The present invention relates to an 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.
[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, an object of the present invention is to provide an 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 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 steel sheet, wherein the plating layer is formed on the surface of the steel sheet and includes: an alloying layer comprising at least one of Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 ; and an aluminum layer formed on the alloying layer and having a thickness of less than 10% of the plating layer thickness, wherein the plating layer has a thickness of 5 to 20 μm, and oxygen measured by GDS at a depth of 0.1 μm from the surface of the plating layer 10% by weight or less.
[11]
The plating layer is 100% by weight, excluding the Fe content diffused from the base steel sheet, Si: more than 4% and 15% or less, the remainder may include Al and other unavoidable impurities.
[12]
The plating layer may further include Mg: 1.1% or less by weight%.
[13]
The base steel sheet is C: 0.04 to 0.5% by weight, 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 by weight%: 0.01 to 4.0%, the sum of at least one selected from the group consisting of Ti, Nb, Zr and V: 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]
A hot press-formed member according to another aspect of the present invention is a hot-press-formed member obtained by hot press-forming the above-described aluminum-based plated steel sheet, and a diffusion layer made of at least one of FeAl(Si) and Fe 3Al is formed on the base steel sheet. and the thickness of the diffusion layer may be 90% or more of the total thickness of the plating layer.
[16]
The hot press-formed member may have a diffusible hydrogen content of 0.1 ppm or less in the member, and a spot welding current range of the hot press-formed member may be 1 kA or more.
[17]
A method for manufacturing an aluminum-based plated steel sheet according to another aspect of the present invention comprises the steps of preparing a base steel sheet; Obtaining an aluminum-plated steel sheet by immersing the base steel sheet in an aluminum plating bath containing Si: more than 4% and 15% or less by weight, the remainder Al and other unavoidable impurities, and plating at a plating amount of 10 to 40 g/m2 based on one side ; performing initial cooling at a cooling rate of 0.1 to 5° C./sec to a temperature of 640° C. or higher immediately after aluminum plating; and obtaining an aluminum-based plated steel sheet through on-line alloying in which heat treatment is performed by continuously maintaining for 1 to 20 seconds in a heating temperature range of 670 to 900° C. after the initial cooling.
[18]
The aluminum plating bath may further include Mg: 1.1% or less by weight%.
Effects of the Invention
[19]
As described above, in the present invention, in the aluminum-based plated steel sheet before hot press forming, by adjusting the Si content of the aluminum plating bath and appropriately controlling the thickness of the plating layer, and alloying it, the majority on the base steel sheet in the hot press forming member There is an effect of improving the hydrogen resistance and spot weldability of a hot press-formed member by forming a plating layer made of a diffusion layer.
[20]
In addition, by controlling the Si content in the composition of the plating bath and forming the thickness of the plating layer to be thin, it enables on-line alloying heat treatment to be performed immediately after forming the plating layer, thereby reducing the manufacturing cost and improving the productivity of aluminum. There is an effect that can provide a method for manufacturing a plated steel sheet.
Brief description of the drawing
[21]
1 schematically shows a manufacturing apparatus in which a manufacturing method according to an aspect of the present invention is implemented.
[22]
2 is a photograph of a cross-section of the aluminum-based plated steel sheet prepared in Inventive Example 4 observed with a scanning electron microscope (SEM).
[23]
3 is a photograph of a cross-section of the aluminum-based plated steel sheet prepared in Comparative Example 7 observed under an optical microscope.
[24]
FIG. 4 is a photograph of a cross-section of plating after hot press forming of the aluminum-based plated steel sheet manufactured in Inventive Example 4 with a scanning electron microscope (SEM).
[25]
5 is a photograph of a plating cross-section after hot press forming of the aluminum-based plated steel sheet prepared in Comparative Example 7 under an optical microscope.
Best mode for carrying out the invention
[26]
Hereinafter, an aluminum-based plated steel sheet for hot pressing 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.
[27]
[Aluminum plated steel sheet]
[28]
The 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 formed on the surface of the base steel sheet and is Fe 3Al, FeAl(Si), Fe 2Al 5 and An alloying layer comprising at least one of FeAl 3 , and an aluminum layer formed on the alloying layer and having a thickness of less than 10% of the thickness of the plating layer, wherein the thickness of the plating layer is 5 to 20 μm, and from the surface of the plating layer It is characterized in that the oxygen content measured by GDS at a depth of 0.1 μm is 10% by weight or less.
[29]
First, the 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. In addition, the plating layer is formed on the surface of the base steel sheet and Fe 3Al, FeAl (Si), Fe 2Al 5 and FeAl 3 An alloying layer comprising at least one, and an aluminum layer formed on the alloying layer.
[30]
According to an embodiment of the present invention, the alloying layer may include at least one of Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 . In addition, the alloying layer may mainly include at least one of Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 .
[31]
Specifically, according to an embodiment of the present invention, the alloying layer may contain 50% or more of one or more of Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 , preferably 80% or more. and more preferably 90% or more, and most preferably 95% or more.
[32]
That is, according to an embodiment of the present invention, the alloying layer mainly includes one or more alloy phases of Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 , but unavoidably included impurities and room to be included in the plating bath It may also contain small amounts of other elements with
[33]
For example, in the present invention , when Mg is added, Mg may be partially included in the Al-Fe-based alloy phase in the alloying layer, and the alloying layer may include other alloy phases including the Al-Fe-Mg-based alloy phase.
[34]
When the alloying heat treatment is performed after aluminum plating on the base steel sheet, Fe of the base steel sheet is diffused into the aluminum plating layer having a high Al content. As a result, an alloying layer mainly composed of an intermetallic compound of Al and Fe formed as a result of diffusion may be formed on the base steel sheet. Although not limited thereto, the alloy phase of the Al-Fe-based intermetallic compound mainly constituting the alloying layer may include Fe 3Al, FeAl(Si), Fe 2Al 5 , FeAl 3 and the like. On the above-mentioned alloying layer, an aluminum layer containing Fe that is the same as the original plating layer component or diffused in a small amount from the steel sheet may be present, and in some cases, the aluminum layer may not exist due to complete alloying.
[35]
The thickness of the plating layer may be 5 ~ 20㎛. If the thickness of the plating layer is less than 5㎛, corrosion resistance is inferior, whereas if the thickness of the plating layer exceeds 20㎛, there is a problem that the weldability is lowered. Therefore, in the present invention, it is preferable to limit the thickness of the alloy plating layer to a thickness of 5 to 20 μm. On the other hand, the thickness of the plating layer may be preferably 6.2 ~ 19.5㎛, more preferably 5 ~ 15㎛ thickness.
[36]
Meanwhile, the oxygen content measured by GDS (Glow Discharge Spectrometer) at a depth of 0.1 μm from the surface of the plating layer may be 10% by weight or less, and more preferably, 7.4% by weight. That is, in the present invention, the lower the oxygen measured by GDS at a depth of 0.1 μm from the surface of the plating layer, the better, so the lower limit may not be separately limited. However, according to one embodiment of the present invention, oxygen measured by GDS at a depth of 0.1 μm from the surface of the plating layer including the error range may be greater than 0% and less than 10% by weight, or greater than 0% and less than or equal to 7.4%. .
[37]
In the present invention, when the aluminum plating layer is alloyed, it is possible to effectively suppress the increase in the oxygen content on the surface of the plating layer because the alloying heat treatment is performed in a short time by increasing the temperature without cooling after the hot-dip aluminum plating. If the oxygen content on the surface of the plating layer exceeds 10 wt%, the surface quality of the coated steel sheet may be inferior. On the other hand, since it is advantageous as the oxygen content on the surface of the plating layer is small, the lower limit may not be separately limited.
[38]
Meanwhile, an aluminum layer mainly made of aluminum may be formed on the surface side of the plating layer and on the alloying layer. In the present invention, the thickness of the aluminum layer may be controlled to be less than 10% of the thickness of the plating layer, and in some cases, sufficient alloying may be made so that the aluminum layer does not exist (that is, it includes 0% of the thickness of the plating layer) . Since the interface between the aluminum layer and the alloying layer in the aluminum-based plated steel sheet is unstable, if the thickness of the aluminum layer is 10% or more of the thickness of the plating layer, peeling of the aluminum layer may occur during winding after alloying heat treatment. Meanwhile, since it is preferable that the thickness of the aluminum layer is smaller, the lower limit thereof may not be separately limited. On the other hand, since it is preferable that the thickness of the aluminum layer is smaller, more preferably, the thickness of the aluminum layer may be less than 5%, more preferably less than 1%, and most preferably 0%.
[39]
According to an embodiment of the present invention, the plating layer is 100% by weight except for the Fe content diffused from the base steel sheet, Si: more than 4% and 15% or less, the remainder Al and other unavoidable impurities. can do. The Si serves to uniformly alloy with Fe in the plating layer, and should be included in an amount exceeding at least 4% in order to obtain such an effect. On the other hand, since Si also serves to suppress the diffusion of Fe, when contained in an amount exceeding 15%, Fe diffusion is excessively suppressed, so that it may not be possible to obtain a desired plating layer structure in the present invention. The Si content may be preferably 4.5 to 14.1%, more preferably 6 to 13%, and most preferably 8 to 11%.
[40]
In addition, as a non-limiting embodiment, the plating layer may optionally further include 1.1% or less of Mg by weight. When Mg is added, it serves to improve the corrosion resistance of the plated steel sheet, and an effect of increasing the alloying rate can also be obtained. However, if the Mg content exceeds 1.1%, a large amount of Mg oxide is generated on the surface after alloying and/or after hot press forming, which may cause a problem in that weldability is deteriorated, so in the present invention, the Mg content is set to 1.1% or less can be limited In addition, more preferably, the Mg may be limited to 0.9% or less, and in some cases, the Mg may be limited to 0.1% or less. Also, in some cases, the plating layer may not include Mg.
[41]
According to an embodiment of the present invention, the base steel sheet is a steel sheet for hot press forming, and may not be particularly limited as long as it is used 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.
[42]
C: 0.04 to 0.5%
[43]
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.
[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: 0.01~10%
[47]
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.
[48]
Al: 0.001~1.0%
[49]
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.
[50]
P: 0.05% or less
[51]
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%.
[52]
S: 0.02% or less
[53]
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%.
[54]
N: 0.02% or less
[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. 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.
[56]
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%, the sum of at least one from the group consisting of Ti, Nb, Zr and V One or more 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% may be additionally added.
[57]
The sum of one or more selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%
[58]
Since Cr, Mo, and W can improve hardenability and secure strength and grain refinement through the precipitation strengthening effect, one or more 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 is set 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.
[59]
The sum of one or more selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%
[60]
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.
[61]
Cu + Ni: 0.005~2.0%
[62]
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.
[63]
Sb + Sn: 0.001~1.0%
[64]
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.
[65]
B: 0.0001~0.01%
[66]
B is an element that can improve hardenability even with a small amount of addition, and can suppress the brittleness of the hot press-formed member due to segregation at the grain boundary of P and/or S by segregation at the grain boundary of prior austenite. 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.
[67]
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.
[68]
When an aluminum-based plated steel sheet having the plated layer of the above configuration is heat-treated in a temperature range of 880 to 950° C. for 3 to 10 minutes and then hot press-formed to manufacture a hot press-formed member, more than 90% of the plated layer is made of FeAlSi and Fe 3Al. Since it may be formed as a diffusion layer made of one or more, hydrogen that has penetrated into the steel during hot press forming easily escapes, and the diffusible hydrogen content in the steel satisfies 0.1 ppm or less, thereby improving hydrogen resistance. In addition, the spot welding current range satisfies 1 kA or more, so that the spot weldability can be improved.
[69]
Next, a method of manufacturing an 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 aluminum-based plated steel sheet for hot press forming is only an example, and the 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 manufacturing method It should be noted that there is no problem in using the method to implement each embodiment of the present invention as long as it satisfies the claims of the present invention.
[70]
[Manufacturing method of aluminum-based plated steel sheet]
[71]
The aluminum-based plated steel sheet according to another aspect of the present invention uses an aluminum plating bath containing Si: more than 4% and 15% or less, the remainder Al and other unavoidable impurities in weight% on the surface of the hot-rolled or cold-rolled base steel sheet. It can be obtained by performing hot-dip aluminum plating with a plating amount of 10 to 40 g/m2 on a single-sided basis, initial cooling in succession to the plating process, and then performing an on-line alloying treatment in which heat treatment is performed immediately.
[72]
Steps to obtain aluminized steel sheet
[73]
In one embodiment of the present invention, a steel sheet is prepared, and the steel sheet is immersed in an aluminum plating bath containing more than 4% and 15% or less of Si: more than 4% and less than 15% by weight, and the remainder Al and other unavoidable impurities on the surface of the steel sheet. An aluminum-coated steel sheet can be obtained by plating aluminum with a standard coating amount of 10 to 40 g/m2. Meanwhile, more preferably, the plating amount may be 15 to 38 g/m 2 . In addition, optionally, an annealing treatment may be performed on the steel sheet before plating.
[74]
The Si is an element that serves to uniformly alloy with Fe in the plating layer, and may be included in an amount exceeding at least 4% in order to obtain the above effect. However, since Si serves to suppress the diffusion of Fe, when it is contained in excess of 15%, the alloying rate is lowered, so that it is difficult to obtain sufficient alloying. Therefore, in the present invention, the Si content included in the plating bath may be limited to more than 4% and 15% or less. Meanwhile, the Si content may be preferably 4.5 to 14.1%, more preferably 6 to 13%, and most preferably 8 to 11%.
[75]
Meanwhile, as a non-limiting embodiment, Mg may be selectively added to the aluminum plating bath. The Mg serves to improve the corrosion resistance of the aluminum-based plated steel sheet, and also serves to increase the alloying rate. However, when the Mg content exceeds 1.1%, a large amount of Mg oxide is generated on the surface after alloying and/or hot press forming, which may cause a problem in that weldability is deteriorated. Therefore, the Mg content optionally included in the present invention may be limited to 1.1% or less. Meanwhile, more preferably, the Mg may be limited to 0.9% or less, and in some cases, the Mg may be limited to 0.1% or less. Also, in some cases, the plating bath may not contain Mg.
[76]
initial cooling stage
[77]
After the aluminum plating, initial cooling may be performed at a cooling rate of 0.1 to 5°C/sec to a temperature range of 640°C or higher. In addition, the initial cooling may be more preferably carried out in a temperature range of 640 ~ 680 ℃, the cooling rate may be 1 ~ 4 ℃ / sec.
[78]
In the present invention, initial cooling after aluminum plating is important in that it is a means of forming a uniform alloy layer. If the cooling end temperature is less than 640 ℃, there is a problem that equipment load may occur because more power must be applied for alloying in the subsequent on-line alloying heat treatment.
[79]
On the other hand, if the cooling rate is less than 0.1 ℃ / sec, the solidification layer is not sufficiently formed on the plating surface, non-uniform alloying proceeds during online alloying, which may cause a problem in that the surface properties of the steel sheet are inferior. On the other hand, when the cooling rate exceeds 5° C./sec, the plating layer is excessively cooled, and the equipment load and time are lengthened to secure a predetermined temperature for alloying, thereby impairing productivity.
[80]
Obtaining an aluminum-based plated steel sheet by alloying heat treatment
[81]
After the initial cooling, an on-line alloying treatment in which heat treatment is performed continuously may be performed. In addition, the heating temperature range during the alloying heat treatment may be 670 ~ 900 ℃, the holding time may be 1 ~ 20 seconds.
[82]
In the present invention, the on-line alloying process refers to a process of heat treatment by increasing the temperature after molten aluminum plating, as can be seen in the schematic diagram shown in FIG. 1 . In the online alloying heat treatment method according to the present invention, since the heat treatment for alloying starts before the plating layer is cooled and hardened after hot-dip aluminum plating, alloying is possible in a short time. In the conventionally known plating layer composition system of aluminum-coated steel sheet, it was difficult to apply an on-line alloying method in which heat treatment was performed immediately after plating because the alloying rate was slow and sufficient alloying could not be completed in a short time. 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 thinning the plating bath component, particularly the Si content and thickness of the plating layer, which affects the alloying rate.
[83]
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° C., there may be a problem that the alloying becomes insufficient. On the other hand, if the heating temperature exceeds 900 ℃, it is difficult to cool after alloying, and if the cooling rate is increased, there may be a problem that the strength of the base steel sheet is too high. Therefore, the heating temperature during the alloying heat treatment is preferably limited to 670 ~ 900 ℃, more preferably 680 ~ 880 ℃, may be most preferably 700 ~ 800 ℃.
[84]
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. Accordingly, the holding time during the alloying heat treatment is preferably limited to 1 to 20 seconds, more preferably 1.5 to 18 seconds, and most preferably 1 to 10 seconds.
[85]
After the alloying is completed as described above, it can be manufactured as a molded member by performing hot press forming. In this case, the hot press forming may use a method generally used in the art, for example, using a press after heating the aluminum-based plated steel sheet according to the present invention in a temperature range of 880 to 950° C. for 3 to 10 minutes. Thus, the heated steel sheet may be hot formed into a desired shape, but is not limited thereto. 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 aluminum-based plated steel sheet described above.
Modes for carrying out the invention
[86]
Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only provided to illustrate the present invention in more detail, and the present invention It should be noted that it is not intended to limit the scope of the rights of This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.
[87]
(Example)
[88]
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 aluminum plating was performed on the surface of the base steel sheet at a plating bath composition and a plating bath temperature of 660°C shown in Table 2 below. Thereafter, initial cooling and alloying heat treatment were performed under the conditions of initial cooling and alloying heat treatment shown in Table 2 below.
[89]
And after the alloying heat treatment and cooling, the structure of the alloying plating layer of the aluminum-plated steel sheet obtained by the above method was observed with an optical microscope or a scanning electron microscope (SEM) to confirm the thickness of the plating layer and the alloying layer.
[90]
In addition, whether the plating layer peels off is indicated by X if peeling of the plating layer occurs at a bending angle of 30 degrees when a three-point bending test is performed on a 60 mm × 60 mm specimen using a punch having a diameter of 5 mm, and O if it does not occur.
[91]
In addition, the oxygen content was measured by GDS (using GDS 850A, LECO, USA) at a depth of 0.1 μm from the surface of the plating layer, and is shown in Table 3 below.
[92]
[Table 1]
[93]

[94]
[Table 2]
[95]

[96]
Thereafter, each 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 thickness ratio of the diffusion layer was measured by observing the structure of the plating layer of the member, and the diffusion hydrogen content and spot weldability were measured, and are 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.
[97]
[Table 3]
[98]

[99]
As can be seen in Tables 1 to 3, Inventive Examples 1 to 9 satisfy all of the aluminum plating conditions, plating layer and aluminum layer thickness conditions, and alloying heat treatment conditions presented in the present invention, so that the diffusion layer thickness ratio in the member is 90 % or more, and thus the diffusible hydrogen content in the member is 0.1 ppm or less, and the spot welding current range is 1 kA or more, thereby confirming excellent hydrogen delayed fracture characteristics and spot weldability.
[100]
However, in Comparative Example 1, the initial cooling was excessively cooled outside the scope of the present invention, and accordingly, the alloying heat treatment was performed at a low temperature of less than 670 °C, resulting in insufficient alloying. As a result, peeling of the plating layer was observed, and the diffusible hydrogen content in the member exceeded 0.1 ppm. On the other hand, in Comparative Example 2, the alloying heat treatment temperature was too high, alloying was sufficiently performed, but the oxygen content in the surface layer was too high, and the spot weldability was inferior.
[101]
Meanwhile, in Comparative Examples 3 to 6, the alloying heat treatment temperature or time is out of the scope of the present invention. In Comparative Examples 3 and 4, the alloying heat treatment temperature was low or the time was not sufficient, and the plating layer peeling was observed, and the alloying did not occur sufficiently, so that the aluminum layer thickness was formed to be 10% or more of the plating layer thickness. Comparative Examples 5 and 6 were cases in which excessive alloying heat treatment was performed, and although sufficient alloying was performed, it can be seen that spot weldability was deteriorated.
[102]
Comparative Example 7 was a case in which aluminum was plated with an excessive plating amount, and the plating layer thickness was too thick, so that alloying was not performed sufficiently.
[103]
Meanwhile, in Comparative Example 8, when the Mg content was excessively added, a large amount of Mg oxide was generated on the surface, and the surface oxygen content was measured to be high, and it was confirmed that the spot weldability was very poor as 0.2 kA. In Comparative Example 9, the Si content was insufficient, the alloying layer was thickly formed, and the spot weldability was also inferior.
[104]
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.
[105]
[106]
[Explanation of code]
[107]
1: Heat treatment furnace
[108]
2: Aluminum plating bath
[109]
3: Initial cooling device
[110]
4: alloying heat treatment device
Claims
[Claim 1]
An 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 steel sheet, wherein the plating layer is formed on the surface of the steel sheet and includes: an alloying layer comprising at least one of Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 ; and an aluminum layer formed on the alloying layer and having a thickness of less than 10% of the thickness of the plating layer; Including, the thickness of the plating layer is 5 ~ 20㎛, the oxygen measured by GDS at a depth of 0.1㎛ from the surface of the plating layer 10% by weight or less aluminum-based plated steel sheet.
[Claim 2]
According to claim 1, wherein the plating layer is, when the alloy composition is 100% excluding the Fe content diffused from the steel sheet by weight, Si: more than 4% and 15% or less, the remainder Al and other unavoidable impurities are included. Aluminum-based plated steel sheet.
[Claim 3]
The aluminum-based plated steel sheet according to claim 2, wherein the plating layer further comprises 1.1% or less of Mg by weight%.
[Claim 4]
According to claim 1, 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, and the remainder Fe and other unavoidable impurities.
[Claim 5]
According to claim 4, wherein the base steel sheet, by weight, Cr, Mo, and at least one sum selected from the group consisting of W: 0.01 ~ 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 an aluminum-based plated steel sheet further comprising at least one.
[Claim 6]
A hot press-formed member obtained by hot press forming the aluminum-based plated steel sheet according to any one of claims 1 to 5, wherein a diffusion layer made of at least one of FeAl(Si) and Fe 3Al is formed on a base steel sheet, and the diffusion layer A hot press-formed member having a thickness of 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 aluminum-based plated steel sheet used for hot press forming, the method comprising: preparing a base steel sheet; Obtaining an aluminum-plated steel sheet by immersing the base steel sheet in an aluminum plating bath containing Si: more than 4% and 15% or less by weight, the remainder Al and other unavoidable impurities, and plating at a plating amount of 10 to 40 g/m2 based on one side ; performing initial cooling at a cooling rate of 0.1 to 5° C./sec to a temperature of 640° C. or higher immediately after aluminum plating; and obtaining an aluminum-based plated steel sheet through on-line alloying in which heat treatment is performed by continuously maintaining 1 to 20 seconds in a heating temperature range of 670 to 900° C. after the initial cooling; A method of manufacturing an aluminum-based plated steel sheet comprising a.
[Claim 9]
The method of claim 8, wherein the aluminum plating bath further comprises 1.1% or less of Mg by weight%.
[Claim 10]
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, and the remainder Fe and other unavoidable impurities.
[Claim 11]
11. The method of claim 10, wherein the base steel sheet by weight, Cr, Mo, and at least one sum selected from the group consisting of W: 0.01 ~ 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: A method of manufacturing an aluminum-based plated steel sheet, characterized in that it further comprises at least one of 0.0001 to 0.01%.