Hot-rolled steel sheet

Abstract

A hot-rolled steel sheet which: comprises a steel sheet and a scale layer formed on the surface of the steel sheet, the steel sheet having a chemical composition comprising, by mass%, 0.05 to 0.20% of C, 0.005 to 0.30% of Si, 0.05 to 1.20% of Mn, 0.001 to 0.010% of P, 0.010% or less of S, 0.05 to 0.25% of Sn, 0.001 to 0.010% of N, 0.001 to 0.040% of Al, 0 to 0.30% of Cu, 0 to 0.15% of Ni, 0 to 0.30% of W, 0 to 0.30% of Mo, 0 to 0.20% of Cr, 0 to 0.020% of V, 0 to 0.020% of Ti, 0 to 0.025% of Nb, and the balance Fe and impurities; satisfies [0.50 < 4.6 x C + 1.5 x Si - 1.2 x Sn + 0.5 x (Cu + Ni + W + Mo) - 0.2 x (Cr + V + Ti + Nb) < 1.10]; has a magnetite volume fraction of 60% or more in the scale layer; and has an Sn-concentrated layer having an Sn content of 0.01 to 0.85% in the scale layer on the interface side with the steel sheet.

Description

[Technical field]

[0001] The present invention relates to a hot-rolled steel sheet. [Background technology]

[0002] Conventionally, containers used for railroad vehicles or land or sea transport require materials that are lightweight and have excellent corrosion resistance. For this reason, aluminum has been the mainstream material for these containers. However, aluminum has problems in that it is expensive and has low strength. For this reason, there has been a demand for steel materials that have high strength and excellent corrosion resistance as container materials.

[0003] Conventionally, the above railcars and containers have a tensile strength of 50 kgf / mm 2 In the past, highly corrosion-resistant rolled steel of the corrosion resistance class (490 MPa) and coated steel have been used.

[0004] However, as the use of maritime containers has increased, the environments in which the containers are used have become even harsher, and the problem has arisen that even painted containers manufactured using the above-mentioned highly corrosion-resistant rolled steel materials suffer from localized corrosion and are unable to withstand long-term use.

[0005] Research has been conducted to improve corrosion resistance in a corrosive environment containing chlorides. For example, Patent Document 1 discloses a hot-rolled steel sheet containing Cu, Sn, etc., and Patent Document 2 discloses a hot-rolled steel sheet containing Si, Cu, Cr, etc.

[0006] Meanwhile, a scale layer is formed on the surface of hot-rolled steel sheets during the manufacturing process. The scale layer (mill scale) formed at high temperatures generally has excellent corrosion resistance, so in the civil engineering and construction fields, some steel materials are used with the mill scale remaining. However, if the mill scale remains, there is a risk that localized corrosion will progress if a scale defect occurs.

[0007] Therefore, containers and the like generally use steel materials obtained by applying anticorrosive coating to hot-rolled steel sheets. Here, when applying anticorrosive coating, if the coating is applied over mill scale, the anticorrosive coating may peel off together with the scale. Therefore, when coating hot-rolled steel sheets, it is usually necessary to remove the mill scale by shot blasting or the like before coating. Therefore, it is desirable for hot-rolled steel sheets used in applications where anticorrosive coating is applied to have good scale removability.

[0008] Usually, when removing mill scale formed during the hot rolling process, the surface of the hot rolled steel sheet is treated by shot blasting. In this case, if mill scale with high adhesion is formed, the shot blasting treatment needs to be repeated many times.

[0009] Patent Document 3 discloses a hot-rolled steel sheet that has excellent scale removal properties and excellent corrosion resistance in a corrosive environment containing chlorides. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 7-157841 [Patent Document 2] JP 2002-105596 A [Patent Document 3] International Publication No. 2017 / 183133 Summary of the Invention [Problem to be solved by the invention]

[0011] The hot-rolled steel sheet described in Patent Document 3 has excellent scale removability by shot blasting and excellent corrosion resistance in a corrosive environment containing chlorides. However, when shot blasting is used, the steel sheet surface becomes rough, and a thick coating is required. Therefore, there is a demand for scale removability by pickling that does not cause roughness of the steel sheet surface. As a result of further investigations by the present inventors, it was found that the hot-rolled steel sheet described in Patent Document 3 has room for improvement in terms of scale removability by pickling.

[0012] The present invention aims to provide a hot-rolled steel sheet that has excellent descaling properties by pickling, excellent appearance after descaling by pickling, and excellent corrosion resistance in a corrosive environment containing chlorides. In the present invention, "excellent appearance after descaling by pickling" means that the steel sheet has a smooth surface with few scale defects. In addition, "excellent corrosion resistance" means that the hot-rolled steel sheet is excellent in both bare corrosion resistance and post-painting corrosion resistance in consideration of the case where the hot-rolled steel sheet is used as a container or the like. [Means for solving the problem]

[0013] The present inventors have conducted extensive research to solve the above problems and have come to the following findings.

[0014] (a) The addition of Sn to steel sheet improves corrosion resistance, but at the same time reduces the ability to remove scale during pickling. In order to improve the ability to remove scale during pickling, it is necessary to control the acid resistance of the steel sheet surface by adjusting the content of alloying elements.

[0015] (b) As a result of extensive investigations using various steel materials, the inventors have found that it is possible to achieve both corrosion resistance and scale removal properties during pickling by appropriately adjusting the contents of the following alloying elements: C, Si, Sn, Cu, Ni, W, Mo, Cr, V, Ti and Nb.

[0016] (c) In addition, by making the structure of the scale layer mainly composed of magnetite and forming a Sn-enriched layer in the scale layer at the interface with the base metal, it is possible to improve the peelability of the scale during pickling.

[0017] The present invention has been made based on the above findings, and the gist of the present invention is the following hot-rolled steel sheet.

[0018] (1) A steel plate and a scale layer formed on a surface of the steel plate, The chemical composition of the steel sheet is, in mass%, C: 0.05-0.20%, Si: 0.005 to 0.30%, Mn: 0.05-1.20%, P: 0.001 to 0.010%, S: 0.010% or less, Sn: 0.05-0.25%, N: 0.001 to 0.010%, Al: 0.001 to 0.040%, Cu: 0-0.30%, Ni: 0-0.15%, W: 0~0.30%, Mo: 0-0.30%, Cr: 0~0.20%, V: 0~0.020%, Ti: 0 to 0.020%, Nb: 0 to 0.025%, The balance is Fe and impurities. The following formula (i) is satisfied: The volume fraction of magnetite in the scale layer is 60% or more, The scale layer has an Sn-enriched layer having an Sn content of 0.01 to 0.85% at an interface between the steel sheet and the scale layer. Hot rolled steel plate. 0.50<(4.6×C+1.5×Si-1.2×Sn+0.5×(Cu+Ni+W+Mo)-0.2×(Cr+V+Ti+Nb))<1.10 ···(i) In the above formula (i), each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained. Effect of the Invention

[0019] According to the present invention, it is possible to obtain a hot-rolled steel sheet that has excellent scale removal properties by pickling, excellent appearance after scale removal by pickling, and excellent corrosion resistance in a corrosive environment containing chlorides. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Each of the features of the present invention will be described in detail below.

[0021] (A) Chemical composition of steel plate The reasons for limiting the content of each element are as follows. In the following description, "%" for the content means "mass %."

[0022] C: 0.05 to 0.20% C is an element necessary to ensure the strength of the material. In addition, C has the effect of suppressing the excessive diffusion of Sn into the scale layer during scale formation, thereby reducing acid resistance and improving scale peelability during pickling. Furthermore, if the C content is less than 0.05%, the scale grows significantly and the layer containing wustite becomes excessive, resulting in a decrease in the volume fraction of magnetite.

[0023] However, if the C content exceeds 0.20%, the weldability is significantly reduced. In addition, as the C content increases, the amount of cementite that acts as a cathode in an environment where the pH level decreases and promotes corrosion increases, thereby reducing the corrosion resistance. Furthermore, if C is excessively contained, scale blisters are generated during finish rolling and the subsequent cooling process, and the scale layer is partially peeled off. As a result, the volume fraction of magnetite in the scale layer decreases, the scale peelability during pickling is deteriorated, and the aesthetic appearance is deteriorated. For this reason, the C content is set to 0.05 to 0.20%. The C content is preferably 0.06% or more. In addition, the C content is preferably 0.18% or less, and more preferably 0.16% or less.

[0024] Silicon: 0.005 to 0.30% Si is an element necessary for deoxidation during steelmaking. Si also has the effect of improving corrosion resistance. In addition, Si has the effect of reducing acid resistance and improving scale peelability during pickling by diffusing Sn into the scale layer during scale formation. If the Si content is less than 0.005%, these effects cannot be obtained sufficiently. However, if the Si content exceeds 0.30%, the steel sheet surface becomes fayalite (2FeO·SiO2), and fine Fe2O3 remains on the outermost surface, causing red scale to form. In addition, the formation of fayalite reduces the volume fraction of magnetite, and furthermore, the Sn content in the Sn-enriched layer increases, so that scale peelability during pickling deteriorates. For this reason, the Si content is set to 0.005 to 0.30%. The Si content is preferably 0.01% or more, and preferably 0.25% or less.

[0025] Mn: 0.05 to 1.20% Mn is an element necessary for increasing the strength of steel sheet. If the Mn content is less than 0.05%, this effect cannot be sufficiently obtained. However, if the Mn content exceeds 1.20%, it becomes difficult to maintain workability. For this reason, the Mn content is set to 0.05 to 1.20%. The Mn content is preferably 0.10% or more and 1.00% or less.

[0026] P: 0.001 to 0.010% P is an element that is effective in increasing strength and is beneficial in improving corrosion resistance. It also has the effect of suppressing the formation of fayalite, and as a result, preventing the occurrence of red scale. If the P content is less than 0.001%, these effects cannot be sufficiently obtained. However, if the P content exceeds 0.010%, it causes slab embrittlement (cracks) during steel plate production. Therefore, the P content is set to 0.001 to 0.010%. The P content is preferably 0.008% or less.

[0027] S: 0.010% or less S combines with Mn to form the sulfide MnS. This sulfide is easily deformed, elongates during rolling, and exists in the steel, deteriorating the bendability and workability of the steel. In particular, in high-strength steel, the S content must be as low as possible to increase crack susceptibility, with the upper limit set at 0.010%. The S content is preferably 0.005% or less. On the other hand, excessive reduction can cause a decrease in economic efficiency, so the S content is preferably 0.001% or more.

[0028] Sn: 0.05 to 0.25% Sn is an element that has the effect of significantly improving corrosion resistance in an atmospheric environment containing chlorides and an acidic environment. In addition, it forms a Sn-enriched layer at the interface between the steel sheet and the scale layer in the scale layer, improving the scale peelability. If the Sn content is less than 0.05%, these effects cannot be obtained sufficiently. However, if the Sn content exceeds 0.25%, the dissolution reaction of the steel sheet surface by acid during pickling is suppressed, that is, the acid resistance is improved and the scale peelability during pickling is reduced. Furthermore, the scale peelability during pickling is also reduced by the excessive Sn content in the Sn-enriched layer formed in the scale layer. In addition, although the reason is unclear, when Sn is contained in the base material, red scale tends to be easily formed. For this reason, the Sn content is set to 0.05 to 0.25%. The Sn content is preferably 0.10% or more, and preferably 0.20% or less.

[0029] N: 0.001 to 0.010% N becomes ammonia and dissolves in the aqueous solution. In an environment with a high amount of airborne salt, Fe 3+ It has the effect of improving the corrosion resistance of steel sheets in salty environments by suppressing the decrease in pH due to hydrolysis of N. If the N content is less than 0.001%, this effect cannot be sufficiently obtained. However, if the N content exceeds 0.010%, not only does the effect saturate, but the toughness of the steel sheet is also deteriorated. For this reason, the N content is set to 0.001 to 0.010%.

[0030] Al: 0.001 to 0.040% Al is an element that improves the corrosion resistance of steel. In addition, like P, it also has the effect of suppressing the formation of fayalite, and as a result, preventing the occurrence of red scale. If the Al content is less than 0.001%, these effects cannot be sufficiently obtained. However, if the Al content exceeds 0.040%, not only does the effect saturate, but the steel sheet becomes easily embrittled, and the corrosion resistance also worsens. For this reason, the Al content is set to 0.001 to 0.040%. The Al content is preferably 0.005% or more, and more preferably 0.010% or more. The Al content is preferably 0.030% or less, and more preferably 0.020% or less.

[0031] Cu: 0-0.30% Ni: 0 to 0.15% W: 0~0.30% Mo: 0 to 0.30% Cu, Ni, W and Mo are elements that have the effect of improving corrosion resistance. However, these elements improve acid resistance and reduce scale removal during pickling. Therefore, the contents of each element are set as follows: Cu: 0.30% or less, Ni: 0.15% or less, W: 0.30% or less, and Mo: 0.30% or less. The Cu content is preferably 0.25% or less, the Ni content is preferably 0.10% or less, the W content is preferably 0.25% or less, and the Mo content is preferably 0.25% or less.

[0032] In order to obtain the above-mentioned effects more reliably, the Cu content is preferably 0.01% or more, and more preferably 0.02% or more, the Ni content is preferably 0.01% or more, and more preferably 0.02% or more, the W content is preferably 0.01% or more, and more preferably 0.05% or more, and the Mo content is preferably 0.01% or more, and more preferably 0.03% or more.

[0033] Cr: 0~0.20% V: 0~0.020% Ti: 0 to 0.020% Nb: 0 to 0.025% Cr, V, Ti and Nb are elements that have the effect of improving corrosion resistance. In addition, they also have the effect of reducing acid resistance and improving scale removal during pickling. However, excessive content of these elements leads to excessive growth of scale, which in turn deteriorates scale removal. Therefore, the content of each element is set to Cr: 0.20% or less, V: 0.020% or less, Ti: 0.020% or less, and Nb: 0.025% or less. The Cr content is preferably 0.15% or less, the V content is preferably 0.015% or less, the Ti content is preferably 0.015% or less, and the Nb content is preferably 0.020% or less.

[0034] In order to obtain the above-mentioned effects more reliably, the Cr content is preferably 0.01% or more, and more preferably 0.02% or more, the V content is preferably 0.001% or more, and more preferably 0.005% or more, the Ti content is preferably 0.005% or more, and more preferably 0.015% or more, and the Nb content is preferably 0.002% or more, and more preferably 0.005% or more.

[0035] In the chemical composition of the hot-rolled steel sheet of the present invention, the balance is Fe and impurities. Here, the term "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps and manufacturing processes during industrial production of steel, and are permissible within a range that does not adversely affect the present invention.

[0036] Furthermore, as described above, it has been found that in the present invention, by appropriately adjusting the contents of C, Si, Sn, Cu, Ni, W, Mo, Cr, V, Ti and Nb, both corrosion resistance and scale strippability during pickling can be achieved. Specifically, in addition to setting the contents of each element within the above ranges, it is necessary to satisfy the following formula (i). By setting the central value in formula (i) to more than 0.50, scale strippability can be improved, and by setting the central value in formula (i) to less than 1.10, corrosion resistance can be improved. The central value in formula (i) is preferably 0.60 or more and preferably 0.80 or less. 0.50<(4.6×C+1.5×Si-1.2×Sn+0.5×(Cu+Ni+W+Mo)-0.2×(Cr+V+Ti+Nb))<1.10 ···(i) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0037] (B) Scale layer The scale layer formed on the surface of a steel plate is generally composed of magnetite seams, wustite, magnetite, and hematite, and is layered in the following order from the steel surface in the thickness direction: magnetite seam, wustite, magnetite, and hematite. In general, when removing the scale layer by shot blasting, it is necessary to mechanically destroy the scale layer. Therefore, if a strong magnetite seam exists at the interface between the base material and the scale layer, it becomes difficult to remove the scale layer, and therefore it has been necessary to reduce the volume fraction of magnetite in the scale layer.

[0038] On the other hand, in the case of removing the scale by pickling, which is the subject of the present invention, the solubility of the scale layer in acid can be increased by making the structure of the scale layer mainly composed of magnetite. As a result, the scale removal property during pickling can be improved. Specifically, the volume fraction of magnetite contained in the scale layer is set to 60% or more. The volume fraction of magnetite contained in the scale layer is preferably 90% or less. The other iron oxides contained in the scale layer may include one or more selected from wustite and hematite.

[0039] As described above, the scale layer is laminated in the thickness direction from the steel surface in the order of magnetite seam, wustite, magnetite, and hematite. In the present invention, the volume fraction of magnetite means the total of magnetite and magnetite seam in the scale layer. In addition, the volume fraction of magnetite of 60% or more also includes the case where magnetite seam accounts for the majority. However, as described later, if a Sn-enriched layer with a Sn content of 0.01 to 0.85% is present in the scale layer, excellent scale peelability can be obtained.

[0040] The volume fraction of magnetite in the scale layer is measured by the following method. First, the scale layer formed on the steel plate surface is scraped off using a hammer, a cutter knife, etc. until the steel plate surface can be confirmed, and the scale layer is completely sampled and crushed to obtain a powder sample. In this case, the scale layer is sampled from five or more locations, and the volume fraction of magnetite is measured using each powder sample, and the average value is adopted.

[0041] Next, a calibration curve to be used for quantitative analysis by X-ray diffraction is obtained. Specifically, magnetite, wustite, and hematite are mixed in different weights to a total of 20 g, and then 0.4 g of zinc oxide is added as an internal standard to prepare multiple standard samples. The multiple standard samples are then analyzed by X-ray diffraction, and the calibration curve is obtained by measuring the area of ​​the X-ray peaks of magnetite, wustite, and hematite relative to the internal standard.

[0042] Then, 20 g of the powder sample taken from the above steel plate is mixed with 0.4 g of zinc oxide as an internal standard, and then analyzed by X-ray diffraction to determine the volume fraction of magnetite based on the above calibration curve.

[0043] The hot-rolled steel sheet according to the present invention has a Sn-enriched layer with a Sn content of 0.01 to 0.85% on the interface side of the scale layer between the steel sheet and the scale layer. Here, "the interface side of the scale layer between the steel sheet and the scale layer" means the interface side from the center position in the thickness direction of the scale layer. As described above, the higher the volume fraction of magnetite in the scale layer, the better the scale peelability during pickling. However, when magnetite seams grow in a layered and strong manner to cover the surface of the base steel sheet by hot rolling at high temperatures, Sn is excessively concentrated between the magnetite seams and the base steel sheet, which makes it easier for partial peeling of the scale layer to occur during hot rolling, causing scale defects.

[0044] Therefore, by setting appropriate manufacturing conditions, Sn in the steel sheet is dissolved and diffused into the scale layer before strong magnetite seams are formed. As a result, a Sn-enriched layer is formed in the scale layer at the interface with the base steel sheet. Although the detailed mechanism is not clear, the Sn-enriched layer is formed in advance, which suppresses the oxidation of Fe in the scale layer and the formation of strong magnetite seams. This also suppresses excessive concentration of Sn, making it difficult for the scale to partially peel off during hot rolling, improving the scale peelability by pickling and the aesthetic appearance after scale peeling by pickling.

[0045] In order to obtain excellent scale peelability by pickling, it is necessary to have an Sn-enriched layer with an Sn content of 0.01 to 0.85%. As described later, in the present invention, the region in the scale with an Sn content of 0.01% or more is defined as the Sn-enriched layer. When the Sn-enriched layer is not formed, that is, when the Sn content in the scale is less than 0.01%, the formation of wustite in the scale layer is promoted, and as a result, a strong magnetite layer is formed in the scale layer, so that the peelability of the scale layer is reduced and the aesthetics are also deteriorated. On the other hand, when the Sn content in the Sn-enriched layer exceeds 0.85%, the scale layer is likely to peel off during hot rolling, which is a cause of scale defects in the roll and the hot-rolled steel sheet, and this is not preferable.

[0046] If the scale layer is partially peeled off during the process up to pickling due to peeling of the scale layer during hot rolling, scale cracking due to rapid cooling after finish rolling, generation of scale blisters during cooling, etc., the thickness of the scale layer becomes non-uniform. Furthermore, if a layer containing magnetite in the scale layer is removed due to partial peeling of the scale layer, a portion in which the volume fraction of magnetite in the scale layer is reduced is generated. As a result, there are portions in which the dissolution reaction of the steel sheet base material is excessive and portions in which the dissolution reaction is insufficient during pickling, and the scale cannot be removed uniformly, resulting in a deterioration in aesthetic appearance.

[0047] There is no particular need to limit the thickness of the Sn-enriched layer. However, in order to further improve the scale peelability by pickling, the thicker the Sn-enriched layer, the more preferable it is, specifically, preferably 0.1 μm or more, more preferably 0.5 μm or more or 1.0 μm or more, and even more preferably 2.0 μm or more. There is no particular limit to the thickness of the scale layer, but it is preferably 5 μm or more on average, and more preferably 10 μm or more. On the other hand, the thickness of the scale layer is preferably 40 μm or less on average, and more preferably 30 μm or less.

[0048] The Sn content in the Sn-enriched layer is determined as follows. That is, a sample taken from a hot-rolled steel sheet is embedded in resin, and a cross section in the thickness direction is mirror-finished by wet polishing. Then, a line analysis of Sn from the scale to the steel sheet is performed on the sample at a magnification of 500 times using an electron probe microanalyzer (EPMA), and the Sn content from the interface between the steel sheet and the scale to the scale layer side is measured. The measurement pitch in the line analysis is 10 nm. The region where the Sn content measured on the line is 0.01% or more is defined as the Sn-enriched layer, and the thickness of the region is defined as the thickness of the Sn-enriched layer. The maximum value of the Sn content measured on the line is defined as the Sn content in the Sn-enriched layer.

[0049] Here, the EPMA can obtain the reflected X-ray intensity of Sn, but cannot directly quantify the Sn content. Therefore, in the present invention, first, a powder sample taken from the scale layer is dissolved in acid, and the average Sn content in the scale layer is measured by inductively coupled plasma (ICP) atomic emission spectrometry. Next, the average value of the reflected X-ray intensity of Sn over the entire thickness of the scale layer is measured by EPMA. Then, the reflected X-ray intensity of Sn is converted to the Sn content based on the relationship between the two, and quantified.

[0050] The thickness of the scale layer is obtained from a cross-sectional photograph of the scale layer. Specifically, a cross-sectional photograph of the scale layer is first obtained, in which the horizontal direction is parallel to the surface of the hot-rolled steel sheet and the vertical direction is the thickness direction of the hot-rolled steel sheet. The area of ​​the scale layer within the observation field is then obtained from the cross-sectional photograph, and the thickness is calculated by dividing the area by the width of the observation field.

[0051] In the present invention, excellent scale removal property during pickling means that the area ratio of remaining scale is 22% or less after a 100 mm × 50 mm test piece is immersed in 1% HCl at 80°C for 14 seconds. The area ratio of remaining scale is calculated by dividing the area of ​​remaining scale on the test piece by the total area of ​​the test piece (5000 mm 2) is obtained. From the viewpoint of scale peelability, there is no particular lower limit for the area ratio of remaining scale. However, if the area ratio of remaining scale is less than 5%, there is a risk that the corrosion resistance of the steel sheet cannot be ensured. Therefore, it is preferable that the area ratio of remaining scale is 5% or more.

[0052] (C) Manufacturing method The inventors have confirmed through their research that the hot-rolled steel sheet of the present invention can be manufactured by carrying out the manufacturing steps shown below in order.

[0053] Specifically, the steel having the above-mentioned chemical composition is rolled under conditions where the finish rolling end temperature is 800 to 950°C, and immediately thereafter, the first cooling is performed at a cooling rate of 12°C / s or more and less than 25°C / s to a temperature range of 700 to 750°C. Then, the second cooling is started from the temperature range of 700 to 750°C, and is stopped after cooling to a temperature range of 520 to 670°C at a cooling rate of 25 to 90°C / s. Then, after coiling at a temperature range of 500 to 650°C, the coil is slowly cooled under conditions where the cooling rate at the center of the coil from the coiling temperature to 300°C is 0.0005 to 0.004°C / s.

[0054] Each manufacturing process will be described in detail below.

[0055] <Hot rolling process> The slab obtained by a conventional method is heated and then hot-rolled. The heating temperature of the slab is preferably 1200°C or higher in order to uniformly heat the slab to the austenite region. On the other hand, the heating temperature is preferably 1250°C or lower in order to avoid deterioration of the surface properties due to scale formation. The rolling start temperature is preferably 1000°C or higher.

[0056] In the hot rolling process, rolling is performed under the condition that the finish rolling end temperature is 800 to 950°C. If the rolling end temperature is less than 800°C, there is a risk of indentation defects occurring due to rolling at a low temperature. In addition, austenite grains are flattened, which may cause variations in mechanical properties between the rolling direction and the width direction, resulting in poor workability. In order to make the precipitates finer, the rolling end temperature is preferably 800°C or higher, and more preferably 850°C or higher.

[0057] On the other hand, if rolling is completed at a temperature exceeding 950°C, the magnetite seam grows due to the growth of the scale after rolling, so the volume fraction of magnetite increases. In addition, the formation of the magnetite seam suppresses the diffusion of Sn into the scale layer, so the Sn content in the Sn-enriched layer increases, and the scale layer partially peels off. Due to such uneven surface properties, it becomes difficult to remove the scale layer uniformly during pickling, and the aesthetic appearance after pickling deteriorates. In addition, there are problems such as the coarsening of crystal grains and the tendency for scale defects to occur. Therefore, the rolling completion temperature is set to 950°C or less, more preferably 900°C or less.

[0058] When hot rolling, it is preferable to set the cumulative reduction rate in the temperature range from the rolling start temperature to the rolling end temperature to 60% or more. If the cumulative reduction rate is less than 60%, the austenite grains may not be sufficiently refined, and the toughness may deteriorate. Therefore, it is preferable to set the cumulative reduction rate in the temperature range from the rolling start temperature to the rolling end temperature to 60% or more. There is no particular upper limit to the cumulative reduction rate, but it is preferable to set the cumulative reduction rate to 65% or less.

[0059] <Cooling process> Immediately after the hot rolling process, the first cooling is performed at a cooling rate of 12°C / s or more and less than 25°C / s to a temperature range of 700 to 750°C. The first cooling may be air cooling or water cooling. If the first cooling rate is less than 12°C / s, the scale growth at high temperatures advances and magnetite seams grow, resulting in a high volume fraction of magnetite. In addition, the formation of magnetite seams suppresses the diffusion of Sn into the scale layer, resulting in a high Sn content in the Sn-enriched layer and partial peeling of the scale layer. Such non-uniform surface properties make it difficult to remove the scale uniformly during pickling, resulting in a deterioration in aesthetic appearance.

[0060] On the other hand, if the first cooling rate is 25°C / s or more, the scale layer will partially peel off due to the difference in expansion coefficient between the base material and the scale layer, which will reduce the volume fraction of magnetite and prevent uniform scale removal during pickling, resulting in poor aesthetics.

[0061] After that, the second cooling is started from a temperature range of 700 to 750°C. When the rolling end temperature is cooled to a temperature range below 700°C at a cooling rate of less than 25°C / s, the scale grows thicker during that time, and magnetite seams grow. Therefore, the volume fraction of magnetite increases. In addition, the formation of magnetite seams suppresses the diffusion of Sn into the scale layer, so the Sn content in the Sn-enriched layer increases, and the scale layer partially peels off. Due to such non-uniform surface properties, it becomes difficult to remove the scale uniformly during pickling, and the aesthetics deteriorate.

[0062] On the other hand, when cooling from a temperature range exceeding 750°C at a cooling rate of 25°C or faster, the scale layer partially peels off during cooling due to the difference in the expansion coefficient between the base material and the scale layer. This reduces the volume fraction of magnetite and also makes it difficult to remove the scale uniformly during pickling, resulting in a deterioration in aesthetic appearance.

[0063] For this reason, from the temperature range of 700 to 750°C, the second cooling is performed at a cooling rate of 25 to 90°C / s. The second cooling is usually performed using cooling water, but air cooling may also be used. If the second cooling rate is less than 25°C / s, a thin scale of iron oxide containing moisture during water cooling grows and becomes hematite or wustite, so the volume fraction of magnetite decreases. This deteriorates the scale peelability during pickling and also deteriorates the aesthetics. On the other hand, if the second cooling rate exceeds 90°C / s, scale cracks occur during cooling and the volume fraction of magnetite decreases. In addition, scale cannot be removed uniformly during pickling, and the surface properties after pickling become non-uniform, resulting in poor aesthetics.

[0064] Then, the second cooling is stopped after cooling to a temperature range of 520 to 670°C. If the second cooling stop temperature is less than 520°C, scale cracks occur during cooling, and the volume fraction of magnetite decreases. In addition, scale cannot be removed uniformly during pickling, and the surface properties after pickling become non-uniform, resulting in poor aesthetics. On the other hand, if the second cooling stop temperature exceeds 670°C, a thin scale of iron oxide containing moisture during water cooling grows and becomes hematite or wustite, resulting in a low volume fraction of magnetite. This deteriorates the scale peelability during pickling.

[0065] <Winding process> Next, the steel sheet is wound at a temperature range of 500 to 650°C. If the winding temperature is less than 500°C, the scale grows before the steel sheet is wound, and the scale peels off during winding, resulting in non-uniform surface properties. As a result, the volume fraction of magnetite decreases, and the scale cannot be removed uniformly during pickling, resulting in deterioration of the appearance. On the other hand, if the winding temperature exceeds 650°C, the steel sheet reheats in the wound coil, and the scale slowly grows and transforms into hematite or wustite, resulting in a decrease in the volume fraction of magnetite. This deteriorates the scale peelability during pickling.

[0066] Thereafter, the coil is slowly cooled (naturally allowed to cool) under conditions where the cooling rate at the center of the coil from the winding temperature to 300°C is 0.0005 to 0.004°C / s. If the natural cooling rate is less than 0.0005°C / s, the slow growth and transformation of scale causes hematite or wustite, resulting in a decrease in the volume fraction of magnetite. As a result, the scale cannot be removed uniformly during pickling, and the aesthetics deteriorate. On the other hand, if the natural cooling rate exceeds 0.004°C / s, a temperature difference occurs between the inside and outside of the coil, so the scale grows non-uniformly and the volume fraction of magnetite decreases. In addition, if the natural cooling rate is excessively fast, Sn cannot be sufficiently diffused, the Sn content of the Sn-enriched layer becomes excessively high, and the thickness of the Sn-enriched layer becomes thin. As a result, the scale peelability during pickling deteriorates, and the aesthetics also deteriorate.

[0067] In the manufacture of the hot-rolled steel sheet according to the present invention, high-pressure descaling may be utilized, and there is no problem in utilizing continuous hot rolling for rolling the bar joining material after rough rolling. By using these facilities, it is possible to prevent the occurrence of scale defects and improve the yield while improving the temperature process capacity.

[0068] Although there is no particular limitation on the thickness of the hot-rolled steel sheet according to the present invention, it is preferable that the thickness is 2 to 16 mm. When the hot-rolled steel sheet is used for a container, the thickness is more preferably 10 mm or less.

[0069] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. EXAMPLES

[0070] A hot-rolled steel sheet having the chemical composition shown in Table 1 was produced under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 3.2 mm and a width of 1000 mm. Here, the first cooling was air cooling, and the second cooling was with cooling water.

[0071] [Table 1]

[0072] [Table 2]

[0073] Thereafter, for each of the hot-rolled steel sheets, the volume fraction of magnetite in the scale layer and the Sn concentration in the Sn-enriched layer were measured by the following method.

[0074] First, the scale layer formed on the surface of each hot-rolled steel sheet was scraped off using a hammer and a cutter knife until the steel sheet surface could be confirmed, and the scale layer was completely sampled and pulverized to prepare a powder sample. The scale layer was sampled from five locations.

[0075] Next, magnetite, wustite, and hematite were mixed in different weights so that the total amount was 20 g, and then 0.4 g of zinc oxide was added as an internal standard to prepare multiple standard samples. The multiple standard samples were analyzed by X-ray diffraction, and the area of ​​the X-ray peaks of magnetite, wustite, and hematite relative to the internal standard was measured to obtain a calibration curve to be used for quantitative analysis by X-ray diffraction.

[0076] Then, 20 g of the powder sample taken from the above steel plate was mixed with 0.4 g of zinc oxide as an internal standard, and then analyzed by X-ray diffraction to determine the volume fraction of magnetite based on the above calibration curve. The volume fraction of magnetite was measured using each of the powder samples taken from the five locations, and the average value was calculated to determine the volume fraction of magnetite in the scale layer.

[0077] Furthermore, samples taken from each hot-rolled steel sheet were embedded in resin, and the cross section in the thickness direction was mirror-finished by wet polishing. Then, a line analysis of Sn from the scale to the steel sheet was performed on the sample at a magnification of 500 times using an EPMA, and the Sn content from the interface between the steel sheet and the scale to the scale layer side was measured. The measurement pitch in the line analysis was 10 nm. The region where the measured Sn content was 0.01% or more was determined as the Sn-enriched layer, and the thickness of the region was determined as the thickness of the Sn-enriched layer. The maximum value of the measured Sn content was determined as the Sn content in the Sn-enriched layer.

[0078] In addition, when measuring the Sn content by EPMA, the reflected X-ray intensity of Sn was converted to the Sn content based on the relationship between the average Sn content in the scale layer measured by ICP atomic emission spectrometry after dissolving a powder sample of the scale layer in acid and the average value of the reflected X-ray intensity of Sn over the entire thickness of the scale layer measured by EPMA.

[0079] In addition, the thickness of the scale layer was determined by determining the area of ​​the scale layer within the observation field from a cross-sectional photograph of the scale layer, in which the horizontal direction is parallel to the surface of the hot-rolled steel plate and the vertical direction is the thickness direction of the hot-rolled steel plate, and dividing this area by the horizontal width of the observation field.

[0080] Next, the scale removal property by pickling and the aesthetics of the steel sheet surface after pickling were evaluated. Specifically, 100 mm × 50 mm test pieces were taken from each hot-rolled steel sheet, and the test pieces were subjected to the following descaling steps in order: first, immersion in 1% HCl at 80°C for 14 seconds, second, immersion in 4% HCl containing 4.2% inhibitor at 90°C for 16 seconds and then scrubbing the surface with a brush, and third, immersion in 8% HCl containing 8.2% inhibitor at 90°C for 16 seconds and then scrubbing the surface with a brush.

[0081] Then, the area ratio of the remaining scale after the primary descaling was measured. When it was 22% or less, it was judged that the scale peelability after pickling was excellent. Also, after the tertiary descaling was completed, the surface of the steel plate was photographed, and the obtained image was subjected to binary black-and-white processing with a brightness of 128 as the threshold value. By image processing, the area ratio of the black portion corresponding to the scale remaining in the defective portion was measured. And when the area ratio of the black portion was 2% or more, it was judged that the aesthetic property was inferior (non-conforming), and when it was less than 2%, it was judged that the aesthetic property was excellent (conforming).

[0082] Subsequently, the corrosion resistance was evaluated based on the SAE J2334 test and the ISO12944 test. Each test will be described.

[0083] <SAE J2334 Test> Test pieces of 100 mm × 60 mm were taken from each hot-rolled steel plate, and the surface was shot-blasted. At this time, shot grains made of martensitic steel with a particle size of 180 to 355 μm were used, and it was carried out under the conditions that the surface rust removal degree was ISO Sa2 1 / 2 and the average roughness Rzjis was 50 μm. Then, a wetting process of holding for 6 hours in an environment of 50°C and a relative humidity of 100%RH, a salt adhesion process of immersing for 15 minutes in an aqueous solution of pH 8 containing 0.5% NaCl, 0.1% CaCl2 and 0.075% NaHCO3, and a drying process of holding for 17.75 hours in an environment of 60°C and 50%RH were repeatedly carried out. And when the thickness reduction amount after 80 cycles was 0.50 mm or less, it was judged as qualified.

[0084] <ISO12944 Test> A test piece of 100 mm x 60 mm was taken from each hot-rolled steel sheet, and the surface was shot blasted. At this time, shot particles made of martensitic steel with a particle size of 180 to 355 μm were used, and the surface was subjected to a rust removal degree of ISO Sa2 1 / 2 and an average roughness Rzjis of 50 μm. Then, a zinc-rich primer (Cerabond 2000 manufactured by Chugoku Paint Co., Ltd.) with a thickness of 30 μm was applied to the surface as a corrosion-resistant base, and then a corrosion-resistant coating (Banno #200 manufactured by Chugoku Paint Co., Ltd.) with a thickness of 90 μm was applied. Then, scratches with a length of 50 mm reaching the steel material were applied to the coating film using a PVC cutter, and the coating was subjected to ISO12944 testing.

[0085] The ISO12944 test is composed of a cycle of three days of ultraviolet irradiation test, three days of salt spray test, and one day of low-temperature freezing test, totaling one week. The ultraviolet irradiation test is a test in which ultraviolet irradiation time is four hours and dark dew time is four hours repeatedly using an ultraviolet fluorescent lamp that irradiates UV-A as specified in ISO16474-3. The salt spray test is a test in which a 5% sodium chloride aqueous solution is sprayed as specified in ISO9227. Furthermore, the low-temperature freezing test is a test in which the sample is held in a thermostatic chamber that can be controlled at a low temperature of -20°C. After 48 cycles, the peeled part of the paint that had progressed due to corrosion from the paint defect was removed with a cutter, and the paint peeled area was obtained by image binarization processing. In this embodiment, if the peeled area was 25% or less, it was considered to have passed.

[0086] The results are shown in Table 3. When no Sn-enriched layer was present in the scale layer, the maximum Sn content in the scale layer was shown in parentheses in the column for the Sn content in the Sn-enriched layer.

[0087] [Table 3]

[0088] As can be seen from Table 3, Test Nos. 1 to 9, which satisfy all the requirements of the present invention, were excellent in scale removal by pickling and in appearance after scale removal, as well as in corrosion resistance. In contrast, Test Nos. 10 to 30, which are comparative examples, were inferior in at least one of scale removal by pickling and appearance after pickling.

[0089] In test No. 10, the C content was below the specified value, and the diffusion of Sn into the scale layer was not easily hindered by C, so the Sn content in the scale layer was high at 0.90%. In addition, because the C content was below the specified value, the scale grew significantly and the magnetite volume fraction was low. As a result, partial scale peeling progressed during acid cleaning, the remaining scale area was high at 60%, and the aesthetics were further reduced due to scale defects.

[0090] In test No. 11, the C content was higher than the specified value, and the diffusion of Sn into the scale layer was suppressed by C, resulting in a low Sn content in the scale layer. Furthermore, scale blisters occurred during the finish rolling and subsequent cooling process, and the scale peeled off partially, resulting in an uneven surface condition. As a result, the magnetite volume fraction was low at 45%, which not only deteriorated the scale peelability but also reduced the aesthetics. In addition, since the C content was higher than the specified value, there was a concern that the dissolution reaction of the steel base material during pickling would be promoted, so Cu, Ni, and Mo were added, but the value of the middle value in equation (i) was excessive. As a result, the corrosion resistance in the corrosion test was reduced.

[0091] In test No. 12, the Si content was below the specified value, so the scale grew significantly, making it difficult to form a Sn-enriched layer in the scale layer, and the Sn content was low. As a result, the area ratio of the remaining scale after pickling exceeded the target value of 22%.

[0092] In test No. 13, the Si content was higher than the standard, and Si-based iron oxides such as fayalite, which have high adhesion, were formed at the interface between the scale and the steel substrate, which reduced the volume fraction of magnetite. In addition, a Sn-enriched layer with a very high Sn content was formed. Therefore, the effect of the Si-based iron oxides reduced the scale removal ability during pickling.

[0093] In test No. 14, the Sn content was below the specified value, so the Sn content in the scale layer was low, and the corrosion resistance in the corrosion test was reduced. In test No. 15, the Sn content was higher than the specified value, so the Sn content in the scale layer was higher than the specified value. During pickling, the high Sn content suppressed the dissolution reaction of the steel base, and the remaining scale area exceeded 22%.

[0094] In test No. 16, the central value of equation (i) was excessive, which resulted in a decrease in corrosion resistance in the corrosion test.

[0095] In Test Nos. 17 to 30, the chemical composition of the steel was satisfactory, but the manufacturing conditions did not satisfy the preferred range, and therefore the scale removal property and / or the aesthetic appearance after pickling were deteriorated.

[0096] In test No. 17, the finish rolling temperature was high at 1000°C, and the scale growth was significant up until the cooling process. In addition, the finish rolling was performed at a high temperature, which promoted the growth of magnetite seams. The growth of magnetite seams inhibited the diffusion of Sn into the scale layer, and Sn was concentrated at the interface between the magnetite seam and the base material, resulting in a significantly high Sn content in the Sn-concentrated layer. As a result, the scale layer peeled off during the cooling process, and the high temperature of the steel sheet created an uneven surface condition, which prevented uniform descaling during pickling. The remaining scale area was high at 65%, and the aesthetics were also the worst.

[0097] In test No. 18, the first cooling rate between the end of finish rolling and the start of the second cooling was slow, which led to the growth of scale at high temperatures and excessive growth of magnetite seams. The growth of magnetite seams inhibited the diffusion of Sn into the scale layer, and Sn became concentrated at the interface between the magnetite seam and the base material, resulting in a high Sn content in the Sn-concentrated layer. After that, scale peeling progressed during cooling, resulting in an uneven surface condition. As a result, uniform descaling could not be achieved during acid washing, the remaining scale area was 30%, and the steel surface became uneven, reducing the aesthetics.

[0098] In test No. 19, the first cooling rate between the end of finish rolling and the start of second cooling was fast, and water cooling was performed immediately after finish rolling, so partial spalling of the scale layer occurred due to the difference in the expansion coefficient between the steel plate base material and the scale layer. When partial spalling of the scale occurs, the oxidation reaction proceeds again and wüstite is formed, reducing the volume fraction of magnetite. As a result, the magnetite volume fraction decreased and the surface condition became non-uniform. Although the remaining scale area after pickling was 22% or less, the steel base surface became uneven and the aesthetics were reduced.

[0099] In test No. 20, the second cooling start temperature was low at 680°C, and scale growth was significant. In addition, magnetite seam growth was excessive. On the other hand, the diffusion of Sn into the scale layer was suppressed, and the Sn content in the Sn-enriched layer became significantly high. As a result, partial peeling of the scale layer occurred during the cooling process, and the high temperature of the steel sheet caused an uneven surface condition, making it impossible to descale uniformly during pickling. The remaining scale area was high at 50%, and the aesthetics were also poor.

[0100] In test No. 21, the second cooling start temperature was high at 780°C, and the difference in the expansion coefficient between the steel base and the scale layer caused partial spalling of the scale layer. As a result, the magnetite volume fraction decreased and the surface condition became non-uniform. Although the remaining scale area after pickling was 22% or less, the steel base surface became uneven and the aesthetics were reduced.

[0101] In test No. 22, the second cooling end temperature was low at 510°C, and scale spalling occurred during water cooling, the volume fraction of magnetite decreased, and the surface condition became non-uniform. As a result, uniform descaling was not possible during pickling, the remaining scale area was 24%, and the aesthetics were also poor.

[0102] In test No. 23, the second cooling end temperature was high at 680°C, and a thin scale containing iron oxides containing moisture during water cooling grew, decreasing the volume fraction of magnetite. As a result, uniform descaling was not achieved during pickling, and the remaining scale area was 30%.

[0103] In test No. 24, the second cooling rate was slow at 20°C / s, and a thin scale containing iron oxide containing moisture during water cooling grew, decreasing the volume fraction of magnetite. As a result, uniform descaling was not possible during pickling, the remaining scale area was 32%, and the aesthetics were also poor.

[0104] In test No. 25, the second cooling rate was very fast at 100℃ / s, which caused scale spalling during water cooling, a decrease in the volume fraction of magnetite, and an uneven surface condition. As a result, uniform descaling was not possible during pickling, the remaining scale area was 25%, and the aesthetics were also poor.

[0105] In test No. 26, the coiling start temperature was low at 450°C, and the scale grew before coiling, and the scale peeled off during coiling, resulting in an uneven surface condition. As a result, the volume fraction of magnetite decreased, uniform descaling was not possible during pickling, the remaining scale area was 27%, and the aesthetics were also poor.

[0106] In test No. 27, the coiling start temperature was high at 660°C, and the steel sheet reheated in the coil after coiling, which caused slow scale growth and transformation during the cooling process, resulting in a decrease in the volume fraction of magnetite. As a result, uniform descaling was not achieved during pickling, and the remaining scale area was 32%.

[0107] In test No. 28, the cooling rate after coiling was slow, which caused slow scale growth and transformation, resulting in a decrease in the volume fraction of magnetite. As a result, uniform descaling was not possible during pickling, the remaining scale area was 25%, and the appearance was also poor.

[0108] In Tests No. 29 and 30, the cooling rate after coiling was fast, which caused a temperature difference between the inside and outside of the coil, and the growth state of the scale varied depending on the location. As a result, the transformation of the scale progressed rapidly in some locations, the volume fraction of magnetite decreased, and uniform descaling during pickling was not possible, with the remaining scale area being 24% and 30%, and the aesthetics were also poor. In addition, the cooling rate was too fast, which prevented Sn from diffusing sufficiently, resulting in an excessively high Sn content in the Sn-enriched layer and a thin Sn-enriched layer. [Industrial Applicability]

[0109] According to the present invention, it is possible to obtain a hot-rolled steel sheet that has excellent scale removability by pickling and excellent corrosion resistance in a corrosive environment containing chlorides.

Claims

[Claim 1] A steel plate and a scale layer formed on a surface of the steel plate, The chemical composition of the steel sheet is, in mass%, C: 0.05-0.20%, Si: 0.005-0.30%, Mn: 0.05-1.20%, P: 0.001-0.010%, S: 0.010% or less, Sn: 0.05-0.25%, N: 0.001 to 0.010%, Al: 0.001-0.040%, Cu: 0 to 0.30%, Ni: 0 to 0.15%, W: 0-0.30%, Mo: 0 to 0.30%, Cr: 0-0.20%, V: 0 to 0.020%, Ti: 0 to 0.020%, Nb: 0 to 0.025%, The balance is Fe and impurities. The following formula (i) is satisfied: The volume fraction of magnetite in the scale layer is 60 to 83%, The thickness of the scale layer is 25 μm or less on average, The scale layer has an Sn-enriched layer having an Sn content of 0.01 to 0.85% at an interface between the steel sheet and the scale layer. Hot rolled steel plate. 0.50<(4.6×C+1.5×Si-1.2×Sn+0.5×(Cu+Ni+W+Mo)-0.2×(Cr+V+Ti+Nb))<1.10...(i) In the above formula (i), each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

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