KR101439621B1 - Manufacturing method for hot press formed products and hot press formed products using the same - Google Patents

Abstract

The present invention relates to a method of manufacturing a hot press molded product for a plated hot press forming steel product, and a hot press molded product which can be suitably used for automobile parts and the like, and a hot press molded product using the same.
To this end, in the present invention, the above object can be achieved by forming a buffer layer on the plating layer of the plated steel.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a hot press molded article,

The present invention relates to a method of manufacturing a hot press molded product for a plated hot press forming steel product, and a hot press molded product which can be suitably used for automobile parts and the like, and a hot press molded product using the same.

BACKGROUND OF THE INVENTION Hot press forming (HPF) technology is a method of manufacturing a high strength member having a complicated shape and is used for manufacturing automotive parts and the like.

Generally, the HPF technology is a technique of heating a steel sheet at a temperature before and after 900 ° C and then press-molding it in a mold, which is simultaneously subjected to molding and quenching heat treatment.

The HPF technology uses a steel having a high crush strength, and the steel has a high content of carbon (C) and manganese (Mn). When HPF technology is applied to such a steel sheet, hot-dip galvanizing or gold plating can be applied to protect the surface of the steel sheet from high temperature heating and to improve corrosion resistance.

At present, a hot press member to which zinc plating is applied has a high strength, can be molded into a complicated shape, and exhibits high corrosion resistance characteristics, and is therefore attracting attention as an automotive part.

However, in the hot-press forming (HPF) of a plated steel material having a zinc-based plated layer formed of a hot-dip galvanizing or a hot-dip galvanizing, the surface stress in a deep drawing region, There is a problem that minute cracks are generated. The size of cracks generated is of the order of several μm to several tens of μm, and these cracks are formed from the plating layer to the surface of the base steel sheet.

Therefore, it is necessary to completely eliminate or minimize the occurrence of cracks because fine cracks generated when HPF molding of a plated steel material having a zinc-based plated layer as a component may hinder durability of parts.

One aspect of the present invention is to optimize the interfacial structure of the base steel sheet and the plating layer during hot press forming by using the hot-pressing plated steel material so that micro cracks (microcracks) generated in the plating layer are propagated to the base steel sheet And to provide a hot press molded article using the above method.

According to an aspect of the present invention, A plating layer formed on the surface of the base steel sheet; And a buffer layer formed on an interface between the base steel sheet and the plating layer,

The buffer layer provides a hot press molded product having an average thickness of 0.3 to 5.0 占 퐉.

According to another aspect of the present invention, there is provided a method of manufacturing a coated steel sheet, Heating the plated steel material in a heating furnace; And a step of hot-pressing and cooling the heated plated steel material, the method comprising the steps of:

The holding time is 6 to 15 minutes and 2 to 10 minutes when the total holding time in the heating furnace and the surface temperature of the plated steel material reach 880 to 980 DEG C in the heating step,

And a buffer layer formed by diffusing therefrom is present at an interface between the base steel sheet and the plating layer after the heating.

According to another aspect of the present invention, there is provided a method of manufacturing a coated steel sheet, comprising the steps of: preparing a plated steel sheet comprising a base steel sheet and a plating layer formed on the base steel sheet; Heating the plated steel material in a heating furnace; And a step of hot-pressing and cooling the heated plated steel material, the method comprising the steps of:

Wherein the temperature and time for heating the plated steel material in the heating furnace are set so that the buffer layer is formed to have a thickness of 0.3 to 5.0 mu m by diffusion from the interface between the base steel sheet and the plated layer.

According to the present invention, in the hot press forming of the plated steel material, the heating time in the heating furnace can be suitably controlled to have a buffer layer on the interface between the base steel sheet and the plating layer, and the buffer layer can be formed by hot- And serves to prevent microcracks generated in the plating layer at the machining site from propagating to the steel sheet.

Therefore, the hot press member manufactured according to the present invention can secure excellent durability.

Fig. 1 shows an example of a shape used in hot press forming.
FIG. 2A shows a process of performing hot press forming in the shape of FIG. 1, and FIG. 2B is a graph showing cooling rates of regions A and B before and after hot press forming according to FIG. 2A.
3 shows the results of observing the cross-section of Inventive Example 4 with a scanning electron microscope (SEM).
4 shows the result of observation of a section of Inventive Example 2 with a transmission electron microscope (TEM).
Fig. 5 is a graph showing the results of observing the generation of microcracks with a scanning electron microscope (SEM) by observing the cross sections of Inventive Example 3 and Comparative Example 2. Fig.

The inventors of the present invention have proposed a solution to solve the problem of micro crack occurring in a deep processing region where the degree of machining is high and the surface friction between the metal mold and the die is severe in the case of performing the hot press forming process using the plated steel material do.

Micro cracks cause cracks in the plating layer due to the frictional force generated on the surface of the plating layer by the die during hot press forming of the plated steel. At this time, the crack propagates to the base steel sheet through the interface between the plating layer and the base steel sheet. If the crack starting from the plating layer is not dispersed at the interface or can not be absorbed from the surface of the base steel sheet, propagation .

More specifically, the mechanism by which a micro crack is generated in the plating layer during hot press forming of the plated steel can be explained with reference to FIGS. 2A and 2B.

When the shape of Fig. 1 is used as an example in the hot press forming, the plated steel material heated in the heating furnace can be positioned in the metal mold as shown in Fig. 2A. 2A, when the plated steel material is placed in the mold, the area A before the application of the HPF is in the cooling state under the influence of the mold at room temperature, and the area B is relatively more Since the mold is separated from the mold, the cooling progress progresses more slowly than in the area A. That is, the cooling progresses differently for each region of the plated steel material placed in the mold. Particularly, in the region A, the surface layer is more closely adhered to the mold than the center portion, which is more influential.

When the HPF is applied to the plated steel, the cooling pattern of the area A and the area B becomes different. That is, since the area A has already been cooled due to the influence of the mold temperature before application of the HPF, the temperature at the time of application of the HPF is lower than that of the area B, which can be confirmed from the graph of FIG. Since the ferrite region is enlarged in the state diagram when machining is started, a soft ferrite phase is formed in the surface layer portion of the A region-bearing steel sheet at the beginning of the process deformation. At this time, a micro crack formed in the plating layer of the A region forms a ferrite- (See the cross-sectional result of region A in Fig. 2B).

That is, the portion of the region A that belongs to the cut direction is a deep drawing region that is severely subjected to processing during molding. Since this region is highly processed and has a large surface friction with the mold, (micro crack) is generated. At this time, microcracks generated in the plated layer in the region A are propagated to the steel sheet. At this time, since the surface layer portion of the steel sheet is formed into a soft ferrite phase, minute cracks are easily propagated. However, in the case of the B region, the entire martensite structure is formed, so that even if a micro crack propagates to the steel sheet, the propagation can be blocked in the surface layer portion (see the sectional view of the region B in FIG.

In order to solve the above-mentioned problems, the inventors of the present invention have determined that if a buffer layer of a thin layer capable of alleviating the impact on the interface structure of the plated layer and the base steel sheet is formed, crack propagation can be prevented. It is necessary to control the production of the steel and the heating conditions of the steel.

Here, the buffer layer means a layer capable of mitigating the spread of fine cracks originating from the plating layer into the steel sheet due to friction with the metal mold during hot press forming, and the buffer layer has mechanical properties And chemical composition. More specifically, when measuring the microhardness at room temperature, the hardness value of the buffer layer is higher than the hardness value of the base steel sheet, and the buffer layer may contain 2 to 12 wt% of Zn in the chemical composition.

Hereinafter, the present invention will be described in detail.

[Method of Manufacturing Hot Press Molded Parts]

First, a method of manufacturing a hot press molded article which is one aspect of the present invention will be described in detail.

The method for manufacturing a hot press formed article according to the present invention comprises a step of preparing a plated steel material having a plated layer formed on a base steel sheet, heating the plated steel material by controlling the conditions of the heating furnace, .

The base steel sheet to be applied to the method of the present invention is not particularly limited, and a steel sheet applicable to general hot press forming is sufficient.

(Si), boron (B), or the like, and further contains, for example, 0.19 to 0.30 wt% of carbon (C) and 0.5 to 4.0 wt% of manganese A steel material having an appropriate component system capable of obtaining physical properties such as a material strength of 1300 MPa or more can be used as well as a steel material having a C content of 0.01 to 0.10% by weight is subjected to hot press application to a material having a material strength of about 500 to 800 MPa .

Preferably, the base steel sheet is plated with a plated steel material before the base steel sheet is produced as a hot press formed product, and the plated steel material is a zinc-based plated steel material, and the plating method is a hot-dip galvanizing method, And the like, and is not particularly limited.

The plating layer formed after plating may be a hot-dip galvanized layer or a hot-dip galvanized layer.

The plated steel material may include a zinc-based plated layer on a base steel sheet and an aluminum (Al) -concentrated layer between the base steel and the plated layer.

The Al-enriched layer is an important factor for forming a buffer layer in the subsequent heating of the plated steel material in a heating furnace. When the plated steel material is heated for hot press forming, the Al- So as to form a dense and thin oxide layer.

More specifically, as will be described later, the buffer layer is formed at the interface where Fe of the base steel sheet and Zn of the plated layer are mutually diffused when the plated steel material is heated. At this time, when the Al-enriched layer is formed finely and uniformly, the Al-enriched layer is easily decomposed when heating the plated steel material, so that the mutual diffusion of Fe and Zn proceeds quickly and uniformly to obtain a uniform buffer layer. However, when the Al-enriched layer is uneven, the alloying is partly changed, which interferes with the diffusion of Fe and Zn, so that the buffer layer is formed unevenly.

The Al concentrated layer is a, the fine particles have a composition containing at least 30 wt% Al had a shape formed continuously, in the present invention, an average of 15 per a fine particle size diameter of more than 500nm in order to form a uniform buffer layer 100μm ² Preferably less than. As a method of controlling the grain size of the Al-enriched layer, it is possible to control the annealing temperature in the range of 500 to 700 占 폚 when annealing the pre-plating steel sheet, or to perform pre-treatment plating with a specific metal or the like before annealing, There is a possibility that the formation rate of the Al-enriched layer is controlled.

The occupied area ratio of the Al-enriched layer as described above at the interface between the base steel sheet and the plating layer is preferably 90% or more, and if it is not satisfied, the alloying becomes uneven and the buffer layer is unevenly formed, Embrittlement may occur.

The present invention has a step of heating the prepared plated steel material in a heating furnace, wherein a buffer layer can be formed between the base steel sheet and the plating layer interface by the heating.

At this time, it is preferable that the heating condition is set so as to form a buffer layer having a desired thickness, and the heating condition, that is, the temperature and the time may be set differently depending on the kind of the applied base steel sheet or the plating layer.

For example, in heating the plated steel material so as to form a buffer layer having a desired thickness, the total holding time in the heating furnace and the surface temperature of the plated steel material reach 880 to 980 DEG C while setting the target temperature to 880 to 980 DEG C The holding time can be controlled.

That is, the heating time at the time of heating is a time during which the material is held from being charged into the heating furnace until the material is introduced into the furnace, and the time during which the temperature of the steel material charged into the furnace reaches a target temperature (for example, 880 to 980 ° C) . At this time, the total holding time in the heating furnace is preferably 6 to 15 minutes, and the holding time after reaching the target temperature is preferably 2 to 10 minutes. More preferably, the holding time after reaching the target temperature is preferably set to 4 to 8 minutes.

If the target temperature is less than 880 DEG C, alloying of the plating layer may not be sufficiently performed. If the total holding time is less than 6 minutes or the holding time is less than 2 minutes after reaching the target temperature, Zn diffusion in the plating layer becomes insufficient, It is difficult to form the buffer layer. On the other hand, if the target temperature exceeds 950 DEG C, the plating layer may be thermally damaged. If the total holding time exceeds 15 minutes or the holding time exceeds 10 minutes after reaching the target temperature, the thermal damage or productivity . ≪ / RTI > Therefore, when the above conditions are satisfied during heating, a desired buffer layer can be formed.

The formation of the buffer layer between the base steel sheet and the plating layer interface by heating as described above can be explained by the following principle.

In heating the plated steel with zinc-based plated layer, Zn in the plated layer diffuses down to the surface of the base steel sheet as the heating time becomes longer, and the physical properties of the surface layer portion of the base steel sheet, So that a buffer layer having a predetermined thickness is formed.

The buffer layer formed by the above-described principle may contain one or more components selected from the group consisting of Fe, Mn, Si, and Al as the components in the plating layer with Zn and the remainder being 2 to 12 wt%.

It is also preferable that the structure of the buffer layer includes epsilon martensite structure. Usually, the coated steel sheet is composed of lath martensite, while the buffer layer according to the present invention contains 30 to 100% of epsilon martensite. It is considered that such epsilon martensite is formed during quenching due to the expansion of the crystal lattice as Zn diffuses into the steel sheet at high temperature and is dissolved.

The buffer layer formed between the base steel sheet and the plating layer interface can prevent the micro cracks generated in the plating layer from being propagated to the base steel sheet during the subsequent hot press forming. That is, when a micro crack generated in the plating layer propagates in the direction of the base steel sheet and reaches the buffer layer, the buffer layer absorbs the stress that propagates the crack, and the propagation to the base steel sheet below the buffer layer is blocked. In addition, since the end portion of the crack remaining in the buffer layer has a sharp and smooth shape, it has an effect of suppressing propagation of fatigue crack due to cracking.

In order to obtain the above effect, it is necessary to form the average thickness of the buffer layer to 0.3 탆 or more, more preferably 0.8 탆 or more, and preferably 5 탆 to the maximum.

The buffer layer is preferably formed continuously along the surface of the steel sheet, and it is preferable that the average thickness of the buffer layer is 70% or more when the cross section is observed. If the buffer layer is formed to a thickness of less than 0.3 袖 m or less than 70% of the cross-sectional area of the buffer layer, the property of preventing crack propagation due to the buffer layer is decreased. On the other hand, There is a fear that the Zn in the plating layer is excessively diffused to the steel sheet and corrosion resistance is deteriorated.

On the other hand, the plating layer above the buffer layer has an average thickness of 10 to 25 占 퐉 and Zn content in the plating layer is 20 to 40 占 퐉 in average to maintain the corrosion resistance of the base steel sheet even if Zn is diffused for forming the buffer layer during heating. %.

If the average thickness of the plated layer is less than 10 占 퐉 or the content of Zn in the plated layer is less than 20%, the corrosion resistance becomes insufficient. On the other hand, if the average thickness is less than 20 占 퐉 or the content of Zn in the plated layer exceeds 40% There is a fear that a brittle fracture (Liquid Metal Embrittlement) phenomenon due to the liquid metal may occur during molding.

Thereafter, the plated steel material having the buffer layer formed at the interface between the base steel sheet and the plated layer by the above heating is hot-pressed and cooled.

At this time, the above-mentioned molding and cooling is sufficient according to a conventional hot press forming method, and thus the present invention does not limit it.

[Hot press molded article]

Hereinafter, a hot press molded article which is another aspect of the present invention will be described in detail.

The hot press formed article according to the present invention comprises a base steel sheet; A plating layer formed on the surface of the base steel sheet; And a buffer layer formed on the interface between the base steel sheet and the plating layer.

The hot-pressed product of the present invention has a buffer layer between the interface between the base steel sheet and the plating layer, so that micro cracks of the plating layer after final molding can be prevented from propagating into the base steel sheet.

That is, when the conventional plated steel material is hot-pressed, a micro crack formed in the plated layer propagates to the base steel sheet. However, in the present invention, by forming a buffer layer containing Zn between the base steel sheet and the plating layer interface, it is possible to prevent the micro crack from propagating to the steel sheet.

For this purpose, the buffer layer in the hot press-molded article preferably has a thickness of 0.3 탆 or more, more preferably 0.3 to 5.0 탆.

The buffer layer may contain zinc (Zn) in an amount of 2 to 12 wt%, and the balance may contain one or more components selected from the group consisting of Fe, Mn, Si and Al. May be in a form containing 30 to 100% epsilon martensite in a fraction.

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 the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

The base steel sheet for the practice of the present invention preferably contains 0.23% of C, 0.25% of Si, 1.25% of Mn, 0.03% of Sol.Al, 0.0026% of B, 0.031% of Ti, 0.0044% And a steel sheet composed of the remaining Fe and unavoidable impurities was used.

The surface of the base steel sheet having the above composition was subjected to hot-dip galvanizing to produce a plated steel material, which was then heated in a heating furnace. At the time of heating, the target temperature was set at 880 to 950 ° C, and the holding time at the target temperature was set as shown in Table 1 below. Thereafter, the hot plated steel sheet was subjected to hot press forming using the shape shown in Fig.

During the hot press forming, the thickness of the plated steel material was 1.5 mm.

Table 1 below shows the results of measurement of plating conditions, heat treatment conditions, formation of a buffer layer and depth of micro cracks.

At this time, the diameter of the Al-enriched layer was determined by dissolving only the plated layer of the plated steel with an acid solution, observing the Al-enriched layer with an electron microscope (SEM), selecting three or more dense layers and then using an image analyzer And the particle size and occupied area ratio of these thickened layers were measured.

The time during which the plated steel material was charged into the heating furnace at the time of the heat treatment was measured and a thermo couple was attached to the surface of the plated steel material to measure the heat history that the temperature of the steel reached and reached the target temperature. Based on this, the time to maintain the target temperature after reaching the target temperature was measured separately. Also, the total time during which the plated steel material was charged into the heating furnace, that is, the ash time, was also measured.

The buffer layer formed at the interface between the substrate steel and the plated layer was observed with an electron microscope (SEM and TEM) for the cross section of the specimen. The observation results are shown in FIGS. 3 and 4.

Since micro cracks in the plating layer occur perpendicularly to the cut direction of the shape of FIG. 1, the specimens were taken parallel to the cut direction and observed by a scanning electron microscope (SEM). At this time, The depth of a micro crack formed in the direction of the base steel sheet was measured and shown in Table 1 and FIG.

division Plated
Adhesion Al concentrated layer Heating furnace
goal
Temperature all
maintain
time Holding time after reaching target temperature Buffer layer
Average thickness Buffer layer
Formation rate Smile crack maximum depth Number of particles Occupancy Inventory 1 50 g / m ² 5.1 94.3% 880 ℃ 12 minutes 8 minutes 1.8 m 89% 3.5 m Inventory 2 50 g / m ² 5.1 94.3% 900 ℃ 8 minutes 4 minutes 0.9 m 75% 8μm Inventory 3 60 g / m ² 4.5 96.2% 930 ° C 9 minutes 4.5 minutes 1.1 m 78% 9μm Honorable 4 40 g / m ² 3.4 95.4% 950 ℃ 6 minutes 2 minutes 0.4μm 95% 7μm Comparative Example 1 50 g / m ² 5.1 94.3% 900 ℃ 5 minutes 0.5 minutes - - 21 μm Comparative Example 2 50 g / m ² 5.1 94.3% 930 ° C 5 minutes 0 minutes 0.1 m 34% 30μm Comparative Example 3 70 g / m ² 38 84% 910 ° C 8 minutes 4.5 minutes 0.8 m 55% 15μm

(In the above Table 1, the amount of plating adhered is measured by measuring the amount of plating on one side of the base steel sheet, and the number of particles of the Al-enriched layer particle having a diameter of 500 nm or more per 100 μm ² of the surface was measured. .

FIG. 3 shows the results obtained by mounting an inventive example 4 shown in Table 1, polishing the surface by etching, and observing it with an electron microscope (SEM).

As shown in FIG. 3, it can be confirmed that a buffer layer formed under the plating layer exists. As a result of hardness measurement using a microhardness meter, it was confirmed that the hardness value of the buffer layer was about 10 to 70% higher than the hardness value of the base steel sheet.

Fig. 4 shows the result of observation of a cross-section of Inventive Example 2 having a buffer layer formed thereon by a transmission electron microscope (TEM).

As shown in Fig. 4, a buffer layer formed between the base steel sheet and the plating layer can be observed. As a result of measuring the content of Zn using EDS, Zn was not found in the base steel sheet, but 5 ~ 10% of Zn was detected in the buffer layer.

Observation of the structure through the TEM results revealed that the base steel sheet is formed of acicular martensite, but the buffer layer has Epsilon martensite structure. As a result of the TEM, it is difficult to grasp the ratio of epsilon martensite in the buffer layer, but it is estimated that about 10 to 90% is composed of epsilon martensite as a whole.

Since the buffer layer formed at the interface between the base steel sheet and the plating layer has different physical properties from the base steel sheet and the plating layer, micro cracks originating from the plating layer can be dispersed in the buffer layer. As a result, It is judged that it inhibits propagation into the steel sheet. In addition, it is judged that the propagation control of the micro crack is thicker and the effect is greater as the buffer layer is thicker.

As shown in Table 1, the average thicknesses of the buffer layers of the inventive examples are all 0.3 탆 or more, and when the continuity of these buffer layers is 70% or more, the micro cracks are formed to a depth of 10 탆 or less.

On the other hand, in the case of the comparative examples in which the Al-enriched layer does not satisfy the requirements of the present invention or the holding time in the heating furnace does not satisfy the requirements of the present invention, it is confirmed that the depth of micro crack generated is not less than 15 탆 .

The micro crack generation patterns of Examples 3 and Comparative Example 2 were measured and shown in FIG.

As shown in Fig. 5, in Comparative Example 2, the maximum depth of the microcracks was 30 mu m, the depth was very deep, and the ends of the microcracks were sharp. On the other hand, in the case of Inventive Example 3, So that the maximum depth of minute cracks is formed within 10 占 퐉, and the end portions are clumped to suppress crack propagation.

Claims (9)

Base steel sheet;
A plating layer formed on the surface of the base steel sheet; And
And a buffer layer formed on the interface between the base steel sheet and the plating layer,
Wherein the buffer layer has an average thickness of 0.3 to 5.0 占 퐉 and the microstructure of the buffer layer comprises 30 to 100% epsilon martensite.
The method according to claim 1,
Wherein the buffer layer contains 2 to 12% by weight of zinc (Zn), and the balance contains one or more components selected from the group consisting of Fe, Mn, Si and Al.
delete The method according to claim 1,
Wherein the plating layer is a hot-dip galvanized layer or a hot-dip galvanized layer.
The method according to claim 1,
Wherein the plating layer has an average thickness of 10 to 25 占 퐉 and a Zn content in the plating layer of 20 to 40%.
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