JP7006257B2 - A method for manufacturing a hot stamped body and a hot stamped body - Google Patents

Description

The present invention relates to a hot stamped body and a method for manufacturing a hot stamped body.

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, on the surface of a hot stamped molded body using a hot-dip zinc-plated steel plate, a Fe-Zn solid-soluble phase was formed in a 2.8 μm square field at a magnification of 40,000 times using a TEM (transmission electron microscope). The Al-based oxide film existing at the interface with the Zn oxide film was observed, the crystal structure of the Al-based oxide film was identified at 5 or more points at intervals of 200 nm per field, and the total was measured for 5 or more fields. A score of 25 or more was measured, and the score identified as γ-Al ₂ O ₃ whose crystal structure was consistent with γ-Al ₂ O ₃ and whose atomic fraction of the metal element excluding oxygen was 80% or more of Al was obtained. When G and the total number of measurement points are P, 0.20 ≦ G / P <1.00, and the Al concentration is 80% or more in terms of the atomic fraction of the metal element under the Zn oxide film. A hot stamped body having excellent weldability, which is characterized by the presence of a film.

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

Further, during annealing, the steel sheet is annealed at a steel sheet temperature in the range of ²⁰⁰ ° C. to 600 ° C. with a dew point of -30 ° C to 20 ° C. A hot-dip galvanized steel sheet is subjected to hot-dip galvanizing, and the steel sheet is blanked to the required size, heated to a range of 700 ° C to 1200 ° C, press-formed, and annealed and cooled. It is a method for manufacturing a molded body.

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 the γ-Al ₂ O ₃ ratio after hot stamping, and the welding resistance after hot stamping. It is a figure which shows the relationship between the γ-Al ₂ O ₃ ratio after hot stamping, and the annealing dew point. It is a figure which shows the relationship between the annealing dew point and the plating basis weight.

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. .. 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 in a very dense and high temperature oxidizing atmosphere to suppress the oxidation of Zn. In contrast, the composite Al oxide containing Zn, Mn, etc. is very effective as a barrier layer, but since it contains Zn, etc., it is easy for Zn, etc. to permeate and suppress the oxidation of Zn. It is considered that it does not function sufficiently as a barrier layer.

FIG. 6 shows the results of investigation of the relationship between the oxide formation state after hot stamping and the welding resistance after hot stamping. There is a strong correlation between the two. That is, among the Al-based oxides composed of the composite Al oxide containing Zn and the like and the single Al oxide containing Al alone, the single Al oxide phase containing Al alone occupies a large proportion. It was clarified that the welding resistance after hot stamping was reduced and the weldability could be improved.

Here, the method for measuring the ratio of the single Al oxide phase is as follows. Using FIB (focused ion beam method), a cross-sectional thin film sample of the outermost surface layer of the hot stamping material was prepared, and using the above TEM (transmission electron microscope), Fe- The Al-based oxide film existing at the interface between the Zn solid-soluble phase and the Zn oxide film is observed, and EDX element mapping is performed to identify the position of the Al-based oxide film. The measurement surface may be either the LD (rolling) direction or the TD (rolling direction and vertical in the plate surface) direction. After that, an electron beam is applied to the Al-based oxide film, a fraction is taken, and component analysis is performed to identify the crystal structure of the Al-based oxide. As the crystal structure of the Al-based oxide of this material, there is a possibility that a composite Al oxide such as ZnAl ₂ O ₄ may exist in addition to γ-Al ₂ O ₃ which is a single Al oxide, but per field of view. The crystal structure of the Al-based oxide was identified at 5 or more points at 200 nm intervals, and a total of 25 or more points were measured by measuring it in 5 or more fields. The crystal structure was consistent with γ-Al ₂ O ₃ and oxygen. When the score identified as γ-Al ₂ O ₃ , that is, the single Al oxide in which Al is 80% or more in the atomic fraction of the metal element excluding the above, is G, and the total measurement score is P, G / P Evaluate by ratio. The TEM observation method is not particularly specified, but in the embodiment, observation is performed with JEOL-200CX or JEM-2100 manufactured by JEOL at an acceleration voltage of 200 kV, and the phase and crystal structure are observed by structure observation, elemental analysis, electron diffraction, etc. Identified.

As shown in FIG. 6, when G / P is 0.20 or more, the effect of suppressing Zn diffusion by the single Al oxide appears, and the amount of Zn oxide film formed is suppressed, so that the welding resistance is reduced and welding is performed. Sex improves. Desirably, G / P is preferably 0.5 or more. However, when G / P becomes very close to 1.00, the effect of suppressing the Zn oxide film can be obtained on the surface of the material on which the single Al oxide is formed, but the Fe oxide film is also generated on the surface of the steel sheet. Therefore, it is desirable that the G / P is less than 1.00 because the welding resistance there increases. Therefore, the upper limit of the appropriate G / P is G / P <1.00, more preferably G / P ≦ 0.99, and even more preferably G / P ≦ 0.95.

When 0.20 ≦ G / P <1.00, for example, adjustment of atmospheric conditions in annealing is effective. As shown in FIG. 7, the annealing dew point is set to -30 ° C or higher to set the G / P to 0.20 or higher, and the annealing dew point is set to 20 ° C or lower to set the G / P to less than 1.00. It will be possible. When the annealing dew point exceeds 20 ° C., the oxidation of Fe in the steel sheet becomes remarkable, and a large amount of active Fe is generated by the subsequent reduction, and Al becomes excessively concentrated, so that the G / P becomes 1. To make the G / P 0.20 or more, it is possible if the annealing dew point is −30 ° C. or higher. When the quenching dew point is lower than -30 ° C, internal oxidation of Mn and Si on the surface of the steel plate does not occur sufficiently, and a large amount of Mn and Si remain as a metal on the surface of the steel plate. Therefore, these elements are contained during Al oxidation. It becomes easy, and it becomes difficult to form a single Al oxide.

The present inventors have only an atmosphere during annealing as a method for forming an Al-based oxide morphology after such hot stamping, and by extension, a Si or Mn-deficient layer on the surface of the steel sheet before plating, which is the basis thereof. However, it was clarified that the temperature during heating and the rate of temperature rise are particularly important, and that it is effective even if the amount of hot-dip galvanizing is changed.

That is, if the H ₂ concentration in the annealed atmosphere is 1 to 15% in terms of volume fraction, the temperature of the steel sheet during annealing is in the range of 200 ° C to 600 ° C, and the dew point is in the range of -30 ° C to 20 ° C as shown in FIG. For example, when the amount of plating is 45 g / m ² or more, the welding resistance is reduced and the weldability is good. In the plot in FIG. 8, the region where the weld resistance exceeds 50 mΩ is defined as the region where the weld resistance is significantly deteriorated, and the region where the weld resistance is 30 to 50 mΩ within the range where the weldability is acceptable is Δ. The region where the welding resistance is 10 to 30 mΩ is plotted as a desirable condition for further improving the weldability, and the region where the weld resistance is 10 mΩ or less is plotted as a desirable condition for further improving the weldability.

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 above 600 ° C., 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.

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 N ₂ or a gas containing 1 to 15% H ₂ in N ₂ with steam, and introducing the gas into the annealing furnace to control the dew point.

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 / m ² to 120 g / m ² , which is the adhesion amount 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: 0.211% (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, according to the conditions in Table 1. It was annealed and plated. At that time, after various changes in the atmosphere at the time of annealing, plating was performed. The value of the annealing dew point is such that the temperature of the steel sheet during annealing is between 200 ° C and 600 ° C. 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 + 20 ° C. or higher. 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 stamps with a built-in water-cooled pipe 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.

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 by taking an embodiment as an example, the present invention is not limited to the above embodiment and can be various modes.

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)

On the surface of a hot stamped molded body using a hot-dip zinc-plated steel plate, a TEM (transmissive electron microscope) was used at a magnification of 40,000 times and a 2.8 μm square field of view at the interface between the Fe-Zn solid-soluble phase and the Zn oxide film. The existing Al-based oxide film was observed, the crystal structure of the Al-based oxide film was identified at 5 points or more at intervals of 200 nm per field, and the crystal structure of the Al-based oxide film was measured at 5 points or more, and a total of 25 points or more were measured. The score identified for γ-Al ₂ O ₃ whose crystal structure matches γ-Al ₂ O ₃ and whose atomic fraction of the metal element excluding oxygen is 80% or more of Al is G, and the total number of measured points is P. When A hot stamped body with excellent weldability. The hot stamp molded body having excellent weldability according to claim 1, wherein the hot-dip galvanized steel sheet is alloyed. During annealing, the steel sheet is annealed at a steel sheet temperature in the range of 200 ° C to 600 ° C with a dew point of -2 ° C to 20 ° C, then hot-dip galvanized with a grain size of 30 g / m ² or more, and tempered and rolled. A hot-dip galvanized steel sheet with excellent weldability, which is characterized by blanking the steel sheet to a required size, heating it to a range of 700 ° C to 1200 ° C, press-molding it, and annealing and cooling it. Manufacturing method. The method for producing a hot stamped molded product having excellent weldability according to claim 3, wherein the alloying treatment is performed after the hot-dip galvanizing.

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