Patent Application
20240368736
The present disclosure relates to a high-corrosion resistance plated steel sheet having excellent corrosion resistance and weldability and a method for manufacturing the same.
When a galvanized steel sheet is exposed to a corrosive environment, a galvanized steel sheet may have sacrificial properties in which zinc, having a lower redox potential than that of iron, is corroded first, such that corrosion of a steel material may be prevented. Also, as zinc in a plated layer oxidizes, dense corrosion products may be formed on the surface of a steel material, thereby blocking the steel material from the oxidizing atmosphere and improving corrosion resistance of the steel material. Due to the advantageous properties, the application of a galvanized steel sheet has been increased to include steel sheets for construction materials, home appliances, and automobiles.
However, the corrosion environment has gradually deteriorated due to an increase in air pollution due to industrial advancement, and due to strict regulations on resource and energy conservation, the need for the development of a steel material having improved corrosion resistance than a conventional galvanized steel material has been increased.
To address this issue, various studies have been conducted on the technique of manufacturing a galvanized alloy plated steel sheet, which may improve corrosion resistance of a steel material by adding elements such as aluminum (Al) and magnesium (Mg) to a galvanizing bath. As a representative example, a Zn—Mg—Al galvanized alloy steel sheet in which Mg is further added to a Zn—Al plating composition system has been used.
Meanwhile, a Zn—Mg—Al-based galvanized alloy steel sheet may be often used by being painted or welded. When a large amount of Al is distributed on the surface of a plated layer, phosphatability required for painting pre-treatment may decrease due to Al-based oxide, such that painting adhesion may deteriorate. Also, there may be a problem in which pores may remain in a welded metal due to influence of Al vapor during arc welding, which may cause a decrease in strength of the welded zone.
Also, a galvanized steel sheet after processing may be often provided on the periphery of products, but surface damage factors such as partial peeling due to paint adhesion deterioration and deformation of a welded zone due to welding pores may be added, such that surface quality may be poor, and accordingly, exterior quality may need to be improved.
However, a technique which meets the demand for high-end products having excellent properties such as corrosion resistance, weldability and chemical treatment has not been developed.
(Cited document 1) Korean Laid-Open Patent Publication No. 2010-0073819
An aspect of the present disclosure is to provide a plated steel sheet having excellent corrosion resistance and weldability and a method for manufacturing the same.
Also, another aspect of the present disclosure is to provide a plated steel sheet having excellent corrosion resistance, weldability and phosphatability and a method for manufacturing the same.
The purpose of present disclosure is not limited to the above aspects. Anyone having ordinary knowledge in the technical field to which the present disclosure belongs may have no difficulty in understanding the additional purpose of the invention from the overall description of the present disclosure.
An aspect of the present disclosure provides
(In relational expression 1, [Zn]1/10t represents a weight % content of Zn in a 1/10t position (t is a total thickness of the plated layer) in a thickness direction from a surface of the plated layer, and [Zn]s represents a weight % content of Zn on a surface of the plated layer.)
(In relational expression 2, [Al]1/10t represents a weight % content of Al in a 1/10t position (t is a total thickness of the plated layer) in a thickness direction from a surface of the plated layer, and [Al]s represents a weight % content of Al on a surface of the plated layer.)
Another aspect of the present disclosure provides
According to an aspect of the present disclosure, a plated steel sheet having excellent corrosion resistance and weldability and a method for manufacturing the same may be provided.
Also, according to another aspect of the present disclosure, a plated steel sheet having excellent corrosion resistance, weldability and phosphatability and a method for manufacturing the same may be provided.
The various and beneficial advantages and effects of the present disclosure are not limited to the above, and will be more easily understood in the course of describing specific embodiments of the present disclosure.
The terms used in this specification are intended to describe specific embodiments and are not intended to limit the present disclosure. Also, an expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context.
The term “comprise” used in the specification specifies components and does not exclude the presence or addition of other components.
Unless otherwise defined, the entirety of terms, including technical and scientific terms, used in this specification may refer to the same as those generally understood by those skilled in the art in the technical field to which the present disclosure belongs. Terms defined in the dictionary are interpreted to have meanings consistent with related technical document and current disclosure.
Hereinafter, [plated steel sheet] according to an aspect of the present disclosure will be described in detail. When indicating the content of each element in the present disclosure, unless specifically defined otherwise, the content may be indicated in weight %.
In a conventional Zn—Mg—Al-based galvanized alloy steel sheet related technology, Mg may be added to improve corrosion resistance. However, when excessive Mg is added, floating dross may increase in a plating bath, such that the dross may need to be removed frequently, which may be problematic, and an upper limit of Mg content may be limited to 3%.
Accordingly, research has been conducted to further improve corrosion resistance by increasing a Mg content from 3%, but as more Mg is added, floating dross may significantly increase in a plating bath, such that it may be difficult to manage the plating bath, and as an exterior of a product is darkened and surface damage factors are added, it may be difficult to assure exterior quality, which may be problematic.
Accordingly, to solve the above-mentioned problem, the technical attempts have been conducted, such as, by increasing the Al content simultaneously as the Mg content increases, Al oxide may be preferentially formed on the plating bath surface rather than Mg oxide, such that Mg oxide-based floating dross may be prevented, and the exterior of the product may be prevented from being easily darkened.
However, even the composition system is determined to have a high Al content and plating workability and exterior quality are assured, due to Al oxide inevitably formed on the surface of the plated layer, as surface damage factors such as partial peeling due to paint adhesion deterioration and deformation of a welded zone due to welding pores are added, such that there may be a problem in which it may be difficult to assure exterior quality after painting and a welding process.
Accordingly, in the prior art, it has been technically difficult to provide a Zn—Mg—Al-based galvanized alloy steel sheet assuring excellent corrosion resistance and also excellent weldability and/or phosphatability.
Accordingly, the present inventors conducted intensive studies to address the above-mentioned problems and to provide a plated steel sheet having excellent corrosion resistance and also excellent weldability and/or phosphatability. As a result, it was found that, after the effect of oxidation on the plated layer was excluded, the content change of each component (e.g., Zn and Al, and further comprising Mg) in the surface layer region from the plated layer surface to the 1/10t position in a thickness direction (t is a total thickness of the plated layer) may be an important factor, and completed the present disclosure.
Accordingly, in the description below, a plated steel sheet having excellent corrosion resistance, and also having excellent weldability and/or phosphatability will be described in detail.
First, the plated steel sheet according to the present disclosure may comprise a base steel sheet; and a Zn—Mg—Al-based plated layer provided on at least one surface of the base steel sheet.
In the present disclosure, the type of base steel sheet may not be particularly limited. For example, the base steel sheet may be implemented as an Fe-based base steel sheet used as a base steel sheet of general galvanized steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, but an embodiment thereof is not limited thereto. Alternatively, the base steel sheet may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as a material for construction, home appliances, and automobiles.
However, as an example, the base steel sheet may have a composition of, in weight %, C: more than 0% (more preferably, 0.001% or more) and 0.18% or less, Si: more than 0% (more preferably, 0.001% or more) and 1.5% or less, Mn: 0.01 to 2.7%, P: more than 0% (more preferably, 0.001% or more) and 0.07% or less, S: more than 0% (more preferably, 0.001% or more) and 0.015% or less, Al: more than 0% (more preferably, 0.001% or more) and 0.5% or less, Nb: more than 0% (more preferably, 0.001% or more) and 0.06% or less, Cr: more than 0% (more preferably, 0.001% or more) and 1.1% or less, Ti: more than 0% (more preferably, 0.001% or more) and 0.06% or less, B: more than 0% (more preferably, 0.001% or more) and 0.03% or less and a balance of Fe and inevitable impurities.
Although not particularly limited, according to an embodiment of the present disclosure, at least one surface of the base steel sheet may be provided with a Zn—Mg—Al-based plated layer consisted of a Zn—Mg—Al-based alloy. The plated layer may be formed on only one surface of the base steel sheet, or may be formed on both surfaces of the base steel sheet. In this case, the Zn—Mg—Al-based plated layer may refer to a plated layer comprising Mg and Al and mainly comprising Zn (comprising more than 50% of Zn).
Although not particularly limited, according to an embodiment of the present disclosure, a thickness of the Zn—Mg—Al-based plated layer may be 9 to 100 μm, more preferably 20 to 90 μm. When the thickness of the plated layer is less than 9 μm, the thickness of the plated layer may excessively decrease locally due to errors resulting from the thickness deviation of the plated layer, such that corrosion resistance may deteriorate. When the thickness of the plated layer exceeds 100 μm, cooling of the molten plated layer may be delayed, a solidification defect such as a flow pattern, for example, may occur on the surface of the plated layer, and productivity of the steel sheet may decrease to solidify the plated layer.
Meanwhile, according to an embodiment of the present disclosure, although not particularly limited, an Fe—Al-based inhibition layer may be further comprised between the base steel sheet and the Zn—Mg—Al-based plated layer. The Fe—Al-based inhibition layer may mainly comprise an intermetallic compound of Fe and Al (e.g., more than 60%), and examples of the intermetallic compound of Fe and Al may comprise FeAl, FeAl3, and Fe2Al5. Also, some components derived from the plated layer, such as Zn and Mg, may be further comprised, for example, up to 40%. The inhibition layer may be formed due to alloying by Fe and plating bath components diffused from the base steel sheet at an initial stage of plating. The inhibition layer may improve adhesion between the base steel sheet and the plated layer, and may also prevent Fe diffusion from the base steel sheet to the plated layer. The inhibition layer may be formed continuously between the base steel sheet and the Zn—Mg—Al-based plated layer, or may be formed discontinuously. In this case, other than the above-mentioned description, regarding the inhibition layer, a commonly known description in the relevant technical field may be applied.
Although not particularly limited, according to an embodiment of the present disclosure, a thickness of the inhibition layer may be 0.01 to 2.50 μm. The inhibition layer may assure corrosion resistance by preventing alloying, but as the layer is brittle, the layer may affect processability, and accordingly, a thickness thereof may be determined to be 2.50 μm or less. However, to function as a inhibition layer, the thickness may be 0.01 μm or more. In terms of further improving the above-mentioned effect, preferably, an upper limit of the thickness of the inhibition layer may be 1.80 μm. Also, a lower limit of the inhibition layer thickness may be 0.02 μm. In this case, the thickness of the inhibition layer may refer to a minimum thickness in the direction perpendicular to an interfacial surface of the base steel sheet.
Meanwhile, according to an embodiment of the present disclosure, although not particularly limited, the Zn—Mg—Al based plated layer may comprise, in weight %, Mg: 4.0 to 7.0%, Al: 8.2 to 19.5%, a balance of Zn and inevitable impurities. In the description below, each component will be described in detail.
Mg may improve corrosion resistance of a plated steel material. In the present disclosure, a Mg content in the plated layer may be controlled to 4.0% or more to assure desired excellent corrosion resistance. When Mg is added excessively, dross may occur, and accordingly, the Mg content may be controlled to 7.0% or less. Meanwhile, more preferably, in terms of maximizing the above-mentioned effect, a lower limit of the Mg content may be 4.7%, or an upper limit of the Mg content may be 6.0%.
Generally, when Mg is added at 1% or more, the effect of improving corrosion resistance may be exhibited, but when Mg is added at 2% or more, floating dross may increase in a plating bath due to oxidation of Mg in the plating bath, such that it may be necessary to reduce dross. Due to this problem, in the prior art, by adding more than 1.0% of Mg in Zn—Mg—Al-based zinc alloy plating, corrosion resistance may be assured, and an upper limit of Mg content may be determined to be 3.0% for commercialization. However, as mentioned above, to further improve corrosion resistance, it may be necessary to increase the Mg content to 4% or more. However, when the plated layer comprises 4% or more of Mg, dross may occur due to oxidation of Mg in the plating bath, which may be problematic, such that it may be necessary to add 8.2% or more of Al. When Al is added excessively to suppress dross, a melting point of the plating bath may increase and an operating temperature may thus increase excessively, such that problems caused by high-temperature work, such as erosion of the plating bath structure and deterioration of the steel material, may occur. Also, when the Al content in the plating bath is excessive, Al may react with Fe of base iron and may not contribute to the formation of the Fe—Al inhibition layer, and the reaction between Al and Zn may occur rapidly, such that a lump-shaped outburst phase may be excessively formed, and corrosion resistance may actually deteriorate. Accordingly, it may be preferable to control an upper limit of Al content in the plated layer to 19.5%. Meanwhile, more preferably, in terms of maximizing the above-mentioned effect, a lower limit of the Al content may be 11.0%, or an upper limit of the Al content may be 18.0%.
In addition to the composition of the plated layer described above, a balance of Zn and other inevitable impurities may be further comprised. Inevitable impurities may comprise components unintentionally mixed in a process of manufacturing general hot-dip galvanized steel sheet, and a person skilled in the relevant technical field may easily understand the addition.
According to the present disclosure, the plated layer may satisfy relational expressions 1 and 2 below.
(In relational expression 1, [Zn]1/10t represents a weight % content of Zn in a 1/10t position (t is a total thickness of the plated layer) in a thickness direction from a surface of the plated layer, and [Zn]s represents a weight % content of Zn on a surface of the plated layer.)
(In relational expression 2, [Al]1/10t represents a weight % content of Al in a 1/10t position (t is a total thickness of the plated layer) in a thickness direction from a surface of the plated layer, and [Al], represents a weight % content of Al on a surface of the plated layer.)
The present inventors conducted a thorough study to assure further improved corrosion resistance as compared to the prior art and also to assure weldability and/or phosphatability. As a result, it was found that, excluding the effect of oxidation on the plated layer, the content change of each component (e.g., Zn and Al, and further comprising Mg) in the surface layer region from the plated layer surface to the 1/10t position in a thickness direction (t is a total thickness of the plated layer) may be an important factor.
In the present disclosure, the Zn—Mg—Al-based plated layer may comprise a Zn single phase and a Zn—MgZn2—Al-based ternary eutectic phase, which are common phases in a high-corrosion resistance plated steel sheet. Generally, as the Al and Mg content in the plated layer decreases, the amount of Zn single phase and Zn—MgZn2—Al ternary eutectic phase may be increased in the entire plated layer, and as the Al and Mg content in the plated layer increases, the amount of MgZn2 phase and Al single phase may increase. In this case, in a plating composition system as in the present disclosure in which the Mg content is 4% or more, as the Mg content increases, the Al content may also need to increase simultaneously to suppress dross, such that the Al single phase may also be present. Accordingly, through repeated research, the present inventors have determined that, by suppressing nucleation and growth of the aforementioned MgZn2 phase and Al single phase on the surface of the plated layer and promoting the same in the plated layer, distribution of Zn single phase, Al—Zn binary eutectic phase and Zn—MgZn2—Al ternary eutectic phase may be increased on the plated layer surface, and a plated layer surface having a high Zn component content and also a low Al component content may be formed.
In other words, the present inventors confirmed that, while assuring the aforementioned corrosion resistance, the region directly affecting weldability and phosphatability may be a surface layer region from the plated layer surface to the 1/10t position in the thickness direction. Accordingly, it was found that, by controlling a change in Zn content and a change in Al content in the surface region of the plated layer to satisfy the above-mentioned relational expressions 1 and 2, a plated steel sheet having excellent weldability and/or phosphatability and also excellent corrosion resistance may be effectively obtained.
Meanwhile, according to an embodiment of the present disclosure, although not particularly limited, in terms of maximizing the above-mentioned effect, more preferably, a lower limit of the [Zn]1/10t—[Zn]s value defined in relational expression 1 above may be −9.9, or an upper limit of the [Zn]1/10t—[Zn]s value may be −5.3 (most preferably, −7.0). Also, a lower limit of the [Al]1/10t—[Al]s value defined in relational expression 2 may be +6.4, or an upper limit of the [Al]1/10t—[Al]s value may be +11.1.
Also, according to an embodiment of the present disclosure, although not particularly limited, the present inventors conducted intensive studies to further improve corrosion resistance and, optionally, it was further found that, by controlling a profile of Mg content in the thickness direction to further satisfy relational expression 3 below, corrosion was carried out uniformly in the thickness direction, which has the effect of improving corrosion resistance to a higher level.
(In relational expression 3, [Mg]1/10t represents a weight % content of Mg in a 1/10t position (t is a total thickness of the plated layer) in a thickness direction from a surface of the plated layer, and [Mg]s represents a weight % content of Mg on a surface of the plated layer.)
According to an embodiment of the present disclosure, in a sacrificial anti-corrosion cell, which acts as a corrosion resistance mechanism of the Zn—Mg—Al-based plated layer, a potential of MgZn2 may be −1.2V on hydrogen reduction potential, and a potential of Al is −0.7V on hydrogen reduction potential, such that a relatively large potential difference may be assured, and accordingly, the potentials may act as an anode and a cathode, respectively, and may form a galvanic cell between microstructures of the adjacent MgZn2 phase and the Al single phase. In other words, when corrosion is carried out and the anode (MgZn2) of the sacrificial anti-corrosion cell in which elution occurs is formed irregularly in a depth direction of the plated layer, a potential difference between the anode and the cathode of the galvanic cell, which has the excellent corrosion resistance and sacrificial anti-corrosion properties described above, is formed differently depending on a position of the plate, such that specific portions may corrode first and corrosion may progress unevenly by position. Accordingly, by controlling the change in Mg content of the surface layer of the plated layer to be low so as to satisfy the above relational expression 3, uniform corrosion may be promoted in the thickness direction, such that overall corrosion resistance may further improve.
Meanwhile, according to an embodiment of the present disclosure, although not particularly limited, in terms of maximizing the above-mentioned effect, more preferably, a lower limit of the [Mg]1/10t—[Mg]s value defined in relational expression 3 above may be −0.8, or an upper limit of the [Mg]1/10t—[Mg]s value may be +0.8.
The method of measuring the content of each component in the 1/10t position defined in the above-mentioned relational expressions 1 to 3 and the content of each component on the surface of the plated layer may not be particularly limited, and for example, the contents may be measured as below.
In other words, the plated steel material may be cut in the vertical direction, and the distribution of content of each component such as Zn, Al, Mg and Fe in the cross-section of the plated layer may be measured using a glow discharge optical emission spectrometry (GDS). Subsequently, to exclude influence of oxidation of the plated layer, the region up to 0.1 μm in the thickness direction from the outermost surface of the plated layer may be excluded in the present disclosure. Accordingly, in this specification, the above-mentioned ‘surface of the plated layer’ may refer to a point which may be 0.1 μm position from an outermost surface of the plated layer in the thickness direction, excluding the region affected by oxidation. Accordingly, a content of each component on the surface of the plated layer defined in the above relational expressions 1 to 3 may be defined as a content of each component (Zn, Al and Mg) in a position of 0.1 μm from an outermost surface of the plated layer.
Meanwhile, to define the content of each component in the 1/10t position in the thickness direction from the above-mentioned plated layer surface, as for a thickness (t) of the total plated layer, a distance from the plated layer surface to a position in which the Zn and Fe contents match each other may be defined as t with respect to a profile of each component measured using a GDS. Accordingly, the 1/10t position for the total thickness t of the plated layer may be defined, such that the content of each component (Zn, Al and Mg) measured by GDS at the 1/10t point may be measured.
For example, in the case of embodiment 7 defined in
Also, according to an embodiment of the present disclosure, although not particularly limited, the weight % content of Mg in the 2/3t position in the thickness direction from the surface of the plated layer may be in the range of +0.5 wt % based on an average weight % content of Mg in the plated layer. By satisfying this condition, corrosion may be carried out uniformly in the thickness direction, such that corrosion resistance may further improve.
Meanwhile, although not particularly limited, in terms of maximizing the above-mentioned effect, more preferably, the weight % content of Mg in the 2/3t position in the thickness direction from the surface of the plated layer may satisfy a ±0.3 wt % range based on an average weight % content of Mg in the plated layer.
In this case, the method of measuring the weight % average content of Mg in the plated layer is not particularly limited. However, as an example, with respect to the change in Mg content in the thickness direction measured using the above-described GDS, excluding the region up to 0.1 μm from the plated layer outermost surface, the Mg content at each point may be measured every 0.5 μm up to the shortest distance t, and an average value calculated therefrom may be obtained.
Also, as for the weight % content of Mg at the 2/3t position in the thickness direction from the surface of the plated layer, excluding the region up to 0.1 μm from the plated layer outermost surface, the Mg content in the 2/3t position may be obtained based on the previously defined t.
Meanwhile, according to an embodiment of the present disclosure, although not particularly limited, optionally, an area ratio of the Al single phase having a Zn solid-solution rate of 27 atomic % or more on the surface of the plated layer may be 2.0 to 10.1%. When the area ratio of the Al single phase having a Zn solid-solution rate of 27 atomic % or more is less than 2.0% or more than 10.0%, one or more properties of weldability and phosphatability may be deteriorated. In terms of further improving the above-described effect, preferably, a lower limit of the area ratio of the Al single phase on which the Zn solid-solution rate is 27 atomic % or more may be 2.2%, or an upper limit of the area ratio of the Al single phase having the Zn solid-solution rate of 27 atomic % or more may be 4.0%.
In this case, in this specification, the Al single phase may refer to a phase mainly formed of Al, regardless of the Zn solid-solution rate, and may refer to a phase in which a balance of Al is comprised, excluding impurities such as dissolved Zn and inevitably comprised Mg. Specifically, in the prior art, it has been generally known that only an Al single phase having a low Zn solid-solution rate of less than 27% may be present as an Al single phase. However, as a result of careful examination by the present inventors, as the Al single phase present on the surface of the plated layer, an Al single phase having a low Zn solid-solution rate of less than 27%, and an Al single phase having a high Zn solid-solution rate of more than 27% may be present. Also, to additionally improve the aforementioned corrosion resistance, weldability and phosphatability, the area ratio of the Al single phase having a Zn solid-solution rate of 27% or more may be controlled low, and may be comprised 2.0 area % or more, which may be an effective factor in reducing Al oxide generated on the surface of the plated layer, which adversely affects weldability and phosphatability.
Also, the method of measuring the area ratio of the Al single phase having a Zn solid-solution rate of 27 atomic % or more on the surface of the plated layer is not particularly limited. However, as an example, images derived under scanning electron microscope (SEM) measurement conditions may be analyzed using an automatic image generation software based on the super-pixel algorithm of microstructure phase analysis software (RISA) of the Research Institute of Industrial Science and Technology (RIST). The superpixel algorithm may be a mechanism of dividing the entire image into thousands to tens of thousands of regions (superpixels) and measuring similarity by comparing superpixels with similar patterns or features, calculating a histogram of the brightness values of a pixel, and automatically selecting superpixels when the similarity is greater than a predefined threshold. As an example of specifying a predefined threshold, as for a boundary of the Al single phase in the image derived by SEM, by predefining each phase based on the Zn solid-solution rate of 27 atomic % employed in the Al single-phase organization using an EDS, histogramming and structure differentiation of brightness values on soft images may be possible. The technical idea of the above-mentioned RISA (microstructure fraction analysis software) may be indicated in Korean Laid-Open Patent Publication No. 2019-0078331.
In the description below, the [method of manufacturing plated steel sheet] according to another aspect of the present disclosure will be described in detail. However, the plated steel sheet in the present disclosure may need to be manufactured by the manufacturing method below.
According to an embodiment of the present disclosure, preparing a base steel sheet may be further comprised, and the type of the base steel sheet is not particularly limited. A Fe-based base steel sheet used as the base steel sheet of the general hot-dip galvanized steel sheet may be a hot-rolled steel sheet or a cold-rolled steel sheet, but an embodiment thereof is not limited thereto. Also, the base steel sheet may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as a material for construction, home appliances, and automobiles, but an embodiment thereof is not limited thereto. In this case, the above description may be applied to the base steel sheet.
According to an embodiment of the present disclosure, as for a steel sheet processed in a shot blasting cabinet in which a metal material is circulated at a rotation speed of 1200 to 2000 mpm, a primary shot blasting treatment may be performed such that metal balls with a particle size of 0.6 to 1.0 mm are projected onto the surface of the base steel sheet at 400 to 1,200 kg/min. By satisfying the conditions of the primary shot blasting treatment, the surface oxide of the base steel sheet before plating may be primarily removed, thereby ensuring the effect of reducing the effect of the oxide.
Subsequently to the primary shot blasting treatment, with respect to the steel sheet processing in a shot blasting cabinet in which the metal material is circulated at a rotation speed of 1200 to 2000 mpm, a secondary shot blasting treatment may be performed such that metal balls with a particle size of 0.2 to 0.5 mm are projected onto the surface of the base steel sheet at 400 to 1200 kg/min. By satisfying the conditions of the secondary shot blasting treatment, by applying fine plastic deformation to the surface of the steel sheet, dislocation density in the base iron structure may be increased, such that the effect of activating the plating reaction may be secured.
Subsequently, the base steel sheet having gone through the primary and secondary shot blasting treatment may comprise, by weight %, Mg: 4.0 to 7.0%, Al: 8.2 to 19.5%, and a balance of Zn and inevitable impurities, and may be immersed in the in a plating bath maintained at a temperature of Ts+20° C. to Ts+80° C. as compared to the solidification start temperature (Ts) in the phase diagram and may be hot-dip galvanized.
In this case, according to an embodiment of the present disclosure, as for the reason for adding components and limiting content in the plating bath described above, excluding the small amount of Fe content which may flow from the base steel sheet, the description of the components of the plated layer described above may be applied.
Meanwhile, to manufacture a plating bath of the above-described composition, a composite ingot comprising predetermined Zn, Al and Mg or a Zn—Mg or a Zn—Al ingot comprising individual components may be used. To replenish the plating bath consumed by hot-dip plating, the ingot may be further melted and supplied. In this case, the ingot may be dissolved by being directly immersed in a plating bath, or the ingot may be melted in a separate pot and the molten metal may be added to the plating bath.
Also, the temperature of the plating bath may be maintained at a temperature 20 to 80° C. higher than the solidification start temperature (Ts) on the phase diagram (Ts+20° C. to Ts+80° C.). In this case, although not particularly limited, the solidification start temperature in the equilibrium phase diagram may be in the range of 390 to 460° C., or the temperature of the plating bath may be maintained in the range of 440 to 520° C. As the temperature of the plating bath increases, fluidity may be assured in the plating bath and a uniform composition may be formed, and the amount of floating dross may be reduced. When the temperature of the plating bath is less than Ts+20° C., the dissolution of the ingot may be extremely slow and viscosity of the plating bath may be relatively high, it may be difficult to assure excellent surface quality of the plated layer. Meanwhile, when the temperature of the plating bath exceeds Ts+80° C., ash defects due to Zn evaporation may occur on the plating surface.
Meanwhile, the hot-dip galvanized steel sheet may be cooled using an inert gas at an average cooling rate of 2 to 8° C./s from the solidification start temperature to the solidification end temperature with reference to the surface temperature. According to the present disclosure, the slow cooling may contribute to promoting nucleation and growth in the plated layer rather than on the surface of the plated layer. Accordingly, when the above-mentioned average cooling rate is less than 2° C./s, the MgZn2 structure may be formed too coarsely on the surface and the entire plated layer may become brittle, which may increase cracks and may be disadvantageous in assuring uniform corrosion resistance and processability. Meanwhile, when the average cooling rate exceeds 8° C./s, the change from the liquid phase to the solid phase may occur too rapidly during the hot-dip plating process, and uneven phases may be formed locally on the surface of the plated layer, such that color deviation in the width direction and corrosion resistance of the plated steel sheet may be reduced.
Also, according to an embodiment of the present disclosure, although not particularly limited, optionally, before performing the hot-dip galvanizing, performing pre-temper rolling by applying a roll reduction of 100 to 400 tons to the surface of the base steel sheet having gone through the shot blasting treatment using a dull roll having a surface roughness Ra of 1.8 to 2.8 μm may be further comprised.
As a result of careful study to further improve weldability and/or phosphatability, the present inventors found that, by performing a pre-temper rolling (SPM) treatment using a dull roll satisfying the above-mentioned conditions, the surface shape of the base steel sheet may be controlled irregularly, and accordingly that the effect of maximizing the creation site of solidification nuclei in the plated layer may be obtained by the subsequent plating process.
Specifically, when the surface roughness Ra of the dull roll is less than 1.8 μm, there may be a problem of promoting nucleation in the surface layer of the plated layer rather than nucleation in the plated layer. Meanwhile, when the surface roughness Ra of the dull roll exceeds 2.8 μm, excessive dents may occur in the base steel sheet, such that the thickness of the molten plated layer formed by the subsequent plating process may become uneven in the width direction.
Also, when the roll reduction of the dull roll is less than 100 tons, the shape control effect of the base steel sheet may be low, such that it may be difficult to expect the effect of promoting solidification nucleation in the plated layer, and when the roll reduction of the dull roll exceeds 400 tons, there may be a risk of inducing C bending of the base steel sheet, and it may be difficult to expect the effect of contributing to uniform formation of the plated layer in the thickness direction.
As described above, by precisely controlling the plating composition and manufacturing conditions, a plated steel sheet having excellent corrosion resistance and also weldability and/or phosphatability may be effectively provided.
Hereinafter, the present disclosure will be described in greater detail through embodiments. However, it may be important to note that the embodiments may be only intended to describe the present disclosure through examples and is not intended to limit the scope of rights of the present disclosure. This is because the scope of rights in present disclosure is determined by matters stated in the claims and matters reasonably inferred therefrom.
Primary and secondary shot blasting treatment was performed on a base steel sheet comprising C: 0.018%, Si: 0.01%, Mn: 0.2%, P: 0.009%, S: 0.005%, Al: 0.1%, Nb: 0.02%, Cr: 0.2%, Ti: 0.02%, B: 0.015%, and a balance of Fe and inevitable impurities under the conditions shown in Table 1 below. Thereafter, pre-temper rolling (SPM) treatment was performed to satisfy the conditions in Table 1 below. Thereafter, hot-dip galvanizing was performed to satisfy the plating bath composition (a balance of Zn and inevitable impurities) and plating bath temperature in Table 2 below, and cooling was performed using inert gas N2 to satisfy the conditions in Table 2 below.
A sample of the plated steel sheet described above was manufactured, the plated layer was dissolved in a hydrochloric acid solution, the dissolved liquid was analyzed by wet analysis (ICP) to measure the composition of the plated layer, and was listed in Table 3 below (however, a balance of Zn and inevitable impurities were comprised).
Also, a cross-sectional sample cut in the thickness direction of the steel sheet (in the direction perpendicular to the rolling direction) was manufactured such that the plated layer and the base iron interfacial surface were able to be observed, was imaged using a scanning electron microscope (SEM), and it was confirmed that a Fe—Al-based inhibition layer having a thickness of 0.02 μm was formed between the base steel sheet and the Zn—Mg—Al-based plated layer.
Also, component analysis was performed from the surface of the plated layer in the thickness direction using a GDS measurement device, the content in the 1/10t position for each component of Zn, Al and Mg was measured, and excluding the region up to 0.1 μm from the surface, the content at the surface at the 0.1 μm point was measured to exclude the influence of oxidation, and was listed in Table 3 below.
Also, the weight % content of Mg was measured in a 2/3t position in the thickness direction from the surface of the plated layer in the same manner as described above in this specification. Similarly, the average weight % content of Mg in the plated layer was also measured in the same manner as described above in this specification.
Also, the surface of the plated layer was imaged using an SEM and the area ratio of the Al single phase having a Zn solid-solution rate of 27 atomic % or more was measured. The values were listed in Table 4 below.
For each embodiment and comparative example, properties were evaluated on the basis of the criteria as below, and the evaluation results of properties were listed in Table 5 below.
Corrosion resistance was evaluated according to the following criteria using a salt spray tester and a test method in accordance with ISO14993.
⊚: The time taken for red rust is 40 times longer than that of Zn plating having the same thickness.
∘: The time taken for red rust is more than 30 times and less than 40 times than that of Zn plating having the same thickness.
Δ: The time taken for red rust is more than 20 times and less than 30 times than that of Zn plating of the same thickness.
x: The time taken for red rust is less than 20 times than that of Zn plating of the same thickness.
To evaluate phosphatability, the steel sheet went through the processes in the order of degreasing-washing-surface adjustment-phosphate treatment, the free acidity in the phosphate solution was 0.7 to 0.9, and total acidity was 19 to 21, and accelerator was 4 to 4.5. Also, using a scanning electron microscope (SEM), three random points on the surface of the steel sheet were observed at a magnification of 1,000 times, and 20 phosphate particles were selected within one observation in order of increasing longitudinal size of phosphate particles, an average thereof was obtained, an average value of the three points was calculated, and phosphatability was evaluated according to the values as below.
∘: Phosphate particle longitudinal length <5 μm
Δ: Phosphate particle longitudinal length 5 to 8 μm
x: Phosphate particle longitudinal length >8 μm
To evaluate weldability, the amount of spatter generation, which affects welding workability, and a pore rate, which affects tensile strength of the weld zone, were evaluated. As for the weldability evaluation, gas metal arc (GMA) welding was performed under the conditions of gas CO2, welding material KC-28 solid wire, current 150 A, voltage 20V, and welding speed 0.6 m/min. The amount of spatter generation was compared by imaging 5 times every 5 seconds immediately after the start of welding, and the welding pore rate was measured by measuring the distribution rate (%) of pore defects on the weld line after radioactive non-destructive testing of the welded zone.
∘: Spatter amount less than 5 times as compared to Zn plating of the same thickness, pore rate less than 15%
Δ: Spatter amount 5 times or more and less than 15 times as compared to Zn plating of the same thickness, pore rate 15% or more and less than 30%
x: Spatter amount 15 times or more as compared to Zn plating of the same thickness, pore rate more than 30%
Also, corrosion resistance, phosphatability, and weldability were evaluated for the steel sheets obtained from each embodiment and comparative example, and the evaluation results are listed in Table 5 below.
As indicated in Table 5 above, in embodiments 1 to 9 satisfying both the plating composition and manufacturing conditions of the present disclosure, by satisfying relational expressions 1 and 2 specified in the present disclosure, one or more properties of the plated steel sheet, comprising corrosion resistance, phosphatability and weldability, were superior to the comparative examples.
In particular, in embodiments 7 to 9, the weight % content of Mg from the surface of the plated layer to the 2/3t position in the thickness direction satisfied ±0.5 wt % based on an average weight % content of Mg in the plated layer, corrosion resistance was superior to that of embodiments 1 to 6.
Meanwhile, in comparative examples 1 to 4, which did not satisfy the plating composition of the present disclosure, one or more of the above-mentioned properties of corrosion resistance, phosphatability and weldability were poor.
Specifically, in comparative examples 1 and 3, as the Mg content of the plated layer was insufficient, corrosion resistance was the poorest, and as the value of [Al]1/10t—[Al]s defined from relational expression 2 was insufficient, phosphatability and weldability were poor.
Also, comparative examples 2 and 4 satisfied the Mg content of the plated layer and ensured corrosion resistance, but did not satisfy relational expressions 1 and 2, such that the Al content was excessive on the surface of the plated layer, and phosphatability and weldability were extremely poor.
Also, in comparative example 5 to 8, which satisfies the plating composition specified in the present disclosure but not satisfying the manufacturing conditions, the plating composition was satisfied such that corrosion resistance was assured, but as relational expressions 1 and 2 were not satisfied, phosphatability and weldability were poor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2021-0129894 | Sep 2021 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2022/014553 | 9/28/2022 | WO |
