Method for manufacturing annealed and pickled steel plate

Abstract

The present invention provides a method for producing annealed and pickled steel sheets that can consistently and stably produce annealed and pickled steel sheets that are excellent in chemical conversion treatability, corrosion resistance after painting, and surface appearance. In the present disclosure, the annealed and pickled steel sheets are continuously produced by continuously performing the following steps: an annealing step in which a cold-rolled steel sheet is passed through an annealing furnace to anneal the cold-rolled steel sheet in the annealing furnace to obtain an annealed steel sheet; a pickling step in which the annealed steel sheet discharged from the annealing furnace is passed through a mixed acid tank containing a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid to pickle the annealed steel sheet with the mixed acid solution; and a re-pickling step in which the annealed steel sheet discharged from the mixed acid tank is passed through an acid tank containing an acid solution containing a non-oxidizing third acid to re-pickle the annealed steel sheet with the acid solution to obtain an annealed and pickled steel sheet. Here, the Fe concentration in the mixed acid solution in the mixed acid tank is measured, and the lower limit of the ferrite fraction in the surface layer of the annealed steel sheet supplied to the pickling step is set based on the measured Fe concentration.

Description

Technical Field

[0001] The present invention relates to a method for continuously manufacturing annealed and pickled steel sheets by annealing, pickling and re-pickling cold-rolled steel sheets. Background Art

[0002] In recent years, from the perspective of protecting the global environment, there has been a strong demand for improved fuel efficiency and collision safety in automobiles, and for lighter and stronger automobile bodies. To meet these demands, efforts are underway to simultaneously achieve both lighter and stronger automobile bodies by increasing the strength and reducing the thickness (lighter) of cold-rolled steel sheets, the raw materials for automotive parts. However, since most automotive parts are manufactured by forming cold-rolled steel sheets, the raw materials for these parts are required to have both high strength and excellent formability.

[0003] One method for increasing the strength of cold-rolled steel sheets without significantly compromising formability is solid solution strengthening by adding Si. However, when a large amount of Si is added to cold-rolled steel sheets, a large amount of Si-containing oxides such as SiO2 and Si-Mn composite oxides are formed on the steel sheet surface during annealing after cold rolling, resulting in poor chemical conversion treatability and corrosion resistance after painting.

[0004] As a technology to address this problem, Patent Document 1 describes a method for continuously producing annealed and pickled steel sheets by continuously performing the following steps: a step of annealing a cold-rolled steel sheet to obtain an annealed steel sheet; a pickling step of immersing the annealed steel sheet in a mixed acid solution containing an oxidizing acid such as nitric acid and a non-oxidizing acid such as hydrochloric acid or hydrofluoric acid for pickling; and a re-pickling step of immersing the annealed steel sheet in an acid solution containing a non-oxidizing acid such as hydrochloric acid or sulfuric acid for re-pickling to obtain the annealed and pickled steel sheet. This method removes Si-containing oxides from the steel sheet surface generated by continuous annealing in the pickling step, and removes iron-based oxides generated in the pickling step in the re-pickling step. This method enables the production of annealed and pickled steel sheets with excellent chemical conversion treatability and corrosion resistance after painting.

[0005] However, when continuously producing annealed and pickled steel sheets using a two-step pickling process as described in Patent Document 1, the Fe concentration and temperature of the mixed acid solution increase over time, leading to an excessively fast pickling rate and, consequently, a problem of poor surface appearance of the produced annealed and pickled steel sheets. On the other hand, Patent Document 2 describes a method for continuously producing annealed and pickled steel sheets having excellent surface appearance by decreasing the concentration of the oxidizing acid in the mixed acid solution and increasing the concentration of the non-oxidizing acid as the Fe concentration in the mixed acid solution increases, thereby suppressing the increase in the mixed acid solution temperature.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2012-132092

[0009] Patent Document 2: International Publication No. 2017 / 007036 Summary of the Invention

[0010] However, the present inventors have conducted research and discovered that when continuously producing annealed and pickled steel sheets using a two-step pickling process as described in Patent Document 1, even when the method described in Patent Document 2 is applied, the surface appearance of the produced annealed and pickled steel sheets can deteriorate over time while maintaining the temperature of the mixed acid solution within an appropriate range. This deterioration in surface appearance has been found to result in a blackening of the steel sheet surface immediately after the pickling step, and this discoloration cannot be removed in the re-pickling step. Furthermore, it has been found that this discolored portion easily peels off from the steel sheet surface, forming a black powder that can easily cause equipment contamination.

[0011] Therefore, in view of the above problems, an object of the present invention is to provide a method for producing an annealed and pickled steel sheet capable of continuously and stably producing an annealed and pickled steel sheet excellent in chemical conversion treatability, corrosion resistance after painting, and surface appearance quality.

[0012] The present inventors have conducted research and discovered a correlation between the ferrite fraction in the surface layer of an annealed steel sheet submitted to the pickling process, the Fe concentration in the mixed acid solution during the pickling process, and the surface appearance quality of the resulting annealed and pickled steel sheet. Specifically, when an annealed steel sheet with a low ferrite fraction in the surface layer is submitted to the pickling process, Fe gradually dissolves from the annealed steel sheet into the mixed acid solution during the pickling process. As the Fe concentration in the mixed acid solution increases, the surface of the steel sheet after the pickling process tends to become black.

[0013] The present inventors investigated the reason and found that, compared with martensite and retained austenite, ferrite dissolves faster during pickling in a mixed acid solution, and this difference becomes more obvious when the Fe concentration in the mixed acid solution is high. Therefore, it can be seen that when an annealed steel sheet with a low ferrite fraction in the surface layer is supplied to a pickling process, as the Fe concentration in the mixed acid solution increases, the entire surface layer cannot be completely removed, and martensite and retained austenite remain on the surface of the steel sheet after the pickling process, forming surface unevenness, and the steel sheet surface appears black. In addition, due to the peeling of the martensite and retained austenite remaining on the surface of the steel sheet, black powder is generated. Therefore, in order to prevent the surface appearance quality from deteriorating even if the Fe concentration in the mixed acid solution increases, it is necessary to set the lower limit of the ferrite fraction in the surface layer of the annealed steel sheet supplied to the pickling process according to the Fe concentration in the mixed acid solution. It should be noted that in the present invention, "Fe concentration" refers to the iron ion concentration, specifically, the sum of the concentrations of iron (II) ions and iron (III) ions.

[0014] The gist of the present invention completed based on the above findings is as follows.

[0015] [1] A method for manufacturing an annealed and pickled steel sheet, characterized in that the annealed and pickled steel sheet is continuously manufactured by continuously performing the following steps:

[0016] Annealing process: passing the cold rolled steel sheet through an annealing furnace, annealing the cold rolled steel sheet in the annealing furnace to obtain an annealed steel sheet.

[0017] a pickling step of passing the annealed steel sheet discharged from the annealing furnace through a mixed acid tank containing a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid, and pickling the annealed steel sheet in the mixed acid solution; and

[0018] a re-pickling step of passing the annealed steel sheet discharged from the mixed acid tank into an acid tank containing an acid solution containing a non-oxidizing third acid, and re-pickling the annealed steel sheet in the acid solution to obtain an annealed pickled steel sheet;

[0019] wherein the Fe concentration in the mixed acid solution in the mixed acid tank is measured,

[0020] The lower limit value of the ferrite fraction in the surface layer of the annealed steel sheet subjected to the pickling step is set based on the measured Fe concentration.

[0021] [2] The method for manufacturing an annealed and pickled steel sheet according to [1], wherein the lower limit of the ferrite fraction in the surface layer of the annealed steel sheet subjected to the pickling step is set higher as the Fe concentration increases.

[0022] [3] The method for manufacturing an annealed and pickled steel sheet according to [1] or [2], wherein the lower limit of the ferrite fraction is set by adjusting the annealing conditions in the annealing step.

[0023] [4] The method for manufacturing an annealed and pickled steel sheet according to [3], wherein the annealing condition is a dew point in a soaking zone of the annealing furnace.

[0024] [5] The method for producing an annealed and pickled steel sheet according to any one of [1] to [4], wherein the first acid is nitric acid.

[0025] [6] The method for manufacturing an annealed and pickled steel sheet according to any one of [1] to [5] above, wherein the second acid is one or more selected from hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid.

[0026] [7] The method for manufacturing an annealed and pickled steel sheet according to any one of [1] to [6] above, wherein the third acid is one or more selected from hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid and oxalic acid.

[0027] [8] The method for manufacturing an annealed and pickled steel sheet according to any one of [1] to [7], wherein the cold-rolled steel sheet has a composition containing 0.50 to 3.00 mass% of Si.

[0028] [9] The method for manufacturing an annealed and pickled steel sheet according to the above [8], wherein the above-mentioned composition, in terms of mass %, contains C: 0.03-0.45%, Si: 0.50-3.00%, Mn: 0.5-5.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.001-0.060%, N: 0.005% or less and B: 0.001-0.005%, and the remainder is Fe and unavoidable impurities.

[0029]

[10] The method for manufacturing an annealed and pickled steel sheet according to [9], wherein the above-mentioned component composition, in terms of mass %, further contains at least one selected from the group consisting of Cu: 1.00% or less, Nb: 0.050% or less, Ti: 0.080% or less, V: 0.5% or less, Mo: 1.00% or less, Cr: 1.000% or less, and Ni: 1.00% or less.

[0030] According to the method for producing an annealed and pickled steel sheet of the present invention, an annealed and pickled steel sheet having excellent chemical conversion treatability, corrosion resistance after painting, and surface appearance quality can be produced continuously and stably. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of an annealing and pickling facility capable of implementing the method for producing an annealed and pickled steel sheet according to one embodiment of the present invention.

[0032] Figure 2 This is a graph showing the relationship between the Fe concentration in the mixed acid solution, the ferrite fraction in the surface layer of the annealed steel sheet, and the surface appearance quality of the annealed and pickled steel sheet in Examples. DETAILED DESCRIPTION

[0033] A method for producing an annealed and pickled steel sheet according to an embodiment of the present invention continuously produces the annealed and pickled steel sheet by continuously performing an annealing step, a pickling step, and a re-pickling step, which will be described in detail below.

[0034] In this embodiment, the annealing process, the pickling process and the re-pickling process are performed by Figure 1 The annealing and pickling equipment shown are carried out continuously on the same line. Figure 1The exemplary annealing and pickling equipment includes, in order from upstream to downstream in the direction of steel sheet travel, a continuous annealing furnace (CAL) 10, a water tank 20 containing water, a mixed acid tank 30 containing a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid, a water tank 40 containing water, an acid tank 50 containing an acid solution containing a non-oxidizing third acid, and a water tank 60. A sheet feeding system including a plurality of rollers 70 can sequentially feed the steel sheet through the continuous annealing furnace 10 and the five tanks described above.

[0035] [Annealing process]

[0036] refer to Figure 1 In the annealing process, the cold rolled steel sheet S1 is passed through the continuous annealing furnace 10, and the cold rolled steel sheet S1 is annealed in the continuous annealing furnace 10 to obtain the annealed steel sheet S2. The annealing process is performed to give the cold rolled steel sheet S1 the desired structure, strength and workability. The annealing furnace 10 can have multiple zones. Figure 1 In the example of FIG, there are a heating zone 12, a soaking zone 14 and a cooling zone 16 in order from the upstream of the plate passing direction. The structure of the continuous annealing furnace is not limited to Figure 1 For example, a preheating zone may be provided upstream of the heating zone 12 , the cooling zone 16 may include multiple cooling zones, and an over-aging zone may be provided downstream of the cooling zone 16 .

[0037] In the heating zone 12, the cold-rolled steel sheet S1 can be directly heated using a burner, or indirectly heated using a radiant tube (RT) or an electric heater. The internal average temperature of the heating zone 12 is preferably 500 to 800°C. While the gas from the soaking zone 14 flows into the heating zone 12, a non-oxidizing or reducing gas is supplied from other paths. As a non-oxidizing gas, for example, N2 gas can be used, and as a reducing gas, for example, an H2-N2 mixed gas can be used. The dew point of the heating zone 12 is preferably in the range of -50 to 20°C.

[0038] In the soaking zone 14, a radiant tube (RT) can be used to indirectly heat the cold-rolled steel sheet S1. The average temperature (soaking temperature) inside the soaking zone 14 is preferably 600 to 950°C. A non-oxidizing or reducing gas is supplied to the soaking zone 14. As the non-oxidizing gas, for example, N2 gas can be used, and as the reducing gas, for example, an H2-N2 mixed gas can be used. The dew point of the soaking zone 14 is preferably within the range of -50 to 20°C.

[0039] The cold-rolled steel sheet S1 is cooled in the cooling zone 16. The cold-rolled steel sheet S1 is cooled to about 100 to 400°C at the stage of leaving the continuous annealing furnace 10.

[0040] [Pickling process]

[0041] refer to Figure 1 In the pickling process, the annealed steel sheet S2 discharged from the annealing furnace 10 is passed through a mixed acid tank 30 containing a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid, and is pickled in the mixed acid solution.

[0042] As mentioned above, during the annealing process, a non-oxidizing or reducing gas can be used as the atmosphere gas, and its dew point is strictly controlled. Therefore, in ordinary cold-rolled steel sheets with a small amount of alloying additions, oxidation of the steel sheet surface is suppressed. However, in the case of cold-rolled steel sheets containing Si and Mn, elements that are more easily oxidized than Fe, even if the composition and dew point of the atmosphere gas during annealing are strictly controlled, Si and Mn are selectively oxidized, forming Si-containing oxides such as Si oxide (SiO2) and Si-Mn composite oxides on the steel sheet surface. In other words, the surface layer of the annealed steel sheet becomes a Si-containing oxide layer, which leads to deterioration of chemical conversion treatability and corrosion resistance after painting.

[0043] Therefore, in the pickling step of this embodiment, the annealed steel sheet S2 is continuously immersed in a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid to remove the Si-containing oxide layer on the surface of the annealed steel sheet S2. The thickness of the Si-containing oxide layer varies depending on the steel sheet composition and annealing conditions (temperature, time, and atmosphere), but is generally about 1 μm from the steel sheet surface.

[0044] Nitric acid is an example of an oxidizing first acid. The reason a first acid is required in the mixed acid solution is that, among Si-containing oxides, Si-Mn composite oxides readily dissolve in acid, but SiO2 is poorly soluble. Therefore, to remove it, an oxidizing acid such as nitric acid is required to remove the Si-containing oxide layer on the steel sheet surface along with the underlying iron.

[0045] From the perspective of effectively removing the Si-containing oxide layer, the concentration of the first acid in the mixed acid solution is preferably 100 g / L or higher, and more preferably 110 g / L or higher. On the other hand, if the concentration of the first acid is too high, the iron-based oxides will be difficult to dissolve in the subsequent re-pickling step. Therefore, the concentration of the first acid in the mixed acid solution is preferably 150 g / L or lower, and more preferably 140 g / L or lower.

[0046] The non-oxidizing second acid is preferably selected from one or more of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid, and particularly preferably selected from one or more of hydrochloric acid, sulfuric acid, and hydrofluoric acid. The reason for using such a non-oxidizing acid is to suppress the formation of iron-based oxides precipitated on the surface of the steel plate during pickling with the oxidizing first acid.

[0047] From the perspective of facilitating the dissolution of iron-based oxides in the subsequent re-pickling step, the concentration of the second acid in the mixed acid solution is preferably 4.5 g / L or higher, more preferably 6.5 g / L or higher. On the other hand, if the concentration of the second acid is too high, the pickling weight loss per unit time decreases. Because there is a concern about residual SiO2 on the surface of the steel plate, the concentration of the second acid in the mixed acid solution is preferably 12.5 g / L or lower, more preferably 8.5 g / L or lower.

[0048] The optimal pickling time in the pickling step is determined by the weight loss required to remove the Si-containing oxide layer formed during the annealing step, the pickling efficiency determined by the composition of the mixed acid solution, and the pickling duration. Generally, the temperature of the mixed acid solution is around 30-60°C, and the pickling time is around 10 seconds.

[0049] [Re-pickling process]

[0050] refer to Figure 1 In the re-pickling process, the annealed steel sheet S2 discharged from the mixed acid tank 30 is passed through the acid tank 50 containing the acid solution containing the non-oxidizing third acid, and the annealed steel sheet S2 is re-pickled in the acid solution to obtain the annealed pickled steel sheet S3.

[0051] During the pickling process, Fe dissolved from the surface of the steel sheet generates iron oxides, which precipitate on the surface of the steel sheet and cover the surface of the steel sheet, thereby reducing the chemical conversion treatability. Therefore, in this embodiment, after the pickling process, the annealed steel sheet S2 is continuously immersed in an acid solution containing a non-oxidizing third acid to remove the iron oxides. "Iron oxides" refer to oxides mainly composed of iron, in which the atomic concentration ratio of iron among the elements constituting the oxide other than oxygen is 30% or more. These iron oxides are present on the surface of the steel sheet with uneven thickness, and are different from the natural oxide film that exists in a layered form with a uniform thickness of several nanometers. It should be noted that the iron oxides generated on the surface of the annealed steel sheet S2 are amorphous, as shown by the results of observation using a transmission electron microscope (TEM) and analysis of the diffraction pattern (diffraction pattern) by electron beam diffraction.

[0052] The non-oxidizing third acid is preferably selected from one or more of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid and oxalic acid, and is particularly preferably selected from one or more of hydrochloric acid, sulfuric acid and hydrofluoric acid. Wherein, since hydrochloric acid is a volatile acid, it is not easy to leave residues such as sulfate radicals on the steel plate surface like sulfuric acid, and the destructive effect of chloride ions on iron-based oxides is large, so it is preferred. In addition, a mixed acid of hydrochloric acid and sulfuric acid can also be used. In addition, the second acid used in the pickling process and the third acid used in this process can be the same type of acid or different types of acid. However, from the viewpoint that manufacturing equipment can be universalized, the same type of acid is preferably used.

[0053] From the perspective of fully dissolving iron-based oxides, the concentration of the third acid in the acid solution is preferably 4.5 g / L or higher, and more preferably 6.5 g / L. On the other hand, if the concentration of the third acid is too high, there is a concern that the acid solution may remain on the surface of the steel sheet and cause discoloration. Therefore, the concentration of the third acid in the acid solution is preferably 12.5 g / L or lower, and more preferably 8.5 g / L or lower.

[0054] The optimal pickling time for the re-pickling step is determined by the pickling efficiency and pickling length, which are determined by the weight loss and acid composition required to remove the iron-based oxides produced in the first pickling step. Generally speaking, the acid solution temperature is around 30-60°C and the pickling time is around 10 seconds.

[0055] The total weight loss of pickling in the pickling step and the re-pickling step is preferably 8 g / m 2 If the total pickling weight loss is 8g / m 2 As described above, Si-containing oxides and iron-based oxides are less likely to remain on the surface of the steel sheet, and thus higher chemical conversion treatability can be obtained.

[0056] [Process after the pickling process]

[0057] The annealed and pickled steel sheet S3 obtained after the pickling step is subjected to subsequent general processing steps such as temper rolling and leveling mill processing to obtain a steel sheet as a product.

[0058] [Water washing process]

[0059] As in this embodiment, it is preferred to provide a water tank 40 between the mixed acid tank 30 and the acid tank 50. Between the pickling step and the re-pickling step, the annealed steel sheet S2 is passed through the water tank 40 and rinsed with water. This prevents the mixed acid solution carried by the annealed steel sheet S2 from the mixed acid tank 30 from mixing with the acid solution in the acid tank 50. Therefore, re-pickling in the acid tank 50 is preferred because it reliably removes iron-based oxides. Furthermore, it is preferred to provide a water tank 20 upstream of the mixed acid tank 30 and pass the annealed steel sheet S2 through the water tank 20 and rinse it with water before the pickling step. This removes impurities from the surface of the annealed steel sheet S2 and prevents impurities from mixing with the mixed acid solution in the mixed acid tank 30. Furthermore, it is preferred to provide a water tank 60 downstream of the acid tank 50 and pass the annealed steel sheet S2 through the water tank 60 and rinse it after the re-pickling step. Thus, the acid liquid carried out from the acid tank 50 by the annealed steel sheet S2 can be removed, and rust on the surface of the annealed steel sheet S2 can be prevented.

[0060] [Determination of Fe concentration]

[0061] The Fe concentration in the mixed acid solution in the mixed acid tank 30 is zero in its fresh state, before being used for pickling the annealed steel sheet S2. However, during the pickling process, Fe gradually dissolves from the annealed steel sheet S2, and the Fe concentration in the mixed acid solution gradually increases. Therefore, in this embodiment, the Fe concentration in the mixed acid solution in the mixed acid tank 30 is measured over time. This measurement can be performed continuously or intermittently at regular intervals.

[0062] Figure 1 The annealing and pickling equipment illustrated in the example includes an Fe concentration meter 80 for measuring the Fe concentration in the mixed acid solution in the mixed acid tank 30. As the Fe concentration meter 80, for example, an analyzer using near-infrared spectroscopy can be used, which irradiates the mixed acid solution with near-infrared rays at intervals of one minute and calculates the Fe concentration in the mixed acid solution based on the change in the spectrum after irradiation.

[0063] The Fe concentration of the mixed acid solution is preferably maintained within the range of 0 to 50 g / L. If the Fe concentration exceeds 50 g / L, the mixed acid solution's oxidizing power becomes excessive, causing yellowing on the steel plate surface. Therefore, it is preferable to maintain the Fe concentration of the mixed acid solution at 50 g / L. Alternatively, the mixed acid solution in the mixed acid tank 30 should be completely discarded before this point, and fresh mixed acid solution should be supplied to the mixed acid tank 30 to reduce the Fe concentration of the mixed acid solution to zero. Alternatively, a portion of the mixed acid solution in the mixed acid tank 30 should be discarded, and fresh mixed acid solution should be added to the mixed acid tank 30 to reduce the Fe concentration of the mixed acid solution.

[0064] [Setting the lower limit of the surface ferrite fraction]

[0065] As mentioned above, the thickness of the Si-containing oxide layer after continuous annealing is generally about 1 μm from the surface of the steel sheet, and this Si-containing oxide layer needs to be removed during the pickling process. However, within this Si-containing oxide layer, the Si-containing oxide is concentrated within the topmost layer, within a few tens of nanometers. In the region below this, the Si-containing oxide is contained within the steel structure. Regarding this steel structure, ferrite dissolves faster during pickling in a mixed acid solution than martensite and retained austenite, and this difference is more pronounced when the Fe concentration in the mixed acid solution is high. Therefore, when an annealed steel sheet with a low ferrite fraction in the surface layer is supplied to the pickling process, as the Fe concentration in the mixed acid solution increases, the entire surface layer cannot be completely removed, and martensite and retained austenite remain on the surface of the steel sheet after the pickling process, forming surface irregularities and giving the steel sheet a black surface appearance.

[0066] Therefore, in this embodiment, it is important to set the lower limit of the ferrite fraction in the surface layer of the annealed steel sheet S2 supplied to the pickling process based on the measured Fe concentration in the mixed acid solution. Specifically, as the measured Fe concentration in the mixed acid solution increases, the lower limit of the ferrite fraction in the surface layer of the annealed steel sheet S2 supplied to the pickling process is set higher. Furthermore, only annealed steel sheets with a ferrite fraction in the surface layer exceeding the set lower limit are supplied to the pickling process. This prevents the surface of the steel sheet from becoming blackened after the pickling process, even if the Fe concentration in the mixed acid solution increases. This allows for the consistent and stable production of annealed and pickled steel sheets with excellent chemical conversion treatability, corrosion resistance after painting, and surface appearance.

[0067] For example, as in the examples described later, the relationship between the Fe concentration in the mixed acid solution and the lower limit of the surface ferrite fraction that can achieve excellent surface appearance quality at this Fe concentration is determined in advance (for example, Figure 2 ), using this relationship, the lower limit of the ferrite fraction in the surface layer of the annealed steel sheet S2 supplied to the pickling process can be set based on the Fe concentration in the measured mixed acid solution. Figure 2 Based on the relationship shown in FIG. 1 , the lower limit of the surface ferrite fraction may be set according to the measured Fe concentration so that the surface appearance quality is “◯” or “◎”.

[0068] The ferrite fraction (volume fraction) in the surface layer of the annealed steel sheet can be measured as follows. That is, a test piece is collected from any position relative to the width direction and the rolling direction of the steel sheet, a longitudinal section parallel to the rolling direction is ground, and the metal structure that appears by corrosion with a nitric acid solution is observed using an SEM within a range from the surface of the test piece to 1 μm. In the SEM observation, more than 10 fields of view are observed at a magnification of 3000 times, and the area fraction of the low grayscale area in the observed image is measured as ferrite, and the average value is calculated. There is no structural change in the direction perpendicular to the rolling direction (width direction). Since the area fraction of the section parallel to the rolling direction is equal to the volume fraction, the area fraction can be regarded as the volume fraction.

[0069] However, because the annealing, pickling, and re-pickling steps are performed continuously on the same line, in actual operation, it is difficult to measure the ferrite fraction in the surface layer of the annealed steel sheet S2 after the annealing step and before the pickling step. The ferrite fraction in the surface layer of the annealed steel sheet S2 is determined by the chemical composition of the cold-rolled steel sheet S1 before annealing and the annealing conditions during the annealing step. Once the chemical composition is determined, it is determined by the annealing conditions. Therefore, the lower limit of the ferrite fraction can be set by adjusting the annealing conditions during the annealing step. In other words, based on the chemical composition of the cold-rolled steel sheet S1, the relationship between the annealing conditions during the annealing step and the ferrite fraction obtained under these annealing conditions is determined in advance. The annealing step can then be performed under annealing conditions that achieve a ferrite fraction above the set lower limit.

[0070] The annealing conditions that affect the ferrite fraction in the surface layer of the annealed steel sheet S2 mainly include the soaking temperature and the dew point in the soaking zone. The higher the soaking temperature, the higher the ferrite fraction. Furthermore, the higher the dew point in the soaking zone, the higher the ferrite fraction. Therefore, the annealing conditions adjusted to achieve the desired ferrite fraction can be either the soaking temperature or the dew point in the soaking zone, or both.

[0071] However, since the soaking temperature also affects the mechanical properties of the final annealed and pickled steel sheet, it should not be changed during operation. Therefore, the annealing conditions adjusted to achieve the desired ferrite fraction are preferably the dew point of the soaking zone of the annealing furnace. For example, by fixing the annealing conditions except for the dew point, including the soaking temperature, and determining in advance the relationship between the dew point of the soaking zone during the annealing process and the ferrite fraction achieved at that dew point, the annealing process can be performed at a dew point that achieves a ferrite fraction above the set lower limit.

[0072] [Composition of cold-rolled steel sheets]

[0073] Hereinafter, the chemical composition of the cold-rolled steel sheet S1 will be described. The unit of the content of each element is "mass %" and is simply expressed as "%.

[0074] Si: 0.50~3.00%

[0075] The composition of the cold-rolled steel sheet S1 is not particularly limited, but preferably has a composition containing 0.50 to 3.00 mass% of Si. Since Si has a large effect on improving the strength of steel (solid solution strengthening ability) and does not seriously damage the workability, it is an effective element for achieving high strength of steel, but it is also an element that has an adverse effect on chemical conversion treatability and corrosion resistance after painting. From the viewpoint of adding Si to achieve high strength, the amount of Si is preferably 0.50% or more, more preferably 0.80% or more. On the other hand, when the amount of Si is too much, the hot rolling and cold rolling properties are greatly reduced, which has an adverse effect on productivity and causes the ductility of the steel sheet itself to decrease. Therefore, the amount of Si is preferably 3.00% or less, more preferably 2.50% or less.

[0076] Components other than Si are not particularly limited as long as they are within the composition range of ordinary cold-rolled steel sheets. However, the following composition is preferred.

[0077] C: 0.03~0.45%

[0078] C is an element effective in adjusting the strength of steel. From this perspective, the C content is preferably 0.03% or more, more preferably 0.05% or more. On the other hand, from the perspective of not degrading weldability, the C content is preferably 0.45% or less, more preferably 0.20% or less.

[0079] Mn: 0.5-5.0%

[0080] Mn is an element effective in improving strength and hardenability. From this point of view, the Mn content is preferably 0.5% or more, more preferably 1.0% or more. On the other hand, from the viewpoint of not reducing ductility and weldability, the Mn content is preferably 5.0% or less, more preferably 3.0% or less.

[0081] P: 0.05% or less

[0082] P is an unavoidable element. To avoid local ductility degradation, the P content is preferably 0.05% or less, more preferably 0.02% or less. It is preferable to minimize the P content, and there is no lower limit. However, from the perspective of dephosphorization costs, the P content may be 0.005% or more.

[0083] S: 0.005% or less

[0084] S is an unavoidable element. To avoid degrading weldability, the S content is preferably 0.005% or less. It is preferable to minimize the S content, and there is no lower limit. However, from the perspective of desulfurization costs, the S content can be 0.0001% or more.

[0085] Al: 0.001~0.060%

[0086] Al is an element effective for deoxidation of molten steel. From this viewpoint, the Al content is preferably 0.001% or more, more preferably 0.020% or more. On the other hand, from the viewpoint of cost, the Al content is preferably 0.060% or less.

[0087] N: 0.005% or less

[0088] Nitrogen forms coarse precipitates, which deteriorates bendability. Therefore, the amount of Nitrogen is preferably 0.005% or less. It is preferable to reduce the amount of Nitrogen as much as possible, and there is no lower limit. However, in industrial applications, the amount of Nitrogen can be 0.001% or more.

[0089] B: 0.001~0.005%

[0090] B is an element effective in improving hardenability, and from this viewpoint, the B content is preferably 0.001% or more. On the other hand, if the B content is too high, the hardenability-improving effect is saturated, so the B content is preferably 0.005% or less.

[0091] In the chemical composition of the cold-rolled steel sheet S1, the remainder other than the above-mentioned components is Fe and inevitable impurities. However, at least one of the following components may be optionally contained.

[0092] Cu: 1.00% or less

[0093] Cu promotes the formation of retained γ phase, contributing to effective improvement in strength. From this perspective, the Cu content is preferably 0.05% or more. On the other hand, from a cost perspective, when Cu is added, the Cu content is set to 1.00% or less.

[0094] Nb: 0.050% or less

[0095] Nb contributes to improving strength. From this viewpoint, the Nb content is preferably 0.005% or more. On the other hand, from the viewpoint of cost, when Nb is added, the Nb content is set to 0.050% or less.

[0096] Ti: 0.080% or less

[0097] Ti contributes to improving strength. From this perspective, the Ti content is preferably 0.005% or more. On the other hand, to prevent deterioration of chemical conversion treatability, when Ti is added, the Ti content is set to 0.080% or less.

[0098] V: 0.5% or less

[0099] V is effective in improving delayed fracture resistance. From this perspective, the V content is preferably 0.004% or more. On the other hand, to avoid deteriorating the strength-ductility balance, when V is added, the V content is set to 0.5% or less, preferably 0.1% or less, and more preferably 0.05% or less.

[0100] Mo: 1.00% or less

[0101] Mo contributes to improving strength. From this viewpoint, the amount of Mo is preferably 0.05% or more. On the other hand, from the viewpoint of cost, when Mo is added, the amount of Mo is set to 1.00% or less.

[0102] Cr: 1.000% or less

[0103] Cr contributes to improving hardenability. From this perspective, the Cr content is preferably 0.001% or more. On the other hand, to prevent deterioration of weldability, when Cr is added, the Cr content is set to 1.000% or less.

[0104] Ni: 1.00% or less

[0105] Ni promotes the formation of a retained γ phase. From this perspective, the Ni content is preferably 0.05% or more. On the other hand, from a cost perspective, when Ni is added, the Ni content is set to 1.00% or less.

[0106] Example

[0107] Cold-rolled steel sheets having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) were used as test pieces and annealed. Annealing conditions included a furnace atmosphere with a dew point of the values ​​shown in Table 2, containing 10% hydrogen by volume, and the balance being nitrogen. The soaking temperature shown in Table 2 was set for a soaking time of 120 seconds. This setting was based on the assumption of annealing in the soaking zone of the CAL.

[0108] The ferrite fraction in the surface layer of the obtained annealed steel sheet test piece was determined by the above-mentioned method. The results are shown in Table 2.

[0109] The annealed steel sheet test pieces were then pickled by immersion in the mixed acid solution shown in Table 3 (nitric acid concentration: 125 g / L, hydrochloric acid or hydrofluoric acid concentration: 7.5 g / L, solution temperature: 30°C) for 15 seconds. Furthermore, the test pieces were pickled again by immersion in hydrochloric acid (concentration: 8.0 g / L) for 8 seconds. The annealed and pickled steel sheet test pieces obtained in this manner were rinsed with water and dried.

[0110] <Surface appearance quality evaluation>

[0111] Cellophane tape (registered trademark) was applied to the surface of a test piece of annealed and pickled steel sheet and pulled apart. The tape was then attached to a piece of white paper. The whiteness (L value) was measured in this state to evaluate the surface appearance quality. Lower whiteness values, with black powder adhering to the tape, indicate poorer surface appearance quality. Surface appearance quality was evaluated according to the following criteria, with ◎ and ○ indicating good. The results are shown in Table 3.

[0112] ◎: L value is 90 or more

[0113] ○: L value is 75 or more and less than 90

[0114] ×: L value is less than 75

[0115] <Evaluation of chemical conversion treatability>

[0116] Test pieces of the annealed and pickled steel sheets were zinc phosphate treated to evaluate chemical conversion treatability.

[0117] (1) Zinc phosphate treatment liquid: Palbond PB-L3065 manufactured by Nihon Parkerizing Co., Ltd.

[0118] (2) Chemical conversion treatment process: degreasing → water washing → surface adjustment → zinc phosphate treatment → water washing

[0119] The surface of the test piece after chemical conversion treatment was observed under a SEM at 1000x magnification, and the formation of phosphate crystals was examined in 10 fields of view. A score of "◎" was given when phosphate crystals were uniformly formed in all 10 fields of view; "○" was given when phosphate crystals were not formed in one field of view; "Δ" was given when phosphate crystals were not formed in 2 to 5 fields of view; and "×" was given when phosphate crystals were not formed in 5 or more fields of view. The results are shown in Table 3.

[0120] <Evaluation of Corrosion Resistance after Painting>

[0121] Annealed and pickled steel sheet test pieces were chemically converted under the above conditions. Furthermore, the surface of the chemical conversion coating was electroplated with V-50, an electroplating coating manufactured by Nippon Paint Co., Ltd., to a thickness of 25 μm. A 45 mm long transverse cut was made on the surface of the test piece with a knife. The test piece was then subjected to a corrosion test repeating 90 cycles of salt spray (5% by mass NaCl aqueous solution at 35°C, relative humidity: 98%) for 2 hours, then drying (60°C, relative humidity: 30%) for 2 hours, then wetting (50°C, relative humidity: 95%) for 2 hours. After rinsing and drying, the cut site was subjected to a tape peel test. The maximum full width of the peel was measured on the left and right sides of the cut site. If the maximum full width was 6.0 mm or less, the post-coating corrosion resistance was considered good. A maximum full width of 6.0 mm or less was rated "○," while a maximum full width of 6.0 mm or more was rated "X." The results are shown in Table 3.

[0122]

[0123] Table 2

[0124]

[0125] Table 3

[0126]

[0127] Based on the results in Table 3, for each Fe concentration in the mixed acid solution, the surface ferrite fraction at the boundary between the surface appearance quality of "○" and "×" and the surface ferrite fraction at the boundary between the surface appearance quality of "◎" and "○" are summarized in Table 4. In addition, based on the results in Table 3, Figure 2 Graphs showing the relationship between the Fe concentration in the mixed acid solution, the ferrite fraction in the surface layer of the annealed steel sheet, and the surface appearance quality of the annealed and pickled steel sheet are shown in FIG.

[0128] Table 4

[0129]

[0130] According to Table 4 and Figure 2It is shown that, in the case of a cold-rolled steel sheet having the composition shown in Table 1, the following standards can be determined, for example, (1) if the Fe concentration is in the range of 5 g / L or less, the surface ferrite fraction is preferably set to 60% or more, more preferably 75% or more, (2) if the Fe concentration is in the range of more than 5 g / L and 10 g / L, the surface ferrite fraction is preferably set to 75% or more, more preferably 85% or more, (3) if the Fe concentration is in the range of more than 10 g / L and 20 g / L, the surface ferrite fraction is preferably set to 90% or more, more preferably 95% or more, (4) if the Fe concentration is in the range of more than 20 g / L and 30 g / L, the surface ferrite fraction is preferably set to 95% or more, more preferably 99% or more, (5) if the Fe concentration is in the range of more than 30 g / L and 50 g / L, the surface ferrite fraction is preferably set to 99% or more. Alternatively, using Figure 2 , the surface ferrite fraction is set according to the measured Fe concentration so that the surface appearance quality is "○" or "◎".

[0131] Furthermore, as an example of an operation for achieving the standards shown in Table 4, when the soaking temperature is 820°C, the following standards can be used: (1) If the Fe concentration is in the range of 5 g / L or less, the dew point is set to be above -15°C; (2) If the Fe concentration exceeds 5 g / L and is below 10 g / L, the dew point is set to be above -10°C; (3) If the Fe concentration exceeds 10 g / L and is below 20 g / L, the dew point is set to be above -5°C; (4) If the Fe concentration exceeds 20 g / L and is below 30 g / L, the dew point is set to be above 5°C; (5) If the Fe concentration exceeds 30 g / L and is below 50 g / L, the dew point is set to be above 15°C.

[0132] Industrial applicability

[0133] The method for producing annealed and pickled steel sheets according to the present invention enables the consistent and stable production of annealed and pickled steel sheets that exhibit excellent chemical conversion properties, corrosion resistance after painting, and surface appearance. Therefore, the annealed and pickled steel sheets produced according to the present invention are suitable for automotive body parts, home appliance components, and architectural components.

[0134] Explanation of symbols

[0135] 10 Continuous annealing furnace

[0136] 12 heating zones

[0137] 14. Soaking zone

[0138] 16 Cooling Zone

[0139] 20 sink

[0140] 30 mixed acid tank

[0141] 40 sink

[0142] 50 Acid Tank

[0143] 60 sink

[0144] 70 rollers (plate passing equipment)

[0145] 80 Fe concentration meter

[0146] S1 Cold-rolled steel sheet

[0147] S2 annealed steel plate

[0148] S3 annealed and pickled steel plate.

Claims

1. A method for manufacturing annealed and pickled steel sheets, characterized in that: The method is to continuously manufacture the annealed and pickled steel plate by continuously performing the following steps: Annealing process: passing the cold rolled steel sheet through an annealing furnace, annealing the cold rolled steel sheet in the annealing furnace to obtain an annealed steel sheet. A pickling step of passing the annealed steel sheet discharged from the annealing furnace through a mixed acid tank containing a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid, and pickling the annealed steel sheet with the mixed acid solution; as well as a re-pickling step of passing the annealed steel sheet discharged from the mixed acid tank into an acid tank containing an acid solution containing a non-oxidizing third acid, and re-pickling the annealed steel sheet with the acid solution to obtain an annealed pickled steel sheet; wherein the Fe concentration in the mixed acid solution in the mixed acid tank is measured, As the measured Fe concentration increases, the lower limit of the ferrite fraction of the surface layer of the annealed steel sheet supplied to the pickling step is set higher. Only the annealed steel sheet having a ferrite fraction in the surface layer equal to or greater than a set lower limit is subjected to the pickling step.

2. The method for manufacturing an annealed and pickled steel sheet according to claim 1, wherein: The lower limit of the ferrite fraction is set by adjusting the annealing conditions in the annealing step.

3. The method for manufacturing an annealed and pickled steel sheet according to claim 2, wherein: The annealing condition is the dew point in the soaking zone of the annealing furnace.

4. The method for producing an annealed and pickled steel sheet according to any one of claims 1 to 3, wherein: The first acid is nitric acid.

5. The method for producing an annealed and pickled steel sheet according to any one of claims 1 to 3, wherein: The second acid is one or more selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid.

6. The method for producing an annealed and pickled steel sheet according to any one of claims 1 to 3, wherein: The third acid is one or more selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid.

7. The method for producing an annealed and pickled steel sheet according to any one of claims 1 to 3, wherein: The cold-rolled steel sheet has a composition containing 0.50 to 3.00 mass % of Si.

8. The method for manufacturing an annealed and pickled steel sheet according to claim 7, wherein: The composition contains, in mass%, C: 0.03-0.45%, Si: 0.50-3.00%, Mn: 0.5-5.0%, P: less than 0.05%, S: less than 0.005%, Al: 0.001-0.060%, N: less than 0.005% and B: 0.001-0.005%, with the remainder being Fe and unavoidable impurities.

9. The method for manufacturing an annealed and pickled steel sheet according to claim 8, wherein: The component composition further contains, in mass%, at least one selected from the group consisting of Cu: 1.00% or less, Nb: 0.050% or less, Ti: 0.080% or less, V: 0.5% or less, Mo: 1.00% or less, Cr: 1.000% or less, and Ni: 1.00% or less.

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