The present invention relates to an iron-aluminum-based plated steel sheet for hot pressing excellent in hydrogen delayed fracture characteristics and spot weldability, 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 in 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 must be heated to a high temperature, the surface of the steel sheet is oxidized. Patent Document 1 has been proposed as a method for solving these problems. In the present invention, 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'). Since the aluminum plating layer is present on the surface of the steel sheet, the steel sheet is not oxidized when heated, but there is a problem in that the spot weldability of the hot press-formed member is deteriorated when the thickness of the plating layer is increased.
[6]
On the other hand, when subjected to hot press forming, the steel sheet may have a strength of 1000 MPa or more, and in some cases, 1400 MPa or more. However, when the strength of the steel sheet increases, it becomes sensitive to hydrogen-delayed fracture, and even if it contains a small amount of hydrogen, the steel sheet may break. In addition, when hot press forming of an aluminum-coated steel sheet, Fe diffusion occurs from the base iron of the steel sheet to the plating layer on the surface and alloying occurs in the plating layer. There is a problem in that the hydrogen resistance of the member is inferior.
[7]
(Patent Document 1) US Patent Publication No. 6,296,805
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[8]
According to one aspect of the present invention, it is possible to provide an iron-aluminum-based plated steel sheet for hot press forming excellent in hydrogen delayed fracture characteristics and spot weldability 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]
An iron-aluminum-based plated steel sheet according to an aspect of the present invention, a base steel sheet; and a plating layer formed on the surface of the base steel sheet, wherein the plating layer is a diffusion layer containing an Fe-Al-based intermetallic compound having a cubic structure; and an alloying layer formed on the diffusion layer and having a crystal structure different from the cubic structure, wherein the diffusion layer has a thickness of 3 to 20 μm, and the thickness of the diffusion layer may be greater than 50% of the total thickness of the plating layer.
[11]
The thickness of the plating layer may be 5 ~ 20㎛.
[12]
The plating layer may include Si: 0.0001 to 7%, Mg: 1.1 to 15% when the remaining alloy composition is 100% except for the Fe content diffused from the base steel sheet by weight%.
[13]
The base steel sheet is in wt%, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.01 to 10%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the balance may contain Fe and other unavoidable impurities.
[14]
The base steel sheet is the sum of at least one selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%, the sum of at least one selected from the group consisting of Ti, Nb, Zr and V by weight%: 0.001 to 0.4%, Cu +Ni: 0.005 to 2.0%, Sb+Sn: 0.001 to 1.0%, and B: may further include one or more of 0.0001 to 0.01%.
[15]
The hot press formed member according to another aspect of the present invention is obtained by hot press forming the iron-aluminum-based plated steel sheet, and the thickness of the diffusion layer may be 90% or more of the total thickness of the plating layer.
[16]
According to another aspect of the present invention, there is provided a method for manufacturing an iron-aluminum-based plated steel sheet, comprising the steps of: preparing a base steel sheet; The base steel sheet, by weight, is immersed in an aluminum plating bath containing Si: 0.0001 to 7%, Mg: 1.1 to 15%, the remainder Al and other unavoidable impurities, and plated with a plating amount of 10 to 40 g/m2 based on one side. obtaining an aluminized steel sheet; obtaining an iron-aluminum-based plated steel sheet through on-line alloying in which the aluminum-plated steel sheet is continuously heated for 1 to 20 seconds in a heating temperature range of 670 to 900° C. without cooling after plating; includes
Effects of the Invention
[17]
As described above, the present invention is a hot press formed member by forming a stable diffusion layer mainly composed of an Fe-Al-based intermetallic compound having a cubic structure on the surface of the plated steel sheet before hot press forming in excess of 50% of the total thickness of the plating layer. It has the effect of remarkably improving the hydrogen delayed fracture characteristics and spot weldability of
[18]
In addition, the present invention provides a stable diffusion layer mainly composed of an Fe-Al-based intermetallic compound having a cubic structure by appropriately controlling the process conditions of Si, Mg components and alloying heat treatment in the plating bath, and directly heat-treating without cooling after molten aluminum plating. It has the effect of providing a method of manufacturing an iron-aluminum-based plated steel sheet with improved productivity and reduced manufacturing cost while at the same time being able to form a steel sheet.
Brief description of the drawing
[19]
1 schematically shows a manufacturing apparatus in which a manufacturing method according to an aspect of the present invention is implemented.
[20]
2 is a photograph of a cross-section of the iron-aluminum-based plated steel sheet prepared in Inventive Example 1 observed with a scanning electron microscope (SEM).
[21]
3 is a photograph of a cross-section of an iron-aluminum-based plated steel sheet prepared in Comparative Example 8 observed under an optical microscope.
[22]
4 is a photograph of a cross-section of the iron-aluminum-based plated steel sheet prepared in Inventive Example 1 after hot press forming was observed with a scanning electron microscope (SEM).
[23]
FIG. 5 is a photograph of an iron-aluminum-based plated steel sheet prepared in Comparative Example 8 observed under an optical microscope after hot press forming;
Best mode for carrying out the invention
[24]
Hereinafter, the present invention will be described in detail.
[25]
In the present invention, when expressing the content of each element, it is necessary to note that '%' means '% by weight' unless otherwise specified. In addition, the ratio of crystals or tissues is based on the area unless otherwise indicated.
[26]
The present inventors have studied in depth the alloy phase of several layers composed of an Fe-Al-based intermetallic compound formed on an aluminum-plated steel sheet during conventional hot press forming. As a result, the alloy phase (eg, FeAl(Si), Fe 3Al, etc.) having a cubic structure among the Fe-Al-based intermetallic compounds is stable, but other alloy phases (eg, FeAl 3, Fe 2Al 5 etc.) were found to be brittle.
[27]
As a result of a more in-depth study on this, the present inventors found that hydrogen is removed from the member after hot press forming, and the mode in which hydrogen is removed varies greatly depending on what kind of plating phase is formed on the surface of the steel sheet before hot forming. , in particular, found that when an orthorhombic crystal phase such as Fe 2Al 5 is formed in the plating layer on the formed alloy, the movement of hydrogen is blocked and the hydrogen in the steel sheet cannot be discharged to the outside. Based on these results, the present inventors found that when a diffusion layer mainly composed of an Fe-Al-based intermetallic compound having a cubic structure is formed to be more than 50% of the total thickness of the plating layer, 90% or more of the diffusion layer is formed in the member after hot press formation, resulting in excellent Based on the fact that hydrogen resistance can be secured, the present invention has been completed.
[28]
Hereinafter, an iron-aluminum-based plated steel sheet according to an aspect of the present invention will be described in detail.
[29]
[Iron-aluminum plated steel sheet]
[30]
An iron-aluminum-based plated steel sheet according to an embodiment of the present invention includes a base steel sheet; and a plating layer formed on the surface of the base steel sheet, wherein the plating layer is a diffusion layer containing an Fe-Al-based intermetallic compound having a cubic structure; and an alloying layer formed on the diffusion layer and having a crystal structure different from the cubic structure, wherein the diffusion layer has a thickness of 3 to 20 μm, and the thickness of the diffusion layer is greater than 50% of the total thickness of the plating layer. do.
[31]
In general, when hot press forming is performed on an aluminum plated steel sheet, Fe of the base steel sheet is diffused into the aluminum plating layer having a high Al content, and various layers of various hard alloy phases of Fe-Al-based intermetallic compounds appear. In this case, on the side close to the base steel sheet, a layer mainly composed of an Fe-Al-based intermetallic compound having a cubic structure with excellent hydrogen embrittlement resistance is formed and stable, but toward the surface, it has a crystal structure such as orthorhombic. A gold phase is formed, and when such a crystal phase is formed in the plating layer, the movement of hydrogen is blocked, so that hydrogen in the steel sheet cannot be discharged to the outside, and the hydrogen resistance property is inferior.
[32]
In order to solve this conventional problem, in the iron-aluminum-based plated steel sheet according to an aspect of the present invention, as shown in FIG. 2, the thickness of the diffusion layer made of the Fe-Al-based intermetallic compound having a cubic structure is increased It is formed to satisfy the conditions of 3 to 20 μm and more than 50% of the total thickness of the plating layer.
[33]
First, according to one embodiment of the present invention, the diffusion layer may include an Fe-Al-based intermetallic compound having a cubic structure. In addition, the diffusion layer may mainly include an Fe-Al-based intermetallic compound having a cubic structure.
[34]
Specifically, according to an embodiment of the present invention, the diffusion layer may contain 50% or more of the Fe-Al-based intermetallic compound having a cubic structure, preferably 80% or more, and more preferably 90% or more. % or more, and most preferably 95% or more.
[35]
In addition, according to one embodiment of the present invention, the diffusion layer mainly includes the Fe-Al-based intermetallic compound having a cubic structure, but may also include a small amount of impurities and other elements that may be included in the plating bath. have.
[36]
For example, when Mg is added, Mg may be partially included in the alloy phase of the Fe-Al-based intermetallic compound in the diffusion layer, and the diffusion layer may include other alloy phases including the Fe-Al-Mg-based alloy phase. have.
[37]
It may be made of an Fe-Al-based intermetallic compound having a cubic structure. In such an Fe-Al-based intermetallic compound, the cubic structure is formed in a region having a relatively high Fe content, and is formed by diffusion of Fe of the base steel sheet into the aluminum plating layer during alloying heat treatment. In addition, although not limited thereto, examples of the alloy phase of the Fe-Al-based intermetallic compound having a cubic structure include FeAl(Si), Fe 3Al, and the like.
[38]
When the thickness of the diffusion layer is less than 3 μm, corrosion resistance is inferior, whereas when the thickness of the diffusion layer exceeds 20 μm, there is a problem in that weldability is deteriorated. Therefore, it is preferable to limit the thickness of the diffusion layer to a thickness of 3 to 20 μm. More preferably, the thickness may be 3.7 to 17.9 μm.
[39]
In addition, the thickness of the diffusion layer may be more than 50% of the total thickness of the plating layer including the alloying layer, or may be 54% or more. Preferably, it may be 70% or more, and more preferably, it may be 90% or more. When the thickness of the diffusion layer exceeds 50% of the total thickness of the plating layer, it is possible to easily obtain a plating layer structure in which the thickness of the Fe-Al-based intermetallic compound having a cubic structure accounts for 90% or more in the plating layer of the hot press-formed member thereafter. From the viewpoint of hydrogen resistance, it is preferable that the ratio of the Fe-Al-based intermetallic compound having a cubic structure is higher, so the upper limit thereof may not be limited.
[40]
In addition, the thickness of the plating layer may be 4.5 ~ 20㎛. If the thickness of the plating layer is less than 4.5 μm, corrosion resistance is inferior, whereas if the thickness of the plating layer exceeds 20 μm, it is difficult to secure the diffusion layer in excess of 50% in the plating layer before hot press forming, even if the diffusion layer is secured in excess of 50% Even then, the thickness of the plating layer becomes too thick after hot press forming, which causes a problem in that it is difficult to secure spot weldability. Therefore, in the present invention, the thickness of the plating layer may be 4.5 ~ 20㎛, more preferably 4.5 ~ 18.9㎛.
[41]
According to one embodiment of the present invention, the plating layer is in weight%, when the remaining alloy composition except for the Fe content diffused from the base steel sheet is 100%, Si: 0.0001-7%, Mg: 1.1-15%, the balance Al and other unavoidable impurities.
[42]
More specifically, in an embodiment of the present invention, Si may be included in an amount of 0.0001 to 7%. The Si serves to uniformly alloy with Fe in the plating layer, and should be contained in at least 0.0001% or more in order to obtain such an effect. On the other hand, since Si also serves to suppress the diffusion of Fe, when it is contained in excess of 7%, Fe diffusion is excessively suppressed, so that it may not be possible to obtain a desired plating structure in the present invention. The Si content may be 0.03 to 7%, preferably 1 to 7%, and more preferably 4 to 7%.
[43]
On the other hand, Mg serves to improve the corrosion resistance of the plated steel sheet, and also has the effect of increasing the alloying rate. In order to obtain the above effect, it should be contained at least 1.1% or more, on the other hand, if it is contained in excess of 15%, there may be a problem in that weldability and paintability are inferior. It may be preferably 1.2 to 12.5%, more preferably 1.1 to 10%, and most preferably 1.1 to 5%. In addition, since Mg in the plating layer tends to diffuse toward the surface, the Mg content measured by a glow discharge spectrometer (GDS) at a depth of 0.5 μm from the surface of the plating layer may be 1 to 20 wt%.
[44]
According to one embodiment of the present invention, oxygen measured by a glow discharge spectrometer (GDS) at a depth of 0.1 μm from the surface of the plating layer may be 10% by weight or less, and the GDS is GDS 850A (device name) of LECO Corporation in the United States. can be measured If oxygen on the surface of the plated layer exceeds 10% by weight, spots may appear on the surface of the plated steel sheet, resulting in poor surface quality. On the other hand, since less oxygen on the surface of the plating layer is advantageous, the lower limit thereof may not be limited.
[45]
According to one embodiment of the present invention, the base steel sheet (base iron) may not be particularly limited as long as it is used for hot press forming as a steel sheet for hot press forming. However, if one non-limiting example is given, the base steel sheet in weight %, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.01 to 10%, Al: 0.001 to 1.0%, P: 0.05% or less, It may have a composition including S: 0.02% or less and N: 0.02% or less.
[46]
C: 0.04 to 0.5%
[47]
The C may be added in an appropriate amount as an essential element in order to increase the strength of the heat treatment member. 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.
[48]
Si: 0.01~2%
[49]
The Si not only needs 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 a 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 generated 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.
[50]
Mn: 0.01~10%
[51]
The Mn may be added in an amount of 0.01% or more in order to not only secure a solid solution strengthening effect, but also to lower a critical cooling rate for securing martensite in a hot press-formed member. In addition, the Mn content may be limited to 10% 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. Preferably, the Mn content may be 9% or less, and in some cases, 8% or less.
[52]
Al: 0.001~1.0%
[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 1.0% 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]
P: 0.05% or less
[55]
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%.
[56]
S: 0.02% or less
[57]
The S is an impurity in steel, and since it is an element that inhibits 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%.
[58]
N: 0.02% or less
[59]
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. There is no need to set a lower limit in particular, but considering an increase in manufacturing cost, the N content may be set to 0.001% or more.
[60]
In the present invention, optionally, in addition to the above-described steel composition, the sum of at least one selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%, Ti, Nb, Zr and V, if necessary. One or more of the sum of at least one in the Ruin group: 0.001 to 0.4%, Cu + Ni: 0.005 to 2.0%, Sb + Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01% may be additionally added.
[61]
The sum of one or more selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%
[62]
The Cr, Mo, and W can improve hardenability and secure strength and grain refinement through the precipitation strengthening effect, so that at least one of these may be added in an amount of 0.01% or more based on the total content. In addition, in order to secure the weldability of the member, the content may be limited to 4.0% or less. In addition, if the content of these elements exceeds 4.0%, the effect may be saturated, so the content may be limited to 4.0% or less.
[63]
The sum of one or more selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%
[64]
The Ti, Nb and V are effective in improving the steel sheet of the heat treatment member by forming fine precipitates and stabilizing the retained austenite and improving the impact toughness by crystal grain refinement. have. However, if the added amount exceeds 0.4%, the effect is not only saturated, but also may cause cost increase due to excessive addition of ferroalloy.
[65]
Cu + Ni: 0.005~2.0%
[66]
The Cu and Ni are elements that improve strength by forming fine precipitates. In order to obtain the above-described effect, the sum of one or more of these components may be 0.005% or more. However, if the value exceeds 2.0%, the upper limit may be set to 2.0% because excessive cost increases.
[67]
Sb + Sn: 0.001~1.0%,
[68]
The Sb and Sn are concentrated on the surface during annealing heat treatment for Al-Si plating to suppress the formation of Si or Mn oxide on the surface, thereby improving plating properties. In order to obtain such an effect, 0.001% or more may be added. However, if the amount exceeds 1.0%, the upper limit is set to 1.0% because not only excessive cost of ferroalloy is required, but also it is dissolved in the slab grain boundary and may cause coil edge cracks during hot rolling.
[69]
B: 0.0001~0.01%
[70]
B is an element that can improve hardenability even with a small amount of addition, and is segregated at the prior austenite grain boundary 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 0.01%, and in one embodiment, the B content may be 0.005% or less.
[71]
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.
[72]
When an iron-aluminum-based plated steel sheet composed of a plating layer having the above-described layer structure is hot-pressed after heat treatment in a temperature range of 880 to 950° C. for 3 to 10 minutes to produce a hot press-formed member, More than 90% is composed of an Fe-Al-based intermetallic compound with a cubic structure, so hydrogen that has penetrated into the steel during hot press forming can easily escape, so that the diffusible hydrogen content in the steel can be reduced to 0.1 ppm or less. Hydrogen resistance may be improved. In addition, the spot welding current range satisfies 1 kA or more, so that the spot weldability can be improved.
[73]
Hereinafter, a method of manufacturing an iron-aluminum-based plated steel sheet for hot press forming according to another aspect of the present invention will be described in detail. However, the following manufacturing method of the iron-aluminum-based plated steel sheet for hot press forming is only an example, and the iron-aluminum-based 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 using it to implement each embodiment of the present invention.
[74]
[Manufacturing method of iron-aluminum plated steel sheet]
[75]
The method for manufacturing an iron-aluminum-based plated steel sheet according to another aspect of the present invention is to perform hot-dip aluminum plating on the surface of a hot-rolled or cold-rolled base steel sheet at a plating amount of 10 to 40 g/m 2 on a single side basis, and continuous to the plating process It can be obtained by performing an on-line alloying treatment in which heat treatment is performed immediately.
[76]
Steps to obtain aluminized steel sheet
[77]
In one embodiment of the present invention, a steel sheet is prepared, and the steel sheet is immersed in an aluminum plating bath containing the weight %, Si: 0.0001 to 7%, Mg: 1.1 to 15%, the remainder Al and other unavoidable impurities. An aluminum-coated steel sheet can be obtained by plating the surface of the base steel sheet with aluminum at a coating amount of 10 to 40 g/m2 based on one side. On the other hand, more preferably, the plating amount may be 11 to 38 g/m 2 based on one side. In addition, optionally, an annealing treatment may be performed on the steel sheet before plating.
[78]
step of spraying aluminum powder
[79]
After the aluminum plating, if necessary, aluminum powder may be sprayed on the surface of the aluminum-plated steel sheet. Aluminum powder can locally cool the surface, as well as refine surface spangles. At this time, if only the surface is locally cooled by the aluminum powder, the diffusion of Mg in the plating layer to the surface during the subsequent online alloying process is more suppressed, and the Mg oxide generated by Mg diffusion to the surface after hot press forming can be reduced. Weldability can be improved. In addition, there is an advantage in that the surface can be uniformly generated after hot press molding by refining the surface spangles.
[80]
The average particle diameter of the aluminum powder may be 5 to 40 μm, more preferably 10 to 30 μm. When the average particle diameter of the aluminum powder is less than 5 μm, the effect of surface cooling and sequin refinement is insufficient. On the other hand, when the average particle diameter exceeds 40 μm, it is not sufficiently dissolved in the plating layer and remains on the surface, causing surface quality problems.
[81]
In the present invention, the injection amount of the aluminum powder may be determined within the limit that satisfies the condition that the surface temperature does not fall below 640°C after the powder injection. If the surface temperature of the steel sheet falls below 640°C after powder spraying, equipment load may occur because more output must be applied for alloying in the subsequent online alloying heat treatment. The injection amount of aluminum powder is related to the surface temperature of the steel sheet, but the surface temperature of the steel sheet cannot be uniformly determined because it can vary greatly depending on the process conditions, equipment, environmental conditions, etc. at the time of implementation. Therefore, since the aluminum powder injection amount is sufficient to satisfy the above conditions, the specific injection amount range may not be particularly limited. However, as a non-limiting embodiment, the aluminum powder may be sprayed within the range of 0.01 to 10 g per 1 m 2 of the aluminum plated steel sheet.
[82]
Obtaining an iron-aluminum-based plated steel sheet by alloying heat treatment
[83]
After the aluminum plating is subjected to minimal air cooling, an on-line alloying treatment in which heat treatment is performed continuously may be performed immediately. In addition, when aluminum powder is selectively sprayed after aluminum plating, the on-line alloying treatment may be continuously performed immediately after powder spraying. At this time, the heating temperature range during the alloying heat treatment may be 670 ~ 900 ℃, the holding time may be 1 ~ 20 seconds.
[84]
In the present invention, the on-line alloying process refers to a process of heat-treating by increasing the temperature after minimal air cooling after hot-dip aluminum plating, or hot-dip aluminum plating and aluminum powder spraying, as shown in FIG. 1 . In the on-line alloying method according to the present invention, since heat treatment is started before the plating layer is cooled and hardened after hot-dip aluminum plating, there is no need for a separate temperature rise process, so heat treatment can be performed in a short time. However, since alloying could not be completed in a short time due to the thickness of the conventional aluminum plated steel sheet having a thick plating layer, it was difficult to apply an on-line alloying method of heat treatment immediately after plating. However, in the present invention, the alloying of the aluminum plating layer can be effectively completed despite the short heat treatment time of 1 to 20 seconds by controlling the plating amount of the aluminum plating layer to 10 to 40 g/m2 based on one side in addition to the adjustment of the above-described plating bath components.
[85]
The heating temperature is based on the surface temperature of the steel sheet to be heat treated. If the heating temperature is less than 670 ℃, the problem of insufficient alloying may occur. On the other hand, if the heating temperature exceeds 900 ℃, it is difficult to cool after alloying. can Therefore, the heating temperature during the alloying heat treatment is preferably limited to 670 ~ 900 ℃, more preferably may be 680 ~ 880 ℃, most preferably may be 700 ~ 800 ℃.
[86]
Meanwhile, during the alloying heat treatment, the holding time may be limited to 1 to 20 seconds. In the present invention, the holding time means the time during which the heating temperature (including deviation ±10° C.) is maintained in the steel sheet. If the holding time is less than 1 second, the heating time is too short, so that sufficient alloying is not achieved. On the other hand, if the holding time exceeds 20 seconds, there may be a problem in that productivity is too reduced. Therefore, the holding time during the alloying heat treatment is preferably limited to 1 to 20 seconds, more preferably 1 to 12 seconds, and most preferably 1 to 10 seconds.
[87]
The formation of the diffusion layer through the alloying heat treatment depends on the heat treatment temperature and the holding time, but at the same time is also affected by the content of Si and Mg contained in the aluminum plating layer. As the amount of Si contained in the aluminum plating layer increases and the amount of Mg increases, the alloying rate increases, so that the thickness of the diffusion layer may be increased. When the on-line heat treatment is performed as in the present invention, since the heat treatment time is relatively short compared to the phase annealing method, If the process conditions are not carefully controlled, a diffusion layer of sufficient thickness cannot be obtained. Therefore, the present inventors can effectively obtain a diffusion layer of sufficient thickness despite a short heat treatment time of 1 to 20 seconds by controlling the Si and Mg content and heat treatment conditions.
[88]
Meanwhile, according to another exemplary embodiment of the present invention, a hot press-formed member obtained by hot press forming the iron-aluminum-based plated steel sheet of the present invention as described above may be provided. In this case, the hot press forming may use a method generally used in the art, for example, after heating the iron-aluminum-based plated steel sheet according to the present invention in a temperature range of 880 to 950° C. for 3 to 10 minutes, press The heated steel sheet may be hot formed into a desired shape by using, but is not limited thereto. In addition, in the hot press-formed member of the present invention, the thickness of the diffusion layer made of the Fe-Al-based intermetallic compound having a cubic structure on the surface of the steel sheet may be 90% or more of the total thickness of the plating layer. In addition, the composition of the base steel sheet of the hot press-formed member may be the same as the composition of the base steel sheet of the iron-aluminum-based alloy little steel sheet described above.
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]
(Example)
[91]
First, prepare a cold-rolled steel sheet for hot press forming having the composition of Table 1 as a base steel sheet, and aluminum plating and alloying with the plating bath composition shown in Table 2 below, plating bath temperature 660° C. and alloying heat treatment conditions on the surface of the base steel sheet Heat treatment was performed.
[92]
And after alloying heat treatment and cooling, the structure of the alloyed plating layer of the iron-aluminum plated steel sheet obtained by the above method was observed with an optical microscope and a scanning electron microscope (SEM) to confirm the thickness of the plating layer and the diffusion layer.
[93]
In FIG. 2, where the alloy layer was observed with a scanning electron microscope (SEM), EDS analysis was performed on the diffusion layer, and Fe 3Al and FeAl having a cubic structure were confirmed.
[94]
In addition, in FIG. 2 , an EDS analysis was performed on the portion of the alloy layer formed on the diffusion layer, and Al: 48%, Fe: 50%, Si: 2% were detected by weight %, and this phase is not a cubic structure but an orthogonal It was confirmed that Fe 2Al 5 having an orthorhombic structure.
[95]
[Table 1]
Element C Si Mn Al P S N Cr Ti B
Content (%) 0.22 0.20 1.2 0.03 0.01 0.002 0.0054 0.2 0.03 0.0025
[96]
[Table 2]
Category Aluminum plating conditions Al powder average particle size (㎛) Alloying heat treatment conditions Steel plate plating layer
Coating amount (g/m 2) Si content (Wt.%) Mg content (Wt.%) Temperature (℃) Time (sec) Plating layer thickness (㎛) Diffusion layer thickness (㎛) Diffusion layer thickness ratio (%)
Invention Example 1 38 6.8 12.5 25 680 10 15.1 8.2 54
Invention Example 2 38 6.8 12.5 25 800 10 16.8 10.2 61
Invention Example 3 38 6.8 12.5 25 880 10 17.8 12.5 70
Comparative Example 1 38 6.8 12.5 25 600 10 13.3 6.1 46
Comparative Example 2 38 6.8 12.5 25 950 10 23.3 21.2 91
Invention Example 4 37 1.2 10.7 30 680 1 16.2 10.8 67
Invention Example 5 37 1.2 10.7 30 800 1 17.8 13.5 76
Inventive Example 6 37 1.2 10.7 30 880 1 18.9 17.9 95
Comparative Example 3 37 1.2 10.7 30 680 0.1 15.4 7.2 47
Comparative Example 4 37 1.2 10.7 30 600 5 15.1 2.9 19
Comparative Example 5 37 1.2 10.7 30 880 25 22.7 19.4 85
Comparative Example 6 37 1.2 10.7 30 950 5 27.4 26.8 98
Invention Example 7 15 4.2 5.3 10 680 3 5.5 3.8 69
Invention Example 8 15 4.2 5.3 10 800 3 5.7 4.7 82
Invention Example 9 15 4.2 5.3 10 880 3 5.8 5.6 97
Inventive Example 10 11 0.03 1.2 16 680 12 4.5 3.7 82
Invention Example 11 11 0.03 1.2 16 800 12 5.2 4.5 87
Inventive Example 12 11 0.03 1.2 16 880 12 5.7 5.4 95
Comparative Example 7 35 5.5 0 3 800 10 5.7 2.8 49
Comparative Example 8 50 6.5 7.8 18 800 10 26.7 5.5 21
Comparative Example 9 35 13.4 1.5 55 800 10 17.8 2.4 13
Comparative Example 10 35 0 17.8 22 800 10 34.7 18.9 54
[97]
Thereafter, each iron-aluminum-based plated steel sheet was heated at 930° C. for 6 minutes in an atmospheric atmosphere, and then hot press forming was performed to obtain a hot press-formed member. Thereafter, the structure of the plating layer of the member was observed, and the diffusible hydrogen content and spot weldability were measured and shown in Table 3 below. The diffusible hydrogen content was measured by heating the specimen to 300°C using a gas chromatography technique to measure the emitted hydrogen content, and the spot weldability was evaluated according to ISO 18278 and the current range was analyzed.
[98]
[Table 3]
Classification Iron-aluminum plated steel sheet hot press formed member
Diffused layer thickness ratio (%) Diffusible hydrogen content (ppm) Diffused layer thickness ratio (%) Diffuse layer thickness ratio (ppm) Spot welding current range (kA)
Invention Example 1 54 0.01 99 0.05 1.6
Inventive Example 2 61 0.02 100 0.04 1.6
Inventive Example 3 70 0.01 100 0.02 1.4
Comparative Example 1 46 0.02 82 0.24 1.8
Comparative Example 2 91 0.02 100 0.03 0.6
Invention Example 4 67 0.01 96 0.08 1.6
Inventive Example 5 76 0.007 96 0.07 1.6
Inventive Example 6 95 0.01 97 0.06 1.4
Comparative Example 3 47 0.01 75 0.28 1.8
Comparative Example 4 19 0.01 52 0.52 1.8
Comparative Example 5 85 0.02 100 0.05 0.4
Comparative Example 6 98 0.01 100 0.05 0.2
Inventive Example 7 69 0.02 97 0.06 2.2
Inventive Example 8 82 0.01 100 0.02 2.0
Inventive Example 9 97 0.02 100 0.03 2.0
Inventive Example 10 82 0.01 100 0.03 1.6
Inventive Example 11 87 0.008 100 0.02 1.6
Inventive Example 12 95 0.01 100 0.02 1.6
Comparative Example 7 49 0.01 48 0.58 2.0
Comparative Example 8 21 0.004 64 0.34 1.6
Comparative Example 9 13 0.02 35 0.6 0.4
Comparative Example 10 54 0.01 94 0.08 0.8
[99]
As can be seen in Tables 1 to 3, Inventive Examples 1 to 12 satisfy all of the plating bath components and alloying heat treatment conditions presented in the present invention, and thus are alloys of Fe-Al-based intermetallic compounds having a cubic structure in the plated steel sheet. The thickness ratio of the diffusion layer containing the phase was 50% or more.
[100]
In addition, when a hot press-formed member is manufactured, the diffusible hydrogen content in the steel is 0.1 ppm or less, and the spot welding current range is 1 kA or more, so it can be confirmed that the hydrogen delayed fracture characteristics and spot weldability are excellent.
[101]
However, in Comparative Examples 1 and 4, the alloying heat treatment temperature was less than 670 ° C. In Comparative Example 1, the diffusion layer was not sufficiently formed, so that the diffusion layer thickness ratio was 50% or less, and in Comparative Example 4, the diffusion layer was formed to have a thickness of less than 3 μm. Accordingly, in the hot press-formed members manufactured from the plated steel sheets of Comparative Examples 1 and 4, the diffusion layer thickness ratio was less than 90%, and the diffusion hydrogen content was 0.1 ppm or more because hydrogen did not easily escape, and the hydrogen resistance property was inferior. did it
[102]
In Comparative Examples 2 and 6, the alloying heat treatment temperature exceeded 900° C., and the thickness of the plating layer and the diffusion layer exceeded 20 μm and was excessively formed. Accordingly, in the hot press-formed member, the spot welding point current range was less than 1 kA, and the spot welding characteristics were inferior.
[103]
Meanwhile, in Comparative Examples 3 and 5, the holding time during the alloying heat treatment was outside the scope of the present invention. In Comparative Example 3, the heat treatment time was very short, and the diffusion layer was not sufficiently formed, so that the diffusion layer thickness ratio of the hot press-formed member was 75%. As a result, the hydrogen resistance property was deteriorated. In addition, in the case of Comparative Example 6, the heat treatment time was long as 25 seconds, and the thickness of the plating layer exceeded 20 μm, and thus the spot weldability was inferior.
[104]
Comparative Examples 7, 9, and 10 are examples in which the content of Si or Mg in the aluminum plating bath component does not satisfy the conditions of the present invention. Comparative Example 7 is a case in which Mg is not added, and Comparative Example 9 is a case in which Si is added in excess of 7%, and the diffusion layer was not sufficiently formed due to a slow alloying rate. As the hydrogen content increased, the hydrogen resistance decreased. In Comparative Example 10, Mg was added in excess of 15% to form a plating layer having a thickness of more than 20 μm, and thus the spot weldability was inferior.
[105]
In Comparative Example 8, the aluminum plating amount was outside the range of the present invention, and the thickness of the plating layer was increased to 26.7 μm, and the diffusion layer thickness ratio was reduced, resulting in inferior hydrogen resistance.
[106]
Although described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the following claims. will be able
[107]
[108]
[Explanation of code]
[109]
1: Heat treatment furnace
[110]
2: Aluminum plating bath
[111]
3: Aluminum powder injection device
[112]
4: alloying heat treatment device
Claims
[Claim 1]
An iron-aluminum-based plated steel sheet used for hot press forming, comprising: a base steel sheet; and a plating layer formed on the surface of the base steel sheet; Including, wherein the plating layer is a diffusion layer comprising a cubic structure (Cubic structure) Fe-Al-based intermetallic compound; and formed on the diffusion layer. and an alloying layer having a crystal structure different from that of the cubic structure; Containing, the thickness of the diffusion layer is 3 ~ 20㎛, the thickness of the diffusion layer is more than 50% of the total thickness of the plated iron-aluminum-based plated steel sheet.
[Claim 2]
The iron-aluminum-based plated steel sheet according to claim 1, wherein the plating layer has a thickness of 5 to 20 μm.
[Claim 3]
The method according to claim 1, wherein the plating layer is 100% by weight, excluding the Fe content diffused from the steel sheet, Si: 0.0001 to 7%, Mg: 1.1 to 15%, the remainder Al and other unavoidable impurities. An iron-aluminum-based plated steel sheet comprising a.
[Claim 4]
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: 0.01 to 10%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, iron-aluminum-based plated steel sheet, characterized in that it contains the remainder Fe and other unavoidable impurities.
[Claim 5]
According to claim 4, wherein the base steel sheet is, by weight%, Cr, Mo, and at least one sum selected from the group consisting of W: 0.01 to 4.0%, Ti, Nb, Zr, and one or more sums from the group consisting of: 0.001 to 0.4%, Cu+Ni: 0.005 to 2.0%, Sb+Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01% of iron-aluminum-based plated steel sheet.
[Claim 6]
A hot press formed member obtained by hot press forming the iron-aluminum plated steel sheet according to any one of claims 1 to 5, wherein the thickness of the diffusion layer is 90% or more of the total thickness of the plating layer.
[Claim 7]
The hot press formed member according to claim 6, wherein the diffusible hydrogen content in the hot press formed member is 0.1 ppm or less, and the spot welding current of the hot press formed member is 1 kA or more.
[Claim 8]
A method of manufacturing an iron-aluminum-based plated steel sheet used for hot press forming, the method comprising: preparing a base steel sheet; The base steel sheet, by weight, is immersed in an aluminum plating bath containing Si: 0.0001 to 7%, Mg: 1.1 to 15%, the remainder Al and other unavoidable impurities, and plated with a plating amount of 10 to 40 g/m2 based on one side. obtaining an aluminized steel sheet; and obtaining an iron-aluminum-based plated steel sheet through on-line alloying in which the aluminum-plated steel sheet is heat treated by maintaining the aluminum-plated steel sheet in a heating temperature range of 670 to 900° C. for 1 to 20 seconds after plating; A method of manufacturing an iron-aluminum-based plated steel sheet comprising a.
[Claim 9]
The method of claim 8 , further comprising spraying aluminum powder on the surface of the aluminum-coated steel sheet after the step of obtaining the aluminum-coated steel sheet.
[Claim 10]
10. The method of claim 9, wherein the aluminum powder has an average particle diameter of 5 to 40 μm.
[Claim 11]
According to claim 8, wherein the base steel sheet is C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.01 to 10%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.02 by weight% % or less, N: 0.02% or less, the balance Fe and other unavoidable impurities, characterized in that it contains an iron-aluminum-based plated steel sheet manufacturing method.
[Claim 12]
12. The method of claim 11, wherein the base steel sheet is, by weight %, Cr, Mo, and at least one sum selected from the group consisting of W: 0.01 to 4.0%, Ti, Nb, Zr, and one or more sums from the group consisting of: 0.001~0.4%, Cu+Ni: 0.005~2.0%, Sb+Sn: 0.001~1.0%, and B: 0.0001~0.01% of iron-a method of manufacturing an aluminum-based plated steel sheet further comprising at least one.