The present invention relates to an aluminum-iron alloy plated steel sheet for hot forming excellent in corrosion resistance and heat resistance, a hot press formed member manufactured using the same, and a method for manufacturing the same.
background
[2]
Recently, due to the depletion of petroleum energy resources and high interest in the environment, regulations on improving fuel efficiency of automobiles are getting stronger day by day. In terms of material, reducing the thickness of the steel sheet used as a method for improving the fuel efficiency of a vehicle may be mentioned. should be
[3]
For this reason, the demand for high-strength steel sheets has been continuously generated, and various types of steel sheets have been developed. However, since these steel sheets have high strength by themselves, there is a problem in that the workability is poor. That is, since the product of the strength and the elongation for each grade of the steel sheet always tends to have a constant value, when the strength of the steel sheet increases, there is a problem that the elongation, which is an index of workability, decreases.
[4]
In order to solve this problem, a hot press forming method has been proposed. The hot press forming method is a method of increasing the strength of the final product by forming a low-temperature structure such as martensite in the steel sheet by processing the steel sheet at a high temperature suitable for processing and then rapidly cooling it to a low temperature. In this case, there is an advantage that the problem of workability can be minimized when manufacturing a member having high strength.
[5]
However, in the case of the hot press forming method, since the steel sheet is heated to a high temperature, the surface of the steel sheet is oxidized, and thus there is a problem that a process of removing oxides from the surface of the steel sheet must be added after press forming. Patent Document 1 has been proposed as a method for solving this problem. In Patent Document 1, an aluminum-plated steel sheet is used in a process of heating and quenching after hot press forming or room temperature forming (simply 'post heat treatment'), and since the aluminum plating layer is present on the surface of the steel sheet, the steel sheet is oxidized during heating. it doesn't happen
[6]
However, even if the steel sheet is not oxidized during heating due to the presence of an aluminum plating layer on the surface as in Patent Document 1, the member obtained after heating and forming is still exposed to a corrosive environment. In particular, in the process of heating the plated steel sheet, the base iron is diffused into the aluminum plating layer to form a hard alloy layer of Fe and Al on the surface of the steel sheet. There is a problem in that the base steel sheet is exposed to a corrosive environment and the corrosion resistance is lowered accordingly.
[7]
(Patent Document 1) US Patent Publication No. 6,296,805
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[8]
An object of the present invention is to provide an aluminum-iron alloy plated steel sheet capable of manufacturing a hot press-formed member having excellent corrosion resistance and heat resistance, a hot press-formed member using the same, and a method for manufacturing the same.
[9]
The object of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the general description of the present invention.
means of solving the problem
[10]
One aspect of the present invention is an aluminum-iron alloy plated steel sheet comprising a base steel sheet and an alloy plating layer formed on the base steel sheet, wherein the alloy plating layer is formed on the base steel sheet, and Al: 5 to 30% by weight an alloying layer comprising (I); an alloying layer (II) formed on the alloying layer (I) and comprising Al: 30-60% by weight; and an alloying layer (III) formed on the alloying layer (II) and comprising Al: 20-50% by weight, in the alloying layer (II) by weight, Al: 20-50% % and Si: FeAl(Si) alloy phase containing 5 to 20% is dispersed and distributed, and the number density of the FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less is 10 3 pieces/mm 2 or more Aluminum for hot forming - It is an iron alloy plated steel plate.
[11]
The alloy plating layer may further include an alloying layer (IV) formed on the alloying layer (III) and including 30 to 60% of Al by weight.
[12]
The base steel sheet is in wt%, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.1 to 5%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.001 to 1%, N: 0.001 to 0.02%, the balance may include Fe and other impurities.
[13]
The base steel sheet may further include one or more of B: 0.001 to 0.01%, Cr: 0.01 to 1%, and Ti: 0.001 to 0.2% by weight%.
[14]
Another aspect of the present invention is a hot press-formed member obtained by hot press forming the above-described aluminum-iron alloy plated steel sheet, in the alloying layer (II) in weight%, Al: 20-50% and Si: 5~ It is a hot press-formed member in which a FeAl(Si) alloy phase containing 20% ​​is dispersed and distributed, and the number density of an FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less is 10 4 pieces/mm 2 or more.
[15]
Another aspect of the present invention is to obtain an aluminum-plated steel sheet by aluminum plating and winding the surface of the steel sheet; annealing the aluminized steel sheet to obtain an aluminum-iron alloy coated steel sheet; and cooling the aluminum-iron alloy plated steel sheet, wherein the aluminum plating amount is 30-200 g/m 2 based on one side of the steel sheet, and up to 250° C. after aluminum plating The cooling rate is 20 ° C./sec or less, and the winding tension is 0.5 to 5 kg/mm ​​2 during winding, and the annealing is carried out for 30 minutes to 50 hours in a heating temperature range of 550 to 750 ° C in an upper annealing furnace, When heating from room temperature to the heating temperature during annealing, the average temperature increase rate is 10 ~ 100 ℃ / h, but the average temperature increase rate of 400 ~ 500 ℃ section is 1 ~ 15 ℃ / h, the atmospheric temperature in the upper annealing furnace It is a method of manufacturing an aluminum-iron alloy plated steel sheet for hot forming in which the difference between the temperature and the steel sheet temperature is 5 to 80°C.
[16]
The base steel sheet is weight %, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.1 to 5%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.001 to 1%, N: 0.001 to 0.02%, the balance may include Fe and other impurities.
[17]
The base steel sheet may further include one or more of B: 0.001 to 0.01%, Cr: 0.01 to 1%, and Ti: 0.001 to 0.2% by weight%.
[18]
Another aspect of the present invention is to heat-treat an aluminum-iron alloy plated steel sheet for hot forming manufactured by the above-described manufacturing method in a temperature range of Ac3 to 950° C. for 1 to 15 minutes, followed by hot press forming. way.
Effects of the Invention
[19]
The aluminum-iron alloy plated steel sheet according to one aspect of the present invention has an alloying layer (I) to (III) or an alloying layer (I) to (IV) on the base steel sheet and does not melt even when heated for hot forming. Since the plating layer is formed, there is an effect of excellent heat resistance.
[20]
In addition, in the aluminum-iron alloy plated steel sheet according to an aspect of the present invention, a fine FeAl(Si) alloy phase is dispersed and distributed in the alloying layer (II), thereby effectively preventing crack formation occurring in the hard alloying layer (II). It can be suppressed, and there is an effect that excellent corrosion resistance can be obtained.
[21]
Various and advantageous advantages and effects of the present invention are not limited to the above, and will be more easily understood in the course of describing specific embodiments of the present invention.
Brief description of the drawing
[22]
1 is a scanning electron microscope photograph of observing a cross section of a plating layer of an aluminum-iron alloy plated steel sheet prepared according to Inventive Example 1. FIG.
[23]
2 is a scanning electron microscope photograph of a cross-section of the plating layer of the aluminum-iron alloy plated steel sheet manufactured according to Invention Example 2 as a modified example in which the alloying layer (IV) is formed on the alloying layer (III).
[24]
3 is a scanning electron microscope photograph of observing a cross section of a plating layer of an aluminum-iron alloy plated steel sheet prepared in Comparative Example 1. FIG.
[25]
4 is a scanning electron microscope photograph of observing a cross section of a plating layer after hot press forming on the aluminum-iron alloy plated steel sheet manufactured according to Invention Example 1. FIG.
[26]
5 is a scanning electron microscope photograph of observing a cross section of a plating layer after hot press forming on the aluminum-iron alloy plated steel sheet prepared in Comparative Example 1. FIG.
[27]
6 is a cross-sectional view of a mold used for evaluation of properties of a plating layer and corrosion resistance of a member after hot forming in Examples.
Best mode for carrying out the invention
[28]
Hereinafter, an aluminum-iron alloy plated steel sheet according to an aspect of the present invention will be described in detail. In the present invention, when expressing the content of each element, it should be noted that, unless otherwise specified, it means weight %. In addition, the ratio of crystals or tissues is based on the area unless otherwise indicated.
[29]
[Aluminum-iron alloy plated steel sheet]
[30]
An aluminum-iron alloy plated steel sheet according to an aspect of the present invention includes a base steel sheet and an alloy plating layer formed on the base steel sheet, wherein the alloy plating layer is formed on the base plate and has an Al content of 5 to 30% ( I), an alloying layer (II) formed on the alloying layer (I) and having an Al content of 30-60%; and an alloying layer (III) formed on the alloying layer (II) and having an Al content of 20 to 50%. In addition, as a modification of the present invention, an alloying layer (IV) having the same alloy composition as that of the alloying layer (II) may be further included on the alloying layer (III).
[31]
On the other hand, according to one aspect of the present invention, by weight, the alloying layer (I) may include Al: 5 to 30%, the alloying layer (II) may include Al: 40 to 60% And, the alloying layer (III) may include Al: 20 to 40%.
[32]
Also, the present invention According to one aspect, in weight %, the alloying layer (I) may include silver Al: 5-30%, the alloying layer (II) may include Al: 45-60%, and The alloying layer (III) may include Al: 20-40%.
[33]
When heat treatment is performed after plating aluminum on the base steel sheet, Fe of the base steel sheet is diffused into the aluminum plating layer with a high Al content. At this time, alloying between Al and Fe is made in the plating layer, and a layer structure consisting of alloying layers (I) to (III) is formed depending on the alloying degree of Fe.
[34]
In addition, as a modification of the present invention, the alloying layer (IV) may be formed on the alloying layer (III) according to the conditions of the alloying heat treatment. The formation of the alloying layer (IV) is affected by the alloying behavior of Fe of the base steel sheet and Al, Si of the plating layer, and whether or not to be formed is determined according to the temperature and time of the phase annealing process performed after aluminum plating in the present invention. In general, it can be mainly formed when heat treatment is performed under heat treatment conditions in which the phase annealing time is short, and the alloying layer (II) has substantially the same configuration as the alloy composition range. Although the reason why the alloying layer (IV) is formed is not precisely known, the alloying layer (II) and the alloying layer (IV) are phases with little solid solubility of Si, and in the case of Si present in the plating layer during alloying, AlFe It is thought that the (Si) phase moves to the alloying layer (III), and the alloying layer (II) and the alloying layer (IV) having no Si solid solubility are formed around it. At this time, if the phase annealing time is sufficient, it is considered that the alloying layer (III) is relatively grown in the surface layer direction due to the continuous alloying reaction, so that the alloying layer (IV) does not exist.
[35]
Meanwhile, in the aluminum-iron alloy plated steel sheet according to an aspect of the present invention, a fine FeAl(Si) alloy phase containing Al: 20-50% and Si: 5-20% is dispersed in the alloying layer (II) in weight %. is distributed. The FeAl(Si) alloy phase is an Al-Fe-Si compound that can be formed in the manufacturing process of the aluminum-iron alloy plated steel sheet of the present invention, and a wide variety of phases can be formed depending on the heat treatment conditions and the composition ratio of each component. , the location where it is formed is also different. In addition, the FeAl(Si) alloy phase has a soft characteristic having a lower hardness than that of the alloying layer (II).
[36]
Also, according to an aspect of the present invention, in the FeAl(Si) alloy phase, the Al content may be 22 to 35%, and the Si content may be 5 to 16%.
[37]
In addition, the present invention is characterized in that the number density of FeAl(Si) alloy phases having an equivalent circle diameter of 5 μm or less in the alloying layer (II) is 10 3 pieces/mm 2 or more. On the other hand, more preferably, the number density of the FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less in the alloying layer (II) may be 5 × 10 3 pieces/mm 2 or more, more preferably 10 4 pieces/mm It can be 2 or more.
[38]
Here, the equivalent circle diameter of the FeAl(Si) alloy phase means the diameter of a circle having the same area as the area of ​​particles when observing a cross section perpendicular to the plating layer, and the equivalent circle diameter is a scanning electron microscope in a cross section perpendicular to the plating layer. It can be easily measured by observation. In general, when heating for hot forming, the number and fraction of FeAl(Si) alloy phases present in the alloying layer (II) increase due to the alloying reaction between the plating layer and the base iron, but FeAl(Si) alloy in the plated steel sheet before hot forming. The number density of the phase plays an important role in the distribution and number density of the FeAl(Si) alloy phase formed during hot forming. In addition, in the case of the alloying layer (II), which is the main constituent phase of the alloying plating layer, cracks are easily generated in the alloying plating layer in a region having a complex shape such as a curvature during molding due to high hardness. FeAl(Si) alloy having relatively ductility When the phase is dispersed and distributed more than a certain amount, it is possible to effectively suppress the cracking of the alloy plating layer, and thus, it is possible to prevent deterioration in corrosion resistance due to cracking of the plating layer in the curvature portion.
[39]
It is important to control the number density of the FeAl(Si) alloy phase in the hard alloying layer (II) to a certain amount or more. The effect of suppressing cracks in the plating layer of the curved portion due to the dispersion distribution of the (Si) alloy phase is insufficient, and corrosion resistance in the curved portion may be deteriorated. Therefore, in the present invention, the number density of the FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less may be limited to 10 3 /mm 2 or more. Since the higher the number density of the FeAl(Si) alloy phase is, the upper limit may not be specifically limited, but as a non-limiting embodiment, it may be 10 8 pieces/mm 2 or less. This is because when the number density of the FeAl(Si) alloy phase exceeds 10 8 /mm 2 , the fraction of soft phase in the plating layer is increased and scratch resistance of the plating layer may be deteriorated.
[40]
When the alloying layer (IV) is formed on the alloying layer (III) according to a modification of the present invention, an FeAl(Si) alloy phase may be included in the alloying layer (IV). However, if the alloying layer (IV) is formed too thinly, the FeAl(Si) alloy phase may not be included due to insufficient space.
[41]
Meanwhile, the base steel sheet of the present invention is a steel sheet for hot press forming, and its composition is not particularly limited if used for hot press forming. However, according to one aspect of the present invention, in weight%, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.1 to 5%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.001~1%, N: 0.001~0.02%, the balance contains Fe and other impurities. Hereinafter, each component system will be described in detail.
[42]
C: 0.04~0.5%
[43]
The C is an essential element in order to increase the strength of the heat treatment member, and may be added in an appropriate amount. That is, in order to sufficiently secure the strength of the heat treatment member, 0.04% or more of C may be added. Preferably, the lower limit of the C content may be 0.1% or more. However, if the content is too high, in the case of producing a cold rolled material, the strength of the hot rolled material is too high when the hot rolled material is cold rolled, so not only the cold rolling property is greatly inferior but also the spot weldability is greatly reduced. In order to secure weldability, 0.5% or less may be added. In addition, the C content may be 0.45% or less, and more preferably, the C content may be limited to 0.4% or less.
[44]
Si: 0.01~2%
[45]
The Si not only has to be added as a deoxidizer in steelmaking, but also serves to suppress the formation of carbides, which have the greatest influence on the strength of the hot press-formed member. In the present invention, it may be added in an amount of 0.01% or more in order to secure retained austenite by concentrating carbon at the martensite lath grain boundary after martensite is formed in hot press forming. In addition, the upper limit of the Si content may be set to 2% in order to secure sufficient plating property when aluminum plating is performed on the steel sheet after rolling. Preferably, the Si content may be limited to 1.5% or less.
[46]
Mn: 0.1~5%
[47]
The Mn may be added in an amount of 0.1% or more in order to not only secure a solid solution strengthening effect, but also lower a critical cooling rate for securing martensite in a hot press-formed member. In addition, the Mn content may be limited to 5% or less in terms of securing the workability of the hot press forming process by properly maintaining the strength of the steel sheet, reducing the manufacturing cost, and improving the spot weldability.
[48]
P: 0.001~0.05%
[49]
The P is present as an impurity in the steel, and it is advantageous as the content thereof is as small as possible. Therefore, in the present invention, the P content may be limited to 0.05% or less, preferably limited to 0.03% or less. Since P is an impurity element which is advantageous as it is small, it is not necessary to specifically set an upper limit of its content. However, in order to excessively lower the P content, there is a concern that the manufacturing cost may increase, and in this case, the lower limit may be set to 0.001%.
[50]
S: 0.0001~0.02%
[51]
S is an impurity in steel, and since it is an element that impairs ductility, impact properties, and weldability of members, the maximum content is limited to 0.02%, preferably 0.01% or less. In addition, if the minimum content is less than 0.0001%, the manufacturing cost may increase, so the lower limit of the content may be 0.0001%.
[52]
Al: 0.001~1%
[53]
The Al may increase the cleanliness of the steel by deoxidizing it together with Si, and may be added in an amount of 0.001% or more to obtain the above effect. In addition, the Al content may be limited to 1% or less in order to prevent the Ac3 temperature from becoming too high so that heating required for hot press molding can be performed in an appropriate temperature range.
[54]
N: 0.001~0.02%
[55]
The N is an element included as an impurity in steel, and in order to reduce the sensitivity to cracking during continuous casting of the slab, and to secure impact properties, the lower the content, the more advantageous, and thus may be included in 0.02% or less. Although it is necessary to specifically set the lower limit, the N content may be set to 0.001% or more in consideration of the increase in manufacturing cost.
[56]
The aluminum-iron alloy plated steel sheet according to an aspect of the present invention may further include one or more of B: 0.001 to 0.01%, Cr: 0.01 to 1%, and Ti: 0.001 to 0.2% in addition to the alloy composition described above. can
[57]
B: 0.001~0.01%
[58]
B is an element capable of not only improving hardenability even with a small amount of addition, but also segregating at prior austenite grain boundaries to suppress the brittleness of the hot press-formed member due to grain boundary segregation of P and/or S. Therefore, B may be added in an amount of 0.0001% or more. However, 0 When it exceeds .01%, the effect is saturated, and since it causes brittleness in hot rolling, the upper limit may be set to 0.01%, and preferably, the B content may be 0.005% or less.
[59]
Cr: 0.01~1%
[60]
The Cr is an element added to improve the solid solution strengthening effect and hardenability during hot forming similarly to Mn, and may be added in an amount of 0.01% or more to obtain the above effect. However, in order to secure the weldability of the member, the content may be limited to 1% or less, and if it exceeds 1%, the effect of improving hardenability compared to the amount added is also weak, which is disadvantageous in terms of cost.
[61]
Ti: 0.001~0.2%
[62]
The Ti is effective in increasing the strength of the heat-treated member by forming fine precipitates and improving the collision performance of the member due to grain refinement. . In order to obtain the above effect, Ti may be added in an amount of 0.001% or more. However, since coarse TiN formation caused by increasing the Ti content deteriorates the collision performance of the member, the content may be limited to 0.2% or less.
[63]
The remainder other than the above-mentioned components may include iron (Fe) and unavoidable impurities, and additional addition is not particularly limited as long as the components may be included in the steel sheet for hot press forming.
[64]
Conventional aluminum-plated steel sheet for hot forming has insufficient heat resistance because the melting point of the aluminum plating layer is lower than the heating temperature for hot forming. This has the disadvantage of being impossible. However, in the case of the steel sheet for hot press forming manufactured according to the present invention, it has an aluminum-iron alloy plated layer, and the melting point of the alloy plated layer is about 1160° C. or higher, which is higher than the heating temperature for hot forming, so it can exhibit excellent heat resistance.
[65]
An aluminum-iron alloy plated steel sheet composed of an alloy plated layer and a base steel sheet having the above-described alloy composition and layer structure is subjected to heat treatment in a temperature range of Ac3 to 950 ° C. for 1 to 15 minutes, followed by hot press forming to manufacture a hot press formed member. have.
[66]
Specifically, another aspect of the present invention is a hot press formed member obtained by hot press forming the above-described aluminum-iron alloy plated steel sheet, in the alloying layer (II) in weight%, Al: 20-50% and Si : A hot press-formed member in which a FeAl(Si) alloy phase containing 5 to 20% is dispersed and distributed, and the number density of an FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less is 10 4 pieces/mm 2 or more.
[67]
That is, the hot press-formed member manufactured by hot press forming the aluminum-iron alloy plated steel sheet according to the present invention has an equivalent circle diameter of 5 μm or less and the soft FeAl(Si) alloy phase is 10 4 in the hard alloying layer (II). Corrosion resistance can be improved by dispersing and distributing at a number density of pieces/mm 2 or more to suppress the formation of cracks in the plating layer in areas with complex shapes such as curvatures during hot forming.
[68]
On the other hand, in the hot press-formed member, the number density of the FeAl(Si) alloy phase having an equivalent circle diameter of 5 µm or less is preferably 2×10 4 pieces/mm 2 or more, more preferably 3×10 4 pieces/mm It can be 2 or more.
[69]
In addition, since the higher the number density of the FeAl(Si) alloy phase in the hot press-formed member, the upper limit may not be specifically limited, but as a non-limiting embodiment, it may be 10 9 pieces/mm 2 or less. When the number density of the FeAl(Si) alloy phase in the molded member exceeds 10 9 /mm 2 , there is a possibility that the chipping resistance may be deteriorated due to a decrease in the hardness of the plating layer.
[70]
In addition, although not particularly limited, according to an aspect of the present invention, in the FeAl(Si) alloy phase, the Al content may be 22 to 35%, and the Si content may be 5 to 16%.
[71]
[72]
Hereinafter, a method of manufacturing an aluminum-iron alloy plated steel sheet according to another aspect of the present invention will be described in detail. However, the following manufacturing method of the aluminum-iron alloy plated steel sheet for hot press forming is only an example, and the aluminum-iron alloy plated steel sheet for hot press forming of the present invention does not necessarily have to be manufactured by this manufacturing method, and any It should be noted that even if the manufacturing method is a method that satisfies the claims of the present invention, there is no problem in implementing each embodiment of the present invention.
[73]
[Method for manufacturing aluminum-iron alloy plated steel sheet]
[74]
The aluminum-iron alloy plated steel sheet of the present invention can be obtained by preparing a hot-rolled or cold-rolled base steel sheet, performing hot-dip aluminum plating on the surface of the base steel sheet, and then annealing the plated steel sheet for alloying.
[75]
aluminum plating process
[76]
A process of obtaining an aluminum-plated steel sheet (coil) is performed by preparing a base steel sheet having the above-described alloy composition, aluminum plating on the surface of the base steel sheet under appropriate conditions, and winding it.
[77]
First, aluminum plating can be performed on the surface of the rolled steel sheet with a coating amount of 30 to 200 g/m 2 on one side. Aluminum plating is usually AlSi plating, which is called type I (contains 80% or more of Al and 5 to 20% of Si, additional elements may be included if necessary), but contains 90% or more of Al called type II and is required Depending on the type of plating, any plating containing additional elements may be used. To form a plating layer, hot-dip aluminum plating may be performed, and an annealing treatment may be performed on the steel sheet before plating. Appropriate coating amount for plating is 30~200g/m 2 based on one side. If the amount of plating is too large, it may take too much time to alloy to the surface. On the contrary, if the amount of plating is too small, it is difficult to obtain sufficient corrosion resistance.
[78]
Next, after aluminum plating, cooling can be performed at a cooling rate of up to 250° C. of 20° C./sec or less. The cooling rate after aluminum plating affects the formation of the diffusion suppression layer between the plating layer and the base iron. If the cooling rate after aluminum plating is too fast, the diffusion suppression layer cannot be formed uniformly, so the alloying behavior of the coil during the subsequent annealing treatment is reduced. may become non-uniform. Therefore, the cooling rate up to 250°C after aluminum plating can be 20°C/sec or less.
[79]
When a coil is obtained by winding a steel sheet after plating, the winding tension of the coil can be adjusted to 0.5-5 kg/mm ​​2 . According to the adjustment of the winding tension of the coil, the alloying behavior and surface quality of the coil may be changed during the subsequent annealing treatment.
[80]
Annealing process
[81]
An aluminum-iron alloy plated steel sheet can be obtained by subjecting the aluminum-plated steel sheet to an annealing treatment under the following conditions.
[82]
The aluminum-coated steel sheet (coil) is heated in a batch annealing furnace (BAF). When heating a steel sheet, the target heat treatment temperature and holding time are 30 minutes to 50 minutes within the range of 550 to 750° C. It is desirable to keep time. On the other hand, more preferably, the heat treatment target temperature may be in the range of 600 ~ 750 ℃, the holding time may be 30 minutes ~ 10 hours.
[83]
Here, the holding time is the time from when the coil temperature reaches the target temperature to the start of cooling. If the alloying is not made sufficiently, the plating layer may be peeled off during roll leveling, so the heating temperature may be set to 550° C. or higher for sufficient alloying. In addition, in order to prevent excessive formation of oxides on the surface layer and secure spot weldability, the heating temperature may be 750° C. or less. In addition, in order to sufficiently secure the plating layer and to prevent a decrease in productivity, the holding time may be set to 30 minutes to 50 hours. In some cases, the temperature of the steel sheet may have a heating pattern in which the temperature continues to rise without a cooling process until the heating temperature is reached, and a heating pattern in which the temperature is raised after maintaining the temperature below the target temperature for a certain period of time is applied You may.
[84]
When heating a steel sheet to the above-mentioned heating temperature, in order to ensure sufficient productivity and uniformly alloy the plating layer in the entire steel sheet (coil), the steel sheet (coil) temperature for the entire temperature section (the section from room temperature to the heating temperature) The average temperature increase rate may be 10 to 100° C./h. On the other hand, the average temperature increase rate may be more preferably 10 ~ 50 ℃ / h, most preferably 10 ~ 30 ℃ / h. The overall average temperature increase rate can be controlled within the above numerical range, but in one embodiment of the present invention, the rolling oil remains in the temperature section at which the mixed rolling oil is vaporized during rolling, preventing surface stains, etc., while preventing sufficient productivity In order to secure the temperature, it can be heated at an average temperature increase rate of 1 to 15 °C/h in the 400 to 500 °C section. On the other hand, the average temperature increase rate in the 400 ~ 500 ℃ section during the temperature increase may be more preferably 2 ~ 10 ℃ / h.
[85]
The difference between the ambient temperature in the upper annealing furnace and the temperature of the steel sheet can be 5 to 80°C. On the other hand, the difference between the atmospheric temperature and the steel sheet temperature in the upper annealing furnace may be more preferably 5 to 50 ℃, most preferably 5 to 30 ℃.
[86]
The general heating of the upper annealing furnace takes the method of heating the steel sheet (coil) through an increase in the atmospheric temperature in the annealing furnace rather than the method of directly heating the steel sheet (coil). In this case, the difference between the ambient temperature and the coil temperature is unavoidable, but in order to minimize the material and plating quality deviation by location in the steel plate, the difference between the ambient temperature and the steel plate temperature should be 80°C or less based on the time when the target heat treatment temperature is reached. have. It is ideal to make the temperature difference as small as possible, but it may be difficult to satisfy the overall average temperature increase rate condition by slowing the temperature increase rate. Here, the temperature of the steel plate is the temperature of the loaded steel plate (coil) bottom (meaning the lowest part of the coil). It means that the degree is measured, and the ambient temperature means the temperature measured at the center of the inner space of the heating furnace.
[87]
hot press forming process
[88]
A hot press-formed member can be manufactured by performing hot press forming on the aluminum-iron alloy plated steel sheet for hot forming manufactured by the above-described manufacturing method. At this time, the hot press forming may use a method generally used in the art, and as a non-limiting example, after heat treatment at a temperature range of Ac3 to 950° C. for 1 to 15 minutes, hot press forming may be performed.
Modes for carrying out the invention
[89]
Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.
[90]
[91]
(Example)
[92]
First, a cold-rolled steel sheet for hot press forming having the composition shown in Table 1 below was prepared as a base steel sheet, and the surface of the steel sheet was plated with a type I plating bath having a composition of Al-9%Si-2.5%Fe on the surface of the steel sheet. During plating, the plating amount was adjusted to 70 g/m 2 per side, and after aluminum plating, the cooling rate to 250° C. was cooled to 12° C./sec, and the winding tension was adjusted to 2.2 kg/mm ​​2 and wound up. The plated steel sheet in this state was used as Comparative Example 1, and a photograph obtained by observing the cross section of the plating layer of Comparative Example 1 with a scanning electron microscope is shown in FIG. 3 . In addition, the melting point of the plating layer of Comparative Example 1 was measured to be about 660 ℃.
[93]
[Table 1]
Element C Si Mn Al P S N Cr Ti B Ac3
Content (%) 0.22 0.18 1.17 0.03 0.008 0.0013 0.0045 0.17 0.028 0.0025 825℃
[94]
Thereafter, the plated steel sheet was heated to 650° C. in an upper annealing furnace under the following conditions.
[95]
- Overall average heating rate up to 650℃: 18℃/h
[96]
- Average temperature increase rate in 400~500℃ temperature section: 10℃/h
[97]
- Temperature difference between atmosphere and steel plate at heating temperature: 20℃
[98]
After heating, it was maintained at the same temperature for 10 hours, and then the steel sheet was air-cooled to obtain an aluminum-iron alloy plated steel sheet for hot forming, which was referred to as Invention Example 1. And by observing the cross section of the alloy-coated steel sheet with a scanning electron microscope (see Fig. 1), the number density of the FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less in the plating layer structure and the alloying layer (II) was confirmed, and this is shown in Table 3 indicated. In addition, the melting point of the alloying layer (II) of Inventive Example 1 was measured, and it was confirmed to have a melting point of about 1160°C.
[99]
Meanwhile, a cold-rolled steel sheet for hot press forming having the composition shown in Table 2 below was prepared as a base steel sheet, and the surface of the steel sheet was plated with a type I plating bath having Al-8%Si-1.5%Fe composition on the surface of the steel sheet. During plating, the plating amount was adjusted to 60 g/m 2 per side, and after aluminum plating, the cooling rate to 250° C. was cooled to 7.5° C./sec, and then the winding tension was adjusted to 3.5 kg/mm ​​2 and wound. The plated steel sheet in this state was referred to as Comparative Example 2.
[100]
[Table 2]
Element C Si Mn Al P S N Cr Ti B Ac3
Content (%) 0.24 0.25 1.55 0.02 0.01 0.0024 0.009 0.2 0.04 0.003 821℃
[101]
Thereafter, the plated steel sheet was heated to 670°C in an upper annealing furnace under the following conditions.
[102]
- Overall average heating rate up to 670℃: 12℃/h
[103]
- Average temperature increase rate in 400~500℃ temperature section: 5℃/h
[104]
- Temperature difference between atmosphere and steel plate at heating temperature: 15℃
[105]
After heating, it was maintained at the same temperature for 1 hour, and then the steel sheet was air-cooled to obtain an aluminum-iron alloy plated steel sheet for hot forming, which was referred to as Invention Example 2. And by observing the cross section of the alloy-coated steel sheet with a scanning electron microscope (see Fig. 2), the number density of the FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less in the plating layer structure and the alloying layer (II) was confirmed, and this is shown in Table 3 indicated.
[106]
[Table 3]
Category Al content in alloying layer (wt.%) Number density of FeAl(Si) alloy phases with an equivalent circle diameter of 5㎛ or less in alloying layer (II) (pieces/mm 2)
alloying layer (I) alloying layer (II) alloying layer (III) alloying layer (IV)
Invention Example 1 26.9 57.3 37.2 - 1.65×10 4
Invention Example 2 29.0 55.5 31.8 46.9 3.73×10 4
[107]
In addition, the cross-sections of the plated steel sheets of Comparative Examples 1 and 2 were observed with a scanning electron microscope to confirm the plated layer structure, and the Al content in each plated layer was measured and shown in Table 4 below.
[108]
[Table 4]
Category Al content (wt.%)
alloy layer Al layer
Comparative Example 1 56.2 97.6
Comparative Example 2 58.7 96.8
[109]
As shown in Tables 3 and 4, in the case of Inventive Examples 1 and 2 according to the present invention, as shown in FIG. 2, alloying layers (I), (II), (III) are sequentially formed on the base steel sheet. had been On the other hand, in the case of Comparative Examples 1 and 2, as shown in FIG. 3, an alloying layer made of Fe and Al is formed on the base steel sheet, and an aluminum layer containing 95% or more of Al is formed on the alloying layer. Confirmed.
[110]
Therefore, in the case of the plated steel sheets corresponding to Comparative Examples 1 and 2, there was no layer corresponding to the alloying layer (II) formed on the alloying layer (I) of the present invention, and therefore the equivalent circle diameter in the alloying layer (II) was The number density of the FeAl(Si) alloy phase of 5 μm or less could not be measured either.
[111]
In order to evaluate the heat resistance of the plated steel sheets obtained according to Inventive Examples 1 and 2 and Comparative Examples 1 and 2, heat treatment was performed at a temperature of 900°C. In the case of Inventive Examples 1 and 2, since the melting point of the alloy plating layer was higher than 900°C, melting did not occur and the heat resistance was excellent. On the other hand, in Comparative Examples 1 and 2, it was confirmed that the melting point of the Al layer was lower than 900 ° C.
[112]
On the other hand, the alloy plated steel sheets of Inventive Examples 1 and 2 and the plated steel sheets of Comparative Examples 1 and 2 were heated at 900° C. for 6 minutes, and then hot press formed with the mold of FIG. 6 to obtain a hot press formed member.
[113]
A part of the obtained member was taken and the cross section was observed with a scanning electron microscope, and the size and number density of an FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less in the alloying layer (II) of the member were measured, and this is shown in Table 5 It was.
[114]
On the other hand, a scanning electron microscope photograph observing the cross section of the plated layer after hot press forming of the plated steel sheet of Inventive Example 1 is shown in FIG. It was confirmed that , (II), (III) were sequentially formed.
[115]
In addition, a scanning electron microscope photograph observing the cross section of the plating layer after hot press forming of the plated steel sheet of Comparative Example 1 is shown in FIG. 5, and the hot press formed member according to Comparative Example 1 is on the base steel sheet, three alloying layers was formed in this order.
[116]
At this time, for the evaluation of corrosion resistance, the weight loss per unit area was measured for the member after 26 CCT cycles, and if the weight loss per unit area was 10 mg/cm 2 or more, it was judged to be inferior.
[117]
In addition, for the evaluation of heat resistance, the presence or absence of melting of the alloy plating layer was observed during hot press forming, and it was determined that the melting of the alloy plating layer was poor.
[118]
[Table 5]
Category FeAl(Si) alloy phase in alloying layer (II) Weight loss per unit area (mg/cm2) Corrosion resistance Heat resistance
Al (wt%) Si (wt%) Number density of FeAl(Si) alloy phase with an equivalent circle diameter of 5㎛ or less (pcs/mm 2)
Invention Example 1 25.8 9.8 4.16×10 4 4.47 Good Good
Invention Example 2 23.2 8.8 7.07×10 4 5.89 Good Good
Comparative Example 1 28.1 11.7 9.8×10 3 12.38 Defective Defective
Comparative Example 2 35.5 13.8 6.4×10 3 15.01 Defective Defective
[119]
As can be seen in Table 5, the hot press-formed members manufactured using the aluminum-iron alloy plated steel sheets according to Inventive Examples 1 and 2 showed good corrosion resistance, but in Comparative Examples 1 and 2, weight loss per unit area It was confirmed that the corrosion resistance was poor at this 10 mg/cm 2 or more.
[120]
In addition, in Inventive Examples 1 and 2, since the melting point of the alloyed plating layer was higher than the heating temperature for hot press forming, it was confirmed that the alloyed plating layer did not melt in the final manufactured hot press formed member and had excellent heat resistance.
[121]
On the other hand, in Comparative Examples 1 and 2, since the melting point of the alloying plating layer was lower than the heating temperature for hot press forming, the alloying plating layer was melted during heating for hot press forming to contaminate the rolls in the heating furnace, or rapid heating This was impossible.
[122]
As described above, in the detailed description of the present invention, a preferred embodiment of the present invention has been described, but those of ordinary skill in the art to which the present invention pertains may make various modifications without departing from the scope of the present invention. Of course, this is possible. Therefore, the scope of the present invention should not be limited to the described embodiments and should be defined by the claims to be described later, as well as equivalents thereof.
Claims
[Claim 1]
As an aluminum-iron alloy plated steel sheet comprising a base steel sheet and an alloy plating layer formed on the base steel sheet, the alloy plating layer is formed on the base steel sheet, and Al: 5-30% by weight of an alloying layer comprising ( I); an alloying layer (II) formed on the alloying layer (I) and comprising Al: 30-60% by weight; and an alloying layer (III) formed on the alloying layer (II) and comprising Al: 20-50% by weight, in the alloying layer (II) by weight, Al: 20-50% % and Si: FeAl (Si) alloy phase containing 5 to 20% is dispersed and distributed, , An aluminum-iron alloy plated steel sheet for hot forming with an equivalent circle diameter of 5 μm or less and a FeAl(Si) alloy phase having a number density of 10 3 pieces/mm 2 or more.
[Claim 2]
According to claim 1, wherein the alloy plating layer, is formed on the alloying layer (III), Al: for hot forming, characterized in that it further comprises an alloying layer (IV) containing 30-60% by weight of Al: Aluminum-iron alloy plated steel sheet.
[Claim 3]
The method according to claim 1, wherein the base steel sheet is, by weight, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.1 to 5%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al : 0.001 to 1%, N: 0.001 to 0.02%, the remainder of the aluminum-iron alloy plated steel sheet for hot forming, characterized in that it contains Fe and other impurities.
[Claim 4]
The aluminum for hot forming according to claim 3, wherein the base steel sheet further comprises at least one of B: 0.001 to 0.01%, Cr: 0.01 to 1%, and Ti: 0.001 to 0.2% by weight%. - Ferrous alloy plated steel plate.
[Claim 5]
A hot press-formed member obtained by hot press-forming an aluminum-iron alloy plated steel sheet according to any one of claims 1 to 4, wherein the alloying layer (II) contains Al: 20-50% and A hot press-formed member in which a FeAl(Si) alloy phase containing 5 to 20% of Si is dispersed and distributed, and the number density of an FeAl(Si) alloy phase having an equivalent circle diameter of 5 μm or less is 10 4 pieces/mm 2 or more.
[Claim 6]
Obtaining an aluminum-plated steel sheet by aluminum plating and winding the surface of the base steel sheet; annealing the aluminized steel sheet to obtain an aluminum-iron alloy coated steel sheet; and cooling the aluminum-iron alloy plated steel sheet, wherein the aluminum plating amount is 30 to 200 g/m 2 based on one side of the steel sheet, and up to 250° C. after aluminum plating The cooling rate is 20 ° C / sec or less, the winding tension is 0.5 to 5 kg / mm 2 during winding, and the annealing is carried out for 30 minutes to 50 hours in a heating temperature range of 550 to 750 ° C in an upper annealing furnace, When heating from room temperature to the heating temperature during annealing, the average temperature increase rate is 10 to 100 ° C / h, but the average temperature increase rate in the 400 to 500 ° C section is 1 to 15 ° C / h, and the atmospheric temperature in the upper annealing furnace A method of manufacturing an aluminum-iron alloy plated steel sheet for hot forming in which the difference between the temperature and the steel sheet temperature is 5 to 80°C.
[Claim 7]
7. The method of claim 6, wherein the base steel sheet in weight %, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.1 to 5%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al : 0.001 to 1%, N: 0.001 to 0.02%, the remainder Fe and other impurities for hot forming, characterized in that it contains an aluminum-iron alloy plated steel sheet manufacturing method.
[Claim 8]
The aluminum for hot forming according to claim 7, wherein the base steel sheet further comprises at least one of B: 0.001 to 0.01%, Cr: 0.01 to 1%, and Ti: 0.001 to 0.2% by weight%. - Manufacturing method of iron alloy plated steel sheet.
[Claim 9]
The manufacture of a hot press-formed member by heat-treating the aluminum-iron alloy plated steel sheet for hot forming prepared according to any one of claims 5 to 8 at a temperature range of Ac3 to 950° C. for 1 to 15 minutes and then hot press forming Way.