Bake-hardening type hot-dip galvanized steel sheet having excellent powdering resistance and method for manufacturing the same

Abstract

The present invention provides a bake hardening type hot-dip galvanized steel sheet having excellent chipping resistance and a method for manufacturing the same. The bake hardening type hot-dip galvanized steel sheet of the present invention contains, in terms of weight %, C: 0.0001-0.005 %, Mn: 0.1-1.2 %, Si: 0.02 % or less (except 0 %), P: 0.01-0.04 %, S: 0.01 % or less (except 0 %), N: 0.01 % or less (except 0 %), sol.Al: 0.01-0.06 %, Nb: 0.003-0.015 %, B: 0.0005-0.0035 %, Cr: 0.01-0.1 %, Mo: 0.005-0.05 %, Ti: 0.003 % or less (except 0 %), the balance of Fe and inevitable impurities, and satisfies the following relation 1 to relation 2, the fine structure of the hot-dip galvanized steel sheet is ferrite single phase, and satisfies the following relation 3.

Description

TECHNICAL FIELD

[0001] The present invention relates to a manufacturing method of a bake hardening type hot-dip galvanized steel sheet having excellent powdering resistance, and more particularly, to a bake hardening type hot-dip galvanized steel sheet and a manufacturing method thereof, in which the annealing temperature, the hot-dip galvanizing alloying temperature, the elongation of a skin pass mill (SPM), and the like, which are main process factors of a hot-dip galvanizing line, are controlled in a bake hardening type hot-dip galvanized steel sheet in which the content of C is at a level of 0.005% by weight or less and the content of P is controlled to be 400 ppm or less, thereby enabling excellent powdering resistance. BACKGROUND

[0002] In recent years, in the automobile field, in developed countries headed by Europe, research into lightening the weight of the vehicle body is actively being conducted for the reasons of fuel efficiency regulation and performance improvement, and in the case of iron steel parts, in order to correspond to the lightening needs of automobile companies, efforts are being made to increase the strength at the same grade and further reduce the thickness of the steel sheet compared to competing materials (Mg, Al, CFRP, etc.). In addition, in order to be used as a material for an automobile outer panel, it is required to have a certain level or more of bake hardenability. The bake hardening phenomenon is a phenomenon in which solid-solved carbon and nitrogen activated at the time of paint baking are fixed to dislocations generated during the processing of a stamped part, thereby increasing the yield strength, and a steel having excellent bake hardenability has the characteristics of being easily formed before paint baking and having improved dent resistance of the final product, and thus is very ideal as a material for an automobile outer panel. Also, in order to be used as a material for an automobile outer panel, it is also required to have a certain level of age resistance to secure aging for a certain period of time.

[0003] Generally, as a method of manufacturing a cold-rolled steel sheet having bake hardenability, an Al-killed steel to which P is simply added at a low carbon is coiled at a low temperature, that is, a steel having a bake hardening amount of about 40-50 MPa at a level according to a phase annealing method is mainly used by utilizing low-temperature coiling at a temperature range of 400-500°C of hot rolling. This is because it is possible to easily have both formability and bake hardenability by annealing. In the case of an Al-killed steel to which P is added according to a continuous annealing method, a relatively fast cooling speed is utilized, and thus it is easy to secure bake hardenability, but there is a problem in that formability is deteriorated due to rapid heating and short-time annealing, and thus it is limited to automobile outer panels that do not require formability. In recent years, due to the rapid development of steelmaking technology, it is possible to control the amount of appropriate solid-solved elements in steel, and a bake hardening type cold-rolled steel sheet having excellent formability is manufactured by using an Al-killed steel sheet to which a strong carbonitride forming element such as Ti or Nb is added, and thus there is a tendency for the use of the cold-rolled steel sheet to increase in automobile outer panel materials requiring dent resistance.

[0004] In addition, a technology of manufacturing a bake hardening type hot-dip galvanized steel sheet of a Micro Alloy Free for Exposed (MAFE) concept having fine grains by using a nano-sized CuS / MnS precipitate has been proposed, but there is a problem of poor surface quality due to high P content at a level of 500 ppm. In addition, due to the addition of P, delayed alloying occurs, the hot-dip galvanizing temperature needs to be increased and operated, and thus the brittle Γ phase having high hardness in the plated layer increases, and a powdering problem in which the plated layer is detached in the form of powder due to press stress at the time of forming a press part can occur. This powdering defect causes poor corrosion resistance due to the plated layer peeling, and the peeled powder adheres in a die, thereby possibly causing defects such as dents in the steel sheet.

[0005] [Related Art Documents]

[0006] (Patent Document 1) Korean Patent Laid-Open No. KR2014-0048668 (Published on April 24, 2014)

[0007] (Patent Document 2) Korean Patent Laid-Open No. KR2011-0005414 (Published on January 18, 2011) SUMMARY

[0008] Technical Problem to be Solved

[0009] An object of the present application is to provide a bake hardening type hot-dip galvanized steel sheet and a method of manufacturing the same, in which the steel sheet has excellent powdering resistance in a bake hardening type hot-dip galvanized steel sheet having a C content at a level of 0.005 by weight or less and a P content controlled to be 400 ppm or less, by strictly controlling the composition of the steel and controlling the main process conditions of a hot-dip galvanizing line.

[0010] In addition, the technical problem to be solved in the present application is not limited to the technical problem described above, and other technical problems not mentioned can be clearly understood by those skilled in the art to which the present application pertains from the following description.

[0011] TECHNICAL SOLUTION

[0012] Accordingly, one aspect of the present application relates to a bake hardening type alloyed hot-dip galvanized steel sheet having excellent powdering resistance, comprising, in weight %, carbon (C): 0.0001-0.005 %, manganese (Mn): 0.1-1.2 %, silicon (Si): 0.02 % or less (except 0 %), phosphorus (P): 0.01-0.04 %, sulfur (S): 0.01 % or less (except 0 %), nitrogen (N): 0.01 % or less (except 0 %), aluminum (sol.Al): 0.01-0.06 %, niobium (Nb): 0.003-0.015 %, boron (B): 0.0005-0.0035 %, chromium (Cr): 0.01-0.1 %, molybdenum (Mo): 0.005-0.05 %, the balance of Fe and inevitable impurities, and satisfying the following relational expression 1 to relational expression 2, the microstructure of the alloyed hot-dip galvanized steel sheet is ferrite single phase, and satisfies the following relational expression 3.

[0013] [Relational expression 1]

[0014] [Nb] / ((93 / 12) x [C]) < 0.55

[0015] [Relational expression 2]

[0016] [Mn] / ((55 / 28) x [Si]) > 10

[0017] [Relational expression 3]

[0018] [TS] x [El.] x [Upper-BH] > 500000 (MPa x %)

[0019] wherein [Upper-BH] is an Upper Yield Stress at 170 °C for 20 minutes after 2 % pre-strain.

[0020] Further, another aspect of the present application relates to a method of manufacturing a bake hardening type alloyed hot-dip galvanized steel sheet having excellent powdering resistance, comprising the steps of:

[0021] The steel slab, which contains, in terms of weight %, carbon (C): 0.0001-0.005 %, manganese (Mn): 0.1-1.2 %, silicon (Si): 0.02 % or less (except 0 %), phosphorus (P): 0.01-0.04 %, sulfur (S): 0.01 % or less (except 0 %), nitrogen (N): 0.01 % or less (except 0 %), aluminum (sol. Al): 0.01-0.06 %, niobium (Nb): 0.003-0.015 %, boron (B): 0.0005-0.0035 %, chromium (Cr): 0.01-0.1 %, molybdenum (Mo): 0.005-0.05 %, the balance of Fe and inevitable impurities, and satisfies the following relation expression 1 to relation expression 2, is reheated at a temperature of 1000-1250 °C;

[0022] The reheated steel slab is hot-rolled at a temperature in the range of 900-1100 °C;

[0023] The hot-rolled steel plate is cooled to a temperature in the range of 500-700 °C at an average cooling rate of 10-70 °C / sec (sec) and then coiled;

[0024] The coiled hot-rolled steel plate is cold-rolled at a reduction of 70-90 % and then continuously annealed in the temperature range of 740-850 °C;

[0025] The continuously annealed cold-rolled steel plate is immersed in a molten zinc-based plating bath and then alloyed in the temperature range of 450-540 °C, thereby manufacturing a hot-dip zinc-based steel plate; and

[0026] The manufactured hot-dip galvanized steel plate is temper-rolled in the range of a reduction of 0.5-2.0 %, the alloyed hot-dip galvanized steel plate satisfying the following relation expression 4.

[0027] [relation expression 1]

[0028] [Nb] / ((93 / 12)×[C]) < 0.55

[0029] [relation expression 2]

[0030] [Mn] / ((55 / 28)×[Si]) > 10

[0031] [relation expression 4]

[0032] Powdering = [-14.2 + 0.0362×(GA) - 0.970×(CGL SPM El.)] < 3

[0033] wherein (GA) is a hot-dip galvanizing alloying temperature, and (CGL SPM El.) is an elongation of temper-rolling of a continuous hot-dip galvanizing line.

[0034] The hot-dip galvanized steel sheet subjected to temper rolling can satisfy the following relation 3.

[0035] [Relation 3]

[0036] [TS] x [El.] x [Upper Baking Hardening] > 500000 (MPa x %)

[0037] wherein [Upper Baking Hardening] is an upper yield stress at the time of baking at 170°C for 20 minutes after 2% pre-strain.

[0038] Advantageous Effects

[0039] According to the present application configured as described above, by appropriately controlling the composition of the added alloying elements and the manufacturing method, it is possible to effectively provide a hot-dip galvanized steel sheet having excellent baking hardening characteristics, which has an Upper Baking Hardening (Upper BH) value of an evaluation baking hardenability of 500000 (MPa x %) or more, and TS x El. x Upper Baking Hardening value, which is a comprehensive index of strength, elongation as a basic tensile test, of 3 or less. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 are diagrams respectively showing BSE images (x7000) and EDS component analysis results of the alloyed plated layer after hot-dip galvanizing of the hot-dip galvanized steel sheets of Inventive Example 1 and Comparative Example 4 in the present embodiment.

[0041] BEST MODE FOR CARRYING OUT THE INVENTION

[0042] Hereinafter, the present application will be described.

[0043] The present inventors have completed the present invention by confirming through experiments that the steel composition, annealing and plating operation conditions satisfy specific relationships to ensure desired physical properties. Specifically, the present invention relates to a bake hardening type hot-dip galvanized steel sheet on which a hot-dip galvanized layer is formed, the base steel sheet comprising, in terms of weight %, carbon (C): 0.0001-0.005 %, manganese (Mn): 0.1-1.2 %, silicon (Si): 0.02 % or less (except 0 %), phosphorus (P): 0.01-0.04 %, sulfur (S): 0.01 % or less (except 0 %), nitrogen (N): 0.01 % or less (except 0 %), aluminum (sol. Al): 0.01-0.06 %, niobium (Nb): 0.003-0.015 %, boron (B): 0.0005-0.0035 %, chromium (Cr): 0.01-0.1 %, molybdenum (Mo): 0.005-0.05 %, the balance of Fe and inevitable impurities, and satisfying the following relationship formula 1 to relationship formula 2, the microstructure of the hot-dip galvanized steel sheet being ferrite single phase, and satisfying the following relationship formula 3.

[0044] Hereinafter, first, the reasons for limiting the steel composition and the content thereof of the base steel sheet constituting the hot-dip galvanized steel sheet of the present invention are described, wherein, unless otherwise defined, "%" means "wt.%".

[0045] Carbon (C): 0.0001-0.005 %

[0046] Carbon (C): 0.0001-0.005 %

[0047] Manganese (Mn): 0.1-1.2 %

[0048] Manganese is a solid solution strengthening element, which not only helps to increase the strength, but also plays a role in precipitating S as MnS in the steel. When the content of Mn is less than 0.1%, MnS cannot be effectively precipitated, and thus the drawability is reduced, but when the content of Mn exceeds 1.2%, even though the strength is increased, excessive Mn is solid-solved, and thus there is a problem in that the drawability is reduced, and thus the content of Mn is preferably limited to 0.1-1.2%. Preferably, the content of Mn is controlled to be in the range of 0.2-1.0%.

[0049] Silicon (Si): 0.02% or less (except 0%)

[0050] Silicon helps to increase the strength of the steel sheet through solid solution strengthening, but when 0.02% or more of silicon is added, there is a problem in that surface defects are caused. In particular, when the [Mn] / [Si] atomic ratio of the following relation expression 2 exceeds 10, a hot-dip galvanized steel sheet having excellent surface quality can be manufactured.

[0051] Phosphorus (P): 0.01-0.04%

[0052] Phosphorus is an element in which the solid solution strengthening effect is the most excellent in ultra-low carbon steel and effectively ensures the strength of the steel without significantly impairing the drawability. In particular, since the P is easily segregated on the grain boundary, the grain growth is hindered at the time of annealing, and with the refinement of the grain, the resistance to aging at room temperature is helped to increase. However, when the content of P is less than 0.01%, the desired strength cannot be ensured, but when the content of P exceeds 0.04%, since the segregation of P in the surface layer portion, defects in the form of a knife line are caused on the surface after hot-dip galvanizing. In addition, a high P content delays the alloying of hot-dip galvanizing, and thus it is necessary to increase the alloying temperature, and thus brittle Fe-Zn intermetallic compounds (Γ) in the plated layer increase, and thus there is a problem in that the powdering property is deteriorated. Preferably, the content of P is controlled to be in the range of 0.015-0.04%.

[0053] In the present application, other alloying elements and process manufacturing conditions are optimized to control the content of P as a core element and to ensure the bake hardenability and the powdering resistance.

[0054] Sulfur (S): 0.01% or less (except 0%)

[0055] Sulfur (S) is an impurity that is inevitably contained in steel, and it is preferable to control the content of sulfur as low as possible. In particular, the sulfur in the steel increases the possibility of red short brittleness, and thus the content of sulfur is controlled to be 0.01% or less.

[0056] Nitrogen (N): 0.01% or less (except 0%)

[0057] Nitrogen is an impurity inevitably contained in steel, and it is important to control the content of nitrogen as low as possible, but for this, there is a problem that the refining cost of steel increases sharply, and therefore the content of nitrogen is controlled to be 0.01% or less as a range of operable conditions.

[0058] Sol. Al: 0.01-0.06%

[0059] Acid-soluble aluminum is an element added for grain refinement and deoxidation, and when the content of acid-soluble aluminum is less than 0.01%, it is not possible to manufacture aluminum killed steel in a normal stable state, on the other hand, when the content of acid-soluble aluminum exceeds 0.06%, it is advantageous to increase the strength due to the grain refinement effect, but excessive inclusions are formed during steelmaking and continuous casting operations, and therefore the possibility of surface defects of plated steel sheets increases, and there is a problem that the manufacturing cost increases sharply. Therefore, in the present invention, the content of acid-soluble aluminum is controlled to be 0.01-0.06%.

[0060] Niobium (Nb): 0.003-0.015%

[0061] Niobium combines with carbon in steel and precipitates as NbC during hot rolling, thereby reducing the solid-solution carbon and affecting the bake hardenability and the resistance to aging. As the content of C in steel precipitated as NbC increases, the content of solid-solution C decreases, and although it is advantageous in terms of resistance to aging characteristics, the bake hardenability decreases. Control of the appropriate level of solid-solution carbon can obtain excellent bake hardenability under the premise of ensuring the resistance to aging at room temperature, and the important element to control such solid-solution carbon is niobium.

[0062] When the content of niobium is less than 0.003%, almost no carbon is precipitated as NbC, and C in the steel remains mostly as solid-solution carbon, and therefore it is advantageous to the bake hardenability, but the problem of poor resistance to aging at room temperature occurs, and therefore there is a limitation in terms of part applications. In addition, when the content of niobium exceeds 0.015%, on the contrary, C in the steel is mostly precipitated as NbC, and the content of solid-solution C is absolutely insufficient, and although it is advantageous to the resistance to aging at room temperature, it is not possible to ensure the upper bake hardening (BH) value of 30 MPa or more required in GA steel sheets. Therefore, in the present invention, it is preferable to control the content of Nb to be 0.003-0.015%. More preferably, the content of Nb is controlled to be in the range of 0.0035-0.010%.

[0063] In addition, as shown in the following relation 1, when the atomic ratio of [Nb] / [C] is less than 0.55, it is possible to have excellent bake hardenability without hindering the resistance to aging.

[0064] Boron (B): 0.0005-0.0035%

[0065] Boron is an element added to prevent secondary working brittleness caused by grain boundary embrittlement in ultra-low carbon steel containing a large amount of P component. In general, B has a high tendency of grain boundary segregation compared to other elements, and by adding boron, P segregation at the grain boundary is suppressed, thereby playing a role in preventing secondary working brittleness. In addition, when boron in the range of the above invention is added, the interaction with dislocations is increased at the time of baking, thereby the bake hardenability can be ensured. However, when the content of B exceeds 0.0035%, the plating layer of the hot-dip galvanized steel sheet can be peeled off, and thus it is preferable to limit the content of B to 0.0005-0.0035%. More preferably, the content of B is controlled in the range of 0.0005-0.003%.

[0066] Chromium (Cr): 0.01-0.1%

[0067] Chromium is a component added to improve the hardenability of steel and ensure high strength, and is an element that plays a very important role in the formation of martensite, but in the case of the bake hardening type steel sheet of the present invention, as a ferrite stabilizing element, it is added to increase strength by controlling the content of P. When Cr is added beyond the range of the present invention, there can be a problem of corrosion resistance of the spiral plate, and there can be a problem of excessive increase in alloy iron cost, and thus it is added at a level within the range of the present invention. More preferably, the content of Cr is controlled in the range of 0.02-0.08%.

[0068] Molybdenum (Mo): 0.005-0.05%

[0069] Like chromium, molybdenum is also an element that improves the hardenability of steel, and is an element that can obtain a high hardenability effect by adding about 0.005-0.05%. In the case of the bake hardening type steel sheet of the present invention, the Mo is a ferrite stabilizing element, and when molybdenum in the range of the present invention is added, Mo-P compounds are formed during rolling, and thus P segregation bands are reduced, thereby the surface quality of the hot-dip galvanized steel sheet can be improved. When Mo is added above the above range, the alloy iron cost is excessively increased, and thus Mo is added at a level within the range of the present invention. More preferably, the content of Mo is controlled in the range of 0.01-0.04%.

[0070] Relationship 1 and Relationship 2

[0071] In the present invention, it is necessary to control the atomic ratio defined by the following Relationship 1 to be less than 0.55. When the atomic ratio of [Nb] and [C] is 0.55 or more, the amount of NbC precipitated increases, and the content of solid solution C is absolutely insufficient, and thus it is difficult to ensure the upper bake hardening value of 30 MPa or more required in the GA steel sheet.

[0072] Further, in the present application, the atomic ratio of [Mn] and [Si] defined by the following relation 2 needs to exceed 10. When the atomic ratio is 10 or less, a surface Mn-Si complex oxide is formed at the time of annealing, resulting in a possibility of poor surface quality of the steel sheet.

[0073] [relation 1]

[0074] [Nb] / ((93 / 12) x [C]) < 0.55

[0075] [relation 2]

[0076] [Mn] / ((55 / 28) x [Si]) > 10

[0077] The remaining components of the present application are iron (Fe). However, in the usual manufacturing process, undesirable impurities can be inevitably mixed from raw materials or the surrounding environment, and thus these impurities cannot be excluded. These impurities are well known to those skilled in the art of the conventional manufacturing process, and thus all of their contents are not particularly mentioned in the present specification.

[0078] Further, the fine structure of the base steel sheet constituting the hot-dip galvanized steel sheet of the present application is a ferrite single phase, and satisfies the following relation 3.

[0079] [relation 3]

[0080] [TS] x [El.] x [post-bake hardenability] > 500000 (MPa x %)

[0081] Here, [post-bake hardenability] is the post-yield stress at 170°C for 20 minutes after 2% pre-strain.

[0082] The relation 3 is a formula related to the [post-bake hardenability] value as an index of bake hardenability and tensile strength, elongation as basic tensile physical properties. When the relation 3 is 500000 (MPa x %) or less, the generally required tensile strength and elongation, U-BH value exceed the lower limit value, and thus the physical properties can be poor.

[0083] Next, the method for manufacturing the hot-dip galvanized steel sheet excellent in the powdering resistance and the bake hardenability of the present application will be explained in detail.

[0084] The method of manufacturing a hot-dip galvanized steel sheet according to the present application includes the steps of: reheating a steel billet satisfying the steel composition components and the following relational expression 1 to relational expression 2 at a temperature of 1000 to 1250°C; hot-rolling the reheated steel billet at a temperature in the range of 900 to 1100°C; coiling the hot-rolled steel sheet after cooling the hot-rolled steel sheet to a temperature in the range of 500 to 700°C at an average cooling rate of 10 to 70°C / sec; cold-rolling the coiled hot-rolled steel sheet at a reduction rate of 70 to 90% and then continuously annealing the cold-rolled steel sheet at a temperature in the range of 740 to 850°C; alloying the continuously annealed cold-rolled steel sheet by immersing the steel sheet in a molten zinc-based plating bath and then at a temperature in the range of 450 to 540°C; and manufacturing a hot-dip zinc-based steel sheet satisfying the following relational expression 4 by flat-rolling the manufactured hot-dip galvanized steel sheet at a reduction rate in the range of 0.5 to 2.0%.

[0085] First, a steel billet having the above-described composition system is reheated at a temperature of 1000 to 1250°C. This process is performed to smoothly perform a subsequent hot-rolling process and to sufficiently obtain desired physical properties of a steel sheet. At this time, when the reheating temperature is lower than 1000°C, a slab inclusion or the like cannot be sufficiently remelted, and can become a cause of material deviation and surface defects after hot-rolling, and when the reheating temperature exceeds 1250°C, strength is reduced due to abnormal grain growth of austenite grains, and thus the reheating temperature is preferably limited to 1000 to 1250°C.

[0086] Next, in the present application, the reheated steel billet is hot-rolled at a temperature in the range of 900 to 1100°C. At this time, when hot-rolling is started at a temperature higher than 1100°C, the temperature of the hot-rolled steel sheet is increased, so that the grain size becomes coarse, and the surface quality of the hot-rolled steel sheet is deteriorated. In addition, when hot-rolling is completed at a temperature lower than 900°C, elongated grains are developed due to excessively delayed recrystallization, and a high yield / tensile strength ratio is obtained, so that cold-rolling properties are deteriorated, and shear workability is also deteriorated.

[0087] Also, in the present application, the hot-rolled steel sheet is coiled after being cooled to a temperature in the range of 500 to 700°C at an average cooling rate of 10 to 70°C / sec.

[0088] At this time, when coiling is performed after being cooled to lower than 500°C, the shape of the steel sheet is deteriorated due to a too low coiling temperature, and ductility is expected to be deteriorated due to formation of fine grains. On the other hand, when coiling is performed after being cooled to more than 700°C, coarse ferrite grains are formed, and coarse carbides and nitrides are easily formed, so that the material quality of the steel can be deteriorated.

[0089] Further, when the average cooling rate at the time of cooling is less than 10°C / sec, coarse ferrite grains are formed, so the fine structure becomes non-uniform, and when the average cooling rate at the time of cooling exceeds 70°C / sec, not only does the shape of the plate become distorted, etc., but the fine structure in the thickness direction of the plate also becomes non-uniform, so the shear workability of the steel can become poor.

[0090] Next, in the present application, the hot-rolled steel plate that has been coiled is cold-rolled at a reduction ratio of 70-90%, and then continuously annealed in a temperature range of 740-850°C.

[0091] In the present application, when the hot-rolled steel plate that has been coiled is cold-rolled, the cold-rolling reduction ratio is preferably in the range of 70-90%. When the cold-rolling reduction ratio is less than 70%, it can be difficult to ensure the target thickness, and it can be difficult to correct the shape of the steel plate. On the other hand, when the cold-rolling reduction ratio exceeds 90%, cracks can occur in the edge portion of the steel plate, and cold-rolling load can be caused.

[0092] In addition, at this time, rolling is performed using a rolling mill consisting mainly of 5-6 stands, and the reduction ratio of the first stand is set to 20-40% and a cold-rolled steel plate is produced. This is because, when the reduction ratio of the first stand is less than 20%, there are limitations in controlling the shape of the hot-rolled steel plate due to the low reduction ratio, and when the reduction ratio of the first stand exceeds 40%, there is an increase in the load on the equipment due to the increased reduction ratio of the first stand. The reduction ratio of the first stand is further preferably 25-35%.

[0093] Also, the cold-rolled steel plate that has been cold-rolled is continuously annealed in a continuous annealing line including a plating line, and the annealing temperature is preferably controlled in the range of 740-850°C. When the annealing temperature is less than 740°C, the structure at the time of cold-rolling is not sufficiently completed in terms of ferrite recrystallization, so a mixed grain structure is produced, and when the annealing temperature exceeds 850°C, the annealing temperature is excessively high, so the possibility of failure of the on-site equipment becomes very high, and the grains become excessively coarse, so the characteristics required in the present application cannot be ensured. After the continuous annealing, cooling is performed under conventional operating conditions.

[0094] Subsequently, in the present application, the cold-rolled steel plate that has been continuously annealed is immersed in a molten zinc-based plating bath, and then alloyed in a temperature range of 450-540°C, thereby producing a hot-dip zinc-based steel plate.

[0095] In the case of a hot-dip zinc material, it can be performed under conventional conditions in the range of 440-480°C as the temperature of the molten zinc plating bath.

[0096] Also, after the hot-dip galvanizing, alloying is preferably performed in a temperature range of 450 to 540°C. When the alloying temperature is lower than 450°C, non-coating sites can occur over the entire width of the annealed steel sheet, and when the alloying temperature exceeds 540°C, the powdering property is deteriorated due to the influence of brittle Fe-Zn intermetallic compound (Γ) caused by excessive alloying.

[0097] Also, in the present application, after the hot-dip galvanizing, skin pass rolling is performed in a range of 0.5 to 2.0%. When the elongation of the skin pass rolling is 0.5% or less, sufficient dislocation cannot be formed, and in terms of the shape of the sheet, it is not favorable, and coating surface defects can occur. On the other hand, when the elongation of the skin pass rolling exceeds 2%, due to high load, equipment load can occur, and side effects such as sheet breakage can occur.

[0098] In addition, in the present application, in order to manufacture a bake hardening type hot-dip galvanized steel sheet having excellent powdering resistance, the hot-dip galvanizing alloying temperature (GA) and the elongation of the skin pass rolling of the continuous hot-dip galvanizing line (CGL SPM El.) are preferably controlled to satisfy the following relational expression 4. The following relational expression 3 is a regression relational expression of each variable with respect to powdering resistance, and when the value defined by the relational expression 3 satisfies less than 3, excellent coating quality can be obtained.

[0099] [Relational Expression 4]

[0100] Powdering = [-14.2 + 0.0362 x (GA) - 0.970 x (CGL SPM El.)] < 3

[0101] where (GA) is the hot-dip galvanizing alloying temperature, and (CGL SPM El.) is the elongation of the rough rolling of the continuous hot-dip galvanizing line DETAILED DESCRIPTION

[0102] Hereinafter, the present application will be described more specifically through examples.

[0103] (Example)

[0104] A steel billet showing the chemical composition of alloying additive elements shown in Table 1 below was manufactured. Then, for the steel billet, a bake hardening type cold rolled steel sheet was manufactured using a process of manufacturing a conventional bake hardening type cold rolled steel sheet. Specifically, the steel billet reheating temperature was about 1100°C, the hot finish rolling temperature was 900 to 940°C or more than the Ar3 temperature, and then the hot rolled steel sheet was cooled at a cooling rate of 30 to 50°C / sec, and then coiled at 580 to 620°C. Then, the hot rolled sheet was pickled using hydrochloric acid, and then cold rolled at a cold rolling rate of 75 to 80% to manufacture a cold rolled steel sheet.

[0105] The cold-rolled steel sheet thus completed was annealed at the annealing temperature shown in Table 2 below, and then cooled under a conventional condition. Subsequently, in order to produce a hot-dip galvanized steel sheet, the cold-rolled steel sheet was immersed in a hot-dip galvanizing pot maintained at a temperature of about 460°C as a conventional condition to perform hot-dip galvanizing, and then, as shown in Table 2 below, an alloying treatment was performed while changing the alloying temperature. Also, the plated steel sheet thus completed with hot-dip galvanizing was given a skin pass rolling rate under the conditions of Table 2 below, and finally a hot-dip galvanized steel sheet was produced.

[0106] For each of the produced hot-dip galvanized steel sheets, a tensile test was performed in a rolling perpendicular direction using a JIS standard to measure the yield strength (YP), tensile strength (TS), and elongation (El.) of the plated steel sheet, and the results thereof are shown in Table 2 below. Also, for the same standard, the upper yield stress [U-BH] was measured after 2% pre-strain and baking at 170°C for 20 minutes, and the results thereof are also shown in Table 2 below.

[0107] Also, it was confirmed whether the above-described relational expression 3 to relational expression 4 of the present application were satisfied, and when satisfied, marked as O, and when not satisfied, marked as X.

[0108] [Table 1]

[0109]

[0110] *In Table 1, the residual components are Fe and unavoidable impurities.

[0111] [Table 2]

[0112]

[0113]

[0114] *In Table 2, SS indicates a continuous annealing temperature, GA indicates an alloying temperature, and SPM El. indicates an elongation of skin pass rolling. Also, relational expression 3 indicates [TS] x [El.] x [upper bake hardening] > 500000 (MPa x %), and relational expression 4 indicates powdering = [-14.2 + 0.0362 x (GA) - 0.970 x (CGL SPM El.)] < 3.

[0115] As shown in the above Tables 1 to 2, it was confirmed that each of Inventive Examples 1 to 7 satisfying the element content range, process conditions, and relational expressions 1 to 4 of the present application can produce a bake hardening type hot-dip galvanized steel sheet having excellent surface quality while ensuring excellent powdering resistance properties as well as basic tensile physical properties.

[0116] On the other hand, the powdering resistance and mechanical properties of Comparative Examples 1 to 7, in which the steel composition is within the scope of the present application but the plated steel sheet manufacturing process conditions are not within the scope of the present application, are not excellent. Specifically, Comparative Example 1, Comparative Example 4, and Comparative Example 5 are cases in which the relationship 4 is not satisfied, and the brittle Γ phase is formed in the alloyed plated layer due to the relatively high alloying temperature, so the powdering resistance is poor. In addition, in Comparative Examples 2 to 3 and Comparative Examples 6 to 7, when the SPM elongation is high to 2% or more, the yield strength rises due to solidification caused by the formation of excessive mobile dislocations, but the elongation decreases, and the BH does not sufficiently rise, so the bake hardening value is poor.

[0117] In addition, in Comparative Example 8, Comparative Example 9, Comparative Example 10, and Comparative Example 12, the brittle Γ phase is formed in the alloyed plated layer due to the high GA temperature, so the powdering resistance is basically poorer than in the inventive examples.

[0118] In addition, in Comparative Examples 9 to 14, the Nb / C atomic ratio is high and the relationship 1 is not satisfied, so the solid-soluted C in the steel is insufficient, and the U-BH value is poor, and Comparative Examples 9 and 13 do not satisfy the relationship 3.

[0119] Also, in Comparative Example 8 and Comparative Example 10, B and Mo are not within the component range of the present application, and Comparative Example 8 does not satisfy the [Mn], [Si] formula of the relationship 2, so the surface quality is not excellent.

[0120] Also, Comparative Example 15 is a case in which a steel sheet in which the manufacturing conditions of the plated steel sheet are within the scope of the present application but the steel composition does not satisfy the relationship 2, and the plated surface quality is not excellent due to the formation of Mn-Si complex oxides at the time of annealing.

[0121] In addition, Figure 1 are graphs showing the BSE images (x7000) and EDS component analysis results of the alloyed plated layer after hot-dip galvanizing of the hot-dip galvanized steel sheet of Inventive Example 1 and Comparative Example 4, respectively.

[0122] When Inventive Example 1 and Comparative Example 4 are compared, the basic composition is the same, but the Fe content in the alloyed plated layer is different due to the difference in the GA temperature. The pt2, pt3 results of the point EDS analysis of the middle portion of the alloyed plated layer were confirmed, and in the case of Inventive Example 1, the Fe content was at the level of 13-14%, and in the case of Comparative Example 4, the Fe content was at the level of 19-20%. In general, when the Fe content is 7-12%, the δ intermetallic compound of FeZn7 is formed, and when the Fe content is 17-20%, the Fe5Zn21 intermetallic compound is formed. In the case of Comparative Example 4, the Fe content is high, so the Fe5Zn21 intermetallic compound is formed, and the powdering resistance is poor. 21Γ intermetallic compound. Since Γ is harder than δ, the thicker Γ phase in the alloyed plating layer, the worse the powder resistance, resulting in an increase in peeling of plating. That is, in the case of Comparative Example 4, it is known that Γ phase forming a thick alloyed plating layer is formed, and thus the powder resistance is poor, compared to Inventive Example 1.

[0123] As described above, the preferred embodiments of the present application are explained in detail in the detailed description of the present application, but those skilled in the art can make various modifications without departing from the scope of the present application. Therefore, the scope of the present application is not limited to the described embodiments, but should be determined by the claims and their equivalents.

Claims

1. A bake hardening type alloyed hot-dip galvanized steel sheet having excellent powdering resistance, consisting of, in mass%, carbon (C): 0.0001-0.005%, manganese (Mn): 0.1-1.2%, silicon (Si): 0.02% or less except 0%, phosphorus (P): 0.01-0.04%, sulfur (S): 0.01% or less except 0%, nitrogen (N): 0.01% or less except 0%, aluminum (sol.Al): 0.01-0.06%, niobium (Nb): 0.003-0.015%, boron (B): 0.0005-0.0035%, chromium (Cr): 0.01-0.1%, molybdenum (Mo): 0.005-0.05%, the balance of Fe and inevitable impurities, and satisfying the following relation 1 to relation 2, a microstructure of the alloyed hot-dip galvanized steel sheet is ferrite single phase, and satisfying the following relation 3, [relation 1] [Nb] / ((93 / 12)×[C]) < 0.55 [relation 2] [Mn] / ((55 / 28)×[Si]) > 10 [relation 3] [TS]×[El.]×[upper bake hardening] > 500000 MPa×MPa×% [upper bake hardening] is an upper yield stress at 170℃ for 20 minutes after 2% pre-strain. 2.A method of manufacturing a bake hardening type alloyed hot-dip galvanized steel sheet having excellent powdering resistance, comprising the steps of: reheating a steel billet consisting of, in mass%, carbon (C): 0.0001-0.005%, manganese (Mn): 0.1-1.2%, silicon (Si): 0.02% or less except 0%, phosphorus (P): 0.01-0.04%, sulfur (S): 0.01% or less except 0%, nitrogen (N): 0.01% or less except 0%, aluminum (sol.Al): 0.01-0.06%, niobium (Nb): 0.003-0.015%, boron (B): 0.0005-0.0035%, chromium (Cr): 0.01-0.1%, molybdenum (Mo): 0.005-0.05%, the balance of Fe and inevitable impurities, and satisfying the following relation 1 to relation 2, at a temperature of 1000-1250℃; hot-rolling the reheated steel billet at a temperature in the range of 900-1100℃; cooling the hot-rolled steel sheet to a temperature in the range of 500-700℃ at an average cooling rate of 10-70℃ / sec after coiling; cold-rolling the coiled hot-rolled steel sheet at a reduction rate of 70-90%, and then continuously annealing in the temperature range of 740-850℃; immersing the continuously annealed cold-rolled steel sheet in a molten zinc-based plating bath, and then alloying in the temperature range of 450-540℃, thereby manufacturing a hot-dip galvanized steel sheet; and temper-rolling the manufactured hot-dip galvanized steel sheet in the range of elongation of 0.5-2.0% of temper-rolling, the alloyed hot-dip galvanized steel sheet satisfying the following relation 4, [relation 1] [relation 4] [TS]×[El.]×[upper bake hardening] > 500000 MPa×MPa×% [upper bake hardening] is an upper yield stress at 170℃ for 20 minutes after 2% pre-strain. ​ ​ ​ ​ wherein ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ [Nb] / ((93 / 12) x [C]) < 0.55 [Relationship 2] [Mn] / ((55 / 28) x [Si]) > 10 [Relationship 4] Powdering = [-14.2 + 0.0362 x GA - 0.970 x CGL SPM El.] < 3 wherein GA is the hot-dip galvanizing alloying temperature, in °C, and CGL SPM El. is the elongation of the skin pass rolling of the continuous hot-dip galvanizing line, in %.

3. The method of producing a bake-hardening type alloyed hot-dip galvanized steel sheet having excellent powdering resistance according to claim 2, characterized by, The hot-dip galvanizing steel sheet of the skin pass rolling satisfies the following Relationship 3, [Relationship 3] [TS] x [El.] x [post-bake hardening] > 500000 MPa x MPa x % wherein [post-bake hardening] is the post-yield stress at 170°C for 20 minutes after 2% pre-strain.

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