KR101630991B1 - Steel for warm press forming with excellent formability and weldability, forming part, and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a coated steel plate for warm press forming, a forming member for warm press forming, and a manufacturing method of same. More specifically, the coated steel plate comprises: a base plate; and a complex coating layer formed at least on one side of the base plate. The complex coating layer consists of alternating layers of a manganese (Mn) coated layer, and an aluminum (Al) coated layer wherein an uppermost layer of the complex coating layer has an aluminum (Al) coated layer. The entire width of the complex coating layer is 5-30 μm, wherein the width of the manganese (Mn) coated layers constitute 50-60% of the entire width of the complex coating layer. The coated steel plate has excellent spot weldability, and is able to secure excellent corrosion resistance by comprising an alloy layer as a sacrificial protection layer.

Description

TECHNICAL FIELD [0001] The present invention relates to a coated steel sheet for warm press forming having excellent weldability and corrosion resistance, a molded member, and a method of manufacturing the same. [0002]

More particularly, the present invention relates to a coated steel sheet for hot press forming which is excellent in weldability and corrosion resistance applicable to automotive parts, etc., a molded member using the same, and a method of manufacturing the same will be.

In recent years, the use of high-strength steels has been increasing to lighten the weight of automobiles. However, such high-strength steels are easily broken at room temperature, and springback phenomenon occurs during machining, there is a problem.

In order to solve such a problem, a hot molding method has been proposed as a method having high strength and securing excellent moldability and excellent shape control ability.

In this way, Korean Unexamined Patent Application Publication No. 2007-0057689 and U.S. Patent No. 6,296,805 disclose a method in which heat treatment and press molding are performed in a single phase of austenite using low strength and high processability before heat treatment, There has been proposed a production method of obtaining an ultrahigh strength steel sheet having martensite as the main phase in the final product.

However, in the case of the non-plated material due to the high heating temperature of the austenite single phase, the above-mentioned technique has a problem that a surface oxidation scale is required to be removed after manufacture and a large cost is required to secure a high temperature.

In order to prevent such a problem, that is, to prevent the oxide scale from being formed on the surface of the steel sheet by high temperature heating and to secure the corrosion resistance of the molded member, there is a method of using a plated steel sheet having a Zn or Al plating layer as a material, 2006-022395 discloses a steel sheet excellent in corrosion resistance because an alloy phase having zinc of 70% or more is present on the surface of the steel sheet when the steel sheet is hot-rolled after hot-dip galvanizing. However, when a galvanized steel sheet such as a hot-dip galvanized steel sheet is heated in air at a high temperature, zinc oxide is formed on the surface of the galvanized steel sheet, and this zinc oxide acts as a resistance which interrupts current conduction in a welding process such as spot welding after molding Therefore, there is a problem that the weldability is heated. In order to improve the spot weldability, there is a method of additionally performing a step of removing zinc oxide present on the surface of the steel sheet after molding, but this poses a problem that the manufacturing cost is increased.

Further, when a plated steel sheet having a Zn or Al plating layer is used as a material, since the melting point of Zn is not more than 500 DEG C and the melting point of Al is not more than 700 DEG C, if the heat treatment is performed at a high temperature, There is a problem that the resin is adhered to the molding die at the time of molding and adversely affects the molding.

In order to solve this problem, Warm Press Forming (WPF) has recently been applied as a preferable method for processing a high strength steel. Warm press forming is a method of forming by heating only austenite and ferrite anisotropy so as to alleviate the problem of melting and volatilization of the plating layer by heating at high temperature beyond the austenite single phase.

Such a steel sheet for warm press forming and a molding method are disclosed in Korean Patent Laid-Open Publication No. 2014-0019299. The steel sheet is subjected to Zn-Ni plating and heated to 800 ° C or lower, which is an austenite and ferrite anomaly temperature, and then molded to produce a hot pressed member.

Since the Zn-Ni plated steel sheet is used at a relatively low temperature, it is advantageous in that the melting of the plated layer is suppressed as compared with the hot press forming method. However, since the Zn-Ni plated steel sheet has no sacrificial corrosion resistance to iron, There is a downside to this.

Therefore, there is a demand for development of a plated steel sheet for warm press forming which is economical, excellent in moldability, excellent in weldability and corrosion resistance, a molded member using the same, and a manufacturing method thereof.

Korean Patent Publication No. 2007-0057689 US registered patent US6296805 Korean Patent Publication No. 2014-0019299

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide an alloy layer that is superior in spot weldability due to thin oxide on the surface thereof, The present invention has been made to provide a hot press forming steel plate for hot press forming which has excellent ability to secure corrosion resistance and has a sacrificial capacity for preventing corrosion of steel sheet.

In one aspect, the present invention provides a method of manufacturing a steel sheet, And a composite plating layer formed on at least one side of the base steel sheet, wherein the composite plating layer is formed by alternately forming a manganese (Mn) plating layer and an aluminum (Al) plating layer, and an aluminum (Al) The total thickness of the composite plating layer is 5 to 30 μm and the thickness of the manganese (Mn) plating layer is 5 to 60% of the total thickness of the composite plating layer.

The manganese (Mn) plating layer may contain at least one selected from the group consisting of chromium (Cr), zinc (Zn), beryllium (Be), magnesium (Mg), calcium (Ca), and titanium % (Excluding 0% by weight).

The aluminum (Al) plating layer contains 20 wt% (excluding 0 wt%) of at least one selected from the group consisting of zinc (Zn), silicon (Si), magnesium (Mg), and manganese can do.

On the other hand, the base steel sheet contains 0.1 to 0.4% by weight of carbon (C), 0.05 to 3% by weight of silicon (Si), 3 to 15% by weight of manganese (Mn), and the balance of Fe and other unavoidable impurities .

The base steel sheet may further contain 0.001 to 0.02 wt% of nitrogen (N), 0.0001 to 0.01 wt% of boron (B), 0.001 to 0.1 wt% of titanium (Ti), 0.001 to 0.1 wt% of niobium (Nb) 0.001 to 0.01 wt% of vanadium (V), 0.001 to 1.0 wt% of chromium (Cr), 0.001 to 1.0 wt% of molybdenum (Mo), 0.001 to 0.1 wt% of antimony (Sb) To 0.3% by weight, based on the total weight of the composition.

Meanwhile, the present invention also provides a warm press-formed member which is manufactured by warm press-forming the plated steel sheet, and includes a base steel sheet and an alloy layer formed on at least one side of the base steel sheet.

The alloy layer is formed of a plurality of alloy phases. More specifically, the alloy layer is composed of an iron (Fe) -manganese (Mn) alloy phase, an iron (Fe) -aluminum An alloy phase of iron (Fe) -aluminum (Al), and a phase of aluminum (Al) -manganese (Mn) alloy.

Alternatively, the alloy layer may be formed of an iron (Fe) -manganese (Mn) alloy phase, an iron (Fe) -Aluminum (Al) Aluminum (Al) -manganese (Mn) alloy phase, zinc (Zn) -manganese (Mn) alloy phase, zinc (Zn) (Al) -magnesium (Mg) -based alloy phase, and an iron (Fe) -manganese (Mn) -Aluminum (Al) -silicon have.

The uppermost layer of the alloy layer is preferably an alloy phase containing aluminum (Al).

On the other hand, the thickness of the surface oxide formed on the outermost surface of the alloy layer is preferably less than 1 mu m.

In another aspect, the present invention provides a method of manufacturing a steel sheet, comprising: preparing a base steel sheet; And forming a composite plating layer on at least one side of the prepared base steel sheet, wherein the composite plating layer is formed by alternately forming a manganese (Mn) plating layer and an aluminum (Al) plating layer so that an aluminum (Al) Wherein the total thickness of the composite plated layer is 5 to 30 μm and the thickness of the manganese (Mn) plated layer is 5 to 60% of the total thickness of the composite plated layer. to provide.

On the other hand, the composite plating layer is preferably formed by a dry plating method.

Meanwhile, the present invention provides a method for manufacturing a coated steel sheet, comprising the steps of: heating a coated steel sheet produced by the above manufacturing method to a temperature range of Ac ₁ to Ac ₃ at an average heating rate of 3 to 200 ° C / s; Warm-forming the heated plated steel sheet at the temperature; And a step of cooling the warm-formed coated steel sheet.

At this time, the manufacturing method of the warm pressed member of the present invention may further include the step of maintaining the heated coated steel sheet at the temperature for not more than 240 seconds after the heating step.

In the coated steel sheet of the present invention, an aluminum (Al) plating layer is disposed on the uppermost layer of the composite plating layer. The molded member manufactured by warm press molding has an advantage of being excellent in spot weldability because the oxide is thin on the surface.

Further, in the molding member manufactured by hot pressing the coated steel sheet of the present invention, the composite coating layer and the base steel are alloyed to form iron (Fe) -manganese (Mn), iron (Fe) (Mn), iron (Fe) -aluminum (Al) and aluminum (Al) -manganese (Mn) are formed on the surface of the substrate. The ability to sacrifice to prevent it has the advantage of excellent corrosion resistance.

1 (a), 1 (b) and 1 (c) are cross-sectional schematic views of a hot-press plated steel sheet having a composite plated layer according to an embodiment of the present invention.
2 is a schematic cross-sectional view of a warm pressed member having an oxide layer according to an embodiment of the present invention.
Fig. 3 is a graph showing the thermal history of the conventional hot press forming
Fig. 4 is a graph showing the heat history related to the warm press forming of the present invention

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art. The shape and size of elements in the drawings may be exaggerated for clarity.

Aluminum-plated steel sheet has excellent heat-resistant properties, and the oxide is very thin on the surface after heating and molding, so that the spot weldability of molded parts is excellent. On the other hand, the electrochemical potential of the steel- It can not base enough to protect it, and it can not do the sacrifice of iron. Therefore, the characteristics of the plating layer for use in the hot press forming steel sheet are heat-resistant, and it is necessary that an alloy phase that is sufficiently strong enough to protect the base iron after heating and molding is present in the alloy layer.

The inventors of the present invention have conducted extensive research to achieve the above object and have found that a manganese (Mn) plating layer and an aluminum (Al) plating layer are alternately formed on a base steel sheet, and an aluminum (Mn) plating layer accounts for 60% or less of the total thickness of the composite plating layer, the composite coating layer and the composite coating layer are formed in the process of heating the coated steel sheet, (Fe) -manganese (Mn), iron (Fe) -aluminum (Al) -manganese (Mn), iron (Fe) -Aluminum (Al) ) System. As a result, it has been found that not only excellent spot weldability but also excellent corrosion resistance is obtained because the surface of the alloy is very small on the surface after press forming, thereby inventing the present invention.

More specifically, the coated steel sheet for warm press forming of the present invention comprises a base steel sheet; And a composite plating layer formed on at least one side of the base steel sheet, wherein the composite plating layer is formed by alternately forming a manganese (Mn) plating layer and an aluminum (Al) plating layer, and an aluminum (Al) The total thickness of the composite plating layer is 5 to 30 占 퐉, and the thickness of the manganese (Mn) plating layer is 5 to 60% of the total thickness of the composite plating layer.

The base steel sheet of the present invention may be made of ordinary carbon steel, and preferably 0.1 to 0.4 wt% of carbon (C), 0.05 to 3 wt% of silicon (Si), 3 to 15 wt% of manganese (Mn) Weight percent, balance iron (Fe), and other inevitable impurities, but are not limited to these components.

Carbon (C): 0.1 to 0.4 wt%

The above C is not only an essential element for increasing the strength of the steel sheet but also it is necessary to appropriately add C in order to secure the retained austenite to be implemented in the present invention. When the C content is less than 0.01%, sufficient strength can not be obtained. In addition, it is difficult to secure a residual austenite of 3 vol% or more in the warm press forming of the member. Therefore, in order to exhibit the above characteristics, , More preferably 0.05% or more.

If it is contained in an amount exceeding 0.5%, not only the cold rolling property of the hot-rolled steel sheet is deteriorated, but too high strength is obtained and it is difficult to secure a desired elongation and the weldability tends to be lowered. , More preferably not more than 0.4%, still more preferably not more than 0.3%.

Silicon (Si): 0.05 to 3 wt%

Silicon is used as a deoxidizing agent and is an element added for strength enhancement by solid solution strengthening. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.05 wt% or more. However, if the content of silicon exceeds 3%, it is difficult to pick up the hot-rolled sheet, which may cause a scaly surface defect due to the hot-rolled steel sheet and the precipitated oxide.

Manganese (Mn): 3 to 15 wt%

The Mn plays a very important role in the present invention. Mn not only enhances the solid solution strengthening element but also lowers the Ms temperature (martensitic transformation start temperature), thereby enhancing the stability of the austenite at room temperature. Mn is an important element for the warm press molding pursued by the present invention because it lowers the Ac1 and Ac3 temperatures. In addition, Mn is diffused into the austenite produced during the warm press forming heat treatment at the temperatures Ac1 to Ac3, and the stability of the austenite at room temperature can be further enhanced. When the Mn content is less than 3%, it is difficult to secure sufficient strength and it is insufficient for enhancing the stability at room temperature of the austenite. Therefore, the Mn content is preferably 3% or more, more preferably 4% %.

On the other hand, when the content exceeds 15%, not only the production cost is increased but also excessive retained austenite is generated and the elongation can be sufficiently increased. However, since it is difficult to ensure sufficient strength, Is not more than 13%, more preferably not more than 11%.

The remaining component of the inventive ground steel sheet is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

In order to further improve the properties such as strength, toughness and weldability, it is preferable that the base steel sheet of the present invention has a nitrogen (N) content in the range of 0.001 - 0.001 to 0.1 wt.% Of niobium (Nb), 0.001 to 0.01 wt.% Of vanadium (V), 0.001 to 0.1 wt.% Of boron (B) 0.001 to 1.0% by weight of molybdenum (Mo), 0.001 to 0.1% by weight of antimony (Sb), and 0.001 to 0.3% by weight of tungsten (W) .

Nitrogen (N): 0.001 to 0.02 wt%

Nitrogen acts in the austenite grains during the coagulation process to precipitate fine nitrides to precipitate twinning, thereby improving the strength and ductility of the steel sheet. However, as the nitrogen content increases, the nitride is excessively precipitated, It is preferable to control the nitrogen content because it lowers the elongation. When the content of nitrogen is less than 0.001% by weight, there is a problem that the production cost for controlling nitrogen in the steelmaking process increases significantly. When the content of nitrogen is more than 0.02% by weight, excessive nitrides are precipitated and hot workability, elongation and cracks do.

Boron (B): 0.0001 to 0.01 wt%

Boron is an effective element to retard ferrite transformation in austenite. In order to exhibit the above-mentioned effects, the present invention preferably contains 0.0001 wt% or more. When the content of boron exceeds 0.01% by weight, there is a problem that the warm processability is lowered.

0.001 to 0.1 wt% of titanium (Ti), niobium (Nb), and vanadium (V)

Titanium, niobium, and vanadium are effective elements for increasing the strength of the steel sheet, making the grain size finer, and improving the heat treatability. In order to exhibit the above-mentioned effects, it is preferable that each of them contains 0.001 wt% or more. However, when the content of each of the above elements exceeds 0.1 wt%, it is difficult to secure strength and yield strength to be secured due to an increase in manufacturing cost and generation of excess carbon and nitride.

Chromium (Cr) and molybdenum (Mo): 0.001 to 1.0 wt%

Chromium and molybdenum are effective elements for increasing the hardenability as well as increasing the toughness of the steel sheet. Therefore, the effect is more remarkable when it is added to a steel sheet requiring a high impact energy characteristic. However, if the above content is less than 0.001%, the above effect can not be sufficiently obtained, and if it exceeds 1.0%, the effect is saturated and the production cost is increased.

Antimony (Sb): 0.001 to 0.1 wt%

Antimony is an element that plays a role in suppressing the selective oxidation of the grain boundary during hot rolling, thereby making the scale uniform and improving the pickling resistance of the hot rolling material. In order to exhibit the above-described effects, it is preferable that the content of the present invention is 0.001% by weight or more. If the content of antimony is more than 0.1% by weight, the effect is not only saturated but also the production cost is increased and brittleness is caused in warm working.

Tungsten (W): 0.001-0.3 wt%

Tungsten is an element that improves the heat-curing ability of a steel sheet, and tungsten-containing precipitates are elements that act to secure strength. In order to exhibit the above-described effects, it is preferable that the content of the present invention is 0.001% by weight or more. When the content of tungsten exceeds 0.3 wt%, the effect is saturated and the production cost increases.

Molybdenum (Mo): 0.01 to 1.0 wt%

The addition of a small amount of Mo can greatly improve the hardenability. Therefore, it is necessary to add Mo at a content of 0.01% or more, but if the Mo content exceeds 1.0%, the hardness of the weld portion is excessively increased, Therefore, it is preferable to add 1.0% or less.

Nickel (Ni): 0.01 to 2.0 wt%

Since Ni is an element capable of simultaneously improving the strength and toughness of the base material, it is necessary to add at least 0.01% by weight in order to sufficiently obtain the effect. However, since Ni is an expensive element, addition of an amount exceeding 2.0% And the weldability is lowered, so that the addition amount is preferably 0.01 to 2.0%.

Next, in the composite plating layer of the present invention, a manganese (Mn) plating layer and an aluminum (Al) plating layer are alternately formed, and an aluminum (Al) plating layer is formed on the uppermost layer. The total thickness of the composite plating layer is 5 to 30 탆 , And the thickness of the manganese (Mn) plating layer is preferably 5 to 60% of the total thickness of the composite plating layer.

Manganese (Mn) is an element electrochemically lower than iron and alloyed with aluminum (Al) or iron (F) to form iron (Fe) -manganese (Mn) (Al) or an aluminum (Al) -iron (Fe) alloy phase when an alloy phase such as manganese (Mn) or aluminum (Al) . Therefore, as the manganese (Mn) ratio in the plated layer increases, the potential of the alloy layer after heating and molding becomes low, thereby increasing the ability to sacrifice iron. However, when manganese (Mn) exceeds 60% of the total thickness of the plating layer, manganese (Mn) diffuses to the surface layer to form manganese (Mn) oxide even if the ability to sacrifice iron is excellent, The manganese (Mn) oxide has a problem of deteriorating spot weldability because it has a large electrical resistance to metal. Therefore, in the present invention, it is preferable that the manganese (Mn) plating layer is formed to be 60% or less of the total thickness of the composite plating layer.

When the manganese (Mn) plating layer is less than 5% of the total thickness, iron (Fe) -manganese (Mn), iron (Fe) -aluminum produced by alloying with aluminum (Al) -manganese (Mn) alloy and aluminum (Al) -manganese (Mn) alloy phase are not produced or are so small that the corrosion resistance of the alloy layer is almost insufficient and the corrosion resistance is insufficient.

If the thickness of the composite plating layer is less than 5 mu m, the corrosion resistance can not be sufficiently secured. On the other hand, when the thickness exceeds 30 mu m, corrosion resistance can be sufficiently secured. At this time, the thickness of the plating layer is based on one side.

In addition, the composite plating layer may include a total of two layers including a manganese (Mn) plating layer and an aluminum (Al) plating layer in total, or alternatively, three or more layers in total. In the uppermost layer of the composite plating layer, aluminum Al) plating layer.

For example, as shown in FIG. 1, when the composite plating layer of the present invention is composed of (a) two layers, a manganese (Mn) plating layer / aluminum (Al) (Al) plating layer / manganese (Mn) plating layer / aluminum (Al) plating layer in the order of the aluminum (Al) (Mn) plating layer / aluminum (Al) plating layer.

If the manganese (Mn) plating layer is included in the uppermost layer of the composite plating layer, there is a problem that the spot weldability is poor since an oxide containing manganese (Mn) as a main component is formed in the surface layer portion of the steel sheet after warm press forming.

On the other hand, the manganese (Mn) plating layer is composed of at least one of chromium (Cr), zinc (Zn), beryllium (Be), magnesium (Mg), calcium (Ca) Ti) in an amount of 20 wt% or less (excluding 0 wt%). Also in this case, it is possible to obtain excellent weldability and corrosion resistance after warm press forming to be obtained in the present invention.

For the purpose of controlling the hardness of the aluminum (Al) plating layer and the control of the flowability of the plating bath, the aluminum (Al) plating layer may be formed of a material selected from the group consisting of zinc (Zn), silicon (Si), magnesium (Mg), and manganese And at least 20% by weight (excluding 0% by weight) of at least one selected. Also in this case, it is possible to obtain excellent weldability and corrosion resistance after warm press forming to be obtained in the present invention.

Meanwhile, in the case of heating the steel sheet at a temperature raising rate of 3 to 200 ° C / s to a temperature range of Ac ₁ to Ac ₃ in order to warm press the steel sheet during warm press forming, the composite coating layer and the iron- (Fe) -manganese (Mn) system, iron (Fe) -aluminum (Al) -manganese (Mn) system, iron (Fe) -aluminum It is preferable to form at least two selected alloy phases. In this case, since the oxide is very little on the surface after hot press forming, it has excellent spot weldability as well as excellent corrosion resistance.

Alternatively, the hot-dip galvanized steel sheet may be formed by hot pressing the Ac ₁ When heated to a temperature of ~ Ac _3, iron (Fe) - manganese (Mn) based, iron (Fe) - aluminum (Al) - manganese (Mn) based, iron (Fe) - aluminum (Al) based, aluminum (Al) -manganese (Mn) system, zinc (Zn) -manganese (Mn) system, zinc (Zn), iron (Fe) system, iron (Fe) ) System, and iron (Fe) -manganese (Mn) -aluminum (Al) -silicon (Si) system. In this case as well, since the oxide is very little on the surface after warm press forming, it has excellent spot weldability as well as excellent corrosion resistance.

Hereinafter, a method of manufacturing the coated steel sheet of the present invention will be described.

A method for manufacturing a coated steel sheet for warm press forming according to the present invention comprises the steps of: preparing a base steel sheet; And forming a composite plating layer on at least one side of the prepared base steel sheet, wherein the composite plating layer is formed by alternately forming a manganese (Mn) plating layer and an aluminum (Al) plating layer so that an aluminum (Al) The total thickness of the composite plating layer is 5 to 30 μm and the thickness of the manganese (Mn) plating layer is 5 to 60% of the total thickness of the composite plating layer.

First, if the base steel sheet is a base steel sheet containing the above-described components, the production method thereof and the like are not particularly limited and can be prepared and manufactured by a method known in the art. Alternatively, commercially available conventional steel sheets containing the above-described components may be used.

Alternatively, a manganese (Mn) plated layer and an aluminum (Al) plated layer are alternately formed on at least one side of the prepared ground steel sheet to form an aluminum (Al) plated layer on the uppermost layer to form a composite plated layer. At this time, it is preferable that the total thickness of the composite plating layer is 5 to 30 μm and the thickness ratio of the manganese (Mn) plating layer is 5 to 60%.

Meanwhile, the composite plating layer can be formed by a known plating method. However, in the case of the present invention, it is preferable to use a dry plating method in consideration of easiness of plating operation, ease of plating efficiency, and ease of deposition amount. More specifically, the manganese (Mn) plating method is more reasonable than the deposition plating method, and the aluminum (Al) plating method is more reasonable than the hot-dip plating method or the evaporation plating method.

Hereinafter, a warm press molded article manufactured using the coated steel sheet of the present invention and a method for producing the same will be described.

First, the warm press formed member of the present invention can be obtained by hot press forming the plated steel sheet, and includes a base steel sheet and an alloy layer formed on at least one surface of the base steel sheet. At this time, as the base steel sheet, a conventional base steel sheet having the above-mentioned composition can be used.

On the other hand, the alloy layer is formed by alloying the composite plating layer and the base iron of the coated steel sheet as described above, and as a result, the alloy layer is formed of a plurality of alloy phases. More specifically, the alloy layer is formed of an alloy of iron (Fe) and manganese (Mn), an alloy of iron (Al) Phase, and an aluminum (Al) -manganese (Mn) alloy phase.

For example, the alloy layer of the present invention comprises (a) an alloy phase of iron (Fe) -aluminum (Al) and an alloy of iron (Al) (Al) -manganese (Mn) alloy phase and an iron (Fe) -Aluminum (Al) -manganese (Mn) alloy phase, (Al) -magnesium alloy phase, iron (Fe) -aluminum (Al) -manganese alloy phase, iron (Fe) (Mn) based alloy phase. However, this is only a schematic example for explaining the alloy layer of the present invention, and the alloy layer of the present invention is not limited to the above-mentioned combination. Also, the order of the layers including the alloy phase is not limited thereto.

On the other hand, as shown in FIG. 2, the molded member of the present invention may have a surface oxide formed on the outermost surface of the alloy layer. At this time, the thickness of the surface oxide formed on the outermost surface of the alloy layer is preferably less than 1 mu m. This is because the oxide on the surface is thin and the spot weldability is excellent.

In addition, the uppermost layer of the alloy layer preferably includes an alloy phase containing aluminum (Al). In this case, such a thin oxide may be formed.

Next, the method for manufacturing a warm pressed member of the present invention comprises the steps of: heating the coated steel sheet to a temperature range of Ac ₁ to Ac ₃ at an average heating rate of 3 to 200 ° C / s; Warm-forming the heated plated steel sheet at the temperature; And cooling the hot-formed plated steel sheet.

For example, heating with an electric furnace or a gas furnace, flame heating, energization heating, high frequency heating, induction heating, or the like can be used as the heating step, But is not limited thereto.

At this time, it is preferable that the heating temperature is a temperature range of Ac ₁ to Ac ₃ . In the process of heating the coated steel sheet according to the present invention, the base steel and the plating layer are alloyed to form iron (Fe) -manganese (Mn), iron (Fe) -aluminum (Fe) -aluminum (Al) system, and aluminum (Al) -manganese (Mn) system.

In the present invention, the heating temperature is limited to a temperature range of Ac ₁ to Ac ₃ , and heating only to austenite and ferritic anomalies does not cause a problem that the plating layer is melted and volatilized.

It is preferable that the heating step has an average heating rate of 3 to 200 DEG C / s. If the temperature is less than 3 占 폚 per second, it takes a long time to reach the target temperature, so that the oxide of the surface layer can be formed to be 1 占 퐉 or more by long heating. Further, the diffusion of the ferrous iron into the plating layer is increased, and as a result, the iron content is increased in the alloy layer, which is disadvantageous to the corrosion resistance and the productivity may be lowered.

In the meantime, according to the present invention, the heated plated steel sheet may be maintained at the temperature for a predetermined time for securing the target material, if necessary, after the heating step. At this time, if the holding time exceeds 240 seconds, the time for forming manganese (Mn) or pearlite in the lower layer diffuses to the surface layer and the oxide is formed, and the oxide thickness is increased. As a result, the spot weldability is lowered . Therefore, in order to obtain a target oxide thickness of less than 1 mu m in the present invention, the holding time is preferably 240 seconds or less.

Meanwhile, the forming step may be a method commonly used in the related art. For example, the steel sheet may be warm-formed to a desired shape by using a press while maintaining the heating temperature. However, But is not limited thereto.

On the other hand, it is preferable that the cooling step is performed at a cooling rate of at least 10 ° C / s to 100 ° C. When the cooling rate is lower than 10 ° C / s, there is a problem that the percentage of transformation of austenite into ferrite increases and the strength after cooling decreases.

Hereinafter, the present invention will be described more specifically by way of examples.

( Example )

First, base steel sheets A and B which are 1.5 mm in thickness and cold-rolled steel sheets for ordinary warm press forming were prepared. The base steel sheet A had 0.22 wt% of C, 0.24 wt% of Si, 5.56 wt% of Mn, 0.012 wt% of N, 0.0028 wt% of B, 0.01 wt% of Cr, 0.03 wt% of Ti, And other unavoidable impurities, and the base steel sheet B contains 0.22 wt% of C, 0.24 wt% of Si, 5.56 wt% of Mn, the balance Fe and other unavoidable impurities.

Next, Mn was deposited on the backing sheet A or B to have a plating thickness as shown in Table 1 below, and a part of the sheets was alloyed with Zn and Mg. Subsequently, Al was vapor-deposited on the substrate, and a part of the substrate was alloyed with Si and Mg. For comparison, a coated steel sheet plated with Al, Mn or Zn on the base steel sheet A or B was also prepared. Table 1 shows the total thickness of the plated layer obtained by dissolving the plated layers of each of the prepared plated steel sheets and analyzing the amount of deposited plated layers.

division Plated layer
Configuration The first plating layer The second plated layer Plated layer
Overall Thickness
(One side, 탆) Plated layer
Thickness ratio
(%) Possession
Steel plate Plating species Plating Thickness
(One side, 탆) Plating species Plating Thickness
(One side, 탆) Inventive Steel 1 A Second floor Mn 100% 1.5 Al 100% 5.1 6.6 22.7 Invention river 2 A Second floor Mn 100% 3.3 Al 100% 8.9 12.2 27 Invention steel 3 A Second floor Mn 100% 6.4 Al 100% 13.9 20.3 31.5 Inventive Steel 4 A Second floor Mn 100% 6.2 Al 100% 15.1 21.3 29.1 Invention steel 5 B Second floor Mn 100% 6.8 Al 100% 8.1 14.9 45.6 Invention steel 6 A Second floor Mn 100% 9.5 Al 100% 10.1 19.6 48.5 Invention steel 7 B Second floor Mn 95%
Zn 5% 2.2 Al 100% 6.9 9.1 24.2 Inventive Steel 8 B Second floor Mn 81%
Zn 19% 4.8 Al 100% 12.4 17.2 27.9 Invention river 9 A Second floor Mn 100% 5.1 Al 92%, Si 8% 12.3 17.4 29.3 Invented Steel 10 A Second floor Mn 100% 5.2 Al 88%, Si 8%, Mg 4% 12.5 17.7 29.4 Comparative River 1 A fault Al 100% 15 - - 15 100 Comparative River 2 A fault Mn 100% 13.8 - - 13.8 100 Comparative Steel 3 A fault Zn 99.4%
Al 0.6% 9.5 - - 9.5 100 Comparative Steel 4 A Second floor Mn 100% 13.6 Al 100% 1.4 15 90.7 Comparative Steel 5 B Second floor Al 100% 13.5 Mn 100% 2.4 15.9 84.9 Comparative Steel 6 B Second floor Mn 100% 1.2 Al 100% 10.1 11.3 10.6 Comparative Steel 7 B Second floor Mn 100% 5.2 Al 100% 7.1 12.3 42.3 Comparative Steel 8 A Second floor Mn 100% 0.6 Al 100% 15.5 16.1 3.73

(In Table 1, the plating layer thickness ratio is calculated as {(first plating layer thickness / total plating layer thickness) x 100}.

Next, each of the coated steel sheets was subjected to hot press forming under the conditions shown in Table 2 below to produce a molded article, and the properties of the produced molded article were evaluated and shown in Table 2 below. For the corrosion resistance evaluation, Salt Spray Test (SST) was performed for 1200 hours, and then the maximum corrosion depth of iron oxide was measured.

division Heating condition Cooling
speed
(° C / sec) Alloy composition
(Excluding oxide) Surface oxide thickness
(탆) Corrosion resistance, (SST, corrosion depth, mm) Heating temperature
(° C) Average heating rate
(° C / sec) Retention time
(sec) Inventory 1 750 10 150 30 Fe-Al, Fe-Mn-Al 0.1 0.16 Inventory 2 733 6 100 30 Fe-Al, Al-Mn, Fe-Mn-Al 0.17 0.11 Inventory 3 780 120 150 25 Fe-Al, Fe-Mn, Al-Mn, Fe-Mn-Al 0.19 ≪ 0.1 Honorable 4 730 30 30 60 Fe-Al, Fe-Mn, Al-Mn, Fe-Mn-Al 0.12 ≪ 0.1 Inventory 5 800 10 200 50 Fe-Al, Fe-Mn, Al-Mn, Fe-Mn-Al 0.33 ≪ 0.1 Inventory 6 750 10 100 30 Fe-Al, Fe-Mn, Al-Mn, Fe-Mn-Al 0.17 ≪ 0.1 Honorable 7 710 10 150 30 Fe-Al, Al-Mn, Fe-Mn-Al, Fe-Mn- 0.28 0.11 Honors 8 730 10 150 20 Fe-Al, Al-Mn, Fe-Mn-Al, Zn-Mn 0.15 ≪ 0.1 Proposition 9 730 10 150 20 Fe-Al, Al-Mn, Fe-Mn-Al, Fe-Mn- 0.08 ≪ 0.1 Inventory 10 720 10 150 20 Fe-Al, Al-Mn, Fe-Mn-Al, Fe-Mn- 0.43 ≪ 0.1 Comparative Example 1 730 10 150 30 Fe-Al 0.08 0.58 Comparative Example 2 750 10 150 30 Fe-Mn > 2 0.21 Comparative Example 3 730 10 150 30 Fe-Zn > 2 0.31 Comparative Example 4 750 10 150 30 Fe-Mn. Al-Mn > 2 0.20 Comparative Example 5 750 10 150 30 Fe-Al, Al-Mn, Fe-Mn-Al > 2 0.43 Comparative Example 6 750 One 200 30 Fe-Al, Fe-Mn, Al-Mn, Fe-Mn-Al 1.05 0.24 Comparative Example 7 800 6 400 30 Fe-Al, Fe-Mn, Al-Mn, Fe-Mn-Al 1.32 ≪ 0.1 Comparative Example 8 730 6 150 30 Fe-Al, Fe-Mn-Al 0.19 0.49

As shown in Tables 1 and 2, the warm press-formed members of Inventive Examples 1 to 6 were made of Fe-Mn, Fe-Al-Mn, Fe-Al, And the uppermost layer thereof contains an Al-based alloy phase, and the surface oxide is formed to have a thickness of less than 1 占 퐉. After the salt spray test, the substrate iron corrosion depth is less than 0.16 mm .

The warm press-formed members of the present invention examples 7 and 8 are made of Fe-Mn, Fe-Al-Mn, Fe-Al, Al-Mn, Fe-Mn- -Mn-based alloy phase is formed on the uppermost layer, and the Al-based alloy phase is included in the uppermost layer. The thickness of the surface oxide is less than 1 占 퐉, and after the salt spray test, Corrosion depth was 0.11mm or less and corrosion resistance was excellent.

The warm press-formed members of the present invention examples 9 and 10 may have a structure in which at least two of the alloy phases of Fe-Al, Al-Mn, Fe-Al-Mn, Fe-Mn-Al- Alloy layer is formed on the uppermost layer and an Al-based alloy phase is contained in the uppermost layer, the surface oxide is formed to have a thickness of less than 1 占 퐉 and the depth of the iron oxide after the salt spray test is less than 0.1 mm Respectively.

On the other hand, in the case of Comparative Example 1, a single layer of Al was plated to a thickness of 15 mu m. In the manufactured warm press-formed member, a Fe-Al alloy phase having heat resistance was formed on the refractory iron, . However, since the Al plating layer does not have the ability to sacrifice the iron oxide, the corrosion resistance is poor due to the depth of iron oxide corrosion of 0.58 mm after the salt spray test.

Further, in the case of Comparative Example 2, Mn was coated to a thickness of 13.8 占 퐉. In the manufactured hot press formed member, an Fe-Mn alloy phase was formed on the upper surface of the base iron. An oxide was formed. However, since the Fe-Mn based alloy phase has a sacrificial corrosion resistance against the ground iron, the corrosion resistance is relatively good as the corrosion depth of the iron oxide after the salt spray test is 0.21 mm.

In the case of Comparative Example 3, in the case of a hot-dip galvanized steel sheet, zinc was oxidized at a high temperature to make a thick oxide exceeding 2 μm on the surface, and the deep iron corrosion depth was relatively deep as 0.31 mm.

In the case of Comparative Example 4, the lower layer was subjected to Mn plating and the upper layer was subjected to Al plating according to the present invention, but when the ratio of the total thickness of the lower layer Mn plating layer in the entire plating layer exceeded 60% , Mn of the lower layer diffused into some surface layer in the heat treatment process to form oxides, so that the surface oxide thickness exceeded 1 탆 defined in the present invention. However, since the alloy layer has the ability to sacrifice iron content, the depth of iron oxide was 0.2 mm, which is relatively good.

In the case of Comparative Example 5, the case where the lower layer was plated with Al and the upper layer was subjected to Mn plating, and since the upper layer of Mn formed oxides on the surface in the heat treatment process, the surface oxide thickness was limited to 1 탆 And the Mn content in the upper layer is mostly oxidized and the Mn content in the alloy layer is low. Therefore, the corrosion resistance of the iron oxide is lowered and the corrosion resistance of the iron oxide corrosion depth is 0.43mm.

In the case of Comparative Examples 6 and 7, Mn was plated so that the total thickness of Mn plating was 60% or less in total plating thickness, and Al was plated thereon. The total plating thickness was 5 to 30 탆, However, since the average temperature raising rate and the holding time are out of the range defined in the present invention, the oxide of the surface layer exceeds 1 탆 defined in the present invention due to long heating. However, the alloy layer contains at least two alloy phases of Fe-Mn, Fe-Al-Mn, Fe-Al, and Al-Mn alloy phases in the present invention, Therefore, the corrosion depth of the substrate iron was 0.24 mm or less.

In the case of Comparative Example 8, the lower layer was subjected to Mn plating by the present invention,

Al plating. However, when the ratio of the Mn plating thickness of the lower layer among the entire plating layers is less than 5% defined in the present invention, since the Mn plating thickness is small, most of the Fe-Al phase and the thin Fe- -Al alloy phase is formed, the corrosion resistance is poor and the ability of sacrificial method is insufficient in the corrosion test.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, It will be obvious to those of ordinary skill in the art.

Claims (13)

Base steel sheet; And
And a composite plating layer formed on at least one surface of the backing steel sheet,
The composite plating layer is formed by alternately forming a manganese (Mn) plating layer and an aluminum (Al) plating layer, and an aluminum (Al) plating layer is formed on the uppermost layer,
Wherein the total thickness of the composite plated layer is 5 to 30 占 퐉 and the thickness of the manganese (Mn) plated layer is 5 to 60% of the total thickness of the composite plated layer.
The method according to claim 1,
The manganese (Mn) plating layer contains at least 20% by weight of at least one selected from the group consisting of Cr, Zn, Ber, Mg, Ca and Ti. 0% by weight or less).
The method according to claim 1,
The aluminum (Al) plating layer is formed of a mixture containing at least 20 wt% (excluding 0 wt%) selected from the group consisting of zinc (Zn), silicon (Si), magnesium (Mg), and manganese Plated steel plates for press forming.
The method according to claim 1,
The base steel sheet comprises a tempered steel sheet containing 0.1 to 0.4% by weight of carbon (C), 0.05 to 3% by weight of silicon (Si), 3 to 15% by weight of manganese (Mn) Plated steel plates for press forming.
5. The method of claim 4,
Wherein said base steel sheet comprises 0.001 to 0.02 weight% of nitrogen (N), 0.0001 to 0.01 weight% of boron (B), 0.001 to 0.1 weight% of titanium (Ti), 0.001 to 0.1 weight% of niobium (Nb) 0.001 to 0.01 wt% of chromium (Cr), 0.001 to 1.0 wt% of molybdenum (Mo), 0.001 to 0.1 wt% of antimony (Sb) and 0.001 to 0.1 wt% of tungsten By weight of at least one member selected from the group consisting of iron and iron.
A warm press-formed member manufactured by warm press-forming a coated steel sheet for warm-pressing according to any one of claims 1 to 5,
Base steel sheet; And
And an alloy layer formed on at least one surface of the base steel sheet,
The alloy layer may be formed of at least one selected from the group consisting of iron (Fe) -manganese (Mn) alloy phase, iron (Fe) -Aluminum (Al) (Al) -manganese (Mn) -based alloy phase.
The method according to claim 6,
The alloy layer may be formed of one selected from the group consisting of zinc (Zn) -manganese (Mn) alloy phase, zinc (Zn) -iron (Fe) alloy phase, iron (Fe) -manganese Wherein the hot pressed formed member further comprises at least one alloy phase selected from the group consisting of iron (Fe) -manganese (Mn) -aluminum (Al) -silicon (Si) alloy phase.
The method according to claim 6,
Wherein the uppermost layer of the alloy layer is an alloy phase containing aluminum (Al).
The method according to claim 6,
Wherein the thickness of the surface oxide formed on the outermost surface of the alloy layer is less than 1 占 퐉.
Preparing a base steel sheet; And
And forming a composite plating layer on at least one side of the prepared ground steel sheet,
(Mn) plating layer and an aluminum (Al) plating layer are alternately formed so that an aluminum (Al) plating layer is formed on the uppermost layer of the composite plating layer, the total thickness of the composite plating layer is 5 to 30 탆, Is 5 to 60% of the total thickness of the composite plated layer.
11. The method of claim 10,
Wherein the composite plated layer is formed by a dry plating method.
Heating the coated steel sheet produced by the manufacturing method of claim 10 or 11 to a temperature range of Ac ₁ to Ac ₃ at an average heating rate of 3 to 200 ° C / s;
Warm-forming the heated plated steel sheet at the temperature; And
And cooling the hot-formed plated steel sheet.
13. The method of claim 12,
Further comprising the step of maintaining the heated plated steel sheet at the temperature for not more than 240 seconds after the heating step.

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