KR20200076254A - Zinc plated steel sheet having excellent surface property and spot weldability and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a galvanized steel sheet having excellent surface quality and spot weldability, and a method for manufacturing the same. According to an aspect of the present invention, a galvanized steel sheet includes a plated steel sheet and a zinc-based plating layer formed on the plated steel sheet, and the ratio (a/b) between Ceq (a) at a surface layer and Ceq (b) at a point of 1/4 of the thickness of the plated steel sheet may be 0.7 or less, where Ceq is expressed by equation 1 below, and the surface layer Ceq refers to a Ceq value measured at a depth of 10 μm from the interface between the zinc-based plating layer and the plated steel sheet. [Equation 1] Ceq = C + Mn/6 + Si/24 + Cr/5 + Mo/4 + V/16 (%) (C, Mn, Si, Cr, Mo, and V each means the content (wt%) of the corresponding element).

Description

Zinc plated steel sheet with excellent surface quality and spot weldability and its manufacturing method{ZINC PLATED STEEL SHEET HAVING EXCELLENT SURFACE PROPERTY AND SPOT WELDABILITY AND MANUFACTURING METHOD THEREOF}

The present invention relates to a galvanized steel sheet excellent in surface quality and spot weldability and a method for manufacturing the same.

Due to environmental pollution, regulations on automobile emissions and fuel economy are being strengthened day by day. As a result, there is a strong demand for reduction of fuel consumption through weight reduction of automobile steel sheets, and thus, various types of high-strength steel sheets having high strength per unit thickness have been developed and released.

High-strength steel usually means a steel having a strength of 490 MPa or more, but is not limited thereto, transformation induced plasticity (TRIP) steel, twin induced plasticity (TWIP) steel, and abnormal structure ( This may include dual phase (DP) steel, complex phase (CP) steel, and the like.

On the other hand, automotive steel is supplied in the form of a plated steel plate that has been plated on the surface to ensure corrosion resistance. Among them, zinc plated steel plate (GI steel plate) or alloyed galvanized steel plate (GA) uses zinc sacrificial corrosion resistance to provide high corrosion resistance. Because it has, it is often used as a material for automobiles.

However, when the surface of the high-strength steel sheet is plated with zinc, there is a problem that spot weldability becomes weak. That is, in the case of high-strength steel, since the tensile strength and the yield strength are high, it is difficult to solve the tensile stress generated during welding through plastic deformation, and thus there is a high possibility of micro-cracking on the surface. When welding a high-strength galvanized steel sheet, zinc with a low melting point penetrates into the micro-cracks of the steel sheet, and as a result, a phenomenon called liquid metal embrittlement (LME) occurs, leading to a problem that the steel sheet leads to destruction. It can act as a major stumbling block to high strength of the steel sheet.

According to one aspect of the present invention, a galvanized steel sheet excellent in surface quality and spot weldability and a method for manufacturing the same are provided.

The subject of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding additional problems of the present invention from the general contents of the present specification.

A galvanized steel sheet according to one aspect of the present invention is a galvanized steel sheet including a steel sheet and a zinc-based plated layer formed on the steel sheet, Ceq(a) of the surface layer portion and Ceq(b) of 1/4 of the thickness of the steel sheet The liver ratio (a/b) may be 0.7 or less.

However, Ceq is represented by Equation 1 below, and the surface layer portion Ceq refers to a Ceq value measured at a position of 10 µm deep from the interface between the zinc-based plated layer and the steel plate.

[Equation 1]

Ceq = C + Mn/6 + Si/24 + Cr/5 + Mo/4 + V/16 (%)

(In the formula, C, Mn, Si, Cr, Mo, V each means the content (% by weight) of the corresponding element)

A method of manufacturing a hot dip galvanized steel sheet according to another aspect of the present invention includes: hot rolling a steel slab to obtain a hot rolled steel sheet; Winding the hot rolled steel sheet at a temperature of 590 to 750° C. to obtain a hot rolled steel sheet; Heating the edge portion of the wound hot-rolled steel sheet at 600 to 800°C for 5 to 24 hours; Cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; Annealing the cold rolled steel sheet at a mailing speed of 40 to 130 mpm while annealing at an atmosphere of a dew point of -10 to 30°C at 650 to 900°C; And hot-dip galvanizing the annealed cold rolled steel sheet.

As described above, the present invention can provide a galvanized steel sheet having excellent surface quality and excellent LME resistance when welding resistance points by forming a layer having a low Ceq under the surface layer portion directly under the high strength plating layer.

1 is a photograph showing the location of each crack by crack type.

Hereinafter, the present invention will be described in detail.

It should be noted that the term “galvanized steel sheet” in the present invention is a concept including not only galvanized steel sheet (GI steel sheet) but also alloyed galvanized steel sheet (GA) as well as galvanized steel sheet mainly containing zinc. Mainly containing zinc means that the proportion of zinc among the elements included in the plating layer is the highest. However, in the alloyed galvanized steel sheet, the proportion of iron may be higher than that of zinc, and among the remaining components except iron, the proportion of zinc may be the highest.

The inventors of the present invention have focused on the fact that the liquid metal embrittlement (LME) generated during welding is caused by micro-cracks generated from the surface of the steel sheet, and research the means for suppressing the micro-cracks on the surface. It has been found that it is necessary to soften the material and the present invention has been reached.

That is, in one embodiment of the present invention, the ratio (a/b) of the Ceq value (b) at 1/4 position of the thickness of the steel sheet in the direction of the thickness of the steel sheet relative to the Ceq value (a) of the surface layer portion of the steel sheet is controlled to be 0.7 or less. . In the present invention, Ceq can be defined by the following equation.

[Equation 1] Ceq = C + Mn/6 + Si/24 + Cr/5 + Mo/4 + V/16 (%)

Here, C, Mn, Si, Cr, Mo, V means the content (% by weight) of each element, and when the element is not included, the value of each element can be substituted into the formula as 0%.

In general, in the case of high-strength steel, a large amount of elements such as C, Mn, Si, Cr, Mo, and V may be included in order to secure hardenability or austenite stability of the steel, and these elements serve to increase the susceptibility to cracks in steel Do it. Therefore, the steel containing a large amount of these elements easily causes micro-cracks, which ultimately causes the embrittlement of liquid metal during welding. According to the research results of the present inventors, the behavior of micro-cracks is closely related to Ceq, and since the cracks are generated from the surface and propagate to the inside, the probability of micro-cracks is increased when the surface Ceq is high.

Therefore, in one embodiment of the present invention, the overall composition of the steel (which may be represented by the composition of a quarter of the thickness of the steel sheet in the thickness direction) is such that it has a high Ceq for high strength, but the surface layer portion, which is a point where cracks occur. Ceq has a low Ceq so that it can be resistant to cracks.

In the present invention, the surface layer means a 10 µm point in the depth direction from the surface of the steel sheet. Therefore, the Ceq of the surface layer portion means a Ceq value measured at a 10 µm depth of the steel sheet from the surface of the steel plate directly under the plated layer (ie, the interface between the plated layer and the plated steel plate).

In addition, since the Ceq value may vary depending on the width direction position of the steel sheet even at the same depth, the ratio (a/b) of the surface layer part Ceq(a) and Ceq(b) at a quarter of the thickness of the steel sheet is the same width direction position. It is preferable to calculate by the value measured in.

In one embodiment of the present invention, the ratio (a/b) of the Ceq may be based on a value measured at the center of the width direction. However, since the Ceq ratio (a/b) of the edge portion in the width direction is often higher than that of the center portion in the width direction of the steel sheet, the Ceq ratio (a/b) at the edge portion is defined by the present invention. If satisfied, the spot weldability can be further improved.

As described above, in one embodiment of the present invention, the Ceq ratio (a/b) may be 0.7 or less in order to secure sufficient spot weldability through prevention of LME generation. According to another embodiment of the present invention, the Ceq ratio (a/b) may be 0.6 or less, and in another embodiment, the Ceq ratio (a/b) may be 0.5 or less.

The lower the Ceq ratio (a/b) is advantageous, the lower limit of the Ceq ratio (a/b) need not be particularly limited. However, according to one embodiment of the present invention, the lower limit of the Ceq ratio (a/b) may be set to 0.02, and in other embodiments, the lower limit of the Ceq ratio (a/b) may be set to 0.1, and another In embodiments, the lower limit of the Ceq ratio (a/b) may be set to 0.2.

In addition, according to one embodiment of the present invention, the Ceq of the surface layer portion needs to be 0.5% or less. That is, the lower the Ceq of the surface layer, the less likely the occurrence of micro-cracks, so in one embodiment of the present invention, the value is limited to 0.5% or less. According to another embodiment of the present invention, the Ceq of the surface layer portion may be limited to 0.3% or less, and in another embodiment, the Ceq of the surface layer portion may be limited to 0.2% or less.

Since the lower the Ceq of the surface layer portion, the lower the Ceq of the surface layer portion is not particularly limited. However, in order to realize the strength of a high-strength steel sheet that may be targeted in one embodiment of the present invention, that is, a high-strength steel sheet having a tensile strength of 490 MPa or more, bulk (in other words, it may be represented by a quarter thickness point in the thickness direction of the steel sheet). Since Ceq of is high, when considering the value, the Ceq of the surface layer may be 0.1% or more, and in some cases, may be 0.15% or more.

Ceq (bulk Ceq) at a quarter of the thickness of the steel sheet is not specifically limited in the present invention as a design specification of the steel sheet. However, in one embodiment of the present invention, considering the high strength of the steel sheet, the Ceq of the quarter of the thickness of the steel sheet may be 0.24% or more, and in some cases, 0.3% or 0.5% or more. If the Ceq at the quarter of the thickness of the steel sheet is too high, the brittleness of the steel sheet may increase, so in one embodiment of the present invention, the Ceq at the quarter of the thickness of the steel sheet is 7.4% or less, and in some cases 6.0% or less, or It can be set to 5.0% or less.

In one embodiment of the present invention, the Ceq ratio (a/b) may be based on a value measured at the center of the width direction of the steel sheet. That is, after cutting the steel sheet in the width direction, the Ceq ratio (a/b) can be obtained by obtaining Ceq(a) at the center of the width direction and Ceq(b) at a quarter of the thickness of the steel sheet.

In addition, according to another embodiment of the present invention, since the surface portion Ceq(a) of the edge portion of the width direction of the steel sheet may have a lower value than the surface portion Ceq(a) of the center portion in the width direction, the Ceq ratio (a/b) is more strict. In terms of management, the Ceq ratio (a/b) may be a value measured at the edge portion in the width direction. That is, in another embodiment of the present invention, the ratio (a/b) of the surface layer portion Ceq(a) measured at the edge portion in the width direction and Ceq(b) at a quarter point in the thickness direction after cutting the steel plate in the width direction is It may have the above-described range. Here, the edge portion in the width direction refers to both end points of the cross-section in which the steel sheet is cut in the width direction, but when there is a problem in the integrity of the specimen, such as contamination occurring at the point, 1 mm inside the width direction from the end point Can mean the point of

According to one embodiment of the present invention, an inner oxide may be present in the surface layer portion of the steel sheet. The internal oxide may include at least one or more of Si, Mn, Al, and Fe, and may further include additional elements derived from the composition of the steel sheet.

The steel sheet to be targeted in the present invention is not limited as long as it is a high-strength steel sheet having a strength of 490 MPa or more. However, although not necessarily limited to this, the steel sheet targeted in the present invention is in a weight ratio, C: 0.05 to 1.5%, Si: 2.0% or less, Mn: 1.0 to 30%, S-Al (acid soluble aluminum): 3% or less, Cr: 2.5% or less, Mo: 1% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less, Sb+Sn+Bi: 0.1% or less, N: It may have a composition containing 0.01% or less. The rest of the components are iron and other impurities, and other elements not listed above, but are not excluded until they further contain elements that can be included in the steel in a total range of 1.0% or less. In the present invention, the content of each component element is indicated by weight unless otherwise specified. The above-mentioned composition means the bulk composition of the steel sheet, that is, the composition at a quarter of the thickness of the steel sheet (hereinafter, the same).

In some embodiments of the present invention, the high-strength steel sheet may be targeted to TRIP steel or the like. Each steel can have the following composition.

Steel composition 1: C: 0.05 to 0.30% (preferably 0.10 to 0.25%), Si: 0.5 to 2.5% (preferably 1.0 to 1.8%), Mn: 1.5 to 4.0% (preferably 2.0 to 3.0%) ), S-Al: 1.0% or less (preferably 0.05% or less), Cr: 2.0% or less (preferably 1.0% or less), Mo: 0.2% or less (preferably 0.1% or less), B: 0.005% Or less (preferably 0.004% or less), Nb: 0.1% or less (preferably 0.05% or less), Ti: 0.1% or less (preferably 0.001 to 0.05%), Sb+Sn+Bi: 0.05% or less, N : 0.01% or less, including residual Fe and unavoidable impurities. In some cases, the elements that are not listed above but can be included in the steel may further contain a total of 1.0% or less.

Steel composition 2: C: 0.05 to 0.30% (preferably 0.10 to 0.2%), Si: 0.5% or less (preferably 0.3% or less), Mn: 4.0 to 10.0% (preferably 5.0 to 9.0%), S-Al: 0.05% or less (preferably 0.001 to 0.04%), Cr: 2.0% or less (preferably 1.0% or less), Mo: 0.5% or less (preferably 0.1 to 0.35%), B: 0.005% Or less (preferably 0.004% or less), Nb: 0.1% or less (preferably 0.05% or less), Ti: 0.15% or less (preferably 0.001 to 0.1%), Sb+Sn+Bi: 0.05% or less, N : 0.01% or less, including residual Fe and unavoidable impurities. In some cases, the elements that are not listed above but can be included in the steel may further contain a total of 1.0% or less.

In addition, when the lower limit of the content of each of the above-mentioned component elements is not limited, it means that they may be regarded as arbitrary elements and the content may be 0%.

According to one embodiment of the present invention, the surface of the steel sheet may include one or more plating layers, and the plating layer may be a zinc-based plating layer including GI (Galvanized) or GA (Galva-annealed). In the present invention, since the Ceq ratio is appropriately controlled as described above, even if a zinc-based plating layer is formed on the surface of the steel sheet, the problem of liquid metal embrittlement occurring during spot welding can be suppressed.

When the zinc-based plating layer is a GA layer, the alloying degree (meaning the content of Fe in the plating layer) can be controlled to 8 to 13% by weight, preferably 10 to 12% by weight. If the degree of alloying is not sufficient, there is a possibility that the zinc in the zinc-based plating layer penetrates into the micro-cracks and causes a problem of embrittlement of the liquid metal. On the contrary, when the degree of alloying is too high, problems such as powdering may occur. .

In addition, the plating amount of the zinc-based plating layer may be 30 to 70 g/m ² . If the plating adhesion amount is too small, it is difficult to obtain sufficient corrosion resistance. On the other hand, when the plating adhesion amount is too large, problems in manufacturing cost increase and liquid metal embrittlement may occur, so that the plating is controlled within the above range. A more preferable range of the plating adhesion amount may be 40 to 60 g/m ² . This plating adhesion amount refers to the amount of the plating layer attached to the final product. When the plating layer is a GA layer, the weight of plating adhesion increases due to alloying, so its weight may decrease slightly before alloying, depending on the degree of alloying. Therefore, although not necessarily limited to this, the amount of adhesion before alloying (that is, the amount of plating attached from the plating bath) may be reduced by about 10%.

Hereinafter, one embodiment of manufacturing the steel sheet of the present invention will be described. However, it is necessary to note that the steel sheet of the present invention is not necessarily manufactured by the following embodiment, and the following embodiment is one preferred method for manufacturing the steel sheet of the present invention. According to this embodiment, oxidizing elements such as C, Mn, and Si in the surface layer portion of the steel sheet can be removed by reacting with the atmosphere outside the steel sheet during manufacture. As a result, the Ceq of the surface layer part is the bulk Ceq (Ceq of 1/4 point in the thickness direction). It can have a lower value, so that it can satisfy the preferred range of the present invention.

First, a hot rolled steel sheet can be produced by hot rolling a steel slab of the above-described composition and then winding it up. Conditions for heating the slab (temperature control in the case of direct rolling) or hot rolling are not particularly limited, but in one embodiment of the present invention, the winding temperature may be limited as follows.

Winding temperature: 590~750℃

The hot rolled steel sheet is then wound up and stored in a coil form, and the wound steel sheet undergoes a slow cooling process. Oxidizing elements included in the surface layer portion of the steel sheet are removed by the above process. When the coiling temperature of the slab is too low, the coil is annealed at a temperature lower than the temperature required for the oxidation removal of these elements, so it is difficult to obtain a sufficient effect. Conversely, if the coiling temperature is too high, the temperature deviation between the center portion and the edge portion in the width direction becomes large and the material deviation increases accordingly. In this case, the cold rolling property is inferior, and the strength of the final product is not only lowered, but also the moldability may be deteriorated. In addition, from the viewpoint of surface oxidation, if the coiling temperature is too high, re-oxidation of the scale may occur and Fe ₂ O ₃ may be generated, in which case surface quality may be deteriorated. Therefore, in one embodiment of the present invention, the upper limit of the coiling temperature may be set to 750°C.

Heated edge of hot-rolled coil: 5 to 24 hours at 600 to 800℃

In one embodiment of the present invention, the hot-rolled coil edge is heated to further lower the Ceq ratio (a/b) of the edge. The heating of the hot-rolled coil edge portion means heating both ends of the coiled coil in the width direction, that is, the edge portion, whereby the edge portion is first heated to a temperature suitable for oxidation by heating the edge portion. That is, the coiled coil is maintained at a high temperature inside, but the edge portion is cooled relatively quickly, thereby shortening the time to be maintained at a temperature suitable for internal oxidation at the edge portion. Therefore, the removal of the oxidizing element at the edge portion becomes less active than the center portion in the width direction. Edge heating may be used as one method for removing oxidizing elements at the edge.

That is, in the case of heating the edge portion, as opposed to the case of cooling after winding, the edge portion is first heated, and accordingly, the temperature of the edge portion in the width direction is appropriately maintained for internal oxidation. As a result, the thickness of the inner oxide layer of the edge portion increases. To this end, the heating temperature of the edge portion needs to be 600°C or higher (based on the temperature of the edge portion of the steel sheet). However, if the temperature is too high, the surface may be deteriorated after pickling due to excessive scale formation on the edge portion during heating or formation of porous high oxidation scale (hematite), so the edge portion temperature may be 800°C or less. A more preferable edge heating temperature is 600 to 750°C.

In addition, in order to eliminate the unevenness of the Ceq value of the surface layer portion between the edge portion and the center portion in the width direction during winding, the edge portion heating time needs to be 5 hours or more. However, if the edge portion heating time is too long, the scale may be excessively formed, or rather, the Ceq value (a) of the surface portion of the edge portion may be too low. Therefore, the edge portion heating time may be 24 hours or less.

According to one embodiment of the present invention, the edge heating may be performed by a combustion heating method through air-fuel ratio control. That is, the oxygen fraction in the atmosphere may be changed by adjusting the air-fuel ratio. The higher the oxygen partial pressure, the greater the concentration of oxygen in contact with the surface layer of the steel sheet may increase decarburization and internal oxidation. Although not necessarily limited to this, the present invention can be controlled to a nitrogen atmosphere containing 0.5 to 2% by volume of oxygen by adjusting the air-fuel ratio in one embodiment. Those skilled in the art to which the present invention pertains may control the oxygen fraction by adjusting the air-fuel ratio without particular difficulty, so this will not be described separately.

The hot rolled steel sheet subjected to the above-described process may be subjected to pickling as necessary, and then cold rolled. After the cold rolling process described above, the process of annealing the steel sheet may be followed. In the annealing process of the steel sheet, the surface layer portion Ceq(a) may vary greatly, so in one embodiment of the present invention, the annealing process can be controlled under conditions to properly control the surface layer portion Ceq(a). The dew point can be controlled under the following conditions.

Mail order speed: 40~130mpm

In order to secure sufficient productivity, the sheet speed of the cold rolled steel sheet needs to be 40MPm or more. However, if the mail order speed is excessively fast, it may be disadvantageous in terms of securing the material, and according to one embodiment of the present invention, the upper limit of the mail order speed may be set to 130 mpm. In addition, since the plate speed affects the thickness of the inner oxide layer, the thickness of the inner oxide increases and the carbon content tends to decrease as the plate speed decreases, so an excessively fast plate speed is sufficient for the thickness of the inner oxide layer. It can be a factor that prevents formation. According to an example, the inner oxide layer may be formed to a thickness of 3 μm or less at a plate speed of more than 130 mpm.

Annealing conditions: 650~900℃, -10~30℃ dew point atmosphere

In the present invention, the temperature at which annealing is performed may be 650° C. or higher, which is a temperature at which a sufficient internal oxidation effect is exhibited. However, when the temperature is too high, surface oxides such as Si are formed to prevent oxygen from diffusing inside, and austenite is excessively generated during heating of the crack zone, resulting in a decrease in the carbon diffusion rate and thereby decarburization degree. The temperature controlling the dew point may be 900° C. or less because it may be reduced and may also cause a problem of shortening equipment life and increasing process cost by generating an annealing furnace load. In the present invention, the annealing temperature means the temperature of the crack zone.

At this time, it is advantageous to control the dew point of the atmosphere in the annealing furnace in order to form a sufficient and uniform internal oxide layer. When the dew point is too low, there is a possibility that oxide such as Si or Mn is generated on the surface due to surface oxidation rather than internal oxidation. Therefore, it is necessary to control the dew point to -10°C or higher. Conversely, when the dew point is too high, there is a possibility that oxidation of Fe occurs, so the dew point needs to be controlled to 30°C or less.

At this time, the dew point can be adjusted by adding wet nitrogen (N ₂ +H ₂ O) containing 1 to 10% by volume of hydrogen into an annealing furnace.

The steel sheet annealed by this process is reheated to a plating bath temperature or higher (460 to 500°C) and then immersed in a plating bath to perform hot dip galvanization. According to one embodiment of the present invention, the thickness of the annealed steel sheet immersed in the plating bath may be adjusted to 1.0 to 2.0 mm. According to one embodiment of the present invention, the plating bath may include 50% by weight or more of Zn as a zinc-based plating bath.

The hot-dip galvanized steel sheet plated by the above-described process may then be subjected to an alloying heat treatment process as necessary. Preferred conditions for the alloying heat treatment are as follows.

Alloying (GA) temperature: 480~560℃

Below 480℃, the amount of Fe diffusion is small, so the degree of alloying is insufficient, so the plating properties may not be good. If it exceeds 560℃, powdering problems due to excessive alloying may occur, and ferrite transformation of residual austenite Since the furnace material may deteriorate, the alloying temperature is set in the above-described range.

In one embodiment of the present invention, in order to secure the sufficient degree of alloying, the alloying heat treatment time may be 1 second or more. However, when the alloying heat treatment time is too long, the alloying degree may exceed the range specified in the present invention, so the upper limit of the alloying heat treatment time may be set to 5 seconds.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention and not to limit the scope of the present invention. This is because the scope of the present invention is determined by the items described in the claims and the items reasonably inferred therefrom.

(Example)

Steel slab having the composition shown in Table 1 below (the remaining components not listed in the table are Fe and impurities that are inevitably included. In the table, B and N are expressed in ppm units, and the remaining components are expressed in weight %) After hot rolling, the edge portion was heated in a nitrogen atmosphere containing oxygen to the hot rolled coil, and after pickling, the obtained cold rolled steel plate was annealed in an annealing furnace, and the steel plate was reheated to 480°C and Al was 0.13. Hot dip galvanizing was performed by immersion in a zinc-based plating bath containing weight percent. The obtained hot-dip galvanized steel sheet was subjected to an alloying (GA) heat treatment for 4 seconds as necessary to finally obtain an alloyed hot-dip galvanized steel sheet.

When the hot-dip galvanized steel sheet is obtained without alloying, the cold-rolled steel sheet is annealed in an annealing furnace and reheated, followed by plating by dipping in a zinc-based plating bath containing 0.24% by weight of Al. After cooling the steel sheet finally to obtain a hot dip galvanized (GI) steel sheet.

In all examples, in order to obtain a steel sheet having a thickness of 1.6 mm, the rolling reduction during cold rolling was set to 52%, the crack zone temperature during annealing was set to 830°C, and the proportion of hydrogen contained in wet nitrogen in the annealing furnace was set to 5.0% by volume. Did. The conditions for each other example are as described in Table 2 (B and N in the table are expressed in ppm units, and the remaining components are expressed in weight percent).

Steel C Si Mn S-Al Cr Mo B Nb Ti V Sb Sn Bi N A 0.295 1.3 2.24 0.0035 0 0 27 0 0.0037 0 0 0 0 17 B 0.201 1.395 2.321 0.0321 0.0019 0.051 17 0.035 0.021 0 0.017 0 0 21 C 0.165 1.47 2.65 0.0027 0.047 0 13 0 0.031 0 0 0.012 0 11 D 0.217 0.324 6.54 0.0211 0.001 0 14 0.021 0.031 0 0.021 0 0 18 E 0.12 0.05 4.65 0.0031 0 0 17 0 0.047 0.05 0 0 0.017 31

Steel division Winding temperature Edge heating Annealing Alloying (℃) Heating temperature Heating time Oxygen fraction Mail order speed 650~900℃ Temperature (℃) (time ) (%) (mpm) Dew point at (℃) (℃) A Inventive Example 1 620 700 12 0.8 79 13 523 D Comparative Example 1 691 550 12 0.79 67 19 527 C Inventive Example 2 680 650 7 1.4 84 17 527 A Comparative Example 2 603 707 11 1.2 65 11 574 B Comparative Example 3 842 720 10 1.2 60 17 515 B Comparative Example 4 623 711 12 5.4 74 17 525 B Comparative Example 5 597 704 12 1.1 74 -14 GI E Comparative Example 6 592 701 36 0.89 62 14 475 A Comparative Example 7 507 708 11 0.75 85 14 527 B Inventive Example 3 597 704 12 1.1 74 11 GI E Comparative Example 8 611 704 5 1.4 152 21 514 B Comparative Example 9 621 714 12 0.8 74 15 470 E Inventive Example 4 598 715 6 1.4 51 21 480 A Comparative Example 10 620 695 11 0.02 64 16 524 A Inventive Example 5 604 701 10 0.8 64 10 GI A Comparative Example 11 604 701 10 0.8 64 -10 GI D Inventive Example 6 691 634 8 0.84 45 19 480 C Comparative Example 12 602 668 12 0.8 27 15 524 E Comparative Example 13 598 715 6 1.4 51 42 482 B Inventive Example 7 612 720 10 1.2 60 14 515 D Comparative Example 14 624 712 12 0.9 67 41 GI D Inventive Example 8 624 712 12 0.9 67 18 GI C Inventive Example 9 612 695 11 1.4 84 14 GI C Comparative Example 15 612 695 11 1.4 84 45 GI E Comparative Example 16 609 874 13 0.7 67 5 GI

Table 3 shows the results of measuring the properties of the alloyed hot-dip galvanized (GA) steel sheet manufactured by the above-described process and observing whether liquid metal embrittlement (LME occurred) during spot welding. It was cut in the direction and carried out along each cut edge area.The spot welding current was applied twice and the hold time of 1 cycle was maintained after the energization.The spot welding was performed in three different layers Evaluation Materials-Evaluation Materials-GA 980DP 1.4 The spot welding was performed by laminating in the order of t. When spot welding, the new electrode was welded 15 times to a soft material, and after the electrode was worn, the upper limit current at which expulsion occurred in the spot welding target material was measured. After measuring the current, spot welding is performed 8 times per welding current at currents 0.5 and 1.0 kA lower than the upper limit current, and the cross section of the spot weld is precisely processed by electric discharge machining, then polished by epoxy mounting, and the crack length is measured by an optical microscope. When observing the optical microscope, the magnification was designated as 100 times, and if no crack was found at the magnification, it was determined that liquid metal embrittlement did not occur, and when cracks were found, the length was measured by image analysis software. As a representative form of each type of crack occurring in the case, in the case of a B-type crack occurring in the shoulder portion of a spot weld, it was judged as bad when a crack having a length exceeding 100 μm was present, otherwise it was judged as good. If a C-type crack was observed (there is no limitation on the length), it was judged to be bad, otherwise it was judged to be good.If any defect occurred in each specimen, the resistance to LME (point weldability) during welding was not good. It can be judged as not.

Ceq was calculated using concentration values for each depth using GDOES. The surface layer part Ceq(a) took the minimum value of Ceq within a range of 30 µm from the surface of the small iron. Ceq(b) at the 1/4t point took Ceq at that point. Tensile strength was measured through a tensile test by making a C-direction sample of JIS-5 standard. The alloying degree and plating adhesion amount were measured using a wet dissolution method using a hydrochloric acid solution. The sealer adhesion was confirmed by attaching the structural adhesive D-type for automobiles to the plating surface, and then bending the steel plate 90 degrees to see if the plating fell off. After powdering, the plated material was bent to 90 degrees, and the tape was adhered to the bent portion, and then peeled off to confirm how many millimeters of the plated layer dropped off the tape. Flaking was checked in the form of'U' and then the plated layer was removed from the processed part.

division Surface part Ceq (a) 1/4t Ceq (b) a/b Tensile strength (Mpa) Plating type Fe alloying degree Plating adhesion (g/m2) Powdering Flaking Sealer adhesion LME occurrence B-type length (㎛) A-type length (㎛) Inventive Example 1 0.21 0.71 0.3 1192 GA 10.8 52 3 Good - 32 0 Comparative Example 1 1.07 1.28 0.84 1152 GA 10.4 46 2 Good - 105 104 Inventive Example 2 0.27 0.64 0.42 1204 GA 10.1 47 2 Good - 45 0 Comparative Example 2 0.31 0.72 0.43 1184 GA 16.1 46 10 Good - 34 0 Comparative Example 3 0.14 0.66 0.21 450 GA 9.7 45 One Good - 24 0 Comparative Example 4 0.24 0.66 0.36 954 GA 10.1 48 One Good - 24 0 Comparative Example 5 0.68 0.66 1.03 945 GI - 59 - - Bad 104 121 Comparative Example 6 0.21 0.9 0.23 1104 GA 9.4 46 3 Good - 21 0 Comparative Example 7 0.72 0.72 One 1192 GA 10.6 51 2 Good - 121 421 Inventive Example 3 0.24 0.66 0.36 975 GI - 59 - - Good 34 0 Comparative Example 8 0.72 0.9 0.8 1141 GA 9.7 54 One Good - 142 245 Comparative Example 9 0.24 0.66 0.36 975 GA 6.4 52 2 Bad - 54 0 Inventive Example 4 0.21 0.9 0.23 1165 GA 10.4 49 3 Good - 21 0 Comparative Example 10 0.67 0.72 0.93 1142 GA 10.9 49 3 Good - 153 241 Inventive Example 5 0.2 0.72 0.28 1178 GI - 57 - - Good 47 0 Comparative Example 11 0.67 0.72 0.93 1165 GI - 61 - - Bad 221 534 Inventive Example 6 0.42 1.32 0.32 1124 GA 9.4 54 2 Good - 21 0 Comparative Example 12 0.14 0.67 0.21 442 GA 10.4 47 3 Good - 47 0 Comparative Example 13 0.21 0.9 0.23 1145 GA 9.1 46 3 Good - 24 0 Inventive Example 7 0.41 0.66 0.62 954 GA 10.4 47 2 Good - 45 0 Comparative Example 14 0.27 1.32 0.2 1154 GI - 60 - - Bad 142 412 Inventive Example 8 0.43 1.32 0.33 1134 GI - 64 - - Good 34 0 Inventive Example 9 0.24 0.67 0.36 945 GI - 61 - - Good 32 0 Comparative Example 16 0.17 0.67 0.25 931 GI - 59 - - Bad 47 0

※1/4t Ceq means the Ceq at the point of 1/4 of the thickness of the steel sheet. The units of each Ceq in the table are% by weight.

Inventive Examples 1, 2, 3, 4, 5, 6, 7, 8, and 9 satisfy the range suggested by the present invention, and the manufacturing method also satisfies the scope of the present invention. Tensile strength, plating It was confirmed that various physical properties such as quality, plating adhesion amount and spot welding LME crack length were good.

In Comparative Example 1, the heating temperature of the heat treatment furnace was lower than that of the present invention. A sufficient internal oxide layer was not formed during hot rolling, and thus it was poor because it did not satisfy the criteria in the evaluation of spot welding LME cracks.

Comparative Example 2 is a case in which the alloying temperature in the GA alloying process exceeds the range suggested by the present invention. The color of the Fe alloy was high, so the color was dark and the surface quality was poor. Powdering occurred excessively when evaluating GA powdering.

Comparative Example 3 is a case in which the coiling temperature during the hot rolling process was higher than the range suggested by the present invention. Therefore, although the internal oxidation occurring during the hot rolling process was sufficiently generated, the LME characteristics were good and the plating quality was good, but the temperature of the hot rolled coil was excessively high, resulting in softening of the hot rolled material and not recovering after annealing.

Comparative Example 4 is a case in which the heating temperature and time of the heat treatment furnace satisfy the range suggested by the present invention, but the oxygen fraction exceeds the range. During the heat treatment process, peroxidation occurred at the edge portion to form a red hematite-based scale on the surface, and the thickness of the scale layer became excessively thick. During the pickling process after hot rolling, the edge portion was excessively picked up, resulting in high surface roughness, resulting in uneven surface shape after plating, and color nonuniformity defects in which the surface color was different from the central portion.

In Comparative Examples 5 and 11, the dew point in the furnace during annealing was lower than the range suggested by the present invention. Even if a sufficient internal oxidation layer was generated for the entire width during the heating process by hot rolling and heat treatment, the point welding LME crack length was poor at the full width because the dew point was not sufficiently high during the annealing process after cold rolling, so that sufficient internal oxidation layer could not be formed after component homogenization occurred. In the case of GI material, since the dew point was low and sufficient internal oxidation was not generated, the surface oxide was excessively generated, resulting in poor plating adhesion.

In Comparative Example 6, the heating temperature of the heat treatment furnace satisfies the scope of the present invention, but when the heating time is exceeded, peroxidation occurs at the edge portion during the heat treatment process to form a red hematite-based scale on the surface. , The thickness of the scale has become excessively thick. During the pickling process after hot rolling, the edge portion was excessively picked up, resulting in high surface roughness, resulting in uneven surface shape after plating, and color nonuniformity defects in which the surface color was different from the central portion.

Comparative Example 7 is a case in which the coiling temperature during the hot rolling process was lower than the range suggested by the present invention. Therefore, even if the dew point is high during annealing because the internal oxidation generated during the hot rolling process is not sufficiently generated, the surface layer part Ceq is formed at 0.7 or higher, and the a/b value is high at 1.0, so it is the standard for spot welding LME evaluation even if the plating surface quality is excellent. The above crack occurred and LME resistance was inferior.

In Comparative Example 8, the sheet speed of the steel sheet in the annealing was controlled higher than the range suggested by the present invention. Since sufficient time was not given for the internal oxidation reaction in which the water vapor and the steel sheet reacted in the annealing furnace, the carbon level of the surface layer of the steel sheet after annealing was high, and thus exceeded the criteria when evaluating the spot welding LME crack.

Comparative Example 9 is a case in which the alloying temperature in the GA alloying process was lower than the range suggested by the present invention. Since the Fe alloyation degree of the plating layer was formed lower than the standard, the surface quality was poor because the surface was too bright, and flaking occurred, resulting in inferior plating surface quality.

In Comparative Example 10, the heat treatment furnace heating temperature and time satisfy the range suggested by the present invention, but the oxygen fraction was controlled lower than the range. The hot rolled internal oxidation was not sufficiently formed, so it was not good enough to meet the criteria when evaluating cracks in spot welding LME.

Comparative Example 12 is a case where the sheet speed of the steel sheet in the annealing was lower than the range suggested by the present invention. Sufficient time was given for the internal oxidation reaction in which the water vapor and the steel sheet reacted in the annealing furnace to satisfy the criteria in the evaluation of the spot welding LME crack, but the material was inferior because the microstructure for securing the material could not be annealed.

In Comparative Examples 13, 14 and 15, the dew point in the furnace during annealing was higher than the range suggested by the present invention. In addition to the internal oxidation during hot rolling, the internal oxidation during annealing was sufficiently generated, so the LME resistance and plating surface quality were good, but the oxide was physically attached to the hearth roll surface in the annealing furnace while Mn-based oxide was excessively generated. Inducing dent was poor in operability.

Comparative Example 16 is a case where the heat treatment furnace temperature exceeds the range suggested in the present invention, peroxidation occurs in the edge portion during the heat treatment process to form a red hematite-based scale on the surface, and the thickness is It became too deep. During the pickling process after hot rolling, the edge portion was excessively picked up, resulting in high surface roughness, resulting in uneven surface shape after plating, and color nonuniformity defects in which the surface color was different from the central portion.

Claims (11)

As a galvanized steel sheet comprising a steel sheet and a zinc-based plating layer formed on the substrate steel plate,
Galvanized steel sheet with excellent surface quality and spot weldability with a ratio (a/b) of Ceq(a) at the quarter of the surface layer portion and Ceq(b) at a quarter of the thickness of the steel plate.
However, Ceq is represented by Equation 1 below, and the surface layer portion Ceq refers to a Ceq value measured at a position of 10 µm deep from the interface between the zinc-based plated layer and the steel plate.
[Equation 1]
Ceq = C + Mn/6 + Si/24 + Cr/5 + Mo/4 + V/16 (%)
(In the formula, C, Mn, Si, Cr, Mo, V each means the content (% by weight) of the corresponding element)
The galvanized steel sheet of claim 1, wherein the surface layer portion Ceq is 0.5% or less and has excellent surface quality and spot weldability.
The galvanized steel sheet according to claim 1, wherein the Ceq at a quarter of the thickness of the steel sheet is 0.24% or more and has excellent surface quality and spot weldability.
The galvanized steel sheet of claim 1, wherein the tensile strength is superior to a surface quality of 490 Mpa or higher and spot weldability.
The zinc-plated steel sheet according to claim 1, wherein the zinc-based plating layer has a plating adhesion amount of 30 to 70 g/m2.
The zinc-plated steel sheet according to claim 1, which is an alloyed hot dip galvanized (GA) layer having an alloying degree of 8 to 13% by weight.
The steel sheet according to any one of claims 1 to 6, wherein the steel sheet is C: 0.05 to 1.5%, Si: 2.0% or less, Mn: 1.0 to 30%, S-Al (acid soluble aluminum): 3% or less, Cr: 2.5% or less, Mo: 1% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less, Sb+Sn+Bi: 0.1% or less, N: 0.01% A galvanized steel sheet excellent in spot weldability having a composition including the following.
Hot rolling a steel slab to obtain a hot rolled steel sheet;
Winding the hot rolled steel sheet at a temperature of 590 to 750° C. to obtain a hot rolled steel sheet;
Heating the edge portion of the wound hot-rolled steel sheet at 600 to 800°C for 5 to 24 hours;
Cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet;
Annealing the cold rolled steel sheet at a mailing speed of 40 to 130 mpm while annealing in an atmosphere of a dew point of -10 to 30°C at 650 to 900°C; And
Hot-dip galvanizing the annealed cold rolled steel sheet
Method for producing a galvanized steel sheet excellent in spot weldability comprising a.
The method of claim 7, further comprising the step of alloying heat treatment of the hot-dip galvanized cold rolled steel sheet.
10. The method of claim 8, wherein the alloying heat treatment is performed at a temperature of 480 to 560°C, and the method of manufacturing a galvanized steel sheet having excellent weldability.
The method according to any one of claims 8 to 10, wherein the steel slab is C: 0.05 to 1.5%, Si: 2.0% or less, Mn: 1.0 to 30%, S-Al (acid soluble aluminum): 3% or less , Cr: 2.5% or less, Mo: 1% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less, Sb+Sn+Bi: 0.1% or less, N: 0.01 % Galvanized steel sheet with excellent weldability.

Images