot-formed steel

[0041]

[0042] Continuous casting: the total oxygen value of the molten steel in the tundish is 260-370 ppm, the tundish temperature is 1510°C-1530°C; the billet drawing speed is 1.0-1.5 m / min, and the continuous casting billet thickness is 200-30 mm; see Table 4 for details.

[0043] Table 4 Continuous casting process parameters of 1800MPa hot-formed steel

[0044]

[0045]

[0046] Rolling system: billet heating temperature 1180 ~ 1250 ℃, finishing rolling temperature 1000 ~ 1100 ℃, final rolling temperature 880 ~ 940 ℃, coiling temperature 530 ~ 580 ℃; pickling speed is controlled at 60 ~ 120m / min; cold rolling reduction rate is controlled at 40% ~ 65%;

[0047] Hot-dip system: the annealing temperature is 770-810°C, the plate temperature entering the aluminum pot is 650-680°C, and the plating solution temperature is 650-660°C; see Table 5 for details.

[0048] Table 51 Rolling and hot-dip process parameters of 800MPa grade hot-formed steel

[0049]

[0050] Hot stamping forming system: the aluminum-silicon coated steel plate is heated to 910-930℃, kept at this temperature for 3-6min, air-cooled for 8-13s, and then cooled in the mold at a cooling rate of >50℃ / s, and the holding time is 5-8s. See Table 6 for details.

[0051] Table 6 Hot forming process parameters of 1800MPa grade hot forming steel

[0052]

[0053]

[0054] It can be seen from Tables 3 to 6 that oxygen-containing hot-formed steel mainly controls the oxygen content in the steel by passing oxygen through the LF furnace in the steelmaking process. The O value is significantly increased compared with traditional hot-formed steel, and a large number of tiny oxygen-containing inclusion particles are formed in the final product, which can form permanent H traps.

[0055] Table 7 Delayed cracking measurement fracture time of steel plate (PH = 1, four-point bending prestress is 1 times yield strength)

[0056]

[0057] It can be seen from Table 7 that in the embodiments, the four-point bending prestressing loading of 1 times the yield strength is adopted, and the steel plates are immersed in a hydrochloric acid solution with a pH of 1, and the cracking time is greater than 130 hours.

[0058] Based on the original technology, this patent increases the number of inclusions inside the steel plate by increasing the O content inside the steel plate, setting more traps for the diffusion of H elements in the steel, inhibiting the diffusion and aggregation of H atoms, and strictly controlling the size of inclusions to avoid affecting the mechanical properties; in the hot forming and stamping process, the residual austenite content is increased, and the low diffusion rate of H atoms in the retained austenite is used to assist in inhibiting the diffusion and aggregation of H atoms, thereby further improving the delayed cracking resistance of the steel plate.

[0059] It is clear here that the examples listed are intended to illustrate the technical concept and main features of the present invention, and are not intended to be a limiting description of the invention. Any equivalent substitution or improvement that does not deviate from the essence of the present invention shall be deemed to be included in the protection scope of the present invention.

Claims

1. A 1800MPa grade delayed cracking resistant hot-formed aluminum-silicon coated steel plate, characterized in that: The chemical composition of the steel substrate is as follows by mass percentage: C: 0.23-0.3, Si: 0.2-0.3, Mn: 1.0-1.5, S: 0.005-0.015, P: 0.005-0.016, Nb: 0.02-0.05, Ti: 0.02-0.05, Als: ≤0.01, Ca: 0.003-0.01, B: 0.001-0.003, Cr: 0.12-0.17, V: 0.08-0.2, O: 0.025-0.035, and the remainder is Fe and unavoidable impurities; the steel contains inclusions of composite oxides and MnS, the size of which is less than 1500nm, and the distribution is 10-40 / mm calculated by the cross section. 2 .

2. The 1800MPa delayed cracking resistant hot-formed aluminum-silicon coated steel plate according to claim 1, characterized in that: The inclusions of the composite oxides are Mn-Nb-O(C), Mn-Ti-O(C, Ca), Mn-Nb-O-Al(Si, C, Ca), Mn-VO(C, Ca), and Mn-Ti-O-Al(Si, C, Ca). The steel contains composite oxides and MnS inclusions of 3 or more kinds.

3. The 1800MPa delayed cracking resistant hot-formed aluminum-silicon coated steel plate according to claim 1, characterized in that: The inclusion sizes are 50-800 nm, accounting for 60-70%, 801-1200 nm, accounting for 25-35%, and 1201-1500 nm, accounting for <10%.

4. The 1800MPa delayed cracking resistant hot-formed aluminum-silicon coated steel plate according to claim 1, characterized in that: The volume percentage of retained austenite in the steel plate is 2% to 5%, and the rest is martensite and a small amount of ferrite + cementite.

5. The 1800MPa delayed cracking resistant hot-formed aluminum-silicon coated steel plate according to claim 1, characterized in that: The inclusions exist in a substantially circular shape.

6. The 1800MPa delayed cracking resistant hot-formed aluminum-silicon coated steel plate according to claim 1, characterized in that: Using four-point bending prestressing loading of 1 times the yield strength and immersion in a hydrochloric acid solution with a pH of 1, the steel plate cracking time is greater than 130 hours.

7. A method for manufacturing a 1800MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet according to any one of claims 1 to 6, comprising refining, continuous casting, hot rolling, cold rolling, hot dip coating and hot stamping, characterized in that: (1) Refining: When the molten steel enters the LF furnace, the temperature at which it enters is controlled at 1620-1660°C, and the total oxygen value is maintained at 400-600ppm. The temperature at which the molten steel exits the LF furnace is 1620-1640°C. During this period, oxygen blowing is used to increase the temperature, ensuring that the total oxygen content of the molten steel remains at 260-400ppm when it exits the LF furnace. (2) Continuous casting: The total oxygen value of the molten steel in the tundish is 260-370 ppm, and the tundish temperature is 1510-1530°C; (3) Hot rolling: billet heating temperature 1180 ~ 1250 ° C, finishing rolling temperature 1000 ~ 1100 ° C, final rolling temperature 880 ~ 940 ° C, coiling temperature 530 ~ 580 ° C; (4) Cold rolling: the pickling speed is controlled at 60-120 m / min; the cold rolling reduction rate is controlled at 40%-65%; (5) Hot dip: The annealing temperature is 770-810°C, the plate temperature entering the aluminum pot is 650-680°C, and the bath temperature is 650-660°C; (6) Hot stamping: The aluminum-silicon coated steel plate is heated to 910°C-930°C, kept at this temperature for 3-6 min, air-cooled for 8-13 s, and then cooled in the mold at a cooling rate of >50°C / s for 5-8 s.

8. The method for manufacturing the 1800MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet according to claim 7, characterized in that: After LF refining, the molten steel is fed with Si-Ca wire, the wire feeding speed is ≥3.5m / s, and the wire feeding amount is 250-450m / tank.

9. The method for manufacturing the 1800MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet according to claim 6, characterized in that: The billet drawing speed is 1.0-1.5 m / min, and the continuous casting billet thickness is 200-300 mm.

Images