KR102312424B1 - Manufacturing method of galvanized steel sheet with excellent weldability and galvanized steel sheet - Google Patents

Abstract

A method of manufacturing a galvanized steel sheet having excellent weldability according to one aspect is, in weight%, carbon (C): 0.12 to 0.22%, silicon (Si): 1.6 to 2.4%, manganese (Mn): 2.0 to 3.0%, phosphorus (P): greater than 0 and less than or equal to 0.02%, sulfur (S): greater than zero and less than or equal to 0.005%, aluminum (Al): 0.01 to 0.05%, chromium (Cr): greater than 0 and less than or equal to 0.05%, titanium (Ti): greater than 0, 0.05 % or less, molybdenum (Mo): more than 0 and 0.05% or less, niobium (Nb): more than 0 and not more than 0.05%, preparing a hot-rolled or cold-rolled steel sheet containing the remainder iron (Fe) and other unavoidable impurities, steel sheet annealing heat treatment at 870 ~ 900 ℃, primary cooling the steel sheet to a temperature of 700 ~ 800 ℃, secondary cooling the steel sheet to 200 ~ 300 ℃, reheating and maintaining the steel sheet to 370 ~ 430 ℃ and performing hot-dip galvanizing treatment on the steel sheet.

Description

Manufacturing method of galvanized steel sheet with excellent weldability and galvanized steel sheet

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a method for manufacturing an ultra-high strength galvanized steel sheet having excellent weldability and a galvanized steel sheet.

From the viewpoint of global environmental conservation, in order to reduce CO2 emissions, it has always been an important task in the automobile industry to reduce the weight of automobile bodies while maintaining their strength and to improve fuel efficiency of automobiles. In order to achieve weight reduction while maintaining the strength of the automobile body, it is effective to reduce the thickness of the steel sheet by increasing the strength of the steel sheet used as a material for automobile parts. On the other hand, most of the automobile parts using a steel plate as a raw material are formed by press working, burring, or the like. For this reason, in addition to having a desired intensity|strength, the outstanding formability is calculated|required of the high strength steel plate used as a raw material for automobile parts.

In recent years, the application of ultra-high-strength steel sheet having a tensile strength TS of 1180 MPa beginner level as a material for a skeleton of an automobile body is expanding. However, such an ultra-high strength steel sheet has a high forming difficulty, and since it is difficult to apply conventional press forming as it is, bending-based processing such as roll forming is often performed. For this reason, bending workability is one of the most important characteristics in applying a 1180 MPa elementary grade ultra-high strength steel sheet. On the other hand, in the component material for automobiles, one of the important characteristics is impact resistance. In the event of a car crash, the deformation rate of each part made of a steel plate reaches up to about 103/s. For this reason, for example, in automobile parts such as pillars, members, and bumpers, impact resistance necessary for ensuring the safety of occupants in the event that the automobile collides while driving is required. That is, it is necessary to secure the collision safety of the vehicle by applying a high-strength steel sheet having impact resistance that exhibits excellent impact energy absorption ability even when subjected to the above high deformation rate during a collision.

As a technology related thereto, there is Republic of Korea Patent Publication No. 10-2014663 (registered on Aug. 20, 2019, high-strength steel sheet and its manufacturing method).

The problem to be solved by the present invention is to provide an excellent method for manufacturing an ultra-high strength galvanized steel sheet and a galvanized steel sheet.

The method for manufacturing a galvanized steel sheet having excellent weldability according to an aspect of the present invention is, by weight, carbon (C): 0.12 to 0.22%, silicon (Si): 1.6 to 2.4%, manganese (Mn): 2.0 to 3.0 %, phosphorus (P): more than 0 and less than 0.02%, sulfur (S): more than 0 and less than or equal to 0.005%, aluminum (Al): 0.01 to 0.05%, chromium (Cr): more than 0 and less than or equal to 0.05%, titanium (Ti): To prepare a hot-rolled or cold-rolled steel sheet containing more than 0 and 0.05% or less, molybdenum (Mo): more than 0 and 0.05% or less, niobium (Nb): more than 0 and 0.05% or less, and the remainder iron (Fe) and other unavoidable impurities. step; annealing the steel sheet at 870 to 900° C.; First cooling the steel sheet to a temperature of 700 ~ 800 ℃; Secondary cooling of the steel sheet to 200 ~ 300 ℃; Reheating and maintaining the steel sheet to 370 ~ 430 ℃; and performing hot-dip galvanizing treatment on the steel sheet.

In the annealing heat treatment step, the dew point in the annealing furnace is -40 ~ -20 ℃,

The moisture concentration in the annealing furnace is preferably about 120 to 6,000 ppm.

The annealing heat treatment may be performed in a furnace in a hydrogen atmosphere of 5 to 7%.

The galvanized steel sheet has a ferrite single-phase microstructure, tensile strength (TS): 850 MPa or more, yield strength (YP): 1,180 MPa or more, and elongation (EL): 14% or more.

The galvanized steel sheet may have an applicable welding current range of 5.5 kA to 7.5 kA.

A galvanized steel sheet having excellent weldability according to another aspect of the present invention, by weight, carbon (C): 0.12 to 0.22%, silicon (Si): 1.6 to 2.4%, manganese (Mn): 2.0 to 3.0%, phosphorus (P): greater than 0 and less than or equal to 0.02%, sulfur (S): greater than zero and less than or equal to 0.005%, aluminum (Al): 0.01 to 0.05%, chromium (Cr): greater than 0 and less than or equal to 0.05%, titanium (Ti): greater than 0, 0.05 % or less, molybdenum (Mo): greater than 0 and less than or equal to 0.05%, niobium (Nb): more than 0 and less than or equal to 0.05%, including the remainder iron (Fe) and other unavoidable impurities, having a ferrite single-phase microstructure, tensile Strength (TS): 850 MPa or more, yield strength (YP): 1,180 MPa or more, elongation (EL): 14% or more of physical properties.

The galvanized steel sheet may have an applicable welding current range of 5.5 kA to 7.5 kA.

According to the present invention, by applying an appropriately controlled alloy composition and high dew point annealing heat treatment process, austenite generation in the surface layer, which is the cause of LME cracks in the welded part, is suppressed, thereby securing strength even under existing welding conditions and expanding the total weldable range. Defects in the welding process can be suppressed and productivity can be improved.

1 and 2 are photographs of microscopic observation of the surface layer structure according to the temperature of the annealing furnace.
3A to 4B are photographs showing the elongation and crack test results of the steel sheet heat treated in an annealing furnace having a general dew point and a high dew point.

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.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, the present inventors discovered the following, as a result of repeating earnest research and experiment from the viewpoint of the component composition of a steel plate, a structure|tissue, and a manufacturing method.

In the case of resistance spot welding of ultra-high strength hot-dip galvanized steel sheet, in the case of hot-dip galvanized steel sheet (GI), the melting point of the plated layer is very low, about 420°C, and in the case of alloyed galvanized steel sheet, the capping reaction (Γ→L) near 780°C. +α) forms liquid zinc. The formed liquid zinc (Liquid Zn) penetrates along the grain boundary of the base material in the region where the load by the welding electrode occurs at high temperature, and the strength of the base material is rapidly inferior. The phase transformation from Y→αFe(Zn) at high temperature by zinc (Zn) diffused toward the base material around the LME crack, and the brittleness is further accelerated by the αFe(Zn) phase with very brittle properties. is being reported

As a result of internal evaluation, the frequency of occurrence of LME by steel type is more sensitive as the amount of retained austenite increases.

On the other hand, the phase fraction control of steel products is made in the process of reheating/cooling/annealing with alloy components, and the austenite phase formed in the component system and heating step is transformed into martensite/bainite or remains as austenite in the cooling and reheating process to form a complex award will be created. The temperature of the heating stage is the region where the austenite phase is formed, but if the dew point temperature of the atmosphere in the annealing furnace is increased to -40°C or higher, the austenite stabilizing element carbon (C) is oxidized on the steel surface and volatilized in the form of carbon monoxide. The decarburization reaction proceeds by the reaction.

If carbon (C) in the surface layer is depleted due to the continuation of this decarburization reaction, and the austenite in the surface layer is already transformed into ferrite even in the heating stage, austenite is not formed in the surface layer in the future cooling/reheating process and ferrite even at room temperature, which is the final step. A single-phase structure can be secured. Austenite, which is sensitive to LME cracks, can be avoided by the ferrite single-phase structure formed in the surface layer, so that the range of welding current can be expanded and welding strength can be increased.

1 and 2 are photographs of microscopic observation of the surface layer structure according to the temperature of the annealing furnace. is a photograph of the surface structure of the base material after annealing heat treatment at a high dew point of -40°C to -20°C.

1 and 2, in the case of FIG. 2 conducted at the high dew point, carbon (C) is depleted due to the continuous decarburization reaction, and it can be seen that the depth of the austenite layer in the surface layer is reduced compared to FIG. 1 conducted at the general dew point. have.

Annealing heat treatment was performed at a temperature of 700~900℃ for a 1200MPa class QP hot-dip galvanized steel sheet having the same alloy composition to examine the occurrence of cracks and changes in elongation of the steel. At this time, the temperature increase rate was set to 500 °C / s and the holding time was set to 1 second, respectively, and the occurrence of LME cracks observed at each temperature is shown in Table 1 below, and the elongation and crack test results are shown in FIGS. 3a to 4b. .

division 700℃ 750℃ 800℃ 850℃ 900℃ general stall LME LME LME LME LME Gono Branch No LME No LME No LME LME LME

As shown in Table 1, when annealing heat treatment was performed at a general dew point, LME cracks occurred at an annealing heat treatment temperature of 700 ° C., but it can be seen that LME cracks occurred from 850 ° C. when carried out at a high dew point.

In addition, referring to FIGS. 3A and 3B , it can be seen that LME cracks occurred at 800° C. when carried out at a general dew point, and the elongation of the steel was lowered due to the cracks.

Referring to FIGS. 4A and 4B , it can be seen that, when carried out at a high dew point, LME cracks did not occur at 800° C. and the elongation rate of the steel due to cracks did not decrease.

As such, even with the same alloy composition, when the annealing heat treatment process is performed at a high dew point, the austenite sensitive to LME cracks can be avoided by the ferrite single-phase structure formed in the surface layer due to the decarburization action, thereby expanding the weldable current range. Weld strength can be increased.

Hereinafter, a galvanized steel sheet having excellent weldability and a method for manufacturing the same according to a preferred aspect of the present invention will be described in detail.

Ultra-high-strength galvanized steel sheet with excellent weldability

The ultra-high strength galvanized steel sheet excellent in weldability according to an aspect of the present invention, by weight, carbon (C): 0.12 to 0.22%, silicon (Si): 1.6 to 2.4%, manganese (Mn): 2.0 to 3.0% , phosphorus (P): more than 0 and less than 0.02%, sulfur (S): more than 0 and less than or equal to 0.005%, aluminum (Al): 0.01 to 0.05%, chromium (Cr): more than 0 and less than or equal to 0.05%, titanium (Ti): 0 It contains more than 0.05%, molybdenum (Mo): more than 0 and not more than 0.05%, niobium (Nb): more than 0 and not more than 0.05%, and contains the remainder iron (Fe) and other unavoidable impurities.

The galvanized steel sheet according to the present invention having the above alloy composition, after hot stamping, has tensile strength (TS): 850 MPa or more, yield strength (YS): 1,180 MPa or more, elongation (EL): 14%, and weldable current range: 5.5 Aim for kA to 7.5 kA.

Hereinafter, the role and content of each component included in the essential alloy composition of the ultra-high strength galvanized steel sheet excellent in weldability according to the present invention will be described in more detail.

Carbon (C): 0.12 to 0.22 wt%

In the present invention, carbon (C) is the most rigid and effective alloying component to generate martensite to secure the strength of steel. Carbon (C) is preferably added in a content ratio of 0.12 to 0.22% by weight of the total weight of the steel according to the present invention. When carbon (C) is added in an amount of 0.12 wt% or less, it may be good in terms of toughness, but since the effect of strengthening the steel by combining with Nb, V or Ti is very small, it is necessary to add 0.12 wt% or more to secure strength have. On the other hand, when the content of carbon (C) exceeds 0.22% by weight, the strength of the steel increases, but there is a problem in that low-temperature impact toughness and weldability are lowered, so it is preferable to limit the carbon (C) content to 0.12 to 0.22% by weight. .

Silicon (Si): 1.6 to 2.4 wt%

In the present invention, silicon (Si) is added as a deoxidizer for removing oxygen in steel during steelmaking. In addition, silicon (Si) is an effective component for solid solution strengthening. In order to obtain the above effect, it is preferable to add 1.6 wt% or more of silicon (Si). However, when it exceeds 2.4 wt%, since weldability and toughness are reduced, it is preferable to add Si in the range of 1.6 to 2.4 wt%.

Manganese (Mn): 2.0 to 3.0 wt%

In the present invention, manganese (Mn) improves strength and toughness by solid-solution strengthening of steel. In addition, it is an element that suppresses ferrite transformation or bainite transformation to generate martensite, thereby increasing tensile strength (TS). In order to obtain this effect, when it is added in less than 2.0% by weight, it is difficult to secure the target strength in the present invention, and when it exceeds 3.0% by weight, it promotes center segregation during playing to reduce low-temperature DWTT resistance. . Therefore, the content of the manganese (Mn) is preferably limited to 2.0 to 3.0% by weight.

Phosphorus (P): greater than 0 0.03% by weight or less

Phosphorus (P) is an impurity that is unavoidably contained during manufacturing, and is included in steel to reduce weldability and toughness, and has a problem of segregation in the slab center and austenite grain boundaries during solidification, so it is desirable to control it as low as possible. When the content of phosphorus (P) exceeds 0.03% by weight, it promotes grain boundary segregation to reduce low-temperature DWTT resistance as well as reduce weldability, so that the content of phosphorus (P) is controlled to 0.03% by weight or less desirable.

Sulfur (S): more than 0 0.005% by weight or less

Sulfur (S) is a component that greatly reduces brittleness by reacting with Mn in steel to form MnS, and when it exceeds 0.005 wt%, it greatly reduces low-temperature low-temperature impact toughness. Therefore, the content of the sulfur (S) is preferably controlled to 0.005% by weight or less.

Aluminum (Al): 0.01 to 0.05 wt%

Aluminum (Al) is a component that deoxidizes together with silicon (Si). When it is added in an amount of less than 0.01% by weight, it is difficult to obtain a deoxidation effect, and when it exceeds 0.05% by weight, the alumina aggregate is increased to improve low-temperature impact toughness. Therefore, the content of the aluminum (Al) is preferably limited to 0.01 to 0.05% by weight.

Chromium (Cr): More than 0 0.05 wt% or less

Chromium (Cr) is an element with high hardenability and is added to increase strength through transformation strengthening. That is, by suppressing ferrite transformation or bainite transformation to generate martensite, the tensile strength (TS) of steel is increased. However, when the chromium (Cr) exceeds 0.05% by weight, the toughness decreases due to the overall non-uniformity while forming a structure such as upper bainite, so that the content is controlled to 0.05% by weight or less desirable.

Titanium (Ti): greater than 0 and less than or equal to 0.05 wt%

Titanium (Ti) is precipitated as TiN in the steel and suppresses the grain growth of austenite during reheating to obtain high strength and excellent impact toughness. However, when the content of titanium (Ti) in the carbon range of the present invention exceeds 0.05% by weight, the effect is saturated and rather coarse TiN may be generated to inhibit low-temperature toughness, so that the titanium (Ti) It is preferable to control the content to 0.05% by weight or less.

Molybdenum (Mo): more than 0 0.05% by weight or less

Molybdenum (Mo) is an element having a greater hardenability than chromium (Cr) and is added to increase strength through transformation strengthening. When it exceeds 0.05% by weight within the range of the carbon (C) component of the present invention, toughness is lowered due to the formation of a large amount of light secondary phases such as martensite/austenite (MA) phase, so the content is controlled to 0.05% by weight it is preferable

Niobium (Nb): greater than 0 and less than or equal to 0.05 wt%

Niobium (Nb) is an element that contributes to improving the strength of the base material through precipitation strengthening effect by forming NbC precipitates through bonding with carbon (C) by adding a small amount. In the carbon (C) range of the present invention, niobium (Nb) may cause a decrease in low-temperature toughness and weldability due to a large amount of precipitates when it exceeds 0.05 wt% in the carbon (C) range of the present invention, so its content is preferably controlled to 0.05 wt% or less.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all details are not specifically mentioned.

The ultra-high-strength galvanized steel sheet having excellent weldability of the present invention having the above alloy components may be manufactured by the following manufacturing process. Hereinafter, a method for manufacturing an ultra-high strength galvanized steel sheet having excellent weldability according to another preferred aspect of the present invention will be described.

Manufacturing method of ultra-high-strength galvanized steel sheet with excellent weldability

The method for manufacturing an ultra-high strength galvanized steel sheet having excellent weldability according to another preferred aspect of the present invention is, in weight %, carbon (C): 0.12 to 0.22%, silicon (Si): 1.6 to 2.4%, manganese (Mn): 2.0 ~3.0%, phosphorus (P): more than 0 and less than 0.02%, sulfur (S): more than 0 and less than 0.005%, aluminum (Al): 0.01 to 0.05%, chromium (Cr): more than 0 and less than 0.05%, titanium (Ti) ): greater than 0 and less than or equal to 0.05%, molybdenum (Mo): greater than zero and less than or equal to 0.05%, niobium (Nb): greater than zero and less than or equal to 0.05%, and the remainder of iron (Fe) and other unavoidable impurities. preparing, annealing the steel sheet at 870 to 900° C., primary cooling the steel sheet to a temperature of 700 to 800° C., secondary cooling the steel sheet to 200 to 300° C., 370 to 430 Reheating and maintaining to ℃, and finally heating to 450 ~ 470 ℃, and performing hot-dip galvanizing treatment.

Steps to prepare the grater

In the present invention, the steel sheet prepared for the annealing heat treatment is not particularly limited, and for example, it may be manufactured by the following method. In the case of preparing a hot-rolled steel sheet, a starting steel sheet may be manufactured by hot-rolling a steel slab having the above alloy composition. The said steel slab can be obtained by melting and casting the steel whose component composition was adjusted to the said range according to a conventional method. The hot rolling may be performed, for example, by heating the steel slab to a heating temperature of 1150° C. or higher and then rolling under the condition of a finish rolling temperature (FRT): 850° C. or higher. The rolled steel sheet, for example, may be wound after cooling to a coiling temperature: 350 to 550 °C at an average cooling rate of 30 °C/sec or more. A hot-rolled steel sheet can be obtained by performing hot rolling and winding under the said conditions.

Moreover, when preparing a cold-rolled steel plate, a cold-rolled steel plate can be manufactured by hot-rolling the steel slab which has the said component composition, then cold-rolling again, and then heat-processing. The said steel slab can be obtained by melting and casting the steel whose component composition was adjusted to the said range according to a conventional method. The hot rolling may be performed, for example, by heating the steel slab to a heating temperature of 1150° C. or higher, and then rolling under the condition of a finish rolling temperature (FRT): 850° C. or higher. The rolled steel sheet, for example, can be wound after cooling to a coiling temperature: 600° C. to 700° C. at an average cooling rate of 30° C./sec or more. Next, after pickling the obtained hot-rolled steel sheet, cold rolling is carried out. For the pickling, for example, hydrochloric acid can be used. Moreover, it is preferable that the rolling-reduction|draft ratio in the said cold rolling shall be 40 % or more. Further, heat treatment is performed on the obtained cold-rolled steel sheet. In the said heat treatment, after heating a cold-rolled steel sheet to the soaking temperature, it maintains at the soaking temperature, then cools to the cooling-stop temperature, and maintains it at the cooling-stop temperature. By performing such heat treatment, a cold-rolled steel sheet for annealing heat treatment can be obtained.

Annealing heat treatment

Next, the steel sheet is subjected to annealing heat treatment at 870 to 900°C. Through annealing heat treatment, high strength and ductility of steel and good shape quality can be secured at the same time. If the heat treatment temperature is less than 870 ℃, it is not possible to secure a sufficient amount of retained austenite, there may be a problem that the total elongation is lowered after the final annealing, if it exceeds 900 ℃, the final annealing structure is made of martensite, high It is easy to secure tensile strength, but a problem of lowering elongation may occur.

In addition, the annealing heat treatment of the present invention may be performed in a furnace atmosphere of 5 to 7% hydrogen concentration. When the hydrogen concentration is less than 5%, the possibility of surface concentration of elements having high oxygen affinity such as Si, Mn, and B contained in steel increases, thereby causing dent defects and plating defects. When the hydrogen concentration exceeds 7%, the defect suppression effect by Si, Mn and B may reach a limit and cause excessive increase in manufacturing cost. In particular, by increasing the temperature of the dew point in the annealing furnace where the annealing heat treatment is performed to -40°C or higher, the austenite stabilizing element carbon (C) volatilizes in the form of carbon monoxide to induce a continuous decarburization reaction on the surface of the steel, and accordingly the heating step By transforming the austenite in the surface layer to ferrite, austenite is not formed in the surface layer in the future cooling/reheating process and a ferrite single-phase structure can be secured even at room temperature, which is the final step. Austenite, which is sensitive to LME cracks, can be avoided by the ferrite single-phase structure formed in the surface layer, thereby expanding the range of welding current and increasing welding strength. However, if the dew point in the annealing furnace is higher than 0°C, problems such as corrosion of equipment in the furnace may occur. . At this time, the moisture concentration in the furnace is about 120 ~ 6,000ppm, and the thickness of the decarburized layer is about 5 ~ 25㎛.

Meanwhile, the annealing heat treatment may be performed through a continuous annealing process, thereby improving productivity. Of course, steel grades containing a large amount of manganese (Mn) may have a high tensile strength and a high elongation when the final annealing is heat treated for 30 minutes or longer. It is a batch-type annealing method, and when using the batch-type annealing method, there is a disadvantage in that the steel sheet is curved in the rolling length direction after heat treatment.

primary cooling

The annealed steel sheet can be first cooled to a temperature range of 700 to 800°C at an average cooling rate of 5 to 10°C/s. When the average cooling rate of the primary cooling is less than 5℃/s or the cooling end temperature of the primary cooling exceeds 800℃, the grain boundary segregation effect of B cannot be sufficiently exhibited because the crystal grains of the single-phase ferrite structure are too coarse. . In addition, when the average cooling rate of the primary cooling exceeds 10°C/s or the cooling end temperature of the primary cooling is less than 700°C, excessive equipment temperature imbalance occurs before and after the cooling process, which may cause equipment load.

secondary cooling

The primary cooled steel sheet can be secondary cooled to a temperature range of 200 to 300 °C at an average cooling rate of 50 °C / s or more. In the present invention, the cooling rate of the secondary cooling does not significantly affect the physical properties of the steel sheet, but the secondary cooling rate is controlled within a certain range in order to secure an excellent shape of the steel sheet. When the cooling rate of the secondary cooling is less than 50°C/s, it may be disadvantageous in terms of economical efficiency due to the slow cooling rate.

reheat

Reheat the cooled steel plate to 370~430℃ and hold it for 100~250 seconds. If the holding time is preferably 100 seconds or more, if it is shorter than this, the recovery and recrystallization of the steel sheet may not be sufficiently performed, resulting in material defects and deviations, and the plating property may be poor because the oxide layer on the surface is not sufficiently reduced. Afterwards, it is finally heated to 440~480℃, which is the entry temperature of the plating bath.

plating bath immersion

A zinc-based plating layer can be formed by immersing the secondary cooled and reheated steel sheet in a zinc (Zn)-based plating bath at 440 to 480°C. The zinc-based plating bath is a pure zinc (Zn) plating bath, or a silicon (Si ), aluminum (Al), magnesium (Mg), etc. may be a zinc-based alloy plating bath. In addition, if necessary, alloying heat treatment may be performed on the zinc-based plated steel sheet, and the alloying heat treatment may be performed in a temperature range of 500 to 600°C.

The galvanized steel sheet of the present invention manufactured by the above manufacturing process is used as a component material for automobiles, has a microstructure of ferrite single phase after hot stamping, tensile strength (TS): 850 MPa or more, yield strength (YP): 1,180 MPa Above, elongation (EL): shows excellent physical properties of 14% or more. In addition, the applicable welding current range is 5.5kA~7.5kA, which can be greatly expanded compared to the existing 5.5kA~6.0kA.

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

Presented in the present invention, by weight, carbon (C): 0.12 to 0.22%, silicon (Si): 1.6 to 2.4%, manganese (Mn): 2.0 to 3.0%, phosphorus (P): more than 0 and 0.02% or less , Sulfur (S): more than 0 and less than 0.005%, Aluminum (Al): 0.01 to 0.05%, Chromium (Cr): more than 0 and less than 0.05%, Titanium (Ti): more than 0 and less than or equal to 0.05%, Molybdenum (Mo): 0 Cold-rolled steel sheets of Comparative Examples and Examples conforming to the alloy composition containing more than 0.05% or less, niobium (Nb): more than 0 and 0.05% or less, and containing the remaining iron (Fe) and other unavoidable impurities under normal process conditions Then, the thickness of the decarburized layer was measured after heat treatment was performed at an annealing temperature and an annealing furnace atmosphere shown in Table 2 below.

division Annealing temperature Annealing atmosphere cold rolled material hydrogen concentration Ronae dew point Moisture Concentration in Furnace Decarburized layer thickness (㎛) Comparative Example 1 875℃ 5-7% -60℃ about 10ppm - Comparative Example 2 875℃ 5-7% -50℃ about 40 ppm - Example 1 875℃ 5-7% -40℃ about 120ppm 5 Example 2 875℃ 5-7% -20℃ about 1000ppm 18 Example 3 875℃ 5-7% 0℃ About 6000ppm 25

Referring to Table 2, it can be seen that at the same temperature, the decarburized layer was formed when the dew point of the annealing furnace was -40 or more, and the thickness of the decarburized layer became thicker as the dew point was higher. However, when the dew point is 0°C or higher, problems such as corrosion of equipment in the furnace may be induced. Therefore, it can be said that it is appropriate that the dew point of the annealing furnace is -40 to -20°C.

Next, cracks were observed by spot welding on the plated steel sheet manufactured by applying the alloy composition and high dew point suggested in the present invention and the plated steel sheet manufactured by applying the general dew point. Welding conditions are shown in Table 3 below, and welding results are shown in Table 4, respectively.

Weld Control electrode tip pressing force number of pulses welding time holding time DC

6mm

6mm 3.5 kN One 300ms 100ms

welding current 5.5 kA 6.0 kA 6.5 kA 7.0 kA 7.5 kA 8.0 kA division burglar
(N) crack
length burglar
(N) crack
length burglar
(N) crack
length burglar
(N) crack
length burglar
(N) crack
length burglar
(N) crack
length general stall 17785 - 18420 - 17058 54 16081 84 Ex 162 Ex 208 Gono Branch 17869 - 18443 - 19055 - 19791 - 20245 - Ex 51

In Table 4, the crack length (μm) represents the maximum crack length, and Ex represents the expulsion. Referring to Table 4, the welding current range was 5.5kA ~ 6.0kA for the steel sheet to which the normal dew point was applied, whereas the range for the steel sheet to which the high dew point was applied was 5.5kA ~ 7.5kA. Therefore, when a high dew point is applied, the range of welding current is expanded, and it is possible to weld at a higher current than a steel sheet to which a general dew point is applied, thereby improving productivity.

According to the present invention described above, by applying an appropriately controlled alloy composition and high dew point annealing heat treatment process to suppress the formation of austenite in the surface layer, which is the cause of LME cracks in the welded part, the strength is secured even under the existing welding conditions and the total weldable range is expanded. Thus, defects in the spot welding process can be suppressed and productivity can be improved.

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)

Carbon (C): 0.12 to 0.22%, Silicon (Si): 1.6 to 2.4%, Manganese (Mn): 2.0 to 3.0%, Phosphorus (P): 0 to 0.02% or less, Sulfur (S): 0 to 0.005% or less, aluminum (Al): 0.01 to 0.05%, chromium (Cr): more than 0, 0.05% or less, titanium (Ti): more than 0, 0.05% or less, molybdenum (Mo): more than 0, 0.05% or less, niobium (Nb): Preparing a hot-rolled or cold-rolled steel sheet containing more than 0 and 0.05% or less, and containing the remaining iron (Fe) and other unavoidable impurities;
As a step of annealing the steel sheet at 870 ~ 900 ℃, the dew point is -40 ~ -20 ℃, the moisture concentration is 120 ~ 6,000 ppm, is carried out in a furnace in a hydrogen atmosphere of 5 to 7%, the surface of the steel sheet Inducing a continuous decarburization reaction to transform austenite present in the surface layer of the steel sheet into ferrite, the annealing heat treatment step;
first cooling the steel sheet to a temperature of 700 to 800°C at an average cooling rate of 5 to 10°C/s;
Secondary cooling the steel sheet to 200 ~ 300 ℃ at an average cooling rate of 50 ℃ / s or more;
Reheating the steel sheet to 370 ~ 430 ℃ and holding for 100 ~ 250 seconds; and
and performing hot-dip galvanizing treatment on the steel sheet;
After the hot-dip galvanizing treatment, the steel sheet has a ferrite single-phase microstructure on the surface layer,
The hot-dip galvanized steel sheet is characterized in that the applicable welding current range is 5.5 kA ~ 7.5 kA,
A method for manufacturing a galvanized steel sheet with excellent weldability.

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