Steel sheet and enameled product

Abstract

This steel sheet has a specific chemical composition; has a ferrite phase area percentage of least 98% at a position at a depth of 1 / 4 from the sheet surface in the direction of thickness; and contains one or more oxides selected from the group consisting of Mn, Cr, and Al, where the number density of inclusions exceeding 0.5 μm but no greater than 10 μm is 300 to 800 inclusions / mm2.

Description

Steel plates and enamel products

[0001] This invention relates to steel plates and enamel products. This application claims priority based on Japanese Patent Application No. 2024-157944, filed in Japan on September 12, 2024, the contents of which are incorporated herein by reference.

[0002] Enameled products are made by firing a vitreous (glazed) material onto the surface of a steel plate. Because enamel products have heat resistance, weather resistance, chemical resistance, and water resistance, they have traditionally been widely used as materials for kitchenware such as pots and sinks, as well as building materials. Generally, such enamel products are manufactured by processing a steel plate into a predetermined shape, assembling it into the product shape by welding or other means, and then applying an enamel treatment (firing treatment) to bake on the vitreous material.

[0003] Steel sheets used as materials for enamel products (enamel steel sheets) are required to have properties such as resistance to firing distortion, resistance to nail breakage after enamel treatment, enamel adhesion, and resistance to bubbles and black spot defects after enamel treatment.

[0004] "Claw breakage" is a phenomenon in which the enamel layer is damaged within about a week after firing, causing crescent-shaped fragments to peel off from the enamel product. The reason for claw breakage is thought to be that hydrogen, which penetrates the steel plate during processes such as enamel treatment and is dissolved in it, turns into a gas after cooling and accumulates at the interface between the steel plate and the glaze, and the pressure from the hydrogen gas destroys the enamel layer. To prevent claw breakage, it is thought that improving the hydrogen storage capacity of the steel plate by increasing the amount of inclusions in the steel plate is effective.

[0005] Regarding enamel technology, for example, Patent Document 1 proposes a method for manufacturing enamel steel by increasing the oxygen content in the steel and performing continuous casting. Patent Document 1 discloses that by using low carbon and high oxygen, high-quality enamel steel with fewer chipping issues, etc., can be manufactured with a high yield, without surface defects such as pinholes and slivers.

[0006] Furthermore, Patent Document 2 describes a steel plate with excellent resistance to nail breakage, in which, as is generally done, deoxidation with Al or Si is performed in the initial stages of secondary refining, but by changing the method to add one or more of Mn and Cr to the molten steel after decarburization is complete and before deoxidation with Al or Si, MnO and Cr are added to the steel as fine inclusions. ２ O ３ A method for dispersing a large quantity of it has been disclosed.

[0007] Japanese Public Gazette No. 57-49089, International Publication No. 2021 / 193953

[0008] The technology described in Patent Document 1 involves adding Al to deoxidize the steel and adjust the amount of oxygen in it. However, when adjusting the amount of oxygen in steel with Al in this way, the Al added to the molten steel often forms many large nonmetallic inclusions, for example, 10 μm or larger, making it difficult to sufficiently generate fine oxides that are highly effective for hydrogen storage.

[0009] Furthermore, the technology described in Patent Document 2 is only applicable when there is a high oxygen content, such as in high-oxygen enamelware. When the oxygen content is low, it is not possible to sufficiently generate inclusions.

[0010] The present invention has been made in view of the above circumstances, and aims to provide a steel plate having excellent enamel properties (resistance to nail breakage, enamel adhesion, and appearance after enamel treatment) after enamel treatment, and an enamel product having excellent enamel properties equipped with this steel plate.

[0011] In medium-oxygen enamelware, the oxygen content is lower compared to high-oxygen enamelware, so simply changing the order of deoxidation with Cr, Mn, or Al, as described in Patent Document 2, was not sufficient to obtain sufficiently fine inclusions. The inventors have found that in medium-oxygen enamelware, retention after rough rolling is important for the formation of fine inclusions that serve as hydrogen trapping sites. Specifically, the inclusions are crushed after rough rolling, and then Cr is released by solid-phase diffusion. ２ O ３ The inventors have discovered that the growth of this material can generate inclusions that serve as hydrogen trapping sites.

[0012] The gist of the present invention made based on the above findings is as follows. [1] A steel sheet according to one embodiment of the present invention has, in mass%, C: 0 to 0.0050%, Si: 0 to 0.050%, Mn: 0.020 to 0.50%, P: 0.003 to 0.050%, S: 0.005 to 0.050%, Al: 0 to 0.010%, O: 0.0100 to 0.0400%, Cu: 0.010 to 0.060%, N: 0 to 0.0050%, Cr: 0.01 to 0.20%, B: 0.0010 to 0.0050%, and the balance Fe and impurities. On a plane parallel to the surface at a position 1 / 4 of the sheet thickness in the sheet thickness direction from the surface of the steel sheet, the area ratio of the ferrite phase is 98% or more, it contains an oxide having one or more selected from the group consisting of Mn, Cr, and Al, and the number density of inclusions having a diameter exceeding 0.5 μm and not exceeding 10 μm is 300 to 800 pieces / mm ２ is as follows. [2] The steel sheet according to [1] above may further contain, in mass%, a total of one or more selected from the group consisting of Ni, Nb, As, Ti, V, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg: more than 0 to 0.100%. [3] The steel sheet according to [1] or [2] above contains an oxide having one or more selected from the group consisting of Mn, Cr, and Al on a plane parallel to the surface at a position 1 / 4 of the sheet thickness in the sheet thickness direction from the surface of the steel sheet, and the number density of inclusions having a major diameter exceeding 1.0 μm and not exceeding 10 μm may be less than 500 pieces / mm ２ . [4] The steel sheet according to any one of [1] to [3] above may have an AI value defined by formula (1) of 30 MPa or less. AI value = σ ｙ１ −σ ｙ２ …(1) formula In the above formula (1), σ ｙ１ is the yield point after aging treatment in MPa, and σ ｙ２ is the yield point before aging treatment in MPa. [5] The steel sheet according to any one of [1] to [4] above may be a cold-rolled steel sheet. [6] The steel sheet according to any one of [1] to [5] above may be a steel sheet for enamelware.

[0013] [7] Another embodiment of the present invention is an enamel product comprising a steel plate as described in any of [1] to [6] above, and a glassy layer disposed on the steel plate.

[0014] According to the present invention, it is possible to obtain a steel sheet having excellent enamel properties after enamel treatment, and an enamel product having excellent enamel properties equipped with this steel sheet.

[0015] The steel sheet according to one embodiment of the present invention has the following composition in mass%, C: 0 to 0.0050%, Si: 0 to 0.050%, Mn: 0.020 to 0.50%, P: 0.003 to 0.050%, S: 0.005 to 0.050%, Al: 0 to 0.010%, O: 0.0100 to 0.0400%, Cu: 0.010 to 0.060%, N: 0 to 0.0050%, Cr: 0.01 to 0.20%, B: 0.0 The chemical composition consists of 0.10 to 0.0050%, with the remainder being Fe and impurities. On a plane parallel to the surface of the steel sheet at a position 1 / 4 of the sheet thickness from the surface, the area ratio of the ferrite phase is 98% or more. The material contains an oxide having one or more elements selected from the group consisting of Mn, Cr, and Al, and the number density of inclusions with a diameter of more than 0.5 μm and less than or equal to 10 μm is 300 to 800 pieces / mm². ２ This will be explained in detail below. The surface of a steel plate refers to the plate surface, that is, the surface whose normality is in the direction of the plate thickness.

[0016] <Chemical Composition of Steel Sheets> First, we will explain the chemical composition of steel sheets. In the following explanation of the chemical composition of steel sheets, the percentages used refer to "mass percent" unless otherwise specified. Also, numerical ranges indicated by "~" include the values ​​at both ends as upper and lower limits. On the other hand, when indicated as "greater than" or "less than," those values ​​are not included as upper or lower limits.

[0017] C: 0-0.0050% The higher the carbon (C) content, the more likely enamel foam defects are to occur. Also, residual solid-solution carbon causes strain aging, and press workability deteriorates. Therefore, the C content should be 0.0050% or less. Preferably, the C content is 0.0020% or less. From a product performance standpoint, a lower C content is preferable, and it may even be 0%, but reducing C requires longer processing time in the steelmaking stage, which increases steelmaking costs. Preferably, the C content is 0.0003% or more.

[0018] Si: 0-0.050% If the Si (silicon) content is high, the enamel properties are inhibited, and at the same time, a large amount of Si oxide is formed during hot rolling, which may reduce the resistance to nail breakage. This effect becomes significant when the Si content exceeds 0.050%, so the Si content should be kept below 0.050%. From the perspective of improving foam resistance and black spot resistance, and obtaining even better surface properties after enamel treatment, it is preferable that the Si content be 0.008% or less.

[0019] Mn: 0.020-0.50% Manganese (Mn) is an element that generates oxygen-containing inclusions and contributes to improving enamel properties. Mn also has the effect of preventing hot embrittlement caused by sulfur (S). To obtain these effects, the Mn content should be 0.020% or more. Preferably, the Mn content is 0.100% or more. On the other hand, Mn is also an element that lowers the transformation point of steel, and if the Mn content is excessive, transformation will occur in the firing temperature range of the enamel treatment, causing firing strain and deformation of the product. Also, if the Mn content is excessive, the workability of the steel will deteriorate. For this reason, the Mn content should be 0.50% or less. Preferably, the Mn content is 0.40% or less.

[0020] P: 0.003 to 0.050% P (phosphorus) is an element that has the effect of increasing the pickling weight loss of the steel sheet during pickling, which is a pretreatment for the galvanizing process. If the P content is less than 0.003%, pickling will be insufficient, and the adhesion of the galvanizing will be impaired. Therefore, the P content is set to 0.003% or more. The P content is preferably 0.005% or more. On the other hand, if the P content exceeds 0.050%, the pickling weight loss becomes excessive, and bubbles and black spot defects are likely to occur after the galvanizing process. To avoid these, the P content is set to 0.050% or less. The P content is preferably 0.035% or less.

[0021] S: 0.005 to 0.050% S is an element that accelerates the pickling speed, roughens the surface of the steel sheet after pickling, and contributes to the improvement of the galvanizing adhesion. To obtain this effect, the S content is set to 0.005% or more. The S content is preferably 0.010% or more, and more preferably 0.015% or more. On the other hand, if the S content becomes excessive, the effect of Mn required for controlling the oxides in the steel may decrease. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.040% or less, and more preferably 0.030% or less.

[0022] Al: 0 to 0.010% Al (aluminum) is a strong deoxidizing element, and in the steel sheet according to this embodiment, it is necessary to carefully control the Al content. If the Al content exceeds 0.010%, it becomes difficult to retain the required amount of O in the steel, and the control of the oxides described below, which is effective for anti-chip-flaking property, becomes difficult. Therefore, the Al content is set to 0.010% or less. The Al content is preferably 0.005% or less. The lower limit of the Al content does not need to be limited and may be 0%, or the Al content may be 0.001% or more.

[0023] O: 0.0100 - 0.0400% O (oxygen) is a constituent element of fine inclusions that capture hydrogen in steel and improve anti-peeling properties, and is an important element in enamel steel sheets. In the steel sheet according to this embodiment, in order to ensure desired enamel properties, the O content is 0.0100% or more. When the O content is less than 0.0100%, the number of inclusions becomes insufficient and peeling occurs. The O content is preferably 0.0200% or more. On the other hand, when the O content increases, the manufacturing cost in steelmaking rises, so the upper limit is 0.0400% or less. The O content is preferably 0.0390% or less, more preferably 0.0380% or less, still more preferably less than 0.0300%, and even more preferably 0.0290% or less. When the O content is less than 0.0300%, the risk of refractory erosion by oxygen can be reduced, so the wear of refractories in the steelmaking process is suppressed, and the replacement frequency and usage amount of refractories are reduced.

[0024] Cu: 0.010 - 0.060% Cu (copper) is an element that reduces pickling loss and improves enamel adhesion by forming fine irregularities on the surface of the steel sheet after pickling. To obtain this effect, the Cu content is 0.010% or more. When the Cu content is less than 0.010%, the effect of improving enamel adhesion is not sufficient. The Cu content is preferably 0.020% or more. On the other hand, when the Cu content exceeds 0.060%, the dissolution rate in steel pickling decreases too much, and sufficient irregularities are not formed on the surface of the steel sheet after pickling. As a result, good adhesion cannot be obtained. Therefore, the Cu content is 0.060% or less. The Cu content is preferably 0.050% or less.

[0025] N: 0 to 0.0050% Nitrogen (N) is an impurity and an element that causes strain aging. When strain aging occurs, the workability of the steel sheet is impaired. To maintain workability, the N content should be 0.0050% or less. Preferably, the N content is 0.0040% or less, and more preferably 0.0035% or less. To maintain workability, a lower N content is preferable, and it may even be 0%, but reducing N requires a longer processing time in the steelmaking stage, which increases steelmaking costs. Therefore, preferably, the N content is 0.0008% or more, and more preferably 0.0010% or more.

[0026] Cr: 0.01-0.20% Cr (chromium) is an element that generates inclusions containing oxygen and contributes to improving enamel properties. In particular, when both Cr and Mn are included, the size of the inclusions becomes appropriate compared to when only Cr or Mn is included, and the occurrence of nail chipping after enamel treatment is suppressed. If the Cr content is less than 0.01%, the effect of the composite oxide with Mn is not obtained. For this reason, the Cr content should be 0.01% or more. Preferably, the Cr content is 0.03% or more. On the other hand, if the Cr content exceeds 0.20%, the processability deteriorates and the resistance to black spots is also impaired. For this reason, the Cr content should be 0.20% or less. Preferably, the Cr content is 0.15% or less, more preferably 0.10% or less.

[0027] B: 0.0010 to 0.0050% Boron (B) fixes nitrogen, improves deep drawability, and prevents aging, thus providing high workability to the steel sheet. In addition, B combines with oxygen in the steel to form oxides, preventing chipping. To obtain these effects, the B content should be 0.0010% or more. Preferably, the B content is 0.0012% or more, more preferably 0.0015% or more, and even more preferably 0.0020% or more. On the other hand, if the B content is excessive, deoxidation proceeds excessively, making it difficult to retain oxides in the steel. Furthermore, if the B content is excessive, foam resistance and black spot resistance deteriorate. Therefore, the B content should be 0.0050% or less. Preferably, the B content is 0.0040% or less, and more preferably 0.0030% or less.

[0028] Other elements such as Ni, Nb, As, Ti, V, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg do not necessarily need to be included, but may be included as needed. Therefore, the total content of these elements may be 0% or greater than 0%. However, if these elements are included in excess, the reaction with oxide-forming elements becomes significant, making it difficult to control the desired oxide. Therefore, it is preferable that the total content of one or more elements selected from the group consisting of Ni, Nb, As, Ti, V, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg be 0.100% or less. More preferably, the total content is 0.050% or less, and even more preferably 0.010% or less. Furthermore, if these aforementioned elements act as deoxidizing elements, they may affect the free oxygen level, making it difficult to adjust the free oxygen level. Therefore, it is preferable to set the upper limit of the content of each element within a range that does not affect the free oxygen value during the casting process.

[0029] Remainder: Fe and impurities. The remainder, excluding the elements listed above, consists of Fe and impurities. Impurities are elements that are introduced from the steel raw materials and / or during the manufacturing process, and are permitted to the extent that they do not impair the properties of the steel sheet.

[0030] The chemical composition of steel sheets is measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, test pieces taken from the steel sheets are measured using a predetermined measuring device under conditions based on a pre-established calibration curve. C and S are measured using the combustion-infrared absorption method, and N and O are measured using the inert gas fusion-thermal conductivity method. When measuring the chemical composition of enamel products, the following procedure is followed. First, the enamel product to be analyzed is prepared, and the glassy enamel layer formed on the surface is removed by physical means. Shot blasting is preferably used to remove the enamel layer. Specifically, an air blast method using steel balls or alumina-based abrasives is used to project abrasive material at high pressure onto the enamel layer, thereby crushing and peeling it off. In shot blasting, the enamel layer can be efficiently removed while suppressing damage to the steel sheet substrate by appropriately adjusting the particle size of the abrasive material, the projection pressure, and the processing time. After the enamel layer is removed, ultrasonic cleaning or wiping with an organic solvent is performed to remove any remaining fine particles or contaminants from the steel sheet surface. Subsequently, the following component analysis of the steel sheet is performed.

[0031] <Metal structure of steel sheet> In the steel sheet according to this embodiment, the area ratio of the ferrite phase is 98% or more on a surface parallel to the surface of the steel sheet, at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel sheet. If pearlite structure remains, the cementite in the pearlite decomposes into gas during firing, causing bubbles and chipping, which leads to poor appearance. The area ratio of the ferrite phase on the above surface is preferably 99% or more. On the other hand, the area ratio of the ferrite phase on the surface parallel to the surface of the steel sheet, at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel sheet, may be, for example, 100% or less, or 99% or less. Also, the area ratio of pearlite may be less than 2%.

[0032] The area ratio of the ferrite phase is measured by the following method. Specifically, a sample is taken from a steel plate, and the surface is polished from the surface of the steel plate to a point 1 / 4 of the plate thickness in the thickness direction until a surface parallel to the surface is created, and this surface is used as the observation surface. This observation surface is etched with nital, and 10 random images of a 100 μm × 100 μm field of view are taken. An optical microscope is used for imaging, with a magnification of 100 to 1000 times. The area ratio of the microstructure is determined by the point counting method, where low-contrast areas are considered to be pearlite, and the remaining microstructure is taken as the area ratio of the ferrite phase. Specifically, for each obtained microstructure image, grid points are placed at intervals of 10 μm vertically and 10 μm horizontally, and a total of 100 measurement points are determined. The microstructure present below each grid point is identified, and the area ratio is calculated from the proportion of points corresponding to the ferrite phase. The average value of the area ratios obtained from the 10 images is taken as the area ratio of the ferrite phase contained in the steel plate.

[0033] <Inclusions> The steel sheet according to this embodiment contains inclusions. These inclusions contain oxides having one or more selected from the group consisting of Mn, Cr, and Al. Hereinafter, oxides containing one or more selected from the group consisting of Mn, Cr, and Al will simply be referred to as oxides. The inclusions are MnO, Cr ２ O ３ and Al ２ O ３The inclusions may be oxides in which at least one of the elements has precipitated individually, or they may be composite oxides of two or more elements selected from the group consisting of Mn, Cr, and Al. The oxide may be substantially composed of Mn, Cr, and O. Furthermore, the inclusions may be composite oxides of at least one of Mn, Cr, and Al with other elements. In addition, the inclusions may be composite precipitates of compounds other than oxides, such as sulfides like MnS, and oxides having one or more elements selected from the group consisting of Mn, Cr, and Al. The inclusions contained in the steel sheet contain, for example, 10% by mass or more of oxides containing at least one of Mn, Cr, or Al. The content of oxides containing at least one of Mn, Cr, or Al is measured using an EPMA (Electron Probe Micro Analyzer). Specifically, given a steel plate thickness of t, the oxide present in a 3 mm x 3 mm area on a surface parallel to the surface of the steel plate, at a position t / 4 (t: plate thickness) from the surface in the thickness direction, is analyzed. The observation surface is mirror-polished with 1 μm diamond. Within the observation area, the elements of the oxide are identified by EPMA, and the content of oxides containing at least one of Mn, Cr, or Al relative to the total oxide is quantified in mass percent. Note that the oxide does not need to contain Nb.

[0034] Oxides having one or more elements selected from the group consisting of fine Mn, Cr, and Al generate voids around them during cold rolling. These voids trap hydrogen, and inclusions near the surface of the steel sheet improve the sheet's resistance to cracking. However, inclusions smaller than 0.5 μm have little effect on improving crack resistance. On the other hand, if there are many coarse inclusions, their number density decreases, reducing the hydrogen absorption effect and thus the effect on improving crack resistance. Furthermore, coarse inclusions are prone to becoming the starting point for cracks during processing and reduce ductility. Therefore, in this embodiment, the number density of inclusions with a diameter greater than 0.5 μm and less than or equal to 10 μm near the surface of the steel sheet is controlled. Specifically, on a plane parallel to the surface at a position 1 / 4 of the sheet thickness from the surface of the steel sheet in the thickness direction, the number density of the above-mentioned inclusions with a diameter greater than 0.5 μm and less than or equal to 10 μm is set to 300 to 800 pieces / mm２ Hereafter, the surface parallel to the surface of the steel plate, at a position 1 / 4 of the plate thickness in the thickness direction from the surface, will be referred to as the measurement surface. The number density of inclusions with a diameter of more than 0.5 μm and less than or equal to 10 μm on the measurement surface is 300 to 800 pieces / mm². ２ The steel plate has excellent resistance to nail breakage. The number density of inclusions on the measurement surface is preferably 400 to 700 pieces / mm². ２ This is the case. Furthermore, particles that do not contain oxides having at least one of Mn, Cr, and Al, such as MnS existing alone, are not considered in the calculation of the number density. In this embodiment, the number density of inclusions containing oxides selected from the group consisting of Mn, Cr, and Al, with a major axis of more than 1.0 μm and less than or equal to 10 μm, on a plane parallel to the surface of the steel plate at a position 1 / 4 of the plate thickness in the thickness direction from the surface of the steel plate, is 500 particles / mm². ２ It is preferable that the oxygen content be less than or equal to 0. Such steel sheets have low oxygen content and therefore offer excellent manufacturability.

[0035] The number density of the inclusions described above is measured using an EPMA (Electron Probe Micro Analyzer). Specifically, given that the thickness of the steel plate is t, the oxide present in a 3 mm × 3 mm area on a surface parallel to the surface of the steel plate, at a position t / 4 (t: plate thickness) from the surface in the thickness direction, is analyzed. However, to improve the accuracy of the number density, the area may be extended to 10 mm × 10 mm. The observation surface was mirror-polished with diamond particles of 1 μm. The diameter of the inclusion is defined as the average diameter equivalent to a circle. The major axis of the inclusion refers to the dimension in the maximum length direction when the shape of the inclusion is elliptical or irregular. Specifically, in the observation image obtained by electron microscopy, the major axis is defined as the length of the longest side of the smallest circumscribing rectangle surrounding the outer shape of the inclusion. In the case of a shape close to a circle, it may be considered equivalent to the diameter equivalent to a circle. The elements of the oxide are also identified by EPMA. Furthermore, the number density of inclusions is specifically determined by the following method: The observation area within the target surface after polishing is observed using a scanning electron microscope (SEM) equipped with compositional analysis capabilities. The observation area is a 3 mm × 3 mm area on a plane parallel to the surface of the steel plate, at a position t / 4 (t: plate thickness) from the surface in the plate thickness direction. During observation, 100 non-overlapping fields of view of 150 μm × 150 μm are randomly selected, and each field of view is observed at a magnification of 500x. Within the observation area, particles (precipitates or inclusions) with an equivalent circle diameter of 1.0 μm or more are identified based on the Z contrast of the backscattered electron image. In the backscattered electron image, particles are shown with a darker contrast compared to the matrix. The equivalent circle diameter refers to the diameter of a circle when the area of ​​the particle is converted to a circle of the same area. For each identified particle, elemental concentration analysis is performed using energy-dispersive X-ray spectroscopy (EDX). In the EDX analysis (elemental concentration analysis), the standardless method is used. Furthermore, the acceleration voltage is set to 20 kV, and the quantitative elements are Si, Mn, P, S, Cr, Ti, Nb, Cu, Ni, Ca, N, O, Al, and Mg.In the EDX analysis results of each particle, when the total mass percentage content of the above-mentioned quantitative elements is set to 100%, inclusions containing one or more oxides selected from the group consisting of Mn, Cr, and Al are identified as such if the sum of the mass percentage content of Mn, Cr, Al, and O is 10% or more, and the sum of the mass percentage content of Mn and S is 90% or less. The total number of inclusions containing one or more oxides selected from the group consisting of Mn, Cr, and Al with an equivalent circle diameter of more than 0.5 μm and less than or equal to 10 μm is determined in each observation region. Based on the total number of inclusions obtained and the total area of ​​each observation region, the number density (particles / mm) of inclusions containing one or more oxides selected from the group consisting of Mn, Cr, and Al with an equivalent circle diameter of more than 0.5 μm and less than or equal to 10 μm is calculated. ２ ) is determined. Similarly, the number density of inclusions with a major axis greater than 1.0 μm and less than or equal to 10 μm is determined.

[0036] Cold rolling creates voids at the interface between the oxide in the inclusions and the base material, improving the resistance to nail breakage after enamel treatment. For this reason, the steel sheet according to this embodiment is preferably a cold-rolled steel sheet. Whether or not it is a cold-rolled steel sheet can be determined by the sheet thickness, which is, for example, 0.5 to 2 mm.

[0037] Furthermore, the steel sheet according to this embodiment has excellent enamel properties. For this reason, it is preferable to use it as an enamel steel sheet, which is the material for enamel products.

[0038] Furthermore, an enamel product according to one embodiment of the present invention comprises a steel sheet according to the above-described embodiment. For example, it is an enamel product obtained by applying an enamel treatment to a steel sheet according to the embodiment of the present invention and processing it as necessary. Therefore, the enamel product according to this embodiment comprises a steel sheet according to the embodiment of the present invention and a glassy layer disposed on the steel sheet. The enamel layer, which is a glassy layer, has a different color from the steel sheet. Therefore, the enamel layer can be identified by observing the surface of the cross-section of the enamel product after mirror polishing with an optical microscope, based on the difference in color.

[0039] <Method for Manufacturing Steel Sheets> A preferred method for manufacturing steel sheets according to the embodiment of the present invention will be described. The steel sheets according to this embodiment can be manufactured by producing a steel billet having the above-described chemical composition by melting, refining, and casting, and then, if necessary, performing hot rolling, cold rolling, annealing, and temper rolling on this steel billet. Each step may be set according to conventional methods, except for the conditions shown below.

[0040] [Smelting Process] When manufacturing enamel steel sheets, generally, Al or Si is added to the molten steel in the early stages of secondary smelting to perform deoxidation. However, in the steel sheet manufacturing method according to this embodiment, one or more of Mn and Cr are added to the molten steel after decarburization is complete and before deoxidation with Al or Si. By adding Mn and Cr individually or as an alloy to the molten steel after decarburization is complete and before deoxidation with Al or Si, a large amount of Cr is removed during the molten steel stage. ２ O ３ The fine inclusions act as nuclei, and after deoxidation, MnO or Al ２ O ３ Composite inclusions coated with such materials are formed. Furthermore, by adding both Mn and Cr to the molten steel after decarburization is complete and before deoxidation with Al or Si, MnO and Cr ２ O ３ The activity of the compound can be reduced compared to when it is added alone, and it can stably produce 300 to 800 oxide particles / mm² with a diameter greater than 0.5 μm and less than or equal to 10 μm. ２ It can be obtained with the following number density.

[0041] [Hot Rolling Process] In the hot rolling process, it is preferable to perform a rough rolling process in which the slab is roughly rolled at a temperature of 1150°C or higher with a total reduction ratio of 80% or higher. The rough rolling completion temperature is preferably 1050°C or higher. It is preferable to hold the temperature at 1050°C or higher for 30 seconds or more after the rough rolling is completed. Furthermore, it is preferable to start finish rolling at a temperature of 1040°C or higher, and more preferably to start finish rolling at 1050°C or higher. The steel sheet after finish rolling is wound at a winding temperature of, for example, 600 to 700°C. Cr produced in the refining process ２ O ３ Al ２ O ３ Alternatively, composite inclusions coated with MnO are Cr ２ O ３From the perspective of growth, MnO and Al ２ O ３ However, it acts as a barrier to the solid-phase diffusion of Cr. Therefore, it is preferable to crush the complex oxide to promote the solid-phase diffusion of Cr. Specifically, MnO or Al produced in the refining process ２ O ３ Cr coated ２ O ３ The composite inclusions containing Cr are crushed in the rough rolling process. ２ O ３ When exposed at the base metal interface, Cr diffuses in solid phase from the base material portion of the steel plate, and O is supplied from the covering MnO, Cr ２ O ３ The growth of these particles is promoted, resulting in the acquisition of inclusions larger than 0.50 μm. The method for manufacturing steel sheets according to this embodiment has now been described.

[0042] [Cold Rolling Process] When manufacturing cold-rolled steel sheets, a cold rolling process is carried out. The conditions for the cold rolling process should be set according to conventional methods. For example, the total reduction ratio (cold rolling ratio) may be 50 to 90%.

[0043] [Annealing Process] The conditions for the annealing process should be set according to conventional methods. For example, it is preferable to perform continuous annealing with a soaking temperature of 650 to 850°C and an annealing time of 20 seconds or more.

[0044] [Temper Rolling Process] When performing the temper rolling process, the conditions should be set according to conventional methods. For example, the rolling ratio may be 0 to 3.5%.

[0045] <Method for Manufacturing Enameled Products> Enameled products according to the embodiment of the present invention are obtained by processing a steel plate according to this embodiment into a predetermined shape, assembling it into a product shape by welding or the like, and then applying an enamel treatment (firing treatment). For example, the enamel treatment can be carried out by heating a steel plate coated with glaze to a predetermined temperature and holding it for a predetermined time, thereby causing the glassy substance of the glaze to adhere closely to the steel plate. This forms a glassy layer on the steel plate.

[0046] Preferred firing conditions include, for example, a firing temperature of 750 to 900°C and a firing time of 1.5 to 10 minutes (in the furnace). In addition, in the manufacture of enamel products, the glaze may be applied multiple times. In this case, the firing process is performed after each application. Therefore, the firing process may be performed multiple times for multiple applications of glaze. By performing the firing process under these conditions, grain growth during the enamel treatment can be suppressed by solid solution carbon and iron carbides, thereby suppressing a decrease in strength. The firing conditions shown here are merely examples and do not limit the conditions for enamel treatment of steel plates according to this embodiment.

[0047] The embodiments of the present invention have been described above. The embodiments described above are merely illustrative, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and achieves similar effects is included within the technical scope of the present invention.

[0048] The present invention will be described in more detail below with reference to examples. The examples described below are merely examples of the present invention and do not limit it.

[0049] Steel with the chemical composition shown in Tables 1 and 2 (the remainder being Fe and impurities) was melted in a converter, and Cr, Mn, and Al were added during secondary refining. As shown in Table 3, these elements were added in the following order: first Mn and Cr were added, followed by Al (refining No. B1), or first Al was added, followed by Mn and Cr (refining No. b1). Subsequently, the molten steel after secondary refining was formed into slabs by continuous casting. Underlined text in the tables indicates that it is outside the scope of the present invention.

[0050]

[0051]

[0052]

[0053] Hot rolling, cold rolling, annealing, and temper rolling were performed under the conditions shown in Table 4. As a rough rolling process, these slabs were heated to a temperature of 1150 to 1250°C (slab heating temperature), and then rolled at the rough rolling end temperature and total reduction ratio shown in Table 4. After holding for the time shown in Table 4 from the end of rough rolling to the start of finish rolling, hot rolling was performed at the finish rolling start temperature and finish temperature shown in Table 4 to obtain a coiled hot-rolled steel sheet at 600 to 700°C. Then, after pickling the hot-rolled steel sheet, cold rolling was performed at the rolling ratio shown in Table 4 to obtain a cold-rolled steel sheet, and after continuous annealing at the soaking temperature shown in Table 4, temper rolling was performed to obtain a steel sheet (cold-rolled steel sheet) with a thickness of 0.7 mm. The soaking time in continuous annealing was 30 seconds. C2 was held at a temperature of 1050°C or higher for 30 seconds or more after the end of rough rolling.

[0054]

[0055] The area percentage of the ferrite phase in each obtained steel sheet was measured. Specifically, a sample was taken from each steel sheet, and the sheet was polished from the surface to a point 1 / 4 of the thickness in the thickness direction until a surface parallel to the surface was created. This surface was designated as the observation surface, and this observation surface was etched with nital to create an observation area of ​​100 μm × 100 μm. This observation area was observed with a light microscope at 100 to 1000 times magnification, and low-contrast areas in the observation image were considered to be pearlite using the point counting method, with the remaining structure being defined as the area percentage of the ferrite phase.

[0056] Furthermore, for each obtained steel sheet, the number density of inclusions with a diameter greater than 0.5 μm and less than or equal to 10 μm, and the number density of inclusions with a major axis greater than 1.0 μm and less than or equal to 10 μm, located in a 3 mm × 3 mm area on a surface parallel to the surface of the steel sheet at a position t / 4 in the thickness direction from the surface (t: sheet thickness), was measured using EPMA in the manner described above. The results are shown in Tables 5 and 6. In addition, the enamel properties (resistance to nail breakage, enamel adhesion, and appearance after enamel treatment) of the obtained steel sheets were evaluated in the following manner.

[0057] [Nail-Splitting Resistance] As a pretreatment, four 150mm x 100mm samples were taken from each steel plate. Each sample was degreased with alkali, immersed in a 15g / l nickel sulfate solution at 70°C for 7 minutes, and then neutralized. After that, 100μm of Nippon Fellow #102 glaze was applied to both sides, and fired at 860°C for 5 minutes in an atmosphere with a dew point of 35°C. After firing, the samples were heated at 150°C for 20 hours, and the occurrence of nail-splitting was observed and evaluated visually. The occurrence was evaluated as the average of the four samples taken from each steel plate. The evaluation criteria were as follows: A: Excellent, B: Normal, C: Problematic, with C being a failure. A: 10 or fewer nail-splitting occurrences per surface B: 11 to 20 nail-splitting occurrences per surface C: 21 or more nail-splitting occurrences per surface

[0058] [Enamel Adhesion] As a pretreatment, four 150 mm x 100 mm samples were taken from each steel plate. After alkaline degreasing, the samples were immersed in a 10% sulfuric acid solution at 70°C for 10 minutes, then immersed in a 15 g / l nickel sulfate solution at 70°C for 7 minutes, and then neutralized. Furthermore, 100 μm of Nippon Fellow #102 glaze was applied to both sides, and the samples were fired at 860°C for 5 minutes in an atmosphere with a dew point of 35°C. After firing, a 2 kg spherical weight was dropped onto each sample from a height of 1 m, and the degree of enamel peeling in the deformed area was measured with 169 palpation needles and evaluated by the area percentage of the unpeeled area. The area percentage was evaluated as the average of the four samples. The evaluation criteria were as follows: A: Excellent, B: Normal, C: Problematic, with C being a failure. A: Area ratio of unpeeled area is 90% or more. B: Area ratio of unpeeled area is 40% or more, but less than 90%. C: Area ratio of unpeeled area is less than 40%.

[0059] [Appearance] As a pretreatment, 10 samples measuring 150 mm x 100 mm were taken from each steel plate. Each sample was degreased with alkali, immersed in a 15 g / l nickel sulfate solution at 70°C for 7 minutes, and then neutralized. Furthermore, 100 μm of Nippon Fellow #102 glaze was applied to both sides, and the samples were fired at 860°C for 5 minutes in an atmosphere with a dew point of 35°C. After firing, the appearance of each sample was visually observed, and the presence or absence of bubbles and black spots was evaluated. If bubbles or black spots were present in 3 or more out of 10 samples, it was considered problematic, and if bubbles or black spots were present in 2 or fewer out of 10 samples, it was considered acceptable.

[0060] [Aging Characteristics] The aging characteristics of the obtained steel sheet were evaluated in the following manner. To evaluate the aging properties of the steel sheet of the present invention, a tensile test was conducted in accordance with JIS Z 2241:2011. Test specimens were taken from JIS No. 5 specimens along the rolling direction of the steel sheet. The test was performed in the following procedure: (1) A pre-strain (plastic strain) of 10% was applied to the test specimen. (2) Then, artificial aging treatment was performed at 100°C for 1 hour. (3) The lower yield point before and after the aging treatment was measured, and the difference was calculated as the AI ​​value (Aging Index) defined by the following formula: AI value = σ ｙ１ -σ ｙ２ Here, σ ｙ１ σ is the lower yield point after aging treatment at unit MPa. ｙ２This is the lower yield point before aging treatment in unit MPa. In this invention, a steel sheet with an AI value of 30 MPa or less was determined to be non-aging, and a value exceeding 30 MPa was evaluated as aging. The rolling direction of the steel sheet was determined by the following method. The rolling direction is obvious if the material is a coil or if the rolling direction is recorded. For steel sheets from which test pieces are cut, if the steel sheet is cut from a coil while maintaining its width dimension, and the width dimension is known, the rolling direction was determined from the dimensions of the steel sheet. Also, if at least one of the end faces of the steel sheet had a coil end face in the width direction remaining, the width direction and rolling direction were determined from information indicating the condition of the coil end face (presence or absence of edge drop, presence or absence of plating). If the above information is lost, the rolling direction was determined by the following method. The Z-plane of the plate (the plane parallel to both the longitudinal and width directions of the plate) was polished to a 1 / 4 thickness position and finished with mirror polishing. After finishing, a Mn concentration map of a 1000 μm × 1000 μm area was obtained on the surface using EPMA. When Mn solidification segregation was measured in streaks, the longitudinal direction of the streaks was determined to be the rolling direction. For steel plates with the same dimensions in the width direction and rolling direction, if there was information indicating the condition of the coil end face on the steel plate, the rolling direction was determined from that information. If there was no such information, the rolling direction was determined by observing the microstructure using the method described above. Note that if at least one of the end faces of the steel plate retains the end face in the width direction of the coil, a person skilled in the art can easily determine the width direction and rolling direction from the information indicating the condition of the coil end face. Furthermore, even when the steel plate is processed into an enamel product, the rolling direction can be determined using the above method for lightly processed parts of the part, such as relatively unprocessed flat parts.

[0061] [Manufacturability] The manufacturability of the invention examples was evaluated. In the manufacture of steel plates, nozzle erosion can occur easily during casting, and in such cases, the frequency of nozzle replacement increases, reducing manufacturability. In addition, inclusions may become fixed on the surface of the slab, and if there are many such inclusions, maintenance is required to remove the inclusions formed on the surface, reducing manufacturability. Therefore, the condition of nozzle erosion and the slab surface were visually inspected. The results are shown in Tables 5 and 6.

[0062]

[0063]

[0064] As shown in Tables 1-6, the composition in mass percent is as follows: C: 0-0.0050%, Si: 0-0.050%, Mn: 0.020-0.50%, P: 0.003-0.050%, S: 0.005-0.050%, Al: 0-0.010%, O: 0.0100-0.0400%, Cu: 0.010-0.060%, N: 0-0.0050%, Cr: 0.01-0.20%, B: 0. The chemical composition consists of 0.0010 to 0.0050%, with the remainder being Fe and impurities. On a plane parallel to the surface of the steel plate at a position 1 / 4 of the plate thickness from the surface, the area ratio of the ferrite phase is 98% or more. The material contains one or more elements selected from the group consisting of Mn, Cr, and Al, and the number density of inclusions with a diameter of more than 0.5 μm and less than or equal to 10 μm is 300 to 800 pieces / mm². ２ Steel plates D1 to D25 exhibited excellent enamel properties, and enamel products made from these steel plates also exhibited excellent enamel properties. Furthermore, in the examples of the present invention with an O content of 0.030 mass% or more, compared to the examples of the present invention with an O content of less than 0.030 mass%, there was greater erosion of the nozzle used during casting, or a greater number of inclusions requiring maintenance were observed on the surface of the slab. In other words, the examples with an O content of less than 0.030 mass% exhibited superior manufacturability.

[0065] Furthermore, the examples of d1 using steel a1 with an excessive C content, d16 using steel a16 with an insufficient B content, and d22 using steel a18 without B content exhibited inferior aging characteristics (they aged).

Claims

1. The chemical composition is as follows (by mass%): C: 0-0.0050%, Si: 0-0.050%, Mn: 0.020-0.50%, P: 0.003-0.050%, S: 0.005-0.050%, Al: 0-0.010%, O: 0.0100-0.0400%, Cu: 0.010-0.060%, N: 0-0.0050%, Cr: 0.01-0.20%, B: 0.0010-0.0050%, with the remainder being Fe and impurities; and the area ratio of the ferrite phase is 98% or more on a surface parallel to the surface of the steel plate at a position 1 / 4 of the plate thickness in the thickness direction from the surface of the steel plate. The material contains an oxide having one or more elements selected from the group consisting of Mn, Cr, and Al, and the number density of inclusions with a diameter greater than 0.5 μm and less than or equal to 10 μm is 300 to 800 particles / mm². ２ It is a steel plate.

2. The steel sheet according to claim 1, further containing, by mass%, a total of one or more elements selected from the group consisting of Ni, Nb, As, Ti, V, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg, in an amount greater than 0 to 0.100%.

3. On a plane parallel to the surface of the steel plate at a position 1 / 4 of the plate thickness in the thickness direction from the surface of the steel plate, the number density of inclusions containing one or more oxides selected from the group consisting of Mn, Cr, and Al, with a major axis of more than 1.0 μm and less than or equal to 10 μm, is 500 pieces / mm². ２ A steel plate according to claim 1 or 2, which is less than [amount missing].

4. A steel plate according to any one of claims 1 to 3, wherein the AI ​​value defined by formula (1) is 30 MPa or less. AI value = σ ｙ１ -σ ｙ２ ...(1) Equation (1) In the above equation (1), σ ｙ１ σ is the lower yield point after aging treatment at unit MPa. ｙ２ This is the lower yield point before aging treatment at a unit MPa.

5. A steel sheet according to any one of claims 1 to 4, which is a cold-rolled steel sheet.

6. A steel sheet for enamel coating, as described in any one of claims 1 to 5.

7. An enamel product comprising a steel plate according to any one of claims 1 to 6, and a glassy layer disposed on the steel plate.

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