KR20190078001A - Steel sheet plated with fe-al alloy for hot press forming, manufacturing method thereof and hot press formed part manufactured therefrom - Google Patents

Abstract

The present invention relates to a steel-aluminum alloy-plated steel sheet for hot press forming, a manufacturing method for the same, and a hot press formed component manufactured using the same. According to an aspect of the present invention, the steel-aluminum alloy-plated steel sheet for hot press forming is used for hot press forming. The steel-aluminum alloy-plated steel sheet for hot press forming includes: a basis material steel sheet; and an Fe-Al alloy plated layer formed on the surface of the basis material steel sheet. A standard deviation ratio of the content of Al is calculated by the relation of the depth and concentration of Al in a range from a point of the depth 1μm from the surface of the plated layer to the depth in which the content of Al is 3%. The standard deviation ratio of the content of Al in accordance with a position indicated by following equation 1 is equal to or less than 2%. [equation 1] (Herein, x_i means actual concentration of Al at a point I, and x_ei means the concentration of Al at point i calculated by a linear expression approximating relation of the content of Al in accordance with the depth in the plated layer, and n means the number of whole data.)

Description

TECHNICAL FIELD [0001] The present invention relates to an iron-aluminum alloy-plated steel sheet for a hot press, a method for manufacturing the same, and a hot-pressed formed part for a hot press,

The present invention relates to an iron-aluminum alloy-plated steel sheet for hot press, a method for producing the same, and a hot-press formed part manufactured from the coated steel sheet.

Recently, due to the depletion of petroleum energy resources and the high interest in the environment, regulations on the improvement of fuel efficiency of automobiles are getting stronger day by day.

One of the ways to improve the fuel economy of automobiles in terms of material is to reduce the thickness of the steel sheet used. However, if the thickness is reduced, the safety of the automobile may be problematic. .

For this reason, demand for high-strength steel sheets has been continuously generated, and various types of steel sheets have been developed. However, since these steel sheets themselves have high strength, there is a problem that the workability is poor. That is, since the product of strength and elongation tends to always have a constant value depending on the grade of the steel sheet, there is a problem that when the strength of the steel sheet is increased, the elongation as an index of workability is decreased.

In order to solve such a problem, a hot press forming method has been proposed. The hot press forming method is a method of improving the strength of a final product by forming a cold structure such as martensite in a steel sheet by processing the steel sheet at a high temperature which is good for processing and then rapidly cooling the steel sheet to a low temperature. In this case, there is an advantage that the problem of workability can be minimized when a member having high strength is manufactured.

However, in the case of the hot press forming method, there is a problem that since the steel sheet must be heated to a high temperature, the surface of the steel sheet is oxidized, and therefore, a process of removing the oxide on the surface of the steel sheet after the press molding has to be added.

As a method for solving such a problem, U.S. Patent No. 6,296,805 has been proposed. In the above-described invention, a steel sheet subjected to aluminum plating is subjected to a hot press forming process or a cold forming process (hereinafter, simply referred to as post-heat treatment). Since the aluminum plating layer is present on the surface of the steel sheet, the steel sheet is not oxidized at the time of heating.

However, in the case of an aluminum-plated steel sheet, two or more materials may be welded to each other to form a single blank, such as TWB (Tailor Welded Blank), which is then used for hot press forming.

Since TWB technology uses two or more materials welded, it is advantageous to design different strength and shock absorbing ability for each part by different material characteristics.

However, when the aluminum-plated steel sheet is welded, the aluminum present in the plating layer is segregated in the welded portion, so that the aluminum content of the welded portion can be greatly increased. When the content of aluminum is increased as described above, martensite structure may not be obtained even if the blank (or a part) is quenched by the press during hot press forming. If the martensite structure is not sufficiently obtained, the strength of the welded portion may be reduced compared to the peripheral portion, and the welded portion may be broken.

In order to prevent this, there has been proposed a method of welding after removing the aluminum plating layer before welding. However, when this method is used, there is a problem that the manufacturing cost is increased due to an increase in the number of processes.

According to one aspect of the present invention, there is provided an iron-aluminum alloy-plated steel sheet for hot press forming, in which a welded portion can have high strength when welding two materials and performing hot press forming without a special additional process, and hot- Can be provided.

That is, the iron-aluminum alloy plated steel sheet provided in the present invention means a material for a blank used for welding, and means an iron-aluminum plated steel sheet capable of having a high weld strength after hot press forming.

The object of the present invention is not limited to the above description. Those skilled in the art will appreciate that there is no difficulty in understanding the additional objects of the present invention from those described in the specification of the present invention.

An iron-aluminum alloy-plated steel sheet according to one aspect of the present invention is an iron-aluminum alloy-plated steel sheet used for hot press forming, comprising a base steel sheet; And an Fe-Al alloy plating layer formed on the surface of the base steel sheet, wherein an Al content in a range from a depth of 1 탆 at the surface of the plating layer to a depth at which an Al content is 3% Standard deviation ratio may be less than 2%.

[Equation 1]

는 i 지점의 실제 Al 농도를

는 도금층 내 깊이에 따른 Al 함량의 관계를 근사한 1차식에 의하여 계산되는 i 지점의 Al 농도를, 그리고 n은 전체 데이터의 개수를 의미한다.)(However, here

The actual Al concentration at i

Represents the Al concentration at the i-point calculated by a linear equation approximating the relationship of the Al content with the depth in the plating layer, and n denotes the total number of data).

In another aspect of the present invention, there is provided a method of producing an iron-aluminum alloy-plated steel sheet, comprising: a step of obtaining an aluminum-plated steel sheet by aluminum plating and winding the surface of the steel sheet; Annealing the aluminum-plated steel sheet to obtain an iron-aluminum-alloy-plated steel sheet; And a step of cooling the iron-aluminum based alloy coated steel sheet, wherein the amount of aluminum plating is 5 to 40 g / m < ² > based on one side of the steel sheet and the winding tension 0.5 to 5 kg / mm < ^{2 &} gt ;, and the annealing is carried out in a heating furnace at 650 to 800 DEG C for 1 to 100 hours, preferably 5 to 30 hours, The average heating rate is 1 to 100 ° C / h and the average heating rate in the range of 400 to 500 ° C is set to 1 to 15 ° C / h, and the difference between the atmospheric temperature and the steel sheet temperature Lt; RTI ID = 0.0 > 80 C < / RTI >

Another aspect of the present invention is a hot press formed part obtained by hot press forming an iron-aluminum alloy plated steel sheet of the present invention and measuring the hardness from a center of the weld to a point spaced 15 mm in both directions The standard deviation of the hardness can be 15 or less.

As described above, the iron-aluminum alloy-plated steel sheet of the present invention is not only limited to a certain level of the aluminum content in the plating layer as a whole but also has a form in which the aluminum content decreases toward the interface of the base steel sheet. It is unlikely that aluminum will get into the welds. As a result, the aluminum content of the welded portion can be limited to a certain level or less, and the martensite ratio in the welded portion can be increased, so that the strength reduction of the welded portion can be effectively prevented.

FIG. 1 is a graph showing the results of GDS analysis of changes in contents of Al and Fe in the thickness (depth) direction from the surface of the plated layer of the aluminum-plated steel sheet produced in Comparative Example 1. FIG.
FIG. 2 is a graph showing the results of GDS analysis of changes in the content of Al and Fe in the thickness (depth) direction from the surface of the coating layer of the iron-aluminum alloy-plated steel sheet produced in Inventive Example 1. FIG.
Fig. 3 is a result of mapping a component to an Al component around a welded portion obtained by hot-pressing the blank after obtaining a blank by laser welding the iron-aluminum alloy-plated steel sheet produced in Inventive Example 1 .
FIG. 4 is a graph showing a change in hardness around a welded portion obtained by laser-welding a steel sheet of an iron-aluminum alloy alloy produced by Inventive Example 1 to obtain a blank and then hot-pressing the blank.
Fig. 5 is a result of mapping the Al component around the welded portion obtained by hot-pressing the blank after obtaining a blank by laser welding the aluminum alloy plated steel sheet produced in Comparative Example 1. Fig.
6 is a graph showing a change in hardness around a welded portion obtained by laser-welding an aluminum alloy plated steel sheet produced by Comparative Example 1 to obtain a blank, and then hot-pressing the blank.
7 is a graph showing the results of GDS analysis of changes in the content of Al and Fe in the thickness (depth) direction from the surface of the plating layer of the iron-aluminum plated steel sheet produced in Comparative Example 2. FIG.
8 is a graph showing changes in hardness around a welded portion obtained by laser-welding a steel sheet coated with iron-aluminum alloy produced in Comparative Example 2 to obtain a blank and then hot-pressing the blank.

Hereinafter, the present invention will be described in detail.

In the present invention, it is to be noted that each element refers to the content by weight unless otherwise specified.

The inventors of the present invention paid attention to the fact that preventing the aluminum plating layer present on the surface of the steel sheet from being mixed into the welded portion is effective for securing the strength of the welded portion in order to prevent oxidation of the steel sheet during hot press forming, And to propose an effective method.

That is, the coated steel sheet according to one embodiment of the present invention is an iron-aluminum alloy coated steel sheet used for hot press forming, which comprises a base steel sheet and an Fe-Al alloy plating layer formed on the surface of the base steel sheet, The Al content is monotonously decreased in the direction of the thickness direction.

As shown in FIG. 1, the aluminum-coated steel sheet generally has no particular change in the aluminum content from the surface to a certain depth, and the aluminum content is greatly reduced due to the diffusion of iron near the interface between the steel sheet and the plating layer It shows the content profile of the form.

However, in one embodiment of the present invention, the plating layer is an Fe-Al alloy plating layer mainly composed of iron and aluminum. As shown in FIG. 2, the aluminum content is the highest on the surface of the plating layer, This shows that monotonous tendency to decrease. As the aluminum content approaches the steel sheet, the aluminum content decreases, so that the space for incorporating aluminum into the weld is reduced during welding, and even if a part of the aluminum is mixed, the strength of the weld portion is not seriously affected.

In the present invention, monotonic reduction means that data representing a GDS (Glow Discharge Spectrometer) profile is represented by a linear equation of position and Al content (where y = ax + b; y is a position and x is an Al content) 1 means that the standard deviation ratio is 2% or less.

는 i 지점의 실제 Al 농도를

는 도금층 내 깊이에 따른 Al 함량의 관계를 근사한 1차식에 의하여 계산되는 i 지점의 Al 농도를, 그리고 n은 전체 데이터의 개수를 의미한다. 1차식은 도금층의 표면에서부터 Al 함량이 3%인 깊이까지의 범위의 Al 농도와 깊이의 관계로부터 구해지는 것으로서, 통상 사용되는 다양한 분석 방법에 의해 구할 수 있으며, 한가지 제한적이지 않은 예를 들면 최소자승법(least square method)을 이용하여 구할 수 있다. 1차식은 y=ax+b의 관계를 가질 수 있다(y: 깊이, x: Al 함량, a: 계수, b: 상수).However,

The actual Al concentration at i

Represents the Al concentration at the i-point calculated by a first-order approximation of the relationship of the Al content with the depth in the plating layer, and n denotes the total number of data. The first equation is obtained from the relationship between the Al concentration and the depth in the range from the surface of the plating layer to the depth of 3% of the Al content, and can be obtained by various analytical methods that are commonly used. For example, (least square method). The first order equation can have the relationship y = ax + b (y: depth, x: Al content, a: coefficient, b: constant).

However, according to one embodiment of the present invention, the Al content within 1 占 퐉 from the outermost surface can be obtained by the following equation Can be excluded when calculating the standard deviation ratio of Equation (1). Therefore, according to one embodiment of the present invention, the standard deviation ratio of the formula (1) can be obtained from a region from a depth of 1 m from the outermost surface to a point where the Al content is 3 wt%.

According to one embodiment of the present invention, the maximum value of the Al content in the GDS analysis profile can be less than 40% by weight. By controlling the maximum Al content to be 40 wt% or less, the content of Al incorporated into the welded portion can be further restricted, and as a result, the strength of the welded portion can be prevented from being lowered during hot press forming. Here, the maximum content of Al means the content in the plating layer excluding the surface oxide layer.

According to another embodiment of the present invention, the thickness of the plating layer of the aluminum alloy plated steel sheet can be set to 40 탆 or less. By limiting the thickness of the plating layer to a predetermined value or less, it is possible to prevent the aluminum from being mixed into the welded portion, and as a result, the strength of the welded portion can be prevented from dropping. The thickness of the plating layer may be 30 占 퐉 or less, and in some cases, it may be 25 占 퐉 or less. Further, in order to prevent oxidation of the steel sheet during hot press forming, the thickness of the plating layer may be 3 탆 or more in some embodiments of the present invention, and may be 5 탆 or more or 7 탆 or more in some cases.

In one embodiment of the present invention, the total content of Fe and Al may be 90% by weight or more. Since the plating layer may be composed entirely of Fe and Al, the upper limit of the total content of Fe and Al is determined to be 100% . Further, the alloy plating layer of the present invention contains Fe and Al as main components, and may further contain components such as Mn, Si, Zn, and Mg, if necessary. Here, Mn may serve to improve the corrosion resistance, and in one embodiment of the present invention may be included in the range of 0.01 to 10%. In addition, Si can serve to uniformize the alloying with Fe in the plating layer, and in one embodiment of the present invention, it can be included in the range of 0.01 to 2%. Mg may serve to improve the corrosion resistance of the plating layer, which may be included in the range of 0.01 to 10% in one embodiment of the present invention. Zn may serve to improve the corrosion resistance of the plating layer, which may be included in the range of 0.01 to 30% in one embodiment of the present invention.

When the hardness of a part obtained by hot-pressing a blank obtained by welding two or more pieces of the hot-rolled steel-plated steel sheet for hot press forming including the above-described plating layer to a point 15 mm away from the center of the weld is measured, Can be 15 or less. Unlike the prior art in which the aluminum plating layer near the welded joint is removed and welded in order to secure the strength of the welded part of the part, the steel sheet produced by one embodiment of the present invention is formed by removing the plated layer of the welded part An aluminum alloy plating layer may be present on the base material adjacent to the welded portion of the component manufactured according to one embodiment of the present invention, so that the corrosion resistance of the component can be further secured. In this case, the hot press forming method for obtaining the component can be a conventional hot press forming method, and the method is not particularly limited, but the blank is heated to 800 to 1000 DEG C and maintained for 4 to 10 minutes, And cooling it at a cooling rate of 25 DEG C / s or higher while pressing and molding it immediately.

As a result, the difference in tensile strength between the hot press formed part manufactured according to one embodiment of the present invention and the plated steel sheet used in the hot press forming can be more than 500 MPa (the difference in tensile strength is caused by the difference between the material- It can be defined as the difference in tensile strength of a part obtained by heating at 930 DEG C in an atmospheric environment (total heating time of 5 minutes) and cooling at a cooling rate of 100 DEG C / s while molding in a press. At this time, The elapsed time from the start of the molding can be set to 10 seconds). In one embodiment of the present invention, the difference in tensile strength may be 700 MPa or more. The larger the difference in tensile strength is, the more advantageous is that the work of blanking or the like in the steel sheet manufacturing step or the processing of other steel sheet is advantageous because the strength of the final product is increased. Therefore, although it is not necessary to specifically determine the upper limit of the above-described difference in tensile strength, it is usually not higher than 1500 MPa, so that if the upper limit is set to 1500 MPa,

The base steel sheet included in the coated steel sheet of the present invention is not particularly limited as long as it is used for hot press forming. However, the base steel sheet having the following composition can be used for one non-limiting example.

According to one embodiment of the present invention, the coated steel sheet of the present invention contains 0.04 to 0.5% of C, 0.01 to 2% of Si, 0.01 to 10% of Mn, 0.001 To 1.0%, P: not more than 0.05%, S: not more than 0.02%, and N: not more than 0.02%.

C: 0.04 to 0.5%

The C may be added in an appropriate amount as an indispensable element to raise the strength of the heat treatment member. That is, in order to ensure sufficient strength of the heat treatment member, C may be added in an amount of 0.04% or more. In one embodiment, the lower limit of the C content may be 0.1%. However, if the content is too high, the cold rolled steel sheet may have a too high thermal strength when cold rolling the hot rolled sheet, resulting in a significant reduction in cold rolling property and a marked reduction in spot weldability. It may be added at 0.5% or less in order to secure weldability. The content of C may be limited to 0.45% or less and 0.4% or less.

Si: 0.01 to 2%

The Si not only needs to be added as a deoxidizing agent in steelmaking but also suppresses the generation of carbides which have the greatest influence on the strength of the hot press formed member and also suppresses the formation of carbides in martensite lath after formation of martensite in hot press forming May be added in an amount of 0.01% or more in order to concentrate carbon to secure retained austenite. Further, the upper limit of the Si content can be set to 2% in order to ensure sufficient plating ability when aluminum plating is performed on the steel sheet after rolling. In one embodiment of the present invention, the Si content may be limited to 1.5% or less.

Mn: 0.01 to 10%

The Mn can be added in an amount of not less than 0.01% to lower the critical cooling rate for securing martensite in the hot pressed member as well as to secure the solid solution strengthening effect. Further, the Mn content can be set to 10% or less from the viewpoint of ensuring workability of the hot press forming process by appropriately maintaining the strength of the steel sheet, reducing the manufacturing cost, and improving the spot weldability. In one embodiment , It may be 9% or less, or 8% or less.

Al: 0.001 to 1.0%

Al may be added at a content of 0.001% or more because it can deoxidize the steel in addition to Si to improve the cleanliness of the steel. In order to prevent the Ac3 temperature from becoming too high, the content of Al may be set to 1.0% or less so that the heating required for the hot press forming can be performed in an appropriate temperature range.

P: not more than 0.05%

The P exists as an impurity in the steel, and the lower the content, the more advantageous it is. Thus, in one embodiment of the invention, P may be included in an amount of up to 0.05%. In another embodiment of the present invention, P may be limited to 0.03% or less. The lower the value of P, the more favorable the impurity element is, so the upper limit of the content is not particularly required. However, in order to excessively lower the P content, there is a fear that the manufacturing cost may rise. If this is considered, the lower limit may be set to 0.001%.

S: not more than 0.02%

The S content is an impurity in the steel and inhibits the ductility, impact properties and weldability of the member, so the maximum content is 0.02% (preferably 0.01% or less). In addition, if the minimum content is less than 0.0001%, the production cost may increase, and therefore, in one embodiment of the present invention, the lower limit of the content can be 0.0001%.

N: 0.02% or less

The N is an element contained in the steel as an impurity. It is advantageous to decrease the sensitivity to cracking at the time of continuous casting of the slab, and to ensure the impact property, the lower the content, the lower the content of N is 0.02% or less. Although it is necessary to set the lower limit specifically, it is also possible to set the N content to 0.001% or more in one embodiment in consideration of an increase in manufacturing cost and the like.

In the present invention, 0.01 to 4.0% of at least one selected from the group consisting of Cr, Mo and W, and at least one of Ti, Nb, Zr, At least one of Cu + Ni: 0.005 to 2.0%, Sb + Sn: 0.001 to 1.0% and B: 0.0001 to 0.01% may be further added.

At least one selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%

The Cr, Mo and W can secure hardenability and grain refinement through the improvement of the hardenability and the precipitation strengthening effect, so that at least one of them can be added in an amount of 0.01% or more based on the total amount. In order to secure the weldability of the member, the content thereof may be limited to 4.0% or less. Further, if the content of these elements exceeds 4.0%, the further increase in effect is insignificant. Therefore, if the content is limited to 4.0% or less, the increase in cost due to the addition of additional elements may be prevented.

Ti, Nb, Zr and V: 0.001 to 0.4%

Since Ti, Nb and V are effective in improving the steel sheet of the heat treated member due to the formation of fine precipitates and stabilizing retained austenite by improving grain refinement and improving impact toughness, at least one of these contents can be added in an amount of 0.001% or more have. However, if the addition amount exceeds 0.4%, the effect is saturated and excessive iron alloy addition may increase the cost.

Cu + Ni: 0.005 to 2.0%

The Cu and Ni are elements that form fine precipitates and improve the strength. 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 excessive cost increases, so the upper limit is set to 2.0%.

Sb + Sn: 0.001 to 1.0%

The Sb and Sn can be concentrated on the surface during the annealing heat treatment for Al-Si plating to suppress the formation of Si or Mn oxide on the surface, and the plating ability can be improved. In order to obtain such effect, 0.001% or more can be added. However, when the addition amount exceeds 1.0%, the upper limit is set to 1.0% since it is possible to cause not only the excessive amount of the iron alloy but also the edge of the steel during the hot rolling by being incorporated in the slab grain boundary.

B: 0.0001 to 0.01%

The B is an element capable of improving the hardenability even with a small amount of addition, and being capable of restraining the brittleness of the hot pressed member due to grain boundary segregation of P and / or S segregated in the old austenite grain boundary system. Therefore, B can be added in an amount of 0.0001% or more. However, when it exceeds 0.01%, the effect is saturated and brittleness is caused in hot rolling so that the upper limit can be set to 0.01%, and in one embodiment, the B content can be set to 0.005% or less.

The remainder other than the above-mentioned components include iron and unavoidable impurities, and the addition is not particularly limited as long as it can be included in the steel sheet for hot forming.

Hereinafter, an example of a method of manufacturing a hot-rolled steel sheet according to one aspect of the present invention will be described. However, it should be noted that the method for producing a hot-press-forming steel sheet described below does not necessarily mean that the steel sheet for hot-press forming according to the present invention is necessarily manufactured by the present manufacturing method, It should be noted that there is no problem in using it to implement each implementation of the present invention.

The steel sheet of the present invention can be obtained by using hot-rolled or cold-rolled base steel sheet and subjecting the surface of the base steel sheet to molten aluminum plating and annealing the coated steel sheet.

[Aluminum plating process]

In one embodiment of the present invention, a process for preparing a base steel sheet, coating the surface of the base steel sheet with aluminum under appropriate conditions, and winding the same to obtain an aluminum-plated steel sheet (coil).

One side  5 to 40 g / m ² The surface of the base steel sheet was plated with aluminum

The surface of the rolled steel sheet can be subjected to an aluminum plating treatment. Aluminum plating is an AlSi plating usually called type I (including more than 80% of Al and 5 to 20% of Si, which may include additional elements as needed), but it contains more than 90% of Al called type II , All of the plating including the additional element may be used. Molten aluminum plating may be performed to form a plated layer, and the steel sheet may be annealed before plating. Proper coating weight during the plating is 5 ~ 40g / m ² in one side reference. If the amount of plating is too large, it may take too much time to alloy the surface, and if the amount of plating is too small, it is difficult to obtain sufficient corrosion resistance. The coating weight is preferably may be 5 ~ 29g / m ^2, may be more preferably 10 ~ 25g / m ²

After plating Coiling  Tension is 0.5 ~ 5 kg / mm ²  As

When the coil is obtained by winding the steel sheet after plating, the winding tension of the coil can be adjusted. Alloying behavior and surface quality of the coils may be changed during the subsequent annealing treatment by adjusting the winding tension of the coils.

[Annealing treatment process]

The aluminum-plated steel sheet is subjected to the annealing treatment under the following conditions to obtain an aluminum alloy-plated steel sheet by the above-described procedure.

Appeal On the road  650 ~ 800 ℃ for 1 to 50 hours

The aluminum coated steel (coil) is heated in a batch annealing furnace. When the steel sheet is heated, the target temperature for the heat treatment and the holding time are in the range of 650 to 800 ° C (in the present invention, the maximum temperature reached by the material in this temperature range is referred to as the heating temperature) It is preferable to keep the time. Here, the term "holding time" refers to a period of time from the time when the coil temperature reaches the target temperature to the start of cooling. In one embodiment of the present invention, if the alloying is not sufficiently performed, the plating layer may be peeled off during roll leveling, so that the heating temperature can be set to 550 ° C or more for sufficient alloying. In addition, the heating temperature may be set to 800 DEG C or less in order to prevent excessive formation of oxides on the surface layer and to secure the spot weldability. Further, in order to sufficiently secure the plating layer and prevent deterioration of the productivity, the holding time may be set to 1 hour to 100 hours, preferably 5 hours to 30 hours. In one embodiment of the present invention, the temperature of the steel sheet may have a heating pattern in which the temperature continuously rises without cooling until the heating temperature is reached.

Average Heating  The heating is performed at a heating rate of 5 to 100 ° C / h

When the steel sheet is heated at the above-mentioned heating temperature, in order to ensure sufficient productivity and uniformly alloy the plating layer in the steel sheet (coil), it is necessary to heat the entire steel sheet (coil) temperature range The average temperature raising rate can be set to 5 to 100 ° C / h. In addition, the overall average temperature raising rate can be controlled within the above-described numerical range. However, in one embodiment of the present invention, the temperature raising rate of a specific temperature section is also controlled as described later, thereby achieving the object of the present invention. According to one embodiment of the present invention, the lower limit of the average temperature raising rate can be set at 5 캜 / h, more preferably at 10 캜 / h. In addition, the upper limit of the average heating rate can be set to 80 ° C / h, more preferably 60 ° C / h.

Upon warming  Average of 400 ~ 500 ℃ Heating  Heat at a rate of 1 ~ 15 ℃ / h

In one embodiment of the present invention, in order to ensure sufficient productivity while preventing rolling oil from remaining in the temperature range at which the rolling oil is vaporized during rolling and causing surface unevenness, an average heating rate in a range of 400 to 500 ° C 1 to 15 ° C / h.

Average of 600 ~ 650 ℃ Heating  The heating rate is 1 to 30 ° C / h

In one embodiment of the present invention, the average heating rate in the range of 600 to 650 占 폚 may be set to 1 占 폚 / h or more in order to improve the productivity. Further, when the temperature raising rate is too high, uniform alloying can be achieved, so that the average temperature raising rate in the section can be made 30 deg. C / h or less.

My apology  Atmosphere temperature and steel plate Between temperature  Difference between 5 and 80 ° C

In general, the heating of the steel plate is performed by heating the steel plate (coil) by raising the atmospheric temperature in the annealing furnace rather than directly heating the steel plate (coil). In this case, the difference between the atmospheric temperature and the coil temperature can not be avoided. However, in order to minimize the quality variation of the material and plating quality at each position in the steel sheet, the difference between the atmospheric temperature and the steel sheet temperature have. Ideally, the temperature difference should be as small as possible, but it may be difficult to meet the overall average rate of temperature rise by slowing the rate of temperature rise, so it can be above 5 ° C if this is taken into account. Here, the temperature of the steel sheet means the temperature of the bottom portion of the loaded steel (coil) (which means the lowest portion of the coil), and the atmospheric temperature means the temperature measured at the center of the inner space of the furnace .

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate and specify the present invention and not to limit the scope of the present invention. Since the scope of the present invention is determined by the matters set forth in the claims and the matters reasonably deduced therefrom.

(Example)

Honor  One

A cold-rolled steel sheet for hot press forming having the composition shown in the following Table 1 was prepared. The surface of the steel sheet was plated with a type I plating bath having an Al-8% Si-2% Fe composition on the surface of the steel sheet. The amount of plating at the time of plating was adjusted to 20 g / m ² per side, and the coil was wound by regulating the winding tension after plating to 2.2 kg / mm ² .

element C Si Mn Al P S N Additional element content(%) 0.21 0.2 1.2 0.03 0.01 0.003 0.004 Ti 0.03, B 0.003, Cr 0.2

The plated steel sheet was heated to 730 DEG C in a furnace under the following conditions.

Total average heating rate up to 730 캜: 20 캜 / h

Average temperature raising rate of temperature range from 400 to 500 ° C: 10 ° C / h

600 ° C to 650 ° C Average temperature raising rate in the temperature section: 10 ° C / h

Temperature difference between atmosphere and coil at heating temperature: 30 ℃

After heating, the steel sheet was held at the same temperature for 10 hours, and then the steel sheet was cooled to an average cooling rate of 40 ° C / h up to 500 ° C and then cooled to 100 ° C at an average cooling rate of 55 ° C / h to obtain a hot- . The tensile strength of the obtained coated steel sheet was 480 MPa.

As a result of analyzing the plated layer of the steel sheet by the GDS analyzer, it was possible to obtain the component profile of the shape as shown in FIG. 2, and the Al content was monotonously decreased from the surface of the plated layer to the boundary with the steel sheet. The standard deviation ratio was 0.3%. The linear equation can be obtained by approximating the Al content data according to the depth in the plating layer up to the point where the Al content is 3% except for the data from the surface to the depth of 1 μm by the least squares method. It was confirmed that the total thickness of the plated layer was about 17 탆. As can be seen from the figure, the maximum Al content in the plated layer was about 21%. It was also confirmed that the average of the total content of Fe and Al in the entire plating layer from the surface to the depth of 1 탆 and the Al content to the depth of 3% was 95.8% by weight.

As described above, the steel plate manufactured by the above method was butted and laser welding was performed. In general, in the case of welding an aluminum-plated steel sheet, an operation of removing the aluminum layer on the surface of the steel sheet (referred to as ablation or alliation) is performed to prevent aluminum from being mixed into the welded portion. The two steel plates were butt welded immediately.

The laser-welded steel sheet was heated to 930 캜 under the following conditions and press-molded.

Atmosphere during heating: Atmosphere

Total heating time: 5 minutes

Air cooling time: 10 seconds (take out from the press and start press molding

Press cooling rate: 100 ° C / sec

FIG. 3 shows the result of mapping the component to the Al component at the cross section of the welded part using the EPMA. As can be seen in the figure, almost no Al segregated in the weld was observed. Also, the base material and the welded part (the center of the welded part in the drawing is 15 mm). And the Vickers hardness was measured at intervals of 0.2 mm with a load of 500 g. As can be seen from the figure, the standard deviation of the 30 hardness values was 8.2, indicating a uniform hardness distribution. The tensile strength of the obtained part was 1596 MPa.

Comparative Example  One

The aluminum-plated steel sheet was plated in the same manner as in the case of the above-described example except for the amount of plating, but only plated and not heated and cooled after winding. In this case, the amount of plating at the time of plating was adjusted to 80 g / m ² on one side, and the plating layer of the steel sheet was analyzed with a GDS analyzer. As a result, a component profile of the shape shown in FIG. 1 was obtained. The tensile strength of the obtained coated steel sheet was 534 MPa.

In addition, the results of EPMA mapping for the Al component after laser welding and hot pressing without removing the plating layer of the welded portion as in the case of the inventive example are shown in Fig. As can be seen in the figure, a large number of Al segregations were observed in the welds. The Vickers hardness was measured in the same manner as in Inventive Example 1 along the base material and welded portion, and the results are shown in FIG. As can be seen from the figure, the standard deviation of 30 hardness values was 33.1, which showed a nonuniform hardness distribution. If the hardness is uneven, the load may concentrate in one place, causing problems such as premature rupture and the like, which is not suitable. The tensile strength of the obtained part was confirmed to be 1408 MPa.

Comparative Example  2

The coated steel sheet obtained in the same manner as in the above-described example was heated to 690 캜 under the following conditions in a furnace.

Total average heating rate up to 690 캜: 20 캜 / h

Average temperature raising rate of temperature range from 400 to 500 ° C: 10 ° C / h

600 ° C to 650 ° C Average temperature raising rate in the temperature section: 10 ° C / h

Temperature difference between atmosphere and coil at heating temperature: 30 ℃

After heating, the steel sheet was held at the same temperature for 50 minutes. After that, the steel sheet was cooled to an average cooling rate of 40 ° C / h up to 500 ° C and then cooled to 100 ° C at an average cooling rate of 55 ° C / h to obtain a hot- .

Analysis of the coating layer of the steel sheet using a GDS analyzer showed that the composition profile of the form as shown in FIG. 7 was obtained, and the standard deviation ratio according to Equation 1 was 2.9%.

In addition, the Vickers hardness was measured in the same manner as in Example 1, except that the plating layer of the welded portion was not removed, and the Vickers hardness was measured along the base material and the welded portion after laser welding and hot pressing. As can be seen from the figure, the standard deviation of 30 hardness values was 18.1, showing a nonuniform hardness distribution.

As described above, when the conditions of the plated layer of the hot-dip galvanized steel sheet are controlled as in the present invention, hardness is not lowered in the welded portion, and hot press formed parts having uniform physical properties can be obtained.

Therefore, the advantageous effects of the present invention can be confirmed.

Claims (12)

As an iron-aluminum alloy-plated steel sheet used for hot press forming,
Base steel sheet; And
And an Fe-Al alloy plating layer formed on the surface of the base steel sheet,
Wherein the standard deviation ratio of the Al content is 2% or less in a range from a depth of 1 탆 from the surface of the plating layer to a depth of 3% of the Al content according to a position expressed by the following formula (1).
[Equation 1]

(However, here

The actual Al concentration at i

Represents the Al concentration at the i-point calculated by a linear equation approximating the relationship of the Al content with the depth in the plating layer, and n denotes the total number of data).
2. The iron-aluminum alloy-plated steel sheet according to claim 1, wherein the thickness of the plating layer is 3 to 40 占 퐉.
The iron-aluminum alloy-plated steel sheet according to claim 1, wherein the maximum content of Al in the plating layer is 40 wt% or less.
4. The iron-aluminum alloy-plated steel sheet according to claim 3, wherein the total content of Fe and Al in the plating layer is 90% by weight or more.
The steel sheet according to claim 1, wherein the base steel sheet comprises 0.04 to 0.5% of C, 0.01 to 2% of Si, 0.01 to 10% of Mn, 0.001 to 1.0% of Al, 0.05% % Or less, N: 0.02% or less, the balance Fe and other unavoidable impurities.
The steel sheet according to claim 5, wherein the composition of the base steel sheet is 0.01 to 4.0% by weight of at least one selected from the group consisting of Cr, Mo and W, at least one member selected from the group consisting of Ti, Nb, Zr and V, Aluminum alloy-plated steel sheet having excellent plating adhesion, further comprising at least one of 0.001 to 0.4% of Cu, 0.005 to 2.0% of Cu, 0.001 to 1.0% of Sb + Sn and 0.0001 to 0.01% of B, .
The iron-aluminum alloy-plated steel sheet according to claim 5, wherein the iron-aluminum alloy-plated steel sheet is maintained at 930 캜 for 5 minutes, and then immediately pressed and molded at a cooling rate of 25 캜 / s.
Plating the surface of the base steel sheet with aluminum and winding to obtain an aluminum-plated steel sheet;
Annealing the aluminum-plated steel sheet to obtain an iron-aluminum-alloy-plated steel sheet; And
A method for producing an iron-aluminum alloy-plated steel sheet, comprising the steps of cooling an iron-aluminum-based alloy-coated steel sheet,
The aluminum plating amount is 5 to 40 g / m < ² > based on one side of the steel sheet,
The winding tension at the time of winding is 0.5 to 5 kg / mm ² ,
The above-mentioned annealing is carried out by keeping the furnace at a heating temperature of 650 to 800 ° C for 1 to 100 hours in a furnace,
The average heating rate is set to 1 to 100 ° C / h, the average heating rate in the range of 400 to 500 ° C is set to 1 to 15 ° C / h,
Wherein the difference between the atmospheric temperature and the steel sheet temperature in the furnace is 5 to 80 占 폚.
The method for producing an iron-aluminum alloy-plated steel sheet according to claim 8, wherein the annealing temperature is in the range of 5 to 20 占 폚 / h at an average heating rate of 600 to 650 占 폚.
The steel sheet according to claim 8, wherein the base steel sheet comprises 0.04 to 0.5% of C, 0.01 to 2% of Si, 0.01 to 10% of Mn, 0.001 to 1.0% of Al, 0.05% % Or less, N: 0.02% or less, the balance Fe and other unavoidable impurities.
The steel sheet according to claim 10, wherein the composition of the base steel sheet is 0.01 to 4.0% by weight of at least one selected from the group consisting of Cr, Mo and W, at least one of Ti, Nb, Zr and V, Aluminum alloy-plated steel sheet further comprising at least one of 0.001 to 0.4% of Cu, 0.005 to 2.0% of Cu + Ni, 0.001 to 1.0% of Sb + Sn and 0.0001 to 0.01% of B;
A hot-press formed part obtained by hot-pressing an iron-aluminum alloy-plated steel sheet according to any one of claims 1 to 7,
A hot press formed part having a standard deviation of hardness of 15 or less when measuring the hardness to a point 15 mm away from the center of the weld.

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