Cr-nb-based 590mpa-grade high-paintability steel sheet for automotive exterior parts and method for manufacturing the same

Abstract

The application provides a Cr-Nb series 590MPa grade high-coating steel plate for automobile exposed parts and a preparation method thereof, and belongs to the field of hot-rolled pickling plates.The composition of the application comprises C, Si, Mn, P, S, Als, Cr, Nb, Fe and inevitable impurity elements;1.7%<=Mn+Cr<=2.1%, 2.0<=Mn / Cr<=3.0, 19<=Cr / Nb<=30 are controlled;in production, the heating, rolling and pickling processes are optimized.Compared with the prior art, the application adopts Cr-Nb composite micro-alloying, cooperates with Mn / Cr and Cr / Nb control, improves the strength and formability;the Cr-Nb composite addition makes the iron oxide scale on the surface of the steel plate thinner and more uniform, the production process is optimized, and the high strength, high formability and good coating performance are obtained.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of hot-rolled pickling plate, and particularly relates to a Cr-Nb system 590MPa grade high paintability steel plate for automobile exposed parts and a preparation method thereof. BACKGROUND

[0002] With the continuous advancement of the process of automobile lightening, the automobile parts have put forward more stringent requirements on the strength, formability and coating performance of the materials. In addition, for automobile exposed chassis parts and other parts, not only is the material required to have good formability, but also the appearance quality after coating, the coating adhesion and the corrosion resistance are extremely high requirements. The traditional high-strength steel has a too thick surface oxide scale or a too smooth surface after pickling, which leads to insufficient coating adhesion, easy paint film peeling and corrosion spreading, and is difficult to meet the appearance quality requirements of high-end vehicles.

[0003] A patent with the publication number CN107746939A disclosed on March 2, 2018 discloses a 590MPa grade high-strength low-alloy hot-rolled pickling strip steel and a production method thereof. The weight percentage of the components is: C: 0.05-0.1%, Si≤0.1%, Mn: 0.8-1.5%, P≤0.02%, S≤0.008%, Nb: 0.01-0.05%, and the rest is Fe and inevitable impurities. By adopting appropriate C and Mn element content and hot rolling and cooling process system, a special type of ferrite-pearlite structure is formed, the volume ratio of ferrite structure is greater than 95%, the ferrite grain size is below 6μm, the strip steel tensile strength is≥590MPa, and the hole expansion rate λ is≥65%. According to the scheme example, the yield strength of the strip steel in the transverse direction is 551MPa, the tensile strength is 608MPa, and the yield strength ratio is more than 0.9. The yield strength ratio of the scheme is high, and the complex parts are easy to crack during forming. Meanwhile, the parts made of the steel plate have insufficient coating performance.

[0004] In the prior art, the hot-rolled pickling steel plate with a tensile strength of 590MPa has been widely used in automobile chassis systems, but there are still problems in the coordination of key performance such as strength, formability and coating effect. SUMMARY

[0005] The primary purpose of the present application is to overcome the shortcomings of the prior art, provide a Cr-Nb system 590MPa grade high paintability steel plate for automobile exposed parts and a preparation method thereof, and provide a Cr-Nb system hot-rolled pickling steel plate with excellent coating adhesion, high corrosion resistance, 590MPa grade high strength and high hole expansion rate. The steel plate is particularly suitable for manufacturing automobile exposed parts with strict appearance and corrosion resistance requirements.

[0006] The specific technical scheme of the present application is as follows:

[0007] The application provides a Cr-Nb system 590MPa grade high-coating steel plate for automobile exposed parts, which contains Cr-Nb, and the content of Cr is greater than or equal to 0.40%. The Cr-Nb system 590MPa grade high-coating steel plate for automobile exposed parts retains a Cr-Fe composite oxide layer with a thickness of less than or equal to 0.5 microns on the surface after pickling, the oxide layer has a porous structure, and a micron-level concave-convex morphology is formed on the surface of the steel plate.

[0008] Preferably, the steel plate retains a Cr-Fe composite oxide with a thickness of 0.05-0.5 microns on the surface after pickling, a micron-level concave-convex structure is formed on the surface of the pickled steel plate, and the Cr-Fe composite oxide has a porous structure.

[0009] In the micron-level concave-convex morphology, the height of the convex part is 0.05-0.3 microns, and the area ratio is 30-50%; the depth of the concave part is 0.05-0.5 microns, and the area ratio is 20-50%; the average pore size of the porous structure is 0.01-0.10 microns, and the porosity is 30%-80%.

[0010] The Cr-Nb system 590MPa grade high-coating steel plate for automobile exposed parts comprises the following components by mass percentage:

[0011] Cr: 0.40%-0.70%, Nb: 0.015%-0.030%, C: 0.05-0.07%, Si: 0.09%-0.17%, Mn: 1.10%-1.70%, P: less than or equal to 0.020%, S: less than or equal to 0.010%, Al: 0.03%-0.07%, and the balance is Fe and inevitable impurity elements.

[0012] The components of the Cr-Nb system 590MPa grade high-coating steel plate for automobile exposed parts satisfy the following conditions: 1.7%≤Mn+Cr≤2.1%, 2.0≤Mn / Cr≤3.0, and 19≤Cr / Nb≤30.

[0013] In the components of the Cr-Nb system 590MPa grade high-coating steel plate for automobile exposed parts, the impurity elements are controlled as follows: Ni: less than or equal to 0.02%, Cu: less than or equal to 0.03%, and Ca: less than or equal to 0.004%.

[0014] The metallographic structure of the Cr-Nb system 590MPa grade high paintability steel plate for automobile exposed parts is ferrite + granular bainite + dispersed carbide, the granular bainite and the carbide are dispersedly distributed on the ferrite matrix, wherein the volume fraction of the granular bainite is 23-27%, and the grain size of the ferrite is 12.5-13.5 grade.

[0015] The Cr-Nb system 590MPa grade high paintability steel plate for automobile exposed parts also has high corrosion resistance and high strength and high hole expansion rate, the thickness of the Cr-Nb system 590MPa grade high paintability steel plate for automobile exposed parts is 2.0-2.5mm, the yield strength is 480-520MPa, the tensile strength is 590-625MPa, the yield strength ratio is ≤0.86, the A 50 The volume fraction of the granular bainite is 23-28%, the hole expansion rate is ≥75%, and the surface quality grade is FB.

[0016] The Cr-Nb system 590MPa grade high paintability steel plate for automobile exposed parts forms a Cr-Fe composite oxide transition layer with a thickness of <1.2μm between the oxide scale and the steel plate matrix after hot rolling.

[0017] The preparation method of the above-mentioned Cr-Nb system 590MPa grade high paintability steel plate for automobile exposed parts provided by the application comprises the following process flow:

[0018] Smelting → continuous casting → hot rolling → pickling.

[0019] The smelting realizes the chemical composition control of C, Si, Mn, Cr and Nb through the processes of molten iron pretreatment, converter smelting, LF furnace refining and Ca treatment, and has high efficiency and low cost.

[0020] The continuous casting adopts continuous casting to produce the casting blank, and dynamic soft reduction is used to ensure the quality of the casting blank, and the thickness of the casting blank is 230mm.

[0021] The hot rolling is as follows: the billet tapping temperature is 1220-1310 DEG C, the furnace time is 200-230 min; the rough rolling is carried out in 3+3 or 3+5 passes, the entrance descaling water is fully opened, and the rough rolling is carried out to the 30-40 mm thickness intermediate billet; the finish rolling is carried out in 7 passes, the rolling-in temperature is 1000-1100 DEG C, the finish rolling temperature is 870-885 DEG C; the front cooling mode is adopted, the cooling speed is 90-140 DEG C / s, and the coiling temperature is 570-595 DEG C. Through the above process control of heating, rolling and cooling, the structure is ensured to be polygonal ferrite+granular bainite and dispersedly distributed nanometer carbide. The product has higher elongation and hole expansion rate, and the iron oxide scale thickness of the hot-rolled steel coil is less than 12 mu m.

[0022] The present application adopts relatively low finish rolling temperature (870-885 DEG C) and relatively high coiling temperature (570-595 DEG C) in combination with the front cooling mode (cooling speed 90-140 DEG C / s). The process combination is beneficial to obtain ideal polygonal ferrite+granular bainite structure+dispersed carbide, and avoid forming coarse carbide or full bainite structure which is not conducive to the hole expansion performance.

[0023] The coiling is carried out at the coiling temperature control CPK>7.0, and the corresponding temperature standard deviation is less than 1 DEG C.

[0024] After the hot rolling, a Cr-Fe composite oxide transition layer with a thickness less than 1.2 mu m is formed between the iron oxide scale and the steel plate matrix.

[0025] The pickling is carried out by adopting continuous pickling, the acid liquid temperature is 70-85 DEG C, the acid liquid concentration is 110-190 g / L, the pickling line speed is 120-190 m / min, and the stretch straightening elongation is 0.45-2.2%. Higher acid liquid temperature is adopted to improve the pickling effect, and the pickling speed is also increased to prevent over-pickling.

[0026] After the pickling, the steel plate surface is reserved with a Cr-Fe composite oxide with a thickness of 0.05-0.5 mu m, a micron-level concave-convex structure is formed on the pickled steel plate surface, the convex height is 0.05-0.3 mu m, the area ratio is 30-50%; the concave depth is 0.05-0.5 mu m, the area ratio is 20-50%; and the Cr-Fe composite oxide is a porous structure, the average pore size is 0.01-0.10 mu m, and the porosity is 30%-80%.

[0027] The design idea of the present application is as follows:

[0028] C: the most effective element for improving the strength of the steel, the present application adopts a low-carbon design route, when the carbon content is higher than 0.07%, the pearlite content of the steel increases, and the flanging and hole expansion performance of the steel decreases; when the C content is lower than 0.05%, the strength of the steel is difficult to guarantee. In order to guarantee the strength and flanging and hole expansion performance of the steel, the C content is controlled to be 0.05-0.07%, and preferably 0.06%.

[0029] Si: ferrite solid solution strengthening element, accelerates C segregation to austenite, and has "cleaning" and "purification" effect on solid solution carbon in ferrite. Studies have shown that in C-Mn steel, when the Si content exceeds 0.15%, the red rust on the surface of hot-rolled steel strip increases significantly. In the present application, alloy element Cr is added to inhibit the formation of red rust, therefore, the Si content of the present application is 0.09%-0.17%, preferably 0.15%.

[0030] Mn: is a solid solution strengthening element, and can also refine ferrite grains. Compared with fine-grain strengthening and precipitation strengthening, the solid solution strengthening method has the lowest rate of improving the yield ratio, so using solid solution strengthening to ensure strength is the most beneficial to obtaining a low yield ratio, but Mn is a strong center segregation element, and center segregation has a significant impact on forming cracking. The Mn content is controlled at Mn: 1.10%-1.70%, preferably 1.50%.

[0031] P: is a harmful element in steel, which can easily cause serious segregation, reduce the toughness of the steel plate, and lead to brittle fracture. In addition, excessive P content will significantly reduce the welding performance of the steel, and generally should be removed. Therefore, P is controlled to be less than or equal to 0.020%.

[0032] S: is a harmful element in steel, which can easily form sulfide inclusions with Mn and other elements in the steel, and cause organization segregation, reduce the strength and toughness of the steel, and worsen the fatigue and welding performance, so its content should be reduced as much as possible. Therefore, the S content is controlled to be less than or equal to 0.010%.

[0033] Als: has a similar effect on the austenite morphology in the critical zone heating chamber as Si, and can also form AlN precipitation to refine the grains to a certain extent. The Als content is controlled at 0.03%-0.07%, preferably 0.05%.

[0034] Cr: inhibits pearlite transformation, increases the undercooling ability of austenite, promotes the formation of bainite, and is beneficial to the improvement of tensile strength; the high diffusion rate of Cr promotes the uniform distribution of elements such as Mn and P, inhibits the formation of dendritic segregation, and reduces the tendency of band structure; in addition, Cr is a ferrite forming element that can promote the diffusion of C to austenite, purify ferrite, and improve plasticity. Cr can react with oxygen at the interface of the oxide scale and the iron matrix and aggregate to form a dense Cr-iron composite oxide, reducing the formation of oxide scale. Combined with the grain refinement effect of Nb, the oxide scale on the surface of the steel plate is thinner and more uniform, and the oxide scale is not easy to peel off during hot rolling, reducing the possibility of foreign object indentation causing pit defects. By controlling the pickling process, a uniform layer of Cr-Fe composite oxide is left on the surface of the pickled steel plate, forming small protrusions and depressions on the surface of the pickled steel plate, increasing the roughness of the steel plate surface. The microscopic rough structure provides more mechanical bite points for the coating, allowing the coating to better adhere to the steel plate surface. Cr ions in the oxide of Cr can form a passivation film at the interface between the coating and the steel plate, inhibiting the occurrence of corrosion reaction, thereby improving the corrosion resistance of the coating. If the content of Cr is too low, the Cr-Fe composite oxide on the surface of the iron matrix is not dense enough to prevent the formation of oxide scale, and the oxide scale is thick and uneven, which is easy to peel off during hot rolling; and the residual Cr-Fe composite oxide on the surface of the steel plate after pickling is not uniform enough to improve the coating performance and inhibit the corrosion reaction. If the content of Cr is too high, it will increase the brittleness of the material and reduce the elongation. The content of Cr is controlled at 0.40%-0.70%, preferably 0.60%.

[0035] Nb: Nb in solid solution significantly inhibits dynamic recrystallization and subsequent static recrystallization during thermal deformation through solute drag effect and precipitation phase synergy, increases recrystallization termination temperature, increases strain accumulation in the rear rack of hot continuous rolling, promotes the formation of high dislocation density austenite, and then through strain-induced ferrite transformation and pinning effect, the ferrite grains are refined. However, excessive Nb will lead to the precipitation of coarse NbC and phase transformation delay. Therefore, the content of Nb is controlled at Nb: 0.015%-0.030%, preferably 0.025%.

[0036] The Cr-Nb system 590 MPa grade high paintability steel plate for automobile exposed parts and the preparation method thereof of the application, by adding Cr-Nb, through the control of rolling, coiling and pickling process, the pickled steel coil with good elongation, hole expansion rate and coating performance is obtained.

[0037] The present application relates to a Cr-Nb system 590MPa grade high-coating steel plate for automobile exposed parts, and the composition of the steel plate satisfies 1.7%<=Mn+Cr<=2.1%, 2.0<=Mn / Cr<=3.0, 19<=Cr / Nb<=30, through the above control, the balance of strengthening effect and formability can be achieved, if Mn+Cr is lower than 1.7%, the total amount is too low, the solid solution strengthening is insufficient, and it is difficult to stabilize austenite to form a uniform strong and tough structure, if Mn+Cr is higher than 2.1%, the total amount is too high, on the one hand, the austenite stability is too strong, and it is easy to form hard and brittle martensite or island-shaped structure after phase transition, the brittleness is increased, and the elongation is reduced, on the other hand, too much alloying element can increase the hardenability of the steel, and the stress concentration of the structure is caused in the rolling / cooling process, and the hole expansion rate is reduced, the present application balances the strength and plasticity by coordinating the ratio of Mn and Cr, and ensures that the structure is uniform ferrite+granular bainite+dispersed carbide, the ferrite+granular bainite is mainly used, the hard and brittle phase and segregation are reduced, and a stable structure environment is created for subsequent Nb precipitation strengthening, through the ratio control of Cr and Nb, the precipitation strengthening is enhanced, the damage of plasticity caused by coarse particles is avoided, the grain is refined, and the strength and toughness are further improved, if Cr / Nb<19, the precipitation amount of Nb may exceed saturation, coarse Nb(C,N) particles are formed, the plasticity and the hole expansion rate are reduced, meanwhile, too much Nb excessively inhibits austenite recrystallization, the grain is coarse, and the structure uniformity is damaged, if Cr / Nb>30, Cr cannot effectively promote Nb precipitation, the precipitation strengthening effect is weakened, more solid solution strengthening is needed, and too much Cr easily forms Cr-rich carbide, and the proportion of brittle phase is increased, the present application balances the strength, formability and elongation by controlling the above relationship of Mn, Cr and Nb.

[0038] Compared with the prior art, the present application has the following positive effects:

[0039] One key feature of the present application is that Cr-Nb is added in combination, a Cr-Fe composite oxide transition layer with a thickness of <1.2 microns is formed between the iron oxide scale and the matrix, and through accurate control of the pickling process, an extremely thin (<=0.5 microns) and uniform Cr-Fe composite oxide layer is retained in situ on the surface of the steel plate. The oxide layer has a porous structure and forms a specific micron-level concave-convex morphology on the surface. This unique surface structure not only can store lubricant to improve the stamping performance, but more importantly, it can provide a large specific surface area and mechanical interlocking (Anchor Effect) effect for the subsequent coating, significantly improving the adhesion of the coating. At the same time, the Cr element in the oxide can form a passivation film at the interface, which fundamentally enhances the corrosion resistance. Details are as follows:

[0040] The Cr-Nb composite micro-alloying is adopted, and the Mn / Cr ratio and the Cr / Nb ratio are accurately controlled to realize the synergistic effect of fine-grain strengthening and second-phase strengthening. The yield strength is 480-520 MPa, the tensile strength is 590-625 MPa, the A 50 The yield strength is 480-520 MPa, the tensile strength is 590-625 MPa, the A

[0041] The Cr-Nb composite addition forms a Cr-Fe composite oxide transition layer with a thickness of <1.2 μm between the oxide scale and the matrix, which is combined with the grain refinement effect of Nb, and the oxide scale on the surface of the steel sheet is thinner and more uniform.

[0042] By adjusting the tapping temperature, in-furnace time, and descaling process, the thickness of the oxide scale is controlled, so that the oxide scale is not easy to peel off during hot rolling, and the possibility of pit defects caused by the pressing of the oxide scale is reduced, and the surface of the steel sheet after pickling is smoother and more uniform.

[0043] The acid pickling efficiency is higher by controlling the acid temperature and pickling speed, and the surface of the steel sheet after pickling retains a Cr-Fe composite oxide with a thickness of ≤0.5 μm, which forms a micron-level concave-convex structure on the surface of the pickled steel sheet, and the Cr-Fe composite oxide has a porous structure, so the steel sheet surface can store more lubricating oil, and in the stamping process, the lubricating oil is continuously released from the micro-depressions to form a dynamic oil film, reducing the friction coefficient.

[0044] The micro-convex and concave structures formed by the Cr-Fe composite oxide on the surface of the steel base and its own porous structure provide more mechanical bite points for the coating, allowing the coating to better adhere to the surface of the steel sheet. The ISO 2409 grid test verifies that the anti-peeling performance of the coating reaches the highest level (0 level): there is no coating peeling or delamination in the cutting grid intersection area, the edge of the scratch is complete and smooth, and after the adhesive tape is peeled off, the coating and the substrate interface have no visible defects.

[0045] The Cr ions in the oxide of Cr form a passivation film at the interface between the coating and the steel sheet, which inhibits the occurrence of corrosion reaction, thereby improving the corrosion resistance of the coating.

[0046] The post-rolling process adopts the front-stage cooling and medium-temperature coiling process, the coiling temperature is controlled to CPK>7.0, and the corresponding temperature standard deviation is <1℃, and the microstructure and performance stability in the length direction of the steel coil is good. CPK (Process Capability Index) is a statistical index for measuring the ability of a production process to stably produce products meeting the specification requirements (i.e. tolerance range). The larger the value is, the better it is. Statistically, CPK=7.0 means that the coiling temperature of the hot-rolled plate is infinitely accurately concentrated around the target value, with almost no fluctuation, thereby ensuring the high uniformity and absolute reliability of the mechanical properties of the final steel plate. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The mill scale thickness of the hot-rolled steel coil of Example 1 is <10 μm;

[0048] Figure 2 The main chemical elements and contents of the base body and the mill scale at different positions of the hot-rolled steel coil of Example 1, and the Cr element content of the transition layer between the mill scale and the base body are increased;

[0049] Figure 3 The microstructure photograph of the hot-rolled pickled steel plate of Example 1 under a metallographic microscope shows that the ferrite microstructure is relatively uniform, and the granular bainite and carbides are dispersedly distributed on the ferrite base body;

[0050] Figure 4 The microstructure photograph of the hot-rolled pickled steel plate of Example 1 under a scanning electron microscope clearly shows the ferrite and the dispersedly distributed granular bainite and carbides;

[0051] Figure 5 The surface micro-morphology of the finished product of the hot-rolled pickled steel plate of Example 1 is a fine and dense concave-convex structure on the surface of the steel plate after pickling;

[0052] Figure 6 The surface photograph of the finished product of the hot-rolled pickled steel plate of Example 1 shows that the surface quality is good, which is FB level;

[0053] Figure 7 The photograph of the hot-rolled pickled steel plate of Example 1 after electrophoretic coating shows good coating effect;

[0054] Figure 8 The microstructure photograph of the hot-rolled pickled steel plate of Comparative Example 1 under a metallographic microscope shows that the microstructure is ferrite + pearlite;

[0055] Figure 9 The microstructure photograph of the hot-rolled pickled steel plate of Comparative Example 1 under a scanning electron microscope clearly shows the morphologies of the ferrite and pearlite microstructures. DETAILED DESCRIPTION

[0056] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0057] Embodiments 1-5

[0058] A Cr-Nb system 590 MPa grade high-paintability steel plate for an exposed part of an automobile comprises the following mass percentage components: as shown in Table 1, the balance not shown in Table 1 is Fe and inevitable impurities.

[0059] Comparative Examples 1-4

[0060] A steel plate comprises the following mass percentage components: as shown in Table 1, the balance not shown in Table 1 is Fe and inevitable impurities.

[0061] Table 1: Components and mass percentages of the steel plates in the embodiments and comparative examples

[0062]

[0063] Comparative Example 1 is changed to Embodiment 1 of CN 109750219A;

[0064] Comparative Example 2 is Comparative Example 4 of CN112195398A;

[0065] Comparative Example 3 is Embodiment 2 of CN112195398A;

[0066] Comparative Example 4 is Comparative Example 2 of CN112195398A.

[0067] Each of the embodiments and comparative examples controls the residual impurities Ni≤0.02%, Cu≤0.03%, and Ca≤0.004%.

[0068] The preparation method of the steel plate in each of the above embodiments and comparative examples comprises the following process flow: smelting→continuous casting→hot rolling→pickling.

[0069] The smelting: through the processes of hot metal pretreatment, converter smelting, LF furnace refining and Ca treatment, the chemical composition control of C, Si, Mn, Cr and Nb is realized on the basis of high efficiency and low cost.

[0070] The continuous casting: the continuous casting is used to produce the casting blank, and the dynamic soft reduction is used to ensure the quality of the casting blank, and the thickness of the casting blank is 230 mm.

[0071] The hot rolling: billet heating temperature 1220-1310℃, holding for 200-230min; rough rolling is carried out in 3+3 or 3+5 passes, the entrance descaling water is fully opened, and the rough rolling is carried out to 30-40mm thickness intermediate billet; the finish rolling is 7 passes continuous rolling, the opening rolling temperature is 1000-1100℃, the final rolling temperature is 870-885℃; the front cooling mode is adopted, the cooling speed is 90-140℃ / s, and the coiling temperature is 570-595℃. Through the above process control of heating, descaling and cooling, the thickness of the iron oxide scale of the hot-rolled steel coil is ensured to be less than 12μm. The coiling temperature control CPK is greater than 7.0, the corresponding temperature standard deviation is less than 1℃, and the microstructure and performance stability in the length direction of the steel coil is good.

[0072] The pickling: continuous pickling is adopted, the acid liquid temperature is 70-85℃, the acid liquid concentration is 110-190g / L, the pickling line speed is 120-190m / min, and the stretch-reducing elongation is 0.45-2.2%; higher acid liquid temperature is adopted to improve the pickling effect, and the pickling speed is increased to prevent over-pickling.

[0073] The main production parameters of the billet heating, hot rolling and laminar cooling of each example and the comparative example are shown in Table 2. The main parameters of pickling are shown in Table 3.

[0074] Table 2 Main parameters of billet heating, hot rolling and laminar cooling of each example and the comparative example

[0075]

[0076] Table 3 Main parameters of pickling of each example and the comparative example

[0077]

[0078] The micron-level concave-convex structure formed on the surface of the pickled steel plate of each example meets: the convex height is 0.05-0.3μm, the proportion is 30-50%; the concave depth is 0.05-0.5μm, the proportion is 20-50%; and the Cr-Fe composite oxide is a porous structure, the average pore size is 0.01-0.10μm, and the porosity is 30%-80%.

[0079] Figure 1 It can be seen that a Cr-Fe composite oxide transition layer with a thickness of less than 1.2μm is covered between the iron oxide scale and the substrate after the steel plate is hot-rolled.

[0080] The steel plates produced in each example and the comparative example are detected, the standard for the mechanical property detection is GB / T228.1, the standard for the hole expansion rate test is GB / T 15825.4, and the results are shown in Table 4.

[0081] Table 4 Performance test results of each example and the comparative example

[0082]

[0083] In Table 4, F is ferrite, B is bainite, P is pearlite, and M is martensite.

[0084] Each of the embodiments and the comparative examples is verified by ISO 2409 grid test, and the anti-peeling performance of the coating reaches the highest grade (0 grade).

[0085] As can be seen from the data in Table 4, the hole expansion ratio of Comparative Example 1 is not given, and according to the analysis of the microstructure characteristics, the hole expansion ratio of the product of the scheme is ≤60%, which cannot meet the requirement of the automobile chassis system on the high hole expansion and flanging performance, and the coiling temperature is relatively low, the surface cooling water of the strip steel is difficult to evaporate dry, and the steel coil is easy to rust and form iron oxide scale under the combined action of temperature, air and accumulated water, the oxide film in the subsequent pickling process may be pressed into the surface of the steel plate to form a pitting defect, thereby affecting the surface quality of the steel plate.

[0086] Comparative Examples 2-3 adopt a low finishing mill outlet temperature, constant speed rolling, segmented cooling and low temperature coiling process, and since the constant speed rolling speed is slow and the production efficiency is low, and the finishing mill outlet temperature is low, the rolling force fluctuation is large when rolling the thin gauge steel plate containing Nb, and the steel is easy to be stacked. In addition, the Mn content in Comparative Example 3 is low, Mn+Cr<1.7% and Mn / Cr<2.0, and the solid solution strengthening is insufficient, and the strength of the product will be insufficient by using the controlled rolling and controlled cooling process of the present application.

[0087] Comparative Example 4 is a non-Nb-containing steel, and adopts a low finishing mill outlet temperature, constant speed rolling, segmented cooling and low temperature coiling process, but the air cooling temperature is low (640℃), and a pearlite structure is formed, the tensile strength is low, and the product does not contain Nb, and the hole expansion and forming performance of the product is poor.

[0088] The present application adopts a ferrite + granular bainite + dispersed carbide organization design, the granular bainite and the carbide are dispersedly distributed on the ferrite matrix, the material obtains higher strength and elongation, and the flanging and hole expanding performance is good.The present application adopts composite addition of Cr-Nb, the oxidation tendency of Cr is higher, a transition layer containing Cr-Fe composite oxide is formed between the FeO layer and the matrix, and the oxidation scale on the surface of the steel plate is thinner and more uniform in combination with the grain refining effect of Nb.In production, the thickness of the oxidation scale is controlled by adjusting the tapping temperature, in-furnace time and descaling process, so that the oxidation scale is not easy to peel off during hot rolling, the possibility of pit defects caused by the pressing of the oxidation scale is reduced, and the surface of the steel plate is smoother and more uniform after pickling.By adjusting the acid liquid temperature, acid liquid concentration and pickling speed, a uniform layer of Cr-Fe composite oxide is reserved on the surface of the pickled steel plate, a uniform concave-convex surface is formed, and the Cr-Fe composite oxide has a porous structure, therefore, the steel plate surface can store more lubricating oil, and in the stamping process, the lubricating oil is continuously released from the micro concave, a dynamic oil film is formed, and the friction coefficient is reduced.The micro convex and concave formed by the Cr-Fe composite oxide on the surface of the steel base and the porous structure of the Cr-Fe composite oxide itself provide more mechanical bite points for the coating, so that the coating can be better attached to the surface of the steel plate, and the anti-peeling performance of the coating is improved.The Cr ions in the Cr oxide can form a passivation film at the interface between the coating and the steel plate, inhibit the occurrence of corrosion reaction, and improve the corrosion resistance of the coating.

[0089] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the application. Those skilled in the art can easily make various modifications to the embodiments, and apply the general principles described herein to other embodiments without creative labor. Therefore, the present application is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present application without departing from the scope of the present application should be within the scope of protection of the present application.

Claims

1. A Cr-Nb series 590MPa grade high paintability steel sheet for exposed automotive parts, characterized in that, The Cr-Nb-based 590MPa grade high paintability steel sheet used for exposed automotive parts contains Cr: 0.40%-0.70%, Nb: 0.015%-0.030%, and Mn: 1.10%-1.70%, and the composition also meets the following requirements: 1.7%≤Mn+Cr≤2.1%, 2.0≤Mn / Cr≤3.0, and 19≤Cr / Nb≤30. After pickling, the surface of the Cr-Nb-based 590MPa grade high paintability steel sheet used for exposed automotive parts retains a Cr-Fe composite oxide layer with a thickness of ≤0.5μm. This oxide layer has a porous structure and forms a micron-level uneven morphology on the surface of the steel sheet. In the micron-scale uneven morphology, the height of the protrusions is 0.05-0.3 μm, accounting for 30-50% of the area; the depth of the depressions is 0.05-0.5 μm, accounting for 20-50% of the area; the average pore size of the porous structure is 0.01-0.10 μm, and the porosity is 30%-80%. The metallographic structure of the steel plate is ferrite + granular bainite + dispersed carbides, with granular bainite and carbides dispersed on the ferrite matrix. The volume fraction of granular bainite is 23-27%, and the grain size of ferrite is 12.5-13.

5. The Cr-Nb series 590MPa grade high paintability steel sheet used for exposed automotive parts has been verified by ISO 2409 cross-cut test, and the coating anti-peeling performance has reached the highest level 0.

2. The Cr-Nb series 590MPa grade high paintability steel sheet for exposed automotive parts according to claim 1, characterized in that... After hot rolling, the steel plate is covered with a Cr-Fe composite oxide transition layer with a thickness of <1.2μm between the iron oxide scale and the substrate.

3. The Cr-Nb series 590MPa grade high paintability steel sheet for exposed automotive parts according to claim 1 or 2, characterized in that, The steel plate comprises the following components by weight percentage: Cr: 0.40%-0.70%, Nb: 0.015%-0.030%, C: 0.05%-0.07%, Si: 0.09%-0.17%, Mn: 1.10%-1.70%, P≤0.020%, S≤0.010%, Als: 0.03%-0.07%, balance being Fe and unavoidable impurity elements; The steel plate also has the following composition requirements: 1.7%≤Mn+Cr≤2.1%, 2.0≤Mn / Cr≤3.0, and 19≤Cr / Nb≤30.

4. The Cr-Nb series 590MPa grade high paintability steel sheet for exposed automotive parts according to claim 3, characterized in that, The steel plate has a yield strength of 480-520 MPa, a tensile strength of 590-625 MPa, and a yield-to-tensile ratio ≤0.

86. 50 The surface quality grade is FB, with a porosity of 23-28% and a hole expansion rate of ≥75%.

5. A method for preparing Cr-Nb-based 590MPa grade high paintability steel sheet for exposed automotive parts as described in claim 3, characterized in that, The preparation method includes the following process flow: smelting → continuous casting → hot rolling → pickling.

6. The preparation method according to claim 5, characterized in that, The hot rolling process involves: a billet exiting the furnace at a temperature of 1220-1310℃ and a furnace time of 200-230 minutes; roughing is performed in 3+3 or 3+5 passes with the descaling water fully open at the inlet, and the intermediate billet is rough-rolled to a thickness of 30-40mm; finishing is performed in 7 consecutive passes with an initial rolling temperature of 1000℃-1100℃ and a final rolling temperature of 870-885℃; a front-stage cooling mode is adopted with a cooling rate of 90-140℃ / s and a coiling temperature of 570-595℃.

7. The preparation method according to claim 5 or 6, characterized in that, The thickness of the iron oxide scale on hot-rolled steel coils is <12μm.

8. The preparation method according to claim 6, characterized in that, The winding temperature is controlled at CPK > 7.0, corresponding to a temperature standard deviation of < 1℃.

9. The preparation method according to claim 5, characterized in that, The pickling process is continuous, with an acid temperature of 70-85℃, an acid concentration of 110-190 g / L, a pickling line speed of 120-190 m / min, and a tensile elongation of 0.45-2.2%.

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