KR20210079722A - Alloyed got dip galvannealed steel sheet and method of manufacturing the same - Google Patents

Abstract

The present invention is to provide an alloyed hot-dip galvannealed (GA) steel sheet and a method of manufacturing the same, which can improve the plating delamination of an alloyed hot-dip galvannealed (GA) steel sheet and secure press workability as well. A method of manufacturing an alloyed hot-dip galvannealed (GA) steel sheet according to an aspect of the present invention includes the steps of preparing a steel sheet having a predetermined alloy composition, heating and cracking the steel sheet, immersing the cracked steel sheet in a hot-dip galvanizing bath of 450 to 560°C to form a hot-dip galvannealed layer in which less than 10 wt% of the gamma (Γ) phase, 60 to 80 wt% of the delta (σ) phase, and 10 to 30 wt% of the zeta (ζ) phase with respect to the entire plating layer are sequentially disposed from the surface of the steel sheet, and performing the alloying heat treatment on the steel sheet at a temperature of 460 to 520℃.

Description

ALLOYED GOT DIP GALVANNEALED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to an alloyed hot-dip galvanized steel sheet having excellent workability and plating peelability suitable for automobile parts and a method for manufacturing the same.

As competition intensifies in the automobile industry, the demand for luxury and diversification of automobile quality is increasing, and in order to satisfy the stricter safety and environmental regulations, it is necessary to increase the strength of the steel sheet and reduce the weight to improve fuel efficiency. Efforts continue. Recently, the steel industry and the automobile industry are interested in and research is focused on high strength and light weight. As automobile designs become more complex and consumers' desires diversify, high strength steel with excellent processability and formability is required.

In general, steel sheets used in automobiles need to improve corrosion resistance in order to extend the life of the automobile, and for this purpose, hot-dip galvanized steel sheets are used. Hot-dip galvanized steel sheet has excellent corrosion resistance and is widely used in building materials, structures, home appliances and automobile bodies. The hot-dip galvanized steel sheets currently most used are hot-dip galvanized steel sheets (GI steel sheets) and alloyed hot-dip galvanized steel sheets (GA steel sheets). It is excellent and is widely used as a material for automobiles themselves.

As a technology related thereto, there is Korean Patent Laid-Open Publication No. 2019-0178955 (published on July 3, 2019, high-strength hot-dip galvanized steel sheet with excellent surface quality and plating adhesion and a manufacturing method thereof).

An object of the present invention is to provide an alloyed hot-dip galvanized (GA) steel sheet and a method for manufacturing the same, which improve the plating peeling phenomenon of the alloyed hot-dip galvanized (GA) steel sheet and secure press workability.

An alloyed hot-dip galvanized (GA) steel sheet according to an aspect of the present invention includes a steel sheet having a predetermined alloy composition; and a hot-dip galvanized layer formed on the steel sheet, wherein the hot-dip galvanized layer comprises less than 10% by weight of a gamma (Γ) phase, 60 to 80% by weight of a delta (σ) phase, with respect to the entire plating layer from the surface of the steel sheet, and It is characterized in that 10 to 30% by weight of the zeta (ζ) phase is sequentially arranged.

The degree of alloying of the hot-dip galvanized layer is preferably 7 to 11% by weight.

It may further include a phosphate-based coating layer disposed on the surface of the hot-dip galvanizing layer.

According to an aspect of the present invention, there is provided a method for manufacturing an alloyed hot-dip galvanized (GA) steel sheet, comprising the steps of: (a) preparing a steel sheet having a predetermined alloy composition; (b) heating and cracking the steel sheet; (c) immersing the cracked steel sheet in a hot-dip galvanizing bath at 450 to 560 ° C., from the surface of the steel sheet to a gamma (Γ) phase of less than 10 wt%, and a delta (σ) of 60 to 80 wt% with respect to the entire plating layer ) phase, and forming a hot-dip galvanizing layer in which 10 to 30 wt% of a zeta (ζ) phase is sequentially disposed; And (d) characterized in that it comprises the step of alloying heat treatment at a temperature of 460 ~ 520 ℃ the steel sheet.

In step (c), the aluminum (Al) concentration of the plating bath is preferably controlled to 0.13 to 0.14 wt%.

In step (c), the degree of alloying of the hot-dip galvanized layer is preferably 7 to 11 wt%.

After the step (d), the method may further include forming a phosphate-based coating layer on the surface of the hot-dip galvanizing layer.

According to the present invention, excellent workability and plating resistance can be secured at the same time by controlling the alloy phase fraction in the plating layer to be zeta (ζ) phase: 10 to 30 wt% and gamma (Γ) phase: less than 10 wt%.

1 is a schematic view schematically showing an alloy phase in a plating layer of a galvanized steel sheet.
2 is a view showing a comparison of the alloy phase fraction in the conventional GA steel sheet and the GA steel sheet according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art to which the present invention pertains can easily practice it. The present invention may be embodied in several different forms, and is not limited to the embodiments described herein. The same reference numerals are assigned to the same or similar components throughout this specification. In addition, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the gist of the present invention will be omitted.

Galvanneled (GA) steel sheet is widely used as steel sheet for construction materials, home appliances, and automobiles because it has superior weldability, paint adhesion, and corrosion resistance after painting compared to general hot-dip galvanized steel sheet. GA steel sheet is manufactured by hot-dip plating zinc (Zn) on a base steel sheet and then forming an Fe-Zn alloy phase in the plating layer through heat treatment. More specifically, the primary steel sheet is hot-dip galvanized to form galvanizing, and the steel sheet is continuously reheated in a temperature range of about 460 to 520° C. to prevent the mutual diffusion of iron (Fe) and zinc (Zn). A GA steel sheet is manufactured by forming Fe-Zn-based intermetallic compounds such as zeta (ζ), delta (δ), and gamma (Γ) phases in the plating layer.

It is known that the quality of such GA steel sheet has a close relationship with the composition of the alloy phase in the plating layer. Looking at the characteristics of each alloy phase in the GA plating layer, in the case of the gamma (Γ) phase that exists at the interface with the base iron, it acts as a stress concentration part during press working and acts as a crack initiation point of the material, worsening workability, and causing powdering plating peeling. known to be a major factor. The delta (δ) phase existing in the upper layer of the gamma (Γ) phase has superior processability and a low coefficient of friction compared to the gamma (Γ) phase. And, in the case of the zeta (ζ) phase present in the uppermost layer, the friction coefficient is high, which deteriorates press workability and causes flaking plating peeling. However, it is known to be advantageous for powdering plating and peeling according to the effect of absorbing and dispersing external stress in the soft zeta (ζ) phase.

Therefore, in general, in the case of GA steel sheet, in order to secure press workability and improve plating peeling (powdering, flaking) phenomenon, the growth of zeta (ζ) and gamma (Γ) phases in the plating layer are suppressed, and the delta (δ) phase is dominant. GA steel sheet composed of an alloy phase of Recently, the surface friction coefficient has been greatly reduced through the application of a phosphate-based lubricating film coating to the GA steel sheet, and accordingly, it is necessary to re-establish the composition of the alloy phase in the plating layer for GA quality optimization.

According to the present invention, by appropriately adjusting each fraction of an iron (Fe)-zinc (Zn) alloy phase in the plating layer of the GA steel sheet, it is possible to improve the plating peeling phenomenon, which is known as a disadvantage of the GA steel sheet, and secure press workability. Through this, problems such as powdering, flaking, and processing cracks of the GA steel sheet can be improved.

According to one embodiment of the present invention, as an alloyed hot-dip galvanized (GA) steel sheet used as an inner or outer plate of a vehicle, the alloy phase constituting the plating layer is zeta (ζ) phase: 10 to 30 wt%, delta (δ) Phase: 60 to 80% by weight, gamma (Γ) phase: consists of less than 10% by weight. In addition, in order to prevent an increase in frictional force as the fraction of the zeta (ζ) phase increases, it is preferable that a phosphate-based lubricating film coating is applied to the surface of the GA steel sheet.

In the GA steel sheet of the present invention, when the fraction of the zeta (ζ) phase satisfies 10 to 30 wt%, it is possible to simultaneously secure workability and plating resistance. When the fraction of the zeta (ζ) phase is less than 10% by weight of the total plating layer, the effect of absorbing and dispersing stress due to the zeta (ζ) phase in the plating layer is reduced, and thus powdering and flaking plating peeling increase. Conversely, when the fraction of the zeta (ζ) phase exceeds 30% by weight of the total plating layer, the plating peel resistance decreases, but the workability deteriorates as the surface friction coefficient increases. Therefore, according to a preferred embodiment of the present invention, when the alloy phase fraction in the plating layer is controlled to zeta (ζ) phase: 10 to 30 wt% and gamma (Γ) phase: less than 10 wt%, excellent workability and plating resistance at the same time can be obtained

1 is a schematic view schematically showing an alloy phase in a plating layer of a galvanized steel sheet, and FIG. 2 is a view showing a comparison of the alloy phase fraction in a conventional GA steel sheet and a GA steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, (B) shows the structure of the plating layer of a general hot-dip galvanized (GI) steel sheet, and it can be seen that it is composed of an Fe layer and a Ze-Fe alloy layer. (A) shows the structure of the plating layer of a conventional alloyed hot-dip galvanized (GA) steel sheet, consisting of an Fe layer-gamma layer (Zn-Fe)-delta layer (Zn-Fe)-zeta layer (Zn-Fe). can see. And, (C) shows the plating layer structure of the alloyed hot-dip galvanized (GA) steel sheet having the improved structure of the present invention, Fe layer-gamma layer (Zn-Fe)-delta layer (Zn-Fe)-zeta layer Although composed of (Zn-Fe), as in the comparative drawing of FIG. 2 , in the case of the GA steel sheet of the present invention shown in the right (E), the delta (δ) phase is thinner than in the conventional GA steel sheet shown in (D) on the left. It can be seen that the thickness of the zeta (ζ) phase has become much thicker.

On the other hand, it is known that the surface quality (workability, powdering property, paintability, corrosion resistance) of an alloyed hot-dip galvanized (GA) steel sheet has a close relationship with the degree of alloying in the plating layer. When the alloying degree is low, a large amount of the zeta (ζ) phase is generated and the generation of the gamma (Γ) phase is suppressed. As a result, the zeta (ζ) phase becomes thick and the gamma (Γ) phase becomes thin. On the other hand, when the alloying degree is high, the gamma (Γ) phase is generated in a large amount and the generation of the zeta (ζ) phase is suppressed. As a result, the gamma (Γ) phase becomes thick and the zeta (ζ) phase becomes thin. When the alloying degree is high, since the gamma (Γ) phase grows thickly at the interface with the base layer, it causes powdering that occurs during press forming of the alloy steel sheet.

Considering these contents, the preferred degree of alloying presented in the present invention is 7 to 11% by weight. The degree of alloying indicates the content of iron (Fe) in the plating layer in weight %, and the degree of alloying is determined according to the alloying temperature during the alloying treatment. When the alloying degree exceeds 11 wt%, powdering and flaking plating peeling according to gamma (Γ) phase growth increases, and when the alloying degree is less than 7 wt%, the eta (η) phase remains in the plating layer, and the friction coefficient There is a problem that the formability deteriorates as the increase increases, and the quality of the surface appearance deteriorates due to non-alloying. Therefore, the GA steel sheet of the present invention has an alloying degree of 7 to 11% by weight, and when it has the above-mentioned alloy phase fraction, excellent workability and plating peelability can be secured.

Hereinafter, another aspect of the present invention, a method for manufacturing an ultra-high strength hot-dip galvanized steel sheet excellent in plating property will be described in detail.

Another aspect of the present invention, a method for manufacturing an ultra-high strength hot-dip galvanized steel sheet excellent in plating property, comprises the steps of preparing a base steel sheet, heating the base steel sheet in an atmospheric gas containing hydrogen and the remainder nitrogen and unavoidable impurities in a volume ratio. Step, cracking the heated steel sheet, and immersing the cracked steel sheet in a hot-dip galvanizing bath at 450 to 560 ° C. to plate the steel sheet, wherein the base steel sheet is manufactured by performing hot rolling, winding and cooling, and , the cooling comprises the step of cooling at a cooling rate of 0.05 ~ 4.0 ℃ / sec in a slow cooling box having a dew point temperature of -20 ~ 20 ℃.

First, the base steel sheet usable in the present invention is not necessarily limited thereto, but as an example, in weight %, silicon (Si): 0.5 to 2.5%, manganese (Mn): 1.0 to 10.0%, aluminum (Al) : 0.5 to 2.5%, including the remainder iron (Fe) and other unavoidable impurities, and may have a composition in which the sum of two or more components among Si, Mn, and Al satisfies 1 to 15%. A steel sheet having such a composition can improve the strength of the steel sheet, but since it is a steel sheet in which the problem of non-plating becomes serious when plated by a conventional method, the effect of the present invention can be more advantageously exhibited.

It is preferable that the base steel sheet is subjected to a hot finish rolling of steel satisfying the composition and component ranges described above, and the hot finish rolled steel sheet is wound. It is preferable to control the temperature of the steel sheet when winding the steel sheet. Such temperature control acts as a driving force for oxygen contained in external air to penetrate into the steel sheet to form internal oxides, thereby affecting the oxygen flux. According to an example of the present invention, it is preferable to control the coiling temperature to 600 ~ 900 ℃ in order to obtain the intended effect. When the coiling temperature is less than 600° C., the flux of oxygen to penetrate into the steel sheet decreases and the depth of the internal oxide becomes shallow, which effectively suppresses the surface concentration of Si, Mn and Al contained in the steel in the annealing process to be described later. Therefore, there is a problem of causing poor plating properties during hot-dip plating. On the other hand, when it exceeds 900° C., the flux of oxygen to penetrate into the steel sheet increases and the depth of the internal oxide becomes thick. When the thickness of the internal oxide becomes excessively thick as described above, the elongation does not reach a target value, and the material deteriorates during bending molding.

It is preferable to cool the wound steel sheet as described above in a slow cooling box having a dew point temperature of -20 to 20° C. at a cooling rate of 0.05 to 4.0° C./sec.

After selectively performing cold rolling on the cooled steel sheet as described above, a subsequent process may be performed. It is preferable to heat-treat the cooled steel sheet as described above in order to secure excellent material properties such as tensile strength or elongation. At this time, the heat treatment is preferably performed at a temperature of 750 to 850 °C. When the temperature at which the steel sheet is heated is less than 750° C., the crystal growth and recovery process does not occur sufficiently, so it is difficult to secure a material at the level required in the present invention. On the other hand, when it exceeds 850°C, fuel and energy consumption used to increase the temperature of the annealing furnace increases, and it is impossible to secure a steel sheet having excellent tensile strength or elongation of the steel by secondary recrystallization.

Having the component composition as described above, when the preparation of the base steel sheet manufactured by the manufacturing method as described above is completed, the base steel sheet is subjected to a pretreatment process of washing and drying with water, and then enters into the annealing furnace for the annealing process to perform annealing heat treatment. can

In the present invention, it is preferable to heat the base steel sheet to 700 to 850° C. under an atmosphere gas containing 3 to 20% of hydrogen and the remainder nitrogen and unavoidable impurities in the annealing furnace at the time of annealing heat treatment in the present invention. The heating is preferably performed in a reducing atmosphere. More preferably, in a volume ratio, heating is performed including 3 to 20% of hydrogen, the remainder nitrogen, and unavoidable impurities.

It is preferable to heat the base steel sheet to 700 ~ 850 °C in an annealing furnace that satisfies the above conditions. When the temperature of the base steel sheet is less than 700 ℃, the internal oxide does not function as a prevention layer to suppress the surface thickening of Si, Mn and Al, so the crystal growth and recovery process does not occur sufficiently, so that the material of the level required in the present invention is secured hard to do On the other hand, when it exceeds 850° C., fuel and energy consumption used to increase the annealing furnace temperature increases, and it is impossible to secure a steel sheet having excellent tensile strength or elongation of the steel by secondary recrystallization. More preferably, it heats to the temperature of 750-820 degreeC.

It is preferable to crack the base steel sheet heated to 700 ~ 850 ° C., and immerse the cracked base steel sheet in a hot-dip galvanizing bath of 450 to 560 ° C. to perform plating. When the temperature of the plating bath is less than 450° C., the viscosity of the plating bath increases and the mobility of a roll that winds the steel sheet decreases, thereby causing a slip between the steel sheet and the roll, thereby causing defects. On the other hand, when the temperature of the plating bath exceeds 560° C., the dissolution of the steel sheet is accelerated, thereby accelerating the generation of dross in the form of Fe—Zn compound, thereby causing non-plating. In addition, the zinc plating bath contains 0.13 to 0.14 wt % of aluminum (Al) and the remainder zinc (Zn) and other unavoidable impurities. When the Al content in the plating bath is less than 0.13% by weight, there is a disadvantage in that the formation of the Fe-Al alloy phase formed at the interface between the base iron and the plating layer is suppressed, and when it exceeds 0.14% by weight, the Al content in the plating layer increases and weldability is improved. There is a problem with dropping.

After plating as described above, it is preferable to perform alloying heat treatment at 460 to 520°C. When the alloying heat treatment temperature is 460° C. or higher, the effect of preventing the powdering phenomenon in which the plating layer falls off by passing through the single or composite oxide layer of Si, Mn and Al formed on the surface layer of the steel sheet to ensure sufficient Fe content in the galvanized layer have. The upper limit does not need to be particularly limited, but it is preferable to control the temperature to 520° C. in consideration of productivity.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only examples for explaining the present invention in more detail, and do not limit the scope of the present invention.

Example

By weight %, Si: 1.5%, Mn: 1.6%, and Al: 0.4%, a final finish hot rolling of a steel sheet having a total content of 3.5% by weight, followed by winding, having a dew point temperature and a cooling rate It was cooled to room temperature in an annealing box. Thereafter, cold rolling was performed at room temperature to prepare a base steel sheet. The surface of the steel sheet prepared as described above was cleanly pretreated, heated to a cracking temperature in a reducing atmosphere containing 15% hydrogen by volume, the remainder nitrogen, and unavoidable impurities, and then cracked for 60 seconds. Thereafter, the steel sheet was cooled to a temperature of 480° C., and plated by immersion in a hot-dip galvanizing bath of 450° C. to prepare a hot-dip galvanized steel sheet as shown in Table 1 below. After that, the base steel sheet was immersed in the plating bath for 3 seconds, and then the amount of plating applied to the surface was maintained at a level of 60 g/m 2 through air wiping.

In order to evaluate the plating property of the steel sheet after the plating process, the alloy phase fraction, alloying degree, powdering grade, flaking grade, and workability grade were respectively measured and shown in Table 1 below.

The alloy phase fraction was analyzed using the Rietveld quantitative analysis method for the diffraction pattern obtained through XRD (X-Ray Diffraction) measurement, and the plating amount and the alloying degree were analyzed using the wet analysis method.

Powdering property was evaluated by the V-bending experiment (standard: Grade 1 (excellent): less than 4.0 mm, Grade 2 (inferiority): 4.0 mm or more). The V-bending experiment was carried out using an automatic powdering vision tester by processing a specimen in a size of 40 mm×70mm×0.7mm. In the case of the V-bending test, first, the evaluation specimen is bent using a V-mold (60o, 1R) with a pressing force of 5 tons, and then unbent again to prepare a specimen for V-bending evaluation. After attaching an adhesive cellophane tape to the prepared specimen, it is removed again, and the maximum width of the plating layer peeled off the tape is measured to evaluate the powdering plating peeling.

Flaking property was evaluated by HAT-bead experiment (standard: 1st grade (excellent): less than 30, 2nd (poor): 30 or more). The HAT test method used to evaluate the flaking property, the specimen was machined to a size of 50 mm × 245 mm × 0.7 mm, and was carried out using an automatic GA steel plate flaking evaluation equipment. In the case of HAT test, after placing the specimen in the mold, fixing the evaluation specimen with a clamping force of 3 tons, forming the specimen to a height of 65 mm at a punch speed of 300 mm/s, and attaching it to the part passing through the bead surface After attaching the cellophane tape for flaking, it is removed again and the thickness of the plating layer peeled off the tape is measured to evaluate the flaking plating peeling. The unit hci used for HAT evaluation represents a quantitative value analyzing the degree of shading through a scan, and it has a larger value as the amount of fragments that fall off increases.

And, the workability was evaluated by the LDH experiment (standard: Grade 1 (excellent): 49.0 mm or more, Grade 2 (inferiority): less than 49.0).

Referring to Table 1, when the fraction of the zeta (ζ) phase in the plating layer satisfies 10 to 30 wt%, the powdering grade f, the flaking grade, and the machinability grade are all grade 1, and the properties of workability and plating peeling resistance are excellent. can be known When the zeta (ζ) phase is less than 10% by weight, powdering and flaking plating peeling increases, and when it exceeds 30% by weight, the plating peeling resistance is excellent, but workability is deteriorated. Therefore, it can be confirmed that when the fraction of the zeta (ζ) phase in the plating layer satisfies 10 to 30% by weight, excellent press workability and plating peeling resistance can be secured.

Although the above description has been focused on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention without departing from the scope of the present invention. Accordingly, the scope of the present invention should be judged by the claims described below.

Claims (7)

a steel sheet having a predetermined alloy composition; and
A hot-dip galvanizing layer formed on the steel sheet,
The hot-dip galvanized layer,
From the surface of the steel sheet, less than 10% by weight of a gamma (Γ) phase, a delta (σ) phase of 60 to 80% by weight, and a zeta (ζ) phase of 10 to 30% by weight with respect to the entire plating layer are sequentially arranged to do,
Alloyed hot-dip galvanized steel sheet. According to claim 1,
The hot-dip galvanized layer is characterized in that the alloying degree is 7 to 11 wt%,
Alloyed hot-dip galvanized steel sheet. According to claim 1,
Characterized in that it further comprises a phosphate-based coating layer disposed on the surface of the hot-dip galvanizing layer,
Alloyed hot-dip galvanized steel sheet. (a) preparing a steel sheet having a predetermined alloy composition;
(b) heating and cracking the steel sheet;
(c) immersing the cracked steel sheet in a hot-dip galvanizing bath at 450 to 560° C., from the surface of the steel sheet to a gamma (Γ) phase of less than 10% by weight, and a delta (σ) of 60 to 80% by weight with respect to the entire plating layer ) phase, and forming a hot-dip galvanizing layer in which 10 to 30 wt% of a zeta (ζ) phase is sequentially disposed; and
(d) characterized in that it comprises the step of alloying heat treatment at a temperature of 460 ~ 520 ℃ the steel sheet,
A method of manufacturing an alloyed hot-dip galvanized steel sheet. 5. The method of claim 4,
In the step (c), the aluminum (Al) concentration of the plating bath is characterized in that it is controlled to 0.13 ~ 0.14% by weight,
A method of manufacturing an alloyed hot-dip galvanized steel sheet. 5. The method of claim 4,
In the step (c), characterized in that the alloying degree of the hot-dip galvanized layer is 7 to 11% by weight,
A method of manufacturing an alloyed hot-dip galvanized steel sheet. 5. The method of claim 4,
After step (d),
It characterized in that it further comprises the step of forming a phosphate-based coating layer on the surface of the hot-dip galvanizing layer,
A method of manufacturing an alloyed hot-dip galvanized steel sheet.

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