Pickling method for steel sheet

Abstract

To provide a pickling method capable of effectively removing a reduced iron layer while suppressing dissolution of an internal oxide layer of a hot-rolled steel sheet.SOLUTION: A method for pickling a hot-rolled steel sheet, the method including a pickling step of pickling the hot-rolled steel sheet by sequentially immersing the hot-rolled steel sheet in n (n is 2 or more) pickling tanks arranged in order from a first pickling tank to an n-th pickling tank, wherein a temperature of a pickling solution contained in each of the pickling tanks satisfies the following formulas (1-1), (1-2), and (2): Tave (1 - k)> Tave (k + 1 - n) (when n = 2, 3) (1-1) Tave (1 - k) ≥ Tave (k + 1 - n) (when n ≥ 4) (1-2) Here, Tave is the average temperature of the pickling solution. Tave (k + 1 to n)> T n (where n ≥ 4) (2) Here, k is n / 2 when n is an even number and (n + 1) / 2 when n is an odd number, and T n is the temperature (°C) of the pickling solution in the nth pickling tank at the final stage.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a method for pickling a steel sheet. [Background technology]

[0002] There has been a demand for improved occupant safety in vehicles, and for this purpose, the strength of vehicle bodies has been improved. On the other hand, against the backdrop of worsening issues such as global warming, efforts to improve the fuel efficiency of automobiles are accelerating. It is known that reducing the weight of vehicle bodies is an effective way to improve fuel efficiency.

[0003] In order to achieve collision safety while reducing the weight of automobiles, steel sheets have been further strengthened, and for example, high-strength cold-rolled steel sheets with a tensile strength of 980 MPa or more are being put into practical use. To achieve a tensile strength of 980 MPa or more, it is necessary to add solid-solution strengthening elements to the steel, and examples of such elements include Si and Mn.

[0004] In the production of high-strength steel sheet containing solute Si, a cast ingot is hot-rolled, pickled, cold-rolled, and annealed. A scale layer composed of iron-based oxides is formed on the surface of the hot-rolled hot-rolled steel sheet, and subsequent coiling at high temperature results in a hot-rolled steel sheet 10 having a layer structure as shown schematically in FIG. 1. The hot-rolled steel sheet 10 shown in FIG. 1 includes a steel sheet substrate 11, an internal oxide layer 12, a scale layer 13, and a reduced iron layer 14. The steel sheet substrate 11 contains solute Si, and the internal oxide layer 12 formed on its surface contains oxides (SiO2) formed from the solute Si. The surface of the internal oxide layer 12 is covered with the scale layer 13 and the reduced iron layer 14.

[0005] This layer structure is due to the presence of Si, an easily oxidizable element, in solid solution in the steel. When a hot-rolled steel sheet is coiled at a high temperature and slowly cooled after hot rolling, oxygen (O) contained in the scale layer 13 formed immediately after hot rolling diffuses toward the interior of the steel sheet. Because Si has a higher oxygen affinity than iron, oxygen combines with the Si solid-solubilized in the steel to form SiO2. This layer containing SiO2 is the internal oxide layer 12. Meanwhile, the oxygen in the scale layer 13 combines with the dissolved Si, reducing the scale layer 13, and as a result, a reduced iron layer 14 is formed on the surface of the scale layer 13.

[0006] The hot-rolled steel sheet 10 is subjected to pickling before cold rolling to remove the scale layer 13, and further to remove the internal oxide layer 12 as required. A continuous pickling apparatus including multiple pickling tanks arranged in series in the direction of steel sheet passage is widely used for pickling treatment. The pickling solution supplied to the continuous pickling apparatus is introduced into the last pickling tank, flows toward the first pickling tank, and is discharged from the first pickling tank. In this way, the pickling efficiency is improved by opposing the direction of steel sheet passage and the flow direction of the pickling solution. Because the acid contained in the pickling solution is consumed by dissolving the scale layer 13 and the like, the acid concentration of the pickling solution is generally highest in the last pickling tank and lowest in the first pickling tank. Therefore, the pickling capacity (ability to dissolve the scale layer 13 and the like) of the last pickling tank is high, while the pickling capacity of the first pickling tank is low.

[0007] If the scale layer 13 and the like are not sufficiently removed by the pickling treatment, it may cause a poor appearance of the final product (cold-rolled steel sheet) or have other adverse effects such as preventing the optional surface treatment of the cold-rolled steel sheet from being performed sufficiently. For example, chemical conversion treatment is sometimes performed as a surface treatment for cold-rolled steel sheets. It is known that if an internal oxide layer 12 remains on the cold-rolled steel sheet, the chemical conversion treatability deteriorates. Therefore, when producing a cold-rolled steel sheet to be chemically treated, it is desirable to thoroughly remove the internal oxide layer 12 of the hot-rolled steel sheet 10 by pickling, but this causes an increase in the pickling time. Therefore, Patent Document 1 considers a pickling method that removes the internal oxide layer 12 in a short time.

[0008] Patent Document 1 discloses that when pickling a hot-rolled steel sheet using a pickling solution containing an acid and an inhibitor in a continuous pickling apparatus having three or more pickling tanks, the ratio of the inhibitor concentration to the acid concentration of the pickling solution in each pickling tank (hereinafter referred to as the "concentration ratio of the pickling solution" or "concentration ratio") is appropriately controlled. The concentration ratio of the pickling solution affects the dissolution rate of the scale layer and the internal oxide layer (referred to as the "intergranular oxide layer" in Patent Document 1), and the higher the concentration ratio, the slower the dissolution rate. The concentration ratio has a significant effect on the dissolution rate of the internal oxide layer in particular. Therefore, Patent Document 1 discloses that the concentration ratio of the pickling solution in each of the pickling tanks (excluding the first and last pickling tanks) where the internal oxide layer is mainly dissolved is lower than the concentration ratio of the pickling solution in the last pickling tank, thereby efficiently removing the internal oxide layer and suppressing over-pickling of the hot-rolled steel sheet. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6453691 Summary of the Invention [Problem to be solved by the invention]

[0010] When galvanizing is performed as a surface treatment for cold-rolled steel sheets, a high amount of dissolved Si on the surface of the steel sheet to be treated reduces the galvanizability. In the above-mentioned internal oxide layer 12 (see FIG. 1), at least a portion of the dissolved Si is oxidized to SiO2, so the concentration of dissolved Si on the surface of the internal oxide layer 12 is low. Therefore, when producing cold-rolled steel sheets to be used for galvanized steel sheets, it is preferable to pickle the hot-rolled steel sheet 10 so as to remove only the scale layer 13 and reduced iron layer 14 and leave the internal oxide layer 12.

[0011] If pickling is performed while suppressing the pickling capacity so as to leave the internal oxide layer 12, the scale layer 13 dissolves relatively easily in the pickling solution, but the reduced iron layer 14 may not dissolve sufficiently. As a result, the reduced iron layer 14 may remain on the surface of the internal oxide layer 12 of the hot-rolled steel sheet after pickling. Furthermore, although the reduced iron layer 14 is temporarily peeled off and removed from the hot-rolled steel sheet by pickling, the peeled reduced iron layer 14 may float in the pickling solution and re-adhere to another position on the hot-rolled steel sheet as it passes through the pickling tank. The reduced iron layer 14 remaining or redeposited on the surface of the hot-rolled steel sheet also remains on the cold-rolled steel sheet produced from that hot-rolled steel sheet, and can cause poor appearance of the galvanized steel sheet when the cold-rolled steel sheet is galvanized.

[0012] In order to completely remove the reduced iron layer 14 from the hot-rolled steel sheet 10, it is effective to improve the pickling ability during pickling, but there is a risk that the internal oxide layer 12 may also be dissolved and removed. Patent Document 1 describes a pickling method for efficiently removing the internal oxide layer, and does not take into consideration pickling that allows the internal oxide layer 12 to remain, nor does it take into consideration insufficient removal of the reduced iron layer 14 in such pickling, or redeposition of the reduced iron layer 14.

[0013] Therefore, an object of an embodiment of the present invention is to provide a pickling method that can effectively remove a reduced iron layer while suppressing dissolution of an internal oxide layer of a hot-rolled steel sheet. [Means for solving the problem]

[0014] Aspect 1 of the present invention is A method for pickling a hot-rolled steel sheet, comprising a pickling step of pickling the hot-rolled steel sheet by immersing it in n pickling tanks (n is 2 or more) arranged in order from a first pickling tank to an nth pickling tank, In the method for pickling a hot-rolled steel sheet, the temperature of the pickling solution contained in each of the pickling tanks satisfies the following formulas (1-1), (1-2), and (2): T ave(1~k) >T ave(k+1~n) (n=2, 3) (1-1) T ave(1~k) ≧Tave(k+1~n) (When n≧4) (1-2) where T ave is the average temperature of the pickling solution. T ave(k+1~n) >T n (However, when n≧4) (2) where: k is n / 2 when n is even, and (n+1) / 2 when n is odd, T n is the temperature (°C) of the pickling solution in the final stage nth pickling tank.

[0015] Aspect 2 of the present invention is Aspect 2 is a pickling method for a hot-rolled steel sheet according to aspect 1, wherein the number n of the pickling tanks is 3 or 4.

[0016] Aspect 3 of the present invention is In the pickling step, The hot-rolled steel sheet is continuously passed through each of the pickling tanks in order from the first pickling tank to the nth pickling tank, In the method for pickling a hot-rolled steel sheet according to aspect 1 or 2, the pickling solution is introduced into the nth pickling tank, passes through each pickling tank in order from the nth pickling tank to the first pickling tank in a direction opposite to a passing direction of the hot-rolled steel sheet, and is discharged from the first pickling tank.

[0017] A fourth aspect of the present invention is In the method for pickling a hot-rolled steel sheet according to any one of Aspects 1 to 3, the pickling solution contains at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid.

[0018] A fifth aspect of the present invention is The method for pickling a hot-rolled steel sheet according to any one of Aspects 1 to 4, wherein the hot-rolled steel sheet contains 1.0 mass % or more of Si and 1.5 mass % or more of Mn. [Effects of the Invention]

[0019] According to an embodiment of the present invention, it is possible to provide a pickling method that can effectively remove a reduced iron layer while suppressing dissolution of an internal oxide layer of a hot-rolled steel sheet. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 2 is a cross-sectional schematic view of a hot-rolled steel sheet before pickling. [Figure 2] 1 is a schematic diagram of a continuous pickling apparatus. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present inventors have conducted extensive research into a method for effectively removing the scale layer and the reduced iron layer while suppressing dissolution and removal of the internal oxide layer in a pickling treatment of a hot-rolled steel sheet on which an internal oxide layer, a scale layer, and a reduced iron layer have been formed, and have obtained the following findings.

[0022] As described above, in a typical continuous pickling apparatus, the pickling solution is introduced into the last pickling tank, passes through each tank in order toward the first pickling tank, and is discharged from the first pickling tank. In this way, by aligning the direction of travel of the hot-rolled steel sheet and the direction of flow of the pickling solution opposite to each other, the efficiency of pickling can be improved.

[0023] The pickling solution pickles the hot-rolled steel sheet while flowing from the last pickling tank to the first pickling tank, and the acid in the pickling solution is consumed in this process, so the acid concentration of the pickling solution in the first pickling tank is the lowest. As a result, the pickling capacity of the first pickling tank may be too low to sufficiently remove the reduced iron layer, and the reduced iron layer is removed in a subsequent pickling tank (e.g., the last pickling tank). If the reduced iron layer is peeled off and removed in the last pickling tank and then re-adheres to the hot-rolled steel sheet passing through the last pickling tank, the pickling ends with the reduced iron layer still re-adhered to the hot-rolled steel sheet, and the hot-rolled steel sheet is used as is for producing cold-rolled steel sheet.

[0024] Increasing the pickling capacity of the pickling tank is an effective measure to prevent the redeposition of reduced iron layers, but conventional methods for controlling the pickling capacity result in an increase in the pickling capacity of all pickling tanks. After the scale layer and reduced iron layer are removed in the upstream pickling tanks, the hot-rolled steel sheet passes through the final pickling tank, which has the highest acid concentration and the highest pickling capacity, causing the internal oxide layer to dissolve excessively.

[0025] The present inventors have found for the first time that it is possible to effectively remove the scale layer and the reduced iron layer while suppressing dissolution and removal of the internal oxide layer of the hot-rolled steel sheet by reducing the difference in pickling capacity between the "low pickling capacity" in the upstream pickling tank and the "high pickling capacity" in the downstream pickling tank, or by reversing the relationship between the pickling capacities. Specifically, the temperature of the pickling solution in the upstream pickling tank is increased to increase the pickling capacity of the upstream pickling tank compared to the conventional method, while the temperature of the pickling solution in the downstream pickling tank is decreased to decrease the pickling capacity of the downstream pickling tank compared to the conventional method.

[0026] In this specification, the term "pickling solution" refers to an acidic liquid supplied to and filled in each pickling tank. The pickling solution contains an acid and may further contain an inhibitor.

[0027] In this specification, the term "scale layer" refers to a layer whose matrix is ​​made of iron oxides. The iron oxides that make up the scale layer include iron-based oxides such as hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO), as well as silicon-based oxides such as fayalite (Fe2SiO4). The scale layer is formed during hot rolling, but can be removed by subsequent pickling.

[0028] In this specification, the term "internal oxide layer" refers to a steel layer whose matrix is ​​composed of Si-Mn oxides such as SiO2, MnSiO3, and Mn2SiO4. This internal oxide layer is formed when Si and Mn located near the surface of the base steel sheet react with oxygen that has diffused through the grain boundaries during the cooling process after hot-rolled steel sheet containing large amounts of Si and Mn is coiled at high temperatures (e.g., 550°C or higher).

[0029] Hereinafter, a method for pickling a hot-rolled steel sheet according to an embodiment of the present invention will be described with reference to the drawings.

[0030] The pickling method according to this embodiment includes a pickling step in which a hot-rolled steel sheet is pickled by immersing it in n (n is 2 or more) pickling tanks arranged in order from a first pickling tank to an nth pickling tank. The pickling step is preferably carried out using a continuous pickling apparatus 20 as shown in Fig. 2, for example. The continuous pickling apparatus 20 shown in FIG. 2 is equipped with four pickling tanks 1A, 1B, 1C, and 1D in this order, and the hot-rolled steel sheet 10 passes through the first pickling tank 1A (the first pickling tank in which the hot-rolled steel sheet 10 is immersed), the second pickling tank 1B (the second pickling tank in which the hot-rolled steel sheet 10 is immersed), the third pickling tank 1C (the third pickling tank in which the hot-rolled steel sheet 10 is immersed), and the fourth pickling tank 1D (the fourth pickling tank in which the hot-rolled steel sheet 10 is immersed) in the final stage.

[0031] In pickling using the continuous pickling apparatus 20 (continuous pickling), a hot-rolled steel sheet 10 unwound from a coil is passed through two or more consecutive pickling tanks 1A to 1D in sequence. Continuous pickling differs from batch-type pickling in that when a portion of the hot-rolled steel sheet 10 passes through the first pickling tank 1A and is immersed in the second pickling tank 1B, another portion of the hot-rolled steel sheet 10 may subsequently be immersed in the first pickling tank 1A (see FIG. 2). In other words, in continuous pickling using the continuous pickling apparatus 20, one hot-rolled steel sheet 10 is passed through the multiple pickling tanks 1A to 1D continuously along the sheet passing direction 10A, and therefore different portions of the hot-rolled steel sheet 10 may be immersed in different pickling tanks.

[0032] The pickling solution 2 is filled in each of a plurality of pickling tanks (pickling tanks 1A, 1B, 1C and 1D), and when the hot-rolled steel sheet 10 passes through the pickling tanks 1A, 1B, 1C and 1D, the pickling solution 2 dissolves the scale layer 13 present on the surface of the hot-rolled steel sheet 10.

[0033] The pickling solution 2 is preferably introduced into the nth pickling tank at the final stage (the fourth pickling tank 1D in FIG. 2), passes through each pickling tank in order from the nth pickling tank to the first pickling tank 1A in the direction opposite to the sheet passing direction 10A of the hot-rolled steel sheet 10, and is then discharged from the first pickling tank 1A. This allows the acid in the pickling solution to be used efficiently.

[0034] In the example of a continuous pickling apparatus 20 shown in FIG. 2, the pickling solution 2 is introduced from the pickling solution supply tank 5 into the fourth pickling tank 1D, the final tank. The pickling solution 2 then moves from the fourth pickling tank 1D to the third pickling tank 1C along arrow 2A, from the third pickling tank 1C to the second pickling tank 1B along arrow 2B, and from the second pickling tank 1B to the first pickling tank 1A along arrow 2C. The pickling solution 2 is then discharged from the first pickling tank 1A. The discharged pickling solution 2 is recovered in the pickling solution recovery tank 4, where its concentration is readjusted and it is returned to the pickling solution supply tank 5 for reuse.

[0035] In this embodiment, the arithmetic mean (average temperature T ave ) satisfies the following formulas (1-1), (1-2), and (2). When the number of pickling tanks n is 2 or 3, it is sufficient to satisfy formula (1-1), and when the number of pickling tanks n is 4 or more, it is sufficient to satisfy formulas (1-2) and (2). The temperature of the pickling solution 2 in each pickling tank is measured near the center of the pickling tank when viewed from above. T ave(1~k) >T ave(k+1~n) (n=2, 3) (1-1) T ave(1~k) ≧T ave(k+1~n) (When n≧4) (1-2) where T aveis the average temperature of the pickling solution. T ave(k+1~n) >T n (However, when n≧4) (2) where: k is n / 2 when n is even, and (n+1) / 2 when n is odd, T n is the temperature (°C) of the pickling solution in the final stage nth pickling tank.

[0036] "T ave(1~k) " is the average temperature of the pickling solution from the first pickling tank to the kth pickling tank. ave(k+1~n) " is the average value (average temperature) of the temperatures of the pickling solutions in the (k+1)th pickling tank to the nth pickling tank.

[0037] In the examples described below, n=2 or 4, so that formulas (1-1), (1-2), and (2) can be rewritten as formulas (1a), (1b), and (2b), respectively. When n=2 T1>T2(1a) When n=4 T ave(1,2) ≧T ave(3,4) (1b) T ave(3,4) >T4(2b) T1 is the temperature of the pickling solution in the first pickling tank, and T2 is the temperature of the pickling solution in the second pickling tank.

[0038] As defined by the formulas (1-1) and (1-2), the average temperature T of the pickling solution 2 in each of the pickling tanks from the first pickling tank 1A to the kth pickling tank (in the example of FIG. 2, from the first pickling tank 1A to the second pickling tank 1B) is ave(1~k) The average temperature T of the pickling solution 2 in each pickling tank from the (k+1)th pickling tank to the final nth pickling tank (in Figure 2, from the third pickling tank 1C to the fourth pickling tank 1D) ave(k+1~n)By making it higher than (when n=2 or 3) or more (when n≧4), the pickling capacity of the first pickling tank 1A to the kth pickling tank can be improved, and the pickling capacity of the (k+1)th tank to the last pickling tank can be reduced, thereby reducing the difference in pickling capacity between the previous and next tanks.

[0039] Furthermore, as defined by the formula (2), the average temperature T of the pickling solution 2 in each pickling tank from the (k+1)th pickling tank to the final nth pickling tank (in FIG. 2, from the third pickling tank 1C to the fourth pickling tank 1D) ave(k+1~n) However, by controlling the temperature of the pickling solution 2 so that it is higher than the temperature of the pickling solution 2 in the nth pickling tank (the fourth pickling tank 1D in Figure 2), which is located at the final stage, it is possible to remove the scale layer and the reduced iron layer and suppress the dissolution of the internal oxide layer.

[0040] By controlling the temperature of the pickling solution 2 in each pickling tank so as to satisfy the formulas (1-1), (1-2), and (2), it is possible to enhance the ability to dissolve and remove the scale layer and reduced iron layer in the upstream pickling tank and to suppress the dissolution of the internal oxide layer in the downstream pickling tank.

[0041] As a method for controlling the temperature of the pickling solution 2 in each pickling tank, for example, a heating means (heater or the like) for heating the pickling solution and a thermometer for measuring the temperature of the pickling solution may be installed in each pickling tank, and the pickling solution may be heated to a desired temperature while measuring the temperature of the pickling solution 2.

[0042] The temperature of the pickling solution 2 is preferably 60° C. or higher and 95° C. or lower, and more preferably 70° C. or higher and 90° C. or lower. The "temperature of the pickling solution 2" varies depending on which pickling tank the temperature of the pickling solution is measured in, but it is preferable that the temperatures of all the pickling solutions 2 are within the above temperature range.

[0043] The acid used for pickling is not particularly limited as long as it is an acid normally used for removing scale layers, and for example, mineral acids such as hydrochloric acid, sulfuric acid, nitric acid, etc. In other words, the pickling solution may contain one or more acids selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid. Considering the economy and pickling speed, the use of hydrochloric acid is preferred. The acid concentration of the pickling solution 2 is preferably 1.0 mass% or more and 20.0 mass% or less, and more preferably 1.5 mass% or more and 18.0 mass% or less. Note that the "acid concentration of the pickling solution 2" may vary depending on which pickling tank the acid concentration of the pickling solution is measured in, but it is preferable that all the acid concentrations are within the above-mentioned acid concentration range.

[0044] The pickling solution 2 may contain known additive components in addition to the acid, such as an inhibitor that has the effect of suppressing corrosion of the steel sheet, and a pickling accelerator for improving the dissolution rate of the scale layer.

[0045] The number (n) of pickling tanks is preferably 3 or more (for example, 3, 4, or more). This allows the redeposited reduced iron layer 14 to be peeled off again in the third pickling tank 1C (and fourth pickling tank 1D) on the downstream side when the peeled off reduced iron layer 14 re-adheres to the hot-rolled steel sheet 10 in the first pickling tank 1A or the second pickling tank 1B on the upstream side.

[0046] In this embodiment, the hot-rolled steel sheet 10 to be pickled includes a reduced iron layer 14, a scale layer 13, and an internal oxide layer 12 on the surface side of the hot-rolled steel sheet 10, as illustrated in FIG. 1 . An example of a hot-rolled steel sheet 10 having such a layer structure is a hot-rolled steel sheet obtained by hot-rolling steel containing large amounts of Si and Mn. For example, a hot-rolled steel sheet manufactured from steel containing 1.0 mass % or more of Si and 1.5 mass % or more of Mn can be cited. As described above, solute Si present on the surface of a cold-rolled steel sheet reduces the galvanization properties of the galvanized steel. Therefore, it is preferable that an internal oxide layer 12 containing SiO2 remains on the surface of the cold-rolled steel sheet to be hot-dip galvanized.

[0047] The pickling method for a hot-rolled steel sheet according to this embodiment can sufficiently remove the reduced iron layer 14 and the scale layer 13, which deteriorate the galvanizability, while leaving the internal oxide layer 12, which improves the galvanizability. Therefore, this method is suitable for pickling a hot-rolled steel sheet made of steel containing a large amount of Si and Mn.

[0048] Other conditions for the pickling treatment are not particularly limited and may be set in accordance with the various conditions employed in conventional pickling treatments. For example, the sheet threading speed may be set based on the required pickling time and the number of pickling tanks. The pickling treatment may also be performed in a predetermined atmosphere, such as air. [Example]

[0049] [Example 1 and Comparative Example 1] The pickling test was carried out using two pickling tanks.

[0050] (1) Measurement of the peeling time of the reduced iron layer A slab containing 1.7% by mass of Si and 2.0% by mass of Mn was heated to 1200°C, hot rolled at a finishing temperature of 930°C, coiled at a coiling temperature of 660°C, and cooled in air to produce a hot-rolled steel sheet (sample 1). Test pieces (50 mm × 50 mm) were taken from a hot-rolled steel sheet (Test Material 1). Pickling solutions with acid concentrations C1 and C2 (mass%) and temperatures T1 and T2 (°C) shown in Table 1 were prepared in two pickling tanks (first and second pickling tanks). Hydrochloric acid was used as the acid. After pickling for 30 seconds in the first pickling tank, the test pieces were immersed in the second pickling tank for 30 seconds (total immersion time (pickling time) 60 seconds). The test pieces immersed in each container were visually observed to measure the time it took for the reduced iron layer to peel off, and the results are shown in the "Reduced iron layer peeling time" column in Table 1.

[0051] (2) Measurement of pickling dissolution amount A slab containing 2.0 mass% Si and 2.6 mass% Mn was heated to 1200°C, hot rolled at a finishing temperature of 930°C, coiled at a coiling temperature of 660°C, and cooled in air to produce a hot-rolled steel sheet (sample 2). Test pieces were taken from the hot-rolled steel sheet (sample 2). Pickling solutions with the acid concentrations and temperatures shown in Table 1 were prepared in two containers (first pickling tank and second pickling tank). Hydrochloric acid was used as the acid. After pickling for 20 seconds in the first pickling tank, the test pieces were immersed in the second pickling tank for 20 seconds (total immersion time (pickling time) 40 seconds). The dry mass (g) of the test piece before and after pickling was measured, and the difference between them was calculated as the area (m 2 ) is divided by the "pickling dissolution amount (g / m 2 )" and listed in Table 1.

[0052] [Table 1]

[0053] As can be seen from the results in Table 1, Example 1, in which pickling was performed under conditions satisfying the requirements of the embodiment of the present invention, was able to shorten the time required to strip the reduced iron layer compared to Comparative Example 1, in which pickling was performed under conditions not satisfying the requirements of the embodiment. On the other hand, the pickling solubility in Example 1 was similar to that of Comparative Example 1. This confirmed that the reduced iron layer could be stripped in a short time without causing excessive dissolution of the internal oxide layer.

[0054] [Example 2 and Comparative Example 2] Pickling tests were carried out using a continuous pickling equipment with four pickling tanks. A hot-rolled steel sheet (Sample 1) was produced under the same conditions as those for Sample 1 of Example 1 and Comparative Example 1.

[0055] (3) Whether or not the reduced iron layer remains after pickling A hot-rolled steel sheet (specimen 1) was prepared and pickled (continuous pickling) by passing it continuously through four pickling tanks using a continuous pickling apparatus 20 equipped with four pickling tanks 1A to 1D as shown in Figure 2. Hydrochloric acid was used as the acid. The pickling conditions for each specimen (acid concentrations C1 to C4 (mass%) and temperatures T1 to T4 (°C) of the pickling solution in each pickling tank) were as shown in Table 2. The surface of the hot-rolled steel sheet (over the entire length and width) after pickling was observed with the naked eye to confirm whether or not a reduced iron layer remained.

[0056] (4) Measurement of the remaining amount of the internal oxide layer Hot-rolled steel sheets (test material 1) were prepared and subjected to continuous pickling using the same pickling conditions and pickling equipment as in (3) above. Test pieces (33 mm × 33 mm) were taken from each of the hot-rolled steel sheets after continuous pickling, and further pickling was performed (additional pickling) in a pickling solution with a temperature of 80°C and a hydrochloric acid concentration of 10 mass % for varying times. The amount of pickling dissolution (g / m) was calculated from the difference in the dry mass of the hot-rolled steel sheet before and after the additional pickling. 2 The amount of the pickling solution at the inflection point of the graph was calculated as the remaining amount of the internal oxide layer (g / m 2 ) was calculated.

[0057] [Table 2]

[0058] As can be seen from the results in Table 2, in Example 2, in which pickling was performed using a continuous pickling apparatus under conditions satisfying the requirements of the embodiment of the present invention, the reduced iron layer was successfully removed from the hot-rolled steel sheet. On the other hand, in Comparative Example 2, in which pickling was performed under conditions not satisfying the requirements of the embodiment of the present invention, the reduced iron layer remained on the hot-rolled steel sheet. The remaining amount of the internal oxide layer was similar between Example 2 and Comparative Example 2. This confirmed that the reduced iron layer could be reliably peeled off without causing excessive dissolution of the internal oxide layer. [Explanation of symbols]

[0059] 1A 1st pickling tank 1B 2nd pickling tank 1C 3rd pickling tank 1D 4th pickling tank 2 Pickling solution 20 Continuous pickling equipment 2A~2C: Flow of pickling solution T1 Temperature of the pickling solution in the first pickling tank T2 Temperature of the pickling solution in the second pickling tank T3 Temperature of the pickling solution in the third pickling tank T4 Temperature of pickling solution in the fourth pickling tank C1 Acid concentration of pickling solution in the first pickling tank C2 Acid concentration of pickling solution in the second pickling tank C3 Acid concentration of pickling solution in the third pickling tank C4 Acid concentration of pickling solution in No. 4 pickling tank 4. Pickling solution recovery tank 5 Pickling liquid supply tank 10 Hot rolled steel plate 11 Steel plate substrate 12 Internal oxide layer 13 Scale Layer 14 Reduced iron layer

Claims

1. A method for pickling a hot-rolled steel sheet, comprising a pickling step of pickling a hot-rolled steel sheet by sequentially immersing the hot-rolled steel sheet in n pickling tanks (n is 2 or more) arranged in order from a first pickling tank to an nth pickling tank, The method for pickling a hot-rolled steel sheet, wherein the temperatures of the pickling solutions contained in each of the pickling tanks satisfy the following formulas (1-1), (1-2), and (2): T ave(1～k) >T ave(k+1～ｎ) (When n = 2 or 3) (1-1) T ave(1～k) ≧T ave(k+1～ｎ) (When n≧4) (1-2) Here, T ave is the average temperature of the pickling solution. T ave(ｋ+1～ｎ) >T ｎ (However, when n≧4) (2) where: k is n / 2 when n is even, and (n+1) / 2 when n is odd, T ｎ is the temperature (°C) of the pickling solution in the final stage nth pickling tank.

2. The method for pickling a hot-rolled steel sheet according to claim 1, wherein the number n of the pickling tanks is 3 or 4.

3. In the pickling step, The hot-rolled steel sheet is continuously passed through each pickling tank in order from the first pickling tank to the nth pickling tank, 2. The method for pickling a hot-rolled steel sheet according to claim 1, wherein the pickling solution is introduced into the nth pickling tank, passes through each pickling tank in turn from the nth pickling tank to the first pickling tank in a direction opposite to a running direction of the hot-rolled steel sheet, and is discharged from the first pickling tank.

4. The method for pickling a hot-rolled steel sheet according to claim 1, wherein the pickling solution contains at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid.

5. The method for pickling a hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains 1.0 mass % or more of Si and 1.5 mass % or more of Mn.

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