High delay fracture resistant 1700 mpa grade aluminum alloy coated hot stamped steel sheet and production method, hot stamped steel component and application

Abstract

The application provides a high anti-delay fracture 1700MPa grade aluminum alloy coated hot forming steel plate and a production method, a hot forming steel component and an application, the composition is Si 0.10-0.50%, Mn 1.00-1.50%, Cr 0.10-0.80%, Ti 0.01-0.10%, Nb 0.01-0.10%, Mn+Cr+Si≤2.00%, Ti+Nb≤0.12%; compared with the prior art, the application controls the composition and process, thereby controlling the oxidation state of the substrate surface layer before hot forming, the thickness fluctuation of the FeAlSi inhibition layer, and finally controlling the size and quantity of the Cottrell holes after hot forming; the micro-alloying elements are added to refine the martensite structure, the hydrogen content in the raw material and the hydrogen absorption reaction during hot forming are controlled, the diffusible hydrogen content in the hot forming steel component is controlled, and the hot forming steel component has high anti-delay fracture performance, cold bending performance and coating corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to high-resistance delayed fracture 1700MPa grade aluminum alloy coated hot-formed steel plates and their production methods, hot-formed steel components and their applications. Background Technology

[0002] In recent years, with increasingly stringent requirements for safety, energy conservation, and emissions in the automotive industry, lightweighting and improved safety performance have become key concerns. In this context, hot stamping technology has emerged. This technology separates forming and strengthening into two steps to produce ultra-high-strength automotive parts, offering advantages such as ultra-high strength, ease of forming, and high forming precision. However, during the hot stamping process, if bare sheet steel is used, oxidation and decarburization inevitably occur on the steel sheet surface, affecting its strength. Furthermore, hot-formed parts require shot peening or pickling, which impacts the product's dimensional accuracy.

[0003] To address the above issues, coated hot-formed steel has become the mainstream product in the market. Currently, the most maturely applied coating product is the Al-Si product proposed by Arcelor Mittal, which has a strength level of 1500 MPa.

[0004] To further reduce weight, automakers have developed and applied higher-strength aluminum-silicon coated hot-formed steel. For example, Anmi's 2000MPa grade aluminum-silicon product was the first in the world to be used in Dongfeng Voyah in 2021, and Tangsteel's 2000MPa grade bare steel was used in Great Wall's third-generation Haval in 2020. However, higher-strength hot-formed steel exhibits higher susceptibility to hydrogen-induced delayed fracture, significantly reducing vehicle safety performance.

[0005] ArcelorMittal's patent publication CN112955572A, published on June 11, 2021, discloses a press-hardened component with high resistance to delayed fracture and its manufacturing method. The product has a strength of 1400-2000 MPa. The coating contains (FexAly) intermetallic compounds generated by iron diffusion into the pre-coated aluminum-based alloy or aluminum alloy. Magnesium is a particularly important element in the steel matrix, with a magnesium content between 0.002% and 0.007%. The generated MgO, MgO-Al2O3, or fine MgOTixOy effectively induce the formation of bainite and / or ferrite during the cooling step of the component in hot pressing, refining the martensitic lath structure and significantly improving resistance to delayed fracture.

[0006] However, the hydrogen-induced delayed fracture susceptibility of pre-coated aluminum alloy high-strength hot-formed steel is not only related to the microstructure formed after hot forming, but also closely related to the coating structure. When the tensile strength of pre-coated aluminum alloy high-strength hot-formed steel exceeds 1600 MPa after hot forming, the material's hydrogen-induced delayed fracture susceptibility increases significantly. Using GMW17508, under 100% yield stress at four-point bending in a 0.1 mol / L (pH=1) hydrochloric acid solution, the fracture time is difficult to exceed 80 hours. This is related to the formation of large and numerous Kirkendal pores in the coating after hot forming. Furthermore, when the coating after hot forming has large and numerous Kirkendal pores, it significantly deteriorates the cold bending performance and corrosion resistance of the coating after hot forming. Summary of the Invention

[0007] The purpose of this invention is to provide a high-resistance delayed fracture 1700MPa grade aluminum alloy coated hot-formed steel sheet and its production method. By matching the composition design and production process, the coating and substrate state of the raw material steel sheet are controlled, thereby controlling the size and number of Kirkendall holes, ensuring that the hot-formed steel components have high resistance to delayed fracture, cold bending performance and coating corrosion resistance.

[0008] Another objective of this invention is to provide hot-formed steel components, which are obtained by hot-forming the above-mentioned high-delay fracture 1700MPa grade aluminum alloy coated hot-formed steel sheets. After hot forming, the number of Kirkendal pores with a diameter of 1.0μm or more in the interdiffusion layer is ≤12 / 100μm, the hydrogen-induced delayed fracture acid fracturing time is ≥120h, the cold bending angle of the hot-formed steel component after baking is ≥50°, and the maximum corrosion spread width is ≤4mm after scratch corrosion testing of the hot-formed steel component after coating (phosphating, electrophoresis), and the tensile strength of the hot-formed steel component is between 1600 and 1750MPa.

[0009] Another object of the present invention is to provide applications for hot-formed steel components, for automotive parts, especially for high-strength automotive parts.

[0010] The specific technical solution of this invention is as follows:

[0011] High-strength delayed fracture 1700MPa grade aluminum alloy coated hot-formed steel sheet, including base steel sheet and aluminum alloy coating;

[0012] The base steel plate comprises the following components by weight percentage:

[0013] C: 0.23–0.29%, Si: 0.10–0.50%, Mn: 1.00–1.50%, Cr: 0.10–0.80%, P: ≤0.03%, S: ≤0.03%, N: ≤0.01%, Al: 0.01–0.10%, B: 0.001–0.01%, Ti: 0.01–0.10%, Nb: 0.01–0.10%, with the remainder being Fe and unavoidable impurities.

[0014] Preferably, the base steel plate comprises the following components by weight percentage:

[0015] C: 0.24–0.28%, Si: 0.10–0.35%, Mn: 1.00–1.30%, Cr: 0.10–0.55%, P: ≤0.03%, S: ≤0.03%, N: ≤0.01%, Al: 0.01–0.06%, B: 0.001–0.005%, Ti: 0.01–0.06%, Nb: 0.01–0.06%, with the remainder being Fe and unavoidable impurities.

[0016] The composition of the base steel plate of the high delayed fracture 1700MPa grade aluminum alloy coated hot-formed steel plate also meets the following requirement: Mn+Cr+Si≤2.00%.

[0017] The composition of the steel substrate of the high-delay fracture 1700MPa grade aluminum alloy coated hot-formed steel sheet also meets the following requirements: Ti+Nb≤0.12%;

[0018] The aluminum alloy coating comprises a FeAl alloy layer, a FeAlSi suppression layer, and an Al alloy layer; from the base steel plate to the surface layer, the layers are sequentially a FeAl alloy layer (thickness < 1 μm), a FeAlSi suppression layer, and an Al alloy layer on its outer side.

[0019] The high-delay fracture-resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet has no surface oxidation within 5μm of the base steel sheet surface, the FeAlS inhibition layer thickness is controlled at 3~7μm, and the FeAlSi inhibition layer thickness fluctuation is ≤40%.

[0020] The design principles for the alloying elements and their contents in the steel plate substrate of this invention are as follows:

[0021] 0.23% ≤ C ≤ 0.29%: Carbon (C) is the most important element for ensuring strength after hot forming. When the C content is between 0.23% and 0.29%, it ensures good hardenability during the cooling process of hot forming and good mechanical strength after hot forming. When the C content is below 0.23%, the hardenability during cooling is insufficient, resulting in the formation of more ferrite structure after hot forming, which significantly reduces mechanical strength. In addition, the low-carbon martensite formed at lower C contents also has insufficient mechanical strength, resulting in a tensile strength that does not reach 1600 MPa. When the C content is above 0.29%, the strength of the martensite formed after hot forming is too high, with a tensile strength exceeding 1750 MPa, and the toughness of the steel plate decreases sharply. Therefore, the C content is determined to be 0.23%–0.29%.

[0022] 0.10% ≤ Si ≤ 0.50%: Si plays a deoxidizing role in liquid steel. When the Si content is below 0.10%, the deoxidizing effect is not significant. However, when the Si content is above 0.50%, a certain amount of Si oxidation and enrichment will occur on the surface layer (including the surface) of the hot-formed steel matrix during hot rolling and annealing, causing incomplete plating or porosity after hot forming. These problems are particularly pronounced when the hot rolling temperature, coiling temperature, annealing temperature, or dew point is too high. Therefore, the Si content is determined to be 0.10–0.50%.

[0023] 1.00% ≤ Mn ≤ 1.50%: Mn plays a crucial role in ensuring hardenability and mechanical strength after hot forming. When the Mn content is below 1.00%, hardenability is insufficient during hot forming and cooling, resulting in a significant reduction in mechanical strength, with tensile strength failing to reach 1600 MPa. However, if the Mn content is too high, such as above 1.50%, Mn oxidation and enrichment will occur on the surface layer (including the outer surface) of the hot-formed steel matrix during hot rolling and annealing, causing incomplete plating or voids after hot forming. These problems are particularly pronounced when the hot rolling temperature, coiling temperature, annealing temperature, or dew point is too high. Therefore, the Mn content is determined to be 1.00–1.50%.

[0024] 0.10% ≤ Cr ≤ 0.80%: Besides C and Mn, which ensure hardenability and mechanical strength after hot forming, Cr also plays a crucial role. To guarantee a tensile strength of 1600 MPa after hot forming, material hardenability must be ensured. When the Cr content is below 0.10%, hardenability is insufficient during cooling after hot forming, resulting in a significant decrease in mechanical strength and a failure to reach 1600 MPa in tensile strength. When the Cr content is above 0.80%, the effect of improving hardenability becomes less significant. Furthermore, when the Cr content is above 0.80%, Cr oxidation and enrichment will occur on the surface layer (including the surface) of the hot-formed steel matrix during hot rolling and annealing, causing incomplete plating or voids after hot forming. These problems are particularly pronounced when the hot rolling temperature, coiling temperature, annealing temperature, or dew point is too high. Therefore, the Cr content is determined to be 0.10–0.80%.

[0025] Mn+Cr+Si≤2.00%: Studies have found that Mn, Cr, and Si are particularly prone to oxidation enrichment on the surface of the steel matrix during hot rolling and annealing. This phenomenon is exacerbated when the hot rolling temperature, coiling temperature, annealing temperature, or dew point is too high. Subsequent hot-dip galvanizing results in poor surface wettability, leading to incomplete plating and significant porosity defects after hot forming, thus reducing the corrosion resistance and weldability of the hot-formed parts. Research also shows that the degree of oxidation enrichment of Mn, Cr, and Si on the matrix surface is affected by the C content of the matrix. During hot rolling and annealing, decarburization and oxidation enrichment of Mn, Cr, and Si occur simultaneously on the matrix surface. Both decarburization and oxidation of alloying elements involve reactions with oxygen in the environment, and these two processes compete with each other. Different C contents and different Mn, Cr, and Si contents result in different decarburization and oxidation enrichment reaction rates. For low-strength hot-formed steels such as 500MPa and 1000MPa, the C content is relatively low, and the tendency for Mn, Cr, and Si to oxidize and enrich on the matrix surface is relatively high. However, to ensure high strength, a higher Mn content is generally added, such as greater than 1.5%. In this invention, the C content is relatively high, ranging from 0.23% to 0.29%. C is the most important element for ensuring strength after hot forming, so a smaller amount of Mn is generally added, ranging from 1.00% to 1.50% in this invention. At this point, the tendency for Mn to oxidize and enrich on the matrix surface is relatively low. However, the inventors have found that when the Mn content is low, Si and Cr are more easily enriched on the surface of the matrix. In this case, to reduce the enrichment degree of alloying elements on the matrix surface, the overall content of Si and Cr needs to be controlled, and the content needs to meet the following requirement: Mn + Cr + Si ≤ 2.00%.

[0026] P≤0.03%, S≤0.03%: Excessive sulfur and phosphorus lead to decreased toughness and significantly reduce the material's resistance to hydrogen-induced delayed fracture and cold bending performance. The P content is 0-0.03% and the S content is 0-0.03%.

[0027] N ≤ 0.01%: When the N content is higher than 0.01%, it easily forms numerous and large-sized Ti and Nb nitrides or carbonitrides, which is detrimental to the toughness of the product and significantly reduces the material's resistance to hydrogen-induced delayed fracture and cold bending performance. Therefore, the N content is determined to be 0–0.01%.

[0028] 0.01% ≤ Al ≤ 0.10%: Al has deoxidizing and nitrogen-precipitating effects. Al is a ferrite stabilizing element. When the Al content is higher than 0.10%, steel is prone to forming δ-ferrite in the high-temperature zone during hot rolling, which deteriorates product performance. In this invention, the Al content is determined to be 0.01% to 0.10%.

[0029] 0.001% ≤ B ≤ 0.010%: Boolean (B) significantly improves the hardenability of steel. When the B content is below 0.001%, its effect on improving hardenability is not fully realized; when the B content is above 0.010%, its effect on improving hardenability no longer increases. Therefore, the B content is determined to be 0.001–0.010%.

[0030] 0.01% ≤ Ti ≤ 0.10%: The main purpose of adding Ti to steel is to solidify nitrogen (N) and prevent boron (B) from forming bimetallic nitrogen (BN), thus allowing B to fully utilize its role in improving hardenability. When the Ti content is below 0.01%, N cannot be sufficiently solidified. When the Ti content is above 0.10%, a large number of large-sized Ti carbides, nitrides, or carbonitrides form in the steel, which is detrimental to the toughness of the product. Therefore, the Ti content is determined to be 0.01%–0.10%.

[0031] 0.01% ≤ Nb ≤ 0.10%: The higher the strength of hot-formed steel, the worse its toughness. Generally, the cold bending angle of 1500MPa grade aluminum-silicon coated hot-formed steel is only 50-55° (VDA238-100), while 1600MPa hot-formed steel, due to its higher strength, has poorer toughness. Ensuring its toughness is one of the key factors. Nb is a strong carbide-forming element, forming carbides or complex carbides in steel. These carbides or complex carbides are fine and dispersed in the steel matrix, which refine the grains and improve strength and toughness. When the Nb content is below 0.01%, the effect of improving strength and toughness is not obvious. When the Nb content is above 0.10%, the effect of improving strength and toughness tends to saturate. At this point, a large number of large-sized Nb and Ti carbides, nitrides, or carbonitrides are formed in the steel. These types of carbides, nitrides, or carbonitrides are detrimental to the toughness of the product. More importantly, grain refinement significantly improves the material's resistance to hydrogen-induced delayed fracture. Therefore, the Nb content was determined to be 0.01–0.10%.

[0032] Ti + Nb ≤ 0.12%: Both Ti and Nb readily form carbides, nitrides, or complex carbonitrides. Studies have found that when Ti + Nb > 0.12%, a large number and size of carbides, nitrides, or complex carbonitrides are easily formed in the steel, significantly deteriorating the material's resistance to hydrogen-induced delayed fracture and cold bending performance. Therefore, Ti + Nb ≤ 0.12%.

[0033] The production method of high-delay fracture resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet provided by the present invention includes the following process flow: steelmaking → continuous casting → hot rolling → pickling and cold rolling → substrate cleaning → annealing → coating → finishing → coiling.

[0034] Specifically:

[0035] 1) Steelmaking: Steelmaking is carried out in accordance with the composition requirements of the steel substrate of the high-delay fracture 1700MPa grade aluminum alloy coated hot-formed steel sheet;

[0036] 2) Continuous casting: The refined molten steel is poured into the tundish, and the tundish then distributes the molten steel to each crystallizer. After the casting is formed and crystallized, the casting is pulled out and cut into slabs of a certain length.

[0037] 3) Hot rolling: The slab is heated in a heating furnace, rolled after exiting the furnace, and then coiled at a temperature between 450 and 580°C.

[0038] It should be noted that hot rolling coiling temperature control is one of the key processes to prevent significant Si, Mn, and Cr oxidation enrichment on the surface of the steel substrate. This invention, based on limiting the Si, Mn, and Cr element content in the steel substrate, specifies an upper limit for the hot rolling coiling temperature, which significantly reduces the tendency of Si, Mn, and Cr oxidation enrichment on the surface of the steel substrate, ensuring good resistance to hydrogen-induced delayed fracture, cold bending performance, and coating corrosion resistance of the final product. In addition, the coiling temperature should not be lower than 450℃. If the coiling temperature is lower than 450℃, more hard phases of martensite and bainite will be generated in the hot-rolled coil, which significantly increases the strength of the hot-rolled coil and makes subsequent acid rolling difficult.

[0039] When oxidation occurs on the surface of the substrate, the oxidized area is generally concentrated within 5 μm of the surface. Through energy dispersive spectroscopy or glow discharge spectroscopy, obvious enrichment regions or points of O, Si, Mn, and Cr elements are found in this oxidized area. The content of O, Si, Mn, and Cr elements in the enriched regions or points is significantly higher than that in the unoxidized area in the center of the substrate. The content of Si, Mn, and Cr elements in the unoxidized area in the center of the substrate is controlled according to the present invention: Si: 0.10–0.50%, Mn: 1.00–1.50%, Cr: 0.10–0.80%.

[0040] 4) Pickling and cold rolling:

[0041] Hot-rolled steel sheets are further pickled and cold-rolled to obtain pickled and cold-rolled steel sheets. This process removes the iron oxide scale generated on the surface of the steel sheet during hot rolling. To ensure good surface quality after plating, the residual oil content on one side of the hard-rolled coil must be ≤250mg / m² after pickling and cold rolling. 2 Residual iron content ≤100mg / m³ 2 Due to temperature differences on different parts of the steel plate surface during hot rolling, the thickness of the iron oxide scale formed in different parts is uneven, resulting in an uneven surface after pickling. If there is a certain degree of alloy element oxidation enrichment in the substrate surface layer at this time, pickling cannot completely remove the alloy element oxides from the substrate surface layer. After pickling, pits form on the surface of the alloy element oxidation areas of the substrate surface. The higher the pickling reduction rate, the more pits there are. The pitted areas are uneven with the normal areas, and coupled with the alloy elements that were not pickled away in the pitted areas, the Fe-Al reaction rate differs between the pitted areas and the normal areas during hot-dip galvanizing. This results in a large difference in the thickness of the FeAlSi inhibition layer between the pitted areas and the normal areas during hot-dip galvanizing. The large fluctuation in the thickness of the FeAlSi inhibition layer causes differences in the diffusion degree in different parts during hot forming, exacerbating the formation of Kirkendal voids. This invention achieves a pickling reduction rate ≤58%, ensuring that the FeAlSi inhibition layer thickness fluctuation after hot-dip galvanizing is ≤40%. The FeAlSi suppression layer thickness fluctuation is calculated as: |Maximum or Minimum Thickness - Average Thickness| / Average Thickness × 100%, where Average Thickness = (Maximum Thickness + Minimum Thickness) / 2. It should be noted that oxidation and enrichment of alloying elements on the substrate surface can also cause excessive fluctuations in the FeAlSi suppression layer thickness after hot-dip plating. To ensure that the FeAlSi suppression layer thickness fluctuation after hot-dip plating is ≤40%, this invention controls the hot-rolling coiling temperature to ≤580℃, the annealing temperature to ≤800℃, and the annealing dew point to ≤-10℃.

[0042] 5) Substrate Cleaning: Substrate cleaning includes: alkaline washing → alkaline brushing → alkaline washing → water brushing → electrolytic cleaning → rinsing → drying. To ensure good surface quality after plating, the residual oil content on the steel plate surface after cleaning should be ≤20mg / m². 2 Single-sided residual iron ≤10mg / m 2 ;

[0043] 6) Annealing: The present invention controls the annealing temperature to 700-800℃. The annealing temperature includes the temperature of the annealing heating section and the temperature of the annealing soaking section, both of which are controlled to meet the requirement of 700-800℃; the dew point inside the annealing furnace is ≤-10℃, that is, the dew point of the heating section and the soaking section does not exceed -10℃.

[0044] The main purpose of the annealing process is to allow the hard-rolled coil to recrystallize, eliminate residual stress, and control the microstructure and properties of the finished coil. The annealing temperature should not be lower than 700℃. If the annealing temperature is too low, the hard-rolled coil will not recrystallize sufficiently, which will be detrimental to the properties of the finished coil.

[0045] The annealing section heating temperature and the soaking section temperature of this invention do not exceed 800℃. Furthermore, the dew point inside the annealing furnace is controlled by adjusting the amount of steam introduced, with the dew point in the heating and soaking sections not exceeding -10℃. The atmosphere inside the annealing furnace is N2+H2, where the volume percentage of H2 is 5-10%. Introducing 5-10% H2 into the furnace reduces iron oxides formed by Fe reacting with H2O and O2, thereby ensuring good coating quality before hot forming. Additionally, the oxygen content in the heating and soaking sections is controlled below 50ppm to further reduce steel substrate oxidation.

[0046] It should be noted that the annealing process control in this invention is also one of the key processes to prevent significant oxidation enrichment of Si, Mn, Cr, etc. on the surface of the steel substrate. Based on limiting the content of Si, Mn, Cr, etc. in the steel substrate, this invention specifies the heating temperature, soaking temperature, dew point, and upper limit of oxygen content, further reducing the tendency of Si, Mn, Cr, etc., to accumulate on the surface of the steel substrate, ensuring good resistance to hydrogen-induced delayed fracture, cold bending performance, and coating corrosion resistance of the final product. Furthermore, an annealing temperature ≤800℃ and a dew point ≤-10℃ significantly reduce the hydrogen content entering the steel substrate during annealing, thereby reducing the diffusible hydrogen content in the hot-formed steel components and further improving the product's resistance to hydrogen-induced delayed fracture.

[0047] 7) Coating: The plating solution contains aluminum alloy and unavoidable impurities. Typical plating solution composition includes the following percentages by mass: 5–11% Si, 2–4% Fe, with the balance being Al and unavoidable impurities. The hot-dip plating solution temperature is between 600 and 680°C. The substrate temperature should be kept as consistent as possible with the hot-dip plating solution temperature when immersed to reduce steel strip dissolution and aluminum dross formation. The immersion time is 2–10 seconds. After hot-dip plating, nitrogen or compressed air is used to control the coating thickness. The pre-coating thickness is controlled at 7–19 μm on one side, and the FeAlSi inhibition layer thickness is controlled at 3–7 μm, with a FeAlSi inhibition layer thickness fluctuation ≤40%.

[0048] The hot-dip galvanizing bath temperature is between 600 and 680°C. When the hot-dip galvanizing bath temperature exceeds 680°C, the thickness fluctuation of the FeAlSi suppression layer formed between the galvanizing bath and the steel substrate increases significantly during hot-dip galvanizing. This large fluctuation in the FeAlSi suppression layer thickness causes differences in the diffusion degree in different parts during hot forming, exacerbating the formation of Kirkendal voids. Furthermore, the melting point of aluminum-silicon alloy is approximately 600°C, and the hot-dip galvanizing bath temperature should not be lower than 600°C. In this invention, the FeAlSi suppression layer thickness fluctuation is ≤40%. It should be noted that to achieve a FeAlSi suppression layer thickness fluctuation of ≤40%, this invention not only requires controlling the hot-dip galvanizing bath temperature to ≤680°C and the acid rolling reduction rate to ≤58%, but also requires controlling the oxidation and enrichment of alloying elements on the substrate surface. Therefore, it is necessary to control the hot rolling coiling temperature to ≤580°C, the annealing temperature to ≤800°C, and the annealing dew point to ≤-10°C.

[0049] The FeAlSi suppression layer thickness fluctuation of this invention is ≤40%.

[0050] The pre-coating thickness should not be less than 7 μm. The inventors have discovered that the thinner the initial coating, the more pronounced the Kirkendal voids become. This is because a thinner initial coating shortens the interdiffusion path of Fe and Al, accelerating the interdiffusion rate. However, the thinner coating reduces the relative Al content in the coating, decreasing the Al available to fill Fe vacancies, thus further exacerbating the formation of large-sized Kirkendal voids. Furthermore, a coating thickness less than 7 μm is prone to incomplete plating defects.

[0051] The pre-coating thickness should not exceed 19μm. If the coating is too thick, it will reduce the cold bending performance of the final product and increase production costs.

[0052] 8) Finishing: The coated steel strip is finished to improve the shape of the strip and control the surface roughness of the coating.

[0053] 9) Winding: Winding and unwinding the steel strip.

[0054] The hot-formed steel component provided by the present invention is obtained by hot forming of the above-mentioned high-delay fracture-resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet.

[0055] The hot-formed steel component, after hot forming, has ≤12 Kirkendall pores with a diameter of 1.0 μm or more in the interdiffusion layer, and the number of such pores per 100 μm is ≤12.

[0056] The diffusible hydrogen content in the hot-formed steel component does not exceed 0.50 ppm.

[0057] The hot-formed steel component has a hydrogen-induced delayed fracture time of ≥120h, a cold bending angle of ≥50° after baking, and a maximum corrosion spread width of ≤4mm after scratch corrosion testing after coating. The tensile strength of the hot-formed steel component is between 1600 and 1750 MPa.

[0058] The specific process of thermoforming includes the following steps: blanking → heat treatment → hot stamping.

[0059] Blanking: The above-mentioned high-delay fracture 1700MPa grade aluminum alloy coated hot-formed steel sheet is punched or cut into a blank of the required shape for the hot-formed part.

[0060] The heat treatment involves placing the billet in a heating furnace and heating and holding it at a temperature of 840–970°C. The furnace atmosphere is either air or nitrogen. The billet remains in the furnace for 2–10 minutes, with a furnace dew point ≤ -5°C. Maintaining the furnace dew point below -5°C effectively reduces the hydrogen absorption reaction between the aluminum coating and water vapor, thereby reducing the hydrogen content entering the steel matrix and ultimately lowering the diffusible hydrogen content in the hot-formed steel components, significantly improving the product's resistance to hydrogen-induced delayed fracture. When the furnace dew point exceeds -5°C, the product's resistance to hydrogen-induced delayed fracture decreases sharply.

[0061] Currently, commonly used heating furnaces include box-type heating furnaces and roller bottom heating furnaces. When using a box-type heating furnace, heating is carried out at a fixed temperature, while when using a roller bottom heating furnace, heating is carried out in sections. In this case, the heating furnace temperature mentioned above refers to the highest heating temperature of the roller bottom heating furnace.

[0062] The hot stamping process involves rapidly transferring the heat-treated blank into a mold for stamping and cooling. The transfer time is no more than 15 seconds, the stamping holding time is 5 to 15 seconds, the cooling and demolding temperature is no more than 250°C, and the cooling rate is ≥30°C / s.

[0063] The application of the hot-formed steel components provided by this invention is in the manufacture of high-strength automotive parts.

[0064] The design concept of this invention is as follows:

[0065] The susceptibility of high-strength hot-formed steel to hydrogen-induced delayed fracture (H-DEF) is related to material strength, diffusible hydrogen content, and stress state. Material strength is mainly related to the martensitic microstructure; the higher the martensitic strength, the higher the susceptibility to H-DEF. This can be addressed by refining the martensitic microstructure, such as by adding microalloying elements like Nb and Mo. The inventors found that when the tensile strength of high-strength hot-formed steel with a pre-coated aluminum alloy layer exceeds 1600 MPa after hot forming, the susceptibility to H-DEF increases significantly. Using GMW17508, under 100% yield stress at four-point bending in a 0.1 mol / L (pH=1) hydrochloric acid solution, the fracture time is difficult to exceed 80 hours. This invention refines the martensitic microstructure by adding microalloying elements, ensuring that the original austenite grain size does not exceed 10 μm (the smaller the original austenite grain size, the finer the martensitic microstructure), guaranteeing that the tensile strength of the hot-formed steel component is between 1600 MPa and 1750 MPa, and that the H-DEF test meets the requirement of 120 hours without fracture.

[0066] The higher the diffusible hydrogen content, the higher the susceptibility to hydrogen-induced delayed fracture. Studies have found that when the diffusible hydrogen content in hot-formed steel components exceeds 0.50 ppm, the susceptibility to hydrogen-induced delayed fracture increases sharply. The diffusible hydrogen content is related to the hydrogen content of the raw materials and the hydrogen absorption during the hot forming heating process. During the production process, the raw materials mainly contain a high content of water vapor in the annealing furnace. H2O reacts with C, Fe, etc. in the steel matrix to generate H, which enters the steel matrix. In addition, during the hot forming heating process, H2O reacts chemically with the Al coating to produce elemental H, which enters the matrix and diffuses along the grain boundaries, selectively reacting with carbon, etc. The resulting hydrogen molecules and methane gas create enormous pressure, leading to cracking. This invention controls the hydrogen content in the raw materials by controlling the annealing process in the annealing furnace and the dew point in the hot forming heating furnace, thereby controlling the hydrogen absorption reaction during the heating process. Ultimately, the diffusible hydrogen content in the hot-formed steel components is controlled to not exceed 0.50 ppm, thus ensuring that the hydrogen-induced delayed fracture test meets the requirement of no fracture for 120 hours.

[0067] The stress state includes the stress level of the hot-formed steel component and the hydrogen-induced crack propagation rate. The stress level of the hot-formed steel component is mainly related to its service condition. When the hot-formed steel component is subjected to a certain stress level, the sensitivity to hydrogen-induced delayed fracture is affected by the hydrogen-induced crack propagation rate. The inventors found that the hydrogen-induced crack propagation rate is not only related to the material strength and the content of diffusing hydrogen, but is also significantly affected by the coating structure after hot forming. The inventors found that during the heating process of aluminum alloy coated hot-formed steel, due to the large difference in the diffusion rates of Fe and Al, Kirkendal voids are easily formed on the substrate surface. Furthermore, when there is a certain degree of oxidation on the substrate surface (including the surface) before hot forming, the Kirkendal void situation (size and number) is significantly aggravated. This may be because the mutual diffusion of Fe and Al is hindered at the oxidation site, and after Fe diffusion forms vacancies, it is more difficult for Al to fill the vacancies. Furthermore, as the coating thins, the interdiffusion path of Fe and Al shortens and the interdiffusion rate accelerates. However, due to the reduced Al content in the coating, the amount of Al available to fill Fe vacancies decreases, further exacerbating the formation of large Kirkendal voids. The inventors also discovered that during the hot forming heating process, the interdiffusion between the steel substrate and the FeAlSi inhibition layer near the substrate is most intense. The uniformity of the FeAlSi inhibition layer thickness in the pre-coated Al-Si coating also significantly affects the formation of Kirkendal voids. When the thickness of the FeAlSi inhibition layer in the pre-coating fluctuates greatly, the diffusion rate differs in different parts of the coating, further intensifying the formation of large Kirkendal voids. When significant Kirkendal voids exist in the hot-formed steel component, hydrogen-induced cracks easily form in the void area under stress, rapidly accelerating the propagation rate of these cracks and causing hydrogen-induced fracture of the steel component.

[0068] The hydrogen-induced delayed fracture susceptibility of pre-coated aluminum alloy high-strength hot-formed steel is related not only to the microstructure formed after hot forming but also to the coating structure. When hot-formed steel is pre-coated with an Al-Si coating, Kirkendal voids easily form between the coating and the substrate. The size and number of these voids are related to the state of the raw material coating or the steel substrate. Larger and more numerous Kirkendal voids in the hot-formed coating significantly increase the hydrogen-induced delayed fracture susceptibility. Currently, GMW17508 is widely used to evaluate hydrogen-induced delayed fracture susceptibility, requiring no fracture after immersion in a 0.1 mol / L (pH=1) hydrochloric acid solution for 120 hours under 100% yield stress in a four-point bend. Studies have found that currently, for hot-formed steel with a pre-coated aluminum alloy layer and a tensile strength of 1500 MPa after hot forming, the material generally meets the 120-hour hydrogen-induced delayed fracture test requirement without fracture. However, when the tensile strength exceeds 1600 MPa after hot forming, the material's susceptibility to hydrogen-induced delayed fracture increases significantly. Using the aforementioned hydrogen embrittlement test, the fracture time is unlikely to exceed 80 hours. This is related to the formation of large and numerous Kirkendal pores in the coating after hot forming. Furthermore, when the coating after hot forming has large and numerous Kirkendal pores, it significantly deteriorates the cold bending performance and corrosion resistance of the coating after hot forming.

[0069] It should be noted that pores may either reduce or improve cold bending performance. When pores exist between the substrate and coating after hot forming, the substrate surface in the pore area undergoes decarburization during the hot forming heating process. The decarburized substrate surface has lower strength and better ductility than the undecarburized area. However, when the pore size is small and the number is small, the decarburized area is small, and decarburization has little effect on improving cold bending performance. However, due to the presence of pores, cracks in the pore area will rapidly propagate from the coating to the substrate when bent under stress, significantly reducing cold bending performance. On the other hand, when the pore size is large and the number of pores is large, especially when the pores are interconnected to form large pore areas, the substrate surface in the pore areas undergoes significant decarburization during the hot forming heating process. In this case, the decarburized area is large, which can improve cold bending performance. However, large pore areas greatly deteriorate the coating corrosion resistance of the hot-formed part. In other words, when the pre-coating layer is thin, to ensure that the hot-formed aluminum alloy coated parts have high resistance to delayed fracture, cold bending performance, and coating corrosion resistance, the size and number of Kirkendal pores need to be strictly controlled. This requires comprehensive control of the oxidation state of the substrate surface before hot forming and the thickness fluctuation of the FeAlSi inhibitory layer. The oxidation state of the substrate surface before hot forming is mainly related to the substrate's chemical composition and production process, which mainly includes hot rolling and annealing. The thickness fluctuation of the FeAlSi inhibitory layer is mainly related to the pickling and rolling process and the hot-dip galvanizing temperature. Furthermore, this invention refines the martensitic structure by adding microalloying elements, ensuring that the tensile strength of the hot-formed steel components is between 1600 MPa and 1750 MPa. Simultaneously, it controls the annealing process in the annealing furnace to control the hydrogen content in the raw materials and the dew point in the hot forming furnace to control the hydrogen absorption reaction during heating. Ultimately, it controls the diffusible hydrogen content in the hot-formed steel components, thus ensuring that the hydrogen-induced delayed fracture test meets the requirement of no fracture after 120 hours.

[0070] Compared with the prior art, the present invention controls the oxidation state of the substrate surface and the thickness fluctuation of the FeAlSi inhibition layer before hot forming by controlling the above components and production process parameters. Finally, it controls the size and number of Kirkendal holes after hot forming, ensuring that the hot-formed steel components have high resistance to delayed fracture, cold bending performance and coating corrosion resistance. Furthermore, this invention refines the martensitic structure by adding microalloying elements, ensuring that the tensile strength of hot-formed steel components is between 1600 MPa and 1750 MPa. Simultaneously, it controls the annealing process in the annealing furnace to control the hydrogen content in the raw materials and the dew point in the hot-forming heating furnace to control the hydrogen absorption reaction during heating. Ultimately, it controls the diffusible hydrogen content in the hot-formed steel components, ensuring that the hydrogen-induced delayed fracture (HICF) fracturing time is ≥120 h. After hot forming, the number of Kirkendal pores with a diameter greater than 1.0 μm in the interdiffusion layer is ≤12 per 100 μm, the HICF fracturing time is ≥120 h, the cold bending angle of the hot-formed steel components after baking is ≥50°, and the maximum corrosion spread width is ≤4 mm after scratch corrosion testing. Finally, the tensile strength of the hot-formed steel components is between 1600 and 1750 MPa. Attached Figure Description

[0071] Figure 1 This is a diagram showing the oxidation state of the steel substrate surface and the thickness fluctuation of the FeAlSi suppression layer before hot forming in Example 1.

[0072] Figure 2 This is a diagram of large precipitates in the steel matrix before hot forming, as shown in Comparative Example 6.

[0073] Figure 3 This is a diagram of the original austenite grains in Example 1;

[0074] Figure 4 The diagram shows the oxidation state of the steel substrate surface and the thickness fluctuation of the FeAlSi inhibition layer before hot forming in Comparative Example 1.

[0075] Figure 5 This is a diagram showing the state of the Kirkendal holes after thermoforming, as shown in Comparative Example 1. Detailed Implementation

[0076] The invention will be further described in detail below with reference to examples.

[0077] The production method of high-delay fracture resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet provided by the present invention includes the following process flow: steelmaking → continuous casting → hot rolling → pickling and cold rolling → substrate cleaning → annealing → coating → finishing → coiling.

[0078] 1) Steelmaking: The steelmaking composition of the steel plate matrix is ​​controlled by mass as follows: C: 0.23~0.29%, Si: 0.10~0.50%, Mn: 1.00~1.50%, Cr: 0.10~0.80%, P: ≤0.03%, S: ≤0.03%, N: ≤0.01%, Al: 0.01~0.10%, B: 0.001~0.01%, Ti: 0.01~0.10%, Nb: 0.01~0.10%, Mn+Cr+Si≤2.00%, Ti+Nb≤0.12%, with the remainder being Fe and unavoidable impurities.

[0079] As an example, the composition of the base steel is shown in Table 1. To control the oxidation and enrichment of Mn, Cr, and Si on the surface of the steel matrix during hot rolling and annealing, this invention specifically limits the content of these three elements: Mn: 1.00–1.50%, Cr: 0.10–0.80%, Si: 0.10–0.50%, and Mn+Cr+Si ≤ 2.00%. Furthermore, to ensure high resistance to delayed fracture, P: ≤ 0.03%, S: ≤ 0.03%, Ti: 0.01–0.10%, Nb: 0.01–0.10%, N: ≤ 0.01%, and Ti+Nb ≤ 0.12%. Embodiment Steel 1 and Embodiment Steel 2 are the compositions of this invention. In Comparative Steel 1, Mn+Cr+Si = 2.38% > 2.00%, exceeding the upper limit of this invention; in Comparative Steel 2, Ti+Nb = 0.14% > 0.12%, exceeding the upper limit of this invention.

[0080] Table 1. Composition of the base steel (wt%)

[0081]

[0082] 2) Continuous casting

[0083] The refined molten steel is poured into the tundish, which then distributes the molten steel to each crystallizer. After the casting is formed and crystallized, the casting is pulled out and cut into slabs of a certain length.

[0084] 3) Hot rolling

[0085] The slab is heated in a furnace, then rolled after exiting the furnace, and finally coiled at a temperature between 450 and 580°C.

[0086] It should be noted that hot rolling temperature control is one of the key processes to prevent significant Si, Mn, and Cr oxidation enrichment on the surface of the steel substrate. This invention, by limiting the Si, Mn, and Cr content in the steel substrate, specifies an upper limit for the hot rolling temperature, significantly reducing the tendency for Si, Mn, and Cr oxidation enrichment on the steel substrate surface, and ensuring good resistance to hydrogen-induced delayed fracture, cold bending performance, and coating corrosion resistance in the final product. Furthermore, the coiling temperature should not be lower than 450℃. Below 450℃, a large amount of hard martensite and bainite phases will be generated in the hot-rolled coil, significantly increasing its strength and making subsequent acid rolling difficult.

[0087] When oxidation occurs on the surface of the substrate, the oxidized area is generally concentrated within 5 μm of the surface. Through energy dispersive spectroscopy or glow discharge spectroscopy, obvious enrichment regions or points of O, Si, Mn, and Cr elements are found in this oxidized area. The content of O, Si, Mn, and Cr elements in the enriched regions or points is significantly higher than that in the unoxidized area in the center of the substrate. The content of Si, Mn, and Cr elements in the unoxidized area in the center of the substrate is controlled according to the present invention: Si: 0.10–0.50%, Mn: 1.00–1.50%, Cr: 0.10–0.80%.

[0088] 4) Pickling and cold rolling

[0089] Hot-rolled steel sheets are further pickled and cold-rolled to obtain pickled and cold-rolled steel sheets. This process removes the iron oxide scale generated on the surface of the steel sheet during hot rolling. To ensure good surface quality after plating, the residual oil content on one side of the hard-rolled coil must be ≤250mg / m² after pickling and cold rolling. 2 Residual iron content ≤100mg / m³ 2Due to temperature differences on different parts of the steel plate surface during hot rolling, the thickness of the iron oxide scale formed in different parts is uneven, resulting in an uneven surface after pickling. If there is a certain degree of alloy element oxidation enrichment in the substrate surface layer at this time, pickling cannot completely remove the alloy element oxides from the substrate surface layer. After pickling, pits form on the surface of the alloy element oxidation areas of the substrate surface. The higher the pickling reduction rate, the more pits there are. The pitted areas are uneven with the normal areas, and coupled with the alloy elements that were not pickled away in the pitted areas, the Fe-Al reaction rate differs between the pitted areas and the normal areas during hot-dip galvanizing. This results in a large difference in the thickness of the FeAlSi inhibition layer between the pitted areas and the normal areas during hot-dip galvanizing. The large fluctuation in the thickness of the FeAlSi inhibition layer causes differences in the diffusion degree in different parts during hot forming, exacerbating the formation of Kirkendal voids. This invention achieves a pickling reduction rate ≤58%, ensuring that the FeAlSi inhibition layer thickness fluctuation after hot-dip galvanizing is ≤40%. The FeAlSi suppression layer thickness fluctuation is calculated as: |Maximum or Minimum Thickness - Average Thickness| / Average Thickness × 100%, where Average Thickness = (Maximum Thickness + Minimum Thickness) / 2. It should be noted that oxidation and enrichment of alloying elements on the substrate surface can also cause excessive fluctuations in the FeAlSi suppression layer thickness after hot-dip plating. To ensure that the FeAlSi suppression layer thickness fluctuation after hot-dip plating is ≤40%, this invention controls the hot-rolling coiling temperature to ≤580℃, the annealing temperature to ≤800℃, and the annealing dew point to ≤-10℃.

[0090] 5) Substrate cleaning

[0091] Substrate cleaning includes: alkaline washing → alkaline brushing → alkaline washing → water brushing → electrolytic cleaning → rinsing → drying. To ensure good surface quality after plating, the residual oil content on one side of the steel plate after cleaning should be ≤20mg / m². 2 Single-sided residual iron ≤10mg / m 2 .

[0092] 6) Annealing

[0093] The main purpose of the annealing process is to allow the hard-rolled coil to recrystallize, eliminate residual stress, and control the microstructure and properties of the finished coil. The annealing temperature should not be lower than 700℃. If the annealing temperature is too low, the hard-rolled coil will not recrystallize sufficiently, which will be detrimental to the properties of the finished coil.

[0094] The annealing section heating temperature and the soaking section temperature of this invention do not exceed 800℃. Furthermore, the dew point inside the annealing furnace is controlled by adjusting the amount of steam introduced, with the dew point in the heating and soaking sections not exceeding -10℃. The atmosphere inside the annealing furnace is N2+H2, where the volume percentage of H2 is 5-10%. Introducing 5-10% H2 into the furnace reduces iron oxides formed by Fe reacting with H2O and O2, thereby ensuring good coating quality before hot forming. Additionally, the oxygen content in the heating and soaking sections is controlled below 50ppm to further reduce steel substrate oxidation.

[0095] It should be noted that the annealing process control in this invention is also one of the key processes to prevent significant oxidation enrichment of Si, Mn, Cr, etc. on the surface of the steel substrate. Based on limiting the content of Si, Mn, Cr, etc. in the steel substrate, this invention specifies the heating temperature, soaking temperature, dew point, and upper limit of oxygen content, further reducing the tendency of Si, Mn, Cr, etc., to accumulate on the surface of the steel substrate, ensuring good resistance to hydrogen-induced delayed fracture, cold bending performance, and coating corrosion resistance of the final product. Furthermore, an annealing temperature ≤800℃ and a dew point ≤-10℃ significantly reduce the hydrogen content entering the steel substrate during annealing, thereby reducing the diffusible hydrogen content in the hot-formed steel components and further improving the product's resistance to hydrogen-induced delayed fracture.

[0096] 7) Coating

[0097] The plating bath consists of aluminum alloy and unavoidable impurities. The target content of the plating bath is 8-10% Si, 2-4% Fe, with the balance being Al and unavoidable impurities. The hot-dip plating bath temperature is between 600-680℃. The substrate temperature should be kept as consistent as possible with the hot-dip plating bath temperature when it is immersed in the bath to reduce the dissolution of the steel strip and the formation of aluminum dross. The immersion time is 2-10 seconds. After hot-dip plating, nitrogen or compressed air is used to purge the coating thickness. The pre-coating thickness is controlled at 7-19 μm on one side, the FeAlSi inhibition layer thickness is 3-7 μm, and the FeAlSi inhibition layer thickness fluctuation is ≤40%.

[0098] The hot-dip galvanizing bath temperature is between 600 and 680°C. When the hot-dip galvanizing bath temperature exceeds 680°C, the thickness fluctuation of the FeAlSi suppression layer formed between the galvanizing bath and the steel substrate increases significantly during hot-dip galvanizing. This large fluctuation in the FeAlSi suppression layer thickness causes differences in the diffusion degree in different parts during hot forming, exacerbating the formation of Kirkendal voids. Furthermore, the melting point of aluminum-silicon alloy is approximately 600°C, and the hot-dip galvanizing bath temperature should not be lower than 600°C. In this invention, the FeAlSi suppression layer thickness fluctuation is ≤40%. It should be noted that to achieve a FeAlSi suppression layer thickness fluctuation of ≤40%, this invention not only requires controlling the hot-dip galvanizing bath temperature to ≤680°C and the acid rolling reduction rate to ≤58%, but also requires controlling the oxidation and enrichment of alloying elements on the substrate surface. Therefore, it is necessary to control the hot rolling coiling temperature to ≤580°C, the annealing temperature to ≤800°C, and the annealing dew point to ≤-10°C.

[0099] The FeAlSi suppression layer thickness fluctuation of this invention is ≤40%.

[0100] The pre-coating thickness should not be less than 7 μm. The inventors have discovered that the thinner the initial coating, the more pronounced the Kirkendal voids become. This is because a thinner initial coating shortens the interdiffusion path of Fe and Al, accelerating the interdiffusion rate. However, the thinner coating reduces the relative Al content in the coating, decreasing the Al available to fill Fe vacancies, thus further exacerbating the formation of large-sized Kirkendal voids. Furthermore, a coating thickness less than 7 μm is prone to incomplete plating defects.

[0101] The pre-coating thickness should not exceed 19μm. If the coating is too thick, it will reduce the cold bending performance of the final product and increase production costs.

[0102] 8) Smoothing

[0103] After coating, the steel strip is finished to improve the shape of the strip and control the surface roughness of the coating.

[0104] 9) Winding

[0105] The steel strip is wound up and unloaded.

[0106] The main process parameters for producing high-delay fracture 1700MPa grade aluminum alloy coated hot-formed steel sheets according to the above method are shown in Table 2.

[0107] The above-mentioned high-delay fracture resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet is used to manufacture hot-formed components. The specific process flow is as follows:

[0108] Blanking → Heat treatment → Hot stamping.

[0109] The blanking process involves punching or cutting the aforementioned high-delay fracture-resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet into a blank of the required shape for the hot-formed part. The typical thickness of the steel sheet in this invention is 1.4mm, and the steel sheet is processed into a template with dimensions of 150×300mm.

[0110] The heat treatment involves placing the billet in a heating furnace and heating and holding it at a temperature of 840–970°C. The furnace atmosphere is either air or nitrogen, and the billet remains in the furnace for 2–10 minutes. The furnace dew point is ≤-5°C. Controlling the furnace dew point below -5°C effectively reduces the degree of hydrogen absorption reaction between the aluminum coating and water vapor, thereby reducing the hydrogen content entering the steel matrix and ultimately reducing the diffusible hydrogen content in the hot-formed steel components, significantly improving the product's resistance to hydrogen-induced delayed fracture. When the furnace dew point is >-5°C, the product's resistance to hydrogen-induced delayed fracture decreases sharply.

[0111] Currently, commonly used heating furnaces include box-type heating furnaces and roller hearth heating furnaces. When using a box-type heating furnace, heating is performed at a fixed temperature, while when using a roller hearth heating furnace, segmented heating is employed. In this case, the heating furnace temperature mentioned above refers to the highest heating temperature of the roller hearth heating furnace. This invention example uses a box-type resistance heating furnace to heat a pre-coated steel plate, employing a typical heating process, namely a heating temperature of 930℃ and a heating time of 5 minutes.

[0112] The hot stamping process involves rapidly transferring the heat-treated billet into a mold for stamping and cooling. The transfer time is no more than 15 seconds, the stamping holding time is 5–15 seconds, the cooling and demolding temperature is no more than 250°C, and the cooling rate is ≥30°C / s. In this invention, the heat-treated steel plate is placed on a flat quenching mold, pressed, and held under pressure for a certain time. Cooling water is circulated inside the mold to cool the steel plate.

[0113] Table 2 shows the pre-coating thickness and FeAlSi thickness fluctuation of the hot-formed steel components produced according to the above method.

[0114] Table 2 Production process parameters, pre-coating thickness, and FeAlSi thickness fluctuation

[0115]

[0116]

[0117] The oxidation state, coating thickness, and FeAlSi layer thickness of the pre-coated aluminum alloy-coated hot-formed steel plate substrate were observed and analyzed. The porosity of the hot-formed steel components was observed and analyzed. The diffusible hydrogen content and resistance to hydrogen-induced delayed fracture of the hot-formed steel components were tested. The cold bending performance of the hot-formed steel components was tested. The scratch corrosion test was performed on the hot-formed steel components after coating. The mechanical properties of the hot-formed steel components were tested.

[0118] The oxidation state of the substrate surface before thermoforming is mainly observed by scanning electron microscopy, with a focus on the oxidation state within 5 μm of the substrate surface, as this area strongly affects the porosity after thermoforming. When oxidation is present, it is mainly concentrated near the grain boundaries of the substrate. The composition of this area is analyzed by energy dispersive spectroscopy. When the oxidation is severe, it can even form a crack-like morphology.

[0119] The porosity of hot-formed steel components was observed and analyzed using a scanning electron microscope (SEM). The porosity was mainly found within the interdiffusion layer, which is connected to the steel substrate and is generally composed of αFe + Fe3Al, with an Fe content of not less than 80%. Large-sized porosity, such as those with a diameter greater than 1.0 μm, requires special attention, as these porosity significantly affect the corrosion resistance of coatings and weldability. This invention statistically analyzes the number of large-sized porosity within the interdiffusion layer. The porosity diameter is determined by measuring the longest and shortest diameters of the porosity under the same field of view, and taking half of their sum as the porosity diameter. The number of porosity is determined by counting porosity within a 100 μm range along the surface of the substrate steel within the SEM field of view. It is important to note that when the surface of the raw material substrate is severely oxidized, the pores in the coating may connect after hot forming, forming a pore area. In this case, the method for determining the number of pores is as follows: In the field of view of a scanning electron microscope, along the surface of the steel substrate, count the area of ​​the pore area within a range of 100 μm in length. The pore area / 1 μm 2 That is, the number of holes.

[0120] The diffusible hydrogen content was detected by TDS thermal desorption mass spectrometry, with a heating rate of 100℃ / h and a heating temperature of 300℃.

[0121] The hydrogen-induced delayed fracture resistance test of hot-formed steel components uses GMW17508, which requires immersing the sample in a 0.1 mol / L (pH=1) hydrochloric acid solution under 100% yield stress at four-point bending, observing whether the sample fractures, and recording the fracture time. Generally, a fracture time ≥120 h is considered to meet the requirements.

[0122] The yield stress of hot-formed steel components is determined by reference to the mechanical property test results, and the test standard adopted is GB / T228.1-2010.

[0123] The hot-formed steel components simulate the baking process in automobile painting. The baking temperature is 170℃ and the baking time is 20 minutes. After baking, the hot-formed steel components are subjected to cold bending performance test, and the test standard adopts VDA238-100.

[0124] After coating, the hot-formed steel components underwent scratch corrosion testing. The coating process included phosphating and electrophoresis. The coating was then subjected to scratch corrosion testing to evaluate paint adhesion and corrosion resistance (a maximum corrosion spread width of no more than 4 mm was considered satisfactory). This invention selected three hot-formed steel plates under the same conditions for scratch corrosion testing, and the average of the maximum corrosion spread widths was used to evaluate paint adhesion and corrosion resistance. The phosphating agents and test parameters listed in Table 3 were used to phosphate the hot-formed samples. Subsequently, the resulting phosphated plates were subjected to electrophoresis (electrophoresis paint model: Kansai HT-8000C), with a dry film thickness of approximately 18 μm. Subsequently, a cyclic corrosion method was used. Each cycle consisted of 8 hours of ambient temperature maintenance (25±3℃, during which 4 sprays of salt solution were performed for 3 minutes each, with the salt solution composition being: 0.9wt% NaCl, 0.1wt% CaCl2, and 0.075wt% NaHCO3), followed by 8 hours of wet heat treatment (49±2℃, 100% RH), and finally 8 hours of drying (60±2℃, <30% RH). A total of 26 cycles were performed to evaluate corrosion resistance.

[0125] Table 3 Phosphating process parameters

[0126]

[0127] Mechanical property testing of hot-formed steel components shall be conducted in accordance with the standard GB / T228.1-2010.

[0128] The oxidation state, porosity, diffusible hydrogen content, resistance to hydrogen-induced delayed fracture, cold bending performance, scratch corrosion test, and mechanical property test results of the hot-formed steel components produced in the above embodiments and comparative examples are shown in Table 4.

[0129] Table 4. Results of oxidation state, porosity, diffusible hydrogen content, resistance to hydrogen-induced delayed fracture, cold bending performance, scratch corrosion test, and mechanical property test.

[0130]

[0131] This invention controls the oxidation state of the substrate surface and the thickness fluctuation of the FeAlSi inhibition layer before hot forming by controlling the chemical composition of the raw material matrix and the production process. Ultimately, it controls the size and number of Kirkendal pores after hot forming, ensuring that the hot-formed steel components possess high resistance to delayed fracture, cold bending performance, and corrosion resistance in coating. Furthermore, this invention refines the martensitic structure by adding microalloying elements, ensuring the tensile strength of the hot-formed steel components is between 1600 MPa and 1750 MPa. Simultaneously, it controls the annealing process in the annealing furnace to control the hydrogen content in the raw materials and the dew point in the hot forming furnace to control the hydrogen absorption reaction during heating. Ultimately, it controls the diffusible hydrogen content in the hot-formed steel components to not exceed 0.50 ppm, thus ensuring that the hydrogen-induced delayed fracture test meets the requirement of no fracture after 120 hours.

[0132] Specifically, within the interdiffusion layer after hot forming, the number of Kirkendal pores with a diameter greater than 1.0 μm is ≤12 / 100 μm, the hydrogen-induced delayed fracture time in acid fracturing is ≥120 h, the cold bending angle of the hot-formed steel component after baking is ≥50°, and the maximum corrosion spread width of the hot-formed steel component after coating (phosphating, electrophoresis) is ≤4 mm, and the tensile strength of the hot-formed steel component is between 1600 and 1750 MPa.

[0133] Specifically:

[0134] 1) Matrix chemical composition: The steelmaking composition of this invention is controlled by mass as follows: C: 0.23-0.29%, Si: 0.10-0.50%, Mn: 1.00-1.50%, Cr: 0.10-0.80%, P: ≤0.03%, S: ≤0.03%, N: ≤0.01%, Al: 0.01-0.10%, B: 0.001-0.01%, Ti: 0.01-0.10%, Nb: 0.01-0.10%, Mn+Cr+Si≤2.00%, Ti+Nb≤0.12%, with the remainder being Fe and unavoidable impurities.

[0135] To control the oxidation and enrichment of Mn, Cr, and Si on the surface of the steel matrix during hot rolling and annealing, this invention specifically limits the content of these three elements: Mn: 1.00–1.50%, Cr: 0.10–0.80%, Si: 0.10–0.50%, and Mn+Cr+Si ≤ 2.00%. Furthermore, to ensure high resistance to delayed fracture, P: ≤ 0.03%, S: ≤ 0.03%, Ti: 0.01–0.10%, Nb: 0.01–0.10%, N: ≤ 0.01%, and Ti+Nb ≤ 0.12%. Comparative steels 1 and 2 are the compositions of this invention. In comparative steel 1, Mn+Cr+Si = 2.38% > 2.00%, exceeding the upper limit of this invention; in comparative steel 2, Ti+Nb = 0.14% > 0.12%, also exceeding the upper limit of this invention.

[0136] This invention controls the oxidation state of the substrate surface and the thickness fluctuation of the FeAlSi inhibition layer before hot forming by controlling the chemical composition of the raw material matrix and the production process. Ultimately, it controls the size and number of Kirkendal pores after hot forming, ensuring that the hot-formed steel components have high resistance to delayed fracture, cold bending performance, and corrosion resistance in coating. For the control of the matrix chemical composition, the contents of three elements—Mn, Cr, and Si—are specifically limited: Mn: 1.00–1.50%, Cr: 0.10–0.80%, Si: 0.10–0.50%, Mn+Cr+Si≤2.00%. The production process control mainly includes hot rolling coiling temperature ≤580℃, pickling reduction rate ≤58%, annealing temperature ≤800℃, annealing dew point ≤-10℃, hot-dip galvanizing bath temperature ≤680℃, and hot forming furnace dew point ≤-5℃. Through the above controls, it is ensured that the surface of the raw material substrate is free of oxidation (the surface within 5μm of the substrate steel plate surface is free of oxidation), and the thickness of the FeAlSi inhibition layer is controlled to be 3-7μm, with the FeAlSi inhibition layer thickness fluctuation ≤40% (total pre-coating thickness 7-19μm). In the final hot-formed interdiffusion layer, the number of Kirkendal pores with a diameter greater than 1.0μm does not exceed 12 / 100μm, the hydrogen-induced delayed fracture acid fracture time is ≥120h, the cold bending angle of the hot-formed steel component after baking is ≥50°, and the maximum corrosion expansion width is ≤4mm after the hot-formed steel component is coated (phosphating, electrophoresis) and subjected to scratch corrosion test.

[0137] Specifically, when using the matrix composition of the present invention, namely Examples 1 / 2 / 3 / 4 / 5 and Comparative Examples 1 / 2 / 3 / 4, Examples 1 / 2 / 3 / 4 / 5 employ the production process of the present invention (hot rolling, pickling, annealing, hot-dip galvanizing, and hot forming heating), it is ensured that there is no obvious oxidation on the surface of the raw material matrix, and the FeAlSi inhibition layer thickness fluctuation is controlled to ≤40% (see...). Figure 1Example 1: The substrate surface layer is 5μm free of oxidation, the FeAlSi inhibition layer thickness is 4-6μm (FeAlSi inhibition layer thickness fluctuates by 20%), and after final hot forming, the number of Kirkendal pores with a diameter of 1.0μm or more in the interdiffusion layer does not exceed 12 / 100μm, the hydrogen-induced delayed fracture time is ≥120h, the cold bending angle of the hot-formed steel component after baking is ≥50°, and the maximum corrosion spread width is ≤4mm after the hot-formed steel component is coated (phosphating, electrophoresis). Comparative Examples 1 / 2 / 3 / 4 did not employ the production process of this invention (hot rolling, pickling, annealing, hot-dip galvanizing, and hot forming heating). Specifically, in Comparative Examples 1 / 2 / 3, the number of Kirkendal pores with a diameter greater than 1.0 μm in the interdiffusion layer after final hot forming exceeded 12 per 100 μm, the hydrogen-induced delayed fracture (HICF) fracturing time was <120 h, and the cold bending angle of the hot-formed steel component could not be stably controlled to be no less than 50° after baking. After coating (phosphating, electrophoresis), the maximum corrosion spread width of the hot-formed steel component was greater than 4 mm in the scratch corrosion test. In Comparative Example 4, although the number of Kirkendal pores with a diameter greater than 1.0 μm did not exceed 12 per 100 μm, the dew point of the heat treatment furnace was too high (5°C > -5°C, exceeding the upper limit of this invention), resulting in an excessively high diffusible hydrogen content in the hot-formed steel component and a HICF fracturing time of <120 h.

[0138] When the matrix composition of this invention was not used, i.e., Comparative Examples 5 and 6, specifically, in Comparative Example 5, the Mn+Cr+Si content in steel 1 was 2.38% > 2.00%, exceeding the upper limit of this invention; and in Comparative Example 6, the Ti+Nb content in steel 2 was 0.14% > 0.12%, exceeding the upper limit of this invention. Even with the production process of this invention, Comparative Example 5 still exhibited significant oxidation on the surface of the raw material matrix, resulting in more than 12 Kirkendal pores with a diameter greater than 1.0 μm / 100 μm in the interdiffusion layer after hot forming. The hydrogen-induced delayed fracture fracturing time was <120 h, and the maximum corrosion spread width was greater than 4 mm after scratch corrosion testing of the hot-formed steel component after coating (phosphating, electrophoresis). While Comparative Example 6, although the number of Kirkendal pores with a diameter greater than 1.0 μm did not exceed 12 / 100 μm, the Ti+Nb content in the steel matrix was 0.14% > 0.12%, leading to the formation of large-sized Ti / Nb carbides / nitrides in the raw material (see...). Figure 2 (See Table 5 for composition) and length > 5 μm (generally, a length ≥ 1 μm is considered a large-size precipitate). In this case, during the hydrogen-induced delayed fracture foaming acid test, the large-size Ti / Nb carbide / nitride regions act as stress sources, exacerbating hydrogen diffusion and causing failure fracture. At this time, the hydrogen-induced delayed fracture foaming acid fracture time is < 120 h, and the cold bending performance decreases significantly, with a cold bending angle < 50°.

[0139] This invention improves the material's resistance to hydrogen-induced delayed fracture and its toughness by refining the martensitic structure through the addition of microalloying elements and controlling the size and quantity of inclusions. Specifically, using the hot-formed steel with the matrix composition of this invention, by controlling the composition of Nb, Ti, etc., fine and dispersed carbides, nitrides, and carbonitrides are formed in the steel matrix, greatly refining the grains and significantly improving the material's resistance to hydrogen-induced delayed fracture and its toughness. Figure 3 The image shows the original austenite grain size after hot forming in Example 1, which is 8.68 μm, while the original austenite grain size in conventional products is above 10 μm. Furthermore, this invention strictly controls P, S, and N to avoid the formation of large inclusions or nitrides, carbonitrides, etc., further ensuring good resistance to hydrogen-induced delayed fracture and toughness of the material.

[0140] The hot-formed steel using the matrix composition of this invention has good hardenability and mechanical strength after hot forming, with a tensile strength between 1600 and 1750 MPa after hot forming.

[0141] Table 5 Comparative Example 6 Figure 2 Energy dispersive spectroscopy (EDS) results

[0142]

[0143] 2) Production Process: Controlling the hot rolling and annealing processes is crucial to preventing significant oxidation enrichment of Si, Mn, and Cr on the surface of the steel substrate. Specifically, in the hot rolling process, the coiling temperature should be ≤580℃, and in the annealing process, the annealing temperature should be ≤800℃, with an annealing dew point ≤-10℃. Controlling the thickness fluctuation of the FeAlSi suppression layer is related not only to the above processes but also to the acid rolling reduction rate and the hot-dip galvanizing bath temperature. Specifically, the acid rolling reduction rate should be ≤58%, and the galvanizing bath temperature ≤680℃. The annealing process and the hot forming heating dew point are key to controlling the diffusible hydrogen content in the hot-formed steel components, with the hot forming heating dew point ≤-5℃.

[0144] It can be seen that the matrix composition of this invention was used, but the production process of this invention was not used, namely Comparative Examples 1 / 2 / 3 / 4. Specifically, the hot rolling coiling temperature of Comparative Example 1 was too high (645℃ > 580℃), and the annealing dew point of Comparative Example 2 was too high (-5℃ > -10℃), resulting in obvious oxidation on the surface of the raw material matrix and large fluctuations in the thickness of the FeAlSi inhibition layer (45% > 40%) (see...). Figure 4 Comparative Example 1, energy dispersive spectroscopy (EDS) analysis points / regions 29, 30, 31, and 32 on the oxidized surface of the substrate, and point / region 33 on the unoxidized surface of the substrate (component analysis results are shown in Table 6). After thermoforming, the number of Kirkendal pores with a diameter greater than 1.0 μm in the interdiffusion layer exceeded 12 per 100 μm (see Table 6). Figure 5Comparative Example 1) shows that hydrogen-induced delayed fracture in acid fracturing time is <120h. Comparative Example 2, in addition to the influence of Kirkendal pores, also shows the influence of excessively high diffusible hydrogen content (0.70ppm > 0.50ppm). After coating (phosphating, electrophoresis), the scratch corrosion test of the hot-formed steel component shows that the maximum corrosion expansion width is greater than 4mm. In contrast, Comparative Example 3 shows that the hot-dip galvanizing bath temperature is too high (700℃ > 680℃), causing the FeAlSi inhibition layer thickness to fluctuate by more than 40%. After hot forming, the number of Kirkendal pores with a diameter greater than 1.0μm in the interdiffusion layer is significantly higher. The hydrogen-induced delayed fracture (HICF) fracture time was <120h for Kirkendal pores with a diameter greater than 1.0μm and a foaming acid fracture time of <120h for hot-formed steel components after coating (phosphating, electrophoresis). The maximum corrosion spread width was greater than 4mm. However, although the number of Kirkendal pores with a diameter greater than 1.0μm in Comparative Example 4 did not exceed 12 per 100μm, the high dew point of the heating furnace (5℃ > -5℃, exceeding the upper limit of this invention) resulted in an excessively high diffusible hydrogen content (0.75ppm > 0.50ppm) in the hot-formed steel components, causing the HICF fracture time to be <120h.

[0145] Table 6 Comparative Example 1 Figure 4 Energy dispersive spectroscopy (EDS) results

[0146]

[0147] The key to this invention lies in controlling the oxidation of the surface region of the substrate. Different points indicate whether oxidation is present or absent. For example, in Table 6, points / regions 29-32 indicate that the surface of the substrate is oxidized (oxygen content is greater than 0), while 33 indicates that the center of the substrate is not oxidized (oxygen content is 0). In the embodiment, there is no obvious oxidation on the surface of the substrate, and the oxygen content is 0.

[0148] In summary, this invention controls the oxidation state of the substrate surface and the thickness fluctuation of the FeAlSi inhibition layer before hot forming by controlling the chemical composition of the raw material matrix and the production process. Ultimately, it controls the size and number of Kirkendal pores after hot forming, ensuring that the hot-formed steel components possess high resistance to delayed fracture, cold bending performance, and corrosion resistance in coating. Furthermore, this invention refines the martensitic structure by adding microalloying elements, ensuring the tensile strength of the hot-formed steel components is between 1600 MPa and 1750 MPa. Simultaneously, it controls the annealing process in the annealing furnace to control the hydrogen content in the raw materials and the dew point in the hot forming furnace to control the hydrogen absorption reaction during heating. Ultimately, it controls the diffusible hydrogen content in the hot-formed steel components, ensuring that the hydrogen-induced delayed fracture test meets the requirement of 120 hours without fracture.

[0149] Specifically, within the interdiffusion layer after hot forming, the number of Kirkendal pores with a diameter greater than 1.0 μm is ≤12 / 100 μm, the hydrogen-induced delayed fracture time in acid fracturing is ≥120 h, the cold bending angle of the hot-formed steel component after baking is ≥50°, and the maximum corrosion spread width of the hot-formed steel component after coating (phosphating, electrophoresis) is ≤4 mm, and the tensile strength of the hot-formed steel component is between 1600 and 1750 MPa.

[0150] The above embodiments have described in detail the purpose and effects of the present invention. It should be understood that the above embodiments are only specific embodiments of the present invention, and the present invention is not limited to the above methods. All modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention or using the technical concept and technical solution of the present invention are within the protection scope of the present invention.

Claims

1. A hot-formed steel sheet with a high delayed fracture resistance of 1700MPa aluminum alloy coating, characterized in that, The high-delay fracture-resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet includes a base steel sheet and an aluminum alloy coating. The base steel plate comprises the following components by weight percentage: C: 0.23~0.29%, Si: 0.10~0.50%, Mn: 1.00~1.50%, Cr: 0.10~0.80%, P: ≤0.03%, S: ≤0.03%, N: ≤0.01%, Al: 0.01~0.10%, B: 0.001~0.01%, Ti: 0.01~0.10%, Nb: 0.01~0.10%, with the remainder being Fe and unavoidable impurities; The composition of the base steel plate also satisfies: Mn+Cr+Si≤2.00%, Ti+Nb≤0.12%; The high-delay fracture resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet has no surface oxidation within 5μm of the base steel sheet surface; the aluminum alloy coating thickness is 7~19μm, and the thickness fluctuation of the FeAlSi suppression layer in the aluminum alloy coating is ≤40%. The hot-formed steel components obtained by hot forming using the high-delay fracture-resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet have ≤12 Kirkendall pores with a diameter of 1.0μm or more in the interdiffusion layer after hot forming. The diffusible hydrogen content in the hot-formed steel component does not exceed 0.50 ppm; The hot-formed steel component has a hydrogen-induced delayed fracture time of ≥120h, a cold bending angle of ≥50° after baking, and a maximum corrosion spread width of ≤4mm after scratch corrosion testing after coating. The tensile strength of the hot-formed steel component is between 1600 and 1750 MPa.

2. The high-resistance delayed fracture 1700MPa grade aluminum alloy coated hot-formed steel sheet according to claim 1, characterized in that, The base steel plate comprises the following components by weight percentage: C: 0.24~0.28%, Si: 0.10~0.35%, Mn: 1.00~1.30%, Cr: 0.10~0.55%, P: ≤0.03%, S: ≤0.03%, N: ≤0.01%, Al: 0.01~0.06%, B: 0.001~0.005%, Ti: 0.01~0.06%, Nb: 0.01~0.06%, with the remainder being Fe and unavoidable impurities.

3. A method for producing a high-resistance delayed fracture 1700MPa grade aluminum alloy coated hot-formed steel sheet as described in any one of claims 1-2, characterized in that, The production process is as follows: steelmaking → continuous casting → hot rolling → pickling and cold rolling → substrate cleaning → annealing → coating → finishing → coiling.

4. The production method according to claim 3, characterized in that, The hot rolling process involves rolling followed by coiling at a temperature of 450~580℃.

5. The production method according to claim 3, characterized in that, The pickling and cold rolling process has a pickling and cold rolling reduction rate of ≤58%.

6. The production method according to claim 3, characterized in that, The annealing process involves an annealing temperature of 700~800℃ and an annealing dew point of ≤-10℃.

7. The production method according to claim 3, characterized in that, The coating process involves a plating solution temperature of 600~680℃.

8. A hot-formed steel component, characterized in that, The high-delay fracture-resistant 1700MPa grade aluminum alloy coated hot-formed steel sheet according to any one of claims 1-2 is obtained by hot forming.

9. The hot-formed steel component according to claim 8, characterized in that, The thermoforming includes heat treatment, in which the dew point inside the heating furnace is ≤-5℃.

10. The hot-formed steel component according to claim 8 or 9, characterized in that, The diffusible hydrogen content in the hot-formed steel component does not exceed 0.50 ppm.

11. The hot-formed steel component according to claim 8 or 9, characterized in that, The hot-formed steel component, after hot forming, has ≤12 Kirkendall pores with a diameter of 1.0 μm or more in the interdiffusion layer, per 100 μm.

12. The hot-formed steel component according to claim 10, characterized in that, The hot-formed steel component, after hot forming, has ≤12 Kirkendall pores with a diameter of 1.0 μm or more in the interdiffusion layer, per 100 μm.

13. The hot-formed steel component according to any one of claims 8, 9, or 12, characterized in that, The hot-formed steel component has a hydrogen-induced delayed fracture time of ≥120h, a cold bending angle of ≥50° after baking, a maximum corrosion spread width of ≤4mm, and a tensile strength between 1600 and 1750MPa.

14. The hot-formed steel component according to claim 11, characterized in that, The hot-formed steel component has a hydrogen-induced delayed fracture time of ≥120h, a cold bending angle of ≥50° after baking, a maximum corrosion spread width of ≤4mm, and a tensile strength between 1600 and 1750MPa.

15. An application of the hot-formed steel component according to any one of claims 8-14, characterized in that, The application of the hot-formed steel components in the manufacture of high-strength automotive parts.

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