2000MPa-grade delayed-cracking-resistant hot-forming aluminum-silicon coated steel plate and manufacturing method thereof

Abstract

The invention discloses a 2000MPa-grade delayed-cracking-resistant hot-pressed aluminum-silicon coated steel plate and a manufacturing method thereof. A steel substrate comprises the following chemical components in percentage by weight: 0.28 to 0.38 percent of C, 0.2 to 0.3 percent of Si, 0.08 to 1.5 percent of Mn, 0.005 to 0.015 percent of S, 0.005 to 0.015 percent of P, 0.02 to 0.05 percent of Nb, 0.02 to 0.05 percent of Ti, less than or equal to 0.01 percent of Als, 0.003 to 0.01 percent of Ca, 0.035 to 0.04 percent of O, 0.001 to 0.003 percent of B, 0.12 to 0.2 percent of Cr, 0.08 to 0.2 percent of V and the balance of Fe and inevitable impurities. The steel contains composite oxide and MnS inclusions, the size of the steel is smaller than 2000 nm, and 20-50 inclusions are distributed per square millimeter according to the cross section. The manufacturing method of the steel plate comprises the steps of refining, continuous casting, hot rolling, acid pickling, 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 2000MPa 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 macroscopic residual stress generated during the rapid cooling deformation process are the main ones.

[0004] Among all phases of iron-based sheet, martensite is the least sensitive to hydrogen embrittlement. When the strength exceeds 1000MPa, the hot-formed sheet contains more than 70vt% martensite. For example, the martensite content of 2000MPa hot-formed steel after quenching is as high as 96vt%. 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, the original austenite grain size is controlled by adding Nb, V, and Ti to form various carbide precipitation phases, thereby obtaining a smaller grain size and more grain boundaries. At the same time, hydrogen traps of varying degrees are formed, 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 its potential risk is very high. For 2000MPa hot-formed steel plates, the C and Mn content is extremely high, plus Nb, V, Ti micro-alloying, the complete austenitization temperature is only 830-860℃, and the Ms point is also relatively high. The martensite content after quenching can reach more than 98vt%. Compared with 1500MPa and 1800MPa hot-formed steels, 2000MPa-level shear martensite has finer structure, more grain boundaries, and greater internal residual stress. The diffusion and aggregation of H atoms are more likely to lead to a decrease in the bonding force between grains, which makes the number of microcracks increase sharply and expand, leading to delayed cracking.

[0007] The diffusion coefficient of H atoms in martensite is very high, and the internal stress generated by a large amount of martensite formed by shear 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 2000MPa 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 a 2000MPa 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 2000MPa grade delayed cracking resistant hot-pressed aluminum-silicon coated steel plate, wherein the chemical composition of the steel substrate is as follows by mass percentage: C: 0.28-0.38, Si: 0.2-0.3, Mn: 0.08-1.5, S: 0.005-0.015, P: 0.005-0.015, Nb: 0.02-0.05, Ti: 0.02-0.05, Als: ≤0.01, Ca: 0.003-0.01, O: 0.035-0.04, B: 0.001-0.003, Cr: 0.12-0.2, V: 0.08-0.2, and the remainder is Fe and unavoidable impurities; the steel contains inclusions of composite oxides and MnS, the size of which is less than 2000nm, and the distribution is 20-50 / 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, Ca, C), Mn-Ti-O-Al(Si, Ca, C), and Mn-VO(C, Ca), and the steel contains three or more inclusions of composite oxides and MnS.

[0012] Furthermore, the inclusions have a size of 50-1000 nm accounting for 60-70%, 1001-1600 nm accounting for 25-35%, 1601-2000 nm accounting for <10%, and are approximately circular in shape.

[0013] Furthermore, the volume percentage of retained austenite in the steel plate is 4% to 6%, 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 2000MPa grade delayed cracking resistant hot-pressed 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 500-650ppm, and the temperature out of the LF furnace 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 370-500ppm 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 360-450ppm, 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 910 ~ 940 ℃, coiling temperature 500 ~ 560 ℃;

[0019] 4. Pickling: The pickling speed is controlled at 60-120 m / min;

[0020] 5. Cold rolling: The cold rolling reduction rate is controlled at 40% to 60%;

[0021] 6. 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℃;

[0022] 7. Hot stamping: In the preheating stage, the aluminum-silicon coated steel plate is heated to 900-920°C and kept warm for 3-5 minutes. After the insulation is completed, the steel plate is quickly air-cooled, and the cooling time is controlled at 6-10 seconds. Subsequently, the steel plate is placed in the mold for stamping and stamping at a cooling rate of >50°C / 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 3-6 seconds.

[0023] 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 300-500 m / tank.

[0024] 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.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] Based on the original technology, this patent 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 material, and strictly controls the type and size of the inclusions. Through the change of the surrounding dislocations by the inclusions, more hydrogen traps are created, the hydrogen precipitation time is delayed, and the delayed fracture resistance of the finished material is improved. Micro inclusions can cause a large amount of distortion of the surrounding lattice and produce a large number of dislocation defects. 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. At the same time, in the hot forming process, by controlling the heating temperature, holding time, air cooling time and holding time, a certain amount of residual austenite is retained in the steel plate. The residual 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. In addition to the traditional Nb, Ti, and V precipitation phases, the hot forming 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

[0027] 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.

[0028] 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.

[0029] The total oxygen content in the hot-formed aluminum-silicon coated steel plate of the present invention is 350-400ppm; the inclusions contained in the steel are Mn-Nb-O(C), Mn-Ti-O(C, Ca), Mn-Nb-O-Al(Si, Ca, C), Mn-Ti-O-Al(Si, Ca, C), Mn-VO(C, Ca) and MnS inclusions. The size of the inclusions in the steel plate is less than 2000nm, of which 50-1000nm accounts for 60-70%, 1001-1600nm accounts for 25-35%, 1601-2000nm accounts for <10%, and exists in a shape that is approximately circular; the inclusion distribution is 20-50 / mm calculated on a plane. 2 , see Table 1 for details.

[0030] Table 1 Inclusions in hot-formed steel

[0031]

[0032]

[0033] The mass percentage of each element in the substrate of the hot-formed aluminum-silicon coated steel plate is: C: 0.28-0.38, Si: 0.2-0.3, Mn: 0.08-1.5, S: 0.005-0.015, P: 0.005-0.015, Nb: 0.02-0.05, Ti: 0.02-0.05, Als: ≤0.01, Ca: 0.003-0.01, O: 0.035-0.04, B: 0.001-0.003, Cr: 0.12-0.2, V: 0.08-0.2, 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.

[0034] Table 2 Chemical composition of 2000MPa grade hot-formed steel (wt%)

[0035]

[0036] 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.

[0037] A method for manufacturing a 2000MPa delayed cracking resistant 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:

[0038] Molten steel refining: The temperature of the molten steel entering the LF furnace is 1620-1660℃, the total oxygen value is 500-650ppm, 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 370-500ppm 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 300-500m / tank. See Table 3 for details.

[0039] Table 3 Steelmaking process parameters for 2000MPa hot-formed steel

[0040]

[0041] Continuous casting: The total oxygen value of the molten steel in the tundish is 360-450ppm, the tundish temperature is 1510-1530℃; the billet drawing speed is 1.0-1.5m / min, and the continuous casting billet thickness is 200-300mm. See Table 4 for details.

[0042] Table 4 Continuous casting process parameters of 1800MPa hot-formed steel

[0043]

[0044]

[0045] Rolling system: billet heating temperature 1180 ~ 1250 ℃, finishing rolling temperature 1000 ~ 1100 ℃, final rolling temperature 910 ~ 940 ℃, coiling temperature 500 ~ 560 ℃; pickling speed controlled at 60 ~ 120m / min; cold rolling reduction rate controlled at 40% ~ 60%;

[0046] Hot-dip system: annealing temperature is 770-810℃, plate temperature entering aluminum pot is 650-680℃, plating solution temperature is 650-660℃.

[0047] Table 5 Rolling and hot-dip process parameters of 2000MPa grade hot-formed steel

[0048]

[0049] Hot stamping forming system: the aluminum-silicon coated steel plate is heated to 900-920°C, kept warm for 3-5 minutes, air-cooled for 6-10 seconds, and then cooled in the mold at a cooling rate of >50°C / s, and the holding time is 3-6 seconds.

[0050] Table 6 Hot forming process parameters of 2000MPa grade hot forming steel

[0051]

[0052]

[0053] Table 7 Delayed cracking measurement fracture time of steel plate (PH = 1, four-point bending prestress is 1 times yield strength)

[0054]

[0055] 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.

[0056] 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.

[0057] 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 2000MPa grade delayed cracking resistant hot-pressed aluminum-silicon coated steel plate, characterized in that: The chemical composition of the steel substrate is as follows by mass percentage: C: 0.28-0.38, Si: 0.2-0.3, Mn: 0.08-1.5, S: 0.005-0.015, P: 0.005-0.015, Nb: 0.02-0.05, Ti: 0.02-0.05, Als: ≤0.01, Ca: 0.003-0.01, O: 0.035-0.04, B: 0.001-0.003, Cr: 0.12-0.2, V: 0.08-0.2, and the remainder is Fe and unavoidable impurities; the steel contains inclusions of composite oxides and MnS, the size of which is less than 2000nm, and the distribution is 20-50 pieces / mm calculated by the cross section. 2 .

2. The 2000MPa delayed cracking resistant hot-pressed 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, Ca, C), Mn-Ti-O-Al(Si, Ca, C), and Mn-VO(C, Ca). The steel contains composite oxides and MnS inclusions of 3 or more kinds.

3. The 2000MPa delayed cracking resistant hot-pressed aluminum-silicon coated steel plate according to claim 1, characterized in that: The inclusion sizes are 50-1000 nm accounting for 60-70%, 1001-1600 nm accounting for 25-35%, and 1601-2000 nm accounting for <10%.

4. The 2000MPa delayed cracking resistant hot-pressed aluminum-silicon coated steel plate according to claim 1, characterized in that: The volume percentage of retained austenite in the steel plate is 4% to 6%, and the rest is martensite and a small amount of ferrite + cementite.

5. The 2000MPa delayed cracking resistant hot-pressed aluminum-silicon coated steel plate according to claim 1, characterized in that: The inclusions exist in a substantially circular shape.

6. The 2000MPa delayed cracking resistant hot-pressed 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 2000MPa delayed cracking resistant hot-pressed aluminum-silicon coated steel sheet according to any one of claims 1 to 6, comprising refining, continuous casting, hot rolling, pickling, cold rolling, hot-dip galvanizing and hot stamping, characterized in that: (1) Refining: When the molten steel enters the LF furnace, the entry temperature is controlled at 1620-1660°C, the total oxygen value is maintained at 500-650ppm, and the LF exit temperature is 1610-1630°C. 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 370-500ppm when it is removed from the LF furnace; (2) Continuous casting: The total oxygen value of the molten steel in the tundish is 360-450 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 910 ~ 940 ° C, coiling temperature 500 ~ 560 ° C; (4) Pickling: The pickling speed is controlled at 60-120 m / min; (5) Cold rolling: The cold rolling reduction rate is controlled at 40% to 60%; (6) Hot dip: 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; (7) Hot stamping: The aluminum-silicon coated steel plate is heated to 900-920°C, kept at this temperature for 3-5 min, air-cooled for 6-10 s, and then cooled in the mold at a cooling rate of >50°C / s for 3-6 s.

8. The method for manufacturing a 2000MPa delayed cracking resistant hot press 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 300-500m / tank.

9. The method for manufacturing a 2000MPa delayed cracking resistant hot press 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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