KR102031459B1 - Ultra high strength high manganese galvanized steel sheet having excellent coatability and method of manufacturing the same - Google Patents

Abstract

The present invention relates to an ultra-high strength high manganese hot-dip galvanized steel sheet excellent in plating properties and a method of manufacturing the same.
One embodiment of the present invention is by weight, C: 0.3-0.9%, Mn: 10-25%, Ti: 0.01-0.5%, Si: 0.05% or less, Al: 0.05% or less, residual Fe and other unavoidable impurities Holding steel sheet comprising a; A zinc-based plating layer formed on at least one surface of the base steel sheet; And a Fe—Zn—Mn based alloy layer formed at an interface between the base steel sheet and the zinc based plating layer, wherein the zinc based plating layer contains less than 0.01% by weight of Al, and the balance includes Zn and the Fe—Zn -Mn-based alloy layer provides an ultra-high strength high manganese hot-dip galvanized steel sheet and a method of manufacturing the same excellent plating properties composed of a two-layer structure of the upper alloy layer and the lower alloy layer.

Description

Ultra-high strength, high manganese hot-dip galvanized steel sheet with excellent plating properties and manufacturing method thereof

The present invention relates to an ultra-high strength high manganese hot-dip galvanized steel sheet excellent in plating properties and a method of manufacturing the same.

Recently, as safety regulations of automobiles are strengthened and as part of environmentally-friendly efforts to reduce greenhouse gas emissions, demand for high strength and light weight of automotive steel sheets is increasing. To this end, research on high-strength steels such as DP (Dual Phase) steel, TRIP (Transformation Induced Plasticity) steel, and TWIP (Twinning Induced Plasticity) steel containing a large amount of non-plating elements such as Si, Mn or Al is being actively conducted. .

On the other hand, automotive steel sheet is exposed to a corrosive environment during use, it is required to have excellent corrosion resistance, accordingly to form a plated layer on its surface is generally used in the form of plated steel sheet.

The current mainstream is hot-dip galvanized steel sheet, which is not only easy to mass-produce, but also excellent in corrosion resistance and sacrificial corrosion resistance. By the way, in the case of hot-dip galvanized steel sheet having a high-strength steel has a disadvantage of poor plating properties. The hot-dip galvanized steel sheet is a plated layer formed by immersing the steel plate in an Al-containing galvanizing bath.Al contained in the plating bath is made of Fe ₂ Al _{5 at the} interface between the base steel plate and the galvanizing layer while suppressing the elution of Fe from the base steel sheet. The alloying inhibitory layer is formed, and the alloying inhibitory layer serves to increase the adhesion between the base steel sheet and the zinc plating layer. However, in the case of high strength steel containing a large amount of Si, Mn or Al, the unplated phenomenon appears due to poor zinc wetting due to the diffusion of Si, Mn or Al to the surface of the steel sheet to form an oxide in the portion where the alloying inhibitory layer is not formed. As a result, peeling of the plating layer occurs.

In order to solve this problem, studies have been conducted to improve the plating property of high strength steel by adding a trace component in the plating bath as in Non-Patent Document 1. However, most of these trace components have a problem that the melting point is significantly higher than the melting point of zinc, making it difficult to contain the trace components in the plating bath. In addition, even if such a minor component is contained, there is a limit in controlling the content thereof.

Therefore, it is a very urgent time for the technology to be able to secure the plating property of the ultra-high strength hot-dip galvanized steel sheet.

 "Influence of Mischmetal on Wetting of Steel by Zn + 5wt% Al Alloys", Y. Yun et al., Journal of Rare Earths, Vol 11, pp. 130 (1993)

One aspect of the present invention is to provide an ultra-high strength high manganese hot-dip galvanized steel sheet excellent in plating properties and a method of manufacturing the same.

One embodiment of the present invention is by weight, C: 0.3-0.9%, Mn: 10-25%, Ti: 0.01-0.5%, Si: 0.05% or less, Al: 0.05% or less, residual Fe and other unavoidable impurities Holding steel sheet comprising a; A zinc-based plating layer formed on at least one surface of the base steel sheet; And a Fe—Zn—Mn based alloy layer formed at an interface between the base steel sheet and the zinc based plating layer, wherein the zinc based plating layer contains less than 0.01% by weight of Al, and the balance includes Zn and the Fe—Zn -Mn-based alloy layer provides a super high strength high manganese hot-dip galvanized steel sheet including a two-layer structure of the upper alloy layer and the lower alloy layer.

Other embodiments of the invention are by weight, C: 0.3-0.9%, Mn: 10-25%, Ti: 0.01-0.5%, Si: 0.05% or less, Al: 0.05% or less, residual Fe and other unavoidable impurities Preparing a steel sheet comprising a; And immersing the base steel sheet in a hot dip galvanizing bath to obtain a hot dip galvanizing steel sheet, wherein the hot dip galvanizing bath contains Al in an amount less than 0.01% by weight, and the balance is excellent in plating property including Zn. Provided is a method for producing a high strength high manganese hot dip galvanized steel sheet.

According to one aspect of the present invention, the hot-dip galvanized steel sheet produced by the present invention has a very high coating layer coverage area, it is possible to secure excellent plating properties irrespective of the steel component properties of ultra-high strength steel containing a large amount of alloying elements There is this.

1 is a cross-sectional photograph of Inventive Example 1 according to an embodiment of the present invention.

The present inventors have found that the leaching of Fe generated in the initial stage of immersion in a hot dip galvanizing bath is inevitable because oxides, which are highly oxidizing elements such as Si, Mn, and Al contained in steel, are inevitably diffused to the surface of a steel sheet during annealing heat treatment. By maximizing the phenomena, the surface oxides drop out along with Fe elution, that is, a lift-off phenomenon, and at this time, Al is not added to the hot dip galvanizing bath to maximize Fe elution. The present invention has been completed by recognizing that excellent plating property can be secured by controlling Al to a minimum.

To this end, the present invention, in one embodiment, in weight%, C: 0.3-0.9%, Mn: 10-25%, Ti: 0.01-0.5%, Si: 0.05% or less, Al: 0.05% or less, balance Fe and A steel sheet containing other unavoidable impurities; A zinc-based plating layer formed on at least one surface of the base steel sheet; And a Fe—Zn—Mn based alloy layer formed at an interface between the base steel sheet and the zinc based plating layer, wherein the zinc based plating layer contains less than 0.01% of Al, and the balance includes Zn and the Fe—Zn— The Mn-based alloy layer provides an ultra-high strength high manganese hot-dip galvanized steel sheet having excellent plating properties including a two-layer structure of an upper alloy layer and a lower alloy layer.

Hereinafter, the present invention will be described. First, the alloy composition of the steel sheet of the present invention will be described. The content of the base steel alloy composition described below means by weight.

C: 0.3 ~ 0.9%

Carbon is an element contributing to stabilization of austenite structure, and as the content thereof increases, there is an advantage in securing austenite structure. In addition, carbon increases the lamination defect energy of the steel and simultaneously increases tensile strength and elongation. If the carbon content is less than 0.3%, there is a problem in that the α '(alpha) -martensite structure is formed by decarburization during the high temperature processing of the steel sheet, which makes it vulnerable to delayed fracture, and also secures the target tensile strength and elongation. There is a difficult problem. On the other hand, when the content exceeds 0.9%, the electrical resistivity increases, which may degrade the weldability. Therefore, in the present invention, it is preferable to limit the carbon content to 0.3 to 0.9%.

Mn: 10-25%

Manganese, along with carbon, is an element that stabilizes austenite structures. If the content is less than 10%, it is difficult to obtain a stable austenite structure due to the formation of α '(alpha) -martensite structure during deformation, whereas when the content exceeds 25%, the effect of strength improvement is saturated. There is a disadvantage that the manufacturing cost increases. Therefore, in the present invention, it is preferable to limit the manganese content to 10-25%.

Ti: 0.01 ~ 0.5%

Titanium improves the formability of the steel by reacting with nitrogen in the steel to form nitride, and improves the strength of the steel by reacting with carbon in the steel to form carbide. In order to obtain such an effect in the present invention, the titanium content is preferably 0.01% or more. However, if the content exceeds 0.5%, there is a problem in that the precipitate is excessively formed to deteriorate the fatigue property of the steel. Therefore, in the present invention, it is preferable to limit the titanium content to 0.01 to 0.5%.

Si: 0.05% or less

Silicon is commonly known as an element used as a deoxidizer in steel. In the present invention, when Si is excessively contained in steel, Si is segregated in the form of an oxide at the grain boundary of the steel sheet to infer plateability and spot welding LME crack resistance. Therefore, in the present invention, it is preferable to limit the silicon content to 0.05% or less. On the other hand, the content of Si is more preferably 0.03% or less, and even more preferably 0.02% or less.

Al: 0.05% or less

Aluminum is an element usually added for deoxidation of steel, but in the present invention, when Al is excessively added, the tensile strength of the steel is lowered, the castability is deteriorated, and the surface quality is degraded due to the deep oxidation of the steel during hot rolling. Similarly to Si, there is a problem of segregation in the form of an oxide at the grain boundary of the steel sheet to infer plateability and spot welding LME crack resistance. Therefore, in the present invention, it is preferable to limit the aluminum content to 0.05% or less.

The remaining components of the steel sheet of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

Hot-dip galvanized steel sheet of the present invention includes a zinc-based plating layer formed on at least one surface of the base steel sheet described above. At this time, it is preferable that the said zinc-based plating layer contains less than 0.01%, and remainder consists of Zn. When Al is contained in an amount of 0.01% or more, the Al is contained in the plating layer in the form of an Fe-Al intermetallic compound, and is unevenly formed on the surface of the steel sheet, which is likely to make plating adhesion inferior. In addition, the Fe-Al intermetallic compound has a high specific resistance, which may result in inferior LME properties due to its high spot welding LME cracking sensitivity. Therefore, Al in the zinc-based plating layer is preferably included less than 0.01%.

The zinc-based plating layer is preferably more than 50g / ㎡ 60g / ㎡ or less. When the adhesion amount of the zinc-based plating layer is less than 50g / ㎡ it may be difficult to secure excellent corrosion, if it exceeds 60g / ㎡ there is a disadvantage of cost increase and spot welding LME characteristics may be inferior.

In addition, the hot-dip galvanized steel sheet of the present invention preferably comprises a Fe-Zn-Mn-based alloy layer formed at the interface between the base steel sheet and the zinc-based plating layer, wherein the Fe-Zn-Mn-based alloy layer is an upper portion It is preferable to include the two-layer structure of an alloy layer and a lower alloy layer. The base steel sheet is non-uniformly formed of Si, Mn, Al alone or complex oxide on the surface of the steel sheet by Si, Mn, Al contained in the steel during the annealing heat treatment process. Such an oxide forms an upper alloy layer including a non-dense amorphous alloy phase because the difference in Fe diffusion for each site is shown as an obstacle to Fe diffusion. In the present invention, in order to suppress excessive Fe diffusion of the Si, Mn, Al alone or complex oxide, it is possible to secure the adhesion by appropriately controlling the thickness of the lower alloy layer and the Fe alloying degree in the lower alloy layer. That is, in the present invention, the degree of Fe diffusion by Si, Mn, Al alone or a composite oxide may be adjusted to form a two-layer structure of a dense amorphous upper alloy layer and a lower alloy layer having a dense structure, thereby improving plating property. .

The upper alloy layer preferably has a thickness of 0.5 ~ 2㎛, it is preferable that the content of Fe is more than 4% by weight to less than 8% by weight. If the thickness of the upper alloy layer is less than 0.5㎛ or Fe content is less than 4% by weight Fe is not diffused smoothly can not effectively drop off the oxide formed on the surface of the steel sheet may lead to plating and adhesion inferior by the residual oxide. . On the other hand, in the case of more than 2㎛ or more than 8% by weight, brittle Γ alloy phase is formed by excessive Fe diffusion may cause plating peeling (powdering).

In addition, the upper alloy layer may have a line fraction on the SEM image of 80% or less. The measurement of the line fraction on the SEM image was performed by scanning electron microscope (SEM, Scanning Electron Microscopy) photographing 10 sections of plated layers at 5000 times magnification, and drawing a straight line parallel to the upper alloy layer to measure the line fraction of the alloy phase. This can be done by.

Preferably, the lower alloy layer has a thickness of 0.5 to 2 μm, the Fe content is 8 to 12 wt%, and the Mn content is 1 to 6 wt%. If the thickness of the lower alloy layer is less than 0.5㎛, the Fe content is less than 8% by weight, or the Mn content is less than 1% by weight Fe is not diffused smoothly do not effectively drop the oxide formed on the surface of the steel sheet remaining Oxides can cause plating and adhesion inferiority. On the other hand, when the content of Fe exceeds 2 µm, the content of Fe exceeds 12% by weight, or the content of Mn exceeds 6% by weight, a highly brittle Γ alloy phase is formed by excessive Fe diffusion, resulting in plating peeling (powdering). May cause.

In addition, the lower alloy layer preferably has a line fraction on the SEM image of 99% or more. When the lower alloy layer has a fraction of 99% on the SEM image, the surface of the plating may be inferior due to the occurrence of excessive Fe diffusion due to locally existing oxides and localized excessive alloying defects.

On the other hand, the hot-dip galvanized steel sheet of the present invention, the zinc-based plating layer, in particular, the alloying degree of the lower plating layer is preferably 6% or more and less than 9%. The low degree of alloying means that the surface layer oxide delayed Fe diffusion in the initial stage of the plating bath immersion, and when the alloying degree is less than 6%, residual oxide may remain on the surface layer of the steel sheet and the plating property may be inferior. If the alloying degree is 9% or more, Fe diffusion occurs from the steel sheet Fe to the entire plating layer, and the entire plating layer is alloyed, so that the surface appearance is not a hot-dip galvanized steel sheet, but an alloyed hot-dip galvanized steel sheet. It will not be secured.

Hereinafter, the manufacturing method of the hot-dip galvanized steel sheet of this invention is demonstrated.

When steel or the like is immersed in the hot dip galvanizing bath for the production of plated steel sheet, Fe is diffused from the steel into the hot dip galvanizing bath to increase the Fe concentration in the hot dip galvanizing bath, which is called Fe dissolution phenomenon. The present invention proposes the optimal conditions for producing ultra-high strength high manganese hot-dip galvanized steel sheet using the Fe leaching phenomenon.

The manufacturing method of the present invention includes preparing a steel sheet having the alloy composition described above, and then immersing the steel sheet in a hot dip galvanizing bath to obtain a hot dip galvanized steel sheet. At this time, the hot dip galvanizing bath preferably contains less than 0.01%, and the balance is composed of Zn. When the Al content is 0.01% by weight or more, the steel sheet Fe and Al in the plating bath react first, so that the Fe-eluting phenomenon is suppressed. There is a fear that plating peeling may occur due to the reduced and remaining surface oxide.

The hot dip galvanizing bath is preferably a temperature of 420 ~ 500 ℃. If the temperature of the hot dip galvanizing bath is less than 420 ° C., solidification of the zinc-based plating bath is started to increase the viscosity of the plating bath, thereby reducing the mobility of the roll winding the steel sheet, Slips between the rolls can lead to defects in the steel sheet. On the other hand, when it exceeds 500 ℃, it is possible to accelerate the dissolution of the steel sheet to accelerate the generation of dross in the form of iron (Fe) -zinc (Al) compound may cause unplating. On the other hand, the temperature of the hot dip galvanizing bath is preferably maintained not only when preparing the plating bath, but also until the plating proceeds by dipping the base steel sheet.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only examples for describing the present invention in more detail, and do not limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

After cold rolling the steel having the alloy composition shown in Table 1, and cleaning the surface of the steel sheet through a degreasing and pickling process, blowing nitrogen gas containing 5% by volume of hydrogen in a reduction furnace for 40 seconds at a temperature of 800 ℃ Annealing process was performed. Thereafter, after cooling the cold rolled steel sheet, the plating was performed by immersing for 5 seconds in a hot dip galvanizing bath under the conditions shown in Table 2 below. Thereafter, the coating weight was adjusted to 60 g / m 2 through air wipping. At this time, the plating bath inlet temperature of the cold rolled steel sheet was set to about 20 ° C. higher than the plating bath temperature, and immersed in the hot dip galvanizing bath to plate. The thickness, components, and the like of the plating layer were measured (average value) of the hot-dip galvanized steel sheet thus prepared, and the coating area ratio of the plating layer, the plating layer peeling or the like of the entire surface of the steel sheet were evaluated, and the results are shown in Table 2 below. . In order to measure the distribution of the annealing oxide formed at the interface between the alloy layer and the base steel sheet, the specimen was cut and 10 sections were analyzed at 2,000 magnification by SEM (Transmission Electron Microscopy) to perform SEM analysis on the SEM image. The ratio calculated by measuring the length of the alloy layer to the length of the x-axis is defined as the line fraction of the alloy layer and the average values are shown in Table 2 below. In addition, after bending the specimen 60 to 180 degrees in order to evaluate the adhesion of the plating, it was measured by taping the bent portion and then detaching the plating layer on the tape when detaching. It is shown in Table 2 below.

division Alloy composition (% by weight) C Mn Ti Si Al Inventive Steel 1 0.41 22.7 0.06 0.025 0.03 Inventive Steel 2 0.35 23.1 0.05 0.03 0.04 Invention Steel 3 0.44 22.5 0.05 0.03 0.03 Inventive Steel 4 0.45 22.2 0.06 0.02 0.02 Comparative Steel 1 0.1 2.5 0.05 0.1 0.05 Comparative Steel 2 0.3 18.5 0.07 0.06 0.06 Comparative Steel 3 0.1 15.2 0.06 0.05 0.1 Comparative Steel 4 0.15 3 0.04 0.2 0.2

division Steel grade no. Plating bath Upper alloy layer Temperature (℃) Al content (wt%) Thickness (㎛) Fe content (wt%) Inventive Example 1 Inventive Steel 1 456 0.008 0.6 5.5 Inventive Example 2 Inventive Steel 2 450 0.005 0.8 5.8 Inventive Example 3 Invention Steel 3 445 0.009 1.8 7.5 Inventive Example 4 Inventive Steel 4 465 0.006 1.1 6.6 Comparative Example 1 Inventive Steel 1 540 0.21 0.2 0 Comparative Example 2 Inventive Steel 2 451 0.24 2.3 0 Comparative Example 3 Invention Steel 3 565 0.13 3.5 0 Comparative Example 4 Inventive Steel 4 460 0.17 0.6 0 Comparative Example 5 Comparative Steel 1 462 0.007 0.5 5.1 Comparative Example 6 Comparative Steel 2 455 0.006 0.5 5.4 Comparative Example 7 Comparative Steel 3 445 0.008 0.6 5.2 Comparative Example 8 Comparative Steel 4 460 0.007 0.6 5.9

division
Bottom alloy layer Plating
Fraction (area%) Plating layer
Peel off thickness
(Μm) Fe content
(weight%) Mn content
(weight%) SEM image award
% Segment Inventive Example 1 0.8 8.2 4.5 99.6 98.5 Non-peel Inventive Example 2 1.5 10.2 5.2 99.7 99.0 Non-peel Inventive Example 3 1.1 9.6 4.9 99.4 98.8 Non-peel Inventive Example 4 1.6 10.8 5.8 99.8 99.2 Non-peel Comparative Example 1 1.5 30.0 0.8 89.5 89.0 Exfoliation Comparative Example 2 1.3 28.0 0.6 88.5 87.5 Exfoliation Comparative Example 3 2.0 25.5 0.5 90.5 93.0 Peeling Comparative Example 4 1.8 24.0 0.5 91.0 93.5 Peeling Comparative Example 5 0.8 8.1 0.15 87.0 88.5 Exfoliation Comparative Example 6 0.9 8.6 0.25 88.0 87.0 Exfoliation Comparative Example 7 1.3 9.5 0.30 89.5 93.5 Peeling Comparative Example 8 1.0 9.1 0.31 90.8 94.0 Peeling

As can be seen through Tables 1 to 3, in the case of Inventive Examples 1 to 4 satisfying the alloy composition and manufacturing conditions of the present invention, the Fe-Zn-Mn-based alloy layer is formed at the interface between the base steel plate and the zinc-based plating layer. , The Fe-Zn-Mn-based alloy layer is not only formed in a two-layer structure, the Fe-Zn-Mn-based alloy layer of the two-layer structure, such as the thickness, Fe content, line fraction on the SEM image proposed by the present invention It is understood that excellent plating adhesion is secured by satisfying the conditions.

On the other hand, Comparative Examples 1 to 4 is a case where the alloy composition proposed by the present invention is satisfied, but not the manufacturing conditions, the Fe-Zn-Mn-based alloy layer on the thickness, Fe content, SEM image proposed by the present invention It turns out that plating adhesiveness is inferior because it does not satisfy | fill conditions, such as a line fraction.

In Comparative Examples 5 to 8, the manufacturing conditions proposed by the present invention are satisfied, but the alloy composition is not satisfied. The Mn content of the lower alloy layer in the Fe-Zn-Mn-based alloy layer and the line fraction on the SEM image are It does not satisfy the proposed condition, and it turns out that plating adhesiveness is not favorable. This is because the oxide formed at the interface between the base steel sheet and the zinc-based plating layer exhibits a Si-rich layer form, and thus Fe dissolution is suppressed and it is determined that the oxide is caused by the remaining oxide.

1 is a cross-sectional photograph of Inventive Example 1. FIG. As can be seen in Figure 1, A corresponds to the lower alloy layer of the present invention, B can be seen that corresponds to the upper alloy layer, the Fe content of the A point was 8.2%, Mn content was 4.5% , Fe content at point B was 5.5%.

Claims (10)

By weight%, C: 0.3-0.9%, Mn: 10-25%, Ti: 0.01-0.5%, Si: 0.05% or less (except 0%), Al: 0.05% or less (except 0%), remainder A steel sheet containing Fe and other unavoidable impurities;
A zinc-based plating layer formed on at least one surface of the base steel sheet; And
It includes a Fe-Zn-Mn-based alloy layer formed at the interface between the base steel sheet and the zinc-based plating layer,
The zinc-based plating layer comprises Al in less than 0.01% by weight (excluding 0%), the balance is composed of Zn,
The Fe-Zn-Mn-based alloy layer includes a two-layer structure of the upper alloy layer and the lower alloy layer,
The upper alloy layer has a thickness of 0.5 ~ 2㎛, Fe content of more than 4% by weight to less than 8% by weight,
The lower alloy layer has a thickness of 0.5 ~ 2㎛, ultra-high strength high manganese hot-dip galvanized steel sheet having excellent plating properties of 8 to 12% by weight of Fe, 1 to 6% by weight of Mn.
delete delete The method according to claim 1,
The upper alloy layer is an ultra-high strength high manganese hot-dip galvanized steel sheet having excellent plating properties of the line fraction on the SEM image is 80% or less (excluding 0%).
delete delete The method according to claim 1,
The lower alloy layer is an ultra-high strength high manganese hot-dip galvanized steel sheet having excellent plating properties of more than 99% of the line fraction on the SEM image.
The method according to claim 1,
The hot-dip galvanized steel sheet is ultra-high strength high manganese hot-dip galvanized steel sheet having excellent plating properties of the alloying degree of the zinc-based plating layer is 6% or more and less than 9%.
By weight%, C: 0.3-0.9%, Mn: 10-25%, Ti: 0.01-0.5%, Si: 0.05% or less (except 0%), Al: 0.05% or less (except 0%), remainder Preparing a steel sheet containing Fe and other unavoidable impurities; And
Immersing the base steel sheet in a hot dip galvanizing bath to obtain a hot dip galvanized steel sheet,
The hot dip galvanizing bath contains less than 0.01% by weight of Al (except 0%), and the balance of Zn is excellent.
The method according to claim 9,
The hot dip galvanizing bath is a method of manufacturing a super high strength high manganese hot dip galvanized steel sheet having a plating property of 420 ~ 500 ℃ temperature.

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