Title of the invention: Hot-rolled hot-dip galvanized steel sheet with excellent surface appearance and its manufacturing method
Technical field
[One]
The present invention relates to a hot-rolled hot-dip galvanized steel sheet and a method for manufacturing the same, and more particularly, to a hot-rolled hot-dip galvanized steel sheet and a manufacturing method having an excellent surface appearance by effectively suppressing the surface defects of the plating layer.
Background
[2]
Hot-dip galvanized steel sheet refers to a steel sheet in which a galvanized layer is formed on the surface of the holding steel sheet by hot-dip plating, and hot-rolled hot-dip galvanized steel sheet refers to a steel sheet in which a zinc plated layer is formed on the surface of a hot-rolled steel sheet by hot-dip plating. In general, the hot-rolled hot-dip galvanized steel sheet can be manufactured by a series of processes such as scale breaking, pickling, heat treatment, immersion in a plating bath, and air wiping.
[3]
[4]
The pickling process is a process performed to remove the oxidized scale formed in the hot rolling process, and a chemical pickling process is mainly used to remove the oxidized scale on the surface of the hot rolled steel sheet using an acid solution. In the case of such chemical pickling, since a strong acidic solution such as sulfuric acid or hydrochloric acid is mainly used, it is harmful to the environment, and there is a problem of severely corroding the base material as the reaction time increases. Accordingly, there is a need for a scale removal technique capable of effectively removing scale on the surface of a hot-rolled steel sheet and minimizing environmental pollution problems.
[5]
[6]
In addition, the air wiping process is an essential process of controlling the amount of plating on the surface of the plated steel sheet by spraying a high-pressure fluid after immersion in the plating bath. It can cause surface defects such as pattern defects. An oxide film having relatively low fluidity is formed on the surface portion of the plating layer, and molten zinc having relatively high fluidity is present inside the plating layer, so a difference in fluidity occurs along the thickness direction of the plating layer. The high-pressure fluid sprayed from the air knife device reaches the surface layer of the plating layer and causes crack formation in the oxide film, and the molten zinc inside the plating layer may be exposed to the outside through cracks formed in the oxide film. Immediately after passing through the air knife device, the plating layer is rapidly solidified, and as a result, flow pattern defects in which the shapes of valleys and floors are continuously appearing occur on the surface of the plating layer.
[7]
[8]
Conventionally, in order to prevent flow pattern defects, a technology has been proposed to prevent the formation of an oxide film itself by introducing a sealing box for creating a non-oxidizing atmosphere. However, the formation of an oxide film on the surface layer of the plating layer can be suppressed to some extent by the introduction of the sealing box. Due to the reaction between the outside air and the plating bath, an excessive amount of dross was generated on the surface of the plating bath, resulting in a problem of lowering the surface quality of the plated steel sheet.
[9]
[10]
Patent Document 1 proposes a technique for mechanically removing the flow pattern defect by temper rolling after forming the plating layer, rather than suppressing the occurrence of the flow pattern defect itself. However, in order to remove the flow pattern, it is necessary to press the steel sheet with as high a pressing force as possible, and there is a high risk of damage to the normal plating layer and peeling over the plating layer. Therefore, there is an urgent need to introduce a technology capable of effectively suppressing flow pattern defects on the surface of the plating layer and preventing deterioration of the surface quality of the coated steel sheet.
[11]
[12]
(Patent Document 1) Republic of Korea Patent Publication No. 10-2001-0060423 (published on July 7, 2001)
[13]
Detailed description of the invention
Technical challenge
[14]
According to an aspect of the present invention, a hot-rolled hot-dip galvanized steel sheet having excellent surface appearance and a method of manufacturing the same can be provided.
[15]
The subject of the present invention is not limited to the above. Those of ordinary skill in the art will have no difficulty in understanding the additional subject of the present invention from the general contents of the present specification.
Means of solving the task
[16]
The hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention includes a holding steel sheet and a galvanized layer, and the galvanized layer may include a continuous Al enriched layer formed within 100 nm from the surface of the galvanized layer.
[17]
The Al enriched layer may be an aluminum oxide (Al 2 O 3 ) layer.
[18]
The thickness of the Al enriched layer may be 50 nm or less (excluding 0 nm).
[19]
The surface roughness of the surface of the holding steel sheet forming an interface with the galvanized layer may be 0.7 to 2.5 μm based on a center line average roughness (Ra).
[20]
The galvanized layer may include, by weight %, Al: 0.2 to 0.6%, the remaining Zn and other unavoidable impurities.
[21]
Hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention, by stretching the holding steel sheet at a first elongation to cause a crack in the scale of the holding steel sheet surface; Mechanically removing scale from the surface of the holding steel plate by applying a physical impact to the surface of the holding steel plate; Chemically pickling the surface of the holding steel sheet by reacting the surface of the holding steel sheet with an acidic solution; Temper rolling the holding steel sheet at a second elongation to planarize the surface of the holding steel sheet; By weight %, Al: 0.2 to 0.4%, it can be prepared by immersing the base steel sheet in a hot-dip galvanizing bath containing the remaining Zn and other inevitable impurities to form a galvanized layer.
[22]
The first elongation may be 0.2 to 1.5%.
[23]
The surface of the holding steel sheet may be shot blasted to mechanically remove scale from the surface of the holding steel sheet.
[24]
In the shot blasting, a shot ball having an average diameter of 0.18 to 0.6 mm can be projected at an average projection amount of 800 to 1800 kg/min and an average projection speed of 65 to 90 m/s.
[25]
The surface of the holding steel sheet may be chemically pickled by immersing the holding steel sheet for 15 to 35 in an aqueous hydrochloric acid solution of 5 to 20% concentration provided in a temperature range of 70 to 85°C.
[26]
The base steel sheet may be temper rolled by Brightrol having a surface roughness of 0.1 to 0.8 μm based on the average roughness of the center line (Ra).
[27]
The surface roughness of the temper-rolled base steel sheet may be 0.7 to 2.5 μm based on a centerline average roughness (Ra).
[28]
The second elongation may be 0.5 to 2.5%.
[29]
The sum of the first elongation and the second elongation may be 0.7 to 4.0%.
[30]
The second elongation may be greater than the first elongation.
[31]
The immersion temperature of the hot-dip galvanizing bath of the holding steel sheet may be 450 to 500°C.
Effects of the Invention
[32]
According to an aspect of the present invention, it is possible to provide a hot-rolled hot-dip galvanized steel sheet having a beautiful surface appearance and a method of manufacturing the same, which effectively suppresses the occurrence of flow pattern defects on the surface of the galvanized layer.
[33]
According to an aspect of the present invention, mechanical scale removal and chemical pickling are sequentially performed on the holding steel sheet, and hot-rolled hot-dip galvanizing plating capable of effectively removing residual scale on the surface of the holding steel sheet while minimizing the use of a chemical solution used for chemical pickling. A method of manufacturing a steel plate can be provided.
Brief description of the drawing
[34]
1 to 3 are results of analyzing the surface layer portion of a hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention using FIB-TEM.
[35]
4 to 6 are results of analyzing the surface layer portion of a hot-rolled hot-dip galvanized steel sheet having a discontinuous aluminum oxide layer using FIB-TEM.
Best mode for carrying out the invention
[36]
The present invention relates to a hot-rolled hot-dip galvanized steel sheet having excellent surface appearance and a method for manufacturing the same, and hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to explain the present invention in more detail to those of ordinary skill in the art to which the present invention pertains.
[37]
[38]
The hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention may include a base steel sheet and a galvanized layer formed on the surface of the base steel sheet. The holding steel sheet of the present invention may be a hot-rolled steel sheet, but is not limited thereto, and may be interpreted as including all types of steel sheets capable of plating.
[39]
[40]
The hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention may include an Al enriched layer continuously distributed at a depth within 100 nm from the surface of the galvanized layer in the galvanized layer. That is, the Al-rich layer of the present invention may be continuously distributed along a direction parallel to the surface of the plated steel plate at a certain depth from the surface of the plated steel plate. In addition, the thickness of the Al-rich layer of the present invention may be 50 nm or less (excluding 0 nm). That is, the Al enriched layer of the present invention is formed at a certain depth from the surface of the plated steel sheet, is formed to a certain thickness, and can be continuously distributed along a direction parallel to the surface of the plated steel sheet.
[41]
[42]
The Al enriched layer of the present invention may be an aluminum oxide (Al 2 O 3 ) layer. Although the content of Al in the galvanized layer of the present invention is relatively small compared to the content of Zn, a layer in which aluminum oxide (Al 2 O 3 ) is concentrated may be continuously formed in the surface portion of the galvanized layer . This is because Al is an element having a high oxygen affinity compared to Zn, and when the plating layer is formed, Al in the plating layer moves to the surface layer of the plating layer and combines with oxygen to form an oxide. That is, in the surface layer portion of the galvanized layer of the present invention, the aluminum oxide layer may be formed before the zinc oxide, and as the continuous aluminum oxide layer is formed, zinc oxidation on the surface of the aluminum oxide layer may be suppressed.
[43]
[44]
The holding steel sheet of the molten galvanized steel sheet according to an aspect of the present invention may have a surface roughness of 0.7 to 2.5 μm based on a centerline average roughness (Ra). That is, the holding steel sheet of the present invention has a flat surface having a surface roughness of 0.7 to 2.5 μm based on the centerline average roughness (Ra), so that the variation in oxidation degree of the surface of the holding steel sheet can be minimized. It is possible to form an aluminum oxide layer distributed in a continuous form.
[45]
[46]
1 to 3 are results of analyzing the surface layer portion of a hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention using FIB-TEM. 4 to 6 are results of analyzing the surface layer portion of a hot-rolled hot-dip galvanized steel sheet having a discontinuous aluminum oxide layer using FIB-TEM.
[47]
[48]
1 to 3 are results of analysis using FIB-TEM for the same cross section of the hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention, respectively, showing distributions of Zn, Al, and O. As shown in FIGS. 2 and 3, Al and O are continuously distributed on the surface side of the galvanized layer, and it can be seen that an aluminum oxide-type concentrated layer is continuously formed on the surface side of the galvanized layer. In addition, as shown in FIG. 1, since Zn is located under the aluminum oxide, it can be seen that almost no Zn exists on the surface of the aluminum oxide layer. That is, it can be seen that a Zn-deficient layer is formed on the surface of the aluminum oxide layer, and thus zinc oxide hardly exists on the surface side of the galvanized layer.
[49]
[50]
On the other hand, as shown in FIGS. 4 to 6, in the case of the zinc plating layer in which the discontinuous aluminum oxide layer is formed, Al and O are intermittently distributed on the surface side of the plating layer, so that the aluminum oxide layer is intermittently formed. . That is, it can be seen that the reaction between Zn and O occurs through the point where the aluminum oxide layer is interrupted, and thus, non-uniform zinc oxide is exposed on the surface of the plating layer. Therefore, if the aluminum oxide layer intermittently exists on the surface side of the plating layer, cracks of the zinc oxide exposed to the outside of the plating layer are caused in the air wiping operation after immersion in the plating bath, and the molten zinc inside the plating layer leaks out of the plating layer. Can cause flow pattern defects.
[51]
[52]
In addition, the galvanized layer of the present invention may contain, by weight %, Al: 0.2 to 0.6%, the remaining Zn and other unavoidable impurities. Components of the galvanized layer of the present invention are affected by the components of the hot-dip galvanizing bath to be described later, and the description of the composition content of the galvanized layer of the present invention will be replaced with a description of the components of the hot-dip galvanizing bath to be described later. However, in general hot dip galvanizing, the Al content in the plating layer is slightly higher than the Al content in the plating bath, and the Al content in the galvanized layer of the present invention is Al in the hot-dip galvanizing bath. It may be contained at a somewhat higher level than the content. That is, the upper limit of 0.6% of the Al content contained in the galvanized layer of the present invention is a content range in consideration of the above.
[53]
[54]
The hot-rolled hot-dip galvanized steel sheet according to one aspect of the present invention has a continuous aluminum oxide (Al 2 O 3 ) layer on the surface of the galvanized layer, effectively preventing the formation of uneven zinc oxide on the surface of the galvanized layer. In this way, flow pattern defects formed on the surface of the hot-dip galvanized steel sheet can be effectively suppressed.
[55]
[56]
Hereinafter, the manufacturing method of the present invention will be described in more detail.
[57]
[58]
Hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention, by stretching a holding steel sheet at a first elongation to generate a crack in the surface scale of the holding steel sheet; Mechanically removing scale from the surface of the holding steel plate by applying a physical impact to the surface of the holding steel plate; Chemically pickling the surface of the holding steel sheet by reacting the surface of the holding steel sheet with an acidic solution; Temper rolling the holding steel sheet at a second elongation to planarize the surface of the holding steel sheet; By weight %, Al: 0.2 to 0.4%, it can be prepared by immersing the base steel sheet in a hot-dip galvanizing bath containing the remaining Zn and other inevitable impurities to form a galvanized layer.
[59]
[60]
In a method of manufacturing a hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention, the base steel sheet is stretched at a first elongation in a scale breaker to remove scale from the surface of the base steel sheet, and by shot blasting. Scale remaining on the surface of the holding steel plate is mechanically removed, and the scale remaining on the surface of the holding steel plate can be pickled by immersing the shot blasted holding steel plate in an acidic solution. Accordingly, hot-rolled scale generated in the manufacturing process of the hot-rolled steel sheet can be effectively removed, and thus, the cleanliness of the surface of the holding steel sheet can be effectively secured.
[61]
[62]
In a method of manufacturing a hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention, the base steel sheet from which scale is removed is tempered to provide roughness to flatten the surface of the base steel sheet, and then the base steel sheet is immersed in a hot-dip galvanizing bath. A galvanized layer can be formed. That is, since the surface of the holding steel sheet is secured to a certain level or higher by temper rolling, it is possible to effectively reduce the variation in oxidation degree on the surface of the holding steel sheet, thereby forming an Al enriched layer that is continuously distributed. have.
[63]
[64]
Hereinafter, each of the process conditions constituting the manufacturing method of the present invention will be described in more detail.
[65]
[66]
Scale breaking
[67]
Scale breaking can be performed as a pre-process for removing hot-rolled oxides formed on the surface of the hot-rolled steel sheet, which is the holding steel sheet. In scale breaking, since the holding steel sheet is stretched at the first elongation, cracks may occur in the hot-rolled oxide formed on the surface of the holding steel sheet. Accordingly, cracks are caused in the hot-rolled oxide on the surface of the steel sheet through scale breaking, so that the efficiency of removing the scale in the subsequent mechanical scale removal and chemical pickling can be effectively improved.
[68]
[69]
In order to sufficiently obtain the effects of mechanical scale removal after scale breaking and residual scale removal in chemical pickling, the elongation in scale breaking must be at least a certain level. Even by mechanical scale removal and chemical pickling, when a certain amount of remaining scale or more is present, peeling of the unplated or plated layer may occur, and the first elongation of the present invention may be 0.2% or more. On the other hand, when the first elongation exceeds a certain level, the material of the holding steel sheet is hardened, and since the flattening effect cannot be sufficiently obtained even by temper rolling, the first elongation of the present invention may be limited to 1.5% or less.
[70]
[71]
Mechanical scale removal
[72]
Shot blasting treatment can be performed on the base steel sheet after scale breaking has been completed. Shot blasting can be carried out by projecting a fine shot ball onto the surface of the holding steel plate. The impact of the projected shotball accelerates the growth of cracks formed on the scale of the holding steel plate, and accordingly, the scale remaining on the surface of the holding steel plate may fall off from the surface of the holding steel plate.
[73]
[74]
The diameter of the shot ball used for shot blasting of the present invention may be 0.18 to 0.6 mm. If the diameter of the short ball is less than 0.18mm, the amount of impact applied to the holding steel sheet is insufficient, and the scale removal efficiency decreases.If the diameter of the short ball exceeds 0.60mm, not only the amount of impact required to remove the scale is exceeded, but also the impact part of the steel sheet This is because local irregularities can intensify.
[75]
[76]
In the shot blasting of the present invention, the average projection amount of the shot ball may be 800 to 1800 kg/min. When the average projection amount of the short ball is insufficient, the effect of removing residual scale cannot be expected as the probability of collision with the steel plate is lowered, so the average projection amount of the short ball may be 800 kg/min or more. On the other hand, when the average projection amount of the short ball is excessive, an excessive cost may be required compared to the increase in the efficiency of scale removal, and the average projection amount of the short ball may be less than 1800 kg/min.
[77]
[78]
In the shot blasting of the present invention, the average throwing speed of the shot ball may be 65 to 90 m/s. When the average throwing speed of the short ball is less than a certain level, the kinetic energy of the individual projectiles is reduced, so that the amount of impact transmitted to the holding steel plate does not reach a certain level, the average throwing speed of the short ball may be 65 m/s or more. However, if the average throwing speed of the short ball is excessive, an amount of impact that is more than necessary may be transmitted to the holding steel plate and the surface irregularities may be intensified, and the average throwing speed of the short ball may be 90 m/s or less.
[79]
[80]
Chemical pickling
[81]
The surface of the base steel sheet after the shot blasting treatment is completed can be chemically pickled by reacting with an acidic solution. The pickling efficiency is mainly influenced by factors such as concentration, temperature and reaction time of the pickling solution, and chemical pickling efficiency can be optimally managed by appropriately controlling these factors. In general, hydrochloric acid or sulfuric acid may be used as the pickling solution, but hydrochloric acid has the advantages of strong erosion compared to sulfuric acid, excellent surface scale removal ability, and small degree of hydrogen embrittlement. In the chemical pickling of the present invention, hydrochloric acid Solutions can be used.
[82]
[83]
The hydrochloric acid solution used in the chemical washing of the present invention may contain hydrochloric acid in a concentration of 5% or more in consideration of pickling efficiency. On the other hand, when the concentration of hydrochloric acid is excessively high, the reaction is stopped due to the concentration of iron chloride (FeCl 2 ) reaching a supersaturated state. The hydrochloric acid solution used in the chemical pickling of the present invention may contain hydrochloric acid having a concentration of 20% or less.
[84]
[85]
The chemical pickling of the present invention may be carried out in a temperature range of 70° C. or higher to secure pickling ability. On the other hand, when the temperature of chemical pickling is excessively high, the degree of improvement of the pickling ability is insufficient, but excessive corrosion of the base steel sheet may be caused by overpickling, and the evaporation amount of the acidic solution increases rapidly, which is not desirable in terms of economy. Bar, the chemical pickling of the present invention can be carried out at a temperature of 85 ℃ or less.
[86]
[87]
The chemical pickling of the present invention may be carried out for 15 seconds or more to provide a sufficient time to remove scale remaining on the surface of the holding steel sheet. However, when chemical pickling is performed excessively for a long time, excessive corrosion of the holding steel sheet is caused by over-pickling, and since it takes a relatively long time to remove chlorine ions in a subsequent process, it is undesirable in terms of efficiency. Chemical pickling can be carried out in less than 35 seconds.
[88]
[89]
The method of manufacturing a hot-rolled hot-dip galvanized steel sheet according to an aspect of the present invention is carried out by mixing mechanical scale removal by shot blasting and chemical pickling with a hydrochloric acid solution, which removes scale in a short time compared to the case of chemical pickling alone. And the amount of acidic solution used can be effectively reduced. In addition, the method of manufacturing a hot-rolled hot-dip galvanized steel sheet having an excellent surface appearance according to an aspect of the present invention is a combination of removing mechanical scale by shot blasting and chemical pickling with a hydrochloric acid solution, so that it remains on the surface of the base steel sheet. By effectively removing the scale, it is possible to effectively secure the cleanliness of the surface of the holding steel sheet.
[90]
[91]
Temper rolling
[92]
After mechanical scale removal and chemical pickling, temper rolling can be performed by pressing the surface of the steel sheet with Brightrol. It can have an average roughness of 0.1 ~ 0.8㎛ based on the average roughness (Ra) of the center line of the surface of Brightrol, and the holding steel sheet can be stretched at a second elongation by pressing of Brightrol.
[93]
[94]
The lower limit of the second elongation for securing the flatness of the surface of the holding steel sheet may be 0.4% or more. However, if the second elongation is excessive, the effect of flattening the roughness of the surface of the holding steel sheet is saturated, while the shape deformation of the holding steel sheet and hardening of the material due to excessive elongation are problematic.The upper limit of the second elongation will be limited to 2.5%. I can.
[95]
[96]
In the above-described scale breaking, the holding steel sheet is stretched at the first elongation to cause cracks in the surface scale of the holding steel sheet, whereas in temper rolling, the holding steel sheet is stretched at the second elongation to secure the surface flatness of the holding steel sheet. . Accordingly, in order to achieve an effective roughness flattening effect, the base steel sheet can be stretched by applying a second elongation greater than the first elongation of scale breaking during temper rolling. This is because when the first elongation is greater than the second elongation, it is difficult to secure sufficient surface roughness by temper rolling due to the hardening of the material generated by scale breaking.
[97]
[98]
In addition, the sum of the first and second elongation may be 0.7% or more in order to achieve surface cleanliness and roughness flattening of the holding steel sheet. However, if the sum of the first elongation and the second elongation exceeds a certain level, excessive rolling load may be caused in the temper mill, which shortens the roll service life, and the material may be deformed due to excessive reduction. And the sum of the second elongation may be 4% or less.
[99]
[100]
The roughness of the surface of the base steel sheet after temper rolling may be at a level of 0.7 to 2.5 μm based on the average roughness of the center line (Ra). In the initial stage of immersion in the hot-dip galvanizing bath, Al, which has high reactivity, first reacts with Fe in the holding steel sheet to form an Fe-Al alloy phase, thereby inhibiting the growth of hard Fe-Zn intermetallic compounds. Therefore, when the effective reaction surface area of ​​the holding steel sheet is increased, the growth of the Fe-Zn intermetallic compound is suppressed, contributing to the improvement of the mechanical properties of the plating layer, and thus plating peeling can be effectively prevented. Therefore, in order to achieve this effect, temper rolling may be performed such that the surface roughness of the holding steel sheet satisfies 0.7 μm or more based on the centerline average roughness (Ra). On the other hand, when the roughness variation of the holding steel sheet is excessive, the oxidation of Al is concentrated in a region where the roughness variation is large, and accordingly, aluminum oxide is formed locally, thereby forming an intermittent aluminum oxide layer. Therefore, in order to secure an aluminum oxide layer continuously distributed on the surface of the plating layer, temper rolling may be performed so that the surface roughness of the holding steel sheet satisfies 2.5 μm or less based on the centerline average roughness (Ra).
[101]
[102]
Dipping in hot-dip zinc plating bath
[103]
The base steel sheet on which the temper rolling is completed may be immersed in a zinc-based plating bath, thereby forming a zinc plating layer. The hot-dip galvanizing bath of the present invention may contain, by weight %, Al: 0.2 to 0.4%, the remaining Zn and other inevitable impurities. Al imparts fluidity to the plating bath and is an element contributing to the improvement of the bonding strength between the galvanized layer and the base steel sheet. The hot-dip galvanizing bath of the present invention may contain 0.2% or more of Al. However, when the Al content is excessively added, the effect of improving the fluidity of the plating bath is saturated, whereas the generation of dross increases due to the acceleration of Fe erosion. have. The preferred Al content of the hot-dip galvanizing bath may be 0.2 to 0.24%.
[104]
[105]
The holding steel sheet of the present invention may be immersed in a hot-dip galvanizing bath at an immersion temperature of 450 to 500°C. If the temperature of the base steel sheet is lower than the temperature of the hot-dip galvanizing bath, the fluidity of the hot-dip galvanizing bath is lowered and the possibility of flow pattern defects is increased. It is preferred to be immersed in the bath. Accordingly, the holding steel sheet of the present invention can be immersed in a hot-dip galvanizing bath at an immersion temperature of 450°C or higher. In addition, when the immersion temperature of the base steel sheet is higher than the temperature of the hot-dip galvanizing bath, Fe elution is accelerated to increase dross generation, and may cause surface defects such as dross stamping on the surface of the plating layer. Accordingly, the holding steel sheet of the present invention can be immersed in a hot dip galvanizing bath at an immersion temperature of 500° C. or less, and accordingly, a galvanized layer can be formed on the holding steel sheet.
[106]
[107]
Accordingly, the hot-rolled hot-dip galvanized steel sheet of the present invention manufactured through the above manufacturing method effectively suppresses the occurrence of flow pattern defects on the surface of the galvanized layer, and thus can have a beautiful surface appearance.
[108]
[109]
Further, in the above manufacturing method, mechanical scale removal and chemical pickling are sequentially performed on the holding steel sheet, so that residual scale on the surface of the holding steel sheet can be effectively removed while minimizing the use of a chemical solution used for chemical pickling.
[110]
Mode for carrying out the invention
[111]
Hereinafter, the present invention will be described in more detail through examples. The following examples are intended to describe preferred examples of the present invention in more detail, and it should be noted that the scope of the present invention is not necessarily limited by the following examples.
[112]
[113]
3.2mm thick JS-SPHC (specimen 1, tensile strength 350MPa) and 2.9mm thick JS-SAPH400 (specimen 2, tensile strength 400MPa) were selected as test specimens. Although the present invention is not necessarily applied to a hot-rolled steel sheet of a thick material level, a harsh environment was given to improvement of flow pattern defects by selecting a hot-rolled steel sheet having a thickness of 3 mm. The scale on the surface of the test piece was removed by applying the first elongation, mechanical scale removal, and chemical pickling shown in Table 1 below, and temper rolling of each test piece using Brightrol with a roughness (Ra) of 0.2 μm to obtain a second elongation. It was stretched. After temper rolling, the rolling oil was degreased and dried, and the surface of the test piece was galvanized. Plating was performed using Iwatani Corp., Multi Functional Process Simulator, and hot-dip galvanized specimens were prepared by setting heat treatment and plating conditions as shown in Table 2 below. When manufacturing the hot-dip galvanized specimen, the temperature conditions for each section (PHS, DFF, HRS, GJS, TDS) were all the same, but only the immersion temperature of the base steel plate was set differently.
[114]
[115]
[Table 1]
division Psalter 1st elongation (%) Short blast Pickling (hydrochloric acid) 2nd elongation (%) Sum of elongation (%) Plated
Projection amount (kg/min) Projection speed (m/s) density(%) Temperature(℃) Processing time (seconds) Plating bath Al (%) Immersion temperature (℃)
Example 1 One 0.40 1200 80 15 80 28 0.51 0.91 0.22 470
Example 2 One 0.25 1200 80 15 80 28 0.51 0.76 0.22 470
Comparative Example 1 One 0.15 1200 80 15 80 28 0.51 0.66 0.22 470
Example 3 One 0.40 1200 80 15 80 28 0.83 1.23 0.22 470
Example 4 One 0.40 1200 80 15 80 28 1.23 1.63 0.22 470
Example 5 One 0.40 1200 80 15 80 28 1.61 2.01 0.22 470
Comparative Example 2 One 0.25 1200 80 15 80 28 0.25 0.50 0.22 470
Example 6 One 0.40 950 80 15 80 28 1.23 1.63 0.22 470
Comparative Example 3 One 0.40 750 80 15 80 28 1.23 1.63 0.22 470
Example 7 One 0.40 1550 80 15 80 28 1.23 1.63 0.22 470
Comparative Example 4 One 0.40 1850 80 15 80 28 1.23 1.63 0.22 470
Example 8 One 0.40 1200 70 15 80 28 1.23 1.63 0.22 470
Comparative Example 5 One 0.40 1200 60 15 80 28 1.23 1.63 0.22 470
Example 9 One 0.40 1200 85 15 80 28 1.23 1.63 0.22 470
Comparative Example 6 One 0.40 1200 95 15 80 28 1.23 1.63 0.22 470
Example 10 One 0.40 1200 80 15 80 18 1.23 1.63 0.22 470
Comparative Example 7 One 0.40 1200 80 15 80 13 1.23 1.63 0.22 470
Comparative Example 8 One 0.40 1200 80 15 80 28 1.23 1.63 0.22 440
Comparative Example 9 One 2.00 1200 80 15 80 28 0.51 2.51 0.22 470
Comparative Example 10 One 2.00 1200 80 15 80 28 1.23 3.23 0.22 470
Example 11 2 0.60 1200 80 15 80 28 0.76 1.36 0.22 470
Example 12 2 0.35 1200 80 15 80 28 0.76 1.11 0.22 470
Comparative Example 11 2 0.15 1200 80 15 80 28 0.76 0.91 0.22 470
Example 13 2 0.60 1200 80 15 80 28 1.08 1.68 0.22 470
Example 14 2 0.60 1200 80 15 80 28 1.70 2.30 0.22 470
Example 15 2 0.60 1200 80 15 80 28 2.45 3.05 0.22 470
Comparative Example 12 2 0.60 1200 80 15 80 28 2.76 3.36 0.22 470
Comparative Example 13 2 0.60 1200 80 15 80 28 0.27 0.87 0.22 470
Comparative Example 14 2 2.20 1200 80 15 80 28 0.76 2.96 0.22 470
Comparative Example 15 2 2.20 1200 80 15 80 28 1.70 3.90 0.22 470
[116]
[117]
[Table 2]
Starting temperature (℃) Preheating zone (℃) Direct furnace (℃) Reduction zone (℃) Gas jet cooling zone (℃) Turndown section (℃) Plating bath temperature (℃)
20 265 592 630 506 465 460
[118]
[119]
After plating, air wiping was performed under the same conditions, and the results of observing the surface and plating layer of each hot dip galvanized specimen are shown in Table 3 below. For each specimen, the surface quality, the distribution range of the Al-rich layer, the continuous distribution, and the maximum thickness were measured, respectively. The surface quality was evaluated by visually observing each specimen, and specifically, it means ○ (excellent, flow pattern defect or non-plating), X (poor, flow pattern defect or non-plating). The distribution range, continuous distribution, and maximum thickness of the Al enriched layer were analyzed by TEM (transmission electron microscope) after FIB (ion beam accelerator) processing. In addition, before plating of each specimen, the residual scale of each steel plate was evaluated, and the results are also shown in Table 3. To evaluate the residual scale of the holding steel sheet, the fraction of the scale area on the image was calculated with an image analyzer after checking the SEM image at 200 times magnification in the back-scattering mode.
[120]
[121]
[Table 3]
division Surface quality Surface roughness of pre-plated steel sheet Remaining scale of plated steel sheet (%) Al enriched layer distribution range (nm) Al enriched layer continuity Maximum Al thickened layer thickness (nm)
Example 1 ○ 2.05 Less than 1% Less than 60 continuity 30
Example 2 ○ 2.15 2.5% Less than 80 continuity 40
Comparative Example 1 X 1,95 4% More than 100 discontinuity 400
Example 3 ○ 1.65 Less than 1% Less than 50 continuity 10
Example 4 ○ 1.15 Less than 1% Less than 40 continuity 10
Example 5 ○ 0.8 Less than 1% Less than 40 continuity 10
Comparative Example 2 X 3.10 Less than 1% More than 100 discontinuity 60
Example 6 ○ 1.20 3% Less than 60 continuity 10
Comparative Example 3 X 1.65 18% More than 100 discontinuity 500
Example 7 ○ 1.55 Less than 1% Less than 80 continuity 30
Comparative Example 4 X 2.35 Less than 1% More than 100 discontinuity 60
Example 8 ○ 1.30 Less than 1% Less than 60 continuity 30
Comparative Example 5 X 1.50 3% More than 100 discontinuity 100
Example 9 ○ 1.15 Less than 1% Less than 60 continuity 20
Comparative Example 6 X 1.75 2.5% More than 100 discontinuity 100
Example 10 ○ 1.25 2% Less than 50 continuity 40
Comparative Example 7 X 1.65 5% More than 100 discontinuity 200
Comparative Example 8 X 1.65 Less than 1% More than 100 discontinuity 80
Comparative Example 9 X 3.65 Less than 1% More than 100 discontinuity 500
Comparative Example 10 X 2.85 Less than 1% More than 100 discontinuity 500
Example 11 ○ 2.40 Less than 1% Less than 60 continuity 30
Example 12 ○ 2.25 3% Less than 60 continuity 40
Comparative Example 11 X 2.55 12% More than 100 discontinuity 500
Example 13 ○ 1.90 Less than 1% Less than 50 continuity 10
Example 14 ○ 1.25 Less than 1% Less than 40 continuity 10
Example 15 ○ 0.75 Less than 1% Less than 35 continuity 10
Comparative Example 12 X 0.65 Less than 1% Less than 60 continuity 30
Comparative Example 13 X 3.60 Less than 1% More than 100 discontinuity 100
Comparative Example 14 X 4.15 Less than 1% More than 100 discontinuity 700
Comparative Example 15 X 3.25 Less than 1% More than 100 discontinuity 700
[122]
[123]
In Examples 1 to 15 satisfying the conditions of the present invention, it can be seen that a continuous Al enriched layer was formed within 100 nm from the surface of the galvanized layer, and the maximum thickness of the Al enriched layer did not exceed 50 nm. In addition, in the case of Examples 1 to 15, it can be seen that the residual scale of the holding steel sheet before plating secures excellent surface cleanliness at a level of 3% or less, and accordingly, a continuous Al enriched layer is formed. Therefore, in the case of Examples 1 to 15, it can be confirmed that a flow pattern defect or a surface defect such as non-plating did not occur, and an excellent surface appearance was provided.
[124]
[125]
On the other hand, in the case of Comparative Examples 1 to 15 that do not satisfy the conditions of the present invention, it can be seen that the Al enriched layer is intermittently formed, and the maximum thickness of the Al enriched layer exceeds 50 nm. That is, it can be seen that the non-uniform and discontinuous Al enriched layer was formed, so that the zinc oxide was non-uniformly formed on the surface of the plating layer, and accordingly, flow pattern defects or surface defects of non-plating occurred.
[126]
[127]
Although the present invention has been described in detail through examples above, other types of examples are also possible. Therefore, the technical spirit and scope of the claims described below are not limited to the embodiments.
Claims
[Claim 1]
A hot-rolled hot-dip galvanized steel sheet having excellent surface appearance, comprising a holding steel sheet and a galvanized layer, wherein the galvanized layer comprises a continuous Al enriched layer formed within 100 nm from the surface of the galvanized layer.
[Claim 2]
The hot-rolled hot-dip galvanized steel sheet according to claim 1, wherein the Al enriched layer is an aluminum oxide (Al 2 O 3 ) layer.
[Claim 3]
The hot-rolled hot-dip galvanized steel sheet according to claim 1, wherein the Al-rich layer has a thickness of 50 nm or less (excluding 0 nm).
[Claim 4]
The hot-rolled hot-dip galvanized steel sheet according to claim 1, wherein the surface roughness of the base steel sheet forming the interface with the galvanized layer is 0.7 to 2.5 µm based on a center line average roughness (Ra).
[Claim 5]
The hot-rolled hot-dip galvanized steel sheet according to claim 1, wherein the galvanized layer contains 0.2 to 0.6% of Al, and the remaining Zn and other unavoidable impurities in weight %.
[Claim 6]
Stretching the holding steel sheet at a first elongation to induce a crack in the scale of the surface of the holding steel sheet; Mechanically removing scale from the surface of the holding steel plate by applying a physical impact to the surface of the holding steel plate; Chemically pickling the surface of the holding steel plate by reacting the surface of the holding steel plate with an acidic solution; Temper rolling the holding steel sheet at a second elongation to planarize the surface of the holding steel sheet; A method of manufacturing a hot-rolled hot-dip galvanized steel sheet having excellent surface appearance by immersing the base steel sheet in a hot-dip galvanizing bath containing 0.2 to 0.4% of Al: 0.2 to 0.4% of the remaining Zn and other unavoidable impurities to form a galvanized layer.
[Claim 7]
The method according to claim 6, wherein the first elongation is 0.2 to 1.5%, and the surface appearance is excellent.
[Claim 8]
The method for manufacturing a hot-rolled hot-dip galvanized steel sheet having excellent surface appearance according to claim 6, wherein the surface of the base steel sheet is subjected to shot blasting to mechanically remove scale from the surface of the base steel sheet.
[Claim 9]
The hot-rolled hot-dip galvanizing method of claim 6, wherein the shot blasting projects a shot ball having an average diameter of 0.18 to 0.6 mm at an average projection amount of 800 to 1800 kg/min and an average projection speed of 65 to 90 m/s. Method of manufacturing a steel plate.
[Claim 10]
The method of claim 6, wherein the holding steel sheet is chemically pickled by immersing the holding steel sheet for 15 to 35 hours in a 5-20% concentration of hydrochloric acid aqueous solution provided in a temperature range of 70 to 85°C. Manufacturing method of hot-rolled hot-dip galvanized steel sheet.
[Claim 11]
The method for manufacturing a hot-rolled hot-dip galvanized steel sheet having excellent surface appearance according to claim 6, wherein the base steel sheet is temper-rolled by Brightrol having a surface roughness of 0.1 to 0.8 µm based on a centerline average roughness (Ra).
[Claim 12]
The method of claim 6, wherein the temper-rolled base steel sheet has a surface roughness of 0.7 to 2.5 µm based on a centerline average roughness (Ra).
[Claim 13]
The method according to claim 6, wherein the second elongation is 0.5 to 2.5%, and the method of manufacturing a hot-rolled hot-dip galvanized steel sheet having excellent surface appearance.
[Claim 14]
The method of claim 6, wherein the sum of the first elongation and the second elongation is 0.7 to 4.0%, and has excellent surface appearance.
[Claim 15]
The method of claim 6, wherein the second elongation is greater than the first elongation, and has excellent surface appearance.
[Claim 16]
The method of claim 6, wherein the immersion temperature of the hot-dip galvanizing bath of the holding steel sheet is 450 to 500°C.