EP3428302A1

EP3428302A1 - Hot-dip galvanized steel sheet with superior bake hardenability and aging resistance, and manufacturing method thereof

Info

Publication number: EP3428302A1
Application number: EP17763524.0A
Authority: EP
Inventors: Sang-Ho Han; Yeon-Sang Ahn
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-03-08
Filing date: 2017-03-07
Publication date: 2019-01-16
Also published as: EP3428302A4; JP2019512600A; CN108779537A; WO2017155263A1; US20190071746A1; KR101767818B1

Abstract

Disclosed are a cold-rolled steel sheet having excellent bake hardenability and aging resistance, and a manufacturing method thereof. The cold-rolled steel sheet comprises, by weight %, 0.02-0.08% of C; 1.3-2.1% of Mn; 0.3% or less (excluding 0%) of Si; 1.0% or less (excluding 0%) of Cr; 0.1% or less (excluding 0%) of P; 0.01% or less (excluding 0%) of S; 0.01% or less (excluding 0%) of N; and 0.01-0.06% of sol.Al; at least one selected from the group consisting of 0.2% or less (excluding 0%) of Mo and 0.003% or less (excluding 0%) of B; and the balance being Fe and other inevitable impurities, wherein the steel sheet contains, as a microstructure thereof, 90-99% of ferrite and 1-10% of martensite in an area fraction, wherein a ratio (a/b) of an average carbon concentration a in the martensite and an average carbon concentration b in the ferrite located in an imaginary circle having a major axis of the martensite as a diameter, at a point of the sheet thickness of 1/4 t is 1.4 or less, and a ratio (d/c) of an average manganese concentration c in the martensite and an average manganese concentration d in the ferrite located in an imaginary circle having a major axis of the martensite as a diameter, at a point of a sheet thickness of 1/4 t is 0.9 or less.

Description

[Technical Field]

The present disclosure relates to a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, and a manufacturing method thereof, and more particularly, to a steel sheet hot dip plated with zinc based layer, having excellent bake hardenability and aging resistance, preferably capable of being used as a material for external automobile panels, and a manufacturing method thereof.

[Background Art]

As impact stability regulations of automobiles and fuel efficiency are emphasized, high tensile steel has been actively used to satisfy requirements for both weight reductions and high strength in automobile bodies. In accordance with this trend, the application of high-strength steel to external automobile panels has also been extended.
Currently, most 340 MPa-grade bake hardened steel has been used as external automobile panels, but a portion of 490 MPa-grade steel sheets are also being applied, which will be expected to be extended to 590 MPa-grade steel sheets.
As described above, when such steel sheets having increased strength are applied as an external panel, weight reduction and dent resistance may be improved. On the other hand, as strength increases, there is a disadvantage that formability may be deteriorated. Accordingly, recently, customers are demanding a steel sheet having a relatively low yield ratio (YR = YS/TS) and relatively high ductility, in order to supplement poor workability while high-strength steel may be applied to use in an external panel.
In addition, it is necessary to have bake hardenability at a certain level or higher in order for a material to be applied to use in external automobile panels. A phenomenon of bake hardenability is a phenomenon in which yield strength is increased due to fixing solid solution carbon and nitrogen, which are activated during the press, onto dislocations at the time of the baking of paint. Steel having excellent bake hardenability is easy to form before the baking of paint, and final products thereof have enhanced dent resistance. Therefore, such steel is very ideal as a material for external automobile panels. In addition, in order to be a material applied to use in external automobile panels, it is necessary to have a certain level of aging resistance to guarantee aging for a certain period or longer.
Japanese Patent Publication No. 2005-264176 discloses a steel sheet having a complex phase mainly composed of martensite as a conventional technique for improving workability in a high-strength steel sheet. In order to improve workability, a method of manufacturing a high-strength steel sheet in which a fine Cu precipitate has a grain size of 1 to 100 nm is disclosed. However, in this technique, it is necessary to add excessive amounts of Cu of 2 to 5% in order to precipitate fine Cu particles. In this case, hot shortness attributable to Cu may occur, and manufacturing costs may be excessively increased.
Japanese Patent Publication No. 2004-292891 discloses a steel sheet having a complex phase including ferrite as a main phase and residual austenite and bainite and martensite which are low temperature transformation phases as secondary phases, and a method for improving ductility and stretch flangeability of the steel sheet. However, this technique has problems in that it may be difficult to secure plating quality, and to secure surface quality in a process for making steel and a continuous casting process, since large amounts of Si and Al are added to secure the residual austenite phase. In addition, there is a disadvantage in that yield ratio may be high because an initial YS value is high due to transformation induced plasticity.
Korean Patent Publication No. 10-2012-0073564 discloses a technique for providing a high tensile hot-dip galvanized steel sheet having good workability. A steel sheet comprising soft ferrite and hard martensite as a microstructure, and a manufacturing method for improving an elongation and an r value (a Lankford value) of the steel sheet are disclosed. However, this technology has a problem that it is difficult to secure good plating quality, since large amounts of Si are added, and a problem that manufacturing costs increase due to the addition of large amounts of Ti and Mo.

[Disclosure]

[Technical Problem]

One of the objects of the present disclosure is to provide a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, and a manufacturing method thereof.

[Technical Solution]

According to an aspect of the present disclosure, a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, comprises a cold-rolled steel sheet and a zinc based plating layer formed on a surface of the cold-rolled steel sheet, wherein the cold-rolled steel sheet comprises, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprises one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprises a remainder of iron (Fe) and unavoidable impurities, and comprises, by area, 90 to 99% of ferrite and 1 to 10% of martensite as a microstructure, wherein a ratio (a/b) of an average carbon concentration a in the martensite and an average carbon concentration b in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of 1/4t of a sheet thickness of the cold-rolled steel sheet is 1.4 or less, and wherein a ratio (d/c) of an average manganese concentration c in the martensite and an average manganese concentration d in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of 1/4t of a sheet thickness of the cold-rolled steel sheet is 0.9 or less.
According to another aspect of the present disclosure, a method of manufacturing a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, comprises reheating a steel slab comprising, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprising one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprising a remainder of iron (Fe) and unavoidable impurities;
hot-rolling the reheated steel slab in a single phase temperature region of austenite to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at a temperature in a range of 760 to 850°C; firstly cooling the continuously annealed cold-rolled steel sheet to a temperature in a range of 630 to 670°C at an average cooling rate of 2 to 14°C/sec; secondly cooling the firstly cooled cold-rolled steel sheet to a temperature in a range of (Ms + 20) - (Ms + 50) °C at an average cooling rate of 3 to 12°C/sec; thirdly cooling the secondly cold-rolled steel sheet to a temperature in a range of 440 to 480°C at a rate of 4 to 8°C/sec; immersing the thirdly cooled cold-rolled steel sheet in a zinc based hot bath to obtain a steel sheet hot-dip plated with zinc based layer; and finally cooling the steel sheet hot-dip plated with zinc based layer to a temperature in a range of (Ms-100)°C or lower at an average cooling rate of 3°C/sec or higher.

[Advantageous Effects]

As one of various effects of the present disclosure, the galvanized steel sheet according to an embodiment of the present disclosure may be suitably applied to a material for external automobile panels, because of its excellent bake hardenability and aging resistance.

[Best Mode for Invention]

Hereinafter, exemplary embodiments of the present disclosure will be described in detail.
The inventors of the present disclosure have conducted intensive research into providing a steel sheet hot-dip plated with zinc based layer securing excellent strength and ductility simultaneously to have excellent formability, as well as excellent bake hardenability and aging resistance, so as to be suitable as a material for external automobile panels. As a result, it became possible to provide a steel sheet hot-dip plated with zinc based layer which satisfies the intended properties by optimally controlling a composition range of a cold-rolled steel sheet, a substrate, and optimizing production conditions thereof. Finally, the present disclosure has been accomplished based on this finding.
Hereinafter, a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, an aspect of the present disclosure, will be described in detail.
The steel sheet hot-dip plated with zinc based layer of the present disclosure may include a cold-rolled steel sheet and a zinc based hot-dip plating layer formed on one or both surfaces of the cold-rolled steel sheet. In the present disclosure, a composition of the zinc based hot-dip plating layer is not particularly limited, and may be a pure Zinc plating layer, or a Zinc based alloy plating layer containing Si, Al, Mg, or the like. The zinc based hot-dip plating layer may be agalva-annealed layer.
Hereinafter, the alloying element and the preferable content range thereof of the cold-rolled steel sheet as a substrate will be described in detail. It is to be noted in advance that the content of each component described below is on a weight basis unless otherwise specified.

Carbon (C): 0.02 to 0.08%

Carbon may be an indispensable element to be added to secure the desired complex phase in the present disclosure. Generally, carbon is advantageous for producing a complex phase since martensite may be easily formed as the content of carbon increases. However, to secure the intended strength and yield ratio (yield strength/tensile strength), it is necessary to control the content in a proper amount. When the content of carbon is less than 0.02%, it may be difficult to achieve the desired strength in the present disclosure, and formation of an appropriate level of martensite may be difficult. On the other hand, when the content thereof exceeds 0.08%, the formation of bainite at the grain boundary may be promoted during cooling after annealing to increase the yield ratio of the steel, and bending and surface defects may be easily caused in machining into automobile parts. Therefore, in the present disclosure, the content of carbon may be controlled to be 0.02 to 0.08%, and more preferably 0.03 to 0.06%.

Manganese (Mn): 1.3 to 2.1%

Manganese may be an element which improves the hardenability in the complex phase steel, and, in particular, plays an important role in forming martensite. When the content of manganese is less than 1.3%, the formation of martensite may be impossible, and complex phase steel may be difficult to be produced. On the other hand, when the content of manganese exceeds 2.1%, martensite may be excessively formed to make a material property unstable, and there may be a problem that the risk of processing crack and strip breakage is significantly increased due to the formation of a band of manganese in the structure. In addition, there may be a problem that the manganese oxide is precipitated on the surface upon annealing, which significantly deteriorates plating characteristics. Therefore, in the present disclosure, the content of manganese may be controlled to be 1.3 to 2.1%, and more preferably to 1.4 to 1.8%.

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

Silicon may contribute to an increase in the strength of the steel sheet by solid solution strengthening, but may be not intentionally added in the present disclosure. Further, there may be no problem in securing the properties without adding silicon. However, 0% may be excluded in consideration of an amount that is inevitably added in the manufacturing process. On the other hand, when the content of silicon exceeds 0.3%, there may be a problem that the surface properties of the plating may be poor. Therefore, the content of silicon may be controlled to be 0.3% or less in the present disclosure.

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

Chromium may be a component having characteristics similar to manganese, and may be an element added to improve hardenability of steel, and to improve strength of steel. In addition, chromium may assist in forming martensite. Further, since an occurrence of yield stretch YP-El is suppressed by precipitating solid solute carbon to be under a certain level which is proper amount of solute carbon in the steel through forming coarse Cr-based carbides such as Cr23C6 during hot-rolling, chromium may be an element favorable for the production of complex phase steel having a relatively low yield ratio. In addition, chromium is an element advantageous for manufacturing high strength complex phase steel having a relatively high ductility by relatively reducing ductility drop compared with the increase in strength. However, when the content thereof exceeds 1.0%, the martensite structure fraction may be excessively increased to cause a decrease in strength and elongation. In the present disclosure, the content of chromium may be controlled to be 1.0% or less.

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

Phosphorus is the most advantageous element in securing strength without significantly impairing formability. However, the possibility of the occurrence of brittle fracture significantly increases when the element is excessively added, the possibility of strip breakage of a slab significantly increases during hot-rolling, and the surface properties of a plated layer may be deteriorated. Therefore, in the present disclosure, the content of phosphorus may be controlled to be 0.1%.

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

Sulfur may be an impurity to be inevitably contained in the steel. It may be desirable to control the content of sulfur to be as low as possible. In particular, sulfur in the steel may increase the possibility of generating hot shortness, and the content thereof may be controlled to be 0.01% or less.

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

Nitrogen may be an impurity to be inevitably contained in the steel. It may be desirable to control the content of nitrogen as low as possible. However, since the steel refining cost rises sharply to reduce the content of nitrogen, the content thereof may be controlled to be 0.01% or less, a possible range of operation conditions.

Acid soluble aluminum (sol.Al): 0.01 to 0.06%

Acid soluble aluminum is an element to be added for grain refinement and deoxidation. When the content thereof is less than 0.01%, aluminum-killed (Al-killed) steel may not be produced in a normal stable state. Meanwhile, when the content thereof exceeds 0.06%, it is advantageous to increase the strength due to the grain refinement effect. On the other hand, when the steelmaking operation in a continuous casting process is carried out, the inclusions may be excessively formed. In this case, the possibility of surface defects of a plated steel sheet may increase, and a sharp rise in manufacturing costs may occur. Therefore, in the present disclosure, the content of acid soluble aluminum may be controlled to be 0.01 to 0.06%.
One or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B)
Molybdenum may be an element added to delay transformation of austenite into pearlite, and to improve ferrite refinement and steel strength. Molybdenum may also assist in improving hardenability of steel. However, when the content of molybdenum exceeds 0.1%, there may be a problem in that manufacturing costs are rapidly increased to lower economical efficiency and to lower ductility of steel. In the present disclosure, the content of molybdenum may be controlled to be 0.1% or less.
In addition, boron may be an element added to prevent secondary work embrittlement caused by phosphorous in the steel. There may be no problem in securing the properties without adding boron. Meanwhile, when the content of boron exceeds 0.003%, there may be a problem that ductility of the steel is lowered. In the present disclosure, the content of boron may be controlled to be 0.003% or less.
In addition, iron (Fe) and unavoidable impurities may be further included as a remainder. However, in the ordinary manufacturing process, impurities that are not intended from raw materials or surrounding environments may be inevitably incorporated, such that it may not be excluded. Such impurities are not specifically mentioned in this specification, as they are known to one of ordinary skill in the art. In addition, the addition of an effective component other than the above-mentioned composition may be not excluded.
The cold-rolled steel sheet of the present disclosure may include, by area, 90 to 99% of ferrite and 1 to 10% of martensite as a microstructure.
When an area ratio of the martensite is less than 1%, it may be difficult to form a complex phase and it may be difficult to obtain a steel sheet having a relatively low yield ratio. On the other hand, when the area ratio exceeds 10%, the strength may be excessively increased. Therefore, an area ratio of martensite is preferably 1 to 10%, more preferably 2 to 5%, by area.
In the cold-rolled steel sheet of the present disclosure, a ratio (a/b) of an average carbon concentration a in the martensite and an average carbon concentration b in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of 1/4t of a sheet thickness thereof may be a value of 1.4 or less.
In the present disclosure, fine martensite in a ferrite matrix may be appropriately distributed. At the same time, a ratio of the carbon concentration in an interior of martensite and in an interior of ferrite in a periphery of the martensite may be appropriately controlled. In accordance therewith, it may be designed such that the carbon intensively present in martensite can easily diffuse into surrounding ferrite by the conventional baking treatment (about 170°C, about 20 minute). When the ratio (a/b) of the average carbon concentration exceeds 1.4, the content of the solid solution carbon present in ferrite is too low to secure the desired bake hardenability. Meanwhile, as the ratio (a/b) of the average carbon concentration lowers, the securing of bake hardenability may be relatively high. Therefore, the lower limit is not particularly limited in the present disclosure.
In the cold-rolled steel sheet of the present disclosure, a ratio (d/c) of an average manganese concentration c in the martensite and an average manganese concentration d in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of 1/4t of a sheet thickness thereof may be a value of 0.9 or less, more preferably a value of 0.8 or less. When the ratio (d/c) of the average manganese concentration exceeds 0.9, the content of manganese present in ferrite is too high to facilitate the formation of a manganese band in the structure. The possibility of processing cracks in forming may increase due to the decrease in ductility of steel. Meanwhile, as the ratio (d/c) of the average manganese concentration lowers, the securing of ductility may be relatively high. Therefore the lower limit is not particularly limited in the present disclosure.
According to an embodiment, an occupancy ratio (M) of martensite having an average circle equivalent diameter of 5 µm or less (excluding 0 µm) present at ferrite grain boundaries (including grain boundary triple points) defined by the following Relationship 1 may be 90% or more: $M = \{M_{gb} / (M_{gb} + Min)\} x 100$
where M_gb refers to the number of martensite having an average circle equivalent diameter of 5 µm or less (excluding 0 µm) present at ferrite grain boundaries, and M_in refers to the number of martensite having an average circle equivalent diameter of 5 µm or less (excluding 0 µm) present inside ferrite crystal grains.
That is, as the fine martensite having an average circle equivalent diameter of 5 µm or less (excluding 0 µm) is mainly present at ferrite grain boundaries rather than inside ferrite crystal grains, it may be advantageous in improving ductility with maintaining a relatively low yield ratio. When the occupancy ratio (M) of martensite is less than 90%, martensite formed in the crystal grains may increase yield strength during tensile deformation to increase yield ratio. In this case, it may be difficult to control the yield ratio through temper rolling. In addition, martensite existing in the crystal grains may significantly inhibit a moving of dislocation during processing and weaken ductility of ferrite, such that a reduction of elongation may be caused.
In the meantime, the cold-rolled steel sheet of the present disclosure may partially contain bainite in addition to the above-mentioned ferrite and martensite. Since solid solute carbon and solid solute nitrogen existing inside the grains of bainite may easily adhere to a dislocation, interfere with the displacement of the dislocation, and exhibit a discontinuous yield behavior to remarkably increase a yield ratio of steel. Therefore, in the present disclosure, the formation of bainite is preferred to be inhibited as much as possible.
According to an embodiment, an area ratio (B) of the bainite defined by the following Relationship 2 may be 3 or less. When the area ratio (B) of the bainite exceeds 3, the carbon concentration around the bainite may increase to deteriorate ductility of steel, and a yield ratio may rise sharply: $B = \{A_{B} / (A_{F} + A_{M} + A_{B})\} x 100$
where A_F refers to an area ratio of ferrite, A_m refers to an area ratio of martensite, and A_B refers to an area ratio of bainite.
According to an embodiment, a plated layer may be formed on a surface of the cold-rolled steel sheet of the present disclosure. Such a plated layer may be any one of a hot-dip galvanized layer or a galva-annealed layer. As described above, when a cold-rolled steel sheet is formed with the plated layer on its surface, corrosion resistance may be remarkably improved.
The steel sheet hot-dip plated with zinc based layer of the present disclosure described above may be produced by various methods, and the production method thereof is not particularly limited. As a preferable example, it may be produced by the following methods.
Hereinafter, a method of producing a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, another aspect of the present disclosure, will be described in detail.
First, a steel slab having the above-mentioned component system may be reheated. This operation may be carried out to smoothly perform the subsequent hot-rolling operation, and to sufficiently obtain the targeted properties of the steel sheet. In the present disclosure, process conditions of the reheating operation are not particularly limited, and may be normal conditions. As an example, a reheating operation may be performed in a temperature range of 1100 to 1300°C.
Next, the reheated steel slab may be hot-rolled in a single phase temperature region of austenite to obtain a hot-rolled steel sheet. The reason why a hot-rolling operation is carried out in the single phase temperature region of austenite may be to increase the uniformity of the structure.
According to an embodiment, during hot-rolling, a finish rolling temperature may be within a range of (Ar3 + 50) to 950°C. When the finish rolling temperature is lower than (Ar3 + 50)°C, ferrite and austenite two-phase region rolling is highly likely to cause non-uniformity of material. On the other hand, when the temperature exceeds 950°C, non-uniformity of material due to coarse grain caused by high-temperature rolling may occur, and a coil twisting phenomenon may occur during cooling of the hot-rolled steel sheet. For reference, a theoretical temperature of an Ar3 point may be obtained by the following Relationship 3: $Ar 3 (° C) = 910 - 310 [C] - 80 [Mn] - 20 [Cu] - 15 [Cr] - 55 [Ni] - 80 [Mo]$
where each of [C], [Mn], [Cu], [Cr]. [Ni] and [Mo] refers to weight% of the respective elements.
Next, the hot-rolled steel sheet may be coiled.
According to an embodiment, the coiling temperature may be within a range of 450 to 700°C. When the coiling temperature is lower than 450°C, excess formation of martensite or bainite may lead to an excessive increase in strength of the hot-rolled steel sheet, which may cause problems such as poor shape, and the like, due to the subsequent load during cold-rolling. On the other hand, when the coiling temperature exceeds 700°C, surface enrichment of elements which lower wettability of hot-dip galvanized steel such as Si, Mn, B, and the like in the steel may be significantly increased.
Next, the rolled hot-rolled steel sheet may be cold-rolled to obtain a cold-rolled steel sheet.
According to an embodiment, in the cold-rolling operation, a cold-rolling reduction ratio in the cold-rolling operation may be 40 to 80%. When the cold-rolling reduction ratio is less than 40%, it may be difficult to secure the target thickness, and it may be also difficult to correct a shape of the steel sheet. On the other hand, when the cold-rolling reduction ratio exceeds 80%, cracks may occur at an edge portion of the steel sheet, and a cold-rolling load may be caused.
Next, the cold-rolled steel sheet may be continuously annealed. This operation may be performed to form ferrite and austenite simultaneously with recrystallization, and to distribute carbon therein.
At this time, an annealing temperature may preferably be within a range of 760 to 850°C. When the annealing temperature is lower than 760°C, sufficient recrystallization may be not achieved, and sufficient formation of austenite may be difficult, which make it difficult to secure the desired strength in the present disclosure. On the other hand, when the temperature exceeds 850°C, the productivity may be lowered, austenite may be excessively formed, bainite may be formed in the subsequent cooling operation, and ductility of steel may be deteriorated.
Meanwhile, the above annealing temperature range may correspond to a two-phase region (ferrite + austenite) temperature range, but annealing is preferably carried out at a temperature range containing as much ferrite as possible. This is why as initial ferrite at the annealing temperature of the two-phase region is relatively more, a growth of crystal grain after annealing may be promoted to enhance ductility. Further, a degree of carbon enrichment in austenite may be increased to lower a martensitic transformation starting temperature (Ms). In this case, it is possible to form martensite upon cooling after plating process, the subsequent operation. In accordance therewith, it is possible to produce a steel sheet having a relatively low yield ratio and a relatively high ductility, since fine and uniform martensite is distributed in crystal grains as much as possible. In consideration of this, the annealing temperature may more preferably be within a range of 770 to 810°C.
Next, the cold-rolled steel sheet subjected to the continuously annealing operation may be firstly cooled in a temperature range of 630 to 670°C at an average cooling rate of 2 to 14°C/sec. In the present disclosure, as the firstly cooling end temperature is controlled to be relatively high, or the firstly cooling rate is controlled to be relatively slow, tendency of uniformity and coarsening of ferrite may be enhanced, advantageous for ensuring ductility of steel. In addition, in the present disclosure, a sufficient time may be provided to allow carbon to diffuse into austenite during the firstly cooling operation, which is significant in the present disclosure. More specifically, in the two-phase temperature region, carbon may diffuse into austenite having a high degree of carbon enrichment. As the temperature thereof is relatively high, a degree of the diffusion may increase. When the firstly cooling end temperature is lower than 630°C, such an excessively low temperature may result in a relatively low carbon diffusion activity. In this case, carbon concentration in ferrite may increase to result in an increase in yield ratio and an increase in a tendency toward cracking during processing. On the other hand, when the firstly cooling end temperature exceeds 670°C, it may be advantageous in terms of diffusion of carbon, but require an excessively high cooling rate in a secondly cooling operation of the subsequent process. When the firstly cooling rate is lower than 2°C/sec, it may be disadvantageous in terms of productivity. On the other hand, when the firstly cooling rate exceeds 14°C/sec, diffusion of carbon may not sufficiently occur, thereby being not preferred.
Next, the firstly cooled cold-rolled steel sheet may be secondly cooled to a temperature in a range of (Ms + 20) to (Ms + 50)°C at an average cooling rate of 3 to 12°C/sec. According to the studies of the present inventors, when martensite is produced before going through a range of 440 to 480°C, the temperature range of a conventional hot-dip galvanizing bath, coarse martensite may be formed on the cold-rolled steel sheet to be finally obtained, thereby a low yield ratio may be not achieved. When the secondly cooling end temperature is lower than (Ms + 20)°C, martensite may be generated during the secondly cooling operation. In the meantime, when the secondly cooling end temperature is higher than (Ms + 50)°C, a cooling rate before introducing into the plating bath after the secondly cooling, that is, a thirdly cooling rate should be controlled to be relatively high. In addition, there is a high possibility that martensite is formed before immersing in the plating bath. When the secondary cooling rate is lower than 3°C/sec, martensite may be not formed, but it is disadvantageous in terms of productivity. On the other hand, when the rate exceeds 12°C/sec, the overall speed of passing a sheet may be increased to generate problems such as shape warping of a sheet. For reference, the theoretical temperature of Ms can be obtained by the following Relationship 4: $Ms (° C) = 539 - 423 [C] - 30.4 [Mn] - 12.1 [Cr] - 17.7 [Ni] - 7.5 [Mo]$
where each of [C], [Mn], [Cr]. [Ni] and [Mo] refers to weight% of the respective elements.
Next, the secondly cooled cold-rolled steel sheet may be thirdly cooled to a temperature range of 440 to 480°C at a rate of 4 to 8°C/sec. The above temperature range may be a temperature range of a conventional galvanizing bath, and this operation may be carried out to prevent formation of a martensite structure before the cold-rolled steel sheet is immersed in the galvanizing bath. When the thirdly cooling rate is lower than 4°C/sec, martensite may be not formed, but it is disadvantageous in terms of productivity. On the other hand, when the rate exceeds 8°C/sec, martensite may be partially formed and bainite may be partially formed in the grains. In this case, ductility may be deteriorated, as well as an increase in yield strength.
Next, the thirdly cooled cold-rolled steel sheet may be immersed in a zinc based hot bath to obtain a steel sheet hot-dip plated with zinc based layer. In the present disclosure, a composition of the zinc based hot bath is not particularly limited, and may be a pure galvanizing bath or an alloyed galvanizing bath containing Si, Al, Mg, or the like.
Next, the hot-dip galvanized steel sheet may be finally cooled to a temperature in a range of (Ms-100)°C or lower at an average cooling rate of 3°C/sec or higher. When the final cooling end temperature is lower than (Ms-100)°C, not only fine martensite may not be obtained, but also a defective problem regarding a plate shape may be caused. Further, when the average cooling rate is lower than 3°C/sec, martensite may be irregularly formed in the grain boundaries or in the crystal grains, due to the excessively slow cooling rate. In addition, since a ratio of martensite formation in the crystal grains to martensite formation in the grain boundaries is relatively low, steel having a relatively low yield ratio may be not manufactured.
Meanwhile, when necessary, the steel sheet hot-dip plated with zinc based layer may be subjected to an alloying heat treatment before the final cooling to obtain a galva-annealed steel sheet. In the present disclosure, conditions of the alloying heat treatment process are not particularly limited, and may be conventional conditions. As an example, an alloying heat treatment process may be performed in a temperature range of 480 to 600°C.
Next, when necessary, the final cooled steel sheet plated with zinc based layer or the galva-annealed steel sheet is subjected to temper rolling to form large amounts of dislocations in ferrite disposed around martensite, thereby further improving bake hardenability.
At this time, a reduction ratio is preferably 0.3 to 1.6%, more preferably 0.5 to 1.4%. When the reduction ratio is less than 0.3%, sufficient dislocations may be not formed and it is disadvantageous from the viewpoint of a plate form. In particular, defects of the plated surface may occur. On the other hand, when the reduction ratio exceeds 1.6%, it is advantageous in terms of formation of dislocation, but it may cause side effects such as occurrence of strip breakage due to facility capability limit.

[Mode for Invention]

Hereinafter, the present disclosure will be described in more detail by way of examples. However, the following examples are only illustrative of the present disclosure in more detail, and do not limit the scope of the present disclosure.
After preparing a steel slab having an alloy composition shown in Table 1 below, a hot-dip galvanized steel sheet (GI steel sheet) or a galva-annealed steel sheet (GA steel sheet) was prepared using a manufacturing process described in Table 2 below. For reference, inventive steels 1, 2, 4 and 5 and comparative examples 1 and 2 correspond to galva-annealed steel sheets in Table 1, and invention steels 3 and 6 correspond to hot-dip galvanized steel sheets. Meanwhile, in a preparation of each specimen, a firstly cooling end temperature was constantly set to be 650°C, a secondly cooling end temperature was constantly set to be 560°C, a thirdly cooling end temperature was constantly set to be 460°C, and a plating bath temperature was constantly set to be 480°C.
Thereafter, microstructures were observed on each of the produced plated steel sheets, and the properties thereof were evaluated. The results therefrom were shown in Table 3 below.
In Table 3, fractions of microstructures and concentration ratios of C and Mn were results from analysis of structures at the point of 1/4t of a sheet thickness of the steel sheet. First, the fractions of microstructures were measured by observing martensite and bainite through Lepera etching using an optical microscope, observing them with SEM (3,000 times), and measuring size and distribution of martensite at three times averages through Count Point operation. Meanwhile, the concentration ratios of C and Mn were performed by preferentially measuring concentrations of C and Mn existing on the respective phases by a CPS (Count Per Sec) method, in a line and point manner using a TEM and an EDS (Energy Dispersive Spectroscopy) analysis method, thereby quantitatively measuring the ratios. At this time, as a criterion for measuring concentrations of C and Mn in ferrite and martensite, concentrations of C and Mn measured in a position in contact with a virtual circle having a diameter corresponding to a short axis of martensite were taken as an average carbon concentration in martensite, and concentrations of C and Mn measured in a ferrite in contact with a virtual circle having a diameter corresponding to a short axis of martensite were taken as an average carbon concentration in ferrite.

Tensile test for each specimen in Table 3 was performed in a C direction using the JIS standard. In the meantime, the bake hardenability was evaluated by a difference in yield strength after maintaining the specimen at 170°C for 20 minutes, based on the strength after a 2% pre-strain. The aging resistance was evaluated by measuring YP-El(%) at the time of tensile test after maintaining the specimen at 100 for 2 hours.

[Table 1]Classification	Cold-Rolled Steel Sheet Composition (wt%)
[Table 1]Classification	C	Mn	Si	Cr	P	S	N	sol. Al	Mo	B
Inventive steel 1	0.023	1.7	0.05	0.80	0.05	0.005	0.003	0.018	0.15	0.0006
Inventive steel 2	0.038	1.72	0.04	0.48	0.05	0.005	0.003	0.04	0.12	0.0009
Inventive steel 3	0.052	1.51	0.10	0.43	0.03	0.007	0.004	0.05	0.13	-
Inventive steel 4	0.051	1.54	0.15	0.81	0.04	0.004	0.003	0.041	0.15	0.0021
Inventive steel 5	0.069	1.43	0.22	0.87	0.02	0.003	0.004	0.052	0.18	-
Inventive steel 6	0.075	1.32	0.21	0.08	0.03	0.004	0.008	0.025	0.08	0.0012
Comparative stteel 1	0.096	1.21	0.62	1.18	0.12	0.006	0.003	0.042	0.45	0.004
Comparative stteel 2	0.098	1.26	0.81	1.21	0.12	0.007	0.005	0.05	0.38	0.0041

[Table 2]

Classification	Manufacturing Conditions									Note
Classification	Reheating Temperature (°C)	Finish Rolling Temperature (°C)	Coiling Temperature (°C)	Cooling Reduction Ratio (%)	Annealing Temperature (°C)	1st Cooling Rate (°C/sec)	2nd Cooling Rate (°C/sec)	3rd Cooling Rate (°C/sec)	Final Cooling Rate (°C/sec)
Inventive Steel 1	1184	882	598	48	766	2.5	4.1	4.5	4.5	Inventive example 1
Inventive Steel 1	1187	895	556	54	764	2.4	4.5	4.6	5.7	Inventive example 2
Inventive Steel 2	1183	912	465	63	777	3.4	3.4	5.1	6.2	Inventive example 3
Inventive Steel 2	1183	921	472	64	779	3.6	3.5	5.5	6.3	Inventive example 4
Inventive Steel 3	1200	891	682	71	811	4.9	6.3	6.3	9.2	Inventive example 5
Inventive Steel 3	1203	896	645	72	815	4.2	6.8	6.2	9.6	Inventive example 6
Inventive Steel 4	1197	935	580	75	741	5.6	9.1	7.8	5.3	Comparative example 1
Inventive Steel 4	1198	942	585	79	821	5.8	10.6	7.5	7.8	Inventive example 7
Inventive Steel 5	1185	923	652	63	857	6.8	11.4	9.2	7.2	Comparative example 2
Inventive Steel 5	1185	912	632	65	839	8.5	12.6	7.1	6.4	Comparative example 3
Inventive Steel 6	1209	897	682	35	841	7.5	8.5	9.2	5.2	Comparative example 4
Inventive Steel 6	1205	890	647	68	835	16.5	7.8	9.5	8.9	Comparative example 5
Comparative steel 1	1203	897	660	72	802	2.8	6.5	11.5	5.3	Comparative example 6
Comparative steel 2	1199	892	672	75	802	3.8	6.5	6.8	5.2	Comparative example 7
Comparative steel 2	1187	885	682	78	779	4.1	7.8	8.3	3.8	Comparative example 8

[Table 3]

Classificat ion	Microstructure					Properties					Note
Classificat ion	①	②	③	④	⑤	YP-El (%)	L-BH (MPa)	El (%)	TS (MPa)	YR
Inventive steel 1	2.2	1.4	92.2	1.25	0.75	0	42	34	476	0.55	Inventive example 1
Inventive steel 1	1.9	1.7	91.4	1.2	0.69	0	41	34	468	0.56	Inventive example 2
Inventive steel 2	3.3	-	90.6	1.12	0.65	0	48	36	502	0.55	Inventive example 3
Inventive steel 2	3.5	0.5	92.1	0.98	0.79	0	38	35	505	0.56	Inventive example 4
Inventive steel 3	4.5	-	92.3	0.89	0.63	0	51	35	496	0.55	Inventive example 5
Inventive steel 3	5.0	0.1	91.5	1.12	0.75	0	43	35	513	0.56	Inventive example 6
Inventive steel 4	0.7	3.5	90.6	1.75	0.93	0	28.5	26	582	0.59	Comparative example 1
Inventive steel 4	6.8	2.1	90.5	1.08	0.71	0	39	33	612	0.56	Inventive example 7
Inventive steel 5	4.1	0.3	77	1.08	0.63	0.4	55	33	531	0.62	Comparative example 2
Inventive steel 5	6.2	1.2	78	1.55	0.83	0.3	54	28	535	0.61	Comparative example 3
Inventive steel 6	1.8	0.5	93	1.72	0.92	0	28	32	532	0.59	Comparative example 4
Inventive steel 6	9.8	1.5	76	1.71	0.93	0	26	26	528	0.59	Comparative example 5
Comparat ive steel 1	4.1	3.5	77	1.67	0.92	0.3	52	26	554	0.63	Comparative example 6
Comparat ive steel 2	4.3	3.1	81	1.58	0.93	0	36	25	555	0.71	Comparative example 7
Comparat ive steel 2	4.2	3.3	83	1.21	0.91	0	43	26	548	0.70	Comparative example 8
Wherein ① refers to area ratios (%) of martensite, ② refers to area ratios (%) of bainite, ③ refers to area ratios (%) of ferrite, ④ refers to (a/b) values, and ⑤ refers to (d/c) values.

Referring to Table 3, in cases of Inventive Examples 1 to 7, which satisfy the alloy composition and manufacturing conditions proposed in the present disclosure, tensile strengths of 450 to 650 MPa were obtained and strengths were thus excellent, yield ratios of 0.57 or less were obtained and yield ratios were thus relatively low, elongations of 33% or more were obtained and were thus excellent in ductility, amounts of bake hardenability (BH) of 35 MPa or more were obtained and were thus excellent in bake hardenability, and an YP-El value of 0% was obtained and was thus excellent in aging resistance.
On the other hand, in Comparative Example 1, since the annealing temperature thereof was lower than the range proposed in the present disclosure, austenite was not sufficiently formed during the annealing operation, and martensite was not sufficiently formed in a final structure. Thus, the desired ductility and bake hardenability could not be obtained. In Comparative Example 2, the annealing temperature exceeded the range proposed in the present disclosure. In this case, bake hardenability was secured by a formation of a martensite structure, but an aging problem was caused. Further, in Comparative Examples 3 and 4, the secondly or thirdly cooling rate exceeded the range proposed in the present disclosure. In these cases, the intended curing properties were not secured, or aging problems were caused. In Comparative Example 5, the firstly cooling rate exceeded the range suggested in the present disclosure. In this case, a diffusion of carbon during cooling operation could not sufficiently occur, and the desired bake hardenability in the present disclosure could not be secured. In addition, in Comparative Examples 6 to 8, since contents of C and Cr in the steel were relatively high, large amounts of bainite were formed on the whole, and elongations thereof were relatively low.
While exemplary aspects have been shown and described above, 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.

Claims

A steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, comprising a cold-rolled steel sheet and a zinc based plating layer formed on a surface of the cold-rolled steel sheet,
wherein the cold-rolled steel sheet comprises, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprises one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprises a remainder of iron (Fe) and unavoidable impurities, and comprises, by area, 90 to 99% of ferrite and 1 to 10% of martensite as a microstructure,
wherein a ratio (a/b) of an average carbon concentration a in the martensite and an average carbon concentration b in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of 1/4t of a sheet thickness of the cold-rolled steel sheet is 1.4 or less, and
wherein a ratio (d/c) of an average manganese concentration c in the martensite and an average manganese concentration d in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of 1/4t of a sheet thickness of the cold-rolled steel sheet is 0.9 or less.
The steel sheet hot-dip plated with zinc based layer according to claim 1,
wherein an occupancy ratio (M) of martensite having an average circle equivalent diameter of 5 µm or less (excluding 0 µm) present at ferrite grain boundaries (including grain boundary triple points) defined by the following Relationship 1, in the cold-rolled steel sheet, is 90% or more: $M = \{M_{gb} / (M_{gb} + M_{in})\} x 100$
(Where M_gb refers to the number of martensite having an average circle equivalent diameter of 5 µm or less (excluding 0 µm) present at ferrite grain boundaries, and M_in refers to the number of martensite having an average circle equivalent diameter of 5 µm or less (excluding 0 µm) present in ferrite crystal grains)
The steel sheet hot-dip plated with zinc based layer according to claim 1,
wherein the cold-rolled steel sheet further comprises bainite as a microstructure, and an area ratio (B) of the bainite defined by the following Relationship 2 is 3 or less:
[Relationship 2] B = {A_B / (A_F + A_M + A_B)} x 100

(Where A_F refers to an area ratio of ferrite, A_M refers to an area ratio of martensite, and A_B refers to an area ratio of bainite)
The steel sheet hot-dip plated with zinc based layer according to claim 1,
wherein the zinc based plating layer is a galva-annealed layer.
The steel sheet hot-dip plated with zinc based layer according to claim 1,
wherein the steel sheet hot-dip plated with zinc based layer has the bake hardenability (BH) of 35 MPa or more.
The steel sheet hot-dip plated with zinc based layer according to claim 1,
wherein the steel sheet hot-dip plated with zinc based layer has a yield ratio of 0.57 or less and an elongation of 33% or less.
A method of manufacturing a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, comprising:
reheating a steel slab comprising, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprising one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprising a remainder of iron (Fe) and unavoidable impurities;

hot-rolling the reheated steel slab in a single phase temperature region of austenite to obtain a hot-rolled steel sheet;

coiling the hot-rolled steel sheet;

cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;

continuously annealing the cold-rolled steel sheet at a temperature in a range of 760 to 850°C;

firstly cooling the continuously annealed cold-rolled steel sheet to a temperature in a range of 630 to 670°C at an average cooling rate of 2 to 14°C/sec;

secondly cooling the firstly cooled cold-rolled steel sheet to a temperature in a range of (Ms + 20) to (Ms + 50)°C at an average cooling rate of 3 to 12°C/sec;

thirdly cooling the secondly cold-rolled steel sheet to a temperature in a range of 440 to 480°C at a rate of 4 to 8°C/sec;

immersing the thirdly cooled cold-rolled steel sheet in a zinc based hot bath to obtain a steel sheet hot-dip plated with zinc based layer; and

finally cooling the steel sheet hot-dip plated with zinc based layer to a temperature in a range of (Ms-100)°C or lower at an average cooling rate of 3°C/sec or higher.
The method according to claim 7, wherein the reheating temperature is within a range of 1100 to 1300°C at the time of reheating the slab.
The method according to claim 7, wherein a finish rolling temperature at the time of the hot-rolling is within a range of (Ar3 + 50) to 950°C.
The method according to claim 7, wherein the coiling temperature at the time of the coiling is within a range of 450 to 700°C.
The method according to claim 7, wherein a cold-reduction ratio at the time of the cold-rolling is 40 to 80%.
The method according to claim 7, wherein the annealing temperature at the time of the continuously annealing is within a range of 770 to 810°C.
The method according to claim 7, wherein a temperature of the zinc based hot bath is within a range of 440 to 480°C.
The method according to claim 7, further comprising subjecting the steel sheet hot-dip plated with zinc based layer to an alloying heat treatment at a temperature in a range of 480 to 600°C, before the final cooling.
The method according to claim 7, further comprising temper rolling at a reduction ratio of 0.3 to 1.6%, after the final cooling.