800mpa grade hot-dip galvannealed multiphase steel sheet and method for manufacturing the same

Abstract

The application provides an 800Mpa-grade hot-based galvanizing composite steel plate and a manufacturing method thereof. The chemical composition of the composite steel plate contains, in mass fraction, C: 0.05%-0.10%, Si: ≤0.45%, Mn: 1.4-1.8%, Al: 0.02-0.08%, Cr: ≤0.5%, Ti: 0.05-0.15%, Nb: ≤0.05%, V: ≤0.2%, Fe and other inevitable impurities. The hot-based galvanizing composite steel plate has the advantages of high strength grade, good forming performance, excellent corrosion resistance and good bake hardening characteristics, can meet the technical requirements of automobile complex part flanging, stamping and automobile light weight, and is especially suitable for the manufacturing of automobile chassis parts and structural parts.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of duplex steel metallurgy, and particularly relates to an 800MPa-grade hot-rolled substrate hot-dip galvanized duplex steel and a manufacturing method thereof. BACKGROUND

[0002] With the development of automobile lightweight technology, the proportion of advanced high-strength steel in automobile structural parts is also increasing. Duplex steel has ultra-high strength and good forming performance, and has become the first choice for control arms, torsion beams and other chassis parts.

[0003] At present, patents such as CN103290320B, CN113549821A, CN113481436A, CN112575267A and CN111961960A disclose hot-rolled pickled duplex steels with different technical characteristics and properties, but these products are all without coating, have low corrosion resistance, and have short life cycle in harsh use environment, so hot-base galvanized duplex steel plate is one of the effective ways to improve the corrosion resistance of automobile chassis.

[0004] Patent CN104513930A discloses an ultra-high strength hot-rolled duplex steel plate and strip with good bending and hole expansion performance and a manufacturing method thereof, which focuses on the performance design and manufacturing method of the hot-rolled pickled plate, and does not disclose the influence of the hot-dip galvanizing process on the performance of the hot-dip galvanized steel plate.

[0005] Patent CN109055867A discloses a method for producing a high-tensile strength hole expansion hot-dip galvanized plate, but the strength grade is only 540MPa.

[0006] Patent CN113215485A discloses a 780MPa hot-base coated dual-phase steel and a manufacturing method thereof, and the chemical composition thereof is: C: 0.04%~0.08%, Si: 0.5%~0.8%, Mn: 1.4%~2.1%, Al: 0.02%~0.08%, Nb: 0.04~0.07%, Ti: 0.05~0.14%, Cr: 0~0.25%, Cu: 0~0.01%, Ni: 0~0.01%, B: 0~0.001%, P: 0~0.015%, S: 0~0.001%, and the balance is Fe and inevitable impurities. The Si content of the patent is relatively high, and red scale is easily formed on the surface during hot rolling, the galvanized surface quality is poor, and the corrosion resistance is affected. SUMMARY

[0007] In order to solve the technical problems of low strength grade and insufficient corrosion resistance of existing duplex steels, the present application provides an 800MPa-grade hot-base galvanized duplex steel, which has high strength grade, good forming performance, excellent corrosion resistance and good bake hardening characteristics, and is particularly suitable for the manufacture of automobile chassis parts.

[0008] The present application aims to provide a hot-rolled substrate hot-dip galvanized steel plate of 800MPa level and a manufacturing method thereof, the hot-dip galvanized steel plate having high strength, high formability, high corrosion resistance and good bake hardening performance, and can be used for automobile structural parts and chassis parts, and can also be used in other application fields requiring high strength, weight reduction and corrosion resistance.

[0009] In order to achieve the above-mentioned purpose, the present application provides a hot-dip galvanized steel plate of 800MPa level, the chemical composition of the hot-dip galvanized steel plate comprises, in mass fraction:

[0010] C: 0.05%-0.10%, Si: ≤0.45%, Mn: 1.4-1.8%, Al: 0.02-0.08%, Cr: ≤0.5%, Ti: 0.05-0.15%, Nb: ≤0.05%, V: ≤0.2%, Fe and other inevitable impurities.

[0011] The final microstructure of the hot-dip galvanized steel plate shows that the microstructure is composed of bainite, ferrite and martensite. In volume fraction, the ferrite is 30-45%, the martensite is 5-15%, and the bainite is 40-60%.

[0012] The longitudinal yield strength of the hot-dip galvanized steel plate is ≥680MPa, the tensile strength is ≥800MPa, and the elongation A 50 ≥20%.

[0013] The bake hardening value BH2 of the hot-dip galvanized steel plate is ≥60MPa.

[0014] Optionally, the grain size of the ferrite is not greater than 6μm, the grain size of the bainite is not greater than 2μm, and the grain size of the martensite is not greater than 1μm.

[0015] Optionally, the thickness of the hot-dip galvanized steel plate is 1.8mm-3.5mm.

[0016] Optionally, the hot-dip galvanized steel plate comprises a substrate and a zinc plating layer on at least one surface thereof, and the weight average value of the single side of the plating layer is 20-380g / m 2 .

[0017] The present application also provides a manufacturing method of a hot-dip galvanized steel plate of 800MPa level, the manufacturing method comprising:

[0018] obtaining the hot-dip galvanized steel plate of 800MPa level;

[0019] heating the slab to 1230-1280℃;

[0020] The slab after the heat preservation is hot-rolled, and then post-rolled cooling is performed to obtain a hot-rolled coil;

[0021] The hot-rolled coil is pickled to remove the surface iron oxide scale to obtain a pickled coil;

[0022] The pickled coil is annealed and hot-dip galvanized to obtain an 800MPa-level hot-based galvanized multiphase steel.

[0023] The above hot-rolling includes: controlling the hot-rolled finishing rolling outlet temperature to be 840-880℃;

[0024] The above post-rolled cooling includes: water-cooling the hot-rolled coil to 680-730℃ at a cooling rate of 80-150℃ / s, air-cooling for 2-6s, and then water-cooling to 440-520℃ at a cooling rate of ≥50℃ / s for coiling after the air-cooling is completed.

[0025] The above annealing and hot-dip galvanizing process includes: directly performing the pickled coil in a combustion non-oxidizing continuous annealing furnace, the annealing soaking temperature is 690-740℃, the annealing soaking section holding time is 30-120s, the cooling rate after the soaking is completed is ≥20℃ / s, the temperature after the hot-dip galvanizing zinc pot is 440-480℃, and the flattening rate after the galvanizing is 0.05-1.3%.

[0026] Optionally, the slab thickness is 220mm-250mm.

[0027] Optionally, the cumulative deformation of the rough rolling and the finishing rolling in the hot-rolling process is all ≥90%, wherein the rough rolling opening rolling temperature is controlled to be 1130-1190℃, and the finishing rolling opening rolling temperature is controlled to be 980-1080℃.

[0028] Optionally, after the coiling process is completed, the hot coil is placed in a holding pit for slow cooling, and the average cooling rate is ≤15℃ / h.

[0029] Optionally, the pickling stretch elongation is 0.2-2%, the pickling speed is 60-150m / min, the temperature of the last pickling acid tank in the pickling process is controlled to be 80-90℃, and the iron ion concentration is controlled to be 30-40g / L.

[0030] The beneficial effects of the present application are:

[0031] (1) The hot-based galvanized multiphase steel of the present application has better corrosion resistance compared with the conventional hot-rolled pickled plate, and is suitable for conventional hot continuous rolling production lines and hot-dip galvanizing production lines. Compared with the traditional hot-dip galvanized plate production method, the present application eliminates the cold rolling process, shortens the process flow, improves the production efficiency, and saves energy.

[0032] (2) The present application is an 800MPa grade hot-dip galvanized multiphase steel sheet. By optimizing and controlling the chemical composition and microstructure, using Nb, Ti, V micro-alloying, on the one hand, the grains are refined during hot rolling, on the other hand, the ferrite is strengthened by precipitation and separation during annealing and hot-dip galvanizing. In addition, by obtaining uniform and fine microstructure through fast cooling speed and low coiling temperature after hot rolling, controlling the proportion of ferrite and bainite during air cooling, and further obtaining a small amount of martensite to improve strength during annealing and hot-dip galvanizing, ultra-high strength and good plasticity have been obtained.

[0033] (3) The hot-dip galvanized steel sheet of the present application has the characteristics of high strength, high formability, high corrosion resistance, etc., the longitudinal yield strength is ≥680MPa, the tensile strength is ≥800MPa, the elongation A 50 ≥20%, in addition, the present application has good bake hardening performance, which can further improve the strength during the coating process after part forming, and has a positive effect on the fatigue performance and safety margin of the part. The present application can be used as automobile body structural parts and automobile chassis parts, and can also be used in other application fields requiring high strength and weight reduction. DETAILED DESCRIPTION

[0034] The present application will be specifically described below in conjunction with specific embodiments and examples, but those skilled in the art should understand that these specific embodiments and examples are used to illustrate the present application, not to limit the present application.

[0035] In the entire specification, unless otherwise specifically stated, the terms used herein are understood as having the meanings as generally used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meanings as generally understood by those skilled in the art to which the present application belongs. If there is a conflict, the present specification takes precedence. The present application provides the following technical solutions to obtain the above technical effects:

[0036] According to a typical embodiment of the present application, an 800MPa grade hot-dip galvanized multiphase steel sheet is provided, the chemical composition of the multiphase steel sheet comprises, in mass fraction:

[0037] C: 0.05%-0.10%, Si: ≤0.45%, Mn: 1.4-1.8%, Al: 0.02-0.08%, Cr: ≤0.5%, Ti: 0.05-0.15%, Nb: ≤0.05%, V: ≤0.2%, Fe and other unavoidable impurities; wherein preferably P ≤0.02%, S ≤0.005%, N ≤0.005%.

[0038] The final microstructure of the hot-dip galvanized multiphase steel sheet shows that the microstructure thereof is composed of bainite, ferrite and martensite. In volume fraction, the ferrite is 30-45%, the martensite is 5-15%, and the bainite is 40-60%.

[0039] The hot-dip galvanizing complex phase steel plate has a longitudinal yield strength of ≥680 MPa, a tensile strength of ≥800 MPa, and an elongation A50 of ≥20%.

[0040] The hot-dip galvanizing complex phase steel plate has a baking hardening value BH2 of ≥60 MPa.

[0041] The control principle of each chemical component of the present application is as follows:

[0042] C: C is the most basic element of steel and the most commonly used strengthening element. The content of carbon determines the tensile strength level of the steel plate to a large extent. Carbon can stabilize austenite and promote the formation of bainite and other structures; carbon and micro-alloying elements form sufficient precipitated strengthening phases to ensure the strength of the steel. However, the increase of the mass percentage of carbon will lead to the deterioration of the forming and welding performance. In the technical solution described in the present application, the mass percentage of C is controlled to be 0.05-0.10%.

[0043] Si: Silicon plays a role of solid solution strengthening to improve the strength of the steel plate. Silicon is an important ferrite forming element, which can promote ferrite phase transition, prevent the precipitation of carbides, and delay the formation of pearlite. During the galvanizing annealing process, silicon can inhibit the decomposition of bainite and the formation of cementite, ensuring the strength of the steel plate. However, a high silicon content in the steel is prone to form iron olivine (2FeO-SiO2) iron oxide scale on the surface during slab heating, which has an adverse effect on the surface quality of the zinc layer. Therefore, in the technical solution described in the present application, the mass percentage of silicon is controlled to be ≤0.45%.

[0044] Mn: Manganese is a solid solution strengthening element, which improves the hardenability, delays pearlite transformation and reduces bainite transformation temperature, and refines bainite structure. A low mass percentage of manganese will lead to insufficient strength, but a high mass percentage of manganese will reduce the plasticity of the steel plate and easily cause segregation. Therefore, the mass percentage of Mn in the present application is set to be 1.4-1.8%.

[0045] Al: Al is a deoxidizing element of steel, which reduces oxide inclusions in steel, purifies steel, and is beneficial to improve the forming performance of the steel plate. However, a high mass percentage of aluminum will produce oxidation, further affecting continuous casting production. Therefore, in the technical solution described in the present application, the mass percentage of Al is controlled to be 0.02-0.08%.

[0046] Cr: Chromium has the effect of improving the hardenability and solid solution strengthening, can inhibit the generation of pearlite, is beneficial to the formation of bainite organization element, in addition, chromium is not easy to segregate. When the mass percentage of Cr is less than 0.15%, the influence on phase change is not significant, but when the mass percentage of Cr is higher, it will lead to cost increase, and it is easy to produce more martensite organization. Therefore, in the technical scheme described in the application, the mass percentage of Cr is controlled to be ≤0.5%.

[0047] Ti: Titanium is one of the important precipitation strengthening and fine-grain strengthening elements, especially in the process of hot galvanizing annealing, Ti will precipitate in the form of TiC, improve the strength of ferrite matrix, and is beneficial to improve the yield ratio and the increase of elongation. Therefore, in the technical scheme described in the application, the mass percentage of Ti is controlled to be Ti: 0.05-0.15%.

[0048] Nb: Niobium is one of the important precipitation strengthening and fine-grain strengthening elements, but when the mass percentage of Nb is higher than 0.05%, the strengthening effect of Nb is close to saturation, and the cost is higher. Therefore, in the high hole expansion hot galvanizing steel plate described in the application, the mass percentage of Nb is controlled to be Nb≤0.05%.

[0049] V: Vanadium is one of the important precipitation strengthening and fine-grain strengthening elements, and the carbide of vanadium precipitates in ferrite, which is beneficial to improve the strength of ferrite. But when the mass percentage of V is higher than 0.2%, the strengthening effect is close to saturation, and the cost is higher. Therefore, in the high hole expansion hot galvanizing steel plate described in the application, the mass percentage of V is controlled to be V≤0.2%.

[0050] In the hot-based galvanizing complex phase steel plate described in the application, among other unavoidable impurities, P≤0.02%, S≤0.005%, N≤0.005%.

[0051] The final microstructure of the hot-based galvanizing complex phase steel plate is composed of bainite, ferrite and martensite. In terms of volume fraction, ferrite is 30-45%, martensite is 5-15%, and bainite is 40-60%.

[0052] The following explains the limited range of microstructure of the application:

[0053] Ferrite 30-45%

[0054] Ferrite is relatively soft, with low strength and good plasticity. In the deformation process, ferrite bears most of the strain, when the proportion of ferrite is less than 30%, the elongation of the steel plate cannot be guaranteed to be ≥20%; when the proportion of ferrite is greater than 45%, the martensite and bainite are reduced, and it is difficult to guarantee the tensile strength above 800MPa.

[0055] Martensite 5-15%

[0056] Martensite strength is high, which is generated in the process of annealing hot galvanizing cooling, and when the content is more than 5%, the tensile strength can be ensured to be more than 800MPa, but when the annealing soaking temperature is increased, the content of martensite can be more than 15%, but at the same time, coarse ferrite is formed, and the carbon content in the ferrite is reduced, which leads to the softening of the ferrite, and the yield strength of the steel plate cannot be ensured to be more than 680MPa.

[0057] The bainite is 40-60%.

[0058] The bainite is a hard phase organization in the complex phase steel, and the formation of appropriate proportion of bainite in the hot rolling process is helpful to improve the uniformity of the steel plate and balance the hardness difference between the phases. When the content of bainite is more than 60%, the elongation after fracture of the complex phase steel is difficult to reach more than 20%.

[0059] The application also provides a manufacturing method of the 800Mpa grade hot base galvanizing complex phase steel plate, and the manufacturing method comprises the following steps:

[0060] 1) obtaining the 800Mpa grade hot base galvanizing slab;

[0061] 2) heating the slab to 1230-1280℃;

[0062] 3) hot rolling the slab after the end of the holding, and then carrying out post-rolling cooling to obtain a hot-rolled coil;

[0063] 4) pickling the hot-rolled coil to remove the surface iron oxide scale to obtain a pickled coil;

[0064] 5) annealing and hot galvanizing the pickled coil to obtain the 800Mpa grade hot base galvanizing complex phase steel.

[0065] In the above step 3), the hot rolling comprises that the hot rolling finishing rolling outlet temperature is controlled to be 840-880℃.

[0066] In the above step 3), the post-rolling cooling comprises that the hot-rolled coil is water-cooled to 680-730℃ at a cooling rate of 80-150℃ / s, air-cooled for 2-6s, and then water-cooled to 440-520℃ at a cooling rate of more than 50℃ / s for coiling after the end of the air-cooling.

[0067] In the above step 4), the annealing and hot galvanizing process comprises that the steel coil is directly subjected to the process in a combustion non-oxidizing continuous annealing furnace after pickling, the annealing soaking temperature is 690-740℃, the annealing soaking section holding time is 30-120s, the cooling rate after the end of the soaking is more than 20℃ / s, the cooling is to the hot galvanizing zinc pot temperature of 440-480℃, and the flattening rate after galvanizing is 0.05-1.3%.

[0068] Optionally, the thickness of the slab is 220mm-250mm.

[0069] Optionally, the cumulative deformation of rough rolling and finish rolling in the hot rolling process is all ≥ 90%, wherein the rough rolling starting temperature is controlled to be 1130-1190℃, and the finish rolling starting temperature is controlled to be 980-1080℃.

[0070] Optionally, after the coiling process is finished, the hot coil is put into a holding pit for slow cooling, and the average cooling rate is ≤ 15℃ / h.

[0071] Optionally, the pickling straightening elongation is 0.2-2%, the pickling speed is 60-150m / min, the temperature of the last pickling acid tank in the pickling process is controlled to be 80-90℃, and the iron ion concentration is controlled to be 30-40g / L.

[0072] The manufacturing process design points and reasons of the application are as follows:

[0073] The heating temperature of the slab in the above step 2) is particularly important for performance and surface. The heating temperature is set to be ≥ 1200℃, and the purpose is to dissolve (Ti, Nb)(C, N) generated in the continuous casting process as much as possible, so that the microalloy Ti in the austenite can be precipitated in the form of nanoscale second phase particles during the hot rolling and coiling, especially during the annealing and hot galvanizing process, which can effectively improve the strength of the steel plate. However, when the temperature is higher, the austenite grains are coarsened, which is not conducive to the toughness of the steel plate; at the same time, the iron oxide scale is thicker, which is not conducive to the dephosphorization of the iron oxide scale, and finally affects the surface quality of the hot galvanizing, so the heating temperature is preferably set to be 1230-1280℃. In this stage, the slab is usually kept at the heating temperature for 1-3 hours.

[0074] In the above step 3), the rough rolling temperature control in the hot rolling process has a greater impact on Ti and other microalloys. Ti carbide and carbonitride will be precipitated at a lower rough rolling temperature and during the finish rolling process, and the size of the precipitated process is larger, which is not conducive to the improvement of the final strength. Therefore, in the present application, the rough rolling starting temperature is preferably controlled to be 1130-1190℃, and the finish rolling starting temperature is controlled to be 980-1080℃.

[0075] In the above step 3), the finish rolling exit temperature of the hot rolling process has a greater impact on the microstructure. Lower finish rolling exit temperature can obtain pie-shaped austenite grains, which accumulate a large amount of deformation energy, which is conducive to obtaining fine grain size in the subsequent cooling process. However, when the finish rolling exit temperature is too low, massive ferrite will be produced. Therefore, the finish rolling exit temperature of the hot rolling is controlled to be 840-880℃.

[0076] In the above step 3), the first-stage cooling rate after rolling is 80-150°C / s, so that the hot-rolled steel plate rapidly enters the ferrite transformation zone. When the water cooling rate is less than 80°C / s, the austenite structure has a tendency to grow, and a fast cooling rate is beneficial to improve the phase change driving force and increase the nucleation rate to refine the ferrite grains. The intermediate point temperature of the first-stage water cooling is 680-730°C, which is the strongest area of the micro-alloy element precipitation, and the precipitation of the second phase in the ferrite is beneficial to improve the strength of the ferrite structure and increase the yield strength of the steel plate. The ideal proportion of ferrite can be obtained when the air cooling time is 2-6s. When the air cooling time is less than 2s, the proportion of ferrite is insufficient, and when the air cooling time exceeds 6s, the proportion of ferrite is high and the strength of the steel plate is insufficient. The second-stage cooling rate is ≥50°C / s, so as to avoid the pearlite transformation zone, and a faster rate is beneficial to obtain smaller grain size. The coiling temperature after cooling is 440-520°C, so as to obtain suitable bainite structure. When the coiling temperature is higher, pearlite is easily formed and the grain size is larger, and when the coiling temperature is lower, martensite is formed, the precipitation is insufficient, and the strength after hot-dip galvanizing annealing is less than 800MPa.

[0077] In the above step 4), the elongation of the pickling and straightening is preferably 0.2-2%. When the elongation of the pickling and straightening is less than 0.2% or more than 2%, it may have an adverse effect on the plate shape, thereby resulting in poor surface quality after hot-dip galvanizing. The pickling speed is 60-150m / min, and the temperature of the last pickling acid tank in the pickling process is controlled to be 80-90°C. A lower pickling speed and a higher acid tank temperature will result in over-pickling, and a higher pickling speed and a lower acid tank temperature will result in under-pickling, both of which will result in surface problems of the pickled plate and affect the surface quality after hot-dip galvanizing.

[0078] In the above step 5), the annealing soaking temperature is 690-740°C. When the annealing temperature is less than 690°C, the austenite content is less during the soaking process, and the martensite content is insufficient after cooling, which also results in a lower BH value. When the soaking temperature exceeds 740°C, more ferrite is produced, resulting in a yield strength of less than 680MPa.

[0079] In the above step 5), the holding time of the annealing soaking section is 30-120s. When the holding time of the annealing soaking section is less than 30s, the internal stress of the steel plate during hot rolling cannot be completely eliminated, and the surface state of the steel plate is poor, so that a good hot-dip galvanizing surface cannot be obtained. However, when the soaking temperature exceeds 120s, a large amount of second phase particles are dissolved, resulting in a decrease in the strength of the ferrite and an inability to guarantee the yield strength of the steel plate.

[0080] In the above step 5), the cooling rate after the soaking is ≥20°C / s. When the cooling rate is less than 20°C / s, the content of the martensite structure is less, and the tensile strength of the steel plate cannot be guaranteed.

[0081] In the above step 5), it is preferable to cool to a temperature of 440-480°C of the hot-dip galvanizing pot, because the wettability of the steel sheet surface with the zinc liquid is good in this temperature range, and below or above this temperature range, the wettability of the steel sheet surface is poor, resulting in poor toxic surface after surface.

[0082] In the above step 5), the flatness rate after the hot-dip galvanizing is 0.05-1.3%. When the flatness rate is less than 0.05%, the surface quality of the zinc layer is poor, and when the flatness rate is greater than 1.3%, a large work hardening is generated, reducing the elongation of the steel sheet.

[0083] The present application is further described in detail below with reference to examples and comparative examples and experimental data.

[0084] Examples

[0085] Different component steels shown in Table 1 were smelted, heated, hot-rolled, pickled, annealed and galvanized according to Table 2 to obtain steel sheets with a thickness of ≤5mm. JIS 5# tensile samples along the longitudinal direction were taken to measure the yield and tensile strength, and the mechanical property test was performed according to GB / T 228.1-2010 standard, wherein the BH value test was performed according to GB / T 24174-2009 standard.

[0086] Examples 1-4, 7-9 and 15-18 of Table 2 adopted the design idea of the present application, i.e. Cr improved the bainite transformation, and the annealing process had the anti-tempering softening ability, Ti, Nb and V elements were added to increase the precipitation strengthening effect during the annealing process. At the same time, a two-stage cooling method and a two-phase annealing process were used to obtain an ideal organization ratio, and through the control of key parameters such as hot rolling final rolling temperature and cooling rate, the grain was refined, the yield strength of the steel sheet was improved, and finally the yield strength of the hot-dip galvanized multiphase steel sheet was ≥680MPa, the tensile strength was ≥800MPa, the elongation A50 was ≥20%, and the BH value was ≥60MPa.

[0087] Compared with Examples 1-4, Comparative Example 5 used the same hot rolling process, and the annealing soaking temperature was higher than the upper limit, resulting in a lower BH value due to less phase change during the hot-dip galvanizing annealing process. The annealing soaking temperature of Comparative Example 6 was higher than the upper limit, resulting in a higher ferrite ratio and lower yield strength of the steel sheet.

[0088] Compared with Examples 7-9, the hot-rolled stage of Comparative Example 10 is insufficient in solid solution of Nb and Ti due to low heating temperature, and a large amount of precipitation occurs after coiling due to high coiling temperature, so that the second phase particles grow rapidly during hot-dip galvanizing annealing, the strengthening effect is weakened, and the ferrite strength is reduced, resulting in a yield strength of the steel sheet less than 680 MPa. The final rolling temperature of Comparative Example 11 is lower than the lower limit, resulting in the occurrence of ferrite during hot rolling, which reduces the strength of the steel sheet. The slab temperature, final rolling temperature and intermediate point temperature of Comparative Example 12 are high, resulting in an increase in grain size, and a reduction in the yield strength of the steel sheet. The intermediate point temperature of Comparative Example 13 is low, and the coiling temperature is high, so that the ideal phase ratio cannot be obtained, resulting in a low strength of the steel sheet.

[0089] Comparative Example 18 has a low C content, resulting in insufficient strength.

[0090] Comparative Example 19 has a low Mn content, which reduces the strength of the steel sheet.

[0091] Comparative Example 20 has a high Mn content, resulting in a high martensite content and a tensile elongation less than 20%.

[0092] Comparative Example 21 has a high C content, resulting in a high martensite content and a tensile elongation less than 20%.

[0093]

[0094]

[0095] Table 3 Microstructure and mechanical properties of steel sheets

[0096]

[0097] As shown in the above table, the hot-base galvanizing multi-phase steel sheet of the present application has excellent mechanical properties, and is particularly suitable for automobile body structural parts and automobile chassis parts, and can also be used in other application fields requiring high strength and weight reduction.

[0098] In addition, the combination of the technical features in the present case is not limited to the combination manner described in the specific embodiments and examples, and all the technical features described in the present case can be freely combined or combined in any manner, unless contradictory to each other. All the modifications directly derived or thought by those skilled in the art from the disclosure of the present application shall fall within the protection scope of the present application.

Claims

1. An 800 MPa grade hot-dipped galvannealed multi-phase steel sheet, characterized by, The chemical composition of the multiphase steel plate in the hot-based galvanizing multiphase steel plate is, in mass fraction: C: 0.05%-0.10%, Si: ≤0.45%, Mn: 1.4-1.8%, Al: 0.02-0.08%, Cr: ≤0.5%, Ti: 0.05-0.15%, Nb: ≤0.05%, V: ≤0.2%, the balance being Fe and other inevitable impurities; The final microstructure of the multiphase steel plate consists of, in volume fraction, 30-42% ferrite, 5-15% martensite, and 48-60% bainite, The grain size of the ferrite is not greater than 6μm, the grain size of the bainite is not greater than 2μm, and the grain size of the martensite is not greater than 1μm; The multiphase steel plate is obtained by the following method: 1) smelting and casting the chemical composition to obtain a hot-based galvanizing slab; 2) heating the slab to 1230-1280℃; 3) hot-rolling the slab after the end of the holding, then water-cooling the hot-rolled coil to 680-730℃ at a cooling rate of 80-150℃ / s, air-cooling for 2-6s, and water-cooling to 440-520℃ at a cooling rate of ≥50℃ / s after the end of the air-cooling to coiling, to obtain a hot-rolled coil; wherein the hot-rolling finishing rolling exit temperature is controlled to be 840-880℃; 4) pickling the hot-rolled coil to obtain a pickled coil; 5) annealing and hot-dip galvanizing the pickled coil to obtain an 800MPa grade hot-based galvanizing multiphase steel; wherein the annealing soaking temperature is 690-740℃, the annealing soaking section holding time is 30-120s, and the cooling rate after the end of the soaking is ≥20℃ / s.

2. The hot-dip galvannealed multiphase steel sheet according to claim 1, characterized in that, In inevitable impurities, P≤0.02%, S≤0.005%, N≤0.005%.

3. The hot-dip galvannealed multiphase steel sheet according to claim 1, characterized in that, C：0.05%-0.072%。 4. The hot-dip galvannealed multiphase steel sheet according to claim 1, characterized in that, Si: ≤0.13%.

5. The hot-dip galvannealed multiphase steel sheet according to claim 1, characterized in that, Mn: 1.4-1.72%.

6. The hot-dip galvannealed multiphase steel sheet according to claim 1, characterized in that, Cr：≤0.38%。 7. The hot-dip galvannealed multiphase steel sheet according to any one of claims 1 to 6, characterized in that, The longitudinal yield strength of the hot-dip galvanized multiphase steel sheet is ≥680MPa, the tensile strength is ≥800MPa, and the elongation is A. 50 ≥20%.

8. The hot-dip galvannealed multiphase steel sheet according to any one of claims 1 to 6, characterized in that, The hot-based galvanizing multiphase steel plate has a bake hardening value BH2≥60MPa.

9. The hot-dip galvannealed multiphase steel sheet according to any one of claims 1 to 6, characterized in that, The hot-based galvanizing multiphase steel plate has a thickness of 1.8mm-3.5mm.

10. The hot-dip galvannealed multiphase steel sheet according to any one of claims 1 to 6, characterized by, The hot-dip galvannealed multiphase steel sheet includes a multiphase steel sheet as a base sheet and a galvannealed layer on at least one surface thereof, the weight average of one side of the galvannealed layer being 20 to 380 g / m 2 .

11. A manufacturing method of an 800Mpa grade hot-based galvanizing multiphase steel plate, comprising: 1) smelting and casting the chemical composition according to any one of claims 1-6 to obtain a hot-based galvanizing slab; 2) heating the slab to 1230-1280℃; 3) hot-rolling the slab after the end of the holding, then water-cooling the hot-rolled coil to 680-730℃ at a cooling rate of 80-150℃ / s, air-cooling for 2-6s, and water-cooling to 440-520℃ at a cooling rate of ≥50℃ / s after the end of the air-cooling to coiling, to obtain a hot-rolled coil; wherein the hot-rolling finishing rolling exit temperature is controlled to be 840-880℃; 4) pickling the hot-rolled coil to obtain a pickled coil; 5) annealing and hot-dip galvanizing the pickled coil to obtain an 800MPa grade hot-based galvanizing multiphase steel; wherein the annealing soaking temperature is 690-740℃, the annealing soaking section holding time is 30-120s, and the cooling rate after the end of the soaking is ≥20℃ / s.

12. The method of producing a hot-dip galvannealed multiphase steel sheet according to claim 11, characterized in that, The slab has a thickness of 220mm-250mm.

13. The method of producing a hot-dip galvannealed multiphase steel sheet according to claim 11 or 12, characterized in that, In step 3), the cumulative deformation of rough rolling and finish rolling in the hot rolling process is all ≥ 90%, wherein the rough rolling starting temperature is controlled to be 1130-1190 ℃, and the finish rolling starting temperature is controlled to be 980-1080 ℃.

14. The method of producing a hot-dip galvannealed multiphase steel sheet according to claim 11 or 12, characterized in that, In step 3), after the coiling process is completed, the hot coil is placed in a soaking pit for slow cooling, and the average cooling rate is ≤ 15 ℃ / h.

15. The method of producing a hot-dip galvannealed multiphase steel sheet according to claim 11 or 12, characterized in that, In step 4), the pickling straightening elongation is 0.2-2%, the pickling speed is 60-150 m / min, the temperature of the last pickling acid tank in the pickling process is controlled to be 80-90 ℃, and the iron ion concentration is controlled to be 30-40 g / L.

16. The method of producing a hot-dip galvannealed multiphase steel sheet according to claim 11 or 12, characterized in that, In step 5), the cooling is to the hot galvanizing zinc pot temperature 440-480 ℃, and the leveling rate after plating is 0.05-1.3%.

17. An automotive underbody component characterized by, The hot-dip galvanizing multiphase steel plate according to any one of claims 1-10 is used as a component.

18. An automotive structural member characterized by comprising: The hot-dip galvanizing multiphase steel plate according to any one of claims 1-10 is used as a component.

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