KR20130002227A - Plated steel sheet for hot press forming having superior stability of plating layer - Google Patents

Abstract

PURPOSE: A plated steel sheet with the excellent stability of a plating layer for a hot press molding is provided to form an oxide layer by moving Al, which is included in a compound layer, to a surface layer in heating, thereby preventing the volatilization of Zn, which is included in the plating layer, and the growth of the oxide so that the Zn content inside an alloy layer is controlled in an optimal range. CONSTITUTION: A plated steel sheet for hot press molding comprises a steel substrate, an Al thickening layer, and a galvanized plating layer. The Al thickening layer, which is formed on the upper side of the steel substrate, of which the Al content is more than 30 wt%. The galvanized plating layer is formed on the Al thickening layer. Among the particles which form the Al thickening layer, the average number of particles of which the size is more than 500 nanometers or less than 15 per 100 square micrometers. The occupied area rate by the Al thickening layer is more than 88% on the interface between the steel substrate and the galvanized plating layer. The thickness of the Al thickening layer is 0.1 to 1.0 micrometer. The surface diffusion layer of a metal is included within the depth of one micrometer from the surface of the substrate, and the Gibbs free energy decrement per one mole of oxygen of the metal is less than that of Cr.

Description

Plated steel sheet for hot press molding with excellent plating layer stability {PLATED STEEL SHEET FOR HOT PRESS FORMING HAVING SUPERIOR STABILITY OF PLATING LAYER}

The present invention relates to a plated steel sheet for hot press forming excellent in the stability of the plated layer, and more particularly, liquid metal embrittlement (LME) phenomenon is suppressed by the zinc enriched region included in the plated layer during hot press forming. The present invention relates to a hot plated steel sheet for excellent stability of the plating layer.

Recently, the demand for high strength steel sheet is rapidly increasing in order to reduce fuel consumption of automobiles due to 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 850 ~ 930 ℃, when heating the surface of the steel sheet is oxidized to generate a scale. Therefore, a separate process such as a short blast to remove the scale after forming the product is required, and the corrosion resistance of the product is also inferior to the plating material.

Therefore, in order to solve this problem, as Al-based plating is applied to the surface of the steel sheet as in the US Patent US Pat. No. 6,296,805, while maintaining the plating layer in the heating furnace, it suppresses the oxidation reaction on the surface of the steel sheet and the corrosion resistance by using the passivation film formation of Al. Increasing products have been developed and commercialized.

However, the Al plating material has excellent heat resistance at high temperature, but is inferior to corrosion resistance compared to the sacrificial anode type Zn plating, and has a disadvantage in that the manufacturing cost increases.

However, in the case of Zn, Zn plated steel sheet manufactured by a conventional method is inferior to Al in heat resistance at a high temperature, and the plating layer is non-uniformly formed by alloying and high temperature oxidation of the Zn layer at a high temperature of 850 to 930 ° C. The ratio of is lowered to less than 20% there is a problem that the function as a plating material is reduced in terms of corrosion resistance.

In addition, during the hot press after heating, an alloy phase having a high Zn ratio in the plating layer is present in the liquid phase, and when performing a hot press, the liquid phase in the plating layer comes into contact with the base steel sheet, causing a so-called liquid metal embrittlement phenomenon, which weakens the steel surface. As a result, steel sheet damage may occur, such as cracks.

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 to improve the 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; 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 an average of 15 or less per 100 μm ² , and Al thickening at an interface between the base steel sheet and the zinc plating layer. The occupancy area of the layer is characterized by being 88% or more.

At this time, it is preferable that the thickness of the said Al thickened layer is 0.1-1.0 micrometer.

In addition, the base steel sheet is advantageous to form a preferred Al thickening layer of the present invention further comprises 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.

At this time, the amount of metal Gibbs free energy reduction per mole of oxygen during the oxidation reaction is preferably one or two or more selected from Ni, Fe, Co, Cu, Sn and Sb.

And, the thickness of the annealing oxide formed on the surface of the steel sheet is advantageously 150nm or less.

In addition, it is more preferable that the occupied area ratio of the Al thickened layer at the interface between the base steel sheet and the zinc plated layer is 95% or more.

In addition, the galvanized layer preferably contains 15.0 wt% or less of Fe.

In addition, the zinc plated layer is Fe: less than 15.0% by weight, the amount of Gibbs free energy reduction per mole of oxygen during the oxidation reaction less than Cr: less than 2.0% by weight, the remainder is effective to have a composition containing Zn and other unavoidable impurities. .

In addition, the base steel sheet may be used in any of the compositions used in the steel sheet for hot press molding, but in the present invention, C: 0.1 to 0.4%, Si: 2.0% or less (excluding 0%), Mn: 0.1 to 4.0 It is more preferable to have the composition which consists of%, remainder Fe, and other unavoidable impurities.

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: If it further comprises at least one selected from the group consisting of 0.001 ~ 1.0%, Sb: 0.001 ~ 0.1% and W: 0.001 ~ 0.3% can obtain excellent properties.

The plated steel sheet for hot press forming provided by the present invention can uniformly finely control the particle size of the compound layer on the surface, thereby uniformly alloying the entire plating layer upon heating for hot press forming, and thus the concentration of zinc It is possible to suppress the occurrence of high liquid zinc concentration (Zn-rich) of more than 40% by weight Zn to prevent the steel sheet damage due to liquid metal embrittlement during hot press molding.

In addition, the aluminum contained in the compound layer is easily decomposed upon heating to move to the surface layer to form an oxide layer, thereby preventing volatilization and oxide growth of the zinc component included in the plating layer to control the zinc content in the alloy layer to an appropriate range. The corrosion resistance of the press-molded parts can be secured.

1 is a scanning electron micrograph observing the Al thickening layer existing between the steel plate and the zinc plated layer of the hot-dip galvanized steel sheet prepared by Example 2 of the present invention,
2 is a scanning electron micrograph of observing the Al thickening layer existing between the steel plate and the zinc plated layer of the hot-dip galvanized steel sheet prepared in Comparative Example 1,
3 is a photograph of the surface of a steel sheet (part) hot press-formed according to Inventive Example 2 with an electron microscope;
4 is a photograph observing the surface of a steel sheet (part) hot press-formed by Comparative Example 1 with an electron microscope,
5 is a photograph of the hot press-formed part of Comparative Example 1 and the cut surface was observed with a transmission electron microscope,
6 is a result of analyzing the cross section and the components at each point of the hot press molded parts manufactured according to Inventive Example 1,
7 is a result of analyzing the cross section and the components at each point of the hot press molded parts manufactured according to Comparative Example 1 by EDS, and
8 is a result of analyzing the cross section and the components at each point of the hot press molded parts manufactured according to Comparative Example 3 by EDS.

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 can suppress the volatilization of zinc in the plating layer and prevent oxide growth when the Al-based thickening layer including Al is more than 30% by weight at the interface between the steel sheet and zinc as a zinc-based plated steel sheet. The discovery that one alloy layer could be obtained led to the present invention. That is, the plated steel sheet of the present invention is a steel sheet; An Al thickening layer containing 30 wt% or more of Al formed on the base steel sheet; And a galvanized layer formed on the Al thickening layer.

The Al thickening layer is Al ₂ O ₃ is the main component (for example, Al ₂ O ₃ content of 90% by weight or more) by Al diffused to the surface of the plating layer to move to the surface of the plating layer and selectively oxidized when heating for hot press in the future It serves to form a dense and thin oxide layer. 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. In addition, since the Al thickened layer is uniformly decomposed during the heating process for hot press, the progress of alloying may also occur quickly and uniformly. As a result, the liquid phase during hot press may be caused by partial liquefaction due to non-uniformity of Zn concentration in the plating layer. The occurrence of metal embrittlement can be effectively suppressed. 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, and more preferably 95% or more. . When the occupied area ratio of the Al enriched layer is low, alloying may occur unevenly, so that liquid embrittlement may occur during hot press working, and an oxide layer may not be sufficiently formed on the surface of the plating layer, thereby preventing sufficient volatilization of zinc.

In addition, it is preferable that the particle | grains which comprise the said Al thickening layer have many particle | grains whose particle size (The method of defining particle size is various, but this invention sets it as the diameter when the particle | grain is averaged) is 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 thickening 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, and the rapid and uniformity of Fe into the plating layer. This is because alloying can be induced. 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 an area of 88% or more, more preferably 95% or more, at the interface of the zinc plated layer.

The method of analyzing the Al thickened layer formed on the surface of the steel sheet is as follows. That is, since the Al thickening layer exists at the interface between the base steel sheet and the galvanized layer as described above, it is difficult to confirm the area distribution unless the galvanized layer is removed. That is, the cross-sectional cutting method can confirm the layer distribution on the cross section, but the particle size distribution is difficult to confirm. Therefore, in order to confirm the particle size distribution, it is necessary to analyze the Al thickened layer after chemically dissolving the Zn plating layer thereon without damaging the Al thickened layer formed on the surface of the base steel sheet. As an example for dissolving the Zn plating layer, it is dissolved in a HNO ₃ + CrO ₃ + ZnSO ₄ solution, and the Zn plating layer is melted, leaving only an Al enriched layer. The scanning electron microscope (SEM) photograph is taken at 20,000 to 50,000 times magnification of the Al enriched layer, and the area ratio of the Al enriched layer and the particle size of the Al enriched layer are analyzed through photo analysis. Here but may be a method of measuring the particle size more than 500nm per 100㎛ ² are a number of, for example, as the average number per this 100㎛ ² after measuring the number of the particle size or more present per unit area of 5,000 ~ 500nm particle 10,000㎛ ² Representation can be obtained by using the method of obtaining. In this case, when it is difficult to observe the particle size in one large area, it is possible to use a method of observing several times and then adding both the area and the number of particles having an observed particle size of 500 μm or more.

Also, the surface occupancy ratio of the Al enriched layer was measured according to the Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count method of ASTM E 562-08 for 20,000 to 50,000 times scanning electron micrographs. In addition, the particle size measurement of the Al enriched layer is measured by image processing the boundary of the particles appearing on the plane observed by using the image scanning electron micrograph using Image Analyze technique. As an example, the method used herein measured particle size using an image analyzer LEICA Q 550 (LEICA).

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. For this purpose, the average thickness of the oxide formed on the surface of the steel sheet during annealing is preferably within 150nm.

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.

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 is dissolved in the Fe-Zn phase during heating for hot forming to prevent excessive diffusion of the components dissolved in the steel sheet into the plating layer, and to suppress excessive diffusion of Zn in the zinc plating layer into the steel sheet. can do. When Zn of the zinc plated layer is excessively 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. Thus, the amount of Zn consumed is reduced. By doing so, it is possible to secure a large amount (for example, 25 to 35% by weight) of Zn, which can contribute to the improvement of corrosion resistance of the steel sheet.

In addition, in the presence of the surface diffusion layer, the Fe component is uniformly diffused 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, 780 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 should be This is because the higher amount of Al can be concentrated on the surface diffusion layer by reacting with the metal. In addition, the content of the metal is preferably limited to 30% or less. This is because if the metal content exceeds 30%, too fast alloying may proceed in the initial stage of alloying, which may inhibit alloying uniformity in the plating layer. 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% or more, preferably included in more than 1.0% by weight can effectively prevent deterioration of the galvanized layer, more preferably more than 3.0% by weight can contribute to ensure the heat resistance of the galvanized layer more excellently. .

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 effect on the formation of the Al concentration layer. At this time, the area where the Al content of the Al-concentrated layer and the surface-diffusion layer in which the amount of reduction of Gibbs free energy per one mole of oxygen during the oxidation reaction is less than 5% by weight of the metal content of Cr is overlapped in the entire surface diffusion layer and the Al concentration layer. It is preferably less than 10%, wherein the overlapping portion means that the metal and Al have alloyed to form an alloy phase. When Al is present in the alloy state with the metal, it is not easy to diffuse to the surface of the plating layer during press heating.₂O₃ The amount of Al which can contribute to forming a continuous oxide film is substantially reduced. Therefore, in the EPMA analysis, when the overlapping portion is 10% or less, Al that does not exist in the alloy state is sufficiently located in the thickened layer.₂O₃ The oxide film is effectively formed.

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 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 preferably added to the galvanized layer. This is to increase the melting point of Zn by Fe is 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, Fe in the plating layer is mainly formed by diffusion into the steel sheet during the plating process, the plating layer becomes brittle as the plating layer becomes delta or gamma phase as the Fe content increases, so the content of Fe in the plating layer is the upper limit. It is preferable to have, but in the present invention, the upper limit is limited to 15.0% by weight. More preferably, 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 less than Cr: less than 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 ensure that the three-phase phase is sufficiently formed to impart heat resistance to the plating layer during press heating, the amount of reduction in Gibbs free energy per mole of oxygen is 0.01 wt% or more during the total oxidation reaction of the plating layer and the Al concentration layer. It is necessary to include, and it is preferable to set the upper limit to 2.0% by weight from the economic point of view.

In addition, Al constituting the Al thickening layer having the above-described characteristics of the present invention can be supplied in a variety of ways, but if you want to provide from the plating layer, the plating layer more preferably contains 0.05 to 0.5% by 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 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. By the way, the annealing oxide not only acts as a diffusion barrier to prevent alloying of Fe, Mn, etc., which are members of the hot dip galvanized layer and the steel sheet, but also adversely affects uniformly forming an Al enriched layer having a particle distribution defined by the present invention. Therefore, it is more preferable not to be formed as thin as possible. 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.

That is, when the thickness of the annealing oxide exceeds 150nm, the Al alloying layer may not be formed well due to the influence of the annealing oxide, and thus an unplating phenomenon may occur, resulting in unevenness of particles in forming the Al alloying layer. Can be. In this case, alloying of the plating layer is delayed at the beginning of hot press heating, and thus sufficient heat resistance cannot be secured at high temperature. 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 average thickness of the annealing oxide may be controlled to 100 nm or less, and more preferably, the plating thickness, heat resistance, and the like may be maximized by controlling the average 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. If a hot rolled steel sheet or a steel sheet which has already undergone recrystallization heat treatment is used, the annealing heat treatment temperature is preferably 500 to 700 ° C.

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, when quenched with water, oil or a cooled press mold after heating to the austenite region, any one can be used as long as the tensile strength is 1400 MPa or more, preferably 1470 MPa or more.

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 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.

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 contain 0.001 to 1.0%, Sb: 0.001 to 0.1% and W: 0.001 to 0.3%.

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. This is because the scope of the present invention is determined 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. In addition, in order to observe the particle size distribution of the Al thickening layer existing on the surface of the base steel sheet and the galvanized layer, a part of the plated steel sheet was taken, and only the plated layer was selectively removed with an acid solution (HNO ₃ + CrO ₃ + ZnSO ₄ ). The particle size distribution was observed using an analyzer and the results are shown. In Table 2 below, as a criterion of the evaluation result of the corrosion, when the surface corrosion hardly occurred, the surface of the good corrosion occurred, but intermittently occurred, when the corrosion depth is less than 100㎛, the failure occurs entirely corrosion Or a point where the corrosion depth exceeds 100 μm is found. In addition, a coating metal content means the result converted from GDS profile.

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 (㎛) Hot press heating temperature (℃) Hot press heating time (minutes) Surface cracks during hot press heating (liquid metal embrittlement cracks) Corrosion evaluation results for processed specimens (after SST 480 hours) Inventory 1 River 1 Ni 1.7 6.5 91.5 8 900 5 No crack Great Inventive Example 2 River 1 Ni 3.0 5.1 94.3 8 910 5 Homology Great Inventory 3 River 2 Fe-Ni 3.2 2.3 90.1 8 850 7 Homology Great Honorable 4 River 1 Cu 3.5 3.5 90.5 10 930 5 Homology Great Comparative Example 1 River 1 - - 16 87 10 900 5 Crack Good Comparative Example 2 River 2 - - 21 75 10 880 5 Homology Good Comparative Example 3 River 2 - - 18 81 7 910 5 No crack Inferior

Inventive Examples 1 to 4 show a case in which a metal having a Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction is applied to control the particle size distribution of the Al enriched layer to a range limited in the present application, and less than Cr. Examples 1-3 show the case where the special operation for particle size distribution is not performed.

In the case of Inventive Examples 1 to 4, LME crack did not occur during hot press processing, and after corrosion test (SST) spraying 5% NaCl aqueous solution for 480 hours, the corrosion resistance of the steel sheet hardly occurred. Showed. However, in Comparative Examples 1 and 2, cracks were generated during hot pressing. In Comparative Example 3, as shown in FIG. 8, Zn content in the plating layer was rapidly decreased, resulting in a thick oxide on the surface, and also extremely poor corrosion resistance. For this reason, corrosion resistance was over 300㎛ on the steel surface after SST test, and the corrosion resistance was extremely inferior.

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. After the hot pressing process, the plated layer was analyzed for oxides formed on the surface and alloy phases in the plated layer through XRD and GOEDS analysis. For reference, the thickness of the plating layer was measured by the length from the surface of the plating layer to the point where the content of Zn in the plating layer in the vertical direction to 25% by weight or more.

As shown in Table 2, when the diffusion metal layer formed by the metal having a smaller amount of Gibbs free energy per mole of oxygen during the oxidation reaction was formed than the Cr, the particle distribution conditions in the Al enriched layer correspond to the conditions of the present invention. In the examples, it was confirmed that the particle distribution conditions did not meet the conditions of the present invention. 1 and 2 show electron micrographs of the Al thickened layers obtained by Inventive Example 2 and Comparative Example 1, respectively. As can be seen in the figure, in the case of the invention example 2 Al concentration layer is fine and uniform, in the case of Comparative Example 1 it can be seen that the Al concentration layer is non-uniform and includes a large number of coarse particles of 500nm or more in diameter.

The particle size distribution of such particles has a great influence on hot press formability. That is, as shown in Table 2, when the Al thickening layer contains a large amount of coarse particles, the liquid metal embrittlement phenomenon occurs during hot press forming, causing cracks in the steel sheet.

In addition, in order to compare the cause of such a phenomenon more reliably, the result of having observed the cut surface of the steel plate (part) hot-press-formed by Inventive Example 2 and Comparative Example 1 by the transmission scanning electron microscope is shown in FIG. 3 and FIG. 4, respectively. . In the drawing, the cracks are not generated on the surface of the invention as a concentrated portion during hot pressing in the case of Inventive Example 2, whereas in the case of FIG. It can be seen.

FIG. 5 is an EDS analysis of the Zn enriched layer formed on the cracks by expanding the cracks penetrated into the steel sheet as a basis for confirming the presence of zinc penetrated into the steel sheet crack portion of FIG. 4. It was confirmed that the middle Zn of the crack penetrated.

It was judged that the uniform alloying did not proceed as the heating proceeded, so that the partially alloyed phase with Fe and the liquid phase coexisted with Zn, and the liquid Zn penetrated into the grain boundary of the steel sheet, causing embrittlement. .

In addition, in order to clarify the comparison, the results of the EDS analysis of the cross section and components at each point of the hot press molded parts manufactured according to Inventive Example 1 are shown in FIG. 6 and Table 3, and Comparative Example 1 and comparison. The results of the EDS analysis of the cross sections and the components at each point of the hot press molded parts manufactured according to Example 3 are shown in FIGS. 7 and 8 and Tables 4 and 5, respectively. However, in the case of the nature of the EDS analysis, for the components with a content within 0.5% by weight, the analytical value may be unclear to the actual present value.

According to FIG. 6 and Table 3, in the case of the invention example, a stable plating film having a Fe content in the range of 25 to 35% is formed after hot pressing, and the plating film can be clearly distinguished from the base steel sheet. At this time, oxides within 5 μm in thickness are uniformly formed on the steel plating surface.

On the other hand, in the case of Fig. 7 and Table 4, in the case of Comparative Example 1, the Zn concentration region (arrow ②) having a Zn content of more than 40% exists in the plating layer after hot pressing, so that the Zn concentration region causes LME crack rate during processing. do. In addition, the formation of surface oxides is uneven. In the case of Comparative Example 3, which is the case of FIG. 8 and Table 5, after hot pressing, a thick oxide of more than 5 μm was formed on the surface, and the boundary between the plating layer and the metal base is unknown. In addition, the content of Zn in the plating layer is not seen as a plating layer that can exhibit substantially corrosion resistance within 20% by weight, and thus, most of the galvanized layer may be lost and diffused into a portion of the steel sheet. In this case, sufficient corrosion resistance cannot be secured when using the parts.

Category (% by weight) ① ② ③ Mn - - - Si - - - Fe 68.6 69.8 73.4 Zn 31.4 30.2 26.6

Category (% by weight) ① ② ③ Mn - - 1.2 Si - - - Fe 67.9 27.4 67.7 Zn 32.1 72.6 31.1

Category (% by weight) ① ② ③ Mn - - 1.8 Si - - - Fe 81.5 84.4 96.1 Zn 18.5 15.6 2.1

Claims (10)

Steel plate;
An Al thickening layer containing 30 wt% or more of Al formed on the base steel sheet; And
It includes a galvanized layer formed on the Al thickening layer,
Among the particles forming the Al thickening layer, the number of particles having a particle size of 500 nm or more is an average of 15 or less per 100 μm ² , and the plating area for hot press forming having an area ratio of the Al thickening layer of 88% or more at the interface between the base steel sheet and the galvanized layer. Grater.
The plated steel sheet for hot press forming according to claim 1, wherein the Al thickening layer has a thickness of 0.1 to 1.0 µm.
The plated steel sheet for hot press forming according to claim 1, wherein the base steel sheet further includes a surface diffusion layer of a metal having a Gibbs free energy reduction amount per mole of oxygen less than Cr when oxidizing within 1 μm of a surface thereof.
The plated steel sheet for hot press forming according to claim 3, wherein the amount of Gibbs free energy reduction per mole of oxygen during the oxidation reaction is smaller than Cr is one or two or more selected from Ni, Fe, Co, Cu, Sn, and Sb.
The plated steel sheet for hot press forming according to claim 1, wherein the thickness of the annealing oxide formed on the surface of the base steel sheet is 150 nm or less.
The plated steel sheet for hot press forming according to claim 1, wherein the area ratio of the Al thickened layer at an interface between the base steel sheet and the zinc plated layer is 95% or more.
The plated steel sheet for hot press forming according to claim 1, wherein the galvanized layer contains 15.0 wt% or less of Fe.
According to claim 3, The galvanized layer is Fe: less than 15.0% by weight, the amount of Gibbs free energy reduction per mole of oxygen during the oxidation reaction less than Cr: 0.01 to 2.0% by weight, the remainder containing Zn and other unavoidable impurities Plated steel sheet for hot press forming.
The hot rolled steel sheet according to claim 1, wherein the base steel sheet has a composition consisting of 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. Plated steel sheet for press forming.
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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