Cold rolled high strength steel sheet having excellent phosphating performance and method for manufacturing the same

Abstract

Disclosed is a cold-rolled high-strength steel sheet with excellent phosphating performance, which has a mass percentage-depth distribution curve of elements in a depth range of 0-1000 nm from the surface of the steel sheet, wherein the mass percentage peak value Mn p of Mn element is ≤10.0%, the mass percentage peak value Si p of Si element is ≤4.0%, and the mass percentage peak value Mn p of Mn element and the mass percentage peak value Si p of Si element are both located at a depth position ≥50 nm from the surface of the steel sheet. Correspondingly, a manufacturing method of the cold-rolled high-strength steel sheet is also disclosed, which adopts a unique continuous annealing process.

Description

Technical Field

[0001] This invention relates to a steel plate and a method for manufacturing the same, and more particularly to a method for manufacturing a cold-rolled steel plate. Background Technology

[0002] High-strength steel is a material that is being used more and more widely. Cold-rolled high-strength steel usually requires the addition of high amounts of alloying elements such as C, Si, Mn, Cr, and Al, combined with continuous annealing processes, and utilizes phase transformation strengthening to achieve certain strength and formability.

[0003] Phosphating is a pretreatment process before automotive electrophoretic coating. It involves immersing steel plates or parts made of steel plates in a phosphating solution to form a layer of insoluble phosphating film on the surface of the steel plates or parts, thereby improving the adhesion of the paint film.

[0004] However, when using continuous annealing to produce cold-rolled high-strength steel, alloying elements such as Si and Mn in the steel will form external oxides on the surface of the annealed strip. These Si and Mn external oxides on the steel surface affect the nucleation and growth of the phosphating film, thus impacting the coating quality of automotive parts. Therefore, in recent years, the phosphating performance of high-strength steel has gradually attracted attention.

[0005] For example, Chinese patent CN107419185A, published on December 1, 2017, entitled "A cold-rolled steel sheet with excellent phosphating properties and its production method", mainly produces cold-rolled sheets with excellent phosphating properties by precisely controlling the composition and simultaneously producing them through hot rolling, pickling, five-stand full six-roll cold continuous rolling and continuous annealing and leveling processes.

[0006] For example, Chinese patent CN111910123A, published on November 10, 2020, entitled "A Cold-Rolled Continuous Annealed Ultra-High Strength Steel with Excellent Phosphating Properties and Its Preparation Method", discloses an annealing process for obtaining excellent phosphating properties of cold-rolled continuous annealed ultra-high strength steel. It limits the H2 content of the annealing atmosphere to 6% to 15%, the dew point to -45℃ to -41℃, and the oxygen content to 2ppm to 5ppm. The purpose is to suppress the selective oxidation of alloying elements through the above parameters, thereby obtaining excellent phosphating properties.

[0007] Therefore, it can be seen that controlling the content of alloying elements is one method in the existing technology to obtain excellent phosphating performance, but limiting the content of alloying elements will affect the performance of the material.

[0008] In addition, controlling the annealing atmosphere is another method to improve the phosphating performance of high-alloy cold-rolled steel sheets. However, controlling both the dew point and oxygen content to a relatively low level is difficult and costly; while controlling the dew point to a high level can cause decarburization on the steel sheet surface, so this method also has certain limitations.

[0009] Based on this, it is desirable to provide a new cold-rolled high-strength steel sheet that achieves excellent phosphating performance. Summary of the Invention

[0010] One of the objectives of this invention is to provide a cold-rolled high-strength steel sheet with excellent phosphating performance, which achieves excellent phosphating performance by controlling the Mn and Si elements to prevent them from accumulating on the surface of the steel sheet.

[0011] Based on the above-mentioned objectives, this invention provides a cold-rolled high-strength steel sheet with excellent phosphating properties. The peak mass percentage of Mn in the Mn content-depth distribution curve within a depth range of 0-1000 nm from the steel sheet surface is [missing information - likely a specific value]. p ≤10.0% peak Si mass percentage p ≤4.0%, and the peak mass percentage of Mn element is Mn p and the peak mass percentage of Si p All are located at a depth of ≥50nm from the surface of the steel plate.

[0012] In this invention, the peak mass percentage of Mn on the depth distribution curve of the elemental mass percentage content within the 0-1000 nm depth range from the surface of the steel plate, as detected by glow discharge emission spectroscopy (GDOES), is Mn. p ≤10.0% peak Si mass percentage p A concentration of ≤4.0% is crucial for achieving excellent phosphating performance in cold-rolled high-strength steel. This is because, in cold-rolled high-strength steel produced through continuous annealing, alloying elements such as Mn and Si form external oxides on the surface of the annealed steel sheet. This results in a significant enrichment of Si and Mn elements on the surface, with the peak mass percentage of Mn or Si on the steel sheet surface reaching several to tens of times that of the matrix Mn or Si. These oxides hinder the reaction between the phosphating solution and the steel sheet, thus affecting the phosphating performance of the high-strength steel.

[0013] The inventors discovered through research that, within a depth range of 0-1000 nm from the surface of the steel plate, the peak mass percentage of Mn on the depth distribution curve is Mn. p >10% or peak Si mass percentage p When Mn >4%, the phosphate performance of the steel plate is poor. Therefore, this invention controls Mn... p ≤10%, Si p ≤4%.

[0014] To achieve excellent phosphating performance, this invention controls Mn p Si p Depth t pMn ≥50nm, t pSi≥50nm. This is because the inventors discovered through research that when Mn p Si p When the depth is <50nm, even Mn p ≤10% or Si p ≤4%, localized oxidation of Si and Mn is still unavoidable, resulting in poor localized phosphating performance.

[0015] Furthermore, in the cold-rolled high-strength steel sheet with excellent phosphating properties described in this invention, the peak mass percentage of Mn element is Mn p and the peak mass percentage of Si p They are all located at a depth of 50-500 nm from the surface of the steel plate.

[0016] Although when t pMn >500nm, t pSi At a wavelength >500nm, cold-rolled high-strength steel also exhibits excellent phosphating properties, but in order to obtain t pMn >500nm, t pSi For wavelengths >500nm, higher pre-oxidation temperatures and longer pre-oxidation times are required. Therefore, the preferred pre-oxidation temperature in this invention is [not specified]. pMn ≤500nm, t pSi ≤500nm.

[0017] Furthermore, in the cold-rolled high-strength steel sheet with excellent phosphating properties described in this invention, on the elemental mass percentage content-depth distribution curve within a depth range of 0-1000 nm from the steel sheet surface, the peak mass percentage of Fe element is Fe. p ≥85%, and the peak mass percentage of Fe element is Fe p Located at a distance of 0-t from the surface of the steel plate pSi Within the depth range, where t pSi Peak Si mass percentage p The depth location where it appears.

[0018] When the Fe p When the content is ≥85%, it can be ensured that there is enough Fe on the surface of cold-rolled high-strength steel to react with the phosphating solution during phosphating treatment, thereby obtaining excellent phosphating performance.

[0019] Further preferred, when Fe p Phosphating performance will be better when the content is ≥90%.

[0020] Furthermore, the cold-rolled high-strength steel sheet with excellent phosphating properties described in this invention includes a base layer, a surface layer, and a transition layer located between the base layer and the surface layer in the thickness direction of the steel; the microstructure of the surface layer is ferrite.

[0021] Furthermore, in the cold-rolled high-strength steel sheet with excellent phosphating properties described in this invention, the thickness of the surface layer is 50-500 nm.

[0022] When the thickness of the surface layer, i.e., the ferrite layer, is 50-500 nm, it can ensure that the peak Mn and Si mass percentages on the elemental mass percentage-depth distribution curve obtained by glow discharge emission spectroscopy (GDOES) are equal. p Si p The depth t p The range is 50-500nm.

[0023] In the high-strength steel with excellent phosphating properties described in this invention, when the surface layer thickness is 50-500 nm, the Mn and Si elements diffused outward from the matrix during annealing are mainly enriched in the transition layer. The ferrite surface layer can effectively prevent alloying elements such as Mn and Si from diffusing to the ferrite surface, thereby ensuring that the peak Mn and Si mass percentages on the element mass percentage-depth distribution curve obtained by glow discharge emission spectroscopy (GDOES) are Mn. p Si p Depth t p The range is 50-500nm.

[0024] Furthermore, in the cold-rolled high-strength steel sheet with excellent phosphating performance described in this invention, the microstructure of the base layer is martensite + ferrite, or martensite + ferrite + retained austenite.

[0025] Furthermore, in the cold-rolled high-strength steel with excellent phosphating properties described in this invention, the transition layer contains oxides of Mn, Si, and Fe and / or composite oxides of Mn, Si, and Fe.

[0026] Furthermore, the oxides of Mn, Si, and Fe are at least one of the following: MnO, MnO2, SiO2, FeO, Fe2O3, Fe3O4; and the composite oxides of Mn, Si, and Fe are at least one of the following: MnSiO3, Mn2SiO4, Mn2FeO3, Mn2FeO4, FeSiO3, Fe2SiO3, Fe2SiO4.

[0027] Furthermore, the cold-rolled high-strength steel sheet with excellent phosphating properties described in this invention has a tensile strength ≥780MPa.

[0028] Furthermore, the cold-rolled high-strength steel sheet with excellent phosphating properties described in this invention contains the following elements in the following mass percentages:

[0029] C: 0.07-0.30%, Si: 0.07-3.0%, Mn: 1.5-5.0%.

[0030] Furthermore, the chemical element mass percentage content of the cold-rolled high-strength steel sheet with excellent phosphating properties described in this invention is as follows:

[0031] C: 0.07-0.30%, Si: 0.07-3.0%, Mn: 1.5-5.0%, and one or more of Nb, Ti, Al, B, Cr, Mo, Ni, and V, with the total mass percentage of Nb, Ti, Al, B, Cr, Mo, Ni, and V less than 2%; the balance being Fe and unavoidable impurities.

[0032] The unavoidable impurities are mainly S and P, and it is desirable for their content to be as low as possible. In some implementations, P ≤ 0.05% and S ≤ 0.02% can be controlled.

[0033] The design principles of the above elements are as follows:

[0034] C is a solid solution strengthening element necessary for steel to obtain strength. When the C content is too low, the strength of the steel is low, while when the C content is too high, the weldability of the steel is poor. Therefore, this invention controls the C mass percentage content to be 0.07-0.30%.

[0035] Si has the effect of simultaneously improving the strength and formability of steel, but when the Si content is too high, the manufacturing difficulty increases. Therefore, this invention controls the Si mass percentage content to be 0.07-3.0%.

[0036] Mn (nitrogen) is an element that increases the stability of austenite, reduces the critical cooling rate during steel quenching, and improves the hardenability of steel. Mn also enhances the work hardening properties of steel, thereby increasing its strength. Therefore, this invention controls the Mn mass percentage to be 1.5-5.0%.

[0037] Elements such as Nb, Ti, Al, B, Cr, Mo, Ni, and V have the effects of solid solution strengthening, refining microstructure, and improving hardenability. Adding small amounts helps to further improve the strength and toughness of steel. Therefore, this invention also contains one or more of Nb, Ti, Al, B, Cr, Mo, Ni, and V, and controls their total amount, by mass percentage, to be less than or equal to 2.0%.

[0038] Another objective of this invention is to provide a method for manufacturing cold-rolled high-strength steel sheets with excellent phosphating performance. This method achieves excellent phosphating performance by employing a unique continuous annealing process to control the enrichment location and amount of Mn and Si elements in the steel sheet.

[0039] Based on the above-mentioned objectives, the present invention provides a method for manufacturing cold-rolled high-strength steel sheets with excellent phosphating properties, comprising the steps of smelting, casting, hot rolling, pickling, cold rolling, and continuous annealing; wherein in the continuous annealing step: the cold-rolled sheet is first heated in an oxidizing atmosphere, then heated and homogenized in a reducing atmosphere, and then cooled and coiled.

[0040] In the manufacturing method described in this invention, smelting, casting, hot rolling, pickling, and cold rolling all adopt existing conventional processes, so they will not be described in detail here.

[0041] In the continuous annealing process, the purpose of selecting an oxidizing atmosphere for the first stage of heating is to form iron oxide on the surface of the steel plate during the first stage of heating. The purpose of selecting a reducing atmosphere for the second stage of heating and soaking is to reduce the iron oxide formed during the first stage of heating to form ferrite, thereby forming a surface layer that is entirely composed of ferrite, so that Mn and Si elements cannot be enriched on the surface of the steel plate.

[0042] Furthermore, in some embodiments of the manufacturing method described in this invention, the oxidizing atmosphere for the first stage of heating is obtained by controlling the air-fuel ratio of the direct-fired furnace to be 0.95-1.05.

[0043] In some implementations, when the first stage of heating uses a direct-fired furnace, an oxidizing atmosphere can be achieved by adjusting the air-fuel ratio. When the air-fuel ratio is <0.95, oxidation is insufficient, and when the air-fuel ratio is >1.05, excessive oxidation occurs. Therefore, the air-fuel ratio is controlled between 0.95 and 1.05.

[0044] Furthermore, in some other embodiments of the manufacturing method described in this invention, the oxidizing atmosphere for the first stage of heating is a N2+O2 mixed gas, wherein the volume percentage of O2 is 0.01-1%.

[0045] In other embodiments, when the first stage of heating uses an infrared radiation furnace, an oxidizing atmosphere is achieved by using a mixture of N2 and O2. When the O2 volume percentage is <0.01%, oxidation is insufficient; when the O2 volume percentage is >1%, further addition of O2 will result in over-oxidation. Therefore, the O2 volume percentage is controlled at 0.01-1%.

[0046] Furthermore, in the manufacturing method described in this invention, the heating rate of the first stage of heating is 1-50℃ / s, the first stage of heating is from room temperature to T1, and the mass percentage of T1 and Si satisfies: 570+4000×Si≤T1≤600+9000×Si.

[0047] The thickness of the oxide layer on the steel plate surface increases with the increase of the end temperature T1 of the first heating stage. Therefore, to obtain a suitable oxide layer thickness, it is necessary to control the upper and lower limits of the T1 temperature. Our research has found that, at the same T1 temperature, the iron oxide thickness on the steel plate surface is also affected by the steel plate composition, with the Si content having the most significant impact. When the Si content in the steel plate is high, a higher T1 temperature is required to obtain the same iron oxide thickness in the first heating stage. A suitable iron oxide thickness can be obtained when the following relationship is satisfied between the T1 temperature and the Si content of the steel plate: 570 + 4000 × Si ≤ T1 ≤ 600 + 9000 × Si.

[0048] Furthermore, in the manufacturing method described in this invention, the dew point of the oxidizing atmosphere in the first stage of heating is ≥-40°C.

[0049] Furthermore, in the manufacturing method described in this invention, the dew point of the reducing atmosphere used for the second stage of heating and homogenization is ≤-20°C.

[0050] It should be noted that, in this invention, the dew point of the atmosphere during the first stage of heating, the second stage of heating, and the homogenization is not a necessary technical feature. In other words, even without special control over the dew point of the atmosphere during the first stage of heating, the second stage of heating, and the homogenization, the peak Mn mass percentage on the depth distribution curve within the 0-1000nm depth range from the steel plate surface can be obtained. p ≤10.0% peak Si mass percentage p The product characteristic is ≤4.0%. This is because, during the first stage of heating, when iron oxide is formed, some Mn in the matrix oxidizes together with Fe, but the oxide formed by Si is mainly located on the transition layer; during the second stage of heating and homogenization, when reduced iron is reduced, regardless of the dew point of the atmosphere, the selective oxidation of alloying elements such as Mn and Si in the matrix is ​​mainly concentrated below the ferrite formed after the reduction of iron oxide, i.e., in the transition layer, and will not accumulate on the surface layer of ferrite. Therefore, this invention differs from existing technologies that utilize increasing the dew point of the reducing atmosphere to promote the formation of internal oxidation of alloying elements and reduce external oxidation.

[0051] However, the inventors have discovered that increasing the dew point of the oxidizing atmosphere in the first stage of heating is advantageous for lowering the T1 temperature. This is because increasing the dew point of the first stage heating atmosphere increases the internal oxidation of alloying elements, thereby reducing the oxidation of alloying elements at the iron oxide / substrate interface, especially the oxidation of Si. Therefore, preferably, the dew point of the oxidizing atmosphere in the first stage of heating can be controlled to ≥-40°C. Since excessively high dew points in the second stage heating and homogenization stages can lead to decarburization, preferably, the dew points of the second stage heating and homogenization atmospheres can be controlled to ≤-20°C.

[0052] Furthermore, in the manufacturing method described in this invention, the reducing atmosphere for the second stage of heating and homogenization is a N2+H2 mixture, wherein the volume percentage of H2 is 0.5-20%.

[0053] Furthermore, in the manufacturing method described in this invention, the heating rate of the second stage of heating is 1-20℃ / s, and the second stage of heating continues from T1 to a homogenization temperature of 720-920℃, with a homogenization time of 30-200s.

[0054] The homogenization temperature T2 is controlled at 720-920℃, and the homogenization time is controlled at 30-200s. This is mainly to obtain suitable mechanical properties of cold-rolled high-strength steel, which is not closely related to phosphating performance.

[0055] The cold-rolled high-strength steel sheet with excellent phosphating properties described in this invention has the following beneficial effects:

[0056] This invention solves the problem of poor phosphating performance of cold-rolled high-strength steel by controlling the peak values ​​of Mn and Si mass percentages on the depth distribution curve of elements within the steel plate and their corresponding depth positions, thereby improving the coating performance of cold-rolled high-strength steel.

[0057] In some preferred embodiments, the phosphated steel sheet also has a tensile strength of ≥780 MPa. Attached Figure Description

[0058] Figure 1 This is a comparison of the depth distribution curves of Mn elements within 0-1000 nm from the surface of the steel plate in Embodiment A1 and Comparative Example A1 of the present invention.

[0059] Figure 2 This is a comparison of the depth distribution curves of Si elements within 0-1000 nm from the surface of the steel plate in Embodiment A1 and Comparative Example A1 of the present invention.

[0060] Figure 3 This is a comparison of the depth distribution curves of Fe element within 0-1000nm from the surface of the steel plate in Embodiment A1 and Comparative Example A1 of the present invention.

[0061] Figure 4 This is a comparison of the depth distribution curves of Mn elements within 0-1000 nm from the surface of the steel plate in Embodiment B1 and Comparative Example B1 of the present invention.

[0062] Figure 5 This is a comparison of the depth distribution curves of Si elements within 0-1000 nm from the surface of the steel plate in Embodiment B1 and Comparative Example B1 of the present invention.

[0063] Figure 6 This is a comparison of the depth distribution curves of Fe element within 0-1000nm from the surface of the steel plate in Embodiment B1 and Comparative Example B1 of the present invention. Detailed Implementation

[0064] The following will further explain and illustrate the cold-rolled high-strength steel sheet with excellent phosphating properties and its manufacturing method according to the present invention, with reference to specific embodiments and accompanying drawings. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0065] Table 1 lists the steel composition ratios used in the various embodiments and comparative examples of the present invention.

[0066] Table 1 (wt%, balance Fe)

[0067] Steel plate code C Si <![CDATA[M n ]]> P S Al Nb Ti B <![CDATA[C r ]]> <![CDATA[M o ]]> V Ni A 0.08 0.4 2.2 0.012 0.002 - 0.04 0.03 - - - - - B 0.2 1.8 2.3 0.007 0.001 0.03 - 0.02 - - - - - C 0.07 0.07 2.1 0.010 0.0008 0.6 - 0.01 0.0006 - 0.2 - - D 0.13 0.26 2.4 0.011 0.0017 - - - - 0.4 - - - E 0.22 1.8 2.8 0.014 0.0014 0.03 - 0.02 - - - - - F 0.11 1.2 1.5 0.006 0.0007 - - - - - - -- - G 0.3 3 3 0.022 0.005 1.5 - - - 0.5 - - - H 0.09 0.8 1.8 0.0091 0.02 0.03 - - 0.0004 0.03 - 0.0003 0.07 I 0.2 0.5 5 0.05 0.0017 1.5 - - - - - 0.02 -

[0068] Based on the steel plate composition in Table 1, the steel plates of the various embodiments and comparative examples of the present invention were prepared according to the following steps. The specific process parameters of the continuous annealing steps of the various embodiments and comparative examples are listed in Table 2:

[0069] (1) Conventional smelting methods are adopted;

[0070] (2) Conventional casting is adopted;

[0071] (3) Conventional hot rolling is adopted;

[0072] (4) Conventional pickling and cold rolling are adopted;

[0073] (5) Continuous annealing; the cold-rolled sheet is heated in an oxidizing atmosphere for the first stage; the heating rate of the first stage is 1-50℃ / s, the first stage heating is from room temperature to T1, and the mass percentage of T1 and Si satisfies: 570+4000×Si≤T1≤600+9000×Si; then the second stage heating and homogenization are carried out in a reducing atmosphere, the reducing atmosphere of the second stage heating is a N2+H2 mixture, where the volume percentage of H2 is 0.5-20%; then cooling and winding are performed.

[0074] In some embodiments, the oxidizing atmosphere for the first stage of heating is a mixture of N2 and O2 gases, wherein the volume percentage of O2 is 0.01-1%.

[0075] In other embodiments, the oxidizing atmosphere for the first stage of heating is obtained by controlling the air-fuel ratio of the direct-fired furnace to be 0.95-1.05.

[0076] In some embodiments, the dew point of the oxidizing atmosphere in the first stage of heating can be controlled to be ≥-40°C, and the dew point of the reducing atmosphere in the second stage of heating can be controlled to be ≤-20°C.

[0077] In some embodiments, the heating rate of the second stage of heating is 1-20℃ / s, and the second stage of heating continues from T1 to a uniform heating temperature of 720-920℃, with a uniform heating time of 30-200s.

[0078] Table 2.

[0079]

[0080]

[0081]

[0082] In Table 2 above, examples where the air-fuel ratio is indicated by " / " indicate that the oxidizing atmosphere used in that example was a N2+O2 mixture. Comparative examples A1-A4, D1-D2, E1-E3, F1, and G1-G2 also use " / " to indicate the air-fuel ratio and oxygen content, indicating that the first stage of heating in these comparative examples did not use an oxidizing atmosphere. The codes "A", "B"..."H", and "I" in Table 2 for each example and comparative example indicate the steel plate composition corresponding to the numbers in Table 1 used.

[0083] Table 3 lists the steel plate composition using AI and the characteristics of the steel plates obtained from each embodiment and comparative example prepared using the process parameters listed in Table 2, as well as the corresponding phosphating properties of each steel plate. Among these, Mn... p Si p t pSi t pMn Fe p Detected using GDOES.

[0084] Table 3.

[0085]

[0086]

[0087] Figure 1 , Figure 2 and Figure 3 The mass percentage content of Mn, Si, and Fe elements at a depth of 0-1000 nm from the surface of the steel plate in Example A1 and Comparative Example A1 are shown respectively. The characteristic value of Mn is marked on the curve. p Si p Fe p and t p . Figure 1 t in p Indicates t pMn , Figure 2 t in p Indicates t pSi , Figure 3 t in p Indicates tpSi .

[0088] As shown in Table 2, Example A1 underwent a first stage of heating in an oxidizing atmosphere, with a heating rate of 10°C / s to 580°C. The oxidizing atmosphere was a mixture of N2 and O2, with O2 comprising 0.2% by volume and a dew point of -50°C. A second stage of heating and homogenization was then performed in a reducing atmosphere, with a heating rate of 4°C / s, a homogenization temperature of 820°C, and a homogenization time of 100s. The reducing atmosphere for the second stage of heating and homogenization was a mixture of N2 and H2, with H2 comprising 2% by volume and a dew point of -50°C.

[0089] The elemental depth distribution on the surface of the annealed steel plate was detected using GDOES, and the results were as follows: Figures 1 to 3 The depth distribution curves of Mn, Si, and Fe elements are shown in the depth range of 0-1000 nm.

[0090] Comparative Example A1 underwent the first stage of heating, the second stage of heating, and homogenization in a reducing atmosphere, with the heating rate, temperature, and time being the same as in Example A1. The depth distribution curves of Mn, Si, and Fe elements on the surface of Comparative Example A1 after annealing, within a depth range of 0-1000 nm, are shown below. Figures 1 to 3 As shown.

[0091] Phosphating treatment was performed on Example A1 and Comparative Example A1 under the same conditions. The phosphating film on the surface of Example A1 was uniform and dense, and the phosphating performance met the requirements (indicated by "○" in Table 2). However, the phosphating film on the surface of Comparative Example A1 was sparse, with a large area covered by the phosphating film, and the phosphating performance did not meet the requirements (indicated by "×" in Table 2).

[0092] Figure 4 , Figure 5 , Figure 6 The depth distribution curves of Mn, Si, and Fe elements within a depth range of 0-1000 nm from the surface of the steel plate are shown for Example B1 and Comparative Example B1, respectively. The characteristic value of Mn is marked on the curve. p Si p Fe p and t p . Figure 4 t in p Indicates t pMn , Figure 5 t in p Indicates t pSi , Figure 6 t in p Indicates t psi .

[0093] Example B1 involves a first stage of heating in an oxidizing atmosphere, with a heating rate of 50°C / s to 650°C. The oxidizing atmosphere is achieved through a direct-fired furnace, with the air-fuel ratio controlled at 0.95 and the dew point at 50°C. A second stage of heating and homogenization is then performed in a reducing atmosphere, with a heating rate of 5°C / s, a homogenization temperature of 870°C, and a homogenization time of 180s. The atmosphere for the second stage of heating and homogenization is a N2+H2 mixture, with H2 comprising 2% by volume and a dew point of -40°C.

[0094] The elemental depth distribution on the surface of the annealed steel plate was detected using GDOES, and the results were as follows: Figures 4-6 The depth distribution curves of Mn, Si, and Fe elements within the depth range of 0-1000 nm are shown. The sample in Example B1 was subjected to phosphating treatment; the phosphating film was uniform and dense, and the phosphating performance met the requirements.

[0095] Comparative Example B1 underwent a first-stage heating, a second-stage heating, and homogenization in a reducing atmosphere. The reducing atmosphere was a N2-H2 mixture, with H2 comprising 2% by volume, and an atmosphere dew point of -20°C. The homogenization temperature and time were the same as in Example B1. The depth distribution curves of Mn, Si, and Fe elements within a depth range of 0-1000 nm from the surface of the steel plate after annealing in Comparative Example B1 are shown below. Figures 4-6 As shown, when the comparative example B1 sample was phosphated, the phosphate film was uneven and not dense, with some areas lacking phosphate film coverage, and the phosphate performance did not meet the requirements.

[0096] The inventors also used scanning electron microscopy to observe the cross-sectional metallographic structure after etching with nitric acid alcohol solution, and tested the microstructure of each embodiment and comparative example. They also used focused ion beam microscopy to prepare cross-sectional transmission electron microscopy samples of the surface layer and transition layer. The thickness of the surface layer was measured under the transmission electron microscope, the types of oxides in the transition layer were analyzed, and the test results are listed in Table 4.

[0097] Table 4.

[0098]

[0099]

[0100] Note: In Table 4, F represents ferrite, M represents martensite, and R represents retained austenite. The surface layer of each comparative example is indicated by "-", meaning that the comparative examples do not have a surface layer.

[0101] The inventors also tested the tensile strength of each embodiment and comparative example according to the tensile test of the national standard GB / T 228.1, and listed the test results in Table 5.

[0102] Table 5.

[0103]

[0104]

[0105] It can be seen that the cold-rolled steel sheet manufactured using the present invention has both excellent phosphating properties and high strength.

[0106] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0107] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A cold-rolled high-strength steel sheet with excellent phosphating properties, characterized in that, On the element mass percentage content-depth distribution curve within a depth range of 0-1000 nm from the steel plate surface, the peak mass percentage of Mn is Mn. p ≤10.0%, peak Si mass percentage p ≤4.0%, and the peak mass percentage of Mn element is Mn p and the peak mass percentage of Si p All are located at a depth of ≥50 nm from the surface of the steel plate; the peak mass percentage of Fe is Fe. p ≥85%, and the peak mass percentage of Fe element is Fe p Located at a distance of 0-t from the surface of the steel plate pSi Within the depth range, where t pSi Peak Si mass percentage p The depth location where it appears; The cold-rolled high-strength steel sheet contains the following elements in the following mass percentages: C: 0.07-0.30%, Si: 0.07-0.5%, Mn: 1.5-5.0%.

2. The cold-rolled high-strength steel sheet with excellent phosphating properties as described in claim 1, characterized in that, Mn element mass percentage peak Mn p and the peak mass percentage of Si p They are all located at a depth of 50-500 nm from the surface of the steel plate.

3. The cold-rolled high-strength steel sheet with excellent phosphating properties as described in claim 1, characterized in that, It comprises a base layer and a surface layer and a transition layer between the base layer and the surface layer in the thickness direction of the steel; the microstructure of the surface layer is ferrite.

4. The cold-rolled high-strength steel sheet with excellent phosphating properties as described in claim 3, characterized in that, The thickness of the surface layer is 50-500 nm.

5. The cold-rolled high-strength steel sheet with excellent phosphating properties as described in claim 3, characterized in that, The microstructure of the base layer is martensite + ferrite, or martensite + ferrite + retained austenite.

6. The cold-rolled high-strength steel sheet with excellent phosphating properties as described in claim 3, characterized in that, The transition layer contains oxides of Mn, Si, and Fe and / or composite oxides of Mn, Si, and Fe.

7. The cold-rolled high-strength steel sheet with excellent phosphating properties as described in claim 6, characterized in that, The oxides of Mn, Si, and Fe are at least one of the following: MnO, MnO2, SiO2, FeO, Fe2O3, Fe3O4; the composite oxides of Mn, Si, and Fe are at least one of the following: MnSiO3, Mn2SiO4, Mn2FeO3, Mn2FeO4, FeSiO3, Fe2SiO3, Fe2SiO4.

8. The cold-rolled high-strength steel sheet with excellent phosphating properties as described in claim 1, characterized in that, Its tensile strength is ≥780MPa.

9. The cold-rolled high-strength steel sheet with excellent phosphating properties as described in claim 1, characterized in that, Its chemical element mass percentage content is: C: 0.07-0.30%, Si: 0.07-0.5%, Mn: 1.5-5.0%, and one or more of Nb, Ti, Al, B, Cr, Mo, Ni, and V, with the total mass percentage of Nb, Ti, Al, B, Cr, Mo, Ni, and V less than 2.0%; the balance being Fe and unavoidable impurities.

10. A method for manufacturing a cold-rolled high-strength steel sheet with excellent phosphating properties as described in any one of claims 1-9, comprising the steps of: smelting, casting, hot rolling, pickling, cold rolling, and continuous annealing; characterized in that, In the continuous annealing step: the cold-rolled sheet is heated in an oxidizing atmosphere for the first stage, then heated and homogenized in a reducing atmosphere for the second stage, and then cooled and rolled; the heating rate of the first stage heating is 1-50℃ / s, the first stage heating is heated from room temperature to T1, and the mass percentage of T1 and Si satisfies: 570+4000×Si≤T1≤600+9000×Si, and the dew point of the oxidizing atmosphere for the first stage heating is ≥-40℃.

11. The manufacturing method as described in claim 10, characterized in that, The oxidizing atmosphere for the first stage of heating is obtained by controlling the air-fuel ratio of the direct-fired furnace to be 0.95-1.

05.

12. The manufacturing method as described in claim 10, characterized in that, The oxidizing atmosphere in the first stage of heating is a mixture of N2 and O2 gases, in which the volume percentage of O2 is 0.01-1%.

13. The manufacturing method as described in claim 10, characterized in that, The reducing atmosphere for the second stage of heating and homogenization is a mixture of N2 and H2 gases, in which the volume percentage of H2 is 0.5-20%.

14. The manufacturing method as described in claim 10, characterized in that, The dew point of the reducing atmosphere used for the second stage of heating and homogenization is ≤-20℃.

15. The manufacturing method as described in claim 10, characterized in that, The heating rate of the second stage is 1-20℃ / s. The second stage heating continues from T1 to the homogenization temperature of 720-920℃, and the homogenization time is 30-200 s.

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