Cold-rolled high-tensile steel sheet with excellent phosphate treatment properties and method for manufacturing the same

Abstract

This invention discloses a cold-rolled high-tensile steel sheet with excellent phosphate treatment properties, and in the elemental mass percentage content-depth distribution curve within a depth range of 0 to 1000 nm from the surface of the steel sheet, the Mn elemental mass percentage peak Mn p ≤10.0%, Si mass percentage peak Si p ≤4.0%, and the peak Mn element mass percentage. p and Si element mass percentage peak Si p These are all located at a depth of 50 nm or more from the surface of the steel sheet. Accordingly, the present invention further discloses a method for manufacturing the cold-rolled high-tensile steel sheet, and this manufacturing method employs a unique continuous annealing process.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and particularly to a cold-rolled steel sheet and a method for manufacturing the same.

Background Art

[0002] High-strength steel is a material that is being applied more and more widely at present. Cold-rolled high-strength steel usually needs to add alloy elements such as C, Si, Mn, Cr, Al, etc. at relatively high contents, and in combination with a continuous annealing process, utilize phase transformation strengthening, etc. to achieve a certain strength and formability.

[0003] Phosphate treatment is a pretreatment process before automotive electrocoating. By immersing a steel sheet or a part manufactured from a steel sheet in a phosphate treatment solution to form an insoluble phosphate film on the surface of the steel sheet or part, it plays a role in improving the adhesion of the coating film.

[0004] However, when manufacturing cold-rolled high-strength steel using continuous annealing, alloy elements such as Si and Mn in the steel cause external oxidation on the surface of the annealed strip steel. The external oxidation of Si and Mn on the steel sheet surface affects the nucleation and growth of the phosphate film, and further affects the coating quality of automotive parts. Therefore, in recent years, the phosphate treatment characteristics of high-strength steel have been increasingly noticed.

[0005] For example, a Chinese patent application with a disclosure number of CN107419185A, a disclosure date of December 1, 2017, and a disclosure name of "Cold-rolled Steel Sheet with Excellent Phosphate Treatment Characteristics and Its Manufacturing Method" mainly precisely manages the components, and at the same time, through hot rolling, pickling, 5-stand total 6-roll cold continuous rolling, continuous annealing, and temper rolling processes, manufactures a cold-rolled steel sheet with excellent phosphate treatment characteristics.

[0006] Furthermore, a Chinese patent application with disclosure number CN111910123A, disclosure date November 10, 2020, and disclosure title "Cold-rolled continuous annealed ultra-high-strength steel having excellent phosphate treatment properties and method for manufacturing the same" discloses an annealing process for obtaining excellent phosphate treatment properties of cold-rolled continuous annealed ultra-high-strength steel. This process limits the H2 content of the annealing atmosphere to 6% to 15%, the dew point to -45°C to -41°C, and the oxygen content to 2 ppm to 5 ppm. The purpose of this process is to obtain excellent phosphate treatment properties by suppressing the selective oxidation of alloying elements using the above parameters.

[0007] Therefore, while controlling the alloying element content is a method for obtaining excellent phosphate treatment properties in existing technologies, limiting the alloying element content affects the performance of material properties.

[0008] Furthermore, controlling the annealing atmosphere is another method for improving the phosphate treatment properties of high-alloy content cold-rolled steel sheets. However, methods that control both the dew point and oxygen content to relatively low levels are difficult and costly, while methods that control the dew point to a relatively high level cause decarburization of the steel sheet surface. Therefore, these methods also have certain limitations.

[0009] Based on this, it is desirable to provide a new cold-rolled high-tensile steel sheet that achieves excellent phosphate treatment properties. [Overview of the project] [Problems that the invention aims to solve]

[0010] The present invention aims to provide a cold-rolled, high-tensile steel sheet with excellent phosphate treatment properties, achieved by controlling the enrichment of Mn and Si elements on the steel sheet surface. [Means for solving the problem]

[0011] Based on the above object of the invention, the present invention provides a cold-rolled high-tensile steel sheet excellent in phosphate treatment characteristics. In the elemental mass percent content-depth distribution curve within a depth range of 0 to 1000 nm from the steel sheet surface, the Mn elemental mass percent peak Mn p ≤ 10.0%, the Si mass percent peak Si p ≤ 4.0%, and the Mn elemental mass percent peak Mn p and the Si elemental mass percent peak Si p are both at a depth position of 50 nm or more from the steel sheet surface. In this specification, the Mn elemental mass percent peak Mn p and the Si elemental mass percent peak Si p are detected by glow discharge optical emission spectrometry (GDOES).

[0012] In the present invention, in the elemental mass percent content-depth distribution curve within a depth range of 0 to 1000 nm from the steel sheet surface detected by glow discharge optical emission spectrometry (GDOES), the Mn mass percent peak Mn p ≤ 10.0%, the Si mass percent peak Si p ≤ 4.0% are the key to obtaining excellent phosphate treatment characteristics for the cold-rolled high-tensile steel. This is because in the cold-rolled high-tensile steel produced by continuous annealing, alloy elements represented by Mn and Si generate external oxidation on the surface of the annealed steel sheet, and the Si and Mn elements are significantly enriched on the surface, and the mass percent peak of Mn or Si on the steel sheet surface reaches several times to dozens of times that of Mn or Si in the base material. These oxides inhibit the reaction between the phosphate treatment solution and the steel sheet, thus affecting the phosphate treatment characteristics of the high-tensile steel.

[0013] Through research, the inventor found that when the Mn mass percent peak Mn p > 10% or the Si mass percent peak Si p > 4% in the elemental mass percent content-depth distribution curve within a depth range of 0 to 1000 nm from the steel sheet surface, the phosphate treatment characteristics of the steel sheet are inferior. Therefore, the present invention controls Mn p ≤ 10%, Si p ≤ 4%. In some embodiments, the present invention provides Mnp ≤8.5%, Si p Control to ≤3.0%. In some embodiments, Mn p It is 5.0-10.0%, Si p The percentage is 0.1-3.0%.

[0014] In order to achieve excellent phosphate treatment characteristics, the present invention uses Mn p Si p A certain depth t pMn ≥50nm, t pSi Controlled to ≥50nm. This is because the inventor, through research, found that Mn p Si p For a certain depth <50nm, Mn p ≤10% or Si p Even at ≤4%, we found that local external oxidation of Si and Mn could not be avoided, resulting in poor local phosphate treatment characteristics.

[0015] Furthermore, in the cold-rolled high-tensile steel sheet with excellent phosphate treatment characteristics according to the present invention, the Mn element mass percent peak Mn p and Si element mass percentage peak Si p These are all located at a depth of 50 to 500 nm from the surface of the steel plate.

[0016] t pMn >500nm, t pSi Even at >500nm, the phosphate treatment characteristics of cold-rolled high-tensile steel are excellent, but t pMn >500nm, t pSi To obtain >500nm, a higher pre-oxidation temperature and a longer pre-oxidation time are required. Therefore, the present invention t pMn ≤500nm, t pSi ≤500nm is preferred. In some embodiments, the present invention is t pMn The wavelength is controlled from 100nm to 450nm, pSi The wavelength is controlled between 100nm and 450nm.

[0017] Furthermore, in the cold-rolled high-tensile steel sheet with excellent phosphate treatment characteristics according to the present invention, in the elemental mass percentage content-depth distribution curve within a depth range of 0 to 1000 nm from the steel sheet surface, the Fe elemental mass percentage peak Fe p ≥85%, and Fe elemental mass percentage peak Fe p From the surface of the steel plate, 0~t pSi It is within the depth range, where t pSi The peak is the Si element mass percentage. p This indicates the depth position at which it appears. In this invention, Fe p This refers to the distance from the surface of the steel plate from 0 to t. pSi This refers to the Fe element mass percentage peak within the specified depth range.

[0018] The above p When the value is ≥85%, it is possible to ensure that sufficient Fe reacts with the phosphate treatment solution on the surface when phosphate treatment is performed on cold-rolled high-tensile steel, resulting in excellent phosphate treatment properties.

[0019] More preferably, Fe p When the phosphate treatment ratio is ≥90%, the phosphate treatment properties are superior.

[0020] Furthermore, the cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to the present invention includes a base layer, 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.

[0021] Furthermore, in the cold-rolled high-tensile steel sheet with excellent phosphate treatment characteristics according to the present invention, the thickness of the surface layer is 50 to 500 nm.

[0022] When the thickness of the surface layer, i.e., the ferrite layer, is 50-500 nm, the Mn and Si mass percentage peaks in the elemental mass percentage content-depth distribution curve detected by glow discharge emission spectrometry (GDOES) are Mn. p Si p A certain depth t p This ensures that the wavelength is between 50 and 500 nm.

[0023] In the high-tensile steel with excellent phosphate treatment properties according to the present invention, when the surface layer thickness is 50 to 500 nm, the Mn and Si elements that diffuse from the substrate to the outside during annealing are mainly enriched in the transition layer, and the ferrite surface layer effectively prevents the diffusion of alloying elements such as Mn and Si to the ferrite surface. p Si p A certain depth t p We can guarantee that it is between 50 and 500 nm.

[0024] Furthermore, in the cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to the present invention, the microstructure of the base layer is martensite + ferrite, or martensite + ferrite + retained austenite.

[0025] Furthermore, in the cold-rolled high-tensile steel with excellent phosphate treatment properties according to the present invention, the transition layer contains oxides of Mn, Si, and Fe and / or composite oxides of Mn, Si, and Fe. Oxides of Mn, Si, and Fe refer to oxides of Mn, Si, or Fe. Composite oxides of Mn, Si, and Fe refer to oxides of two or three of the elements of Mn, Si, and Fe.

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

[0027] Furthermore, the cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to the present invention has a tensile strength of ≥ 780 MPa. Furthermore, the cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to the present invention has a tensile strength of ≥ 980 MPa. Furthermore, the cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to the present invention has a tensile strength of ≥ 1180 MPa.

[0028] This application does not particularly limit the type and elemental composition of cold-rolled high-tensile steel, as various cold-rolled high-tensile steels known in the art are manufactured by the method of the present invention and thus have the structure (e.g., elemental distribution) and phosphate treatment characteristics described herein.

[0029] In some embodiments, the cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to the present invention contains the following elements in the following mass percentages: Specifically, it contains C: 0.07-0.30%, Si: 0.07-3.0%, and Mn: 1.5-5.0%.

[0030] Furthermore, the chemical element mass percentage content of the cold-rolled high-tensile steel sheet with excellent phosphate treatment characteristics according to the present invention is: The composition is 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. The total mass percentage content of Nb, Ti, Al, B, Cr, Mo, Ni, and V is 2.0% or less, with the remainder being Fe and unavoidable impurities.

[0031] The unavoidable impurities in question are mainly S and P, and the lower their content, the better. In some embodiments, the content is controlled to P ≤ 0.05% and S ≤ 0.02%.

[0032] The design principles for each of the above elements are as follows:

[0033] C is an essential solid solution strengthening element for obtaining strength in steel. If the C content is too low, the strength of the steel will be low, and if the C content is too high, the weldability of the steel will be poor. Therefore, this invention controls the C mass percentage content to 0.07 to 0.30%.

[0034] Si: While it has the effect of simultaneously improving the strength and formability of steel, if the Si content is too high, the difficulty of manufacturing increases. Therefore, in this invention, the Si mass percentage content is controlled to 0.07 to 3.0%.

[0035] Mn is an element that increases the stability of austenite, reduces the critical cooling rate during steel quenching, and improves the hardenability of steel. Mn further improves the work hardening properties of steel, thereby increasing its strength. For this reason, the present invention controls the Mn mass percentage content to 1.5 to 5.0%.

[0036] The elements Nb, Ti, Al, B, Cr, Mo, Ni, and V have effects such as solid solution strengthening, microstructure refinement, and improved hardenability, and even small amounts of their addition can lead to further improvements in the strength and toughness of steel. Therefore, the present invention may further contain one or more of the elements Nb, Ti, Al, B, Cr, Mo, Ni, and V, and their total amount is controlled to 2.0% or less by mass percentage. In some embodiments, when included, the Nb content is 0.01-0.1%, the Ti content is 0.01-0.05%, the Al content is 0.01-1.5%, the B content is 0.0002-0.0008%, the Cr content is 0.01-0.5%, the Mo content is 0.1-0.3%, the Ni content is 0.0001-0.03%, and the Ni content is 0.01-0.1%.

[0037] The present invention further aims to provide a method for manufacturing cold-rolled high-tensile steel sheets with excellent phosphate treatment properties, by employing a unique continuous annealing process and controlling the enrichment location and amount of Mn and Si elements in the steel sheet.

[0038] Based on the above-mentioned objectives of the invention, the present invention provides a method for manufacturing a cold-rolled high-tensile steel sheet with excellent phosphate treatment characteristics, comprising the steps of steelmaking, casting, hot rolling, pickling, cold rolling, and continuous annealing, wherein in the continuous annealing step, the cold-rolled steel sheet is heated once in an oxidizing atmosphere, then heated a second time and soaked in a reducing atmosphere, and then cooled and coiled.

[0039] In the manufacturing method according to the present invention, steelmaking, casting, hot rolling, pickling, and cold rolling all employ existing general processes. In an exemplary method, in the hot rolling process, the heating temperature is controlled to 1150-1260°C, the finish rolling start temperature to 1100-1220°C, and the finish rolling end temperature to 900-950°C. In the cold rolling process, the cold rolling reduction ratio is controlled to be ≥ 50%. In some embodiments, the annealing process of this application can be carried out using commercially available cold-rolled steel.

[0040] In the continuous annealing process, the purpose of selecting an oxidizing atmosphere for the first heating is to form iron oxide on the surface of the steel sheet during the first heating process. The purpose of selecting a reducing atmosphere for the second heating and soaking is to reduce the iron oxide formed during the first heating process to form ferrite, thereby forming a surface layer that is entirely ferrite, so that the surface of the steel sheet is not enriched with Mn and Si elements.

[0041] Furthermore, in some embodiments of the manufacturing method according to the present invention, the oxidizing atmosphere for the first heating is obtained by controlling the air-fuel ratio of the direct-fired furnace to 0.95 to 1.05.

[0042] In some embodiments, when the first heating is performed in a direct-fired furnace, an oxidizing atmosphere is achieved by adjusting the air-fuel ratio. If the air-fuel ratio is <0.95, oxidation is insufficient, and if the air-fuel ratio is >1.05, excessive oxidation occurs, so the air-fuel ratio is controlled to be between 0.95 and 1.05.

[0043] Furthermore, in some other embodiments of the manufacturing method according to the present invention, the oxidizing atmosphere for the first heating is an N2+O2 mixed gas, of which the volume percentage of O2 is 0.01 to 1%.

[0044] In some other embodiments, when the first heating is performed in an infrared radiation furnace, an oxidizing atmosphere is created by using an N2+O2 mixed gas. If the O2 volume percentage is <0.01%, oxidation is insufficient, and if the O2 volume percentage is >1%, increasing the O2 further will cause excessive oxidation, so the O2 volume percentage is controlled to 0.01-1%.

[0045] Furthermore, in the manufacturing method according to the present invention, the heating rate for the first heating is 1 to 50°C / s, the first heating is performed from room temperature to the heating completion temperature T1, and the mass percentage content of T1 and Si satisfies 570 + 4000 × Si ≤ T1 ≤ 600 + 9000 × Si.

[0046] Since the thickness of the oxide layer on the steel sheet surface increases with increasing first heating completion temperature T1, it is necessary to control the upper and lower limits of the T1 temperature to obtain an appropriate oxide layer thickness. Through research, the inventors have discovered that when the T1 temperature is the same, the thickness of the iron oxide on the steel sheet surface is also affected by the steel sheet components, with the effect of the Si content being the most significant. When the Si content in the steel sheet is high, a higher T1 temperature is required to obtain the same thickness of iron oxide in the first heating. An appropriate iron oxide thickness can be obtained when the T1 temperature and the Si content of the steel sheet satisfy the following relationship, namely 570 + 4000 × Si ≤ T1 ≤ 600 + 9000 × Si. In some embodiments, T1 is 580 to 800°C. Preferably, the thickness of the oxide layer formed in the first heating is 50 to 500 nm.

[0047] Furthermore, in the manufacturing method according to the present invention, the dew point of the oxidizing atmosphere during the first heating is ≥ -50°C, preferably ≥ -40°C. In some embodiments, the dew point of the oxidizing atmosphere during the first heating is -50°C to 50°C. In some embodiments, the dew point of the oxidizing atmosphere during the first heating is -50°C to 0°C.

[0048] Furthermore, in the manufacturing method according to the present invention, the dew point of the reducing atmosphere during the second heating and soaking is ≤10°C, for example, ≤0°C, ≤-10°C, or ≤-20°C. In some embodiments, the dew point of the reducing atmosphere during the second heating and soaking is 10°C to -50°C, for example, 0°C to -50°C or -20°C to -50°C.

[0049] What needs to be specifically explained here is that, in this invention, the atmospheric dew points for the first heating, second heating, and soaking are not technical features required for this application. In other words, even without special control of the atmospheric dew points for the first heating, second heating, and soaking, the elemental mass percentage content-depth distribution curve within a depth range of 0 to 1000 nm from the steel plate surface shows a Mn mass percentage peak. p ≤10.0%, Si mass percentage peak Si p Product characteristics of ≤4.0% can be obtained. This is because, during the first heating to form iron oxide, some Mn in the substrate oxidizes together with Fe, but the oxide formed by Si is mainly in the transition layer. During the second heating and soaking, when reduced iron is reduced, regardless of the dew point of the atmosphere, the selective oxidation formed by alloying elements such as Mn and Si in the substrate is mainly concentrated in the lower part of the ferrite formed after the reduction of iron oxide, i.e., the transition layer, and is not enriched in the ferrite surface layer. Therefore, the present invention differs from existing technologies that promote internal oxidation of alloying elements and reduce external oxidation by increasing the dew point of the reducing atmosphere.

[0050] However, the inventors have discovered through research that increasing the dew point of the oxidizing atmosphere during the first heating leads to a decrease in the T1 temperature. This is because increasing the dew point of the atmosphere during the first heating increases the internal oxidation of the alloying elements, reducing the oxidation of the alloying elements at the iron oxide / substrate interface, particularly the oxidation of Si. Therefore, it is preferable to control the dew point of the oxidizing atmosphere during the first heating to ≥-40°C. If the dew point of the second heating and soaking is too high, decarburization will occur, so it is preferable to control the dew point of the atmosphere during the second heating and soaking to ≤-20°C.

[0051] Furthermore, in the manufacturing method according to the present invention, the reducing atmosphere for the second heating and soaking is an N2+H2 mixed gas, of which the volume percentage of H2 is 0.5-20%.

[0052] Furthermore, in the manufacturing method according to the present invention, the heating rate for the second heating is 1 to 20°C / s, the second heating is carried out continuously from T1 to a soaking temperature of 720 to 920°C, and the soaking time is 30 to 200 s.

[0053] The soaking temperature T2 is controlled to 720-920°C and the soaking time to 30-200 s primarily to obtain appropriate mechanical properties for cold-rolled high-tensile steel, and is not significantly related to phosphate treatment properties. [Effects of the Invention]

[0054] The cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to the present invention has the following advantages.

[0055] This invention solves the problem of poor phosphate treatment characteristics of cold-rolled high-tensile steel by controlling the Mn and Si mass percentage peaks and their depth positions in the elemental mass percentage content-depth distribution curve inside the steel sheet, thereby improving the coating characteristics of cold-rolled high-tensile steel.

[0056] In some more preferred embodiments, the steel sheet with excellent phosphate treatment properties further has a tensile strength of ≥780 MPa. [Brief explanation of the drawing]

[0057] [Figure 1] This shows a comparison of the mass percentage content-depth distribution curves of the element Mn in the range of 0 to 1000 nm from the surface of the steel sheet for Example A1 and Comparative Example A1 of the present invention. [Figure 2] This shows a comparison of the mass percentage content-depth distribution curves of the Si element in the range of 0 to 1000 nm from the surface of the steel sheet for Example A1 and Comparative Example A1 of the present invention. [Figure 3] This shows a comparison of the mass percentage content-depth distribution curves of the element Fe in the range of 0 to 1000 nm from the surface of the steel sheet for Example A1 and Comparative Example A1 of the present invention. [Figure 4]This shows a comparison of the mass percentage content-depth distribution curves of the element Mn in the range of 0 to 1000 nm from the surface of the steel sheet for Example B1 and Comparative Example B1 of the present invention. [Figure 5] This shows a comparison of the mass percentage content-depth distribution curves of the Si element in the range of 0 to 1000 nm from the surface of the steel sheet for Example B1 and Comparative Example B1 of the present invention. [Figure 6] This shows a comparison of the mass percentage content-depth distribution curves of Fe element in the range of 0 to 1000 nm from the surface of the steel sheet for Example B1 and Comparative Example B1 of the present invention. [Modes for carrying out the invention]

[0058] The following interpretation and explanation will be made in conjunction with specific examples and the drawings of the specification, and will further describe the cold-rolled high-tensile steel sheet with excellent phosphate treatment properties and the method for manufacturing the same according to the present invention, but this interpretation and explanation will not inappropriately limit the embodiments of the present invention.

[0059] Table 1 shows the component ratios of the steel types used in each example and comparative example of the present invention.

[0060] [Table 1]

[0061] Based on the steel sheet composition shown in Table 1, steel sheets for each example and comparative example of the present invention were manufactured according to the following steps, and the specific process parameters for the continuous annealing process for each example and comparative example are shown in Table 2. (1) Conventional steelmaking methods will be used. (2) Conventional casting will be used. (3) Conventional hot rolling is used. (4) Conventional pickling and cold rolling are used. (5) Continuous annealing is employed. Cold-rolled steel sheet is heated for the first time in an oxidizing atmosphere. The heating rate for the first heating is 1 to 50°C / s, the first heating is performed from room temperature to T1, and the mass percentage content of T1 and Si satisfies 570 + 4000 × Si ≤ T1 ≤ 600 + 9000 × Si. Subsequently, a second heating and soaking are performed in a reducing atmosphere, the reducing atmosphere for the second heating being a mixed gas of N2 + H2, of which the volume percentage of H2 is 0.5 to 20%. After that, cooling and winding are performed.

[0062] In some of these examples, the oxidizing atmosphere for the first heating was a mixed gas of N2 + O2, with O2 accounting for 0.01 to 1% by volume.

[0063] In several other embodiments, the oxidizing atmosphere for the first heating is obtained by controlling the air-fuel ratio of the direct-fired furnace to 0.95-1.05.

[0064] In some of these examples, the dew point of the oxidizing atmosphere during the first heating was controlled to ≥-40°C, and the dew point of the reducing atmosphere during the second heating was controlled to ≤-20°C.

[0065] In some examples, the heating rate for the second heating was 1 to 20°C / s, and the second heating continued from T1 to a soaking temperature of 720 to 920°C, with a soaking time of 30 to 200 s.

[0066] [Table 2-1]

[0067] [Table 2-2]

[0068] In Table 2 above, the examples where the air-fuel ratio is " / " indicate that the oxidizing atmosphere used in that example was an N2+O2 mixed gas. The fact that the air-fuel ratio and oxygen content of Comparative Examples A1-A4, D1-D2, E1-E3, F1, and G1-G2 are all shown as " / " indicates that the first heating in these comparative examples did not involve an oxidizing atmosphere. The symbols "A", "B", ... "H", and "I" for each example and comparative example in Table 2 indicate that the steel sheet components corresponding to the numbers in Table 1 were used.

[0069] Table 3 shows the steel sheet properties of each example and comparative example manufactured using the steel sheet components A to I and the process parameters shown in Table 2, as well as the corresponding phosphate treatment properties of each steel sheet. p Si p t pSi t pMn Fe p This was detected by GDOES.

[0070] [Table 3-1]

[0071] [Table 3-2]

[0072] [Table 3-3]

[0073] Figures 1, 2, and 3 show the mass percentage content-depth distribution curves of Mn, Si, and Fe elements in the depth range of 0 to 1000 nm from the surface of the steel plate for Example A1 and Comparative Example A1, respectively. The curves are plotted with characteristic values ​​of Mn. p Si p Fe p and t p It is stated that t in Figure 1 p is t pMn This shows that t in Figure 2 p is tpSi This shows that t in Figure 3 p is t pSi This indicates.

[0074] According to Table 2, Example A1 underwent a first heating in an oxidizing atmosphere, heating to 580°C at a heating rate of 10°C / s. The oxidizing atmosphere was a mixed gas of N2 + O2, with an O2 volume percentage of 0.2% and a dew point of -50°C. Subsequently, a second heating and soaking was performed in a reducing atmosphere at a heating rate of 4°C / s, with a soaking temperature of 820°C and a soaking time of 100 s. The reducing atmosphere for the second heating and soaking was a mixed gas of N2 + H2, with an H2 volume percentage of 2% and an atmospheric dew point of -50°C.

[0075] Using GDOES, elemental depth distribution detection was performed on the surface of the annealed steel sheet, and the mass percentage content-depth distribution curves of Mn, Si, and Fe elements within the depth range of 0 to 1000 nm were obtained, as shown in Figures 1 to 3.

[0076] Comparative Example A1 underwent first heating, second heating, and soaking in a reducing atmosphere, with the heating rate, temperature, and time being the same as in Example A1. The mass percentage content-depth distribution curves of Mn, Si, and Fe elements in the 0-1000 nm depth range of the surface of Comparative Example A1 after annealing are shown in Figures 1 to 3.

[0077] When phosphate treatment was performed on Example A1 and Comparative Example A1 under the same conditions, Example A1 showed a uniform and dense phosphate film on the surface, and its phosphate treatment characteristics met the requirements (indicated by "○" in Table 2-2), while Comparative Example A1 showed a sparse phosphate film on the surface, with a large area not covered by the phosphate film, and its phosphate treatment characteristics did not meet the requirements (indicated by "×" in Table 2-2). In this specification, the superiority or inferiority of phosphate treatment characteristics is determined by observing the phosphate film on the surface of the steel sheet after phosphate treatment. Among these, phosphate film coverage rate 100%, phosphate crystal size <12 μm, phosphate film weight >2 g / m 2In this case, the phosphate treatment characteristics will meet the requirements. The phosphate treatment will be performed using a commercially available treatment agent, and the specific steps and processes are as follows: (1) Pre-degreasing spray, 45-55°C, 1 min. (2) Degreasing spray, 45-55°C, 2.0 min. (3) Water rinse spray twice, room temperature, 1 min. (4) Surface conditioning spray, room temperature, 1 min. (5) Phosphate treatment spray, 50-60°C, 2.0 min. (6) Water rinse spray, room temperature, 1 min. (7) Drying.

[0078] Figures 4, 5, and 6 show the mass percentage content-depth distribution curves of Mn, Si, and Fe elements in the depth range of 0 to 1000 nm from the surface of the steel plate in Example B1 and Comparative Example B1, respectively, with the characteristic value Mn p Si p Fe p and t p It is stated that t in Figure 4 p is t pMn This shows that t in Figure 5 p is t pSi This shows that t in Figure 6 p is t psi This indicates.

[0079] Example B1 underwent a first heating in an oxidizing atmosphere, heating to 650°C at a heating rate of 50°C / s. The oxidizing atmosphere was created by a direct-fired heating furnace, with the air-fuel ratio controlled to 0.95 and the dew point to 50°C. Subsequently, a second heating and soaking was performed in a reducing atmosphere with a heating rate of 5°C / s, a soaking temperature of 870°C, and a soaking time of 180 s. The atmosphere for the second heating and soaking was an N2+H2 mixed gas, with an H2 volume percentage of 2% and an atmospheric dew point of -40°C.

[0080] Elemental depth distribution detection was performed on the surface of the annealed steel sheet using GDOES, and the mass percentage content-depth distribution curves of Mn, Si, and Fe elements within the depth range of 0 to 1000 nm, as shown in Figures 4 to 6, were obtained. When the sample from Example B1 was treated with phosphate, the phosphate film was uniform and dense, and the phosphate treatment characteristics met the requirements.

[0081] Comparative Example B1 underwent first heating, second heating, and soaking in a reducing atmosphere. The reducing atmosphere was an N2-H2 mixed gas with a volume percentage of H2 of 2%, and the atmospheric dew point was -20°C. The soaking temperature and soaking time were the same as in Example B1. The mass percentage content-depth distribution curves of Mn, Si, and Fe elements in the depth range of 0 to 1000 nm from the surface of the annealed steel sheet of Comparative Example B1 are shown in Figures 4 to 6. When the sample of Comparative Example B1 was treated with phosphate, the phosphate film was uneven and not dense, with localized areas not covered by the phosphate film, and the phosphate treatment characteristics did not meet the requirements.

[0082] Furthermore, the inventors detected the microstructure of each example and comparative example by observing the cross-sectional metal structure, which had been corroded with a nitric acid alcohol solution, using a scanning electron microscope. Cross-sectional transmission electron microscope samples of the surface and transition layers were prepared using a focused ion beam microscope, the thickness of the surface layer was measured using a transmission electron microscope, and the types of oxides in the transition layer were analyzed. The detection results are shown in Table 4.

[0083] [Table 4-1]

[0084] [Table 4-2]

[0085] [Table 4-3]

[0086] [Table 4-4]

[0087] Furthermore, the inventors detected the tensile strength of each example and comparative example according to the tensile test of national standard GB / T 228.1, and the results are shown in Table 5.

[0088] [Table 5-1]

[0089] [Table 5-2]

[0090] [Table 5-3]

[0091] As a result, the cold-rolled steel sheet produced by this invention has excellent phosphate treatment properties and high strength.

[0092] Furthermore, the combination methods of each technical feature in this application are not limited to the combination methods described in the claims of this application or the combination methods described in the specific embodiments. All technical features described in this application can be freely combined or combined in any manner, as long as they do not contradict each other.

[0093] Furthermore, the embodiments described above are merely specific examples of the present invention. The present invention is not limited to the embodiments described above, and similar changes or modifications can be obtained from or easily conceived by those skilled in the art from the content disclosed herein, and are within the scope of protection of the present invention.

Claims

1. In the elemental mass percentage content-depth distribution curve within a depth range of 0 to 1000 nm from the surface of the steel plate, the Mn elemental mass percentage peak is Mn p ≤10.0%, Si mass percentage peak Si p ≤4.0%, and the peak Mn element mass percentage. p and Si element mass percentage peak Si p The cold-rolled high-tensile steel sheet is characterized by having phosphates located at a depth of 50 nm or more from the surface of the steel sheet, and exhibiting excellent phosphate treatment properties.

2. The aforementioned Mn elemental mass percent peak Mn p and Si element mass percentage peak Si p The cold-rolled high-tensile steel sheet with excellent phosphate treatment properties as described in claim 1, characterized in that all of these are located at a depth of 50 to 500 nm from the surface of the steel sheet.

3. In the elemental mass percentage-depth distribution curve within the depth range of 0 to 1000 nm from the surface of the steel sheet, the elemental mass percentage peak of Fe element, Fe p ≧85%, and the elemental mass percentage peak of Fe element, Fe p is within the depth range of 0 to t pSi from the surface of the steel sheet, where t pSi represents the depth position at which the elemental mass percentage peak of Si element, Si p appears. The cold-rolled high-tensile steel sheet excellent in phosphate treatment characteristics according to claim 1, characterized by the above.

4. The cold-rolled high-tensile steel sheet having excellent phosphate treatment properties, as described in any one of claims 1 to 3, wherein the steel sheet comprises a base layer, a surface layer, and a transition layer between the base layer and the surface layer in the thickness direction, and the microstructure of the surface layer is ferrite.

5. The cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to claim 4, characterized in that the thickness of the surface layer is 50 to 500 nm.

6. The cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to claim 4, characterized in that the microstructure of the base layer is martensite + ferrite, or martensite + ferrite + retained austenite.

7. The cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to claim 4, characterized in that the transition layer contains an oxide of Mn, Si, Fe and / or a composite oxide of Mn, Si, Fe.

8. The aforementioned oxides of Mn, Si, and Fe are MnO, MnO 2 SiO 2 Fe disaster, Fe 2 O 3 Fe 3 O 4 At least one of the above, and the composite oxide of Mn, Si, and Fe is MnSiO 3 Mn 2 SiO 4 Mn 2 FeO 3 Mn 2 FeO 4 FeSiO 3 Fe 2 SiO 3 Fe 2 SiO 4 A cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to claim 7, characterized in that it is at least one of the above.

9. A cold-rolled high-tensile steel sheet with excellent phosphate treatment properties as described in claim 1, characterized in that its tensile strength is ≥ 780 MPa.

10. Each of the following elements is present in the following mass percentages: That is, a cold-rolled high-tensile steel sheet with excellent phosphate treatment properties as described in claim 1, characterized by containing C: 0.07 to 0.30%, Si: 0.07 to 3.0%, and Mn: 1.5 to 5.0%.

11. The mass percentage content of that chemical element is, The cold-rolled high-tensile steel sheet with excellent phosphate treatment properties according to claim 10, characterized in that the composition is C: 0.07 to 0.30%, Si: 0.07 to 3.0%, Mn: 1.5 to 5.0%, and one or more of Nb, Ti, Al, B, Cr, Mo, Ni, and V, with the total mass percentage content of Nb, Ti, Al, B, Cr, Mo, Ni, and V being 2.0% or less, and the remainder being Fe and unavoidable impurities.

12. A method for manufacturing a cold-rolled high-tensile steel sheet with excellent phosphate treatment characteristics according to any one of claims 1 to 11, comprising the steps of steelmaking, casting, hot rolling, pickling, cold rolling, and continuous annealing, characterized in that in the continuous annealing step, the cold-rolled steel sheet is heated once in an oxidizing atmosphere, then heated a second time and soaked in a reducing atmosphere, and then cooled and coiled.

13. The oxidizing atmosphere for the first heating can be obtained by controlling the air-fuel ratio of the direct-fired furnace to 0.95-1.05, or the oxidizing atmosphere for the first heating can be N 2 +O 2 It is a gas mixture, of which O 2 The manufacturing method according to claim 12, characterized in that the volume percentage of is 0.01 to 1%.

14. The heating rate for the first heating cycle is 1 to 50°C / s, and the first heating cycle starts from room temperature. 1 Heated to and T 1 And the mass percentage content of Si is 570 + 4000 × Si ≤ T 1 The manufacturing method according to claim 12, characterized in that it satisfies ≤600 + 9000 × Si, and preferably T1 is 580 to 800°C.

15. The following characteristics, namely, The dew point of the oxidizing atmosphere during the first heating must be ≥ -50°C. The reducing atmosphere for the second heating and soaking is N 2 +H 2 It is a gas mixture, of which H 2 The volume percentage is between 0.5% and 20%. The dew point of the reducing atmosphere during the second heating and soaking must be ≤10°C. The heating rate for the second heating is 1-20°C / s, and the second heating is T 1 The manufacturing method according to claim 12, characterized by having one or more of the following: being continuously heated from to a soaking temperature of 720 to 920°C, and having a soaking time of 30 to 200 s.

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