The present invention relates to a hot press-formed member having excellent resistance to hydrogen embrittlement and a method for manufacturing the same.
background
[2]
Due to the recent 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]
[4]
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.
[5]
[6]
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.
[7]
[8]
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 these problems. In Patent Document 1, the aluminum-plated steel sheet is used for hot press forming or room temperature forming, followed by heating and rapid cooling (simply 'post heat treatment'), and since the aluminum plating layer exists on the surface of the steel sheet, the steel sheet is oxidized during heating. it doesn't happen
[9]
[10]
On the other hand, in the case of high-strength members, so-called hydrogen embrittlement is often a problem. That is, when a member comes into contact with a highly corrosive aqueous solution such as calcium chloride, hydrogen permeates into the steel sheet and then accumulates therein to apply a high pressure to the member, which may cause hydrogen embrittlement, causing the member to be destroyed.
[11]
[12]
In general, since the hot press forming method is a method of processing a material at a high temperature in which the ductility of the material (blank) increases, a member manufactured by the hot press forming method has a smaller internal residual stress than a member manufactured by the cold forming method. As a result, even if hydrogen is accumulated inside and pressure is generated, it is not destroyed, which has the advantage of exhibiting excellent resistance to hydrogen embrittlement. However, as the demand for strength of automobile members increases, the sensitivity to hydrogen embrittlement of hot press-formed members is also increasing. Also, there is a trend that the need to improve the hydrogen embrittlement resistance of the hot press-formed member is increasing more and more.
[13]
[14]
[Prior art literature]
[15]
[Patent Literature]
[16]
(Patent Document 1) US Patent Publication No. 6,296,805
[17]
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[18]
According to one aspect of the present invention, it is possible to provide a hot press-formed member having excellent hydrogen embrittlement resistance and a method for manufacturing the same.
[19]
[20]
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.
[21]
means of solving the problem
[22]
One aspect of the present invention includes a base steel sheet and an alloy plating layer formed on the surface of the base steel sheet, and when viewed from a cross-section cut in the thickness direction, the alloy plating layer is occupied by pores having a size of 5 μm or less compared to the area of ​​the alloy plating layer Provided is a hot press-formed member having excellent resistance to hydrogen embrittlement including pores so that the area ratio is 3 to 30%.
[23]
[24]
Another aspect of the present invention includes a steel base plate and an alloy plating layer formed on the surface of the base steel plate, and when viewed from a cross-section cut in the thickness direction of the member, the alloy plating layer has an area of ​​the alloy plating layer of pores having a size of 5 μm or less. Provided is a hot press-formed member having excellent resistance to hydrogen embrittlement including pores so that the number density of pores, which is a value divided by the number, is 5 x 10 3 to 2 x 10 6 pieces/mm 2 .
[25]
[26]
Another aspect of the present invention, the step of obtaining 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 heat-treating an aluminum-iron alloy plated steel sheet for hot forming for 1 to 15 minutes in a temperature range of Ac3 to 950° C. and then hot press forming, wherein the aluminum plating amount is one side of the steel sheet. 30-200 g/m 2 on a cotton basis, the cooling rate to 250° C. after aluminum plating is 20° C./sec or less, and the winding tension during winding is 0.5-5 kg/mm ​​2, and the annealing is hydrogen by volume It is carried out for 30 minutes to 50 hours in a heating temperature range of 550 to 750 ° C in an upper annealing furnace containing 50% or more of the furnace, and when heating from room temperature to the heating temperature during the annealing, the average temperature increase rate is 10 to 100 ° C / h Hot press forming with excellent resistance to hydrogen embrittlement in which the average temperature increase rate in the 400 to 500 ° C section is 1 to 15 ° C / h, and the difference between the atmospheric temperature in the upper annealing furnace and the steel sheet temperature is 5 to 80 ° C. A method for manufacturing a member is provided.
[27]
Effects of the Invention
[28]
The hot press-formed member and its manufacturing method according to an aspect of the present invention can effectively prevent hydrogen from penetrating into the base steel sheet by appropriately controlling the shape of the pores in the alloy plating layer formed on the surface of the forming member, thereby preventing hydrogen embrittlement. A hot press-formed member having excellent resistance can be provided.
[29]
[30]
Various and beneficial 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.
[31]
Brief description of the drawing
[32]
1 is an apparatus for applying a bending stress to a specimen according to the ISO 7539-2 test method;
[33]
2 is a photograph of a cross section of Comparative Example 1 (a) and Inventive Example 1 (b) observed with an optical microscope;
[34]
3 is a photograph observed with a scanning electron microscope of a cross section processed with a FIB (Focused Ion Beam) of Comparative Example 1 and Inventive Example 1, and
[35]
4 is a photograph observing whether fracture occurred in the specimen when the stress corrosion cracking test was performed for Comparative Example 1 and Inventive Example 1. FIG.
[36]
Best mode for carrying out the invention
[37]
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 is necessary to note that unless otherwise specified, it means weight %. In addition, the ratio of crystals or tissues is based on the area unless otherwise indicated.
[38]
[39]
The hot press-formed member includes an alloy layer formed by the alloying reaction between the base steel sheet and the plating layer in the heating process for forming or in the steel plate preparation process before that on the surface. In other words, the hot press-formed member of the present invention includes an alloy plating layer formed on the surface of the base steel plate and the base steel plate.
[40]
[41]
According to the research results of the inventors of the present invention, if the alloy layer formed on the surface is well controlled, it is possible to effectively prevent hydrogen from penetrating into the base steel sheet, so that a hot press-formed member with excellent resistance to hydrogen embrittlement (hydrogen embrittlement resistance) can be obtained. can
[42]
[43]
Hydrogen induces hydrogen embrittlement by generating pressure by being accumulated in the steel sheet by the following process (however, the following description is only for explaining the approximate hydrogen embrittlement phenomenon, and for limiting the scope of the present invention It should be noted that this is not the case).
[44]
[45]
: ① After the water in the solution is decomposed into hydrogen and oxygen on the surface of the steel plate, ② after the hydrogen penetrates into the steel plate in an atomic state, ③ hydrogen accumulates in the steel plate to generate pressure.
[46]
[47]
An object of the present invention is to provide a hot press-formed member having excellent resistance to hydrogen embrittlement by maximally blocking the process of hydrogen permeation into the base steel sheet in the above-described hydrogen embrittlement mechanism.
[48]
[49]
That is, in one embodiment of the present invention, as a hot press-formed member including a steel sheet and an alloy plating layer formed on the surface of the steel sheet, the alloy plating layer may include pores. According to the research results of the present inventors, even if hydrogen is generated on the surface of the member and moves to the base steel sheet, when pores are present in the movement process, a large amount of hydrogen is accumulated (trapped) in the pores of the alloy plating layer, resulting in the base steel sheet The amount of hydrogen reached can be greatly reduced.
[50]
[51]
In addition, there is no special residual stress in the alloy plating layer, and even if the pressure of hydrogen acts, it does not lead to the destruction of the entire member unlike the action in the base steel sheet.
[52]
[53]
Therefore, in the present invention, in order to obtain such an effect, it is intended to form a large amount of micropores capable of accumulating hydrogen in the alloy plating layer. In the present invention, when the pores are analyzed with an image analyzer, the size means 5 μm or less. If a single coarse pore is formed, it may be destroyed during use and may not function as a pore, and may not be suitable for capturing hydrogen due to its small specific surface area. The smaller the size of the pores, the more advantageous it is, so the above limitation is not particularly limited. However, when considering a general case, the size of the pores may be usually 0.1 μm or more. Also, such In order to obtain the effect, at least one factor among the fraction (area ratio) and the number of micropores must be appropriately controlled, which will be described in detail below. According to one embodiment of the present invention, the size of the pores may be based on the equivalent circle diameter.
[54]
[55]
Fraction of pores: 3~30% of the area of ​​the entire alloy plating layer
[56]
When the member is observed from a cross section cut in the thickness direction, the fraction of pores may have a ratio of 3% or more to the area of ​​the entire alloy plating layer. A sufficient hydrogen accumulation effect can be obtained by making the fraction of pores into the above-mentioned ratio. However, when the proportion of pores is too high, since there is a problem in that the alloy plating layer becomes weak, in one embodiment of the present invention, the proportion of pores may be set to 30% or less. In another embodiment of the present invention, the fraction of the pores may be set to 5 to 20%.
[57]
[58]
Number density of pores: 5 x 10 3 ~ 2 x 10 6 pieces/mm 2
[59]
In order to provide a site for hydrogen accumulation, the pores are preferably 5 x 10 3 pieces/mm 2 or more. However, when the number of pores is excessive, since there is a problem that the alloy plating layer becomes weak, in one embodiment of the present invention, the number of pores may be limited to 2×10 6 pieces/mm 2 or less. In the present invention, the number density of pores means a value obtained by dividing the number of pores by the area of ​​the alloy plating layer. In another embodiment of the present invention, the number density of the pores may be set to 9 x 10 3 ~ 1 x 10 6 pieces/mm 2 .
[60]
[61]
The micropores of the present invention are formed in the alloy layer, and if the above-described conditions are satisfied, the distribution or existence form thereof is not limited in principle. However, if the micropores exist close to the steel sheet, even if hydrogen is trapped by the pores, there is a possibility of moving back to the steel sheet. More than 70% of pores may be present in the surface layer portion of the alloy plating layer. By doing this, it is possible to further block the possibility that once trapped hydrogen moves to the steel sheet. In terms of preventing the movement of trapped hydrogen, the higher the ratio of the area of ​​the pores present in the surface layer is, the more advantageous, so the upper limit is not particularly limited (including 100%). In another embodiment of the present invention, the area ratio of the pores present in the surface layer may be set to 80% or more.
[62]
[63]
In one embodiment of the present invention, the surface layer portion of the alloy plating layer may mean a portion above the center line of the thickness, that is, a portion close to the free surface, when viewed in the thickness direction of the alloy plating layer. Even if the surface of the alloy plating layer or the interface between the alloy plating layer and the base steel sheet is not flat, the center line can be obtained by connecting the center points in the thickness direction at each point.
[64]
[65]
According to one embodiment of the present invention, the alloy plating layer may mean a plating layer formed by mainly diffusion of Fe in the base steel sheet to the aluminum plating layer, and 30 to 55% of Al, 35 to 60% of Fe, and Components derived from the remaining plating layer or the base steel sheet may be further included.
[66]
[67]
As described above, the hot press-formed member of the present invention may have an excellent resistance to hydrogen embrittlement by controlling the pores of the alloy plating layer as comprising a base steel sheet and an alloy plating layer formed on the surface of the base steel sheet. The base steel sheet included in the hot press-formed member of the present invention is not particularly limited as long as it has a composition of the base steel plate suitable for the hot press-formed member, but the base steel plate according to one embodiment of the present invention is C: 0.04 to 0 by weight . 5%, Si: 0.01~2%, Mn: 0.1~5%, P: 0.001~0.05%, S: 0.0001~0.02%, Al: 0.001~1%, N: 0.001~0.02%, balance Fe and other impurities It may have a composition comprising Hereinafter, the reason for determining the content of each element will be described in detail.
[68]
[69]
C: 0.04~0.5%
[70]
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 cold-rolled materials, the strength of the hot-rolled material is too high when cold-rolling the hot-rolled material, so that not only the cold-rollability 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.
[71]
[72]
Si: 0.01~2%
[73]
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 the formation of martensite 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.
[74]
[75]
Mn: 0.1~5%
[76]
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.
[77]
[78]
P: 0.001~0.05%
[79]
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 that 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%.
[80]
[81]
S: 0.0001~0.02%
[82]
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%.
[83]
[84]
Al: 0.001~1%
[85]
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.
[86]
[87]
N: 0.001~0.02%
[88]
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, so it can 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.
[89]
[90]
The aluminum-iron alloy plated steel sheet according to an aspect of the present invention may further include one or more of B: 0.0001 to 0.01%, Cr: 0.01 to 1%, and Ti: 0.001 to 0.2% in addition to the alloy composition described above. can
[91]
[92]
B: 0.0001~0.01%
[93]
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, if it exceeds 0.01%, the effect is not only saturated, but also causes brittleness in hot rolling, so the upper limit may be set to 0.01%, and preferably, the B content may be 0.005% or less.
[94]
[95]
Cr: 0.01~1%
[96]
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 addition amount is also weak, which is disadvantageous in terms of cost.
[97]
[98]
Ti: 0.001~0.2%
[99]
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 the formation of coarse TiN caused by an increase in the Ti content deteriorates the collision performance of the member, the content may be limited to 0.2% or less.
[100]
[101]
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.
[102]
[103]
Conventional aluminum-plated steel sheet for hot forming lacks heat resistance because the melting point of the aluminum plating layer is lower than the heating temperature for hot forming, and this For hot forming, the plating layer is melted during heating to contaminate the rolls in the heating furnace, or rapid heating is 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 plating layer, and the melting point of the alloy plating layer is about 1160 ° C. or higher, which is higher than the heating temperature for hot forming, so it can exhibit excellent heat resistance.
[104]
[105]
Hereinafter, a method for manufacturing a hot press-formed member according to another aspect of the present invention will be described in detail. However, the following method of manufacturing a hot press-formed member is only an example, and the hot press-formed member of the present invention does not necessarily have to be manufactured by the present method, and any manufacturing method satisfies the claims of the present invention. It should be noted that there is no problem in implementing each embodiment of the present invention. In order to manufacture a hot press-formed member, it is necessary to go through a step of manufacturing a steel sheet used for hot press forming and a step of hot press forming.
[106]
[107]
[Manufacturing method of aluminum-iron alloy plated steel sheet]
[108]
According to one embodiment of the present invention, unlike a conventional hot press forming process using an aluminum plated steel sheet, an aluminum-iron alloy plated steel sheet is used in the hot press forming process to provide an advantageous hot press formed member of the present invention. can As such, the aluminum-iron alloy plated steel sheet suitable for the hot press-formed member of the present invention is prepared by preparing a hot-rolled or cold-rolled base steel plate, and after performing hot-dip aluminum plating on the surface of the base steel plate, annealing for alloying on the plated steel plate It can be obtained by processing. Hereinafter, each process will be described in detail.
[109]
[110]
aluminum plating process
[111]
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.
[112]
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. In order to form a plating layer, hot-dip aluminum plating may be performed, and you may perform annealing process with respect to a steel plate 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.
[113]
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 uneven. Accordingly, the cooling rate up to 250°C after aluminum plating can be set to 20°C/sec or less.
[114]
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.
[115]
[116]
Annealing process
[117]
An aluminum-iron alloy plated steel sheet can be obtained by performing annealing treatment on the aluminum-plated steel sheet under the following conditions.
[118]
The aluminum-coated steel sheet (coil) is heated in a batch annealing furnace (BAF). When heating the 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. 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, the heating temperature may be 750° C. or less in order to prevent excessive formation of oxides on the surface layer and secure spot weldability. 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.
[119]
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. 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.
[120]
The difference between the ambient temperature in the upper annealing furnace and the steel plate temperature can be 5 to 80°C. 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 variation in material and plating quality for each 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 the heat treatment target 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 means measuring the temperature of the bottom (meaning the lowest part of the coil) charged steel plate (coil), and the ambient temperature means the temperature measured at the center of the internal space of the heating furnace. .
[121]
According to one embodiment of the present invention, in order to form a large number of pores in the hot press-formed member, the atmosphere during annealing may be adjusted to a hydrogen atmosphere. According to the research results of the present inventors, pores can be more easily formed by using a hydrogen atmosphere. In the present invention, the hydrogen atmosphere means an atmosphere in which the volume ratio of hydrogen is 50% or more (including 100%), and the gas other than hydrogen is not particularly limited, but may include nitrogen or an inert gas.
[122]
[123]
[Hot Press Forming Process]
[124]
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. In this case, 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.
[125]
Modes for carrying out the invention
[126]
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.
[127]
[128]
(Example)
[129]
First, prepare a cold-rolled steel sheet for hot press forming having the composition (Ac3: 830℃) in Table 1 as a base steel sheet, and use a type I plating bath having a composition of Al-8%Si-2.5%Fe on the surface of the steel sheet. was plated. During plating, the plating amount was adjusted to 65 g/m 2 per side, and after aluminum plating, the cooling rate to 250° C. was cooled to 8° C./sec, and the winding tension was adjusted to 2.4 kg/mm ​​2 to obtain an aluminum-plated steel sheet. .
[130]
[131]
[Table 1]
Element C Si Mn Al P S N Cr Ti B
Content (%) 0.2 0.25 1.23 0.04 0.012 0.0022 0.0055 0.22 0.03 0.003
[132]
[133]
Thereafter, the plated steel sheet was subjected to alloying heat treatment and hot press forming in an upper annealing furnace under the conditions shown in Table 2 below to obtain a hot-formed member (Invention Examples 1-3, Comparative Examples 3). In Table 2, the atmosphere was adjusted to be nitrogen (N 2) except for H 2 in the atmosphere.
[134]
[135]
On the other hand, as an additional comparative example, hot press forming was performed under the conditions shown in Table 2 below without performing a separate alloying heat treatment on the above-described aluminum plated steel sheet to obtain hot formed members (Comparative Examples 1 and 2).
[136]
[137]
[Table 2]
Category Alloying Heat Treatment Conditions Hot Press Forming Conditions
Temperature (℃) Average heating rate (℃/h) Average heating rate in 400~500℃ temperature range (℃/h) Temperature difference between atmosphere and steel plate at heating temperature (℃) H2 fraction in atmosphere (%) Time ( h) temperature (°C) time (min)
Invention Example 1 650 25 12 30 100 10 900 6
Invention example 2 600 22 14 35 100 18 930 5
Invention Example 3 680 30 15 38 100 6 930 6
Invention Example 4 590 18 8 25 55 28 930 7
Comparative Example 1 - - - - - - 900 6
Comparative Example 2 - - - - - - 930 5
Comparative Example 3 520 27 12 25 100 6 930 5
[138]
[139]
after The cross section of the hot-formed member obtained in each invention example and comparative example was observed with a scanning electron microscope to measure the fraction of pores in the alloy plating layer (ratio of the area of ​​pores to the total area of ​​the alloy plating layer) and the number density, and are shown in Table 3. In addition, in order to evaluate whether hydrogen embrittlement occurred for the hot-formed member obtained in each Inventive Example and Comparative Example, three specimens were taken from the flat part, and as shown in FIG. 1, the radius of curvature 50R according to the ISO 7539-2 test method After U-bending with a furnace, immersion in 0.1N HCl solution was performed to evaluate stress corrosion cracking under the condition of maintaining 300 hours, and the weight loss of the member and the presence or absence of cracks were visually confirmed, and the results are shown in Table 3.
[140]
[141]
[Table 3]
Classification Fraction of internal pores in the plating layer (%) Number of pores in the plating layer (pcs/mm 2) Proportion of pores in the surface layer (%) Stress corrosion cracking evaluation
Weight reduction (g) Crack occurrence
Inventive Example 1 9.4 1.9 x 10 4 87.0 1.87 Non-occurrence
Inventive Example 2 8.2 1.6 x 10 4 90.1 2.23 Non-occurrence
Invention Example 3 17.6 3.6 x 10 4 91.2 2.53 Non-occurrence
Inventive Example 4 3.9 9.6 x 10 3 88.7 1.96 Non-occurrence
Comparative Example 1 0.5 0.9 x 10 3 56.3 1.85 crack
Comparative Example 2 1.4 1.1 x 10 3 37.7 2.12 crack
Comparative Example 3 2.3 1.4 x 10 3 42.3 2.22 crack
[142]
[143]
2 and 3 are photographs of observing the cross-sections of the hot press-formed members of Comparative Example 1 (a) and Invention Example 1 (b) of the present invention, which are typical shapes of cross-sections shown in Comparative Examples and Invention Examples. am. As can be seen from FIG. 2 , it can be seen that, in Comparative Example 1, there are not many pores in the alloy plating layer, whereas in Inventive Example 1, a large number of pores exist in the alloy plating layer. As shown in Table 3, the difference in the degree of pore formation is represented by the difference in the degree of stress corrosion cracking (the degree of hydrogen embrittlement). Although there was a slight difference between the respective invention examples and the comparative examples, the amount of weight loss was similar, and although there was no particular difference in the degree of corrosion that causes hydrogen generation, it can be seen that there was a large difference in hydrogen embrittlement.
[144]
[145]
That is, as can be seen in Table 3, in the case of Inventive Examples 1 to 3, in which the fraction of pores in the plating layer or the number density of pores in the plating layer satisfy the scope of the present invention, cracks due to stress corrosion cracking, which are a measure of hydrogen embrittlement Although it did not occur at all, in Comparative Examples 1 to 3 in which sufficient pores were not formed, it was found that cracks occurred in all cases.
[146]
[147]
4 shows the stress corrosion cracking results of Comparative Example 1 and Inventive Example 1, in the case of Comparative Example 1, two of the three specimens were fractured after the stress corrosion cracking test, but in the case of Inventive Example 1, the fracture of the specimen was not at all. indicates that it did not happen.
[148]
[149]
In addition, in the case of the invention example, it was also confirmed that the possibility that the trapped hydrogen was transmitted to the base steel sheet was minimized because a large number of pores were present in the surface layer.
[150]
[151]
Therefore, the advantageous effect of the present invention could be confirmed.
Claims
[Claim 1]
A base steel sheet and an alloy plating layer formed on the surface of the base steel sheet, and when viewed from a cross-section cut in the thickness direction, the alloy plating layer has a ratio of the area occupied by pores having a size of 5 μm or less to the area of ​​the alloy plating layer 3~ A hot press-formed member with excellent resistance to hydrogen embrittlement including pores to be 30%.
[Claim 2]
A base steel sheet and an alloy plating layer formed on the surface of the base steel sheet, and when viewed from a cross-section cut in the thickness direction, the alloy plating layer is a value obtained by dividing the area of ​​the alloy plating layer by the number of pores having a size of 5 μm or less The number density of pores A hot press-formed member with excellent resistance to hydrogen embrittlement including pores so that is 5 x 10 3 ~ 2 x 10 6 pieces/mm 2 .
[Claim 3]
The hot press-formed member having excellent resistance to hydrogen embrittlement according to claim 1, wherein 70% or more of the pores with a size of 5 μm or less exist in the surface layer of the alloy plating layer based on the area.
[Claim 4]
The hot press-formed member having excellent resistance to hydrogen embrittlement according to claim 2, wherein 70% or more of the pores with a size of 5 μm or less exist in the surface layer of the alloy plating layer based on the area.
[Claim 5]
The hydrogen containing pores according to claim 2, wherein the alloy plating layer has pores such that the ratio of the area occupied by pores having a size of 5 μm or less to the area of ​​the alloy plating layer is 3 to 30% when the member is cut in the thickness direction. Hot press-formed parts with excellent resistance to brittleness.
[Claim 6]
The method according to any one of claims 1 to 5, 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: 0.0001~0.02%, Al: 0.001~1%, N: 0.001~0.02%, the balance Fe and other impurities, characterized in that it contains excellent resistance to hydrogen embrittlement hot press formed member.
[Claim 7]
According to claim 6, wherein the base steel sheet, B: 0.0001 ~ 0.01% by weight, Cr: 0.01 ~ 1%, Ti: Hydrogen embrittlement, characterized in that it further comprises at least one of 0.001 ~ 0.2% Hot press-formed parts with excellent resistance.
[Claim 8]
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 heat-treating an aluminum-iron alloy plated steel sheet for hot forming for 1 to 15 minutes in a temperature range of Ac3 to 950° C. and then hot press forming, wherein the aluminum plating amount is one side of the steel sheet. 30-200 g/m 2 on a cotton basis, the cooling rate to 250° C. after aluminum plating is 20° C./sec or less, and the winding tension during winding is 0.5-5 kg/mm ​​2, and the annealing is hydrogen by volume It is carried out for 30 minutes to 50 hours in a heating temperature range of 550 to 750 ° C in an upper annealing furnace containing 50% or more of the furnace, and when heating from room temperature to the heating temperature during the annealing, the average temperature increase rate is 10 to 100 ° C / h Hot press forming with excellent resistance to hydrogen embrittlement in which the average temperature increase rate in the 400 to 500 ° C section is 1 to 15 ° C / h, and the difference between the atmospheric temperature in the upper annealing furnace and the steel sheet temperature is 5 to 80 ° C. A method for manufacturing a member.
[Claim 9]
The method of claim 8, 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 : 0.001~1%, N: 0.001~0.02%, a method of manufacturing a hot press-formed member having excellent resistance to hydrogen embrittlement including the remainder Fe and other impurities.
[Claim 10]
10. The method of claim 9, wherein the base steel sheet by weight, B: 0.0001 ~ 0.01%, Cr: 0.01 ~ 1%, Ti: excellent resistance to hydrogen embrittlement further comprising at least one of 0.001 ~ 0.2% hot hot A method for manufacturing a press-formed member.