1500MPa-grade delayed-cracking-resistant hot-forming aluminum-silicon coated steel plate and manufacturing method thereof

Abstract

The invention discloses a 1500MPa-grade delayed-cracking-resistant hot-forming aluminum-silicon coated steel plate and a manufacturing method thereof. A steel substrate comprises the following chemical components in percentage by weight: 0.17 to 0.25 percent of C, 0.2 to 0.3 percent of Si, 1.0 to 1.6 percent of Mn, 0.005 to 0.015 percent of S, 0.005 to 0.016 percent of P, 0 to 0.03 percent of Nb, 0.02 to 0.06 percent of Ti, less than or equal to 0.01 percent of Als, 0.003 to 0.01 percent of Ca, 0.015 to 0.02 percent of O, 0.001 to 0.003 percent of B and 0.12 to 0.18 percent of Cr. The steel contains inclusions of composite oxides and MnS, the size of the inclusions is smaller than 1100 nm, and the distribution number of the inclusions is 10-30 / mm < 2 >. The manufacturing method comprises the steps that the temperature of molten steel entering an LF is 1620-1660 DEG C, the total oxygen value is 400-600 ppm, the LF moving-out temperature is 1610-1630 DEG C, oxygen blowing heating is conducted during the period, the total oxygen content of the molten steel is 200-250 ppm, the total oxygen value of tundish molten steel is 180-230 ppm, the temperature is 1510-1535 DEG C, and then continuous casting, hot rolling, cold rolling, hot dipping and hot stamping forming are conducted. 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 oxygen-containing hot forming with excellent 1500MPa-level delayed cracking resistance, 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 large. These stresses mainly come from the phase transformation process of the plate, mainly the martensitic shear stress and the macro residual stress during the rapid cooling deformation process. Martensitic structure is the least sensitive to hydrogen embrittlement. When the strength exceeds 1500MPa, more than 90vt% of the hot-formed plate contains martensitic structure. Therefore, the finished parts after hot forming are prone to hydrogen-induced delayed cracking during long-term storage.

[0004] 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 hydrogen elements and residual stress are the main reasons for delayed cracking of high-strength steel plates. Therefore, controlling the diffusion and aggregation of hydrogen elements 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. The smaller the grain size, the more grain boundaries, and when the small-angle grain boundaries are dominant, the more uniform the distribution of hydrogen atoms in the grain boundaries and the less likely they are to aggregate, the lower the probability of hydrogen precipitation; when the vacancies and precipitated phases are small in size, 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, smaller grain sizes and more grain boundaries are obtained by controlling the original austenite grains; various carbide precipitation phases are formed by adding Nb, V, and Ti, which not only inhibits grain growth, but also forms hydrogen traps to varying degrees.

[0005] However, from the actual use situation, the martensite content in hot-formed steel above 1500MPa level reaches more than 90%, and the rest is cementite particles, retaining only a very small amount of ferrite structure. It is difficult to inhibit the diffusion of H atoms through the ferrite structure, and its internal residual stress usually reaches hundreds of MPa. The diffusion coefficient of H in martensite is the fastest among all phases, which is most likely to cause the aggregation of H atoms to form bubbles and even microcracks. Therefore, it is far from enough to rely solely on a small amount of Nb, Ti C, N compounds to set up hydrogen traps.

[0006] 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. Introducing a certain amount of austenite in high-strength and ultra-high-strength steels can effectively improve the delayed cracking resistance. Inclusions as harmful substances are generally avoided as much as possible, but 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 carbides of Nb, Ti, and V, and their size is larger than that of the precipitated phase, the degree of lattice change is more significant, and the effect of forming hydrogen traps is also better. Summary of the invention

[0007] The object of the present invention is to provide a 1500MPa grade delayed cracking resistant hot-formed 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 to produce H traps, inhibit the diffusion of H atoms, and further improve the delayed cracking resistance of the steel plate.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] One of the technical solutions of the present invention is to provide a 1500MPa grade delayed cracking resistant hot-formed aluminum-silicon coated steel plate, wherein the mass percentage of each element in the steel substrate is: C: 0.17% to 0.25%, Si: 0.2% to 0.3%, Mn: 1.0% to 1.6%, S: 0.005% to 0.015%, P: 0.005% to 0.016%, Nb: 0 to 0.03%, Ti: 0.02% to 0.06%, Als: ≤0.01%, Ca: 0.003% to 0.01%, O: 0.015% to 0.02%, B: 0.001% to 0.003%, Cr: 0.12% to 0.18%, and the remainder is Fe and unavoidable impurities; the steel contains inclusions of composite oxides and MnS, the size of which is less than 1100nm, and the distribution number is 10 to 30 pieces / mm calculated on a plane. 2 .

[0010] Furthermore, the inclusions of the composite oxide are Mn-Nb-O(C), Mn-Ti-O(C, Ca), Mn-Nb-O-Al(Si, C, Ca), and Mn-Ti-O-Al(Si, C, Ca), and the steel contains three or more inclusions of composite oxide and MnS.

[0011] Furthermore, among the inclusions, those with a size of 50 to 700 nm account for 60 to 80%, and those with a size of 701 to 1100 nm account for 20 to 40%.

[0012] Furthermore, the volume percentage of retained austenite in the steel plate is greater than 2%, and the rest is martensite and a small amount of ferrite + cementite.

[0013] Furthermore, the inclusion exists in a shape approximately like a circle.

[0014] Furthermore, when four-point bending prestressing with 1 times the yield strength is applied and the steel is immersed in a hydrochloric acid solution with a pH of 1, the cracking time is greater than 140 hours.

[0015] One of the technical solutions of the present invention is to provide a method for manufacturing a 1500MPa grade delayed cracking resistant hot-formed aluminum-silicon coated steel plate, including molten steel refining, continuous casting, hot rolling, cold rolling, hot dip and hot stamping forming, specifically:

[0016] 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 200-250ppm when it is out of the LF furnace;

[0017] Continuous casting: The total oxygen value of the molten steel in the tundish is 180-230ppm, and the tundish temperature is 1510-1535℃;

[0018] Hot rolling: billet heating temperature 1180 ~ 1250 ℃, finishing rolling temperature 1000 ~ 1100 ℃, final rolling temperature 880 ~ 950 ℃, coiling temperature 530 ~ 620 ℃;

[0019] Cold rolling: The cold rolling reduction rate is controlled at 50% to 75%;

[0020] Hot dip: annealing temperature 770-820℃, plate temperature entering aluminum pot 650-680℃, bath temperature 650-660℃;

[0021] Hot stamping: The aluminum-silicon coated steel plate is heated to 930-950°C, kept warm for 4-8 minutes, air-cooled for 6-10 seconds, and then cooled in the mold at a cooling rate of >50°C / s, with a holding time of 3-6 seconds.

[0022] Furthermore, 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 150-350m / tank.

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

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

[0025] Based on the existing technology of setting H traps through Nb and Ti precipitated phases, the present invention increases the number of inclusions inside the plate by adding a certain amount of O element, that is, increasing the O content of the finished product, and strictly controls the type and size of the inclusions to avoid affecting the mechanical properties.

[0026] By changing the dislocations around the inclusions, more hydrogen traps are created, the hydrogen precipitation time is delayed, and the delayed fracture resistance of the finished material is improved. Tiny inclusions can cause a large amount of distortion of 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 and it is difficult to move, which can effectively avoid the aggregation of H atoms.

[0027] In addition, the C and Mn contents in 1500MPa grade hot-formed steel are relatively high, and it is difficult to retain effective ferrite content. The hot-forming process parameters can be controlled to increase a certain amount of retained austenite content. The diffusion coefficient of H atoms in austenite is much smaller than that in martensite, and retained austenite can even become a permanent and stable hydrogen trap to inhibit H diffusion.

[0028] In addition to the traditional Nb and Ti 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 retain a certain amount of residual austenite as a barrier to the diffusion of H atoms, thereby improving the delayed cracking performance of the steel plate after hot forming. DETAILED DESCRIPTION

[0029] 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.

[0030] The present invention is described in more detail below through examples.

[0031] The total oxygen content in the hot-formed aluminum-silicon coated steel plate of the present invention is 150-200 ppm; the inclusions contained in the steel are Mn-Nb-O(C), Mn-Ti-O(C, Ca), Mn-Nb-O-Al(Si, C, Ca), Mn-Ti-O-Al(Si, C, Ca) composite oxides, and MnS; the size of the inclusions in the steel plate is less than 1100 nm, of which the size of 50-700 nm accounts for 60%-80%, the size of 701-1100 nm accounts for 20-40%, and exists in a shape approximately round; the inclusion distribution is 10-30 / mm calculated on a plane. 2 See Table 1 for details.

[0032] Table 1 Inclusions in oxygen-containing hot-formed steel

[0033]

[0034]

[0035] The mass percentage of each element in the substrate of the hot-formed aluminum-silicon coated steel plate is: C: 0.17% to 0.25%, Si: 0.2% to 0.3%, Mn: 1.0% to 1.6%, S: 0.005% to 0.015%, P: 0.005% to 0.016%, Nb: 0 to 0.03%, Ti: 0.02% to 0.06%, Als: ≤ 0.01%, Ca: 0.003% to 0.01%, O: 0.015% to 0.02%, B: 0.001% to 0.003%, Cr: 0.12% to 0.18%, see Table 2 for details. The content of retained austenite in the steel plate is > 2% (volume content), and the rest is martensite and a small amount of ferrite + cementite, see Table 6 for details.

[0036] Table 2 Chemical composition of 1500MPa grade oxygen-containing hot-formed steel (wt%)

[0037]

[0038] It can be seen from Tables 1 and 2 that, in addition to the traditional Nb and Ti 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. It can be seen from the composition 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.

[0039] A method for manufacturing a 1500MPa 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 and hot stamping forming, specifically:

[0040] 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 200-250ppm 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 150-350m / tank. See Table 3 for details.

[0041] Table 3 Steelmaking process parameters of 1500MPa oxygen-containing hot-formed steel

[0042]

[0043] Continuous casting: The total oxygen value of the molten steel in the tundish is 180-230ppm, the tundish temperature is 1510-1535℃; the billet drawing speed is 1.0-1.5m / min, and the continuous casting billet thickness is 200-300mm. See Table 4 for details.

[0044] Table 4 Continuous casting process parameters of 1500MPa hot-formed steel

[0045]

[0046] Hot-dip rolling: billet heating temperature 1180-1250℃, finishing rolling temperature 1000-1100℃, final rolling temperature 880-950℃, coiling temperature 530-620℃; cold rolling reduction rate controlled at 50%-75%; continuous annealing temperature 770-820℃, plate entering aluminum pot temperature 650-680℃, plating solution temperature 650-660℃. See Table 5 for details.

[0047] Table 5 Rolling and hot-dip coating process of 1500MPa grade oxygen-containing hot-formed steel

[0048]

[0049]

[0050] Hot stamping: The aluminum-silicon coated steel plate is heated to 930℃~950℃, kept at this temperature for 4~8min, air-cooled for 6~10s, and then cooled in the mold at a cooling rate of >50℃ / s, and the holding time is 3~6s. See Table 6 for details.

[0051] Table 6 Hot forming process of 1500MPa grade oxygen-containing hot forming steel

[0052]

[0053] 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.

[0054] The coated steel plate was prestressed by four-point bending with 1 times the yield strength and immersed in a hydrochloric acid solution with a pH of 1. The cracking time was greater than 140 hours. See Table 7 for details.

[0055] Table 7 Delayed cracking measurement fracture time h PH = 1 four-point bending - prestress is 1 times the yield strength

[0056]

[0057] The present invention is based on the existing technology of setting H traps through Nb and Ti precipitation phases, increases the number of inclusions in the steel plate by increasing the O content in the steel plate, sets more traps for the diffusion of H elements in the steel, inhibits the diffusion and aggregation of H atoms, and strictly controls the size of inclusions to avoid affecting the mechanical properties; in the hot forming and stamping process, the content of retained austenite is increased, and the low diffusion rate of H atoms in the retained austenite is utilized to assist in inhibiting the diffusion and aggregation of H atoms, thereby further improving the delayed cracking resistance of the steel plate.

[0058] The above embodiments are only for illustrating the technical concept and features of the present invention, but not for limiting the present invention. The purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A 1500MPa grade delayed cracking resistant hot-formed aluminum-silicon coated steel plate, characterized in that: The mass percentage of each element in the steel substrate is: C: 0.17% to 0.25%, Si: 0.2% to 0.3%, Mn: 1.0% to 1.6%, S: 0.005% to 0.015%, P: 0.005% to 0.016%, Nb: 0 to 0.03%, Ti: 0.02% to 0.06%, Als: ≤ 0.01%, Ca: 0.003% to 0.01%, O: 0.015% to 0.02%, B: 0.001% to 0.003%, Cr: 0.12% to 0.18%, and the balance is Fe and unavoidable impurities; the steel contains inclusions of composite oxides and MnS, the size of which is less than 1100nm, and the distribution number is 10 to 30 / mm calculated on the plane. 2 .

2. The 1500MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet 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), and Mn-Ti-O-Al(Si, C, Ca). The steel contains composite oxides and MnS inclusions of 3 or more kinds.

3. The 1500MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet according to claim 1, characterized in that: Among the inclusions, those with a size of 50 to 700 nm account for 60% to 80%, and those with a size of 701 to 1100 nm account for 20% to 40%.

4. The 1500MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet according to claim 1, characterized in that: The volume percentage of retained austenite in the steel plate is >2%, and the rest is martensite and a small amount of ferrite + cementite.

5. The 1500MPa 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 1500MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet according to claim 1, characterized in that: Using four-point bending prestressing loading of 1 times the yield strength and immersion in a PH=1 hydrochloric acid solution, the cracking time is greater than 140 hours.

7. A method for manufacturing a 1500MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet according to any one of claims 1 to 6, characterized in that: It includes molten steel refining, continuous casting, hot rolling, cold rolling, hot dip galvanizing and hot stamping forming, and is characterized in that: 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 200-250ppm when it is out of the LF furnace; Continuous casting: The total oxygen value of the molten steel in the tundish is 180-230ppm, and the tundish temperature is 1510-1535℃; Hot rolling: billet heating temperature 1180 ~ 1250 ℃, finishing rolling temperature 1000 ~ 1100 ℃, final rolling temperature 880 ~ 950 ℃, coiling temperature 530 ~ 620 ℃; Cold rolling: The cold rolling reduction rate is controlled at 50% to 75%; Hot dip: annealing temperature 770-820℃, plate temperature entering aluminum pot 650-680℃, bath temperature 650-660℃; Hot stamping: The aluminum-silicon coated steel plate is heated to 930-950°C, kept warm for 4-8 minutes, air-cooled for 6-10 seconds, and then cooled in the mold at a cooling rate of >50°C / s, with a holding time of 3-6 seconds.

8. The method for manufacturing a 1500MPa 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 150-350m / tank.

9. The method for manufacturing a 1500MPa delayed cracking resistant hot-formed aluminum-silicon coated steel sheet according to claim 7, characterized in that: The billet drawing speed is 1.0-1.5 m / min, and the continuous casting billet thickness is 200-300 mm.

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