KR101726094B1 - Hot pressed part with reduced microcrack and method for manufacturing same - Google Patents

Abstract

The present invention discloses a hot press forming (HPF) product manufactured by applying a heat press forming (HPF) technique to a base material, and a zinc-based galvanized steel sheet with a zinc-based galvanized layer formed on a surface of the base steel sheet. The zinc-based galvanized layer contains one or more types of the following elements selected from a group composed of Sb, Sn, and Bi; totaling 0.05-2.0 wt%, and the remainder consisting of Zn and inevitable impurities. 70 wt% or more of the one or more types of the elements selected from the group composed of Sb, Sn, and Bi are thickened in an area within 3 m from a surface of an alloyed zinc-based galvanized layer of the HPF product formed in such a manner that the zinc-based galvanized layer is alloyed. As such, the present invention has an advantage of excellent durability in a manner which effectively restrains a microcrack, generated in a galvanized layer when performing an HPF technique, from being spread to a base steel sheet.

Description

TECHNICAL FIELD [0001] The present invention relates to a hot press formed article in which micro cracks are suppressed and a method of manufacturing the same.

The present invention relates to a hot press formed article in which micro cracks are suppressed and a method for producing the same.

In recent years, the use of high-strength steels has been increasing to lighten automobiles, but such high-strength steels are easily worn or fractured at room temperature. In addition, due to springback phenomenon during machining, it is difficult to precisely dimension work, which makes molding of complex products difficult. Accordingly, Hot Press Forming (HPF) has been applied as a preferable method for machining high strength steel.

The hot press forming (HPF) is a method of processing a steel sheet into a complicated shape at a high temperature by using a property of softening and high ductility at a high temperature, more specifically, At the same time, quenching is performed to transform the structure of the steel sheet into martensite to form a product having high strength and precise shape.

However, when the steel material is heated to a high temperature, a phenomenon such as corrosion or decarburization may occur on the surface of the steel material. To prevent this, there are many plated steel materials having a zinc- Is used. In particular, a galvanized steel sheet having a zinc-based plated layer is a steel material improved in corrosion resistance by utilizing the self-sacrificing corrosion resistance of zinc.

However, when such a plated steel material is subjected to hot press forming, cracks are generated in the plated layer at the deeply processed portion where the metal surface and the plated layer are in direct contact with each other, and the cracks generated in the plated layer cause fine cracks A problem has been discovered.

In order to solve such a problem, Patent Document 1 proposes a technique of performing Al-based plating on the surface of a steel sheet on the surface of a steel sheet. As proposed by the above Patent Document 1, there is an advantage in that corrosion resistance is enhanced by using the Al-based plating to keep the plating layer in the heating furnace while suppressing the oxidation reaction on the surface of the steel sheet and utilizing the formation of a passive film of Al. There has been a problem that the corrosion resistance is drastically reduced.

In order to solve such a problem, studies on Zn-plated hot-pressed steel sheets have been reexamined, but the plated steels have a high work temperature exceeding 900 占 폚, There is a problem that a micro-crack occurs to the surface of the steel sheet due to stress caused by friction. Such fine cracks may act as a starting point for propagation of cracks in the base steel sheet or may cause fatigue cracks, which may hinder the durability of parts.

U.S. Patent No. 6,296,805

One of the objects of the present invention is to provide a hot press formed article in which micro cracks are suppressed and a method of manufacturing the same.

According to an aspect of the present invention, there is provided a hot press formed product obtained by hot pressing a zinc-based plated steel sheet including a base steel sheet and a zinc-based plated layer formed on a surface of the base steel sheet, wherein the zinc- : 0.05 to 2.0% by weight in total, Zn and inevitable impurities, and at least 70% by weight of at least one element selected from the group consisting of Sb, Sn and Bi is contained in the zinc- The hot press formed article is concentrated in an area of 3 占 퐉 or less from the surface of the alloyed zinc plated layer of the hot press formed product formed by alloying the plating layer.

According to another aspect of the present invention, there is provided a method of manufacturing a galvanized steel sheet, comprising the steps of: preparing a zinc plated steel sheet; first heating the zinc plated steel sheet to a temperature of 640 to 680 ° C at a rate of 3.5 to 4.2 ° C / sec; Heating the zinc plated steel sheet to a temperature of 900 to 930 캜 at a rate of 1.1 to 1.6 캜 / sec, maintaining the secondary heated zinc plated steel sheet at a constant temperature for 1 to 5 minutes, Wherein the zinc-plated steel sheet is formed on the surface of the base steel sheet and the base steel sheet, and is characterized in that the zinc-base galvanized steel sheet has a thickness of 1 < RTI ID = 0.0 > And a zinc-based plated layer containing 0.05 to 2.0% by weight in total of at least two kinds of elements.

As one of various effects of the present invention, the hot-press molded article according to one embodiment of the present invention is advantageous in that the fine cracks in the plated layer generated during the hot press forming are effectively suppressed from propagating to the base steel sheet, and the durability is excellent.

It should be understood, however, that the various and advantageous advantages and effects of the present invention are not limited to those described above, and may be more readily understood in the course of describing a specific embodiment of the present invention.

Fig. 1 (a) is a graph showing microcracks of Comparative Example 1, Fig. 1 (b) is a graph showing microcracks of Inventive Example 1, Fig. 1 4, and FIG. 1 (e) is a graph showing the micro cracks in Inventive Example 5. FIG.
FIG. 2 (a) is GDS data obtained by analyzing the content of Al, Mg and Sn according to the depth of the plating layer in Inventive Example 1, and FIG. 2 (b) FIG. 2 (c) is GDS data obtained by analyzing the content of Al, Mg and Sn according to the depth of the plating layer in Inventive Example 5. FIG.

The present inventors have studied in depth to provide a hot press formed article in which micro cracks are suppressed. As a result, a zinc-based plated steel sheet having a zinc-based plated layer containing an appropriate amount of grain boundary elements is used as a material for hot press forming, It has been found that the fine cracks in the plating layer can be effectively prevented from propagating to the base steel sheet by appropriately controlling the heating conditions during the hot press forming to thicken the grain boundary segregated elements in the surface layer of the plating layer.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a hot press molded article which is one aspect of the present invention will be described in detail.

The hot press formed product as one aspect of the present invention is produced by subjecting a zinc plated steel sheet including a base steel sheet and a zinc-based plated layer formed on the surface of the base steel sheet to hot press molding.

In the present invention, the kind of the base steel sheet is not particularly limited and may be, for example, a hot-rolled steel sheet or a cold-rolled steel sheet used as a base of a conventional zinc plated steel sheet. However, in the case of a hot-rolled steel sheet, a large amount of oxide scale is present on the surface of the hot-rolled steel sheet. Such an oxide scale has a problem of deteriorating the plating adhesion and deteriorating the plating quality. Therefore, More preferable.

On the other hand, the zinc-based plated layer is formed on one side or both sides of the base steel sheet, and the zinc-based plated layer is alloyed at the time of heat treatment for hot press forming to change into an alloyed zinc-based plated layer.

The zinc-based plated layer preferably contains 0.05 to 2.0% by weight in total of at least one element selected from the group consisting of Sb, Sn and Bi, the remainder Zn and unavoidable impurities.

Sb, Sn, and Bi are intergranular segregated elements that act to inhibit the formation of internal oxides due to penetration of oxygen into the grain boundary in a high temperature working environment. In order to exhibit such effects in the present invention, the sum of the contents of the above elements is preferably 0.05% by weight or more, more preferably 0.3% by weight or more. However, if the content is excessive, the formation of the aluminum oxide film formed on the surface of the plating layer may be hindered, and the barrier function of aluminum may be impaired. Therefore, the sum of the sum of the above elements is preferably 2.0 wt% or less, more preferably 1.5 wt% or less.

According to one example, the zinc-based plated layer may further contain 0.1 to 5.0% by weight of Mg and 0.1 to 7.5% by weight of Al.

Mg is an element that plays a role in improving the corrosion resistance of a hot press formed product. In order to exhibit such an effect in the present invention, it is preferable to contain 0.1 wt% or more, more preferably 1 wt% or more. However, if the content is excessive, there is a problem of occurrence of plating bath loss due to Mg oxidation in the plating bath. Therefore, the upper limit of the magnesium content is preferably 5.0 wt%, more preferably 4.0 wt%, and even more preferably 3.0 wt%.

Al plays a role of suppressing Mg oxide dross. If the content is too low, the effect of Mg oxidation prevention in the plating bath is insignificant. Therefore, the lower limit of the aluminum content is preferably 0.1 wt%, more preferably 1.5 wt%. However, when the content is excessive, there is a problem that the plating bath temperature must be increased. If the plating bath temperature is high, the plating facility may be eroded. Therefore, the upper limit of the aluminum content is preferably 7.5% by weight, more preferably 7.2% by weight.

According to one example, the Fe alloying degree of the alloyed zinc-based plated layer formed by alloying the zinc-based plated layer is preferably 30 to 85%, more preferably 45 to 78%, still more preferably 50 to 75% . When the Fe content is in the above range, it is possible to effectively prevent surface cracking during hot pressing, and it is advantageous that the corrosion resistance by the sacrificial method is excellent. If the Fe alloying degree is less than 30%, the region in which a part of Zn in the plating layer is concentrated may exist as a liquid phase, which may cause liquid brittle cracking during processing. On the other hand, when the Fe alloyed degree is more than 85%, the corrosion resistance may decrease.

The hot-pressed product of the present invention is characterized in that at least 70% by weight of at least one element selected from the group consisting of Sb, Sn and Bi is concentrated in a region within 3 占 퐉 from the surface of the alloying-zinc-based plating layer.

As described above, when Sb, Sn and Bi are concentrated in a large amount on the surface of the alloying zinc plated layer, Sb, Sn and Bi are located on the surface of the plating layer before oxygen penetrates from the surface of the plating layer to cause grain boundary segregation, It is possible to prevent formation of intergranular cracks in the plating layer by inhibiting the formation of oxides, thereby preventing the propagation of micro cracks to the substrate member. Moreover, micro cracks are mainly generated in areas where the friction between the mold and the plating layer is severe. The oxide of Sb, Sn, and Bi concentrated on the surface reduces the friction coefficient between the mold and the plating layer to reduce the occurrence of micro cracks, The durability can be further improved.

In the present invention, a specific method for measuring the content of at least one element selected from the group consisting of Sb, Sn and Bi concentrated in a region within 3 占 퐉 from the surface of the alloying zinc plated layer is not particularly limited, , The following method can be used. That is, after the hot press-molded article was cut vertically, the distribution of at least one element selected from the group consisting of Sb, Sn and Bi in the section of the plating layer was measured by using GDS (Glow Discharge Emission Spectrometry) Sb, Sb and Bi concentrated in a region within 3 占 퐉 from the surface of the alloying zinc plated layer by integrating the area in a graph relating to the content of at least one element selected from the group consisting of Sb, Sn and Bi relative to the depth from the surface of the plating layer, The content of at least one element selected from the group consisting of Sn and Bi can be measured.

The hot-pressed product of the present invention described above can be produced by various methods, and the production method thereof is not particularly limited. However, it can be produced by the following method as one embodiment thereof.

Hereinafter, a method for manufacturing a hot press molded article having excellent durability, which is another aspect of the present invention, will be described in detail.

First, a galvanized steel sheet having the above-described alloy composition is prepared. In the present invention, a specific method for preparing a zinc-based plated steel sheet is not particularly limited, and it can be produced by a conventional method for producing a zinc-based galvanized steel sheet. For example, in a zinc-based plating bath having the above- The zinc plated steel sheet can be prepared by immersing the steel sheet and cooling it.

However, in order to further maximize the desired effects of the present invention, it is preferable to perform bubbling by supplying an inert gas in advance in the zinc-based plating bath before immersing the substrate steel in the zinc-based plating bath. At this time, the inert gas may be one or more selected from the group consisting of nitrogen (N ₂ ), argon (Ar) and helium (He).

When performing bubbling in the zinc-based plating bath prior to performing the plating as described above, not only Sb, Sn and Bi are more uniformly distributed in the zinc-based plating bath, Helping to distribute Sb, Sn and Bi more uniformly in the interconnected plated layer and also helping to concentrate Sb, Sn and Bi on the surface of the alloyed zinc plated layer of the resulting hot press formed product. This is because the more uniform the distribution of Sb, Sn and Bi in the pre-heating plating layer for hot press forming, the more easily the surface concentration of Sb, Sn and Bi becomes.

On the other hand, in order to obtain the above effect, the supply of the inert gas is preferably maintained for 1 hour or more, and more preferably for 3 hours or more. On the other hand, as the supply time of the inert gas is increased, it is more advantageous to evenly distribute the components in the plating bath, and therefore the upper limit is not particularly limited.

Next, the plated zinc plated steel sheet is firstly heated to be processed into a molded product. This step is performed in order to sufficiently provide the zinc content of the plating layer in the step of increasing the melting point by alloying with zinc after the zinc in the plating layer is oxidized in the atmosphere and then heating it.

During the first heating, the average heating rate is preferably 3.5 to 4.2 DEG C / sec. If the temperature is less than 3.5 ° C / sec, the rise time is prolonged, and the effect of the increase of the melting point due to alloying is delayed to cause excessive oxidation of zinc. On the other hand, There is a fear that the surface of the plating layer may be oxidized by melting first.

During the first heating, the first heating end temperature is preferably 640 to 680 ° C. If the temperature is lower than 650 ° C, the diffusion coefficient in the plating layer is low due to the low temperature, and the plating layer may not be uniformly alloyed. On the other hand, if the temperature exceeds 680 ° C, the plating layer is liquefied beyond the melting point of zinc delta, Thereby causing loss of the plating layer.

Next, the firstly heated zinc plated steel sheet is secondarily heated. This step is performed to inhibit microcracks by preventing the internal oxidation inhibitors added to the Fe-alpha phase while the plating layer changed to a sufficiently delta phase is stabilized.

During the secondary heating, the average heating rate is preferably 1.1 to 1.6 DEG C / sec. If the temperature is less than 1.1 ° C / sec, the alloying time to Fe-alpha phase becomes long and there is a fear of oxygen calculation due to oxygen rather than grain boundary segregation element. On the other hand, There is a possibility of quality due to a uniform surface due to the occurrence of liquefaction of the plating layer.

In the secondary heating, the secondary heating end temperature is preferably 900 to 930 ° C. If the temperature is lower than 900 ° C., sufficient austenite transformation of the material can not be achieved, which makes it difficult to secure the strength of the final product. When the temperature exceeds 930 ° C., the plating layer is liquefied and the fine crack- .

Next, the secondarily heated zinc plated steel sheet is kept at the second heating end temperature for 1 to 5 minutes. If the holding time is less than 1 minute, it may be difficult to secure a sufficient time for transformation of the austenite of the material due to the shortage of the total heating time. On the other hand, if the holding time exceeds 5 minutes, excessive galvannealing of the plating layer causes deterioration Resulting in deterioration of corrosion resistance.

Next, the secondarily heated zinc plated steel sheet is molded by the die and quenched at the same time. At this time, the molding and quenching by the mold is sufficient according to a conventional hot press forming method, and therefore, the present invention does not particularly limit it.

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. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

A low carbon cold-rolled steel sheet having a thickness of 0.8 mm, a width of 100 mm and a length of 200 mm was prepared as a test piece for plating, and the above-ground steel sheet was immersed in acetone and ultrasonically cleaned to remove foreign substances such as rolling oil present on the surface. Thereafter, the steel sheet was subjected to a heat treatment in a reducing atmosphere at 750 캜 for securing the mechanical properties of the steel sheet at the general hot-dip coating site, and then immersed in a zinc-based plating bath having the composition shown in the following Table 1 to prepare a plated steel material. Thereafter, each of the produced plated steel materials was gas-wiped to adjust the amount of plating adhered to 70 g / m ² per one side and cooled at a rate of 12 ° C / sec.

Thereafter, each cooled plated steel material was heated under the conditions shown in Table 2, and hot press molded to obtain a hot press molded product.

Then, each of the hot press-molded articles was cut vertically, and the distribution of intergranular segregation elements in the plated layer was measured by GDS analysis. The results are shown in Table 2 below. The specific measurement method is as described above.

Then, the maximum depth of micro cracks was measured at the portion where tensile and surface friction were most severe during molding, and the results are shown in Table 2 below.

Bath type Plating bath composition (% by weight) (* remaining amount is Zn) Sb Sn Bi Al Mg Bath 1 - - - 0.21 - Bath 2 - 0.7 - 1.50 1.0 Bath 3 - 0.4 - 2.5 1.0 Bath 4 - - - 2.5 1.0 Bath 5 - 0.3 - 7.0 3.0 Bath 6 0.3 - - 1.5 1.0 Bath 7 - - 0.5 2.5 1.0

No. Bath type Primary heating Secondary heating maintain ① Maximum crack depth
(μm) Remarks Speed (° C / s) End temperature (캜) Speed (° C / s) End temperature (캜) Time (minutes) One Bath 1 3.8 640 1.2 900 5 - 32.0 Comparative Example 1 2 Bath 1 4.2 670 1.6 910 5 - 29.0 Comparative Example 2 3 Bath 2 3.5 650 1.2 900 5 78 2.5 Inventory 1 4 Bath 2 3.9 670 1.4 910 5 85 3.8 Inventory 2 5 Bath 3 3.7 650 1.3 910 5 99 3.0 Inventory 3 6 Bath 3 4.0 660 1.1 900 5 92 3.7 Honorable 4 7 Bath 4 4.0 660 1.3 900 5 - 28.0 Comparative Example 3 8 Bath 4 3.9 660 1.5 915 5 - 26.0 Comparative Example 4 9 Bath 5 3.7 650 1.2 910 5 77 7.0 Inventory 5 10 Bath 5 3.8 640 1.4 910 5 79 7.3 Inventory 6 11 Bath 6 4.0 650 1.5 900 5 83 5.2 Honorable 7 12 Bath 6 4.1 640 1.3 920 5 82 6.0 Honors 8 13 Bath 7 3.9 650 1.3 910 5 89 8.0 Proposition 9 14 Bath 7 3.5 640 1.2 930 5 91 7.8 Inventory 10 Here, (1) means the content (% by weight) of at least one element selected from the group consisting of Sb, Sn and Bi concentrated in a region within 3 탆 from the surface of the alloying zinc plated layer.

Referring to Table 2, it can be confirmed that the maximum depth of fine cracks in Inventive Examples 1 to 10 satisfying all the conditions of the present invention was suppressed to 10 μm or less.

1 (a) is a graph showing microcracks in Inventive Example 1, FIG. 1 (d) is a graph showing microcracks observed in Inventive Example 3, and FIG. 1 And FIG. 1E is a view showing the fine cracks of Inventive Example 5 observed. Referring to FIGS. 1A to 1E, it can be seen that the propagation of fine cracks in the plating layer to the steel sheet is effectively blocked in the case of the inventive examples.

2 (a) is GDS data obtained by analyzing the content of Al, Mg and Sn according to the depth of the plating layer in Inventive Example 1, and FIG. 2 (b) 2 shows the GDS data obtained by analyzing the content of Al, Mg and Sn according to the depth of the plating layer in Inventive Example 5. FIG.

Claims (7)

A hot press formed article produced by hot press forming a zinc plated steel sheet comprising a base steel sheet and a zinc-based plated layer formed on the surface of the base steel sheet,
Wherein the zinc-based electroplating layer contains 0.05 to 2.0% by weight in total of at least one element selected from the group consisting of Sb, Sn and Bi, the remainder Zn and unavoidable impurities,
Wherein at least 70 wt% of at least one element selected from the group consisting of Sb, Sn and Bi is hot pressed into a region within 3 mu m from the surface of the alloyed zinc plated layer of the hot press formed product formed by alloying the zinc plated layer, Shaped article.
The method according to claim 1,
Wherein the zinc-based plated layer contains 0.3 to 1.5 wt% of at least one element selected from the group consisting of Sb, Sn and Bi in total.
The method according to claim 1,
Wherein the zinc-based plated layer further contains 0.1 to 5.0% of Al and 0.1 to 5.0% of Mg in weight percent.
The method according to claim 1,
Wherein the alloyed zinc-plated layer has an Fe alloyed degree of 30 to 85%.
Preparing a zinc plated steel sheet;
Heating the zinc plated steel sheet to a temperature of 640 to 680 ° C at a rate of 3.5 to 4.2 ° C / sec;
Heating the primary heated zinc plated steel sheet to a temperature of 900 to 930 캜 at a rate of 1.1 to 1.6 캜 / sec;
Maintaining the secondarily heated zinc plated steel sheet at a constant temperature for 1 to 5 minutes; And
And a step of forming the galvanized steel sheet kept at a constant temperature by a mold and quenching it,
The zinc-plated steel sheet includes a base steel sheet and a hot press formed on the surface of the base steel sheet and including a zinc-based plated layer containing a total of at least one element selected from the group consisting of Sb, Sn and Bi in a total amount of 0.05 to 2.0% A method of manufacturing a molded article.
6. The method of claim 5,
Wherein the zinc-based plated layer contains 0.3 to 1.5 wt% of at least one element selected from the group consisting of Sb, Sn and Bi in total.
6. The method of claim 5,
Wherein the zinc-based plated layer further contains 0.1 to 5.0% of Al and 0.1 to 5.0% of Mg in terms of weight%.

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