KR20130002226A - 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 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: When a plated steel sheet for a hot press molding is heated at the temperature of 900°C, the Zn content of 25 to 35 wt% of a part in a plating layer is more than 90% with volume ratio. At the temperature of 750°C, the plating layer has a Fe thickening area of which the Zn content is less than 40 wt%, and a Zn thickening area of which the Zn content is more than 40 wt%. At the temperature of 750°C, the plating layer has a Fe thickening area of which the Zn content is 30 to 40 wt%, and a Zn thickening area of which the Zn content is 60 to 90 wt%. A plated steel sheet for hot press molding includes a steel substrate, an Al thickening layer, which is formed on the upper side of the steel substrate, of which the Al content is more than 30 wt%, and a galvanized plating layer which is formed on the Al thickening layer. [Reference numerals] (AA) Liquid or liquid-containing area; (BB) Solid area; (CC) Liquid; (DD) aFe+liquid; (EE) Route 2; (FF) Area 1; (GG) Temperature(°C); (HH) Route 1; (II) Zn weight fraction(Zn/(Zn+Fe))

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 800 ~ 950 ℃, when heating the surface of the steel sheet is oxidized to produce 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, the Zn plated steel sheet manufactured by the conventional method is inferior to Al in heat resistance at a high temperature, and the plating layer is nonuniformly formed by alloying and high temperature oxidation of the Zn layer at a high temperature of 800 to 900 ° C. The ratio of is lowered to less than 30% 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, liquid zinc flows into the grain boundary of the steel sheet, causing the so-called liquid metal embrittlement (LME) phenomenon, which weakens the interface, and consequently when the stress is applied to the steel sheet in the subsequent press process. It may also cause damage to the steel sheet.

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.

The plated steel sheet according to one aspect for solving the problems of the present invention, when heated to 900 ℃, the portion of the Zn content of 25 to 35% by weight of the coating layer is characterized in that more than 90% by volume ratio.

At this time, it is advantageous to have a Fe enriched region having a Zn content of 40 wt% or less and a Zn enriched region having a Zn content of more than 40 wt% in the plating layer at 750 ° C. to obtain a desirable alloy layer composition at the final heating temperature.

In addition, it is more preferable to have a Fe enriched region having a Zn content of 30 to 40 wt% and a Zn enriched region having a Zn content of 60 to 90 wt% at 750 ° C.

In addition, the plated steel sheet for hot press molding of the present invention is a steel sheet; It is more preferable to have a layer structure including 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.

In addition, the base steel sheet further includes a surface diffusion layer of a metal having a reduced Gibbs free energy per mole of oxygen less than Cr when oxidized within 1 µm from the surface to prevent rapid alloying, thereby reducing the alloy layer composition at the final hot press forming temperature. It is advantageous to control.

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

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 uniformly and finely controls the particle size of the Al thickening 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) region to prevent damage to the steel sheet due to the liquid metal embrittlement during hot press molding.

In addition, the aluminum contained in the compound layer moves to the surface layer upon heating to form an oxide layer, thereby preventing volatilization and oxide growth of the zinc component included in the plating layer, thereby controlling the zinc content in the alloy layer to an appropriate range. Corrosion resistance of the finished parts can be secured.

1 is a state diagram illustrating a phenomenon in which phase separation between a Fe thickened region and a Zn thickened region is performed when a conventional galvanized steel sheet is heated to a hot press forming temperature.
2 is a state diagram showing a heating path in which the content of Zn is excessively lowered during hot press molding;
3 is a state diagram showing a heating path of the steel sheet for hot press forming according to the present invention;
4 is a cross-sectional photograph of a plating layer after the plated steel sheet of Inventive Example 1 is heated to a hot press temperature and quenched;
5 is a cross-sectional photograph of the plating layer after the plated steel sheet of Comparative Example 1 is heated to a hot press temperature and quenched;
6 is a cross-sectional photograph of the plating layer after the plated steel sheet of Comparative Example 2 is heated to a hot press temperature and quenched;
7 is a surface photograph of a hot press working part of Inventive Example 1, and
8 is a surface photograph of a hot press working part of Comparative Example 1;

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 are a zinc-based galvanized steel sheet because the liquid phase does not exist in the plated layer of the steel sheet at the temperature for hot press forming, the liquid metal embrittlement phenomenon does not occur during press molding, and also due to excessive alloying and zinc volatilization loss The present invention has been found that alloying should be performed in an appropriate path according to a heating pass in order to prevent a decrease in corrosion resistance as the zinc content in the plating layer decreases.

That is, the galvanized layer is required to have a uniform alloy composition in the temperature range of 780 ~ 950 ℃ hot press forming temperature. In other words, even though the amounts of Fe and Zn present in the entire plating layer are the same, alloying according to the heating path does not proceed uniformly as shown in FIG. 1, so that the Zn enriched areas area2 in the Fe enriched areas area1. It may exist as such. In this case, the Zn enriched region (area2) is composed of a solid alloy region having a Zn fraction of about 0.4 (40 wt%) and a liquid region having a Zn fraction of about 0.9 (90 wt%) or more. It is divided into three regions: Fe concentration region, solid state alloy region among Zn concentration regions, and liquid phase region among Zn concentration regions.

At this time, the liquid metal in the Zn enriched region is in contact with the base steel sheet during hot press processing and penetrates into the grain boundary of the base steel sheet, which is relatively weak at high temperature, and causes grain boundary embrittlement. .

Therefore, the first implementation means of the present invention is such that the liquid region does not exist in the plating layer of the plated steel sheet even at the hot press molding temperature.

As a result of the research of the present inventors, in order to obtain such an effect, the content of Zn contained in the plating layer at 900 ° C, which is the temperature of one point in the hot press molding temperature section, needs to be 35% by weight or less at substantially all positions. That is, as described above, if the Zn content is excessively high, there is a risk of generating a liquid phase, which is not preferable. When the Zn content at the above temperature is in the above-mentioned range, that is, alloying with Fe sufficiently occurs that embrittlement by the liquid metal, that is, LME phenomenon, at a normal hot press forming temperature of 780 to 950 ° C. can be prevented. Means that. Therefore, the Zn content at this temperature is taken as a reference.

However, as shown in FIG. 2, when the Zn content in the galvanized layer is excessively reduced, since the alloying ratio is high, the risk of embrittlement of the liquid metal may be reduced, but the corrosion resistance of the molded part may be seriously impaired. . That is, as described above, the zinc plated material has higher corrosion resistance than the aluminum plated material or the non-plated steel sheet by suppressing oxidation of the steel sheet in the sacrificial anode method, and thus may be advantageously used for press molding. However, when Fe is excessively alloyed into the galvanized layer, not only the Zn content in the alloy layer is reduced, but also the Zn activity is greatly reduced by interaction with Fe. When the activity of Zn decreases, the oxidation reaction of Zn becomes inactive and eventually does not fully function as a sacrificial anode.

Therefore, excessive alloying also needs to be avoided. According to the results of the inventors of the present invention, in order for Zn included in the alloy layer to serve as a sacrificial anode, the Zn needs to be included at 25 wt% or more at 900 ° C.

Therefore, when the hot-pressed plated steel sheet of the present invention is heated to 900 ℃, it is characterized in that the content of Zn in the plating layer at 25 to 35% by weight in substantially all positions. In the present invention, virtually all positions refer to the alloying plating layer (ie, the plating layer obtained by heating and alloying at 900 ° C. and then quenching and obtained at room temperature) to have a volume ratio in the portion having the alloy composition described above. It is 90% or more, More preferably, it means 95% or more.

In this case, as an analysis method of the volume ratio of the alloy composition, the surface of the hot pressed specimen may be mirror polished, slightly etched with a nital solution, and then observed with an optical microscope or a scanning electron microscope. In this case, the Zn content of 25% to 35% and the other areas are physically distinct, and the image of the plating layer may be analyzed by an image analyzer having an image analysis function.

However, since the degree of progress of alloying may vary depending on the heating process and the heating time, in the present invention, the average heating rate from room temperature to the measurement temperature (900 ° C.) is 2-20 in consideration of the heating process of a conventional hot press molding process. It means the alloy layer conditions at the time of quenching, after making into 0 degreeC / second and holding time in final heating temperature (measurement temperature) for 0 to 6 minutes.

However, the galvanized layer, as can be seen in the Fe-Zn state diagram of FIG. 1 described above, as the temperature rises due to the presence of many intermediate phases, the alloying proceeds, and inevitably, before the final heating temperature is reached under normal heating conditions. It is divided into areas. Here, the normal heating condition is a heating condition for hot press molding. If one example is given, the internal temperature of the furnace is in the range of 780 ° C to 950 ° C, and the average heating rate during heating means a speed of 2 to 20 ° C / sec. do.

At this time, the composition of each phase divided needs to be the most suitable composition so that the alloy layer finally corresponds to the desired composition at the hot press molding temperature. As shown in FIG. 3, when the alloy is divided into two regions at a relatively low temperature at the initial stage of alloying, it is very difficult to finally converge the composition of the two regions into the same composition range. It is separated into two regions at the forming temperature, and as a result, the liquid metal is present in the plating layer, resulting in liquid metal embrittlement.

As a result of the study of the present inventors, such phase separation is caused by rapid alloying, and if the conditions of the steel sheet are controlled so that the alloying occurs as uniformly and quickly as possible, the composition of the alloy layer at the hot press forming temperature can be converged into one region. The measure of uniform and gentle alloying as described above can be confirmed from the phase composition at 750 ° C., ie, the zinc-plated layer at this temperature has a Zn content of 40 wt% or less, preferably 30-40 wt% Fe thickened region. And Zn content of more than 40% by weight, preferably divided into a Zn thickening zone of 60 to 90% by weight. At this time, if the Zn content in the Fe thickened zone is too low, the Zn content in the alloy layer is too low even at the hot press forming temperature, which is disadvantageous in securing corrosion resistance, and it is difficult to obtain a uniform alloy layer, and the Zn content in the Zn thickened zone is too high. In this case, too, since the composition of the two regions at the hot press molding temperature is difficult to converge in one range, a large amount of the liquid region is present, which may cause liquid metal embrittlement. In the case of the more preferable Zn concentration region, the Zn content is composed of about 60 to 90% by weight, which is considered to be due to the decomposition of a part of the phase in the plating layer into a delta (δ) or gamma (Г) phase during cooling.

Therefore, according to a more preferred aspect of the present invention, the hot-rolled plated steel sheet of the present invention also has a Fe-concentrated region having a Zn content of 40% by weight or less and a Zn-rich region having a Zn content of more than 40% by weight at 750 ° C. It can be characterized by three kinds. However, since the progress of alloying may vary depending on the heating process and the heating time, in the present invention, the average heating rate from room temperature to the measurement temperature (750 ° C.) is 2 to 20 in consideration of the heating process of a conventional hot press molding process. It means the alloy layer conditions at the time of carrying out quenching, after making holding time in 0 degreeC / second and holding time in measurement temperature) 0-6 minutes. The alloy layer formation history during the heating of the plated steel sheet for hot press forming of the present invention which satisfies the advantageous conditions described above is shown in the state diagram of FIG. 3.

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, the hot press plated plated steel sheet that satisfies the heating path described above is more advantageous in that the alloying proceeds rapidly while simultaneously increasing the melting point of the plating layer and minimizing the volatilization loss of zinc in the plating layer. An example of a plated steel sheet having a structure will be briefly described.

The inventors of the present invention can suppress the volatilization of zinc in the plating layer and prevent oxide growth when the Al thickening layer containing 30 wt% or more of Al is homogeneously formed at the interface between the steel sheet and zinc, and obtain a uniform alloy layer. And the present invention has been reached. That is, the plated steel sheet according to a preferred aspect 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 a dense and thin oxide layer in which Al 2 O 3 is the main component (for example, Al 2 O 3 content of 90% by weight or more) by Al diffusion to the plating layer surface and selective oxidation during heating for hot press molding in the future. It serves to form. 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 a more preferred aspect of the present application, the plated steel sheet of the base 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 advantageous to further have a feature of distributing at an area ratio of 88% or more at the interface of 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.

Moreover, as another method, the method of plating after spraying Al-type compound particle whose particle size was controlled on the steel plate surface is mentioned.

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.

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 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 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 ratio of delta (δ) or gamma (Г) in the plating layer is increased, and thus the particles are easily embrittled in the plating layer. Therefore, the Fe content is preferably limited to 15% 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 ensure that the three-phase phase is sufficiently formed to impart heat resistance to the plating layer during press heating, a metal having a Gib free energy reduction amount per mole of oxygen less than Cr at a high temperature heating for hot press work in a galvanized steel sheet is 0.01 wt% or more. It is necessary to include, and in terms of economics, the upper limit is preferably set to 2.0% by weight.

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. 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. 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. 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%) Galvanized layer thickness (㎛) Heating time up to 900 ℃ (minutes) 900 ℃, Zn content in plating layer
(weight%) Volume fraction (%) of phase with Zn 25 ~ 35% or more in plating layer 5% or more of Zn 35 ~ 90% area in plating layer Hot press heating temperature (℃) 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 8 5 30.3 > 99 radish 900 Not occurring Great Inventive Example 2 River 1 Ni 3.0 8 5 34.1 > 99 radish 910 Not occurring Great Inventory 3 River 2 Fe-Ni 3.2 8 7 32.2 > 99 radish 850 Not occurring Great Honorable 4 River 1 Cu 3.5 10 5 33.3 > 99 radish 930 Not occurring Great Comparative Example 1 River 1 - - 7 5 14.5 - radish 910 Not occurring Inferior Comparative Example 2 River 2 - - 10 5 37.2 72 U 900 Occur Good

Inventive Examples 1 to 4 are applied by annealing by applying a metal having a Gib free energy reduction amount less than Cr per mole of oxygen during oxidation, which is one preferable method to prevent cracking due to liquid metal embrittlement during hot press processing and to provide excellent corrosion resistance. After plating, the base steel sheet contains a diffusion layer of the above metal, and Comparative Examples 1 and 2 show a case where no special operation such as metal coating and diffusion layer formation is performed. In this case, in the case of the invention examples 1 to 4, the number of particles having a particle size of 500 nm or more on the surface is within 15 pieces per 100 μm ^{2 on} average, and it was confirmed that the ratio is also 88 area% or more. However, in Comparative Examples 1 and 2, the number of particles having a thickness of 500 nm or more exceeded an average of 15 per 100 μm ² (18, 20 levels), and the occupied area ratio was also 70%. In order to observe whether the plated steel sheet is likely to cause liquid metal embrittlement at the hot press forming temperature, the steel sheet was heated and quenched under the conditions of Table 2 above, and then Zn content and phase analysis were performed in the galvanized layer. The average heating rate of the steel sheet at the time of heating was set at 4 ° C / sec. As can be seen in the drawings, the steel sheet provided by the invention example of the present invention had a ratio of 25 to 35% by weight of Zn mostly at 900 ℃, the steel sheet provided by the comparative example has a large amount of phase Zn content liquid metal It was confirmed that the possibility of causing embrittlement was high. In addition, although the results are not shown in Table 2, the steel plate was extracted at 750 ° C. and quenched to analyze the plating layer. As a result, it was confirmed that the Fe-enriched region and the Zn-concentrated region existed at the same time.

In order to confirm whether the above conditions of Table 2 are connected to the liquid metal embrittlement phenomenon during the actual hot press molding, a hot press process was performed on the hot-dip galvanized steel sheet under the conditions shown in Table 2. At this time, the hot press heating furnace was atmosphere controlled in air | atmosphere. During press molding, the average heating rate was controlled at 4 ° C / sec, and the total holding time in the furnace was set at 5 minutes. 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 a result of EDS analysis on the cross section of the plating layer after hot pressing, in the case of Inventive Examples 1 to 4, the Zn concentration in the plating layer included 5% or less of Zn concentration within 35% by weight to 90% by weight.

As a result, the proportion of the phase having a Zn content within 35 to 90% by weight was hardly observed, and the proportion of the phase having a Zn content of 25 to 35% by weight was 99% or more in the plating layer. In Table 2, the average Zn content in the plated layer was measured after the EDS was measured at five equal intervals from the top to the bottom of the plated layer with respect to the cross section of the plated layer.

As described above, when the Zn content in the plating layer is included within 5% by volume, and more preferably within 1% by volume, embrittlement by the liquid metal can be suppressed during hot pressing. It will have a big impact on press forming and the use of parts. That is, as shown in Table 2, when 95% by volume or more of the plating layer is composed of 25 to 35% by weight of Zn at a heating temperature in the range of 780 to 950 ° C, liquid metal embrittlement is suppressed during hot press forming. Cracks will not occur.

As described above, the Zn ratio in the plating layer is stably distributed within 25 to 35% by weight, so that corrosion resistance of the steel sheet hardly occurs after 480 hours of corrosion experiment (SST) spraying a 5% NaCl solution. Showed.

In the case of Comparative Example 1, as shown in FIG. 5, the Zn content in the plating layer rapidly decreases to generate a thick oxide on the surface, and also the corrosion resistance is extremely inferior, resulting in corrosion depth of 300 μm or more on the steel surface after the SST test. Corrosiveness was extremely inferior.

4 is a cross-sectional photograph of Inventive Example 1, and Table 3 shows the results of EDS analysis for each plated layer of Inventive Example 1. FIG. The plating layer is clearly distinguished from the metal base layer, and the EDS analysis of each part shows 25 to 35% by weight. In this case, no crack was generated in the hot press working part, and the corrosion resistance of the steel sheet was shown to be superior to corrosion resistance. In the plating layer of FIG. 4, the alloying process of Fe in the plating layer during hot press heating follows the process of FIG. 1.

5 is a cross-sectional photograph of Comparative Example 1, Table 4 shows the results of EDS analysis for each plated layer of Comparative Example 1. Thick oxide is formed on the plating layer, it can be seen that the Zn content in the plating layer is sharply lowered to less than 20%. In this case, as shown in Table 2, corrosion resistance is particularly inferior. In the plating layer of FIG. 5, the alloying process of Fe during hot press heating follows the process of FIG. 2.

6 is a cross-sectional photograph of Comparative Example 2, Table 5 shows the results of EDS analysis for each plated layer of Comparative Example 2. It is observed that the Fe thickening zone having a Zn content of 25 to 35% and the Zn thickening zone having a Zn content of 35 to 90% are mixed in the plating layer. The presence of such a Zn thickening zone includes a liquefaction zone when heated to 780 ~ 950 ℃ causes cracks during hot press heating. In the plating layer of FIG. 6, the alloying process of Fe during hot press heating follows the process of FIG. 3.

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 - - 1.8 Si - - - Fe 81.5 84.4 96.1 Zn 18.5 15.6 2.1

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

7 is a surface photograph of a hot press working part of Inventive Example 1 shown in Table 2. FIG. No crack is observed on the surface.

8 is a surface photograph of a hot press working part of Comparative Example 1 of Table 2. FIG. Cracks due to liquid metal embrittlement are observed on the surface.

Claims (8)

The plated steel sheet for hot press forming when the portion of the Zn content in the plating layer of 25 to 35% by weight when heated to 900 ℃ 90% or more by volume ratio.
The plated steel sheet for hot press forming according to claim 1, further comprising a Fe thickened region having a Zn content of 40 wt% or less and a Zn thickened region having a Zn content of more than 40 wt% at 750 ° C.
The plated steel sheet for hot press forming according to claim 2, wherein the plated steel sheet has a Fe-concentrated region having a Zn content of 30 to 40 wt% and a Zn thickening region having a Zn content of 60 to 90 wt% at 750 ° C.
The method according to any one of claims 1 to 3,
Steel plate;
Al thickening layer containing 30 wt% or more of Al formed on the upper steel sheet and
Plated steel sheet for hot press molding comprising a zinc plated layer formed on the Al thickening layer.
The plated steel sheet for hot press forming according to claim 3, wherein the base steel sheet further comprises a surface diffusion layer of a metal having a reduced Gibbs free energy per mole of oxygen less than Cr when oxidized within a depth of 1 μm from a surface thereof.
The plated steel sheet for hot press forming according to claim 3, wherein the thickness of the annealing oxide formed on the surface of the base steel sheet is 150 nm or less.
The hot rolled steel sheet according to claim 3, 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 6, 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%.

Images