KR101304621B1 - Method for manufacturing hot press forming parts having different strengths by area - Google Patents

Abstract

The present invention relates to a method for manufacturing a molded article having different strengths for each region by press molding, which is one of the molding methods for automobile parts, and more particularly, by giving various temperature differences to parts of the blank before molding or during molding. By using the difference in heat treatment characteristics, it has a multi-strength region having various strengths after hot press molding, and manufactures a press-formed product which prevents the liquid metal embrittlement phenomenon which may occur during heating or the lack of corrosion resistance by volatilization of the plating layer. It is about how to.
Method for producing a press-formed article according to one aspect of the present invention is a blank from a plated steel sheet comprising a steel plate, 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. Collecting step; Heating the blank; And hot pressing the heated blank, and characterized in that it has a different thermal history for each area of the blank in at least one of the heating and hot press molding.

Description

Manufacturing method of press-formed product having different strength for each field {METHOD FOR MANUFACTURING HOT PRESS FORMING PARTS HAVING DIFFERENT STRENGTHS BY AREA}

The present invention relates to a method for manufacturing a molded article having different strengths for each region by press molding, which is one of the molding methods for automobile parts, and more particularly, by giving various temperature differences to parts of the blank before molding or during molding. By using the difference in heat treatment characteristics, it has a multi-strength region having various strengths after hot press molding, and manufactures a press-formed product which prevents the liquid metal embrittlement phenomenon which may occur during heating or the lack of corrosion resistance by volatilization of the plating layer. It is about how to.

Recently, each automobile manufacturer has been increasing the use of high-strength materials in order to respond to the social demands for eco-friendly fuel economy and lightweight in applying parts to automobiles. However, the molding of high-strength material has problems such as spring back and dimensional freezing, and the reality is that its use is limited due to the difficulty of molding.

This molding problem can be solved by molding a material at a high temperature with good moldability and simultaneously quenching it in a mold to manufacture a high strength component. This method is called a hot press molding process. According to such a process, the component which has the intensity | strength of 1500 MPa can be formed normally.

However, the parts having only a single strength through the hot press molding process have a low degree of freedom in terms of design to satisfy the required performance such as impact performance. The hot press molding technology incorporating the 3) technology has been developed. However, this TWB method has a disadvantage in that a blank welding process is added, and since the integrity of the weld can affect the performance of the part, there are many difficulties in process management.

In addition, the hot press molded article is produced by pressing the plated unplated plate, a large amount of oxidizing scale is formed on the surface adversely affects the surface properties of the final product. 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 base steel sheet, causing a so-called liquid metal embrittlement phenomenon (LME), which weakens the interface. When stress is applied, it may cause damage to the steel sheet.

In addition, after forming in cold without necessarily forming in hot, there may be a case in which a high strength is applied by applying a heat history to the molded part. In this case, too, since the heat treatment is performed at a high temperature, there may be a problem that the surface of the steel sheet is oxidized when heated in a strip state.

Therefore, in this case, too, a plating layer is required on the surface of the steel sheet, but as described above, Al plating has difficulty in exhibiting sufficient corrosion resistance, and Zn plating has a problem in that volatilization and oxidation loss tend to occur.

According to one aspect of the present invention, when heated to a high temperature in order to give strength to the part, there is provided a method for producing a press-formed article that can prevent the loss of volatilization and oxidation of the galvanized layer.

In addition, according to another aspect of the present invention, the liquid metal embrittlement phenomenon is prevented when the press molding is carried out hot, the alloying and high temperature stability of the galvanized layer is improved, and different strength for each region under the same hot press molding and cooling conditions And it provides a method for producing a hot press molded article that can obtain physical properties.

According to one aspect of the present invention, there is provided a method for manufacturing a press-formed product, including: a steel sheet, an Al thickening layer including 30 wt% or more of Al formed on the upper steel sheet, and a zinc plating layer formed on the Al thickening layer. Collecting a blank from the plated steel sheet; Heating the blank; And hot pressing the heated blank, and characterized in that it has a different thermal history for each area of the blank in at least one of the heating and hot press molding.

At this time, according to one preferred embodiment of the present invention, the thermal history different for each of the regions is characterized by being obtained by setting the temperature of the hot press die differently for each region.

At this time, according to another preferred embodiment of the present invention, the thermal history different for each region, the step of heating the blank is the step of heating the blank to the first temperature; And heating a portion of the heated blank to a second temperature higher than the first temperature.

In addition, during hot press forming, it is preferable to hot press form the remaining portion of the blank, not the portion heated to the second temperature, above the Ac1 temperature.

According to another embodiment of the present invention, the thermal history different for each of the regions may include heating the entire blank to a first temperature; And locally cooling a portion of the heated blank to a second temperature lower than the first temperature.

At this time, it is preferable to hot press form a part of the heated blank which is locally cooled to the second temperature during hot press forming at an Ac1 temperature or higher.

According to yet another aspect of the present invention, the method comprising: collecting a blank from a plated steel sheet including a plated steel sheet, an Al enriched layer including 30 wt% or more of Al formed on the base plate and a zinc plated layer formed on the Al enriched layer; Press molding the blank to obtain a molded article; Heating the molded article; And cooling the heated molded article, and preferably at least one of the heating and cooling steps have different thermal history for each region of the molded article.

At this time, in one preferred embodiment of the present invention according to the above aspect, different thermal history for each region can be obtained by setting different cooling rates for each region of the molded article.

Further, in another preferred embodiment of the present invention, the thermal history different for each of the regions, the step of heating the molded article is the step of heating the entire molded article to a first temperature; And heating a part of the heated molded article to a second temperature higher than the first temperature.

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

In addition, the base steel sheet is effective to further include a surface diffusion layer of a metal having an absolute value of the Gibbs free energy reduction per mole of oxygen in the oxidation reaction within a depth of 1㎛ from the surface less than Cr.

In addition, it is advantageous that the thickness of the annealing oxide formed on the surface of the base steel sheet is 150 nm or less for uniform alloying and zinc volatilization prevention.

In addition, the steel sheet has a composition consisting of C: 0.1 to 0.4% by weight, Si: 2.0% or less (excluding 0%), Mn: 0.1 to 4.0%, balance Fe and other unavoidable impurities. It is advantageous.

At this time, 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%.

According to the present invention, the rapid alloying is prevented during press heating, thereby suppressing the presence of the liquid metal in the plating layer at the press working temperature, so that embrittlement due to the liquid metal does not occur, and thus the production yield is high. The effect of being able to implement all of the various designed strengths can be obtained.

1 is an example of a hot press molded article having a strength distribution for each site according to the present invention.
2 is an example of a CCT curve of hot press forming steel.
Figure 3 is a graph showing the concept of a pattern of the second heating part of the present invention after heating the whole blank.
4 is a graph showing an example in the case of secondary heating a part after the uniform heating of the present invention.
5 is a conceptual diagram showing a graph of temperature and time as an example of the method proposed in the present embodiment.
FIG. 6A is a graph showing expansion with respect to the temperature of the part which performed secondary heating after uniform heating, and FIG. 6B is a graph which shows the expansion of the part which performed only uniform heating.
7 is a photograph showing the tissue of FIGS. 6A and 6B, respectively.
8 is a graph showing the results of measuring the Vickers hardness of the additionally heated portion above the A3 temperature and the cooled portion after being heated and maintained below the A1 temperature.
9 is a graph showing the results of measuring Vickers hardness and tensile strength of specimens subjected to hot press molding after different heat treatment conditions.
FIG. 10 is a graph showing tensile strength, standard deviation, and range when secondary heating is performed with different holding time after uniform heating.
11 is a stress-strain curve of the molded article produced by Example 4. FIG.
12 is a stress-strain curve of the molded article prepared in Example 5. FIG.

Hereinafter, the present invention will be described 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 have found that when an Al thickening layer containing 30 wt% or more of Al is formed at the interface between the steel sheet and zinc, it is possible to suppress volatilization of zinc in the plating layer and to prevent oxide growth. . In addition, it has been found that the Al thickening layer described above also has an effect that liquid metal embrittlement phenomenon (LME) may be obtained even if press molding is performed at a high temperature by obtaining an alloy layer at a high temperature. 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 dense and thin in which Al 2 O 3 is the main component (for example, the Al 2 O 3 content is 90% by weight or more) by Al diffusion to the surface of the plating layer and selective oxidation during heat treatment after hot press molding or press molding. Serves to form an oxide layer. The oxide layer formed on the surface of the plating layer during hot press molding or heat treatment serves to prevent volatilization of zinc so that zinc in the alloy layer is sufficiently present during the alloying process by heating to faithfully serve as a sacrificial anode. In addition, since the oxide layer is uniformly coated on the surface of the galvanized layer, the progress of alloying may also occur uniformly. As a result, the liquid metal embrittlement due to the unalloyation may occur effectively when the press forming is performed in a hot state. It can be suppressed. The Al thickening layer is preferably present in combination with a ratio close to the stoichiometric ratio of Fe and the intermetallic compound, for example, in the form of Fe ₂ Al ₅ .

The concentrated layer may further contain Zn within 10% by weight. When the Zn content is 10% by weight or more, the shape of the Al thickened layer becomes nonuniform, thereby reducing the effect of the homogeneous alloying. In this case, the Al thickening layer has a form in which fine particles are continuously formed. In the present invention, the area ratio occupied by the Al thickening layer at the interface between the steel plate and the plating layer is preferably 88% or more. When the occupied area ratio of the Al enriched layer is low, the oxide layer may not be sufficiently formed on the surface of the plating layer, thereby preventing the volatilization of zinc from being sufficient.

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

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

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

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

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

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

For this purpose, it is preferable that the content of the metal whose absolute value of the Gibbs free energy reduction amount per mole of oxygen in the oxidation reaction included in the surface diffusion layer is smaller than Cr 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. Moreover, it is preferable to limit content of the said metal to 30 weight% or less. This is because when the metal content exceeds 30% by weight, 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 the coating of metal as described above, in order to secure the heat resistance of the galvanized layer, the absolute value of the Gibbs free energy reduction amount per mole of oxygen is less than Cr in the oxidation reaction within 1 μm from the surface of the steel sheet. If the metal should be present at least 0.1% by weight, preferably at least 1.0% by weight, it is possible to effectively prevent deterioration of the galvanized layer, and more preferably at least 3.0% by weight is more excellent in securing the heat resistance of the galvanized layer. Can contribute.

In addition, if the absolute value of the amount of Gibbs free energy reduction per mole of oxygen in the oxidation reaction is less than Cr contains a surface diffusion layer of metal or the absolute value of the Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction in the zinc plating layer is less than Cr When included, the Al thickening layer may further include a metal having an absolute value of Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction smaller than Cr, and the content thereof may be 5 wt% or less of the entire Al thickening layer. More preferably, it may be 0.1 to 5% by weight. Metals having an absolute value of the Gibbs free energy reduction per mole of oxygen during the oxidation reaction smaller than Cr may also 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, in the EPMA analysis, the area where the Al content of the Al-diffusion layer and the surface-diffusion layer overlap a portion where the absolute content of the Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction is less than 5% by weight of the Cr content, and the entire surface diffusion layer and Al 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.

Ni is a representative example of a metal in which the absolute value of the Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction is smaller than Cr, 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 the absolute value of Gibbs free energy reduction per mole of oxygen less than Cr in the oxidation reaction is most preferably present as a surface diffusion layer, but is not necessarily limited thereto and is plated together with zinc in the plating bath to be present in the zinc plating layer. It is also possible.

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 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 rest 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, in order to sufficiently form the three-phase phase in order to provide heat resistance to the plated layer during press heating, a zinc-plated steel sheet containing 0.01 wt% or more of a metal whose absolute value of Gibbs free energy reduction amount per mole of oxygen during oxidation is less than Cr. It is necessary to set the upper limit to 2.0% by weight from the economic point of view.

In addition, Al constituting the Al thickening layer having the above-described characteristics of the present invention can be supplied in a variety of ways, but if you want to provide from the plating layer, the plating layer more preferably contains 0.05 to 0.5% by weight of Al. If the Al content is less than 0.05%, the plating layer is easily formed non-uniformly, and if the Al content is more than 0.5%, an activation layer is formed thickly at the interface of the Zn plating layer, and thus the reaction is initiated in a hot press heating furnace. Since the diffusion rate of Fe, Mn, etc. into the Zn layer decreases and the alloying in the heating furnace is delayed, the amount of Al is limited to 0.5% or less, and more preferably 0.25% or less. More effective.

In addition, the thickness of the galvanized layer should be 3㎛ or more to ensure the heat resistance at high temperature, if the thickness is less than 3㎛ may cause uneven thickness of the plating layer or lower the corrosion resistance, more preferably It is effective that is 5 micrometers or more. In addition, the thicker the thickness of the plating layer, the better the corrosion resistance. However, if the thickness is about 30 μm, sufficient corrosion resistance can be obtained. From the viewpoint of economics, the upper limit of the thickness of the zinc plating layer is preferably set to 30 μm, and more preferably, the thickness of the plating layer. It is also possible to suppress the LME cracks that may occur on the surface during press working by controlling the within 15 μm to ensure a high ratio 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 can 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%.

In the case of using a plated steel sheet having the above conditions, an alloy plating layer having a relatively uniform zinc content can be obtained at a high temperature, so that volatilization and oxidation loss of the zinc plated layer can be minimized. The breakage of the plate cut to the size for pressing can be suppressed, and the error rate of the product can be increased. Since the galvanized steel sheet is manufactured as a press-formed product by the following method, it has a different strength for each region, so that no additional softening heat treatment is required. In the present invention, it will be divided into a hot press molding method and a heat treatment method after press molding.

(Hot press forming)

In the present invention, a method of having a component having multiple strengths in the production of a hot press molded product has a different thermal history for each region in at least one of a heating step or a press molding step for press molding. These thermal histories can be broadly divided into various ways, such as multiplexing the cooling rate (providing the heat history in the press molding step), multiplexing the heating temperature (providing the heat history in the heating step), multiplexing the cooling rate and the heating temperature. .

Multiplexing of Cooling Speed

Hot-dipped galvanized steel sheet obtains different properties according to the cooling rate during the cooling process after it is austenitic. 1 shows a CCT curve of a material for hot press molding, and as shown in FIG. 1, when a sufficient cooling rate is secured, a martensite structure is obtained to obtain sufficient strength, and under low cooling rates, ferrite, pearlite, and the like are low. It will contain tissue with strength, resulting in low strength.

If such a property is used, a difference in physical properties after cooling can be obtained due to the difference in cooling rate even if the cooling rate is sufficiently different. In order to differentiate the cooling rate for each region, a method of setting the mold temperature differently for each region may be used.

To this end, it is necessary to first take a blank from the steel sheet and heat the collected blank to a high temperature. The heating temperature may be any heating temperature as long as it is a heating temperature necessary for normal hot press molding, and more preferably, heats up to an austenite region (commonly referred to as a temperature of A3 or higher, and the upper limit is a temperature at which delta ferrite is not formed). It is good to, and more specifically, it is good to heat in the range of 780-950 degreeC. After heating, the temperature of the blank is maintained for a certain time. In this embodiment, maintaining the heated blank for a certain time is to ensure uniformity of the temperature in the blank and to promote the diffusion of carbon (C) in the blank material to secure the increase in strength and uniformity. The holding time may be set differently according to the steel grade of the blank material, the size of the blank, and the like, and can be easily set by anyone having ordinary skill in the art. One preferred example of the holding time is 180 seconds or more. However, if the holding time exceeds 10 minutes, it is not preferable in terms of equipment and energy efficiency of the process.

The heated blank is then subjected to hot press molding. During hot press molding, rapid cooling occurs at the same time as the shape change by the low-temperature mold, and the cooling rate and the cooling stop temperature may be changed by varying the temperature for each mold. Thereby, the hot press-molded product which has different intensity | strength for every area can be obtained. The temperature of the mold may be set differently according to the strength, but preferably, the high strength region (for example, 1470 MPa or more) may be set below MS (martensite production temperature), and the low intensity region (for example, less than 1470 MPa, More preferably, less than 600 MPa) may be set to 400 ~ 500 ℃.

Accordingly, one aspect of the method for producing a hot press molded article of the present invention comprises the steps of: extracting a blank from the steel sheet of the present invention under the advantageous conditions described above; Heating the blank in a heating furnace; And hot pressing the heated blank, and setting the temperature of the hot press die differently for each region so that the hot press molded article has different strengths for each region.

Multiplexing of heating temperature

However, as described above, in order to make a difference in the cooling rate, the mold may need to be heated to about 400 to 500 ° C. or use a separate means in order to obtain a slow cooling rate.

Accordingly, the inventors of the present invention, in the method of manufacturing a molded article obtained by heating the blank and then hot press molding, the entire blank before the hot press molding (for example, A1 temperature (Austenite transformation does not occur) Any temperature below the boundary temperature of the ferrite region and the region where the austenite / ferrite two phases coexist), and then a portion of the second temperature higher than the first temperature (any temperature higher than A3 at which austenite transformation occurs). If the blank with temperature distribution is secured through the second heating before hot press molding from the two-stage heating method that further heats up to), and hot press molding is performed, the same mold cooling is finally performed in the same part as shown in FIG. 1. We realized that we can obtain molded parts with different strength distributions in this paper.

That is, in the present embodiment, first, prior to hot press molding, the entire blank is first heated in the heating furnace to the first temperature. At this time, it is preferable to set the range of heating from Ms (martensite transformation start temperature) to less than A1 temperature.

After heating, the temperature of the blank is maintained for a certain time. Maintaining the heated blank for a certain time in this embodiment ensures uniformity of the temperature in the blank, promotes diffusion of carbon in the blank material and increases strength when quenched after a short secondary heating time. This is to ensure uniformity. The holding time varies depending on the steel grade of the blank material and the size of the blank. One preferred example of the holding time is 180 seconds or more. However, if the holding time exceeds 10 minutes, it is not preferable in terms of equipment and energy efficiency of the process.

And further heating at least one or more portions of the blank held for a certain time to a second temperature (for example, the temperature of the austenite region) higher than the first temperature. In a preferred embodiment this additionally heated portion undergoes sufficient austenite transformation, which is then cooled after hot press forming to have martensitic structure and thus high tensile strength. The heating at this time is preferably at least A3 temperature (austenite region). The reason for heating to a temperature of A3 or more is that heating to a temperature between A1 and A3 does not cause sufficient austenite transformation, and therefore, even if quenching does not sufficiently secure martensite structure, high strength cannot be obtained.

Preferably it is heated to the temperature of A3 or more and maintained for a certain time to achieve a sufficient austenite transformation. At this time, the holding time is preferably 5 seconds or more. At this time, one minute is sufficient for sufficient austenite transformation to occur in the additionally heated portion, and it is not preferable because the austenite transformation may occur even in the unheated portion.

A step of performing hot press molding and cooling the entire blank in which the portion is further heated. According to this method, since the temperature of the initial blank is different, the portion heated above A3 has martensite structure after cooling, even though it has similar rapid cooling rate, and the portion heated below A1 is cooled after ferrite, pearlite, and bay. You will have a knight's organization and you'll get parts with different final properties.

3 is a conceptual diagram illustrating an example of a method proposed in this embodiment. That is, the entire blank is uniformly heated in the furnace below the A1 temperature, and only a part of the outside of the furnace is heated secondly above the A3 temperature, and then maintained for as long as the austenite transformation is completed, followed by molding and cooling in the mold. Conduct. As a result, the portion heated above the A3 temperature has martensite structure and has a high strength of about 1500 MPa, and the portion heated to the temperature below A1 has the structure of ferrite, pearlite, bainite, and so on. Molded articles can be produced.

Moreover, the following method is mentioned as a method of multiplexing a heating temperature separately from the said method. That is, the method of hot press molding after heating to a part after local heating at the same temperature is also mentioned.

In this embodiment, first, the hot forming blank is prepared, and the first heat treatment is performed at a first temperature for heating the entire hot forming blank. The first temperature is preferably heated above the austenite transformation point (Ac3) of the blank is a steel material.

Heating above the austenite transformation point is to obtain a high strength molded article by martensite strengthening through subsequent hot forming. Thereafter, as described later, the locally cooled portion can obtain low strength. In more detail, the hot-formed portion of the blank heated above the austenite transformation point is maintained in a high temperature state and reinforced with the austenite-martensite transformation, and the locally cooled portion is a composite structure including ferrite or pearlite. Is formed, it is possible to form relatively low strength or desired strength.

The primary heating method is not particularly limited, and various methods such as heating by an induction furnace and induction heating may be applied. For example, the heating by a heating furnace loads and heats a blank in the heating furnace heated to more than an austenite transformation point, Preferably it is 900 degreeC or more. At this time, in consideration of economical efficiency, it is preferable to heat within 180 seconds, and the average heating rate at this time is preferably 5 ° C / s or more.

It is more preferable to maintain the heated blank for a certain time. The reason is to ensure uniformity of the temperature in the blank, to promote the diffusion of carbon (C) in the blank material, and to secure the increase in strength and uniformity when forming and cooling after some cooling. The holding time varies depending on the steel grade of the blank material and the size of the blank. A preferable example of the holding time is 30 seconds or more, and when the holding time exceeds 10 minutes, it is not preferable in terms of equipment and energy efficiency of the process, and therefore does not exceed 10 minutes.

Local cooling is performed to cool one or two or more portions of the primary heated blank to a second temperature (preferably one point below the austenite transformation temperature). The cooled part is sufficiently transformed, and then cooled after hot forming to have a softer structure (eg, ferrite, pearlite, etc.) than martensite, and thus have a relatively low tensile strength. Ductility has the property of improving.

It does not specifically limit the said method of local cooling, It is preferable to perform the said cooling rate at the cooling rate of 40-100 degreeC / s. When the cooling rate is cooled at a cooling rate of less than 40 ℃ / s, because the temperature transition region is too wide between the portion to be cooled and the portion where the high temperature is maintained, there is a fear that unnecessary areas may occur, 100 ℃ / s If the cooling is over, since martensite transformation may occur in the portion to be cooled, the temperature range is preferably 40 to 100 ° C / s.

The local cooling is preferably cooled to a temperature range of less than the austenite transformation point (Ac3), the martensite transformation start temperature (Ms). The reason for cooling below Ac3 is that if the temperature above Ac3 is cooled, martensite transformation occurs during the subsequent hot forming, so that a product having a different strength distribution cannot be formed, and if cooled below Ms, martens before hot forming By forming the site structure, the ductility may be reduced, which may adversely affect the formability, and different strength distributions may not be formed in the final product.

On the other hand, it is preferable to hold the locally cooled blank for 25 seconds or longer. The blank at this time contains both the part which has Ac3 or more, and the part which has a temperature below Ac3 ~ Ms or more. The reason for maintaining the above 25 seconds is not preferable because it is sufficient for more than 25 seconds to cause a sufficient transformation in the cooled portion, and if it does not exceed this, subsequent hot forming is not performed to obtain a low strength structure.

The heat treatment for the local cooling may use a separate device. The apparatus preferably includes a portion of the first heat-treated blank that maintains the first heat treatment temperature, that is, cooling means capable of cooling the temperature of the portion to be cooled and the temperature of the portion to be cooled.

The whole of the locally cooled blank is hot formed and cooled. It is possible to secure the shape of the desired part through hot forming, and the part at which the temperature of Ac3 or more is maintained in the blank has a martensite structure through cooling after hot forming to secure high strength, and the locally cooled part is ferrite. The structure is relatively low in strength due to the structure of pearlite and bainite, but high ductility can be ensured.

In this case, it is preferable that the portion of the blank having a low temperature (ie, the portion not heated secondly or the portion which has been locally cooled) is press-molded at a temperature higher than the ferrite / austenite transformation temperature (Ac1). That is, since the transformation is no longer occurring as the ferrite region, the variation in strength in the part is excessive because almost no increase in strength is expected even when quenching during press molding. Therefore, it is preferable to perform hot press molding at a temperature of Ac1 or more (preferably, ferrite / austenite or higher region temperature) which is a temperature at which a partial increase in strength can be expected. If the part with low temperature becomes Ac3 or more, it is not preferable because the strength difference from the high temperature region cannot be obtained.

5 is a conceptual diagram showing a graph of temperature and time as an example of the method proposed in the present embodiment. That is, the whole blank is heated to Ac3 or more, maintained for a certain time, and then part B is locally cooled to a temperature of Ms or more of less than Ac3, and the heat-treated blank is molded and cooled. An example of a part manufactured accordingly is shown in FIG. 2, where part A maintained at a temperature of Ac3 or higher has a high tensile strength TS, and some cooled parts B have a somewhat low tensile strength but high It has an elongation (EL).

In addition, the method of multiplexing the cooling rate and the method of multiplexing the heating temperature need not be mutually exclusive. That is, after multiplexing the heating temperature for each region it may be used to multiplex the cooling rate.

(Heat treatment after press molding)

The press molding here is a concept including both cold press molding and warm (meaning a method of pressing the entire blank in a ferrite region). Each press molding method is not particularly limited in the present invention because it is well known in the art.

Similarly to the method of manufacturing the hot press-molded product described above, the heat treatment method may use various methods such as multiplexing the cooling rate, multiplexing the heating temperature, multiplexing the cooling rate and the heating temperature, and the like.

Multiplexing of Cooling Speed

As described above, if the cooling rate of the material is sufficiently different, even after heating to the same temperature, the difference in physical properties after cooling can be obtained by the difference in cooling rate. In order to differentiate the cooling rate for each region, a method of setting the mold temperature differently for each region may be used.

To this end, it is necessary to first take a blank from the steel sheet, press-molded the collected blank to obtain a molded article, and then heat it to a high temperature. The heating temperature is preferably heated to the austenite region, more specifically in the range of 780 ~ 950 ℃. After heating, the temperature of the molded product is maintained for a certain time. Maintaining the heated molded article for a certain time in this embodiment is to ensure uniformity of the temperature in the molded article, to promote the diffusion of carbon (C) in the molded article to secure the increase in strength and uniformity. The holding time may be set differently according to the steel grade of the molded article material, the size of the molded article, and the like, and can be easily set by anyone skilled in the art to which the present embodiment belongs. One preferred example of the holding time is 180 seconds or more. However, if the holding time exceeds 10 minutes, it is not preferable in terms of equipment and energy efficiency of the process.

The heated molded article then undergoes a quenching process. In this case, the press-molded product having different strengths for each region may be obtained by changing the flow rate or type of the cooling medium for each region. Each cooling rate is easily determined by referring to the critical cooling rate that causes martensite transformation or bainite transformation for each steel material.

Thus, another aspect of the method for producing a press-formed product of the present invention comprises the steps of: extracting a blank from the steel sheet of the present invention under the advantageous conditions described above; Press molding the blank to obtain a molded article; And heating the molded article. Cooling the heated molded article at different cooling rates for each region.

Multiplexing of heating temperature

However, as described above, in order to make a difference in cooling speed, different refrigerants need to be added or flow rates are different for each region, and it is not easy to give different cooling patterns to the same component.

Therefore, in the present embodiment, in the method of manufacturing a molded article by post-heat treatment of the molded article after press molding, the whole molded article after press molding is subjected to a first temperature (for example, A1 temperature at which austenite transformation does not occur (ferrite region and austenite / Heating to a certain temperature) below the boundary temperature of the region where the ferrite two phases coexist), and then further heating a portion to a second temperature higher than the first temperature (any temperature above the A3 temperature at which austenite transformation occurs). If the molded product having the temperature distribution is secured through the second heating before cooling from the two-stage heating method, then the molded product having the different strength distribution is finally obtained in the same part even through the same cooling pattern.

That is, the present embodiment includes the step of first obtaining a molded article by press molding and then heating it to a first temperature in a heating furnace. At this time, it is preferable to set the range of heating from Ms (martensite transformation start temperature) to less than A1 temperature.

After the heating, the temperature of the molded article is maintained for a predetermined time. Maintaining the heated molded article for a predetermined time in this embodiment ensures uniformity of temperature in the molded article, promotes diffusion of carbon (C) in the molded article material, and increases strength when rapidly cooling after a short secondary heating time. This is to ensure uniformity. The holding time is different depending on the steel grade of the molded article material and the size of the molded article. One preferred example of the holding time is 180 seconds or more. However, if the holding time exceeds 10 minutes, it is not preferable in terms of equipment and energy efficiency of the process.

The method further comprises heating one or two or more portions of the molded article held for a predetermined time to a second temperature higher than the first temperature (for example, the temperature of the austenite region). In a preferred embodiment this additionally heated portion is subjected to sufficient austenite transformation, which is then cooled to have martensitic structure resulting in high tensile strength. The heating at this time is preferably at least A3 temperature (austenite region). The reason for heating to a temperature of A3 or more is that heating to a temperature between A1 and A3 does not cause sufficient austenite transformation, and therefore, even if quenching does not sufficiently secure martensite structure, high strength cannot be obtained.

Preferably it is heated to the temperature of A3 or more and maintained for a certain time to achieve a sufficient austenite transformation. At this time, the holding time is preferably 5 seconds or more. At this time, one minute is sufficient for sufficient austenite transformation to occur in the additionally heated portion, and it is not preferable because the austenite transformation may occur even in the unheated portion.

A step of cooling the entire molded article in which the portion is further heated is performed. According to this method, since the temperature of the initial molded part is different, even if it has similar rapid cooling rate, the part heated above A3 has martensite structure after cooling, and the part heated below A1 is cooled after ferrite, pearlite, and bay. You will have a knight's organization and you'll get parts with different final properties.

3 is a conceptual diagram illustrating an example of a method proposed in this embodiment. In other words, the entire molded product is uniformly heated in the furnace below the A1 temperature, and only a part of the outside of the furnace is heated secondly above the A3 temperature, and then maintained for as long as the austenite transformation is completed. Conduct. As a result, the part heated above A3 temperature has martensitic structure and has high strength of about 1500 MPa, and the part heated to temperature below A1 has the structure of ferrite, pearlite and bainite, and has low strength. Can be prepared.

Moreover, the following method is mentioned as a method of multiplexing a heating temperature separately from the said method. That is, after heating to the same temperature, the method of cooling locally after only partially cooling is also mentioned.

In this embodiment, first, a molded article is prepared, and the first heat treatment is performed at a first temperature for heating the entire molded article. The first temperature is preferably heated above the austenite transformation point (Ac3) of the molded article is a steel material.

Heating above the austenite transformation point is to obtain a high strength molded article by martensite strengthening through a subsequent cooling process. Thereafter, as described later, the locally cooled portion can obtain low strength. In more detail, the portion heated above the austenite transformation point is strengthened by the austenite-martensite transformation, and the locally cooled portion forms a complex structure including ferrite or pearlite, so that a relatively low or desired strength is achieved. Formation is possible.

The primary heating method is not particularly limited, and various methods such as heating by an induction furnace and induction heating may be applied. For example, the heating by a heating furnace charges and heats a molded article in the heating furnace heated to the austenite transformation point or more, Preferably it is 900 degreeC or more. At this time, in consideration of economical efficiency, it is preferable to heat within 180 seconds, and the average heating rate at this time is preferably 5 ° C / s or more.

It is more preferable to keep the heated molded article for a certain time. The reason for this is to ensure uniformity of temperature in the molded article, to promote the diffusion of carbon (C) in the molded article material, and to secure an increase in strength and uniformity when forming and cooling after partial cooling. The holding time is different depending on the steel grade of the molded article material and the size of the molded article. A preferable example of the holding time is 30 seconds or more, and when the holding time exceeds 10 minutes, it is not preferable in terms of equipment and energy efficiency of the process, and therefore does not exceed 10 minutes.

Local cooling is performed to cool one or two or more portions of the primary heated molded article to a second temperature (preferably one point below the austenite transformation temperature). The cooled part is sufficiently transformed, and then cooled to have a softer structure than martensite (eg, ferrite, pearlite, etc.), and thus have a relatively low tensile strength. Ductility has the property of improving.

It does not specifically limit the said method of local cooling, It is preferable to perform the said cooling rate at the cooling rate of 40-100 degreeC / s. When the cooling rate is cooled at a cooling rate of less than 40 ℃ / s, because the temperature transition region is too wide between the portion to be cooled and the portion where the high temperature is maintained, there is a fear that unnecessary areas may occur, 100 ℃ / s If the cooling is over, since martensite transformation may occur in the portion to be cooled, the temperature range is preferably 40 to 100 ° C / s.

The local cooling is preferably cooled to a temperature range of less than the austenite transformation point (Ac3), the martensite transformation start temperature (Ms). The reason for cooling below Ac3 is that if the temperature above Ac3 is cooled, all martensite transformation occurs even in the locally cooled region in the subsequent cooling process, so that a product having a different intensity distribution cannot be formed, and cooled below Ms. Once again, martensitic structures are already formed prior to cooling, making it impossible to form different intensity distributions in the final product. The local cooling temperature of the molded article is more preferably at least Ac1.

On the other hand, it is preferable to hold the locally cooled molded article for 25 seconds or longer. The molded article at this time contains both the part which has Ac3 or more, and the part which has a temperature below Ac3 ~ Ms or more. The reason for maintaining the above 25 seconds is that it is difficult to obtain a high-strength tissue by cooling in a state in which the transformation is not completed if it is sufficient for 25 seconds or more in order to cause sufficient transformation in the cooled portion.

The local cooling may use a separate device. It is preferred that the apparatus comprises a means for retaining a portion of the primary heated molded article, i.e. cooling means capable of cooling the portion that needs to be cooled.

Thereafter, the entire locally cooled molded article is cooled. The part where the temperature of Ac3 or more is maintained in the molded article may have high martensite structure through rapid cooling, and the locally cooled part may have a structure of ferrite, pearlite, bainite, etc. High ductility can be secured. Each cooling rate is easily determined by referring to the critical cooling rate that causes martensite transformation or bainite transformation for each steel material.

In addition, in the method of heat treatment after press molding, the method of multiplexing the cooling rate and the method of multiplexing the heating temperature need not be mutually exclusive. That is, after multiplexing the heating temperature for each region it may be used to multiplex the cooling rate.

The press-molded product produced by the advantageous method of the present invention as described above has the following characteristics.

That is, the press-molded article according to the present invention is a press-formed steel sheet having a different strength for each region; A zinc plated layer including a Fe—Zn phase in which an absolute value of a Gibbs free energy reduction amount per mole of oxygen is dissolved in an amount of 0.008% by weight or more during the oxidation reaction formed on at least a portion of the base steel sheet in a solid solution; And an oxide layer having an average thickness of 0.01 to 5 μm formed on the zinc plated layer. The term 'at least a portion of the base steel sheet' of the present invention means a region in which the base steel sheet is heated to an austenite temperature range. This is because the temperature range is a high temperature, in particular, a region in which problems such as volatilization loss of zinc and liquid metal embrittlement can occur a lot, and in the case of a plated steel sheet having a layer structure of the present invention, a particularly advantageous effect can be obtained in the above temperature range.

In the hot dip forming or press forming and heat treatment, 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 per mole of oxygen less than Cr is dissolved in an Fe-Zn phase of at least 0.008% by weight. That is, the metal layer having an absolute value of the Gibbs free energy reduction amount per mole of oxygen during the oxidation reaction in the plating layer before the hot press or heat treatment is contained in an amount of 0.01% by weight or more, and the cast per mole of oxygen during the oxidation reaction by hot press heating. If the absolute value of the free energy reduction amount is less than Cr, the metal is dissolved in the Fe-Zn phase. If the absolute value of the Gibbs free energy reduction amount per mole of oxygen is less than 0.008 wt. It is possible to prevent the diffusion of the plated layer from being prevented, and to suppress the diffusion of Zn of the zinc plated layer into the base 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.

At this time, the oxide layer preferably comprises a continuous film having an average thickness of 10 ~ 300nm consisting of one or more oxides selected from the group consisting of SiO ₂ and Al ₂ O ₃ . In particular, Al ₂ O ₃ Oxides are mainly formed, Al ₂ O ₃ The oxide may be formed alone, and some SiO ₂ Oxides may be included. Since the oxide layer is dense and chemically very stable, it serves to protect the surface of the plating layer at a high temperature even in the form of a very thin film. In particular, in order to effectively protect the plating layer by preventing volatilization of Zn, it is preferable that the oxide film is formed in a continuous form. This can cause problems that cannot be properly protected.

In addition, the present inventors have found that not only the heat resistance of the plating layer but also the coating property and the coating film adhesion during the electrodeposition coating process when the continuous film is formed on the oxide layer as described above. Conventionally, due to the phenomenon of poor paintability or peeling of the formed coating film during electrodeposition coating treatment, it was forced to undergo phosphate treatment. However, when the oxide layer including the continuous coating film is formed on the plating layer as in the present invention, it is possible to secure electrodeposition paintability and coating film adhesion even without performing a separate phosphate treatment, which can bring great improvement in terms of economic efficiency and manufacturing efficiency. .

In addition, at least one oxide selected from the group consisting of SiO ₂ and Al ₂ O ₃ is preferably not only continuous but also having a thickness of 10 to 300 nm. If less than 10 nm, the thickness is too thin to form the continuous film. Not only is it difficult to perform, but also has a problem that it is difficult to fully perform the role of preventing the volatilization of Zn, if the thickness exceeds 300nm because the amount is too large, such as the problem of deterioration of weldability, the thickness is 10 ~ 300nm It is desirable to limit.

In addition, the oxide layer includes ZnO, more preferably 0.01 to 50% by weight of at least one oxide selected from the group consisting of MnO, SiO ₂ and Al ₂ O ₃ . Since oxides made of ZnO grow at a high temperature and grow rapidly at high temperatures, the plating layer cannot be protected, so that oxides made of MnO, SiO ₂ , and Al ₂ O ₃ other than ZnO are contained in an amount of 0.01 wt% or more, thereby suppressing the growth of the oxide layer. While being able to function as a protective oxide film that can protect the plating layer. However, since the weldability may be inhibited when the content exceeds 50% by weight, the upper limit is preferably limited to 50% by weight.

At this time, an oxide containing ZnO and MnO is formed on the continuous film, and the content of MnO is more preferably smaller than ZnO. Since MnO oxide was formed on the surface of the plating layer after Mn component was diffused from the base steel plate to the plating layer, more MnO oxides were formed than ZnO oxide, which means that the diffusion was excessive and the surface oxide was formed rapidly. In addition, since ZnO has excellent electrical conductivity, which is advantageous for electrodeposition coating and phosphate treatment, the content of MnO is preferably smaller than ZnO.

In addition, the oxide layer is preferably FeO of 10% by weight or less. When the ratio of FeO to the oxide layer exceeds 10%, this means that a large amount of Fe diffused from the steel sheet to the plated layer to form an oxide, thereby forming a uniform plating layer having a Zn content of 30% or more. There is a fear that the continuity of the protective oxide film formed of Al ₂ O ₃ or SiO ₂ formed on the surface may be broken by diffusion of Fe. Therefore, the ratio of FeO in the oxide formed on the surface of the press-formed product obtained in the present invention is suitably less than 10%. There is no restriction on the lower limit because the smaller the amount of FeO, the better.

On the other hand, it is preferable that the zinc diffusion phase is discontinuously present on the upper portion of the steel sheet. In general, when the hot-dip galvanized steel sheet is charged into a heating furnace for hot pressing or heat treatment after pressing, 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. Due to excessive alloying, the Zn content in the plating layer is not sufficient, which means that the heat resistance is not good. Accordingly, the zinc plated layer may not properly exhibit the corrosion resistance effect. Therefore, the zinc diffusion phase is discontinuously formed in order to secure heat resistance and corrosion resistance. desirable.

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.

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

In the present invention, the content of Zn contained in the zinc diffusion phase is less than 20 % by weight. Since the portion of the Zn content of 20 % 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 is less than 20 % by weight of the zinc diffusion phase, and thus the zinc plated layer and the base steel sheet The distinction becomes unclear.

According to the above it is possible to ensure the zinc plating layer stably by securing the Zn content of the hot-dip galvanized layer 30% by weight or more after hot press molding or press and heat treatment of the present invention. 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 or press and heat treatment as described above, the zinc plated layer can be stably maintained and the Zn content of the plated layer can satisfy 25% or more. have. If the Zn content of the plating layer is less than 25%, it is impossible to form a uniform plating layer, and the sacrificial anode properties of the plating layer are deteriorated, and thus corrosion resistance is easily deteriorated.

At this time, after the hot press molding or press and heat treatment (in the present invention, both the hot press molding and post-heat treatment operations will be referred to simply as 'press molding') that the thickness of the hot-dip galvanized layer is 1.5 times or more than before press molding. More preferred. In general, in the hot pressing process, the Fe is more diffused from the base iron by heating, and the plating layer becomes thicker than before the press forming. In particular, the present invention has a Zn content of 25% in the plating layer from the surface of the steel sheet where the press forming is completed. When the above point is referred to as the thickness of the galvanized layer, the thickness is controlled to be 1.5 times or more than before the press molding in order to secure sufficient corrosion resistance.

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 rapidly advance the alloying at the initial stage of press heating, and to suppress the alloying on the contrary when it becomes 750 ° C. or more, it is preferable to maintain the galvanized layer, but the present invention controls both to secure heat resistance.

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

(Example)

Example 1

In the oxidation reaction, Ni, one of the metals whose Gibbs free energy reduction per mole of oxygen is smaller than Cr, is 0.20% by weight, Si: 0.5% by weight, Mn: 1.5% by weight, B: 0.003% by weight and other unavoidable impurities. It was coated on the surface of the cold-rolled steel sheet comprising a and annealing in an atmosphere of 800 ℃, dew point temperature of -12 ℃ to form a metal diffusion layer containing 3.5% by weight of the Ni within 1 ㎛ depth. The plated layer was formed by immersing the base steel sheet on which the metal diffusion layer was formed in a molten zinc plating bath containing 0.2 wt% Al. In addition, as a result of cutting the plated steel sheet and analyzing the interface with an electron microscope (EDS), it was confirmed that an Al thickening layer (about 40% by weight of Al content) was present between the plated layer and the base steel sheet with a thickness of 0.5 μm. .

After preparing a blank for forming a hot forming blank is added boron having a width of 1mm, a length of 4mm and a height of 10mm from the plated steel sheet, as shown in Figure 4 and heated to a temperature of 10 ℃ per second to 700 ℃ below A1 temperature and maintained for 180 seconds A portion of the blank was then heated second to a temperature rise rate of 50 ° C. per second to 900 ° C., above the A3 temperature, held for 5 seconds and then cooled to a cooling rate of 50 ° C. per second.

The expansion test result with respect to the temperature of the part which performed the secondary heating of the said press molding blank is shown to FIG. 6A. The expansion was measured using a dilatometer. As can be seen in Figure 6a it can be seen that the transformation of martensite by austenite and cooling is achieved only with a heating temperature of 900 ℃ and a holding time of 5 seconds.

FIG. 6B is a graph showing the results of observation of the temperature and dilation of the portion cooled at a cooling rate of 50 ° C. immediately after holding at 700 ° C. for 180 seconds in the cold rolling press blank. As shown in FIG. 6B, the part cooled after being maintained at or below A1 temperature does not have austenite transformation, and thus, it can be seen that there is no phase transformation in the heating and cooling process, so that the initial phase is almost intact.

7 are optical micrographs of portions of FIGS. 6A and 6B of the blanks after hot press molding, respectively. The additionally heated part above the A3 temperature (parts with a heating temperature of 900 ° C. and a holding time of 5 seconds) shows martensite structure after cooling and the cooled blank (180 seconds at 700 ° C. after being heated and held below the A1 temperature). After cooling, the part cooled at a cooling rate of 50 ° C. per second) shows that the austenite transformation does not occur, and thus there is no phase transformation in the heating and cooling process, so that it has almost the initial phases of ferrite and pearlite structures.

FIG. 8 shows an additionally heated portion above A3 temperature (part having a heating temperature of 900 ° C. and a holding time of 5 seconds) and a cooled blank after being kept below A1 temperature (50 seconds per second after holding at 700 ° C. for 180 seconds). Vickers hardness of the cooled portion at a cooling rate of ℃) is measured and the results are shown.

The additionally heated portion above the A3 temperature shows a martensite structure after cooling, which has a large Vickers hardness value, and when it is converted into tensile strength, it can be seen that it has 1500 MPa or more. It can be seen that the low hardness value, about 580MPa when converted into tensile strength.

In addition, the tensile test was performed after the heat treatment of the tensile specimens in a continuous annealing equipment to determine the tensile properties after heat treatment. The heat treatment conditions were similar to the dilatometer test conditions, but the temperature was raised to about 10 ° C. per second when the temperature was raised above A3, and the holding time at 900 ° C. was 5 seconds. In addition, the conditions which hold | maintain the heating temperature conditions of A1 or less for 180 second at 700 degreeC were the same. Cooling conditions were also performed at 50 ° C. per second, similar to the dilatometer test. Specimen maintained above A3 temperature showed tensile strength of about 1700MPa, and specimens maintained below A1 temperature showed tensile strength of about 580MPa. The above contents are summarized in Table 1 below.

Heat treatment condition YS (MPa) TS (MPa) U.EL% T.EL% Cooling after maintaining 700 ℃ -180 seconds and maintaining 900 ℃ -5 seconds 1203 1691 3.8 4.9 Cooling after maintaining 700 ℃ -180 seconds 441 580 15.3 23.0

Example 2

After the same heat treatment conditions of the same plated steel sheet of Example 1 was subjected to hot press molding and cooled to a cooling rate of 50 ℃ per second, the respective Vickers hardness and tensile strength was measured and the results are shown in FIG. Indicated. Tensile strength at this time represents the tensile strength converted from Vickers hardness. As shown in Figure 9 it can be seen that when maintained for 5 seconds or more at 850 ℃ tensile strength of more than 1500MPa can be obtained.

Example 3

FIG. 10 is a blank taken from the same plated steel sheet as that of Example 1, except that the cold rolled steel sheet was not cold rolled, and the blanks not maintained for 180 seconds at 700 ° C. and the retained blanks, respectively. In addition, after the heat treatment at 900 ℃ for 5 seconds or more, the hot pressing molding and the tensile strength of the cooled blank is measured, showing the standard deviation and range.

As shown in FIG. 10, the blank not further maintained at 700 ° C. even before heating to 900 ° C. for 5 seconds or more is low in tensile strength and has a high deviation.

9 and 10, the blank using the hot rolled steel sheet of FIG. 10 requires higher temperature and holding time for sufficient austenite transformation than the blank using the cold rolled steel sheet of FIG. 9. do.

Example 4

Blanks were taken from the same plated steel sheet as in Example 1. The blank is heated to a temperature of 10 ° C. per second to 900 ° C. or higher at a temperature of 3 ° C. and maintained for 60 seconds, and then a part of the blank is transformed by local cooling in a region of less than Ar 3 and 600 ° C. or higher. Was carried out.

The blanks partially heat-treated at 900 ° C. and 600 ° C. by the above method were held for 30 seconds, and transferred to a mold to perform hot press molding and cooling to manufacture parts.

Physical properties of the parts were measured, and the results are shown in FIG. 11. That is, in one part, it can be seen that the portion maintained at a temperature of 900 ℃ has a high stress against the strain, has a high strength, the portion maintained at 600 ℃ can secure excellent strain even if the strength is low It can be seen that excellent ductility can be obtained.

Therefore, through the method of the present invention, it was possible to manufacture one component having heterogeneous physical properties through heat treatment before hot press molding.

Example 5

Blanks were taken from the same plated steel sheet as in Example 1. After heating the blank at a temperature increase rate of 10 ° C. per second to 900 ° C. or higher, which is equal to or greater than Ac 3, and maintaining it for 60 seconds, various temperatures below Ar 3 for parts of the blank (700, 650, 600, 550, 500, 450 ℃) transformation was carried out through local cooling.

The blanks locally cooled at 900 ° C. and various temperatures were maintained for 30 seconds by the above method, and transferred to a mold for hot forming and cooling to manufacture molded articles. The strain-stress of each part was measured in the manufactured molded article and shown in FIG. 12. As shown in Figure 12, it can be seen that a combination of various physical properties can be obtained according to the temperature conditions of the local cooling.

For example, the portion where the local cooling is performed in the 550 ° C. temperature range can obtain a slightly higher strength than the 600 ° C. temperature range, but the strain that can be secured is slightly lowered.

In Example 4, it was confirmed that all the oxides (including 0.05 wt% of ZnO) containing Al 2 O 3 as the main component were continuously present at 100 nm thickness on all surfaces of the steel of the press-formed molded product, and the zinc content of the plating layer was sacrificed as about 32%. It was confirmed that the corrosion resistance secured by the anode may be excellent. On the surface of the Al 2 O 3 layer of the press-molded article, an oxide layer having an average thickness of 1.3 μm having a Zn oxide as a main component was formed. At this time, the zinc content of the plating layer was about 32%, and it was confirmed that the corrosion resistance by the sacrificial anode was excellent. A zinc diffusion phase having a zinc content of about 15% by weight was present at a thickness of 2 μm under the zinc plating layer. It was confirmed that the layer was not formed continuously. In addition, in the plating layer, a Zn-rich region having a Zn content of 35 to 90% was not observed, and surface cracks of the processed portion did not occur.

Claims (15)

Collecting a blank from a plated steel sheet including a steel sheet, an Al thickening layer including 30 wt% or more of Al formed on the upper steel sheet, and a zinc plating layer formed on the Al thickening layer;
Heating the blank; And
Hot pressing the heated blank;
Method of manufacturing a press-molded product to have a different thermal history for each area of the blank in at least one step of the heating step and hot press molding step.
The method of claim 1,
The method of manufacturing a press-formed product obtained by setting the temperature of the hot press die differently for each region is different for each region.
The method of claim 1,
Different thermal history for each region,
Heating the blank comprises heating the entire blank to a first temperature; And heating a part of the heated blank to a second temperature higher than the first temperature.
The method of manufacturing a press-molded product according to claim 3, wherein the remaining part of the blank, which is not a part heated to the second temperature, is hot press-molded at an Ac1 temperature or more during hot press molding.
The method of claim 1,
Different thermal history for each region,
Heating the entire blank to a first temperature; And locally cooling a portion of the heated blank to a second temperature lower than the first temperature.
The method of manufacturing a press-formed product according to claim 5, wherein a part of the heated blanks locally cooled to the second temperature during hot press molding is hot press formed at an Ac1 temperature or more.
Collecting a blank from a plated steel sheet including a base steel sheet, an Al thickening layer including 30 wt% or more of Al formed on the base steel sheet, and a zinc plating layer formed on the Al thickening layer;
Press molding the blank to obtain a molded article;
Heating the molded article; And
Cooling the heated molded article,
The method of manufacturing a press-molded article to have a different thermal history for each region of the molded article in at least one of the heating step and the cooling step.
The method of claim 7, wherein
A method of manufacturing a press-molded product obtained by setting different cooling histories for each region by setting different cooling rates for each region of the molded article.
The method of claim 7, wherein
Different thermal history for each region,
Heating the molded article may heat the entire molded article to a first temperature; And heating a part of the heated molded article to a second temperature higher than the first temperature.
The method of claim 7, wherein
Different thermal history for each region,
Heating the entire molded article to a first temperature; And locally cooling a portion of the heated molded article to a second temperature lower than the first temperature.
The said Al thickening layer is a manufacturing method of the press-molded object of any one of Claims 1-10 whose thickness is 0.1-1.0 micrometer.
The press according to any one of claims 1 to 10, wherein the base steel sheet further comprises a surface diffusion layer of a metal having an absolute value of a Gibbs free energy reduction amount per mole of oxygen less than Cr when oxidized within 1 µm from the surface thereof. Manufacturing method of the molded article.
The method for producing a press-formed product according to any one of claims 1 to 10, wherein annealing oxide is formed on the surface of the steel sheet and the thickness of the annealing oxide is 150 nm or less.
The steel sheet according to any one of claims 1 to 10, wherein the base steel sheet has a weight percent of C: 0.1 to 0.4%, Si: 2.0% or less (excluding 0%), Mn: 0.1 to 4.0%, balance Fe and A method for producing a press-formed product having a composition composed of other unavoidable impurities.
15. The method according to claim 14, wherein the steel sheet is in weight% 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: Method for producing a press-molded article further comprising at least one selected from the group consisting of 0.001 to 1.0%, Mo: 0.001 to 1.0%, Sb: 0.001 to 0.1% and W: 0.001 to 0.3%.

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