KR101359161B1 - Plated steel sheet for hot press forming having corrosion resistance - Google Patents

Abstract

The present invention relates to a plated steel sheet for hot press molding excellent in corrosion resistance, and more particularly, a hot press capable of preventing corrosion and volatilization of excessive zinc contained in a plated layer during hot press forming to have sufficient zinc content in the plated layer to maintain corrosion resistance. It relates to a galvanized steel sheet.
Plated steel sheet according to one aspect of the invention the steel sheet; And a galvanized layer formed on the base steel sheet, wherein the plated steel sheet includes a continuous oxide layer having a thickness of 10 to 200 nm on the surface of the plated layer in a temperature range of 950 ° C. or less when heated to 400 ° C. or more. It features.

Description

Plated steel sheet for hot press molding excellent in corrosion resistance {PLATED STEEL SHEET FOR HOT PRESS FORMING HAVING CORROSION RESISTANCE}

The present invention relates to a plated steel sheet for hot press molding excellent in corrosion resistance, and more particularly, a hot press capable of preventing corrosion and volatilization of excessive zinc contained in a plated layer during hot press forming to have sufficient zinc content in the plated layer to maintain corrosion resistance. It relates to a galvanized steel sheet.

Recently, demand for high strength steel plates has been increasing rapidly in order to reduce automobile fuel consumption in accordance with environmental regulations. As automobile steel sheet is strengthened, wear, fracture, etc. occur easily during press molding, and complicated product molding is difficult. Therefore, in order to solve this problem, the production of products by the hot press process of forming a steel sheet in a hot state by heating the steel sheet has been greatly increased.

Hot press steel sheet is usually subjected to press working in a state heated to 800 ~ 900 ℃, the surface scale of the center of the Zn oxide is formed thick, or the problem of deterioration of the plating layer that the plating layer disappears due to the volatilization of Zn is seriously raised. In this case, the Zn plating layer is nonuniform and the appearance quality is reduced, and the portion where the Zn layer disappears greatly reduces the corrosion resistance. Therefore, the surface must be protected from excessive oxidation or volatilization in the furnace, especially during elevated temperatures.

According to one aspect of the invention, there is provided a plated steel sheet for hot press molding to prevent the liquid metal embrittlement phenomenon and improved alloying and high temperature stability of the galvanized layer.

Plated steel sheet according to one aspect for solving the problem of the present invention is a steel sheet; And a galvanized layer formed on the base steel sheet, wherein the plated steel sheet includes a continuous oxide layer having a thickness of 10 to 200 nm on the surface of the plated layer in a temperature range of 950 ° C. or less when heated to 400 ° C. or more. It features.

At this time, the plated steel sheet is formed an oxide containing 1 ~ 5㎛ Zn oxide and Mn oxide on the continuous film at a temperature between 780 ~ 950 ℃, the content of Zn oxide in the oxide in the weight percent It is preferably at least 80% of the oxides.

As one more preferable form of the above-described advantageous plated steel sheet, it is preferable to further include an Al thickening layer having a uniform shape containing 30% by weight or more of Al between the base steel sheet and the galvanized layer.

In addition, the base steel sheet is advantageous to further include a surface diffusion layer of a metal less than the amount of Gibbs free energy per mole of oxygen during the oxidation reaction within 1㎛ depth from the surface.

As another preferred form of the plated steel sheet of the present invention, it is advantageous to further include a powder layer of the metal or metal compound forming the oxide layer on top of the galvanized layer before being heated to 400 ° C. or higher.

At this time, the base steel sheet has a composition consisting of C: 0.1 to 0.4% by weight, Si: 2.0% or less (excluding 0%), Mn: 0.1 to 4.0%, balance Fe and other unavoidable impurities after hot press molding. It is advantageous to secure desirable physical properties.

In addition, the base steel sheet is N: 0.001-0.02%, B: 0.0001-0.01%, Ti: 0.001-0.1%, Nb: 0.001-0.1%, V: 0.001-0.1%, Cr: 0.001-1.0%, Mo: It is preferable to further include at least one selected from the group consisting of 0.001 to 1.0%, Sb: 0.001 to 0.1% and W: 0.001 to 0.3%.

The steel sheet provided by the present invention is formed with a uniform oxide layer on the surface during heating for hot press molding, thereby preventing the volatilization of Zn, thereby ensuring a sufficient amount of plating layer to ensure corrosion resistance, It becomes possible to heat-treat the plated steel sheet at a high temperature.

1 is a cross-sectional view of the surface of the plating layer of the member of the galvanized steel sheet of the present invention in a heating furnace heated to 900 ℃ in one embodiment of the present invention by removing the steel sheet from the heating furnace at the time of 3 minutes and 30 seconds to cool in the mold This is the result of observing with a transmission electron microscope (TEM). The steel plate surface temperature when the steel sheet was taken out of the heating furnace was 820 ° C. In the photograph of Fig. 1, the bright part is an Al2O3 layer and the lower layer is a galvanized layer.
2 is a graph showing the results of analyzing the plated steel sheet layer of FIG. 1 according to depth.
3 is a cross section of the surface of the plated layer of the member cooled by the mold by inserting the same steel sheet as in FIG. 1 into the heating furnace heated to 900 ° C and removing the steel sheet from the heating furnace at 6 minutes. This is the result of mapping the components. At this time, the temperature of the steel sheet reaches about 900 ° C, which is the same as the internal temperature of the heating furnace. It can be seen that a thin Al 2 O 3 layer is formed directly on the Zn plating layer, and the Zn oxide is formed on the Zn oxide to a thickness of about 2 μm.
4 is an electron micrograph of the plated steel sheet of Inventive Example 1 subjected to intermediate extraction at 750 ° C. to observe the formation of surface oxides.
FIG. 5 is an electron micrograph of the plated steel sheet of Comparative Example 2 obtained by intermediate extraction at 750 ° C. to observe a result of forming a surface oxide. FIG.

EMBODIMENT OF THE INVENTION Hereinafter, the steel plate of this invention is demonstrated in detail.

The present invention is directed to galvanized steel sheet. In general, a galvanized steel sheet is a steel sheet having a plating layer containing zinc as a main component (for example, Zn ≧ 50% by weight), and the corrosion resistance of the steel sheet may be greatly improved by the sacrificial anode effect of zinc.

The inventors of the present invention find that the zinc-based plated steel sheet needs to efficiently form an effective Al ₂ O ₃ oxide layer on the surface during heating in order to minimize oxidation or volatilization of the plating layer at a temperature for hot press forming. Discovered and led to the present invention.

That is, Zn is a metal having a very low melting point and a boiling point among the metals for plating. As a result, Zn can be volatilized and oxidized as described above at a hot press forming temperature of 780 to 950 ° C., so hot galvanized steel sheets are usually hot pressed. When it is used for molding, the zinc content in the plating layer is excessively reduced due to the loss of Zn, and thus, the zinc plating layer cannot exhibit its unique function of providing corrosion resistance by sacrificial anode method.

However, the steel material of the present invention, as can be seen from the micrograph of FIG. 1 and the component analysis result of FIG. 2, the temperature of the steel sheet is higher than 400 ° C. or more in the middle of the process of rising to the working temperature of 780-950 ° C., the heating target temperature. When it reaches, the oxide film starts to be actively formed on the surface. At this time, since the plating surface is protected at a high temperature according to the quality and thickness of the generated oxide, the formation of the oxide becomes very important for securing the heat resistance of the zinc plating material.

That is, the oxides generated on the surface during heating serve as a barrier to volatilization and oxidation of zinc in the plating layer until the hot press forming temperature is reached. In this case, since zinc volatilization and oxidation do not occur at a high temperature, the zinc content in the plating layer can be maintained in an appropriate range (for example, 25 to 35% by weight), which is advantageous for securing corrosion resistance. That is, the steel of the present invention is characterized by forming a continuous oxide layer on the surface of the plating layer at a temperature of 400 ℃ or more. Continuous in the present invention means that the oxides formed on the entire surface of the galvanized layer are connected in a substantially segmental manner. If there was a segment of oxide, new oxide is formed from the segmented site, so the segment can be seen by observing the cross section. At this time, it is more preferable that the average heating rate until reaching a target temperature at the time of heating has a range of about 2-20 degreeC / sec according to a normal hot press molding process. The oxide layer is an Al 2 O 3 or SiO 2 layer, and most preferably 90% or more Al 2 O 3, so that a dense layer is formed on the galvanized layer so that zinc does not come into contact with the atmosphere. At this time, in order for the oxide layer to act as a barrier by contact between Zn and the atmosphere, the thickness of the oxide layer is preferably 10 nm or more. If the thickness of the oxide layer is too small, even if the oxide layer does not exist continuously or continuously, it is segmented by small stress and diffusion of the material, and eventually Zn comes into contact with the atmosphere. In addition, when the thickness of the oxide layer is too large, it tends to be brittle, so that it is difficult to act as a sufficient barrier, and therefore, an appropriate oxide thickness is preferably 200 nm or less.

At this time, since the oxide is preferably formed in an oxidizing atmosphere, for example, the oxygen partial pressure is preferably set to 10 ^-40 atm or more, preferably 10 ^-5 atm or more, and the product is carried out in the air. Most preferred in terms of economics of manufacture. In this case, technically, oxide formation as described above is possible in pure oxygen atmosphere, and effective oxide formation is possible even under a condition where the oxygen partial pressure exceeds 1 atm.

Therefore, the plated steel sheet for hot press forming according to an aspect of the present invention is characterized by forming a continuous oxide having a thickness of 10 to 200 nm in the surface layer at 400 ° C. or higher. In particular, whether the steel forms an oxide having a thickness of 10 to 200 nm on the surface layer may be judged by the result of observing the cut surface of the steel sheet quenched at 750 ° C. The thickness of the oxide layer is more preferably limited to 30 to 150 nm, most preferably 50 to 120 nm.

The plated steel sheet for hot press forming, which satisfies the advantageous conditions of the present invention described above, may be various, and is not particularly limited in the present invention. However, since the hot-rolled plated steel sheet which satisfies the above-described heating path is alloyed uniformly while the alloying proceeds rapidly, it is more advantageous to minimize the volatilization loss of zinc in the plated layer. Briefly explain.

The inventors of the present invention, if the Al thickening layer containing more than 30% by weight of Al at the interface between the steel sheet and zinc is formed homogeneously at 400 ℃ or higher as Al is diffused in the Al thickened layer and moved to the surface By forming an oxide on the surface of the plating layer at a temperature, it has been found that volatilization of zinc in the plating layer can be suppressed, oxide growth can be prevented, and a uniform alloy layer can be obtained. That is, one preferred side of the plated steel sheet of the present invention is characterized in that it further comprises a homogeneous Al thickening layer containing at least 30% by weight of Al between the base steel sheet and the galvanized layer.

In the Al thickening layer, Al is diffused to the surface of the plating layer during hot press molding in the future to move to the surface of the plating layer to selectively oxidize to form a dense and thin oxide layer having Al 2 O 3 as a main component (eg, 90% by weight or more of Al 2 O 3). It plays a role. The oxide layer formed on the surface of the plating layer during hot press molding serves to prevent the volatilization of zinc so that zinc in the alloy layer is sufficiently present during the alloying process by heating to faithfully serve as a sacrificial anode.

The Al thickening layer is preferably present in combination with a ratio close to the stoichiometric ratio of Fe and the intermetallic compound, for example, in the form of Fe ₂ Al ₅ .

The concentrated layer may include some Zn, but the content is preferably limited to within 10% by weight. When the Zn content is 10% by weight or more, the shape of the Al thickened layer becomes nonuniform, thereby reducing the effect of the homogeneous alloying.

In this case, the Al thickening layer has a form in which fine particles are continuously formed. In the present invention, the area ratio occupied by the Al thickening layer at the interface between the steel plate and the plating layer is preferably 88% or more. When the occupied area ratio of the Al enriched layer is low, the oxide layer may not be sufficiently formed on the surface of the plating layer, thereby preventing the volatilization of zinc from being sufficient.

In addition, it is preferable that the particle | grains which comprise the said Al thickened layer make many particle | grains whose particle size (it defines with the maximum length of the particle | grains in this invention, although there are many methods of defining particle size) are 500 nm or less. The reason why the ratio of the fine particles having a particle size of 500 nm or less should be high is that the Al-concentrated layer must be easily decomposed upon heating for press molding in order to quickly move to the surface of the plating layer to form an oxide layer. In other words, as the fine particles are distributed in a large amount, the interface is increased and the compound particles are thermodynamically unstable, and thus can be easily decomposed.

For this purpose, when observed using a particle size analyzer such as an image analyzer, for example, it is preferable that the number of particles having a particle size of 500 nm or more is present within an average of 15 particles per 100 μm ² . The number of particles having a particle size of 500 nm or more is higher than this, indicating that the particle size is not uniform as a whole, and the above-described concentration layer is not easily decomposed, which is less favorable for preventing volatilization of zinc and liquid embrittlement during hot pressing.

Therefore, according to one aspect of the present application, the plated steel sheet of the holding steel sheet; An Al thickening layer containing 30 wt% or more of Al formed on the base steel sheet; And a zinc plating layer formed on the Al thickening layer, wherein the number of particles having a particle size of 500 nm or more among the particles constituting the Al thickening layer is preferably present in an average of 15 or less per 100 μm ² , and the thickening layer is a steel sheet It is preferable to distribute at 88% or more of the occupied area ratio at the interface between the zinc plated layer and the zinc plated layer.

Since there may be a variety of methods for adjusting the particle size distribution of the particles forming the Al enriched layer, the independent claims of the present invention do not particularly limit it. However, for example, when the diffusion barrier layer is formed as described below or the dew point temperature is controlled during annealing, the elements dissolved in the steel sheet are internally oxidized to prevent the diffusion and oxidation to the surface to prevent oxidation. Can be obtained.

At this time, it is more preferable that the Al thickening layer has a thickness of 0.1 to 1.0 μm. If the thickness of the Al thickened layer is less than 0.1 μm, the amount is too short to form the oxide film continuously. If the thickness exceeds 1.0 μm, the thickness of the oxide film may be too thick. It is preferable to limit it to -1.0 micrometer. There may be various methods for adjusting the thickness of the concentrated layer, but if one of them is mentioned, there may be mentioned a method for adjusting the Al content in the plating layer. If hot dip galvanizing is used, the Al content in the galvanizing bath can be controlled. In addition, a thickening layer can be obtained even when zinc plating is applied after the aluminum compound is coated on the surface of the steel sheet. In this case, the thickening layer can be adjusted by adjusting the coating thickness of the aluminum compound.

There may be various methods of forming the Al enriched layer. For example, when providing from a plating layer, it is more preferable that a plating layer contains 0.05 to 0.5 weight% of Al. If the Al content is less than 0.05%, the plating layer is easily formed non-uniformly, and if the Al content is more than 0.5%, an activation layer is formed thickly at the interface of the Zn plating layer, and thus the reaction is initiated in a hot press heating furnace. Since the diffusion rate of Fe, Mn, etc. into the Zn layer decreases and the alloying in the heating furnace is delayed, the amount of Al is limited to 0.5% or less, and more preferably 0.25% or less. More effective.

In another embodiment of forming an oxide layer on the surface of the plating layer, a galvanized steel sheet in which a galvanized layer is formed on a base steel sheet and a powder layer of a metal or metal compound that forms an oxide on the galvanized layer is heated before being heated to 400 ° C. or more. Can be mentioned.

Another embodiment of forming an oxide is a so-called zinc alloyed (GA) steel sheet, which is 5 to 80% by weight of Fe, preferably 5 to 12% by weight, as a composition before heating to 400 ° C or more. In the zinc alloy plating layer included may have a plating layer containing 0.1 to 0.3% by weight of Al on the surface.

The metal powder layer preferably includes Al or Si, which is an intermediate structure for forming oxides such as Al ₂ O ₃ and SiO ₂ on the plating layer after press heating, and the Al or Si powder is deposited on the galvanized layer as described above. When formed, the Al ₂ O ₃ or SiO ₂ oxide is formed on the plating layer by press heating, and the oxide serves to stably maintain the galvanized layer without deterioration even at a high temperature at which the zinc plating layer performs hot press molding as described above.

In addition, the average thickness of the metal powder layer is preferably from 0.1 to 5.0㎛, if the thickness is less than 0.1㎛ the thickness is too thin, even if hot press heating to form a sufficient Al ₂ O ₃ or SiO ₂ oxide It may not be enough to play a protective role for the galvanized layer. That is, when the thickness of the layer is less than 0.1㎛, even if the Al ₂ O ₃ or SiO ₂ oxide film is formed through hot press heating afterwards, the amount is very small and can substantially prevent the galvanized layer from deteriorating. Since there is a possibility that the adhesion state of the metal may also occur unevenly, in order to ensure excellent heat resistance, it is effective to attach the metal powder sufficiently above the thickness in the metal coating step, and more preferably, to have a thickness of 0.3 μm or more. In this case, heat resistance can be more excellent. On the contrary, if the thickness exceeds 5.0 μm, the thickness of the oxide layer formed after hot press heating becomes excessive, resulting in deterioration of weldability or peeling of the oxide layer. In this case, the thickness of the Al 2 O 3 or SiO 2 main oxide formed on the surface of the coated steel sheet during hot press heating is also preferably about 10 to 200 nm.

At this time, the content of Al or Si in the metal powder layer is preferably at least 30% by weight. The metal powder includes Al or Si, and the content of Al or Si should be at least 30% by weight so that the amount of Al ₂ O ₃ or SiO ₂ oxide which substantially serves to protect the galvanized layer in the oxide layer formed after hot press heating. We can secure enough.

Moreover, as for the said metal powder, it is more preferable that average diameter is 5 micrometers or less. If the diameter is large, the gap between the powders is too large and the oxide film is likely to be unevenly formed after hot press heating. Therefore, by controlling the diameter of the metal powder to 5 μm or less, the oxide film is formed continuously and tightly after press heating. It is preferable to ensure heat resistance.

In order to manufacture a plated steel sheet according to the present embodiment, a method of applying a powder of a metal or a metal compound forming the oxide may be used to perform zinc plating on a surface of a steel sheet and form a fine powder. That is, when the hot-dip galvanized steel sheet is manufactured by applying the powder of the metal or metal compound forming the oxide to the surface of the steel sheet after immersing the steel sheet in the galvanizing bath, the metal oxide is then applied to the surface of the steel sheet by press heating. Recognizing that the role of the protective film to prevent the deterioration of the plating layer is formed, the invention is to achieve the invention that can ensure a stable plating layer even at high temperatures. Preferably, the metal powder preferably contains Al or Si, which may be included alone or in the form of an alloy such as AlSi, FeAl or FeSi.

In manufacturing the galvanized steel sheet and hot press molded part of this embodiment, there is no particular limitation on the type of galvanizing method. That is, hot dip galvanizing may be applied, or electro galvanizing may be applied, or dry plating using plasma or high temperature liquid Zn spraying may be performed. One side of the present invention may be a hot dip galvanizing method as an example of the above zinc plating method. To explain.

At this time, the step of immersing in the zinc plating bath is preferably carried out in a zinc plating bath having a temperature range of 430 ~ 500 ℃. In order for the plating bath to have sufficient fluidity, a temperature of 430 ° C. or more is required, and in order to sufficiently enrich Ni and Co elements and Al at the interface between the plating layer and the base steel sheet, it is more preferable to control the temperature to 460 ° C. or more, but 500 ° C. When exceeding, the occurrence of dross in the plating bath is frequently generated, which is not preferable in terms of mass production efficiency. Therefore, the upper limit of the plating bath temperature is preferably limited to 500 ° C.

In addition, the immersion in the zinc plating bath is preferably performed in a zinc plating bath containing less than 0.5% by weight of Al, Zn and the inevitable impurities. As a method of producing an oxide layer serving as a protective film of the plating layer, the above method of directly applying a metal (for example, Al or Si) forming an oxide is preferable, and when Al is further included in the plating bath, Al Concentrated at the interface between the base steel plate and the plating layer, the concentrated Al is diffused out of the plating layer during press heating, Al ₂ O ₃ It can contribute to forming a film.

That is, this is not an essential process, but in addition to the formation of an oxide film by the application of the Al metal, it may be a method that can be selected to maximize the effect, wherein the Al contained in the galvanizing bath is 0.5% by weight or less. desirable. If the amount of Al contained in the galvanizing bath exceeds 0.5% by weight, the oxide film formed on the plating layer after the press heating may be too thick, which may cause peeling of the oxide layer. Therefore, the upper limit is 0.5% by weight. It is preferable.

In addition, the step of applying the metal powder is more preferably performed when the surface temperature of the steel plate after the immersion is 430 ℃ or more. If the metal powder is applied while the surface temperature is less than 430 ° C., solidification of the galvanized layer begins to occur, so that the metal powder may be scattered without being attached to the surface of the galvanized layer. It is necessary to contain the metal powder before falling below this temperature.

Meanwhile, the method may further include performing an alloying heat treatment after applying the metal powder. At this time, the step of performing the alloying heat treatment is preferably performed at a temperature range of 400 ~ 600 ℃. In order to sufficiently obtain the alloying effect, it is necessary to heat at a temperature of at least 400 ° C., but when it exceeds 600 ° C., the plating layer is alloyed to increase heat resistance in the hot press heating furnace, but cracking may occur due to embrittlement of the plating layer. Since the growth of scale on the surface of the plating layer increases in the furnace, the alloying heat treatment temperature is limited to 600 ° C. or less, preferably 500 ° C. or less, thereby suppressing Fe in the plating layer to 5% by weight or less, thereby generating fine cracks in the plating layer. Can be effectively prevented, and more preferably, it can be controlled to 450 ° C or less to suppress the occurrence of fine cracks as much as possible.

Plated steel sheet according to an embodiment of the present invention having the above-described characteristics can form an oxide layer of 10 ~ 200nm thickness on the surface at a temperature of 400 ℃ or more.

The plated steel sheet of the present invention obtained by the various embodiments as described above is further heated to an oxide of a metal other than the oxide on the oxide layer at a press molding temperature (for example, 780 ~ 950 ℃) 1 ~ 5㎛ thickness It can be formed as. Preferably, the oxide layer contains 80 wt% or more of Zn oxide (eg, ZnO). In this case, it is possible to obtain an effect of having excellent electrical conductivity and advantageous for electrodeposition coating and phosphate treatment. In addition to the Zn oxide, Mn oxide (eg, MnO) may be included, and trace amounts of oxides of Fe may be included. In this case, when the thickness of the oxide is too thin, the phosphate adhesion or electrodeposition coating property is inferior. On the contrary, when the thickness is too thick, the oxide may be peeled off and the operation may be hindered by the oxide contamination in the subsequent process after hot pressing.

In addition, according to a more preferred aspect of the present invention, the base steel sheet is a metal surface diffusion layer (hereinafter, simply referred to as a 'surface diffusion layer') of the metal less than the amount of Gibbs free energy per mol of oxygen during the oxidation reaction within 1㎛ depth from the surface It is preferable to include. The surface diffusion layer may be dissolved in the Fe-Zn phase during heating for hot forming to prevent the component dissolved in the steel sheet from diffusing into the plating layer, and at the same time, the Zn of the zinc plating layer may be prevented from diffusing into the steel sheet. When Zn of the zinc plated layer is diffused into the base steel sheet, Zn diffused into the base steel sheet has a lower ratio than the steel sheet component, and thus hardly contributes to the improvement of corrosion resistance of the steel sheet. By reducing the amount of Zn consumed, the steel sheet is reduced. It is possible to secure a large amount of Zn (for example, 25 to 35% by weight) that can contribute to the improvement of corrosion resistance. In addition, in the presence of the surface diffusion layer, the Fe component uniformly diffuses from the steel sheet to assist alloying. Due to this role, the plating layer has a liquid metal even at a hot press forming temperature (for example, 750 to 950 ° C). It is not present so that liquid metal embrittlement (LME) can be effectively suppressed. In addition, since the surface diffusion layer allows the Al thickening layer of the present invention to be more easily formed, the surface diffusion layer serves to simultaneously generate the particles forming the Al thickening layer, and as a result, as described above, The particle size distribution conditions of the particles forming the preferred Al thickened layer can be satisfied.

To this end, it is preferable that the amount of metal having a Gibbs free energy reduction amount less than Cr per mole of oxygen during the oxidation reaction included in the surface diffusion layer is 0.1 wt% or more. That is, the metal is diffused into the base material during the annealing heat treatment after coating to lower the concentration of the surface. As a result of the study, when the metal content is 0.1 wt% or more within 1 μm from the surface, the Al in the plating bath is galvanized. This is because the higher amount of Al can be concentrated on the surface diffusion layer by reacting with the metal. However, it is preferable that the content of the metal is contained within 10% by weight within 1 μm of the surface. If the Ni content is increased, the manufacturing cost increases, and even if more than 10% by weight, further effects are difficult to expect.

Therefore, in order to prevent the galvanized layer from being decomposed at a high temperature by coating the metal as described above, in order to secure the heat resistance of the galvanized layer, a metal having a Gib free energy reduction amount per mole of oxygen less than Cr is less than 0.1 when oxidized within 1 μm from the surface of the steel sheet. It should be present in the weight percent or more, preferably included in more than 1.0% by weight can effectively prevent the deterioration of the galvanized layer, more preferably 3.0 to 10% by weight or more will contribute to the excellent heat resistance of the galvanized layer. Can be.

Further, if the amount of Gibbs free energy reduction per mole of oxygen in the oxidation reaction is less than Cr includes a surface diffusion layer or the zinc-plated layer includes a metal in which Gibbs free energy reduction per mole of oxygen during the oxidation reaction is less than Cr is Al The concentrated layer may further include a metal having a reduced amount of Gibbs free energy per mole of oxygen during the oxidation reaction, less than Cr, the content of which may be 5 wt% or less of the entire Al enriched layer, and more preferably 0.1 to 5 wt%. Can be Metals having a reduced Gibbs free energy reduction per mole of oxygen during the oxidation reaction may be derived from a steel sheet.

In particular, when the surface diffusion layer is formed, Al is more concentrated on the surface diffusion layer through the interfacial reaction, so that the surface diffusion layer has an important influence on the formation of the Al concentration layer. At this time, the area where the Al content of the Al and the surface diffusion layers overlaps the portion of the Gibbs free energy reduction amount per mole of oxygen less than Cr by 5% by weight or more in the entire surface diffusion layer and the Al enrichment layer during EPMA analysis. It is preferably less than 10%, wherein the overlapping portion means that the metal and Al have alloyed to form an alloy phase. Once this way when the Al exists as the metals and alloys, it does not easily spread to the surface of the plating layer during press heating, the portion present in the alloy state lot Al ₂ O ₃ The amount of Al which can contribute to forming a continuous oxide film is substantially reduced. Therefore, in the EPMA analysis, the overlapping portion should be 10% or less, so that Al which does not exist in an alloy state is sufficiently positioned in the thickened layer to effectively form an Al ₂ O ₃ oxide film.

Representative examples of metals having a reduced Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction may include Ni, and in addition, Fe, Co, Cu, Sn, and Sb may be applied. Ni is an element with less oxygen affinity than Fe, and when Ni surface diffusion layer is coated on the surface of steel sheet, Ni does not oxidize during the annealing process after coating and inhibits oxidation of Mn, Si, etc. Done. Fe, Co, Cu, Sn, Sb also shows similar characteristics when coated on the metal surface. At this time, Fe is more preferably used in an alloy state with Ni and the like than using alone. In addition, the metal having a less Gib free energy reduction per mole of oxygen during the oxidation reaction is most preferably present as a surface diffusion layer, but is not necessarily limited thereto, and may be present in the zinc plating layer by being plated with zinc in the plating bath. Do.

In addition, the type of zinc plating layer of the present invention is not particularly limited, and may include both hot dip galvanization, electro zinc plating, dry zinc plating by plasma, and zinc plating layer by high temperature liquid Zn spray.

Fe is more preferably added to the galvanized layer, which is sufficient to increase the melting point of Zn by Fe being sufficiently diffused into the galvanized layer to form a Fe—Zn alloy phase, which corresponds to a very important configuration for securing heat resistance. However, more preferably, when Fe is excessively added, the delta (δ) or gamma (Г) phase metaphor in the plating layer becomes high, so that it is easily embrittled in the plating layer. Therefore, the Fe content is preferably limited to 15.0% by weight or less. In addition, when Fe is added in an amount of 5.0 wt% or less, fine cracks that may occur in the plating layer may be further reduced.

In addition, the zinc plating layer is Fe: 15.0% by weight or less, the amount of Gibbs free energy reduction per mole of oxygen during the oxidation reaction is less than Cr: 0.01 to 2.0% by weight, the remainder preferably contains Zn and other unavoidable impurities. In the oxidation reaction included in the hot dip galvanized layer, a metal having a Gib free energy reduction amount per mole of oxygen less than Cr is diffused into the plating layer during hot press heating to be included in the plating layer, and in particular, when the oxidation reaction is performed on Fe-Zn during hot press heating. Metals having a smaller Gis free energy reduction per mole of oxygen are dissolved to form a ternary phase, thereby reducing the diffusion of Fe, etc., of the ferrous iron into the plating layer during press heating, thereby not decomposing the galvanized layer. It plays a key role in forming a single plating layer. Therefore, in order to sufficiently form the three-phase phase in order to provide heat resistance to the plated layer during press heating, the zinc-plated steel sheet needs to include 0.01 wt% or more of metal having a Gibbs free energy reduction amount less than Cr per mole of oxygen during oxidation. In terms of economics, the upper limit is preferably set at 2.0% by weight.

In addition, the thickness of the galvanized layer should be 3㎛ or more to ensure the heat resistance at high temperature, if the thickness is less than 3㎛ may cause uneven thickness of the plating layer or lower the corrosion resistance, more preferably It is effective that is 5 micrometers or more. In addition, the thicker the thickness of the plating layer, the better the corrosion resistance. However, if the thickness is about 30 μm, sufficient corrosion resistance may be obtained. It is also possible to control the cracks due to LME that may occur on the surface at the time of press working as much as possible by controlling within 15 μm to ensure a high proportion of the alloy phase in which the Fe becomes 60 wt% or more in the plating layer after the hot pressing process.

In addition, the annealing heat treatment may be performed depending on the type of the plated steel sheet. At this time, the annealing oxide may be formed on the surface of the steel sheet. The annealing oxide is discontinuously distributed on the surface diffusion layer, and a part of the annealing layer may be included in the Al thickening layer. However, the annealing oxide not only acts as a diffusion barrier to prevent alloying of Fe, Mn, etc., which is a member of the hot dip galvanized layer and the steel sheet, but also adversely affects the formation of an Al enriched layer having a particle distribution defined by the present invention. For this reason, it is more preferable that it be formed as thin as possible or not formed. In the present invention, the thickness of the annealing oxide is 150 nm or less, thereby facilitating alloying of the hot dip galvanized layer, improving heat resistance and plating adhesion after press molding, and suppressing liquid metal embrittlement.

In other words, when the thickness of the annealing oxide exceeds 150nm, plating may not be performed well due to the effect of the annealing oxide, and thus an unplating phenomenon may occur, and alloying of the plating layer may be delayed at the beginning of hot press heating, thereby sufficient heat resistance at high temperature heating. Cannot be secured. At this time, the thickness of the annealing oxide may vary depending on the content of Si, Mn, etc. of the steel sheet, the thickness of the annealing oxide is 150nm or less to secure the plating property and heat resistance, it is possible to suppress the liquid metal embrittlement phenomenon.

Preferably, the thickness of the annealing oxide may be controlled to 100 nm or less, and more preferably, the plating property and the heat resistance may be maximized by controlling the thickness of the annealing oxide to 50 nm or less.

In this case, the annealing heat treatment is preferably performed at a temperature in the range of 700 to 900 ° C. in order to promote uniform alloying and secure heat resistance by not forming the annealing oxide, and at the same time to obtain a particle distribution of the Al enriched layer. Do. When the annealing heat treatment temperature is less than 700 ℃ annealing temperature is too low to secure the material properties of the steel, when the temperature exceeds 900 ℃, the growth rate of the oxide is faster between the steel sheet and the hot dip galvanized layer in the present invention It becomes difficult to form a thin oxide film.

The dew point temperature of the annealing atmosphere is more preferably -10 ° C or lower. The mixed gas is a hydrogen (H ₂ ) gas is a ratio of 3 to 15% by volume, the balance is a nitrogen (N ₂ ) gas is preferably a mixed gas. If the ratio of H ₂ is less than 3%, the reducing power of the atmosphere gas is reduced, and the production of oxides is easy. If the ratio of H ₂ is more than 15%, the reducing power is improved, but it is too much due to the increase in manufacturing cost, compared to the increase in reducing power. It is economically disadvantageous.

In addition, if the base steel sheet is used as a steel sheet for hot press forming, any type of hot rolled steel sheet or cold rolled steel sheet can be used as well as any kind, and the steel sheet for hot press forming is well known in the art. Therefore, the present invention is not particularly limited. However, as the property, any one may be used as long as the tensile strength is 1400 MPa or more, preferably 1470 MPa or more when it is quenched with water after heating to an austenite region.

However, for example, the base steel sheet is made of weight% C: 0.1 to 0.4%, Si: 2.0% or less (excluding 0%), Mn: 0.1 to 4.0%, balance Fe and other unavoidable impurities. It may be more consistent with the subject matter of the invention, but is not necessarily limited thereto.

Hereinafter, the composition of the base steel sheet of the present invention will be described. It is noted that the content of each component described below is based on weight unless otherwise specified.

C: 0.1 to 0.4%

C is a key element for increasing the strength of steel sheet, and produces a hard phase of austenite and martensite. In the case where the content of C is less than 0.1%, even if hot pressing is performed in the austenitic single-phase zone, it is difficult to secure the target strength, and therefore, it is preferable to add the content of C at least 0.1%. If the content of C exceeds 0.4%, the likelihood of deterioration of toughness and weldability is increased, and the strength is excessively high, so that there is a disadvantage in the manufacturing process, such as impairing the flowability in the annealing and plating process, so the upper limit of C is 0.4% or less. Limited to

Mn: 0.1 ~ 4.0%

Mn, as a solid solution strengthening element, not only contributes greatly to the strength increase, but also plays an important role in delaying the transformation from austenite to ferrite. If the Mn content is less than 0.1%, the ferrite transformation temperature (Ae3) is increased in austenite, so that a high heat treatment temperature is required to press-process the steel sheet on the austenite single phase. On the other hand, when the content of Mn exceeds 4.0%, weldability, hot rolling property and the like may deteriorate, which is not preferable. At this time, the content of Mn is more preferably 0.5% or more in order to sufficiently reduce the ferrite transformation temperature (Ae3) and hardenability due to Mn.

Si: 2% or less (except 0%)

Si is a component added for the purpose of deoxidation. If the content of Si exceeds 2%, it is difficult to pickle the hot rolled sheet, which may cause scale surface defects due to hot pickled sheet and unpickled oxide. Since SiO ₂ oxide may be generated on the steel surface and unplating may occur, the upper limit of Si is preferably limited to 2%. More preferably, the addition of more than 0.3% is more effective to maximize the deoxidation action.

In addition, the steel sheet is N: 0.001-0.02%, B: 0.0001-0.01%, Ti: 0.001-0.1%, Nb: 0.001-0.1%, V: 0.001-0.1%, Cr: 0.001-1.0%, Mo: It is preferable to further include at least one selected from the group consisting of 0.001 to 1.0%, Sb: 0.001 to 0.1% and W: 0.001 to 0.3%.

In addition, the steel of the present invention may include some unavoidable impurities, which are not specifically mentioned in the present invention since the impurities are obvious to those skilled in the art. One example of the impurity is Al, but if the amount of Al increases, steelmaking cracks may occur, so it is not added as much as possible. In the present invention, it is more preferable to manage it at 0.05% or less. Other impurities may include PS, but do not exclude impurities common in the steel field.

N: 0.001 to 0.02%

If the N is less than 0.001%, the manufacturing cost for controlling N in the steelmaking process can significantly increase, so the lower limit is set to 0.001%. When the N content is more than 0.02%, it is difficult to dissolve and perform the steel sheet in the manufacturing process, so that the manufacturing cost may increase, and slab cracking due to AlN is likely to occur, so the upper limit thereof is made 0.02%.

B: 0.0001 to 0.01%

B is an element that delays ferrite transformation in austenite, and its content is difficult to achieve sufficiently when the content is less than 0.0001%, and when the content of B is more than 0.01%, the effect is not only saturated but also degrades hot workability. It is preferable to limit the upper limit to 0.01%.

Ti, Nb or V: 0.001 to 0.1%

Ti, Nb and V are effective elements for increasing the strength of the steel sheet, miniaturizing the grain size and improving heat treatment properties. If the content is less than 0.001%, the above effect cannot be sufficiently obtained. If the content exceeds 0.1%, the effect of the desired strength and yield strength increase cannot be expected due to an increase in manufacturing cost and excessive carbon and nitride production. Therefore, the upper limit is limited to 0.1%. desirable.

Cr or Mo: 0.001-1.0%

Since Cr and Mo not only increase the hardenability but also increase the toughness of the heat-treated steel sheet, the effect is greater when added to a steel sheet that requires high impact energy characteristics, and the effect is lower when the content is less than 0.001%. It is preferable to limit the upper limit to 1.0% because it cannot be sufficiently obtained and the effect is saturated not only in 1.0% but also the manufacturing cost increases.

Sb: 0.001-0.1%

Sb is an element which plays a role of making the generation of scale uniform by suppressing selective oxidation of grain boundaries during hot rolling and improving pickling properties of hot rolled materials. If the Sb content is less than 0.001%, the effect is difficult to achieve, and if the Sb content is more than 0.1%, the effect is not only saturated, but the manufacturing cost may be increased and brittleness may occur during hot working, so the upper limit is limited to 0.1%. desirable.

W: 0.001 to 0.3%

W is an element that improves the heat treatment hardenability of the steel sheet and at the same time, W-containing precipitates are advantageous for securing strength, and when the content is less than 0.001%, the above effect cannot be sufficiently obtained, and the content is 0.3% When exceeded, the effect is not only saturated, but also increases the manufacturing cost, the content is preferably limited to 0.001 ~ 0.3%.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are intended to illustrate the invention and not to limit the scope of the invention. And the scope of the present invention is defined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

First, the steel plate which cold-rolled the steel material which has a composition of Table 1 was tested.

Category (% by weight) C Si Mn P S Al River 1 0.23 0.035 2.2 0.008 0.0015 0.035 River 2 0.22 0.8 1.7 0.007 0.001 0.03

When the oxidation reaction is performed on the surface of the steel sheet before annealing, one of the metals whose Gibbs free energy reduction per mole of oxygen is smaller than Cr is a predetermined metal of the type shown in Table 2 (when not otherwise described in Table 2, The amount of Gibbs free energy reduction per mole of oxygen is not applied to the metal less than Cr), and the annealing treatment is performed at a temperature of 785 ° C, and the plating treatment is performed in a Zn plating bath containing 0.21% by weight of Al. To obtain a plated steel sheet. At this time, Fe in the plating bath has a small amount of Fe dissolved in the steel sheet, but is not particularly controlled unless dross occurs in the plating bath, which does not interfere with the operation.

After taking a specimen of a part of the hot-dip galvanized steel sheet, the thickness of the metal coating layer, the amount of metal concentrated to the depth of 1 μm from the surface, and the thickness of the Zn plating layer were measured by GOEDS analysis, and in order to increase the accuracy of the data. Comparison was verified by SEM, TEM observation, wet analysis and electrospectrochemical analysis (ESCA) of the cross sections.

division Steel grade Coating metal Coated metal content in the diffusion layer (wt%) Number of Al-enriched layer particles over 500 nm in diameter per 100 μm ^{2 of} surface Share of surface Al thickening layer (%) Galvanized layer thickness (㎛) Furnace temperature (℃) Hot press heating time (minutes) Whether there is a continuous Al2O3 oxide layer Continuous Al2O3 Oxide Layer Thickness (nm) Inventory 1 River 1 Ni 1.7 6.5 91.5 8 900 3 continuity 50 Inventive Example 2 River 1 Ni 3.0 5.1 94.3 8 910 4 continuity 110 Inventory 3 River 2 Fe-Ni 3.2 2.3 90.1 8 850 4 continuity 70 Comparative Example 1 River 1 - - 16 87 10 900 4 discontinuity - Comparative Example 2 River 2 - - 18 81 7 910 3 discontinuity -

Inventive Examples 1 to 3 are applied after annealing by applying a metal having a Gib free energy reduction amount per mole of oxygen less than Cr during oxidation, which is one preferred method for homogeneously forming a surface protective oxide film during heat treatment for hot press processing. The case where the base steel sheet contains the diffusion layer of the said metal is shown, and the comparative examples 1 and 2 show the case where the special operation | movement, such as metal coating and diffusion layer formation, is not performed.

After plating, the hot-dipped galvanized steel sheet was subjected to a hot press process under the conditions shown in Table 2, and the hot press heating furnace was controlled in the atmosphere in the air. The heating time was 3 to 4 minutes, and the plated steel sheet was taken out of the heating furnace and cooled in a mold before the plated steel sheet reached the furnace atmosphere temperature. In order to measure the surface oxide continuity and oxide thickness, the surface and cross section of the specimen after the hot pressing process were observed by scanning electron microscope (SEM), transmission electron microscope (TEM), and the surface was analyzed by XRD and GOEDS analysis. The formed oxide and the alloy phase in the plating layer were analyzed.

4 (Invention Example 1) and Figure 5 (Comparative Example 2) are for observing the surface properties during the heating process, the intermediate extraction of the steel sheet at the intermediate temperature, not the final target temperature to observe the surface oxide formation result An electron micrograph. In the heating process, when the oxides formed on the plating surface are not continuous, a new kind of oxide such as ZnO is generated from the portion where the Al 2 O 3 film loses the continuity, so that the plating surface is coated with ZnO or partially ZnO is formed. 4 is an example in which the Al 2 O 3 film maintains continuity, and the plating surface has a flat shape. At this time, the film was confirmed to have a thickness of about 50nm. 5 shows an example in which the Al ₂ O ₃ film loses continuity, grains of ZnO are formed on the Al ₂ O ₃ which loses continuity.

Inventive Examples 1 to 3, if the oxide film on the plating surface maintains the continuity, it can be seen that the thickness of the film is between 10 ~ 200nm. On the other hand, in the case of 1 and 2, ZnO was formed on the Al2O3 oxide film which was formed continuously, thus losing continuity.

On the other hand, after the steel sheet was investigated for the oxide structure after reaching the heating target temperature of 780 ~ 950 ℃ and heated for 7 minutes at the temperature of 900 ℃ for the specimens of Example 1 and Comparative Example 1 in Table 2 The weight ratio of ZnO in the oxide was investigated from the analytical value of the component elements in the oxide by analyzing the oxide GDS formed on the Al 2 O 3 film. As a result, it can be seen that the ZnO ratio of Inventive Example 1 was 87% while the ZnO ratio of Comparative Example 1 decreased to 67%.

Claims (9)

Base steel sheet;
A galvanized layer formed on the base steel sheet; And
As a plated steel sheet comprising an Al thickening layer containing 30 wt% or more of Al between the base steel sheet and the galvanized layer,
The plated steel sheet includes a continuous oxide layer having a thickness of 10 to 200 nm on the surface of the plated layer in a temperature range of 950 ° C. or less when heated to 400 ° C. or more.
The holding steel sheet further comprises a surface diffusion layer of a metal having an absolute value of the Gibbs free energy reduction amount per mol of oxygen less than Cr in the oxidation reaction within a depth of 1㎛ from the surface of the steel sheet for hot press forming.
The oxide of claim 1, wherein an oxide including 1-5 μm thick Zn oxide and Mn oxide is formed on the continuous oxide layer at a temperature between 780 ° C. and 950 ° C., wherein the content of Zn oxide is in weight percent. Plated steel sheet for hot press forming, which is at least 80% of the total oxide.
The plated steel sheet for hot press forming according to claim 1, wherein the continuous oxide layer has a thickness of 30 to 150 nm.
delete delete 4. The hot press forming according to any one of claims 1 to 3, wherein the galvanized layer comprises 5 to 80% by weight Fe, 0.1 to 0.3% by weight Al and the balance Zn before heating to 400 ° C or more. Plated steel sheet.
The plated steel sheet according to any one of claims 1 to 3, further comprising a powder layer of a metal or a metal compound forming the oxide layer on the galvanized layer before being heated to 400 ° C. or higher.
The steel sheet according to any one of claims 1 to 3, wherein the base steel sheet has a weight% of C: 0.1 to 0.4%, Si: 2.0% or less (excluding 0%), Mn: 0.1 to 4.0%, balance Fe and Plated steel sheet for hot press forming having a composition composed of other unavoidable impurities.
According to claim 8, wherein the steel sheet is N: 0.001 ~ 0.02%, B: 0.0001 ~ 0.01%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, V: 0.001 ~ 0.1%, Cr: 0.001 ~ 1.0 Plated steel sheet for hot press forming further comprising at least one selected from the group consisting of%, Mo: 0.001 to 1.0%, Sb: 0.001 to 0.1%, and W: 0.001 to 0.3%.

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