Manufacturing method of annealed and pickled steel sheet

Abstract

To provide a production method of an annealed acid cleaned steel plate capable of stably and continuously producing an annealed acid cleaned steel plate which has excellent surface appearance quality.SOLUTION: In an acid cleaning step for plate leaping of an annealed steel plate S2 into an acid cleaning tank 20 which stores a mixed acid solution including first acid having oxidizability and second acid having non-oxidizability, for acid cleaning the annealed steel plate S2 by the mixed acid solution, the mixed acid solution is circulated between the acid cleaning tank 20 and the circulation tank 30, and a cooling facility 26 cools the mixed acid solution for suppressing temperature rise of the mixed acid solution caused by reaction heat of the mixed acid solution and the annealed steel plate. In addition, if density of the first acid and density of second acid in the mixed acid solution decrease, by entry of water sprayed from a spray nozzle 32 to the annealed steel plate S2 into the acid cleaning tank 20, from a first stock solution tank 40 and a second stock solution tank 50, a stock solution of the first acid and a stock solution of the second acid are supplied to the circulation tank 30. In synchronization with the supply of the stock solutions, output of the cooling facility 26 is increased temporarily, for suppressing temperature rise of the mixed acid solution caused by dissolution heat of the stock solutions of the first acid and the second acid.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for continuously producing an annealed and pickled steel sheet by annealing, pickling, and re-pickling a cold-rolled steel sheet. [Background technology]

[0002] In recent years, from the perspective of protecting the global environment, there has been a strong demand for improved fuel efficiency and crashworthiness of automobiles, resulting in a demand for lighter and stronger automobile bodies. To meet these demands, efforts are being actively made to simultaneously achieve lighter and stronger automobile bodies by increasing the strength and thinning (lightweighting) of cold-rolled steel sheets, which are used as the raw material for automobile components. However, since many automobile components are manufactured by forming cold-rolled steel sheets, the raw material for these components is required to have not only high strength but also excellent formability.

[0003] One way to increase the strength of cold-rolled steel sheets without significantly impairing formability is through 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, is formed on the steel sheet surface during annealing after cold rolling, which results in poor chemical conversion treatability and corrosion resistance after painting.

[0004] To solve this problem, Patent Document 1 describes a method for continuously producing an annealed and pickled steel sheet by successively performing the following steps: annealing a cold-rolled steel sheet to obtain an annealed steel sheet; pickling the annealed steel sheet by immersing it 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; and then re-pickling the annealed steel sheet by immersing it in an acid solution containing a non-oxidizing acid such as hydrochloric acid or sulfuric acid. This method removes Si-containing oxides formed on the steel sheet surface during continuous annealing in the pickling step, and removes iron-based oxides formed during the pickling step in the re-pickling step, thereby enabling the production of an annealed and pickled steel sheet with excellent chemical conversion treatability and corrosion resistance after painting.

[0005] In the pickling process (first pickling stage) of such two-stage pickling, as shown in Patent Document 2 (see FIG. 1), an annealed steel sheet is passed through the pickling tank while a mixed acid solution is circulated between the pickling tank and a circulation tank, and the annealed steel sheet is pickled with the mixed acid solution. During this process, the circulating mixed acid solution is cooled using a cooling system (heat exchanger) to suppress a temperature rise in the mixed acid solution due to the heat of reaction between the mixed acid solution and the annealed steel sheet. However, the following problem has been encountered. Over time, Fe gradually leaches out of the cold-rolled steel sheet, increasing the Fe concentration in the mixed acid solution. As a result, the pickling rate increases, and the generated reaction heat exceeds the capacity of the cooling system, raising the temperature of the mixed acid solution. This causes a deterioration in the surface appearance quality of the annealed and pickled steel sheet produced. Therefore, in Patent Document 2, the concentration of the oxidizing acid in the mixed acid solution is decreased and the concentration of the non-oxidizing acid is increased as the Fe concentration in the mixed acid solution increases. This allows the pickling speed to be slowed down and the temperature rise of the mixed acid solution due to the heat of reaction to be sufficiently suppressed by the cooling equipment, thereby maintaining the mixed acid solution within a predetermined temperature range and enabling the continuous production of annealed and pickled steel sheets with excellent surface appearance quality. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-132092 [Patent Document 2] International Publication No. 2017 / 007036 Summary of the Invention [Problem to be solved by the invention]

[0007] However, as a result of further investigations by the present inventors, it has been found that there are cases where annealed and pickled steel sheets with excellent surface appearance quality cannot be continuously and stably produced simply by operating cooling equipment at a constant output in order to suppress the temperature rise of the mixed acid solution due to the heat of reaction between the mixed acid solution and the annealed steel sheet as described above.

[0008] In view of the above problems, the present invention aims to provide a method for producing annealed and pickled steel sheets that can continuously and stably produce annealed and pickled steel sheets with excellent surface appearance quality. [Means for solving the problem]

[0009] The present inventors conducted extensive research to solve the above-mentioned problems and discovered the following. Recently, in two-stage pickling such as the above, a spraying process is sometimes performed between the pickling process and the re-pickling process. This spraying process involves spraying water onto the annealed steel sheet to wet it immediately after the annealed steel sheet is removed from the pickling tank. This spraying process prevents discoloration due to the dried mixed acid solution on the annealed steel sheet. Because the spray nozzle is located above the pickling tank, the water sprayed onto the annealed steel sheet gets mixed into the pickling tank, diluting the acid in the mixed acid solution and gradually reducing the acid concentration. Therefore, to maintain the acid concentration in the mixed acid solution within an appropriate range, a stock solution addition process is performed during the pickling process. This stock solution addition process involves intermittently supplying a high-acid-concentration acid solution (stock solution) to a circulation tank and adding it to the mixed acid solution when the acid concentration in the mixed acid solution has decreased to a certain extent. It has been found that annealed and pickled steel sheets produced by performing the pickling process immediately after the stock solution addition process have poor surface appearance quality. This is because the temperature of the mixed acid solution temporarily rises immediately after the acid solution is added due to the heat of dissolution of the acid solution. Simply operating a cooling system at a constant output to suppress the temperature rise of the mixed acid solution due to the heat of reaction between the mixed acid solution and the annealed steel sheet is insufficient to suppress this temporary temperature rise of the mixed acid solution due to the heat of dissolution, and in fact, it is not possible to constantly maintain the mixed acid solution within the suitable temperature range. If the temperature of the mixed acid solution exceeds the suitable temperature range, the steel sheet will be over-pickled. This excessive pickling causes stains and scratches due to the adhesion of iron powder on the steel sheet surface, impairing the surface appearance quality.

[0010] Therefore, the inventors came up with the idea of ​​temporarily increasing the output of the cooling equipment in synchronization with the execution of the acid stock solution charging step to strengthen the cooling of the mixed acid solution. This makes it possible to suppress a temporary temperature rise in the mixed acid solution due to the heat of dissolution of the acid stock solution, and to constantly maintain the mixed acid solution within a suitable temperature range. As a result, it was found that over-pickling can be suppressed and annealed and pickled steel sheets with excellent surface appearance quality can be continuously and stably produced.

[0011] The gist and configuration of the present invention, which has been completed based on the above findings, is as follows. [1] An annealing step in which a cold-rolled steel sheet is passed through an annealing furnace and annealed 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 pickling 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; a spraying step of spraying water onto the annealed steel sheet discharged from the pickling tank from a spray nozzle located above the pickling tank; Thereafter, a re-pickling step of passing the annealed steel sheet through a re-pickling tank containing an acid solution containing a non-oxidizing third acid, and re-pickling the annealed steel sheet with the acid solution; A method for continuously producing an annealed and pickled steel sheet by continuously carrying out the above steps, In the pickling step, circulating the mixed acid solution between the pickling tank and a circulation tank; cooling the mixed acid solution by a cooling facility provided between the pickling tank and the circulation tank in order to suppress a temperature rise of the mixed acid solution due to heat of reaction between the mixed acid solution and the annealed steel sheet; When the concentrations of the first acid and the second acid in the mixed acid solution decrease due to the water sprayed onto the annealed steel sheet in the spraying step being mixed into the pickling tank, a stock solution introducing step is carried out in which the first acid stock solution and the second acid stock solution are supplied to the circulation tank from a first stock solution tank and a second stock solution tank, which respectively contain the first acid stock solution and the second acid stock solution; a cooling intensification process is performed in synchronization with the execution of the stock solution adding process, in which an output of the cooling equipment is temporarily increased to suppress a temperature rise of the mixed acid solution due to heat of dissolution of the first acid stock solution and the second acid stock solution.

[0012] [2] In the cooling strengthening process, measuring the temperature of the mixed acid solution with a thermometer installed downstream of the cooling equipment; [1] The method for producing an annealed and pickled steel sheet according to the above-mentioned [1], wherein, after the raw solution charging step is carried out, an output of the cooling equipment is temporarily increased so as to compensate for a difference between the temperature of the mixed acid solution measured by the thermometer and a target temperature of the mixed acid solution.

[0013] [3] In the cooling strengthening process, the time from the start of the raw solution introduction step to the start of the temperature increase of the mixed acid solution due to the heat of dissolution; Stock solution injection process and the amount of temperature increase of the mixed acid solution due to the heat of dissolution calculated from the amounts of the first acid stock solution and the second acid stock solution added in the above step, The method for producing an annealed and pickled steel sheet according to [1] above, wherein an output of the cooling equipment is temporarily increased so as to offset a temperature rise of the mixed acid solution due to the heat of dissolution.

[0014] [4] The method for producing an annealed and pickled steel sheet according to any one of the above [1] to [3], wherein the first acid is nitric acid.

[0015] [5] The method for producing an annealed and pickled steel sheet according to any one of the above [1] to [4], wherein the second acid is at least one selected from hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid.

[0016] [6] The method for producing an annealed and pickled steel sheet according to any one of the above [1] to [5], 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.

[0017] [7] The method for producing an annealed and pickled steel sheet according to any one of the above [1] to [6], wherein the cold-rolled steel sheet has a chemical composition containing 0.50 to 3.00 mass % of Si.

[0018] [8] The method for producing an annealed and pickled steel sheet according to the above [7], wherein the chemical composition contains, in mass%, C: 0.03 to 0.45%, Si: 0.50 to 3.00%, Mn: 0.5 to 5.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.001 to 0.060%, and N: 0.005% or less, with the balance being Fe and unavoidable impurities.

[0019] [9] The method for producing an annealed and pickled steel sheet according to [8] above, wherein the chemical composition further contains, in mass%, at least one selected from B: 0.005% or less, 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. [Effects of the Invention]

[0020] According to the method for producing an annealed and pickled steel sheet of the present invention, it is possible to continuously and stably produce an annealed and pickled steel sheet having excellent surface appearance quality. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic diagram of an annealing and pickling facility 100 according to an embodiment of the present invention. [Figure 2] 1 is a schematic graph showing the change over time in the mixed acid solution temperature and the cooling equipment output after the raw solution is charged in a comparative example. [Figure 3] 1 is a schematic graph showing the change over time in the mixed acid solution temperature and the cooling equipment output after the raw solution is charged in Example 1 of the present invention. [Figure 4] 1 is a schematic graph showing the change over time in the mixed acid solution temperature and the cooling equipment output after the raw solution is charged in Example 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] In a method for producing an annealed and pickled steel sheet according to one embodiment of the present invention, an annealing step, a pickling step, a spraying step, and a re-pickling step, which will be described in detail below, are successively carried out to continuously produce an annealed and pickled steel sheet.

[0023] In this embodiment, the annealing process, pickling process, and re-pickling process are continuously performed on the same line, for example, by an annealing and pickling facility 100 shown in Fig. 1. The annealing and pickling facility 100 shown in Fig. 1 includes, in this order from upstream to downstream in the traveling direction of the steel sheet, a continuous annealing line (CAL) 10, a pickling tank 20 containing a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid, a re-pickling tank 80 containing an acid solution containing a non-oxidizing third acid, and a rinsing tank 90 containing water. A strip threading facility including a plurality of rolls 92 can sequentially thread the steel sheet through the continuous annealing furnace 10 and the above three tanks.

[0024] [Annealing process] Referring to FIG. 1 , in the annealing step, a cold-rolled steel sheet S1 is passed through a continuous annealing furnace 10 and annealed in the continuous annealing furnace 10 to obtain an annealed steel sheet S2. The annealing step is performed to impart desired structure, strength, and workability to the cold-rolled steel sheet S1. The annealing furnace 10 may have multiple zones, and in the example of FIG. 1 , it has, from upstream in the sheet passing direction, a heating zone 12, a soaking zone 14, and a cooling zone 16. The configuration of the continuous annealing furnace is not limited to that shown in FIG. 1 . For example, a pre-heating zone may be provided upstream of the heating zone 12, the cooling zone 16 may include multiple cooling zones, and an overaging zone may be provided downstream of the cooling zone 16.

[0025] 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 average temperature inside the heating zone 12 is preferably 500 to 800°C. A non-oxidizing or reducing gas is separately supplied to the heating zone 12 at the same time as the gas from the soaking zone 14 flows into the heating zone 12. For example, N2 gas is used as the non-oxidizing gas, and for example, a H2-N2 mixed gas is used as the reducing gas. The dew point of the heating zone 12 is preferably within a range of -50 to 20°C.

[0026] In the soaking zone 14, the cold-rolled steel sheet S1 can be indirectly heated using a radiant tube (RT). The average temperature inside the soaking zone 14 (soaking temperature) is preferably 600 to 950°C. A non-oxidizing or reducing gas is supplied to the soaking zone 14. For example, N2 gas is used as the non-oxidizing gas, and for example, H2-N2 mixed gas is used as the reducing gas. The dew point of the soaking zone 14 is preferably within a range of -50 to 20°C.

[0027] 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 when it leaves the continuous annealing furnace .

[0028] [Pickling process] 1, in the pickling process, an annealed steel sheet S2 discharged from a continuous annealing furnace 10 is passed through a pickling tank 20 containing a mixed acid solution (mixed acid aqueous solution) containing a first oxidizing acid and a second non-oxidizing acid, and the annealed steel sheet S2 is pickled with the mixed acid solution. In FIG. 1, an example is shown in which nitric acid is used as the first acid and hydrochloric acid is used as the second acid, and the pickling tank 20 contains nitric acid and hydrochloric acid.

[0029] As described above, in the annealing process, a non-oxidizing or reducing gas is used as the atmospheric gas, and the dew point of the gas is strictly controlled. Therefore, oxidation of the steel sheet surface is suppressed in general cold-rolled steel sheets with low alloying content. However, in the case of cold-rolled steel sheets containing Si and Mn, which are more easily oxidized than Fe, even if the composition and dew point of the atmospheric gas during annealing are strictly controlled, Si and Mn are selectively oxidized, and Si-containing oxides such as Si oxide (SiO2) and Si-Mn composite oxides are formed on the steel sheet surface. In other words, the surface layer of the annealed steel sheet S2 becomes a Si-containing oxide layer, which deteriorates the chemical conversion treatability and corrosion resistance after painting.

[0030] Therefore, in the pickling process of this embodiment, the annealed steel sheet S2 is continuously immersed in a mixed acid solution containing a first oxidizing acid and a second non-oxidizing 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 components and annealing conditions (temperature, time, atmosphere), but is usually about 1 μm from the steel sheet surface.

[0031] An example of an oxidizing first acid is nitric acid. The reason why the first acid is necessary in the mixed acid solution is that, among Si-containing oxides, Si-Mn composite oxides dissolve easily in acid, but SiO2 is difficult to dissolve, and to remove it, it is necessary to remove the Si-containing oxide layer on the steel sheet surface together with the base steel using an oxidizing acid such as nitric acid.

[0032] From the viewpoint of efficiently removing the Si-containing oxide layer, the concentration of the first acid in the mixed acid solution is preferably 100 g / L or more, more preferably 110 g / L or more. On the other hand, if the concentration of the first acid is too high, it becomes difficult to dissolve iron-based oxides in the subsequent re-pickling step, so the concentration of the first acid in the mixed acid solution is preferably 150 g / L or less, more preferably 140 g / L or less.

[0033] The non-oxidizing second acid is preferably one or more selected from hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid, and is particularly preferably one or more selected from 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 that precipitate on the steel sheet surface during pickling with the above-mentioned oxidizing first acid.

[0034] The concentration of the second acid in the mixed acid solution is preferably 4.5 g / L or more, more preferably 6.5 g / L or more, from the viewpoint of facilitating dissolution of iron-based oxides in the subsequent re-pickling step. On the other hand, if the concentration of the second acid is too high, the pickling weight loss per unit time decreases and there is a concern that SiO2 may remain on the surface layer of the steel sheet. Therefore, the concentration of the second acid in the mixed acid solution is preferably 12.5 g / L or less, more preferably 8.5 g / L or less.

[0035] The suitable pickling time in the pickling step is determined by the pickling weight loss required to remove the Si-containing oxide layer formed in the annealing step, the pickling efficiency determined by the composition of the mixed acid solution, and the pickling duration. Generally, the mixed acid solution temperature is about 30 to 60°C, and the pickling time is about 6 to 10 seconds.

[0036] [Spray process] 1, in the spraying step, water is sprayed from a spray nozzle 32 positioned above the pickling tank 20 onto the annealed steel sheet S2 discharged from the pickling tank 20, to wet the surface of the annealed steel sheet S2. This is because if the mixed acid solution dries on the annealed steel sheet S2 immediately after the annealed steel sheet S2 is pulled out of the pickling tank 20, discoloration will occur in the annealed and pickled steel sheet S3 that is finally obtained. The amount of water sprayed from the spray nozzle 32 is not particularly limited, but is generally 1 to 15 m 3 / hr. In this case, however, the water sprayed onto the annealed steel sheet S2 is mixed into the pickling tank 20, diluting the acid in the mixed acid solution and gradually reducing the acid concentration.

[0037] [Re-pickling process] 1, in the re-pickling step, the annealed steel sheet S2 discharged from the pickling tank 20 is passed through a re-pickling tank 80 containing an acid solution (acid aqueous solution) containing a non-oxidizing third acid, and the annealed steel sheet S2 is re-pickled with the acid solution. This produces an annealed and pickled steel sheet S3. In FIG. 1, an example is shown in which hydrochloric acid is used as the third acid and the re-pickling tank 80 contains hydrochloric acid.

[0038] During the pickling process, Fe dissolved from the steel sheet surface generates iron-based oxides, which precipitate and cover the steel sheet surface, resulting in reduced chemical 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-based oxides. The term "iron-based oxides" refers to oxides primarily composed of iron, in which the atomic concentration ratio of iron among elements other than oxygen that constitute the oxide is 30% or more. These iron-based oxides exist on the steel sheet surface with a non-uniform thickness and are different from the natural oxide film, which exists uniformly and in a layer with a thickness of several nanometers. The iron-based oxides formed on the surface of the annealed steel sheet S2 have been found to be amorphous based on observations using a transmission electron microscope (TEM) and analysis of diffraction patterns using electron beam diffraction.

[0039] The non-oxidizing third acid is preferably one or more selected from hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid, and particularly preferably one or more selected from hydrochloric acid, sulfuric acid, and hydrofluoric acid. Among these, hydrochloric acid is preferred because it is a volatile acid and therefore does not easily leave residues such as sulfate radicals on the steel sheet surface, unlike sulfuric acid, and because it has a significant effect of destroying iron-based oxides through chloride ions. Alternatively, a mixture of hydrochloric acid and sulfuric acid may be used. The second acid used in the pickling step and the third acid used in this step may be the same or different acids. However, from the viewpoint of sharing manufacturing equipment, it is preferable that they are the same type of acid.

[0040] The concentration of the third acid in the acid solution is preferably 4.5 g / L or more, more preferably 6.5 g / L or more, from the viewpoint of sufficiently dissolving iron-based oxides. 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 steel sheet surface and cause discoloration. Therefore, the concentration of the third acid in the acid solution is preferably 12.5 g / L or less, more preferably 8.5 g / L or less.

[0041] The suitable pickling time for the second pickling step is determined by the pickling weight loss required to remove iron oxides formed in the first pickling step, the pickling efficiency determined by the acid composition, and the pickling duration. Generally, the temperature of the acid solution is about 30 to 60°C, and the pickling time is about 2 to 10 seconds.

[0042] The total pickling weight loss in the pickling process and the re-pickling process was 8 g / m 2 The total pickling loss is preferably 8 g / m or more. 2 If the above conditions are met, Si-containing oxides and iron-based oxides are less likely to remain on the steel sheet surface, and therefore higher chemical conversion treatability can be achieved.

[0043] [Process after re-pickling process] The annealed and pickled steel sheet S3 obtained after the re-pickling step can then be made into a steel sheet product through ordinary processing steps such as temper rolling and leveling.

[0044] [Rinse process] As shown in Fig. 1, it is preferable to provide a rinsing tank 90 downstream of the re-pickling tank 80, and after the re-pickling step, pass the annealed and pickled steel sheet S3 through the rinsing tank 90 to rinse it with water. This makes it possible to remove the acid solution carried away by the annealed and pickled steel sheet S3 from the re-pickling tank 80, and to prevent rust on the surface of the annealed and pickled steel sheet S3.

[0045] Furthermore, although not shown, a rinse tank containing water may be provided between the pickling tank 20 and the re-pickling tank 80, and the annealed steel sheet S2 may be passed through the rinse tank between the pickling step and the re-pickling step to be rinsed with water. This prevents the mixed acid solution carried by the annealed steel sheet S2 from the pickling tank 20 from being mixed into the acid solution in the re-pickling tank 80. This is therefore preferable because iron-based oxides can be reliably removed by re-pickling in the re-pickling tank 80.

[0046] Furthermore, although not shown, a rinse tank containing water may be provided upstream of the pickling tank 20, and the annealed steel sheet S2 may be passed through the rinse tank and rinsed with water before the pickling step. This makes it possible to remove impurities from the surface of the annealed steel sheet S2 and prevent impurities from being mixed into the mixed acid solution in the pickling tank 20.

[0047] [Details of the pickling process] The pickling process will be described in detail with reference to Fig. 1. First, in the pickling process, a mixed acid solution is circulated between a pickling tank 20 and a circulation tank 30. Specifically, the pickling tank 20 and the circulation tank 30 are connected by two lines of pipes 22A and 22B. A first pump 24 is installed in the pipe 22B, and by operating the first pump 24, the mixed acid solution flows from the pickling tank 20 to the circulation tank 30 via the pipe 22A, and from the circulation tank 30 to the pickling tank 20 via the pipe 22B.

[0048] At the start of operation, the concentration of the first acid in the mixed acid solution is set to a specific value (e.g., 130 g / L) within the aforementioned preferred range (100 to 150 g / L), and the concentration of the second acid in the mixed acid solution is set to a specific value (e.g., 6.5 g / L) within the aforementioned preferred range (4.5 to 12.5 g / L). The amount of mixed acid solution circulated is not particularly limited, but may be, for example, 200 to 500 m 3 During operation, the mixed acid solution prepared to the predetermined concentration is circulated between the pickling tank 20 and the circulation tank 30, and the raw solution is only charged intermittently.

[0049] The first stock solution tank 40 contains a stock solution of a first acid (nitric acid is exemplified in FIG. 1 ), and the second stock solution tank 50 contains a stock solution of a second acid (hydrochloric acid is exemplified in FIG. 1 ). The nitric acid stock solution can be, for example, dilute nitric acid (62 mass % nitric acid aqueous solution), and the hydrochloric acid stock solution can be, for example, a 35 mass % hydrochloric acid aqueous solution. The first stock solution tank 40 and the circulation tank 30 are connected by a pipe 42. The second stock solution tank 50 and the circulation tank 30 are connected by a pipe 52. A pump 44 is installed in the pipe 42, and by operating the pump 44, a predetermined amount of the first acid stock solution can be supplied from the first stock solution tank 40 to the circulation tank 30. The pipe 52 is also installed with a pump 54, and by operating the pump 54, a predetermined amount of the second acid stock solution can be supplied from the second stock solution tank 50 to the circulation tank 30.

[0050] In this embodiment, when the water sprayed onto the annealed steel sheet S2 in the spraying process is mixed into the pickling tank 20, causing a decrease in the concentrations of the first acid and the second acid in the mixed acid solution, a concentrate introduction process is performed to supply the first acid concentrate and the second acid concentrate from the first concentrate tank 40 and the second concentrate tank 50 to the circulation tank 30, respectively. Specifically, an example of the concentrate introduction process can be performed as follows: The acid concentrations (concentrations of the first acid and the second acid) of the mixed acid solution in the pickling tank 20 are continuously or intermittently measured using an acid concentration measuring device 60. When the measured acid concentration reaches a preset threshold, the concentrate introduction process is performed. For example, with regard to the concentration of the first acid, the threshold is set to a predetermined value (e.g., 110 g / L) within the aforementioned preferred range (100 to 150 g / L), and a decrease from the set concentration (e.g., 130 g / L) at the start of operation is monitored in situ. Similarly, for the concentration of the second acid, the threshold is set to a predetermined value (e.g., 5.0 g / L) within the aforementioned preferred range (4.5 to 12.5 g / L), and the decrease from the set concentration (e.g., 6.5 g / L) at the start of operation is monitored in situ. When one of the first acid concentration and the second acid concentration measured by the acid concentration measuring device 60 reaches the predetermined threshold, the stock solution addition step is performed. The amounts of the first acid stock solution and the second acid stock solution added at that time are set to be amounts that can return the first acid concentration and the second acid concentration at that time to the set concentrations (e.g., 130 g / L and 6.5 g / L, respectively) at the start of operation. Stock solution injection process 1, the control unit 70 operates the pumps 44 and 54 based on the measured value output by the acid concentration measuring device 60, thereby supplying predetermined amounts of the first acid concentrate and the second acid concentrate to the circulation tank 30. The control unit 70 can be realized by a central processing unit (CPU) inside a computer. Alternatively, Stock solution injection process This can also be done by an operator operating pumps 44 and 54 based on the measurement value of the acid concentration measuring device 60 to supply predetermined amounts of the first acid stock solution and the second acid stock solution to the circulation tank 30.

[0051] A cooling device 26 is provided between the pickling tank 20 and the circulation tank 30. For example, a heat exchanger is installed in the pipe 22B as the cooling device 26. The cooling device 26 operates at a constant output to cool the mixed acid circulating between the pickling tank 20 and the circulation tank 30, thereby suppressing a temperature rise of the mixed acid due to the heat of reaction between the mixed acid and the annealed steel sheet S2. As described above, the temperature of the mixed acid is preferably about 30 to 60°C, and therefore, for example, the target temperature of the mixed acid is set to 32°C. The cooling device 26 operates at a constant output to suppress a temperature rise of the mixed acid due to the heat of reaction, thereby maintaining the temperature of the mixed acid at the target temperature of 32°C. The temperature of the mixed acid can be measured continuously or intermittently by a thermometer 28 provided downstream of the cooling device 26.

[0052] Here, the above Stock solution injection process When the above-mentioned process is performed, even if the cooling equipment is operated at a constant output to suppress the temperature rise of the mixed acid due to the heat of reaction, the heat of dissolution of the first and second acid concentrates causes a temporary temperature rise of the mixed acid. This leads to the temporary production of annealed and pickled steel sheets with poor surface appearance quality, i.e., a reduced yield. Therefore, in this embodiment, a cooling enhancement process is performed in which the output of the cooling equipment 26 is temporarily increased in synchronization with the execution of the concentrate solution addition process to suppress the temperature rise of the mixed acid due to the heat of dissolution of the first and second acid concentrate solutions. In this way, when the acid concentrates are added to the pickling tank 20 containing the mixed acid to restore the acid concentration reduced by the inclusion of water, the temperature rise of the mixed acid due to the heat of dissolution of the concentrate solutions is suppressed, thereby enabling the continuous and stable production of annealed and pickled steel sheets with excellent surface appearance quality.

[0053] In a first example of a specific embodiment of the cooling enhancement step, the temperature of the mixed acid solution is measured using a thermometer 28 installed downstream of the cooling device 26. After the raw solution introduction step is performed, the output of the cooling device 26 can be temporarily increased to compensate for the difference between the temperature of the mixed acid solution measured by the thermometer 28 and the target temperature (e.g., 32°C) of the mixed acid solution. An example of the time-dependent changes in the mixed acid solution temperature and the cooling device output after the raw solution introduction in this case is shown in FIG. 3. This first example can be performed by the control unit 70 increasing the output of the cooling device 26 based on the measurement value of the thermometer 28 to compensate for the difference between the measured value and the target solution temperature, and repeating this in a closed loop. This is feedback control by the control unit 70. Alternatively, an operator may increase the output of the cooling device 26 based on the measurement value of the thermometer 28 to compensate for the difference between the measured value and the target solution temperature.

[0054] As a specific embodiment of the cooling strengthening step, in a second example, the time from the start of the raw solution introduction step to the start of the temperature rise of the mixed acid solution due to the dissolution heat, Stock solution injection process The amount of temperature rise of the mixed acid solution due to the heat of dissolution, which is calculated from the amounts of the first and second acid concentrate solutions added in step 1, can be predicted, and the output of the cooling equipment 26 can be temporarily increased to offset the temperature rise of the mixed acid solution due to the heat of dissolution. FIG. 4 shows an example of the time-dependent changes in the mixed acid solution temperature and the cooling equipment output after the concentrate solutions are added in this case. In the first example, feedback control is used, so a slight increase in the mixed acid solution temperature is tolerated, whereas in the second example, the output of the cooling equipment 26 is increased in anticipation of the temperature rise of the mixed acid solution due to the heat of dissolution, making it possible to continuously and more stably produce annealed and pickled steel sheets with excellent surface appearance quality.

[0055] The time from the start of the raw solution charging step until the temperature of the mixed acid solution (the temperature measured by the thermometer 28) starts to rise due to the heat of dissolution is affected by the scale of the equipment (the size of the pickling tank 20, the circulation tank 30, the pipes 22A and 22B) and the amount of mixed acid solution circulated, but can be empirically determined by investigating past operations. Stock solution injection process The temperature rise of the mixed acid solution due to Stock solution injection processIt can be calculated based on the heat of dissolution calculated from the amounts of the first acid stock solution and the second acid stock solution charged in step 1 and the amount of the original mixed acid solution circulated.

[0056] In this second example, the control unit 70: Stock solution injection process The controller 70 performs a feedforward control. Alternatively, the controller 70 may perform the above-described steps by an operator. The controller 70 may perform a feedforward control. Alternatively, the controller 70 may perform the above-described steps by an operator. The controller 70 may perform a feedforward control. The controller 70 may perform the ...

[0057] [Cold-rolled steel sheet composition] The chemical composition of the cold-rolled steel sheet S1 will be described below. The unit of the content of each element is "mass %", but it will be expressed simply as "%".

[0058] Si: 0.50 to 3.00% The chemical composition of the cold-rolled steel sheet S1 is not particularly limited, but preferably contains 0.50 to 3.00 mass% of Si. Si is an effective element for achieving high strength in steel because it has a significant effect of increasing the strength of steel (solid-solution strengthening ability) without significantly impairing workability. However, it is also an element that adversely affects chemical conversion treatability and corrosion resistance after painting. From the viewpoint of achieving high strength by adding Si, the Si content is preferably 0.50% or more, and more preferably 0.80% or more. On the other hand, if the Si content is excessive, the hot rolling property and cold rolling property are significantly reduced, adversely affecting productivity and reducing the ductility of the steel sheet itself. Therefore, the Si content is preferably 3.00% or less, and more preferably 2.50% or less.

[0059] The elements other than Si are not particularly limited and are permissible as long as they fall within the composition range of a normal cold-rolled steel sheet, but the following element composition is preferred.

[0060] C: 0.03 to 0.45% C is an element effective for adjusting the strength of steel, and from this viewpoint, the C content is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, from the viewpoint of not deteriorating weldability, the C content is preferably 0.45% or less, and more preferably 0.20% or less.

[0061] Mn: 0.5 to 5.0% Mn is an element effective in improving strength and hardenability, and from this viewpoint, the Mn content is preferably 0.5% or more, and more preferably 1.0% or more. On the other hand, from the viewpoint of not deteriorating ductility and weldability, the Mn content is preferably 5.0% or less, and more preferably 3.0% or less.

[0062] P:0.05% or less P is one of the elements that is inevitably contained, but from the viewpoint of not deteriorating local ductility, the P content is preferably 0.05% or less, more preferably 0.02% or less. It is preferable to reduce the P content as much as possible, and there is no lower limit. However, from the viewpoint of dephosphorization costs, the P content may be 0.005% or more.

[0063] S: 0.005% or less S is one of the elements that is inevitably contained, but from the viewpoint of not deteriorating weldability, the S content is preferably 0.005% or less. It is preferable to reduce the S content as much as possible, and there is no lower limit. However, from the viewpoint of desulfurization costs, the S content may be 0.0001% or more.

[0064] Al: 0.001 to 0.060% Al is an element effective in deoxidizing molten steel, and from this viewpoint, the Al content is preferably 0.001% or more, and more preferably 0.020% or more. On the other hand, from the viewpoint of cost, the Al content is preferably 0.060% or less.

[0065] N: 0.005% or less Nitrogen forms coarse precipitates and deteriorates bendability. Therefore, the N content is preferably 0.005% or less. It is preferable to reduce the N content as much as possible, and there is no lower limit. However, industrially, the N content can be 0.001% or more.

[0066] The balance of the cold-rolled steel sheet S1 other than the above-mentioned components is Fe and unavoidable impurities, but may optionally contain at least one of the following components:

[0067] B: 0.005% or less B is an element effective in improving hardenability, and from this viewpoint, the B content is preferably 0.0001% or more. On the other hand, if the B content is excessive, the effect of improving hardenability becomes saturated, so if B is added, the B content is set to 0.005% or less.

[0068] Cu:1.00% or less Cu promotes the formation of residual γ phase and effectively contributes to improving strength. From this viewpoint, the Cu content is preferably 0.05% or more. On the other hand, from the viewpoint of cost, when Cu is added, the Cu content is set to 1.00% or less.

[0069] Nb: 0.050% or less 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.

[0070] Ti: 0.080% or less Ti contributes to improving strength. From this viewpoint, the Ti content is preferably 0.005% or more. On the other hand, from the viewpoint of not deteriorating chemical conversion treatability, when Ti is added, the Ti content is set to 0.080% or less.

[0071] V: 0.5% or less V is effective in improving delayed fracture resistance. From this viewpoint, the V content is preferably 0.004% or more. On the other hand, from the viewpoint of not deteriorating the strength-ductility balance, when V is added, the V content is 0.5% or less, preferably 0.1% or less, and more preferably 0.05% or less.

[0072] Mo: 1.00% or less Mo contributes to improving strength. From this viewpoint, the Mo content is preferably 0.05% or more. On the other hand, from the viewpoint of cost, when Mo is added, the Mo content is set to 1.00% or less.

[0073] Cr:1.000% or less Cr contributes to improving hardenability. From this viewpoint, the Cr content is preferably 0.001% or more. On the other hand, from the viewpoint of not deteriorating weldability, if Cr is added, the Cr content is set to 1.000% or less.

[0074] Ni: 1.00% or less Ni promotes the formation of residual γ phase. From this viewpoint, the Ni content is preferably 0.05% or more. On the other hand, from the viewpoint of cost, when Ni is added, the Ni content is set to 1.00% or less. [Example]

[0075] Using the manufacturing equipment shown in Fig. 1, operations were carried out according to the following Comparative Example and Invention Examples 1 and 2. A cold-rolled steel sheet having a composition containing, by mass%, C: 0.17%, Si: 1.20%, Mn: 1.95%, P: 0.01%, S: 0.001%, Al: 0.03%, N: 0.002%, and B: 0.0001%, with the balance being Fe and unavoidable impurities, was continuously annealed under predetermined conditions to obtain an annealed steel sheet.

[0076] [Pickling process] The pickling tank contained a mixed acid solution containing nitric acid and hydrochloric acid (nitric acid concentration: 120 g / L, hydrochloric acid concentration: 6.5 g / L), which was circulated between the tank and the circulation tank. The circulation volume of the mixed acid solution was 260 m 3 / hr. By operating the cooling equipment at a constant output, the temperature of the mixed acid solution was maintained at the target solution temperature of 32°C. The annealed steel sheet was passed through the pickling tank and pickled with the mixed acid solution. The pickling time was 6 seconds. In addition, a spraying process (spray volume: 8 m) was performed in which water was sprayed onto the annealed steel sheet discharged from the pickling tank. 3 / hr) was also performed.

[0077] Dilute nitric acid (62% by mass nitric acid aqueous solution) was used as the nitric acid stock solution, and 35% by mass hydrochloric acid aqueous solution was used as the hydrochloric acid stock solution. The acid concentration of the mixed acid solution decreases over time due to the water mixed into the pickling tank during the spraying process. Therefore, the acid concentration of the mixed acid solution in the pickling tank was constantly measured using an acid concentration meter, and the stock solution introduction process was carried out when the nitric acid concentration decreased to 110 g / L or the hydrochloric acid concentration decreased to 5.5 g / L. Specifically, the nitric acid stock solution and the hydrochloric acid stock solution were supplied to the circulation tank, and the acid concentrations in the mixed acid solution were returned to the initial set concentrations (nitric acid concentration: 120 g / L, hydrochloric acid concentration: 6.5 g / L).

[0078] [Re-pickling process] The re-pickling tank contained an acid solution containing hydrochloric acid (hydrochloric acid concentration: 5 g / L), and the annealed steel sheet after the pickling process was passed through the re-pickling tank to be re-pickled with the acid solution. The temperature of the acid solution was maintained at 45°C, and the pickling time was 2 seconds.

[0079] [Rinse process] Thereafter, the annealed steel sheet was passed through a rinse tank, washed with water, and dried to obtain an annealed pickled steel sheet.

[0080] (Comparative Example) After the raw solution addition process, the cooling equipment continued to operate at a constant output, and no intensive cooling process was carried out. Figure 2 shows an example of the time-dependent changes in the mixed acid temperature and cooling equipment output after the raw solution addition process.

[0081] (Example 1) In synchronization with the raw material solution charging process, the operator performed the cooling enhancement process. Specifically, feedback control was performed to temporarily increase the output of the cooling equipment to compensate for the difference between the mixed acid solution temperature measured by a thermometer installed downstream of the cooling equipment and the target temperature (32°C) of the mixed acid solution. Figure 3 shows an example of the time-dependent changes in the mixed acid solution temperature and cooling equipment output after the raw material solution was charged. In this case, as a result of feedback control, the output began to increase 90 seconds after the raw material was charged, and the output increased by up to 15%, as shown in the profile in Figure 3.

[0082] (Example 2) The cooling intensification process was carried out by the operator in synchronization with the raw material introduction process. Specifically, the time from the introduction of the raw material introduction process until the temperature of the mixed acid solution began to rise due to the heat of dissolution and the amount of temperature rise of the mixed acid solution due to the heat of dissolution, calculated from the amounts of raw nitric acid solution and raw hydrochloric acid solution introduced in the raw material introduction process, were predicted, and the output of the cooling equipment was temporarily increased to offset the temperature rise of the mixed acid solution due to the heat of dissolution. Figure 4 shows an example of the time-dependent changes in the mixed acid solution temperature and cooling equipment output after the raw material introduction in this case. In this case, the output increase began 70 seconds after the raw material introduction, and the output was increased by a maximum of 15%, as shown in the profile in Figure 4.

[0083] [Evaluation of surface appearance quality] In all of the Comparative Example, Inventive Example 1, and Inventive Example 2, it was possible to continuously produce annealed and pickled steel sheets that were basically excellent in surface appearance quality. However, in the Comparative Example, Stock solution injection process In the annealed and pickled steel sheets that were subjected to the pickling process within a predetermined period immediately after the annealing, stains and scratches occurred on the steel sheet surface due to the adhesion of iron powder, and the surface appearance quality was impaired. Stock solution injection process Annealed and pickled steel sheets with excellent surface appearance quality could be continuously and stably produced, including for a predetermined period immediately after the treatment. In particular, in Example 2, the output of the cooling equipment was increased in anticipation of a temperature rise in the mixed acid solution due to the heat of dissolution, Stock solution injection process Since the temperature of the mixed acid solution could be maintained at the target temperature throughout the entire period, annealed and pickled steel sheets with excellent surface appearance quality could be continuously and stably produced. [Industrial Applicability]

[0084] According to the method for producing an annealed and pickled steel sheet of the present invention, it is possible to continuously and stably produce an annealed and pickled steel sheet having excellent surface appearance quality. Therefore, the annealed and pickled steel sheet produced by the present invention can be suitably used as a component for an automobile body, a component for a home appliance, a building component, etc. [Explanation of symbols]

[0085] 100 Annealing and pickling equipment 10 Continuous annealing furnace 12 Heating Zone 14. Equal Temperature 16 Cooling Zone 20 Pickling tank (mixed acid of nitric acid and hydrochloric acid) 22A piping 22B Piping 24 First Pump 26 Cooling equipment 28 Thermometer 30 Circulation Tank 32 spray nozzle 40 First concentrate tank (nitric acid concentrate) 42 Piping 44 Pump 50 Second concentrate tank (hydrochloric acid concentrate) 52 Piping 54 Pump 60 Acid concentration meter 70 Control Unit 80 Re-pickling tank (hydrochloric acid) 90 Rinse tank (water) 92 Roll (threading equipment) S1 cold rolled steel plate S2 annealed steel plate S3 Annealed and pickled steel plate

Claims

1. an annealing step of passing the cold-rolled steel sheet through an annealing furnace and 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 pickling 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; a spraying step of spraying water onto the annealed steel sheet discharged from the pickling tank from a spray nozzle located above the pickling tank; Thereafter, a re-pickling step of passing the annealed steel sheet through a re-pickling tank containing an acid solution containing a non-oxidizing third acid, and re-pickling the annealed steel sheet with the acid solution; A method for continuously producing an annealed and pickled steel sheet by continuously carrying out the above steps, In the pickling step, circulating the mixed acid solution between the pickling tank and a circulation tank; cooling the mixed acid solution by a cooling facility provided between the pickling tank and the circulation tank in order to suppress a temperature rise of the mixed acid solution due to heat of reaction between the mixed acid solution and the annealed steel sheet; When the concentrations of the first acid and the second acid in the mixed acid solution decrease due to the water sprayed onto the annealed steel sheet in the spraying step being mixed into the pickling tank, a stock solution introducing step is carried out in which the stock solutions of the first acid and the second acid are supplied to the circulation tank from a first stock solution tank and a second stock solution tank, which respectively contain the stock solutions of the first acid and the second acid, a cooling intensification process is performed in synchronization with the execution of the stock solution adding process, to temporarily increase output of the cooling equipment and suppress a temperature rise of the mixed acid solution due to heat of dissolution of the first acid stock solution and the second acid stock solution.

2. In the cooling strengthening step, measuring the temperature of the mixed acid solution with a thermometer installed downstream of the cooling equipment; 2. The method for producing an annealed and pickled steel sheet according to claim 1, wherein after the raw solution charging step is performed, an output of the cooling facility is temporarily increased so as to compensate for a difference between the temperature of the mixed acid solution measured by the thermometer and a target temperature of the mixed acid solution.

3. In the cooling strengthening step, predicting a time from the start of the stock solution introduction step to the start of a temperature increase of the mixed acid solution due to the heat of dissolution, and an amount of temperature increase of the mixed acid solution due to the heat of dissolution calculated from the amounts of the first acid stock solution and the second acid stock solution introduced in the stock solution introduction step; The method for producing an annealed and pickled steel sheet according to claim 1, wherein an output of the cooling facility is temporarily increased so as to offset a temperature rise of the mixed acid solution due to the heat of dissolution.

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 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 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 component composition containing 0.50 to 3.00 mass% of Si.

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

9. 9. The method for producing an annealed and pickled steel sheet according to claim 8, wherein the component composition further contains, in mass%, at least one selected from B: 0.005% or less, 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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