JP2019116655A - Hot stamp molding and method for manufacturing the same - Google Patents

Abstract

To provide a technique capable of dramatically improving weldability after heating almost without increasing cost in a hot stamp zinc based steel sheet.SOLUTION: In the hot stamp molding excellent in weldability, a crystal structure obtained by: observing an Al based oxide film existing on an interface between an Fe-Zn solid solution phase and a Zn oxide film in a visual field of a 40000 magnification and a 2.8 μm square using a transmission electron microscope on the surface of the hot stamp molding using a hot-dip galvanized steel sheet; identifying the crystal structure of the Al based oxide film at five or more points at an interval of 200 nm per one visual field; and measuring the crystal structure at 25 points or more in total measured in five or more visual fields is coincident with γ-AlO. When the number of points identified as γ-AlOhaving Al of 80% or more at atomic fractions of metal elements except oxygen is G, and the total number of measured points is P, 0.20≤G/P<1.00 is satisfied, and the independent Al oxide film having an Al concentration of 80% or more at the atomic fractions of the metal elements exists under the Zn oxide film.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a hot stamped molded body and a method of manufacturing the hot stamped molded body.

  Various kinds of automobile parts constituting the car body of the automobile are required to improve various performances and characteristics 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 sites except for. Therefore, a method of manufacturing a hot stamp member by forming a hardened high strength steel hot stamp (represented also as a hot press in some documents) only on a portion corresponding to a specific portion in the automobile part requiring reinforcement is partially adopted. There is.

  Under the present circumstances, if the cold-rolled steel plate which is not surface-treated is used, the oxidation scale of iron will generate | occur | produce on a steel plate surface during heating. This iron oxide scale peels off during molding and wears the mold, or becomes a habit of the steel plate itself, and if it remains on the surface of the steel plate after molding, it causes welding defects in the later welding process, or the coating process May cause adhesion failure of 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, thereby suppressing the oxidation of iron and significantly improving weldability and paintability.

  However, although the weldability is improved compared to the iron oxide scale, the zinc oxide film has lower electric conductivity than the unheated cold-rolled steel plate or plated steel plate having no oxide film on the surface, so the weldability is improved. descend. Therefore, even with a hot stamp material using a zinc-based plated steel sheet, the weldability is not sufficient, and it is required to improve the weldability even by a small amount.

  Several proposals have been made for improvement measures, and in Patent Document 2, it is proposed that welding resistance be reduced and weldability be improved by removing the zinc oxide film that causes welding resistance by heating after cleaning. ing. In this method, the weldability is improved, but after heating, a cleaning process at a car maker or parts maker is required, resulting in an increase in cost. Further, in Patent Document 3, improvement of weldability is achieved by specifying the steel plate components, the Mn concentration of the steel plate surface, the plating adhesion amount, the Al amount in the plating film, the Al concentration in the plating film, the metal structure of the steel plate surface, etc. I'm trying. Even with this method, some improvement can be expected, but the dramatic improvement effect is difficult.

Patent No. 4039548 Patent No. 5880321 Patent No. 5720856 gazette

  Thus, in the prior art, there has been no technology for dramatically improving the weldability of a zinc-based hot stamping steel sheet.

  The present invention is an invention made under such background, and an object of the present invention is to provide a technique for dramatically improving the weldability after heating with little increase in cost in a steel sheet for zinc-based hot stamping It is.

In order to solve the above problems, on the surface of a hot stamped steel sheet on which a hot-dip galvanized steel sheet is used, an Fe-Zn solid solution phase with a magnification of 40000 times and a 2.8 μm square field of view using TEM (transmission electron microscope) The Al-based oxide film present at the interface with the Zn oxide film is observed, and the crystal structure of the Al-based oxide film is identified at five or more points at an interval of 200 nm per field of view. measured over 25 points, consistent crystal structure with _gamma-Al 2 _{O 3,} a score Al was identified in a 80% _gamma-Al 2 _{O 3} in atomic percent of metallic elements except for oxygen G, when the total number of measurement points is P, it is 0.20 ≦ G / P <1.00 and a single Al oxide having an Al concentration of 80% or more in atomic fraction of metal element under Zn oxide film Excellent weldability characterized by the presence of a coating It was a hot stamping body.

  Moreover, it is preferable that the hot-dip galvanized steel sheet is configured to be subjected to an alloying treatment.

In addition, annealing is performed with a steel sheet temperature in the range of 200 ° C. to 600 ° C. with a dew point of −30 ° C. to 20 ° C., followed by hot dip galvanization with a coating weight of 30 g / m ² or more, and temper rolling Hot stamp with excellent weldability characterized by applying hot-dip galvanized steel sheet, blanking the steel sheet to required size, heating to a range of 700 ° C to 1200 ° C, press forming and quenching cooling It is set as the manufacturing method of a molded object.

  Moreover, it is preferable to perform an alloying process after performing the said hot dip galvanization in the said manufacturing method.

  According to the present invention, it is possible to provide a technique for dramatically improving the weldability after heating, with little increase in cost, in a zinc-based hot stamping steel sheet.

It is a figure showing the relationship between the annealing dew point in the first half of the annealing process of the steel plate before plating, and the welding resistance after a hot stamp. It is a schematic diagram of the plating film cross-section structure before the hot stamp heating of conditions of this invention. It is a schematic diagram of the film cross-section after hot-stamping the plating of the conditions of the present invention shown in FIG. It is a schematic diagram of the plating film cross-section structure before the hot stamp heating of normal conditions. It is a schematic diagram of the film cross-section after hot-stamping the plating of the normal conditions shown in FIG. Is a diagram showing the relationship between the welding resistance of the hot gamma-Al ₂ O ₃ ratio after stamping and after hot stamping. Is a diagram representing the relationship between γ-Al ₂ O ₃ ratio after hot stamping and annealing dew point. It is a figure showing the relationship of an annealing dew point and a plating weight.

  The inventors of the present invention show in detail the relationship between the state of the steel plate before annealing plating, the annealing conditions before plating, the plating conditions, etc. and the weldability (welding resistance) after hot stamping, as a result of investigation in detail. As described above, it was found that by setting the atmosphere of annealing in the first half of the annealing process of the steel plate before plating, in particular, the dew point as the micro oxidation condition, the welding resistance after hot stamping is significantly reduced and the weldability is greatly improved.

Although the detailed mechanism by which the weldability is improved by the annealing in which the dew point is set to the slight oxidation atmosphere is unknown as described above, if the annealing atmosphere before plating is controlled as described later, It is considered that the state of concentration of oxides such as Si and Mn and the metal element changes, and the state of the Fe-Al-concentrated layer formed at the initial stage of plating changes. Under the influence of that, Al that is oxidized at the time of subsequent hot stamping becomes complex oxide with Si and Mn under the normal conditions, but under the micro oxidation conditions of the present invention, the Al concentration is the atomic fraction of the metal element It becomes a single Al oxide called γ-Al ₂ O ₃ which becomes 80% or more, and is presumed to contribute to improvement of weldability.

The details are as follows. Examples of oxides containing Al include γ-Al ₂ O _{3 in} which the Al concentration is 80 or more in the atomic fraction of metal elements, and complex oxides containing Zn, Mn, Fe, etc. in addition to Al. . In the present invention, hereinafter, an oxide having an Al concentration of 80% or more as an atomic fraction of a metal element is a single Al oxide (γ-Al ₂ O ₃ , γ-alumina: crystal structure is also called Al ₂ O ₃ ) And Al, as well as a large amount of Zn, Mn, etc., with an atomic fraction of metal element and an oxide having an Al concentration of less than 80% (a crystal structure such as ZnAl ₂ O ₄ or MnAl ₂ O ₄ (Zn , Mn) Al ₂ O _4, etc., a single Al oxide and a composite Al oxide are collectively referred to as an Al-based oxide. In addition, in order to be influenced by surrounding elements as analysis accuracy, if Al is an atomic fraction and Al concentration is 80% or more among metal components excluding oxygen, single Al oxide, if less than 80%, complex Al oxide It can be regarded as a thing.

  2 compares the schematic view of the cross-sectional structure of the plated film before hot stamp heating under the conditions of the present invention, FIG. 3 compares the schematic view of the cross section of the film after hot stamping the plating under the conditions of the present invention, and FIG. FIG. 5 is a schematic view of the cross-sectional structure of the plated film before the hot stamp heating under the normal conditions, and FIG. 5 is a schematic view of the cross-sectional structure of the coating after the hot stamping of the plating under the normal conditions. The numbers in the figure are respectively 1: base steel plate 2: Zn-based plating film before heating 3: thick thick solid single Al oxide film of the plating surface according to the invention 4: thin and sparse plating surface under normal conditions Composite Al oxide film, 5: Fe-Zn solid solution phase after hot stamp heating, 6: thick solid single Al oxide film after hot stamp heating under the conditions of the present invention, 7: after hot stamp heating under the conditions of the present invention Thin Zn oxide film, 8: thin complex Al oxide film containing Zn, Mn, etc. after hot stamp heating under ordinary conditions, 9: thick Zn oxide film after hot stamp heating under ordinary conditions.

  The Zn-based oxides (7 in FIG. 3 and 9 in FIG. 5), which are formed on the outermost surface after hot stamping and serve as welding resistance as shown in the schematic views in FIGS. Any of these single Al oxide films (3 in FIG. 2) or composite Al oxide films (4 in FIG. 4) made of Al and Zn or Mn formed at the time of stamping are all plated through Al-based oxides. Since Zn (2 in the figure) is formed by diffusion toward the plating surface, the formation amount of Zn oxide differs depending on the formation state of the Al-based oxide film, and the amount of Zn oxide is small. It is thought that the weldability improves. That is, since the single Al oxide film does not contain other elements, other metallic elements such as Zn diffuse and permeate through the single Al oxide film in a very dense and high temperature oxidizing atmosphere to suppress the oxidation of Zn. While the effect is very large as a barrier layer, in the case of a composite Al oxide containing Zn, Mn, etc., since Zn etc. is contained in itself, Zn etc. is easily transmitted and the oxidation of Zn is suppressed. It is considered that it does not function sufficiently as a barrier layer.

  The investigation result of the relationship between the formation state of the oxide after hot stamping and the welding resistance after hot stamping is shown in FIG. There is a strong correlation between the two. That is, a ratio of a single Al oxide phase containing Al alone among the Al-based oxides composed of a single Al oxide containing Zn and the like alone and a composite Al oxide containing Zn etc. is large It became clear that welding resistance after hot stamping decreased and weldability could be improved.

Here, the method of measuring the ratio of a single Al oxide phase is as follows. A cross-sectional thin film sample of the outermost surface layer of the hot stamping material is prepared using FIB (focused ion beam method), and Fe— with a field of view of 2.8 μm square with a magnification of 40000 × using the above TEM (transmission electron microscope). The Al-based oxide film present at the interface between the Zn solid solution phase and the Zn oxide film is observed, EDX element mapping is performed, and the position of the Al-based oxide film is specified. The measurement surface may be either the LD (rolling) direction or the TD (rolling direction and in-plane vertical) direction. Thereafter, an electron beam is applied to the Al-based oxide film, and the di-fraction is taken to analyze the component to identify the crystal structure of the Al-based oxide. As the crystal structure of the Al-based oxide of this material, there may be a complex Al oxide such as ZnAl ₂ O ₄ other than γ-Al ₂ O ₃ which is a single Al oxide, The crystal structure of the Al-based oxide is identified at five points or more at 200 nm intervals, and a total of 25 points or more measured in five or more fields of view are measured, and the crystal structure matches γ-Al ₂ O ₃ and oxygen The atomic fraction of the metal element excluding Al is 80% or more, that is, γ-Al ₂ O ₃ , that is, the point identified as a single Al oxide is G and the total measurement point is P, G / P Evaluate by ratio. The TEM observation method is not particularly defined, but in the embodiment, observation is performed with an acceleration voltage of 200 kV with JEM-200 CX or JEM-2100 manufactured by JEOL, and phase or crystal structure is observed by structure observation, elemental analysis, electron beam diffraction, etc. Identified.

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

  For the above 0.20 ≦ G / P <1.00, for example, adjustment of the atmosphere conditions in annealing is effective. As shown in FIG. 7, in order to set G / P to 0.20 or more, the annealing dew point is set to -30.degree. C. or more, and to set G / P to less than 1.00, the annealing dew point is set to 20.degree. It becomes possible. When the annealing dew point is over 20 ° C., the oxidation of Fe in the steel sheet becomes remarkable, and a large amount of active Fe is generated in the subsequent reduction to excessively concentrate Al, so G / P becomes 1. In order to make G / P 0.20 or more, it will become possible if an annealing dew point shall be -30 ° C or more. When the annealing dew point is lower than -30 ° C., internal oxidation of Mn and Si does not occur sufficiently on the steel sheet surface, so many Mn and Si remain as metal on the steel sheet surface, and these elements are contained during Al oxidation It becomes easy and it becomes difficult to form a single Al oxide.

  The present inventors further set only the atmosphere during annealing as a method for forming an Al-based oxide form after such hot stamping and, consequently, a Si or Mn depleted layer on the steel plate surface before the original plating. In addition, the temperature and heating rate during heating were particularly important, and it was clarified that changing the coating amount of hot-dip galvanization is effective.

That is, if the H ₂ concentration in the annealing atmosphere is in the range of 200 ° C. to 600 ° C. in the range of 200 ° C. to 600 ° C., the dew point is in the range of −30 ° C. to 20 ° C. as shown in FIG. For example, when the coating weight is 45 g / m ² or more, the welding resistance is reduced and the weldability is good. In the plot in FIG. 8, a region where the welding resistance exceeds 50 mΩ as a region having high welding resistance where the weldability is significantly deteriorated is x, and a region where the welding resistance is 30 to 50 mΩ in a region where the weldability passes is Δ. An area where the welding resistance is 10 to 30 mΩ is plotted as ○ as a desirable condition that can further improve the weldability, and an area where the welding resistance is 10 mΩ or less is plotted as ● as a desirable condition that can improve the weldability.

  The steel plate temperature is set as described above because, when the temperature of the steel plate is lower than 200 ° C., the temperature is too low and Si and Mn in the steel hardly diffuse, so the Si and Mn deficiency layer on the steel plate surface Can not be formed, and diffusion occurs sufficiently at temperatures higher than 600 ° C, but the annealing time including the subsequent holding time becomes short, and a sufficient time for sufficient formation of a Si or Mn depleted layer is obtained. It is because there is not. Thus, the steel sheet temperature is most preferably in the range of 200 ° C. to 600 ° C. from the viewpoint of the diffusion rate of Si and Mn and the annealing time, and more preferably in the range of 300 ° C. to 500 ° C.

As for the atmosphere, most of the atmosphere except H ₂ is N ₂ gas, and the others are unavoidable impurities. If the H ₂ concentration is 1% or less, the reduction of the steel plate becomes insufficient, which causes non-plating and the like. On the other hand, if it is 15% or more, the consumption of H ₂ during annealing increases and there is a problem in cost. Therefore, the H ₂ concentration is preferably 1 to 15%, more preferably 2 to 12%. As a dew point at the time of annealing, if it is less than -30 ° C, Si and Mn in steel can not be oxidized enough, and it can not form a lack layer of Si and Mn. Further, if the temperature is higher than + 20 ° C., it is not preferable because oxidation of Fe in the steel sheet occurs to cause non-plating and the like. Accordingly, the annealing dew point is preferably -30 ° C to 20 ° C, more preferably -20 ° C to 10 ° C, and still more preferably -15 to 0 ° C. In order to adjust the dew point, a gas containing N ₂ or N ₂ containing 1 to 15% of H ₂ is humidified with water vapor, and the gas is introduced into the annealing furnace for control.

  The conditions of the 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, radiation heating, induction heating, electric current heating, etc. are used to 700 ° C to 1200 ° C. After heating to the range, it is preferable to press-mold and quench-quench.

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 mainly containing zinc, but galvanized steel sheet (abbreviated GI), alloyed galvanized steel sheet (abbreviated GA), molten steel It includes zinc-aluminum alloy plating, hot-dip zinc-aluminium-silicon alloy plating, hot-dip zinc-aluminium-magnesium alloy plating and the like, and the detailed elemental composition of each is not particularly limited. Not limited particularly also the basis weight of the plating, but the adhesion amount of common hot stamping plating, can be used up to 30g / m ² ~120g / m ² approximately. Although the material of the steel plate is not particularly limited, a hardened high-strength steel used as a hot stamp application is generally used.

The steel composition is C: 0.211% (mass%, the same below), Si: 0.033%, Mn: 1.2%, P: 0.01%, S: 0.007 at a thickness of 1.0 mm. %, Cr: 0.2%, Ti: 0.02%, B: 0.003%, balance Fe and galvanized steel sheet and hot-dip galvanized steel sheet consisting of Fe and impurities according to the conditions shown in Table 1 Annealing and plating. At that time, plating was performed after changing the atmosphere at the time of annealing variously. In addition, the value of the annealing dew point is a hot plate temperature between 200 degreeC-600 degreeC during annealing. The coating weight of plating was 50/50 g / m ^{2 on the} front / back side. In the case of annealing conditions, for example, when the dew point is + 20 ° C. or more, there was a steel sheet after plating in which non-plating was observed. The non-plating was a place where the wettability of the plating deteriorated when the oxidation of the surface of the base material before plating progressed too much, and the plating repelled and was not attached, and the generated one was regarded as defective.

  After cutting out the steel plate after plating to a size of 100 mm square, heat the electric furnace in the atmosphere to 900 ° C., heat it for 4 minutes in it and take it out, and rapidly quench it by sandwiching it in a flat plate with water cooling piping. I got a hot stamp high strength material. Welding resistance was measured as an indicator of weldability. The welding resistance was measured under the following conditions: 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. However, if the welding resistance is 50 mΩ or less, the generation of dust during welding is almost eliminated and the welding strength is also stable. Therefore, 50 mΩ or less is preferable, more preferably 25 mΩ or less.

  As shown in Table 1, in the range of the annealing atmosphere, the effect of improving weldability can not be obtained when the annealing dew point is lower than -30 ° C. On the other hand, when the annealing dew point is higher than 20 ° C., the base material before plating is oxidized too much to cause non-plating, which is not preferable because appearance defects occur.

  Although the present invention has been described above by taking the embodiment as an example, the present invention is not limited to the above embodiment, and various aspects can be made.

1 base metal steel plate 2 Zn-based plating film before heating 3 thick solid single Al oxide film on plating surface according to the present invention 4 thin and sparse composite Al oxide film on plating surface under ordinary conditions 5 Fe-Zn after hot stamp heating Solid solution phase 6 thick solid single Al oxide film 7 after hot stamp heating under the conditions of the present invention thin Zn oxide film 8 after hot stamp heating under the conditions of the present invention 8 thin Zn, Mn, etc. after hot stamp heating under normal conditions Composite Al oxide film 9 containing thick Zn oxide film after hot stamp heating under normal conditions

Claims (4)

On the surface of a hot stamped steel sheet on which hot-dip galvanized steel sheet is used, the interface between the Fe-Zn solid phase and the Zn oxide film in a visual field of 2.8 μm square with 40000 magnification using TEM (transmission electron microscope) The existing Al-based oxide film is observed, the crystal structure of the Al-based oxide film is identified at five or more points at 200 nm intervals in one field of view, and a total of 25 or more points measured in five or more fields of view are measured. G is the number of points identified in γ-Al ₂ O ₃ that have a crystal structure that matches γ-Al ₂ O ₃ and the atomic fraction of metal elements excluding oxygen is at least 80%, and the total number of measurement points is P When it is set, it is characterized that 0.20 ≦ G / P <1.00, and there is a single Al oxide film having an Al concentration of 80% or more in the atomic fraction of the metal element under the Zn oxide film. Hot-stamped compact with excellent weldability   The hot-stamped steel excellent in weldability according to claim 1, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment. Annealing is performed at a steel sheet temperature of 200 ° C. to 600 ° C. with a dew point of −30 ° C. to 20 ° C., and then hot dip galvanization with a coating weight of 30 g / m ² or more is performed and temper rolling is performed Hot-stamped steel excellent in weldability characterized by hot-dip galvanized steel sheet, blanking the steel sheet to a required size, heating to a range of 700 ° C. to 1200 ° C., press forming and quenching cooling. Manufacturing method.   The method for producing a hot-stamped steel excellent in weldability according to claim 3, characterized in that an alloying treatment is carried out after the hot-dip galvanizing.