Hot-dip galvanized steel sheet

Abstract

Even if the steel sheet contains boron, ensure that the adhesion of plating can be secured. 【Solution means】 It is a hot-dip galvanized steel sheet with a plating layer applied to the surface of the steel sheet, where the steel sheet consists of C: 0.05 to 0.50% by mass, Si: 0.01 to 2.0% by mass, Mn: 0.1 to 2.5% by mass, P: 0.00 to 0.03% by mass, S: 0.00 to 0.01% by mass, Al: 0.00 to 1.00% by mass, N: 0.00 to 0.01% by mass, Ti: 0.00 to 0.20% by mass, B: 0.0005 to 0.0050% by mass and Fe and impurities, the plating layer has an adhesion amount on one side: 30 g / m 2 ~120 g / m 2 and an Fe content: 0.4 g / m 2 or more and 3.0 g / m 2 or less, and it is a hot-dip galvanized steel sheet that is detected such that the maximum value of the Ni emission intensity in the GDS analysis is 10 times or more the average value of the Ni emission intensity of the steel sheet.

Description

[Technical Field]

[0001] This invention relates to a hot-dip galvanized steel sheet. [Background technology]

[0002] Hot-dip galvanized steel sheets are steel sheets that have been coated with zinc by passing them through a hot-dip galvanizing bath. Because hot-dip galvanized steel sheets have excellent corrosion resistance, they are used in a wide range of applications, including home appliances, building materials, and automobiles. However, they are formed and processed to suit their respective applications. Therefore, it is crucial that the plating does not peel off during processing, and hot-dip galvanized steel sheets require high plating adhesion (adhesion between the plating layer and the steel sheet). On the other hand, it is known that when high-strength steel sheets (high-tensile steel) are used as the base material for plating, plating adhesion tends to decrease. This is because elements such as Si (silicon) and Mn (manganese), which are added to improve the mechanical properties of high-strength steel sheets, are easily oxidized. During the annealing process, these elements are selectively oxidized on the surface of the steel sheet, and the presence of oxides of silicon and manganese on the outermost layer of the steel sheet reduces the wettability with molten zinc. While these problems can be avoided by removing additives such as silicon, steel sheets used in automobiles and other applications require high strength, making it impossible to completely eliminate the desire to include Si, Mn, and B (boron (hereinafter sometimes referred to as boron)) to modify the steel material. For this reason, technologies have been developed to improve the adhesion of the plating on hot-dip galvanized steel sheets containing Si (see Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2001-323355

[0004] Against the background described above, while researching hot-dip galvanized steel sheets that satisfy both the modification of the steel material and the avoidance of a decrease in plating adhesion, the inventors found that when steel sheets containing boron are used as the base material for plating, there is a tendency for the plating adhesion to decrease. [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] This invention was made against this background, and the problem that this invention aims to solve is to ensure plating adhesion even when the steel sheet contains boron. [Means for solving the problem]

[0006] To solve the above problems, a hot-dip galvanized steel sheet is provided, in which a plating layer is applied to the surface of the steel sheet, wherein the steel sheet consists of C: 0.05~0.50 mass%, Si: 0.01~2.0 mass%, Mn: 0.1~2.5 mass%, P: 0.00~0.03 mass%, S: 0.00~0.01 mass%, Al: 0.00~1.00 mass%, N: 0.00~0.01 mass%, Ti: 0.00~0.20 mass%, B: 0.0005~0.0050 mass%, Fe, and impurities, and the plating layer has an adhesion amount of 30 g / m² on one side. 2 ~120g / m 2 , Fe content: 0.4g / m 2 More than 3.0g / m 2 The following describes a hot-dip galvanized steel sheet characterized in that, in high-frequency glow discharge emission analysis (GDS analysis), the maximum value of Ni emission intensity is detected to be 10 times or more the average value of Ni emission intensity of the steel sheet.

[0007] Furthermore, the Al barrier index = Al content in the plating layer [mg / m²] 2 ]-(Plating adhesion amount [mg / m 2 ]-Fe content in the plating layer [mg / m 2 It is preferable to have a configuration that satisfies ]) × 0.002 ≥ 150.

[0008] Moreover, it is preferable that the surface layer region within a depth of 5 μm from the surface of the steel sheet contains at least one of austenite phases containing Fe3Ni, FeNi, FeNi3, and Ni.

Advantages of the Invention

[0009] When the present invention is used, even if boron is contained in the steel sheet, the adhesion of plating can be ensured.

Brief Description of the Drawings

[0010] [Figure 1] It is a conceptual diagram of a hot-dip galvanized steel sheet.

Embodiments for Carrying Out the Invention

[0011] Embodiments for carrying out the invention are shown below. As understood from FIG. 1, in the hot-dip galvanized steel sheet 1 of the present embodiment, a plating layer 3 is provided on the surface of the steel sheet 2. This hot-dip galvanized steel sheet 1 is composed of C: 0.05 to 0.50% by mass, Si: 0.01 to 2.0% by mass, Mn: 0.1 to 2.5% by mass, P: 0.00 to 0.03% by mass, S: 0.00 to 0.01% by mass, Al: 0.00 to 1.00% by mass, N: 0.00 to 0.01% by mass, Ti: 0.00 to 0.20% by mass, B: 0.0005 to 0.0050% by mass, and Fe and impurities in the steel sheet 2. The plating layer 3 provided on the surface of this steel sheet 2 has a single-sided adhesion amount of 30 g / m 2 ~120 g / m 2 and an Fe content of 0.4 g / m 2 or more and 3.0 g / m 2 or less. Further, this hot-dip galvanized steel sheet 1 is detected in GDS analysis such that the maximum value of the Ni emission intensity is 10 times or more the average value of the Ni emission intensity of the steel sheet 2. Therefore, even if boron is contained in the steel sheet 2, it is possible to ensure the adhesion of plating.

[0012] Here, the basic matters of the hot-dip galvanized steel sheet 1 will be described. The hot-dip galvanized steel sheet 1 is manufactured on a continuous hot-dip galvanizing line called CGL (Continuous Galvanizing Line). In order to ensure the plating adhesion, it is important to control the surface state of the steel sheet 2 before it is immersed in the hot-dip galvanizing bath. By appropriately accumulating Ni on the surface of the steel sheet 2, it is possible to ensure the plating wettability when immersed in the hot-dip galvanizing bath and improve the plating adhesion.

[0013] In the present invention, among the steel sheets 2 containing boron in the chemical composition, a specific steel sheet 2 is targeted. Therefore, the composition of the steel sheet 2 will be described.

[0014] C (carbon) contained in the steel sheet 2 is 0.05 to 0.50% by mass. Also, it is preferable that carbon is 0.07 to 0.45% by mass. Further, it is preferable that carbon is 0.10 to 0.40% by mass. Carbon is an element that increases the strength of the steel sheet 2. If the carbon content is too low, it is difficult to manufacture a high-strength steel sheet 2. However, if the carbon content is too high, the toughness of the steel sheet 2 may be reduced.

[0015] Si (silicon) contained in the steel sheet 2 is 0.01 to 2.0% by mass. Also, it is preferable that silicon is 0.02 to 1.8% by mass. Further, it is preferable that silicon is 0.05 to 1.5% by mass. Silicon is an element that increases the strength of steel by solid solution strengthening and structure strengthening. However, if the silicon content is too high, oxides may be formed on the surface of the steel sheet 2, reducing the plating wettability. Also, silicon is generally utilized as a deoxidizer, and if the silicon content is too low, it becomes difficult to manufacture industrially and technically.

[0016] The manganese (Mn) content in steel sheet 2 should be 0.1 to 2.5% by mass. Preferably, the manganese content should be 0.3% to 2.3% by mass. Even more preferably, it should be 0.5% to 2.0% by mass. Manganese is an element that enhances the strength and hardenability of steel. If the manganese content is too low, it becomes difficult to manufacture high-strength steel sheet 2. On the other hand, if the manganese content is too high, the workability of the steel tends to deteriorate.

[0017] The steel sheet 2 does not need to contain phosphorus (P), but it may contain it. However, the amount of phosphorus in the steel sheet 2 should be 0.03% by mass or less. In other words, the amount of phosphorus in the steel sheet 2 should be 0.00 to 0.03% by mass. Furthermore, it is preferable that the amount of phosphorus be 0.02% by mass or less. Phosphorus segregates at the grain boundaries, reducing the toughness of the steel and decreasing its resistance to delayed fracture. While there is no lower limit, the phosphorus content may be 0.001% or more from the viewpoint of suppressing increases in manufacturing costs.

[0018] Steel plate 2 does not need to contain sulfur (S), but it may contain it. However, the amount of sulfur in steel plate 2 should be 0.01% by mass or less. In other words, the amount of sulfur in steel plate 2 should be 0.00 to 0.01% by mass. Furthermore, it is preferable that the amount of sulfur be 0.005% by mass or less. Sulfur forms sulfides, which reduce the toughness of the steel and decrease its resistance to delayed fracture. Although there is no lower limit, the sulfur content may be 0.0001% or more from the viewpoint of suppressing increases in manufacturing costs.

[0019] Steel plate 2 does not necessarily need to contain aluminum (Al), but it may contain it. However, the amount of aluminum in steel plate 2 should be 1.00% by mass or less. In other words, the amount of aluminum in steel plate 2 should be between 0.00% and 1.00% by mass. Aluminum is generally used for deoxidation of steel, and in that case, it is also contained in steel plate 2. On the other hand, Al is easily oxidized, and an increase in inclusions can easily degrade the workability of the steel. Furthermore, although there is no lower limit, the aluminum content may be 0.005% or more. Note that the aluminum content in this specification refers to the content of acid-soluble aluminum.

[0020] Steel plate 2 does not need to contain nitrogen (N), but it may contain it. However, the amount of nitrogen in steel plate 2 should be 0.01% by mass or less. In other words, the amount of titanium in steel plate 2 should be 0.00 to 0.01% by mass. Nitrogen forms nitrides, which reduce the toughness of the steel. If boron is present, nitrogen combines with boron to reduce the amount of solid-solution boron. For this reason, nitrogen reduces the hardenability of steel plate 2.

[0021] Steel plate 2 does not necessarily need to contain titanium (Ti), but it may be included. However, the amount of titanium in steel plate 2 should be 0.20% by mass or less. In other words, the amount of titanium in steel plate 2 should be between 0.00% and 0.20% by mass. Since titanium combines with nitrogen to form nitrides, the bonding of boron with nitrogen is suppressed, and the decrease in hardenability due to the formation of boron nitrides can be suppressed. However, if the titanium content is too high, the above effect will saturate, and there is a concern that the toughness of the steel will decrease due to the excessive precipitation of titanium nitrides.

[0022] The amount of boron (B) in steel sheet 2 should be 0.0005 to 0.0050 by mass. Preferably, the boron content should be 0.0010 to 0.0040 by mass. Even more preferably, the boron content should be 0.0015 to 0.0030 by mass. Boron contributes to increasing the hardenability of steel and the strength of steel sheet 2. However, if the boron content is too high, there is a concern that its effect will saturate and that it will affect the oxidation behavior of the surface of steel sheet 2 during annealing heating, leading to a decrease in plating wettability.

[0023] The remainder of steel plate 2 consists of Fe (iron) and impurities. These impurities tend to be numerous when electric furnace steel, produced by melting raw materials such as iron scrap in an electric furnace, is used as steel plate 2. Examples of impurities include Cr (chromium), Mo (molybdenum), Nb (niobium), Ni (nickel), V (vanadium), and Cu (copper).

[0024] The plating layer 3 is usually applied similarly to the entire surface of the steel sheet 2, but the amount of plating on one side is 30 g / m². 2 ~120g / m 2 Make it so that it becomes 30g / m 2 If it is less than 120 g / m², there is a risk of impairing corrosion resistance. 2 This is because exceeding this amount would be uneconomical. Furthermore, from the standpoint of corrosion resistance and cost-effectiveness, the amount of plating applied to one side should be 45 g / m². 2 ~100g / m 2 It is preferable to have the plating adhesion amount on one side set to 60 g / m². 2 ~90g / m 2 It is preferable to do so.

[0025] The plating layer 3 does not contain only zinc (Zn). For example, Fe is included in the plating, but the Fe content in the plating layer 3 is 0.4 g / m². 2 More than 3.0g / m 2 The following applies: 0.4g / m 2 If it is less than 3.0 g / m², there is concern about the reactivity between the steel plate 2 and the plating layer 3. 2 This is because exceeding this level makes it easier for red rust to form, raising concerns about corrosion resistance. Furthermore, from the viewpoint of the reactivity and corrosion resistance of the plating layer 3, the Fe content in the plating layer 3 should be 0.5 g / m². 2 ~2.5g / m 2 It is preferable that the Fe content in the plating layer 3 be 0.6 g / m 2 ~2.0g / m 2 It is preferable to do so.

[0026] The inventors have discovered that by accumulating Ni on the surface layer 6 of the hot-dip galvanized steel sheet 1, the plating can be made less likely to peel off even if the steel sheet 2 contains boron. However, GDS analysis revealed that such an effect could not be observed when the maximum Ni emission intensity was less than 10 times the average Ni emission intensity of the steel sheet 2, and it was found that it is preferable for the maximum Ni emission intensity to be 10 times or more the average Ni emission intensity of the steel sheet 2. Furthermore, it is even more preferable for the maximum Ni emission intensity to be 30 times or more the average Ni emission intensity of the steel sheet 2.

[0027] Preferably, the hot-dip galvanized steel sheet 1 contains one or more of the following austenite phases: Fe3Ni, FeNi, FeNi3, and Ni, in the surface layer region within a depth of 5 μm from the surface of the steel sheet 2. This is because the presence of a Ni-enriched region on the surface layer 6 of the steel sheet increases reactivity in the hot-dip galvanizing bath, thereby improving the adhesion between the steel sheet 2 and the plating layer 3. Whether or not these are present can be confirmed by dissolving the plating layer 3 in dilute hydrochloric acid containing an inhibitor and analyzing it from the surface direction of the steel sheet 2 using EPMA and EBSD.

[0028] The plating layer 3 of the hot-dip galvanized steel sheet 1 contains aluminum, which may originate from the steel sheet 2 or from components in the plating bath. In any case, the inventors have found that by appropriately forming an Al barrier layer 5 by accumulating Al at the boundary between the Zn plating layer 4 and the steel sheet 2, the plating can be made less likely to peel off even if the steel sheet 2 contains boron. Furthermore, the inventors have found that the Al content in the plating layer 3 [mg / m³] serves as an indicator to check how much Al is accumulated at the boundary between the Zn plating layer 4 and the steel sheet 2. 2 ]-(Plating adhesion amount [mg / m 2 ]-Fe content in plating layer 3 [mg / m 2 We also found that if the value obtained by the formula ]) × 0.002 (a judgment formula representing the concentration of aluminum in the barrier layer: Al barrier index) is 150 or higher, it is possible to secure an acceptable level for plating adhesion.

[0029] In other words, if the above determination formula ≥ 150 is satisfied, the plating can be made less likely to peel off. Furthermore, it is preferable to satisfy the determination formula ≥ 175, because it is considered that the Al barrier layer 5 is more sufficiently formed. Moreover, it is preferable to satisfy 450 ≥ determination formula ≥ 200, because if the Al barrier index becomes too high, there is a concern that aluminum oxide will form on the outermost layer of the plating layer 3, degrading the chemical conversion treatment properties.

[0030] Furthermore, to accumulate Al at the boundary between the Zn plating layer 4 and the steel sheet 2, it is effective to pre-deposit Ni onto the steel sheet 2, thereby accumulating Ni on the steel sheet surface layer 6. Of course, various methods other than pre-deposition of Ni can be used to accumulate Ni on the steel sheet surface layer 6.

[0031] Areas where components such as Ni accumulate can be identified using GDS analysis, which allows for spectroscopic measurement of atomic emission within an Ar plasma using sputtering in a glow discharge region.

[0032] Glow discharge optical emission spectrometry (GD-OES), used in GDS analysis, is a well-known technique, but a brief explanation is provided to facilitate understanding of the following explanation. GDS analysis measures the abundance of an element by detecting the emission intensity corresponding to the abundance of that element. Furthermore, since the analysis is performed by gradually drilling into the sample surface, the analysis is performed at positions deeper in the sample from the surface as time passes after the start of measurement. Therefore, for example, by comparing the measurement time until the maximum emission intensity is detected (measurement time until the peak) for each element, it is possible to determine which elements are concentrated at deeper positions from the sample surface.

[0033] Specifically, if the measurement time at which the maximum emission intensity of a certain element is detected is longer than the measurement time from the start of measurement to the detection of the maximum emission intensity of other elements, it indicates that the element is concentrated at a greater distance (deeper position) from the sample surface than other elements.

[0034] It is preferable that Ni (nickel) accumulates on the surface layer 6 of the steel sheet, and it is preferable that the depth to which Ni is accumulated is the same as or shallower than the depth to which Mn is accumulated. In other words, it is preferable that the hot-dip galvanized steel sheet 1 is in a state in which the measurement time at which the maximum value of Ni emission intensity is detected in GDS analysis is less than or equal to the measurement time at which the maximum value of Mn emission intensity is detected. [Examples]

[0035] Here, we will describe the examples. The steel sheet 2 in the examples satisfies the chemical composition described above, but the detailed composition ratios will be omitted. In the steel sheet 2 of the examples, in addition to boron, manganese, silicon, carbon, aluminum, and titanium are included, but as mentioned above, aluminum and titanium do not need to be included in the steel sheet 2, and phosphorus, sulfur, and nitrogen may also be included in the steel sheet 2.

[0036] The amount of plating layer 3 was measured by the following method. A sample containing plating layer 3 was taken from each hot-dip galvanized steel sheet 1. Plating layer 3 was dissolved using 5% hydrochloric acid containing 0.1 g / L of an inhibitor that suppresses the dissolution of iron (manufactured by Asahi Chemical Industry Co., Ltd., product name: Ibit 700A). The completion of the dissolution of plating layer 3 was determined from the way foaming occurred as the plating layer 3 dissolved. The amount of plating adhesion (g / m²) was calculated based on the sample weight before dissolution, the sample weight after dissolution, and the area where plating layer 3 was formed. 2 ) was sought. The ICP analysis used for the component analysis of plating layer 3 was performed using an ICPS-8100 manufactured by Shimadzu Corporation. For the analysis, the hydrochloric acid in which plating layer 3 was dissolved was used, diluted as appropriate, to determine the amount of plating layer 3 present. To perform concentration analysis of Al and Fe, calibration curves were created using Al and Fe standard solutions, and the concentrations of the sample were quantified.

[0037] Furthermore, GDS analysis was performed using a GD-Profiler2 manufactured by Horiba, Ltd. The discharge was performed under conditions set to a power of 35W, an Ar pressure of 600Pa, and a discharge diameter of 4mmφ. The measurement time was set to 300 seconds, with a measurement interval of 0.5 seconds, but the measurement time may be extended as needed for samples with a large amount of plating. In addition, the measurement time at which the maximum value of the emission intensity of each element is detected will be determined from 2.0 seconds onwards. This is because there is a concern that the elements being analyzed may be detected at high values ​​due to the influence of dirt and atmospheric oxidation on the outermost surface. Furthermore, the average value of the Ni emission intensity of steel plate 2 was calculated as the average value of the Ni emission intensity between 250 seconds and 300 seconds of analysis time, in order to calculate the value from a region sufficiently deep in the thickness direction from the surface of steel plate 2, without being influenced by the plating layer 3 and the steel plate surface layer 6.

[0038] Observations using EBSD (electron backscatter diffraction) with a scanning electron microscope (SEM) were performed using a Hitachi SU70. Observations were conducted over a 100 μm × 100 μm area at 0.50 μm intervals, and the diffraction patterns of ferrite, austenite, Fe3Ni, FeNi, and FeNi3 were analyzed.

[0039] EPMA analysis was performed using a Shimadzu EPMA1610 with the same field of view as EBSD. The analysis was performed with an acceleration voltage of 15.0 kV and an irradiation current of 2.0 × 10⁻¹⁴. -8 A. The measurement is performed under conditions where the measurement time is 50 ms / point and the measurement area is 100 μm × 100 μm.

[0040] To perform EBSD and EPMA analysis in the same field of view, the plating layer 3 was dissolved in dilute hydrochloric acid containing an inhibitor, and then indentations were made near the four corners of the analysis area using a Vickers hardness tester to serve as markers.

[0041] Plating adhesion was determined using a ball impact test. More specifically, a ball was dropped from a height corresponding to the thickness of the hot-dip galvanized steel sheet 1 to be evaluated, creating a protrusion. Tape was then applied to the protrusion, removed, and the degree of plating peeling at that location was observed and evaluated using the ball impact test. Specifically, a ball was dropped from a height of 350 mm for sheet thicknesses less than 0.8 mm, 400 mm for sheet thicknesses between 0.8 mm and 1.0 mm, 500 mm for sheet thicknesses between 1.0 mm and 1.2 mm, 600 mm for sheet thicknesses between 1.2 mm and 1.5 mm, 800 mm for sheet thicknesses between 1.5 mm and 1.8 mm, and 950 mm for sheet thicknesses of 1.8 mm or more, using a punch weighing 25 kg, a punch tip shape of φ12.7 mm, and a die hole diameter of 20 mm. Plating adhesion was evaluated based on the area ratio of the peeled plating layer 3 attached to the tape. If the area ratio was less than 1%, the plating adhesion was considered very good and was evaluated as Excellent. If the area ratio was between 1% and less than 5%, the plating adhesion was considered good and was evaluated as Good. If the area ratio was 5% or more, there was a problem with the plating adhesion and it was evaluated as Not Good.

[0042] [Table 1]

[0043] As can be seen from these results, the adhesion of the plating varies regardless of the amount of plating applied or the iron content in the plating. However, in the GDS analysis, when the maximum value of the Ni emission intensity was 10 times or more the average value of the Ni emission intensity of steel plate 2, the plating adhesion was good, and when it was less than 10 times, the plating adhesion was poor.

[0044] Furthermore, it was found that the plating adhesion was improved when the surface layer region within a depth of 5 μm from the surface of the steel plate 2 contained one or more of the austenite phases containing Fe3Ni, FeNi, FeNi3, and Ni.

[0045] Furthermore, the Al content in the plating layer 3 [mg / m 2 ]-(Plating adhesion amount [mg / m 2 ]-Fe content in plating layer 3 [mg / m 2 If the calculation result in the formula (judgment formula) ) × 0.002 is 150 or higher, the plating adhesion is good; if it is less than 150, the plating adhesion is not good.

[0046] Although the present invention has been described above, focusing on embodiments, the present invention is not limited to the above embodiments and can be implemented in various forms. [Explanation of Symbols]

[0047] 1. Hot-dip galvanized steel sheet 2 steel plate 3 Plating layer 4 Zn plating layer 5. Al barrier layer 6. Steel plate surface layer (Ni accumulation region)

Claims

1. A hot-dip galvanized steel sheet having a plating layer applied to its surface, Steel plate C: 0.05 to 0.50% by mass, Si: 0.01 to 2.0% by mass, Mn: 0.1 to 2.5% by mass, P: 0.00 to 0.03% by mass, S: 0.00 to 0.01% by mass, Al: 0.00 to 1.00% by mass, N: 0.00 to 0.01% by mass, Ti: 0.00 to 0.20% by mass, B: 0.0005 to 0.0050% by mass And Fe and impurities, Plating Adhesion amount on one side: 30 g / m ２ ~120g / m ２ Fe content: 0.4 g / m ２ The above 3.0 g / m ２ the following And, A hot-dip galvanized steel sheet characterized in that, in high-frequency glow discharge emission analysis (GDS analysis), the maximum value of Ni emission intensity is detected to be 10 times or more the average value of Ni emission intensity of the steel sheet.

2. Al barrier index = Al content in the plating layer [mg / m²] ２ ]- (Plating adhesion amount [mg / m ２ ]- Fe content in the plating layer [mg / m ２ ]) × 0.002 ≥ 150 The hot-dip galvanized steel sheet according to claim 1, characterized in that it satisfies the requirements.

3. In the surface layer region within 5 μm from the surface of the steel sheet, Fe ３ Ni, FeNi, FeNi ３ , and the hot-dip galvanized steel sheet according to claim 1 or 2 containing any one or more of austenite phases containing Ni.

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