US20130177780A1

US20130177780A1 - Hot Dip Plated Steel Sheet Having Excellent Plating Adhesiveness and Method of Manufacturing the Same

Info

Publication number: US20130177780A1
Application number: US13/824,525
Authority: US
Inventors: Rho-Bum Park; Jong-sang Kim; Joong-chul Park; Young-Ra Lee; Hong-Jong Yoo; Pan-Woo Seon; Sang-Kug Han
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2010-12-28
Filing date: 2011-12-05
Publication date: 2013-07-11
Also published as: WO2012091310A3; EP2659016A4; CN103282532A; EP2659016A2; JP2016084543A; WO2012091310A2; KR20120075260A; JP2014505168A

Abstract

Provided is a hot dip plated steel sheet used in automotive materials and a method of manufacturing the same, and more particularly, to a hot dip plated steel sheet having excellent platability and plating adhesiveness in which a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures is used as an underlying steel sheet, and a method of manufacturing the same. According to the present invention, a hot dip plated steel sheet having excellent platability and plating adhesiveness is provided, in which a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures is used as an underlying steel sheet, and thus, limitations in added amounts of silicon (Si), manganese (Mn), or aluminum (Al) may be mitigated. Therefore, development of new steels may be accelerated.

Description

TECHNICAL FIELD

The present invention relates to a hot dip plated steel sheet used in automotive materials and a method of manufacturing the same, and more particularly, to a hot dip plated steel sheet having excellent platability and plating adhesiveness, and a method of manufacturing the same.

BACKGROUND ART

Vehicle fuel economy regulations have recently been reinforced as environmental issues have become more important and accordingly, various, weight reduction methods for automobiles have been sought as measures to improve fuel economy.
For this purpose, steel manufacturers have tended to make great efforts to manufacture high strength steels in order to reduce the weight of steel sheets used in automotive materials, while assuring passenger safety.
In accordance with this trend, demand for high strength hot dip zinc (Zn) plated steel sheets for automotive bodies has greatly increased in recent years. However, although a typical method of manufacturing high strength steels using solid solution strengthening elements such as phosphorous (P) and manganese (Mn) is somewhat helpful for strengthening and lightening of plated steel sheets for automotive bodies, there are limitations in that the foregoing high strength steels are processed into automotive components requiring various shapes. Therefore, steels able to be formed as automotive components having complex shapes due to excellent processability, as well as having characteristics demonstrating high strength after forming, are required.
Such steels may include an advanced high strength steel (AHSS) or the like, such as a dual phase (DP) steel and a transformation induced plasticity (TRIP) steel, which have recently been developed and partially commercialized.
Advanced high strength steel is characterized by containing large amounts of elements, such as silicon (Si), manganese (Mn), and aluminum (Al), and for example, since Si acts as an element to maintain ductility without significantly reducing the strength thereof, Si is frequently added to obtain the foregoing property.
However, a hot dip Zn plated steel sheet manufactured through a typical method of hot dip Zn plating by using a steel sheet as an underlying steel sheet, in which Si is added as an alloy element in an amount of about 0.1 wt % or more, may have limitations in that bare spots or appearance defects may easily be generated.
The foregoing limitations are due to an environment of an annealing process, one of processes of manufacturing the hot dip Zn plated steel sheet. In the annealing process, a heat treatment is performed at a high temperature of about 800° C. while a reducing environment including 5% or more of hydrogen and nitrogen as a remainder is maintained (see JP1999-323443 and U.S. Pat. No. 5,137,586), and Si diffuses to a surface of the steel sheet during the foregoing high-temperature heat treatment process.
Therefore, a concentration of Si in the steel surface will be about 10 to 100 times higher than an average concentration of Si in the overall steel, and the Si concentrated on the surface of the steel sheet may react with moisture or impurities in a furnace atmosphere to form a SiO₂coating layer.
The SiO₂coating layer formed on the surface of the steel sheet may significantly decrease wettability of the steel sheet, and eventually, excellent platability may be difficult to secure. That is, the SiO₂coating layer formed on the surface of the steel sheet may generate a phenomenon of multiple bare spots or may significantly deteriorate plating adhesiveness even in the case that plating is possible, and therefore, may act as a cause of a plating delamination phenomenon, in which a plating layer is delaminated when the steel sheet is processed into a component.
Various techniques have been suggested for preventing the foregoing phenomenon of bare spots or the deterioration of plating adhesiveness caused by the foregoing oxide coating layer.
An example thereof may be a technique, in which amounts of alloy layers of Zn—Fe—Al—Si and Fe—Al—Si systems at an interface of underlying iron and plating layers are increased by increasing an added amount of Al in a hot dip Zn plating bath.
Since such alloy layers may cause a reduction of an oxide layer of an alloying element, the decrease of wettability due to the oxide coating layer at the interface may be prevented. However, a method of increasing Al in the plating bath may be inappropriate. The reason for this is that the increase in Al may be a cause of intergranular corrosion, together with lead (Pb) inevitably added in the plating bath as an impurity during the manufacturing of a mini spangle steel sheet.
Since intergranular corrosion may generate plating delamination and the increase of Al in the plating bath may further deteriorate weldability during processing of the steel sheet, there may be many difficulties in actually applying the foregoing related art.
Another related art plating process may include a technique in which an oxide coating layer is formed by introducing excessive air into a direct fired furnace in order to improve the platability of a Si-added steel, and a reduction treatment is then performed in a heating furnace having a reducing environment, to significantly improve platability.
An example of the foregoing technique was disclosed in Japanese Patent Application Laid-Open Publication No. 2001-226742, and according to the technique, stable platability may be secured because a pure iron layer is formed on the surface of a steel sheet, when a reduction heat treatment is performed after a thickness of iron oxide is increased by increasing an air ratio in a direct fired furnace from a typical value of 0.9 to 1.05.
However, in the related art, plating delamination may be instead be generated by the thick coating layer when the thickness of the oxide coating layer is not precisely controlled.
On the other hand, when the oxide coating layer is completely reduced by the reduction treatment because the oxide coating layer is thin, Si is concentrated as it is on the surface of the steel sheet and thus, a Zn plating layer is not strongly adhered to the surface of the steel sheet or bare spots may be generated.
Therefore, the thickness of the iron oxide in the direct fired furnace may be very precisely controlled.
Another related art may include a technique disclosed in Japanese Patent Application Laid-Open Publication No. 2010-1156590, the technique being a method of manufacturing a hot dip Zn plated steel sheet by oxidizing, reduction annealing, and hot dip Zn plating a cold-rolled steel sheet containing alloying elements such as Si and Mn, wherein the steel sheet is heated to obtain a steel sheet temperature of 550° C. or more in an environment, in which fuel gas including H₂and CH₄in a total amount of 50 vol % or more burns at an air-fuel ratio range of 1.01 to 1.5 during oxidation, and the steel sheet is also heated in an environment, which includes 1 vol % to 20 vol % of hydrogen having a dew point range of −50° C. to −10° C. and nitrogen as a remainder, during reduction annealing.
However, plating adhesiveness may also be insufficiently secured by means of the foregoing related art.

DISCLOSURE

Technical Problem

An aspect of the present invention provides a hot dip plated steel sheet having excellent platability and plating adhesiveness, in which a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures is used as an underlying steel sheet, and a method of manufacturing the hot dip plated steel sheet.

Technical Solution

According to an aspect of the present invention, there is provided a hot dip plated steel sheet having excellent plating adhesiveness including: a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures as an underlying steel sheet; and a plating material plated on the underlying steel sheet, wherein a discontinuous reduced iron (Fe) layer and a Fe-plating material alloy phase are formed at an interface of the underlying steel sheet and a plating layer.
Examples of the underlying steel sheet may be an annealed or a full hard cold-rolled steel sheet including one or more of silicon (Si), manganese (Mn), and aluminum (Al) in addition to Fe.
An example of the underlying steel sheet may be a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel, a complex phase (CP) steel, a martensitic (MART) steel, a twinning induced plasticity (TWIP) steel, etc.
The plating material may be a material including one or more of zinc (Zn), aluminum (Al), and magnesium (Mg) as main components.
Also, examples of the hot dip plated steel may be a hot dip Zn plated steel sheet, a hot dip Al plated steel sheet, a hot dip Zn—Al plated steel sheet, and hot dip alloy plated steel sheets of Al—Zn—Mg—Si, Al—Zn—Mg, Al—Mg, Zn—Mg, and the like.
According to another aspect of the present invention, there is provided a method of manufacturing a hot dip plated steel sheet having excellent plating adhesiveness including: oxidizing a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures by using a direct fired furnace at an air/fuel ratio or an air/gas ratio range of about 1.01 to about 1.5 and a steel sheet temperature range at an exit of the direct fired furnace of about 550° C. to about 750° C.; performing reduction annealing and hot dip plating; and performing a slight annealing treatment in a temperature range of 400° C. to about 550° C.
Examples of the underlying steel plate may be an annealed or a full hard cold-rolled steel sheet including one or more of Si, Mn, and Al in addition to Fe.
An example of the underlying steel sheet may be a DP steel, a TRIP steel, a CP steel, a MART steel, a TWIP steel, or the like.
Examples of the direct fired furnace may be a direct fired furnace (DFF) or a direct flame impingement (DFI) furnace.
The plating material may be a material including one or more of Zn, Al, and Mg as a main component.
Also, examples of the hot dip plated steel sheet may be a hot dip Zn plated steel sheet, a hot dip Al plated steel sheet, a hot dip Zn—Al plated steel sheet, and hot dip alloy plated steel sheets of Al—Zn—Mg—Si, Al—Zn—Mg, Al—Mg, Zn—Mg, and the like.

Advantageous Effects

According to the present invention, a hot dip plated steel sheet having excellent platability and plating adhesiveness, in which a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures is used as an underlying steel sheet, is provided, and thus, limitations in added amounts of Si, Mn, or Al may be mitigated. Therefore, development of new steels may be accelerated.

DESCRIPTION OF DRAWINGS

The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view illustrating a cross-section in a thickness direction of a hot dip zinc plated steel sheet manufactured according to a conventional method;

FIG. 2 is a schematic view illustrating a cross-section in a thickness direction of a hot dip zinc plated steel sheet manufactured according to an example of the present invention;

FIG. 3 are micrographs showing cross-sections in a thickness direction of a hot dip zinc plated steel sheet after a slight annealing treatment according to an example of the present invention;

FIG. 4 is a micrograph showing a cross-section in a thickness direction of a hot dip zinc plated steel sheet manufactured according to a conventional method; and

FIG. 5 is a micrograph showing a cross-section in a thickness direction of a hot dip zinc plated steel sheet manufactured according to an example of the present invention.

BEST MODE

Hereinafter, the present invention is described in more detail.
As described above, there are various types of hot dip plated steel sheets which may be applicable to the present invention, and hereinafter, a hot dip zinc (Zn) plated steel sheet, known as the most typical hot dip plated steel sheet, is described as an example.
When a steel sheet containing alloying elements, such as silicon (Si) and manganese (Mn), easily forming oxides on a surface of the steel sheet at high temperatures, is used as an underlying steel sheet, a iron (Fe) oxide layer is formed on the surface of the steel sheet in a direct fired furnace as in the foregoing related art, a reduced Fe layer is formed by reducing the Fe oxide layer in a reduction annealing process, and hot dip Zn plating is then performed, an oxide layer is continuously or mostly formed between the underlying steel sheet and a zinc plating layer as shown in FIG. 1.
The oxide layer formed at this time is not necessarily formed continuously, and any form may be applicable so long as the oxide layer may cause bare spots or plating delamination even in the case that the oxide layer is discontinuous.
Examples of the oxide layer formed between the underlying steel sheet and the Zn plating layer may be a oxide layer formed of mono oxide, in which Si, Mn, or aluminum (Al) are singly combined with oxygen, or a complex oxide, in which two or more of Si, Mn, or Al are combined with oxygen by being mixed with each other.
Hereinafter, the present invention and the related art are described according to an example of an underlying steel sheet containing Si and Mn as alloying elements among components easily forming oxides on the surface of the steel sheet.
The oxide layer formed between the underlying steel sheet and the Zn plating layer denotes an oxide layer formed singly of mono oxides such as SiO₂and MnO or complex oxides such as Mn₂SiO₄and MnSiO₃, or formed by mixing two or more thereof.
Herein, an oxide layer, formed by continuous or discontinuous concentration of the foregoing oxides on the surface, is described by through denotation as an Si—Mn oxide layer for convenience, as shown in FIGS. 1, 2, 4, and 5.
In the related art, although zinc platability is improved because diffusion of a Si—Mn oxide layer to a surface layer portion of a steel sheet is prevented due to a reduced Fe layer, delamination at an interface between the Si—Mn oxide layer and the reduced Fe layer may be facilitated as shown in FIG. 1 because the Si—Mn oxide layer and the reduced Fe layer lack adhesiveness.
The objective of the present invention is to improve plating adhesiveness as well as platability by using a hot dip Zn plated steel sheet, in which a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures is used as an underlying steel sheet and a plating material (hereinafter, Zn is described as an example) is plated on the underlying steel sheet, and forming discontinuous reduced Fe layer and Fe—Zn alloy phase, as shown in FIG. 2, at or near an interface of the underlying steel sheet and the hot dip Zn plating layer.
The Fe—Zn alloy phase, as shown in FIG. 2, is distributed from the interface of the plating layer/underlying steel sheet in a direction of a surface of the plating layer, and may be distributed so as not to be optically observed at the surface of the plated steel sheet.
For example, the Fe—Zn alloy phase, as shown in FIG. 2, is distributed from the interface of the plating layer/underlying steel sheet Pinto the zinc plating layer by penetrating a discontinuous oxide layer.
The Fe—Zn alloy phase may be distributed within a thickness of 60% of a total thickness of the plating layer from the interface of the plating layer/underlying steel sheet to a direction of the surface of the plating layer.
When the thickness of the Fe—Zn alloy phase is greater than 60% of the total thickness of the plating layer, corrosion resistance of the steel sheet may decrease because the alloy phase may be partially formed on the surface of the plating layer.
The hot dip Zn plated steel sheet of the present invention may include a discontinuous Si—Mn oxide layer and an Al—Fe inhibition layer in addition to the discontinuous reduced Fe layer and the Fe—Zn alloy phase between the underlying steel sheet and the Zn plating layer.
The discontinuous Si—Mn oxide layer and Al—Fe inhibition layer were layers observed in a relatively continuous form in the related art as in FIG. 1, but the Si—Mn oxide layer and Al—Fe inhibition layer are observed in a discontinuous form because the foregoing layers are partially destructed or diffused into the zinc plating layer while the Fe—Zn alloy phase is formed as in FIG. 2.
The hot dip Zn plated steel sheet of the present invention may include Si—Mn internal oxide, for example, up to a depth of 7 pm from the interface of the plating layer/underlying steel sheet in an inner direction of the underlying steel sheet.
Also, examples of the underlying steel sheet may be an annealed or full hard cold-rolled steel sheet including one or more of Si, Mn, and Al in addition to Fe.
An example of the underlying steel sheet may be a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel, a complex phase (CP) steel, a martensitic (MART) steel, a twinning induced plasticity (TWIP) steel, etc.
The plating material may be a material including one or more of Zn, Al, and magnesium (Mg) as main components.
Also, examples of the hot dip plated steel may be a hot dip Zn plated steel sheet, a hot dip Al plated steel sheet, a hot dip Zn—Al plated steel sheet, and hot dip alloy plated steel sheets of Al—Zn—Mg—Si, Al—Zn—Mg, Al—Mg, Zn—Mg, and the like.
As described above, excellent plating adhesiveness may be secured in the hot dip Zn plated steel sheet of the present invention, because wettability is improved due to the prevention of a diffusion amount of the Si—Mn oxide to the surface portion of the steel sheet by the reduced Fe layer, and also, after plating as shown in FIG. 3, the Fe—Zn alloy phase acts as a bridge between the steel sheet and the plating layer by destroying the continuous oxide layer and being formed inside the zinc plating layer from the interface of the plating layer/underlying steel sheet.
Hereinafter, a method of manufacturing a hot dip Zn plated steel sheet having excellent plating adhesiveness is described according to the present invention.
The method of manufacturing the hot dip Zn plated steel sheet of the present invention includes a process of oxidizing an underlying steel sheet by using a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures as the underlying steel sheet, a process of reduction annealing, a process of hot dip Zn plating, and a process of performing a slight annealing treatment.
Examples of the underlying steel plate may be an annealed or full hard cold-rolled steel sheet including one or more of Si, Mn, and Al in addition to Fe.
An example of the underlying steel sheet may be a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel, a complex phase (CP) steel, a martensitic (MART) steel, a twinning induced plasticity (TWIP) steel, etc.
The plating material may be a material including one or more of Zn, Al, and Mg as main components.
Also, examples of the hot dip plated steel may be a hot dip Zn plated steel sheet, a hot dip Al plated steel sheet, a hot dip Zn—Al plated steel sheet, and hot dip alloy plated steel sheets of Al—Zn—Mg—Si, Al—Zn—Mg, Al—Mg, Zn—Mg, and the like.
The oxidation process is performed under conditions of an air-fuel ratio of 1.01 to 1.5 in a direct fired furnace and a temperature of the steel sheet at an exit of the direct fired furnace is within a range of 550° C. to 750° C.
Examples of the direct fired furnace may be a direct fired furnace (DFF) or a direct flame impingement (DFI) furnace.
When the air-fuel ratio in the direct fired furnace is less than 1.01, oxidation of Fe may be insufficient, and when the air-fuel ratio is greater than 1.5, a backfire phenomenon of a heating apparatus of the direct fired furnace may occur. Therefore, the air-fuel ratio in the direct fired furnace may be limited to within a range of 1.01 to 1.5.
When the temperature of the steel sheet at the exit of the direct fired furnace is less than 550° C., prevention of the diffusion of oxide to a surface thereof may be difficult because the generation of internal oxidation of the steel sheet is insufficient, and when the temperature of the steel sheet at the exit of the direct fired furnace is greater than 750° C., an amount of a concentrated product may be too large because a diffusion rate of the oxide to the surface thereof is excessively increased while an amount of production may decrease because a line speed must be reduced in order to attain a target temperature. Therefore, the temperature of the steel sheet at the exit of the direct fired furnace may be limited to within a range of 550° C. to 750° C.
For example, when an inner portion of the direct fired furnace is divided into four regions, an air-fuel ratio of a third region from an entrance of an incoming steel sheet among the four regions may be within a range of 1.1 to 1.4, and an air-fuel ratio of a fourth region may be within a range of 1.1 to 1.3.
At this time, a DFF or DFI apparatus, the direct fired furnace described in the present invention, is mainly positioned at a first half part of a heating section. However, any position may be used so long as a targeted heat treatment temperature may be secured and a Fe oxide layer may be formed.
When the steel sheet is oxidized in the direct fired furnace as described above, a Fe oxide layer is formed on the surface of the steel sheet and the Fe oxide layer is reduced in a reduction annealing process to be formed as a reduced Fe layer.
Diffusion of oxides of alloying elements contained in the underlying steel sheet, for example, Si—Mn oxides, is prevented by forming a reduced Fe layer and therefore, excellent platability may be secured.
The reduction annealing process in the present invention is not particularly limited and the reduction annealing process is irrelevant to apparatus types and operating conditions so long as mechanical properties of the steel sheet may be secured and the reduced Fe layer may be formed by continuously performing heat treatments.
For example, a general annealing heat treatment apparatus is composed of a preheating section, a heating section, a soaking section, a fast cooling section, a slow cooling section, an overaging section, or a reheating section, and the arrangement and numbers of the foregoing heat treatment sections may be changed if necessary.
At this time, when oxidation and a reduction annealing heat treatment are performed by using a steel sheet containing alloying elements forming oxides on the surface of the steel sheet at high temperatures as an underlying steel sheet, diffusion rates of Si—Mn oxides to the surface portion of the steel sheet increase as temperature increases.
For example, with respect to a hot dip Zn plating process, a section having the highest heat treatment temperature is a soaking section and a soaking heat treatment is performed while the temperature thereof is typically maintained at a range of 780° C. to 850° C. for 50 seconds to 100 seconds.
The hot dip Zn plating process is not particularly limited in the present invention, and the plating is typically performed by dipping for about 3 seconds to 5 seconds in a plating bath having a composition of 0.12% to 0.25% of Al and Zn as a remainder and a temperature of 460° C. to 470° C.
At this time, there is a general operating condition with respect to a hot dip plated steel sheet having a different plating material. For example, the operating condition may be changed according to melting points of alloying elements, such as a plating bath having 5% to 12% of Si and Al as a remainder at a temperature of 660° C. to 680° C. with respect to a hot dip Al plated steel sheet and a plating bath having 50% to 60% of Al and 40% to 50% of Zn at a temperature of 590° C. to 610° C. with respect to a hot dip Al—Zn plated steel sheet, and capacity of an apparatus.
Also, in the present invention, the underlying steel sheet is hot dip Zn plated as described above, and a slight annealing treatment is then performed in order to form the reduced Fe layer and Fe—Zn alloy phase at the interface between the underlying steel plate and hot dip Zn plating layer. At this time, a temperature of the slight annealing treatment may be limited within a range of 400° C. to 550° C.
When the slight annealing treatment temperature is less than 400° C., plating delamination may be generated because a bridging effect_between the underlying steel sheet and the plating layer is insufficient due to the less formation of the Fe—Zn alloy phase at the interface of the underlying steel sheet/plating layer. When the slight annealing treatment temperature is greater than 550° C., a decrease in corrosion resistance and a non-uniform appearance of the steel sheet may occur because the thickness of the Fe—Zn alloy phase is greater than 60% of the total thickness of the plating layer and the Fe—Zn alloy phase is partially formed at the surface of the plating layer.
The slight annealing treatment temperature, for example, may be within a range of 440° C. to 500° C.
Any apparatus may be used for the slight annealing treatment so long as the apparatus may perform a slight annealing reaction by being positioned at a certain position past an air knife and heating the steel sheet to a temperature range of 400° C. to 550° C. within a few seconds after hot dip plating. However, it may be effective to perform the slight annealing treatment by using a GA heater installed in a typical plating line or an apparatus having a function similar thereto.
Also, a discontinuous Si—Mn oxide layer and an Al—Fe inhibition layer may exist at the interface between the plating layer and the underlying steel sheet.
Hereinafter, the present invention is described in more detail according to an example.

Example

In the present invention, 1.0 mm thick full hard cold-rolled steel sheets having compositions of the following Table 1 were used as underlying steel sheets and oxidized under the conditions of the following Table 2 in a direct fired furnace, and reduction annealing and hot dip plating were then performed under typical operating conditions.
At this time, with respect to sample numbers 1 to 5 in the following Table 1, the reduction annealing was performed in a mixed gas environment of 10% H₂-90% N₂having a dew point of −45° C., and in particular, a heat treatment condition of a soaking section was maintained at 800° C. for 1 minute.
Meanwhile, with respect to sample numbers 6 to 8 in the following Table 1, the reduction annealing was performed in a mixed gas environment of 25% H₂-75% N₂having a dew point of −45° C., and a heat treatment condition of a soaking section was also maintained at 800° C. for 1 minute.
Also, the hot dip plating was performed by dipping in a plating bath, which had plating material compositions of the following Table 1 and was sufficiently melted, for 5 seconds.
Further, hot dip plated steel sheets manufactured under the foregoing conditions were subjected to slight annealing treatments with the conditions of Table 2 to manufacture respective hot dip Zn plated steel sheets.
The presence of bare spots and plating adhesiveness were investigated on the hot dip Zn plated steel sheets thus manufactured and the results thereof are presented in the following Table 2.
Herein, methods and criteria of evaluating plating quality are as below, respectively.
Platability: as a property of easily coating a hot dip plating material on an underlying steel sheet, an appearance of a plated steel sheet was optically observed and evaluation criteria are as below.
Grades 1 to 2: no bare spots, a level for an automotive outer panel
Grades 3 to 5: observation of ultra fine bare spots, a level for an automotive inner panel and other products
Non-grade: observation of small bare spots, not a product level
Plating adhesiveness: as a property of evaluating whether a plating layer adhered to an underlying steel sheet generates a delamination phenomenon or not, whether or not a delaminated plating layer peeled off with a tape was observed when the tape was adhered to a bending portion and detached after O-T bending of a plated steel sheet, and evaluation criteria are as below.
⊚: very good, no overall delamination

- ◯: good, generation of delamination only within spots 5 mm away from an edge portion (it is fine if a side trimming treatment is performed during production of products)

X: generation of overall delamination
In consideration of other operability and stability, cases of the generation of particular problems were also observed and evaluation criteria are as below.
◯: fine
X: generation of problems (a back fire phenomenon in a direct fired furnace)
Meanwhile, a cross-sectional micrograph of a hot dip Zn plated steel sheet manufactured according to a conventional example using a conventional method in a thickness direction was observed, and the result thereof is presented in FIG. 4. A cross-sectional micrograph of a hot dip Zn plated steel sheet manufactured according to an example of the present invention in a thickness direction was observed, and the result thereof is presented in FIG. 5.

	TABLE 1

	Underlying Steel Sheet	Hot Dip Plated Steel Sheet

	Chemical		Plating
Sample	Composition (wt %)		Material Composition

No.	C	Mn	Si	Al	Steels	Zn	Al	Mg	Si	Type

1	0.2	1.5	1.5	0.05	TRIP	99.8	0.2	—	—	Hot dip Zn plated
										steel sheet
2	0.1	2.0	0.1	0.05	DP	99.8	0.2	—	—	Hot dip Zn plated
										steel sheet
3	0.2	2.5	0.2	0.05	CP	99.8	0.2	—	—	Hot dip Zn plated
										steel sheet
4	0.2	1.6	0.05	0.02	MART	99.8	0.2	—	—	Hot dip Zn plated
										steel sheet
5	0.7	15.0	0.5	2.0	TWIP	99.8	0.2	—	—	Hot dip Zn plated
										steel sheet
6	0.2	1.5	1.5	0.05	TRIP	—	91	—	9	Hot dip Al plated
										steel sheet
7	0.2	1.5	1.5	0.05	TRIP	43	55	—	2	Hot dip Al—Zn
										plated steel
										sheet
8	0.2	1.5	1.5	0.05	TRIP	90	6	3	1	Hot dip alloy
										plated steel
										sheet

	TABLE 2

	Direct fired furnace	Slight

Air-fuel

annealing

Plating quality

ratio

Steel sheet

Surface

	Sample	3^rd	4^th	temperature	temperature	appearance	Plating	Others
Example No.	No.	region	region	(exit, ° C.)	(exit, ° C.)	(grade)	adhesiveness	(operability)

Conventional	1	0.98	0.98	580	No	No grade	X	◯
Example1 1					treatment
Conventional	1	1.3	1.1	620	No	1-2	X	◯
Example1 2					treatment
Inventive	1	1.01	1.3	620	460	1-2	⊚	◯
Example1 1
Inventive	1	1.3	1.01	620	460	1-2	⊚	◯
Example1 2
Inventive	1	1.50	1.1	620	460	1-2	⊚	◯
Example1 3
Comparative	1	1.55	1.1	620	460	—	—	X(backfire)
Example 1
Comparative	1	1.3	1.1	530	460	3-5	X	◯
Example 2
Inventive	1	1.3	1.1	550	460	1-2	⊚	◯
Example1 4
Inventive	1	1.3	1.1	750	460	1-2	◯	◯
Example1 5
Comparative	1	1.3	1.1	770	460	1-3	X	◯
Example 3
Comparative	1	1.3	1.1	620	380	1-3	X	◯
Example 4
Inventive	1	1.3	1.1	620	400	1-2	◯	◯
Example1 6
Inventive	1	1.3	1.1	620	550	3-5	⊚	◯
Example1 7
Comparative	1	1.3	1.1	620	570	No grade	⊚	◯
Example 5
Inventive	2	1.3	1.1	620	460	1-2	⊚	◯
Example1 8
Inventive	3	1.3	1.1	620	460	1-2	⊚	◯
Example1 9
Inventive	4	1.3	1.1	620	460	1-2	◯	◯
Example1 10
Inventive	5	1.3	1.1	620	460	3-5	◯	◯
Example1 11
Inventive	6	1.3	1.1	620	460	3-5	⊚	◯
Example1 12
Inventive	7	1.3	1.1	620	460	3-5	⊚	◯
Example1 13
Inventive	8	1.3	1.1	620	460	3-5	◯	◯
Example1 14

As shown in Table 2, with respect to the hot dip Zn plated steel sheet manufactured according to Conventional Example 2 using a conventional method, it may be understood that a surface appearance of the plated steel sheet was grades 1 to 3 such that platability was good, but plating adhesiveness was not.
Meanwhile, with respect to the hot dip Zn plated steel sheet manufactured according to Inventive Examples corresponding to the present invention, it may be understood that the plated steel sheet had a good surface appearance as well as having good plating adhesiveness.
Also, as shown in FIG. 4, a continuous and dense Si—Mn oxide layer was observed in the hot dip Zn plated steel sheet manufactured according to a conventional example, while it may be understood that a Fe—Zn alloy phase and a discontinuous Si—Mn oxide layer were observed in the hot dip Zn plated steel sheet manufactured according to an inventive example as shown in FIG. 5.
As shown in FIG. 5, in the present inventive example, the Si—Mn oxide layer was destroyed by the formation of the Fe—Zn alloy phase and became discontinuous or weakened, and therefore, plating adhesiveness of the hot dip Zn plated steel sheet may be more improved.
While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A hot dip plated steel sheet having excellent plating adhesiveness comprising:

a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures as an underlying steel sheet; and

a plating material plated on the underlying steel sheet,

wherein a discontinuous reduced iron (Fe) layer and a Fe-plating material alloy phase are formed at an interface of the underlying steel sheet and a plating layer.

2. The hot dip plated steel sheet having excellent plating adhesiveness of claim 1, wherein the underlying steel sheet comprises one or more alloying elements of silicon (Si), manganese (Mn), and aluminum (Al).

3. The hot dip plated steel sheet having excellent plating adhesiveness of claim 1, wherein the underlying steel sheet is any one of a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel, a complex phase (CP) steel, a martensitic (MART) steel; and a twinning induced plasticity (TWIP) steel.

4. The hot dip plated steel sheet having excellent plating adhesiveness of claim 1, wherein the plating material comprises one or more of zinc (Zn), aluminum (Al), and magnesium (Mg) as main components.

5. The hot dip plated steel sheet having excellent plating adhesiveness of claim 1, wherein a discontinuous Si—Mn oxide layer is formed between the underlying steel sheet and the plating layer.

6. The hot dip plated steel sheet having excellent plating adhesiveness of claim 1, wherein the Fe-plating material alloy phase is distributed within a thickness of 60% of a total thickness of the plating layer from the interface of the plating layer and the underlying steel sheet to a direction of a surface of the plating layer.

7. The hot dip plated steel sheet having excellent plating adhesiveness of claim 1, wherein a discontinuous Al—Fe inhibition layer is distributed between the underlying steel sheet and the plating layer.

8. The hot dip plated steel sheet having excellent plating adhesiveness of claim 1, wherein a Si—Mn internal oxide is distributed up to a depth of 7 μm from the interface of the plating layer and the underlying steel sheet to an inner direction of the underlying steel sheet.

9. A method of manufacturing a hot dip plated steel sheet having excellent plating adhesiveness, the method comprising:

oxidizing a steel sheet containing alloying elements forming oxides on a surface of the steel sheet at high temperatures by using a direct fired furnace at an air/fuel ratio or an air/gas ratio range of about 1.01 to about 1.5 and a steel sheet temperature range at an exit of the direct fired furnace of about 550° C. to about 750° C.;

performing reduction annealing and hot dip plating; and

performing a slight annealing treatment in a temperature range of 400° C. to about 550° C.

10. The method of claim 9, wherein an underlying steel sheet comprises one or more of silicon (Si), manganese (Mn), and aluminum (Al).

11. The method of claim 9, wherein the underlying steel sheet is any one of a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel, a complex phase (CP) steel, a martensitic (MART) steel, and a twinning induced plasticity (TWIP) steel.

12. The method of claim 9, wherein a temperature of the slight annealing treatment is within a range of 440° C. to about 500° C.

13. The method of claim 9, wherein the plating material comprises one or more of zinc (Zn), aluminum (Al), and magnesium (Mg) as a main component.