Patent Description:
As a zinc plating method which may prevent corrosion of iron through a cathodic protection method has excellent method performance and is highly economical, the method has been widely used for manufacturing a steel material having high corrosion resistance properties, and the consumption of a galvanized steel material plated with zinc has increased in overall industrial fields of vehicles, home appliances, construction materials, and the like.

A galvanized steel material may have sacrificial corrosion protection properties such that, when a galvanized steel material is exposed to a corrosion environment, zinc having oxidation-reduction potential lower than oxidation-reduction potential of iron may be corroded first, and corrosion of iron may be prevented. Also, a galvanized steel material may form a dense corrosion product on a surface of the steel material as zinc of a plating layer is oxidized, and the steel material may be protected from an oxidization atmosphere such that corrosion resistance of the steel material may improve.

However, air pollution has increased and a corrosion environment has been increasingly deteriorated due to high industrialization, and the demand for developing a steel material having more improved corrosion resistance than that of a conventional hot-dip galvanized steel material has increased due to strict regulations on saving resources and energy. Accordingly, various studies of a technique of manufacturing a zinc alloy plated steel material which may improve corrosion resistance of a steel material by adding elements such as magnesium (Mg), and the like, to a plating layer have been conducted.

Meanwhile, generally, a galvanized steel material or an zinc alloy plated steel material (hereinafter, referred to as "zinc-based plated steel") may be processed to a component through processing, and the like, may be welded through spot-welding, and may be used as a product. However, in the case of a zinc-based plated steel using a high strength steel material including austenite or retained austenite as a microstructure, a high P added high strength IF (Interstitial Free) steel material, and the like, as a base, there may be the problem of LME (liquid metal embrittlement) in which hot-dip state zinc may permeate along a grain boundary of a base steel during spot-welding and may cause embrittlement cracks.

Patent documents <CIT>, <CIT> and <CIT> disclose zinc alloy plated steels comprising: a base steel; a Zn plated layer formed on the base steel; and a Zn-Mg alloy plated layer formed on the Zn plated layer and obtained through mutual diffusion of Zn and Mg. A method for making this coating using PVD techniques is also disclosed in these documents.

<FIG> is an image obtained by magnifying and observing a welded portion of a welding member in which LME cracks occur by a spot-welding process. In <FIG>, cracks created on upper and lower portions of nugget may be referred to as Type A cracks, cracks created on a welding shoulder portion may be referred to as Type B cracks, and cracks created in a steel sheet by misalignment of electrodes during welding may be referred to as Type C cracks. As Type B and C cracks may greatly affect stiffness of a material, the prevention of the cracks during welding may be a key requirement matter in the corresponding technical field.

An aspect of the present disclosure is to provide a zinc alloy plated steel having excellent weldability and corrosion resistance.

According to an aspect of the present invention, a zinc alloy plated steel is provided, the zinc alloy plated steel including a base steel, a Zn plated layer formed on the base steel, and a Zn-Mg alloy plated layer formed on the Zn plated layer and obtained through mutual diffusion of Zn and Mg, and a ratio of weight of Mg contained in the Zn-Mg alloy plated layer based on a total weight of the Zn plated layer and the Zn-Mg alloy plated layer is <NUM> to <NUM>, and a sum of coating amounts of the Zn plated layer and the Zn-Mg alloy plated layer is <NUM> to <NUM>/m<NUM>.

According to another aspect of the present disclosure, a method of manufacturing a zinc alloy plated steel according to claim <NUM> is provided, the method including preparing a Zn plated steel sheet including a Zn plated layer formed thereon, generating deposition vapor by lifting and heating a coating material by electromagnetic force in a vacuum chamber, and forming an Mg deposition layer by induction-spraying the deposition vapor on a surface of the Zn plated layer, forming a Zn-Mg alloy layer by performing a heat treatment on the Zn plated steel sheet including the Mg deposition layer formed thereon at a temperature of <NUM> or higher to less than <NUM>, and a ratio of a weight of the Mg deposition layer based on a total weight of the Zn plated layer and the Mg deposition layer is <NUM> to <NUM>, and a sum of coating amounts of the Zn plated layer and the Zn-Mg alloy plated layer is <NUM> to <NUM>/m2.

As one of effects of the present invention, a zinc alloy plated steel according to the present disclosure may have an advantage in that the zinc alloy plated steel may have excellent weldability, and accordingly, even when the zinc alloy plated steel uses a high strength steel material including austenite or retained austenite as a microstructure, a high P added high strength Interstitial Free (IF) steel material, and the like, as a base, liquid metal embrittlement (LME) may be effectively prevented.

Also, a multilayer zinc alloy plated steel according to the present disclosure may secure excellent corrosion resistance with even a small coating amount, and may thus be eco-friendly and highly economical, which may also be advantageous.

The advantages and effect of the present disclosure are not limited to those described above, and may be more easily understood in the process of describing specific example embodiments.

In the case of a Zn-Mg alloy plated steel, an increased content of Mg may be more advantageous to corrosion resistance, but may be disadvantageous to weldability. Accordingly, generally, a content of Mg in a plated layer has been managed to be <NUM> wt% as a maximum content. That is because a Zn-Mg based intermetallic compound having a low melting point in a Zn-Mg alloy plated layer may be easily dissolved and may cause liquid metal embrittlement. However, as a result of additional research of the inventors of the present disclosure, the inventors have found that, even when a content of Mg in a plated layer exceeds <NUM> wt%, if an average content thereof is within a certain range, and an average grain size of grains forming the Zn-Mg alloy plated layer is within a certain range, weldability may rather significantly improve, and the present invention is suggested.

In the description below, a zinc alloy plated steel having excellent weldability and corrosion resistance according to the claim <NUM> will be described in detail.

The zinc alloy plated steel includes a base steel and a Zn plated layer and a Zn-Mg alloy plated layer sequentially formed on the base steel. In the present invention, a form of the base steel may not be particularly limited. For example, the base steel may be a steel sheet or a steel wire rod.

Also, in the present disclosure, a composition of the base steel may not be particularly limited as well. As an example, the base steel may include, by wt%, <NUM> to <NUM>% of C, <NUM> to <NUM>% of Si, <NUM> to <NUM>% of Mn, <NUM> to <NUM>% of Al, <NUM>% or less of P (excluding <NUM>%), <NUM>% or less of S (excluding <NUM>%), and a balance of Fe and inevitable impurities. In this case, the contents of C, Si, Mn, P, and S may satisfy Relational Expression <NUM> below. Meanwhile, the base steel having the composition as described above may include austenite or retained austenite as a microstructure. <MAT> (where [C], [Mn], [Si], [P], and [S] each indicate a content (wt%) of each element).

When the base steel has the above-described alloy composition and microstructure, liquid metal embrittlement (LME) may become a problem during spot-welding, and the reason is as follows. An austenite or retained austenite microstructure may have a vulnerable grain boundary as compared to another microstructure. When stress is applied by spot-welding, liquid molten zinc may permeate a grain boundary of an austenite or retained austenite microstructure on a welded portion and may cause cracks, and liquid metal embrittlement may occur.

However, as described below, in the present disclosure, as the time for which liquid molten zinc remains is significantly reduced, even when a zinc alloy plated steel is manufactured using a steel material having the above-described alloy composition and microstructure as a base, liquid metal embrittlement may be effectively prevented. Also, the present disclosure may also be applied even when an alloy composition of the base steel does not satisfy the above-described range.

The Zn plated layer may be formed on the base steel and may protect the base steel from a corrosion environment. The Zn plated layer may be formed by one of an electroplating method and a physical vapor deposition (PVD) method.

When the Zn plated layer is formed by a hot-dip plating method, Fe<NUM>Al<NUM> with high resistance may be inevitably present on an interfacial surface between the base steel and the Zn plated layer, non-conductive Al<NUM>O<NUM> may be created on electrodes during welding, and a thickness deviation of the plated layer may relatively increase, which may be disadvantageous in terms of spot-weldability. In consideration thereof, the Zn plated layer may be an electroplated layer or a plated layer formed by a physical vapor deposition method.

The Zn-Mg alloy plated layer is formed on the Zn plated layer, and is obtained by mutual diffusion between Zn and Mg in the Zn plated layer and an Mg deposition layer as described below.

In the present invention, a ratio of a weight of Mg contained in the Zn-Mg alloy plated layer based on a total weight of the Zn plated layer and the Zn-Mg alloy plated layer is <NUM> to <NUM>. A more preferable ratio of a weight of Mg may be <NUM> to <NUM>.

The Zn-Mg alloy plated layer may include a Zn single phase, an Mg single phase, an Mg<NUM>Zn<NUM> alloy phase, an MgZn<NUM> alloy phase, an MgZn alloy phase, an Mg<NUM>Zn<NUM> alloy phase, and the like. The inventors of the present disclosure have found that, when a ratio of a weight of Mg contained in the Zn-Mg alloy plated layer based on a total weight of the Zn plated layer and the Zn-Mg alloy plated layer is controlled to be within the above-described range, the Zn plated layer and the Zn-Mg alloy plated layer on a welded portion may be melted and transformed to an alloy layer formed as a single layer including an MgZn<NUM> alloy phase of <NUM> area% or higher (including <NUM> area%) during spot-welding, and that, in this case, liquid metal embrittlement (LME) may be effectively prevented. Presumably, that is because the time for which the melted plated layer remains as liquid may be significantly reduced as a melding point of the plated layer is high, as indicated in <FIG> illustrating a phase diagram of Mg-Zn binary-system alloy. Meanwhile, in the present disclosure, a residual microstructure other than an MgZn<NUM> alloy phase in the plated layer on the welded portion may not be particularly limited. However, according to an example embodiment, but not limited thereto, a residual microstructure other than an MgZn<NUM> alloy phase may be an Mg<NUM>Zn<NUM> alloy phase.

A phase fraction may be analyzed and measured by a standardless Rietveld quantitative analysis method using a general XRD and also using a more accurate TEM-ASTAR (TEM-based crystal orientation mapping technique), but the method may not be limited thereto. Meanwhile, a phase transformation process of the the Zn-Mg alloy plated layer may be analyzed using a high-temperature in-situ radiation XRD. More specifically, a phase transformation process of the Zn-Mg alloy plated layer may be analyzed by, while heating a sample at heating rates of <NUM>/sec and <NUM>/sec, and at a heating temperature of <NUM>, consecutively measuring an XRD spectrum of each frame per <NUM> second throughout <NUM> frames during a heating and cooling thermal cycle. However, an example analysis method is mot limited thereto.

According to the result of additional research of the inventors of the present invention, an average grain size of grains forming the Zn-Mg alloy plated layer may significantly affect corrosion resistance of a plated steel material. <FIG> is a diagram illustrating a process of corrosion of a plated steel material. <FIG> is a diagram illustrating an example in which a grain size is fine, and.

<FIG> is an example in which a grain size is coarse. Referring to <FIG>, it has been found that, when a grain size is fine, a relatively dense and uniform corrosion product may be formed during corrosion, which may be relatively helpful to delay corrosion.

Also, an average grain size of grains forming the Zn-Mg alloy plated layer may significantly affect weldability of a plated steel material. When an average grain size of grains is a certain level or lower, the creation of Type B cracks may be significantly reduced, and presumably, that is because the active movement of atoms in the melted plated layer may be advantageous to securing an aimed microstructure.

As described above, considering both corrosion resistance and weldability of the plated steel material, it may be required to appropriately manage an upper limit of an average grain size of grains forming the Zn-Mg alloy plated layer, and it may be preferable to control an average grain size of grains forming the Zn-Mg alloy plated layer to be <NUM> or less. The average grain size may refer to an average long diameter of grains detected by observing a cross-sectional surface of the plated layer taken in a thickness direction.

According to the invention, a sum of coating amounts of the Zn plated layer and the Zn-Mg alloy plated layer is <NUM> to <NUM>/m2. The greater the sum of coating amounts of the Zn plated layer and the Zn-Mg alloy plated layer, it may be more advantageous in terms of corrosion resistance. However, due to an increased coating amount, liquid metal embrittlement (LME) may be caused during spot-welding. Thus, considering the aspect of weldability, an upper limit thereof may be limited as above. Meanwhile, a more preferable range of a sum of coating amounts of the Zn plated layer and the Zn-Mg alloy plated layer may be <NUM> to <NUM>/m2.

The zinc alloy plated steel described above is manufactured by the method as below.

Foreign objects on a surface of the base steel may be removed by washing, rinsing, and drying the surface using an HCl solution of <NUM> weight% or higher, a natural oxide film may be removed using a plasma and an ion beam, and the surface is plated with Zn, thereby preparing a Zn plated steel material including a Zn plated layer formed thereon. As described above, the Zn plated layer on the base steel may be formed by an electroplating method or a physical vapor deposition method.

An Mg deposition layer is formed on the Zn plated layer. The Mg deposition layer is formed by an electromagnetic levitation physical vapor deposition method having an electromagnetic stirring effect preferably.

The electromagnetic levitation physical vapor deposition method refers to a method using a phenomenon in which, when electromagnetic force is generated by applying radio frequency power to a pair of electromagnetic coils generating an alternating current electromagnetic field, a coating material (Mg in the present disclosure) may be lifted in the air in the space surrounded by an alternating current electromagnetic field without any external help, and the lifted coating material may generate a large amount of deposition vapor (metal vapor). <FIG> is a diagram illustrating a device for such electromagnetic levitation physical vapor deposition. Referring to <FIG>, a large amount of deposition vapor formed by the above-described method may be sprayed onto a surface of a material to be coated through a plurality of nozzles of a vapor distribution box at a high speed and may form a deposition layer.

In a general vacuum deposition device, a coating material may be provided in a crucible, and vaporization of the coating material may be performed by heating the crucible including the coating material. In this case, there may be a difficulty in providing sufficient thermal energy to the coating material because of dissolution of the crucible, heat loss caused by the crucible, and the like. Accordingly, a deposition speed may be slow, and there may be a certain limitation in refining a size of grains forming the deposition layer.

However, when the deposition is performed by an electromagnetic levitation physical vapor deposition method, differently from a general vacuum deposition method, there may be no constraint conditions related to a temperature, and the coating material may be exposed to a higher temperature. Accordingly, a high speed deposition may be performed, and consequently, the refinement of a size of grains forming the formed deposition layer may be achieved, which may be advantageous.

During the deposition process, a degree of vacuum in the vacuum chamber is adjusted to <NUM> ×<NUM>-<NUM>mbar to <NUM> ×<NUM>-<NUM>mbar. In this case, an increase of embrittlement and degradation of properties caused by an oxide formed during forming the deposition layer may be effectively prevented.

During the deposition process, it is adjusted the temperature of the lifted coating material to be <NUM> or higher, and it may be more preferable to adjust the temperature to be <NUM> or higher, and it may be even more preferable to adjust the temperature to be <NUM> or higher. When the temperature is less than <NUM>, there may be the problem in which an effect of refinement of grains may not be sufficiently secured. Meanwhile, the higher the temperature of the lifted coating material, it may be more advantageous to achieve a technical effect aimed in the present disclosure, and thus, in the present disclosure, an upper limit thereof may not be particularly limited. However, when the temperature is a certain level or higher, the effect thereof may be saturated, and process costs may significantly increase. In consideration thereof, an upper temperature limited to <NUM>.

It may be preferable to adjust a temperature of the Zn plated steel material to be <NUM> or lower before and after the deposition. When the temperature exceeds <NUM>, the maintained degree of vacuum may be interfered with by bending in a width direction caused by unevenness of a temperature of a width steel sheet while passing through a multiple-stage differentiated pressure reducing system at an exit side.

A heat treatment is performed on the Zn plated steel material including the Mg deposition layer formed thereon at <NUM> or higher, and a Zn-Mg alloy plated layer may be formed. The reason why the heat treatment temperature is limited to <NUM> or higher is that, when the heat treatment temperature is less than <NUM>, mutual diffusion between Zn and Mg in the Zn plated layer and the Mg deposition layer may not be performed easily. Meanwhile, an upper limit of the heat treatment temperature may not be particularly limited in the present disclosure, but when the temperature is <NUM> or higher, a brittle zinc and icon alloy phase may be formed on an interfacial surface between the base steel and the Zn plated layer, and it may be disadvantageous in terms of sealer adhesion properties. In consideration thereof, an upper limit thereof is limited to less than <NUM>.

In the present invention, the heat treatment method may be an induction heat method or an infrared heating method, and a heating time may be <NUM> seconds to <NUM> seconds. When the heating time is less than <NUM> seconds, the alloying may not be sufficiently performed such that the Mg deposition layer may partially remain. When the heating time exceeds <NUM> seconds, the steel sheet and the Zn plated layer may be alloyed with each other.

In the description below, an embodiment of the present disclosure will be described in detail. The below embodiment is provided for understanding of the present disclosure, and will not limit the present disclosure.

An electro-galvanized plated steel sheet having an electro-galvanized layer formed thereon and having a thickness as in Table <NUM> below was prepared on a high strength cold-rolled steel sheet used for vehicles, including, by wt%, <NUM>% of C, <NUM>% of Si, <NUM>% of Mn, <NUM>% of Al, <NUM>% of P, <NUM>% of S, and a balance of Fe and inevitable impurities, and having a thickness of <NUM>, and an Mg deposition layer having a thickness as in Table <NUM> below was formed using a device (a degree of vacuum, <NUM> ×<NUM>-<NUM>mbar) illustrated in <FIG>. In overall embodiments, when the Mg deposition layer is formed, a current applied to a pair of electromagnetic coils was <NUM>. 2kA, a frequency applied to a pair of electromagnetic coils was <NUM> with reference to <NUM> of a deposition material, a temperature of a lifted coating material was <NUM>, and a temperature of vapor distribution box was <NUM>, constantly. Also, a temperature of the base steel before and after the deposition was maintained at <NUM>. The electro-galvanized plated steel sheet including the Mg deposition layer passed through an exit strip-lock and was discharged into the air, and an alloying heat treatment was performed on the electro-galvanized plated steel sheet in a heat treatment zone using an induction heating process. In overall embodiments, a heat treatment temperature was <NUM>, and a heat treatment time was <NUM> seconds, constantly.

Thereafter, a total coating amount and a ratio of a weight of Mg of the manufactured zinc alloy plated steel were measured by an ICP (inductively coupled plasma) method. More specifically, the steel material was cut into a sample of a size of 80mmX80mm, a surface was degreased, and a primary weighing process (W1: <NUM>) was performed using a high precision scale. Then, an O-Ring of <NUM> dia dedicated column was attached with a clamp on a front portion of specimen and was adhered to prevent leakage of a solution. A 30cc of <NUM>:<NUM> HCl solution was input, and <NUM> or <NUM> drops of inhibitor were input. After the generation of a H<NUM> gas was terminated, the solution was collected in a 100cc mass flask. All residue amount of the solution on the surface was collected using a wash bottle, and 100cc or less of the solution was collected. The sample was completely dried, a secondary weighing process (W2) was performed, and a value obtained by dividing a difference between a primary weighing value and a secondary weighing value by a unit area was determined as a total coating amount of the zinc alloy plated steel material. A content of Mg in the collected solution was measured by a ICP method, and the obtained result was determined as a ratio of a content of Mg.

An average grain size of grains forming the Zn-Mg alloy plated layer was measured. As a result of the measurement, an average grain size of grains forming the Zn-Mg alloy plated layer was <NUM> or less.

Weldability and corrosion resistance of the manufactured zinc alloy plated steel were tested, and the results were listed in Table <NUM> below as well.

More specifically, as for weldability, the steel material was cut into a sample of a size of 40mmX120mm in accordance with SEP <NUM>-<NUM> standard, spot-welding was performed <NUM> times on each sample, a presence and a size of Type B crack were measured, and the results were examined in accordance with the references as below.

As for corrosion resistance, each of the multilayer zinc alloy plated steels was cut into a sample of a size of 75mmX150mm, a salt spray test was performed under JIS Z2371, an initial red rust generation time was measured, and the result was examined in accordance with the references as below.

Referring to Table <NUM>, inventive example <NUM> to <NUM> satisfying the overall conditions suggested in the present disclosure had excellent corrosion resistance and spot-weldability. Further, to secure more excellent spot-weldability, it has been found that a preferable ratio of a content of Mg may be <NUM> to <NUM>, and that it may be preferable to control a sum of coating amounts of the Zn plated layer and the Zn-Mg alloy plated layer to be <NUM>/m<NUM> or lower.

As for comparative examples <NUM> to <NUM>, a ratio of a content of Mg was beyond the range suggested in the present disclosure, and spot-weldability was deteriorated.

<FIG> is an image obtained by observing a welded portion after spot-welding the zinc alloy plated steel material of inventive example <NUM>. Referring to <FIG>, it may be visually observed that Type B cracks and Type C cracks were not created at all in the welded portion in the zinc alloy plated steel material.

Claim 1:
A zinc alloy plated steel, comprising:
a base steel;
a Zn plated layer formed on the base steel; and
a Zn-Mg alloy plated layer formed on the Zn plated layer,
wherein a ratio of weight of Mg contained in the Zn-Mg alloy plated layer based on a total weight of the Zn plated layer and the Zn-Mg alloy plated layer is <NUM> to <NUM>, and a sum of coating amounts of the Zn plated layer and the Zn-Mg alloy plated layer is <NUM> to <NUM>/m<NUM>,
wherein an average grain size of grains forming the Zn-Mg alloy plated layer is <NUM> or less,
wherein a total coating amount and a ratio of a weight of Mg of the zinc alloy plated steel are measured by an ICP method and the grain size is detected by observing a cross-sectional surface of the plated layer taken in a thickness direction.