JP7006256B2 - Manufacturing method of hot-stamped hot-dip galvanized steel sheet and hot-stamped hot-dip galvanized steel sheet - Google Patents

Description

The present invention relates to a galvanized steel sheet for hot stamping, which is a high-strength member having excellent weldability after hot stamping, which manufactures a high-strength member by heat quenching, and a method for manufacturing the same.

Various automobile parts constituting the vehicle body of an automobile are required to have various performances and characteristics improved from various viewpoints such as static strength, dynamic strength, collision safety, and weight reduction. For example, in automobile parts such as A-pillar reinforcement, B-pillar reinforcement, bumper reinforcement, tunnel reinforcement, side sill reinforcement, roof reinforcement or floor cross member, only a specific part in each automobile part is this specific part. It is required to have higher strength than general parts except for. Therefore, a method of manufacturing hot stamp members by quenching high-strength steel hot stamping (also referred to as hot stamping depending on the literature) is partially adopted only for the part corresponding to a specific part of an automobile part that needs reinforcement. There is.

At this time, if a cold-rolled steel sheet that has not been surface-treated is used, iron oxide scale is generated on the surface of the steel sheet during heating. This iron oxide scale peels off during molding and wears the mold, causes defects in the steel sheet itself, and if it remains on the surface of the steel sheet after molding, it may cause welding defects in the subsequent welding process or the painting process. May cause poor adhesion of the paint. Therefore, in order to prevent this iron oxide scale, a zinc-based plated steel sheet may be used as in Patent Document 1. By using a zinc-based plated steel sheet, a small amount of zinc is oxidized before iron, so that the oxidation of iron can be suppressed and the weldability and coatability can be significantly improved.

However, although the weldability is improved compared to the iron oxide scale, the weldability is lower than that of unheated cold-rolled steel sheet or plated steel sheet without an oxide film on the surface, even if it is a zinc oxide film. descend. Therefore, even a hot stamping material using a galvanized steel sheet does not have sufficient weldability, and it is required to improve the weldability as much as possible.

Several improvement measures have been proposed, and Patent Document 2 proposes to reduce the welding resistance and improve the weldability by removing the zinc oxide film that causes an increase in welding resistance by cleaning after heating. ing. Although this method improves weldability, it requires a cleaning process by an automobile manufacturer or a parts manufacturer after heating, which increases the cost. Further, Patent Document 3 improves weldability by defining the steel sheet component, the Mn concentration on the surface of the steel sheet, the amount of adhesion to the plating, the amount of Al in the plating film, the concentration of Al in the plating film, the metal structure on the surface of the steel sheet, and the like. I'm trying. Although some improvement can be expected with this method, dramatic improvement effect is difficult.

Japanese Patent No. 4039548 Japanese Patent No. 5880321 Japanese Patent No. 5720856

As described above, in the conventional technique, there is no technique for dramatically improving the weldability of the zinc-based hot stamping steel sheet.

The present invention is an invention made in such a background, and an object of the present invention is to provide a technique for dramatically improving weldability after heating in a zinc-based hot stamping steel sheet with almost no cost increase. That is.

In order to solve the above problems, when the hot-dip galvanized outermost layer before hot stamp heating is measured by sputtering from the surface layer with a glow discharge emission analyzer (GDS), the average Al weight concentration at a depth of 0 to 5 nm of the plated surface layer is measured. A, where the average Al concentration at a depth of 5 nm to 10 nm is B, 2.5 ≧ A / B ≧ 1.5, and the Al concentration is 80% or more in terms of the atomic fraction of the metal element. A hot-dip galvanized steel sheet having excellent weldability, characterized in that the depth of the plating surface layer is 0 to 5 nm more than the range of the depth of 5 nm to 10 nm from the plating surface layer.

Further, the hot-dip galvanized steel sheet is preferably configured to be alloyed.

Further, after annealing in an annealing furnace in which an atmospheric gas having a dew point of −30 ° C. to 20 ° C. is introduced into the annealing furnace in a steel sheet temperature range of at least 200 ° C. to 600 ° C., hot dip galvanizing is performed to prepare the steel sheet. It is a method for manufacturing a hot-dip galvanized steel sheet for hot stamping, which is characterized by performing quality rolling and has excellent weldability.

Further, in the production method, it is preferable to perform the alloying treatment after performing the hot-dip galvanizing.

According to the present invention, it is possible to provide a technique for dramatically improving weldability after heating in a zinc-based hot stamping steel sheet with almost no cost increase.

It is a figure which showed the relationship between the annealing dew point in the first half of the annealing process of the steel sheet before plating, and the welding resistance after hot stamping. It is a schematic diagram of the cross-sectional structure of the plating film before hot stamp heating under the conditions of the present invention. It is a schematic diagram of the cross-sectional structure of the film after hot stamping the plating under the conditions of the present invention shown in FIG. 2. It is a schematic diagram of the cross-sectional structure of the plating film before hot stamp heating under normal conditions. It is a schematic diagram of the cross-sectional structure of the film after hot stamping the plating under the normal conditions shown in FIG. 4. It is a figure which shows the relationship between A / B and welding resistance after hot stamping. It is a figure which shows the relationship between A / B and an annealing dew point.

The present inventors have investigated in detail the relationship between the state of the steel sheet before annealing, the annealing conditions before plating, the plating conditions, and the weldability (welding resistance) after hot stamping, and the results are shown in FIG. As described above, it was found that the annealing atmosphere in the first half of the annealing process of the steel sheet before plating, particularly the dew point is set as a slight oxidation condition, the welding resistance after hot stamping is remarkably reduced and the weldability is greatly improved.

Although the detailed mechanism for improving weldability by annealing with the dew point as a slightly oxidized atmosphere is unknown, as will be described later, if the annealing atmosphere before plating is controlled, Fe on the surface and inside of the steel sheet can be used. It is considered that the concentrated state of oxides and metal elements such as Si and Mn changes, and the state of the Fe—Al concentrated layer formed at the initial stage of plating changes due to the change. Under the influence of this, Al that is oxidized during subsequent hot stamping becomes a composite oxide with Si and Mn under normal conditions, but under the slight oxidation conditions of the present invention, the Al concentration is the atomic fraction of the metal element. It is presumed that it becomes a single Al oxide called γ-Al ₂ O ₃ which becomes 80% or more and contributes to the improvement of weldability.

The details are as follows. Oxides containing Al include γ-Al ₂ O3 having an Al concentration of 80% or more in terms of the atomic fraction of the metal element _, and composite oxides containing Zn, Mn, Fe, etc. in addition to Al. be. In the present invention, the oxide having an Al concentration of 80% or more in terms of the atomic fraction of the metal element is referred to as a single Al oxide (γ-Al ₂ O ₃ , γ-alumina: the crystal structure is also referred to as Al ₂ O ₃ ). , Not only Al, but also a composite Al oxide containing a large amount of Zn, Mn, etc. and an Al concentration of less than 80% in terms of the atomic fraction of the metal element (the crystal structure is, for example, ZnAl ₂ O ₄ or Mn Al ₂ O ₄ , (Zn). , Mn) Al ₂ O ₄ etc.), the generic term for single Al oxides and composite Al oxides is referred to as Al-based oxides. In addition, since the analysis accuracy is affected by the surrounding elements, if Al is an atomic fraction and the Al concentration is 80% or more among the metal components excluding oxygen, it is a single Al oxide, and if it is less than 80%, it is a composite Al oxidation. It can be regarded as a thing.

FIG. 2 is a schematic diagram of the cross-sectional structure of the plating film before hot stamping under the conditions of the present invention, and FIG. 3 is a schematic diagram of the cross-sectional structure of the plating film after hot stamping the plating under the conditions of the present invention. A schematic diagram of the cross-sectional structure of the plating film before hot stamping under normal conditions is shown, and FIG. 5 shows a schematic diagram of the cross-sectional structure of the plating film after hot stamping the plating under normal conditions. The numbers in the figure are 1: base steel sheet, 2: Zn-based plating film before heating, 3: thick and strong single Al oxide film on the plating surface according to the present invention, 4: thin and sparse plating surface under normal conditions. Composite Al oxide film, 5: Fe—Zn solid-soluble phase after hot stamp heating, 6: Thick and strong single Al oxide film after hot stamp heating under the conditions of the present invention, 7: After hot stamp heating under the present invention conditions 8: Thin Zn, composite Al oxide film containing Zn, Mn, etc. after hot stamp heating under normal conditions, 9: Thick Zn oxide film after hot stamp heating under normal conditions.

As shown in the schematic diagram in FIGS. 2 to 5, the Zn-based oxides (7 in FIG. 3 and 9 in FIG. 5) formed on the outermost surface after hot stamping and serving as welding resistance are hot from the inside of the Zn plating layer. Both of these single Al oxide films (Fig. 2-3) formed at the time of stamping or composite Al oxide films composed of Al, Zn, Mn, etc. (Fig. 4-4) are plated through Al-based oxides. Since Zn (2 in the figure) is formed by diffusing toward the plating surface, the amount of Zn oxide formed differs depending on the difference in the formation state of this Al-based oxide film, and the amount of this Zn oxide is small. It is considered that the weldability is improved. That is, since the single Al oxide film does not contain other elements, other metal elements such as Zn diffuse and permeate the single Al oxide film to form Zn oxide in a very dense and high temperature oxidizing atmosphere. In contrast, the composite Al oxide containing Zn, Mn, etc. is very effective as a barrier layer that suppresses zinc, but since it contains Zn, etc., it is easy for Zn, etc. to permeate and oxidation of Zn. It is considered that it does not function sufficiently as a barrier layer that suppresses zinc.

When the dew point during annealing was actually controlled within the range of the present invention, in hot-dip galvanizing, a phenomenon was observed in which Al contained in a small amount in the film was extremely concentrated on the surface layer of the plating surface layer before hot stamp heating. .. Measured while sputtering from the surface layer with a glow discharge emission analyzer (GDS), and when the average Al weight concentration at a depth of 0 to 5 nm on the plated surface layer is A and the average Al concentration at 5 nm to 10 nm is B, it is referred to as A / B. The results shown in FIG. 6 were obtained with the welding resistance after hot stamping. In addition, the results shown in FIG. 7 were obtained between the A / B and the annealing dew point. That is, the A / B changes as shown in FIG. 7 depending on the dew point at the time of annealing before plating. As shown in FIG. 6, when the A / B is 1.5 or more and 2.5 or less, the welding resistance after hot stamping is 50 mΩ or less, which is good. This is because when A / B is 1.5 or less, a sufficient amount of strong single Al oxide film is not formed on the surface layer, whereas when A / B is 1.5 or more, sufficiently strong single Al oxide film is formed on the plated surface. It is considered that the oxide film is formed and the oxidation of Zn can be suppressed during the subsequent hot stamp heating to improve the weldability.

In order to obtain such an A / B range, for example, adjustment of atmospheric conditions in annealing is effective. As shown in FIG. 7, the annealing dew point is −30 ° C. or higher to set the A / B to 1.5 or higher, and the annealing dew point is set to 20 ° C. or lower to set the A / B to 2.5 or lower. Under the condition that the dew point is lower than -30 ° C, which is the normal annealing condition, no remarkable surface thickening of Al is observed on the plated surface, and A / B <1.5, but the dew point which is the condition of the present invention- It was found that when the temperature was between 30 ° C and 20 ° C, Al was concentrated on the plating surface and A / B ≧ 1.5. If the dew point is + 20 ° C. or higher, problems such as non-plating may occur during plating, which is not preferable.

Although the GDS measurement method is not limited, in the embodiment, the analysis current is 900 V, 20 mA, the measurement diameter is 4 mmφ, the analysis time is 2000 s, the data rate is 5%, and the analysis element is Fe. C, O, Al, Si, P, Cr, Mn and Zn were analyzed, and the weight% concentration of Al was measured.

It is considered that Al-based oxides are formed even in the alloyed hot-dip galvanized steel sheet before hot stamping, but since it is very thin, detailed composition and crystals can be analyzed by TEM (transmission electron microscope). The structure could not be identified. However, as described above, by defining the annealing atmosphere of the present invention, it is possible to increase the surface concentration of Al already before the hot stamping, and the Al-based oxide is plated during the subsequent hot stamping. It is considered that the oxidation of Zn can be suppressed by growing into a thick and strong single Al oxide while absorbing Al in the film. The mechanism by which the surface concentration of Al is promoted or becomes an oxide of Al alone is presumed as follows. That is, under the slight oxidation condition where the dew point is in a specific range, Si and Mn, which are easily oxidizing elements, are consumed as oxides on the surface layer of the steel plate. It is presumed that it becomes difficult to become and becomes a single Al oxide.

On the other hand, at the dew point at the time of normal annealing such as less than -30 ° C, as described above, ZnAl ₂ O ₄ containing Zn, Mn and the like, and composite Al oxide such as MnAl ₂ O ₄ are formed, and Zn becomes It is presumed that oxidation cannot be sufficiently suppressed.

Further, as shown in FIGS. 3 and 5 above, the structure of the film after hot-stamping the alloyed hot-dip galvanized steel sheet is usually generally a Zn oxide film (7 or 9) and ferrite from the surface side. The Fe—Zn solid-dissolved phase (5) in which Zn is solid-dissolved in Fe and the steel plate base material (1) are formed in this order. As a result of investigating this film structure in more detail using component analysis by TEM and crystal structure analysis by diffraction image, the present inventors have found a Zn oxide film (7 or 9) and a Fe—Zn solid-soluble phase (5). ) Is very thin, but an Al-based oxide film (6 or 8) of a single Al oxide or a composite Al oxide is formed, and an Al-based oxide composed of the single Al oxide or the composite Al oxide is formed. It was found that the crystal structure of the film changes depending on the manufacturing conditions, and that the structure greatly affects the weldability of the hot stamping material.

That is, in the case of a hot stamp molded product manufactured under normal hot-dip galvanizing conditions, as shown in FIG. 8, a large amount of composite oxide containing not only Al but also Mn, Zn, etc. is formed. On the other hand, in the hot stamp molded product according to the present invention, as shown in FIG. 6, a thick and strong single Al oxide film such as γ-Al ₂ O ₃ containing no Mn or Zn is formed. It turned out to be. Therefore, it is considered that this single Al oxide film suppresses Zn oxidation in hot stamping.

Regarding the reason why the Al-based oxide does not form a composite oxide when the steel plate is annealed under the dew point condition during annealing of the present invention, in the dew point range of the present invention, Mn and Si in the steel, which are easily oxidizing elements, are first. By being oxidized, Mn, Si, etc. do not dissolve in the Fe—Al alloy layer formed at the interface between the plating and the steel plate at the initial stage of plating, and a single Al oxide containing no Mn, Si, etc. is formed during subsequent oxidation. Presumed to be formed.

Further, the reason why the single Al oxide can suppress the oxidation of the surface layer Zn as compared with the composite Al oxide containing Mn and Si is that the composite Al oxide of Mn and Zn has a coarse structure, and Zn and the like are also included. Since it contains a large amount of Zn, it diffuses quickly inside, and Zn easily diffuses from the lower layer to the surface layer to promote oxidation. On the other hand, in the case of a single Al oxide, the crystal structure is dense and it does not contain Zn. It is presumed that Zn is difficult to diffuse inside, and it is a barrier when Zn comes out from the lower layer to the surface layer and is oxidized.

Regarding the dew point range of the present invention, under the condition that the dew point is lower than −30 ° C., which is a general annealing condition, only a small amount of Mn and Si near the surface of the steel sheet is externally oxidized on the surface of the steel sheet. It is considered that a large amount of Mn and Si remain in the steel and are contained in the Fe—Al layer at the time of plating. On the other hand, under the dew point condition in which the internal oxidation state is obtained according to the present invention, a sufficiently large amount of oxygen is supplied and penetrates into the inside of the steel to oxide Mn and Si to a considerable depth in the steel, and the steel is no longer in a metallic state. It is considered that the Fe-Al layer at that time does not dissolve in solid solution.

Due to such a film structure, especially when the heating before hot stamping is furnace heating with an electric heater or combustion gas, that is, even under conditions where heating takes time and Zn is easily diffused, the above-mentioned barrier is used. From the effect, the diffusion of Zn is suppressed, and as a result, it is possible to contribute to avoiding deterioration of weldability. The outline of the film structure of the present invention that avoids deterioration of weldability even in the case of heating in the furnace is as shown in FIGS. 2 and 3 described above.

Under normal conditions, the composite Al oxide film 4 on the surface under normal conditions is thin and sparse in FIG. 4, so that the function as a diffusion barrier is weak during the subsequent hot stamp heating in FIG. 5, and the diffusion of Zn from the lower layer increases. , A thick Zn oxide film 9 is formed on the thick Zn oxide film 9. On the other hand, in the present invention, since the single Al oxide film 3 on the surface is thick and strong before the heating in FIG. 2, it functions as a diffusion barrier after the hot stamp heating in FIG. 3 and suppresses the diffusion of Zn from the lower layer to the upper layer. It is considered that the amount of Zn oxide film 7 formed on the surface layer can be suppressed and the weldability can be improved.

The TEM observation method is not particularly specified, but in the embodiment, the JEOL-200CX or JEM-2100 manufactured by JEOL is used for observation at an acceleration voltage of 200 kV, and the phase or crystal is observed by structure observation, elemental analysis, electron diffraction, or the like. The structure was identified.

As for the atmosphere, most of them are N ₂ gas except H ₂ , and other than that, they are unavoidable impurities. If the H2 concentration is ₁ % or less, the reduction of the steel sheet becomes insufficient, which causes non-plating and the like. On the other hand, if it is 15% or _more , the consumption of H2 during annealing increases, which causes a problem in terms of cost. Therefore, the H ₂ concentration is preferably 1 to 15%, more preferably 2 to 12%. As the dew point at the time of annealing, if it is less than −30 ° C., Si and Mn in the steel cannot be sufficiently oxidized and a depletion layer of Si and Mn cannot be formed. Further, if the temperature exceeds + 20 ° C., oxidation of Fe in the steel sheet occurs, which causes non-plating and the like, which is not preferable. Therefore, the annealing dew point is preferably −30 ° C. to 20 ° C., more preferably −20 ° C. to 10 ° C., and even more preferably −15 to 0 ° C.

The dew point is adjusted by humidifying N2 or a gas containing 1 to 15% H2 in N2 with steam, and introducing the gas into the annealing furnace to control the dew point. For its introduction, it is necessary to introduce in the section where the steel plate temperature is at least between 200 ° C and 600 ° C. The reason why the steel sheet temperature is set as described above is that when the temperature of the steel sheet is lower than 200 ° C., the temperature is too low and Si and Mn hardly diffuse in the steel, so that the Si and Mn-deficient layer is formed on the surface of the steel sheet. However, diffusion occurs sufficiently at 600 ° C. or higher, but the annealing time including the subsequent holding time is shortened, and sufficient time is obtained to form a Si or Mn-deficient layer. Because there is no such thing. As described above, the steel sheet temperature in the range of 200 ° C. to 600 ° C. is most desirable from the viewpoint of the diffusion rate of Si and Mn and the annealing time, but more preferably in the range of 300 ° C. to 500 ° C.

The conditions for hot stamping are not particularly specified, but after the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet is blanked to a required size, radiant heating, induction heating, energization heating, etc. are used at 700 ° C to 1200 ° C. After heating to the above range, it is preferable to press-mold and quench-cool.

The type of zinc-based plated steel sheet that can be used in the present invention is not particularly limited as long as it is a plated steel sheet containing zinc as a main component, but is a zinc-plated steel sheet (abbreviated as GI), an alloyed zinc-plated steel sheet (abbreviated as GA), or molten. It includes zinc-aluminum alloy plating, hot-dip zinc-aluminum-silicon alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, and the detailed element composition of each is not particularly limited. The basis weight of the plating is not particularly limited, but it can be used up to about 30 g / m2 to 120 g / m2, which is the amount of adhesion of general hot stamping plating. The material of the steel sheet is not particularly limited, but hardened high-strength steel used for hot stamping is generally used.

The thickness is 1.0 mm, and the steel composition is C: 0.21% (mass%, the same applies hereinafter), Si: 0.033%, Mn: 1.2%, P: 0.01%, S: 0.007. %, Cr: 0.2%, Ti: 0.02%, B: 0.003%, hot-dip galvanized steel sheet for hot stamping and alloyed hot-dip galvanized steel sheet consisting of balance Fe and impurities are annealed and plated. did. At that time, after various changes in the atmosphere at the time of annealing, plating was performed. The basis weight of the plating was 50/50 g / m ² on the front / back. In some steel sheets after plating, non-plating was observed when the annealing conditions were, for example, a dew point of more than + 20 ° C. Non-plating is a place where the wettability of the plating deteriorates when the surface of the base metal before plating progresses too much, and the plating is repelled and the plating is not attached.

The plated steel sheet is cut into a size of 100 mm square, then heated to 900 ° C in an electric furnace with an atmospheric atmosphere, heated in it for 4 minutes, then taken out, and immediately sandwiched between flat plate presses with built-in water cooling pipes and rapidly cooled. A high-strength material for hot stamping was obtained. Welding resistance was measured as an index of weldability. The welding resistance measurement conditions were: electrode: CF type φ6R40, load: 250 kgf, current value: 2 A, and the resistance value at that time was measured. The smaller the welding resistance, the better, but if it is 50 mΩ or less, dust generation during welding is almost eliminated and the welding strength is stable. Therefore, 50 mΩ or less is good, and more preferably 25 mΩ or less. The results are shown in Table 1.

As shown in Table 1, in the range of the annealing atmosphere, when the annealing dew point is lower than −30 ° C., the effect of improving weldability cannot be obtained. On the other hand, if the annealing dew point exceeds 20 ° C., the pre-plating base material is overoxidized and non-plating occurs, which is not preferable because poor appearance or the like occurs.

Although the present invention has been described above with a focus on the embodiments, the present invention is not limited to the above embodiments and can be in various embodiments.

1 Base steel sheet 2 Zn-based plating film before heating 3 Thick and strong single Al oxide film on the plating surface according to the present invention 4 Thin and sparse composite Al oxide film on the plating surface under normal conditions 5 Fe-Zn after hot stamp heating Solid-dissolved phase 6 Thick and strong single Al oxide film after hot stamping under the conditions of the present invention 7 Thin Zn oxide film after hot stamping under the conditions of the present invention 8 Thin Zn, Mn, etc. after hot stamping under normal conditions Containing composite Al oxide film 9 Thick Zn oxide film after hot stamp heating under normal conditions

Claims (4)

When the outermost surface layer of hot-dip galvanizing before hot stamp heating is measured by sputtering from the surface layer with a glow discharge emission analyzer (GDS), the average Al weight concentration at a depth of 0 to 5 nm of the plating surface layer is A, 5 nm to 10 nm depth. Assuming that the average Al concentration is B, a single Al oxide having an Al concentration of 80% or more in terms of the atomic fraction of the metal element having 2.5 ≧ A / B ≧ 1.5 is obtained from the plating surface layer. A hot-dip galvanized steel sheet having excellent weldability, characterized in that the plating surface layer has a depth of 0 to 5 nm more than a depth of 5 nm to 10 nm. The hot-dip galvanized steel sheet for hot stamping according to claim 1, wherein the hot-dip galvanized steel sheet is alloyed. An atmosphere gas with a dew point of -3 ° C to 20 ° C is introduced into the annealing furnace at a steel sheet temperature in the range of at least 200 ° C to 600 ° C. A method for manufacturing a hot-dip galvanized steel sheet for hot stamping, which is characterized by having excellent weldability. The method for producing a hot-dip galvanized steel sheet having excellent weldability according to claim 3, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanized coating.

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