KR101674771B1 - Galvannealed steel sheet and method for manufacturing the same having excellent workability - Google Patents

Abstract

The present invention relates to a steel sheet and an alloy plating layer formed on at least one surface of the steel sheet, wherein the alloy plating layer contains 8-14 wt% of Fe, 0.17-0.9 wt% of Al, 0.1-1.5 wt% of Mg, [Fe], [Al] and [Mg] are 0.02 < ([Al The present invention provides an alloyed hot-dip galvanized steel sheet excellent in workability that satisfies the following formula: [Mg + Mg]) / [Fe] < 0.2, There is an effect of improving the characteristics.

Description

TECHNICAL FIELD The present invention relates to a galvannealed steel sheet and a method of manufacturing same,

The present invention relates to an alloyed hot-dip galvanized steel sheet excellent in workability and a method of manufacturing the same.

Alloyed galvanized steel sheets (GA steel sheets) are manufactured by alloying with iron of the ferrous iron and zinc of the plating bath during the alloying process. Depending on the degree of alloying, it is divided into gamma phase, delta phase and zeta phase in the direction of the upper side of the zinc plated layer, and has a different hardness for each phase. The hardness of such a plated layer is an important factor for determining the frictional properties and resistance to powdering of the GA steel plate.

In order to improve the processability of the GA steel sheet, the friction characteristics of the surface must be improved, which can be achieved by increasing the hardness of the surface of the plating layer. The hardness of the plated layer becomes higher as the iron content is increased by progressing the alloying. If the iron content is high, the formability of the steel sheet can be enhanced, but the iron content near the interface between the base steel sheet and the plating layer, which is the lower part of the plating layer, becomes larger than the surface and the gamma phase is formed. In such a situation, the surface of the plating layer improves the friction characteristics and improves the flaking property. However, there is a contradiction point that the powdering phenomenon in which the plating layer is separated from the gamma phase at the interface between the plated iron and the ferrite iron occurs and the formability deteriorates.

Therefore, there is a need to unconditionally control the alloying temperature and time by appropriately considering the friction characteristics, powdering and flaking properties, rather than raising the content of iron contained in the plating layer. Generally, it is manufactured by optimizing the process conditions to make the most of the plating layer to the delta phase in accordance with the purpose of the GA steel sheet. However, the hard gamma phase of the base iron and the plating layer which is inevitably generated is inferior in the adhesion of the plating layer And the soft zeta phase on top of the plated layer reduces the friction characteristics during processing.

In order to prevent this, a method of increasing the aluminum concentration in the zinc plating bath at the time of hot-dipping is higher than the normal concentration is used. This improves the adhesion of the plating film by controlling the rate at which the galvanizing is alloyed to ensure the uniformity of the plating layer. However, if the aluminum concentration is increased, dross may be formed on the plating bath to cause other defects on the surface of the coated steel sheet. In addition, the alloying temperature for alloying is high and precise control is required, which is another cause of defects.

In order to solve the contradiction between the frictional characteristics and the plating adhesion, in the production of the galvannealed galvanized steel sheet, the degree of alloying of the plated layer is kept low so that the gamma phase at the interface between the plated steel and the plated layer is reduced, To provide an alloyed hot-dip galvanized steel sheet excellent in workability at the time of press forming and a method of manufacturing the same.

According to an embodiment of the present invention, there is provided a steel sheet comprising a steel sheet and an alloy plating layer formed on at least one surface of the steel sheet, wherein the alloy plating layer contains 8-14 wt% of Fe, 0.17-0.9 wt% of Al, [Al] and [Mg] in the case where the mass% of Fe, Al and Mg are respectively defined as [Fe], [Al] and [Mg] The present invention provides an alloyed hot-dip galvanized steel sheet excellent in workability satisfying 0.02 < ([Al] + [Mg]) / [Fe] < 0.2.

The alloy plating layer may have a plating adhesion amount of 20 g / m ^{2 or} more.

The alloy plating layer may further include at least one metal material selected from tin (Sn), antimony (Sb), bismuth (Bi), and silicon (Si).

 The content of the metal material may be 0.2 wt% or less.

The area fraction of the gamma phase at the interface between the steel sheet and the plating layer may be 70% or less.

According to another embodiment of the present invention, there is provided a method of manufacturing a steel sheet, comprising: a heating step of heating a steel sheet in a reducing atmosphere; The steel sheet is immersed in an alloy plating bath containing 0.1 to 0.5% by weight of Al, 0.1 to 2.0% by weight of Mg and the balance of Zn and unavoidable impurities to form a plating layer, and an alloying step of alloying the plating layer by heat treatment The present invention also provides a process for producing an alloyed hot-dip galvanized steel sheet having excellent processability.

The heating temperature in the heating step may be 700 to 950 ° C.

The alloy plating bath may have a temperature of 430 to 480 캜.

After the plating layer formation step, the plating layer may further include a step of controlling the plating adhesion amount to 20 g / m ² or more.

In the alloying step, the heating rate may be 20 to 55 ° C / sec.

The alloying step may be a step of alloying the steel sheet so that the area fraction of the gamma phase at the interface between the steel sheet and the plating layer is 80% or less.

The present invention provides an alloyed hot-dip galvanized steel sheet having excellent workability and a method of manufacturing the same, and has an effect of improving the hardness of a galvanized steel sheet and forming a uniform delta phase to improve plating adhesion and surface friction characteristics .

1 is a photograph of a section of a galvannealed galvanized steel sheet produced in Examples.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below.

The present inventors have continuously observed that the degree of alloying, surface hardness and coefficient of friction of the plating layer greatly affect the workability of the steel sheet, and in order to improve the workability of the steel sheet, the degree of alloying at the surface between the iron- The gamma phase should be suppressed, but in order to improve the friction characteristics, the inventors proceeded to the present invention to solve the contradiction that the degree of alloying of the plating layer should be increased.

When the steel sheet is immersed in the plating bath, Al contained in the zinc plating bath may be combined with Fe of the steel sheet during plating to form an Fe alloying inhibition layer such as Fe ₂ Al ₅ . However, since the inhibiting layer is not uniformly formed at the interface between the plating layer and the steel sheet, the Fe-diffused zeta-phase can be unevenly mixed at the interface where the inhibiting layer is not formed. When the alloying progresses in this state, as the diffusion of Fe proceeds unevenly, a portion of hard and non-uniform gamma phase having a high Fe content is formed at a high ratio in the plating layer and the base iron interface, resulting in poor plating adhesion .

In order to solve this problem, magnesium is added to the plating bath and the Mg-Zn-Al process phase is preferentially formed at the interface between the plating layer and the steel sheet when the steel sheet is immersed in the plating bath. The amount of aluminum pick-up is increased compared to the case where magnesium is not present by the above process, and the process phase is preferentially formed between the plating layer and the ferrous iron interface and concentrated at the bottom of the plating layer, so that it can be uniformly distributed in the plating layer and the ferrous iron interface . These effects work together to form a thin and uniform inhibiting layer.

When the plating layer formed with the uniform suppression layer is alloyed in the alloying furnace, the Fe diffusion becomes uniform and the alloying degree as a whole is lowered, and the plating layer becomes a uniform and crackless delta phase. As a result, the proportion of the gamma phase which causes flaking and powder ring is reduced between the plating layer and the base metal, so that the plating adhesion can be remarkably improved.

Further, magnesium contained in the plating layer plays a role of improving the hardness of the plating layer, so that the hardness of the plating layer can be increased by addition of magnesium even though the degree of alloying of Fe is lowered. Further, magnesium selectively oxidized on the surface during the alloying process forms a thin oxide film on the surface of the plating layer, and the oxide film has a solid lubricating action on the surface to improve the friction characteristics, so that a galvannealed steel sheet excellent in workability can be manufactured have.

Hereinafter, the steel sheet of the present invention will be described in detail.

One embodiment of the present invention includes a steel sheet and an alloy plating layer formed on at least one surface of the steel sheet, wherein the alloy plating layer contains 8-14 wt% of Fe, 0.17-0.9 wt% of Al, 0.1-1.5 wt% of Mg, [Al] and [Mg] are 0.02 [Fe], [Al] and [Mg], respectively, when the mass% of Fe, Al and Mg is defined as [Fe] The present invention provides an alloyed hot-dip galvanized steel sheet excellent in workability satisfying the formula <([Al] + [Mg]) / [Fe] <0.2.

Fe and Zn in the plating layer serve to increase the hardness of the plating layer by forming an alloy. The Fe is preferably contained in the plating layer in an amount of 8 to 14% by weight of Fe, more preferably 9 to 12% by weight . If the content of Fe is less than 8 wt%, the weldability is deteriorated and the hardness of the plating layer is too low, so that it may be difficult to secure the basic friction characteristics of the coated steel sheet even if other compositions are controlled. Phase control becomes difficult and a problem of poor moldability during processing occurs.

The Al content of the plated layer is preferably 0.17 to 0.9 wt%, more preferably 0.17 to 0.32 wt%. If the content of aluminum in the plating layer is less than 0.17% by weight, the Fe-Al alloy layer (inhibition layer) may not be formed between the base steel sheet and the plating layer interface or the surface may be unevenly formed, resulting in a thick gamma phase. If it exceeds 0.9% by weight, the inhibition layer is excessively formed, and the alloying itself is inhibited.

The Mg content of the plating layer is preferably 0.1 to 1.5% by weight, more preferably 0.2 to 0.9% by weight of Mg. When the content of magnesium in the plated layer is less than 0.1 wt%, there is no improvement in processability. When the content is more than 1.5 wt%, the surface quality is lowered due to the influence of magnesium oxide on the surface.

[Fe], [Al] and [Mg] satisfy 0.02 < (Al) + [Mg] < ) / [Fe] &lt; 0.2. Thus, it is possible to improve the hardness of the plating layer of the galvannealed galvanized steel sheet and to form a uniform delta phase, thereby improving the plating adhesion and the surface friction characteristic.

The alloy plating layer may further include at least one metal material selected from tin (Sn), antimony (Sb), bismuth (Bi) and silicon (Si) to control the surface oxide. The content is preferably 0.2% by weight or less, more preferably 0.001 to 0.2% by weight. If the content of the metal material is more than 0.2% by weight, the zinc-phase sequins become excessively coarse, and the appearance of the surface may be poor after the alloying, or excessive oxides may be formed on the surface.

The plated steel sheet of the present invention comprises a base steel sheet and a plated layer, and the area fraction of the gamma phase at the interface between the base steel sheet and the plated layer may be 70% or less, more preferably 50% or less. If the gamma phase fraction exceeds 70%, a powdering phenomenon in which the steel sheet and the plating layer are separated may occur, resulting in a drawback that the formability is deteriorated. Another phase other than the gamma phase existing at the interface between the base steel sheet and the plated layer is preferably a delta phase.

Hereinafter, the production method of the present steel sheet will be described in detail.

According to an embodiment of the present invention, there is provided a method of manufacturing a steel sheet, comprising: a heating step of heating a steel sheet in a reducing atmosphere; The steel sheet is immersed in an alloy plating bath containing 0.1 to 0.5% by weight of Al, 0.1 to 2.0% by weight of Mg and the balance of Zn and unavoidable impurities to form a plating layer, and an alloying step of alloying the plating layer by heat treatment The present invention also provides a process for producing an alloyed hot-dip galvanized steel sheet having excellent processability.

In the heating step, the heating temperature is preferably 700 to 950 ° C. If the heating temperature is less than 700 ° C., sufficient recrystallization can not be performed and the material properties of the steel sheet are inferior. If the heating temperature is more than 950 ° C., As costs are generated, the utility becomes ineffective. The atmosphere of the heating furnace can be maintained in a reducing atmosphere containing 5% of hydrogen, in which the oxide on the surface of the rare-earth iron is reduced and the Fe can be immersed in the plating bath with clean Fe.

After the steel sheet is heated, it is immersed in an alloy plating bath containing 0.1 to 0.5% by weight of Al, 0.1 to 2.0% by weight of Mg and the remainder of Zn and unavoidable impurities to form a plating layer.

The Mg and Al contained in the alloy plating bath may become a member of the plating layer when the steel sheet is immersed and plated. At this time, Al having good reactivity with Fe moves to the interface near the substrate iron in the plating layer, and Mg moves to the interface as well. Al, which is highly reactive with Fe, is picked up by the plating layer more than the concentration contained in the alloy plating bath, and Mg moves with Al.

The Al and Mg are present in the plating interface at the interface as the deposition amount is controlled and cooled by the air knife. In this way, the Al and Mg contents are observed at the interface between the substrate and the plated layer higher than the other plated layers. This process effect results in the formation of a uniformly alloyed plated layer of plated iron in a plating bath containing Mg rather than Mg. In addition, magnesium plays a role of improving the hardness of the zinc plated layer, so that it maintains high hardness even when the same degree of alloying is maintained in order to control gamma phase, thereby improving workability.

The amount of Al contained in the alloy plating bath is preferably 0.1 to 0.5 wt%, and if the content of Al is less than 0.1 wt%, the amount of bottom dross is increased in the plating bath, The alloying inhibition layer is increased in the plated layer, making alloying difficult.

The content of Mg contained in the alloy plating bath is preferably 0.1 to 2.0 wt%, and if the content of Mg is less than 0.1 wt%, it is not possible to secure a sufficient amount of Mg adhered to the plating layer so as to have an effect on workability, If the amount is more than 2.0% by weight, the amount of top dross generated in the plating bath increases, which adversely affects the workability.

The temperature of the alloy plating bath is preferably 430 to 480 DEG C, and if the temperature of the plating bath is less than 430 DEG C, the viscosity of the plating bath is lowered and the roll in the plating bath is difficult to drive, resulting in slip, If the temperature exceeds 480 DEG C, erosion of equipment such as sink roll due to zinc occurs, excessive dross occurs in the plating bath, and the evaporated zinc flakes are deposited on the steel sheet, Lt; / RTI &gt;

After the plating layer is formed on one side of the steel sheet, the plating layer may have an amount of plating adhered by an air knife, and the plating adhesion amount is preferably 20 g / m ² or more. If the amount of plating is less than 20 g / m ^2, sufficient corrosion resistance can not be secured.

The steel sheet can be alloyed by heat-treating the coated steel sheet whose plating adhesion amount is controlled. When the steel sheet is alloyed, it is preferable that the temperature raising rate is 20 to 55 ° C / sec. It is preferable to heat the plated layer by heating at the heating rate to 510 ° C to alloy the plated layer, but the final temperature may vary depending on the steel type. If the temperature raising rate is less than 20 캜 / sec, the line speed must be slowed down to achieve sufficient alloying. Thus, when the heating rate exceeds 55 캜 / sec, the surface quality is degraded due to rapid magnesium oxidation.

In the alloying step, the alloy is preferably alloyed so that the percentage of the gamma phase at the interface between the steel sheet and the delta phase is 70% or less, and the gamma phase fraction is more preferably 50% or less. If the gamma phase fraction exceeds 70%, a powdering phenomenon in which the steel sheet and the plating layer are separated may occur, resulting in a drawback that the formability is deteriorated.

Hereinafter, the present invention will be described in detail with reference to the following examples. However, the scope of the present invention is not limited by the following specific examples.

Example

There is no restriction on the steel sheet material of the present invention. In this experiment, a low carbon cold-rolled steel sheet having a thickness of 0.65 mm suitable for automobiles was used. The steel sheet was annealed at 850 ° C in an atmosphere of 5% H ₂ -N ₂ , maintained for 50 seconds, cooled, and immersed at a plating bath temperature of 465 ° C for 3 seconds. The concentration of Al was maintained while the concentration of Al was maintained, and after immersing in a hot dip galvanizing bath, the amount of deposition was adjusted with an air knife immediately after coming out of the plating bath, and then heated to 510 ° C. using an induction heater. Galvanized steel sheets were prepared and the plating bath conditions are shown in Table 1. Alloying temperature was controlled by precise control by attaching a thermocouple to the steel sheet. The composition of the plating bath was measured by inductively coupled plasma (ICP) method, and the contents of Al and Mg were measured and expressed in weight%.

division Plating bath Al concentration (% by weight) Plating bath Mg concentration (% by weight) Plating bath temperature (캜) Reference Example 1 0.132 0.114 465 Reference Example 2 0.131 0.224 465 Inventory 1 0.130 0.439 465 Inventory 2 0.127 0.958 465 Comparative Example 1 0.130 0 465

Table 2 below is a table in which the plating layer is analyzed by the ICP method. In order to investigate the composition of the plating bath and the composition of the plating layer, the plating layer was dissolved in the plated steel sheet and the content of each composition was examined by ICP method.

division Zn concentration in plating layer (% by weight) Coating Al concentration (% by weight) Plating layer Mg concentration (% by weight) Plated layer Fe concentration (% by weight) Plating amount
(g / m ² ) Reference Example 1 88.52 0.21 0.1 11.17 72 Reference Example 2 88.07 0.22 0.19 11.52 70 Inventory 1 88.99 0.24 0.34 10.43 75 Inventory 2 88.52 0.28 0.77 10.43 66 Comparative Example 1 89.10 0.17 - 10.7 50

As shown in Tables 1 and 2, the content of aluminum in the plating layer is higher than that of aluminum in the plating bath, which is caused by diffusion of aluminum, which has good reactivity with the ferrous iron, into the ferric iron interface to generate intermetallic compounds. On the other hand, in magnesium, the content in the plating layer is lower than that of the plating bath because the magnesium can not be alloyed with iron, unlike aluminum, and only in the Zn-Al-Mg process So that the composition is similar to or slightly lower than the composition of the plating bath.

In addition, although the content of aluminum in the plating bath is similar, the content of aluminum in the plating layer gradually increases as the magnesium content of the plating bath is increased. The magnesium has an effect of picking up aluminum in the plating layer. With this effect, a process condition is uniformly formed between the base iron and the plating layer, and a condition is provided in which a uniform and thin inhibition layer can be formed.

Therefore, the alloying degree (Fe concentration) of the plating layer is lowered as the content of magnesium in the plating bath is higher even though the same alloying condition is applied, and the lowered degree of alloying is an effect of inhibiting the gamma phase formation relatively. As shown in FIG. 1, it was confirmed that the higher the amount of magnesium in the plating bath, the lower the gamma phase at the interface between the plating layer and the substrate iron.

Table 3 shows the hardness, friction coefficient, flaking and powdering of the plated layer. The judging criteria of the flaking were expressed as? (Extremely excellent),? (Excellent),? (Normal) and X (poor).

division Hardness Coefficient of friction Play King Reference Example 1 437 0.1222 △ Reference Example 2 438 0.1189 ○ Inventory 1 454 0.1183 ◎ Inventory 2 455 0.1092 ◎ Comparative Example 1 428 - △

As shown in Table 3, as the content of magnesium in the plating composition increases, the gamma phase that causes flaking decreases, and the degree of alloying in the plating layer decreases, but the hardness of the plating layer increases due to the effect of magnesium rather. Moreover, the addition of magnesium forms a magnesium oxide film on the surface, and a solid lubricating action is added to increase the coefficient of friction. By such a combined action, the plating adhesion of the plating layer during molding was strengthened, and flaking was remarkably reduced, thereby confirming that the processability was improved. In the case of Reference Example 1, it was confirmed that the effect of flaking was normal but the coefficient of friction and hardness were higher than that of Comparative Example 1. [

On the other hand, when the magnesium content exceeds 2% by weight, excessive aluminum concentration is promoted at the interface, and the alloying is excessively suppressed. A steel sheet having a magnesium content exceeding 2% by weight is required to be precisely controlled by alloying, thereby affecting productivity. In addition, the dross formed on the surface of the plating bath tends to be excessively produced if the magnesium content exceeds 2 wt%, so that the content of 2 wt% is defined as the upper limit of the present invention.

division The area fraction (%) of the gamma phase at the interface between the steel sheet and the plated layer Reference Example 1 50 Reference Example 2 42 Inventory 1 20 Inventory 2 18 Comparative Example 1 86

As shown in FIG. 1, the higher the amount of magnesium in the plating bath, the lower the gamma phase at the interface between the plating layer and the substrate iron. In Comparative Example 1 not containing magnesium, the gamma phase fraction exceeded 80% did. As a result, in Comparative Example 1, a powdering phenomenon in which the steel sheet and the plating layer are separated from each other may occur, resulting in a drawback that the formability is deteriorated.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, It will be obvious to those of ordinary skill in the art.

Claims (11)

Steel plate and
And an alloy plating layer formed on at least one surface of the steel sheet,
Wherein the alloy plating layer contains 8 to 10.43% by weight of Fe, 0.17 to 0.9% by weight of Al, 0.34 to 1.5% by weight of Mg, the balance of Zn and unavoidable impurities,
[Fe], [Al] and [Mg] satisfy 0.02 < (Al) + [Mg] < ) / [Fe] &lt; 0.2,
The alloy plating layer is uniformly distributed with Mg,
Wherein the hardness of the alloy plating layer is 437 HV or more, and the alloyed hot-dip galvanized steel sheet excellent in workability.
The galvannealed steel sheet according to claim 1, wherein the alloy plating layer has a plating adhesion amount of 20 g / m ² or more.
The galvannealed steel sheet according to claim 1, wherein the alloy plating layer further comprises at least one metallic material selected from tin (Sn), antimony (Sb), bismuth (Bi), and silicon (Si).
4. The galvannealed steel sheet according to claim 3, wherein the metal material is 0.2% by weight or less.
The galvannealed steel sheet according to claim 1, wherein an area fraction of a gamma phase at an interface between the steel sheet and the plating layer is 70% or less.
A heating step of heating the steel sheet;
A plating layer forming step of immersing the steel sheet in an alloy plating bath containing 0.1 to 0.5% by weight of Al, 0.439 to 0.958% by weight of Mg, and the remainder of Zn and unavoidable impurities to form a plating layer; And
And an alloying step of alloying the plating layer by heat treatment at a heating rate of 20 to 55 DEG C / sec,
The alloyed plated layer is uniformly distributed in Mg,
Wherein the hardness of the alloyed plated layer is 437 HV or more.
The method of claim 6, wherein the heating is performed at a temperature of 700 to 950 캜.
The method according to claim 6, wherein the alloy plating bath has a high temperature of 430 to 480 캜.
7. The method of claim 6, wherein the plating layer forming step comprises forming a plating layer on the steel sheet at 20 g / m &lt; ² &gt; or more per one side.
delete [7] The method of claim 6, wherein the alloying step comprises alloying the steel sheet so that the area fraction of the gamma phase is 80% or less at the interface between the steel sheet and the plating layer.

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