KR20190078032A - Hot press formed part having improved resistance for corrosion and crack propagation and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a hot press formed member excellent in corrosion resistance and crack propagation resistance.
According to one aspect of the present invention, there is provided a hot press forming member comprising: a base steel sheet; And a zinc-iron-based alloy plating layer formed on the surface of the base steel sheet, wherein the zinc-iron-based alloy plating layer includes a soft first layer in contact with the base steel sheet and a hard second layer formed on the first layer .

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot press formed member having excellent corrosion resistance and crack propagation resistance,

The present invention relates to a hot press formed member excellent in corrosion resistance and crack propagation resistance.

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. 6,564,604 discloses a method of hot-pressing a galvanized steel sheet. Zinc has excellent sacrificial performance and has excellent corrosion resistance compared to aluminum plating.

However, when the galvanized steel sheet is hot-pressed, there is a problem that micro cracks are formed on the steel sheets of the parts to cause defects. In order to solve such a problem, there has been proposed a technique of extracting a blank from a heating furnace and then preventing cracks from occurring by previously cooling an area susceptible to micro cracks. However, this technique requires additional equipment, In addition, since the heating temperature is high, there is a problem that the oxidation amount of the surface of the plating layer is increased and a process of removing the surface oxide layer is required.

According to one aspect of the present invention, there is provided a hot press formed part and a method of manufacturing the same, which can have high corrosion resistance while suppressing generation of micro cracks without any additional operation.

The object of the present invention is not limited to the above description. It will be apparent to those skilled in the art that the present invention may be practiced without departing from the spirit and scope of the invention.

According to one aspect of the present invention, there is provided a hot press forming member comprising: a base steel sheet; And a zinc-iron-based alloy plating layer formed on the surface of the base steel sheet, wherein the zinc-iron-based alloy plating layer includes a soft first layer in contact with the base steel sheet and a hard second layer formed on the first layer .

According to another aspect of the present invention, there is provided a method of manufacturing a hot press formed member, comprising the steps of: preparing a galvanized steel sheet plated with a plated amount of 20 to 140 g / m ² per side of a base steel sheet; Heating the galvanized steel sheet; And heating the galvanized steel sheet while heating the galvanized steel sheet, wherein the maximum heating temperature is from Ac 3 + 10 ° C to 800 ° C, and from room temperature to 440 ° C the average temperature rise rate is 3.0 ~ 4.8 ℃ / sec, and an average temperature rising rate from 440 ℃ to 500 ℃ 1.0 ~ 3.9 ℃ / sec, the change in the rate of temperature rise at 500 ℃ to the target heating temperature of 0.023 ~ 0.050 ℃ / sec ² days .

As described above, the present invention forms a two-layered alloy plating layer of a hard second layer and a soft first layer on the surface of a hot press formed member, thereby reducing the depth of crack propagation to the inside of the steel sheet while maintaining excellent corrosion resistance It is possible to provide a hot press formed member having excellent crack propagation resistance.

FIG. 1 is a Zn composition mapping result obtained by analyzing a plating layer of a member manufactured by Inventive Example 1 with an EPMA analyzer. FIG.
2 is a Zn and Fe profile obtained by analyzing a member manufactured by Inventive Example 1 with a GDS analyzer.
3 is a result of analyzing the plating layer by a transmission electron microscope (TEM) in order to confirm the structure of the plating layer and the diffraction pattern of the member manufactured by the inventive example 1. Fig.
4 is a result of analysis using a SEM and a nano hardness tester to confirm the hardness of the coating layer of the member manufactured in Inventive Example 1. Fig.
Fig. 5 is a result of Zn component mapping obtained by analyzing the plating layer of the member manufactured in Inventive Example 2 with an EPMA analyzer.
6 is a Zn and Fe profile obtained by analyzing a member manufactured by Inventive Example 2 with a GDS analyzer.
FIG. 7 is a result of mapping Zn component by analyzing the plating layer of the member manufactured in Comparative Example 1 with an EPMA analyzer.
8 is a Zn and Fe profile obtained by analyzing a member manufactured by Comparative Example 1 with a GDS analyzer.
9 is a graph showing the Zn component mapping result of the plating layer of the member manufactured in Comparative Example 3 analyzed by an EPMA analyzer.
10 is a Zn and Fe profile obtained by analyzing a member manufactured by Comparative Example 3 with a GDS analyzer.
11 is a result of analyzing the plating layer by a transmission electron microscope (TEM) to confirm the structure of the plating layer and the diffraction pattern of the member manufactured by the comparative example 3. Fig.
12 is a scanning electron microscope photograph showing the hardness of the plating layer of the member manufactured in Comparative Example 3 and showing the hardness of the plating layer.
13 is a partial cross-sectional view of a photograph and a member observing the shape of the hot press formed member manufactured by the inventive and comparative examples of the present invention.

Hereinafter, the present invention will be described in detail.

Unless specifically stated otherwise in the present invention, the content units of the components of the steel sheet or plating layer are based on weight. Also, the percentages of crystals or tissues are based on area unless otherwise specified.

The inventors of the present invention have analyzed the cause of micro cracks in the hot press formed member manufactured from the galvanized steel sheet and found that micro cracks are generated in the steel strip of the member as a result of cracking from the outermost surface of the plated layer, Cracks are transferred, leading to the present invention.

The present invention is based on the knowledge of the inventors of the present invention as described above. In one embodiment of the present invention, the hot press forming member includes a base steel sheet and a zinc-iron alloy plating layer formed on the surface of the base steel sheet, Based alloy plated layer includes a soft first layer in contact with the base steel sheet and a hard second layer formed on the first layer.

That is, the plating layer of the present invention includes a soft first layer and a hard second layer, the physical properties of which are clearly distinguished. The hard second layer is the outermost layer of the plating layer, which is a portion directly contacting the metal mold during press forming. When the hard second layer comes into contact with the press mold on the surface, the hard second layer not only lubs and lubricates while increasing the number of cracks at the place where the shear stress concentrates, and disperses the load transferred from the press can do. Therefore, the depth of the crack formed in the hot press-formed member can be reduced. However, this does not mean that the hard second layer functions as the outermost layer in the plating layer, and excludes various layers other than the plating layer which can be formed on the surface of the oxide layer, the coating layer, and other hot press forming members.

The soft first layer existing below the hard second layer, that is, the soft first layer present on the hard-coated steel sheet, effectively stops the load or crack propagating through the second layer even if it propagates. Therefore, the hot press forming member of the present invention having such a layer structure has excellent characteristics of stopping crack propagation.

According to one embodiment of the present invention, the hard second layer may have a hardness of at least 150 Hv. That is, in order to achieve sufficient load distribution and lubrication, the hardness of the hard second layer should be at least 150, and even if the hardness is high, there is no particular problem, but considering the hardness range of a typical zinc- Can be set to 500 Hv.

Further, the soft second layer may have a hardness of less than 150 Hv, preferably not more than 140 Hv. It is not necessary to specifically determine the lower limit of the hardness of the soft second layer, but the lower limit of the hardness of the soft second layer can be set to 50 Hv or less in consideration of the hardness range of the ordinary zinc-iron alloy.

In one embodiment of the present invention, when the plated steel sheet is cut in the thickness direction, the hard first layer and the hard second layer may be divided into layers in an optical microscope or an electron microscope image, And may be a hardness measured at the central portion in the thickness direction of the corresponding region among the divided layers.

According to one embodiment of the present invention, the hard second layer may be a plating layer having a Zn content of 40% or more, and the soft first layer may be a plating layer having a Zn content of less than 40%. According to one embodiment of the present invention, the upper limit of the Zn content of the hard second layer can be set at 95%. The lower limit of the Zn content of the soft first layer may be set at 5%. The Zn content of these layers refers to the content of the central portion in the thickness direction of each layer observed by a scanning electron microscope when measured by EDS.

In addition, the plating layer of the hot-press-molded member of the present invention may include crystals of BCC (body center cubic lattice) structure as a whole in an area ratio of 90% or more. When the plated layer contains 90% or more of the body-centered cubic lattice, the resistance to hydrogen retardation can be improved when heated for hot press forming.

According to one embodiment of the present invention, the hard second layer may have a thickness of 10% or more of the thickness of the entire coating layer. When the thickness of the hard second layer is 10% or more of the thickness of the entire coating layer, Can be effectively prevented. The thickness ratio of the hard second layer to the total plating layer thickness may be preferably at least 15%, more preferably at least 20%. However, when the thickness of the hard second layer is too large, the thickness of the soft first layer is relatively small, so that the plating adhesion is reduced and the crack can not be effectively prevented from being transferred to the steel sheet. The thickness ratio of the second layer may be set to 90% or less, preferably 80% or less.

Also, according to one embodiment of the present invention, the hard second layer may contain at least 90% of the area of the gamma gamma (?) Phase. The capital gamma phase is characterized by a BCC structure, the hardness satisfying the range according to one embodiment of the present invention, and the zinc content being relatively high. That is, when the capping gamma phase is present as the second layer, it has a proper hardness value and serves to disperse the load and lubricate it as described above to stop the propagation of the cracks. In addition, due to the relatively high Zn content, It can be made to have sacrifice performance. The upper limit of the ratio of the capital gamma phase in the hard second layer is not particularly defined, and may be up to 100%.

The soft first layer may contain 90% or more of the? -Fe phase in an area fraction. The α-Fe phase is a phase of the BCC structure, the hardness range satisfies the hardness range according to one embodiment of the present invention, and the adhesion between the α-Fe phase and the base steel sheet is excellent so that the plating layer is not easily peeled off from the base steel sheet, . The upper limit of the ratio of the? -Fe phase in the soft first layer is not particularly defined, and may be up to 100%.

In one embodiment of the present invention, the sum of the Fe and Zn contents in the hard second layer and the soft first layer may be at least 90%. The upper limit of the total content of Fe and Zn is not particularly limited and may be 100%.

In the present invention, the kind of the base steel sheet is not particularly limited as long as it can be used as the base steel sheet of the hot press formed member. However, in one embodiment of the present invention, it is possible to use 3.1 to 15% by weight of Mn as the base steel sheet.

That is, Mn not only secures the solid solution strengthening effect but also serves to reduce the austenitization temperature for securing martensite in the hot pressed member. One of the factors for forming the plated layer of the two-layer structure according to one embodiment of the present invention is the heating temperature at the time of hot press forming. It is preferable that the heating temperature during hot press forming is 800 占 폚 or lower . In addition, since the strength of the steel sheet can be appropriately maintained to ensure the workability of the hot press forming process, the manufacturing cost can be reduced, the spot weldability can be improved, and martensite can be produced in an appropriate amount, The content can be less than or equal to 15%, and in one embodiment of the present invention, less than or equal to 12%, or less than or equal to 10%.

In one embodiment of the present invention, the base steel sheet contains 0.04 to 0.3% of C, 0.01 to 2% of Si, 0.001 to 1.0% of Al, not more than 0.05% of P, 0.02% or less, and N: 0.02% or less. Hereinafter, the reasons for limiting the content of each element will be briefly described.

C: 0.04 to 0.3%

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. 0.3% or less may be added to secure weldability. The content of C may be limited to 0.25% or less and 0.2% 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, when zinc or zinc alloy plating is applied to the steel sheet after rolling, the upper limit of the Si content can be set to 2% in order to ensure sufficient plating ability. In one embodiment of the present invention, the Si content may be limited to 1.5% 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 element which inhibits ductility, impact properties and weldability of the member as impurities in steel, so that the maximum content can be 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 an effect, the total content of these elements may be added so as to be 0.001% or more. However, if the total content exceeds 1.0%, the upper limit is set to 1.0% since it is possible to cause not only an excessive amount of the iron alloy but also a crack at the edge of the coil 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. Further, the addition is not particularly limited as long as it is a component that can be included in the steel sheet for hot forming.

In the present invention, the base steel sheet refers to a steel region of a hot press-formed member manufactured in a state of parts through hot press forming, and does not mean only a steel sheet in a plate form, It means. In one embodiment of the present invention, the hot press formed member including the base steel sheet may have a tensile strength of 1000 MPa or more, preferably 1,200 MPa or more, and more preferably 1,400 MPa or more.

The method for producing the zinc or zinc alloy plated steel sheet of the present invention is not particularly limited as long as the zinc or zinc alloy plated steel sheet of the above-described conditions can be produced. However, one non-limiting example is as follows.

Hereinafter, one example of a method of manufacturing a hot press formed member according to one aspect of the present invention will be described. However, the method of manufacturing a hot-press-molded member described below is not necessarily an example in which the hot-pressed member of the present invention is necessarily manufactured by the present manufacturing method, and any manufacturing method may be applied to the hot- It should be noted that there is no problem in using it to implement each embodiment of the present invention.

That is, the hot press formed member of the present invention can be manufactured by preparing a galvanized steel sheet subjected to galvanization, and then subjecting the galvanized steel sheet to hot press forming. Specific conditions will be described below.

[Preparation of galvanized steel sheet]

The galvanized steel sheet to be used in the hot pressing step of the present invention is not necessarily limited to this, but it can be prepared to satisfy the following conditions.

20 ~ 140g / m per side of base steel sheet ² Plated galvanized steel sheet with plating amount

It is necessary to prepare a galvanized steel sheet to form a plating layer having the above-described conditions on the hot press-formed member. In one embodiment of the present invention, a galvanized (GI) plating or a galvanizing bath in which a base steel sheet is immersed in a galvanizing bath and immersed in a galvanized steel sheet, A galvanized (GA) plated steel sheet, or a plated steel sheet obtained by galvanically electroplating the surface of a base steel sheet can be used. In the galvanized plating or galvanized plating, the plating bath may further include a small amount of additive component generally used in the technical field of the present invention, for example, Al, etc. As a conventional art, There will be no difficulty in identifying the type and content of the product. In addition, in the case of zinc-based electroplating, an alloy plating layer such as Zn-Fe or Zn-Ni system can be used in addition to pure Zn. According to one embodiment of the present invention, the plating amount attached to the base steel sheet may be 20 to 140 g / m ² on one side. In order to form a hard second layer on the surface by inducing sufficient alloying during heating for hot press forming and to form a liquid metal embrittlement (LME) with unalloyed liquid Zn, the plating amount is set to 140 g / m ² or less And the plating amount can be set to 20 g / m ² or more in order to secure sufficient corrosion resistance. The plating amount may be preferably 30 to 120 g / m ² , and more preferably 40 to 100 g / m ² .

[Heating process]

In one embodiment of the present invention, a process for heating the prepared coated steel sheet to an appropriate temperature range and heating conditions is carried out in order to press-form the prepared steel sheet under appropriate conditions.

The maximum heating temperature is set above Ac3 + 10 ℃ and below 800 ℃

In order to ensure the strength of the steel sheet, it is necessary to austenitize the steel sheet so that it can be transformed into martensite upon cooling. Taking this into consideration, the maximum heating temperature can be set to Ac 3 + 10 ° C or higher. Further, in order to secure the desired plating layer structure in the present invention, and to prevent a large amount of oxide from being formed on the surface of the member to deteriorate the spot weldability, the target heating temperature may be set to 800 DEG C or less. At this time, in one embodiment of the present invention, the temperature raising rate for each temperature interval is adjusted as follows to obtain the advantageous effect of the present invention.

The average heating rate from room temperature to 440 ℃ is set to 3.0 ~ 4.8 ℃ / sec.

In heating the coated steel sheet, the average heating rate can be limited up to 440 캜 from room temperature to ensure excellent corrosion resistance and crack propagation resistance, which is an object of the present invention. The above average heating rate can be performed to make the structure of the plating layer two layers. If the average heating rate is less than 3.0 ° C / sec, it is difficult to secure productivity in the hot forming process. If the average heating rate is less than 3.0 ° C / It is difficult to secure it continuously.

The average heating rate from 440 ° C to 500 ° C is set to 1.0 to 3.9 ° C / second

Blanks heated to 440 ° C may limit the average rate of temperature rise to 500 ° C. If the average temperature raising rate is less than 1.0 캜 / sec, it is advantageous to secure the desired plating layer structure in the present invention, but productivity may be deteriorated. At a temperature higher than 3.9 캜 / second, And it may be difficult to secure the crack resistance.

The change in the heating rate from 500 ° C to the target heating temperature was 0.023 to 0.050 ° C / ² As

Blank heated to 500 ° C may limit the rate of temperature rise to the target heating temperature. Here, the change in the heating rate means a constant value obtained when a temperature rise curve (time-temperature) is obtained from a quadratic equation from 500 ° C to a target temperature and then subjected to second-order differentiation. If the change in the heating rate is less than 0.023 ° C / sec ² , productivity can be improved due to rapid heating, but nonuniform alloying can occur again. If the change is 0.050 ° C / sec ² or more, the object of the present invention can be easily achieved , The productivity may be deteriorated.

[Press forming and cooling process]

Thereafter, the hot-dip galvanized steel sheet is press-molded and cooled in the mold. The press forming and cooling can be performed by a conventional hot press forming method, and thus the present invention is not particularly limited. However, in one embodiment of the present invention, the hot press forming start temperature may be Ac1 or higher, The rate may be above the critical cooling rate at which martensite is produced. The upper limit of the cooling rate is not particularly limited and can be determined by the cooling performance of the press.

Hereinafter, the present invention will be described in more detail by way of examples. It should be noted, however, that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

(Example)

Inventory 1

A cold-rolled steel sheet having the composition shown in Table 1 below was prepared. The surface of the steel sheet was plated with a plating bath having a Zn-0.22% Al composition on the surface of the steel sheet. Plating amount was adjusted to 70 g / m ² per side, and then the coil was wound.

element C Si Mn Al P S N Additional element content(%) 0.13 0.1 6.7 0.03 0.009 0.003 0.0076 impurities

The plated steel sheet was heated to 720 占 폚 under the following conditions and press-formed.

Atmosphere during heating: Atmosphere

Room temperature to 440 캜 Temperature rise rate: 4.0 캜 / sec

Temperature rise rate from 440 to 500 ° C: 2.4 ° C / sec

Temperature change rate from 500 to 720 ° C: 0.031 ° C / second ²

Total heating time: 8 minutes

Air cooling time: 10 seconds (time taken from press to start press forming)

Press cooling rate: 100 ° C / sec

As a result of analyzing the plated layer of the press-molded member with the EPMA analyzer, the Zn component mapping result as shown in FIG. 1 was obtained. From the surface of the plated layer to the boundary with the base steel sheet, Layer. In addition, the ratio of the thickness of the hard second layer (Zn plated layer (II)) having a high Zn content to the thickness of the entire plating layer was found to be 44.6% from the result of SEM analysis.

In addition, by analyzing the above member with a GDS analyzer, Zn and Fe profiles as shown in FIG. 2 were obtained, and it was confirmed that the two layers are similar to the EPMA results.

The plating layer structure and the representative diffraction pattern as shown in FIG. 3 were obtained by analyzing the plating layer with a transmission electron microscope (TEM) to confirm the structure of the plating layer of the member. As a result, phase analysis was performed for each position, The results of the EDS component and diffraction pattern analysis at each position are shown in Table 2. At this time, the diffraction pattern was measured with a camera length of 400 mm at a voltage of 200 KeV. As can be seen in Table 2, positions 1, 2, 3 and 4 represent the hard second layer, and positions 5 and 6 represent the soft first layer.

location Phase Crystal structure Zn (% by weight) Fe (% by weight) Mn (% by weight) One Capital Gamma BCC 57.9 39.0 2.9 2 Capital Gamma BCC 67.7 29.1 3.0 3 Capital Gamma BCC 67.0 29.5 3.0 4 Capital Gamma BCC 67.5 29.1 3.2 5 α-Fe (Zn) BCC 31.4 65.0 3.4 6 α-Fe (Zn) BCC 24.4 71.4 3.9

In order to confirm the hardness of the plating layer of the above member, the results of analysis using SEM and nano hardness analyzer are shown in FIG. At this time, the load was 2 g when measuring the hardness. The hardness of the hard second layer (plated layer (II)) having a high Zn content was 347, which was 2 to 4 times higher than that of the soft first layer (plated layer (I)) having a low Zn content. Further, in the hard second layer, the gamma-phase of the capital was substantially dotted over the entire region, and the soft first layer substantially occupied the entire region of the alpha -Fe phase.

At this time, the tensile strength of the hot-formed member was 1658 MPa, and the ultra high strength was secured even at a low heating temperature.

Inventory 2

A cold-rolled steel sheet for hot press forming having the composition shown in Table 3 below was prepared. The surface of the steel sheet was plated with a plating bath having a Zn-0.14% Al composition on the surface of the steel sheet. The amount of plating at the time of plating was adjusted to 70 g / m ² on one side, and then the alloy was alloyed at 530 ° C. After that, the coil was wound.

element C Si Mn Al P S N Additional element content(%) 0.13 0.1 6.7 0.03 0.009 0.003 0.0076 Ti: 0.03, B: 0.003

The plated steel sheet was heated to 760 占 폚 under the following conditions and press-formed.

Atmosphere during heating: Atmosphere

Temperature rise rate from room temperature to 440 캜: 4.2 캜 / sec

Temperature rise rate from 440 to 500 ° C: 2.6 ° C / sec

Temperature change rate from 500 to 760 캜: 0.035 캜 / sec ²

Total heating time: 7 minutes

Air cooling time: 10 seconds (time taken from press to start press forming)

Press cooling rate: 100 ° C / sec

As a result of analyzing the plated layer of the press-formed member with an EPMA analyzer, it was found that the Zn component mapping result as shown in FIG. 5 was obtained. The Zn component mapping from the surface of the plated layer to the boundary between the substrate and the steel sheet, And the thickness of the hard second layer (Zn plating layer (II)) was reduced. In addition, the ratio of the thickness of the hard second layer (Zn plated layer (II)) having a large Zn content to the total plating layer thickness was 29.6% as a result of SEM analysis.

In addition, by analyzing the above member with a GDS analyzer, Zn and Fe profiles as shown in FIG. 6 were obtained. As a result, it was confirmed that the film had two layers similar to the EPMA results. The hardness of the hard second layer in the thickness direction center portion was 325 Hv and the hardness of the soft first layer in the thickness direction center portion was 117 Hv. Further, in the hard second layer, the gamma-phase of the capital was substantially dotted over the entire region, and the soft first layer substantially occupied the entire region of the alpha -Fe phase.

In this case, the tensile strength of the hot-formed member was 1682 MPa, and thus it was possible to secure ultra-high strength even at a low heating temperature.

Comparative Example 1

A cold-rolled steel sheet for hot press forming having the composition shown in Table 3 was prepared. The surface of the steel sheet was plated with a plating bath having a Zn-0.14% Al composition on the surface of the steel sheet. The amount of plating at the time of plating was adjusted to 70 g / m ² on one side, and then the alloy was alloyed at 530 ° C. After that, the coil was wound.

The plated steel sheet was heated to 900 under the following conditions and press-molded.

Atmosphere during heating: Atmosphere

Room temperature to 440 ℃ Temperature rise rate: 4.9 ℃ / sec

Temperature rise rate from 440 to 500 ° C: 4.0 ° C / sec

Change in heating rate from 500 to 900 ° C: 0.022 ° C / sec ²

Total heating time: 6 minutes

Air cooling time: 10 seconds (time taken from press to start press forming)

Press cooling rate: 100 / sec

As a result of analyzing the plated layer of the press-formed member with an EPMA analyzer, the Zn component mapping result of the shape as shown in FIG. 7 was obtained. The content of Zn component from the surface of the plated layer to the boundary between the substrate and the non- And the ratio of the thickness of the hard second layer (Zn plating layer (II)) to the thickness of the soft first layer (Zn plating layer (I)) was close to zero .

The Zn and Fe profiles as shown in FIG. 8 were obtained by analyzing the above member with a GDS analyzer, and the Zn content in the center of the plating layer was about 30 to 40%.

At this time, the tensile strength of the hot-formed member was 1,574 MPa, and the super high strength could be secured. However, the plated layer was formed only of the soft first layer (Zn plating layer (I)) having a low Zn content. The hardness of the soft first layer in the thickness direction center portion was 109 Hv. Further, the soft first layer occupied substantially the entire area of the? -Fe phase.

Comparative Example 2

A cold-rolled steel sheet for hot press forming having the composition shown in Table 4 was prepared. The surface of the steel sheet was plated with a plating bath having a Zn-0.23% Al composition on the surface of the steel sheet. The amount of plating at the time of plating was adjusted to 70 g / m ² per side, and then the coils were wound

element C Si Mn Al P S N Additional element content(%) 0.22 0.25 1.3 0.04 0.012 0.002 0.051 Ti: 0.02, B: 0.003

The plated steel sheet was heated to 740 占 폚 under the following conditions and press-formed.

Atmosphere during heating: Atmosphere

Temperature rise rate from room temperature to 440 캜: 4.1 캜 / sec

Temperature rise rate from 440 to 500 ° C: 2.5 ° C / sec

Temperature change rate from 500 to 740 캜: 0.034 캜 / sec ²

Total heating time: 8 minutes

Air cooling time: 10 seconds (time taken from press to start press forming)

Press cooling rate: 100 / sec

As described above, the tensile strength of the press-formed member was 631 MPa, and thus the ultra high strength pursued by the present invention could not be secured.

Comparative Example 3

A cold-rolled steel sheet for hot press forming having the composition shown in Table 4 was prepared. The surface of the steel sheet was plated with a plating bath having a Zn-0.23% Al composition on the surface of the steel sheet. The amount of plating at the time of plating was adjusted to 70 g / m ² per side, and then the coils were wound

The plated steel sheet was heated to 900 DEG C under the following conditions and press-molded.

Atmosphere during heating: Atmosphere

Temperature rise rate from room temperature to 440 캜: 5.1 캜 / sec

Temperature rise rate from 440 to 500 ° C: 4.3 ° C / sec

Change in heating rate from 500 to 740 캜: 0.020 캜 / sec ²

Total heating time: 6 minutes

Air cooling time: 10 seconds (time taken from press to start press forming)

Press cooling rate: 100 ° C / sec

As a result of analyzing the plating layer of the press-molded member by an EPMA analyzer, the Zn component mapping result of the shape as shown in FIG. 9 was obtained. The content of Zn component from the surface of the plating layer to the boundary with the substrate steel was found to be 1 And that the thickness of the hard second layer (Zn-plated layer (II)) was close to zero with respect to the thickness of the entire plating layer.

In addition, Zn and Fe profiles as shown in FIG. 10 were obtained by analyzing the above member with a GDS analyzer, and the Zn content was about 10 to 40% at the center of the plating layer.

As a result of analyzing the plating layer by a transmission electron microscope (TEM) in order to confirm the structure of the plating layer of the member, a plating layer structure and a representative diffraction pattern as shown in FIG. 11 were obtained. The results of the EDS component and diffraction pattern analysis at each position are shown in Table 5. At this time, the diffraction pattern was measured under a condition that the camera length was 400 mm at a voltage of 200 KeV.

location Phase Crystal structure Zn (% by weight) Fe (% by weight) Mn (% by weight) One α-Fe (Zn) BCC 35.5 64.2 - 2 α-Fe (Zn) BCC 37.1 62.5 - 3 α-Fe (Zn) BCC 34.2 65.6 -

In order to confirm the hardness of the plated layer of the member, the results of analysis using SEM and nano hardness analyzer are shown in FIG. At this time, the load was 2 g when measuring the hardness. The hardness of the soft first layer (the small plated layer (I)) of Zn content was as low as 116. Further, the soft first layer occupied substantially the entire area of the? -Fe phase.

In the flat part of the member having the HAT shape as shown in FIG. 13 obtained from the inventive and comparative examples having a predetermined tensile strength of 1000 MPa or more after hot forming, a cyclic corrosion test was performed after painting, And the degree of the occurrence of the field was observed. In the composite corrosion test, one cycle was 24 hours of exposure for 2 hours in a wet atmosphere, 2 hours in a salt water spray exposure, 1 hour in a dry condition, 6 hours in a wet atmosphere, and 2 hours in a wet atmosphere. 50 cycles were maintained. Also, in the side portion of the HAT-shaped member as shown in FIG. 13, the crack length penetrating through the substrate was optically measured at five sites, and the maximum length was analyzed.

division Maximum blister width (mm) Maximum crack depth (㎛) Inventory 1 0.6 6.2 Inventory 2 0.9 8.4 Comparative Example 1 1.9 18.6 Comparative Example 3 2.2 19.3

In Examples 1 and 2, it was confirmed that the maximum blister width was 1 mm or less, and the maximum crack depth was 10 탆 or less, which was excellent in corrosion resistance and crack resistance. On the other hand, in Comparative Examples 1 and 3, the maximum blister width was 1.9 and 2.2 mm, respectively, and not only the corrosion resistance after coating was lowered but also the maximum crack depth penetrating through the substrate iron was 18.6 and 19.3 μm, respectively, I could.

As described above, when the components of the hot-dip coated steel sheet and the hot forming heating conditions are controlled as in the present invention, cracking of the coating layer is suppressed, and hot press formed parts having excellent corrosion resistance after painting can be obtained. Therefore, the advantageous effects of the present invention can be confirmed.

Claims (15)

Base steel sheet; And
The zinc-iron-based alloy plating layer
/ RTI >
Wherein the zinc-iron-based alloy plating layer comprises a soft first layer in contact with the base steel sheet and a hard second layer formed on the first layer
Hot press formed member.
The method according to claim 1,
Wherein the hard second layer has a hardness of 150 Hv or more and the soft second layer has a hardness of less than 150 Hv.
The hot press forming member according to claim 1, wherein the hard second layer is a plating layer having a Zn content of 40% or more, and the soft first layer is a plating layer having a Zn content of less than 40%.
The hot press forming member according to claim 1, wherein the plating layer of the hot press-formed member contains crystals of a BCC structure in an area ratio of 90% or more.
The hot press forming member according to claim 1, wherein the hard second layer has a thickness of 10% or more of the thickness of the entire plating layer.
The hot press forming member according to claim 1, wherein the hard second layer contains at least 90% of the area of the gamma (Γ) phase.
The hot press forming member according to claim 1, wherein the soft first layer contains 90% or more of? -Fe in an area fraction.
The hot press forming member according to any one of claims 1 to 7, wherein the base steel sheet comprises 3.1 to 15% by weight of Mn.
The steel sheet according to claim 8, wherein the base steel sheet comprises 0.04 to 0.3% of C, 0.01 to 2% of Si, 3.1 to 15% of Mn, 0.001 to 1.0% of Al, 0.02% or less and N: 0.02% or less.
The steel sheet according to claim 9, wherein the base steel sheet comprises 0.01 to 4.0% of at least one selected from the group consisting of Cr, Mo and W in weight percent, at least one 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: 0.0001 to 0.01%.
Preparing a galvanized steel sheet plated with a plated amount of 20 to 140 g / m ² per side of the base steel sheet;
Heating the galvanized steel sheet;
A step of cooling the galvanized steel sheet in a mold while hot-pressing the galvanized steel sheet
Lt; / RTI >
In the step of heating the galvanized steel sheet,
The maximum heating temperature is not less than Ac3 + 10 ℃ and not more than 800 ℃,
An average rate of temperature rise from room temperature to 440 占 폚 is 3.0 to 4.8 占 폚 / sec,
An average temperature raising rate from 440 ° C to 500 ° C is 1.0 to 3.9 ° C /
At 500 ℃ the change in the rate of temperature increase up to the target heating temperature of 0.023 ~ 0.050 ℃ / sec ²
A method of manufacturing a hot press formed member.
The method of manufacturing a hot press forming member according to claim 11, wherein the galvanized steel sheet has a plating amount of 30 to 120 g / m ² .
8. The method of manufacturing a hot press formed member according to any one of claims 1 to 7, wherein the base steel sheet comprises 3.1 to 15% by weight of Mn.
The steel sheet according to claim 13, wherein the base steel sheet comprises 0.04 to 0.3% of C, 0.01 to 2% of Si, 3.1 to 15% of Mn, 0.001 to 1.0% of Al, 0.02% or less and N: 0.02% or less.
The steel sheet according to claim 14, wherein the base steel sheet comprises 0.01 to 4.0% of at least one selected from the group consisting of Cr, Mo and W in weight percent, at least one 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: 0.0001 to 0.01%.

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