Hot-dip galvanized steel plate with excellent bake hardenability and anti-aging property at room temperature

Abstract

Provided is a hot-dip galvanized steel plate which is applicable as a material for a vehicle outer panel and has an excellent bake hardenability and anti-aging property at room temperature, and a method for manufacturing a hot-dip galvanized steel plate, the method comprising a process including the steps of: winding a hot-rolled steel plate, followed by cooling at a speed of 0.002-0.027° C./sec; cold-rolling the cooled steel plate; continuously annealing the cold-rolled steel plate; and cooling the annealed steel plate in the multi-stage manner of primary to third rounds.

Claims

1. A hot-dip galvanized steel sheet comprising: a cold-rolled steel sheet; and a hot-dip galvanized layer formed on the cold-rolled steel sheet, wherein the cold-rolled steel sheet comprises, by weight %, 0.005 to 0.08% of C, 1.3 to 2.3% of Mn, 1.0% or less of Cr excluding 0%, 0.1% or less of P excluding 0%, 0.01% or less of S excluding 0%, 0.01% or less of N excluding 0%, 0.01 to 0.06% of sol.Al, one or more of 0.3% or less of Si, 0.2% or less of Mo and 0.003% or less of B, and a balance of Fe and inevitable impurities, wherein ferrite and martensite are included as a microstructure of the cold-rolled steel sheet, and wherein, K, an average concentration ratio of solid solution carbon before and after baking determined by Expression 1 below, is 0.2 to 0.7,
K=C.sub.1/C.sub.2  [Expression 1] where C.sub.1 and C.sub.2 are an average concentration of solid solution C before and after baking performed for 20 minutes at 140° C., respectively, in an area within 100 nm in a direction of a ferrite grain from a grain boundary of ferrite and martensite at a t/4 position (t=a thickness of the cold-rolled sheet) from a surface of the cold-rolled steel sheet.

2. The hot-dip galvanized steel sheet of claim 1, wherein the cold the microstructure comprises: 90 to 99 area % of the ferrite and 1 to 10 area % of the martensite.

3. The hot-dip galvanized steel sheet of claim 1, wherein the microstructure further comprises bainite, and an area ratio of bainite, B, defined by Expression 2 below, is 3 or less,
B={A.sub.B/(A.sub.F+A.sub.M+A.sub.B)}×100  [Expression 2] where A.sub.F indicates an area ratio of ferrite, A.sub.M indicates an area ratio of martensite, and A.sub.B is an area ratio of bainite.

4. The hot-dip galvanized steel sheet of claim 1, wherein the cold-rolled steel sheet satisfies Relational Expression 1 below,
0.001≤[Mn]/150+[Cr]/100+[Mo]/90≤0.026  [Relational Expression 1] where Mn, Cr, and Mo indicate a content of each element by weight %.

5. The hot-dip galvanized steel sheet of claim 1, wherein the hot-dip galvanized layer comprises: one or more elements selected from the group consisting of Si, Al, and Mg.

6. The hot-dip galvanized steel sheet of claim 1, wherein, after the baking, an amount of bake hardenability, BH, is 35 MPa or higher, and when a heat treatment is performed for 1 hour at 100° C., an yield point, YP-El, is 0.2% or less, including 0%.

Description

DESCRIPTION OF DRAWINGS

(1) (a) of FIG. 1 is an image obtained by observing a process in which solid solution carbon is dispersed into a ferrite grain from martensite and ferrite grain boundaries before and after baking of inventive steel 1 using an APT in atom unit, and (b) of FIG. 1 is graphs representing the process in profile.

BEST MODE FOR INVENTION

(2) The inventors conducted intensive research to provide a hot-dip galvanized steel sheet which may be preferably used as a material for external panels of automobiles. As a result, it has been found that, when a microstructure of a cold-rolled steel sheet, a substrate, is controlled to have a two-phase structure of ferrite and martensite, and distribution of solid solution carbon near ferrite and martensite grain boundaries in the two-phase structure is appropriately controlled, the steel sheet appears to have excellent bake hardenability and excellent room temperature aging resistance at the same time in a wide range of temperature (approximately 140 to 190° C.), and accordingly, the steel sheet may be preferably applied as a material for external panels of vehicles.

(3) In the description below, a hot-dip galvanized steel sheet having excellent bake hardenability and room temperature aging resistance will be described in detail.

(4) A hot-dip galvanized steel sheet of the present disclosure may include a cold-rolled steel sheet, and a hot-dip galvanized layer formed on one surface or both surfaces of the cold-rolled steel sheet. In the present disclosure, a composition of the hot-dip galvanized layer is not particularly limited. The hot-dip galvanized layer may be a pure zinc plated layer or may be a zinc-based alloy plating layer including Si, Al, Mg, and the like. The hot-dip galvanized layer may also be an alloyed hot-dip galvanized layer.

(5) An alloy composition and preferable content ranges thereof of a cold-rolled steel sheet, a substrate, will be described in detail. A content of each element described below is based on a weight unless otherwise indicated.

(6) C: 0.005 to 0.08%

(7) Carbon is an essential element which may be added to secure a duplex structure aimed in the present disclosure. The more the content of carbon increases, the easier the formation of martensite may be, which may be advantageous to manufacturing duplex structure steel. However, to secure intended strength and an yield ratio (yield strength/tensile strength), it may be required to appropriately control a content of carbon. When a content of carbon is lower than 0.005%, it may be difficult to secure the strength aimed in the present disclosure, and it may be difficult to form an appropriate level of martensite. When a content of carbon exceeds 0.08%, the formation of grain boundary bainite may be facilitated during cooling after annealing such that a yield ratio of steel may increase, and indentations and surface defects may easily occur when processing the steel into a vehicle component, which may be disadvantages. Thus, in the present disclosure, a content of carbon may be controlled to be 0.005 to 0.08%, or may be controlled to be 0.007 to 0.06% more preferably.

(8) Mn: 1.3 to 2.3%

(9) Manganese is an element which may improve hardenability in duplex structure steel, and may be an important element in relation to the formation of martensite. When a content of manganese is lower than 1.3%, martensite may not be formed such that it may be difficult to manufacture duplex structure steel. When a content of manganese exceeds 2.3%, martensite may be excessively formed, such that a material may become unstable, and a manganese band may be formed in a structure such that a risk of process cracks and breakage may significantly increase. Also, manganese oxide may be dissolved onto a surface during annealing, which may greatly degrade coatability. Thus, in the present disclosure, a content of manganese may be controlled to be 1.3 to 2.3%, and may be controlled to be 1.7 to 2.1% more preferably.

(10) Cr: 1.0% or less (excluding 0%)

(11) Chromium may have a characteristic similar to that of manganese, and may be added to improve strength of steel along with hardenability of steel. Also, chromium may help in the formation of martensite, and may precipitate solid solution carbon in steel to be an appropriate level of amount or less by forming coarse Cr-based carbide such as Cr.sub.23C.sub.6 during hot-rolling such that chromium may prevent the generation of yield point elongation (YP-El). Thus, chromium may be advantageous to manufacturing duplex structure steel having a low yield ratio. Also, chromium may minimize a decrease of ductility as compared to an increase of strength such that chromium may also be advantageous to manufacturing high strength duplex structure steel having high ductility. However, when a content of chromium exceeds 1.0%, a martensite structure fraction may excessively increase, which may degrade strength and an elongation. Thus, in the present disclosure, a content of chromium may be controlled to be 1.0% or less.

(12) P: 0.1% or less (excluding 0%)

(13) Phosphorus may be the most advantageous element to secure strength without greatly deteriorating formability, but when phosphorus is excessively included, the possibility of brittleness breakage may significantly increase such that the possibility of breakage of a slab may greatly increase during hot-rolling. Also, phosphorus may degrade plating surface properties, and thus, in the present disclosure, a content of phosphorus may be controlled to be 0.1% or less.

(14) S: 0.01% or less (excluding 0%)

(15) Sulfur is one of impurities which may be inevitably included in steel, and it may be preferable to control a content of sulfur to be low as possible. In particular, sulfur in steel may increase the possibility of hot shortness, and thus, a content of sulfur may be controlled to be 0.01% or less.

(16) N: 0.01% or less (excluding 0%)

(17) Nitride is one of impurities which may be inevitably included in steel, and it may be important to control a content of nitride to be low as possible. To this end, there may be a problem in which the cost of refining steel may rapidly increase, and thus, a content of nitride may be controlled to be 0.01% or less, a range in which operational conditions may be able to be implemented.

(18) Sol.Al: 0.01 to 0.06%

(19) Sol.Al may be added for gain refinement and deoxidation. When a content of sol.Al is lower than 0.01%, generally used stable al-killed steel may not be manufactured. When a content of sol.Al exceeds 0.06%, the content may be advantageous to increasing strength due to a grain refinement effect, but inclusions may be excessively formed during a continuous casting process in steel-making such that the possibility of surface defects of a plating steel sheet may increase, and manufacturing costs may rapidly increase. Thus, in the present disclosure, a content of sol.Al may be controlled to be 0.01 to 0.06%.

(20) A remainder other than the above-described composition is Fe. However, in a general manufacturing process, inevitable impurities may be inevitably added from raw materials or a surrounding environment, and thus, impurities may not be excluded. A person skilled in the art may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.

(21) An effective component other than the above-described composition is not excluded. For example, one or more elements among Si, Mo, and B may further be included.

(22) Si: 0.3% or less (excluding 0%)

(23) Silicon may contribute to increasing strength of a steel sheet by strengthening solid solution, but in the present disclosure, silicon may not be intentionally added. Even if silicon is not added, there may be no difficulty to secure property. When a content of silicon exceeds 0.3%, plating surface properties may be deteriorated, and thus, in the present disclosure, a content of silicon may be controlled to be 0.3% or less.

(24) Mo: 0.2% or less (excluding 0%)

(25) Molybdenum may delay the transformation of austenite to pearlite and may refine ferrite at the same time, and may also contribute to increasing hardenability and strength of steel. However, even though molybdenum is not added, there may be no difficulty in securing properties. When a content of molybdenum exceeds 0.2%, manufacturing costs may rapidly increase, which may be economically disadvantageous, and ductility of steel may also degrade. Thus, in the present disclosure, a content of molybdenum is controlled to be 0.2% or less.

(26) B: 0.003% or less (excluding 0%)

(27) Boron may prevent secondary process brittleness caused by phosphorus in steel, but even though boron is not added, there may be no difficulty in securing properties. When a content of boron exceeds 0.003%, there may be a problem of degradation of ductility of steel, and thus, in the present disclosure, a content of boron may be controlled to be 0.003% or less.

(28) When manufacturing steel having the above-described composition ranges, it may be preferable to control contents of Mn, Cr, and Mo to satisfy Relational Expression 1 below.
0.001≤[Mn]/150+[Cr]/100+[Mo]/90≤0.026  [Relational Expression 1] (where Mn, Cr, and Mo indicate a content of each element by weight %)

(29) The elements above may improve hardenability of steel. When the contents of the elements do not satisfy Relational Expression 1, martensite may be formed in a hot-rolling state such that cold-rolling load may increase, and a size of martensite formed during a cooling process after cold-rolling and annealing processes may also be affected. Accordingly, a size of martensite in a final structure may not be uniform such that it may be difficult to secure aimed bake hardenability and room temperature aging resistance. A more preferable lower limit may be 0.009, and an even more preferable upper limit may be 0.020.

(30) In the description below, a microstructure of a cold-rolled steel sheet, a substrate, will be described in detail.

(31) A cold-rolled steel sheet, a substrate of a hot-dip galvanized steel sheet, may include ferrite and martensite as a microstructure, and K, an average solid solution carbon concentration ratio before and after baking determined by Expression 1 below, may be 0.2 to 0.7. In Expression 1 below, C.sub.1 is a result of measurement obtained at room temperature (approximately 25° C.) before baking, and C.sub.2 is a result of measurement obtained at room temperature (approximately 25° C.) after baking.
K=C.sub.1/C.sub.2  [Expression 1] (where C.sub.1 is an average concentration of solid solution C present within 100 nm in a direction of a ferrite grain from ferrite and martensite grain boundaries at a t/4 position taken in a sheet thickness direction before baking, where t indicates a thickness of the cold-rolled steel sheet, and C.sub.2 is an average concentration of solid solution C present within 100 nm in a direction of a ferrite grain from ferrite and martensite grain boundaries at a t/4 position taken in a sheet thickness direction after baking performed for 20 minutes at 140° C.)

(32) As described above, the inventors has found that, when a microstructure of a cold-rolled steel sheet is controlled to have a two-phase structure of ferrite and martensite, and the distribution of solid solution carbon near ferrite and martensite grain boundaries in the two-phase structure is appropriately controlled, a hot-dip galvanized steel sheet appearing to have excellent bake hardenability and excellent room temperature aging resistance at the same time in a wide range of temperature (approximately 140 to 190° C.) may be provided.

(33) More specifically, when K is lower than 0.2 in Expression 1, a content of solid solution carbon in martensite positioned near a grain boundary may not be sufficient such that a content of solid solution carbon which may be dispersed into a ferrite grain by baking may be absolutely insufficient, and thus, it may be difficult to secure bake hardenability aimed in the present disclosure. When K exceeds 0.7 in Expression 1, a content of solid solution carbon in ferrite positioned near a grain boundary may be excessive such that an yield point elongation (YP-El) phenomenon may occur in a room temperature toughness test such that it may be difficult to guarantee 6 months of longer of aging at room temperature, and surface defects may occur during processing a component. Also, as a content of solid solution carbon in ferrite is excessive, solid solution carbon may not be smoothly dispersed during baking, and thus, it may be difficult to secure bake hardenability aimed in the present disclosure.

(34) According to an example embodiment, the cold-rolled steel sheet may include ferrite of 90 to 99 area % and martensite of 1 to 10 area % as a microstructure. If an area ratio of martensite is less than 1%, or an area ratio of ferrite exceeds 99%, solid solution carbon may not be sufficiently dispersed into ferrite during baking of paint, and a content of solid solution carbon in entire ferrite may be relatively insufficient such that it may be difficult to secure bake hardenability aimed in the present disclosure. When an area ratio of martensite exceeds 10%, or an area ratio of ferrite is lower than 90%, excessive solid solution carbon may be present in ferrite such that it may be difficult to secure aimed aging resistance at room temperature.

(35) According to an example embodiment, the cold-rolled steel sheet may further include bainite as a microstructure, and a bainite area ratio (B) defined by Expression 2 below is 3 or less (including 0). When an area ratio (B) of bainite exceeds 3, carbon concentration around bainite may increase such that ductility of steel may be deteriorated, and an yield ratio may rapidly increase.
B={A.sub.B/(A.sub.F+A.sub.M+A.sub.B)}×100  [Expression 2] (where A.sub.F indicates an area ratio of ferrite, A.sub.M indicates an area ratio of martensite, and A.sub.B is an area ratio of bainite)

(36) The hot-dip galvanized steel sheet in the present disclosure may have an advantage of excellent bake hardenability, and even when the hot-dip galvanized steel sheet is baked at 170° C., a general baking temperature in a vehicle process, and also at 140 to 190° C., a relatively broad annealing temperature range including 170° C., aimed bake hardenability may be secured. According to an example embodiment, but not limited thereto, the hot-dip galvanized steel sheet may have 35 MPa or higher of bake hardenability amount (BH) when the hot-dip galvanized steel sheet is baked for 20 minutes at 140° C.

(37) Also, the hot-dip galvanized steel sheet in the present disclosure may have an advantage of excellent room temperature aging resistance. According to an example embodiment, but not limited thereto, the hot-dip galvanized steel sheet may have an yield point elongation (YP-El) of 0.2% or less (including 0%) after a 1 hour of heat treatment at 100° C.

(38) The hot-dip galvanized steel sheet described above may be manufactured by various methods, and the manufacturing method is not particularly limited. However, as a preferable example embodiment, the hot-dip galvanized steel sheet may be manufactured by the method as below.

(39) In the description below, a method of manufacturing a hot-dip galvanized steel sheet having excellent bake hardenability and room temperature aging resistance, another aspect of the present disclosure, will be described.

(40) A steel slab having the above described composition system may be reheated. The reheating process may be performed to smoothly perform a hot-rolling process, performed later, and to sufficiently obtain aimed property of a steel sheet. In the present disclosure, process conditions of the reheating process will not be particularly limited, and general conditions may be used. As an example, the reheating process may be performed at a temperature range of 1100 to 1300° C.

(41) The reheated steel slab may be hot-rolled in an austenite single phase region, thereby obtaining a hot-rolled steel sheet. The hot-rolling may be performed in an austenite single phase region to increase uniformity of a structure.

(42) According to an example embodiment, during the hot-rolling, a finish rolling temperature in the hot-rolling may be Ar3+50° C. to 950° C. When the finish rolling temperature is lower than Ar3+50° C., it may be highly likely that a ferrite and austenite two-phase region may be rolled, which may cause non-uniformity of a material. When the finish rolling temperature exceeds 950° C., non-uniformity of a material may occur due to the formation of an abnormal coarse grain caused by a high temperature hot-rolling, and accordingly, a coil may be distorted during cooling the hot-rolled steel sheet. A theoretical temperature of Ar3 may be obtained by Expression 3 below.
Ar3(° C.)=910−310[C]−80[Mn]−20[Cu]−15[Cr]−55[Ni]−80[Mo]  [Expression 3] (where [C], [Mn], [Cu], [Cr], [Ni], and [Mo] indicate a content of each element by weight %)

(43) The hot-rolled steel sheet may be coiled.

(44) According to an example embodiment, a coiling temperature may be 450 to 700° C. When the coiling temperature is lower than 450° C., martensite or bainite may be excessively formed such that strength of the hot-rolled steel sheet may excessive increase, and the problem of shape defects may be caused due to load during the cold-rolling. When the coiling temperature exceeds 700° C., elements which may degrade wetness of a hot-dip zinc plating, such as Si, Mn, B, and the like, in steel, may be greatly thickened on a surface.

(45) The coiled hot-rolled steel sheet may be cooled. In the present disclosure, it may be preferable that there is no martensite in a structure in a hot-rolling state, and that the structure is formed of ferrite and partially formed of pearlite. That is because, if martensite is present in a hot-rolling state, steel may be hardened, and a cold-rolling load may increase such that a shape of the sheet may be distorted after cold-rolling, and rolling properties may significantly degrade. Also, a size of martensite formed during a cooling process after cold-rolling and annealing processes may be affected, and a size of martensite may not be uniformly formed such that it may be difficult to secure aimed bake hardenability and room temperature aging resistance. Considering the issue above, it may be preferable to control a cooling speed during the cooling of the coiled hot-rolled steel sheet to be 0.027° C./sec or less, and it may be more preferable to control the speed to be 0.02° C./sec or less. If the cooling speed is too slow, productivity may degrade, and carbon may be excessively dispersed, such that even a small amount of solid solution carbon may be precipitated as carbide, and accordingly, ductility may degrade. Considering this, it may be preferable to control the cooling speed during the cooling of the coiled hot-rolled steel sheet to be 0.002° C./sec or more, and it may be more preferable to control the speed to be 0.01° C./sec or more. In the present disclosure, a cooling terminating temperature during the cooling of the coiled hot-rolled steel sheet is not particularly limited, and the cooling may be performed to 200 to 300° C., a temperature range in which transformation is completed.

(46) The coiled hot-rolled steel sheet may be cold-rolled, thereby obtaining a cold-rolled steel sheet.

(47) According to an example embodiment, a cold reduction ratio may be 40 to 80% during the cold-rolling. When the cold reduction ratio is lower than 40%, it may be difficult to secure a target thickness and to correct a shape of the steel sheet. When the cold reduction ratio exceeds 80%, cracks may occur in an edge of the steel sheet, and cold-rolling load may be caused.

(48) Thereafter, the cold-rolled steel sheet may be continuously annealed. The process may be performed to achieve recrystallization and to form ferrite and austenite at the same time, and to distribute carbon.

(49) A preferable annealing temperature may be Ac1+20° C. to Ac3−20° C. When the annealing temperature is lower than Ac1+20° C., fine martensite may not be sufficiently formed during the cooling after a final annealing as an austenite fraction is not sufficient in a low two-phase region (ferrite+austenite) temperature, and accordingly, bake hardenability required in the present disclosure may not be obtained. When the annealing temperature exceeds Ac3−20° C., an austenite fraction may be excessively high during the two-phase region annealing, and a size of martensite after the annealing cooling may become coarse, and a fraction thereof may exceeds 10% such that strength may rapidly increase, and the possibility of process cracks may increase during a component forming process. Thus, it may be preferable to control an appropriate annealing temperature range to be Ac1+20° C. to Ac3−20° C. Theoretical temperatures of Ac1 and Ac3 may be obtained by Expressions 4 and 5 below.
Ac1(° C.)=723−10.7[Mn]−16.9[Ni]+29.1[Si]+16.9[Cr]  [Expression 4]
Ac3(° C.)=910−203v[C]−15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]  [Expression 5] (where [Mn], [Ni], [Si], [Cr]. [C], [V], [Mo], and [W] indicate a content of each element by weight %)

(50) Thereafter, the continuously annealed cold-rolled steel sheet may be primarily cooled to 630 to 670° C. at an average cooling speed of 2 to 14° C./sec.

(51) In the present disclosure, the higher the primary cooling terminating temperature is controlled to be, or the slower the primary cooling speed is controlled to be, uniformity and coarsening tendency of ferrite may increase, which may be advantageous to securing ductility of steel. Also, in the present disclosure, to endow a sufficient time for carbon to be dispersed to austenite during the primary cooling may be a main feature. This feature may be significant in the present disclosure. More specifically, carbon may be dispersed and moved to austenite of which a carbon concentration degree is high in a two-phase region, and the higher the temperature thereof, and the longer the time thereof, the degree of the dispersion may further increase. When the primary cooling terminating temperature is lower than 630° C., a dispersion action degree of carbon may be low due to excessively low temperature such that carbon concentration in ferrite may increase, an yield ratio may increase, and it may be highly likely that cracks may increase during processing. When the primary cooling terminating temperature exceeds 670° C., it may be advantageous in terms of dispersion of carbon, but an excessively high cooling speed may be required during a secondary cooling, which may be a disadvantage. Also, when the primary cooling speed is lower than 2° C./sec, it may be disadvantageous in terms of productivity, whereas when the speed exceeds 14° C./sec, carbon may not be sufficiently dispersed, which may not be preferable.

(52) The primarily cooled cold-rolled steel sheet may be secondarily cooled to Ms+20° C. to Ms+50° C. at an average cooling speed of 3 to 12° C./sec.

(53) According to the research of the inventors, if martensite is formed before passing through 440 to 480° C., a general temperature range of a hot-dip zinc plating bath, a coarse martensite may be formed on a finally obtained cold-rolled steel sheet such that a low yield ratio may not be achieved. When the secondary cooling termination temperature is lower than Ms+20° C., martensite may be formed during the secondary cooling. When the temperature exceeds Ms+50° C., it may be required to control a cooling speed before entering a plating bath after the secondary cooling, a tertiary cooling speed, to be relatively high. Further, it may be highly likely that martensite may be formed before being submerged into the plating bath. When the secondary cooling speed is lower than 3° C./sec, martensite may not be formed, but it may be disadvantageous in terms of productivity. When the speed exceeds 12° C./sec, an overall passing speed of the sheet may increase such that the problem of distortion of a shape of a sheet, and the like, may occur. A theoretical temperature of Ms may be obtained by Expression 6 below.
Ms(° C.)=539−423[C]−30.4[Mn]−12.1[Cr]−17.7[Ni]−7.5[Mo]  [Expression 6] (where [C], [Mn], [Cr], [Ni], and [Mo] indicate a content of each element by weight %)

(54) Thereafter, the secondarily cooled cold-rolled steel sheet may be tertiarily cooled to 440° C. to 480° C. at a speed of 4 to 8° C./sec.

(55) The temperature range is a general temperature range of a zinc-based plating bath, and the process may be performed to prevent a martensite structure from being formed before the cold-rolled steel sheet is submerged into the zinc-based plating bath. When the tertiarily cooling speed is lower than 4° C./sec, martensite may not be formed, but it may be disadvantageous in terms of productivity. When the speed exceeds 8° C./sec, martensite may be partially formed in a grain, and bainite may be partially formed, and accordingly, yield strength may increase and ductility may be deteriorated.

(56) The tertiarily cooled cold-rolled steel sheet may be submerged into a hot-dip zinc-based plating bath, thereby obtaining a hot-dip galvanized steel sheet. A composition of the hot-dip zinc-based plating bath is not particularly limited in the present disclosure, and the zinc-based plating bath may be a pure zinc plating bath or a zinc-based alloy plating bath including Si, Al, Mg, and the like.

(57) The hot-dip galvanized steel sheet may be finally cooled to Ms-100° C. or less at an average cooling speed of 3° C./sec or higher. When the final cooling terminating temperature is lower than Ms-100° C., fine martensite may not be obtained, and the problem of sheet shape defects may occur. Also, an average cooling speed is lower than 3° C./sec, martensite may be irregularly formed in a grain boundary or in a grain due to the too slow cooling speed, and also, steel having a low yield ratio may not be manufactured as a formation ratio of martensite in a grain boundary is too low as compared to that of martensite in a grain.

(58) If necessary, an alloyed hot-dip galvanized steel sheet may be obtained by performing an alloying heat treatment on the hot-dip galvanized steel sheet before the final cooling. In the present disclosure, process conditions of the alloying heat treatment are not particularly limited, and general conditions may be used. As an example, the alloying heat treatment process may be performed at a temperature range of 500 to 540° C.

(59) Thereafter, if necessary, the finally cooled hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet may be skin-pass rolled to form a large amount of dislocation in ferrite positioned around martensite, thereby further improving bake hardenability.

(60) In this case, a preferable reduction ratio may be 0.3 to 1.6%, and a more preferable reduction ratio may be 0.5 to 1.4%. When the reduction ratio is lower than 0.3%, sufficient dislocation may not be formed, it may be disadvantageous in terms of a sheet shape, and particularly, plating surface defects may occur. When the reduction ratio exceeds 1.6%, it may be advantageous in terms of the formation of dislocation, but defects such as breakage may occur due to a limitation in facility capability.

(61) In the description below, an example embodiment of the present disclosure will be described in greater detail. It should be noted that the exemplary embodiments are provided to describe the present disclosure in greater detail, and to not limit the scope of rights of the present disclosure. The scope of rights of the present disclosure may be determined on the basis of the subject matters recited in the claims and the matters reasonably inferred from the subject matters.

MODE FOR INVENTION

Embodiment

(62) A steel slab having an alloy composition listed in Table 1 below was prepared, and a hot-dip galvanized steel sheet (GI steel sheet) or an alloyed hot-dip galvanized steel sheet (GA steel sheet) was manufactured using manufacturing processes listed in Table 2. In Table 1, inventive steels 1, 2, 4, and 5 and comparative steels 1 and 2 are alloyed hot-dip galvanized steel sheets, and inventive steels 3, 6, and 7 may be hot-dip galvanized steel sheets. When manufacturing each sample, a primary cooling terminating temperature was 650° C., a secondary cooling terminating temperature was 510° C., a tertiary cooling terminating temperature was 460° C., and a plating bath temperature was 460° C. in all samples.

(63) Thereafter, a microstructure of each of the manufactured steel sheets was observed, properties thereof were examined, and the results were listed in Table 3 below.

(64) In Table 3, a microstructure fraction was obtained by observing martensite and bainite through Lepelar corrosion at a ¼ t point of a sheet thickness using an optical microscope and observing martensite and bainite again using an SEM (3,000 magnification), and measuring sizes and distribution amounts of martensite and bainite based on a three-time averaged value through a count point task. A phase other than the above-described structures were assumed as a content of ferrite. In Table 3, a tensile test for each sample were conducted in a C direction (a direction perpendicular to a rolling direction) using a JIS standard.

(65) Bake hardenability was measured through a baking process performed for 20 minutes at 140° C., and was examined based on an yield strength difference after 2% pre-strain. Aging resistance was examined by measuring YP-El (%) in a tensile test after maintaining the steel sheet for 1 hour at 100° C.

(66) Also, to analyze a value K (C.sub.1/C.sub.2) based on room temperature and baking temperature, an APT devices which may visualize distribution of compositions three-dimensionally with sub-nanometer spatial resolution was used.

(67) To examine a degree of dispersion of solid solution carbon into ferrite from martensite and ferrite grain boundaries, the numbers of solid solution C within a square of a length of 100 nm were measured by a count method with reference to a grain boundary using an atom probe tomography (APT) carbon profile, and the numbers of solid solution C were three-time averaged.

(68) TABLE-US-00001 TABLE 1 Steel Cold-Rolled Steel Sheet Alloy Composition (weight %) Type C Mn Si Cr P S N sol.Al Mo B Inventive 0.007 2.21 0.05 0.80 0.005 0.005 0.003 0.018 0.15 0.0006 Steel 1 Inventive 0.023 1.96 — 0.48 0.005 0.005 0.003 0.04 0.12 0.0009 Steel 2 Inventive 0.050 1.73 0.10 0.43 0.003 0.007 0.004 0.05 0.13 — Steel 3 Inventive 0.061 1.64 0.15 0.81 0.002 0.004 0.003 0.041 — 0.0021 Steel 4 Inventive 0.069 1.43 0.22 0.57 0.002 0.003 0.004 0.052 0.18 — Steel 5 Inventive 0.075 1.55 0.21 0.08 0.003 0.004 0.008 0.025 0.08 0.0012 Steel 6 Inventive 0.077 1.61 0.13 0.02 0.005 0.006 0.003 0.042 0.15 0.0025 Steel 7 Comparative 0.0021 0.15 0.05 0.005 0.006 0.004 0.002 0.032 — 0.0008 Steel 1 Comparative 0.12 1.26 0.81 1.21 0.006 0.007 0.005 0.05 0.38 0.0041 Steel 2

(69) TABLE-US-00002 TABLE 2 Cooling Finish Speed Cold Primary Secondary Tertiary Final Reheating Rolling Coiling After Reduction Annealing Cooling Cooling Cooling Cooling Steel Temperature Temperature Temperature Coiling Ratio Temperature Speed Speed Speed SPeed Type (° C.) (° C.) (° C.) (° C./s) (%) (° C.) (° C./s) (° C./s) (° C./s) (° C./s) Note Inventive 1184 882 596 0.012 51 768 2.3 4.4 4.3 4.9 Embodiment Steel 1 1 1191 894 556 0.013 54 767 2.4 4.5 4.6 5.2 Embodiment 2 Inventive 1185 910 466 0.018 61 774 3.1 3.1 5.2 6.2 Embodiment Steel 2 3 1184 920 472 0.019 63 792 3.6 3.5 5.5 6.3 Embodiment 4 Inventive 1201 894 686 0.014 73 811 4.9 6.1 6.3 9.1 Embodiment Steel 3 5 1202 899 643 0.014 71 812 4.1 6.8 6.1 9.6 Embodiment 6 Inventive 1196 915 584 0.035 73 725 5.6 9.1 7.8 5.2 Comparative Steel 4 Example 1 1198 932 589 0.012 73 821 5.8 10.2 7.5 7.8 Embodiment 7 Inventive 1185 912 631 0.0013 63 839 8.5 12.6 7.1 6.4 Comparative Steel 5 Example 2 Inventive 1196 897 686 0.023 33 841 7.3 8.5 9.2 5.3 Comparative Steel 6 Example 3 1195 897 643 0.0015 68 834 16.5 7.2 9.2 8.9 Comparative Example 4 Inventive 1201 895 684 0.015 35 846 7.5 8.5 9.3 5.2 Comparative Steel 7 Example 5 1203 893 642 0.014 68 835 17.5 7.2 9.1 8.9 Comparative Example 6 Comparative 1203 923 672 0.013 72 802 2.8 6.1 11.5 5.3 Comparative Steel 1 Example 7 Comparative 1199 894 356 0.015 76 802 3.7 6.5 6.8 5.4 Comparative Steel 2 Example 8 1187 885 684 0.016 78 779 4.1 7.8 8.3 3.8 Comparative Example 9

(70) TABLE-US-00003 TABLE 3 Steel YP-El L-BH El YS Type {circle around (1)} {circle around (2)} {circle around (3)} {circle around (4)} (%) (MPa) (%) (MPa) Note Inventive 2.7 0.1 91.2 0.31 0 42 37 238 Inventive Steel 1 Steel 1 1.2 0.1 98.7 0.28 0 44 34 242 Inventive Steel 2 Inventive 3.3 0.1 96.6 0.45 0 48 38 283 Inventive Steel 2 Steel 3 3.2 0.1 96.7 0.58 0 39 35 278 Inventive Steel 4 Inventive 4.2 0.1 95.7 0.29 0 50 36 292 Inventive Steel 3 Steel 5 5.3 0.1 94.6 0.65 0 47 35 289 Inventive Steel 6 Inventive 0 0 100 0.05 0.52 23.5 33 292 Comparative Steel 4 Steel 1 2.1 0.3 97.6 0.45 0 39 33 289 Inventive Steel 7 Inventive 2.2 0 97.8 0.14 0.3 31 32 265 Comparative Steel 5 Steel 2 Inventive 1.8 0.6 97.6 0.15 0.35 43 32 278 Comparative Steel 6 Steel 3 10.2 0 89.8 0.85 0.61 26 31 281 Comparative Steel 4 Inventive 1.8 0.5 97.7 0.15 0.28 28 32 265 Comparative Steel 7 Steel 5 2.1 1.1 96.8 0.83 0 26 33 262 Comparative Steel 6 Comparative 0 0 100 0 0.3 27 38 213 Comparative Steel 1 Steel 7 Comparative 12.3 4.1 83.6 0.85 0.32 45 30 321 Comparative Steel 2 Steel 8 13.1 4.3 82.6 0.89 0.31 38 32 313 Comparative Steel 9 In the table, {circle around (1)} indicates a martensite area ratio (%), {circle around (2)} indicates a bainite area ratio (%), {circle around (3)} indicates a ferrite area ratio (%), {circle around (4)} indicates a value K, and YP-El 0% indicates that room temperature aging resistance may be guaranteed.

(71) Referring to Table 3, as for inventive steels 1 to 7 which satisfied alloy compositions and manufacturing conditions suggested in the present disclosure, aging resistance was secured at room temperature (an yield point elongation (YP-El) was 0.2% or less after a heat treatment for 1 hour at 100° C.), and 350 MPa or higher bake hardenability appeared at a baking temperature of 140° C.

(72) As for comparative steel 1, an annealing temperature was lower than the range suggested in the present disclosure, and accordingly, martensite was partially formed in a hot-rolling state as a cooling speed was high after hot-rolling and coiling, but austenite was not sufficiently formed during a two-phase region annealing after cold-rolling such that martensite was not sufficiently formed in a final structure. Accordingly, aimed bake hardenability was not secured.

(73) As for comparative steel 2, a cooling speed after coiling was lower than the range suggested in the present disclosure, and a secondary cooling speed exceeded the range suggested in the present disclosure. Thus, aimed aging resistance and bake hardenability were not obtained.

(74) As for comparative steels 3 to 6, the primary or tertiary cooling speed was beyond the suggested range, or a cold reduction ratio was lower than the suggested range. As a result, aging resistance or bake hardenability was not sufficient. In comparative steel 4, a content of martensite in a structure was excessive.

(75) As for comparative steel 7, a composition system of comparative steel 7 was different from that of the present disclosure in that comparative steel 7 was the sample in which bake hardenability steel was manufactured using general extremely low carbon steel. Accordingly, martensite was not sufficiently formed in a structure, and there was a problem in aging resistance and bake hardenability.

(76) As for comparative steel 8, when a coiling temperature is too low, or a baking temperature is too high, a content of martensite in a steel structure was excessive, which may cause a problem in aging resistance. In comparative steel 9, when a tertiary cooling speed is too high, a content of martensite in a structure was too high, and there was a problem in aging resistance.

(77) FIG. 1(a) is an image obtained by observing a process in which solid solution carbon is dispersed into a ferrite grain from martensite and ferrite grain boundaries before and after baking of inventive steel 1 using an APT in atom unit. FIG. 1(b) is graphs representing the process in profile.

(78) While exemplary embodiments have been shown and described above, the scope of the present disclosure is not limited thereto, and it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.