High delay fracture resistant 1800 mpa grade aluminum alloy coated hot stamped steel sheet and production method, hot stamped steel component and application

Abstract

The application provides a high-delay fracture resistance 1800MPa grade hot forming steel plate and a production method, a hot forming steel component and an application, the composition comprises Si 0.10-0.35%, Mn 0.80-1.20%, Cr 0.10-0.35%, Nb 0.01-0.06%, Mo 0.05-0.25%, Mn+Cr+Si≤1.60%, Nb / N≤50; compared with the prior art, the application controls the base body chemical composition, production control, thereby controlling the base body surface layer oxidation state before hot forming, the thickness fluctuation of FeAlSi inhibition layer, finally controlling the Cottrell hole size and quantity after hot forming; adding micro-alloying elements to refine the martensite structure, controlling hydrogen content and hydrogen absorption reaction in the heating process of hot forming, finally controlling the diffusible hydrogen content in the hot forming steel component, and ensuring that the hot forming steel component has high-delay fracture resistance, cold bending performance and coating corrosion resistance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metallurgy, and relates to a 1800MPa-grade high-delay-fracture-resistant aluminum alloy coated hot-formed steel plate, a production method thereof, a hot-formed steel component and application. BACKGROUND

[0002] The main direction of the current development of the automobile industry is light weight. In order to reduce the weight of the automobile body, automobile manufacturers use advanced structural design techniques such as hydroforming and laser welding to reduce the amount of material used in the automobile body while ensuring the safety of the automobile body structure. On the other hand, high-strength steel plates are used to reduce the amount of steel used. Hot-formed steel is used after being heated at high temperature, which avoids the problem of large springback and poor dimensional accuracy in cold forming. In addition, the product has high strength after rapid cooling to form a martensitic structure. The aluminum-silicon coated hot-formed steel avoids oxidation and decarburization of the bare plate during heating and has high corrosion resistance, and is currently the mainstream hot-formed steel product on the market.

[0003] The most common strength grade of aluminum-silicon coated hot-formed steel is currently 1500MPa. However, in order to further reduce the weight, major steel companies at home and abroad have developed higher strength grade products, such as Am Usibor2000, Tanggang 1800MPa hot-formed steel, and Magang 1800MPa hot-formed steel. However, with the increase in strength, the toughness and hydrogen-induced delayed fracture performance of the material decrease sharply, which cannot meet the increasingly stringent requirements for automobile safety performance. When the strength of the aluminum-silicon coated hot-formed steel exceeds 1800MPa, the cold bending angle of the material is generally only 45°, and the hydrogen-induced delayed fracture sensitivity is very high. Currently, GMW17508 is often used to evaluate the hydrogen-induced delayed fracture sensitivity, which requires that the steel does not break after being immersed in 0.1 mol / L (pH=1) hydrochloric acid solution for 120 hours at 100% yield stress in four-point bending. However, when the tensile strength of the hot-formed steel exceeds 1800MPa, it is difficult for the fracture time to exceed 50 hours.

[0004] The patent with publication number CN106164319A disclosed on November 23, 2016, has a martensitic steel with resistance to delayed fracture and a manufacturing method. It proposes a martensitic steel plate directly obtained after cold rolling, annealing and cooling, which obtains 100% tempered martensite through composition and process control, the original austenite grain size is less than 20μm, and the steel plate has at least 24 hours of resistance to delayed fracture and at least 1700MPa of tensile strength during acid immersion U-bending test.

[0005] Patent with publication number CN111424212A published on July 17, 2020 discloses an anti-tensile strength 1800MPa grade aluminum plated steel sheet and its manufacturing method and hot forming parts. After hot forming, the material meets the performance requirement of tensile strength ≥1800MPa, and has excellent comprehensive mechanical properties. Using such a steel sheet to manufacture hot forming parts can eliminate the uneven appearance formed after electrophoresis of traditional aluminum-silicon steel sheet, and improve the adhesion and corrosion resistance after electrophoretic coating.

[0006] However, the hydrogen-induced delayed fracture sensitivity of high-strength hot forming steel, the coating corrosion resistance performance and the coating structure after hot forming are closely related. The patent with publication number CN106164319A proposes a bare plate hot forming steel, which does not consider the influence of the coating structure after hot forming on the hydrogen-induced delayed fracture sensitivity. The patent with publication number CN111424212A does not consider the influence of the coating structure after hot forming on the coating corrosion resistance performance.

[0007] How to balance the anti-delayed fracture performance, cold bending performance and coating corrosion resistance performance of high-strength hot forming steel sheet is a problem to be solved. SUMMARY

[0008] The purpose of the present application is to provide a high anti-delayed fracture 1800MPa grade aluminum alloy coated hot forming steel sheet and a production method. By controlling the chemical composition of the raw material steel sheet substrate and the production process, the oxidation state of the substrate surface layer before hot forming, the thickness fluctuation of the FeAlSi suppression layer are controlled, and finally the size and number of Cottrell holes after hot forming are controlled, to ensure that the hot forming steel component has high anti-delayed fracture performance, cold bending performance and coating corrosion resistance performance. In addition, by controlling the composition, the martensite structure is refined, and the raw material production process and the hot forming production process are controlled, so that the hydrogen-induced delayed fracture test meets 60 hours without fracture.

[0009] Another purpose of the present application is to provide a hot forming steel component obtained by hot forming the above-mentioned high anti-delayed fracture 1800MPa grade aluminum alloy coated hot forming steel sheet. The hot forming steel component, after hot forming, has a mutual diffusion layer, the number of Cottrell holes with a diameter of 1.0μm or more is ≤10 / 100μm, the hydrogen-induced delayed fracture bubble acid fracture time is ≥60h, the cold bending angle of the hot forming steel component after baking is ≥50°, the maximum corrosion propagation width of the hot forming steel component after coating (phosphating, electrophoresis) and scratch corrosion test is ≤4mm, and the tensile strength of the hot forming steel component is ≥1800MPa.

[0010] The last purpose of the present application is to provide the application of the hot forming steel component to automobile manufacturing.

[0011] The specific technical solutions of the present application are as follows:

[0012] A high-resistance delayed fracture 1800MPa grade aluminum alloy coated hot forming steel sheet, the high-resistance delayed fracture 1800MPa grade aluminum alloy coated hot forming steel sheet comprising a base steel sheet and an aluminum alloy coating;

[0013] The base steel sheet comprises the following mass percentage components:

[0014] C: 0.30-0.34%, Si: 0.10-0.35%, Mn: 0.80-1.20%, Cr: 0.10-0.35%, P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, Al: 0.01-0.06%, Nb: 0.01-0.06%, Mo: 0.05-0.25%, the rest being Fe and inevitable impurities.

[0015] Preferably, the base steel sheet comprises the following mass percentage components:

[0016] C: 0.30-0.32%, Si: 0.15-0.35%, Mn: 0.90-1.10%, Cr: 0.15-0.35%, P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, Al: 0.01-0.06%, Nb: 0.01-0.06%, Mo: 0.10-0.20%, the rest being Fe and inevitable impurities.

[0017] The components of the base steel sheet also satisfy: Mn+Cr+Si ≤1.60%;

[0018] The components of the base steel sheet also satisfy: Nb / N ≤50.

[0019] The aluminum alloy coating comprises a FeAl alloy layer, a FeAlSi barrier layer and an Al alloy layer, from the base steel sheet to the surface layer, FeAl alloy (thickness <1 μm), FeAlSi barrier layer and Al alloy layer outside.

[0020] The high-resistance delayed fracture 1800MPa grade aluminum alloy coated hot forming steel sheet has no oxidation within 5 μm from the surface of the base steel sheet, the thickness of the FeAlSi barrier layer is controlled at 3-7 μm, and the thickness fluctuation of the FeAlSi barrier layer is ≤40%.

[0021] The design principles of each alloying element and content in the steel substrate are as follows:

[0022] 0.30%≤C≤0.34%: C is the most important strength guarantee element after hot forming. When the content of C is between 0.30% and 0.34%, the steel plate has good hardenability during hot forming cooling and good mechanical strength after hot forming. When the content of C is lower than 0.30%, the hardenability during hot forming cooling is insufficient, and a large amount of ferrite is generated after hot forming, which causes the mechanical strength to be obviously reduced. In addition, the low-carbon martensite generated by low content of C also has insufficient mechanical strength, which causes the tensile strength to be less than 1800 MPa. When the content of C is higher than 0.34%, the strength of the martensite generated after hot forming is too high, and the toughness of the steel plate is sharply reduced. Therefore, the content of C is determined to be 0.30%-0.34%.

[0023] 0.10%≤Si≤0.35%: Si element has the function of deoxidization in liquid steel. When the content of Si is lower than 0.10%, the deoxidization function is not obvious. However, when the content of Si is high, there is a certain Si oxidation enrichment on the surface layer (including the surface) of the hot forming steel matrix during hot rolling and annealing, which causes plating leakage or holes after hot forming. Especially when the hot rolling heating temperature is too high, the coiling temperature is too high, or the annealing temperature is too high, or the dew point is too high, the above problems are more obvious. Generally, the content of Si is not higher than 0.50%, and the present application further controls it to be not higher than 0.35%, which further reduces the tendency of Si surface layer oxidation enrichment. Therefore, the content of Si is determined to be 0.10%-0.35%.

[0024] 0.80%≤Mn≤1.20%: Mn also has a very important role in ensuring hardenability and mechanical strength after hot forming. When the content of Mn is lower than 0.80%, the hardenability during hot forming cooling is insufficient, and the mechanical strength is obviously reduced, and the tensile strength cannot reach 1800 MPa. However, when the content of Mn is too high, there is a certain Mn oxidation enrichment on the surface layer (including the surface) of the hot forming steel matrix during hot rolling and annealing, which causes plating leakage or holes after hot forming. Especially when the hot rolling heating temperature is too high, the coiling temperature is too high, or the annealing temperature is too high, or the dew point is too high, the above problems are more obvious. Generally, the content of Mn is not higher than 1.50%, and the present application further controls it to be not higher than 1.20%, which further reduces the tendency of Mn surface layer oxidation enrichment. Therefore, the content of Mn is determined to be 0.80%-1.20%.

[0025] 0.10%≤ Cr ≤ 0.35%: In addition to the C and Mn elements having the effect of ensuring the hardenability and the mechanical strength after hot forming, Cr also has the above-mentioned effect, and the material hardenability must be ensured to ensure that the tensile strength after hot forming reaches 1800 MPa. When the Cr content is lower than 0.10%, the hardenability is insufficient during hot forming cooling, and the mechanical strength is significantly reduced, and the tensile strength cannot reach 1800 MPa. In addition, when the Cr content is higher than 0.50%, there is a certain Cr oxidation enrichment in the surface layer (including the surface) of the hot forming steel matrix during hot rolling and annealing, causing plating leakage or holes after hot forming. Especially when the hot rolling heating temperature is too high, the coiling temperature is too high, or the annealing temperature is too high, or the dew point is too high, the above problems are more obvious. The present application further controls Cr to be not higher than 0.35%, further reducing the tendency of Cr surface layer oxidation enrichment. Therefore, the Cr content is determined to be 0.10-0.35%.

[0026] Mn + Cr + Si ≤ 1.60%: Research has found that Mn, Cr, and Si are three elements that are particularly prone to form oxidation enrichment of the above-mentioned elements in the surface layer of the steel matrix during hot rolling and annealing, especially when the hot rolling heating temperature is too high, the coiling temperature is too high, or the annealing temperature is too high, or the dew point is too high, the above-mentioned phenomenon is more obvious, the surface wettability is poor during subsequent hot dipping, causing plating leakage, and causing obvious hole defects after hot forming, reducing the hydrogen embrittlement resistance, cold bending performance, and coating corrosion resistance of the hot formed part. The present application limits Si: 0.10-0.35%, limits Mn: 0.80-1.20%, limits Cr: 0.10-0.35%, thereby controlling the oxidation enrichment degree of Mn, Cr, and Si in the matrix surface layer, and limiting Mn + Cr + Si ≤ 1.60%, further reducing the tendency of Mn, Cr, and Si to oxidize and enrich in the matrix surface layer.

[0027] P ≤ 0.015%, S ≤ 0.005%: Due to the high strength of the 1800 MPa hot forming steel, the material hydrogen embrittlement resistance and cold bending performance are greatly affected by inclusions, and excessive P and S significantly worsen the hydrogen embrittlement resistance and cold bending performance, especially when P and S are segregated at the grain boundary. The present application strictly controls the P and S content, with P content of 0-0.015% and S content of 0-0.005%.

[0028] N ≤ 0.005%: In order to ensure the toughness of the 1800 MPa hot forming steel, the size and quantity of N-containing precipitates in the matrix steel must be strictly controlled. When the N content is higher than 0.005%, Ti and Nb are prone to form Ti and Nb nitrides or carbonitrides with a large number and size, which is not conducive to the hydrogen embrittlement resistance and cold bending performance of the product. Therefore, the N content is determined to be 0-0.005%.

[0029] 0.01%≤Al≤0.06%: Al has the effect of deoxidation and nitrogen precipitation, Al is a ferrite stabilizing element, when the Al element is higher than 0.06%, the steel is easy to form delta ferrite at high temperature zone in hot rolling, which deteriorates the product performance. The Al content of the application is determined to be 0.01-0.10%.

[0030] 0.01%≤Nb≤0.06%: The higher the strength of the hot forming steel, the worse the material toughness, generally the cold bending angle of 1500MPa grade aluminum silicon plated hot forming steel is only 50-55° (VDA238-100), and the cold bending angle of 1800MPa grade aluminum silicon plated hot forming steel is only 45-50° (VDA238-100), and the 1800MPa hot forming steel has higher strength and worse material toughness, how to ensure the material toughness is one of the keys. Nb is a strong carbide forming element, which generates carbide or composite carbide in the steel, and the carbide or composite carbide is fine and dispersed in the steel matrix, which has the effect of refining grains, improving strength and toughness, and improving hydrogen embrittlement resistance. When the Nb content is lower than 0.01%, the above effect is not obvious, and when the Nb content is higher than 0.06%, the effect of improving strength and toughness tends to be saturated, and when the Nb content is high, the steel is easy to form a large number of Nb carbides, nitrides or carbonitrides with large size, which strongly reduces the hydrogen embrittlement resistance and cold bending performance of the product. Therefore, the Nb content is determined to be 0.01-0.06%.

[0031] Nb / N≤50: The addition of 0.01-0.06% Nb in the steel mainly plays the role of refining grains and improving the hydrogen embrittlement resistance and cold bending performance of the product. Research shows that when Nb / N>50, Nb in the steel is easy to segregate and grow to form large-size Nb nitrides, carbonitrides and the like, which strongly reduces the hydrogen embrittlement resistance and cold bending performance of the product. The application controls Nb / N≤50.

[0032] 0.05%≤Mo≤0.25%: The addition of Mo element can form fine precipitates on one hand, which can further refine grains and improve grain boundary strength, thereby ensuring the hydrogen embrittlement resistance and cold bending performance of the material. When the Mo content is less than 0.05%, the above effect is not obvious. On the other hand, Mo is a hardenability improving element, when the Mo content is higher than 0.25%, its effect on improving the hardenability of the steel is no longer increased, and the Mo content is too high, the cost is obviously increased. In addition, the addition of Mo element can appropriately reduce the addition of Mn element, thereby reducing the banded structure and ensuring good material toughness. Therefore, the Mo content is determined to be 0.05-0.25%.

[0033] The production method of the high anti-delay fracture 1800MPa grade aluminum alloy coated hot forming steel plate provided by the application comprises the following process flow: steelmaking→continuous casting→hot rolling→pickling and cold rolling→base plate cleaning→annealing→coating→finishing→coiling.

[0034] The steelmaking is according to the component requirement of the steel plate base body of the high-delay fracture resistance 1800MPa-grade aluminum alloy coated hot-formed steel plate;

[0035] The continuous casting is to inject the refined molten steel into a tundish, and the tundish distributes the molten steel into each crystallizer, and after the casting is shaped and crystallized, the casting is pulled out and cut into a slab of a certain length.

[0036] The hot rolling is to place the slab into a heating furnace for heating, and after the slab is discharged, the slab is rolled and coiled, and the coiling temperature is between 450-570℃.

[0037] It should be noted that the hot rolling coiling temperature control is one of the key processes for preventing the formation of obvious Si, Mn and Cr oxidation enrichment on the surface layer of the steel base body. The present application defines the upper limit of the hot rolling coiling temperature on the basis of limiting the Si, Mn and Cr element content of the steel base body, significantly reduces the oxidation enrichment trend of Si, Mn and Cr on the surface layer of the steel base body, and ensures the good hydrogen-induced delayed fracture resistance, 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℃, a large amount of martensite and bainite hard phases will be generated in the hot rolling coil, which significantly increases the strength of the hot rolling coil and causes difficulty in subsequent pickling rolling.

[0038] When the base body surface layer is oxidized, the oxidation zone is generally concentrated in the surface layer within 5μm from the surface. Through energy spectrum analysis or glow spectrum analysis, there are obvious O, Si, Mn and Cr element enrichment areas or points in the oxidation zone. The O, Si, Mn and Cr element content in the above enrichment areas or points is obviously higher than that in the unoxidized central area of the base body. The Si, Mn and Cr element content in the unoxidized central area of the base body is controlled according to the present application, i.e. Si: 0.10-0.35%, Mn: 0.80-1.20%, and Cr: 0.10-0.35%.

[0039] The pickling cold rolling is to further pickling cold roll the hot rolled steel plate to obtain a pickling cold rolled steel plate. This process can remove the iron oxide scale generated on the surface of the steel plate during the hot rolling process. In order to ensure good surface quality after plating, the residual oil content on one side of the hard coil after pickling cold rolling should be ≤250mg / m 2 , and the residual iron content should be ≤100mg / m 2Due to the temperature difference of different parts of the steel plate surface in the hot rolling process, the thickness of the oxide skin formed by different parts is uneven, and the surface of the steel plate after pickling is uneven. If there is a certain degree of alloy element oxidation enrichment in the surface layer of the substrate at this time, the pickling cannot completely remove the oxide of the alloy element in the surface layer of the substrate, and after the pickling and rolling, the surface of the alloy element oxidation area of the substrate layer forms a pit pattern. When the pickling and rolling reduction is greater, the number of pits is more. The pit area and the normal area itself are uneven, and in addition, the pit area has not been pickled off the alloy element, the pit area and the normal area have different Fe-Al reaction rates during hot dipping, which causes the thickness difference of the FeAlSi inhibition layer during hot dipping to be large, and the thickness fluctuation of the FeAlSi inhibition layer is large, which causes the difference in diffusion degree of different parts during hot forming, and aggravates the formation of Cordenal holes. The pickling and rolling reduction of the present application is ≤57%, which ensures that the thickness of the FeAlSi inhibition layer after hot dipping is controlled to be 3-7μm, and the thickness fluctuation of the FeAlSi inhibition layer is ≤40%. Among them, the thickness fluctuation of the FeAlSi inhibition layer = | maximum thickness or minimum thickness - average thickness | / average thickness x 100%, and the average thickness = (maximum thickness + minimum thickness) / 2. It should be noted that the oxidation and enrichment of alloy elements in the surface layer of the substrate will also cause the thickness fluctuation of the FeAlSi inhibition layer after hot dipping to be too large. In order to ensure that the thickness fluctuation of the FeAlSi inhibition layer after hot dipping is ≤40%, the present application controls the hot rolling coiling temperature ≤570℃, the annealing temperature ≤790℃, and the annealing dew point ≤-10℃.

[0040] The substrate cleaning includes: alkali washing → alkali brushing → alkali washing → water brushing → electrolytic cleaning → rinsing → drying. In order to ensure good surface quality after plating, the residual oil amount of the steel plate after cleaning is ≤20mg / m 2 , and the residual iron of one side is ≤10mg / m 2 .

[0041] The annealing: the annealing temperature of the present application is controlled to be 700-790℃, and the annealing temperature includes the heating section temperature and the soaking section temperature, which are both controlled to meet the requirement of 700-790℃; the dew point in the annealing furnace is ≤-10℃, that is, the dew points of the heating section and the soaking section do not exceed -10℃.

[0042] The main purpose of the annealing process is to make the hard-rolled coil recover and recrystallize, eliminate residual stress, and control the organization and performance of the finished coil. The annealing temperature should not be lower than 700℃, and if the annealing temperature is too low, the hard-rolled coil will not be fully recovered and recrystallized, which is not conducive to the performance of the finished coil.

[0043] The heating temperature of the annealing section is not more than 790 DEG C, and the temperature of the soaking section is not more than 790 DEG C. In addition, the annealing furnace controls the dew point in the furnace by adjusting the amount of water vapor, and the dew point of the heating section and the soaking section is not more than -10 DEG C. The atmosphere in the annealing furnace is N2+H2, and the volume percentage of H2 is 5-10%. The 5-10% H2 in the furnace can reduce the iron oxide generated by Fe and H2O, O2, etc., so as to ensure the good quality of the pre-coating for hot forming, and the oxygen content in the heating section and the soaking section is controlled to be less than 50 ppm, which further reduces the oxidation of the steel matrix.

[0044] It should be noted that the annealing process control is one of the key processes for preventing the formation of obvious Si, Mn, Cr and other oxidized enrichment on the surface of the steel matrix. On the basis of limiting the content of Si, Mn, Cr and other elements in the steel matrix, the upper limit of the heating temperature, the soaking temperature, the dew point and the oxygen content is specified, which further reduces the oxidation enrichment trend of Si, Mn, Cr and other elements on the surface of the steel matrix, and ensures the good hydrogen-induced delayed fracture resistance, cold bending performance and coating corrosion resistance of the final product. In addition, the annealing temperature is less than or equal to 790 DEG C, and the dew point is less than or equal to -10 DEG C, which significantly reduces the hydrogen content entering the steel matrix during the annealing process, thereby reducing the diffusion hydrogen content in the hot-formed steel member, and further improving the hydrogen-induced delayed fracture resistance of the product.

[0045] The plating solution is an aluminum alloy and inevitable impurities, and a typical plating solution includes the following components by mass percentage: 5-11% Si, 2-4% Fe, and the balance of Al and inevitable impurities. The hot-dip plating solution temperature is between 600-670 DEG C, and the temperature of the substrate entering the plating solution should be consistent with the hot-dip plating solution temperature as much as possible to reduce the dissolution of the steel strip and the formation of aluminum slag. The immersion plating time is 2-10 s, and nitrogen gas or compressed air is used to control the coating thickness after hot-dip plating. The pre-coating thickness is controlled to be 7-19 mu m on a single side, the FeAlSi suppression layer thickness is 3-7 mu m, and the thickness fluctuation of the FeAlSi suppression layer is less than or equal to 40%.

[0046] The hot-dip plating solution temperature is between 600-670 DEG C. When the hot-dip plating solution temperature is higher than 670 DEG C, the thickness fluctuation of the FeAlSi suppression layer formed by the plating solution and the steel matrix is significantly increased during hot-dip plating. The large thickness fluctuation of the FeAlSi suppression layer causes differences in the diffusion degree of different parts during hot forming, which aggravates the formation of Kirkendall holes. In addition, the melting point of aluminum-silicon alloy is about 600 DEG C, and the hot-dip plating solution temperature should be not less than 600 DEG C. The thickness fluctuation of the FeAlSi suppression layer is less than or equal to 40% according to the present application. It should be noted that in order to achieve a thickness fluctuation of the FeAlSi suppression layer of less than or equal to 40%, it is necessary to control the hot-dip plating solution temperature to be less than or equal to 670 DEG C, the acid rolling reduction rate to be less than or equal to 57%, and the hot-rolled coiling temperature to be less than or equal to 570 DEG C, the annealing temperature to be less than or equal to 790 DEG C, and the annealing dew point to be less than or equal to -10 DEG C.

[0047] The thickness fluctuation of the FeAlSi inhibition layer is less than or equal to 40%.

[0048] The thickness of the pre-coating layer is not less than 7 microns. The inventor has found that the thinner the initial coating layer, the more obvious the Cottrell hole, because the thinner the initial coating layer, the shorter the diffusion distance of Fe and Al, and the faster the mutual diffusion rate, but because the coating is thinned, the relative Al content in the coating is reduced, and the Al available for supplementing Fe vacancies is reduced, which further aggravates the formation of large-size Cottrell holes. In addition, when the thickness of the coating is less than 7 microns, plating defects are prone to occur.

[0049] The thickness of the pre-coating layer is not greater than 19 microns. The coating is too thick, which reduces the cold bending performance of the final product, and the coating is too thick, which increases the production cost.

[0050] The finishing: the steel strip is finished after coating to improve the plate shape and control the surface roughness of the coating.

[0051] The coiling: the steel strip is coiled and offline.

[0052] The hot-formed steel member is prepared by hot forming a high-delay fracture-resistant 1800MPa-grade aluminum alloy coated hot-formed steel plate.

[0053] The diffusible hydrogen content in the hot-formed steel member is not more than 0.50ppm.

[0054] In the hot-formed steel member, the number of Cottrell holes with a diameter of 1.0 microns or more in the interdiffusion layer after hot forming is not more than 10 per 100 microns.

[0055] The hot-formed steel member has a hydrogen-induced delayed fracture bubble fracture time of not less than 60 hours, a cold bending angle of not less than 50 degrees after baking, a maximum corrosion propagation width of not more than 4mm after scratch corrosion testing, and a tensile strength of not less than 1800MPa.

[0056] The specific hot-formed steel member preparation includes the following process flow: blanking, heat treatment, and hot stamping.

[0057] In the blanking, the high-delay fracture-resistant 1800MPa-grade aluminum alloy coated hot-formed steel plate is punched or cut into a blank with a shape required by the hot-formed part.

[0058] The heat treatment: the blank is heated and kept in a heating furnace, the heating furnace temperature is 840-970 DEG C, the heating furnace atmosphere adopts air or nitrogen, the blank stays in the heating furnace for 2-10 min, and the dew point in the furnace is less than or equal to-5 DEG C. The dew point in the furnace is controlled below-5 DEG C, which can effectively reduce the hydrogen absorption reaction degree of the aluminum coating and water vapor, thereby reducing the hydrogen content in the steel matrix, finally reducing the diffusible hydrogen content in the hot-formed steel member, and significantly improving the hydrogen-induced delayed fracture resistance of the product. When the dew point in the furnace is greater than-5 DEG C, the hydrogen-induced delayed fracture resistance of the product is sharply reduced.

[0059] At present, the commonly used heating furnace is a box-type heating furnace and a roller bottom heating furnace. When the box-type heating furnace is used, the heating is fixed temperature, and when the roller bottom heating furnace is used, the heating is segmented. At this time, the heating furnace temperature refers to the highest heating temperature of the roller bottom heating furnace.

[0060] The hot stamping: the blank after heat treatment is quickly transferred to the die for stamping forming and cooling, wherein the transfer time is not more than 15 s, the stamping forming pressure holding time is 5-15 s, the cooling ejection temperature is not more than 250 DEG C, and the cooling speed is greater than or equal to 30 DEG C / s.

[0061] The application of the hot-formed steel member provided by the application is used for automobile manufacturing, especially for manufacturing high-strength automobile parts.

[0062] The inventors have found that after the hot-formed steel is pre-coated with an aluminum alloy coating, the hydrogen-induced delayed fracture performance, cold bending performance and coating corrosion resistance of the product are not only related to the organization state of the steel matrix, but also closely related to the coating structure after hot forming. Research has found that after the hot-formed steel is pre-coated with an aluminum alloy coating, Cottrell holes are easily formed between the coating and the matrix after hot forming, and the size and number of the Cottrell holes are related to the state of the raw material coating or the steel matrix. When the coating after hot forming has larger and more Cottrell holes, the hydrogen-induced delayed fracture sensitivity is greatly increased, and the cold bending performance and coating corrosion resistance of the product are reduced. The naked plate hot-formed steel disclosed in the prior art with publication number CN106164319A does not consider the influence of the coating structure after hot forming on the hydrogen-induced delayed fracture sensitivity, and the patent with publication number CN111424212A does not consider the influence of the coating structure after hot forming on the coating corrosion resistance. Therefore, how to consider the delayed fracture resistance, cold bending performance and coating corrosion resistance of the high-strength hot-formed steel plate, the inventors have carried out the following research:

[0063] The hydrogen-induced delayed fracture sensitivity of high-strength hot-formed steel is related to material strength, diffusible hydrogen content and stress state. The material strength is mainly related to the state of martensite structure. The higher the strength of martensite, the higher the hydrogen-induced delayed fracture sensitivity. At this time, the martensite structure can be refined by adding micro-alloying elements such as Nb and Mo. The present inventors found that when the tensile strength of the hot-formed steel plate pre-coated with an aluminum alloy coating exceeds 1800 MPa, the hydrogen embrittlement sensitivity increases sharply, and it is difficult to meet the 50-hour continuous fracture. The present application refines the martensite structure by adding micro-alloying elements, so that the original austenite grain size is not more than 10 μm, the tensile strength of the hot-formed steel component is ≥1800 MPa, and the hydrogen-induced delayed fracture test meets the 60-hour continuous fracture.

[0064] The higher the diffusible hydrogen content, the higher the hydrogen-induced delayed fracture sensitivity. It is found that when the diffusible hydrogen content in the hot-formed steel component exceeds 0.50 ppm, the hydrogen-induced delayed fracture sensitivity of the steel component increases sharply. The diffusible hydrogen content is related to the hydrogen content of the raw material and the hydrogen absorption during the hot forming heating process. During the production process of the raw material, a high content of water vapor exists in the annealing furnace, H2O reacts with C and Fe in the steel matrix to form H into the steel matrix. In addition, during the heating process of hot forming, H2O reacts with Al coating to produce elemental H, which enters the matrix and diffuses along the grain boundary, and selectively reacts with carbon to form hydrogen molecules and methane gas, which forms a huge pressure and causes cracking. The present application controls the hydrogen content in the raw material by controlling the annealing process in the annealing furnace and controls the dew point in the heating furnace during hot forming to control the hydrogen absorption reaction during heating, and finally controls the diffusible hydrogen content in the hot-formed steel component to not more than 0.50 ppm, so as to ensure that the hydrogen-induced delayed fracture test meets the 60-hour continuous fracture.

[0065] The stress state includes the stress level of the hot-formed steel member and the hydrogen-induced crack propagation rate. The stress level of the hot-formed steel member is mainly related to the service state of the hot-formed member, and when the hot-formed steel member is subjected to a certain stress level, the hydrogen-induced delayed fracture sensitivity is affected by the hydrogen-induced crack propagation rate. The inventors have found that the hydrogen-induced crack propagation rate is not only related to the material strength and the diffusion hydrogen content, but is also significantly affected by the coating structure after hot forming. The inventors have found that due to the large difference in diffusion speed of Fe and Al, Kirkendall voids are easily formed in the surface layer of the aluminum alloy coated hot-formed steel during heating, and when there is a certain oxidation in the surface layer (including the surface) of the substrate before hot forming, the Kirkendall voids (size, number) will be significantly intensified, which may be due to the fact that the mutual diffusion of Fe and Al is blocked at the oxidation site, and after the diffusion of Fe forms vacancies, it is more difficult for Al to supplement the vacancies. In addition, after the coating is thinned, the diffusion distance of Fe and Al is shortened, and the mutual diffusion rate is accelerated, but due to the reduction of the relative Al content in the coating after the coating is thinned, the Al available for supplementing the Fe vacancies is reduced, which further intensifies the formation of large-size Kirkendall voids. Furthermore, the inventors have found that during the heating process of hot forming, the mutual diffusion between the steel substrate and the FeAlSi inhibition layer close to the substrate is the most intense, and the uniformity of the thickness of the FeAlSi inhibition layer in the pre-coated Al-Si coating also has a significant influence on the formation of Kirkendall voids. When the thickness of the FeAlSi inhibition layer in the pre-coated layer fluctuates greatly, the diffusion rates at different parts of the coating are different, which also intensifies the formation of large-size Kirkendall voids. When the hot-formed steel member has obvious Kirkendall voids, hydrogen-induced cracks are easily formed in the void region when the steel member is subjected to stress, and the hydrogen-induced crack propagation rate is accelerated, causing hydrogen-induced fracture of the steel member. Compared with 1500MPa hot-formed steel members, when the tensile strength of the hot-formed steel member is ≥1800MPa, in order to ensure that the hydrogen-induced delayed fracture test meets the 60-hour non-fracture requirement, it is necessary to more strictly control the size and number of Kirkendall voids, and the number of Kirkendall voids with a diameter of 1.0μm or more is controlled to be ≤10 per 100μm.

[0066] In addition, the Kirkendall hole also affects the cold bending performance of the hot formed part. It should be noted that the hole can reduce the cold bending performance or improve the cold bending performance. When there is a hole between the substrate and the coating after hot forming, the substrate surface layer in the hole area decarburizes during the hot forming heating process. The decarburized substrate surface layer is lower in strength and better in plasticity and toughness than the non-decarburized area, but when the hole size is small and the number is small, the decarburized area is small, and the decarburization has little effect on the improvement of the cold bending performance, but at this time, due to the existence of the hole, the crack in the hole area will rapidly expand from the coating to the substrate under stress bending, significantly reducing the cold bending performance; but when the hole size is large and the number is large, especially when the holes are connected to form a large area of hole area, the substrate surface layer in the hole area decarburizes significantly during the hot forming heating process. At this time, the decarburized area is large, which can improve the cold bending performance, but the large area of holes greatly deteriorates the coating corrosion resistance of the hot formed part.

[0067] That is, when the pre-coating is thin, in order to ensure that the aluminum alloy coated hot formed part has high resistance to delayed fracture, cold bending performance and coating corrosion resistance, the size and number of Kirkendall holes need to be strictly controlled, at this time, the oxidation state of the substrate surface layer before hot forming, the thickness fluctuation of the FeAlSi inhibitor layer need to be comprehensively controlled. Among them, the oxidation state of the substrate surface layer before hot forming is mainly related to the chemical composition of the substrate and the production process, the production process mainly includes hot rolling process, annealing process, and the thickness fluctuation of the FeAlSi inhibitor layer is mainly related to the acid rolling process and hot dipping temperature. The number of Kirkendall holes with a diameter of 1.0 μm or more is controlled to be ≤10 / 100 μm, in addition, the present application refines the martensite structure by adding micro-alloying elements, ensures that the tensile strength of the hot formed steel member is ≥1800 MPa, controls the annealing process in the annealing furnace to control the hydrogen content in the raw material and the dew point in the hot forming heating furnace to control the hydrogen absorption reaction in the heating process, and finally controls the diffusible hydrogen content in the hot formed steel member, so as to ensure that the hydrogen-induced delayed fracture test meets 60 hours without breaking.

[0068] Compared with the prior art, the application controls the surface layer oxidation state of the base body before hot forming, the thickness fluctuation of the FeAlSi inhibition layer, the size and quantity of the Cottrell holes after hot forming, and guarantees that the hot-formed steel member has high resistance to delayed fracture, cold bending performance and coating corrosion resistance. In addition, the application refines the martensite structure by adding micro-alloying elements, guarantees that the tensile strength of the hot-formed steel member is greater than or equal to 1800 MPa, controls the annealing process in the annealing furnace to control the hydrogen content in the raw material and the dew point in the hot forming heating furnace to control the hydrogen absorption reaction in the heating process, and finally controls the diffusible hydrogen content in the hot-formed steel member, so as to guarantee that the hydrogen-induced delayed fracture test meets 60 hours without fracture. The hot-formed steel member made of the high delayed fracture resistance 1800 MPa grade aluminum alloy coated hot-formed steel plate after hot forming, in the interdiffusion layer after hot forming, the number of Cottrell holes with a diameter of greater than or equal to 1.0 μm is less than or equal to 10 per 100 μm, the hydrogen-induced delayed fracture bubble acid fracture time is greater than or equal to 60 h, the cold bending angle of the hot-formed steel member after baking is greater than or equal to 50°, the maximum corrosion propagation width of the hot-formed steel member after coating (phosphating, electrophoresis) and scratch corrosion test is less than or equal to 4 mm, and the tensile strength of the hot-formed steel member is greater than or equal to 1800 MPa. BRIEF DESCRIPTION OF DRAWINGS

[0069] Figure 1 The figure is the steel base body surface layer oxidation state and FeAlSi inhibition layer thickness fluctuation diagram before hot forming of Example 3.

[0070] Figure 2 The figure is the original austenite grain diagram of Example 1.

[0071] Figure 3 The figure is the precipitate of large-size Nb in the steel base body before hot forming of Comparative Example 6.

[0072] Figure 4 The figure is the steel base body surface layer oxidation state and FeAlSi inhibition layer thickness fluctuation diagram before hot forming of Comparative Example 1.

[0073] Figure 5 The figure is the Cottrell hole state diagram after hot forming of Comparative Example 1. DETAILED DESCRIPTION

[0074] The application will be further described in detail in combination with cases.

[0075] The production method of the high delayed fracture resistance 1800 MPa grade aluminum alloy coated hot-formed steel plate provided by the application comprises the following process flow: steelmaking, continuous casting, hot rolling, pickling and cold rolling, base plate cleaning, annealing, coating, finishing and coiling.

[0076] 1) Steelmaking: The steel plate base steel composition is controlled as follows in mass: C: 0.30-0.34%, Si: 0.10-0.35%, Mn: 0.80-1.20%, Cr: 0.10-0.35%, P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, Al: 0.01-0.06%, Nb: 0.01-0.06%, Mo: 0.05-0.25%, Mn+Cr+Si ≤1.60%, Nb / N ≤50, and the balance being Fe and unavoidable impurities.

[0077] As an example, the base steel composition is shown in Table 1. The present application controls the oxidation enrichment of Mn, Cr, and Si in the surface layer of the steel base during hot rolling and annealing, and the content of the three elements is specially limited as follows: Mn: 0.80-1.20%, Cr: 0.10-0.35%, Si: 0.10-0.35%, and Mn+Cr+Si ≤1.60%. In addition, to ensure high resistance to delayed fracture and cold bending performance, P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, Nb: 0.01-0.06%, Mo: 0.05-0.25%, and Nb / N ≤50. Steel 1 and Steel 2 are the compositions of the present application, and in Comparative Steel 1, Nb / N = 60 > 50, which exceeds the upper limit of the present application, and in Comparative Steel 2, Mn+Cr+Si = 1.90% > 1.60%, which exceeds the upper limit of the present application.

[0078] Table 1 Base Steel Composition (wt%)

[0079]

[0080] 2) Continuous Casting

[0081] The refined molten steel is injected into a tundish, which in turn distributes the molten steel to each crystallizer, and after the casting is formed and crystallized, the casting is drawn out and cut into a certain length of slab.

[0082] 3) Hot Rolling

[0083] The slab is heated in a heating furnace, and after being discharged, it is rolled and coiled at a temperature of 450-570°C.

[0084] It should be noted that the hot rolling coiling temperature control is one of the key processes for preventing the formation of obvious Si, Mn and Cr oxidation enrichment on the surface layer of the steel matrix. The present application defines the upper limit of the hot rolling coiling temperature on the basis of limiting the Si, Mn and Cr element content of the steel matrix, significantly reduces the oxidation enrichment trend of Si, Mn and Cr on the surface layer of the steel matrix, and ensures the good hydrogen-induced delayed fracture resistance, 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 martensite and bainite hard phases will be generated in the hot rolling coil, which significantly increases the strength of the hot rolling coil and causes the subsequent pickling and rolling difficulties.

[0085] When the substrate surface layer is oxidized, the oxidation zone is generally concentrated in the surface layer within 5μm from the surface. Through energy spectrum analysis or glow spectrum analysis, there are obvious O, Si, Mn and Cr element enrichment areas or points in the oxidation zone. The O, Si, Mn and Cr element content in the above enrichment areas or points is obviously higher than that in the unoxidized central zone of the substrate. The Si, Mn and Cr element content in the unoxidized central zone of the substrate is controlled according to the present application, i.e. Si: 0.10-0.35%, Mn: 0.80-1.20%, and Cr: 0.10-0.35%.

[0086] 4) Pickling and cold rolling

[0087] The hot rolled steel plate is further pickled and cold rolled to obtain a pickled and cold rolled steel plate. This process can remove the iron oxide scale generated on the surface of the steel plate during hot rolling. In order to ensure good surface quality after plating, the residual oil content on one side of the hard coil after pickling and cold rolling should be ≤250mg / m 2 , and the residual iron content should be ≤100mg / m 2Because of the temperature difference of different parts of the steel plate surface in the hot rolling process, the thickness of the oxide skin formed by different parts is uneven, and the surface of the steel plate after pickling is uneven. If there is a certain degree of alloy element oxidation enrichment on the surface layer of the substrate at this time, the pickling cannot completely remove the alloy element oxides on the surface layer of the substrate, and after the pickling and rolling, the surface of the alloy element oxidation area of the substrate surface layer forms a pit pattern. When the reduction rate of pickling and rolling is greater, the number of pits is more. The pit area and the normal area itself are uneven, and in addition, the pit area has not been pickled off the alloy elements, the pit area and the normal area have different Fe-Al reaction rates during hot dipping, which causes the thickness difference of the FeAlSi inhibition layer during hot dipping to be large, and the thickness fluctuation of the FeAlSi inhibition layer is large, which causes the difference in diffusion degree of different parts during hot forming, and aggravates the formation of Cordenal holes. The reduction rate of pickling and rolling is ≤57%, which ensures that the thickness fluctuation of the FeAlSi inhibition layer after hot dipping is ≤40%. Among them, the thickness fluctuation of the FeAlSi inhibition layer = | maximum thickness or minimum thickness - average thickness | / average thickness x 100%, the average thickness = (maximum thickness + minimum thickness) / 2. It should be noted that the oxidation enrichment of alloy elements on the surface layer of the substrate will also cause the thickness fluctuation of the FeAlSi inhibition layer after hot dipping to be too large, in order to ensure that the thickness fluctuation of the FeAlSi inhibition layer after hot dipping is ≤40%, the present application controls the hot rolling coiling temperature ≤570℃, the annealing temperature ≤790℃, and the annealing dew point ≤-10℃.

[0088] 5) substrate cleaning

[0089] The substrate cleaning includes: alkali washing → alkali brushing → alkali washing → water brushing → electrolytic cleaning → rinsing → drying, in order to ensure good surface quality after plating, the residual oil amount of the steel plate after cleaning is ≤20mg / m 2 , and the residual iron of one side is ≤10mg / m 2 .

[0090] 6) annealing

[0091] The main purpose of the annealing process is to make the hard-rolled coil recover and recrystallize, eliminate residual stress, and control the structure and performance of the finished coil, and the annealing temperature should not be lower than 700℃. If the annealing temperature is too low, the hard-rolled coil will not be fully recovered and recrystallized, which is not conducive to the performance of the finished coil.

[0092] The heating temperature of the annealing section of the present application is not more than 790℃, and the soaking temperature is not more than 790℃. In addition, the annealing furnace controls the dew point in the furnace by adjusting the amount of water vapor, and the dew point of the heating section and the soaking section is not more than-10℃. The atmosphere in the annealing furnace is N2+H2, and the volume percentage of H2 is 5-10%. The introduction of 5-10% H2 in the furnace can reduce the oxides of iron generated by Fe and H2O, O2, etc., so as to ensure good quality of the pre-coating before hot forming, and the oxygen content in the heating section and the soaking section is controlled below 50ppm to further reduce the oxidation of the steel substrate.

[0093] It should be noted that the annealing process control of the present application is also one of the key processes for preventing the formation of obvious Si, Mn, Cr and other oxidation enrichment on the surface layer of the steel matrix. On the basis of limiting the content of Si, Mn, Cr and other elements in the steel matrix, the present application specifies the heating temperature, the soaking temperature, the dew point, and the upper limit of oxygen content, further reduces the oxidation enrichment trend of Si, Mn, Cr and other elements on the surface layer of the steel matrix, and ensures the good hydrogen-induced delayed fracture resistance, cold bending performance and coating corrosion resistance of the final product. Moreover, the annealing temperature is ≤790℃, and the dew point is ≤-10℃, which significantly reduces the hydrogen content entering the steel matrix during the annealing process, thereby reducing the diffusible hydrogen content in the hot-formed steel member, and further improving the hydrogen-induced delayed fracture resistance of the product.

[0094] 7) Coating

[0095] The plating solution is an aluminum alloy and inevitable impurities, and the target control of the plating solution is 8-10% Si, 2-4% Fe, and the balance of Fe is Al and inevitable impurities. The hot-dip plating solution temperature is between 600-670℃, and the temperature of the substrate entering the plating solution needs to be consistent with the hot-dip plating solution temperature as much as possible to reduce the dissolution of the steel strip and the formation of aluminum slag. The immersion plating time is 2-10s, and nitrogen gas or compressed air is used to control the coating thickness after hot-dip plating. The pre-coating thickness is controlled to be 7-19μm on one side, the FeAlSi suppression layer thickness is 3-7μm, and the thickness fluctuation of the FeAlSi suppression layer is ≤40%.

[0096] The hot-dip plating solution temperature is between 600-670℃. When the hot-dip plating solution temperature is higher than 670℃, the thickness fluctuation of the FeAlSi suppression layer formed by the plating solution and the steel matrix is significantly increased during hot-dip plating. The large thickness fluctuation of the FeAlSi suppression layer causes differences in the diffusion degree of different parts during hot forming, which aggravates the formation of Kirkendall holes. In addition, the melting point of aluminum-silicon alloy is about 600℃, and the hot-dip plating solution temperature should not be lower than 600℃. The thickness fluctuation of the FeAlSi suppression layer of the present application is ≤40%. It should be noted that in order to achieve a thickness fluctuation of the FeAlSi suppression layer ≤40%, not only the hot-dip plating solution temperature ≤670℃ and the acid rolling reduction rate ≤57% need to be controlled, but also the alloy element oxidation enrichment on the surface layer of the substrate needs to be controlled. Therefore, the hot rolling coiling temperature ≤570℃, the annealing temperature ≤790℃, and the annealing dew point ≤-10℃ need to be controlled.

[0097] The thickness fluctuation of the FeAlSi suppression layer of the present application is ≤40%.

[0098] The thickness of the pre-coating layer should not be less than 7 μm. The inventors have found that the thinner the initial coating layer, the more obvious the Cottrell holes, because the thinner the initial coating layer, the shorter the diffusion distance of Fe and Al and the faster the diffusion rate, but because the coating layer is thinned, the relative Al content in the coating layer is reduced, and the Al available to supplement the Fe vacancies is reduced, which at this time further aggravates the formation of large-size Cottrell holes. In addition, when the thickness of the coating layer is less than 7 μm, plating defects are prone to occur.

[0099] The thickness of the pre-coating layer should not be greater than 19 μm. A coating layer that is too thick reduces the cold bending performance of the final product, and a coating layer that is too thick increases the production cost.

[0100] 8) Finishing

[0101] The steel strip is finished after plating to improve the plate shape and control the surface roughness of the coating layer.

[0102] 8) Coiling

[0103] The steel strip is coiled and discharged.

[0104] The main process parameters for producing the high-delay fracture resistance 1800 MPa grade aluminum alloy coated hot forming steel sheet according to the above Table 1 composition and the above method are shown in Table 2.

[0105] The high-delay fracture resistance 1800 MPa grade aluminum alloy coated hot forming steel sheet produced above is used to make a hot forming steel component after hot forming, and the specific process is: blanking → heat treatment → hot stamping.

[0106] Specifically as follows:

[0107] Blanking:

[0108] The above tensile strength 1800 MPa grade aluminum alloy coated pre-coated aluminum alloy coated hot forming steel sheet is blanked or cut into a blank of the shape required for the hot forming component. The thickness of the steel sheet is selected as a typical value of 1.4 mm, and the steel sheet is processed into a sample plate with a size of 150 x 300 mm.

[0109] Heat treatment:

[0110] The blank is heated and held in a heating furnace, the heating furnace temperature is 840-970 ℃, the heating furnace atmosphere is air or nitrogen, and the blank stays in the heating furnace for 2-10 min. The dew point in the furnace is ≤-5 ℃. Controlling the dew point in the furnace to be below -5 ℃ can effectively reduce the degree of hydrogen absorption reaction between the aluminum coating and water vapor, thereby reducing the hydrogen content entering the steel matrix, ultimately reducing the diffusible hydrogen content in the hot forming steel component, and significantly improving the hydrogen-induced delayed fracture resistance of the product. When the dew point in the furnace is greater than -5 ℃, the hydrogen-induced delayed fracture resistance of the product is dramatically reduced.

[0111] The currently used heating furnace includes a box-type heating furnace and a roller bottom heating furnace. When the box-type heating furnace is used, fixed temperature heating is adopted, and when the roller bottom heating furnace is used, segmented heating is adopted. The temperature of the above-mentioned heating furnace refers to the highest heating temperature of the roller bottom heating furnace. The present application adopts the box-type resistance heating furnace to heat the pre-coated steel plate, and adopts the typical heating process, i.e. the heating temperature is 930℃ and the heating time is 5min.

[0112] hot stamping:

[0113] The hot-treated blank is quickly transferred to the die for stamping forming and cooling, wherein the transfer time is not more than 15s, the stamping forming pressure maintaining time is 5-15s, the cooling ejection temperature is not more than 250℃, and the cooling speed is ≥30℃ / s. The present application places the hot-treated steel plate on the flat plate quenching die for pressing and pressure maintaining for a certain time, and the die is connected with cooling water to cool the steel plate.

[0114] According to the above process flow, the hot-formed steel member is prepared, and the pre-coating layer thickness and FeAlSi thickness fluctuation are shown in Table 2.

[0115] Table 2 Production process parameters, pre-coating layer thickness, FeAlSi thickness fluctuation

[0116]

[0117]

[0118] The surface layer oxidation state of the above-mentioned pre-coated aluminum alloy coating hot-formed steel plate base body, the coating thickness, the FeAlSi layer thickness are observed and analyzed, the hole condition of the hot-formed steel member is observed and analyzed, the diffusible hydrogen content of the hot-formed steel member, the hydrogen-induced delayed fracture resistance performance is tested, the cold bending performance of the hot-formed steel member is tested, the scratch corrosion test of the hot-formed steel member after coating is carried out, and the mechanical properties of the hot-formed steel member are tested.

[0119] Among them, the surface layer oxidation state of the base body before hot forming is mainly observed by scanning electron microscope, and the oxidation state within 5μm of the surface layer of the base body is mainly focused on, because this part of the area strongly affects the hole condition after hot forming. When oxidation exists, the oxidation is mainly concentrated near the grain boundary of the base body. The composition of the area is analyzed by energy spectrum analyzer. When the oxidation is serious, even a crack-like morphology is formed.

[0120] The hole condition of the hot-formed steel member is observed and analyzed by a scanning electron microscope. The holes mainly exist in the interdiffusion layer, wherein the interdiffusion layer is connected with the steel matrix and is generally composed of αFe+Fe3Al, and the content of Fe is not less than 80%. It is necessary to focus on the large-size holes, such as the holes with a diameter greater than 1.0 μm, and such holes have a greater influence on the coating corrosion resistance and welding performance. The present application counts the number of large-size holes in the interdiffusion layer, and the determination method of the diameter of the hole is as follows: in the same field of view, the longest diameter and the shortest diameter of the hole are measured, and half of the sum of the two is taken as the diameter of the hole. The determination method of the number of holes is as follows: in the field of view of the scanning electron microscope, the holes in the range of 100 μm along the surface of the base steel are counted. It should be noted that when the surface layer of the raw material base is seriously oxidized, the holes in the coating layer after hot forming may be connected to form a hole area. At this time, the determination method of the number of holes is as follows: in the field of view of the scanning electron microscope, the area of the hole region in the range of 100 μm along the surface of the base steel is counted, and the hole region area / 1 μm 2 That is, the number of holes.

[0121] The content of diffusible hydrogen is detected by TDS thermal desorption mass spectrometry, the heating rate is 100℃ / h, and the heating is to 300℃.

[0122] The hydrogen-induced delayed fracture resistance performance test of the hot-formed steel member adopts GMW17508, which requires that the sample is immersed in 0.1 mol / L (pH=1) hydrochloric acid solution under the four-point bending 100% yield stress, and whether the sample is fractured is observed, and the fracture time is recorded. The yield stress of the hot-formed steel member is referred to the test results of the mechanical properties, and the test standard adopts GB / T228.1-2010.

[0123] The hot-formed steel member simulates the baking process in the automobile paint spraying process, the baking temperature is 170℃, and the baking time is 20 min. The cold bending performance test of the hot-formed steel member after baking is carried out, and the test standard adopts VDA238-100.

[0124] The hot-formed steel member is subjected to scratch corrosion test after coating, which includes phosphating, electrophoresis, and scratch corrosion test on the coated layer to evaluate the paint adhesion and corrosion resistance (satisfactory when the maximum corrosion propagation width is not greater than 4 mm). The present application selects three hot-formed steel plates under the same conditions to perform scratch corrosion test, and takes the average value of the maximum corrosion propagation width to evaluate the paint adhesion and corrosion resistance. The hot-formed samples are subjected to phosphating treatment using the phosphating agent and test parameters in Table 3, and then the obtained phosphating plate is subjected to electrophoresis (electrophoresis paint type: Kansai HT-8000C), and the dry film thickness of electrophoresis is about 18 μm. Then, the corrosion resistance is evaluated using the cyclic corrosion method, and a single cycle includes 8 h of normal temperature maintenance (25±3℃, 4 times of spraying salt solution for 3 min each time during the period, and the salt solution composition is: 0.9wt% of NaCl, 0.1wt% of CaCl2, and 0.075wt% of NaHCO3), then 8 hours of wet heat (49±2℃, 100% RH), and finally 8 h of drying (60±2℃, <30% RH), a total of 26 cycles.

[0125] Table 3 Phosphating process parameters

[0126]

[0127]

[0128] The mechanical properties of the hot-formed steel member are tested according to GB / T228.1-2010.

[0129] The results of the oxidation state, pore, diffusible hydrogen content, hydrogen-induced delayed fracture resistance, cold bending performance, scratch corrosion test, and mechanical property test are shown in Table 4.

[0130] Table 4 Results of oxidation state, pore, diffusible hydrogen content, hydrogen-induced delayed fracture, cold bending performance, scratch corrosion test, and mechanical property test

[0131]

[0132]

[0133] The present application controls the oxidation state of the substrate surface layer before hot forming, the thickness fluctuation of the FeAlSi inhibition layer, the size and quantity of the Cottrell holes after hot forming, and guarantees that the hot-formed steel member has high resistance to delayed fracture, cold bending performance and coating corrosion resistance. In addition, the present application refines the martensite structure by adding micro-alloying elements, guarantees that the hot-formed steel member has a tensile strength of ≥1800MPa, controls the annealing process in the annealing furnace to control the hydrogen content in the raw material and the dew point in the hot forming heating furnace to control the hydrogen absorption reaction during heating, and finally controls the diffusible hydrogen content in the hot-formed steel member to guarantee that the hydrogen-induced delayed fracture test meets 60 hours of non-fracture.

[0134] Specifically, after hot forming, the number of Cottrell holes with a diameter of 1.0 μm or more in the interdiffusion layer is ≤10 / 100 μm, the hydrogen-induced delayed fracture bubble fracture time is ≥60 h, the hot-formed steel member has a cold bending angle of ≥50° after baking, the maximum corrosion propagation width is ≤4 mm after the hot-formed steel member is coated (phosphating, electrophoresis), and the tensile strength of the hot-formed steel member is ≥1800MPa.

[0135] Specifically:

[0136] 1) The chemical composition of the substrate is controlled as follows: C: 0.30-0.34%, Si: 0.10-0.35%, Mn: 0.80-1.20%, Cr: 0.10-0.35%, P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, Al: 0.01-0.06%, Nb: 0.01-0.06%, Mo: 0.05-0.25%, Mn+Cr+Si≤1.60%, Nb / N≤50, and the rest is Fe and unavoidable impurities.

[0137] The present application controls the oxidation enrichment of Mn, Cr and Si on the surface layer of the steel substrate during hot rolling and annealing, and particularly limits the content of the three elements as follows: Mn: 0.80-1.20%, Cr: 0.10-0.35%, Si: 0.10-0.35%, Mn+Cr+Si≤1.60%. In addition, to guarantee high resistance to delayed fracture and cold bending performance, P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, Nb: 0.01-0.06%, Mo: 0.05-0.25%, and Nb / N≤50. The composition of Steel 1 and Steel 2 of the present application, and the Nb / N of Comparative Steel 1 is 60>50, which exceeds the upper limit of the present application, and the Mn+Cr+Si of Comparative Steel 2 is 1.90%>1.60%, which exceeds the upper limit of the present application.

[0138] The present application improves the hydrogen-induced delayed fracture resistance and toughness of the material by adding micro-alloying elements to refine the martensite structure, and controlling the size and quantity of inclusions. Specifically, the hot-formed steel with the base composition of the present application, by controlling the Nb and Mo contents, i.e. Nb: 0.01-0.06%, Mo: 0.05-0.25%, ensures that the carbides, nitrides, carbonitrides, etc. formed are small and dispersed in the steel matrix, greatly refining the grains and significantly improving the hydrogen-induced delayed fracture resistance and toughness of the material. For example, Figure 2 The figure of the original austenite grain size after hot forming of Example 1 is 8.72 μm, while the original austenite grain size of conventional products is above 10 μm. In addition, to avoid the formation of large-size inclusions or nitrides, carbonitrides, etc., the present application strictly controls the P, S, and N contents, i.e. P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, and Nb / N ≤50, further ensuring the high hydrogen-induced delayed fracture resistance and toughness of the material.

[0139] The present application controls the base surface layer oxidation state and the thickness fluctuation of the FeAlSi inhibition layer before hot forming by controlling the base chemical composition of the raw material and the production process, and finally controls the size and quantity of the Kirkendall holes after hot forming, ensuring that the hot-formed steel member has high delayed fracture resistance, cold bending performance, and coating corrosion resistance. For the base chemical composition control, the contents of Mn, Cr, and Si are particularly limited, i.e. Mn: 0.80-1.20%, Cr: 0.10-0.35%, Si: 0.10-0.35%, and Mn+Cr+Si ≤1.60%; the production process control mainly includes hot rolling coiling temperature ≤570°C, pickling reduction ratio ≤57%, annealing temperature ≤790°C, annealing dew point ≤-10°C, hot dip plating bath temperature ≤670°C, and hot forming heating furnace dew point ≤-5°C. Through the above controls, the base surface layer is not significantly oxidized before hot forming, the thickness fluctuation of the FeAlSi inhibition layer is ≤40% (total thickness of the pre-coating layer is 7-19 μm), and finally in the inter-diffusion layer after hot forming, the number of Kirkendall holes with a diameter of above 1.0 μm is not more than 10 / 100 μm, the hydrogen-induced delayed fracture bubble fracture time is ≥60 h, the cold bending angle of the hot-formed steel member after baking is ≥50°, and the maximum corrosion propagation width is ≤4 mm after the scratch corrosion test of the hot-formed steel member after coating (phosphating, electrophoresis).

[0140] Specifically, when the base composition of the present application is used, i.e. Examples 1 / 2 / 3 / 4 / 5 / 6 and Comparative Examples 1 / 2 / 3 / 4 / 5, and the production process of the present application (hot rolling, pickling, annealing, hot dip plating, and hot forming heating) is used for Examples 1 / 2 / 3 / 4 / 5 / 6, the base surface layer of the raw material is not significantly oxidized, and the thickness fluctuation of the FeAlSi inhibition layer is controlled to be ≤40% (see Figure 1, the example 3, the substrate surface layer 5 μm no oxidation, FeAlSi inhibition layer thickness 3 ~ 6 μm, FeAlSi inhibition layer thickness fluctuation 33%, in the interdiffusion layer after final hot forming, the number of Cottrell holes with diameter above 1.0 μm is not more than 10 per 100 μm, the hydrogen-induced delayed fracture blister acid fracture time is ≥60 h, the hot formed steel member baking after cold bending angle is ≥50°, the hot formed steel member after coating (phosphating, electrophoresis) is subjected to scratch corrosion test, the maximum corrosion propagation width is ≤4 mm. While the comparative example 1 / 2 / 3 / 4 does not use the production process of the present application (hot rolling, pickling, annealing, hot dipping), in the interdiffusion layer after final hot forming, the number of Cottrell holes with diameter above 1.0 μm is more than 10 per 100 μm, the hydrogen-induced delayed fracture blister acid fracture time is <60 h, the cold bending angle of the hot formed steel member after baking cannot be stably controlled to be not less than 50°, the hot formed steel member after coating (phosphating, electrophoresis) is subjected to scratch corrosion test, the maximum corrosion propagation width is greater than 4 mm. (The cold bending angle of the comparative example 1-2 is greater than 50°, because a large number of holes are formed in the coating, causing the substrate surface layer to be significantly decarburized); the comparative example 5 has a dew point of the hot forming heating furnace that is too high, 5 ℃ > -5 ℃ (exceeding the upper limit of the present application), the diffusion hydrogen content in the hot formed steel member is too high, 0.75 ppm > 0.50 ppm, causing the hydrogen-induced delayed fracture blister acid fracture time to be <60 h.

[0141] When the substrate composition of the present application is not used, i.e. the comparative example 6, 7, specifically, the comparative example 6 (comparative steel 1) has Nb / N = 60 > 50, exceeding the upper limit of the present application, at this time although the production process of the present application is used, large-size Nb carbide / nitride is generated in the raw material (see Figure 3 , the composition is shown in Table 5), the length > 5 μm (generally the length ≥1 μm is a large-size precipitate), at this time the large-size Nb carbide / nitride region is a stress source during the hydrogen-induced delayed fracture blister acid test, aggravating hydrogen diffusion to cause failure fracture, at this time the hydrogen-induced delayed fracture blister acid fracture time is <60 h, and the cold bending performance is significantly decreased, the cold bending angle is <50°; the comparative example 7 (comparative steel 2) has Mn+Cr+Si = 1.90% > 1.60%, exceeding the upper limit of the present application, at this time although the production process of the present application is used, because the overall content of Mn+Cr+Si is high, the substrate surface layer of the raw material still has significant oxidation, and the FeAlSi inhibition layer thickness fluctuation exceeds 40%, causing the number of Cottrell holes with diameter above 1.0 μm in the interdiffusion layer after hot forming to be more than 10 per 100 μm, at this time the hydrogen-induced delayed fracture blister acid fracture time is <60 h, the cold bending angle is <50°, the hot formed steel member after coating (phosphating, electrophoresis) is subjected to scratch corrosion test, the maximum corrosion propagation width is greater than 4 mm.

[0142] In addition, the hot formed steel using the substrate composition of the present application has good hardenability and mechanical strength after hot forming, the tensile strength after hot forming is ≥1800 MPa.

[0143] Table 5 Comparative Example 6 Figure 3 Energy spectrum analysis results in Comparative Example 1

[0144]

[0145] 2) Production process: The hot rolling process and the annealing process control are the key processes for preventing the formation of obvious oxidation enrichment of Si, Mn, Cr and the like on the surface layer of the steel base. In the hot rolling process, the coiling temperature is ≤570℃, and in the annealing process, the annealing temperature is ≤790℃ and the annealing dew point is ≤-10℃. Controlling the thickness fluctuation of the FeAlSi inhibition layer is related to the above processes, the acid rolling reduction rate and the hot dip plating bath temperature, wherein the acid rolling reduction rate is ≤57% and the plating bath temperature is ≤670℃. The annealing process and the hot forming heating dew point are the key processes for controlling the content of diffused hydrogen in the steel member after hot forming, wherein the hot forming heating dew point is ≤-5℃.

[0146] It can be seen that the base component of the present application is used, but the production process of the present application is not used, i.e. Comparative Examples 1 / 2 / 3 / 4 / 5, specifically, the hot rolling coiling temperature of Comparative Example 1 is too high (625℃>570℃), the annealing heating and soaking dew point of Comparative Example 2 is too high (0℃>-10℃), which causes obvious oxidation on the surface layer of the raw material base, and the thickness fluctuation of the FeAlSi inhibition layer exceeds 40% (see Table 5). Figure 4 In Comparative Example 1, the composition analysis results of the energy spectrum analysis points / areas 1, 2 and the unoxidized points / areas 3 in the base are shown in Table 6, and the number of Cottrell holes with a diameter of more than 1.0 μm in the interdiffusion layer after hot forming is more than 10 / 100 μm (see Table 5). Figure 5 The hydrogen-induced delayed fracture blister acid fracture time of Comparative Example 1 is <60h, the hot formed steel member is coated (phosphating, electrophoresis) and then subjected to scratch corrosion test, and the maximum corrosion propagation width is greater than 4mm. It should be noted that the cold bending angle is abnormally high at this time, which is related to the decarburization of the base surface layer in the hole area due to the formation of a large number of holes in the heating process; the acid rolling reduction rate of Comparative Example 3 is too high (65%>57%), the hot dip plating bath temperature of Comparative Example 4 is too high (700℃>670℃) and the heating furnace dew point of the heat treatment is too high (5℃>-5℃), at this time the thickness fluctuation of the FeAlSi inhibition layer exceeds 40%, which causes the number of Cottrell holes with a diameter of more than 1.0 μm in the interdiffusion layer after hot forming to be more than 10 / 100 μm, at this time the hydrogen-induced delayed fracture blister acid fracture time is <60h, the cold bending angle is <50°, the hot formed steel member is coated (phosphating, electrophoresis) and then subjected to scratch corrosion test, and the maximum corrosion propagation width is greater than 4mm; the heating furnace dew point of Comparative Example 5 is too high, 5℃>-5℃ (exceeding the upper limit of the present application), the content of diffused hydrogen in the hot formed steel member is too high, 0.75ppm>0.50ppm, which causes the hydrogen-induced delayed fracture blister acid fracture time to be <60h.

[0147] Table 6 Comparative Example 1 Figure 4 ) in the energy spectrum analysis results

[0148]

[0149] In summary, the present application controls the base material matrix chemical composition and production process, thereby controlling the base surface layer oxidation state before hot forming, the FeAlSi suppression layer thickness fluctuation, and finally controlling the Cokendal hole size and number after hot forming, ensuring that the hot-formed steel member has high resistance to delayed fracture performance, cold bending performance and coating corrosion resistance. In addition, the present application refines the martensite structure by adding micro-alloying elements, ensures that the hot-formed steel member has a tensile strength ≥1800MPa, controls the annealing process in the annealing furnace to control the hydrogen content in the raw material and the dew point in the hot forming heating furnace to control the hydrogen absorption reaction during heating, and finally controls the diffusible hydrogen content in the hot-formed steel member, thereby ensuring that the hydrogen-induced delayed fracture test meets 60 hours without fracture.

[0150] Specifically, in the interdiffusion layer after hot forming, the number of Cokendal holes with a diameter of 1.0 μm or more is ≤10 / 100 μm, the hydrogen-induced delayed fracture bubble fracture time is ≥60 h, the hot-formed steel member has a cold bending angle of ≥50° after baking, the hot-formed steel member has a maximum corrosion propagation width of ≤4 mm after coating (phosphating, electrophoresis) and scratch corrosion testing, and the hot-formed steel member has a tensile strength of ≥1800MPa.

[0151] The above embodiments have described the purpose and implementation effects of the present application in detail, and it should be understood that the above embodiments are only specific embodiments of the present application, and the present application is not limited by the above methods, and any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application or using the technical concept and technical scheme of the present application are within the protection scope of the present application.

Claims

1. A high delay fracture resistance 1800 MPa grade aluminum alloy coated hot formed steel sheet characterized in that, The high anti-delayed fracture 1800MPa grade aluminum alloy coating hot forming steel plate comprises a base steel plate and an aluminum alloy coating. The base steel plate comprises the following mass percentage components: C: 0.30-0.34%, Si: 0.10-0.35%, Mn: 0.80-1.20%, Cr: 0.10-0.35%, P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, Al: 0.01-0.06%, Nb: 0.01-0.06%, Mo: 0.05-0.25%, and the rest is Fe and inevitable impurities; The components of the base steel plate satisfy the following requirements: Mn+Cr+Si≤1.60%; Nb / N≤50; The production method of the high anti-delayed fracture 1800MPa grade aluminum alloy coating hot forming steel plate comprises the following technological process: steelmaking→continuous casting→hot rolling→pickling cold rolling→base plate cleaning→annealing→coating→finishing→coiling. The hot rolling is rolled and then coiled, and the coiling temperature is 450-570°C. The pickling cold rolling has a reduction of ≤57%. The annealing temperature is 700-790°C, and the annealing dew point is ≤-10°C. The coating has a plating liquid temperature of 600-670°C. The aluminum alloy coating of the high anti-delayed fracture 1800MPa grade aluminum alloy coating hot forming steel plate has a thickness of 7-19μm, and the thickness fluctuation of the FeAlSi inhibition layer in the aluminum alloy coating is ≤40%. The hot forming steel member prepared by hot forming the high anti-delayed fracture 1800MPa grade aluminum alloy coating hot forming steel plate has ≤10 Cottrell holes with a diameter of ≥1.0μm per 100μm in the inter-diffusion layer after hot forming.

2. The high delay-fracture resistance 1800 MPa grade aluminum alloy coated hot stamped steel sheet according to claim 1 characterized in that, The base steel plate comprises the following mass percentage components: C: 0.30-0.32%, Si: 0.15-0.35%, Mn: 0.90-1.10%, Cr: 0.15-0.35%, P: ≤0.015%, S: ≤0.005%, N: ≤0.005%, Al: 0.01-0.06%, Nb: 0.01-0.06%, Mo: 0.10-0.20%, and the rest is Fe and inevitable impurities.

3. A method of producing a high delay-fracture resistance 1800 MPa grade aluminum alloy coated hot-formed steel sheet according to any one of claims 1 to 2, characterized in that, The production method comprises the following technological process: steelmaking→continuous casting→hot rolling→pickling cold rolling→base plate cleaning→annealing→coating→finishing→coiling The hot rolling is rolled and then coiled, and the coiling temperature is 450-570°C. The pickling cold rolling has a reduction of ≤57%. The annealing temperature is 700-790°C, and the annealing dew point is ≤-10°C. The coating has a plating liquid temperature of 600-670°C.

4. A hot-formed steel component, characterized in that The high anti-delayed fracture 1800MPa grade aluminum alloy coating hot forming steel plate prepared by hot forming the high anti-delayed fracture 1800MPa grade aluminum alloy coating hot forming steel plate according to any one of claims 1-2.

5. A hot-formed steel component according to claim 4, characterised in that, The hot forming comprises heat treatment, and the dew point in the heating furnace is ≤-5°C.

6. A hot-formed steel component according to claim 4 or 5, characterised in that, The diffusible hydrogen content in the hot forming steel member is not more than 0.50ppm.

7. A hot-formed steel component according to either one of claims 4 or 5, characterised in that, The hot-formed steel member has a hydrogen delayed fracture blister acid fracture time of ≥ 60 h, a hot-formed steel member cold-bent angle after baking of ≥ 50°, a maximum corrosion propagation width of ≤ 4 mm, and a hot-formed steel member tensile strength of ≥ 1800 MPa.

8. The hot-formed steel component according to claim 6, characterized in that, The hot-formed steel member has a hydrogen delayed fracture blister acid fracture time of ≥ 60 h, a hot-formed steel member cold-bent angle after baking of ≥ 50°, a maximum corrosion propagation width of ≤ 4 mm, and a hot-formed steel member tensile strength of ≥ 1800 MPa.

9. Use of a hot-formed steel part according to any one of claims 4 to 8, characterized in that, For automobile manufacturing.

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