(19)
(11)EP 3 733 919 A1

(12)EUROPEAN PATENT APPLICATION
published in accordance with Art. 153(4) EPC

(43)Date of publication:
04.11.2020 Bulletin 2020/45

(21)Application number: 18894000.1

(22)Date of filing:  06.12.2018
(51)International Patent Classification (IPC): 
C23C 2/06(2006.01)
C23G 1/08(2006.01)
B21B 45/06(2006.01)
B24C 1/08(2006.01)
C23C 2/02(2006.01)
C22C 18/04(2006.01)
B21B 1/26(2006.01)
(86)International application number:
PCT/KR2018/015366
(87)International publication number:
WO 2019/132293 (04.07.2019 Gazette  2019/27)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
KH MA MD TN

(30)Priority: 26.12.2017 KR 20170180298

(71)Applicant: POSCO
Gyeongsangbuk-do 37859 (KR)

(72)Inventors:
  • KWON, Moon-Jae
    Pohang-si, Gyeongsangbuk-do 37877 (KR)
  • KIM, Su-Young
    Pohang-si, Gyeongsangbuk-do 37877 (KR)
  • CHOI, Won-Suk
    Pohang-si, Gyeongsangbuk-do 37877 (KR)

(74)Representative: Zech, Stefan Markus 
Meissner Bolte Patentanwälte Rechtsanwälte Partnerschaft mbB Postfach 86 06 24
81633 München
81633 München (DE)

  


(54)HOT DIP GALVANIZED STEEL SHEET HAVING EXCELLENT SURFACE APPEARANCE, AND MANUFACTURING METHOD THEREFOR


(57) A hot dip galvanized steel sheet having an excellent surface appearance, according to one aspect of the present invention, comprises a base steel sheet and a zinc plating layer, wherein the zinc plating layer includes a continuous Al-rich layer formed within 100 nm from a surface of the zinc plating layer.




Description

[Technical Field]



[0001] The present invention relates to a hot-dip galvanized hot-rolled steel sheet and a method of manufacturing the same, and specifically, a hot-dip galvanized hot-rolled steel sheet which may have excellent surface appearance by effectively preventing a surface defect of a galvanized layer, and a method of manufacturing the same.

[Background Art]



[0002] A hot-dip galvanized steel sheet may refer to a steel sheet obtained by forming a galvanized layer on a surface of a base steel sheet by hot-dip galvanizing, and a hot-dip galvanized hot-rolled steel sheet may refer to a steel sheet obtained by forming a galvanized layer on a surface of a hot-rolled steel sheet by hot-dip galvanizing. Generally, a hot-dip galvanized hot-rolled steel sheet may be manufactured in a series of processes including scale breaking, pickling, a heat treatment, submersion in a galvanizing bath, an air-wiping treatment, and so on.

[0003] A pickling process may be performed to remove oxidized scale formed in a hot-rolling process, and a chemical pickling process for removing oxidized scale on a surface of a hot-rolled steel sheet using an acid solution may generally be used. In the case of such a chemical pickling process, as a strongly acid solution such as sulfuric acid or hydrochloric acid is generally used, such an acid solution may be detrimental to the environment, and may greatly corrode a base material according to an increase of a reaction time. Thus, there has been demand for a scale removing technique which may effectively remove scale on a surface of a hot-rolled steel sheet and may also reduce the problem of environmental pollution.

[0004] Also, an air-wiping process is an essential process for adjusting an amount of coating on a surface of a galvanized steel sheet by spraying highly pressured fluid after submersion in a galvanizing bath, but as the highly pressured fluid is sprayed towards a galvanized layer while a galvanized layer has not been completely solidified, a surface defect such as a flow pattern defect may occur. As an oxide film having relatively low fluidity may be formed on a surface layer of a galvanized layer, and molten zinc having relatively high fluidity may be present in a galvanized layer, there may be a difference in fluidities in a thickness direction of the galvanized layer. Highly pressured fluid sprayed from an air-knife device may reach a surface layer of the galvanized layer and may form cracks on an oxide film, and molten zinc in the galvanized layer may be externally exposed through cracks formed on the oxide film. The galvanized layer may be rapidly solidified directly after passing through the air-knife device, and accordingly, a flow pattern defect in which valleys and peaks are continuously exhibited may occur on the surface layer of the galvanized layer.

[0005] To prevent the flow pattern defect, a technique of preventing the formation of an oxide film by introducing a sealing box for making a non-oxidative atmosphere has been suggested in the prior art. The formation of an oxide film on the surface layer of a galvanized layer may be prevented to some extent by introducing a sealing box. However, a facility structure may become highly complex, and when there is a gap between a galvanizing bath and the sealing box to easily emit vaporized zinc, a large amount of dross may be formed on a surface of the galvanizing bath due to reaction between external air and the galvanizing bath such that surface quality of a galvanized steel sheet may rather degrade, which may be a problem.

[0006] Reference 1 suggests a technique of mechanically removing a flow pattern defect by skin-pass rolling after forming a galvanized layer, rather than preventing the formation of a flow pattern defect. However, as a steel sheet should be pressured by a relatively high reduction force to remove a flow pattern, a risk of damage to a normal galvanized layer and aging peeling of the galvanized layer may be high. Thus, it may be highly necessary to introduce a technique which may prevent degradation of surface quality of a galvanized steel sheet by effectively preventing a flow pattern defect on a surface of a galvanized layer.

(Reference 1) Korean Laid-Open Patent Publication No. 10-2001-0060423 (Publicized on July 7, 2001)


[Disclosure]


[Technical Problem]



[0007] According to an aspect of the present invention, a hot-dip galvanized hot-rolled steel sheet having excellent surface appearance and a method of manufacturing the same may be provided.

[0008] A purpose of the present invention is not limited to the above-mentioned matter. There may be no difficulty for a person skilled in the art to understand an additional purpose of the present invention based on the overall descriptions of the present specification.

[Technical Solution]



[0009] A hot-dip galvanized hot-rolled steel sheet according to an aspect of the present invention may include a base steel sheet and a galvanized layer, and the galvanized layer includes a continuous Al thickened layer formed within 100nm from a surface of the galvanized layer.

[0010] The Al thickened layer may be an aluminum oxide (Al2O3) layer.

[0011] A thickness of the Al thickened layer may be 50nm or less (excluding 0nm).

[0012] Surface roughness of a surface of the base steel sheet forming an interfacial surface with the galvanized layer may be 0.7-2.5 µm with reference to center line average roughness (Ra).

[0013] The galvanized layer may include, by weight%, 0.2-0.6% of Al and a balance of Zn and other impurities.

[0014] A hot-dip galvanized hot-rolled steel sheet according to an aspect of the present invention may be manufactured by creating cracks in scale on a surface of a base steel sheet by elongating the base steel sheet at a first elongation rate; mechanically removing scale from the surface of the base steel sheet by providing physical impacts to the surface of the base steel sheet; chemically pickling the surface of the base steel sheet by allowing the surface of the base steel sheet to react with an acid solution; flattening the surface of the base steel sheet by skin pass rolling the base steel sheet at a second elongation rate; and forming a galvanized layer by submerging the base steel sheet in a hot-dip galvanizing bath including, by weight%, 0.2-0.4% of Al and a balance of Zn and other impurities.

[0015] The first elongation rate may be 0.2-1.5%.

[0016] Scale on the surface of the base steel sheet may be mechanically removed by performing a shot-blasting treatment on the surface of the base steel sheet.

[0017] The shot-blasting may include projecting a shot ball having an average diameter of 0.18-0.6mm by an average projection amount of 800-1800kg/min and at an average projection velocity of 65-90m/s.

[0018] The surface of the base steel sheet may be chemically pickled by submerging the base steel sheet in hydrochloric acid aqueous solution of a concentration of 5-20% provided within a temperature range of 70-85°C for 15-35 seconds.

[0019] The base steel sheet may be skin-pass rolled by a bright roll having surface roughness of 0.1-0.8 µm with reference to center line average roughness (Ra).

[0020] Surface roughness of the skin-pass rolled base steel sheet may be 0.7-2.5 µm with reference to center line average roughness (Ra).

[0021] The second elongation rate may be 0.5-2.5%.

[0022] A sum of the first elongation rate and the second elongation rate may be 0.7-4.0%.

[0023] The second elongation rate may be greater than the first elongation rate.

[0024] A submersion temperature of the hot-dip galvanizing bath of the base steel sheet may be 450-500°C.

[Advantageous Effects]



[0025] According to an aspect of the present invention, a flow pattern defect of a surface of a galvanized layer may be effectively prevented such that a hot dip galvanized hot-rolled steel sheet having a beautified surface appearance and a method of manufacturing the same may be provided.

[0026] According an aspect of the present invention, as mechanical scale removal and chemical pickling may be performed in sequence on a base steel sheet, a method of manufacturing a hot dip galvanized hot-rolled steel sheet, which may significantly reduce the use of chemical solution used in chemical pickling and may effectively remove remaining scale on a surface of the base steel sheet may be provided.

[Description of Drawings]



[0027] 

FIGS. 1 to 3 are results of analyzing a surface layer of a hot dip galvanized hot-rolled steel sheet using a FIB-TEM according to an aspect of the present invention;

FIGS. 4 to 6 are results of analyzing a surface layer of a hot dip galvanized hot-rolled steel sheet having a discontinuous aluminum oxide layer using a FIB-TEM.


[Best Mode for Invention]



[0028] The present invention relates to a hot dip galvanized hot-rolled steel sheet having an excellent surface appearance and a method of manufacturing the same, and in the description below, preferable embodiments of the present invention will be described. The embodiments of the present invention may be modified to various forms, and it should be understood that the scope of the present invention is not limited to the embodiments described below. The embodiments are provided to describe the present invention in greater detail to a person having ordinary skill in the art to which the present invention belongs to.

[0029] The hot-dip galvanized hot-rolled steel sheet according to one aspect of the present invention may include a base steel sheet, and a galvanized layer formed on a surface of the base steel sheet. The base steel sheet of the present invention may be a hot-rolled steel sheet, but an example embodiment thereof is not limited thereto, and it may be understood that the base steel sheet may include all possible types of steel sheets able to be coated.

[0030] The hot-dip galvanized hot-rolled steel sheet according to an aspect of the present invention may include an Al thickened layer continuously distributed at a depth of within 100nm from a surface of a galvanized layer in the galvanized layer. In other words, the Al thickened layer of the present invention may be continuously distributed in a direction parallel to a surface of the galvanized steel sheet at a certain depth from the surface of the galvanized steel sheet. Also, a thickness of the Al thickened layer of the present invention may be 50nm or less (excluding 0nm). In other words, the Al thickened layer of the present invention may be formed at a certain depth from the surface of the galvanized steel sheet, and may have a certain thickness such that the Al thickened layer may be continuously distributed in a direction parallel to the surface of the galvanized steel sheet.

[0031] The Al thickened layer of the present invention may be an aluminum oxide (Al2O3) layer. Although a content of Al in the galvanized layer of the present invention is relatively smaller than a content of Zn, an aluminum oxide (Al2O3) layer thickened layer may be continuously formed on a surface layer of the galvanized layer. That is because Al has oxygen affinity higher than that of Zn, and while the galvanized layer is formed, Al in the galvanized layer may move to the surface layer of the galvanized layer, may be combined with oxygen, and may form oxide. Accordingly, an aluminum oxide layer may be formed earlier than zinc oxide on the surface layer of the galvanized layer, and as the continuous aluminum oxide layer is formed, zinc oxidization on the surface of the aluminum oxide layer may be prevented.

[0032] In the base steel sheet of the hot-dip galvanized steel sheet according to one aspect of the present invention, surface roughness of the surface of the base steel sheet forming an interfacial surface may be 0.7-2.5 µm with reference to center line average roughness (Ra). Accordingly, as the base steel sheet of the present invention has a flattened surface having surface roughness of 0.7-2.5 µm with reference to center line average roughness (Ra), a deviation of oxidization degree of the surface of the base steel sheet may be significantly reduced, and accordingly, the aluminum oxide layer distributed in a continuous form may be formed while the galvanized layer is formed.

[0033] FIGS. 1 to 3 are results of analyzing a surface layer of a hot dip galvanized hot-rolled steel sheet using a FIB-TEM according to an aspect of the present invention. FIGS. 4 to 6 are results of analyzing a surface layer of a hot dip galvanized hot-rolled steel sheet having a discontinuous aluminum oxide layer using a FIB-TEM.

[0034] FIGS. 1 to 3 are results of analyzing the same cross-sectional surface of a hot dip galvanized hot-rolled steel sheet using a FIB-TEM according to an aspect of the present invention, and show distribution of Zn, Al, and O, respectively. As indicated in FIGS. 2 and 3, it has been confirmed that, as Al and O were continuously distributed on a surface side of the galvanized layer, a thickened layer in a form of an aluminum oxide was continuously formed on the surface side of the galvanized layer. Also, as indicated in FIG. 1, it has been confirmed that, as Zn is disposed below an aluminum oxide, Zn was rarely present on the surface of the aluminum oxide layer. Thus, it has been confirmed that a Zn deficient layer was formed on the surface of the aluminum oxide layer, and accordingly that zinc oxide was rarely present on the surface side of the galvanized layer.

[0035] Meanwhile, as shown in FIGS. 4 and 6, it has been confirmed that, in the case of the galvanized layer on which a discontinuous aluminum oxide layer was formed, Al and O were intermittently distributed on the surface side of the galvanized layer, the aluminum oxide layer was intermittently formed. In other words, reaction between Zn and O occurred through the part in which the aluminum oxide layer was intermittent, and accordingly, non-uniform zinc oxide was exposed on the surface of the galvanized layer. Accordingly, when the aluminum oxide layer is intermittently present on the surface side of the galvanized layer, cracks may be created on zinc oxide exposed externally of the galvanized layer in an air-wiping operation after submersion in a galvanizing bath, and molten zinc in the galvanized layer may be exposed externally of the galvanized layer and may cause a flow pattern defect.

[0036] Also, the galvanized layer of the present invention may include, by weight%, 0.2-0.6% of Al and a balance of Zn and other impurities. As a composition of the galvanized layer of the present invention is affected from a composition of a hot-dip galvanizing bath described later, the description of a composition content of the galvanized layer of the present invention will be replaced with the description of a composition of the hot-dip galvanizing bath described later. However, as a content of Al included in the galvanized layer may be relatively higher than a content of Al included in the galvanizing bath in a general hot-dip galvanizing process, a content of Al included in the galvanized layer of the present invention may be higher than a content of Al of the hot-dip galvanizing bath. Thus, an upper limit 0.6% of a content of Al included in the galvanized layer of the present invention may be a content range determined in consideration of the above-described matters.

[0037] As the hot-dip galvanized hot-rolled steel sheet according to one aspect of the present invention includes a continuous aluminum oxide (Al2O3) layer on the surface layer of the galvanized layer, non-uniform zinc oxide may be effectively prevented from being formed on the surface of the galvanized layer, and accordingly, a flow pattern defect formed on the surface of the hot-dip galvanized steel sheet may be effectively prevented.

[0038] In the description below, a manufacturing method of the present invention will be described in greater detail.

[0039] A hot-dip galvanized hot-rolled steel sheet according to one aspect of the present invention may be manufactured by creating cracks on scale of the surface of a base steel sheet by elongating the base steel sheet at a first elongation rate; mechanically removing scale from the surface of the base steel sheet by providing physical impact on the surface of the base steel sheet; chemically pickling the surface of the base steel sheet by allowing the surface of the base steel sheet to react with acid solution; flattening the surface of the base steel sheet by skin pass rolling the base steel sheet at a second elongation rate; and forming a galvanized layer by submerging the base steel sheet in a hot-dip galvanizing bath including, by weight%, 0.2-0.4% of Al and a balance of Zn and other impurities.

[0040] The method of manufacturing the hot-dip galvanized hot-rolled steel sheet according to one aspect of the present invention may include removing scale on the surface of the base steel sheet by elongating the base steel sheet at the first elongation rate in a scale breaker, mechanically removing scale remaining on the surface of the base steel sheet by shot blasting, and pickling scale remaining on the surface of the base steel sheet by submerging the base steel sheet on which the shot blasting treatment has been performed in acid solution. Accordingly, hot-rolling scale formed in a process of manufacturing the hot-rolled steel sheet may be effectively removed, and accordingly, cleanliness of the surface of the base steel sheet may be effectively secured.

[0041] The method of manufacturing the hot-dip galvanized hot-rolled steel sheet according to one aspect of the present invention may include flattening the surface of the base steel sheet by providing roughness by skin pass rolling the base steel sheet from which scale has been removed, and submerging the base steel sheet in a hot-dip galvanizing bath, thereby forming a galvanized layer. Accordingly, a certain level or higher of flatness of the surface of the base steel sheet may be secured by skin pass rolling such that deviation of oxidation on the surface of the base steel sheet may be effectively reduced, and accordingly, a continuously distributed Al thickened layer may be formed.

[0042] In the description below, each of the process conditions included in the manufacturing method of the present invention will be described in greater detail.

Scale Breaking



[0043] As a preliminary process for removing hot-rolled oxide formed on the surface of a hot-rolled steel sheet, a base steel sheet, scale breaking may be performed. As the base steel sheet may be elongated at the first elongation rate in the scale breaking, cracks may be created on the hot-rolled oxide formed on the surface of the base steel sheet. Accordingly, as cracks are created on the hot-rolled oxide on the surface of the base steel sheet through the scale breaking, efficiency of mechanical scale removal and scale removal in chemical pickling, subsequently performed, may effectively improve.

[0044] To sufficiently obtain the effect of the mechanical scale removal and remaining scale removal in the chemical pickling, performed after the scale breaking, an elongation rate in the scale breaking should be a certain level or higher. When a certain or greater amount of remaining scale is present even after performing the mechanical scale removal and the chemical pickling, non-galvanizing or peeling of the galvanized layer may occur. Thus, the first elongation rate in the present invention may be 0.2% or higher. When the first elongation rate exceeds a certain level, mechanical property of the base steel sheet may be hardened, and the effect of flattening may not be sufficiently obtained even through the skin pass rolling. Therefore, the first elongation rate in the present invention may be limited to 1.5% or lower.

Mechanical Scale Removing



[0045] A shot blasting treatment may be performed on the base steel sheet on which the scale breaking has been terminated. The shot blasting may be performed by projecting a fine shot ball on the surface of the base steel sheet. Growth of cracks formed on scale of the surface of the base steel sheet may be accelerated by collision between projected shot balls, and accordingly, scale remaining on the surface of the base steel sheet may be separated from the surface of the base steel sheet.

[0046] A diameter of a shot ball used in the shot blasting in the present invention may be 0.18-0.6mm. When a diameter of the shot ball is less than 0.18mm, the amount of impact applied to the base steel sheet may be insignificant such that scale removing efficiency may degrade. When a diameter of a shot ball exceeds 0.60mm, the amount of impact required for the scale removal may be exceeded, and local serrations in an impact zone of the steel sheet may also be intensified.

[0047] In the shot blasting in the present invention, the average amount of projection of a shot ball may be 800-1800kg/min. When the average amount of projection of a shot ball is excessively small, a possibility of collision with the steel sheet may decrease such that it may be impossible to expect the effect of removing remaining scale. Thus, the average amount of projection of a shot ball may be 800kg/min or higher. When the average amount of projection of a shot ball is excessive, excessive costs may be consumed for an increase of efficiency of scale removal, and thus, the average amount of projection of a shot ball may be 1800kg/min or less.

[0048] An average projection velocity of a shot ball in the shot blasting in the present invention may be 65-90m/s. When an average projection velocity of a shot ball is a certain level or lower, kinetic energy of an individual projectile may decrease such that an amount of impact transferred to the base steel sheet may not meet a certain level. Thus, an average projection velocity of a shot ball may be 65m/s or higher. When an average projection velocity of a shot ball is excessive, an unnecessarily high amount of impact may be transferred to the base steel sheet such that surface uneveness may increase. Thus, an average projection velocity of a shot ball may be 90m/s or lower.

Chemical Pickling



[0049] The base steel sheet on which the shot blasting treatment has been terminated may be chemically pickled by allowing the surface of the base steel sheet to react with acid solution. As pickling efficiency may be affected from factors such as a concentration, a temperature, and reaction time, and the like, of picking solution, chemical pickling efficiency may be managed to be optimal by appropriately controlling the factors mentioned above. Generally, as pickling solution, hydrochloric acid or sulfuric acid may be used. Hydrochloric acid may have erosion properties stronger than that of sulfuric acid, may have excellent surface scale removing capability, and may not greatly cause hydrogen embrittlement, which are advantages, and thus, in the chemical pickling in the present invention, hydrochloric acid solution may be used.

[0050] Hydrochloric acid solution used in chemical washing in the present invention may include hydrochloric acid in 5% or higher concentration in consideration of pickling efficiency. When a concentration of hydrochloric acid is excessively high, a concentration of iron chloride (FeCl2) may reach a supersaturated state such that reaction may be stopped, and also, there may be a problem in which pickling efficiency around a precipitation point may be constant or may rather decrease. Thus, hydrochloric acid solution used in the chemical washing in the present invention may include hydrochloric acid in 20% or lower concentration.

[0051] The chemical pickling in the present invention may be performed in a temperature range of 70°C or higher to secure picking ability. When a temperature of the chemical pickling is excessively high, improvement of picking ability may be insignificant, but the base steel sheet may be excessively corroded due to over-pickling, and the amount of vaporized acid solution may rapidly increase, which may not be economically preferable. Thus, the chemical pickling may be performed at a temperature of 85°C or lower.

[0052] The chemical pickling in the present invention may be performed for 15 seconds or longer to provide the sufficient time for removing the scale remaining on the surface of the base steel sheet. When the chemical pickling is performed for the excessively long period time, the base steel sheet may be excessively corroded due to over-pickling, and the relatively long period of time may be spent on removing chlorine ions in a subsequent process, which may not be preferable in terms of efficiency. Thus, the chemical pickling in the present invention may be performed for 35 seconds or shorter.

[0053] As for the method of manufacturing the hot-dip galvanized hot-rolled steel sheet according to an aspect of the present invention, the mechanical scale removal by the shot blasting and the chemical pickling by hydrochloric acid solution may be performed in a mixed manner, the scale may be removed for a short period of time as compared to the case in which only the chemical pickling is performed, and the use amount of acid solution may be effectively decreased. Also, as for the method of manufacturing the hot-dip galvanized hot-rolled steel sheet having an excellent surface appearance according to an aspect of the present invention, the mechanical scale removal by the shot blasting and the chemical pickling by hydrochloric acid solution may be performed in a mixed manner, the scale remaining on the surface of the base steel sheet may be effectively removed such that cleanliness of the surface of the base steel sheet may be effectively secured.

Skin Pass Rolling



[0054] The skin pass rolling may be performed by pressuring the surface of the base steel sheet using a bright roll after the mechanical scale removal and the chemical pickling. The skin-pass rolled base steel sheet may have average roughness of 0.1-0.8 µm with reference to center line average roughness (Ra) of the surface of the bright roll, and the base steel sheet may be elongated at a second elongation rate by pressure applied by the bright roll.

[0055] A lower limit of the second elongation rate to secure flatness of the surface of the base steel sheet may be 0.4% or higher. When the second elongation rate is excessively high, the effect of roughness flattening of the surface of the base steel sheet may be saturated, and there may be a problem of deformation of a shape and hardening of mechanical property of the base steel sheet due to excessive elongation. Thus, an upper limit of the second elongation rate may be limited to 2.5%.

[0056] In the scale breaking mentioned above, an elongating process may be performed on the base steel sheet at a first elongation rate to create cracks on the surface scale of the base steel sheet, whereas in the skin pass rolling, an elongating process may be performed on the base steel sheet at the second elongation rate to secure surface flatness of the base steel sheet. Thus, to effectively achieve the effect of roughness flattening, in the skin pass rolling, an elongating process may be performed on the base steel sheet by applying the second elongation rate higher than the first elongation rate of the scale breaking. When the first elongation rate is higher than the second elongation rate, it may be difficult to secure sufficient surface roughness obtained by the skin pass rolling due to the hardening of mechanical property occurring in the scale breaking.

[0057] Also, to achieve surface cleanliness and roughness flattening of the base steel sheet, a sum of the first and second elongation rates may be 0.7% or higher. When a sum of the first and second elongation rates exceeds a certain level, an excessive rolling load may be applied to a skin pass mill such that lifespan of the roll may be shortened, and the material may be deformed due to excessive reduction. Thus, a sum of the first and second elongation rates may be 4% or lower.

[0058] Roughness of the surface of the base steel sheet after the skin pass rolling may be 0.7-2.5 µm level with reference to center line average roughness (Ra). At an initial stage of the submersion in the hot-dip galvanizing bath, Al having high reactivity may firstly react with Fe of the base steel sheet such that an Fe-Al alloy phase may be formed, and accordingly, growth of a hard Fe-Zn intermetallic compound may be prevented. Therefore, when a surface area of effective reaction of the base steel sheet is widened, growth of an Fe-Zn intermetallic compound may be inhibited, which may contribute to improvement of mechanical property, and accordingly, plating peeling may be effectively prevented. Thus, to achieve such an effect, the skin pass rolling may be performed for surface roughness of the base steel sheet to satisfy 0.7µm or greater with reference to center line average roughness (Ra). When roughness deviation of the base steel sheet is excessive, oxidation of Al may be concentrated on a region in which roughness deviation is high, and accordingly, aluminum oxide may be locally formed such that an intermittent aluminum oxide layer may be formed. Thus, to secure an aluminum oxide layer continuously distributed on a surface side of the galvanized layer, the skin pass rolling may be performed for surface roughness of the base steel sheet to satisfy 2.5 µm or less with reference to center line average roughness (Ra).

Submersion in Hot-Dip Galvanizing Bath



[0059] The base steel sheet on which the skin pass rolling has been completed may be submerged in a zinc-based plating bath, and accordingly, a galvanized layer may be formed. The hot-dip galvanizing bath may include, by weight%, 0.2-0.4% of Al and a balance of Zn and other impurities. As Al may provide fluidity to the galvanizing bath, and may contribute to improvement of cohesion force between the galvanized layer and the base steel sheet, the hot-dip galvanizing bath in the present invention may include 0.2% or higher of Al. When a content of added Al is excessive, the effect of improvement in fluidity of the galvanizing bath may be saturated, whereas dross may increasingly occur due to facilitation of Fe aggression. Thus, a content of Al in the hot-dip galvanizing bath in the present invention may be 0.4% or lower. A preferable content of Al in the hot-dip galvanizing bath may be 0.2-0.24%.

[0060] The base steel sheet of the present invention may be submerged in the hot-dip galvanizing bath at a submersion temperature of 450-500°C. When a temperature of the base steel sheet is lower than a temperature of the hot-dip galvanizing bath, fluidity of the hot-dip galvanizing bath may degrade such that it may be highly likely that a flow pattern defect may occur. Thus, preferably, the base steel sheet may be submerged in the galvanizing bath by being maintained and heated at a temperature higher than a temperature of the galvanizing bath. Thus, the base steel sheet of the present invention may be submerged in the hot-dip galvanizing bath at a submersion temperature of 450°C or higher. Also, when a submersion temperature of the base steel sheet is higher than a temperature of the hot-dip galvanizing bath, dissolution of Fe may be accelerated such that dross may increasingly occur, and a surface defect such as dross stamping, or the like, may occur. Thus, the base steel sheet of the present invention may be submerged in the hot-dip galvanizing bath at a submersion temperature of 500°C or lower, and accordingly, a galvanized layer may be formed on the base steel sheet.

[0061] The hot-dip galvanized hot-rolled steel sheet manufactured by the manufacturing method as above may effectively prevent a flow pattern defect on the surface of the galvanized layer, and may thus have beautified surface appearance.

[0062] Also, as for the manufacturing method as above, as the mechanical scale removal and the chemical pickling for the base steel sheet may be performed in sequence, the use of chemical solution used in the chemical pickling may be significantly reduced, and remaining scale on the surface of the base steel sheet may be effectively removed.

[Mode for Invention]



[0063] In the description below, an example embodiment of the present disclosure will be described in greater detail. It should be noted that the example embodiments are provided to describe the present disclosure in greater detail, and the scope of the present invention is not necessarily limited to the embodiments described below.

[0064] JS-SPHC (sample 1, tensile strength of 350MPa) having a thickness of 3.2mm and JS-SAPH400 (sample 2, tensile strength of 400MPa) having a thickness of 2.9mm were selected as samples. The present invention is not necessarily applied to a thick steel sheet level of a hot-rolled steel sheet, but by selecting a hot-rolled steel sheet having a thickness of 3mm level, a harsh environment for improvement of a flow pattern defect was provided. Scale on a surface of the sample was removed by applying a first elongation rate, mechanical scale removal, and chemical pickling, listed in Table 1 below, and each sample was elongated at a second elongation rate by skin pass rolling the samples using a bright roll having roughness (Ra) of 0.2µm. After the skin pass rolling, a rolling oil was degreased and dried, and the surface of the sample was galvanized. The galvanizing was performed using a hot-dip galvanizing simulator (Iwatani Corp., multi functional process simulator) of Iwatani, and heat treatment and galvanizing conditions were set as in Table 2 below, thereby manufacturing a hot-dip galvanized sample. While the hot-dip galvanized sample was manufactured, temperature conditions (PHS, DFF, HRS, GJS, and TDS) at each section were applied the same, and only temperatures of submersion of the base steel sheet were different.
[Table 1]
ClassificationSampleFirst Elongation Rate (%)Shot BlastPickling (Hydrochloric Acid)Second Elongation Rate (%)Sum of Elongation Rates (%)Galvanizing
Projection Amount (kg/min )Projection Velocity (m/s)Concentration (%)Temperature (°C)Treatment Time (Sec onds )Galvanizing Bath Al (%)Submersion Temperature (°C)
Embodiment 1 1 0.40 1200 80 15 80 28 0.51 0.91 0.22 470
Embodiment 2 1 0.25 1200 80 15 80 28 0.51 0.76 0.22 470
Comparative Example 1 1 0.15 1200 80 15 80 28 0.51 0.66 0.22 470
Embodiment 3 1 0.40 1200 80 15 80 28 0.83 1.23 0.22 470
Embodiment 4 1 0.40 1200 80 15 80 28 1.23 1.63 0.22 470
Embodiment 5 1 0.40 1200 80 15 80 28 1.61 2.01 0.22 470
Comparative Example 2 1 0.25 1200 80 15 80 28 0.25 0.50 0.22 470
Embodiment 6 1 0.40 950 80 15 80 28 1.23 1.63 0.22 470
Comparative Example 3 1 0.40 750 80 15 80 28 1.23 1.63 0.22 470
Embodiment 7 1 0.40 1550 80 15 80 28 1.23 1.63 0.22 470
Comparative Example 4 1 0.40 1850 80 15 80 28 1.23 1.63 0.22 470
Embodiment 8 1 0.40 1200 70 15 80 28 1.23 1.63 0.22 470
Comparative Example 5 1 0.40 1200 60 15 80 28 1.23 1.63 0.22 470
Embodiment 9 1 0.40 1200 85 15 80 28 1.23 1.63 0.22 470
Comparative Example 6 1 0.40 1200 95 15 80 28 1.23 1.63 0.22 470
Embodiment 10 1 0.40 1200 80 15 80 18 1.23 1.63 0.22 470
Comparative Example 7 1 0.40 1200 80 15 80 13 1.23 1.63 0.22 470
Comparative Example 8 1 0.40 1200 80 15 80 28 1.23 1.63 0.22 440
Comparative Example 9 1 2.00 1200 80 15 80 28 0.51 2.51 0.22 470
Comparative Example 10 1 2.00 1200 80 15 80 28 1.23 3.23 0.22 470
Embodiment 11 2 0.60 1200 80 15 80 28 0.76 1.36 0.22 470
Embodiment 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
Embodiment 13 2 0.60 1200 80 15 80 28 1. 08 1.68 0.22 470
Embodiment 14 2 0.60 1200 80 15 80 28 1.70 2.30 0.22 470
Embodiment 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
[Table 2]
Starting Temperature (°C)Preheating Zone (°C)Direct Fired Furnace (°C)Reducing Zone (°C)Gas Jet Cooling Zone (°C)Turn Down Section (°C)Galvanizing Bath Temperature (°C)
20 265 592 630 506 465 460


[0065] After the galvanizing, an air-wiping was performed under the same condition, and results of observation of a surface and a galvanized layer of each hot-dip galvanized sample were as in Table 3. Surface quality, a distribution range of an Al thickened layer, whether the Al thickened layer was continuously distributed, and a maximum thickness thereof were measured. Surface quality was examined by observing each sample with the naked eye, and specifically, surface quality was determined as "○" (good, no flow pattern defect or no non-galvanizing occurred) or "X" (poor, a flow pattern defect or non-galvanizing occurred). A distribution range of the Al thickened layer, whether the Al thickened layer was continuously distributed, and a maximum thickness thereof were analyzed using a transmission electron microscope (TEM) after focused ion beams (FIB) processing. Also, remaining scale of each base steel sheet before galvanizing each sample was examined, and a result thereof is as in Table 3 below. As for the examination of the remaining scale of the base steel sheet, an SEM image at 200x magnification was checked in a back-scattering mode, and a fraction of a scale region on the image was calculated using an image analyzer.
[Table 3]
ClassificationSurface QualitySurface Roughness of Base Steel Sheet Before Galvanizing (µm)Remaining Scale of Base Steel Sheet Before Galvanizing (%)Distribution Range of Al Thickened Layer (nm)Continuity of Al Thickened LayerMaximum Thickness of Al Thickened Layer (nm)
Embodiment 1 2.05 Less than 1% Less than 60 Continuous 30
Embodiment 2 2.15 2.5% Less than 80 Continuous 40
Comparative Example 1 X 1,95 4% Greater than 100 Discontinuous 400
Embodiment 3 1.65 Less than 1% Less than 50 Continuous 10
Embodiment 4 1.15 Less than 1% Less than 40 Continuous 10
Embodiment 5 0.8 Less than 1% Less than 40 Continuous 10
Comparative Example 2 X 3.10 Less than 1% Greater than 100 Discontinuous 60
Embodiment 6 1.20 3% Less than 60 Continuous 10
Comparative Example 3 X 1.65 18% Greater than 100 Discontinuous 500
Embodiment 7 1.55 Less than 1% Less than 80 Continuous 30
Comparative Example 4 X 2.35 Less than 1% Greater than 100 Discontinuous 60
Embodiment 8 1.30 Less than 1% Less than 60 Continuous 30
Comparative Example 5 X 1.50 3% Greater than 100 Discontinuous 100
Embodiment 9 1.15 Less than 1% Less than 60 Continuous 20
Comparative Example 6 X 1.75 2.5% Greater than 100 Discontinuous 100
Embodiment 10 1.25 2% Less than 50 Continuous 40
Comparative Example 7 X 1.65 5% Greater than 100 Discontinuous 200
Comparative Example 8 X 1.65 Less than 1% Greater than 100 Discontinuous 80
Comparative Example 9 X 3.65 Less than 1% Greater than 100 Discontinuous 500
Comparative Example 10 X 2.85 Less than 1% Greater than 100 Discontinuous 500
Embodiment 11 2.40 Less than 1% Less than 60 Continuous 30
Embodiment 12 2.25 3% Less than 60 Continuous 40
Comparative Example 11 X 2.55 12% Greater than 100 Discontinuous 500
Embodiment 13 1.90 Less than 1% Less than 50 Continuous 10
Embodiment 14 1.25 Less than 1% Less than 40 Continuous 10
Embodiment 15 0.75 Less than 1% Less than 35 Continuous 10
Comparative Example 12 X 0.65 Less than 1% Less than 60 Continuous 30
Comparative Example 13 X 3.60 Less than 1% Greater than 100 Discontinuous 100
Comparative Example 14 X 4.15 Less than 1% Greater than 100 Discontinuous 700
Comparative Example 15 X 3.25 Less than 1% Greater than 100 Discontinuous 700


[0066] As for embodiments 1 to 15 which satisfied the conditions of the present invention, it has been confirmed that a continuous Al thickened layer was formed within 100nm from a surface of the galvanized layer, and that a maximum thickness of the Al thickened layer did not exceed 50nm. Also, it has been confirmed that embodiments 1 to 15 secured excellent surface cleanliness as remaining scale of the base steel sheet before the galvanizing was 3% or lower, and accordingly that a continuous Al thickened layer was formed. Thus, it has been confirmed that embodiments 1 to 15 had excellent surface appearance as a surface defect such as a flow pattern defect or non-galvanizing did not occur.

[0067] As for comparative examples 1 to 15 which did not satisfy the conditions of the present invention, it has been indicated that an Al thickened layer was intermittently formed, and a maximum thickness of the Al thickened layer exceeded 50nm. Thus, as a non-uniform and discontinuous Al thickened layer was formed, zinc oxide was non-uniformly formed on the surface of the galvanized layer, and accordingly, a surface defect such as a flow pattern defect or non-galvanizing occurred.

[0068] While the present invention has been described in detail through embodiments, it will be apparent that embodiments different from the above embodiments could be made. Thus, the technical idea and the scope of the claims set forth below will not be limited to the embodiments.


Claims

1. A hot-dip galvanized hot-rolled steel sheet having an excellent surface appearance, comprising:

a base steel sheet and a galvanized layer,

wherein the galvanized layer includes a continuous Al thickened layer formed within 100nm from a surface of the galvanized layer.


 
2. The hot-dip galvanized hot-rolled steel sheet of claim 1, wherein the Al thickened layer is an aluminum oxide (Al2O3) layer.
 
3. The hot-dip galvanized hot-rolled steel sheet of claim 1, wherein a thickness of the Al thickened layer is 50nm or less (excluding 0nm).
 
4. The hot-dip galvanized hot-rolled steel sheet of claim 1, wherein surface roughness of a surface of the base steel sheet forming an interfacial surface with the galvanized layer is 0.7-2.5 µm with reference to center line average roughness (Ra).
 
5. The hot-dip galvanized hot-rolled steel sheet of claim 1, wherein the galvanized layer includes, by weight%, 0.2-0.6% of Al and a balance of Zn and other impurities.
 
6. A method of manufacturing a hot-dip galvanized hot-rolled steel sheet having an excellent surface appearance, the method comprising:

creating cracks in scale on a surface of a base steel sheet by elongating the base steel sheet at a first elongation rate;

mechanically removing scale from the surface of the base steel sheet by providing physical impact on the surface of the base steel sheet;

chemically pickling the surface of the base steel sheet by allowing the surface of the base steel sheet to react with acid solution;

flattening the surface of the base steel sheet by skin pass rolling the base steel sheet at a second elongation rate; and

forming a galvanized layer by submerging the base steel sheet in a hot-dip galvanizing bath including, by weight%, 0.2-0.4% of Al and a balance of Zn and other impurities.


 
7. The method of claim 6, wherein the first elongation rate is 0.2-1.5%.
 
8. The method of claim 6, wherein scale on the surface of the base steel sheet is mechanically removed by performing a shot-blasting treatment on the surface of the base steel sheet.
 
9. The method of claim 6, wherein the shot-blasting includes projecting a shot ball having an average diameter of 0.18-0.6mm by an average projection amount of 800-1800kg/min and at an average projection velocity of 65-90m/s.
 
10. The method of claim 6, wherein the surface of the base steel sheet is chemically pickled by submerging the base steel sheet in hydrochloric acid aqueous solution of a concentration of 5-20% provided within a temperature range of 70-85°C for 15-35 seconds.
 
11. The method of claim 6, wherein the base steel sheet is skin-pass rolled by a bright roll having surface roughness of 0.1-0.8 µm with reference to center line average roughness (Ra) .
 
12. The method of claim 6, wherein surface roughness of the skin-pass rolled base steel sheet is 0.7-2.5 µm with reference to center line average roughness (Ra).
 
13. The method of claim 6, wherein the second elongation rate is 0.5-2.5%.
 
14. The method of claim 6, wherein a sum of the first elongation rate and the second elongation rate is 0.7-4.0%.
 
15. The method of claim 6, wherein the second elongation rate is greater than the first elongation rate.
 
16. The method of claim 6, wherein a submersion temperature of the hot-dip galvanizing bath of the base steel sheet is 450-500°C.
 




Drawing













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