KR101657422B1

KR101657422B1 - Manufacturing method for plated steel sheet

Info

Publication number: KR101657422B1
Application number: KR1020150134331A
Authority: KR
Inventors: 한영철; 강춘구; 구관모; 김주형; 이보룡; 한창문
Original assignee: 현대제철 주식회사
Priority date: 2015-09-23
Filing date: 2015-09-23
Publication date: 2016-09-13

Abstract

The invention relates to a method for manufacturing a plated steel sheet. A method for manufacturing a coated steel sheet according to the present invention comprises: 0.001 to 0.005% by weight of carbon; 0.1 to 0.6% by weight of manganese (Mn); 0.1 to 0.6% 0.01 to 0.08% by weight of sulfur, 0.008 to 0.008% by weight of sulfur, 0.002 to 0.08% by weight of aluminum (Al), 0.005 to 0.015% by weight of niobium (Nb) A steel slab containing 0.001 to 0.004% by weight of nitrogen (N), iron and other inevitable impurities in a remaining amount, to produce a cold-rolled steel sheet; Annealing the cold rolled steel sheet; Dipping the cold-rolled steel sheet subjected to the annealing heat treatment in a zinc plating solution to perform plating; Transferring the plated steel sheet to an alloying heat treatment facility, and sequentially performing alloying heat treatment through a heating stand and a support stand; And cooling the alloyed heat treated steel sheet by cooling it through a transfer roll, wherein the temperature of the transfer roll is controlled to be equal to or lower than the temperature of the steel sheet transferred to the holding.

Description

Technical Field [0001] The present invention relates to a method of manufacturing a coated steel sheet,

The present invention relates to a method for manufacturing a coated steel sheet. More particularly, the present invention relates to a method for producing a coated steel sheet excellent in dent resistance, endurance and hardening properties.

In recent years, efforts have been made to increase the strength of materials applied to automobile bodies in accordance with the improvement of automobile safety and the trend of lightweight.

The automobile exterior panel part is composed of parts such as a hood, a fender, a door, a roof, a trunk lid and a side outer. As the plate material parts are subjected to a strain of about 2 ~ 4% during molding, the resistance to bending property is required to secure shape durability and it is necessary to have dent resistance which is not easily deformed after external stress. The sintered hardened steel which is developed by reflecting this demand is a material suitable for automotive exterior sheathing due to its excellent hardenability due to few elements to be welded and its yield strength in low temperature heat treatment after painting. As a principle, heat treatment is performed for 20 minutes at a temperature of 160 ° C to 170 ° C for drying after press molding, and the yield strength of activated carbon is increased by interaction with dislocations.

BACKGROUND ART [0002] The background art relating to the present invention is disclosed in Korean Patent Registration No. 10-0742818 (published on Jul. 25, 2007, entitled Cold Rolled Steel Sheet having excellent processability and its manufacturing method).

According to one embodiment of the present invention, there is provided a method for producing a coated steel sheet excellent in dent resistance and anti-aging properties.

According to one embodiment of the present invention, there is provided a method of manufacturing a coated steel sheet excellent in surface characteristics and moldability.

According to one embodiment of the present invention, there is provided a method of manufacturing a coated steel sheet excellent in hardening of the bake portion.

One aspect of the present invention relates to a method of manufacturing a coated steel sheet. In one embodiment, the method of producing the coated steel sheet comprises 0.001 to 0.005% by weight of carbon (C), 0.1 to 0.1% by weight of silicon (Si), 0.1 to 0.6% by weight of manganese (Mn) 0.001 to 0.08 wt% sulfur (S), 0.008 wt% or less of sulfur (S), 0.02 to 0.08 wt% of aluminum (Al), 0.005 to 0.015 wt% of niobium (Nb) 0.001 to 0.004% by weight of boron, 0.001 to 0.008% by weight of nitrogen, 0.001 to 0.004% by weight of boron (B) and 0.0005 to 0.003% by weight of boron and the balance of iron and other unavoidable impurities; Annealing the cold rolled steel sheet; Dipping the cold-rolled steel sheet subjected to the annealing heat treatment in a zinc plating solution to perform plating; Transferring the plated steel sheet to an alloying heat treatment facility, and sequentially performing alloying heat treatment through a heating stand and a support stand; And cooling the alloyed heat treated steel sheet by cooling it through a transfer roll, wherein the temperature of the transfer roll is controlled to be equal to or lower than the temperature of the steel sheet transferred to the holding.

In one embodiment, the annealing heat treatment may be performed at 780 ° C to 880 ° C.

In one embodiment, the alloying heat treatment is performed by heating the steel sheet at 450 ° C to 600 ° C in the heating stand; And transferring the heated steel sheet to the holding unit to maintain the temperature at 450 ° C to 600 ° C.

In one embodiment, the temperature of the transfer roll may be adjusted from 110 ° C to 260 ° C.

In one embodiment, the cold-rolled steel sheet comprises: reheating the steel slab; Hot-rolling the reheated steel slab at a finish rolling temperature of 880 캜 to 930 캜; Winding the hot-rolled steel slab to produce a hot-rolled coil; And cold rolling the hot rolled coil by uncoiling the hot rolled coil.

In one embodiment, the coiling may take place at a coiling temperature of 600 ° C to 750 ° C.

In one embodiment, the reheating is performed by first heating the steel slab to 1200 ° C to 1250 ° C; And heating and heating the primary heated steel slab at a temperature of 1150 ° C to 1200 ° C.

The coated steel sheet produced by applying the method of the present invention is excellent in dent resistance and endurance, and can prevent the occurrence of local stretcher strain in the steel sheet when it is conveyed after cooling after the alloying heat treatment, And can be excellent in moldability and curing hardenability.

1 shows a method of manufacturing a coated steel sheet according to one embodiment of the present invention.
FIG. 2 illustrates a heat treatment process of alloying in the production of a coated steel sheet according to one embodiment of the present invention.
Fig. 3 (a) shows the surface of the coated steel sheet according to the embodiment of the present invention, and Fig. 3 (b) shows the surface of the coated steel sheet according to the comparative example of the present invention.

Hereinafter, the present invention will be described in detail. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to be exemplary, self-explanatory, allowing for equivalent explanations of the present invention.

One aspect of the present invention relates to a method of manufacturing a coated steel sheet. 1 shows a method of manufacturing a coated steel sheet according to one embodiment of the present invention. Referring to FIG. 1, the method for manufacturing a coated steel sheet comprises the steps of: (S10) preparing a cold rolled steel sheet; (S20) Annealing heat treatment step; (S30) a hot dip galvanizing step; (S40) an alloying heat treatment step; And (S50) a cooling step.

More specifically, the method for producing a coated steel sheet according to the present invention comprises the steps of: (C) 0.001 to 0.005% by weight of carbon; 0.1 to 0.6% by weight of silicon; 0.1 to 0.6% (Ti): 0.01 to 0.08 wt%, sulfur (S): more than 0 wt% to 0.008 wt%, aluminum (Al): 0.02 to 0.08 wt%, niobium (Nb) : 0.005 to 0.008 wt.%, Nitrogen (N): 0.001 to 0.004 wt.%, Boron (B): 0.0005 to 0.003 wt.% And a balance of iron and other unavoidable impurities to produce cold rolled steel sheets ; (S20) annealing the cold-rolled steel sheet; (S30) dipping the cold-rolled steel sheet annealed and annealed in a zinc plating solution for plating; (S40) transferring the plated steel sheet to an alloying heat treatment facility, and performing an alloying heat treatment sequentially through a heating stand and a support stand; And (S50) cooling the alloyed heat-treated steel sheet by cooling it through a transfer roll to cool it.

Hereinafter, a method for manufacturing a coated steel sheet according to the present invention will be described in detail.

(S10) Cold rolled steel sheet manufacturing step

Wherein said step comprises: 0.001 to 0.005% by weight of carbon; 0.1 to 0.1% by weight of silicon; 0.1 to 0.6% of manganese; 0.01 to 0.08% of phosphorus; Sulfur (S): more than 0 wt% to 0.008 wt%, aluminum (Al): 0.02 to 0.08 wt%, niobium (Nb): 0.005 to 0.015 wt%, titanium (Ti): 0.005 to 0.008 wt% ): 0.001 to 0.004% by weight, boron (B): 0.0005 to 0.003% by weight, and the balance of iron and other unavoidable impurities.

Hereinafter, components constituting the steel slab will be described in detail.

Carbon (C)

The carbon (C) is an important element that causes hardening of the resin and it is an essential element to control its content in order to control the natural aging. Since the moldability is deteriorated or the natural aging is accelerated depending on the carbon content, strong carbonitride forming elements such as Nb and Ti are added to control the solid nitrogen. The carbon may be included in an amount of 0.001 to 0.005% by weight based on the total weight of the steel slab. When it is included in the above range, it can be excellent in economical efficiency, hardening hardenability and endurance. If the content of carbon is less than 0.001 wt%, the hardenability and the antistatic property can not be ensured. If the carbon content is more than 0.005 wt%, the addition of expensive alloying elements is required. For example, 0.001 to 0.003% by weight.

silicon( Si )

The silicon (Si) pushes carbon (C) out of the cementite and improves the hardening hardenability by increasing the solid carbon. The silicon is contained in an amount of 0.1% by weight or less based on the total weight of the steel slab. When the content is in the above range, excellent elongation and excellent mechanical strength can be secured. When the silicon is contained in an amount exceeding 0.1% by weight, elongation at yield point occurs and strength is increased, but deterioration of ductility occurs. For example, from 0% by weight to 0.1% by weight or less.

Manganese (Mn)

The manganese (Mn) binds with sulfur (S) contained in the steel slab to precipitate MnS, thereby preventing hot brittleness due to the formation of FeS, and refining the particles without damaging the ductility.

The manganese is contained in an amount of 0.1 to 0.6% by weight based on the total weight of the steel slab. Within the above range, the coated steel sheet may have excellent rigidity and moldability. When the manganese content is less than 0.1 wt%, the mechanical strength of the coated steel sheet is deteriorated. When the manganese content is more than 0.6 wt%, the Mn-C dipole formation deteriorates the formability and sintering hardenability. And conversely, accelerates the formation of cementite to reduce the amount of solid carbon to deteriorate the hardening of the bake hardenability. In the production of the hot-dip coated steel sheet, a large amount of oxide such as MnO is generated on the surface in the annealing process to deteriorate the plating adhesion , And stripe-shaped plating defects may occur, which may degrade the surface quality.

In (P)

The phosphorus (P) plays the role of improving the in-plane anisotropy and enhancing the strength of the substitutional alloying element having the largest solubility enhancement effect. As phosphorus is added to the carbon, the carbon hardening hardening property is increased. The phosphorus is contained in an amount of 0.01 to 0.08% by weight based on the total weight of the steel slab. When the content is in the above range, the effect of sintering hardening and grain refinement can be excellent. If the content of phosphorus is less than 0.01% by weight, the hardening ability of the coated steel sheet may be deteriorated. If the content of the phosphorus exceeds 0.08% by weight, the secondary process embrittlement of the coated steel sheet may occur.

Sulfur (S)

The sulfur (S) forms MnS or the like to reduce the effective manganese content and cause surface defects by the MnS. The sulfur is contained in an amount of more than 0% by weight and 0.008% by weight or less based on the total weight of the steel slab. Within the above range, deterioration of surface properties, ductility and moldability can be prevented. When sulfur is contained in an amount exceeding 0.008% by weight, the hot brittleness of the coated steel sheet is generated, and the size of the MnS precipitates is increased to cause surface defects.

Aluminum (Al)

The aluminum (Al) can be used not only as a deoxidizer but also have a crystal grain inhibiting effect by precipitating nitrogen (N) and nitride AlN during hot rolling.

The aluminum is contained in an amount of 0.02 to 0.08% by weight based on the total weight of the steel slab. When it is included in the above range, the strength can be excellent. When the content of aluminum is less than 0.02% by weight, the added amount thereof may be insufficient and it may be difficult to secure deoxidation effect. When the aluminum is contained in an amount exceeding 0.08% by weight, the inclusions are excessively formed during the steelmaking operation and the possibility of occurrence of defects on the plating surface is increased.

Niobium ( Nb )

The niobium (Nb) is an important alloying element controlling the amount of solid carbon. According to the present invention, even if 10 to 35 ppm remains due to the control of the solid carbon activity of the micro precipitates at room temperature and the refinement of grain refinement, the endurance can be guaranteed.

The niobium is contained in an amount of 0.005 to 0.015% by weight based on the total weight of the steel slab. When the content is in the above range, the effect of strengthening the strength and grain refinement of the coated steel sheet can be excellent. When the content of niobium is less than 0.005 wt%, the strength of the coated steel sheet is lowered, and when it is more than 0.015 wt%, the precipitate size may increase.

titanium( Ti )

The titanium (Ti) is a carbonitride-forming element that forms sulfides and nitrides together with nitrogen (N) and sulfur (S) at high temperatures or forms carbides during the hot rolling process.

The titanium is contained in an amount of 0.005 to 0.008% by weight based on the total weight of the steel slab. When it is included in the above range, the strength and the sintering ability of the present invention can be excellent. When the amount of titanium is less than 0.005% by weight, the strength and hardenability of the coated steel sheet deteriorate. When the amount of titanium exceeds 0.008% by weight, TiC is formed to decrease the hardening amount of sintering and increase the size of nitride and sulphide precipitates .

Nitrogen (N)

The nitrogen (N) is an alloying element that causes hardening and aging of the resin together with carbon. The nitrogen (N) has an ability to improve the hardening of carbonization as compared with carbon, but deterioration of elongation and formability and aging phenomenon are rapidly increased, It is an alloying element which is difficult to apply for hardening of resin.

The nitrogen is contained in an amount of 0.001 to 0.004% by weight based on the total weight of the steel slab. When the content is in the above range, the curing property of the resin can be excellent. When the amount of nitrogen is less than 0.001 wt%, the hardening property of the resin is lowered. When the amount of nitrogen is more than 0.004 wt%, the size of the nitride rapidly increases and the formability may be deteriorated.

In one embodiment, the atomic ratio (Al / N) of aluminum and nitrogen may be 5 or less. When the amount is in the above range, the fine precipitate effect can be excellent.

Unavoidable impurities: Boron (B)

The boron (B) may be included as an unavoidable impurity. The boron suppresses the secondary machining brittleness of the phosphorus (P) added steel as an element with a large effect on the addition amount. In addition, it has a high affinity with carbon and a high ratio in the grain boundaries, thereby enhancing the effect of carbon (C) diffusion through the grain boundary, and improving the hardening properties of C due to C.

The boron may be contained in an amount of 0.0005 to 0.003% by weight based on the total weight of the steel slab. When the content is in the above range, the bake hardenability can be excellent.

In one embodiment, the cold-rolled steel sheet comprises a steel slab reheating step; A hot rolling step; A winding step; And a cold rolling step.

In the steel slab reheating step, the cold-rolled steel sheet reheats the steel slab. In one embodiment, the reheating is performed by first heating the steel slab to 1200 to 1250 占 폚; And heating and heating the primary heated steel slab at a temperature of 1150 to 1200 ° C.

During the primary heating. The austenite structure is uniformized at 1200 to 1250 占 폚 in a heating zone, and after the precipitate is reused, the steel slab is maintained at 1150 to 1200 占 폚 for 10 minutes to 30 minutes before the steel slab is extracted to finely disperse the nuclei of the precipitate . In one embodiment, the reheating time of the steel slab including the primary heating and heating holding times may be 1 to 2 hours.

The hot rolling step is a step of hot-rolling the reheated steel slab at a finish rolling temperature of 880 to 930 ° C. During the hot rolling under the above-mentioned finish rolling temperature condition, the grain size of the plated steel sheet is suitably formed, the strength is excellent, uniform microstructure is generated, and the formability can be excellent.

The winding step is a step of winding the hot-rolled steel slab to produce a hot-rolled coil. The winding may be performed at a winding temperature of 600 to 750 캜. And can be excellent in endurance, moldability, and strength at the above temperature.

The cold rolling step is a step of uncoiling the hot rolled coil, pickling it, and then cold rolling it. The cold rolling may be performed at a reduction ratio of 70 to 80%. <110> // Development of RD rolled aggregate structure and 111 // ND recrystallized aggregate structure that optimizes formability after annealing can be developed at the above mentioned reduction rate, have.

(S20) Annealing Heat treatment step

The step of annealing the cold-rolled steel sheet is a step of annealing. In one embodiment, continuous annealing may be performed by a conventional method. In one embodiment, the annealing heat treatment may be performed at 780 to 880 ° C. For example, 800 to 850 ° C.

In one embodiment, the annealing heat treatment may be carried out by sequentially charging the cold-rolled steel sheet into the annealing furnace, and then heating, cooling, and over-annealing.

In the heating section, a mixed gas of air and fuel is burned at the surface of the steel sheet and heated to be heated. In the specific example, the cold-rolled steel sheet is rapidly heated at a heating rate of 10 to 20 ° C / s by using a direct fired furnace (DFF) in the heating zone, and then the rapidly- (RTF) in a reducing gas atmosphere containing a reducing gas.

In one embodiment, the cold-rolled steel sheet may be heated to 780 ° C to 880 ° C through a heating section. When the steel sheet is heated to the above-mentioned temperature range, the surface of the steel sheet is excellent in the effect of reducing the oxide, so that the steel sheet is excellent in plating ability and crystal grains can be made finer, whereby the physical properties and workability of the steel sheet can be improved. For example, 800 to 850 ° C.

In the cooling section, the cold-rolled steel sheet can be cooled down and quenched. The slow cooling may be carried out at a cooling rate of 3 to 15 DEG C / s. Upon cooling under the above conditions, the steel sheet may have excellent moldability. The quenching may be performed by quenching the cold-rolled steel sheet to a temperature in the martensite temperature range. The cooling may be carried out at a cooling rate of 10 to 35 DEG C / s. When cooled under the above conditions, the plated steel sheet of the present invention can have excellent moldability.

In the embodiment, the cold rolling and quenching may be performed by using a roll quenching method or a gas jet method.

During the overexposure period, the cooled cold-rolled steel sheet may be subjected to aging treatment at 420 ° C to 500 ° C for 20 seconds to 200 seconds. The carbon (C) solidified under the above conditions can be precipitated to the maximum to improve the physical properties and the plating adhesion of the steel sheet.

(S30) The hot-dip galvanizing step

The above step is a step of dipping the cold-rolled steel sheet subjected to the annealing heat treatment in a zinc plating solution to perform hot-dip galvanizing. In one embodiment, the cold-rolled steel sheet subjected to the annealing heat treatment may be dipped in a hot-dip galvanizing solution to form a plating layer on the surface of the steel sheet.

In one embodiment, the hot dip galvanizing may be performed at 450-520 < 0 > C. The corrosion resistance of the steel sheet can be secured through the hot dip galvanizing. For example, at 460 to 480 [deg.] C.

(S40) Alloying Heat treatment step

The above step is a step of alloying heat treatment of the hot-dip galvanized steel sheet.

FIG. 2 illustrates a heat treatment process of alloying in the production of a coated steel sheet according to one embodiment of the present invention. Referring to FIG. 2, the plated steel sheet s is transferred to an alloying heat treatment facility, and the heat plate 20 and the supporter 22 are sequentially subjected to alloying heat treatment.

In one embodiment, the alloying heat treatment may be performed at 450 to 600 < 0 > C. In one embodiment, the alloying heat treatment is performed by heating the steel sheet s at 450 ° C to 600 ° C in a heating stand 20; And transferring the heated steel sheet to the supporter 22 to maintain the temperature at 450 캜 to 600 캜.

(S50) Cooling step

In this step, the alloyed heat-treated steel sheet (hot-dip galvanized steel sheet) is cooled and transferred to the cooling roll through the transfer roll. 2, the steel sheet s which has been subjected to the alloying heat treatment by sequentially passing through the heating table 20 and the holding table 22 is passed through the first conveying roll 12 and the second conveying roll 13, 24).

On the other hand, in the case of the local aging defect (Stretcher Strain), due to the temperature rise of the transport roll, when the transport roll cooling is not controlled when the transport roll of the steel strip passes, the transport roll temperature continuously increases, The temperature of the transfer roll tends to be slower than that of the end portion, and the diameter of the central portion of the transfer roll increases due to thermal expansion.

For this reason, some abnormal deformation occurs in the process of being transported by the transport roll, and a potential may be formed at the deformation site. At this time, if the temperature of the transfer roll is maintained at a high temperature, the carbon employed in the steel is fixed to the potential formed by the abnormal deformation, and local yield point stretching is caused even before the hardening of the resin so as to cause a stressor strain on the surface during molding do.

The temperature of the transport roll is adjusted to be equal to or lower than the temperature of the supporter. In one embodiment, the temperature of the transfer roll is controlled at 260 占 폚 or less. It is possible to prevent a phenomenon that the surface of the steel sheet conveyed by the deformation of the conveying roll is locally deformed below the temperature. It is also possible to limit the diffusion of activated carbon at high temperatures, to prevent the formation of tops formed by local deformation, and thus to produce automobile shells with excellent surface quality. Thus, defects not found in the form of steel plates It is possible to prevent the local aging defect from occurring during the molding, and the productivity can be improved.

When the temperature of the transfer path is higher than 260 ° C, as the diffusion of dissolved carbon contained in the transferred steel sheet subjected to the alloying heat treatment is increased, carbon is stuck to form a cottrell atmosphere and local unsteady deformation occurs in the steel sheet .

For example, the temperature of the transfer roll may be 110 to 260 ° C. As another example, the temperature of the transfer roll may be 150 to 220 캜. When the temperature of the conveying roll is adjusted to less than 110 캜, the cost required for cooling the conveying roll is excessively increased and the production cost is increased, which is not preferable.

In one embodiment, the temperature of the first feed roll 12 can be adjusted below 260 占 폚. For example, the temperature of the first transfer roll 12 may be 110 to 260 캜. As another example, the temperature of the first transfer roll 12 may be 150 to 220 캜.

In one embodiment, the steel sheet cooled through the cooling table 24 is conveyed through the third conveyance roll 14, and can be subjected to temper rolling using a skin pass mill or the like.

Another aspect of the present invention relates to a coated steel sheet produced by the above method for producing a coated steel sheet. In one embodiment, the coated steel sheet comprises 0.001 to 0.005 wt% of carbon (C), 0.1 wt% to 0.1 wt% of silicon (Si), 0.1 to 0.6 wt% of manganese (Mn) 0.008 to 0.008 wt% of aluminum (Al), 0.005 to 0.015 wt% of niobium (Nb), and 0.005 to 0.08 wt% of titanium (Ti) 0.001 to 0.004% by weight of nitrogen (N), 0.0005 to 0.003% by weight of boron (B), and a balance of iron and other unavoidable impurities; And a plating layer formed on the surface of the cold-rolled steel base material.

The coated steel sheet may have a microstructure in which precipitates of 100 nm or less are finely distributed in the steel and solid carbon is stabilized around the precipitates.

In one embodiment, the tensile strength of the coated steel sheet may be 340 MPa or more. For example, 340 to 380 MPa.

In one embodiment, the plastic anisotropy index (R value) of the coated steel sheet may be 1.8 or more. For example, 1.8 to 2.1.

In one embodiment, the hardening amount of the plated steel sheet upon baking may be 40 MPa or more. For example, from 40 to 50 MPa.

In one embodiment, the aging index during the artificial aging treatment of the coated steel sheet may be 30 MPa or less. For example, 20 to 30 MPa.

In addition, low-temperature annealing is possible and the grain refinement by fine precipitates can secure the tensile strength and the yield strength stably, the amount of solid carbon after annealing can be stably secured, and the grain refinement property can be obtained by the grain refinement due to the grain refinement , When the annealing is carried out by the presence of fine precipitates (NbC, TiN, MnS, AlN) and the constraint of the solid carbon in the solid carbon in the steel, Γ-fiber, which is a recrystallization aggregate that improves moldability, can be minimized.

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Example One

A steel slab containing an alloy component having the content shown in Table 1 below and iron and other unavoidable impurities in the remaining amount was first heated at 1230 占 폚 and heated and held at 1170 占 폚 to perform reheating. The reheated steel slab was hot-rolled at a finish rolling temperature of 910 ° C, and the hot-rolled steel slab was wound at a coiling temperature of 640 ° C to produce a hot-rolled coil. The hot-rolled coil was uncoiled and pickled, then cold-rolled at a reduction ratio of 75% to produce a cold-rolled steel sheet.

The cold-rolled steel sheet thus produced was subjected to annealing at 830 캜, and the cold-rolled steel sheet subjected to the annealing heat treatment as shown in Fig. 2 was immersed in a zinc plating solution (1) by a sink roll (11) Respectively. Thereafter, the hot-dip galvanized steel sheet is transferred to an alloying heat treatment facility, the steel sheet is heated in the heating stand 20 to 450 to 600 ° C, and then the heated steel sheet is transferred to the supporter 22, The alloy was annealed at a temperature of ~ 600 ℃. The alloyed heat treated steel sheet s was transferred to the cooling stand 24 through the first conveying roll 12 and the second conveying roll 13. At this time, the temperature of the first transport roll 12 was adjusted to 160 캜.

Example 2

A coated steel sheet was prepared in the same manner as in Example 1, except that an alloy component having the content shown in the following Table 1 was used.

Comparative Example One

A coated steel sheet was prepared in the same manner as in Example 1, except that the alloy component having the content shown in Table 1 and the temperature of the first transfer roll 12 were adjusted to 350 캜.

division
(Unit: wt%) C Si Mn P S Al Nb Ti N Example 1 0.003 0.03 0.3 0.07 0.006 0.03 0.008 0.008 0.004 Example 2 0.004 0.06 0.4 0.04 0.006 0.03 0.007 0.008 0.004 Comparative Example 1 0.003 0.03 0.3 0.07 0.006 0.03 0.008 0.008 0.004

Tensile strength, yield strength, r value (plastic anisotropy index), BH (hardening hardening amount) and AI (aging index) were measured for the coated steel sheets of Examples 1 and 2 and Comparative Example 1, .

The tensile strength
(Mpa) Yield strength
(Mpa) r value BH (Mpa) AI (Mpa) Example 1 365 227 2.01 49 24 Example 2 365 225 1.92 45 26 Comparative Example 1 351 223 1.93 39 23

With reference to Table 2 above, it is possible to obtain a steel sheet excellent in moldability at 40 MPa or higher through the composition and the steps of the present invention, and at the same time, it is possible to obtain a steel sheet excellent in moldability, Hyosung can be secured.

Fig. 3 (a) shows the surface of the coated steel sheet of Example 1, and Fig. 3 (b) is a photograph showing the surface of the coated steel sheet of Comparative Example 1. Referring to Tables 1, 3 (a) and 3 (b), the surface of the coated steel sheet according to Example 1 according to the present invention was free from surface defects, It was found that the steel sheet of Comparative Example 1 transported by the roll had surface defects due to local unsteady deformation.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

1: zinc plating solution 11: sink roll
12: first conveying roll 13: second conveying roll
14: Third conveying roll 20:
22: Maintaining stand 24: Cooling stand
s: steel plate

Claims

(C): 0.001 to 0.005 wt%, silicon (Si): more than 0 wt% to 0.1 wt%, manganese (Mn): 0.1 to 0.6 wt%, phosphorus (P): 0.01 to 0.08 wt% (N): 0.005 to 0.015 wt%, Ti: 0.005 to 0.008 wt%, and nitrogen (N): 0.001 to 0.005 wt% To 0.004% by weight, and a remaining amount of iron and other unavoidable impurities to produce a cold-rolled steel sheet;
Annealing the cold rolled steel sheet;
Dipping the cold-rolled steel sheet subjected to the annealing heat treatment in a zinc plating solution to perform plating;
Transferring the plated steel sheet to an alloying heat treatment facility, and sequentially performing alloying heat treatment through a heating stand and a support stand; And
And transferring the alloyed heat-treated steel sheet through the transfer roll to the cooling to cool the alloyed heat-
The temperature of the conveying roll is controlled to be equal to or lower than the temperature of the steel plate conveyed to the holding,
The alloying heat treatment may include heating the steel sheet at 450 ° C to 600 ° C in the heating stand; And
And transferring the heated steel sheet to the holding unit to maintain the temperature at a temperature of 450 ° C to 600 ° C,
Wherein the temperature of the transfer roll is controlled at 150 ° C to 260 ° C.

The method according to claim 1,
Wherein the annealing heat treatment is performed at 780 캜 to 880 캜.

delete

The method according to claim 1,
Before the annealing heat treatment, the cold-rolled steel sheet reheats the steel slab;
Hot-rolling the reheated steel slab at a finish rolling temperature of 880 캜 to 930 캜;
Winding the hot-rolled steel slab to produce a hot-rolled coil; And
And annealing the hot-rolled coil to cold-roll the hot-rolled coil.

6. The method of claim 5,
The reheating may be performed by first heating the steel slab to 1200 ° C to 1250 ° C; And
And heating and holding the primary heated steel slab at a temperature of 1150 ° C to 1200 ° C.

6. The method of claim 5,
Wherein the coiling is performed at a coiling temperature of 600 캜 to 750 캜.