KR101197891B1 - Hot pressed parts having excellent heat resistance and corrosion resistance - Google Patents

Abstract

One side of the present invention is a steel sheet; A zinc plated layer including a Fe—Zn phase in which an absolute value of the Gibbs free energy reduction amount per mole of oxygen is more than 0.008% by weight or more in solid solution in the oxidation reaction formed on the base steel sheet; And an oxide layer having an average thickness of 0.01 to 5 μm formed on the zinc plated layer, by providing a hot press molded part having excellent heat resistance and corrosion resistance in which a zinc diffusion phase is discontinuously present on an upper portion of the base steel sheet.
Before annealing of the galvanized steel sheet, a metal having a low oxygen affinity is coated to an effective thickness to suppress the formation of annealing oxide on the surface of the steel sheet to form a uniform galvanized layer. By increasing the melting temperature in a short time, it is possible to prevent deterioration of the plating layer and to minimize the generation of oxides formed after hot press molding.

Description

HOT PRESSED PARTS HAVING EXCELLENT HEAT RESISTANCE AND CORROSION RESISTANCE}

The present invention relates to a hot press molded part excellent in heat resistance and corrosion resistance, and more particularly, to a hot press molded part which secures heat resistance and corrosion resistance of a plated layer by controlling a zinc diffusion phase diffused from a plating layer to a base steel sheet.

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 ~ 900 ℃, 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 is excellent in heat resistance at high temperature but inferior in corrosion resistance compared to the sacrificial anode type Zn plating 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 formed unevenly by alloying and high temperature oxidation of Zn layer at 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.

One side of the present invention is to prevent the deterioration of the galvanized layer during hot press molding of the plating material using zinc plating, hot press molding excellent heat resistance and corrosion resistance that can minimize the generation of oxide formed on the surface of the plating layer after hot press molding To provide a part.

One side of the present invention is a steel sheet; A zinc plated layer including a Fe—Zn phase in which an absolute value of the Gibbs free energy reduction amount per mole of oxygen is more than 0.008% by weight or more in solid solution in the oxidation reaction formed on the base steel sheet; And an oxide layer having an average thickness of 0.01 to 5 μm formed on the zinc plated layer, and providing a hot press molded part having excellent heat resistance and corrosion resistance in which a zinc diffusion phase is discontinuously present on an upper portion of the base steel sheet.

In this case, the metal of which the absolute value of the Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction is smaller than Cr is preferably Ni or Fe—Ni.

Moreover, it is preferable that the average thickness of the said zinc diffusion phase is 5 micrometers or less.

In addition, the Zn content of the galvanized layer is preferably 30% by weight or more.

At this time, the thickness of the galvanized layer is more preferably 1.5 times or more than the thickness before hot press molding.

According to one aspect of the present invention, before the annealing of the galvanized steel sheet by coating a metal having a low oxygen affinity to an effective thickness to suppress the formation of annealing oxide on the surface of the steel sheet to form a uniform galvanized layer, the zinc plated layer during press heat treatment The alloying of the metal plate is promoted and the melting temperature of the zinc plated layer rises within a short time, thereby preventing deterioration of the plated layer and minimizing the generation of oxides formed after hot press molding.

1 is a cross-sectional view of a hot press molded part manufactured according to the invention example.
2 shows a cross section of a hot press molded part manufactured according to a comparative example.

EMBODIMENT OF THE INVENTION Hereinafter, the molded part of this invention is demonstrated.

One side of the present invention is a steel sheet; A zinc plated layer including a Fe—Zn phase in which an absolute value of the Gibbs free energy reduction amount per mole of oxygen is more than 0.008% by weight or more in solid solution in the oxidation reaction formed on the base steel sheet; And an oxide layer having an average thickness of 0.01 to 5 μm formed on the zinc plated layer, and providing a hot press molded part having excellent heat resistance and corrosion resistance in which a zinc diffusion phase is discontinuously present on an upper portion of the base steel sheet.

After the hot press molding, the hot dip galvanized layer is preferably in a Fe-Zn phase in which a metal having an absolute value of the Gibbs free energy reduction amount per mole of oxygen less than Cr is dissolved in an Fe-Zn phase of at least 0.008% by weight. That is, before the hot press, the plating layer contains 0.01 wt% or more of the metal whose absolute value of Gibbs free energy reduction amount per mole of oxygen is less than Cr during oxidation, and the Gibbs free energy per mole of oxygen during the oxidation reaction by hot press heating. If the absolute value of the decrease is less than Cr, the solid solution is formed in the Fe-Zn phase. If the metal is less than 0.008% by weight of the Gib free energy reduction amount per mole of oxygen in the ternary phase, the plating layer of the base steel sheet is included. At the same time, it is possible to prevent the diffusion of Zn from the zinc plated layer into the steel sheet.

It is preferable that the thickness of the said oxide layer is 0.01-5 micrometers or less. When the thickness of the oxide layer formed on the surface of the hot-dip galvanized layer exceeds 5 μm, the oxide is brittle and the growth stress is concentrated, so that the oxide is easily peeled off from the surface. Thus, an oxide removing process such as shot blasting is performed after product molding. As necessary, it is necessary to manage the thickness of the oxide layer to 5 µm or less. However, since the volatilization of Zn in the said plating layer cannot be suppressed when the said thickness is less than 0.01 micrometer, it is preferable to limit the minimum of the said thickness to 0.01 micrometer.

In addition, the zinc diffusion phase is preferably present discontinuously on the upper portion of the base steel sheet. In general, when the hot-dip galvanized steel sheet is applied to a hot press heating furnace, zinc contained in the plating layer is diffused into the steel sheet to form a zinc diffusion phase having a predetermined thickness on the upper portion of the steel sheet, which is caused by excessive alloying. It means that the Zn content is not sufficient, so that the heat resistance is not good, and thus the zinc plated layer may not properly exhibit the corrosion resistance effect. Therefore, the zinc diffusion phase is preferably formed discontinuously to ensure heat resistance and corrosion resistance.

According to the present invention, at the interface between the plated layer and the base steel sheet, Zn, Fe and ternary phases of metals having an absolute value of Gibbs free energy reduction per mole of oxygen during the oxidation reaction are smaller than Cr are formed to prevent diffusion of the plated layer components. At the same time, since the Zn contained in the plating layer is suppressed from being diffused into the base steel sheet, the zinc diffusion phase is discontinuously formed, which means that prevention of separation of the Zn in the plating layer is good, thereby ensuring excellent corrosion resistance. do.

When the zinc diffusion phase is too thick, like the continuous zinc diffusion phase, it means that zinc contained in the plating layer is diffused into the base steel sheet by hot press heating, and in this case, there is a limit to securing excellent heat resistance and corrosion resistance. That is, in order to secure heat resistance and corrosion resistance of the hot press molded part, it is necessary to control the average thickness of the zinc diffusion phase to 5 μm or less. The zinc diffusion phase is preferably not formed continuously over 1000 μm along the surface of the base steel sheet, and the average thickness refers to the average of the thicknesses of the alloy phases observed within a certain distance of the surface of 2000 μm or more.

In the present invention, the zinc diffusion phase is a zinc-containing phase in the hot dip galvanized steel sheet, the zinc plating layer and the zinc diffusion phase, the acid when the steel sheet is immersed in an acidic solution such as HCl solution with an inhibitor It does not dissolve and contains Zn on the surface of the base steel sheet and the remaining part becomes zinc diffusion phase. Therefore, by dissolving the zinc-plated steel sheet as an acidic solution as described above, by measuring the thickness of the remaining zinc diffusion phase or the content of Zn contained therein, the presence and composition of the zinc diffusion phase can be confirmed.

The content of Zn contained in the zinc diffusion phase in the present invention is less than 30% by weight. Since the portion of the Zn content of 30% by weight or more constitutes a part of the galvanized layer, a large amount of Fe is diffused so that the portion of the Zn content of less than 30% by weight becomes a zinc diffusion phase, and thus the zinc plated layer and the base steel sheet The distinction becomes unclear.

After the hot press molding of the present invention according to the above to ensure the Zn content of the hot-dip galvanized layer 30% by weight or more can maintain the galvanized layer stably. That is, since the Zn loss of the zinc plated layer can be suppressed by the ternary phase and the oxide layer formed after hot press forming as described above, the zinc plated layer can be stably maintained and the Zn content of the plated layer can satisfy 30% or more. If the Zn content of the plating layer is less than 30%, it is impossible to form a uniform plating layer, and the sacrificial anode property of the plating layer is deteriorated, and thus corrosion resistance is easily deteriorated.

At this time, the thickness of the hot dip galvanizing layer after the hot press molding is more preferably 1.5 times or more than before hot press molding. In general, in the hot pressing process, the Fe is more diffused from the base steel by heating, and the plating layer becomes thicker than before the hot pressing process. In particular, the present invention has a Zn content of 30 in the plating layer from the surface of the steel sheet on which the hot pressing is completed. When it is referred to as the thickness of the galvanized layer up to the point of more than%, the thickness is controlled to be 1.5 times or more than before the press molding in order to secure sufficient corrosion resistance.

In the oxidation reaction, the metal having the absolute value of Gibbs free energy reduction per mole of oxygen less than Cr is Ni or Fe—Ni. 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 also shows similar properties 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.

Hereinafter, an example of a hot-dip galvanized steel sheet that can be used for manufacturing a hot press molded part of the present invention will be described.

One aspect of the present invention is a steel sheet including a surface diffusion layer of a metal having an absolute value of the amount of Gibbs free energy reduction per mole of oxygen in the oxidation reaction within a depth of 1㎛ from the surface less than Cr; An Al thickening layer containing 30 wt% or more of Al formed on the surface diffusion layer of a metal having an absolute value of Gibbs free energy reduction per mole of oxygen during the oxidation reaction less than Cr; And a galvanized layer formed on the Al thickening layer,

Annealing oxides having an average thickness of 150 nm or less are discontinuously distributed between the surface diffusion layer and the Al thickening layer, and the absolute value of the Gibbs free energy reduction per mole of oxygen during the oxidation reaction is less than 1 μm from the surface of the base steel sheet. It provides a hot-dip galvanized steel sheet having excellent surface properties with a smaller metal content of 0.1% by weight or more.

The base steel sheet may be applied to both a hot rolled steel sheet and a cold rolled steel sheet, and the annealing oxide serves as a diffusion barrier to prevent alloying of Fe, Mn, etc., which are members of the hot dip galvanized layer and the steel sheet. In the present invention, the thickness of the annealing oxide is 150 nm or less, thereby facilitating alloying of the hot dip galvanized layer, thereby improving heat resistance and plating adhesion after press molding. 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.

It is preferable that the thickness of the said annealing oxide is 150 nm or less. The annealing oxide is formed in the process of annealing heat treatment after the metal coating, as shown in the following manufacturing process. When the thickness of the annealing oxide exceeds 150nm, plating may not be performed well due to the influence of the annealing oxide, and thus plating may occur, and alloying of the plating layer is delayed at the initial stage of hot press heating, thereby securing sufficient heat resistance during high temperature heating. You will not be able to. 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 ensure the plating property and heat resistance.

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

The hot-dip galvanized steel sheet of the present invention has a surface diffusion layer of a metal whose absolute value of Gibbs free energy reduction per mole of oxygen is less than Cr in the oxidation reaction within 1 µm from the surface of the steel sheet, and within 1 µm from the surface of the base steel sheet. It is preferable that the content of the metal is 0.1% by weight or more.

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% by weight or more within 1 μm from the surface, the Al in the plating bath is not changed. Reacting with the metal allows a greater amount of Al to be concentrated on the surface diffusion layer. The concentrated Al diffuses to the surface layer portion in the press heating process and then selectively oxidizes to form a dense and thin Al ₂ O ₃ oxide film, thereby suppressing volatilization and oxide growth of Zn, thereby increasing the surface diffusion layer as described above. It is desirable to increase the Al concentration through.

That is, in order to prevent the galvanized layer from being decomposed at a high temperature by the coating of the metal as described above, in order to secure the heat resistance of the galvanized layer, the absolute value of the Gibbs free energy reduction per mole of oxygen is less than Cr in the oxidation reaction within 1 μm from the surface of the steel sheet. If gold is present in an amount of at least 0.1% by weight, preferably in an amount of at least 1.0% by weight, it is possible to effectively prevent deterioration of the galvanized layer. Can contribute.

In this case, the zinc plated layer is Fe: 15.0% by weight or less, the absolute value of the Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction is less than Cr: 0.01 to 2.0% by weight, the remainder containing Zn and other unavoidable impurities desirable. Metals having an absolute value of the Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction included in the hot dip galvanized layer are diffused into the plating layer during hot press heating to be included in the plating layer. In the oxidation reaction, a metal in which the absolute value of the Gibbs free energy reduction amount per mole of oxygen is smaller than Cr is dissolved to form a ternary phase, thereby reducing the diffusion of Fe, etc., of the ferrous iron into the plating layer during press heating. The galvanized layer does not decompose and plays a key role in forming a single plated layer. Therefore, if the absolute value of the amount of Gibbs free energy reduction per mole of oxygen in the galvanized steel sheet is less than 0.01% by weight of metal containing less than Cr, the amount of the three-phase phase during press heating is insignificant, making it difficult to secure proper heat resistance. In this regard, the upper limit is preferably set at 2.0% by weight in view of economic efficiency.

The type of galvanized steel sheet of the present invention is not particularly limited, and may include all hot dip galvanized steel sheets, electrogalvanized steel sheets, dry galvanized steel sheets by plasma, and galvanized steel sheets by high temperature liquid Zn spray.

In addition, it is preferable that Fe be added to the galvanized layer at 15.0 wt% or less, which is to increase the melting point of Zn by Fe being sufficiently diffused into the galvanized layer to form a Fe-Zn alloy phase, and very high for securing heat resistance. It is an important configuration. 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.

The metal is a metal in which the absolute value of the Gibbs free energy reduction amount is less than Cr in oxide formation of the metal per mole of oxygen, and typically Ni or Fe-Ni. 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 also shows similar properties 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 thickness of the Al thickening layer is 0.1 ~ 1㎛, the area where the portion of the Al enriched layer and the metal content of the surface diffusion layer overlaps more than 5% by weight in the EPMA analysis for the surface diffusion layer and Al enriched layer It is preferable that it is 10% or less. After immersion in the zinc plating bath containing Al, an Al thickening layer is formed on the surface diffusion layer to a thickness of 0.1 ~ 1.0㎛, which can be adjusted according to the content of Al. 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.

Al contained in the concentrated layer diffuses to the surface layer portion in the press heating process and then selectively oxidizes to form a dense and thin Al ₂ O ₃ oxide film, thereby suppressing volatilization and oxide growth of Zn. Therefore, in order to obtain the surface state of the hot press-formed part of the present invention, the process of forming the Al thickening layer after the plating bath may be essential. 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, in the EPMA analysis, the area where the Al content layer and the surface diffusion layer overlap an area where the absolute content of the Gibbs free energy reduction amount per mole of oxygen less than Cr is 5 wt% or more in the oxidation reaction of the Al diffusion layer and the surface diffusion layer. The concentration is preferably 10% or less with respect to the concentrated layer, and the overlapping portion means that the metal and Al have alloyed to form an alloy phase. In this way, 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. Therefore, when there are many parts in the alloy state, the Al ₂ O ₃ continuous oxide film is formed. The amount of Al that can contribute 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.

As a result, in the early stage of press heating, by controlling the average thickness of the oxides discontinuously distributed on the metal surface diffusion layer on the top of the base steel sheet to 150 nm or less, the alloying is promoted, thereby rapidly increasing the melting point of the galvanized layer to ensure heat resistance. Preferably, when the press heating continues to reach 750 ° C. or more, the metal is concentrated in the Zn-Fe phase as described above to form a ternary phase to prevent excessive alloying, thereby stably maintaining the zinc plated layer. That is, it is advantageous to advance alloying rapidly at the beginning of press heating, and when it becomes 750 degreeC or more, it is preferable to hold | maintain a galvanized layer on the contrary.

Hereinafter, an example of the manufacturing method of the hot press molding part of this invention is demonstrated and demonstrated.

Another aspect of the present invention comprises the steps of coating a steel sheet with a metal of less than Cr the absolute value of Gibbs free energy reduction per mole of oxygen during the oxidation reaction; Annealing the coated steel sheet at 700 to 900 ° C .; Immersing the annealed steel sheet in a hot dip galvanizing bath containing Al: 0.05 to 0.5% by weight, balance Zn and other unavoidable impurities, and having a temperature range of 430 to 500 ° C; Heating the steel sheet immersed in the hot dip galvanizing bath to 750 to 950 ° C. at an elevated temperature rate of 2 to 10 ° C./sec in an oxidizing atmosphere, and then maintaining the steel sheet immersed in 10 minutes or less; And it provides a method for producing a hot press molded part comprising the step of pressing the steel sheet maintained after the heating at a temperature range of 600 ~ 900 ℃.

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

First, in the present invention, a steel sheet for hot press forming is coated with a metal whose absolute value of Gibbs free energy reduction per mole of oxygen is less than Cr during oxidation reaction. As described above, when the melting temperature of Zn is placed in a hot press heating furnace at 420 ° C. up to 800 ° C. to 900 ° C., the plating layer may be lost. Therefore, while the temperature of the initial steel sheet increases in the heating furnace, it is necessary to rapidly alloy Fe, Mn, etc., constituent elements of the steel sheet, to the Zn layer to increase the melting temperature of the Zn layer.

When the steel sheet is exposed to too high temperature or exposed to high temperature for a long time, the plating layer is oxidized, and when thick ZnO is formed on the surface of the plating layer, consumption of the plating layer is severe, and mutual diffusion of Zn of the plating layer and the base material of the steel sheet is active. Since Zn content in a plating layer becomes small, there exists a possibility that corrosion resistance may fall. Therefore, it is necessary to minimize the growth of oxide on the surface of the plating layer and maintain the Zn content in the plating layer at a certain amount.

In order to achieve the above object, it is necessary to coat a metal having an absolute value of Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction on the surface of the steel sheet before charging the annealing furnace of the steel sheet. The role of the coating is to minimize the production of annealing oxides formed on the surface of the cold rolled steel sheet in the annealing furnace. Annealed oxide acts as a diffusion barrier to prevent alloying of Fe and Mn, which is a constituent element of the Zn plating layer and steel sheet, and alloying Fe and Mn into the Zn layer when the metal is coated to minimize the formation of the annealing oxide. Is promoted so that the plating layer may have heat resistance in the furnace.

The annealing heat treatment is preferably performed at a temperature range of 700 to 900 ° C. in a mixed gas atmosphere in which nitrogen and hydrogen are mixed. As for the dew point temperature of the said atmosphere, -10 degrees C or less is preferable. 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.

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. In addition, when the dew point temperature of the atmosphere exceeds −10 ° C., the growth rate of the oxide is similarly increased.

In addition, the hot dip galvanizing is more preferably performed by immersing in a plating bath containing 0.05 to 0.5% by weight of Al, Zn and unavoidable impurities, and having a temperature range of 430 to 500 占 폚 based on the annealed steel sheet. . 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.

Other plating conditions are by a conventional method, the temperature of the plating bath is preferably performed in the plating operation within the range of 430 ~ 500 ℃. If the plating bath temperature is less than 430 ℃ the plating bath does not have sufficient fluidity, on the contrary, if the plating bath temperature exceeds 500 ℃ dross occurs frequently in the plating bath to reduce the production efficiency, the plating bath temperature is 430 It is preferable to control to -500 degreeC. More preferably, when the temperature is 460 ° C. or more, it is more effective to sufficiently concentrate Al and a metal having a less oxidizing property than Cr at the interface between the plating layer and the base steel sheet.

The hot dip galvanizing is performed so as to have a thickness of 5 to 30 µm. If the thickness of the hot-dip galvanized layer is less than 5㎛ the alloy in the plating layer in the hot press heating furnace is excessive, the amount of Zn in the plating layer after the hot press working significantly falls, when the thickness of the plating layer exceeds 30㎛, hot press heating furnace In this case, the alloying of the plating layer is delayed, so that the oxide grows rapidly on the surface of the plating layer, and it is also disadvantageous in terms of manufacturing cost.

At this time, the step of coating the metal of the absolute value of the Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction less than Cr is preferably coated with Ni or Fe-Ni with an average thickness of 1 ~ 1000nm. The metal applied to the coating should be composed of a metal whose absolute value of the amount of Gibbs free energy reduction is smaller than that of Cr in forming oxides of metal per mole of oxygen. If the absolute value of the Gibbs free energy reduction is greater than Cr, the coated metal itself is oxidized and there is no improvement effect. Ni and Fe are typically applied as the metal. Although these may be applied in a mixed or alloyed state, Fe is more preferably applied in an alloy state.

At this time, the coating thickness of the metal is preferably 1 to 1000nm. When the coating thickness is less than 1 nm, the annealing oxide suppression function is not sufficient, and when the coating thickness is more than 1000 nm, oxide suppression by metal coating is possible, but it is limited economically due to the increase in manufacturing cost, so it is limited to within 1000 nm. Therefore, it is preferable to control the thickness at 1 to 1000 nm, and more preferably at 10 to 200 nm, the oxide formation inhibitory effect can be more excellently secured and economical at the same time.

In addition, after the immersion in the hot dip galvanizing bath may further comprise the step of alloying heat treatment in a temperature range of less than 600 ℃. When performing the alloying heat treatment after the plating, the temperature of the alloying heat treatment is limited to 600 ° C or less. If the temperature exceeds 600 ° C, alloying of the plating layer proceeds to increase the heat resistance in the hot press heating furnace, but the alloying heat treatment temperature may be increased because cracking may occur due to embrittlement of the plating layer and scale growth increases on the surface of the plating layer in the heating furnace. By limiting to less than 600 ℃, preferably less than 500 ℃ by suppressing the Fe in the plating layer to 5% by weight or less, it is possible to effectively prevent the occurrence of fine cracks in the plating layer, if the temperature is suppressed to 450 ℃ or less It is more preferable to suppress the occurrence of fine cracks.

After the hot-dip galvanized steel sheet is manufactured, the hot pressing process is performed. First, the hot-dip galvanized steel sheet is subjected to a heat treatment process. The heat treatment step is preferably heated to 750 ~ 950 ℃ in an oxidizing atmosphere at a temperature rising rate of 2 ~ 10 ℃ / second and maintained for 10 minutes or less. When the temperature increase rate is less than 2 ℃ / sec, the plating time is easy to deteriorate due to excessive time in the heating furnace, and when the temperature increase rate exceeds 10 ℃ / second, the galvanized layer is not sufficiently alloyed This is because there is a risk that the galvanized layer deteriorates due to excessive temperature rise.

When heating, the maximum temperature is 750 ~ 950 ℃ and the holding time at the maximum temperature is preferably within 10 minutes. When the maximum temperature is less than 750 ° C, the steel microstructure is not sufficiently transformed into the austenite region, and thus it is not easy to secure the strength. In addition, when the holding time at the above temperature is too long, the surface quality of the plating may be lowered. Therefore, it should not exceed 30 minutes, and more preferably within 10 minutes is effective.

In particular, when heated to 750 ~ 950 ℃ in an oxidizing atmosphere to form an Al ₂ O ₃ layer on the surface of the steel sheet to act as a protective layer to suppress the volatilization of Zn of the plating layer, in order to form such a protective layer continuously It is advantageous that the oxygen partial pressure in the heating atmosphere is 10 ^-40 atm or more, and more preferably, the protective layer can be more smoothly formed in the case of 10 ^-5 atm or more.

After the heat treatment, press molding is performed at a temperature range of 600 to 900 ° C. to manufacture hot press molded parts. When the temperature is less than 600 ℃, it is difficult to secure sufficient strength even if the austenite is transformed to ferrite to perform a hot press, it is preferable to limit the upper limit to 900 ℃ in terms of economics.

Hereinafter, the present invention will be described in detail by way of examples, which are intended for a more complete description of the present invention, and the scope of the present invention is not limited by the following individual examples.

( 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 0.24 0.04 2.3 0.008 0.0015 0.025 River 2 0.22 1.0 1.7 0.01 0.001 0.04

Then, a predetermined metal was applied to the surface of the steel sheet before annealing within 200nm and then subjected to annealing at a temperature of 785 ℃ and Zn plating to prepare a hot-dip galvanized steel sheet. Through the GOEDS analysis, 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. In order to increase the accuracy of the data, SEM, TEM observation, wet analysis, and electron spectrochemical Verification was made by comparison by assay (ESCA).

Then, a hot press process was performed on the hot-dip galvanized steel sheet, the temperature of the hot press furnace was carried out at 750 ~ 950 ℃ and the atmosphere of the furnace was 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 by XRD and GOEDS analysis. The thickness of the plated layer, continuity and thickness of the zinc diffusion phase were measured through cross-sectional analysis of the specimen. For reference, the thickness of the plating layer was measured by the length from the surface of the plating layer to the point up to the point of the content of Zn in the plating layer 30% by weight or more in the vertical direction, each experimental condition or measurement results are shown in Table 2 below.

division Steel grade Type of coated metal Metal coating thickness
(nm) Surface layer
Within 1㎛
thickening
Metal amount
(wt%) zinc
Plated layer
thickness
(Μm) Alloying
Temperature
(℃) Hot
Press heating
Temperature (℃) Hot
Press
heating
time
(minute) Hot Gulf
Leshoo
Plated layer
thickness
(Μm) Continuity of Zinc Diffusion Phase Zinc Diffusion Phase Thickness
(Μm) Inventory 1 River 1 Ni 25 2.2 8 - 910 6 20 discontinuity 3 Inventive Example 2 River 1 Ni 25 2.2 8 500 910 6 21 discontinuity 3 Inventory 3 River 2 Fe-Ni 50 4.5 12 - 900 4 21 discontinuity 2 Honorable 4 River 2 Ni 20 1.8 10 - 930 7 27 discontinuity 4 Comparative Example 1 River 1 - - - 8 - 910 6 - continuity 19 Comparative Example 2 River 2 - - - 10 - 900 5 - continuity 22 Comparative Example 3 River 2 - - - 10 560 900 6 - continuity 23

First, Inventive Examples 1 to 4 suppressed the diffusion of Zn-containing steel sheet by forming a ternary phase of Fe-Zn-Ni in the plating layer during hot press heating through Ni coating, so that the zinc diffusion phase appeared in a discontinuous form. The thickness of the zinc diffusion phase was also thinly suppressed to 3 µm or less. Therefore, the heat resistance is ensured and the Zn plating layer is stably maintained, so that the plating layer becomes thicker after heating, whereby the corrosion resistance of the plating layer can be excellently exhibited.

On the other hand, in Comparative Examples 1 to 3, Ni lead was not performed, and Zn of the plating layer rapidly diffused into the base steel sheet during hot press heating, so that a zinc diffusion phase was formed continuously and thickly. Accordingly, it can be confirmed that after the press heating, the Zn plating layer is all lost and heat resistance is not secured, and thus, it is impossible to secure corrosion resistance, which is the purpose of using galvanized steel.

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 part manufactured according to Inventive Example 1 are shown in FIG. 1 and Table 3, and according to Comparative Example 1 2 and Table 4 show the results of analyzing the cross section and the components at each point of the manufactured hot press molded part by EDS.

Category (% by weight) ① ② ③ ④ Mn - - - 2.2 Si - - - 0.3 Fe 67.65 67.85 68.05 97.5 Zn 32.35 32.15 31.95 -

Category (% by weight) ① ② ③ Mn - - 1.66 Si - - - Fe 80.47 83.71 96.16 Zn 19.08 16.29 2.18

First, referring to Figure 1, it can be seen that the zinc diffusion phase is hardly formed on the upper portion of the base steel sheet, the distinction between the plated layer and the base steel sheet is clear. That is, even after hot press heating, the plating layer is not lost and is stably maintained. As shown in Table 3, ①, ②, and ③ points are in the stable plating layer with the ratio of Zn exceeding 30% by weight, and ④ point is the upper part of the steel sheet, and Zn is hardly shown, so the formation of zinc diffusion phase is very small. It can be seen that. Therefore, the heat resistance of the plating layer is excellently secured, and thus corrosion resistance may be effectively expressed.

On the contrary, referring to FIG. 2, it can be seen that zinc diffusion occurs excessively to make it difficult to distinguish the plated layer from the steel sheet. That is, most of the Zn of the plating layer is lost to the base steel sheet and thus the heat resistance is not secured. Even in Table 4, since the Zn content is less than 20% by weight at the point ① and ②, which were points in the plated layer before press heating, it cannot be seen as a plated layer that can exhibit substantial corrosion resistance, and eventually, most of the galvanized layer is lost. It can be seen that it spreads in part.

Claims (5)

Steel plate;
A zinc plated layer including a Fe—Zn phase in which Ni or Fe—Ni formed on the base steel sheet is dissolved in 0.008 to 2.0 wt%; And
A hot press molded part comprising an Al ₂ O ₃ oxide film having an average thickness of 0.01 to 5 μm formed on the zinc plated layer, and having a zinc diffusion phase discontinuously present on the upper portion of the base steel sheet.
delete The method according to claim 1,
The hot press molded part excellent in heat resistance and corrosion resistance whose average thickness of a zinc diffusion phase is 5 micrometers or less. The method according to claim 1 or 3,
Hot press molded parts having excellent heat resistance and corrosion resistance of Zn content of the galvanized layer of 30% by weight or more.
The method of claim 4,
The thickness of the galvanized layer is a hot press molded part excellent in heat resistance and corrosion resistance that is 1.5 times or more than the thickness before hot press molding.

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